3 @setfilename coreutils.info
4 @settitle GNU Coreutils
9 @include constants.texi
11 @c Define new indices.
15 @c Put everything in one index (arbitrarily chosen to be the concept index).
25 * Coreutils: (coreutils). Core GNU (file, text, shell) utilities.
26 * Common options: (coreutils)Common options.
27 * File permissions: (coreutils)File permissions. Access modes.
28 * Date input formats: (coreutils)Date input formats.
31 @c FIXME: the following need documentation
32 @c * [: (coreutils)[ invocation. File/string tests.
33 @c * pinky: (coreutils)pinky invocation. FIXME.
35 @dircategory Individual utilities
37 * arch: (coreutils)arch invocation. Print machine hardware name.
38 * base64: (coreutils)base64 invocation. Base64 encode/decode data.
39 * basename: (coreutils)basename invocation. Strip directory and suffix.
40 * cat: (coreutils)cat invocation. Concatenate and write files.
41 * chcon: (coreutils)chcon invocation. Change SELinux CTX of files.
42 * chgrp: (coreutils)chgrp invocation. Change file groups.
43 * chmod: (coreutils)chmod invocation. Change access permissions.
44 * chown: (coreutils)chown invocation. Change file owners and groups.
45 * chroot: (coreutils)chroot invocation. Specify the root directory.
46 * cksum: (coreutils)cksum invocation. Print POSIX CRC checksum.
47 * comm: (coreutils)comm invocation. Compare sorted files by line.
48 * cp: (coreutils)cp invocation. Copy files.
49 * csplit: (coreutils)csplit invocation. Split by context.
50 * cut: (coreutils)cut invocation. Print selected parts of lines.
51 * date: (coreutils)date invocation. Print/set system date and time.
52 * dd: (coreutils)dd invocation. Copy and convert a file.
53 * df: (coreutils)df invocation. Report file system disk usage.
54 * dir: (coreutils)dir invocation. List directories briefly.
55 * dircolors: (coreutils)dircolors invocation. Color setup for ls.
56 * dirname: (coreutils)dirname invocation. Strip last file name component.
57 * du: (coreutils)du invocation. Report on disk usage.
58 * echo: (coreutils)echo invocation. Print a line of text.
59 * env: (coreutils)env invocation. Modify the environment.
60 * expand: (coreutils)expand invocation. Convert tabs to spaces.
61 * expr: (coreutils)expr invocation. Evaluate expressions.
62 * factor: (coreutils)factor invocation. Print prime factors
63 * false: (coreutils)false invocation. Do nothing, unsuccessfully.
64 * fmt: (coreutils)fmt invocation. Reformat paragraph text.
65 * fold: (coreutils)fold invocation. Wrap long input lines.
66 * groups: (coreutils)groups invocation. Print group names a user is in.
67 * head: (coreutils)head invocation. Output the first part of files.
68 * hostid: (coreutils)hostid invocation. Print numeric host identifier.
69 * hostname: (coreutils)hostname invocation. Print or set system name.
70 * id: (coreutils)id invocation. Print user identity.
71 * install: (coreutils)install invocation. Copy and change attributes.
72 * join: (coreutils)join invocation. Join lines on a common field.
73 * kill: (coreutils)kill invocation. Send a signal to processes.
74 * link: (coreutils)link invocation. Make hard links between files.
75 * ln: (coreutils)ln invocation. Make links between files.
76 * logname: (coreutils)logname invocation. Print current login name.
77 * ls: (coreutils)ls invocation. List directory contents.
78 * md5sum: (coreutils)md5sum invocation. Print or check MD5 digests.
79 * mkdir: (coreutils)mkdir invocation. Create directories.
80 * mkfifo: (coreutils)mkfifo invocation. Create FIFOs (named pipes).
81 * mknod: (coreutils)mknod invocation. Create special files.
82 * mktemp: (coreutils)mktemp invocation. Create temporary files.
83 * mv: (coreutils)mv invocation. Rename files.
84 * nice: (coreutils)nice invocation. Modify niceness.
85 * nl: (coreutils)nl invocation. Number lines and write files.
86 * nohup: (coreutils)nohup invocation. Immunize to hangups.
87 * nproc: (coreutils)nproc invocation. Print the number of processors.
88 * numfmt: (coreutils)numfmt invocation. Reformat numbers.
89 * od: (coreutils)od invocation. Dump files in octal, etc.
90 * paste: (coreutils)paste invocation. Merge lines of files.
91 * pathchk: (coreutils)pathchk invocation. Check file name portability.
92 * pr: (coreutils)pr invocation. Paginate or columnate files.
93 * printenv: (coreutils)printenv invocation. Print environment variables.
94 * printf: (coreutils)printf invocation. Format and print data.
95 * ptx: (coreutils)ptx invocation. Produce permuted indexes.
96 * pwd: (coreutils)pwd invocation. Print working directory.
97 * readlink: (coreutils)readlink invocation. Print referent of a symlink.
98 * realpath: (coreutils)readpath invocation. Print resolved file names.
99 * rm: (coreutils)rm invocation. Remove files.
100 * rmdir: (coreutils)rmdir invocation. Remove empty directories.
101 * runcon: (coreutils)runcon invocation. Run in specified SELinux CTX.
102 * seq: (coreutils)seq invocation. Print numeric sequences
103 * sha1sum: (coreutils)sha1sum invocation. Print or check SHA-1 digests.
104 * sha2: (coreutils)sha2 utilities. Print or check SHA-2 digests.
105 * shred: (coreutils)shred invocation. Remove files more securely.
106 * shuf: (coreutils)shuf invocation. Shuffling text files.
107 * sleep: (coreutils)sleep invocation. Delay for a specified time.
108 * sort: (coreutils)sort invocation. Sort text files.
109 * split: (coreutils)split invocation. Split into pieces.
110 * stat: (coreutils)stat invocation. Report file(system) status.
111 * stdbuf: (coreutils)stdbuf invocation. Modify stdio buffering.
112 * stty: (coreutils)stty invocation. Print/change terminal settings.
113 * sum: (coreutils)sum invocation. Print traditional checksum.
114 * sync: (coreutils)sync invocation. Synchronize memory and disk.
115 * tac: (coreutils)tac invocation. Reverse files.
116 * tail: (coreutils)tail invocation. Output the last part of files.
117 * tee: (coreutils)tee invocation. Redirect to multiple files.
118 * test: (coreutils)test invocation. File/string tests.
119 * timeout: (coreutils)timeout invocation. Run with time limit.
120 * touch: (coreutils)touch invocation. Change file timestamps.
121 * tr: (coreutils)tr invocation. Translate characters.
122 * true: (coreutils)true invocation. Do nothing, successfully.
123 * truncate: (coreutils)truncate invocation. Shrink/extend size of a file.
124 * tsort: (coreutils)tsort invocation. Topological sort.
125 * tty: (coreutils)tty invocation. Print terminal name.
126 * uname: (coreutils)uname invocation. Print system information.
127 * unexpand: (coreutils)unexpand invocation. Convert spaces to tabs.
128 * uniq: (coreutils)uniq invocation. Uniquify files.
129 * unlink: (coreutils)unlink invocation. Removal via unlink(2).
130 * uptime: (coreutils)uptime invocation. Print uptime and load.
131 * users: (coreutils)users invocation. Print current user names.
132 * vdir: (coreutils)vdir invocation. List directories verbosely.
133 * wc: (coreutils)wc invocation. Line, word, and byte counts.
134 * who: (coreutils)who invocation. Print who is logged in.
135 * whoami: (coreutils)whoami invocation. Print effective user ID.
136 * yes: (coreutils)yes invocation. Print a string indefinitely.
140 This manual documents version @value{VERSION} of the GNU core
141 utilities, including the standard programs for text and file manipulation.
143 Copyright @copyright{} 1994-2013 Free Software Foundation, Inc.
146 Permission is granted to copy, distribute and/or modify this document
147 under the terms of the GNU Free Documentation License, Version 1.3 or
148 any later version published by the Free Software Foundation; with no
149 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
150 Texts. A copy of the license is included in the section entitled ``GNU
151 Free Documentation License''.
156 @title GNU @code{Coreutils}
157 @subtitle Core GNU utilities
158 @subtitle for version @value{VERSION}, @value{UPDATED}
159 @author David MacKenzie et al.
162 @vskip 0pt plus 1filll
175 @cindex core utilities
176 @cindex text utilities
177 @cindex shell utilities
178 @cindex file utilities
181 * Introduction:: Caveats, overview, and authors
182 * Common options:: Common options
183 * Output of entire files:: cat tac nl od base64
184 * Formatting file contents:: fmt pr fold
185 * Output of parts of files:: head tail split csplit
186 * Summarizing files:: wc sum cksum md5sum sha1sum sha2
187 * Operating on sorted files:: sort shuf uniq comm ptx tsort
188 * Operating on fields:: cut paste join
189 * Operating on characters:: tr expand unexpand
190 * Directory listing:: ls dir vdir dircolors
191 * Basic operations:: cp dd install mv rm shred
192 * Special file types:: mkdir rmdir unlink mkfifo mknod ln link readlink
193 * Changing file attributes:: chgrp chmod chown touch
194 * Disk usage:: df du stat sync truncate
195 * Printing text:: echo printf yes
196 * Conditions:: false true test expr
198 * File name manipulation:: dirname basename pathchk mktemp realpath
199 * Working context:: pwd stty printenv tty
200 * User information:: id logname whoami groups users who
201 * System context:: date arch nproc uname hostname hostid uptime
202 * SELinux context:: chcon runcon
203 * Modified command invocation:: chroot env nice nohup stdbuf timeout
204 * Process control:: kill
206 * Numeric operations:: factor seq
207 * File permissions:: Access modes
208 * Date input formats:: Specifying date strings
209 * Opening the software toolbox:: The software tools philosophy
210 * GNU Free Documentation License:: Copying and sharing this manual
211 * Concept index:: General index
214 --- The Detailed Node Listing ---
218 * Exit status:: Indicating program success or failure
219 * Backup options:: Backup options
220 * Block size:: Block size
221 * Floating point:: Floating point number representation
222 * Signal specifications:: Specifying signals
223 * Disambiguating names and IDs:: chgrp and chown owner and group syntax
224 * Random sources:: Sources of random data
225 * Target directory:: Target directory
226 * Trailing slashes:: Trailing slashes
227 * Traversing symlinks:: Traversing symlinks to directories
228 * Treating / specially:: Treating / specially
229 * Standards conformance:: Standards conformance
231 Output of entire files
233 * cat invocation:: Concatenate and write files
234 * tac invocation:: Concatenate and write files in reverse
235 * nl invocation:: Number lines and write files
236 * od invocation:: Write files in octal or other formats
237 * base64 invocation:: Transform data into printable data
239 Formatting file contents
241 * fmt invocation:: Reformat paragraph text
242 * numfmt invocation:: Reformat numbers
243 * pr invocation:: Paginate or columnate files for printing
244 * fold invocation:: Wrap input lines to fit in specified width
246 @command{numfmt}: General Options, Units
248 Output of parts of files
250 * head invocation:: Output the first part of files
251 * tail invocation:: Output the last part of files
252 * split invocation:: Split a file into fixed-size pieces
253 * csplit invocation:: Split a file into context-determined pieces
257 * wc invocation:: Print newline, word, and byte counts
258 * sum invocation:: Print checksum and block counts
259 * cksum invocation:: Print CRC checksum and byte counts
260 * md5sum invocation:: Print or check MD5 digests
261 * sha1sum invocation:: Print or check SHA-1 digests
262 * sha2 utilities:: Print or check SHA-2 digests
264 Operating on sorted files
266 * sort invocation:: Sort text files
267 * shuf invocation:: Shuffle text files
268 * uniq invocation:: Uniquify files
269 * comm invocation:: Compare two sorted files line by line
270 * ptx invocation:: Produce a permuted index of file contents
271 * tsort invocation:: Topological sort
273 @command{ptx}: Produce permuted indexes
275 * General options in ptx:: Options which affect general program behavior
276 * Charset selection in ptx:: Underlying character set considerations
277 * Input processing in ptx:: Input fields, contexts, and keyword selection
278 * Output formatting in ptx:: Types of output format, and sizing the fields
279 * Compatibility in ptx:: The GNU extensions to @command{ptx}
283 * cut invocation:: Print selected parts of lines
284 * paste invocation:: Merge lines of files
285 * join invocation:: Join lines on a common field
287 Operating on characters
289 * tr invocation:: Translate, squeeze, and/or delete characters
290 * expand invocation:: Convert tabs to spaces
291 * unexpand invocation:: Convert spaces to tabs
293 @command{tr}: Translate, squeeze, and/or delete characters
295 * Character sets:: Specifying sets of characters
296 * Translating:: Changing one set of characters to another
297 * Squeezing:: Squeezing repeats and deleting
301 * ls invocation:: List directory contents
302 * dir invocation:: Briefly list directory contents
303 * vdir invocation:: Verbosely list directory contents
304 * dircolors invocation:: Color setup for @command{ls}
306 @command{ls}: List directory contents
308 * Which files are listed:: Which files are listed
309 * What information is listed:: What information is listed
310 * Sorting the output:: Sorting the output
311 * Details about version sort:: More details about version sort
312 * General output formatting:: General output formatting
313 * Formatting the file names:: Formatting the file names
317 * cp invocation:: Copy files and directories
318 * dd invocation:: Convert and copy a file
319 * install invocation:: Copy files and set attributes
320 * mv invocation:: Move (rename) files
321 * rm invocation:: Remove files or directories
322 * shred invocation:: Remove files more securely
326 * link invocation:: Make a hard link via the link syscall
327 * ln invocation:: Make links between files
328 * mkdir invocation:: Make directories
329 * mkfifo invocation:: Make FIFOs (named pipes)
330 * mknod invocation:: Make block or character special files
331 * readlink invocation:: Print value of a symlink or canonical file name
332 * rmdir invocation:: Remove empty directories
333 * unlink invocation:: Remove files via unlink syscall
335 Changing file attributes
337 * chown invocation:: Change file owner and group
338 * chgrp invocation:: Change group ownership
339 * chmod invocation:: Change access permissions
340 * touch invocation:: Change file timestamps
344 * df invocation:: Report file system disk space usage
345 * du invocation:: Estimate file space usage
346 * stat invocation:: Report file or file system status
347 * sync invocation:: Synchronize data on disk with memory
348 * truncate invocation:: Shrink or extend the size of a file
352 * echo invocation:: Print a line of text
353 * printf invocation:: Format and print data
354 * yes invocation:: Print a string until interrupted
358 * false invocation:: Do nothing, unsuccessfully
359 * true invocation:: Do nothing, successfully
360 * test invocation:: Check file types and compare values
361 * expr invocation:: Evaluate expressions
363 @command{test}: Check file types and compare values
365 * File type tests:: File type tests
366 * Access permission tests:: Access permission tests
367 * File characteristic tests:: File characteristic tests
368 * String tests:: String tests
369 * Numeric tests:: Numeric tests
371 @command{expr}: Evaluate expression
373 * String expressions:: + : match substr index length
374 * Numeric expressions:: + - * / %
375 * Relations for expr:: | & < <= = == != >= >
376 * Examples of expr:: Examples of using @command{expr}
380 * tee invocation:: Redirect output to multiple files or processes
382 File name manipulation
384 * basename invocation:: Strip directory and suffix from a file name
385 * dirname invocation:: Strip last file name component
386 * pathchk invocation:: Check file name validity and portability
387 * mktemp invocation:: Create temporary file or directory
388 * realpath invocation:: Print resolved file names
392 * pwd invocation:: Print working directory
393 * stty invocation:: Print or change terminal characteristics
394 * printenv invocation:: Print all or some environment variables
395 * tty invocation:: Print file name of terminal on standard input
397 @command{stty}: Print or change terminal characteristics
399 * Control:: Control settings
400 * Input:: Input settings
401 * Output:: Output settings
402 * Local:: Local settings
403 * Combination:: Combination settings
404 * Characters:: Special characters
405 * Special:: Special settings
409 * id invocation:: Print user identity
410 * logname invocation:: Print current login name
411 * whoami invocation:: Print effective user ID
412 * groups invocation:: Print group names a user is in
413 * users invocation:: Print login names of users currently logged in
414 * who invocation:: Print who is currently logged in
418 * arch invocation:: Print machine hardware name
419 * date invocation:: Print or set system date and time
420 * nproc invocation:: Print the number of processors
421 * uname invocation:: Print system information
422 * hostname invocation:: Print or set system name
423 * hostid invocation:: Print numeric host identifier
424 * uptime invocation:: Print system uptime and load
426 @command{date}: Print or set system date and time
428 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
429 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
430 * Literal conversion specifiers:: %[%nt]
431 * Padding and other flags:: Pad with zeros, spaces, etc.
432 * Setting the time:: Changing the system clock
433 * Options for date:: Instead of the current time
434 * Date input formats:: Specifying date strings
435 * Examples of date:: Examples
439 * chcon invocation:: Change SELinux context of file
440 * runcon invocation:: Run a command in specified SELinux context
442 Modified command invocation
444 * chroot invocation:: Run a command with a different root directory
445 * env invocation:: Run a command in a modified environment
446 * nice invocation:: Run a command with modified niceness
447 * nohup invocation:: Run a command immune to hangups
448 * stdbuf invocation:: Run a command with modified I/O buffering
449 * timeout invocation:: Run a command with a time limit
453 * kill invocation:: Sending a signal to processes.
457 * sleep invocation:: Delay for a specified time
461 * factor invocation:: Print prime factors
462 * seq invocation:: Print numeric sequences
466 * Mode Structure:: Structure of file mode bits
467 * Symbolic Modes:: Mnemonic representation of file mode bits
468 * Numeric Modes:: File mode bits as octal numbers
469 * Directory Setuid and Setgid:: Set-user-ID and set-group-ID on directories
473 * General date syntax:: Common rules
474 * Calendar date items:: 19 Dec 1994
475 * Time of day items:: 9:20pm
476 * Time zone items:: EST, PDT, UTC, @dots{}
477 * Combined date and time of day items:: 1972-09-24T20:02:00,000000-0500
478 * Day of week items:: Monday and others
479 * Relative items in date strings:: next tuesday, 2 years ago
480 * Pure numbers in date strings:: 19931219, 1440
481 * Seconds since the Epoch:: @@1078100502
482 * Specifying time zone rules:: TZ="America/New_York", TZ="UTC0"
483 * Authors of parse_datetime:: Bellovin, Eggert, Salz, Berets, et al.
485 Opening the software toolbox
487 * Toolbox introduction:: Toolbox introduction
488 * I/O redirection:: I/O redirection
489 * The who command:: The @command{who} command
490 * The cut command:: The @command{cut} command
491 * The sort command:: The @command{sort} command
492 * The uniq command:: The @command{uniq} command
493 * Putting the tools together:: Putting the tools together
497 * GNU Free Documentation License:: Copying and sharing this manual
504 @chapter Introduction
506 This manual is a work in progress: many sections make no attempt to explain
507 basic concepts in a way suitable for novices. Thus, if you are interested,
508 please get involved in improving this manual. The entire GNU community
512 The GNU utilities documented here are mostly compatible with the
514 @cindex bugs, reporting
515 Please report bugs to @email{bug-coreutils@@gnu.org}. Remember
516 to include the version number, machine architecture, input files, and
517 any other information needed to reproduce the bug: your input, what you
518 expected, what you got, and why it is wrong. Diffs are welcome, but
519 please include a description of the problem as well, since this is
520 sometimes difficult to infer. @xref{Bugs, , , gcc, Using and Porting GNU CC}.
526 @cindex MacKenzie, D.
529 This manual was originally derived from the Unix man pages in the
530 distributions, which were written by David MacKenzie and updated by Jim
531 Meyering. What you are reading now is the authoritative documentation
532 for these utilities; the man pages are no longer being maintained. The
533 original @command{fmt} man page was written by Ross Paterson. Fran@,{c}ois
534 Pinard did the initial conversion to Texinfo format. Karl Berry did the
535 indexing, some reorganization, and editing of the results. Brian
536 Youmans of the Free Software Foundation office staff combined the
537 manuals for textutils, fileutils, and sh-utils to produce the present
538 omnibus manual. Richard Stallman contributed his usual invaluable
539 insights to the overall process.
542 @chapter Common options
546 @itemx @w{@kbd{--backup}[=@var{method}]}
549 @vindex VERSION_CONTROL
550 @cindex backups, making
551 @xref{Backup options}.
552 Make a backup of each file that would otherwise be overwritten or removed.
555 @macro optBackupSuffix
556 @item -S @var{suffix}
557 @itemx --suffix=@var{suffix}
560 Append @var{suffix} to each backup file made with @option{-b}.
561 @xref{Backup options}.
564 @macro optTargetDirectory
565 @item -t @var{directory}
566 @itemx @w{@kbd{--target-directory}=@var{directory}}
568 @opindex --target-directory
569 @cindex target directory
570 @cindex destination directory
571 Specify the destination @var{directory}.
572 @xref{Target directory}.
575 @macro optNoTargetDirectory
577 @itemx --no-target-directory
579 @opindex --no-target-directory
580 @cindex target directory
581 @cindex destination directory
582 Do not treat the last operand specially when it is a directory or a
583 symbolic link to a directory. @xref{Target directory}.
591 @cindex output NUL-byte-terminated lines
592 Output a zero byte (ASCII NUL) at the end of each line,
593 rather than a newline. This option enables other programs to parse the
594 output of @command{\cmd\} even when that output would contain data
595 with embedded newlines.
602 Append an SI-style abbreviation to each size, such as @samp{M} for
603 megabytes. Powers of 1000 are used, not 1024; @samp{M} stands for
604 1,000,000 bytes. This option is equivalent to
605 @option{--block-size=si}. Use the @option{-h} or
606 @option{--human-readable} option if
607 you prefer powers of 1024.
610 @macro optHumanReadable
612 @itemx --human-readable
614 @opindex --human-readable
615 @cindex human-readable output
616 Append a size letter to each size, such as @samp{M} for mebibytes.
617 Powers of 1024 are used, not 1000; @samp{M} stands for 1,048,576 bytes.
618 This option is equivalent to @option{--block-size=human-readable}.
619 Use the @option{--si} option if you prefer powers of 1000.
622 @macro optStripTrailingSlashes
623 @item @w{@kbd{--strip-trailing-slashes}}
624 @opindex --strip-trailing-slashes
625 @cindex stripping trailing slashes
626 Remove any trailing slashes from each @var{source} argument.
627 @xref{Trailing slashes}.
630 @macro mayConflictWithShellBuiltIn{cmd}
631 @cindex conflicts with shell built-ins
632 @cindex built-in shell commands, conflicts with
633 Due to shell aliases and built-in @command{\cmd\} functions, using an
634 unadorned @command{\cmd\} interactively or in a script may get you
635 different functionality than that described here. Invoke it via
636 @command{env} (i.e., @code{env \cmd\ @dots{}}) to avoid interference
641 @macro multiplierSuffixes{varName}
642 @var{\varName\} may be, or may be an integer optionally followed by,
643 one of the following multiplicative suffixes:
645 @samp{b} => 512 ("blocks")
646 @samp{KB} => 1000 (KiloBytes)
647 @samp{K} => 1024 (KibiBytes)
648 @samp{MB} => 1000*1000 (MegaBytes)
649 @samp{M} => 1024*1024 (MebiBytes)
650 @samp{GB} => 1000*1000*1000 (GigaBytes)
651 @samp{G} => 1024*1024*1024 (GibiBytes)
653 and so on for @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}.
656 @c FIXME: same as above, but no ``blocks'' line.
657 @macro multiplierSuffixesNoBlocks{varName}
658 @var{\varName\} may be, or may be an integer optionally followed by,
659 one of the following multiplicative suffixes:
661 @samp{KB} => 1000 (KiloBytes)
662 @samp{K} => 1024 (KibiBytes)
663 @samp{MB} => 1000*1000 (MegaBytes)
664 @samp{M} => 1024*1024 (MebiBytes)
665 @samp{GB} => 1000*1000*1000 (GigaBytes)
666 @samp{G} => 1024*1024*1024 (GibiBytes)
668 and so on for @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}.
671 @cindex common options
673 Certain options are available in all of these programs. Rather than
674 writing identical descriptions for each of the programs, they are
675 described here. (In fact, every GNU program accepts (or should accept)
678 @vindex POSIXLY_CORRECT
679 Normally options and operands can appear in any order, and programs act
680 as if all the options appear before any operands. For example,
681 @samp{sort -r passwd -t :} acts like @samp{sort -r -t : passwd}, since
682 @samp{:} is an option-argument of @option{-t}. However, if the
683 @env{POSIXLY_CORRECT} environment variable is set, options must appear
684 before operands, unless otherwise specified for a particular command.
686 A few programs can usefully have trailing operands with leading
687 @samp{-}. With such a program, options must precede operands even if
688 @env{POSIXLY_CORRECT} is not set, and this fact is noted in the
689 program description. For example, the @command{env} command's options
690 must appear before its operands, since in some cases the operands
691 specify a command that itself contains options.
693 Most programs that accept long options recognize unambiguous
694 abbreviations of those options. For example, @samp{rmdir
695 --ignore-fail-on-non-empty} can be invoked as @samp{rmdir
696 --ignore-fail} or even @samp{rmdir --i}. Ambiguous options, such as
697 @samp{ls --h}, are identified as such.
699 Some of these programs recognize the @option{--help} and @option{--version}
700 options only when one of them is the sole command line argument. For
701 these programs, abbreviations of the long options are not always recognized.
708 Print a usage message listing all available options, then exit successfully.
712 @cindex version number, finding
713 Print the version number, then exit successfully.
717 @cindex option delimiter
718 Delimit the option list. Later arguments, if any, are treated as
719 operands even if they begin with @samp{-}. For example, @samp{sort --
720 -r} reads from the file named @file{-r}.
724 @cindex standard input
725 @cindex standard output
726 A single @samp{-} operand is not really an option, though it looks like one. It
727 stands for standard input, or for standard output if that is clear from
728 the context. For example, @samp{sort -} reads from standard input,
729 and is equivalent to plain @samp{sort}, and @samp{tee -} writes an
730 extra copy of its input to standard output. Unless otherwise
731 specified, @samp{-} can appear as any operand that requires a file
735 * Exit status:: Indicating program success or failure.
736 * Backup options:: -b -S, in some programs.
737 * Block size:: BLOCK_SIZE and --block-size, in some programs.
738 * Floating point:: Floating point number representation.
739 * Signal specifications:: Specifying signals using the --signal option.
740 * Disambiguating names and IDs:: chgrp and chown owner and group syntax
741 * Random sources:: --random-source, in some programs.
742 * Target directory:: Specifying a target directory, in some programs.
743 * Trailing slashes:: --strip-trailing-slashes, in some programs.
744 * Traversing symlinks:: -H, -L, or -P, in some programs.
745 * Treating / specially:: --preserve-root and --no-preserve-root.
746 * Special built-in utilities:: @command{break}, @command{:}, @dots{}
747 * Standards conformance:: Conformance to the POSIX standard.
755 An exit status of zero indicates success,
756 and a nonzero value indicates failure.
759 Nearly every command invocation yields an integral @dfn{exit status}
760 that can be used to change how other commands work.
761 For the vast majority of commands, an exit status of zero indicates
762 success. Failure is indicated by a nonzero value---typically
763 @samp{1}, though it may differ on unusual platforms as POSIX
764 requires only that it be nonzero.
766 However, some of the programs documented here do produce
767 other exit status values and a few associate different
768 meanings with the values @samp{0} and @samp{1}.
769 Here are some of the exceptions:
770 @command{chroot}, @command{env}, @command{expr}, @command{nice},
771 @command{nohup}, @command{numfmt}, @command{printenv}, @command{sort},
772 @command{stdbuf}, @command{test}, @command{timeout}, @command{tty}.
776 @section Backup options
778 @cindex backup options
780 Some GNU programs (at least @command{cp}, @command{install},
781 @command{ln}, and @command{mv}) optionally make backups of files
782 before writing new versions.
783 These options control the details of these backups. The options are also
784 briefly mentioned in the descriptions of the particular programs.
789 @itemx @w{@kbd{--backup}[=@var{method}]}
792 @vindex VERSION_CONTROL
793 @cindex backups, making
794 Make a backup of each file that would otherwise be overwritten or removed.
795 Without this option, the original versions are destroyed.
796 Use @var{method} to determine the type of backups to make.
797 When this option is used but @var{method} is not specified,
798 then the value of the @env{VERSION_CONTROL}
799 environment variable is used. And if @env{VERSION_CONTROL} is not set,
800 the default backup type is @samp{existing}.
802 Note that the short form of this option, @option{-b} does not accept any
803 argument. Using @option{-b} is equivalent to using @option{--backup=existing}.
805 @vindex version-control @r{Emacs variable}
806 This option corresponds to the Emacs variable @samp{version-control};
807 the values for @var{method} are the same as those used in Emacs.
808 This option also accepts more descriptive names.
809 The valid @var{method}s are (unique abbreviations are accepted):
814 @opindex none @r{backup method}
819 @opindex numbered @r{backup method}
820 Always make numbered backups.
824 @opindex existing @r{backup method}
825 Make numbered backups of files that already have them, simple backups
830 @opindex simple @r{backup method}
831 Always make simple backups. Please note @samp{never} is not to be
832 confused with @samp{none}.
836 @item -S @var{suffix}
837 @itemx --suffix=@var{suffix}
840 @cindex backup suffix
841 @vindex SIMPLE_BACKUP_SUFFIX
842 Append @var{suffix} to each backup file made with @option{-b}. If this
843 option is not specified, the value of the @env{SIMPLE_BACKUP_SUFFIX}
844 environment variable is used. And if @env{SIMPLE_BACKUP_SUFFIX} is not
845 set, the default is @samp{~}, just as in Emacs.
854 Some GNU programs (at least @command{df}, @command{du}, and
855 @command{ls}) display sizes in ``blocks''. You can adjust the block size
856 and method of display to make sizes easier to read. The block size
857 used for display is independent of any file system block size.
858 Fractional block counts are rounded up to the nearest integer.
860 @opindex --block-size=@var{size}
863 @vindex DF_BLOCK_SIZE
864 @vindex DU_BLOCK_SIZE
865 @vindex LS_BLOCK_SIZE
866 @vindex POSIXLY_CORRECT@r{, and block size}
868 The default block size is chosen by examining the following environment
869 variables in turn; the first one that is set determines the block size.
874 This specifies the default block size for the @command{df} command.
875 Similarly, @env{DU_BLOCK_SIZE} specifies the default for @command{du} and
876 @env{LS_BLOCK_SIZE} for @command{ls}.
879 This specifies the default block size for all three commands, if the
880 above command-specific environment variables are not set.
883 This specifies the default block size for all values that are normally
884 printed as blocks, if neither @env{BLOCK_SIZE} nor the above
885 command-specific environment variables are set. Unlike the other
886 environment variables, @env{BLOCKSIZE} does not affect values that are
887 normally printed as byte counts, e.g., the file sizes contained in
890 @item POSIXLY_CORRECT
891 If neither @env{@var{command}_BLOCK_SIZE}, nor @env{BLOCK_SIZE}, nor
892 @env{BLOCKSIZE} is set, but this variable is set, the block size
897 If none of the above environment variables are set, the block size
898 currently defaults to 1024 bytes in most contexts, but this number may
899 change in the future. For @command{ls} file sizes, the block size
902 @cindex human-readable output
905 A block size specification can be a positive integer specifying the number
906 of bytes per block, or it can be @code{human-readable} or @code{si} to
907 select a human-readable format. Integers may be followed by suffixes
908 that are upward compatible with the
909 @uref{http://www.bipm.org/en/si/si_brochure/chapter3/prefixes.html, SI prefixes}
910 for decimal multiples and with the
911 @uref{http://physics.nist.gov/cuu/Units/binary.html, ISO/IEC 80000-13
912 (formerly IEC 60027-2) prefixes} for binary multiples.
914 With human-readable formats, output sizes are followed by a size letter
915 such as @samp{M} for megabytes. @code{BLOCK_SIZE=human-readable} uses
916 powers of 1024; @samp{M} stands for 1,048,576 bytes.
917 @code{BLOCK_SIZE=si} is similar, but uses powers of 1000 and appends
918 @samp{B}; @samp{MB} stands for 1,000,000 bytes.
921 A block size specification preceded by @samp{'} causes output sizes to
922 be displayed with thousands separators. The @env{LC_NUMERIC} locale
923 specifies the thousands separator and grouping. For example, in an
924 American English locale, @samp{--block-size="'1kB"} would cause a size
925 of 1234000 bytes to be displayed as @samp{1,234}. In the default C
926 locale, there is no thousands separator so a leading @samp{'} has no
929 An integer block size can be followed by a suffix to specify a
930 multiple of that size. A bare size letter,
931 or one followed by @samp{iB}, specifies
932 a multiple using powers of 1024. A size letter followed by @samp{B}
933 specifies powers of 1000 instead. For example, @samp{1M} and
934 @samp{1MiB} are equivalent to @samp{1048576}, whereas @samp{1MB} is
935 equivalent to @samp{1000000}.
937 A plain suffix without a preceding integer acts as if @samp{1} were
938 prepended, except that it causes a size indication to be appended to
939 the output. For example, @samp{--block-size="kB"} displays 3000 as
942 The following suffixes are defined. Large sizes like @code{1Y}
943 may be rejected by your computer due to limitations of its arithmetic.
947 @cindex kilobyte, definition of
948 kilobyte: @math{10^3 = 1000}.
952 @cindex kibibyte, definition of
953 kibibyte: @math{2^{10} = 1024}. @samp{K} is special: the SI prefix is
954 @samp{k} and the ISO/IEC 80000-13 prefix is @samp{Ki}, but tradition and
955 POSIX use @samp{k} to mean @samp{KiB}.
957 @cindex megabyte, definition of
958 megabyte: @math{10^6 = 1,000,000}.
961 @cindex mebibyte, definition of
962 mebibyte: @math{2^{20} = 1,048,576}.
964 @cindex gigabyte, definition of
965 gigabyte: @math{10^9 = 1,000,000,000}.
968 @cindex gibibyte, definition of
969 gibibyte: @math{2^{30} = 1,073,741,824}.
971 @cindex terabyte, definition of
972 terabyte: @math{10^{12} = 1,000,000,000,000}.
975 @cindex tebibyte, definition of
976 tebibyte: @math{2^{40} = 1,099,511,627,776}.
978 @cindex petabyte, definition of
979 petabyte: @math{10^{15} = 1,000,000,000,000,000}.
982 @cindex pebibyte, definition of
983 pebibyte: @math{2^{50} = 1,125,899,906,842,624}.
985 @cindex exabyte, definition of
986 exabyte: @math{10^{18} = 1,000,000,000,000,000,000}.
989 @cindex exbibyte, definition of
990 exbibyte: @math{2^{60} = 1,152,921,504,606,846,976}.
992 @cindex zettabyte, definition of
993 zettabyte: @math{10^{21} = 1,000,000,000,000,000,000,000}
996 @math{2^{70} = 1,180,591,620,717,411,303,424}.
998 @cindex yottabyte, definition of
999 yottabyte: @math{10^{24} = 1,000,000,000,000,000,000,000,000}.
1002 @math{2^{80} = 1,208,925,819,614,629,174,706,176}.
1007 @opindex --block-size
1008 @opindex --human-readable
1011 Block size defaults can be overridden by an explicit
1012 @option{--block-size=@var{size}} option. The @option{-k}
1013 option is equivalent to @option{--block-size=1K}, which
1014 is the default unless the @env{POSIXLY_CORRECT} environment variable is
1015 set. The @option{-h} or @option{--human-readable} option is equivalent to
1016 @option{--block-size=human-readable}. The @option{--si} option is
1017 equivalent to @option{--block-size=si}.
1019 @node Floating point
1020 @section Floating point numbers
1021 @cindex floating point
1022 @cindex IEEE floating point
1024 Commands that accept or produce floating point numbers employ the
1025 floating point representation of the underlying system, and suffer
1026 from rounding error, overflow, and similar floating-point issues.
1027 Almost all modern systems use IEEE-754 floating point, and it is
1028 typically portable to assume IEEE-754 behavior these days. IEEE-754
1029 has positive and negative infinity, distinguishes positive from
1030 negative zero, and uses special values called NaNs to represent
1031 invalid computations such as dividing zero by itself. For more
1032 information, please see David Goldberg's paper
1033 @uref{http://@/www.validlab.com/@/goldberg/@/paper.pdf, What Every
1034 Computer Scientist Should Know About Floating-Point Arithmetic}.
1037 Commands that accept floating point numbers as options, operands or
1038 input use the standard C functions @code{strtod} and @code{strtold} to
1039 convert from text to floating point numbers. These floating point
1040 numbers therefore can use scientific notation like @code{1.0e-34} and
1041 @code{-10e100}. Modern C implementations also accept hexadecimal
1042 floating point numbers such as @code{-0x.ep-3}, which stands for
1043 @minus{}14/16 times @math{2^-3}, which equals @minus{}0.109375. The
1044 @env{LC_NUMERIC} locale determines the decimal-point character.
1045 @xref{Parsing of Floats,,, libc, The GNU C Library Reference Manual}.
1047 @node Signal specifications
1048 @section Signal specifications
1049 @cindex signals, specifying
1051 A @var{signal} may be a signal name like @samp{HUP}, or a signal
1052 number like @samp{1}, or an exit status of a process terminated by the
1053 signal. A signal name can be given in canonical form or prefixed by
1054 @samp{SIG}@. The case of the letters is ignored. The following signal names
1055 and numbers are supported on all POSIX compliant systems:
1061 2. Terminal interrupt.
1067 9. Kill (cannot be caught or ignored).
1075 Other supported signal names have system-dependent corresponding
1076 numbers. All systems conforming to POSIX 1003.1-2001 also
1077 support the following signals:
1081 Access to an undefined portion of a memory object.
1083 Child process terminated, stopped, or continued.
1085 Continue executing, if stopped.
1087 Erroneous arithmetic operation.
1089 Illegal Instruction.
1091 Write on a pipe with no one to read it.
1093 Invalid memory reference.
1095 Stop executing (cannot be caught or ignored).
1099 Background process attempting read.
1101 Background process attempting write.
1103 High bandwidth data is available at a socket.
1105 User-defined signal 1.
1107 User-defined signal 2.
1111 POSIX 1003.1-2001 systems that support the XSI extension
1112 also support the following signals:
1118 Profiling timer expired.
1122 Trace/breakpoint trap.
1124 Virtual timer expired.
1126 CPU time limit exceeded.
1128 File size limit exceeded.
1132 POSIX 1003.1-2001 systems that support the XRT extension
1133 also support at least eight real-time signals called @samp{RTMIN},
1134 @samp{RTMIN+1}, @dots{}, @samp{RTMAX-1}, @samp{RTMAX}.
1136 @node Disambiguating names and IDs
1137 @section chown and chgrp: Disambiguating user names and IDs
1138 @cindex user names, disambiguating
1139 @cindex user IDs, disambiguating
1140 @cindex group names, disambiguating
1141 @cindex group IDs, disambiguating
1142 @cindex disambiguating group names and IDs
1144 Since the @var{owner} and @var{group} arguments to @command{chown} and
1145 @command{chgrp} may be specified as names or numeric IDs, there is an
1147 What if a user or group @emph{name} is a string of digits?
1148 @footnote{Using a number as a user name is common in some environments.}
1149 Should the command interpret it as a user name or as an ID@?
1150 POSIX requires that @command{chown} and @command{chgrp}
1151 first attempt to resolve the specified string as a name, and
1152 only once that fails, then try to interpret it as an ID@.
1153 This is troublesome when you want to specify a numeric ID, say 42,
1154 and it must work even in a pathological situation where
1155 @samp{42} is a user name that maps to some other user ID, say 1000.
1156 Simply invoking @code{chown 42 F}, will set @file{F}s owner ID to
1157 1000---not what you intended.
1159 GNU @command{chown} and @command{chgrp} provide a way to work around this,
1160 that at the same time may result in a significant performance improvement
1161 by eliminating a database look-up.
1162 Simply precede each numeric user ID and/or group ID with a @samp{+},
1163 in order to force its interpretation as an integer:
1167 chgrp +$numeric_group_id another-file
1171 GNU @command{chown} and @command{chgrp}
1172 skip the name look-up process for each @samp{+}-prefixed string,
1173 because a string containing @samp{+} is never a valid user or group name.
1174 This syntax is accepted on most common Unix systems, but not on Solaris 10.
1176 @node Random sources
1177 @section Sources of random data
1179 @cindex random sources
1181 The @command{shuf}, @command{shred}, and @command{sort} commands
1182 sometimes need random data to do their work. For example, @samp{sort
1183 -R} must choose a hash function at random, and it needs random data to
1184 make this selection.
1186 By default these commands use an internal pseudorandom generator
1187 initialized by a small amount of entropy, but can be directed to use
1188 an external source with the @option{--random-source=@var{file}} option.
1189 An error is reported if @var{file} does not contain enough bytes.
1191 For example, the device file @file{/dev/urandom} could be used as the
1192 source of random data. Typically, this device gathers environmental
1193 noise from device drivers and other sources into an entropy pool, and
1194 uses the pool to generate random bits. If the pool is short of data,
1195 the device reuses the internal pool to produce more bits, using a
1196 cryptographically secure pseudorandom number generator. But be aware
1197 that this device is not designed for bulk random data generation
1198 and is relatively slow.
1200 @file{/dev/urandom} suffices for most practical uses, but applications
1201 requiring high-value or long-term protection of private data may
1202 require an alternate data source like @file{/dev/random} or
1203 @file{/dev/arandom}. The set of available sources depends on your
1206 To reproduce the results of an earlier invocation of a command, you
1207 can save some random data into a file and then use that file as the
1208 random source in earlier and later invocations of the command.
1210 @node Target directory
1211 @section Target directory
1213 @cindex target directory
1215 The @command{cp}, @command{install}, @command{ln}, and @command{mv}
1216 commands normally treat the last operand specially when it is a
1217 directory or a symbolic link to a directory. For example, @samp{cp
1218 source dest} is equivalent to @samp{cp source dest/source} if
1219 @file{dest} is a directory. Sometimes this behavior is not exactly
1220 what is wanted, so these commands support the following options to
1221 allow more fine-grained control:
1226 @itemx --no-target-directory
1227 @opindex --no-target-directory
1228 @cindex target directory
1229 @cindex destination directory
1230 Do not treat the last operand specially when it is a directory or a
1231 symbolic link to a directory. This can help avoid race conditions in
1232 programs that operate in a shared area. For example, when the command
1233 @samp{mv /tmp/source /tmp/dest} succeeds, there is no guarantee that
1234 @file{/tmp/source} was renamed to @file{/tmp/dest}: it could have been
1235 renamed to @file{/tmp/dest/source} instead, if some other process
1236 created @file{/tmp/dest} as a directory. However, if @file{mv
1237 -T /tmp/source /tmp/dest} succeeds, there is no
1238 question that @file{/tmp/source} was renamed to @file{/tmp/dest}.
1240 In the opposite situation, where you want the last operand to be
1241 treated as a directory and want a diagnostic otherwise, you can use
1242 the @option{--target-directory} (@option{-t}) option.
1244 @item -t @var{directory}
1245 @itemx @w{@kbd{--target-directory}=@var{directory}}
1246 @opindex --target-directory
1247 @cindex target directory
1248 @cindex destination directory
1249 Use @var{directory} as the directory component of each destination
1252 The interface for most programs is that after processing options and a
1253 finite (possibly zero) number of fixed-position arguments, the remaining
1254 argument list is either expected to be empty, or is a list of items
1255 (usually files) that will all be handled identically. The @command{xargs}
1256 program is designed to work well with this convention.
1258 The commands in the @command{mv}-family are unusual in that they take
1259 a variable number of arguments with a special case at the @emph{end}
1260 (namely, the target directory). This makes it nontrivial to perform some
1261 operations, e.g., ``move all files from here to ../d/'', because
1262 @code{mv * ../d/} might exhaust the argument space, and @code{ls | xargs ...}
1263 doesn't have a clean way to specify an extra final argument for each
1264 invocation of the subject command. (It can be done by going through a
1265 shell command, but that requires more human labor and brain power than
1268 The @w{@kbd{--target-directory}} (@option{-t}) option allows the @command{cp},
1269 @command{install}, @command{ln}, and @command{mv} programs to be used
1270 conveniently with @command{xargs}. For example, you can move the files
1271 from the current directory to a sibling directory, @code{d} like this:
1274 ls | xargs mv -t ../d --
1277 However, this doesn't move files whose names begin with @samp{.}.
1278 If you use the GNU @command{find} program, you can move those
1279 files too, with this command:
1282 find . -mindepth 1 -maxdepth 1 \
1286 But both of the above approaches fail if there are no files in the
1287 current directory, or if any file has a name containing a blank or
1288 some other special characters.
1289 The following example removes those limitations and requires both
1290 GNU @command{find} and GNU @command{xargs}:
1293 find . -mindepth 1 -maxdepth 1 -print0 \
1294 | xargs --null --no-run-if-empty \
1301 The @option{--target-directory} (@option{-t}) and
1302 @option{--no-target-directory} (@option{-T})
1303 options cannot be combined.
1305 @node Trailing slashes
1306 @section Trailing slashes
1308 @cindex trailing slashes
1310 Some GNU programs (at least @command{cp} and @command{mv}) allow you to
1311 remove any trailing slashes from each @var{source} argument before
1312 operating on it. The @w{@kbd{--strip-trailing-slashes}} option enables
1315 This is useful when a @var{source} argument may have a trailing slash and
1316 @c FIXME: mv's behavior in this case is system-dependent
1317 specify a symbolic link to a directory. This scenario is in fact rather
1318 common because some shells can automatically append a trailing slash when
1319 performing file name completion on such symbolic links. Without this
1320 option, @command{mv}, for example, (via the system's rename function) must
1321 interpret a trailing slash as a request to dereference the symbolic link
1322 and so must rename the indirectly referenced @emph{directory} and not
1323 the symbolic link. Although it may seem surprising that such behavior
1324 be the default, it is required by POSIX and is consistent with
1325 other parts of that standard.
1327 @node Traversing symlinks
1328 @section Traversing symlinks
1330 @cindex symbolic link to directory, controlling traversal of
1332 The following options modify how @command{chown} and @command{chgrp}
1333 @c FIXME: note that 'du' has these options, too, but they have slightly
1334 @c different meaning.
1335 traverse a hierarchy when the @option{--recursive} (@option{-R})
1336 option is also specified.
1337 If more than one of the following options is specified, only the final
1339 These options specify whether processing a symbolic link to a directory
1340 entails operating on just the symbolic link or on all files in the
1341 hierarchy rooted at that directory.
1343 These options are independent of @option{--dereference} and
1344 @option{--no-dereference} (@option{-h}), which control whether to modify
1345 a symlink or its referent.
1352 @cindex symbolic link to directory, traverse if on the command line
1353 If @option{--recursive} (@option{-R}) is specified and
1354 a command line argument is a symbolic link to a directory, traverse it.
1361 @cindex symbolic link to directory, traverse each that is encountered
1362 In a recursive traversal, traverse every symbolic link to a directory
1363 that is encountered.
1370 @cindex symbolic link to directory, never traverse
1371 Do not traverse any symbolic links.
1372 This is the default if none of @option{-H}, @option{-L},
1373 or @option{-P} is specified.
1380 @node Treating / specially
1381 @section Treating @file{/} specially
1383 Certain commands can operate destructively on entire hierarchies.
1384 For example, if a user with appropriate privileges mistakenly runs
1385 @samp{rm -rf / tmp/junk}, that may remove
1386 all files on the entire system. Since there are so few
1387 legitimate uses for such a command,
1388 GNU @command{rm} normally declines to operate on any directory
1389 that resolves to @file{/}. If you really want to try to remove all
1390 the files on your system, you can use the @option{--no-preserve-root}
1391 option, but the default behavior, specified by the
1392 @option{--preserve-root} option, is safer for most purposes.
1394 The commands @command{chgrp}, @command{chmod} and @command{chown}
1395 can also operate destructively on entire hierarchies, so they too
1396 support these options. Although, unlike @command{rm}, they don't
1397 actually unlink files, these commands are arguably more dangerous
1398 when operating recursively on @file{/}, since they often work much
1399 more quickly, and hence damage more files before an alert user can
1400 interrupt them. Tradition and POSIX require these commands
1401 to operate recursively on @file{/}, so they default to
1402 @option{--no-preserve-root}, but using the @option{--preserve-root}
1403 option makes them safer for most purposes. For convenience you can
1404 specify @option{--preserve-root} in an alias or in a shell function.
1406 Note that the @option{--preserve-root} option also ensures
1407 that @command{chgrp} and @command{chown} do not modify @file{/}
1408 even when dereferencing a symlink pointing to @file{/}.
1410 @node Special built-in utilities
1411 @section Special built-in utilities
1413 Some programs like @command{nice} can invoke other programs; for
1414 example, the command @samp{nice cat file} invokes the program
1415 @command{cat} by executing the command @samp{cat file}. However,
1416 @dfn{special built-in utilities} like @command{exit} cannot be invoked
1417 this way. For example, the command @samp{nice exit} does not have a
1418 well-defined behavior: it may generate an error message instead of
1421 Here is a list of the special built-in utilities that are standardized
1422 by POSIX 1003.1-2004.
1425 @t{.@: : break continue eval exec exit export readonly
1426 return set shift times trap unset}
1429 For example, because @samp{.}, @samp{:}, and @samp{exec} are special,
1430 the commands @samp{nice . foo.sh}, @samp{nice :}, and @samp{nice exec
1431 pwd} do not work as you might expect.
1433 Many shells extend this list. For example, Bash has several extra
1434 special built-in utilities like @command{history}, and
1435 @command{suspend}, and with Bash the command @samp{nice suspend}
1436 generates an error message instead of suspending.
1438 @node Standards conformance
1439 @section Standards conformance
1441 @vindex POSIXLY_CORRECT
1442 In a few cases, the GNU utilities' default behavior is
1443 incompatible with the POSIX standard. To suppress these
1444 incompatibilities, define the @env{POSIXLY_CORRECT} environment
1445 variable. Unless you are checking for POSIX conformance, you
1446 probably do not need to define @env{POSIXLY_CORRECT}.
1448 Newer versions of POSIX are occasionally incompatible with older
1449 versions. For example, older versions of POSIX required the
1450 command @samp{sort +1} to sort based on the second and succeeding
1451 fields in each input line, but starting with POSIX 1003.1-2001
1452 the same command is required to sort the file named @file{+1}, and you
1453 must instead use the command @samp{sort -k 2} to get the field-based
1456 @vindex _POSIX2_VERSION
1457 The GNU utilities normally conform to the version of POSIX
1458 that is standard for your system. To cause them to conform to a
1459 different version of POSIX, define the @env{_POSIX2_VERSION}
1460 environment variable to a value of the form @var{yyyymm} specifying
1461 the year and month the standard was adopted. Three values are currently
1462 supported for @env{_POSIX2_VERSION}: @samp{199209} stands for
1463 POSIX 1003.2-1992, @samp{200112} stands for POSIX
1464 1003.1-2001, and @samp{200809} stands for POSIX 1003.1-2008.
1465 For example, if you have a newer system but are running software
1466 that assumes an older version of POSIX and uses @samp{sort +1}
1467 or @samp{tail +10}, you can work around any compatibility problems by setting
1468 @samp{_POSIX2_VERSION=199209} in your environment.
1470 @node Output of entire files
1471 @chapter Output of entire files
1473 @cindex output of entire files
1474 @cindex entire files, output of
1476 These commands read and write entire files, possibly transforming them
1480 * cat invocation:: Concatenate and write files.
1481 * tac invocation:: Concatenate and write files in reverse.
1482 * nl invocation:: Number lines and write files.
1483 * od invocation:: Write files in octal or other formats.
1484 * base64 invocation:: Transform data into printable data.
1487 @node cat invocation
1488 @section @command{cat}: Concatenate and write files
1491 @cindex concatenate and write files
1492 @cindex copying files
1494 @command{cat} copies each @var{file} (@samp{-} means standard input), or
1495 standard input if none are given, to standard output. Synopsis:
1498 cat [@var{option}] [@var{file}]@dots{}
1501 The program accepts the following options. Also see @ref{Common options}.
1509 Equivalent to @option{-vET}.
1512 @itemx --number-nonblank
1514 @opindex --number-nonblank
1515 Number all nonempty output lines, starting with 1.
1519 Equivalent to @option{-vE}.
1524 @opindex --show-ends
1525 Display a @samp{$} after the end of each line.
1531 Number all output lines, starting with 1. This option is ignored
1532 if @option{-b} is in effect.
1535 @itemx --squeeze-blank
1537 @opindex --squeeze-blank
1538 @cindex squeezing empty lines
1539 Suppress repeated adjacent empty lines; output just one empty line
1544 Equivalent to @option{-vT}.
1549 @opindex --show-tabs
1550 Display TAB characters as @samp{^I}.
1554 Ignored; for POSIX compatibility.
1557 @itemx --show-nonprinting
1559 @opindex --show-nonprinting
1560 Display control characters except for LFD and TAB using
1561 @samp{^} notation and precede characters that have the high bit set with
1566 On systems like MS-DOS that distinguish between text and binary files,
1567 @command{cat} normally reads and writes in binary mode. However,
1568 @command{cat} reads in text mode if one of the options
1569 @option{-bensAE} is used or if @command{cat} is reading from standard
1570 input and standard input is a terminal. Similarly, @command{cat}
1571 writes in text mode if one of the options @option{-bensAE} is used or
1572 if standard output is a terminal.
1579 # Output f's contents, then standard input, then g's contents.
1582 # Copy standard input to standard output.
1587 @node tac invocation
1588 @section @command{tac}: Concatenate and write files in reverse
1591 @cindex reversing files
1593 @command{tac} copies each @var{file} (@samp{-} means standard input), or
1594 standard input if none are given, to standard output, reversing the
1595 records (lines by default) in each separately. Synopsis:
1598 tac [@var{option}]@dots{} [@var{file}]@dots{}
1601 @dfn{Records} are separated by instances of a string (newline by
1602 default). By default, this separator string is attached to the end of
1603 the record that it follows in the file.
1605 The program accepts the following options. Also see @ref{Common options}.
1613 The separator is attached to the beginning of the record that it
1614 precedes in the file.
1620 Treat the separator string as a regular expression.
1622 @item -s @var{separator}
1623 @itemx --separator=@var{separator}
1625 @opindex --separator
1626 Use @var{separator} as the record separator, instead of newline.
1630 On systems like MS-DOS that distinguish between text and binary files,
1631 @command{tac} reads and writes in binary mode.
1638 # Reverse a file character by character.
1644 @section @command{nl}: Number lines and write files
1647 @cindex numbering lines
1648 @cindex line numbering
1650 @command{nl} writes each @var{file} (@samp{-} means standard input), or
1651 standard input if none are given, to standard output, with line numbers
1652 added to some or all of the lines. Synopsis:
1655 nl [@var{option}]@dots{} [@var{file}]@dots{}
1658 @cindex logical pages, numbering on
1659 @command{nl} decomposes its input into (logical) pages; by default, the
1660 line number is reset to 1 at the top of each logical page. @command{nl}
1661 treats all of the input files as a single document; it does not reset
1662 line numbers or logical pages between files.
1664 @cindex headers, numbering
1665 @cindex body, numbering
1666 @cindex footers, numbering
1667 A logical page consists of three sections: header, body, and footer.
1668 Any of the sections can be empty. Each can be numbered in a different
1669 style from the others.
1671 The beginnings of the sections of logical pages are indicated in the
1672 input file by a line containing exactly one of these delimiter strings:
1683 The two characters from which these strings are made can be changed from
1684 @samp{\} and @samp{:} via options (see below), but the pattern and
1685 length of each string cannot be changed.
1687 A section delimiter is replaced by an empty line on output. Any text
1688 that comes before the first section delimiter string in the input file
1689 is considered to be part of a body section, so @command{nl} treats a
1690 file that contains no section delimiters as a single body section.
1692 The program accepts the following options. Also see @ref{Common options}.
1696 @item -b @var{style}
1697 @itemx --body-numbering=@var{style}
1699 @opindex --body-numbering
1700 Select the numbering style for lines in the body section of each
1701 logical page. When a line is not numbered, the current line number
1702 is not incremented, but the line number separator character is still
1703 prepended to the line. The styles are:
1709 number only nonempty lines (default for body),
1711 do not number lines (default for header and footer),
1713 number only lines that contain a match for the basic regular
1714 expression @var{bre}.
1715 @xref{Regular Expressions, , Regular Expressions, grep, The GNU Grep Manual}.
1719 @itemx --section-delimiter=@var{cd}
1721 @opindex --section-delimiter
1722 @cindex section delimiters of pages
1723 Set the section delimiter characters to @var{cd}; default is
1724 @samp{\:}. If only @var{c} is given, the second remains @samp{:}.
1725 (Remember to protect @samp{\} or other metacharacters from shell
1726 expansion with quotes or extra backslashes.)
1728 @item -f @var{style}
1729 @itemx --footer-numbering=@var{style}
1731 @opindex --footer-numbering
1732 Analogous to @option{--body-numbering}.
1734 @item -h @var{style}
1735 @itemx --header-numbering=@var{style}
1737 @opindex --header-numbering
1738 Analogous to @option{--body-numbering}.
1740 @item -i @var{number}
1741 @itemx --line-increment=@var{number}
1743 @opindex --line-increment
1744 Increment line numbers by @var{number} (default 1).
1746 @item -l @var{number}
1747 @itemx --join-blank-lines=@var{number}
1749 @opindex --join-blank-lines
1750 @cindex empty lines, numbering
1751 @cindex blank lines, numbering
1752 Consider @var{number} (default 1) consecutive empty lines to be one
1753 logical line for numbering, and only number the last one. Where fewer
1754 than @var{number} consecutive empty lines occur, do not number them.
1755 An empty line is one that contains no characters, not even spaces
1758 @item -n @var{format}
1759 @itemx --number-format=@var{format}
1761 @opindex --number-format
1762 Select the line numbering format (default is @code{rn}):
1766 @opindex ln @r{format for @command{nl}}
1767 left justified, no leading zeros;
1769 @opindex rn @r{format for @command{nl}}
1770 right justified, no leading zeros;
1772 @opindex rz @r{format for @command{nl}}
1773 right justified, leading zeros.
1777 @itemx --no-renumber
1779 @opindex --no-renumber
1780 Do not reset the line number at the start of a logical page.
1782 @item -s @var{string}
1783 @itemx --number-separator=@var{string}
1785 @opindex --number-separator
1786 Separate the line number from the text line in the output with
1787 @var{string} (default is the TAB character).
1789 @item -v @var{number}
1790 @itemx --starting-line-number=@var{number}
1792 @opindex --starting-line-number
1793 Set the initial line number on each logical page to @var{number} (default 1).
1795 @item -w @var{number}
1796 @itemx --number-width=@var{number}
1798 @opindex --number-width
1799 Use @var{number} characters for line numbers (default 6).
1807 @section @command{od}: Write files in octal or other formats
1810 @cindex octal dump of files
1811 @cindex hex dump of files
1812 @cindex ASCII dump of files
1813 @cindex file contents, dumping unambiguously
1815 @command{od} writes an unambiguous representation of each @var{file}
1816 (@samp{-} means standard input), or standard input if none are given.
1820 od [@var{option}]@dots{} [@var{file}]@dots{}
1821 od [-abcdfilosx]@dots{} [@var{file}] [[+]@var{offset}[.][b]]
1822 od [@var{option}]@dots{} --traditional [@var{file}]@c
1823 [[+]@var{offset}[.][b] [[+]@var{label}[.][b]]]
1826 Each line of output consists of the offset in the input, followed by
1827 groups of data from the file. By default, @command{od} prints the offset in
1828 octal, and each group of file data is a C @code{short int}'s worth of input
1829 printed as a single octal number.
1831 If @var{offset} is given, it specifies how many input bytes to skip
1832 before formatting and writing. By default, it is interpreted as an
1833 octal number, but the optional trailing decimal point causes it to be
1834 interpreted as decimal. If no decimal is specified and the offset
1835 begins with @samp{0x} or @samp{0X} it is interpreted as a hexadecimal
1836 number. If there is a trailing @samp{b}, the number of bytes skipped
1837 will be @var{offset} multiplied by 512.
1839 If a command is of both the first and second forms, the second form is
1840 assumed if the last operand begins with @samp{+} or (if there are two
1841 operands) a digit. For example, in @samp{od foo 10} and @samp{od +10}
1842 the @samp{10} is an offset, whereas in @samp{od 10} the @samp{10} is a
1845 The program accepts the following options. Also see @ref{Common options}.
1849 @item -A @var{radix}
1850 @itemx --address-radix=@var{radix}
1852 @opindex --address-radix
1853 @cindex radix for file offsets
1854 @cindex file offset radix
1855 Select the base in which file offsets are printed. @var{radix} can
1856 be one of the following:
1866 none (do not print offsets).
1869 The default is octal.
1871 @item -j @var{bytes}
1872 @itemx --skip-bytes=@var{bytes}
1874 @opindex --skip-bytes
1875 Skip @var{bytes} input bytes before formatting and writing. If
1876 @var{bytes} begins with @samp{0x} or @samp{0X}, it is interpreted in
1877 hexadecimal; otherwise, if it begins with @samp{0}, in octal; otherwise,
1879 @multiplierSuffixes{bytes}
1881 @item -N @var{bytes}
1882 @itemx --read-bytes=@var{bytes}
1884 @opindex --read-bytes
1885 Output at most @var{bytes} bytes of the input. Prefixes and suffixes on
1886 @code{bytes} are interpreted as for the @option{-j} option.
1888 @item -S @var{bytes}
1889 @itemx --strings[=@var{bytes}]
1892 @cindex string constants, outputting
1893 Instead of the normal output, output only @dfn{string constants}: at
1894 least @var{bytes} consecutive ASCII graphic characters,
1895 followed by a zero byte (ASCII NUL).
1896 Prefixes and suffixes on @var{bytes} are interpreted as for the
1899 If @var{bytes} is omitted with @option{--strings}, the default is 3.
1902 @itemx --format=@var{type}
1905 Select the format in which to output the file data. @var{type} is a
1906 string of one or more of the below type indicator characters. If you
1907 include more than one type indicator character in a single @var{type}
1908 string, or use this option more than once, @command{od} writes one copy
1909 of each output line using each of the data types that you specified,
1910 in the order that you specified.
1912 Adding a trailing ``z'' to any type specification appends a display
1913 of the ASCII character representation of the printable characters
1914 to the output line generated by the type specification.
1918 named character, ignoring high-order bit
1920 ASCII character or backslash escape,
1924 floating point (@pxref{Floating point})
1933 The type @code{a} outputs things like @samp{sp} for space, @samp{nl} for
1934 newline, and @samp{nul} for a zero byte. Only the least significant
1935 seven bits of each byte is used; the high-order bit is ignored.
1936 Type @code{c} outputs
1937 @samp{ }, @samp{\n}, and @code{\0}, respectively.
1940 Except for types @samp{a} and @samp{c}, you can specify the number
1941 of bytes to use in interpreting each number in the given data type
1942 by following the type indicator character with a decimal integer.
1943 Alternately, you can specify the size of one of the C compiler's
1944 built-in data types by following the type indicator character with
1945 one of the following characters. For integers (@samp{d}, @samp{o},
1946 @samp{u}, @samp{x}):
1959 For floating point (@code{f}):
1971 @itemx --output-duplicates
1973 @opindex --output-duplicates
1974 Output consecutive lines that are identical. By default, when two or
1975 more consecutive output lines would be identical, @command{od} outputs only
1976 the first line, and puts just an asterisk on the following line to
1977 indicate the elision.
1980 @itemx --width[=@var{n}]
1983 Dump @code{n} input bytes per output line. This must be a multiple of
1984 the least common multiple of the sizes associated with the specified
1987 If this option is not given at all, the default is 16. If @var{n} is
1988 omitted, the default is 32.
1992 The next several options are shorthands for format specifications.
1993 GNU @command{od} accepts any combination of shorthands and format
1994 specification options. These options accumulate.
2000 Output as named characters. Equivalent to @samp{-t a}.
2004 Output as octal bytes. Equivalent to @samp{-t o1}.
2008 Output as ASCII characters or backslash escapes. Equivalent to
2013 Output as unsigned decimal two-byte units. Equivalent to @samp{-t u2}.
2017 Output as floats. Equivalent to @samp{-t fF}.
2021 Output as decimal ints. Equivalent to @samp{-t dI}.
2025 Output as decimal long ints. Equivalent to @samp{-t dL}.
2029 Output as octal two-byte units. Equivalent to @option{-t o2}.
2033 Output as decimal two-byte units. Equivalent to @option{-t d2}.
2037 Output as hexadecimal two-byte units. Equivalent to @samp{-t x2}.
2040 @opindex --traditional
2041 Recognize the non-option label argument that traditional @command{od}
2042 accepted. The following syntax:
2045 od --traditional [@var{file}] [[+]@var{offset}[.][b] [[+]@var{label}[.][b]]]
2049 can be used to specify at most one file and optional arguments
2050 specifying an offset and a pseudo-start address, @var{label}.
2051 The @var{label} argument is interpreted
2052 just like @var{offset}, but it specifies an initial pseudo-address. The
2053 pseudo-addresses are displayed in parentheses following any normal
2060 @node base64 invocation
2061 @section @command{base64}: Transform data into printable data
2064 @cindex base64 encoding
2066 @command{base64} transforms data read from a file, or standard input,
2067 into (or from) base64 encoded form. The base64 encoded form uses
2068 printable ASCII characters to represent binary data.
2072 base64 [@var{option}]@dots{} [@var{file}]
2073 base64 --decode [@var{option}]@dots{} [@var{file}]
2076 The base64 encoding expands data to roughly 133% of the original.
2077 The format conforms to
2078 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc4648.txt, RFC 4648}.
2080 The program accepts the following options. Also see @ref{Common options}.
2085 @itemx --wrap=@var{cols}
2089 @cindex column to wrap data after
2090 During encoding, wrap lines after @var{cols} characters. This must be
2093 The default is to wrap after 76 characters. Use the value 0 to
2094 disable line wrapping altogether.
2100 @cindex Decode base64 data
2101 @cindex Base64 decoding
2102 Change the mode of operation, from the default of encoding data, to
2103 decoding data. Input is expected to be base64 encoded data, and the
2104 output will be the original data.
2107 @itemx --ignore-garbage
2109 @opindex --ignore-garbage
2110 @cindex Ignore garbage in base64 stream
2111 When decoding, newlines are always accepted.
2112 During decoding, ignore unrecognized bytes,
2113 to permit distorted data to be decoded.
2120 @node Formatting file contents
2121 @chapter Formatting file contents
2123 @cindex formatting file contents
2125 These commands reformat the contents of files.
2128 * fmt invocation:: Reformat paragraph text.
2129 * numfmt invocation:: Reformat numbers.
2130 * pr invocation:: Paginate or columnate files for printing.
2131 * fold invocation:: Wrap input lines to fit in specified width.
2135 @node fmt invocation
2136 @section @command{fmt}: Reformat paragraph text
2139 @cindex reformatting paragraph text
2140 @cindex paragraphs, reformatting
2141 @cindex text, reformatting
2143 @command{fmt} fills and joins lines to produce output lines of (at most)
2144 a given number of characters (75 by default). Synopsis:
2147 fmt [@var{option}]@dots{} [@var{file}]@dots{}
2150 @command{fmt} reads from the specified @var{file} arguments (or standard
2151 input if none are given), and writes to standard output.
2153 By default, blank lines, spaces between words, and indentation are
2154 preserved in the output; successive input lines with different
2155 indentation are not joined; tabs are expanded on input and introduced on
2158 @cindex line-breaking
2159 @cindex sentences and line-breaking
2160 @cindex Knuth, Donald E.
2161 @cindex Plass, Michael F.
2162 @command{fmt} prefers breaking lines at the end of a sentence, and tries to
2163 avoid line breaks after the first word of a sentence or before the last
2164 word of a sentence. A @dfn{sentence break} is defined as either the end
2165 of a paragraph or a word ending in any of @samp{.?!}, followed by two
2166 spaces or end of line, ignoring any intervening parentheses or quotes.
2167 Like @TeX{}, @command{fmt} reads entire ``paragraphs'' before choosing line
2168 breaks; the algorithm is a variant of that given by Donald E. Knuth
2169 and Michael F. Plass in ``Breaking Paragraphs Into Lines'',
2170 @cite{Software---Practice & Experience} @b{11}, 11 (November 1981),
2173 The program accepts the following options. Also see @ref{Common options}.
2178 @itemx --crown-margin
2180 @opindex --crown-margin
2181 @cindex crown margin
2182 @dfn{Crown margin} mode: preserve the indentation of the first two
2183 lines within a paragraph, and align the left margin of each subsequent
2184 line with that of the second line.
2187 @itemx --tagged-paragraph
2189 @opindex --tagged-paragraph
2190 @cindex tagged paragraphs
2191 @dfn{Tagged paragraph} mode: like crown margin mode, except that if
2192 indentation of the first line of a paragraph is the same as the
2193 indentation of the second, the first line is treated as a one-line
2199 @opindex --split-only
2200 Split lines only. Do not join short lines to form longer ones. This
2201 prevents sample lines of code, and other such ``formatted'' text from
2202 being unduly combined.
2205 @itemx --uniform-spacing
2207 @opindex --uniform-spacing
2208 Uniform spacing. Reduce spacing between words to one space, and spacing
2209 between sentences to two spaces.
2212 @itemx -w @var{width}
2213 @itemx --width=@var{width}
2214 @opindex -@var{width}
2217 Fill output lines up to @var{width} characters (default 75 or @var{goal}
2218 plus 10, if @var{goal} is provided).
2221 @itemx --goal=@var{goal}
2224 @command{fmt} initially tries to make lines @var{goal} characters wide.
2225 By default, this is 7% shorter than @var{width}.
2227 @item -p @var{prefix}
2228 @itemx --prefix=@var{prefix}
2229 Only lines beginning with @var{prefix} (possibly preceded by whitespace)
2230 are subject to formatting. The prefix and any preceding whitespace are
2231 stripped for the formatting and then re-attached to each formatted output
2232 line. One use is to format certain kinds of program comments, while
2233 leaving the code unchanged.
2239 @node numfmt invocation
2240 @section @command{numfmt}: Reformat numbers
2244 @command{numfmt} reads numbers in various representations and reformats them
2245 as requested. The most common usage is converting numbers to/from @emph{human}
2246 representation (e.g. @samp{4G} @expansion{} @samp{4,000,000,000}).
2249 numfmt [@var{option}]@dots{} [@var{number}]
2252 @command{numfmt} converts each @var{number} on the command-line according to the
2253 specified options (see below). If no @var{number}s are given, it reads numbers
2254 from standard input. @command{numfmt} can optionally extract numbers from
2255 specific columns, maintaining proper line padding and alignment.
2259 See @option{--invalid} for additional information regarding exit status.
2261 @subsection General options
2263 The program accepts the following options. Also see @ref{Common options}.
2267 @item --from=@var{unit}
2269 Auto-scales input numbers according to @var{unit}. See @emph{Units} below.
2270 The default is no scaling, meaning suffixes (e.g. @samp{M}, @samp{G}) will
2273 @item --from-unit=@var{n}
2274 @opindex --from-unit
2275 Specify the input unit size (instead of the default 1). Use this option when the
2276 input numbers represent other units (e.g. if the input number @samp{10}
2277 represents 10 units of 512 bytes, use @samp{--from=unit=512}).
2279 @item --to=@var{unit}
2281 Auto-scales output numbers according to @var{unit}. See @emph{Units} below.
2282 The default is no scaling, meaning all the digits of the number are printed.
2284 @item --to-unit=@var{n}
2286 Specify the output unit size (instead of the default 1). Use this option when
2287 the output numbers represent other units (e.g. to represent @samp{4,000,000}
2288 bytes in blocks of 1KB, use @samp{--to=si --to=units=1000}).
2290 @item --round=@var{method}
2293 @opindex --round=down
2294 @opindex --round=from-zero
2295 @opindex --round=towards-zero
2296 @opindex --round=nearest
2297 When converting number representations, round the number according to
2298 @var{method}, which can be @samp{up}, @samp{down},
2299 @samp{from-zero} (the default), @samp{towards-zero}, @samp{nearest}.
2301 @item --suffix=@var{suffix}
2303 Add @samp{SUFFIX} to the output numbers, and accept optional @samp{SUFFIX} in
2306 @item --padding=@var{n}
2308 Pad the output numbers to @var{n} characters, by adding spaces. If @var{n} is
2309 a positive number, numbers will be right-aligned. If @var{n} is a negative
2310 number, numbers will be left-aligned. By default, numbers are automatically
2311 aligned based on the input line's width (only with the default delimiter).
2315 Group digits in output numbers according to the current locale's grouping rules
2316 (e.g @emph{Thousands Separator} character, commonly @samp{.} (dot) or @samp{,}
2317 comma). This option has no effect in @samp{POSIX/C} locale.
2319 @item --header[=@var{n}]
2322 Print the first @var{n} (default: 1) lines without any conversion.
2324 @item --field=@var{n}
2326 Convert the number in input field @var{n} (default: 1).
2328 @item --format=@var{format}
2330 Use printf-style floating FORMAT string. The @var{format} string must contain
2331 one @samp{%f} directive, optionally with @samp{'}, @samp{-}, or width
2332 modifiers. @samp{'} modified will enable @option{--grouping}. @samp{-} modifier
2333 will enable left-aligned @option{--padding}. Width modifier will enable
2334 right-aligned @option{--padding}.
2336 @item --invalid=@var{mode}
2338 The default action on input errors is to exit immediately with status code 2.
2339 @option{--invalid=@samp{abort}} explicitly specifies this default mode.
2340 With a @var{mode} of @samp{fail}, print a warning for @emph{each} conversion
2341 error, and exit with status 2. With a @var{mode} of @samp{warn}, exit with
2342 status 0, even in the presence of conversion errors, and with a @var{mode} of
2343 @samp{ignore} do not even print diagnostics.
2346 @itemx --delimiter=@var{d}
2348 @opindex --delimiter
2349 Use the character @var{d} as input field separator (default: whitespace).
2350 @emph{Note}: Using non-default delimiter turns off automatic padding.
2354 Print (to standard error) warning messages about possible erroneous usage.
2358 @subsection Possible @var{unit}s:
2360 The following are the possible @var{unit} options with @option{--from=UNITS} and
2361 @option{--to=UNITS}:
2366 No scaling is performed. For input numbers, no suffixes are accepted, and any
2367 trailing characters following the number will trigger an error. For output
2368 numbers, all digits of the numbers will be printed.
2371 Auto-scale numbers according to the @emph{International System of Units (SI)}
2373 For input numbers, accept one of the following suffixes.
2374 For output numbers, values larger than 1000 will be rounded, and printed with
2375 one of the following suffixes:
2378 @samp{K} => @math{1000^1 = 10^3} (Kilo)
2379 @samp{M} => @math{1000^2 = 10^6} (Mega)
2380 @samp{G} => @math{1000^3 = 10^9} (Giga)
2381 @samp{T} => @math{1000^4 = 10^{12}} (Tera)
2382 @samp{P} => @math{1000^5 = 10^{15}} (Peta)
2383 @samp{E} => @math{1000^6 = 10^{18}} (Exa)
2384 @samp{Z} => @math{1000^7 = 10^{21}} (Zetta)
2385 @samp{Y} => @math{1000^8 = 10^{24}} (Yotta)
2389 Auto-scale numbers according to the @emph{International Electronical
2390 Commission (IEC)} standard.
2391 For input numbers, accept one of the following suffixes.
2392 For output numbers, values larger than 1024 will be rounded, and printed with
2393 one of the following suffixes:
2396 @samp{K} => @math{1024^1 = 2^{10}} (Kibi)
2397 @samp{M} => @math{1024^2 = 2^{20}} (Mebi)
2398 @samp{G} => @math{1024^3 = 2^{30}} (Gibi)
2399 @samp{T} => @math{1024^4 = 2^{40}} (Tebi)
2400 @samp{P} => @math{1024^5 = 2^{50}} (Pebi)
2401 @samp{E} => @math{1024^6 = 2^{60}} (Exbi)
2402 @samp{Z} => @math{1024^7 = 2^{70}} (Zebi)
2403 @samp{Y} => @math{1024^8 = 2^{80}} (Yobi)
2406 The @option{iec} option uses a single letter suffix (e.g. @samp{G}), which is
2407 not fully standard, as the @emph{iec} standard recommends a two-letter symbol
2408 (e.g @samp{Gi}) - but in practice, this method common. Compare with
2409 the @option{iec-i} option.
2412 Auto-scale numbers according to the @emph{International Electronical
2413 Commission (IEC)} standard.
2414 For input numbers, accept one of the following suffixes.
2415 For output numbers, values larger than 1024 will be rounded, and printed with
2416 one of the following suffixes:
2419 @samp{Ki} => @math{1024^1 = 2^{10}} (Kibi)
2420 @samp{Mi} => @math{1024^2 = 2^{20}} (Mebi)
2421 @samp{Gi} => @math{1024^3 = 2^{30}} (Gibi)
2422 @samp{Ti} => @math{1024^4 = 2^{40}} (Tebi)
2423 @samp{Pi} => @math{1024^5 = 2^{50}} (Pebi)
2424 @samp{Ei} => @math{1024^6 = 2^{60}} (Exbi)
2425 @samp{Zi} => @math{1024^7 = 2^{70}} (Zebi)
2426 @samp{Yi} => @math{1024^8 = 2^{80}} (Yobi)
2429 The @option{iec-i} option uses a two-letter suffix symbol (e.g. @samp{Gi}),
2430 as the @emph{iec} standard recommends, but this is not always common in
2431 practice. Compare with the @option{iec} option.
2434 @samp{auto} can only be used with @option{--from}. With this method, numbers
2435 with @samp{K},@samp{M},@samp{G},@samp{T},@samp{P},@samp{E},@samp{Z},@samp{Y}
2436 suffixes are interpreted as @emph{SI} values, and numbers with @samp{Ki},
2437 @samp{Mi},@samp{Gi},@samp{Ti},@samp{Pi},@samp{Ei},@samp{Zi},@samp{Yi} suffixes
2438 are interpreted as @emph{IEC} values.
2442 @subsection Examples of using @command{numfmt}
2444 Converting a single number from/to @emph{human} representation:
2446 $ nunfmt --to=si 500000
2449 $ numfmt --to=iec 500000
2452 $ numfmt --to=iec-i 500000
2455 $ numfmt --from=si 1M
2458 $ numfmt --from=iec 1M
2461 # with '--from=auto', M=Mega, Mi=Mebi
2462 $ numfmt --from=auto 1M
2464 $ numfmt --from=auto 1Mi
2468 Converting from @samp{SI} to @samp{IEC} scales (e.g. when a harddisk capacity is
2469 advertised as @samp{1TB}, while checking the drive's capacity gives lower
2473 $ numfmt --from=si --to=iec 1T
2478 Converting a single field from an input file / piped input (these contrived
2479 examples are for demonstration purposes only, as both @command{ls} and
2480 @command{df} support the @option{--human-readable} option to
2481 output sizes in human-readable format):
2484 # Third field (file size) will be shown in SI representation
2485 $ ls -log | numfmt --field 3 --header --to=si | head -n4
2486 -rw-r--r-- 1 94K Aug 23 2011 ABOUT-NLS
2487 -rw-r--r-- 1 3.7K Jan 7 16:15 AUTHORS
2488 -rw-r--r-- 1 36K Jun 1 2011 COPYING
2489 -rw-r--r-- 1 0 Jan 7 15:15 ChangeLog
2491 # Second field (size) will be shown in IEC representation
2492 $ df --block-size=1 | numfmt --field 2 --header --to=iec | head -n4
2493 File system 1B-blocks Used Available Use% Mounted on
2494 rootfs 132G 104741408 26554036 80% /
2495 tmpfs 794M 7580 804960 1% /run/shm
2496 /dev/sdb1 694G 651424756 46074696 94% /home
2500 Output can be tweaked using @option{--padding} or @option{--format}:
2503 # Pad to 10 characters, right-aligned
2504 $ du -s * | numfmt --to=si --padding=10
2510 # Pad to 10 characters, left-aligned
2511 $ du -s * | numfmt --to=si --padding=-10
2517 # Pad to 10 characters, left-aligned, using 'format'
2518 $ du -s * | numfmt --to=si --format="%10f"
2524 # Pad to 10 characters, left-aligned, using 'format'
2525 $ du -s * | numfmt --to=si --padding="%-10f"
2532 With locales that support grouping digits, using @option{--grouping} or
2533 @option{--format} enables grouping. In @samp{POSIX} locale, grouping is silently
2537 $ LC_ALL=C numfmt --from=iec --grouping 2G
2540 $ LC_ALL=en_US.utf8 numfmt --from=iec --grouping 2G
2543 $ LC_ALL=ta_IN numfmt --from=iec --grouping 2G
2546 $ LC_ALL=C ./src/numfmt --from=iec --format="==%'15f==" 2G
2549 $ LC_ALL=en_US.utf8 ./src/numfmt --from=iec --format="==%'15f==" 2G
2552 $ LC_ALL=en_US.utf8 ./src/numfmt --from=iec --format="==%'-15f==" 2G
2555 $ LC_ALL=ta_IN ./src/numfmt --from=iec --format="==%'15f==" 2G
2560 @section @command{pr}: Paginate or columnate files for printing
2563 @cindex printing, preparing files for
2564 @cindex multicolumn output, generating
2565 @cindex merging files in parallel
2567 @command{pr} writes each @var{file} (@samp{-} means standard input), or
2568 standard input if none are given, to standard output, paginating and
2569 optionally outputting in multicolumn format; optionally merges all
2570 @var{file}s, printing all in parallel, one per column. Synopsis:
2573 pr [@var{option}]@dots{} [@var{file}]@dots{}
2577 By default, a 5-line header is printed at each page: two blank lines;
2578 a line with the date, the file name, and the page count; and two more
2579 blank lines. A footer of five blank lines is also printed.
2580 The default @var{page_length} is 66
2581 lines. The default number of text lines is therefore 56.
2582 The text line of the header takes the form
2583 @samp{@var{date} @var{string} @var{page}}, with spaces inserted around
2584 @var{string} so that the line takes up the full @var{page_width}. Here,
2585 @var{date} is the date (see the @option{-D} or @option{--date-format}
2586 option for details), @var{string} is the centered header string, and
2587 @var{page} identifies the page number. The @env{LC_MESSAGES} locale
2588 category affects the spelling of @var{page}; in the default C locale, it
2589 is @samp{Page @var{number}} where @var{number} is the decimal page
2592 Form feeds in the input cause page breaks in the output. Multiple form
2593 feeds produce empty pages.
2595 Columns are of equal width, separated by an optional string (default
2596 is @samp{space}). For multicolumn output, lines will always be truncated to
2597 @var{page_width} (default 72), unless you use the @option{-J} option.
2599 column output no line truncation occurs by default. Use @option{-W} option to
2600 truncate lines in that case.
2602 The program accepts the following options. Also see @ref{Common options}.
2606 @item +@var{first_page}[:@var{last_page}]
2607 @itemx --pages=@var{first_page}[:@var{last_page}]
2608 @c The two following @opindex lines evoke warnings because they contain ':'
2609 @c The 'info' spec does not permit that. If we use those lines, we end
2610 @c up with truncated index entries that don't work.
2611 @c @opindex +@var{first_page}[:@var{last_page}]
2612 @c @opindex --pages=@var{first_page}[:@var{last_page}]
2613 @opindex +@var{page_range}
2614 @opindex --pages=@var{page_range}
2615 Begin printing with page @var{first_page} and stop with @var{last_page}.
2616 Missing @samp{:@var{last_page}} implies end of file. While estimating
2617 the number of skipped pages each form feed in the input file results
2618 in a new page. Page counting with and without @samp{+@var{first_page}}
2619 is identical. By default, counting starts with the first page of input
2620 file (not first page printed). Line numbering may be altered by @option{-N}
2624 @itemx --columns=@var{column}
2625 @opindex -@var{column}
2627 @cindex down columns
2628 With each single @var{file}, produce @var{column} columns of output
2629 (default is 1) and print columns down, unless @option{-a} is used. The
2630 column width is automatically decreased as @var{column} increases; unless
2631 you use the @option{-W/-w} option to increase @var{page_width} as well.
2632 This option might well cause some lines to be truncated. The number of
2633 lines in the columns on each page are balanced. The options @option{-e}
2634 and @option{-i} are on for multiple text-column output. Together with
2635 @option{-J} option column alignment and line truncation is turned off.
2636 Lines of full length are joined in a free field format and @option{-S}
2637 option may set field separators. @option{-@var{column}} may not be used
2638 with @option{-m} option.
2644 @cindex across columns
2645 With each single @var{file}, print columns across rather than down. The
2646 @option{-@var{column}} option must be given with @var{column} greater than one.
2647 If a line is too long to fit in a column, it is truncated.
2650 @itemx --show-control-chars
2652 @opindex --show-control-chars
2653 Print control characters using hat notation (e.g., @samp{^G}); print
2654 other nonprinting characters in octal backslash notation. By default,
2655 nonprinting characters are not changed.
2658 @itemx --double-space
2660 @opindex --double-space
2661 @cindex double spacing
2662 Double space the output.
2664 @item -D @var{format}
2665 @itemx --date-format=@var{format}
2666 @cindex time formats
2667 @cindex formatting times
2668 Format header dates using @var{format}, using the same conventions as
2669 for the command @samp{date +@var{format}}. @xref{date invocation}.
2670 Except for directives, which start with
2671 @samp{%}, characters in @var{format} are printed unchanged. You can use
2672 this option to specify an arbitrary string in place of the header date,
2673 e.g., @option{--date-format="Monday morning"}.
2675 @vindex POSIXLY_CORRECT
2677 The default date format is @samp{%Y-%m-%d %H:%M} (for example,
2678 @samp{2001-12-04 23:59});
2679 but if the @env{POSIXLY_CORRECT} environment variable is set
2680 and the @env{LC_TIME} locale category specifies the POSIX
2681 locale, the default is @samp{%b %e %H:%M %Y} (for example,
2682 @samp{Dec@ @ 4 23:59 2001}.
2685 Time stamps are listed according to the time zone rules specified by
2686 the @env{TZ} environment variable, or by the system default rules if
2687 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
2688 with @env{TZ}, libc, The GNU C Library Reference Manual}.
2690 @item -e[@var{in-tabchar}[@var{in-tabwidth}]]
2691 @itemx --expand-tabs[=@var{in-tabchar}[@var{in-tabwidth}]]
2693 @opindex --expand-tabs
2695 Expand @var{tab}s to spaces on input. Optional argument @var{in-tabchar} is
2696 the input tab character (default is the TAB character). Second optional
2697 argument @var{in-tabwidth} is the input tab character's width (default
2705 @opindex --form-feed
2706 Use a form feed instead of newlines to separate output pages. This does
2707 not alter the default page length of 66 lines.
2709 @item -h @var{header}
2710 @itemx --header=@var{header}
2713 Replace the file name in the header with the centered string @var{header}.
2714 When using the shell, @var{header} should be quoted and should be
2715 separated from @option{-h} by a space.
2717 @item -i[@var{out-tabchar}[@var{out-tabwidth}]]
2718 @itemx --output-tabs[=@var{out-tabchar}[@var{out-tabwidth}]]
2720 @opindex --output-tabs
2722 Replace spaces with @var{tab}s on output. Optional argument @var{out-tabchar}
2723 is the output tab character (default is the TAB character). Second optional
2724 argument @var{out-tabwidth} is the output tab character's width (default
2730 @opindex --join-lines
2731 Merge lines of full length. Used together with the column options
2732 @option{-@var{column}}, @option{-a -@var{column}} or @option{-m}. Turns off
2733 @option{-W/-w} line truncation;
2734 no column alignment used; may be used with
2735 @option{--sep-string[=@var{string}]}. @option{-J} has been introduced
2736 (together with @option{-W} and @option{--sep-string})
2737 to disentangle the old (POSIX-compliant) options @option{-w} and
2738 @option{-s} along with the three column options.
2741 @item -l @var{page_length}
2742 @itemx --length=@var{page_length}
2745 Set the page length to @var{page_length} (default 66) lines, including
2746 the lines of the header [and the footer]. If @var{page_length} is less
2747 than or equal to 10, the header and footer are omitted, as if the
2748 @option{-t} option had been given.
2754 Merge and print all @var{file}s in parallel, one in each column. If a
2755 line is too long to fit in a column, it is truncated, unless the @option{-J}
2756 option is used. @option{--sep-string[=@var{string}]} may be used.
2758 some @var{file}s (form feeds set) produce empty columns, still marked
2759 by @var{string}. The result is a continuous line numbering and column
2760 marking throughout the whole merged file. Completely empty merged pages
2761 show no separators or line numbers. The default header becomes
2762 @samp{@var{date} @var{page}} with spaces inserted in the middle; this
2763 may be used with the @option{-h} or @option{--header} option to fill up
2764 the middle blank part.
2766 @item -n[@var{number-separator}[@var{digits}]]
2767 @itemx --number-lines[=@var{number-separator}[@var{digits}]]
2769 @opindex --number-lines
2770 Provide @var{digits} digit line numbering (default for @var{digits} is
2771 5). With multicolumn output the number occupies the first @var{digits}
2772 column positions of each text column or only each line of @option{-m}
2773 output. With single column output the number precedes each line just as
2774 @option{-m} does. Default counting of the line numbers starts with the
2775 first line of the input file (not the first line printed, compare the
2776 @option{--page} option and @option{-N} option).
2777 Optional argument @var{number-separator} is the character appended to
2778 the line number to separate it from the text followed. The default
2779 separator is the TAB character. In a strict sense a TAB is always
2780 printed with single column output only. The TAB width varies
2781 with the TAB position, e.g., with the left @var{margin} specified
2782 by @option{-o} option. With multicolumn output priority is given to
2783 @samp{equal width of output columns} (a POSIX specification).
2784 The TAB width is fixed to the value of the first column and does
2785 not change with different values of left @var{margin}. That means a
2786 fixed number of spaces is always printed in the place of the
2787 @var{number-separator} TAB@. The tabification depends upon the output
2790 @item -N @var{line_number}
2791 @itemx --first-line-number=@var{line_number}
2793 @opindex --first-line-number
2794 Start line counting with the number @var{line_number} at first line of
2795 first page printed (in most cases not the first line of the input file).
2797 @item -o @var{margin}
2798 @itemx --indent=@var{margin}
2801 @cindex indenting lines
2803 Indent each line with a margin @var{margin} spaces wide (default is zero).
2804 The total page width is the size of the margin plus the @var{page_width}
2805 set with the @option{-W/-w} option. A limited overflow may occur with
2806 numbered single column output (compare @option{-n} option).
2809 @itemx --no-file-warnings
2811 @opindex --no-file-warnings
2812 Do not print a warning message when an argument @var{file} cannot be
2813 opened. (The exit status will still be nonzero, however.)
2815 @item -s[@var{char}]
2816 @itemx --separator[=@var{char}]
2818 @opindex --separator
2819 Separate columns by a single character @var{char}. The default for
2820 @var{char} is the TAB character without @option{-w} and @samp{no
2821 character} with @option{-w}. Without @option{-s} the default separator
2822 @samp{space} is set. @option{-s[char]} turns off line truncation of all
2823 three column options (@option{-COLUMN}|@option{-a -COLUMN}|@option{-m}) unless
2824 @option{-w} is set. This is a POSIX-compliant formulation.
2827 @item -S[@var{string}]
2828 @itemx --sep-string[=@var{string}]
2830 @opindex --sep-string
2831 Use @var{string} to separate output columns. The @option{-S} option doesn't
2832 affect the @option{-W/-w} option, unlike the @option{-s} option which does. It
2833 does not affect line truncation or column alignment.
2834 Without @option{-S}, and with @option{-J}, @command{pr} uses the default output
2836 Without @option{-S} or @option{-J}, @command{pr} uses a @samp{space}
2837 (same as @option{-S"@w{ }"}).
2838 If no @samp{@var{string}} argument is specified, @samp{""} is assumed.
2841 @itemx --omit-header
2843 @opindex --omit-header
2844 Do not print the usual header [and footer] on each page, and do not fill
2845 out the bottom of pages (with blank lines or a form feed). No page
2846 structure is produced, but form feeds set in the input files are retained.
2847 The predefined pagination is not changed. @option{-t} or @option{-T} may be
2848 useful together with other options; e.g.: @option{-t -e4}, expand TAB characters
2849 in the input file to 4 spaces but don't make any other changes. Use of
2850 @option{-t} overrides @option{-h}.
2853 @itemx --omit-pagination
2855 @opindex --omit-pagination
2856 Do not print header [and footer]. In addition eliminate all form feeds
2857 set in the input files.
2860 @itemx --show-nonprinting
2862 @opindex --show-nonprinting
2863 Print nonprinting characters in octal backslash notation.
2865 @item -w @var{page_width}
2866 @itemx --width=@var{page_width}
2869 Set page width to @var{page_width} characters for multiple text-column
2870 output only (default for @var{page_width} is 72). @option{-s[CHAR]} turns
2871 off the default page width and any line truncation and column alignment.
2872 Lines of full length are merged, regardless of the column options
2873 set. No @var{page_width} setting is possible with single column output.
2874 A POSIX-compliant formulation.
2876 @item -W @var{page_width}
2877 @itemx --page_width=@var{page_width}
2879 @opindex --page_width
2880 Set the page width to @var{page_width} characters. That's valid with and
2881 without a column option. Text lines are truncated, unless @option{-J}
2882 is used. Together with one of the three column options
2883 (@option{-@var{column}}, @option{-a -@var{column}} or @option{-m}) column
2884 alignment is always used. The separator options @option{-S} or @option{-s}
2885 don't affect the @option{-W} option. Default is 72 characters. Without
2886 @option{-W @var{page_width}} and without any of the column options NO line
2887 truncation is used (defined to keep downward compatibility and to meet
2888 most frequent tasks). That's equivalent to @option{-W 72 -J}@. The header
2889 line is never truncated.
2896 @node fold invocation
2897 @section @command{fold}: Wrap input lines to fit in specified width
2900 @cindex wrapping long input lines
2901 @cindex folding long input lines
2903 @command{fold} writes each @var{file} (@option{-} means standard input), or
2904 standard input if none are given, to standard output, breaking long
2908 fold [@var{option}]@dots{} [@var{file}]@dots{}
2911 By default, @command{fold} breaks lines wider than 80 columns. The output
2912 is split into as many lines as necessary.
2914 @cindex screen columns
2915 @command{fold} counts screen columns by default; thus, a tab may count more
2916 than one column, backspace decreases the column count, and carriage
2917 return sets the column to zero.
2919 The program accepts the following options. Also see @ref{Common options}.
2927 Count bytes rather than columns, so that tabs, backspaces, and carriage
2928 returns are each counted as taking up one column, just like other
2935 Break at word boundaries: the line is broken after the last blank before
2936 the maximum line length. If the line contains no such blanks, the line
2937 is broken at the maximum line length as usual.
2939 @item -w @var{width}
2940 @itemx --width=@var{width}
2943 Use a maximum line length of @var{width} columns instead of 80.
2945 For compatibility @command{fold} supports an obsolete option syntax
2946 @option{-@var{width}}. New scripts should use @option{-w @var{width}}
2954 @node Output of parts of files
2955 @chapter Output of parts of files
2957 @cindex output of parts of files
2958 @cindex parts of files, output of
2960 These commands output pieces of the input.
2963 * head invocation:: Output the first part of files.
2964 * tail invocation:: Output the last part of files.
2965 * split invocation:: Split a file into pieces.
2966 * csplit invocation:: Split a file into context-determined pieces.
2969 @node head invocation
2970 @section @command{head}: Output the first part of files
2973 @cindex initial part of files, outputting
2974 @cindex first part of files, outputting
2976 @command{head} prints the first part (10 lines by default) of each
2977 @var{file}; it reads from standard input if no files are given or
2978 when given a @var{file} of @option{-}. Synopsis:
2981 head [@var{option}]@dots{} [@var{file}]@dots{}
2984 If more than one @var{file} is specified, @command{head} prints a
2985 one-line header consisting of:
2988 ==> @var{file name} <==
2992 before the output for each @var{file}.
2994 The program accepts the following options. Also see @ref{Common options}.
2999 @itemx --bytes=@var{k}
3002 Print the first @var{k} bytes, instead of initial lines.
3003 However, if @var{k} starts with a @samp{-},
3004 print all but the last @var{k} bytes of each file.
3005 @multiplierSuffixes{k}
3008 @itemx --lines=@var{k}
3011 Output the first @var{k} lines.
3012 However, if @var{k} starts with a @samp{-},
3013 print all but the last @var{k} lines of each file.
3014 Size multiplier suffixes are the same as with the @option{-c} option.
3022 Never print file name headers.
3028 Always print file name headers.
3032 For compatibility @command{head} also supports an obsolete option syntax
3033 @option{-@var{count}@var{options}}, which is recognized only if it is
3034 specified first. @var{count} is a decimal number optionally followed
3035 by a size letter (@samp{b}, @samp{k}, @samp{m}) as in @option{-c}, or
3036 @samp{l} to mean count by lines, or other option letters (@samp{cqv}).
3037 Scripts intended for standard hosts should use @option{-c @var{count}}
3038 or @option{-n @var{count}} instead. If your script must also run on
3039 hosts that support only the obsolete syntax, it is usually simpler to
3040 avoid @command{head}, e.g., by using @samp{sed 5q} instead of
3046 @node tail invocation
3047 @section @command{tail}: Output the last part of files
3050 @cindex last part of files, outputting
3052 @command{tail} prints the last part (10 lines by default) of each
3053 @var{file}; it reads from standard input if no files are given or
3054 when given a @var{file} of @samp{-}. Synopsis:
3057 tail [@var{option}]@dots{} [@var{file}]@dots{}
3060 If more than one @var{file} is specified, @command{tail} prints a
3061 one-line header consisting of:
3064 ==> @var{file name} <==
3068 before the output for each @var{file}.
3070 @cindex BSD @command{tail}
3071 GNU @command{tail} can output any amount of data (some other versions of
3072 @command{tail} cannot). It also has no @option{-r} option (print in
3073 reverse), since reversing a file is really a different job from printing
3074 the end of a file; BSD @command{tail} (which is the one with @option{-r}) can
3075 only reverse files that are at most as large as its buffer, which is
3076 typically 32 KiB@. A more reliable and versatile way to reverse files is
3077 the GNU @command{tac} command.
3079 The program accepts the following options. Also see @ref{Common options}.
3084 @itemx --bytes=@var{k}
3087 Output the last @var{k} bytes, instead of final lines.
3088 However, if @var{k} starts with a @samp{+}, start printing with the
3089 @var{k}th byte from the start of each file, instead of from the end.
3090 @multiplierSuffixes{k}
3093 @itemx --follow[=@var{how}]
3096 @cindex growing files
3097 @vindex name @r{follow option}
3098 @vindex descriptor @r{follow option}
3099 Loop forever trying to read more characters at the end of the file,
3100 presumably because the file is growing.
3101 If more than one file is given, @command{tail} prints a header whenever it
3102 gets output from a different file, to indicate which file that output is
3105 There are two ways to specify how you'd like to track files with this option,
3106 but that difference is noticeable only when a followed file is removed or
3108 If you'd like to continue to track the end of a growing file even after
3109 it has been unlinked, use @option{--follow=descriptor}. This is the default
3110 behavior, but it is not useful if you're tracking a log file that may be
3111 rotated (removed or renamed, then reopened). In that case, use
3112 @option{--follow=name} to track the named file, perhaps by reopening it
3113 periodically to see if it has been removed and recreated by some other program.
3114 Note that the inotify-based implementation handles this case without
3115 the need for any periodic reopening.
3117 No matter which method you use, if the tracked file is determined to have
3118 shrunk, @command{tail} prints a message saying the file has been truncated
3119 and resumes tracking the end of the file from the newly-determined endpoint.
3121 When a file is removed, @command{tail}'s behavior depends on whether it is
3122 following the name or the descriptor. When following by name, tail can
3123 detect that a file has been removed and gives a message to that effect,
3124 and if @option{--retry} has been specified it will continue checking
3125 periodically to see if the file reappears.
3126 When following a descriptor, tail does not detect that the file has
3127 been unlinked or renamed and issues no message; even though the file
3128 may no longer be accessible via its original name, it may still be
3131 The option values @samp{descriptor} and @samp{name} may be specified only
3132 with the long form of the option, not with @option{-f}.
3134 The @option{-f} option is ignored if
3135 no @var{file} operand is specified and standard input is a FIFO or a pipe.
3136 Likewise, the @option{-f} option has no effect for any
3137 operand specified as @samp{-}, when standard input is a FIFO or a pipe.
3139 With kernel inotify support, output is triggered by file changes
3140 and is generally very prompt.
3141 Otherwise, @command{tail} sleeps for one second between checks---
3142 use @option{--sleep-interval=@var{n}} to change that default---which can
3143 make the output appear slightly less responsive or bursty.
3144 When using tail without inotify support, you can make it more responsive
3145 by using a sub-second sleep interval, e.g., via an alias like this:
3148 alias tail='tail -s.1'
3153 This option is the same as @option{--follow=name --retry}. That is, tail
3154 will attempt to reopen a file when it is removed. Should this fail, tail
3155 will keep trying until it becomes accessible again.
3159 This option is useful mainly when following by name (i.e., with
3160 @option{--follow=name}).
3161 Without this option, when tail encounters a file that doesn't
3162 exist or is otherwise inaccessible, it reports that fact and
3163 never checks it again.
3165 @item --sleep-interval=@var{number}
3166 @opindex --sleep-interval
3167 Change the number of seconds to wait between iterations (the default is 1.0).
3168 During one iteration, every specified file is checked to see if it has
3170 Historical implementations of @command{tail} have required that
3171 @var{number} be an integer. However, GNU @command{tail} accepts
3172 an arbitrary floating point number. @xref{Floating point}.
3173 When @command{tail} uses inotify, this polling-related option
3174 is usually ignored. However, if you also specify @option{--pid=@var{p}},
3175 @command{tail} checks whether process @var{p} is alive at least
3176 every @var{number} seconds.
3178 @item --pid=@var{pid}
3180 When following by name or by descriptor, you may specify the process ID,
3181 @var{pid}, of the sole writer of all @var{file} arguments. Then, shortly
3182 after that process terminates, tail will also terminate. This will
3183 work properly only if the writer and the tailing process are running on
3184 the same machine. For example, to save the output of a build in a file
3185 and to watch the file grow, if you invoke @command{make} and @command{tail}
3186 like this then the tail process will stop when your build completes.
3187 Without this option, you would have had to kill the @code{tail -f}
3191 $ make >& makerr & tail --pid=$! -f makerr
3194 If you specify a @var{pid} that is not in use or that does not correspond
3195 to the process that is writing to the tailed files, then @command{tail}
3196 may terminate long before any @var{file}s stop growing or it may not
3197 terminate until long after the real writer has terminated.
3198 Note that @option{--pid} cannot be supported on some systems; @command{tail}
3199 will print a warning if this is the case.
3201 @item --max-unchanged-stats=@var{n}
3202 @opindex --max-unchanged-stats
3203 When tailing a file by name, if there have been @var{n} (default
3204 n=@value{DEFAULT_MAX_N_UNCHANGED_STATS_BETWEEN_OPENS}) consecutive
3205 iterations for which the file has not changed, then
3206 @code{open}/@code{fstat} the file to determine if that file name is
3207 still associated with the same device/inode-number pair as before.
3208 When following a log file that is rotated, this is approximately the
3209 number of seconds between when tail prints the last pre-rotation lines
3210 and when it prints the lines that have accumulated in the new log file.
3211 This option is meaningful only when polling (i.e., without inotify)
3212 and when following by name.
3215 @itemx --lines=@var{k}
3218 Output the last @var{k} lines.
3219 However, if @var{k} starts with a @samp{+}, start printing with the
3220 @var{k}th line from the start of each file, instead of from the end.
3221 Size multiplier suffixes are the same as with the @option{-c} option.
3229 Never print file name headers.
3235 Always print file name headers.
3239 For compatibility @command{tail} also supports an obsolete usage
3240 @samp{tail -[@var{count}][bcl][f] [@var{file}]}, which is recognized
3241 only if it does not conflict with the usage described
3242 above. This obsolete form uses exactly one option and at most one
3243 file. In the option, @var{count} is an optional decimal number optionally
3244 followed by a size letter (@samp{b}, @samp{c}, @samp{l}) to mean count
3245 by 512-byte blocks, bytes, or lines, optionally followed by @samp{f}
3246 which has the same meaning as @option{-f}.
3248 @vindex _POSIX2_VERSION
3249 On older systems, the leading @samp{-} can be replaced by @samp{+} in
3250 the obsolete option syntax with the same meaning as in counts, and
3251 obsolete usage overrides normal usage when the two conflict.
3252 This obsolete behavior can be enabled or disabled with the
3253 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
3256 Scripts intended for use on standard hosts should avoid obsolete
3257 syntax and should use @option{-c @var{count}[b]}, @option{-n
3258 @var{count}}, and/or @option{-f} instead. If your script must also
3259 run on hosts that support only the obsolete syntax, you can often
3260 rewrite it to avoid problematic usages, e.g., by using @samp{sed -n
3261 '$p'} rather than @samp{tail -1}. If that's not possible, the script
3262 can use a test like @samp{if tail -c +1 </dev/null >/dev/null 2>&1;
3263 then @dots{}} to decide which syntax to use.
3265 Even if your script assumes the standard behavior, you should still
3266 beware usages whose behaviors differ depending on the POSIX
3267 version. For example, avoid @samp{tail - main.c}, since it might be
3268 interpreted as either @samp{tail main.c} or as @samp{tail -- -
3269 main.c}; avoid @samp{tail -c 4}, since it might mean either @samp{tail
3270 -c4} or @samp{tail -c 10 4}; and avoid @samp{tail +4}, since it might
3271 mean either @samp{tail ./+4} or @samp{tail -n +4}.
3276 @node split invocation
3277 @section @command{split}: Split a file into pieces.
3280 @cindex splitting a file into pieces
3281 @cindex pieces, splitting a file into
3283 @command{split} creates output files containing consecutive or interleaved
3284 sections of @var{input} (standard input if none is given or @var{input}
3285 is @samp{-}). Synopsis:
3288 split [@var{option}] [@var{input} [@var{prefix}]]
3291 By default, @command{split} puts 1000 lines of @var{input} (or whatever is
3292 left over for the last section), into each output file.
3294 @cindex output file name prefix
3295 The output files' names consist of @var{prefix} (@samp{x} by default)
3296 followed by a group of characters (@samp{aa}, @samp{ab}, @dots{} by
3297 default), such that concatenating the output files in traditional
3298 sorted order by file name produces the original input file (except
3299 @option{-nr/@var{n}}). By default split will initially create files
3300 with two generated suffix characters, and will increase this width by two
3301 when the next most significant position reaches the last character.
3302 (@samp{yz}, @samp{zaaa}, @samp{zaab}, @dots{}). In this way an arbitrary
3303 number of output files are supported, which sort as described above,
3304 even in the presence of an @option{--additional-suffix} option.
3305 If the @option{-a} option is specified and the output file names are
3306 exhausted, @command{split} reports an error without deleting the
3307 output files that it did create.
3309 The program accepts the following options. Also see @ref{Common options}.
3313 @item -l @var{lines}
3314 @itemx --lines=@var{lines}
3317 Put @var{lines} lines of @var{input} into each output file.
3319 For compatibility @command{split} also supports an obsolete
3320 option syntax @option{-@var{lines}}. New scripts should use
3321 @option{-l @var{lines}} instead.
3324 @itemx --bytes=@var{size}
3327 Put @var{size} bytes of @var{input} into each output file.
3328 @multiplierSuffixes{size}
3331 @itemx --line-bytes=@var{size}
3333 @opindex --line-bytes
3334 Put into each output file as many complete lines of @var{input} as
3335 possible without exceeding @var{size} bytes. Individual lines longer than
3336 @var{size} bytes are broken into multiple files.
3337 @var{size} has the same format as for the @option{--bytes} option.
3339 @item --filter=@var{command}
3341 With this option, rather than simply writing to each output file,
3342 write through a pipe to the specified shell @var{command} for each output file.
3343 @var{command} should use the $FILE environment variable, which is set
3344 to a different output file name for each invocation of the command.
3345 For example, imagine that you have a 1TiB compressed file
3346 that, if uncompressed, would be too large to reside on disk,
3347 yet you must split it into individually-compressed pieces
3348 of a more manageable size.
3349 To do that, you might run this command:
3352 xz -dc BIG.xz | split -b200G --filter='xz > $FILE.xz' - big-
3355 Assuming a 10:1 compression ratio, that would create about fifty 20GiB files
3356 with names @file{big-aa.xz}, @file{big-ab.xz}, @file{big-ac.xz}, etc.
3358 @item -n @var{chunks}
3359 @itemx --number=@var{chunks}
3363 Split @var{input} to @var{chunks} output files where @var{chunks} may be:
3366 @var{n} generate @var{n} files based on current size of @var{input}
3367 @var{k}/@var{n} only output @var{k}th of @var{n} to stdout
3368 l/@var{n} generate @var{n} files without splitting lines
3369 l/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
3370 r/@var{n} like @samp{l} but use round robin distribution
3371 r/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
3374 Any excess bytes remaining after dividing the @var{input}
3375 into @var{n} chunks, are assigned to the last chunk.
3376 Any excess bytes appearing after the initial calculation are discarded
3377 (except when using @samp{r} mode).
3379 All @var{n} files are created even if there are fewer than @var{n} lines,
3380 or the @var{input} is truncated.
3382 For @samp{l} mode, chunks are approximately @var{input} size / @var{n}.
3383 The @var{input} is partitioned into @var{n} equal sized portions, with
3384 the last assigned any excess. If a line @emph{starts} within a partition
3385 it is written completely to the corresponding file. Since lines
3386 are not split even if they overlap a partition, the files written
3387 can be larger or smaller than the partition size, and even empty
3388 if a line is so long as to completely overlap the partition.
3390 For @samp{r} mode, the size of @var{input} is irrelevant,
3391 and so can be a pipe for example.
3393 @item -a @var{length}
3394 @itemx --suffix-length=@var{length}
3396 @opindex --suffix-length
3397 Use suffixes of length @var{length}. If a @var{length} of 0 is specified,
3398 this is the same as if (any previous) @option{-a} was not specified, and
3399 thus enables the default behavior, which starts the suffix length at 2,
3400 and unless @option{-n} or @option{--numeric-suffixes=@var{from}} is
3401 specified, will auto increase the length by 2 as required.
3404 @itemx --numeric-suffixes[=@var{from}]
3406 @opindex --numeric-suffixes
3407 Use digits in suffixes rather than lower-case letters. The numerical
3408 suffix counts from @var{from} if specified, 0 otherwise.
3409 Note specifying a @var{from} value also disables the default
3410 auto suffix length expansion described above, and so you may also
3411 want to specify @option{-a} to allow suffixes beyond @samp{99}.
3413 @item --additional-suffix=@var{suffix}
3414 @opindex --additional-suffix
3415 Append an additional @var{suffix} to output file names. @var{suffix}
3416 must not contain slash.
3419 @itemx --elide-empty-files
3421 @opindex --elide-empty-files
3422 Suppress the generation of zero-length output files. This can happen
3423 with the @option{--number} option if a file is (truncated to be) shorter
3424 than the number requested, or if a line is so long as to completely
3425 span a chunk. The output file sequence numbers, always run consecutively
3426 even when this option is specified.
3431 @opindex --unbuffered
3432 Immediately copy input to output in @option{--number r/@dots{}} mode,
3433 which is a much slower mode of operation.
3437 Write a diagnostic just before each output file is opened.
3443 Here are a few examples to illustrate how the
3444 @option{--number} (@option{-n}) option works:
3446 Notice how, by default, one line may be split onto two or more:
3449 $ seq -w 6 10 > k; split -n3 k; head xa?
3462 Use the "l/" modifier to suppress that:
3465 $ seq -w 6 10 > k; split -nl/3 k; head xa?
3478 Use the "r/" modifier to distribute lines in a round-robin fashion:
3481 $ seq -w 6 10 > k; split -nr/3 k; head xa?
3494 You can also extract just the Kth chunk.
3495 This extracts and prints just the 7th "chunk" of 33:
3498 $ seq 100 > k; split -nl/7/33 k
3505 @node csplit invocation
3506 @section @command{csplit}: Split a file into context-determined pieces
3509 @cindex context splitting
3510 @cindex splitting a file into pieces by context
3512 @command{csplit} creates zero or more output files containing sections of
3513 @var{input} (standard input if @var{input} is @samp{-}). Synopsis:
3516 csplit [@var{option}]@dots{} @var{input} @var{pattern}@dots{}
3519 The contents of the output files are determined by the @var{pattern}
3520 arguments, as detailed below. An error occurs if a @var{pattern}
3521 argument refers to a nonexistent line of the input file (e.g., if no
3522 remaining line matches a given regular expression). After every
3523 @var{pattern} has been matched, any remaining input is copied into one
3526 By default, @command{csplit} prints the number of bytes written to each
3527 output file after it has been created.
3529 The types of pattern arguments are:
3534 Create an output file containing the input up to but not including line
3535 @var{n} (a positive integer). If followed by a repeat count, also
3536 create an output file containing the next @var{n} lines of the input
3537 file once for each repeat.
3539 @item /@var{regexp}/[@var{offset}]
3540 Create an output file containing the current line up to (but not
3541 including) the next line of the input file that contains a match for
3542 @var{regexp}. The optional @var{offset} is an integer.
3543 If it is given, the input up to (but not including) the
3544 matching line plus or minus @var{offset} is put into the output file,
3545 and the line after that begins the next section of input.
3547 @item %@var{regexp}%[@var{offset}]
3548 Like the previous type, except that it does not create an output
3549 file, so that section of the input file is effectively ignored.
3551 @item @{@var{repeat-count}@}
3552 Repeat the previous pattern @var{repeat-count} additional
3553 times. The @var{repeat-count} can either be a positive integer or an
3554 asterisk, meaning repeat as many times as necessary until the input is
3559 The output files' names consist of a prefix (@samp{xx} by default)
3560 followed by a suffix. By default, the suffix is an ascending sequence
3561 of two-digit decimal numbers from @samp{00} to @samp{99}. In any case,
3562 concatenating the output files in sorted order by file name produces the
3563 original input file.
3565 By default, if @command{csplit} encounters an error or receives a hangup,
3566 interrupt, quit, or terminate signal, it removes any output files
3567 that it has created so far before it exits.
3569 The program accepts the following options. Also see @ref{Common options}.
3573 @item -f @var{prefix}
3574 @itemx --prefix=@var{prefix}
3577 @cindex output file name prefix
3578 Use @var{prefix} as the output file name prefix.
3580 @item -b @var{suffix}
3581 @itemx --suffix=@var{suffix}
3584 @cindex output file name suffix
3585 Use @var{suffix} as the output file name suffix. When this option is
3586 specified, the suffix string must include exactly one
3587 @code{printf(3)}-style conversion specification, possibly including
3588 format specification flags, a field width, a precision specifications,
3589 or all of these kinds of modifiers. The format letter must convert a
3590 binary unsigned integer argument to readable form. The format letters
3591 @samp{d} and @samp{i} are aliases for @samp{u}, and the
3592 @samp{u}, @samp{o}, @samp{x}, and @samp{X} conversions are allowed. The
3593 entire @var{suffix} is given (with the current output file number) to
3594 @code{sprintf(3)} to form the file name suffixes for each of the
3595 individual output files in turn. If this option is used, the
3596 @option{--digits} option is ignored.
3598 @item -n @var{digits}
3599 @itemx --digits=@var{digits}
3602 Use output file names containing numbers that are @var{digits} digits
3603 long instead of the default 2.
3608 @opindex --keep-files
3609 Do not remove output files when errors are encountered.
3612 @itemx --elide-empty-files
3614 @opindex --elide-empty-files
3615 Suppress the generation of zero-length output files. (In cases where
3616 the section delimiters of the input file are supposed to mark the first
3617 lines of each of the sections, the first output file will generally be a
3618 zero-length file unless you use this option.) The output file sequence
3619 numbers always run consecutively starting from 0, even when this option
3630 Do not print counts of output file sizes.
3636 Here is an example of its usage.
3637 First, create an empty directory for the exercise,
3644 Now, split the sequence of 1..14 on lines that end with 0 or 5:
3647 $ seq 14 | csplit - '/[05]$/' '@{*@}'
3653 Each number printed above is the size of an output
3654 file that csplit has just created.
3655 List the names of those output files:
3662 Use @command{head} to show their contents:
3687 @node Summarizing files
3688 @chapter Summarizing files
3690 @cindex summarizing files
3692 These commands generate just a few numbers representing entire
3696 * wc invocation:: Print newline, word, and byte counts.
3697 * sum invocation:: Print checksum and block counts.
3698 * cksum invocation:: Print CRC checksum and byte counts.
3699 * md5sum invocation:: Print or check MD5 digests.
3700 * sha1sum invocation:: Print or check SHA-1 digests.
3701 * sha2 utilities:: Print or check SHA-2 digests.
3706 @section @command{wc}: Print newline, word, and byte counts
3710 @cindex character count
3714 @command{wc} counts the number of bytes, characters, whitespace-separated
3715 words, and newlines in each given @var{file}, or standard input if none
3716 are given or for a @var{file} of @samp{-}. Synopsis:
3719 wc [@var{option}]@dots{} [@var{file}]@dots{}
3722 @cindex total counts
3723 @command{wc} prints one line of counts for each file, and if the file was
3724 given as an argument, it prints the file name following the counts. If
3725 more than one @var{file} is given, @command{wc} prints a final line
3726 containing the cumulative counts, with the file name @file{total}. The
3727 counts are printed in this order: newlines, words, characters, bytes,
3728 maximum line length.
3729 Each count is printed right-justified in a field with at least one
3730 space between fields so that the numbers and file names normally line
3731 up nicely in columns. The width of the count fields varies depending
3732 on the inputs, so you should not depend on a particular field width.
3733 However, as a GNU extension, if only one count is printed,
3734 it is guaranteed to be printed without leading spaces.
3736 By default, @command{wc} prints three counts: the newline, words, and byte
3737 counts. Options can specify that only certain counts be printed.
3738 Options do not undo others previously given, so
3745 prints both the byte counts and the word counts.
3747 With the @option{--max-line-length} option, @command{wc} prints the length
3748 of the longest line per file, and if there is more than one file it
3749 prints the maximum (not the sum) of those lengths. The line lengths here
3750 are measured in screen columns, according to the current locale and
3751 assuming tab positions in every 8th column.
3753 The program accepts the following options. Also see @ref{Common options}.
3761 Print only the byte counts.
3767 Print only the character counts.
3773 Print only the word counts.
3779 Print only the newline counts.
3782 @itemx --max-line-length
3784 @opindex --max-line-length
3785 Print only the maximum line lengths.
3787 @macro filesZeroFromOption{cmd,withTotalOption,subListOutput}
3788 @item --files0-from=@var{file}
3789 @opindex --files0-from=@var{file}
3790 @c This is commented out to avoid a texi2dvi failure.
3791 @c texi2dvi (GNU Texinfo 4.11) 1.104
3792 @c @cindex including files from @command{\cmd\}
3793 Disallow processing files named on the command line, and instead process
3794 those named in file @var{file}; each name being terminated by a zero byte
3796 This is useful \withTotalOption\
3797 when the list of file names is so long that it may exceed a command line
3799 In such cases, running @command{\cmd\} via @command{xargs} is undesirable
3800 because it splits the list into pieces and makes @command{\cmd\} print
3801 \subListOutput\ for each sublist rather than for the entire list.
3802 One way to produce a list of ASCII NUL terminated file
3804 @command{find}, using its @option{-print0} predicate.
3805 If @var{file} is @samp{-} then the ASCII NUL terminated
3806 file names are read from standard input.
3808 @filesZeroFromOption{wc,,a total}
3810 For example, to find the length of the longest line in any @file{.c} or
3811 @file{.h} file in the current hierarchy, do this:
3814 find . -name '*.[ch]' -print0 |
3815 wc -L --files0-from=- | tail -n1
3823 @node sum invocation
3824 @section @command{sum}: Print checksum and block counts
3827 @cindex 16-bit checksum
3828 @cindex checksum, 16-bit
3830 @command{sum} computes a 16-bit checksum for each given @var{file}, or
3831 standard input if none are given or for a @var{file} of @samp{-}. Synopsis:
3834 sum [@var{option}]@dots{} [@var{file}]@dots{}
3837 @command{sum} prints the checksum for each @var{file} followed by the
3838 number of blocks in the file (rounded up). If more than one @var{file}
3839 is given, file names are also printed (by default). (With the
3840 @option{--sysv} option, corresponding file names are printed when there is
3841 at least one file argument.)
3843 By default, GNU @command{sum} computes checksums using an algorithm
3844 compatible with BSD @command{sum} and prints file sizes in units of
3847 The program accepts the following options. Also see @ref{Common options}.
3853 @cindex BSD @command{sum}
3854 Use the default (BSD compatible) algorithm. This option is included for
3855 compatibility with the System V @command{sum}. Unless @option{-s} was also
3856 given, it has no effect.
3862 @cindex System V @command{sum}
3863 Compute checksums using an algorithm compatible with System V
3864 @command{sum}'s default, and print file sizes in units of 512-byte blocks.
3868 @command{sum} is provided for compatibility; the @command{cksum} program (see
3869 next section) is preferable in new applications.
3874 @node cksum invocation
3875 @section @command{cksum}: Print CRC checksum and byte counts
3878 @cindex cyclic redundancy check
3879 @cindex CRC checksum
3881 @command{cksum} computes a cyclic redundancy check (CRC) checksum for each
3882 given @var{file}, or standard input if none are given or for a
3883 @var{file} of @samp{-}. Synopsis:
3886 cksum [@var{option}]@dots{} [@var{file}]@dots{}
3889 @command{cksum} prints the CRC checksum for each file along with the number
3890 of bytes in the file, and the file name unless no arguments were given.
3892 @command{cksum} is typically used to ensure that files
3893 transferred by unreliable means (e.g., netnews) have not been corrupted,
3894 by comparing the @command{cksum} output for the received files with the
3895 @command{cksum} output for the original files (typically given in the
3898 The CRC algorithm is specified by the POSIX standard. It is not
3899 compatible with the BSD or System V @command{sum} algorithms (see the
3900 previous section); it is more robust.
3902 The only options are @option{--help} and @option{--version}. @xref{Common
3908 @node md5sum invocation
3909 @section @command{md5sum}: Print or check MD5 digests
3913 @cindex 128-bit checksum
3914 @cindex checksum, 128-bit
3915 @cindex fingerprint, 128-bit
3916 @cindex message-digest, 128-bit
3918 @command{md5sum} computes a 128-bit checksum (or @dfn{fingerprint} or
3919 @dfn{message-digest}) for each specified @var{file}.
3921 Note: The MD5 digest is more reliable than a simple CRC (provided by
3922 the @command{cksum} command) for detecting accidental file corruption,
3923 as the chances of accidentally having two files with identical MD5
3924 are vanishingly small. However, it should not be considered secure
3925 against malicious tampering: although finding a file with a given MD5
3926 fingerprint is considered infeasible at the moment, it is known how
3927 to modify certain files, including digital certificates, so that they
3928 appear valid when signed with an MD5 digest.
3929 For more secure hashes, consider using SHA-2. @xref{sha2 utilities}.
3931 If a @var{file} is specified as @samp{-} or if no files are given
3932 @command{md5sum} computes the checksum for the standard input.
3933 @command{md5sum} can also determine whether a file and checksum are
3934 consistent. Synopsis:
3937 md5sum [@var{option}]@dots{} [@var{file}]@dots{}
3940 For each @var{file}, @samp{md5sum} outputs the MD5 checksum, a flag
3941 indicating binary or text input mode, and the file name.
3942 If @var{file} contains a backslash or newline, the
3943 line is started with a backslash, and each problematic character in
3944 the file name is escaped with a backslash, making the output
3945 unambiguous even in the presence of arbitrary file names.
3946 If @var{file} is omitted or specified as @samp{-}, standard input is read.
3948 The program accepts the following options. Also see @ref{Common options}.
3956 @cindex binary input files
3957 Treat each input file as binary, by reading it in binary mode and
3958 outputting a @samp{*} flag. This is the inverse of @option{--text}.
3959 On systems like GNU that do not distinguish between binary
3960 and text files, this option merely flags each input mode as binary:
3961 the MD5 checksum is unaffected. This option is the default on systems
3962 like MS-DOS that distinguish between binary and text files, except
3963 for reading standard input when standard input is a terminal.
3967 Read file names and checksum information (not data) from each
3968 @var{file} (or from stdin if no @var{file} was specified) and report
3969 whether the checksums match the contents of the named files.
3970 The input to this mode of @command{md5sum} is usually the output of
3971 a prior, checksum-generating run of @samp{md5sum}.
3972 Each valid line of input consists of an MD5 checksum, a binary/text
3973 flag, and then a file name.
3974 Binary mode is indicated with @samp{*}, text with @samp{ } (space).
3975 For each such line, @command{md5sum} reads the named file and computes its
3976 MD5 checksum. Then, if the computed message digest does not match the
3977 one on the line with the file name, the file is noted as having
3978 failed the test. Otherwise, the file passes the test.
3979 By default, for each valid line, one line is written to standard
3980 output indicating whether the named file passed the test.
3981 After all checks have been performed, if there were any failures,
3982 a warning is issued to standard error.
3983 Use the @option{--status} option to inhibit that output.
3984 If any listed file cannot be opened or read, if any valid line has
3985 an MD5 checksum inconsistent with the associated file, or if no valid
3986 line is found, @command{md5sum} exits with nonzero status. Otherwise,
3987 it exits successfully.
3991 @cindex verifying MD5 checksums
3992 This option is useful only when verifying checksums.
3993 When verifying checksums, don't generate an 'OK' message per successfully
3994 checked file. Files that fail the verification are reported in the
3995 default one-line-per-file format. If there is any checksum mismatch,
3996 print a warning summarizing the failures to standard error.
4000 @cindex verifying MD5 checksums
4001 This option is useful only when verifying checksums.
4002 When verifying checksums, don't generate the default one-line-per-file
4003 diagnostic and don't output the warning summarizing any failures.
4004 Failures to open or read a file still evoke individual diagnostics to
4006 If all listed files are readable and are consistent with the associated
4007 MD5 checksums, exit successfully. Otherwise exit with a status code
4008 indicating there was a failure.
4013 Output BSD style checksums, which indicate the checksum algorithm used.
4014 As a GNU extension, file names with problematic characters
4015 are escaped as described above, with the same escaping indicator of @samp{\}
4016 at the start of the line, being used.
4017 The @option{--tag} option implies binary mode, and is disallowed with
4018 @option{--text} mode as supporting that would unnecessarily complicate
4019 the output format, while providing little benefit.
4025 @cindex text input files
4026 Treat each input file as text, by reading it in text mode and
4027 outputting a @samp{ } flag. This is the inverse of @option{--binary}.
4028 This option is the default on systems like GNU that do not
4029 distinguish between binary and text files. On other systems, it is
4030 the default for reading standard input when standard input is a
4031 terminal. This mode is never defaulted to if @option{--tag} is used.
4037 @cindex verifying MD5 checksums
4038 When verifying checksums, warn about improperly formatted MD5 checksum lines.
4039 This option is useful only if all but a few lines in the checked input
4044 @cindex verifying MD5 checksums
4045 When verifying checksums,
4046 if one or more input line is invalid,
4047 exit nonzero after all warnings have been issued.
4054 @node sha1sum invocation
4055 @section @command{sha1sum}: Print or check SHA-1 digests
4059 @cindex 160-bit checksum
4060 @cindex checksum, 160-bit
4061 @cindex fingerprint, 160-bit
4062 @cindex message-digest, 160-bit
4064 @command{sha1sum} computes a 160-bit checksum for each specified
4065 @var{file}. The usage and options of this command are precisely the
4066 same as for @command{md5sum}. @xref{md5sum invocation}.
4068 Note: The SHA-1 digest is more secure than MD5, and no collisions of
4069 it are known (different files having the same fingerprint). However,
4070 it is known that they can be produced with considerable, but not
4071 unreasonable, resources. For this reason, it is generally considered
4072 that SHA-1 should be gradually phased out in favor of the more secure
4073 SHA-2 hash algorithms. @xref{sha2 utilities}.
4076 @node sha2 utilities
4077 @section sha2 utilities: Print or check SHA-2 digests
4084 @cindex 224-bit checksum
4085 @cindex 256-bit checksum
4086 @cindex 384-bit checksum
4087 @cindex 512-bit checksum
4088 @cindex checksum, 224-bit
4089 @cindex checksum, 256-bit
4090 @cindex checksum, 384-bit
4091 @cindex checksum, 512-bit
4092 @cindex fingerprint, 224-bit
4093 @cindex fingerprint, 256-bit
4094 @cindex fingerprint, 384-bit
4095 @cindex fingerprint, 512-bit
4096 @cindex message-digest, 224-bit
4097 @cindex message-digest, 256-bit
4098 @cindex message-digest, 384-bit
4099 @cindex message-digest, 512-bit
4101 The commands @command{sha224sum}, @command{sha256sum},
4102 @command{sha384sum} and @command{sha512sum} compute checksums of
4103 various lengths (respectively 224, 256, 384 and 512 bits),
4104 collectively known as the SHA-2 hashes. The usage and options of
4105 these commands are precisely the same as for @command{md5sum}.
4106 @xref{md5sum invocation}.
4108 Note: The SHA384 and SHA512 digests are considerably slower to
4109 compute, especially on 32-bit computers, than SHA224 or SHA256.
4112 @node Operating on sorted files
4113 @chapter Operating on sorted files
4115 @cindex operating on sorted files
4116 @cindex sorted files, operations on
4118 These commands work with (or produce) sorted files.
4121 * sort invocation:: Sort text files.
4122 * shuf invocation:: Shuffle text files.
4123 * uniq invocation:: Uniquify files.
4124 * comm invocation:: Compare two sorted files line by line.
4125 * ptx invocation:: Produce a permuted index of file contents.
4126 * tsort invocation:: Topological sort.
4130 @node sort invocation
4131 @section @command{sort}: Sort text files
4134 @cindex sorting files
4136 @command{sort} sorts, merges, or compares all the lines from the given
4137 files, or standard input if none are given or for a @var{file} of
4138 @samp{-}. By default, @command{sort} writes the results to standard
4142 sort [@var{option}]@dots{} [@var{file}]@dots{}
4145 @command{sort} has three modes of operation: sort (the default), merge,
4146 and check for sortedness. The following options change the operation
4153 @itemx --check=diagnose-first
4156 @cindex checking for sortedness
4157 Check whether the given file is already sorted: if it is not all
4158 sorted, print a diagnostic containing the first out-of-order line and
4159 exit with a status of 1.
4160 Otherwise, exit successfully.
4161 At most one input file can be given.
4164 @itemx --check=quiet
4165 @itemx --check=silent
4168 @cindex checking for sortedness
4169 Exit successfully if the given file is already sorted, and
4170 exit with status 1 otherwise.
4171 At most one input file can be given.
4172 This is like @option{-c}, except it does not print a diagnostic.
4178 @cindex merging sorted files
4179 Merge the given files by sorting them as a group. Each input file must
4180 always be individually sorted. It always works to sort instead of
4181 merge; merging is provided because it is faster, in the case where it
4186 @cindex sort stability
4187 @cindex sort's last-resort comparison
4188 A pair of lines is compared as follows:
4189 @command{sort} compares each pair of fields, in the
4190 order specified on the command line, according to the associated
4191 ordering options, until a difference is found or no fields are left.
4192 If no key fields are specified, @command{sort} uses a default key of
4193 the entire line. Finally, as a last resort when all keys compare
4194 equal, @command{sort} compares entire lines as if no ordering options
4195 other than @option{--reverse} (@option{-r}) were specified. The
4196 @option{--stable} (@option{-s}) option disables this @dfn{last-resort
4197 comparison} so that lines in which all fields compare equal are left
4198 in their original relative order. The @option{--unique}
4199 (@option{-u}) option also disables the last-resort comparison.
4203 Unless otherwise specified, all comparisons use the character collating
4204 sequence specified by the @env{LC_COLLATE} locale.@footnote{If you
4205 use a non-POSIX locale (e.g., by setting @env{LC_ALL}
4206 to @samp{en_US}), then @command{sort} may produce output that is sorted
4207 differently than you're accustomed to. In that case, set the @env{LC_ALL}
4208 environment variable to @samp{C}@. Note that setting only @env{LC_COLLATE}
4209 has two problems. First, it is ineffective if @env{LC_ALL} is also set.
4210 Second, it has undefined behavior if @env{LC_CTYPE} (or @env{LANG}, if
4211 @env{LC_CTYPE} is unset) is set to an incompatible value. For example,
4212 you get undefined behavior if @env{LC_CTYPE} is @code{ja_JP.PCK} but
4213 @env{LC_COLLATE} is @code{en_US.UTF-8}.}
4215 GNU @command{sort} (as specified for all GNU utilities) has no
4216 limit on input line length or restrictions on bytes allowed within lines.
4217 In addition, if the final byte of an input file is not a newline, GNU
4218 @command{sort} silently supplies one. A line's trailing newline is not
4219 part of the line for comparison purposes.
4221 @cindex exit status of @command{sort}
4225 0 if no error occurred
4226 1 if invoked with @option{-c} or @option{-C} and the input is not sorted
4227 2 if an error occurred
4231 If the environment variable @env{TMPDIR} is set, @command{sort} uses its
4232 value as the directory for temporary files instead of @file{/tmp}. The
4233 @option{--temporary-directory} (@option{-T}) option in turn overrides
4234 the environment variable.
4236 The following options affect the ordering of output lines. They may be
4237 specified globally or as part of a specific key field. If no key
4238 fields are specified, global options apply to comparison of entire
4239 lines; otherwise the global options are inherited by key fields that do
4240 not specify any special options of their own. In pre-POSIX
4241 versions of @command{sort}, global options affect only later key fields,
4242 so portable shell scripts should specify global options first.
4247 @itemx --ignore-leading-blanks
4249 @opindex --ignore-leading-blanks
4250 @cindex blanks, ignoring leading
4252 Ignore leading blanks when finding sort keys in each line.
4253 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4254 can change this. Note blanks may be ignored by your locale's collating
4255 rules, but without this option they will be significant for character
4256 positions specified in keys with the @option{-k} option.
4259 @itemx --dictionary-order
4261 @opindex --dictionary-order
4262 @cindex dictionary order
4263 @cindex phone directory order
4264 @cindex telephone directory order
4266 Sort in @dfn{phone directory} order: ignore all characters except
4267 letters, digits and blanks when sorting.
4268 By default letters and digits are those of ASCII and a blank
4269 is a space or a tab, but the @env{LC_CTYPE} locale can change this.
4272 @itemx --ignore-case
4274 @opindex --ignore-case
4275 @cindex ignoring case
4276 @cindex case folding
4278 Fold lowercase characters into the equivalent uppercase characters when
4279 comparing so that, for example, @samp{b} and @samp{B} sort as equal.
4280 The @env{LC_CTYPE} locale determines character types.
4281 When used with @option{--unique} those lower case equivalent lines are
4282 thrown away. (There is currently no way to throw away the upper case
4283 equivalent instead. (Any @option{--reverse} given would only affect
4284 the final result, after the throwing away.))
4287 @itemx --general-numeric-sort
4288 @itemx --sort=general-numeric
4290 @opindex --general-numeric-sort
4292 @cindex general numeric sort
4294 Sort numerically, converting a prefix of each line to a long
4295 double-precision floating point number. @xref{Floating point}.
4296 Do not report overflow, underflow, or conversion errors.
4297 Use the following collating sequence:
4301 Lines that do not start with numbers (all considered to be equal).
4303 NaNs (``Not a Number'' values, in IEEE floating point arithmetic)
4304 in a consistent but machine-dependent order.
4308 Finite numbers in ascending numeric order (with @math{-0} and @math{+0} equal).
4313 Use this option only if there is no alternative; it is much slower than
4314 @option{--numeric-sort} (@option{-n}) and it can lose information when
4315 converting to floating point.
4318 @itemx --human-numeric-sort
4319 @itemx --sort=human-numeric
4321 @opindex --human-numeric-sort
4323 @cindex human numeric sort
4325 Sort numerically, first by numeric sign (negative, zero, or positive);
4326 then by SI suffix (either empty, or @samp{k} or @samp{K}, or
4327 one of @samp{MGTPEZY}, in that order; @pxref{Block size}); and finally
4328 by numeric value. For example, @samp{1023M} sorts before @samp{1G}
4329 because @samp{M} (mega) precedes @samp{G} (giga) as an SI
4330 suffix. This option sorts values that are consistently scaled to the
4331 nearest suffix, regardless of whether suffixes denote powers of 1000
4332 or 1024, and it therefore sorts the output of any single invocation of
4333 the @command{df}, @command{du}, or @command{ls} commands that are
4334 invoked with their @option{--human-readable} or @option{--si} options.
4335 The syntax for numbers is the same as for the @option{--numeric-sort}
4336 option; the SI suffix must immediately follow the number.
4339 @itemx --ignore-nonprinting
4341 @opindex --ignore-nonprinting
4342 @cindex nonprinting characters, ignoring
4343 @cindex unprintable characters, ignoring
4345 Ignore nonprinting characters.
4346 The @env{LC_CTYPE} locale determines character types.
4347 This option has no effect if the stronger @option{--dictionary-order}
4348 (@option{-d}) option is also given.
4354 @opindex --month-sort
4356 @cindex months, sorting by
4358 An initial string, consisting of any amount of blanks, followed
4359 by a month name abbreviation, is folded to UPPER case and
4360 compared in the order @samp{JAN} < @samp{FEB} < @dots{} < @samp{DEC}@.
4361 Invalid names compare low to valid names. The @env{LC_TIME} locale
4362 category determines the month spellings.
4363 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4367 @itemx --numeric-sort
4368 @itemx --sort=numeric
4370 @opindex --numeric-sort
4372 @cindex numeric sort
4374 Sort numerically. The number begins each line and consists
4375 of optional blanks, an optional @samp{-} sign, and zero or more
4376 digits possibly separated by thousands separators, optionally followed
4377 by a decimal-point character and zero or more digits. An empty
4378 number is treated as @samp{0}. The @env{LC_NUMERIC}
4379 locale specifies the decimal-point character and thousands separator.
4380 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4383 Comparison is exact; there is no rounding error.
4385 Neither a leading @samp{+} nor exponential notation is recognized.
4386 To compare such strings numerically, use the
4387 @option{--general-numeric-sort} (@option{-g}) option.
4390 @itemx --version-sort
4392 @opindex --version-sort
4393 @cindex version number sort
4394 Sort by version name and number. It behaves like a standard sort,
4395 except that each sequence of decimal digits is treated numerically
4396 as an index/version number. (@xref{Details about version sort}.)
4402 @cindex reverse sorting
4403 Reverse the result of comparison, so that lines with greater key values
4404 appear earlier in the output instead of later.
4407 @itemx --random-sort
4408 @itemx --sort=random
4410 @opindex --random-sort
4413 Sort by hashing the input keys and then sorting the hash values.
4414 Choose the hash function at random, ensuring that it is free of
4415 collisions so that differing keys have differing hash values. This is
4416 like a random permutation of the inputs (@pxref{shuf invocation}),
4417 except that keys with the same value sort together.
4419 If multiple random sort fields are specified, the same random hash
4420 function is used for all fields. To use different random hash
4421 functions for different fields, you can invoke @command{sort} more
4424 The choice of hash function is affected by the
4425 @option{--random-source} option.
4433 @item --compress-program=@var{prog}
4434 Compress any temporary files with the program @var{prog}.
4436 With no arguments, @var{prog} must compress standard input to standard
4437 output, and when given the @option{-d} option it must decompress
4438 standard input to standard output.
4440 Terminate with an error if @var{prog} exits with nonzero status.
4442 White space and the backslash character should not appear in
4443 @var{prog}; they are reserved for future use.
4445 @filesZeroFromOption{sort,,sorted output}
4447 @item -k @var{pos1}[,@var{pos2}]
4448 @itemx --key=@var{pos1}[,@var{pos2}]
4452 Specify a sort field that consists of the part of the line between
4453 @var{pos1} and @var{pos2} (or the end of the line, if @var{pos2} is
4454 omitted), @emph{inclusive}.
4456 Each @var{pos} has the form @samp{@var{f}[.@var{c}][@var{opts}]},
4457 where @var{f} is the number of the field to use, and @var{c} is the number
4458 of the first character from the beginning of the field. Fields and character
4459 positions are numbered starting with 1; a character position of zero in
4460 @var{pos2} indicates the field's last character. If @samp{.@var{c}} is
4461 omitted from @var{pos1}, it defaults to 1 (the beginning of the field);
4462 if omitted from @var{pos2}, it defaults to 0 (the end of the field).
4463 @var{opts} are ordering options, allowing individual keys to be sorted
4464 according to different rules; see below for details. Keys can span
4467 Example: To sort on the second field, use @option{--key=2,2}
4468 (@option{-k 2,2}). See below for more notes on keys and more examples.
4469 See also the @option{--debug} option to help determine the part
4470 of the line being used in the sort.
4473 Highlight the portion of each line used for sorting.
4474 Also issue warnings about questionable usage to stderr.
4476 @item --batch-size=@var{nmerge}
4477 @opindex --batch-size
4478 @cindex number of inputs to merge, nmerge
4479 Merge at most @var{nmerge} inputs at once.
4481 When @command{sort} has to merge more than @var{nmerge} inputs,
4482 it merges them in groups of @var{nmerge}, saving the result in
4483 a temporary file, which is then used as an input in a subsequent merge.
4485 A large value of @var{nmerge} may improve merge performance and decrease
4486 temporary storage utilization at the expense of increased memory usage
4487 and I/O@. Conversely a small value of @var{nmerge} may reduce memory
4488 requirements and I/O at the expense of temporary storage consumption and
4491 The value of @var{nmerge} must be at least 2. The default value is
4492 currently 16, but this is implementation-dependent and may change in
4495 The value of @var{nmerge} may be bounded by a resource limit for open
4496 file descriptors. The commands @samp{ulimit -n} or @samp{getconf
4497 OPEN_MAX} may display limits for your systems; these limits may be
4498 modified further if your program already has some files open, or if
4499 the operating system has other limits on the number of open files. If
4500 the value of @var{nmerge} exceeds the resource limit, @command{sort}
4501 silently uses a smaller value.
4503 @item -o @var{output-file}
4504 @itemx --output=@var{output-file}
4507 @cindex overwriting of input, allowed
4508 Write output to @var{output-file} instead of standard output.
4509 Normally, @command{sort} reads all input before opening
4510 @var{output-file}, so you can safely sort a file in place by using
4511 commands like @code{sort -o F F} and @code{cat F | sort -o F}@.
4512 However, @command{sort} with @option{--merge} (@option{-m}) can open
4513 the output file before reading all input, so a command like @code{cat
4514 F | sort -m -o F - G} is not safe as @command{sort} might start
4515 writing @file{F} before @command{cat} is done reading it.
4517 @vindex POSIXLY_CORRECT
4518 On newer systems, @option{-o} cannot appear after an input file if
4519 @env{POSIXLY_CORRECT} is set, e.g., @samp{sort F -o F}@. Portable
4520 scripts should specify @option{-o @var{output-file}} before any input
4523 @item --random-source=@var{file}
4524 @opindex --random-source
4525 @cindex random source for sorting
4526 Use @var{file} as a source of random data used to determine which
4527 random hash function to use with the @option{-R} option. @xref{Random
4534 @cindex sort stability
4535 @cindex sort's last-resort comparison
4537 Make @command{sort} stable by disabling its last-resort comparison.
4538 This option has no effect if no fields or global ordering options
4539 other than @option{--reverse} (@option{-r}) are specified.
4542 @itemx --buffer-size=@var{size}
4544 @opindex --buffer-size
4545 @cindex size for main memory sorting
4546 Use a main-memory sort buffer of the given @var{size}. By default,
4547 @var{size} is in units of 1024 bytes. Appending @samp{%} causes
4548 @var{size} to be interpreted as a percentage of physical memory.
4549 Appending @samp{K} multiplies @var{size} by 1024 (the default),
4550 @samp{M} by 1,048,576, @samp{G} by 1,073,741,824, and so on for
4551 @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}@. Appending
4552 @samp{b} causes @var{size} to be interpreted as a byte count, with no
4555 This option can improve the performance of @command{sort} by causing it
4556 to start with a larger or smaller sort buffer than the default.
4557 However, this option affects only the initial buffer size. The buffer
4558 grows beyond @var{size} if @command{sort} encounters input lines larger
4561 @item -t @var{separator}
4562 @itemx --field-separator=@var{separator}
4564 @opindex --field-separator
4565 @cindex field separator character
4566 Use character @var{separator} as the field separator when finding the
4567 sort keys in each line. By default, fields are separated by the empty
4568 string between a non-blank character and a blank character.
4569 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4572 That is, given the input line @w{@samp{ foo bar}}, @command{sort} breaks it
4573 into fields @w{@samp{ foo}} and @w{@samp{ bar}}. The field separator is
4574 not considered to be part of either the field preceding or the field
4575 following, so with @samp{sort @w{-t " "}} the same input line has
4576 three fields: an empty field, @samp{foo}, and @samp{bar}.
4577 However, fields that extend to the end of the line,
4578 as @option{-k 2}, or fields consisting of a range, as @option{-k 2,3},
4579 retain the field separators present between the endpoints of the range.
4581 To specify ASCII NUL as the field separator,
4582 use the two-character string @samp{\0}, e.g., @samp{sort -t '\0'}.
4584 @item -T @var{tempdir}
4585 @itemx --temporary-directory=@var{tempdir}
4587 @opindex --temporary-directory
4588 @cindex temporary directory
4590 Use directory @var{tempdir} to store temporary files, overriding the
4591 @env{TMPDIR} environment variable. If this option is given more than
4592 once, temporary files are stored in all the directories given. If you
4593 have a large sort or merge that is I/O-bound, you can often improve
4594 performance by using this option to specify directories on different
4595 disks and controllers.
4597 @item --parallel=@var{n}
4599 @cindex multithreaded sort
4600 Set the number of sorts run in parallel to @var{n}. By default,
4601 @var{n} is set to the number of available processors, but limited
4602 to 8, as there are diminishing performance gains after that.
4603 Note also that using @var{n} threads increases the memory usage by
4604 a factor of log @var{n}. Also see @ref{nproc invocation}.
4610 @cindex uniquifying output
4612 Normally, output only the first of a sequence of lines that compare
4613 equal. For the @option{--check} (@option{-c} or @option{-C}) option,
4614 check that no pair of consecutive lines compares equal.
4616 This option also disables the default last-resort comparison.
4618 The commands @code{sort -u} and @code{sort | uniq} are equivalent, but
4619 this equivalence does not extend to arbitrary @command{sort} options.
4620 For example, @code{sort -n -u} inspects only the value of the initial
4621 numeric string when checking for uniqueness, whereas @code{sort -n |
4622 uniq} inspects the entire line. @xref{uniq invocation}.
4624 @macro zeroTerminatedOption
4626 @itemx --zero-terminated
4628 @opindex --zero-terminated
4629 @cindex process zero-terminated items
4630 Delimit items with a zero byte rather than a newline (ASCII LF).
4631 I.e., treat input as items separated by ASCII NUL
4632 and terminate output items with ASCII NUL.
4633 This option can be useful in conjunction with @samp{perl -0} or
4634 @samp{find -print0} and @samp{xargs -0} which do the same in order to
4635 reliably handle arbitrary file names (even those containing blanks
4636 or other special characters).
4638 @zeroTerminatedOption
4642 Historical (BSD and System V) implementations of @command{sort} have
4643 differed in their interpretation of some options, particularly
4644 @option{-b}, @option{-f}, and @option{-n}.
4645 GNU sort follows the POSIX
4646 behavior, which is usually (but not always!) like the System V behavior.
4647 According to POSIX, @option{-n} no longer implies @option{-b}. For
4648 consistency, @option{-M} has been changed in the same way. This may
4649 affect the meaning of character positions in field specifications in
4650 obscure cases. The only fix is to add an explicit @option{-b}.
4652 A position in a sort field specified with @option{-k} may have any
4653 of the option letters @samp{MbdfghinRrV} appended to it, in which case no
4654 global ordering options are inherited by that particular field. The
4655 @option{-b} option may be independently attached to either or both of
4656 the start and end positions of a field specification, and if it is
4657 inherited from the global options it will be attached to both.
4658 If input lines can contain leading or adjacent blanks and @option{-t}
4659 is not used, then @option{-k} is typically combined with @option{-b} or
4660 an option that implicitly ignores leading blanks (@samp{Mghn}) as otherwise
4661 the varying numbers of leading blanks in fields can cause confusing results.
4663 If the start position in a sort field specifier falls after the end of
4664 the line or after the end field, the field is empty. If the @option{-b}
4665 option was specified, the @samp{.@var{c}} part of a field specification
4666 is counted from the first nonblank character of the field.
4668 @vindex _POSIX2_VERSION
4669 @vindex POSIXLY_CORRECT
4670 On older systems, @command{sort} supports an obsolete origin-zero
4671 syntax @samp{+@var{pos1} [-@var{pos2}]} for specifying sort keys.
4672 The obsolete sequence @samp{sort +@var{a}.@var{x} -@var{b}.@var{y}}
4673 is equivalent to @samp{sort -k @var{a+1}.@var{x+1},@var{b}} if @var{y}
4674 is @samp{0} or absent, otherwise it is equivalent to @samp{sort -k
4675 @var{a+1}.@var{x+1},@var{b+1}.@var{y}}.
4677 This obsolete behavior can be enabled or disabled with the
4678 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4679 conformance}); it can also be enabled when @env{POSIXLY_CORRECT} is
4680 not set by using the obsolete syntax with @samp{-@var{pos2}} present.
4682 Scripts intended for use on standard hosts should avoid obsolete
4683 syntax and should use @option{-k} instead. For example, avoid
4684 @samp{sort +2}, since it might be interpreted as either @samp{sort
4685 ./+2} or @samp{sort -k 3}. If your script must also run on hosts that
4686 support only the obsolete syntax, it can use a test like @samp{if sort
4687 -k 1 </dev/null >/dev/null 2>&1; then @dots{}} to decide which syntax
4690 Here are some examples to illustrate various combinations of options.
4695 Sort in descending (reverse) numeric order.
4702 Run no more than 4 sorts concurrently, using a buffer size of 10M.
4705 sort --parallel=4 -S 10M
4709 Sort alphabetically, omitting the first and second fields
4710 and the blanks at the start of the third field.
4711 This uses a single key composed of the characters beginning
4712 at the start of the first nonblank character in field three
4713 and extending to the end of each line.
4720 Sort numerically on the second field and resolve ties by sorting
4721 alphabetically on the third and fourth characters of field five.
4722 Use @samp{:} as the field delimiter.
4725 sort -t : -k 2,2n -k 5.3,5.4
4728 Note that if you had written @option{-k 2n} instead of @option{-k 2,2n}
4729 @command{sort} would have used all characters beginning in the second field
4730 and extending to the end of the line as the primary @emph{numeric}
4731 key. For the large majority of applications, treating keys spanning
4732 more than one field as numeric will not do what you expect.
4734 Also note that the @samp{n} modifier was applied to the field-end
4735 specifier for the first key. It would have been equivalent to
4736 specify @option{-k 2n,2} or @option{-k 2n,2n}. All modifiers except
4737 @samp{b} apply to the associated @emph{field}, regardless of whether
4738 the modifier character is attached to the field-start and/or the
4739 field-end part of the key specifier.
4742 Sort the password file on the fifth field and ignore any
4743 leading blanks. Sort lines with equal values in field five
4744 on the numeric user ID in field three. Fields are separated
4748 sort -t : -k 5b,5 -k 3,3n /etc/passwd
4749 sort -t : -n -k 5b,5 -k 3,3 /etc/passwd
4750 sort -t : -b -k 5,5 -k 3,3n /etc/passwd
4753 These three commands have equivalent effect. The first specifies that
4754 the first key's start position ignores leading blanks and the second
4755 key is sorted numerically. The other two commands rely on global
4756 options being inherited by sort keys that lack modifiers. The inheritance
4757 works in this case because @option{-k 5b,5b} and @option{-k 5b,5} are
4758 equivalent, as the location of a field-end lacking a @samp{.@var{c}}
4759 character position is not affected by whether initial blanks are
4763 Sort a set of log files, primarily by IPv4 address and secondarily by
4764 time stamp. If two lines' primary and secondary keys are identical,
4765 output the lines in the same order that they were input. The log
4766 files contain lines that look like this:
4769 4.150.156.3 - - [01/Apr/2004:06:31:51 +0000] message 1
4770 211.24.3.231 - - [24/Apr/2004:20:17:39 +0000] message 2
4773 Fields are separated by exactly one space. Sort IPv4 addresses
4774 lexicographically, e.g., 212.61.52.2 sorts before 212.129.233.201
4775 because 61 is less than 129.
4778 sort -s -t ' ' -k 4.9n -k 4.5M -k 4.2n -k 4.14,4.21 file*.log |
4779 sort -s -t '.' -k 1,1n -k 2,2n -k 3,3n -k 4,4n
4782 This example cannot be done with a single @command{sort} invocation,
4783 since IPv4 address components are separated by @samp{.} while dates
4784 come just after a space. So it is broken down into two invocations of
4785 @command{sort}: the first sorts by time stamp and the second by IPv4
4786 address. The time stamp is sorted by year, then month, then day, and
4787 finally by hour-minute-second field, using @option{-k} to isolate each
4788 field. Except for hour-minute-second there's no need to specify the
4789 end of each key field, since the @samp{n} and @samp{M} modifiers sort
4790 based on leading prefixes that cannot cross field boundaries. The
4791 IPv4 addresses are sorted lexicographically. The second sort uses
4792 @samp{-s} so that ties in the primary key are broken by the secondary
4793 key; the first sort uses @samp{-s} so that the combination of the two
4797 Generate a tags file in case-insensitive sorted order.
4800 find src -type f -print0 | sort -z -f | xargs -0 etags --append
4803 The use of @option{-print0}, @option{-z}, and @option{-0} in this case means
4804 that file names that contain blanks or other special characters are
4806 by the sort operation.
4808 @c This example is a bit contrived and needs more explanation.
4810 @c Sort records separated by an arbitrary string by using a pipe to convert
4811 @c each record delimiter string to @samp{\0}, then using sort's -z option,
4812 @c and converting each @samp{\0} back to the original record delimiter.
4815 @c printf 'c\n\nb\n\na\n' |
4816 @c perl -0pe 's/\n\n/\n\0/g' |
4818 @c perl -0pe 's/\0/\n/g'
4822 Use the common DSU, Decorate Sort Undecorate idiom to
4823 sort lines according to their length.
4826 awk '@{print length, $0@}' /etc/passwd | sort -n | cut -f2- -d' '
4829 In general this technique can be used to sort data that the @command{sort}
4830 command does not support, or is inefficient at, sorting directly.
4833 Shuffle a list of directories, but preserve the order of files within
4834 each directory. For instance, one could use this to generate a music
4835 playlist in which albums are shuffled but the songs of each album are
4839 ls */* | sort -t / -k 1,1R -k 2,2
4845 @node shuf invocation
4846 @section @command{shuf}: Shuffling text
4849 @cindex shuffling files
4851 @command{shuf} shuffles its input by outputting a random permutation
4852 of its input lines. Each output permutation is equally likely.
4856 shuf [@var{option}]@dots{} [@var{file}]
4857 shuf -e [@var{option}]@dots{} [@var{arg}]@dots{}
4858 shuf -i @var{lo}-@var{hi} [@var{option}]@dots{}
4861 @command{shuf} has three modes of operation that affect where it
4862 obtains its input lines. By default, it reads lines from standard
4863 input. The following options change the operation mode:
4871 @cindex command-line operands to shuffle
4872 Treat each command-line operand as an input line.
4874 @item -i @var{lo}-@var{hi}
4875 @itemx --input-range=@var{lo}-@var{hi}
4877 @opindex --input-range
4878 @cindex input range to shuffle
4879 Act as if input came from a file containing the range of unsigned
4880 decimal integers @var{lo}@dots{}@var{hi}, one per line.
4884 @command{shuf}'s other options can affect its behavior in all
4889 @item -n @var{lines}
4890 @itemx --head-count=@var{count}
4892 @opindex --head-count
4893 @cindex head of output
4894 Output at most @var{count} lines. By default, all input lines are
4897 @item -o @var{output-file}
4898 @itemx --output=@var{output-file}
4901 @cindex overwriting of input, allowed
4902 Write output to @var{output-file} instead of standard output.
4903 @command{shuf} reads all input before opening
4904 @var{output-file}, so you can safely shuffle a file in place by using
4905 commands like @code{shuf -o F <F} and @code{cat F | shuf -o F}.
4907 @item --random-source=@var{file}
4908 @opindex --random-source
4909 @cindex random source for shuffling
4910 Use @var{file} as a source of random data used to determine which
4911 permutation to generate. @xref{Random sources}.
4913 @zeroTerminatedOption
4929 might produce the output
4939 Similarly, the command:
4942 shuf -e clubs hearts diamonds spades
4956 and the command @samp{shuf -i 1-4} might output:
4966 These examples all have four input lines, so @command{shuf} might
4967 produce any of the twenty-four possible permutations of the input. In
4968 general, if there are @var{n} input lines, there are @var{n}! (i.e.,
4969 @var{n} factorial, or @var{n} * (@var{n} - 1) * @dots{} * 1) possible
4970 output permutations.
4975 @node uniq invocation
4976 @section @command{uniq}: Uniquify files
4979 @cindex uniquify files
4981 @command{uniq} writes the unique lines in the given @file{input}, or
4982 standard input if nothing is given or for an @var{input} name of
4986 uniq [@var{option}]@dots{} [@var{input} [@var{output}]]
4989 By default, @command{uniq} prints its input lines, except that
4990 it discards all but the first of adjacent repeated lines, so that
4991 no output lines are repeated. Optionally, it can instead discard
4992 lines that are not repeated, or all repeated lines.
4994 The input need not be sorted, but repeated input lines are detected
4995 only if they are adjacent. If you want to discard non-adjacent
4996 duplicate lines, perhaps you want to use @code{sort -u}.
4997 @xref{sort invocation}.
5000 Comparisons honor the rules specified by the @env{LC_COLLATE}
5003 If no @var{output} file is specified, @command{uniq} writes to standard
5006 The program accepts the following options. Also see @ref{Common options}.
5011 @itemx --skip-fields=@var{n}
5013 @opindex --skip-fields
5014 Skip @var{n} fields on each line before checking for uniqueness. Use
5015 a null string for comparison if a line has fewer than @var{n} fields. Fields
5016 are sequences of non-space non-tab characters that are separated from
5017 each other by at least one space or tab.
5019 For compatibility @command{uniq} supports an obsolete option syntax
5020 @option{-@var{n}}. New scripts should use @option{-f @var{n}} instead.
5023 @itemx --skip-chars=@var{n}
5025 @opindex --skip-chars
5026 Skip @var{n} characters before checking for uniqueness. Use a null string
5027 for comparison if a line has fewer than @var{n} characters. If you use both
5028 the field and character skipping options, fields are skipped over first.
5030 @vindex _POSIX2_VERSION
5031 On older systems, @command{uniq} supports an obsolete option syntax
5033 This obsolete behavior can be enabled or disabled with the
5034 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
5035 conformance}), but portable scripts should avoid commands whose
5036 behavior depends on this variable.
5037 For example, use @samp{uniq ./+10} or @samp{uniq -s 10} rather than
5038 the ambiguous @samp{uniq +10}.
5044 Print the number of times each line occurred along with the line.
5047 @itemx --ignore-case
5049 @opindex --ignore-case
5050 Ignore differences in case when comparing lines.
5056 @cindex repeated lines, outputting
5057 Discard lines that are not repeated. When used by itself, this option
5058 causes @command{uniq} to print the first copy of each repeated line,
5062 @itemx --all-repeated[=@var{delimit-method}]
5064 @opindex --all-repeated
5065 @cindex all repeated lines, outputting
5066 Do not discard the second and subsequent repeated input lines,
5067 but discard lines that are not repeated.
5068 This option is useful mainly in conjunction with other options e.g.,
5069 to ignore case or to compare only selected fields.
5070 The optional @var{delimit-method} tells how to delimit
5071 groups of repeated lines, and must be one of the following:
5076 Do not delimit groups of repeated lines.
5077 This is equivalent to @option{--all-repeated} (@option{-D}).
5080 Output a newline before each group of repeated lines.
5081 With @option{--zero-terminated} (@option{-z}), use a zero
5082 byte (ASCII NUL) instead of a newline.
5085 Separate groups of repeated lines with a single newline.
5086 With @option{--zero-terminated} (@option{-z}), use a zero
5087 byte (ASCII NUL) instead of a newline.
5088 This is the same as using @samp{prepend}, except that
5089 no delimiter is inserted before the first group, and hence
5090 may be better suited for output direct to users.
5093 Note that when groups are delimited and the input stream contains
5094 two or more consecutive blank lines, then the output is ambiguous.
5095 To avoid that, filter the input through @samp{tr -s '\n'} to replace
5096 each sequence of consecutive newlines with a single newline.
5098 This is a GNU extension.
5099 @c FIXME: give an example showing *how* it's useful
5105 @cindex unique lines, outputting
5106 Discard the first repeated line. When used by itself, this option
5107 causes @command{uniq} to print unique lines, and nothing else.
5110 @itemx --check-chars=@var{n}
5112 @opindex --check-chars
5113 Compare at most @var{n} characters on each line (after skipping any specified
5114 fields and characters). By default the entire rest of the lines are
5117 @zeroTerminatedOption
5124 @node comm invocation
5125 @section @command{comm}: Compare two sorted files line by line
5128 @cindex line-by-line comparison
5129 @cindex comparing sorted files
5131 @command{comm} writes to standard output lines that are common, and lines
5132 that are unique, to two input files; a file name of @samp{-} means
5133 standard input. Synopsis:
5136 comm [@var{option}]@dots{} @var{file1} @var{file2}
5140 Before @command{comm} can be used, the input files must be sorted using the
5141 collating sequence specified by the @env{LC_COLLATE} locale.
5142 If an input file ends in a non-newline
5143 character, a newline is silently appended. The @command{sort} command with
5144 no options always outputs a file that is suitable input to @command{comm}.
5146 @cindex differing lines
5147 @cindex common lines
5148 With no options, @command{comm} produces three-column output. Column one
5149 contains lines unique to @var{file1}, column two contains lines unique
5150 to @var{file2}, and column three contains lines common to both files.
5151 Columns are separated by a single TAB character.
5152 @c FIXME: when there's an option to supply an alternative separator
5153 @c string, append "by default" to the above sentence.
5158 The options @option{-1}, @option{-2}, and @option{-3} suppress printing of
5159 the corresponding columns (and separators). Also see @ref{Common options}.
5161 Unlike some other comparison utilities, @command{comm} has an exit
5162 status that does not depend on the result of the comparison.
5163 Upon normal completion @command{comm} produces an exit code of zero.
5164 If there is an error it exits with nonzero status.
5166 @macro checkOrderOption{cmd}
5167 If the @option{--check-order} option is given, unsorted inputs will
5168 cause a fatal error message. If the option @option{--nocheck-order}
5169 is given, unsorted inputs will never cause an error message. If neither
5170 of these options is given, wrongly sorted inputs are diagnosed
5171 only if an input file is found to contain unpairable
5173 lines, and when both input files are non empty.
5175 @ifclear JOIN_COMMAND
5178 If an input file is diagnosed as being unsorted, the @command{\cmd\}
5179 command will exit with a nonzero status (and the output should not be used).
5181 Forcing @command{\cmd\} to process wrongly sorted input files
5182 containing unpairable lines by specifying @option{--nocheck-order} is
5183 not guaranteed to produce any particular output. The output will
5184 probably not correspond with whatever you hoped it would be.
5186 @checkOrderOption{comm}
5191 Fail with an error message if either input file is wrongly ordered.
5193 @item --nocheck-order
5194 Do not check that both input files are in sorted order.
5198 @item --output-delimiter=@var{str}
5199 Print @var{str} between adjacent output columns,
5200 rather than the default of a single TAB character.
5202 The delimiter @var{str} may not be empty.
5206 @node ptx invocation
5207 @section @command{ptx}: Produce permuted indexes
5211 @command{ptx} reads a text file and essentially produces a permuted index, with
5212 each keyword in its context. The calling sketch is either one of:
5215 ptx [@var{option} @dots{}] [@var{file} @dots{}]
5216 ptx -G [@var{option} @dots{}] [@var{input} [@var{output}]]
5219 The @option{-G} (or its equivalent: @option{--traditional}) option disables
5220 all GNU extensions and reverts to traditional mode, thus introducing some
5221 limitations and changing several of the program's default option values.
5222 When @option{-G} is not specified, GNU extensions are always enabled.
5223 GNU extensions to @command{ptx} are documented wherever appropriate in this
5224 document. @xref{Compatibility in ptx}, for the full list.
5226 Individual options are explained in the following sections.
5228 When GNU extensions are enabled, there may be zero, one or several
5229 @var{file}s after the options. If there is no @var{file}, the program
5230 reads the standard input. If there is one or several @var{file}s, they
5231 give the name of input files which are all read in turn, as if all the
5232 input files were concatenated. However, there is a full contextual
5233 break between each file and, when automatic referencing is requested,
5234 file names and line numbers refer to individual text input files. In
5235 all cases, the program outputs the permuted index to the standard
5238 When GNU extensions are @emph{not} enabled, that is, when the program
5239 operates in traditional mode, there may be zero, one or two parameters
5240 besides the options. If there are no parameters, the program reads the
5241 standard input and outputs the permuted index to the standard output.
5242 If there is only one parameter, it names the text @var{input} to be read
5243 instead of the standard input. If two parameters are given, they give
5244 respectively the name of the @var{input} file to read and the name of
5245 the @var{output} file to produce. @emph{Be very careful} to note that,
5246 in this case, the contents of file given by the second parameter is
5247 destroyed. This behavior is dictated by System V @command{ptx}
5248 compatibility; GNU Standards normally discourage output parameters not
5249 introduced by an option.
5251 Note that for @emph{any} file named as the value of an option or as an
5252 input text file, a single dash @kbd{-} may be used, in which case
5253 standard input is assumed. However, it would not make sense to use this
5254 convention more than once per program invocation.
5257 * General options in ptx:: Options which affect general program behavior.
5258 * Charset selection in ptx:: Underlying character set considerations.
5259 * Input processing in ptx:: Input fields, contexts, and keyword selection.
5260 * Output formatting in ptx:: Types of output format, and sizing the fields.
5261 * Compatibility in ptx::
5265 @node General options in ptx
5266 @subsection General options
5271 @itemx --traditional
5272 As already explained, this option disables all GNU extensions to
5273 @command{ptx} and switches to traditional mode.
5276 Print a short help on standard output, then exit without further
5280 Print the program version on standard output, then exit without further
5288 @node Charset selection in ptx
5289 @subsection Charset selection
5291 @c FIXME: People don't necessarily know what an IBM-PC was these days.
5292 As it is set up now, the program assumes that the input file is coded
5293 using 8-bit ISO 8859-1 code, also known as Latin-1 character set,
5294 @emph{unless} it is compiled for MS-DOS, in which case it uses the
5295 character set of the IBM-PC@. (GNU @command{ptx} is not known to work on
5296 smaller MS-DOS machines anymore.) Compared to 7-bit ASCII, the set
5297 of characters which are letters is different; this alters the behavior
5298 of regular expression matching. Thus, the default regular expression
5299 for a keyword allows foreign or diacriticized letters. Keyword sorting,
5300 however, is still crude; it obeys the underlying character set ordering
5306 @itemx --ignore-case
5307 Fold lower case letters to upper case for sorting.
5312 @node Input processing in ptx
5313 @subsection Word selection and input processing
5318 @itemx --break-file=@var{file}
5320 This option provides an alternative (to @option{-W}) method of describing
5321 which characters make up words. It introduces the name of a
5322 file which contains a list of characters which can@emph{not} be part of
5323 one word; this file is called the @dfn{Break file}. Any character which
5324 is not part of the Break file is a word constituent. If both options
5325 @option{-b} and @option{-W} are specified, then @option{-W} has precedence and
5326 @option{-b} is ignored.
5328 When GNU extensions are enabled, the only way to avoid newline as a
5329 break character is to write all the break characters in the file with no
5330 newline at all, not even at the end of the file. When GNU extensions
5331 are disabled, spaces, tabs and newlines are always considered as break
5332 characters even if not included in the Break file.
5335 @itemx --ignore-file=@var{file}
5337 The file associated with this option contains a list of words which will
5338 never be taken as keywords in concordance output. It is called the
5339 @dfn{Ignore file}. The file contains exactly one word in each line; the
5340 end of line separation of words is not subject to the value of the
5344 @itemx --only-file=@var{file}
5346 The file associated with this option contains a list of words which will
5347 be retained in concordance output; any word not mentioned in this file
5348 is ignored. The file is called the @dfn{Only file}. The file contains
5349 exactly one word in each line; the end of line separation of words is
5350 not subject to the value of the @option{-S} option.
5352 There is no default for the Only file. When both an Only file and an
5353 Ignore file are specified, a word is considered a keyword only
5354 if it is listed in the Only file and not in the Ignore file.
5359 On each input line, the leading sequence of non-white space characters will be
5360 taken to be a reference that has the purpose of identifying this input
5361 line in the resulting permuted index.
5362 @xref{Output formatting in ptx},
5363 for more information about reference production.
5364 Using this option changes the default value for option @option{-S}.
5366 Using this option, the program does not try very hard to remove
5367 references from contexts in output, but it succeeds in doing so
5368 @emph{when} the context ends exactly at the newline. If option
5369 @option{-r} is used with @option{-S} default value, or when GNU extensions
5370 are disabled, this condition is always met and references are completely
5371 excluded from the output contexts.
5373 @item -S @var{regexp}
5374 @itemx --sentence-regexp=@var{regexp}
5376 This option selects which regular expression will describe the end of a
5377 line or the end of a sentence. In fact, this regular expression is not
5378 the only distinction between end of lines or end of sentences, and input
5379 line boundaries have no special significance outside this option. By
5380 default, when GNU extensions are enabled and if @option{-r} option is not
5381 used, end of sentences are used. In this case, this @var{regex} is
5382 imported from GNU Emacs:
5385 [.?!][]\"')@}]*\\($\\|\t\\| \\)[ \t\n]*
5388 Whenever GNU extensions are disabled or if @option{-r} option is used, end
5389 of lines are used; in this case, the default @var{regexp} is just:
5395 Using an empty @var{regexp} is equivalent to completely disabling end of
5396 line or end of sentence recognition. In this case, the whole file is
5397 considered to be a single big line or sentence. The user might want to
5398 disallow all truncation flag generation as well, through option @option{-F
5399 ""}. @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
5402 When the keywords happen to be near the beginning of the input line or
5403 sentence, this often creates an unused area at the beginning of the
5404 output context line; when the keywords happen to be near the end of the
5405 input line or sentence, this often creates an unused area at the end of
5406 the output context line. The program tries to fill those unused areas
5407 by wrapping around context in them; the tail of the input line or
5408 sentence is used to fill the unused area on the left of the output line;
5409 the head of the input line or sentence is used to fill the unused area
5410 on the right of the output line.
5412 As a matter of convenience to the user, many usual backslashed escape
5413 sequences from the C language are recognized and converted to the
5414 corresponding characters by @command{ptx} itself.
5416 @item -W @var{regexp}
5417 @itemx --word-regexp=@var{regexp}
5419 This option selects which regular expression will describe each keyword.
5420 By default, if GNU extensions are enabled, a word is a sequence of
5421 letters; the @var{regexp} used is @samp{\w+}. When GNU extensions are
5422 disabled, a word is by default anything which ends with a space, a tab
5423 or a newline; the @var{regexp} used is @samp{[^ \t\n]+}.
5425 An empty @var{regexp} is equivalent to not using this option.
5426 @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
5429 As a matter of convenience to the user, many usual backslashed escape
5430 sequences, as found in the C language, are recognized and converted to
5431 the corresponding characters by @command{ptx} itself.
5436 @node Output formatting in ptx
5437 @subsection Output formatting
5439 Output format is mainly controlled by the @option{-O} and @option{-T} options
5440 described in the table below. When neither @option{-O} nor @option{-T} are
5441 selected, and if GNU extensions are enabled, the program chooses an
5442 output format suitable for a dumb terminal. Each keyword occurrence is
5443 output to the center of one line, surrounded by its left and right
5444 contexts. Each field is properly justified, so the concordance output
5445 can be readily observed. As a special feature, if automatic
5446 references are selected by option @option{-A} and are output before the
5447 left context, that is, if option @option{-R} is @emph{not} selected, then
5448 a colon is added after the reference; this nicely interfaces with GNU
5449 Emacs @code{next-error} processing. In this default output format, each
5450 white space character, like newline and tab, is merely changed to
5451 exactly one space, with no special attempt to compress consecutive
5452 spaces. This might change in the future. Except for those white space
5453 characters, every other character of the underlying set of 256
5454 characters is transmitted verbatim.
5456 Output format is further controlled by the following options.
5460 @item -g @var{number}
5461 @itemx --gap-size=@var{number}
5463 Select the size of the minimum white space gap between the fields on the
5466 @item -w @var{number}
5467 @itemx --width=@var{number}
5469 Select the maximum output width of each final line. If references are
5470 used, they are included or excluded from the maximum output width
5471 depending on the value of option @option{-R}@. If this option is not
5472 selected, that is, when references are output before the left context,
5473 the maximum output width takes into account the maximum length of all
5474 references. If this option is selected, that is, when references are
5475 output after the right context, the maximum output width does not take
5476 into account the space taken by references, nor the gap that precedes
5480 @itemx --auto-reference
5482 Select automatic references. Each input line will have an automatic
5483 reference made up of the file name and the line ordinal, with a single
5484 colon between them. However, the file name will be empty when standard
5485 input is being read. If both @option{-A} and @option{-r} are selected, then
5486 the input reference is still read and skipped, but the automatic
5487 reference is used at output time, overriding the input reference.
5490 @itemx --right-side-refs
5492 In the default output format, when option @option{-R} is not used, any
5493 references produced by the effect of options @option{-r} or @option{-A} are
5494 placed to the far right of output lines, after the right context. With
5495 default output format, when the @option{-R} option is specified, references
5496 are rather placed at the beginning of each output line, before the left
5497 context. For any other output format, option @option{-R} is
5498 ignored, with one exception: with @option{-R} the width of references
5499 is @emph{not} taken into account in total output width given by @option{-w}.
5501 This option is automatically selected whenever GNU extensions are
5504 @item -F @var{string}
5505 @itemx --flac-truncation=@var{string}
5507 This option will request that any truncation in the output be reported
5508 using the string @var{string}. Most output fields theoretically extend
5509 towards the beginning or the end of the current line, or current
5510 sentence, as selected with option @option{-S}@. But there is a maximum
5511 allowed output line width, changeable through option @option{-w}, which is
5512 further divided into space for various output fields. When a field has
5513 to be truncated because it cannot extend beyond the beginning or the end of
5514 the current line to fit in, then a truncation occurs. By default,
5515 the string used is a single slash, as in @option{-F /}.
5517 @var{string} may have more than one character, as in @option{-F @dots{}}.
5518 Also, in the particular case when @var{string} is empty (@option{-F ""}),
5519 truncation flagging is disabled, and no truncation marks are appended in
5522 As a matter of convenience to the user, many usual backslashed escape
5523 sequences, as found in the C language, are recognized and converted to
5524 the corresponding characters by @command{ptx} itself.
5526 @item -M @var{string}
5527 @itemx --macro-name=@var{string}
5529 Select another @var{string} to be used instead of @samp{xx}, while
5530 generating output suitable for @command{nroff}, @command{troff} or @TeX{}.
5533 @itemx --format=roff
5535 Choose an output format suitable for @command{nroff} or @command{troff}
5536 processing. Each output line will look like:
5539 .xx "@var{tail}" "@var{before}" "@var{keyword_and_after}"@c
5540 "@var{head}" "@var{ref}"
5543 so it will be possible to write a @samp{.xx} roff macro to take care of
5544 the output typesetting. This is the default output format when GNU
5545 extensions are disabled. Option @option{-M} can be used to change
5546 @samp{xx} to another macro name.
5548 In this output format, each non-graphical character, like newline and
5549 tab, is merely changed to exactly one space, with no special attempt to
5550 compress consecutive spaces. Each quote character: @kbd{"} is doubled
5551 so it will be correctly processed by @command{nroff} or @command{troff}.
5556 Choose an output format suitable for @TeX{} processing. Each output
5557 line will look like:
5560 \xx @{@var{tail}@}@{@var{before}@}@{@var{keyword}@}@c
5561 @{@var{after}@}@{@var{head}@}@{@var{ref}@}
5565 so it will be possible to write a @code{\xx} definition to take care of
5566 the output typesetting. Note that when references are not being
5567 produced, that is, neither option @option{-A} nor option @option{-r} is
5568 selected, the last parameter of each @code{\xx} call is inhibited.
5569 Option @option{-M} can be used to change @samp{xx} to another macro
5572 In this output format, some special characters, like @kbd{$}, @kbd{%},
5573 @kbd{&}, @kbd{#} and @kbd{_} are automatically protected with a
5574 backslash. Curly brackets @kbd{@{}, @kbd{@}} are protected with a
5575 backslash and a pair of dollar signs (to force mathematical mode). The
5576 backslash itself produces the sequence @code{\backslash@{@}}.
5577 Circumflex and tilde diacritical marks produce the sequence @code{^\@{ @}} and
5578 @code{~\@{ @}} respectively. Other diacriticized characters of the
5579 underlying character set produce an appropriate @TeX{} sequence as far
5580 as possible. The other non-graphical characters, like newline and tab,
5581 and all other characters which are not part of ASCII, are merely
5582 changed to exactly one space, with no special attempt to compress
5583 consecutive spaces. Let me know how to improve this special character
5584 processing for @TeX{}.
5589 @node Compatibility in ptx
5590 @subsection The GNU extensions to @command{ptx}
5592 This version of @command{ptx} contains a few features which do not exist in
5593 System V @command{ptx}. These extra features are suppressed by using the
5594 @option{-G} command line option, unless overridden by other command line
5595 options. Some GNU extensions cannot be recovered by overriding, so the
5596 simple rule is to avoid @option{-G} if you care about GNU extensions.
5597 Here are the differences between this program and System V @command{ptx}.
5602 This program can read many input files at once, it always writes the
5603 resulting concordance on standard output. On the other hand, System V
5604 @command{ptx} reads only one file and sends the result to standard output
5605 or, if a second @var{file} parameter is given on the command, to that
5608 Having output parameters not introduced by options is a dangerous
5609 practice which GNU avoids as far as possible. So, for using @command{ptx}
5610 portably between GNU and System V, you should always use it with a
5611 single input file, and always expect the result on standard output. You
5612 might also want to automatically configure in a @option{-G} option to
5613 @command{ptx} calls in products using @command{ptx}, if the configurator finds
5614 that the installed @command{ptx} accepts @option{-G}.
5617 The only options available in System V @command{ptx} are options @option{-b},
5618 @option{-f}, @option{-g}, @option{-i}, @option{-o}, @option{-r}, @option{-t} and
5619 @option{-w}. All other options are GNU extensions and are not repeated in
5620 this enumeration. Moreover, some options have a slightly different
5621 meaning when GNU extensions are enabled, as explained below.
5624 By default, concordance output is not formatted for @command{troff} or
5625 @command{nroff}. It is rather formatted for a dumb terminal. @command{troff}
5626 or @command{nroff} output may still be selected through option @option{-O}.
5629 Unless @option{-R} option is used, the maximum reference width is
5630 subtracted from the total output line width. With GNU extensions
5631 disabled, width of references is not taken into account in the output
5632 line width computations.
5635 All 256 bytes, even ASCII NUL bytes, are always read and
5636 processed from input file with no adverse effect, even if GNU extensions
5637 are disabled. However, System V @command{ptx} does not accept 8-bit
5638 characters, a few control characters are rejected, and the tilde
5639 @kbd{~} is also rejected.
5642 Input line length is only limited by available memory, even if GNU
5643 extensions are disabled. However, System V @command{ptx} processes only
5644 the first 200 characters in each line.
5647 The break (non-word) characters default to be every character except all
5648 letters of the underlying character set, diacriticized or not. When GNU
5649 extensions are disabled, the break characters default to space, tab and
5653 The program makes better use of output line width. If GNU extensions
5654 are disabled, the program rather tries to imitate System V @command{ptx},
5655 but still, there are some slight disposition glitches this program does
5656 not completely reproduce.
5659 The user can specify both an Ignore file and an Only file. This is not
5660 allowed with System V @command{ptx}.
5665 @node tsort invocation
5666 @section @command{tsort}: Topological sort
5669 @cindex topological sort
5671 @command{tsort} performs a topological sort on the given @var{file}, or
5672 standard input if no input file is given or for a @var{file} of
5673 @samp{-}. For more details and some history, see @ref{tsort background}.
5677 tsort [@var{option}] [@var{file}]
5680 @command{tsort} reads its input as pairs of strings, separated by blanks,
5681 indicating a partial ordering. The output is a total ordering that
5682 corresponds to the given partial ordering.
5696 will produce the output
5707 Consider a more realistic example.
5708 You have a large set of functions all in one file, and they may all be
5709 declared static except one. Currently that one (say @code{main}) is the
5710 first function defined in the file, and the ones it calls directly follow
5711 it, followed by those they call, etc. Let's say that you are determined
5712 to take advantage of prototypes, so you have to choose between declaring
5713 all of those functions (which means duplicating a lot of information from
5714 the definitions) and rearranging the functions so that as many as possible
5715 are defined before they are used. One way to automate the latter process
5716 is to get a list for each function of the functions it calls directly.
5717 Many programs can generate such lists. They describe a call graph.
5718 Consider the following list, in which a given line indicates that the
5719 function on the left calls the one on the right directly.
5725 tail_file pretty_name
5726 tail_file write_header
5728 tail_forever recheck
5729 tail_forever pretty_name
5730 tail_forever write_header
5731 tail_forever dump_remainder
5734 tail_lines start_lines
5735 tail_lines dump_remainder
5736 tail_lines file_lines
5737 tail_lines pipe_lines
5739 tail_bytes start_bytes
5740 tail_bytes dump_remainder
5741 tail_bytes pipe_bytes
5742 file_lines dump_remainder
5746 then you can use @command{tsort} to produce an ordering of those
5747 functions that satisfies your requirement.
5750 example$ tsort call-graph | tac
5770 @command{tsort} detects any cycles in the input and writes the first cycle
5771 encountered to standard error.
5773 Note that for a given partial ordering, generally there is no unique
5774 total ordering. In the context of the call graph above, the function
5775 @code{parse_options} may be placed anywhere in the list as long as it
5776 precedes @code{main}.
5778 The only options are @option{--help} and @option{--version}. @xref{Common
5784 * tsort background:: Where tsort came from.
5787 @node tsort background
5788 @subsection @command{tsort}: Background
5790 @command{tsort} exists because very early versions of the Unix linker processed
5791 an archive file exactly once, and in order. As @command{ld} read each object
5792 in the archive, it decided whether it was needed in the program based on
5793 whether it defined any symbols which were undefined at that point in
5796 This meant that dependencies within the archive had to be handled
5797 specially. For example, @code{scanf} probably calls @code{read}. That means
5798 that in a single pass through an archive, it was important for @code{scanf.o}
5799 to appear before read.o, because otherwise a program which calls
5800 @code{scanf} but not @code{read} might end up with an unexpected unresolved
5801 reference to @code{read}.
5803 The way to address this problem was to first generate a set of
5804 dependencies of one object file on another. This was done by a shell
5805 script called @command{lorder}. The GNU tools don't provide a version of
5806 lorder, as far as I know, but you can still find it in BSD
5809 Then you ran @command{tsort} over the @command{lorder} output, and you used the
5810 resulting sort to define the order in which you added objects to the archive.
5812 This whole procedure has been obsolete since about 1980, because
5813 Unix archives now contain a symbol table (traditionally built by
5814 @command{ranlib}, now generally built by @command{ar} itself), and the Unix
5815 linker uses the symbol table to effectively make multiple passes over
5818 Anyhow, that's where tsort came from. To solve an old problem with
5819 the way the linker handled archive files, which has since been solved
5823 @node Operating on fields
5824 @chapter Operating on fields
5827 * cut invocation:: Print selected parts of lines.
5828 * paste invocation:: Merge lines of files.
5829 * join invocation:: Join lines on a common field.
5833 @node cut invocation
5834 @section @command{cut}: Print selected parts of lines
5837 @command{cut} writes to standard output selected parts of each line of each
5838 input file, or standard input if no files are given or for a file name of
5842 cut @var{option}@dots{} [@var{file}]@dots{}
5845 In the table which follows, the @var{byte-list}, @var{character-list},
5846 and @var{field-list} are one or more numbers or ranges (two numbers
5847 separated by a dash) separated by commas. Bytes, characters, and
5848 fields are numbered starting at 1. Incomplete ranges may be
5849 given: @option{-@var{m}} means @samp{1-@var{m}}; @samp{@var{n}-} means
5850 @samp{@var{n}} through end of line or last field. The list elements
5851 can be repeated, can overlap, and can be specified in any order; but
5852 the selected input is written in the same order that it is read, and
5853 is written exactly once.
5855 The program accepts the following options. Also see @ref{Common
5860 @item -b @var{byte-list}
5861 @itemx --bytes=@var{byte-list}
5864 Select for printing only the bytes in positions listed in
5865 @var{byte-list}. Tabs and backspaces are treated like any other
5866 character; they take up 1 byte. If an output delimiter is specified,
5867 (see the description of @option{--output-delimiter}), then output that
5868 string between ranges of selected bytes.
5870 @item -c @var{character-list}
5871 @itemx --characters=@var{character-list}
5873 @opindex --characters
5874 Select for printing only the characters in positions listed in
5875 @var{character-list}. The same as @option{-b} for now, but
5876 internationalization will change that. Tabs and backspaces are
5877 treated like any other character; they take up 1 character. If an
5878 output delimiter is specified, (see the description of
5879 @option{--output-delimiter}), then output that string between ranges
5882 @item -f @var{field-list}
5883 @itemx --fields=@var{field-list}
5886 Select for printing only the fields listed in @var{field-list}.
5887 Fields are separated by a TAB character by default. Also print any
5888 line that contains no delimiter character, unless the
5889 @option{--only-delimited} (@option{-s}) option is specified.
5891 Note @command{awk} supports more sophisticated field processing,
5892 and by default will use (and discard) runs of blank characters to
5893 separate fields, and ignore leading and trailing blanks.
5896 awk '{print $2}' # print the second field
5897 awk '{print $NF-1}' # print the penultimate field
5898 awk '{print $2,$1}' # reorder the first two fields
5902 In the unlikely event that @command{awk} is unavailable,
5903 one can use the @command{join} command, to process blank
5904 characters as @command{awk} does above.
5907 join -a1 -o 1.2 - /dev/null # print the second field
5908 join -a1 -o 1.2,1.1 - /dev/null # reorder the first two fields
5912 @item -d @var{input_delim_byte}
5913 @itemx --delimiter=@var{input_delim_byte}
5915 @opindex --delimiter
5916 With @option{-f}, use the first byte of @var{input_delim_byte} as
5917 the input fields separator (default is TAB).
5921 Do not split multi-byte characters (no-op for now).
5924 @itemx --only-delimited
5926 @opindex --only-delimited
5927 For @option{-f}, do not print lines that do not contain the field separator
5928 character. Normally, any line without a field separator is printed verbatim.
5930 @item --output-delimiter=@var{output_delim_string}
5931 @opindex --output-delimiter
5932 With @option{-f}, output fields are separated by @var{output_delim_string}.
5933 The default with @option{-f} is to use the input delimiter.
5934 When using @option{-b} or @option{-c} to select ranges of byte or
5935 character offsets (as opposed to ranges of fields),
5936 output @var{output_delim_string} between non-overlapping
5937 ranges of selected bytes.
5940 @opindex --complement
5941 This option is a GNU extension.
5942 Select for printing the complement of the bytes, characters or fields
5943 selected with the @option{-b}, @option{-c} or @option{-f} options.
5944 In other words, do @emph{not} print the bytes, characters or fields
5945 specified via those options. This option is useful when you have
5946 many fields and want to print all but a few of them.
5953 @node paste invocation
5954 @section @command{paste}: Merge lines of files
5957 @cindex merging files
5959 @command{paste} writes to standard output lines consisting of sequentially
5960 corresponding lines of each given file, separated by a TAB character.
5961 Standard input is used for a file name of @samp{-} or if no input files
5983 paste [@var{option}]@dots{} [@var{file}]@dots{}
5986 The program accepts the following options. Also see @ref{Common options}.
5994 Paste the lines of one file at a time rather than one line from each
5995 file. Using the above example data:
5998 $ paste -s num2 let3
6003 @item -d @var{delim-list}
6004 @itemx --delimiters=@var{delim-list}
6006 @opindex --delimiters
6007 Consecutively use the characters in @var{delim-list} instead of
6008 TAB to separate merged lines. When @var{delim-list} is
6009 exhausted, start again at its beginning. Using the above example data:
6012 $ paste -d '%_' num2 let3 num2
6023 @node join invocation
6024 @section @command{join}: Join lines on a common field
6027 @cindex common field, joining on
6029 @command{join} writes to standard output a line for each pair of input
6030 lines that have identical join fields. Synopsis:
6033 join [@var{option}]@dots{} @var{file1} @var{file2}
6036 Either @var{file1} or @var{file2} (but not both) can be @samp{-},
6037 meaning standard input. @var{file1} and @var{file2} should be
6038 sorted on the join fields.
6041 Normally, the sort order is that of the
6042 collating sequence specified by the @env{LC_COLLATE} locale. Unless
6043 the @option{-t} option is given, the sort comparison ignores blanks at
6044 the start of the join field, as in @code{sort -b}. If the
6045 @option{--ignore-case} option is given, the sort comparison ignores
6046 the case of characters in the join field, as in @code{sort -f}.
6048 The @command{sort} and @command{join} commands should use consistent
6049 locales and options if the output of @command{sort} is fed to
6050 @command{join}. You can use a command like @samp{sort -k 1b,1} to
6051 sort a file on its default join field, but if you select a non-default
6052 locale, join field, separator, or comparison options, then you should
6053 do so consistently between @command{join} and @command{sort}.
6054 If @samp{join -t ''} is specified then the whole line is considered which
6055 matches the default operation of sort.
6057 If the input has no unpairable lines, a GNU extension is
6058 available; the sort order can be any order that considers two fields
6059 to be equal if and only if the sort comparison described above
6060 considers them to be equal. For example:
6078 @checkOrderOption{join}
6083 @item the join field is the first field in each line;
6084 @item fields in the input are separated by one or more blanks, with leading
6085 blanks on the line ignored;
6086 @item fields in the output are separated by a space;
6087 @item each output line consists of the join field, the remaining
6088 fields from @var{file1}, then the remaining fields from @var{file2}.
6091 The program accepts the following options. Also see @ref{Common options}.
6095 @item -a @var{file-number}
6097 Print a line for each unpairable line in file @var{file-number} (either
6098 @samp{1} or @samp{2}), in addition to the normal output.
6101 Fail with an error message if either input file is wrongly ordered.
6103 @item --nocheck-order
6104 Do not check that both input files are in sorted order. This is the default.
6106 @item -e @var{string}
6108 Replace those output fields that are missing in the input with @var{string}.
6109 I.e., missing fields specified with the @option{-12jo} options.
6113 Treat the first line of each input file as a header line. The header lines
6114 will be joined and printed as the first output line. If @option{-o} is used to
6115 specify output format, the header line will be printed according to the
6116 specified format. The header lines will not be checked for ordering even if
6117 @option{--check-order} is specified. Also if the header lines from each file
6118 do not match, the heading fields from the first file will be used.
6121 @itemx --ignore-case
6123 @opindex --ignore-case
6124 Ignore differences in case when comparing keys.
6125 With this option, the lines of the input files must be ordered in the same way.
6126 Use @samp{sort -f} to produce this ordering.
6128 @item -1 @var{field}
6130 Join on field @var{field} (a positive integer) of file 1.
6132 @item -2 @var{field}
6134 Join on field @var{field} (a positive integer) of file 2.
6136 @item -j @var{field}
6137 Equivalent to @option{-1 @var{field} -2 @var{field}}.
6139 @item -o @var{field-list}
6141 If the keyword @samp{auto} is specified, infer the output format from
6142 the first line in each file. This is the same as the default output format
6143 but also ensures the same number of fields are output for each line.
6144 Missing fields are replaced with the @option{-e} option and extra fields
6147 Otherwise, construct each output line according to the format in
6148 @var{field-list}. Each element in @var{field-list} is either the single
6149 character @samp{0} or has the form @var{m.n} where the file number, @var{m},
6150 is @samp{1} or @samp{2} and @var{n} is a positive field number.
6152 A field specification of @samp{0} denotes the join field.
6153 In most cases, the functionality of the @samp{0} field spec
6154 may be reproduced using the explicit @var{m.n} that corresponds
6155 to the join field. However, when printing unpairable lines
6156 (using either of the @option{-a} or @option{-v} options), there is no way
6157 to specify the join field using @var{m.n} in @var{field-list}
6158 if there are unpairable lines in both files.
6159 To give @command{join} that functionality, POSIX invented the @samp{0}
6160 field specification notation.
6162 The elements in @var{field-list}
6163 are separated by commas or blanks.
6164 Blank separators typically need to be quoted for the shell. For
6165 example, the commands @samp{join -o 1.2,2.2} and @samp{join -o '1.2
6166 2.2'} are equivalent.
6168 All output lines---including those printed because of any -a or -v
6169 option---are subject to the specified @var{field-list}.
6172 Use character @var{char} as the input and output field separator.
6173 Treat as significant each occurrence of @var{char} in the input file.
6174 Use @samp{sort -t @var{char}}, without the @option{-b} option of
6175 @samp{sort}, to produce this ordering. If @samp{join -t ''} is specified,
6176 the whole line is considered, matching the default operation of sort.
6177 If @samp{-t '\0'} is specified then the ASCII NUL
6178 character is used to delimit the fields.
6180 @item -v @var{file-number}
6181 Print a line for each unpairable line in file @var{file-number}
6182 (either @samp{1} or @samp{2}), instead of the normal output.
6189 @node Operating on characters
6190 @chapter Operating on characters
6192 @cindex operating on characters
6194 This commands operate on individual characters.
6197 * tr invocation:: Translate, squeeze, and/or delete characters.
6198 * expand invocation:: Convert tabs to spaces.
6199 * unexpand invocation:: Convert spaces to tabs.
6204 @section @command{tr}: Translate, squeeze, and/or delete characters
6211 tr [@var{option}]@dots{} @var{set1} [@var{set2}]
6214 @command{tr} copies standard input to standard output, performing
6215 one of the following operations:
6219 translate, and optionally squeeze repeated characters in the result,
6221 squeeze repeated characters,
6225 delete characters, then squeeze repeated characters from the result.
6228 The @var{set1} and (if given) @var{set2} arguments define ordered
6229 sets of characters, referred to below as @var{set1} and @var{set2}. These
6230 sets are the characters of the input that @command{tr} operates on.
6231 The @option{--complement} (@option{-c}, @option{-C}) option replaces
6233 complement (all of the characters that are not in @var{set1}).
6235 Currently @command{tr} fully supports only single-byte characters.
6236 Eventually it will support multibyte characters; when it does, the
6237 @option{-C} option will cause it to complement the set of characters,
6238 whereas @option{-c} will cause it to complement the set of values.
6239 This distinction will matter only when some values are not characters,
6240 and this is possible only in locales using multibyte encodings when
6241 the input contains encoding errors.
6243 The program accepts the @option{--help} and @option{--version}
6244 options. @xref{Common options}. Options must precede operands.
6249 * Character sets:: Specifying sets of characters.
6250 * Translating:: Changing one set of characters to another.
6251 * Squeezing:: Squeezing repeats and deleting.
6255 @node Character sets
6256 @subsection Specifying sets of characters
6258 @cindex specifying sets of characters
6260 The format of the @var{set1} and @var{set2} arguments resembles
6261 the format of regular expressions; however, they are not regular
6262 expressions, only lists of characters. Most characters simply
6263 represent themselves in these strings, but the strings can contain
6264 the shorthands listed below, for convenience. Some of them can be
6265 used only in @var{set1} or @var{set2}, as noted below.
6269 @item Backslash escapes
6270 @cindex backslash escapes
6272 The following backslash escape sequences are recognized:
6290 The 8-bit character with the value given by @var{ooo}, which is 1 to 3
6291 octal digits. Note that @samp{\400} is interpreted as the two-byte
6292 sequence, @samp{\040} @samp{0}.
6297 While a backslash followed by a character not listed above is
6298 interpreted as that character, the backslash also effectively
6299 removes any special significance, so it is useful to escape
6300 @samp{[}, @samp{]}, @samp{*}, and @samp{-}.
6305 The notation @samp{@var{m}-@var{n}} expands to all of the characters
6306 from @var{m} through @var{n}, in ascending order. @var{m} should
6307 collate before @var{n}; if it doesn't, an error results. As an example,
6308 @samp{0-9} is the same as @samp{0123456789}.
6310 GNU @command{tr} does not support the System V syntax that uses square
6311 brackets to enclose ranges. Translations specified in that format
6312 sometimes work as expected, since the brackets are often transliterated
6313 to themselves. However, they should be avoided because they sometimes
6314 behave unexpectedly. For example, @samp{tr -d '[0-9]'} deletes brackets
6317 Many historically common and even accepted uses of ranges are not
6318 portable. For example, on EBCDIC hosts using the @samp{A-Z}
6319 range will not do what most would expect because @samp{A} through @samp{Z}
6320 are not contiguous as they are in ASCII@.
6321 If you can rely on a POSIX compliant version of @command{tr}, then
6322 the best way to work around this is to use character classes (see below).
6323 Otherwise, it is most portable (and most ugly) to enumerate the members
6326 @item Repeated characters
6327 @cindex repeated characters
6329 The notation @samp{[@var{c}*@var{n}]} in @var{set2} expands to @var{n}
6330 copies of character @var{c}. Thus, @samp{[y*6]} is the same as
6331 @samp{yyyyyy}. The notation @samp{[@var{c}*]} in @var{string2} expands
6332 to as many copies of @var{c} as are needed to make @var{set2} as long as
6333 @var{set1}. If @var{n} begins with @samp{0}, it is interpreted in
6334 octal, otherwise in decimal.
6336 @item Character classes
6337 @cindex character classes
6339 The notation @samp{[:@var{class}:]} expands to all of the characters in
6340 the (predefined) class @var{class}. The characters expand in no
6341 particular order, except for the @code{upper} and @code{lower} classes,
6342 which expand in ascending order. When the @option{--delete} (@option{-d})
6343 and @option{--squeeze-repeats} (@option{-s}) options are both given, any
6344 character class can be used in @var{set2}. Otherwise, only the
6345 character classes @code{lower} and @code{upper} are accepted in
6346 @var{set2}, and then only if the corresponding character class
6347 (@code{upper} and @code{lower}, respectively) is specified in the same
6348 relative position in @var{set1}. Doing this specifies case conversion.
6349 The class names are given below; an error results when an invalid class
6361 Horizontal whitespace.
6370 Printable characters, not including space.
6376 Printable characters, including space.
6379 Punctuation characters.
6382 Horizontal or vertical whitespace.
6391 @item Equivalence classes
6392 @cindex equivalence classes
6394 The syntax @samp{[=@var{c}=]} expands to all of the characters that are
6395 equivalent to @var{c}, in no particular order. Equivalence classes are
6396 a relatively recent invention intended to support non-English alphabets.
6397 But there seems to be no standard way to define them or determine their
6398 contents. Therefore, they are not fully implemented in GNU @command{tr};
6399 each character's equivalence class consists only of that character,
6400 which is of no particular use.
6406 @subsection Translating
6408 @cindex translating characters
6410 @command{tr} performs translation when @var{set1} and @var{set2} are
6411 both given and the @option{--delete} (@option{-d}) option is not given.
6412 @command{tr} translates each character of its input that is in @var{set1}
6413 to the corresponding character in @var{set2}. Characters not in
6414 @var{set1} are passed through unchanged. When a character appears more
6415 than once in @var{set1} and the corresponding characters in @var{set2}
6416 are not all the same, only the final one is used. For example, these
6417 two commands are equivalent:
6424 A common use of @command{tr} is to convert lowercase characters to
6425 uppercase. This can be done in many ways. Here are three of them:
6428 tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
6430 tr '[:lower:]' '[:upper:]'
6434 But note that using ranges like @code{a-z} above is not portable.
6436 When @command{tr} is performing translation, @var{set1} and @var{set2}
6437 typically have the same length. If @var{set1} is shorter than
6438 @var{set2}, the extra characters at the end of @var{set2} are ignored.
6440 On the other hand, making @var{set1} longer than @var{set2} is not
6441 portable; POSIX says that the result is undefined. In this situation,
6442 BSD @command{tr} pads @var{set2} to the length of @var{set1} by repeating
6443 the last character of @var{set2} as many times as necessary. System V
6444 @command{tr} truncates @var{set1} to the length of @var{set2}.
6446 By default, GNU @command{tr} handles this case like BSD @command{tr}.
6447 When the @option{--truncate-set1} (@option{-t}) option is given,
6448 GNU @command{tr} handles this case like the System V @command{tr}
6449 instead. This option is ignored for operations other than translation.
6451 Acting like System V @command{tr} in this case breaks the relatively common
6455 tr -cs A-Za-z0-9 '\012'
6459 because it converts only zero bytes (the first element in the
6460 complement of @var{set1}), rather than all non-alphanumerics, to
6464 By the way, the above idiom is not portable because it uses ranges, and
6465 it assumes that the octal code for newline is 012.
6466 Assuming a POSIX compliant @command{tr}, here is a better
6470 tr -cs '[:alnum:]' '[\n*]'
6475 @subsection Squeezing repeats and deleting
6477 @cindex squeezing repeat characters
6478 @cindex deleting characters
6480 When given just the @option{--delete} (@option{-d}) option, @command{tr}
6481 removes any input characters that are in @var{set1}.
6483 When given just the @option{--squeeze-repeats} (@option{-s}) option,
6484 @command{tr} replaces each input sequence of a repeated character that
6485 is in @var{set1} with a single occurrence of that character.
6487 When given both @option{--delete} and @option{--squeeze-repeats}, @command{tr}
6488 first performs any deletions using @var{set1}, then squeezes repeats
6489 from any remaining characters using @var{set2}.
6491 The @option{--squeeze-repeats} option may also be used when translating,
6492 in which case @command{tr} first performs translation, then squeezes
6493 repeats from any remaining characters using @var{set2}.
6495 Here are some examples to illustrate various combinations of options:
6500 Remove all zero bytes:
6507 Put all words on lines by themselves. This converts all
6508 non-alphanumeric characters to newlines, then squeezes each string
6509 of repeated newlines into a single newline:
6512 tr -cs '[:alnum:]' '[\n*]'
6516 Convert each sequence of repeated newlines to a single newline:
6523 Find doubled occurrences of words in a document.
6524 @c Separate the following two "the"s, so typo checkers don't complain.
6525 For example, people often write ``the @w{}the'' with the repeated words
6526 separated by a newline. The Bourne shell script below works first
6527 by converting each sequence of punctuation and blank characters to a
6528 single newline. That puts each ``word'' on a line by itself.
6529 Next it maps all uppercase characters to lower case, and finally it
6530 runs @command{uniq} with the @option{-d} option to print out only the words
6536 | tr -s '[:punct:][:blank:]' '[\n*]' \
6537 | tr '[:upper:]' '[:lower:]' \
6542 Deleting a small set of characters is usually straightforward. For example,
6543 to remove all @samp{a}s, @samp{x}s, and @samp{M}s you would do this:
6549 However, when @samp{-} is one of those characters, it can be tricky because
6550 @samp{-} has special meanings. Performing the same task as above but also
6551 removing all @samp{-} characters, we might try @code{tr -d -axM}, but
6552 that would fail because @command{tr} would try to interpret @option{-a} as
6553 a command-line option. Alternatively, we could try putting the hyphen
6554 inside the string, @code{tr -d a-xM}, but that wouldn't work either because
6555 it would make @command{tr} interpret @code{a-x} as the range of characters
6556 @samp{a}@dots{}@samp{x} rather than the three.
6557 One way to solve the problem is to put the hyphen at the end of the list
6564 Or you can use @samp{--} to terminate option processing:
6570 More generally, use the character class notation @code{[=c=]}
6571 with @samp{-} (or any other character) in place of the @samp{c}:
6577 Note how single quotes are used in the above example to protect the
6578 square brackets from interpretation by a shell.
6583 @node expand invocation
6584 @section @command{expand}: Convert tabs to spaces
6587 @cindex tabs to spaces, converting
6588 @cindex converting tabs to spaces
6590 @command{expand} writes the contents of each given @var{file}, or standard
6591 input if none are given or for a @var{file} of @samp{-}, to standard
6592 output, with tab characters converted to the appropriate number of
6596 expand [@var{option}]@dots{} [@var{file}]@dots{}
6599 By default, @command{expand} converts all tabs to spaces. It preserves
6600 backspace characters in the output; they decrement the column count for
6601 tab calculations. The default action is equivalent to @option{-t 8} (set
6602 tabs every 8 columns).
6604 The program accepts the following options. Also see @ref{Common options}.
6608 @item -t @var{tab1}[,@var{tab2}]@dots{}
6609 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6612 @cindex tab stops, setting
6613 If only one tab stop is given, set the tabs @var{tab1} spaces apart
6614 (default is 8). Otherwise, set the tabs at columns @var{tab1},
6615 @var{tab2}, @dots{} (numbered from 0), and replace any tabs beyond the
6616 last tab stop given with single spaces. Tab stops can be separated by
6617 blanks as well as by commas.
6619 For compatibility, GNU @command{expand} also accepts the obsolete
6620 option syntax, @option{-@var{t1}[,@var{t2}]@dots{}}. New scripts
6621 should use @option{-t @var{t1}[,@var{t2}]@dots{}} instead.
6627 @cindex initial tabs, converting
6628 Only convert initial tabs (those that precede all non-space or non-tab
6629 characters) on each line to spaces.
6636 @node unexpand invocation
6637 @section @command{unexpand}: Convert spaces to tabs
6641 @command{unexpand} writes the contents of each given @var{file}, or
6642 standard input if none are given or for a @var{file} of @samp{-}, to
6643 standard output, converting blanks at the beginning of each line into
6644 as many tab characters as needed. In the default POSIX
6645 locale, a @dfn{blank} is a space or a tab; other locales may specify
6646 additional blank characters. Synopsis:
6649 unexpand [@var{option}]@dots{} [@var{file}]@dots{}
6652 By default, @command{unexpand} converts only initial blanks (those
6653 that precede all non-blank characters) on each line. It
6654 preserves backspace characters in the output; they decrement the column
6655 count for tab calculations. By default, tabs are set at every 8th
6658 The program accepts the following options. Also see @ref{Common options}.
6662 @item -t @var{tab1}[,@var{tab2}]@dots{}
6663 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6666 If only one tab stop is given, set the tabs @var{tab1} columns apart
6667 instead of the default 8. Otherwise, set the tabs at columns
6668 @var{tab1}, @var{tab2}, @dots{} (numbered from 0), and leave blanks
6669 beyond the tab stops given unchanged. Tab stops can be separated by
6670 blanks as well as by commas. This option implies the @option{-a} option.
6672 For compatibility, GNU @command{unexpand} supports the obsolete option syntax,
6673 @option{-@var{tab1}[,@var{tab2}]@dots{}}, where tab stops must be
6674 separated by commas. (Unlike @option{-t}, this obsolete option does
6675 not imply @option{-a}.) New scripts should use @option{--first-only -t
6676 @var{tab1}[,@var{tab2}]@dots{}} instead.
6682 Also convert all sequences of two or more blanks just before a tab stop,
6683 even if they occur after non-blank characters in a line.
6690 @node Directory listing
6691 @chapter Directory listing
6693 This chapter describes the @command{ls} command and its variants @command{dir}
6694 and @command{vdir}, which list information about files.
6697 * ls invocation:: List directory contents.
6698 * dir invocation:: Briefly ls.
6699 * vdir invocation:: Verbosely ls.
6700 * dircolors invocation:: Color setup for ls, etc.
6705 @section @command{ls}: List directory contents
6708 @cindex directory listing
6710 The @command{ls} program lists information about files (of any type,
6711 including directories). Options and file arguments can be intermixed
6712 arbitrarily, as usual.
6714 For non-option command-line arguments that are directories, by default
6715 @command{ls} lists the contents of directories, not recursively, and
6716 omitting files with names beginning with @samp{.}. For other non-option
6717 arguments, by default @command{ls} lists just the file name. If no
6718 non-option argument is specified, @command{ls} operates on the current
6719 directory, acting as if it had been invoked with a single argument of @samp{.}.
6722 By default, the output is sorted alphabetically, according to the locale
6723 settings in effect.@footnote{If you use a non-POSIX
6724 locale (e.g., by setting @env{LC_ALL} to @samp{en_US}), then @command{ls} may
6725 produce output that is sorted differently than you're accustomed to.
6726 In that case, set the @env{LC_ALL} environment variable to @samp{C}.}
6727 If standard output is
6728 a terminal, the output is in columns (sorted vertically) and control
6729 characters are output as question marks; otherwise, the output is listed
6730 one per line and control characters are output as-is.
6732 Because @command{ls} is such a fundamental program, it has accumulated many
6733 options over the years. They are described in the subsections below;
6734 within each section, options are listed alphabetically (ignoring case).
6735 The division of options into the subsections is not absolute, since some
6736 options affect more than one aspect of @command{ls}'s operation.
6738 @cindex exit status of @command{ls}
6743 1 minor problems (e.g., failure to access a file or directory not
6744 specified as a command line argument. This happens when listing a
6745 directory in which entries are actively being removed or renamed.)
6746 2 serious trouble (e.g., memory exhausted, invalid option, failure
6747 to access a file or directory specified as a command line argument
6748 or a directory loop)
6751 Also see @ref{Common options}.
6754 * Which files are listed::
6755 * What information is listed::
6756 * Sorting the output::
6757 * Details about version sort::
6758 * General output formatting::
6759 * Formatting file timestamps::
6760 * Formatting the file names::
6764 @node Which files are listed
6765 @subsection Which files are listed
6767 These options determine which files @command{ls} lists information for.
6768 By default, @command{ls} lists files and the contents of any
6769 directories on the command line, except that in directories it ignores
6770 files whose names start with @samp{.}.
6778 In directories, do not ignore file names that start with @samp{.}.
6783 @opindex --almost-all
6784 In directories, do not ignore all file names that start with @samp{.};
6785 ignore only @file{.} and @file{..}. The @option{--all} (@option{-a})
6786 option overrides this option.
6789 @itemx --ignore-backups
6791 @opindex --ignore-backups
6792 @cindex backup files, ignoring
6793 In directories, ignore files that end with @samp{~}. This option is
6794 equivalent to @samp{--ignore='*~' --ignore='.*~'}.
6799 @opindex --directory
6800 List just the names of directories, as with other types of files, rather
6801 than listing their contents.
6802 @c The following sentence is the same as the one for -F.
6803 Do not follow symbolic links listed on the
6804 command line unless the @option{--dereference-command-line} (@option{-H}),
6805 @option{--dereference} (@option{-L}), or
6806 @option{--dereference-command-line-symlink-to-dir} options are specified.
6809 @itemx --dereference-command-line
6811 @opindex --dereference-command-line
6812 @cindex symbolic links, dereferencing
6813 If a command line argument specifies a symbolic link, show information
6814 for the file the link references rather than for the link itself.
6816 @item --dereference-command-line-symlink-to-dir
6817 @opindex --dereference-command-line-symlink-to-dir
6818 @cindex symbolic links, dereferencing
6819 Do not dereference symbolic links, with one exception:
6820 if a command line argument specifies a symbolic link that refers to
6821 a directory, show information for that directory rather than for the
6823 This is the default behavior when no other dereferencing-related
6824 option has been specified (@option{--classify} (@option{-F}),
6825 @option{--directory} (@option{-d}),
6827 @option{--dereference} (@option{-L}), or
6828 @option{--dereference-command-line} (@option{-H})).
6830 @item --group-directories-first
6831 @opindex --group-directories-first
6832 Group all the directories before the files and then sort the
6833 directories and the files separately using the selected sort key
6834 (see --sort option).
6835 That is, this option specifies a primary sort key,
6836 and the --sort option specifies a secondary key.
6837 However, any use of @option{--sort=none}
6838 (@option{-U}) disables this option altogether.
6840 @item --hide=PATTERN
6841 @opindex --hide=@var{pattern}
6842 In directories, ignore files whose names match the shell pattern
6843 @var{pattern}, unless the @option{--all} (@option{-a}) or
6844 @option{--almost-all} (@option{-A}) is also given. This
6845 option acts like @option{--ignore=@var{pattern}} except that it has no
6846 effect if @option{--all} (@option{-a}) or @option{--almost-all}
6847 (@option{-A}) is also given.
6849 This option can be useful in shell aliases. For example, if
6850 @command{lx} is an alias for @samp{ls --hide='*~'} and @command{ly} is
6851 an alias for @samp{ls --ignore='*~'}, then the command @samp{lx -A}
6852 lists the file @file{README~} even though @samp{ly -A} would not.
6854 @item -I @var{pattern}
6855 @itemx --ignore=@var{pattern}
6857 @opindex --ignore=@var{pattern}
6858 In directories, ignore files whose names match the shell pattern
6859 (not regular expression) @var{pattern}. As
6860 in the shell, an initial @samp{.} in a file name does not match a
6861 wildcard at the start of @var{pattern}. Sometimes it is useful
6862 to give this option several times. For example,
6865 $ ls --ignore='.??*' --ignore='.[^.]' --ignore='#*'
6868 The first option ignores names of length 3 or more that start with @samp{.},
6869 the second ignores all two-character names that start with @samp{.}
6870 except @samp{..}, and the third ignores names that start with @samp{#}.
6873 @itemx --dereference
6875 @opindex --dereference
6876 @cindex symbolic links, dereferencing
6877 When showing file information for a symbolic link, show information
6878 for the file the link references rather than the link itself.
6879 However, even with this option, @command{ls} still prints the name
6880 of the link itself, not the name of the file that the link points to.
6885 @opindex --recursive
6886 @cindex recursive directory listing
6887 @cindex directory listing, recursive
6888 List the contents of all directories recursively.
6893 @node What information is listed
6894 @subsection What information is listed
6896 These options affect the information that @command{ls} displays. By
6897 default, only file names are shown.
6903 @cindex hurd, author, printing
6904 List each file's author when producing long format directory listings.
6905 In GNU/Hurd, file authors can differ from their owners, but in other
6906 operating systems the two are the same.
6912 @cindex dired Emacs mode support
6913 With the long listing (@option{-l}) format, print an additional line after
6917 //DIRED// @var{beg1} @var{end1} @var{beg2} @var{end2} @dots{}
6921 The @var{begn} and @var{endn} are unsigned integers that record the
6922 byte position of the beginning and end of each file name in the output.
6923 This makes it easy for Emacs to find the names, even when they contain
6924 unusual characters such as space or newline, without fancy searching.
6926 If directories are being listed recursively (@option{-R}), output a similar
6927 line with offsets for each subdirectory name:
6930 //SUBDIRED// @var{beg1} @var{end1} @dots{}
6933 Finally, output a line of the form:
6936 //DIRED-OPTIONS// --quoting-style=@var{word}
6940 where @var{word} is the quoting style (@pxref{Formatting the file names}).
6942 Here is an actual example:
6945 $ mkdir -p a/sub/deeper a/sub2
6947 $ touch a/sub/deeper/file
6948 $ ls -gloRF --dired a
6951 -rw-r--r-- 1 0 Jun 10 12:27 f1
6952 -rw-r--r-- 1 0 Jun 10 12:27 f2
6953 drwxr-xr-x 3 4096 Jun 10 12:27 sub/
6954 drwxr-xr-x 2 4096 Jun 10 12:27 sub2/
6958 drwxr-xr-x 2 4096 Jun 10 12:27 deeper/
6962 -rw-r--r-- 1 0 Jun 10 12:27 file
6966 //DIRED// 48 50 84 86 120 123 158 162 217 223 282 286
6967 //SUBDIRED// 2 3 167 172 228 240 290 296
6968 //DIRED-OPTIONS// --quoting-style=literal
6971 Note that the pairs of offsets on the @samp{//DIRED//} line above delimit
6972 these names: @file{f1}, @file{f2}, @file{sub}, @file{sub2}, @file{deeper},
6974 The offsets on the @samp{//SUBDIRED//} line delimit the following
6975 directory names: @file{a}, @file{a/sub}, @file{a/sub/deeper}, @file{a/sub2}.
6977 Here is an example of how to extract the fifth entry name, @samp{deeper},
6978 corresponding to the pair of offsets, 222 and 228:
6981 $ ls -gloRF --dired a > out
6982 $ dd bs=1 skip=222 count=6 < out 2>/dev/null; echo
6986 Note that although the listing above includes a trailing slash
6987 for the @samp{deeper} entry, the offsets select the name without
6988 the trailing slash. However, if you invoke @command{ls} with @option{--dired}
6989 along with an option like @option{--escape} (aka @option{-b}) and operate
6990 on a file whose name contains special characters, notice that the backslash
6995 $ ls -blog --dired 'a b'
6996 -rw-r--r-- 1 0 Jun 10 12:28 a\ b
6998 //DIRED-OPTIONS// --quoting-style=escape
7001 If you use a quoting style that adds quote marks
7002 (e.g., @option{--quoting-style=c}), then the offsets include the quote marks.
7003 So beware that the user may select the quoting style via the environment
7004 variable @env{QUOTING_STYLE}@. Hence, applications using @option{--dired}
7005 should either specify an explicit @option{--quoting-style=literal} option
7006 (aka @option{-N} or @option{--literal}) on the command line, or else be
7007 prepared to parse the escaped names.
7010 @opindex --full-time
7011 Produce long format directory listings, and list times in full. It is
7012 equivalent to using @option{--format=long} with
7013 @option{--time-style=full-iso} (@pxref{Formatting file timestamps}).
7017 Produce long format directory listings, but don't display owner information.
7023 Inhibit display of group information in a long format directory listing.
7024 (This is the default in some non-GNU versions of @command{ls}, so we
7025 provide this option for compatibility.)
7033 @cindex inode number, printing
7034 Print the inode number (also called the file serial number and index
7035 number) of each file to the left of the file name. (This number
7036 uniquely identifies each file within a particular file system.)
7039 @itemx --format=long
7040 @itemx --format=verbose
7043 @opindex long ls @r{format}
7044 @opindex verbose ls @r{format}
7045 In addition to the name of each file, print the file type, file mode bits,
7046 number of hard links, owner name, group name, size, and
7047 timestamp (@pxref{Formatting file timestamps}), normally
7048 the modification time. Print question marks for information that
7049 cannot be determined.
7051 Normally the size is printed as a byte count without punctuation, but
7052 this can be overridden (@pxref{Block size}). For example, @option{-h}
7053 prints an abbreviated, human-readable count, and
7054 @samp{--block-size="'1"} prints a byte count with the thousands
7055 separator of the current locale.
7057 For each directory that is listed, preface the files with a line
7058 @samp{total @var{blocks}}, where @var{blocks} is the total disk allocation
7059 for all files in that directory. The block size currently defaults to 1024
7060 bytes, but this can be overridden (@pxref{Block size}).
7061 The @var{blocks} computed counts each hard link separately;
7062 this is arguably a deficiency.
7064 The file type is one of the following characters:
7066 @c The commented-out entries are ones we're not sure about.
7074 character special file
7076 high performance (``contiguous data'') file
7080 door (Solaris 2.5 and up)
7082 @c semaphore, if this is a distinct file type
7086 @c multiplexed file (7th edition Unix; obsolete)
7088 off-line (``migrated'') file (Cray DMF)
7090 network special file (HP-UX)
7094 port (Solaris 10 and up)
7096 @c message queue, if this is a distinct file type
7100 @c shared memory object, if this is a distinct file type
7102 @c typed memory object, if this is a distinct file type
7104 @c whiteout (4.4BSD; not implemented)
7106 some other file type
7109 @cindex permissions, output by @command{ls}
7110 The file mode bits listed are similar to symbolic mode specifications
7111 (@pxref{Symbolic Modes}). But @command{ls} combines multiple bits into the
7112 third character of each set of permissions as follows:
7116 If the set-user-ID or set-group-ID bit and the corresponding executable bit
7120 If the set-user-ID or set-group-ID bit is set but the corresponding
7121 executable bit is not set.
7124 If the restricted deletion flag or sticky bit, and the
7125 other-executable bit, are both set. The restricted deletion flag is
7126 another name for the sticky bit. @xref{Mode Structure}.
7129 If the restricted deletion flag or sticky bit is set but the
7130 other-executable bit is not set.
7133 If the executable bit is set and none of the above apply.
7139 Following the file mode bits is a single character that specifies
7140 whether an alternate access method such as an access control list
7141 applies to the file. When the character following the file mode bits is a
7142 space, there is no alternate access method. When it is a printing
7143 character, then there is such a method.
7145 GNU @command{ls} uses a @samp{.} character to indicate a file
7146 with an SELinux security context, but no other alternate access method.
7148 A file with any other combination of alternate access methods
7149 is marked with a @samp{+} character.
7152 @itemx --numeric-uid-gid
7154 @opindex --numeric-uid-gid
7155 @cindex numeric uid and gid
7156 @cindex numeric user and group IDs
7157 Produce long format directory listings, but
7158 display numeric user and group IDs instead of the owner and group names.
7162 Produce long format directory listings, but don't display group information.
7163 It is equivalent to using @option{--format=long} with @option{--no-group} .
7169 @cindex disk allocation
7170 @cindex size of files, reporting
7171 Print the disk allocation of each file to the left of the file name.
7172 This is the amount of disk space used by the file, which is usually a
7173 bit more than the file's size, but it can be less if the file has holes.
7175 Normally the disk allocation is printed in units of
7176 1024 bytes, but this can be overridden (@pxref{Block size}).
7178 @cindex NFS mounts from BSD to HP-UX
7179 For files that are NFS-mounted from an HP-UX system to a BSD system,
7180 this option reports sizes that are half the correct values. On HP-UX
7181 systems, it reports sizes that are twice the correct values for files
7182 that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX;
7183 it also affects the HP-UX @command{ls} program.
7192 @cindex security context
7193 Display the SELinux security context or @samp{?} if none is found.
7194 When used with the @option{-l} option, print the security context
7195 to the left of the size column.
7200 @node Sorting the output
7201 @subsection Sorting the output
7203 @cindex sorting @command{ls} output
7204 These options change the order in which @command{ls} sorts the information
7205 it outputs. By default, sorting is done by character code
7206 (e.g., ASCII order).
7212 @itemx --time=status
7215 @opindex ctime@r{, printing or sorting by}
7216 @opindex status time@r{, printing or sorting by}
7217 @opindex use time@r{, printing or sorting files by}
7218 If the long listing format (e.g., @option{-l}, @option{-o}) is being used,
7219 print the status change time (the @samp{ctime} in the inode) instead of
7220 the modification time.
7221 When explicitly sorting by time (@option{--sort=time} or @option{-t})
7222 or when not using a long listing format,
7223 sort according to the status change time.
7227 @cindex unsorted directory listing
7228 @cindex directory order, listing by
7229 Primarily, like @option{-U}---do not sort; list the files in whatever
7230 order they are stored in the directory. But also enable @option{-a} (list
7231 all files) and disable @option{-l}, @option{--color}, and @option{-s} (if they
7232 were specified before the @option{-f}).
7238 @cindex reverse sorting
7239 Reverse whatever the sorting method is---e.g., list files in reverse
7240 alphabetical order, youngest first, smallest first, or whatever.
7246 @opindex size of files@r{, sorting files by}
7247 Sort by file size, largest first.
7253 @opindex modification time@r{, sorting files by}
7254 Sort by modification time (the @samp{mtime} in the inode), newest first.
7258 @itemx --time=access
7262 @opindex use time@r{, printing or sorting files by}
7263 @opindex atime@r{, printing or sorting files by}
7264 @opindex access time@r{, printing or sorting files by}
7265 If the long listing format (e.g., @option{--format=long}) is being used,
7266 print the last access time (the @samp{atime} in the inode).
7267 When explicitly sorting by time (@option{--sort=time} or @option{-t})
7268 or when not using a long listing format, sort according to the access time.
7274 @opindex none@r{, sorting option for @command{ls}}
7275 Do not sort; list the files in whatever order they are
7276 stored in the directory. (Do not do any of the other unrelated things
7277 that @option{-f} does.) This is especially useful when listing very large
7278 directories, since not doing any sorting can be noticeably faster.
7281 @itemx --sort=version
7284 @opindex version@r{, sorting option for @command{ls}}
7285 Sort by version name and number, lowest first. It behaves like a default
7286 sort, except that each sequence of decimal digits is treated numerically
7287 as an index/version number. (@xref{Details about version sort}.)
7290 @itemx --sort=extension
7293 @opindex extension@r{, sorting files by}
7294 Sort directory contents alphabetically by file extension (characters
7295 after the last @samp{.}); files with no extension are sorted first.
7300 @node Details about version sort
7301 @subsection Details about version sort
7303 Version sorting handles the fact that file names frequently include indices or
7304 version numbers. Standard sorting usually does not produce the order that one
7305 expects because comparisons are made on a character-by-character basis.
7306 Version sorting is especially useful when browsing directories that contain
7307 many files with indices/version numbers in their names:
7311 abc.zml-1.gz abc.zml-1.gz
7312 abc.zml-12.gz abc.zml-2.gz
7313 abc.zml-2.gz abc.zml-12.gz
7316 Version-sorted strings are compared such that if @var{ver1} and @var{ver2}
7317 are version numbers and @var{prefix} and @var{suffix} (@var{suffix} matching
7318 the regular expression @samp{(\.[A-Za-z~][A-Za-z0-9~]*)*}) are strings then
7319 @var{ver1} < @var{ver2} implies that the name composed of
7320 ``@var{prefix} @var{ver1} @var{suffix}'' sorts before
7321 ``@var{prefix} @var{ver2} @var{suffix}''.
7323 Note also that leading zeros of numeric parts are ignored:
7327 abc-1.007.tgz abc-1.01a.tgz
7328 abc-1.012b.tgz abc-1.007.tgz
7329 abc-1.01a.tgz abc-1.012b.tgz
7332 This functionality is implemented using gnulib's @code{filevercmp} function,
7333 which has some caveats worth noting.
7336 @item @env{LC_COLLATE} is ignored, which means @samp{ls -v} and @samp{sort -V}
7337 will sort non-numeric prefixes as if the @env{LC_COLLATE} locale category
7338 was set to @samp{C}@.
7339 @item Some suffixes will not be matched by the regular
7340 expression mentioned above. Consequently these examples may
7341 not sort as you expect:
7349 abc-1.2.3.4.x86_64.rpm
7350 abc-1.2.3.x86_64.rpm
7354 @node General output formatting
7355 @subsection General output formatting
7357 These options affect the appearance of the overall output.
7362 @itemx --format=single-column
7365 @opindex single-column @r{output of files}
7366 List one file per line. This is the default for @command{ls} when standard
7367 output is not a terminal.
7370 @itemx --format=vertical
7373 @opindex vertical @r{sorted files in columns}
7374 List files in columns, sorted vertically. This is the default for
7375 @command{ls} if standard output is a terminal. It is always the default
7376 for the @command{dir} program.
7377 GNU @command{ls} uses variable width columns to display as many files as
7378 possible in the fewest lines.
7380 @item --color [=@var{when}]
7382 @cindex color, distinguishing file types with
7383 Specify whether to use color for distinguishing file types. @var{when}
7384 may be omitted, or one of:
7387 @vindex none @r{color option}
7388 - Do not use color at all. This is the default.
7390 @vindex auto @r{color option}
7391 @cindex terminal, using color iff
7392 - Only use color if standard output is a terminal.
7394 @vindex always @r{color option}
7397 Specifying @option{--color} and no @var{when} is equivalent to
7398 @option{--color=always}.
7399 Piping a colorized listing through a pager like @command{more} or
7400 @command{less} usually produces unreadable results. However, using
7401 @code{more -f} does seem to work.
7404 @vindex SHELL @r{environment variable, and color}
7405 Note that using the @option{--color} option may incur a noticeable
7406 performance penalty when run in a directory with very many entries,
7407 because the default settings require that @command{ls} @code{stat} every
7408 single file it lists.
7409 However, if you would like most of the file-type coloring
7410 but can live without the other coloring options (e.g.,
7411 executable, orphan, sticky, other-writable, capability), use
7412 @command{dircolors} to set the @env{LS_COLORS} environment variable like this,
7414 eval $(dircolors -p | perl -pe \
7415 's/^((CAP|S[ET]|O[TR]|M|E)\w+).*/$1 00/' | dircolors -)
7417 and on a @code{dirent.d_type}-capable file system, @command{ls}
7418 will perform only one @code{stat} call per command line argument.
7422 @itemx --indicator-style=classify
7425 @opindex --indicator-style
7426 @cindex file type and executables, marking
7427 @cindex executables and file type, marking
7428 Append a character to each file name indicating the file type. Also,
7429 for regular files that are executable, append @samp{*}. The file type
7430 indicators are @samp{/} for directories, @samp{@@} for symbolic links,
7431 @samp{|} for FIFOs, @samp{=} for sockets, @samp{>} for doors,
7432 and nothing for regular files.
7433 @c The following sentence is the same as the one for -d.
7434 Do not follow symbolic links listed on the
7435 command line unless the @option{--dereference-command-line} (@option{-H}),
7436 @option{--dereference} (@option{-L}), or
7437 @option{--dereference-command-line-symlink-to-dir} options are specified.
7440 @itemx --indicator-style=file-type
7441 @opindex --file-type
7442 @opindex --indicator-style
7443 @cindex file type, marking
7444 Append a character to each file name indicating the file type. This is
7445 like @option{-F}, except that executables are not marked.
7447 @item --indicator-style=@var{word}
7448 @opindex --indicator-style
7449 Append a character indicator with style @var{word} to entry names,
7454 Do not append any character indicator; this is the default.
7456 Append @samp{/} for directories. This is the same as the @option{-p}
7459 Append @samp{/} for directories, @samp{@@} for symbolic links, @samp{|}
7460 for FIFOs, @samp{=} for sockets, and nothing for regular files. This is
7461 the same as the @option{--file-type} option.
7463 Append @samp{*} for executable regular files, otherwise behave as for
7464 @samp{file-type}. This is the same as the @option{-F} or
7465 @option{--classify} option.
7471 @opindex --kibibytes
7472 Set the default block size to its normal value of 1024 bytes,
7473 overriding any contrary specification in environment variables
7474 (@pxref{Block size}). This option is in turn overridden by the
7475 @option{--block-size}, @option{-h} or @option{--human-readable}, and
7476 @option{--si} options.
7478 The @option{-k} or @option{--kibibytes} option affects the
7479 per-directory block count written by the @option{-l} and similar
7480 options, and the size written by the @option{-s} or @option{--size}
7481 option. It does not affect the file size written by @option{-l}.
7484 @itemx --format=commas
7487 @opindex commas@r{, outputting between files}
7488 List files horizontally, with as many as will fit on each line,
7489 separated by @samp{, } (a comma and a space).
7492 @itemx --indicator-style=slash
7494 @opindex --indicator-style
7495 @cindex file type, marking
7496 Append a @samp{/} to directory names.
7499 @itemx --format=across
7500 @itemx --format=horizontal
7503 @opindex across@r{, listing files}
7504 @opindex horizontal@r{, listing files}
7505 List the files in columns, sorted horizontally.
7508 @itemx --tabsize=@var{cols}
7511 Assume that each tab stop is @var{cols} columns wide. The default is 8.
7512 @command{ls} uses tabs where possible in the output, for efficiency. If
7513 @var{cols} is zero, do not use tabs at all.
7515 @c FIXME: remove in 2009, if Apple Terminal has been fixed for long enough.
7516 Some terminal emulators (at least Apple Terminal 1.5 (133) from Mac OS X 10.4.8)
7517 do not properly align columns to the right of a TAB following a
7518 non-ASCII byte. If you use such a terminal emulator, use the
7519 @option{-T0} option or put @code{TABSIZE=0} in your environment to tell
7520 @command{ls} to align using spaces, not tabs.
7523 @itemx --width=@var{cols}
7527 Assume the screen is @var{cols} columns wide. The default is taken
7528 from the terminal settings if possible; otherwise the environment
7529 variable @env{COLUMNS} is used if it is set; otherwise the default
7535 @node Formatting file timestamps
7536 @subsection Formatting file timestamps
7538 By default, file timestamps are listed in abbreviated form, using
7539 a date like @samp{Mar 30@ @ 2002} for non-recent timestamps, and a
7540 date-without-year and time like @samp{Mar 30 23:45} for recent timestamps.
7541 This format can change depending on the current locale as detailed below.
7544 A timestamp is considered to be @dfn{recent} if it is less than six
7545 months old, and is not dated in the future. If a timestamp dated
7546 today is not listed in recent form, the timestamp is in the future,
7547 which means you probably have clock skew problems which may break
7548 programs like @command{make} that rely on file timestamps.
7551 Time stamps are listed according to the time zone rules specified by
7552 the @env{TZ} environment variable, or by the system default rules if
7553 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
7554 with @env{TZ}, libc, The GNU C Library Reference Manual}.
7556 The following option changes how file timestamps are printed.
7559 @item --time-style=@var{style}
7560 @opindex --time-style
7562 List timestamps in style @var{style}. The @var{style} should
7563 be one of the following:
7568 List timestamps using @var{format}, where @var{format} is interpreted
7569 like the format argument of @command{date} (@pxref{date invocation}).
7570 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
7571 @command{ls} to list timestamps like @samp{2002-03-30 23:45:56}. As
7572 with @command{date}, @var{format}'s interpretation is affected by the
7573 @env{LC_TIME} locale category.
7575 If @var{format} contains two format strings separated by a newline,
7576 the former is used for non-recent files and the latter for recent
7577 files; if you want output columns to line up, you may need to insert
7578 spaces in one of the two formats.
7581 List timestamps in full using ISO 8601 date, time, and time zone
7582 format with nanosecond precision, e.g., @samp{2002-03-30
7583 23:45:56.477817180 -0700}. This style is equivalent to
7584 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
7586 This is useful because the time output includes all the information that
7587 is available from the operating system. For example, this can help
7588 explain @command{make}'s behavior, since GNU @command{make}
7589 uses the full timestamp to determine whether a file is out of date.
7592 List ISO 8601 date and time in minutes, e.g.,
7593 @samp{2002-03-30 23:45}. These timestamps are shorter than
7594 @samp{full-iso} timestamps, and are usually good enough for everyday
7595 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
7598 List ISO 8601 dates for non-recent timestamps (e.g.,
7599 @samp{2002-03-30@ }), and ISO 8601 month, day, hour, and
7600 minute for recent timestamps (e.g., @samp{03-30 23:45}). These
7601 timestamps are uglier than @samp{long-iso} timestamps, but they carry
7602 nearly the same information in a smaller space and their brevity helps
7603 @command{ls} output fit within traditional 80-column output lines.
7604 The following two @command{ls} invocations are equivalent:
7609 ls -l --time-style="+%Y-%m-%d $newline%m-%d %H:%M"
7610 ls -l --time-style="iso"
7615 List timestamps in a locale-dependent form. For example, a Finnish
7616 locale might list non-recent timestamps like @samp{maalis 30@ @ 2002}
7617 and recent timestamps like @samp{maalis 30 23:45}. Locale-dependent
7618 timestamps typically consume more space than @samp{iso} timestamps and
7619 are harder for programs to parse because locale conventions vary so
7620 widely, but they are easier for many people to read.
7622 The @env{LC_TIME} locale category specifies the timestamp format. The
7623 default POSIX locale uses timestamps like @samp{Mar 30@
7624 @ 2002} and @samp{Mar 30 23:45}; in this locale, the following two
7625 @command{ls} invocations are equivalent:
7630 ls -l --time-style="+%b %e %Y$newline%b %e %H:%M"
7631 ls -l --time-style="locale"
7634 Other locales behave differently. For example, in a German locale,
7635 @option{--time-style="locale"} might be equivalent to
7636 @option{--time-style="+%e. %b %Y $newline%e. %b %H:%M"}
7637 and might generate timestamps like @samp{30. M@"ar 2002@ } and
7638 @samp{30. M@"ar 23:45}.
7640 @item posix-@var{style}
7642 List POSIX-locale timestamps if the @env{LC_TIME} locale
7643 category is POSIX, @var{style} timestamps otherwise. For
7644 example, the @samp{posix-long-iso} style lists
7645 timestamps like @samp{Mar 30@ @ 2002} and @samp{Mar 30 23:45} when in
7646 the POSIX locale, and like @samp{2002-03-30 23:45} otherwise.
7651 You can specify the default value of the @option{--time-style} option
7652 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
7653 the default style is @samp{locale}. GNU Emacs 21.3 and
7654 later use the @option{--dired} option and therefore can parse any date
7655 format, but if you are using Emacs 21.1 or 21.2 and specify a
7656 non-POSIX locale you may need to set
7657 @samp{TIME_STYLE="posix-long-iso"}.
7659 To avoid certain denial-of-service attacks, timestamps that would be
7660 longer than 1000 bytes may be treated as errors.
7663 @node Formatting the file names
7664 @subsection Formatting the file names
7666 These options change how file names themselves are printed.
7672 @itemx --quoting-style=escape
7675 @opindex --quoting-style
7676 @cindex backslash sequences for file names
7677 Quote nongraphic characters in file names using alphabetic and octal
7678 backslash sequences like those used in C.
7682 @itemx --quoting-style=literal
7685 @opindex --quoting-style
7686 Do not quote file names. However, with @command{ls} nongraphic
7687 characters are still printed as question marks if the output is a
7688 terminal and you do not specify the @option{--show-control-chars}
7692 @itemx --hide-control-chars
7694 @opindex --hide-control-chars
7695 Print question marks instead of nongraphic characters in file names.
7696 This is the default if the output is a terminal and the program is
7701 @itemx --quoting-style=c
7703 @opindex --quote-name
7704 @opindex --quoting-style
7705 Enclose file names in double quotes and quote nongraphic characters as
7708 @item --quoting-style=@var{word}
7709 @opindex --quoting-style
7710 @cindex quoting style
7711 Use style @var{word} to quote file names and other strings that may
7712 contain arbitrary characters. The @var{word} should
7713 be one of the following:
7717 Output strings as-is; this is the same as the @option{-N} or
7718 @option{--literal} option.
7720 Quote strings for the shell if they contain shell metacharacters or would
7721 cause ambiguous output.
7722 The quoting is suitable for POSIX-compatible shells like
7723 @command{bash}, but it does not always work for incompatible shells
7726 Quote strings for the shell, even if they would normally not require quoting.
7728 Quote strings as for C character string literals, including the
7729 surrounding double-quote characters; this is the same as the
7730 @option{-Q} or @option{--quote-name} option.
7732 Quote strings as for C character string literals, except omit the
7733 surrounding double-quote
7734 characters; this is the same as the @option{-b} or @option{--escape} option.
7736 Quote strings as for C character string literals, except use
7737 surrounding quotation marks appropriate for the
7740 @c Use @t instead of @samp to avoid duplicate quoting in some output styles.
7741 Quote strings as for C character string literals, except use
7742 surrounding quotation marks appropriate for the locale, and quote
7743 @t{'like this'} instead of @t{"like
7744 this"} in the default C locale. This looks nicer on many displays.
7747 You can specify the default value of the @option{--quoting-style} option
7748 with the environment variable @env{QUOTING_STYLE}@. If that environment
7749 variable is not set, the default value is @samp{literal}, but this
7750 default may change to @samp{shell} in a future version of this package.
7752 @item --show-control-chars
7753 @opindex --show-control-chars
7754 Print nongraphic characters as-is in file names.
7755 This is the default unless the output is a terminal and the program is
7761 @node dir invocation
7762 @section @command{dir}: Briefly list directory contents
7765 @cindex directory listing, brief
7767 @command{dir} is equivalent to @code{ls -C
7768 -b}; that is, by default files are listed in columns, sorted vertically,
7769 and special characters are represented by backslash escape sequences.
7771 @xref{ls invocation, @command{ls}}.
7774 @node vdir invocation
7775 @section @command{vdir}: Verbosely list directory contents
7778 @cindex directory listing, verbose
7780 @command{vdir} is equivalent to @code{ls -l
7781 -b}; that is, by default files are listed in long format and special
7782 characters are represented by backslash escape sequences.
7784 @node dircolors invocation
7785 @section @command{dircolors}: Color setup for @command{ls}
7789 @cindex setup for color
7791 @command{dircolors} outputs a sequence of shell commands to set up the
7792 terminal for color output from @command{ls} (and @command{dir}, etc.).
7796 eval "$(dircolors [@var{option}]@dots{} [@var{file}])"
7799 If @var{file} is specified, @command{dircolors} reads it to determine which
7800 colors to use for which file types and extensions. Otherwise, a
7801 precompiled database is used. For details on the format of these files,
7802 run @samp{dircolors --print-database}.
7804 To make @command{dircolors} read a @file{~/.dircolors} file if it
7805 exists, you can put the following lines in your @file{~/.bashrc} (or
7806 adapt them to your favorite shell):
7810 test -r $d && eval "$(dircolors $d)"
7814 @vindex SHELL @r{environment variable, and color}
7815 The output is a shell command to set the @env{LS_COLORS} environment
7816 variable. You can specify the shell syntax to use on the command line,
7817 or @command{dircolors} will guess it from the value of the @env{SHELL}
7818 environment variable.
7820 The program accepts the following options. Also see @ref{Common options}.
7825 @itemx --bourne-shell
7828 @opindex --bourne-shell
7829 @cindex Bourne shell syntax for color setup
7830 @cindex @command{sh} syntax for color setup
7831 Output Bourne shell commands. This is the default if the @env{SHELL}
7832 environment variable is set and does not end with @samp{csh} or
7841 @cindex C shell syntax for color setup
7842 @cindex @command{csh} syntax for color setup
7843 Output C shell commands. This is the default if @code{SHELL} ends with
7844 @command{csh} or @command{tcsh}.
7847 @itemx --print-database
7849 @opindex --print-database
7850 @cindex color database, printing
7851 @cindex database for color setup, printing
7852 @cindex printing color database
7853 Print the (compiled-in) default color configuration database. This
7854 output is itself a valid configuration file, and is fairly descriptive
7855 of the possibilities.
7862 @node Basic operations
7863 @chapter Basic operations
7865 @cindex manipulating files
7867 This chapter describes the commands for basic file manipulation:
7868 copying, moving (renaming), and deleting (removing).
7871 * cp invocation:: Copy files.
7872 * dd invocation:: Convert and copy a file.
7873 * install invocation:: Copy files and set attributes.
7874 * mv invocation:: Move (rename) files.
7875 * rm invocation:: Remove files or directories.
7876 * shred invocation:: Remove files more securely.
7881 @section @command{cp}: Copy files and directories
7884 @cindex copying files and directories
7885 @cindex files, copying
7886 @cindex directories, copying
7888 @command{cp} copies files (or, optionally, directories). The copy is
7889 completely independent of the original. You can either copy one file to
7890 another, or copy arbitrarily many files to a destination directory.
7894 cp [@var{option}]@dots{} [-T] @var{source} @var{dest}
7895 cp [@var{option}]@dots{} @var{source}@dots{} @var{directory}
7896 cp [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
7901 If two file names are given, @command{cp} copies the first file to the
7905 If the @option{--target-directory} (@option{-t}) option is given, or
7906 failing that if the last file is a directory and the
7907 @option{--no-target-directory} (@option{-T}) option is not given,
7908 @command{cp} copies each @var{source} file to the specified directory,
7909 using the @var{source}s' names.
7912 Generally, files are written just as they are read. For exceptions,
7913 see the @option{--sparse} option below.
7915 By default, @command{cp} does not copy directories. However, the
7916 @option{-R}, @option{-a}, and @option{-r} options cause @command{cp} to
7917 copy recursively by descending into source directories and copying files
7918 to corresponding destination directories.
7920 When copying from a symbolic link, @command{cp} normally follows the
7921 link only when not copying
7922 recursively. This default can be overridden with the
7923 @option{--archive} (@option{-a}), @option{-d}, @option{--dereference}
7924 (@option{-L}), @option{--no-dereference} (@option{-P}), and
7925 @option{-H} options. If more than one of these options is specified,
7926 the last one silently overrides the others.
7928 When copying to a symbolic link, @command{cp} follows the
7929 link only when it refers to an existing regular file.
7930 However, when copying to a dangling symbolic link, @command{cp}
7931 refuses by default, and fails with a diagnostic, since the operation
7932 is inherently dangerous. This behavior is contrary to historical
7933 practice and to POSIX@.
7934 Set @env{POSIXLY_CORRECT} to make @command{cp} attempt to create
7935 the target of a dangling destination symlink, in spite of the possible risk.
7936 Also, when an option like
7937 @option{--backup} or @option{--link} acts to rename or remove the
7938 destination before copying, @command{cp} renames or removes the
7939 symbolic link rather than the file it points to.
7941 By default, @command{cp} copies the contents of special files only
7942 when not copying recursively. This default can be overridden with the
7943 @option{--copy-contents} option.
7945 @cindex self-backups
7946 @cindex backups, making only
7947 @command{cp} generally refuses to copy a file onto itself, with the
7948 following exception: if @option{--force --backup} is specified with
7949 @var{source} and @var{dest} identical, and referring to a regular file,
7950 @command{cp} will make a backup file, either regular or numbered, as
7951 specified in the usual ways (@pxref{Backup options}). This is useful when
7952 you simply want to make a backup of an existing file before changing it.
7954 The program accepts the following options. Also see @ref{Common options}.
7961 Preserve as much as possible of the structure and attributes of the
7962 original files in the copy (but do not attempt to preserve internal
7963 directory structure; i.e., @samp{ls -U} may list the entries in a copied
7964 directory in a different order).
7965 Try to preserve SELinux security context and extended attributes (xattr),
7966 but ignore any failure to do that and print no corresponding diagnostic.
7967 Equivalent to @option{-dR --preserve=all} with the reduced diagnostics.
7969 @item --attributes-only
7970 @opindex --attributes-only
7971 Copy only the specified attributes of the source file to the destination.
7972 If the destination already exists, do not alter its contents.
7973 See the @option{--preserve} option for controlling which attributes to copy.
7976 @itemx @w{@kbd{--backup}[=@var{method}]}
7979 @vindex VERSION_CONTROL
7980 @cindex backups, making
7981 @xref{Backup options}.
7982 Make a backup of each file that would otherwise be overwritten or removed.
7983 As a special case, @command{cp} makes a backup of @var{source} when the force
7984 and backup options are given and @var{source} and @var{dest} are the same
7985 name for an existing, regular file. One useful application of this
7986 combination of options is this tiny Bourne shell script:
7990 # Usage: backup FILE...
7991 # Create a GNU-style backup of each listed FILE.
7994 cp --backup --force --preserve=all -- "$i" "$i" || fail=1
7999 @item --copy-contents
8000 @cindex directories, copying recursively
8001 @cindex copying directories recursively
8002 @cindex recursively copying directories
8003 @cindex non-directories, copying as special files
8004 If copying recursively, copy the contents of any special files (e.g.,
8005 FIFOs and device files) as if they were regular files. This means
8006 trying to read the data in each source file and writing it to the
8007 destination. It is usually a mistake to use this option, as it
8008 normally has undesirable effects on special files like FIFOs and the
8009 ones typically found in the @file{/dev} directory. In most cases,
8010 @code{cp -R --copy-contents} will hang indefinitely trying to read
8011 from FIFOs and special files like @file{/dev/console}, and it will
8012 fill up your destination disk if you use it to copy @file{/dev/zero}.
8013 This option has no effect unless copying recursively, and it does not
8014 affect the copying of symbolic links.
8018 @cindex symbolic links, copying
8019 @cindex hard links, preserving
8020 Copy symbolic links as symbolic links rather than copying the files that
8021 they point to, and preserve hard links between source files in the copies.
8022 Equivalent to @option{--no-dereference --preserve=links}.
8028 When copying without this option and an existing destination file cannot
8029 be opened for writing, the copy fails. However, with @option{--force},
8030 when a destination file cannot be opened, @command{cp} then removes it and
8031 tries to open it again. Contrast this behavior with that enabled by
8032 @option{--link} and @option{--symbolic-link}, whereby the destination file
8033 is never opened but rather is removed unconditionally. Also see the
8034 description of @option{--remove-destination}.
8036 This option is independent of the @option{--interactive} or
8037 @option{-i} option: neither cancels the effect of the other.
8039 This option is ignored when the @option{--no-clobber} or @option{-n} option
8044 If a command line argument specifies a symbolic link, then copy the
8045 file it points to rather than the symbolic link itself. However,
8046 copy (preserving its nature) any symbolic link that is encountered
8047 via recursive traversal.
8050 @itemx --interactive
8052 @opindex --interactive
8053 When copying a file other than a directory, prompt whether to
8054 overwrite an existing destination file. The @option{-i} option overrides
8055 a previous @option{-n} option.
8061 Make hard links instead of copies of non-directories.
8064 @itemx --dereference
8066 @opindex --dereference
8067 Follow symbolic links when copying from them.
8068 With this option, @command{cp} cannot create a symbolic link.
8069 For example, a symlink (to regular file) in the source tree will be copied to
8070 a regular file in the destination tree.
8075 @opindex --no-clobber
8076 Do not overwrite an existing file. The @option{-n} option overrides a previous
8077 @option{-i} option. This option is mutually exclusive with @option{-b} or
8078 @option{--backup} option.
8081 @itemx --no-dereference
8083 @opindex --no-dereference
8084 @cindex symbolic links, copying
8085 Copy symbolic links as symbolic links rather than copying the files that
8086 they point to. This option affects only symbolic links in the source;
8087 symbolic links in the destination are always followed if possible.
8090 @itemx @w{@kbd{--preserve}[=@var{attribute_list}]}
8093 @cindex file information, preserving, extended attributes, xattr
8094 Preserve the specified attributes of the original files.
8095 If specified, the @var{attribute_list} must be a comma-separated list
8096 of one or more of the following strings:
8100 Preserve the file mode bits and access control lists.
8102 Preserve the owner and group. On most modern systems,
8103 only users with appropriate privileges may change the owner of a file,
8105 may preserve the group ownership of a file only if they happen to be
8106 a member of the desired group.
8108 Preserve the times of last access and last modification, when possible.
8109 On older systems, it is not possible to preserve these attributes
8110 when the affected file is a symbolic link.
8111 However, many systems now provide the @code{utimensat} function,
8112 which makes it possible even for symbolic links.
8114 Preserve in the destination files
8115 any links between corresponding source files.
8116 Note that with @option{-L} or @option{-H}, this option can convert
8117 symbolic links to hard links. For example,
8119 $ mkdir c; : > a; ln -s a b; cp -aH a b c; ls -i1 c
8124 Note the inputs: @file{b} is a symlink to regular file @file{a},
8125 yet the files in destination directory, @file{c/}, are hard-linked.
8126 Since @option{-a} implies @option{--preserve=links}, and since @option{-H}
8127 tells @command{cp} to dereference command line arguments, it sees two files
8128 with the same inode number, and preserves the perceived hard link.
8130 Here is a similar example that exercises @command{cp}'s @option{-L} option:
8132 $ mkdir b c; (cd b; : > a; ln -s a b); cp -aL b c; ls -i1 c/b
8138 Preserve SELinux security context of the file, or fail with full diagnostics.
8140 Preserve extended attributes of the file, or fail with full diagnostics.
8141 If @command{cp} is built without xattr support, ignore this option.
8142 If SELinux context, ACLs or Capabilities are implemented using xattrs,
8143 they are preserved implicitly by this option as well, i.e., even without
8144 specifying @option{--preserve=mode} or @option{--preserve=context}.
8146 Preserve all file attributes.
8147 Equivalent to specifying all of the above, but with the difference
8148 that failure to preserve SELinux security context or extended attributes
8149 does not change @command{cp}'s exit status. In contrast to @option{-a},
8150 all but @samp{Operation not supported} warnings are output.
8153 Using @option{--preserve} with no @var{attribute_list} is equivalent
8154 to @option{--preserve=mode,ownership,timestamps}.
8156 In the absence of this option, each destination file is created with the
8157 mode bits of the corresponding source file, minus the bits set in the
8158 umask and minus the set-user-ID and set-group-ID bits.
8159 @xref{File permissions}.
8161 @item @w{@kbd{--no-preserve}=@var{attribute_list}}
8162 @cindex file information, preserving
8163 Do not preserve the specified attributes. The @var{attribute_list}
8164 has the same form as for @option{--preserve}.
8168 @cindex parent directories and @command{cp}
8169 Form the name of each destination file by appending to the target
8170 directory a slash and the specified name of the source file. The last
8171 argument given to @command{cp} must be the name of an existing directory.
8172 For example, the command:
8175 cp --parents a/b/c existing_dir
8179 copies the file @file{a/b/c} to @file{existing_dir/a/b/c}, creating
8180 any missing intermediate directories.
8187 @opindex --recursive
8188 @cindex directories, copying recursively
8189 @cindex copying directories recursively
8190 @cindex recursively copying directories
8191 @cindex non-directories, copying as special files
8192 Copy directories recursively. By default, do not follow symbolic
8193 links in the source; see the @option{--archive} (@option{-a}), @option{-d},
8194 @option{--dereference} (@option{-L}), @option{--no-dereference}
8195 (@option{-P}), and @option{-H} options. Special files are copied by
8196 creating a destination file of the same type as the source; see the
8197 @option{--copy-contents} option. It is not portable to use
8198 @option{-r} to copy symbolic links or special files. On some
8199 non-GNU systems, @option{-r} implies the equivalent of
8200 @option{-L} and @option{--copy-contents} for historical reasons.
8201 Also, it is not portable to use @option{-R} to copy symbolic links
8202 unless you also specify @option{-P}, as POSIX allows
8203 implementations that dereference symbolic links by default.
8205 @item --reflink[=@var{when}]
8206 @opindex --reflink[=@var{when}]
8209 @cindex copy on write
8210 Perform a lightweight, copy-on-write (COW) copy, if supported by the
8211 file system. Once it has succeeded, beware that the source and destination
8212 files share the same disk data blocks as long as they remain unmodified.
8213 Thus, if a disk I/O error affects data blocks of one of the files,
8214 the other suffers the same fate.
8216 The @var{when} value can be one of the following:
8220 The default behavior: if the copy-on-write operation is not supported
8221 then report the failure for each file and exit with a failure status.
8224 If the copy-on-write operation is not supported then fall back
8225 to the standard copy behaviour.
8228 This option is overridden by the @option{--link}, @option{--symbolic-link}
8229 and @option{--attributes-only} options, thus allowing it to be used
8230 to configure the default data copying behavior for @command{cp}.
8231 For example, with the following alias, @command{cp} will use the
8232 minimum amount of space supported by the file system.
8235 alias cp='cp --reflink=auto --sparse=always'
8238 @item --remove-destination
8239 @opindex --remove-destination
8240 Remove each existing destination file before attempting to open it
8241 (contrast with @option{-f} above).
8243 @item --sparse=@var{when}
8244 @opindex --sparse=@var{when}
8245 @cindex sparse files, copying
8246 @cindex holes, copying files with
8247 @findex read @r{system call, and holes}
8248 A @dfn{sparse file} contains @dfn{holes}---a sequence of zero bytes that
8249 does not occupy any physical disk blocks; the @samp{read} system call
8250 reads these as zeros. This can both save considerable disk space and
8251 increase speed, since many binary files contain lots of consecutive zero
8252 bytes. By default, @command{cp} detects holes in input source files via a crude
8253 heuristic and makes the corresponding output file sparse as well.
8254 Only regular files may be sparse.
8256 The @var{when} value can be one of the following:
8260 The default behavior: if the input file is sparse, attempt to make
8261 the output file sparse, too. However, if an output file exists but
8262 refers to a non-regular file, then do not attempt to make it sparse.
8265 For each sufficiently long sequence of zero bytes in the input file,
8266 attempt to create a corresponding hole in the output file, even if the
8267 input file does not appear to be sparse.
8268 This is useful when the input file resides on a file system
8269 that does not support sparse files
8270 (for example, @samp{efs} file systems in SGI IRIX 5.3 and earlier),
8271 but the output file is on a type of file system that does support them.
8272 Holes may be created only in regular files, so if the destination file
8273 is of some other type, @command{cp} does not even try to make it sparse.
8276 Never make the output file sparse.
8277 This is useful in creating a file for use with the @command{mkswap} command,
8278 since such a file must not have any holes.
8281 @optStripTrailingSlashes
8284 @itemx --symbolic-link
8286 @opindex --symbolic-link
8287 @cindex symbolic links, copying with
8288 Make symbolic links instead of copies of non-directories. All source
8289 file names must be absolute (starting with @samp{/}) unless the
8290 destination files are in the current directory. This option merely
8291 results in an error message on systems that do not support symbolic links.
8297 @optNoTargetDirectory
8303 @cindex newer files, copying only
8304 Do not copy a non-directory that has an existing destination with the
8305 same or newer modification time. If time stamps are being preserved,
8306 the comparison is to the source time stamp truncated to the
8307 resolutions of the destination file system and of the system calls
8308 used to update time stamps; this avoids duplicate work if several
8309 @samp{cp -pu} commands are executed with the same source and destination.
8310 If @option{--preserve=links} is also specified (like with @samp{cp -au}
8311 for example), that will take precedence. Consequently, depending on the
8312 order that files are processed from the source, newer files in the destination
8313 may be replaced, to mirror hard links in the source.
8319 Print the name of each file before copying it.
8322 @itemx --one-file-system
8324 @opindex --one-file-system
8325 @cindex file systems, omitting copying to different
8326 Skip subdirectories that are on different file systems from the one that
8327 the copy started on.
8328 However, mount point directories @emph{are} copied.
8336 @section @command{dd}: Convert and copy a file
8339 @cindex converting while copying a file
8341 @command{dd} copies a file (from standard input to standard output, by
8342 default) with a changeable I/O block size, while optionally performing
8343 conversions on it. Synopses:
8346 dd [@var{operand}]@dots{}
8350 The only options are @option{--help} and @option{--version}.
8351 @xref{Common options}. @command{dd} accepts the following operands,
8352 whose syntax was inspired by the DD (data definition) statement of
8359 Read from @var{file} instead of standard input.
8363 Write to @var{file} instead of standard output. Unless
8364 @samp{conv=notrunc} is given, @command{dd} truncates @var{file} to zero
8365 bytes (or the size specified with @samp{seek=}).
8367 @item ibs=@var{bytes}
8369 @cindex block size of input
8370 @cindex input block size
8371 Set the input block size to @var{bytes}.
8372 This makes @command{dd} read @var{bytes} per block.
8373 The default is 512 bytes.
8375 @item obs=@var{bytes}
8377 @cindex block size of output
8378 @cindex output block size
8379 Set the output block size to @var{bytes}.
8380 This makes @command{dd} write @var{bytes} per block.
8381 The default is 512 bytes.
8383 @item bs=@var{bytes}
8386 Set both input and output block sizes to @var{bytes}.
8387 This makes @command{dd} read and write @var{bytes} per block,
8388 overriding any @samp{ibs} and @samp{obs} settings.
8389 In addition, if no data-transforming @option{conv} option is specified,
8390 input is copied to the output as soon as it's read,
8391 even if it is smaller than the block size.
8393 @item cbs=@var{bytes}
8395 @cindex block size of conversion
8396 @cindex conversion block size
8397 @cindex fixed-length records, converting to variable-length
8398 @cindex variable-length records, converting to fixed-length
8399 Set the conversion block size to @var{bytes}.
8400 When converting variable-length records to fixed-length ones
8401 (@option{conv=block}) or the reverse (@option{conv=unblock}),
8402 use @var{bytes} as the fixed record length.
8406 Skip @var{n} @samp{ibs}-byte blocks in the input file before copying.
8407 If @samp{iflag=skip_bytes} is specified, @var{n} is interpreted
8408 as a byte count rather than a block count.
8412 Skip @var{n} @samp{obs}-byte blocks in the output file before copying.
8413 if @samp{oflag=seek_bytes} is specified, @var{n} is interpreted
8414 as a byte count rather than a block count.
8418 Copy @var{n} @samp{ibs}-byte blocks from the input file, instead
8419 of everything until the end of the file.
8420 if @samp{iflag=count_bytes} is specified, @var{n} is interpreted
8421 as a byte count rather than a block count.
8422 Note if the input may return short reads as could be the case
8423 when reading from a pipe for example, @samp{iflag=fullblock}
8424 will ensure that @samp{count=} corresponds to complete input blocks
8425 rather than the traditional POSIX specified behavior of counting
8426 input read operations.
8428 @item status=@var{which}
8430 Transfer information is normally output to stderr upon
8431 receipt of the @samp{INFO} signal or when @command{dd} exits.
8432 Specifying @var{which} will identify which information to suppress.
8437 @opindex noxfer @r{dd status=}
8438 Do not print the transfer rate and volume statistics
8439 that normally make up the last status line.
8442 @opindex none @r{dd status=}
8443 Do not print any informational messages to stderr.
8444 Error messages are output as normal.
8448 @item conv=@var{conversion}[,@var{conversion}]@dots{}
8450 Convert the file as specified by the @var{conversion} argument(s).
8451 (No spaces around any comma(s).)
8458 @opindex ascii@r{, converting to}
8459 Convert EBCDIC to ASCII,
8460 using the conversion table specified by POSIX@.
8461 This provides a 1:1 translation for all 256 bytes.
8464 @opindex ebcdic@r{, converting to}
8465 Convert ASCII to EBCDIC@.
8466 This is the inverse of the @samp{ascii} conversion.
8469 @opindex alternate ebcdic@r{, converting to}
8470 Convert ASCII to alternate EBCDIC,
8471 using the alternate conversion table specified by POSIX@.
8472 This is not a 1:1 translation, but reflects common historical practice
8473 for @samp{~}, @samp{[}, and @samp{]}.
8475 The @samp{ascii}, @samp{ebcdic}, and @samp{ibm} conversions are
8479 @opindex block @r{(space-padding)}
8480 For each line in the input, output @samp{cbs} bytes, replacing the
8481 input newline with a space and padding with spaces as necessary.
8485 Remove any trailing spaces in each @samp{cbs}-sized input block,
8486 and append a newline.
8488 The @samp{block} and @samp{unblock} conversions are mutually exclusive.
8491 @opindex lcase@r{, converting to}
8492 Change uppercase letters to lowercase.
8495 @opindex ucase@r{, converting to}
8496 Change lowercase letters to uppercase.
8498 The @samp{lcase} and @samp{ucase} conversions are mutually exclusive.
8502 Try to seek rather than write NUL output blocks.
8503 On a file system that supports sparse files, this will create
8504 sparse output when extending the output file.
8505 Be careful when using this option in conjunction with
8506 @samp{conv=notrunc} or @samp{oflag=append}.
8507 With @samp{conv=notrunc}, existing data in the output file
8508 corresponding to NUL blocks from the input, will be untouched.
8509 With @samp{oflag=append} the seeks performed will be ineffective.
8510 Similarly, when the output is a device rather than a file,
8511 NUL input blocks are not copied, and therefore this option
8512 is most useful with virtual or pre zeroed devices.
8515 @opindex swab @r{(byte-swapping)}
8516 @cindex byte-swapping
8517 Swap every pair of input bytes. GNU @command{dd}, unlike others, works
8518 when an odd number of bytes are read---the last byte is simply copied
8519 (since there is nothing to swap it with).
8522 @opindex sync @r{(padding with ASCII NULs)}
8523 Pad every input block to size of @samp{ibs} with trailing zero bytes.
8524 When used with @samp{block} or @samp{unblock}, pad with spaces instead of
8529 The following ``conversions'' are really file flags
8530 and don't affect internal processing:
8535 @cindex creating output file, requiring
8536 Fail if the output file already exists; @command{dd} must create the
8541 @cindex creating output file, avoiding
8542 Do not create the output file; the output file must already exist.
8544 The @samp{excl} and @samp{nocreat} conversions are mutually exclusive.
8548 @cindex truncating output file, avoiding
8549 Do not truncate the output file.
8553 @cindex read errors, ignoring
8554 Continue after read errors.
8558 @cindex synchronized data writes, before finishing
8559 Synchronize output data just before finishing. This forces a physical
8560 write of output data.
8564 @cindex synchronized data and metadata writes, before finishing
8565 Synchronize output data and metadata just before finishing. This
8566 forces a physical write of output data and metadata.
8570 @item iflag=@var{flag}[,@var{flag}]@dots{}
8572 Access the input file using the flags specified by the @var{flag}
8573 argument(s). (No spaces around any comma(s).)
8575 @item oflag=@var{flag}[,@var{flag}]@dots{}
8577 Access the output file using the flags specified by the @var{flag}
8578 argument(s). (No spaces around any comma(s).)
8580 Here are the flags. Not every flag is supported on every operating
8587 @cindex appending to the output file
8588 Write in append mode, so that even if some other process is writing to
8589 this file, every @command{dd} write will append to the current
8590 contents of the file. This flag makes sense only for output.
8591 If you combine this flag with the @samp{of=@var{file}} operand,
8592 you should also specify @samp{conv=notrunc} unless you want the
8593 output file to be truncated before being appended to.
8597 @cindex concurrent I/O
8598 Use concurrent I/O mode for data. This mode performs direct I/O
8599 and drops the POSIX requirement to serialize all I/O to the same file.
8600 A file cannot be opened in CIO mode and with a standard open at the
8606 Use direct I/O for data, avoiding the buffer cache.
8607 Note that the kernel may impose restrictions on read or write buffer sizes.
8608 For example, with an ext4 destination file system and a linux-based kernel,
8609 using @samp{oflag=direct} will cause writes to fail with @code{EINVAL} if the
8610 output buffer size is not a multiple of 512.
8614 @cindex directory I/O
8616 Fail unless the file is a directory. Most operating systems do not
8617 allow I/O to a directory, so this flag has limited utility.
8621 @cindex synchronized data reads
8622 Use synchronized I/O for data. For the output file, this forces a
8623 physical write of output data on each write. For the input file,
8624 this flag can matter when reading from a remote file that has been
8625 written to synchronously by some other process. Metadata (e.g.,
8626 last-access and last-modified time) is not necessarily synchronized.
8630 @cindex synchronized data and metadata I/O
8631 Use synchronized I/O for both data and metadata.
8635 @cindex discarding file cache
8636 Discard the data cache for a file.
8637 When count=0 all cache is discarded,
8638 otherwise the cache is dropped for the processed
8639 portion of the file. Also when count=0
8640 failure to discard the cache is diagnosed
8641 and reflected in the exit status.
8642 Here as some usage examples:
8645 # Advise to drop cache for whole file
8646 dd if=ifile iflag=nocache count=0
8648 # Ensure drop cache for the whole file
8649 dd of=ofile oflag=nocache conv=notrunc,fdatasync count=0
8651 # Drop cache for part of file
8652 dd if=ifile iflag=nocache skip=10 count=10 of=/dev/null
8654 # Stream data using just the read-ahead cache
8655 dd if=ifile of=ofile iflag=nocache oflag=nocache
8660 @cindex nonblocking I/O
8661 Use non-blocking I/O.
8666 Do not update the file's access time.
8667 Some older file systems silently ignore this flag, so it is a good
8668 idea to test it on your files before relying on it.
8672 @cindex controlling terminal
8673 Do not assign the file to be a controlling terminal for @command{dd}.
8674 This has no effect when the file is not a terminal.
8675 On many hosts (e.g., GNU/Linux hosts), this option has no effect
8680 @cindex symbolic links, following
8681 Do not follow symbolic links.
8686 Fail if the file has multiple hard links.
8691 Use binary I/O@. This option has an effect only on nonstandard
8692 platforms that distinguish binary from text I/O.
8697 Use text I/O@. Like @samp{binary}, this option has no effect on
8702 Accumulate full blocks from input. The @code{read} system call
8703 may return early if a full block is not available.
8704 When that happens, continue calling @code{read} to fill the remainder
8706 This flag can be used only with @code{iflag}.
8707 This flag is useful with pipes for example
8708 as they may return short reads. In that case,
8709 this flag is needed to ensure that a @samp{count=} argument is
8710 interpreted as a block count rather than a count of read operations.
8713 @opindex count_bytes
8714 Interpret the @samp{count=} operand as a byte count,
8715 rather than a block count, which allows specifying
8716 a length that is not a multiple of the I/O block size.
8717 This flag can be used only with @code{iflag}.
8721 Interpret the @samp{skip=} operand as a byte count,
8722 rather than a block count, which allows specifying
8723 an offset that is not a multiple of the I/O block size.
8724 This flag can be used only with @code{iflag}.
8728 Interpret the @samp{seek=} operand as a byte count,
8729 rather than a block count, which allows specifying
8730 an offset that is not a multiple of the I/O block size.
8731 This flag can be used only with @code{oflag}.
8735 These flags are not supported on all systems, and @samp{dd} rejects
8736 attempts to use them when they are not supported. When reading from
8737 standard input or writing to standard output, the @samp{nofollow} and
8738 @samp{noctty} flags should not be specified, and the other flags
8739 (e.g., @samp{nonblock}) can affect how other processes behave with the
8740 affected file descriptors, even after @command{dd} exits.
8744 @cindex multipliers after numbers
8745 The numeric-valued strings above (@var{n} and @var{bytes})
8746 can be followed by a multiplier: @samp{b}=512, @samp{c}=1,
8747 @samp{w}=2, @samp{x@var{m}}=@var{m}, or any of the
8748 standard block size suffixes like @samp{k}=1024 (@pxref{Block size}).
8750 Any block size you specify via @samp{bs=}, @samp{ibs=}, @samp{obs=}, @samp{cbs=}
8751 should not be too large---values larger than a few megabytes
8752 are generally wasteful or (as in the gigabyte..exabyte case) downright
8753 counterproductive or error-inducing.
8755 To process data that is at an offset or size that is not a
8756 multiple of the I/O@ block size, you can use the @samp{skip_bytes},
8757 @samp{seek_bytes} and @samp{count_bytes} flags. Alternatively
8758 the traditional method of separate @command{dd} invocations can be used.
8759 For example, the following shell commands copy data
8760 in 512 KiB blocks between a disk and a tape, but do not save
8761 or restore a 4 KiB label at the start of the disk:
8764 disk=/dev/rdsk/c0t1d0s2
8767 # Copy all but the label from disk to tape.
8768 (dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape
8770 # Copy from tape back to disk, but leave the disk label alone.
8771 (dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk
8774 Sending an @samp{INFO} signal to a running @command{dd}
8775 process makes it print I/O statistics to standard error
8776 and then resume copying. In the example below,
8777 @command{dd} is run in the background to copy 10 million blocks.
8778 The @command{kill} command makes it output intermediate I/O statistics,
8779 and when @command{dd} completes normally or is killed by the
8780 @code{SIGINT} signal, it outputs the final statistics.
8783 $ dd if=/dev/zero of=/dev/null count=10MB & pid=$!
8784 $ kill -s INFO $pid; wait $pid
8785 3385223+0 records in
8786 3385223+0 records out
8787 1733234176 bytes (1.7 GB) copied, 6.42173 seconds, 270 MB/s
8788 10000000+0 records in
8789 10000000+0 records out
8790 5120000000 bytes (5.1 GB) copied, 18.913 seconds, 271 MB/s
8793 @vindex POSIXLY_CORRECT
8794 On systems lacking the @samp{INFO} signal @command{dd} responds to the
8795 @samp{USR1} signal instead, unless the @env{POSIXLY_CORRECT}
8796 environment variable is set.
8801 @node install invocation
8802 @section @command{install}: Copy files and set attributes
8805 @cindex copying files and setting attributes
8807 @command{install} copies files while setting their file mode bits and, if
8808 possible, their owner and group. Synopses:
8811 install [@var{option}]@dots{} [-T] @var{source} @var{dest}
8812 install [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8813 install [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8814 install [@var{option}]@dots{} -d @var{directory}@dots{}
8819 If two file names are given, @command{install} copies the first file to the
8823 If the @option{--target-directory} (@option{-t}) option is given, or
8824 failing that if the last file is a directory and the
8825 @option{--no-target-directory} (@option{-T}) option is not given,
8826 @command{install} copies each @var{source} file to the specified
8827 directory, using the @var{source}s' names.
8830 If the @option{--directory} (@option{-d}) option is given,
8831 @command{install} creates each @var{directory} and any missing parent
8832 directories. Parent directories are created with mode
8833 @samp{u=rwx,go=rx} (755), regardless of the @option{-m} option or the
8834 current umask. @xref{Directory Setuid and Setgid}, for how the
8835 set-user-ID and set-group-ID bits of parent directories are inherited.
8838 @cindex Makefiles, installing programs in
8839 @command{install} is similar to @command{cp}, but allows you to control the
8840 attributes of destination files. It is typically used in Makefiles to
8841 copy programs into their destination directories. It refuses to copy
8842 files onto themselves.
8844 @cindex extended attributes, xattr
8845 @command{install} never preserves extended attributes (xattr).
8847 The program accepts the following options. Also see @ref{Common options}.
8857 Compare each pair of source and destination files, and if the destination has
8858 identical content and any specified owner, group, permissions, and possibly
8859 SELinux context, then do not modify the destination at all.
8863 Ignored; for compatibility with old Unix versions of @command{install}.
8867 Create any missing parent directories of @var{dest},
8868 then copy @var{source} to @var{dest}.
8869 This option is ignored if a destination directory is specified
8870 via @option{--target-directory=DIR}.
8875 @opindex --directory
8876 @cindex directories, creating with given attributes
8877 @cindex parent directories, creating missing
8878 @cindex leading directories, creating missing
8879 Create any missing parent directories, giving them the default
8880 attributes. Then create each given directory, setting their owner,
8881 group and mode as given on the command line or to the defaults.
8883 @item -g @var{group}
8884 @itemx --group=@var{group}
8887 @cindex group ownership of installed files, setting
8888 Set the group ownership of installed files or directories to
8889 @var{group}. The default is the process's current group. @var{group}
8890 may be either a group name or a numeric group ID.
8893 @itemx --mode=@var{mode}
8896 @cindex permissions of installed files, setting
8897 Set the file mode bits for the installed file or directory to @var{mode},
8898 which can be either an octal number, or a symbolic mode as in
8899 @command{chmod}, with @samp{a=} (no access allowed to anyone) as the
8900 point of departure (@pxref{File permissions}).
8901 The default mode is @samp{u=rwx,go=rx,a-s}---read, write, and
8902 execute for the owner, read and execute for group and other, and with
8903 set-user-ID and set-group-ID disabled.
8904 This default is not quite the same as @samp{755}, since it disables
8905 instead of preserving set-user-ID and set-group-ID on directories.
8906 @xref{Directory Setuid and Setgid}.
8908 @item -o @var{owner}
8909 @itemx --owner=@var{owner}
8912 @cindex ownership of installed files, setting
8913 @cindex appropriate privileges
8914 @vindex root @r{as default owner}
8915 If @command{install} has appropriate privileges (is run as root), set the
8916 ownership of installed files or directories to @var{owner}. The default
8917 is @code{root}. @var{owner} may be either a user name or a numeric user
8920 @item --preserve-context
8921 @opindex --preserve-context
8923 @cindex security context
8924 Preserve the SELinux security context of files and directories.
8925 Failure to preserve the context in all of the files or directories
8926 will result in an exit status of 1. If SELinux is disabled then
8927 print a warning and ignore the option.
8930 @itemx --preserve-timestamps
8932 @opindex --preserve-timestamps
8933 @cindex timestamps of installed files, preserving
8934 Set the time of last access and the time of last modification of each
8935 installed file to match those of each corresponding original file.
8936 When a file is installed without this option, its last access and
8937 last modification times are both set to the time of installation.
8938 This option is useful if you want to use the last modification times
8939 of installed files to keep track of when they were last built as opposed
8940 to when they were last installed.
8946 @cindex symbol table information, stripping
8947 @cindex stripping symbol table information
8948 Strip the symbol tables from installed binary executables.
8950 @item --strip-program=@var{program}
8951 @opindex --strip-program
8952 @cindex symbol table information, stripping, program
8953 Program used to strip binaries.
8959 @optNoTargetDirectory
8965 Print the name of each file before copying it.
8967 @item -Z @var{context}
8968 @itemx --context=@var{context}
8972 @cindex security context
8973 Set the default SELinux security context to be used for any
8974 created files and directories. If SELinux is disabled then
8975 print a warning and ignore the option.
8983 @section @command{mv}: Move (rename) files
8987 @command{mv} moves or renames files (or directories). Synopses:
8990 mv [@var{option}]@dots{} [-T] @var{source} @var{dest}
8991 mv [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8992 mv [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8997 If two file names are given, @command{mv} moves the first file to the
9001 If the @option{--target-directory} (@option{-t}) option is given, or
9002 failing that if the last file is a directory and the
9003 @option{--no-target-directory} (@option{-T}) option is not given,
9004 @command{mv} moves each @var{source} file to the specified
9005 directory, using the @var{source}s' names.
9008 @command{mv} can move any type of file from one file system to another.
9009 Prior to version @code{4.0} of the fileutils,
9010 @command{mv} could move only regular files between file systems.
9011 For example, now @command{mv} can move an entire directory hierarchy
9012 including special device files from one partition to another. It first
9013 uses some of the same code that's used by @code{cp -a} to copy the
9014 requested directories and files, then (assuming the copy succeeded)
9015 it removes the originals. If the copy fails, then the part that was
9016 copied to the destination partition is removed. If you were to copy
9017 three directories from one partition to another and the copy of the first
9018 directory succeeded, but the second didn't, the first would be left on
9019 the destination partition and the second and third would be left on the
9022 @cindex extended attributes, xattr
9023 @command{mv} always tries to copy extended attributes (xattr), which may
9024 include SELinux context, ACLs or Capabilities.
9025 Upon failure all but @samp{Operation not supported} warnings are output.
9027 @cindex prompting, and @command{mv}
9028 If a destination file exists but is normally unwritable, standard input
9029 is a terminal, and the @option{-f} or @option{--force} option is not given,
9030 @command{mv} prompts the user for whether to replace the file. (You might
9031 own the file, or have write permission on its directory.) If the
9032 response is not affirmative, the file is skipped.
9034 @emph{Warning}: Avoid specifying a source name with a trailing slash,
9035 when it might be a symlink to a directory.
9036 Otherwise, @command{mv} may do something very surprising, since
9037 its behavior depends on the underlying rename system call.
9038 On a system with a modern Linux-based kernel, it fails with
9039 @code{errno=ENOTDIR}@.
9040 However, on other systems (at least FreeBSD 6.1 and Solaris 10) it silently
9041 renames not the symlink but rather the directory referenced by the symlink.
9042 @xref{Trailing slashes}.
9044 The program accepts the following options. Also see @ref{Common options}.
9054 @cindex prompts, omitting
9055 Do not prompt the user before removing a destination file.
9057 If you specify more than one of the @option{-i}, @option{-f}, @option{-n}
9058 options, only the final one takes effect.
9063 @itemx --interactive
9065 @opindex --interactive
9066 @cindex prompts, forcing
9067 Prompt whether to overwrite each existing destination file, regardless
9069 If the response is not affirmative, the file is skipped.
9075 @opindex --no-clobber
9076 @cindex prompts, omitting
9077 Do not overwrite an existing file.
9079 This option is mutually exclusive with @option{-b} or @option{--backup} option.
9085 @cindex newer files, moving only
9086 Do not move a non-directory that has an existing destination with the
9087 same or newer modification time.
9088 If the move is across file system boundaries, the comparison is to the
9089 source time stamp truncated to the resolutions of the destination file
9090 system and of the system calls used to update time stamps; this avoids
9091 duplicate work if several @samp{mv -u} commands are executed with the
9092 same source and destination.
9098 Print the name of each file before moving it.
9100 @optStripTrailingSlashes
9106 @optNoTargetDirectory
9114 @section @command{rm}: Remove files or directories
9117 @cindex removing files or directories
9119 @command{rm} removes each given @var{file}. By default, it does not remove
9120 directories. Synopsis:
9123 rm [@var{option}]@dots{} [@var{file}]@dots{}
9126 @cindex prompting, and @command{rm}
9127 If the @option{-I} or @option{--interactive=once} option is given,
9128 and there are more than three files or the @option{-r}, @option{-R},
9129 or @option{--recursive} are given, then @command{rm} prompts the user
9130 for whether to proceed with the entire operation. If the response is
9131 not affirmative, the entire command is aborted.
9133 Otherwise, if a file is unwritable, standard input is a terminal, and
9134 the @option{-f} or @option{--force} option is not given, or the
9135 @option{-i} or @option{--interactive=always} option @emph{is} given,
9136 @command{rm} prompts the user for whether to remove the file.
9137 If the response is not affirmative, the file is skipped.
9139 Any attempt to remove a file whose last file name component is
9140 @file{.} or @file{..} is rejected without any prompting.
9142 @emph{Warning}: If you use @command{rm} to remove a file, it is usually
9143 possible to recover the contents of that file. If you want more assurance
9144 that the contents are truly unrecoverable, consider using @command{shred}.
9146 The program accepts the following options. Also see @ref{Common options}.
9154 @cindex directories, removing
9155 Remove the listed directories if they are empty.
9161 Ignore nonexistent files and missing operands, and never prompt the user.
9162 Ignore any previous @option{--interactive} (@option{-i}) option.
9166 Prompt whether to remove each file.
9167 If the response is not affirmative, the file is skipped.
9168 Ignore any previous @option{--force} (@option{-f}) option.
9169 Equivalent to @option{--interactive=always}.
9173 Prompt once whether to proceed with the command, if more than three
9174 files are named or if a recursive removal is requested. Ignore any
9175 previous @option{--force} (@option{-f}) option. Equivalent to
9176 @option{--interactive=once}.
9178 @item --interactive [=@var{when}]
9179 @opindex --interactive
9180 Specify when to issue an interactive prompt. @var{when} may be
9184 @vindex never @r{interactive option}
9185 - Do not prompt at all.
9187 @vindex once @r{interactive option}
9188 - Prompt once if more than three files are named or if a recursive
9189 removal is requested. Equivalent to @option{-I}.
9191 @vindex always @r{interactive option}
9192 - Prompt for every file being removed. Equivalent to @option{-i}.
9194 @option{--interactive} with no @var{when} is equivalent to
9195 @option{--interactive=always}.
9197 @item --one-file-system
9198 @opindex --one-file-system
9199 @cindex one file system, restricting @command{rm} to
9200 When removing a hierarchy recursively, skip any directory that is on a
9201 file system different from that of the corresponding command line argument.
9204 This option is useful when removing a build ``chroot'' hierarchy,
9205 which normally contains no valuable data. However, it is not uncommon
9206 to bind-mount @file{/home} into such a hierarchy, to make it easier to
9207 use one's start-up file. The catch is that it's easy to forget to
9208 unmount @file{/home}. Then, when you use @command{rm -rf} to remove
9209 your normally throw-away chroot, that command will remove everything
9210 under @file{/home}, too.
9211 Use the @option{--one-file-system} option, and it will
9212 warn about and skip directories on other file systems.
9213 Of course, this will not save your @file{/home} if it and your
9214 chroot happen to be on the same file system.
9216 @item --preserve-root
9217 @opindex --preserve-root
9218 @cindex root directory, disallow recursive destruction
9219 Fail upon any attempt to remove the root directory, @file{/},
9220 when used with the @option{--recursive} option.
9221 This is the default behavior.
9222 @xref{Treating / specially}.
9224 @item --no-preserve-root
9225 @opindex --no-preserve-root
9226 @cindex root directory, allow recursive destruction
9227 Do not treat @file{/} specially when removing recursively.
9228 This option is not recommended unless you really want to
9229 remove all the files on your computer.
9230 @xref{Treating / specially}.
9237 @opindex --recursive
9238 @cindex directories, removing (recursively)
9239 Remove the listed directories and their contents recursively.
9245 Print the name of each file before removing it.
9249 @cindex files beginning with @samp{-}, removing
9250 @cindex @samp{-}, removing files beginning with
9251 One common question is how to remove files whose names begin with a
9252 @samp{-}. GNU @command{rm}, like every program that uses the @code{getopt}
9253 function to parse its arguments, lets you use the @samp{--} option to
9254 indicate that all following arguments are non-options. To remove a file
9255 called @file{-f} in the current directory, you could type either:
9268 @opindex - @r{and Unix @command{rm}}
9269 The Unix @command{rm} program's use of a single @samp{-} for this purpose
9270 predates the development of the getopt standard syntax.
9275 @node shred invocation
9276 @section @command{shred}: Remove files more securely
9279 @cindex data, erasing
9280 @cindex erasing data
9282 @command{shred} overwrites devices or files, to help prevent even
9283 very expensive hardware from recovering the data.
9285 Ordinarily when you remove a file (@pxref{rm invocation}), the data is
9286 not actually destroyed. Only the index listing where the file is
9287 stored is destroyed, and the storage is made available for reuse.
9288 There are undelete utilities that will attempt to reconstruct the index
9289 and can bring the file back if the parts were not reused.
9291 On a busy system with a nearly-full drive, space can get reused in a few
9292 seconds. But there is no way to know for sure. If you have sensitive
9293 data, you may want to be sure that recovery is not possible by actually
9294 overwriting the file with non-sensitive data.
9296 However, even after doing that, it is possible to take the disk back
9297 to a laboratory and use a lot of sensitive (and expensive) equipment
9298 to look for the faint ``echoes'' of the original data underneath the
9299 overwritten data. If the data has only been overwritten once, it's not
9302 The best way to remove something irretrievably is to destroy the media
9303 it's on with acid, melt it down, or the like. For cheap removable media
9304 like floppy disks, this is the preferred method. However, hard drives
9305 are expensive and hard to melt, so the @command{shred} utility tries
9306 to achieve a similar effect non-destructively.
9308 This uses many overwrite passes, with the data patterns chosen to
9309 maximize the damage they do to the old data. While this will work on
9310 floppies, the patterns are designed for best effect on hard drives.
9311 For more details, see the source code and Peter Gutmann's paper
9312 @uref{http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html,
9313 @cite{Secure Deletion of Data from Magnetic and Solid-State Memory}},
9314 from the proceedings of the Sixth USENIX Security Symposium (San Jose,
9315 California, July 22--25, 1996).
9317 @strong{Please note} that @command{shred} relies on a very important assumption:
9318 that the file system overwrites data in place. This is the traditional
9319 way to do things, but many modern file system designs do not satisfy this
9320 assumption. Exceptions include:
9325 Log-structured or journaled file systems, such as those supplied with
9326 AIX and Solaris, and JFS, ReiserFS, XFS, Ext3 (in @code{data=journal} mode),
9327 BFS, NTFS, etc., when they are configured to journal @emph{data}.
9330 File systems that write redundant data and carry on even if some writes
9331 fail, such as RAID-based file systems.
9334 File systems that make snapshots, such as Network Appliance's NFS server.
9337 File systems that cache in temporary locations, such as NFS version 3
9341 Compressed file systems.
9344 In the particular case of ext3 file systems, the above disclaimer applies (and
9345 @command{shred} is thus of limited effectiveness) only in @code{data=journal}
9346 mode, which journals file data in addition to just metadata. In both
9347 the @code{data=ordered} (default) and @code{data=writeback} modes,
9348 @command{shred} works as usual. Ext3 journaling modes can be changed
9349 by adding the @code{data=something} option to the mount options for a
9350 particular file system in the @file{/etc/fstab} file, as documented in
9351 the mount man page (man mount).
9353 If you are not sure how your file system operates, then you should assume
9354 that it does not overwrite data in place, which means that shred cannot
9355 reliably operate on regular files in your file system.
9357 Generally speaking, it is more reliable to shred a device than a file,
9358 since this bypasses the problem of file system design mentioned above.
9359 However, even shredding devices is not always completely reliable. For
9360 example, most disks map out bad sectors invisibly to the application; if
9361 the bad sectors contain sensitive data, @command{shred} won't be able to
9364 @command{shred} makes no attempt to detect or report this problem, just as
9365 it makes no attempt to do anything about backups. However, since it is
9366 more reliable to shred devices than files, @command{shred} by default does
9367 not truncate or remove the output file. This default is more suitable
9368 for devices, which typically cannot be truncated and should not be
9371 Finally, consider the risk of backups and mirrors.
9372 File system backups and remote mirrors may contain copies of the
9373 file that cannot be removed, and that will allow a shredded file
9374 to be recovered later. So if you keep any data you may later want
9375 to destroy using @command{shred}, be sure that it is not backed up or mirrored.
9378 shred [@var{option}]@dots{} @var{file}[@dots{}]
9381 The program accepts the following options. Also see @ref{Common options}.
9389 @cindex force deletion
9390 Override file permissions if necessary to allow overwriting.
9393 @itemx -n @var{number}
9394 @itemx --iterations=@var{number}
9395 @opindex -n @var{number}
9396 @opindex --iterations=@var{number}
9397 @cindex iterations, selecting the number of
9398 By default, @command{shred} uses @value{SHRED_DEFAULT_PASSES} passes of
9399 overwrite. You can reduce this to save time, or increase it if you think it's
9400 appropriate. After 25 passes all of the internal overwrite patterns will have
9401 been used at least once.
9403 @item --random-source=@var{file}
9404 @opindex --random-source
9405 @cindex random source for shredding
9406 Use @var{file} as a source of random data used to overwrite and to
9407 choose pass ordering. @xref{Random sources}.
9409 @item -s @var{bytes}
9410 @itemx --size=@var{bytes}
9411 @opindex -s @var{bytes}
9412 @opindex --size=@var{bytes}
9413 @cindex size of file to shred
9414 Shred the first @var{bytes} bytes of the file. The default is to shred
9415 the whole file. @var{bytes} can be followed by a size specification like
9416 @samp{K}, @samp{M}, or @samp{G} to specify a multiple. @xref{Block size}.
9422 @cindex removing files after shredding
9423 After shredding a file, truncate it (if possible) and then remove it.
9424 If a file has multiple links, only the named links will be removed.
9430 Display to standard error all status updates as sterilization proceeds.
9436 By default, @command{shred} rounds the size of a regular file up to the next
9437 multiple of the file system block size to fully erase the last block
9439 Use @option{--exact} to suppress that behavior.
9440 Thus, by default if you shred a 10-byte regular file on a system with 512-byte
9441 blocks, the resulting file will be 512 bytes long. With this option,
9442 shred does not increase the apparent size of the file.
9448 Normally, the last pass that @command{shred} writes is made up of
9449 random data. If this would be conspicuous on your hard drive (for
9450 example, because it looks like encrypted data), or you just think
9451 it's tidier, the @option{--zero} option adds an additional overwrite pass with
9452 all zero bits. This is in addition to the number of passes specified
9453 by the @option{--iterations} option.
9457 You might use the following command to erase all trace of the
9458 file system you'd created on the floppy disk in your first drive.
9459 That command takes about 20 minutes to erase a ``1.44MB'' (actually
9463 shred --verbose /dev/fd0
9466 Similarly, to erase all data on a selected partition of
9467 your hard disk, you could give a command like this:
9470 shred --verbose /dev/sda5
9473 On modern disks, a single pass should be adequate,
9474 and it will take one third the time of the default three-pass approach.
9477 # 1 pass, write pseudo-random data; 3x faster than the default
9478 shred --verbose -n1 /dev/sda5
9481 To be on the safe side, use at least one pass that overwrites using
9482 pseudo-random data. I.e., don't be tempted to use @samp{-n0 --zero},
9483 in case some disk controller optimizes the process of writing blocks
9484 of all zeros, and thereby does not clear all bytes in a block.
9485 Some SSDs may do just that.
9487 A @var{file} of @samp{-} denotes standard output.
9488 The intended use of this is to shred a removed temporary file.
9495 echo "Hello, world" >&3
9500 However, the command @samp{shred - >file} does not shred the contents
9501 of @var{file}, since the shell truncates @var{file} before invoking
9502 @command{shred}. Use the command @samp{shred file} or (if using a
9503 Bourne-compatible shell) the command @samp{shred - 1<>file} instead.
9508 @node Special file types
9509 @chapter Special file types
9511 @cindex special file types
9512 @cindex file types, special
9514 This chapter describes commands which create special types of files (and
9515 @command{rmdir}, which removes directories, one special file type).
9517 @cindex special file types
9519 Although Unix-like operating systems have markedly fewer special file
9520 types than others, not @emph{everything} can be treated only as the
9521 undifferentiated byte stream of @dfn{normal files}. For example, when a
9522 file is created or removed, the system must record this information,
9523 which it does in a @dfn{directory}---a special type of file. Although
9524 you can read directories as normal files, if you're curious, in order
9525 for the system to do its job it must impose a structure, a certain
9526 order, on the bytes of the file. Thus it is a ``special'' type of file.
9528 Besides directories, other special file types include named pipes
9529 (FIFOs), symbolic links, sockets, and so-called @dfn{special files}.
9532 * link invocation:: Make a hard link via the link syscall
9533 * ln invocation:: Make links between files.
9534 * mkdir invocation:: Make directories.
9535 * mkfifo invocation:: Make FIFOs (named pipes).
9536 * mknod invocation:: Make block or character special files.
9537 * readlink invocation:: Print value of a symlink or canonical file name.
9538 * rmdir invocation:: Remove empty directories.
9539 * unlink invocation:: Remove files via the unlink syscall
9543 @node link invocation
9544 @section @command{link}: Make a hard link via the link syscall
9547 @cindex links, creating
9548 @cindex hard links, creating
9549 @cindex creating links (hard only)
9551 @command{link} creates a single hard link at a time.
9552 It is a minimalist interface to the system-provided
9553 @code{link} function. @xref{Hard Links, , , libc,
9554 The GNU C Library Reference Manual}.
9555 It avoids the bells and whistles of the more commonly-used
9556 @command{ln} command (@pxref{ln invocation}).
9560 link @var{filename} @var{linkname}
9563 @var{filename} must specify an existing file, and @var{linkname}
9564 must specify a nonexistent entry in an existing directory.
9565 @command{link} simply calls @code{link (@var{filename}, @var{linkname})}
9568 On a GNU system, this command acts like @samp{ln --directory
9569 --no-target-directory @var{filename} @var{linkname}}. However, the
9570 @option{--directory} and @option{--no-target-directory} options are
9571 not specified by POSIX, and the @command{link} command is
9572 more portable in practice.
9574 If @var{filename} is a symbolic link, it is unspecified whether
9575 @var{linkname} will be a hard link to the symbolic link or to the
9576 target of the symbolic link. Use @command{ln -P} or @command{ln -L}
9577 to specify which behavior is desired.
9583 @section @command{ln}: Make links between files
9586 @cindex links, creating
9587 @cindex hard links, creating
9588 @cindex symbolic (soft) links, creating
9589 @cindex creating links (hard or soft)
9591 @cindex file systems and hard links
9592 @command{ln} makes links between files. By default, it makes hard links;
9593 with the @option{-s} option, it makes symbolic (or @dfn{soft}) links.
9597 ln [@var{option}]@dots{} [-T] @var{target} @var{linkname}
9598 ln [@var{option}]@dots{} @var{target}
9599 ln [@var{option}]@dots{} @var{target}@dots{} @var{directory}
9600 ln [@var{option}]@dots{} -t @var{directory} @var{target}@dots{}
9606 If two file names are given, @command{ln} creates a link to the first
9607 file from the second.
9610 If one @var{target} is given, @command{ln} creates a link to that file
9611 in the current directory.
9614 If the @option{--target-directory} (@option{-t}) option is given, or
9615 failing that if the last file is a directory and the
9616 @option{--no-target-directory} (@option{-T}) option is not given,
9617 @command{ln} creates a link to each @var{target} file in the specified
9618 directory, using the @var{target}s' names.
9622 Normally @command{ln} does not remove existing files. Use the
9623 @option{--force} (@option{-f}) option to remove them unconditionally,
9624 the @option{--interactive} (@option{-i}) option to remove them
9625 conditionally, and the @option{--backup} (@option{-b}) option to
9628 @cindex hard link, defined
9629 @cindex inode, and hard links
9630 A @dfn{hard link} is another name for an existing file; the link and the
9631 original are indistinguishable. Technically speaking, they share the
9632 same inode, and the inode contains all the information about a
9633 file---indeed, it is not incorrect to say that the inode @emph{is} the
9634 file. Most systems prohibit making a hard link to
9635 a directory; on those where it is allowed, only the super-user can do
9636 so (and with caution, since creating a cycle will cause problems to many
9637 other utilities). Hard links cannot cross file system boundaries. (These
9638 restrictions are not mandated by POSIX, however.)
9640 @cindex dereferencing symbolic links
9641 @cindex symbolic link, defined
9642 @dfn{Symbolic links} (@dfn{symlinks} for short), on the other hand, are
9643 a special file type (which not all kernels support: System V release 3
9644 (and older) systems lack symlinks) in which the link file actually
9645 refers to a different file, by name. When most operations (opening,
9646 reading, writing, and so on) are passed the symbolic link file, the
9647 kernel automatically @dfn{dereferences} the link and operates on the
9648 target of the link. But some operations (e.g., removing) work on the
9649 link file itself, rather than on its target. The owner and group of a
9650 symlink are not significant to file access performed through
9651 the link, but do have implications on deleting a symbolic link from a
9652 directory with the restricted deletion bit set. On the GNU system,
9653 the mode of a symlink has no significance and cannot be changed, but
9654 on some BSD systems, the mode can be changed and will affect whether
9655 the symlink will be traversed in file name resolution. @xref{Symbolic Links,,,
9656 libc, The GNU C Library Reference Manual}.
9658 Symbolic links can contain arbitrary strings; a @dfn{dangling symlink}
9659 occurs when the string in the symlink does not resolve to a file.
9660 There are no restrictions against creating dangling symbolic links.
9661 There are trade-offs to using absolute or relative symlinks. An
9662 absolute symlink always points to the same file, even if the directory
9663 containing the link is moved. However, if the symlink is visible from
9664 more than one machine (such as on a networked file system), the file
9665 pointed to might not always be the same. A relative symbolic link is
9666 resolved in relation to the directory that contains the link, and is
9667 often useful in referring to files on the same device without regards
9668 to what name that device is mounted on when accessed via networked
9671 When creating a relative symlink in a different location than the
9672 current directory, the resolution of the symlink will be different
9673 than the resolution of the same string from the current directory.
9674 Therefore, many users prefer to first change directories to the
9675 location where the relative symlink will be created, so that
9676 tab-completion or other file resolution will find the same target as
9677 what will be placed in the symlink.
9679 The program accepts the following options. Also see @ref{Common options}.
9690 @opindex --directory
9691 @cindex hard links to directories
9692 Allow users with appropriate privileges to attempt to make hard links
9694 However, note that this will probably fail due to
9695 system restrictions, even for the super-user.
9701 Remove existing destination files.
9704 @itemx --interactive
9706 @opindex --interactive
9707 @cindex prompting, and @command{ln}
9708 Prompt whether to remove existing destination files.
9714 If @option{-s} is not in effect, and the source file is a symbolic
9715 link, create the hard link to the file referred to by the symbolic
9716 link, rather than the symbolic link itself.
9719 @itemx --no-dereference
9721 @opindex --no-dereference
9722 Do not treat the last operand specially when it is a symbolic link to
9723 a directory. Instead, treat it as if it were a normal file.
9725 When the destination is an actual directory (not a symlink to one),
9726 there is no ambiguity. The link is created in that directory.
9727 But when the specified destination is a symlink to a directory,
9728 there are two ways to treat the user's request. @command{ln} can
9729 treat the destination just as it would a normal directory and create
9730 the link in it. On the other hand, the destination can be viewed as a
9731 non-directory---as the symlink itself. In that case, @command{ln}
9732 must delete or backup that symlink before creating the new link.
9733 The default is to treat a destination that is a symlink to a directory
9734 just like a directory.
9736 This option is weaker than the @option{--no-target-directory}
9737 (@option{-T}) option, so it has no effect if both options are given.
9743 If @option{-s} is not in effect, and the source file is a symbolic
9744 link, create the hard link to the symbolic link itself. On platforms
9745 where this is not supported by the kernel, this option creates a
9746 symbolic link with identical contents; since symbolic link contents
9747 cannot be edited, any file name resolution performed through either
9748 link will be the same as if a hard link had been created.
9754 Make symbolic links relative to the link location.
9759 ln -srv /a/file /tmp
9760 '/tmp/file' -> '../a/file'
9763 @xref{realpath invocation}, which gives greater control
9764 over relative file name generation.
9770 Make symbolic links instead of hard links. This option merely produces
9771 an error message on systems that do not support symbolic links.
9777 @optNoTargetDirectory
9783 Print the name of each file after linking it successfully.
9787 @cindex hard links to symbolic links
9788 @cindex symbolic links and @command{ln}
9789 If @option{-L} and @option{-P} are both given, the last one takes
9790 precedence. If @option{-s} is also given, @option{-L} and @option{-P}
9791 are silently ignored. If neither option is given, then this
9792 implementation defaults to @option{-P} if the system @code{link} supports
9793 hard links to symbolic links (such as the GNU system), and @option{-L}
9794 if @code{link} follows symbolic links (such as on BSD).
9803 # Create link ../a pointing to a in that directory.
9804 # Not really useful because it points to itself.
9809 # Change to the target before creating symlinks to avoid being confused.
9815 # Hard coded file names don't move well.
9816 ln -s $(pwd)/a /some/dir/
9820 # Relative file names survive directory moves and also
9821 # work across networked file systems.
9822 ln -s afile anotherfile
9823 ln -s ../adir/afile yetanotherfile
9827 @node mkdir invocation
9828 @section @command{mkdir}: Make directories
9831 @cindex directories, creating
9832 @cindex creating directories
9834 @command{mkdir} creates directories with the specified names. Synopsis:
9837 mkdir [@var{option}]@dots{} @var{name}@dots{}
9840 @command{mkdir} creates each directory @var{name} in the order given.
9841 It reports an error if @var{name} already exists, unless the
9842 @option{-p} option is given and @var{name} is a directory.
9844 The program accepts the following options. Also see @ref{Common options}.
9849 @itemx --mode=@var{mode}
9852 @cindex modes of created directories, setting
9853 Set the file permission bits of created directories to @var{mode},
9854 which uses the same syntax as
9855 in @command{chmod} and uses @samp{a=rwx} (read, write and execute allowed for
9856 everyone) for the point of the departure. @xref{File permissions}.
9858 Normally the directory has the desired file mode bits at the moment it
9859 is created. As a GNU extension, @var{mode} may also mention
9860 special mode bits, but in this case there may be a temporary window
9861 during which the directory exists but its special mode bits are
9862 incorrect. @xref{Directory Setuid and Setgid}, for how the
9863 set-user-ID and set-group-ID bits of directories are inherited unless
9864 overridden in this way.
9870 @cindex parent directories, creating
9871 Make any missing parent directories for each argument, setting their
9872 file permission bits to the umask modified by @samp{u+wx}. Ignore
9873 existing parent directories, and do not change their file permission
9876 To set the file permission bits of any newly-created parent
9877 directories to a value that includes @samp{u+wx}, you can set the
9878 umask before invoking @command{mkdir}. For example, if the shell
9879 command @samp{(umask u=rwx,go=rx; mkdir -p P/Q)} creates the parent
9880 @file{P} it sets the parent's permission bits to @samp{u=rwx,go=rx}.
9881 To set a parent's special mode bits as well, you can invoke
9882 @command{chmod} after @command{mkdir}. @xref{Directory Setuid and
9883 Setgid}, for how the set-user-ID and set-group-ID bits of
9884 newly-created parent directories are inherited.
9890 Print a message for each created directory. This is most useful with
9893 @item -Z @var{context}
9894 @itemx --context=@var{context}
9898 @cindex security context
9899 Set the default SELinux security context to be used for created directories.
9906 @node mkfifo invocation
9907 @section @command{mkfifo}: Make FIFOs (named pipes)
9910 @cindex FIFOs, creating
9911 @cindex named pipes, creating
9912 @cindex creating FIFOs (named pipes)
9914 @command{mkfifo} creates FIFOs (also called @dfn{named pipes}) with the
9915 specified names. Synopsis:
9918 mkfifo [@var{option}] @var{name}@dots{}
9921 A @dfn{FIFO} is a special file type that permits independent processes
9922 to communicate. One process opens the FIFO file for writing, and
9923 another for reading, after which data can flow as with the usual
9924 anonymous pipe in shells or elsewhere.
9926 The program accepts the following option. Also see @ref{Common options}.
9931 @itemx --mode=@var{mode}
9934 @cindex modes of created FIFOs, setting
9935 Set the mode of created FIFOs to @var{mode}, which is symbolic as in
9936 @command{chmod} and uses @samp{a=rw} (read and write allowed for everyone)
9937 for the point of departure. @var{mode} should specify only file
9938 permission bits. @xref{File permissions}.
9940 @item -Z @var{context}
9941 @itemx --context=@var{context}
9945 @cindex security context
9946 Set the default SELinux security context to be used for created FIFOs.
9953 @node mknod invocation
9954 @section @command{mknod}: Make block or character special files
9957 @cindex block special files, creating
9958 @cindex character special files, creating
9960 @command{mknod} creates a FIFO, character special file, or block special
9961 file with the specified name. Synopsis:
9964 mknod [@var{option}]@dots{} @var{name} @var{type} [@var{major} @var{minor}]
9967 @cindex special files
9968 @cindex block special files
9969 @cindex character special files
9970 Unlike the phrase ``special file type'' above, the term @dfn{special
9971 file} has a technical meaning on Unix: something that can generate or
9972 receive data. Usually this corresponds to a physical piece of hardware,
9973 e.g., a printer or a disk. (These files are typically created at
9974 system-configuration time.) The @command{mknod} command is what creates
9975 files of this type. Such devices can be read either a character at a
9976 time or a ``block'' (many characters) at a time, hence we say there are
9977 @dfn{block special} files and @dfn{character special} files.
9979 @c mknod is a shell built-in at least with OpenBSD's /bin/sh
9980 @mayConflictWithShellBuiltIn{mknod}
9982 The arguments after @var{name} specify the type of file to make:
9987 @opindex p @r{for FIFO file}
9991 @opindex b @r{for block special file}
9992 for a block special file
9995 @c Don't document the 'u' option -- it's just a synonym for 'c'.
9996 @c Do *any* versions of mknod still use it?
9998 @opindex c @r{for character special file}
9999 @c @opindex u @r{for character special file}
10000 for a character special file
10004 When making a block or character special file, the major and minor
10005 device numbers must be given after the file type.
10006 If a major or minor device number begins with @samp{0x} or @samp{0X},
10007 it is interpreted as hexadecimal; otherwise, if it begins with @samp{0},
10008 as octal; otherwise, as decimal.
10010 The program accepts the following option. Also see @ref{Common options}.
10014 @item -m @var{mode}
10015 @itemx --mode=@var{mode}
10018 Set the mode of created files to @var{mode}, which is symbolic as in
10019 @command{chmod} and uses @samp{a=rw} as the point of departure.
10020 @var{mode} should specify only file permission bits.
10021 @xref{File permissions}.
10023 @item -Z @var{context}
10024 @itemx --context=@var{context}
10028 @cindex security context
10029 Set the default SELinux security context to be used for created files.
10036 @node readlink invocation
10037 @section @command{readlink}: Print value of a symlink or canonical file name
10040 @cindex displaying value of a symbolic link
10041 @cindex canonical file name
10042 @cindex canonicalize a file name
10045 @command{readlink} may work in one of two supported modes:
10049 @item Readlink mode
10051 @command{readlink} outputs the value of the given symbolic links.
10052 If @command{readlink} is invoked with an argument other than the name
10053 of a symbolic link, it produces no output and exits with a nonzero exit code.
10055 @item Canonicalize mode
10057 @command{readlink} outputs the absolute name of the given files which contain
10058 no @file{.}, @file{..} components nor any repeated separators
10059 (@file{/}) or symbolic links.
10064 readlink [@var{option}]@dots{} @var{file}@dots{}
10067 By default, @command{readlink} operates in readlink mode.
10069 The program accepts the following options. Also see @ref{Common options}.
10074 @itemx --canonicalize
10076 @opindex --canonicalize
10077 Activate canonicalize mode.
10078 If any component of the file name except the last one is missing or unavailable,
10079 @command{readlink} produces no output and exits with a nonzero exit
10080 code. A trailing slash is ignored.
10083 @itemx --canonicalize-existing
10085 @opindex --canonicalize-existing
10086 Activate canonicalize mode.
10087 If any component is missing or unavailable, @command{readlink} produces
10088 no output and exits with a nonzero exit code. A trailing slash
10089 requires that the name resolve to a directory.
10092 @itemx --canonicalize-missing
10094 @opindex --canonicalize-missing
10095 Activate canonicalize mode.
10096 If any component is missing or unavailable, @command{readlink} treats it
10100 @itemx --no-newline
10102 @opindex --no-newline
10103 Do not print the output delimiter, when a single @var{file} is specified.
10104 Print a warning if specified along with multiple @var{file}s.
10114 Suppress most error messages.
10120 Report error messages.
10126 Separate output items with NUL characters.
10130 The @command{readlink} utility first appeared in OpenBSD 2.1.
10132 The @command{realpath} command without options, operates like
10133 @command{readlink} in canonicalize mode.
10138 @node rmdir invocation
10139 @section @command{rmdir}: Remove empty directories
10142 @cindex removing empty directories
10143 @cindex directories, removing empty
10145 @command{rmdir} removes empty directories. Synopsis:
10148 rmdir [@var{option}]@dots{} @var{directory}@dots{}
10151 If any @var{directory} argument does not refer to an existing empty
10152 directory, it is an error.
10154 The program accepts the following options. Also see @ref{Common options}.
10158 @item --ignore-fail-on-non-empty
10159 @opindex --ignore-fail-on-non-empty
10160 @cindex directory deletion, ignoring failures
10161 Ignore each failure to remove a directory that is solely because
10162 the directory is non-empty.
10168 @cindex parent directories, removing
10169 Remove @var{directory}, then try to remove each component of @var{directory}.
10170 So, for example, @samp{rmdir -p a/b/c} is similar to @samp{rmdir a/b/c a/b a}.
10171 As such, it fails if any of those directories turns out not to be empty.
10172 Use the @option{--ignore-fail-on-non-empty} option to make it so such
10173 a failure does not evoke a diagnostic and does not cause @command{rmdir} to
10174 exit unsuccessfully.
10180 @cindex directory deletion, reporting
10181 Give a diagnostic for each successful removal.
10182 @var{directory} is removed.
10186 @xref{rm invocation}, for how to remove non-empty directories (recursively).
10191 @node unlink invocation
10192 @section @command{unlink}: Remove files via the unlink syscall
10195 @cindex removing files or directories (via the unlink syscall)
10197 @command{unlink} deletes a single specified file name.
10198 It is a minimalist interface to the system-provided
10199 @code{unlink} function. @xref{Deleting Files, , , libc,
10200 The GNU C Library Reference Manual}. Synopsis:
10201 It avoids the bells and whistles of the more commonly-used
10202 @command{rm} command (@pxref{rm invocation}).
10205 unlink @var{filename}
10208 On some systems @code{unlink} can be used to delete the name of a
10209 directory. On others, it can be used that way only by a privileged user.
10210 In the GNU system @code{unlink} can never delete the name of a directory.
10212 The @command{unlink} command honors the @option{--help} and
10213 @option{--version} options. To remove a file whose name begins with
10214 @samp{-}, prefix the name with @samp{./}, e.g., @samp{unlink ./--help}.
10219 @node Changing file attributes
10220 @chapter Changing file attributes
10222 @cindex changing file attributes
10223 @cindex file attributes, changing
10224 @cindex attributes, file
10226 A file is not merely its contents, a name, and a file type
10227 (@pxref{Special file types}). A file also has an owner (a user ID), a
10228 group (a group ID), permissions (what the owner can do with the file,
10229 what people in the group can do, and what everyone else can do), various
10230 timestamps, and other information. Collectively, we call these a file's
10233 These commands change file attributes.
10236 * chown invocation:: Change file owners and groups.
10237 * chgrp invocation:: Change file groups.
10238 * chmod invocation:: Change access permissions.
10239 * touch invocation:: Change file timestamps.
10243 @node chown invocation
10244 @section @command{chown}: Change file owner and group
10247 @cindex file ownership, changing
10248 @cindex group ownership, changing
10249 @cindex changing file ownership
10250 @cindex changing group ownership
10252 @command{chown} changes the user and/or group ownership of each given @var{file}
10253 to @var{new-owner} or to the user and group of an existing reference file.
10257 chown [@var{option}]@dots{} @{@var{new-owner} | --reference=@var{ref_file}@}@c
10261 If used, @var{new-owner} specifies the new owner and/or group as follows
10262 (with no embedded white space):
10265 [@var{owner}] [ : [@var{group}] ]
10272 If only an @var{owner} (a user name or numeric user ID) is given, that
10273 user is made the owner of each given file, and the files' group is not
10276 @item owner@samp{:}group
10277 If the @var{owner} is followed by a colon and a @var{group} (a
10278 group name or numeric group ID), with no spaces between them, the group
10279 ownership of the files is changed as well (to @var{group}).
10281 @item owner@samp{:}
10282 If a colon but no group name follows @var{owner}, that user is
10283 made the owner of the files and the group of the files is changed to
10284 @var{owner}'s login group.
10286 @item @samp{:}group
10287 If the colon and following @var{group} are given, but the owner
10288 is omitted, only the group of the files is changed; in this case,
10289 @command{chown} performs the same function as @command{chgrp}.
10292 If only a colon is given, or if @var{new-owner} is empty, neither the
10293 owner nor the group is changed.
10297 If @var{owner} or @var{group} is intended to represent a numeric user
10298 or group ID, then you may specify it with a leading @samp{+}.
10299 @xref{Disambiguating names and IDs}.
10301 Some older scripts may still use @samp{.} in place of the @samp{:} separator.
10302 POSIX 1003.1-2001 (@pxref{Standards conformance}) does not
10303 require support for that, but for backward compatibility GNU
10304 @command{chown} supports @samp{.} so long as no ambiguity results.
10305 New scripts should avoid the use of @samp{.} because it is not
10306 portable, and because it has undesirable results if the entire
10307 @var{owner@samp{.}group} happens to identify a user whose name
10310 The @command{chown} command sometimes clears the set-user-ID or
10311 set-group-ID permission bits. This behavior depends on the policy and
10312 functionality of the underlying @code{chown} system call, which may
10313 make system-dependent file mode modifications outside the control of
10314 the @command{chown} command. For example, the @command{chown} command
10315 might not affect those bits when invoked by a user with appropriate
10316 privileges, or when the
10317 bits signify some function other than executable permission (e.g.,
10318 mandatory locking).
10319 When in doubt, check the underlying system behavior.
10321 The program accepts the following options. Also see @ref{Common options}.
10329 @cindex changed owners, verbosely describing
10330 Verbosely describe the action for each @var{file} whose ownership
10339 @cindex error messages, omitting
10340 Do not print error messages about files whose ownership cannot be
10343 @item @w{@kbd{--from}=@var{old-owner}}
10345 @cindex symbolic links, changing owner
10346 Change a @var{file}'s ownership only if it has current attributes specified
10347 by @var{old-owner}. @var{old-owner} has the same form as @var{new-owner}
10349 This option is useful primarily from a security standpoint in that
10350 it narrows considerably the window of potential abuse.
10351 For example, to reflect a user ID numbering change for one user's files
10352 without an option like this, @code{root} might run
10355 find / -owner OLDUSER -print0 | xargs -0 chown -h NEWUSER
10358 But that is dangerous because the interval between when the @command{find}
10359 tests the existing file's owner and when the @command{chown} is actually run
10360 may be quite large.
10361 One way to narrow the gap would be to invoke chown for each file
10365 find / -owner OLDUSER -exec chown -h NEWUSER @{@} \;
10368 But that is very slow if there are many affected files.
10369 With this option, it is safer (the gap is narrower still)
10370 though still not perfect:
10373 chown -h -R --from=OLDUSER NEWUSER /
10376 @item --dereference
10377 @opindex --dereference
10378 @cindex symbolic links, changing owner
10380 Do not act on symbolic links themselves but rather on what they point to.
10381 This is the default.
10384 @itemx --no-dereference
10386 @opindex --no-dereference
10387 @cindex symbolic links, changing owner
10389 Act on symbolic links themselves instead of what they point to.
10390 This mode relies on the @code{lchown} system call.
10391 On systems that do not provide the @code{lchown} system call,
10392 @command{chown} fails when a file specified on the command line
10393 is a symbolic link.
10394 By default, no diagnostic is issued for symbolic links encountered
10395 during a recursive traversal, but see @option{--verbose}.
10397 @item --preserve-root
10398 @opindex --preserve-root
10399 @cindex root directory, disallow recursive modification
10400 Fail upon any attempt to recursively change the root directory, @file{/}.
10401 Without @option{--recursive}, this option has no effect.
10402 @xref{Treating / specially}.
10404 @item --no-preserve-root
10405 @opindex --no-preserve-root
10406 @cindex root directory, allow recursive modification
10407 Cancel the effect of any preceding @option{--preserve-root} option.
10408 @xref{Treating / specially}.
10410 @item --reference=@var{ref_file}
10411 @opindex --reference
10412 Change the user and group of each @var{file} to be the same as those of
10413 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
10414 user and group of the symbolic link, but rather those of the file it
10421 Output a diagnostic for every file processed.
10422 If a symbolic link is encountered during a recursive traversal
10423 on a system without the @code{lchown} system call, and @option{--no-dereference}
10424 is in effect, then issue a diagnostic saying neither the symbolic link nor
10425 its referent is being changed.
10430 @opindex --recursive
10431 @cindex recursively changing file ownership
10432 Recursively change ownership of directories and their contents.
10435 @xref{Traversing symlinks}.
10438 @xref{Traversing symlinks}.
10441 @xref{Traversing symlinks}.
10450 # Change the owner of /u to "root".
10453 # Likewise, but also change its group to "staff".
10454 chown root:staff /u
10456 # Change the owner of /u and subfiles to "root".
10461 @node chgrp invocation
10462 @section @command{chgrp}: Change group ownership
10465 @cindex group ownership, changing
10466 @cindex changing group ownership
10468 @command{chgrp} changes the group ownership of each given @var{file}
10469 to @var{group} (which can be either a group name or a numeric group ID)
10470 or to the group of an existing reference file. Synopsis:
10473 chgrp [@var{option}]@dots{} @{@var{group} | --reference=@var{ref_file}@}@c
10477 If @var{group} is intended to represent a
10478 numeric group ID, then you may specify it with a leading @samp{+}.
10479 @xref{Disambiguating names and IDs}.
10481 The program accepts the following options. Also see @ref{Common options}.
10489 @cindex changed files, verbosely describing
10490 Verbosely describe the action for each @var{file} whose group actually
10499 @cindex error messages, omitting
10500 Do not print error messages about files whose group cannot be
10503 @item --dereference
10504 @opindex --dereference
10505 @cindex symbolic links, changing owner
10507 Do not act on symbolic links themselves but rather on what they point to.
10508 This is the default.
10511 @itemx --no-dereference
10513 @opindex --no-dereference
10514 @cindex symbolic links, changing group
10516 Act on symbolic links themselves instead of what they point to.
10517 This mode relies on the @code{lchown} system call.
10518 On systems that do not provide the @code{lchown} system call,
10519 @command{chgrp} fails when a file specified on the command line
10520 is a symbolic link.
10521 By default, no diagnostic is issued for symbolic links encountered
10522 during a recursive traversal, but see @option{--verbose}.
10524 @item --preserve-root
10525 @opindex --preserve-root
10526 @cindex root directory, disallow recursive modification
10527 Fail upon any attempt to recursively change the root directory, @file{/}.
10528 Without @option{--recursive}, this option has no effect.
10529 @xref{Treating / specially}.
10531 @item --no-preserve-root
10532 @opindex --no-preserve-root
10533 @cindex root directory, allow recursive modification
10534 Cancel the effect of any preceding @option{--preserve-root} option.
10535 @xref{Treating / specially}.
10537 @item --reference=@var{ref_file}
10538 @opindex --reference
10539 Change the group of each @var{file} to be the same as that of
10540 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
10541 group of the symbolic link, but rather that of the file it refers to.
10547 Output a diagnostic for every file processed.
10548 If a symbolic link is encountered during a recursive traversal
10549 on a system without the @code{lchown} system call, and @option{--no-dereference}
10550 is in effect, then issue a diagnostic saying neither the symbolic link nor
10551 its referent is being changed.
10556 @opindex --recursive
10557 @cindex recursively changing group ownership
10558 Recursively change the group ownership of directories and their contents.
10561 @xref{Traversing symlinks}.
10564 @xref{Traversing symlinks}.
10567 @xref{Traversing symlinks}.
10576 # Change the group of /u to "staff".
10579 # Change the group of /u and subfiles to "staff".
10584 @node chmod invocation
10585 @section @command{chmod}: Change access permissions
10588 @cindex changing access permissions
10589 @cindex access permissions, changing
10590 @cindex permissions, changing access
10592 @command{chmod} changes the access permissions of the named files. Synopsis:
10595 chmod [@var{option}]@dots{} @{@var{mode} | --reference=@var{ref_file}@}@c
10599 @cindex symbolic links, permissions of
10600 @command{chmod} never changes the permissions of symbolic links, since
10601 the @command{chmod} system call cannot change their permissions.
10602 This is not a problem since the permissions of symbolic links are
10603 never used. However, for each symbolic link listed on the command
10604 line, @command{chmod} changes the permissions of the pointed-to file.
10605 In contrast, @command{chmod} ignores symbolic links encountered during
10606 recursive directory traversals.
10608 A successful use of @command{chmod} clears the set-group-ID bit of a
10609 regular file if the file's group ID does not match the user's
10610 effective group ID or one of the user's supplementary group IDs,
10611 unless the user has appropriate privileges. Additional restrictions
10612 may cause the set-user-ID and set-group-ID bits of @var{mode} or
10613 @var{ref_file} to be ignored. This behavior depends on the policy and
10614 functionality of the underlying @code{chmod} system call. When in
10615 doubt, check the underlying system behavior.
10617 If used, @var{mode} specifies the new file mode bits.
10618 For details, see the section on @ref{File permissions}.
10619 If you really want @var{mode} to have a leading @samp{-}, you should
10620 use @option{--} first, e.g., @samp{chmod -- -w file}. Typically,
10621 though, @samp{chmod a-w file} is preferable, and @command{chmod -w
10622 file} (without the @option{--}) complains if it behaves differently
10623 from what @samp{chmod a-w file} would do.
10625 The program accepts the following options. Also see @ref{Common options}.
10633 Verbosely describe the action for each @var{file} whose permissions
10642 @cindex error messages, omitting
10643 Do not print error messages about files whose permissions cannot be
10646 @item --preserve-root
10647 @opindex --preserve-root
10648 @cindex root directory, disallow recursive modification
10649 Fail upon any attempt to recursively change the root directory, @file{/}.
10650 Without @option{--recursive}, this option has no effect.
10651 @xref{Treating / specially}.
10653 @item --no-preserve-root
10654 @opindex --no-preserve-root
10655 @cindex root directory, allow recursive modification
10656 Cancel the effect of any preceding @option{--preserve-root} option.
10657 @xref{Treating / specially}.
10663 Verbosely describe the action or non-action taken for every @var{file}.
10665 @item --reference=@var{ref_file}
10666 @opindex --reference
10667 Change the mode of each @var{file} to be the same as that of @var{ref_file}.
10668 @xref{File permissions}.
10669 If @var{ref_file} is a symbolic link, do not use the mode
10670 of the symbolic link, but rather that of the file it refers to.
10675 @opindex --recursive
10676 @cindex recursively changing access permissions
10677 Recursively change permissions of directories and their contents.
10684 @node touch invocation
10685 @section @command{touch}: Change file timestamps
10688 @cindex changing file timestamps
10689 @cindex file timestamps, changing
10690 @cindex timestamps, changing file
10692 @command{touch} changes the access and/or modification times of the
10693 specified files. Synopsis:
10696 touch [@var{option}]@dots{} @var{file}@dots{}
10699 @cindex empty files, creating
10700 Any @var{file} argument that does not exist is created empty, unless
10701 option @option{--no-create} (@option{-c}) or @option{--no-dereference}
10702 (@option{-h}) was in effect.
10704 A @var{file} argument string of @samp{-} is handled specially and
10705 causes @command{touch} to change the times of the file associated with
10709 By default, @command{touch} sets file timestamps to the current time.
10710 Because @command{touch} acts on its operands left to right, the
10711 resulting timestamps of earlier and later operands may disagree.
10712 Also, the determination of what time is ``current'' depends on the
10713 platform. Platforms with network file systems often use different
10714 clocks for the operating system and for file systems; because
10715 @command{touch} typically uses file systems' clocks by default, clock
10716 skew can cause the resulting file timestamps to appear to be in a
10717 program's ``future'' or ``past''.
10719 @cindex file timestamp resolution
10720 The @command{touch} command sets the file's timestamp to the greatest
10721 representable value that is not greater than the requested time. This
10722 can differ from the requested time for several reasons. First, the
10723 requested time may have a higher resolution than supported. Second, a
10724 file system may use different resolutions for different types of
10725 times. Third, file timestamps may use a different resolution than
10726 operating system timestamps. Fourth, the operating system primitives
10727 used to update timestamps may employ yet a different resolution. For
10728 example, in theory a file system might use 10-microsecond resolution
10729 for access time and 100-nanosecond resolution for modification time,
10730 and the operating system might use nanosecond resolution for the
10731 current time and microsecond resolution for the primitive that
10732 @command{touch} uses to set a file's timestamp to an arbitrary value.
10734 @cindex permissions, for changing file timestamps
10735 When setting file timestamps to the current time, @command{touch} can
10736 change the timestamps for files that the user does not own but has
10737 write permission for. Otherwise, the user must own the files. Some
10738 older systems have a further restriction: the user must own the files
10739 unless both the access and modification times are being set to the
10742 Although @command{touch} provides options for changing two of the times---the
10743 times of last access and modification---of a file, there is actually
10744 a standard third one as well: the inode change time. This is often
10745 referred to as a file's @code{ctime}.
10746 The inode change time represents the time when the file's meta-information
10747 last changed. One common example of this is when the permissions of a
10748 file change. Changing the permissions doesn't access the file, so
10749 the atime doesn't change, nor does it modify the file, so the mtime
10750 doesn't change. Yet, something about the file itself has changed,
10751 and this must be noted somewhere. This is the job of the ctime field.
10752 This is necessary, so that, for example, a backup program can make a
10753 fresh copy of the file, including the new permissions value.
10754 Another operation that modifies a file's ctime without affecting
10755 the others is renaming. In any case, it is not possible, in normal
10756 operations, for a user to change the ctime field to a user-specified value.
10757 Some operating systems and file systems support a fourth time: the
10758 birth time, when the file was first created; by definition, this
10759 timestamp never changes.
10762 Time stamps assume the time zone rules specified by the @env{TZ}
10763 environment variable, or by the system default rules if @env{TZ} is
10764 not set. @xref{TZ Variable,, Specifying the Time Zone with @env{TZ},
10765 libc, The GNU C Library Reference Manual}.
10766 You can avoid ambiguities during
10767 daylight saving transitions by using UTC time stamps.
10769 The program accepts the following options. Also see @ref{Common options}.
10774 @itemx --time=atime
10775 @itemx --time=access
10779 @opindex atime@r{, changing}
10780 @opindex access @r{time, changing}
10781 @opindex use @r{time, changing}
10782 Change the access time only.
10787 @opindex --no-create
10788 Do not warn about or create files that do not exist.
10791 @itemx --date=@var{time}
10795 Use @var{time} instead of the current time. It can contain month names,
10796 time zones, @samp{am} and @samp{pm}, @samp{yesterday}, etc. For
10797 example, @option{--date="2004-02-27 14:19:13.489392193 +0530"}
10798 specifies the instant of time that is 489,392,193 nanoseconds after
10799 February 27, 2004 at 2:19:13 PM in a time zone that is 5 hours and 30
10800 minutes east of UTC@. @xref{Date input formats}.
10801 File systems that do not support high-resolution time stamps
10802 silently ignore any excess precision here.
10806 @cindex BSD @command{touch} compatibility
10807 Ignored; for compatibility with BSD versions of @command{touch}.
10810 @itemx --no-dereference
10812 @opindex --no-dereference
10813 @cindex symbolic links, changing time
10815 Attempt to change the timestamps of a symbolic link, rather than what
10816 the link refers to. When using this option, empty files are not
10817 created, but option @option{-c} must also be used to avoid warning
10818 about files that do not exist. Not all systems support changing the
10819 timestamps of symlinks, since underlying system support for this
10820 action was not required until POSIX 2008. Also, on some
10821 systems, the mere act of examining a symbolic link changes the access
10822 time, such that only changes to the modification time will persist
10823 long enough to be observable. When coupled with option @option{-r}, a
10824 reference timestamp is taken from a symbolic link rather than the file
10828 @itemx --time=mtime
10829 @itemx --time=modify
10832 @opindex mtime@r{, changing}
10833 @opindex modify @r{time, changing}
10834 Change the modification time only.
10836 @item -r @var{file}
10837 @itemx --reference=@var{file}
10839 @opindex --reference
10840 Use the times of the reference @var{file} instead of the current time.
10841 If this option is combined with the @option{--date=@var{time}}
10842 (@option{-d @var{time}}) option, the reference @var{file}'s time is
10843 the origin for any relative @var{time}s given, but is otherwise ignored.
10844 For example, @samp{-r foo -d '-5 seconds'} specifies a time stamp
10845 equal to five seconds before the corresponding time stamp for @file{foo}.
10846 If @var{file} is a symbolic link, the reference timestamp is taken
10847 from the target of the symlink, unless @option{-h} was also in effect.
10849 @item -t [[@var{cc}]@var{yy}]@var{mmddhhmm}[.@var{ss}]
10850 @cindex leap seconds
10851 Use the argument (optional four-digit or two-digit years, months,
10852 days, hours, minutes, optional seconds) instead of the current time.
10853 If the year is specified with only two digits, then @var{cc}
10854 is 20 for years in the range 0 @dots{} 68, and 19 for years in
10855 69 @dots{} 99. If no digits of the year are specified,
10856 the argument is interpreted as a date in the current year.
10857 On the atypical systems that support leap seconds, @var{ss} may be
10862 @vindex _POSIX2_VERSION
10863 On older systems, @command{touch} supports an obsolete syntax, as follows.
10864 If no timestamp is given with any of the @option{-d}, @option{-r}, or
10865 @option{-t} options, and if there are two or more @var{file}s and the
10866 first @var{file} is of the form @samp{@var{mmddhhmm}[@var{yy}]} and this
10867 would be a valid argument to the @option{-t} option (if the @var{yy}, if
10868 any, were moved to the front), and if the represented year
10869 is in the range 1969--1999, that argument is interpreted as the time
10870 for the other files instead of as a file name.
10871 This obsolete behavior can be enabled or disabled with the
10872 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
10873 conformance}), but portable scripts should avoid commands whose
10874 behavior depends on this variable.
10875 For example, use @samp{touch ./12312359 main.c} or @samp{touch -t
10876 12312359 main.c} rather than the ambiguous @samp{touch 12312359 main.c}.
10882 @chapter Disk usage
10886 No disk can hold an infinite amount of data. These commands report
10887 how much disk storage is in use or available, report other file and
10888 file status information, and write buffers to disk.
10891 * df invocation:: Report file system disk space usage.
10892 * du invocation:: Estimate file space usage.
10893 * stat invocation:: Report file or file system status.
10894 * sync invocation:: Synchronize memory and disk.
10895 * truncate invocation:: Shrink or extend the size of a file.
10899 @node df invocation
10900 @section @command{df}: Report file system disk space usage
10903 @cindex file system disk usage
10904 @cindex disk usage by file system
10906 @command{df} reports the amount of disk space used and available on
10907 file systems. Synopsis:
10910 df [@var{option}]@dots{} [@var{file}]@dots{}
10913 With no arguments, @command{df} reports the space used and available on all
10914 currently mounted file systems (of all types). Otherwise, @command{df}
10915 reports on the file system containing each argument @var{file}.
10917 Normally the disk space is printed in units of
10918 1024 bytes, but this can be overridden (@pxref{Block size}).
10919 Non-integer quantities are rounded up to the next higher unit.
10921 For bind mounts and without arguments, @command{df} only outputs the statistics
10922 for that device with the shortest mount point name in the list of file systems
10923 (@var{mtab}), i.e., it hides duplicate entries, unless the @option{-a} option is
10926 With the same logic, @command{df} elides a mount entry of a dummy pseude device
10927 if there is another mount entry of a real block device for that mount point with
10928 the same device number, e.g. the early-boot pseudo file system @samp{rootfs} is
10929 not shown per default when already the real root device has been mounted.
10931 @cindex disk device file
10932 @cindex device file, disk
10933 If an argument @var{file} is a disk device file containing a mounted
10934 file system, @command{df} shows the space available on that file system
10935 rather than on the file system containing the device node (i.e., the root
10936 file system). GNU @command{df} does not attempt to determine the
10938 on unmounted file systems, because on most kinds of systems doing so
10939 requires extremely nonportable intimate knowledge of file system
10942 The program accepts the following options. Also see @ref{Common options}.
10950 @cindex automounter file systems
10951 @cindex ignore file systems
10952 Include in the listing dummy file systems, which
10953 are omitted by default. Such file systems are typically special-purpose
10954 pseudo-file-systems, such as automounter entries.
10956 @item -B @var{size}
10957 @itemx --block-size=@var{size}
10959 @opindex --block-size
10960 @cindex file system sizes
10961 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10962 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10966 @cindex grand total of disk size, usage and available space
10967 Print a grand total of all arguments after all arguments have
10968 been processed. This can be used to find out the total disk size, usage
10969 and available space of all listed devices.
10971 For the grand total line, @command{df} prints @samp{"total"} into the
10972 @var{source} column, and @samp{"-"} into the @var{target} column.
10973 If there is no @var{source} column (see @option{--output}), then
10974 @command{df} prints @samp{"total"} into the @var{target} column,
10981 Equivalent to @option{--si}.
10987 @cindex inode usage
10988 List inode usage information instead of block usage. An inode (short
10989 for index node) contains information about a file such as its owner,
10990 permissions, timestamps, and location on the disk.
10994 @cindex kibibytes for file system sizes
10995 Print sizes in 1024-byte blocks, overriding the default block size
10996 (@pxref{Block size}).
10997 This option is equivalent to @option{--block-size=1K}.
11003 @cindex file system types, limiting output to certain
11004 Limit the listing to local file systems. By default, remote file systems
11009 @cindex file system space, retrieving old data more quickly
11010 Do not invoke the @code{sync} system call before getting any usage data.
11011 This may make @command{df} run significantly faster on systems with many
11012 disks, but on some systems (notably SunOS) the results may be slightly
11013 out of date. This is the default.
11016 @itemx @w{@kbd{--output}[=@var{field_list}]}
11018 Use the output format defined by @var{field_list}, or print all fields if
11019 @var{field_list} is omitted. In the latter case, the order of the columns
11020 conforms to the order of the field descriptions below.
11022 The use of the @option{--output} together with each of the options @option{-i},
11023 @option{-P}, and @option{-T} is mutually exclusive.
11025 FIELD_LIST is a comma-separated list of columns to be included in @command{df}'s
11026 output and therefore effectively controls the order of output columns.
11027 Each field can thus be used at the place of choice, but yet must only be
11030 Valid field names in the @var{field_list} are:
11033 The source of the mount point, usually a device.
11038 Total number of inodes.
11040 Number of used inodes.
11042 Number of available inodes.
11044 Percentage of @var{iused} divided by @var{itotal}.
11047 Total number of blocks.
11049 Number of used blocks.
11051 Number of available blocks.
11053 Percentage of @var{used} divided by @var{size}.
11059 The fields for block and inodes statistics are affected by the scaling
11060 options like @option{-h} as usual.
11062 The definition of the @var{field_list} can even be splitted among several
11063 @option{--output} uses.
11067 # Print the TARGET (i.e., the mount point) along with their percentage
11068 # statistic regarding the blocks and the inodes.
11069 df --out=target --output=pcent,ipcent
11071 # Print all available fields.
11077 @itemx --portability
11079 @opindex --portability
11080 @cindex one-line output format
11081 @cindex POSIX output format
11082 @cindex portable output format
11083 @cindex output format, portable
11084 Use the POSIX output format. This is like the default format except
11089 The information about each file system is always printed on exactly
11090 one line; a mount device is never put on a line by itself. This means
11091 that if the mount device name is more than 20 characters long (e.g., for
11092 some network mounts), the columns are misaligned.
11095 The labels in the header output line are changed to conform to POSIX.
11098 The default block size and output format are unaffected by the
11099 @env{DF_BLOCK_SIZE}, @env{BLOCK_SIZE} and @env{BLOCKSIZE} environment
11100 variables. However, the default block size is still affected by
11101 @env{POSIXLY_CORRECT}: it is 512 if @env{POSIXLY_CORRECT} is set, 1024
11102 otherwise. @xref{Block size}.
11109 @cindex file system space, retrieving current data more slowly
11110 Invoke the @code{sync} system call before getting any usage data. On
11111 some systems (notably SunOS), doing this yields more up to date results,
11112 but in general this option makes @command{df} much slower, especially when
11113 there are many or very busy file systems.
11115 @item -t @var{fstype}
11116 @itemx --type=@var{fstype}
11119 @cindex file system types, limiting output to certain
11120 Limit the listing to file systems of type @var{fstype}. Multiple
11121 file system types can be specified by giving multiple @option{-t} options.
11122 By default, nothing is omitted.
11125 @itemx --print-type
11127 @opindex --print-type
11128 @cindex file system types, printing
11129 Print each file system's type. The types printed here are the same ones
11130 you can include or exclude with @option{-t} and @option{-x}. The particular
11131 types printed are whatever is supported by the system. Here are some of
11132 the common names (this list is certainly not exhaustive):
11137 @cindex NFS file system type
11138 An NFS file system, i.e., one mounted over a network from another
11139 machine. This is the one type name which seems to be used uniformly by
11142 @item 4.2@r{, }ufs@r{, }efs@dots{}
11143 @cindex Linux file system types
11144 @cindex local file system types
11145 @opindex 4.2 @r{file system type}
11146 @opindex ufs @r{file system type}
11147 @opindex efs @r{file system type}
11148 A file system on a locally-mounted hard disk. (The system might even
11149 support more than one type here; Linux does.)
11151 @item hsfs@r{, }cdfs
11152 @cindex CD-ROM file system type
11153 @cindex High Sierra file system
11154 @opindex hsfs @r{file system type}
11155 @opindex cdfs @r{file system type}
11156 A file system on a CD-ROM drive. HP-UX uses @samp{cdfs}, most other
11157 systems use @samp{hsfs} (@samp{hs} for ``High Sierra'').
11160 @cindex PC file system
11161 @cindex DOS file system
11162 @cindex MS-DOS file system
11163 @cindex diskette file system
11165 An MS-DOS file system, usually on a diskette.
11169 @item -x @var{fstype}
11170 @itemx --exclude-type=@var{fstype}
11172 @opindex --exclude-type
11173 Limit the listing to file systems not of type @var{fstype}.
11174 Multiple file system types can be eliminated by giving multiple
11175 @option{-x} options. By default, no file system types are omitted.
11178 Ignored; for compatibility with System V versions of @command{df}.
11183 Failure includes the case where no output is generated, so you can
11184 inspect the exit status of a command like @samp{df -t ext3 -t reiserfs
11185 @var{dir}} to test whether @var{dir} is on a file system of type
11186 @samp{ext3} or @samp{reiserfs}.
11188 Since the list of file systems (@var{mtab}) is needed to determine the
11189 file system type, failure includes the cases when that list cannot
11190 be read and one or more of the options @option{-a}, @option{-l}, @option{-t}
11191 or @option{-x} is used together with a file name argument.
11194 @node du invocation
11195 @section @command{du}: Estimate file space usage
11198 @cindex file space usage
11199 @cindex disk usage for files
11201 @command{du} reports the amount of disk space used by the specified files
11202 and for each subdirectory (of directory arguments). Synopsis:
11205 du [@var{option}]@dots{} [@var{file}]@dots{}
11208 With no arguments, @command{du} reports the disk space for the current
11209 directory. Normally the disk space is printed in units of
11210 1024 bytes, but this can be overridden (@pxref{Block size}).
11211 Non-integer quantities are rounded up to the next higher unit.
11213 If two or more hard links point to the same file, only one of the hard
11214 links is counted. The @var{file} argument order affects which links
11215 are counted, and changing the argument order may change the numbers
11216 that @command{du} outputs.
11218 The program accepts the following options. Also see @ref{Common options}.
11228 Show counts for all files, not just directories.
11230 @item --apparent-size
11231 @opindex --apparent-size
11232 Print apparent sizes, rather than disk usage. The apparent size of a
11233 file is the number of bytes reported by @code{wc -c} on regular files,
11234 or more generally, @code{ls -l --block-size=1} or @code{stat --format=%s}.
11235 For example, a file containing the word @samp{zoo} with no newline would,
11236 of course, have an apparent size of 3. Such a small file may require
11237 anywhere from 0 to 16 KiB or more of disk space, depending on
11238 the type and configuration of the file system on which the file resides.
11239 However, a sparse file created with this command:
11242 dd bs=1 seek=2GiB if=/dev/null of=big
11246 has an apparent size of 2 GiB, yet on most modern
11247 systems, it actually uses almost no disk space.
11249 @item -B @var{size}
11250 @itemx --block-size=@var{size}
11252 @opindex --block-size
11254 Scale sizes by @var{size} before printing them (@pxref{Block size}).
11255 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
11261 Equivalent to @code{--apparent-size --block-size=1}.
11267 @cindex grand total of disk space
11268 Print a grand total of all arguments after all arguments have
11269 been processed. This can be used to find out the total disk usage of
11270 a given set of files or directories.
11273 @itemx --dereference-args
11275 @opindex --dereference-args
11276 Dereference symbolic links that are command line arguments.
11277 Does not affect other symbolic links. This is helpful for finding
11278 out the disk usage of directories, such as @file{/usr/tmp}, which
11279 are often symbolic links.
11281 @item -d @var{depth}
11282 @itemx --max-depth=@var{depth}
11283 @opindex -d @var{depth}
11284 @opindex --max-depth=@var{depth}
11285 @cindex limiting output of @command{du}
11286 Show the total for each directory (and file if --all) that is at
11287 most MAX_DEPTH levels down from the root of the hierarchy. The root
11288 is at level 0, so @code{du --max-depth=0} is equivalent to @code{du -s}.
11290 @c --files0-from=FILE
11291 @filesZeroFromOption{du,, with the @option{--total} (@option{-c}) option}
11295 Equivalent to @option{--dereference-args} (@option{-D}).
11301 @cindex kibibytes for file sizes
11302 Print sizes in 1024-byte blocks, overriding the default block size
11303 (@pxref{Block size}).
11304 This option is equivalent to @option{--block-size=1K}.
11307 @itemx --dereference
11309 @opindex --dereference
11310 @cindex symbolic links, dereferencing in @command{du}
11311 Dereference symbolic links (show the disk space used by the file
11312 or directory that the link points to instead of the space used by
11316 @itemx --count-links
11318 @opindex --count-links
11319 @cindex hard links, counting in @command{du}
11320 Count the size of all files, even if they have appeared already (as a
11325 @cindex mebibytes for file sizes
11326 Print sizes in 1,048,576-byte blocks, overriding the default block size
11327 (@pxref{Block size}).
11328 This option is equivalent to @option{--block-size=1M}.
11331 @itemx --no-dereference
11333 @opindex --no-dereference
11334 @cindex symbolic links, dereferencing in @command{du}
11335 For each symbolic links encountered by @command{du},
11336 consider the disk space used by the symbolic link.
11339 @itemx --separate-dirs
11341 @opindex --separate-dirs
11342 Normally, in the output of @command{du} (when not using @option{--summarize}),
11343 the size listed next to a directory name, @var{d}, represents the sum
11344 of sizes of all entries beneath @var{d} as well as the size of @var{d} itself.
11345 With @option{--separate-dirs}, the size reported for a directory name,
11346 @var{d}, is merely the @code{stat.st_size}-derived size of the directory
11354 @opindex --summarize
11355 Display only a total for each argument.
11357 @item -t @var{size}
11358 @itemx --threshold=@var{size}
11360 @opindex --threshold
11361 Exclude entries based on a given @var{size} (@pxref{Block size}).
11363 If @var{size} is positive, then @command{du} will only print entries with a size
11364 greater than or equal to that.
11366 If @var{size} is negative, then @command{du} will only print entries with a size
11367 smaller than or equal to that.
11369 Although GNU @command{find} can be used to find files of a certain size,
11370 @command{du}'s @option{--threshold} option can be used to also filter
11371 directories based on a given size.
11373 Please note that the @option{--threshold} option can be combined with the
11374 @option{--apparent-size} option, and in this case would elide entries based on
11377 Here's how you would use @option{--threshold} to find directories with a size
11378 greater than or equal to 200 megabytes:
11381 du --threshold=200MB
11384 Here's how you would use @option{--threshold} to find directories and files -
11385 note the @option{-a} - with an apparent size smaller than or equal to 500 bytes:
11388 du -a -t -500 --apparent-size
11394 @cindex last modified dates, displaying in @command{du}
11395 Show time of the most recent modification of any file in the directory,
11396 or any of its subdirectories.
11399 @itemx --time=status
11402 @opindex ctime@r{, show the most recent}
11403 @opindex status time@r{, show the most recent}
11404 @opindex use time@r{, show the most recent}
11405 Show the most recent status change time (the @samp{ctime} in the inode) of
11406 any file in the directory, instead of the modification time.
11409 @itemx --time=access
11411 @opindex atime@r{, show the most recent}
11412 @opindex access time@r{, show the most recent}
11413 Show the most recent access time (the @samp{atime} in the inode) of
11414 any file in the directory, instead of the modification time.
11416 @item --time-style=@var{style}
11417 @opindex --time-style
11419 List timestamps in style @var{style}. This option has an effect only if
11420 the @option{--time} option is also specified. The @var{style} should
11421 be one of the following:
11424 @item +@var{format}
11426 List timestamps using @var{format}, where @var{format} is interpreted
11427 like the format argument of @command{date} (@pxref{date invocation}).
11428 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
11429 @command{du} to list timestamps like @samp{2002-03-30 23:45:56}. As
11430 with @command{date}, @var{format}'s interpretation is affected by the
11431 @env{LC_TIME} locale category.
11434 List timestamps in full using ISO 8601 date, time, and time zone
11435 format with nanosecond precision, e.g., @samp{2002-03-30
11436 23:45:56.477817180 -0700}. This style is equivalent to
11437 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
11440 List ISO 8601 date and time in minutes, e.g.,
11441 @samp{2002-03-30 23:45}. These timestamps are shorter than
11442 @samp{full-iso} timestamps, and are usually good enough for everyday
11443 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
11446 List ISO 8601 dates for timestamps, e.g., @samp{2002-03-30}.
11447 This style is equivalent to @samp{+%Y-%m-%d}.
11451 You can specify the default value of the @option{--time-style} option
11452 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
11453 the default style is @samp{long-iso}. For compatibility with @command{ls},
11454 if @env{TIME_STYLE} begins with @samp{+} and contains a newline,
11455 the newline and any later characters are ignored; if @env{TIME_STYLE}
11456 begins with @samp{posix-} the @samp{posix-} is ignored; and if
11457 @env{TIME_STYLE} is @samp{locale} it is ignored.
11459 @item -X @var{file}
11460 @itemx --exclude-from=@var{file}
11461 @opindex -X @var{file}
11462 @opindex --exclude-from=@var{file}
11463 @cindex excluding files from @command{du}
11464 Like @option{--exclude}, except take the patterns to exclude from @var{file},
11465 one per line. If @var{file} is @samp{-}, take the patterns from standard
11468 @item --exclude=@var{pattern}
11469 @opindex --exclude=@var{pattern}
11470 @cindex excluding files from @command{du}
11471 When recursing, skip subdirectories or files matching @var{pattern}.
11472 For example, @code{du --exclude='*.o'} excludes files whose names
11476 @itemx --one-file-system
11478 @opindex --one-file-system
11479 @cindex one file system, restricting @command{du} to
11480 Skip directories that are on different file systems from the one that
11481 the argument being processed is on.
11485 @cindex NFS mounts from BSD to HP-UX
11486 On BSD systems, @command{du} reports sizes that are half the correct
11487 values for files that are NFS-mounted from HP-UX systems. On HP-UX
11488 systems, it reports sizes that are twice the correct values for
11489 files that are NFS-mounted from BSD systems. This is due to a flaw
11490 in HP-UX; it also affects the HP-UX @command{du} program.
11495 @node stat invocation
11496 @section @command{stat}: Report file or file system status
11499 @cindex file status
11500 @cindex file system status
11502 @command{stat} displays information about the specified file(s). Synopsis:
11505 stat [@var{option}]@dots{} [@var{file}]@dots{}
11508 With no option, @command{stat} reports all information about the given files.
11509 But it also can be used to report the information of the file systems the
11510 given files are located on. If the files are links, @command{stat} can
11511 also give information about the files the links point to.
11513 @mayConflictWithShellBuiltIn{stat}
11518 @itemx --dereference
11520 @opindex --dereference
11521 @cindex symbolic links, dereferencing in @command{stat}
11522 Change how @command{stat} treats symbolic links.
11523 With this option, @command{stat} acts on the file referenced
11524 by each symbolic link argument.
11525 Without it, @command{stat} acts on any symbolic link argument directly.
11528 @itemx --file-system
11530 @opindex --file-system
11531 @cindex file systems
11532 Report information about the file systems where the given files are located
11533 instead of information about the files themselves.
11534 This option implies the @option{-L} option.
11537 @itemx --format=@var{format}
11539 @opindex --format=@var{format}
11540 @cindex output format
11541 Use @var{format} rather than the default format.
11542 @var{format} is automatically newline-terminated, so
11543 running a command like the following with two or more @var{file}
11544 operands produces a line of output for each operand:
11546 $ stat --format=%d:%i / /usr
11551 @item --printf=@var{format}
11552 @opindex --printf=@var{format}
11553 @cindex output format
11554 Use @var{format} rather than the default format.
11555 Like @option{--format}, but interpret backslash escapes,
11556 and do not output a mandatory trailing newline.
11557 If you want a newline, include @samp{\n} in the @var{format}.
11558 Here's how you would use @option{--printf} to print the device
11559 and inode numbers of @file{/} and @file{/usr}:
11561 $ stat --printf='%d:%i\n' / /usr
11570 @cindex terse output
11571 Print the information in terse form, suitable for parsing by other programs.
11575 The valid @var{format} directives for files with @option{--format} and
11576 @option{--printf} are:
11579 @item %a - Access rights in octal
11580 @item %A - Access rights in human readable form
11581 @item %b - Number of blocks allocated (see @samp{%B})
11582 @item %B - The size in bytes of each block reported by @samp{%b}
11583 @item %C - The SELinux security context of a file, if available
11584 @item %d - Device number in decimal
11585 @item %D - Device number in hex
11586 @item %f - Raw mode in hex
11587 @item %F - File type
11588 @item %g - Group ID of owner
11589 @item %G - Group name of owner
11590 @item %h - Number of hard links
11591 @item %i - Inode number
11592 @item %m - Mount point (See note below)
11593 @item %n - File name
11594 @item %N - Quoted file name with dereference if symbolic link
11595 @item %o - Optimal I/O transfer size hint
11596 @item %s - Total size, in bytes
11597 @item %t - Major device type in hex
11598 @item %T - Minor device type in hex
11599 @item %u - User ID of owner
11600 @item %U - User name of owner
11601 @item %w - Time of file birth, or @samp{-} if unknown
11602 @item %W - Time of file birth as seconds since Epoch, or @samp{0}
11603 @item %x - Time of last access
11604 @item %X - Time of last access as seconds since Epoch
11605 @item %y - Time of last modification
11606 @item %Y - Time of last modification as seconds since Epoch
11607 @item %z - Time of last change
11608 @item %Z - Time of last change as seconds since Epoch
11611 The @samp{%W}, @samp{%X}, @samp{%Y}, and @samp{%Z} formats accept a
11612 precision preceded by a period to specify the number of digits to
11613 print after the decimal point. For example, @samp{%.3X} outputs the
11614 last access time to millisecond precision. If a period is given but no
11615 precision, @command{stat} uses 9 digits, so @samp{%.X} is equivalent to
11616 @samp{%.9X}@. When discarding excess precision, time stamps are truncated
11617 toward minus infinity.
11621 $ stat -c '[%015Y]' /usr
11624 $ stat -c '[%15Y]' /usr
11626 $ stat -c '[%-15Y]' /usr
11629 $ stat -c '[%.3Y]' /usr
11631 $ stat -c '[%.Y]' /usr
11632 [1288929712.114951834]
11635 The mount point printed by @samp{%m} is similar to that output
11636 by @command{df}, except that:
11639 stat does not dereference symlinks by default
11640 (unless @option{-L} is specified)
11642 stat does not search for specified device nodes in the
11643 file system list, instead operating on them directly
11646 stat outputs the alias for a bind mounted file, rather than
11647 the initial mount point of its backing device.
11648 One can recursively call stat until there is no change in output,
11649 to get the current base mount point
11652 When listing file system information (@option{--file-system} (@option{-f})),
11653 you must use a different set of @var{format} directives:
11656 @item %a - Free blocks available to non-super-user
11657 @item %b - Total data blocks in file system
11658 @item %c - Total file nodes in file system
11659 @item %d - Free file nodes in file system
11660 @item %f - Free blocks in file system
11661 @item %i - File System ID in hex
11662 @item %l - Maximum length of file names
11663 @item %n - File name
11664 @item %s - Block size (for faster transfers)
11665 @item %S - Fundamental block size (for block counts)
11666 @item %t - Type in hex
11667 @item %T - Type in human readable form
11671 Time stamps are listed according to the time zone rules specified by
11672 the @env{TZ} environment variable, or by the system default rules if
11673 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
11674 with @env{TZ}, libc, The GNU C Library Reference Manual}.
11679 @node sync invocation
11680 @section @command{sync}: Synchronize data on disk with memory
11683 @cindex synchronize disk and memory
11685 @cindex superblock, writing
11686 @cindex inodes, written buffered
11687 @command{sync} writes any data buffered in memory out to disk. This can
11688 include (but is not limited to) modified superblocks, modified inodes,
11689 and delayed reads and writes. This must be implemented by the kernel;
11690 The @command{sync} program does nothing but exercise the @code{sync} system
11693 @cindex crashes and corruption
11694 The kernel keeps data in memory to avoid doing (relatively slow) disk
11695 reads and writes. This improves performance, but if the computer
11696 crashes, data may be lost or the file system corrupted as a
11697 result. The @command{sync} command ensures everything in memory
11698 is written to disk.
11700 Any arguments are ignored, except for a lone @option{--help} or
11701 @option{--version} (@pxref{Common options}).
11706 @node truncate invocation
11707 @section @command{truncate}: Shrink or extend the size of a file
11710 @cindex truncating, file sizes
11712 @command{truncate} shrinks or extends the size of each @var{file} to the
11713 specified size. Synopsis:
11716 truncate @var{option}@dots{} @var{file}@dots{}
11719 @cindex files, creating
11720 Any @var{file} that does not exist is created.
11722 @cindex sparse files, creating
11723 @cindex holes, creating files with
11724 If a @var{file} is larger than the specified size, the extra data is lost.
11725 If a @var{file} is shorter, it is extended and the extended part (or hole)
11726 reads as zero bytes.
11728 The program accepts the following options. Also see @ref{Common options}.
11735 @opindex --no-create
11736 Do not create files that do not exist.
11741 @opindex --io-blocks
11742 Treat @var{size} as number of I/O blocks of the @var{file} rather than bytes.
11744 @item -r @var{rfile}
11745 @itemx --reference=@var{rfile}
11747 @opindex --reference
11748 Base the size of each @var{file} on the size of @var{rfile}.
11750 @item -s @var{size}
11751 @itemx --size=@var{size}
11754 Set or adjust the size of each @var{file} according to @var{size}.
11755 @multiplierSuffixesNoBlocks{size}
11757 @var{size} may also be prefixed by one of the following to adjust
11758 the size of each @var{file} based on their current size:
11760 @samp{+} => extend by
11761 @samp{-} => reduce by
11762 @samp{<} => at most
11763 @samp{>} => at least
11764 @samp{/} => round down to multiple of
11765 @samp{%} => round up to multiple of
11773 @node Printing text
11774 @chapter Printing text
11776 @cindex printing text, commands for
11777 @cindex commands for printing text
11779 This section describes commands that display text strings.
11782 * echo invocation:: Print a line of text.
11783 * printf invocation:: Format and print data.
11784 * yes invocation:: Print a string until interrupted.
11788 @node echo invocation
11789 @section @command{echo}: Print a line of text
11792 @cindex displaying text
11793 @cindex printing text
11794 @cindex text, displaying
11795 @cindex arbitrary text, displaying
11797 @command{echo} writes each given @var{string} to standard output, with a
11798 space between each and a newline after the last one. Synopsis:
11801 echo [@var{option}]@dots{} [@var{string}]@dots{}
11804 @mayConflictWithShellBuiltIn{echo}
11806 The program accepts the following options. Also see @ref{Common options}.
11807 Options must precede operands, and the normally-special argument
11808 @samp{--} has no special meaning and is treated like any other
11814 Do not output the trailing newline.
11818 @cindex backslash escapes
11819 Enable interpretation of the following backslash-escaped characters in
11828 produce no further output
11844 the eight-bit value that is the octal number @var{nnn}
11845 (zero to three octal digits), if @var{nnn} is
11846 a nine-bit value, the ninth bit is ignored
11848 the eight-bit value that is the octal number @var{nnn}
11849 (one to three octal digits), if @var{nnn} is
11850 a nine-bit value, the ninth bit is ignored
11852 the eight-bit value that is the hexadecimal number @var{hh}
11853 (one or two hexadecimal digits)
11858 @cindex backslash escapes
11859 Disable interpretation of backslash escapes in each @var{string}.
11860 This is the default. If @option{-e} and @option{-E} are both
11861 specified, the last one given takes effect.
11865 @vindex POSIXLY_CORRECT
11866 If the @env{POSIXLY_CORRECT} environment variable is set, then when
11867 @command{echo}'s first argument is not @option{-n} it outputs
11868 option-like arguments instead of treating them as options. For
11869 example, @code{echo -ne hello} outputs @samp{-ne hello} instead of
11870 plain @samp{hello}.
11872 POSIX does not require support for any options, and says
11873 that the behavior of @command{echo} is implementation-defined if any
11874 @var{string} contains a backslash or if the first argument is
11875 @option{-n}. Portable programs can use the @command{printf} command
11876 if they need to omit trailing newlines or output control characters or
11877 backslashes. @xref{printf invocation}.
11882 @node printf invocation
11883 @section @command{printf}: Format and print data
11886 @command{printf} does formatted printing of text. Synopsis:
11889 printf @var{format} [@var{argument}]@dots{}
11892 @command{printf} prints the @var{format} string, interpreting @samp{%}
11893 directives and @samp{\} escapes to format numeric and string arguments
11894 in a way that is mostly similar to the C @samp{printf} function.
11895 @xref{Output Conversion Syntax,, @command{printf} format directives,
11896 libc, The GNU C Library Reference Manual}, for details.
11897 The differences are listed below.
11899 @mayConflictWithShellBuiltIn{printf}
11904 The @var{format} argument is reused as necessary to convert all the
11905 given @var{argument}s. For example, the command @samp{printf %s a b}
11909 Missing @var{argument}s are treated as null strings or as zeros,
11910 depending on whether the context expects a string or a number. For
11911 example, the command @samp{printf %sx%d} prints @samp{x0}.
11915 An additional escape, @samp{\c}, causes @command{printf} to produce no
11916 further output. For example, the command @samp{printf 'A%sC\cD%sF' B
11917 E} prints @samp{ABC}.
11920 The hexadecimal escape sequence @samp{\x@var{hh}} has at most two
11921 digits, as opposed to C where it can have an unlimited number of
11922 digits. For example, the command @samp{printf '\x07e'} prints two
11923 bytes, whereas the C statement @samp{printf ("\x07e")} prints just
11928 @command{printf} has an additional directive, @samp{%b}, which prints its
11929 argument string with @samp{\} escapes interpreted in the same way as in
11930 the @var{format} string, except that octal escapes are of the form
11931 @samp{\0@var{ooo}} where @var{ooo} is 0 to 3 octal digits. If
11932 @samp{\@var{ooo}} is nine-bit value, ignore the ninth bit.
11933 If a precision is also given, it limits the number of bytes printed
11934 from the converted string.
11937 Numeric arguments must be single C constants, possibly with leading
11938 @samp{+} or @samp{-}. For example, @samp{printf %.4d -3} outputs
11942 @vindex POSIXLY_CORRECT
11943 If the leading character of a numeric argument is @samp{"} or @samp{'}
11944 then its value is the numeric value of the immediately following
11945 character. Any remaining characters are silently ignored if the
11946 @env{POSIXLY_CORRECT} environment variable is set; otherwise, a
11947 warning is printed. For example, @samp{printf "%d" "'a"} outputs
11948 @samp{97} on hosts that use the ASCII character set, since
11949 @samp{a} has the numeric value 97 in ASCII.
11954 A floating-point argument must use a period before any fractional
11955 digits, but is printed according to the @env{LC_NUMERIC} category of the
11956 current locale. For example, in a locale whose radix character is a
11957 comma, the command @samp{printf %g 3.14} outputs @samp{3,14} whereas
11958 the command @samp{printf %g 3,14} is an error.
11959 @xref{Floating point}.
11963 @command{printf} interprets @samp{\@var{ooo}} in @var{format} as an octal number
11964 (if @var{ooo} is 1 to 3 octal digits) specifying a byte to print,
11965 and @samp{\x@var{hh}} as a hexadecimal number (if @var{hh} is 1 to 2 hex
11966 digits) specifying a character to print.
11967 Note however that when @samp{\@var{ooo}} specifies a number larger than 255,
11968 @command{printf} ignores the ninth bit.
11969 For example, @samp{printf '\400'} is equivalent to @samp{printf '\0'}.
11974 @cindex ISO/IEC 10646
11976 @command{printf} interprets two character syntaxes introduced in
11978 @samp{\u} for 16-bit Unicode (ISO/IEC 10646)
11979 characters, specified as
11980 four hexadecimal digits @var{hhhh}, and @samp{\U} for 32-bit Unicode
11981 characters, specified as eight hexadecimal digits @var{hhhhhhhh}.
11982 @command{printf} outputs the Unicode characters
11983 according to the @env{LC_CTYPE} locale. Unicode characters in the ranges
11984 U+0000@dots{}U+009F, U+D800@dots{}U+DFFF cannot be specified by this syntax,
11985 except for U+0024 ($), U+0040 (@@), and U+0060 (@`).
11987 The processing of @samp{\u} and @samp{\U} requires a full-featured
11988 @code{iconv} facility. It is activated on systems with glibc 2.2 (or newer),
11989 or when @code{libiconv} is installed prior to this package. Otherwise
11990 @samp{\u} and @samp{\U} will print as-is.
11992 The only options are a lone @option{--help} or
11993 @option{--version}. @xref{Common options}.
11994 Options must precede operands.
11996 The Unicode character syntaxes are useful for writing strings in a locale
11997 independent way. For example, a string containing the Euro currency symbol
12000 $ env printf '\u20AC 14.95'
12004 will be output correctly in all locales supporting the Euro symbol
12005 (ISO-8859-15, UTF-8, and others). Similarly, a Chinese string
12008 $ env printf '\u4e2d\u6587'
12012 will be output correctly in all Chinese locales (GB2312, BIG5, UTF-8, etc).
12014 Note that in these examples, the @command{printf} command has been
12015 invoked via @command{env} to ensure that we run the program found via
12016 your shell's search path, and not a shell alias or a built-in function.
12018 For larger strings, you don't need to look up the hexadecimal code
12019 values of each character one by one. ASCII characters mixed with \u
12020 escape sequences is also known as the JAVA source file encoding. You can
12021 use GNU recode 3.5c (or newer) to convert strings to this encoding. Here
12022 is how to convert a piece of text into a shell script which will output
12023 this text in a locale-independent way:
12026 $ LC_CTYPE=zh_CN.big5 /usr/local/bin/printf \
12027 '\u4e2d\u6587\n' > sample.txt
12028 $ recode BIG5..JAVA < sample.txt \
12029 | sed -e "s|^|/usr/local/bin/printf '|" -e "s|$|\\\\n'|" \
12036 @node yes invocation
12037 @section @command{yes}: Print a string until interrupted
12040 @cindex repeated output of a string
12042 @command{yes} prints the command line arguments, separated by spaces and
12043 followed by a newline, forever until it is killed. If no arguments are
12044 given, it prints @samp{y} followed by a newline forever until killed.
12046 Upon a write error, @command{yes} exits with status @samp{1}.
12048 The only options are a lone @option{--help} or @option{--version}.
12049 To output an argument that begins with
12050 @samp{-}, precede it with @option{--}, e.g., @samp{yes -- --help}.
12051 @xref{Common options}.
12055 @chapter Conditions
12058 @cindex commands for exit status
12059 @cindex exit status commands
12061 This section describes commands that are primarily useful for their exit
12062 status, rather than their output. Thus, they are often used as the
12063 condition of shell @code{if} statements, or as the last command in a
12067 * false invocation:: Do nothing, unsuccessfully.
12068 * true invocation:: Do nothing, successfully.
12069 * test invocation:: Check file types and compare values.
12070 * expr invocation:: Evaluate expressions.
12074 @node false invocation
12075 @section @command{false}: Do nothing, unsuccessfully
12078 @cindex do nothing, unsuccessfully
12079 @cindex failure exit status
12080 @cindex exit status of @command{false}
12082 @command{false} does nothing except return an exit status of 1, meaning
12083 @dfn{failure}. It can be used as a place holder in shell scripts
12084 where an unsuccessful command is needed.
12085 In most modern shells, @command{false} is a built-in command, so when
12086 you use @samp{false} in a script, you're probably using the built-in
12087 command, not the one documented here.
12089 @command{false} honors the @option{--help} and @option{--version} options.
12091 This version of @command{false} is implemented as a C program, and is thus
12092 more secure and faster than a shell script implementation, and may safely
12093 be used as a dummy shell for the purpose of disabling accounts.
12095 Note that @command{false} (unlike all other programs documented herein)
12096 exits unsuccessfully, even when invoked with
12097 @option{--help} or @option{--version}.
12099 Portable programs should not assume that the exit status of
12100 @command{false} is 1, as it is greater than 1 on some
12104 @node true invocation
12105 @section @command{true}: Do nothing, successfully
12108 @cindex do nothing, successfully
12110 @cindex successful exit
12111 @cindex exit status of @command{true}
12113 @command{true} does nothing except return an exit status of 0, meaning
12114 @dfn{success}. It can be used as a place holder in shell scripts
12115 where a successful command is needed, although the shell built-in
12116 command @code{:} (colon) may do the same thing faster.
12117 In most modern shells, @command{true} is a built-in command, so when
12118 you use @samp{true} in a script, you're probably using the built-in
12119 command, not the one documented here.
12121 @command{true} honors the @option{--help} and @option{--version} options.
12123 Note, however, that it is possible to cause @command{true}
12124 to exit with nonzero status: with the @option{--help} or @option{--version}
12125 option, and with standard
12126 output already closed or redirected to a file that evokes an I/O error.
12127 For example, using a Bourne-compatible shell:
12130 $ ./true --version >&-
12131 ./true: write error: Bad file number
12132 $ ./true --version > /dev/full
12133 ./true: write error: No space left on device
12136 This version of @command{true} is implemented as a C program, and is thus
12137 more secure and faster than a shell script implementation, and may safely
12138 be used as a dummy shell for the purpose of disabling accounts.
12140 @node test invocation
12141 @section @command{test}: Check file types and compare values
12144 @cindex check file types
12145 @cindex compare values
12146 @cindex expression evaluation
12148 @command{test} returns a status of 0 (true) or 1 (false) depending on the
12149 evaluation of the conditional expression @var{expr}. Each part of the
12150 expression must be a separate argument.
12152 @command{test} has file status checks, string operators, and numeric
12153 comparison operators.
12155 @command{test} has an alternate form that uses opening and closing
12156 square brackets instead a leading @samp{test}. For example, instead
12157 of @samp{test -d /}, you can write @samp{[ -d / ]}. The square
12158 brackets must be separate arguments; for example, @samp{[-d /]} does
12159 not have the desired effect. Since @samp{test @var{expr}} and @samp{[
12160 @var{expr} ]} have the same meaning, only the former form is discussed
12166 test @var{expression}
12168 [ @var{expression} ]
12173 @mayConflictWithShellBuiltIn{test}
12175 If @var{expression} is omitted, @command{test} returns false.
12176 If @var{expression} is a single argument,
12177 @command{test} returns false if the argument is null and true
12178 otherwise. The argument
12179 can be any string, including strings like @samp{-d}, @samp{-1},
12180 @samp{--}, @samp{--help}, and @samp{--version} that most other
12181 programs would treat as options. To get help and version information,
12182 invoke the commands @samp{[ --help} and @samp{[ --version}, without
12183 the usual closing brackets. @xref{Common options}.
12185 @cindex exit status of @command{test}
12189 0 if the expression is true,
12190 1 if the expression is false,
12191 2 if an error occurred.
12195 * File type tests:: -[bcdfhLpSt]
12196 * Access permission tests:: -[gkruwxOG]
12197 * File characteristic tests:: -e -s -nt -ot -ef
12198 * String tests:: -z -n = == !=
12199 * Numeric tests:: -eq -ne -lt -le -gt -ge
12200 * Connectives for test:: ! -a -o
12204 @node File type tests
12205 @subsection File type tests
12207 @cindex file type tests
12209 These options test for particular types of files. (Everything's a file,
12210 but not all files are the same!)
12214 @item -b @var{file}
12216 @cindex block special check
12217 True if @var{file} exists and is a block special device.
12219 @item -c @var{file}
12221 @cindex character special check
12222 True if @var{file} exists and is a character special device.
12224 @item -d @var{file}
12226 @cindex directory check
12227 True if @var{file} exists and is a directory.
12229 @item -f @var{file}
12231 @cindex regular file check
12232 True if @var{file} exists and is a regular file.
12234 @item -h @var{file}
12235 @itemx -L @var{file}
12238 @cindex symbolic link check
12239 True if @var{file} exists and is a symbolic link.
12240 Unlike all other file-related tests, this test does not dereference
12241 @var{file} if it is a symbolic link.
12243 @item -p @var{file}
12245 @cindex named pipe check
12246 True if @var{file} exists and is a named pipe.
12248 @item -S @var{file}
12250 @cindex socket check
12251 True if @var{file} exists and is a socket.
12255 @cindex terminal check
12256 True if @var{fd} is a file descriptor that is associated with a
12262 @node Access permission tests
12263 @subsection Access permission tests
12265 @cindex access permission tests
12266 @cindex permission tests
12268 These options test for particular access permissions.
12272 @item -g @var{file}
12274 @cindex set-group-ID check
12275 True if @var{file} exists and has its set-group-ID bit set.
12277 @item -k @var{file}
12279 @cindex sticky bit check
12280 True if @var{file} exists and has its @dfn{sticky} bit set.
12282 @item -r @var{file}
12284 @cindex readable file check
12285 True if @var{file} exists and read permission is granted.
12287 @item -u @var{file}
12289 @cindex set-user-ID check
12290 True if @var{file} exists and has its set-user-ID bit set.
12292 @item -w @var{file}
12294 @cindex writable file check
12295 True if @var{file} exists and write permission is granted.
12297 @item -x @var{file}
12299 @cindex executable file check
12300 True if @var{file} exists and execute permission is granted
12301 (or search permission, if it is a directory).
12303 @item -O @var{file}
12305 @cindex owned by effective user ID check
12306 True if @var{file} exists and is owned by the current effective user ID.
12308 @item -G @var{file}
12310 @cindex owned by effective group ID check
12311 True if @var{file} exists and is owned by the current effective group ID.
12315 @node File characteristic tests
12316 @subsection File characteristic tests
12318 @cindex file characteristic tests
12320 These options test other file characteristics.
12324 @item -e @var{file}
12326 @cindex existence-of-file check
12327 True if @var{file} exists.
12329 @item -s @var{file}
12331 @cindex nonempty file check
12332 True if @var{file} exists and has a size greater than zero.
12334 @item @var{file1} -nt @var{file2}
12336 @cindex newer-than file check
12337 True if @var{file1} is newer (according to modification date) than
12338 @var{file2}, or if @var{file1} exists and @var{file2} does not.
12340 @item @var{file1} -ot @var{file2}
12342 @cindex older-than file check
12343 True if @var{file1} is older (according to modification date) than
12344 @var{file2}, or if @var{file2} exists and @var{file1} does not.
12346 @item @var{file1} -ef @var{file2}
12348 @cindex same file check
12349 @cindex hard link check
12350 True if @var{file1} and @var{file2} have the same device and inode
12351 numbers, i.e., if they are hard links to each other.
12357 @subsection String tests
12359 @cindex string tests
12361 These options test string characteristics. You may need to quote
12362 @var{string} arguments for the shell. For example:
12368 The quotes here prevent the wrong arguments from being passed to
12369 @command{test} if @samp{$V} is empty or contains special characters.
12373 @item -z @var{string}
12375 @cindex zero-length string check
12376 True if the length of @var{string} is zero.
12378 @item -n @var{string}
12379 @itemx @var{string}
12381 @cindex nonzero-length string check
12382 True if the length of @var{string} is nonzero.
12384 @item @var{string1} = @var{string2}
12386 @cindex equal string check
12387 True if the strings are equal.
12389 @item @var{string1} == @var{string2}
12391 @cindex equal string check
12392 True if the strings are equal (synonym for =).
12394 @item @var{string1} != @var{string2}
12396 @cindex not-equal string check
12397 True if the strings are not equal.
12402 @node Numeric tests
12403 @subsection Numeric tests
12405 @cindex numeric tests
12406 @cindex arithmetic tests
12408 Numeric relational operators. The arguments must be entirely numeric
12409 (possibly negative), or the special expression @w{@code{-l @var{string}}},
12410 which evaluates to the length of @var{string}.
12414 @item @var{arg1} -eq @var{arg2}
12415 @itemx @var{arg1} -ne @var{arg2}
12416 @itemx @var{arg1} -lt @var{arg2}
12417 @itemx @var{arg1} -le @var{arg2}
12418 @itemx @var{arg1} -gt @var{arg2}
12419 @itemx @var{arg1} -ge @var{arg2}
12426 These arithmetic binary operators return true if @var{arg1} is equal,
12427 not-equal, less-than, less-than-or-equal, greater-than, or
12428 greater-than-or-equal than @var{arg2}, respectively.
12435 test -1 -gt -2 && echo yes
12437 test -l abc -gt 1 && echo yes
12440 @error{} test: integer expression expected before -eq
12444 @node Connectives for test
12445 @subsection Connectives for @command{test}
12447 @cindex logical connectives
12448 @cindex connectives, logical
12450 The usual logical connectives.
12456 True if @var{expr} is false.
12458 @item @var{expr1} -a @var{expr2}
12460 @cindex logical and operator
12461 @cindex and operator
12462 True if both @var{expr1} and @var{expr2} are true.
12464 @item @var{expr1} -o @var{expr2}
12466 @cindex logical or operator
12467 @cindex or operator
12468 True if either @var{expr1} or @var{expr2} is true.
12473 @node expr invocation
12474 @section @command{expr}: Evaluate expressions
12477 @cindex expression evaluation
12478 @cindex evaluation of expressions
12480 @command{expr} evaluates an expression and writes the result on standard
12481 output. Each token of the expression must be a separate argument.
12483 Operands are either integers or strings. Integers consist of one or
12484 more decimal digits, with an optional leading @samp{-}.
12485 @command{expr} converts
12486 anything appearing in an operand position to an integer or a string
12487 depending on the operation being applied to it.
12489 Strings are not quoted for @command{expr} itself, though you may need to
12490 quote them to protect characters with special meaning to the shell,
12491 e.g., spaces. However, regardless of whether it is quoted, a string
12492 operand should not be a parenthesis or any of @command{expr}'s
12493 operators like @code{+}, so you cannot safely pass an arbitrary string
12494 @code{$str} to expr merely by quoting it to the shell. One way to
12495 work around this is to use the GNU extension @code{+},
12496 (e.g., @code{+ "$str" = foo}); a more portable way is to use
12497 @code{@w{" $str"}} and to adjust the rest of the expression to take
12498 the leading space into account (e.g., @code{@w{" $str" = " foo"}}).
12500 You should not pass a negative integer or a string with leading
12501 @samp{-} as @command{expr}'s first argument, as it might be
12502 misinterpreted as an option; this can be avoided by parenthesization.
12503 Also, portable scripts should not use a string operand that happens to
12504 take the form of an integer; this can be worked around by inserting
12505 leading spaces as mentioned above.
12507 @cindex parentheses for grouping
12508 Operators may be given as infix symbols or prefix keywords. Parentheses
12509 may be used for grouping in the usual manner. You must quote
12510 parentheses and many operators to avoid the shell evaluating them,
12513 When built with support for the GNU MP library, @command{expr} uses
12514 arbitrary-precision arithmetic; otherwise, it uses native arithmetic
12515 types and may fail due to arithmetic overflow.
12517 The only options are @option{--help} and @option{--version}. @xref{Common
12518 options}. Options must precede operands.
12520 @cindex exit status of @command{expr}
12524 0 if the expression is neither null nor 0,
12525 1 if the expression is null or 0,
12526 2 if the expression is invalid,
12527 3 if an internal error occurred (e.g., arithmetic overflow).
12531 * String expressions:: + : match substr index length
12532 * Numeric expressions:: + - * / %
12533 * Relations for expr:: | & < <= = == != >= >
12534 * Examples of expr:: Examples.
12538 @node String expressions
12539 @subsection String expressions
12541 @cindex string expressions
12542 @cindex expressions, string
12544 @command{expr} supports pattern matching and other string operators. These
12545 have higher precedence than both the numeric and relational operators (in
12546 the next sections).
12550 @item @var{string} : @var{regex}
12551 @cindex pattern matching
12552 @cindex regular expression matching
12553 @cindex matching patterns
12554 Perform pattern matching. The arguments are converted to strings and the
12555 second is considered to be a (basic, a la GNU @code{grep}) regular
12556 expression, with a @code{^} implicitly prepended. The first argument is
12557 then matched against this regular expression.
12559 If the match succeeds and @var{regex} uses @samp{\(} and @samp{\)}, the
12560 @code{:} expression returns the part of @var{string} that matched the
12561 subexpression; otherwise, it returns the number of characters matched.
12563 If the match fails, the @code{:} operator returns the null string if
12564 @samp{\(} and @samp{\)} are used in @var{regex}, otherwise 0.
12566 @kindex \( @r{regexp operator}
12567 Only the first @samp{\( @dots{} \)} pair is relevant to the return
12568 value; additional pairs are meaningful only for grouping the regular
12569 expression operators.
12571 @kindex \+ @r{regexp operator}
12572 @kindex \? @r{regexp operator}
12573 @kindex \| @r{regexp operator}
12574 In the regular expression, @code{\+}, @code{\?}, and @code{\|} are
12575 operators which respectively match one or more, zero or one, or separate
12576 alternatives. SunOS and other @command{expr}'s treat these as regular
12577 characters. (POSIX allows either behavior.)
12578 @xref{Top, , Regular Expression Library, regex, Regex}, for details of
12579 regular expression syntax. Some examples are in @ref{Examples of expr}.
12581 @item match @var{string} @var{regex}
12583 An alternative way to do pattern matching. This is the same as
12584 @w{@samp{@var{string} : @var{regex}}}.
12586 @item substr @var{string} @var{position} @var{length}
12588 Returns the substring of @var{string} beginning at @var{position}
12589 with length at most @var{length}. If either @var{position} or
12590 @var{length} is negative, zero, or non-numeric, returns the null string.
12592 @item index @var{string} @var{charset}
12594 Returns the first position in @var{string} where the first character in
12595 @var{charset} was found. If no character in @var{charset} is found in
12596 @var{string}, return 0.
12598 @item length @var{string}
12600 Returns the length of @var{string}.
12602 @item + @var{token}
12604 Interpret @var{token} as a string, even if it is a keyword like @var{match}
12605 or an operator like @code{/}.
12606 This makes it possible to test @code{expr length + "$x"} or
12607 @code{expr + "$x" : '.*/\(.\)'} and have it do the right thing even if
12608 the value of @var{$x} happens to be (for example) @code{/} or @code{index}.
12609 This operator is a GNU extension. Portable shell scripts should use
12610 @code{@w{" $token"} : @w{' \(.*\)'}} instead of @code{+ "$token"}.
12614 To make @command{expr} interpret keywords as strings, you must use the
12615 @code{quote} operator.
12618 @node Numeric expressions
12619 @subsection Numeric expressions
12621 @cindex numeric expressions
12622 @cindex expressions, numeric
12624 @command{expr} supports the usual numeric operators, in order of increasing
12625 precedence. These numeric operators have lower precedence than the
12626 string operators described in the previous section, and higher precedence
12627 than the connectives (next section).
12635 @cindex subtraction
12636 Addition and subtraction. Both arguments are converted to integers;
12637 an error occurs if this cannot be done.
12643 @cindex multiplication
12646 Multiplication, division, remainder. Both arguments are converted to
12647 integers; an error occurs if this cannot be done.
12652 @node Relations for expr
12653 @subsection Relations for @command{expr}
12655 @cindex connectives, logical
12656 @cindex logical connectives
12657 @cindex relations, numeric or string
12659 @command{expr} supports the usual logical connectives and relations. These
12660 have lower precedence than the string and numeric operators
12661 (previous sections). Here is the list, lowest-precedence operator first.
12667 @cindex logical or operator
12668 @cindex or operator
12669 Returns its first argument if that is neither null nor zero, otherwise
12670 its second argument if it is neither null nor zero, otherwise 0. It
12671 does not evaluate its second argument if its first argument is neither
12676 @cindex logical and operator
12677 @cindex and operator
12678 Return its first argument if neither argument is null or zero, otherwise
12679 0. It does not evaluate its second argument if its first argument is
12682 @item < <= = == != >= >
12689 @cindex comparison operators
12691 Compare the arguments and return 1 if the relation is true, 0 otherwise.
12692 @code{==} is a synonym for @code{=}. @command{expr} first tries to convert
12693 both arguments to integers and do a numeric comparison; if either
12694 conversion fails, it does a lexicographic comparison using the character
12695 collating sequence specified by the @env{LC_COLLATE} locale.
12700 @node Examples of expr
12701 @subsection Examples of using @command{expr}
12703 @cindex examples of @command{expr}
12704 Here are a few examples, including quoting for shell metacharacters.
12706 To add 1 to the shell variable @code{foo}, in Bourne-compatible shells:
12709 foo=$(expr $foo + 1)
12712 To print the non-directory part of the file name stored in
12713 @code{$fname}, which need not contain a @code{/}:
12716 expr $fname : '.*/\(.*\)' '|' $fname
12719 An example showing that @code{\+} is an operator:
12727 expr abc : 'a\(.\)c'
12729 expr index abcdef cz
12732 @error{} expr: syntax error
12733 expr index + index a
12739 @chapter Redirection
12741 @cindex redirection
12742 @cindex commands for redirection
12744 Unix shells commonly provide several forms of @dfn{redirection}---ways
12745 to change the input source or output destination of a command. But one
12746 useful redirection is performed by a separate command, not by the shell;
12747 it's described here.
12750 * tee invocation:: Redirect output to multiple files or processes.
12754 @node tee invocation
12755 @section @command{tee}: Redirect output to multiple files or processes
12758 @cindex pipe fitting
12759 @cindex destinations, multiple output
12760 @cindex read from stdin and write to stdout and files
12762 The @command{tee} command copies standard input to standard output and also
12763 to any files given as arguments. This is useful when you want not only
12764 to send some data down a pipe, but also to save a copy. Synopsis:
12767 tee [@var{option}]@dots{} [@var{file}]@dots{}
12770 If a file being written to does not already exist, it is created. If a
12771 file being written to already exists, the data it previously contained
12772 is overwritten unless the @option{-a} option is used.
12774 A @var{file} of @samp{-} causes @command{tee} to send another copy of
12775 input to standard output, but this is typically not that useful as the
12776 copies are interleaved.
12778 The program accepts the following options. Also see @ref{Common options}.
12785 Append standard input to the given files rather than overwriting
12789 @itemx --ignore-interrupts
12791 @opindex --ignore-interrupts
12792 Ignore interrupt signals.
12796 The @command{tee} command is useful when you happen to be transferring a large
12797 amount of data and also want to summarize that data without reading
12798 it a second time. For example, when you are downloading a DVD image,
12799 you often want to verify its signature or checksum right away.
12800 The inefficient way to do it is simply:
12803 wget http://example.com/some.iso && sha1sum some.iso
12806 One problem with the above is that it makes you wait for the
12807 download to complete before starting the time-consuming SHA1 computation.
12808 Perhaps even more importantly, the above requires reading
12809 the DVD image a second time (the first was from the network).
12811 The efficient way to do it is to interleave the download
12812 and SHA1 computation. Then, you'll get the checksum for
12813 free, because the entire process parallelizes so well:
12816 # slightly contrived, to demonstrate process substitution
12817 wget -O - http://example.com/dvd.iso \
12818 | tee >(sha1sum > dvd.sha1) > dvd.iso
12821 That makes @command{tee} write not just to the expected output file,
12822 but also to a pipe running @command{sha1sum} and saving the final
12823 checksum in a file named @file{dvd.sha1}.
12825 Note, however, that this example relies on a feature of modern shells
12826 called @dfn{process substitution}
12827 (the @samp{>(command)} syntax, above;
12828 @xref{Process Substitution,,Process Substitution, bash,
12829 The Bash Reference Manual}.),
12830 so it works with @command{zsh}, @command{bash}, and @command{ksh},
12831 but not with @command{/bin/sh}. So if you write code like this
12832 in a shell script, be sure to start the script with @samp{#!/bin/bash}.
12834 Since the above example writes to one file and one process,
12835 a more conventional and portable use of @command{tee} is even better:
12838 wget -O - http://example.com/dvd.iso \
12839 | tee dvd.iso | sha1sum > dvd.sha1
12842 You can extend this example to make @command{tee} write to two processes,
12843 computing MD5 and SHA1 checksums in parallel. In this case,
12844 process substitution is required:
12847 wget -O - http://example.com/dvd.iso \
12848 | tee >(sha1sum > dvd.sha1) \
12849 >(md5sum > dvd.md5) \
12853 This technique is also useful when you want to make a @emph{compressed}
12854 copy of the contents of a pipe.
12855 Consider a tool to graphically summarize disk usage data from @samp{du -ak}.
12856 For a large hierarchy, @samp{du -ak} can run for a long time,
12857 and can easily produce terabytes of data, so you won't want to
12858 rerun the command unnecessarily. Nor will you want to save
12859 the uncompressed output.
12861 Doing it the inefficient way, you can't even start the GUI
12862 until after you've compressed all of the @command{du} output:
12865 du -ak | gzip -9 > /tmp/du.gz
12866 gzip -d /tmp/du.gz | xdiskusage -a
12869 With @command{tee} and process substitution, you start the GUI
12870 right away and eliminate the decompression completely:
12873 du -ak | tee >(gzip -9 > /tmp/du.gz) | xdiskusage -a
12876 Finally, if you regularly create more than one type of
12877 compressed tarball at once, for example when @code{make dist} creates
12878 both @command{gzip}-compressed and @command{bzip2}-compressed tarballs,
12879 there may be a better way.
12880 Typical @command{automake}-generated @file{Makefile} rules create
12881 the two compressed tar archives with commands in sequence, like this
12882 (slightly simplified):
12885 tardir=your-pkg-M.N
12886 tar chof - "$tardir" | gzip -9 -c > your-pkg-M.N.tar.gz
12887 tar chof - "$tardir" | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12890 However, if the hierarchy you are archiving and compressing is larger
12891 than a couple megabytes, and especially if you are using a multi-processor
12892 system with plenty of memory, then you can do much better by reading the
12893 directory contents only once and running the compression programs in parallel:
12896 tardir=your-pkg-M.N
12897 tar chof - "$tardir" \
12898 | tee >(gzip -9 -c > your-pkg-M.N.tar.gz) \
12899 | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12905 @node File name manipulation
12906 @chapter File name manipulation
12908 @cindex file name manipulation
12909 @cindex manipulation of file names
12910 @cindex commands for file name manipulation
12912 This section describes commands that manipulate file names.
12915 * basename invocation:: Strip directory and suffix from a file name.
12916 * dirname invocation:: Strip last file name component.
12917 * pathchk invocation:: Check file name validity and portability.
12918 * mktemp invocation:: Create temporary file or directory.
12919 * realpath invocation:: Print resolved file names.
12923 @node basename invocation
12924 @section @command{basename}: Strip directory and suffix from a file name
12927 @cindex strip directory and suffix from file names
12928 @cindex directory, stripping from file names
12929 @cindex suffix, stripping from file names
12930 @cindex file names, stripping directory and suffix
12931 @cindex leading directory components, stripping
12933 @command{basename} removes any leading directory components from
12934 @var{name}. Synopsis:
12937 basename @var{name} [@var{suffix}]
12938 basename @var{option}@dots{} @var{name}@dots{}
12941 If @var{suffix} is specified and is identical to the end of @var{name},
12942 it is removed from @var{name} as well. Note that since trailing slashes
12943 are removed prior to suffix matching, @var{suffix} will do nothing if it
12944 contains slashes. @command{basename} prints the result on standard
12947 @c This test is used both here and in the section on dirname.
12948 @macro basenameAndDirname
12949 Together, @command{basename} and @command{dirname} are designed such
12950 that if @samp{ls "$name"} succeeds, then the command sequence @samp{cd
12951 "$(dirname "$name")"; ls "$(basename "$name")"} will, too. This works
12952 for everything except file names containing a trailing newline.
12954 @basenameAndDirname
12956 POSIX allows the implementation to define the results if
12957 @var{name} is empty or @samp{//}. In the former case, GNU
12958 @command{basename} returns the empty string. In the latter case, the
12959 result is @samp{//} on platforms where @var{//} is distinct from
12960 @var{/}, and @samp{/} on platforms where there is no difference.
12962 The program accepts the following options. Also see @ref{Common options}.
12963 Options must precede operands.
12970 @opindex --multiple
12971 Support more than one argument. Treat every argument as a @var{name}.
12972 With this, an optional @var{suffix} must be specified using the
12973 @option{-s} option.
12975 @item -s @var{suffix}
12976 @itemx --suffix=@var{suffix}
12979 Remove a trailing @var{suffix}.
12980 This option implies the @option{-a} option.
12986 Separate output items with NUL characters.
12996 basename /usr/bin/sort
12999 basename include/stdio.h .h
13002 basename -s .h include/stdio.h
13004 # Output "stdio" followed by "stdlib"
13005 basename -a -s .h include/stdio.h include/stdlib.h
13009 @node dirname invocation
13010 @section @command{dirname}: Strip last file name component
13013 @cindex directory components, printing
13014 @cindex stripping non-directory suffix
13015 @cindex non-directory suffix, stripping
13017 @command{dirname} prints all but the final slash-delimited component
13018 of each @var{name}. Slashes on either side of the final component are
13019 also removed. If the string contains no slash, @command{dirname}
13020 prints @samp{.} (meaning the current directory). Synopsis:
13023 dirname [@var{option}] @var{name}@dots{}
13026 @var{name} need not be a file name, but if it is, this operation
13027 effectively lists the directory that contains the final component,
13028 including the case when the final component is itself a directory.
13030 @basenameAndDirname
13032 POSIX allows the implementation to define the results if
13033 @var{name} is @samp{//}. With GNU @command{dirname}, the
13034 result is @samp{//} on platforms where @var{//} is distinct from
13035 @var{/}, and @samp{/} on platforms where there is no difference.
13037 The program accepts the following option. Also see @ref{Common options}.
13045 Separate output items with NUL characters.
13054 # Output "/usr/bin".
13055 dirname /usr/bin/sort
13056 dirname /usr/bin//.//
13058 # Output "dir1" followed by "dir2"
13059 dirname dir1/str dir2/str
13066 @node pathchk invocation
13067 @section @command{pathchk}: Check file name validity and portability
13070 @cindex file names, checking validity and portability
13071 @cindex valid file names, checking for
13072 @cindex portable file names, checking for
13074 @command{pathchk} checks validity and portability of file names. Synopsis:
13077 pathchk [@var{option}]@dots{} @var{name}@dots{}
13080 For each @var{name}, @command{pathchk} prints an error message if any of
13081 these conditions is true:
13085 One of the existing directories in @var{name} does not have search
13086 (execute) permission,
13088 The length of @var{name} is larger than the maximum supported by the
13091 The length of one component of @var{name} is longer than
13092 its file system's maximum.
13095 A nonexistent @var{name} is not an error, so long a file with that
13096 name could be created under the above conditions.
13098 The program accepts the following options. Also see @ref{Common options}.
13099 Options must precede operands.
13105 Instead of performing checks based on the underlying file system,
13106 print an error message if any of these conditions is true:
13110 A file name is empty.
13113 A file name contains a character outside the POSIX portable file
13114 name character set, namely, the ASCII letters and digits, @samp{.},
13115 @samp{_}, @samp{-}, and @samp{/}.
13118 The length of a file name or one of its components exceeds the
13119 POSIX minimum limits for portability.
13124 Print an error message if a file name is empty, or if it contains a component
13125 that begins with @samp{-}.
13127 @item --portability
13128 @opindex --portability
13129 Print an error message if a file name is not portable to all POSIX
13130 hosts. This option is equivalent to @samp{-p -P}.
13134 @cindex exit status of @command{pathchk}
13138 0 if all specified file names passed all checks,
13142 @node mktemp invocation
13143 @section @command{mktemp}: Create temporary file or directory
13146 @cindex file names, creating temporary
13147 @cindex directory, creating temporary
13148 @cindex temporary files and directories
13150 @command{mktemp} manages the creation of temporary files and
13151 directories. Synopsis:
13154 mktemp [@var{option}]@dots{} [@var{template}]
13157 Safely create a temporary file or directory based on @var{template},
13158 and print its name. If given, @var{template} must include at least
13159 three consecutive @samp{X}s in the last component. If omitted, the template
13160 @samp{tmp.XXXXXXXXXX} is used, and option @option{--tmpdir} is
13161 implied. The final run of @samp{X}s in the @var{template} will be replaced
13162 by alpha-numeric characters; thus, on a case-sensitive file system,
13163 and with a @var{template} including a run of @var{n} instances of @samp{X},
13164 there are @samp{62**@var{n}} potential file names.
13166 Older scripts used to create temporary files by simply joining the
13167 name of the program with the process id (@samp{$$}) as a suffix.
13168 However, that naming scheme is easily predictable, and suffers from a
13169 race condition where the attacker can create an appropriately named
13170 symbolic link, such that when the script then opens a handle to what
13171 it thought was an unused file, it is instead modifying an existing
13172 file. Using the same scheme to create a directory is slightly safer,
13173 since the @command{mkdir} will fail if the target already exists, but
13174 it is still inferior because it allows for denial of service attacks.
13175 Therefore, modern scripts should use the @command{mktemp} command to
13176 guarantee that the generated name will be unpredictable, and that
13177 knowledge of the temporary file name implies that the file was created
13178 by the current script and cannot be modified by other users.
13180 When creating a file, the resulting file has read and write
13181 permissions for the current user, but no permissions for the group or
13182 others; these permissions are reduced if the current umask is more
13185 Here are some examples (although note that if you repeat them, you
13186 will most likely get different file names):
13191 Create a temporary file in the current directory.
13198 Create a temporary file with a known suffix.
13200 $ mktemp --suffix=.txt file-XXXX
13202 $ mktemp file-XXXX-XXXX.txt
13207 Create a secure fifo relative to the user's choice of @env{TMPDIR},
13208 but falling back to the current directory rather than @file{/tmp}.
13209 Note that @command{mktemp} does not create fifos, but can create a
13210 secure directory in which the fifo can live. Exit the shell if the
13211 directory or fifo could not be created.
13213 $ dir=$(mktemp -p "$@{TMPDIR:-.@}" -d dir-XXXX) || exit 1
13215 $ mkfifo "$fifo" || @{ rmdir "$dir"; exit 1; @}
13219 Create and use a temporary file if possible, but ignore failure. The
13220 file will reside in the directory named by @env{TMPDIR}, if specified,
13221 or else in @file{/tmp}.
13223 $ file=$(mktemp -q) && @{
13224 > # Safe to use $file only within this block. Use quotes,
13225 > # since $TMPDIR, and thus $file, may contain whitespace.
13226 > echo ... > "$file"
13232 Act as a semi-random character generator (it is not fully random,
13233 since it is impacted by the contents of the current directory). To
13234 avoid security holes, do not use the resulting names to create a file.
13244 The program accepts the following options. Also see @ref{Common options}.
13251 @opindex --directory
13252 Create a directory rather than a file. The directory will have read,
13253 write, and search permissions for the current user, but no permissions
13254 for the group or others; these permissions are reduced if the current
13255 umask is more restrictive.
13261 Suppress diagnostics about failure to create a file or directory. The
13262 exit status will still reflect whether a file was created.
13268 Generate a temporary name that does not name an existing file, without
13269 changing the file system contents. Using the output of this command
13270 to create a new file is inherently unsafe, as there is a window of
13271 time between generating the name and using it where another process
13272 can create an object by the same name.
13275 @itemx --tmpdir[=@var{dir}]
13278 Treat @var{template} relative to the directory @var{dir}. If
13279 @var{dir} is not specified (only possible with the long option
13280 @option{--tmpdir}) or is the empty string, use the value of
13281 @env{TMPDIR} if available, otherwise use @samp{/tmp}. If this is
13282 specified, @var{template} must not be absolute. However,
13283 @var{template} can still contain slashes, although intermediate
13284 directories must already exist.
13286 @item --suffix=@var{suffix}
13288 Append @var{suffix} to the @var{template}. @var{suffix} must not
13289 contain slash. If @option{--suffix} is specified, @var{template} must
13290 end in @samp{X}; if it is not specified, then an appropriate
13291 @option{--suffix} is inferred by finding the last @samp{X} in
13292 @var{template}. This option exists for use with the default
13293 @var{template} and for the creation of a @var{suffix} that starts with
13298 Treat @var{template} as a single file relative to the value of
13299 @env{TMPDIR} if available, or to the directory specified by
13300 @option{-p}, otherwise to @samp{/tmp}. @var{template} must not
13301 contain slashes. This option is deprecated; the use of @option{-p}
13302 without @option{-t} offers better defaults (by favoring the command
13303 line over @env{TMPDIR}) and more flexibility (by allowing intermediate
13308 @cindex exit status of @command{mktemp}
13312 0 if the file was created,
13317 @node realpath invocation
13318 @section @command{realpath}: Print the resolved file name.
13321 @cindex file names, canonicalization
13322 @cindex symlinks, resolution
13323 @cindex canonical file name
13324 @cindex canonicalize a file name
13328 @command{realpath} expands all symbolic links and resolves references to
13329 @samp{/./}, @samp{/../} and extra @samp{/} characters. By default,
13330 all but the last component of the specified files must exist. Synopsis:
13333 realpath [@var{option}]@dots{} @var{file}@dots{}
13336 The program accepts the following options. Also see @ref{Common options}.
13341 @itemx --canonicalize-existing
13343 @opindex --canonicalize-existing
13344 Ensure that all components of the specified file names exist.
13345 If any component is missing or unavailable, @command{realpath} will output
13346 a diagnostic unless the @option{-q} option is specified, and exit with a
13347 nonzero exit code. A trailing slash requires that the name resolve to a
13351 @itemx --canonicalize-missing
13353 @opindex --canonicalize-missing
13354 If any component of a specified file name is missing or unavailable,
13355 treat it as a directory.
13361 Symbolic links are resolved in the specified file names,
13362 but they are resolved after any subsequent @samp{..} components are processed.
13367 @opindex --physical
13368 Symbolic links are resolved in the specified file names,
13369 and they are resolved before any subsequent @samp{..} components are processed.
13370 This is the default mode of operation.
13376 Suppress diagnostic messages for specified file names.
13380 @itemx --no-symlinks
13383 @opindex --no-symlinks
13384 Do not resolve symbolic links. Only resolve references to
13385 @samp{/./}, @samp{/../} and remove extra @samp{/} characters.
13386 When combined with the @option{-m} option, realpath operates
13387 only on the file name, and does not touch any actual file.
13393 Separate output items with NUL characters.
13395 @item --relative-to=@var{file}
13396 @opindex --relative-to
13398 Print the resolved file names relative to the specified file.
13399 Note this option honors the @option{-m} and @option{-e} options
13400 pertaining to file existence.
13402 @item --relative-base=@var{base}
13403 @opindex --relative-base
13404 This option is valid when used with @option{--relative-to}, and will restrict
13405 the output of @option{--relative-to} so that relative names are output,
13406 only when @var{file}s are descendants of @var{base}. Otherwise output the
13407 absolute file name. If @option{--relative-to} was not specified, then
13408 the descendants of @var{base} are printed relative to @var{base}. If
13409 @option{--relative-to} is specified, then that directory must be a
13410 descendant of @var{base} for this option to have an effect.
13411 Note: this option honors the @option{-m} and @option{-e}
13412 options pertaining to file existence. For example:
13415 realpath --relative-to=/usr /tmp /usr/bin
13418 realpath --relative-base=/usr /tmp /usr/bin
13425 @cindex exit status of @command{realpath}
13429 0 if all file names were printed without issue.
13434 @node Working context
13435 @chapter Working context
13437 @cindex working context
13438 @cindex commands for printing the working context
13440 This section describes commands that display or alter the context in
13441 which you are working: the current directory, the terminal settings, and
13442 so forth. See also the user-related commands in the next section.
13445 * pwd invocation:: Print working directory.
13446 * stty invocation:: Print or change terminal characteristics.
13447 * printenv invocation:: Print environment variables.
13448 * tty invocation:: Print file name of terminal on standard input.
13452 @node pwd invocation
13453 @section @command{pwd}: Print working directory
13456 @cindex print name of current directory
13457 @cindex current working directory, printing
13458 @cindex working directory, printing
13461 @command{pwd} prints the name of the current directory. Synopsis:
13464 pwd [@var{option}]@dots{}
13467 The program accepts the following options. Also see @ref{Common options}.
13474 If the contents of the environment variable @env{PWD} provide an
13475 absolute name of the current directory with no @samp{.} or @samp{..}
13476 components, but possibly with symbolic links, then output those
13477 contents. Otherwise, fall back to default @option{-P} handling.
13482 @opindex --physical
13483 Print a fully resolved name for the current directory. That is, all
13484 components of the printed name will be actual directory names---none
13485 will be symbolic links.
13488 @cindex symbolic links and @command{pwd}
13489 If @option{-L} and @option{-P} are both given, the last one takes
13490 precedence. If neither option is given, then this implementation uses
13491 @option{-P} as the default unless the @env{POSIXLY_CORRECT}
13492 environment variable is set.
13494 @mayConflictWithShellBuiltIn{pwd}
13499 @node stty invocation
13500 @section @command{stty}: Print or change terminal characteristics
13503 @cindex change or print terminal settings
13504 @cindex terminal settings
13505 @cindex line settings of terminal
13507 @command{stty} prints or changes terminal characteristics, such as baud rate.
13511 stty [@var{option}] [@var{setting}]@dots{}
13512 stty [@var{option}]
13515 If given no line settings, @command{stty} prints the baud rate, line
13516 discipline number (on systems that support it), and line settings
13517 that have been changed from the values set by @samp{stty sane}.
13518 By default, mode reading and setting are performed on the tty line
13519 connected to standard input, although this can be modified by the
13520 @option{--file} option.
13522 @command{stty} accepts many non-option arguments that change aspects of
13523 the terminal line operation, as described below.
13525 The program accepts the following options. Also see @ref{Common options}.
13532 Print all current settings in human-readable form. This option may not
13533 be used in combination with any line settings.
13535 @item -F @var{device}
13536 @itemx --file=@var{device}
13539 Set the line opened by the file name specified in @var{device} instead of
13540 the tty line connected to standard input. This option is necessary
13541 because opening a POSIX tty requires use of the
13542 @code{O_NONDELAY} flag to prevent a POSIX tty from blocking
13543 until the carrier detect line is high if
13544 the @code{clocal} flag is not set. Hence, it is not always possible
13545 to allow the shell to open the device in the traditional manner.
13551 @cindex machine-readable @command{stty} output
13552 Print all current settings in a form that can be used as an argument to
13553 another @command{stty} command to restore the current settings. This option
13554 may not be used in combination with any line settings.
13558 Many settings can be turned off by preceding them with a @samp{-}.
13559 Such arguments are marked below with ``May be negated'' in their
13560 description. The descriptions themselves refer to the positive
13561 case, that is, when @emph{not} negated (unless stated otherwise,
13564 Some settings are not available on all POSIX systems, since they use
13565 extensions. Such arguments are marked below with
13566 ``Non-POSIX'' in their description. On non-POSIX
13567 systems, those or other settings also may not
13568 be available, but it's not feasible to document all the variations: just
13574 * Control:: Control settings
13575 * Input:: Input settings
13576 * Output:: Output settings
13577 * Local:: Local settings
13578 * Combination:: Combination settings
13579 * Characters:: Special characters
13580 * Special:: Special settings
13585 @subsection Control settings
13587 @cindex control settings
13593 @cindex two-way parity
13594 Generate parity bit in output and expect parity bit in input.
13600 @cindex even parity
13601 Set odd parity (even if negated). May be negated.
13608 @cindex character size
13609 @cindex eight-bit characters
13610 Set character size to 5, 6, 7, or 8 bits.
13615 Send a hangup signal when the last process closes the tty. May be
13621 Use two stop bits per character (one if negated). May be negated.
13625 Allow input to be received. May be negated.
13629 @cindex modem control
13630 Disable modem control signals. May be negated.
13634 @cindex hardware flow control
13635 @cindex flow control, hardware
13636 @cindex RTS/CTS flow control
13637 Enable RTS/CTS flow control. Non-POSIX@. May be negated.
13642 @subsection Input settings
13644 @cindex input settings
13645 These settings control operations on data received from the terminal.
13650 @cindex breaks, ignoring
13651 Ignore break characters. May be negated.
13655 @cindex breaks, cause interrupts
13656 Make breaks cause an interrupt signal. May be negated.
13660 @cindex parity, ignoring
13661 Ignore characters with parity errors. May be negated.
13665 @cindex parity errors, marking
13666 Mark parity errors (with a 255-0-character sequence). May be negated.
13670 Enable input parity checking. May be negated.
13674 @cindex eight-bit input
13675 Clear high (8th) bit of input characters. May be negated.
13679 @cindex newline, translating to return
13680 Translate newline to carriage return. May be negated.
13684 @cindex return, ignoring
13685 Ignore carriage return. May be negated.
13689 @cindex return, translating to newline
13690 Translate carriage return to newline. May be negated.
13694 @cindex input encoding, UTF-8
13695 Assume input characters are UTF-8 encoded. May be negated.
13699 @kindex C-s/C-q flow control
13700 @cindex XON/XOFF flow control
13701 Enable XON/XOFF flow control (that is, @kbd{CTRL-S}/@kbd{CTRL-Q}). May
13708 @cindex software flow control
13709 @cindex flow control, software
13710 Enable sending of @code{stop} character when the system input buffer
13711 is almost full, and @code{start} character when it becomes almost
13712 empty again. May be negated.
13716 @cindex uppercase, translating to lowercase
13717 Translate uppercase characters to lowercase. Non-POSIX@. May be
13718 negated. Note ilcuc is not implemented, as one would not be able to issue
13719 almost any (lowercase) Unix command, after invoking it.
13723 Allow any character to restart output (only the start character
13724 if negated). Non-POSIX@. May be negated.
13728 @cindex beeping at input buffer full
13729 Enable beeping and not flushing input buffer if a character arrives
13730 when the input buffer is full. Non-POSIX@. May be negated.
13735 @subsection Output settings
13737 @cindex output settings
13738 These settings control operations on data sent to the terminal.
13743 Postprocess output. May be negated.
13747 @cindex lowercase, translating to output
13748 Translate lowercase characters to uppercase. Non-POSIX@. May be
13749 negated. (Note ouclc is not currently implemented.)
13753 @cindex return, translating to newline
13754 Translate carriage return to newline. Non-POSIX@. May be negated.
13758 @cindex newline, translating to crlf
13759 Translate newline to carriage return-newline. Non-POSIX@. May be
13764 Do not print carriage returns in the first column. Non-POSIX@.
13769 Newline performs a carriage return. Non-POSIX@. May be negated.
13773 @cindex pad instead of timing for delaying
13774 Use fill (padding) characters instead of timing for delays.
13780 @cindex pad character
13781 Use ASCII DEL characters for fill instead of
13782 ASCII NUL characters. Non-POSIX@.
13788 Newline delay style. Non-POSIX.
13795 Carriage return delay style. Non-POSIX.
13801 @opindex tab@var{n}
13802 Horizontal tab delay style. Non-POSIX.
13807 Backspace delay style. Non-POSIX.
13812 Vertical tab delay style. Non-POSIX.
13817 Form feed delay style. Non-POSIX.
13822 @subsection Local settings
13824 @cindex local settings
13829 Enable @code{interrupt}, @code{quit}, and @code{suspend} special
13830 characters. May be negated.
13834 Enable @code{erase}, @code{kill}, @code{werase}, and @code{rprnt}
13835 special characters. May be negated.
13839 Enable non-POSIX special characters. May be negated.
13843 Echo input characters. May be negated.
13849 Echo @code{erase} characters as backspace-space-backspace. May be
13854 @cindex newline echoing after @code{kill}
13855 Echo a newline after a @code{kill} character. May be negated.
13859 @cindex newline, echoing
13860 Echo newline even if not echoing other characters. May be negated.
13864 @cindex flushing, disabling
13865 Disable flushing after @code{interrupt} and @code{quit} special
13866 characters. May be negated.
13870 @cindex case translation
13871 Enable input and output of uppercase characters by preceding their
13872 lowercase equivalents with @samp{\}, when @code{icanon} is set.
13873 Non-POSIX@. May be negated.
13877 @cindex background jobs, stopping at terminal write
13878 Stop background jobs that try to write to the terminal. Non-POSIX@.
13885 Echo erased characters backward, between @samp{\} and @samp{/}.
13886 Non-POSIX@. May be negated.
13892 @cindex control characters, using @samp{^@var{c}}
13893 @cindex hat notation for control characters
13894 Echo control characters in hat notation (@samp{^@var{c}}) instead
13895 of literally. Non-POSIX@. May be negated.
13901 Echo the @code{kill} special character by erasing each character on
13902 the line as indicated by the @code{echoprt} and @code{echoe} settings,
13903 instead of by the @code{echoctl} and @code{echok} settings.
13910 @subsection Combination settings
13912 @cindex combination settings
13913 Combination settings:
13920 Same as @code{parenb -parodd cs7}. May be negated. If negated, same
13921 as @code{-parenb cs8}.
13925 Same as @code{parenb parodd cs7}. May be negated. If negated, same
13926 as @code{-parenb cs8}.
13930 Same as @code{-icrnl -onlcr}. May be negated. If negated, same as
13931 @code{icrnl -inlcr -igncr onlcr -ocrnl -onlret}.
13935 Reset the @code{erase} and @code{kill} special characters to their default
13942 @c This is too long to write inline.
13944 cread -ignbrk brkint -inlcr -igncr icrnl -ixoff
13945 -iuclc -ixany imaxbel opost -olcuc -ocrnl onlcr
13946 -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0
13947 ff0 isig icanon iexten echo echoe echok -echonl
13948 -noflsh -xcase -tostop -echoprt echoctl echoke
13952 and also sets all special characters to their default values.
13956 Same as @code{brkint ignpar istrip icrnl ixon opost isig icanon}, plus
13957 sets the @code{eof} and @code{eol} characters to their default values
13958 if they are the same as the @code{min} and @code{time} characters.
13959 May be negated. If negated, same as @code{raw}.
13966 -ignbrk -brkint -ignpar -parmrk -inpck -istrip
13967 -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany
13968 -imaxbel -opost -isig -icanon -xcase min 1 time 0
13972 May be negated. If negated, same as @code{cooked}.
13976 Same as @option{-icanon}. May be negated. If negated, same as
13981 @cindex eight-bit characters
13982 Same as @code{-parenb -istrip cs8}. May be negated. If negated,
13983 same as @code{parenb istrip cs7}.
13987 Same as @option{-parenb -istrip -opost cs8}. May be negated.
13988 If negated, same as @code{parenb istrip opost cs7}.
13992 Same as @option{-ixany}. Non-POSIX@. May be negated.
13996 Same as @code{tab0}. Non-POSIX@. May be negated. If negated, same
14003 Same as @code{xcase iuclc olcuc}. Non-POSIX@. May be negated.
14004 (Used for terminals with uppercase characters only.)
14008 Same as @code{echoe echoctl echoke}.
14012 Same as @code{echoe echoctl echoke -ixany intr ^C erase ^? kill C-u}.
14017 @subsection Special characters
14019 @cindex special characters
14020 @cindex characters, special
14022 The special characters' default values vary from system to system.
14023 They are set with the syntax @samp{name value}, where the names are
14024 listed below and the value can be given either literally, in hat
14025 notation (@samp{^@var{c}}), or as an integer which may start with
14026 @samp{0x} to indicate hexadecimal, @samp{0} to indicate octal, or
14027 any other digit to indicate decimal.
14029 @cindex disabling special characters
14030 @kindex u@r{, and disabling special characters}
14031 For GNU stty, giving a value of @code{^-} or @code{undef} disables that
14032 special character. (This is incompatible with Ultrix @command{stty},
14033 which uses a value of @samp{u} to disable a special character. GNU
14034 @command{stty} treats a value @samp{u} like any other, namely to set that
14035 special character to @key{U}.)
14041 Send an interrupt signal.
14045 Send a quit signal.
14049 Erase the last character typed.
14053 Erase the current line.
14057 Send an end of file (terminate the input).
14065 Alternate character to end the line. Non-POSIX.
14069 Switch to a different shell layer. Non-POSIX.
14073 Restart the output after stopping it.
14081 Send a terminal stop signal.
14085 Send a terminal stop signal after flushing the input. Non-POSIX.
14089 Redraw the current line. Non-POSIX.
14093 Erase the last word typed. Non-POSIX.
14097 Enter the next character typed literally, even if it is a special
14098 character. Non-POSIX.
14103 @subsection Special settings
14105 @cindex special settings
14110 Set the minimum number of characters that will satisfy a read until
14111 the time value has expired, when @option{-icanon} is set.
14115 Set the number of tenths of a second before reads time out if the minimum
14116 number of characters have not been read, when @option{-icanon} is set.
14118 @item ispeed @var{n}
14120 Set the input speed to @var{n}.
14122 @item ospeed @var{n}
14124 Set the output speed to @var{n}.
14128 Tell the tty kernel driver that the terminal has @var{n} rows.
14132 @itemx columns @var{n}
14135 Tell the kernel that the terminal has @var{n} columns. Non-POSIX.
14141 Print the number of rows and columns that the kernel thinks the
14142 terminal has. (Systems that don't support rows and columns in the kernel
14143 typically use the environment variables @env{LINES} and @env{COLUMNS}
14144 instead; however, GNU @command{stty} does not know anything about them.)
14149 Use line discipline @var{n}. Non-POSIX.
14153 Print the terminal speed.
14156 @cindex baud rate, setting
14157 Set the input and output speeds to @var{n}. @var{n} can be one of: 0
14158 50 75 110 134 134.5 150 200 300 600 1200 1800 2400 4800 9600 19200
14159 38400 @code{exta} @code{extb}. @code{exta} is the same as 19200;
14160 @code{extb} is the same as 38400. Many systems, including GNU/Linux,
14161 support higher speeds. The @command{stty} command includes support
14178 4000000 where the system supports these.
14179 0 hangs up the line if @option{-clocal} is set.
14183 @node printenv invocation
14184 @section @command{printenv}: Print all or some environment variables
14187 @cindex printing all or some environment variables
14188 @cindex environment variables, printing
14190 @command{printenv} prints environment variable values. Synopsis:
14193 printenv [@var{option}] [@var{variable}]@dots{}
14196 If no @var{variable}s are specified, @command{printenv} prints the value of
14197 every environment variable. Otherwise, it prints the value of each
14198 @var{variable} that is set, and nothing for those that are not set.
14200 The program accepts the following option. Also see @ref{Common options}.
14208 @cindex exit status of @command{printenv}
14212 0 if all variables specified were found
14213 1 if at least one specified variable was not found
14214 2 if a write error occurred
14218 @node tty invocation
14219 @section @command{tty}: Print file name of terminal on standard input
14222 @cindex print terminal file name
14223 @cindex terminal file name, printing
14225 @command{tty} prints the file name of the terminal connected to its standard
14226 input. It prints @samp{not a tty} if standard input is not a terminal.
14230 tty [@var{option}]@dots{}
14233 The program accepts the following option. Also see @ref{Common options}.
14243 Print nothing; only return an exit status.
14247 @cindex exit status of @command{tty}
14251 0 if standard input is a terminal
14252 1 if standard input is not a terminal
14253 2 if given incorrect arguments
14254 3 if a write error occurs
14258 @node User information
14259 @chapter User information
14261 @cindex user information, commands for
14262 @cindex commands for printing user information
14264 This section describes commands that print user-related information:
14265 logins, groups, and so forth.
14268 * id invocation:: Print user identity.
14269 * logname invocation:: Print current login name.
14270 * whoami invocation:: Print effective user ID.
14271 * groups invocation:: Print group names a user is in.
14272 * users invocation:: Print login names of users currently logged in.
14273 * who invocation:: Print who is currently logged in.
14277 @node id invocation
14278 @section @command{id}: Print user identity
14281 @cindex real user and group IDs, printing
14282 @cindex effective user and group IDs, printing
14283 @cindex printing real and effective user and group IDs
14285 @command{id} prints information about the given user, or the process
14286 running it if no user is specified. Synopsis:
14289 id [@var{option}]@dots{} [@var{username}]
14292 @vindex POSIXLY_CORRECT
14293 By default, it prints the real user ID, real group ID, effective user ID
14294 if different from the real user ID, effective group ID if different from
14295 the real group ID, and supplemental group IDs.
14296 In addition, if SELinux
14297 is enabled and the @env{POSIXLY_CORRECT} environment variable is not set,
14298 then print @samp{context=@var{c}}, where @var{c} is the security context.
14300 Each of these numeric values is preceded by an identifying string and
14301 followed by the corresponding user or group name in parentheses.
14303 The options cause @command{id} to print only part of the above information.
14304 Also see @ref{Common options}.
14311 Print only the group ID.
14317 Print only the group ID and the supplementary groups.
14323 Print the user or group name instead of the ID number. Requires
14324 @option{-u}, @option{-g}, or @option{-G}.
14330 Print the real, instead of effective, user or group ID@. Requires
14331 @option{-u}, @option{-g}, or @option{-G}.
14337 Print only the user ID.
14344 @cindex security context
14345 Print only the security context of the current user.
14346 If SELinux is disabled then print a warning and
14347 set the exit status to 1.
14353 @macro primaryAndSupplementaryGroups{cmd,arg}
14354 Primary and supplementary groups for a process are normally inherited
14355 from its parent and are usually unchanged since login. This means
14356 that if you change the group database after logging in, @command{\cmd\}
14357 will not reflect your changes within your existing login session.
14358 Running @command{\cmd\} with a \arg\ causes the user and group
14359 database to be consulted afresh, and so will give a different result.
14361 @primaryAndSupplementaryGroups{id,user argument}
14363 @node logname invocation
14364 @section @command{logname}: Print current login name
14367 @cindex printing user's login name
14368 @cindex login name, printing
14369 @cindex user name, printing
14372 @command{logname} prints the calling user's name, as found in a
14373 system-maintained file (often @file{/var/run/utmp} or
14374 @file{/etc/utmp}), and exits with a status of 0. If there is no entry
14375 for the calling process, @command{logname} prints
14376 an error message and exits with a status of 1.
14378 The only options are @option{--help} and @option{--version}. @xref{Common
14384 @node whoami invocation
14385 @section @command{whoami}: Print effective user ID
14388 @cindex effective user ID, printing
14389 @cindex printing the effective user ID
14391 @command{whoami} prints the user name associated with the current
14392 effective user ID@. It is equivalent to the command @samp{id -un}.
14394 The only options are @option{--help} and @option{--version}. @xref{Common
14400 @node groups invocation
14401 @section @command{groups}: Print group names a user is in
14404 @cindex printing groups a user is in
14405 @cindex supplementary groups, printing
14407 @command{groups} prints the names of the primary and any supplementary
14408 groups for each given @var{username}, or the current process if no names
14409 are given. If more than one name is given, the name of each user is
14411 the list of that user's groups and the user name is separated from the
14412 group list by a colon. Synopsis:
14415 groups [@var{username}]@dots{}
14418 The group lists are equivalent to the output of the command @samp{id -Gn}.
14420 @primaryAndSupplementaryGroups{groups,list of users}
14422 The only options are @option{--help} and @option{--version}. @xref{Common
14428 @node users invocation
14429 @section @command{users}: Print login names of users currently logged in
14432 @cindex printing current usernames
14433 @cindex usernames, printing current
14435 @cindex login sessions, printing users with
14436 @command{users} prints on a single line a blank-separated list of user
14437 names of users currently logged in to the current host. Each user name
14438 corresponds to a login session, so if a user has more than one login
14439 session, that user's name will appear the same number of times in the
14448 With no @var{file} argument, @command{users} extracts its information from
14449 a system-maintained file (often @file{/var/run/utmp} or
14450 @file{/etc/utmp}). If a file argument is given, @command{users} uses
14451 that file instead. A common choice is @file{/var/log/wtmp}.
14453 The only options are @option{--help} and @option{--version}. @xref{Common
14459 @node who invocation
14460 @section @command{who}: Print who is currently logged in
14463 @cindex printing current user information
14464 @cindex information, about current users
14466 @command{who} prints information about users who are currently logged on.
14470 @command{who} [@var{option}] [@var{file}] [am i]
14473 @cindex terminal lines, currently used
14475 @cindex remote hostname
14476 If given no non-option arguments, @command{who} prints the following
14477 information for each user currently logged on: login name, terminal
14478 line, login time, and remote hostname or X display.
14482 If given one non-option argument, @command{who} uses that instead of
14483 a default system-maintained file (often @file{/var/run/utmp} or
14484 @file{/etc/utmp}) as the name of the file containing the record of
14485 users logged on. @file{/var/log/wtmp} is commonly given as an argument
14486 to @command{who} to look at who has previously logged on.
14490 If given two non-option arguments, @command{who} prints only the entry
14491 for the user running it (determined from its standard input), preceded
14492 by the hostname. Traditionally, the two arguments given are @samp{am
14493 i}, as in @samp{who am i}.
14496 Time stamps are listed according to the time zone rules specified by
14497 the @env{TZ} environment variable, or by the system default rules if
14498 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
14499 with @env{TZ}, libc, The GNU C Library Reference Manual}.
14501 The program accepts the following options. Also see @ref{Common options}.
14509 Same as @samp{-b -d --login -p -r -t -T -u}.
14515 Print the date and time of last system boot.
14521 Print information corresponding to dead processes.
14527 Print a line of column headings.
14533 List only the entries that correspond to processes via which the
14534 system is waiting for a user to login. The user name is always @samp{LOGIN}.
14538 Attempt to canonicalize hostnames found in utmp through a DNS lookup. This
14539 is not the default because it can cause significant delays on systems with
14540 automatic dial-up internet access.
14544 Same as @samp{who am i}.
14550 List active processes spawned by init.
14556 Print only the login names and the number of users logged on.
14557 Overrides all other options.
14562 @opindex --runlevel
14563 Print the current (and maybe previous) run-level of the init process.
14567 Ignored; for compatibility with other versions of @command{who}.
14573 Print last system clock change.
14578 After the login time, print the number of hours and minutes that the
14579 user has been idle. @samp{.} means the user was active in the last minute.
14580 @samp{old} means the user has been idle for more than 24 hours.
14591 @opindex --writable
14592 @cindex message status
14593 @pindex write@r{, allowed}
14594 After each login name print a character indicating the user's message status:
14597 @samp{+} allowing @code{write} messages
14598 @samp{-} disallowing @code{write} messages
14599 @samp{?} cannot find terminal device
14607 @node System context
14608 @chapter System context
14610 @cindex system context
14611 @cindex context, system
14612 @cindex commands for system context
14614 This section describes commands that print or change system-wide
14618 * date invocation:: Print or set system date and time.
14619 * arch invocation:: Print machine hardware name.
14620 * nproc invocation:: Print the number of processors.
14621 * uname invocation:: Print system information.
14622 * hostname invocation:: Print or set system name.
14623 * hostid invocation:: Print numeric host identifier.
14624 * uptime invocation:: Print system uptime and load.
14627 @node date invocation
14628 @section @command{date}: Print or set system date and time
14631 @cindex time, printing or setting
14632 @cindex printing the current time
14637 date [@var{option}]@dots{} [+@var{format}]
14638 date [-u|--utc|--universal] @c this avoids a newline in the output
14639 [ MMDDhhmm[[CC]YY][.ss] ]
14643 Invoking @command{date} with no @var{format} argument is equivalent to invoking
14644 it with a default format that depends on the @env{LC_TIME} locale category.
14645 In the default C locale, this format is @samp{'+%a %b %e %H:%M:%S %Z %Y'},
14646 so the output looks like @samp{Thu Mar @ 3 13:47:51 PST 2005}.
14649 Normally, @command{date} uses the time zone rules indicated by the
14650 @env{TZ} environment variable, or the system default rules if @env{TZ}
14651 is not set. @xref{TZ Variable,, Specifying the Time Zone with
14652 @env{TZ}, libc, The GNU C Library Reference Manual}.
14654 @findex strftime @r{and @command{date}}
14655 @cindex time formats
14656 @cindex formatting times
14657 If given an argument that starts with a @samp{+}, @command{date} prints the
14658 current date and time (or the date and time specified by the
14659 @option{--date} option, see below) in the format defined by that argument,
14660 which is similar to that of the @code{strftime} function. Except for
14661 conversion specifiers, which start with @samp{%}, characters in the
14662 format string are printed unchanged. The conversion specifiers are
14668 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
14669 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
14670 * Literal conversion specifiers:: %[%nt]
14671 * Padding and other flags:: Pad with zeros, spaces, etc.
14672 * Setting the time:: Changing the system clock.
14673 * Options for date:: Instead of the current time.
14675 * Date input formats:: Specifying date strings.
14677 * Examples of date:: Examples.
14680 @node Time conversion specifiers
14681 @subsection Time conversion specifiers
14683 @cindex time conversion specifiers
14684 @cindex conversion specifiers, time
14686 @command{date} conversion specifiers related to times.
14690 hour (@samp{00}@dots{}@samp{23})
14692 hour (@samp{01}@dots{}@samp{12})
14694 hour, space padded (@samp{ 0}@dots{}@samp{23}); equivalent to @samp{%_H}@.
14695 This is a GNU extension.
14697 hour, space padded (@samp{ 1}@dots{}@samp{12}); equivalent to @samp{%_I}@.
14698 This is a GNU extension.
14700 minute (@samp{00}@dots{}@samp{59})
14702 nanoseconds (@samp{000000000}@dots{}@samp{999999999}).
14703 This is a GNU extension.
14705 locale's equivalent of either @samp{AM} or @samp{PM};
14706 blank in many locales.
14707 Noon is treated as @samp{PM} and midnight as @samp{AM}.
14709 like @samp{%p}, except lower case.
14710 This is a GNU extension.
14712 locale's 12-hour clock time (e.g., @samp{11:11:04 PM})
14714 24-hour hour and minute. Same as @samp{%H:%M}.
14716 @cindex epoch, seconds since
14717 @cindex seconds since the epoch
14718 @cindex beginning of time
14719 @cindex leap seconds
14720 seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC@.
14721 Leap seconds are not counted unless leap second support is available.
14722 @xref{%s-examples}, for examples.
14723 This is a GNU extension.
14725 @cindex leap seconds
14726 second (@samp{00}@dots{}@samp{60}).
14727 This may be @samp{60} if leap seconds are supported.
14729 24-hour hour, minute, and second. Same as @samp{%H:%M:%S}.
14731 locale's time representation (e.g., @samp{23:13:48})
14733 @w{RFC 2822/ISO 8601} style numeric time zone
14734 (e.g., @samp{-0600} or @samp{+0530}), or nothing if no
14735 time zone is determinable. This value reflects the numeric time zone
14736 appropriate for the current time, using the time zone rules specified
14737 by the @env{TZ} environment variable.
14738 The time (and optionally, the time zone rules) can be overridden
14739 by the @option{--date} option.
14741 @w{RFC 3339/ISO 8601} style numeric time zone with
14742 @samp{:} (e.g., @samp{-06:00} or @samp{+05:30}), or nothing if no time
14743 zone is determinable.
14744 This is a GNU extension.
14746 Numeric time zone to the nearest second with @samp{:} (e.g.,
14747 @samp{-06:00:00} or @samp{+05:30:00}), or nothing if no time zone is
14749 This is a GNU extension.
14751 Numeric time zone with @samp{:} using the minimum necessary precision
14752 (e.g., @samp{-06}, @samp{+05:30}, or @samp{-04:56:02}), or nothing if
14753 no time zone is determinable.
14754 This is a GNU extension.
14756 alphabetic time zone abbreviation (e.g., @samp{EDT}), or nothing if no
14757 time zone is determinable. See @samp{%z} for how it is determined.
14761 @node Date conversion specifiers
14762 @subsection Date conversion specifiers
14764 @cindex date conversion specifiers
14765 @cindex conversion specifiers, date
14767 @command{date} conversion specifiers related to dates.
14771 locale's abbreviated weekday name (e.g., @samp{Sun})
14773 locale's full weekday name, variable length (e.g., @samp{Sunday})
14775 locale's abbreviated month name (e.g., @samp{Jan})
14777 locale's full month name, variable length (e.g., @samp{January})
14779 locale's date and time (e.g., @samp{Thu Mar @ 3 23:05:25 2005})
14781 century. This is like @samp{%Y}, except the last two digits are omitted.
14782 For example, it is @samp{20} if @samp{%Y} is @samp{2000},
14783 and is @samp{-0} if @samp{%Y} is @samp{-001}.
14784 It is normally at least two characters, but it may be more.
14786 day of month (e.g., @samp{01})
14788 date; same as @samp{%m/%d/%y}
14790 day of month, space padded; same as @samp{%_d}
14792 full date in ISO 8601 format; same as @samp{%Y-%m-%d}.
14793 This is a good choice for a date format, as it is standard and
14794 is easy to sort in the usual case where years are in the range
14797 year corresponding to the ISO week number, but without the century
14798 (range @samp{00} through @samp{99}). This has the same format and value
14799 as @samp{%y}, except that if the ISO week number (see
14801 to the previous or next year, that year is used instead.
14803 year corresponding to the ISO week number. This has the
14804 same format and value as @samp{%Y}, except that if the ISO
14806 @samp{%V}) belongs to the previous or next year, that year is used
14808 It is normally useful only if @samp{%V} is also used;
14809 for example, the format @samp{%G-%m-%d} is probably a mistake,
14810 since it combines the ISO week number year with the conventional month and day.
14814 day of year (@samp{001}@dots{}@samp{366})
14816 month (@samp{01}@dots{}@samp{12})
14818 day of week (@samp{1}@dots{}@samp{7}) with @samp{1} corresponding to Monday
14820 week number of year, with Sunday as the first day of the week
14821 (@samp{00}@dots{}@samp{53}).
14822 Days in a new year preceding the first Sunday are in week zero.
14824 ISO week number, that is, the
14825 week number of year, with Monday as the first day of the week
14826 (@samp{01}@dots{}@samp{53}).
14827 If the week containing January 1 has four or more days in
14828 the new year, then it is considered week 1; otherwise, it is week 53 of
14829 the previous year, and the next week is week 1. (See the ISO 8601
14832 day of week (@samp{0}@dots{}@samp{6}) with 0 corresponding to Sunday
14834 week number of year, with Monday as first day of week
14835 (@samp{00}@dots{}@samp{53}).
14836 Days in a new year preceding the first Monday are in week zero.
14838 locale's date representation (e.g., @samp{12/31/99})
14840 last two digits of year (@samp{00}@dots{}@samp{99})
14842 year. This is normally at least four characters, but it may be more.
14843 Year @samp{0000} precedes year @samp{0001}, and year @samp{-001}
14844 precedes year @samp{0000}.
14848 @node Literal conversion specifiers
14849 @subsection Literal conversion specifiers
14851 @cindex literal conversion specifiers
14852 @cindex conversion specifiers, literal
14854 @command{date} conversion specifiers that produce literal strings.
14866 @node Padding and other flags
14867 @subsection Padding and other flags
14869 @cindex numeric field padding
14870 @cindex padding of numeric fields
14871 @cindex fields, padding numeric
14873 Unless otherwise specified, @command{date} normally pads numeric fields
14874 with zeros, so that, for
14875 example, numeric months are always output as two digits.
14876 Seconds since the epoch are not padded, though,
14877 since there is no natural width for them.
14879 As a GNU extension, @command{date} recognizes any of the
14880 following optional flags after the @samp{%}:
14884 (hyphen) Do not pad the field; useful if the output is intended for
14887 (underscore) Pad with spaces; useful if you need a fixed
14888 number of characters in the output, but zeros are too distracting.
14890 (zero) Pad with zeros even if the conversion specifier
14891 would normally pad with spaces.
14893 Use upper case characters if possible.
14895 Use opposite case characters if possible.
14896 A field that is normally upper case becomes lower case, and vice versa.
14900 Here are some examples of padding:
14903 date +%d/%m -d "Feb 1"
14905 date +%-d/%-m -d "Feb 1"
14907 date +%_d/%_m -d "Feb 1"
14911 As a GNU extension, you can specify the field width
14912 (after any flag, if present) as a decimal number. If the natural size of the
14913 output of the field has less than the specified number of characters,
14914 the result is written right adjusted and padded to the given
14915 size. For example, @samp{%9B} prints the right adjusted month name in
14916 a field of width 9.
14918 An optional modifier can follow the optional flag and width
14919 specification. The modifiers are:
14923 Use the locale's alternate representation for date and time. This
14924 modifier applies to the @samp{%c}, @samp{%C}, @samp{%x}, @samp{%X},
14925 @samp{%y} and @samp{%Y} conversion specifiers. In a Japanese locale, for
14926 example, @samp{%Ex} might yield a date format based on the Japanese
14930 Use the locale's alternate numeric symbols for numbers. This modifier
14931 applies only to numeric conversion specifiers.
14934 If the format supports the modifier but no alternate representation
14935 is available, it is ignored.
14938 @node Setting the time
14939 @subsection Setting the time
14941 @cindex setting the time
14942 @cindex time setting
14943 @cindex appropriate privileges
14945 If given an argument that does not start with @samp{+}, @command{date} sets
14946 the system clock to the date and time specified by that argument (as
14947 described below). You must have appropriate privileges to set the
14948 system clock. Note for changes to persist across a reboot, the
14949 hardware clock may need to be updated from the system clock, which
14950 might not happen automatically on your system.
14952 The argument must consist entirely of digits, which have the following
14965 first two digits of year (optional)
14967 last two digits of year (optional)
14972 Note, the @option{--date} and @option{--set} options may not be used with an
14973 argument in the above format. The @option{--universal} option may be used
14974 with such an argument to indicate that the specified date and time are
14975 relative to Coordinated Universal Time rather than to the local time zone.
14978 @node Options for date
14979 @subsection Options for @command{date}
14981 @cindex @command{date} options
14982 @cindex options for @command{date}
14984 The program accepts the following options. Also see @ref{Common options}.
14988 @item -d @var{datestr}
14989 @itemx --date=@var{datestr}
14992 @cindex parsing date strings
14993 @cindex date strings, parsing
14994 @cindex arbitrary date strings, parsing
14997 @opindex next @var{day}
14998 @opindex last @var{day}
14999 Display the date and time specified in @var{datestr} instead of the
15000 current date and time. @var{datestr} can be in almost any common
15001 format. It can contain month names, time zones, @samp{am} and @samp{pm},
15002 @samp{yesterday}, etc. For example, @option{--date="2004-02-27
15003 14:19:13.489392193 +0530"} specifies the instant of time that is
15004 489,392,193 nanoseconds after February 27, 2004 at 2:19:13 PM in a
15005 time zone that is 5 hours and 30 minutes east of UTC.@*
15006 Note: input currently must be in locale independent format. E.g., the
15007 LC_TIME=C below is needed to print back the correct date in many locales:
15009 date -d "$(LC_TIME=C date)"
15011 @xref{Date input formats}.
15013 @item -f @var{datefile}
15014 @itemx --file=@var{datefile}
15017 Parse each line in @var{datefile} as with @option{-d} and display the
15018 resulting date and time. If @var{datefile} is @samp{-}, use standard
15019 input. This is useful when you have many dates to process, because the
15020 system overhead of starting up the @command{date} executable many times can
15023 @item -I[@var{timespec}]
15024 @itemx --iso-8601[=@var{timespec}]
15025 @opindex -I[@var{timespec}]
15026 @opindex --iso-8601[=@var{timespec}]
15027 Display the date using the ISO 8601 format, @samp{%Y-%m-%d}.
15029 The argument @var{timespec} specifies the number of additional
15030 terms of the time to include. It can be one of the following:
15033 Print just the date. This is the default if @var{timespec} is omitted.
15036 Append the hour of the day to the date.
15039 Append the hours and minutes.
15042 Append the hours, minutes and seconds.
15045 Append the hours, minutes, seconds and nanoseconds.
15048 If showing any time terms, then include the time zone using the format
15051 @item -r @var{file}
15052 @itemx --reference=@var{file}
15054 @opindex --reference
15055 Display the date and time of the last modification of @var{file},
15056 instead of the current date and time.
15063 @opindex --rfc-2822
15064 Display the date and time using the format @samp{%a, %d %b %Y %H:%M:%S
15065 %z}, evaluated in the C locale so abbreviations are always in English.
15069 Fri, 09 Sep 2005 13:51:39 -0700
15072 This format conforms to
15073 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt, Internet
15075 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc822.txt, 822}, the
15076 current and previous standards for Internet email.
15078 @item --rfc-3339=@var{timespec}
15079 @opindex --rfc-3339=@var{timespec}
15080 Display the date using a format specified by
15081 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt, Internet
15082 RFC 3339}. This is a subset of the ISO 8601
15083 format, except that it also permits applications to use a space rather
15084 than a @samp{T} to separate dates from times. Unlike the other
15085 standard formats, RFC 3339 format is always suitable as
15086 input for the @option{--date} (@option{-d}) and @option{--file}
15087 (@option{-f}) options, regardless of the current locale.
15089 The argument @var{timespec} specifies how much of the time to include.
15090 It can be one of the following:
15094 Print just the full-date, e.g., @samp{2005-09-14}.
15095 This is equivalent to the format @samp{%Y-%m-%d}.
15098 Print the full-date and full-time separated by a space, e.g.,
15099 @samp{2005-09-14 00:56:06+05:30}. The output ends with a numeric
15100 time-offset; here the @samp{+05:30} means that local time is five
15101 hours and thirty minutes east of UTC@. This is equivalent to
15102 the format @samp{%Y-%m-%d %H:%M:%S%:z}.
15105 Like @samp{seconds}, but also print nanoseconds, e.g.,
15106 @samp{2005-09-14 00:56:06.998458565+05:30}.
15107 This is equivalent to the format @samp{%Y-%m-%d %H:%M:%S.%N%:z}.
15111 @item -s @var{datestr}
15112 @itemx --set=@var{datestr}
15115 Set the date and time to @var{datestr}. See @option{-d} above.
15116 See also @ref{Setting the time}.
15123 @opindex --universal
15124 @cindex Coordinated Universal Time
15126 @cindex Greenwich Mean Time
15128 @cindex leap seconds
15130 Use Coordinated Universal Time (UTC) by operating as if the
15131 @env{TZ} environment variable were set to the string @samp{UTC0}.
15133 Universal Time is often called ``Greenwich Mean Time'' (GMT) for
15134 historical reasons.
15135 Typically, systems ignore leap seconds and thus implement an
15136 approximation to UTC rather than true UTC.
15140 @node Examples of date
15141 @subsection Examples of @command{date}
15143 @cindex examples of @command{date}
15145 Here are a few examples. Also see the documentation for the @option{-d}
15146 option in the previous section.
15151 To print the date of the day before yesterday:
15154 date --date='2 days ago'
15158 To print the date of the day three months and one day hence:
15161 date --date='3 months 1 day'
15165 To print the day of year of Christmas in the current year:
15168 date --date='25 Dec' +%j
15172 To print the current full month name and the day of the month:
15178 But this may not be what you want because for the first nine days of
15179 the month, the @samp{%d} expands to a zero-padded two-digit field,
15180 for example @samp{date -d 1may '+%B %d'} will print @samp{May 01}.
15183 To print a date without the leading zero for one-digit days
15184 of the month, you can use the (GNU extension)
15185 @samp{-} flag to suppress
15186 the padding altogether:
15189 date -d 1may '+%B %-d
15193 To print the current date and time in the format required by many
15194 non-GNU versions of @command{date} when setting the system clock:
15197 date +%m%d%H%M%Y.%S
15201 To set the system clock forward by two minutes:
15204 date --set='+2 minutes'
15208 To print the date in RFC 2822 format,
15209 use @samp{date --rfc-2822}. Here is some example output:
15212 Fri, 09 Sep 2005 13:51:39 -0700
15215 @anchor{%s-examples}
15217 To convert a date string to the number of seconds since the epoch
15218 (which is 1970-01-01 00:00:00 UTC), use the @option{--date} option with
15219 the @samp{%s} format. That can be useful in sorting and/or graphing
15220 and/or comparing data by date. The following command outputs the
15221 number of the seconds since the epoch for the time two minutes after the
15225 date --date='1970-01-01 00:02:00 +0000' +%s
15229 If you do not specify time zone information in the date string,
15230 @command{date} uses your computer's idea of the time zone when
15231 interpreting the string. For example, if your computer's time zone is
15232 that of Cambridge, Massachusetts, which was then 5 hours (i.e., 18,000
15233 seconds) behind UTC:
15236 # local time zone used
15237 date --date='1970-01-01 00:02:00' +%s
15242 If you're sorting or graphing dated data, your raw date values may be
15243 represented as seconds since the epoch. But few people can look at
15244 the date @samp{946684800} and casually note ``Oh, that's the first second
15245 of the year 2000 in Greenwich, England.''
15248 date --date='2000-01-01 UTC' +%s
15252 An alternative is to use the @option{--utc} (@option{-u}) option.
15253 Then you may omit @samp{UTC} from the date string. Although this
15254 produces the same result for @samp{%s} and many other format sequences,
15255 with a time zone offset different from zero, it would give a different
15256 result for zone-dependent formats like @samp{%z}.
15259 date -u --date=2000-01-01 +%s
15263 To convert such an unwieldy number of seconds back to
15264 a more readable form, use a command like this:
15267 # local time zone used
15268 date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
15269 1999-12-31 19:00:00 -0500
15272 Or if you do not mind depending on the @samp{@@} feature present since
15273 coreutils 5.3.0, you could shorten this to:
15276 date -d @@946684800 +"%F %T %z"
15277 1999-12-31 19:00:00 -0500
15280 Often it is better to output UTC-relative date and time:
15283 date -u -d '1970-01-01 946684800 seconds' +"%Y-%m-%d %T %z"
15284 2000-01-01 00:00:00 +0000
15288 @cindex leap seconds
15289 Typically the seconds count omits leap seconds, but some systems are
15290 exceptions. Because leap seconds are not predictable, the mapping
15291 between the seconds count and a future timestamp is not reliable on
15292 the atypical systems that include leap seconds in their counts.
15294 Here is how the two kinds of systems handle the leap second at
15295 2012-06-30 23:59:60 UTC:
15298 # Typical systems ignore leap seconds:
15299 date --date='2012-06-30 23:59:59 +0000' +%s
15301 date --date='2012-06-30 23:59:60 +0000' +%s
15302 date: invalid date '2012-06-30 23:59:60 +0000'
15303 date --date='2012-07-01 00:00:00 +0000' +%s
15308 # Atypical systems count leap seconds:
15309 date --date='2012-06-30 23:59:59 +0000' +%s
15311 date --date='2012-06-30 23:59:60 +0000' +%s
15313 date --date='2012-07-01 00:00:00 +0000' +%s
15320 @node arch invocation
15321 @section @command{arch}: Print machine hardware name
15324 @cindex print machine hardware name
15325 @cindex system information, printing
15327 @command{arch} prints the machine hardware name,
15328 and is equivalent to @samp{uname -m}.
15332 arch [@var{option}]
15335 The program accepts the @ref{Common options} only.
15340 @node nproc invocation
15341 @section @command{nproc}: Print the number of available processors
15344 @cindex Print the number of processors
15345 @cindex system information, printing
15347 Print the number of processing units available to the current process,
15348 which may be less than the number of online processors.
15349 If this information is not accessible, then print the number of
15350 processors installed. If the @env{OMP_NUM_THREADS} environment variable is
15351 set, then it will determine the returned value. The result is guaranteed to be
15352 greater than zero. Synopsis:
15355 nproc [@var{option}]
15358 The program accepts the following options. Also see @ref{Common options}.
15364 Print the number of installed processors on the system, which may
15365 be greater than the number online or available to the current process.
15366 The @env{OMP_NUM_THREADS} environment variable is not honored in this case.
15368 @item --ignore=@var{number}
15370 If possible, exclude this @var{number} of processing units.
15377 @node uname invocation
15378 @section @command{uname}: Print system information
15381 @cindex print system information
15382 @cindex system information, printing
15384 @command{uname} prints information about the machine and operating system
15385 it is run on. If no options are given, @command{uname} acts as if the
15386 @option{-s} option were given. Synopsis:
15389 uname [@var{option}]@dots{}
15392 If multiple options or @option{-a} are given, the selected information is
15393 printed in this order:
15396 @var{kernel-name} @var{nodename} @var{kernel-release} @var{kernel-version}
15397 @var{machine} @var{processor} @var{hardware-platform} @var{operating-system}
15400 The information may contain internal spaces, so such output cannot be
15401 parsed reliably. In the following example, @var{release} is
15402 @samp{2.2.18ss.e820-bda652a #4 SMP Tue Jun 5 11:24:08 PDT 2001}:
15406 @result{} Linux dumdum 2.2.18 #4 SMP Tue Jun 5 11:24:08 PDT 2001 i686@c
15407 unknown unknown GNU/Linux
15411 The program accepts the following options. Also see @ref{Common options}.
15419 Print all of the below information, except omit the processor type
15420 and the hardware platform name if they are unknown.
15423 @itemx --hardware-platform
15425 @opindex --hardware-platform
15426 @cindex implementation, hardware
15427 @cindex hardware platform
15428 @cindex platform, hardware
15429 Print the hardware platform name
15430 (sometimes called the hardware implementation).
15431 Print @samp{unknown} if the kernel does not make this information
15432 easily available, as is the case with Linux kernels.
15438 @cindex machine type
15439 @cindex hardware class
15440 @cindex hardware type
15441 Print the machine hardware name (sometimes called the hardware class
15447 @opindex --nodename
15450 @cindex network node name
15451 Print the network node hostname.
15456 @opindex --processor
15457 @cindex host processor type
15458 Print the processor type (sometimes called the instruction set
15459 architecture or ISA).
15460 Print @samp{unknown} if the kernel does not make this information
15461 easily available, as is the case with Linux kernels.
15464 @itemx --operating-system
15466 @opindex --operating-system
15467 @cindex operating system name
15468 Print the name of the operating system.
15471 @itemx --kernel-release
15473 @opindex --kernel-release
15474 @cindex kernel release
15475 @cindex release of kernel
15476 Print the kernel release.
15479 @itemx --kernel-name
15481 @opindex --kernel-name
15482 @cindex kernel name
15483 @cindex name of kernel
15484 Print the kernel name.
15485 POSIX 1003.1-2001 (@pxref{Standards conformance}) calls this
15486 ``the implementation of the operating system'', because the
15487 POSIX specification itself has no notion of ``kernel''.
15488 The kernel name might be the same as the operating system name printed
15489 by the @option{-o} or @option{--operating-system} option, but it might
15490 differ. Some operating systems (e.g., FreeBSD, HP-UX) have the same
15491 name as their underlying kernels; others (e.g., GNU/Linux, Solaris)
15495 @itemx --kernel-version
15497 @opindex --kernel-version
15498 @cindex kernel version
15499 @cindex version of kernel
15500 Print the kernel version.
15507 @node hostname invocation
15508 @section @command{hostname}: Print or set system name
15511 @cindex setting the hostname
15512 @cindex printing the hostname
15513 @cindex system name, printing
15514 @cindex appropriate privileges
15516 With no arguments, @command{hostname} prints the name of the current host
15517 system. With one argument, it sets the current host name to the
15518 specified string. You must have appropriate privileges to set the host
15522 hostname [@var{name}]
15525 The only options are @option{--help} and @option{--version}. @xref{Common
15531 @node hostid invocation
15532 @section @command{hostid}: Print numeric host identifier
15535 @cindex printing the host identifier
15537 @command{hostid} prints the numeric identifier of the current host
15538 in hexadecimal. This command accepts no arguments.
15539 The only options are @option{--help} and @option{--version}.
15540 @xref{Common options}.
15542 For example, here's what it prints on one system I use:
15549 On that system, the 32-bit quantity happens to be closely
15550 related to the system's Internet address, but that isn't always
15555 @node uptime invocation
15556 @section @command{uptime}: Print system uptime and load
15559 @cindex printing the system uptime and load
15561 @command{uptime} prints the current time, the system's uptime, the
15562 number of logged-in users and the current load average.
15564 If an argument is specified, it is used as the file to be read
15565 to discover how many users are logged in. If no argument is
15566 specified, a system default is used (@command{uptime --help} indicates
15567 the default setting).
15569 The only options are @option{--help} and @option{--version}.
15570 @xref{Common options}.
15572 For example, here's what it prints right now on one system I use:
15576 14:07 up 3:35, 3 users, load average: 1.39, 1.15, 1.04
15579 The precise method of calculation of load average varies somewhat
15580 between systems. Some systems calculate it as the average number of
15581 runnable processes over the last 1, 5 and 15 minutes, but some systems
15582 also include processes in the uninterruptible sleep state (that is,
15583 those processes which are waiting for disk I/O). The Linux kernel
15584 includes uninterruptible processes.
15586 @node SELinux context
15587 @chapter SELinux context
15589 @cindex SELinux context
15590 @cindex SELinux, context
15591 @cindex commands for SELinux context
15593 This section describes commands for operations with SELinux
15597 * chcon invocation:: Change SELinux context of file
15598 * runcon invocation:: Run a command in specified SELinux context
15601 @node chcon invocation
15602 @section @command{chcon}: Change SELinux context of file
15605 @cindex changing security context
15606 @cindex change SELinux context
15608 @command{chcon} changes the SELinux security context of the selected files.
15612 chcon [@var{option}]@dots{} @var{context} @var{file}@dots{}
15613 chcon [@var{option}]@dots{} [-u @var{user}] [-r @var{role}] [-l @var{range}]@c
15614 [-t @var{type}] @var{file}@dots{}
15615 chcon [@var{option}]@dots{} --reference=@var{rfile} @var{file}@dots{}
15618 Change the SELinux security context of each @var{file} to @var{context}.
15619 With @option{--reference}, change the security context of each @var{file}
15620 to that of @var{rfile}.
15622 The program accepts the following options. Also see @ref{Common options}.
15626 @item --dereference
15627 @opindex --dereference
15628 Do not affect symbolic links but what they refer to; this is the default.
15631 @itemx --no-dereference
15633 @opindex --no-dereference
15634 @cindex no dereference
15635 Affect the symbolic links themselves instead of any referenced file.
15637 @item --reference=@var{rfile}
15638 @opindex --reference
15639 @cindex reference file
15640 Use @var{rfile}'s security context rather than specifying a @var{context} value.
15645 @opindex --recursive
15646 Operate on files and directories recursively.
15648 @item --preserve-root
15649 @opindex --preserve-root
15650 Refuse to operate recursively on the root directory, @file{/},
15651 when used together with the @option{--recursive} option.
15652 @xref{Treating / specially}.
15654 @item --no-preserve-root
15655 @opindex --no-preserve-root
15656 Do not treat the root directory, @file{/}, specially when operating
15657 recursively; this is the default.
15658 @xref{Treating / specially}.
15661 @xref{Traversing symlinks}.
15664 @xref{Traversing symlinks}.
15667 @xref{Traversing symlinks}.
15674 Output a diagnostic for every file processed.
15676 @item -u @var{user}
15677 @itemx --user=@var{user}
15680 Set user @var{user} in the target security context.
15682 @item -r @var{role}
15683 @itemx --role=@var{role}
15686 Set role @var{role} in the target security context.
15688 @item -t @var{type}
15689 @itemx --type=@var{type}
15692 Set type @var{type} in the target security context.
15694 @item -l @var{range}
15695 @itemx --range=@var{range}
15698 Set range @var{range} in the target security context.
15704 @node runcon invocation
15705 @section @command{runcon}: Run a command in specified SELinux context
15708 @cindex run with security context
15711 @command{runcon} runs file in specified SELinux security context.
15715 runcon @var{context} @var{command} [@var{args}]
15716 runcon [ -c ] [-u @var{user}] [-r @var{role}] [-t @var{type}]@c
15717 [-l @var{range}] @var{command} [@var{args}]
15720 Run @var{command} with completely-specified @var{context}, or with
15721 current or transitioned security context modified by one or more of @var{level},
15722 @var{role}, @var{type} and @var{user}.
15724 If none of @option{-c}, @option{-t}, @option{-u}, @option{-r}, or @option{-l}
15725 is specified, the first argument is used as the complete context.
15726 Any additional arguments after @var{command}
15727 are interpreted as arguments to the command.
15729 With neither @var{context} nor @var{command}, print the current
15732 The program accepts the following options. Also see @ref{Common options}.
15740 Compute process transition context before modifying.
15742 @item -u @var{user}
15743 @itemx --user=@var{user}
15746 Set user @var{user} in the target security context.
15748 @item -r @var{role}
15749 @itemx --role=@var{role}
15752 Set role @var{role} in the target security context.
15754 @item -t @var{type}
15755 @itemx --type=@var{type}
15758 Set type @var{type} in the target security context.
15760 @item -l @var{range}
15761 @itemx --range=@var{range}
15764 Set range @var{range} in the target security context.
15768 @cindex exit status of @command{runcon}
15772 126 if @var{command} is found but cannot be invoked
15773 127 if @command{runcon} itself fails or if @var{command} cannot be found
15774 the exit status of @var{command} otherwise
15777 @node Modified command invocation
15778 @chapter Modified command invocation
15780 @cindex modified command invocation
15781 @cindex invocation of commands, modified
15782 @cindex commands for invoking other commands
15784 This section describes commands that run other commands in some context
15785 different than the current one: a modified environment, as a different
15789 * chroot invocation:: Modify the root directory.
15790 * env invocation:: Modify environment variables.
15791 * nice invocation:: Modify niceness.
15792 * nohup invocation:: Immunize to hangups.
15793 * stdbuf invocation:: Modify buffering of standard streams.
15794 * timeout invocation:: Run with time limit.
15798 @node chroot invocation
15799 @section @command{chroot}: Run a command with a different root directory
15802 @cindex running a program in a specified root directory
15803 @cindex root directory, running a program in a specified
15805 @command{chroot} runs a command with a specified root directory.
15806 On many systems, only the super-user can do this.@footnote{However,
15807 some systems (e.g., FreeBSD) can be configured to allow certain regular
15808 users to use the @code{chroot} system call, and hence to run this program.
15809 Also, on Cygwin, anyone can run the @command{chroot} command, because the
15810 underlying function is non-privileged due to lack of support in MS-Windows.}
15814 chroot @var{option} @var{newroot} [@var{command} [@var{args}]@dots{}]
15815 chroot @var{option}
15818 Ordinarily, file names are looked up starting at the root of the
15819 directory structure, i.e., @file{/}. @command{chroot} changes the root to
15820 the directory @var{newroot} (which must exist) and then runs
15821 @var{command} with optional @var{args}. If @var{command} is not
15822 specified, the default is the value of the @env{SHELL} environment
15823 variable or @command{/bin/sh} if not set, invoked with the @option{-i} option.
15824 @var{command} must not be a special built-in utility
15825 (@pxref{Special built-in utilities}).
15827 The program accepts the following options. Also see @ref{Common options}.
15828 Options must precede operands.
15832 @item --userspec=@var{user}[:@var{group}]
15833 @opindex --userspec
15834 By default, @var{command} is run with the same credentials
15835 as the invoking process.
15836 Use this option to run it as a different @var{user} and/or with a
15837 different primary @var{group}.
15839 @item --groups=@var{groups}
15841 Use this option to specify the supplementary @var{groups} to be
15842 used by the new process.
15843 The items in the list (names or numeric IDs) must be separated by commas.
15847 Here are a few tips to help avoid common problems in using chroot.
15848 To start with a simple example, make @var{command} refer to a statically
15849 linked binary. If you were to use a dynamically linked executable, then
15850 you'd have to arrange to have the shared libraries in the right place under
15851 your new root directory.
15853 For example, if you create a statically linked @command{ls} executable,
15854 and put it in @file{/tmp/empty}, you can run this command as root:
15857 $ chroot /tmp/empty /ls -Rl /
15860 Then you'll see output like this:
15865 -rwxr-xr-x 1 0 0 1041745 Aug 16 11:17 ls
15868 If you want to use a dynamically linked executable, say @command{bash},
15869 then first run @samp{ldd bash} to see what shared objects it needs.
15870 Then, in addition to copying the actual binary, also copy the listed
15871 files to the required positions under your intended new root directory.
15872 Finally, if the executable requires any other files (e.g., data, state,
15873 device files), copy them into place, too.
15875 @cindex exit status of @command{chroot}
15879 125 if @command{chroot} itself fails
15880 126 if @var{command} is found but cannot be invoked
15881 127 if @var{command} cannot be found
15882 the exit status of @var{command} otherwise
15886 @node env invocation
15887 @section @command{env}: Run a command in a modified environment
15890 @cindex environment, running a program in a modified
15891 @cindex modified environment, running a program in a
15892 @cindex running a program in a modified environment
15894 @command{env} runs a command with a modified environment. Synopses:
15897 env [@var{option}]@dots{} [@var{name}=@var{value}]@dots{} @c
15898 [@var{command} [@var{args}]@dots{}]
15902 Operands of the form @samp{@var{variable}=@var{value}} set
15903 the environment variable @var{variable} to value @var{value}.
15904 @var{value} may be empty (@samp{@var{variable}=}). Setting a variable
15905 to an empty value is different from unsetting it.
15906 These operands are evaluated left-to-right, so if two operands
15907 mention the same variable the earlier is ignored.
15909 Environment variable names can be empty, and can contain any
15910 characters other than @samp{=} and ASCII NUL.
15911 However, it is wise to limit yourself to names that
15912 consist solely of underscores, digits, and ASCII letters,
15913 and that begin with a non-digit, as applications like the shell do not
15914 work well with other names.
15917 The first operand that does not contain the character @samp{=}
15918 specifies the program to invoke; it is
15919 searched for according to the @env{PATH} environment variable. Any
15920 remaining arguments are passed as arguments to that program.
15921 The program should not be a special built-in utility
15922 (@pxref{Special built-in utilities}).
15924 Modifications to @env{PATH} take effect prior to searching for
15925 @var{command}. Use caution when reducing @env{PATH}; behavior is
15926 not portable when @env{PATH} is undefined or omits key directories
15927 such as @file{/bin}.
15929 In the rare case that a utility contains a @samp{=} in the name, the
15930 only way to disambiguate it from a variable assignment is to use an
15931 intermediate command for @var{command}, and pass the problematic
15932 program name via @var{args}. For example, if @file{./prog=} is an
15933 executable in the current @env{PATH}:
15936 env prog= true # runs 'true', with prog= in environment
15937 env ./prog= true # runs 'true', with ./prog= in environment
15938 env -- prog= true # runs 'true', with prog= in environment
15939 env sh -c '\prog= true' # runs 'prog=' with argument 'true'
15940 env sh -c 'exec "$@@"' sh prog= true # also runs 'prog='
15943 @cindex environment, printing
15945 If no command name is specified following the environment
15946 specifications, the resulting environment is printed. This is like
15947 specifying the @command{printenv} program.
15949 For some examples, suppose the environment passed to @command{env}
15950 contains @samp{LOGNAME=rms}, @samp{EDITOR=emacs}, and
15951 @samp{PATH=.:/gnubin:/hacks}:
15956 Output the current environment.
15958 $ env | LC_ALL=C sort
15961 PATH=.:/gnubin:/hacks
15965 Run @command{foo} with a reduced environment, preserving only the
15966 original @env{PATH} to avoid problems in locating @command{foo}.
15968 env - PATH="$PATH" foo
15972 Run @command{foo} with the environment containing @samp{LOGNAME=rms},
15973 @samp{EDITOR=emacs}, and @samp{PATH=.:/gnubin:/hacks}, and guarantees
15974 that @command{foo} was found in the file system rather than as a shell
15981 Run @command{nemacs} with the environment containing @samp{LOGNAME=foo},
15982 @samp{EDITOR=emacs}, @samp{PATH=.:/gnubin:/hacks}, and
15983 @samp{DISPLAY=gnu:0}.
15985 env DISPLAY=gnu:0 LOGNAME=foo nemacs
15989 Attempt to run the program @command{/energy/--} (as that is the only
15990 possible path search result); if the command exists, the environment
15991 will contain @samp{LOGNAME=rms} and @samp{PATH=/energy}, and the
15992 arguments will be @samp{e=mc2}, @samp{bar}, and @samp{baz}.
15994 env -u EDITOR PATH=/energy -- e=mc2 bar baz
16000 The program accepts the following options. Also see @ref{Common options}.
16001 Options must precede operands.
16007 @item -u @var{name}
16008 @itemx --unset=@var{name}
16011 Remove variable @var{name} from the environment, if it was in the
16016 @itemx --ignore-environment
16019 @opindex --ignore-environment
16020 Start with an empty environment, ignoring the inherited environment.
16024 @cindex exit status of @command{env}
16028 0 if no @var{command} is specified and the environment is output
16029 125 if @command{env} itself fails
16030 126 if @var{command} is found but cannot be invoked
16031 127 if @var{command} cannot be found
16032 the exit status of @var{command} otherwise
16036 @node nice invocation
16037 @section @command{nice}: Run a command with modified niceness
16041 @cindex scheduling, affecting
16042 @cindex appropriate privileges
16044 @command{nice} prints a process's @dfn{niceness}, or runs
16045 a command with modified niceness. @dfn{niceness} affects how
16046 favorably the process is scheduled in the system.
16050 nice [@var{option}]@dots{} [@var{command} [@var{arg}]@dots{}]
16053 If no arguments are given, @command{nice} prints the current niceness.
16054 Otherwise, @command{nice} runs the given @var{command} with its
16055 niceness adjusted. By default, its niceness is incremented by 10.
16057 Niceness values range at least from @minus{}20 (process has high priority
16058 and gets more resources, thus slowing down other processes) through 19
16059 (process has lower priority and runs slowly itself, but has less impact
16060 on the speed of other running processes). Some systems
16061 may have a wider range of niceness values; conversely, other systems may
16062 enforce more restrictive limits. An attempt to set the niceness
16063 outside the supported range is treated as an attempt to use the
16064 minimum or maximum supported value.
16066 A niceness should not be confused with a scheduling priority, which
16067 lets applications determine the order in which threads are scheduled
16068 to run. Unlike a priority, a niceness is merely advice to the
16069 scheduler, which the scheduler is free to ignore. Also, as a point of
16070 terminology, POSIX defines the behavior of @command{nice} in
16071 terms of a @dfn{nice value}, which is the nonnegative difference
16072 between a niceness and the minimum niceness. Though @command{nice}
16073 conforms to POSIX, its documentation and diagnostics use the
16074 term ``niceness'' for compatibility with historical practice.
16076 @var{command} must not be a special built-in utility (@pxref{Special
16077 built-in utilities}).
16079 @mayConflictWithShellBuiltIn{nice}
16081 Note to change the @dfn{niceness} of an existing process,
16082 one needs to use the @command{renice} command.
16084 The program accepts the following option. Also see @ref{Common options}.
16085 Options must precede operands.
16088 @item -n @var{adjustment}
16089 @itemx --adjustment=@var{adjustment}
16091 @opindex --adjustment
16092 Add @var{adjustment} instead of 10 to the command's niceness. If
16093 @var{adjustment} is negative and you lack appropriate privileges,
16094 @command{nice} issues a warning but otherwise acts as if you specified
16097 For compatibility @command{nice} also supports an obsolete
16098 option syntax @option{-@var{adjustment}}. New scripts should use
16099 @option{-n @var{adjustment}} instead.
16103 @cindex exit status of @command{nice}
16107 0 if no @var{command} is specified and the niceness is output
16108 125 if @command{nice} itself fails
16109 126 if @var{command} is found but cannot be invoked
16110 127 if @var{command} cannot be found
16111 the exit status of @var{command} otherwise
16114 It is sometimes useful to run a non-interactive program with reduced niceness.
16117 $ nice factor 4611686018427387903
16120 Since @command{nice} prints the current niceness,
16121 you can invoke it through itself to demonstrate how it works.
16123 The default behavior is to increase the niceness by @samp{10}:
16134 The @var{adjustment} is relative to the current niceness. In the
16135 next example, the first @command{nice} invocation runs the second one
16136 with niceness 10, and it in turn runs the final one with a niceness
16140 $ nice nice -n 3 nice
16144 Specifying a niceness larger than the supported range
16145 is the same as specifying the maximum supported value:
16148 $ nice -n 10000000000 nice
16152 Only a privileged user may run a process with lower niceness:
16156 nice: cannot set niceness: Permission denied
16158 $ sudo nice -n -1 nice
16163 @node nohup invocation
16164 @section @command{nohup}: Run a command immune to hangups
16167 @cindex hangups, immunity to
16168 @cindex immunity to hangups
16169 @cindex logging out and continuing to run
16172 @command{nohup} runs the given @var{command} with hangup signals ignored,
16173 so that the command can continue running in the background after you log
16177 nohup @var{command} [@var{arg}]@dots{}
16180 If standard input is a terminal, it is redirected from
16181 @file{/dev/null} so that terminal sessions do not mistakenly consider
16182 the terminal to be used by the command. This is a GNU
16183 extension; programs intended to be portable to non-GNU hosts
16184 should use @samp{nohup @var{command} [@var{arg}]@dots{} </dev/null}
16188 If standard output is a terminal, the command's standard output is appended
16189 to the file @file{nohup.out}; if that cannot be written to, it is appended
16190 to the file @file{$HOME/nohup.out}; and if that cannot be written to, the
16191 command is not run.
16192 Any @file{nohup.out} or @file{$HOME/nohup.out} file created by
16193 @command{nohup} is made readable and writable only to the user,
16194 regardless of the current umask settings.
16196 If standard error is a terminal, it is normally redirected to the same file
16197 descriptor as the (possibly-redirected) standard output.
16198 However, if standard output is closed, standard error terminal output
16199 is instead appended to the file @file{nohup.out} or
16200 @file{$HOME/nohup.out} as above.
16202 To capture the command's output to a file other than @file{nohup.out}
16203 you can redirect it. For example, to capture the output of
16207 nohup make > make.log
16210 @command{nohup} does not automatically put the command it runs in the
16211 background; you must do that explicitly, by ending the command line
16212 with an @samp{&}. Also, @command{nohup} does not alter the
16213 niceness of @var{command}; use @command{nice} for that,
16214 e.g., @samp{nohup nice @var{command}}.
16216 @var{command} must not be a special built-in utility (@pxref{Special
16217 built-in utilities}).
16219 The only options are @option{--help} and @option{--version}. @xref{Common
16220 options}. Options must precede operands.
16222 @cindex exit status of @command{nohup}
16226 125 if @command{nohup} itself fails, and @env{POSIXLY_CORRECT} is not set
16227 126 if @var{command} is found but cannot be invoked
16228 127 if @var{command} cannot be found
16229 the exit status of @var{command} otherwise
16232 If @env{POSIXLY_CORRECT} is set, internal failures give status 127
16236 @node stdbuf invocation
16237 @section @command{stdbuf}: Run a command with modified I/O stream buffering
16240 @cindex standard streams, buffering
16241 @cindex line buffered
16243 @command{stdbuf} allows one to modify the buffering operations of the
16244 three standard I/O streams associated with a program. Synopsis:
16247 stdbuf @var{option}@dots{} @var{command}
16250 @var{command} must start with the name of a program that
16253 uses the ISO C @code{FILE} streams for input/output (note the
16254 programs @command{dd} and @command{cat} don't do that),
16257 does not adjust the buffering of its standard streams (note the
16258 program @command{tee} is not in this category).
16261 Any additional @var{arg}s are passed as additional arguments to the
16264 The program accepts the following options. Also see @ref{Common options}.
16268 @item -i @var{mode}
16269 @itemx --input=@var{mode}
16272 Adjust the standard input stream buffering.
16274 @item -o @var{mode}
16275 @itemx --output=@var{mode}
16278 Adjust the standard output stream buffering.
16280 @item -e @var{mode}
16281 @itemx --error=@var{mode}
16284 Adjust the standard error stream buffering.
16288 The @var{mode} can be specified as follows:
16293 Set the stream to line buffered mode.
16294 In this mode data is coalesced until a newline is output or
16295 input is read from any stream attached to a terminal device.
16296 This option is invalid with standard input.
16299 Disable buffering of the selected stream.
16300 In this mode, data is output immediately and only the
16301 amount of data requested is read from input.
16302 Note the difference in function for input and output.
16303 Disabling buffering for input will not influence the responsiveness
16304 or blocking behavior of the stream input functions.
16305 For example @code{fread} will still block until @code{EOF} or error,
16306 even if the underlying @code{read} returns less data than requested.
16309 Specify the size of the buffer to use in fully buffered mode.
16310 @multiplierSuffixesNoBlocks{size}
16314 @cindex exit status of @command{stdbuf}
16318 125 if @command{stdbuf} itself fails
16319 126 if @var{command} is found but cannot be invoked
16320 127 if @var{command} cannot be found
16321 the exit status of @var{command} otherwise
16325 @node timeout invocation
16326 @section @command{timeout}: Run a command with a time limit
16330 @cindex run commands with bounded time
16332 @command{timeout} runs the given @var{command} and kills it if it is
16333 still running after the specified time interval. Synopsis:
16336 timeout [@var{option}] @var{duration} @var{command} [@var{arg}]@dots{}
16339 @var{command} must not be a special built-in utility (@pxref{Special
16340 built-in utilities}).
16342 The program accepts the following options. Also see @ref{Common options}.
16343 Options must precede operands.
16346 @item --preserve-status
16347 @opindex --preserve-status
16348 Return the exit status of the managed @var{command} on timeout, rather than
16349 a specific exit status indicating a timeout. This is useful if the
16350 managed @var{command} supports running for an indeterminite amount of time.
16353 @opindex --foreground
16354 Don't create a separate background program group, so that
16355 the managed @var{command} can use the foreground TTY normally.
16356 This is needed to support timing out commands not started
16357 directly from an interactive shell, in two situations.
16360 @var{command} is interactive and needs to read from the terminal for example
16362 the user wants to support sending signals directly to @var{command}
16363 from the terminal (like Ctrl-C for example)
16366 Note in this mode of operation, any children of @var{command}
16367 will not be timed out.
16369 @item -k @var{duration}
16370 @itemx --kill-after=@var{duration}
16372 @opindex --kill-after
16373 Ensure the monitored @var{command} is killed by also sending a @samp{KILL}
16374 signal, after the specified @var{duration}. Without this option, if the
16375 selected signal proves not to be fatal, @command{timeout} does not kill
16378 @item -s @var{signal}
16379 @itemx --signal=@var{signal}
16382 Send this @var{signal} to @var{command} on timeout, rather than the
16383 default @samp{TERM} signal. @var{signal} may be a name like @samp{HUP}
16384 or a number. @xref{Signal specifications}.
16388 @var{duration} is a floating point number followed by an optional unit:
16390 @samp{s} for seconds (the default)
16391 @samp{m} for minutes
16395 A duration of 0 disables the associated timeout.
16396 Note that the actual timeout duration is dependent on system conditions,
16397 which should be especially considered when specifying sub-second timeouts.
16399 @cindex exit status of @command{timeout}
16403 124 if @var{command} times out
16404 125 if @command{timeout} itself fails
16405 126 if @var{command} is found but cannot be invoked
16406 127 if @var{command} cannot be found
16407 137 if @var{command} is sent the KILL(9) signal (128+9)
16408 the exit status of @var{command} otherwise
16412 @node Process control
16413 @chapter Process control
16415 @cindex processes, commands for controlling
16416 @cindex commands for controlling processes
16419 * kill invocation:: Sending a signal to processes.
16423 @node kill invocation
16424 @section @command{kill}: Send a signal to processes
16427 @cindex send a signal to processes
16429 The @command{kill} command sends a signal to processes, causing them
16430 to terminate or otherwise act upon receiving the signal in some way.
16431 Alternatively, it lists information about signals. Synopses:
16434 kill [-s @var{signal} | --signal @var{signal} | -@var{signal}] @var{pid}@dots{}
16435 kill [-l | --list | -t | --table] [@var{signal}]@dots{}
16438 @mayConflictWithShellBuiltIn{kill}
16440 The first form of the @command{kill} command sends a signal to all
16441 @var{pid} arguments. The default signal to send if none is specified
16442 is @samp{TERM}@. The special signal number @samp{0} does not denote a
16443 valid signal, but can be used to test whether the @var{pid} arguments
16444 specify processes to which a signal could be sent.
16446 If @var{pid} is positive, the signal is sent to the process with the
16447 process ID @var{pid}. If @var{pid} is zero, the signal is sent to all
16448 processes in the process group of the current process. If @var{pid}
16449 is @minus{}1, the signal is sent to all processes for which the user has
16450 permission to send a signal. If @var{pid} is less than @minus{}1, the signal
16451 is sent to all processes in the process group that equals the absolute
16452 value of @var{pid}.
16454 If @var{pid} is not positive, a system-dependent set of system
16455 processes is excluded from the list of processes to which the signal
16458 If a negative @var{pid} argument is desired as the first one, it
16459 should be preceded by @option{--}. However, as a common extension to
16460 POSIX, @option{--} is not required with @samp{kill
16461 -@var{signal} -@var{pid}}. The following commands are equivalent:
16470 The first form of the @command{kill} command succeeds if every @var{pid}
16471 argument specifies at least one process that the signal was sent to.
16473 The second form of the @command{kill} command lists signal information.
16474 Either the @option{-l} or @option{--list} option, or the @option{-t}
16475 or @option{--table} option must be specified. Without any
16476 @var{signal} argument, all supported signals are listed. The output
16477 of @option{-l} or @option{--list} is a list of the signal names, one
16478 per line; if @var{signal} is already a name, the signal number is
16479 printed instead. The output of @option{-t} or @option{--table} is a
16480 table of signal numbers, names, and descriptions. This form of the
16481 @command{kill} command succeeds if all @var{signal} arguments are valid
16482 and if there is no output error.
16484 The @command{kill} command also supports the @option{--help} and
16485 @option{--version} options. @xref{Common options}.
16487 A @var{signal} may be a signal name like @samp{HUP}, or a signal
16488 number like @samp{1}, or an exit status of a process terminated by the
16489 signal. A signal name can be given in canonical form or prefixed by
16490 @samp{SIG}@. The case of the letters is ignored, except for the
16491 @option{-@var{signal}} option which must use upper case to avoid
16492 ambiguity with lower case option letters.
16493 @xref{Signal specifications}, for a list of supported
16494 signal names and numbers.
16499 @cindex delaying commands
16500 @cindex commands for delaying
16502 @c Perhaps @command{wait} or other commands should be described here also?
16505 * sleep invocation:: Delay for a specified time.
16509 @node sleep invocation
16510 @section @command{sleep}: Delay for a specified time
16513 @cindex delay for a specified time
16515 @command{sleep} pauses for an amount of time specified by the sum of
16516 the values of the command line arguments.
16520 sleep @var{number}[smhd]@dots{}
16524 Each argument is a number followed by an optional unit; the default
16525 is seconds. The units are:
16538 Historical implementations of @command{sleep} have required that
16539 @var{number} be an integer, and only accepted a single argument
16540 without a suffix. However, GNU @command{sleep} accepts
16541 arbitrary floating point numbers. @xref{Floating point}.
16543 The only options are @option{--help} and @option{--version}. @xref{Common
16546 @c sleep is a shell built-in at least with Solaris 11's /bin/sh
16547 @mayConflictWithShellBuiltIn{sleep}
16552 @node Numeric operations
16553 @chapter Numeric operations
16555 @cindex numeric operations
16556 These programs do numerically-related operations.
16559 * factor invocation:: Show factors of numbers.
16560 * seq invocation:: Print sequences of numbers.
16564 @node factor invocation
16565 @section @command{factor}: Print prime factors
16568 @cindex prime factors
16570 @command{factor} prints prime factors. Synopses:
16573 factor [@var{number}]@dots{}
16574 factor @var{option}
16577 If no @var{number} is specified on the command line, @command{factor} reads
16578 numbers from standard input, delimited by newlines, tabs, or spaces.
16580 The @command{factor} command supports only a small number of options:
16584 Print a short help on standard output, then exit without further
16588 Print the program version on standard output, then exit without further
16592 Factoring the product of the eighth and ninth Mersenne primes
16593 takes about 30 milliseconds of CPU time on a 2.2 GHz Athlon.
16596 M8=$(echo 2^31-1|bc)
16597 M9=$(echo 2^61-1|bc)
16598 n=$(echo "$M8 * $M9" | bc)
16599 /usr/bin/time -f %U factor $n
16600 4951760154835678088235319297: 2147483647 2305843009213693951
16604 Similarly, factoring the eighth Fermat number @math{2^{256}+1} takes
16605 about 20 seconds on the same machine.
16607 Factoring large numbers is, in general, hard. The Pollard Rho
16608 algorithm used by @command{factor} is particularly effective for
16609 numbers with relatively small factors. If you wish to factor large
16610 numbers which do not have small factors (for example, numbers which
16611 are the product of two large primes), other methods are far better.
16613 If @command{factor} is built without using GNU MP, only
16614 single-precision arithmetic is available, and so large numbers
16615 (typically @math{2^{64}} and above) will not be supported. The single-precision
16616 code uses an algorithm which is designed for factoring smaller
16622 @node seq invocation
16623 @section @command{seq}: Print numeric sequences
16626 @cindex numeric sequences
16627 @cindex sequence of numbers
16629 @command{seq} prints a sequence of numbers to standard output. Synopses:
16632 seq [@var{option}]@dots{} @var{last}
16633 seq [@var{option}]@dots{} @var{first} @var{last}
16634 seq [@var{option}]@dots{} @var{first} @var{increment} @var{last}
16637 @command{seq} prints the numbers from @var{first} to @var{last} by
16638 @var{increment}. By default, each number is printed on a separate line.
16639 When @var{increment} is not specified, it defaults to @samp{1},
16640 even when @var{first} is larger than @var{last}.
16641 @var{first} also defaults to @samp{1}. So @code{seq 1} prints
16642 @samp{1}, but @code{seq 0} and @code{seq 10 5} produce no output.
16643 Floating-point numbers may be specified. @xref{Floating point}.
16645 The program accepts the following options. Also see @ref{Common options}.
16646 Options must precede operands.
16649 @item -f @var{format}
16650 @itemx --format=@var{format}
16651 @opindex -f @var{format}
16652 @opindex --format=@var{format}
16653 @cindex formatting of numbers in @command{seq}
16654 Print all numbers using @var{format}.
16655 @var{format} must contain exactly one of the @samp{printf}-style
16656 floating point conversion specifications @samp{%a}, @samp{%e},
16657 @samp{%f}, @samp{%g}, @samp{%A}, @samp{%E}, @samp{%F}, @samp{%G}@.
16658 The @samp{%} may be followed by zero or more flags taken from the set
16659 @samp{-+#0 '}, then an optional width containing one or more digits,
16660 then an optional precision consisting of a @samp{.} followed by zero
16661 or more digits. @var{format} may also contain any number of @samp{%%}
16662 conversion specifications. All conversion specifications have the
16663 same meaning as with @samp{printf}.
16665 The default format is derived from @var{first}, @var{step}, and
16666 @var{last}. If these all use a fixed point decimal representation,
16667 the default format is @samp{%.@var{p}f}, where @var{p} is the minimum
16668 precision that can represent the output numbers exactly. Otherwise,
16669 the default format is @samp{%g}.
16671 @item -s @var{string}
16672 @itemx --separator=@var{string}
16673 @cindex separator for numbers in @command{seq}
16674 Separate numbers with @var{string}; default is a newline.
16675 The output always terminates with a newline.
16678 @itemx --equal-width
16679 Print all numbers with the same width, by padding with leading zeros.
16680 @var{first}, @var{step}, and @var{last} should all use a fixed point
16681 decimal representation.
16682 (To have other kinds of padding, use @option{--format}).
16686 You can get finer-grained control over output with @option{-f}:
16689 $ seq -f '(%9.2E)' -9e5 1.1e6 1.3e6
16695 If you want hexadecimal integer output, you can use @command{printf}
16696 to perform the conversion:
16699 $ printf '%x\n' $(seq 1048575 1024 1050623)
16705 For very long lists of numbers, use xargs to avoid
16706 system limitations on the length of an argument list:
16709 $ seq 1000000 | xargs printf '%x\n' | tail -n 3
16715 To generate octal output, use the printf @code{%o} format instead
16718 On most systems, seq can produce whole-number output for values up to
16719 at least @math{2^{53}}. Larger integers are approximated. The details
16720 differ depending on your floating-point implementation.
16721 @xref{Floating point}. A common
16722 case is that @command{seq} works with integers through @math{2^{64}},
16723 and larger integers may not be numerically correct:
16726 $ seq 50000000000000000000 2 50000000000000000004
16727 50000000000000000000
16728 50000000000000000000
16729 50000000000000000004
16732 However, note that when limited to non-negative whole numbers,
16733 an increment of 1 and no format-specifying option, seq can print
16734 arbitrarily large numbers.
16736 Be careful when using @command{seq} with outlandish values: otherwise
16737 you may see surprising results, as @command{seq} uses floating point
16738 internally. For example, on the x86 platform, where the internal
16739 representation uses a 64-bit fraction, the command:
16742 seq 1 0.0000000000000000001 1.0000000000000000009
16745 outputs 1.0000000000000000007 twice and skips 1.0000000000000000008.
16750 @node File permissions
16751 @chapter File permissions
16754 @include parse-datetime.texi
16758 @node Opening the software toolbox
16759 @chapter Opening the Software Toolbox
16761 An earlier version of this chapter appeared in
16762 @uref{http://www.linuxjournal.com/article.php?sid=2762, the
16763 @cite{What's GNU@?} column of the June 1994 @cite{Linux Journal}}.
16764 It was written by Arnold Robbins.
16767 * Toolbox introduction:: Toolbox introduction
16768 * I/O redirection:: I/O redirection
16769 * The who command:: The @command{who} command
16770 * The cut command:: The @command{cut} command
16771 * The sort command:: The @command{sort} command
16772 * The uniq command:: The @command{uniq} command
16773 * Putting the tools together:: Putting the tools together
16777 @node Toolbox introduction
16778 @unnumberedsec Toolbox Introduction
16780 This month's column is only peripherally related to the GNU Project, in
16781 that it describes a number of the GNU tools on your GNU/Linux system
16783 might be used. What it's really about is the ``Software Tools'' philosophy
16784 of program development and usage.
16786 The software tools philosophy was an important and integral concept
16787 in the initial design and development of Unix (of which Linux and GNU are
16788 essentially clones). Unfortunately, in the modern day press of
16789 Internetworking and flashy GUIs, it seems to have fallen by the
16790 wayside. This is a shame, since it provides a powerful mental model
16791 for solving many kinds of problems.
16793 Many people carry a Swiss Army knife around in their pants pockets (or
16794 purse). A Swiss Army knife is a handy tool to have: it has several knife
16795 blades, a screwdriver, tweezers, toothpick, nail file, corkscrew, and perhaps
16796 a number of other things on it. For the everyday, small miscellaneous jobs
16797 where you need a simple, general purpose tool, it's just the thing.
16799 On the other hand, an experienced carpenter doesn't build a house using
16800 a Swiss Army knife. Instead, he has a toolbox chock full of specialized
16801 tools---a saw, a hammer, a screwdriver, a plane, and so on. And he knows
16802 exactly when and where to use each tool; you won't catch him hammering nails
16803 with the handle of his screwdriver.
16805 The Unix developers at Bell Labs were all professional programmers and trained
16806 computer scientists. They had found that while a one-size-fits-all program
16807 might appeal to a user because there's only one program to use, in practice
16812 difficult to write,
16815 difficult to maintain and
16819 difficult to extend to meet new situations.
16822 Instead, they felt that programs should be specialized tools. In short, each
16823 program ``should do one thing well.'' No more and no less. Such programs are
16824 simpler to design, write, and get right---they only do one thing.
16826 Furthermore, they found that with the right machinery for hooking programs
16827 together, that the whole was greater than the sum of the parts. By combining
16828 several special purpose programs, you could accomplish a specific task
16829 that none of the programs was designed for, and accomplish it much more
16830 quickly and easily than if you had to write a special purpose program.
16831 We will see some (classic) examples of this further on in the column.
16832 (An important additional point was that, if necessary, take a detour
16833 and build any software tools you may need first, if you don't already
16834 have something appropriate in the toolbox.)
16836 @node I/O redirection
16837 @unnumberedsec I/O Redirection
16839 Hopefully, you are familiar with the basics of I/O redirection in the
16840 shell, in particular the concepts of ``standard input,'' ``standard output,''
16841 and ``standard error''. Briefly, ``standard input'' is a data source, where
16842 data comes from. A program should not need to either know or care if the
16843 data source is a disk file, a keyboard, a magnetic tape, or even a punched
16844 card reader. Similarly, ``standard output'' is a data sink, where data goes
16845 to. The program should neither know nor care where this might be.
16846 Programs that only read their standard input, do something to the data,
16847 and then send it on, are called @dfn{filters}, by analogy to filters in a
16850 With the Unix shell, it's very easy to set up data pipelines:
16853 program_to_create_data | filter1 | ... | filterN > final.pretty.data
16856 We start out by creating the raw data; each filter applies some successive
16857 transformation to the data, until by the time it comes out of the pipeline,
16858 it is in the desired form.
16860 This is fine and good for standard input and standard output. Where does the
16861 standard error come in to play? Well, think about @command{filter1} in
16862 the pipeline above. What happens if it encounters an error in the data it
16863 sees? If it writes an error message to standard output, it will just
16864 disappear down the pipeline into @command{filter2}'s input, and the
16865 user will probably never see it. So programs need a place where they can send
16866 error messages so that the user will notice them. This is standard error,
16867 and it is usually connected to your console or window, even if you have
16868 redirected standard output of your program away from your screen.
16870 For filter programs to work together, the format of the data has to be
16871 agreed upon. The most straightforward and easiest format to use is simply
16872 lines of text. Unix data files are generally just streams of bytes, with
16873 lines delimited by the ASCII LF (Line Feed) character,
16874 conventionally called a ``newline'' in the Unix literature. (This is
16875 @code{'\n'} if you're a C programmer.) This is the format used by all
16876 the traditional filtering programs. (Many earlier operating systems
16877 had elaborate facilities and special purpose programs for managing
16878 binary data. Unix has always shied away from such things, under the
16879 philosophy that it's easiest to simply be able to view and edit your
16880 data with a text editor.)
16882 OK, enough introduction. Let's take a look at some of the tools, and then
16883 we'll see how to hook them together in interesting ways. In the following
16884 discussion, we will only present those command line options that interest
16885 us. As you should always do, double check your system documentation
16886 for the full story.
16888 @node The who command
16889 @unnumberedsec The @command{who} Command
16891 The first program is the @command{who} command. By itself, it generates a
16892 list of the users who are currently logged in. Although I'm writing
16893 this on a single-user system, we'll pretend that several people are
16898 @print{} arnold console Jan 22 19:57
16899 @print{} miriam ttyp0 Jan 23 14:19(:0.0)
16900 @print{} bill ttyp1 Jan 21 09:32(:0.0)
16901 @print{} arnold ttyp2 Jan 23 20:48(:0.0)
16904 Here, the @samp{$} is the usual shell prompt, at which I typed @samp{who}.
16905 There are three people logged in, and I am logged in twice. On traditional
16906 Unix systems, user names are never more than eight characters long. This
16907 little bit of trivia will be useful later. The output of @command{who} is nice,
16908 but the data is not all that exciting.
16910 @node The cut command
16911 @unnumberedsec The @command{cut} Command
16913 The next program we'll look at is the @command{cut} command. This program
16914 cuts out columns or fields of input data. For example, we can tell it
16915 to print just the login name and full name from the @file{/etc/passwd}
16916 file. The @file{/etc/passwd} file has seven fields, separated by
16920 arnold:xyzzy:2076:10:Arnold D. Robbins:/home/arnold:/bin/bash
16923 To get the first and fifth fields, we would use @command{cut} like this:
16926 $ cut -d: -f1,5 /etc/passwd
16927 @print{} root:Operator
16929 @print{} arnold:Arnold D. Robbins
16930 @print{} miriam:Miriam A. Robbins
16934 With the @option{-c} option, @command{cut} will cut out specific characters
16935 (i.e., columns) in the input lines. This is useful for input data
16936 that has fixed width fields, and does not have a field separator. For
16937 example, list the Monday dates for the current month:
16939 @c Is using cal ok? Looked at gcal, but I don't like it.
16950 @node The sort command
16951 @unnumberedsec The @command{sort} Command
16953 Next we'll look at the @command{sort} command. This is one of the most
16954 powerful commands on a Unix-style system; one that you will often find
16955 yourself using when setting up fancy data plumbing.
16958 command reads and sorts each file named on the command line. It then
16959 merges the sorted data and writes it to standard output. It will read
16960 standard input if no files are given on the command line (thus
16961 making it into a filter). The sort is based on the character collating
16962 sequence or based on user-supplied ordering criteria.
16965 @node The uniq command
16966 @unnumberedsec The @command{uniq} Command
16968 Finally (at least for now), we'll look at the @command{uniq} program. When
16969 sorting data, you will often end up with duplicate lines, lines that
16970 are identical. Usually, all you need is one instance of each line.
16971 This is where @command{uniq} comes in. The @command{uniq} program reads its
16972 standard input. It prints only one
16973 copy of each repeated line. It does have several options. Later on,
16974 we'll use the @option{-c} option, which prints each unique line, preceded
16975 by a count of the number of times that line occurred in the input.
16978 @node Putting the tools together
16979 @unnumberedsec Putting the Tools Together
16981 Now, let's suppose this is a large ISP server system with dozens of users
16982 logged in. The management wants the system administrator to write a
16984 generate a sorted list of logged in users. Furthermore, even if a user
16985 is logged in multiple times, his or her name should only show up in the
16988 The administrator could sit down with the system documentation and write a C
16989 program that did this. It would take perhaps a couple of hundred lines
16990 of code and about two hours to write it, test it, and debug it.
16991 However, knowing the software toolbox, the administrator can instead start out
16992 by generating just a list of logged on users:
17002 Next, sort the list:
17005 $ who | cut -c1-8 | sort
17012 Finally, run the sorted list through @command{uniq}, to weed out duplicates:
17015 $ who | cut -c1-8 | sort | uniq
17021 The @command{sort} command actually has a @option{-u} option that does what
17022 @command{uniq} does. However, @command{uniq} has other uses for which one
17023 cannot substitute @samp{sort -u}.
17025 The administrator puts this pipeline into a shell script, and makes it
17027 all the users on the system (@samp{#} is the system administrator,
17028 or @code{root}, prompt):
17031 # cat > /usr/local/bin/listusers
17032 who | cut -c1-8 | sort | uniq
17034 # chmod +x /usr/local/bin/listusers
17037 There are four major points to note here. First, with just four
17038 programs, on one command line, the administrator was able to save about two
17039 hours worth of work. Furthermore, the shell pipeline is just about as
17040 efficient as the C program would be, and it is much more efficient in
17041 terms of programmer time. People time is much more expensive than
17042 computer time, and in our modern ``there's never enough time to do
17043 everything'' society, saving two hours of programmer time is no mean
17046 Second, it is also important to emphasize that with the
17047 @emph{combination} of the tools, it is possible to do a special
17048 purpose job never imagined by the authors of the individual programs.
17050 Third, it is also valuable to build up your pipeline in stages, as we did here.
17051 This allows you to view the data at each stage in the pipeline, which helps
17052 you acquire the confidence that you are indeed using these tools correctly.
17054 Finally, by bundling the pipeline in a shell script, other users can use
17055 your command, without having to remember the fancy plumbing you set up for
17056 them. In terms of how you run them, shell scripts and compiled programs are
17059 After the previous warm-up exercise, we'll look at two additional, more
17060 complicated pipelines. For them, we need to introduce two more tools.
17062 The first is the @command{tr} command, which stands for ``transliterate.''
17063 The @command{tr} command works on a character-by-character basis, changing
17064 characters. Normally it is used for things like mapping upper case to
17068 $ echo ThIs ExAmPlE HaS MIXED case! | tr '[:upper:]' '[:lower:]'
17069 @print{} this example has mixed case!
17072 There are several options of interest:
17076 work on the complement of the listed characters, i.e.,
17077 operations apply to characters not in the given set
17080 delete characters in the first set from the output
17083 squeeze repeated characters in the output into just one character.
17086 We will be using all three options in a moment.
17088 The other command we'll look at is @command{comm}. The @command{comm}
17089 command takes two sorted input files as input data, and prints out the
17090 files' lines in three columns. The output columns are the data lines
17091 unique to the first file, the data lines unique to the second file, and
17092 the data lines that are common to both. The @option{-1}, @option{-2}, and
17093 @option{-3} command line options @emph{omit} the respective columns. (This is
17094 non-intuitive and takes a little getting used to.) For example:
17116 The file name @file{-} tells @command{comm} to read standard input
17117 instead of a regular file.
17119 Now we're ready to build a fancy pipeline. The first application is a word
17120 frequency counter. This helps an author determine if he or she is over-using
17123 The first step is to change the case of all the letters in our input file
17124 to one case. ``The'' and ``the'' are the same word when doing counting.
17127 $ tr '[:upper:]' '[:lower:]' < whats.gnu | ...
17130 The next step is to get rid of punctuation. Quoted words and unquoted words
17131 should be treated identically; it's easiest to just get the punctuation out of
17135 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | ...
17138 The second @command{tr} command operates on the complement of the listed
17139 characters, which are all the letters, the digits, the underscore, and
17140 the blank. The @samp{\n} represents the newline character; it has to
17141 be left alone. (The ASCII tab character should also be included for
17142 good measure in a production script.)
17144 At this point, we have data consisting of words separated by blank space.
17145 The words only contain alphanumeric characters (and the underscore). The
17146 next step is break the data apart so that we have one word per line. This
17147 makes the counting operation much easier, as we will see shortly.
17150 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
17151 > tr -s ' ' '\n' | ...
17154 This command turns blanks into newlines. The @option{-s} option squeezes
17155 multiple newline characters in the output into just one. This helps us
17156 avoid blank lines. (The @samp{>} is the shell's ``secondary prompt.''
17157 This is what the shell prints when it notices you haven't finished
17158 typing in all of a command.)
17160 We now have data consisting of one word per line, no punctuation, all one
17161 case. We're ready to count each word:
17164 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
17165 > tr -s ' ' '\n' | sort | uniq -c | ...
17168 At this point, the data might look something like this:
17181 The output is sorted by word, not by count! What we want is the most
17182 frequently used words first. Fortunately, this is easy to accomplish,
17183 with the help of two more @command{sort} options:
17187 do a numeric sort, not a textual one
17190 reverse the order of the sort
17193 The final pipeline looks like this:
17196 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
17197 > tr -s ' ' '\n' | sort | uniq -c | sort -n -r
17206 Whew! That's a lot to digest. Yet, the same principles apply. With six
17207 commands, on two lines (really one long one split for convenience), we've
17208 created a program that does something interesting and useful, in much
17209 less time than we could have written a C program to do the same thing.
17211 A minor modification to the above pipeline can give us a simple spelling
17212 checker! To determine if you've spelled a word correctly, all you have to
17213 do is look it up in a dictionary. If it is not there, then chances are
17214 that your spelling is incorrect. So, we need a dictionary.
17215 The conventional location for a dictionary is @file{/usr/dict/words}.
17216 On my GNU/Linux system,@footnote{Redhat Linux 6.1, for the November 2000
17217 revision of this article.}
17218 this is a sorted, 45,402 word dictionary.
17220 Now, how to compare our file with the dictionary? As before, we generate
17221 a sorted list of words, one per line:
17224 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
17225 > tr -s ' ' '\n' | sort -u | ...
17228 Now, all we need is a list of words that are @emph{not} in the
17229 dictionary. Here is where the @command{comm} command comes in.
17232 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
17233 > tr -s ' ' '\n' | sort -u |
17234 > comm -23 - /usr/dict/words
17237 The @option{-2} and @option{-3} options eliminate lines that are only in the
17238 dictionary (the second file), and lines that are in both files. Lines
17239 only in the first file (standard input, our stream of words), are
17240 words that are not in the dictionary. These are likely candidates for
17241 spelling errors. This pipeline was the first cut at a production
17242 spelling checker on Unix.
17244 There are some other tools that deserve brief mention.
17248 search files for text that matches a regular expression
17251 count lines, words, characters
17254 a T-fitting for data pipes, copies data to files and to standard output
17257 the stream editor, an advanced tool
17260 a data manipulation language, another advanced tool
17263 The software tools philosophy also espoused the following bit of
17264 advice: ``Let someone else do the hard part.'' This means, take
17265 something that gives you most of what you need, and then massage it the
17266 rest of the way until it's in the form that you want.
17272 Each program should do one thing well. No more, no less.
17275 Combining programs with appropriate plumbing leads to results where
17276 the whole is greater than the sum of the parts. It also leads to novel
17277 uses of programs that the authors might never have imagined.
17280 Programs should never print extraneous header or trailer data, since these
17281 could get sent on down a pipeline. (A point we didn't mention earlier.)
17284 Let someone else do the hard part.
17287 Know your toolbox! Use each program appropriately. If you don't have an
17288 appropriate tool, build one.
17291 As of this writing, all the programs we've discussed are available via
17292 anonymous @command{ftp} from: @*
17293 @uref{ftp://gnudist.gnu.org/textutils/textutils-1.22.tar.gz}. (There may
17294 be more recent versions available now.)
17296 None of what I have presented in this column is new. The Software Tools
17297 philosophy was first introduced in the book @cite{Software Tools}, by
17298 Brian Kernighan and P.J. Plauger (Addison-Wesley, ISBN 0-201-03669-X).
17299 This book showed how to write and use software tools. It was written in
17300 1976, using a preprocessor for FORTRAN named @command{ratfor} (RATional
17301 FORtran). At the time, C was not as ubiquitous as it is now; FORTRAN
17302 was. The last chapter presented a @command{ratfor} to FORTRAN
17303 processor, written in @command{ratfor}. @command{ratfor} looks an awful
17304 lot like C; if you know C, you won't have any problem following the
17307 In 1981, the book was updated and made available as @cite{Software Tools
17308 in Pascal} (Addison-Wesley, ISBN 0-201-10342-7). Both books are
17309 still in print and are well worth
17310 reading if you're a programmer. They certainly made a major change in
17311 how I view programming.
17313 The programs in both books are available from
17314 @uref{http://cm.bell-labs.com/who/bwk, Brian Kernighan's home page}.
17315 For a number of years, there was an active
17316 Software Tools Users Group, whose members had ported the original
17317 @command{ratfor} programs to essentially every computer system with a
17318 FORTRAN compiler. The popularity of the group waned in the middle 1980s
17319 as Unix began to spread beyond universities.
17321 With the current proliferation of GNU code and other clones of Unix programs,
17322 these programs now receive little attention; modern C versions are
17323 much more efficient and do more than these programs do. Nevertheless, as
17324 exposition of good programming style, and evangelism for a still-valuable
17325 philosophy, these books are unparalleled, and I recommend them highly.
17327 Acknowledgment: I would like to express my gratitude to Brian Kernighan
17328 of Bell Labs, the original Software Toolsmith, for reviewing this column.
17330 @node GNU Free Documentation License
17331 @appendix GNU Free Documentation License
17335 @node Concept index
17342 @c Local variables:
17343 @c texinfo-column-for-description: 32