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. 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). @command{fmt}
2207 initially tries to make lines about 7% shorter than this, to give it
2208 room to balance line lengths.
2210 @item -p @var{prefix}
2211 @itemx --prefix=@var{prefix}
2212 Only lines beginning with @var{prefix} (possibly preceded by whitespace)
2213 are subject to formatting. The prefix and any preceding whitespace are
2214 stripped for the formatting and then re-attached to each formatted output
2215 line. One use is to format certain kinds of program comments, while
2216 leaving the code unchanged.
2224 @section @command{pr}: Paginate or columnate files for printing
2227 @cindex printing, preparing files for
2228 @cindex multicolumn output, generating
2229 @cindex merging files in parallel
2231 @command{pr} writes each @var{file} (@samp{-} means standard input), or
2232 standard input if none are given, to standard output, paginating and
2233 optionally outputting in multicolumn format; optionally merges all
2234 @var{file}s, printing all in parallel, one per column. Synopsis:
2237 pr [@var{option}]@dots{} [@var{file}]@dots{}
2241 By default, a 5-line header is printed at each page: two blank lines;
2242 a line with the date, the file name, and the page count; and two more
2243 blank lines. A footer of five blank lines is also printed.
2244 The default @var{page_length} is 66
2245 lines. The default number of text lines is therefore 56.
2246 The text line of the header takes the form
2247 @samp{@var{date} @var{string} @var{page}}, with spaces inserted around
2248 @var{string} so that the line takes up the full @var{page_width}. Here,
2249 @var{date} is the date (see the @option{-D} or @option{--date-format}
2250 option for details), @var{string} is the centered header string, and
2251 @var{page} identifies the page number. The @env{LC_MESSAGES} locale
2252 category affects the spelling of @var{page}; in the default C locale, it
2253 is @samp{Page @var{number}} where @var{number} is the decimal page
2256 Form feeds in the input cause page breaks in the output. Multiple form
2257 feeds produce empty pages.
2259 Columns are of equal width, separated by an optional string (default
2260 is @samp{space}). For multicolumn output, lines will always be truncated to
2261 @var{page_width} (default 72), unless you use the @option{-J} option.
2263 column output no line truncation occurs by default. Use @option{-W} option to
2264 truncate lines in that case.
2266 The following changes were made in version 1.22i and apply to later
2267 versions of @command{pr}:
2268 @c FIXME: this whole section here sounds very awkward to me. I
2269 @c made a few small changes, but really it all needs to be redone. - Brian
2270 @c OK, I fixed another sentence or two, but some of it I just don't understand.
2275 Some small @var{letter options} (@option{-s}, @option{-w}) have been
2276 redefined for better @acronym{POSIX} compliance. The output of some further
2277 cases has been adapted to other Unix systems. These changes are not
2278 compatible with earlier versions of the program.
2281 Some @var{new capital letter} options (@option{-J}, @option{-S}, @option{-W})
2282 have been introduced to turn off unexpected interferences of small letter
2283 options. The @option{-N} option and the second argument @var{last_page}
2284 of @samp{+FIRST_PAGE} offer more flexibility. The detailed handling of
2285 form feeds set in the input files requires the @option{-T} option.
2288 Capital letter options override small letter ones.
2291 Some of the option-arguments (compare @option{-s}, @option{-e},
2292 @option{-i}, @option{-n}) cannot be specified as separate arguments from the
2293 preceding option letter (already stated in the @acronym{POSIX} specification).
2296 The program accepts the following options. Also see @ref{Common options}.
2300 @item +@var{first_page}[:@var{last_page}]
2301 @itemx --pages=@var{first_page}[:@var{last_page}]
2302 @c The two following @opindex lines evoke warnings because they contain ':'
2303 @c The 'info' spec does not permit that. If we use those lines, we end
2304 @c up with truncated index entries that don't work.
2305 @c @opindex +@var{first_page}[:@var{last_page}]
2306 @c @opindex --pages=@var{first_page}[:@var{last_page}]
2307 @opindex +@var{page_range}
2308 @opindex --pages=@var{page_range}
2309 Begin printing with page @var{first_page} and stop with @var{last_page}.
2310 Missing @samp{:@var{last_page}} implies end of file. While estimating
2311 the number of skipped pages each form feed in the input file results
2312 in a new page. Page counting with and without @samp{+@var{first_page}}
2313 is identical. By default, counting starts with the first page of input
2314 file (not first page printed). Line numbering may be altered by @option{-N}
2318 @itemx --columns=@var{column}
2319 @opindex -@var{column}
2321 @cindex down columns
2322 With each single @var{file}, produce @var{column} columns of output
2323 (default is 1) and print columns down, unless @option{-a} is used. The
2324 column width is automatically decreased as @var{column} increases; unless
2325 you use the @option{-W/-w} option to increase @var{page_width} as well.
2326 This option might well cause some lines to be truncated. The number of
2327 lines in the columns on each page are balanced. The options @option{-e}
2328 and @option{-i} are on for multiple text-column output. Together with
2329 @option{-J} option column alignment and line truncation is turned off.
2330 Lines of full length are joined in a free field format and @option{-S}
2331 option may set field separators. @option{-@var{column}} may not be used
2332 with @option{-m} option.
2338 @cindex across columns
2339 With each single @var{file}, print columns across rather than down. The
2340 @option{-@var{column}} option must be given with @var{column} greater than one.
2341 If a line is too long to fit in a column, it is truncated.
2344 @itemx --show-control-chars
2346 @opindex --show-control-chars
2347 Print control characters using hat notation (e.g., @samp{^G}); print
2348 other nonprinting characters in octal backslash notation. By default,
2349 nonprinting characters are not changed.
2352 @itemx --double-space
2354 @opindex --double-space
2355 @cindex double spacing
2356 Double space the output.
2358 @item -D @var{format}
2359 @itemx --date-format=@var{format}
2360 @cindex time formats
2361 @cindex formatting times
2362 Format header dates using @var{format}, using the same conventions as
2363 for the command @samp{date +@var{format}}; @xref{date invocation}.
2364 Except for directives, which start with
2365 @samp{%}, characters in @var{format} are printed unchanged. You can use
2366 this option to specify an arbitrary string in place of the header date,
2367 e.g., @option{--date-format="Monday morning"}.
2369 @vindex POSIXLY_CORRECT
2371 The default date format is @samp{%Y-%m-%d %H:%M} (for example,
2372 @samp{2001-12-04 23:59});
2373 but if the @env{POSIXLY_CORRECT} environment variable is set
2374 and the @env{LC_TIME} locale category specifies the @acronym{POSIX}
2375 locale, the default is @samp{%b %e %H:%M %Y} (for example,
2376 @samp{Dec@ @ 4 23:59 2001}.
2379 Time stamps are listed according to the time zone rules specified by
2380 the @env{TZ} environment variable, or by the system default rules if
2381 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
2382 with @env{TZ}, libc, The GNU C Library Reference Manual}.
2384 @item -e[@var{in-tabchar}[@var{in-tabwidth}]]
2385 @itemx --expand-tabs[=@var{in-tabchar}[@var{in-tabwidth}]]
2387 @opindex --expand-tabs
2389 Expand @var{tab}s to spaces on input. Optional argument @var{in-tabchar} is
2390 the input tab character (default is the TAB character). Second optional
2391 argument @var{in-tabwidth} is the input tab character's width (default
2399 @opindex --form-feed
2400 Use a form feed instead of newlines to separate output pages. This does
2401 not alter the default page length of 66 lines.
2403 @item -h @var{header}
2404 @itemx --header=@var{header}
2407 Replace the file name in the header with the centered string @var{header}.
2408 When using the shell, @var{header} should be quoted and should be
2409 separated from @option{-h} by a space.
2411 @item -i[@var{out-tabchar}[@var{out-tabwidth}]]
2412 @itemx --output-tabs[=@var{out-tabchar}[@var{out-tabwidth}]]
2414 @opindex --output-tabs
2416 Replace spaces with @var{tab}s on output. Optional argument @var{out-tabchar}
2417 is the output tab character (default is the TAB character). Second optional
2418 argument @var{out-tabwidth} is the output tab character's width (default
2424 @opindex --join-lines
2425 Merge lines of full length. Used together with the column options
2426 @option{-@var{column}}, @option{-a -@var{column}} or @option{-m}. Turns off
2427 @option{-W/-w} line truncation;
2428 no column alignment used; may be used with
2429 @option{--sep-string[=@var{string}]}. @option{-J} has been introduced
2430 (together with @option{-W} and @option{--sep-string})
2431 to disentangle the old (@acronym{POSIX}-compliant) options @option{-w} and
2432 @option{-s} along with the three column options.
2435 @item -l @var{page_length}
2436 @itemx --length=@var{page_length}
2439 Set the page length to @var{page_length} (default 66) lines, including
2440 the lines of the header [and the footer]. If @var{page_length} is less
2441 than or equal to 10, the header and footer are omitted, as if the
2442 @option{-t} option had been given.
2448 Merge and print all @var{file}s in parallel, one in each column. If a
2449 line is too long to fit in a column, it is truncated, unless the @option{-J}
2450 option is used. @option{--sep-string[=@var{string}]} may be used.
2452 some @var{file}s (form feeds set) produce empty columns, still marked
2453 by @var{string}. The result is a continuous line numbering and column
2454 marking throughout the whole merged file. Completely empty merged pages
2455 show no separators or line numbers. The default header becomes
2456 @samp{@var{date} @var{page}} with spaces inserted in the middle; this
2457 may be used with the @option{-h} or @option{--header} option to fill up
2458 the middle blank part.
2460 @item -n[@var{number-separator}[@var{digits}]]
2461 @itemx --number-lines[=@var{number-separator}[@var{digits}]]
2463 @opindex --number-lines
2464 Provide @var{digits} digit line numbering (default for @var{digits} is
2465 5). With multicolumn output the number occupies the first @var{digits}
2466 column positions of each text column or only each line of @option{-m}
2467 output. With single column output the number precedes each line just as
2468 @option{-m} does. Default counting of the line numbers starts with the
2469 first line of the input file (not the first line printed, compare the
2470 @option{--page} option and @option{-N} option).
2471 Optional argument @var{number-separator} is the character appended to
2472 the line number to separate it from the text followed. The default
2473 separator is the TAB character. In a strict sense a TAB is always
2474 printed with single column output only. The TAB width varies
2475 with the TAB position, e.g., with the left @var{margin} specified
2476 by @option{-o} option. With multicolumn output priority is given to
2477 @samp{equal width of output columns} (a @acronym{POSIX} specification).
2478 The TAB width is fixed to the value of the first column and does
2479 not change with different values of left @var{margin}. That means a
2480 fixed number of spaces is always printed in the place of the
2481 @var{number-separator} TAB. The tabification depends upon the output
2484 @item -N @var{line_number}
2485 @itemx --first-line-number=@var{line_number}
2487 @opindex --first-line-number
2488 Start line counting with the number @var{line_number} at first line of
2489 first page printed (in most cases not the first line of the input file).
2491 @item -o @var{margin}
2492 @itemx --indent=@var{margin}
2495 @cindex indenting lines
2497 Indent each line with a margin @var{margin} spaces wide (default is zero).
2498 The total page width is the size of the margin plus the @var{page_width}
2499 set with the @option{-W/-w} option. A limited overflow may occur with
2500 numbered single column output (compare @option{-n} option).
2503 @itemx --no-file-warnings
2505 @opindex --no-file-warnings
2506 Do not print a warning message when an argument @var{file} cannot be
2507 opened. (The exit status will still be nonzero, however.)
2509 @item -s[@var{char}]
2510 @itemx --separator[=@var{char}]
2512 @opindex --separator
2513 Separate columns by a single character @var{char}. The default for
2514 @var{char} is the TAB character without @option{-w} and @samp{no
2515 character} with @option{-w}. Without @option{-s} the default separator
2516 @samp{space} is set. @option{-s[char]} turns off line truncation of all
2517 three column options (@option{-COLUMN}|@option{-a -COLUMN}|@option{-m}) unless
2518 @option{-w} is set. This is a @acronym{POSIX}-compliant formulation.
2521 @item -S[@var{string}]
2522 @itemx --sep-string[=@var{string}]
2524 @opindex --sep-string
2525 Use @var{string} to separate output columns. The @option{-S} option doesn't
2526 affect the @option{-W/-w} option, unlike the @option{-s} option which does. It
2527 does not affect line truncation or column alignment.
2528 Without @option{-S}, and with @option{-J}, @command{pr} uses the default output
2530 Without @option{-S} or @option{-J}, @command{pr} uses a @samp{space}
2531 (same as @option{-S"@w{ }"}).
2532 If no @samp{@var{string}} argument is specified, @samp{""} is assumed.
2535 @itemx --omit-header
2537 @opindex --omit-header
2538 Do not print the usual header [and footer] on each page, and do not fill
2539 out the bottom of pages (with blank lines or a form feed). No page
2540 structure is produced, but form feeds set in the input files are retained.
2541 The predefined pagination is not changed. @option{-t} or @option{-T} may be
2542 useful together with other options; e.g.: @option{-t -e4}, expand TAB characters
2543 in the input file to 4 spaces but don't make any other changes. Use of
2544 @option{-t} overrides @option{-h}.
2547 @itemx --omit-pagination
2549 @opindex --omit-pagination
2550 Do not print header [and footer]. In addition eliminate all form feeds
2551 set in the input files.
2554 @itemx --show-nonprinting
2556 @opindex --show-nonprinting
2557 Print nonprinting characters in octal backslash notation.
2559 @item -w @var{page_width}
2560 @itemx --width=@var{page_width}
2563 Set page width to @var{page_width} characters for multiple text-column
2564 output only (default for @var{page_width} is 72). @option{-s[CHAR]} turns
2565 off the default page width and any line truncation and column alignment.
2566 Lines of full length are merged, regardless of the column options
2567 set. No @var{page_width} setting is possible with single column output.
2568 A @acronym{POSIX}-compliant formulation.
2570 @item -W @var{page_width}
2571 @itemx --page_width=@var{page_width}
2573 @opindex --page_width
2574 Set the page width to @var{page_width} characters. That's valid with and
2575 without a column option. Text lines are truncated, unless @option{-J}
2576 is used. Together with one of the three column options
2577 (@option{-@var{column}}, @option{-a -@var{column}} or @option{-m}) column
2578 alignment is always used. The separator options @option{-S} or @option{-s}
2579 don't affect the @option{-W} option. Default is 72 characters. Without
2580 @option{-W @var{page_width}} and without any of the column options NO line
2581 truncation is used (defined to keep downward compatibility and to meet
2582 most frequent tasks). That's equivalent to @option{-W 72 -J}. The header
2583 line is never truncated.
2590 @node fold invocation
2591 @section @command{fold}: Wrap input lines to fit in specified width
2594 @cindex wrapping long input lines
2595 @cindex folding long input lines
2597 @command{fold} writes each @var{file} (@option{-} means standard input), or
2598 standard input if none are given, to standard output, breaking long
2602 fold [@var{option}]@dots{} [@var{file}]@dots{}
2605 By default, @command{fold} breaks lines wider than 80 columns. The output
2606 is split into as many lines as necessary.
2608 @cindex screen columns
2609 @command{fold} counts screen columns by default; thus, a tab may count more
2610 than one column, backspace decreases the column count, and carriage
2611 return sets the column to zero.
2613 The program accepts the following options. Also see @ref{Common options}.
2621 Count bytes rather than columns, so that tabs, backspaces, and carriage
2622 returns are each counted as taking up one column, just like other
2629 Break at word boundaries: the line is broken after the last blank before
2630 the maximum line length. If the line contains no such blanks, the line
2631 is broken at the maximum line length as usual.
2633 @item -w @var{width}
2634 @itemx --width=@var{width}
2637 Use a maximum line length of @var{width} columns instead of 80.
2639 For compatibility @command{fold} supports an obsolete option syntax
2640 @option{-@var{width}}. New scripts should use @option{-w @var{width}}
2648 @node Output of parts of files
2649 @chapter Output of parts of files
2651 @cindex output of parts of files
2652 @cindex parts of files, output of
2654 These commands output pieces of the input.
2657 * head invocation:: Output the first part of files.
2658 * tail invocation:: Output the last part of files.
2659 * split invocation:: Split a file into pieces.
2660 * csplit invocation:: Split a file into context-determined pieces.
2663 @node head invocation
2664 @section @command{head}: Output the first part of files
2667 @cindex initial part of files, outputting
2668 @cindex first part of files, outputting
2670 @command{head} prints the first part (10 lines by default) of each
2671 @var{file}; it reads from standard input if no files are given or
2672 when given a @var{file} of @option{-}. Synopsis:
2675 head [@var{option}]@dots{} [@var{file}]@dots{}
2678 If more than one @var{file} is specified, @command{head} prints a
2679 one-line header consisting of:
2682 ==> @var{file name} <==
2686 before the output for each @var{file}.
2688 The program accepts the following options. Also see @ref{Common options}.
2693 @itemx --bytes=@var{k}
2696 Print the first @var{k} bytes, instead of initial lines.
2697 However, if @var{k} starts with a @samp{-},
2698 print all but the last @var{k} bytes of each file.
2699 @multiplierSuffixes{k}
2702 @itemx --lines=@var{k}
2705 Output the first @var{k} lines.
2706 However, if @var{k} starts with a @samp{-},
2707 print all but the last @var{k} lines of each file.
2708 Size multiplier suffixes are the same as with the @option{-c} option.
2716 Never print file name headers.
2722 Always print file name headers.
2726 For compatibility @command{head} also supports an obsolete option syntax
2727 @option{-@var{count}@var{options}}, which is recognized only if it is
2728 specified first. @var{count} is a decimal number optionally followed
2729 by a size letter (@samp{b}, @samp{k}, @samp{m}) as in @option{-c}, or
2730 @samp{l} to mean count by lines, or other option letters (@samp{cqv}).
2731 Scripts intended for standard hosts should use @option{-c @var{count}}
2732 or @option{-n @var{count}} instead. If your script must also run on
2733 hosts that support only the obsolete syntax, it is usually simpler to
2734 avoid @command{head}, e.g., by using @samp{sed 5q} instead of
2740 @node tail invocation
2741 @section @command{tail}: Output the last part of files
2744 @cindex last part of files, outputting
2746 @command{tail} prints the last part (10 lines by default) of each
2747 @var{file}; it reads from standard input if no files are given or
2748 when given a @var{file} of @samp{-}. Synopsis:
2751 tail [@var{option}]@dots{} [@var{file}]@dots{}
2754 If more than one @var{file} is specified, @command{tail} prints a
2755 one-line header consisting of:
2758 ==> @var{file name} <==
2762 before the output for each @var{file}.
2764 @cindex BSD @command{tail}
2765 @sc{gnu} @command{tail} can output any amount of data (some other versions of
2766 @command{tail} cannot). It also has no @option{-r} option (print in
2767 reverse), since reversing a file is really a different job from printing
2768 the end of a file; BSD @command{tail} (which is the one with @option{-r}) can
2769 only reverse files that are at most as large as its buffer, which is
2770 typically 32 KiB@. A more reliable and versatile way to reverse files is
2771 the @sc{gnu} @command{tac} command.
2773 The program accepts the following options. Also see @ref{Common options}.
2778 @itemx --bytes=@var{k}
2781 Output the last @var{k} bytes, instead of final lines.
2782 However, if @var{k} starts with a @samp{+}, start printing with the
2783 @var{k}th byte from the start of each file, instead of from the end.
2784 @multiplierSuffixes{k}
2787 @itemx --follow[=@var{how}]
2790 @cindex growing files
2791 @vindex name @r{follow option}
2792 @vindex descriptor @r{follow option}
2793 Loop forever trying to read more characters at the end of the file,
2794 presumably because the file is growing.
2795 If more than one file is given, @command{tail} prints a header whenever it
2796 gets output from a different file, to indicate which file that output is
2799 There are two ways to specify how you'd like to track files with this option,
2800 but that difference is noticeable only when a followed file is removed or
2802 If you'd like to continue to track the end of a growing file even after
2803 it has been unlinked, use @option{--follow=descriptor}. This is the default
2804 behavior, but it is not useful if you're tracking a log file that may be
2805 rotated (removed or renamed, then reopened). In that case, use
2806 @option{--follow=name} to track the named file, perhaps by reopening it
2807 periodically to see if it has been removed and recreated by some other program.
2808 Note that the inotify-based implementation handles this case without
2809 the need for any periodic reopening.
2811 No matter which method you use, if the tracked file is determined to have
2812 shrunk, @command{tail} prints a message saying the file has been truncated
2813 and resumes tracking the end of the file from the newly-determined endpoint.
2815 When a file is removed, @command{tail}'s behavior depends on whether it is
2816 following the name or the descriptor. When following by name, tail can
2817 detect that a file has been removed and gives a message to that effect,
2818 and if @option{--retry} has been specified it will continue checking
2819 periodically to see if the file reappears.
2820 When following a descriptor, tail does not detect that the file has
2821 been unlinked or renamed and issues no message; even though the file
2822 may no longer be accessible via its original name, it may still be
2825 The option values @samp{descriptor} and @samp{name} may be specified only
2826 with the long form of the option, not with @option{-f}.
2828 The @option{-f} option is ignored if
2829 no @var{file} operand is specified and standard input is a FIFO or a pipe.
2830 Likewise, the @option{-f} option has no effect for any
2831 operand specified as @samp{-}, when standard input is a FIFO or a pipe.
2833 With kernel inotify support, output is triggered by file changes
2834 and is generally very prompt.
2835 Otherwise, @command{tail} sleeps for one second between checks---
2836 use @option{--sleep-interval=@var{n}} to change that default---which can
2837 make the output appear slightly less responsive or bursty.
2838 When using tail without inotify support, you can make it more responsive
2839 by using a sub-second sleep interval, e.g., via an alias like this:
2842 alias tail='tail -s.1'
2847 This option is the same as @option{--follow=name --retry}. That is, tail
2848 will attempt to reopen a file when it is removed. Should this fail, tail
2849 will keep trying until it becomes accessible again.
2853 This option is useful mainly when following by name (i.e., with
2854 @option{--follow=name}).
2855 Without this option, when tail encounters a file that doesn't
2856 exist or is otherwise inaccessible, it reports that fact and
2857 never checks it again.
2859 @itemx --sleep-interval=@var{number}
2860 @opindex --sleep-interval
2861 Change the number of seconds to wait between iterations (the default is 1.0).
2862 During one iteration, every specified file is checked to see if it has
2864 Historical implementations of @command{tail} have required that
2865 @var{number} be an integer. However, GNU @command{tail} accepts
2866 an arbitrary floating point number. @xref{Floating point}.
2867 When @command{tail} uses inotify, this polling-related option
2868 is usually ignored. However, if you also specify @option{--pid=@var{p}},
2869 @command{tail} checks whether process @var{p} is alive at least
2870 every @var{number} seconds.
2872 @itemx --pid=@var{pid}
2874 When following by name or by descriptor, you may specify the process ID,
2875 @var{pid}, of the sole writer of all @var{file} arguments. Then, shortly
2876 after that process terminates, tail will also terminate. This will
2877 work properly only if the writer and the tailing process are running on
2878 the same machine. For example, to save the output of a build in a file
2879 and to watch the file grow, if you invoke @command{make} and @command{tail}
2880 like this then the tail process will stop when your build completes.
2881 Without this option, you would have had to kill the @code{tail -f}
2885 $ make >& makerr & tail --pid=$! -f makerr
2888 If you specify a @var{pid} that is not in use or that does not correspond
2889 to the process that is writing to the tailed files, then @command{tail}
2890 may terminate long before any @var{file}s stop growing or it may not
2891 terminate until long after the real writer has terminated.
2892 Note that @option{--pid} cannot be supported on some systems; @command{tail}
2893 will print a warning if this is the case.
2895 @itemx --max-unchanged-stats=@var{n}
2896 @opindex --max-unchanged-stats
2897 When tailing a file by name, if there have been @var{n} (default
2898 n=@value{DEFAULT_MAX_N_UNCHANGED_STATS_BETWEEN_OPENS}) consecutive
2899 iterations for which the file has not changed, then
2900 @code{open}/@code{fstat} the file to determine if that file name is
2901 still associated with the same device/inode-number pair as before.
2902 When following a log file that is rotated, this is approximately the
2903 number of seconds between when tail prints the last pre-rotation lines
2904 and when it prints the lines that have accumulated in the new log file.
2905 This option is meaningful only when polling (i.e., without inotify)
2906 and when following by name.
2909 @itemx --lines=@var{k}
2912 Output the last @var{k} lines.
2913 However, if @var{k} starts with a @samp{+}, start printing with the
2914 @var{k}th line from the start of each file, instead of from the end.
2915 Size multiplier suffixes are the same as with the @option{-c} option.
2923 Never print file name headers.
2929 Always print file name headers.
2933 For compatibility @command{tail} also supports an obsolete usage
2934 @samp{tail -[@var{count}][bcl][f] [@var{file}]}, which is recognized
2935 only if it does not conflict with the usage described
2936 above. This obsolete form uses exactly one option and at most one
2937 file. In the option, @var{count} is an optional decimal number optionally
2938 followed by a size letter (@samp{b}, @samp{c}, @samp{l}) to mean count
2939 by 512-byte blocks, bytes, or lines, optionally followed by @samp{f}
2940 which has the same meaning as @option{-f}.
2942 @vindex _POSIX2_VERSION
2943 On older systems, the leading @samp{-} can be replaced by @samp{+} in
2944 the obsolete option syntax with the same meaning as in counts, and
2945 obsolete usage overrides normal usage when the two conflict.
2946 This obsolete behavior can be enabled or disabled with the
2947 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
2950 Scripts intended for use on standard hosts should avoid obsolete
2951 syntax and should use @option{-c @var{count}[b]}, @option{-n
2952 @var{count}}, and/or @option{-f} instead. If your script must also
2953 run on hosts that support only the obsolete syntax, you can often
2954 rewrite it to avoid problematic usages, e.g., by using @samp{sed -n
2955 '$p'} rather than @samp{tail -1}. If that's not possible, the script
2956 can use a test like @samp{if tail -c +1 </dev/null >/dev/null 2>&1;
2957 then @dots{}} to decide which syntax to use.
2959 Even if your script assumes the standard behavior, you should still
2960 beware usages whose behaviors differ depending on the @acronym{POSIX}
2961 version. For example, avoid @samp{tail - main.c}, since it might be
2962 interpreted as either @samp{tail main.c} or as @samp{tail -- -
2963 main.c}; avoid @samp{tail -c 4}, since it might mean either @samp{tail
2964 -c4} or @samp{tail -c 10 4}; and avoid @samp{tail +4}, since it might
2965 mean either @samp{tail ./+4} or @samp{tail -n +4}.
2970 @node split invocation
2971 @section @command{split}: Split a file into pieces.
2974 @cindex splitting a file into pieces
2975 @cindex pieces, splitting a file into
2977 @command{split} creates output files containing consecutive or interleaved
2978 sections of @var{input} (standard input if none is given or @var{input}
2979 is @samp{-}). Synopsis:
2982 split [@var{option}] [@var{input} [@var{prefix}]]
2985 By default, @command{split} puts 1000 lines of @var{input} (or whatever is
2986 left over for the last section), into each output file.
2988 @cindex output file name prefix
2989 The output files' names consist of @var{prefix} (@samp{x} by default)
2990 followed by a group of characters (@samp{aa}, @samp{ab}, @dots{} by
2991 default), such that concatenating the output files in traditional
2992 sorted order by file name produces the original input file (except
2993 @option{-nr/@var{n}}). By default split will initially create files
2994 with two generated suffix characters, and will increase this width by two
2995 when the next most significant position reaches the last character.
2996 (@samp{yz}, @samp{zaaa}, @samp{zaab}, @dots{}). In this way an arbitrary
2997 number of output files are supported, which sort as described above,
2998 even in the presence of an @option{--additional-suffix} option.
2999 If the @option{-a} option is specified and the output file names are
3000 exhausted, @command{split} reports an error without deleting the
3001 output files that it did create.
3003 The program accepts the following options. Also see @ref{Common options}.
3007 @item -l @var{lines}
3008 @itemx --lines=@var{lines}
3011 Put @var{lines} lines of @var{input} into each output file.
3013 For compatibility @command{split} also supports an obsolete
3014 option syntax @option{-@var{lines}}. New scripts should use
3015 @option{-l @var{lines}} instead.
3018 @itemx --bytes=@var{size}
3021 Put @var{size} bytes of @var{input} into each output file.
3022 @multiplierSuffixes{size}
3025 @itemx --line-bytes=@var{size}
3027 @opindex --line-bytes
3028 Put into each output file as many complete lines of @var{input} as
3029 possible without exceeding @var{size} bytes. Individual lines longer than
3030 @var{size} bytes are broken into multiple files.
3031 @var{size} has the same format as for the @option{--bytes} option.
3033 @itemx --filter=@var{command}
3035 With this option, rather than simply writing to each output file,
3036 write through a pipe to the specified shell @var{command} for each output file.
3037 @var{command} should use the $FILE environment variable, which is set
3038 to a different output file name for each invocation of the command.
3039 For example, imagine that you have a 1TiB compressed file
3040 that, if uncompressed, would be too large to reside on disk,
3041 yet you must split it into individually-compressed pieces
3042 of a more manageable size.
3043 To do that, you might run this command:
3046 xz -dc BIG.xz | split -b200G --filter='xz > $FILE.xz' - big-
3049 Assuming a 10:1 compression ratio, that would create about fifty 20GiB files
3050 with names @file{big-aa.xz}, @file{big-ab.xz}, @file{big-ac.xz}, etc.
3052 @item -n @var{chunks}
3053 @itemx --number=@var{chunks}
3057 Split @var{input} to @var{chunks} output files where @var{chunks} may be:
3060 @var{n} generate @var{n} files based on current size of @var{input}
3061 @var{k}/@var{n} only output @var{k}th of @var{n} to stdout
3062 l/@var{n} generate @var{n} files without splitting lines
3063 l/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
3064 r/@var{n} like @samp{l} but use round robin distribution
3065 r/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
3068 Any excess bytes remaining after dividing the @var{input}
3069 into @var{n} chunks, are assigned to the last chunk.
3070 Any excess bytes appearing after the initial calculation are discarded
3071 (except when using @samp{r} mode).
3073 All @var{n} files are created even if there are fewer than @var{n} lines,
3074 or the @var{input} is truncated.
3076 For @samp{l} mode, chunks are approximately @var{input} size / @var{n}.
3077 The @var{input} is partitioned into @var{n} equal sized portions, with
3078 the last assigned any excess. If a line @emph{starts} within a partition
3079 it is written completely to the corresponding file. Since lines
3080 are not split even if they overlap a partition, the files written
3081 can be larger or smaller than the partition size, and even empty
3082 if a line is so long as to completely overlap the partition.
3084 For @samp{r} mode, the size of @var{input} is irrelevant,
3085 and so can be a pipe for example.
3087 @item -a @var{length}
3088 @itemx --suffix-length=@var{length}
3090 @opindex --suffix-length
3091 Use suffixes of length @var{length}. If a @var{length} of 0 is specified,
3092 this is the same as if (any previous) @option{-a} was not specified, and
3093 thus enables the default behavior, which starts the suffix length at 2,
3094 and unless @option{-n} or @option{--numeric-suffixes=@var{from}} are
3095 specified, will auto increase the length by 2 as required.
3098 @itemx --numeric-suffixes[=@var{from}]
3100 @opindex --numeric-suffixes
3101 Use digits in suffixes rather than lower-case letters. The numerical
3102 suffix counts from @var{from} if specified, 0 otherwise.
3103 Note specifying a @var{from} value also disables the default
3104 auto suffix length expansion described above, and so you may also
3105 want to specify @option{-a} to allow suffixes beyond @samp{99}.
3107 @itemx --additional-suffix=@var{suffix}
3108 @opindex --additional-suffix
3109 Append an additional @var{suffix} to output file names. @var{suffix}
3110 must not contain slash.
3113 @itemx --elide-empty-files
3115 @opindex --elide-empty-files
3116 Suppress the generation of zero-length output files. This can happen
3117 with the @option{--number} option if a file is (truncated to be) shorter
3118 than the number requested, or if a line is so long as to completely
3119 span a chunk. The output file sequence numbers, always run consecutively
3120 even when this option is specified.
3125 @opindex --unbuffered
3126 Immediately copy input to output in @option{--number r/...} mode,
3127 which is a much slower mode of operation.
3131 Write a diagnostic just before each output file is opened.
3137 Here are a few examples to illustrate how the
3138 @option{--number} (@option{-n}) option works:
3140 Notice how, by default, one line may be split onto two or more:
3143 $ seq -w 6 10 > k; split -n3 k; head xa?
3156 Use the "l/" modifier to suppress that:
3159 $ seq -w 6 10 > k; split -nl/3 k; head xa?
3172 Use the "r/" modifier to distribute lines in a round-robin fashion:
3175 $ seq -w 6 10 > k; split -nr/3 k; head xa?
3188 You can also extract just the Kth chunk.
3189 This extracts and prints just the 7th "chunk" of 33:
3192 $ seq 100 > k; split -nl/7/33 k
3199 @node csplit invocation
3200 @section @command{csplit}: Split a file into context-determined pieces
3203 @cindex context splitting
3204 @cindex splitting a file into pieces by context
3206 @command{csplit} creates zero or more output files containing sections of
3207 @var{input} (standard input if @var{input} is @samp{-}). Synopsis:
3210 csplit [@var{option}]@dots{} @var{input} @var{pattern}@dots{}
3213 The contents of the output files are determined by the @var{pattern}
3214 arguments, as detailed below. An error occurs if a @var{pattern}
3215 argument refers to a nonexistent line of the input file (e.g., if no
3216 remaining line matches a given regular expression). After every
3217 @var{pattern} has been matched, any remaining input is copied into one
3220 By default, @command{csplit} prints the number of bytes written to each
3221 output file after it has been created.
3223 The types of pattern arguments are:
3228 Create an output file containing the input up to but not including line
3229 @var{n} (a positive integer). If followed by a repeat count, also
3230 create an output file containing the next @var{n} lines of the input
3231 file once for each repeat.
3233 @item /@var{regexp}/[@var{offset}]
3234 Create an output file containing the current line up to (but not
3235 including) the next line of the input file that contains a match for
3236 @var{regexp}. The optional @var{offset} is an integer.
3237 If it is given, the input up to (but not including) the
3238 matching line plus or minus @var{offset} is put into the output file,
3239 and the line after that begins the next section of input.
3241 @item %@var{regexp}%[@var{offset}]
3242 Like the previous type, except that it does not create an output
3243 file, so that section of the input file is effectively ignored.
3245 @item @{@var{repeat-count}@}
3246 Repeat the previous pattern @var{repeat-count} additional
3247 times. The @var{repeat-count} can either be a positive integer or an
3248 asterisk, meaning repeat as many times as necessary until the input is
3253 The output files' names consist of a prefix (@samp{xx} by default)
3254 followed by a suffix. By default, the suffix is an ascending sequence
3255 of two-digit decimal numbers from @samp{00} to @samp{99}. In any case,
3256 concatenating the output files in sorted order by file name produces the
3257 original input file.
3259 By default, if @command{csplit} encounters an error or receives a hangup,
3260 interrupt, quit, or terminate signal, it removes any output files
3261 that it has created so far before it exits.
3263 The program accepts the following options. Also see @ref{Common options}.
3267 @item -f @var{prefix}
3268 @itemx --prefix=@var{prefix}
3271 @cindex output file name prefix
3272 Use @var{prefix} as the output file name prefix.
3274 @item -b @var{suffix}
3275 @itemx --suffix=@var{suffix}
3278 @cindex output file name suffix
3279 Use @var{suffix} as the output file name suffix. When this option is
3280 specified, the suffix string must include exactly one
3281 @code{printf(3)}-style conversion specification, possibly including
3282 format specification flags, a field width, a precision specifications,
3283 or all of these kinds of modifiers. The format letter must convert a
3284 binary unsigned integer argument to readable form. The format letters
3285 @samp{d} and @samp{i} are aliases for @samp{u}, and the
3286 @samp{u}, @samp{o}, @samp{x}, and @samp{X} conversions are allowed. The
3287 entire @var{suffix} is given (with the current output file number) to
3288 @code{sprintf(3)} to form the file name suffixes for each of the
3289 individual output files in turn. If this option is used, the
3290 @option{--digits} option is ignored.
3292 @item -n @var{digits}
3293 @itemx --digits=@var{digits}
3296 Use output file names containing numbers that are @var{digits} digits
3297 long instead of the default 2.
3302 @opindex --keep-files
3303 Do not remove output files when errors are encountered.
3306 @itemx --elide-empty-files
3308 @opindex --elide-empty-files
3309 Suppress the generation of zero-length output files. (In cases where
3310 the section delimiters of the input file are supposed to mark the first
3311 lines of each of the sections, the first output file will generally be a
3312 zero-length file unless you use this option.) The output file sequence
3313 numbers always run consecutively starting from 0, even when this option
3324 Do not print counts of output file sizes.
3330 Here is an example of its usage.
3331 First, create an empty directory for the exercise,
3338 Now, split the sequence of 1..14 on lines that end with 0 or 5:
3341 $ seq 14 | csplit - '/[05]$/' '@{*@}'
3347 Each number printed above is the size of an output
3348 file that csplit has just created.
3349 List the names of those output files:
3356 Use @command{head} to show their contents:
3381 @node Summarizing files
3382 @chapter Summarizing files
3384 @cindex summarizing files
3386 These commands generate just a few numbers representing entire
3390 * wc invocation:: Print newline, word, and byte counts.
3391 * sum invocation:: Print checksum and block counts.
3392 * cksum invocation:: Print CRC checksum and byte counts.
3393 * md5sum invocation:: Print or check MD5 digests.
3394 * sha1sum invocation:: Print or check SHA-1 digests.
3395 * sha2 utilities:: Print or check SHA-2 digests.
3400 @section @command{wc}: Print newline, word, and byte counts
3404 @cindex character count
3408 @command{wc} counts the number of bytes, characters, whitespace-separated
3409 words, and newlines in each given @var{file}, or standard input if none
3410 are given or for a @var{file} of @samp{-}. Synopsis:
3413 wc [@var{option}]@dots{} [@var{file}]@dots{}
3416 @cindex total counts
3417 @command{wc} prints one line of counts for each file, and if the file was
3418 given as an argument, it prints the file name following the counts. If
3419 more than one @var{file} is given, @command{wc} prints a final line
3420 containing the cumulative counts, with the file name @file{total}. The
3421 counts are printed in this order: newlines, words, characters, bytes,
3422 maximum line length.
3423 Each count is printed right-justified in a field with at least one
3424 space between fields so that the numbers and file names normally line
3425 up nicely in columns. The width of the count fields varies depending
3426 on the inputs, so you should not depend on a particular field width.
3427 However, as a @acronym{GNU} extension, if only one count is printed,
3428 it is guaranteed to be printed without leading spaces.
3430 By default, @command{wc} prints three counts: the newline, words, and byte
3431 counts. Options can specify that only certain counts be printed.
3432 Options do not undo others previously given, so
3439 prints both the byte counts and the word counts.
3441 With the @option{--max-line-length} option, @command{wc} prints the length
3442 of the longest line per file, and if there is more than one file it
3443 prints the maximum (not the sum) of those lengths. The line lengths here
3444 are measured in screen columns, according to the current locale and
3445 assuming tab positions in every 8th column.
3447 The program accepts the following options. Also see @ref{Common options}.
3455 Print only the byte counts.
3461 Print only the character counts.
3467 Print only the word counts.
3473 Print only the newline counts.
3476 @itemx --max-line-length
3478 @opindex --max-line-length
3479 Print only the maximum line lengths.
3481 @macro filesZeroFromOption{cmd,withTotalOption,subListOutput}
3482 @itemx --files0-from=@var{file}
3483 @opindex --files0-from=@var{file}
3484 @c This is commented out to avoid a texi2dvi failure.
3485 @c texi2dvi (GNU Texinfo 4.11) 1.104
3486 @c @cindex including files from @command{\cmd\}
3487 Disallow processing files named on the command line, and instead process
3488 those named in file @var{file}; each name being terminated by a zero byte
3489 (@acronym{ASCII} @sc{nul}).
3490 This is useful \withTotalOption\
3491 when the list of file names is so long that it may exceed a command line
3493 In such cases, running @command{\cmd\} via @command{xargs} is undesirable
3494 because it splits the list into pieces and makes @command{\cmd\} print
3495 \subListOutput\ for each sublist rather than for the entire list.
3496 One way to produce a list of @acronym{ASCII} @sc{nul} terminated file
3497 names is with @sc{gnu}
3498 @command{find}, using its @option{-print0} predicate.
3499 If @var{file} is @samp{-} then the @acronym{ASCII} @sc{nul} terminated
3500 file names are read from standard input.
3502 @filesZeroFromOption{wc,,a total}
3504 For example, to find the length of the longest line in any @file{.c} or
3505 @file{.h} file in the current hierarchy, do this:
3508 find . -name '*.[ch]' -print0 |
3509 wc -L --files0-from=- | tail -n1
3517 @node sum invocation
3518 @section @command{sum}: Print checksum and block counts
3521 @cindex 16-bit checksum
3522 @cindex checksum, 16-bit
3524 @command{sum} computes a 16-bit checksum for each given @var{file}, or
3525 standard input if none are given or for a @var{file} of @samp{-}. Synopsis:
3528 sum [@var{option}]@dots{} [@var{file}]@dots{}
3531 @command{sum} prints the checksum for each @var{file} followed by the
3532 number of blocks in the file (rounded up). If more than one @var{file}
3533 is given, file names are also printed (by default). (With the
3534 @option{--sysv} option, corresponding file names are printed when there is
3535 at least one file argument.)
3537 By default, @sc{gnu} @command{sum} computes checksums using an algorithm
3538 compatible with BSD @command{sum} and prints file sizes in units of
3541 The program accepts the following options. Also see @ref{Common options}.
3547 @cindex BSD @command{sum}
3548 Use the default (BSD compatible) algorithm. This option is included for
3549 compatibility with the System V @command{sum}. Unless @option{-s} was also
3550 given, it has no effect.
3556 @cindex System V @command{sum}
3557 Compute checksums using an algorithm compatible with System V
3558 @command{sum}'s default, and print file sizes in units of 512-byte blocks.
3562 @command{sum} is provided for compatibility; the @command{cksum} program (see
3563 next section) is preferable in new applications.
3568 @node cksum invocation
3569 @section @command{cksum}: Print CRC checksum and byte counts
3572 @cindex cyclic redundancy check
3573 @cindex CRC checksum
3575 @command{cksum} computes a cyclic redundancy check (CRC) checksum for each
3576 given @var{file}, or standard input if none are given or for a
3577 @var{file} of @samp{-}. Synopsis:
3580 cksum [@var{option}]@dots{} [@var{file}]@dots{}
3583 @command{cksum} prints the CRC checksum for each file along with the number
3584 of bytes in the file, and the file name unless no arguments were given.
3586 @command{cksum} is typically used to ensure that files
3587 transferred by unreliable means (e.g., netnews) have not been corrupted,
3588 by comparing the @command{cksum} output for the received files with the
3589 @command{cksum} output for the original files (typically given in the
3592 The CRC algorithm is specified by the @acronym{POSIX} standard. It is not
3593 compatible with the BSD or System V @command{sum} algorithms (see the
3594 previous section); it is more robust.
3596 The only options are @option{--help} and @option{--version}. @xref{Common
3602 @node md5sum invocation
3603 @section @command{md5sum}: Print or check MD5 digests
3607 @cindex 128-bit checksum
3608 @cindex checksum, 128-bit
3609 @cindex fingerprint, 128-bit
3610 @cindex message-digest, 128-bit
3612 @command{md5sum} computes a 128-bit checksum (or @dfn{fingerprint} or
3613 @dfn{message-digest}) for each specified @var{file}.
3615 Note: The MD5 digest is more reliable than a simple CRC (provided by
3616 the @command{cksum} command) for detecting accidental file corruption,
3617 as the chances of accidentally having two files with identical MD5
3618 are vanishingly small. However, it should not be considered secure
3619 against malicious tampering: although finding a file with a given MD5
3620 fingerprint is considered infeasible at the moment, it is known how
3621 to modify certain files, including digital certificates, so that they
3622 appear valid when signed with an MD5 digest.
3623 For more secure hashes, consider using SHA-2. @xref{sha2 utilities}.
3625 If a @var{file} is specified as @samp{-} or if no files are given
3626 @command{md5sum} computes the checksum for the standard input.
3627 @command{md5sum} can also determine whether a file and checksum are
3628 consistent. Synopsis:
3631 md5sum [@var{option}]@dots{} [@var{file}]@dots{}
3634 For each @var{file}, @samp{md5sum} outputs the MD5 checksum, a flag
3635 indicating binary or text input mode, and the file name.
3636 If @var{file} contains a backslash or newline, the
3637 line is started with a backslash, and each problematic character in
3638 the file name is escaped with a backslash, making the output
3639 unambiguous even in the presence of arbitrary file names.
3640 If @var{file} is omitted or specified as @samp{-}, standard input is read.
3642 The program accepts the following options. Also see @ref{Common options}.
3650 @cindex binary input files
3651 Treat each input file as binary, by reading it in binary mode and
3652 outputting a @samp{*} flag. This is the inverse of @option{--text}.
3653 On systems like @acronym{GNU} that do not distinguish between binary
3654 and text files, this option merely flags each input mode as binary:
3655 the MD5 checksum is unaffected. This option is the default on systems
3656 like MS-DOS that distinguish between binary and text files, except
3657 for reading standard input when standard input is a terminal.
3661 Read file names and checksum information (not data) from each
3662 @var{file} (or from stdin if no @var{file} was specified) and report
3663 whether the checksums match the contents of the named files.
3664 The input to this mode of @command{md5sum} is usually the output of
3665 a prior, checksum-generating run of @samp{md5sum}.
3666 Each valid line of input consists of an MD5 checksum, a binary/text
3667 flag, and then a file name.
3668 Binary mode is indicated with @samp{*}, text with @samp{ } (space).
3669 For each such line, @command{md5sum} reads the named file and computes its
3670 MD5 checksum. Then, if the computed message digest does not match the
3671 one on the line with the file name, the file is noted as having
3672 failed the test. Otherwise, the file passes the test.
3673 By default, for each valid line, one line is written to standard
3674 output indicating whether the named file passed the test.
3675 After all checks have been performed, if there were any failures,
3676 a warning is issued to standard error.
3677 Use the @option{--status} option to inhibit that output.
3678 If any listed file cannot be opened or read, if any valid line has
3679 an MD5 checksum inconsistent with the associated file, or if no valid
3680 line is found, @command{md5sum} exits with nonzero status. Otherwise,
3681 it exits successfully.
3685 @cindex verifying MD5 checksums
3686 This option is useful only when verifying checksums.
3687 When verifying checksums, don't generate an 'OK' message per successfully
3688 checked file. Files that fail the verification are reported in the
3689 default one-line-per-file format. If there is any checksum mismatch,
3690 print a warning summarizing the failures to standard error.
3694 @cindex verifying MD5 checksums
3695 This option is useful only when verifying checksums.
3696 When verifying checksums, don't generate the default one-line-per-file
3697 diagnostic and don't output the warning summarizing any failures.
3698 Failures to open or read a file still evoke individual diagnostics to
3700 If all listed files are readable and are consistent with the associated
3701 MD5 checksums, exit successfully. Otherwise exit with a status code
3702 indicating there was a failure.
3708 @cindex text input files
3709 Treat each input file as text, by reading it in text mode and
3710 outputting a @samp{ } flag. This is the inverse of @option{--binary}.
3711 This option is the default on systems like @acronym{GNU} that do not
3712 distinguish between binary and text files. On other systems, it is
3713 the default for reading standard input when standard input is a
3720 @cindex verifying MD5 checksums
3721 When verifying checksums, warn about improperly formatted MD5 checksum lines.
3722 This option is useful only if all but a few lines in the checked input
3727 @cindex verifying MD5 checksums
3728 When verifying checksums,
3729 if one or more input line is invalid,
3730 exit nonzero after all warnings have been issued.
3737 @node sha1sum invocation
3738 @section @command{sha1sum}: Print or check SHA-1 digests
3742 @cindex 160-bit checksum
3743 @cindex checksum, 160-bit
3744 @cindex fingerprint, 160-bit
3745 @cindex message-digest, 160-bit
3747 @command{sha1sum} computes a 160-bit checksum for each specified
3748 @var{file}. The usage and options of this command are precisely the
3749 same as for @command{md5sum}. @xref{md5sum invocation}.
3751 Note: The SHA-1 digest is more secure than MD5, and no collisions of
3752 it are known (different files having the same fingerprint). However,
3753 it is known that they can be produced with considerable, but not
3754 unreasonable, resources. For this reason, it is generally considered
3755 that SHA-1 should be gradually phased out in favor of the more secure
3756 SHA-2 hash algorithms. @xref{sha2 utilities}.
3759 @node sha2 utilities
3760 @section sha2 utilities: Print or check SHA-2 digests
3767 @cindex 224-bit checksum
3768 @cindex 256-bit checksum
3769 @cindex 384-bit checksum
3770 @cindex 512-bit checksum
3771 @cindex checksum, 224-bit
3772 @cindex checksum, 256-bit
3773 @cindex checksum, 384-bit
3774 @cindex checksum, 512-bit
3775 @cindex fingerprint, 224-bit
3776 @cindex fingerprint, 256-bit
3777 @cindex fingerprint, 384-bit
3778 @cindex fingerprint, 512-bit
3779 @cindex message-digest, 224-bit
3780 @cindex message-digest, 256-bit
3781 @cindex message-digest, 384-bit
3782 @cindex message-digest, 512-bit
3784 The commands @command{sha224sum}, @command{sha256sum},
3785 @command{sha384sum} and @command{sha512sum} compute checksums of
3786 various lengths (respectively 224, 256, 384 and 512 bits),
3787 collectively known as the SHA-2 hashes. The usage and options of
3788 these commands are precisely the same as for @command{md5sum}.
3789 @xref{md5sum invocation}.
3791 Note: The SHA384 and SHA512 digests are considerably slower to
3792 compute, especially on 32-bit computers, than SHA224 or SHA256.
3795 @node Operating on sorted files
3796 @chapter Operating on sorted files
3798 @cindex operating on sorted files
3799 @cindex sorted files, operations on
3801 These commands work with (or produce) sorted files.
3804 * sort invocation:: Sort text files.
3805 * shuf invocation:: Shuffle text files.
3806 * uniq invocation:: Uniquify files.
3807 * comm invocation:: Compare two sorted files line by line.
3808 * ptx invocation:: Produce a permuted index of file contents.
3809 * tsort invocation:: Topological sort.
3813 @node sort invocation
3814 @section @command{sort}: Sort text files
3817 @cindex sorting files
3819 @command{sort} sorts, merges, or compares all the lines from the given
3820 files, or standard input if none are given or for a @var{file} of
3821 @samp{-}. By default, @command{sort} writes the results to standard
3825 sort [@var{option}]@dots{} [@var{file}]@dots{}
3828 @command{sort} has three modes of operation: sort (the default), merge,
3829 and check for sortedness. The following options change the operation
3836 @itemx --check=diagnose-first
3839 @cindex checking for sortedness
3840 Check whether the given file is already sorted: if it is not all
3841 sorted, print a diagnostic containing the first out-of-order line and
3842 exit with a status of 1.
3843 Otherwise, exit successfully.
3844 At most one input file can be given.
3847 @itemx --check=quiet
3848 @itemx --check=silent
3851 @cindex checking for sortedness
3852 Exit successfully if the given file is already sorted, and
3853 exit with status 1 otherwise.
3854 At most one input file can be given.
3855 This is like @option{-c}, except it does not print a diagnostic.
3861 @cindex merging sorted files
3862 Merge the given files by sorting them as a group. Each input file must
3863 always be individually sorted. It always works to sort instead of
3864 merge; merging is provided because it is faster, in the case where it
3869 @cindex sort stability
3870 @cindex sort's last-resort comparison
3871 A pair of lines is compared as follows:
3872 @command{sort} compares each pair of fields, in the
3873 order specified on the command line, according to the associated
3874 ordering options, until a difference is found or no fields are left.
3875 If no key fields are specified, @command{sort} uses a default key of
3876 the entire line. Finally, as a last resort when all keys compare
3877 equal, @command{sort} compares entire lines as if no ordering options
3878 other than @option{--reverse} (@option{-r}) were specified. The
3879 @option{--stable} (@option{-s}) option disables this @dfn{last-resort
3880 comparison} so that lines in which all fields compare equal are left
3881 in their original relative order. The @option{--unique}
3882 (@option{-u}) option also disables the last-resort comparison.
3886 Unless otherwise specified, all comparisons use the character collating
3887 sequence specified by the @env{LC_COLLATE} locale.@footnote{If you
3888 use a non-@acronym{POSIX} locale (e.g., by setting @env{LC_ALL}
3889 to @samp{en_US}), then @command{sort} may produce output that is sorted
3890 differently than you're accustomed to. In that case, set the @env{LC_ALL}
3891 environment variable to @samp{C}. Note that setting only @env{LC_COLLATE}
3892 has two problems. First, it is ineffective if @env{LC_ALL} is also set.
3893 Second, it has undefined behavior if @env{LC_CTYPE} (or @env{LANG}, if
3894 @env{LC_CTYPE} is unset) is set to an incompatible value. For example,
3895 you get undefined behavior if @env{LC_CTYPE} is @code{ja_JP.PCK} but
3896 @env{LC_COLLATE} is @code{en_US.UTF-8}.}
3898 @sc{gnu} @command{sort} (as specified for all @sc{gnu} utilities) has no
3899 limit on input line length or restrictions on bytes allowed within lines.
3900 In addition, if the final byte of an input file is not a newline, @sc{gnu}
3901 @command{sort} silently supplies one. A line's trailing newline is not
3902 part of the line for comparison purposes.
3904 @cindex exit status of @command{sort}
3908 0 if no error occurred
3909 1 if invoked with @option{-c} or @option{-C} and the input is not sorted
3910 2 if an error occurred
3914 If the environment variable @env{TMPDIR} is set, @command{sort} uses its
3915 value as the directory for temporary files instead of @file{/tmp}. The
3916 @option{--temporary-directory} (@option{-T}) option in turn overrides
3917 the environment variable.
3919 The following options affect the ordering of output lines. They may be
3920 specified globally or as part of a specific key field. If no key
3921 fields are specified, global options apply to comparison of entire
3922 lines; otherwise the global options are inherited by key fields that do
3923 not specify any special options of their own. In pre-@acronym{POSIX}
3924 versions of @command{sort}, global options affect only later key fields,
3925 so portable shell scripts should specify global options first.
3930 @itemx --ignore-leading-blanks
3932 @opindex --ignore-leading-blanks
3933 @cindex blanks, ignoring leading
3935 Ignore leading blanks when finding sort keys in each line.
3936 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3937 can change this. Note blanks may be ignored by your locale's collating
3938 rules, but without this option they will be significant for character
3939 positions specified in keys with the @option{-k} option.
3942 @itemx --dictionary-order
3944 @opindex --dictionary-order
3945 @cindex dictionary order
3946 @cindex phone directory order
3947 @cindex telephone directory order
3949 Sort in @dfn{phone directory} order: ignore all characters except
3950 letters, digits and blanks when sorting.
3951 By default letters and digits are those of @acronym{ASCII} and a blank
3952 is a space or a tab, but the @env{LC_CTYPE} locale can change this.
3955 @itemx --ignore-case
3957 @opindex --ignore-case
3958 @cindex ignoring case
3959 @cindex case folding
3961 Fold lowercase characters into the equivalent uppercase characters when
3962 comparing so that, for example, @samp{b} and @samp{B} sort as equal.
3963 The @env{LC_CTYPE} locale determines character types.
3964 When used with @option{--unique} those lower case equivalent lines are
3965 thrown away. (There is currently no way to throw away the upper case
3966 equivalent instead. (Any @option{--reverse} given would only affect
3967 the final result, after the throwing away.))
3970 @itemx --general-numeric-sort
3971 @itemx --sort=general-numeric
3973 @opindex --general-numeric-sort
3975 @cindex general numeric sort
3977 Sort numerically, converting a prefix of each line to a long
3978 double-precision floating point number. @xref{Floating point}.
3979 Do not report overflow, underflow, or conversion errors.
3980 Use the following collating sequence:
3984 Lines that do not start with numbers (all considered to be equal).
3986 NaNs (``Not a Number'' values, in IEEE floating point arithmetic)
3987 in a consistent but machine-dependent order.
3991 Finite numbers in ascending numeric order (with @math{-0} and @math{+0} equal).
3996 Use this option only if there is no alternative; it is much slower than
3997 @option{--numeric-sort} (@option{-n}) and it can lose information when
3998 converting to floating point.
4001 @itemx --human-numeric-sort
4002 @itemx --sort=human-numeric
4004 @opindex --human-numeric-sort
4006 @cindex human numeric sort
4008 Sort numerically, first by numeric sign (negative, zero, or positive);
4009 then by @acronym{SI} suffix (either empty, or @samp{k} or @samp{K}, or
4010 one of @samp{MGTPEZY}, in that order; @pxref{Block size}); and finally
4011 by numeric value. For example, @samp{1023M} sorts before @samp{1G}
4012 because @samp{M} (mega) precedes @samp{G} (giga) as an @acronym{SI}
4013 suffix. This option sorts values that are consistently scaled to the
4014 nearest suffix, regardless of whether suffixes denote powers of 1000
4015 or 1024, and it therefore sorts the output of any single invocation of
4016 the @command{df}, @command{du}, or @command{ls} commands that are
4017 invoked with their @option{--human-readable} or @option{--si} options.
4018 The syntax for numbers is the same as for the @option{--numeric-sort}
4019 option; the @acronym{SI} suffix must immediately follow the number.
4022 @itemx --ignore-nonprinting
4024 @opindex --ignore-nonprinting
4025 @cindex nonprinting characters, ignoring
4026 @cindex unprintable characters, ignoring
4028 Ignore nonprinting characters.
4029 The @env{LC_CTYPE} locale determines character types.
4030 This option has no effect if the stronger @option{--dictionary-order}
4031 (@option{-d}) option is also given.
4037 @opindex --month-sort
4039 @cindex months, sorting by
4041 An initial string, consisting of any amount of blanks, followed
4042 by a month name abbreviation, is folded to UPPER case and
4043 compared in the order @samp{JAN} < @samp{FEB} < @dots{} < @samp{DEC}.
4044 Invalid names compare low to valid names. The @env{LC_TIME} locale
4045 category determines the month spellings.
4046 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4050 @itemx --numeric-sort
4051 @itemx --sort=numeric
4053 @opindex --numeric-sort
4055 @cindex numeric sort
4057 Sort numerically. The number begins each line and consists
4058 of optional blanks, an optional @samp{-} sign, and zero or more
4059 digits possibly separated by thousands separators, optionally followed
4060 by a decimal-point character and zero or more digits. An empty
4061 number is treated as @samp{0}. The @env{LC_NUMERIC}
4062 locale specifies the decimal-point character and thousands separator.
4063 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4066 Comparison is exact; there is no rounding error.
4068 Neither a leading @samp{+} nor exponential notation is recognized.
4069 To compare such strings numerically, use the
4070 @option{--general-numeric-sort} (@option{-g}) option.
4073 @itemx --version-sort
4075 @opindex --version-sort
4076 @cindex version number sort
4077 Sort by version name and number. It behaves like a standard sort,
4078 except that each sequence of decimal digits is treated numerically
4079 as an index/version number. (@xref{Details about version sort}.)
4085 @cindex reverse sorting
4086 Reverse the result of comparison, so that lines with greater key values
4087 appear earlier in the output instead of later.
4090 @itemx --random-sort
4091 @itemx --sort=random
4093 @opindex --random-sort
4096 Sort by hashing the input keys and then sorting the hash values.
4097 Choose the hash function at random, ensuring that it is free of
4098 collisions so that differing keys have differing hash values. This is
4099 like a random permutation of the inputs (@pxref{shuf invocation}),
4100 except that keys with the same value sort together.
4102 If multiple random sort fields are specified, the same random hash
4103 function is used for all fields. To use different random hash
4104 functions for different fields, you can invoke @command{sort} more
4107 The choice of hash function is affected by the
4108 @option{--random-source} option.
4116 @item --compress-program=@var{prog}
4117 Compress any temporary files with the program @var{prog}.
4119 With no arguments, @var{prog} must compress standard input to standard
4120 output, and when given the @option{-d} option it must decompress
4121 standard input to standard output.
4123 Terminate with an error if @var{prog} exits with nonzero status.
4125 White space and the backslash character should not appear in
4126 @var{prog}; they are reserved for future use.
4128 @filesZeroFromOption{sort,,sorted output}
4130 @item -k @var{pos1}[,@var{pos2}]
4131 @itemx --key=@var{pos1}[,@var{pos2}]
4135 Specify a sort field that consists of the part of the line between
4136 @var{pos1} and @var{pos2} (or the end of the line, if @var{pos2} is
4137 omitted), @emph{inclusive}.
4139 Each @var{pos} has the form @samp{@var{f}[.@var{c}][@var{opts}]},
4140 where @var{f} is the number of the field to use, and @var{c} is the number
4141 of the first character from the beginning of the field. Fields and character
4142 positions are numbered starting with 1; a character position of zero in
4143 @var{pos2} indicates the field's last character. If @samp{.@var{c}} is
4144 omitted from @var{pos1}, it defaults to 1 (the beginning of the field);
4145 if omitted from @var{pos2}, it defaults to 0 (the end of the field).
4146 @var{opts} are ordering options, allowing individual keys to be sorted
4147 according to different rules; see below for details. Keys can span
4150 Example: To sort on the second field, use @option{--key=2,2}
4151 (@option{-k 2,2}). See below for more notes on keys and more examples.
4152 See also the @option{--debug} option to help determine the part
4153 of the line being used in the sort.
4156 Highlight the portion of each line used for sorting.
4157 Also issue warnings about questionable usage to stderr.
4159 @item --batch-size=@var{nmerge}
4160 @opindex --batch-size
4161 @cindex number of inputs to merge, nmerge
4162 Merge at most @var{nmerge} inputs at once.
4164 When @command{sort} has to merge more than @var{nmerge} inputs,
4165 it merges them in groups of @var{nmerge}, saving the result in
4166 a temporary file, which is then used as an input in a subsequent merge.
4168 A large value of @var{nmerge} may improve merge performance and decrease
4169 temporary storage utilization at the expense of increased memory usage
4170 and I/O. Conversely a small value of @var{nmerge} may reduce memory
4171 requirements and I/O at the expense of temporary storage consumption and
4174 The value of @var{nmerge} must be at least 2. The default value is
4175 currently 16, but this is implementation-dependent and may change in
4178 The value of @var{nmerge} may be bounded by a resource limit for open
4179 file descriptors. The commands @samp{ulimit -n} or @samp{getconf
4180 OPEN_MAX} may display limits for your systems; these limits may be
4181 modified further if your program already has some files open, or if
4182 the operating system has other limits on the number of open files. If
4183 the value of @var{nmerge} exceeds the resource limit, @command{sort}
4184 silently uses a smaller value.
4186 @item -o @var{output-file}
4187 @itemx --output=@var{output-file}
4190 @cindex overwriting of input, allowed
4191 Write output to @var{output-file} instead of standard output.
4192 Normally, @command{sort} reads all input before opening
4193 @var{output-file}, so you can safely sort a file in place by using
4194 commands like @code{sort -o F F} and @code{cat F | sort -o F}.
4195 However, @command{sort} with @option{--merge} (@option{-m}) can open
4196 the output file before reading all input, so a command like @code{cat
4197 F | sort -m -o F - G} is not safe as @command{sort} might start
4198 writing @file{F} before @command{cat} is done reading it.
4200 @vindex POSIXLY_CORRECT
4201 On newer systems, @option{-o} cannot appear after an input file if
4202 @env{POSIXLY_CORRECT} is set, e.g., @samp{sort F -o F}. Portable
4203 scripts should specify @option{-o @var{output-file}} before any input
4206 @item --random-source=@var{file}
4207 @opindex --random-source
4208 @cindex random source for sorting
4209 Use @var{file} as a source of random data used to determine which
4210 random hash function to use with the @option{-R} option. @xref{Random
4217 @cindex sort stability
4218 @cindex sort's last-resort comparison
4220 Make @command{sort} stable by disabling its last-resort comparison.
4221 This option has no effect if no fields or global ordering options
4222 other than @option{--reverse} (@option{-r}) are specified.
4225 @itemx --buffer-size=@var{size}
4227 @opindex --buffer-size
4228 @cindex size for main memory sorting
4229 Use a main-memory sort buffer of the given @var{size}. By default,
4230 @var{size} is in units of 1024 bytes. Appending @samp{%} causes
4231 @var{size} to be interpreted as a percentage of physical memory.
4232 Appending @samp{K} multiplies @var{size} by 1024 (the default),
4233 @samp{M} by 1,048,576, @samp{G} by 1,073,741,824, and so on for
4234 @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}. Appending
4235 @samp{b} causes @var{size} to be interpreted as a byte count, with no
4238 This option can improve the performance of @command{sort} by causing it
4239 to start with a larger or smaller sort buffer than the default.
4240 However, this option affects only the initial buffer size. The buffer
4241 grows beyond @var{size} if @command{sort} encounters input lines larger
4244 @item -t @var{separator}
4245 @itemx --field-separator=@var{separator}
4247 @opindex --field-separator
4248 @cindex field separator character
4249 Use character @var{separator} as the field separator when finding the
4250 sort keys in each line. By default, fields are separated by the empty
4251 string between a non-blank character and a blank character.
4252 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4255 That is, given the input line @w{@samp{ foo bar}}, @command{sort} breaks it
4256 into fields @w{@samp{ foo}} and @w{@samp{ bar}}. The field separator is
4257 not considered to be part of either the field preceding or the field
4258 following, so with @samp{sort @w{-t " "}} the same input line has
4259 three fields: an empty field, @samp{foo}, and @samp{bar}.
4260 However, fields that extend to the end of the line,
4261 as @option{-k 2}, or fields consisting of a range, as @option{-k 2,3},
4262 retain the field separators present between the endpoints of the range.
4264 To specify @acronym{ASCII} @sc{nul} as the field separator,
4265 use the two-character string @samp{\0}, e.g., @samp{sort -t '\0'}.
4267 @item -T @var{tempdir}
4268 @itemx --temporary-directory=@var{tempdir}
4270 @opindex --temporary-directory
4271 @cindex temporary directory
4273 Use directory @var{tempdir} to store temporary files, overriding the
4274 @env{TMPDIR} environment variable. If this option is given more than
4275 once, temporary files are stored in all the directories given. If you
4276 have a large sort or merge that is I/O-bound, you can often improve
4277 performance by using this option to specify directories on different
4278 disks and controllers.
4280 @item --parallel=@var{n}
4282 @cindex multithreaded sort
4283 Set the number of sorts run in parallel to @var{n}. By default,
4284 @var{n} is set to the number of available processors, but limited
4285 to 8, as there are diminishing performance gains after that.
4286 Note also that using @var{n} threads increases the memory usage by
4287 a factor of log @var{n}. Also see @ref{nproc invocation}.
4293 @cindex uniquifying output
4295 Normally, output only the first of a sequence of lines that compare
4296 equal. For the @option{--check} (@option{-c} or @option{-C}) option,
4297 check that no pair of consecutive lines compares equal.
4299 This option also disables the default last-resort comparison.
4301 The commands @code{sort -u} and @code{sort | uniq} are equivalent, but
4302 this equivalence does not extend to arbitrary @command{sort} options.
4303 For example, @code{sort -n -u} inspects only the value of the initial
4304 numeric string when checking for uniqueness, whereas @code{sort -n |
4305 uniq} inspects the entire line. @xref{uniq invocation}.
4307 @macro zeroTerminatedOption
4309 @itemx --zero-terminated
4311 @opindex --zero-terminated
4312 @cindex process zero-terminated items
4313 Delimit items with a zero byte rather than a newline (@acronym{ASCII} @sc{lf}).
4314 I.E. treat input as items separated by @acronym{ASCII} @sc{nul}
4315 and terminate output items with @acronym{ASCII} @sc{nul}.
4316 This option can be useful in conjunction with @samp{perl -0} or
4317 @samp{find -print0} and @samp{xargs -0} which do the same in order to
4318 reliably handle arbitrary file names (even those containing blanks
4319 or other special characters).
4321 @zeroTerminatedOption
4325 Historical (BSD and System V) implementations of @command{sort} have
4326 differed in their interpretation of some options, particularly
4327 @option{-b}, @option{-f}, and @option{-n}.
4328 @sc{gnu} sort follows the @acronym{POSIX}
4329 behavior, which is usually (but not always!) like the System V behavior.
4330 According to @acronym{POSIX}, @option{-n} no longer implies @option{-b}. For
4331 consistency, @option{-M} has been changed in the same way. This may
4332 affect the meaning of character positions in field specifications in
4333 obscure cases. The only fix is to add an explicit @option{-b}.
4335 A position in a sort field specified with @option{-k} may have any
4336 of the option letters @samp{MbdfghinRrV} appended to it, in which case no
4337 global ordering options are inherited by that particular field. The
4338 @option{-b} option may be independently attached to either or both of
4339 the start and end positions of a field specification, and if it is
4340 inherited from the global options it will be attached to both.
4341 If input lines can contain leading or adjacent blanks and @option{-t}
4342 is not used, then @option{-k} is typically combined with @option{-b} or
4343 an option that implicitly ignores leading blanks (@samp{Mghn}) as otherwise
4344 the varying numbers of leading blanks in fields can cause confusing results.
4346 If the start position in a sort field specifier falls after the end of
4347 the line or after the end field, the field is empty. If the @option{-b}
4348 option was specified, the @samp{.@var{c}} part of a field specification
4349 is counted from the first nonblank character of the field.
4351 @vindex _POSIX2_VERSION
4352 @vindex POSIXLY_CORRECT
4353 On older systems, @command{sort} supports an obsolete origin-zero
4354 syntax @samp{+@var{pos1} [-@var{pos2}]} for specifying sort keys.
4355 The obsolete sequence @samp{sort +@var{a}.@var{x} -@var{b}.@var{y}}
4356 is equivalent to @samp{sort -k @var{a+1}.@var{x+1},@var{b}} if @var{y}
4357 is @samp{0} or absent, otherwise it is equivalent to @samp{sort -k
4358 @var{a+1}.@var{x+1},@var{b+1}.@var{y}}.
4360 This obsolete behavior can be enabled or disabled with the
4361 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4362 conformance}); it can also be enabled when @env{POSIXLY_CORRECT} is
4363 not set by using the obsolete syntax with @samp{-@var{pos2}} present.
4365 Scripts intended for use on standard hosts should avoid obsolete
4366 syntax and should use @option{-k} instead. For example, avoid
4367 @samp{sort +2}, since it might be interpreted as either @samp{sort
4368 ./+2} or @samp{sort -k 3}. If your script must also run on hosts that
4369 support only the obsolete syntax, it can use a test like @samp{if sort
4370 -k 1 </dev/null >/dev/null 2>&1; then @dots{}} to decide which syntax
4373 Here are some examples to illustrate various combinations of options.
4378 Sort in descending (reverse) numeric order.
4385 Run no more than 4 sorts concurrently, using a buffer size of 10M.
4388 sort --parallel=4 -S 10M
4392 Sort alphabetically, omitting the first and second fields
4393 and the blanks at the start of the third field.
4394 This uses a single key composed of the characters beginning
4395 at the start of the first nonblank character in field three
4396 and extending to the end of each line.
4403 Sort numerically on the second field and resolve ties by sorting
4404 alphabetically on the third and fourth characters of field five.
4405 Use @samp{:} as the field delimiter.
4408 sort -t : -k 2,2n -k 5.3,5.4
4411 Note that if you had written @option{-k 2n} instead of @option{-k 2,2n}
4412 @command{sort} would have used all characters beginning in the second field
4413 and extending to the end of the line as the primary @emph{numeric}
4414 key. For the large majority of applications, treating keys spanning
4415 more than one field as numeric will not do what you expect.
4417 Also note that the @samp{n} modifier was applied to the field-end
4418 specifier for the first key. It would have been equivalent to
4419 specify @option{-k 2n,2} or @option{-k 2n,2n}. All modifiers except
4420 @samp{b} apply to the associated @emph{field}, regardless of whether
4421 the modifier character is attached to the field-start and/or the
4422 field-end part of the key specifier.
4425 Sort the password file on the fifth field and ignore any
4426 leading blanks. Sort lines with equal values in field five
4427 on the numeric user ID in field three. Fields are separated
4431 sort -t : -k 5b,5 -k 3,3n /etc/passwd
4432 sort -t : -n -k 5b,5 -k 3,3 /etc/passwd
4433 sort -t : -b -k 5,5 -k 3,3n /etc/passwd
4436 These three commands have equivalent effect. The first specifies that
4437 the first key's start position ignores leading blanks and the second
4438 key is sorted numerically. The other two commands rely on global
4439 options being inherited by sort keys that lack modifiers. The inheritance
4440 works in this case because @option{-k 5b,5b} and @option{-k 5b,5} are
4441 equivalent, as the location of a field-end lacking a @samp{.@var{c}}
4442 character position is not affected by whether initial blanks are
4446 Sort a set of log files, primarily by IPv4 address and secondarily by
4447 time stamp. If two lines' primary and secondary keys are identical,
4448 output the lines in the same order that they were input. The log
4449 files contain lines that look like this:
4452 4.150.156.3 - - [01/Apr/2004:06:31:51 +0000] message 1
4453 211.24.3.231 - - [24/Apr/2004:20:17:39 +0000] message 2
4456 Fields are separated by exactly one space. Sort IPv4 addresses
4457 lexicographically, e.g., 212.61.52.2 sorts before 212.129.233.201
4458 because 61 is less than 129.
4461 sort -s -t ' ' -k 4.9n -k 4.5M -k 4.2n -k 4.14,4.21 file*.log |
4462 sort -s -t '.' -k 1,1n -k 2,2n -k 3,3n -k 4,4n
4465 This example cannot be done with a single @command{sort} invocation,
4466 since IPv4 address components are separated by @samp{.} while dates
4467 come just after a space. So it is broken down into two invocations of
4468 @command{sort}: the first sorts by time stamp and the second by IPv4
4469 address. The time stamp is sorted by year, then month, then day, and
4470 finally by hour-minute-second field, using @option{-k} to isolate each
4471 field. Except for hour-minute-second there's no need to specify the
4472 end of each key field, since the @samp{n} and @samp{M} modifiers sort
4473 based on leading prefixes that cannot cross field boundaries. The
4474 IPv4 addresses are sorted lexicographically. The second sort uses
4475 @samp{-s} so that ties in the primary key are broken by the secondary
4476 key; the first sort uses @samp{-s} so that the combination of the two
4480 Generate a tags file in case-insensitive sorted order.
4483 find src -type f -print0 | sort -z -f | xargs -0 etags --append
4486 The use of @option{-print0}, @option{-z}, and @option{-0} in this case means
4487 that file names that contain blanks or other special characters are
4489 by the sort operation.
4491 @c This example is a bit contrived and needs more explanation.
4493 @c Sort records separated by an arbitrary string by using a pipe to convert
4494 @c each record delimiter string to @samp{\0}, then using sort's -z option,
4495 @c and converting each @samp{\0} back to the original record delimiter.
4498 @c printf 'c\n\nb\n\na\n' |
4499 @c perl -0pe 's/\n\n/\n\0/g' |
4501 @c perl -0pe 's/\0/\n/g'
4505 Use the common @acronym{DSU, Decorate Sort Undecorate} idiom to
4506 sort lines according to their length.
4509 awk '@{print length, $0@}' /etc/passwd | sort -n | cut -f2- -d' '
4512 In general this technique can be used to sort data that the @command{sort}
4513 command does not support, or is inefficient at, sorting directly.
4516 Shuffle a list of directories, but preserve the order of files within
4517 each directory. For instance, one could use this to generate a music
4518 playlist in which albums are shuffled but the songs of each album are
4522 ls */* | sort -t / -k 1,1R -k 2,2
4528 @node shuf invocation
4529 @section @command{shuf}: Shuffling text
4532 @cindex shuffling files
4534 @command{shuf} shuffles its input by outputting a random permutation
4535 of its input lines. Each output permutation is equally likely.
4539 shuf [@var{option}]@dots{} [@var{file}]
4540 shuf -e [@var{option}]@dots{} [@var{arg}]@dots{}
4541 shuf -i @var{lo}-@var{hi} [@var{option}]@dots{}
4544 @command{shuf} has three modes of operation that affect where it
4545 obtains its input lines. By default, it reads lines from standard
4546 input. The following options change the operation mode:
4554 @cindex command-line operands to shuffle
4555 Treat each command-line operand as an input line.
4557 @item -i @var{lo}-@var{hi}
4558 @itemx --input-range=@var{lo}-@var{hi}
4560 @opindex --input-range
4561 @cindex input range to shuffle
4562 Act as if input came from a file containing the range of unsigned
4563 decimal integers @var{lo}@dots{}@var{hi}, one per line.
4567 @command{shuf}'s other options can affect its behavior in all
4572 @item -n @var{lines}
4573 @itemx --head-count=@var{count}
4575 @opindex --head-count
4576 @cindex head of output
4577 Output at most @var{count} lines. By default, all input lines are
4580 @item -o @var{output-file}
4581 @itemx --output=@var{output-file}
4584 @cindex overwriting of input, allowed
4585 Write output to @var{output-file} instead of standard output.
4586 @command{shuf} reads all input before opening
4587 @var{output-file}, so you can safely shuffle a file in place by using
4588 commands like @code{shuf -o F <F} and @code{cat F | shuf -o F}.
4590 @item --random-source=@var{file}
4591 @opindex --random-source
4592 @cindex random source for shuffling
4593 Use @var{file} as a source of random data used to determine which
4594 permutation to generate. @xref{Random sources}.
4596 @zeroTerminatedOption
4612 might produce the output
4622 Similarly, the command:
4625 shuf -e clubs hearts diamonds spades
4639 and the command @samp{shuf -i 1-4} might output:
4649 These examples all have four input lines, so @command{shuf} might
4650 produce any of the twenty-four possible permutations of the input. In
4651 general, if there are @var{n} input lines, there are @var{n}! (i.e.,
4652 @var{n} factorial, or @var{n} * (@var{n} - 1) * @dots{} * 1) possible
4653 output permutations.
4658 @node uniq invocation
4659 @section @command{uniq}: Uniquify files
4662 @cindex uniquify files
4664 @command{uniq} writes the unique lines in the given @file{input}, or
4665 standard input if nothing is given or for an @var{input} name of
4669 uniq [@var{option}]@dots{} [@var{input} [@var{output}]]
4672 By default, @command{uniq} prints its input lines, except that
4673 it discards all but the first of adjacent repeated lines, so that
4674 no output lines are repeated. Optionally, it can instead discard
4675 lines that are not repeated, or all repeated lines.
4677 The input need not be sorted, but repeated input lines are detected
4678 only if they are adjacent. If you want to discard non-adjacent
4679 duplicate lines, perhaps you want to use @code{sort -u}.
4680 @xref{sort invocation}.
4683 Comparisons honor the rules specified by the @env{LC_COLLATE}
4686 If no @var{output} file is specified, @command{uniq} writes to standard
4689 The program accepts the following options. Also see @ref{Common options}.
4694 @itemx --skip-fields=@var{n}
4696 @opindex --skip-fields
4697 Skip @var{n} fields on each line before checking for uniqueness. Use
4698 a null string for comparison if a line has fewer than @var{n} fields. Fields
4699 are sequences of non-space non-tab characters that are separated from
4700 each other by at least one space or tab.
4702 For compatibility @command{uniq} supports an obsolete option syntax
4703 @option{-@var{n}}. New scripts should use @option{-f @var{n}} instead.
4706 @itemx --skip-chars=@var{n}
4708 @opindex --skip-chars
4709 Skip @var{n} characters before checking for uniqueness. Use a null string
4710 for comparison if a line has fewer than @var{n} characters. If you use both
4711 the field and character skipping options, fields are skipped over first.
4713 @vindex _POSIX2_VERSION
4714 On older systems, @command{uniq} supports an obsolete option syntax
4716 This obsolete behavior can be enabled or disabled with the
4717 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4718 conformance}), but portable scripts should avoid commands whose
4719 behavior depends on this variable.
4720 For example, use @samp{uniq ./+10} or @samp{uniq -s 10} rather than
4721 the ambiguous @samp{uniq +10}.
4727 Print the number of times each line occurred along with the line.
4730 @itemx --ignore-case
4732 @opindex --ignore-case
4733 Ignore differences in case when comparing lines.
4739 @cindex repeated lines, outputting
4740 Discard lines that are not repeated. When used by itself, this option
4741 causes @command{uniq} to print the first copy of each repeated line,
4745 @itemx --all-repeated[=@var{delimit-method}]
4747 @opindex --all-repeated
4748 @cindex all repeated lines, outputting
4749 Do not discard the second and subsequent repeated input lines,
4750 but discard lines that are not repeated.
4751 This option is useful mainly in conjunction with other options e.g.,
4752 to ignore case or to compare only selected fields.
4753 The optional @var{delimit-method} tells how to delimit
4754 groups of repeated lines, and must be one of the following:
4759 Do not delimit groups of repeated lines.
4760 This is equivalent to @option{--all-repeated} (@option{-D}).
4763 Output a newline before each group of repeated lines.
4764 With @option{--zero-terminated} (@option{-z}), use a zero
4765 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4768 Separate groups of repeated lines with a single newline.
4769 With @option{--zero-terminated} (@option{-z}), use a zero
4770 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4771 This is the same as using @samp{prepend}, except that
4772 no delimiter is inserted before the first group, and hence
4773 may be better suited for output direct to users.
4776 Note that when groups are delimited and the input stream contains
4777 two or more consecutive blank lines, then the output is ambiguous.
4778 To avoid that, filter the input through @samp{tr -s '\n'} to replace
4779 each sequence of consecutive newlines with a single newline.
4781 This is a @sc{gnu} extension.
4782 @c FIXME: give an example showing *how* it's useful
4788 @cindex unique lines, outputting
4789 Discard the first repeated line. When used by itself, this option
4790 causes @command{uniq} to print unique lines, and nothing else.
4793 @itemx --check-chars=@var{n}
4795 @opindex --check-chars
4796 Compare at most @var{n} characters on each line (after skipping any specified
4797 fields and characters). By default the entire rest of the lines are
4800 @zeroTerminatedOption
4807 @node comm invocation
4808 @section @command{comm}: Compare two sorted files line by line
4811 @cindex line-by-line comparison
4812 @cindex comparing sorted files
4814 @command{comm} writes to standard output lines that are common, and lines
4815 that are unique, to two input files; a file name of @samp{-} means
4816 standard input. Synopsis:
4819 comm [@var{option}]@dots{} @var{file1} @var{file2}
4823 Before @command{comm} can be used, the input files must be sorted using the
4824 collating sequence specified by the @env{LC_COLLATE} locale.
4825 If an input file ends in a non-newline
4826 character, a newline is silently appended. The @command{sort} command with
4827 no options always outputs a file that is suitable input to @command{comm}.
4829 @cindex differing lines
4830 @cindex common lines
4831 With no options, @command{comm} produces three-column output. Column one
4832 contains lines unique to @var{file1}, column two contains lines unique
4833 to @var{file2}, and column three contains lines common to both files.
4834 Columns are separated by a single TAB character.
4835 @c FIXME: when there's an option to supply an alternative separator
4836 @c string, append "by default" to the above sentence.
4841 The options @option{-1}, @option{-2}, and @option{-3} suppress printing of
4842 the corresponding columns (and separators). Also see @ref{Common options}.
4844 Unlike some other comparison utilities, @command{comm} has an exit
4845 status that does not depend on the result of the comparison.
4846 Upon normal completion @command{comm} produces an exit code of zero.
4847 If there is an error it exits with nonzero status.
4849 @macro checkOrderOption{cmd}
4850 If the @option{--check-order} option is given, unsorted inputs will
4851 cause a fatal error message. If the option @option{--nocheck-order}
4852 is given, unsorted inputs will never cause an error message. If neither
4853 of these options is given, wrongly sorted inputs are diagnosed
4854 only if an input file is found to contain unpairable
4856 lines, and when both input files are non empty.
4858 @ifclear JOIN_COMMAND
4861 If an input file is diagnosed as being unsorted, the @command{\cmd\}
4862 command will exit with a nonzero status (and the output should not be used).
4864 Forcing @command{\cmd\} to process wrongly sorted input files
4865 containing unpairable lines by specifying @option{--nocheck-order} is
4866 not guaranteed to produce any particular output. The output will
4867 probably not correspond with whatever you hoped it would be.
4869 @checkOrderOption{comm}
4874 Fail with an error message if either input file is wrongly ordered.
4876 @item --nocheck-order
4877 Do not check that both input files are in sorted order.
4881 @item --output-delimiter=@var{str}
4882 Print @var{str} between adjacent output columns,
4883 rather than the default of a single TAB character.
4885 The delimiter @var{str} may not be empty.
4889 @node ptx invocation
4890 @section @command{ptx}: Produce permuted indexes
4894 @command{ptx} reads a text file and essentially produces a permuted index, with
4895 each keyword in its context. The calling sketch is either one of:
4898 ptx [@var{option} @dots{}] [@var{file} @dots{}]
4899 ptx -G [@var{option} @dots{}] [@var{input} [@var{output}]]
4902 The @option{-G} (or its equivalent: @option{--traditional}) option disables
4903 all @sc{gnu} extensions and reverts to traditional mode, thus introducing some
4904 limitations and changing several of the program's default option values.
4905 When @option{-G} is not specified, @sc{gnu} extensions are always enabled.
4906 @sc{gnu} extensions to @command{ptx} are documented wherever appropriate in this
4907 document. For the full list, see @xref{Compatibility in ptx}.
4909 Individual options are explained in the following sections.
4911 When @sc{gnu} extensions are enabled, there may be zero, one or several
4912 @var{file}s after the options. If there is no @var{file}, the program
4913 reads the standard input. If there is one or several @var{file}s, they
4914 give the name of input files which are all read in turn, as if all the
4915 input files were concatenated. However, there is a full contextual
4916 break between each file and, when automatic referencing is requested,
4917 file names and line numbers refer to individual text input files. In
4918 all cases, the program outputs the permuted index to the standard
4921 When @sc{gnu} extensions are @emph{not} enabled, that is, when the program
4922 operates in traditional mode, there may be zero, one or two parameters
4923 besides the options. If there are no parameters, the program reads the
4924 standard input and outputs the permuted index to the standard output.
4925 If there is only one parameter, it names the text @var{input} to be read
4926 instead of the standard input. If two parameters are given, they give
4927 respectively the name of the @var{input} file to read and the name of
4928 the @var{output} file to produce. @emph{Be very careful} to note that,
4929 in this case, the contents of file given by the second parameter is
4930 destroyed. This behavior is dictated by System V @command{ptx}
4931 compatibility; @sc{gnu} Standards normally discourage output parameters not
4932 introduced by an option.
4934 Note that for @emph{any} file named as the value of an option or as an
4935 input text file, a single dash @kbd{-} may be used, in which case
4936 standard input is assumed. However, it would not make sense to use this
4937 convention more than once per program invocation.
4940 * General options in ptx:: Options which affect general program behavior.
4941 * Charset selection in ptx:: Underlying character set considerations.
4942 * Input processing in ptx:: Input fields, contexts, and keyword selection.
4943 * Output formatting in ptx:: Types of output format, and sizing the fields.
4944 * Compatibility in ptx::
4948 @node General options in ptx
4949 @subsection General options
4954 @itemx --traditional
4955 As already explained, this option disables all @sc{gnu} extensions to
4956 @command{ptx} and switches to traditional mode.
4959 Print a short help on standard output, then exit without further
4963 Print the program version on standard output, then exit without further
4971 @node Charset selection in ptx
4972 @subsection Charset selection
4974 @c FIXME: People don't necessarily know what an IBM-PC was these days.
4975 As it is set up now, the program assumes that the input file is coded
4976 using 8-bit @acronym{ISO} 8859-1 code, also known as Latin-1 character set,
4977 @emph{unless} it is compiled for MS-DOS, in which case it uses the
4978 character set of the IBM-PC@. (@sc{gnu} @command{ptx} is not known to work on
4979 smaller MS-DOS machines anymore.) Compared to 7-bit @acronym{ASCII}, the set
4980 of characters which are letters is different; this alters the behavior
4981 of regular expression matching. Thus, the default regular expression
4982 for a keyword allows foreign or diacriticized letters. Keyword sorting,
4983 however, is still crude; it obeys the underlying character set ordering
4989 @itemx --ignore-case
4990 Fold lower case letters to upper case for sorting.
4995 @node Input processing in ptx
4996 @subsection Word selection and input processing
5001 @itemx --break-file=@var{file}
5003 This option provides an alternative (to @option{-W}) method of describing
5004 which characters make up words. It introduces the name of a
5005 file which contains a list of characters which can@emph{not} be part of
5006 one word; this file is called the @dfn{Break file}. Any character which
5007 is not part of the Break file is a word constituent. If both options
5008 @option{-b} and @option{-W} are specified, then @option{-W} has precedence and
5009 @option{-b} is ignored.
5011 When @sc{gnu} extensions are enabled, the only way to avoid newline as a
5012 break character is to write all the break characters in the file with no
5013 newline at all, not even at the end of the file. When @sc{gnu} extensions
5014 are disabled, spaces, tabs and newlines are always considered as break
5015 characters even if not included in the Break file.
5018 @itemx --ignore-file=@var{file}
5020 The file associated with this option contains a list of words which will
5021 never be taken as keywords in concordance output. It is called the
5022 @dfn{Ignore file}. The file contains exactly one word in each line; the
5023 end of line separation of words is not subject to the value of the
5027 @itemx --only-file=@var{file}
5029 The file associated with this option contains a list of words which will
5030 be retained in concordance output; any word not mentioned in this file
5031 is ignored. The file is called the @dfn{Only file}. The file contains
5032 exactly one word in each line; the end of line separation of words is
5033 not subject to the value of the @option{-S} option.
5035 There is no default for the Only file. When both an Only file and an
5036 Ignore file are specified, a word is considered a keyword only
5037 if it is listed in the Only file and not in the Ignore file.
5042 On each input line, the leading sequence of non-white space characters will be
5043 taken to be a reference that has the purpose of identifying this input
5044 line in the resulting permuted index. For more information about reference
5045 production, see @xref{Output formatting in ptx}.
5046 Using this option changes the default value for option @option{-S}.
5048 Using this option, the program does not try very hard to remove
5049 references from contexts in output, but it succeeds in doing so
5050 @emph{when} the context ends exactly at the newline. If option
5051 @option{-r} is used with @option{-S} default value, or when @sc{gnu} extensions
5052 are disabled, this condition is always met and references are completely
5053 excluded from the output contexts.
5055 @item -S @var{regexp}
5056 @itemx --sentence-regexp=@var{regexp}
5058 This option selects which regular expression will describe the end of a
5059 line or the end of a sentence. In fact, this regular expression is not
5060 the only distinction between end of lines or end of sentences, and input
5061 line boundaries have no special significance outside this option. By
5062 default, when @sc{gnu} extensions are enabled and if @option{-r} option is not
5063 used, end of sentences are used. In this case, this @var{regex} is
5064 imported from @sc{gnu} Emacs:
5067 [.?!][]\"')@}]*\\($\\|\t\\| \\)[ \t\n]*
5070 Whenever @sc{gnu} extensions are disabled or if @option{-r} option is used, end
5071 of lines are used; in this case, the default @var{regexp} is just:
5077 Using an empty @var{regexp} is equivalent to completely disabling end of
5078 line or end of sentence recognition. In this case, the whole file is
5079 considered to be a single big line or sentence. The user might want to
5080 disallow all truncation flag generation as well, through option @option{-F
5081 ""}. @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
5084 When the keywords happen to be near the beginning of the input line or
5085 sentence, this often creates an unused area at the beginning of the
5086 output context line; when the keywords happen to be near the end of the
5087 input line or sentence, this often creates an unused area at the end of
5088 the output context line. The program tries to fill those unused areas
5089 by wrapping around context in them; the tail of the input line or
5090 sentence is used to fill the unused area on the left of the output line;
5091 the head of the input line or sentence is used to fill the unused area
5092 on the right of the output line.
5094 As a matter of convenience to the user, many usual backslashed escape
5095 sequences from the C language are recognized and converted to the
5096 corresponding characters by @command{ptx} itself.
5098 @item -W @var{regexp}
5099 @itemx --word-regexp=@var{regexp}
5101 This option selects which regular expression will describe each keyword.
5102 By default, if @sc{gnu} extensions are enabled, a word is a sequence of
5103 letters; the @var{regexp} used is @samp{\w+}. When @sc{gnu} extensions are
5104 disabled, a word is by default anything which ends with a space, a tab
5105 or a newline; the @var{regexp} used is @samp{[^ \t\n]+}.
5107 An empty @var{regexp} is equivalent to not using this option.
5108 @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
5111 As a matter of convenience to the user, many usual backslashed escape
5112 sequences, as found in the C language, are recognized and converted to
5113 the corresponding characters by @command{ptx} itself.
5118 @node Output formatting in ptx
5119 @subsection Output formatting
5121 Output format is mainly controlled by the @option{-O} and @option{-T} options
5122 described in the table below. When neither @option{-O} nor @option{-T} are
5123 selected, and if @sc{gnu} extensions are enabled, the program chooses an
5124 output format suitable for a dumb terminal. Each keyword occurrence is
5125 output to the center of one line, surrounded by its left and right
5126 contexts. Each field is properly justified, so the concordance output
5127 can be readily observed. As a special feature, if automatic
5128 references are selected by option @option{-A} and are output before the
5129 left context, that is, if option @option{-R} is @emph{not} selected, then
5130 a colon is added after the reference; this nicely interfaces with @sc{gnu}
5131 Emacs @code{next-error} processing. In this default output format, each
5132 white space character, like newline and tab, is merely changed to
5133 exactly one space, with no special attempt to compress consecutive
5134 spaces. This might change in the future. Except for those white space
5135 characters, every other character of the underlying set of 256
5136 characters is transmitted verbatim.
5138 Output format is further controlled by the following options.
5142 @item -g @var{number}
5143 @itemx --gap-size=@var{number}
5145 Select the size of the minimum white space gap between the fields on the
5148 @item -w @var{number}
5149 @itemx --width=@var{number}
5151 Select the maximum output width of each final line. If references are
5152 used, they are included or excluded from the maximum output width
5153 depending on the value of option @option{-R}. If this option is not
5154 selected, that is, when references are output before the left context,
5155 the maximum output width takes into account the maximum length of all
5156 references. If this option is selected, that is, when references are
5157 output after the right context, the maximum output width does not take
5158 into account the space taken by references, nor the gap that precedes
5162 @itemx --auto-reference
5164 Select automatic references. Each input line will have an automatic
5165 reference made up of the file name and the line ordinal, with a single
5166 colon between them. However, the file name will be empty when standard
5167 input is being read. If both @option{-A} and @option{-r} are selected, then
5168 the input reference is still read and skipped, but the automatic
5169 reference is used at output time, overriding the input reference.
5172 @itemx --right-side-refs
5174 In the default output format, when option @option{-R} is not used, any
5175 references produced by the effect of options @option{-r} or @option{-A} are
5176 placed to the far right of output lines, after the right context. With
5177 default output format, when the @option{-R} option is specified, references
5178 are rather placed at the beginning of each output line, before the left
5179 context. For any other output format, option @option{-R} is
5180 ignored, with one exception: with @option{-R} the width of references
5181 is @emph{not} taken into account in total output width given by @option{-w}.
5183 This option is automatically selected whenever @sc{gnu} extensions are
5186 @item -F @var{string}
5187 @itemx --flac-truncation=@var{string}
5189 This option will request that any truncation in the output be reported
5190 using the string @var{string}. Most output fields theoretically extend
5191 towards the beginning or the end of the current line, or current
5192 sentence, as selected with option @option{-S}. But there is a maximum
5193 allowed output line width, changeable through option @option{-w}, which is
5194 further divided into space for various output fields. When a field has
5195 to be truncated because it cannot extend beyond the beginning or the end of
5196 the current line to fit in, then a truncation occurs. By default,
5197 the string used is a single slash, as in @option{-F /}.
5199 @var{string} may have more than one character, as in @option{-F ...}.
5200 Also, in the particular case when @var{string} is empty (@option{-F ""}),
5201 truncation flagging is disabled, and no truncation marks are appended in
5204 As a matter of convenience to the user, many usual backslashed escape
5205 sequences, as found in the C language, are recognized and converted to
5206 the corresponding characters by @command{ptx} itself.
5208 @item -M @var{string}
5209 @itemx --macro-name=@var{string}
5211 Select another @var{string} to be used instead of @samp{xx}, while
5212 generating output suitable for @command{nroff}, @command{troff} or @TeX{}.
5215 @itemx --format=roff
5217 Choose an output format suitable for @command{nroff} or @command{troff}
5218 processing. Each output line will look like:
5221 .xx "@var{tail}" "@var{before}" "@var{keyword_and_after}"@c
5222 "@var{head}" "@var{ref}"
5225 so it will be possible to write a @samp{.xx} roff macro to take care of
5226 the output typesetting. This is the default output format when @sc{gnu}
5227 extensions are disabled. Option @option{-M} can be used to change
5228 @samp{xx} to another macro name.
5230 In this output format, each non-graphical character, like newline and
5231 tab, is merely changed to exactly one space, with no special attempt to
5232 compress consecutive spaces. Each quote character: @kbd{"} is doubled
5233 so it will be correctly processed by @command{nroff} or @command{troff}.
5238 Choose an output format suitable for @TeX{} processing. Each output
5239 line will look like:
5242 \xx @{@var{tail}@}@{@var{before}@}@{@var{keyword}@}@c
5243 @{@var{after}@}@{@var{head}@}@{@var{ref}@}
5247 so it will be possible to write a @code{\xx} definition to take care of
5248 the output typesetting. Note that when references are not being
5249 produced, that is, neither option @option{-A} nor option @option{-r} is
5250 selected, the last parameter of each @code{\xx} call is inhibited.
5251 Option @option{-M} can be used to change @samp{xx} to another macro
5254 In this output format, some special characters, like @kbd{$}, @kbd{%},
5255 @kbd{&}, @kbd{#} and @kbd{_} are automatically protected with a
5256 backslash. Curly brackets @kbd{@{}, @kbd{@}} are protected with a
5257 backslash and a pair of dollar signs (to force mathematical mode). The
5258 backslash itself produces the sequence @code{\backslash@{@}}.
5259 Circumflex and tilde diacritical marks produce the sequence @code{^\@{ @}} and
5260 @code{~\@{ @}} respectively. Other diacriticized characters of the
5261 underlying character set produce an appropriate @TeX{} sequence as far
5262 as possible. The other non-graphical characters, like newline and tab,
5263 and all other characters which are not part of @acronym{ASCII}, are merely
5264 changed to exactly one space, with no special attempt to compress
5265 consecutive spaces. Let me know how to improve this special character
5266 processing for @TeX{}.
5271 @node Compatibility in ptx
5272 @subsection The @sc{gnu} extensions to @command{ptx}
5274 This version of @command{ptx} contains a few features which do not exist in
5275 System V @command{ptx}. These extra features are suppressed by using the
5276 @option{-G} command line option, unless overridden by other command line
5277 options. Some @sc{gnu} extensions cannot be recovered by overriding, so the
5278 simple rule is to avoid @option{-G} if you care about @sc{gnu} extensions.
5279 Here are the differences between this program and System V @command{ptx}.
5284 This program can read many input files at once, it always writes the
5285 resulting concordance on standard output. On the other hand, System V
5286 @command{ptx} reads only one file and sends the result to standard output
5287 or, if a second @var{file} parameter is given on the command, to that
5290 Having output parameters not introduced by options is a dangerous
5291 practice which @sc{gnu} avoids as far as possible. So, for using @command{ptx}
5292 portably between @sc{gnu} and System V, you should always use it with a
5293 single input file, and always expect the result on standard output. You
5294 might also want to automatically configure in a @option{-G} option to
5295 @command{ptx} calls in products using @command{ptx}, if the configurator finds
5296 that the installed @command{ptx} accepts @option{-G}.
5299 The only options available in System V @command{ptx} are options @option{-b},
5300 @option{-f}, @option{-g}, @option{-i}, @option{-o}, @option{-r}, @option{-t} and
5301 @option{-w}. All other options are @sc{gnu} extensions and are not repeated in
5302 this enumeration. Moreover, some options have a slightly different
5303 meaning when @sc{gnu} extensions are enabled, as explained below.
5306 By default, concordance output is not formatted for @command{troff} or
5307 @command{nroff}. It is rather formatted for a dumb terminal. @command{troff}
5308 or @command{nroff} output may still be selected through option @option{-O}.
5311 Unless @option{-R} option is used, the maximum reference width is
5312 subtracted from the total output line width. With @sc{gnu} extensions
5313 disabled, width of references is not taken into account in the output
5314 line width computations.
5317 All 256 bytes, even @acronym{ASCII} @sc{nul} bytes, are always read and
5318 processed from input file with no adverse effect, even if @sc{gnu} extensions
5319 are disabled. However, System V @command{ptx} does not accept 8-bit characters,
5320 a few control characters are rejected, and the tilde @kbd{~} is also rejected.
5323 Input line length is only limited by available memory, even if @sc{gnu}
5324 extensions are disabled. However, System V @command{ptx} processes only
5325 the first 200 characters in each line.
5328 The break (non-word) characters default to be every character except all
5329 letters of the underlying character set, diacriticized or not. When @sc{gnu}
5330 extensions are disabled, the break characters default to space, tab and
5334 The program makes better use of output line width. If @sc{gnu} extensions
5335 are disabled, the program rather tries to imitate System V @command{ptx},
5336 but still, there are some slight disposition glitches this program does
5337 not completely reproduce.
5340 The user can specify both an Ignore file and an Only file. This is not
5341 allowed with System V @command{ptx}.
5346 @node tsort invocation
5347 @section @command{tsort}: Topological sort
5350 @cindex topological sort
5352 @command{tsort} performs a topological sort on the given @var{file}, or
5353 standard input if no input file is given or for a @var{file} of
5354 @samp{-}. For more details and some history, see @ref{tsort background}.
5358 tsort [@var{option}] [@var{file}]
5361 @command{tsort} reads its input as pairs of strings, separated by blanks,
5362 indicating a partial ordering. The output is a total ordering that
5363 corresponds to the given partial ordering.
5377 will produce the output
5388 Consider a more realistic example.
5389 You have a large set of functions all in one file, and they may all be
5390 declared static except one. Currently that one (say @code{main}) is the
5391 first function defined in the file, and the ones it calls directly follow
5392 it, followed by those they call, etc. Let's say that you are determined
5393 to take advantage of prototypes, so you have to choose between declaring
5394 all of those functions (which means duplicating a lot of information from
5395 the definitions) and rearranging the functions so that as many as possible
5396 are defined before they are used. One way to automate the latter process
5397 is to get a list for each function of the functions it calls directly.
5398 Many programs can generate such lists. They describe a call graph.
5399 Consider the following list, in which a given line indicates that the
5400 function on the left calls the one on the right directly.
5406 tail_file pretty_name
5407 tail_file write_header
5409 tail_forever recheck
5410 tail_forever pretty_name
5411 tail_forever write_header
5412 tail_forever dump_remainder
5415 tail_lines start_lines
5416 tail_lines dump_remainder
5417 tail_lines file_lines
5418 tail_lines pipe_lines
5420 tail_bytes start_bytes
5421 tail_bytes dump_remainder
5422 tail_bytes pipe_bytes
5423 file_lines dump_remainder
5427 then you can use @command{tsort} to produce an ordering of those
5428 functions that satisfies your requirement.
5431 example$ tsort call-graph | tac
5451 @command{tsort} detects any cycles in the input and writes the first cycle
5452 encountered to standard error.
5454 Note that for a given partial ordering, generally there is no unique
5455 total ordering. In the context of the call graph above, the function
5456 @code{parse_options} may be placed anywhere in the list as long as it
5457 precedes @code{main}.
5459 The only options are @option{--help} and @option{--version}. @xref{Common
5465 * tsort background:: Where tsort came from.
5468 @node tsort background
5469 @subsection @command{tsort}: Background
5471 @command{tsort} exists because very early versions of the Unix linker processed
5472 an archive file exactly once, and in order. As @command{ld} read each object
5473 in the archive, it decided whether it was needed in the program based on
5474 whether it defined any symbols which were undefined at that point in
5477 This meant that dependencies within the archive had to be handled
5478 specially. For example, @code{scanf} probably calls @code{read}. That means
5479 that in a single pass through an archive, it was important for @code{scanf.o}
5480 to appear before read.o, because otherwise a program which calls
5481 @code{scanf} but not @code{read} might end up with an unexpected unresolved
5482 reference to @code{read}.
5484 The way to address this problem was to first generate a set of
5485 dependencies of one object file on another. This was done by a shell
5486 script called @command{lorder}. The GNU tools don't provide a version of
5487 lorder, as far as I know, but you can still find it in BSD
5490 Then you ran @command{tsort} over the @command{lorder} output, and you used the
5491 resulting sort to define the order in which you added objects to the archive.
5493 This whole procedure has been obsolete since about 1980, because
5494 Unix archives now contain a symbol table (traditionally built by
5495 @command{ranlib}, now generally built by @command{ar} itself), and the Unix
5496 linker uses the symbol table to effectively make multiple passes over
5499 Anyhow, that's where tsort came from. To solve an old problem with
5500 the way the linker handled archive files, which has since been solved
5504 @node Operating on fields
5505 @chapter Operating on fields
5508 * cut invocation:: Print selected parts of lines.
5509 * paste invocation:: Merge lines of files.
5510 * join invocation:: Join lines on a common field.
5514 @node cut invocation
5515 @section @command{cut}: Print selected parts of lines
5518 @command{cut} writes to standard output selected parts of each line of each
5519 input file, or standard input if no files are given or for a file name of
5523 cut @var{option}@dots{} [@var{file}]@dots{}
5526 In the table which follows, the @var{byte-list}, @var{character-list},
5527 and @var{field-list} are one or more numbers or ranges (two numbers
5528 separated by a dash) separated by commas. Bytes, characters, and
5529 fields are numbered starting at 1. Incomplete ranges may be
5530 given: @option{-@var{m}} means @samp{1-@var{m}}; @samp{@var{n}-} means
5531 @samp{@var{n}} through end of line or last field. The list elements
5532 can be repeated, can overlap, and can be specified in any order; but
5533 the selected input is written in the same order that it is read, and
5534 is written exactly once.
5536 The program accepts the following options. Also see @ref{Common
5541 @item -b @var{byte-list}
5542 @itemx --bytes=@var{byte-list}
5545 Select for printing only the bytes in positions listed in
5546 @var{byte-list}. Tabs and backspaces are treated like any other
5547 character; they take up 1 byte. If an output delimiter is specified,
5548 (see the description of @option{--output-delimiter}), then output that
5549 string between ranges of selected bytes.
5551 @item -c @var{character-list}
5552 @itemx --characters=@var{character-list}
5554 @opindex --characters
5555 Select for printing only the characters in positions listed in
5556 @var{character-list}. The same as @option{-b} for now, but
5557 internationalization will change that. Tabs and backspaces are
5558 treated like any other character; they take up 1 character. If an
5559 output delimiter is specified, (see the description of
5560 @option{--output-delimiter}), then output that string between ranges
5563 @item -f @var{field-list}
5564 @itemx --fields=@var{field-list}
5567 Select for printing only the fields listed in @var{field-list}.
5568 Fields are separated by a TAB character by default. Also print any
5569 line that contains no delimiter character, unless the
5570 @option{--only-delimited} (@option{-s}) option is specified.
5572 Note @command{awk} supports more sophisticated field processing,
5573 and by default will use (and discard) runs of blank characters to
5574 separate fields, and ignore leading and trailing blanks.
5577 awk '{print $2}' # print the second field
5578 awk '{print $NF-1}' # print the penultimate field
5579 awk '{print $2,$1}' # reorder the first two fields
5583 In the unlikely event that @command{awk} is unavailable,
5584 one can use the @command{join} command, to process blank
5585 characters as @command{awk} does above.
5588 join -a1 -o 1.2 - /dev/null # print the second field
5589 join -a1 -o 1.2,1.1 - /dev/null # reorder the first two fields
5593 @item -d @var{input_delim_byte}
5594 @itemx --delimiter=@var{input_delim_byte}
5596 @opindex --delimiter
5597 With @option{-f}, use the first byte of @var{input_delim_byte} as
5598 the input fields separator (default is TAB).
5602 Do not split multi-byte characters (no-op for now).
5605 @itemx --only-delimited
5607 @opindex --only-delimited
5608 For @option{-f}, do not print lines that do not contain the field separator
5609 character. Normally, any line without a field separator is printed verbatim.
5611 @item --output-delimiter=@var{output_delim_string}
5612 @opindex --output-delimiter
5613 With @option{-f}, output fields are separated by @var{output_delim_string}.
5614 The default with @option{-f} is to use the input delimiter.
5615 When using @option{-b} or @option{-c} to select ranges of byte or
5616 character offsets (as opposed to ranges of fields),
5617 output @var{output_delim_string} between non-overlapping
5618 ranges of selected bytes.
5621 @opindex --complement
5622 This option is a @acronym{GNU} extension.
5623 Select for printing the complement of the bytes, characters or fields
5624 selected with the @option{-b}, @option{-c} or @option{-f} options.
5625 In other words, do @emph{not} print the bytes, characters or fields
5626 specified via those options. This option is useful when you have
5627 many fields and want to print all but a few of them.
5634 @node paste invocation
5635 @section @command{paste}: Merge lines of files
5638 @cindex merging files
5640 @command{paste} writes to standard output lines consisting of sequentially
5641 corresponding lines of each given file, separated by a TAB character.
5642 Standard input is used for a file name of @samp{-} or if no input files
5664 paste [@var{option}]@dots{} [@var{file}]@dots{}
5667 The program accepts the following options. Also see @ref{Common options}.
5675 Paste the lines of one file at a time rather than one line from each
5676 file. Using the above example data:
5679 $ paste -s num2 let3
5684 @item -d @var{delim-list}
5685 @itemx --delimiters=@var{delim-list}
5687 @opindex --delimiters
5688 Consecutively use the characters in @var{delim-list} instead of
5689 TAB to separate merged lines. When @var{delim-list} is
5690 exhausted, start again at its beginning. Using the above example data:
5693 $ paste -d '%_' num2 let3 num2
5704 @node join invocation
5705 @section @command{join}: Join lines on a common field
5708 @cindex common field, joining on
5710 @command{join} writes to standard output a line for each pair of input
5711 lines that have identical join fields. Synopsis:
5714 join [@var{option}]@dots{} @var{file1} @var{file2}
5717 Either @var{file1} or @var{file2} (but not both) can be @samp{-},
5718 meaning standard input. @var{file1} and @var{file2} should be
5719 sorted on the join fields.
5722 Normally, the sort order is that of the
5723 collating sequence specified by the @env{LC_COLLATE} locale. Unless
5724 the @option{-t} option is given, the sort comparison ignores blanks at
5725 the start of the join field, as in @code{sort -b}. If the
5726 @option{--ignore-case} option is given, the sort comparison ignores
5727 the case of characters in the join field, as in @code{sort -f}.
5729 The @command{sort} and @command{join} commands should use consistent
5730 locales and options if the output of @command{sort} is fed to
5731 @command{join}. You can use a command like @samp{sort -k 1b,1} to
5732 sort a file on its default join field, but if you select a non-default
5733 locale, join field, separator, or comparison options, then you should
5734 do so consistently between @command{join} and @command{sort}.
5735 If @samp{join -t ''} is specified then the whole line is considered which
5736 matches the default operation of sort.
5738 If the input has no unpairable lines, a @acronym{GNU} extension is
5739 available; the sort order can be any order that considers two fields
5740 to be equal if and only if the sort comparison described above
5741 considers them to be equal. For example:
5759 @checkOrderOption{join}
5764 @item the join field is the first field in each line;
5765 @item fields in the input are separated by one or more blanks, with leading
5766 blanks on the line ignored;
5767 @item fields in the output are separated by a space;
5768 @item each output line consists of the join field, the remaining
5769 fields from @var{file1}, then the remaining fields from @var{file2}.
5772 The program accepts the following options. Also see @ref{Common options}.
5776 @item -a @var{file-number}
5778 Print a line for each unpairable line in file @var{file-number} (either
5779 @samp{1} or @samp{2}), in addition to the normal output.
5782 Fail with an error message if either input file is wrongly ordered.
5784 @item --nocheck-order
5785 Do not check that both input files are in sorted order. This is the default.
5787 @item -e @var{string}
5789 Replace those output fields that are missing in the input with @var{string}.
5790 I.E. missing fields specified with the @option{-12jo} options.
5794 Treat the first line of each input file as a header line. The header lines will
5795 be joined and printed as the first output line. If @option{-o} is used to
5796 specify output format, the header line will be printed according to the
5797 specified format. The header lines will not be checked for ordering even if
5798 @option{--check-order} is specified. Also if the header lines from each file
5799 do not match, the heading fields from the first file will be used.
5802 @itemx --ignore-case
5804 @opindex --ignore-case
5805 Ignore differences in case when comparing keys.
5806 With this option, the lines of the input files must be ordered in the same way.
5807 Use @samp{sort -f} to produce this ordering.
5809 @item -1 @var{field}
5811 Join on field @var{field} (a positive integer) of file 1.
5813 @item -2 @var{field}
5815 Join on field @var{field} (a positive integer) of file 2.
5817 @item -j @var{field}
5818 Equivalent to @option{-1 @var{field} -2 @var{field}}.
5820 @item -o @var{field-list}
5822 If the keyword @samp{auto} is specified, infer the output format from
5823 the first line in each file. This is the same as the default output format
5824 but also ensures the same number of fields are output for each line.
5825 Missing fields are replaced with the @option{-e} option and extra fields
5828 Otherwise, construct each output line according to the format in
5829 @var{field-list}. Each element in @var{field-list} is either the single
5830 character @samp{0} or has the form @var{m.n} where the file number, @var{m},
5831 is @samp{1} or @samp{2} and @var{n} is a positive field number.
5833 A field specification of @samp{0} denotes the join field.
5834 In most cases, the functionality of the @samp{0} field spec
5835 may be reproduced using the explicit @var{m.n} that corresponds
5836 to the join field. However, when printing unpairable lines
5837 (using either of the @option{-a} or @option{-v} options), there is no way
5838 to specify the join field using @var{m.n} in @var{field-list}
5839 if there are unpairable lines in both files.
5840 To give @command{join} that functionality, @acronym{POSIX} invented the @samp{0}
5841 field specification notation.
5843 The elements in @var{field-list}
5844 are separated by commas or blanks.
5845 Blank separators typically need to be quoted for the shell. For
5846 example, the commands @samp{join -o 1.2,2.2} and @samp{join -o '1.2
5847 2.2'} are equivalent.
5849 All output lines---including those printed because of any -a or -v
5850 option---are subject to the specified @var{field-list}.
5853 Use character @var{char} as the input and output field separator.
5854 Treat as significant each occurrence of @var{char} in the input file.
5855 Use @samp{sort -t @var{char}}, without the @option{-b} option of
5856 @samp{sort}, to produce this ordering. If @samp{join -t ''} is specified,
5857 the whole line is considered, matching the default operation of sort.
5858 If @samp{-t '\0'} is specified then the @acronym{ASCII} @sc{nul}
5859 character is used to delimit the fields.
5861 @item -v @var{file-number}
5862 Print a line for each unpairable line in file @var{file-number}
5863 (either @samp{1} or @samp{2}), instead of the normal output.
5870 @node Operating on characters
5871 @chapter Operating on characters
5873 @cindex operating on characters
5875 This commands operate on individual characters.
5878 * tr invocation:: Translate, squeeze, and/or delete characters.
5879 * expand invocation:: Convert tabs to spaces.
5880 * unexpand invocation:: Convert spaces to tabs.
5885 @section @command{tr}: Translate, squeeze, and/or delete characters
5892 tr [@var{option}]@dots{} @var{set1} [@var{set2}]
5895 @command{tr} copies standard input to standard output, performing
5896 one of the following operations:
5900 translate, and optionally squeeze repeated characters in the result,
5902 squeeze repeated characters,
5906 delete characters, then squeeze repeated characters from the result.
5909 The @var{set1} and (if given) @var{set2} arguments define ordered
5910 sets of characters, referred to below as @var{set1} and @var{set2}. These
5911 sets are the characters of the input that @command{tr} operates on.
5912 The @option{--complement} (@option{-c}, @option{-C}) option replaces
5914 complement (all of the characters that are not in @var{set1}).
5916 Currently @command{tr} fully supports only single-byte characters.
5917 Eventually it will support multibyte characters; when it does, the
5918 @option{-C} option will cause it to complement the set of characters,
5919 whereas @option{-c} will cause it to complement the set of values.
5920 This distinction will matter only when some values are not characters,
5921 and this is possible only in locales using multibyte encodings when
5922 the input contains encoding errors.
5924 The program accepts the @option{--help} and @option{--version}
5925 options. @xref{Common options}. Options must precede operands.
5930 * Character sets:: Specifying sets of characters.
5931 * Translating:: Changing one set of characters to another.
5932 * Squeezing:: Squeezing repeats and deleting.
5936 @node Character sets
5937 @subsection Specifying sets of characters
5939 @cindex specifying sets of characters
5941 The format of the @var{set1} and @var{set2} arguments resembles
5942 the format of regular expressions; however, they are not regular
5943 expressions, only lists of characters. Most characters simply
5944 represent themselves in these strings, but the strings can contain
5945 the shorthands listed below, for convenience. Some of them can be
5946 used only in @var{set1} or @var{set2}, as noted below.
5950 @item Backslash escapes
5951 @cindex backslash escapes
5953 The following backslash escape sequences are recognized:
5971 The 8-bit character with the value given by @var{ooo}, which is 1 to 3
5972 octal digits. Note that @samp{\400} is interpreted as the two-byte
5973 sequence, @samp{\040} @samp{0}.
5978 While a backslash followed by a character not listed above is
5979 interpreted as that character, the backslash also effectively
5980 removes any special significance, so it is useful to escape
5981 @samp{[}, @samp{]}, @samp{*}, and @samp{-}.
5986 The notation @samp{@var{m}-@var{n}} expands to all of the characters
5987 from @var{m} through @var{n}, in ascending order. @var{m} should
5988 collate before @var{n}; if it doesn't, an error results. As an example,
5989 @samp{0-9} is the same as @samp{0123456789}.
5991 @sc{gnu} @command{tr} does not support the System V syntax that uses square
5992 brackets to enclose ranges. Translations specified in that format
5993 sometimes work as expected, since the brackets are often transliterated
5994 to themselves. However, they should be avoided because they sometimes
5995 behave unexpectedly. For example, @samp{tr -d '[0-9]'} deletes brackets
5998 Many historically common and even accepted uses of ranges are not
5999 portable. For example, on @acronym{EBCDIC} hosts using the @samp{A-Z}
6000 range will not do what most would expect because @samp{A} through @samp{Z}
6001 are not contiguous as they are in @acronym{ASCII}.
6002 If you can rely on a @acronym{POSIX} compliant version of @command{tr}, then
6003 the best way to work around this is to use character classes (see below).
6004 Otherwise, it is most portable (and most ugly) to enumerate the members
6007 @item Repeated characters
6008 @cindex repeated characters
6010 The notation @samp{[@var{c}*@var{n}]} in @var{set2} expands to @var{n}
6011 copies of character @var{c}. Thus, @samp{[y*6]} is the same as
6012 @samp{yyyyyy}. The notation @samp{[@var{c}*]} in @var{string2} expands
6013 to as many copies of @var{c} as are needed to make @var{set2} as long as
6014 @var{set1}. If @var{n} begins with @samp{0}, it is interpreted in
6015 octal, otherwise in decimal.
6017 @item Character classes
6018 @cindex character classes
6020 The notation @samp{[:@var{class}:]} expands to all of the characters in
6021 the (predefined) class @var{class}. The characters expand in no
6022 particular order, except for the @code{upper} and @code{lower} classes,
6023 which expand in ascending order. When the @option{--delete} (@option{-d})
6024 and @option{--squeeze-repeats} (@option{-s}) options are both given, any
6025 character class can be used in @var{set2}. Otherwise, only the
6026 character classes @code{lower} and @code{upper} are accepted in
6027 @var{set2}, and then only if the corresponding character class
6028 (@code{upper} and @code{lower}, respectively) is specified in the same
6029 relative position in @var{set1}. Doing this specifies case conversion.
6030 The class names are given below; an error results when an invalid class
6042 Horizontal whitespace.
6051 Printable characters, not including space.
6057 Printable characters, including space.
6060 Punctuation characters.
6063 Horizontal or vertical whitespace.
6072 @item Equivalence classes
6073 @cindex equivalence classes
6075 The syntax @samp{[=@var{c}=]} expands to all of the characters that are
6076 equivalent to @var{c}, in no particular order. Equivalence classes are
6077 a relatively recent invention intended to support non-English alphabets.
6078 But there seems to be no standard way to define them or determine their
6079 contents. Therefore, they are not fully implemented in @sc{gnu} @command{tr};
6080 each character's equivalence class consists only of that character,
6081 which is of no particular use.
6087 @subsection Translating
6089 @cindex translating characters
6091 @command{tr} performs translation when @var{set1} and @var{set2} are
6092 both given and the @option{--delete} (@option{-d}) option is not given.
6093 @command{tr} translates each character of its input that is in @var{set1}
6094 to the corresponding character in @var{set2}. Characters not in
6095 @var{set1} are passed through unchanged. When a character appears more
6096 than once in @var{set1} and the corresponding characters in @var{set2}
6097 are not all the same, only the final one is used. For example, these
6098 two commands are equivalent:
6105 A common use of @command{tr} is to convert lowercase characters to
6106 uppercase. This can be done in many ways. Here are three of them:
6109 tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
6111 tr '[:lower:]' '[:upper:]'
6115 But note that using ranges like @code{a-z} above is not portable.
6117 When @command{tr} is performing translation, @var{set1} and @var{set2}
6118 typically have the same length. If @var{set1} is shorter than
6119 @var{set2}, the extra characters at the end of @var{set2} are ignored.
6121 On the other hand, making @var{set1} longer than @var{set2} is not
6122 portable; @acronym{POSIX} says that the result is undefined. In this situation,
6123 BSD @command{tr} pads @var{set2} to the length of @var{set1} by repeating
6124 the last character of @var{set2} as many times as necessary. System V
6125 @command{tr} truncates @var{set1} to the length of @var{set2}.
6127 By default, @sc{gnu} @command{tr} handles this case like BSD @command{tr}.
6128 When the @option{--truncate-set1} (@option{-t}) option is given,
6129 @sc{gnu} @command{tr} handles this case like the System V @command{tr}
6130 instead. This option is ignored for operations other than translation.
6132 Acting like System V @command{tr} in this case breaks the relatively common
6136 tr -cs A-Za-z0-9 '\012'
6140 because it converts only zero bytes (the first element in the
6141 complement of @var{set1}), rather than all non-alphanumerics, to
6145 By the way, the above idiom is not portable because it uses ranges, and
6146 it assumes that the octal code for newline is 012.
6147 Assuming a @acronym{POSIX} compliant @command{tr}, here is a better
6151 tr -cs '[:alnum:]' '[\n*]'
6156 @subsection Squeezing repeats and deleting
6158 @cindex squeezing repeat characters
6159 @cindex deleting characters
6161 When given just the @option{--delete} (@option{-d}) option, @command{tr}
6162 removes any input characters that are in @var{set1}.
6164 When given just the @option{--squeeze-repeats} (@option{-s}) option,
6165 @command{tr} replaces each input sequence of a repeated character that
6166 is in @var{set1} with a single occurrence of that character.
6168 When given both @option{--delete} and @option{--squeeze-repeats}, @command{tr}
6169 first performs any deletions using @var{set1}, then squeezes repeats
6170 from any remaining characters using @var{set2}.
6172 The @option{--squeeze-repeats} option may also be used when translating,
6173 in which case @command{tr} first performs translation, then squeezes
6174 repeats from any remaining characters using @var{set2}.
6176 Here are some examples to illustrate various combinations of options:
6181 Remove all zero bytes:
6188 Put all words on lines by themselves. This converts all
6189 non-alphanumeric characters to newlines, then squeezes each string
6190 of repeated newlines into a single newline:
6193 tr -cs '[:alnum:]' '[\n*]'
6197 Convert each sequence of repeated newlines to a single newline:
6204 Find doubled occurrences of words in a document.
6205 @c Separate the following two "the"s, so typo checkers don't complain.
6206 For example, people often write ``the @w{}the'' with the repeated words
6207 separated by a newline. The Bourne shell script below works first
6208 by converting each sequence of punctuation and blank characters to a
6209 single newline. That puts each ``word'' on a line by itself.
6210 Next it maps all uppercase characters to lower case, and finally it
6211 runs @command{uniq} with the @option{-d} option to print out only the words
6217 | tr -s '[:punct:][:blank:]' '[\n*]' \
6218 | tr '[:upper:]' '[:lower:]' \
6223 Deleting a small set of characters is usually straightforward. For example,
6224 to remove all @samp{a}s, @samp{x}s, and @samp{M}s you would do this:
6230 However, when @samp{-} is one of those characters, it can be tricky because
6231 @samp{-} has special meanings. Performing the same task as above but also
6232 removing all @samp{-} characters, we might try @code{tr -d -axM}, but
6233 that would fail because @command{tr} would try to interpret @option{-a} as
6234 a command-line option. Alternatively, we could try putting the hyphen
6235 inside the string, @code{tr -d a-xM}, but that wouldn't work either because
6236 it would make @command{tr} interpret @code{a-x} as the range of characters
6237 @samp{a}@dots{}@samp{x} rather than the three.
6238 One way to solve the problem is to put the hyphen at the end of the list
6245 Or you can use @samp{--} to terminate option processing:
6251 More generally, use the character class notation @code{[=c=]}
6252 with @samp{-} (or any other character) in place of the @samp{c}:
6258 Note how single quotes are used in the above example to protect the
6259 square brackets from interpretation by a shell.
6264 @node expand invocation
6265 @section @command{expand}: Convert tabs to spaces
6268 @cindex tabs to spaces, converting
6269 @cindex converting tabs to spaces
6271 @command{expand} writes the contents of each given @var{file}, or standard
6272 input if none are given or for a @var{file} of @samp{-}, to standard
6273 output, with tab characters converted to the appropriate number of
6277 expand [@var{option}]@dots{} [@var{file}]@dots{}
6280 By default, @command{expand} converts all tabs to spaces. It preserves
6281 backspace characters in the output; they decrement the column count for
6282 tab calculations. The default action is equivalent to @option{-t 8} (set
6283 tabs every 8 columns).
6285 The program accepts the following options. Also see @ref{Common options}.
6289 @item -t @var{tab1}[,@var{tab2}]@dots{}
6290 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6293 @cindex tab stops, setting
6294 If only one tab stop is given, set the tabs @var{tab1} spaces apart
6295 (default is 8). Otherwise, set the tabs at columns @var{tab1},
6296 @var{tab2}, @dots{} (numbered from 0), and replace any tabs beyond the
6297 last tab stop given with single spaces. Tab stops can be separated by
6298 blanks as well as by commas.
6300 For compatibility, GNU @command{expand} also accepts the obsolete
6301 option syntax, @option{-@var{t1}[,@var{t2}]@dots{}}. New scripts
6302 should use @option{-t @var{t1}[,@var{t2}]@dots{}} instead.
6308 @cindex initial tabs, converting
6309 Only convert initial tabs (those that precede all non-space or non-tab
6310 characters) on each line to spaces.
6317 @node unexpand invocation
6318 @section @command{unexpand}: Convert spaces to tabs
6322 @command{unexpand} writes the contents of each given @var{file}, or
6323 standard input if none are given or for a @var{file} of @samp{-}, to
6324 standard output, converting blanks at the beginning of each line into
6325 as many tab characters as needed. In the default @acronym{POSIX}
6326 locale, a @dfn{blank} is a space or a tab; other locales may specify
6327 additional blank characters. Synopsis:
6330 unexpand [@var{option}]@dots{} [@var{file}]@dots{}
6333 By default, @command{unexpand} converts only initial blanks (those
6334 that precede all non-blank characters) on each line. It
6335 preserves backspace characters in the output; they decrement the column
6336 count for tab calculations. By default, tabs are set at every 8th
6339 The program accepts the following options. Also see @ref{Common options}.
6343 @item -t @var{tab1}[,@var{tab2}]@dots{}
6344 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6347 If only one tab stop is given, set the tabs @var{tab1} columns apart
6348 instead of the default 8. Otherwise, set the tabs at columns
6349 @var{tab1}, @var{tab2}, @dots{} (numbered from 0), and leave blanks
6350 beyond the tab stops given unchanged. Tab stops can be separated by
6351 blanks as well as by commas. This option implies the @option{-a} option.
6353 For compatibility, GNU @command{unexpand} supports the obsolete option syntax,
6354 @option{-@var{tab1}[,@var{tab2}]@dots{}}, where tab stops must be
6355 separated by commas. (Unlike @option{-t}, this obsolete option does
6356 not imply @option{-a}.) New scripts should use @option{--first-only -t
6357 @var{tab1}[,@var{tab2}]@dots{}} instead.
6363 Also convert all sequences of two or more blanks just before a tab stop,
6364 even if they occur after non-blank characters in a line.
6371 @node Directory listing
6372 @chapter Directory listing
6374 This chapter describes the @command{ls} command and its variants @command{dir}
6375 and @command{vdir}, which list information about files.
6378 * ls invocation:: List directory contents.
6379 * dir invocation:: Briefly ls.
6380 * vdir invocation:: Verbosely ls.
6381 * dircolors invocation:: Color setup for ls, etc.
6386 @section @command{ls}: List directory contents
6389 @cindex directory listing
6391 The @command{ls} program lists information about files (of any type,
6392 including directories). Options and file arguments can be intermixed
6393 arbitrarily, as usual.
6395 For non-option command-line arguments that are directories, by default
6396 @command{ls} lists the contents of directories, not recursively, and
6397 omitting files with names beginning with @samp{.}. For other non-option
6398 arguments, by default @command{ls} lists just the file name. If no
6399 non-option argument is specified, @command{ls} operates on the current
6400 directory, acting as if it had been invoked with a single argument of @samp{.}.
6403 By default, the output is sorted alphabetically, according to the locale
6404 settings in effect.@footnote{If you use a non-@acronym{POSIX}
6405 locale (e.g., by setting @env{LC_ALL} to @samp{en_US}), then @command{ls} may
6406 produce output that is sorted differently than you're accustomed to.
6407 In that case, set the @env{LC_ALL} environment variable to @samp{C}.}
6408 If standard output is
6409 a terminal, the output is in columns (sorted vertically) and control
6410 characters are output as question marks; otherwise, the output is listed
6411 one per line and control characters are output as-is.
6413 Because @command{ls} is such a fundamental program, it has accumulated many
6414 options over the years. They are described in the subsections below;
6415 within each section, options are listed alphabetically (ignoring case).
6416 The division of options into the subsections is not absolute, since some
6417 options affect more than one aspect of @command{ls}'s operation.
6419 @cindex exit status of @command{ls}
6424 1 minor problems (e.g., failure to access a file or directory not
6425 specified as a command line argument. This happens when listing a
6426 directory in which entries are actively being removed or renamed.)
6427 2 serious trouble (e.g., memory exhausted, invalid option, failure
6428 to access a file or directory specified as a command line argument
6429 or a directory loop)
6432 Also see @ref{Common options}.
6435 * Which files are listed::
6436 * What information is listed::
6437 * Sorting the output::
6438 * Details about version sort::
6439 * General output formatting::
6440 * Formatting file timestamps::
6441 * Formatting the file names::
6445 @node Which files are listed
6446 @subsection Which files are listed
6448 These options determine which files @command{ls} lists information for.
6449 By default, @command{ls} lists files and the contents of any
6450 directories on the command line, except that in directories it ignores
6451 files whose names start with @samp{.}.
6459 In directories, do not ignore file names that start with @samp{.}.
6464 @opindex --almost-all
6465 In directories, do not ignore all file names that start with @samp{.};
6466 ignore only @file{.} and @file{..}. The @option{--all} (@option{-a})
6467 option overrides this option.
6470 @itemx --ignore-backups
6472 @opindex --ignore-backups
6473 @cindex backup files, ignoring
6474 In directories, ignore files that end with @samp{~}. This option is
6475 equivalent to @samp{--ignore='*~' --ignore='.*~'}.
6480 @opindex --directory
6481 List just the names of directories, as with other types of files, rather
6482 than listing their contents.
6483 @c The following sentence is the same as the one for -F.
6484 Do not follow symbolic links listed on the
6485 command line unless the @option{--dereference-command-line} (@option{-H}),
6486 @option{--dereference} (@option{-L}), or
6487 @option{--dereference-command-line-symlink-to-dir} options are specified.
6490 @itemx --dereference-command-line
6492 @opindex --dereference-command-line
6493 @cindex symbolic links, dereferencing
6494 If a command line argument specifies a symbolic link, show information
6495 for the file the link references rather than for the link itself.
6497 @itemx --dereference-command-line-symlink-to-dir
6498 @opindex --dereference-command-line-symlink-to-dir
6499 @cindex symbolic links, dereferencing
6500 Do not dereference symbolic links, with one exception:
6501 if a command line argument specifies a symbolic link that refers to
6502 a directory, show information for that directory rather than for the
6504 This is the default behavior when no other dereferencing-related
6505 option has been specified (@option{--classify} (@option{-F}),
6506 @option{--directory} (@option{-d}),
6508 @option{--dereference} (@option{-L}), or
6509 @option{--dereference-command-line} (@option{-H})).
6511 @item --group-directories-first
6512 @opindex --group-directories-first
6513 Group all the directories before the files and then sort the
6514 directories and the files separately using the selected sort key
6515 (see --sort option).
6516 That is, this option specifies a primary sort key,
6517 and the --sort option specifies a secondary key.
6518 However, any use of @option{--sort=none}
6519 (@option{-U}) disables this option altogether.
6521 @item --hide=PATTERN
6522 @opindex --hide=@var{pattern}
6523 In directories, ignore files whose names match the shell pattern
6524 @var{pattern}, unless the @option{--all} (@option{-a}) or
6525 @option{--almost-all} (@option{-A}) is also given. This
6526 option acts like @option{--ignore=@var{pattern}} except that it has no
6527 effect if @option{--all} (@option{-a}) or @option{--almost-all}
6528 (@option{-A}) is also given.
6530 This option can be useful in shell aliases. For example, if
6531 @command{lx} is an alias for @samp{ls --hide='*~'} and @command{ly} is
6532 an alias for @samp{ls --ignore='*~'}, then the command @samp{lx -A}
6533 lists the file @file{README~} even though @samp{ly -A} would not.
6535 @item -I @var{pattern}
6536 @itemx --ignore=@var{pattern}
6538 @opindex --ignore=@var{pattern}
6539 In directories, ignore files whose names match the shell pattern
6540 (not regular expression) @var{pattern}. As
6541 in the shell, an initial @samp{.} in a file name does not match a
6542 wildcard at the start of @var{pattern}. Sometimes it is useful
6543 to give this option several times. For example,
6546 $ ls --ignore='.??*' --ignore='.[^.]' --ignore='#*'
6549 The first option ignores names of length 3 or more that start with @samp{.},
6550 the second ignores all two-character names that start with @samp{.}
6551 except @samp{..}, and the third ignores names that start with @samp{#}.
6554 @itemx --dereference
6556 @opindex --dereference
6557 @cindex symbolic links, dereferencing
6558 When showing file information for a symbolic link, show information
6559 for the file the link references rather than the link itself.
6560 However, even with this option, @command{ls} still prints the name
6561 of the link itself, not the name of the file that the link points to.
6566 @opindex --recursive
6567 @cindex recursive directory listing
6568 @cindex directory listing, recursive
6569 List the contents of all directories recursively.
6574 @node What information is listed
6575 @subsection What information is listed
6577 These options affect the information that @command{ls} displays. By
6578 default, only file names are shown.
6584 @cindex hurd, author, printing
6585 List each file's author when producing long format directory listings.
6586 In GNU/Hurd, file authors can differ from their owners, but in other
6587 operating systems the two are the same.
6593 @cindex dired Emacs mode support
6594 With the long listing (@option{-l}) format, print an additional line after
6598 //DIRED// @var{beg1} @var{end1} @var{beg2} @var{end2} @dots{}
6602 The @var{begn} and @var{endn} are unsigned integers that record the
6603 byte position of the beginning and end of each file name in the output.
6604 This makes it easy for Emacs to find the names, even when they contain
6605 unusual characters such as space or newline, without fancy searching.
6607 If directories are being listed recursively (@option{-R}), output a similar
6608 line with offsets for each subdirectory name:
6611 //SUBDIRED// @var{beg1} @var{end1} @dots{}
6614 Finally, output a line of the form:
6617 //DIRED-OPTIONS// --quoting-style=@var{word}
6621 where @var{word} is the quoting style (@pxref{Formatting the file names}).
6623 Here is an actual example:
6626 $ mkdir -p a/sub/deeper a/sub2
6628 $ touch a/sub/deeper/file
6629 $ ls -gloRF --dired a
6632 -rw-r--r-- 1 0 Jun 10 12:27 f1
6633 -rw-r--r-- 1 0 Jun 10 12:27 f2
6634 drwxr-xr-x 3 4096 Jun 10 12:27 sub/
6635 drwxr-xr-x 2 4096 Jun 10 12:27 sub2/
6639 drwxr-xr-x 2 4096 Jun 10 12:27 deeper/
6643 -rw-r--r-- 1 0 Jun 10 12:27 file
6647 //DIRED// 48 50 84 86 120 123 158 162 217 223 282 286
6648 //SUBDIRED// 2 3 167 172 228 240 290 296
6649 //DIRED-OPTIONS// --quoting-style=literal
6652 Note that the pairs of offsets on the @samp{//DIRED//} line above delimit
6653 these names: @file{f1}, @file{f2}, @file{sub}, @file{sub2}, @file{deeper},
6655 The offsets on the @samp{//SUBDIRED//} line delimit the following
6656 directory names: @file{a}, @file{a/sub}, @file{a/sub/deeper}, @file{a/sub2}.
6658 Here is an example of how to extract the fifth entry name, @samp{deeper},
6659 corresponding to the pair of offsets, 222 and 228:
6662 $ ls -gloRF --dired a > out
6663 $ dd bs=1 skip=222 count=6 < out 2>/dev/null; echo
6667 Note that although the listing above includes a trailing slash
6668 for the @samp{deeper} entry, the offsets select the name without
6669 the trailing slash. However, if you invoke @command{ls} with @option{--dired}
6670 along with an option like @option{--escape} (aka @option{-b}) and operate
6671 on a file whose name contains special characters, notice that the backslash
6676 $ ls -blog --dired 'a b'
6677 -rw-r--r-- 1 0 Jun 10 12:28 a\ b
6679 //DIRED-OPTIONS// --quoting-style=escape
6682 If you use a quoting style that adds quote marks
6683 (e.g., @option{--quoting-style=c}), then the offsets include the quote marks.
6684 So beware that the user may select the quoting style via the environment
6685 variable @env{QUOTING_STYLE}. Hence, applications using @option{--dired}
6686 should either specify an explicit @option{--quoting-style=literal} option
6687 (aka @option{-N} or @option{--literal}) on the command line, or else be
6688 prepared to parse the escaped names.
6691 @opindex --full-time
6692 Produce long format directory listings, and list times in full. It is
6693 equivalent to using @option{--format=long} with
6694 @option{--time-style=full-iso} (@pxref{Formatting file timestamps}).
6698 Produce long format directory listings, but don't display owner information.
6704 Inhibit display of group information in a long format directory listing.
6705 (This is the default in some non-@sc{gnu} versions of @command{ls}, so we
6706 provide this option for compatibility.)
6714 @cindex inode number, printing
6715 Print the inode number (also called the file serial number and index
6716 number) of each file to the left of the file name. (This number
6717 uniquely identifies each file within a particular file system.)
6720 @itemx --format=long
6721 @itemx --format=verbose
6724 @opindex long ls @r{format}
6725 @opindex verbose ls @r{format}
6726 In addition to the name of each file, print the file type, file mode bits,
6727 number of hard links, owner name, group name, size, and
6728 timestamp (@pxref{Formatting file timestamps}), normally
6729 the modification time. Print question marks for information that
6730 cannot be determined.
6732 Normally the size is printed as a byte count without punctuation, but
6733 this can be overridden (@pxref{Block size}). For example, @option{-h}
6734 prints an abbreviated, human-readable count, and
6735 @samp{--block-size="'1"} prints a byte count with the thousands
6736 separator of the current locale.
6738 For each directory that is listed, preface the files with a line
6739 @samp{total @var{blocks}}, where @var{blocks} is the total disk allocation
6740 for all files in that directory. The block size currently defaults to 1024
6741 bytes, but this can be overridden (@pxref{Block size}).
6742 The @var{blocks} computed counts each hard link separately;
6743 this is arguably a deficiency.
6745 The file type is one of the following characters:
6747 @c The commented-out entries are ones we're not sure about.
6755 character special file
6757 high performance (``contiguous data'') file
6761 door (Solaris 2.5 and up)
6763 @c semaphore, if this is a distinct file type
6767 @c multiplexed file (7th edition Unix; obsolete)
6769 off-line (``migrated'') file (Cray DMF)
6771 network special file (HP-UX)
6775 port (Solaris 10 and up)
6777 @c message queue, if this is a distinct file type
6781 @c shared memory object, if this is a distinct file type
6783 @c typed memory object, if this is a distinct file type
6785 @c whiteout (4.4BSD; not implemented)
6787 some other file type
6790 @cindex permissions, output by @command{ls}
6791 The file mode bits listed are similar to symbolic mode specifications
6792 (@pxref{Symbolic Modes}). But @command{ls} combines multiple bits into the
6793 third character of each set of permissions as follows:
6797 If the set-user-ID or set-group-ID bit and the corresponding executable bit
6801 If the set-user-ID or set-group-ID bit is set but the corresponding
6802 executable bit is not set.
6805 If the restricted deletion flag or sticky bit, and the
6806 other-executable bit, are both set. The restricted deletion flag is
6807 another name for the sticky bit. @xref{Mode Structure}.
6810 If the restricted deletion flag or sticky bit is set but the
6811 other-executable bit is not set.
6814 If the executable bit is set and none of the above apply.
6820 Following the file mode bits is a single character that specifies
6821 whether an alternate access method such as an access control list
6822 applies to the file. When the character following the file mode bits is a
6823 space, there is no alternate access method. When it is a printing
6824 character, then there is such a method.
6826 GNU @command{ls} uses a @samp{.} character to indicate a file
6827 with an SELinux security context, but no other alternate access method.
6829 A file with any other combination of alternate access methods
6830 is marked with a @samp{+} character.
6833 @itemx --numeric-uid-gid
6835 @opindex --numeric-uid-gid
6836 @cindex numeric uid and gid
6837 @cindex numeric user and group IDs
6838 Produce long format directory listings, but
6839 display numeric user and group IDs instead of the owner and group names.
6843 Produce long format directory listings, but don't display group information.
6844 It is equivalent to using @option{--format=long} with @option{--no-group} .
6850 @cindex disk allocation
6851 @cindex size of files, reporting
6852 Print the disk allocation of each file to the left of the file name.
6853 This is the amount of disk space used by the file, which is usually a
6854 bit more than the file's size, but it can be less if the file has holes.
6856 Normally the disk allocation is printed in units of
6857 1024 bytes, but this can be overridden (@pxref{Block size}).
6859 @cindex NFS mounts from BSD to HP-UX
6860 For files that are NFS-mounted from an HP-UX system to a BSD system,
6861 this option reports sizes that are half the correct values. On HP-UX
6862 systems, it reports sizes that are twice the correct values for files
6863 that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX;
6864 it also affects the HP-UX @command{ls} program.
6873 @cindex security context
6874 Display the SELinux security context or @samp{?} if none is found.
6875 When used with the @option{-l} option, print the security context
6876 to the left of the size column.
6881 @node Sorting the output
6882 @subsection Sorting the output
6884 @cindex sorting @command{ls} output
6885 These options change the order in which @command{ls} sorts the information
6886 it outputs. By default, sorting is done by character code
6887 (e.g., @acronym{ASCII} order).
6893 @itemx --time=status
6896 @opindex ctime@r{, printing or sorting by}
6897 @opindex status time@r{, printing or sorting by}
6898 @opindex use time@r{, printing or sorting files by}
6899 If the long listing format (e.g., @option{-l}, @option{-o}) is being used,
6900 print the status change time (the @samp{ctime} in the inode) instead of
6901 the modification time.
6902 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6903 or when not using a long listing format,
6904 sort according to the status change time.
6908 @cindex unsorted directory listing
6909 @cindex directory order, listing by
6910 Primarily, like @option{-U}---do not sort; list the files in whatever
6911 order they are stored in the directory. But also enable @option{-a} (list
6912 all files) and disable @option{-l}, @option{--color}, and @option{-s} (if they
6913 were specified before the @option{-f}).
6919 @cindex reverse sorting
6920 Reverse whatever the sorting method is---e.g., list files in reverse
6921 alphabetical order, youngest first, smallest first, or whatever.
6927 @opindex size of files@r{, sorting files by}
6928 Sort by file size, largest first.
6934 @opindex modification time@r{, sorting files by}
6935 Sort by modification time (the @samp{mtime} in the inode), newest first.
6939 @itemx --time=access
6943 @opindex use time@r{, printing or sorting files by}
6944 @opindex atime@r{, printing or sorting files by}
6945 @opindex access time@r{, printing or sorting files by}
6946 If the long listing format (e.g., @option{--format=long}) is being used,
6947 print the last access time (the @samp{atime} in the inode).
6948 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6949 or when not using a long listing format, sort according to the access time.
6955 @opindex none@r{, sorting option for @command{ls}}
6956 Do not sort; list the files in whatever order they are
6957 stored in the directory. (Do not do any of the other unrelated things
6958 that @option{-f} does.) This is especially useful when listing very large
6959 directories, since not doing any sorting can be noticeably faster.
6962 @itemx --sort=version
6965 @opindex version@r{, sorting option for @command{ls}}
6966 Sort by version name and number, lowest first. It behaves like a default
6967 sort, except that each sequence of decimal digits is treated numerically
6968 as an index/version number. (@xref{Details about version sort}.)
6971 @itemx --sort=extension
6974 @opindex extension@r{, sorting files by}
6975 Sort directory contents alphabetically by file extension (characters
6976 after the last @samp{.}); files with no extension are sorted first.
6981 @node Details about version sort
6982 @subsection Details about version sort
6984 Version sorting handles the fact that file names frequently include indices or
6985 version numbers. Standard sorting usually does not produce the order that one
6986 expects because comparisons are made on a character-by-character basis.
6987 Version sorting is especially useful when browsing directories that contain
6988 many files with indices/version numbers in their names:
6992 abc.zml-1.gz abc.zml-1.gz
6993 abc.zml-12.gz abc.zml-2.gz
6994 abc.zml-2.gz abc.zml-12.gz
6997 Version-sorted strings are compared such that if @var{ver1} and @var{ver2}
6998 are version numbers and @var{prefix} and @var{suffix} (@var{suffix} matching
6999 the regular expression @samp{(\.[A-Za-z~][A-Za-z0-9~]*)*}) are strings then
7000 @var{ver1} < @var{ver2} implies that the name composed of
7001 ``@var{prefix} @var{ver1} @var{suffix}'' sorts before
7002 ``@var{prefix} @var{ver2} @var{suffix}''.
7004 Note also that leading zeros of numeric parts are ignored:
7008 abc-1.007.tgz abc-1.01a.tgz
7009 abc-1.012b.tgz abc-1.007.tgz
7010 abc-1.01a.tgz abc-1.012b.tgz
7013 This functionality is implemented using gnulib's @code{filevercmp} function,
7014 which has some caveats worth noting.
7017 @item @env{LC_COLLATE} is ignored, which means @samp{ls -v} and @samp{sort -V}
7018 will sort non-numeric prefixes as if the @env{LC_COLLATE} locale category
7019 was set to @samp{C}.
7020 @item Some suffixes will not be matched by the regular
7021 expression mentioned above. Consequently these examples may
7022 not sort as you expect:
7030 abc-1.2.3.4.x86_64.rpm
7031 abc-1.2.3.x86_64.rpm
7035 @node General output formatting
7036 @subsection General output formatting
7038 These options affect the appearance of the overall output.
7043 @itemx --format=single-column
7046 @opindex single-column @r{output of files}
7047 List one file per line. This is the default for @command{ls} when standard
7048 output is not a terminal.
7051 @itemx --format=vertical
7054 @opindex vertical @r{sorted files in columns}
7055 List files in columns, sorted vertically. This is the default for
7056 @command{ls} if standard output is a terminal. It is always the default
7057 for the @command{dir} program.
7058 @sc{gnu} @command{ls} uses variable width columns to display as many files as
7059 possible in the fewest lines.
7061 @item --color [=@var{when}]
7063 @cindex color, distinguishing file types with
7064 Specify whether to use color for distinguishing file types. @var{when}
7065 may be omitted, or one of:
7068 @vindex none @r{color option}
7069 - Do not use color at all. This is the default.
7071 @vindex auto @r{color option}
7072 @cindex terminal, using color iff
7073 - Only use color if standard output is a terminal.
7075 @vindex always @r{color option}
7078 Specifying @option{--color} and no @var{when} is equivalent to
7079 @option{--color=always}.
7080 Piping a colorized listing through a pager like @command{more} or
7081 @command{less} usually produces unreadable results. However, using
7082 @code{more -f} does seem to work.
7085 @vindex SHELL @r{environment variable, and color}
7086 Note that using the @option{--color} option may incur a noticeable
7087 performance penalty when run in a directory with very many entries,
7088 because the default settings require that @command{ls} @code{stat} every
7089 single file it lists.
7090 However, if you would like most of the file-type coloring
7091 but can live without the other coloring options (e.g.,
7092 executable, orphan, sticky, other-writable, capability), use
7093 @command{dircolors} to set the @env{LS_COLORS} environment variable like this,
7095 eval $(dircolors -p | perl -pe \
7096 's/^((CAP|S[ET]|O[TR]|M|E)\w+).*/$1 00/' | dircolors -)
7098 and on a @code{dirent.d_type}-capable file system, @command{ls}
7099 will perform only one @code{stat} call per command line argument.
7103 @itemx --indicator-style=classify
7106 @opindex --indicator-style
7107 @cindex file type and executables, marking
7108 @cindex executables and file type, marking
7109 Append a character to each file name indicating the file type. Also,
7110 for regular files that are executable, append @samp{*}. The file type
7111 indicators are @samp{/} for directories, @samp{@@} for symbolic links,
7112 @samp{|} for FIFOs, @samp{=} for sockets, @samp{>} for doors,
7113 and nothing for regular files.
7114 @c The following sentence is the same as the one for -d.
7115 Do not follow symbolic links listed on the
7116 command line unless the @option{--dereference-command-line} (@option{-H}),
7117 @option{--dereference} (@option{-L}), or
7118 @option{--dereference-command-line-symlink-to-dir} options are specified.
7121 @itemx --indicator-style=file-type
7122 @opindex --file-type
7123 @opindex --indicator-style
7124 @cindex file type, marking
7125 Append a character to each file name indicating the file type. This is
7126 like @option{-F}, except that executables are not marked.
7128 @item --indicator-style=@var{word}
7129 @opindex --indicator-style
7130 Append a character indicator with style @var{word} to entry names,
7135 Do not append any character indicator; this is the default.
7137 Append @samp{/} for directories. This is the same as the @option{-p}
7140 Append @samp{/} for directories, @samp{@@} for symbolic links, @samp{|}
7141 for FIFOs, @samp{=} for sockets, and nothing for regular files. This is
7142 the same as the @option{--file-type} option.
7144 Append @samp{*} for executable regular files, otherwise behave as for
7145 @samp{file-type}. This is the same as the @option{-F} or
7146 @option{--classify} option.
7152 @opindex --kibibytes
7153 Set the default block size to its normal value of 1024 bytes,
7154 overriding any contrary specification in environment variables
7155 (@pxref{Block size}). This option is in turn overridden by the
7156 @option{--block-size}, @option{-h} or @option{--human-readable}, and
7157 @option{--si} options.
7159 The @option{-k} or @option{--kibibytes} option affects the
7160 per-directory block count written by the @option{-l} and similar
7161 options, and the size written by the @option{-s} or @option{--size}
7162 option. It does not affect the file size written by @option{-l}.
7165 @itemx --format=commas
7168 @opindex commas@r{, outputting between files}
7169 List files horizontally, with as many as will fit on each line,
7170 separated by @samp{, } (a comma and a space).
7173 @itemx --indicator-style=slash
7175 @opindex --indicator-style
7176 @cindex file type, marking
7177 Append a @samp{/} to directory names.
7180 @itemx --format=across
7181 @itemx --format=horizontal
7184 @opindex across@r{, listing files}
7185 @opindex horizontal@r{, listing files}
7186 List the files in columns, sorted horizontally.
7189 @itemx --tabsize=@var{cols}
7192 Assume that each tab stop is @var{cols} columns wide. The default is 8.
7193 @command{ls} uses tabs where possible in the output, for efficiency. If
7194 @var{cols} is zero, do not use tabs at all.
7196 @c FIXME: remove in 2009, if Apple Terminal has been fixed for long enough.
7197 Some terminal emulators (at least Apple Terminal 1.5 (133) from Mac OS X 10.4.8)
7198 do not properly align columns to the right of a TAB following a
7199 non-@acronym{ASCII} byte. If you use such a terminal emulator, use the
7200 @option{-T0} option or put @code{TABSIZE=0} in your environment to tell
7201 @command{ls} to align using spaces, not tabs.
7204 @itemx --width=@var{cols}
7208 Assume the screen is @var{cols} columns wide. The default is taken
7209 from the terminal settings if possible; otherwise the environment
7210 variable @env{COLUMNS} is used if it is set; otherwise the default
7216 @node Formatting file timestamps
7217 @subsection Formatting file timestamps
7219 By default, file timestamps are listed in abbreviated form, using
7220 a date like @samp{Mar 30@ @ 2002} for non-recent timestamps, and a
7221 date-without-year and time like @samp{Mar 30 23:45} for recent timestamps.
7222 This format can change depending on the current locale as detailed below.
7225 A timestamp is considered to be @dfn{recent} if it is less than six
7226 months old, and is not dated in the future. If a timestamp dated
7227 today is not listed in recent form, the timestamp is in the future,
7228 which means you probably have clock skew problems which may break
7229 programs like @command{make} that rely on file timestamps.
7232 Time stamps are listed according to the time zone rules specified by
7233 the @env{TZ} environment variable, or by the system default rules if
7234 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
7235 with @env{TZ}, libc, The GNU C Library Reference Manual}.
7237 The following option changes how file timestamps are printed.
7240 @item --time-style=@var{style}
7241 @opindex --time-style
7243 List timestamps in style @var{style}. The @var{style} should
7244 be one of the following:
7249 List timestamps using @var{format}, where @var{format} is interpreted
7250 like the format argument of @command{date} (@pxref{date invocation}).
7251 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
7252 @command{ls} to list timestamps like @samp{2002-03-30 23:45:56}. As
7253 with @command{date}, @var{format}'s interpretation is affected by the
7254 @env{LC_TIME} locale category.
7256 If @var{format} contains two format strings separated by a newline,
7257 the former is used for non-recent files and the latter for recent
7258 files; if you want output columns to line up, you may need to insert
7259 spaces in one of the two formats.
7262 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
7263 format with nanosecond precision, e.g., @samp{2002-03-30
7264 23:45:56.477817180 -0700}. This style is equivalent to
7265 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
7267 This is useful because the time output includes all the information that
7268 is available from the operating system. For example, this can help
7269 explain @command{make}'s behavior, since @acronym{GNU} @command{make}
7270 uses the full timestamp to determine whether a file is out of date.
7273 List @acronym{ISO} 8601 date and time in minutes, e.g.,
7274 @samp{2002-03-30 23:45}. These timestamps are shorter than
7275 @samp{full-iso} timestamps, and are usually good enough for everyday
7276 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
7279 List @acronym{ISO} 8601 dates for non-recent timestamps (e.g.,
7280 @samp{2002-03-30@ }), and @acronym{ISO} 8601 month, day, hour, and
7281 minute for recent timestamps (e.g., @samp{03-30 23:45}). These
7282 timestamps are uglier than @samp{long-iso} timestamps, but they carry
7283 nearly the same information in a smaller space and their brevity helps
7284 @command{ls} output fit within traditional 80-column output lines.
7285 The following two @command{ls} invocations are equivalent:
7290 ls -l --time-style="+%Y-%m-%d $newline%m-%d %H:%M"
7291 ls -l --time-style="iso"
7296 List timestamps in a locale-dependent form. For example, a Finnish
7297 locale might list non-recent timestamps like @samp{maalis 30@ @ 2002}
7298 and recent timestamps like @samp{maalis 30 23:45}. Locale-dependent
7299 timestamps typically consume more space than @samp{iso} timestamps and
7300 are harder for programs to parse because locale conventions vary so
7301 widely, but they are easier for many people to read.
7303 The @env{LC_TIME} locale category specifies the timestamp format. The
7304 default @acronym{POSIX} locale uses timestamps like @samp{Mar 30@
7305 @ 2002} and @samp{Mar 30 23:45}; in this locale, the following two
7306 @command{ls} invocations are equivalent:
7311 ls -l --time-style="+%b %e %Y$newline%b %e %H:%M"
7312 ls -l --time-style="locale"
7315 Other locales behave differently. For example, in a German locale,
7316 @option{--time-style="locale"} might be equivalent to
7317 @option{--time-style="+%e. %b %Y $newline%e. %b %H:%M"}
7318 and might generate timestamps like @samp{30. M@"ar 2002@ } and
7319 @samp{30. M@"ar 23:45}.
7321 @item posix-@var{style}
7323 List @acronym{POSIX}-locale timestamps if the @env{LC_TIME} locale
7324 category is @acronym{POSIX}, @var{style} timestamps otherwise. For
7325 example, the @samp{posix-long-iso} style lists
7326 timestamps like @samp{Mar 30@ @ 2002} and @samp{Mar 30 23:45} when in
7327 the @acronym{POSIX} locale, and like @samp{2002-03-30 23:45} otherwise.
7332 You can specify the default value of the @option{--time-style} option
7333 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
7334 the default style is @samp{locale}. @acronym{GNU} Emacs 21.3 and
7335 later use the @option{--dired} option and therefore can parse any date
7336 format, but if you are using Emacs 21.1 or 21.2 and specify a
7337 non-@acronym{POSIX} locale you may need to set
7338 @samp{TIME_STYLE="posix-long-iso"}.
7340 To avoid certain denial-of-service attacks, timestamps that would be
7341 longer than 1000 bytes may be treated as errors.
7344 @node Formatting the file names
7345 @subsection Formatting the file names
7347 These options change how file names themselves are printed.
7353 @itemx --quoting-style=escape
7356 @opindex --quoting-style
7357 @cindex backslash sequences for file names
7358 Quote nongraphic characters in file names using alphabetic and octal
7359 backslash sequences like those used in C.
7363 @itemx --quoting-style=literal
7366 @opindex --quoting-style
7367 Do not quote file names. However, with @command{ls} nongraphic
7368 characters are still printed as question marks if the output is a
7369 terminal and you do not specify the @option{--show-control-chars}
7373 @itemx --hide-control-chars
7375 @opindex --hide-control-chars
7376 Print question marks instead of nongraphic characters in file names.
7377 This is the default if the output is a terminal and the program is
7382 @itemx --quoting-style=c
7384 @opindex --quote-name
7385 @opindex --quoting-style
7386 Enclose file names in double quotes and quote nongraphic characters as
7389 @item --quoting-style=@var{word}
7390 @opindex --quoting-style
7391 @cindex quoting style
7392 Use style @var{word} to quote file names and other strings that may
7393 contain arbitrary characters. The @var{word} should
7394 be one of the following:
7398 Output strings as-is; this is the same as the @option{-N} or
7399 @option{--literal} option.
7401 Quote strings for the shell if they contain shell metacharacters or would
7402 cause ambiguous output.
7403 The quoting is suitable for @acronym{POSIX}-compatible shells like
7404 @command{bash}, but it does not always work for incompatible shells
7407 Quote strings for the shell, even if they would normally not require quoting.
7409 Quote strings as for C character string literals, including the
7410 surrounding double-quote characters; this is the same as the
7411 @option{-Q} or @option{--quote-name} option.
7413 Quote strings as for C character string literals, except omit the
7414 surrounding double-quote
7415 characters; this is the same as the @option{-b} or @option{--escape} option.
7417 Quote strings as for C character string literals, except use
7418 surrounding quotation marks appropriate for the
7421 @c Use @t instead of @samp to avoid duplicate quoting in some output styles.
7422 Quote strings as for C character string literals, except use
7423 surrounding quotation marks appropriate for the locale, and quote
7424 @t{'like this'} instead of @t{"like
7425 this"} in the default C locale. This looks nicer on many displays.
7428 You can specify the default value of the @option{--quoting-style} option
7429 with the environment variable @env{QUOTING_STYLE}. If that environment
7430 variable is not set, the default value is @samp{literal}, but this
7431 default may change to @samp{shell} in a future version of this package.
7433 @item --show-control-chars
7434 @opindex --show-control-chars
7435 Print nongraphic characters as-is in file names.
7436 This is the default unless the output is a terminal and the program is
7442 @node dir invocation
7443 @section @command{dir}: Briefly list directory contents
7446 @cindex directory listing, brief
7448 @command{dir} is equivalent to @code{ls -C
7449 -b}; that is, by default files are listed in columns, sorted vertically,
7450 and special characters are represented by backslash escape sequences.
7452 @xref{ls invocation, @command{ls}}.
7455 @node vdir invocation
7456 @section @command{vdir}: Verbosely list directory contents
7459 @cindex directory listing, verbose
7461 @command{vdir} is equivalent to @code{ls -l
7462 -b}; that is, by default files are listed in long format and special
7463 characters are represented by backslash escape sequences.
7465 @node dircolors invocation
7466 @section @command{dircolors}: Color setup for @command{ls}
7470 @cindex setup for color
7472 @command{dircolors} outputs a sequence of shell commands to set up the
7473 terminal for color output from @command{ls} (and @command{dir}, etc.).
7477 eval "`dircolors [@var{option}]@dots{} [@var{file}]`"
7480 If @var{file} is specified, @command{dircolors} reads it to determine which
7481 colors to use for which file types and extensions. Otherwise, a
7482 precompiled database is used. For details on the format of these files,
7483 run @samp{dircolors --print-database}.
7485 To make @command{dircolors} read a @file{~/.dircolors} file if it
7486 exists, you can put the following lines in your @file{~/.bashrc} (or
7487 adapt them to your favorite shell):
7491 test -r $d && eval "$(dircolors $d)"
7495 @vindex SHELL @r{environment variable, and color}
7496 The output is a shell command to set the @env{LS_COLORS} environment
7497 variable. You can specify the shell syntax to use on the command line,
7498 or @command{dircolors} will guess it from the value of the @env{SHELL}
7499 environment variable.
7501 The program accepts the following options. Also see @ref{Common options}.
7506 @itemx --bourne-shell
7509 @opindex --bourne-shell
7510 @cindex Bourne shell syntax for color setup
7511 @cindex @command{sh} syntax for color setup
7512 Output Bourne shell commands. This is the default if the @env{SHELL}
7513 environment variable is set and does not end with @samp{csh} or
7522 @cindex C shell syntax for color setup
7523 @cindex @command{csh} syntax for color setup
7524 Output C shell commands. This is the default if @code{SHELL} ends with
7525 @command{csh} or @command{tcsh}.
7528 @itemx --print-database
7530 @opindex --print-database
7531 @cindex color database, printing
7532 @cindex database for color setup, printing
7533 @cindex printing color database
7534 Print the (compiled-in) default color configuration database. This
7535 output is itself a valid configuration file, and is fairly descriptive
7536 of the possibilities.
7543 @node Basic operations
7544 @chapter Basic operations
7546 @cindex manipulating files
7548 This chapter describes the commands for basic file manipulation:
7549 copying, moving (renaming), and deleting (removing).
7552 * cp invocation:: Copy files.
7553 * dd invocation:: Convert and copy a file.
7554 * install invocation:: Copy files and set attributes.
7555 * mv invocation:: Move (rename) files.
7556 * rm invocation:: Remove files or directories.
7557 * shred invocation:: Remove files more securely.
7562 @section @command{cp}: Copy files and directories
7565 @cindex copying files and directories
7566 @cindex files, copying
7567 @cindex directories, copying
7569 @command{cp} copies files (or, optionally, directories). The copy is
7570 completely independent of the original. You can either copy one file to
7571 another, or copy arbitrarily many files to a destination directory.
7575 cp [@var{option}]@dots{} [-T] @var{source} @var{dest}
7576 cp [@var{option}]@dots{} @var{source}@dots{} @var{directory}
7577 cp [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
7582 If two file names are given, @command{cp} copies the first file to the
7586 If the @option{--target-directory} (@option{-t}) option is given, or
7587 failing that if the last file is a directory and the
7588 @option{--no-target-directory} (@option{-T}) option is not given,
7589 @command{cp} copies each @var{source} file to the specified directory,
7590 using the @var{source}s' names.
7593 Generally, files are written just as they are read. For exceptions,
7594 see the @option{--sparse} option below.
7596 By default, @command{cp} does not copy directories. However, the
7597 @option{-R}, @option{-a}, and @option{-r} options cause @command{cp} to
7598 copy recursively by descending into source directories and copying files
7599 to corresponding destination directories.
7601 When copying from a symbolic link, @command{cp} normally follows the
7602 link only when not copying
7603 recursively. This default can be overridden with the
7604 @option{--archive} (@option{-a}), @option{-d}, @option{--dereference}
7605 (@option{-L}), @option{--no-dereference} (@option{-P}), and
7606 @option{-H} options. If more than one of these options is specified,
7607 the last one silently overrides the others.
7609 When copying to a symbolic link, @command{cp} follows the
7610 link only when it refers to an existing regular file.
7611 However, when copying to a dangling symbolic link, @command{cp}
7612 refuses by default, and fails with a diagnostic, since the operation
7613 is inherently dangerous. This behavior is contrary to historical
7614 practice and to @acronym{POSIX}.
7615 Set @env{POSIXLY_CORRECT} to make @command{cp} attempt to create
7616 the target of a dangling destination symlink, in spite of the possible risk.
7617 Also, when an option like
7618 @option{--backup} or @option{--link} acts to rename or remove the
7619 destination before copying, @command{cp} renames or removes the
7620 symbolic link rather than the file it points to.
7622 By default, @command{cp} copies the contents of special files only
7623 when not copying recursively. This default can be overridden with the
7624 @option{--copy-contents} option.
7626 @cindex self-backups
7627 @cindex backups, making only
7628 @command{cp} generally refuses to copy a file onto itself, with the
7629 following exception: if @option{--force --backup} is specified with
7630 @var{source} and @var{dest} identical, and referring to a regular file,
7631 @command{cp} will make a backup file, either regular or numbered, as
7632 specified in the usual ways (@pxref{Backup options}). This is useful when
7633 you simply want to make a backup of an existing file before changing it.
7635 The program accepts the following options. Also see @ref{Common options}.
7642 Preserve as much as possible of the structure and attributes of the
7643 original files in the copy (but do not attempt to preserve internal
7644 directory structure; i.e., @samp{ls -U} may list the entries in a copied
7645 directory in a different order).
7646 Try to preserve SELinux security context and extended attributes (xattr),
7647 but ignore any failure to do that and print no corresponding diagnostic.
7648 Equivalent to @option{-dR --preserve=all} with the reduced diagnostics.
7650 @itemx --attributes-only
7651 @opindex --attributes-only
7652 Preserve the specified attributes of the original files in the copy,
7653 but do not copy any data. See the @option{--preserve} option for
7654 controlling which attributes to copy.
7657 @itemx @w{@kbd{--backup}[=@var{method}]}
7660 @vindex VERSION_CONTROL
7661 @cindex backups, making
7662 @xref{Backup options}.
7663 Make a backup of each file that would otherwise be overwritten or removed.
7664 As a special case, @command{cp} makes a backup of @var{source} when the force
7665 and backup options are given and @var{source} and @var{dest} are the same
7666 name for an existing, regular file. One useful application of this
7667 combination of options is this tiny Bourne shell script:
7671 # Usage: backup FILE...
7672 # Create a @sc{gnu}-style backup of each listed FILE.
7674 cp --backup --force -- "$i" "$i"
7678 @item --copy-contents
7679 @cindex directories, copying recursively
7680 @cindex copying directories recursively
7681 @cindex recursively copying directories
7682 @cindex non-directories, copying as special files
7683 If copying recursively, copy the contents of any special files (e.g.,
7684 FIFOs and device files) as if they were regular files. This means
7685 trying to read the data in each source file and writing it to the
7686 destination. It is usually a mistake to use this option, as it
7687 normally has undesirable effects on special files like FIFOs and the
7688 ones typically found in the @file{/dev} directory. In most cases,
7689 @code{cp -R --copy-contents} will hang indefinitely trying to read
7690 from FIFOs and special files like @file{/dev/console}, and it will
7691 fill up your destination disk if you use it to copy @file{/dev/zero}.
7692 This option has no effect unless copying recursively, and it does not
7693 affect the copying of symbolic links.
7697 @cindex symbolic links, copying
7698 @cindex hard links, preserving
7699 Copy symbolic links as symbolic links rather than copying the files that
7700 they point to, and preserve hard links between source files in the copies.
7701 Equivalent to @option{--no-dereference --preserve=links}.
7707 When copying without this option and an existing destination file cannot
7708 be opened for writing, the copy fails. However, with @option{--force}),
7709 when a destination file cannot be opened, @command{cp} then removes it and
7710 tries to open it again. Contrast this behavior with that enabled by
7711 @option{--link} and @option{--symbolic-link}, whereby the destination file
7712 is never opened but rather is removed unconditionally. Also see the
7713 description of @option{--remove-destination}.
7715 This option is independent of the @option{--interactive} or
7716 @option{-i} option: neither cancels the effect of the other.
7718 This option is redundant if the @option{--no-clobber} or @option{-n} option is
7723 If a command line argument specifies a symbolic link, then copy the
7724 file it points to rather than the symbolic link itself. However,
7725 copy (preserving its nature) any symbolic link that is encountered
7726 via recursive traversal.
7729 @itemx --interactive
7731 @opindex --interactive
7732 When copying a file other than a directory, prompt whether to
7733 overwrite an existing destination file. The @option{-i} option overrides
7734 a previous @option{-n} option.
7740 Make hard links instead of copies of non-directories.
7743 @itemx --dereference
7745 @opindex --dereference
7746 Follow symbolic links when copying from them.
7747 With this option, @command{cp} cannot create a symbolic link.
7748 For example, a symlink (to regular file) in the source tree will be copied to
7749 a regular file in the destination tree.
7754 @opindex --no-clobber
7755 Do not overwrite an existing file. The @option{-n} option overrides a previous
7756 @option{-i} option. This option is mutually exclusive with @option{-b} or
7757 @option{--backup} option.
7760 @itemx --no-dereference
7762 @opindex --no-dereference
7763 @cindex symbolic links, copying
7764 Copy symbolic links as symbolic links rather than copying the files that
7765 they point to. This option affects only symbolic links in the source;
7766 symbolic links in the destination are always followed if possible.
7769 @itemx @w{@kbd{--preserve}[=@var{attribute_list}]}
7772 @cindex file information, preserving, extended attributes, xattr
7773 Preserve the specified attributes of the original files.
7774 If specified, the @var{attribute_list} must be a comma-separated list
7775 of one or more of the following strings:
7779 Preserve the file mode bits and access control lists.
7781 Preserve the owner and group. On most modern systems,
7782 only users with appropriate privileges may change the owner of a file,
7784 may preserve the group ownership of a file only if they happen to be
7785 a member of the desired group.
7787 Preserve the times of last access and last modification, when possible.
7788 On older systems, it is not possible to preserve these attributes
7789 when the affected file is a symbolic link.
7790 However, many systems now provide the @code{utimensat} function,
7791 which makes it possible even for symbolic links.
7793 Preserve in the destination files
7794 any links between corresponding source files.
7795 Note that with @option{-L} or @option{-H}, this option can convert
7796 symbolic links to hard links. For example,
7798 $ mkdir c; : > a; ln -s a b; cp -aH a b c; ls -i1 c
7803 Note the inputs: @file{b} is a symlink to regular file @file{a},
7804 yet the files in destination directory, @file{c/}, are hard-linked.
7805 Since @option{-a} implies @option{--preserve=links}, and since @option{-H}
7806 tells @command{cp} to dereference command line arguments, it sees two files
7807 with the same inode number, and preserves the perceived hard link.
7809 Here is a similar example that exercises @command{cp}'s @option{-L} option:
7811 $ mkdir b c; (cd b; : > a; ln -s a b); cp -aL b c; ls -i1 c/b
7817 Preserve SELinux security context of the file, or fail with full diagnostics.
7819 Preserve extended attributes of the file, or fail with full diagnostics.
7820 If @command{cp} is built without xattr support, ignore this option.
7821 If SELinux context, ACLs or Capabilities are implemented using xattrs,
7822 they are preserved by this option as well.
7824 Preserve all file attributes.
7825 Equivalent to specifying all of the above, but with the difference
7826 that failure to preserve SELinux security context or extended attributes
7827 does not change @command{cp}'s exit status. In contrast to @option{-a},
7828 all but @samp{Operation not supported} warnings are output.
7831 Using @option{--preserve} with no @var{attribute_list} is equivalent
7832 to @option{--preserve=mode,ownership,timestamps}.
7834 In the absence of this option, each destination file is created with the
7835 mode bits of the corresponding source file, minus the bits set in the
7836 umask and minus the set-user-ID and set-group-ID bits.
7837 @xref{File permissions}.
7839 @itemx @w{@kbd{--no-preserve}=@var{attribute_list}}
7840 @cindex file information, preserving
7841 Do not preserve the specified attributes. The @var{attribute_list}
7842 has the same form as for @option{--preserve}.
7846 @cindex parent directories and @command{cp}
7847 Form the name of each destination file by appending to the target
7848 directory a slash and the specified name of the source file. The last
7849 argument given to @command{cp} must be the name of an existing directory.
7850 For example, the command:
7853 cp --parents a/b/c existing_dir
7857 copies the file @file{a/b/c} to @file{existing_dir/a/b/c}, creating
7858 any missing intermediate directories.
7865 @opindex --recursive
7866 @cindex directories, copying recursively
7867 @cindex copying directories recursively
7868 @cindex recursively copying directories
7869 @cindex non-directories, copying as special files
7870 Copy directories recursively. By default, do not follow symbolic
7871 links in the source; see the @option{--archive} (@option{-a}), @option{-d},
7872 @option{--dereference} (@option{-L}), @option{--no-dereference}
7873 (@option{-P}), and @option{-H} options. Special files are copied by
7874 creating a destination file of the same type as the source; see the
7875 @option{--copy-contents} option. It is not portable to use
7876 @option{-r} to copy symbolic links or special files. On some
7877 non-@sc{gnu} systems, @option{-r} implies the equivalent of
7878 @option{-L} and @option{--copy-contents} for historical reasons.
7879 Also, it is not portable to use @option{-R} to copy symbolic links
7880 unless you also specify @option{-P}, as @acronym{POSIX} allows
7881 implementations that dereference symbolic links by default.
7883 @item --reflink[=@var{when}]
7884 @opindex --reflink[=@var{when}]
7887 @cindex copy on write
7888 Perform a lightweight, copy-on-write (COW) copy, if supported by the
7889 file system. Once it has succeeded, beware that the source and destination
7890 files share the same disk data blocks as long as they remain unmodified.
7891 Thus, if a disk I/O error affects data blocks of one of the files,
7892 the other suffers the same fate.
7894 The @var{when} value can be one of the following:
7898 The default behavior: if the copy-on-write operation is not supported
7899 then report the failure for each file and exit with a failure status.
7902 If the copy-on-write operation is not supported then fall back
7903 to the standard copy behaviour.
7906 This option is overridden by the @option{--link}, @option{--symbolic-link}
7907 and @option{--attributes-only} options, thus allowing it to be used
7908 to configure the default data copying behavior for @command{cp}.
7909 For example, with the following alias, @command{cp} will use the
7910 minimum amount of space supported by the file system.
7913 alias cp='cp --reflink=auto --sparse=always'
7916 @item --remove-destination
7917 @opindex --remove-destination
7918 Remove each existing destination file before attempting to open it
7919 (contrast with @option{-f} above).
7921 @item --sparse=@var{when}
7922 @opindex --sparse=@var{when}
7923 @cindex sparse files, copying
7924 @cindex holes, copying files with
7925 @findex read @r{system call, and holes}
7926 A @dfn{sparse file} contains @dfn{holes}---a sequence of zero bytes that
7927 does not occupy any physical disk blocks; the @samp{read} system call
7928 reads these as zeros. This can both save considerable disk space and
7929 increase speed, since many binary files contain lots of consecutive zero
7930 bytes. By default, @command{cp} detects holes in input source files via a crude
7931 heuristic and makes the corresponding output file sparse as well.
7932 Only regular files may be sparse.
7934 The @var{when} value can be one of the following:
7938 The default behavior: if the input file is sparse, attempt to make
7939 the output file sparse, too. However, if an output file exists but
7940 refers to a non-regular file, then do not attempt to make it sparse.
7943 For each sufficiently long sequence of zero bytes in the input file,
7944 attempt to create a corresponding hole in the output file, even if the
7945 input file does not appear to be sparse.
7946 This is useful when the input file resides on a file system
7947 that does not support sparse files
7948 (for example, @samp{efs} file systems in SGI IRIX 5.3 and earlier),
7949 but the output file is on a type of file system that does support them.
7950 Holes may be created only in regular files, so if the destination file
7951 is of some other type, @command{cp} does not even try to make it sparse.
7954 Never make the output file sparse.
7955 This is useful in creating a file for use with the @command{mkswap} command,
7956 since such a file must not have any holes.
7959 @optStripTrailingSlashes
7962 @itemx --symbolic-link
7964 @opindex --symbolic-link
7965 @cindex symbolic links, copying with
7966 Make symbolic links instead of copies of non-directories. All source
7967 file names must be absolute (starting with @samp{/}) unless the
7968 destination files are in the current directory. This option merely
7969 results in an error message on systems that do not support symbolic links.
7975 @optNoTargetDirectory
7981 @cindex newer files, copying only
7982 Do not copy a non-directory that has an existing destination with the
7983 same or newer modification time. If time stamps are being preserved,
7984 the comparison is to the source time stamp truncated to the
7985 resolutions of the destination file system and of the system calls
7986 used to update time stamps; this avoids duplicate work if several
7987 @samp{cp -pu} commands are executed with the same source and destination.
7988 If @option{--preserve=links} is also specified (like with @samp{cp -au}
7989 for example), that will take precedence. Consequently, depending on the
7990 order that files are processed from the source, newer files in the destination
7991 may be replaced, to mirror hard links in the source.
7997 Print the name of each file before copying it.
8000 @itemx --one-file-system
8002 @opindex --one-file-system
8003 @cindex file systems, omitting copying to different
8004 Skip subdirectories that are on different file systems from the one that
8005 the copy started on.
8006 However, mount point directories @emph{are} copied.
8014 @section @command{dd}: Convert and copy a file
8017 @cindex converting while copying a file
8019 @command{dd} copies a file (from standard input to standard output, by
8020 default) with a changeable I/O block size, while optionally performing
8021 conversions on it. Synopses:
8024 dd [@var{operand}]@dots{}
8028 The only options are @option{--help} and @option{--version}.
8029 @xref{Common options}. @command{dd} accepts the following operands.
8035 Read from @var{file} instead of standard input.
8039 Write to @var{file} instead of standard output. Unless
8040 @samp{conv=notrunc} is given, @command{dd} truncates @var{file} to zero
8041 bytes (or the size specified with @samp{seek=}).
8043 @item ibs=@var{bytes}
8045 @cindex block size of input
8046 @cindex input block size
8047 Set the input block size to @var{bytes}.
8048 This makes @command{dd} read @var{bytes} per block.
8049 The default is 512 bytes.
8051 @item obs=@var{bytes}
8053 @cindex block size of output
8054 @cindex output block size
8055 Set the output block size to @var{bytes}.
8056 This makes @command{dd} write @var{bytes} per block.
8057 The default is 512 bytes.
8059 @item bs=@var{bytes}
8062 Set both input and output block sizes to @var{bytes}.
8063 This makes @command{dd} read and write @var{bytes} per block,
8064 overriding any @samp{ibs} and @samp{obs} settings.
8065 In addition, if no data-transforming @option{conv} option is specified,
8066 input is copied to the output as soon as it's read,
8067 even if it is smaller than the block size.
8069 @item cbs=@var{bytes}
8071 @cindex block size of conversion
8072 @cindex conversion block size
8073 @cindex fixed-length records, converting to variable-length
8074 @cindex variable-length records, converting to fixed-length
8075 Set the conversion block size to @var{bytes}.
8076 When converting variable-length records to fixed-length ones
8077 (@option{conv=block}) or the reverse (@option{conv=unblock}),
8078 use @var{bytes} as the fixed record length.
8082 Skip @var{n} @samp{ibs}-byte blocks in the input file before copying.
8083 If @samp{iflag=skip_bytes} is specified, @var{n} is interpreted
8084 as a byte count rather than a block count.
8088 Skip @var{n} @samp{obs}-byte blocks in the output file before copying.
8089 if @samp{oflag=seek_bytes} is specified, @var{n} is interpreted
8090 as a byte count rather than a block count.
8094 Copy @var{n} @samp{ibs}-byte blocks from the input file, instead
8095 of everything until the end of the file.
8096 if @samp{iflag=count_bytes} is specified, @var{n} is interpreted
8097 as a byte count rather than a block count.
8101 Do not print the overall transfer rate and volume statistics
8102 that normally make up the third status line when @command{dd} exits.
8104 @item conv=@var{conversion}[,@var{conversion}]@dots{}
8106 Convert the file as specified by the @var{conversion} argument(s).
8107 (No spaces around any comma(s).)
8114 @opindex ascii@r{, converting to}
8115 Convert @acronym{EBCDIC} to @acronym{ASCII},
8116 using the conversion table specified by @acronym{POSIX}.
8117 This provides a 1:1 translation for all 256 bytes.
8120 @opindex ebcdic@r{, converting to}
8121 Convert @acronym{ASCII} to @acronym{EBCDIC}.
8122 This is the inverse of the @samp{ascii} conversion.
8125 @opindex alternate ebcdic@r{, converting to}
8126 Convert @acronym{ASCII} to alternate @acronym{EBCDIC},
8127 using the alternate conversion table specified by @acronym{POSIX}.
8128 This is not a 1:1 translation, but reflects common historical practice
8129 for @samp{~}, @samp{[}, and @samp{]}.
8131 The @samp{ascii}, @samp{ebcdic}, and @samp{ibm} conversions are
8135 @opindex block @r{(space-padding)}
8136 For each line in the input, output @samp{cbs} bytes, replacing the
8137 input newline with a space and padding with spaces as necessary.
8141 Remove any trailing spaces in each @samp{cbs}-sized input block,
8142 and append a newline.
8144 The @samp{block} and @samp{unblock} conversions are mutually exclusive.
8147 @opindex lcase@r{, converting to}
8148 Change uppercase letters to lowercase.
8151 @opindex ucase@r{, converting to}
8152 Change lowercase letters to uppercase.
8154 The @samp{lcase} and @samp{ucase} conversions are mutually exclusive.
8158 Try to seek rather than write @sc{nul} output blocks.
8159 On a file system that supports sparse files, this will create
8160 sparse output when extending the output file.
8161 Be careful when using this option in conjunction with
8162 @samp{conv=notrunc} or @samp{oflag=append}.
8163 With @samp{conv=notrunc}, existing data in the output
8164 corresponding to @sc{nul} blocks from the input, will be untouched.
8165 With @samp{oflag=append} the seeks performed will be ineffective.
8168 @opindex swab @r{(byte-swapping)}
8169 @cindex byte-swapping
8170 Swap every pair of input bytes. @sc{gnu} @command{dd}, unlike others, works
8171 when an odd number of bytes are read---the last byte is simply copied
8172 (since there is nothing to swap it with).
8175 @opindex sync @r{(padding with @acronym{ASCII} @sc{nul}s)}
8176 Pad every input block to size of @samp{ibs} with trailing zero bytes.
8177 When used with @samp{block} or @samp{unblock}, pad with spaces instead of
8182 The following ``conversions'' are really file flags
8183 and don't affect internal processing:
8188 @cindex creating output file, requiring
8189 Fail if the output file already exists; @command{dd} must create the
8194 @cindex creating output file, avoiding
8195 Do not create the output file; the output file must already exist.
8197 The @samp{excl} and @samp{nocreat} conversions are mutually exclusive.
8201 @cindex truncating output file, avoiding
8202 Do not truncate the output file.
8206 @cindex read errors, ignoring
8207 Continue after read errors.
8211 @cindex synchronized data writes, before finishing
8212 Synchronize output data just before finishing. This forces a physical
8213 write of output data.
8217 @cindex synchronized data and metadata writes, before finishing
8218 Synchronize output data and metadata just before finishing. This
8219 forces a physical write of output data and metadata.
8223 @item iflag=@var{flag}[,@var{flag}]@dots{}
8225 Access the input file using the flags specified by the @var{flag}
8226 argument(s). (No spaces around any comma(s).)
8228 @item oflag=@var{flag}[,@var{flag}]@dots{}
8230 Access the output file using the flags specified by the @var{flag}
8231 argument(s). (No spaces around any comma(s).)
8233 Here are the flags. Not every flag is supported on every operating
8240 @cindex appending to the output file
8241 Write in append mode, so that even if some other process is writing to
8242 this file, every @command{dd} write will append to the current
8243 contents of the file. This flag makes sense only for output.
8244 If you combine this flag with the @samp{of=@var{file}} operand,
8245 you should also specify @samp{conv=notrunc} unless you want the
8246 output file to be truncated before being appended to.
8250 @cindex concurrent I/O
8251 Use concurrent I/O mode for data. This mode performs direct I/O
8252 and drops the @acronym{POSIX} requirement to serialize all I/O to the same file.
8253 A file cannot be opened in CIO mode and with a standard open at the
8259 Use direct I/O for data, avoiding the buffer cache.
8260 Note that the kernel may impose restrictions on read or write buffer sizes.
8261 For example, with an ext4 destination file system and a linux-based kernel,
8262 using @samp{oflag=direct} will cause writes to fail with @code{EINVAL} if the
8263 output buffer size is not a multiple of 512.
8267 @cindex directory I/O
8269 Fail unless the file is a directory. Most operating systems do not
8270 allow I/O to a directory, so this flag has limited utility.
8274 @cindex synchronized data reads
8275 Use synchronized I/O for data. For the output file, this forces a
8276 physical write of output data on each write. For the input file,
8277 this flag can matter when reading from a remote file that has been
8278 written to synchronously by some other process. Metadata (e.g.,
8279 last-access and last-modified time) is not necessarily synchronized.
8283 @cindex synchronized data and metadata I/O
8284 Use synchronized I/O for both data and metadata.
8288 @cindex discarding file cache
8289 Discard the data cache for a file.
8290 When count=0 all cache is discarded,
8291 otherwise the cache is dropped for the processed
8292 portion of the file. Also when count=0
8293 failure to discard the cache is diagnosed
8294 and reflected in the exit status.
8295 Here as some usage examples:
8298 # Advise to drop cache for whole file
8299 dd if=ifile iflag=nocache count=0
8301 # Ensure drop cache for the whole file
8302 dd of=ofile oflag=nocache conv=notrunc,fdatasync count=0
8304 # Drop cache for part of file
8305 dd if=ifile iflag=nocache skip=10 count=10 of=/dev/null
8307 # Stream data using just the read-ahead cache
8308 dd if=ifile of=ofile iflag=nocache oflag=nocache
8313 @cindex nonblocking I/O
8314 Use non-blocking I/O.
8319 Do not update the file's access time.
8320 Some older file systems silently ignore this flag, so it is a good
8321 idea to test it on your files before relying on it.
8325 @cindex controlling terminal
8326 Do not assign the file to be a controlling terminal for @command{dd}.
8327 This has no effect when the file is not a terminal.
8328 On many hosts (e.g., @acronym{GNU}/Linux hosts), this option has no effect
8333 @cindex symbolic links, following
8334 Do not follow symbolic links.
8339 Fail if the file has multiple hard links.
8344 Use binary I/O. This option has an effect only on nonstandard
8345 platforms that distinguish binary from text I/O.
8350 Use text I/O. Like @samp{binary}, this option has no effect on
8355 Accumulate full blocks from input. The @code{read} system call
8356 may return early if a full block is not available.
8357 When that happens, continue calling @code{read} to fill the remainder
8359 This flag can be used only with @code{iflag}.
8362 @opindex count_bytes
8363 Interpret the @samp{count=} operand as a byte count,
8364 rather than a block count, which allows specifying
8365 a length that is not a multiple of the I/O block size.
8366 This flag can be used only with @code{iflag}.
8370 Interpret the @samp{skip=} operand as a byte count,
8371 rather than a block count, which allows specifying
8372 an offset that is not a multiple of the I/O block size.
8373 This flag can be used only with @code{iflag}.
8377 Interpret the @samp{seek=} operand as a byte count,
8378 rather than a block count, which allows specifying
8379 an offset that is not a multiple of the I/O block size.
8380 This flag can be used only with @code{oflag}.
8384 These flags are not supported on all systems, and @samp{dd} rejects
8385 attempts to use them when they are not supported. When reading from
8386 standard input or writing to standard output, the @samp{nofollow} and
8387 @samp{noctty} flags should not be specified, and the other flags
8388 (e.g., @samp{nonblock}) can affect how other processes behave with the
8389 affected file descriptors, even after @command{dd} exits.
8393 @cindex multipliers after numbers
8394 The numeric-valued strings above (@var{bytes} and @var{blocks}) can be
8395 followed by a multiplier: @samp{b}=512, @samp{c}=1,
8396 @samp{w}=2, @samp{x@var{m}}=@var{m}, or any of the
8397 standard block size suffixes like @samp{k}=1024 (@pxref{Block size}).
8399 Any block size you specify via @samp{bs=}, @samp{ibs=}, @samp{obs=}, @samp{cbs=}
8400 should not be too large---values larger than a few megabytes
8401 are generally wasteful or (as in the gigabyte..exabyte case) downright
8402 counterproductive or error-inducing.
8404 To process data that is at an offset or size that is not a
8405 multiple of the I/O@ block size, you can use the @samp{skip_bytes},
8406 @samp{seek_bytes} and @samp{count_bytes} flags. Alternatively
8407 the traditional method of separate @command{dd} invocations can be used.
8408 For example, the following shell commands copy data
8409 in 512 KiB blocks between a disk and a tape, but do not save
8410 or restore a 4 KiB label at the start of the disk:
8413 disk=/dev/rdsk/c0t1d0s2
8416 # Copy all but the label from disk to tape.
8417 (dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape
8419 # Copy from tape back to disk, but leave the disk label alone.
8420 (dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk
8423 Sending an @samp{INFO} signal to a running @command{dd}
8424 process makes it print I/O statistics to standard error
8425 and then resume copying. In the example below,
8426 @command{dd} is run in the background to copy 10 million blocks.
8427 The @command{kill} command makes it output intermediate I/O statistics,
8428 and when @command{dd} completes normally or is killed by the
8429 @code{SIGINT} signal, it outputs the final statistics.
8432 $ dd if=/dev/zero of=/dev/null count=10MB & pid=$!
8433 $ kill -s INFO $pid; wait $pid
8434 3385223+0 records in
8435 3385223+0 records out
8436 1733234176 bytes (1.7 GB) copied, 6.42173 seconds, 270 MB/s
8437 10000000+0 records in
8438 10000000+0 records out
8439 5120000000 bytes (5.1 GB) copied, 18.913 seconds, 271 MB/s
8442 @vindex POSIXLY_CORRECT
8443 On systems lacking the @samp{INFO} signal @command{dd} responds to the
8444 @samp{USR1} signal instead, unless the @env{POSIXLY_CORRECT}
8445 environment variable is set.
8450 @node install invocation
8451 @section @command{install}: Copy files and set attributes
8454 @cindex copying files and setting attributes
8456 @command{install} copies files while setting their file mode bits and, if
8457 possible, their owner and group. Synopses:
8460 install [@var{option}]@dots{} [-T] @var{source} @var{dest}
8461 install [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8462 install [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8463 install [@var{option}]@dots{} -d @var{directory}@dots{}
8468 If two file names are given, @command{install} copies the first file to the
8472 If the @option{--target-directory} (@option{-t}) option is given, or
8473 failing that if the last file is a directory and the
8474 @option{--no-target-directory} (@option{-T}) option is not given,
8475 @command{install} copies each @var{source} file to the specified
8476 directory, using the @var{source}s' names.
8479 If the @option{--directory} (@option{-d}) option is given,
8480 @command{install} creates each @var{directory} and any missing parent
8481 directories. Parent directories are created with mode
8482 @samp{u=rwx,go=rx} (755), regardless of the @option{-m} option or the
8483 current umask. @xref{Directory Setuid and Setgid}, for how the
8484 set-user-ID and set-group-ID bits of parent directories are inherited.
8487 @cindex Makefiles, installing programs in
8488 @command{install} is similar to @command{cp}, but allows you to control the
8489 attributes of destination files. It is typically used in Makefiles to
8490 copy programs into their destination directories. It refuses to copy
8491 files onto themselves.
8493 @cindex extended attributes, xattr
8494 @command{install} never preserves extended attributes (xattr).
8496 The program accepts the following options. Also see @ref{Common options}.
8506 Compare each pair of source and destination files, and if the destination has
8507 identical content and any specified owner, group, permissions, and possibly
8508 SELinux context, then do not modify the destination at all.
8512 Ignored; for compatibility with old Unix versions of @command{install}.
8516 Create any missing parent directories of @var{dest},
8517 then copy @var{source} to @var{dest}.
8518 This option is ignored if a destination directory is specified
8519 via @option{--target-directory=DIR}.
8524 @opindex --directory
8525 @cindex directories, creating with given attributes
8526 @cindex parent directories, creating missing
8527 @cindex leading directories, creating missing
8528 Create any missing parent directories, giving them the default
8529 attributes. Then create each given directory, setting their owner,
8530 group and mode as given on the command line or to the defaults.
8532 @item -g @var{group}
8533 @itemx --group=@var{group}
8536 @cindex group ownership of installed files, setting
8537 Set the group ownership of installed files or directories to
8538 @var{group}. The default is the process's current group. @var{group}
8539 may be either a group name or a numeric group ID.
8542 @itemx --mode=@var{mode}
8545 @cindex permissions of installed files, setting
8546 Set the file mode bits for the installed file or directory to @var{mode},
8547 which can be either an octal number, or a symbolic mode as in
8548 @command{chmod}, with @samp{a=} (no access allowed to anyone) as the
8549 point of departure (@pxref{File permissions}).
8550 The default mode is @samp{u=rwx,go=rx,a-s}---read, write, and
8551 execute for the owner, read and execute for group and other, and with
8552 set-user-ID and set-group-ID disabled.
8553 This default is not quite the same as @samp{755}, since it disables
8554 instead of preserving set-user-ID and set-group-ID on directories.
8555 @xref{Directory Setuid and Setgid}.
8557 @item -o @var{owner}
8558 @itemx --owner=@var{owner}
8561 @cindex ownership of installed files, setting
8562 @cindex appropriate privileges
8563 @vindex root @r{as default owner}
8564 If @command{install} has appropriate privileges (is run as root), set the
8565 ownership of installed files or directories to @var{owner}. The default
8566 is @code{root}. @var{owner} may be either a user name or a numeric user
8569 @item --preserve-context
8570 @opindex --preserve-context
8572 @cindex security context
8573 Preserve the SELinux security context of files and directories.
8574 Failure to preserve the context in all of the files or directories
8575 will result in an exit status of 1. If SELinux is disabled then
8576 print a warning and ignore the option.
8579 @itemx --preserve-timestamps
8581 @opindex --preserve-timestamps
8582 @cindex timestamps of installed files, preserving
8583 Set the time of last access and the time of last modification of each
8584 installed file to match those of each corresponding original file.
8585 When a file is installed without this option, its last access and
8586 last modification times are both set to the time of installation.
8587 This option is useful if you want to use the last modification times
8588 of installed files to keep track of when they were last built as opposed
8589 to when they were last installed.
8595 @cindex symbol table information, stripping
8596 @cindex stripping symbol table information
8597 Strip the symbol tables from installed binary executables.
8599 @itemx --strip-program=@var{program}
8600 @opindex --strip-program
8601 @cindex symbol table information, stripping, program
8602 Program used to strip binaries.
8608 @optNoTargetDirectory
8614 Print the name of each file before copying it.
8616 @item -Z @var{context}
8617 @itemx --context=@var{context}
8621 @cindex security context
8622 Set the default SELinux security context to be used for any
8623 created files and directories. If SELinux is disabled then
8624 print a warning and ignore the option.
8632 @section @command{mv}: Move (rename) files
8636 @command{mv} moves or renames files (or directories). Synopses:
8639 mv [@var{option}]@dots{} [-T] @var{source} @var{dest}
8640 mv [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8641 mv [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8646 If two file names are given, @command{mv} moves the first file to the
8650 If the @option{--target-directory} (@option{-t}) option is given, or
8651 failing that if the last file is a directory and the
8652 @option{--no-target-directory} (@option{-T}) option is not given,
8653 @command{mv} moves each @var{source} file to the specified
8654 directory, using the @var{source}s' names.
8657 @command{mv} can move any type of file from one file system to another.
8658 Prior to version @code{4.0} of the fileutils,
8659 @command{mv} could move only regular files between file systems.
8660 For example, now @command{mv} can move an entire directory hierarchy
8661 including special device files from one partition to another. It first
8662 uses some of the same code that's used by @code{cp -a} to copy the
8663 requested directories and files, then (assuming the copy succeeded)
8664 it removes the originals. If the copy fails, then the part that was
8665 copied to the destination partition is removed. If you were to copy
8666 three directories from one partition to another and the copy of the first
8667 directory succeeded, but the second didn't, the first would be left on
8668 the destination partition and the second and third would be left on the
8671 @cindex extended attributes, xattr
8672 @command{mv} always tries to copy extended attributes (xattr), which may
8673 include SELinux context, ACLs or Capabilities.
8674 Upon failure all but @samp{Operation not supported} warnings are output.
8676 @cindex prompting, and @command{mv}
8677 If a destination file exists but is normally unwritable, standard input
8678 is a terminal, and the @option{-f} or @option{--force} option is not given,
8679 @command{mv} prompts the user for whether to replace the file. (You might
8680 own the file, or have write permission on its directory.) If the
8681 response is not affirmative, the file is skipped.
8683 @emph{Warning}: Avoid specifying a source name with a trailing slash,
8684 when it might be a symlink to a directory.
8685 Otherwise, @command{mv} may do something very surprising, since
8686 its behavior depends on the underlying rename system call.
8687 On a system with a modern Linux-based kernel, it fails with
8688 @code{errno=ENOTDIR}.
8689 However, on other systems (at least FreeBSD 6.1 and Solaris 10) it silently
8690 renames not the symlink but rather the directory referenced by the symlink.
8691 @xref{Trailing slashes}.
8693 The program accepts the following options. Also see @ref{Common options}.
8703 @cindex prompts, omitting
8704 Do not prompt the user before removing a destination file.
8706 If you specify more than one of the @option{-i}, @option{-f}, @option{-n}
8707 options, only the final one takes effect.
8712 @itemx --interactive
8714 @opindex --interactive
8715 @cindex prompts, forcing
8716 Prompt whether to overwrite each existing destination file, regardless
8718 If the response is not affirmative, the file is skipped.
8724 @opindex --no-clobber
8725 @cindex prompts, omitting
8726 Do not overwrite an existing file.
8728 This option is mutually exclusive with @option{-b} or @option{--backup} option.
8734 @cindex newer files, moving only
8735 Do not move a non-directory that has an existing destination with the
8736 same or newer modification time.
8737 If the move is across file system boundaries, the comparison is to the
8738 source time stamp truncated to the resolutions of the destination file
8739 system and of the system calls used to update time stamps; this avoids
8740 duplicate work if several @samp{mv -u} commands are executed with the
8741 same source and destination.
8747 Print the name of each file before moving it.
8749 @optStripTrailingSlashes
8755 @optNoTargetDirectory
8763 @section @command{rm}: Remove files or directories
8766 @cindex removing files or directories
8768 @command{rm} removes each given @var{file}. By default, it does not remove
8769 directories. Synopsis:
8772 rm [@var{option}]@dots{} [@var{file}]@dots{}
8775 @cindex prompting, and @command{rm}
8776 If the @option{-I} or @option{--interactive=once} option is given,
8777 and there are more than three files or the @option{-r}, @option{-R},
8778 or @option{--recursive} are given, then @command{rm} prompts the user
8779 for whether to proceed with the entire operation. If the response is
8780 not affirmative, the entire command is aborted.
8782 Otherwise, if a file is unwritable, standard input is a terminal, and
8783 the @option{-f} or @option{--force} option is not given, or the
8784 @option{-i} or @option{--interactive=always} option @emph{is} given,
8785 @command{rm} prompts the user for whether to remove the file.
8786 If the response is not affirmative, the file is skipped.
8788 Any attempt to remove a file whose last file name component is
8789 @file{.} or @file{..} is rejected without any prompting.
8791 @emph{Warning}: If you use @command{rm} to remove a file, it is usually
8792 possible to recover the contents of that file. If you want more assurance
8793 that the contents are truly unrecoverable, consider using @command{shred}.
8795 The program accepts the following options. Also see @ref{Common options}.
8803 Ignore nonexistent files and missing operands, and never prompt the user.
8804 Ignore any previous @option{--interactive} (@option{-i}) option.
8808 Prompt whether to remove each file.
8809 If the response is not affirmative, the file is skipped.
8810 Ignore any previous @option{--force} (@option{-f}) option.
8811 Equivalent to @option{--interactive=always}.
8815 Prompt once whether to proceed with the command, if more than three
8816 files are named or if a recursive removal is requested. Ignore any
8817 previous @option{--force} (@option{-f}) option. Equivalent to
8818 @option{--interactive=once}.
8820 @itemx --interactive [=@var{when}]
8821 @opindex --interactive
8822 Specify when to issue an interactive prompt. @var{when} may be
8826 @vindex never @r{interactive option}
8827 - Do not prompt at all.
8829 @vindex once @r{interactive option}
8830 - Prompt once if more than three files are named or if a recursive
8831 removal is requested. Equivalent to @option{-I}.
8833 @vindex always @r{interactive option}
8834 - Prompt for every file being removed. Equivalent to @option{-i}.
8836 @option{--interactive} with no @var{when} is equivalent to
8837 @option{--interactive=always}.
8839 @itemx --one-file-system
8840 @opindex --one-file-system
8841 @cindex one file system, restricting @command{rm} to
8842 When removing a hierarchy recursively, skip any directory that is on a
8843 file system different from that of the corresponding command line argument.
8846 This option is useful when removing a build ``chroot'' hierarchy,
8847 which normally contains no valuable data. However, it is not uncommon
8848 to bind-mount @file{/home} into such a hierarchy, to make it easier to
8849 use one's start-up file. The catch is that it's easy to forget to
8850 unmount @file{/home}. Then, when you use @command{rm -rf} to remove
8851 your normally throw-away chroot, that command will remove everything
8852 under @file{/home}, too.
8853 Use the @option{--one-file-system} option, and it will
8854 warn about and skip directories on other file systems.
8855 Of course, this will not save your @file{/home} if it and your
8856 chroot happen to be on the same file system.
8858 @itemx --preserve-root
8859 @opindex --preserve-root
8860 @cindex root directory, disallow recursive destruction
8861 Fail upon any attempt to remove the root directory, @file{/},
8862 when used with the @option{--recursive} option.
8863 This is the default behavior.
8864 @xref{Treating / specially}.
8866 @itemx --no-preserve-root
8867 @opindex --no-preserve-root
8868 @cindex root directory, allow recursive destruction
8869 Do not treat @file{/} specially when removing recursively.
8870 This option is not recommended unless you really want to
8871 remove all the files on your computer.
8872 @xref{Treating / specially}.
8879 @opindex --recursive
8880 @cindex directories, removing (recursively)
8881 Remove the listed directories and their contents recursively.
8887 Print the name of each file before removing it.
8891 @cindex files beginning with @samp{-}, removing
8892 @cindex @samp{-}, removing files beginning with
8893 One common question is how to remove files whose names begin with a
8894 @samp{-}. @sc{gnu} @command{rm}, like every program that uses the @code{getopt}
8895 function to parse its arguments, lets you use the @samp{--} option to
8896 indicate that all following arguments are non-options. To remove a file
8897 called @file{-f} in the current directory, you could type either:
8910 @opindex - @r{and Unix @command{rm}}
8911 The Unix @command{rm} program's use of a single @samp{-} for this purpose
8912 predates the development of the getopt standard syntax.
8917 @node shred invocation
8918 @section @command{shred}: Remove files more securely
8921 @cindex data, erasing
8922 @cindex erasing data
8924 @command{shred} overwrites devices or files, to help prevent even
8925 very expensive hardware from recovering the data.
8927 Ordinarily when you remove a file (@pxref{rm invocation}), the data is
8928 not actually destroyed. Only the index listing where the file is
8929 stored is destroyed, and the storage is made available for reuse.
8930 There are undelete utilities that will attempt to reconstruct the index
8931 and can bring the file back if the parts were not reused.
8933 On a busy system with a nearly-full drive, space can get reused in a few
8934 seconds. But there is no way to know for sure. If you have sensitive
8935 data, you may want to be sure that recovery is not possible by actually
8936 overwriting the file with non-sensitive data.
8938 However, even after doing that, it is possible to take the disk back
8939 to a laboratory and use a lot of sensitive (and expensive) equipment
8940 to look for the faint ``echoes'' of the original data underneath the
8941 overwritten data. If the data has only been overwritten once, it's not
8944 The best way to remove something irretrievably is to destroy the media
8945 it's on with acid, melt it down, or the like. For cheap removable media
8946 like floppy disks, this is the preferred method. However, hard drives
8947 are expensive and hard to melt, so the @command{shred} utility tries
8948 to achieve a similar effect non-destructively.
8950 This uses many overwrite passes, with the data patterns chosen to
8951 maximize the damage they do to the old data. While this will work on
8952 floppies, the patterns are designed for best effect on hard drives.
8953 For more details, see the source code and Peter Gutmann's paper
8954 @uref{http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html,
8955 @cite{Secure Deletion of Data from Magnetic and Solid-State Memory}},
8956 from the proceedings of the Sixth @acronym{USENIX} Security Symposium (San Jose,
8957 California, July 22--25, 1996).
8959 @strong{Please note} that @command{shred} relies on a very important assumption:
8960 that the file system overwrites data in place. This is the traditional
8961 way to do things, but many modern file system designs do not satisfy this
8962 assumption. Exceptions include:
8967 Log-structured or journaled file systems, such as those supplied with
8968 AIX and Solaris, and JFS, ReiserFS, XFS, Ext3 (in @code{data=journal} mode),
8969 BFS, NTFS, etc.@: when they are configured to journal @emph{data}.
8972 File systems that write redundant data and carry on even if some writes
8973 fail, such as RAID-based file systems.
8976 File systems that make snapshots, such as Network Appliance's NFS server.
8979 File systems that cache in temporary locations, such as NFS version 3
8983 Compressed file systems.
8986 In the particular case of ext3 file systems, the above disclaimer applies (and
8987 @command{shred} is thus of limited effectiveness) only in @code{data=journal}
8988 mode, which journals file data in addition to just metadata. In both
8989 the @code{data=ordered} (default) and @code{data=writeback} modes,
8990 @command{shred} works as usual. Ext3 journaling modes can be changed
8991 by adding the @code{data=something} option to the mount options for a
8992 particular file system in the @file{/etc/fstab} file, as documented in
8993 the mount man page (man mount).
8995 If you are not sure how your file system operates, then you should assume
8996 that it does not overwrite data in place, which means that shred cannot
8997 reliably operate on regular files in your file system.
8999 Generally speaking, it is more reliable to shred a device than a file,
9000 since this bypasses the problem of file system design mentioned above.
9001 However, even shredding devices is not always completely reliable. For
9002 example, most disks map out bad sectors invisibly to the application; if
9003 the bad sectors contain sensitive data, @command{shred} won't be able to
9006 @command{shred} makes no attempt to detect or report this problem, just as
9007 it makes no attempt to do anything about backups. However, since it is
9008 more reliable to shred devices than files, @command{shred} by default does
9009 not truncate or remove the output file. This default is more suitable
9010 for devices, which typically cannot be truncated and should not be
9013 Finally, consider the risk of backups and mirrors.
9014 File system backups and remote mirrors may contain copies of the
9015 file that cannot be removed, and that will allow a shredded file
9016 to be recovered later. So if you keep any data you may later want
9017 to destroy using @command{shred}, be sure that it is not backed up or mirrored.
9020 shred [@var{option}]@dots{} @var{file}[@dots{}]
9023 The program accepts the following options. Also see @ref{Common options}.
9031 @cindex force deletion
9032 Override file permissions if necessary to allow overwriting.
9035 @itemx -n @var{number}
9036 @itemx --iterations=@var{number}
9037 @opindex -n @var{number}
9038 @opindex --iterations=@var{number}
9039 @cindex iterations, selecting the number of
9040 By default, @command{shred} uses @value{SHRED_DEFAULT_PASSES} passes of
9041 overwrite. You can reduce this to save time, or increase it if you think it's
9042 appropriate. After 25 passes all of the internal overwrite patterns will have
9043 been used at least once.
9045 @item --random-source=@var{file}
9046 @opindex --random-source
9047 @cindex random source for shredding
9048 Use @var{file} as a source of random data used to overwrite and to
9049 choose pass ordering. @xref{Random sources}.
9051 @item -s @var{bytes}
9052 @itemx --size=@var{bytes}
9053 @opindex -s @var{bytes}
9054 @opindex --size=@var{bytes}
9055 @cindex size of file to shred
9056 Shred the first @var{bytes} bytes of the file. The default is to shred
9057 the whole file. @var{bytes} can be followed by a size specification like
9058 @samp{K}, @samp{M}, or @samp{G} to specify a multiple. @xref{Block size}.
9064 @cindex removing files after shredding
9065 After shredding a file, truncate it (if possible) and then remove it.
9066 If a file has multiple links, only the named links will be removed.
9072 Display to standard error all status updates as sterilization proceeds.
9078 By default, @command{shred} rounds the size of a regular file up to the next
9079 multiple of the file system block size to fully erase the last block
9081 Use @option{--exact} to suppress that behavior.
9082 Thus, by default if you shred a 10-byte regular file on a system with 512-byte
9083 blocks, the resulting file will be 512 bytes long. With this option,
9084 shred does not increase the apparent size of the file.
9090 Normally, the last pass that @command{shred} writes is made up of
9091 random data. If this would be conspicuous on your hard drive (for
9092 example, because it looks like encrypted data), or you just think
9093 it's tidier, the @option{--zero} option adds an additional overwrite pass with
9094 all zero bits. This is in addition to the number of passes specified
9095 by the @option{--iterations} option.
9099 You might use the following command to erase all trace of the
9100 file system you'd created on the floppy disk in your first drive.
9101 That command takes about 20 minutes to erase a ``1.44MB'' (actually
9105 shred --verbose /dev/fd0
9108 Similarly, to erase all data on a selected partition of
9109 your hard disk, you could give a command like this:
9112 shred --verbose /dev/sda5
9115 On modern disks, a single pass should be adequate,
9116 and it will take one third the time of the default three-pass approach.
9119 # 1 pass, write pseudo-random data; 3x faster than the default
9120 shred --verbose -n1 /dev/sda5
9123 To be on the safe side, use at least one pass that overwrites using
9124 pseudo-random data. I.e., don't be tempted to use @samp{-n0 --zero},
9125 in case some disk controller optimizes the process of writing blocks
9126 of all zeros, and thereby does not clear all bytes in a block.
9127 Some SSDs may do just that.
9129 A @var{file} of @samp{-} denotes standard output.
9130 The intended use of this is to shred a removed temporary file.
9137 echo "Hello, world" >&3
9142 However, the command @samp{shred - >file} does not shred the contents
9143 of @var{file}, since the shell truncates @var{file} before invoking
9144 @command{shred}. Use the command @samp{shred file} or (if using a
9145 Bourne-compatible shell) the command @samp{shred - 1<>file} instead.
9150 @node Special file types
9151 @chapter Special file types
9153 @cindex special file types
9154 @cindex file types, special
9156 This chapter describes commands which create special types of files (and
9157 @command{rmdir}, which removes directories, one special file type).
9159 @cindex special file types
9161 Although Unix-like operating systems have markedly fewer special file
9162 types than others, not @emph{everything} can be treated only as the
9163 undifferentiated byte stream of @dfn{normal files}. For example, when a
9164 file is created or removed, the system must record this information,
9165 which it does in a @dfn{directory}---a special type of file. Although
9166 you can read directories as normal files, if you're curious, in order
9167 for the system to do its job it must impose a structure, a certain
9168 order, on the bytes of the file. Thus it is a ``special'' type of file.
9170 Besides directories, other special file types include named pipes
9171 (FIFOs), symbolic links, sockets, and so-called @dfn{special files}.
9174 * link invocation:: Make a hard link via the link syscall
9175 * ln invocation:: Make links between files.
9176 * mkdir invocation:: Make directories.
9177 * mkfifo invocation:: Make FIFOs (named pipes).
9178 * mknod invocation:: Make block or character special files.
9179 * readlink invocation:: Print value of a symlink or canonical file name.
9180 * rmdir invocation:: Remove empty directories.
9181 * unlink invocation:: Remove files via the unlink syscall
9185 @node link invocation
9186 @section @command{link}: Make a hard link via the link syscall
9189 @cindex links, creating
9190 @cindex hard links, creating
9191 @cindex creating links (hard only)
9193 @command{link} creates a single hard link at a time.
9194 It is a minimalist interface to the system-provided
9195 @code{link} function. @xref{Hard Links, , , libc,
9196 The GNU C Library Reference Manual}.
9197 It avoids the bells and whistles of the more commonly-used
9198 @command{ln} command (@pxref{ln invocation}).
9202 link @var{filename} @var{linkname}
9205 @var{filename} must specify an existing file, and @var{linkname}
9206 must specify a nonexistent entry in an existing directory.
9207 @command{link} simply calls @code{link (@var{filename}, @var{linkname})}
9210 On a @acronym{GNU} system, this command acts like @samp{ln --directory
9211 --no-target-directory @var{filename} @var{linkname}}. However, the
9212 @option{--directory} and @option{--no-target-directory} options are
9213 not specified by @acronym{POSIX}, and the @command{link} command is
9214 more portable in practice.
9216 If @var{filename} is a symbolic link, it is unspecified whether
9217 @var{linkname} will be a hard link to the symbolic link or to the
9218 target of the symbolic link. Use @command{ln -P} or @command{ln -L}
9219 to specify which behavior is desired.
9225 @section @command{ln}: Make links between files
9228 @cindex links, creating
9229 @cindex hard links, creating
9230 @cindex symbolic (soft) links, creating
9231 @cindex creating links (hard or soft)
9233 @cindex file systems and hard links
9234 @command{ln} makes links between files. By default, it makes hard links;
9235 with the @option{-s} option, it makes symbolic (or @dfn{soft}) links.
9239 ln [@var{option}]@dots{} [-T] @var{target} @var{linkname}
9240 ln [@var{option}]@dots{} @var{target}
9241 ln [@var{option}]@dots{} @var{target}@dots{} @var{directory}
9242 ln [@var{option}]@dots{} -t @var{directory} @var{target}@dots{}
9248 If two file names are given, @command{ln} creates a link to the first
9249 file from the second.
9252 If one @var{target} is given, @command{ln} creates a link to that file
9253 in the current directory.
9256 If the @option{--target-directory} (@option{-t}) option is given, or
9257 failing that if the last file is a directory and the
9258 @option{--no-target-directory} (@option{-T}) option is not given,
9259 @command{ln} creates a link to each @var{target} file in the specified
9260 directory, using the @var{target}s' names.
9264 Normally @command{ln} does not remove existing files. Use the
9265 @option{--force} (@option{-f}) option to remove them unconditionally,
9266 the @option{--interactive} (@option{-i}) option to remove them
9267 conditionally, and the @option{--backup} (@option{-b}) option to
9270 @cindex hard link, defined
9271 @cindex inode, and hard links
9272 A @dfn{hard link} is another name for an existing file; the link and the
9273 original are indistinguishable. Technically speaking, they share the
9274 same inode, and the inode contains all the information about a
9275 file---indeed, it is not incorrect to say that the inode @emph{is} the
9276 file. Most systems prohibit making a hard link to
9277 a directory; on those where it is allowed, only the super-user can do
9278 so (and with caution, since creating a cycle will cause problems to many
9279 other utilities). Hard links cannot cross file system boundaries. (These
9280 restrictions are not mandated by @acronym{POSIX}, however.)
9282 @cindex dereferencing symbolic links
9283 @cindex symbolic link, defined
9284 @dfn{Symbolic links} (@dfn{symlinks} for short), on the other hand, are
9285 a special file type (which not all kernels support: System V release 3
9286 (and older) systems lack symlinks) in which the link file actually
9287 refers to a different file, by name. When most operations (opening,
9288 reading, writing, and so on) are passed the symbolic link file, the
9289 kernel automatically @dfn{dereferences} the link and operates on the
9290 target of the link. But some operations (e.g., removing) work on the
9291 link file itself, rather than on its target. The owner and group of a
9292 symlink are not significant to file access performed through
9293 the link, but do have implications on deleting a symbolic link from a
9294 directory with the restricted deletion bit set. On the GNU system,
9295 the mode of a symlink has no significance and cannot be changed, but
9296 on some BSD systems, the mode can be changed and will affect whether
9297 the symlink will be traversed in file name resolution. @xref{Symbolic Links,,,
9298 libc, The GNU C Library Reference Manual}.
9300 Symbolic links can contain arbitrary strings; a @dfn{dangling symlink}
9301 occurs when the string in the symlink does not resolve to a file.
9302 There are no restrictions against creating dangling symbolic links.
9303 There are trade-offs to using absolute or relative symlinks. An
9304 absolute symlink always points to the same file, even if the directory
9305 containing the link is moved. However, if the symlink is visible from
9306 more than one machine (such as on a networked file system), the file
9307 pointed to might not always be the same. A relative symbolic link is
9308 resolved in relation to the directory that contains the link, and is
9309 often useful in referring to files on the same device without regards
9310 to what name that device is mounted on when accessed via networked
9313 When creating a relative symlink in a different location than the
9314 current directory, the resolution of the symlink will be different
9315 than the resolution of the same string from the current directory.
9316 Therefore, many users prefer to first change directories to the
9317 location where the relative symlink will be created, so that
9318 tab-completion or other file resolution will find the same target as
9319 what will be placed in the symlink.
9321 The program accepts the following options. Also see @ref{Common options}.
9332 @opindex --directory
9333 @cindex hard links to directories
9334 Allow users with appropriate privileges to attempt to make hard links
9336 However, note that this will probably fail due to
9337 system restrictions, even for the super-user.
9343 Remove existing destination files.
9346 @itemx --interactive
9348 @opindex --interactive
9349 @cindex prompting, and @command{ln}
9350 Prompt whether to remove existing destination files.
9356 If @option{-s} is not in effect, and the source file is a symbolic
9357 link, create the hard link to the file referred to by the symbolic
9358 link, rather than the symbolic link itself.
9361 @itemx --no-dereference
9363 @opindex --no-dereference
9364 Do not treat the last operand specially when it is a symbolic link to
9365 a directory. Instead, treat it as if it were a normal file.
9367 When the destination is an actual directory (not a symlink to one),
9368 there is no ambiguity. The link is created in that directory.
9369 But when the specified destination is a symlink to a directory,
9370 there are two ways to treat the user's request. @command{ln} can
9371 treat the destination just as it would a normal directory and create
9372 the link in it. On the other hand, the destination can be viewed as a
9373 non-directory---as the symlink itself. In that case, @command{ln}
9374 must delete or backup that symlink before creating the new link.
9375 The default is to treat a destination that is a symlink to a directory
9376 just like a directory.
9378 This option is weaker than the @option{--no-target-directory}
9379 (@option{-T}) option, so it has no effect if both options are given.
9385 If @option{-s} is not in effect, and the source file is a symbolic
9386 link, create the hard link to the symbolic link itself. On platforms
9387 where this is not supported by the kernel, this option creates a
9388 symbolic link with identical contents; since symbolic link contents
9389 cannot be edited, any file name resolution performed through either
9390 link will be the same as if a hard link had been created.
9396 Make symbolic links relative to the link location.
9401 ln -srv /a/file /tmp
9402 '/tmp/file' -> '../a/file'
9405 @xref{realpath invocation}, which gives greater control
9406 over relative path generation.
9412 Make symbolic links instead of hard links. This option merely produces
9413 an error message on systems that do not support symbolic links.
9419 @optNoTargetDirectory
9425 Print the name of each file after linking it successfully.
9429 @cindex hard links to symbolic links
9430 @cindex symbolic links and @command{ln}
9431 If @option{-L} and @option{-P} are both given, the last one takes
9432 precedence. If @option{-s} is also given, @option{-L} and @option{-P}
9433 are silently ignored. If neither option is given, then this
9434 implementation defaults to @option{-P} if the system @code{link} supports
9435 hard links to symbolic links (such as the GNU system), and @option{-L}
9436 if @code{link} follows symbolic links (such as on BSD).
9445 # Create link ../a pointing to a in that directory.
9446 # Not really useful because it points to itself.
9451 # Change to the target before creating symlinks to avoid being confused.
9457 # Hard coded file names don't move well.
9458 ln -s $(pwd)/a /some/dir/
9462 # Relative file names survive directory moves and also
9463 # work across networked file systems.
9464 ln -s afile anotherfile
9465 ln -s ../adir/afile yetanotherfile
9469 @node mkdir invocation
9470 @section @command{mkdir}: Make directories
9473 @cindex directories, creating
9474 @cindex creating directories
9476 @command{mkdir} creates directories with the specified names. Synopsis:
9479 mkdir [@var{option}]@dots{} @var{name}@dots{}
9482 @command{mkdir} creates each directory @var{name} in the order given.
9483 It reports an error if @var{name} already exists, unless the
9484 @option{-p} option is given and @var{name} is a directory.
9486 The program accepts the following options. Also see @ref{Common options}.
9491 @itemx --mode=@var{mode}
9494 @cindex modes of created directories, setting
9495 Set the file permission bits of created directories to @var{mode},
9496 which uses the same syntax as
9497 in @command{chmod} and uses @samp{a=rwx} (read, write and execute allowed for
9498 everyone) for the point of the departure. @xref{File permissions}.
9500 Normally the directory has the desired file mode bits at the moment it
9501 is created. As a @acronym{GNU} extension, @var{mode} may also mention
9502 special mode bits, but in this case there may be a temporary window
9503 during which the directory exists but its special mode bits are
9504 incorrect. @xref{Directory Setuid and Setgid}, for how the
9505 set-user-ID and set-group-ID bits of directories are inherited unless
9506 overridden in this way.
9512 @cindex parent directories, creating
9513 Make any missing parent directories for each argument, setting their
9514 file permission bits to the umask modified by @samp{u+wx}. Ignore
9515 existing parent directories, and do not change their file permission
9518 To set the file permission bits of any newly-created parent
9519 directories to a value that includes @samp{u+wx}, you can set the
9520 umask before invoking @command{mkdir}. For example, if the shell
9521 command @samp{(umask u=rwx,go=rx; mkdir -p P/Q)} creates the parent
9522 @file{P} it sets the parent's permission bits to @samp{u=rwx,go=rx}.
9523 To set a parent's special mode bits as well, you can invoke
9524 @command{chmod} after @command{mkdir}. @xref{Directory Setuid and
9525 Setgid}, for how the set-user-ID and set-group-ID bits of
9526 newly-created parent directories are inherited.
9532 Print a message for each created directory. This is most useful with
9535 @item -Z @var{context}
9536 @itemx --context=@var{context}
9540 @cindex security context
9541 Set the default SELinux security context to be used for created directories.
9548 @node mkfifo invocation
9549 @section @command{mkfifo}: Make FIFOs (named pipes)
9552 @cindex FIFOs, creating
9553 @cindex named pipes, creating
9554 @cindex creating FIFOs (named pipes)
9556 @command{mkfifo} creates FIFOs (also called @dfn{named pipes}) with the
9557 specified names. Synopsis:
9560 mkfifo [@var{option}] @var{name}@dots{}
9563 A @dfn{FIFO} is a special file type that permits independent processes
9564 to communicate. One process opens the FIFO file for writing, and
9565 another for reading, after which data can flow as with the usual
9566 anonymous pipe in shells or elsewhere.
9568 The program accepts the following option. Also see @ref{Common options}.
9573 @itemx --mode=@var{mode}
9576 @cindex modes of created FIFOs, setting
9577 Set the mode of created FIFOs to @var{mode}, which is symbolic as in
9578 @command{chmod} and uses @samp{a=rw} (read and write allowed for everyone)
9579 for the point of departure. @var{mode} should specify only file
9580 permission bits. @xref{File permissions}.
9582 @item -Z @var{context}
9583 @itemx --context=@var{context}
9587 @cindex security context
9588 Set the default SELinux security context to be used for created FIFOs.
9595 @node mknod invocation
9596 @section @command{mknod}: Make block or character special files
9599 @cindex block special files, creating
9600 @cindex character special files, creating
9602 @command{mknod} creates a FIFO, character special file, or block special
9603 file with the specified name. Synopsis:
9606 mknod [@var{option}]@dots{} @var{name} @var{type} [@var{major} @var{minor}]
9609 @cindex special files
9610 @cindex block special files
9611 @cindex character special files
9612 Unlike the phrase ``special file type'' above, the term @dfn{special
9613 file} has a technical meaning on Unix: something that can generate or
9614 receive data. Usually this corresponds to a physical piece of hardware,
9615 e.g., a printer or a disk. (These files are typically created at
9616 system-configuration time.) The @command{mknod} command is what creates
9617 files of this type. Such devices can be read either a character at a
9618 time or a ``block'' (many characters) at a time, hence we say there are
9619 @dfn{block special} files and @dfn{character special} files.
9621 @c mknod is a shell built-in at least with OpenBSD's /bin/sh
9622 @mayConflictWithShellBuiltIn{mknod}
9624 The arguments after @var{name} specify the type of file to make:
9629 @opindex p @r{for FIFO file}
9633 @opindex b @r{for block special file}
9634 for a block special file
9637 @c Don't document the 'u' option -- it's just a synonym for 'c'.
9638 @c Do *any* versions of mknod still use it?
9640 @opindex c @r{for character special file}
9641 @c @opindex u @r{for character special file}
9642 for a character special file
9646 When making a block or character special file, the major and minor
9647 device numbers must be given after the file type.
9648 If a major or minor device number begins with @samp{0x} or @samp{0X},
9649 it is interpreted as hexadecimal; otherwise, if it begins with @samp{0},
9650 as octal; otherwise, as decimal.
9652 The program accepts the following option. Also see @ref{Common options}.
9657 @itemx --mode=@var{mode}
9660 Set the mode of created files to @var{mode}, which is symbolic as in
9661 @command{chmod} and uses @samp{a=rw} as the point of departure.
9662 @var{mode} should specify only file permission bits.
9663 @xref{File permissions}.
9665 @item -Z @var{context}
9666 @itemx --context=@var{context}
9670 @cindex security context
9671 Set the default SELinux security context to be used for created files.
9678 @node readlink invocation
9679 @section @command{readlink}: Print value of a symlink or canonical file name
9682 @cindex displaying value of a symbolic link
9683 @cindex canonical file name
9684 @cindex canonicalize a file name
9687 @command{readlink} may work in one of two supported modes:
9693 @command{readlink} outputs the value of the given symbolic link.
9694 If @command{readlink} is invoked with an argument other than the name
9695 of a symbolic link, it produces no output and exits with a nonzero exit code.
9697 @item Canonicalize mode
9699 @command{readlink} outputs the absolute name of the given file which contains
9700 no @file{.}, @file{..} components nor any repeated separators
9701 (@file{/}) or symbolic links.
9706 readlink [@var{option}] @var{file}
9709 By default, @command{readlink} operates in readlink mode.
9711 The program accepts the following options. Also see @ref{Common options}.
9716 @itemx --canonicalize
9718 @opindex --canonicalize
9719 Activate canonicalize mode.
9720 If any component of the file name except the last one is missing or unavailable,
9721 @command{readlink} produces no output and exits with a nonzero exit
9722 code. A trailing slash is ignored.
9725 @itemx --canonicalize-existing
9727 @opindex --canonicalize-existing
9728 Activate canonicalize mode.
9729 If any component is missing or unavailable, @command{readlink} produces
9730 no output and exits with a nonzero exit code. A trailing slash
9731 requires that the name resolve to a directory.
9734 @itemx --canonicalize-missing
9736 @opindex --canonicalize-missing
9737 Activate canonicalize mode.
9738 If any component is missing or unavailable, @command{readlink} treats it
9744 @opindex --no-newline
9745 Do not output the trailing newline.
9755 Suppress most error messages.
9761 Report error messages.
9765 The @command{readlink} utility first appeared in OpenBSD 2.1.
9767 The @command{realpath} command without options, operates like
9768 @command{readlink} in canonicalize mode.
9773 @node rmdir invocation
9774 @section @command{rmdir}: Remove empty directories
9777 @cindex removing empty directories
9778 @cindex directories, removing empty
9780 @command{rmdir} removes empty directories. Synopsis:
9783 rmdir [@var{option}]@dots{} @var{directory}@dots{}
9786 If any @var{directory} argument does not refer to an existing empty
9787 directory, it is an error.
9789 The program accepts the following options. Also see @ref{Common options}.
9793 @item --ignore-fail-on-non-empty
9794 @opindex --ignore-fail-on-non-empty
9795 @cindex directory deletion, ignoring failures
9796 Ignore each failure to remove a directory that is solely because
9797 the directory is non-empty.
9803 @cindex parent directories, removing
9804 Remove @var{directory}, then try to remove each component of @var{directory}.
9805 So, for example, @samp{rmdir -p a/b/c} is similar to @samp{rmdir a/b/c a/b a}.
9806 As such, it fails if any of those directories turns out not to be empty.
9807 Use the @option{--ignore-fail-on-non-empty} option to make it so such
9808 a failure does not evoke a diagnostic and does not cause @command{rmdir} to
9809 exit unsuccessfully.
9815 @cindex directory deletion, reporting
9816 Give a diagnostic for each successful removal.
9817 @var{directory} is removed.
9821 @xref{rm invocation}, for how to remove non-empty directories (recursively).
9826 @node unlink invocation
9827 @section @command{unlink}: Remove files via the unlink syscall
9830 @cindex removing files or directories (via the unlink syscall)
9832 @command{unlink} deletes a single specified file name.
9833 It is a minimalist interface to the system-provided
9834 @code{unlink} function. @xref{Deleting Files, , , libc,
9835 The GNU C Library Reference Manual}. Synopsis:
9836 It avoids the bells and whistles of the more commonly-used
9837 @command{rm} command (@pxref{rm invocation}).
9840 unlink @var{filename}
9843 On some systems @code{unlink} can be used to delete the name of a
9844 directory. On others, it can be used that way only by a privileged user.
9845 In the GNU system @code{unlink} can never delete the name of a directory.
9847 The @command{unlink} command honors the @option{--help} and
9848 @option{--version} options. To remove a file whose name begins with
9849 @samp{-}, prefix the name with @samp{./}, e.g., @samp{unlink ./--help}.
9854 @node Changing file attributes
9855 @chapter Changing file attributes
9857 @cindex changing file attributes
9858 @cindex file attributes, changing
9859 @cindex attributes, file
9861 A file is not merely its contents, a name, and a file type
9862 (@pxref{Special file types}). A file also has an owner (a user ID), a
9863 group (a group ID), permissions (what the owner can do with the file,
9864 what people in the group can do, and what everyone else can do), various
9865 timestamps, and other information. Collectively, we call these a file's
9868 These commands change file attributes.
9871 * chgrp invocation:: Change file groups.
9872 * chmod invocation:: Change access permissions.
9873 * chown invocation:: Change file owners and groups.
9874 * touch invocation:: Change file timestamps.
9878 @node chown invocation
9879 @section @command{chown}: Change file owner and group
9882 @cindex file ownership, changing
9883 @cindex group ownership, changing
9884 @cindex changing file ownership
9885 @cindex changing group ownership
9887 @command{chown} changes the user and/or group ownership of each given @var{file}
9888 to @var{new-owner} or to the user and group of an existing reference file.
9892 chown [@var{option}]@dots{} @{@var{new-owner} | --reference=@var{ref_file}@}@c
9896 If used, @var{new-owner} specifies the new owner and/or group as follows
9897 (with no embedded white space):
9900 [@var{owner}] [ : [@var{group}] ]
9907 If only an @var{owner} (a user name or numeric user ID) is given, that
9908 user is made the owner of each given file, and the files' group is not
9911 @item owner@samp{:}group
9912 If the @var{owner} is followed by a colon and a @var{group} (a
9913 group name or numeric group ID), with no spaces between them, the group
9914 ownership of the files is changed as well (to @var{group}).
9917 If a colon but no group name follows @var{owner}, that user is
9918 made the owner of the files and the group of the files is changed to
9919 @var{owner}'s login group.
9922 If the colon and following @var{group} are given, but the owner
9923 is omitted, only the group of the files is changed; in this case,
9924 @command{chown} performs the same function as @command{chgrp}.
9927 If only a colon is given, or if @var{new-owner} is empty, neither the
9928 owner nor the group is changed.
9932 If @var{owner} or @var{group} is intended to represent a numeric user
9933 or group ID, then you may specify it with a leading @samp{+}.
9934 @xref{Disambiguating names and IDs}.
9936 Some older scripts may still use @samp{.} in place of the @samp{:} separator.
9937 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) does not
9938 require support for that, but for backward compatibility @acronym{GNU}
9939 @command{chown} supports @samp{.} so long as no ambiguity results.
9940 New scripts should avoid the use of @samp{.} because it is not
9941 portable, and because it has undesirable results if the entire
9942 @var{owner@samp{.}group} happens to identify a user whose name
9945 The @command{chown} command sometimes clears the set-user-ID or
9946 set-group-ID permission bits. This behavior depends on the policy and
9947 functionality of the underlying @code{chown} system call, which may
9948 make system-dependent file mode modifications outside the control of
9949 the @command{chown} command. For example, the @command{chown} command
9950 might not affect those bits when invoked by a user with appropriate
9951 privileges, or when the
9952 bits signify some function other than executable permission (e.g.,
9954 When in doubt, check the underlying system behavior.
9956 The program accepts the following options. Also see @ref{Common options}.
9964 @cindex changed owners, verbosely describing
9965 Verbosely describe the action for each @var{file} whose ownership
9974 @cindex error messages, omitting
9975 Do not print error messages about files whose ownership cannot be
9978 @itemx @w{@kbd{--from}=@var{old-owner}}
9980 @cindex symbolic links, changing owner
9981 Change a @var{file}'s ownership only if it has current attributes specified
9982 by @var{old-owner}. @var{old-owner} has the same form as @var{new-owner}
9984 This option is useful primarily from a security standpoint in that
9985 it narrows considerably the window of potential abuse.
9986 For example, to reflect a user ID numbering change for one user's files
9987 without an option like this, @code{root} might run
9990 find / -owner OLDUSER -print0 | xargs -0 chown -h NEWUSER
9993 But that is dangerous because the interval between when the @command{find}
9994 tests the existing file's owner and when the @command{chown} is actually run
9996 One way to narrow the gap would be to invoke chown for each file
10000 find / -owner OLDUSER -exec chown -h NEWUSER @{@} \;
10003 But that is very slow if there are many affected files.
10004 With this option, it is safer (the gap is narrower still)
10005 though still not perfect:
10008 chown -h -R --from=OLDUSER NEWUSER /
10011 @item --dereference
10012 @opindex --dereference
10013 @cindex symbolic links, changing owner
10015 Do not act on symbolic links themselves but rather on what they point to.
10016 This is the default.
10019 @itemx --no-dereference
10021 @opindex --no-dereference
10022 @cindex symbolic links, changing owner
10024 Act on symbolic links themselves instead of what they point to.
10025 This mode relies on the @code{lchown} system call.
10026 On systems that do not provide the @code{lchown} system call,
10027 @command{chown} fails when a file specified on the command line
10028 is a symbolic link.
10029 By default, no diagnostic is issued for symbolic links encountered
10030 during a recursive traversal, but see @option{--verbose}.
10032 @itemx --preserve-root
10033 @opindex --preserve-root
10034 @cindex root directory, disallow recursive modification
10035 Fail upon any attempt to recursively change the root directory, @file{/}.
10036 Without @option{--recursive}, this option has no effect.
10037 @xref{Treating / specially}.
10039 @itemx --no-preserve-root
10040 @opindex --no-preserve-root
10041 @cindex root directory, allow recursive modification
10042 Cancel the effect of any preceding @option{--preserve-root} option.
10043 @xref{Treating / specially}.
10045 @item --reference=@var{ref_file}
10046 @opindex --reference
10047 Change the user and group of each @var{file} to be the same as those of
10048 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
10049 user and group of the symbolic link, but rather those of the file it
10056 Output a diagnostic for every file processed.
10057 If a symbolic link is encountered during a recursive traversal
10058 on a system without the @code{lchown} system call, and @option{--no-dereference}
10059 is in effect, then issue a diagnostic saying neither the symbolic link nor
10060 its referent is being changed.
10065 @opindex --recursive
10066 @cindex recursively changing file ownership
10067 Recursively change ownership of directories and their contents.
10070 @xref{Traversing symlinks}.
10073 @xref{Traversing symlinks}.
10076 @xref{Traversing symlinks}.
10085 # Change the owner of /u to "root".
10088 # Likewise, but also change its group to "staff".
10089 chown root:staff /u
10091 # Change the owner of /u and subfiles to "root".
10096 @node chgrp invocation
10097 @section @command{chgrp}: Change group ownership
10100 @cindex group ownership, changing
10101 @cindex changing group ownership
10103 @command{chgrp} changes the group ownership of each given @var{file}
10104 to @var{group} (which can be either a group name or a numeric group ID)
10105 or to the group of an existing reference file. Synopsis:
10108 chgrp [@var{option}]@dots{} @{@var{group} | --reference=@var{ref_file}@}@c
10112 If @var{group} is intended to represent a
10113 numeric group ID, then you may specify it with a leading @samp{+}.
10114 @xref{Disambiguating names and IDs}.
10116 The program accepts the following options. Also see @ref{Common options}.
10124 @cindex changed files, verbosely describing
10125 Verbosely describe the action for each @var{file} whose group actually
10134 @cindex error messages, omitting
10135 Do not print error messages about files whose group cannot be
10138 @item --dereference
10139 @opindex --dereference
10140 @cindex symbolic links, changing owner
10142 Do not act on symbolic links themselves but rather on what they point to.
10143 This is the default.
10146 @itemx --no-dereference
10148 @opindex --no-dereference
10149 @cindex symbolic links, changing group
10151 Act on symbolic links themselves instead of what they point to.
10152 This mode relies on the @code{lchown} system call.
10153 On systems that do not provide the @code{lchown} system call,
10154 @command{chgrp} fails when a file specified on the command line
10155 is a symbolic link.
10156 By default, no diagnostic is issued for symbolic links encountered
10157 during a recursive traversal, but see @option{--verbose}.
10159 @itemx --preserve-root
10160 @opindex --preserve-root
10161 @cindex root directory, disallow recursive modification
10162 Fail upon any attempt to recursively change the root directory, @file{/}.
10163 Without @option{--recursive}, this option has no effect.
10164 @xref{Treating / specially}.
10166 @itemx --no-preserve-root
10167 @opindex --no-preserve-root
10168 @cindex root directory, allow recursive modification
10169 Cancel the effect of any preceding @option{--preserve-root} option.
10170 @xref{Treating / specially}.
10172 @item --reference=@var{ref_file}
10173 @opindex --reference
10174 Change the group of each @var{file} to be the same as that of
10175 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
10176 group of the symbolic link, but rather that of the file it refers to.
10182 Output a diagnostic for every file processed.
10183 If a symbolic link is encountered during a recursive traversal
10184 on a system without the @code{lchown} system call, and @option{--no-dereference}
10185 is in effect, then issue a diagnostic saying neither the symbolic link nor
10186 its referent is being changed.
10191 @opindex --recursive
10192 @cindex recursively changing group ownership
10193 Recursively change the group ownership of directories and their contents.
10196 @xref{Traversing symlinks}.
10199 @xref{Traversing symlinks}.
10202 @xref{Traversing symlinks}.
10211 # Change the group of /u to "staff".
10214 # Change the group of /u and subfiles to "staff".
10219 @node chmod invocation
10220 @section @command{chmod}: Change access permissions
10223 @cindex changing access permissions
10224 @cindex access permissions, changing
10225 @cindex permissions, changing access
10227 @command{chmod} changes the access permissions of the named files. Synopsis:
10230 chmod [@var{option}]@dots{} @{@var{mode} | --reference=@var{ref_file}@}@c
10234 @cindex symbolic links, permissions of
10235 @command{chmod} never changes the permissions of symbolic links, since
10236 the @command{chmod} system call cannot change their permissions.
10237 This is not a problem since the permissions of symbolic links are
10238 never used. However, for each symbolic link listed on the command
10239 line, @command{chmod} changes the permissions of the pointed-to file.
10240 In contrast, @command{chmod} ignores symbolic links encountered during
10241 recursive directory traversals.
10243 A successful use of @command{chmod} clears the set-group-ID bit of a
10244 regular file if the file's group ID does not match the user's
10245 effective group ID or one of the user's supplementary group IDs,
10246 unless the user has appropriate privileges. Additional restrictions
10247 may cause the set-user-ID and set-group-ID bits of @var{mode} or
10248 @var{ref_file} to be ignored. This behavior depends on the policy and
10249 functionality of the underlying @code{chmod} system call. When in
10250 doubt, check the underlying system behavior.
10252 If used, @var{mode} specifies the new file mode bits.
10253 For details, see the section on @ref{File permissions}.
10254 If you really want @var{mode} to have a leading @samp{-}, you should
10255 use @option{--} first, e.g., @samp{chmod -- -w file}. Typically,
10256 though, @samp{chmod a-w file} is preferable, and @command{chmod -w
10257 file} (without the @option{--}) complains if it behaves differently
10258 from what @samp{chmod a-w file} would do.
10260 The program accepts the following options. Also see @ref{Common options}.
10268 Verbosely describe the action for each @var{file} whose permissions
10277 @cindex error messages, omitting
10278 Do not print error messages about files whose permissions cannot be
10281 @itemx --preserve-root
10282 @opindex --preserve-root
10283 @cindex root directory, disallow recursive modification
10284 Fail upon any attempt to recursively change the root directory, @file{/}.
10285 Without @option{--recursive}, this option has no effect.
10286 @xref{Treating / specially}.
10288 @itemx --no-preserve-root
10289 @opindex --no-preserve-root
10290 @cindex root directory, allow recursive modification
10291 Cancel the effect of any preceding @option{--preserve-root} option.
10292 @xref{Treating / specially}.
10298 Verbosely describe the action or non-action taken for every @var{file}.
10300 @item --reference=@var{ref_file}
10301 @opindex --reference
10302 Change the mode of each @var{file} to be the same as that of @var{ref_file}.
10303 @xref{File permissions}.
10304 If @var{ref_file} is a symbolic link, do not use the mode
10305 of the symbolic link, but rather that of the file it refers to.
10310 @opindex --recursive
10311 @cindex recursively changing access permissions
10312 Recursively change permissions of directories and their contents.
10319 @node touch invocation
10320 @section @command{touch}: Change file timestamps
10323 @cindex changing file timestamps
10324 @cindex file timestamps, changing
10325 @cindex timestamps, changing file
10327 @command{touch} changes the access and/or modification times of the
10328 specified files. Synopsis:
10331 touch [@var{option}]@dots{} @var{file}@dots{}
10334 @cindex empty files, creating
10335 Any @var{file} argument that does not exist is created empty, unless
10336 option @option{--no-create} (@option{-c}) or @option{--no-dereference}
10337 (@option{-h}) was in effect.
10339 A @var{file} argument string of @samp{-} is handled specially and
10340 causes @command{touch} to change the times of the file associated with
10344 By default, @command{touch} sets file timestamps to the current time.
10345 Because @command{touch} acts on its operands left to right, the
10346 resulting timestamps of earlier and later operands may disagree.
10347 Also, the determination of what time is ``current'' depends on the
10348 platform. Platforms with network file systems often use different
10349 clocks for the operating system and for file systems; because
10350 @command{touch} typically uses file systems' clocks by default, clock
10351 skew can cause the resulting file timestamps to appear to be in a
10352 program's ``future'' or ``past''.
10354 @cindex file timestamp resolution
10355 The @command{touch} command sets the file's timestamp to the greatest
10356 representable value that is not greater than the requested time. This
10357 can differ from the requested time for several reasons. First, the
10358 requested time may have a higher resolution than supported. Second, a
10359 file system may use different resolutions for different types of
10360 times. Third, file timestamps may use a different resolution than
10361 operating system timestamps. Fourth, the operating system primitives
10362 used to update timestamps may employ yet a different resolution. For
10363 example, in theory a file system might use 10-microsecond resolution
10364 for access time and 100-nanosecond resolution for modification time,
10365 and the operating system might use nanosecond resolution for the
10366 current time and microsecond resolution for the primitive that
10367 @command{touch} uses to set a file's timestamp to an arbitrary value.
10369 @cindex permissions, for changing file timestamps
10370 When setting file timestamps to the current time, @command{touch} can
10371 change the timestamps for files that the user does not own but has
10372 write permission for. Otherwise, the user must own the files. Some
10373 older systems have a further restriction: the user must own the files
10374 unless both the access and modification times are being set to the
10377 Although @command{touch} provides options for changing two of the times---the
10378 times of last access and modification---of a file, there is actually
10379 a standard third one as well: the inode change time. This is often
10380 referred to as a file's @code{ctime}.
10381 The inode change time represents the time when the file's meta-information
10382 last changed. One common example of this is when the permissions of a
10383 file change. Changing the permissions doesn't access the file, so
10384 the atime doesn't change, nor does it modify the file, so the mtime
10385 doesn't change. Yet, something about the file itself has changed,
10386 and this must be noted somewhere. This is the job of the ctime field.
10387 This is necessary, so that, for example, a backup program can make a
10388 fresh copy of the file, including the new permissions value.
10389 Another operation that modifies a file's ctime without affecting
10390 the others is renaming. In any case, it is not possible, in normal
10391 operations, for a user to change the ctime field to a user-specified value.
10392 Some operating systems and file systems support a fourth time: the
10393 birth time, when the file was first created; by definition, this
10394 timestamp never changes.
10397 Time stamps assume the time zone rules specified by the @env{TZ}
10398 environment variable, or by the system default rules if @env{TZ} is
10399 not set. @xref{TZ Variable,, Specifying the Time Zone with @env{TZ},
10400 libc, The GNU C Library Reference Manual}.
10401 You can avoid ambiguities during
10402 daylight saving transitions by using @sc{utc} time stamps.
10404 The program accepts the following options. Also see @ref{Common options}.
10409 @itemx --time=atime
10410 @itemx --time=access
10414 @opindex atime@r{, changing}
10415 @opindex access @r{time, changing}
10416 @opindex use @r{time, changing}
10417 Change the access time only.
10422 @opindex --no-create
10423 Do not warn about or create files that do not exist.
10426 @itemx --date=@var{time}
10430 Use @var{time} instead of the current time. It can contain month names,
10431 time zones, @samp{am} and @samp{pm}, @samp{yesterday}, etc. For
10432 example, @option{--date="2004-02-27 14:19:13.489392193 +0530"}
10433 specifies the instant of time that is 489,392,193 nanoseconds after
10434 February 27, 2004 at 2:19:13 PM in a time zone that is 5 hours and 30
10435 minutes east of @acronym{UTC}. @xref{Date input formats}.
10436 File systems that do not support high-resolution time stamps
10437 silently ignore any excess precision here.
10441 @cindex BSD @command{touch} compatibility
10442 Ignored; for compatibility with BSD versions of @command{touch}.
10445 @itemx --no-dereference
10447 @opindex --no-dereference
10448 @cindex symbolic links, changing time
10450 Attempt to change the timestamps of a symbolic link, rather than what
10451 the link refers to. When using this option, empty files are not
10452 created, but option @option{-c} must also be used to avoid warning
10453 about files that do not exist. Not all systems support changing the
10454 timestamps of symlinks, since underlying system support for this
10455 action was not required until @acronym{POSIX} 2008. Also, on some
10456 systems, the mere act of examining a symbolic link changes the access
10457 time, such that only changes to the modification time will persist
10458 long enough to be observable. When coupled with option @option{-r}, a
10459 reference timestamp is taken from a symbolic link rather than the file
10463 @itemx --time=mtime
10464 @itemx --time=modify
10467 @opindex mtime@r{, changing}
10468 @opindex modify @r{time, changing}
10469 Change the modification time only.
10471 @item -r @var{file}
10472 @itemx --reference=@var{file}
10474 @opindex --reference
10475 Use the times of the reference @var{file} instead of the current time.
10476 If this option is combined with the @option{--date=@var{time}}
10477 (@option{-d @var{time}}) option, the reference @var{file}'s time is
10478 the origin for any relative @var{time}s given, but is otherwise ignored.
10479 For example, @samp{-r foo -d '-5 seconds'} specifies a time stamp
10480 equal to five seconds before the corresponding time stamp for @file{foo}.
10481 If @var{file} is a symbolic link, the reference timestamp is taken
10482 from the target of the symlink, unless @option{-h} was also in effect.
10484 @item -t [[@var{cc}]@var{yy}]@var{mmddhhmm}[.@var{ss}]
10485 Use the argument (optional four-digit or two-digit years, months,
10486 days, hours, minutes, optional seconds) instead of the current time.
10487 If the year is specified with only two digits, then @var{cc}
10488 is 20 for years in the range 0 @dots{} 68, and 19 for years in
10489 69 @dots{} 99. If no digits of the year are specified,
10490 the argument is interpreted as a date in the current year.
10491 Note that @var{ss} may be @samp{60}, to accommodate leap seconds.
10495 @vindex _POSIX2_VERSION
10496 On older systems, @command{touch} supports an obsolete syntax, as follows.
10497 If no timestamp is given with any of the @option{-d}, @option{-r}, or
10498 @option{-t} options, and if there are two or more @var{file}s and the
10499 first @var{file} is of the form @samp{@var{mmddhhmm}[@var{yy}]} and this
10500 would be a valid argument to the @option{-t} option (if the @var{yy}, if
10501 any, were moved to the front), and if the represented year
10502 is in the range 1969--1999, that argument is interpreted as the time
10503 for the other files instead of as a file name.
10504 This obsolete behavior can be enabled or disabled with the
10505 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
10506 conformance}), but portable scripts should avoid commands whose
10507 behavior depends on this variable.
10508 For example, use @samp{touch ./12312359 main.c} or @samp{touch -t
10509 12312359 main.c} rather than the ambiguous @samp{touch 12312359 main.c}.
10515 @chapter Disk usage
10519 No disk can hold an infinite amount of data. These commands report
10520 how much disk storage is in use or available, report other file and
10521 file status information, and write buffers to disk.
10524 * df invocation:: Report file system disk space usage.
10525 * du invocation:: Estimate file space usage.
10526 * stat invocation:: Report file or file system status.
10527 * sync invocation:: Synchronize memory and disk.
10528 * truncate invocation:: Shrink or extend the size of a file.
10532 @node df invocation
10533 @section @command{df}: Report file system disk space usage
10536 @cindex file system disk usage
10537 @cindex disk usage by file system
10539 @command{df} reports the amount of disk space used and available on
10540 file systems. Synopsis:
10543 df [@var{option}]@dots{} [@var{file}]@dots{}
10546 With no arguments, @command{df} reports the space used and available on all
10547 currently mounted file systems (of all types). Otherwise, @command{df}
10548 reports on the file system containing each argument @var{file}.
10550 Normally the disk space is printed in units of
10551 1024 bytes, but this can be overridden (@pxref{Block size}).
10552 Non-integer quantities are rounded up to the next higher unit.
10554 @cindex disk device file
10555 @cindex device file, disk
10556 If an argument @var{file} is a disk device file containing a mounted
10557 file system, @command{df} shows the space available on that file system
10558 rather than on the file system containing the device node (i.e., the root
10559 file system). @sc{gnu} @command{df} does not attempt to determine the
10561 on unmounted file systems, because on most kinds of systems doing so
10562 requires extremely nonportable intimate knowledge of file system
10565 The program accepts the following options. Also see @ref{Common options}.
10573 @cindex automounter file systems
10574 @cindex ignore file systems
10575 Include in the listing dummy file systems, which
10576 are omitted by default. Such file systems are typically special-purpose
10577 pseudo-file-systems, such as automounter entries.
10579 @item -B @var{size}
10580 @itemx --block-size=@var{size}
10582 @opindex --block-size
10583 @cindex file system sizes
10584 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10585 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10589 @cindex grand total of disk size, usage and available space
10590 Print a grand total of all arguments after all arguments have
10591 been processed. This can be used to find out the total disk size, usage
10592 and available space of all listed devices.
10598 Equivalent to @option{--si}.
10604 @cindex inode usage
10605 List inode usage information instead of block usage. An inode (short
10606 for index node) contains information about a file such as its owner,
10607 permissions, timestamps, and location on the disk.
10611 @cindex kibibytes for file system sizes
10612 Print sizes in 1024-byte blocks, overriding the default block size
10613 (@pxref{Block size}).
10614 This option is equivalent to @option{--block-size=1K}.
10620 @cindex file system types, limiting output to certain
10621 Limit the listing to local file systems. By default, remote file systems
10626 @cindex file system space, retrieving old data more quickly
10627 Do not invoke the @code{sync} system call before getting any usage data.
10628 This may make @command{df} run significantly faster on systems with many
10629 disks, but on some systems (notably SunOS) the results may be slightly
10630 out of date. This is the default.
10633 @itemx --portability
10635 @opindex --portability
10636 @cindex one-line output format
10637 @cindex @acronym{POSIX} output format
10638 @cindex portable output format
10639 @cindex output format, portable
10640 Use the @acronym{POSIX} output format. This is like the default format except
10645 The information about each file system is always printed on exactly
10646 one line; a mount device is never put on a line by itself. This means
10647 that if the mount device name is more than 20 characters long (e.g., for
10648 some network mounts), the columns are misaligned.
10651 The labels in the header output line are changed to conform to @acronym{POSIX}.
10654 The default block size and output format are unaffected by the
10655 @env{DF_BLOCK_SIZE}, @env{BLOCK_SIZE} and @env{BLOCKSIZE} environment
10656 variables. However, the default block size is still affected by
10657 @env{POSIXLY_CORRECT}: it is 512 if @env{POSIXLY_CORRECT} is set, 1024
10658 otherwise. @xref{Block size}.
10665 @cindex file system space, retrieving current data more slowly
10666 Invoke the @code{sync} system call before getting any usage data. On
10667 some systems (notably SunOS), doing this yields more up to date results,
10668 but in general this option makes @command{df} much slower, especially when
10669 there are many or very busy file systems.
10671 @item -t @var{fstype}
10672 @itemx --type=@var{fstype}
10675 @cindex file system types, limiting output to certain
10676 Limit the listing to file systems of type @var{fstype}. Multiple
10677 file system types can be specified by giving multiple @option{-t} options.
10678 By default, nothing is omitted.
10681 @itemx --print-type
10683 @opindex --print-type
10684 @cindex file system types, printing
10685 Print each file system's type. The types printed here are the same ones
10686 you can include or exclude with @option{-t} and @option{-x}. The particular
10687 types printed are whatever is supported by the system. Here are some of
10688 the common names (this list is certainly not exhaustive):
10693 @cindex @acronym{NFS} file system type
10694 An @acronym{NFS} file system, i.e., one mounted over a network from another
10695 machine. This is the one type name which seems to be used uniformly by
10698 @item 4.2@r{, }ufs@r{, }efs@dots{}
10699 @cindex Linux file system types
10700 @cindex local file system types
10701 @opindex 4.2 @r{file system type}
10702 @opindex ufs @r{file system type}
10703 @opindex efs @r{file system type}
10704 A file system on a locally-mounted hard disk. (The system might even
10705 support more than one type here; Linux does.)
10707 @item hsfs@r{, }cdfs
10708 @cindex CD-ROM file system type
10709 @cindex High Sierra file system
10710 @opindex hsfs @r{file system type}
10711 @opindex cdfs @r{file system type}
10712 A file system on a CD-ROM drive. HP-UX uses @samp{cdfs}, most other
10713 systems use @samp{hsfs} (@samp{hs} for ``High Sierra'').
10716 @cindex PC file system
10717 @cindex DOS file system
10718 @cindex MS-DOS file system
10719 @cindex diskette file system
10721 An MS-DOS file system, usually on a diskette.
10725 @item -x @var{fstype}
10726 @itemx --exclude-type=@var{fstype}
10728 @opindex --exclude-type
10729 Limit the listing to file systems not of type @var{fstype}.
10730 Multiple file system types can be eliminated by giving multiple
10731 @option{-x} options. By default, no file system types are omitted.
10734 Ignored; for compatibility with System V versions of @command{df}.
10739 Failure includes the case where no output is generated, so you can
10740 inspect the exit status of a command like @samp{df -t ext3 -t reiserfs
10741 @var{dir}} to test whether @var{dir} is on a file system of type
10742 @samp{ext3} or @samp{reiserfs}.
10745 @node du invocation
10746 @section @command{du}: Estimate file space usage
10749 @cindex file space usage
10750 @cindex disk usage for files
10752 @command{du} reports the amount of disk space used by the specified files
10753 and for each subdirectory (of directory arguments). Synopsis:
10756 du [@var{option}]@dots{} [@var{file}]@dots{}
10759 With no arguments, @command{du} reports the disk space for the current
10760 directory. Normally the disk space is printed in units of
10761 1024 bytes, but this can be overridden (@pxref{Block size}).
10762 Non-integer quantities are rounded up to the next higher unit.
10764 If two or more hard links point to the same file, only one of the hard
10765 links is counted. The @var{file} argument order affects which links
10766 are counted, and changing the argument order may change the numbers
10767 that @command{du} outputs.
10769 The program accepts the following options. Also see @ref{Common options}.
10777 Show counts for all files, not just directories.
10779 @itemx --apparent-size
10780 @opindex --apparent-size
10781 Print apparent sizes, rather than disk usage. The apparent size of a
10782 file is the number of bytes reported by @code{wc -c} on regular files,
10783 or more generally, @code{ls -l --block-size=1} or @code{stat --format=%s}.
10784 For example, a file containing the word @samp{zoo} with no newline would,
10785 of course, have an apparent size of 3. Such a small file may require
10786 anywhere from 0 to 16 KiB or more of disk space, depending on
10787 the type and configuration of the file system on which the file resides.
10788 However, a sparse file created with this command:
10791 dd bs=1 seek=2GiB if=/dev/null of=big
10795 has an apparent size of 2 GiB, yet on most modern
10796 systems, it actually uses almost no disk space.
10802 Equivalent to @code{--apparent-size --block-size=1}.
10804 @item -B @var{size}
10805 @itemx --block-size=@var{size}
10807 @opindex --block-size
10809 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10810 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10816 @cindex grand total of disk space
10817 Print a grand total of all arguments after all arguments have
10818 been processed. This can be used to find out the total disk usage of
10819 a given set of files or directories.
10822 @itemx --dereference-args
10824 @opindex --dereference-args
10825 Dereference symbolic links that are command line arguments.
10826 Does not affect other symbolic links. This is helpful for finding
10827 out the disk usage of directories, such as @file{/usr/tmp}, which
10828 are often symbolic links.
10830 @c --files0-from=FILE
10831 @filesZeroFromOption{du,, with the @option{--total} (@option{-c}) option}
10837 Equivalent to @option{--dereference-args} (@option{-D}).
10841 @cindex kibibytes for file sizes
10842 Print sizes in 1024-byte blocks, overriding the default block size
10843 (@pxref{Block size}).
10844 This option is equivalent to @option{--block-size=1K}.
10847 @itemx --count-links
10849 @opindex --count-links
10850 @cindex hard links, counting in @command{du}
10851 Count the size of all files, even if they have appeared already (as a
10855 @itemx --dereference
10857 @opindex --dereference
10858 @cindex symbolic links, dereferencing in @command{du}
10859 Dereference symbolic links (show the disk space used by the file
10860 or directory that the link points to instead of the space used by
10865 @cindex mebibytes for file sizes
10866 Print sizes in 1,048,576-byte blocks, overriding the default block size
10867 (@pxref{Block size}).
10868 This option is equivalent to @option{--block-size=1M}.
10871 @itemx --no-dereference
10873 @opindex --no-dereference
10874 @cindex symbolic links, dereferencing in @command{du}
10875 For each symbolic links encountered by @command{du},
10876 consider the disk space used by the symbolic link.
10878 @item -d @var{depth}
10879 @item --max-depth=@var{depth}
10880 @opindex -d @var{depth}
10881 @opindex --max-depth=@var{depth}
10882 @cindex limiting output of @command{du}
10883 Show the total for each directory (and file if --all) that is at
10884 most MAX_DEPTH levels down from the root of the hierarchy. The root
10885 is at level 0, so @code{du --max-depth=0} is equivalent to @code{du -s}.
10894 @opindex --summarize
10895 Display only a total for each argument.
10898 @itemx --separate-dirs
10900 @opindex --separate-dirs
10901 Normally, in the output of @command{du} (when not using @option{--summarize}),
10902 the size listed next to a directory name, @var{d}, represents the sum
10903 of sizes of all entries beneath @var{d} as well as the size of @var{d} itself.
10904 With @option{--separate-dirs}, the size reported for a directory name,
10905 @var{d}, is merely the @code{stat.st_size}-derived size of the directory
10910 @cindex last modified dates, displaying in @command{du}
10911 Show time of the most recent modification of any file in the directory,
10912 or any of its subdirectories.
10914 @itemx --time=ctime
10915 @itemx --time=status
10918 @opindex ctime@r{, show the most recent}
10919 @opindex status time@r{, show the most recent}
10920 @opindex use time@r{, show the most recent}
10921 Show the most recent status change time (the @samp{ctime} in the inode) of
10922 any file in the directory, instead of the modification time.
10924 @itemx --time=atime
10925 @itemx --time=access
10927 @opindex atime@r{, show the most recent}
10928 @opindex access time@r{, show the most recent}
10929 Show the most recent access time (the @samp{atime} in the inode) of
10930 any file in the directory, instead of the modification time.
10932 @item --time-style=@var{style}
10933 @opindex --time-style
10935 List timestamps in style @var{style}. This option has an effect only if
10936 the @option{--time} option is also specified. The @var{style} should
10937 be one of the following:
10940 @item +@var{format}
10942 List timestamps using @var{format}, where @var{format} is interpreted
10943 like the format argument of @command{date} (@pxref{date invocation}).
10944 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
10945 @command{du} to list timestamps like @samp{2002-03-30 23:45:56}. As
10946 with @command{date}, @var{format}'s interpretation is affected by the
10947 @env{LC_TIME} locale category.
10950 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
10951 format with nanosecond precision, e.g., @samp{2002-03-30
10952 23:45:56.477817180 -0700}. This style is equivalent to
10953 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
10956 List @acronym{ISO} 8601 date and time in minutes, e.g.,
10957 @samp{2002-03-30 23:45}. These timestamps are shorter than
10958 @samp{full-iso} timestamps, and are usually good enough for everyday
10959 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
10962 List @acronym{ISO} 8601 dates for timestamps, e.g., @samp{2002-03-30}.
10963 This style is equivalent to @samp{+%Y-%m-%d}.
10967 You can specify the default value of the @option{--time-style} option
10968 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
10969 the default style is @samp{long-iso}. For compatibility with @command{ls},
10970 if @env{TIME_STYLE} begins with @samp{+} and contains a newline,
10971 the newline and any later characters are ignored; if @env{TIME_STYLE}
10972 begins with @samp{posix-} the @samp{posix-} is ignored; and if
10973 @env{TIME_STYLE} is @samp{locale} it is ignored.
10976 @itemx --one-file-system
10978 @opindex --one-file-system
10979 @cindex one file system, restricting @command{du} to
10980 Skip directories that are on different file systems from the one that
10981 the argument being processed is on.
10983 @item --exclude=@var{pattern}
10984 @opindex --exclude=@var{pattern}
10985 @cindex excluding files from @command{du}
10986 When recursing, skip subdirectories or files matching @var{pattern}.
10987 For example, @code{du --exclude='*.o'} excludes files whose names
10990 @item -X @var{file}
10991 @itemx --exclude-from=@var{file}
10992 @opindex -X @var{file}
10993 @opindex --exclude-from=@var{file}
10994 @cindex excluding files from @command{du}
10995 Like @option{--exclude}, except take the patterns to exclude from @var{file},
10996 one per line. If @var{file} is @samp{-}, take the patterns from standard
11001 @cindex NFS mounts from BSD to HP-UX
11002 On BSD systems, @command{du} reports sizes that are half the correct
11003 values for files that are NFS-mounted from HP-UX systems. On HP-UX
11004 systems, it reports sizes that are twice the correct values for
11005 files that are NFS-mounted from BSD systems. This is due to a flaw
11006 in HP-UX; it also affects the HP-UX @command{du} program.
11011 @node stat invocation
11012 @section @command{stat}: Report file or file system status
11015 @cindex file status
11016 @cindex file system status
11018 @command{stat} displays information about the specified file(s). Synopsis:
11021 stat [@var{option}]@dots{} [@var{file}]@dots{}
11024 With no option, @command{stat} reports all information about the given files.
11025 But it also can be used to report the information of the file systems the
11026 given files are located on. If the files are links, @command{stat} can
11027 also give information about the files the links point to.
11029 @mayConflictWithShellBuiltIn{stat}
11034 @itemx --dereference
11036 @opindex --dereference
11037 @cindex symbolic links, dereferencing in @command{stat}
11038 Change how @command{stat} treats symbolic links.
11039 With this option, @command{stat} acts on the file referenced
11040 by each symbolic link argument.
11041 Without it, @command{stat} acts on any symbolic link argument directly.
11044 @itemx --file-system
11046 @opindex --file-system
11047 @cindex file systems
11048 Report information about the file systems where the given files are located
11049 instead of information about the files themselves.
11050 This option implies the @option{-L} option.
11053 @itemx --format=@var{format}
11055 @opindex --format=@var{format}
11056 @cindex output format
11057 Use @var{format} rather than the default format.
11058 @var{format} is automatically newline-terminated, so
11059 running a command like the following with two or more @var{file}
11060 operands produces a line of output for each operand:
11062 $ stat --format=%d:%i / /usr
11067 @itemx --printf=@var{format}
11068 @opindex --printf=@var{format}
11069 @cindex output format
11070 Use @var{format} rather than the default format.
11071 Like @option{--format}, but interpret backslash escapes,
11072 and do not output a mandatory trailing newline.
11073 If you want a newline, include @samp{\n} in the @var{format}.
11074 Here's how you would use @option{--printf} to print the device
11075 and inode numbers of @file{/} and @file{/usr}:
11077 $ stat --printf='%d:%i\n' / /usr
11086 @cindex terse output
11087 Print the information in terse form, suitable for parsing by other programs.
11091 The valid @var{format} directives for files with @option{--format} and
11092 @option{--printf} are:
11095 @item %a - Access rights in octal
11096 @item %A - Access rights in human readable form
11097 @item %b - Number of blocks allocated (see @samp{%B})
11098 @item %B - The size in bytes of each block reported by @samp{%b}
11099 @item %C - The SELinux security context of a file, if available
11100 @item %d - Device number in decimal
11101 @item %D - Device number in hex
11102 @item %f - Raw mode in hex
11103 @item %F - File type
11104 @item %g - Group ID of owner
11105 @item %G - Group name of owner
11106 @item %h - Number of hard links
11107 @item %i - Inode number
11108 @item %m - Mount point (See note below)
11109 @item %n - File name
11110 @item %N - Quoted file name with dereference if symbolic link
11111 @item %o - Optimal I/O transfer size hint
11112 @item %s - Total size, in bytes
11113 @item %t - Major device type in hex
11114 @item %T - Minor device type in hex
11115 @item %u - User ID of owner
11116 @item %U - User name of owner
11117 @item %w - Time of file birth, or @samp{-} if unknown
11118 @item %W - Time of file birth as seconds since Epoch, or @samp{0}
11119 @item %x - Time of last access
11120 @item %X - Time of last access as seconds since Epoch
11121 @item %y - Time of last modification
11122 @item %Y - Time of last modification as seconds since Epoch
11123 @item %z - Time of last change
11124 @item %Z - Time of last change as seconds since Epoch
11127 The @samp{%W}, @samp{%X}, @samp{%Y}, and @samp{%Z} formats accept a
11128 precision preceded by a period to specify the number of digits to
11129 print after the decimal point. For example, @samp{%.3X} outputs the
11130 last access time to millisecond precision. If a period is given but no
11131 precision, @command{stat} uses 9 digits, so @samp{%.X} is equivalent to
11132 @samp{%.9X}. When discarding excess precision, time stamps are truncated
11133 toward minus infinity.
11137 $ stat -c '[%015Y]' /usr
11140 $ stat -c '[%15Y]' /usr
11142 $ stat -c '[%-15Y]' /usr
11145 $ stat -c '[%.3Y]' /usr
11147 $ stat -c '[%.Y]' /usr
11148 [1288929712.114951834]
11151 The mount point printed by @samp{%m} is similar to that output
11152 by @command{df}, except that:
11155 stat does not dereference symlinks by default
11156 (unless @option{-L} is specified)
11158 stat does not search for specified device nodes in the
11159 file system list, instead operating on them directly
11162 stat outputs the alias for a bind mounted file, rather than
11163 the initial mount point of its backing device.
11164 One can recursively call stat until there is no change in output,
11165 to get the current base mount point
11168 When listing file system information (@option{--file-system} (@option{-f})),
11169 you must use a different set of @var{format} directives:
11172 @item %a - Free blocks available to non-super-user
11173 @item %b - Total data blocks in file system
11174 @item %c - Total file nodes in file system
11175 @item %d - Free file nodes in file system
11176 @item %f - Free blocks in file system
11177 @item %i - File System ID in hex
11178 @item %l - Maximum length of file names
11179 @item %n - File name
11180 @item %s - Block size (for faster transfers)
11181 @item %S - Fundamental block size (for block counts)
11182 @item %t - Type in hex
11183 @item %T - Type in human readable form
11187 Time stamps are listed according to the time zone rules specified by
11188 the @env{TZ} environment variable, or by the system default rules if
11189 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
11190 with @env{TZ}, libc, The GNU C Library Reference Manual}.
11195 @node sync invocation
11196 @section @command{sync}: Synchronize data on disk with memory
11199 @cindex synchronize disk and memory
11201 @cindex superblock, writing
11202 @cindex inodes, written buffered
11203 @command{sync} writes any data buffered in memory out to disk. This can
11204 include (but is not limited to) modified superblocks, modified inodes,
11205 and delayed reads and writes. This must be implemented by the kernel;
11206 The @command{sync} program does nothing but exercise the @code{sync} system
11209 @cindex crashes and corruption
11210 The kernel keeps data in memory to avoid doing (relatively slow) disk
11211 reads and writes. This improves performance, but if the computer
11212 crashes, data may be lost or the file system corrupted as a
11213 result. The @command{sync} command ensures everything in memory
11214 is written to disk.
11216 Any arguments are ignored, except for a lone @option{--help} or
11217 @option{--version} (@pxref{Common options}).
11222 @node truncate invocation
11223 @section @command{truncate}: Shrink or extend the size of a file
11226 @cindex truncating, file sizes
11228 @command{truncate} shrinks or extends the size of each @var{file} to the
11229 specified size. Synopsis:
11232 truncate @var{option}@dots{} @var{file}@dots{}
11235 @cindex files, creating
11236 Any @var{file} that does not exist is created.
11238 @cindex sparse files, creating
11239 @cindex holes, creating files with
11240 If a @var{file} is larger than the specified size, the extra data is lost.
11241 If a @var{file} is shorter, it is extended and the extended part (or hole)
11242 reads as zero bytes.
11244 The program accepts the following options. Also see @ref{Common options}.
11251 @opindex --no-create
11252 Do not create files that do not exist.
11257 @opindex --io-blocks
11258 Treat @var{size} as number of I/O blocks of the @var{file} rather than bytes.
11260 @item -r @var{rfile}
11261 @itemx --reference=@var{rfile}
11263 @opindex --reference
11264 Base the size of each @var{file} on the size of @var{rfile}.
11266 @item -s @var{size}
11267 @itemx --size=@var{size}
11270 Set or adjust the size of each @var{file} according to @var{size}.
11271 @multiplierSuffixesNoBlocks{size}
11273 @var{size} may also be prefixed by one of the following to adjust
11274 the size of each @var{file} based on their current size:
11276 @samp{+} => extend by
11277 @samp{-} => reduce by
11278 @samp{<} => at most
11279 @samp{>} => at least
11280 @samp{/} => round down to multiple of
11281 @samp{%} => round up to multiple of
11289 @node Printing text
11290 @chapter Printing text
11292 @cindex printing text, commands for
11293 @cindex commands for printing text
11295 This section describes commands that display text strings.
11298 * echo invocation:: Print a line of text.
11299 * printf invocation:: Format and print data.
11300 * yes invocation:: Print a string until interrupted.
11304 @node echo invocation
11305 @section @command{echo}: Print a line of text
11308 @cindex displaying text
11309 @cindex printing text
11310 @cindex text, displaying
11311 @cindex arbitrary text, displaying
11313 @command{echo} writes each given @var{string} to standard output, with a
11314 space between each and a newline after the last one. Synopsis:
11317 echo [@var{option}]@dots{} [@var{string}]@dots{}
11320 @mayConflictWithShellBuiltIn{echo}
11322 The program accepts the following options. Also see @ref{Common options}.
11323 Options must precede operands, and the normally-special argument
11324 @samp{--} has no special meaning and is treated like any other
11330 Do not output the trailing newline.
11334 @cindex backslash escapes
11335 Enable interpretation of the following backslash-escaped characters in
11344 produce no further output
11360 the eight-bit value that is the octal number @var{nnn}
11361 (zero to three octal digits), if @var{nnn} is
11362 a nine-bit value, the ninth bit is ignored
11364 the eight-bit value that is the octal number @var{nnn}
11365 (one to three octal digits), if @var{nnn} is
11366 a nine-bit value, the ninth bit is ignored
11368 the eight-bit value that is the hexadecimal number @var{hh}
11369 (one or two hexadecimal digits)
11374 @cindex backslash escapes
11375 Disable interpretation of backslash escapes in each @var{string}.
11376 This is the default. If @option{-e} and @option{-E} are both
11377 specified, the last one given takes effect.
11381 @vindex POSIXLY_CORRECT
11382 If the @env{POSIXLY_CORRECT} environment variable is set, then when
11383 @command{echo}'s first argument is not @option{-n} it outputs
11384 option-like arguments instead of treating them as options. For
11385 example, @code{echo -ne hello} outputs @samp{-ne hello} instead of
11386 plain @samp{hello}.
11388 @acronym{POSIX} does not require support for any options, and says
11389 that the behavior of @command{echo} is implementation-defined if any
11390 @var{string} contains a backslash or if the first argument is
11391 @option{-n}. Portable programs can use the @command{printf} command
11392 if they need to omit trailing newlines or output control characters or
11393 backslashes. @xref{printf invocation}.
11398 @node printf invocation
11399 @section @command{printf}: Format and print data
11402 @command{printf} does formatted printing of text. Synopsis:
11405 printf @var{format} [@var{argument}]@dots{}
11408 @command{printf} prints the @var{format} string, interpreting @samp{%}
11409 directives and @samp{\} escapes to format numeric and string arguments
11410 in a way that is mostly similar to the C @samp{printf} function.
11411 @xref{Output Conversion Syntax,, @command{printf} format directives,
11412 libc, The GNU C Library Reference Manual}, for details.
11413 The differences are listed below.
11415 @mayConflictWithShellBuiltIn{printf}
11420 The @var{format} argument is reused as necessary to convert all the
11421 given @var{argument}s. For example, the command @samp{printf %s a b}
11425 Missing @var{argument}s are treated as null strings or as zeros,
11426 depending on whether the context expects a string or a number. For
11427 example, the command @samp{printf %sx%d} prints @samp{x0}.
11431 An additional escape, @samp{\c}, causes @command{printf} to produce no
11432 further output. For example, the command @samp{printf 'A%sC\cD%sF' B
11433 E} prints @samp{ABC}.
11436 The hexadecimal escape sequence @samp{\x@var{hh}} has at most two
11437 digits, as opposed to C where it can have an unlimited number of
11438 digits. For example, the command @samp{printf '\x07e'} prints two
11439 bytes, whereas the C statement @samp{printf ("\x07e")} prints just
11444 @command{printf} has an additional directive, @samp{%b}, which prints its
11445 argument string with @samp{\} escapes interpreted in the same way as in
11446 the @var{format} string, except that octal escapes are of the form
11447 @samp{\0@var{ooo}} where @var{ooo} is 0 to 3 octal digits. If
11448 @samp{\@var{ooo}} is nine-bit value, ignore the ninth bit.
11449 If a precision is also given, it limits the number of bytes printed
11450 from the converted string.
11453 Numeric arguments must be single C constants, possibly with leading
11454 @samp{+} or @samp{-}. For example, @samp{printf %.4d -3} outputs
11458 @vindex POSIXLY_CORRECT
11459 If the leading character of a numeric argument is @samp{"} or @samp{'}
11460 then its value is the numeric value of the immediately following
11461 character. Any remaining characters are silently ignored if the
11462 @env{POSIXLY_CORRECT} environment variable is set; otherwise, a
11463 warning is printed. For example, @samp{printf "%d" "'a"} outputs
11464 @samp{97} on hosts that use the @acronym{ASCII} character set, since
11465 @samp{a} has the numeric value 97 in @acronym{ASCII}.
11470 A floating-point argument must use a period before any fractional
11471 digits, but is printed according to the @env{LC_NUMERIC} category of the
11472 current locale. For example, in a locale whose radix character is a
11473 comma, the command @samp{printf %g 3.14} outputs @samp{3,14} whereas
11474 the command @samp{printf %g 3,14} is an error.
11475 @xref{Floating point}.
11479 @command{printf} interprets @samp{\@var{ooo}} in @var{format} as an octal number
11480 (if @var{ooo} is 1 to 3 octal digits) specifying a byte to print,
11481 and @samp{\x@var{hh}} as a hexadecimal number (if @var{hh} is 1 to 2 hex
11482 digits) specifying a character to print.
11483 Note however that when @samp{\@var{ooo}} specifies a number larger than 255,
11484 @command{printf} ignores the ninth bit.
11485 For example, @samp{printf '\400'} is equivalent to @samp{printf '\0'}.
11490 @cindex ISO/IEC 10646
11492 @command{printf} interprets two character syntaxes introduced in
11493 @acronym{ISO} C 99:
11494 @samp{\u} for 16-bit Unicode (@acronym{ISO}/@acronym{IEC} 10646)
11495 characters, specified as
11496 four hexadecimal digits @var{hhhh}, and @samp{\U} for 32-bit Unicode
11497 characters, specified as eight hexadecimal digits @var{hhhhhhhh}.
11498 @command{printf} outputs the Unicode characters
11499 according to the @env{LC_CTYPE} locale. Unicode characters in the ranges
11500 U+0000...U+009F, U+D800...U+DFFF cannot be specified by this syntax, except
11501 for U+0024 ($), U+0040 (@@), and U+0060 (@`).
11503 The processing of @samp{\u} and @samp{\U} requires a full-featured
11504 @code{iconv} facility. It is activated on systems with glibc 2.2 (or newer),
11505 or when @code{libiconv} is installed prior to this package. Otherwise
11506 @samp{\u} and @samp{\U} will print as-is.
11508 The only options are a lone @option{--help} or
11509 @option{--version}. @xref{Common options}.
11510 Options must precede operands.
11512 The Unicode character syntaxes are useful for writing strings in a locale
11513 independent way. For example, a string containing the Euro currency symbol
11516 $ env printf '\u20AC 14.95'
11520 will be output correctly in all locales supporting the Euro symbol
11521 (@acronym{ISO}-8859-15, UTF-8, and others). Similarly, a Chinese string
11524 $ env printf '\u4e2d\u6587'
11528 will be output correctly in all Chinese locales (GB2312, BIG5, UTF-8, etc).
11530 Note that in these examples, the @command{printf} command has been
11531 invoked via @command{env} to ensure that we run the program found via
11532 your shell's search path, and not a shell alias or a built-in function.
11534 For larger strings, you don't need to look up the hexadecimal code
11535 values of each character one by one. @acronym{ASCII} characters mixed with \u
11536 escape sequences is also known as the JAVA source file encoding. You can
11537 use GNU recode 3.5c (or newer) to convert strings to this encoding. Here
11538 is how to convert a piece of text into a shell script which will output
11539 this text in a locale-independent way:
11542 $ LC_CTYPE=zh_CN.big5 /usr/local/bin/printf \
11543 '\u4e2d\u6587\n' > sample.txt
11544 $ recode BIG5..JAVA < sample.txt \
11545 | sed -e "s|^|/usr/local/bin/printf '|" -e "s|$|\\\\n'|" \
11552 @node yes invocation
11553 @section @command{yes}: Print a string until interrupted
11556 @cindex repeated output of a string
11558 @command{yes} prints the command line arguments, separated by spaces and
11559 followed by a newline, forever until it is killed. If no arguments are
11560 given, it prints @samp{y} followed by a newline forever until killed.
11562 Upon a write error, @command{yes} exits with status @samp{1}.
11564 The only options are a lone @option{--help} or @option{--version}.
11565 To output an argument that begins with
11566 @samp{-}, precede it with @option{--}, e.g., @samp{yes -- --help}.
11567 @xref{Common options}.
11571 @chapter Conditions
11574 @cindex commands for exit status
11575 @cindex exit status commands
11577 This section describes commands that are primarily useful for their exit
11578 status, rather than their output. Thus, they are often used as the
11579 condition of shell @code{if} statements, or as the last command in a
11583 * false invocation:: Do nothing, unsuccessfully.
11584 * true invocation:: Do nothing, successfully.
11585 * test invocation:: Check file types and compare values.
11586 * expr invocation:: Evaluate expressions.
11590 @node false invocation
11591 @section @command{false}: Do nothing, unsuccessfully
11594 @cindex do nothing, unsuccessfully
11595 @cindex failure exit status
11596 @cindex exit status of @command{false}
11598 @command{false} does nothing except return an exit status of 1, meaning
11599 @dfn{failure}. It can be used as a place holder in shell scripts
11600 where an unsuccessful command is needed.
11601 In most modern shells, @command{false} is a built-in command, so when
11602 you use @samp{false} in a script, you're probably using the built-in
11603 command, not the one documented here.
11605 @command{false} honors the @option{--help} and @option{--version} options.
11607 This version of @command{false} is implemented as a C program, and is thus
11608 more secure and faster than a shell script implementation, and may safely
11609 be used as a dummy shell for the purpose of disabling accounts.
11611 Note that @command{false} (unlike all other programs documented herein)
11612 exits unsuccessfully, even when invoked with
11613 @option{--help} or @option{--version}.
11615 Portable programs should not assume that the exit status of
11616 @command{false} is 1, as it is greater than 1 on some
11617 non-@acronym{GNU} hosts.
11620 @node true invocation
11621 @section @command{true}: Do nothing, successfully
11624 @cindex do nothing, successfully
11626 @cindex successful exit
11627 @cindex exit status of @command{true}
11629 @command{true} does nothing except return an exit status of 0, meaning
11630 @dfn{success}. It can be used as a place holder in shell scripts
11631 where a successful command is needed, although the shell built-in
11632 command @code{:} (colon) may do the same thing faster.
11633 In most modern shells, @command{true} is a built-in command, so when
11634 you use @samp{true} in a script, you're probably using the built-in
11635 command, not the one documented here.
11637 @command{true} honors the @option{--help} and @option{--version} options.
11639 Note, however, that it is possible to cause @command{true}
11640 to exit with nonzero status: with the @option{--help} or @option{--version}
11641 option, and with standard
11642 output already closed or redirected to a file that evokes an I/O error.
11643 For example, using a Bourne-compatible shell:
11646 $ ./true --version >&-
11647 ./true: write error: Bad file number
11648 $ ./true --version > /dev/full
11649 ./true: write error: No space left on device
11652 This version of @command{true} is implemented as a C program, and is thus
11653 more secure and faster than a shell script implementation, and may safely
11654 be used as a dummy shell for the purpose of disabling accounts.
11656 @node test invocation
11657 @section @command{test}: Check file types and compare values
11660 @cindex check file types
11661 @cindex compare values
11662 @cindex expression evaluation
11664 @command{test} returns a status of 0 (true) or 1 (false) depending on the
11665 evaluation of the conditional expression @var{expr}. Each part of the
11666 expression must be a separate argument.
11668 @command{test} has file status checks, string operators, and numeric
11669 comparison operators.
11671 @command{test} has an alternate form that uses opening and closing
11672 square brackets instead a leading @samp{test}. For example, instead
11673 of @samp{test -d /}, you can write @samp{[ -d / ]}. The square
11674 brackets must be separate arguments; for example, @samp{[-d /]} does
11675 not have the desired effect. Since @samp{test @var{expr}} and @samp{[
11676 @var{expr} ]} have the same meaning, only the former form is discussed
11682 test @var{expression}
11684 [ @var{expression} ]
11689 @mayConflictWithShellBuiltIn{test}
11691 If @var{expression} is omitted, @command{test} returns false.
11692 If @var{expression} is a single argument,
11693 @command{test} returns false if the argument is null and true
11694 otherwise. The argument
11695 can be any string, including strings like @samp{-d}, @samp{-1},
11696 @samp{--}, @samp{--help}, and @samp{--version} that most other
11697 programs would treat as options. To get help and version information,
11698 invoke the commands @samp{[ --help} and @samp{[ --version}, without
11699 the usual closing brackets. @xref{Common options}.
11701 @cindex exit status of @command{test}
11705 0 if the expression is true,
11706 1 if the expression is false,
11707 2 if an error occurred.
11711 * File type tests:: -[bcdfhLpSt]
11712 * Access permission tests:: -[gkruwxOG]
11713 * File characteristic tests:: -e -s -nt -ot -ef
11714 * String tests:: -z -n = == !=
11715 * Numeric tests:: -eq -ne -lt -le -gt -ge
11716 * Connectives for test:: ! -a -o
11720 @node File type tests
11721 @subsection File type tests
11723 @cindex file type tests
11725 These options test for particular types of files. (Everything's a file,
11726 but not all files are the same!)
11730 @item -b @var{file}
11732 @cindex block special check
11733 True if @var{file} exists and is a block special device.
11735 @item -c @var{file}
11737 @cindex character special check
11738 True if @var{file} exists and is a character special device.
11740 @item -d @var{file}
11742 @cindex directory check
11743 True if @var{file} exists and is a directory.
11745 @item -f @var{file}
11747 @cindex regular file check
11748 True if @var{file} exists and is a regular file.
11750 @item -h @var{file}
11751 @itemx -L @var{file}
11754 @cindex symbolic link check
11755 True if @var{file} exists and is a symbolic link.
11756 Unlike all other file-related tests, this test does not dereference
11757 @var{file} if it is a symbolic link.
11759 @item -p @var{file}
11761 @cindex named pipe check
11762 True if @var{file} exists and is a named pipe.
11764 @item -S @var{file}
11766 @cindex socket check
11767 True if @var{file} exists and is a socket.
11771 @cindex terminal check
11772 True if @var{fd} is a file descriptor that is associated with a
11778 @node Access permission tests
11779 @subsection Access permission tests
11781 @cindex access permission tests
11782 @cindex permission tests
11784 These options test for particular access permissions.
11788 @item -g @var{file}
11790 @cindex set-group-ID check
11791 True if @var{file} exists and has its set-group-ID bit set.
11793 @item -k @var{file}
11795 @cindex sticky bit check
11796 True if @var{file} exists and has its @dfn{sticky} bit set.
11798 @item -r @var{file}
11800 @cindex readable file check
11801 True if @var{file} exists and read permission is granted.
11803 @item -u @var{file}
11805 @cindex set-user-ID check
11806 True if @var{file} exists and has its set-user-ID bit set.
11808 @item -w @var{file}
11810 @cindex writable file check
11811 True if @var{file} exists and write permission is granted.
11813 @item -x @var{file}
11815 @cindex executable file check
11816 True if @var{file} exists and execute permission is granted
11817 (or search permission, if it is a directory).
11819 @item -O @var{file}
11821 @cindex owned by effective user ID check
11822 True if @var{file} exists and is owned by the current effective user ID.
11824 @item -G @var{file}
11826 @cindex owned by effective group ID check
11827 True if @var{file} exists and is owned by the current effective group ID.
11831 @node File characteristic tests
11832 @subsection File characteristic tests
11834 @cindex file characteristic tests
11836 These options test other file characteristics.
11840 @item -e @var{file}
11842 @cindex existence-of-file check
11843 True if @var{file} exists.
11845 @item -s @var{file}
11847 @cindex nonempty file check
11848 True if @var{file} exists and has a size greater than zero.
11850 @item @var{file1} -nt @var{file2}
11852 @cindex newer-than file check
11853 True if @var{file1} is newer (according to modification date) than
11854 @var{file2}, or if @var{file1} exists and @var{file2} does not.
11856 @item @var{file1} -ot @var{file2}
11858 @cindex older-than file check
11859 True if @var{file1} is older (according to modification date) than
11860 @var{file2}, or if @var{file2} exists and @var{file1} does not.
11862 @item @var{file1} -ef @var{file2}
11864 @cindex same file check
11865 @cindex hard link check
11866 True if @var{file1} and @var{file2} have the same device and inode
11867 numbers, i.e., if they are hard links to each other.
11873 @subsection String tests
11875 @cindex string tests
11877 These options test string characteristics. You may need to quote
11878 @var{string} arguments for the shell. For example:
11884 The quotes here prevent the wrong arguments from being passed to
11885 @command{test} if @samp{$V} is empty or contains special characters.
11889 @item -z @var{string}
11891 @cindex zero-length string check
11892 True if the length of @var{string} is zero.
11894 @item -n @var{string}
11895 @itemx @var{string}
11897 @cindex nonzero-length string check
11898 True if the length of @var{string} is nonzero.
11900 @item @var{string1} = @var{string2}
11902 @cindex equal string check
11903 True if the strings are equal.
11905 @item @var{string1} == @var{string2}
11907 @cindex equal string check
11908 True if the strings are equal (synonym for =).
11910 @item @var{string1} != @var{string2}
11912 @cindex not-equal string check
11913 True if the strings are not equal.
11918 @node Numeric tests
11919 @subsection Numeric tests
11921 @cindex numeric tests
11922 @cindex arithmetic tests
11924 Numeric relational operators. The arguments must be entirely numeric
11925 (possibly negative), or the special expression @w{@code{-l @var{string}}},
11926 which evaluates to the length of @var{string}.
11930 @item @var{arg1} -eq @var{arg2}
11931 @itemx @var{arg1} -ne @var{arg2}
11932 @itemx @var{arg1} -lt @var{arg2}
11933 @itemx @var{arg1} -le @var{arg2}
11934 @itemx @var{arg1} -gt @var{arg2}
11935 @itemx @var{arg1} -ge @var{arg2}
11942 These arithmetic binary operators return true if @var{arg1} is equal,
11943 not-equal, less-than, less-than-or-equal, greater-than, or
11944 greater-than-or-equal than @var{arg2}, respectively.
11951 test -1 -gt -2 && echo yes
11953 test -l abc -gt 1 && echo yes
11956 @error{} test: integer expression expected before -eq
11960 @node Connectives for test
11961 @subsection Connectives for @command{test}
11963 @cindex logical connectives
11964 @cindex connectives, logical
11966 The usual logical connectives.
11972 True if @var{expr} is false.
11974 @item @var{expr1} -a @var{expr2}
11976 @cindex logical and operator
11977 @cindex and operator
11978 True if both @var{expr1} and @var{expr2} are true.
11980 @item @var{expr1} -o @var{expr2}
11982 @cindex logical or operator
11983 @cindex or operator
11984 True if either @var{expr1} or @var{expr2} is true.
11989 @node expr invocation
11990 @section @command{expr}: Evaluate expressions
11993 @cindex expression evaluation
11994 @cindex evaluation of expressions
11996 @command{expr} evaluates an expression and writes the result on standard
11997 output. Each token of the expression must be a separate argument.
11999 Operands are either integers or strings. Integers consist of one or
12000 more decimal digits, with an optional leading @samp{-}.
12001 @command{expr} converts
12002 anything appearing in an operand position to an integer or a string
12003 depending on the operation being applied to it.
12005 Strings are not quoted for @command{expr} itself, though you may need to
12006 quote them to protect characters with special meaning to the shell,
12007 e.g., spaces. However, regardless of whether it is quoted, a string
12008 operand should not be a parenthesis or any of @command{expr}'s
12009 operators like @code{+}, so you cannot safely pass an arbitrary string
12010 @code{$str} to expr merely by quoting it to the shell. One way to
12011 work around this is to use the @sc{gnu} extension @code{+},
12012 (e.g., @code{+ "$str" = foo}); a more portable way is to use
12013 @code{@w{" $str"}} and to adjust the rest of the expression to take
12014 the leading space into account (e.g., @code{@w{" $str" = " foo"}}).
12016 You should not pass a negative integer or a string with leading
12017 @samp{-} as @command{expr}'s first argument, as it might be
12018 misinterpreted as an option; this can be avoided by parenthesization.
12019 Also, portable scripts should not use a string operand that happens to
12020 take the form of an integer; this can be worked around by inserting
12021 leading spaces as mentioned above.
12023 @cindex parentheses for grouping
12024 Operators may be given as infix symbols or prefix keywords. Parentheses
12025 may be used for grouping in the usual manner. You must quote
12026 parentheses and many operators to avoid the shell evaluating them,
12029 When built with support for the GNU MP library, @command{expr} uses
12030 arbitrary-precision arithmetic; otherwise, it uses native arithmetic
12031 types and may fail due to arithmetic overflow.
12033 The only options are @option{--help} and @option{--version}. @xref{Common
12034 options}. Options must precede operands.
12036 @cindex exit status of @command{expr}
12040 0 if the expression is neither null nor 0,
12041 1 if the expression is null or 0,
12042 2 if the expression is invalid,
12043 3 if an internal error occurred (e.g., arithmetic overflow).
12047 * String expressions:: + : match substr index length
12048 * Numeric expressions:: + - * / %
12049 * Relations for expr:: | & < <= = == != >= >
12050 * Examples of expr:: Examples.
12054 @node String expressions
12055 @subsection String expressions
12057 @cindex string expressions
12058 @cindex expressions, string
12060 @command{expr} supports pattern matching and other string operators. These
12061 have higher precedence than both the numeric and relational operators (in
12062 the next sections).
12066 @item @var{string} : @var{regex}
12067 @cindex pattern matching
12068 @cindex regular expression matching
12069 @cindex matching patterns
12070 Perform pattern matching. The arguments are converted to strings and the
12071 second is considered to be a (basic, a la GNU @code{grep}) regular
12072 expression, with a @code{^} implicitly prepended. The first argument is
12073 then matched against this regular expression.
12075 If the match succeeds and @var{regex} uses @samp{\(} and @samp{\)}, the
12076 @code{:} expression returns the part of @var{string} that matched the
12077 subexpression; otherwise, it returns the number of characters matched.
12079 If the match fails, the @code{:} operator returns the null string if
12080 @samp{\(} and @samp{\)} are used in @var{regex}, otherwise 0.
12082 @kindex \( @r{regexp operator}
12083 Only the first @samp{\( @dots{} \)} pair is relevant to the return
12084 value; additional pairs are meaningful only for grouping the regular
12085 expression operators.
12087 @kindex \+ @r{regexp operator}
12088 @kindex \? @r{regexp operator}
12089 @kindex \| @r{regexp operator}
12090 In the regular expression, @code{\+}, @code{\?}, and @code{\|} are
12091 operators which respectively match one or more, zero or one, or separate
12092 alternatives. SunOS and other @command{expr}'s treat these as regular
12093 characters. (@acronym{POSIX} allows either behavior.)
12094 @xref{Top, , Regular Expression Library, regex, Regex}, for details of
12095 regular expression syntax. Some examples are in @ref{Examples of expr}.
12097 @item match @var{string} @var{regex}
12099 An alternative way to do pattern matching. This is the same as
12100 @w{@samp{@var{string} : @var{regex}}}.
12102 @item substr @var{string} @var{position} @var{length}
12104 Returns the substring of @var{string} beginning at @var{position}
12105 with length at most @var{length}. If either @var{position} or
12106 @var{length} is negative, zero, or non-numeric, returns the null string.
12108 @item index @var{string} @var{charset}
12110 Returns the first position in @var{string} where the first character in
12111 @var{charset} was found. If no character in @var{charset} is found in
12112 @var{string}, return 0.
12114 @item length @var{string}
12116 Returns the length of @var{string}.
12118 @item + @var{token}
12120 Interpret @var{token} as a string, even if it is a keyword like @var{match}
12121 or an operator like @code{/}.
12122 This makes it possible to test @code{expr length + "$x"} or
12123 @code{expr + "$x" : '.*/\(.\)'} and have it do the right thing even if
12124 the value of @var{$x} happens to be (for example) @code{/} or @code{index}.
12125 This operator is a @acronym{GNU} extension. Portable shell scripts should use
12126 @code{@w{" $token"} : @w{' \(.*\)'}} instead of @code{+ "$token"}.
12130 To make @command{expr} interpret keywords as strings, you must use the
12131 @code{quote} operator.
12134 @node Numeric expressions
12135 @subsection Numeric expressions
12137 @cindex numeric expressions
12138 @cindex expressions, numeric
12140 @command{expr} supports the usual numeric operators, in order of increasing
12141 precedence. These numeric operators have lower precedence than the
12142 string operators described in the previous section, and higher precedence
12143 than the connectives (next section).
12151 @cindex subtraction
12152 Addition and subtraction. Both arguments are converted to integers;
12153 an error occurs if this cannot be done.
12159 @cindex multiplication
12162 Multiplication, division, remainder. Both arguments are converted to
12163 integers; an error occurs if this cannot be done.
12168 @node Relations for expr
12169 @subsection Relations for @command{expr}
12171 @cindex connectives, logical
12172 @cindex logical connectives
12173 @cindex relations, numeric or string
12175 @command{expr} supports the usual logical connectives and relations. These
12176 have lower precedence than the string and numeric operators
12177 (previous sections). Here is the list, lowest-precedence operator first.
12183 @cindex logical or operator
12184 @cindex or operator
12185 Returns its first argument if that is neither null nor zero, otherwise
12186 its second argument if it is neither null nor zero, otherwise 0. It
12187 does not evaluate its second argument if its first argument is neither
12192 @cindex logical and operator
12193 @cindex and operator
12194 Return its first argument if neither argument is null or zero, otherwise
12195 0. It does not evaluate its second argument if its first argument is
12198 @item < <= = == != >= >
12205 @cindex comparison operators
12207 Compare the arguments and return 1 if the relation is true, 0 otherwise.
12208 @code{==} is a synonym for @code{=}. @command{expr} first tries to convert
12209 both arguments to integers and do a numeric comparison; if either
12210 conversion fails, it does a lexicographic comparison using the character
12211 collating sequence specified by the @env{LC_COLLATE} locale.
12216 @node Examples of expr
12217 @subsection Examples of using @command{expr}
12219 @cindex examples of @command{expr}
12220 Here are a few examples, including quoting for shell metacharacters.
12222 To add 1 to the shell variable @code{foo}, in Bourne-compatible shells:
12225 foo=`expr $foo + 1`
12228 To print the non-directory part of the file name stored in
12229 @code{$fname}, which need not contain a @code{/}:
12232 expr $fname : '.*/\(.*\)' '|' $fname
12235 An example showing that @code{\+} is an operator:
12243 expr abc : 'a\(.\)c'
12245 expr index abcdef cz
12248 @error{} expr: syntax error
12249 expr index + index a
12255 @chapter Redirection
12257 @cindex redirection
12258 @cindex commands for redirection
12260 Unix shells commonly provide several forms of @dfn{redirection}---ways
12261 to change the input source or output destination of a command. But one
12262 useful redirection is performed by a separate command, not by the shell;
12263 it's described here.
12266 * tee invocation:: Redirect output to multiple files or processes.
12270 @node tee invocation
12271 @section @command{tee}: Redirect output to multiple files or processes
12274 @cindex pipe fitting
12275 @cindex destinations, multiple output
12276 @cindex read from stdin and write to stdout and files
12278 The @command{tee} command copies standard input to standard output and also
12279 to any files given as arguments. This is useful when you want not only
12280 to send some data down a pipe, but also to save a copy. Synopsis:
12283 tee [@var{option}]@dots{} [@var{file}]@dots{}
12286 If a file being written to does not already exist, it is created. If a
12287 file being written to already exists, the data it previously contained
12288 is overwritten unless the @option{-a} option is used.
12290 A @var{file} of @samp{-} causes @command{tee} to send another copy of
12291 input to standard output, but this is typically not that useful as the
12292 copies are interleaved.
12294 The program accepts the following options. Also see @ref{Common options}.
12301 Append standard input to the given files rather than overwriting
12305 @itemx --ignore-interrupts
12307 @opindex --ignore-interrupts
12308 Ignore interrupt signals.
12312 The @command{tee} command is useful when you happen to be transferring a large
12313 amount of data and also want to summarize that data without reading
12314 it a second time. For example, when you are downloading a DVD image,
12315 you often want to verify its signature or checksum right away.
12316 The inefficient way to do it is simply:
12319 wget http://example.com/some.iso && sha1sum some.iso
12322 One problem with the above is that it makes you wait for the
12323 download to complete before starting the time-consuming SHA1 computation.
12324 Perhaps even more importantly, the above requires reading
12325 the DVD image a second time (the first was from the network).
12327 The efficient way to do it is to interleave the download
12328 and SHA1 computation. Then, you'll get the checksum for
12329 free, because the entire process parallelizes so well:
12332 # slightly contrived, to demonstrate process substitution
12333 wget -O - http://example.com/dvd.iso \
12334 | tee >(sha1sum > dvd.sha1) > dvd.iso
12337 That makes @command{tee} write not just to the expected output file,
12338 but also to a pipe running @command{sha1sum} and saving the final
12339 checksum in a file named @file{dvd.sha1}.
12341 Note, however, that this example relies on a feature of modern shells
12342 called @dfn{process substitution}
12343 (the @samp{>(command)} syntax, above;
12344 @xref{Process Substitution,,Process Substitution, bashref,
12345 The Bash Reference Manual}.),
12346 so it works with @command{zsh}, @command{bash}, and @command{ksh},
12347 but not with @command{/bin/sh}. So if you write code like this
12348 in a shell script, be sure to start the script with @samp{#!/bin/bash}.
12350 Since the above example writes to one file and one process,
12351 a more conventional and portable use of @command{tee} is even better:
12354 wget -O - http://example.com/dvd.iso \
12355 | tee dvd.iso | sha1sum > dvd.sha1
12358 You can extend this example to make @command{tee} write to two processes,
12359 computing MD5 and SHA1 checksums in parallel. In this case,
12360 process substitution is required:
12363 wget -O - http://example.com/dvd.iso \
12364 | tee >(sha1sum > dvd.sha1) \
12365 >(md5sum > dvd.md5) \
12369 This technique is also useful when you want to make a @emph{compressed}
12370 copy of the contents of a pipe.
12371 Consider a tool to graphically summarize disk usage data from @samp{du -ak}.
12372 For a large hierarchy, @samp{du -ak} can run for a long time,
12373 and can easily produce terabytes of data, so you won't want to
12374 rerun the command unnecessarily. Nor will you want to save
12375 the uncompressed output.
12377 Doing it the inefficient way, you can't even start the GUI
12378 until after you've compressed all of the @command{du} output:
12381 du -ak | gzip -9 > /tmp/du.gz
12382 gzip -d /tmp/du.gz | xdiskusage -a
12385 With @command{tee} and process substitution, you start the GUI
12386 right away and eliminate the decompression completely:
12389 du -ak | tee >(gzip -9 > /tmp/du.gz) | xdiskusage -a
12392 Finally, if you regularly create more than one type of
12393 compressed tarball at once, for example when @code{make dist} creates
12394 both @command{gzip}-compressed and @command{bzip2}-compressed tarballs,
12395 there may be a better way.
12396 Typical @command{automake}-generated @file{Makefile} rules create
12397 the two compressed tar archives with commands in sequence, like this
12398 (slightly simplified):
12401 tardir=your-pkg-M.N
12402 tar chof - "$tardir" | gzip -9 -c > your-pkg-M.N.tar.gz
12403 tar chof - "$tardir" | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12406 However, if the hierarchy you are archiving and compressing is larger
12407 than a couple megabytes, and especially if you are using a multi-processor
12408 system with plenty of memory, then you can do much better by reading the
12409 directory contents only once and running the compression programs in parallel:
12412 tardir=your-pkg-M.N
12413 tar chof - "$tardir" \
12414 | tee >(gzip -9 -c > your-pkg-M.N.tar.gz) \
12415 | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12421 @node File name manipulation
12422 @chapter File name manipulation
12424 @cindex file name manipulation
12425 @cindex manipulation of file names
12426 @cindex commands for file name manipulation
12428 This section describes commands that manipulate file names.
12431 * basename invocation:: Strip directory and suffix from a file name.
12432 * dirname invocation:: Strip last file name component.
12433 * pathchk invocation:: Check file name validity and portability.
12434 * mktemp invocation:: Create temporary file or directory.
12435 * realpath invocation:: Print resolved file names.
12439 @node basename invocation
12440 @section @command{basename}: Strip directory and suffix from a file name
12443 @cindex strip directory and suffix from file names
12444 @cindex directory, stripping from file names
12445 @cindex suffix, stripping from file names
12446 @cindex file names, stripping directory and suffix
12447 @cindex leading directory components, stripping
12449 @command{basename} removes any leading directory components from
12450 @var{name}. Synopsis:
12453 basename @var{name} [@var{suffix}]
12454 basename @var{option}... @var{name}...
12457 If @var{suffix} is specified and is identical to the end of @var{name},
12458 it is removed from @var{name} as well. Note that since trailing slashes
12459 are removed prior to suffix matching, @var{suffix} will do nothing if it
12460 contains slashes. @command{basename} prints the result on standard
12463 @c This test is used both here and in the section on dirname.
12464 @macro basenameAndDirname
12465 Together, @command{basename} and @command{dirname} are designed such
12466 that if @samp{ls "$name"} succeeds, then the command sequence @samp{cd
12467 "$(dirname "$name")"; ls "$(basename "$name")"} will, too. This works
12468 for everything except file names containing a trailing newline.
12470 @basenameAndDirname
12472 @acronym{POSIX} allows the implementation to define the results if
12473 @var{name} is empty or @samp{//}. In the former case, @acronym{GNU}
12474 @command{basename} returns the empty string. In the latter case, the
12475 result is @samp{//} on platforms where @var{//} is distinct from
12476 @var{/}, and @samp{/} on platforms where there is no difference.
12478 The program accepts the following options. Also see @ref{Common options}.
12479 Options must precede operands.
12486 @opindex --multiple
12487 Support more than one argument. Treat every argument as a @var{name}.
12488 With this, an optional @var{suffix} must be specified using the
12489 @option{-s} option.
12491 @item -s @var{suffix}
12492 @itemx --suffix=@var{suffix}
12495 Remove a trailing @var{suffix}.
12496 This option implies the @option{-a} option.
12502 Separate output items with @sc{nul} characters.
12512 basename /usr/bin/sort
12515 basename include/stdio.h .h
12518 basename -s .h include/stdio.h
12520 # Output "stdio" followed by "stdlib"
12521 basename -a -s .h include/stdio.h include/stdlib.h
12525 @node dirname invocation
12526 @section @command{dirname}: Strip last file name component
12529 @cindex directory components, printing
12530 @cindex stripping non-directory suffix
12531 @cindex non-directory suffix, stripping
12533 @command{dirname} prints all but the final slash-delimited component
12534 of each @var{name}. Slashes on either side of the final component are
12535 also removed. If the string contains no slash, @command{dirname}
12536 prints @samp{.} (meaning the current directory). Synopsis:
12539 dirname [@var{option}] @var{name}...
12542 @var{name} need not be a file name, but if it is, this operation
12543 effectively lists the directory that contains the final component,
12544 including the case when the final component is itself a directory.
12546 @basenameAndDirname
12548 @acronym{POSIX} allows the implementation to define the results if
12549 @var{name} is @samp{//}. With @acronym{GNU} @command{dirname}, the
12550 result is @samp{//} on platforms where @var{//} is distinct from
12551 @var{/}, and @samp{/} on platforms where there is no difference.
12553 The program accepts the following option. Also see @ref{Common options}.
12561 Separate output items with @sc{nul} characters.
12570 # Output "/usr/bin".
12571 dirname /usr/bin/sort
12572 dirname /usr/bin//.//
12574 # Output "dir1" followed by "dir2"
12575 dirname dir1/str dir2/str
12582 @node pathchk invocation
12583 @section @command{pathchk}: Check file name validity and portability
12586 @cindex file names, checking validity and portability
12587 @cindex valid file names, checking for
12588 @cindex portable file names, checking for
12590 @command{pathchk} checks validity and portability of file names. Synopsis:
12593 pathchk [@var{option}]@dots{} @var{name}@dots{}
12596 For each @var{name}, @command{pathchk} prints an error message if any of
12597 these conditions is true:
12601 One of the existing directories in @var{name} does not have search
12602 (execute) permission,
12604 The length of @var{name} is larger than the maximum supported by the
12607 The length of one component of @var{name} is longer than
12608 its file system's maximum.
12611 A nonexistent @var{name} is not an error, so long a file with that
12612 name could be created under the above conditions.
12614 The program accepts the following options. Also see @ref{Common options}.
12615 Options must precede operands.
12621 Instead of performing checks based on the underlying file system,
12622 print an error message if any of these conditions is true:
12626 A file name is empty.
12629 A file name contains a character outside the @acronym{POSIX} portable file
12630 name character set, namely, the ASCII letters and digits, @samp{.},
12631 @samp{_}, @samp{-}, and @samp{/}.
12634 The length of a file name or one of its components exceeds the
12635 @acronym{POSIX} minimum limits for portability.
12640 Print an error message if a file name is empty, or if it contains a component
12641 that begins with @samp{-}.
12643 @item --portability
12644 @opindex --portability
12645 Print an error message if a file name is not portable to all @acronym{POSIX}
12646 hosts. This option is equivalent to @samp{-p -P}.
12650 @cindex exit status of @command{pathchk}
12654 0 if all specified file names passed all checks,
12658 @node mktemp invocation
12659 @section @command{mktemp}: Create temporary file or directory
12662 @cindex file names, creating temporary
12663 @cindex directory, creating temporary
12664 @cindex temporary files and directories
12666 @command{mktemp} manages the creation of temporary files and
12667 directories. Synopsis:
12670 mktemp [@var{option}]@dots{} [@var{template}]
12673 Safely create a temporary file or directory based on @var{template},
12674 and print its name. If given, @var{template} must include at least
12675 three consecutive @samp{X}s in the last component. If omitted, the template
12676 @samp{tmp.XXXXXXXXXX} is used, and option @option{--tmpdir} is
12677 implied. The final run of @samp{X}s in the @var{template} will be replaced
12678 by alpha-numeric characters; thus, on a case-sensitive file system,
12679 and with a @var{template} including a run of @var{n} instances of @samp{X},
12680 there are @samp{62**@var{n}} potential file names.
12682 Older scripts used to create temporary files by simply joining the
12683 name of the program with the process id (@samp{$$}) as a suffix.
12684 However, that naming scheme is easily predictable, and suffers from a
12685 race condition where the attacker can create an appropriately named
12686 symbolic link, such that when the script then opens a handle to what
12687 it thought was an unused file, it is instead modifying an existing
12688 file. Using the same scheme to create a directory is slightly safer,
12689 since the @command{mkdir} will fail if the target already exists, but
12690 it is still inferior because it allows for denial of service attacks.
12691 Therefore, modern scripts should use the @command{mktemp} command to
12692 guarantee that the generated name will be unpredictable, and that
12693 knowledge of the temporary file name implies that the file was created
12694 by the current script and cannot be modified by other users.
12696 When creating a file, the resulting file has read and write
12697 permissions for the current user, but no permissions for the group or
12698 others; these permissions are reduced if the current umask is more
12701 Here are some examples (although note that if you repeat them, you
12702 will most likely get different file names):
12707 Create a temporary file in the current directory.
12714 Create a temporary file with a known suffix.
12716 $ mktemp --suffix=.txt file-XXXX
12718 $ mktemp file-XXXX-XXXX.txt
12723 Create a secure fifo relative to the user's choice of @env{TMPDIR},
12724 but falling back to the current directory rather than @file{/tmp}.
12725 Note that @command{mktemp} does not create fifos, but can create a
12726 secure directory in which the fifo can live. Exit the shell if the
12727 directory or fifo could not be created.
12729 $ dir=$(mktemp -p "$@{TMPDIR:-.@}" -d dir-XXXX) || exit 1
12731 $ mkfifo "$fifo" || @{ rmdir "$dir"; exit 1; @}
12735 Create and use a temporary file if possible, but ignore failure. The
12736 file will reside in the directory named by @env{TMPDIR}, if specified,
12737 or else in @file{/tmp}.
12739 $ file=$(mktemp -q) && @{
12740 > # Safe to use $file only within this block. Use quotes,
12741 > # since $TMPDIR, and thus $file, may contain whitespace.
12742 > echo ... > "$file"
12748 Act as a semi-random character generator (it is not fully random,
12749 since it is impacted by the contents of the current directory). To
12750 avoid security holes, do not use the resulting names to create a file.
12760 The program accepts the following options. Also see @ref{Common options}.
12767 @opindex --directory
12768 Create a directory rather than a file. The directory will have read,
12769 write, and search permissions for the current user, but no permissions
12770 for the group or others; these permissions are reduced if the current
12771 umask is more restrictive.
12777 Suppress diagnostics about failure to create a file or directory. The
12778 exit status will still reflect whether a file was created.
12784 Generate a temporary name that does not name an existing file, without
12785 changing the file system contents. Using the output of this command
12786 to create a new file is inherently unsafe, as there is a window of
12787 time between generating the name and using it where another process
12788 can create an object by the same name.
12791 @itemx --tmpdir[=@var{dir}]
12794 Treat @var{template} relative to the directory @var{dir}. If
12795 @var{dir} is not specified (only possible with the long option
12796 @option{--tmpdir}) or is the empty string, use the value of
12797 @env{TMPDIR} if available, otherwise use @samp{/tmp}. If this is
12798 specified, @var{template} must not be absolute. However,
12799 @var{template} can still contain slashes, although intermediate
12800 directories must already exist.
12802 @item --suffix=@var{suffix}
12804 Append @var{suffix} to the @var{template}. @var{suffix} must not
12805 contain slash. If @option{--suffix} is specified, @var{template} must
12806 end in @samp{X}; if it is not specified, then an appropriate
12807 @option{--suffix} is inferred by finding the last @samp{X} in
12808 @var{template}. This option exists for use with the default
12809 @var{template} and for the creation of a @var{suffix} that starts with
12814 Treat @var{template} as a single file relative to the value of
12815 @env{TMPDIR} if available, or to the directory specified by
12816 @option{-p}, otherwise to @samp{/tmp}. @var{template} must not
12817 contain slashes. This option is deprecated; the use of @option{-p}
12818 without @option{-t} offers better defaults (by favoring the command
12819 line over @env{TMPDIR}) and more flexibility (by allowing intermediate
12824 @cindex exit status of @command{mktemp}
12828 0 if the file was created,
12833 @node realpath invocation
12834 @section @command{realpath}: Print the resolved file name.
12837 @cindex file names, canonicalization
12838 @cindex symlinks, resolution
12839 @cindex canonical file name
12840 @cindex canonicalize a file name
12844 @command{realpath} expands all symbolic links and resolves references to
12845 @samp{/./}, @samp{/../} and extra @samp{/} characters. By default,
12846 all but the last component of the specified files must exist. Synopsis:
12849 realpath [@var{option}]@dots{} @var{file}@dots{}
12852 The program accepts the following options. Also see @ref{Common options}.
12857 @itemx --canonicalize-existing
12859 @opindex --canonicalize-existing
12860 Ensure that all components of the specified file names exist.
12861 If any component is missing or unavailable, @command{realpath} will output
12862 a diagnostic unless the @option{-q} option is specified, and exit with a
12863 nonzero exit code. A trailing slash requires that the name resolve to a
12867 @itemx --canonicalize-missing
12869 @opindex --canonicalize-missing
12870 If any component of a specified file name is missing or unavailable,
12871 treat it as a directory.
12877 Symbolic links are resolved in the specified file names,
12878 but they are resolved after any subsequent @samp{..} components are processed.
12883 @opindex --physical
12884 Symbolic links are resolved in the specified file names,
12885 and they are resolved before any subsequent @samp{..} components are processed.
12886 This is the default mode of operation.
12892 Suppress diagnostic messages for specified file names.
12896 @itemx --no-symlinks
12899 @opindex --no-symlinks
12900 Do not resolve symbolic links. Only resolve references to
12901 @samp{/./}, @samp{/../} and remove extra @samp{/} characters.
12902 When combined with the @option{-m} option, realpath operates
12903 only on the file name, and does not touch any actual file.
12909 Separate output items with @sc{nul} characters.
12911 @itemx --relative-to=@var{file}
12912 @opindex --relative-to
12914 Print the resolved file names relative to the specified file.
12915 Note this option honors the @option{-m} and @option{-e} options
12916 pertaining to file existence.
12918 @itemx --relative-base=@var{base}
12919 @opindex --relative-base
12920 This option is valid when used with @option{--relative-to}, and will restrict
12921 the output of @option{--relative-to} so that relative names are output,
12922 only when @var{file}s are descendants of @var{base}. Otherwise output the
12923 absolute file name. If @option{--relative-to} was not specified, then
12924 the descendants of @var{base} are printed relative to @var{base}. If
12925 @option{--relative-to} is specified, then that directory must be a
12926 descendant of @var{base} for this option to have an effect.
12927 Note: this option honors the @option{-m} and @option{-e}
12928 options pertaining to file existence. For example:
12931 realpath --relative-to=/usr /tmp /usr/bin
12934 realpath --relative-base=/usr /tmp /usr/bin
12941 @cindex exit status of @command{realpath}
12945 0 if all file names were printed without issue.
12950 @node Working context
12951 @chapter Working context
12953 @cindex working context
12954 @cindex commands for printing the working context
12956 This section describes commands that display or alter the context in
12957 which you are working: the current directory, the terminal settings, and
12958 so forth. See also the user-related commands in the next section.
12961 * pwd invocation:: Print working directory.
12962 * stty invocation:: Print or change terminal characteristics.
12963 * printenv invocation:: Print environment variables.
12964 * tty invocation:: Print file name of terminal on standard input.
12968 @node pwd invocation
12969 @section @command{pwd}: Print working directory
12972 @cindex print name of current directory
12973 @cindex current working directory, printing
12974 @cindex working directory, printing
12977 @command{pwd} prints the name of the current directory. Synopsis:
12980 pwd [@var{option}]@dots{}
12983 The program accepts the following options. Also see @ref{Common options}.
12990 If the contents of the environment variable @env{PWD} provide an
12991 absolute name of the current directory with no @samp{.} or @samp{..}
12992 components, but possibly with symbolic links, then output those
12993 contents. Otherwise, fall back to default @option{-P} handling.
12998 @opindex --physical
12999 Print a fully resolved name for the current directory. That is, all
13000 components of the printed name will be actual directory names---none
13001 will be symbolic links.
13004 @cindex symbolic links and @command{pwd}
13005 If @option{-L} and @option{-P} are both given, the last one takes
13006 precedence. If neither option is given, then this implementation uses
13007 @option{-P} as the default unless the @env{POSIXLY_CORRECT}
13008 environment variable is set.
13010 @mayConflictWithShellBuiltIn{pwd}
13015 @node stty invocation
13016 @section @command{stty}: Print or change terminal characteristics
13019 @cindex change or print terminal settings
13020 @cindex terminal settings
13021 @cindex line settings of terminal
13023 @command{stty} prints or changes terminal characteristics, such as baud rate.
13027 stty [@var{option}] [@var{setting}]@dots{}
13028 stty [@var{option}]
13031 If given no line settings, @command{stty} prints the baud rate, line
13032 discipline number (on systems that support it), and line settings
13033 that have been changed from the values set by @samp{stty sane}.
13034 By default, mode reading and setting are performed on the tty line
13035 connected to standard input, although this can be modified by the
13036 @option{--file} option.
13038 @command{stty} accepts many non-option arguments that change aspects of
13039 the terminal line operation, as described below.
13041 The program accepts the following options. Also see @ref{Common options}.
13048 Print all current settings in human-readable form. This option may not
13049 be used in combination with any line settings.
13051 @item -F @var{device}
13052 @itemx --file=@var{device}
13055 Set the line opened by the file name specified in @var{device} instead of
13056 the tty line connected to standard input. This option is necessary
13057 because opening a @acronym{POSIX} tty requires use of the
13058 @code{O_NONDELAY} flag to prevent a @acronym{POSIX} tty from blocking
13059 until the carrier detect line is high if
13060 the @code{clocal} flag is not set. Hence, it is not always possible
13061 to allow the shell to open the device in the traditional manner.
13067 @cindex machine-readable @command{stty} output
13068 Print all current settings in a form that can be used as an argument to
13069 another @command{stty} command to restore the current settings. This option
13070 may not be used in combination with any line settings.
13074 Many settings can be turned off by preceding them with a @samp{-}.
13075 Such arguments are marked below with ``May be negated'' in their
13076 description. The descriptions themselves refer to the positive
13077 case, that is, when @emph{not} negated (unless stated otherwise,
13080 Some settings are not available on all @acronym{POSIX} systems, since they use
13081 extensions. Such arguments are marked below with
13082 ``Non-@acronym{POSIX}'' in their description. On non-@acronym{POSIX}
13083 systems, those or other settings also may not
13084 be available, but it's not feasible to document all the variations: just
13090 * Control:: Control settings
13091 * Input:: Input settings
13092 * Output:: Output settings
13093 * Local:: Local settings
13094 * Combination:: Combination settings
13095 * Characters:: Special characters
13096 * Special:: Special settings
13101 @subsection Control settings
13103 @cindex control settings
13109 @cindex two-way parity
13110 Generate parity bit in output and expect parity bit in input.
13116 @cindex even parity
13117 Set odd parity (even if negated). May be negated.
13124 @cindex character size
13125 @cindex eight-bit characters
13126 Set character size to 5, 6, 7, or 8 bits.
13131 Send a hangup signal when the last process closes the tty. May be
13137 Use two stop bits per character (one if negated). May be negated.
13141 Allow input to be received. May be negated.
13145 @cindex modem control
13146 Disable modem control signals. May be negated.
13150 @cindex hardware flow control
13151 @cindex flow control, hardware
13152 @cindex RTS/CTS flow control
13153 Enable RTS/CTS flow control. Non-@acronym{POSIX}. May be negated.
13158 @subsection Input settings
13160 @cindex input settings
13161 These settings control operations on data received from the terminal.
13166 @cindex breaks, ignoring
13167 Ignore break characters. May be negated.
13171 @cindex breaks, cause interrupts
13172 Make breaks cause an interrupt signal. May be negated.
13176 @cindex parity, ignoring
13177 Ignore characters with parity errors. May be negated.
13181 @cindex parity errors, marking
13182 Mark parity errors (with a 255-0-character sequence). May be negated.
13186 Enable input parity checking. May be negated.
13190 @cindex eight-bit input
13191 Clear high (8th) bit of input characters. May be negated.
13195 @cindex newline, translating to return
13196 Translate newline to carriage return. May be negated.
13200 @cindex return, ignoring
13201 Ignore carriage return. May be negated.
13205 @cindex return, translating to newline
13206 Translate carriage return to newline. May be negated.
13210 @cindex input encoding, UTF-8
13211 Assume input characters are UTF-8 encoded. May be negated.
13215 @kindex C-s/C-q flow control
13216 @cindex XON/XOFF flow control
13217 Enable XON/XOFF flow control (that is, @kbd{CTRL-S}/@kbd{CTRL-Q}). May
13224 @cindex software flow control
13225 @cindex flow control, software
13226 Enable sending of @code{stop} character when the system input buffer
13227 is almost full, and @code{start} character when it becomes almost
13228 empty again. May be negated.
13232 @cindex uppercase, translating to lowercase
13233 Translate uppercase characters to lowercase. Non-@acronym{POSIX}. May be
13234 negated. Note ilcuc is not implemented, as one would not be able to issue
13235 almost any (lowercase) Unix command, after invoking it.
13239 Allow any character to restart output (only the start character
13240 if negated). Non-@acronym{POSIX}. May be negated.
13244 @cindex beeping at input buffer full
13245 Enable beeping and not flushing input buffer if a character arrives
13246 when the input buffer is full. Non-@acronym{POSIX}. May be negated.
13251 @subsection Output settings
13253 @cindex output settings
13254 These settings control operations on data sent to the terminal.
13259 Postprocess output. May be negated.
13263 @cindex lowercase, translating to output
13264 Translate lowercase characters to uppercase. Non-@acronym{POSIX}. May be
13265 negated. (Note ouclc is not currently implemented.)
13269 @cindex return, translating to newline
13270 Translate carriage return to newline. Non-@acronym{POSIX}. May be negated.
13274 @cindex newline, translating to crlf
13275 Translate newline to carriage return-newline. Non-@acronym{POSIX}. May be
13280 Do not print carriage returns in the first column. Non-@acronym{POSIX}.
13285 Newline performs a carriage return. Non-@acronym{POSIX}. May be negated.
13289 @cindex pad instead of timing for delaying
13290 Use fill (padding) characters instead of timing for delays.
13291 Non-@acronym{POSIX}.
13296 @cindex pad character
13297 Use @acronym{ASCII} @sc{del} characters for fill instead of
13298 @acronym{ASCII} @sc{nul} characters. Non-@acronym{POSIX}.
13304 Newline delay style. Non-@acronym{POSIX}.
13311 Carriage return delay style. Non-@acronym{POSIX}.
13317 @opindex tab@var{n}
13318 Horizontal tab delay style. Non-@acronym{POSIX}.
13323 Backspace delay style. Non-@acronym{POSIX}.
13328 Vertical tab delay style. Non-@acronym{POSIX}.
13333 Form feed delay style. Non-@acronym{POSIX}.
13338 @subsection Local settings
13340 @cindex local settings
13345 Enable @code{interrupt}, @code{quit}, and @code{suspend} special
13346 characters. May be negated.
13350 Enable @code{erase}, @code{kill}, @code{werase}, and @code{rprnt}
13351 special characters. May be negated.
13355 Enable non-@acronym{POSIX} special characters. May be negated.
13359 Echo input characters. May be negated.
13365 Echo @code{erase} characters as backspace-space-backspace. May be
13370 @cindex newline echoing after @code{kill}
13371 Echo a newline after a @code{kill} character. May be negated.
13375 @cindex newline, echoing
13376 Echo newline even if not echoing other characters. May be negated.
13380 @cindex flushing, disabling
13381 Disable flushing after @code{interrupt} and @code{quit} special
13382 characters. May be negated.
13386 @cindex case translation
13387 Enable input and output of uppercase characters by preceding their
13388 lowercase equivalents with @samp{\}, when @code{icanon} is set.
13389 Non-@acronym{POSIX}. May be negated.
13393 @cindex background jobs, stopping at terminal write
13394 Stop background jobs that try to write to the terminal. Non-@acronym{POSIX}.
13401 Echo erased characters backward, between @samp{\} and @samp{/}.
13402 Non-@acronym{POSIX}. May be negated.
13408 @cindex control characters, using @samp{^@var{c}}
13409 @cindex hat notation for control characters
13410 Echo control characters in hat notation (@samp{^@var{c}}) instead
13411 of literally. Non-@acronym{POSIX}. May be negated.
13417 Echo the @code{kill} special character by erasing each character on
13418 the line as indicated by the @code{echoprt} and @code{echoe} settings,
13419 instead of by the @code{echoctl} and @code{echok} settings.
13420 Non-@acronym{POSIX}.
13426 @subsection Combination settings
13428 @cindex combination settings
13429 Combination settings:
13436 Same as @code{parenb -parodd cs7}. May be negated. If negated, same
13437 as @code{-parenb cs8}.
13441 Same as @code{parenb parodd cs7}. May be negated. If negated, same
13442 as @code{-parenb cs8}.
13446 Same as @code{-icrnl -onlcr}. May be negated. If negated, same as
13447 @code{icrnl -inlcr -igncr onlcr -ocrnl -onlret}.
13451 Reset the @code{erase} and @code{kill} special characters to their default
13458 @c This is too long to write inline.
13460 cread -ignbrk brkint -inlcr -igncr icrnl -ixoff
13461 -iuclc -ixany imaxbel opost -olcuc -ocrnl onlcr
13462 -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0
13463 ff0 isig icanon iexten echo echoe echok -echonl
13464 -noflsh -xcase -tostop -echoprt echoctl echoke
13468 and also sets all special characters to their default values.
13472 Same as @code{brkint ignpar istrip icrnl ixon opost isig icanon}, plus
13473 sets the @code{eof} and @code{eol} characters to their default values
13474 if they are the same as the @code{min} and @code{time} characters.
13475 May be negated. If negated, same as @code{raw}.
13482 -ignbrk -brkint -ignpar -parmrk -inpck -istrip
13483 -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany
13484 -imaxbel -opost -isig -icanon -xcase min 1 time 0
13488 May be negated. If negated, same as @code{cooked}.
13492 Same as @option{-icanon}. May be negated. If negated, same as
13497 @cindex eight-bit characters
13498 Same as @code{-parenb -istrip cs8}. May be negated. If negated,
13499 same as @code{parenb istrip cs7}.
13503 Same as @option{-parenb -istrip -opost cs8}. May be negated.
13504 If negated, same as @code{parenb istrip opost cs7}.
13508 Same as @option{-ixany}. Non-@acronym{POSIX}. May be negated.
13512 Same as @code{tab0}. Non-@acronym{POSIX}. May be negated. If negated, same
13519 Same as @code{xcase iuclc olcuc}. Non-@acronym{POSIX}. May be negated.
13520 (Used for terminals with uppercase characters only.)
13524 Same as @code{echoe echoctl echoke}.
13528 Same as @code{echoe echoctl echoke -ixany intr ^C erase ^? kill C-u}.
13533 @subsection Special characters
13535 @cindex special characters
13536 @cindex characters, special
13538 The special characters' default values vary from system to system.
13539 They are set with the syntax @samp{name value}, where the names are
13540 listed below and the value can be given either literally, in hat
13541 notation (@samp{^@var{c}}), or as an integer which may start with
13542 @samp{0x} to indicate hexadecimal, @samp{0} to indicate octal, or
13543 any other digit to indicate decimal.
13545 @cindex disabling special characters
13546 @kindex u@r{, and disabling special characters}
13547 For GNU stty, giving a value of @code{^-} or @code{undef} disables that
13548 special character. (This is incompatible with Ultrix @command{stty},
13549 which uses a value of @samp{u} to disable a special character. GNU
13550 @command{stty} treats a value @samp{u} like any other, namely to set that
13551 special character to @key{U}.)
13557 Send an interrupt signal.
13561 Send a quit signal.
13565 Erase the last character typed.
13569 Erase the current line.
13573 Send an end of file (terminate the input).
13581 Alternate character to end the line. Non-@acronym{POSIX}.
13585 Switch to a different shell layer. Non-@acronym{POSIX}.
13589 Restart the output after stopping it.
13597 Send a terminal stop signal.
13601 Send a terminal stop signal after flushing the input. Non-@acronym{POSIX}.
13605 Redraw the current line. Non-@acronym{POSIX}.
13609 Erase the last word typed. Non-@acronym{POSIX}.
13613 Enter the next character typed literally, even if it is a special
13614 character. Non-@acronym{POSIX}.
13619 @subsection Special settings
13621 @cindex special settings
13626 Set the minimum number of characters that will satisfy a read until
13627 the time value has expired, when @option{-icanon} is set.
13631 Set the number of tenths of a second before reads time out if the minimum
13632 number of characters have not been read, when @option{-icanon} is set.
13634 @item ispeed @var{n}
13636 Set the input speed to @var{n}.
13638 @item ospeed @var{n}
13640 Set the output speed to @var{n}.
13644 Tell the tty kernel driver that the terminal has @var{n} rows.
13645 Non-@acronym{POSIX}.
13648 @itemx columns @var{n}
13651 Tell the kernel that the terminal has @var{n} columns. Non-@acronym{POSIX}.
13657 Print the number of rows and columns that the kernel thinks the
13658 terminal has. (Systems that don't support rows and columns in the kernel
13659 typically use the environment variables @env{LINES} and @env{COLUMNS}
13660 instead; however, GNU @command{stty} does not know anything about them.)
13661 Non-@acronym{POSIX}.
13665 Use line discipline @var{n}. Non-@acronym{POSIX}.
13669 Print the terminal speed.
13672 @cindex baud rate, setting
13673 Set the input and output speeds to @var{n}. @var{n} can be one of: 0
13674 50 75 110 134 134.5 150 200 300 600 1200 1800 2400 4800 9600 19200
13675 38400 @code{exta} @code{extb}. @code{exta} is the same as 19200;
13676 @code{extb} is the same as 38400. Many systems, including GNU/Linux,
13677 support higher speeds. The @command{stty} command includes support
13694 4000000 where the system supports these.
13695 0 hangs up the line if @option{-clocal} is set.
13699 @node printenv invocation
13700 @section @command{printenv}: Print all or some environment variables
13703 @cindex printing all or some environment variables
13704 @cindex environment variables, printing
13706 @command{printenv} prints environment variable values. Synopsis:
13709 printenv [@var{option}] [@var{variable}]@dots{}
13712 If no @var{variable}s are specified, @command{printenv} prints the value of
13713 every environment variable. Otherwise, it prints the value of each
13714 @var{variable} that is set, and nothing for those that are not set.
13716 The program accepts the following option. Also see @ref{Common options}.
13724 @cindex exit status of @command{printenv}
13728 0 if all variables specified were found
13729 1 if at least one specified variable was not found
13730 2 if a write error occurred
13734 @node tty invocation
13735 @section @command{tty}: Print file name of terminal on standard input
13738 @cindex print terminal file name
13739 @cindex terminal file name, printing
13741 @command{tty} prints the file name of the terminal connected to its standard
13742 input. It prints @samp{not a tty} if standard input is not a terminal.
13746 tty [@var{option}]@dots{}
13749 The program accepts the following option. Also see @ref{Common options}.
13759 Print nothing; only return an exit status.
13763 @cindex exit status of @command{tty}
13767 0 if standard input is a terminal
13768 1 if standard input is not a terminal
13769 2 if given incorrect arguments
13770 3 if a write error occurs
13774 @node User information
13775 @chapter User information
13777 @cindex user information, commands for
13778 @cindex commands for printing user information
13780 This section describes commands that print user-related information:
13781 logins, groups, and so forth.
13784 * id invocation:: Print user identity.
13785 * logname invocation:: Print current login name.
13786 * whoami invocation:: Print effective user ID.
13787 * groups invocation:: Print group names a user is in.
13788 * users invocation:: Print login names of users currently logged in.
13789 * who invocation:: Print who is currently logged in.
13793 @node id invocation
13794 @section @command{id}: Print user identity
13797 @cindex real user and group IDs, printing
13798 @cindex effective user and group IDs, printing
13799 @cindex printing real and effective user and group IDs
13801 @command{id} prints information about the given user, or the process
13802 running it if no user is specified. Synopsis:
13805 id [@var{option}]@dots{} [@var{username}]
13808 @vindex POSIXLY_CORRECT
13809 By default, it prints the real user ID, real group ID, effective user ID
13810 if different from the real user ID, effective group ID if different from
13811 the real group ID, and supplemental group IDs.
13812 In addition, if SELinux
13813 is enabled and the @env{POSIXLY_CORRECT} environment variable is not set,
13814 then print @samp{context=@var{c}}, where @var{c} is the security context.
13816 Each of these numeric values is preceded by an identifying string and
13817 followed by the corresponding user or group name in parentheses.
13819 The options cause @command{id} to print only part of the above information.
13820 Also see @ref{Common options}.
13827 Print only the group ID.
13833 Print only the group ID and the supplementary groups.
13839 Print the user or group name instead of the ID number. Requires
13840 @option{-u}, @option{-g}, or @option{-G}.
13846 Print the real, instead of effective, user or group ID. Requires
13847 @option{-u}, @option{-g}, or @option{-G}.
13853 Print only the user ID.
13860 @cindex security context
13861 Print only the security context of the current user.
13862 If SELinux is disabled then print a warning and
13863 set the exit status to 1.
13869 @macro primaryAndSupplementaryGroups{cmd,arg}
13870 Primary and supplementary groups for a process are normally inherited
13871 from its parent and are usually unchanged since login. This means
13872 that if you change the group database after logging in, @command{\cmd\}
13873 will not reflect your changes within your existing login session.
13874 Running @command{\cmd\} with a \arg\ causes the user and group
13875 database to be consulted afresh, and so will give a different result.
13877 @primaryAndSupplementaryGroups{id,user argument}
13879 @node logname invocation
13880 @section @command{logname}: Print current login name
13883 @cindex printing user's login name
13884 @cindex login name, printing
13885 @cindex user name, printing
13888 @command{logname} prints the calling user's name, as found in a
13889 system-maintained file (often @file{/var/run/utmp} or
13890 @file{/etc/utmp}), and exits with a status of 0. If there is no entry
13891 for the calling process, @command{logname} prints
13892 an error message and exits with a status of 1.
13894 The only options are @option{--help} and @option{--version}. @xref{Common
13900 @node whoami invocation
13901 @section @command{whoami}: Print effective user ID
13904 @cindex effective user ID, printing
13905 @cindex printing the effective user ID
13907 @command{whoami} prints the user name associated with the current
13908 effective user ID. It is equivalent to the command @samp{id -un}.
13910 The only options are @option{--help} and @option{--version}. @xref{Common
13916 @node groups invocation
13917 @section @command{groups}: Print group names a user is in
13920 @cindex printing groups a user is in
13921 @cindex supplementary groups, printing
13923 @command{groups} prints the names of the primary and any supplementary
13924 groups for each given @var{username}, or the current process if no names
13925 are given. If more than one name is given, the name of each user is
13927 the list of that user's groups and the user name is separated from the
13928 group list by a colon. Synopsis:
13931 groups [@var{username}]@dots{}
13934 The group lists are equivalent to the output of the command @samp{id -Gn}.
13936 @primaryAndSupplementaryGroups{groups,list of users}
13938 The only options are @option{--help} and @option{--version}. @xref{Common
13944 @node users invocation
13945 @section @command{users}: Print login names of users currently logged in
13948 @cindex printing current usernames
13949 @cindex usernames, printing current
13951 @cindex login sessions, printing users with
13952 @command{users} prints on a single line a blank-separated list of user
13953 names of users currently logged in to the current host. Each user name
13954 corresponds to a login session, so if a user has more than one login
13955 session, that user's name will appear the same number of times in the
13964 With no @var{file} argument, @command{users} extracts its information from
13965 a system-maintained file (often @file{/var/run/utmp} or
13966 @file{/etc/utmp}). If a file argument is given, @command{users} uses
13967 that file instead. A common choice is @file{/var/log/wtmp}.
13969 The only options are @option{--help} and @option{--version}. @xref{Common
13975 @node who invocation
13976 @section @command{who}: Print who is currently logged in
13979 @cindex printing current user information
13980 @cindex information, about current users
13982 @command{who} prints information about users who are currently logged on.
13986 @command{who} [@var{option}] [@var{file}] [am i]
13989 @cindex terminal lines, currently used
13991 @cindex remote hostname
13992 If given no non-option arguments, @command{who} prints the following
13993 information for each user currently logged on: login name, terminal
13994 line, login time, and remote hostname or X display.
13998 If given one non-option argument, @command{who} uses that instead of
13999 a default system-maintained file (often @file{/var/run/utmp} or
14000 @file{/etc/utmp}) as the name of the file containing the record of
14001 users logged on. @file{/var/log/wtmp} is commonly given as an argument
14002 to @command{who} to look at who has previously logged on.
14006 If given two non-option arguments, @command{who} prints only the entry
14007 for the user running it (determined from its standard input), preceded
14008 by the hostname. Traditionally, the two arguments given are @samp{am
14009 i}, as in @samp{who am i}.
14012 Time stamps are listed according to the time zone rules specified by
14013 the @env{TZ} environment variable, or by the system default rules if
14014 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
14015 with @env{TZ}, libc, The GNU C Library Reference Manual}.
14017 The program accepts the following options. Also see @ref{Common options}.
14025 Same as @samp{-b -d --login -p -r -t -T -u}.
14031 Print the date and time of last system boot.
14037 Print information corresponding to dead processes.
14043 Print a line of column headings.
14049 List only the entries that correspond to processes via which the
14050 system is waiting for a user to login. The user name is always @samp{LOGIN}.
14054 Attempt to canonicalize hostnames found in utmp through a DNS lookup. This
14055 is not the default because it can cause significant delays on systems with
14056 automatic dial-up internet access.
14060 Same as @samp{who am i}.
14066 List active processes spawned by init.
14072 Print only the login names and the number of users logged on.
14073 Overrides all other options.
14078 @opindex --runlevel
14079 Print the current (and maybe previous) run-level of the init process.
14083 Ignored; for compatibility with other versions of @command{who}.
14089 Print last system clock change.
14094 After the login time, print the number of hours and minutes that the
14095 user has been idle. @samp{.} means the user was active in the last minute.
14096 @samp{old} means the user has been idle for more than 24 hours.
14107 @opindex --writable
14108 @cindex message status
14109 @pindex write@r{, allowed}
14110 After each login name print a character indicating the user's message status:
14113 @samp{+} allowing @code{write} messages
14114 @samp{-} disallowing @code{write} messages
14115 @samp{?} cannot find terminal device
14123 @node System context
14124 @chapter System context
14126 @cindex system context
14127 @cindex context, system
14128 @cindex commands for system context
14130 This section describes commands that print or change system-wide
14134 * date invocation:: Print or set system date and time.
14135 * arch invocation:: Print machine hardware name.
14136 * nproc invocation:: Print the number of processors.
14137 * uname invocation:: Print system information.
14138 * hostname invocation:: Print or set system name.
14139 * hostid invocation:: Print numeric host identifier.
14140 * uptime invocation:: Print system uptime and load.
14143 @node date invocation
14144 @section @command{date}: Print or set system date and time
14147 @cindex time, printing or setting
14148 @cindex printing the current time
14153 date [@var{option}]@dots{} [+@var{format}]
14154 date [-u|--utc|--universal] @c this avoids a newline in the output
14155 [ MMDDhhmm[[CC]YY][.ss] ]
14159 Invoking @command{date} with no @var{format} argument is equivalent to invoking
14160 it with a default format that depends on the @env{LC_TIME} locale category.
14161 In the default C locale, this format is @samp{'+%a %b %e %H:%M:%S %Z %Y'},
14162 so the output looks like @samp{Thu Mar @ 3 13:47:51 PST 2005}.
14165 Normally, @command{date} uses the time zone rules indicated by the
14166 @env{TZ} environment variable, or the system default rules if @env{TZ}
14167 is not set. @xref{TZ Variable,, Specifying the Time Zone with
14168 @env{TZ}, libc, The GNU C Library Reference Manual}.
14170 @findex strftime @r{and @command{date}}
14171 @cindex time formats
14172 @cindex formatting times
14173 If given an argument that starts with a @samp{+}, @command{date} prints the
14174 current date and time (or the date and time specified by the
14175 @option{--date} option, see below) in the format defined by that argument,
14176 which is similar to that of the @code{strftime} function. Except for
14177 conversion specifiers, which start with @samp{%}, characters in the
14178 format string are printed unchanged. The conversion specifiers are
14184 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
14185 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
14186 * Literal conversion specifiers:: %[%nt]
14187 * Padding and other flags:: Pad with zeros, spaces, etc.
14188 * Setting the time:: Changing the system clock.
14189 * Options for date:: Instead of the current time.
14191 * Date input formats:: Specifying date strings.
14193 * Examples of date:: Examples.
14196 @node Time conversion specifiers
14197 @subsection Time conversion specifiers
14199 @cindex time conversion specifiers
14200 @cindex conversion specifiers, time
14202 @command{date} conversion specifiers related to times.
14206 hour (@samp{00}@dots{}@samp{23})
14208 hour (@samp{01}@dots{}@samp{12})
14210 hour, space padded (@samp{ 0}@dots{}@samp{23}); equivalent to @samp{%_H}.
14211 This is a @acronym{GNU} extension.
14213 hour, space padded (@samp{ 1}@dots{}@samp{12}); equivalent to @samp{%_I}.
14214 This is a @acronym{GNU} extension.
14216 minute (@samp{00}@dots{}@samp{59})
14218 nanoseconds (@samp{000000000}@dots{}@samp{999999999}).
14219 This is a @acronym{GNU} extension.
14221 locale's equivalent of either @samp{AM} or @samp{PM};
14222 blank in many locales.
14223 Noon is treated as @samp{PM} and midnight as @samp{AM}.
14225 like @samp{%p}, except lower case.
14226 This is a @acronym{GNU} extension.
14228 locale's 12-hour clock time (e.g., @samp{11:11:04 PM})
14230 24-hour hour and minute. Same as @samp{%H:%M}.
14232 @cindex epoch, seconds since
14233 @cindex seconds since the epoch
14234 @cindex beginning of time
14235 seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC.
14236 Leap seconds are not counted unless leap second support is available.
14237 @xref{%s-examples}, for examples.
14238 This is a @acronym{GNU} extension.
14240 second (@samp{00}@dots{}@samp{60}).
14241 This may be @samp{60} if leap seconds are supported.
14243 24-hour hour, minute, and second. Same as @samp{%H:%M:%S}.
14245 locale's time representation (e.g., @samp{23:13:48})
14247 @w{@acronym{RFC} 2822/@acronym{ISO} 8601} style numeric time zone
14248 (e.g., @samp{-0600} or @samp{+0530}), or nothing if no
14249 time zone is determinable. This value reflects the numeric time zone
14250 appropriate for the current time, using the time zone rules specified
14251 by the @env{TZ} environment variable.
14252 The time (and optionally, the time zone rules) can be overridden
14253 by the @option{--date} option.
14255 @w{@acronym{RFC} 3339/@acronym{ISO} 8601} style numeric time zone with
14256 @samp{:} (e.g., @samp{-06:00} or @samp{+05:30}), or nothing if no time
14257 zone is determinable.
14258 This is a @acronym{GNU} extension.
14260 Numeric time zone to the nearest second with @samp{:} (e.g.,
14261 @samp{-06:00:00} or @samp{+05:30:00}), or nothing if no time zone is
14263 This is a @acronym{GNU} extension.
14265 Numeric time zone with @samp{:} using the minimum necessary precision
14266 (e.g., @samp{-06}, @samp{+05:30}, or @samp{-04:56:02}), or nothing if
14267 no time zone is determinable.
14268 This is a @acronym{GNU} extension.
14270 alphabetic time zone abbreviation (e.g., @samp{EDT}), or nothing if no
14271 time zone is determinable. See @samp{%z} for how it is determined.
14275 @node Date conversion specifiers
14276 @subsection Date conversion specifiers
14278 @cindex date conversion specifiers
14279 @cindex conversion specifiers, date
14281 @command{date} conversion specifiers related to dates.
14285 locale's abbreviated weekday name (e.g., @samp{Sun})
14287 locale's full weekday name, variable length (e.g., @samp{Sunday})
14289 locale's abbreviated month name (e.g., @samp{Jan})
14291 locale's full month name, variable length (e.g., @samp{January})
14293 locale's date and time (e.g., @samp{Thu Mar @ 3 23:05:25 2005})
14295 century. This is like @samp{%Y}, except the last two digits are omitted.
14296 For example, it is @samp{20} if @samp{%Y} is @samp{2000},
14297 and is @samp{-0} if @samp{%Y} is @samp{-001}.
14298 It is normally at least two characters, but it may be more.
14300 day of month (e.g., @samp{01})
14302 date; same as @samp{%m/%d/%y}
14304 day of month, space padded; same as @samp{%_d}
14306 full date in @acronym{ISO} 8601 format; same as @samp{%Y-%m-%d}.
14307 This is a good choice for a date format, as it is standard and
14308 is easy to sort in the usual case where years are in the range
14311 year corresponding to the @acronym{ISO} week number, but without the century
14312 (range @samp{00} through @samp{99}). This has the same format and value
14313 as @samp{%y}, except that if the @acronym{ISO} week number (see
14315 to the previous or next year, that year is used instead.
14317 year corresponding to the @acronym{ISO} week number. This has the
14318 same format and value as @samp{%Y}, except that if the @acronym{ISO}
14320 @samp{%V}) belongs to the previous or next year, that year is used
14322 It is normally useful only if @samp{%V} is also used;
14323 for example, the format @samp{%G-%m-%d} is probably a mistake,
14324 since it combines the ISO week number year with the conventional month and day.
14328 day of year (@samp{001}@dots{}@samp{366})
14330 month (@samp{01}@dots{}@samp{12})
14332 day of week (@samp{1}@dots{}@samp{7}) with @samp{1} corresponding to Monday
14334 week number of year, with Sunday as the first day of the week
14335 (@samp{00}@dots{}@samp{53}).
14336 Days in a new year preceding the first Sunday are in week zero.
14338 @acronym{ISO} week number, that is, the
14339 week number of year, with Monday as the first day of the week
14340 (@samp{01}@dots{}@samp{53}).
14341 If the week containing January 1 has four or more days in
14342 the new year, then it is considered week 1; otherwise, it is week 53 of
14343 the previous year, and the next week is week 1. (See the @acronym{ISO} 8601
14346 day of week (@samp{0}@dots{}@samp{6}) with 0 corresponding to Sunday
14348 week number of year, with Monday as first day of week
14349 (@samp{00}@dots{}@samp{53}).
14350 Days in a new year preceding the first Monday are in week zero.
14352 locale's date representation (e.g., @samp{12/31/99})
14354 last two digits of year (@samp{00}@dots{}@samp{99})
14356 year. This is normally at least four characters, but it may be more.
14357 Year @samp{0000} precedes year @samp{0001}, and year @samp{-001}
14358 precedes year @samp{0000}.
14362 @node Literal conversion specifiers
14363 @subsection Literal conversion specifiers
14365 @cindex literal conversion specifiers
14366 @cindex conversion specifiers, literal
14368 @command{date} conversion specifiers that produce literal strings.
14380 @node Padding and other flags
14381 @subsection Padding and other flags
14383 @cindex numeric field padding
14384 @cindex padding of numeric fields
14385 @cindex fields, padding numeric
14387 Unless otherwise specified, @command{date} normally pads numeric fields
14388 with zeros, so that, for
14389 example, numeric months are always output as two digits.
14390 Seconds since the epoch are not padded, though,
14391 since there is no natural width for them.
14393 As a @acronym{GNU} extension, @command{date} recognizes any of the
14394 following optional flags after the @samp{%}:
14398 (hyphen) Do not pad the field; useful if the output is intended for
14401 (underscore) Pad with spaces; useful if you need a fixed
14402 number of characters in the output, but zeros are too distracting.
14404 (zero) Pad with zeros even if the conversion specifier
14405 would normally pad with spaces.
14407 Use upper case characters if possible.
14409 Use opposite case characters if possible.
14410 A field that is normally upper case becomes lower case, and vice versa.
14414 Here are some examples of padding:
14417 date +%d/%m -d "Feb 1"
14419 date +%-d/%-m -d "Feb 1"
14421 date +%_d/%_m -d "Feb 1"
14425 As a @acronym{GNU} extension, you can specify the field width
14426 (after any flag, if present) as a decimal number. If the natural size of the
14427 output of the field has less than the specified number of characters,
14428 the result is written right adjusted and padded to the given
14429 size. For example, @samp{%9B} prints the right adjusted month name in
14430 a field of width 9.
14432 An optional modifier can follow the optional flag and width
14433 specification. The modifiers are:
14437 Use the locale's alternate representation for date and time. This
14438 modifier applies to the @samp{%c}, @samp{%C}, @samp{%x}, @samp{%X},
14439 @samp{%y} and @samp{%Y} conversion specifiers. In a Japanese locale, for
14440 example, @samp{%Ex} might yield a date format based on the Japanese
14444 Use the locale's alternate numeric symbols for numbers. This modifier
14445 applies only to numeric conversion specifiers.
14448 If the format supports the modifier but no alternate representation
14449 is available, it is ignored.
14452 @node Setting the time
14453 @subsection Setting the time
14455 @cindex setting the time
14456 @cindex time setting
14457 @cindex appropriate privileges
14459 If given an argument that does not start with @samp{+}, @command{date} sets
14460 the system clock to the date and time specified by that argument (as
14461 described below). You must have appropriate privileges to set the
14462 system clock. Note for changes to persist across a reboot, the
14463 hardware clock may need to be updated from the system clock, which
14464 might not happen automatically on your system.
14466 The argument must consist entirely of digits, which have the following
14479 first two digits of year (optional)
14481 last two digits of year (optional)
14486 Note, the @option{--date} and @option{--set} options may not be used with an
14487 argument in the above format. The @option{--universal} option may be used
14488 with such an argument to indicate that the specified date and time are
14489 relative to Coordinated Universal Time rather than to the local time zone.
14492 @node Options for date
14493 @subsection Options for @command{date}
14495 @cindex @command{date} options
14496 @cindex options for @command{date}
14498 The program accepts the following options. Also see @ref{Common options}.
14502 @item -d @var{datestr}
14503 @itemx --date=@var{datestr}
14506 @cindex parsing date strings
14507 @cindex date strings, parsing
14508 @cindex arbitrary date strings, parsing
14511 @opindex next @var{day}
14512 @opindex last @var{day}
14513 Display the date and time specified in @var{datestr} instead of the
14514 current date and time. @var{datestr} can be in almost any common
14515 format. It can contain month names, time zones, @samp{am} and @samp{pm},
14516 @samp{yesterday}, etc. For example, @option{--date="2004-02-27
14517 14:19:13.489392193 +0530"} specifies the instant of time that is
14518 489,392,193 nanoseconds after February 27, 2004 at 2:19:13 PM in a
14519 time zone that is 5 hours and 30 minutes east of @acronym{UTC}.@*
14520 Note: input currently must be in locale independent format. E.g., the
14521 LC_TIME=C below is needed to print back the correct date in many locales:
14523 date -d "$(LC_TIME=C date)"
14525 @xref{Date input formats}.
14527 @item -f @var{datefile}
14528 @itemx --file=@var{datefile}
14531 Parse each line in @var{datefile} as with @option{-d} and display the
14532 resulting date and time. If @var{datefile} is @samp{-}, use standard
14533 input. This is useful when you have many dates to process, because the
14534 system overhead of starting up the @command{date} executable many times can
14537 @item -I[@var{timespec}]
14538 @itemx --iso-8601[=@var{timespec}]
14539 @opindex -I[@var{timespec}]
14540 @opindex --iso-8601[=@var{timespec}]
14541 Display the date using the @acronym{ISO} 8601 format, @samp{%Y-%m-%d}.
14543 The argument @var{timespec} specifies the number of additional
14544 terms of the time to include. It can be one of the following:
14547 Print just the date. This is the default if @var{timespec} is omitted.
14550 Append the hour of the day to the date.
14553 Append the hours and minutes.
14556 Append the hours, minutes and seconds.
14559 Append the hours, minutes, seconds and nanoseconds.
14562 If showing any time terms, then include the time zone using the format
14565 @item -r @var{file}
14566 @itemx --reference=@var{file}
14568 @opindex --reference
14569 Display the date and time of the last modification of @var{file},
14570 instead of the current date and time.
14577 @opindex --rfc-2822
14578 Display the date and time using the format @samp{%a, %d %b %Y %H:%M:%S
14579 %z}, evaluated in the C locale so abbreviations are always in English.
14583 Fri, 09 Sep 2005 13:51:39 -0700
14586 This format conforms to
14587 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt, Internet
14588 @acronym{RFCs} 2822} and
14589 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc822.txt, 822}, the
14590 current and previous standards for Internet email.
14592 @item --rfc-3339=@var{timespec}
14593 @opindex --rfc-3339=@var{timespec}
14594 Display the date using a format specified by
14595 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt, Internet
14596 @acronym{RFC} 3339}. This is a subset of the @acronym{ISO} 8601
14597 format, except that it also permits applications to use a space rather
14598 than a @samp{T} to separate dates from times. Unlike the other
14599 standard formats, @acronym{RFC} 3339 format is always suitable as
14600 input for the @option{--date} (@option{-d}) and @option{--file}
14601 (@option{-f}) options, regardless of the current locale.
14603 The argument @var{timespec} specifies how much of the time to include.
14604 It can be one of the following:
14608 Print just the full-date, e.g., @samp{2005-09-14}.
14609 This is equivalent to the format @samp{%Y-%m-%d}.
14612 Print the full-date and full-time separated by a space, e.g.,
14613 @samp{2005-09-14 00:56:06+05:30}. The output ends with a numeric
14614 time-offset; here the @samp{+05:30} means that local time is five
14615 hours and thirty minutes east of @acronym{UTC}. This is equivalent to
14616 the format @samp{%Y-%m-%d %H:%M:%S%:z}.
14619 Like @samp{seconds}, but also print nanoseconds, e.g.,
14620 @samp{2005-09-14 00:56:06.998458565+05:30}.
14621 This is equivalent to the format @samp{%Y-%m-%d %H:%M:%S.%N%:z}.
14625 @item -s @var{datestr}
14626 @itemx --set=@var{datestr}
14629 Set the date and time to @var{datestr}. See @option{-d} above.
14630 See also @ref{Setting the time}.
14637 @opindex --universal
14638 @cindex Coordinated Universal Time
14640 @cindex Greenwich Mean Time
14643 Use Coordinated Universal Time (@acronym{UTC}) by operating as if the
14644 @env{TZ} environment variable were set to the string @samp{UTC0}.
14646 Universal Time is often called ``Greenwich Mean Time'' (@sc{gmt}) for
14647 historical reasons.
14651 @node Examples of date
14652 @subsection Examples of @command{date}
14654 @cindex examples of @command{date}
14656 Here are a few examples. Also see the documentation for the @option{-d}
14657 option in the previous section.
14662 To print the date of the day before yesterday:
14665 date --date='2 days ago'
14669 To print the date of the day three months and one day hence:
14672 date --date='3 months 1 day'
14676 To print the day of year of Christmas in the current year:
14679 date --date='25 Dec' +%j
14683 To print the current full month name and the day of the month:
14689 But this may not be what you want because for the first nine days of
14690 the month, the @samp{%d} expands to a zero-padded two-digit field,
14691 for example @samp{date -d 1may '+%B %d'} will print @samp{May 01}.
14694 To print a date without the leading zero for one-digit days
14695 of the month, you can use the (@acronym{GNU} extension)
14696 @samp{-} flag to suppress
14697 the padding altogether:
14700 date -d 1may '+%B %-d
14704 To print the current date and time in the format required by many
14705 non-@acronym{GNU} versions of @command{date} when setting the system clock:
14708 date +%m%d%H%M%Y.%S
14712 To set the system clock forward by two minutes:
14715 date --set='+2 minutes'
14719 To print the date in @acronym{RFC} 2822 format,
14720 use @samp{date --rfc-2822}. Here is some example output:
14723 Fri, 09 Sep 2005 13:51:39 -0700
14726 @anchor{%s-examples}
14728 To convert a date string to the number of seconds since the epoch
14729 (which is 1970-01-01 00:00:00 UTC), use the @option{--date} option with
14730 the @samp{%s} format. That can be useful in sorting and/or graphing
14731 and/or comparing data by date. The following command outputs the
14732 number of the seconds since the epoch for the time two minutes after the
14736 date --date='1970-01-01 00:02:00 +0000' +%s
14740 If you do not specify time zone information in the date string,
14741 @command{date} uses your computer's idea of the time zone when
14742 interpreting the string. For example, if your computer's time zone is
14743 that of Cambridge, Massachusetts, which was then 5 hours (i.e., 18,000
14744 seconds) behind UTC:
14747 # local time zone used
14748 date --date='1970-01-01 00:02:00' +%s
14753 If you're sorting or graphing dated data, your raw date values may be
14754 represented as seconds since the epoch. But few people can look at
14755 the date @samp{946684800} and casually note ``Oh, that's the first second
14756 of the year 2000 in Greenwich, England.''
14759 date --date='2000-01-01 UTC' +%s
14763 An alternative is to use the @option{--utc} (@option{-u}) option.
14764 Then you may omit @samp{UTC} from the date string. Although this
14765 produces the same result for @samp{%s} and many other format sequences,
14766 with a time zone offset different from zero, it would give a different
14767 result for zone-dependent formats like @samp{%z}.
14770 date -u --date=2000-01-01 +%s
14774 To convert such an unwieldy number of seconds back to
14775 a more readable form, use a command like this:
14778 # local time zone used
14779 date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
14780 1999-12-31 19:00:00 -0500
14783 Or if you do not mind depending on the @samp{@@} feature present since
14784 coreutils 5.3.0, you could shorten this to:
14787 date -d @@946684800 +"%F %T %z"
14788 1999-12-31 19:00:00 -0500
14791 Often it is better to output UTC-relative date and time:
14794 date -u -d '1970-01-01 946684800 seconds' +"%Y-%m-%d %T %z"
14795 2000-01-01 00:00:00 +0000
14801 @node arch invocation
14802 @section @command{arch}: Print machine hardware name
14805 @cindex print machine hardware name
14806 @cindex system information, printing
14808 @command{arch} prints the machine hardware name,
14809 and is equivalent to @samp{uname -m}.
14813 arch [@var{option}]
14816 The program accepts the @ref{Common options} only.
14821 @node nproc invocation
14822 @section @command{nproc}: Print the number of available processors
14825 @cindex Print the number of processors
14826 @cindex system information, printing
14828 Print the number of processing units available to the current process,
14829 which may be less than the number of online processors.
14830 If this information is not accessible, then print the number of
14831 processors installed. If the @env{OMP_NUM_THREADS} environment variable is
14832 set, then it will determine the returned value. The result is guaranteed to be
14833 greater than zero. Synopsis:
14836 nproc [@var{option}]
14839 The program accepts the following options. Also see @ref{Common options}.
14845 Print the number of installed processors on the system, which may
14846 be greater than the number online or available to the current process.
14847 The @env{OMP_NUM_THREADS} environment variable is not honored in this case.
14849 @item --ignore=@var{number}
14851 If possible, exclude this @var{number} of processing units.
14858 @node uname invocation
14859 @section @command{uname}: Print system information
14862 @cindex print system information
14863 @cindex system information, printing
14865 @command{uname} prints information about the machine and operating system
14866 it is run on. If no options are given, @command{uname} acts as if the
14867 @option{-s} option were given. Synopsis:
14870 uname [@var{option}]@dots{}
14873 If multiple options or @option{-a} are given, the selected information is
14874 printed in this order:
14877 @var{kernel-name} @var{nodename} @var{kernel-release} @var{kernel-version}
14878 @var{machine} @var{processor} @var{hardware-platform} @var{operating-system}
14881 The information may contain internal spaces, so such output cannot be
14882 parsed reliably. In the following example, @var{release} is
14883 @samp{2.2.18ss.e820-bda652a #4 SMP Tue Jun 5 11:24:08 PDT 2001}:
14887 @result{} Linux dum 2.2.18 #4 SMP Tue Jun 5 11:24:08 PDT 2001 i686@c
14888 unknown unknown GNU/Linux
14892 The program accepts the following options. Also see @ref{Common options}.
14900 Print all of the below information, except omit the processor type
14901 and the hardware platform name if they are unknown.
14904 @itemx --hardware-platform
14906 @opindex --hardware-platform
14907 @cindex implementation, hardware
14908 @cindex hardware platform
14909 @cindex platform, hardware
14910 Print the hardware platform name
14911 (sometimes called the hardware implementation).
14912 Print @samp{unknown} if the kernel does not make this information
14913 easily available, as is the case with Linux kernels.
14919 @cindex machine type
14920 @cindex hardware class
14921 @cindex hardware type
14922 Print the machine hardware name (sometimes called the hardware class
14928 @opindex --nodename
14931 @cindex network node name
14932 Print the network node hostname.
14937 @opindex --processor
14938 @cindex host processor type
14939 Print the processor type (sometimes called the instruction set
14940 architecture or ISA).
14941 Print @samp{unknown} if the kernel does not make this information
14942 easily available, as is the case with Linux kernels.
14945 @itemx --operating-system
14947 @opindex --operating-system
14948 @cindex operating system name
14949 Print the name of the operating system.
14952 @itemx --kernel-release
14954 @opindex --kernel-release
14955 @cindex kernel release
14956 @cindex release of kernel
14957 Print the kernel release.
14960 @itemx --kernel-name
14962 @opindex --kernel-name
14963 @cindex kernel name
14964 @cindex name of kernel
14965 Print the kernel name.
14966 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) calls this
14967 ``the implementation of the operating system'', because the
14968 @acronym{POSIX} specification itself has no notion of ``kernel''.
14969 The kernel name might be the same as the operating system name printed
14970 by the @option{-o} or @option{--operating-system} option, but it might
14971 differ. Some operating systems (e.g., FreeBSD, HP-UX) have the same
14972 name as their underlying kernels; others (e.g., GNU/Linux, Solaris)
14976 @itemx --kernel-version
14978 @opindex --kernel-version
14979 @cindex kernel version
14980 @cindex version of kernel
14981 Print the kernel version.
14988 @node hostname invocation
14989 @section @command{hostname}: Print or set system name
14992 @cindex setting the hostname
14993 @cindex printing the hostname
14994 @cindex system name, printing
14995 @cindex appropriate privileges
14997 With no arguments, @command{hostname} prints the name of the current host
14998 system. With one argument, it sets the current host name to the
14999 specified string. You must have appropriate privileges to set the host
15003 hostname [@var{name}]
15006 The only options are @option{--help} and @option{--version}. @xref{Common
15012 @node hostid invocation
15013 @section @command{hostid}: Print numeric host identifier
15016 @cindex printing the host identifier
15018 @command{hostid} prints the numeric identifier of the current host
15019 in hexadecimal. This command accepts no arguments.
15020 The only options are @option{--help} and @option{--version}.
15021 @xref{Common options}.
15023 For example, here's what it prints on one system I use:
15030 On that system, the 32-bit quantity happens to be closely
15031 related to the system's Internet address, but that isn't always
15036 @node uptime invocation
15037 @section @command{uptime}: Print system uptime and load
15040 @cindex printing the system uptime and load
15042 @command{uptime} prints the current time, the system's uptime, the
15043 number of logged-in users and the current load average.
15045 If an argument is specified, it is used as the file to be read
15046 to discover how many users are logged in. If no argument is
15047 specified, a system default is used (@command{uptime --help} indicates
15048 the default setting).
15050 The only options are @option{--help} and @option{--version}.
15051 @xref{Common options}.
15053 For example, here's what it prints right now on one system I use:
15057 14:07 up 3:35, 3 users, load average: 1.39, 1.15, 1.04
15060 The precise method of calculation of load average varies somewhat
15061 between systems. Some systems calculate it as the average number of
15062 runnable processes over the last 1, 5 and 15 minutes, but some systems
15063 also include processes in the uninterruptible sleep state (that is,
15064 those processes which are waiting for disk I/O). The Linux kernel
15065 includes uninterruptible processes.
15067 @node SELinux context
15068 @chapter SELinux context
15070 @cindex SELinux context
15071 @cindex SELinux, context
15072 @cindex commands for SELinux context
15074 This section describes commands for operations with SELinux
15078 * chcon invocation:: Change SELinux context of file
15079 * runcon invocation:: Run a command in specified SELinux context
15082 @node chcon invocation
15083 @section @command{chcon}: Change SELinux context of file
15086 @cindex changing security context
15087 @cindex change SELinux context
15089 @command{chcon} changes the SELinux security context of the selected files.
15093 chcon [@var{option}]@dots{} @var{context} @var{file}@dots{}
15094 chcon [@var{option}]@dots{} [-u @var{user}] [-r @var{role}] [-l @var{range}]@c
15095 [-t @var{type}] @var{file}@dots{}
15096 chcon [@var{option}]@dots{} --reference=@var{rfile} @var{file}@dots{}
15099 Change the SELinux security context of each @var{file} to @var{context}.
15100 With @option{--reference}, change the security context of each @var{file}
15101 to that of @var{rfile}.
15103 The program accepts the following options. Also see @ref{Common options}.
15108 @itemx --no-dereference
15110 @opindex --no-dereference
15111 @cindex no dereference
15112 Affect symbolic links instead of any referenced file.
15114 @item --reference=@var{rfile}
15115 @opindex --reference
15116 @cindex reference file
15117 Use @var{rfile}'s security context rather than specifying a @var{context} value.
15122 @opindex --recursive
15123 Operate on files and directories recursively.
15126 @xref{Traversing symlinks}.
15129 @xref{Traversing symlinks}.
15132 @xref{Traversing symlinks}.
15139 Output a diagnostic for every file processed.
15141 @item -u @var{user}
15142 @itemx --user=@var{user}
15145 Set user @var{user} in the target security context.
15147 @item -r @var{role}
15148 @itemx --role=@var{role}
15151 Set role @var{role} in the target security context.
15153 @item -t @var{type}
15154 @itemx --type=@var{type}
15157 Set type @var{type} in the target security context.
15159 @item -l @var{range}
15160 @itemx --range=@var{range}
15163 Set range @var{range} in the target security context.
15169 @node runcon invocation
15170 @section @command{runcon}: Run a command in specified SELinux context
15173 @cindex run with security context
15176 @command{runcon} runs file in specified SELinux security context.
15180 runcon @var{context} @var{command} [@var{args}]
15181 runcon [ -c ] [-u @var{user}] [-r @var{role}] [-t @var{type}]@c
15182 [-l @var{range}] @var{command} [@var{args}]
15185 Run @var{command} with completely-specified @var{context}, or with
15186 current or transitioned security context modified by one or more of @var{level},
15187 @var{role}, @var{type} and @var{user}.
15189 If none of @option{-c}, @option{-t}, @option{-u}, @option{-r}, or @option{-l}
15190 is specified, the first argument is used as the complete context.
15191 Any additional arguments after @var{command}
15192 are interpreted as arguments to the command.
15194 With neither @var{context} nor @var{command}, print the current
15197 The program accepts the following options. Also see @ref{Common options}.
15205 Compute process transition context before modifying.
15207 @item -u @var{user}
15208 @itemx --user=@var{user}
15211 Set user @var{user} in the target security context.
15213 @item -r @var{role}
15214 @itemx --role=@var{role}
15217 Set role @var{role} in the target security context.
15219 @item -t @var{type}
15220 @itemx --type=@var{type}
15223 Set type @var{type} in the target security context.
15225 @item -l @var{range}
15226 @itemx --range=@var{range}
15229 Set range @var{range} in the target security context.
15233 @cindex exit status of @command{runcon}
15237 126 if @var{command} is found but cannot be invoked
15238 127 if @command{runcon} itself fails or if @var{command} cannot be found
15239 the exit status of @var{command} otherwise
15242 @node Modified command invocation
15243 @chapter Modified command invocation
15245 @cindex modified command invocation
15246 @cindex invocation of commands, modified
15247 @cindex commands for invoking other commands
15249 This section describes commands that run other commands in some context
15250 different than the current one: a modified environment, as a different
15254 * chroot invocation:: Modify the root directory.
15255 * env invocation:: Modify environment variables.
15256 * nice invocation:: Modify niceness.
15257 * nohup invocation:: Immunize to hangups.
15258 * stdbuf invocation:: Modify buffering of standard streams.
15259 * su invocation:: Modify user and group ID.
15260 * timeout invocation:: Run with time limit.
15264 @node chroot invocation
15265 @section @command{chroot}: Run a command with a different root directory
15268 @cindex running a program in a specified root directory
15269 @cindex root directory, running a program in a specified
15271 @command{chroot} runs a command with a specified root directory.
15272 On many systems, only the super-user can do this.@footnote{However,
15273 some systems (e.g., FreeBSD) can be configured to allow certain regular
15274 users to use the @code{chroot} system call, and hence to run this program.
15275 Also, on Cygwin, anyone can run the @command{chroot} command, because the
15276 underlying function is non-privileged due to lack of support in MS-Windows.}
15280 chroot @var{option} @var{newroot} [@var{command} [@var{args}]@dots{}]
15281 chroot @var{option}
15284 Ordinarily, file names are looked up starting at the root of the
15285 directory structure, i.e., @file{/}. @command{chroot} changes the root to
15286 the directory @var{newroot} (which must exist) and then runs
15287 @var{command} with optional @var{args}. If @var{command} is not
15288 specified, the default is the value of the @env{SHELL} environment
15289 variable or @command{/bin/sh} if not set, invoked with the @option{-i} option.
15290 @var{command} must not be a special built-in utility
15291 (@pxref{Special built-in utilities}).
15293 The program accepts the following options. Also see @ref{Common options}.
15294 Options must precede operands.
15298 @itemx --userspec=@var{user}[:@var{group}]
15299 @opindex --userspec
15300 By default, @var{command} is run with the same credentials
15301 as the invoking process.
15302 Use this option to run it as a different @var{user} and/or with a
15303 different primary @var{group}.
15305 @itemx --groups=@var{groups}
15307 Use this option to specify the supplementary @var{groups} to be
15308 used by the new process.
15309 The items in the list (names or numeric IDs) must be separated by commas.
15313 Here are a few tips to help avoid common problems in using chroot.
15314 To start with a simple example, make @var{command} refer to a statically
15315 linked binary. If you were to use a dynamically linked executable, then
15316 you'd have to arrange to have the shared libraries in the right place under
15317 your new root directory.
15319 For example, if you create a statically linked @command{ls} executable,
15320 and put it in @file{/tmp/empty}, you can run this command as root:
15323 $ chroot /tmp/empty /ls -Rl /
15326 Then you'll see output like this:
15331 -rwxr-xr-x 1 0 0 1041745 Aug 16 11:17 ls
15334 If you want to use a dynamically linked executable, say @command{bash},
15335 then first run @samp{ldd bash} to see what shared objects it needs.
15336 Then, in addition to copying the actual binary, also copy the listed
15337 files to the required positions under your intended new root directory.
15338 Finally, if the executable requires any other files (e.g., data, state,
15339 device files), copy them into place, too.
15341 @cindex exit status of @command{chroot}
15345 125 if @command{chroot} itself fails
15346 126 if @var{command} is found but cannot be invoked
15347 127 if @var{command} cannot be found
15348 the exit status of @var{command} otherwise
15352 @node env invocation
15353 @section @command{env}: Run a command in a modified environment
15356 @cindex environment, running a program in a modified
15357 @cindex modified environment, running a program in a
15358 @cindex running a program in a modified environment
15360 @command{env} runs a command with a modified environment. Synopses:
15363 env [@var{option}]@dots{} [@var{name}=@var{value}]@dots{} @c
15364 [@var{command} [@var{args}]@dots{}]
15368 Operands of the form @samp{@var{variable}=@var{value}} set
15369 the environment variable @var{variable} to value @var{value}.
15370 @var{value} may be empty (@samp{@var{variable}=}). Setting a variable
15371 to an empty value is different from unsetting it.
15372 These operands are evaluated left-to-right, so if two operands
15373 mention the same variable the earlier is ignored.
15375 Environment variable names can be empty, and can contain any
15376 characters other than @samp{=} and @acronym{ASCII} @sc{nul}.
15377 However, it is wise to limit yourself to names that
15378 consist solely of underscores, digits, and @acronym{ASCII} letters,
15379 and that begin with a non-digit, as applications like the shell do not
15380 work well with other names.
15383 The first operand that does not contain the character @samp{=}
15384 specifies the program to invoke; it is
15385 searched for according to the @env{PATH} environment variable. Any
15386 remaining arguments are passed as arguments to that program.
15387 The program should not be a special built-in utility
15388 (@pxref{Special built-in utilities}).
15390 Modifications to @env{PATH} take effect prior to searching for
15391 @var{command}. Use caution when reducing @env{PATH}; behavior is
15392 not portable when @env{PATH} is undefined or omits key directories
15393 such as @file{/bin}.
15395 In the rare case that a utility contains a @samp{=} in the name, the
15396 only way to disambiguate it from a variable assignment is to use an
15397 intermediate command for @var{command}, and pass the problematic
15398 program name via @var{args}. For example, if @file{./prog=} is an
15399 executable in the current @env{PATH}:
15402 env prog= true # runs 'true', with prog= in environment
15403 env ./prog= true # runs 'true', with ./prog= in environment
15404 env -- prog= true # runs 'true', with prog= in environment
15405 env sh -c '\prog= true' # runs 'prog=' with argument 'true'
15406 env sh -c 'exec "$@@"' sh prog= true # also runs 'prog='
15409 @cindex environment, printing
15411 If no command name is specified following the environment
15412 specifications, the resulting environment is printed. This is like
15413 specifying the @command{printenv} program.
15415 For some examples, suppose the environment passed to @command{env}
15416 contains @samp{LOGNAME=rms}, @samp{EDITOR=emacs}, and
15417 @samp{PATH=.:/gnubin:/hacks}:
15422 Output the current environment.
15424 $ env | LC_ALL=C sort
15427 PATH=.:/gnubin:/hacks
15431 Run @command{foo} with a reduced environment, preserving only the
15432 original @env{PATH} to avoid problems in locating @command{foo}.
15434 env - PATH="$PATH" foo
15438 Run @command{foo} with the environment containing @samp{LOGNAME=rms},
15439 @samp{EDITOR=emacs}, and @samp{PATH=.:/gnubin:/hacks}, and guarantees
15440 that @command{foo} was found in the file system rather than as a shell
15447 Run @command{nemacs} with the environment containing @samp{LOGNAME=foo},
15448 @samp{EDITOR=emacs}, @samp{PATH=.:/gnubin:/hacks}, and
15449 @samp{DISPLAY=gnu:0}.
15451 env DISPLAY=gnu:0 LOGNAME=foo nemacs
15455 Attempt to run the program @command{/energy/--} (as that is the only
15456 possible path search result); if the command exists, the environment
15457 will contain @samp{LOGNAME=rms} and @samp{PATH=/energy}, and the
15458 arguments will be @samp{e=mc2}, @samp{bar}, and @samp{baz}.
15460 env -u EDITOR PATH=/energy -- e=mc2 bar baz
15466 The program accepts the following options. Also see @ref{Common options}.
15467 Options must precede operands.
15473 @item -u @var{name}
15474 @itemx --unset=@var{name}
15477 Remove variable @var{name} from the environment, if it was in the
15482 @itemx --ignore-environment
15485 @opindex --ignore-environment
15486 Start with an empty environment, ignoring the inherited environment.
15490 @cindex exit status of @command{env}
15494 0 if no @var{command} is specified and the environment is output
15495 125 if @command{env} itself fails
15496 126 if @var{command} is found but cannot be invoked
15497 127 if @var{command} cannot be found
15498 the exit status of @var{command} otherwise
15502 @node nice invocation
15503 @section @command{nice}: Run a command with modified niceness
15507 @cindex scheduling, affecting
15508 @cindex appropriate privileges
15510 @command{nice} prints or modifies a process's @dfn{niceness},
15511 a parameter that affects whether the process is scheduled favorably.
15515 nice [@var{option}]@dots{} [@var{command} [@var{arg}]@dots{}]
15518 If no arguments are given, @command{nice} prints the current niceness.
15519 Otherwise, @command{nice} runs the given @var{command} with its
15520 niceness adjusted. By default, its niceness is incremented by 10.
15522 Niceness values range at least from @minus{}20 (process has high priority
15523 and gets more resources, thus slowing down other processes) through 19
15524 (process has lower priority and runs slowly itself, but has less impact
15525 on the speed of other running processes). Some systems
15526 may have a wider range of nicenesses; conversely, other systems may
15527 enforce more restrictive limits. An attempt to set the niceness
15528 outside the supported range is treated as an attempt to use the
15529 minimum or maximum supported value.
15531 A niceness should not be confused with a scheduling priority, which
15532 lets applications determine the order in which threads are scheduled
15533 to run. Unlike a priority, a niceness is merely advice to the
15534 scheduler, which the scheduler is free to ignore. Also, as a point of
15535 terminology, @acronym{POSIX} defines the behavior of @command{nice} in
15536 terms of a @dfn{nice value}, which is the nonnegative difference
15537 between a niceness and the minimum niceness. Though @command{nice}
15538 conforms to @acronym{POSIX}, its documentation and diagnostics use the
15539 term ``niceness'' for compatibility with historical practice.
15541 @var{command} must not be a special built-in utility (@pxref{Special
15542 built-in utilities}).
15544 @mayConflictWithShellBuiltIn{nice}
15546 The program accepts the following option. Also see @ref{Common options}.
15547 Options must precede operands.
15550 @item -n @var{adjustment}
15551 @itemx --adjustment=@var{adjustment}
15553 @opindex --adjustment
15554 Add @var{adjustment} instead of 10 to the command's niceness. If
15555 @var{adjustment} is negative and you lack appropriate privileges,
15556 @command{nice} issues a warning but otherwise acts as if you specified
15559 For compatibility @command{nice} also supports an obsolete
15560 option syntax @option{-@var{adjustment}}. New scripts should use
15561 @option{-n @var{adjustment}} instead.
15565 @cindex exit status of @command{nice}
15569 0 if no @var{command} is specified and the niceness is output
15570 125 if @command{nice} itself fails
15571 126 if @var{command} is found but cannot be invoked
15572 127 if @var{command} cannot be found
15573 the exit status of @var{command} otherwise
15576 It is sometimes useful to run a non-interactive program with reduced niceness.
15579 $ nice factor 4611686018427387903
15582 Since @command{nice} prints the current niceness,
15583 you can invoke it through itself to demonstrate how it works.
15585 The default behavior is to increase the niceness by @samp{10}:
15596 The @var{adjustment} is relative to the current niceness. In the
15597 next example, the first @command{nice} invocation runs the second one
15598 with niceness 10, and it in turn runs the final one with a niceness
15602 $ nice nice -n 3 nice
15606 Specifying a niceness larger than the supported range
15607 is the same as specifying the maximum supported value:
15610 $ nice -n 10000000000 nice
15614 Only a privileged user may run a process with lower niceness:
15618 nice: cannot set niceness: Permission denied
15620 $ sudo nice -n -1 nice
15625 @node nohup invocation
15626 @section @command{nohup}: Run a command immune to hangups
15629 @cindex hangups, immunity to
15630 @cindex immunity to hangups
15631 @cindex logging out and continuing to run
15634 @command{nohup} runs the given @var{command} with hangup signals ignored,
15635 so that the command can continue running in the background after you log
15639 nohup @var{command} [@var{arg}]@dots{}
15642 If standard input is a terminal, it is redirected from
15643 @file{/dev/null} so that terminal sessions do not mistakenly consider
15644 the terminal to be used by the command. This is a @acronym{GNU}
15645 extension; programs intended to be portable to non-@acronym{GNU} hosts
15646 should use @samp{nohup @var{command} [@var{arg}]@dots{} </dev/null}
15650 If standard output is a terminal, the command's standard output is appended
15651 to the file @file{nohup.out}; if that cannot be written to, it is appended
15652 to the file @file{$HOME/nohup.out}; and if that cannot be written to, the
15653 command is not run.
15654 Any @file{nohup.out} or @file{$HOME/nohup.out} file created by
15655 @command{nohup} is made readable and writable only to the user,
15656 regardless of the current umask settings.
15658 If standard error is a terminal, it is normally redirected to the same file
15659 descriptor as the (possibly-redirected) standard output.
15660 However, if standard output is closed, standard error terminal output
15661 is instead appended to the file @file{nohup.out} or
15662 @file{$HOME/nohup.out} as above.
15664 To capture the command's output to a file other than @file{nohup.out}
15665 you can redirect it. For example, to capture the output of
15669 nohup make > make.log
15672 @command{nohup} does not automatically put the command it runs in the
15673 background; you must do that explicitly, by ending the command line
15674 with an @samp{&}. Also, @command{nohup} does not alter the
15675 niceness of @var{command}; use @command{nice} for that,
15676 e.g., @samp{nohup nice @var{command}}.
15678 @var{command} must not be a special built-in utility (@pxref{Special
15679 built-in utilities}).
15681 The only options are @option{--help} and @option{--version}. @xref{Common
15682 options}. Options must precede operands.
15684 @cindex exit status of @command{nohup}
15688 125 if @command{nohup} itself fails, and @env{POSIXLY_CORRECT} is not set
15689 126 if @var{command} is found but cannot be invoked
15690 127 if @var{command} cannot be found
15691 the exit status of @var{command} otherwise
15694 If @env{POSIXLY_CORRECT} is set, internal failures give status 127
15698 @node stdbuf invocation
15699 @section @command{stdbuf}: Run a command with modified I/O stream buffering
15702 @cindex standard streams, buffering
15703 @cindex line buffered
15705 @command{stdbuf} allows one to modify the buffering operations of the
15706 three standard I/O streams associated with a program. Synopsis:
15709 stdbuf @var{option}@dots{} @var{command}
15712 @var{command} must start with the name of a program that
15715 uses the ISO C @code{FILE} streams for input/output (note the
15716 programs @command{dd} and @command{cat} don't do that),
15719 does not adjust the buffering of its standard streams (note the
15720 program @command{tee} is not in this category).
15723 Any additional @var{arg}s are passed as additional arguments to the
15726 The program accepts the following options. Also see @ref{Common options}.
15730 @item -i @var{mode}
15731 @itemx --input=@var{mode}
15734 Adjust the standard input stream buffering.
15736 @item -o @var{mode}
15737 @itemx --output=@var{mode}
15740 Adjust the standard output stream buffering.
15742 @item -e @var{mode}
15743 @itemx --error=@var{mode}
15746 Adjust the standard error stream buffering.
15750 The @var{mode} can be specified as follows:
15755 Set the stream to line buffered mode.
15756 In this mode data is coalesced until a newline is output or
15757 input is read from any stream attached to a terminal device.
15758 This option is invalid with standard input.
15761 Disable buffering of the selected stream.
15762 In this mode, data is output immediately and only the
15763 amount of data requested is read from input.
15764 Note the difference in function for input and output.
15765 Disabling buffering for input will not influence the responsiveness
15766 or blocking behavior of the stream input functions.
15767 For example @code{fread} will still block until @code{EOF} or error,
15768 even if the underlying @code{read} returns less data than requested.
15771 Specify the size of the buffer to use in fully buffered mode.
15772 @multiplierSuffixesNoBlocks{size}
15776 @cindex exit status of @command{stdbuf}
15780 125 if @command{stdbuf} itself fails
15781 126 if @var{command} is found but cannot be invoked
15782 127 if @var{command} cannot be found
15783 the exit status of @var{command} otherwise
15787 @node su invocation
15788 @section @command{su}: Run a command with substitute user and group ID
15791 @cindex substitute user and group IDs
15792 @cindex user ID, switching
15793 @cindex super-user, becoming
15794 @cindex root, becoming
15796 @command{su} allows one user to temporarily become another user. It runs a
15797 command (often an interactive shell) with the real and effective user
15798 ID, group ID, and supplemental groups of a given @var{user}. Synopsis:
15801 su [@var{option}]@dots{} [@var{user} [@var{arg}]@dots{}]
15804 @cindex passwd entry, and @command{su} shell
15806 @flindex /etc/passwd
15807 If no @var{user} is given, the default is @code{root}, the super-user.
15808 The shell to use is taken from @var{user}'s @code{passwd} entry, or
15809 @file{/bin/sh} if none is specified there. If @var{user} has a
15810 password, @command{su} prompts for the password unless run by a user with
15811 effective user ID of zero (the super-user).
15817 @cindex login shell
15818 By default, @command{su} does not change the current directory.
15819 It sets the environment variables @env{HOME} and @env{SHELL}
15820 from the password entry for @var{user}, and if @var{user} is not
15821 the super-user, sets @env{USER} and @env{LOGNAME} to @var{user}.
15822 By default, the shell is not a login shell.
15824 Any additional @var{arg}s are passed as additional arguments to the
15827 @cindex @option{-su}
15828 GNU @command{su} does not treat @file{/bin/sh} or any other shells specially
15829 (e.g., by setting @code{argv[0]} to @option{-su}, passing @option{-c} only
15830 to certain shells, etc.).
15833 @command{su} can optionally be compiled to use @code{syslog} to report
15834 failed, and optionally successful, @command{su} attempts. (If the system
15835 supports @code{syslog}.) However, GNU @command{su} does not check if the
15836 user is a member of the @code{wheel} group; see below.
15838 The program accepts the following options. Also see @ref{Common options}.
15841 @item -c @var{command}
15842 @itemx --command=@var{command}
15845 Pass @var{command}, a single command line to run, to the shell with
15846 a @option{-c} option instead of starting an interactive shell.
15853 @cindex file name pattern expansion, disabled
15854 @cindex globbing, disabled
15855 Pass the @option{-f} option to the shell. This probably only makes sense
15856 if the shell run is @command{csh} or @command{tcsh}, for which the @option{-f}
15857 option prevents reading the startup file (@file{.cshrc}). With
15858 Bourne-like shells, the @option{-f} option disables file name pattern
15859 expansion (globbing), which is not likely to be useful.
15867 @c other variables already indexed above
15870 @cindex login shell, creating
15871 Make the shell a login shell. This means the following. Unset all
15872 environment variables except @env{TERM}, @env{HOME}, and @env{SHELL}
15873 (which are set as described above), and @env{USER} and @env{LOGNAME}
15874 (which are set, even for the super-user, as described above), and set
15875 @env{PATH} to a compiled-in default value. Change to @var{user}'s home
15876 directory. Prepend @samp{-} to the shell's name, intended to make it
15877 read its login startup file(s).
15881 @itemx --preserve-environment
15884 @opindex --preserve-environment
15885 @cindex environment, preserving
15886 @flindex /etc/shells
15887 @cindex restricted shell
15888 Do not change the environment variables @env{HOME}, @env{USER},
15889 @env{LOGNAME}, or @env{SHELL}. Run the shell given in the environment
15890 variable @env{SHELL} instead of the shell from @var{user}'s passwd
15891 entry, unless the user running @command{su} is not the super-user and
15892 @var{user}'s shell is restricted. A @dfn{restricted shell} is one that
15893 is not listed in the file @file{/etc/shells}, or in a compiled-in list
15894 if that file does not exist. Parts of what this option does can be
15895 overridden by @option{--login} and @option{--shell}.
15897 @item -s @var{shell}
15898 @itemx --shell=@var{shell}
15901 Run @var{shell} instead of the shell from @var{user}'s passwd entry,
15902 unless the user running @command{su} is not the super-user and @var{user}'s
15903 shell is restricted (see @option{-m} just above).
15907 @cindex exit status of @command{su}
15911 125 if @command{su} itself fails
15912 126 if subshell is found but cannot be invoked
15913 127 if subshell cannot be found
15914 the exit status of the subshell otherwise
15917 @cindex wheel group, not supported
15918 @cindex group wheel, not supported
15920 @subsection Why GNU @command{su} does not support the @samp{wheel} group
15922 (This section is by Richard Stallman.)
15926 Sometimes a few of the users try to hold total power over all the
15927 rest. For example, in 1984, a few users at the MIT AI lab decided to
15928 seize power by changing the operator password on the Twenex system and
15929 keeping it secret from everyone else. (I was able to thwart this coup
15930 and give power back to the users by patching the kernel, but I
15931 wouldn't know how to do that in Unix.)
15933 However, occasionally the rulers do tell someone. Under the usual
15934 @command{su} mechanism, once someone learns the root password who
15935 sympathizes with the ordinary users, he or she can tell the rest. The
15936 ``wheel group'' feature would make this impossible, and thus cement the
15937 power of the rulers.
15939 I'm on the side of the masses, not that of the rulers. If you are
15940 used to supporting the bosses and sysadmins in whatever they do, you
15941 might find this idea strange at first.
15944 @node timeout invocation
15945 @section @command{timeout}: Run a command with a time limit
15949 @cindex run commands with bounded time
15951 @command{timeout} runs the given @var{command} and kills it if it is
15952 still running after the specified time interval. Synopsis:
15955 timeout [@var{option}] @var{duration} @var{command} [@var{arg}]@dots{}
15958 @var{command} must not be a special built-in utility (@pxref{Special
15959 built-in utilities}).
15961 The program accepts the following options. Also see @ref{Common options}.
15962 Options must precede operands.
15965 @itemx --foreground
15966 @opindex --foreground
15967 Don't create a separate background program group, so that
15968 the managed @var{command} can use the foreground TTY normally.
15969 This is needed to support timing out commands not started
15970 directly from an interactive shell, in two situations.
15973 @var{command} is interactive and needs to read from the terminal for example
15975 the user wants to support sending signals directly to @var{command}
15976 from the terminal (like Ctrl-C for example)
15979 Note in this mode of operation, any children of @var{command}
15980 will not be timed out.
15982 @item -k @var{duration}
15983 @itemx --kill-after=@var{duration}
15985 @opindex --kill-after
15986 Ensure the monitored @var{command} is killed by also sending a @samp{KILL}
15987 signal, after the specified @var{duration}. Without this option, if the
15988 selected signal proves not to be fatal, @command{timeout} does not kill
15991 @item -s @var{signal}
15992 @itemx --signal=@var{signal}
15995 Send this @var{signal} to @var{command} on timeout, rather than the
15996 default @samp{TERM} signal. @var{signal} may be a name like @samp{HUP}
15997 or a number. Also see @xref{Signal specifications}.
16001 @var{duration} is a floating point number followed by an optional unit:
16003 @samp{s} for seconds (the default)
16004 @samp{m} for minutes
16008 A duration of 0 disables the associated timeout.
16009 Note that the actual timeout duration is dependent on system conditions,
16010 which should be especially considered when specifying sub-second timeouts.
16012 @cindex exit status of @command{timeout}
16016 124 if @var{command} times out
16017 125 if @command{timeout} itself fails
16018 126 if @var{command} is found but cannot be invoked
16019 127 if @var{command} cannot be found
16020 137 if @var{command} is sent the KILL(9) signal (128+9)
16021 the exit status of @var{command} otherwise
16025 @node Process control
16026 @chapter Process control
16028 @cindex processes, commands for controlling
16029 @cindex commands for controlling processes
16032 * kill invocation:: Sending a signal to processes.
16036 @node kill invocation
16037 @section @command{kill}: Send a signal to processes
16040 @cindex send a signal to processes
16042 The @command{kill} command sends a signal to processes, causing them
16043 to terminate or otherwise act upon receiving the signal in some way.
16044 Alternatively, it lists information about signals. Synopses:
16047 kill [-s @var{signal} | --signal @var{signal} | -@var{signal}] @var{pid}@dots{}
16048 kill [-l | --list | -t | --table] [@var{signal}]@dots{}
16051 @mayConflictWithShellBuiltIn{kill}
16053 The first form of the @command{kill} command sends a signal to all
16054 @var{pid} arguments. The default signal to send if none is specified
16055 is @samp{TERM}. The special signal number @samp{0} does not denote a
16056 valid signal, but can be used to test whether the @var{pid} arguments
16057 specify processes to which a signal could be sent.
16059 If @var{pid} is positive, the signal is sent to the process with the
16060 process ID @var{pid}. If @var{pid} is zero, the signal is sent to all
16061 processes in the process group of the current process. If @var{pid}
16062 is @minus{}1, the signal is sent to all processes for which the user has
16063 permission to send a signal. If @var{pid} is less than @minus{}1, the signal
16064 is sent to all processes in the process group that equals the absolute
16065 value of @var{pid}.
16067 If @var{pid} is not positive, a system-dependent set of system
16068 processes is excluded from the list of processes to which the signal
16071 If a negative @var{pid} argument is desired as the first one, it
16072 should be preceded by @option{--}. However, as a common extension to
16073 @acronym{POSIX}, @option{--} is not required with @samp{kill
16074 -@var{signal} -@var{pid}}. The following commands are equivalent:
16083 The first form of the @command{kill} command succeeds if every @var{pid}
16084 argument specifies at least one process that the signal was sent to.
16086 The second form of the @command{kill} command lists signal information.
16087 Either the @option{-l} or @option{--list} option, or the @option{-t}
16088 or @option{--table} option must be specified. Without any
16089 @var{signal} argument, all supported signals are listed. The output
16090 of @option{-l} or @option{--list} is a list of the signal names, one
16091 per line; if @var{signal} is already a name, the signal number is
16092 printed instead. The output of @option{-t} or @option{--table} is a
16093 table of signal numbers, names, and descriptions. This form of the
16094 @command{kill} command succeeds if all @var{signal} arguments are valid
16095 and if there is no output error.
16097 The @command{kill} command also supports the @option{--help} and
16098 @option{--version} options. @xref{Common options}.
16100 A @var{signal} may be a signal name like @samp{HUP}, or a signal
16101 number like @samp{1}, or an exit status of a process terminated by the
16102 signal. A signal name can be given in canonical form or prefixed by
16103 @samp{SIG}. The case of the letters is ignored, except for the
16104 @option{-@var{signal}} option which must use upper case to avoid
16105 ambiguity with lower case option letters. For a list of supported
16106 signal names and numbers see @xref{Signal specifications}.
16111 @cindex delaying commands
16112 @cindex commands for delaying
16114 @c Perhaps @command{wait} or other commands should be described here also?
16117 * sleep invocation:: Delay for a specified time.
16121 @node sleep invocation
16122 @section @command{sleep}: Delay for a specified time
16125 @cindex delay for a specified time
16127 @command{sleep} pauses for an amount of time specified by the sum of
16128 the values of the command line arguments.
16132 sleep @var{number}[smhd]@dots{}
16136 Each argument is a number followed by an optional unit; the default
16137 is seconds. The units are:
16150 Historical implementations of @command{sleep} have required that
16151 @var{number} be an integer, and only accepted a single argument
16152 without a suffix. However, GNU @command{sleep} accepts
16153 arbitrary floating point numbers. @xref{Floating point}.
16155 The only options are @option{--help} and @option{--version}. @xref{Common
16158 @c sleep is a shell built-in at least with Solaris 11's /bin/sh
16159 @mayConflictWithShellBuiltIn{sleep}
16164 @node Numeric operations
16165 @chapter Numeric operations
16167 @cindex numeric operations
16168 These programs do numerically-related operations.
16171 * factor invocation:: Show factors of numbers.
16172 * seq invocation:: Print sequences of numbers.
16176 @node factor invocation
16177 @section @command{factor}: Print prime factors
16180 @cindex prime factors
16182 @command{factor} prints prime factors. Synopses:
16185 factor [@var{number}]@dots{}
16186 factor @var{option}
16189 If no @var{number} is specified on the command line, @command{factor} reads
16190 numbers from standard input, delimited by newlines, tabs, or spaces.
16192 The @command{factor} command supports only a small number of options:
16196 Print a short help on standard output, then exit without further
16200 Print the program version on standard output, then exit without further
16204 Factoring the product of the eighth and ninth Mersenne primes
16205 takes about 30 milliseconds of CPU time on a 2.2 GHz Athlon.
16208 M8=$(echo 2^31-1|bc)
16209 M9=$(echo 2^61-1|bc)
16210 n=$(echo "$M8 * $M9" | bc)
16211 /usr/bin/time -f %U factor $n
16212 4951760154835678088235319297: 2147483647 2305843009213693951
16216 Similarly, factoring the eighth Fermat number @math{2^{256}+1} takes
16217 about 20 seconds on the same machine.
16219 Factoring large numbers is, in general, hard. The Pollard Rho
16220 algorithm used by @command{factor} is particularly effective for
16221 numbers with relatively small factors. If you wish to factor large
16222 numbers which do not have small factors (for example, numbers which
16223 are the product of two large primes), other methods are far better.
16225 If @command{factor} is built without using GNU MP, only
16226 single-precision arithmetic is available, and so large numbers
16227 (typically @math{2^{64}} and above) will not be supported. The single-precision
16228 code uses an algorithm which is designed for factoring smaller
16234 @node seq invocation
16235 @section @command{seq}: Print numeric sequences
16238 @cindex numeric sequences
16239 @cindex sequence of numbers
16241 @command{seq} prints a sequence of numbers to standard output. Synopses:
16244 seq [@var{option}]@dots{} @var{last}
16245 seq [@var{option}]@dots{} @var{first} @var{last}
16246 seq [@var{option}]@dots{} @var{first} @var{increment} @var{last}
16249 @command{seq} prints the numbers from @var{first} to @var{last} by
16250 @var{increment}. By default, each number is printed on a separate line.
16251 When @var{increment} is not specified, it defaults to @samp{1},
16252 even when @var{first} is larger than @var{last}.
16253 @var{first} also defaults to @samp{1}. So @code{seq 1} prints
16254 @samp{1}, but @code{seq 0} and @code{seq 10 5} produce no output.
16255 Floating-point numbers may be specified. @xref{Floating point}.
16257 The program accepts the following options. Also see @ref{Common options}.
16258 Options must precede operands.
16261 @item -f @var{format}
16262 @itemx --format=@var{format}
16263 @opindex -f @var{format}
16264 @opindex --format=@var{format}
16265 @cindex formatting of numbers in @command{seq}
16266 Print all numbers using @var{format}.
16267 @var{format} must contain exactly one of the @samp{printf}-style
16268 floating point conversion specifications @samp{%a}, @samp{%e},
16269 @samp{%f}, @samp{%g}, @samp{%A}, @samp{%E}, @samp{%F}, @samp{%G}.
16270 The @samp{%} may be followed by zero or more flags taken from the set
16271 @samp{-+#0 '}, then an optional width containing one or more digits,
16272 then an optional precision consisting of a @samp{.} followed by zero
16273 or more digits. @var{format} may also contain any number of @samp{%%}
16274 conversion specifications. All conversion specifications have the
16275 same meaning as with @samp{printf}.
16277 The default format is derived from @var{first}, @var{step}, and
16278 @var{last}. If these all use a fixed point decimal representation,
16279 the default format is @samp{%.@var{p}f}, where @var{p} is the minimum
16280 precision that can represent the output numbers exactly. Otherwise,
16281 the default format is @samp{%g}.
16283 @item -s @var{string}
16284 @itemx --separator=@var{string}
16285 @cindex separator for numbers in @command{seq}
16286 Separate numbers with @var{string}; default is a newline.
16287 The output always terminates with a newline.
16290 @itemx --equal-width
16291 Print all numbers with the same width, by padding with leading zeros.
16292 @var{first}, @var{step}, and @var{last} should all use a fixed point
16293 decimal representation.
16294 (To have other kinds of padding, use @option{--format}).
16298 You can get finer-grained control over output with @option{-f}:
16301 $ seq -f '(%9.2E)' -9e5 1.1e6 1.3e6
16307 If you want hexadecimal integer output, you can use @command{printf}
16308 to perform the conversion:
16311 $ printf '%x\n' `seq 1048575 1024 1050623`
16317 For very long lists of numbers, use xargs to avoid
16318 system limitations on the length of an argument list:
16321 $ seq 1000000 | xargs printf '%x\n' | tail -n 3
16327 To generate octal output, use the printf @code{%o} format instead
16330 On most systems, seq can produce whole-number output for values up to
16331 at least @math{2^{53}}. Larger integers are approximated. The details
16332 differ depending on your floating-point implementation.
16333 @xref{Floating point}. A common
16334 case is that @command{seq} works with integers through @math{2^{64}},
16335 and larger integers may not be numerically correct:
16338 $ seq 18446744073709551616 1 18446744073709551618
16339 18446744073709551616
16340 18446744073709551616
16341 18446744073709551618
16344 Be careful when using @command{seq} with outlandish values: otherwise
16345 you may see surprising results, as @command{seq} uses floating point
16346 internally. For example, on the x86 platform, where the internal
16347 representation uses a 64-bit fraction, the command:
16350 seq 1 0.0000000000000000001 1.0000000000000000009
16353 outputs 1.0000000000000000007 twice and skips 1.0000000000000000008.
16358 @node File permissions
16359 @chapter File permissions
16362 @include parse-datetime.texi
16366 @node Opening the software toolbox
16367 @chapter Opening the Software Toolbox
16369 An earlier version of this chapter appeared in
16370 @uref{http://www.linuxjournal.com/article.php?sid=2762, the
16371 @cite{What's GNU?} column of the June 1994 @cite{Linux Journal}}.
16372 It was written by Arnold Robbins.
16375 * Toolbox introduction:: Toolbox introduction
16376 * I/O redirection:: I/O redirection
16377 * The who command:: The @command{who} command
16378 * The cut command:: The @command{cut} command
16379 * The sort command:: The @command{sort} command
16380 * The uniq command:: The @command{uniq} command
16381 * Putting the tools together:: Putting the tools together
16385 @node Toolbox introduction
16386 @unnumberedsec Toolbox Introduction
16388 This month's column is only peripherally related to the GNU Project, in
16389 that it describes a number of the GNU tools on your GNU/Linux system
16391 might be used. What it's really about is the ``Software Tools'' philosophy
16392 of program development and usage.
16394 The software tools philosophy was an important and integral concept
16395 in the initial design and development of Unix (of which Linux and GNU are
16396 essentially clones). Unfortunately, in the modern day press of
16397 Internetworking and flashy GUIs, it seems to have fallen by the
16398 wayside. This is a shame, since it provides a powerful mental model
16399 for solving many kinds of problems.
16401 Many people carry a Swiss Army knife around in their pants pockets (or
16402 purse). A Swiss Army knife is a handy tool to have: it has several knife
16403 blades, a screwdriver, tweezers, toothpick, nail file, corkscrew, and perhaps
16404 a number of other things on it. For the everyday, small miscellaneous jobs
16405 where you need a simple, general purpose tool, it's just the thing.
16407 On the other hand, an experienced carpenter doesn't build a house using
16408 a Swiss Army knife. Instead, he has a toolbox chock full of specialized
16409 tools---a saw, a hammer, a screwdriver, a plane, and so on. And he knows
16410 exactly when and where to use each tool; you won't catch him hammering nails
16411 with the handle of his screwdriver.
16413 The Unix developers at Bell Labs were all professional programmers and trained
16414 computer scientists. They had found that while a one-size-fits-all program
16415 might appeal to a user because there's only one program to use, in practice
16420 difficult to write,
16423 difficult to maintain and
16427 difficult to extend to meet new situations.
16430 Instead, they felt that programs should be specialized tools. In short, each
16431 program ``should do one thing well.'' No more and no less. Such programs are
16432 simpler to design, write, and get right---they only do one thing.
16434 Furthermore, they found that with the right machinery for hooking programs
16435 together, that the whole was greater than the sum of the parts. By combining
16436 several special purpose programs, you could accomplish a specific task
16437 that none of the programs was designed for, and accomplish it much more
16438 quickly and easily than if you had to write a special purpose program.
16439 We will see some (classic) examples of this further on in the column.
16440 (An important additional point was that, if necessary, take a detour
16441 and build any software tools you may need first, if you don't already
16442 have something appropriate in the toolbox.)
16444 @node I/O redirection
16445 @unnumberedsec I/O Redirection
16447 Hopefully, you are familiar with the basics of I/O redirection in the
16448 shell, in particular the concepts of ``standard input,'' ``standard output,''
16449 and ``standard error''. Briefly, ``standard input'' is a data source, where
16450 data comes from. A program should not need to either know or care if the
16451 data source is a disk file, a keyboard, a magnetic tape, or even a punched
16452 card reader. Similarly, ``standard output'' is a data sink, where data goes
16453 to. The program should neither know nor care where this might be.
16454 Programs that only read their standard input, do something to the data,
16455 and then send it on, are called @dfn{filters}, by analogy to filters in a
16458 With the Unix shell, it's very easy to set up data pipelines:
16461 program_to_create_data | filter1 | ... | filterN > final.pretty.data
16464 We start out by creating the raw data; each filter applies some successive
16465 transformation to the data, until by the time it comes out of the pipeline,
16466 it is in the desired form.
16468 This is fine and good for standard input and standard output. Where does the
16469 standard error come in to play? Well, think about @command{filter1} in
16470 the pipeline above. What happens if it encounters an error in the data it
16471 sees? If it writes an error message to standard output, it will just
16472 disappear down the pipeline into @command{filter2}'s input, and the
16473 user will probably never see it. So programs need a place where they can send
16474 error messages so that the user will notice them. This is standard error,
16475 and it is usually connected to your console or window, even if you have
16476 redirected standard output of your program away from your screen.
16478 For filter programs to work together, the format of the data has to be
16479 agreed upon. The most straightforward and easiest format to use is simply
16480 lines of text. Unix data files are generally just streams of bytes, with
16481 lines delimited by the @acronym{ASCII} @sc{lf} (Line Feed) character,
16482 conventionally called a ``newline'' in the Unix literature. (This is
16483 @code{'\n'} if you're a C programmer.) This is the format used by all
16484 the traditional filtering programs. (Many earlier operating systems
16485 had elaborate facilities and special purpose programs for managing
16486 binary data. Unix has always shied away from such things, under the
16487 philosophy that it's easiest to simply be able to view and edit your
16488 data with a text editor.)
16490 OK, enough introduction. Let's take a look at some of the tools, and then
16491 we'll see how to hook them together in interesting ways. In the following
16492 discussion, we will only present those command line options that interest
16493 us. As you should always do, double check your system documentation
16494 for the full story.
16496 @node The who command
16497 @unnumberedsec The @command{who} Command
16499 The first program is the @command{who} command. By itself, it generates a
16500 list of the users who are currently logged in. Although I'm writing
16501 this on a single-user system, we'll pretend that several people are
16506 @print{} arnold console Jan 22 19:57
16507 @print{} miriam ttyp0 Jan 23 14:19(:0.0)
16508 @print{} bill ttyp1 Jan 21 09:32(:0.0)
16509 @print{} arnold ttyp2 Jan 23 20:48(:0.0)
16512 Here, the @samp{$} is the usual shell prompt, at which I typed @samp{who}.
16513 There are three people logged in, and I am logged in twice. On traditional
16514 Unix systems, user names are never more than eight characters long. This
16515 little bit of trivia will be useful later. The output of @command{who} is nice,
16516 but the data is not all that exciting.
16518 @node The cut command
16519 @unnumberedsec The @command{cut} Command
16521 The next program we'll look at is the @command{cut} command. This program
16522 cuts out columns or fields of input data. For example, we can tell it
16523 to print just the login name and full name from the @file{/etc/passwd}
16524 file. The @file{/etc/passwd} file has seven fields, separated by
16528 arnold:xyzzy:2076:10:Arnold D. Robbins:/home/arnold:/bin/bash
16531 To get the first and fifth fields, we would use @command{cut} like this:
16534 $ cut -d: -f1,5 /etc/passwd
16535 @print{} root:Operator
16537 @print{} arnold:Arnold D. Robbins
16538 @print{} miriam:Miriam A. Robbins
16542 With the @option{-c} option, @command{cut} will cut out specific characters
16543 (i.e., columns) in the input lines. This is useful for input data
16544 that has fixed width fields, and does not have a field separator. For
16545 example, list the Monday dates for the current month:
16547 @c Is using cal ok? Looked at gcal, but I don't like it.
16558 @node The sort command
16559 @unnumberedsec The @command{sort} Command
16561 Next we'll look at the @command{sort} command. This is one of the most
16562 powerful commands on a Unix-style system; one that you will often find
16563 yourself using when setting up fancy data plumbing.
16566 command reads and sorts each file named on the command line. It then
16567 merges the sorted data and writes it to standard output. It will read
16568 standard input if no files are given on the command line (thus
16569 making it into a filter). The sort is based on the character collating
16570 sequence or based on user-supplied ordering criteria.
16573 @node The uniq command
16574 @unnumberedsec The @command{uniq} Command
16576 Finally (at least for now), we'll look at the @command{uniq} program. When
16577 sorting data, you will often end up with duplicate lines, lines that
16578 are identical. Usually, all you need is one instance of each line.
16579 This is where @command{uniq} comes in. The @command{uniq} program reads its
16580 standard input. It prints only one
16581 copy of each repeated line. It does have several options. Later on,
16582 we'll use the @option{-c} option, which prints each unique line, preceded
16583 by a count of the number of times that line occurred in the input.
16586 @node Putting the tools together
16587 @unnumberedsec Putting the Tools Together
16589 Now, let's suppose this is a large ISP server system with dozens of users
16590 logged in. The management wants the system administrator to write a
16592 generate a sorted list of logged in users. Furthermore, even if a user
16593 is logged in multiple times, his or her name should only show up in the
16596 The administrator could sit down with the system documentation and write a C
16597 program that did this. It would take perhaps a couple of hundred lines
16598 of code and about two hours to write it, test it, and debug it.
16599 However, knowing the software toolbox, the administrator can instead start out
16600 by generating just a list of logged on users:
16610 Next, sort the list:
16613 $ who | cut -c1-8 | sort
16620 Finally, run the sorted list through @command{uniq}, to weed out duplicates:
16623 $ who | cut -c1-8 | sort | uniq
16629 The @command{sort} command actually has a @option{-u} option that does what
16630 @command{uniq} does. However, @command{uniq} has other uses for which one
16631 cannot substitute @samp{sort -u}.
16633 The administrator puts this pipeline into a shell script, and makes it
16635 all the users on the system (@samp{#} is the system administrator,
16636 or @code{root}, prompt):
16639 # cat > /usr/local/bin/listusers
16640 who | cut -c1-8 | sort | uniq
16642 # chmod +x /usr/local/bin/listusers
16645 There are four major points to note here. First, with just four
16646 programs, on one command line, the administrator was able to save about two
16647 hours worth of work. Furthermore, the shell pipeline is just about as
16648 efficient as the C program would be, and it is much more efficient in
16649 terms of programmer time. People time is much more expensive than
16650 computer time, and in our modern ``there's never enough time to do
16651 everything'' society, saving two hours of programmer time is no mean
16654 Second, it is also important to emphasize that with the
16655 @emph{combination} of the tools, it is possible to do a special
16656 purpose job never imagined by the authors of the individual programs.
16658 Third, it is also valuable to build up your pipeline in stages, as we did here.
16659 This allows you to view the data at each stage in the pipeline, which helps
16660 you acquire the confidence that you are indeed using these tools correctly.
16662 Finally, by bundling the pipeline in a shell script, other users can use
16663 your command, without having to remember the fancy plumbing you set up for
16664 them. In terms of how you run them, shell scripts and compiled programs are
16667 After the previous warm-up exercise, we'll look at two additional, more
16668 complicated pipelines. For them, we need to introduce two more tools.
16670 The first is the @command{tr} command, which stands for ``transliterate.''
16671 The @command{tr} command works on a character-by-character basis, changing
16672 characters. Normally it is used for things like mapping upper case to
16676 $ echo ThIs ExAmPlE HaS MIXED case! | tr '[:upper:]' '[:lower:]'
16677 @print{} this example has mixed case!
16680 There are several options of interest:
16684 work on the complement of the listed characters, i.e.,
16685 operations apply to characters not in the given set
16688 delete characters in the first set from the output
16691 squeeze repeated characters in the output into just one character.
16694 We will be using all three options in a moment.
16696 The other command we'll look at is @command{comm}. The @command{comm}
16697 command takes two sorted input files as input data, and prints out the
16698 files' lines in three columns. The output columns are the data lines
16699 unique to the first file, the data lines unique to the second file, and
16700 the data lines that are common to both. The @option{-1}, @option{-2}, and
16701 @option{-3} command line options @emph{omit} the respective columns. (This is
16702 non-intuitive and takes a little getting used to.) For example:
16724 The file name @file{-} tells @command{comm} to read standard input
16725 instead of a regular file.
16727 Now we're ready to build a fancy pipeline. The first application is a word
16728 frequency counter. This helps an author determine if he or she is over-using
16731 The first step is to change the case of all the letters in our input file
16732 to one case. ``The'' and ``the'' are the same word when doing counting.
16735 $ tr '[:upper:]' '[:lower:]' < whats.gnu | ...
16738 The next step is to get rid of punctuation. Quoted words and unquoted words
16739 should be treated identically; it's easiest to just get the punctuation out of
16743 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | ...
16746 The second @command{tr} command operates on the complement of the listed
16747 characters, which are all the letters, the digits, the underscore, and
16748 the blank. The @samp{\n} represents the newline character; it has to
16749 be left alone. (The @acronym{ASCII} tab character should also be included for
16750 good measure in a production script.)
16752 At this point, we have data consisting of words separated by blank space.
16753 The words only contain alphanumeric characters (and the underscore). The
16754 next step is break the data apart so that we have one word per line. This
16755 makes the counting operation much easier, as we will see shortly.
16758 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16759 > tr -s ' ' '\n' | ...
16762 This command turns blanks into newlines. The @option{-s} option squeezes
16763 multiple newline characters in the output into just one. This helps us
16764 avoid blank lines. (The @samp{>} is the shell's ``secondary prompt.''
16765 This is what the shell prints when it notices you haven't finished
16766 typing in all of a command.)
16768 We now have data consisting of one word per line, no punctuation, all one
16769 case. We're ready to count each word:
16772 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16773 > tr -s ' ' '\n' | sort | uniq -c | ...
16776 At this point, the data might look something like this:
16789 The output is sorted by word, not by count! What we want is the most
16790 frequently used words first. Fortunately, this is easy to accomplish,
16791 with the help of two more @command{sort} options:
16795 do a numeric sort, not a textual one
16798 reverse the order of the sort
16801 The final pipeline looks like this:
16804 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16805 > tr -s ' ' '\n' | sort | uniq -c | sort -n -r
16814 Whew! That's a lot to digest. Yet, the same principles apply. With six
16815 commands, on two lines (really one long one split for convenience), we've
16816 created a program that does something interesting and useful, in much
16817 less time than we could have written a C program to do the same thing.
16819 A minor modification to the above pipeline can give us a simple spelling
16820 checker! To determine if you've spelled a word correctly, all you have to
16821 do is look it up in a dictionary. If it is not there, then chances are
16822 that your spelling is incorrect. So, we need a dictionary.
16823 The conventional location for a dictionary is @file{/usr/dict/words}.
16824 On my GNU/Linux system,@footnote{Redhat Linux 6.1, for the November 2000
16825 revision of this article.}
16826 this is a sorted, 45,402 word dictionary.
16828 Now, how to compare our file with the dictionary? As before, we generate
16829 a sorted list of words, one per line:
16832 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16833 > tr -s ' ' '\n' | sort -u | ...
16836 Now, all we need is a list of words that are @emph{not} in the
16837 dictionary. Here is where the @command{comm} command comes in.
16840 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16841 > tr -s ' ' '\n' | sort -u |
16842 > comm -23 - /usr/dict/words
16845 The @option{-2} and @option{-3} options eliminate lines that are only in the
16846 dictionary (the second file), and lines that are in both files. Lines
16847 only in the first file (standard input, our stream of words), are
16848 words that are not in the dictionary. These are likely candidates for
16849 spelling errors. This pipeline was the first cut at a production
16850 spelling checker on Unix.
16852 There are some other tools that deserve brief mention.
16856 search files for text that matches a regular expression
16859 count lines, words, characters
16862 a T-fitting for data pipes, copies data to files and to standard output
16865 the stream editor, an advanced tool
16868 a data manipulation language, another advanced tool
16871 The software tools philosophy also espoused the following bit of
16872 advice: ``Let someone else do the hard part.'' This means, take
16873 something that gives you most of what you need, and then massage it the
16874 rest of the way until it's in the form that you want.
16880 Each program should do one thing well. No more, no less.
16883 Combining programs with appropriate plumbing leads to results where
16884 the whole is greater than the sum of the parts. It also leads to novel
16885 uses of programs that the authors might never have imagined.
16888 Programs should never print extraneous header or trailer data, since these
16889 could get sent on down a pipeline. (A point we didn't mention earlier.)
16892 Let someone else do the hard part.
16895 Know your toolbox! Use each program appropriately. If you don't have an
16896 appropriate tool, build one.
16899 As of this writing, all the programs we've discussed are available via
16900 anonymous @command{ftp} from: @*
16901 @uref{ftp://gnudist.gnu.org/textutils/textutils-1.22.tar.gz}. (There may
16902 be more recent versions available now.)
16904 None of what I have presented in this column is new. The Software Tools
16905 philosophy was first introduced in the book @cite{Software Tools}, by
16906 Brian Kernighan and P.J. Plauger (Addison-Wesley, ISBN 0-201-03669-X).
16907 This book showed how to write and use software tools. It was written in
16908 1976, using a preprocessor for FORTRAN named @command{ratfor} (RATional
16909 FORtran). At the time, C was not as ubiquitous as it is now; FORTRAN
16910 was. The last chapter presented a @command{ratfor} to FORTRAN
16911 processor, written in @command{ratfor}. @command{ratfor} looks an awful
16912 lot like C; if you know C, you won't have any problem following the
16915 In 1981, the book was updated and made available as @cite{Software Tools
16916 in Pascal} (Addison-Wesley, ISBN 0-201-10342-7). Both books are
16917 still in print and are well worth
16918 reading if you're a programmer. They certainly made a major change in
16919 how I view programming.
16921 The programs in both books are available from
16922 @uref{http://cm.bell-labs.com/who/bwk, Brian Kernighan's home page}.
16923 For a number of years, there was an active
16924 Software Tools Users Group, whose members had ported the original
16925 @command{ratfor} programs to essentially every computer system with a
16926 FORTRAN compiler. The popularity of the group waned in the middle 1980s
16927 as Unix began to spread beyond universities.
16929 With the current proliferation of GNU code and other clones of Unix programs,
16930 these programs now receive little attention; modern C versions are
16931 much more efficient and do more than these programs do. Nevertheless, as
16932 exposition of good programming style, and evangelism for a still-valuable
16933 philosophy, these books are unparalleled, and I recommend them highly.
16935 Acknowledgment: I would like to express my gratitude to Brian Kernighan
16936 of Bell Labs, the original Software Toolsmith, for reviewing this column.
16938 @node GNU Free Documentation License
16939 @appendix GNU Free Documentation License
16943 @node Concept index
16950 @c Local variables:
16951 @c texinfo-column-for-description: 32