3 @setfilename coreutils.info
4 @settitle @sc{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 file permissions.
44 * chown: (coreutils)chown invocation. Change file owners/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 * od: (coreutils)od invocation. Dump files in octal, etc.
89 * paste: (coreutils)paste invocation. Merge lines of files.
90 * pathchk: (coreutils)pathchk invocation. Check file name portability.
91 * pr: (coreutils)pr invocation. Paginate or columnate files.
92 * printenv: (coreutils)printenv invocation. Print environment variables.
93 * printf: (coreutils)printf invocation. Format and print data.
94 * ptx: (coreutils)ptx invocation. Produce permuted indexes.
95 * pwd: (coreutils)pwd invocation. Print working directory.
96 * readlink: (coreutils)readlink invocation. Print referent of a symlink.
97 * realpath: (coreutils)readpath invocation. Print resolved file names.
98 * rm: (coreutils)rm invocation. Remove files.
99 * rmdir: (coreutils)rmdir invocation. Remove empty directories.
100 * runcon: (coreutils)runcon invocation. Run in specified SELinux CTX.
101 * seq: (coreutils)seq invocation. Print numeric sequences
102 * sha1sum: (coreutils)sha1sum invocation. Print or check SHA-1 digests.
103 * sha2: (coreutils)sha2 utilities. Print or check SHA-2 digests.
104 * shred: (coreutils)shred invocation. Remove files more securely.
105 * shuf: (coreutils)shuf invocation. Shuffling text files.
106 * sleep: (coreutils)sleep invocation. Delay for a specified time.
107 * sort: (coreutils)sort invocation. Sort text files.
108 * split: (coreutils)split invocation. Split into pieces.
109 * stat: (coreutils)stat invocation. Report file(system) status.
110 * stdbuf: (coreutils)stdbuf invocation. Modify stdio buffering.
111 * stty: (coreutils)stty invocation. Print/change terminal settings.
112 * su: (coreutils)su invocation. Modify user and group ID.
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 @sc{gnu} core
141 utilities, including the standard programs for text and file manipulation.
143 Copyright @copyright{} 1994-2012 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 @sc{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 su 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 * pr invocation:: Paginate or columnate files for printing
243 * fold invocation:: Wrap input lines to fit in specified width
245 Output of parts of files
247 * head invocation:: Output the first part of files
248 * tail invocation:: Output the last part of files
249 * split invocation:: Split a file into fixed-size pieces
250 * csplit invocation:: Split a file into context-determined pieces
254 * wc invocation:: Print newline, word, and byte counts
255 * sum invocation:: Print checksum and block counts
256 * cksum invocation:: Print CRC checksum and byte counts
257 * md5sum invocation:: Print or check MD5 digests
258 * sha1sum invocation:: Print or check SHA-1 digests
259 * sha2 utilities:: Print or check SHA-2 digests
261 Operating on sorted files
263 * sort invocation:: Sort text files
264 * shuf invocation:: Shuffle text files
265 * uniq invocation:: Uniquify files
266 * comm invocation:: Compare two sorted files line by line
267 * ptx invocation:: Produce a permuted index of file contents
268 * tsort invocation:: Topological sort
270 @command{ptx}: Produce permuted indexes
272 * General options in ptx:: Options which affect general program behavior
273 * Charset selection in ptx:: Underlying character set considerations
274 * Input processing in ptx:: Input fields, contexts, and keyword selection
275 * Output formatting in ptx:: Types of output format, and sizing the fields
276 * Compatibility in ptx:: The @acronym{GNU} extensions to @command{ptx}
280 * cut invocation:: Print selected parts of lines
281 * paste invocation:: Merge lines of files
282 * join invocation:: Join lines on a common field
284 Operating on characters
286 * tr invocation:: Translate, squeeze, and/or delete characters
287 * expand invocation:: Convert tabs to spaces
288 * unexpand invocation:: Convert spaces to tabs
290 @command{tr}: Translate, squeeze, and/or delete characters
292 * Character sets:: Specifying sets of characters
293 * Translating:: Changing one set of characters to another
294 * Squeezing:: Squeezing repeats and deleting
298 * ls invocation:: List directory contents
299 * dir invocation:: Briefly list directory contents
300 * vdir invocation:: Verbosely list directory contents
301 * dircolors invocation:: Color setup for @command{ls}
303 @command{ls}: List directory contents
305 * Which files are listed:: Which files are listed
306 * What information is listed:: What information is listed
307 * Sorting the output:: Sorting the output
308 * Details about version sort:: More details about version sort
309 * General output formatting:: General output formatting
310 * Formatting the file names:: Formatting the file names
314 * cp invocation:: Copy files and directories
315 * dd invocation:: Convert and copy a file
316 * install invocation:: Copy files and set attributes
317 * mv invocation:: Move (rename) files
318 * rm invocation:: Remove files or directories
319 * shred invocation:: Remove files more securely
323 * link invocation:: Make a hard link via the link syscall
324 * ln invocation:: Make links between files
325 * mkdir invocation:: Make directories
326 * mkfifo invocation:: Make FIFOs (named pipes)
327 * mknod invocation:: Make block or character special files
328 * readlink invocation:: Print value of a symlink or canonical file name
329 * rmdir invocation:: Remove empty directories
330 * unlink invocation:: Remove files via unlink syscall
332 Changing file attributes
334 * chown invocation:: Change file owner and group
335 * chgrp invocation:: Change group ownership
336 * chmod invocation:: Change access permissions
337 * touch invocation:: Change file timestamps
341 * df invocation:: Report file system disk space usage
342 * du invocation:: Estimate file space usage
343 * stat invocation:: Report file or file system status
344 * sync invocation:: Synchronize data on disk with memory
345 * truncate invocation:: Shrink or extend the size of a file
349 * echo invocation:: Print a line of text
350 * printf invocation:: Format and print data
351 * yes invocation:: Print a string until interrupted
355 * false invocation:: Do nothing, unsuccessfully
356 * true invocation:: Do nothing, successfully
357 * test invocation:: Check file types and compare values
358 * expr invocation:: Evaluate expressions
360 @command{test}: Check file types and compare values
362 * File type tests:: File type tests
363 * Access permission tests:: Access permission tests
364 * File characteristic tests:: File characteristic tests
365 * String tests:: String tests
366 * Numeric tests:: Numeric tests
368 @command{expr}: Evaluate expression
370 * String expressions:: + : match substr index length
371 * Numeric expressions:: + - * / %
372 * Relations for expr:: | & < <= = == != >= >
373 * Examples of expr:: Examples of using @command{expr}
377 * tee invocation:: Redirect output to multiple files or processes
379 File name manipulation
381 * basename invocation:: Strip directory and suffix from a file name
382 * dirname invocation:: Strip last file name component
383 * pathchk invocation:: Check file name validity and portability
384 * mktemp invocation:: Create temporary file or directory
385 * realpath invocation:: Print resolved file names
389 * pwd invocation:: Print working directory
390 * stty invocation:: Print or change terminal characteristics
391 * printenv invocation:: Print all or some environment variables
392 * tty invocation:: Print file name of terminal on standard input
394 @command{stty}: Print or change terminal characteristics
396 * Control:: Control settings
397 * Input:: Input settings
398 * Output:: Output settings
399 * Local:: Local settings
400 * Combination:: Combination settings
401 * Characters:: Special characters
402 * Special:: Special settings
406 * id invocation:: Print user identity
407 * logname invocation:: Print current login name
408 * whoami invocation:: Print effective user ID
409 * groups invocation:: Print group names a user is in
410 * users invocation:: Print login names of users currently logged in
411 * who invocation:: Print who is currently logged in
415 * arch invocation:: Print machine hardware name
416 * date invocation:: Print or set system date and time
417 * nproc invocation:: Print the number of processors
418 * uname invocation:: Print system information
419 * hostname invocation:: Print or set system name
420 * hostid invocation:: Print numeric host identifier
421 * uptime invocation:: Print system uptime and load
423 @command{date}: Print or set system date and time
425 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
426 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
427 * Literal conversion specifiers:: %[%nt]
428 * Padding and other flags:: Pad with zeros, spaces, etc.
429 * Setting the time:: Changing the system clock
430 * Options for date:: Instead of the current time
431 * Date input formats:: Specifying date strings
432 * Examples of date:: Examples
436 * chcon invocation:: Change SELinux context of file
437 * runcon invocation:: Run a command in specified SELinux context
439 Modified command invocation
441 * chroot invocation:: Run a command with a different root directory
442 * env invocation:: Run a command in a modified environment
443 * nice invocation:: Run a command with modified niceness
444 * nohup invocation:: Run a command immune to hangups
445 * stdbuf invocation:: Run a command with modified I/O buffering
446 * su invocation:: Run a command with substitute user and group ID
447 * timeout invocation:: Run a command with a time limit
451 * kill invocation:: Sending a signal to processes.
455 * sleep invocation:: Delay for a specified time
459 * factor invocation:: Print prime factors
460 * seq invocation:: Print numeric sequences
464 * Mode Structure:: Structure of file mode bits
465 * Symbolic Modes:: Mnemonic representation of file mode bits
466 * Numeric Modes:: File mode bits as octal numbers
467 * Directory Setuid and Setgid:: Set-user-ID and set-group-ID on directories
471 * General date syntax:: Common rules
472 * Calendar date items:: 19 Dec 1994
473 * Time of day items:: 9:20pm
474 * Time zone items:: @sc{est}, @sc{pdt}, @sc{gmt}
475 * Day of week items:: Monday and others
476 * Relative items in date strings:: next tuesday, 2 years ago
477 * Pure numbers in date strings:: 19931219, 1440
478 * Seconds since the Epoch:: @@1078100502
479 * Specifying time zone rules:: TZ="America/New_York", TZ="UTC0"
480 * Authors of parse_datetime:: Bellovin, Eggert, Salz, Berets, et al
482 Opening the software toolbox
484 * Toolbox introduction:: Toolbox introduction
485 * I/O redirection:: I/O redirection
486 * The who command:: The @command{who} command
487 * The cut command:: The @command{cut} command
488 * The sort command:: The @command{sort} command
489 * The uniq command:: The @command{uniq} command
490 * Putting the tools together:: Putting the tools together
494 * GNU Free Documentation License:: Copying and sharing this manual
501 @chapter Introduction
503 This manual is a work in progress: many sections make no attempt to explain
504 basic concepts in a way suitable for novices. Thus, if you are interested,
505 please get involved in improving this manual. The entire @sc{gnu} community
508 @cindex @acronym{POSIX}
509 The @sc{gnu} utilities documented here are mostly compatible with the
510 @acronym{POSIX} standard.
511 @cindex bugs, reporting
512 Please report bugs to @email{bug-coreutils@@gnu.org}. Remember
513 to include the version number, machine architecture, input files, and
514 any other information needed to reproduce the bug: your input, what you
515 expected, what you got, and why it is wrong. Diffs are welcome, but
516 please include a description of the problem as well, since this is
517 sometimes difficult to infer. @xref{Bugs, , , gcc, Using and Porting GNU CC}.
523 @cindex MacKenzie, D.
526 This manual was originally derived from the Unix man pages in the
527 distributions, which were written by David MacKenzie and updated by Jim
528 Meyering. What you are reading now is the authoritative documentation
529 for these utilities; the man pages are no longer being maintained. The
530 original @command{fmt} man page was written by Ross Paterson. Fran@,{c}ois
531 Pinard did the initial conversion to Texinfo format. Karl Berry did the
532 indexing, some reorganization, and editing of the results. Brian
533 Youmans of the Free Software Foundation office staff combined the
534 manuals for textutils, fileutils, and sh-utils to produce the present
535 omnibus manual. Richard Stallman contributed his usual invaluable
536 insights to the overall process.
539 @chapter Common options
543 @itemx @w{@kbd{--backup}[=@var{method}]}
546 @vindex VERSION_CONTROL
547 @cindex backups, making
548 @xref{Backup options}.
549 Make a backup of each file that would otherwise be overwritten or removed.
552 @macro optBackupSuffix
553 @item -S @var{suffix}
554 @itemx --suffix=@var{suffix}
557 Append @var{suffix} to each backup file made with @option{-b}.
558 @xref{Backup options}.
561 @macro optTargetDirectory
562 @item -t @var{directory}
563 @itemx @w{@kbd{--target-directory}=@var{directory}}
565 @opindex --target-directory
566 @cindex target directory
567 @cindex destination directory
568 Specify the destination @var{directory}.
569 @xref{Target directory}.
572 @macro optNoTargetDirectory
574 @itemx --no-target-directory
576 @opindex --no-target-directory
577 @cindex target directory
578 @cindex destination directory
579 Do not treat the last operand specially when it is a directory or a
580 symbolic link to a directory. @xref{Target directory}.
588 @cindex output @sc{nul}-byte-terminated lines
589 Output a zero byte (@acronym{ASCII} @sc{nul}) at the end of each line,
590 rather than a newline. This option enables other programs to parse the
591 output of @command{\cmd\} even when that output would contain data
592 with embedded newlines.
599 Append an SI-style abbreviation to each size, such as @samp{M} for
600 megabytes. Powers of 1000 are used, not 1024; @samp{M} stands for
601 1,000,000 bytes. This option is equivalent to
602 @option{--block-size=si}. Use the @option{-h} or
603 @option{--human-readable} option if
604 you prefer powers of 1024.
607 @macro optHumanReadable
609 @itemx --human-readable
611 @opindex --human-readable
612 @cindex human-readable output
613 Append a size letter to each size, such as @samp{M} for mebibytes.
614 Powers of 1024 are used, not 1000; @samp{M} stands for 1,048,576 bytes.
615 This option is equivalent to @option{--block-size=human-readable}.
616 Use the @option{--si} option if you prefer powers of 1000.
619 @macro optStripTrailingSlashes
620 @itemx @w{@kbd{--strip-trailing-slashes}}
621 @opindex --strip-trailing-slashes
622 @cindex stripping trailing slashes
623 Remove any trailing slashes from each @var{source} argument.
624 @xref{Trailing slashes}.
627 @macro mayConflictWithShellBuiltIn{cmd}
628 @cindex conflicts with shell built-ins
629 @cindex built-in shell commands, conflicts with
630 Due to shell aliases and built-in @command{\cmd\} functions, using an
631 unadorned @command{\cmd\} interactively or in a script may get you
632 different functionality than that described here. Invoke it via
633 @command{env} (i.e., @code{env \cmd\ @dots{}}) to avoid interference
638 @macro multiplierSuffixes{varName}
639 @var{\varName\} may be, or may be an integer optionally followed by,
640 one of the following multiplicative suffixes:
642 @samp{b} => 512 ("blocks")
643 @samp{KB} => 1000 (KiloBytes)
644 @samp{K} => 1024 (KibiBytes)
645 @samp{MB} => 1000*1000 (MegaBytes)
646 @samp{M} => 1024*1024 (MebiBytes)
647 @samp{GB} => 1000*1000*1000 (GigaBytes)
648 @samp{G} => 1024*1024*1024 (GibiBytes)
650 and so on for @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}.
653 @c FIXME: same as above, but no ``blocks'' line.
654 @macro multiplierSuffixesNoBlocks{varName}
655 @var{\varName\} may be, or may be an integer optionally followed by,
656 one of the following multiplicative suffixes:
658 @samp{KB} => 1000 (KiloBytes)
659 @samp{K} => 1024 (KibiBytes)
660 @samp{MB} => 1000*1000 (MegaBytes)
661 @samp{M} => 1024*1024 (MebiBytes)
662 @samp{GB} => 1000*1000*1000 (GigaBytes)
663 @samp{G} => 1024*1024*1024 (GibiBytes)
665 and so on for @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}.
668 @cindex common options
670 Certain options are available in all of these programs. Rather than
671 writing identical descriptions for each of the programs, they are
672 described here. (In fact, every @sc{gnu} program accepts (or should accept)
675 @vindex POSIXLY_CORRECT
676 Normally options and operands can appear in any order, and programs act
677 as if all the options appear before any operands. For example,
678 @samp{sort -r passwd -t :} acts like @samp{sort -r -t : passwd}, since
679 @samp{:} is an option-argument of @option{-t}. However, if the
680 @env{POSIXLY_CORRECT} environment variable is set, options must appear
681 before operands, unless otherwise specified for a particular command.
683 A few programs can usefully have trailing operands with leading
684 @samp{-}. With such a program, options must precede operands even if
685 @env{POSIXLY_CORRECT} is not set, and this fact is noted in the
686 program description. For example, the @command{env} command's options
687 must appear before its operands, since in some cases the operands
688 specify a command that itself contains options.
690 Most programs that accept long options recognize unambiguous
691 abbreviations of those options. For example, @samp{rmdir
692 --ignore-fail-on-non-empty} can be invoked as @samp{rmdir
693 --ignore-fail} or even @samp{rmdir --i}. Ambiguous options, such as
694 @samp{ls --h}, are identified as such.
696 Some of these programs recognize the @option{--help} and @option{--version}
697 options only when one of them is the sole command line argument. For
698 these programs, abbreviations of the long options are not always recognized.
705 Print a usage message listing all available options, then exit successfully.
709 @cindex version number, finding
710 Print the version number, then exit successfully.
714 @cindex option delimiter
715 Delimit the option list. Later arguments, if any, are treated as
716 operands even if they begin with @samp{-}. For example, @samp{sort --
717 -r} reads from the file named @file{-r}.
721 @cindex standard input
722 @cindex standard output
723 A single @samp{-} operand is not really an option, though it looks like one. It
724 stands for standard input, or for standard output if that is clear from
725 the context. For example, @samp{sort -} reads from standard input,
726 and is equivalent to plain @samp{sort}, and @samp{tee -} writes an
727 extra copy of its input to standard output. Unless otherwise
728 specified, @samp{-} can appear as any operand that requires a file
732 * Exit status:: Indicating program success or failure.
733 * Backup options:: -b -S, in some programs.
734 * Block size:: BLOCK_SIZE and --block-size, in some programs.
735 * Floating point:: Floating point number representation.
736 * Signal specifications:: Specifying signals using the --signal option.
737 * Disambiguating names and IDs:: chgrp and chown owner and group syntax
738 * Random sources:: --random-source, in some programs.
739 * Target directory:: Specifying a target directory, in some programs.
740 * Trailing slashes:: --strip-trailing-slashes, in some programs.
741 * Traversing symlinks:: -H, -L, or -P, in some programs.
742 * Treating / specially:: --preserve-root and --no-preserve-root.
743 * Special built-in utilities:: @command{break}, @command{:}, @dots{}
744 * Standards conformance:: Conformance to the @acronym{POSIX} standard.
752 An exit status of zero indicates success,
753 and a nonzero value indicates failure.
756 Nearly every command invocation yields an integral @dfn{exit status}
757 that can be used to change how other commands work.
758 For the vast majority of commands, an exit status of zero indicates
759 success. Failure is indicated by a nonzero value---typically
760 @samp{1}, though it may differ on unusual platforms as @acronym{POSIX}
761 requires only that it be nonzero.
763 However, some of the programs documented here do produce
764 other exit status values and a few associate different
765 meanings with the values @samp{0} and @samp{1}.
766 Here are some of the exceptions:
767 @command{chroot}, @command{env}, @command{expr}, @command{nice},
768 @command{nohup}, @command{printenv}, @command{sort}, @command{stdbuf},
769 @command{su}, @command{test}, @command{timeout}, @command{tty}.
773 @section Backup options
775 @cindex backup options
777 Some @sc{gnu} programs (at least @command{cp}, @command{install},
778 @command{ln}, and @command{mv}) optionally make backups of files
779 before writing new versions.
780 These options control the details of these backups. The options are also
781 briefly mentioned in the descriptions of the particular programs.
786 @itemx @w{@kbd{--backup}[=@var{method}]}
789 @vindex VERSION_CONTROL
790 @cindex backups, making
791 Make a backup of each file that would otherwise be overwritten or removed.
792 Without this option, the original versions are destroyed.
793 Use @var{method} to determine the type of backups to make.
794 When this option is used but @var{method} is not specified,
795 then the value of the @env{VERSION_CONTROL}
796 environment variable is used. And if @env{VERSION_CONTROL} is not set,
797 the default backup type is @samp{existing}.
799 Note that the short form of this option, @option{-b} does not accept any
800 argument. Using @option{-b} is equivalent to using @option{--backup=existing}.
802 @vindex version-control @r{Emacs variable}
803 This option corresponds to the Emacs variable @samp{version-control};
804 the values for @var{method} are the same as those used in Emacs.
805 This option also accepts more descriptive names.
806 The valid @var{method}s are (unique abbreviations are accepted):
811 @opindex none @r{backup method}
816 @opindex numbered @r{backup method}
817 Always make numbered backups.
821 @opindex existing @r{backup method}
822 Make numbered backups of files that already have them, simple backups
827 @opindex simple @r{backup method}
828 Always make simple backups. Please note @samp{never} is not to be
829 confused with @samp{none}.
833 @item -S @var{suffix}
834 @itemx --suffix=@var{suffix}
837 @cindex backup suffix
838 @vindex SIMPLE_BACKUP_SUFFIX
839 Append @var{suffix} to each backup file made with @option{-b}. If this
840 option is not specified, the value of the @env{SIMPLE_BACKUP_SUFFIX}
841 environment variable is used. And if @env{SIMPLE_BACKUP_SUFFIX} is not
842 set, the default is @samp{~}, just as in Emacs.
851 Some @sc{gnu} programs (at least @command{df}, @command{du}, and
852 @command{ls}) display sizes in ``blocks''. You can adjust the block size
853 and method of display to make sizes easier to read. The block size
854 used for display is independent of any file system block size.
855 Fractional block counts are rounded up to the nearest integer.
857 @opindex --block-size=@var{size}
860 @vindex DF_BLOCK_SIZE
861 @vindex DU_BLOCK_SIZE
862 @vindex LS_BLOCK_SIZE
863 @vindex POSIXLY_CORRECT@r{, and block size}
865 The default block size is chosen by examining the following environment
866 variables in turn; the first one that is set determines the block size.
871 This specifies the default block size for the @command{df} command.
872 Similarly, @env{DU_BLOCK_SIZE} specifies the default for @command{du} and
873 @env{LS_BLOCK_SIZE} for @command{ls}.
876 This specifies the default block size for all three commands, if the
877 above command-specific environment variables are not set.
880 This specifies the default block size for all values that are normally
881 printed as blocks, if neither @env{BLOCK_SIZE} nor the above
882 command-specific environment variables are set. Unlike the other
883 environment variables, @env{BLOCKSIZE} does not affect values that are
884 normally printed as byte counts, e.g., the file sizes contained in
887 @item POSIXLY_CORRECT
888 If neither @env{@var{command}_BLOCK_SIZE}, nor @env{BLOCK_SIZE}, nor
889 @env{BLOCKSIZE} is set, but this variable is set, the block size
894 If none of the above environment variables are set, the block size
895 currently defaults to 1024 bytes in most contexts, but this number may
896 change in the future. For @command{ls} file sizes, the block size
899 @cindex human-readable output
902 A block size specification can be a positive integer specifying the number
903 of bytes per block, or it can be @code{human-readable} or @code{si} to
904 select a human-readable format. Integers may be followed by suffixes
905 that are upward compatible with the
906 @uref{http://www.bipm.org/en/si/si_brochure/chapter3/prefixes.html, SI prefixes}
907 for decimal multiples and with the
908 @uref{http://physics.nist.gov/cuu/Units/binary.html, ISO/IEC 80000-13
909 (formerly IEC 60027-2) prefixes} for binary multiples.
911 With human-readable formats, output sizes are followed by a size letter
912 such as @samp{M} for megabytes. @code{BLOCK_SIZE=human-readable} uses
913 powers of 1024; @samp{M} stands for 1,048,576 bytes.
914 @code{BLOCK_SIZE=si} is similar, but uses powers of 1000 and appends
915 @samp{B}; @samp{MB} stands for 1,000,000 bytes.
918 A block size specification preceded by @samp{'} causes output sizes to
919 be displayed with thousands separators. The @env{LC_NUMERIC} locale
920 specifies the thousands separator and grouping. For example, in an
921 American English locale, @samp{--block-size="'1kB"} would cause a size
922 of 1234000 bytes to be displayed as @samp{1,234}. In the default C
923 locale, there is no thousands separator so a leading @samp{'} has no
926 An integer block size can be followed by a suffix to specify a
927 multiple of that size. A bare size letter,
928 or one followed by @samp{iB}, specifies
929 a multiple using powers of 1024. A size letter followed by @samp{B}
930 specifies powers of 1000 instead. For example, @samp{1M} and
931 @samp{1MiB} are equivalent to @samp{1048576}, whereas @samp{1MB} is
932 equivalent to @samp{1000000}.
934 A plain suffix without a preceding integer acts as if @samp{1} were
935 prepended, except that it causes a size indication to be appended to
936 the output. For example, @samp{--block-size="kB"} displays 3000 as
939 The following suffixes are defined. Large sizes like @code{1Y}
940 may be rejected by your computer due to limitations of its arithmetic.
944 @cindex kilobyte, definition of
945 kilobyte: @math{10^3 = 1000}.
949 @cindex kibibyte, definition of
950 kibibyte: @math{2^{10} = 1024}. @samp{K} is special: the SI prefix is
951 @samp{k} and the ISO/IEC 80000-13 prefix is @samp{Ki}, but tradition and
952 @acronym{POSIX} use @samp{k} to mean @samp{KiB}.
954 @cindex megabyte, definition of
955 megabyte: @math{10^6 = 1,000,000}.
958 @cindex mebibyte, definition of
959 mebibyte: @math{2^{20} = 1,048,576}.
961 @cindex gigabyte, definition of
962 gigabyte: @math{10^9 = 1,000,000,000}.
965 @cindex gibibyte, definition of
966 gibibyte: @math{2^{30} = 1,073,741,824}.
968 @cindex terabyte, definition of
969 terabyte: @math{10^{12} = 1,000,000,000,000}.
972 @cindex tebibyte, definition of
973 tebibyte: @math{2^{40} = 1,099,511,627,776}.
975 @cindex petabyte, definition of
976 petabyte: @math{10^{15} = 1,000,000,000,000,000}.
979 @cindex pebibyte, definition of
980 pebibyte: @math{2^{50} = 1,125,899,906,842,624}.
982 @cindex exabyte, definition of
983 exabyte: @math{10^{18} = 1,000,000,000,000,000,000}.
986 @cindex exbibyte, definition of
987 exbibyte: @math{2^{60} = 1,152,921,504,606,846,976}.
989 @cindex zettabyte, definition of
990 zettabyte: @math{10^{21} = 1,000,000,000,000,000,000,000}
993 @math{2^{70} = 1,180,591,620,717,411,303,424}.
995 @cindex yottabyte, definition of
996 yottabyte: @math{10^{24} = 1,000,000,000,000,000,000,000,000}.
999 @math{2^{80} = 1,208,925,819,614,629,174,706,176}.
1004 @opindex --block-size
1005 @opindex --human-readable
1008 Block size defaults can be overridden by an explicit
1009 @option{--block-size=@var{size}} option. The @option{-k}
1010 option is equivalent to @option{--block-size=1K}, which
1011 is the default unless the @env{POSIXLY_CORRECT} environment variable is
1012 set. The @option{-h} or @option{--human-readable} option is equivalent to
1013 @option{--block-size=human-readable}. The @option{--si} option is
1014 equivalent to @option{--block-size=si}.
1016 @node Floating point
1017 @section Floating point numbers
1018 @cindex floating point
1019 @cindex IEEE floating point
1021 Commands that accept or produce floating point numbers employ the
1022 floating point representation of the underlying system, and suffer
1023 from rounding error, overflow, and similar floating-point issues.
1024 Almost all modern systems use IEEE-754 floating point, and it is
1025 typically portable to assume IEEE-754 behavior these days. IEEE-754
1026 has positive and negative infinity, distinguishes positive from
1027 negative zero, and uses special values called NaNs to represent
1028 invalid computations such as dividing zero by itself. For more
1029 information, please see David Goldberg's paper
1030 @uref{http://@/www.validlab.com/@/goldberg/@/paper.pdf, What Every
1031 Computer Scientist Should Know About Floating-Point Arithmetic}.
1034 Commands that accept floating point numbers as options, operands or
1035 input use the standard C functions @code{strtod} and @code{strtold} to
1036 convert from text to floating point numbers. These floating point
1037 numbers therefore can use scientific notation like @code{1.0e-34} and
1038 @code{-10e100}. Modern C implementations also accept hexadecimal
1039 floating point numbers such as @code{-0x.ep-3}, which stands for
1040 @minus{}14/16 times @math{2^-3}, which equals @minus{}0.109375. The
1041 @env{LC_NUMERIC} locale determines the decimal-point character.
1042 @xref{Parsing of Floats,,, libc, The GNU C Library Reference Manual}.
1044 @node Signal specifications
1045 @section Signal specifications
1046 @cindex signals, specifying
1048 A @var{signal} may be a signal name like @samp{HUP}, or a signal
1049 number like @samp{1}, or an exit status of a process terminated by the
1050 signal. A signal name can be given in canonical form or prefixed by
1051 @samp{SIG}. The case of the letters is ignored. The following signal names
1052 and numbers are supported on all @acronym{POSIX} compliant systems:
1058 2. Terminal interrupt.
1064 9. Kill (cannot be caught or ignored).
1072 Other supported signal names have system-dependent corresponding
1073 numbers. All systems conforming to @acronym{POSIX} 1003.1-2001 also
1074 support the following signals:
1078 Access to an undefined portion of a memory object.
1080 Child process terminated, stopped, or continued.
1082 Continue executing, if stopped.
1084 Erroneous arithmetic operation.
1086 Illegal Instruction.
1088 Write on a pipe with no one to read it.
1090 Invalid memory reference.
1092 Stop executing (cannot be caught or ignored).
1096 Background process attempting read.
1098 Background process attempting write.
1100 High bandwidth data is available at a socket.
1102 User-defined signal 1.
1104 User-defined signal 2.
1108 @acronym{POSIX} 1003.1-2001 systems that support the @acronym{XSI} extension
1109 also support the following signals:
1115 Profiling timer expired.
1119 Trace/breakpoint trap.
1121 Virtual timer expired.
1123 CPU time limit exceeded.
1125 File size limit exceeded.
1129 @acronym{POSIX} 1003.1-2001 systems that support the @acronym{XRT} extension
1130 also support at least eight real-time signals called @samp{RTMIN},
1131 @samp{RTMIN+1}, @dots{}, @samp{RTMAX-1}, @samp{RTMAX}.
1133 @node Disambiguating names and IDs
1134 @section chown and chgrp: Disambiguating user names and IDs
1135 @cindex user names, disambiguating
1136 @cindex user IDs, disambiguating
1137 @cindex group names, disambiguating
1138 @cindex group IDs, disambiguating
1139 @cindex disambiguating group names and IDs
1141 Since the @var{owner} and @var{group} arguments to @command{chown} and
1142 @command{chgrp} may be specified as names or numeric IDs, there is an
1144 What if a user or group @emph{name} is a string of digits?
1145 @footnote{Using a number as a user name is common in some environments.}
1146 Should the command interpret it as a user name or as an ID?
1147 @acronym{POSIX} requires that @command{chown} and @command{chgrp}
1148 first attempt to resolve the specified string as a name, and
1149 only once that fails, then try to interpret it as an ID.
1150 This is troublesome when you want to specify a numeric ID, say 42,
1151 and it must work even in a pathological situation where
1152 @samp{42} is a user name that maps to some other user ID, say 1000.
1153 Simply invoking @code{chown 42 F}, will set @file{F}s owner ID to
1154 1000---not what you intended.
1156 GNU @command{chown} and @command{chgrp} provide a way to work around this,
1157 that at the same time may result in a significant performance improvement
1158 by eliminating a database look-up.
1159 Simply precede each numeric user ID and/or group ID with a @samp{+},
1160 in order to force its interpretation as an integer:
1164 chgrp +$numeric_group_id another-file
1168 GNU @command{chown} and @command{chgrp}
1169 skip the name look-up process for each @samp{+}-prefixed string,
1170 because a string containing @samp{+} is never a valid user or group name.
1171 This syntax is accepted on most common Unix systems, but not on Solaris 10.
1173 @node Random sources
1174 @section Sources of random data
1176 @cindex random sources
1178 The @command{shuf}, @command{shred}, and @command{sort} commands
1179 sometimes need random data to do their work. For example, @samp{sort
1180 -R} must choose a hash function at random, and it needs random data to
1181 make this selection.
1183 By default these commands use an internal pseudorandom generator
1184 initialized by a small amount of entropy, but can be directed to use
1185 an external source with the @option{--random-source=@var{file}} option.
1186 An error is reported if @var{file} does not contain enough bytes.
1188 For example, the device file @file{/dev/urandom} could be used as the
1189 source of random data. Typically, this device gathers environmental
1190 noise from device drivers and other sources into an entropy pool, and
1191 uses the pool to generate random bits. If the pool is short of data,
1192 the device reuses the internal pool to produce more bits, using a
1193 cryptographically secure pseudorandom number generator. But be aware
1194 that this device is not designed for bulk random data generation
1195 and is relatively slow.
1197 @file{/dev/urandom} suffices for most practical uses, but applications
1198 requiring high-value or long-term protection of private data may
1199 require an alternate data source like @file{/dev/random} or
1200 @file{/dev/arandom}. The set of available sources depends on your
1203 To reproduce the results of an earlier invocation of a command, you
1204 can save some random data into a file and then use that file as the
1205 random source in earlier and later invocations of the command.
1207 @node Target directory
1208 @section Target directory
1210 @cindex target directory
1212 The @command{cp}, @command{install}, @command{ln}, and @command{mv}
1213 commands normally treat the last operand specially when it is a
1214 directory or a symbolic link to a directory. For example, @samp{cp
1215 source dest} is equivalent to @samp{cp source dest/source} if
1216 @file{dest} is a directory. Sometimes this behavior is not exactly
1217 what is wanted, so these commands support the following options to
1218 allow more fine-grained control:
1223 @itemx --no-target-directory
1224 @opindex --no-target-directory
1225 @cindex target directory
1226 @cindex destination directory
1227 Do not treat the last operand specially when it is a directory or a
1228 symbolic link to a directory. This can help avoid race conditions in
1229 programs that operate in a shared area. For example, when the command
1230 @samp{mv /tmp/source /tmp/dest} succeeds, there is no guarantee that
1231 @file{/tmp/source} was renamed to @file{/tmp/dest}: it could have been
1232 renamed to @file{/tmp/dest/source} instead, if some other process
1233 created @file{/tmp/dest} as a directory. However, if @file{mv
1234 -T /tmp/source /tmp/dest} succeeds, there is no
1235 question that @file{/tmp/source} was renamed to @file{/tmp/dest}.
1237 In the opposite situation, where you want the last operand to be
1238 treated as a directory and want a diagnostic otherwise, you can use
1239 the @option{--target-directory} (@option{-t}) option.
1241 @item -t @var{directory}
1242 @itemx @w{@kbd{--target-directory}=@var{directory}}
1243 @opindex --target-directory
1244 @cindex target directory
1245 @cindex destination directory
1246 Use @var{directory} as the directory component of each destination
1249 The interface for most programs is that after processing options and a
1250 finite (possibly zero) number of fixed-position arguments, the remaining
1251 argument list is either expected to be empty, or is a list of items
1252 (usually files) that will all be handled identically. The @command{xargs}
1253 program is designed to work well with this convention.
1255 The commands in the @command{mv}-family are unusual in that they take
1256 a variable number of arguments with a special case at the @emph{end}
1257 (namely, the target directory). This makes it nontrivial to perform some
1258 operations, e.g., ``move all files from here to ../d/'', because
1259 @code{mv * ../d/} might exhaust the argument space, and @code{ls | xargs ...}
1260 doesn't have a clean way to specify an extra final argument for each
1261 invocation of the subject command. (It can be done by going through a
1262 shell command, but that requires more human labor and brain power than
1265 The @w{@kbd{--target-directory}} (@option{-t}) option allows the @command{cp},
1266 @command{install}, @command{ln}, and @command{mv} programs to be used
1267 conveniently with @command{xargs}. For example, you can move the files
1268 from the current directory to a sibling directory, @code{d} like this:
1271 ls | xargs mv -t ../d --
1274 However, this doesn't move files whose names begin with @samp{.}.
1275 If you use the @sc{gnu} @command{find} program, you can move those
1276 files too, with this command:
1279 find . -mindepth 1 -maxdepth 1 \
1283 But both of the above approaches fail if there are no files in the
1284 current directory, or if any file has a name containing a blank or
1285 some other special characters.
1286 The following example removes those limitations and requires both
1287 @sc{gnu} @command{find} and @sc{gnu} @command{xargs}:
1290 find . -mindepth 1 -maxdepth 1 -print0 \
1291 | xargs --null --no-run-if-empty \
1298 The @option{--target-directory} (@option{-t}) and
1299 @option{--no-target-directory} (@option{-T})
1300 options cannot be combined.
1302 @node Trailing slashes
1303 @section Trailing slashes
1305 @cindex trailing slashes
1307 Some @sc{gnu} programs (at least @command{cp} and @command{mv}) allow you to
1308 remove any trailing slashes from each @var{source} argument before
1309 operating on it. The @w{@kbd{--strip-trailing-slashes}} option enables
1312 This is useful when a @var{source} argument may have a trailing slash and
1313 @c FIXME: mv's behavior in this case is system-dependent
1314 specify a symbolic link to a directory. This scenario is in fact rather
1315 common because some shells can automatically append a trailing slash when
1316 performing file name completion on such symbolic links. Without this
1317 option, @command{mv}, for example, (via the system's rename function) must
1318 interpret a trailing slash as a request to dereference the symbolic link
1319 and so must rename the indirectly referenced @emph{directory} and not
1320 the symbolic link. Although it may seem surprising that such behavior
1321 be the default, it is required by @acronym{POSIX} and is consistent with
1322 other parts of that standard.
1324 @node Traversing symlinks
1325 @section Traversing symlinks
1327 @cindex symbolic link to directory, controlling traversal of
1329 The following options modify how @command{chown} and @command{chgrp}
1330 @c FIXME: note that 'du' has these options, too, but they have slightly
1331 @c different meaning.
1332 traverse a hierarchy when the @option{--recursive} (@option{-R})
1333 option is also specified.
1334 If more than one of the following options is specified, only the final
1336 These options specify whether processing a symbolic link to a directory
1337 entails operating on just the symbolic link or on all files in the
1338 hierarchy rooted at that directory.
1340 These options are independent of @option{--dereference} and
1341 @option{--no-dereference} (@option{-h}), which control whether to modify
1342 a symlink or its referent.
1349 @cindex symbolic link to directory, traverse if on the command line
1350 If @option{--recursive} (@option{-R}) is specified and
1351 a command line argument is a symbolic link to a directory, traverse it.
1358 @cindex symbolic link to directory, traverse each that is encountered
1359 In a recursive traversal, traverse every symbolic link to a directory
1360 that is encountered.
1367 @cindex symbolic link to directory, never traverse
1368 Do not traverse any symbolic links.
1369 This is the default if none of @option{-H}, @option{-L},
1370 or @option{-P} is specified.
1377 @node Treating / specially
1378 @section Treating @file{/} specially
1380 Certain commands can operate destructively on entire hierarchies.
1381 For example, if a user with appropriate privileges mistakenly runs
1382 @samp{rm -rf / tmp/junk}, that may remove
1383 all files on the entire system. Since there are so few
1384 legitimate uses for such a command,
1385 @sc{gnu} @command{rm} normally declines to operate on any directory
1386 that resolves to @file{/}. If you really want to try to remove all
1387 the files on your system, you can use the @option{--no-preserve-root}
1388 option, but the default behavior, specified by the
1389 @option{--preserve-option}, is safer for most purposes.
1391 The commands @command{chgrp}, @command{chmod} and @command{chown}
1392 can also operate destructively on entire hierarchies, so they too
1393 support these options. Although, unlike @command{rm}, they don't
1394 actually unlink files, these commands are arguably more dangerous
1395 when operating recursively on @file{/}, since they often work much
1396 more quickly, and hence damage more files before an alert user can
1397 interrupt them. Tradition and @acronym{POSIX} require these commands
1398 to operate recursively on @file{/}, so they default to
1399 @option{--no-preserve-root}, but using the @option{--preserve-root}
1400 option makes them safer for most purposes. For convenience you can
1401 specify @option{--preserve-root} in an alias or in a shell function.
1403 Note that the @option{--preserve-root} option also ensures
1404 that @command{chgrp} and @command{chown} do not modify @file{/}
1405 even when dereferencing a symlink pointing to @file{/}.
1407 @node Special built-in utilities
1408 @section Special built-in utilities
1410 Some programs like @command{nice} can invoke other programs; for
1411 example, the command @samp{nice cat file} invokes the program
1412 @command{cat} by executing the command @samp{cat file}. However,
1413 @dfn{special built-in utilities} like @command{exit} cannot be invoked
1414 this way. For example, the command @samp{nice exit} does not have a
1415 well-defined behavior: it may generate an error message instead of
1418 Here is a list of the special built-in utilities that are standardized
1419 by @acronym{POSIX} 1003.1-2004.
1422 @t{.@: : break continue eval exec exit export readonly
1423 return set shift times trap unset}
1426 For example, because @samp{.}, @samp{:}, and @samp{exec} are special,
1427 the commands @samp{nice . foo.sh}, @samp{nice :}, and @samp{nice exec
1428 pwd} do not work as you might expect.
1430 Many shells extend this list. For example, Bash has several extra
1431 special built-in utilities like @command{history}, and
1432 @command{suspend}, and with Bash the command @samp{nice suspend}
1433 generates an error message instead of suspending.
1435 @node Standards conformance
1436 @section Standards conformance
1438 @vindex POSIXLY_CORRECT
1439 In a few cases, the @sc{gnu} utilities' default behavior is
1440 incompatible with the @acronym{POSIX} standard. To suppress these
1441 incompatibilities, define the @env{POSIXLY_CORRECT} environment
1442 variable. Unless you are checking for @acronym{POSIX} conformance, you
1443 probably do not need to define @env{POSIXLY_CORRECT}.
1445 Newer versions of @acronym{POSIX} are occasionally incompatible with older
1446 versions. For example, older versions of @acronym{POSIX} required the
1447 command @samp{sort +1} to sort based on the second and succeeding
1448 fields in each input line, but starting with @acronym{POSIX} 1003.1-2001
1449 the same command is required to sort the file named @file{+1}, and you
1450 must instead use the command @samp{sort -k 2} to get the field-based
1453 @vindex _POSIX2_VERSION
1454 The @sc{gnu} utilities normally conform to the version of @acronym{POSIX}
1455 that is standard for your system. To cause them to conform to a
1456 different version of @acronym{POSIX}, define the @env{_POSIX2_VERSION}
1457 environment variable to a value of the form @var{yyyymm} specifying
1458 the year and month the standard was adopted. Three values are currently
1459 supported for @env{_POSIX2_VERSION}: @samp{199209} stands for
1460 @acronym{POSIX} 1003.2-1992, @samp{200112} stands for @acronym{POSIX}
1461 1003.1-2001, and @samp{200809} stands for @acronym{POSIX} 1003.1-2008.
1462 For example, if you have a newer system but are running software
1463 that assumes an older version of @acronym{POSIX} and uses @samp{sort +1}
1464 or @samp{tail +10}, you can work around any compatibility problems by setting
1465 @samp{_POSIX2_VERSION=199209} in your environment.
1467 @node Output of entire files
1468 @chapter Output of entire files
1470 @cindex output of entire files
1471 @cindex entire files, output of
1473 These commands read and write entire files, possibly transforming them
1477 * cat invocation:: Concatenate and write files.
1478 * tac invocation:: Concatenate and write files in reverse.
1479 * nl invocation:: Number lines and write files.
1480 * od invocation:: Write files in octal or other formats.
1481 * base64 invocation:: Transform data into printable data.
1484 @node cat invocation
1485 @section @command{cat}: Concatenate and write files
1488 @cindex concatenate and write files
1489 @cindex copying files
1491 @command{cat} copies each @var{file} (@samp{-} means standard input), or
1492 standard input if none are given, to standard output. Synopsis:
1495 cat [@var{option}] [@var{file}]@dots{}
1498 The program accepts the following options. Also see @ref{Common options}.
1506 Equivalent to @option{-vET}.
1509 @itemx --number-nonblank
1511 @opindex --number-nonblank
1512 Number all nonempty output lines, starting with 1.
1516 Equivalent to @option{-vE}.
1521 @opindex --show-ends
1522 Display a @samp{$} after the end of each line.
1528 Number all output lines, starting with 1. This option is ignored
1529 if @option{-b} is in effect.
1532 @itemx --squeeze-blank
1534 @opindex --squeeze-blank
1535 @cindex squeezing empty lines
1536 Suppress repeated adjacent empty lines; output just one empty line
1541 Equivalent to @option{-vT}.
1546 @opindex --show-tabs
1547 Display TAB characters as @samp{^I}.
1551 Ignored; for @acronym{POSIX} compatibility.
1554 @itemx --show-nonprinting
1556 @opindex --show-nonprinting
1557 Display control characters except for LFD and TAB using
1558 @samp{^} notation and precede characters that have the high bit set with
1563 On systems like MS-DOS that distinguish between text and binary files,
1564 @command{cat} normally reads and writes in binary mode. However,
1565 @command{cat} reads in text mode if one of the options
1566 @option{-bensAE} is used or if @command{cat} is reading from standard
1567 input and standard input is a terminal. Similarly, @command{cat}
1568 writes in text mode if one of the options @option{-bensAE} is used or
1569 if standard output is a terminal.
1576 # Output f's contents, then standard input, then g's contents.
1579 # Copy standard input to standard output.
1584 @node tac invocation
1585 @section @command{tac}: Concatenate and write files in reverse
1588 @cindex reversing files
1590 @command{tac} copies each @var{file} (@samp{-} means standard input), or
1591 standard input if none are given, to standard output, reversing the
1592 records (lines by default) in each separately. Synopsis:
1595 tac [@var{option}]@dots{} [@var{file}]@dots{}
1598 @dfn{Records} are separated by instances of a string (newline by
1599 default). By default, this separator string is attached to the end of
1600 the record that it follows in the file.
1602 The program accepts the following options. Also see @ref{Common options}.
1610 The separator is attached to the beginning of the record that it
1611 precedes in the file.
1617 Treat the separator string as a regular expression. Users of @command{tac}
1618 on MS-DOS/MS-Windows should note that, since @command{tac} reads files in
1619 binary mode, each line of a text file might end with a CR/LF pair
1620 instead of the Unix-style LF.
1622 @item -s @var{separator}
1623 @itemx --separator=@var{separator}
1625 @opindex --separator
1626 Use @var{separator} as the record separator, instead of newline.
1634 @section @command{nl}: Number lines and write files
1637 @cindex numbering lines
1638 @cindex line numbering
1640 @command{nl} writes each @var{file} (@samp{-} means standard input), or
1641 standard input if none are given, to standard output, with line numbers
1642 added to some or all of the lines. Synopsis:
1645 nl [@var{option}]@dots{} [@var{file}]@dots{}
1648 @cindex logical pages, numbering on
1649 @command{nl} decomposes its input into (logical) pages; by default, the
1650 line number is reset to 1 at the top of each logical page. @command{nl}
1651 treats all of the input files as a single document; it does not reset
1652 line numbers or logical pages between files.
1654 @cindex headers, numbering
1655 @cindex body, numbering
1656 @cindex footers, numbering
1657 A logical page consists of three sections: header, body, and footer.
1658 Any of the sections can be empty. Each can be numbered in a different
1659 style from the others.
1661 The beginnings of the sections of logical pages are indicated in the
1662 input file by a line containing exactly one of these delimiter strings:
1673 The two characters from which these strings are made can be changed from
1674 @samp{\} and @samp{:} via options (see below), but the pattern and
1675 length of each string cannot be changed.
1677 A section delimiter is replaced by an empty line on output. Any text
1678 that comes before the first section delimiter string in the input file
1679 is considered to be part of a body section, so @command{nl} treats a
1680 file that contains no section delimiters as a single body section.
1682 The program accepts the following options. Also see @ref{Common options}.
1686 @item -b @var{style}
1687 @itemx --body-numbering=@var{style}
1689 @opindex --body-numbering
1690 Select the numbering style for lines in the body section of each
1691 logical page. When a line is not numbered, the current line number
1692 is not incremented, but the line number separator character is still
1693 prepended to the line. The styles are:
1699 number only nonempty lines (default for body),
1701 do not number lines (default for header and footer),
1703 number only lines that contain a match for the basic regular
1704 expression @var{bre}.
1705 @xref{Regular Expressions, , Regular Expressions, grep, The GNU Grep Manual}.
1709 @itemx --section-delimiter=@var{cd}
1711 @opindex --section-delimiter
1712 @cindex section delimiters of pages
1713 Set the section delimiter characters to @var{cd}; default is
1714 @samp{\:}. If only @var{c} is given, the second remains @samp{:}.
1715 (Remember to protect @samp{\} or other metacharacters from shell
1716 expansion with quotes or extra backslashes.)
1718 @item -f @var{style}
1719 @itemx --footer-numbering=@var{style}
1721 @opindex --footer-numbering
1722 Analogous to @option{--body-numbering}.
1724 @item -h @var{style}
1725 @itemx --header-numbering=@var{style}
1727 @opindex --header-numbering
1728 Analogous to @option{--body-numbering}.
1730 @item -i @var{number}
1731 @itemx --line-increment=@var{number}
1733 @opindex --line-increment
1734 Increment line numbers by @var{number} (default 1).
1736 @item -l @var{number}
1737 @itemx --join-blank-lines=@var{number}
1739 @opindex --join-blank-lines
1740 @cindex empty lines, numbering
1741 @cindex blank lines, numbering
1742 Consider @var{number} (default 1) consecutive empty lines to be one
1743 logical line for numbering, and only number the last one. Where fewer
1744 than @var{number} consecutive empty lines occur, do not number them.
1745 An empty line is one that contains no characters, not even spaces
1748 @item -n @var{format}
1749 @itemx --number-format=@var{format}
1751 @opindex --number-format
1752 Select the line numbering format (default is @code{rn}):
1756 @opindex ln @r{format for @command{nl}}
1757 left justified, no leading zeros;
1759 @opindex rn @r{format for @command{nl}}
1760 right justified, no leading zeros;
1762 @opindex rz @r{format for @command{nl}}
1763 right justified, leading zeros.
1767 @itemx --no-renumber
1769 @opindex --no-renumber
1770 Do not reset the line number at the start of a logical page.
1772 @item -s @var{string}
1773 @itemx --number-separator=@var{string}
1775 @opindex --number-separator
1776 Separate the line number from the text line in the output with
1777 @var{string} (default is the TAB character).
1779 @item -v @var{number}
1780 @itemx --starting-line-number=@var{number}
1782 @opindex --starting-line-number
1783 Set the initial line number on each logical page to @var{number} (default 1).
1785 @item -w @var{number}
1786 @itemx --number-width=@var{number}
1788 @opindex --number-width
1789 Use @var{number} characters for line numbers (default 6).
1797 @section @command{od}: Write files in octal or other formats
1800 @cindex octal dump of files
1801 @cindex hex dump of files
1802 @cindex ASCII dump of files
1803 @cindex file contents, dumping unambiguously
1805 @command{od} writes an unambiguous representation of each @var{file}
1806 (@samp{-} means standard input), or standard input if none are given.
1810 od [@var{option}]@dots{} [@var{file}]@dots{}
1811 od [-abcdfilosx]@dots{} [@var{file}] [[+]@var{offset}[.][b]]
1812 od [@var{option}]@dots{} --traditional [@var{file}]@c
1813 [[+]@var{offset}[.][b] [[+]@var{label}[.][b]]]
1816 Each line of output consists of the offset in the input, followed by
1817 groups of data from the file. By default, @command{od} prints the offset in
1818 octal, and each group of file data is a C @code{short int}'s worth of input
1819 printed as a single octal number.
1821 If @var{offset} is given, it specifies how many input bytes to skip
1822 before formatting and writing. By default, it is interpreted as an
1823 octal number, but the optional trailing decimal point causes it to be
1824 interpreted as decimal. If no decimal is specified and the offset
1825 begins with @samp{0x} or @samp{0X} it is interpreted as a hexadecimal
1826 number. If there is a trailing @samp{b}, the number of bytes skipped
1827 will be @var{offset} multiplied by 512.
1829 If a command is of both the first and second forms, the second form is
1830 assumed if the last operand begins with @samp{+} or (if there are two
1831 operands) a digit. For example, in @samp{od foo 10} and @samp{od +10}
1832 the @samp{10} is an offset, whereas in @samp{od 10} the @samp{10} is a
1835 The program accepts the following options. Also see @ref{Common options}.
1839 @item -A @var{radix}
1840 @itemx --address-radix=@var{radix}
1842 @opindex --address-radix
1843 @cindex radix for file offsets
1844 @cindex file offset radix
1845 Select the base in which file offsets are printed. @var{radix} can
1846 be one of the following:
1856 none (do not print offsets).
1859 The default is octal.
1861 @item -j @var{bytes}
1862 @itemx --skip-bytes=@var{bytes}
1864 @opindex --skip-bytes
1865 Skip @var{bytes} input bytes before formatting and writing. If
1866 @var{bytes} begins with @samp{0x} or @samp{0X}, it is interpreted in
1867 hexadecimal; otherwise, if it begins with @samp{0}, in octal; otherwise,
1869 @multiplierSuffixes{bytes}
1871 @item -N @var{bytes}
1872 @itemx --read-bytes=@var{bytes}
1874 @opindex --read-bytes
1875 Output at most @var{bytes} bytes of the input. Prefixes and suffixes on
1876 @code{bytes} are interpreted as for the @option{-j} option.
1878 @item -S @var{bytes}
1879 @itemx --strings[=@var{bytes}]
1882 @cindex string constants, outputting
1883 Instead of the normal output, output only @dfn{string constants}: at
1884 least @var{bytes} consecutive @acronym{ASCII} graphic characters,
1885 followed by a zero byte (@acronym{ASCII} @sc{nul}).
1886 Prefixes and suffixes on @var{bytes} are interpreted as for the
1889 If @var{n} is omitted with @option{--strings}, the default is 3.
1892 @itemx --format=@var{type}
1895 Select the format in which to output the file data. @var{type} is a
1896 string of one or more of the below type indicator characters. If you
1897 include more than one type indicator character in a single @var{type}
1898 string, or use this option more than once, @command{od} writes one copy
1899 of each output line using each of the data types that you specified,
1900 in the order that you specified.
1902 Adding a trailing ``z'' to any type specification appends a display
1903 of the @acronym{ASCII} character representation of the printable characters
1904 to the output line generated by the type specification.
1908 named character, ignoring high-order bit
1910 @acronym{ASCII} character or backslash escape,
1914 floating point (@pxref{Floating point})
1923 The type @code{a} outputs things like @samp{sp} for space, @samp{nl} for
1924 newline, and @samp{nul} for a zero byte. Only the least significant
1925 seven bits of each byte is used; the high-order bit is ignored.
1926 Type @code{c} outputs
1927 @samp{ }, @samp{\n}, and @code{\0}, respectively.
1930 Except for types @samp{a} and @samp{c}, you can specify the number
1931 of bytes to use in interpreting each number in the given data type
1932 by following the type indicator character with a decimal integer.
1933 Alternately, you can specify the size of one of the C compiler's
1934 built-in data types by following the type indicator character with
1935 one of the following characters. For integers (@samp{d}, @samp{o},
1936 @samp{u}, @samp{x}):
1949 For floating point (@code{f}):
1961 @itemx --output-duplicates
1963 @opindex --output-duplicates
1964 Output consecutive lines that are identical. By default, when two or
1965 more consecutive output lines would be identical, @command{od} outputs only
1966 the first line, and puts just an asterisk on the following line to
1967 indicate the elision.
1970 @itemx --width[=@var{n}]
1973 Dump @code{n} input bytes per output line. This must be a multiple of
1974 the least common multiple of the sizes associated with the specified
1977 If this option is not given at all, the default is 16. If @var{n} is
1978 omitted, the default is 32.
1982 The next several options are shorthands for format specifications.
1983 @sc{gnu} @command{od} accepts any combination of shorthands and format
1984 specification options. These options accumulate.
1990 Output as named characters. Equivalent to @samp{-t a}.
1994 Output as octal bytes. Equivalent to @samp{-t o1}.
1998 Output as @acronym{ASCII} characters or backslash escapes. Equivalent to
2003 Output as unsigned decimal two-byte units. Equivalent to @samp{-t u2}.
2007 Output as floats. Equivalent to @samp{-t fF}.
2011 Output as decimal ints. Equivalent to @samp{-t dI}.
2015 Output as decimal long ints. Equivalent to @samp{-t dL}.
2019 Output as octal two-byte units. Equivalent to @option{-t o2}.
2023 Output as decimal two-byte units. Equivalent to @option{-t d2}.
2027 Output as hexadecimal two-byte units. Equivalent to @samp{-t x2}.
2030 @opindex --traditional
2031 Recognize the non-option label argument that traditional @command{od}
2032 accepted. The following syntax:
2035 od --traditional [@var{file}] [[+]@var{offset}[.][b] [[+]@var{label}[.][b]]]
2039 can be used to specify at most one file and optional arguments
2040 specifying an offset and a pseudo-start address, @var{label}.
2041 The @var{label} argument is interpreted
2042 just like @var{offset}, but it specifies an initial pseudo-address. The
2043 pseudo-addresses are displayed in parentheses following any normal
2050 @node base64 invocation
2051 @section @command{base64}: Transform data into printable data
2054 @cindex base64 encoding
2056 @command{base64} transforms data read from a file, or standard input,
2057 into (or from) base64 encoded form. The base64 encoded form uses
2058 printable @acronym{ASCII} characters to represent binary data.
2062 base64 [@var{option}]@dots{} [@var{file}]
2063 base64 --decode [@var{option}]@dots{} [@var{file}]
2066 The base64 encoding expands data to roughly 133% of the original.
2067 The format conforms to
2068 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc4648.txt, RFC 4648}.
2070 The program accepts the following options. Also see @ref{Common options}.
2075 @itemx --wrap=@var{cols}
2079 @cindex column to wrap data after
2080 During encoding, wrap lines after @var{cols} characters. This must be
2083 The default is to wrap after 76 characters. Use the value 0 to
2084 disable line wrapping altogether.
2090 @cindex Decode base64 data
2091 @cindex Base64 decoding
2092 Change the mode of operation, from the default of encoding data, to
2093 decoding data. Input is expected to be base64 encoded data, and the
2094 output will be the original data.
2097 @itemx --ignore-garbage
2099 @opindex --ignore-garbage
2100 @cindex Ignore garbage in base64 stream
2101 When decoding, newlines are always accepted.
2102 During decoding, ignore unrecognized bytes,
2103 to permit distorted data to be decoded.
2110 @node Formatting file contents
2111 @chapter Formatting file contents
2113 @cindex formatting file contents
2115 These commands reformat the contents of files.
2118 * fmt invocation:: Reformat paragraph text.
2119 * pr invocation:: Paginate or columnate files for printing.
2120 * fold invocation:: Wrap input lines to fit in specified width.
2124 @node fmt invocation
2125 @section @command{fmt}: Reformat paragraph text
2128 @cindex reformatting paragraph text
2129 @cindex paragraphs, reformatting
2130 @cindex text, reformatting
2132 @command{fmt} fills and joins lines to produce output lines of (at most)
2133 a given number of characters (75 by default). Synopsis:
2136 fmt [@var{option}]@dots{} [@var{file}]@dots{}
2139 @command{fmt} reads from the specified @var{file} arguments (or standard
2140 input if none are given), and writes to standard output.
2142 By default, blank lines, spaces between words, and indentation are
2143 preserved in the output; successive input lines with different
2144 indentation are not joined; tabs are expanded on input and introduced on
2147 @cindex line-breaking
2148 @cindex sentences and line-breaking
2149 @cindex Knuth, Donald E.
2150 @cindex Plass, Michael F.
2151 @command{fmt} prefers breaking lines at the end of a sentence, and tries to
2152 avoid line breaks after the first word of a sentence or before the last
2153 word of a sentence. A @dfn{sentence break} is defined as either the end
2154 of a paragraph or a word ending in any of @samp{.?!}, followed by two
2155 spaces or end of line, ignoring any intervening parentheses or quotes.
2156 Like @TeX{}, @command{fmt} reads entire ``paragraphs'' before choosing line
2157 breaks; the algorithm is a variant of that given by Donald E. Knuth
2158 and Michael F. Plass in ``Breaking Paragraphs Into Lines'',
2159 @cite{Software---Practice & Experience} @b{11}, 11 (November 1981),
2162 The program accepts the following options. Also see @ref{Common options}.
2167 @itemx --crown-margin
2169 @opindex --crown-margin
2170 @cindex crown margin
2171 @dfn{Crown margin} mode: preserve the indentation of the first two
2172 lines within a paragraph, and align the left margin of each subsequent
2173 line with that of the second line.
2176 @itemx --tagged-paragraph
2178 @opindex --tagged-paragraph
2179 @cindex tagged paragraphs
2180 @dfn{Tagged paragraph} mode: like crown margin mode, except that if
2181 indentation of the first line of a paragraph is the same as the
2182 indentation of the second, the first line is treated as a one-line
2188 @opindex --split-only
2189 Split lines only. Do not join short lines to form longer ones. This
2190 prevents sample lines of code, and other such ``formatted'' text from
2191 being unduly combined.
2194 @itemx --uniform-spacing
2196 @opindex --uniform-spacing
2197 Uniform spacing. Reduce spacing between words to one space, and spacing
2198 between sentences to two spaces.
2201 @itemx -w @var{width}
2202 @itemx --width=@var{width}
2203 @opindex -@var{width}
2206 Fill output lines up to @var{width} characters (default 75 or @var{goal}
2207 plus 10, if @var{goal} is provided).
2209 @itemx -g @var{goal}
2210 @itemx --goal=@var{goal}
2213 @command{fmt} initially tries to make lines @var{goal} characters wide.
2214 By default, this is 7% shorter than @var{width}.
2216 @item -p @var{prefix}
2217 @itemx --prefix=@var{prefix}
2218 Only lines beginning with @var{prefix} (possibly preceded by whitespace)
2219 are subject to formatting. The prefix and any preceding whitespace are
2220 stripped for the formatting and then re-attached to each formatted output
2221 line. One use is to format certain kinds of program comments, while
2222 leaving the code unchanged.
2230 @section @command{pr}: Paginate or columnate files for printing
2233 @cindex printing, preparing files for
2234 @cindex multicolumn output, generating
2235 @cindex merging files in parallel
2237 @command{pr} writes each @var{file} (@samp{-} means standard input), or
2238 standard input if none are given, to standard output, paginating and
2239 optionally outputting in multicolumn format; optionally merges all
2240 @var{file}s, printing all in parallel, one per column. Synopsis:
2243 pr [@var{option}]@dots{} [@var{file}]@dots{}
2247 By default, a 5-line header is printed at each page: two blank lines;
2248 a line with the date, the file name, and the page count; and two more
2249 blank lines. A footer of five blank lines is also printed.
2250 The default @var{page_length} is 66
2251 lines. The default number of text lines is therefore 56.
2252 The text line of the header takes the form
2253 @samp{@var{date} @var{string} @var{page}}, with spaces inserted around
2254 @var{string} so that the line takes up the full @var{page_width}. Here,
2255 @var{date} is the date (see the @option{-D} or @option{--date-format}
2256 option for details), @var{string} is the centered header string, and
2257 @var{page} identifies the page number. The @env{LC_MESSAGES} locale
2258 category affects the spelling of @var{page}; in the default C locale, it
2259 is @samp{Page @var{number}} where @var{number} is the decimal page
2262 Form feeds in the input cause page breaks in the output. Multiple form
2263 feeds produce empty pages.
2265 Columns are of equal width, separated by an optional string (default
2266 is @samp{space}). For multicolumn output, lines will always be truncated to
2267 @var{page_width} (default 72), unless you use the @option{-J} option.
2269 column output no line truncation occurs by default. Use @option{-W} option to
2270 truncate lines in that case.
2272 The following changes were made in version 1.22i and apply to later
2273 versions of @command{pr}:
2274 @c FIXME: this whole section here sounds very awkward to me. I
2275 @c made a few small changes, but really it all needs to be redone. - Brian
2276 @c OK, I fixed another sentence or two, but some of it I just don't understand.
2281 Some small @var{letter options} (@option{-s}, @option{-w}) have been
2282 redefined for better @acronym{POSIX} compliance. The output of some further
2283 cases has been adapted to other Unix systems. These changes are not
2284 compatible with earlier versions of the program.
2287 Some @var{new capital letter} options (@option{-J}, @option{-S}, @option{-W})
2288 have been introduced to turn off unexpected interferences of small letter
2289 options. The @option{-N} option and the second argument @var{last_page}
2290 of @samp{+FIRST_PAGE} offer more flexibility. The detailed handling of
2291 form feeds set in the input files requires the @option{-T} option.
2294 Capital letter options override small letter ones.
2297 Some of the option-arguments (compare @option{-s}, @option{-e},
2298 @option{-i}, @option{-n}) cannot be specified as separate arguments from the
2299 preceding option letter (already stated in the @acronym{POSIX} specification).
2302 The program accepts the following options. Also see @ref{Common options}.
2306 @item +@var{first_page}[:@var{last_page}]
2307 @itemx --pages=@var{first_page}[:@var{last_page}]
2308 @c The two following @opindex lines evoke warnings because they contain ':'
2309 @c The 'info' spec does not permit that. If we use those lines, we end
2310 @c up with truncated index entries that don't work.
2311 @c @opindex +@var{first_page}[:@var{last_page}]
2312 @c @opindex --pages=@var{first_page}[:@var{last_page}]
2313 @opindex +@var{page_range}
2314 @opindex --pages=@var{page_range}
2315 Begin printing with page @var{first_page} and stop with @var{last_page}.
2316 Missing @samp{:@var{last_page}} implies end of file. While estimating
2317 the number of skipped pages each form feed in the input file results
2318 in a new page. Page counting with and without @samp{+@var{first_page}}
2319 is identical. By default, counting starts with the first page of input
2320 file (not first page printed). Line numbering may be altered by @option{-N}
2324 @itemx --columns=@var{column}
2325 @opindex -@var{column}
2327 @cindex down columns
2328 With each single @var{file}, produce @var{column} columns of output
2329 (default is 1) and print columns down, unless @option{-a} is used. The
2330 column width is automatically decreased as @var{column} increases; unless
2331 you use the @option{-W/-w} option to increase @var{page_width} as well.
2332 This option might well cause some lines to be truncated. The number of
2333 lines in the columns on each page are balanced. The options @option{-e}
2334 and @option{-i} are on for multiple text-column output. Together with
2335 @option{-J} option column alignment and line truncation is turned off.
2336 Lines of full length are joined in a free field format and @option{-S}
2337 option may set field separators. @option{-@var{column}} may not be used
2338 with @option{-m} option.
2344 @cindex across columns
2345 With each single @var{file}, print columns across rather than down. The
2346 @option{-@var{column}} option must be given with @var{column} greater than one.
2347 If a line is too long to fit in a column, it is truncated.
2350 @itemx --show-control-chars
2352 @opindex --show-control-chars
2353 Print control characters using hat notation (e.g., @samp{^G}); print
2354 other nonprinting characters in octal backslash notation. By default,
2355 nonprinting characters are not changed.
2358 @itemx --double-space
2360 @opindex --double-space
2361 @cindex double spacing
2362 Double space the output.
2364 @item -D @var{format}
2365 @itemx --date-format=@var{format}
2366 @cindex time formats
2367 @cindex formatting times
2368 Format header dates using @var{format}, using the same conventions as
2369 for the command @samp{date +@var{format}}; @xref{date invocation}.
2370 Except for directives, which start with
2371 @samp{%}, characters in @var{format} are printed unchanged. You can use
2372 this option to specify an arbitrary string in place of the header date,
2373 e.g., @option{--date-format="Monday morning"}.
2375 @vindex POSIXLY_CORRECT
2377 The default date format is @samp{%Y-%m-%d %H:%M} (for example,
2378 @samp{2001-12-04 23:59});
2379 but if the @env{POSIXLY_CORRECT} environment variable is set
2380 and the @env{LC_TIME} locale category specifies the @acronym{POSIX}
2381 locale, the default is @samp{%b %e %H:%M %Y} (for example,
2382 @samp{Dec@ @ 4 23:59 2001}.
2385 Time stamps are listed according to the time zone rules specified by
2386 the @env{TZ} environment variable, or by the system default rules if
2387 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
2388 with @env{TZ}, libc, The GNU C Library Reference Manual}.
2390 @item -e[@var{in-tabchar}[@var{in-tabwidth}]]
2391 @itemx --expand-tabs[=@var{in-tabchar}[@var{in-tabwidth}]]
2393 @opindex --expand-tabs
2395 Expand @var{tab}s to spaces on input. Optional argument @var{in-tabchar} is
2396 the input tab character (default is the TAB character). Second optional
2397 argument @var{in-tabwidth} is the input tab character's width (default
2405 @opindex --form-feed
2406 Use a form feed instead of newlines to separate output pages. This does
2407 not alter the default page length of 66 lines.
2409 @item -h @var{header}
2410 @itemx --header=@var{header}
2413 Replace the file name in the header with the centered string @var{header}.
2414 When using the shell, @var{header} should be quoted and should be
2415 separated from @option{-h} by a space.
2417 @item -i[@var{out-tabchar}[@var{out-tabwidth}]]
2418 @itemx --output-tabs[=@var{out-tabchar}[@var{out-tabwidth}]]
2420 @opindex --output-tabs
2422 Replace spaces with @var{tab}s on output. Optional argument @var{out-tabchar}
2423 is the output tab character (default is the TAB character). Second optional
2424 argument @var{out-tabwidth} is the output tab character's width (default
2430 @opindex --join-lines
2431 Merge lines of full length. Used together with the column options
2432 @option{-@var{column}}, @option{-a -@var{column}} or @option{-m}. Turns off
2433 @option{-W/-w} line truncation;
2434 no column alignment used; may be used with
2435 @option{--sep-string[=@var{string}]}. @option{-J} has been introduced
2436 (together with @option{-W} and @option{--sep-string})
2437 to disentangle the old (@acronym{POSIX}-compliant) options @option{-w} and
2438 @option{-s} along with the three column options.
2441 @item -l @var{page_length}
2442 @itemx --length=@var{page_length}
2445 Set the page length to @var{page_length} (default 66) lines, including
2446 the lines of the header [and the footer]. If @var{page_length} is less
2447 than or equal to 10, the header and footer are omitted, as if the
2448 @option{-t} option had been given.
2454 Merge and print all @var{file}s in parallel, one in each column. If a
2455 line is too long to fit in a column, it is truncated, unless the @option{-J}
2456 option is used. @option{--sep-string[=@var{string}]} may be used.
2458 some @var{file}s (form feeds set) produce empty columns, still marked
2459 by @var{string}. The result is a continuous line numbering and column
2460 marking throughout the whole merged file. Completely empty merged pages
2461 show no separators or line numbers. The default header becomes
2462 @samp{@var{date} @var{page}} with spaces inserted in the middle; this
2463 may be used with the @option{-h} or @option{--header} option to fill up
2464 the middle blank part.
2466 @item -n[@var{number-separator}[@var{digits}]]
2467 @itemx --number-lines[=@var{number-separator}[@var{digits}]]
2469 @opindex --number-lines
2470 Provide @var{digits} digit line numbering (default for @var{digits} is
2471 5). With multicolumn output the number occupies the first @var{digits}
2472 column positions of each text column or only each line of @option{-m}
2473 output. With single column output the number precedes each line just as
2474 @option{-m} does. Default counting of the line numbers starts with the
2475 first line of the input file (not the first line printed, compare the
2476 @option{--page} option and @option{-N} option).
2477 Optional argument @var{number-separator} is the character appended to
2478 the line number to separate it from the text followed. The default
2479 separator is the TAB character. In a strict sense a TAB is always
2480 printed with single column output only. The TAB width varies
2481 with the TAB position, e.g., with the left @var{margin} specified
2482 by @option{-o} option. With multicolumn output priority is given to
2483 @samp{equal width of output columns} (a @acronym{POSIX} specification).
2484 The TAB width is fixed to the value of the first column and does
2485 not change with different values of left @var{margin}. That means a
2486 fixed number of spaces is always printed in the place of the
2487 @var{number-separator} TAB. The tabification depends upon the output
2490 @item -N @var{line_number}
2491 @itemx --first-line-number=@var{line_number}
2493 @opindex --first-line-number
2494 Start line counting with the number @var{line_number} at first line of
2495 first page printed (in most cases not the first line of the input file).
2497 @item -o @var{margin}
2498 @itemx --indent=@var{margin}
2501 @cindex indenting lines
2503 Indent each line with a margin @var{margin} spaces wide (default is zero).
2504 The total page width is the size of the margin plus the @var{page_width}
2505 set with the @option{-W/-w} option. A limited overflow may occur with
2506 numbered single column output (compare @option{-n} option).
2509 @itemx --no-file-warnings
2511 @opindex --no-file-warnings
2512 Do not print a warning message when an argument @var{file} cannot be
2513 opened. (The exit status will still be nonzero, however.)
2515 @item -s[@var{char}]
2516 @itemx --separator[=@var{char}]
2518 @opindex --separator
2519 Separate columns by a single character @var{char}. The default for
2520 @var{char} is the TAB character without @option{-w} and @samp{no
2521 character} with @option{-w}. Without @option{-s} the default separator
2522 @samp{space} is set. @option{-s[char]} turns off line truncation of all
2523 three column options (@option{-COLUMN}|@option{-a -COLUMN}|@option{-m}) unless
2524 @option{-w} is set. This is a @acronym{POSIX}-compliant formulation.
2527 @item -S[@var{string}]
2528 @itemx --sep-string[=@var{string}]
2530 @opindex --sep-string
2531 Use @var{string} to separate output columns. The @option{-S} option doesn't
2532 affect the @option{-W/-w} option, unlike the @option{-s} option which does. It
2533 does not affect line truncation or column alignment.
2534 Without @option{-S}, and with @option{-J}, @command{pr} uses the default output
2536 Without @option{-S} or @option{-J}, @command{pr} uses a @samp{space}
2537 (same as @option{-S"@w{ }"}).
2538 If no @samp{@var{string}} argument is specified, @samp{""} is assumed.
2541 @itemx --omit-header
2543 @opindex --omit-header
2544 Do not print the usual header [and footer] on each page, and do not fill
2545 out the bottom of pages (with blank lines or a form feed). No page
2546 structure is produced, but form feeds set in the input files are retained.
2547 The predefined pagination is not changed. @option{-t} or @option{-T} may be
2548 useful together with other options; e.g.: @option{-t -e4}, expand TAB characters
2549 in the input file to 4 spaces but don't make any other changes. Use of
2550 @option{-t} overrides @option{-h}.
2553 @itemx --omit-pagination
2555 @opindex --omit-pagination
2556 Do not print header [and footer]. In addition eliminate all form feeds
2557 set in the input files.
2560 @itemx --show-nonprinting
2562 @opindex --show-nonprinting
2563 Print nonprinting characters in octal backslash notation.
2565 @item -w @var{page_width}
2566 @itemx --width=@var{page_width}
2569 Set page width to @var{page_width} characters for multiple text-column
2570 output only (default for @var{page_width} is 72). @option{-s[CHAR]} turns
2571 off the default page width and any line truncation and column alignment.
2572 Lines of full length are merged, regardless of the column options
2573 set. No @var{page_width} setting is possible with single column output.
2574 A @acronym{POSIX}-compliant formulation.
2576 @item -W @var{page_width}
2577 @itemx --page_width=@var{page_width}
2579 @opindex --page_width
2580 Set the page width to @var{page_width} characters. That's valid with and
2581 without a column option. Text lines are truncated, unless @option{-J}
2582 is used. Together with one of the three column options
2583 (@option{-@var{column}}, @option{-a -@var{column}} or @option{-m}) column
2584 alignment is always used. The separator options @option{-S} or @option{-s}
2585 don't affect the @option{-W} option. Default is 72 characters. Without
2586 @option{-W @var{page_width}} and without any of the column options NO line
2587 truncation is used (defined to keep downward compatibility and to meet
2588 most frequent tasks). That's equivalent to @option{-W 72 -J}. The header
2589 line is never truncated.
2596 @node fold invocation
2597 @section @command{fold}: Wrap input lines to fit in specified width
2600 @cindex wrapping long input lines
2601 @cindex folding long input lines
2603 @command{fold} writes each @var{file} (@option{-} means standard input), or
2604 standard input if none are given, to standard output, breaking long
2608 fold [@var{option}]@dots{} [@var{file}]@dots{}
2611 By default, @command{fold} breaks lines wider than 80 columns. The output
2612 is split into as many lines as necessary.
2614 @cindex screen columns
2615 @command{fold} counts screen columns by default; thus, a tab may count more
2616 than one column, backspace decreases the column count, and carriage
2617 return sets the column to zero.
2619 The program accepts the following options. Also see @ref{Common options}.
2627 Count bytes rather than columns, so that tabs, backspaces, and carriage
2628 returns are each counted as taking up one column, just like other
2635 Break at word boundaries: the line is broken after the last blank before
2636 the maximum line length. If the line contains no such blanks, the line
2637 is broken at the maximum line length as usual.
2639 @item -w @var{width}
2640 @itemx --width=@var{width}
2643 Use a maximum line length of @var{width} columns instead of 80.
2645 For compatibility @command{fold} supports an obsolete option syntax
2646 @option{-@var{width}}. New scripts should use @option{-w @var{width}}
2654 @node Output of parts of files
2655 @chapter Output of parts of files
2657 @cindex output of parts of files
2658 @cindex parts of files, output of
2660 These commands output pieces of the input.
2663 * head invocation:: Output the first part of files.
2664 * tail invocation:: Output the last part of files.
2665 * split invocation:: Split a file into pieces.
2666 * csplit invocation:: Split a file into context-determined pieces.
2669 @node head invocation
2670 @section @command{head}: Output the first part of files
2673 @cindex initial part of files, outputting
2674 @cindex first part of files, outputting
2676 @command{head} prints the first part (10 lines by default) of each
2677 @var{file}; it reads from standard input if no files are given or
2678 when given a @var{file} of @option{-}. Synopsis:
2681 head [@var{option}]@dots{} [@var{file}]@dots{}
2684 If more than one @var{file} is specified, @command{head} prints a
2685 one-line header consisting of:
2688 ==> @var{file name} <==
2692 before the output for each @var{file}.
2694 The program accepts the following options. Also see @ref{Common options}.
2699 @itemx --bytes=@var{k}
2702 Print the first @var{k} bytes, instead of initial lines.
2703 However, if @var{k} starts with a @samp{-},
2704 print all but the last @var{k} bytes of each file.
2705 @multiplierSuffixes{k}
2708 @itemx --lines=@var{k}
2711 Output the first @var{k} lines.
2712 However, if @var{k} starts with a @samp{-},
2713 print all but the last @var{k} lines of each file.
2714 Size multiplier suffixes are the same as with the @option{-c} option.
2722 Never print file name headers.
2728 Always print file name headers.
2732 For compatibility @command{head} also supports an obsolete option syntax
2733 @option{-@var{count}@var{options}}, which is recognized only if it is
2734 specified first. @var{count} is a decimal number optionally followed
2735 by a size letter (@samp{b}, @samp{k}, @samp{m}) as in @option{-c}, or
2736 @samp{l} to mean count by lines, or other option letters (@samp{cqv}).
2737 Scripts intended for standard hosts should use @option{-c @var{count}}
2738 or @option{-n @var{count}} instead. If your script must also run on
2739 hosts that support only the obsolete syntax, it is usually simpler to
2740 avoid @command{head}, e.g., by using @samp{sed 5q} instead of
2746 @node tail invocation
2747 @section @command{tail}: Output the last part of files
2750 @cindex last part of files, outputting
2752 @command{tail} prints the last part (10 lines by default) of each
2753 @var{file}; it reads from standard input if no files are given or
2754 when given a @var{file} of @samp{-}. Synopsis:
2757 tail [@var{option}]@dots{} [@var{file}]@dots{}
2760 If more than one @var{file} is specified, @command{tail} prints a
2761 one-line header consisting of:
2764 ==> @var{file name} <==
2768 before the output for each @var{file}.
2770 @cindex BSD @command{tail}
2771 @sc{gnu} @command{tail} can output any amount of data (some other versions of
2772 @command{tail} cannot). It also has no @option{-r} option (print in
2773 reverse), since reversing a file is really a different job from printing
2774 the end of a file; BSD @command{tail} (which is the one with @option{-r}) can
2775 only reverse files that are at most as large as its buffer, which is
2776 typically 32 KiB@. A more reliable and versatile way to reverse files is
2777 the @sc{gnu} @command{tac} command.
2779 The program accepts the following options. Also see @ref{Common options}.
2784 @itemx --bytes=@var{k}
2787 Output the last @var{k} bytes, instead of final lines.
2788 However, if @var{k} starts with a @samp{+}, start printing with the
2789 @var{k}th byte from the start of each file, instead of from the end.
2790 @multiplierSuffixes{k}
2793 @itemx --follow[=@var{how}]
2796 @cindex growing files
2797 @vindex name @r{follow option}
2798 @vindex descriptor @r{follow option}
2799 Loop forever trying to read more characters at the end of the file,
2800 presumably because the file is growing.
2801 If more than one file is given, @command{tail} prints a header whenever it
2802 gets output from a different file, to indicate which file that output is
2805 There are two ways to specify how you'd like to track files with this option,
2806 but that difference is noticeable only when a followed file is removed or
2808 If you'd like to continue to track the end of a growing file even after
2809 it has been unlinked, use @option{--follow=descriptor}. This is the default
2810 behavior, but it is not useful if you're tracking a log file that may be
2811 rotated (removed or renamed, then reopened). In that case, use
2812 @option{--follow=name} to track the named file, perhaps by reopening it
2813 periodically to see if it has been removed and recreated by some other program.
2814 Note that the inotify-based implementation handles this case without
2815 the need for any periodic reopening.
2817 No matter which method you use, if the tracked file is determined to have
2818 shrunk, @command{tail} prints a message saying the file has been truncated
2819 and resumes tracking the end of the file from the newly-determined endpoint.
2821 When a file is removed, @command{tail}'s behavior depends on whether it is
2822 following the name or the descriptor. When following by name, tail can
2823 detect that a file has been removed and gives a message to that effect,
2824 and if @option{--retry} has been specified it will continue checking
2825 periodically to see if the file reappears.
2826 When following a descriptor, tail does not detect that the file has
2827 been unlinked or renamed and issues no message; even though the file
2828 may no longer be accessible via its original name, it may still be
2831 The option values @samp{descriptor} and @samp{name} may be specified only
2832 with the long form of the option, not with @option{-f}.
2834 The @option{-f} option is ignored if
2835 no @var{file} operand is specified and standard input is a FIFO or a pipe.
2836 Likewise, the @option{-f} option has no effect for any
2837 operand specified as @samp{-}, when standard input is a FIFO or a pipe.
2839 With kernel inotify support, output is triggered by file changes
2840 and is generally very prompt.
2841 Otherwise, @command{tail} sleeps for one second between checks---
2842 use @option{--sleep-interval=@var{n}} to change that default---which can
2843 make the output appear slightly less responsive or bursty.
2844 When using tail without inotify support, you can make it more responsive
2845 by using a sub-second sleep interval, e.g., via an alias like this:
2848 alias tail='tail -s.1'
2853 This option is the same as @option{--follow=name --retry}. That is, tail
2854 will attempt to reopen a file when it is removed. Should this fail, tail
2855 will keep trying until it becomes accessible again.
2859 This option is useful mainly when following by name (i.e., with
2860 @option{--follow=name}).
2861 Without this option, when tail encounters a file that doesn't
2862 exist or is otherwise inaccessible, it reports that fact and
2863 never checks it again.
2865 @itemx --sleep-interval=@var{number}
2866 @opindex --sleep-interval
2867 Change the number of seconds to wait between iterations (the default is 1.0).
2868 During one iteration, every specified file is checked to see if it has
2870 Historical implementations of @command{tail} have required that
2871 @var{number} be an integer. However, GNU @command{tail} accepts
2872 an arbitrary floating point number. @xref{Floating point}.
2873 When @command{tail} uses inotify, this polling-related option
2874 is usually ignored. However, if you also specify @option{--pid=@var{p}},
2875 @command{tail} checks whether process @var{p} is alive at least
2876 every @var{number} seconds.
2878 @itemx --pid=@var{pid}
2880 When following by name or by descriptor, you may specify the process ID,
2881 @var{pid}, of the sole writer of all @var{file} arguments. Then, shortly
2882 after that process terminates, tail will also terminate. This will
2883 work properly only if the writer and the tailing process are running on
2884 the same machine. For example, to save the output of a build in a file
2885 and to watch the file grow, if you invoke @command{make} and @command{tail}
2886 like this then the tail process will stop when your build completes.
2887 Without this option, you would have had to kill the @code{tail -f}
2891 $ make >& makerr & tail --pid=$! -f makerr
2894 If you specify a @var{pid} that is not in use or that does not correspond
2895 to the process that is writing to the tailed files, then @command{tail}
2896 may terminate long before any @var{file}s stop growing or it may not
2897 terminate until long after the real writer has terminated.
2898 Note that @option{--pid} cannot be supported on some systems; @command{tail}
2899 will print a warning if this is the case.
2901 @itemx --max-unchanged-stats=@var{n}
2902 @opindex --max-unchanged-stats
2903 When tailing a file by name, if there have been @var{n} (default
2904 n=@value{DEFAULT_MAX_N_UNCHANGED_STATS_BETWEEN_OPENS}) consecutive
2905 iterations for which the file has not changed, then
2906 @code{open}/@code{fstat} the file to determine if that file name is
2907 still associated with the same device/inode-number pair as before.
2908 When following a log file that is rotated, this is approximately the
2909 number of seconds between when tail prints the last pre-rotation lines
2910 and when it prints the lines that have accumulated in the new log file.
2911 This option is meaningful only when polling (i.e., without inotify)
2912 and when following by name.
2915 @itemx --lines=@var{k}
2918 Output the last @var{k} lines.
2919 However, if @var{k} starts with a @samp{+}, start printing with the
2920 @var{k}th line from the start of each file, instead of from the end.
2921 Size multiplier suffixes are the same as with the @option{-c} option.
2929 Never print file name headers.
2935 Always print file name headers.
2939 For compatibility @command{tail} also supports an obsolete usage
2940 @samp{tail -[@var{count}][bcl][f] [@var{file}]}, which is recognized
2941 only if it does not conflict with the usage described
2942 above. This obsolete form uses exactly one option and at most one
2943 file. In the option, @var{count} is an optional decimal number optionally
2944 followed by a size letter (@samp{b}, @samp{c}, @samp{l}) to mean count
2945 by 512-byte blocks, bytes, or lines, optionally followed by @samp{f}
2946 which has the same meaning as @option{-f}.
2948 @vindex _POSIX2_VERSION
2949 On older systems, the leading @samp{-} can be replaced by @samp{+} in
2950 the obsolete option syntax with the same meaning as in counts, and
2951 obsolete usage overrides normal usage when the two conflict.
2952 This obsolete behavior can be enabled or disabled with the
2953 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
2956 Scripts intended for use on standard hosts should avoid obsolete
2957 syntax and should use @option{-c @var{count}[b]}, @option{-n
2958 @var{count}}, and/or @option{-f} instead. If your script must also
2959 run on hosts that support only the obsolete syntax, you can often
2960 rewrite it to avoid problematic usages, e.g., by using @samp{sed -n
2961 '$p'} rather than @samp{tail -1}. If that's not possible, the script
2962 can use a test like @samp{if tail -c +1 </dev/null >/dev/null 2>&1;
2963 then @dots{}} to decide which syntax to use.
2965 Even if your script assumes the standard behavior, you should still
2966 beware usages whose behaviors differ depending on the @acronym{POSIX}
2967 version. For example, avoid @samp{tail - main.c}, since it might be
2968 interpreted as either @samp{tail main.c} or as @samp{tail -- -
2969 main.c}; avoid @samp{tail -c 4}, since it might mean either @samp{tail
2970 -c4} or @samp{tail -c 10 4}; and avoid @samp{tail +4}, since it might
2971 mean either @samp{tail ./+4} or @samp{tail -n +4}.
2976 @node split invocation
2977 @section @command{split}: Split a file into pieces.
2980 @cindex splitting a file into pieces
2981 @cindex pieces, splitting a file into
2983 @command{split} creates output files containing consecutive or interleaved
2984 sections of @var{input} (standard input if none is given or @var{input}
2985 is @samp{-}). Synopsis:
2988 split [@var{option}] [@var{input} [@var{prefix}]]
2991 By default, @command{split} puts 1000 lines of @var{input} (or whatever is
2992 left over for the last section), into each output file.
2994 @cindex output file name prefix
2995 The output files' names consist of @var{prefix} (@samp{x} by default)
2996 followed by a group of characters (@samp{aa}, @samp{ab}, @dots{} by
2997 default), such that concatenating the output files in traditional
2998 sorted order by file name produces the original input file (except
2999 @option{-nr/@var{n}}). By default split will initially create files
3000 with two generated suffix characters, and will increase this width by two
3001 when the next most significant position reaches the last character.
3002 (@samp{yz}, @samp{zaaa}, @samp{zaab}, @dots{}). In this way an arbitrary
3003 number of output files are supported, which sort as described above,
3004 even in the presence of an @option{--additional-suffix} option.
3005 If the @option{-a} option is specified and the output file names are
3006 exhausted, @command{split} reports an error without deleting the
3007 output files that it did create.
3009 The program accepts the following options. Also see @ref{Common options}.
3013 @item -l @var{lines}
3014 @itemx --lines=@var{lines}
3017 Put @var{lines} lines of @var{input} into each output file.
3019 For compatibility @command{split} also supports an obsolete
3020 option syntax @option{-@var{lines}}. New scripts should use
3021 @option{-l @var{lines}} instead.
3024 @itemx --bytes=@var{size}
3027 Put @var{size} bytes of @var{input} into each output file.
3028 @multiplierSuffixes{size}
3031 @itemx --line-bytes=@var{size}
3033 @opindex --line-bytes
3034 Put into each output file as many complete lines of @var{input} as
3035 possible without exceeding @var{size} bytes. Individual lines longer than
3036 @var{size} bytes are broken into multiple files.
3037 @var{size} has the same format as for the @option{--bytes} option.
3039 @itemx --filter=@var{command}
3041 With this option, rather than simply writing to each output file,
3042 write through a pipe to the specified shell @var{command} for each output file.
3043 @var{command} should use the $FILE environment variable, which is set
3044 to a different output file name for each invocation of the command.
3045 For example, imagine that you have a 1TiB compressed file
3046 that, if uncompressed, would be too large to reside on disk,
3047 yet you must split it into individually-compressed pieces
3048 of a more manageable size.
3049 To do that, you might run this command:
3052 xz -dc BIG.xz | split -b200G --filter='xz > $FILE.xz' - big-
3055 Assuming a 10:1 compression ratio, that would create about fifty 20GiB files
3056 with names @file{big-aa.xz}, @file{big-ab.xz}, @file{big-ac.xz}, etc.
3058 @item -n @var{chunks}
3059 @itemx --number=@var{chunks}
3063 Split @var{input} to @var{chunks} output files where @var{chunks} may be:
3066 @var{n} generate @var{n} files based on current size of @var{input}
3067 @var{k}/@var{n} only output @var{k}th of @var{n} to stdout
3068 l/@var{n} generate @var{n} files without splitting lines
3069 l/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
3070 r/@var{n} like @samp{l} but use round robin distribution
3071 r/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
3074 Any excess bytes remaining after dividing the @var{input}
3075 into @var{n} chunks, are assigned to the last chunk.
3076 Any excess bytes appearing after the initial calculation are discarded
3077 (except when using @samp{r} mode).
3079 All @var{n} files are created even if there are fewer than @var{n} lines,
3080 or the @var{input} is truncated.
3082 For @samp{l} mode, chunks are approximately @var{input} size / @var{n}.
3083 The @var{input} is partitioned into @var{n} equal sized portions, with
3084 the last assigned any excess. If a line @emph{starts} within a partition
3085 it is written completely to the corresponding file. Since lines
3086 are not split even if they overlap a partition, the files written
3087 can be larger or smaller than the partition size, and even empty
3088 if a line is so long as to completely overlap the partition.
3090 For @samp{r} mode, the size of @var{input} is irrelevant,
3091 and so can be a pipe for example.
3093 @item -a @var{length}
3094 @itemx --suffix-length=@var{length}
3096 @opindex --suffix-length
3097 Use suffixes of length @var{length}. If a @var{length} of 0 is specified,
3098 this is the same as if (any previous) @option{-a} was not specified, and
3099 thus enables the default behavior, which starts the suffix length at 2,
3100 and unless @option{-n} or @option{--numeric-suffixes=@var{from}} are
3101 specified, will auto increase the length by 2 as required.
3104 @itemx --numeric-suffixes[=@var{from}]
3106 @opindex --numeric-suffixes
3107 Use digits in suffixes rather than lower-case letters. The numerical
3108 suffix counts from @var{from} if specified, 0 otherwise.
3109 Note specifying a @var{from} value also disables the default
3110 auto suffix length expansion described above, and so you may also
3111 want to specify @option{-a} to allow suffixes beyond @samp{99}.
3113 @itemx --additional-suffix=@var{suffix}
3114 @opindex --additional-suffix
3115 Append an additional @var{suffix} to output file names. @var{suffix}
3116 must not contain slash.
3119 @itemx --elide-empty-files
3121 @opindex --elide-empty-files
3122 Suppress the generation of zero-length output files. This can happen
3123 with the @option{--number} option if a file is (truncated to be) shorter
3124 than the number requested, or if a line is so long as to completely
3125 span a chunk. The output file sequence numbers, always run consecutively
3126 even when this option is specified.
3131 @opindex --unbuffered
3132 Immediately copy input to output in @option{--number r/...} mode,
3133 which is a much slower mode of operation.
3137 Write a diagnostic just before each output file is opened.
3143 Here are a few examples to illustrate how the
3144 @option{--number} (@option{-n}) option works:
3146 Notice how, by default, one line may be split onto two or more:
3149 $ seq -w 6 10 > k; split -n3 k; head xa?
3162 Use the "l/" modifier to suppress that:
3165 $ seq -w 6 10 > k; split -nl/3 k; head xa?
3178 Use the "r/" modifier to distribute lines in a round-robin fashion:
3181 $ seq -w 6 10 > k; split -nr/3 k; head xa?
3194 You can also extract just the Kth chunk.
3195 This extracts and prints just the 7th "chunk" of 33:
3198 $ seq 100 > k; split -nl/7/33 k
3205 @node csplit invocation
3206 @section @command{csplit}: Split a file into context-determined pieces
3209 @cindex context splitting
3210 @cindex splitting a file into pieces by context
3212 @command{csplit} creates zero or more output files containing sections of
3213 @var{input} (standard input if @var{input} is @samp{-}). Synopsis:
3216 csplit [@var{option}]@dots{} @var{input} @var{pattern}@dots{}
3219 The contents of the output files are determined by the @var{pattern}
3220 arguments, as detailed below. An error occurs if a @var{pattern}
3221 argument refers to a nonexistent line of the input file (e.g., if no
3222 remaining line matches a given regular expression). After every
3223 @var{pattern} has been matched, any remaining input is copied into one
3226 By default, @command{csplit} prints the number of bytes written to each
3227 output file after it has been created.
3229 The types of pattern arguments are:
3234 Create an output file containing the input up to but not including line
3235 @var{n} (a positive integer). If followed by a repeat count, also
3236 create an output file containing the next @var{n} lines of the input
3237 file once for each repeat.
3239 @item /@var{regexp}/[@var{offset}]
3240 Create an output file containing the current line up to (but not
3241 including) the next line of the input file that contains a match for
3242 @var{regexp}. The optional @var{offset} is an integer.
3243 If it is given, the input up to (but not including) the
3244 matching line plus or minus @var{offset} is put into the output file,
3245 and the line after that begins the next section of input.
3247 @item %@var{regexp}%[@var{offset}]
3248 Like the previous type, except that it does not create an output
3249 file, so that section of the input file is effectively ignored.
3251 @item @{@var{repeat-count}@}
3252 Repeat the previous pattern @var{repeat-count} additional
3253 times. The @var{repeat-count} can either be a positive integer or an
3254 asterisk, meaning repeat as many times as necessary until the input is
3259 The output files' names consist of a prefix (@samp{xx} by default)
3260 followed by a suffix. By default, the suffix is an ascending sequence
3261 of two-digit decimal numbers from @samp{00} to @samp{99}. In any case,
3262 concatenating the output files in sorted order by file name produces the
3263 original input file.
3265 By default, if @command{csplit} encounters an error or receives a hangup,
3266 interrupt, quit, or terminate signal, it removes any output files
3267 that it has created so far before it exits.
3269 The program accepts the following options. Also see @ref{Common options}.
3273 @item -f @var{prefix}
3274 @itemx --prefix=@var{prefix}
3277 @cindex output file name prefix
3278 Use @var{prefix} as the output file name prefix.
3280 @item -b @var{suffix}
3281 @itemx --suffix=@var{suffix}
3284 @cindex output file name suffix
3285 Use @var{suffix} as the output file name suffix. When this option is
3286 specified, the suffix string must include exactly one
3287 @code{printf(3)}-style conversion specification, possibly including
3288 format specification flags, a field width, a precision specifications,
3289 or all of these kinds of modifiers. The format letter must convert a
3290 binary unsigned integer argument to readable form. The format letters
3291 @samp{d} and @samp{i} are aliases for @samp{u}, and the
3292 @samp{u}, @samp{o}, @samp{x}, and @samp{X} conversions are allowed. The
3293 entire @var{suffix} is given (with the current output file number) to
3294 @code{sprintf(3)} to form the file name suffixes for each of the
3295 individual output files in turn. If this option is used, the
3296 @option{--digits} option is ignored.
3298 @item -n @var{digits}
3299 @itemx --digits=@var{digits}
3302 Use output file names containing numbers that are @var{digits} digits
3303 long instead of the default 2.
3308 @opindex --keep-files
3309 Do not remove output files when errors are encountered.
3312 @itemx --elide-empty-files
3314 @opindex --elide-empty-files
3315 Suppress the generation of zero-length output files. (In cases where
3316 the section delimiters of the input file are supposed to mark the first
3317 lines of each of the sections, the first output file will generally be a
3318 zero-length file unless you use this option.) The output file sequence
3319 numbers always run consecutively starting from 0, even when this option
3330 Do not print counts of output file sizes.
3336 Here is an example of its usage.
3337 First, create an empty directory for the exercise,
3344 Now, split the sequence of 1..14 on lines that end with 0 or 5:
3347 $ seq 14 | csplit - '/[05]$/' '@{*@}'
3353 Each number printed above is the size of an output
3354 file that csplit has just created.
3355 List the names of those output files:
3362 Use @command{head} to show their contents:
3387 @node Summarizing files
3388 @chapter Summarizing files
3390 @cindex summarizing files
3392 These commands generate just a few numbers representing entire
3396 * wc invocation:: Print newline, word, and byte counts.
3397 * sum invocation:: Print checksum and block counts.
3398 * cksum invocation:: Print CRC checksum and byte counts.
3399 * md5sum invocation:: Print or check MD5 digests.
3400 * sha1sum invocation:: Print or check SHA-1 digests.
3401 * sha2 utilities:: Print or check SHA-2 digests.
3406 @section @command{wc}: Print newline, word, and byte counts
3410 @cindex character count
3414 @command{wc} counts the number of bytes, characters, whitespace-separated
3415 words, and newlines in each given @var{file}, or standard input if none
3416 are given or for a @var{file} of @samp{-}. Synopsis:
3419 wc [@var{option}]@dots{} [@var{file}]@dots{}
3422 @cindex total counts
3423 @command{wc} prints one line of counts for each file, and if the file was
3424 given as an argument, it prints the file name following the counts. If
3425 more than one @var{file} is given, @command{wc} prints a final line
3426 containing the cumulative counts, with the file name @file{total}. The
3427 counts are printed in this order: newlines, words, characters, bytes,
3428 maximum line length.
3429 Each count is printed right-justified in a field with at least one
3430 space between fields so that the numbers and file names normally line
3431 up nicely in columns. The width of the count fields varies depending
3432 on the inputs, so you should not depend on a particular field width.
3433 However, as a @acronym{GNU} extension, if only one count is printed,
3434 it is guaranteed to be printed without leading spaces.
3436 By default, @command{wc} prints three counts: the newline, words, and byte
3437 counts. Options can specify that only certain counts be printed.
3438 Options do not undo others previously given, so
3445 prints both the byte counts and the word counts.
3447 With the @option{--max-line-length} option, @command{wc} prints the length
3448 of the longest line per file, and if there is more than one file it
3449 prints the maximum (not the sum) of those lengths. The line lengths here
3450 are measured in screen columns, according to the current locale and
3451 assuming tab positions in every 8th column.
3453 The program accepts the following options. Also see @ref{Common options}.
3461 Print only the byte counts.
3467 Print only the character counts.
3473 Print only the word counts.
3479 Print only the newline counts.
3482 @itemx --max-line-length
3484 @opindex --max-line-length
3485 Print only the maximum line lengths.
3487 @macro filesZeroFromOption{cmd,withTotalOption,subListOutput}
3488 @itemx --files0-from=@var{file}
3489 @opindex --files0-from=@var{file}
3490 @c This is commented out to avoid a texi2dvi failure.
3491 @c texi2dvi (GNU Texinfo 4.11) 1.104
3492 @c @cindex including files from @command{\cmd\}
3493 Disallow processing files named on the command line, and instead process
3494 those named in file @var{file}; each name being terminated by a zero byte
3495 (@acronym{ASCII} @sc{nul}).
3496 This is useful \withTotalOption\
3497 when the list of file names is so long that it may exceed a command line
3499 In such cases, running @command{\cmd\} via @command{xargs} is undesirable
3500 because it splits the list into pieces and makes @command{\cmd\} print
3501 \subListOutput\ for each sublist rather than for the entire list.
3502 One way to produce a list of @acronym{ASCII} @sc{nul} terminated file
3503 names is with @sc{gnu}
3504 @command{find}, using its @option{-print0} predicate.
3505 If @var{file} is @samp{-} then the @acronym{ASCII} @sc{nul} terminated
3506 file names are read from standard input.
3508 @filesZeroFromOption{wc,,a total}
3510 For example, to find the length of the longest line in any @file{.c} or
3511 @file{.h} file in the current hierarchy, do this:
3514 find . -name '*.[ch]' -print0 |
3515 wc -L --files0-from=- | tail -n1
3523 @node sum invocation
3524 @section @command{sum}: Print checksum and block counts
3527 @cindex 16-bit checksum
3528 @cindex checksum, 16-bit
3530 @command{sum} computes a 16-bit checksum for each given @var{file}, or
3531 standard input if none are given or for a @var{file} of @samp{-}. Synopsis:
3534 sum [@var{option}]@dots{} [@var{file}]@dots{}
3537 @command{sum} prints the checksum for each @var{file} followed by the
3538 number of blocks in the file (rounded up). If more than one @var{file}
3539 is given, file names are also printed (by default). (With the
3540 @option{--sysv} option, corresponding file names are printed when there is
3541 at least one file argument.)
3543 By default, @sc{gnu} @command{sum} computes checksums using an algorithm
3544 compatible with BSD @command{sum} and prints file sizes in units of
3547 The program accepts the following options. Also see @ref{Common options}.
3553 @cindex BSD @command{sum}
3554 Use the default (BSD compatible) algorithm. This option is included for
3555 compatibility with the System V @command{sum}. Unless @option{-s} was also
3556 given, it has no effect.
3562 @cindex System V @command{sum}
3563 Compute checksums using an algorithm compatible with System V
3564 @command{sum}'s default, and print file sizes in units of 512-byte blocks.
3568 @command{sum} is provided for compatibility; the @command{cksum} program (see
3569 next section) is preferable in new applications.
3574 @node cksum invocation
3575 @section @command{cksum}: Print CRC checksum and byte counts
3578 @cindex cyclic redundancy check
3579 @cindex CRC checksum
3581 @command{cksum} computes a cyclic redundancy check (CRC) checksum for each
3582 given @var{file}, or standard input if none are given or for a
3583 @var{file} of @samp{-}. Synopsis:
3586 cksum [@var{option}]@dots{} [@var{file}]@dots{}
3589 @command{cksum} prints the CRC checksum for each file along with the number
3590 of bytes in the file, and the file name unless no arguments were given.
3592 @command{cksum} is typically used to ensure that files
3593 transferred by unreliable means (e.g., netnews) have not been corrupted,
3594 by comparing the @command{cksum} output for the received files with the
3595 @command{cksum} output for the original files (typically given in the
3598 The CRC algorithm is specified by the @acronym{POSIX} standard. It is not
3599 compatible with the BSD or System V @command{sum} algorithms (see the
3600 previous section); it is more robust.
3602 The only options are @option{--help} and @option{--version}. @xref{Common
3608 @node md5sum invocation
3609 @section @command{md5sum}: Print or check MD5 digests
3613 @cindex 128-bit checksum
3614 @cindex checksum, 128-bit
3615 @cindex fingerprint, 128-bit
3616 @cindex message-digest, 128-bit
3618 @command{md5sum} computes a 128-bit checksum (or @dfn{fingerprint} or
3619 @dfn{message-digest}) for each specified @var{file}.
3621 Note: The MD5 digest is more reliable than a simple CRC (provided by
3622 the @command{cksum} command) for detecting accidental file corruption,
3623 as the chances of accidentally having two files with identical MD5
3624 are vanishingly small. However, it should not be considered secure
3625 against malicious tampering: although finding a file with a given MD5
3626 fingerprint is considered infeasible at the moment, it is known how
3627 to modify certain files, including digital certificates, so that they
3628 appear valid when signed with an MD5 digest.
3629 For more secure hashes, consider using SHA-2. @xref{sha2 utilities}.
3631 If a @var{file} is specified as @samp{-} or if no files are given
3632 @command{md5sum} computes the checksum for the standard input.
3633 @command{md5sum} can also determine whether a file and checksum are
3634 consistent. Synopsis:
3637 md5sum [@var{option}]@dots{} [@var{file}]@dots{}
3640 For each @var{file}, @samp{md5sum} outputs the MD5 checksum, a flag
3641 indicating binary or text input mode, and the file name.
3642 If @var{file} contains a backslash or newline, the
3643 line is started with a backslash, and each problematic character in
3644 the file name is escaped with a backslash, making the output
3645 unambiguous even in the presence of arbitrary file names.
3646 If @var{file} is omitted or specified as @samp{-}, standard input is read.
3648 The program accepts the following options. Also see @ref{Common options}.
3656 @cindex binary input files
3657 Treat each input file as binary, by reading it in binary mode and
3658 outputting a @samp{*} flag. This is the inverse of @option{--text}.
3659 On systems like @acronym{GNU} that do not distinguish between binary
3660 and text files, this option merely flags each input mode as binary:
3661 the MD5 checksum is unaffected. This option is the default on systems
3662 like MS-DOS that distinguish between binary and text files, except
3663 for reading standard input when standard input is a terminal.
3667 Read file names and checksum information (not data) from each
3668 @var{file} (or from stdin if no @var{file} was specified) and report
3669 whether the checksums match the contents of the named files.
3670 The input to this mode of @command{md5sum} is usually the output of
3671 a prior, checksum-generating run of @samp{md5sum}.
3672 Each valid line of input consists of an MD5 checksum, a binary/text
3673 flag, and then a file name.
3674 Binary mode is indicated with @samp{*}, text with @samp{ } (space).
3675 For each such line, @command{md5sum} reads the named file and computes its
3676 MD5 checksum. Then, if the computed message digest does not match the
3677 one on the line with the file name, the file is noted as having
3678 failed the test. Otherwise, the file passes the test.
3679 By default, for each valid line, one line is written to standard
3680 output indicating whether the named file passed the test.
3681 After all checks have been performed, if there were any failures,
3682 a warning is issued to standard error.
3683 Use the @option{--status} option to inhibit that output.
3684 If any listed file cannot be opened or read, if any valid line has
3685 an MD5 checksum inconsistent with the associated file, or if no valid
3686 line is found, @command{md5sum} exits with nonzero status. Otherwise,
3687 it exits successfully.
3691 @cindex verifying MD5 checksums
3692 This option is useful only when verifying checksums.
3693 When verifying checksums, don't generate an 'OK' message per successfully
3694 checked file. Files that fail the verification are reported in the
3695 default one-line-per-file format. If there is any checksum mismatch,
3696 print a warning summarizing the failures to standard error.
3700 @cindex verifying MD5 checksums
3701 This option is useful only when verifying checksums.
3702 When verifying checksums, don't generate the default one-line-per-file
3703 diagnostic and don't output the warning summarizing any failures.
3704 Failures to open or read a file still evoke individual diagnostics to
3706 If all listed files are readable and are consistent with the associated
3707 MD5 checksums, exit successfully. Otherwise exit with a status code
3708 indicating there was a failure.
3714 @cindex text input files
3715 Treat each input file as text, by reading it in text mode and
3716 outputting a @samp{ } flag. This is the inverse of @option{--binary}.
3717 This option is the default on systems like @acronym{GNU} that do not
3718 distinguish between binary and text files. On other systems, it is
3719 the default for reading standard input when standard input is a
3726 @cindex verifying MD5 checksums
3727 When verifying checksums, warn about improperly formatted MD5 checksum lines.
3728 This option is useful only if all but a few lines in the checked input
3733 @cindex verifying MD5 checksums
3734 When verifying checksums,
3735 if one or more input line is invalid,
3736 exit nonzero after all warnings have been issued.
3743 @node sha1sum invocation
3744 @section @command{sha1sum}: Print or check SHA-1 digests
3748 @cindex 160-bit checksum
3749 @cindex checksum, 160-bit
3750 @cindex fingerprint, 160-bit
3751 @cindex message-digest, 160-bit
3753 @command{sha1sum} computes a 160-bit checksum for each specified
3754 @var{file}. The usage and options of this command are precisely the
3755 same as for @command{md5sum}. @xref{md5sum invocation}.
3757 Note: The SHA-1 digest is more secure than MD5, and no collisions of
3758 it are known (different files having the same fingerprint). However,
3759 it is known that they can be produced with considerable, but not
3760 unreasonable, resources. For this reason, it is generally considered
3761 that SHA-1 should be gradually phased out in favor of the more secure
3762 SHA-2 hash algorithms. @xref{sha2 utilities}.
3765 @node sha2 utilities
3766 @section sha2 utilities: Print or check SHA-2 digests
3773 @cindex 224-bit checksum
3774 @cindex 256-bit checksum
3775 @cindex 384-bit checksum
3776 @cindex 512-bit checksum
3777 @cindex checksum, 224-bit
3778 @cindex checksum, 256-bit
3779 @cindex checksum, 384-bit
3780 @cindex checksum, 512-bit
3781 @cindex fingerprint, 224-bit
3782 @cindex fingerprint, 256-bit
3783 @cindex fingerprint, 384-bit
3784 @cindex fingerprint, 512-bit
3785 @cindex message-digest, 224-bit
3786 @cindex message-digest, 256-bit
3787 @cindex message-digest, 384-bit
3788 @cindex message-digest, 512-bit
3790 The commands @command{sha224sum}, @command{sha256sum},
3791 @command{sha384sum} and @command{sha512sum} compute checksums of
3792 various lengths (respectively 224, 256, 384 and 512 bits),
3793 collectively known as the SHA-2 hashes. The usage and options of
3794 these commands are precisely the same as for @command{md5sum}.
3795 @xref{md5sum invocation}.
3797 Note: The SHA384 and SHA512 digests are considerably slower to
3798 compute, especially on 32-bit computers, than SHA224 or SHA256.
3801 @node Operating on sorted files
3802 @chapter Operating on sorted files
3804 @cindex operating on sorted files
3805 @cindex sorted files, operations on
3807 These commands work with (or produce) sorted files.
3810 * sort invocation:: Sort text files.
3811 * shuf invocation:: Shuffle text files.
3812 * uniq invocation:: Uniquify files.
3813 * comm invocation:: Compare two sorted files line by line.
3814 * ptx invocation:: Produce a permuted index of file contents.
3815 * tsort invocation:: Topological sort.
3819 @node sort invocation
3820 @section @command{sort}: Sort text files
3823 @cindex sorting files
3825 @command{sort} sorts, merges, or compares all the lines from the given
3826 files, or standard input if none are given or for a @var{file} of
3827 @samp{-}. By default, @command{sort} writes the results to standard
3831 sort [@var{option}]@dots{} [@var{file}]@dots{}
3834 @command{sort} has three modes of operation: sort (the default), merge,
3835 and check for sortedness. The following options change the operation
3842 @itemx --check=diagnose-first
3845 @cindex checking for sortedness
3846 Check whether the given file is already sorted: if it is not all
3847 sorted, print a diagnostic containing the first out-of-order line and
3848 exit with a status of 1.
3849 Otherwise, exit successfully.
3850 At most one input file can be given.
3853 @itemx --check=quiet
3854 @itemx --check=silent
3857 @cindex checking for sortedness
3858 Exit successfully if the given file is already sorted, and
3859 exit with status 1 otherwise.
3860 At most one input file can be given.
3861 This is like @option{-c}, except it does not print a diagnostic.
3867 @cindex merging sorted files
3868 Merge the given files by sorting them as a group. Each input file must
3869 always be individually sorted. It always works to sort instead of
3870 merge; merging is provided because it is faster, in the case where it
3875 @cindex sort stability
3876 @cindex sort's last-resort comparison
3877 A pair of lines is compared as follows:
3878 @command{sort} compares each pair of fields, in the
3879 order specified on the command line, according to the associated
3880 ordering options, until a difference is found or no fields are left.
3881 If no key fields are specified, @command{sort} uses a default key of
3882 the entire line. Finally, as a last resort when all keys compare
3883 equal, @command{sort} compares entire lines as if no ordering options
3884 other than @option{--reverse} (@option{-r}) were specified. The
3885 @option{--stable} (@option{-s}) option disables this @dfn{last-resort
3886 comparison} so that lines in which all fields compare equal are left
3887 in their original relative order. The @option{--unique}
3888 (@option{-u}) option also disables the last-resort comparison.
3892 Unless otherwise specified, all comparisons use the character collating
3893 sequence specified by the @env{LC_COLLATE} locale.@footnote{If you
3894 use a non-@acronym{POSIX} locale (e.g., by setting @env{LC_ALL}
3895 to @samp{en_US}), then @command{sort} may produce output that is sorted
3896 differently than you're accustomed to. In that case, set the @env{LC_ALL}
3897 environment variable to @samp{C}. Note that setting only @env{LC_COLLATE}
3898 has two problems. First, it is ineffective if @env{LC_ALL} is also set.
3899 Second, it has undefined behavior if @env{LC_CTYPE} (or @env{LANG}, if
3900 @env{LC_CTYPE} is unset) is set to an incompatible value. For example,
3901 you get undefined behavior if @env{LC_CTYPE} is @code{ja_JP.PCK} but
3902 @env{LC_COLLATE} is @code{en_US.UTF-8}.}
3904 @sc{gnu} @command{sort} (as specified for all @sc{gnu} utilities) has no
3905 limit on input line length or restrictions on bytes allowed within lines.
3906 In addition, if the final byte of an input file is not a newline, @sc{gnu}
3907 @command{sort} silently supplies one. A line's trailing newline is not
3908 part of the line for comparison purposes.
3910 @cindex exit status of @command{sort}
3914 0 if no error occurred
3915 1 if invoked with @option{-c} or @option{-C} and the input is not sorted
3916 2 if an error occurred
3920 If the environment variable @env{TMPDIR} is set, @command{sort} uses its
3921 value as the directory for temporary files instead of @file{/tmp}. The
3922 @option{--temporary-directory} (@option{-T}) option in turn overrides
3923 the environment variable.
3925 The following options affect the ordering of output lines. They may be
3926 specified globally or as part of a specific key field. If no key
3927 fields are specified, global options apply to comparison of entire
3928 lines; otherwise the global options are inherited by key fields that do
3929 not specify any special options of their own. In pre-@acronym{POSIX}
3930 versions of @command{sort}, global options affect only later key fields,
3931 so portable shell scripts should specify global options first.
3936 @itemx --ignore-leading-blanks
3938 @opindex --ignore-leading-blanks
3939 @cindex blanks, ignoring leading
3941 Ignore leading blanks when finding sort keys in each line.
3942 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3943 can change this. Note blanks may be ignored by your locale's collating
3944 rules, but without this option they will be significant for character
3945 positions specified in keys with the @option{-k} option.
3948 @itemx --dictionary-order
3950 @opindex --dictionary-order
3951 @cindex dictionary order
3952 @cindex phone directory order
3953 @cindex telephone directory order
3955 Sort in @dfn{phone directory} order: ignore all characters except
3956 letters, digits and blanks when sorting.
3957 By default letters and digits are those of @acronym{ASCII} and a blank
3958 is a space or a tab, but the @env{LC_CTYPE} locale can change this.
3961 @itemx --ignore-case
3963 @opindex --ignore-case
3964 @cindex ignoring case
3965 @cindex case folding
3967 Fold lowercase characters into the equivalent uppercase characters when
3968 comparing so that, for example, @samp{b} and @samp{B} sort as equal.
3969 The @env{LC_CTYPE} locale determines character types.
3970 When used with @option{--unique} those lower case equivalent lines are
3971 thrown away. (There is currently no way to throw away the upper case
3972 equivalent instead. (Any @option{--reverse} given would only affect
3973 the final result, after the throwing away.))
3976 @itemx --general-numeric-sort
3977 @itemx --sort=general-numeric
3979 @opindex --general-numeric-sort
3981 @cindex general numeric sort
3983 Sort numerically, converting a prefix of each line to a long
3984 double-precision floating point number. @xref{Floating point}.
3985 Do not report overflow, underflow, or conversion errors.
3986 Use the following collating sequence:
3990 Lines that do not start with numbers (all considered to be equal).
3992 NaNs (``Not a Number'' values, in IEEE floating point arithmetic)
3993 in a consistent but machine-dependent order.
3997 Finite numbers in ascending numeric order (with @math{-0} and @math{+0} equal).
4002 Use this option only if there is no alternative; it is much slower than
4003 @option{--numeric-sort} (@option{-n}) and it can lose information when
4004 converting to floating point.
4007 @itemx --human-numeric-sort
4008 @itemx --sort=human-numeric
4010 @opindex --human-numeric-sort
4012 @cindex human numeric sort
4014 Sort numerically, first by numeric sign (negative, zero, or positive);
4015 then by @acronym{SI} suffix (either empty, or @samp{k} or @samp{K}, or
4016 one of @samp{MGTPEZY}, in that order; @pxref{Block size}); and finally
4017 by numeric value. For example, @samp{1023M} sorts before @samp{1G}
4018 because @samp{M} (mega) precedes @samp{G} (giga) as an @acronym{SI}
4019 suffix. This option sorts values that are consistently scaled to the
4020 nearest suffix, regardless of whether suffixes denote powers of 1000
4021 or 1024, and it therefore sorts the output of any single invocation of
4022 the @command{df}, @command{du}, or @command{ls} commands that are
4023 invoked with their @option{--human-readable} or @option{--si} options.
4024 The syntax for numbers is the same as for the @option{--numeric-sort}
4025 option; the @acronym{SI} suffix must immediately follow the number.
4028 @itemx --ignore-nonprinting
4030 @opindex --ignore-nonprinting
4031 @cindex nonprinting characters, ignoring
4032 @cindex unprintable characters, ignoring
4034 Ignore nonprinting characters.
4035 The @env{LC_CTYPE} locale determines character types.
4036 This option has no effect if the stronger @option{--dictionary-order}
4037 (@option{-d}) option is also given.
4043 @opindex --month-sort
4045 @cindex months, sorting by
4047 An initial string, consisting of any amount of blanks, followed
4048 by a month name abbreviation, is folded to UPPER case and
4049 compared in the order @samp{JAN} < @samp{FEB} < @dots{} < @samp{DEC}.
4050 Invalid names compare low to valid names. The @env{LC_TIME} locale
4051 category determines the month spellings.
4052 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4056 @itemx --numeric-sort
4057 @itemx --sort=numeric
4059 @opindex --numeric-sort
4061 @cindex numeric sort
4063 Sort numerically. The number begins each line and consists
4064 of optional blanks, an optional @samp{-} sign, and zero or more
4065 digits possibly separated by thousands separators, optionally followed
4066 by a decimal-point character and zero or more digits. An empty
4067 number is treated as @samp{0}. The @env{LC_NUMERIC}
4068 locale specifies the decimal-point character and thousands separator.
4069 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4072 Comparison is exact; there is no rounding error.
4074 Neither a leading @samp{+} nor exponential notation is recognized.
4075 To compare such strings numerically, use the
4076 @option{--general-numeric-sort} (@option{-g}) option.
4079 @itemx --version-sort
4081 @opindex --version-sort
4082 @cindex version number sort
4083 Sort by version name and number. It behaves like a standard sort,
4084 except that each sequence of decimal digits is treated numerically
4085 as an index/version number. (@xref{Details about version sort}.)
4091 @cindex reverse sorting
4092 Reverse the result of comparison, so that lines with greater key values
4093 appear earlier in the output instead of later.
4096 @itemx --random-sort
4097 @itemx --sort=random
4099 @opindex --random-sort
4102 Sort by hashing the input keys and then sorting the hash values.
4103 Choose the hash function at random, ensuring that it is free of
4104 collisions so that differing keys have differing hash values. This is
4105 like a random permutation of the inputs (@pxref{shuf invocation}),
4106 except that keys with the same value sort together.
4108 If multiple random sort fields are specified, the same random hash
4109 function is used for all fields. To use different random hash
4110 functions for different fields, you can invoke @command{sort} more
4113 The choice of hash function is affected by the
4114 @option{--random-source} option.
4122 @item --compress-program=@var{prog}
4123 Compress any temporary files with the program @var{prog}.
4125 With no arguments, @var{prog} must compress standard input to standard
4126 output, and when given the @option{-d} option it must decompress
4127 standard input to standard output.
4129 Terminate with an error if @var{prog} exits with nonzero status.
4131 White space and the backslash character should not appear in
4132 @var{prog}; they are reserved for future use.
4134 @filesZeroFromOption{sort,,sorted output}
4136 @item -k @var{pos1}[,@var{pos2}]
4137 @itemx --key=@var{pos1}[,@var{pos2}]
4141 Specify a sort field that consists of the part of the line between
4142 @var{pos1} and @var{pos2} (or the end of the line, if @var{pos2} is
4143 omitted), @emph{inclusive}.
4145 Each @var{pos} has the form @samp{@var{f}[.@var{c}][@var{opts}]},
4146 where @var{f} is the number of the field to use, and @var{c} is the number
4147 of the first character from the beginning of the field. Fields and character
4148 positions are numbered starting with 1; a character position of zero in
4149 @var{pos2} indicates the field's last character. If @samp{.@var{c}} is
4150 omitted from @var{pos1}, it defaults to 1 (the beginning of the field);
4151 if omitted from @var{pos2}, it defaults to 0 (the end of the field).
4152 @var{opts} are ordering options, allowing individual keys to be sorted
4153 according to different rules; see below for details. Keys can span
4156 Example: To sort on the second field, use @option{--key=2,2}
4157 (@option{-k 2,2}). See below for more notes on keys and more examples.
4158 See also the @option{--debug} option to help determine the part
4159 of the line being used in the sort.
4162 Highlight the portion of each line used for sorting.
4163 Also issue warnings about questionable usage to stderr.
4165 @item --batch-size=@var{nmerge}
4166 @opindex --batch-size
4167 @cindex number of inputs to merge, nmerge
4168 Merge at most @var{nmerge} inputs at once.
4170 When @command{sort} has to merge more than @var{nmerge} inputs,
4171 it merges them in groups of @var{nmerge}, saving the result in
4172 a temporary file, which is then used as an input in a subsequent merge.
4174 A large value of @var{nmerge} may improve merge performance and decrease
4175 temporary storage utilization at the expense of increased memory usage
4176 and I/O. Conversely a small value of @var{nmerge} may reduce memory
4177 requirements and I/O at the expense of temporary storage consumption and
4180 The value of @var{nmerge} must be at least 2. The default value is
4181 currently 16, but this is implementation-dependent and may change in
4184 The value of @var{nmerge} may be bounded by a resource limit for open
4185 file descriptors. The commands @samp{ulimit -n} or @samp{getconf
4186 OPEN_MAX} may display limits for your systems; these limits may be
4187 modified further if your program already has some files open, or if
4188 the operating system has other limits on the number of open files. If
4189 the value of @var{nmerge} exceeds the resource limit, @command{sort}
4190 silently uses a smaller value.
4192 @item -o @var{output-file}
4193 @itemx --output=@var{output-file}
4196 @cindex overwriting of input, allowed
4197 Write output to @var{output-file} instead of standard output.
4198 Normally, @command{sort} reads all input before opening
4199 @var{output-file}, so you can safely sort a file in place by using
4200 commands like @code{sort -o F F} and @code{cat F | sort -o F}.
4201 However, @command{sort} with @option{--merge} (@option{-m}) can open
4202 the output file before reading all input, so a command like @code{cat
4203 F | sort -m -o F - G} is not safe as @command{sort} might start
4204 writing @file{F} before @command{cat} is done reading it.
4206 @vindex POSIXLY_CORRECT
4207 On newer systems, @option{-o} cannot appear after an input file if
4208 @env{POSIXLY_CORRECT} is set, e.g., @samp{sort F -o F}. Portable
4209 scripts should specify @option{-o @var{output-file}} before any input
4212 @item --random-source=@var{file}
4213 @opindex --random-source
4214 @cindex random source for sorting
4215 Use @var{file} as a source of random data used to determine which
4216 random hash function to use with the @option{-R} option. @xref{Random
4223 @cindex sort stability
4224 @cindex sort's last-resort comparison
4226 Make @command{sort} stable by disabling its last-resort comparison.
4227 This option has no effect if no fields or global ordering options
4228 other than @option{--reverse} (@option{-r}) are specified.
4231 @itemx --buffer-size=@var{size}
4233 @opindex --buffer-size
4234 @cindex size for main memory sorting
4235 Use a main-memory sort buffer of the given @var{size}. By default,
4236 @var{size} is in units of 1024 bytes. Appending @samp{%} causes
4237 @var{size} to be interpreted as a percentage of physical memory.
4238 Appending @samp{K} multiplies @var{size} by 1024 (the default),
4239 @samp{M} by 1,048,576, @samp{G} by 1,073,741,824, and so on for
4240 @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}. Appending
4241 @samp{b} causes @var{size} to be interpreted as a byte count, with no
4244 This option can improve the performance of @command{sort} by causing it
4245 to start with a larger or smaller sort buffer than the default.
4246 However, this option affects only the initial buffer size. The buffer
4247 grows beyond @var{size} if @command{sort} encounters input lines larger
4250 @item -t @var{separator}
4251 @itemx --field-separator=@var{separator}
4253 @opindex --field-separator
4254 @cindex field separator character
4255 Use character @var{separator} as the field separator when finding the
4256 sort keys in each line. By default, fields are separated by the empty
4257 string between a non-blank character and a blank character.
4258 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4261 That is, given the input line @w{@samp{ foo bar}}, @command{sort} breaks it
4262 into fields @w{@samp{ foo}} and @w{@samp{ bar}}. The field separator is
4263 not considered to be part of either the field preceding or the field
4264 following, so with @samp{sort @w{-t " "}} the same input line has
4265 three fields: an empty field, @samp{foo}, and @samp{bar}.
4266 However, fields that extend to the end of the line,
4267 as @option{-k 2}, or fields consisting of a range, as @option{-k 2,3},
4268 retain the field separators present between the endpoints of the range.
4270 To specify @acronym{ASCII} @sc{nul} as the field separator,
4271 use the two-character string @samp{\0}, e.g., @samp{sort -t '\0'}.
4273 @item -T @var{tempdir}
4274 @itemx --temporary-directory=@var{tempdir}
4276 @opindex --temporary-directory
4277 @cindex temporary directory
4279 Use directory @var{tempdir} to store temporary files, overriding the
4280 @env{TMPDIR} environment variable. If this option is given more than
4281 once, temporary files are stored in all the directories given. If you
4282 have a large sort or merge that is I/O-bound, you can often improve
4283 performance by using this option to specify directories on different
4284 disks and controllers.
4286 @item --parallel=@var{n}
4288 @cindex multithreaded sort
4289 Set the number of sorts run in parallel to @var{n}. By default,
4290 @var{n} is set to the number of available processors, but limited
4291 to 8, as there are diminishing performance gains after that.
4292 Note also that using @var{n} threads increases the memory usage by
4293 a factor of log @var{n}. Also see @ref{nproc invocation}.
4299 @cindex uniquifying output
4301 Normally, output only the first of a sequence of lines that compare
4302 equal. For the @option{--check} (@option{-c} or @option{-C}) option,
4303 check that no pair of consecutive lines compares equal.
4305 This option also disables the default last-resort comparison.
4307 The commands @code{sort -u} and @code{sort | uniq} are equivalent, but
4308 this equivalence does not extend to arbitrary @command{sort} options.
4309 For example, @code{sort -n -u} inspects only the value of the initial
4310 numeric string when checking for uniqueness, whereas @code{sort -n |
4311 uniq} inspects the entire line. @xref{uniq invocation}.
4313 @macro zeroTerminatedOption
4315 @itemx --zero-terminated
4317 @opindex --zero-terminated
4318 @cindex process zero-terminated items
4319 Delimit items with a zero byte rather than a newline (@acronym{ASCII} @sc{lf}).
4320 I.E. treat input as items separated by @acronym{ASCII} @sc{nul}
4321 and terminate output items with @acronym{ASCII} @sc{nul}.
4322 This option can be useful in conjunction with @samp{perl -0} or
4323 @samp{find -print0} and @samp{xargs -0} which do the same in order to
4324 reliably handle arbitrary file names (even those containing blanks
4325 or other special characters).
4327 @zeroTerminatedOption
4331 Historical (BSD and System V) implementations of @command{sort} have
4332 differed in their interpretation of some options, particularly
4333 @option{-b}, @option{-f}, and @option{-n}.
4334 @sc{gnu} sort follows the @acronym{POSIX}
4335 behavior, which is usually (but not always!) like the System V behavior.
4336 According to @acronym{POSIX}, @option{-n} no longer implies @option{-b}. For
4337 consistency, @option{-M} has been changed in the same way. This may
4338 affect the meaning of character positions in field specifications in
4339 obscure cases. The only fix is to add an explicit @option{-b}.
4341 A position in a sort field specified with @option{-k} may have any
4342 of the option letters @samp{MbdfghinRrV} appended to it, in which case no
4343 global ordering options are inherited by that particular field. The
4344 @option{-b} option may be independently attached to either or both of
4345 the start and end positions of a field specification, and if it is
4346 inherited from the global options it will be attached to both.
4347 If input lines can contain leading or adjacent blanks and @option{-t}
4348 is not used, then @option{-k} is typically combined with @option{-b} or
4349 an option that implicitly ignores leading blanks (@samp{Mghn}) as otherwise
4350 the varying numbers of leading blanks in fields can cause confusing results.
4352 If the start position in a sort field specifier falls after the end of
4353 the line or after the end field, the field is empty. If the @option{-b}
4354 option was specified, the @samp{.@var{c}} part of a field specification
4355 is counted from the first nonblank character of the field.
4357 @vindex _POSIX2_VERSION
4358 @vindex POSIXLY_CORRECT
4359 On older systems, @command{sort} supports an obsolete origin-zero
4360 syntax @samp{+@var{pos1} [-@var{pos2}]} for specifying sort keys.
4361 The obsolete sequence @samp{sort +@var{a}.@var{x} -@var{b}.@var{y}}
4362 is equivalent to @samp{sort -k @var{a+1}.@var{x+1},@var{b}} if @var{y}
4363 is @samp{0} or absent, otherwise it is equivalent to @samp{sort -k
4364 @var{a+1}.@var{x+1},@var{b+1}.@var{y}}.
4366 This obsolete behavior can be enabled or disabled with the
4367 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4368 conformance}); it can also be enabled when @env{POSIXLY_CORRECT} is
4369 not set by using the obsolete syntax with @samp{-@var{pos2}} present.
4371 Scripts intended for use on standard hosts should avoid obsolete
4372 syntax and should use @option{-k} instead. For example, avoid
4373 @samp{sort +2}, since it might be interpreted as either @samp{sort
4374 ./+2} or @samp{sort -k 3}. If your script must also run on hosts that
4375 support only the obsolete syntax, it can use a test like @samp{if sort
4376 -k 1 </dev/null >/dev/null 2>&1; then @dots{}} to decide which syntax
4379 Here are some examples to illustrate various combinations of options.
4384 Sort in descending (reverse) numeric order.
4391 Run no more than 4 sorts concurrently, using a buffer size of 10M.
4394 sort --parallel=4 -S 10M
4398 Sort alphabetically, omitting the first and second fields
4399 and the blanks at the start of the third field.
4400 This uses a single key composed of the characters beginning
4401 at the start of the first nonblank character in field three
4402 and extending to the end of each line.
4409 Sort numerically on the second field and resolve ties by sorting
4410 alphabetically on the third and fourth characters of field five.
4411 Use @samp{:} as the field delimiter.
4414 sort -t : -k 2,2n -k 5.3,5.4
4417 Note that if you had written @option{-k 2n} instead of @option{-k 2,2n}
4418 @command{sort} would have used all characters beginning in the second field
4419 and extending to the end of the line as the primary @emph{numeric}
4420 key. For the large majority of applications, treating keys spanning
4421 more than one field as numeric will not do what you expect.
4423 Also note that the @samp{n} modifier was applied to the field-end
4424 specifier for the first key. It would have been equivalent to
4425 specify @option{-k 2n,2} or @option{-k 2n,2n}. All modifiers except
4426 @samp{b} apply to the associated @emph{field}, regardless of whether
4427 the modifier character is attached to the field-start and/or the
4428 field-end part of the key specifier.
4431 Sort the password file on the fifth field and ignore any
4432 leading blanks. Sort lines with equal values in field five
4433 on the numeric user ID in field three. Fields are separated
4437 sort -t : -k 5b,5 -k 3,3n /etc/passwd
4438 sort -t : -n -k 5b,5 -k 3,3 /etc/passwd
4439 sort -t : -b -k 5,5 -k 3,3n /etc/passwd
4442 These three commands have equivalent effect. The first specifies that
4443 the first key's start position ignores leading blanks and the second
4444 key is sorted numerically. The other two commands rely on global
4445 options being inherited by sort keys that lack modifiers. The inheritance
4446 works in this case because @option{-k 5b,5b} and @option{-k 5b,5} are
4447 equivalent, as the location of a field-end lacking a @samp{.@var{c}}
4448 character position is not affected by whether initial blanks are
4452 Sort a set of log files, primarily by IPv4 address and secondarily by
4453 time stamp. If two lines' primary and secondary keys are identical,
4454 output the lines in the same order that they were input. The log
4455 files contain lines that look like this:
4458 4.150.156.3 - - [01/Apr/2004:06:31:51 +0000] message 1
4459 211.24.3.231 - - [24/Apr/2004:20:17:39 +0000] message 2
4462 Fields are separated by exactly one space. Sort IPv4 addresses
4463 lexicographically, e.g., 212.61.52.2 sorts before 212.129.233.201
4464 because 61 is less than 129.
4467 sort -s -t ' ' -k 4.9n -k 4.5M -k 4.2n -k 4.14,4.21 file*.log |
4468 sort -s -t '.' -k 1,1n -k 2,2n -k 3,3n -k 4,4n
4471 This example cannot be done with a single @command{sort} invocation,
4472 since IPv4 address components are separated by @samp{.} while dates
4473 come just after a space. So it is broken down into two invocations of
4474 @command{sort}: the first sorts by time stamp and the second by IPv4
4475 address. The time stamp is sorted by year, then month, then day, and
4476 finally by hour-minute-second field, using @option{-k} to isolate each
4477 field. Except for hour-minute-second there's no need to specify the
4478 end of each key field, since the @samp{n} and @samp{M} modifiers sort
4479 based on leading prefixes that cannot cross field boundaries. The
4480 IPv4 addresses are sorted lexicographically. The second sort uses
4481 @samp{-s} so that ties in the primary key are broken by the secondary
4482 key; the first sort uses @samp{-s} so that the combination of the two
4486 Generate a tags file in case-insensitive sorted order.
4489 find src -type f -print0 | sort -z -f | xargs -0 etags --append
4492 The use of @option{-print0}, @option{-z}, and @option{-0} in this case means
4493 that file names that contain blanks or other special characters are
4495 by the sort operation.
4497 @c This example is a bit contrived and needs more explanation.
4499 @c Sort records separated by an arbitrary string by using a pipe to convert
4500 @c each record delimiter string to @samp{\0}, then using sort's -z option,
4501 @c and converting each @samp{\0} back to the original record delimiter.
4504 @c printf 'c\n\nb\n\na\n' |
4505 @c perl -0pe 's/\n\n/\n\0/g' |
4507 @c perl -0pe 's/\0/\n/g'
4511 Use the common @acronym{DSU, Decorate Sort Undecorate} idiom to
4512 sort lines according to their length.
4515 awk '@{print length, $0@}' /etc/passwd | sort -n | cut -f2- -d' '
4518 In general this technique can be used to sort data that the @command{sort}
4519 command does not support, or is inefficient at, sorting directly.
4522 Shuffle a list of directories, but preserve the order of files within
4523 each directory. For instance, one could use this to generate a music
4524 playlist in which albums are shuffled but the songs of each album are
4528 ls */* | sort -t / -k 1,1R -k 2,2
4534 @node shuf invocation
4535 @section @command{shuf}: Shuffling text
4538 @cindex shuffling files
4540 @command{shuf} shuffles its input by outputting a random permutation
4541 of its input lines. Each output permutation is equally likely.
4545 shuf [@var{option}]@dots{} [@var{file}]
4546 shuf -e [@var{option}]@dots{} [@var{arg}]@dots{}
4547 shuf -i @var{lo}-@var{hi} [@var{option}]@dots{}
4550 @command{shuf} has three modes of operation that affect where it
4551 obtains its input lines. By default, it reads lines from standard
4552 input. The following options change the operation mode:
4560 @cindex command-line operands to shuffle
4561 Treat each command-line operand as an input line.
4563 @item -i @var{lo}-@var{hi}
4564 @itemx --input-range=@var{lo}-@var{hi}
4566 @opindex --input-range
4567 @cindex input range to shuffle
4568 Act as if input came from a file containing the range of unsigned
4569 decimal integers @var{lo}@dots{}@var{hi}, one per line.
4573 @command{shuf}'s other options can affect its behavior in all
4578 @item -n @var{lines}
4579 @itemx --head-count=@var{count}
4581 @opindex --head-count
4582 @cindex head of output
4583 Output at most @var{count} lines. By default, all input lines are
4586 @item -o @var{output-file}
4587 @itemx --output=@var{output-file}
4590 @cindex overwriting of input, allowed
4591 Write output to @var{output-file} instead of standard output.
4592 @command{shuf} reads all input before opening
4593 @var{output-file}, so you can safely shuffle a file in place by using
4594 commands like @code{shuf -o F <F} and @code{cat F | shuf -o F}.
4596 @item --random-source=@var{file}
4597 @opindex --random-source
4598 @cindex random source for shuffling
4599 Use @var{file} as a source of random data used to determine which
4600 permutation to generate. @xref{Random sources}.
4602 @zeroTerminatedOption
4618 might produce the output
4628 Similarly, the command:
4631 shuf -e clubs hearts diamonds spades
4645 and the command @samp{shuf -i 1-4} might output:
4655 These examples all have four input lines, so @command{shuf} might
4656 produce any of the twenty-four possible permutations of the input. In
4657 general, if there are @var{n} input lines, there are @var{n}! (i.e.,
4658 @var{n} factorial, or @var{n} * (@var{n} - 1) * @dots{} * 1) possible
4659 output permutations.
4664 @node uniq invocation
4665 @section @command{uniq}: Uniquify files
4668 @cindex uniquify files
4670 @command{uniq} writes the unique lines in the given @file{input}, or
4671 standard input if nothing is given or for an @var{input} name of
4675 uniq [@var{option}]@dots{} [@var{input} [@var{output}]]
4678 By default, @command{uniq} prints its input lines, except that
4679 it discards all but the first of adjacent repeated lines, so that
4680 no output lines are repeated. Optionally, it can instead discard
4681 lines that are not repeated, or all repeated lines.
4683 The input need not be sorted, but repeated input lines are detected
4684 only if they are adjacent. If you want to discard non-adjacent
4685 duplicate lines, perhaps you want to use @code{sort -u}.
4686 @xref{sort invocation}.
4689 Comparisons honor the rules specified by the @env{LC_COLLATE}
4692 If no @var{output} file is specified, @command{uniq} writes to standard
4695 The program accepts the following options. Also see @ref{Common options}.
4700 @itemx --skip-fields=@var{n}
4702 @opindex --skip-fields
4703 Skip @var{n} fields on each line before checking for uniqueness. Use
4704 a null string for comparison if a line has fewer than @var{n} fields. Fields
4705 are sequences of non-space non-tab characters that are separated from
4706 each other by at least one space or tab.
4708 For compatibility @command{uniq} supports an obsolete option syntax
4709 @option{-@var{n}}. New scripts should use @option{-f @var{n}} instead.
4712 @itemx --skip-chars=@var{n}
4714 @opindex --skip-chars
4715 Skip @var{n} characters before checking for uniqueness. Use a null string
4716 for comparison if a line has fewer than @var{n} characters. If you use both
4717 the field and character skipping options, fields are skipped over first.
4719 @vindex _POSIX2_VERSION
4720 On older systems, @command{uniq} supports an obsolete option syntax
4722 This obsolete behavior can be enabled or disabled with the
4723 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4724 conformance}), but portable scripts should avoid commands whose
4725 behavior depends on this variable.
4726 For example, use @samp{uniq ./+10} or @samp{uniq -s 10} rather than
4727 the ambiguous @samp{uniq +10}.
4733 Print the number of times each line occurred along with the line.
4736 @itemx --ignore-case
4738 @opindex --ignore-case
4739 Ignore differences in case when comparing lines.
4745 @cindex repeated lines, outputting
4746 Discard lines that are not repeated. When used by itself, this option
4747 causes @command{uniq} to print the first copy of each repeated line,
4751 @itemx --all-repeated[=@var{delimit-method}]
4753 @opindex --all-repeated
4754 @cindex all repeated lines, outputting
4755 Do not discard the second and subsequent repeated input lines,
4756 but discard lines that are not repeated.
4757 This option is useful mainly in conjunction with other options e.g.,
4758 to ignore case or to compare only selected fields.
4759 The optional @var{delimit-method} tells how to delimit
4760 groups of repeated lines, and must be one of the following:
4765 Do not delimit groups of repeated lines.
4766 This is equivalent to @option{--all-repeated} (@option{-D}).
4769 Output a newline before each group of repeated lines.
4770 With @option{--zero-terminated} (@option{-z}), use a zero
4771 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4774 Separate groups of repeated lines with a single newline.
4775 With @option{--zero-terminated} (@option{-z}), use a zero
4776 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4777 This is the same as using @samp{prepend}, except that
4778 no delimiter is inserted before the first group, and hence
4779 may be better suited for output direct to users.
4782 Note that when groups are delimited and the input stream contains
4783 two or more consecutive blank lines, then the output is ambiguous.
4784 To avoid that, filter the input through @samp{tr -s '\n'} to replace
4785 each sequence of consecutive newlines with a single newline.
4787 This is a @sc{gnu} extension.
4788 @c FIXME: give an example showing *how* it's useful
4794 @cindex unique lines, outputting
4795 Discard the first repeated line. When used by itself, this option
4796 causes @command{uniq} to print unique lines, and nothing else.
4799 @itemx --check-chars=@var{n}
4801 @opindex --check-chars
4802 Compare at most @var{n} characters on each line (after skipping any specified
4803 fields and characters). By default the entire rest of the lines are
4806 @zeroTerminatedOption
4813 @node comm invocation
4814 @section @command{comm}: Compare two sorted files line by line
4817 @cindex line-by-line comparison
4818 @cindex comparing sorted files
4820 @command{comm} writes to standard output lines that are common, and lines
4821 that are unique, to two input files; a file name of @samp{-} means
4822 standard input. Synopsis:
4825 comm [@var{option}]@dots{} @var{file1} @var{file2}
4829 Before @command{comm} can be used, the input files must be sorted using the
4830 collating sequence specified by the @env{LC_COLLATE} locale.
4831 If an input file ends in a non-newline
4832 character, a newline is silently appended. The @command{sort} command with
4833 no options always outputs a file that is suitable input to @command{comm}.
4835 @cindex differing lines
4836 @cindex common lines
4837 With no options, @command{comm} produces three-column output. Column one
4838 contains lines unique to @var{file1}, column two contains lines unique
4839 to @var{file2}, and column three contains lines common to both files.
4840 Columns are separated by a single TAB character.
4841 @c FIXME: when there's an option to supply an alternative separator
4842 @c string, append "by default" to the above sentence.
4847 The options @option{-1}, @option{-2}, and @option{-3} suppress printing of
4848 the corresponding columns (and separators). Also see @ref{Common options}.
4850 Unlike some other comparison utilities, @command{comm} has an exit
4851 status that does not depend on the result of the comparison.
4852 Upon normal completion @command{comm} produces an exit code of zero.
4853 If there is an error it exits with nonzero status.
4855 @macro checkOrderOption{cmd}
4856 If the @option{--check-order} option is given, unsorted inputs will
4857 cause a fatal error message. If the option @option{--nocheck-order}
4858 is given, unsorted inputs will never cause an error message. If neither
4859 of these options is given, wrongly sorted inputs are diagnosed
4860 only if an input file is found to contain unpairable
4862 lines, and when both input files are non empty.
4864 @ifclear JOIN_COMMAND
4867 If an input file is diagnosed as being unsorted, the @command{\cmd\}
4868 command will exit with a nonzero status (and the output should not be used).
4870 Forcing @command{\cmd\} to process wrongly sorted input files
4871 containing unpairable lines by specifying @option{--nocheck-order} is
4872 not guaranteed to produce any particular output. The output will
4873 probably not correspond with whatever you hoped it would be.
4875 @checkOrderOption{comm}
4880 Fail with an error message if either input file is wrongly ordered.
4882 @item --nocheck-order
4883 Do not check that both input files are in sorted order.
4887 @item --output-delimiter=@var{str}
4888 Print @var{str} between adjacent output columns,
4889 rather than the default of a single TAB character.
4891 The delimiter @var{str} may not be empty.
4895 @node ptx invocation
4896 @section @command{ptx}: Produce permuted indexes
4900 @command{ptx} reads a text file and essentially produces a permuted index, with
4901 each keyword in its context. The calling sketch is either one of:
4904 ptx [@var{option} @dots{}] [@var{file} @dots{}]
4905 ptx -G [@var{option} @dots{}] [@var{input} [@var{output}]]
4908 The @option{-G} (or its equivalent: @option{--traditional}) option disables
4909 all @sc{gnu} extensions and reverts to traditional mode, thus introducing some
4910 limitations and changing several of the program's default option values.
4911 When @option{-G} is not specified, @sc{gnu} extensions are always enabled.
4912 @sc{gnu} extensions to @command{ptx} are documented wherever appropriate in this
4913 document. For the full list, see @xref{Compatibility in ptx}.
4915 Individual options are explained in the following sections.
4917 When @sc{gnu} extensions are enabled, there may be zero, one or several
4918 @var{file}s after the options. If there is no @var{file}, the program
4919 reads the standard input. If there is one or several @var{file}s, they
4920 give the name of input files which are all read in turn, as if all the
4921 input files were concatenated. However, there is a full contextual
4922 break between each file and, when automatic referencing is requested,
4923 file names and line numbers refer to individual text input files. In
4924 all cases, the program outputs the permuted index to the standard
4927 When @sc{gnu} extensions are @emph{not} enabled, that is, when the program
4928 operates in traditional mode, there may be zero, one or two parameters
4929 besides the options. If there are no parameters, the program reads the
4930 standard input and outputs the permuted index to the standard output.
4931 If there is only one parameter, it names the text @var{input} to be read
4932 instead of the standard input. If two parameters are given, they give
4933 respectively the name of the @var{input} file to read and the name of
4934 the @var{output} file to produce. @emph{Be very careful} to note that,
4935 in this case, the contents of file given by the second parameter is
4936 destroyed. This behavior is dictated by System V @command{ptx}
4937 compatibility; @sc{gnu} Standards normally discourage output parameters not
4938 introduced by an option.
4940 Note that for @emph{any} file named as the value of an option or as an
4941 input text file, a single dash @kbd{-} may be used, in which case
4942 standard input is assumed. However, it would not make sense to use this
4943 convention more than once per program invocation.
4946 * General options in ptx:: Options which affect general program behavior.
4947 * Charset selection in ptx:: Underlying character set considerations.
4948 * Input processing in ptx:: Input fields, contexts, and keyword selection.
4949 * Output formatting in ptx:: Types of output format, and sizing the fields.
4950 * Compatibility in ptx::
4954 @node General options in ptx
4955 @subsection General options
4960 @itemx --traditional
4961 As already explained, this option disables all @sc{gnu} extensions to
4962 @command{ptx} and switches to traditional mode.
4965 Print a short help on standard output, then exit without further
4969 Print the program version on standard output, then exit without further
4977 @node Charset selection in ptx
4978 @subsection Charset selection
4980 @c FIXME: People don't necessarily know what an IBM-PC was these days.
4981 As it is set up now, the program assumes that the input file is coded
4982 using 8-bit @acronym{ISO} 8859-1 code, also known as Latin-1 character set,
4983 @emph{unless} it is compiled for MS-DOS, in which case it uses the
4984 character set of the IBM-PC@. (@sc{gnu} @command{ptx} is not known to work on
4985 smaller MS-DOS machines anymore.) Compared to 7-bit @acronym{ASCII}, the set
4986 of characters which are letters is different; this alters the behavior
4987 of regular expression matching. Thus, the default regular expression
4988 for a keyword allows foreign or diacriticized letters. Keyword sorting,
4989 however, is still crude; it obeys the underlying character set ordering
4995 @itemx --ignore-case
4996 Fold lower case letters to upper case for sorting.
5001 @node Input processing in ptx
5002 @subsection Word selection and input processing
5007 @itemx --break-file=@var{file}
5009 This option provides an alternative (to @option{-W}) method of describing
5010 which characters make up words. It introduces the name of a
5011 file which contains a list of characters which can@emph{not} be part of
5012 one word; this file is called the @dfn{Break file}. Any character which
5013 is not part of the Break file is a word constituent. If both options
5014 @option{-b} and @option{-W} are specified, then @option{-W} has precedence and
5015 @option{-b} is ignored.
5017 When @sc{gnu} extensions are enabled, the only way to avoid newline as a
5018 break character is to write all the break characters in the file with no
5019 newline at all, not even at the end of the file. When @sc{gnu} extensions
5020 are disabled, spaces, tabs and newlines are always considered as break
5021 characters even if not included in the Break file.
5024 @itemx --ignore-file=@var{file}
5026 The file associated with this option contains a list of words which will
5027 never be taken as keywords in concordance output. It is called the
5028 @dfn{Ignore file}. The file contains exactly one word in each line; the
5029 end of line separation of words is not subject to the value of the
5033 @itemx --only-file=@var{file}
5035 The file associated with this option contains a list of words which will
5036 be retained in concordance output; any word not mentioned in this file
5037 is ignored. The file is called the @dfn{Only file}. The file contains
5038 exactly one word in each line; the end of line separation of words is
5039 not subject to the value of the @option{-S} option.
5041 There is no default for the Only file. When both an Only file and an
5042 Ignore file are specified, a word is considered a keyword only
5043 if it is listed in the Only file and not in the Ignore file.
5048 On each input line, the leading sequence of non-white space characters will be
5049 taken to be a reference that has the purpose of identifying this input
5050 line in the resulting permuted index. For more information about reference
5051 production, see @xref{Output formatting in ptx}.
5052 Using this option changes the default value for option @option{-S}.
5054 Using this option, the program does not try very hard to remove
5055 references from contexts in output, but it succeeds in doing so
5056 @emph{when} the context ends exactly at the newline. If option
5057 @option{-r} is used with @option{-S} default value, or when @sc{gnu} extensions
5058 are disabled, this condition is always met and references are completely
5059 excluded from the output contexts.
5061 @item -S @var{regexp}
5062 @itemx --sentence-regexp=@var{regexp}
5064 This option selects which regular expression will describe the end of a
5065 line or the end of a sentence. In fact, this regular expression is not
5066 the only distinction between end of lines or end of sentences, and input
5067 line boundaries have no special significance outside this option. By
5068 default, when @sc{gnu} extensions are enabled and if @option{-r} option is not
5069 used, end of sentences are used. In this case, this @var{regex} is
5070 imported from @sc{gnu} Emacs:
5073 [.?!][]\"')@}]*\\($\\|\t\\| \\)[ \t\n]*
5076 Whenever @sc{gnu} extensions are disabled or if @option{-r} option is used, end
5077 of lines are used; in this case, the default @var{regexp} is just:
5083 Using an empty @var{regexp} is equivalent to completely disabling end of
5084 line or end of sentence recognition. In this case, the whole file is
5085 considered to be a single big line or sentence. The user might want to
5086 disallow all truncation flag generation as well, through option @option{-F
5087 ""}. @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
5090 When the keywords happen to be near the beginning of the input line or
5091 sentence, this often creates an unused area at the beginning of the
5092 output context line; when the keywords happen to be near the end of the
5093 input line or sentence, this often creates an unused area at the end of
5094 the output context line. The program tries to fill those unused areas
5095 by wrapping around context in them; the tail of the input line or
5096 sentence is used to fill the unused area on the left of the output line;
5097 the head of the input line or sentence is used to fill the unused area
5098 on the right of the output line.
5100 As a matter of convenience to the user, many usual backslashed escape
5101 sequences from the C language are recognized and converted to the
5102 corresponding characters by @command{ptx} itself.
5104 @item -W @var{regexp}
5105 @itemx --word-regexp=@var{regexp}
5107 This option selects which regular expression will describe each keyword.
5108 By default, if @sc{gnu} extensions are enabled, a word is a sequence of
5109 letters; the @var{regexp} used is @samp{\w+}. When @sc{gnu} extensions are
5110 disabled, a word is by default anything which ends with a space, a tab
5111 or a newline; the @var{regexp} used is @samp{[^ \t\n]+}.
5113 An empty @var{regexp} is equivalent to not using this option.
5114 @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
5117 As a matter of convenience to the user, many usual backslashed escape
5118 sequences, as found in the C language, are recognized and converted to
5119 the corresponding characters by @command{ptx} itself.
5124 @node Output formatting in ptx
5125 @subsection Output formatting
5127 Output format is mainly controlled by the @option{-O} and @option{-T} options
5128 described in the table below. When neither @option{-O} nor @option{-T} are
5129 selected, and if @sc{gnu} extensions are enabled, the program chooses an
5130 output format suitable for a dumb terminal. Each keyword occurrence is
5131 output to the center of one line, surrounded by its left and right
5132 contexts. Each field is properly justified, so the concordance output
5133 can be readily observed. As a special feature, if automatic
5134 references are selected by option @option{-A} and are output before the
5135 left context, that is, if option @option{-R} is @emph{not} selected, then
5136 a colon is added after the reference; this nicely interfaces with @sc{gnu}
5137 Emacs @code{next-error} processing. In this default output format, each
5138 white space character, like newline and tab, is merely changed to
5139 exactly one space, with no special attempt to compress consecutive
5140 spaces. This might change in the future. Except for those white space
5141 characters, every other character of the underlying set of 256
5142 characters is transmitted verbatim.
5144 Output format is further controlled by the following options.
5148 @item -g @var{number}
5149 @itemx --gap-size=@var{number}
5151 Select the size of the minimum white space gap between the fields on the
5154 @item -w @var{number}
5155 @itemx --width=@var{number}
5157 Select the maximum output width of each final line. If references are
5158 used, they are included or excluded from the maximum output width
5159 depending on the value of option @option{-R}. If this option is not
5160 selected, that is, when references are output before the left context,
5161 the maximum output width takes into account the maximum length of all
5162 references. If this option is selected, that is, when references are
5163 output after the right context, the maximum output width does not take
5164 into account the space taken by references, nor the gap that precedes
5168 @itemx --auto-reference
5170 Select automatic references. Each input line will have an automatic
5171 reference made up of the file name and the line ordinal, with a single
5172 colon between them. However, the file name will be empty when standard
5173 input is being read. If both @option{-A} and @option{-r} are selected, then
5174 the input reference is still read and skipped, but the automatic
5175 reference is used at output time, overriding the input reference.
5178 @itemx --right-side-refs
5180 In the default output format, when option @option{-R} is not used, any
5181 references produced by the effect of options @option{-r} or @option{-A} are
5182 placed to the far right of output lines, after the right context. With
5183 default output format, when the @option{-R} option is specified, references
5184 are rather placed at the beginning of each output line, before the left
5185 context. For any other output format, option @option{-R} is
5186 ignored, with one exception: with @option{-R} the width of references
5187 is @emph{not} taken into account in total output width given by @option{-w}.
5189 This option is automatically selected whenever @sc{gnu} extensions are
5192 @item -F @var{string}
5193 @itemx --flac-truncation=@var{string}
5195 This option will request that any truncation in the output be reported
5196 using the string @var{string}. Most output fields theoretically extend
5197 towards the beginning or the end of the current line, or current
5198 sentence, as selected with option @option{-S}. But there is a maximum
5199 allowed output line width, changeable through option @option{-w}, which is
5200 further divided into space for various output fields. When a field has
5201 to be truncated because it cannot extend beyond the beginning or the end of
5202 the current line to fit in, then a truncation occurs. By default,
5203 the string used is a single slash, as in @option{-F /}.
5205 @var{string} may have more than one character, as in @option{-F ...}.
5206 Also, in the particular case when @var{string} is empty (@option{-F ""}),
5207 truncation flagging is disabled, and no truncation marks are appended in
5210 As a matter of convenience to the user, many usual backslashed escape
5211 sequences, as found in the C language, are recognized and converted to
5212 the corresponding characters by @command{ptx} itself.
5214 @item -M @var{string}
5215 @itemx --macro-name=@var{string}
5217 Select another @var{string} to be used instead of @samp{xx}, while
5218 generating output suitable for @command{nroff}, @command{troff} or @TeX{}.
5221 @itemx --format=roff
5223 Choose an output format suitable for @command{nroff} or @command{troff}
5224 processing. Each output line will look like:
5227 .xx "@var{tail}" "@var{before}" "@var{keyword_and_after}"@c
5228 "@var{head}" "@var{ref}"
5231 so it will be possible to write a @samp{.xx} roff macro to take care of
5232 the output typesetting. This is the default output format when @sc{gnu}
5233 extensions are disabled. Option @option{-M} can be used to change
5234 @samp{xx} to another macro name.
5236 In this output format, each non-graphical character, like newline and
5237 tab, is merely changed to exactly one space, with no special attempt to
5238 compress consecutive spaces. Each quote character: @kbd{"} is doubled
5239 so it will be correctly processed by @command{nroff} or @command{troff}.
5244 Choose an output format suitable for @TeX{} processing. Each output
5245 line will look like:
5248 \xx @{@var{tail}@}@{@var{before}@}@{@var{keyword}@}@c
5249 @{@var{after}@}@{@var{head}@}@{@var{ref}@}
5253 so it will be possible to write a @code{\xx} definition to take care of
5254 the output typesetting. Note that when references are not being
5255 produced, that is, neither option @option{-A} nor option @option{-r} is
5256 selected, the last parameter of each @code{\xx} call is inhibited.
5257 Option @option{-M} can be used to change @samp{xx} to another macro
5260 In this output format, some special characters, like @kbd{$}, @kbd{%},
5261 @kbd{&}, @kbd{#} and @kbd{_} are automatically protected with a
5262 backslash. Curly brackets @kbd{@{}, @kbd{@}} are protected with a
5263 backslash and a pair of dollar signs (to force mathematical mode). The
5264 backslash itself produces the sequence @code{\backslash@{@}}.
5265 Circumflex and tilde diacritical marks produce the sequence @code{^\@{ @}} and
5266 @code{~\@{ @}} respectively. Other diacriticized characters of the
5267 underlying character set produce an appropriate @TeX{} sequence as far
5268 as possible. The other non-graphical characters, like newline and tab,
5269 and all other characters which are not part of @acronym{ASCII}, are merely
5270 changed to exactly one space, with no special attempt to compress
5271 consecutive spaces. Let me know how to improve this special character
5272 processing for @TeX{}.
5277 @node Compatibility in ptx
5278 @subsection The @sc{gnu} extensions to @command{ptx}
5280 This version of @command{ptx} contains a few features which do not exist in
5281 System V @command{ptx}. These extra features are suppressed by using the
5282 @option{-G} command line option, unless overridden by other command line
5283 options. Some @sc{gnu} extensions cannot be recovered by overriding, so the
5284 simple rule is to avoid @option{-G} if you care about @sc{gnu} extensions.
5285 Here are the differences between this program and System V @command{ptx}.
5290 This program can read many input files at once, it always writes the
5291 resulting concordance on standard output. On the other hand, System V
5292 @command{ptx} reads only one file and sends the result to standard output
5293 or, if a second @var{file} parameter is given on the command, to that
5296 Having output parameters not introduced by options is a dangerous
5297 practice which @sc{gnu} avoids as far as possible. So, for using @command{ptx}
5298 portably between @sc{gnu} and System V, you should always use it with a
5299 single input file, and always expect the result on standard output. You
5300 might also want to automatically configure in a @option{-G} option to
5301 @command{ptx} calls in products using @command{ptx}, if the configurator finds
5302 that the installed @command{ptx} accepts @option{-G}.
5305 The only options available in System V @command{ptx} are options @option{-b},
5306 @option{-f}, @option{-g}, @option{-i}, @option{-o}, @option{-r}, @option{-t} and
5307 @option{-w}. All other options are @sc{gnu} extensions and are not repeated in
5308 this enumeration. Moreover, some options have a slightly different
5309 meaning when @sc{gnu} extensions are enabled, as explained below.
5312 By default, concordance output is not formatted for @command{troff} or
5313 @command{nroff}. It is rather formatted for a dumb terminal. @command{troff}
5314 or @command{nroff} output may still be selected through option @option{-O}.
5317 Unless @option{-R} option is used, the maximum reference width is
5318 subtracted from the total output line width. With @sc{gnu} extensions
5319 disabled, width of references is not taken into account in the output
5320 line width computations.
5323 All 256 bytes, even @acronym{ASCII} @sc{nul} bytes, are always read and
5324 processed from input file with no adverse effect, even if @sc{gnu} extensions
5325 are disabled. However, System V @command{ptx} does not accept 8-bit characters,
5326 a few control characters are rejected, and the tilde @kbd{~} is also rejected.
5329 Input line length is only limited by available memory, even if @sc{gnu}
5330 extensions are disabled. However, System V @command{ptx} processes only
5331 the first 200 characters in each line.
5334 The break (non-word) characters default to be every character except all
5335 letters of the underlying character set, diacriticized or not. When @sc{gnu}
5336 extensions are disabled, the break characters default to space, tab and
5340 The program makes better use of output line width. If @sc{gnu} extensions
5341 are disabled, the program rather tries to imitate System V @command{ptx},
5342 but still, there are some slight disposition glitches this program does
5343 not completely reproduce.
5346 The user can specify both an Ignore file and an Only file. This is not
5347 allowed with System V @command{ptx}.
5352 @node tsort invocation
5353 @section @command{tsort}: Topological sort
5356 @cindex topological sort
5358 @command{tsort} performs a topological sort on the given @var{file}, or
5359 standard input if no input file is given or for a @var{file} of
5360 @samp{-}. For more details and some history, see @ref{tsort background}.
5364 tsort [@var{option}] [@var{file}]
5367 @command{tsort} reads its input as pairs of strings, separated by blanks,
5368 indicating a partial ordering. The output is a total ordering that
5369 corresponds to the given partial ordering.
5383 will produce the output
5394 Consider a more realistic example.
5395 You have a large set of functions all in one file, and they may all be
5396 declared static except one. Currently that one (say @code{main}) is the
5397 first function defined in the file, and the ones it calls directly follow
5398 it, followed by those they call, etc. Let's say that you are determined
5399 to take advantage of prototypes, so you have to choose between declaring
5400 all of those functions (which means duplicating a lot of information from
5401 the definitions) and rearranging the functions so that as many as possible
5402 are defined before they are used. One way to automate the latter process
5403 is to get a list for each function of the functions it calls directly.
5404 Many programs can generate such lists. They describe a call graph.
5405 Consider the following list, in which a given line indicates that the
5406 function on the left calls the one on the right directly.
5412 tail_file pretty_name
5413 tail_file write_header
5415 tail_forever recheck
5416 tail_forever pretty_name
5417 tail_forever write_header
5418 tail_forever dump_remainder
5421 tail_lines start_lines
5422 tail_lines dump_remainder
5423 tail_lines file_lines
5424 tail_lines pipe_lines
5426 tail_bytes start_bytes
5427 tail_bytes dump_remainder
5428 tail_bytes pipe_bytes
5429 file_lines dump_remainder
5433 then you can use @command{tsort} to produce an ordering of those
5434 functions that satisfies your requirement.
5437 example$ tsort call-graph | tac
5457 @command{tsort} detects any cycles in the input and writes the first cycle
5458 encountered to standard error.
5460 Note that for a given partial ordering, generally there is no unique
5461 total ordering. In the context of the call graph above, the function
5462 @code{parse_options} may be placed anywhere in the list as long as it
5463 precedes @code{main}.
5465 The only options are @option{--help} and @option{--version}. @xref{Common
5471 * tsort background:: Where tsort came from.
5474 @node tsort background
5475 @subsection @command{tsort}: Background
5477 @command{tsort} exists because very early versions of the Unix linker processed
5478 an archive file exactly once, and in order. As @command{ld} read each object
5479 in the archive, it decided whether it was needed in the program based on
5480 whether it defined any symbols which were undefined at that point in
5483 This meant that dependencies within the archive had to be handled
5484 specially. For example, @code{scanf} probably calls @code{read}. That means
5485 that in a single pass through an archive, it was important for @code{scanf.o}
5486 to appear before read.o, because otherwise a program which calls
5487 @code{scanf} but not @code{read} might end up with an unexpected unresolved
5488 reference to @code{read}.
5490 The way to address this problem was to first generate a set of
5491 dependencies of one object file on another. This was done by a shell
5492 script called @command{lorder}. The GNU tools don't provide a version of
5493 lorder, as far as I know, but you can still find it in BSD
5496 Then you ran @command{tsort} over the @command{lorder} output, and you used the
5497 resulting sort to define the order in which you added objects to the archive.
5499 This whole procedure has been obsolete since about 1980, because
5500 Unix archives now contain a symbol table (traditionally built by
5501 @command{ranlib}, now generally built by @command{ar} itself), and the Unix
5502 linker uses the symbol table to effectively make multiple passes over
5505 Anyhow, that's where tsort came from. To solve an old problem with
5506 the way the linker handled archive files, which has since been solved
5510 @node Operating on fields
5511 @chapter Operating on fields
5514 * cut invocation:: Print selected parts of lines.
5515 * paste invocation:: Merge lines of files.
5516 * join invocation:: Join lines on a common field.
5520 @node cut invocation
5521 @section @command{cut}: Print selected parts of lines
5524 @command{cut} writes to standard output selected parts of each line of each
5525 input file, or standard input if no files are given or for a file name of
5529 cut @var{option}@dots{} [@var{file}]@dots{}
5532 In the table which follows, the @var{byte-list}, @var{character-list},
5533 and @var{field-list} are one or more numbers or ranges (two numbers
5534 separated by a dash) separated by commas. Bytes, characters, and
5535 fields are numbered starting at 1. Incomplete ranges may be
5536 given: @option{-@var{m}} means @samp{1-@var{m}}; @samp{@var{n}-} means
5537 @samp{@var{n}} through end of line or last field. The list elements
5538 can be repeated, can overlap, and can be specified in any order; but
5539 the selected input is written in the same order that it is read, and
5540 is written exactly once.
5542 The program accepts the following options. Also see @ref{Common
5547 @item -b @var{byte-list}
5548 @itemx --bytes=@var{byte-list}
5551 Select for printing only the bytes in positions listed in
5552 @var{byte-list}. Tabs and backspaces are treated like any other
5553 character; they take up 1 byte. If an output delimiter is specified,
5554 (see the description of @option{--output-delimiter}), then output that
5555 string between ranges of selected bytes.
5557 @item -c @var{character-list}
5558 @itemx --characters=@var{character-list}
5560 @opindex --characters
5561 Select for printing only the characters in positions listed in
5562 @var{character-list}. The same as @option{-b} for now, but
5563 internationalization will change that. Tabs and backspaces are
5564 treated like any other character; they take up 1 character. If an
5565 output delimiter is specified, (see the description of
5566 @option{--output-delimiter}), then output that string between ranges
5569 @item -f @var{field-list}
5570 @itemx --fields=@var{field-list}
5573 Select for printing only the fields listed in @var{field-list}.
5574 Fields are separated by a TAB character by default. Also print any
5575 line that contains no delimiter character, unless the
5576 @option{--only-delimited} (@option{-s}) option is specified.
5578 Note @command{awk} supports more sophisticated field processing,
5579 and by default will use (and discard) runs of blank characters to
5580 separate fields, and ignore leading and trailing blanks.
5583 awk '{print $2}' # print the second field
5584 awk '{print $NF-1}' # print the penultimate field
5585 awk '{print $2,$1}' # reorder the first two fields
5589 In the unlikely event that @command{awk} is unavailable,
5590 one can use the @command{join} command, to process blank
5591 characters as @command{awk} does above.
5594 join -a1 -o 1.2 - /dev/null # print the second field
5595 join -a1 -o 1.2,1.1 - /dev/null # reorder the first two fields
5599 @item -d @var{input_delim_byte}
5600 @itemx --delimiter=@var{input_delim_byte}
5602 @opindex --delimiter
5603 With @option{-f}, use the first byte of @var{input_delim_byte} as
5604 the input fields separator (default is TAB).
5608 Do not split multi-byte characters (no-op for now).
5611 @itemx --only-delimited
5613 @opindex --only-delimited
5614 For @option{-f}, do not print lines that do not contain the field separator
5615 character. Normally, any line without a field separator is printed verbatim.
5617 @item --output-delimiter=@var{output_delim_string}
5618 @opindex --output-delimiter
5619 With @option{-f}, output fields are separated by @var{output_delim_string}.
5620 The default with @option{-f} is to use the input delimiter.
5621 When using @option{-b} or @option{-c} to select ranges of byte or
5622 character offsets (as opposed to ranges of fields),
5623 output @var{output_delim_string} between non-overlapping
5624 ranges of selected bytes.
5627 @opindex --complement
5628 This option is a @acronym{GNU} extension.
5629 Select for printing the complement of the bytes, characters or fields
5630 selected with the @option{-b}, @option{-c} or @option{-f} options.
5631 In other words, do @emph{not} print the bytes, characters or fields
5632 specified via those options. This option is useful when you have
5633 many fields and want to print all but a few of them.
5640 @node paste invocation
5641 @section @command{paste}: Merge lines of files
5644 @cindex merging files
5646 @command{paste} writes to standard output lines consisting of sequentially
5647 corresponding lines of each given file, separated by a TAB character.
5648 Standard input is used for a file name of @samp{-} or if no input files
5670 paste [@var{option}]@dots{} [@var{file}]@dots{}
5673 The program accepts the following options. Also see @ref{Common options}.
5681 Paste the lines of one file at a time rather than one line from each
5682 file. Using the above example data:
5685 $ paste -s num2 let3
5690 @item -d @var{delim-list}
5691 @itemx --delimiters=@var{delim-list}
5693 @opindex --delimiters
5694 Consecutively use the characters in @var{delim-list} instead of
5695 TAB to separate merged lines. When @var{delim-list} is
5696 exhausted, start again at its beginning. Using the above example data:
5699 $ paste -d '%_' num2 let3 num2
5710 @node join invocation
5711 @section @command{join}: Join lines on a common field
5714 @cindex common field, joining on
5716 @command{join} writes to standard output a line for each pair of input
5717 lines that have identical join fields. Synopsis:
5720 join [@var{option}]@dots{} @var{file1} @var{file2}
5723 Either @var{file1} or @var{file2} (but not both) can be @samp{-},
5724 meaning standard input. @var{file1} and @var{file2} should be
5725 sorted on the join fields.
5728 Normally, the sort order is that of the
5729 collating sequence specified by the @env{LC_COLLATE} locale. Unless
5730 the @option{-t} option is given, the sort comparison ignores blanks at
5731 the start of the join field, as in @code{sort -b}. If the
5732 @option{--ignore-case} option is given, the sort comparison ignores
5733 the case of characters in the join field, as in @code{sort -f}.
5735 The @command{sort} and @command{join} commands should use consistent
5736 locales and options if the output of @command{sort} is fed to
5737 @command{join}. You can use a command like @samp{sort -k 1b,1} to
5738 sort a file on its default join field, but if you select a non-default
5739 locale, join field, separator, or comparison options, then you should
5740 do so consistently between @command{join} and @command{sort}.
5741 If @samp{join -t ''} is specified then the whole line is considered which
5742 matches the default operation of sort.
5744 If the input has no unpairable lines, a @acronym{GNU} extension is
5745 available; the sort order can be any order that considers two fields
5746 to be equal if and only if the sort comparison described above
5747 considers them to be equal. For example:
5765 @checkOrderOption{join}
5770 @item the join field is the first field in each line;
5771 @item fields in the input are separated by one or more blanks, with leading
5772 blanks on the line ignored;
5773 @item fields in the output are separated by a space;
5774 @item each output line consists of the join field, the remaining
5775 fields from @var{file1}, then the remaining fields from @var{file2}.
5778 The program accepts the following options. Also see @ref{Common options}.
5782 @item -a @var{file-number}
5784 Print a line for each unpairable line in file @var{file-number} (either
5785 @samp{1} or @samp{2}), in addition to the normal output.
5788 Fail with an error message if either input file is wrongly ordered.
5790 @item --nocheck-order
5791 Do not check that both input files are in sorted order. This is the default.
5793 @item -e @var{string}
5795 Replace those output fields that are missing in the input with @var{string}.
5796 I.E. missing fields specified with the @option{-12jo} options.
5800 Treat the first line of each input file as a header line. The header lines will
5801 be joined and printed as the first output line. If @option{-o} is used to
5802 specify output format, the header line will be printed according to the
5803 specified format. The header lines will not be checked for ordering even if
5804 @option{--check-order} is specified. Also if the header lines from each file
5805 do not match, the heading fields from the first file will be used.
5808 @itemx --ignore-case
5810 @opindex --ignore-case
5811 Ignore differences in case when comparing keys.
5812 With this option, the lines of the input files must be ordered in the same way.
5813 Use @samp{sort -f} to produce this ordering.
5815 @item -1 @var{field}
5817 Join on field @var{field} (a positive integer) of file 1.
5819 @item -2 @var{field}
5821 Join on field @var{field} (a positive integer) of file 2.
5823 @item -j @var{field}
5824 Equivalent to @option{-1 @var{field} -2 @var{field}}.
5826 @item -o @var{field-list}
5828 If the keyword @samp{auto} is specified, infer the output format from
5829 the first line in each file. This is the same as the default output format
5830 but also ensures the same number of fields are output for each line.
5831 Missing fields are replaced with the @option{-e} option and extra fields
5834 Otherwise, construct each output line according to the format in
5835 @var{field-list}. Each element in @var{field-list} is either the single
5836 character @samp{0} or has the form @var{m.n} where the file number, @var{m},
5837 is @samp{1} or @samp{2} and @var{n} is a positive field number.
5839 A field specification of @samp{0} denotes the join field.
5840 In most cases, the functionality of the @samp{0} field spec
5841 may be reproduced using the explicit @var{m.n} that corresponds
5842 to the join field. However, when printing unpairable lines
5843 (using either of the @option{-a} or @option{-v} options), there is no way
5844 to specify the join field using @var{m.n} in @var{field-list}
5845 if there are unpairable lines in both files.
5846 To give @command{join} that functionality, @acronym{POSIX} invented the @samp{0}
5847 field specification notation.
5849 The elements in @var{field-list}
5850 are separated by commas or blanks.
5851 Blank separators typically need to be quoted for the shell. For
5852 example, the commands @samp{join -o 1.2,2.2} and @samp{join -o '1.2
5853 2.2'} are equivalent.
5855 All output lines---including those printed because of any -a or -v
5856 option---are subject to the specified @var{field-list}.
5859 Use character @var{char} as the input and output field separator.
5860 Treat as significant each occurrence of @var{char} in the input file.
5861 Use @samp{sort -t @var{char}}, without the @option{-b} option of
5862 @samp{sort}, to produce this ordering. If @samp{join -t ''} is specified,
5863 the whole line is considered, matching the default operation of sort.
5864 If @samp{-t '\0'} is specified then the @acronym{ASCII} @sc{nul}
5865 character is used to delimit the fields.
5867 @item -v @var{file-number}
5868 Print a line for each unpairable line in file @var{file-number}
5869 (either @samp{1} or @samp{2}), instead of the normal output.
5876 @node Operating on characters
5877 @chapter Operating on characters
5879 @cindex operating on characters
5881 This commands operate on individual characters.
5884 * tr invocation:: Translate, squeeze, and/or delete characters.
5885 * expand invocation:: Convert tabs to spaces.
5886 * unexpand invocation:: Convert spaces to tabs.
5891 @section @command{tr}: Translate, squeeze, and/or delete characters
5898 tr [@var{option}]@dots{} @var{set1} [@var{set2}]
5901 @command{tr} copies standard input to standard output, performing
5902 one of the following operations:
5906 translate, and optionally squeeze repeated characters in the result,
5908 squeeze repeated characters,
5912 delete characters, then squeeze repeated characters from the result.
5915 The @var{set1} and (if given) @var{set2} arguments define ordered
5916 sets of characters, referred to below as @var{set1} and @var{set2}. These
5917 sets are the characters of the input that @command{tr} operates on.
5918 The @option{--complement} (@option{-c}, @option{-C}) option replaces
5920 complement (all of the characters that are not in @var{set1}).
5922 Currently @command{tr} fully supports only single-byte characters.
5923 Eventually it will support multibyte characters; when it does, the
5924 @option{-C} option will cause it to complement the set of characters,
5925 whereas @option{-c} will cause it to complement the set of values.
5926 This distinction will matter only when some values are not characters,
5927 and this is possible only in locales using multibyte encodings when
5928 the input contains encoding errors.
5930 The program accepts the @option{--help} and @option{--version}
5931 options. @xref{Common options}. Options must precede operands.
5936 * Character sets:: Specifying sets of characters.
5937 * Translating:: Changing one set of characters to another.
5938 * Squeezing:: Squeezing repeats and deleting.
5942 @node Character sets
5943 @subsection Specifying sets of characters
5945 @cindex specifying sets of characters
5947 The format of the @var{set1} and @var{set2} arguments resembles
5948 the format of regular expressions; however, they are not regular
5949 expressions, only lists of characters. Most characters simply
5950 represent themselves in these strings, but the strings can contain
5951 the shorthands listed below, for convenience. Some of them can be
5952 used only in @var{set1} or @var{set2}, as noted below.
5956 @item Backslash escapes
5957 @cindex backslash escapes
5959 The following backslash escape sequences are recognized:
5977 The 8-bit character with the value given by @var{ooo}, which is 1 to 3
5978 octal digits. Note that @samp{\400} is interpreted as the two-byte
5979 sequence, @samp{\040} @samp{0}.
5984 While a backslash followed by a character not listed above is
5985 interpreted as that character, the backslash also effectively
5986 removes any special significance, so it is useful to escape
5987 @samp{[}, @samp{]}, @samp{*}, and @samp{-}.
5992 The notation @samp{@var{m}-@var{n}} expands to all of the characters
5993 from @var{m} through @var{n}, in ascending order. @var{m} should
5994 collate before @var{n}; if it doesn't, an error results. As an example,
5995 @samp{0-9} is the same as @samp{0123456789}.
5997 @sc{gnu} @command{tr} does not support the System V syntax that uses square
5998 brackets to enclose ranges. Translations specified in that format
5999 sometimes work as expected, since the brackets are often transliterated
6000 to themselves. However, they should be avoided because they sometimes
6001 behave unexpectedly. For example, @samp{tr -d '[0-9]'} deletes brackets
6004 Many historically common and even accepted uses of ranges are not
6005 portable. For example, on @acronym{EBCDIC} hosts using the @samp{A-Z}
6006 range will not do what most would expect because @samp{A} through @samp{Z}
6007 are not contiguous as they are in @acronym{ASCII}.
6008 If you can rely on a @acronym{POSIX} compliant version of @command{tr}, then
6009 the best way to work around this is to use character classes (see below).
6010 Otherwise, it is most portable (and most ugly) to enumerate the members
6013 @item Repeated characters
6014 @cindex repeated characters
6016 The notation @samp{[@var{c}*@var{n}]} in @var{set2} expands to @var{n}
6017 copies of character @var{c}. Thus, @samp{[y*6]} is the same as
6018 @samp{yyyyyy}. The notation @samp{[@var{c}*]} in @var{string2} expands
6019 to as many copies of @var{c} as are needed to make @var{set2} as long as
6020 @var{set1}. If @var{n} begins with @samp{0}, it is interpreted in
6021 octal, otherwise in decimal.
6023 @item Character classes
6024 @cindex character classes
6026 The notation @samp{[:@var{class}:]} expands to all of the characters in
6027 the (predefined) class @var{class}. The characters expand in no
6028 particular order, except for the @code{upper} and @code{lower} classes,
6029 which expand in ascending order. When the @option{--delete} (@option{-d})
6030 and @option{--squeeze-repeats} (@option{-s}) options are both given, any
6031 character class can be used in @var{set2}. Otherwise, only the
6032 character classes @code{lower} and @code{upper} are accepted in
6033 @var{set2}, and then only if the corresponding character class
6034 (@code{upper} and @code{lower}, respectively) is specified in the same
6035 relative position in @var{set1}. Doing this specifies case conversion.
6036 The class names are given below; an error results when an invalid class
6048 Horizontal whitespace.
6057 Printable characters, not including space.
6063 Printable characters, including space.
6066 Punctuation characters.
6069 Horizontal or vertical whitespace.
6078 @item Equivalence classes
6079 @cindex equivalence classes
6081 The syntax @samp{[=@var{c}=]} expands to all of the characters that are
6082 equivalent to @var{c}, in no particular order. Equivalence classes are
6083 a relatively recent invention intended to support non-English alphabets.
6084 But there seems to be no standard way to define them or determine their
6085 contents. Therefore, they are not fully implemented in @sc{gnu} @command{tr};
6086 each character's equivalence class consists only of that character,
6087 which is of no particular use.
6093 @subsection Translating
6095 @cindex translating characters
6097 @command{tr} performs translation when @var{set1} and @var{set2} are
6098 both given and the @option{--delete} (@option{-d}) option is not given.
6099 @command{tr} translates each character of its input that is in @var{set1}
6100 to the corresponding character in @var{set2}. Characters not in
6101 @var{set1} are passed through unchanged. When a character appears more
6102 than once in @var{set1} and the corresponding characters in @var{set2}
6103 are not all the same, only the final one is used. For example, these
6104 two commands are equivalent:
6111 A common use of @command{tr} is to convert lowercase characters to
6112 uppercase. This can be done in many ways. Here are three of them:
6115 tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
6117 tr '[:lower:]' '[:upper:]'
6121 But note that using ranges like @code{a-z} above is not portable.
6123 When @command{tr} is performing translation, @var{set1} and @var{set2}
6124 typically have the same length. If @var{set1} is shorter than
6125 @var{set2}, the extra characters at the end of @var{set2} are ignored.
6127 On the other hand, making @var{set1} longer than @var{set2} is not
6128 portable; @acronym{POSIX} says that the result is undefined. In this situation,
6129 BSD @command{tr} pads @var{set2} to the length of @var{set1} by repeating
6130 the last character of @var{set2} as many times as necessary. System V
6131 @command{tr} truncates @var{set1} to the length of @var{set2}.
6133 By default, @sc{gnu} @command{tr} handles this case like BSD @command{tr}.
6134 When the @option{--truncate-set1} (@option{-t}) option is given,
6135 @sc{gnu} @command{tr} handles this case like the System V @command{tr}
6136 instead. This option is ignored for operations other than translation.
6138 Acting like System V @command{tr} in this case breaks the relatively common
6142 tr -cs A-Za-z0-9 '\012'
6146 because it converts only zero bytes (the first element in the
6147 complement of @var{set1}), rather than all non-alphanumerics, to
6151 By the way, the above idiom is not portable because it uses ranges, and
6152 it assumes that the octal code for newline is 012.
6153 Assuming a @acronym{POSIX} compliant @command{tr}, here is a better
6157 tr -cs '[:alnum:]' '[\n*]'
6162 @subsection Squeezing repeats and deleting
6164 @cindex squeezing repeat characters
6165 @cindex deleting characters
6167 When given just the @option{--delete} (@option{-d}) option, @command{tr}
6168 removes any input characters that are in @var{set1}.
6170 When given just the @option{--squeeze-repeats} (@option{-s}) option,
6171 @command{tr} replaces each input sequence of a repeated character that
6172 is in @var{set1} with a single occurrence of that character.
6174 When given both @option{--delete} and @option{--squeeze-repeats}, @command{tr}
6175 first performs any deletions using @var{set1}, then squeezes repeats
6176 from any remaining characters using @var{set2}.
6178 The @option{--squeeze-repeats} option may also be used when translating,
6179 in which case @command{tr} first performs translation, then squeezes
6180 repeats from any remaining characters using @var{set2}.
6182 Here are some examples to illustrate various combinations of options:
6187 Remove all zero bytes:
6194 Put all words on lines by themselves. This converts all
6195 non-alphanumeric characters to newlines, then squeezes each string
6196 of repeated newlines into a single newline:
6199 tr -cs '[:alnum:]' '[\n*]'
6203 Convert each sequence of repeated newlines to a single newline:
6210 Find doubled occurrences of words in a document.
6211 @c Separate the following two "the"s, so typo checkers don't complain.
6212 For example, people often write ``the @w{}the'' with the repeated words
6213 separated by a newline. The Bourne shell script below works first
6214 by converting each sequence of punctuation and blank characters to a
6215 single newline. That puts each ``word'' on a line by itself.
6216 Next it maps all uppercase characters to lower case, and finally it
6217 runs @command{uniq} with the @option{-d} option to print out only the words
6223 | tr -s '[:punct:][:blank:]' '[\n*]' \
6224 | tr '[:upper:]' '[:lower:]' \
6229 Deleting a small set of characters is usually straightforward. For example,
6230 to remove all @samp{a}s, @samp{x}s, and @samp{M}s you would do this:
6236 However, when @samp{-} is one of those characters, it can be tricky because
6237 @samp{-} has special meanings. Performing the same task as above but also
6238 removing all @samp{-} characters, we might try @code{tr -d -axM}, but
6239 that would fail because @command{tr} would try to interpret @option{-a} as
6240 a command-line option. Alternatively, we could try putting the hyphen
6241 inside the string, @code{tr -d a-xM}, but that wouldn't work either because
6242 it would make @command{tr} interpret @code{a-x} as the range of characters
6243 @samp{a}@dots{}@samp{x} rather than the three.
6244 One way to solve the problem is to put the hyphen at the end of the list
6251 Or you can use @samp{--} to terminate option processing:
6257 More generally, use the character class notation @code{[=c=]}
6258 with @samp{-} (or any other character) in place of the @samp{c}:
6264 Note how single quotes are used in the above example to protect the
6265 square brackets from interpretation by a shell.
6270 @node expand invocation
6271 @section @command{expand}: Convert tabs to spaces
6274 @cindex tabs to spaces, converting
6275 @cindex converting tabs to spaces
6277 @command{expand} writes the contents of each given @var{file}, or standard
6278 input if none are given or for a @var{file} of @samp{-}, to standard
6279 output, with tab characters converted to the appropriate number of
6283 expand [@var{option}]@dots{} [@var{file}]@dots{}
6286 By default, @command{expand} converts all tabs to spaces. It preserves
6287 backspace characters in the output; they decrement the column count for
6288 tab calculations. The default action is equivalent to @option{-t 8} (set
6289 tabs every 8 columns).
6291 The program accepts the following options. Also see @ref{Common options}.
6295 @item -t @var{tab1}[,@var{tab2}]@dots{}
6296 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6299 @cindex tab stops, setting
6300 If only one tab stop is given, set the tabs @var{tab1} spaces apart
6301 (default is 8). Otherwise, set the tabs at columns @var{tab1},
6302 @var{tab2}, @dots{} (numbered from 0), and replace any tabs beyond the
6303 last tab stop given with single spaces. Tab stops can be separated by
6304 blanks as well as by commas.
6306 For compatibility, GNU @command{expand} also accepts the obsolete
6307 option syntax, @option{-@var{t1}[,@var{t2}]@dots{}}. New scripts
6308 should use @option{-t @var{t1}[,@var{t2}]@dots{}} instead.
6314 @cindex initial tabs, converting
6315 Only convert initial tabs (those that precede all non-space or non-tab
6316 characters) on each line to spaces.
6323 @node unexpand invocation
6324 @section @command{unexpand}: Convert spaces to tabs
6328 @command{unexpand} writes the contents of each given @var{file}, or
6329 standard input if none are given or for a @var{file} of @samp{-}, to
6330 standard output, converting blanks at the beginning of each line into
6331 as many tab characters as needed. In the default @acronym{POSIX}
6332 locale, a @dfn{blank} is a space or a tab; other locales may specify
6333 additional blank characters. Synopsis:
6336 unexpand [@var{option}]@dots{} [@var{file}]@dots{}
6339 By default, @command{unexpand} converts only initial blanks (those
6340 that precede all non-blank characters) on each line. It
6341 preserves backspace characters in the output; they decrement the column
6342 count for tab calculations. By default, tabs are set at every 8th
6345 The program accepts the following options. Also see @ref{Common options}.
6349 @item -t @var{tab1}[,@var{tab2}]@dots{}
6350 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6353 If only one tab stop is given, set the tabs @var{tab1} columns apart
6354 instead of the default 8. Otherwise, set the tabs at columns
6355 @var{tab1}, @var{tab2}, @dots{} (numbered from 0), and leave blanks
6356 beyond the tab stops given unchanged. Tab stops can be separated by
6357 blanks as well as by commas. This option implies the @option{-a} option.
6359 For compatibility, GNU @command{unexpand} supports the obsolete option syntax,
6360 @option{-@var{tab1}[,@var{tab2}]@dots{}}, where tab stops must be
6361 separated by commas. (Unlike @option{-t}, this obsolete option does
6362 not imply @option{-a}.) New scripts should use @option{--first-only -t
6363 @var{tab1}[,@var{tab2}]@dots{}} instead.
6369 Also convert all sequences of two or more blanks just before a tab stop,
6370 even if they occur after non-blank characters in a line.
6377 @node Directory listing
6378 @chapter Directory listing
6380 This chapter describes the @command{ls} command and its variants @command{dir}
6381 and @command{vdir}, which list information about files.
6384 * ls invocation:: List directory contents.
6385 * dir invocation:: Briefly ls.
6386 * vdir invocation:: Verbosely ls.
6387 * dircolors invocation:: Color setup for ls, etc.
6392 @section @command{ls}: List directory contents
6395 @cindex directory listing
6397 The @command{ls} program lists information about files (of any type,
6398 including directories). Options and file arguments can be intermixed
6399 arbitrarily, as usual.
6401 For non-option command-line arguments that are directories, by default
6402 @command{ls} lists the contents of directories, not recursively, and
6403 omitting files with names beginning with @samp{.}. For other non-option
6404 arguments, by default @command{ls} lists just the file name. If no
6405 non-option argument is specified, @command{ls} operates on the current
6406 directory, acting as if it had been invoked with a single argument of @samp{.}.
6409 By default, the output is sorted alphabetically, according to the locale
6410 settings in effect.@footnote{If you use a non-@acronym{POSIX}
6411 locale (e.g., by setting @env{LC_ALL} to @samp{en_US}), then @command{ls} may
6412 produce output that is sorted differently than you're accustomed to.
6413 In that case, set the @env{LC_ALL} environment variable to @samp{C}.}
6414 If standard output is
6415 a terminal, the output is in columns (sorted vertically) and control
6416 characters are output as question marks; otherwise, the output is listed
6417 one per line and control characters are output as-is.
6419 Because @command{ls} is such a fundamental program, it has accumulated many
6420 options over the years. They are described in the subsections below;
6421 within each section, options are listed alphabetically (ignoring case).
6422 The division of options into the subsections is not absolute, since some
6423 options affect more than one aspect of @command{ls}'s operation.
6425 @cindex exit status of @command{ls}
6430 1 minor problems (e.g., failure to access a file or directory not
6431 specified as a command line argument. This happens when listing a
6432 directory in which entries are actively being removed or renamed.)
6433 2 serious trouble (e.g., memory exhausted, invalid option, failure
6434 to access a file or directory specified as a command line argument
6435 or a directory loop)
6438 Also see @ref{Common options}.
6441 * Which files are listed::
6442 * What information is listed::
6443 * Sorting the output::
6444 * Details about version sort::
6445 * General output formatting::
6446 * Formatting file timestamps::
6447 * Formatting the file names::
6451 @node Which files are listed
6452 @subsection Which files are listed
6454 These options determine which files @command{ls} lists information for.
6455 By default, @command{ls} lists files and the contents of any
6456 directories on the command line, except that in directories it ignores
6457 files whose names start with @samp{.}.
6465 In directories, do not ignore file names that start with @samp{.}.
6470 @opindex --almost-all
6471 In directories, do not ignore all file names that start with @samp{.};
6472 ignore only @file{.} and @file{..}. The @option{--all} (@option{-a})
6473 option overrides this option.
6476 @itemx --ignore-backups
6478 @opindex --ignore-backups
6479 @cindex backup files, ignoring
6480 In directories, ignore files that end with @samp{~}. This option is
6481 equivalent to @samp{--ignore='*~' --ignore='.*~'}.
6486 @opindex --directory
6487 List just the names of directories, as with other types of files, rather
6488 than listing their contents.
6489 @c The following sentence is the same as the one for -F.
6490 Do not follow symbolic links listed on the
6491 command line unless the @option{--dereference-command-line} (@option{-H}),
6492 @option{--dereference} (@option{-L}), or
6493 @option{--dereference-command-line-symlink-to-dir} options are specified.
6496 @itemx --dereference-command-line
6498 @opindex --dereference-command-line
6499 @cindex symbolic links, dereferencing
6500 If a command line argument specifies a symbolic link, show information
6501 for the file the link references rather than for the link itself.
6503 @itemx --dereference-command-line-symlink-to-dir
6504 @opindex --dereference-command-line-symlink-to-dir
6505 @cindex symbolic links, dereferencing
6506 Do not dereference symbolic links, with one exception:
6507 if a command line argument specifies a symbolic link that refers to
6508 a directory, show information for that directory rather than for the
6510 This is the default behavior when no other dereferencing-related
6511 option has been specified (@option{--classify} (@option{-F}),
6512 @option{--directory} (@option{-d}),
6514 @option{--dereference} (@option{-L}), or
6515 @option{--dereference-command-line} (@option{-H})).
6517 @item --group-directories-first
6518 @opindex --group-directories-first
6519 Group all the directories before the files and then sort the
6520 directories and the files separately using the selected sort key
6521 (see --sort option).
6522 That is, this option specifies a primary sort key,
6523 and the --sort option specifies a secondary key.
6524 However, any use of @option{--sort=none}
6525 (@option{-U}) disables this option altogether.
6527 @item --hide=PATTERN
6528 @opindex --hide=@var{pattern}
6529 In directories, ignore files whose names match the shell pattern
6530 @var{pattern}, unless the @option{--all} (@option{-a}) or
6531 @option{--almost-all} (@option{-A}) is also given. This
6532 option acts like @option{--ignore=@var{pattern}} except that it has no
6533 effect if @option{--all} (@option{-a}) or @option{--almost-all}
6534 (@option{-A}) is also given.
6536 This option can be useful in shell aliases. For example, if
6537 @command{lx} is an alias for @samp{ls --hide='*~'} and @command{ly} is
6538 an alias for @samp{ls --ignore='*~'}, then the command @samp{lx -A}
6539 lists the file @file{README~} even though @samp{ly -A} would not.
6541 @item -I @var{pattern}
6542 @itemx --ignore=@var{pattern}
6544 @opindex --ignore=@var{pattern}
6545 In directories, ignore files whose names match the shell pattern
6546 (not regular expression) @var{pattern}. As
6547 in the shell, an initial @samp{.} in a file name does not match a
6548 wildcard at the start of @var{pattern}. Sometimes it is useful
6549 to give this option several times. For example,
6552 $ ls --ignore='.??*' --ignore='.[^.]' --ignore='#*'
6555 The first option ignores names of length 3 or more that start with @samp{.},
6556 the second ignores all two-character names that start with @samp{.}
6557 except @samp{..}, and the third ignores names that start with @samp{#}.
6560 @itemx --dereference
6562 @opindex --dereference
6563 @cindex symbolic links, dereferencing
6564 When showing file information for a symbolic link, show information
6565 for the file the link references rather than the link itself.
6566 However, even with this option, @command{ls} still prints the name
6567 of the link itself, not the name of the file that the link points to.
6572 @opindex --recursive
6573 @cindex recursive directory listing
6574 @cindex directory listing, recursive
6575 List the contents of all directories recursively.
6580 @node What information is listed
6581 @subsection What information is listed
6583 These options affect the information that @command{ls} displays. By
6584 default, only file names are shown.
6590 @cindex hurd, author, printing
6591 List each file's author when producing long format directory listings.
6592 In GNU/Hurd, file authors can differ from their owners, but in other
6593 operating systems the two are the same.
6599 @cindex dired Emacs mode support
6600 With the long listing (@option{-l}) format, print an additional line after
6604 //DIRED// @var{beg1} @var{end1} @var{beg2} @var{end2} @dots{}
6608 The @var{begn} and @var{endn} are unsigned integers that record the
6609 byte position of the beginning and end of each file name in the output.
6610 This makes it easy for Emacs to find the names, even when they contain
6611 unusual characters such as space or newline, without fancy searching.
6613 If directories are being listed recursively (@option{-R}), output a similar
6614 line with offsets for each subdirectory name:
6617 //SUBDIRED// @var{beg1} @var{end1} @dots{}
6620 Finally, output a line of the form:
6623 //DIRED-OPTIONS// --quoting-style=@var{word}
6627 where @var{word} is the quoting style (@pxref{Formatting the file names}).
6629 Here is an actual example:
6632 $ mkdir -p a/sub/deeper a/sub2
6634 $ touch a/sub/deeper/file
6635 $ ls -gloRF --dired a
6638 -rw-r--r-- 1 0 Jun 10 12:27 f1
6639 -rw-r--r-- 1 0 Jun 10 12:27 f2
6640 drwxr-xr-x 3 4096 Jun 10 12:27 sub/
6641 drwxr-xr-x 2 4096 Jun 10 12:27 sub2/
6645 drwxr-xr-x 2 4096 Jun 10 12:27 deeper/
6649 -rw-r--r-- 1 0 Jun 10 12:27 file
6653 //DIRED// 48 50 84 86 120 123 158 162 217 223 282 286
6654 //SUBDIRED// 2 3 167 172 228 240 290 296
6655 //DIRED-OPTIONS// --quoting-style=literal
6658 Note that the pairs of offsets on the @samp{//DIRED//} line above delimit
6659 these names: @file{f1}, @file{f2}, @file{sub}, @file{sub2}, @file{deeper},
6661 The offsets on the @samp{//SUBDIRED//} line delimit the following
6662 directory names: @file{a}, @file{a/sub}, @file{a/sub/deeper}, @file{a/sub2}.
6664 Here is an example of how to extract the fifth entry name, @samp{deeper},
6665 corresponding to the pair of offsets, 222 and 228:
6668 $ ls -gloRF --dired a > out
6669 $ dd bs=1 skip=222 count=6 < out 2>/dev/null; echo
6673 Note that although the listing above includes a trailing slash
6674 for the @samp{deeper} entry, the offsets select the name without
6675 the trailing slash. However, if you invoke @command{ls} with @option{--dired}
6676 along with an option like @option{--escape} (aka @option{-b}) and operate
6677 on a file whose name contains special characters, notice that the backslash
6682 $ ls -blog --dired 'a b'
6683 -rw-r--r-- 1 0 Jun 10 12:28 a\ b
6685 //DIRED-OPTIONS// --quoting-style=escape
6688 If you use a quoting style that adds quote marks
6689 (e.g., @option{--quoting-style=c}), then the offsets include the quote marks.
6690 So beware that the user may select the quoting style via the environment
6691 variable @env{QUOTING_STYLE}. Hence, applications using @option{--dired}
6692 should either specify an explicit @option{--quoting-style=literal} option
6693 (aka @option{-N} or @option{--literal}) on the command line, or else be
6694 prepared to parse the escaped names.
6697 @opindex --full-time
6698 Produce long format directory listings, and list times in full. It is
6699 equivalent to using @option{--format=long} with
6700 @option{--time-style=full-iso} (@pxref{Formatting file timestamps}).
6704 Produce long format directory listings, but don't display owner information.
6710 Inhibit display of group information in a long format directory listing.
6711 (This is the default in some non-@sc{gnu} versions of @command{ls}, so we
6712 provide this option for compatibility.)
6720 @cindex inode number, printing
6721 Print the inode number (also called the file serial number and index
6722 number) of each file to the left of the file name. (This number
6723 uniquely identifies each file within a particular file system.)
6726 @itemx --format=long
6727 @itemx --format=verbose
6730 @opindex long ls @r{format}
6731 @opindex verbose ls @r{format}
6732 In addition to the name of each file, print the file type, file mode bits,
6733 number of hard links, owner name, group name, size, and
6734 timestamp (@pxref{Formatting file timestamps}), normally
6735 the modification time. Print question marks for information that
6736 cannot be determined.
6738 Normally the size is printed as a byte count without punctuation, but
6739 this can be overridden (@pxref{Block size}). For example, @option{-h}
6740 prints an abbreviated, human-readable count, and
6741 @samp{--block-size="'1"} prints a byte count with the thousands
6742 separator of the current locale.
6744 For each directory that is listed, preface the files with a line
6745 @samp{total @var{blocks}}, where @var{blocks} is the total disk allocation
6746 for all files in that directory. The block size currently defaults to 1024
6747 bytes, but this can be overridden (@pxref{Block size}).
6748 The @var{blocks} computed counts each hard link separately;
6749 this is arguably a deficiency.
6751 The file type is one of the following characters:
6753 @c The commented-out entries are ones we're not sure about.
6761 character special file
6763 high performance (``contiguous data'') file
6767 door (Solaris 2.5 and up)
6769 @c semaphore, if this is a distinct file type
6773 @c multiplexed file (7th edition Unix; obsolete)
6775 off-line (``migrated'') file (Cray DMF)
6777 network special file (HP-UX)
6781 port (Solaris 10 and up)
6783 @c message queue, if this is a distinct file type
6787 @c shared memory object, if this is a distinct file type
6789 @c typed memory object, if this is a distinct file type
6791 @c whiteout (4.4BSD; not implemented)
6793 some other file type
6796 @cindex permissions, output by @command{ls}
6797 The file mode bits listed are similar to symbolic mode specifications
6798 (@pxref{Symbolic Modes}). But @command{ls} combines multiple bits into the
6799 third character of each set of permissions as follows:
6803 If the set-user-ID or set-group-ID bit and the corresponding executable bit
6807 If the set-user-ID or set-group-ID bit is set but the corresponding
6808 executable bit is not set.
6811 If the restricted deletion flag or sticky bit, and the
6812 other-executable bit, are both set. The restricted deletion flag is
6813 another name for the sticky bit. @xref{Mode Structure}.
6816 If the restricted deletion flag or sticky bit is set but the
6817 other-executable bit is not set.
6820 If the executable bit is set and none of the above apply.
6826 Following the file mode bits is a single character that specifies
6827 whether an alternate access method such as an access control list
6828 applies to the file. When the character following the file mode bits is a
6829 space, there is no alternate access method. When it is a printing
6830 character, then there is such a method.
6832 GNU @command{ls} uses a @samp{.} character to indicate a file
6833 with an SELinux security context, but no other alternate access method.
6835 A file with any other combination of alternate access methods
6836 is marked with a @samp{+} character.
6839 @itemx --numeric-uid-gid
6841 @opindex --numeric-uid-gid
6842 @cindex numeric uid and gid
6843 @cindex numeric user and group IDs
6844 Produce long format directory listings, but
6845 display numeric user and group IDs instead of the owner and group names.
6849 Produce long format directory listings, but don't display group information.
6850 It is equivalent to using @option{--format=long} with @option{--no-group} .
6856 @cindex disk allocation
6857 @cindex size of files, reporting
6858 Print the disk allocation of each file to the left of the file name.
6859 This is the amount of disk space used by the file, which is usually a
6860 bit more than the file's size, but it can be less if the file has holes.
6862 Normally the disk allocation is printed in units of
6863 1024 bytes, but this can be overridden (@pxref{Block size}).
6865 @cindex NFS mounts from BSD to HP-UX
6866 For files that are NFS-mounted from an HP-UX system to a BSD system,
6867 this option reports sizes that are half the correct values. On HP-UX
6868 systems, it reports sizes that are twice the correct values for files
6869 that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX;
6870 it also affects the HP-UX @command{ls} program.
6879 @cindex security context
6880 Display the SELinux security context or @samp{?} if none is found.
6881 When used with the @option{-l} option, print the security context
6882 to the left of the size column.
6887 @node Sorting the output
6888 @subsection Sorting the output
6890 @cindex sorting @command{ls} output
6891 These options change the order in which @command{ls} sorts the information
6892 it outputs. By default, sorting is done by character code
6893 (e.g., @acronym{ASCII} order).
6899 @itemx --time=status
6902 @opindex ctime@r{, printing or sorting by}
6903 @opindex status time@r{, printing or sorting by}
6904 @opindex use time@r{, printing or sorting files by}
6905 If the long listing format (e.g., @option{-l}, @option{-o}) is being used,
6906 print the status change time (the @samp{ctime} in the inode) instead of
6907 the modification time.
6908 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6909 or when not using a long listing format,
6910 sort according to the status change time.
6914 @cindex unsorted directory listing
6915 @cindex directory order, listing by
6916 Primarily, like @option{-U}---do not sort; list the files in whatever
6917 order they are stored in the directory. But also enable @option{-a} (list
6918 all files) and disable @option{-l}, @option{--color}, and @option{-s} (if they
6919 were specified before the @option{-f}).
6925 @cindex reverse sorting
6926 Reverse whatever the sorting method is---e.g., list files in reverse
6927 alphabetical order, youngest first, smallest first, or whatever.
6933 @opindex size of files@r{, sorting files by}
6934 Sort by file size, largest first.
6940 @opindex modification time@r{, sorting files by}
6941 Sort by modification time (the @samp{mtime} in the inode), newest first.
6945 @itemx --time=access
6949 @opindex use time@r{, printing or sorting files by}
6950 @opindex atime@r{, printing or sorting files by}
6951 @opindex access time@r{, printing or sorting files by}
6952 If the long listing format (e.g., @option{--format=long}) is being used,
6953 print the last access time (the @samp{atime} in the inode).
6954 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6955 or when not using a long listing format, sort according to the access time.
6961 @opindex none@r{, sorting option for @command{ls}}
6962 Do not sort; list the files in whatever order they are
6963 stored in the directory. (Do not do any of the other unrelated things
6964 that @option{-f} does.) This is especially useful when listing very large
6965 directories, since not doing any sorting can be noticeably faster.
6968 @itemx --sort=version
6971 @opindex version@r{, sorting option for @command{ls}}
6972 Sort by version name and number, lowest first. It behaves like a default
6973 sort, except that each sequence of decimal digits is treated numerically
6974 as an index/version number. (@xref{Details about version sort}.)
6977 @itemx --sort=extension
6980 @opindex extension@r{, sorting files by}
6981 Sort directory contents alphabetically by file extension (characters
6982 after the last @samp{.}); files with no extension are sorted first.
6987 @node Details about version sort
6988 @subsection Details about version sort
6990 Version sorting handles the fact that file names frequently include indices or
6991 version numbers. Standard sorting usually does not produce the order that one
6992 expects because comparisons are made on a character-by-character basis.
6993 Version sorting is especially useful when browsing directories that contain
6994 many files with indices/version numbers in their names:
6998 abc.zml-1.gz abc.zml-1.gz
6999 abc.zml-12.gz abc.zml-2.gz
7000 abc.zml-2.gz abc.zml-12.gz
7003 Version-sorted strings are compared such that if @var{ver1} and @var{ver2}
7004 are version numbers and @var{prefix} and @var{suffix} (@var{suffix} matching
7005 the regular expression @samp{(\.[A-Za-z~][A-Za-z0-9~]*)*}) are strings then
7006 @var{ver1} < @var{ver2} implies that the name composed of
7007 ``@var{prefix} @var{ver1} @var{suffix}'' sorts before
7008 ``@var{prefix} @var{ver2} @var{suffix}''.
7010 Note also that leading zeros of numeric parts are ignored:
7014 abc-1.007.tgz abc-1.01a.tgz
7015 abc-1.012b.tgz abc-1.007.tgz
7016 abc-1.01a.tgz abc-1.012b.tgz
7019 This functionality is implemented using gnulib's @code{filevercmp} function,
7020 which has some caveats worth noting.
7023 @item @env{LC_COLLATE} is ignored, which means @samp{ls -v} and @samp{sort -V}
7024 will sort non-numeric prefixes as if the @env{LC_COLLATE} locale category
7025 was set to @samp{C}.
7026 @item Some suffixes will not be matched by the regular
7027 expression mentioned above. Consequently these examples may
7028 not sort as you expect:
7036 abc-1.2.3.4.x86_64.rpm
7037 abc-1.2.3.x86_64.rpm
7041 @node General output formatting
7042 @subsection General output formatting
7044 These options affect the appearance of the overall output.
7049 @itemx --format=single-column
7052 @opindex single-column @r{output of files}
7053 List one file per line. This is the default for @command{ls} when standard
7054 output is not a terminal.
7057 @itemx --format=vertical
7060 @opindex vertical @r{sorted files in columns}
7061 List files in columns, sorted vertically. This is the default for
7062 @command{ls} if standard output is a terminal. It is always the default
7063 for the @command{dir} program.
7064 @sc{gnu} @command{ls} uses variable width columns to display as many files as
7065 possible in the fewest lines.
7067 @item --color [=@var{when}]
7069 @cindex color, distinguishing file types with
7070 Specify whether to use color for distinguishing file types. @var{when}
7071 may be omitted, or one of:
7074 @vindex none @r{color option}
7075 - Do not use color at all. This is the default.
7077 @vindex auto @r{color option}
7078 @cindex terminal, using color iff
7079 - Only use color if standard output is a terminal.
7081 @vindex always @r{color option}
7084 Specifying @option{--color} and no @var{when} is equivalent to
7085 @option{--color=always}.
7086 Piping a colorized listing through a pager like @command{more} or
7087 @command{less} usually produces unreadable results. However, using
7088 @code{more -f} does seem to work.
7091 @vindex SHELL @r{environment variable, and color}
7092 Note that using the @option{--color} option may incur a noticeable
7093 performance penalty when run in a directory with very many entries,
7094 because the default settings require that @command{ls} @code{stat} every
7095 single file it lists.
7096 However, if you would like most of the file-type coloring
7097 but can live without the other coloring options (e.g.,
7098 executable, orphan, sticky, other-writable, capability), use
7099 @command{dircolors} to set the @env{LS_COLORS} environment variable like this,
7101 eval $(dircolors -p | perl -pe \
7102 's/^((CAP|S[ET]|O[TR]|M|E)\w+).*/$1 00/' | dircolors -)
7104 and on a @code{dirent.d_type}-capable file system, @command{ls}
7105 will perform only one @code{stat} call per command line argument.
7109 @itemx --indicator-style=classify
7112 @opindex --indicator-style
7113 @cindex file type and executables, marking
7114 @cindex executables and file type, marking
7115 Append a character to each file name indicating the file type. Also,
7116 for regular files that are executable, append @samp{*}. The file type
7117 indicators are @samp{/} for directories, @samp{@@} for symbolic links,
7118 @samp{|} for FIFOs, @samp{=} for sockets, @samp{>} for doors,
7119 and nothing for regular files.
7120 @c The following sentence is the same as the one for -d.
7121 Do not follow symbolic links listed on the
7122 command line unless the @option{--dereference-command-line} (@option{-H}),
7123 @option{--dereference} (@option{-L}), or
7124 @option{--dereference-command-line-symlink-to-dir} options are specified.
7127 @itemx --indicator-style=file-type
7128 @opindex --file-type
7129 @opindex --indicator-style
7130 @cindex file type, marking
7131 Append a character to each file name indicating the file type. This is
7132 like @option{-F}, except that executables are not marked.
7134 @item --indicator-style=@var{word}
7135 @opindex --indicator-style
7136 Append a character indicator with style @var{word} to entry names,
7141 Do not append any character indicator; this is the default.
7143 Append @samp{/} for directories. This is the same as the @option{-p}
7146 Append @samp{/} for directories, @samp{@@} for symbolic links, @samp{|}
7147 for FIFOs, @samp{=} for sockets, and nothing for regular files. This is
7148 the same as the @option{--file-type} option.
7150 Append @samp{*} for executable regular files, otherwise behave as for
7151 @samp{file-type}. This is the same as the @option{-F} or
7152 @option{--classify} option.
7158 @opindex --kibibytes
7159 Set the default block size to its normal value of 1024 bytes,
7160 overriding any contrary specification in environment variables
7161 (@pxref{Block size}). This option is in turn overridden by the
7162 @option{--block-size}, @option{-h} or @option{--human-readable}, and
7163 @option{--si} options.
7165 The @option{-k} or @option{--kibibytes} option affects the
7166 per-directory block count written by the @option{-l} and similar
7167 options, and the size written by the @option{-s} or @option{--size}
7168 option. It does not affect the file size written by @option{-l}.
7171 @itemx --format=commas
7174 @opindex commas@r{, outputting between files}
7175 List files horizontally, with as many as will fit on each line,
7176 separated by @samp{, } (a comma and a space).
7179 @itemx --indicator-style=slash
7181 @opindex --indicator-style
7182 @cindex file type, marking
7183 Append a @samp{/} to directory names.
7186 @itemx --format=across
7187 @itemx --format=horizontal
7190 @opindex across@r{, listing files}
7191 @opindex horizontal@r{, listing files}
7192 List the files in columns, sorted horizontally.
7195 @itemx --tabsize=@var{cols}
7198 Assume that each tab stop is @var{cols} columns wide. The default is 8.
7199 @command{ls} uses tabs where possible in the output, for efficiency. If
7200 @var{cols} is zero, do not use tabs at all.
7202 @c FIXME: remove in 2009, if Apple Terminal has been fixed for long enough.
7203 Some terminal emulators (at least Apple Terminal 1.5 (133) from Mac OS X 10.4.8)
7204 do not properly align columns to the right of a TAB following a
7205 non-@acronym{ASCII} byte. If you use such a terminal emulator, use the
7206 @option{-T0} option or put @code{TABSIZE=0} in your environment to tell
7207 @command{ls} to align using spaces, not tabs.
7210 @itemx --width=@var{cols}
7214 Assume the screen is @var{cols} columns wide. The default is taken
7215 from the terminal settings if possible; otherwise the environment
7216 variable @env{COLUMNS} is used if it is set; otherwise the default
7222 @node Formatting file timestamps
7223 @subsection Formatting file timestamps
7225 By default, file timestamps are listed in abbreviated form, using
7226 a date like @samp{Mar 30@ @ 2002} for non-recent timestamps, and a
7227 date-without-year and time like @samp{Mar 30 23:45} for recent timestamps.
7228 This format can change depending on the current locale as detailed below.
7231 A timestamp is considered to be @dfn{recent} if it is less than six
7232 months old, and is not dated in the future. If a timestamp dated
7233 today is not listed in recent form, the timestamp is in the future,
7234 which means you probably have clock skew problems which may break
7235 programs like @command{make} that rely on file timestamps.
7238 Time stamps are listed according to the time zone rules specified by
7239 the @env{TZ} environment variable, or by the system default rules if
7240 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
7241 with @env{TZ}, libc, The GNU C Library Reference Manual}.
7243 The following option changes how file timestamps are printed.
7246 @item --time-style=@var{style}
7247 @opindex --time-style
7249 List timestamps in style @var{style}. The @var{style} should
7250 be one of the following:
7255 List timestamps using @var{format}, where @var{format} is interpreted
7256 like the format argument of @command{date} (@pxref{date invocation}).
7257 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
7258 @command{ls} to list timestamps like @samp{2002-03-30 23:45:56}. As
7259 with @command{date}, @var{format}'s interpretation is affected by the
7260 @env{LC_TIME} locale category.
7262 If @var{format} contains two format strings separated by a newline,
7263 the former is used for non-recent files and the latter for recent
7264 files; if you want output columns to line up, you may need to insert
7265 spaces in one of the two formats.
7268 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
7269 format with nanosecond precision, e.g., @samp{2002-03-30
7270 23:45:56.477817180 -0700}. This style is equivalent to
7271 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
7273 This is useful because the time output includes all the information that
7274 is available from the operating system. For example, this can help
7275 explain @command{make}'s behavior, since @acronym{GNU} @command{make}
7276 uses the full timestamp to determine whether a file is out of date.
7279 List @acronym{ISO} 8601 date and time in minutes, e.g.,
7280 @samp{2002-03-30 23:45}. These timestamps are shorter than
7281 @samp{full-iso} timestamps, and are usually good enough for everyday
7282 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
7285 List @acronym{ISO} 8601 dates for non-recent timestamps (e.g.,
7286 @samp{2002-03-30@ }), and @acronym{ISO} 8601 month, day, hour, and
7287 minute for recent timestamps (e.g., @samp{03-30 23:45}). These
7288 timestamps are uglier than @samp{long-iso} timestamps, but they carry
7289 nearly the same information in a smaller space and their brevity helps
7290 @command{ls} output fit within traditional 80-column output lines.
7291 The following two @command{ls} invocations are equivalent:
7296 ls -l --time-style="+%Y-%m-%d $newline%m-%d %H:%M"
7297 ls -l --time-style="iso"
7302 List timestamps in a locale-dependent form. For example, a Finnish
7303 locale might list non-recent timestamps like @samp{maalis 30@ @ 2002}
7304 and recent timestamps like @samp{maalis 30 23:45}. Locale-dependent
7305 timestamps typically consume more space than @samp{iso} timestamps and
7306 are harder for programs to parse because locale conventions vary so
7307 widely, but they are easier for many people to read.
7309 The @env{LC_TIME} locale category specifies the timestamp format. The
7310 default @acronym{POSIX} locale uses timestamps like @samp{Mar 30@
7311 @ 2002} and @samp{Mar 30 23:45}; in this locale, the following two
7312 @command{ls} invocations are equivalent:
7317 ls -l --time-style="+%b %e %Y$newline%b %e %H:%M"
7318 ls -l --time-style="locale"
7321 Other locales behave differently. For example, in a German locale,
7322 @option{--time-style="locale"} might be equivalent to
7323 @option{--time-style="+%e. %b %Y $newline%e. %b %H:%M"}
7324 and might generate timestamps like @samp{30. M@"ar 2002@ } and
7325 @samp{30. M@"ar 23:45}.
7327 @item posix-@var{style}
7329 List @acronym{POSIX}-locale timestamps if the @env{LC_TIME} locale
7330 category is @acronym{POSIX}, @var{style} timestamps otherwise. For
7331 example, the @samp{posix-long-iso} style lists
7332 timestamps like @samp{Mar 30@ @ 2002} and @samp{Mar 30 23:45} when in
7333 the @acronym{POSIX} locale, and like @samp{2002-03-30 23:45} otherwise.
7338 You can specify the default value of the @option{--time-style} option
7339 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
7340 the default style is @samp{locale}. @acronym{GNU} Emacs 21.3 and
7341 later use the @option{--dired} option and therefore can parse any date
7342 format, but if you are using Emacs 21.1 or 21.2 and specify a
7343 non-@acronym{POSIX} locale you may need to set
7344 @samp{TIME_STYLE="posix-long-iso"}.
7346 To avoid certain denial-of-service attacks, timestamps that would be
7347 longer than 1000 bytes may be treated as errors.
7350 @node Formatting the file names
7351 @subsection Formatting the file names
7353 These options change how file names themselves are printed.
7359 @itemx --quoting-style=escape
7362 @opindex --quoting-style
7363 @cindex backslash sequences for file names
7364 Quote nongraphic characters in file names using alphabetic and octal
7365 backslash sequences like those used in C.
7369 @itemx --quoting-style=literal
7372 @opindex --quoting-style
7373 Do not quote file names. However, with @command{ls} nongraphic
7374 characters are still printed as question marks if the output is a
7375 terminal and you do not specify the @option{--show-control-chars}
7379 @itemx --hide-control-chars
7381 @opindex --hide-control-chars
7382 Print question marks instead of nongraphic characters in file names.
7383 This is the default if the output is a terminal and the program is
7388 @itemx --quoting-style=c
7390 @opindex --quote-name
7391 @opindex --quoting-style
7392 Enclose file names in double quotes and quote nongraphic characters as
7395 @item --quoting-style=@var{word}
7396 @opindex --quoting-style
7397 @cindex quoting style
7398 Use style @var{word} to quote file names and other strings that may
7399 contain arbitrary characters. The @var{word} should
7400 be one of the following:
7404 Output strings as-is; this is the same as the @option{-N} or
7405 @option{--literal} option.
7407 Quote strings for the shell if they contain shell metacharacters or would
7408 cause ambiguous output.
7409 The quoting is suitable for @acronym{POSIX}-compatible shells like
7410 @command{bash}, but it does not always work for incompatible shells
7413 Quote strings for the shell, even if they would normally not require quoting.
7415 Quote strings as for C character string literals, including the
7416 surrounding double-quote characters; this is the same as the
7417 @option{-Q} or @option{--quote-name} option.
7419 Quote strings as for C character string literals, except omit the
7420 surrounding double-quote
7421 characters; this is the same as the @option{-b} or @option{--escape} option.
7423 Quote strings as for C character string literals, except use
7424 surrounding quotation marks appropriate for the
7427 @c Use @t instead of @samp to avoid duplicate quoting in some output styles.
7428 Quote strings as for C character string literals, except use
7429 surrounding quotation marks appropriate for the locale, and quote
7430 @t{'like this'} instead of @t{"like
7431 this"} in the default C locale. This looks nicer on many displays.
7434 You can specify the default value of the @option{--quoting-style} option
7435 with the environment variable @env{QUOTING_STYLE}. If that environment
7436 variable is not set, the default value is @samp{literal}, but this
7437 default may change to @samp{shell} in a future version of this package.
7439 @item --show-control-chars
7440 @opindex --show-control-chars
7441 Print nongraphic characters as-is in file names.
7442 This is the default unless the output is a terminal and the program is
7448 @node dir invocation
7449 @section @command{dir}: Briefly list directory contents
7452 @cindex directory listing, brief
7454 @command{dir} is equivalent to @code{ls -C
7455 -b}; that is, by default files are listed in columns, sorted vertically,
7456 and special characters are represented by backslash escape sequences.
7458 @xref{ls invocation, @command{ls}}.
7461 @node vdir invocation
7462 @section @command{vdir}: Verbosely list directory contents
7465 @cindex directory listing, verbose
7467 @command{vdir} is equivalent to @code{ls -l
7468 -b}; that is, by default files are listed in long format and special
7469 characters are represented by backslash escape sequences.
7471 @node dircolors invocation
7472 @section @command{dircolors}: Color setup for @command{ls}
7476 @cindex setup for color
7478 @command{dircolors} outputs a sequence of shell commands to set up the
7479 terminal for color output from @command{ls} (and @command{dir}, etc.).
7483 eval "$(dircolors [@var{option}]@dots{} [@var{file}])"
7486 If @var{file} is specified, @command{dircolors} reads it to determine which
7487 colors to use for which file types and extensions. Otherwise, a
7488 precompiled database is used. For details on the format of these files,
7489 run @samp{dircolors --print-database}.
7491 To make @command{dircolors} read a @file{~/.dircolors} file if it
7492 exists, you can put the following lines in your @file{~/.bashrc} (or
7493 adapt them to your favorite shell):
7497 test -r $d && eval "$(dircolors $d)"
7501 @vindex SHELL @r{environment variable, and color}
7502 The output is a shell command to set the @env{LS_COLORS} environment
7503 variable. You can specify the shell syntax to use on the command line,
7504 or @command{dircolors} will guess it from the value of the @env{SHELL}
7505 environment variable.
7507 The program accepts the following options. Also see @ref{Common options}.
7512 @itemx --bourne-shell
7515 @opindex --bourne-shell
7516 @cindex Bourne shell syntax for color setup
7517 @cindex @command{sh} syntax for color setup
7518 Output Bourne shell commands. This is the default if the @env{SHELL}
7519 environment variable is set and does not end with @samp{csh} or
7528 @cindex C shell syntax for color setup
7529 @cindex @command{csh} syntax for color setup
7530 Output C shell commands. This is the default if @code{SHELL} ends with
7531 @command{csh} or @command{tcsh}.
7534 @itemx --print-database
7536 @opindex --print-database
7537 @cindex color database, printing
7538 @cindex database for color setup, printing
7539 @cindex printing color database
7540 Print the (compiled-in) default color configuration database. This
7541 output is itself a valid configuration file, and is fairly descriptive
7542 of the possibilities.
7549 @node Basic operations
7550 @chapter Basic operations
7552 @cindex manipulating files
7554 This chapter describes the commands for basic file manipulation:
7555 copying, moving (renaming), and deleting (removing).
7558 * cp invocation:: Copy files.
7559 * dd invocation:: Convert and copy a file.
7560 * install invocation:: Copy files and set attributes.
7561 * mv invocation:: Move (rename) files.
7562 * rm invocation:: Remove files or directories.
7563 * shred invocation:: Remove files more securely.
7568 @section @command{cp}: Copy files and directories
7571 @cindex copying files and directories
7572 @cindex files, copying
7573 @cindex directories, copying
7575 @command{cp} copies files (or, optionally, directories). The copy is
7576 completely independent of the original. You can either copy one file to
7577 another, or copy arbitrarily many files to a destination directory.
7581 cp [@var{option}]@dots{} [-T] @var{source} @var{dest}
7582 cp [@var{option}]@dots{} @var{source}@dots{} @var{directory}
7583 cp [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
7588 If two file names are given, @command{cp} copies the first file to the
7592 If the @option{--target-directory} (@option{-t}) option is given, or
7593 failing that if the last file is a directory and the
7594 @option{--no-target-directory} (@option{-T}) option is not given,
7595 @command{cp} copies each @var{source} file to the specified directory,
7596 using the @var{source}s' names.
7599 Generally, files are written just as they are read. For exceptions,
7600 see the @option{--sparse} option below.
7602 By default, @command{cp} does not copy directories. However, the
7603 @option{-R}, @option{-a}, and @option{-r} options cause @command{cp} to
7604 copy recursively by descending into source directories and copying files
7605 to corresponding destination directories.
7607 When copying from a symbolic link, @command{cp} normally follows the
7608 link only when not copying
7609 recursively. This default can be overridden with the
7610 @option{--archive} (@option{-a}), @option{-d}, @option{--dereference}
7611 (@option{-L}), @option{--no-dereference} (@option{-P}), and
7612 @option{-H} options. If more than one of these options is specified,
7613 the last one silently overrides the others.
7615 When copying to a symbolic link, @command{cp} follows the
7616 link only when it refers to an existing regular file.
7617 However, when copying to a dangling symbolic link, @command{cp}
7618 refuses by default, and fails with a diagnostic, since the operation
7619 is inherently dangerous. This behavior is contrary to historical
7620 practice and to @acronym{POSIX}.
7621 Set @env{POSIXLY_CORRECT} to make @command{cp} attempt to create
7622 the target of a dangling destination symlink, in spite of the possible risk.
7623 Also, when an option like
7624 @option{--backup} or @option{--link} acts to rename or remove the
7625 destination before copying, @command{cp} renames or removes the
7626 symbolic link rather than the file it points to.
7628 By default, @command{cp} copies the contents of special files only
7629 when not copying recursively. This default can be overridden with the
7630 @option{--copy-contents} option.
7632 @cindex self-backups
7633 @cindex backups, making only
7634 @command{cp} generally refuses to copy a file onto itself, with the
7635 following exception: if @option{--force --backup} is specified with
7636 @var{source} and @var{dest} identical, and referring to a regular file,
7637 @command{cp} will make a backup file, either regular or numbered, as
7638 specified in the usual ways (@pxref{Backup options}). This is useful when
7639 you simply want to make a backup of an existing file before changing it.
7641 The program accepts the following options. Also see @ref{Common options}.
7648 Preserve as much as possible of the structure and attributes of the
7649 original files in the copy (but do not attempt to preserve internal
7650 directory structure; i.e., @samp{ls -U} may list the entries in a copied
7651 directory in a different order).
7652 Try to preserve SELinux security context and extended attributes (xattr),
7653 but ignore any failure to do that and print no corresponding diagnostic.
7654 Equivalent to @option{-dR --preserve=all} with the reduced diagnostics.
7656 @itemx --attributes-only
7657 @opindex --attributes-only
7658 Copy only the specified attributes of the source file to the destination.
7659 If the destination already exists, do not alter its contents.
7660 See the @option{--preserve} option for controlling which attributes to copy.
7663 @itemx @w{@kbd{--backup}[=@var{method}]}
7666 @vindex VERSION_CONTROL
7667 @cindex backups, making
7668 @xref{Backup options}.
7669 Make a backup of each file that would otherwise be overwritten or removed.
7670 As a special case, @command{cp} makes a backup of @var{source} when the force
7671 and backup options are given and @var{source} and @var{dest} are the same
7672 name for an existing, regular file. One useful application of this
7673 combination of options is this tiny Bourne shell script:
7677 # Usage: backup FILE...
7678 # Create a @sc{gnu}-style backup of each listed FILE.
7680 cp --backup --force -- "$i" "$i"
7684 @item --copy-contents
7685 @cindex directories, copying recursively
7686 @cindex copying directories recursively
7687 @cindex recursively copying directories
7688 @cindex non-directories, copying as special files
7689 If copying recursively, copy the contents of any special files (e.g.,
7690 FIFOs and device files) as if they were regular files. This means
7691 trying to read the data in each source file and writing it to the
7692 destination. It is usually a mistake to use this option, as it
7693 normally has undesirable effects on special files like FIFOs and the
7694 ones typically found in the @file{/dev} directory. In most cases,
7695 @code{cp -R --copy-contents} will hang indefinitely trying to read
7696 from FIFOs and special files like @file{/dev/console}, and it will
7697 fill up your destination disk if you use it to copy @file{/dev/zero}.
7698 This option has no effect unless copying recursively, and it does not
7699 affect the copying of symbolic links.
7703 @cindex symbolic links, copying
7704 @cindex hard links, preserving
7705 Copy symbolic links as symbolic links rather than copying the files that
7706 they point to, and preserve hard links between source files in the copies.
7707 Equivalent to @option{--no-dereference --preserve=links}.
7713 When copying without this option and an existing destination file cannot
7714 be opened for writing, the copy fails. However, with @option{--force}),
7715 when a destination file cannot be opened, @command{cp} then removes it and
7716 tries to open it again. Contrast this behavior with that enabled by
7717 @option{--link} and @option{--symbolic-link}, whereby the destination file
7718 is never opened but rather is removed unconditionally. Also see the
7719 description of @option{--remove-destination}.
7721 This option is independent of the @option{--interactive} or
7722 @option{-i} option: neither cancels the effect of the other.
7724 This option is redundant if the @option{--no-clobber} or @option{-n} option is
7729 If a command line argument specifies a symbolic link, then copy the
7730 file it points to rather than the symbolic link itself. However,
7731 copy (preserving its nature) any symbolic link that is encountered
7732 via recursive traversal.
7735 @itemx --interactive
7737 @opindex --interactive
7738 When copying a file other than a directory, prompt whether to
7739 overwrite an existing destination file. The @option{-i} option overrides
7740 a previous @option{-n} option.
7746 Make hard links instead of copies of non-directories.
7749 @itemx --dereference
7751 @opindex --dereference
7752 Follow symbolic links when copying from them.
7753 With this option, @command{cp} cannot create a symbolic link.
7754 For example, a symlink (to regular file) in the source tree will be copied to
7755 a regular file in the destination tree.
7760 @opindex --no-clobber
7761 Do not overwrite an existing file. The @option{-n} option overrides a previous
7762 @option{-i} option. This option is mutually exclusive with @option{-b} or
7763 @option{--backup} option.
7766 @itemx --no-dereference
7768 @opindex --no-dereference
7769 @cindex symbolic links, copying
7770 Copy symbolic links as symbolic links rather than copying the files that
7771 they point to. This option affects only symbolic links in the source;
7772 symbolic links in the destination are always followed if possible.
7775 @itemx @w{@kbd{--preserve}[=@var{attribute_list}]}
7778 @cindex file information, preserving, extended attributes, xattr
7779 Preserve the specified attributes of the original files.
7780 If specified, the @var{attribute_list} must be a comma-separated list
7781 of one or more of the following strings:
7785 Preserve the file mode bits and access control lists.
7787 Preserve the owner and group. On most modern systems,
7788 only users with appropriate privileges may change the owner of a file,
7790 may preserve the group ownership of a file only if they happen to be
7791 a member of the desired group.
7793 Preserve the times of last access and last modification, when possible.
7794 On older systems, it is not possible to preserve these attributes
7795 when the affected file is a symbolic link.
7796 However, many systems now provide the @code{utimensat} function,
7797 which makes it possible even for symbolic links.
7799 Preserve in the destination files
7800 any links between corresponding source files.
7801 Note that with @option{-L} or @option{-H}, this option can convert
7802 symbolic links to hard links. For example,
7804 $ mkdir c; : > a; ln -s a b; cp -aH a b c; ls -i1 c
7809 Note the inputs: @file{b} is a symlink to regular file @file{a},
7810 yet the files in destination directory, @file{c/}, are hard-linked.
7811 Since @option{-a} implies @option{--preserve=links}, and since @option{-H}
7812 tells @command{cp} to dereference command line arguments, it sees two files
7813 with the same inode number, and preserves the perceived hard link.
7815 Here is a similar example that exercises @command{cp}'s @option{-L} option:
7817 $ mkdir b c; (cd b; : > a; ln -s a b); cp -aL b c; ls -i1 c/b
7823 Preserve SELinux security context of the file, or fail with full diagnostics.
7825 Preserve extended attributes of the file, or fail with full diagnostics.
7826 If @command{cp} is built without xattr support, ignore this option.
7827 If SELinux context, ACLs or Capabilities are implemented using xattrs,
7828 they are preserved by this option as well.
7830 Preserve all file attributes.
7831 Equivalent to specifying all of the above, but with the difference
7832 that failure to preserve SELinux security context or extended attributes
7833 does not change @command{cp}'s exit status. In contrast to @option{-a},
7834 all but @samp{Operation not supported} warnings are output.
7837 Using @option{--preserve} with no @var{attribute_list} is equivalent
7838 to @option{--preserve=mode,ownership,timestamps}.
7840 In the absence of this option, each destination file is created with the
7841 mode bits of the corresponding source file, minus the bits set in the
7842 umask and minus the set-user-ID and set-group-ID bits.
7843 @xref{File permissions}.
7845 @itemx @w{@kbd{--no-preserve}=@var{attribute_list}}
7846 @cindex file information, preserving
7847 Do not preserve the specified attributes. The @var{attribute_list}
7848 has the same form as for @option{--preserve}.
7852 @cindex parent directories and @command{cp}
7853 Form the name of each destination file by appending to the target
7854 directory a slash and the specified name of the source file. The last
7855 argument given to @command{cp} must be the name of an existing directory.
7856 For example, the command:
7859 cp --parents a/b/c existing_dir
7863 copies the file @file{a/b/c} to @file{existing_dir/a/b/c}, creating
7864 any missing intermediate directories.
7871 @opindex --recursive
7872 @cindex directories, copying recursively
7873 @cindex copying directories recursively
7874 @cindex recursively copying directories
7875 @cindex non-directories, copying as special files
7876 Copy directories recursively. By default, do not follow symbolic
7877 links in the source; see the @option{--archive} (@option{-a}), @option{-d},
7878 @option{--dereference} (@option{-L}), @option{--no-dereference}
7879 (@option{-P}), and @option{-H} options. Special files are copied by
7880 creating a destination file of the same type as the source; see the
7881 @option{--copy-contents} option. It is not portable to use
7882 @option{-r} to copy symbolic links or special files. On some
7883 non-@sc{gnu} systems, @option{-r} implies the equivalent of
7884 @option{-L} and @option{--copy-contents} for historical reasons.
7885 Also, it is not portable to use @option{-R} to copy symbolic links
7886 unless you also specify @option{-P}, as @acronym{POSIX} allows
7887 implementations that dereference symbolic links by default.
7889 @item --reflink[=@var{when}]
7890 @opindex --reflink[=@var{when}]
7893 @cindex copy on write
7894 Perform a lightweight, copy-on-write (COW) copy, if supported by the
7895 file system. Once it has succeeded, beware that the source and destination
7896 files share the same disk data blocks as long as they remain unmodified.
7897 Thus, if a disk I/O error affects data blocks of one of the files,
7898 the other suffers the same fate.
7900 The @var{when} value can be one of the following:
7904 The default behavior: if the copy-on-write operation is not supported
7905 then report the failure for each file and exit with a failure status.
7908 If the copy-on-write operation is not supported then fall back
7909 to the standard copy behaviour.
7912 This option is overridden by the @option{--link}, @option{--symbolic-link}
7913 and @option{--attributes-only} options, thus allowing it to be used
7914 to configure the default data copying behavior for @command{cp}.
7915 For example, with the following alias, @command{cp} will use the
7916 minimum amount of space supported by the file system.
7919 alias cp='cp --reflink=auto --sparse=always'
7922 @item --remove-destination
7923 @opindex --remove-destination
7924 Remove each existing destination file before attempting to open it
7925 (contrast with @option{-f} above).
7927 @item --sparse=@var{when}
7928 @opindex --sparse=@var{when}
7929 @cindex sparse files, copying
7930 @cindex holes, copying files with
7931 @findex read @r{system call, and holes}
7932 A @dfn{sparse file} contains @dfn{holes}---a sequence of zero bytes that
7933 does not occupy any physical disk blocks; the @samp{read} system call
7934 reads these as zeros. This can both save considerable disk space and
7935 increase speed, since many binary files contain lots of consecutive zero
7936 bytes. By default, @command{cp} detects holes in input source files via a crude
7937 heuristic and makes the corresponding output file sparse as well.
7938 Only regular files may be sparse.
7940 The @var{when} value can be one of the following:
7944 The default behavior: if the input file is sparse, attempt to make
7945 the output file sparse, too. However, if an output file exists but
7946 refers to a non-regular file, then do not attempt to make it sparse.
7949 For each sufficiently long sequence of zero bytes in the input file,
7950 attempt to create a corresponding hole in the output file, even if the
7951 input file does not appear to be sparse.
7952 This is useful when the input file resides on a file system
7953 that does not support sparse files
7954 (for example, @samp{efs} file systems in SGI IRIX 5.3 and earlier),
7955 but the output file is on a type of file system that does support them.
7956 Holes may be created only in regular files, so if the destination file
7957 is of some other type, @command{cp} does not even try to make it sparse.
7960 Never make the output file sparse.
7961 This is useful in creating a file for use with the @command{mkswap} command,
7962 since such a file must not have any holes.
7965 @optStripTrailingSlashes
7968 @itemx --symbolic-link
7970 @opindex --symbolic-link
7971 @cindex symbolic links, copying with
7972 Make symbolic links instead of copies of non-directories. All source
7973 file names must be absolute (starting with @samp{/}) unless the
7974 destination files are in the current directory. This option merely
7975 results in an error message on systems that do not support symbolic links.
7981 @optNoTargetDirectory
7987 @cindex newer files, copying only
7988 Do not copy a non-directory that has an existing destination with the
7989 same or newer modification time. If time stamps are being preserved,
7990 the comparison is to the source time stamp truncated to the
7991 resolutions of the destination file system and of the system calls
7992 used to update time stamps; this avoids duplicate work if several
7993 @samp{cp -pu} commands are executed with the same source and destination.
7994 If @option{--preserve=links} is also specified (like with @samp{cp -au}
7995 for example), that will take precedence. Consequently, depending on the
7996 order that files are processed from the source, newer files in the destination
7997 may be replaced, to mirror hard links in the source.
8003 Print the name of each file before copying it.
8006 @itemx --one-file-system
8008 @opindex --one-file-system
8009 @cindex file systems, omitting copying to different
8010 Skip subdirectories that are on different file systems from the one that
8011 the copy started on.
8012 However, mount point directories @emph{are} copied.
8020 @section @command{dd}: Convert and copy a file
8023 @cindex converting while copying a file
8025 @command{dd} copies a file (from standard input to standard output, by
8026 default) with a changeable I/O block size, while optionally performing
8027 conversions on it. Synopses:
8030 dd [@var{operand}]@dots{}
8034 The only options are @option{--help} and @option{--version}.
8035 @xref{Common options}. @command{dd} accepts the following operands.
8041 Read from @var{file} instead of standard input.
8045 Write to @var{file} instead of standard output. Unless
8046 @samp{conv=notrunc} is given, @command{dd} truncates @var{file} to zero
8047 bytes (or the size specified with @samp{seek=}).
8049 @item ibs=@var{bytes}
8051 @cindex block size of input
8052 @cindex input block size
8053 Set the input block size to @var{bytes}.
8054 This makes @command{dd} read @var{bytes} per block.
8055 The default is 512 bytes.
8057 @item obs=@var{bytes}
8059 @cindex block size of output
8060 @cindex output block size
8061 Set the output block size to @var{bytes}.
8062 This makes @command{dd} write @var{bytes} per block.
8063 The default is 512 bytes.
8065 @item bs=@var{bytes}
8068 Set both input and output block sizes to @var{bytes}.
8069 This makes @command{dd} read and write @var{bytes} per block,
8070 overriding any @samp{ibs} and @samp{obs} settings.
8071 In addition, if no data-transforming @option{conv} option is specified,
8072 input is copied to the output as soon as it's read,
8073 even if it is smaller than the block size.
8075 @item cbs=@var{bytes}
8077 @cindex block size of conversion
8078 @cindex conversion block size
8079 @cindex fixed-length records, converting to variable-length
8080 @cindex variable-length records, converting to fixed-length
8081 Set the conversion block size to @var{bytes}.
8082 When converting variable-length records to fixed-length ones
8083 (@option{conv=block}) or the reverse (@option{conv=unblock}),
8084 use @var{bytes} as the fixed record length.
8088 Skip @var{n} @samp{ibs}-byte blocks in the input file before copying.
8089 If @samp{iflag=skip_bytes} is specified, @var{n} is interpreted
8090 as a byte count rather than a block count.
8094 Skip @var{n} @samp{obs}-byte blocks in the output file before copying.
8095 if @samp{oflag=seek_bytes} is specified, @var{n} is interpreted
8096 as a byte count rather than a block count.
8100 Copy @var{n} @samp{ibs}-byte blocks from the input file, instead
8101 of everything until the end of the file.
8102 if @samp{iflag=count_bytes} is specified, @var{n} is interpreted
8103 as a byte count rather than a block count.
8107 Do not print the overall transfer rate and volume statistics
8108 that normally make up the third status line when @command{dd} exits.
8110 @item conv=@var{conversion}[,@var{conversion}]@dots{}
8112 Convert the file as specified by the @var{conversion} argument(s).
8113 (No spaces around any comma(s).)
8120 @opindex ascii@r{, converting to}
8121 Convert @acronym{EBCDIC} to @acronym{ASCII},
8122 using the conversion table specified by @acronym{POSIX}.
8123 This provides a 1:1 translation for all 256 bytes.
8126 @opindex ebcdic@r{, converting to}
8127 Convert @acronym{ASCII} to @acronym{EBCDIC}.
8128 This is the inverse of the @samp{ascii} conversion.
8131 @opindex alternate ebcdic@r{, converting to}
8132 Convert @acronym{ASCII} to alternate @acronym{EBCDIC},
8133 using the alternate conversion table specified by @acronym{POSIX}.
8134 This is not a 1:1 translation, but reflects common historical practice
8135 for @samp{~}, @samp{[}, and @samp{]}.
8137 The @samp{ascii}, @samp{ebcdic}, and @samp{ibm} conversions are
8141 @opindex block @r{(space-padding)}
8142 For each line in the input, output @samp{cbs} bytes, replacing the
8143 input newline with a space and padding with spaces as necessary.
8147 Remove any trailing spaces in each @samp{cbs}-sized input block,
8148 and append a newline.
8150 The @samp{block} and @samp{unblock} conversions are mutually exclusive.
8153 @opindex lcase@r{, converting to}
8154 Change uppercase letters to lowercase.
8157 @opindex ucase@r{, converting to}
8158 Change lowercase letters to uppercase.
8160 The @samp{lcase} and @samp{ucase} conversions are mutually exclusive.
8164 Try to seek rather than write @sc{nul} output blocks.
8165 On a file system that supports sparse files, this will create
8166 sparse output when extending the output file.
8167 Be careful when using this option in conjunction with
8168 @samp{conv=notrunc} or @samp{oflag=append}.
8169 With @samp{conv=notrunc}, existing data in the output
8170 corresponding to @sc{nul} blocks from the input, will be untouched.
8171 With @samp{oflag=append} the seeks performed will be ineffective.
8174 @opindex swab @r{(byte-swapping)}
8175 @cindex byte-swapping
8176 Swap every pair of input bytes. @sc{gnu} @command{dd}, unlike others, works
8177 when an odd number of bytes are read---the last byte is simply copied
8178 (since there is nothing to swap it with).
8181 @opindex sync @r{(padding with @acronym{ASCII} @sc{nul}s)}
8182 Pad every input block to size of @samp{ibs} with trailing zero bytes.
8183 When used with @samp{block} or @samp{unblock}, pad with spaces instead of
8188 The following ``conversions'' are really file flags
8189 and don't affect internal processing:
8194 @cindex creating output file, requiring
8195 Fail if the output file already exists; @command{dd} must create the
8200 @cindex creating output file, avoiding
8201 Do not create the output file; the output file must already exist.
8203 The @samp{excl} and @samp{nocreat} conversions are mutually exclusive.
8207 @cindex truncating output file, avoiding
8208 Do not truncate the output file.
8212 @cindex read errors, ignoring
8213 Continue after read errors.
8217 @cindex synchronized data writes, before finishing
8218 Synchronize output data just before finishing. This forces a physical
8219 write of output data.
8223 @cindex synchronized data and metadata writes, before finishing
8224 Synchronize output data and metadata just before finishing. This
8225 forces a physical write of output data and metadata.
8229 @item iflag=@var{flag}[,@var{flag}]@dots{}
8231 Access the input file using the flags specified by the @var{flag}
8232 argument(s). (No spaces around any comma(s).)
8234 @item oflag=@var{flag}[,@var{flag}]@dots{}
8236 Access the output file using the flags specified by the @var{flag}
8237 argument(s). (No spaces around any comma(s).)
8239 Here are the flags. Not every flag is supported on every operating
8246 @cindex appending to the output file
8247 Write in append mode, so that even if some other process is writing to
8248 this file, every @command{dd} write will append to the current
8249 contents of the file. This flag makes sense only for output.
8250 If you combine this flag with the @samp{of=@var{file}} operand,
8251 you should also specify @samp{conv=notrunc} unless you want the
8252 output file to be truncated before being appended to.
8256 @cindex concurrent I/O
8257 Use concurrent I/O mode for data. This mode performs direct I/O
8258 and drops the @acronym{POSIX} requirement to serialize all I/O to the same file.
8259 A file cannot be opened in CIO mode and with a standard open at the
8265 Use direct I/O for data, avoiding the buffer cache.
8266 Note that the kernel may impose restrictions on read or write buffer sizes.
8267 For example, with an ext4 destination file system and a linux-based kernel,
8268 using @samp{oflag=direct} will cause writes to fail with @code{EINVAL} if the
8269 output buffer size is not a multiple of 512.
8273 @cindex directory I/O
8275 Fail unless the file is a directory. Most operating systems do not
8276 allow I/O to a directory, so this flag has limited utility.
8280 @cindex synchronized data reads
8281 Use synchronized I/O for data. For the output file, this forces a
8282 physical write of output data on each write. For the input file,
8283 this flag can matter when reading from a remote file that has been
8284 written to synchronously by some other process. Metadata (e.g.,
8285 last-access and last-modified time) is not necessarily synchronized.
8289 @cindex synchronized data and metadata I/O
8290 Use synchronized I/O for both data and metadata.
8294 @cindex discarding file cache
8295 Discard the data cache for a file.
8296 When count=0 all cache is discarded,
8297 otherwise the cache is dropped for the processed
8298 portion of the file. Also when count=0
8299 failure to discard the cache is diagnosed
8300 and reflected in the exit status.
8301 Here as some usage examples:
8304 # Advise to drop cache for whole file
8305 dd if=ifile iflag=nocache count=0
8307 # Ensure drop cache for the whole file
8308 dd of=ofile oflag=nocache conv=notrunc,fdatasync count=0
8310 # Drop cache for part of file
8311 dd if=ifile iflag=nocache skip=10 count=10 of=/dev/null
8313 # Stream data using just the read-ahead cache
8314 dd if=ifile of=ofile iflag=nocache oflag=nocache
8319 @cindex nonblocking I/O
8320 Use non-blocking I/O.
8325 Do not update the file's access time.
8326 Some older file systems silently ignore this flag, so it is a good
8327 idea to test it on your files before relying on it.
8331 @cindex controlling terminal
8332 Do not assign the file to be a controlling terminal for @command{dd}.
8333 This has no effect when the file is not a terminal.
8334 On many hosts (e.g., @acronym{GNU}/Linux hosts), this option has no effect
8339 @cindex symbolic links, following
8340 Do not follow symbolic links.
8345 Fail if the file has multiple hard links.
8350 Use binary I/O. This option has an effect only on nonstandard
8351 platforms that distinguish binary from text I/O.
8356 Use text I/O. Like @samp{binary}, this option has no effect on
8361 Accumulate full blocks from input. The @code{read} system call
8362 may return early if a full block is not available.
8363 When that happens, continue calling @code{read} to fill the remainder
8365 This flag can be used only with @code{iflag}.
8368 @opindex count_bytes
8369 Interpret the @samp{count=} operand as a byte count,
8370 rather than a block count, which allows specifying
8371 a length that is not a multiple of the I/O block size.
8372 This flag can be used only with @code{iflag}.
8376 Interpret the @samp{skip=} operand as a byte count,
8377 rather than a block count, which allows specifying
8378 an offset that is not a multiple of the I/O block size.
8379 This flag can be used only with @code{iflag}.
8383 Interpret the @samp{seek=} operand as a byte count,
8384 rather than a block count, which allows specifying
8385 an offset that is not a multiple of the I/O block size.
8386 This flag can be used only with @code{oflag}.
8390 These flags are not supported on all systems, and @samp{dd} rejects
8391 attempts to use them when they are not supported. When reading from
8392 standard input or writing to standard output, the @samp{nofollow} and
8393 @samp{noctty} flags should not be specified, and the other flags
8394 (e.g., @samp{nonblock}) can affect how other processes behave with the
8395 affected file descriptors, even after @command{dd} exits.
8399 @cindex multipliers after numbers
8400 The numeric-valued strings above (@var{bytes} and @var{blocks}) can be
8401 followed by a multiplier: @samp{b}=512, @samp{c}=1,
8402 @samp{w}=2, @samp{x@var{m}}=@var{m}, or any of the
8403 standard block size suffixes like @samp{k}=1024 (@pxref{Block size}).
8405 Any block size you specify via @samp{bs=}, @samp{ibs=}, @samp{obs=}, @samp{cbs=}
8406 should not be too large---values larger than a few megabytes
8407 are generally wasteful or (as in the gigabyte..exabyte case) downright
8408 counterproductive or error-inducing.
8410 To process data that is at an offset or size that is not a
8411 multiple of the I/O@ block size, you can use the @samp{skip_bytes},
8412 @samp{seek_bytes} and @samp{count_bytes} flags. Alternatively
8413 the traditional method of separate @command{dd} invocations can be used.
8414 For example, the following shell commands copy data
8415 in 512 KiB blocks between a disk and a tape, but do not save
8416 or restore a 4 KiB label at the start of the disk:
8419 disk=/dev/rdsk/c0t1d0s2
8422 # Copy all but the label from disk to tape.
8423 (dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape
8425 # Copy from tape back to disk, but leave the disk label alone.
8426 (dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk
8429 Sending an @samp{INFO} signal to a running @command{dd}
8430 process makes it print I/O statistics to standard error
8431 and then resume copying. In the example below,
8432 @command{dd} is run in the background to copy 10 million blocks.
8433 The @command{kill} command makes it output intermediate I/O statistics,
8434 and when @command{dd} completes normally or is killed by the
8435 @code{SIGINT} signal, it outputs the final statistics.
8438 $ dd if=/dev/zero of=/dev/null count=10MB & pid=$!
8439 $ kill -s INFO $pid; wait $pid
8440 3385223+0 records in
8441 3385223+0 records out
8442 1733234176 bytes (1.7 GB) copied, 6.42173 seconds, 270 MB/s
8443 10000000+0 records in
8444 10000000+0 records out
8445 5120000000 bytes (5.1 GB) copied, 18.913 seconds, 271 MB/s
8448 @vindex POSIXLY_CORRECT
8449 On systems lacking the @samp{INFO} signal @command{dd} responds to the
8450 @samp{USR1} signal instead, unless the @env{POSIXLY_CORRECT}
8451 environment variable is set.
8456 @node install invocation
8457 @section @command{install}: Copy files and set attributes
8460 @cindex copying files and setting attributes
8462 @command{install} copies files while setting their file mode bits and, if
8463 possible, their owner and group. Synopses:
8466 install [@var{option}]@dots{} [-T] @var{source} @var{dest}
8467 install [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8468 install [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8469 install [@var{option}]@dots{} -d @var{directory}@dots{}
8474 If two file names are given, @command{install} copies the first file to the
8478 If the @option{--target-directory} (@option{-t}) option is given, or
8479 failing that if the last file is a directory and the
8480 @option{--no-target-directory} (@option{-T}) option is not given,
8481 @command{install} copies each @var{source} file to the specified
8482 directory, using the @var{source}s' names.
8485 If the @option{--directory} (@option{-d}) option is given,
8486 @command{install} creates each @var{directory} and any missing parent
8487 directories. Parent directories are created with mode
8488 @samp{u=rwx,go=rx} (755), regardless of the @option{-m} option or the
8489 current umask. @xref{Directory Setuid and Setgid}, for how the
8490 set-user-ID and set-group-ID bits of parent directories are inherited.
8493 @cindex Makefiles, installing programs in
8494 @command{install} is similar to @command{cp}, but allows you to control the
8495 attributes of destination files. It is typically used in Makefiles to
8496 copy programs into their destination directories. It refuses to copy
8497 files onto themselves.
8499 @cindex extended attributes, xattr
8500 @command{install} never preserves extended attributes (xattr).
8502 The program accepts the following options. Also see @ref{Common options}.
8512 Compare each pair of source and destination files, and if the destination has
8513 identical content and any specified owner, group, permissions, and possibly
8514 SELinux context, then do not modify the destination at all.
8518 Ignored; for compatibility with old Unix versions of @command{install}.
8522 Create any missing parent directories of @var{dest},
8523 then copy @var{source} to @var{dest}.
8524 This option is ignored if a destination directory is specified
8525 via @option{--target-directory=DIR}.
8530 @opindex --directory
8531 @cindex directories, creating with given attributes
8532 @cindex parent directories, creating missing
8533 @cindex leading directories, creating missing
8534 Create any missing parent directories, giving them the default
8535 attributes. Then create each given directory, setting their owner,
8536 group and mode as given on the command line or to the defaults.
8538 @item -g @var{group}
8539 @itemx --group=@var{group}
8542 @cindex group ownership of installed files, setting
8543 Set the group ownership of installed files or directories to
8544 @var{group}. The default is the process's current group. @var{group}
8545 may be either a group name or a numeric group ID.
8548 @itemx --mode=@var{mode}
8551 @cindex permissions of installed files, setting
8552 Set the file mode bits for the installed file or directory to @var{mode},
8553 which can be either an octal number, or a symbolic mode as in
8554 @command{chmod}, with @samp{a=} (no access allowed to anyone) as the
8555 point of departure (@pxref{File permissions}).
8556 The default mode is @samp{u=rwx,go=rx,a-s}---read, write, and
8557 execute for the owner, read and execute for group and other, and with
8558 set-user-ID and set-group-ID disabled.
8559 This default is not quite the same as @samp{755}, since it disables
8560 instead of preserving set-user-ID and set-group-ID on directories.
8561 @xref{Directory Setuid and Setgid}.
8563 @item -o @var{owner}
8564 @itemx --owner=@var{owner}
8567 @cindex ownership of installed files, setting
8568 @cindex appropriate privileges
8569 @vindex root @r{as default owner}
8570 If @command{install} has appropriate privileges (is run as root), set the
8571 ownership of installed files or directories to @var{owner}. The default
8572 is @code{root}. @var{owner} may be either a user name or a numeric user
8575 @item --preserve-context
8576 @opindex --preserve-context
8578 @cindex security context
8579 Preserve the SELinux security context of files and directories.
8580 Failure to preserve the context in all of the files or directories
8581 will result in an exit status of 1. If SELinux is disabled then
8582 print a warning and ignore the option.
8585 @itemx --preserve-timestamps
8587 @opindex --preserve-timestamps
8588 @cindex timestamps of installed files, preserving
8589 Set the time of last access and the time of last modification of each
8590 installed file to match those of each corresponding original file.
8591 When a file is installed without this option, its last access and
8592 last modification times are both set to the time of installation.
8593 This option is useful if you want to use the last modification times
8594 of installed files to keep track of when they were last built as opposed
8595 to when they were last installed.
8601 @cindex symbol table information, stripping
8602 @cindex stripping symbol table information
8603 Strip the symbol tables from installed binary executables.
8605 @itemx --strip-program=@var{program}
8606 @opindex --strip-program
8607 @cindex symbol table information, stripping, program
8608 Program used to strip binaries.
8614 @optNoTargetDirectory
8620 Print the name of each file before copying it.
8622 @item -Z @var{context}
8623 @itemx --context=@var{context}
8627 @cindex security context
8628 Set the default SELinux security context to be used for any
8629 created files and directories. If SELinux is disabled then
8630 print a warning and ignore the option.
8638 @section @command{mv}: Move (rename) files
8642 @command{mv} moves or renames files (or directories). Synopses:
8645 mv [@var{option}]@dots{} [-T] @var{source} @var{dest}
8646 mv [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8647 mv [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8652 If two file names are given, @command{mv} moves the first file to the
8656 If the @option{--target-directory} (@option{-t}) option is given, or
8657 failing that if the last file is a directory and the
8658 @option{--no-target-directory} (@option{-T}) option is not given,
8659 @command{mv} moves each @var{source} file to the specified
8660 directory, using the @var{source}s' names.
8663 @command{mv} can move any type of file from one file system to another.
8664 Prior to version @code{4.0} of the fileutils,
8665 @command{mv} could move only regular files between file systems.
8666 For example, now @command{mv} can move an entire directory hierarchy
8667 including special device files from one partition to another. It first
8668 uses some of the same code that's used by @code{cp -a} to copy the
8669 requested directories and files, then (assuming the copy succeeded)
8670 it removes the originals. If the copy fails, then the part that was
8671 copied to the destination partition is removed. If you were to copy
8672 three directories from one partition to another and the copy of the first
8673 directory succeeded, but the second didn't, the first would be left on
8674 the destination partition and the second and third would be left on the
8677 @cindex extended attributes, xattr
8678 @command{mv} always tries to copy extended attributes (xattr), which may
8679 include SELinux context, ACLs or Capabilities.
8680 Upon failure all but @samp{Operation not supported} warnings are output.
8682 @cindex prompting, and @command{mv}
8683 If a destination file exists but is normally unwritable, standard input
8684 is a terminal, and the @option{-f} or @option{--force} option is not given,
8685 @command{mv} prompts the user for whether to replace the file. (You might
8686 own the file, or have write permission on its directory.) If the
8687 response is not affirmative, the file is skipped.
8689 @emph{Warning}: Avoid specifying a source name with a trailing slash,
8690 when it might be a symlink to a directory.
8691 Otherwise, @command{mv} may do something very surprising, since
8692 its behavior depends on the underlying rename system call.
8693 On a system with a modern Linux-based kernel, it fails with
8694 @code{errno=ENOTDIR}.
8695 However, on other systems (at least FreeBSD 6.1 and Solaris 10) it silently
8696 renames not the symlink but rather the directory referenced by the symlink.
8697 @xref{Trailing slashes}.
8699 The program accepts the following options. Also see @ref{Common options}.
8709 @cindex prompts, omitting
8710 Do not prompt the user before removing a destination file.
8712 If you specify more than one of the @option{-i}, @option{-f}, @option{-n}
8713 options, only the final one takes effect.
8718 @itemx --interactive
8720 @opindex --interactive
8721 @cindex prompts, forcing
8722 Prompt whether to overwrite each existing destination file, regardless
8724 If the response is not affirmative, the file is skipped.
8730 @opindex --no-clobber
8731 @cindex prompts, omitting
8732 Do not overwrite an existing file.
8734 This option is mutually exclusive with @option{-b} or @option{--backup} option.
8740 @cindex newer files, moving only
8741 Do not move a non-directory that has an existing destination with the
8742 same or newer modification time.
8743 If the move is across file system boundaries, the comparison is to the
8744 source time stamp truncated to the resolutions of the destination file
8745 system and of the system calls used to update time stamps; this avoids
8746 duplicate work if several @samp{mv -u} commands are executed with the
8747 same source and destination.
8753 Print the name of each file before moving it.
8755 @optStripTrailingSlashes
8761 @optNoTargetDirectory
8769 @section @command{rm}: Remove files or directories
8772 @cindex removing files or directories
8774 @command{rm} removes each given @var{file}. By default, it does not remove
8775 directories. Synopsis:
8778 rm [@var{option}]@dots{} [@var{file}]@dots{}
8781 @cindex prompting, and @command{rm}
8782 If the @option{-I} or @option{--interactive=once} option is given,
8783 and there are more than three files or the @option{-r}, @option{-R},
8784 or @option{--recursive} are given, then @command{rm} prompts the user
8785 for whether to proceed with the entire operation. If the response is
8786 not affirmative, the entire command is aborted.
8788 Otherwise, if a file is unwritable, standard input is a terminal, and
8789 the @option{-f} or @option{--force} option is not given, or the
8790 @option{-i} or @option{--interactive=always} option @emph{is} given,
8791 @command{rm} prompts the user for whether to remove the file.
8792 If the response is not affirmative, the file is skipped.
8794 Any attempt to remove a file whose last file name component is
8795 @file{.} or @file{..} is rejected without any prompting.
8797 @emph{Warning}: If you use @command{rm} to remove a file, it is usually
8798 possible to recover the contents of that file. If you want more assurance
8799 that the contents are truly unrecoverable, consider using @command{shred}.
8801 The program accepts the following options. Also see @ref{Common options}.
8809 Ignore nonexistent files and missing operands, and never prompt the user.
8810 Ignore any previous @option{--interactive} (@option{-i}) option.
8814 Prompt whether to remove each file.
8815 If the response is not affirmative, the file is skipped.
8816 Ignore any previous @option{--force} (@option{-f}) option.
8817 Equivalent to @option{--interactive=always}.
8821 Prompt once whether to proceed with the command, if more than three
8822 files are named or if a recursive removal is requested. Ignore any
8823 previous @option{--force} (@option{-f}) option. Equivalent to
8824 @option{--interactive=once}.
8826 @itemx --interactive [=@var{when}]
8827 @opindex --interactive
8828 Specify when to issue an interactive prompt. @var{when} may be
8832 @vindex never @r{interactive option}
8833 - Do not prompt at all.
8835 @vindex once @r{interactive option}
8836 - Prompt once if more than three files are named or if a recursive
8837 removal is requested. Equivalent to @option{-I}.
8839 @vindex always @r{interactive option}
8840 - Prompt for every file being removed. Equivalent to @option{-i}.
8842 @option{--interactive} with no @var{when} is equivalent to
8843 @option{--interactive=always}.
8845 @itemx --one-file-system
8846 @opindex --one-file-system
8847 @cindex one file system, restricting @command{rm} to
8848 When removing a hierarchy recursively, skip any directory that is on a
8849 file system different from that of the corresponding command line argument.
8852 This option is useful when removing a build ``chroot'' hierarchy,
8853 which normally contains no valuable data. However, it is not uncommon
8854 to bind-mount @file{/home} into such a hierarchy, to make it easier to
8855 use one's start-up file. The catch is that it's easy to forget to
8856 unmount @file{/home}. Then, when you use @command{rm -rf} to remove
8857 your normally throw-away chroot, that command will remove everything
8858 under @file{/home}, too.
8859 Use the @option{--one-file-system} option, and it will
8860 warn about and skip directories on other file systems.
8861 Of course, this will not save your @file{/home} if it and your
8862 chroot happen to be on the same file system.
8864 @itemx --preserve-root
8865 @opindex --preserve-root
8866 @cindex root directory, disallow recursive destruction
8867 Fail upon any attempt to remove the root directory, @file{/},
8868 when used with the @option{--recursive} option.
8869 This is the default behavior.
8870 @xref{Treating / specially}.
8872 @itemx --no-preserve-root
8873 @opindex --no-preserve-root
8874 @cindex root directory, allow recursive destruction
8875 Do not treat @file{/} specially when removing recursively.
8876 This option is not recommended unless you really want to
8877 remove all the files on your computer.
8878 @xref{Treating / specially}.
8885 @opindex --recursive
8886 @cindex directories, removing (recursively)
8887 Remove the listed directories and their contents recursively.
8893 Print the name of each file before removing it.
8897 @cindex files beginning with @samp{-}, removing
8898 @cindex @samp{-}, removing files beginning with
8899 One common question is how to remove files whose names begin with a
8900 @samp{-}. @sc{gnu} @command{rm}, like every program that uses the @code{getopt}
8901 function to parse its arguments, lets you use the @samp{--} option to
8902 indicate that all following arguments are non-options. To remove a file
8903 called @file{-f} in the current directory, you could type either:
8916 @opindex - @r{and Unix @command{rm}}
8917 The Unix @command{rm} program's use of a single @samp{-} for this purpose
8918 predates the development of the getopt standard syntax.
8923 @node shred invocation
8924 @section @command{shred}: Remove files more securely
8927 @cindex data, erasing
8928 @cindex erasing data
8930 @command{shred} overwrites devices or files, to help prevent even
8931 very expensive hardware from recovering the data.
8933 Ordinarily when you remove a file (@pxref{rm invocation}), the data is
8934 not actually destroyed. Only the index listing where the file is
8935 stored is destroyed, and the storage is made available for reuse.
8936 There are undelete utilities that will attempt to reconstruct the index
8937 and can bring the file back if the parts were not reused.
8939 On a busy system with a nearly-full drive, space can get reused in a few
8940 seconds. But there is no way to know for sure. If you have sensitive
8941 data, you may want to be sure that recovery is not possible by actually
8942 overwriting the file with non-sensitive data.
8944 However, even after doing that, it is possible to take the disk back
8945 to a laboratory and use a lot of sensitive (and expensive) equipment
8946 to look for the faint ``echoes'' of the original data underneath the
8947 overwritten data. If the data has only been overwritten once, it's not
8950 The best way to remove something irretrievably is to destroy the media
8951 it's on with acid, melt it down, or the like. For cheap removable media
8952 like floppy disks, this is the preferred method. However, hard drives
8953 are expensive and hard to melt, so the @command{shred} utility tries
8954 to achieve a similar effect non-destructively.
8956 This uses many overwrite passes, with the data patterns chosen to
8957 maximize the damage they do to the old data. While this will work on
8958 floppies, the patterns are designed for best effect on hard drives.
8959 For more details, see the source code and Peter Gutmann's paper
8960 @uref{http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html,
8961 @cite{Secure Deletion of Data from Magnetic and Solid-State Memory}},
8962 from the proceedings of the Sixth @acronym{USENIX} Security Symposium (San Jose,
8963 California, July 22--25, 1996).
8965 @strong{Please note} that @command{shred} relies on a very important assumption:
8966 that the file system overwrites data in place. This is the traditional
8967 way to do things, but many modern file system designs do not satisfy this
8968 assumption. Exceptions include:
8973 Log-structured or journaled file systems, such as those supplied with
8974 AIX and Solaris, and JFS, ReiserFS, XFS, Ext3 (in @code{data=journal} mode),
8975 BFS, NTFS, etc.@: when they are configured to journal @emph{data}.
8978 File systems that write redundant data and carry on even if some writes
8979 fail, such as RAID-based file systems.
8982 File systems that make snapshots, such as Network Appliance's NFS server.
8985 File systems that cache in temporary locations, such as NFS version 3
8989 Compressed file systems.
8992 In the particular case of ext3 file systems, the above disclaimer applies (and
8993 @command{shred} is thus of limited effectiveness) only in @code{data=journal}
8994 mode, which journals file data in addition to just metadata. In both
8995 the @code{data=ordered} (default) and @code{data=writeback} modes,
8996 @command{shred} works as usual. Ext3 journaling modes can be changed
8997 by adding the @code{data=something} option to the mount options for a
8998 particular file system in the @file{/etc/fstab} file, as documented in
8999 the mount man page (man mount).
9001 If you are not sure how your file system operates, then you should assume
9002 that it does not overwrite data in place, which means that shred cannot
9003 reliably operate on regular files in your file system.
9005 Generally speaking, it is more reliable to shred a device than a file,
9006 since this bypasses the problem of file system design mentioned above.
9007 However, even shredding devices is not always completely reliable. For
9008 example, most disks map out bad sectors invisibly to the application; if
9009 the bad sectors contain sensitive data, @command{shred} won't be able to
9012 @command{shred} makes no attempt to detect or report this problem, just as
9013 it makes no attempt to do anything about backups. However, since it is
9014 more reliable to shred devices than files, @command{shred} by default does
9015 not truncate or remove the output file. This default is more suitable
9016 for devices, which typically cannot be truncated and should not be
9019 Finally, consider the risk of backups and mirrors.
9020 File system backups and remote mirrors may contain copies of the
9021 file that cannot be removed, and that will allow a shredded file
9022 to be recovered later. So if you keep any data you may later want
9023 to destroy using @command{shred}, be sure that it is not backed up or mirrored.
9026 shred [@var{option}]@dots{} @var{file}[@dots{}]
9029 The program accepts the following options. Also see @ref{Common options}.
9037 @cindex force deletion
9038 Override file permissions if necessary to allow overwriting.
9041 @itemx -n @var{number}
9042 @itemx --iterations=@var{number}
9043 @opindex -n @var{number}
9044 @opindex --iterations=@var{number}
9045 @cindex iterations, selecting the number of
9046 By default, @command{shred} uses @value{SHRED_DEFAULT_PASSES} passes of
9047 overwrite. You can reduce this to save time, or increase it if you think it's
9048 appropriate. After 25 passes all of the internal overwrite patterns will have
9049 been used at least once.
9051 @item --random-source=@var{file}
9052 @opindex --random-source
9053 @cindex random source for shredding
9054 Use @var{file} as a source of random data used to overwrite and to
9055 choose pass ordering. @xref{Random sources}.
9057 @item -s @var{bytes}
9058 @itemx --size=@var{bytes}
9059 @opindex -s @var{bytes}
9060 @opindex --size=@var{bytes}
9061 @cindex size of file to shred
9062 Shred the first @var{bytes} bytes of the file. The default is to shred
9063 the whole file. @var{bytes} can be followed by a size specification like
9064 @samp{K}, @samp{M}, or @samp{G} to specify a multiple. @xref{Block size}.
9070 @cindex removing files after shredding
9071 After shredding a file, truncate it (if possible) and then remove it.
9072 If a file has multiple links, only the named links will be removed.
9078 Display to standard error all status updates as sterilization proceeds.
9084 By default, @command{shred} rounds the size of a regular file up to the next
9085 multiple of the file system block size to fully erase the last block
9087 Use @option{--exact} to suppress that behavior.
9088 Thus, by default if you shred a 10-byte regular file on a system with 512-byte
9089 blocks, the resulting file will be 512 bytes long. With this option,
9090 shred does not increase the apparent size of the file.
9096 Normally, the last pass that @command{shred} writes is made up of
9097 random data. If this would be conspicuous on your hard drive (for
9098 example, because it looks like encrypted data), or you just think
9099 it's tidier, the @option{--zero} option adds an additional overwrite pass with
9100 all zero bits. This is in addition to the number of passes specified
9101 by the @option{--iterations} option.
9105 You might use the following command to erase all trace of the
9106 file system you'd created on the floppy disk in your first drive.
9107 That command takes about 20 minutes to erase a ``1.44MB'' (actually
9111 shred --verbose /dev/fd0
9114 Similarly, to erase all data on a selected partition of
9115 your hard disk, you could give a command like this:
9118 shred --verbose /dev/sda5
9121 On modern disks, a single pass should be adequate,
9122 and it will take one third the time of the default three-pass approach.
9125 # 1 pass, write pseudo-random data; 3x faster than the default
9126 shred --verbose -n1 /dev/sda5
9129 To be on the safe side, use at least one pass that overwrites using
9130 pseudo-random data. I.e., don't be tempted to use @samp{-n0 --zero},
9131 in case some disk controller optimizes the process of writing blocks
9132 of all zeros, and thereby does not clear all bytes in a block.
9133 Some SSDs may do just that.
9135 A @var{file} of @samp{-} denotes standard output.
9136 The intended use of this is to shred a removed temporary file.
9143 echo "Hello, world" >&3
9148 However, the command @samp{shred - >file} does not shred the contents
9149 of @var{file}, since the shell truncates @var{file} before invoking
9150 @command{shred}. Use the command @samp{shred file} or (if using a
9151 Bourne-compatible shell) the command @samp{shred - 1<>file} instead.
9156 @node Special file types
9157 @chapter Special file types
9159 @cindex special file types
9160 @cindex file types, special
9162 This chapter describes commands which create special types of files (and
9163 @command{rmdir}, which removes directories, one special file type).
9165 @cindex special file types
9167 Although Unix-like operating systems have markedly fewer special file
9168 types than others, not @emph{everything} can be treated only as the
9169 undifferentiated byte stream of @dfn{normal files}. For example, when a
9170 file is created or removed, the system must record this information,
9171 which it does in a @dfn{directory}---a special type of file. Although
9172 you can read directories as normal files, if you're curious, in order
9173 for the system to do its job it must impose a structure, a certain
9174 order, on the bytes of the file. Thus it is a ``special'' type of file.
9176 Besides directories, other special file types include named pipes
9177 (FIFOs), symbolic links, sockets, and so-called @dfn{special files}.
9180 * link invocation:: Make a hard link via the link syscall
9181 * ln invocation:: Make links between files.
9182 * mkdir invocation:: Make directories.
9183 * mkfifo invocation:: Make FIFOs (named pipes).
9184 * mknod invocation:: Make block or character special files.
9185 * readlink invocation:: Print value of a symlink or canonical file name.
9186 * rmdir invocation:: Remove empty directories.
9187 * unlink invocation:: Remove files via the unlink syscall
9191 @node link invocation
9192 @section @command{link}: Make a hard link via the link syscall
9195 @cindex links, creating
9196 @cindex hard links, creating
9197 @cindex creating links (hard only)
9199 @command{link} creates a single hard link at a time.
9200 It is a minimalist interface to the system-provided
9201 @code{link} function. @xref{Hard Links, , , libc,
9202 The GNU C Library Reference Manual}.
9203 It avoids the bells and whistles of the more commonly-used
9204 @command{ln} command (@pxref{ln invocation}).
9208 link @var{filename} @var{linkname}
9211 @var{filename} must specify an existing file, and @var{linkname}
9212 must specify a nonexistent entry in an existing directory.
9213 @command{link} simply calls @code{link (@var{filename}, @var{linkname})}
9216 On a @acronym{GNU} system, this command acts like @samp{ln --directory
9217 --no-target-directory @var{filename} @var{linkname}}. However, the
9218 @option{--directory} and @option{--no-target-directory} options are
9219 not specified by @acronym{POSIX}, and the @command{link} command is
9220 more portable in practice.
9222 If @var{filename} is a symbolic link, it is unspecified whether
9223 @var{linkname} will be a hard link to the symbolic link or to the
9224 target of the symbolic link. Use @command{ln -P} or @command{ln -L}
9225 to specify which behavior is desired.
9231 @section @command{ln}: Make links between files
9234 @cindex links, creating
9235 @cindex hard links, creating
9236 @cindex symbolic (soft) links, creating
9237 @cindex creating links (hard or soft)
9239 @cindex file systems and hard links
9240 @command{ln} makes links between files. By default, it makes hard links;
9241 with the @option{-s} option, it makes symbolic (or @dfn{soft}) links.
9245 ln [@var{option}]@dots{} [-T] @var{target} @var{linkname}
9246 ln [@var{option}]@dots{} @var{target}
9247 ln [@var{option}]@dots{} @var{target}@dots{} @var{directory}
9248 ln [@var{option}]@dots{} -t @var{directory} @var{target}@dots{}
9254 If two file names are given, @command{ln} creates a link to the first
9255 file from the second.
9258 If one @var{target} is given, @command{ln} creates a link to that file
9259 in the current directory.
9262 If the @option{--target-directory} (@option{-t}) option is given, or
9263 failing that if the last file is a directory and the
9264 @option{--no-target-directory} (@option{-T}) option is not given,
9265 @command{ln} creates a link to each @var{target} file in the specified
9266 directory, using the @var{target}s' names.
9270 Normally @command{ln} does not remove existing files. Use the
9271 @option{--force} (@option{-f}) option to remove them unconditionally,
9272 the @option{--interactive} (@option{-i}) option to remove them
9273 conditionally, and the @option{--backup} (@option{-b}) option to
9276 @cindex hard link, defined
9277 @cindex inode, and hard links
9278 A @dfn{hard link} is another name for an existing file; the link and the
9279 original are indistinguishable. Technically speaking, they share the
9280 same inode, and the inode contains all the information about a
9281 file---indeed, it is not incorrect to say that the inode @emph{is} the
9282 file. Most systems prohibit making a hard link to
9283 a directory; on those where it is allowed, only the super-user can do
9284 so (and with caution, since creating a cycle will cause problems to many
9285 other utilities). Hard links cannot cross file system boundaries. (These
9286 restrictions are not mandated by @acronym{POSIX}, however.)
9288 @cindex dereferencing symbolic links
9289 @cindex symbolic link, defined
9290 @dfn{Symbolic links} (@dfn{symlinks} for short), on the other hand, are
9291 a special file type (which not all kernels support: System V release 3
9292 (and older) systems lack symlinks) in which the link file actually
9293 refers to a different file, by name. When most operations (opening,
9294 reading, writing, and so on) are passed the symbolic link file, the
9295 kernel automatically @dfn{dereferences} the link and operates on the
9296 target of the link. But some operations (e.g., removing) work on the
9297 link file itself, rather than on its target. The owner and group of a
9298 symlink are not significant to file access performed through
9299 the link, but do have implications on deleting a symbolic link from a
9300 directory with the restricted deletion bit set. On the GNU system,
9301 the mode of a symlink has no significance and cannot be changed, but
9302 on some BSD systems, the mode can be changed and will affect whether
9303 the symlink will be traversed in file name resolution. @xref{Symbolic Links,,,
9304 libc, The GNU C Library Reference Manual}.
9306 Symbolic links can contain arbitrary strings; a @dfn{dangling symlink}
9307 occurs when the string in the symlink does not resolve to a file.
9308 There are no restrictions against creating dangling symbolic links.
9309 There are trade-offs to using absolute or relative symlinks. An
9310 absolute symlink always points to the same file, even if the directory
9311 containing the link is moved. However, if the symlink is visible from
9312 more than one machine (such as on a networked file system), the file
9313 pointed to might not always be the same. A relative symbolic link is
9314 resolved in relation to the directory that contains the link, and is
9315 often useful in referring to files on the same device without regards
9316 to what name that device is mounted on when accessed via networked
9319 When creating a relative symlink in a different location than the
9320 current directory, the resolution of the symlink will be different
9321 than the resolution of the same string from the current directory.
9322 Therefore, many users prefer to first change directories to the
9323 location where the relative symlink will be created, so that
9324 tab-completion or other file resolution will find the same target as
9325 what will be placed in the symlink.
9327 The program accepts the following options. Also see @ref{Common options}.
9338 @opindex --directory
9339 @cindex hard links to directories
9340 Allow users with appropriate privileges to attempt to make hard links
9342 However, note that this will probably fail due to
9343 system restrictions, even for the super-user.
9349 Remove existing destination files.
9352 @itemx --interactive
9354 @opindex --interactive
9355 @cindex prompting, and @command{ln}
9356 Prompt whether to remove existing destination files.
9362 If @option{-s} is not in effect, and the source file is a symbolic
9363 link, create the hard link to the file referred to by the symbolic
9364 link, rather than the symbolic link itself.
9367 @itemx --no-dereference
9369 @opindex --no-dereference
9370 Do not treat the last operand specially when it is a symbolic link to
9371 a directory. Instead, treat it as if it were a normal file.
9373 When the destination is an actual directory (not a symlink to one),
9374 there is no ambiguity. The link is created in that directory.
9375 But when the specified destination is a symlink to a directory,
9376 there are two ways to treat the user's request. @command{ln} can
9377 treat the destination just as it would a normal directory and create
9378 the link in it. On the other hand, the destination can be viewed as a
9379 non-directory---as the symlink itself. In that case, @command{ln}
9380 must delete or backup that symlink before creating the new link.
9381 The default is to treat a destination that is a symlink to a directory
9382 just like a directory.
9384 This option is weaker than the @option{--no-target-directory}
9385 (@option{-T}) option, so it has no effect if both options are given.
9391 If @option{-s} is not in effect, and the source file is a symbolic
9392 link, create the hard link to the symbolic link itself. On platforms
9393 where this is not supported by the kernel, this option creates a
9394 symbolic link with identical contents; since symbolic link contents
9395 cannot be edited, any file name resolution performed through either
9396 link will be the same as if a hard link had been created.
9402 Make symbolic links relative to the link location.
9407 ln -srv /a/file /tmp
9408 '/tmp/file' -> '../a/file'
9411 @xref{realpath invocation}, which gives greater control
9412 over relative file name generation.
9418 Make symbolic links instead of hard links. This option merely produces
9419 an error message on systems that do not support symbolic links.
9425 @optNoTargetDirectory
9431 Print the name of each file after linking it successfully.
9435 @cindex hard links to symbolic links
9436 @cindex symbolic links and @command{ln}
9437 If @option{-L} and @option{-P} are both given, the last one takes
9438 precedence. If @option{-s} is also given, @option{-L} and @option{-P}
9439 are silently ignored. If neither option is given, then this
9440 implementation defaults to @option{-P} if the system @code{link} supports
9441 hard links to symbolic links (such as the GNU system), and @option{-L}
9442 if @code{link} follows symbolic links (such as on BSD).
9451 # Create link ../a pointing to a in that directory.
9452 # Not really useful because it points to itself.
9457 # Change to the target before creating symlinks to avoid being confused.
9463 # Hard coded file names don't move well.
9464 ln -s $(pwd)/a /some/dir/
9468 # Relative file names survive directory moves and also
9469 # work across networked file systems.
9470 ln -s afile anotherfile
9471 ln -s ../adir/afile yetanotherfile
9475 @node mkdir invocation
9476 @section @command{mkdir}: Make directories
9479 @cindex directories, creating
9480 @cindex creating directories
9482 @command{mkdir} creates directories with the specified names. Synopsis:
9485 mkdir [@var{option}]@dots{} @var{name}@dots{}
9488 @command{mkdir} creates each directory @var{name} in the order given.
9489 It reports an error if @var{name} already exists, unless the
9490 @option{-p} option is given and @var{name} is a directory.
9492 The program accepts the following options. Also see @ref{Common options}.
9497 @itemx --mode=@var{mode}
9500 @cindex modes of created directories, setting
9501 Set the file permission bits of created directories to @var{mode},
9502 which uses the same syntax as
9503 in @command{chmod} and uses @samp{a=rwx} (read, write and execute allowed for
9504 everyone) for the point of the departure. @xref{File permissions}.
9506 Normally the directory has the desired file mode bits at the moment it
9507 is created. As a @acronym{GNU} extension, @var{mode} may also mention
9508 special mode bits, but in this case there may be a temporary window
9509 during which the directory exists but its special mode bits are
9510 incorrect. @xref{Directory Setuid and Setgid}, for how the
9511 set-user-ID and set-group-ID bits of directories are inherited unless
9512 overridden in this way.
9518 @cindex parent directories, creating
9519 Make any missing parent directories for each argument, setting their
9520 file permission bits to the umask modified by @samp{u+wx}. Ignore
9521 existing parent directories, and do not change their file permission
9524 To set the file permission bits of any newly-created parent
9525 directories to a value that includes @samp{u+wx}, you can set the
9526 umask before invoking @command{mkdir}. For example, if the shell
9527 command @samp{(umask u=rwx,go=rx; mkdir -p P/Q)} creates the parent
9528 @file{P} it sets the parent's permission bits to @samp{u=rwx,go=rx}.
9529 To set a parent's special mode bits as well, you can invoke
9530 @command{chmod} after @command{mkdir}. @xref{Directory Setuid and
9531 Setgid}, for how the set-user-ID and set-group-ID bits of
9532 newly-created parent directories are inherited.
9538 Print a message for each created directory. This is most useful with
9541 @item -Z @var{context}
9542 @itemx --context=@var{context}
9546 @cindex security context
9547 Set the default SELinux security context to be used for created directories.
9554 @node mkfifo invocation
9555 @section @command{mkfifo}: Make FIFOs (named pipes)
9558 @cindex FIFOs, creating
9559 @cindex named pipes, creating
9560 @cindex creating FIFOs (named pipes)
9562 @command{mkfifo} creates FIFOs (also called @dfn{named pipes}) with the
9563 specified names. Synopsis:
9566 mkfifo [@var{option}] @var{name}@dots{}
9569 A @dfn{FIFO} is a special file type that permits independent processes
9570 to communicate. One process opens the FIFO file for writing, and
9571 another for reading, after which data can flow as with the usual
9572 anonymous pipe in shells or elsewhere.
9574 The program accepts the following option. Also see @ref{Common options}.
9579 @itemx --mode=@var{mode}
9582 @cindex modes of created FIFOs, setting
9583 Set the mode of created FIFOs to @var{mode}, which is symbolic as in
9584 @command{chmod} and uses @samp{a=rw} (read and write allowed for everyone)
9585 for the point of departure. @var{mode} should specify only file
9586 permission bits. @xref{File permissions}.
9588 @item -Z @var{context}
9589 @itemx --context=@var{context}
9593 @cindex security context
9594 Set the default SELinux security context to be used for created FIFOs.
9601 @node mknod invocation
9602 @section @command{mknod}: Make block or character special files
9605 @cindex block special files, creating
9606 @cindex character special files, creating
9608 @command{mknod} creates a FIFO, character special file, or block special
9609 file with the specified name. Synopsis:
9612 mknod [@var{option}]@dots{} @var{name} @var{type} [@var{major} @var{minor}]
9615 @cindex special files
9616 @cindex block special files
9617 @cindex character special files
9618 Unlike the phrase ``special file type'' above, the term @dfn{special
9619 file} has a technical meaning on Unix: something that can generate or
9620 receive data. Usually this corresponds to a physical piece of hardware,
9621 e.g., a printer or a disk. (These files are typically created at
9622 system-configuration time.) The @command{mknod} command is what creates
9623 files of this type. Such devices can be read either a character at a
9624 time or a ``block'' (many characters) at a time, hence we say there are
9625 @dfn{block special} files and @dfn{character special} files.
9627 @c mknod is a shell built-in at least with OpenBSD's /bin/sh
9628 @mayConflictWithShellBuiltIn{mknod}
9630 The arguments after @var{name} specify the type of file to make:
9635 @opindex p @r{for FIFO file}
9639 @opindex b @r{for block special file}
9640 for a block special file
9643 @c Don't document the 'u' option -- it's just a synonym for 'c'.
9644 @c Do *any* versions of mknod still use it?
9646 @opindex c @r{for character special file}
9647 @c @opindex u @r{for character special file}
9648 for a character special file
9652 When making a block or character special file, the major and minor
9653 device numbers must be given after the file type.
9654 If a major or minor device number begins with @samp{0x} or @samp{0X},
9655 it is interpreted as hexadecimal; otherwise, if it begins with @samp{0},
9656 as octal; otherwise, as decimal.
9658 The program accepts the following option. Also see @ref{Common options}.
9663 @itemx --mode=@var{mode}
9666 Set the mode of created files to @var{mode}, which is symbolic as in
9667 @command{chmod} and uses @samp{a=rw} as the point of departure.
9668 @var{mode} should specify only file permission bits.
9669 @xref{File permissions}.
9671 @item -Z @var{context}
9672 @itemx --context=@var{context}
9676 @cindex security context
9677 Set the default SELinux security context to be used for created files.
9684 @node readlink invocation
9685 @section @command{readlink}: Print value of a symlink or canonical file name
9688 @cindex displaying value of a symbolic link
9689 @cindex canonical file name
9690 @cindex canonicalize a file name
9693 @command{readlink} may work in one of two supported modes:
9699 @command{readlink} outputs the value of the given symbolic link.
9700 If @command{readlink} is invoked with an argument other than the name
9701 of a symbolic link, it produces no output and exits with a nonzero exit code.
9703 @item Canonicalize mode
9705 @command{readlink} outputs the absolute name of the given file which contains
9706 no @file{.}, @file{..} components nor any repeated separators
9707 (@file{/}) or symbolic links.
9712 readlink [@var{option}] @var{file}
9715 By default, @command{readlink} operates in readlink mode.
9717 The program accepts the following options. Also see @ref{Common options}.
9722 @itemx --canonicalize
9724 @opindex --canonicalize
9725 Activate canonicalize mode.
9726 If any component of the file name except the last one is missing or unavailable,
9727 @command{readlink} produces no output and exits with a nonzero exit
9728 code. A trailing slash is ignored.
9731 @itemx --canonicalize-existing
9733 @opindex --canonicalize-existing
9734 Activate canonicalize mode.
9735 If any component is missing or unavailable, @command{readlink} produces
9736 no output and exits with a nonzero exit code. A trailing slash
9737 requires that the name resolve to a directory.
9740 @itemx --canonicalize-missing
9742 @opindex --canonicalize-missing
9743 Activate canonicalize mode.
9744 If any component is missing or unavailable, @command{readlink} treats it
9750 @opindex --no-newline
9751 Do not output the trailing newline.
9761 Suppress most error messages.
9767 Report error messages.
9771 The @command{readlink} utility first appeared in OpenBSD 2.1.
9773 The @command{realpath} command without options, operates like
9774 @command{readlink} in canonicalize mode.
9779 @node rmdir invocation
9780 @section @command{rmdir}: Remove empty directories
9783 @cindex removing empty directories
9784 @cindex directories, removing empty
9786 @command{rmdir} removes empty directories. Synopsis:
9789 rmdir [@var{option}]@dots{} @var{directory}@dots{}
9792 If any @var{directory} argument does not refer to an existing empty
9793 directory, it is an error.
9795 The program accepts the following options. Also see @ref{Common options}.
9799 @item --ignore-fail-on-non-empty
9800 @opindex --ignore-fail-on-non-empty
9801 @cindex directory deletion, ignoring failures
9802 Ignore each failure to remove a directory that is solely because
9803 the directory is non-empty.
9809 @cindex parent directories, removing
9810 Remove @var{directory}, then try to remove each component of @var{directory}.
9811 So, for example, @samp{rmdir -p a/b/c} is similar to @samp{rmdir a/b/c a/b a}.
9812 As such, it fails if any of those directories turns out not to be empty.
9813 Use the @option{--ignore-fail-on-non-empty} option to make it so such
9814 a failure does not evoke a diagnostic and does not cause @command{rmdir} to
9815 exit unsuccessfully.
9821 @cindex directory deletion, reporting
9822 Give a diagnostic for each successful removal.
9823 @var{directory} is removed.
9827 @xref{rm invocation}, for how to remove non-empty directories (recursively).
9832 @node unlink invocation
9833 @section @command{unlink}: Remove files via the unlink syscall
9836 @cindex removing files or directories (via the unlink syscall)
9838 @command{unlink} deletes a single specified file name.
9839 It is a minimalist interface to the system-provided
9840 @code{unlink} function. @xref{Deleting Files, , , libc,
9841 The GNU C Library Reference Manual}. Synopsis:
9842 It avoids the bells and whistles of the more commonly-used
9843 @command{rm} command (@pxref{rm invocation}).
9846 unlink @var{filename}
9849 On some systems @code{unlink} can be used to delete the name of a
9850 directory. On others, it can be used that way only by a privileged user.
9851 In the GNU system @code{unlink} can never delete the name of a directory.
9853 The @command{unlink} command honors the @option{--help} and
9854 @option{--version} options. To remove a file whose name begins with
9855 @samp{-}, prefix the name with @samp{./}, e.g., @samp{unlink ./--help}.
9860 @node Changing file attributes
9861 @chapter Changing file attributes
9863 @cindex changing file attributes
9864 @cindex file attributes, changing
9865 @cindex attributes, file
9867 A file is not merely its contents, a name, and a file type
9868 (@pxref{Special file types}). A file also has an owner (a user ID), a
9869 group (a group ID), permissions (what the owner can do with the file,
9870 what people in the group can do, and what everyone else can do), various
9871 timestamps, and other information. Collectively, we call these a file's
9874 These commands change file attributes.
9877 * chgrp invocation:: Change file groups.
9878 * chmod invocation:: Change access permissions.
9879 * chown invocation:: Change file owners and groups.
9880 * touch invocation:: Change file timestamps.
9884 @node chown invocation
9885 @section @command{chown}: Change file owner and group
9888 @cindex file ownership, changing
9889 @cindex group ownership, changing
9890 @cindex changing file ownership
9891 @cindex changing group ownership
9893 @command{chown} changes the user and/or group ownership of each given @var{file}
9894 to @var{new-owner} or to the user and group of an existing reference file.
9898 chown [@var{option}]@dots{} @{@var{new-owner} | --reference=@var{ref_file}@}@c
9902 If used, @var{new-owner} specifies the new owner and/or group as follows
9903 (with no embedded white space):
9906 [@var{owner}] [ : [@var{group}] ]
9913 If only an @var{owner} (a user name or numeric user ID) is given, that
9914 user is made the owner of each given file, and the files' group is not
9917 @item owner@samp{:}group
9918 If the @var{owner} is followed by a colon and a @var{group} (a
9919 group name or numeric group ID), with no spaces between them, the group
9920 ownership of the files is changed as well (to @var{group}).
9923 If a colon but no group name follows @var{owner}, that user is
9924 made the owner of the files and the group of the files is changed to
9925 @var{owner}'s login group.
9928 If the colon and following @var{group} are given, but the owner
9929 is omitted, only the group of the files is changed; in this case,
9930 @command{chown} performs the same function as @command{chgrp}.
9933 If only a colon is given, or if @var{new-owner} is empty, neither the
9934 owner nor the group is changed.
9938 If @var{owner} or @var{group} is intended to represent a numeric user
9939 or group ID, then you may specify it with a leading @samp{+}.
9940 @xref{Disambiguating names and IDs}.
9942 Some older scripts may still use @samp{.} in place of the @samp{:} separator.
9943 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) does not
9944 require support for that, but for backward compatibility @acronym{GNU}
9945 @command{chown} supports @samp{.} so long as no ambiguity results.
9946 New scripts should avoid the use of @samp{.} because it is not
9947 portable, and because it has undesirable results if the entire
9948 @var{owner@samp{.}group} happens to identify a user whose name
9951 The @command{chown} command sometimes clears the set-user-ID or
9952 set-group-ID permission bits. This behavior depends on the policy and
9953 functionality of the underlying @code{chown} system call, which may
9954 make system-dependent file mode modifications outside the control of
9955 the @command{chown} command. For example, the @command{chown} command
9956 might not affect those bits when invoked by a user with appropriate
9957 privileges, or when the
9958 bits signify some function other than executable permission (e.g.,
9960 When in doubt, check the underlying system behavior.
9962 The program accepts the following options. Also see @ref{Common options}.
9970 @cindex changed owners, verbosely describing
9971 Verbosely describe the action for each @var{file} whose ownership
9980 @cindex error messages, omitting
9981 Do not print error messages about files whose ownership cannot be
9984 @itemx @w{@kbd{--from}=@var{old-owner}}
9986 @cindex symbolic links, changing owner
9987 Change a @var{file}'s ownership only if it has current attributes specified
9988 by @var{old-owner}. @var{old-owner} has the same form as @var{new-owner}
9990 This option is useful primarily from a security standpoint in that
9991 it narrows considerably the window of potential abuse.
9992 For example, to reflect a user ID numbering change for one user's files
9993 without an option like this, @code{root} might run
9996 find / -owner OLDUSER -print0 | xargs -0 chown -h NEWUSER
9999 But that is dangerous because the interval between when the @command{find}
10000 tests the existing file's owner and when the @command{chown} is actually run
10001 may be quite large.
10002 One way to narrow the gap would be to invoke chown for each file
10006 find / -owner OLDUSER -exec chown -h NEWUSER @{@} \;
10009 But that is very slow if there are many affected files.
10010 With this option, it is safer (the gap is narrower still)
10011 though still not perfect:
10014 chown -h -R --from=OLDUSER NEWUSER /
10017 @item --dereference
10018 @opindex --dereference
10019 @cindex symbolic links, changing owner
10021 Do not act on symbolic links themselves but rather on what they point to.
10022 This is the default.
10025 @itemx --no-dereference
10027 @opindex --no-dereference
10028 @cindex symbolic links, changing owner
10030 Act on symbolic links themselves instead of what they point to.
10031 This mode relies on the @code{lchown} system call.
10032 On systems that do not provide the @code{lchown} system call,
10033 @command{chown} fails when a file specified on the command line
10034 is a symbolic link.
10035 By default, no diagnostic is issued for symbolic links encountered
10036 during a recursive traversal, but see @option{--verbose}.
10038 @itemx --preserve-root
10039 @opindex --preserve-root
10040 @cindex root directory, disallow recursive modification
10041 Fail upon any attempt to recursively change the root directory, @file{/}.
10042 Without @option{--recursive}, this option has no effect.
10043 @xref{Treating / specially}.
10045 @itemx --no-preserve-root
10046 @opindex --no-preserve-root
10047 @cindex root directory, allow recursive modification
10048 Cancel the effect of any preceding @option{--preserve-root} option.
10049 @xref{Treating / specially}.
10051 @item --reference=@var{ref_file}
10052 @opindex --reference
10053 Change the user and group of each @var{file} to be the same as those of
10054 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
10055 user and group of the symbolic link, but rather those of the file it
10062 Output a diagnostic for every file processed.
10063 If a symbolic link is encountered during a recursive traversal
10064 on a system without the @code{lchown} system call, and @option{--no-dereference}
10065 is in effect, then issue a diagnostic saying neither the symbolic link nor
10066 its referent is being changed.
10071 @opindex --recursive
10072 @cindex recursively changing file ownership
10073 Recursively change ownership of directories and their contents.
10076 @xref{Traversing symlinks}.
10079 @xref{Traversing symlinks}.
10082 @xref{Traversing symlinks}.
10091 # Change the owner of /u to "root".
10094 # Likewise, but also change its group to "staff".
10095 chown root:staff /u
10097 # Change the owner of /u and subfiles to "root".
10102 @node chgrp invocation
10103 @section @command{chgrp}: Change group ownership
10106 @cindex group ownership, changing
10107 @cindex changing group ownership
10109 @command{chgrp} changes the group ownership of each given @var{file}
10110 to @var{group} (which can be either a group name or a numeric group ID)
10111 or to the group of an existing reference file. Synopsis:
10114 chgrp [@var{option}]@dots{} @{@var{group} | --reference=@var{ref_file}@}@c
10118 If @var{group} is intended to represent a
10119 numeric group ID, then you may specify it with a leading @samp{+}.
10120 @xref{Disambiguating names and IDs}.
10122 The program accepts the following options. Also see @ref{Common options}.
10130 @cindex changed files, verbosely describing
10131 Verbosely describe the action for each @var{file} whose group actually
10140 @cindex error messages, omitting
10141 Do not print error messages about files whose group cannot be
10144 @item --dereference
10145 @opindex --dereference
10146 @cindex symbolic links, changing owner
10148 Do not act on symbolic links themselves but rather on what they point to.
10149 This is the default.
10152 @itemx --no-dereference
10154 @opindex --no-dereference
10155 @cindex symbolic links, changing group
10157 Act on symbolic links themselves instead of what they point to.
10158 This mode relies on the @code{lchown} system call.
10159 On systems that do not provide the @code{lchown} system call,
10160 @command{chgrp} fails when a file specified on the command line
10161 is a symbolic link.
10162 By default, no diagnostic is issued for symbolic links encountered
10163 during a recursive traversal, but see @option{--verbose}.
10165 @itemx --preserve-root
10166 @opindex --preserve-root
10167 @cindex root directory, disallow recursive modification
10168 Fail upon any attempt to recursively change the root directory, @file{/}.
10169 Without @option{--recursive}, this option has no effect.
10170 @xref{Treating / specially}.
10172 @itemx --no-preserve-root
10173 @opindex --no-preserve-root
10174 @cindex root directory, allow recursive modification
10175 Cancel the effect of any preceding @option{--preserve-root} option.
10176 @xref{Treating / specially}.
10178 @item --reference=@var{ref_file}
10179 @opindex --reference
10180 Change the group of each @var{file} to be the same as that of
10181 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
10182 group of the symbolic link, but rather that of the file it refers to.
10188 Output a diagnostic for every file processed.
10189 If a symbolic link is encountered during a recursive traversal
10190 on a system without the @code{lchown} system call, and @option{--no-dereference}
10191 is in effect, then issue a diagnostic saying neither the symbolic link nor
10192 its referent is being changed.
10197 @opindex --recursive
10198 @cindex recursively changing group ownership
10199 Recursively change the group ownership of directories and their contents.
10202 @xref{Traversing symlinks}.
10205 @xref{Traversing symlinks}.
10208 @xref{Traversing symlinks}.
10217 # Change the group of /u to "staff".
10220 # Change the group of /u and subfiles to "staff".
10225 @node chmod invocation
10226 @section @command{chmod}: Change access permissions
10229 @cindex changing access permissions
10230 @cindex access permissions, changing
10231 @cindex permissions, changing access
10233 @command{chmod} changes the access permissions of the named files. Synopsis:
10236 chmod [@var{option}]@dots{} @{@var{mode} | --reference=@var{ref_file}@}@c
10240 @cindex symbolic links, permissions of
10241 @command{chmod} never changes the permissions of symbolic links, since
10242 the @command{chmod} system call cannot change their permissions.
10243 This is not a problem since the permissions of symbolic links are
10244 never used. However, for each symbolic link listed on the command
10245 line, @command{chmod} changes the permissions of the pointed-to file.
10246 In contrast, @command{chmod} ignores symbolic links encountered during
10247 recursive directory traversals.
10249 A successful use of @command{chmod} clears the set-group-ID bit of a
10250 regular file if the file's group ID does not match the user's
10251 effective group ID or one of the user's supplementary group IDs,
10252 unless the user has appropriate privileges. Additional restrictions
10253 may cause the set-user-ID and set-group-ID bits of @var{mode} or
10254 @var{ref_file} to be ignored. This behavior depends on the policy and
10255 functionality of the underlying @code{chmod} system call. When in
10256 doubt, check the underlying system behavior.
10258 If used, @var{mode} specifies the new file mode bits.
10259 For details, see the section on @ref{File permissions}.
10260 If you really want @var{mode} to have a leading @samp{-}, you should
10261 use @option{--} first, e.g., @samp{chmod -- -w file}. Typically,
10262 though, @samp{chmod a-w file} is preferable, and @command{chmod -w
10263 file} (without the @option{--}) complains if it behaves differently
10264 from what @samp{chmod a-w file} would do.
10266 The program accepts the following options. Also see @ref{Common options}.
10274 Verbosely describe the action for each @var{file} whose permissions
10283 @cindex error messages, omitting
10284 Do not print error messages about files whose permissions cannot be
10287 @itemx --preserve-root
10288 @opindex --preserve-root
10289 @cindex root directory, disallow recursive modification
10290 Fail upon any attempt to recursively change the root directory, @file{/}.
10291 Without @option{--recursive}, this option has no effect.
10292 @xref{Treating / specially}.
10294 @itemx --no-preserve-root
10295 @opindex --no-preserve-root
10296 @cindex root directory, allow recursive modification
10297 Cancel the effect of any preceding @option{--preserve-root} option.
10298 @xref{Treating / specially}.
10304 Verbosely describe the action or non-action taken for every @var{file}.
10306 @item --reference=@var{ref_file}
10307 @opindex --reference
10308 Change the mode of each @var{file} to be the same as that of @var{ref_file}.
10309 @xref{File permissions}.
10310 If @var{ref_file} is a symbolic link, do not use the mode
10311 of the symbolic link, but rather that of the file it refers to.
10316 @opindex --recursive
10317 @cindex recursively changing access permissions
10318 Recursively change permissions of directories and their contents.
10325 @node touch invocation
10326 @section @command{touch}: Change file timestamps
10329 @cindex changing file timestamps
10330 @cindex file timestamps, changing
10331 @cindex timestamps, changing file
10333 @command{touch} changes the access and/or modification times of the
10334 specified files. Synopsis:
10337 touch [@var{option}]@dots{} @var{file}@dots{}
10340 @cindex empty files, creating
10341 Any @var{file} argument that does not exist is created empty, unless
10342 option @option{--no-create} (@option{-c}) or @option{--no-dereference}
10343 (@option{-h}) was in effect.
10345 A @var{file} argument string of @samp{-} is handled specially and
10346 causes @command{touch} to change the times of the file associated with
10350 By default, @command{touch} sets file timestamps to the current time.
10351 Because @command{touch} acts on its operands left to right, the
10352 resulting timestamps of earlier and later operands may disagree.
10353 Also, the determination of what time is ``current'' depends on the
10354 platform. Platforms with network file systems often use different
10355 clocks for the operating system and for file systems; because
10356 @command{touch} typically uses file systems' clocks by default, clock
10357 skew can cause the resulting file timestamps to appear to be in a
10358 program's ``future'' or ``past''.
10360 @cindex file timestamp resolution
10361 The @command{touch} command sets the file's timestamp to the greatest
10362 representable value that is not greater than the requested time. This
10363 can differ from the requested time for several reasons. First, the
10364 requested time may have a higher resolution than supported. Second, a
10365 file system may use different resolutions for different types of
10366 times. Third, file timestamps may use a different resolution than
10367 operating system timestamps. Fourth, the operating system primitives
10368 used to update timestamps may employ yet a different resolution. For
10369 example, in theory a file system might use 10-microsecond resolution
10370 for access time and 100-nanosecond resolution for modification time,
10371 and the operating system might use nanosecond resolution for the
10372 current time and microsecond resolution for the primitive that
10373 @command{touch} uses to set a file's timestamp to an arbitrary value.
10375 @cindex permissions, for changing file timestamps
10376 When setting file timestamps to the current time, @command{touch} can
10377 change the timestamps for files that the user does not own but has
10378 write permission for. Otherwise, the user must own the files. Some
10379 older systems have a further restriction: the user must own the files
10380 unless both the access and modification times are being set to the
10383 Although @command{touch} provides options for changing two of the times---the
10384 times of last access and modification---of a file, there is actually
10385 a standard third one as well: the inode change time. This is often
10386 referred to as a file's @code{ctime}.
10387 The inode change time represents the time when the file's meta-information
10388 last changed. One common example of this is when the permissions of a
10389 file change. Changing the permissions doesn't access the file, so
10390 the atime doesn't change, nor does it modify the file, so the mtime
10391 doesn't change. Yet, something about the file itself has changed,
10392 and this must be noted somewhere. This is the job of the ctime field.
10393 This is necessary, so that, for example, a backup program can make a
10394 fresh copy of the file, including the new permissions value.
10395 Another operation that modifies a file's ctime without affecting
10396 the others is renaming. In any case, it is not possible, in normal
10397 operations, for a user to change the ctime field to a user-specified value.
10398 Some operating systems and file systems support a fourth time: the
10399 birth time, when the file was first created; by definition, this
10400 timestamp never changes.
10403 Time stamps assume the time zone rules specified by the @env{TZ}
10404 environment variable, or by the system default rules if @env{TZ} is
10405 not set. @xref{TZ Variable,, Specifying the Time Zone with @env{TZ},
10406 libc, The GNU C Library Reference Manual}.
10407 You can avoid ambiguities during
10408 daylight saving transitions by using @sc{utc} time stamps.
10410 The program accepts the following options. Also see @ref{Common options}.
10415 @itemx --time=atime
10416 @itemx --time=access
10420 @opindex atime@r{, changing}
10421 @opindex access @r{time, changing}
10422 @opindex use @r{time, changing}
10423 Change the access time only.
10428 @opindex --no-create
10429 Do not warn about or create files that do not exist.
10432 @itemx --date=@var{time}
10436 Use @var{time} instead of the current time. It can contain month names,
10437 time zones, @samp{am} and @samp{pm}, @samp{yesterday}, etc. For
10438 example, @option{--date="2004-02-27 14:19:13.489392193 +0530"}
10439 specifies the instant of time that is 489,392,193 nanoseconds after
10440 February 27, 2004 at 2:19:13 PM in a time zone that is 5 hours and 30
10441 minutes east of @acronym{UTC}. @xref{Date input formats}.
10442 File systems that do not support high-resolution time stamps
10443 silently ignore any excess precision here.
10447 @cindex BSD @command{touch} compatibility
10448 Ignored; for compatibility with BSD versions of @command{touch}.
10451 @itemx --no-dereference
10453 @opindex --no-dereference
10454 @cindex symbolic links, changing time
10456 Attempt to change the timestamps of a symbolic link, rather than what
10457 the link refers to. When using this option, empty files are not
10458 created, but option @option{-c} must also be used to avoid warning
10459 about files that do not exist. Not all systems support changing the
10460 timestamps of symlinks, since underlying system support for this
10461 action was not required until @acronym{POSIX} 2008. Also, on some
10462 systems, the mere act of examining a symbolic link changes the access
10463 time, such that only changes to the modification time will persist
10464 long enough to be observable. When coupled with option @option{-r}, a
10465 reference timestamp is taken from a symbolic link rather than the file
10469 @itemx --time=mtime
10470 @itemx --time=modify
10473 @opindex mtime@r{, changing}
10474 @opindex modify @r{time, changing}
10475 Change the modification time only.
10477 @item -r @var{file}
10478 @itemx --reference=@var{file}
10480 @opindex --reference
10481 Use the times of the reference @var{file} instead of the current time.
10482 If this option is combined with the @option{--date=@var{time}}
10483 (@option{-d @var{time}}) option, the reference @var{file}'s time is
10484 the origin for any relative @var{time}s given, but is otherwise ignored.
10485 For example, @samp{-r foo -d '-5 seconds'} specifies a time stamp
10486 equal to five seconds before the corresponding time stamp for @file{foo}.
10487 If @var{file} is a symbolic link, the reference timestamp is taken
10488 from the target of the symlink, unless @option{-h} was also in effect.
10490 @item -t [[@var{cc}]@var{yy}]@var{mmddhhmm}[.@var{ss}]
10491 Use the argument (optional four-digit or two-digit years, months,
10492 days, hours, minutes, optional seconds) instead of the current time.
10493 If the year is specified with only two digits, then @var{cc}
10494 is 20 for years in the range 0 @dots{} 68, and 19 for years in
10495 69 @dots{} 99. If no digits of the year are specified,
10496 the argument is interpreted as a date in the current year.
10497 Note that @var{ss} may be @samp{60}, to accommodate leap seconds.
10501 @vindex _POSIX2_VERSION
10502 On older systems, @command{touch} supports an obsolete syntax, as follows.
10503 If no timestamp is given with any of the @option{-d}, @option{-r}, or
10504 @option{-t} options, and if there are two or more @var{file}s and the
10505 first @var{file} is of the form @samp{@var{mmddhhmm}[@var{yy}]} and this
10506 would be a valid argument to the @option{-t} option (if the @var{yy}, if
10507 any, were moved to the front), and if the represented year
10508 is in the range 1969--1999, that argument is interpreted as the time
10509 for the other files instead of as a file name.
10510 This obsolete behavior can be enabled or disabled with the
10511 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
10512 conformance}), but portable scripts should avoid commands whose
10513 behavior depends on this variable.
10514 For example, use @samp{touch ./12312359 main.c} or @samp{touch -t
10515 12312359 main.c} rather than the ambiguous @samp{touch 12312359 main.c}.
10521 @chapter Disk usage
10525 No disk can hold an infinite amount of data. These commands report
10526 how much disk storage is in use or available, report other file and
10527 file status information, and write buffers to disk.
10530 * df invocation:: Report file system disk space usage.
10531 * du invocation:: Estimate file space usage.
10532 * stat invocation:: Report file or file system status.
10533 * sync invocation:: Synchronize memory and disk.
10534 * truncate invocation:: Shrink or extend the size of a file.
10538 @node df invocation
10539 @section @command{df}: Report file system disk space usage
10542 @cindex file system disk usage
10543 @cindex disk usage by file system
10545 @command{df} reports the amount of disk space used and available on
10546 file systems. Synopsis:
10549 df [@var{option}]@dots{} [@var{file}]@dots{}
10552 With no arguments, @command{df} reports the space used and available on all
10553 currently mounted file systems (of all types). Otherwise, @command{df}
10554 reports on the file system containing each argument @var{file}.
10556 Normally the disk space is printed in units of
10557 1024 bytes, but this can be overridden (@pxref{Block size}).
10558 Non-integer quantities are rounded up to the next higher unit.
10560 @cindex disk device file
10561 @cindex device file, disk
10562 If an argument @var{file} is a disk device file containing a mounted
10563 file system, @command{df} shows the space available on that file system
10564 rather than on the file system containing the device node (i.e., the root
10565 file system). @sc{gnu} @command{df} does not attempt to determine the
10567 on unmounted file systems, because on most kinds of systems doing so
10568 requires extremely nonportable intimate knowledge of file system
10571 The program accepts the following options. Also see @ref{Common options}.
10579 @cindex automounter file systems
10580 @cindex ignore file systems
10581 Include in the listing dummy file systems, which
10582 are omitted by default. Such file systems are typically special-purpose
10583 pseudo-file-systems, such as automounter entries.
10585 @item -B @var{size}
10586 @itemx --block-size=@var{size}
10588 @opindex --block-size
10589 @cindex file system sizes
10590 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10591 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10595 @cindex grand total of disk size, usage and available space
10596 Print a grand total of all arguments after all arguments have
10597 been processed. This can be used to find out the total disk size, usage
10598 and available space of all listed devices.
10604 Equivalent to @option{--si}.
10610 @cindex inode usage
10611 List inode usage information instead of block usage. An inode (short
10612 for index node) contains information about a file such as its owner,
10613 permissions, timestamps, and location on the disk.
10617 @cindex kibibytes for file system sizes
10618 Print sizes in 1024-byte blocks, overriding the default block size
10619 (@pxref{Block size}).
10620 This option is equivalent to @option{--block-size=1K}.
10626 @cindex file system types, limiting output to certain
10627 Limit the listing to local file systems. By default, remote file systems
10632 @cindex file system space, retrieving old data more quickly
10633 Do not invoke the @code{sync} system call before getting any usage data.
10634 This may make @command{df} run significantly faster on systems with many
10635 disks, but on some systems (notably SunOS) the results may be slightly
10636 out of date. This is the default.
10639 @itemx --portability
10641 @opindex --portability
10642 @cindex one-line output format
10643 @cindex @acronym{POSIX} output format
10644 @cindex portable output format
10645 @cindex output format, portable
10646 Use the @acronym{POSIX} output format. This is like the default format except
10651 The information about each file system is always printed on exactly
10652 one line; a mount device is never put on a line by itself. This means
10653 that if the mount device name is more than 20 characters long (e.g., for
10654 some network mounts), the columns are misaligned.
10657 The labels in the header output line are changed to conform to @acronym{POSIX}.
10660 The default block size and output format are unaffected by the
10661 @env{DF_BLOCK_SIZE}, @env{BLOCK_SIZE} and @env{BLOCKSIZE} environment
10662 variables. However, the default block size is still affected by
10663 @env{POSIXLY_CORRECT}: it is 512 if @env{POSIXLY_CORRECT} is set, 1024
10664 otherwise. @xref{Block size}.
10671 @cindex file system space, retrieving current data more slowly
10672 Invoke the @code{sync} system call before getting any usage data. On
10673 some systems (notably SunOS), doing this yields more up to date results,
10674 but in general this option makes @command{df} much slower, especially when
10675 there are many or very busy file systems.
10677 @item -t @var{fstype}
10678 @itemx --type=@var{fstype}
10681 @cindex file system types, limiting output to certain
10682 Limit the listing to file systems of type @var{fstype}. Multiple
10683 file system types can be specified by giving multiple @option{-t} options.
10684 By default, nothing is omitted.
10687 @itemx --print-type
10689 @opindex --print-type
10690 @cindex file system types, printing
10691 Print each file system's type. The types printed here are the same ones
10692 you can include or exclude with @option{-t} and @option{-x}. The particular
10693 types printed are whatever is supported by the system. Here are some of
10694 the common names (this list is certainly not exhaustive):
10699 @cindex @acronym{NFS} file system type
10700 An @acronym{NFS} file system, i.e., one mounted over a network from another
10701 machine. This is the one type name which seems to be used uniformly by
10704 @item 4.2@r{, }ufs@r{, }efs@dots{}
10705 @cindex Linux file system types
10706 @cindex local file system types
10707 @opindex 4.2 @r{file system type}
10708 @opindex ufs @r{file system type}
10709 @opindex efs @r{file system type}
10710 A file system on a locally-mounted hard disk. (The system might even
10711 support more than one type here; Linux does.)
10713 @item hsfs@r{, }cdfs
10714 @cindex CD-ROM file system type
10715 @cindex High Sierra file system
10716 @opindex hsfs @r{file system type}
10717 @opindex cdfs @r{file system type}
10718 A file system on a CD-ROM drive. HP-UX uses @samp{cdfs}, most other
10719 systems use @samp{hsfs} (@samp{hs} for ``High Sierra'').
10722 @cindex PC file system
10723 @cindex DOS file system
10724 @cindex MS-DOS file system
10725 @cindex diskette file system
10727 An MS-DOS file system, usually on a diskette.
10731 @item -x @var{fstype}
10732 @itemx --exclude-type=@var{fstype}
10734 @opindex --exclude-type
10735 Limit the listing to file systems not of type @var{fstype}.
10736 Multiple file system types can be eliminated by giving multiple
10737 @option{-x} options. By default, no file system types are omitted.
10740 Ignored; for compatibility with System V versions of @command{df}.
10745 Failure includes the case where no output is generated, so you can
10746 inspect the exit status of a command like @samp{df -t ext3 -t reiserfs
10747 @var{dir}} to test whether @var{dir} is on a file system of type
10748 @samp{ext3} or @samp{reiserfs}.
10751 @node du invocation
10752 @section @command{du}: Estimate file space usage
10755 @cindex file space usage
10756 @cindex disk usage for files
10758 @command{du} reports the amount of disk space used by the specified files
10759 and for each subdirectory (of directory arguments). Synopsis:
10762 du [@var{option}]@dots{} [@var{file}]@dots{}
10765 With no arguments, @command{du} reports the disk space for the current
10766 directory. Normally the disk space is printed in units of
10767 1024 bytes, but this can be overridden (@pxref{Block size}).
10768 Non-integer quantities are rounded up to the next higher unit.
10770 If two or more hard links point to the same file, only one of the hard
10771 links is counted. The @var{file} argument order affects which links
10772 are counted, and changing the argument order may change the numbers
10773 that @command{du} outputs.
10775 The program accepts the following options. Also see @ref{Common options}.
10783 Show counts for all files, not just directories.
10785 @itemx --apparent-size
10786 @opindex --apparent-size
10787 Print apparent sizes, rather than disk usage. The apparent size of a
10788 file is the number of bytes reported by @code{wc -c} on regular files,
10789 or more generally, @code{ls -l --block-size=1} or @code{stat --format=%s}.
10790 For example, a file containing the word @samp{zoo} with no newline would,
10791 of course, have an apparent size of 3. Such a small file may require
10792 anywhere from 0 to 16 KiB or more of disk space, depending on
10793 the type and configuration of the file system on which the file resides.
10794 However, a sparse file created with this command:
10797 dd bs=1 seek=2GiB if=/dev/null of=big
10801 has an apparent size of 2 GiB, yet on most modern
10802 systems, it actually uses almost no disk space.
10808 Equivalent to @code{--apparent-size --block-size=1}.
10810 @item -B @var{size}
10811 @itemx --block-size=@var{size}
10813 @opindex --block-size
10815 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10816 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10822 @cindex grand total of disk space
10823 Print a grand total of all arguments after all arguments have
10824 been processed. This can be used to find out the total disk usage of
10825 a given set of files or directories.
10828 @itemx --dereference-args
10830 @opindex --dereference-args
10831 Dereference symbolic links that are command line arguments.
10832 Does not affect other symbolic links. This is helpful for finding
10833 out the disk usage of directories, such as @file{/usr/tmp}, which
10834 are often symbolic links.
10836 @c --files0-from=FILE
10837 @filesZeroFromOption{du,, with the @option{--total} (@option{-c}) option}
10843 Equivalent to @option{--dereference-args} (@option{-D}).
10847 @cindex kibibytes for file sizes
10848 Print sizes in 1024-byte blocks, overriding the default block size
10849 (@pxref{Block size}).
10850 This option is equivalent to @option{--block-size=1K}.
10853 @itemx --count-links
10855 @opindex --count-links
10856 @cindex hard links, counting in @command{du}
10857 Count the size of all files, even if they have appeared already (as a
10861 @itemx --dereference
10863 @opindex --dereference
10864 @cindex symbolic links, dereferencing in @command{du}
10865 Dereference symbolic links (show the disk space used by the file
10866 or directory that the link points to instead of the space used by
10871 @cindex mebibytes for file sizes
10872 Print sizes in 1,048,576-byte blocks, overriding the default block size
10873 (@pxref{Block size}).
10874 This option is equivalent to @option{--block-size=1M}.
10877 @itemx --no-dereference
10879 @opindex --no-dereference
10880 @cindex symbolic links, dereferencing in @command{du}
10881 For each symbolic links encountered by @command{du},
10882 consider the disk space used by the symbolic link.
10884 @item -d @var{depth}
10885 @item --max-depth=@var{depth}
10886 @opindex -d @var{depth}
10887 @opindex --max-depth=@var{depth}
10888 @cindex limiting output of @command{du}
10889 Show the total for each directory (and file if --all) that is at
10890 most MAX_DEPTH levels down from the root of the hierarchy. The root
10891 is at level 0, so @code{du --max-depth=0} is equivalent to @code{du -s}.
10900 @opindex --summarize
10901 Display only a total for each argument.
10904 @itemx --separate-dirs
10906 @opindex --separate-dirs
10907 Normally, in the output of @command{du} (when not using @option{--summarize}),
10908 the size listed next to a directory name, @var{d}, represents the sum
10909 of sizes of all entries beneath @var{d} as well as the size of @var{d} itself.
10910 With @option{--separate-dirs}, the size reported for a directory name,
10911 @var{d}, is merely the @code{stat.st_size}-derived size of the directory
10916 @cindex last modified dates, displaying in @command{du}
10917 Show time of the most recent modification of any file in the directory,
10918 or any of its subdirectories.
10920 @itemx --time=ctime
10921 @itemx --time=status
10924 @opindex ctime@r{, show the most recent}
10925 @opindex status time@r{, show the most recent}
10926 @opindex use time@r{, show the most recent}
10927 Show the most recent status change time (the @samp{ctime} in the inode) of
10928 any file in the directory, instead of the modification time.
10930 @itemx --time=atime
10931 @itemx --time=access
10933 @opindex atime@r{, show the most recent}
10934 @opindex access time@r{, show the most recent}
10935 Show the most recent access time (the @samp{atime} in the inode) of
10936 any file in the directory, instead of the modification time.
10938 @item --time-style=@var{style}
10939 @opindex --time-style
10941 List timestamps in style @var{style}. This option has an effect only if
10942 the @option{--time} option is also specified. The @var{style} should
10943 be one of the following:
10946 @item +@var{format}
10948 List timestamps using @var{format}, where @var{format} is interpreted
10949 like the format argument of @command{date} (@pxref{date invocation}).
10950 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
10951 @command{du} to list timestamps like @samp{2002-03-30 23:45:56}. As
10952 with @command{date}, @var{format}'s interpretation is affected by the
10953 @env{LC_TIME} locale category.
10956 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
10957 format with nanosecond precision, e.g., @samp{2002-03-30
10958 23:45:56.477817180 -0700}. This style is equivalent to
10959 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
10962 List @acronym{ISO} 8601 date and time in minutes, e.g.,
10963 @samp{2002-03-30 23:45}. These timestamps are shorter than
10964 @samp{full-iso} timestamps, and are usually good enough for everyday
10965 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
10968 List @acronym{ISO} 8601 dates for timestamps, e.g., @samp{2002-03-30}.
10969 This style is equivalent to @samp{+%Y-%m-%d}.
10973 You can specify the default value of the @option{--time-style} option
10974 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
10975 the default style is @samp{long-iso}. For compatibility with @command{ls},
10976 if @env{TIME_STYLE} begins with @samp{+} and contains a newline,
10977 the newline and any later characters are ignored; if @env{TIME_STYLE}
10978 begins with @samp{posix-} the @samp{posix-} is ignored; and if
10979 @env{TIME_STYLE} is @samp{locale} it is ignored.
10982 @itemx --one-file-system
10984 @opindex --one-file-system
10985 @cindex one file system, restricting @command{du} to
10986 Skip directories that are on different file systems from the one that
10987 the argument being processed is on.
10989 @item --exclude=@var{pattern}
10990 @opindex --exclude=@var{pattern}
10991 @cindex excluding files from @command{du}
10992 When recursing, skip subdirectories or files matching @var{pattern}.
10993 For example, @code{du --exclude='*.o'} excludes files whose names
10996 @item -X @var{file}
10997 @itemx --exclude-from=@var{file}
10998 @opindex -X @var{file}
10999 @opindex --exclude-from=@var{file}
11000 @cindex excluding files from @command{du}
11001 Like @option{--exclude}, except take the patterns to exclude from @var{file},
11002 one per line. If @var{file} is @samp{-}, take the patterns from standard
11007 @cindex NFS mounts from BSD to HP-UX
11008 On BSD systems, @command{du} reports sizes that are half the correct
11009 values for files that are NFS-mounted from HP-UX systems. On HP-UX
11010 systems, it reports sizes that are twice the correct values for
11011 files that are NFS-mounted from BSD systems. This is due to a flaw
11012 in HP-UX; it also affects the HP-UX @command{du} program.
11017 @node stat invocation
11018 @section @command{stat}: Report file or file system status
11021 @cindex file status
11022 @cindex file system status
11024 @command{stat} displays information about the specified file(s). Synopsis:
11027 stat [@var{option}]@dots{} [@var{file}]@dots{}
11030 With no option, @command{stat} reports all information about the given files.
11031 But it also can be used to report the information of the file systems the
11032 given files are located on. If the files are links, @command{stat} can
11033 also give information about the files the links point to.
11035 @mayConflictWithShellBuiltIn{stat}
11040 @itemx --dereference
11042 @opindex --dereference
11043 @cindex symbolic links, dereferencing in @command{stat}
11044 Change how @command{stat} treats symbolic links.
11045 With this option, @command{stat} acts on the file referenced
11046 by each symbolic link argument.
11047 Without it, @command{stat} acts on any symbolic link argument directly.
11050 @itemx --file-system
11052 @opindex --file-system
11053 @cindex file systems
11054 Report information about the file systems where the given files are located
11055 instead of information about the files themselves.
11056 This option implies the @option{-L} option.
11059 @itemx --format=@var{format}
11061 @opindex --format=@var{format}
11062 @cindex output format
11063 Use @var{format} rather than the default format.
11064 @var{format} is automatically newline-terminated, so
11065 running a command like the following with two or more @var{file}
11066 operands produces a line of output for each operand:
11068 $ stat --format=%d:%i / /usr
11073 @itemx --printf=@var{format}
11074 @opindex --printf=@var{format}
11075 @cindex output format
11076 Use @var{format} rather than the default format.
11077 Like @option{--format}, but interpret backslash escapes,
11078 and do not output a mandatory trailing newline.
11079 If you want a newline, include @samp{\n} in the @var{format}.
11080 Here's how you would use @option{--printf} to print the device
11081 and inode numbers of @file{/} and @file{/usr}:
11083 $ stat --printf='%d:%i\n' / /usr
11092 @cindex terse output
11093 Print the information in terse form, suitable for parsing by other programs.
11097 The valid @var{format} directives for files with @option{--format} and
11098 @option{--printf} are:
11101 @item %a - Access rights in octal
11102 @item %A - Access rights in human readable form
11103 @item %b - Number of blocks allocated (see @samp{%B})
11104 @item %B - The size in bytes of each block reported by @samp{%b}
11105 @item %C - The SELinux security context of a file, if available
11106 @item %d - Device number in decimal
11107 @item %D - Device number in hex
11108 @item %f - Raw mode in hex
11109 @item %F - File type
11110 @item %g - Group ID of owner
11111 @item %G - Group name of owner
11112 @item %h - Number of hard links
11113 @item %i - Inode number
11114 @item %m - Mount point (See note below)
11115 @item %n - File name
11116 @item %N - Quoted file name with dereference if symbolic link
11117 @item %o - Optimal I/O transfer size hint
11118 @item %s - Total size, in bytes
11119 @item %t - Major device type in hex
11120 @item %T - Minor device type in hex
11121 @item %u - User ID of owner
11122 @item %U - User name of owner
11123 @item %w - Time of file birth, or @samp{-} if unknown
11124 @item %W - Time of file birth as seconds since Epoch, or @samp{0}
11125 @item %x - Time of last access
11126 @item %X - Time of last access as seconds since Epoch
11127 @item %y - Time of last modification
11128 @item %Y - Time of last modification as seconds since Epoch
11129 @item %z - Time of last change
11130 @item %Z - Time of last change as seconds since Epoch
11133 The @samp{%W}, @samp{%X}, @samp{%Y}, and @samp{%Z} formats accept a
11134 precision preceded by a period to specify the number of digits to
11135 print after the decimal point. For example, @samp{%.3X} outputs the
11136 last access time to millisecond precision. If a period is given but no
11137 precision, @command{stat} uses 9 digits, so @samp{%.X} is equivalent to
11138 @samp{%.9X}. When discarding excess precision, time stamps are truncated
11139 toward minus infinity.
11143 $ stat -c '[%015Y]' /usr
11146 $ stat -c '[%15Y]' /usr
11148 $ stat -c '[%-15Y]' /usr
11151 $ stat -c '[%.3Y]' /usr
11153 $ stat -c '[%.Y]' /usr
11154 [1288929712.114951834]
11157 The mount point printed by @samp{%m} is similar to that output
11158 by @command{df}, except that:
11161 stat does not dereference symlinks by default
11162 (unless @option{-L} is specified)
11164 stat does not search for specified device nodes in the
11165 file system list, instead operating on them directly
11168 stat outputs the alias for a bind mounted file, rather than
11169 the initial mount point of its backing device.
11170 One can recursively call stat until there is no change in output,
11171 to get the current base mount point
11174 When listing file system information (@option{--file-system} (@option{-f})),
11175 you must use a different set of @var{format} directives:
11178 @item %a - Free blocks available to non-super-user
11179 @item %b - Total data blocks in file system
11180 @item %c - Total file nodes in file system
11181 @item %d - Free file nodes in file system
11182 @item %f - Free blocks in file system
11183 @item %i - File System ID in hex
11184 @item %l - Maximum length of file names
11185 @item %n - File name
11186 @item %s - Block size (for faster transfers)
11187 @item %S - Fundamental block size (for block counts)
11188 @item %t - Type in hex
11189 @item %T - Type in human readable form
11193 Time stamps are listed according to the time zone rules specified by
11194 the @env{TZ} environment variable, or by the system default rules if
11195 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
11196 with @env{TZ}, libc, The GNU C Library Reference Manual}.
11201 @node sync invocation
11202 @section @command{sync}: Synchronize data on disk with memory
11205 @cindex synchronize disk and memory
11207 @cindex superblock, writing
11208 @cindex inodes, written buffered
11209 @command{sync} writes any data buffered in memory out to disk. This can
11210 include (but is not limited to) modified superblocks, modified inodes,
11211 and delayed reads and writes. This must be implemented by the kernel;
11212 The @command{sync} program does nothing but exercise the @code{sync} system
11215 @cindex crashes and corruption
11216 The kernel keeps data in memory to avoid doing (relatively slow) disk
11217 reads and writes. This improves performance, but if the computer
11218 crashes, data may be lost or the file system corrupted as a
11219 result. The @command{sync} command ensures everything in memory
11220 is written to disk.
11222 Any arguments are ignored, except for a lone @option{--help} or
11223 @option{--version} (@pxref{Common options}).
11228 @node truncate invocation
11229 @section @command{truncate}: Shrink or extend the size of a file
11232 @cindex truncating, file sizes
11234 @command{truncate} shrinks or extends the size of each @var{file} to the
11235 specified size. Synopsis:
11238 truncate @var{option}@dots{} @var{file}@dots{}
11241 @cindex files, creating
11242 Any @var{file} that does not exist is created.
11244 @cindex sparse files, creating
11245 @cindex holes, creating files with
11246 If a @var{file} is larger than the specified size, the extra data is lost.
11247 If a @var{file} is shorter, it is extended and the extended part (or hole)
11248 reads as zero bytes.
11250 The program accepts the following options. Also see @ref{Common options}.
11257 @opindex --no-create
11258 Do not create files that do not exist.
11263 @opindex --io-blocks
11264 Treat @var{size} as number of I/O blocks of the @var{file} rather than bytes.
11266 @item -r @var{rfile}
11267 @itemx --reference=@var{rfile}
11269 @opindex --reference
11270 Base the size of each @var{file} on the size of @var{rfile}.
11272 @item -s @var{size}
11273 @itemx --size=@var{size}
11276 Set or adjust the size of each @var{file} according to @var{size}.
11277 @multiplierSuffixesNoBlocks{size}
11279 @var{size} may also be prefixed by one of the following to adjust
11280 the size of each @var{file} based on their current size:
11282 @samp{+} => extend by
11283 @samp{-} => reduce by
11284 @samp{<} => at most
11285 @samp{>} => at least
11286 @samp{/} => round down to multiple of
11287 @samp{%} => round up to multiple of
11295 @node Printing text
11296 @chapter Printing text
11298 @cindex printing text, commands for
11299 @cindex commands for printing text
11301 This section describes commands that display text strings.
11304 * echo invocation:: Print a line of text.
11305 * printf invocation:: Format and print data.
11306 * yes invocation:: Print a string until interrupted.
11310 @node echo invocation
11311 @section @command{echo}: Print a line of text
11314 @cindex displaying text
11315 @cindex printing text
11316 @cindex text, displaying
11317 @cindex arbitrary text, displaying
11319 @command{echo} writes each given @var{string} to standard output, with a
11320 space between each and a newline after the last one. Synopsis:
11323 echo [@var{option}]@dots{} [@var{string}]@dots{}
11326 @mayConflictWithShellBuiltIn{echo}
11328 The program accepts the following options. Also see @ref{Common options}.
11329 Options must precede operands, and the normally-special argument
11330 @samp{--} has no special meaning and is treated like any other
11336 Do not output the trailing newline.
11340 @cindex backslash escapes
11341 Enable interpretation of the following backslash-escaped characters in
11350 produce no further output
11366 the eight-bit value that is the octal number @var{nnn}
11367 (zero to three octal digits), if @var{nnn} is
11368 a nine-bit value, the ninth bit is ignored
11370 the eight-bit value that is the octal number @var{nnn}
11371 (one to three octal digits), if @var{nnn} is
11372 a nine-bit value, the ninth bit is ignored
11374 the eight-bit value that is the hexadecimal number @var{hh}
11375 (one or two hexadecimal digits)
11380 @cindex backslash escapes
11381 Disable interpretation of backslash escapes in each @var{string}.
11382 This is the default. If @option{-e} and @option{-E} are both
11383 specified, the last one given takes effect.
11387 @vindex POSIXLY_CORRECT
11388 If the @env{POSIXLY_CORRECT} environment variable is set, then when
11389 @command{echo}'s first argument is not @option{-n} it outputs
11390 option-like arguments instead of treating them as options. For
11391 example, @code{echo -ne hello} outputs @samp{-ne hello} instead of
11392 plain @samp{hello}.
11394 @acronym{POSIX} does not require support for any options, and says
11395 that the behavior of @command{echo} is implementation-defined if any
11396 @var{string} contains a backslash or if the first argument is
11397 @option{-n}. Portable programs can use the @command{printf} command
11398 if they need to omit trailing newlines or output control characters or
11399 backslashes. @xref{printf invocation}.
11404 @node printf invocation
11405 @section @command{printf}: Format and print data
11408 @command{printf} does formatted printing of text. Synopsis:
11411 printf @var{format} [@var{argument}]@dots{}
11414 @command{printf} prints the @var{format} string, interpreting @samp{%}
11415 directives and @samp{\} escapes to format numeric and string arguments
11416 in a way that is mostly similar to the C @samp{printf} function.
11417 @xref{Output Conversion Syntax,, @command{printf} format directives,
11418 libc, The GNU C Library Reference Manual}, for details.
11419 The differences are listed below.
11421 @mayConflictWithShellBuiltIn{printf}
11426 The @var{format} argument is reused as necessary to convert all the
11427 given @var{argument}s. For example, the command @samp{printf %s a b}
11431 Missing @var{argument}s are treated as null strings or as zeros,
11432 depending on whether the context expects a string or a number. For
11433 example, the command @samp{printf %sx%d} prints @samp{x0}.
11437 An additional escape, @samp{\c}, causes @command{printf} to produce no
11438 further output. For example, the command @samp{printf 'A%sC\cD%sF' B
11439 E} prints @samp{ABC}.
11442 The hexadecimal escape sequence @samp{\x@var{hh}} has at most two
11443 digits, as opposed to C where it can have an unlimited number of
11444 digits. For example, the command @samp{printf '\x07e'} prints two
11445 bytes, whereas the C statement @samp{printf ("\x07e")} prints just
11450 @command{printf} has an additional directive, @samp{%b}, which prints its
11451 argument string with @samp{\} escapes interpreted in the same way as in
11452 the @var{format} string, except that octal escapes are of the form
11453 @samp{\0@var{ooo}} where @var{ooo} is 0 to 3 octal digits. If
11454 @samp{\@var{ooo}} is nine-bit value, ignore the ninth bit.
11455 If a precision is also given, it limits the number of bytes printed
11456 from the converted string.
11459 Numeric arguments must be single C constants, possibly with leading
11460 @samp{+} or @samp{-}. For example, @samp{printf %.4d -3} outputs
11464 @vindex POSIXLY_CORRECT
11465 If the leading character of a numeric argument is @samp{"} or @samp{'}
11466 then its value is the numeric value of the immediately following
11467 character. Any remaining characters are silently ignored if the
11468 @env{POSIXLY_CORRECT} environment variable is set; otherwise, a
11469 warning is printed. For example, @samp{printf "%d" "'a"} outputs
11470 @samp{97} on hosts that use the @acronym{ASCII} character set, since
11471 @samp{a} has the numeric value 97 in @acronym{ASCII}.
11476 A floating-point argument must use a period before any fractional
11477 digits, but is printed according to the @env{LC_NUMERIC} category of the
11478 current locale. For example, in a locale whose radix character is a
11479 comma, the command @samp{printf %g 3.14} outputs @samp{3,14} whereas
11480 the command @samp{printf %g 3,14} is an error.
11481 @xref{Floating point}.
11485 @command{printf} interprets @samp{\@var{ooo}} in @var{format} as an octal number
11486 (if @var{ooo} is 1 to 3 octal digits) specifying a byte to print,
11487 and @samp{\x@var{hh}} as a hexadecimal number (if @var{hh} is 1 to 2 hex
11488 digits) specifying a character to print.
11489 Note however that when @samp{\@var{ooo}} specifies a number larger than 255,
11490 @command{printf} ignores the ninth bit.
11491 For example, @samp{printf '\400'} is equivalent to @samp{printf '\0'}.
11496 @cindex ISO/IEC 10646
11498 @command{printf} interprets two character syntaxes introduced in
11499 @acronym{ISO} C 99:
11500 @samp{\u} for 16-bit Unicode (@acronym{ISO}/@acronym{IEC} 10646)
11501 characters, specified as
11502 four hexadecimal digits @var{hhhh}, and @samp{\U} for 32-bit Unicode
11503 characters, specified as eight hexadecimal digits @var{hhhhhhhh}.
11504 @command{printf} outputs the Unicode characters
11505 according to the @env{LC_CTYPE} locale. Unicode characters in the ranges
11506 U+0000...U+009F, U+D800...U+DFFF cannot be specified by this syntax, except
11507 for U+0024 ($), U+0040 (@@), and U+0060 (@`).
11509 The processing of @samp{\u} and @samp{\U} requires a full-featured
11510 @code{iconv} facility. It is activated on systems with glibc 2.2 (or newer),
11511 or when @code{libiconv} is installed prior to this package. Otherwise
11512 @samp{\u} and @samp{\U} will print as-is.
11514 The only options are a lone @option{--help} or
11515 @option{--version}. @xref{Common options}.
11516 Options must precede operands.
11518 The Unicode character syntaxes are useful for writing strings in a locale
11519 independent way. For example, a string containing the Euro currency symbol
11522 $ env printf '\u20AC 14.95'
11526 will be output correctly in all locales supporting the Euro symbol
11527 (@acronym{ISO}-8859-15, UTF-8, and others). Similarly, a Chinese string
11530 $ env printf '\u4e2d\u6587'
11534 will be output correctly in all Chinese locales (GB2312, BIG5, UTF-8, etc).
11536 Note that in these examples, the @command{printf} command has been
11537 invoked via @command{env} to ensure that we run the program found via
11538 your shell's search path, and not a shell alias or a built-in function.
11540 For larger strings, you don't need to look up the hexadecimal code
11541 values of each character one by one. @acronym{ASCII} characters mixed with \u
11542 escape sequences is also known as the JAVA source file encoding. You can
11543 use GNU recode 3.5c (or newer) to convert strings to this encoding. Here
11544 is how to convert a piece of text into a shell script which will output
11545 this text in a locale-independent way:
11548 $ LC_CTYPE=zh_CN.big5 /usr/local/bin/printf \
11549 '\u4e2d\u6587\n' > sample.txt
11550 $ recode BIG5..JAVA < sample.txt \
11551 | sed -e "s|^|/usr/local/bin/printf '|" -e "s|$|\\\\n'|" \
11558 @node yes invocation
11559 @section @command{yes}: Print a string until interrupted
11562 @cindex repeated output of a string
11564 @command{yes} prints the command line arguments, separated by spaces and
11565 followed by a newline, forever until it is killed. If no arguments are
11566 given, it prints @samp{y} followed by a newline forever until killed.
11568 Upon a write error, @command{yes} exits with status @samp{1}.
11570 The only options are a lone @option{--help} or @option{--version}.
11571 To output an argument that begins with
11572 @samp{-}, precede it with @option{--}, e.g., @samp{yes -- --help}.
11573 @xref{Common options}.
11577 @chapter Conditions
11580 @cindex commands for exit status
11581 @cindex exit status commands
11583 This section describes commands that are primarily useful for their exit
11584 status, rather than their output. Thus, they are often used as the
11585 condition of shell @code{if} statements, or as the last command in a
11589 * false invocation:: Do nothing, unsuccessfully.
11590 * true invocation:: Do nothing, successfully.
11591 * test invocation:: Check file types and compare values.
11592 * expr invocation:: Evaluate expressions.
11596 @node false invocation
11597 @section @command{false}: Do nothing, unsuccessfully
11600 @cindex do nothing, unsuccessfully
11601 @cindex failure exit status
11602 @cindex exit status of @command{false}
11604 @command{false} does nothing except return an exit status of 1, meaning
11605 @dfn{failure}. It can be used as a place holder in shell scripts
11606 where an unsuccessful command is needed.
11607 In most modern shells, @command{false} is a built-in command, so when
11608 you use @samp{false} in a script, you're probably using the built-in
11609 command, not the one documented here.
11611 @command{false} honors the @option{--help} and @option{--version} options.
11613 This version of @command{false} is implemented as a C program, and is thus
11614 more secure and faster than a shell script implementation, and may safely
11615 be used as a dummy shell for the purpose of disabling accounts.
11617 Note that @command{false} (unlike all other programs documented herein)
11618 exits unsuccessfully, even when invoked with
11619 @option{--help} or @option{--version}.
11621 Portable programs should not assume that the exit status of
11622 @command{false} is 1, as it is greater than 1 on some
11623 non-@acronym{GNU} hosts.
11626 @node true invocation
11627 @section @command{true}: Do nothing, successfully
11630 @cindex do nothing, successfully
11632 @cindex successful exit
11633 @cindex exit status of @command{true}
11635 @command{true} does nothing except return an exit status of 0, meaning
11636 @dfn{success}. It can be used as a place holder in shell scripts
11637 where a successful command is needed, although the shell built-in
11638 command @code{:} (colon) may do the same thing faster.
11639 In most modern shells, @command{true} is a built-in command, so when
11640 you use @samp{true} in a script, you're probably using the built-in
11641 command, not the one documented here.
11643 @command{true} honors the @option{--help} and @option{--version} options.
11645 Note, however, that it is possible to cause @command{true}
11646 to exit with nonzero status: with the @option{--help} or @option{--version}
11647 option, and with standard
11648 output already closed or redirected to a file that evokes an I/O error.
11649 For example, using a Bourne-compatible shell:
11652 $ ./true --version >&-
11653 ./true: write error: Bad file number
11654 $ ./true --version > /dev/full
11655 ./true: write error: No space left on device
11658 This version of @command{true} is implemented as a C program, and is thus
11659 more secure and faster than a shell script implementation, and may safely
11660 be used as a dummy shell for the purpose of disabling accounts.
11662 @node test invocation
11663 @section @command{test}: Check file types and compare values
11666 @cindex check file types
11667 @cindex compare values
11668 @cindex expression evaluation
11670 @command{test} returns a status of 0 (true) or 1 (false) depending on the
11671 evaluation of the conditional expression @var{expr}. Each part of the
11672 expression must be a separate argument.
11674 @command{test} has file status checks, string operators, and numeric
11675 comparison operators.
11677 @command{test} has an alternate form that uses opening and closing
11678 square brackets instead a leading @samp{test}. For example, instead
11679 of @samp{test -d /}, you can write @samp{[ -d / ]}. The square
11680 brackets must be separate arguments; for example, @samp{[-d /]} does
11681 not have the desired effect. Since @samp{test @var{expr}} and @samp{[
11682 @var{expr} ]} have the same meaning, only the former form is discussed
11688 test @var{expression}
11690 [ @var{expression} ]
11695 @mayConflictWithShellBuiltIn{test}
11697 If @var{expression} is omitted, @command{test} returns false.
11698 If @var{expression} is a single argument,
11699 @command{test} returns false if the argument is null and true
11700 otherwise. The argument
11701 can be any string, including strings like @samp{-d}, @samp{-1},
11702 @samp{--}, @samp{--help}, and @samp{--version} that most other
11703 programs would treat as options. To get help and version information,
11704 invoke the commands @samp{[ --help} and @samp{[ --version}, without
11705 the usual closing brackets. @xref{Common options}.
11707 @cindex exit status of @command{test}
11711 0 if the expression is true,
11712 1 if the expression is false,
11713 2 if an error occurred.
11717 * File type tests:: -[bcdfhLpSt]
11718 * Access permission tests:: -[gkruwxOG]
11719 * File characteristic tests:: -e -s -nt -ot -ef
11720 * String tests:: -z -n = == !=
11721 * Numeric tests:: -eq -ne -lt -le -gt -ge
11722 * Connectives for test:: ! -a -o
11726 @node File type tests
11727 @subsection File type tests
11729 @cindex file type tests
11731 These options test for particular types of files. (Everything's a file,
11732 but not all files are the same!)
11736 @item -b @var{file}
11738 @cindex block special check
11739 True if @var{file} exists and is a block special device.
11741 @item -c @var{file}
11743 @cindex character special check
11744 True if @var{file} exists and is a character special device.
11746 @item -d @var{file}
11748 @cindex directory check
11749 True if @var{file} exists and is a directory.
11751 @item -f @var{file}
11753 @cindex regular file check
11754 True if @var{file} exists and is a regular file.
11756 @item -h @var{file}
11757 @itemx -L @var{file}
11760 @cindex symbolic link check
11761 True if @var{file} exists and is a symbolic link.
11762 Unlike all other file-related tests, this test does not dereference
11763 @var{file} if it is a symbolic link.
11765 @item -p @var{file}
11767 @cindex named pipe check
11768 True if @var{file} exists and is a named pipe.
11770 @item -S @var{file}
11772 @cindex socket check
11773 True if @var{file} exists and is a socket.
11777 @cindex terminal check
11778 True if @var{fd} is a file descriptor that is associated with a
11784 @node Access permission tests
11785 @subsection Access permission tests
11787 @cindex access permission tests
11788 @cindex permission tests
11790 These options test for particular access permissions.
11794 @item -g @var{file}
11796 @cindex set-group-ID check
11797 True if @var{file} exists and has its set-group-ID bit set.
11799 @item -k @var{file}
11801 @cindex sticky bit check
11802 True if @var{file} exists and has its @dfn{sticky} bit set.
11804 @item -r @var{file}
11806 @cindex readable file check
11807 True if @var{file} exists and read permission is granted.
11809 @item -u @var{file}
11811 @cindex set-user-ID check
11812 True if @var{file} exists and has its set-user-ID bit set.
11814 @item -w @var{file}
11816 @cindex writable file check
11817 True if @var{file} exists and write permission is granted.
11819 @item -x @var{file}
11821 @cindex executable file check
11822 True if @var{file} exists and execute permission is granted
11823 (or search permission, if it is a directory).
11825 @item -O @var{file}
11827 @cindex owned by effective user ID check
11828 True if @var{file} exists and is owned by the current effective user ID.
11830 @item -G @var{file}
11832 @cindex owned by effective group ID check
11833 True if @var{file} exists and is owned by the current effective group ID.
11837 @node File characteristic tests
11838 @subsection File characteristic tests
11840 @cindex file characteristic tests
11842 These options test other file characteristics.
11846 @item -e @var{file}
11848 @cindex existence-of-file check
11849 True if @var{file} exists.
11851 @item -s @var{file}
11853 @cindex nonempty file check
11854 True if @var{file} exists and has a size greater than zero.
11856 @item @var{file1} -nt @var{file2}
11858 @cindex newer-than file check
11859 True if @var{file1} is newer (according to modification date) than
11860 @var{file2}, or if @var{file1} exists and @var{file2} does not.
11862 @item @var{file1} -ot @var{file2}
11864 @cindex older-than file check
11865 True if @var{file1} is older (according to modification date) than
11866 @var{file2}, or if @var{file2} exists and @var{file1} does not.
11868 @item @var{file1} -ef @var{file2}
11870 @cindex same file check
11871 @cindex hard link check
11872 True if @var{file1} and @var{file2} have the same device and inode
11873 numbers, i.e., if they are hard links to each other.
11879 @subsection String tests
11881 @cindex string tests
11883 These options test string characteristics. You may need to quote
11884 @var{string} arguments for the shell. For example:
11890 The quotes here prevent the wrong arguments from being passed to
11891 @command{test} if @samp{$V} is empty or contains special characters.
11895 @item -z @var{string}
11897 @cindex zero-length string check
11898 True if the length of @var{string} is zero.
11900 @item -n @var{string}
11901 @itemx @var{string}
11903 @cindex nonzero-length string check
11904 True if the length of @var{string} is nonzero.
11906 @item @var{string1} = @var{string2}
11908 @cindex equal string check
11909 True if the strings are equal.
11911 @item @var{string1} == @var{string2}
11913 @cindex equal string check
11914 True if the strings are equal (synonym for =).
11916 @item @var{string1} != @var{string2}
11918 @cindex not-equal string check
11919 True if the strings are not equal.
11924 @node Numeric tests
11925 @subsection Numeric tests
11927 @cindex numeric tests
11928 @cindex arithmetic tests
11930 Numeric relational operators. The arguments must be entirely numeric
11931 (possibly negative), or the special expression @w{@code{-l @var{string}}},
11932 which evaluates to the length of @var{string}.
11936 @item @var{arg1} -eq @var{arg2}
11937 @itemx @var{arg1} -ne @var{arg2}
11938 @itemx @var{arg1} -lt @var{arg2}
11939 @itemx @var{arg1} -le @var{arg2}
11940 @itemx @var{arg1} -gt @var{arg2}
11941 @itemx @var{arg1} -ge @var{arg2}
11948 These arithmetic binary operators return true if @var{arg1} is equal,
11949 not-equal, less-than, less-than-or-equal, greater-than, or
11950 greater-than-or-equal than @var{arg2}, respectively.
11957 test -1 -gt -2 && echo yes
11959 test -l abc -gt 1 && echo yes
11962 @error{} test: integer expression expected before -eq
11966 @node Connectives for test
11967 @subsection Connectives for @command{test}
11969 @cindex logical connectives
11970 @cindex connectives, logical
11972 The usual logical connectives.
11978 True if @var{expr} is false.
11980 @item @var{expr1} -a @var{expr2}
11982 @cindex logical and operator
11983 @cindex and operator
11984 True if both @var{expr1} and @var{expr2} are true.
11986 @item @var{expr1} -o @var{expr2}
11988 @cindex logical or operator
11989 @cindex or operator
11990 True if either @var{expr1} or @var{expr2} is true.
11995 @node expr invocation
11996 @section @command{expr}: Evaluate expressions
11999 @cindex expression evaluation
12000 @cindex evaluation of expressions
12002 @command{expr} evaluates an expression and writes the result on standard
12003 output. Each token of the expression must be a separate argument.
12005 Operands are either integers or strings. Integers consist of one or
12006 more decimal digits, with an optional leading @samp{-}.
12007 @command{expr} converts
12008 anything appearing in an operand position to an integer or a string
12009 depending on the operation being applied to it.
12011 Strings are not quoted for @command{expr} itself, though you may need to
12012 quote them to protect characters with special meaning to the shell,
12013 e.g., spaces. However, regardless of whether it is quoted, a string
12014 operand should not be a parenthesis or any of @command{expr}'s
12015 operators like @code{+}, so you cannot safely pass an arbitrary string
12016 @code{$str} to expr merely by quoting it to the shell. One way to
12017 work around this is to use the @sc{gnu} extension @code{+},
12018 (e.g., @code{+ "$str" = foo}); a more portable way is to use
12019 @code{@w{" $str"}} and to adjust the rest of the expression to take
12020 the leading space into account (e.g., @code{@w{" $str" = " foo"}}).
12022 You should not pass a negative integer or a string with leading
12023 @samp{-} as @command{expr}'s first argument, as it might be
12024 misinterpreted as an option; this can be avoided by parenthesization.
12025 Also, portable scripts should not use a string operand that happens to
12026 take the form of an integer; this can be worked around by inserting
12027 leading spaces as mentioned above.
12029 @cindex parentheses for grouping
12030 Operators may be given as infix symbols or prefix keywords. Parentheses
12031 may be used for grouping in the usual manner. You must quote
12032 parentheses and many operators to avoid the shell evaluating them,
12035 When built with support for the GNU MP library, @command{expr} uses
12036 arbitrary-precision arithmetic; otherwise, it uses native arithmetic
12037 types and may fail due to arithmetic overflow.
12039 The only options are @option{--help} and @option{--version}. @xref{Common
12040 options}. Options must precede operands.
12042 @cindex exit status of @command{expr}
12046 0 if the expression is neither null nor 0,
12047 1 if the expression is null or 0,
12048 2 if the expression is invalid,
12049 3 if an internal error occurred (e.g., arithmetic overflow).
12053 * String expressions:: + : match substr index length
12054 * Numeric expressions:: + - * / %
12055 * Relations for expr:: | & < <= = == != >= >
12056 * Examples of expr:: Examples.
12060 @node String expressions
12061 @subsection String expressions
12063 @cindex string expressions
12064 @cindex expressions, string
12066 @command{expr} supports pattern matching and other string operators. These
12067 have higher precedence than both the numeric and relational operators (in
12068 the next sections).
12072 @item @var{string} : @var{regex}
12073 @cindex pattern matching
12074 @cindex regular expression matching
12075 @cindex matching patterns
12076 Perform pattern matching. The arguments are converted to strings and the
12077 second is considered to be a (basic, a la GNU @code{grep}) regular
12078 expression, with a @code{^} implicitly prepended. The first argument is
12079 then matched against this regular expression.
12081 If the match succeeds and @var{regex} uses @samp{\(} and @samp{\)}, the
12082 @code{:} expression returns the part of @var{string} that matched the
12083 subexpression; otherwise, it returns the number of characters matched.
12085 If the match fails, the @code{:} operator returns the null string if
12086 @samp{\(} and @samp{\)} are used in @var{regex}, otherwise 0.
12088 @kindex \( @r{regexp operator}
12089 Only the first @samp{\( @dots{} \)} pair is relevant to the return
12090 value; additional pairs are meaningful only for grouping the regular
12091 expression operators.
12093 @kindex \+ @r{regexp operator}
12094 @kindex \? @r{regexp operator}
12095 @kindex \| @r{regexp operator}
12096 In the regular expression, @code{\+}, @code{\?}, and @code{\|} are
12097 operators which respectively match one or more, zero or one, or separate
12098 alternatives. SunOS and other @command{expr}'s treat these as regular
12099 characters. (@acronym{POSIX} allows either behavior.)
12100 @xref{Top, , Regular Expression Library, regex, Regex}, for details of
12101 regular expression syntax. Some examples are in @ref{Examples of expr}.
12103 @item match @var{string} @var{regex}
12105 An alternative way to do pattern matching. This is the same as
12106 @w{@samp{@var{string} : @var{regex}}}.
12108 @item substr @var{string} @var{position} @var{length}
12110 Returns the substring of @var{string} beginning at @var{position}
12111 with length at most @var{length}. If either @var{position} or
12112 @var{length} is negative, zero, or non-numeric, returns the null string.
12114 @item index @var{string} @var{charset}
12116 Returns the first position in @var{string} where the first character in
12117 @var{charset} was found. If no character in @var{charset} is found in
12118 @var{string}, return 0.
12120 @item length @var{string}
12122 Returns the length of @var{string}.
12124 @item + @var{token}
12126 Interpret @var{token} as a string, even if it is a keyword like @var{match}
12127 or an operator like @code{/}.
12128 This makes it possible to test @code{expr length + "$x"} or
12129 @code{expr + "$x" : '.*/\(.\)'} and have it do the right thing even if
12130 the value of @var{$x} happens to be (for example) @code{/} or @code{index}.
12131 This operator is a @acronym{GNU} extension. Portable shell scripts should use
12132 @code{@w{" $token"} : @w{' \(.*\)'}} instead of @code{+ "$token"}.
12136 To make @command{expr} interpret keywords as strings, you must use the
12137 @code{quote} operator.
12140 @node Numeric expressions
12141 @subsection Numeric expressions
12143 @cindex numeric expressions
12144 @cindex expressions, numeric
12146 @command{expr} supports the usual numeric operators, in order of increasing
12147 precedence. These numeric operators have lower precedence than the
12148 string operators described in the previous section, and higher precedence
12149 than the connectives (next section).
12157 @cindex subtraction
12158 Addition and subtraction. Both arguments are converted to integers;
12159 an error occurs if this cannot be done.
12165 @cindex multiplication
12168 Multiplication, division, remainder. Both arguments are converted to
12169 integers; an error occurs if this cannot be done.
12174 @node Relations for expr
12175 @subsection Relations for @command{expr}
12177 @cindex connectives, logical
12178 @cindex logical connectives
12179 @cindex relations, numeric or string
12181 @command{expr} supports the usual logical connectives and relations. These
12182 have lower precedence than the string and numeric operators
12183 (previous sections). Here is the list, lowest-precedence operator first.
12189 @cindex logical or operator
12190 @cindex or operator
12191 Returns its first argument if that is neither null nor zero, otherwise
12192 its second argument if it is neither null nor zero, otherwise 0. It
12193 does not evaluate its second argument if its first argument is neither
12198 @cindex logical and operator
12199 @cindex and operator
12200 Return its first argument if neither argument is null or zero, otherwise
12201 0. It does not evaluate its second argument if its first argument is
12204 @item < <= = == != >= >
12211 @cindex comparison operators
12213 Compare the arguments and return 1 if the relation is true, 0 otherwise.
12214 @code{==} is a synonym for @code{=}. @command{expr} first tries to convert
12215 both arguments to integers and do a numeric comparison; if either
12216 conversion fails, it does a lexicographic comparison using the character
12217 collating sequence specified by the @env{LC_COLLATE} locale.
12222 @node Examples of expr
12223 @subsection Examples of using @command{expr}
12225 @cindex examples of @command{expr}
12226 Here are a few examples, including quoting for shell metacharacters.
12228 To add 1 to the shell variable @code{foo}, in Bourne-compatible shells:
12231 foo=$(expr $foo + 1)
12234 To print the non-directory part of the file name stored in
12235 @code{$fname}, which need not contain a @code{/}:
12238 expr $fname : '.*/\(.*\)' '|' $fname
12241 An example showing that @code{\+} is an operator:
12249 expr abc : 'a\(.\)c'
12251 expr index abcdef cz
12254 @error{} expr: syntax error
12255 expr index + index a
12261 @chapter Redirection
12263 @cindex redirection
12264 @cindex commands for redirection
12266 Unix shells commonly provide several forms of @dfn{redirection}---ways
12267 to change the input source or output destination of a command. But one
12268 useful redirection is performed by a separate command, not by the shell;
12269 it's described here.
12272 * tee invocation:: Redirect output to multiple files or processes.
12276 @node tee invocation
12277 @section @command{tee}: Redirect output to multiple files or processes
12280 @cindex pipe fitting
12281 @cindex destinations, multiple output
12282 @cindex read from stdin and write to stdout and files
12284 The @command{tee} command copies standard input to standard output and also
12285 to any files given as arguments. This is useful when you want not only
12286 to send some data down a pipe, but also to save a copy. Synopsis:
12289 tee [@var{option}]@dots{} [@var{file}]@dots{}
12292 If a file being written to does not already exist, it is created. If a
12293 file being written to already exists, the data it previously contained
12294 is overwritten unless the @option{-a} option is used.
12296 A @var{file} of @samp{-} causes @command{tee} to send another copy of
12297 input to standard output, but this is typically not that useful as the
12298 copies are interleaved.
12300 The program accepts the following options. Also see @ref{Common options}.
12307 Append standard input to the given files rather than overwriting
12311 @itemx --ignore-interrupts
12313 @opindex --ignore-interrupts
12314 Ignore interrupt signals.
12318 The @command{tee} command is useful when you happen to be transferring a large
12319 amount of data and also want to summarize that data without reading
12320 it a second time. For example, when you are downloading a DVD image,
12321 you often want to verify its signature or checksum right away.
12322 The inefficient way to do it is simply:
12325 wget http://example.com/some.iso && sha1sum some.iso
12328 One problem with the above is that it makes you wait for the
12329 download to complete before starting the time-consuming SHA1 computation.
12330 Perhaps even more importantly, the above requires reading
12331 the DVD image a second time (the first was from the network).
12333 The efficient way to do it is to interleave the download
12334 and SHA1 computation. Then, you'll get the checksum for
12335 free, because the entire process parallelizes so well:
12338 # slightly contrived, to demonstrate process substitution
12339 wget -O - http://example.com/dvd.iso \
12340 | tee >(sha1sum > dvd.sha1) > dvd.iso
12343 That makes @command{tee} write not just to the expected output file,
12344 but also to a pipe running @command{sha1sum} and saving the final
12345 checksum in a file named @file{dvd.sha1}.
12347 Note, however, that this example relies on a feature of modern shells
12348 called @dfn{process substitution}
12349 (the @samp{>(command)} syntax, above;
12350 @xref{Process Substitution,,Process Substitution, bashref,
12351 The Bash Reference Manual}.),
12352 so it works with @command{zsh}, @command{bash}, and @command{ksh},
12353 but not with @command{/bin/sh}. So if you write code like this
12354 in a shell script, be sure to start the script with @samp{#!/bin/bash}.
12356 Since the above example writes to one file and one process,
12357 a more conventional and portable use of @command{tee} is even better:
12360 wget -O - http://example.com/dvd.iso \
12361 | tee dvd.iso | sha1sum > dvd.sha1
12364 You can extend this example to make @command{tee} write to two processes,
12365 computing MD5 and SHA1 checksums in parallel. In this case,
12366 process substitution is required:
12369 wget -O - http://example.com/dvd.iso \
12370 | tee >(sha1sum > dvd.sha1) \
12371 >(md5sum > dvd.md5) \
12375 This technique is also useful when you want to make a @emph{compressed}
12376 copy of the contents of a pipe.
12377 Consider a tool to graphically summarize disk usage data from @samp{du -ak}.
12378 For a large hierarchy, @samp{du -ak} can run for a long time,
12379 and can easily produce terabytes of data, so you won't want to
12380 rerun the command unnecessarily. Nor will you want to save
12381 the uncompressed output.
12383 Doing it the inefficient way, you can't even start the GUI
12384 until after you've compressed all of the @command{du} output:
12387 du -ak | gzip -9 > /tmp/du.gz
12388 gzip -d /tmp/du.gz | xdiskusage -a
12391 With @command{tee} and process substitution, you start the GUI
12392 right away and eliminate the decompression completely:
12395 du -ak | tee >(gzip -9 > /tmp/du.gz) | xdiskusage -a
12398 Finally, if you regularly create more than one type of
12399 compressed tarball at once, for example when @code{make dist} creates
12400 both @command{gzip}-compressed and @command{bzip2}-compressed tarballs,
12401 there may be a better way.
12402 Typical @command{automake}-generated @file{Makefile} rules create
12403 the two compressed tar archives with commands in sequence, like this
12404 (slightly simplified):
12407 tardir=your-pkg-M.N
12408 tar chof - "$tardir" | gzip -9 -c > your-pkg-M.N.tar.gz
12409 tar chof - "$tardir" | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12412 However, if the hierarchy you are archiving and compressing is larger
12413 than a couple megabytes, and especially if you are using a multi-processor
12414 system with plenty of memory, then you can do much better by reading the
12415 directory contents only once and running the compression programs in parallel:
12418 tardir=your-pkg-M.N
12419 tar chof - "$tardir" \
12420 | tee >(gzip -9 -c > your-pkg-M.N.tar.gz) \
12421 | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12427 @node File name manipulation
12428 @chapter File name manipulation
12430 @cindex file name manipulation
12431 @cindex manipulation of file names
12432 @cindex commands for file name manipulation
12434 This section describes commands that manipulate file names.
12437 * basename invocation:: Strip directory and suffix from a file name.
12438 * dirname invocation:: Strip last file name component.
12439 * pathchk invocation:: Check file name validity and portability.
12440 * mktemp invocation:: Create temporary file or directory.
12441 * realpath invocation:: Print resolved file names.
12445 @node basename invocation
12446 @section @command{basename}: Strip directory and suffix from a file name
12449 @cindex strip directory and suffix from file names
12450 @cindex directory, stripping from file names
12451 @cindex suffix, stripping from file names
12452 @cindex file names, stripping directory and suffix
12453 @cindex leading directory components, stripping
12455 @command{basename} removes any leading directory components from
12456 @var{name}. Synopsis:
12459 basename @var{name} [@var{suffix}]
12460 basename @var{option}... @var{name}...
12463 If @var{suffix} is specified and is identical to the end of @var{name},
12464 it is removed from @var{name} as well. Note that since trailing slashes
12465 are removed prior to suffix matching, @var{suffix} will do nothing if it
12466 contains slashes. @command{basename} prints the result on standard
12469 @c This test is used both here and in the section on dirname.
12470 @macro basenameAndDirname
12471 Together, @command{basename} and @command{dirname} are designed such
12472 that if @samp{ls "$name"} succeeds, then the command sequence @samp{cd
12473 "$(dirname "$name")"; ls "$(basename "$name")"} will, too. This works
12474 for everything except file names containing a trailing newline.
12476 @basenameAndDirname
12478 @acronym{POSIX} allows the implementation to define the results if
12479 @var{name} is empty or @samp{//}. In the former case, @acronym{GNU}
12480 @command{basename} returns the empty string. In the latter case, the
12481 result is @samp{//} on platforms where @var{//} is distinct from
12482 @var{/}, and @samp{/} on platforms where there is no difference.
12484 The program accepts the following options. Also see @ref{Common options}.
12485 Options must precede operands.
12492 @opindex --multiple
12493 Support more than one argument. Treat every argument as a @var{name}.
12494 With this, an optional @var{suffix} must be specified using the
12495 @option{-s} option.
12497 @item -s @var{suffix}
12498 @itemx --suffix=@var{suffix}
12501 Remove a trailing @var{suffix}.
12502 This option implies the @option{-a} option.
12508 Separate output items with @sc{nul} characters.
12518 basename /usr/bin/sort
12521 basename include/stdio.h .h
12524 basename -s .h include/stdio.h
12526 # Output "stdio" followed by "stdlib"
12527 basename -a -s .h include/stdio.h include/stdlib.h
12531 @node dirname invocation
12532 @section @command{dirname}: Strip last file name component
12535 @cindex directory components, printing
12536 @cindex stripping non-directory suffix
12537 @cindex non-directory suffix, stripping
12539 @command{dirname} prints all but the final slash-delimited component
12540 of each @var{name}. Slashes on either side of the final component are
12541 also removed. If the string contains no slash, @command{dirname}
12542 prints @samp{.} (meaning the current directory). Synopsis:
12545 dirname [@var{option}] @var{name}...
12548 @var{name} need not be a file name, but if it is, this operation
12549 effectively lists the directory that contains the final component,
12550 including the case when the final component is itself a directory.
12552 @basenameAndDirname
12554 @acronym{POSIX} allows the implementation to define the results if
12555 @var{name} is @samp{//}. With @acronym{GNU} @command{dirname}, the
12556 result is @samp{//} on platforms where @var{//} is distinct from
12557 @var{/}, and @samp{/} on platforms where there is no difference.
12559 The program accepts the following option. Also see @ref{Common options}.
12567 Separate output items with @sc{nul} characters.
12576 # Output "/usr/bin".
12577 dirname /usr/bin/sort
12578 dirname /usr/bin//.//
12580 # Output "dir1" followed by "dir2"
12581 dirname dir1/str dir2/str
12588 @node pathchk invocation
12589 @section @command{pathchk}: Check file name validity and portability
12592 @cindex file names, checking validity and portability
12593 @cindex valid file names, checking for
12594 @cindex portable file names, checking for
12596 @command{pathchk} checks validity and portability of file names. Synopsis:
12599 pathchk [@var{option}]@dots{} @var{name}@dots{}
12602 For each @var{name}, @command{pathchk} prints an error message if any of
12603 these conditions is true:
12607 One of the existing directories in @var{name} does not have search
12608 (execute) permission,
12610 The length of @var{name} is larger than the maximum supported by the
12613 The length of one component of @var{name} is longer than
12614 its file system's maximum.
12617 A nonexistent @var{name} is not an error, so long a file with that
12618 name could be created under the above conditions.
12620 The program accepts the following options. Also see @ref{Common options}.
12621 Options must precede operands.
12627 Instead of performing checks based on the underlying file system,
12628 print an error message if any of these conditions is true:
12632 A file name is empty.
12635 A file name contains a character outside the @acronym{POSIX} portable file
12636 name character set, namely, the ASCII letters and digits, @samp{.},
12637 @samp{_}, @samp{-}, and @samp{/}.
12640 The length of a file name or one of its components exceeds the
12641 @acronym{POSIX} minimum limits for portability.
12646 Print an error message if a file name is empty, or if it contains a component
12647 that begins with @samp{-}.
12649 @item --portability
12650 @opindex --portability
12651 Print an error message if a file name is not portable to all @acronym{POSIX}
12652 hosts. This option is equivalent to @samp{-p -P}.
12656 @cindex exit status of @command{pathchk}
12660 0 if all specified file names passed all checks,
12664 @node mktemp invocation
12665 @section @command{mktemp}: Create temporary file or directory
12668 @cindex file names, creating temporary
12669 @cindex directory, creating temporary
12670 @cindex temporary files and directories
12672 @command{mktemp} manages the creation of temporary files and
12673 directories. Synopsis:
12676 mktemp [@var{option}]@dots{} [@var{template}]
12679 Safely create a temporary file or directory based on @var{template},
12680 and print its name. If given, @var{template} must include at least
12681 three consecutive @samp{X}s in the last component. If omitted, the template
12682 @samp{tmp.XXXXXXXXXX} is used, and option @option{--tmpdir} is
12683 implied. The final run of @samp{X}s in the @var{template} will be replaced
12684 by alpha-numeric characters; thus, on a case-sensitive file system,
12685 and with a @var{template} including a run of @var{n} instances of @samp{X},
12686 there are @samp{62**@var{n}} potential file names.
12688 Older scripts used to create temporary files by simply joining the
12689 name of the program with the process id (@samp{$$}) as a suffix.
12690 However, that naming scheme is easily predictable, and suffers from a
12691 race condition where the attacker can create an appropriately named
12692 symbolic link, such that when the script then opens a handle to what
12693 it thought was an unused file, it is instead modifying an existing
12694 file. Using the same scheme to create a directory is slightly safer,
12695 since the @command{mkdir} will fail if the target already exists, but
12696 it is still inferior because it allows for denial of service attacks.
12697 Therefore, modern scripts should use the @command{mktemp} command to
12698 guarantee that the generated name will be unpredictable, and that
12699 knowledge of the temporary file name implies that the file was created
12700 by the current script and cannot be modified by other users.
12702 When creating a file, the resulting file has read and write
12703 permissions for the current user, but no permissions for the group or
12704 others; these permissions are reduced if the current umask is more
12707 Here are some examples (although note that if you repeat them, you
12708 will most likely get different file names):
12713 Create a temporary file in the current directory.
12720 Create a temporary file with a known suffix.
12722 $ mktemp --suffix=.txt file-XXXX
12724 $ mktemp file-XXXX-XXXX.txt
12729 Create a secure fifo relative to the user's choice of @env{TMPDIR},
12730 but falling back to the current directory rather than @file{/tmp}.
12731 Note that @command{mktemp} does not create fifos, but can create a
12732 secure directory in which the fifo can live. Exit the shell if the
12733 directory or fifo could not be created.
12735 $ dir=$(mktemp -p "$@{TMPDIR:-.@}" -d dir-XXXX) || exit 1
12737 $ mkfifo "$fifo" || @{ rmdir "$dir"; exit 1; @}
12741 Create and use a temporary file if possible, but ignore failure. The
12742 file will reside in the directory named by @env{TMPDIR}, if specified,
12743 or else in @file{/tmp}.
12745 $ file=$(mktemp -q) && @{
12746 > # Safe to use $file only within this block. Use quotes,
12747 > # since $TMPDIR, and thus $file, may contain whitespace.
12748 > echo ... > "$file"
12754 Act as a semi-random character generator (it is not fully random,
12755 since it is impacted by the contents of the current directory). To
12756 avoid security holes, do not use the resulting names to create a file.
12766 The program accepts the following options. Also see @ref{Common options}.
12773 @opindex --directory
12774 Create a directory rather than a file. The directory will have read,
12775 write, and search permissions for the current user, but no permissions
12776 for the group or others; these permissions are reduced if the current
12777 umask is more restrictive.
12783 Suppress diagnostics about failure to create a file or directory. The
12784 exit status will still reflect whether a file was created.
12790 Generate a temporary name that does not name an existing file, without
12791 changing the file system contents. Using the output of this command
12792 to create a new file is inherently unsafe, as there is a window of
12793 time between generating the name and using it where another process
12794 can create an object by the same name.
12797 @itemx --tmpdir[=@var{dir}]
12800 Treat @var{template} relative to the directory @var{dir}. If
12801 @var{dir} is not specified (only possible with the long option
12802 @option{--tmpdir}) or is the empty string, use the value of
12803 @env{TMPDIR} if available, otherwise use @samp{/tmp}. If this is
12804 specified, @var{template} must not be absolute. However,
12805 @var{template} can still contain slashes, although intermediate
12806 directories must already exist.
12808 @item --suffix=@var{suffix}
12810 Append @var{suffix} to the @var{template}. @var{suffix} must not
12811 contain slash. If @option{--suffix} is specified, @var{template} must
12812 end in @samp{X}; if it is not specified, then an appropriate
12813 @option{--suffix} is inferred by finding the last @samp{X} in
12814 @var{template}. This option exists for use with the default
12815 @var{template} and for the creation of a @var{suffix} that starts with
12820 Treat @var{template} as a single file relative to the value of
12821 @env{TMPDIR} if available, or to the directory specified by
12822 @option{-p}, otherwise to @samp{/tmp}. @var{template} must not
12823 contain slashes. This option is deprecated; the use of @option{-p}
12824 without @option{-t} offers better defaults (by favoring the command
12825 line over @env{TMPDIR}) and more flexibility (by allowing intermediate
12830 @cindex exit status of @command{mktemp}
12834 0 if the file was created,
12839 @node realpath invocation
12840 @section @command{realpath}: Print the resolved file name.
12843 @cindex file names, canonicalization
12844 @cindex symlinks, resolution
12845 @cindex canonical file name
12846 @cindex canonicalize a file name
12850 @command{realpath} expands all symbolic links and resolves references to
12851 @samp{/./}, @samp{/../} and extra @samp{/} characters. By default,
12852 all but the last component of the specified files must exist. Synopsis:
12855 realpath [@var{option}]@dots{} @var{file}@dots{}
12858 The program accepts the following options. Also see @ref{Common options}.
12863 @itemx --canonicalize-existing
12865 @opindex --canonicalize-existing
12866 Ensure that all components of the specified file names exist.
12867 If any component is missing or unavailable, @command{realpath} will output
12868 a diagnostic unless the @option{-q} option is specified, and exit with a
12869 nonzero exit code. A trailing slash requires that the name resolve to a
12873 @itemx --canonicalize-missing
12875 @opindex --canonicalize-missing
12876 If any component of a specified file name is missing or unavailable,
12877 treat it as a directory.
12883 Symbolic links are resolved in the specified file names,
12884 but they are resolved after any subsequent @samp{..} components are processed.
12889 @opindex --physical
12890 Symbolic links are resolved in the specified file names,
12891 and they are resolved before any subsequent @samp{..} components are processed.
12892 This is the default mode of operation.
12898 Suppress diagnostic messages for specified file names.
12902 @itemx --no-symlinks
12905 @opindex --no-symlinks
12906 Do not resolve symbolic links. Only resolve references to
12907 @samp{/./}, @samp{/../} and remove extra @samp{/} characters.
12908 When combined with the @option{-m} option, realpath operates
12909 only on the file name, and does not touch any actual file.
12915 Separate output items with @sc{nul} characters.
12917 @itemx --relative-to=@var{file}
12918 @opindex --relative-to
12920 Print the resolved file names relative to the specified file.
12921 Note this option honors the @option{-m} and @option{-e} options
12922 pertaining to file existence.
12924 @itemx --relative-base=@var{base}
12925 @opindex --relative-base
12926 This option is valid when used with @option{--relative-to}, and will restrict
12927 the output of @option{--relative-to} so that relative names are output,
12928 only when @var{file}s are descendants of @var{base}. Otherwise output the
12929 absolute file name. If @option{--relative-to} was not specified, then
12930 the descendants of @var{base} are printed relative to @var{base}. If
12931 @option{--relative-to} is specified, then that directory must be a
12932 descendant of @var{base} for this option to have an effect.
12933 Note: this option honors the @option{-m} and @option{-e}
12934 options pertaining to file existence. For example:
12937 realpath --relative-to=/usr /tmp /usr/bin
12940 realpath --relative-base=/usr /tmp /usr/bin
12947 @cindex exit status of @command{realpath}
12951 0 if all file names were printed without issue.
12956 @node Working context
12957 @chapter Working context
12959 @cindex working context
12960 @cindex commands for printing the working context
12962 This section describes commands that display or alter the context in
12963 which you are working: the current directory, the terminal settings, and
12964 so forth. See also the user-related commands in the next section.
12967 * pwd invocation:: Print working directory.
12968 * stty invocation:: Print or change terminal characteristics.
12969 * printenv invocation:: Print environment variables.
12970 * tty invocation:: Print file name of terminal on standard input.
12974 @node pwd invocation
12975 @section @command{pwd}: Print working directory
12978 @cindex print name of current directory
12979 @cindex current working directory, printing
12980 @cindex working directory, printing
12983 @command{pwd} prints the name of the current directory. Synopsis:
12986 pwd [@var{option}]@dots{}
12989 The program accepts the following options. Also see @ref{Common options}.
12996 If the contents of the environment variable @env{PWD} provide an
12997 absolute name of the current directory with no @samp{.} or @samp{..}
12998 components, but possibly with symbolic links, then output those
12999 contents. Otherwise, fall back to default @option{-P} handling.
13004 @opindex --physical
13005 Print a fully resolved name for the current directory. That is, all
13006 components of the printed name will be actual directory names---none
13007 will be symbolic links.
13010 @cindex symbolic links and @command{pwd}
13011 If @option{-L} and @option{-P} are both given, the last one takes
13012 precedence. If neither option is given, then this implementation uses
13013 @option{-P} as the default unless the @env{POSIXLY_CORRECT}
13014 environment variable is set.
13016 @mayConflictWithShellBuiltIn{pwd}
13021 @node stty invocation
13022 @section @command{stty}: Print or change terminal characteristics
13025 @cindex change or print terminal settings
13026 @cindex terminal settings
13027 @cindex line settings of terminal
13029 @command{stty} prints or changes terminal characteristics, such as baud rate.
13033 stty [@var{option}] [@var{setting}]@dots{}
13034 stty [@var{option}]
13037 If given no line settings, @command{stty} prints the baud rate, line
13038 discipline number (on systems that support it), and line settings
13039 that have been changed from the values set by @samp{stty sane}.
13040 By default, mode reading and setting are performed on the tty line
13041 connected to standard input, although this can be modified by the
13042 @option{--file} option.
13044 @command{stty} accepts many non-option arguments that change aspects of
13045 the terminal line operation, as described below.
13047 The program accepts the following options. Also see @ref{Common options}.
13054 Print all current settings in human-readable form. This option may not
13055 be used in combination with any line settings.
13057 @item -F @var{device}
13058 @itemx --file=@var{device}
13061 Set the line opened by the file name specified in @var{device} instead of
13062 the tty line connected to standard input. This option is necessary
13063 because opening a @acronym{POSIX} tty requires use of the
13064 @code{O_NONDELAY} flag to prevent a @acronym{POSIX} tty from blocking
13065 until the carrier detect line is high if
13066 the @code{clocal} flag is not set. Hence, it is not always possible
13067 to allow the shell to open the device in the traditional manner.
13073 @cindex machine-readable @command{stty} output
13074 Print all current settings in a form that can be used as an argument to
13075 another @command{stty} command to restore the current settings. This option
13076 may not be used in combination with any line settings.
13080 Many settings can be turned off by preceding them with a @samp{-}.
13081 Such arguments are marked below with ``May be negated'' in their
13082 description. The descriptions themselves refer to the positive
13083 case, that is, when @emph{not} negated (unless stated otherwise,
13086 Some settings are not available on all @acronym{POSIX} systems, since they use
13087 extensions. Such arguments are marked below with
13088 ``Non-@acronym{POSIX}'' in their description. On non-@acronym{POSIX}
13089 systems, those or other settings also may not
13090 be available, but it's not feasible to document all the variations: just
13096 * Control:: Control settings
13097 * Input:: Input settings
13098 * Output:: Output settings
13099 * Local:: Local settings
13100 * Combination:: Combination settings
13101 * Characters:: Special characters
13102 * Special:: Special settings
13107 @subsection Control settings
13109 @cindex control settings
13115 @cindex two-way parity
13116 Generate parity bit in output and expect parity bit in input.
13122 @cindex even parity
13123 Set odd parity (even if negated). May be negated.
13130 @cindex character size
13131 @cindex eight-bit characters
13132 Set character size to 5, 6, 7, or 8 bits.
13137 Send a hangup signal when the last process closes the tty. May be
13143 Use two stop bits per character (one if negated). May be negated.
13147 Allow input to be received. May be negated.
13151 @cindex modem control
13152 Disable modem control signals. May be negated.
13156 @cindex hardware flow control
13157 @cindex flow control, hardware
13158 @cindex RTS/CTS flow control
13159 Enable RTS/CTS flow control. Non-@acronym{POSIX}. May be negated.
13164 @subsection Input settings
13166 @cindex input settings
13167 These settings control operations on data received from the terminal.
13172 @cindex breaks, ignoring
13173 Ignore break characters. May be negated.
13177 @cindex breaks, cause interrupts
13178 Make breaks cause an interrupt signal. May be negated.
13182 @cindex parity, ignoring
13183 Ignore characters with parity errors. May be negated.
13187 @cindex parity errors, marking
13188 Mark parity errors (with a 255-0-character sequence). May be negated.
13192 Enable input parity checking. May be negated.
13196 @cindex eight-bit input
13197 Clear high (8th) bit of input characters. May be negated.
13201 @cindex newline, translating to return
13202 Translate newline to carriage return. May be negated.
13206 @cindex return, ignoring
13207 Ignore carriage return. May be negated.
13211 @cindex return, translating to newline
13212 Translate carriage return to newline. May be negated.
13216 @cindex input encoding, UTF-8
13217 Assume input characters are UTF-8 encoded. May be negated.
13221 @kindex C-s/C-q flow control
13222 @cindex XON/XOFF flow control
13223 Enable XON/XOFF flow control (that is, @kbd{CTRL-S}/@kbd{CTRL-Q}). May
13230 @cindex software flow control
13231 @cindex flow control, software
13232 Enable sending of @code{stop} character when the system input buffer
13233 is almost full, and @code{start} character when it becomes almost
13234 empty again. May be negated.
13238 @cindex uppercase, translating to lowercase
13239 Translate uppercase characters to lowercase. Non-@acronym{POSIX}. May be
13240 negated. Note ilcuc is not implemented, as one would not be able to issue
13241 almost any (lowercase) Unix command, after invoking it.
13245 Allow any character to restart output (only the start character
13246 if negated). Non-@acronym{POSIX}. May be negated.
13250 @cindex beeping at input buffer full
13251 Enable beeping and not flushing input buffer if a character arrives
13252 when the input buffer is full. Non-@acronym{POSIX}. May be negated.
13257 @subsection Output settings
13259 @cindex output settings
13260 These settings control operations on data sent to the terminal.
13265 Postprocess output. May be negated.
13269 @cindex lowercase, translating to output
13270 Translate lowercase characters to uppercase. Non-@acronym{POSIX}. May be
13271 negated. (Note ouclc is not currently implemented.)
13275 @cindex return, translating to newline
13276 Translate carriage return to newline. Non-@acronym{POSIX}. May be negated.
13280 @cindex newline, translating to crlf
13281 Translate newline to carriage return-newline. Non-@acronym{POSIX}. May be
13286 Do not print carriage returns in the first column. Non-@acronym{POSIX}.
13291 Newline performs a carriage return. Non-@acronym{POSIX}. May be negated.
13295 @cindex pad instead of timing for delaying
13296 Use fill (padding) characters instead of timing for delays.
13297 Non-@acronym{POSIX}.
13302 @cindex pad character
13303 Use @acronym{ASCII} @sc{del} characters for fill instead of
13304 @acronym{ASCII} @sc{nul} characters. Non-@acronym{POSIX}.
13310 Newline delay style. Non-@acronym{POSIX}.
13317 Carriage return delay style. Non-@acronym{POSIX}.
13323 @opindex tab@var{n}
13324 Horizontal tab delay style. Non-@acronym{POSIX}.
13329 Backspace delay style. Non-@acronym{POSIX}.
13334 Vertical tab delay style. Non-@acronym{POSIX}.
13339 Form feed delay style. Non-@acronym{POSIX}.
13344 @subsection Local settings
13346 @cindex local settings
13351 Enable @code{interrupt}, @code{quit}, and @code{suspend} special
13352 characters. May be negated.
13356 Enable @code{erase}, @code{kill}, @code{werase}, and @code{rprnt}
13357 special characters. May be negated.
13361 Enable non-@acronym{POSIX} special characters. May be negated.
13365 Echo input characters. May be negated.
13371 Echo @code{erase} characters as backspace-space-backspace. May be
13376 @cindex newline echoing after @code{kill}
13377 Echo a newline after a @code{kill} character. May be negated.
13381 @cindex newline, echoing
13382 Echo newline even if not echoing other characters. May be negated.
13386 @cindex flushing, disabling
13387 Disable flushing after @code{interrupt} and @code{quit} special
13388 characters. May be negated.
13392 @cindex case translation
13393 Enable input and output of uppercase characters by preceding their
13394 lowercase equivalents with @samp{\}, when @code{icanon} is set.
13395 Non-@acronym{POSIX}. May be negated.
13399 @cindex background jobs, stopping at terminal write
13400 Stop background jobs that try to write to the terminal. Non-@acronym{POSIX}.
13407 Echo erased characters backward, between @samp{\} and @samp{/}.
13408 Non-@acronym{POSIX}. May be negated.
13414 @cindex control characters, using @samp{^@var{c}}
13415 @cindex hat notation for control characters
13416 Echo control characters in hat notation (@samp{^@var{c}}) instead
13417 of literally. Non-@acronym{POSIX}. May be negated.
13423 Echo the @code{kill} special character by erasing each character on
13424 the line as indicated by the @code{echoprt} and @code{echoe} settings,
13425 instead of by the @code{echoctl} and @code{echok} settings.
13426 Non-@acronym{POSIX}.
13432 @subsection Combination settings
13434 @cindex combination settings
13435 Combination settings:
13442 Same as @code{parenb -parodd cs7}. May be negated. If negated, same
13443 as @code{-parenb cs8}.
13447 Same as @code{parenb parodd cs7}. May be negated. If negated, same
13448 as @code{-parenb cs8}.
13452 Same as @code{-icrnl -onlcr}. May be negated. If negated, same as
13453 @code{icrnl -inlcr -igncr onlcr -ocrnl -onlret}.
13457 Reset the @code{erase} and @code{kill} special characters to their default
13464 @c This is too long to write inline.
13466 cread -ignbrk brkint -inlcr -igncr icrnl -ixoff
13467 -iuclc -ixany imaxbel opost -olcuc -ocrnl onlcr
13468 -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0
13469 ff0 isig icanon iexten echo echoe echok -echonl
13470 -noflsh -xcase -tostop -echoprt echoctl echoke
13474 and also sets all special characters to their default values.
13478 Same as @code{brkint ignpar istrip icrnl ixon opost isig icanon}, plus
13479 sets the @code{eof} and @code{eol} characters to their default values
13480 if they are the same as the @code{min} and @code{time} characters.
13481 May be negated. If negated, same as @code{raw}.
13488 -ignbrk -brkint -ignpar -parmrk -inpck -istrip
13489 -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany
13490 -imaxbel -opost -isig -icanon -xcase min 1 time 0
13494 May be negated. If negated, same as @code{cooked}.
13498 Same as @option{-icanon}. May be negated. If negated, same as
13503 @cindex eight-bit characters
13504 Same as @code{-parenb -istrip cs8}. May be negated. If negated,
13505 same as @code{parenb istrip cs7}.
13509 Same as @option{-parenb -istrip -opost cs8}. May be negated.
13510 If negated, same as @code{parenb istrip opost cs7}.
13514 Same as @option{-ixany}. Non-@acronym{POSIX}. May be negated.
13518 Same as @code{tab0}. Non-@acronym{POSIX}. May be negated. If negated, same
13525 Same as @code{xcase iuclc olcuc}. Non-@acronym{POSIX}. May be negated.
13526 (Used for terminals with uppercase characters only.)
13530 Same as @code{echoe echoctl echoke}.
13534 Same as @code{echoe echoctl echoke -ixany intr ^C erase ^? kill C-u}.
13539 @subsection Special characters
13541 @cindex special characters
13542 @cindex characters, special
13544 The special characters' default values vary from system to system.
13545 They are set with the syntax @samp{name value}, where the names are
13546 listed below and the value can be given either literally, in hat
13547 notation (@samp{^@var{c}}), or as an integer which may start with
13548 @samp{0x} to indicate hexadecimal, @samp{0} to indicate octal, or
13549 any other digit to indicate decimal.
13551 @cindex disabling special characters
13552 @kindex u@r{, and disabling special characters}
13553 For GNU stty, giving a value of @code{^-} or @code{undef} disables that
13554 special character. (This is incompatible with Ultrix @command{stty},
13555 which uses a value of @samp{u} to disable a special character. GNU
13556 @command{stty} treats a value @samp{u} like any other, namely to set that
13557 special character to @key{U}.)
13563 Send an interrupt signal.
13567 Send a quit signal.
13571 Erase the last character typed.
13575 Erase the current line.
13579 Send an end of file (terminate the input).
13587 Alternate character to end the line. Non-@acronym{POSIX}.
13591 Switch to a different shell layer. Non-@acronym{POSIX}.
13595 Restart the output after stopping it.
13603 Send a terminal stop signal.
13607 Send a terminal stop signal after flushing the input. Non-@acronym{POSIX}.
13611 Redraw the current line. Non-@acronym{POSIX}.
13615 Erase the last word typed. Non-@acronym{POSIX}.
13619 Enter the next character typed literally, even if it is a special
13620 character. Non-@acronym{POSIX}.
13625 @subsection Special settings
13627 @cindex special settings
13632 Set the minimum number of characters that will satisfy a read until
13633 the time value has expired, when @option{-icanon} is set.
13637 Set the number of tenths of a second before reads time out if the minimum
13638 number of characters have not been read, when @option{-icanon} is set.
13640 @item ispeed @var{n}
13642 Set the input speed to @var{n}.
13644 @item ospeed @var{n}
13646 Set the output speed to @var{n}.
13650 Tell the tty kernel driver that the terminal has @var{n} rows.
13651 Non-@acronym{POSIX}.
13654 @itemx columns @var{n}
13657 Tell the kernel that the terminal has @var{n} columns. Non-@acronym{POSIX}.
13663 Print the number of rows and columns that the kernel thinks the
13664 terminal has. (Systems that don't support rows and columns in the kernel
13665 typically use the environment variables @env{LINES} and @env{COLUMNS}
13666 instead; however, GNU @command{stty} does not know anything about them.)
13667 Non-@acronym{POSIX}.
13671 Use line discipline @var{n}. Non-@acronym{POSIX}.
13675 Print the terminal speed.
13678 @cindex baud rate, setting
13679 Set the input and output speeds to @var{n}. @var{n} can be one of: 0
13680 50 75 110 134 134.5 150 200 300 600 1200 1800 2400 4800 9600 19200
13681 38400 @code{exta} @code{extb}. @code{exta} is the same as 19200;
13682 @code{extb} is the same as 38400. Many systems, including GNU/Linux,
13683 support higher speeds. The @command{stty} command includes support
13700 4000000 where the system supports these.
13701 0 hangs up the line if @option{-clocal} is set.
13705 @node printenv invocation
13706 @section @command{printenv}: Print all or some environment variables
13709 @cindex printing all or some environment variables
13710 @cindex environment variables, printing
13712 @command{printenv} prints environment variable values. Synopsis:
13715 printenv [@var{option}] [@var{variable}]@dots{}
13718 If no @var{variable}s are specified, @command{printenv} prints the value of
13719 every environment variable. Otherwise, it prints the value of each
13720 @var{variable} that is set, and nothing for those that are not set.
13722 The program accepts the following option. Also see @ref{Common options}.
13730 @cindex exit status of @command{printenv}
13734 0 if all variables specified were found
13735 1 if at least one specified variable was not found
13736 2 if a write error occurred
13740 @node tty invocation
13741 @section @command{tty}: Print file name of terminal on standard input
13744 @cindex print terminal file name
13745 @cindex terminal file name, printing
13747 @command{tty} prints the file name of the terminal connected to its standard
13748 input. It prints @samp{not a tty} if standard input is not a terminal.
13752 tty [@var{option}]@dots{}
13755 The program accepts the following option. Also see @ref{Common options}.
13765 Print nothing; only return an exit status.
13769 @cindex exit status of @command{tty}
13773 0 if standard input is a terminal
13774 1 if standard input is not a terminal
13775 2 if given incorrect arguments
13776 3 if a write error occurs
13780 @node User information
13781 @chapter User information
13783 @cindex user information, commands for
13784 @cindex commands for printing user information
13786 This section describes commands that print user-related information:
13787 logins, groups, and so forth.
13790 * id invocation:: Print user identity.
13791 * logname invocation:: Print current login name.
13792 * whoami invocation:: Print effective user ID.
13793 * groups invocation:: Print group names a user is in.
13794 * users invocation:: Print login names of users currently logged in.
13795 * who invocation:: Print who is currently logged in.
13799 @node id invocation
13800 @section @command{id}: Print user identity
13803 @cindex real user and group IDs, printing
13804 @cindex effective user and group IDs, printing
13805 @cindex printing real and effective user and group IDs
13807 @command{id} prints information about the given user, or the process
13808 running it if no user is specified. Synopsis:
13811 id [@var{option}]@dots{} [@var{username}]
13814 @vindex POSIXLY_CORRECT
13815 By default, it prints the real user ID, real group ID, effective user ID
13816 if different from the real user ID, effective group ID if different from
13817 the real group ID, and supplemental group IDs.
13818 In addition, if SELinux
13819 is enabled and the @env{POSIXLY_CORRECT} environment variable is not set,
13820 then print @samp{context=@var{c}}, where @var{c} is the security context.
13822 Each of these numeric values is preceded by an identifying string and
13823 followed by the corresponding user or group name in parentheses.
13825 The options cause @command{id} to print only part of the above information.
13826 Also see @ref{Common options}.
13833 Print only the group ID.
13839 Print only the group ID and the supplementary groups.
13845 Print the user or group name instead of the ID number. Requires
13846 @option{-u}, @option{-g}, or @option{-G}.
13852 Print the real, instead of effective, user or group ID. Requires
13853 @option{-u}, @option{-g}, or @option{-G}.
13859 Print only the user ID.
13866 @cindex security context
13867 Print only the security context of the current user.
13868 If SELinux is disabled then print a warning and
13869 set the exit status to 1.
13875 @macro primaryAndSupplementaryGroups{cmd,arg}
13876 Primary and supplementary groups for a process are normally inherited
13877 from its parent and are usually unchanged since login. This means
13878 that if you change the group database after logging in, @command{\cmd\}
13879 will not reflect your changes within your existing login session.
13880 Running @command{\cmd\} with a \arg\ causes the user and group
13881 database to be consulted afresh, and so will give a different result.
13883 @primaryAndSupplementaryGroups{id,user argument}
13885 @node logname invocation
13886 @section @command{logname}: Print current login name
13889 @cindex printing user's login name
13890 @cindex login name, printing
13891 @cindex user name, printing
13894 @command{logname} prints the calling user's name, as found in a
13895 system-maintained file (often @file{/var/run/utmp} or
13896 @file{/etc/utmp}), and exits with a status of 0. If there is no entry
13897 for the calling process, @command{logname} prints
13898 an error message and exits with a status of 1.
13900 The only options are @option{--help} and @option{--version}. @xref{Common
13906 @node whoami invocation
13907 @section @command{whoami}: Print effective user ID
13910 @cindex effective user ID, printing
13911 @cindex printing the effective user ID
13913 @command{whoami} prints the user name associated with the current
13914 effective user ID. It is equivalent to the command @samp{id -un}.
13916 The only options are @option{--help} and @option{--version}. @xref{Common
13922 @node groups invocation
13923 @section @command{groups}: Print group names a user is in
13926 @cindex printing groups a user is in
13927 @cindex supplementary groups, printing
13929 @command{groups} prints the names of the primary and any supplementary
13930 groups for each given @var{username}, or the current process if no names
13931 are given. If more than one name is given, the name of each user is
13933 the list of that user's groups and the user name is separated from the
13934 group list by a colon. Synopsis:
13937 groups [@var{username}]@dots{}
13940 The group lists are equivalent to the output of the command @samp{id -Gn}.
13942 @primaryAndSupplementaryGroups{groups,list of users}
13944 The only options are @option{--help} and @option{--version}. @xref{Common
13950 @node users invocation
13951 @section @command{users}: Print login names of users currently logged in
13954 @cindex printing current usernames
13955 @cindex usernames, printing current
13957 @cindex login sessions, printing users with
13958 @command{users} prints on a single line a blank-separated list of user
13959 names of users currently logged in to the current host. Each user name
13960 corresponds to a login session, so if a user has more than one login
13961 session, that user's name will appear the same number of times in the
13970 With no @var{file} argument, @command{users} extracts its information from
13971 a system-maintained file (often @file{/var/run/utmp} or
13972 @file{/etc/utmp}). If a file argument is given, @command{users} uses
13973 that file instead. A common choice is @file{/var/log/wtmp}.
13975 The only options are @option{--help} and @option{--version}. @xref{Common
13981 @node who invocation
13982 @section @command{who}: Print who is currently logged in
13985 @cindex printing current user information
13986 @cindex information, about current users
13988 @command{who} prints information about users who are currently logged on.
13992 @command{who} [@var{option}] [@var{file}] [am i]
13995 @cindex terminal lines, currently used
13997 @cindex remote hostname
13998 If given no non-option arguments, @command{who} prints the following
13999 information for each user currently logged on: login name, terminal
14000 line, login time, and remote hostname or X display.
14004 If given one non-option argument, @command{who} uses that instead of
14005 a default system-maintained file (often @file{/var/run/utmp} or
14006 @file{/etc/utmp}) as the name of the file containing the record of
14007 users logged on. @file{/var/log/wtmp} is commonly given as an argument
14008 to @command{who} to look at who has previously logged on.
14012 If given two non-option arguments, @command{who} prints only the entry
14013 for the user running it (determined from its standard input), preceded
14014 by the hostname. Traditionally, the two arguments given are @samp{am
14015 i}, as in @samp{who am i}.
14018 Time stamps are listed according to the time zone rules specified by
14019 the @env{TZ} environment variable, or by the system default rules if
14020 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
14021 with @env{TZ}, libc, The GNU C Library Reference Manual}.
14023 The program accepts the following options. Also see @ref{Common options}.
14031 Same as @samp{-b -d --login -p -r -t -T -u}.
14037 Print the date and time of last system boot.
14043 Print information corresponding to dead processes.
14049 Print a line of column headings.
14055 List only the entries that correspond to processes via which the
14056 system is waiting for a user to login. The user name is always @samp{LOGIN}.
14060 Attempt to canonicalize hostnames found in utmp through a DNS lookup. This
14061 is not the default because it can cause significant delays on systems with
14062 automatic dial-up internet access.
14066 Same as @samp{who am i}.
14072 List active processes spawned by init.
14078 Print only the login names and the number of users logged on.
14079 Overrides all other options.
14084 @opindex --runlevel
14085 Print the current (and maybe previous) run-level of the init process.
14089 Ignored; for compatibility with other versions of @command{who}.
14095 Print last system clock change.
14100 After the login time, print the number of hours and minutes that the
14101 user has been idle. @samp{.} means the user was active in the last minute.
14102 @samp{old} means the user has been idle for more than 24 hours.
14113 @opindex --writable
14114 @cindex message status
14115 @pindex write@r{, allowed}
14116 After each login name print a character indicating the user's message status:
14119 @samp{+} allowing @code{write} messages
14120 @samp{-} disallowing @code{write} messages
14121 @samp{?} cannot find terminal device
14129 @node System context
14130 @chapter System context
14132 @cindex system context
14133 @cindex context, system
14134 @cindex commands for system context
14136 This section describes commands that print or change system-wide
14140 * date invocation:: Print or set system date and time.
14141 * arch invocation:: Print machine hardware name.
14142 * nproc invocation:: Print the number of processors.
14143 * uname invocation:: Print system information.
14144 * hostname invocation:: Print or set system name.
14145 * hostid invocation:: Print numeric host identifier.
14146 * uptime invocation:: Print system uptime and load.
14149 @node date invocation
14150 @section @command{date}: Print or set system date and time
14153 @cindex time, printing or setting
14154 @cindex printing the current time
14159 date [@var{option}]@dots{} [+@var{format}]
14160 date [-u|--utc|--universal] @c this avoids a newline in the output
14161 [ MMDDhhmm[[CC]YY][.ss] ]
14165 Invoking @command{date} with no @var{format} argument is equivalent to invoking
14166 it with a default format that depends on the @env{LC_TIME} locale category.
14167 In the default C locale, this format is @samp{'+%a %b %e %H:%M:%S %Z %Y'},
14168 so the output looks like @samp{Thu Mar @ 3 13:47:51 PST 2005}.
14171 Normally, @command{date} uses the time zone rules indicated by the
14172 @env{TZ} environment variable, or the system default rules if @env{TZ}
14173 is not set. @xref{TZ Variable,, Specifying the Time Zone with
14174 @env{TZ}, libc, The GNU C Library Reference Manual}.
14176 @findex strftime @r{and @command{date}}
14177 @cindex time formats
14178 @cindex formatting times
14179 If given an argument that starts with a @samp{+}, @command{date} prints the
14180 current date and time (or the date and time specified by the
14181 @option{--date} option, see below) in the format defined by that argument,
14182 which is similar to that of the @code{strftime} function. Except for
14183 conversion specifiers, which start with @samp{%}, characters in the
14184 format string are printed unchanged. The conversion specifiers are
14190 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
14191 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
14192 * Literal conversion specifiers:: %[%nt]
14193 * Padding and other flags:: Pad with zeros, spaces, etc.
14194 * Setting the time:: Changing the system clock.
14195 * Options for date:: Instead of the current time.
14197 * Date input formats:: Specifying date strings.
14199 * Examples of date:: Examples.
14202 @node Time conversion specifiers
14203 @subsection Time conversion specifiers
14205 @cindex time conversion specifiers
14206 @cindex conversion specifiers, time
14208 @command{date} conversion specifiers related to times.
14212 hour (@samp{00}@dots{}@samp{23})
14214 hour (@samp{01}@dots{}@samp{12})
14216 hour, space padded (@samp{ 0}@dots{}@samp{23}); equivalent to @samp{%_H}.
14217 This is a @acronym{GNU} extension.
14219 hour, space padded (@samp{ 1}@dots{}@samp{12}); equivalent to @samp{%_I}.
14220 This is a @acronym{GNU} extension.
14222 minute (@samp{00}@dots{}@samp{59})
14224 nanoseconds (@samp{000000000}@dots{}@samp{999999999}).
14225 This is a @acronym{GNU} extension.
14227 locale's equivalent of either @samp{AM} or @samp{PM};
14228 blank in many locales.
14229 Noon is treated as @samp{PM} and midnight as @samp{AM}.
14231 like @samp{%p}, except lower case.
14232 This is a @acronym{GNU} extension.
14234 locale's 12-hour clock time (e.g., @samp{11:11:04 PM})
14236 24-hour hour and minute. Same as @samp{%H:%M}.
14238 @cindex epoch, seconds since
14239 @cindex seconds since the epoch
14240 @cindex beginning of time
14241 seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC.
14242 Leap seconds are not counted unless leap second support is available.
14243 @xref{%s-examples}, for examples.
14244 This is a @acronym{GNU} extension.
14246 second (@samp{00}@dots{}@samp{60}).
14247 This may be @samp{60} if leap seconds are supported.
14249 24-hour hour, minute, and second. Same as @samp{%H:%M:%S}.
14251 locale's time representation (e.g., @samp{23:13:48})
14253 @w{@acronym{RFC} 2822/@acronym{ISO} 8601} style numeric time zone
14254 (e.g., @samp{-0600} or @samp{+0530}), or nothing if no
14255 time zone is determinable. This value reflects the numeric time zone
14256 appropriate for the current time, using the time zone rules specified
14257 by the @env{TZ} environment variable.
14258 The time (and optionally, the time zone rules) can be overridden
14259 by the @option{--date} option.
14261 @w{@acronym{RFC} 3339/@acronym{ISO} 8601} style numeric time zone with
14262 @samp{:} (e.g., @samp{-06:00} or @samp{+05:30}), or nothing if no time
14263 zone is determinable.
14264 This is a @acronym{GNU} extension.
14266 Numeric time zone to the nearest second with @samp{:} (e.g.,
14267 @samp{-06:00:00} or @samp{+05:30:00}), or nothing if no time zone is
14269 This is a @acronym{GNU} extension.
14271 Numeric time zone with @samp{:} using the minimum necessary precision
14272 (e.g., @samp{-06}, @samp{+05:30}, or @samp{-04:56:02}), or nothing if
14273 no time zone is determinable.
14274 This is a @acronym{GNU} extension.
14276 alphabetic time zone abbreviation (e.g., @samp{EDT}), or nothing if no
14277 time zone is determinable. See @samp{%z} for how it is determined.
14281 @node Date conversion specifiers
14282 @subsection Date conversion specifiers
14284 @cindex date conversion specifiers
14285 @cindex conversion specifiers, date
14287 @command{date} conversion specifiers related to dates.
14291 locale's abbreviated weekday name (e.g., @samp{Sun})
14293 locale's full weekday name, variable length (e.g., @samp{Sunday})
14295 locale's abbreviated month name (e.g., @samp{Jan})
14297 locale's full month name, variable length (e.g., @samp{January})
14299 locale's date and time (e.g., @samp{Thu Mar @ 3 23:05:25 2005})
14301 century. This is like @samp{%Y}, except the last two digits are omitted.
14302 For example, it is @samp{20} if @samp{%Y} is @samp{2000},
14303 and is @samp{-0} if @samp{%Y} is @samp{-001}.
14304 It is normally at least two characters, but it may be more.
14306 day of month (e.g., @samp{01})
14308 date; same as @samp{%m/%d/%y}
14310 day of month, space padded; same as @samp{%_d}
14312 full date in @acronym{ISO} 8601 format; same as @samp{%Y-%m-%d}.
14313 This is a good choice for a date format, as it is standard and
14314 is easy to sort in the usual case where years are in the range
14317 year corresponding to the @acronym{ISO} week number, but without the century
14318 (range @samp{00} through @samp{99}). This has the same format and value
14319 as @samp{%y}, except that if the @acronym{ISO} week number (see
14321 to the previous or next year, that year is used instead.
14323 year corresponding to the @acronym{ISO} week number. This has the
14324 same format and value as @samp{%Y}, except that if the @acronym{ISO}
14326 @samp{%V}) belongs to the previous or next year, that year is used
14328 It is normally useful only if @samp{%V} is also used;
14329 for example, the format @samp{%G-%m-%d} is probably a mistake,
14330 since it combines the ISO week number year with the conventional month and day.
14334 day of year (@samp{001}@dots{}@samp{366})
14336 month (@samp{01}@dots{}@samp{12})
14338 day of week (@samp{1}@dots{}@samp{7}) with @samp{1} corresponding to Monday
14340 week number of year, with Sunday as the first day of the week
14341 (@samp{00}@dots{}@samp{53}).
14342 Days in a new year preceding the first Sunday are in week zero.
14344 @acronym{ISO} week number, that is, the
14345 week number of year, with Monday as the first day of the week
14346 (@samp{01}@dots{}@samp{53}).
14347 If the week containing January 1 has four or more days in
14348 the new year, then it is considered week 1; otherwise, it is week 53 of
14349 the previous year, and the next week is week 1. (See the @acronym{ISO} 8601
14352 day of week (@samp{0}@dots{}@samp{6}) with 0 corresponding to Sunday
14354 week number of year, with Monday as first day of week
14355 (@samp{00}@dots{}@samp{53}).
14356 Days in a new year preceding the first Monday are in week zero.
14358 locale's date representation (e.g., @samp{12/31/99})
14360 last two digits of year (@samp{00}@dots{}@samp{99})
14362 year. This is normally at least four characters, but it may be more.
14363 Year @samp{0000} precedes year @samp{0001}, and year @samp{-001}
14364 precedes year @samp{0000}.
14368 @node Literal conversion specifiers
14369 @subsection Literal conversion specifiers
14371 @cindex literal conversion specifiers
14372 @cindex conversion specifiers, literal
14374 @command{date} conversion specifiers that produce literal strings.
14386 @node Padding and other flags
14387 @subsection Padding and other flags
14389 @cindex numeric field padding
14390 @cindex padding of numeric fields
14391 @cindex fields, padding numeric
14393 Unless otherwise specified, @command{date} normally pads numeric fields
14394 with zeros, so that, for
14395 example, numeric months are always output as two digits.
14396 Seconds since the epoch are not padded, though,
14397 since there is no natural width for them.
14399 As a @acronym{GNU} extension, @command{date} recognizes any of the
14400 following optional flags after the @samp{%}:
14404 (hyphen) Do not pad the field; useful if the output is intended for
14407 (underscore) Pad with spaces; useful if you need a fixed
14408 number of characters in the output, but zeros are too distracting.
14410 (zero) Pad with zeros even if the conversion specifier
14411 would normally pad with spaces.
14413 Use upper case characters if possible.
14415 Use opposite case characters if possible.
14416 A field that is normally upper case becomes lower case, and vice versa.
14420 Here are some examples of padding:
14423 date +%d/%m -d "Feb 1"
14425 date +%-d/%-m -d "Feb 1"
14427 date +%_d/%_m -d "Feb 1"
14431 As a @acronym{GNU} extension, you can specify the field width
14432 (after any flag, if present) as a decimal number. If the natural size of the
14433 output of the field has less than the specified number of characters,
14434 the result is written right adjusted and padded to the given
14435 size. For example, @samp{%9B} prints the right adjusted month name in
14436 a field of width 9.
14438 An optional modifier can follow the optional flag and width
14439 specification. The modifiers are:
14443 Use the locale's alternate representation for date and time. This
14444 modifier applies to the @samp{%c}, @samp{%C}, @samp{%x}, @samp{%X},
14445 @samp{%y} and @samp{%Y} conversion specifiers. In a Japanese locale, for
14446 example, @samp{%Ex} might yield a date format based on the Japanese
14450 Use the locale's alternate numeric symbols for numbers. This modifier
14451 applies only to numeric conversion specifiers.
14454 If the format supports the modifier but no alternate representation
14455 is available, it is ignored.
14458 @node Setting the time
14459 @subsection Setting the time
14461 @cindex setting the time
14462 @cindex time setting
14463 @cindex appropriate privileges
14465 If given an argument that does not start with @samp{+}, @command{date} sets
14466 the system clock to the date and time specified by that argument (as
14467 described below). You must have appropriate privileges to set the
14468 system clock. Note for changes to persist across a reboot, the
14469 hardware clock may need to be updated from the system clock, which
14470 might not happen automatically on your system.
14472 The argument must consist entirely of digits, which have the following
14485 first two digits of year (optional)
14487 last two digits of year (optional)
14492 Note, the @option{--date} and @option{--set} options may not be used with an
14493 argument in the above format. The @option{--universal} option may be used
14494 with such an argument to indicate that the specified date and time are
14495 relative to Coordinated Universal Time rather than to the local time zone.
14498 @node Options for date
14499 @subsection Options for @command{date}
14501 @cindex @command{date} options
14502 @cindex options for @command{date}
14504 The program accepts the following options. Also see @ref{Common options}.
14508 @item -d @var{datestr}
14509 @itemx --date=@var{datestr}
14512 @cindex parsing date strings
14513 @cindex date strings, parsing
14514 @cindex arbitrary date strings, parsing
14517 @opindex next @var{day}
14518 @opindex last @var{day}
14519 Display the date and time specified in @var{datestr} instead of the
14520 current date and time. @var{datestr} can be in almost any common
14521 format. It can contain month names, time zones, @samp{am} and @samp{pm},
14522 @samp{yesterday}, etc. For example, @option{--date="2004-02-27
14523 14:19:13.489392193 +0530"} specifies the instant of time that is
14524 489,392,193 nanoseconds after February 27, 2004 at 2:19:13 PM in a
14525 time zone that is 5 hours and 30 minutes east of @acronym{UTC}.@*
14526 Note: input currently must be in locale independent format. E.g., the
14527 LC_TIME=C below is needed to print back the correct date in many locales:
14529 date -d "$(LC_TIME=C date)"
14531 @xref{Date input formats}.
14533 @item -f @var{datefile}
14534 @itemx --file=@var{datefile}
14537 Parse each line in @var{datefile} as with @option{-d} and display the
14538 resulting date and time. If @var{datefile} is @samp{-}, use standard
14539 input. This is useful when you have many dates to process, because the
14540 system overhead of starting up the @command{date} executable many times can
14543 @item -I[@var{timespec}]
14544 @itemx --iso-8601[=@var{timespec}]
14545 @opindex -I[@var{timespec}]
14546 @opindex --iso-8601[=@var{timespec}]
14547 Display the date using the @acronym{ISO} 8601 format, @samp{%Y-%m-%d}.
14549 The argument @var{timespec} specifies the number of additional
14550 terms of the time to include. It can be one of the following:
14553 Print just the date. This is the default if @var{timespec} is omitted.
14556 Append the hour of the day to the date.
14559 Append the hours and minutes.
14562 Append the hours, minutes and seconds.
14565 Append the hours, minutes, seconds and nanoseconds.
14568 If showing any time terms, then include the time zone using the format
14571 @item -r @var{file}
14572 @itemx --reference=@var{file}
14574 @opindex --reference
14575 Display the date and time of the last modification of @var{file},
14576 instead of the current date and time.
14583 @opindex --rfc-2822
14584 Display the date and time using the format @samp{%a, %d %b %Y %H:%M:%S
14585 %z}, evaluated in the C locale so abbreviations are always in English.
14589 Fri, 09 Sep 2005 13:51:39 -0700
14592 This format conforms to
14593 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt, Internet
14594 @acronym{RFCs} 2822} and
14595 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc822.txt, 822}, the
14596 current and previous standards for Internet email.
14598 @item --rfc-3339=@var{timespec}
14599 @opindex --rfc-3339=@var{timespec}
14600 Display the date using a format specified by
14601 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt, Internet
14602 @acronym{RFC} 3339}. This is a subset of the @acronym{ISO} 8601
14603 format, except that it also permits applications to use a space rather
14604 than a @samp{T} to separate dates from times. Unlike the other
14605 standard formats, @acronym{RFC} 3339 format is always suitable as
14606 input for the @option{--date} (@option{-d}) and @option{--file}
14607 (@option{-f}) options, regardless of the current locale.
14609 The argument @var{timespec} specifies how much of the time to include.
14610 It can be one of the following:
14614 Print just the full-date, e.g., @samp{2005-09-14}.
14615 This is equivalent to the format @samp{%Y-%m-%d}.
14618 Print the full-date and full-time separated by a space, e.g.,
14619 @samp{2005-09-14 00:56:06+05:30}. The output ends with a numeric
14620 time-offset; here the @samp{+05:30} means that local time is five
14621 hours and thirty minutes east of @acronym{UTC}. This is equivalent to
14622 the format @samp{%Y-%m-%d %H:%M:%S%:z}.
14625 Like @samp{seconds}, but also print nanoseconds, e.g.,
14626 @samp{2005-09-14 00:56:06.998458565+05:30}.
14627 This is equivalent to the format @samp{%Y-%m-%d %H:%M:%S.%N%:z}.
14631 @item -s @var{datestr}
14632 @itemx --set=@var{datestr}
14635 Set the date and time to @var{datestr}. See @option{-d} above.
14636 See also @ref{Setting the time}.
14643 @opindex --universal
14644 @cindex Coordinated Universal Time
14646 @cindex Greenwich Mean Time
14649 Use Coordinated Universal Time (@acronym{UTC}) by operating as if the
14650 @env{TZ} environment variable were set to the string @samp{UTC0}.
14652 Universal Time is often called ``Greenwich Mean Time'' (@sc{gmt}) for
14653 historical reasons.
14657 @node Examples of date
14658 @subsection Examples of @command{date}
14660 @cindex examples of @command{date}
14662 Here are a few examples. Also see the documentation for the @option{-d}
14663 option in the previous section.
14668 To print the date of the day before yesterday:
14671 date --date='2 days ago'
14675 To print the date of the day three months and one day hence:
14678 date --date='3 months 1 day'
14682 To print the day of year of Christmas in the current year:
14685 date --date='25 Dec' +%j
14689 To print the current full month name and the day of the month:
14695 But this may not be what you want because for the first nine days of
14696 the month, the @samp{%d} expands to a zero-padded two-digit field,
14697 for example @samp{date -d 1may '+%B %d'} will print @samp{May 01}.
14700 To print a date without the leading zero for one-digit days
14701 of the month, you can use the (@acronym{GNU} extension)
14702 @samp{-} flag to suppress
14703 the padding altogether:
14706 date -d 1may '+%B %-d
14710 To print the current date and time in the format required by many
14711 non-@acronym{GNU} versions of @command{date} when setting the system clock:
14714 date +%m%d%H%M%Y.%S
14718 To set the system clock forward by two minutes:
14721 date --set='+2 minutes'
14725 To print the date in @acronym{RFC} 2822 format,
14726 use @samp{date --rfc-2822}. Here is some example output:
14729 Fri, 09 Sep 2005 13:51:39 -0700
14732 @anchor{%s-examples}
14734 To convert a date string to the number of seconds since the epoch
14735 (which is 1970-01-01 00:00:00 UTC), use the @option{--date} option with
14736 the @samp{%s} format. That can be useful in sorting and/or graphing
14737 and/or comparing data by date. The following command outputs the
14738 number of the seconds since the epoch for the time two minutes after the
14742 date --date='1970-01-01 00:02:00 +0000' +%s
14746 If you do not specify time zone information in the date string,
14747 @command{date} uses your computer's idea of the time zone when
14748 interpreting the string. For example, if your computer's time zone is
14749 that of Cambridge, Massachusetts, which was then 5 hours (i.e., 18,000
14750 seconds) behind UTC:
14753 # local time zone used
14754 date --date='1970-01-01 00:02:00' +%s
14759 If you're sorting or graphing dated data, your raw date values may be
14760 represented as seconds since the epoch. But few people can look at
14761 the date @samp{946684800} and casually note ``Oh, that's the first second
14762 of the year 2000 in Greenwich, England.''
14765 date --date='2000-01-01 UTC' +%s
14769 An alternative is to use the @option{--utc} (@option{-u}) option.
14770 Then you may omit @samp{UTC} from the date string. Although this
14771 produces the same result for @samp{%s} and many other format sequences,
14772 with a time zone offset different from zero, it would give a different
14773 result for zone-dependent formats like @samp{%z}.
14776 date -u --date=2000-01-01 +%s
14780 To convert such an unwieldy number of seconds back to
14781 a more readable form, use a command like this:
14784 # local time zone used
14785 date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
14786 1999-12-31 19:00:00 -0500
14789 Or if you do not mind depending on the @samp{@@} feature present since
14790 coreutils 5.3.0, you could shorten this to:
14793 date -d @@946684800 +"%F %T %z"
14794 1999-12-31 19:00:00 -0500
14797 Often it is better to output UTC-relative date and time:
14800 date -u -d '1970-01-01 946684800 seconds' +"%Y-%m-%d %T %z"
14801 2000-01-01 00:00:00 +0000
14807 @node arch invocation
14808 @section @command{arch}: Print machine hardware name
14811 @cindex print machine hardware name
14812 @cindex system information, printing
14814 @command{arch} prints the machine hardware name,
14815 and is equivalent to @samp{uname -m}.
14819 arch [@var{option}]
14822 The program accepts the @ref{Common options} only.
14827 @node nproc invocation
14828 @section @command{nproc}: Print the number of available processors
14831 @cindex Print the number of processors
14832 @cindex system information, printing
14834 Print the number of processing units available to the current process,
14835 which may be less than the number of online processors.
14836 If this information is not accessible, then print the number of
14837 processors installed. If the @env{OMP_NUM_THREADS} environment variable is
14838 set, then it will determine the returned value. The result is guaranteed to be
14839 greater than zero. Synopsis:
14842 nproc [@var{option}]
14845 The program accepts the following options. Also see @ref{Common options}.
14851 Print the number of installed processors on the system, which may
14852 be greater than the number online or available to the current process.
14853 The @env{OMP_NUM_THREADS} environment variable is not honored in this case.
14855 @item --ignore=@var{number}
14857 If possible, exclude this @var{number} of processing units.
14864 @node uname invocation
14865 @section @command{uname}: Print system information
14868 @cindex print system information
14869 @cindex system information, printing
14871 @command{uname} prints information about the machine and operating system
14872 it is run on. If no options are given, @command{uname} acts as if the
14873 @option{-s} option were given. Synopsis:
14876 uname [@var{option}]@dots{}
14879 If multiple options or @option{-a} are given, the selected information is
14880 printed in this order:
14883 @var{kernel-name} @var{nodename} @var{kernel-release} @var{kernel-version}
14884 @var{machine} @var{processor} @var{hardware-platform} @var{operating-system}
14887 The information may contain internal spaces, so such output cannot be
14888 parsed reliably. In the following example, @var{release} is
14889 @samp{2.2.18ss.e820-bda652a #4 SMP Tue Jun 5 11:24:08 PDT 2001}:
14893 @result{} Linux dum 2.2.18 #4 SMP Tue Jun 5 11:24:08 PDT 2001 i686@c
14894 unknown unknown GNU/Linux
14898 The program accepts the following options. Also see @ref{Common options}.
14906 Print all of the below information, except omit the processor type
14907 and the hardware platform name if they are unknown.
14910 @itemx --hardware-platform
14912 @opindex --hardware-platform
14913 @cindex implementation, hardware
14914 @cindex hardware platform
14915 @cindex platform, hardware
14916 Print the hardware platform name
14917 (sometimes called the hardware implementation).
14918 Print @samp{unknown} if the kernel does not make this information
14919 easily available, as is the case with Linux kernels.
14925 @cindex machine type
14926 @cindex hardware class
14927 @cindex hardware type
14928 Print the machine hardware name (sometimes called the hardware class
14934 @opindex --nodename
14937 @cindex network node name
14938 Print the network node hostname.
14943 @opindex --processor
14944 @cindex host processor type
14945 Print the processor type (sometimes called the instruction set
14946 architecture or ISA).
14947 Print @samp{unknown} if the kernel does not make this information
14948 easily available, as is the case with Linux kernels.
14951 @itemx --operating-system
14953 @opindex --operating-system
14954 @cindex operating system name
14955 Print the name of the operating system.
14958 @itemx --kernel-release
14960 @opindex --kernel-release
14961 @cindex kernel release
14962 @cindex release of kernel
14963 Print the kernel release.
14966 @itemx --kernel-name
14968 @opindex --kernel-name
14969 @cindex kernel name
14970 @cindex name of kernel
14971 Print the kernel name.
14972 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) calls this
14973 ``the implementation of the operating system'', because the
14974 @acronym{POSIX} specification itself has no notion of ``kernel''.
14975 The kernel name might be the same as the operating system name printed
14976 by the @option{-o} or @option{--operating-system} option, but it might
14977 differ. Some operating systems (e.g., FreeBSD, HP-UX) have the same
14978 name as their underlying kernels; others (e.g., GNU/Linux, Solaris)
14982 @itemx --kernel-version
14984 @opindex --kernel-version
14985 @cindex kernel version
14986 @cindex version of kernel
14987 Print the kernel version.
14994 @node hostname invocation
14995 @section @command{hostname}: Print or set system name
14998 @cindex setting the hostname
14999 @cindex printing the hostname
15000 @cindex system name, printing
15001 @cindex appropriate privileges
15003 With no arguments, @command{hostname} prints the name of the current host
15004 system. With one argument, it sets the current host name to the
15005 specified string. You must have appropriate privileges to set the host
15009 hostname [@var{name}]
15012 The only options are @option{--help} and @option{--version}. @xref{Common
15018 @node hostid invocation
15019 @section @command{hostid}: Print numeric host identifier
15022 @cindex printing the host identifier
15024 @command{hostid} prints the numeric identifier of the current host
15025 in hexadecimal. This command accepts no arguments.
15026 The only options are @option{--help} and @option{--version}.
15027 @xref{Common options}.
15029 For example, here's what it prints on one system I use:
15036 On that system, the 32-bit quantity happens to be closely
15037 related to the system's Internet address, but that isn't always
15042 @node uptime invocation
15043 @section @command{uptime}: Print system uptime and load
15046 @cindex printing the system uptime and load
15048 @command{uptime} prints the current time, the system's uptime, the
15049 number of logged-in users and the current load average.
15051 If an argument is specified, it is used as the file to be read
15052 to discover how many users are logged in. If no argument is
15053 specified, a system default is used (@command{uptime --help} indicates
15054 the default setting).
15056 The only options are @option{--help} and @option{--version}.
15057 @xref{Common options}.
15059 For example, here's what it prints right now on one system I use:
15063 14:07 up 3:35, 3 users, load average: 1.39, 1.15, 1.04
15066 The precise method of calculation of load average varies somewhat
15067 between systems. Some systems calculate it as the average number of
15068 runnable processes over the last 1, 5 and 15 minutes, but some systems
15069 also include processes in the uninterruptible sleep state (that is,
15070 those processes which are waiting for disk I/O). The Linux kernel
15071 includes uninterruptible processes.
15073 @node SELinux context
15074 @chapter SELinux context
15076 @cindex SELinux context
15077 @cindex SELinux, context
15078 @cindex commands for SELinux context
15080 This section describes commands for operations with SELinux
15084 * chcon invocation:: Change SELinux context of file
15085 * runcon invocation:: Run a command in specified SELinux context
15088 @node chcon invocation
15089 @section @command{chcon}: Change SELinux context of file
15092 @cindex changing security context
15093 @cindex change SELinux context
15095 @command{chcon} changes the SELinux security context of the selected files.
15099 chcon [@var{option}]@dots{} @var{context} @var{file}@dots{}
15100 chcon [@var{option}]@dots{} [-u @var{user}] [-r @var{role}] [-l @var{range}]@c
15101 [-t @var{type}] @var{file}@dots{}
15102 chcon [@var{option}]@dots{} --reference=@var{rfile} @var{file}@dots{}
15105 Change the SELinux security context of each @var{file} to @var{context}.
15106 With @option{--reference}, change the security context of each @var{file}
15107 to that of @var{rfile}.
15109 The program accepts the following options. Also see @ref{Common options}.
15114 @itemx --no-dereference
15116 @opindex --no-dereference
15117 @cindex no dereference
15118 Affect symbolic links instead of any referenced file.
15120 @item --reference=@var{rfile}
15121 @opindex --reference
15122 @cindex reference file
15123 Use @var{rfile}'s security context rather than specifying a @var{context} value.
15128 @opindex --recursive
15129 Operate on files and directories recursively.
15132 @xref{Traversing symlinks}.
15135 @xref{Traversing symlinks}.
15138 @xref{Traversing symlinks}.
15145 Output a diagnostic for every file processed.
15147 @item -u @var{user}
15148 @itemx --user=@var{user}
15151 Set user @var{user} in the target security context.
15153 @item -r @var{role}
15154 @itemx --role=@var{role}
15157 Set role @var{role} in the target security context.
15159 @item -t @var{type}
15160 @itemx --type=@var{type}
15163 Set type @var{type} in the target security context.
15165 @item -l @var{range}
15166 @itemx --range=@var{range}
15169 Set range @var{range} in the target security context.
15175 @node runcon invocation
15176 @section @command{runcon}: Run a command in specified SELinux context
15179 @cindex run with security context
15182 @command{runcon} runs file in specified SELinux security context.
15186 runcon @var{context} @var{command} [@var{args}]
15187 runcon [ -c ] [-u @var{user}] [-r @var{role}] [-t @var{type}]@c
15188 [-l @var{range}] @var{command} [@var{args}]
15191 Run @var{command} with completely-specified @var{context}, or with
15192 current or transitioned security context modified by one or more of @var{level},
15193 @var{role}, @var{type} and @var{user}.
15195 If none of @option{-c}, @option{-t}, @option{-u}, @option{-r}, or @option{-l}
15196 is specified, the first argument is used as the complete context.
15197 Any additional arguments after @var{command}
15198 are interpreted as arguments to the command.
15200 With neither @var{context} nor @var{command}, print the current
15203 The program accepts the following options. Also see @ref{Common options}.
15211 Compute process transition context before modifying.
15213 @item -u @var{user}
15214 @itemx --user=@var{user}
15217 Set user @var{user} in the target security context.
15219 @item -r @var{role}
15220 @itemx --role=@var{role}
15223 Set role @var{role} in the target security context.
15225 @item -t @var{type}
15226 @itemx --type=@var{type}
15229 Set type @var{type} in the target security context.
15231 @item -l @var{range}
15232 @itemx --range=@var{range}
15235 Set range @var{range} in the target security context.
15239 @cindex exit status of @command{runcon}
15243 126 if @var{command} is found but cannot be invoked
15244 127 if @command{runcon} itself fails or if @var{command} cannot be found
15245 the exit status of @var{command} otherwise
15248 @node Modified command invocation
15249 @chapter Modified command invocation
15251 @cindex modified command invocation
15252 @cindex invocation of commands, modified
15253 @cindex commands for invoking other commands
15255 This section describes commands that run other commands in some context
15256 different than the current one: a modified environment, as a different
15260 * chroot invocation:: Modify the root directory.
15261 * env invocation:: Modify environment variables.
15262 * nice invocation:: Modify niceness.
15263 * nohup invocation:: Immunize to hangups.
15264 * stdbuf invocation:: Modify buffering of standard streams.
15265 * su invocation:: Modify user and group ID.
15266 * timeout invocation:: Run with time limit.
15270 @node chroot invocation
15271 @section @command{chroot}: Run a command with a different root directory
15274 @cindex running a program in a specified root directory
15275 @cindex root directory, running a program in a specified
15277 @command{chroot} runs a command with a specified root directory.
15278 On many systems, only the super-user can do this.@footnote{However,
15279 some systems (e.g., FreeBSD) can be configured to allow certain regular
15280 users to use the @code{chroot} system call, and hence to run this program.
15281 Also, on Cygwin, anyone can run the @command{chroot} command, because the
15282 underlying function is non-privileged due to lack of support in MS-Windows.}
15286 chroot @var{option} @var{newroot} [@var{command} [@var{args}]@dots{}]
15287 chroot @var{option}
15290 Ordinarily, file names are looked up starting at the root of the
15291 directory structure, i.e., @file{/}. @command{chroot} changes the root to
15292 the directory @var{newroot} (which must exist) and then runs
15293 @var{command} with optional @var{args}. If @var{command} is not
15294 specified, the default is the value of the @env{SHELL} environment
15295 variable or @command{/bin/sh} if not set, invoked with the @option{-i} option.
15296 @var{command} must not be a special built-in utility
15297 (@pxref{Special built-in utilities}).
15299 The program accepts the following options. Also see @ref{Common options}.
15300 Options must precede operands.
15304 @itemx --userspec=@var{user}[:@var{group}]
15305 @opindex --userspec
15306 By default, @var{command} is run with the same credentials
15307 as the invoking process.
15308 Use this option to run it as a different @var{user} and/or with a
15309 different primary @var{group}.
15311 @itemx --groups=@var{groups}
15313 Use this option to specify the supplementary @var{groups} to be
15314 used by the new process.
15315 The items in the list (names or numeric IDs) must be separated by commas.
15319 Here are a few tips to help avoid common problems in using chroot.
15320 To start with a simple example, make @var{command} refer to a statically
15321 linked binary. If you were to use a dynamically linked executable, then
15322 you'd have to arrange to have the shared libraries in the right place under
15323 your new root directory.
15325 For example, if you create a statically linked @command{ls} executable,
15326 and put it in @file{/tmp/empty}, you can run this command as root:
15329 $ chroot /tmp/empty /ls -Rl /
15332 Then you'll see output like this:
15337 -rwxr-xr-x 1 0 0 1041745 Aug 16 11:17 ls
15340 If you want to use a dynamically linked executable, say @command{bash},
15341 then first run @samp{ldd bash} to see what shared objects it needs.
15342 Then, in addition to copying the actual binary, also copy the listed
15343 files to the required positions under your intended new root directory.
15344 Finally, if the executable requires any other files (e.g., data, state,
15345 device files), copy them into place, too.
15347 @cindex exit status of @command{chroot}
15351 125 if @command{chroot} itself fails
15352 126 if @var{command} is found but cannot be invoked
15353 127 if @var{command} cannot be found
15354 the exit status of @var{command} otherwise
15358 @node env invocation
15359 @section @command{env}: Run a command in a modified environment
15362 @cindex environment, running a program in a modified
15363 @cindex modified environment, running a program in a
15364 @cindex running a program in a modified environment
15366 @command{env} runs a command with a modified environment. Synopses:
15369 env [@var{option}]@dots{} [@var{name}=@var{value}]@dots{} @c
15370 [@var{command} [@var{args}]@dots{}]
15374 Operands of the form @samp{@var{variable}=@var{value}} set
15375 the environment variable @var{variable} to value @var{value}.
15376 @var{value} may be empty (@samp{@var{variable}=}). Setting a variable
15377 to an empty value is different from unsetting it.
15378 These operands are evaluated left-to-right, so if two operands
15379 mention the same variable the earlier is ignored.
15381 Environment variable names can be empty, and can contain any
15382 characters other than @samp{=} and @acronym{ASCII} @sc{nul}.
15383 However, it is wise to limit yourself to names that
15384 consist solely of underscores, digits, and @acronym{ASCII} letters,
15385 and that begin with a non-digit, as applications like the shell do not
15386 work well with other names.
15389 The first operand that does not contain the character @samp{=}
15390 specifies the program to invoke; it is
15391 searched for according to the @env{PATH} environment variable. Any
15392 remaining arguments are passed as arguments to that program.
15393 The program should not be a special built-in utility
15394 (@pxref{Special built-in utilities}).
15396 Modifications to @env{PATH} take effect prior to searching for
15397 @var{command}. Use caution when reducing @env{PATH}; behavior is
15398 not portable when @env{PATH} is undefined or omits key directories
15399 such as @file{/bin}.
15401 In the rare case that a utility contains a @samp{=} in the name, the
15402 only way to disambiguate it from a variable assignment is to use an
15403 intermediate command for @var{command}, and pass the problematic
15404 program name via @var{args}. For example, if @file{./prog=} is an
15405 executable in the current @env{PATH}:
15408 env prog= true # runs 'true', with prog= in environment
15409 env ./prog= true # runs 'true', with ./prog= in environment
15410 env -- prog= true # runs 'true', with prog= in environment
15411 env sh -c '\prog= true' # runs 'prog=' with argument 'true'
15412 env sh -c 'exec "$@@"' sh prog= true # also runs 'prog='
15415 @cindex environment, printing
15417 If no command name is specified following the environment
15418 specifications, the resulting environment is printed. This is like
15419 specifying the @command{printenv} program.
15421 For some examples, suppose the environment passed to @command{env}
15422 contains @samp{LOGNAME=rms}, @samp{EDITOR=emacs}, and
15423 @samp{PATH=.:/gnubin:/hacks}:
15428 Output the current environment.
15430 $ env | LC_ALL=C sort
15433 PATH=.:/gnubin:/hacks
15437 Run @command{foo} with a reduced environment, preserving only the
15438 original @env{PATH} to avoid problems in locating @command{foo}.
15440 env - PATH="$PATH" foo
15444 Run @command{foo} with the environment containing @samp{LOGNAME=rms},
15445 @samp{EDITOR=emacs}, and @samp{PATH=.:/gnubin:/hacks}, and guarantees
15446 that @command{foo} was found in the file system rather than as a shell
15453 Run @command{nemacs} with the environment containing @samp{LOGNAME=foo},
15454 @samp{EDITOR=emacs}, @samp{PATH=.:/gnubin:/hacks}, and
15455 @samp{DISPLAY=gnu:0}.
15457 env DISPLAY=gnu:0 LOGNAME=foo nemacs
15461 Attempt to run the program @command{/energy/--} (as that is the only
15462 possible path search result); if the command exists, the environment
15463 will contain @samp{LOGNAME=rms} and @samp{PATH=/energy}, and the
15464 arguments will be @samp{e=mc2}, @samp{bar}, and @samp{baz}.
15466 env -u EDITOR PATH=/energy -- e=mc2 bar baz
15472 The program accepts the following options. Also see @ref{Common options}.
15473 Options must precede operands.
15479 @item -u @var{name}
15480 @itemx --unset=@var{name}
15483 Remove variable @var{name} from the environment, if it was in the
15488 @itemx --ignore-environment
15491 @opindex --ignore-environment
15492 Start with an empty environment, ignoring the inherited environment.
15496 @cindex exit status of @command{env}
15500 0 if no @var{command} is specified and the environment is output
15501 125 if @command{env} itself fails
15502 126 if @var{command} is found but cannot be invoked
15503 127 if @var{command} cannot be found
15504 the exit status of @var{command} otherwise
15508 @node nice invocation
15509 @section @command{nice}: Run a command with modified niceness
15513 @cindex scheduling, affecting
15514 @cindex appropriate privileges
15516 @command{nice} prints or modifies a process's @dfn{niceness},
15517 a parameter that affects whether the process is scheduled favorably.
15521 nice [@var{option}]@dots{} [@var{command} [@var{arg}]@dots{}]
15524 If no arguments are given, @command{nice} prints the current niceness.
15525 Otherwise, @command{nice} runs the given @var{command} with its
15526 niceness adjusted. By default, its niceness is incremented by 10.
15528 Niceness values range at least from @minus{}20 (process has high priority
15529 and gets more resources, thus slowing down other processes) through 19
15530 (process has lower priority and runs slowly itself, but has less impact
15531 on the speed of other running processes). Some systems
15532 may have a wider range of nicenesses; conversely, other systems may
15533 enforce more restrictive limits. An attempt to set the niceness
15534 outside the supported range is treated as an attempt to use the
15535 minimum or maximum supported value.
15537 A niceness should not be confused with a scheduling priority, which
15538 lets applications determine the order in which threads are scheduled
15539 to run. Unlike a priority, a niceness is merely advice to the
15540 scheduler, which the scheduler is free to ignore. Also, as a point of
15541 terminology, @acronym{POSIX} defines the behavior of @command{nice} in
15542 terms of a @dfn{nice value}, which is the nonnegative difference
15543 between a niceness and the minimum niceness. Though @command{nice}
15544 conforms to @acronym{POSIX}, its documentation and diagnostics use the
15545 term ``niceness'' for compatibility with historical practice.
15547 @var{command} must not be a special built-in utility (@pxref{Special
15548 built-in utilities}).
15550 @mayConflictWithShellBuiltIn{nice}
15552 The program accepts the following option. Also see @ref{Common options}.
15553 Options must precede operands.
15556 @item -n @var{adjustment}
15557 @itemx --adjustment=@var{adjustment}
15559 @opindex --adjustment
15560 Add @var{adjustment} instead of 10 to the command's niceness. If
15561 @var{adjustment} is negative and you lack appropriate privileges,
15562 @command{nice} issues a warning but otherwise acts as if you specified
15565 For compatibility @command{nice} also supports an obsolete
15566 option syntax @option{-@var{adjustment}}. New scripts should use
15567 @option{-n @var{adjustment}} instead.
15571 @cindex exit status of @command{nice}
15575 0 if no @var{command} is specified and the niceness is output
15576 125 if @command{nice} itself fails
15577 126 if @var{command} is found but cannot be invoked
15578 127 if @var{command} cannot be found
15579 the exit status of @var{command} otherwise
15582 It is sometimes useful to run a non-interactive program with reduced niceness.
15585 $ nice factor 4611686018427387903
15588 Since @command{nice} prints the current niceness,
15589 you can invoke it through itself to demonstrate how it works.
15591 The default behavior is to increase the niceness by @samp{10}:
15602 The @var{adjustment} is relative to the current niceness. In the
15603 next example, the first @command{nice} invocation runs the second one
15604 with niceness 10, and it in turn runs the final one with a niceness
15608 $ nice nice -n 3 nice
15612 Specifying a niceness larger than the supported range
15613 is the same as specifying the maximum supported value:
15616 $ nice -n 10000000000 nice
15620 Only a privileged user may run a process with lower niceness:
15624 nice: cannot set niceness: Permission denied
15626 $ sudo nice -n -1 nice
15631 @node nohup invocation
15632 @section @command{nohup}: Run a command immune to hangups
15635 @cindex hangups, immunity to
15636 @cindex immunity to hangups
15637 @cindex logging out and continuing to run
15640 @command{nohup} runs the given @var{command} with hangup signals ignored,
15641 so that the command can continue running in the background after you log
15645 nohup @var{command} [@var{arg}]@dots{}
15648 If standard input is a terminal, it is redirected from
15649 @file{/dev/null} so that terminal sessions do not mistakenly consider
15650 the terminal to be used by the command. This is a @acronym{GNU}
15651 extension; programs intended to be portable to non-@acronym{GNU} hosts
15652 should use @samp{nohup @var{command} [@var{arg}]@dots{} </dev/null}
15656 If standard output is a terminal, the command's standard output is appended
15657 to the file @file{nohup.out}; if that cannot be written to, it is appended
15658 to the file @file{$HOME/nohup.out}; and if that cannot be written to, the
15659 command is not run.
15660 Any @file{nohup.out} or @file{$HOME/nohup.out} file created by
15661 @command{nohup} is made readable and writable only to the user,
15662 regardless of the current umask settings.
15664 If standard error is a terminal, it is normally redirected to the same file
15665 descriptor as the (possibly-redirected) standard output.
15666 However, if standard output is closed, standard error terminal output
15667 is instead appended to the file @file{nohup.out} or
15668 @file{$HOME/nohup.out} as above.
15670 To capture the command's output to a file other than @file{nohup.out}
15671 you can redirect it. For example, to capture the output of
15675 nohup make > make.log
15678 @command{nohup} does not automatically put the command it runs in the
15679 background; you must do that explicitly, by ending the command line
15680 with an @samp{&}. Also, @command{nohup} does not alter the
15681 niceness of @var{command}; use @command{nice} for that,
15682 e.g., @samp{nohup nice @var{command}}.
15684 @var{command} must not be a special built-in utility (@pxref{Special
15685 built-in utilities}).
15687 The only options are @option{--help} and @option{--version}. @xref{Common
15688 options}. Options must precede operands.
15690 @cindex exit status of @command{nohup}
15694 125 if @command{nohup} itself fails, and @env{POSIXLY_CORRECT} is not set
15695 126 if @var{command} is found but cannot be invoked
15696 127 if @var{command} cannot be found
15697 the exit status of @var{command} otherwise
15700 If @env{POSIXLY_CORRECT} is set, internal failures give status 127
15704 @node stdbuf invocation
15705 @section @command{stdbuf}: Run a command with modified I/O stream buffering
15708 @cindex standard streams, buffering
15709 @cindex line buffered
15711 @command{stdbuf} allows one to modify the buffering operations of the
15712 three standard I/O streams associated with a program. Synopsis:
15715 stdbuf @var{option}@dots{} @var{command}
15718 @var{command} must start with the name of a program that
15721 uses the ISO C @code{FILE} streams for input/output (note the
15722 programs @command{dd} and @command{cat} don't do that),
15725 does not adjust the buffering of its standard streams (note the
15726 program @command{tee} is not in this category).
15729 Any additional @var{arg}s are passed as additional arguments to the
15732 The program accepts the following options. Also see @ref{Common options}.
15736 @item -i @var{mode}
15737 @itemx --input=@var{mode}
15740 Adjust the standard input stream buffering.
15742 @item -o @var{mode}
15743 @itemx --output=@var{mode}
15746 Adjust the standard output stream buffering.
15748 @item -e @var{mode}
15749 @itemx --error=@var{mode}
15752 Adjust the standard error stream buffering.
15756 The @var{mode} can be specified as follows:
15761 Set the stream to line buffered mode.
15762 In this mode data is coalesced until a newline is output or
15763 input is read from any stream attached to a terminal device.
15764 This option is invalid with standard input.
15767 Disable buffering of the selected stream.
15768 In this mode, data is output immediately and only the
15769 amount of data requested is read from input.
15770 Note the difference in function for input and output.
15771 Disabling buffering for input will not influence the responsiveness
15772 or blocking behavior of the stream input functions.
15773 For example @code{fread} will still block until @code{EOF} or error,
15774 even if the underlying @code{read} returns less data than requested.
15777 Specify the size of the buffer to use in fully buffered mode.
15778 @multiplierSuffixesNoBlocks{size}
15782 @cindex exit status of @command{stdbuf}
15786 125 if @command{stdbuf} itself fails
15787 126 if @var{command} is found but cannot be invoked
15788 127 if @var{command} cannot be found
15789 the exit status of @var{command} otherwise
15793 @node su invocation
15794 @section @command{su}: Run a command with substitute user and group ID
15797 @cindex substitute user and group IDs
15798 @cindex user ID, switching
15799 @cindex super-user, becoming
15800 @cindex root, becoming
15802 @command{su} allows one user to temporarily become another user. It runs a
15803 command (often an interactive shell) with the real and effective user
15804 ID, group ID, and supplemental groups of a given @var{user}. Synopsis:
15807 su [@var{option}]@dots{} [@var{user} [@var{arg}]@dots{}]
15810 @cindex passwd entry, and @command{su} shell
15812 @flindex /etc/passwd
15813 If no @var{user} is given, the default is @code{root}, the super-user.
15814 The shell to use is taken from @var{user}'s @code{passwd} entry, or
15815 @file{/bin/sh} if none is specified there. If @var{user} has a
15816 password, @command{su} prompts for the password unless run by a user with
15817 effective user ID of zero (the super-user).
15823 @cindex login shell
15824 By default, @command{su} does not change the current directory.
15825 It sets the environment variables @env{HOME} and @env{SHELL}
15826 from the password entry for @var{user}, and if @var{user} is not
15827 the super-user, sets @env{USER} and @env{LOGNAME} to @var{user}.
15828 By default, the shell is not a login shell.
15830 Any additional @var{arg}s are passed as additional arguments to the
15833 @cindex @option{-su}
15834 GNU @command{su} does not treat @file{/bin/sh} or any other shells specially
15835 (e.g., by setting @code{argv[0]} to @option{-su}, passing @option{-c} only
15836 to certain shells, etc.).
15839 @command{su} can optionally be compiled to use @code{syslog} to report
15840 failed, and optionally successful, @command{su} attempts. (If the system
15841 supports @code{syslog}.) However, GNU @command{su} does not check if the
15842 user is a member of the @code{wheel} group; see below.
15844 The program accepts the following options. Also see @ref{Common options}.
15847 @item -c @var{command}
15848 @itemx --command=@var{command}
15851 Pass @var{command}, a single command line to run, to the shell with
15852 a @option{-c} option instead of starting an interactive shell.
15859 @cindex file name pattern expansion, disabled
15860 @cindex globbing, disabled
15861 Pass the @option{-f} option to the shell. This probably only makes sense
15862 if the shell run is @command{csh} or @command{tcsh}, for which the @option{-f}
15863 option prevents reading the startup file (@file{.cshrc}). With
15864 Bourne-like shells, the @option{-f} option disables file name pattern
15865 expansion (globbing), which is not likely to be useful.
15873 @c other variables already indexed above
15876 @cindex login shell, creating
15877 Make the shell a login shell. This means the following. Unset all
15878 environment variables except @env{TERM}, @env{HOME}, and @env{SHELL}
15879 (which are set as described above), and @env{USER} and @env{LOGNAME}
15880 (which are set, even for the super-user, as described above), and set
15881 @env{PATH} to a compiled-in default value. Change to @var{user}'s home
15882 directory. Prepend @samp{-} to the shell's name, intended to make it
15883 read its login startup file(s).
15887 @itemx --preserve-environment
15890 @opindex --preserve-environment
15891 @cindex environment, preserving
15892 @flindex /etc/shells
15893 @cindex restricted shell
15894 Do not change the environment variables @env{HOME}, @env{USER},
15895 @env{LOGNAME}, or @env{SHELL}. Run the shell given in the environment
15896 variable @env{SHELL} instead of the shell from @var{user}'s passwd
15897 entry, unless the user running @command{su} is not the super-user and
15898 @var{user}'s shell is restricted. A @dfn{restricted shell} is one that
15899 is not listed in the file @file{/etc/shells}, or in a compiled-in list
15900 if that file does not exist. Parts of what this option does can be
15901 overridden by @option{--login} and @option{--shell}.
15903 @item -s @var{shell}
15904 @itemx --shell=@var{shell}
15907 Run @var{shell} instead of the shell from @var{user}'s passwd entry,
15908 unless the user running @command{su} is not the super-user and @var{user}'s
15909 shell is restricted (see @option{-m} just above).
15913 @cindex exit status of @command{su}
15917 125 if @command{su} itself fails
15918 126 if subshell is found but cannot be invoked
15919 127 if subshell cannot be found
15920 the exit status of the subshell otherwise
15923 @cindex wheel group, not supported
15924 @cindex group wheel, not supported
15926 @subsection Why GNU @command{su} does not support the @samp{wheel} group
15928 (This section is by Richard Stallman.)
15932 Sometimes a few of the users try to hold total power over all the
15933 rest. For example, in 1984, a few users at the MIT AI lab decided to
15934 seize power by changing the operator password on the Twenex system and
15935 keeping it secret from everyone else. (I was able to thwart this coup
15936 and give power back to the users by patching the kernel, but I
15937 wouldn't know how to do that in Unix.)
15939 However, occasionally the rulers do tell someone. Under the usual
15940 @command{su} mechanism, once someone learns the root password who
15941 sympathizes with the ordinary users, he or she can tell the rest. The
15942 ``wheel group'' feature would make this impossible, and thus cement the
15943 power of the rulers.
15945 I'm on the side of the masses, not that of the rulers. If you are
15946 used to supporting the bosses and sysadmins in whatever they do, you
15947 might find this idea strange at first.
15950 @node timeout invocation
15951 @section @command{timeout}: Run a command with a time limit
15955 @cindex run commands with bounded time
15957 @command{timeout} runs the given @var{command} and kills it if it is
15958 still running after the specified time interval. Synopsis:
15961 timeout [@var{option}] @var{duration} @var{command} [@var{arg}]@dots{}
15964 @var{command} must not be a special built-in utility (@pxref{Special
15965 built-in utilities}).
15967 The program accepts the following options. Also see @ref{Common options}.
15968 Options must precede operands.
15971 @itemx --foreground
15972 @opindex --foreground
15973 Don't create a separate background program group, so that
15974 the managed @var{command} can use the foreground TTY normally.
15975 This is needed to support timing out commands not started
15976 directly from an interactive shell, in two situations.
15979 @var{command} is interactive and needs to read from the terminal for example
15981 the user wants to support sending signals directly to @var{command}
15982 from the terminal (like Ctrl-C for example)
15985 Note in this mode of operation, any children of @var{command}
15986 will not be timed out.
15988 @item -k @var{duration}
15989 @itemx --kill-after=@var{duration}
15991 @opindex --kill-after
15992 Ensure the monitored @var{command} is killed by also sending a @samp{KILL}
15993 signal, after the specified @var{duration}. Without this option, if the
15994 selected signal proves not to be fatal, @command{timeout} does not kill
15997 @item -s @var{signal}
15998 @itemx --signal=@var{signal}
16001 Send this @var{signal} to @var{command} on timeout, rather than the
16002 default @samp{TERM} signal. @var{signal} may be a name like @samp{HUP}
16003 or a number. Also see @xref{Signal specifications}.
16007 @var{duration} is a floating point number followed by an optional unit:
16009 @samp{s} for seconds (the default)
16010 @samp{m} for minutes
16014 A duration of 0 disables the associated timeout.
16015 Note that the actual timeout duration is dependent on system conditions,
16016 which should be especially considered when specifying sub-second timeouts.
16018 @cindex exit status of @command{timeout}
16022 124 if @var{command} times out
16023 125 if @command{timeout} itself fails
16024 126 if @var{command} is found but cannot be invoked
16025 127 if @var{command} cannot be found
16026 137 if @var{command} is sent the KILL(9) signal (128+9)
16027 the exit status of @var{command} otherwise
16031 @node Process control
16032 @chapter Process control
16034 @cindex processes, commands for controlling
16035 @cindex commands for controlling processes
16038 * kill invocation:: Sending a signal to processes.
16042 @node kill invocation
16043 @section @command{kill}: Send a signal to processes
16046 @cindex send a signal to processes
16048 The @command{kill} command sends a signal to processes, causing them
16049 to terminate or otherwise act upon receiving the signal in some way.
16050 Alternatively, it lists information about signals. Synopses:
16053 kill [-s @var{signal} | --signal @var{signal} | -@var{signal}] @var{pid}@dots{}
16054 kill [-l | --list | -t | --table] [@var{signal}]@dots{}
16057 @mayConflictWithShellBuiltIn{kill}
16059 The first form of the @command{kill} command sends a signal to all
16060 @var{pid} arguments. The default signal to send if none is specified
16061 is @samp{TERM}. The special signal number @samp{0} does not denote a
16062 valid signal, but can be used to test whether the @var{pid} arguments
16063 specify processes to which a signal could be sent.
16065 If @var{pid} is positive, the signal is sent to the process with the
16066 process ID @var{pid}. If @var{pid} is zero, the signal is sent to all
16067 processes in the process group of the current process. If @var{pid}
16068 is @minus{}1, the signal is sent to all processes for which the user has
16069 permission to send a signal. If @var{pid} is less than @minus{}1, the signal
16070 is sent to all processes in the process group that equals the absolute
16071 value of @var{pid}.
16073 If @var{pid} is not positive, a system-dependent set of system
16074 processes is excluded from the list of processes to which the signal
16077 If a negative @var{pid} argument is desired as the first one, it
16078 should be preceded by @option{--}. However, as a common extension to
16079 @acronym{POSIX}, @option{--} is not required with @samp{kill
16080 -@var{signal} -@var{pid}}. The following commands are equivalent:
16089 The first form of the @command{kill} command succeeds if every @var{pid}
16090 argument specifies at least one process that the signal was sent to.
16092 The second form of the @command{kill} command lists signal information.
16093 Either the @option{-l} or @option{--list} option, or the @option{-t}
16094 or @option{--table} option must be specified. Without any
16095 @var{signal} argument, all supported signals are listed. The output
16096 of @option{-l} or @option{--list} is a list of the signal names, one
16097 per line; if @var{signal} is already a name, the signal number is
16098 printed instead. The output of @option{-t} or @option{--table} is a
16099 table of signal numbers, names, and descriptions. This form of the
16100 @command{kill} command succeeds if all @var{signal} arguments are valid
16101 and if there is no output error.
16103 The @command{kill} command also supports the @option{--help} and
16104 @option{--version} options. @xref{Common options}.
16106 A @var{signal} may be a signal name like @samp{HUP}, or a signal
16107 number like @samp{1}, or an exit status of a process terminated by the
16108 signal. A signal name can be given in canonical form or prefixed by
16109 @samp{SIG}. The case of the letters is ignored, except for the
16110 @option{-@var{signal}} option which must use upper case to avoid
16111 ambiguity with lower case option letters. For a list of supported
16112 signal names and numbers see @xref{Signal specifications}.
16117 @cindex delaying commands
16118 @cindex commands for delaying
16120 @c Perhaps @command{wait} or other commands should be described here also?
16123 * sleep invocation:: Delay for a specified time.
16127 @node sleep invocation
16128 @section @command{sleep}: Delay for a specified time
16131 @cindex delay for a specified time
16133 @command{sleep} pauses for an amount of time specified by the sum of
16134 the values of the command line arguments.
16138 sleep @var{number}[smhd]@dots{}
16142 Each argument is a number followed by an optional unit; the default
16143 is seconds. The units are:
16156 Historical implementations of @command{sleep} have required that
16157 @var{number} be an integer, and only accepted a single argument
16158 without a suffix. However, GNU @command{sleep} accepts
16159 arbitrary floating point numbers. @xref{Floating point}.
16161 The only options are @option{--help} and @option{--version}. @xref{Common
16164 @c sleep is a shell built-in at least with Solaris 11's /bin/sh
16165 @mayConflictWithShellBuiltIn{sleep}
16170 @node Numeric operations
16171 @chapter Numeric operations
16173 @cindex numeric operations
16174 These programs do numerically-related operations.
16177 * factor invocation:: Show factors of numbers.
16178 * seq invocation:: Print sequences of numbers.
16182 @node factor invocation
16183 @section @command{factor}: Print prime factors
16186 @cindex prime factors
16188 @command{factor} prints prime factors. Synopses:
16191 factor [@var{number}]@dots{}
16192 factor @var{option}
16195 If no @var{number} is specified on the command line, @command{factor} reads
16196 numbers from standard input, delimited by newlines, tabs, or spaces.
16198 The @command{factor} command supports only a small number of options:
16202 Print a short help on standard output, then exit without further
16206 Print the program version on standard output, then exit without further
16210 Factoring the product of the eighth and ninth Mersenne primes
16211 takes about 30 milliseconds of CPU time on a 2.2 GHz Athlon.
16214 M8=$(echo 2^31-1|bc)
16215 M9=$(echo 2^61-1|bc)
16216 n=$(echo "$M8 * $M9" | bc)
16217 /usr/bin/time -f %U factor $n
16218 4951760154835678088235319297: 2147483647 2305843009213693951
16222 Similarly, factoring the eighth Fermat number @math{2^{256}+1} takes
16223 about 20 seconds on the same machine.
16225 Factoring large numbers is, in general, hard. The Pollard Rho
16226 algorithm used by @command{factor} is particularly effective for
16227 numbers with relatively small factors. If you wish to factor large
16228 numbers which do not have small factors (for example, numbers which
16229 are the product of two large primes), other methods are far better.
16231 If @command{factor} is built without using GNU MP, only
16232 single-precision arithmetic is available, and so large numbers
16233 (typically @math{2^{64}} and above) will not be supported. The single-precision
16234 code uses an algorithm which is designed for factoring smaller
16240 @node seq invocation
16241 @section @command{seq}: Print numeric sequences
16244 @cindex numeric sequences
16245 @cindex sequence of numbers
16247 @command{seq} prints a sequence of numbers to standard output. Synopses:
16250 seq [@var{option}]@dots{} @var{last}
16251 seq [@var{option}]@dots{} @var{first} @var{last}
16252 seq [@var{option}]@dots{} @var{first} @var{increment} @var{last}
16255 @command{seq} prints the numbers from @var{first} to @var{last} by
16256 @var{increment}. By default, each number is printed on a separate line.
16257 When @var{increment} is not specified, it defaults to @samp{1},
16258 even when @var{first} is larger than @var{last}.
16259 @var{first} also defaults to @samp{1}. So @code{seq 1} prints
16260 @samp{1}, but @code{seq 0} and @code{seq 10 5} produce no output.
16261 Floating-point numbers may be specified. @xref{Floating point}.
16263 The program accepts the following options. Also see @ref{Common options}.
16264 Options must precede operands.
16267 @item -f @var{format}
16268 @itemx --format=@var{format}
16269 @opindex -f @var{format}
16270 @opindex --format=@var{format}
16271 @cindex formatting of numbers in @command{seq}
16272 Print all numbers using @var{format}.
16273 @var{format} must contain exactly one of the @samp{printf}-style
16274 floating point conversion specifications @samp{%a}, @samp{%e},
16275 @samp{%f}, @samp{%g}, @samp{%A}, @samp{%E}, @samp{%F}, @samp{%G}.
16276 The @samp{%} may be followed by zero or more flags taken from the set
16277 @samp{-+#0 '}, then an optional width containing one or more digits,
16278 then an optional precision consisting of a @samp{.} followed by zero
16279 or more digits. @var{format} may also contain any number of @samp{%%}
16280 conversion specifications. All conversion specifications have the
16281 same meaning as with @samp{printf}.
16283 The default format is derived from @var{first}, @var{step}, and
16284 @var{last}. If these all use a fixed point decimal representation,
16285 the default format is @samp{%.@var{p}f}, where @var{p} is the minimum
16286 precision that can represent the output numbers exactly. Otherwise,
16287 the default format is @samp{%g}.
16289 @item -s @var{string}
16290 @itemx --separator=@var{string}
16291 @cindex separator for numbers in @command{seq}
16292 Separate numbers with @var{string}; default is a newline.
16293 The output always terminates with a newline.
16296 @itemx --equal-width
16297 Print all numbers with the same width, by padding with leading zeros.
16298 @var{first}, @var{step}, and @var{last} should all use a fixed point
16299 decimal representation.
16300 (To have other kinds of padding, use @option{--format}).
16304 You can get finer-grained control over output with @option{-f}:
16307 $ seq -f '(%9.2E)' -9e5 1.1e6 1.3e6
16313 If you want hexadecimal integer output, you can use @command{printf}
16314 to perform the conversion:
16317 $ printf '%x\n' $(seq 1048575 1024 1050623)
16323 For very long lists of numbers, use xargs to avoid
16324 system limitations on the length of an argument list:
16327 $ seq 1000000 | xargs printf '%x\n' | tail -n 3
16333 To generate octal output, use the printf @code{%o} format instead
16336 On most systems, seq can produce whole-number output for values up to
16337 at least @math{2^{53}}. Larger integers are approximated. The details
16338 differ depending on your floating-point implementation.
16339 @xref{Floating point}. A common
16340 case is that @command{seq} works with integers through @math{2^{64}},
16341 and larger integers may not be numerically correct:
16344 $ seq 18446744073709551616 1 18446744073709551618
16345 18446744073709551616
16346 18446744073709551616
16347 18446744073709551618
16350 Be careful when using @command{seq} with outlandish values: otherwise
16351 you may see surprising results, as @command{seq} uses floating point
16352 internally. For example, on the x86 platform, where the internal
16353 representation uses a 64-bit fraction, the command:
16356 seq 1 0.0000000000000000001 1.0000000000000000009
16359 outputs 1.0000000000000000007 twice and skips 1.0000000000000000008.
16364 @node File permissions
16365 @chapter File permissions
16368 @include parse-datetime.texi
16372 @node Opening the software toolbox
16373 @chapter Opening the Software Toolbox
16375 An earlier version of this chapter appeared in
16376 @uref{http://www.linuxjournal.com/article.php?sid=2762, the
16377 @cite{What's GNU?} column of the June 1994 @cite{Linux Journal}}.
16378 It was written by Arnold Robbins.
16381 * Toolbox introduction:: Toolbox introduction
16382 * I/O redirection:: I/O redirection
16383 * The who command:: The @command{who} command
16384 * The cut command:: The @command{cut} command
16385 * The sort command:: The @command{sort} command
16386 * The uniq command:: The @command{uniq} command
16387 * Putting the tools together:: Putting the tools together
16391 @node Toolbox introduction
16392 @unnumberedsec Toolbox Introduction
16394 This month's column is only peripherally related to the GNU Project, in
16395 that it describes a number of the GNU tools on your GNU/Linux system
16397 might be used. What it's really about is the ``Software Tools'' philosophy
16398 of program development and usage.
16400 The software tools philosophy was an important and integral concept
16401 in the initial design and development of Unix (of which Linux and GNU are
16402 essentially clones). Unfortunately, in the modern day press of
16403 Internetworking and flashy GUIs, it seems to have fallen by the
16404 wayside. This is a shame, since it provides a powerful mental model
16405 for solving many kinds of problems.
16407 Many people carry a Swiss Army knife around in their pants pockets (or
16408 purse). A Swiss Army knife is a handy tool to have: it has several knife
16409 blades, a screwdriver, tweezers, toothpick, nail file, corkscrew, and perhaps
16410 a number of other things on it. For the everyday, small miscellaneous jobs
16411 where you need a simple, general purpose tool, it's just the thing.
16413 On the other hand, an experienced carpenter doesn't build a house using
16414 a Swiss Army knife. Instead, he has a toolbox chock full of specialized
16415 tools---a saw, a hammer, a screwdriver, a plane, and so on. And he knows
16416 exactly when and where to use each tool; you won't catch him hammering nails
16417 with the handle of his screwdriver.
16419 The Unix developers at Bell Labs were all professional programmers and trained
16420 computer scientists. They had found that while a one-size-fits-all program
16421 might appeal to a user because there's only one program to use, in practice
16426 difficult to write,
16429 difficult to maintain and
16433 difficult to extend to meet new situations.
16436 Instead, they felt that programs should be specialized tools. In short, each
16437 program ``should do one thing well.'' No more and no less. Such programs are
16438 simpler to design, write, and get right---they only do one thing.
16440 Furthermore, they found that with the right machinery for hooking programs
16441 together, that the whole was greater than the sum of the parts. By combining
16442 several special purpose programs, you could accomplish a specific task
16443 that none of the programs was designed for, and accomplish it much more
16444 quickly and easily than if you had to write a special purpose program.
16445 We will see some (classic) examples of this further on in the column.
16446 (An important additional point was that, if necessary, take a detour
16447 and build any software tools you may need first, if you don't already
16448 have something appropriate in the toolbox.)
16450 @node I/O redirection
16451 @unnumberedsec I/O Redirection
16453 Hopefully, you are familiar with the basics of I/O redirection in the
16454 shell, in particular the concepts of ``standard input,'' ``standard output,''
16455 and ``standard error''. Briefly, ``standard input'' is a data source, where
16456 data comes from. A program should not need to either know or care if the
16457 data source is a disk file, a keyboard, a magnetic tape, or even a punched
16458 card reader. Similarly, ``standard output'' is a data sink, where data goes
16459 to. The program should neither know nor care where this might be.
16460 Programs that only read their standard input, do something to the data,
16461 and then send it on, are called @dfn{filters}, by analogy to filters in a
16464 With the Unix shell, it's very easy to set up data pipelines:
16467 program_to_create_data | filter1 | ... | filterN > final.pretty.data
16470 We start out by creating the raw data; each filter applies some successive
16471 transformation to the data, until by the time it comes out of the pipeline,
16472 it is in the desired form.
16474 This is fine and good for standard input and standard output. Where does the
16475 standard error come in to play? Well, think about @command{filter1} in
16476 the pipeline above. What happens if it encounters an error in the data it
16477 sees? If it writes an error message to standard output, it will just
16478 disappear down the pipeline into @command{filter2}'s input, and the
16479 user will probably never see it. So programs need a place where they can send
16480 error messages so that the user will notice them. This is standard error,
16481 and it is usually connected to your console or window, even if you have
16482 redirected standard output of your program away from your screen.
16484 For filter programs to work together, the format of the data has to be
16485 agreed upon. The most straightforward and easiest format to use is simply
16486 lines of text. Unix data files are generally just streams of bytes, with
16487 lines delimited by the @acronym{ASCII} @sc{lf} (Line Feed) character,
16488 conventionally called a ``newline'' in the Unix literature. (This is
16489 @code{'\n'} if you're a C programmer.) This is the format used by all
16490 the traditional filtering programs. (Many earlier operating systems
16491 had elaborate facilities and special purpose programs for managing
16492 binary data. Unix has always shied away from such things, under the
16493 philosophy that it's easiest to simply be able to view and edit your
16494 data with a text editor.)
16496 OK, enough introduction. Let's take a look at some of the tools, and then
16497 we'll see how to hook them together in interesting ways. In the following
16498 discussion, we will only present those command line options that interest
16499 us. As you should always do, double check your system documentation
16500 for the full story.
16502 @node The who command
16503 @unnumberedsec The @command{who} Command
16505 The first program is the @command{who} command. By itself, it generates a
16506 list of the users who are currently logged in. Although I'm writing
16507 this on a single-user system, we'll pretend that several people are
16512 @print{} arnold console Jan 22 19:57
16513 @print{} miriam ttyp0 Jan 23 14:19(:0.0)
16514 @print{} bill ttyp1 Jan 21 09:32(:0.0)
16515 @print{} arnold ttyp2 Jan 23 20:48(:0.0)
16518 Here, the @samp{$} is the usual shell prompt, at which I typed @samp{who}.
16519 There are three people logged in, and I am logged in twice. On traditional
16520 Unix systems, user names are never more than eight characters long. This
16521 little bit of trivia will be useful later. The output of @command{who} is nice,
16522 but the data is not all that exciting.
16524 @node The cut command
16525 @unnumberedsec The @command{cut} Command
16527 The next program we'll look at is the @command{cut} command. This program
16528 cuts out columns or fields of input data. For example, we can tell it
16529 to print just the login name and full name from the @file{/etc/passwd}
16530 file. The @file{/etc/passwd} file has seven fields, separated by
16534 arnold:xyzzy:2076:10:Arnold D. Robbins:/home/arnold:/bin/bash
16537 To get the first and fifth fields, we would use @command{cut} like this:
16540 $ cut -d: -f1,5 /etc/passwd
16541 @print{} root:Operator
16543 @print{} arnold:Arnold D. Robbins
16544 @print{} miriam:Miriam A. Robbins
16548 With the @option{-c} option, @command{cut} will cut out specific characters
16549 (i.e., columns) in the input lines. This is useful for input data
16550 that has fixed width fields, and does not have a field separator. For
16551 example, list the Monday dates for the current month:
16553 @c Is using cal ok? Looked at gcal, but I don't like it.
16564 @node The sort command
16565 @unnumberedsec The @command{sort} Command
16567 Next we'll look at the @command{sort} command. This is one of the most
16568 powerful commands on a Unix-style system; one that you will often find
16569 yourself using when setting up fancy data plumbing.
16572 command reads and sorts each file named on the command line. It then
16573 merges the sorted data and writes it to standard output. It will read
16574 standard input if no files are given on the command line (thus
16575 making it into a filter). The sort is based on the character collating
16576 sequence or based on user-supplied ordering criteria.
16579 @node The uniq command
16580 @unnumberedsec The @command{uniq} Command
16582 Finally (at least for now), we'll look at the @command{uniq} program. When
16583 sorting data, you will often end up with duplicate lines, lines that
16584 are identical. Usually, all you need is one instance of each line.
16585 This is where @command{uniq} comes in. The @command{uniq} program reads its
16586 standard input. It prints only one
16587 copy of each repeated line. It does have several options. Later on,
16588 we'll use the @option{-c} option, which prints each unique line, preceded
16589 by a count of the number of times that line occurred in the input.
16592 @node Putting the tools together
16593 @unnumberedsec Putting the Tools Together
16595 Now, let's suppose this is a large ISP server system with dozens of users
16596 logged in. The management wants the system administrator to write a
16598 generate a sorted list of logged in users. Furthermore, even if a user
16599 is logged in multiple times, his or her name should only show up in the
16602 The administrator could sit down with the system documentation and write a C
16603 program that did this. It would take perhaps a couple of hundred lines
16604 of code and about two hours to write it, test it, and debug it.
16605 However, knowing the software toolbox, the administrator can instead start out
16606 by generating just a list of logged on users:
16616 Next, sort the list:
16619 $ who | cut -c1-8 | sort
16626 Finally, run the sorted list through @command{uniq}, to weed out duplicates:
16629 $ who | cut -c1-8 | sort | uniq
16635 The @command{sort} command actually has a @option{-u} option that does what
16636 @command{uniq} does. However, @command{uniq} has other uses for which one
16637 cannot substitute @samp{sort -u}.
16639 The administrator puts this pipeline into a shell script, and makes it
16641 all the users on the system (@samp{#} is the system administrator,
16642 or @code{root}, prompt):
16645 # cat > /usr/local/bin/listusers
16646 who | cut -c1-8 | sort | uniq
16648 # chmod +x /usr/local/bin/listusers
16651 There are four major points to note here. First, with just four
16652 programs, on one command line, the administrator was able to save about two
16653 hours worth of work. Furthermore, the shell pipeline is just about as
16654 efficient as the C program would be, and it is much more efficient in
16655 terms of programmer time. People time is much more expensive than
16656 computer time, and in our modern ``there's never enough time to do
16657 everything'' society, saving two hours of programmer time is no mean
16660 Second, it is also important to emphasize that with the
16661 @emph{combination} of the tools, it is possible to do a special
16662 purpose job never imagined by the authors of the individual programs.
16664 Third, it is also valuable to build up your pipeline in stages, as we did here.
16665 This allows you to view the data at each stage in the pipeline, which helps
16666 you acquire the confidence that you are indeed using these tools correctly.
16668 Finally, by bundling the pipeline in a shell script, other users can use
16669 your command, without having to remember the fancy plumbing you set up for
16670 them. In terms of how you run them, shell scripts and compiled programs are
16673 After the previous warm-up exercise, we'll look at two additional, more
16674 complicated pipelines. For them, we need to introduce two more tools.
16676 The first is the @command{tr} command, which stands for ``transliterate.''
16677 The @command{tr} command works on a character-by-character basis, changing
16678 characters. Normally it is used for things like mapping upper case to
16682 $ echo ThIs ExAmPlE HaS MIXED case! | tr '[:upper:]' '[:lower:]'
16683 @print{} this example has mixed case!
16686 There are several options of interest:
16690 work on the complement of the listed characters, i.e.,
16691 operations apply to characters not in the given set
16694 delete characters in the first set from the output
16697 squeeze repeated characters in the output into just one character.
16700 We will be using all three options in a moment.
16702 The other command we'll look at is @command{comm}. The @command{comm}
16703 command takes two sorted input files as input data, and prints out the
16704 files' lines in three columns. The output columns are the data lines
16705 unique to the first file, the data lines unique to the second file, and
16706 the data lines that are common to both. The @option{-1}, @option{-2}, and
16707 @option{-3} command line options @emph{omit} the respective columns. (This is
16708 non-intuitive and takes a little getting used to.) For example:
16730 The file name @file{-} tells @command{comm} to read standard input
16731 instead of a regular file.
16733 Now we're ready to build a fancy pipeline. The first application is a word
16734 frequency counter. This helps an author determine if he or she is over-using
16737 The first step is to change the case of all the letters in our input file
16738 to one case. ``The'' and ``the'' are the same word when doing counting.
16741 $ tr '[:upper:]' '[:lower:]' < whats.gnu | ...
16744 The next step is to get rid of punctuation. Quoted words and unquoted words
16745 should be treated identically; it's easiest to just get the punctuation out of
16749 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | ...
16752 The second @command{tr} command operates on the complement of the listed
16753 characters, which are all the letters, the digits, the underscore, and
16754 the blank. The @samp{\n} represents the newline character; it has to
16755 be left alone. (The @acronym{ASCII} tab character should also be included for
16756 good measure in a production script.)
16758 At this point, we have data consisting of words separated by blank space.
16759 The words only contain alphanumeric characters (and the underscore). The
16760 next step is break the data apart so that we have one word per line. This
16761 makes the counting operation much easier, as we will see shortly.
16764 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16765 > tr -s ' ' '\n' | ...
16768 This command turns blanks into newlines. The @option{-s} option squeezes
16769 multiple newline characters in the output into just one. This helps us
16770 avoid blank lines. (The @samp{>} is the shell's ``secondary prompt.''
16771 This is what the shell prints when it notices you haven't finished
16772 typing in all of a command.)
16774 We now have data consisting of one word per line, no punctuation, all one
16775 case. We're ready to count each word:
16778 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16779 > tr -s ' ' '\n' | sort | uniq -c | ...
16782 At this point, the data might look something like this:
16795 The output is sorted by word, not by count! What we want is the most
16796 frequently used words first. Fortunately, this is easy to accomplish,
16797 with the help of two more @command{sort} options:
16801 do a numeric sort, not a textual one
16804 reverse the order of the sort
16807 The final pipeline looks like this:
16810 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16811 > tr -s ' ' '\n' | sort | uniq -c | sort -n -r
16820 Whew! That's a lot to digest. Yet, the same principles apply. With six
16821 commands, on two lines (really one long one split for convenience), we've
16822 created a program that does something interesting and useful, in much
16823 less time than we could have written a C program to do the same thing.
16825 A minor modification to the above pipeline can give us a simple spelling
16826 checker! To determine if you've spelled a word correctly, all you have to
16827 do is look it up in a dictionary. If it is not there, then chances are
16828 that your spelling is incorrect. So, we need a dictionary.
16829 The conventional location for a dictionary is @file{/usr/dict/words}.
16830 On my GNU/Linux system,@footnote{Redhat Linux 6.1, for the November 2000
16831 revision of this article.}
16832 this is a sorted, 45,402 word dictionary.
16834 Now, how to compare our file with the dictionary? As before, we generate
16835 a sorted list of words, one per line:
16838 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16839 > tr -s ' ' '\n' | sort -u | ...
16842 Now, all we need is a list of words that are @emph{not} in the
16843 dictionary. Here is where the @command{comm} command comes in.
16846 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16847 > tr -s ' ' '\n' | sort -u |
16848 > comm -23 - /usr/dict/words
16851 The @option{-2} and @option{-3} options eliminate lines that are only in the
16852 dictionary (the second file), and lines that are in both files. Lines
16853 only in the first file (standard input, our stream of words), are
16854 words that are not in the dictionary. These are likely candidates for
16855 spelling errors. This pipeline was the first cut at a production
16856 spelling checker on Unix.
16858 There are some other tools that deserve brief mention.
16862 search files for text that matches a regular expression
16865 count lines, words, characters
16868 a T-fitting for data pipes, copies data to files and to standard output
16871 the stream editor, an advanced tool
16874 a data manipulation language, another advanced tool
16877 The software tools philosophy also espoused the following bit of
16878 advice: ``Let someone else do the hard part.'' This means, take
16879 something that gives you most of what you need, and then massage it the
16880 rest of the way until it's in the form that you want.
16886 Each program should do one thing well. No more, no less.
16889 Combining programs with appropriate plumbing leads to results where
16890 the whole is greater than the sum of the parts. It also leads to novel
16891 uses of programs that the authors might never have imagined.
16894 Programs should never print extraneous header or trailer data, since these
16895 could get sent on down a pipeline. (A point we didn't mention earlier.)
16898 Let someone else do the hard part.
16901 Know your toolbox! Use each program appropriately. If you don't have an
16902 appropriate tool, build one.
16905 As of this writing, all the programs we've discussed are available via
16906 anonymous @command{ftp} from: @*
16907 @uref{ftp://gnudist.gnu.org/textutils/textutils-1.22.tar.gz}. (There may
16908 be more recent versions available now.)
16910 None of what I have presented in this column is new. The Software Tools
16911 philosophy was first introduced in the book @cite{Software Tools}, by
16912 Brian Kernighan and P.J. Plauger (Addison-Wesley, ISBN 0-201-03669-X).
16913 This book showed how to write and use software tools. It was written in
16914 1976, using a preprocessor for FORTRAN named @command{ratfor} (RATional
16915 FORtran). At the time, C was not as ubiquitous as it is now; FORTRAN
16916 was. The last chapter presented a @command{ratfor} to FORTRAN
16917 processor, written in @command{ratfor}. @command{ratfor} looks an awful
16918 lot like C; if you know C, you won't have any problem following the
16921 In 1981, the book was updated and made available as @cite{Software Tools
16922 in Pascal} (Addison-Wesley, ISBN 0-201-10342-7). Both books are
16923 still in print and are well worth
16924 reading if you're a programmer. They certainly made a major change in
16925 how I view programming.
16927 The programs in both books are available from
16928 @uref{http://cm.bell-labs.com/who/bwk, Brian Kernighan's home page}.
16929 For a number of years, there was an active
16930 Software Tools Users Group, whose members had ported the original
16931 @command{ratfor} programs to essentially every computer system with a
16932 FORTRAN compiler. The popularity of the group waned in the middle 1980s
16933 as Unix began to spread beyond universities.
16935 With the current proliferation of GNU code and other clones of Unix programs,
16936 these programs now receive little attention; modern C versions are
16937 much more efficient and do more than these programs do. Nevertheless, as
16938 exposition of good programming style, and evangelism for a still-valuable
16939 philosophy, these books are unparalleled, and I recommend them highly.
16941 Acknowledgment: I would like to express my gratitude to Brian Kernighan
16942 of Bell Labs, the original Software Toolsmith, for reviewing this column.
16944 @node GNU Free Documentation License
16945 @appendix GNU Free Documentation License
16949 @node Concept index
16956 @c Local variables:
16957 @c texinfo-column-for-description: 32