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 * rm: (coreutils)rm invocation. Remove files.
98 * rmdir: (coreutils)rmdir invocation. Remove empty directories.
99 * runcon: (coreutils)runcon invocation. Run in specified SELinux CTX.
100 * seq: (coreutils)seq invocation. Print numeric sequences
101 * sha1sum: (coreutils)sha1sum invocation. Print or check SHA-1 digests.
102 * sha2: (coreutils)sha2 utilities. Print or check SHA-2 digests.
103 * shred: (coreutils)shred invocation. Remove files more securely.
104 * shuf: (coreutils)shuf invocation. Shuffling text files.
105 * sleep: (coreutils)sleep invocation. Delay for a specified time.
106 * sort: (coreutils)sort invocation. Sort text files.
107 * split: (coreutils)split invocation. Split into pieces.
108 * stat: (coreutils)stat invocation. Report file(system) status.
109 * stdbuf: (coreutils)stdbuf invocation. Modify stdio buffering.
110 * stty: (coreutils)stty invocation. Print/change terminal settings.
111 * su: (coreutils)su invocation. Modify user and group ID.
112 * sum: (coreutils)sum invocation. Print traditional checksum.
113 * sync: (coreutils)sync invocation. Synchronize memory and disk.
114 * tac: (coreutils)tac invocation. Reverse files.
115 * tail: (coreutils)tail invocation. Output the last part of files.
116 * tee: (coreutils)tee invocation. Redirect to multiple files.
117 * test: (coreutils)test invocation. File/string tests.
118 * timeout: (coreutils)timeout invocation. Run with time limit.
119 * touch: (coreutils)touch invocation. Change file timestamps.
120 * tr: (coreutils)tr invocation. Translate characters.
121 * true: (coreutils)true invocation. Do nothing, successfully.
122 * truncate: (coreutils)truncate invocation. Shrink/extend size of a file.
123 * tsort: (coreutils)tsort invocation. Topological sort.
124 * tty: (coreutils)tty invocation. Print terminal name.
125 * uname: (coreutils)uname invocation. Print system information.
126 * unexpand: (coreutils)unexpand invocation. Convert spaces to tabs.
127 * uniq: (coreutils)uniq invocation. Uniquify files.
128 * unlink: (coreutils)unlink invocation. Removal via unlink(2).
129 * uptime: (coreutils)uptime invocation. Print uptime and load.
130 * users: (coreutils)users invocation. Print current user names.
131 * vdir: (coreutils)vdir invocation. List directories verbosely.
132 * wc: (coreutils)wc invocation. Line, word, and byte counts.
133 * who: (coreutils)who invocation. Print who is logged in.
134 * whoami: (coreutils)whoami invocation. Print effective user ID.
135 * yes: (coreutils)yes invocation. Print a string indefinitely.
139 This manual documents version @value{VERSION} of the @sc{gnu} core
140 utilities, including the standard programs for text and file manipulation.
142 Copyright @copyright{} 1994-1996, 2000-2011 Free Software Foundation, Inc.
145 Permission is granted to copy, distribute and/or modify this document
146 under the terms of the GNU Free Documentation License, Version 1.3 or
147 any later version published by the Free Software Foundation; with no
148 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
149 Texts. A copy of the license is included in the section entitled ``GNU
150 Free Documentation License''.
155 @title @sc{gnu} @code{Coreutils}
156 @subtitle Core GNU utilities
157 @subtitle for version @value{VERSION}, @value{UPDATED}
158 @author David MacKenzie et al.
161 @vskip 0pt plus 1filll
174 @cindex core utilities
175 @cindex text utilities
176 @cindex shell utilities
177 @cindex file utilities
180 * Introduction:: Caveats, overview, and authors
181 * Common options:: Common options
182 * Output of entire files:: cat tac nl od base64
183 * Formatting file contents:: fmt pr fold
184 * Output of parts of files:: head tail split csplit
185 * Summarizing files:: wc sum cksum md5sum sha1sum sha2
186 * Operating on sorted files:: sort shuf uniq comm ptx tsort
187 * Operating on fields:: cut paste join
188 * Operating on characters:: tr expand unexpand
189 * Directory listing:: ls dir vdir dircolors
190 * Basic operations:: cp dd install mv rm shred
191 * Special file types:: mkdir rmdir unlink mkfifo mknod ln link readlink
192 * Changing file attributes:: chgrp chmod chown touch
193 * Disk usage:: df du stat sync truncate
194 * Printing text:: echo printf yes
195 * Conditions:: false true test expr
197 * File name manipulation:: dirname basename pathchk mktemp
198 * Working context:: pwd stty printenv tty
199 * User information:: id logname whoami groups users who
200 * System context:: date arch nproc uname hostname hostid uptime
201 * SELinux context:: chcon runcon
202 * Modified command invocation:: chroot env nice nohup stdbuf su timeout
203 * Process control:: kill
205 * Numeric operations:: factor seq
206 * File permissions:: Access modes
207 * Date input formats:: Specifying date strings
208 * Opening the software toolbox:: The software tools philosophy
209 * GNU Free Documentation License:: Copying and sharing this manual
210 * Concept index:: General index
213 --- The Detailed Node Listing ---
217 * Exit status:: Indicating program success or failure
218 * Backup options:: Backup options
219 * Block size:: Block size
220 * Floating point:: Floating point number representation
221 * Signal specifications:: Specifying signals
222 * Disambiguating names and IDs:: chgrp and chown owner and group syntax
223 * Random sources:: Sources of random data
224 * Target directory:: Target directory
225 * Trailing slashes:: Trailing slashes
226 * Traversing symlinks:: Traversing symlinks to directories
227 * Treating / specially:: Treating / specially
228 * Standards conformance:: Standards conformance
230 Output of entire files
232 * cat invocation:: Concatenate and write files
233 * tac invocation:: Concatenate and write files in reverse
234 * nl invocation:: Number lines and write files
235 * od invocation:: Write files in octal or other formats
236 * base64 invocation:: Transform data into printable data
238 Formatting file contents
240 * fmt invocation:: Reformat paragraph text
241 * pr invocation:: Paginate or columnate files for printing
242 * fold invocation:: Wrap input lines to fit in specified width
244 Output of parts of files
246 * head invocation:: Output the first part of files
247 * tail invocation:: Output the last part of files
248 * split invocation:: Split a file into fixed-size pieces
249 * csplit invocation:: Split a file into context-determined pieces
253 * wc invocation:: Print newline, word, and byte counts
254 * sum invocation:: Print checksum and block counts
255 * cksum invocation:: Print CRC checksum and byte counts
256 * md5sum invocation:: Print or check MD5 digests
257 * sha1sum invocation:: Print or check SHA-1 digests
258 * sha2 utilities:: Print or check SHA-2 digests
260 Operating on sorted files
262 * sort invocation:: Sort text files
263 * shuf invocation:: Shuffle text files
264 * uniq invocation:: Uniquify files
265 * comm invocation:: Compare two sorted files line by line
266 * ptx invocation:: Produce a permuted index of file contents
267 * tsort invocation:: Topological sort
269 @command{ptx}: Produce permuted indexes
271 * General options in ptx:: Options which affect general program behavior
272 * Charset selection in ptx:: Underlying character set considerations
273 * Input processing in ptx:: Input fields, contexts, and keyword selection
274 * Output formatting in ptx:: Types of output format, and sizing the fields
275 * Compatibility in ptx:: The @acronym{GNU} extensions to @command{ptx}
279 * cut invocation:: Print selected parts of lines
280 * paste invocation:: Merge lines of files
281 * join invocation:: Join lines on a common field
283 Operating on characters
285 * tr invocation:: Translate, squeeze, and/or delete characters
286 * expand invocation:: Convert tabs to spaces
287 * unexpand invocation:: Convert spaces to tabs
289 @command{tr}: Translate, squeeze, and/or delete characters
291 * Character sets:: Specifying sets of characters
292 * Translating:: Changing one set of characters to another
293 * Squeezing:: Squeezing repeats and deleting
297 * ls invocation:: List directory contents
298 * dir invocation:: Briefly list directory contents
299 * vdir invocation:: Verbosely list directory contents
300 * dircolors invocation:: Color setup for @command{ls}
302 @command{ls}: List directory contents
304 * Which files are listed:: Which files are listed
305 * What information is listed:: What information is listed
306 * Sorting the output:: Sorting the output
307 * Details about version sort:: More details about version sort
308 * General output formatting:: General output formatting
309 * Formatting the file names:: Formatting the file names
313 * cp invocation:: Copy files and directories
314 * dd invocation:: Convert and copy a file
315 * install invocation:: Copy files and set attributes
316 * mv invocation:: Move (rename) files
317 * rm invocation:: Remove files or directories
318 * shred invocation:: Remove files more securely
322 * link invocation:: Make a hard link via the link syscall
323 * ln invocation:: Make links between files
324 * mkdir invocation:: Make directories
325 * mkfifo invocation:: Make FIFOs (named pipes)
326 * mknod invocation:: Make block or character special files
327 * readlink invocation:: Print value of a symlink or canonical file name
328 * rmdir invocation:: Remove empty directories
329 * unlink invocation:: Remove files via unlink syscall
331 Changing file attributes
333 * chown invocation:: Change file owner and group
334 * chgrp invocation:: Change group ownership
335 * chmod invocation:: Change access permissions
336 * touch invocation:: Change file timestamps
340 * df invocation:: Report file system disk space usage
341 * du invocation:: Estimate file space usage
342 * stat invocation:: Report file or file system status
343 * sync invocation:: Synchronize data on disk with memory
344 * truncate invocation:: Shrink or extend the size of a file
348 * echo invocation:: Print a line of text
349 * printf invocation:: Format and print data
350 * yes invocation:: Print a string until interrupted
354 * false invocation:: Do nothing, unsuccessfully
355 * true invocation:: Do nothing, successfully
356 * test invocation:: Check file types and compare values
357 * expr invocation:: Evaluate expressions
359 @command{test}: Check file types and compare values
361 * File type tests:: File type tests
362 * Access permission tests:: Access permission tests
363 * File characteristic tests:: File characteristic tests
364 * String tests:: String tests
365 * Numeric tests:: Numeric tests
367 @command{expr}: Evaluate expression
369 * String expressions:: + : match substr index length
370 * Numeric expressions:: + - * / %
371 * Relations for expr:: | & < <= = == != >= >
372 * Examples of expr:: Examples of using @command{expr}
376 * tee invocation:: Redirect output to multiple files or processes
378 File name manipulation
380 * basename invocation:: Strip directory and suffix from a file name
381 * dirname invocation:: Strip last file name component
382 * pathchk invocation:: Check file name validity and portability
383 * mktemp invocation:: Create temporary file or directory
387 * pwd invocation:: Print working directory
388 * stty invocation:: Print or change terminal characteristics
389 * printenv invocation:: Print all or some environment variables
390 * tty invocation:: Print file name of terminal on standard input
392 @command{stty}: Print or change terminal characteristics
394 * Control:: Control settings
395 * Input:: Input settings
396 * Output:: Output settings
397 * Local:: Local settings
398 * Combination:: Combination settings
399 * Characters:: Special characters
400 * Special:: Special settings
404 * id invocation:: Print user identity
405 * logname invocation:: Print current login name
406 * whoami invocation:: Print effective user ID
407 * groups invocation:: Print group names a user is in
408 * users invocation:: Print login names of users currently logged in
409 * who invocation:: Print who is currently logged in
413 * arch invocation:: Print machine hardware name
414 * date invocation:: Print or set system date and time
415 * nproc invocation:: Print the number of processors
416 * uname invocation:: Print system information
417 * hostname invocation:: Print or set system name
418 * hostid invocation:: Print numeric host identifier
419 * uptime invocation:: Print system uptime and load
421 @command{date}: Print or set system date and time
423 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
424 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
425 * Literal conversion specifiers:: %[%nt]
426 * Padding and other flags:: Pad with zeros, spaces, etc.
427 * Setting the time:: Changing the system clock
428 * Options for date:: Instead of the current time
429 * Date input formats:: Specifying date strings
430 * Examples of date:: Examples
434 * chcon invocation:: Change SELinux context of file
435 * runcon invocation:: Run a command in specified SELinux context
437 Modified command invocation
439 * chroot invocation:: Run a command with a different root directory
440 * env invocation:: Run a command in a modified environment
441 * nice invocation:: Run a command with modified niceness
442 * nohup invocation:: Run a command immune to hangups
443 * stdbuf invocation:: Run a command with modified I/O buffering
444 * su invocation:: Run a command with substitute user and group ID
445 * timeout invocation:: Run a command with a time limit
449 * kill invocation:: Sending a signal to processes.
453 * sleep invocation:: Delay for a specified time
457 * factor invocation:: Print prime factors
458 * seq invocation:: Print numeric sequences
462 * Mode Structure:: Structure of file mode bits
463 * Symbolic Modes:: Mnemonic representation of file mode bits
464 * Numeric Modes:: File mode bits as octal numbers
465 * Directory Setuid and Setgid:: Set-user-ID and set-group-ID on directories
469 * General date syntax:: Common rules
470 * Calendar date items:: 19 Dec 1994
471 * Time of day items:: 9:20pm
472 * Time zone items:: @sc{est}, @sc{pdt}, @sc{gmt}
473 * Day of week items:: Monday and others
474 * Relative items in date strings:: next tuesday, 2 years ago
475 * Pure numbers in date strings:: 19931219, 1440
476 * Seconds since the Epoch:: @@1078100502
477 * Specifying time zone rules:: TZ="America/New_York", TZ="UTC0"
478 * Authors of parse_datetime:: Bellovin, Eggert, Salz, Berets, et al
480 Opening the software toolbox
482 * Toolbox introduction:: Toolbox introduction
483 * I/O redirection:: I/O redirection
484 * The who command:: The @command{who} command
485 * The cut command:: The @command{cut} command
486 * The sort command:: The @command{sort} command
487 * The uniq command:: The @command{uniq} command
488 * Putting the tools together:: Putting the tools together
492 * GNU Free Documentation License:: Copying and sharing this manual
499 @chapter Introduction
501 This manual is a work in progress: many sections make no attempt to explain
502 basic concepts in a way suitable for novices. Thus, if you are interested,
503 please get involved in improving this manual. The entire @sc{gnu} community
506 @cindex @acronym{POSIX}
507 The @sc{gnu} utilities documented here are mostly compatible with the
508 @acronym{POSIX} standard.
509 @cindex bugs, reporting
510 Please report bugs to @email{bug-coreutils@@gnu.org}. Remember
511 to include the version number, machine architecture, input files, and
512 any other information needed to reproduce the bug: your input, what you
513 expected, what you got, and why it is wrong. Diffs are welcome, but
514 please include a description of the problem as well, since this is
515 sometimes difficult to infer. @xref{Bugs, , , gcc, Using and Porting GNU CC}.
521 @cindex MacKenzie, D.
524 This manual was originally derived from the Unix man pages in the
525 distributions, which were written by David MacKenzie and updated by Jim
526 Meyering. What you are reading now is the authoritative documentation
527 for these utilities; the man pages are no longer being maintained. The
528 original @command{fmt} man page was written by Ross Paterson. Fran@,{c}ois
529 Pinard did the initial conversion to Texinfo format. Karl Berry did the
530 indexing, some reorganization, and editing of the results. Brian
531 Youmans of the Free Software Foundation office staff combined the
532 manuals for textutils, fileutils, and sh-utils to produce the present
533 omnibus manual. Richard Stallman contributed his usual invaluable
534 insights to the overall process.
537 @chapter Common options
541 @itemx @w{@kbd{--backup}[=@var{method}]}
544 @vindex VERSION_CONTROL
545 @cindex backups, making
546 @xref{Backup options}.
547 Make a backup of each file that would otherwise be overwritten or removed.
550 @macro optBackupSuffix
551 @item -S @var{suffix}
552 @itemx --suffix=@var{suffix}
555 Append @var{suffix} to each backup file made with @option{-b}.
556 @xref{Backup options}.
559 @macro optTargetDirectory
560 @item -t @var{directory}
561 @itemx @w{@kbd{--target-directory}=@var{directory}}
563 @opindex --target-directory
564 @cindex target directory
565 @cindex destination directory
566 Specify the destination @var{directory}.
567 @xref{Target directory}.
570 @macro optNoTargetDirectory
572 @itemx --no-target-directory
574 @opindex --no-target-directory
575 @cindex target directory
576 @cindex destination directory
577 Do not treat the last operand specially when it is a directory or a
578 symbolic link to a directory. @xref{Target directory}.
586 @cindex output @sc{nul}-byte-terminated lines
587 Output a zero byte (@acronym{ASCII} @sc{nul}) at the end of each line,
588 rather than a newline. This option enables other programs to parse the
589 output of @command{\cmd\} even when that output would contain data
590 with embedded newlines.
597 Append an SI-style abbreviation to each size, such as @samp{M} for
598 megabytes. Powers of 1000 are used, not 1024; @samp{M} stands for
599 1,000,000 bytes. This option is equivalent to
600 @option{--block-size=si}. Use the @option{-h} or
601 @option{--human-readable} option if
602 you prefer powers of 1024.
605 @macro optHumanReadable
607 @itemx --human-readable
609 @opindex --human-readable
610 @cindex human-readable output
611 Append a size letter to each size, such as @samp{M} for mebibytes.
612 Powers of 1024 are used, not 1000; @samp{M} stands for 1,048,576 bytes.
613 This option is equivalent to @option{--block-size=human-readable}.
614 Use the @option{--si} option if you prefer powers of 1000.
617 @macro optStripTrailingSlashes
618 @itemx @w{@kbd{--strip-trailing-slashes}}
619 @opindex --strip-trailing-slashes
620 @cindex stripping trailing slashes
621 Remove any trailing slashes from each @var{source} argument.
622 @xref{Trailing slashes}.
625 @macro mayConflictWithShellBuiltIn{cmd}
626 @cindex conflicts with shell built-ins
627 @cindex built-in shell commands, conflicts with
628 Due to shell aliases and built-in @command{\cmd\} functions, using an
629 unadorned @command{\cmd\} interactively or in a script may get you
630 different functionality than that described here. Invoke it via
631 @command{env} (i.e., @code{env \cmd\ @dots{}}) to avoid interference
636 @macro multiplierSuffixes{varName}
637 @var{\varName\} may be, or may be an integer optionally followed by,
638 one of the following multiplicative suffixes:
640 @samp{b} => 512 ("blocks")
641 @samp{KB} => 1000 (KiloBytes)
642 @samp{K} => 1024 (KibiBytes)
643 @samp{MB} => 1000*1000 (MegaBytes)
644 @samp{M} => 1024*1024 (MebiBytes)
645 @samp{GB} => 1000*1000*1000 (GigaBytes)
646 @samp{G} => 1024*1024*1024 (GibiBytes)
648 and so on for @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}.
651 @c FIXME: same as above, but no ``blocks'' line.
652 @macro multiplierSuffixesNoBlocks{varName}
653 @var{\varName\} may be, or may be an integer optionally followed by,
654 one of the following multiplicative suffixes:
656 @samp{KB} => 1000 (KiloBytes)
657 @samp{K} => 1024 (KibiBytes)
658 @samp{MB} => 1000*1000 (MegaBytes)
659 @samp{M} => 1024*1024 (MebiBytes)
660 @samp{GB} => 1000*1000*1000 (GigaBytes)
661 @samp{G} => 1024*1024*1024 (GibiBytes)
663 and so on for @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}.
666 @cindex common options
668 Certain options are available in all of these programs. Rather than
669 writing identical descriptions for each of the programs, they are
670 described here. (In fact, every @sc{gnu} program accepts (or should accept)
673 @vindex POSIXLY_CORRECT
674 Normally options and operands can appear in any order, and programs act
675 as if all the options appear before any operands. For example,
676 @samp{sort -r passwd -t :} acts like @samp{sort -r -t : passwd}, since
677 @samp{:} is an option-argument of @option{-t}. However, if the
678 @env{POSIXLY_CORRECT} environment variable is set, options must appear
679 before operands, unless otherwise specified for a particular command.
681 A few programs can usefully have trailing operands with leading
682 @samp{-}. With such a program, options must precede operands even if
683 @env{POSIXLY_CORRECT} is not set, and this fact is noted in the
684 program description. For example, the @command{env} command's options
685 must appear before its operands, since in some cases the operands
686 specify a command that itself contains options.
688 Most programs that accept long options recognize unambiguous
689 abbreviations of those options. For example, @samp{rmdir
690 --ignore-fail-on-non-empty} can be invoked as @samp{rmdir
691 --ignore-fail} or even @samp{rmdir --i}. Ambiguous options, such as
692 @samp{ls --h}, are identified as such.
694 Some of these programs recognize the @option{--help} and @option{--version}
695 options only when one of them is the sole command line argument. For
696 these programs, abbreviations of the long options are not always recognized.
703 Print a usage message listing all available options, then exit successfully.
707 @cindex version number, finding
708 Print the version number, then exit successfully.
712 @cindex option delimiter
713 Delimit the option list. Later arguments, if any, are treated as
714 operands even if they begin with @samp{-}. For example, @samp{sort --
715 -r} reads from the file named @file{-r}.
719 @cindex standard input
720 @cindex standard output
721 A single @samp{-} operand is not really an option, though it looks like one. It
722 stands for standard input, or for standard output if that is clear from
723 the context. For example, @samp{sort -} reads from standard input,
724 and is equivalent to plain @samp{sort}, and @samp{tee -} writes an
725 extra copy of its input to standard output. Unless otherwise
726 specified, @samp{-} can appear as any operand that requires a file
730 * Exit status:: Indicating program success or failure.
731 * Backup options:: -b -S, in some programs.
732 * Block size:: BLOCK_SIZE and --block-size, in some programs.
733 * Floating point:: Floating point number representation.
734 * Signal specifications:: Specifying signals using the --signal option.
735 * Disambiguating names and IDs:: chgrp and chown owner and group syntax
736 * Random sources:: --random-source, in some programs.
737 * Target directory:: Specifying a target directory, in some programs.
738 * Trailing slashes:: --strip-trailing-slashes, in some programs.
739 * Traversing symlinks:: -H, -L, or -P, in some programs.
740 * Treating / specially:: --preserve-root and --no-preserve-root.
741 * Special built-in utilities:: @command{break}, @command{:}, @dots{}
742 * Standards conformance:: Conformance to the @acronym{POSIX} standard.
750 An exit status of zero indicates success,
751 and a nonzero value indicates failure.
754 Nearly every command invocation yields an integral @dfn{exit status}
755 that can be used to change how other commands work.
756 For the vast majority of commands, an exit status of zero indicates
757 success. Failure is indicated by a nonzero value---typically
758 @samp{1}, though it may differ on unusual platforms as @acronym{POSIX}
759 requires only that it be nonzero.
761 However, some of the programs documented here do produce
762 other exit status values and a few associate different
763 meanings with the values @samp{0} and @samp{1}.
764 Here are some of the exceptions:
765 @command{chroot}, @command{env}, @command{expr}, @command{nice},
766 @command{nohup}, @command{printenv}, @command{sort}, @command{stdbuf},
767 @command{su}, @command{test}, @command{timeout}, @command{tty}.
771 @section Backup options
773 @cindex backup options
775 Some @sc{gnu} programs (at least @command{cp}, @command{install},
776 @command{ln}, and @command{mv}) optionally make backups of files
777 before writing new versions.
778 These options control the details of these backups. The options are also
779 briefly mentioned in the descriptions of the particular programs.
784 @itemx @w{@kbd{--backup}[=@var{method}]}
787 @vindex VERSION_CONTROL
788 @cindex backups, making
789 Make a backup of each file that would otherwise be overwritten or removed.
790 Without this option, the original versions are destroyed.
791 Use @var{method} to determine the type of backups to make.
792 When this option is used but @var{method} is not specified,
793 then the value of the @env{VERSION_CONTROL}
794 environment variable is used. And if @env{VERSION_CONTROL} is not set,
795 the default backup type is @samp{existing}.
797 Note that the short form of this option, @option{-b} does not accept any
798 argument. Using @option{-b} is equivalent to using @option{--backup=existing}.
800 @vindex version-control @r{Emacs variable}
801 This option corresponds to the Emacs variable @samp{version-control};
802 the values for @var{method} are the same as those used in Emacs.
803 This option also accepts more descriptive names.
804 The valid @var{method}s are (unique abbreviations are accepted):
809 @opindex none @r{backup method}
814 @opindex numbered @r{backup method}
815 Always make numbered backups.
819 @opindex existing @r{backup method}
820 Make numbered backups of files that already have them, simple backups
825 @opindex simple @r{backup method}
826 Always make simple backups. Please note @samp{never} is not to be
827 confused with @samp{none}.
831 @item -S @var{suffix}
832 @itemx --suffix=@var{suffix}
835 @cindex backup suffix
836 @vindex SIMPLE_BACKUP_SUFFIX
837 Append @var{suffix} to each backup file made with @option{-b}. If this
838 option is not specified, the value of the @env{SIMPLE_BACKUP_SUFFIX}
839 environment variable is used. And if @env{SIMPLE_BACKUP_SUFFIX} is not
840 set, the default is @samp{~}, just as in Emacs.
849 Some @sc{gnu} programs (at least @command{df}, @command{du}, and
850 @command{ls}) display sizes in ``blocks''. You can adjust the block size
851 and method of display to make sizes easier to read. The block size
852 used for display is independent of any file system block size.
853 Fractional block counts are rounded up to the nearest integer.
855 @opindex --block-size=@var{size}
858 @vindex DF_BLOCK_SIZE
859 @vindex DU_BLOCK_SIZE
860 @vindex LS_BLOCK_SIZE
861 @vindex POSIXLY_CORRECT@r{, and block size}
863 The default block size is chosen by examining the following environment
864 variables in turn; the first one that is set determines the block size.
869 This specifies the default block size for the @command{df} command.
870 Similarly, @env{DU_BLOCK_SIZE} specifies the default for @command{du} and
871 @env{LS_BLOCK_SIZE} for @command{ls}.
874 This specifies the default block size for all three commands, if the
875 above command-specific environment variables are not set.
878 This specifies the default block size for all values that are normally
879 printed as blocks, if neither @env{BLOCK_SIZE} nor the above
880 command-specific environment variables are set. Unlike the other
881 environment variables, @env{BLOCKSIZE} does not affect values that are
882 normally printed as byte counts, e.g., the file sizes contained in
885 @item POSIXLY_CORRECT
886 If neither @env{@var{command}_BLOCK_SIZE}, nor @env{BLOCK_SIZE}, nor
887 @env{BLOCKSIZE} is set, but this variable is set, the block size
892 If none of the above environment variables are set, the block size
893 currently defaults to 1024 bytes in most contexts, but this number may
894 change in the future. For @command{ls} file sizes, the block size
897 @cindex human-readable output
900 A block size specification can be a positive integer specifying the number
901 of bytes per block, or it can be @code{human-readable} or @code{si} to
902 select a human-readable format. Integers may be followed by suffixes
903 that are upward compatible with the
904 @uref{http://www.bipm.fr/enus/3_SI/si-prefixes.html, SI prefixes}
905 for decimal multiples and with the
906 @uref{http://physics.nist.gov/cuu/Units/binary.html, IEC 60027-2
907 prefixes for binary multiples}.
909 With human-readable formats, output sizes are followed by a size letter
910 such as @samp{M} for megabytes. @code{BLOCK_SIZE=human-readable} uses
911 powers of 1024; @samp{M} stands for 1,048,576 bytes.
912 @code{BLOCK_SIZE=si} is similar, but uses powers of 1000 and appends
913 @samp{B}; @samp{MB} stands for 1,000,000 bytes.
916 A block size specification preceded by @samp{'} causes output sizes to
917 be displayed with thousands separators. The @env{LC_NUMERIC} locale
918 specifies the thousands separator and grouping. For example, in an
919 American English locale, @samp{--block-size="'1kB"} would cause a size
920 of 1234000 bytes to be displayed as @samp{1,234}. In the default C
921 locale, there is no thousands separator so a leading @samp{'} has no
924 An integer block size can be followed by a suffix to specify a
925 multiple of that size. A bare size letter,
926 or one followed by @samp{iB}, specifies
927 a multiple using powers of 1024. A size letter followed by @samp{B}
928 specifies powers of 1000 instead. For example, @samp{1M} and
929 @samp{1MiB} are equivalent to @samp{1048576}, whereas @samp{1MB} is
930 equivalent to @samp{1000000}.
932 A plain suffix without a preceding integer acts as if @samp{1} were
933 prepended, except that it causes a size indication to be appended to
934 the output. For example, @samp{--block-size="kB"} displays 3000 as
937 The following suffixes are defined. Large sizes like @code{1Y}
938 may be rejected by your computer due to limitations of its arithmetic.
942 @cindex kilobyte, definition of
943 kilobyte: @math{10^3 = 1000}.
947 @cindex kibibyte, definition of
948 kibibyte: @math{2^{10} = 1024}. @samp{K} is special: the SI prefix is
949 @samp{k} and the IEC 60027-2 prefix is @samp{Ki}, but tradition and
950 @acronym{POSIX} use @samp{k} to mean @samp{KiB}.
952 @cindex megabyte, definition of
953 megabyte: @math{10^6 = 1,000,000}.
956 @cindex mebibyte, definition of
957 mebibyte: @math{2^{20} = 1,048,576}.
959 @cindex gigabyte, definition of
960 gigabyte: @math{10^9 = 1,000,000,000}.
963 @cindex gibibyte, definition of
964 gibibyte: @math{2^{30} = 1,073,741,824}.
966 @cindex terabyte, definition of
967 terabyte: @math{10^{12} = 1,000,000,000,000}.
970 @cindex tebibyte, definition of
971 tebibyte: @math{2^{40} = 1,099,511,627,776}.
973 @cindex petabyte, definition of
974 petabyte: @math{10^{15} = 1,000,000,000,000,000}.
977 @cindex pebibyte, definition of
978 pebibyte: @math{2^{50} = 1,125,899,906,842,624}.
980 @cindex exabyte, definition of
981 exabyte: @math{10^{18} = 1,000,000,000,000,000,000}.
984 @cindex exbibyte, definition of
985 exbibyte: @math{2^{60} = 1,152,921,504,606,846,976}.
987 @cindex zettabyte, definition of
988 zettabyte: @math{10^{21} = 1,000,000,000,000,000,000,000}
991 @math{2^{70} = 1,180,591,620,717,411,303,424}.
992 (@samp{Zi} is a @acronym{GNU} extension to IEC 60027-2.)
994 @cindex yottabyte, definition of
995 yottabyte: @math{10^{24} = 1,000,000,000,000,000,000,000,000}.
998 @math{2^{80} = 1,208,925,819,614,629,174,706,176}.
999 (@samp{Yi} is a @acronym{GNU} extension to IEC 60027-2.)
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{ }"}). @option{--sep-string} with no
2532 @samp{=@var{string}} is equivalent to @option{--sep-string=""}.
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{-r}). If the output file names are exhausted, @command{split}
2994 reports an error without deleting the output files that it did create.
2996 The program accepts the following options. Also see @ref{Common options}.
3000 @item -l @var{lines}
3001 @itemx --lines=@var{lines}
3004 Put @var{lines} lines of @var{input} into each output file.
3006 For compatibility @command{split} also supports an obsolete
3007 option syntax @option{-@var{lines}}. New scripts should use
3008 @option{-l @var{lines}} instead.
3011 @itemx --bytes=@var{size}
3014 Put @var{size} bytes of @var{input} into each output file.
3015 @multiplierSuffixes{size}
3018 @itemx --line-bytes=@var{size}
3020 @opindex --line-bytes
3021 Put into each output file as many complete lines of @var{input} as
3022 possible without exceeding @var{size} bytes. Individual lines longer than
3023 @var{size} bytes are broken into multiple files.
3024 @var{size} has the same format as for the @option{--bytes} option.
3026 @itemx --filter=@var{command}
3028 With this option, rather than simply writing to each output file,
3029 write through a pipe to the specified shell @var{command} for each output file.
3030 @var{command} should use the $FILE environment variable, which is set
3031 to a different output file name for each invocation of the command.
3032 For example, imagine that you have a 1TiB compressed file
3033 that, if uncompressed, would be too large to reside on disk,
3034 yet you must split it into individually-compressed pieces
3035 of a more manageable size.
3036 To do that, you might run this command:
3039 xz -dc BIG.xz | split -b200G --filter='xz > $FILE.xz' - big-
3042 Assuming a 10:1 compression ratio, that would create about fifty 20GiB files
3043 with names @file{big-xaa.xz}, @file{big-xab.xz}, @file{big-xac.xz}, etc.
3045 @item -n @var{chunks}
3046 @itemx --number=@var{chunks}
3050 Split @var{input} to @var{chunks} output files where @var{chunks} may be:
3053 @var{n} generate @var{n} files based on current size of @var{input}
3054 @var{k}/@var{n} only output @var{k}th of @var{n} to stdout
3055 l/@var{n} generate @var{n} files without splitting lines
3056 l/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
3057 r/@var{n} like @samp{l} but use round robin distribution
3058 r/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
3061 Any excess bytes remaining after dividing the @var{input}
3062 into @var{n} chunks, are assigned to the last chunk.
3063 Any excess bytes appearing after the initial calculation are discarded
3064 (except when using @samp{r} mode).
3066 All @var{n} files are created even if there are fewer than @var{n} lines,
3067 or the @var{input} is truncated.
3069 For @samp{l} mode, chunks are approximately @var{input} size / @var{n}.
3070 The @var{input} is partitioned into @var{n} equal sized portions, with
3071 the last assigned any excess. If a line @emph{starts} within a partition
3072 it is written completely to the corresponding file. Since lines
3073 are not split even if they overlap a partition, the files written
3074 can be larger or smaller than the partition size, and even empty
3075 if a line is so long as to completely overlap the partition.
3077 For @samp{r} mode, the size of @var{input} is irrelevant,
3078 and so can be a pipe for example.
3080 @item -a @var{length}
3081 @itemx --suffix-length=@var{length}
3083 @opindex --suffix-length
3084 Use suffixes of length @var{length}. The default @var{length} is 2.
3087 @itemx --numeric-suffixes
3089 @opindex --numeric-suffixes
3090 Use digits in suffixes rather than lower-case letters.
3093 @itemx --elide-empty-files
3095 @opindex --elide-empty-files
3096 Suppress the generation of zero-length output files. This can happen
3097 with the @option{--number} option if a file is (truncated to be) shorter
3098 than the number requested, or if a line is so long as to completely
3099 span a chunk. The output file sequence numbers, always run consecutively
3100 even when this option is specified.
3105 @opindex --unbuffered
3106 Immediately copy input to output in @option{--number r/...} mode,
3107 which is a much slower mode of operation.
3111 Write a diagnostic just before each output file is opened.
3117 Here are a few examples to illustrate how the
3118 @option{--number} (@option{-n}) option works:
3120 Notice how, by default, one line may be split onto two or more:
3123 $ seq -w 6 10 > k; split -n3 k; head xa?
3136 Use the "l/" modifier to suppress that:
3139 $ seq -w 6 10 > k; split -nl/3 k; head xa?
3152 Use the "r/" modifier to distribute lines in a round-robin fashion:
3155 $ seq -w 6 10 > k; split -nr/3 k; head xa?
3168 You can also extract just the Kth chunk.
3169 This extracts and prints just the 7th "chunk" of 33:
3172 $ seq 100 > k; split -nl/7/33 k
3179 @node csplit invocation
3180 @section @command{csplit}: Split a file into context-determined pieces
3183 @cindex context splitting
3184 @cindex splitting a file into pieces by context
3186 @command{csplit} creates zero or more output files containing sections of
3187 @var{input} (standard input if @var{input} is @samp{-}). Synopsis:
3190 csplit [@var{option}]@dots{} @var{input} @var{pattern}@dots{}
3193 The contents of the output files are determined by the @var{pattern}
3194 arguments, as detailed below. An error occurs if a @var{pattern}
3195 argument refers to a nonexistent line of the input file (e.g., if no
3196 remaining line matches a given regular expression). After every
3197 @var{pattern} has been matched, any remaining input is copied into one
3200 By default, @command{csplit} prints the number of bytes written to each
3201 output file after it has been created.
3203 The types of pattern arguments are:
3208 Create an output file containing the input up to but not including line
3209 @var{n} (a positive integer). If followed by a repeat count, also
3210 create an output file containing the next @var{n} lines of the input
3211 file once for each repeat.
3213 @item /@var{regexp}/[@var{offset}]
3214 Create an output file containing the current line up to (but not
3215 including) the next line of the input file that contains a match for
3216 @var{regexp}. The optional @var{offset} is an integer.
3217 If it is given, the input up to (but not including) the
3218 matching line plus or minus @var{offset} is put into the output file,
3219 and the line after that begins the next section of input.
3221 @item %@var{regexp}%[@var{offset}]
3222 Like the previous type, except that it does not create an output
3223 file, so that section of the input file is effectively ignored.
3225 @item @{@var{repeat-count}@}
3226 Repeat the previous pattern @var{repeat-count} additional
3227 times. The @var{repeat-count} can either be a positive integer or an
3228 asterisk, meaning repeat as many times as necessary until the input is
3233 The output files' names consist of a prefix (@samp{xx} by default)
3234 followed by a suffix. By default, the suffix is an ascending sequence
3235 of two-digit decimal numbers from @samp{00} to @samp{99}. In any case,
3236 concatenating the output files in sorted order by file name produces the
3237 original input file.
3239 By default, if @command{csplit} encounters an error or receives a hangup,
3240 interrupt, quit, or terminate signal, it removes any output files
3241 that it has created so far before it exits.
3243 The program accepts the following options. Also see @ref{Common options}.
3247 @item -f @var{prefix}
3248 @itemx --prefix=@var{prefix}
3251 @cindex output file name prefix
3252 Use @var{prefix} as the output file name prefix.
3254 @item -b @var{suffix}
3255 @itemx --suffix=@var{suffix}
3258 @cindex output file name suffix
3259 Use @var{suffix} as the output file name suffix. When this option is
3260 specified, the suffix string must include exactly one
3261 @code{printf(3)}-style conversion specification, possibly including
3262 format specification flags, a field width, a precision specifications,
3263 or all of these kinds of modifiers. The format letter must convert a
3264 binary unsigned integer argument to readable form. The format letters
3265 @samp{d} and @samp{i} are aliases for @samp{u}, and the
3266 @samp{u}, @samp{o}, @samp{x}, and @samp{X} conversions are allowed. The
3267 entire @var{suffix} is given (with the current output file number) to
3268 @code{sprintf(3)} to form the file name suffixes for each of the
3269 individual output files in turn. If this option is used, the
3270 @option{--digits} option is ignored.
3272 @item -n @var{digits}
3273 @itemx --digits=@var{digits}
3276 Use output file names containing numbers that are @var{digits} digits
3277 long instead of the default 2.
3282 @opindex --keep-files
3283 Do not remove output files when errors are encountered.
3286 @itemx --elide-empty-files
3288 @opindex --elide-empty-files
3289 Suppress the generation of zero-length output files. (In cases where
3290 the section delimiters of the input file are supposed to mark the first
3291 lines of each of the sections, the first output file will generally be a
3292 zero-length file unless you use this option.) The output file sequence
3293 numbers always run consecutively starting from 0, even when this option
3304 Do not print counts of output file sizes.
3310 Here is an example of its usage.
3311 First, create an empty directory for the exercise,
3318 Now, split the sequence of 1..14 on lines that end with 0 or 5:
3321 $ seq 14 | csplit - '/[05]$/' '@{*@}'
3327 Each number printed above is the size of an output
3328 file that csplit has just created.
3329 List the names of those output files:
3336 Use @command{head} to show their contents:
3361 @node Summarizing files
3362 @chapter Summarizing files
3364 @cindex summarizing files
3366 These commands generate just a few numbers representing entire
3370 * wc invocation:: Print newline, word, and byte counts.
3371 * sum invocation:: Print checksum and block counts.
3372 * cksum invocation:: Print CRC checksum and byte counts.
3373 * md5sum invocation:: Print or check MD5 digests.
3374 * sha1sum invocation:: Print or check SHA-1 digests.
3375 * sha2 utilities:: Print or check SHA-2 digests.
3380 @section @command{wc}: Print newline, word, and byte counts
3384 @cindex character count
3388 @command{wc} counts the number of bytes, characters, whitespace-separated
3389 words, and newlines in each given @var{file}, or standard input if none
3390 are given or for a @var{file} of @samp{-}. Synopsis:
3393 wc [@var{option}]@dots{} [@var{file}]@dots{}
3396 @cindex total counts
3397 @command{wc} prints one line of counts for each file, and if the file was
3398 given as an argument, it prints the file name following the counts. If
3399 more than one @var{file} is given, @command{wc} prints a final line
3400 containing the cumulative counts, with the file name @file{total}. The
3401 counts are printed in this order: newlines, words, characters, bytes,
3402 maximum line length.
3403 Each count is printed right-justified in a field with at least one
3404 space between fields so that the numbers and file names normally line
3405 up nicely in columns. The width of the count fields varies depending
3406 on the inputs, so you should not depend on a particular field width.
3407 However, as a @acronym{GNU} extension, if only one count is printed,
3408 it is guaranteed to be printed without leading spaces.
3410 By default, @command{wc} prints three counts: the newline, words, and byte
3411 counts. Options can specify that only certain counts be printed.
3412 Options do not undo others previously given, so
3419 prints both the byte counts and the word counts.
3421 With the @option{--max-line-length} option, @command{wc} prints the length
3422 of the longest line per file, and if there is more than one file it
3423 prints the maximum (not the sum) of those lengths. The line lengths here
3424 are measured in screen columns, according to the current locale and
3425 assuming tab positions in every 8th column.
3427 The program accepts the following options. Also see @ref{Common options}.
3435 Print only the byte counts.
3441 Print only the character counts.
3447 Print only the word counts.
3453 Print only the newline counts.
3456 @itemx --max-line-length
3458 @opindex --max-line-length
3459 Print only the maximum line lengths.
3461 @macro filesZeroFromOption{cmd,withTotalOption,subListOutput}
3462 @itemx --files0-from=@var{file}
3463 @opindex --files0-from=@var{file}
3464 @c This is commented out to avoid a texi2dvi failure.
3465 @c texi2dvi (GNU Texinfo 4.11) 1.104
3466 @c @cindex including files from @command{\cmd\}
3467 Disallow processing files named on the command line, and instead process
3468 those named in file @var{file}; each name being terminated by a zero byte
3469 (@acronym{ASCII} @sc{nul}).
3470 This is useful \withTotalOption\
3471 when the list of file names is so long that it may exceed a command line
3473 In such cases, running @command{\cmd\} via @command{xargs} is undesirable
3474 because it splits the list into pieces and makes @command{\cmd\} print
3475 \subListOutput\ for each sublist rather than for the entire list.
3476 One way to produce a list of @acronym{ASCII} @sc{nul} terminated file
3477 names is with @sc{gnu}
3478 @command{find}, using its @option{-print0} predicate.
3479 If @var{file} is @samp{-} then the @acronym{ASCII} @sc{nul} terminated
3480 file names are read from standard input.
3482 @filesZeroFromOption{wc,,a total}
3484 For example, to find the length of the longest line in any @file{.c} or
3485 @file{.h} file in the current hierarchy, do this:
3488 find . -name '*.[ch]' -print0 |
3489 wc -L --files0-from=- | tail -n1
3497 @node sum invocation
3498 @section @command{sum}: Print checksum and block counts
3501 @cindex 16-bit checksum
3502 @cindex checksum, 16-bit
3504 @command{sum} computes a 16-bit checksum for each given @var{file}, or
3505 standard input if none are given or for a @var{file} of @samp{-}. Synopsis:
3508 sum [@var{option}]@dots{} [@var{file}]@dots{}
3511 @command{sum} prints the checksum for each @var{file} followed by the
3512 number of blocks in the file (rounded up). If more than one @var{file}
3513 is given, file names are also printed (by default). (With the
3514 @option{--sysv} option, corresponding file names are printed when there is
3515 at least one file argument.)
3517 By default, @sc{gnu} @command{sum} computes checksums using an algorithm
3518 compatible with BSD @command{sum} and prints file sizes in units of
3521 The program accepts the following options. Also see @ref{Common options}.
3527 @cindex BSD @command{sum}
3528 Use the default (BSD compatible) algorithm. This option is included for
3529 compatibility with the System V @command{sum}. Unless @option{-s} was also
3530 given, it has no effect.
3536 @cindex System V @command{sum}
3537 Compute checksums using an algorithm compatible with System V
3538 @command{sum}'s default, and print file sizes in units of 512-byte blocks.
3542 @command{sum} is provided for compatibility; the @command{cksum} program (see
3543 next section) is preferable in new applications.
3548 @node cksum invocation
3549 @section @command{cksum}: Print CRC checksum and byte counts
3552 @cindex cyclic redundancy check
3553 @cindex CRC checksum
3555 @command{cksum} computes a cyclic redundancy check (CRC) checksum for each
3556 given @var{file}, or standard input if none are given or for a
3557 @var{file} of @samp{-}. Synopsis:
3560 cksum [@var{option}]@dots{} [@var{file}]@dots{}
3563 @command{cksum} prints the CRC checksum for each file along with the number
3564 of bytes in the file, and the file name unless no arguments were given.
3566 @command{cksum} is typically used to ensure that files
3567 transferred by unreliable means (e.g., netnews) have not been corrupted,
3568 by comparing the @command{cksum} output for the received files with the
3569 @command{cksum} output for the original files (typically given in the
3572 The CRC algorithm is specified by the @acronym{POSIX} standard. It is not
3573 compatible with the BSD or System V @command{sum} algorithms (see the
3574 previous section); it is more robust.
3576 The only options are @option{--help} and @option{--version}. @xref{Common
3582 @node md5sum invocation
3583 @section @command{md5sum}: Print or check MD5 digests
3587 @cindex 128-bit checksum
3588 @cindex checksum, 128-bit
3589 @cindex fingerprint, 128-bit
3590 @cindex message-digest, 128-bit
3592 @command{md5sum} computes a 128-bit checksum (or @dfn{fingerprint} or
3593 @dfn{message-digest}) for each specified @var{file}.
3595 Note: The MD5 digest is more reliable than a simple CRC (provided by
3596 the @command{cksum} command) for detecting accidental file corruption,
3597 as the chances of accidentally having two files with identical MD5
3598 are vanishingly small. However, it should not be considered secure
3599 against malicious tampering: although finding a file with a given MD5
3600 fingerprint is considered infeasible at the moment, it is known how
3601 to modify certain files, including digital certificates, so that they
3602 appear valid when signed with an MD5 digest.
3603 For more secure hashes, consider using SHA-2. @xref{sha2 utilities}.
3605 If a @var{file} is specified as @samp{-} or if no files are given
3606 @command{md5sum} computes the checksum for the standard input.
3607 @command{md5sum} can also determine whether a file and checksum are
3608 consistent. Synopsis:
3611 md5sum [@var{option}]@dots{} [@var{file}]@dots{}
3614 For each @var{file}, @samp{md5sum} outputs the MD5 checksum, a flag
3615 indicating a binary or text input file, and the file name.
3616 If @var{file} contains a backslash or newline, the
3617 line is started with a backslash, and each problematic character in
3618 the file name is escaped with a backslash, making the output
3619 unambiguous even in the presence of arbitrary file names.
3620 If @var{file} is omitted or specified as @samp{-}, standard input is read.
3622 The program accepts the following options. Also see @ref{Common options}.
3630 @cindex binary input files
3631 Treat each input file as binary, by reading it in binary mode and
3632 outputting a @samp{*} flag. This is the inverse of @option{--text}.
3633 On systems like @acronym{GNU} that do not distinguish between binary
3634 and text files, this option merely flags each input file as binary:
3635 the MD5 checksum is unaffected. This option is the default on systems
3636 like MS-DOS that distinguish between binary and text files, except
3637 for reading standard input when standard input is a terminal.
3641 Read file names and checksum information (not data) from each
3642 @var{file} (or from stdin if no @var{file} was specified) and report
3643 whether the checksums match the contents of the named files.
3644 The input to this mode of @command{md5sum} is usually the output of
3645 a prior, checksum-generating run of @samp{md5sum}.
3646 Each valid line of input consists of an MD5 checksum, a binary/text
3647 flag, and then a file name.
3648 Binary files are marked with @samp{*}, text with @samp{ }.
3649 For each such line, @command{md5sum} reads the named file and computes its
3650 MD5 checksum. Then, if the computed message digest does not match the
3651 one on the line with the file name, the file is noted as having
3652 failed the test. Otherwise, the file passes the test.
3653 By default, for each valid line, one line is written to standard
3654 output indicating whether the named file passed the test.
3655 After all checks have been performed, if there were any failures,
3656 a warning is issued to standard error.
3657 Use the @option{--status} option to inhibit that output.
3658 If any listed file cannot be opened or read, if any valid line has
3659 an MD5 checksum inconsistent with the associated file, or if no valid
3660 line is found, @command{md5sum} exits with nonzero status. Otherwise,
3661 it exits successfully.
3665 @cindex verifying MD5 checksums
3666 This option is useful only when verifying checksums.
3667 When verifying checksums, don't generate an 'OK' message per successfully
3668 checked file. Files that fail the verification are reported in the
3669 default one-line-per-file format. If there is any checksum mismatch,
3670 print a warning summarizing the failures to standard error.
3674 @cindex verifying MD5 checksums
3675 This option is useful only when verifying checksums.
3676 When verifying checksums, don't generate the default one-line-per-file
3677 diagnostic and don't output the warning summarizing any failures.
3678 Failures to open or read a file still evoke individual diagnostics to
3680 If all listed files are readable and are consistent with the associated
3681 MD5 checksums, exit successfully. Otherwise exit with a status code
3682 indicating there was a failure.
3688 @cindex text input files
3689 Treat each input file as text, by reading it in text mode and
3690 outputting a @samp{ } flag. This is the inverse of @option{--binary}.
3691 This option is the default on systems like @acronym{GNU} that do not
3692 distinguish between binary and text files. On other systems, it is
3693 the default for reading standard input when standard input is a
3700 @cindex verifying MD5 checksums
3701 When verifying checksums, warn about improperly formatted MD5 checksum lines.
3702 This option is useful only if all but a few lines in the checked input
3710 @node sha1sum invocation
3711 @section @command{sha1sum}: Print or check SHA-1 digests
3715 @cindex 160-bit checksum
3716 @cindex checksum, 160-bit
3717 @cindex fingerprint, 160-bit
3718 @cindex message-digest, 160-bit
3720 @command{sha1sum} computes a 160-bit checksum for each specified
3721 @var{file}. The usage and options of this command are precisely the
3722 same as for @command{md5sum}. @xref{md5sum invocation}.
3724 Note: The SHA-1 digest is more secure than MD5, and no collisions of
3725 it are known (different files having the same fingerprint). However,
3726 it is known that they can be produced with considerable, but not
3727 unreasonable, resources. For this reason, it is generally considered
3728 that SHA-1 should be gradually phased out in favor of the more secure
3729 SHA-2 hash algorithms. @xref{sha2 utilities}.
3732 @node sha2 utilities
3733 @section sha2 utilities: Print or check SHA-2 digests
3740 @cindex 224-bit checksum
3741 @cindex 256-bit checksum
3742 @cindex 384-bit checksum
3743 @cindex 512-bit checksum
3744 @cindex checksum, 224-bit
3745 @cindex checksum, 256-bit
3746 @cindex checksum, 384-bit
3747 @cindex checksum, 512-bit
3748 @cindex fingerprint, 224-bit
3749 @cindex fingerprint, 256-bit
3750 @cindex fingerprint, 384-bit
3751 @cindex fingerprint, 512-bit
3752 @cindex message-digest, 224-bit
3753 @cindex message-digest, 256-bit
3754 @cindex message-digest, 384-bit
3755 @cindex message-digest, 512-bit
3757 The commands @command{sha224sum}, @command{sha256sum},
3758 @command{sha384sum} and @command{sha512sum} compute checksums of
3759 various lengths (respectively 224, 256, 384 and 512 bits),
3760 collectively known as the SHA-2 hashes. The usage and options of
3761 these commands are precisely the same as for @command{md5sum}.
3762 @xref{md5sum invocation}.
3764 Note: The SHA384 and SHA512 digests are considerably slower to
3765 compute, especially on 32-bit computers, than SHA224 or SHA256.
3768 @node Operating on sorted files
3769 @chapter Operating on sorted files
3771 @cindex operating on sorted files
3772 @cindex sorted files, operations on
3774 These commands work with (or produce) sorted files.
3777 * sort invocation:: Sort text files.
3778 * shuf invocation:: Shuffle text files.
3779 * uniq invocation:: Uniquify files.
3780 * comm invocation:: Compare two sorted files line by line.
3781 * ptx invocation:: Produce a permuted index of file contents.
3782 * tsort invocation:: Topological sort.
3786 @node sort invocation
3787 @section @command{sort}: Sort text files
3790 @cindex sorting files
3792 @command{sort} sorts, merges, or compares all the lines from the given
3793 files, or standard input if none are given or for a @var{file} of
3794 @samp{-}. By default, @command{sort} writes the results to standard
3798 sort [@var{option}]@dots{} [@var{file}]@dots{}
3801 @command{sort} has three modes of operation: sort (the default), merge,
3802 and check for sortedness. The following options change the operation
3809 @itemx --check=diagnose-first
3812 @cindex checking for sortedness
3813 Check whether the given file is already sorted: if it is not all
3814 sorted, print a diagnostic containing the first out-of-order line and
3815 exit with a status of 1.
3816 Otherwise, exit successfully.
3817 At most one input file can be given.
3820 @itemx --check=quiet
3821 @itemx --check=silent
3824 @cindex checking for sortedness
3825 Exit successfully if the given file is already sorted, and
3826 exit with status 1 otherwise.
3827 At most one input file can be given.
3828 This is like @option{-c}, except it does not print a diagnostic.
3834 @cindex merging sorted files
3835 Merge the given files by sorting them as a group. Each input file must
3836 always be individually sorted. It always works to sort instead of
3837 merge; merging is provided because it is faster, in the case where it
3842 @cindex sort stability
3843 @cindex sort's last-resort comparison
3844 A pair of lines is compared as follows:
3845 @command{sort} compares each pair of fields, in the
3846 order specified on the command line, according to the associated
3847 ordering options, until a difference is found or no fields are left.
3848 If no key fields are specified, @command{sort} uses a default key of
3849 the entire line. Finally, as a last resort when all keys compare
3850 equal, @command{sort} compares entire lines as if no ordering options
3851 other than @option{--reverse} (@option{-r}) were specified. The
3852 @option{--stable} (@option{-s}) option disables this @dfn{last-resort
3853 comparison} so that lines in which all fields compare equal are left
3854 in their original relative order. The @option{--unique}
3855 (@option{-u}) option also disables the last-resort comparison.
3859 Unless otherwise specified, all comparisons use the character collating
3860 sequence specified by the @env{LC_COLLATE} locale.@footnote{If you
3861 use a non-@acronym{POSIX} locale (e.g., by setting @env{LC_ALL}
3862 to @samp{en_US}), then @command{sort} may produce output that is sorted
3863 differently than you're accustomed to. In that case, set the @env{LC_ALL}
3864 environment variable to @samp{C}. Note that setting only @env{LC_COLLATE}
3865 has two problems. First, it is ineffective if @env{LC_ALL} is also set.
3866 Second, it has undefined behavior if @env{LC_CTYPE} (or @env{LANG}, if
3867 @env{LC_CTYPE} is unset) is set to an incompatible value. For example,
3868 you get undefined behavior if @env{LC_CTYPE} is @code{ja_JP.PCK} but
3869 @env{LC_COLLATE} is @code{en_US.UTF-8}.}
3871 @sc{gnu} @command{sort} (as specified for all @sc{gnu} utilities) has no
3872 limit on input line length or restrictions on bytes allowed within lines.
3873 In addition, if the final byte of an input file is not a newline, @sc{gnu}
3874 @command{sort} silently supplies one. A line's trailing newline is not
3875 part of the line for comparison purposes.
3877 @cindex exit status of @command{sort}
3881 0 if no error occurred
3882 1 if invoked with @option{-c} or @option{-C} and the input is not sorted
3883 2 if an error occurred
3887 If the environment variable @env{TMPDIR} is set, @command{sort} uses its
3888 value as the directory for temporary files instead of @file{/tmp}. The
3889 @option{--temporary-directory} (@option{-T}) option in turn overrides
3890 the environment variable.
3892 The following options affect the ordering of output lines. They may be
3893 specified globally or as part of a specific key field. If no key
3894 fields are specified, global options apply to comparison of entire
3895 lines; otherwise the global options are inherited by key fields that do
3896 not specify any special options of their own. In pre-@acronym{POSIX}
3897 versions of @command{sort}, global options affect only later key fields,
3898 so portable shell scripts should specify global options first.
3903 @itemx --ignore-leading-blanks
3905 @opindex --ignore-leading-blanks
3906 @cindex blanks, ignoring leading
3908 Ignore leading blanks when finding sort keys in each line.
3909 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3910 can change this. Note blanks may be ignored by your locale's collating
3911 rules, but without this option they will be significant for character
3912 positions specified in keys with the @option{-k} option.
3915 @itemx --dictionary-order
3917 @opindex --dictionary-order
3918 @cindex dictionary order
3919 @cindex phone directory order
3920 @cindex telephone directory order
3922 Sort in @dfn{phone directory} order: ignore all characters except
3923 letters, digits and blanks when sorting.
3924 By default letters and digits are those of @acronym{ASCII} and a blank
3925 is a space or a tab, but the @env{LC_CTYPE} locale can change this.
3928 @itemx --ignore-case
3930 @opindex --ignore-case
3931 @cindex ignoring case
3932 @cindex case folding
3934 Fold lowercase characters into the equivalent uppercase characters when
3935 comparing so that, for example, @samp{b} and @samp{B} sort as equal.
3936 The @env{LC_CTYPE} locale determines character types.
3937 When used with @option{--unique} those lower case equivalent lines are
3938 thrown away. (There is currently no way to throw away the upper case
3939 equivalent instead. (Any @option{--reverse} given would only affect
3940 the final result, after the throwing away.))
3943 @itemx --general-numeric-sort
3944 @itemx --sort=general-numeric
3946 @opindex --general-numeric-sort
3948 @cindex general numeric sort
3950 Sort numerically, converting a prefix of each line to a long
3951 double-precision floating point number. @xref{Floating point}.
3952 Do not report overflow, underflow, or conversion errors.
3953 Use the following collating sequence:
3957 Lines that do not start with numbers (all considered to be equal).
3959 NaNs (``Not a Number'' values, in IEEE floating point arithmetic)
3960 in a consistent but machine-dependent order.
3964 Finite numbers in ascending numeric order (with @math{-0} and @math{+0} equal).
3969 Use this option only if there is no alternative; it is much slower than
3970 @option{--numeric-sort} (@option{-n}) and it can lose information when
3971 converting to floating point.
3974 @itemx --human-numeric-sort
3975 @itemx --sort=human-numeric
3977 @opindex --human-numeric-sort
3979 @cindex human numeric sort
3981 Sort numerically, first by numeric sign (negative, zero, or positive);
3982 then by @acronym{SI} suffix (either empty, or @samp{k} or @samp{K}, or
3983 one of @samp{MGTPEZY}, in that order; @pxref{Block size}); and finally
3984 by numeric value. For example, @samp{1023M} sorts before @samp{1G}
3985 because @samp{M} (mega) precedes @samp{G} (giga) as an @acronym{SI}
3986 suffix. This option sorts values that are consistently scaled to the
3987 nearest suffix, regardless of whether suffixes denote powers of 1000
3988 or 1024, and it therefore sorts the output of any single invocation of
3989 the @command{df}, @command{du}, or @command{ls} commands that are
3990 invoked with their @option{--human-readable} or @option{--si} options.
3991 The syntax for numbers is the same as for the @option{--numeric-sort}
3992 option; the @acronym{SI} suffix must immediately follow the number.
3995 @itemx --ignore-nonprinting
3997 @opindex --ignore-nonprinting
3998 @cindex nonprinting characters, ignoring
3999 @cindex unprintable characters, ignoring
4001 Ignore nonprinting characters.
4002 The @env{LC_CTYPE} locale determines character types.
4003 This option has no effect if the stronger @option{--dictionary-order}
4004 (@option{-d}) option is also given.
4010 @opindex --month-sort
4012 @cindex months, sorting by
4014 An initial string, consisting of any amount of blanks, followed
4015 by a month name abbreviation, is folded to UPPER case and
4016 compared in the order @samp{JAN} < @samp{FEB} < @dots{} < @samp{DEC}.
4017 Invalid names compare low to valid names. The @env{LC_TIME} locale
4018 category determines the month spellings.
4019 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4023 @itemx --numeric-sort
4024 @itemx --sort=numeric
4026 @opindex --numeric-sort
4028 @cindex numeric sort
4030 Sort numerically. The number begins each line and consists
4031 of optional blanks, an optional @samp{-} sign, and zero or more
4032 digits possibly separated by thousands separators, optionally followed
4033 by a decimal-point character and zero or more digits. An empty
4034 number is treated as @samp{0}. The @env{LC_NUMERIC}
4035 locale specifies the decimal-point character and thousands separator.
4036 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4039 Comparison is exact; there is no rounding error.
4041 Neither a leading @samp{+} nor exponential notation is recognized.
4042 To compare such strings numerically, use the
4043 @option{--general-numeric-sort} (@option{-g}) option.
4046 @itemx --version-sort
4048 @opindex --version-sort
4049 @cindex version number sort
4050 Sort by version name and number. It behaves like a standard sort,
4051 except that each sequence of decimal digits is treated numerically
4052 as an index/version number. (@xref{Details about version sort}.)
4058 @cindex reverse sorting
4059 Reverse the result of comparison, so that lines with greater key values
4060 appear earlier in the output instead of later.
4063 @itemx --random-sort
4064 @itemx --sort=random
4066 @opindex --random-sort
4069 Sort by hashing the input keys and then sorting the hash values.
4070 Choose the hash function at random, ensuring that it is free of
4071 collisions so that differing keys have differing hash values. This is
4072 like a random permutation of the inputs (@pxref{shuf invocation}),
4073 except that keys with the same value sort together.
4075 If multiple random sort fields are specified, the same random hash
4076 function is used for all fields. To use different random hash
4077 functions for different fields, you can invoke @command{sort} more
4080 The choice of hash function is affected by the
4081 @option{--random-source} option.
4089 @item --compress-program=@var{prog}
4090 Compress any temporary files with the program @var{prog}.
4092 With no arguments, @var{prog} must compress standard input to standard
4093 output, and when given the @option{-d} option it must decompress
4094 standard input to standard output.
4096 Terminate with an error if @var{prog} exits with nonzero status.
4098 White space and the backslash character should not appear in
4099 @var{prog}; they are reserved for future use.
4101 @filesZeroFromOption{sort,,sorted output}
4103 @item -k @var{pos1}[,@var{pos2}]
4104 @itemx --key=@var{pos1}[,@var{pos2}]
4108 Specify a sort field that consists of the part of the line between
4109 @var{pos1} and @var{pos2} (or the end of the line, if @var{pos2} is
4110 omitted), @emph{inclusive}.
4112 Each @var{pos} has the form @samp{@var{f}[.@var{c}][@var{opts}]},
4113 where @var{f} is the number of the field to use, and @var{c} is the number
4114 of the first character from the beginning of the field. Fields and character
4115 positions are numbered starting with 1; a character position of zero in
4116 @var{pos2} indicates the field's last character. If @samp{.@var{c}} is
4117 omitted from @var{pos1}, it defaults to 1 (the beginning of the field);
4118 if omitted from @var{pos2}, it defaults to 0 (the end of the field).
4119 @var{opts} are ordering options, allowing individual keys to be sorted
4120 according to different rules; see below for details. Keys can span
4123 Example: To sort on the second field, use @option{--key=2,2}
4124 (@option{-k 2,2}). See below for more notes on keys and more examples.
4125 See also the @option{--debug} option to help determine the part
4126 of the line being used in the sort.
4129 Highlight the portion of each line used for sorting.
4130 Also issue warnings about questionable usage to stderr.
4132 @item --batch-size=@var{nmerge}
4133 @opindex --batch-size
4134 @cindex number of inputs to merge, nmerge
4135 Merge at most @var{nmerge} inputs at once.
4137 When @command{sort} has to merge more than @var{nmerge} inputs,
4138 it merges them in groups of @var{nmerge}, saving the result in
4139 a temporary file, which is then used as an input in a subsequent merge.
4141 A large value of @var{nmerge} may improve merge performance and decrease
4142 temporary storage utilization at the expense of increased memory usage
4143 and I/0. Conversely a small value of @var{nmerge} may reduce memory
4144 requirements and I/0 at the expense of temporary storage consumption and
4147 The value of @var{nmerge} must be at least 2. The default value is
4148 currently 16, but this is implementation-dependent and may change in
4151 The value of @var{nmerge} may be bounded by a resource limit for open
4152 file descriptors. The commands @samp{ulimit -n} or @samp{getconf
4153 OPEN_MAX} may display limits for your systems; these limits may be
4154 modified further if your program already has some files open, or if
4155 the operating system has other limits on the number of open files. If
4156 the value of @var{nmerge} exceeds the resource limit, @command{sort}
4157 silently uses a smaller value.
4159 @item -o @var{output-file}
4160 @itemx --output=@var{output-file}
4163 @cindex overwriting of input, allowed
4164 Write output to @var{output-file} instead of standard output.
4165 Normally, @command{sort} reads all input before opening
4166 @var{output-file}, so you can safely sort a file in place by using
4167 commands like @code{sort -o F F} and @code{cat F | sort -o F}.
4168 However, @command{sort} with @option{--merge} (@option{-m}) can open
4169 the output file before reading all input, so a command like @code{cat
4170 F | sort -m -o F - G} is not safe as @command{sort} might start
4171 writing @file{F} before @command{cat} is done reading it.
4173 @vindex POSIXLY_CORRECT
4174 On newer systems, @option{-o} cannot appear after an input file if
4175 @env{POSIXLY_CORRECT} is set, e.g., @samp{sort F -o F}. Portable
4176 scripts should specify @option{-o @var{output-file}} before any input
4179 @item --random-source=@var{file}
4180 @opindex --random-source
4181 @cindex random source for sorting
4182 Use @var{file} as a source of random data used to determine which
4183 random hash function to use with the @option{-R} option. @xref{Random
4190 @cindex sort stability
4191 @cindex sort's last-resort comparison
4193 Make @command{sort} stable by disabling its last-resort comparison.
4194 This option has no effect if no fields or global ordering options
4195 other than @option{--reverse} (@option{-r}) are specified.
4198 @itemx --buffer-size=@var{size}
4200 @opindex --buffer-size
4201 @cindex size for main memory sorting
4202 Use a main-memory sort buffer of the given @var{size}. By default,
4203 @var{size} is in units of 1024 bytes. Appending @samp{%} causes
4204 @var{size} to be interpreted as a percentage of physical memory.
4205 Appending @samp{K} multiplies @var{size} by 1024 (the default),
4206 @samp{M} by 1,048,576, @samp{G} by 1,073,741,824, and so on for
4207 @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}. Appending
4208 @samp{b} causes @var{size} to be interpreted as a byte count, with no
4211 This option can improve the performance of @command{sort} by causing it
4212 to start with a larger or smaller sort buffer than the default.
4213 However, this option affects only the initial buffer size. The buffer
4214 grows beyond @var{size} if @command{sort} encounters input lines larger
4217 @item -t @var{separator}
4218 @itemx --field-separator=@var{separator}
4220 @opindex --field-separator
4221 @cindex field separator character
4222 Use character @var{separator} as the field separator when finding the
4223 sort keys in each line. By default, fields are separated by the empty
4224 string between a non-blank character and a blank character.
4225 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4228 That is, given the input line @w{@samp{ foo bar}}, @command{sort} breaks it
4229 into fields @w{@samp{ foo}} and @w{@samp{ bar}}. The field separator is
4230 not considered to be part of either the field preceding or the field
4231 following, so with @samp{sort @w{-t " "}} the same input line has
4232 three fields: an empty field, @samp{foo}, and @samp{bar}.
4233 However, fields that extend to the end of the line,
4234 as @option{-k 2}, or fields consisting of a range, as @option{-k 2,3},
4235 retain the field separators present between the endpoints of the range.
4237 To specify @acronym{ASCII} @sc{nul} as the field separator,
4238 use the two-character string @samp{\0}, e.g., @samp{sort -t '\0'}.
4240 @item -T @var{tempdir}
4241 @itemx --temporary-directory=@var{tempdir}
4243 @opindex --temporary-directory
4244 @cindex temporary directory
4246 Use directory @var{tempdir} to store temporary files, overriding the
4247 @env{TMPDIR} environment variable. If this option is given more than
4248 once, temporary files are stored in all the directories given. If you
4249 have a large sort or merge that is I/O-bound, you can often improve
4250 performance by using this option to specify directories on different
4251 disks and controllers.
4253 @item --parallel=@var{n}
4255 @cindex multithreaded sort
4256 Set the number of sorts run in parallel to @var{n}. By default,
4257 @var{n} is set to the number of available processors, but limited
4258 to 8, as there are diminishing performance gains after that.
4259 Note also that using @var{n} threads increases the memory usage by
4260 a factor of log @var{n}. Also see @ref{nproc invocation}.
4266 @cindex uniquifying output
4268 Normally, output only the first of a sequence of lines that compare
4269 equal. For the @option{--check} (@option{-c} or @option{-C}) option,
4270 check that no pair of consecutive lines compares equal.
4272 This option also disables the default last-resort comparison.
4274 The commands @code{sort -u} and @code{sort | uniq} are equivalent, but
4275 this equivalence does not extend to arbitrary @command{sort} options.
4276 For example, @code{sort -n -u} inspects only the value of the initial
4277 numeric string when checking for uniqueness, whereas @code{sort -n |
4278 uniq} inspects the entire line. @xref{uniq invocation}.
4280 @macro zeroTerminatedOption
4282 @itemx --zero-terminated
4284 @opindex --zero-terminated
4285 @cindex process zero-terminated items
4286 Delimit items with a zero byte rather than a newline (@acronym{ASCII} @sc{lf}).
4287 I.E. treat input as items separated by @acronym{ASCII} @sc{nul}
4288 and terminate output items with @acronym{ASCII} @sc{nul}.
4289 This option can be useful in conjunction with @samp{perl -0} or
4290 @samp{find -print0} and @samp{xargs -0} which do the same in order to
4291 reliably handle arbitrary file names (even those containing blanks
4292 or other special characters).
4294 @zeroTerminatedOption
4298 Historical (BSD and System V) implementations of @command{sort} have
4299 differed in their interpretation of some options, particularly
4300 @option{-b}, @option{-f}, and @option{-n}.
4301 @sc{gnu} sort follows the @acronym{POSIX}
4302 behavior, which is usually (but not always!) like the System V behavior.
4303 According to @acronym{POSIX}, @option{-n} no longer implies @option{-b}. For
4304 consistency, @option{-M} has been changed in the same way. This may
4305 affect the meaning of character positions in field specifications in
4306 obscure cases. The only fix is to add an explicit @option{-b}.
4308 A position in a sort field specified with @option{-k} may have any
4309 of the option letters @samp{MbdfghinRrV} appended to it, in which case no
4310 global ordering options are inherited by that particular field. The
4311 @option{-b} option may be independently attached to either or both of
4312 the start and end positions of a field specification, and if it is
4313 inherited from the global options it will be attached to both.
4314 If input lines can contain leading or adjacent blanks and @option{-t}
4315 is not used, then @option{-k} is typically combined with @option{-b} or
4316 an option that implicitly ignores leading blanks (@samp{Mghn}) as otherwise
4317 the varying numbers of leading blanks in fields can cause confusing results.
4319 If the start position in a sort field specifier falls after the end of
4320 the line or after the end field, the field is empty. If the @option{-b}
4321 option was specified, the @samp{.@var{c}} part of a field specification
4322 is counted from the first nonblank character of the field.
4324 @vindex _POSIX2_VERSION
4325 @vindex POSIXLY_CORRECT
4326 On older systems, @command{sort} supports an obsolete origin-zero
4327 syntax @samp{+@var{pos1} [-@var{pos2}]} for specifying sort keys.
4328 The obsolete sequence @samp{sort +@var{a}.@var{x} -@var{b}.@var{y}}
4329 is equivalent to @samp{sort -k @var{a+1}.@var{x+1},@var{b}} if @var{y}
4330 is @samp{0} or absent, otherwise it is equivalent to @samp{sort -k
4331 @var{a+1}.@var{x+1},@var{b+1}.@var{y}}.
4333 This obsolete behavior can be enabled or disabled with the
4334 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4335 conformance}); it can also be enabled when @env{POSIXLY_CORRECT} is
4336 not set by using the obsolete syntax with @samp{-@var{pos2}} present.
4338 Scripts intended for use on standard hosts should avoid obsolete
4339 syntax and should use @option{-k} instead. For example, avoid
4340 @samp{sort +2}, since it might be interpreted as either @samp{sort
4341 ./+2} or @samp{sort -k 3}. If your script must also run on hosts that
4342 support only the obsolete syntax, it can use a test like @samp{if sort
4343 -k 1 </dev/null >/dev/null 2>&1; then @dots{}} to decide which syntax
4346 Here are some examples to illustrate various combinations of options.
4351 Sort in descending (reverse) numeric order.
4358 Run no more that 4 sorts concurrently, using a buffer size of 10M.
4361 sort --parallel=4 -S 10M
4365 Sort alphabetically, omitting the first and second fields
4366 and the blanks at the start of the third field.
4367 This uses a single key composed of the characters beginning
4368 at the start of the first nonblank character in field three
4369 and extending to the end of each line.
4376 Sort numerically on the second field and resolve ties by sorting
4377 alphabetically on the third and fourth characters of field five.
4378 Use @samp{:} as the field delimiter.
4381 sort -t : -k 2,2n -k 5.3,5.4
4384 Note that if you had written @option{-k 2n} instead of @option{-k 2,2n}
4385 @command{sort} would have used all characters beginning in the second field
4386 and extending to the end of the line as the primary @emph{numeric}
4387 key. For the large majority of applications, treating keys spanning
4388 more than one field as numeric will not do what you expect.
4390 Also note that the @samp{n} modifier was applied to the field-end
4391 specifier for the first key. It would have been equivalent to
4392 specify @option{-k 2n,2} or @option{-k 2n,2n}. All modifiers except
4393 @samp{b} apply to the associated @emph{field}, regardless of whether
4394 the modifier character is attached to the field-start and/or the
4395 field-end part of the key specifier.
4398 Sort the password file on the fifth field and ignore any
4399 leading blanks. Sort lines with equal values in field five
4400 on the numeric user ID in field three. Fields are separated
4404 sort -t : -k 5b,5 -k 3,3n /etc/passwd
4405 sort -t : -n -k 5b,5 -k 3,3 /etc/passwd
4406 sort -t : -b -k 5,5 -k 3,3n /etc/passwd
4409 These three commands have equivalent effect. The first specifies that
4410 the first key's start position ignores leading blanks and the second
4411 key is sorted numerically. The other two commands rely on global
4412 options being inherited by sort keys that lack modifiers. The inheritance
4413 works in this case because @option{-k 5b,5b} and @option{-k 5b,5} are
4414 equivalent, as the location of a field-end lacking a @samp{.@var{c}}
4415 character position is not affected by whether initial blanks are
4419 Sort a set of log files, primarily by IPv4 address and secondarily by
4420 time stamp. If two lines' primary and secondary keys are identical,
4421 output the lines in the same order that they were input. The log
4422 files contain lines that look like this:
4425 4.150.156.3 - - [01/Apr/2004:06:31:51 +0000] message 1
4426 211.24.3.231 - - [24/Apr/2004:20:17:39 +0000] message 2
4429 Fields are separated by exactly one space. Sort IPv4 addresses
4430 lexicographically, e.g., 212.61.52.2 sorts before 212.129.233.201
4431 because 61 is less than 129.
4434 sort -s -t ' ' -k 4.9n -k 4.5M -k 4.2n -k 4.14,4.21 file*.log |
4435 sort -s -t '.' -k 1,1n -k 2,2n -k 3,3n -k 4,4n
4438 This example cannot be done with a single @command{sort} invocation,
4439 since IPv4 address components are separated by @samp{.} while dates
4440 come just after a space. So it is broken down into two invocations of
4441 @command{sort}: the first sorts by time stamp and the second by IPv4
4442 address. The time stamp is sorted by year, then month, then day, and
4443 finally by hour-minute-second field, using @option{-k} to isolate each
4444 field. Except for hour-minute-second there's no need to specify the
4445 end of each key field, since the @samp{n} and @samp{M} modifiers sort
4446 based on leading prefixes that cannot cross field boundaries. The
4447 IPv4 addresses are sorted lexicographically. The second sort uses
4448 @samp{-s} so that ties in the primary key are broken by the secondary
4449 key; the first sort uses @samp{-s} so that the combination of the two
4453 Generate a tags file in case-insensitive sorted order.
4456 find src -type f -print0 | sort -z -f | xargs -0 etags --append
4459 The use of @option{-print0}, @option{-z}, and @option{-0} in this case means
4460 that file names that contain blanks or other special characters are
4462 by the sort operation.
4464 @c This example is a bit contrived and needs more explanation.
4466 @c Sort records separated by an arbitrary string by using a pipe to convert
4467 @c each record delimiter string to @samp{\0}, then using sort's -z option,
4468 @c and converting each @samp{\0} back to the original record delimiter.
4471 @c printf 'c\n\nb\n\na\n' |
4472 @c perl -0pe 's/\n\n/\n\0/g' |
4474 @c perl -0pe 's/\0/\n/g'
4478 Use the common @acronym{DSU, Decorate Sort Undecorate} idiom to
4479 sort lines according to their length.
4482 awk '@{print length, $0@}' /etc/passwd | sort -n | cut -f2- -d' '
4485 In general this technique can be used to sort data that the @command{sort}
4486 command does not support, or is inefficient at, sorting directly.
4489 Shuffle a list of directories, but preserve the order of files within
4490 each directory. For instance, one could use this to generate a music
4491 playlist in which albums are shuffled but the songs of each album are
4495 ls */* | sort -t / -k 1,1R -k 2,2
4501 @node shuf invocation
4502 @section @command{shuf}: Shuffling text
4505 @cindex shuffling files
4507 @command{shuf} shuffles its input by outputting a random permutation
4508 of its input lines. Each output permutation is equally likely.
4512 shuf [@var{option}]@dots{} [@var{file}]
4513 shuf -e [@var{option}]@dots{} [@var{arg}]@dots{}
4514 shuf -i @var{lo}-@var{hi} [@var{option}]@dots{}
4517 @command{shuf} has three modes of operation that affect where it
4518 obtains its input lines. By default, it reads lines from standard
4519 input. The following options change the operation mode:
4527 @cindex command-line operands to shuffle
4528 Treat each command-line operand as an input line.
4530 @item -i @var{lo}-@var{hi}
4531 @itemx --input-range=@var{lo}-@var{hi}
4533 @opindex --input-range
4534 @cindex input range to shuffle
4535 Act as if input came from a file containing the range of unsigned
4536 decimal integers @var{lo}@dots{}@var{hi}, one per line.
4540 @command{shuf}'s other options can affect its behavior in all
4545 @item -n @var{lines}
4546 @itemx --head-count=@var{count}
4548 @opindex --head-count
4549 @cindex head of output
4550 Output at most @var{count} lines. By default, all input lines are
4553 @item -o @var{output-file}
4554 @itemx --output=@var{output-file}
4557 @cindex overwriting of input, allowed
4558 Write output to @var{output-file} instead of standard output.
4559 @command{shuf} reads all input before opening
4560 @var{output-file}, so you can safely shuffle a file in place by using
4561 commands like @code{shuf -o F <F} and @code{cat F | shuf -o F}.
4563 @item --random-source=@var{file}
4564 @opindex --random-source
4565 @cindex random source for shuffling
4566 Use @var{file} as a source of random data used to determine which
4567 permutation to generate. @xref{Random sources}.
4569 @zeroTerminatedOption
4585 might produce the output
4595 Similarly, the command:
4598 shuf -e clubs hearts diamonds spades
4612 and the command @samp{shuf -i 1-4} might output:
4622 These examples all have four input lines, so @command{shuf} might
4623 produce any of the twenty-four possible permutations of the input. In
4624 general, if there are @var{n} input lines, there are @var{n}! (i.e.,
4625 @var{n} factorial, or @var{n} * (@var{n} - 1) * @dots{} * 1) possible
4626 output permutations.
4631 @node uniq invocation
4632 @section @command{uniq}: Uniquify files
4635 @cindex uniquify files
4637 @command{uniq} writes the unique lines in the given @file{input}, or
4638 standard input if nothing is given or for an @var{input} name of
4642 uniq [@var{option}]@dots{} [@var{input} [@var{output}]]
4645 By default, @command{uniq} prints its input lines, except that
4646 it discards all but the first of adjacent repeated lines, so that
4647 no output lines are repeated. Optionally, it can instead discard
4648 lines that are not repeated, or all repeated lines.
4650 The input need not be sorted, but repeated input lines are detected
4651 only if they are adjacent. If you want to discard non-adjacent
4652 duplicate lines, perhaps you want to use @code{sort -u}.
4653 @xref{sort invocation}.
4656 Comparisons honor the rules specified by the @env{LC_COLLATE}
4659 If no @var{output} file is specified, @command{uniq} writes to standard
4662 The program accepts the following options. Also see @ref{Common options}.
4667 @itemx --skip-fields=@var{n}
4669 @opindex --skip-fields
4670 Skip @var{n} fields on each line before checking for uniqueness. Use
4671 a null string for comparison if a line has fewer than @var{n} fields. Fields
4672 are sequences of non-space non-tab characters that are separated from
4673 each other by at least one space or tab.
4675 For compatibility @command{uniq} supports an obsolete option syntax
4676 @option{-@var{n}}. New scripts should use @option{-f @var{n}} instead.
4679 @itemx --skip-chars=@var{n}
4681 @opindex --skip-chars
4682 Skip @var{n} characters before checking for uniqueness. Use a null string
4683 for comparison if a line has fewer than @var{n} characters. If you use both
4684 the field and character skipping options, fields are skipped over first.
4686 @vindex _POSIX2_VERSION
4687 On older systems, @command{uniq} supports an obsolete option syntax
4689 This obsolete behavior can be enabled or disabled with the
4690 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4691 conformance}), but portable scripts should avoid commands whose
4692 behavior depends on this variable.
4693 For example, use @samp{uniq ./+10} or @samp{uniq -s 10} rather than
4694 the ambiguous @samp{uniq +10}.
4700 Print the number of times each line occurred along with the line.
4703 @itemx --ignore-case
4705 @opindex --ignore-case
4706 Ignore differences in case when comparing lines.
4712 @cindex repeated lines, outputting
4713 Discard lines that are not repeated. When used by itself, this option
4714 causes @command{uniq} to print the first copy of each repeated line,
4718 @itemx --all-repeated[=@var{delimit-method}]
4720 @opindex --all-repeated
4721 @cindex all repeated lines, outputting
4722 Do not discard the second and subsequent repeated input lines,
4723 but discard lines that are not repeated.
4724 This option is useful mainly in conjunction with other options e.g.,
4725 to ignore case or to compare only selected fields.
4726 The optional @var{delimit-method} tells how to delimit
4727 groups of repeated lines, and must be one of the following:
4732 Do not delimit groups of repeated lines.
4733 This is equivalent to @option{--all-repeated} (@option{-D}).
4736 Output a newline before each group of repeated lines.
4737 With @option{--zero-terminated} (@option{-z}), use a zero
4738 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4741 Separate groups of repeated lines with a single newline.
4742 With @option{--zero-terminated} (@option{-z}), use a zero
4743 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4744 This is the same as using @samp{prepend}, except that
4745 no delimiter is inserted before the first group, and hence
4746 may be better suited for output direct to users.
4749 Note that when groups are delimited and the input stream contains
4750 two or more consecutive blank lines, then the output is ambiguous.
4751 To avoid that, filter the input through @samp{tr -s '\n'} to replace
4752 each sequence of consecutive newlines with a single newline.
4754 This is a @sc{gnu} extension.
4755 @c FIXME: give an example showing *how* it's useful
4761 @cindex unique lines, outputting
4762 Discard the first repeated line. When used by itself, this option
4763 causes @command{uniq} to print unique lines, and nothing else.
4766 @itemx --check-chars=@var{n}
4768 @opindex --check-chars
4769 Compare at most @var{n} characters on each line (after skipping any specified
4770 fields and characters). By default the entire rest of the lines are
4773 @zeroTerminatedOption
4780 @node comm invocation
4781 @section @command{comm}: Compare two sorted files line by line
4784 @cindex line-by-line comparison
4785 @cindex comparing sorted files
4787 @command{comm} writes to standard output lines that are common, and lines
4788 that are unique, to two input files; a file name of @samp{-} means
4789 standard input. Synopsis:
4792 comm [@var{option}]@dots{} @var{file1} @var{file2}
4796 Before @command{comm} can be used, the input files must be sorted using the
4797 collating sequence specified by the @env{LC_COLLATE} locale.
4798 If an input file ends in a non-newline
4799 character, a newline is silently appended. The @command{sort} command with
4800 no options always outputs a file that is suitable input to @command{comm}.
4802 @cindex differing lines
4803 @cindex common lines
4804 With no options, @command{comm} produces three-column output. Column one
4805 contains lines unique to @var{file1}, column two contains lines unique
4806 to @var{file2}, and column three contains lines common to both files.
4807 Columns are separated by a single TAB character.
4808 @c FIXME: when there's an option to supply an alternative separator
4809 @c string, append `by default' to the above sentence.
4814 The options @option{-1}, @option{-2}, and @option{-3} suppress printing of
4815 the corresponding columns (and separators). Also see @ref{Common options}.
4817 Unlike some other comparison utilities, @command{comm} has an exit
4818 status that does not depend on the result of the comparison.
4819 Upon normal completion @command{comm} produces an exit code of zero.
4820 If there is an error it exits with nonzero status.
4822 @macro checkOrderOption{cmd}
4823 If the @option{--check-order} option is given, unsorted inputs will
4824 cause a fatal error message. If the option @option{--nocheck-order}
4825 is given, unsorted inputs will never cause an error message. If neither
4826 of these options is given, wrongly sorted inputs are diagnosed
4827 only if an input file is found to contain unpairable
4829 lines, and when both input files are non empty.
4831 @ifclear JOIN_COMMAND
4834 If an input file is diagnosed as being unsorted, the @command{\cmd\}
4835 command will exit with a nonzero status (and the output should not be used).
4837 Forcing @command{\cmd\} to process wrongly sorted input files
4838 containing unpairable lines by specifying @option{--nocheck-order} is
4839 not guaranteed to produce any particular output. The output will
4840 probably not correspond with whatever you hoped it would be.
4842 @checkOrderOption{comm}
4847 Fail with an error message if either input file is wrongly ordered.
4849 @item --nocheck-order
4850 Do not check that both input files are in sorted order.
4854 @item --output-delimiter=@var{str}
4855 Print @var{str} between adjacent output columns,
4856 rather than the default of a single TAB character.
4858 The delimiter @var{str} may not be empty.
4862 @node ptx invocation
4863 @section @command{ptx}: Produce permuted indexes
4867 @command{ptx} reads a text file and essentially produces a permuted index, with
4868 each keyword in its context. The calling sketch is either one of:
4871 ptx [@var{option} @dots{}] [@var{file} @dots{}]
4872 ptx -G [@var{option} @dots{}] [@var{input} [@var{output}]]
4875 The @option{-G} (or its equivalent: @option{--traditional}) option disables
4876 all @sc{gnu} extensions and reverts to traditional mode, thus introducing some
4877 limitations and changing several of the program's default option values.
4878 When @option{-G} is not specified, @sc{gnu} extensions are always enabled.
4879 @sc{gnu} extensions to @command{ptx} are documented wherever appropriate in this
4880 document. For the full list, see @xref{Compatibility in ptx}.
4882 Individual options are explained in the following sections.
4884 When @sc{gnu} extensions are enabled, there may be zero, one or several
4885 @var{file}s after the options. If there is no @var{file}, the program
4886 reads the standard input. If there is one or several @var{file}s, they
4887 give the name of input files which are all read in turn, as if all the
4888 input files were concatenated. However, there is a full contextual
4889 break between each file and, when automatic referencing is requested,
4890 file names and line numbers refer to individual text input files. In
4891 all cases, the program outputs the permuted index to the standard
4894 When @sc{gnu} extensions are @emph{not} enabled, that is, when the program
4895 operates in traditional mode, there may be zero, one or two parameters
4896 besides the options. If there are no parameters, the program reads the
4897 standard input and outputs the permuted index to the standard output.
4898 If there is only one parameter, it names the text @var{input} to be read
4899 instead of the standard input. If two parameters are given, they give
4900 respectively the name of the @var{input} file to read and the name of
4901 the @var{output} file to produce. @emph{Be very careful} to note that,
4902 in this case, the contents of file given by the second parameter is
4903 destroyed. This behavior is dictated by System V @command{ptx}
4904 compatibility; @sc{gnu} Standards normally discourage output parameters not
4905 introduced by an option.
4907 Note that for @emph{any} file named as the value of an option or as an
4908 input text file, a single dash @kbd{-} may be used, in which case
4909 standard input is assumed. However, it would not make sense to use this
4910 convention more than once per program invocation.
4913 * General options in ptx:: Options which affect general program behavior.
4914 * Charset selection in ptx:: Underlying character set considerations.
4915 * Input processing in ptx:: Input fields, contexts, and keyword selection.
4916 * Output formatting in ptx:: Types of output format, and sizing the fields.
4917 * Compatibility in ptx::
4921 @node General options in ptx
4922 @subsection General options
4927 @itemx --traditional
4928 As already explained, this option disables all @sc{gnu} extensions to
4929 @command{ptx} and switches to traditional mode.
4932 Print a short help on standard output, then exit without further
4936 Print the program version on standard output, then exit without further
4944 @node Charset selection in ptx
4945 @subsection Charset selection
4947 @c FIXME: People don't necessarily know what an IBM-PC was these days.
4948 As it is set up now, the program assumes that the input file is coded
4949 using 8-bit @acronym{ISO} 8859-1 code, also known as Latin-1 character set,
4950 @emph{unless} it is compiled for MS-DOS, in which case it uses the
4951 character set of the IBM-PC@. (@sc{gnu} @command{ptx} is not known to work on
4952 smaller MS-DOS machines anymore.) Compared to 7-bit @acronym{ASCII}, the set
4953 of characters which are letters is different; this alters the behavior
4954 of regular expression matching. Thus, the default regular expression
4955 for a keyword allows foreign or diacriticized letters. Keyword sorting,
4956 however, is still crude; it obeys the underlying character set ordering
4962 @itemx --ignore-case
4963 Fold lower case letters to upper case for sorting.
4968 @node Input processing in ptx
4969 @subsection Word selection and input processing
4974 @itemx --break-file=@var{file}
4976 This option provides an alternative (to @option{-W}) method of describing
4977 which characters make up words. It introduces the name of a
4978 file which contains a list of characters which can@emph{not} be part of
4979 one word; this file is called the @dfn{Break file}. Any character which
4980 is not part of the Break file is a word constituent. If both options
4981 @option{-b} and @option{-W} are specified, then @option{-W} has precedence and
4982 @option{-b} is ignored.
4984 When @sc{gnu} extensions are enabled, the only way to avoid newline as a
4985 break character is to write all the break characters in the file with no
4986 newline at all, not even at the end of the file. When @sc{gnu} extensions
4987 are disabled, spaces, tabs and newlines are always considered as break
4988 characters even if not included in the Break file.
4991 @itemx --ignore-file=@var{file}
4993 The file associated with this option contains a list of words which will
4994 never be taken as keywords in concordance output. It is called the
4995 @dfn{Ignore file}. The file contains exactly one word in each line; the
4996 end of line separation of words is not subject to the value of the
5000 @itemx --only-file=@var{file}
5002 The file associated with this option contains a list of words which will
5003 be retained in concordance output; any word not mentioned in this file
5004 is ignored. The file is called the @dfn{Only file}. The file contains
5005 exactly one word in each line; the end of line separation of words is
5006 not subject to the value of the @option{-S} option.
5008 There is no default for the Only file. When both an Only file and an
5009 Ignore file are specified, a word is considered a keyword only
5010 if it is listed in the Only file and not in the Ignore file.
5015 On each input line, the leading sequence of non-white space characters will be
5016 taken to be a reference that has the purpose of identifying this input
5017 line in the resulting permuted index. For more information about reference
5018 production, see @xref{Output formatting in ptx}.
5019 Using this option changes the default value for option @option{-S}.
5021 Using this option, the program does not try very hard to remove
5022 references from contexts in output, but it succeeds in doing so
5023 @emph{when} the context ends exactly at the newline. If option
5024 @option{-r} is used with @option{-S} default value, or when @sc{gnu} extensions
5025 are disabled, this condition is always met and references are completely
5026 excluded from the output contexts.
5028 @item -S @var{regexp}
5029 @itemx --sentence-regexp=@var{regexp}
5031 This option selects which regular expression will describe the end of a
5032 line or the end of a sentence. In fact, this regular expression is not
5033 the only distinction between end of lines or end of sentences, and input
5034 line boundaries have no special significance outside this option. By
5035 default, when @sc{gnu} extensions are enabled and if @option{-r} option is not
5036 used, end of sentences are used. In this case, this @var{regex} is
5037 imported from @sc{gnu} Emacs:
5040 [.?!][]\"')@}]*\\($\\|\t\\| \\)[ \t\n]*
5043 Whenever @sc{gnu} extensions are disabled or if @option{-r} option is used, end
5044 of lines are used; in this case, the default @var{regexp} is just:
5050 Using an empty @var{regexp} is equivalent to completely disabling end of
5051 line or end of sentence recognition. In this case, the whole file is
5052 considered to be a single big line or sentence. The user might want to
5053 disallow all truncation flag generation as well, through option @option{-F
5054 ""}. @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
5057 When the keywords happen to be near the beginning of the input line or
5058 sentence, this often creates an unused area at the beginning of the
5059 output context line; when the keywords happen to be near the end of the
5060 input line or sentence, this often creates an unused area at the end of
5061 the output context line. The program tries to fill those unused areas
5062 by wrapping around context in them; the tail of the input line or
5063 sentence is used to fill the unused area on the left of the output line;
5064 the head of the input line or sentence is used to fill the unused area
5065 on the right of the output line.
5067 As a matter of convenience to the user, many usual backslashed escape
5068 sequences from the C language are recognized and converted to the
5069 corresponding characters by @command{ptx} itself.
5071 @item -W @var{regexp}
5072 @itemx --word-regexp=@var{regexp}
5074 This option selects which regular expression will describe each keyword.
5075 By default, if @sc{gnu} extensions are enabled, a word is a sequence of
5076 letters; the @var{regexp} used is @samp{\w+}. When @sc{gnu} extensions are
5077 disabled, a word is by default anything which ends with a space, a tab
5078 or a newline; the @var{regexp} used is @samp{[^ \t\n]+}.
5080 An empty @var{regexp} is equivalent to not using this option.
5081 @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
5084 As a matter of convenience to the user, many usual backslashed escape
5085 sequences, as found in the C language, are recognized and converted to
5086 the corresponding characters by @command{ptx} itself.
5091 @node Output formatting in ptx
5092 @subsection Output formatting
5094 Output format is mainly controlled by the @option{-O} and @option{-T} options
5095 described in the table below. When neither @option{-O} nor @option{-T} are
5096 selected, and if @sc{gnu} extensions are enabled, the program chooses an
5097 output format suitable for a dumb terminal. Each keyword occurrence is
5098 output to the center of one line, surrounded by its left and right
5099 contexts. Each field is properly justified, so the concordance output
5100 can be readily observed. As a special feature, if automatic
5101 references are selected by option @option{-A} and are output before the
5102 left context, that is, if option @option{-R} is @emph{not} selected, then
5103 a colon is added after the reference; this nicely interfaces with @sc{gnu}
5104 Emacs @code{next-error} processing. In this default output format, each
5105 white space character, like newline and tab, is merely changed to
5106 exactly one space, with no special attempt to compress consecutive
5107 spaces. This might change in the future. Except for those white space
5108 characters, every other character of the underlying set of 256
5109 characters is transmitted verbatim.
5111 Output format is further controlled by the following options.
5115 @item -g @var{number}
5116 @itemx --gap-size=@var{number}
5118 Select the size of the minimum white space gap between the fields on the
5121 @item -w @var{number}
5122 @itemx --width=@var{number}
5124 Select the maximum output width of each final line. If references are
5125 used, they are included or excluded from the maximum output width
5126 depending on the value of option @option{-R}. If this option is not
5127 selected, that is, when references are output before the left context,
5128 the maximum output width takes into account the maximum length of all
5129 references. If this option is selected, that is, when references are
5130 output after the right context, the maximum output width does not take
5131 into account the space taken by references, nor the gap that precedes
5135 @itemx --auto-reference
5137 Select automatic references. Each input line will have an automatic
5138 reference made up of the file name and the line ordinal, with a single
5139 colon between them. However, the file name will be empty when standard
5140 input is being read. If both @option{-A} and @option{-r} are selected, then
5141 the input reference is still read and skipped, but the automatic
5142 reference is used at output time, overriding the input reference.
5145 @itemx --right-side-refs
5147 In the default output format, when option @option{-R} is not used, any
5148 references produced by the effect of options @option{-r} or @option{-A} are
5149 placed to the far right of output lines, after the right context. With
5150 default output format, when the @option{-R} option is specified, references
5151 are rather placed at the beginning of each output line, before the left
5152 context. For any other output format, option @option{-R} is
5153 ignored, with one exception: with @option{-R} the width of references
5154 is @emph{not} taken into account in total output width given by @option{-w}.
5156 This option is automatically selected whenever @sc{gnu} extensions are
5159 @item -F @var{string}
5160 @itemx --flac-truncation=@var{string}
5162 This option will request that any truncation in the output be reported
5163 using the string @var{string}. Most output fields theoretically extend
5164 towards the beginning or the end of the current line, or current
5165 sentence, as selected with option @option{-S}. But there is a maximum
5166 allowed output line width, changeable through option @option{-w}, which is
5167 further divided into space for various output fields. When a field has
5168 to be truncated because it cannot extend beyond the beginning or the end of
5169 the current line to fit in, then a truncation occurs. By default,
5170 the string used is a single slash, as in @option{-F /}.
5172 @var{string} may have more than one character, as in @option{-F ...}.
5173 Also, in the particular case when @var{string} is empty (@option{-F ""}),
5174 truncation flagging is disabled, and no truncation marks are appended in
5177 As a matter of convenience to the user, many usual backslashed escape
5178 sequences, as found in the C language, are recognized and converted to
5179 the corresponding characters by @command{ptx} itself.
5181 @item -M @var{string}
5182 @itemx --macro-name=@var{string}
5184 Select another @var{string} to be used instead of @samp{xx}, while
5185 generating output suitable for @command{nroff}, @command{troff} or @TeX{}.
5188 @itemx --format=roff
5190 Choose an output format suitable for @command{nroff} or @command{troff}
5191 processing. Each output line will look like:
5194 .xx "@var{tail}" "@var{before}" "@var{keyword_and_after}"@c
5195 "@var{head}" "@var{ref}"
5198 so it will be possible to write a @samp{.xx} roff macro to take care of
5199 the output typesetting. This is the default output format when @sc{gnu}
5200 extensions are disabled. Option @option{-M} can be used to change
5201 @samp{xx} to another macro name.
5203 In this output format, each non-graphical character, like newline and
5204 tab, is merely changed to exactly one space, with no special attempt to
5205 compress consecutive spaces. Each quote character: @kbd{"} is doubled
5206 so it will be correctly processed by @command{nroff} or @command{troff}.
5211 Choose an output format suitable for @TeX{} processing. Each output
5212 line will look like:
5215 \xx @{@var{tail}@}@{@var{before}@}@{@var{keyword}@}@c
5216 @{@var{after}@}@{@var{head}@}@{@var{ref}@}
5220 so it will be possible to write a @code{\xx} definition to take care of
5221 the output typesetting. Note that when references are not being
5222 produced, that is, neither option @option{-A} nor option @option{-r} is
5223 selected, the last parameter of each @code{\xx} call is inhibited.
5224 Option @option{-M} can be used to change @samp{xx} to another macro
5227 In this output format, some special characters, like @kbd{$}, @kbd{%},
5228 @kbd{&}, @kbd{#} and @kbd{_} are automatically protected with a
5229 backslash. Curly brackets @kbd{@{}, @kbd{@}} are protected with a
5230 backslash and a pair of dollar signs (to force mathematical mode). The
5231 backslash itself produces the sequence @code{\backslash@{@}}.
5232 Circumflex and tilde diacritical marks produce the sequence @code{^\@{ @}} and
5233 @code{~\@{ @}} respectively. Other diacriticized characters of the
5234 underlying character set produce an appropriate @TeX{} sequence as far
5235 as possible. The other non-graphical characters, like newline and tab,
5236 and all other characters which are not part of @acronym{ASCII}, are merely
5237 changed to exactly one space, with no special attempt to compress
5238 consecutive spaces. Let me know how to improve this special character
5239 processing for @TeX{}.
5244 @node Compatibility in ptx
5245 @subsection The @sc{gnu} extensions to @command{ptx}
5247 This version of @command{ptx} contains a few features which do not exist in
5248 System V @command{ptx}. These extra features are suppressed by using the
5249 @option{-G} command line option, unless overridden by other command line
5250 options. Some @sc{gnu} extensions cannot be recovered by overriding, so the
5251 simple rule is to avoid @option{-G} if you care about @sc{gnu} extensions.
5252 Here are the differences between this program and System V @command{ptx}.
5257 This program can read many input files at once, it always writes the
5258 resulting concordance on standard output. On the other hand, System V
5259 @command{ptx} reads only one file and sends the result to standard output
5260 or, if a second @var{file} parameter is given on the command, to that
5263 Having output parameters not introduced by options is a dangerous
5264 practice which @sc{gnu} avoids as far as possible. So, for using @command{ptx}
5265 portably between @sc{gnu} and System V, you should always use it with a
5266 single input file, and always expect the result on standard output. You
5267 might also want to automatically configure in a @option{-G} option to
5268 @command{ptx} calls in products using @command{ptx}, if the configurator finds
5269 that the installed @command{ptx} accepts @option{-G}.
5272 The only options available in System V @command{ptx} are options @option{-b},
5273 @option{-f}, @option{-g}, @option{-i}, @option{-o}, @option{-r}, @option{-t} and
5274 @option{-w}. All other options are @sc{gnu} extensions and are not repeated in
5275 this enumeration. Moreover, some options have a slightly different
5276 meaning when @sc{gnu} extensions are enabled, as explained below.
5279 By default, concordance output is not formatted for @command{troff} or
5280 @command{nroff}. It is rather formatted for a dumb terminal. @command{troff}
5281 or @command{nroff} output may still be selected through option @option{-O}.
5284 Unless @option{-R} option is used, the maximum reference width is
5285 subtracted from the total output line width. With @sc{gnu} extensions
5286 disabled, width of references is not taken into account in the output
5287 line width computations.
5290 All 256 bytes, even @acronym{ASCII} @sc{nul} bytes, are always read and
5291 processed from input file with no adverse effect, even if @sc{gnu} extensions
5292 are disabled. However, System V @command{ptx} does not accept 8-bit characters,
5293 a few control characters are rejected, and the tilde @kbd{~} is also rejected.
5296 Input line length is only limited by available memory, even if @sc{gnu}
5297 extensions are disabled. However, System V @command{ptx} processes only
5298 the first 200 characters in each line.
5301 The break (non-word) characters default to be every character except all
5302 letters of the underlying character set, diacriticized or not. When @sc{gnu}
5303 extensions are disabled, the break characters default to space, tab and
5307 The program makes better use of output line width. If @sc{gnu} extensions
5308 are disabled, the program rather tries to imitate System V @command{ptx},
5309 but still, there are some slight disposition glitches this program does
5310 not completely reproduce.
5313 The user can specify both an Ignore file and an Only file. This is not
5314 allowed with System V @command{ptx}.
5319 @node tsort invocation
5320 @section @command{tsort}: Topological sort
5323 @cindex topological sort
5325 @command{tsort} performs a topological sort on the given @var{file}, or
5326 standard input if no input file is given or for a @var{file} of
5327 @samp{-}. For more details and some history, see @ref{tsort background}.
5331 tsort [@var{option}] [@var{file}]
5334 @command{tsort} reads its input as pairs of strings, separated by blanks,
5335 indicating a partial ordering. The output is a total ordering that
5336 corresponds to the given partial ordering.
5350 will produce the output
5361 Consider a more realistic example.
5362 You have a large set of functions all in one file, and they may all be
5363 declared static except one. Currently that one (say @code{main}) is the
5364 first function defined in the file, and the ones it calls directly follow
5365 it, followed by those they call, etc. Let's say that you are determined
5366 to take advantage of prototypes, so you have to choose between declaring
5367 all of those functions (which means duplicating a lot of information from
5368 the definitions) and rearranging the functions so that as many as possible
5369 are defined before they are used. One way to automate the latter process
5370 is to get a list for each function of the functions it calls directly.
5371 Many programs can generate such lists. They describe a call graph.
5372 Consider the following list, in which a given line indicates that the
5373 function on the left calls the one on the right directly.
5379 tail_file pretty_name
5380 tail_file write_header
5382 tail_forever recheck
5383 tail_forever pretty_name
5384 tail_forever write_header
5385 tail_forever dump_remainder
5388 tail_lines start_lines
5389 tail_lines dump_remainder
5390 tail_lines file_lines
5391 tail_lines pipe_lines
5393 tail_bytes start_bytes
5394 tail_bytes dump_remainder
5395 tail_bytes pipe_bytes
5396 file_lines dump_remainder
5400 then you can use @command{tsort} to produce an ordering of those
5401 functions that satisfies your requirement.
5404 example$ tsort call-graph | tac
5424 @command{tsort} detects any cycles in the input and writes the first cycle
5425 encountered to standard error.
5427 Note that for a given partial ordering, generally there is no unique
5428 total ordering. In the context of the call graph above, the function
5429 @code{parse_options} may be placed anywhere in the list as long as it
5430 precedes @code{main}.
5432 The only options are @option{--help} and @option{--version}. @xref{Common
5438 * tsort background:: Where tsort came from.
5441 @node tsort background
5442 @subsection @command{tsort}: Background
5444 @command{tsort} exists because very early versions of the Unix linker processed
5445 an archive file exactly once, and in order. As @command{ld} read each object
5446 in the archive, it decided whether it was needed in the program based on
5447 whether it defined any symbols which were undefined at that point in
5450 This meant that dependencies within the archive had to be handled
5451 specially. For example, @code{scanf} probably calls @code{read}. That means
5452 that in a single pass through an archive, it was important for @code{scanf.o}
5453 to appear before read.o, because otherwise a program which calls
5454 @code{scanf} but not @code{read} might end up with an unexpected unresolved
5455 reference to @code{read}.
5457 The way to address this problem was to first generate a set of
5458 dependencies of one object file on another. This was done by a shell
5459 script called @command{lorder}. The GNU tools don't provide a version of
5460 lorder, as far as I know, but you can still find it in BSD
5463 Then you ran @command{tsort} over the @command{lorder} output, and you used the
5464 resulting sort to define the order in which you added objects to the archive.
5466 This whole procedure has been obsolete since about 1980, because
5467 Unix archives now contain a symbol table (traditionally built by
5468 @command{ranlib}, now generally built by @command{ar} itself), and the Unix
5469 linker uses the symbol table to effectively make multiple passes over
5472 Anyhow, that's where tsort came from. To solve an old problem with
5473 the way the linker handled archive files, which has since been solved
5477 @node Operating on fields
5478 @chapter Operating on fields
5481 * cut invocation:: Print selected parts of lines.
5482 * paste invocation:: Merge lines of files.
5483 * join invocation:: Join lines on a common field.
5487 @node cut invocation
5488 @section @command{cut}: Print selected parts of lines
5491 @command{cut} writes to standard output selected parts of each line of each
5492 input file, or standard input if no files are given or for a file name of
5496 cut @var{option}@dots{} [@var{file}]@dots{}
5499 In the table which follows, the @var{byte-list}, @var{character-list},
5500 and @var{field-list} are one or more numbers or ranges (two numbers
5501 separated by a dash) separated by commas. Bytes, characters, and
5502 fields are numbered starting at 1. Incomplete ranges may be
5503 given: @option{-@var{m}} means @samp{1-@var{m}}; @samp{@var{n}-} means
5504 @samp{@var{n}} through end of line or last field. The list elements
5505 can be repeated, can overlap, and can be specified in any order; but
5506 the selected input is written in the same order that it is read, and
5507 is written exactly once.
5509 The program accepts the following options. Also see @ref{Common
5514 @item -b @var{byte-list}
5515 @itemx --bytes=@var{byte-list}
5518 Select for printing only the bytes in positions listed in
5519 @var{byte-list}. Tabs and backspaces are treated like any other
5520 character; they take up 1 byte. If an output delimiter is specified,
5521 (see the description of @option{--output-delimiter}), then output that
5522 string between ranges of selected bytes.
5524 @item -c @var{character-list}
5525 @itemx --characters=@var{character-list}
5527 @opindex --characters
5528 Select for printing only the characters in positions listed in
5529 @var{character-list}. The same as @option{-b} for now, but
5530 internationalization will change that. Tabs and backspaces are
5531 treated like any other character; they take up 1 character. If an
5532 output delimiter is specified, (see the description of
5533 @option{--output-delimiter}), then output that string between ranges
5536 @item -f @var{field-list}
5537 @itemx --fields=@var{field-list}
5540 Select for printing only the fields listed in @var{field-list}.
5541 Fields are separated by a TAB character by default. Also print any
5542 line that contains no delimiter character, unless the
5543 @option{--only-delimited} (@option{-s}) option is specified.
5545 Note @command{awk} supports more sophisticated field processing,
5546 and by default will use (and discard) runs of blank characters to
5547 separate fields, and ignore leading and trailing blanks.
5550 awk '{print $2}' # print the second field
5551 awk '{print $NF-1}' # print the penultimate field
5552 awk '{print $2,$1}' # reorder the first two fields
5556 In the unlikely event that @command{awk} is unavailable,
5557 one can use the @command{join} command, to process blank
5558 characters as @command{awk} does above.
5561 join -a1 -o 1.2 - /dev/null # print the second field
5562 join -a1 -o 1.2,1.1 - /dev/null # reorder the first two fields
5566 @item -d @var{input_delim_byte}
5567 @itemx --delimiter=@var{input_delim_byte}
5569 @opindex --delimiter
5570 With @option{-f}, use the first byte of @var{input_delim_byte} as
5571 the input fields separator (default is TAB).
5575 Do not split multi-byte characters (no-op for now).
5578 @itemx --only-delimited
5580 @opindex --only-delimited
5581 For @option{-f}, do not print lines that do not contain the field separator
5582 character. Normally, any line without a field separator is printed verbatim.
5584 @item --output-delimiter=@var{output_delim_string}
5585 @opindex --output-delimiter
5586 With @option{-f}, output fields are separated by @var{output_delim_string}.
5587 The default with @option{-f} is to use the input delimiter.
5588 When using @option{-b} or @option{-c} to select ranges of byte or
5589 character offsets (as opposed to ranges of fields),
5590 output @var{output_delim_string} between non-overlapping
5591 ranges of selected bytes.
5594 @opindex --complement
5595 This option is a @acronym{GNU} extension.
5596 Select for printing the complement of the bytes, characters or fields
5597 selected with the @option{-b}, @option{-c} or @option{-f} options.
5598 In other words, do @emph{not} print the bytes, characters or fields
5599 specified via those options. This option is useful when you have
5600 many fields and want to print all but a few of them.
5607 @node paste invocation
5608 @section @command{paste}: Merge lines of files
5611 @cindex merging files
5613 @command{paste} writes to standard output lines consisting of sequentially
5614 corresponding lines of each given file, separated by a TAB character.
5615 Standard input is used for a file name of @samp{-} or if no input files
5637 paste [@var{option}]@dots{} [@var{file}]@dots{}
5640 The program accepts the following options. Also see @ref{Common options}.
5648 Paste the lines of one file at a time rather than one line from each
5649 file. Using the above example data:
5652 $ paste -s num2 let3
5657 @item -d @var{delim-list}
5658 @itemx --delimiters=@var{delim-list}
5660 @opindex --delimiters
5661 Consecutively use the characters in @var{delim-list} instead of
5662 TAB to separate merged lines. When @var{delim-list} is
5663 exhausted, start again at its beginning. Using the above example data:
5666 $ paste -d '%_' num2 let3 num2
5677 @node join invocation
5678 @section @command{join}: Join lines on a common field
5681 @cindex common field, joining on
5683 @command{join} writes to standard output a line for each pair of input
5684 lines that have identical join fields. Synopsis:
5687 join [@var{option}]@dots{} @var{file1} @var{file2}
5690 Either @var{file1} or @var{file2} (but not both) can be @samp{-},
5691 meaning standard input. @var{file1} and @var{file2} should be
5692 sorted on the join fields.
5695 Normally, the sort order is that of the
5696 collating sequence specified by the @env{LC_COLLATE} locale. Unless
5697 the @option{-t} option is given, the sort comparison ignores blanks at
5698 the start of the join field, as in @code{sort -b}. If the
5699 @option{--ignore-case} option is given, the sort comparison ignores
5700 the case of characters in the join field, as in @code{sort -f}.
5702 The @command{sort} and @command{join} commands should use consistent
5703 locales and options if the output of @command{sort} is fed to
5704 @command{join}. You can use a command like @samp{sort -k 1b,1} to
5705 sort a file on its default join field, but if you select a non-default
5706 locale, join field, separator, or comparison options, then you should
5707 do so consistently between @command{join} and @command{sort}.
5708 If @samp{join -t ''} is specified then the whole line is considered which
5709 matches the default operation of sort.
5711 If the input has no unpairable lines, a @acronym{GNU} extension is
5712 available; the sort order can be any order that considers two fields
5713 to be equal if and only if the sort comparison described above
5714 considers them to be equal. For example:
5732 @checkOrderOption{join}
5737 @item the join field is the first field in each line;
5738 @item fields in the input are separated by one or more blanks, with leading
5739 blanks on the line ignored;
5740 @item fields in the output are separated by a space;
5741 @item each output line consists of the join field, the remaining
5742 fields from @var{file1}, then the remaining fields from @var{file2}.
5745 The program accepts the following options. Also see @ref{Common options}.
5749 @item -a @var{file-number}
5751 Print a line for each unpairable line in file @var{file-number} (either
5752 @samp{1} or @samp{2}), in addition to the normal output.
5755 Fail with an error message if either input file is wrongly ordered.
5757 @item --nocheck-order
5758 Do not check that both input files are in sorted order. This is the default.
5760 @item -e @var{string}
5762 Replace those output fields that are missing in the input with @var{string}.
5763 I.E. missing fields specified with the @option{-12jo} options.
5767 Treat the first line of each input file as a header line. The header lines will
5768 be joined and printed as the first output line. If @option{-o} is used to
5769 specify output format, the header line will be printed according to the
5770 specified format. The header lines will not be checked for ordering even if
5771 @option{--check-order} is specified. Also if the header lines from each file
5772 do not match, the heading fields from the first file will be used.
5775 @itemx --ignore-case
5777 @opindex --ignore-case
5778 Ignore differences in case when comparing keys.
5779 With this option, the lines of the input files must be ordered in the same way.
5780 Use @samp{sort -f} to produce this ordering.
5782 @item -1 @var{field}
5784 Join on field @var{field} (a positive integer) of file 1.
5786 @item -2 @var{field}
5788 Join on field @var{field} (a positive integer) of file 2.
5790 @item -j @var{field}
5791 Equivalent to @option{-1 @var{field} -2 @var{field}}.
5793 @item -o @var{field-list}
5795 If the keyword @samp{auto} is specified, infer the output format from
5796 the first line in each file. This is the same as the default output format
5797 but also ensures the same number of fields are output for each line.
5798 Missing fields are replaced with the @option{-e} option and extra fields
5801 Otherwise, construct each output line according to the format in
5802 @var{field-list}. Each element in @var{field-list} is either the single
5803 character @samp{0} or has the form @var{m.n} where the file number, @var{m},
5804 is @samp{1} or @samp{2} and @var{n} is a positive field number.
5806 A field specification of @samp{0} denotes the join field.
5807 In most cases, the functionality of the @samp{0} field spec
5808 may be reproduced using the explicit @var{m.n} that corresponds
5809 to the join field. However, when printing unpairable lines
5810 (using either of the @option{-a} or @option{-v} options), there is no way
5811 to specify the join field using @var{m.n} in @var{field-list}
5812 if there are unpairable lines in both files.
5813 To give @command{join} that functionality, @acronym{POSIX} invented the @samp{0}
5814 field specification notation.
5816 The elements in @var{field-list}
5817 are separated by commas or blanks.
5818 Blank separators typically need to be quoted for the shell. For
5819 example, the commands @samp{join -o 1.2,2.2} and @samp{join -o '1.2
5820 2.2'} are equivalent.
5822 All output lines---including those printed because of any -a or -v
5823 option---are subject to the specified @var{field-list}.
5826 Use character @var{char} as the input and output field separator.
5827 Treat as significant each occurrence of @var{char} in the input file.
5828 Use @samp{sort -t @var{char}}, without the @option{-b} option of
5829 @samp{sort}, to produce this ordering. If @samp{join -t ''} is specified,
5830 the whole line is considered, matching the default operation of sort.
5831 If @samp{-t '\0'} is specified then the @acronym{ASCII} @sc{nul}
5832 character is used to delimit the fields.
5834 @item -v @var{file-number}
5835 Print a line for each unpairable line in file @var{file-number}
5836 (either @samp{1} or @samp{2}), instead of the normal output.
5843 @node Operating on characters
5844 @chapter Operating on characters
5846 @cindex operating on characters
5848 This commands operate on individual characters.
5851 * tr invocation:: Translate, squeeze, and/or delete characters.
5852 * expand invocation:: Convert tabs to spaces.
5853 * unexpand invocation:: Convert spaces to tabs.
5858 @section @command{tr}: Translate, squeeze, and/or delete characters
5865 tr [@var{option}]@dots{} @var{set1} [@var{set2}]
5868 @command{tr} copies standard input to standard output, performing
5869 one of the following operations:
5873 translate, and optionally squeeze repeated characters in the result,
5875 squeeze repeated characters,
5879 delete characters, then squeeze repeated characters from the result.
5882 The @var{set1} and (if given) @var{set2} arguments define ordered
5883 sets of characters, referred to below as @var{set1} and @var{set2}. These
5884 sets are the characters of the input that @command{tr} operates on.
5885 The @option{--complement} (@option{-c}, @option{-C}) option replaces
5887 complement (all of the characters that are not in @var{set1}).
5889 Currently @command{tr} fully supports only single-byte characters.
5890 Eventually it will support multibyte characters; when it does, the
5891 @option{-C} option will cause it to complement the set of characters,
5892 whereas @option{-c} will cause it to complement the set of values.
5893 This distinction will matter only when some values are not characters,
5894 and this is possible only in locales using multibyte encodings when
5895 the input contains encoding errors.
5897 The program accepts the @option{--help} and @option{--version}
5898 options. @xref{Common options}. Options must precede operands.
5903 * Character sets:: Specifying sets of characters.
5904 * Translating:: Changing one set of characters to another.
5905 * Squeezing:: Squeezing repeats and deleting.
5909 @node Character sets
5910 @subsection Specifying sets of characters
5912 @cindex specifying sets of characters
5914 The format of the @var{set1} and @var{set2} arguments resembles
5915 the format of regular expressions; however, they are not regular
5916 expressions, only lists of characters. Most characters simply
5917 represent themselves in these strings, but the strings can contain
5918 the shorthands listed below, for convenience. Some of them can be
5919 used only in @var{set1} or @var{set2}, as noted below.
5923 @item Backslash escapes
5924 @cindex backslash escapes
5926 The following backslash escape sequences are recognized:
5944 The 8-bit character with the value given by @var{ooo}, which is 1 to 3
5945 octal digits. Note that @samp{\400} is interpreted as the two-byte
5946 sequence, @samp{\040} @samp{0}.
5951 While a backslash followed by a character not listed above is
5952 interpreted as that character, the backslash also effectively
5953 removes any special significance, so it is useful to escape
5954 @samp{[}, @samp{]}, @samp{*}, and @samp{-}.
5959 The notation @samp{@var{m}-@var{n}} expands to all of the characters
5960 from @var{m} through @var{n}, in ascending order. @var{m} should
5961 collate before @var{n}; if it doesn't, an error results. As an example,
5962 @samp{0-9} is the same as @samp{0123456789}.
5964 @sc{gnu} @command{tr} does not support the System V syntax that uses square
5965 brackets to enclose ranges. Translations specified in that format
5966 sometimes work as expected, since the brackets are often transliterated
5967 to themselves. However, they should be avoided because they sometimes
5968 behave unexpectedly. For example, @samp{tr -d '[0-9]'} deletes brackets
5971 Many historically common and even accepted uses of ranges are not
5972 portable. For example, on @acronym{EBCDIC} hosts using the @samp{A-Z}
5973 range will not do what most would expect because @samp{A} through @samp{Z}
5974 are not contiguous as they are in @acronym{ASCII}.
5975 If you can rely on a @acronym{POSIX} compliant version of @command{tr}, then
5976 the best way to work around this is to use character classes (see below).
5977 Otherwise, it is most portable (and most ugly) to enumerate the members
5980 @item Repeated characters
5981 @cindex repeated characters
5983 The notation @samp{[@var{c}*@var{n}]} in @var{set2} expands to @var{n}
5984 copies of character @var{c}. Thus, @samp{[y*6]} is the same as
5985 @samp{yyyyyy}. The notation @samp{[@var{c}*]} in @var{string2} expands
5986 to as many copies of @var{c} as are needed to make @var{set2} as long as
5987 @var{set1}. If @var{n} begins with @samp{0}, it is interpreted in
5988 octal, otherwise in decimal.
5990 @item Character classes
5991 @cindex character classes
5993 The notation @samp{[:@var{class}:]} expands to all of the characters in
5994 the (predefined) class @var{class}. The characters expand in no
5995 particular order, except for the @code{upper} and @code{lower} classes,
5996 which expand in ascending order. When the @option{--delete} (@option{-d})
5997 and @option{--squeeze-repeats} (@option{-s}) options are both given, any
5998 character class can be used in @var{set2}. Otherwise, only the
5999 character classes @code{lower} and @code{upper} are accepted in
6000 @var{set2}, and then only if the corresponding character class
6001 (@code{upper} and @code{lower}, respectively) is specified in the same
6002 relative position in @var{set1}. Doing this specifies case conversion.
6003 The class names are given below; an error results when an invalid class
6015 Horizontal whitespace.
6024 Printable characters, not including space.
6030 Printable characters, including space.
6033 Punctuation characters.
6036 Horizontal or vertical whitespace.
6045 @item Equivalence classes
6046 @cindex equivalence classes
6048 The syntax @samp{[=@var{c}=]} expands to all of the characters that are
6049 equivalent to @var{c}, in no particular order. Equivalence classes are
6050 a relatively recent invention intended to support non-English alphabets.
6051 But there seems to be no standard way to define them or determine their
6052 contents. Therefore, they are not fully implemented in @sc{gnu} @command{tr};
6053 each character's equivalence class consists only of that character,
6054 which is of no particular use.
6060 @subsection Translating
6062 @cindex translating characters
6064 @command{tr} performs translation when @var{set1} and @var{set2} are
6065 both given and the @option{--delete} (@option{-d}) option is not given.
6066 @command{tr} translates each character of its input that is in @var{set1}
6067 to the corresponding character in @var{set2}. Characters not in
6068 @var{set1} are passed through unchanged. When a character appears more
6069 than once in @var{set1} and the corresponding characters in @var{set2}
6070 are not all the same, only the final one is used. For example, these
6071 two commands are equivalent:
6078 A common use of @command{tr} is to convert lowercase characters to
6079 uppercase. This can be done in many ways. Here are three of them:
6082 tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
6084 tr '[:lower:]' '[:upper:]'
6088 But note that using ranges like @code{a-z} above is not portable.
6090 When @command{tr} is performing translation, @var{set1} and @var{set2}
6091 typically have the same length. If @var{set1} is shorter than
6092 @var{set2}, the extra characters at the end of @var{set2} are ignored.
6094 On the other hand, making @var{set1} longer than @var{set2} is not
6095 portable; @acronym{POSIX} says that the result is undefined. In this situation,
6096 BSD @command{tr} pads @var{set2} to the length of @var{set1} by repeating
6097 the last character of @var{set2} as many times as necessary. System V
6098 @command{tr} truncates @var{set1} to the length of @var{set2}.
6100 By default, @sc{gnu} @command{tr} handles this case like BSD @command{tr}.
6101 When the @option{--truncate-set1} (@option{-t}) option is given,
6102 @sc{gnu} @command{tr} handles this case like the System V @command{tr}
6103 instead. This option is ignored for operations other than translation.
6105 Acting like System V @command{tr} in this case breaks the relatively common
6109 tr -cs A-Za-z0-9 '\012'
6113 because it converts only zero bytes (the first element in the
6114 complement of @var{set1}), rather than all non-alphanumerics, to
6118 By the way, the above idiom is not portable because it uses ranges, and
6119 it assumes that the octal code for newline is 012.
6120 Assuming a @acronym{POSIX} compliant @command{tr}, here is a better
6124 tr -cs '[:alnum:]' '[\n*]'
6129 @subsection Squeezing repeats and deleting
6131 @cindex squeezing repeat characters
6132 @cindex deleting characters
6134 When given just the @option{--delete} (@option{-d}) option, @command{tr}
6135 removes any input characters that are in @var{set1}.
6137 When given just the @option{--squeeze-repeats} (@option{-s}) option,
6138 @command{tr} replaces each input sequence of a repeated character that
6139 is in @var{set1} with a single occurrence of that character.
6141 When given both @option{--delete} and @option{--squeeze-repeats}, @command{tr}
6142 first performs any deletions using @var{set1}, then squeezes repeats
6143 from any remaining characters using @var{set2}.
6145 The @option{--squeeze-repeats} option may also be used when translating,
6146 in which case @command{tr} first performs translation, then squeezes
6147 repeats from any remaining characters using @var{set2}.
6149 Here are some examples to illustrate various combinations of options:
6154 Remove all zero bytes:
6161 Put all words on lines by themselves. This converts all
6162 non-alphanumeric characters to newlines, then squeezes each string
6163 of repeated newlines into a single newline:
6166 tr -cs '[:alnum:]' '[\n*]'
6170 Convert each sequence of repeated newlines to a single newline:
6177 Find doubled occurrences of words in a document.
6178 @c Separate the following two "the"s, so typo checkers don't complain.
6179 For example, people often write ``the @w{}the'' with the repeated words
6180 separated by a newline. The Bourne shell script below works first
6181 by converting each sequence of punctuation and blank characters to a
6182 single newline. That puts each ``word'' on a line by itself.
6183 Next it maps all uppercase characters to lower case, and finally it
6184 runs @command{uniq} with the @option{-d} option to print out only the words
6190 | tr -s '[:punct:][:blank:]' '[\n*]' \
6191 | tr '[:upper:]' '[:lower:]' \
6196 Deleting a small set of characters is usually straightforward. For example,
6197 to remove all @samp{a}s, @samp{x}s, and @samp{M}s you would do this:
6203 However, when @samp{-} is one of those characters, it can be tricky because
6204 @samp{-} has special meanings. Performing the same task as above but also
6205 removing all @samp{-} characters, we might try @code{tr -d -axM}, but
6206 that would fail because @command{tr} would try to interpret @option{-a} as
6207 a command-line option. Alternatively, we could try putting the hyphen
6208 inside the string, @code{tr -d a-xM}, but that wouldn't work either because
6209 it would make @command{tr} interpret @code{a-x} as the range of characters
6210 @samp{a}@dots{}@samp{x} rather than the three.
6211 One way to solve the problem is to put the hyphen at the end of the list
6218 Or you can use @samp{--} to terminate option processing:
6224 More generally, use the character class notation @code{[=c=]}
6225 with @samp{-} (or any other character) in place of the @samp{c}:
6231 Note how single quotes are used in the above example to protect the
6232 square brackets from interpretation by a shell.
6237 @node expand invocation
6238 @section @command{expand}: Convert tabs to spaces
6241 @cindex tabs to spaces, converting
6242 @cindex converting tabs to spaces
6244 @command{expand} writes the contents of each given @var{file}, or standard
6245 input if none are given or for a @var{file} of @samp{-}, to standard
6246 output, with tab characters converted to the appropriate number of
6250 expand [@var{option}]@dots{} [@var{file}]@dots{}
6253 By default, @command{expand} converts all tabs to spaces. It preserves
6254 backspace characters in the output; they decrement the column count for
6255 tab calculations. The default action is equivalent to @option{-t 8} (set
6256 tabs every 8 columns).
6258 The program accepts the following options. Also see @ref{Common options}.
6262 @item -t @var{tab1}[,@var{tab2}]@dots{}
6263 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6266 @cindex tab stops, setting
6267 If only one tab stop is given, set the tabs @var{tab1} spaces apart
6268 (default is 8). Otherwise, set the tabs at columns @var{tab1},
6269 @var{tab2}, @dots{} (numbered from 0), and replace any tabs beyond the
6270 last tab stop given with single spaces. Tab stops can be separated by
6271 blanks as well as by commas.
6273 For compatibility, GNU @command{expand} also accepts the obsolete
6274 option syntax, @option{-@var{t1}[,@var{t2}]@dots{}}. New scripts
6275 should use @option{-t @var{t1}[,@var{t2}]@dots{}} instead.
6281 @cindex initial tabs, converting
6282 Only convert initial tabs (those that precede all non-space or non-tab
6283 characters) on each line to spaces.
6290 @node unexpand invocation
6291 @section @command{unexpand}: Convert spaces to tabs
6295 @command{unexpand} writes the contents of each given @var{file}, or
6296 standard input if none are given or for a @var{file} of @samp{-}, to
6297 standard output, converting blanks at the beginning of each line into
6298 as many tab characters as needed. In the default @acronym{POSIX}
6299 locale, a @dfn{blank} is a space or a tab; other locales may specify
6300 additional blank characters. Synopsis:
6303 unexpand [@var{option}]@dots{} [@var{file}]@dots{}
6306 By default, @command{unexpand} converts only initial blanks (those
6307 that precede all non-blank characters) on each line. It
6308 preserves backspace characters in the output; they decrement the column
6309 count for tab calculations. By default, tabs are set at every 8th
6312 The program accepts the following options. Also see @ref{Common options}.
6316 @item -t @var{tab1}[,@var{tab2}]@dots{}
6317 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6320 If only one tab stop is given, set the tabs @var{tab1} columns apart
6321 instead of the default 8. Otherwise, set the tabs at columns
6322 @var{tab1}, @var{tab2}, @dots{} (numbered from 0), and leave blanks
6323 beyond the tab stops given unchanged. Tab stops can be separated by
6324 blanks as well as by commas. This option implies the @option{-a} option.
6326 For compatibility, GNU @command{unexpand} supports the obsolete option syntax,
6327 @option{-@var{tab1}[,@var{tab2}]@dots{}}, where tab stops must be
6328 separated by commas. (Unlike @option{-t}, this obsolete option does
6329 not imply @option{-a}.) New scripts should use @option{--first-only -t
6330 @var{tab1}[,@var{tab2}]@dots{}} instead.
6336 Also convert all sequences of two or more blanks just before a tab stop,
6337 even if they occur after non-blank characters in a line.
6344 @node Directory listing
6345 @chapter Directory listing
6347 This chapter describes the @command{ls} command and its variants @command{dir}
6348 and @command{vdir}, which list information about files.
6351 * ls invocation:: List directory contents.
6352 * dir invocation:: Briefly ls.
6353 * vdir invocation:: Verbosely ls.
6354 * dircolors invocation:: Color setup for ls, etc.
6359 @section @command{ls}: List directory contents
6362 @cindex directory listing
6364 The @command{ls} program lists information about files (of any type,
6365 including directories). Options and file arguments can be intermixed
6366 arbitrarily, as usual.
6368 For non-option command-line arguments that are directories, by default
6369 @command{ls} lists the contents of directories, not recursively, and
6370 omitting files with names beginning with @samp{.}. For other non-option
6371 arguments, by default @command{ls} lists just the file name. If no
6372 non-option argument is specified, @command{ls} operates on the current
6373 directory, acting as if it had been invoked with a single argument of @samp{.}.
6376 By default, the output is sorted alphabetically, according to the locale
6377 settings in effect.@footnote{If you use a non-@acronym{POSIX}
6378 locale (e.g., by setting @env{LC_ALL} to @samp{en_US}), then @command{ls} may
6379 produce output that is sorted differently than you're accustomed to.
6380 In that case, set the @env{LC_ALL} environment variable to @samp{C}.}
6381 If standard output is
6382 a terminal, the output is in columns (sorted vertically) and control
6383 characters are output as question marks; otherwise, the output is listed
6384 one per line and control characters are output as-is.
6386 Because @command{ls} is such a fundamental program, it has accumulated many
6387 options over the years. They are described in the subsections below;
6388 within each section, options are listed alphabetically (ignoring case).
6389 The division of options into the subsections is not absolute, since some
6390 options affect more than one aspect of @command{ls}'s operation.
6392 @cindex exit status of @command{ls}
6397 1 minor problems (e.g., failure to access a file or directory not
6398 specified as a command line argument. This happens when listing a
6399 directory in which entries are actively being removed or renamed.)
6400 2 serious trouble (e.g., memory exhausted, invalid option, failure
6401 to access a file or directory specified as a command line argument
6402 or a directory loop)
6405 Also see @ref{Common options}.
6408 * Which files are listed::
6409 * What information is listed::
6410 * Sorting the output::
6411 * Details about version sort::
6412 * General output formatting::
6413 * Formatting file timestamps::
6414 * Formatting the file names::
6418 @node Which files are listed
6419 @subsection Which files are listed
6421 These options determine which files @command{ls} lists information for.
6422 By default, @command{ls} lists files and the contents of any
6423 directories on the command line, except that in directories it ignores
6424 files whose names start with @samp{.}.
6432 In directories, do not ignore file names that start with @samp{.}.
6437 @opindex --almost-all
6438 In directories, do not ignore all file names that start with @samp{.};
6439 ignore only @file{.} and @file{..}. The @option{--all} (@option{-a})
6440 option overrides this option.
6443 @itemx --ignore-backups
6445 @opindex --ignore-backups
6446 @cindex backup files, ignoring
6447 In directories, ignore files that end with @samp{~}. This option is
6448 equivalent to @samp{--ignore='*~' --ignore='.*~'}.
6453 @opindex --directory
6454 List just the names of directories, as with other types of files, rather
6455 than listing their contents.
6456 @c The following sentence is the same as the one for -F.
6457 Do not follow symbolic links listed on the
6458 command line unless the @option{--dereference-command-line} (@option{-H}),
6459 @option{--dereference} (@option{-L}), or
6460 @option{--dereference-command-line-symlink-to-dir} options are specified.
6463 @itemx --dereference-command-line
6465 @opindex --dereference-command-line
6466 @cindex symbolic links, dereferencing
6467 If a command line argument specifies a symbolic link, show information
6468 for the file the link references rather than for the link itself.
6470 @itemx --dereference-command-line-symlink-to-dir
6471 @opindex --dereference-command-line-symlink-to-dir
6472 @cindex symbolic links, dereferencing
6473 Do not dereference symbolic links, with one exception:
6474 if a command line argument specifies a symbolic link that refers to
6475 a directory, show information for that directory rather than for the
6477 This is the default behavior when no other dereferencing-related
6478 option has been specified (@option{--classify} (@option{-F}),
6479 @option{--directory} (@option{-d}),
6481 @option{--dereference} (@option{-L}), or
6482 @option{--dereference-command-line} (@option{-H})).
6484 @item --group-directories-first
6485 @opindex --group-directories-first
6486 Group all the directories before the files and then sort the
6487 directories and the files separately using the selected sort key
6488 (see --sort option).
6489 That is, this option specifies a primary sort key,
6490 and the --sort option specifies a secondary key.
6491 However, any use of @option{--sort=none}
6492 (@option{-U}) disables this option altogether.
6494 @item --hide=PATTERN
6495 @opindex --hide=@var{pattern}
6496 In directories, ignore files whose names match the shell pattern
6497 @var{pattern}, unless the @option{--all} (@option{-a}) or
6498 @option{--almost-all} (@option{-A}) is also given. This
6499 option acts like @option{--ignore=@var{pattern}} except that it has no
6500 effect if @option{--all} (@option{-a}) or @option{--almost-all}
6501 (@option{-A}) is also given.
6503 This option can be useful in shell aliases. For example, if
6504 @command{lx} is an alias for @samp{ls --hide='*~'} and @command{ly} is
6505 an alias for @samp{ls --ignore='*~'}, then the command @samp{lx -A}
6506 lists the file @file{README~} even though @samp{ly -A} would not.
6508 @item -I @var{pattern}
6509 @itemx --ignore=@var{pattern}
6511 @opindex --ignore=@var{pattern}
6512 In directories, ignore files whose names match the shell pattern
6513 (not regular expression) @var{pattern}. As
6514 in the shell, an initial @samp{.} in a file name does not match a
6515 wildcard at the start of @var{pattern}. Sometimes it is useful
6516 to give this option several times. For example,
6519 $ ls --ignore='.??*' --ignore='.[^.]' --ignore='#*'
6522 The first option ignores names of length 3 or more that start with @samp{.},
6523 the second ignores all two-character names that start with @samp{.}
6524 except @samp{..}, and the third ignores names that start with @samp{#}.
6527 @itemx --dereference
6529 @opindex --dereference
6530 @cindex symbolic links, dereferencing
6531 When showing file information for a symbolic link, show information
6532 for the file the link references rather than the link itself.
6533 However, even with this option, @command{ls} still prints the name
6534 of the link itself, not the name of the file that the link points to.
6539 @opindex --recursive
6540 @cindex recursive directory listing
6541 @cindex directory listing, recursive
6542 List the contents of all directories recursively.
6547 @node What information is listed
6548 @subsection What information is listed
6550 These options affect the information that @command{ls} displays. By
6551 default, only file names are shown.
6557 @cindex hurd, author, printing
6558 List each file's author when producing long format directory listings.
6559 In GNU/Hurd, file authors can differ from their owners, but in other
6560 operating systems the two are the same.
6566 @cindex dired Emacs mode support
6567 With the long listing (@option{-l}) format, print an additional line after
6571 //DIRED// @var{beg1} @var{end1} @var{beg2} @var{end2} @dots{}
6575 The @var{begn} and @var{endn} are unsigned integers that record the
6576 byte position of the beginning and end of each file name in the output.
6577 This makes it easy for Emacs to find the names, even when they contain
6578 unusual characters such as space or newline, without fancy searching.
6580 If directories are being listed recursively (@option{-R}), output a similar
6581 line with offsets for each subdirectory name:
6584 //SUBDIRED// @var{beg1} @var{end1} @dots{}
6587 Finally, output a line of the form:
6590 //DIRED-OPTIONS// --quoting-style=@var{word}
6594 where @var{word} is the quoting style (@pxref{Formatting the file names}).
6596 Here is an actual example:
6599 $ mkdir -p a/sub/deeper a/sub2
6601 $ touch a/sub/deeper/file
6602 $ ls -gloRF --dired a
6605 -rw-r--r-- 1 0 Jun 10 12:27 f1
6606 -rw-r--r-- 1 0 Jun 10 12:27 f2
6607 drwxr-xr-x 3 4096 Jun 10 12:27 sub/
6608 drwxr-xr-x 2 4096 Jun 10 12:27 sub2/
6612 drwxr-xr-x 2 4096 Jun 10 12:27 deeper/
6616 -rw-r--r-- 1 0 Jun 10 12:27 file
6620 //DIRED// 48 50 84 86 120 123 158 162 217 223 282 286
6621 //SUBDIRED// 2 3 167 172 228 240 290 296
6622 //DIRED-OPTIONS// --quoting-style=literal
6625 Note that the pairs of offsets on the @samp{//DIRED//} line above delimit
6626 these names: @file{f1}, @file{f2}, @file{sub}, @file{sub2}, @file{deeper},
6628 The offsets on the @samp{//SUBDIRED//} line delimit the following
6629 directory names: @file{a}, @file{a/sub}, @file{a/sub/deeper}, @file{a/sub2}.
6631 Here is an example of how to extract the fifth entry name, @samp{deeper},
6632 corresponding to the pair of offsets, 222 and 228:
6635 $ ls -gloRF --dired a > out
6636 $ dd bs=1 skip=222 count=6 < out 2>/dev/null; echo
6640 Note that although the listing above includes a trailing slash
6641 for the @samp{deeper} entry, the offsets select the name without
6642 the trailing slash. However, if you invoke @command{ls} with @option{--dired}
6643 along with an option like @option{--escape} (aka @option{-b}) and operate
6644 on a file whose name contains special characters, notice that the backslash
6649 $ ls -blog --dired 'a b'
6650 -rw-r--r-- 1 0 Jun 10 12:28 a\ b
6652 //DIRED-OPTIONS// --quoting-style=escape
6655 If you use a quoting style that adds quote marks
6656 (e.g., @option{--quoting-style=c}), then the offsets include the quote marks.
6657 So beware that the user may select the quoting style via the environment
6658 variable @env{QUOTING_STYLE}. Hence, applications using @option{--dired}
6659 should either specify an explicit @option{--quoting-style=literal} option
6660 (aka @option{-N} or @option{--literal}) on the command line, or else be
6661 prepared to parse the escaped names.
6664 @opindex --full-time
6665 Produce long format directory listings, and list times in full. It is
6666 equivalent to using @option{--format=long} with
6667 @option{--time-style=full-iso} (@pxref{Formatting file timestamps}).
6671 Produce long format directory listings, but don't display owner information.
6677 Inhibit display of group information in a long format directory listing.
6678 (This is the default in some non-@sc{gnu} versions of @command{ls}, so we
6679 provide this option for compatibility.)
6687 @cindex inode number, printing
6688 Print the inode number (also called the file serial number and index
6689 number) of each file to the left of the file name. (This number
6690 uniquely identifies each file within a particular file system.)
6693 @itemx --format=long
6694 @itemx --format=verbose
6697 @opindex long ls @r{format}
6698 @opindex verbose ls @r{format}
6699 In addition to the name of each file, print the file type, file mode bits,
6700 number of hard links, owner name, group name, size, and
6701 timestamp (@pxref{Formatting file timestamps}), normally
6702 the modification time. Print question marks for information that
6703 cannot be determined.
6705 Normally the size is printed as a byte count without punctuation, but
6706 this can be overridden (@pxref{Block size}). For example, @option{-h}
6707 prints an abbreviated, human-readable count, and
6708 @samp{--block-size="'1"} prints a byte count with the thousands
6709 separator of the current locale.
6711 For each directory that is listed, preface the files with a line
6712 @samp{total @var{blocks}}, where @var{blocks} is the total disk allocation
6713 for all files in that directory. The block size currently defaults to 1024
6714 bytes, but this can be overridden (@pxref{Block size}).
6715 The @var{blocks} computed counts each hard link separately;
6716 this is arguably a deficiency.
6718 The file type is one of the following characters:
6720 @c The commented-out entries are ones we're not sure about.
6728 character special file
6730 high performance (``contiguous data'') file
6734 door (Solaris 2.5 and up)
6736 @c semaphore, if this is a distinct file type
6740 @c multiplexed file (7th edition Unix; obsolete)
6742 off-line (``migrated'') file (Cray DMF)
6744 network special file (HP-UX)
6748 port (Solaris 10 and up)
6750 @c message queue, if this is a distinct file type
6754 @c shared memory object, if this is a distinct file type
6756 @c typed memory object, if this is a distinct file type
6758 @c whiteout (4.4BSD; not implemented)
6760 some other file type
6763 @cindex permissions, output by @command{ls}
6764 The file mode bits listed are similar to symbolic mode specifications
6765 (@pxref{Symbolic Modes}). But @command{ls} combines multiple bits into the
6766 third character of each set of permissions as follows:
6770 If the set-user-ID or set-group-ID bit and the corresponding executable bit
6774 If the set-user-ID or set-group-ID bit is set but the corresponding
6775 executable bit is not set.
6778 If the restricted deletion flag or sticky bit, and the
6779 other-executable bit, are both set. The restricted deletion flag is
6780 another name for the sticky bit. @xref{Mode Structure}.
6783 If the restricted deletion flag or sticky bit is set but the
6784 other-executable bit is not set.
6787 If the executable bit is set and none of the above apply.
6793 Following the file mode bits is a single character that specifies
6794 whether an alternate access method such as an access control list
6795 applies to the file. When the character following the file mode bits is a
6796 space, there is no alternate access method. When it is a printing
6797 character, then there is such a method.
6799 GNU @command{ls} uses a @samp{.} character to indicate a file
6800 with an SELinux security context, but no other alternate access method.
6802 A file with any other combination of alternate access methods
6803 is marked with a @samp{+} character.
6806 @itemx --numeric-uid-gid
6808 @opindex --numeric-uid-gid
6809 @cindex numeric uid and gid
6810 @cindex numeric user and group IDs
6811 Produce long format directory listings, but
6812 display numeric user and group IDs instead of the owner and group names.
6816 Produce long format directory listings, but don't display group information.
6817 It is equivalent to using @option{--format=long} with @option{--no-group} .
6823 @cindex disk allocation
6824 @cindex size of files, reporting
6825 Print the disk allocation of each file to the left of the file name.
6826 This is the amount of disk space used by the file, which is usually a
6827 bit more than the file's size, but it can be less if the file has holes.
6829 Normally the disk allocation is printed in units of
6830 1024 bytes, but this can be overridden (@pxref{Block size}).
6832 @cindex NFS mounts from BSD to HP-UX
6833 For files that are NFS-mounted from an HP-UX system to a BSD system,
6834 this option reports sizes that are half the correct values. On HP-UX
6835 systems, it reports sizes that are twice the correct values for files
6836 that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX;
6837 it also affects the HP-UX @command{ls} program.
6846 @cindex security context
6847 Display the SELinux security context or @samp{?} if none is found.
6848 When used with the @option{-l} option, print the security context
6849 to the left of the size column.
6854 @node Sorting the output
6855 @subsection Sorting the output
6857 @cindex sorting @command{ls} output
6858 These options change the order in which @command{ls} sorts the information
6859 it outputs. By default, sorting is done by character code
6860 (e.g., @acronym{ASCII} order).
6866 @itemx --time=status
6869 @opindex ctime@r{, printing or sorting by}
6870 @opindex status time@r{, printing or sorting by}
6871 @opindex use time@r{, printing or sorting files by}
6872 If the long listing format (e.g., @option{-l}, @option{-o}) is being used,
6873 print the status change time (the @samp{ctime} in the inode) instead of
6874 the modification time.
6875 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6876 or when not using a long listing format,
6877 sort according to the status change time.
6881 @cindex unsorted directory listing
6882 @cindex directory order, listing by
6883 Primarily, like @option{-U}---do not sort; list the files in whatever
6884 order they are stored in the directory. But also enable @option{-a} (list
6885 all files) and disable @option{-l}, @option{--color}, and @option{-s} (if they
6886 were specified before the @option{-f}).
6892 @cindex reverse sorting
6893 Reverse whatever the sorting method is---e.g., list files in reverse
6894 alphabetical order, youngest first, smallest first, or whatever.
6900 @opindex size of files@r{, sorting files by}
6901 Sort by file size, largest first.
6907 @opindex modification time@r{, sorting files by}
6908 Sort by modification time (the @samp{mtime} in the inode), newest first.
6912 @itemx --time=access
6916 @opindex use time@r{, printing or sorting files by}
6917 @opindex atime@r{, printing or sorting files by}
6918 @opindex access time@r{, printing or sorting files by}
6919 If the long listing format (e.g., @option{--format=long}) is being used,
6920 print the last access time (the @samp{atime} in the inode).
6921 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6922 or when not using a long listing format, sort according to the access time.
6928 @opindex none@r{, sorting option for @command{ls}}
6929 Do not sort; list the files in whatever order they are
6930 stored in the directory. (Do not do any of the other unrelated things
6931 that @option{-f} does.) This is especially useful when listing very large
6932 directories, since not doing any sorting can be noticeably faster.
6935 @itemx --sort=version
6938 @opindex version@r{, sorting option for @command{ls}}
6939 Sort by version name and number, lowest first. It behaves like a default
6940 sort, except that each sequence of decimal digits is treated numerically
6941 as an index/version number. (@xref{Details about version sort}.)
6944 @itemx --sort=extension
6947 @opindex extension@r{, sorting files by}
6948 Sort directory contents alphabetically by file extension (characters
6949 after the last @samp{.}); files with no extension are sorted first.
6954 @node Details about version sort
6955 @subsection Details about version sort
6957 Version sorting handles the fact that file names frequently include indices or
6958 version numbers. Standard sorting usually does not produce the order that one
6959 expects because comparisons are made on a character-by-character basis.
6960 Version sorting is especially useful when browsing directories that contain
6961 many files with indices/version numbers in their names:
6965 abc.zml-1.gz abc.zml-1.gz
6966 abc.zml-12.gz abc.zml-2.gz
6967 abc.zml-2.gz abc.zml-12.gz
6970 Version-sorted strings are compared such that if @var{ver1} and @var{ver2}
6971 are version numbers and @var{prefix} and @var{suffix} (@var{suffix} matching
6972 the regular expression @samp{(\.[A-Za-z~][A-Za-z0-9~]*)*}) are strings then
6973 @var{ver1} < @var{ver2} implies that the name composed of
6974 ``@var{prefix} @var{ver1} @var{suffix}'' sorts before
6975 ``@var{prefix} @var{ver2} @var{suffix}''.
6977 Note also that leading zeros of numeric parts are ignored:
6981 abc-1.007.tgz abc-1.01a.tgz
6982 abc-1.012b.tgz abc-1.007.tgz
6983 abc-1.01a.tgz abc-1.012b.tgz
6986 This functionality is implemented using gnulib's @code{filevercmp} function,
6987 which has some caveats worth noting.
6990 @item @env{LC_COLLATE} is ignored, which means @samp{ls -v} and @samp{sort -V}
6991 will sort non-numeric prefixes as if the @env{LC_COLLATE} locale category
6992 was set to @samp{C}.
6993 @item Some suffixes will not be matched by the regular
6994 expression mentioned above. Consequently these examples may
6995 not sort as you expect:
7003 abc-1.2.3.4.x86_64.rpm
7004 abc-1.2.3.x86_64.rpm
7008 @node General output formatting
7009 @subsection General output formatting
7011 These options affect the appearance of the overall output.
7016 @itemx --format=single-column
7019 @opindex single-column @r{output of files}
7020 List one file per line. This is the default for @command{ls} when standard
7021 output is not a terminal.
7024 @itemx --format=vertical
7027 @opindex vertical @r{sorted files in columns}
7028 List files in columns, sorted vertically. This is the default for
7029 @command{ls} if standard output is a terminal. It is always the default
7030 for the @command{dir} program.
7031 @sc{gnu} @command{ls} uses variable width columns to display as many files as
7032 possible in the fewest lines.
7034 @item --color [=@var{when}]
7036 @cindex color, distinguishing file types with
7037 Specify whether to use color for distinguishing file types. @var{when}
7038 may be omitted, or one of:
7041 @vindex none @r{color option}
7042 - Do not use color at all. This is the default.
7044 @vindex auto @r{color option}
7045 @cindex terminal, using color iff
7046 - Only use color if standard output is a terminal.
7048 @vindex always @r{color option}
7051 Specifying @option{--color} and no @var{when} is equivalent to
7052 @option{--color=always}.
7053 Piping a colorized listing through a pager like @command{more} or
7054 @command{less} usually produces unreadable results. However, using
7055 @code{more -f} does seem to work.
7058 @vindex SHELL @r{environment variable, and color}
7059 Note that using the @option{--color} option may incur a noticeable
7060 performance penalty when run in a directory with very many entries,
7061 because the default settings require that @command{ls} @code{stat} every
7062 single file it lists.
7063 However, if you would like most of the file-type coloring
7064 but can live without the other coloring options (e.g.,
7065 executable, orphan, sticky, other-writable, capability), use
7066 @command{dircolors} to set the @env{LS_COLORS} environment variable like this,
7068 eval $(dircolors -p | perl -pe \
7069 's/^((CAP|S[ET]|O[TR]|M|E)\w+).*/$1 00/' | dircolors -)
7071 and on a @code{dirent.d_type}-capable file system, @command{ls}
7072 will perform only one @code{stat} call per command line argument.
7076 @itemx --indicator-style=classify
7079 @opindex --indicator-style
7080 @cindex file type and executables, marking
7081 @cindex executables and file type, marking
7082 Append a character to each file name indicating the file type. Also,
7083 for regular files that are executable, append @samp{*}. The file type
7084 indicators are @samp{/} for directories, @samp{@@} for symbolic links,
7085 @samp{|} for FIFOs, @samp{=} for sockets, @samp{>} for doors,
7086 and nothing for regular files.
7087 @c The following sentence is the same as the one for -d.
7088 Do not follow symbolic links listed on the
7089 command line unless the @option{--dereference-command-line} (@option{-H}),
7090 @option{--dereference} (@option{-L}), or
7091 @option{--dereference-command-line-symlink-to-dir} options are specified.
7094 @itemx --indicator-style=file-type
7095 @opindex --file-type
7096 @opindex --indicator-style
7097 @cindex file type, marking
7098 Append a character to each file name indicating the file type. This is
7099 like @option{-F}, except that executables are not marked.
7101 @item --indicator-style=@var{word}
7102 @opindex --indicator-style
7103 Append a character indicator with style @var{word} to entry names,
7108 Do not append any character indicator; this is the default.
7110 Append @samp{/} for directories. This is the same as the @option{-p}
7113 Append @samp{/} for directories, @samp{@@} for symbolic links, @samp{|}
7114 for FIFOs, @samp{=} for sockets, and nothing for regular files. This is
7115 the same as the @option{--file-type} option.
7117 Append @samp{*} for executable regular files, otherwise behave as for
7118 @samp{file-type}. This is the same as the @option{-F} or
7119 @option{--classify} option.
7124 Print file sizes in 1024-byte blocks, overriding the default block
7125 size (@pxref{Block size}).
7126 This option is equivalent to @option{--block-size=1K}.
7129 @itemx --format=commas
7132 @opindex commas@r{, outputting between files}
7133 List files horizontally, with as many as will fit on each line,
7134 separated by @samp{, } (a comma and a space).
7137 @itemx --indicator-style=slash
7139 @opindex --indicator-style
7140 @cindex file type, marking
7141 Append a @samp{/} to directory names.
7144 @itemx --format=across
7145 @itemx --format=horizontal
7148 @opindex across@r{, listing files}
7149 @opindex horizontal@r{, listing files}
7150 List the files in columns, sorted horizontally.
7153 @itemx --tabsize=@var{cols}
7156 Assume that each tab stop is @var{cols} columns wide. The default is 8.
7157 @command{ls} uses tabs where possible in the output, for efficiency. If
7158 @var{cols} is zero, do not use tabs at all.
7160 @c FIXME: remove in 2009, if Apple Terminal has been fixed for long enough.
7161 Some terminal emulators (at least Apple Terminal 1.5 (133) from Mac OS X 10.4.8)
7162 do not properly align columns to the right of a TAB following a
7163 non-@acronym{ASCII} byte. If you use such a terminal emulator, use the
7164 @option{-T0} option or put @code{TABSIZE=0} in your environment to tell
7165 @command{ls} to align using spaces, not tabs.
7168 @itemx --width=@var{cols}
7172 Assume the screen is @var{cols} columns wide. The default is taken
7173 from the terminal settings if possible; otherwise the environment
7174 variable @env{COLUMNS} is used if it is set; otherwise the default
7180 @node Formatting file timestamps
7181 @subsection Formatting file timestamps
7183 By default, file timestamps are listed in abbreviated form, using
7184 a date like @samp{Mar 30@ @ 2002} for non-recent timestamps, and a
7185 date-without-year and time like @samp{Mar 30 23:45} for recent timestamps.
7186 This format can change depending on the current locale as detailed below.
7188 A timestamp is considered to be @dfn{recent} if it is less than six
7189 months old, and is not dated in the future. If a timestamp dated
7190 today is not listed in recent form, the timestamp is in the future,
7191 which means you probably have clock skew problems which may break
7192 programs like @command{make} that rely on file timestamps.
7195 Time stamps are listed according to the time zone rules specified by
7196 the @env{TZ} environment variable, or by the system default rules if
7197 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
7198 with @env{TZ}, libc, The GNU C Library Reference Manual}.
7200 The following option changes how file timestamps are printed.
7203 @item --time-style=@var{style}
7204 @opindex --time-style
7206 List timestamps in style @var{style}. The @var{style} should
7207 be one of the following:
7212 List timestamps using @var{format}, where @var{format} is interpreted
7213 like the format argument of @command{date} (@pxref{date invocation}).
7214 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
7215 @command{ls} to list timestamps like @samp{2002-03-30 23:45:56}. As
7216 with @command{date}, @var{format}'s interpretation is affected by the
7217 @env{LC_TIME} locale category.
7219 If @var{format} contains two format strings separated by a newline,
7220 the former is used for non-recent files and the latter for recent
7221 files; if you want output columns to line up, you may need to insert
7222 spaces in one of the two formats.
7225 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
7226 format with nanosecond precision, e.g., @samp{2002-03-30
7227 23:45:56.477817180 -0700}. This style is equivalent to
7228 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
7230 This is useful because the time output includes all the information that
7231 is available from the operating system. For example, this can help
7232 explain @command{make}'s behavior, since @acronym{GNU} @command{make}
7233 uses the full timestamp to determine whether a file is out of date.
7236 List @acronym{ISO} 8601 date and time in minutes, e.g.,
7237 @samp{2002-03-30 23:45}. These timestamps are shorter than
7238 @samp{full-iso} timestamps, and are usually good enough for everyday
7239 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
7242 List @acronym{ISO} 8601 dates for non-recent timestamps (e.g.,
7243 @samp{2002-03-30@ }), and @acronym{ISO} 8601 month, day, hour, and
7244 minute for recent timestamps (e.g., @samp{03-30 23:45}). These
7245 timestamps are uglier than @samp{long-iso} timestamps, but they carry
7246 nearly the same information in a smaller space and their brevity helps
7247 @command{ls} output fit within traditional 80-column output lines.
7248 The following two @command{ls} invocations are equivalent:
7253 ls -l --time-style="+%Y-%m-%d $newline%m-%d %H:%M"
7254 ls -l --time-style="iso"
7259 List timestamps in a locale-dependent form. For example, a Finnish
7260 locale might list non-recent timestamps like @samp{maalis 30@ @ 2002}
7261 and recent timestamps like @samp{maalis 30 23:45}. Locale-dependent
7262 timestamps typically consume more space than @samp{iso} timestamps and
7263 are harder for programs to parse because locale conventions vary so
7264 widely, but they are easier for many people to read.
7266 The @env{LC_TIME} locale category specifies the timestamp format. The
7267 default @acronym{POSIX} locale uses timestamps like @samp{Mar 30@
7268 @ 2002} and @samp{Mar 30 23:45}; in this locale, the following two
7269 @command{ls} invocations are equivalent:
7274 ls -l --time-style="+%b %e %Y$newline%b %e %H:%M"
7275 ls -l --time-style="locale"
7278 Other locales behave differently. For example, in a German locale,
7279 @option{--time-style="locale"} might be equivalent to
7280 @option{--time-style="+%e. %b %Y $newline%e. %b %H:%M"}
7281 and might generate timestamps like @samp{30. M@"ar 2002@ } and
7282 @samp{30. M@"ar 23:45}.
7284 @item posix-@var{style}
7286 List @acronym{POSIX}-locale timestamps if the @env{LC_TIME} locale
7287 category is @acronym{POSIX}, @var{style} timestamps otherwise. For
7288 example, the @samp{posix-long-iso} style lists
7289 timestamps like @samp{Mar 30@ @ 2002} and @samp{Mar 30 23:45} when in
7290 the @acronym{POSIX} locale, and like @samp{2002-03-30 23:45} otherwise.
7295 You can specify the default value of the @option{--time-style} option
7296 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
7297 the default style is @samp{locale}. @acronym{GNU} Emacs 21.3 and
7298 later use the @option{--dired} option and therefore can parse any date
7299 format, but if you are using Emacs 21.1 or 21.2 and specify a
7300 non-@acronym{POSIX} locale you may need to set
7301 @samp{TIME_STYLE="posix-long-iso"}.
7303 To avoid certain denial-of-service attacks, timestamps that would be
7304 longer than 1000 bytes may be treated as errors.
7307 @node Formatting the file names
7308 @subsection Formatting the file names
7310 These options change how file names themselves are printed.
7316 @itemx --quoting-style=escape
7319 @opindex --quoting-style
7320 @cindex backslash sequences for file names
7321 Quote nongraphic characters in file names using alphabetic and octal
7322 backslash sequences like those used in C.
7326 @itemx --quoting-style=literal
7329 @opindex --quoting-style
7330 Do not quote file names. However, with @command{ls} nongraphic
7331 characters are still printed as question marks if the output is a
7332 terminal and you do not specify the @option{--show-control-chars}
7336 @itemx --hide-control-chars
7338 @opindex --hide-control-chars
7339 Print question marks instead of nongraphic characters in file names.
7340 This is the default if the output is a terminal and the program is
7345 @itemx --quoting-style=c
7347 @opindex --quote-name
7348 @opindex --quoting-style
7349 Enclose file names in double quotes and quote nongraphic characters as
7352 @item --quoting-style=@var{word}
7353 @opindex --quoting-style
7354 @cindex quoting style
7355 Use style @var{word} to quote file names and other strings that may
7356 contain arbitrary characters. The @var{word} should
7357 be one of the following:
7361 Output strings as-is; this is the same as the @option{-N} or
7362 @option{--literal} option.
7364 Quote strings for the shell if they contain shell metacharacters or would
7365 cause ambiguous output.
7366 The quoting is suitable for @acronym{POSIX}-compatible shells like
7367 @command{bash}, but it does not always work for incompatible shells
7370 Quote strings for the shell, even if they would normally not require quoting.
7372 Quote strings as for C character string literals, including the
7373 surrounding double-quote characters; this is the same as the
7374 @option{-Q} or @option{--quote-name} option.
7376 Quote strings as for C character string literals, except omit the
7377 surrounding double-quote
7378 characters; this is the same as the @option{-b} or @option{--escape} option.
7380 Quote strings as for C character string literals, except use
7381 surrounding quotation marks appropriate for the
7384 @c Use @t instead of @samp to avoid duplicate quoting in some output styles.
7385 Quote strings as for C character string literals, except use
7386 surrounding quotation marks appropriate for the locale, and quote
7387 @t{`like this'} instead of @t{"like
7388 this"} in the default C locale. This looks nicer on many displays.
7391 You can specify the default value of the @option{--quoting-style} option
7392 with the environment variable @env{QUOTING_STYLE}. If that environment
7393 variable is not set, the default value is @samp{literal}, but this
7394 default may change to @samp{shell} in a future version of this package.
7396 @item --show-control-chars
7397 @opindex --show-control-chars
7398 Print nongraphic characters as-is in file names.
7399 This is the default unless the output is a terminal and the program is
7405 @node dir invocation
7406 @section @command{dir}: Briefly list directory contents
7409 @cindex directory listing, brief
7411 @command{dir} is equivalent to @code{ls -C
7412 -b}; that is, by default files are listed in columns, sorted vertically,
7413 and special characters are represented by backslash escape sequences.
7415 @xref{ls invocation, @command{ls}}.
7418 @node vdir invocation
7419 @section @command{vdir}: Verbosely list directory contents
7422 @cindex directory listing, verbose
7424 @command{vdir} is equivalent to @code{ls -l
7425 -b}; that is, by default files are listed in long format and special
7426 characters are represented by backslash escape sequences.
7428 @node dircolors invocation
7429 @section @command{dircolors}: Color setup for @command{ls}
7433 @cindex setup for color
7435 @command{dircolors} outputs a sequence of shell commands to set up the
7436 terminal for color output from @command{ls} (and @command{dir}, etc.).
7440 eval "`dircolors [@var{option}]@dots{} [@var{file}]`"
7443 If @var{file} is specified, @command{dircolors} reads it to determine which
7444 colors to use for which file types and extensions. Otherwise, a
7445 precompiled database is used. For details on the format of these files,
7446 run @samp{dircolors --print-database}.
7448 To make @command{dircolors} read a @file{~/.dircolors} file if it
7449 exists, you can put the following lines in your @file{~/.bashrc} (or
7450 adapt them to your favorite shell):
7454 test -r $d && eval "$(dircolors $d)"
7458 @vindex SHELL @r{environment variable, and color}
7459 The output is a shell command to set the @env{LS_COLORS} environment
7460 variable. You can specify the shell syntax to use on the command line,
7461 or @command{dircolors} will guess it from the value of the @env{SHELL}
7462 environment variable.
7464 The program accepts the following options. Also see @ref{Common options}.
7469 @itemx --bourne-shell
7472 @opindex --bourne-shell
7473 @cindex Bourne shell syntax for color setup
7474 @cindex @command{sh} syntax for color setup
7475 Output Bourne shell commands. This is the default if the @env{SHELL}
7476 environment variable is set and does not end with @samp{csh} or
7485 @cindex C shell syntax for color setup
7486 @cindex @command{csh} syntax for color setup
7487 Output C shell commands. This is the default if @code{SHELL} ends with
7488 @command{csh} or @command{tcsh}.
7491 @itemx --print-database
7493 @opindex --print-database
7494 @cindex color database, printing
7495 @cindex database for color setup, printing
7496 @cindex printing color database
7497 Print the (compiled-in) default color configuration database. This
7498 output is itself a valid configuration file, and is fairly descriptive
7499 of the possibilities.
7506 @node Basic operations
7507 @chapter Basic operations
7509 @cindex manipulating files
7511 This chapter describes the commands for basic file manipulation:
7512 copying, moving (renaming), and deleting (removing).
7515 * cp invocation:: Copy files.
7516 * dd invocation:: Convert and copy a file.
7517 * install invocation:: Copy files and set attributes.
7518 * mv invocation:: Move (rename) files.
7519 * rm invocation:: Remove files or directories.
7520 * shred invocation:: Remove files more securely.
7525 @section @command{cp}: Copy files and directories
7528 @cindex copying files and directories
7529 @cindex files, copying
7530 @cindex directories, copying
7532 @command{cp} copies files (or, optionally, directories). The copy is
7533 completely independent of the original. You can either copy one file to
7534 another, or copy arbitrarily many files to a destination directory.
7538 cp [@var{option}]@dots{} [-T] @var{source} @var{dest}
7539 cp [@var{option}]@dots{} @var{source}@dots{} @var{directory}
7540 cp [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
7545 If two file names are given, @command{cp} copies the first file to the
7549 If the @option{--target-directory} (@option{-t}) option is given, or
7550 failing that if the last file is a directory and the
7551 @option{--no-target-directory} (@option{-T}) option is not given,
7552 @command{cp} copies each @var{source} file to the specified directory,
7553 using the @var{source}s' names.
7556 Generally, files are written just as they are read. For exceptions,
7557 see the @option{--sparse} option below.
7559 By default, @command{cp} does not copy directories. However, the
7560 @option{-R}, @option{-a}, and @option{-r} options cause @command{cp} to
7561 copy recursively by descending into source directories and copying files
7562 to corresponding destination directories.
7564 When copying from a symbolic link, @command{cp} normally follows the
7565 link only when not copying
7566 recursively. This default can be overridden with the
7567 @option{--archive} (@option{-a}), @option{-d}, @option{--dereference}
7568 (@option{-L}), @option{--no-dereference} (@option{-P}), and
7569 @option{-H} options. If more than one of these options is specified,
7570 the last one silently overrides the others.
7572 When copying to a symbolic link, @command{cp} follows the
7573 link only when it refers to an existing regular file.
7574 However, when copying to a dangling symbolic link, @command{cp}
7575 refuses by default, and fails with a diagnostic, since the operation
7576 is inherently dangerous. This behavior is contrary to historical
7577 practice and to @acronym{POSIX}.
7578 Set @env{POSIXLY_CORRECT} to make @command{cp} attempt to create
7579 the target of a dangling destination symlink, in spite of the possible risk.
7580 Also, when an option like
7581 @option{--backup} or @option{--link} acts to rename or remove the
7582 destination before copying, @command{cp} renames or removes the
7583 symbolic link rather than the file it points to.
7585 By default, @command{cp} copies the contents of special files only
7586 when not copying recursively. This default can be overridden with the
7587 @option{--copy-contents} option.
7589 @cindex self-backups
7590 @cindex backups, making only
7591 @command{cp} generally refuses to copy a file onto itself, with the
7592 following exception: if @option{--force --backup} is specified with
7593 @var{source} and @var{dest} identical, and referring to a regular file,
7594 @command{cp} will make a backup file, either regular or numbered, as
7595 specified in the usual ways (@pxref{Backup options}). This is useful when
7596 you simply want to make a backup of an existing file before changing it.
7598 The program accepts the following options. Also see @ref{Common options}.
7605 Preserve as much as possible of the structure and attributes of the
7606 original files in the copy (but do not attempt to preserve internal
7607 directory structure; i.e., @samp{ls -U} may list the entries in a copied
7608 directory in a different order).
7609 Try to preserve SELinux security context and extended attributes (xattr),
7610 but ignore any failure to do that and print no corresponding diagnostic.
7611 Equivalent to @option{-dR --preserve=all} with the reduced diagnostics.
7613 @itemx --attributes-only
7614 @opindex --attributes-only
7615 Preserve the specified attributes of the original files in the copy,
7616 but do not copy any data. See the @option{--preserve} option for
7617 controlling which attributes to copy.
7620 @itemx @w{@kbd{--backup}[=@var{method}]}
7623 @vindex VERSION_CONTROL
7624 @cindex backups, making
7625 @xref{Backup options}.
7626 Make a backup of each file that would otherwise be overwritten or removed.
7627 As a special case, @command{cp} makes a backup of @var{source} when the force
7628 and backup options are given and @var{source} and @var{dest} are the same
7629 name for an existing, regular file. One useful application of this
7630 combination of options is this tiny Bourne shell script:
7634 # Usage: backup FILE...
7635 # Create a @sc{gnu}-style backup of each listed FILE.
7637 cp --backup --force -- "$i" "$i"
7641 @item --copy-contents
7642 @cindex directories, copying recursively
7643 @cindex copying directories recursively
7644 @cindex recursively copying directories
7645 @cindex non-directories, copying as special files
7646 If copying recursively, copy the contents of any special files (e.g.,
7647 FIFOs and device files) as if they were regular files. This means
7648 trying to read the data in each source file and writing it to the
7649 destination. It is usually a mistake to use this option, as it
7650 normally has undesirable effects on special files like FIFOs and the
7651 ones typically found in the @file{/dev} directory. In most cases,
7652 @code{cp -R --copy-contents} will hang indefinitely trying to read
7653 from FIFOs and special files like @file{/dev/console}, and it will
7654 fill up your destination disk if you use it to copy @file{/dev/zero}.
7655 This option has no effect unless copying recursively, and it does not
7656 affect the copying of symbolic links.
7660 @cindex symbolic links, copying
7661 @cindex hard links, preserving
7662 Copy symbolic links as symbolic links rather than copying the files that
7663 they point to, and preserve hard links between source files in the copies.
7664 Equivalent to @option{--no-dereference --preserve=links}.
7670 When copying without this option and an existing destination file cannot
7671 be opened for writing, the copy fails. However, with @option{--force}),
7672 when a destination file cannot be opened, @command{cp} then removes it and
7673 tries to open it again. Contrast this behavior with that enabled by
7674 @option{--link} and @option{--symbolic-link}, whereby the destination file
7675 is never opened but rather is removed unconditionally. Also see the
7676 description of @option{--remove-destination}.
7678 This option is independent of the @option{--interactive} or
7679 @option{-i} option: neither cancels the effect of the other.
7681 This option is redundant if the @option{--no-clobber} or @option{-n} option is
7686 If a command line argument specifies a symbolic link, then copy the
7687 file it points to rather than the symbolic link itself. However,
7688 copy (preserving its nature) any symbolic link that is encountered
7689 via recursive traversal.
7692 @itemx --interactive
7694 @opindex --interactive
7695 When copying a file other than a directory, prompt whether to
7696 overwrite an existing destination file. The @option{-i} option overrides
7697 a previous @option{-n} option.
7703 Make hard links instead of copies of non-directories.
7706 @itemx --dereference
7708 @opindex --dereference
7709 Follow symbolic links when copying from them.
7710 With this option, @command{cp} cannot create a symbolic link.
7711 For example, a symlink (to regular file) in the source tree will be copied to
7712 a regular file in the destination tree.
7717 @opindex --no-clobber
7718 Do not overwrite an existing file. The @option{-n} option overrides a previous
7719 @option{-i} option. This option is mutually exclusive with @option{-b} or
7720 @option{--backup} option.
7723 @itemx --no-dereference
7725 @opindex --no-dereference
7726 @cindex symbolic links, copying
7727 Copy symbolic links as symbolic links rather than copying the files that
7728 they point to. This option affects only symbolic links in the source;
7729 symbolic links in the destination are always followed if possible.
7732 @itemx @w{@kbd{--preserve}[=@var{attribute_list}]}
7735 @cindex file information, preserving, extended attributes, xattr
7736 Preserve the specified attributes of the original files.
7737 If specified, the @var{attribute_list} must be a comma-separated list
7738 of one or more of the following strings:
7742 Preserve the file mode bits and access control lists.
7744 Preserve the owner and group. On most modern systems,
7745 only users with appropriate privileges may change the owner of a file,
7747 may preserve the group ownership of a file only if they happen to be
7748 a member of the desired group.
7750 Preserve the times of last access and last modification, when possible.
7751 On older systems, it is not possible to preserve these attributes
7752 when the affected file is a symbolic link.
7753 However, many systems now provide the @code{utimensat} function,
7754 which makes it possible even for symbolic links.
7756 Preserve in the destination files
7757 any links between corresponding source files.
7758 Note that with @option{-L} or @option{-H}, this option can convert
7759 symbolic links to hard links. For example,
7761 $ mkdir c; : > a; ln -s a b; cp -aH a b c; ls -i1 c
7766 Note the inputs: @file{b} is a symlink to regular file @file{a},
7767 yet the files in destination directory, @file{c/}, are hard-linked.
7768 Since @option{-a} implies @option{--preserve=links}, and since @option{-H}
7769 tells @command{cp} to dereference command line arguments, it sees two files
7770 with the same inode number, and preserves the perceived hard link.
7772 Here is a similar example that exercises @command{cp}'s @option{-L} option:
7774 $ mkdir b c; (cd b; : > a; ln -s a b); cp -aL b c; ls -i1 c/b
7780 Preserve SELinux security context of the file, or fail with full diagnostics.
7782 Preserve extended attributes of the file, or fail with full diagnostics.
7783 If @command{cp} is built without xattr support, ignore this option.
7784 If SELinux context, ACLs or Capabilities are implemented using xattrs,
7785 they are preserved by this option as well.
7787 Preserve all file attributes.
7788 Equivalent to specifying all of the above, but with the difference
7789 that failure to preserve SELinux security context or extended attributes
7790 does not change @command{cp}'s exit status. In contrast to @option{-a},
7791 all but @samp{Operation not supported} warnings are output.
7794 Using @option{--preserve} with no @var{attribute_list} is equivalent
7795 to @option{--preserve=mode,ownership,timestamps}.
7797 In the absence of this option, each destination file is created with the
7798 mode bits of the corresponding source file, minus the bits set in the
7799 umask and minus the set-user-ID and set-group-ID bits.
7800 @xref{File permissions}.
7802 @itemx @w{@kbd{--no-preserve}=@var{attribute_list}}
7803 @cindex file information, preserving
7804 Do not preserve the specified attributes. The @var{attribute_list}
7805 has the same form as for @option{--preserve}.
7809 @cindex parent directories and @command{cp}
7810 Form the name of each destination file by appending to the target
7811 directory a slash and the specified name of the source file. The last
7812 argument given to @command{cp} must be the name of an existing directory.
7813 For example, the command:
7816 cp --parents a/b/c existing_dir
7820 copies the file @file{a/b/c} to @file{existing_dir/a/b/c}, creating
7821 any missing intermediate directories.
7828 @opindex --recursive
7829 @cindex directories, copying recursively
7830 @cindex copying directories recursively
7831 @cindex recursively copying directories
7832 @cindex non-directories, copying as special files
7833 Copy directories recursively. By default, do not follow symbolic
7834 links in the source; see the @option{--archive} (@option{-a}), @option{-d},
7835 @option{--dereference} (@option{-L}), @option{--no-dereference}
7836 (@option{-P}), and @option{-H} options. Special files are copied by
7837 creating a destination file of the same type as the source; see the
7838 @option{--copy-contents} option. It is not portable to use
7839 @option{-r} to copy symbolic links or special files. On some
7840 non-@sc{gnu} systems, @option{-r} implies the equivalent of
7841 @option{-L} and @option{--copy-contents} for historical reasons.
7842 Also, it is not portable to use @option{-R} to copy symbolic links
7843 unless you also specify @option{-P}, as @acronym{POSIX} allows
7844 implementations that dereference symbolic links by default.
7846 @item --reflink[=@var{when}]
7847 @opindex --reflink[=@var{when}]
7850 @cindex copy on write
7851 Perform a lightweight, copy-on-write (COW) copy, if supported by the
7852 file system. Once it has succeeded, beware that the source and destination
7853 files share the same disk data blocks as long as they remain unmodified.
7854 Thus, if a disk I/O error affects data blocks of one of the files,
7855 the other suffers the same fate.
7857 The @var{when} value can be one of the following:
7861 The default behavior: if the copy-on-write operation is not supported
7862 then report the failure for each file and exit with a failure status.
7865 If the copy-on-write operation is not supported then fall back
7866 to the standard copy behaviour.
7869 This option is overridden by the @option{--link}, @option{--symbolic-link}
7870 and @option{--attributes-only} options, thus allowing it to be used
7871 to configure the default data copying behavior for @command{cp}.
7872 For example, with the following alias, @command{cp} will use the
7873 minimum amount of space supported by the file system.
7876 alias cp='cp --reflink=auto --sparse=always'
7879 @item --remove-destination
7880 @opindex --remove-destination
7881 Remove each existing destination file before attempting to open it
7882 (contrast with @option{-f} above).
7884 @item --sparse=@var{when}
7885 @opindex --sparse=@var{when}
7886 @cindex sparse files, copying
7887 @cindex holes, copying files with
7888 @findex read @r{system call, and holes}
7889 A @dfn{sparse file} contains @dfn{holes}---a sequence of zero bytes that
7890 does not occupy any physical disk blocks; the @samp{read} system call
7891 reads these as zeros. This can both save considerable disk space and
7892 increase speed, since many binary files contain lots of consecutive zero
7893 bytes. By default, @command{cp} detects holes in input source files via a crude
7894 heuristic and makes the corresponding output file sparse as well.
7895 Only regular files may be sparse.
7897 The @var{when} value can be one of the following:
7901 The default behavior: if the input file is sparse, attempt to make
7902 the output file sparse, too. However, if an output file exists but
7903 refers to a non-regular file, then do not attempt to make it sparse.
7906 For each sufficiently long sequence of zero bytes in the input file,
7907 attempt to create a corresponding hole in the output file, even if the
7908 input file does not appear to be sparse.
7909 This is useful when the input file resides on a file system
7910 that does not support sparse files
7911 (for example, @samp{efs} file systems in SGI IRIX 5.3 and earlier),
7912 but the output file is on a type of file system that does support them.
7913 Holes may be created only in regular files, so if the destination file
7914 is of some other type, @command{cp} does not even try to make it sparse.
7917 Never make the output file sparse.
7918 This is useful in creating a file for use with the @command{mkswap} command,
7919 since such a file must not have any holes.
7922 @optStripTrailingSlashes
7925 @itemx --symbolic-link
7927 @opindex --symbolic-link
7928 @cindex symbolic links, copying with
7929 Make symbolic links instead of copies of non-directories. All source
7930 file names must be absolute (starting with @samp{/}) unless the
7931 destination files are in the current directory. This option merely
7932 results in an error message on systems that do not support symbolic links.
7938 @optNoTargetDirectory
7944 @cindex newer files, copying only
7945 Do not copy a non-directory that has an existing destination with the
7946 same or newer modification time. If time stamps are being preserved,
7947 the comparison is to the source time stamp truncated to the
7948 resolutions of the destination file system and of the system calls
7949 used to update time stamps; this avoids duplicate work if several
7950 @samp{cp -pu} commands are executed with the same source and
7957 Print the name of each file before copying it.
7960 @itemx --one-file-system
7962 @opindex --one-file-system
7963 @cindex file systems, omitting copying to different
7964 Skip subdirectories that are on different file systems from the one that
7965 the copy started on.
7966 However, mount point directories @emph{are} copied.
7974 @section @command{dd}: Convert and copy a file
7977 @cindex converting while copying a file
7979 @command{dd} copies a file (from standard input to standard output, by
7980 default) with a changeable I/O block size, while optionally performing
7981 conversions on it. Synopses:
7984 dd [@var{operand}]@dots{}
7988 The only options are @option{--help} and @option{--version}.
7989 @xref{Common options}. @command{dd} accepts the following operands.
7995 Read from @var{file} instead of standard input.
7999 Write to @var{file} instead of standard output. Unless
8000 @samp{conv=notrunc} is given, @command{dd} truncates @var{file} to zero
8001 bytes (or the size specified with @samp{seek=}).
8003 @item ibs=@var{bytes}
8005 @cindex block size of input
8006 @cindex input block size
8007 Set the input block size to @var{bytes}.
8008 This makes @command{dd} read @var{bytes} per block.
8009 The default is 512 bytes.
8011 @item obs=@var{bytes}
8013 @cindex block size of output
8014 @cindex output block size
8015 Set the output block size to @var{bytes}.
8016 This makes @command{dd} write @var{bytes} per block.
8017 The default is 512 bytes.
8019 @item bs=@var{bytes}
8022 Set both input and output block sizes to @var{bytes}.
8023 This makes @command{dd} read and write @var{bytes} per block,
8024 overriding any @samp{ibs} and @samp{obs} settings.
8025 In addition, if no data-transforming @option{conv} option is specified,
8026 input is copied to the output as soon as it's read,
8027 even if it is smaller than the block size.
8029 @item cbs=@var{bytes}
8031 @cindex block size of conversion
8032 @cindex conversion block size
8033 @cindex fixed-length records, converting to variable-length
8034 @cindex variable-length records, converting to fixed-length
8035 Set the conversion block size to @var{bytes}.
8036 When converting variable-length records to fixed-length ones
8037 (@option{conv=block}) or the reverse (@option{conv=unblock}),
8038 use @var{bytes} as the fixed record length.
8040 @item skip=@var{blocks}
8042 Skip @var{blocks} @samp{ibs}-byte blocks in the input file before copying.
8044 @item seek=@var{blocks}
8046 Skip @var{blocks} @samp{obs}-byte blocks in the output file before copying.
8048 @item count=@var{blocks}
8050 Copy @var{blocks} @samp{ibs}-byte blocks from the input file, instead
8051 of everything until the end of the file.
8055 Do not print the overall transfer rate and volume statistics
8056 that normally make up the third status line when @command{dd} exits.
8058 @item conv=@var{conversion}[,@var{conversion}]@dots{}
8060 Convert the file as specified by the @var{conversion} argument(s).
8061 (No spaces around any comma(s).)
8068 @opindex ascii@r{, converting to}
8069 Convert @acronym{EBCDIC} to @acronym{ASCII},
8070 using the conversion table specified by @acronym{POSIX}.
8071 This provides a 1:1 translation for all 256 bytes.
8074 @opindex ebcdic@r{, converting to}
8075 Convert @acronym{ASCII} to @acronym{EBCDIC}.
8076 This is the inverse of the @samp{ascii} conversion.
8079 @opindex alternate ebcdic@r{, converting to}
8080 Convert @acronym{ASCII} to alternate @acronym{EBCDIC},
8081 using the alternate conversion table specified by @acronym{POSIX}.
8082 This is not a 1:1 translation, but reflects common historical practice
8083 for @samp{~}, @samp{[}, and @samp{]}.
8085 The @samp{ascii}, @samp{ebcdic}, and @samp{ibm} conversions are
8089 @opindex block @r{(space-padding)}
8090 For each line in the input, output @samp{cbs} bytes, replacing the
8091 input newline with a space and padding with spaces as necessary.
8095 Remove any trailing spaces in each @samp{cbs}-sized input block,
8096 and append a newline.
8098 The @samp{block} and @samp{unblock} conversions are mutually exclusive.
8101 @opindex lcase@r{, converting to}
8102 Change uppercase letters to lowercase.
8105 @opindex ucase@r{, converting to}
8106 Change lowercase letters to uppercase.
8108 The @samp{lcase} and @samp{ucase} conversions are mutually exclusive.
8111 @opindex swab @r{(byte-swapping)}
8112 @cindex byte-swapping
8113 Swap every pair of input bytes. @sc{gnu} @command{dd}, unlike others, works
8114 when an odd number of bytes are read---the last byte is simply copied
8115 (since there is nothing to swap it with).
8118 @opindex sync @r{(padding with @acronym{ASCII} @sc{nul}s)}
8119 Pad every input block to size of @samp{ibs} with trailing zero bytes.
8120 When used with @samp{block} or @samp{unblock}, pad with spaces instead of
8125 The following ``conversions'' are really file flags
8126 and don't affect internal processing:
8131 @cindex creating output file, requiring
8132 Fail if the output file already exists; @command{dd} must create the
8137 @cindex creating output file, avoiding
8138 Do not create the output file; the output file must already exist.
8140 The @samp{excl} and @samp{nocreat} conversions are mutually exclusive.
8144 @cindex truncating output file, avoiding
8145 Do not truncate the output file.
8149 @cindex read errors, ignoring
8150 Continue after read errors.
8154 @cindex synchronized data writes, before finishing
8155 Synchronize output data just before finishing. This forces a physical
8156 write of output data.
8160 @cindex synchronized data and metadata writes, before finishing
8161 Synchronize output data and metadata just before finishing. This
8162 forces a physical write of output data and metadata.
8166 @item iflag=@var{flag}[,@var{flag}]@dots{}
8168 Access the input file using the flags specified by the @var{flag}
8169 argument(s). (No spaces around any comma(s).)
8171 @item oflag=@var{flag}[,@var{flag}]@dots{}
8173 Access the output file using the flags specified by the @var{flag}
8174 argument(s). (No spaces around any comma(s).)
8176 Here are the flags. Not every flag is supported on every operating
8183 @cindex appending to the output file
8184 Write in append mode, so that even if some other process is writing to
8185 this file, every @command{dd} write will append to the current
8186 contents of the file. This flag makes sense only for output.
8187 If you combine this flag with the @samp{of=@var{file}} operand,
8188 you should also specify @samp{conv=notrunc} unless you want the
8189 output file to be truncated before being appended to.
8193 @cindex concurrent I/O
8194 Use concurrent I/O mode for data. This mode performs direct I/O
8195 and drops the @acronym{POSIX} requirement to serialize all I/O to the same file.
8196 A file cannot be opened in CIO mode and with a standard open at the
8202 Use direct I/O for data, avoiding the buffer cache.
8203 Note that the kernel may impose restrictions on read or write buffer sizes.
8204 For example, with an ext4 destination file system and a linux-based kernel,
8205 using @samp{oflag=direct} will cause writes to fail with @code{EINVAL} if the
8206 output buffer size is not a multiple of 512.
8210 @cindex directory I/O
8212 Fail unless the file is a directory. Most operating systems do not
8213 allow I/O to a directory, so this flag has limited utility.
8217 @cindex synchronized data reads
8218 Use synchronized I/O for data. For the output file, this forces a
8219 physical write of output data on each write. For the input file,
8220 this flag can matter when reading from a remote file that has been
8221 written to synchronously by some other process. Metadata (e.g.,
8222 last-access and last-modified time) is not necessarily synchronized.
8226 @cindex synchronized data and metadata I/O
8227 Use synchronized I/O for both data and metadata.
8231 @cindex discarding file cache
8232 Discard the data cache for a file.
8233 When count=0 all cache is discarded,
8234 otherwise the cache is dropped for the processed
8235 portion of the file. Also when count=0
8236 failure to discard the cache is diagnosed
8237 and reflected in the exit status.
8238 Here as some usage examples:
8241 # Advise to drop cache for whole file
8242 dd if=ifile iflag=nocache count=0
8244 # Ensure drop cache for the whole file
8245 dd of=ofile oflag=nocache conv=notrunc,fdatasync count=0
8247 # Drop cache for part of file
8248 dd if=ifile iflag=nocache skip=10 count=10 of=/dev/null
8250 # Stream data using just the read-ahead cache
8251 dd if=ifile of=ofile iflag=nocache oflag=nocache
8256 @cindex nonblocking I/O
8257 Use non-blocking I/O.
8262 Do not update the file's access time.
8263 Some older file systems silently ignore this flag, so it is a good
8264 idea to test it on your files before relying on it.
8268 @cindex controlling terminal
8269 Do not assign the file to be a controlling terminal for @command{dd}.
8270 This has no effect when the file is not a terminal.
8271 On many hosts (e.g., @acronym{GNU}/Linux hosts), this option has no effect
8276 @cindex symbolic links, following
8277 Do not follow symbolic links.
8282 Fail if the file has multiple hard links.
8287 Use binary I/O. This option has an effect only on nonstandard
8288 platforms that distinguish binary from text I/O.
8293 Use text I/O. Like @samp{binary}, this option has no effect on
8298 Accumulate full blocks from input. The @code{read} system call
8299 may return early if a full block is not available.
8300 When that happens, continue calling @code{read} to fill the remainder
8302 This flag can be used only with @code{iflag}.
8306 These flags are not supported on all systems, and @samp{dd} rejects
8307 attempts to use them when they are not supported. When reading from
8308 standard input or writing to standard output, the @samp{nofollow} and
8309 @samp{noctty} flags should not be specified, and the other flags
8310 (e.g., @samp{nonblock}) can affect how other processes behave with the
8311 affected file descriptors, even after @command{dd} exits.
8315 @cindex multipliers after numbers
8316 The numeric-valued strings above (@var{bytes} and @var{blocks}) can be
8317 followed by a multiplier: @samp{b}=512, @samp{c}=1,
8318 @samp{w}=2, @samp{x@var{m}}=@var{m}, or any of the
8319 standard block size suffixes like @samp{k}=1024 (@pxref{Block size}).
8321 Any block size you specify via @samp{bs=}, @samp{ibs=}, @samp{obs=}, @samp{cbs=}
8322 should not be too large---values larger than a few megabytes
8323 are generally wasteful or (as in the gigabyte..exabyte case) downright
8324 counterproductive or error-inducing.
8326 Use different @command{dd} invocations to use different block sizes for
8327 skipping and I/O@. For example, the following shell commands copy data
8328 in 512 KiB blocks between a disk and a tape, but do not save or restore a
8329 4 KiB label at the start of the disk:
8332 disk=/dev/rdsk/c0t1d0s2
8335 # Copy all but the label from disk to tape.
8336 (dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape
8338 # Copy from tape back to disk, but leave the disk label alone.
8339 (dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk
8342 Sending an @samp{INFO} signal to a running @command{dd}
8343 process makes it print I/O statistics to standard error
8344 and then resume copying. In the example below,
8345 @command{dd} is run in the background to copy 10 million blocks.
8346 The @command{kill} command makes it output intermediate I/O statistics,
8347 and when @command{dd} completes normally or is killed by the
8348 @code{SIGINT} signal, it outputs the final statistics.
8351 $ dd if=/dev/zero of=/dev/null count=10MB & pid=$!
8352 $ kill -s INFO $pid; wait $pid
8353 3385223+0 records in
8354 3385223+0 records out
8355 1733234176 bytes (1.7 GB) copied, 6.42173 seconds, 270 MB/s
8356 10000000+0 records in
8357 10000000+0 records out
8358 5120000000 bytes (5.1 GB) copied, 18.913 seconds, 271 MB/s
8361 @vindex POSIXLY_CORRECT
8362 On systems lacking the @samp{INFO} signal @command{dd} responds to the
8363 @samp{USR1} signal instead, unless the @env{POSIXLY_CORRECT}
8364 environment variable is set.
8369 @node install invocation
8370 @section @command{install}: Copy files and set attributes
8373 @cindex copying files and setting attributes
8375 @command{install} copies files while setting their file mode bits and, if
8376 possible, their owner and group. Synopses:
8379 install [@var{option}]@dots{} [-T] @var{source} @var{dest}
8380 install [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8381 install [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8382 install [@var{option}]@dots{} -d @var{directory}@dots{}
8387 If two file names are given, @command{install} copies the first file to the
8391 If the @option{--target-directory} (@option{-t}) option is given, or
8392 failing that if the last file is a directory and the
8393 @option{--no-target-directory} (@option{-T}) option is not given,
8394 @command{install} copies each @var{source} file to the specified
8395 directory, using the @var{source}s' names.
8398 If the @option{--directory} (@option{-d}) option is given,
8399 @command{install} creates each @var{directory} and any missing parent
8400 directories. Parent directories are created with mode
8401 @samp{u=rwx,go=rx} (755), regardless of the @option{-m} option or the
8402 current umask. @xref{Directory Setuid and Setgid}, for how the
8403 set-user-ID and set-group-ID bits of parent directories are inherited.
8406 @cindex Makefiles, installing programs in
8407 @command{install} is similar to @command{cp}, but allows you to control the
8408 attributes of destination files. It is typically used in Makefiles to
8409 copy programs into their destination directories. It refuses to copy
8410 files onto themselves.
8412 @cindex extended attributes, xattr
8413 @command{install} never preserves extended attributes (xattr).
8415 The program accepts the following options. Also see @ref{Common options}.
8425 Compare each pair of source and destination files, and if the destination has
8426 identical content and any specified owner, group, permissions, and possibly
8427 SELinux context, then do not modify the destination at all.
8431 Ignored; for compatibility with old Unix versions of @command{install}.
8435 Create any missing parent directories of @var{dest},
8436 then copy @var{source} to @var{dest}.
8437 This option is ignored if a destination directory is specified
8438 via @option{--target-directory=DIR}.
8443 @opindex --directory
8444 @cindex directories, creating with given attributes
8445 @cindex parent directories, creating missing
8446 @cindex leading directories, creating missing
8447 Create any missing parent directories, giving them the default
8448 attributes. Then create each given directory, setting their owner,
8449 group and mode as given on the command line or to the defaults.
8451 @item -g @var{group}
8452 @itemx --group=@var{group}
8455 @cindex group ownership of installed files, setting
8456 Set the group ownership of installed files or directories to
8457 @var{group}. The default is the process's current group. @var{group}
8458 may be either a group name or a numeric group ID.
8461 @itemx --mode=@var{mode}
8464 @cindex permissions of installed files, setting
8465 Set the file mode bits for the installed file or directory to @var{mode},
8466 which can be either an octal number, or a symbolic mode as in
8467 @command{chmod}, with @samp{a=} (no access allowed to anyone) as the
8468 point of departure (@pxref{File permissions}).
8469 The default mode is @samp{u=rwx,go=rx,a-s}---read, write, and
8470 execute for the owner, read and execute for group and other, and with
8471 set-user-ID and set-group-ID disabled.
8472 This default is not quite the same as @samp{755}, since it disables
8473 instead of preserving set-user-ID and set-group-ID on directories.
8474 @xref{Directory Setuid and Setgid}.
8476 @item -o @var{owner}
8477 @itemx --owner=@var{owner}
8480 @cindex ownership of installed files, setting
8481 @cindex appropriate privileges
8482 @vindex root @r{as default owner}
8483 If @command{install} has appropriate privileges (is run as root), set the
8484 ownership of installed files or directories to @var{owner}. The default
8485 is @code{root}. @var{owner} may be either a user name or a numeric user
8488 @item --preserve-context
8489 @opindex --preserve-context
8491 @cindex security context
8492 Preserve the SELinux security context of files and directories.
8493 Failure to preserve the context in all of the files or directories
8494 will result in an exit status of 1. If SELinux is disabled then
8495 print a warning and ignore the option.
8498 @itemx --preserve-timestamps
8500 @opindex --preserve-timestamps
8501 @cindex timestamps of installed files, preserving
8502 Set the time of last access and the time of last modification of each
8503 installed file to match those of each corresponding original file.
8504 When a file is installed without this option, its last access and
8505 last modification times are both set to the time of installation.
8506 This option is useful if you want to use the last modification times
8507 of installed files to keep track of when they were last built as opposed
8508 to when they were last installed.
8514 @cindex symbol table information, stripping
8515 @cindex stripping symbol table information
8516 Strip the symbol tables from installed binary executables.
8518 @itemx --strip-program=@var{program}
8519 @opindex --strip-program
8520 @cindex symbol table information, stripping, program
8521 Program used to strip binaries.
8527 @optNoTargetDirectory
8533 Print the name of each file before copying it.
8535 @item -Z @var{context}
8536 @itemx --context=@var{context}
8540 @cindex security context
8541 Set the default SELinux security context to be used for any
8542 created files and directories. If SELinux is disabled then
8543 print a warning and ignore the option.
8551 @section @command{mv}: Move (rename) files
8555 @command{mv} moves or renames files (or directories). Synopses:
8558 mv [@var{option}]@dots{} [-T] @var{source} @var{dest}
8559 mv [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8560 mv [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8565 If two file names are given, @command{mv} moves the first file to the
8569 If the @option{--target-directory} (@option{-t}) option is given, or
8570 failing that if the last file is a directory and the
8571 @option{--no-target-directory} (@option{-T}) option is not given,
8572 @command{mv} moves each @var{source} file to the specified
8573 directory, using the @var{source}s' names.
8576 @command{mv} can move any type of file from one file system to another.
8577 Prior to version @code{4.0} of the fileutils,
8578 @command{mv} could move only regular files between file systems.
8579 For example, now @command{mv} can move an entire directory hierarchy
8580 including special device files from one partition to another. It first
8581 uses some of the same code that's used by @code{cp -a} to copy the
8582 requested directories and files, then (assuming the copy succeeded)
8583 it removes the originals. If the copy fails, then the part that was
8584 copied to the destination partition is removed. If you were to copy
8585 three directories from one partition to another and the copy of the first
8586 directory succeeded, but the second didn't, the first would be left on
8587 the destination partition and the second and third would be left on the
8590 @cindex extended attributes, xattr
8591 @command{mv} always tries to copy extended attributes (xattr), which may
8592 include SELinux context, ACLs or Capabilities.
8593 Upon failure all but @samp{Operation not supported} warnings are output.
8595 @cindex prompting, and @command{mv}
8596 If a destination file exists but is normally unwritable, standard input
8597 is a terminal, and the @option{-f} or @option{--force} option is not given,
8598 @command{mv} prompts the user for whether to replace the file. (You might
8599 own the file, or have write permission on its directory.) If the
8600 response is not affirmative, the file is skipped.
8602 @emph{Warning}: Avoid specifying a source name with a trailing slash,
8603 when it might be a symlink to a directory.
8604 Otherwise, @command{mv} may do something very surprising, since
8605 its behavior depends on the underlying rename system call.
8606 On a system with a modern Linux-based kernel, it fails with
8607 @code{errno=ENOTDIR}.
8608 However, on other systems (at least FreeBSD 6.1 and Solaris 10) it silently
8609 renames not the symlink but rather the directory referenced by the symlink.
8610 @xref{Trailing slashes}.
8612 The program accepts the following options. Also see @ref{Common options}.
8622 @cindex prompts, omitting
8623 Do not prompt the user before removing a destination file.
8625 If you specify more than one of the @option{-i}, @option{-f}, @option{-n}
8626 options, only the final one takes effect.
8631 @itemx --interactive
8633 @opindex --interactive
8634 @cindex prompts, forcing
8635 Prompt whether to overwrite each existing destination file, regardless
8637 If the response is not affirmative, the file is skipped.
8643 @opindex --no-clobber
8644 @cindex prompts, omitting
8645 Do not overwrite an existing file.
8647 This option is mutually exclusive with @option{-b} or @option{--backup} option.
8653 @cindex newer files, moving only
8654 Do not move a non-directory that has an existing destination with the
8655 same or newer modification time.
8656 If the move is across file system boundaries, the comparison is to the
8657 source time stamp truncated to the resolutions of the destination file
8658 system and of the system calls used to update time stamps; this avoids
8659 duplicate work if several @samp{mv -u} commands are executed with the
8660 same source and destination.
8666 Print the name of each file before moving it.
8668 @optStripTrailingSlashes
8674 @optNoTargetDirectory
8682 @section @command{rm}: Remove files or directories
8685 @cindex removing files or directories
8687 @command{rm} removes each given @var{file}. By default, it does not remove
8688 directories. Synopsis:
8691 rm [@var{option}]@dots{} [@var{file}]@dots{}
8694 @cindex prompting, and @command{rm}
8695 If the @option{-I} or @option{--interactive=once} option is given,
8696 and there are more than three files or the @option{-r}, @option{-R},
8697 or @option{--recursive} are given, then @command{rm} prompts the user
8698 for whether to proceed with the entire operation. If the response is
8699 not affirmative, the entire command is aborted.
8701 Otherwise, if a file is unwritable, standard input is a terminal, and
8702 the @option{-f} or @option{--force} option is not given, or the
8703 @option{-i} or @option{--interactive=always} option @emph{is} given,
8704 @command{rm} prompts the user for whether to remove the file.
8705 If the response is not affirmative, the file is skipped.
8707 Any attempt to remove a file whose last file name component is
8708 @file{.} or @file{..} is rejected without any prompting.
8710 @emph{Warning}: If you use @command{rm} to remove a file, it is usually
8711 possible to recover the contents of that file. If you want more assurance
8712 that the contents are truly unrecoverable, consider using @command{shred}.
8714 The program accepts the following options. Also see @ref{Common options}.
8722 Ignore nonexistent files and never prompt the user.
8723 Ignore any previous @option{--interactive} (@option{-i}) option.
8727 Prompt whether to remove each file.
8728 If the response is not affirmative, the file is skipped.
8729 Ignore any previous @option{--force} (@option{-f}) option.
8730 Equivalent to @option{--interactive=always}.
8734 Prompt once whether to proceed with the command, if more than three
8735 files are named or if a recursive removal is requested. Ignore any
8736 previous @option{--force} (@option{-f}) option. Equivalent to
8737 @option{--interactive=once}.
8739 @itemx --interactive [=@var{when}]
8740 @opindex --interactive
8741 Specify when to issue an interactive prompt. @var{when} may be
8745 @vindex never @r{interactive option}
8746 - Do not prompt at all.
8748 @vindex once @r{interactive option}
8749 - Prompt once if more than three files are named or if a recursive
8750 removal is requested. Equivalent to @option{-I}.
8752 @vindex always @r{interactive option}
8753 - Prompt for every file being removed. Equivalent to @option{-i}.
8755 @option{--interactive} with no @var{when} is equivalent to
8756 @option{--interactive=always}.
8758 @itemx --one-file-system
8759 @opindex --one-file-system
8760 @cindex one file system, restricting @command{rm} to
8761 When removing a hierarchy recursively, skip any directory that is on a
8762 file system different from that of the corresponding command line argument.
8765 This option is useful when removing a build ``chroot'' hierarchy,
8766 which normally contains no valuable data. However, it is not uncommon
8767 to bind-mount @file{/home} into such a hierarchy, to make it easier to
8768 use one's start-up file. The catch is that it's easy to forget to
8769 unmount @file{/home}. Then, when you use @command{rm -rf} to remove
8770 your normally throw-away chroot, that command will remove everything
8771 under @file{/home}, too.
8772 Use the @option{--one-file-system} option, and it will
8773 warn about and skip directories on other file systems.
8774 Of course, this will not save your @file{/home} if it and your
8775 chroot happen to be on the same file system.
8777 @itemx --preserve-root
8778 @opindex --preserve-root
8779 @cindex root directory, disallow recursive destruction
8780 Fail upon any attempt to remove the root directory, @file{/},
8781 when used with the @option{--recursive} option.
8782 This is the default behavior.
8783 @xref{Treating / specially}.
8785 @itemx --no-preserve-root
8786 @opindex --no-preserve-root
8787 @cindex root directory, allow recursive destruction
8788 Do not treat @file{/} specially when removing recursively.
8789 This option is not recommended unless you really want to
8790 remove all the files on your computer.
8791 @xref{Treating / specially}.
8798 @opindex --recursive
8799 @cindex directories, removing (recursively)
8800 Remove the listed directories and their contents recursively.
8806 Print the name of each file before removing it.
8810 @cindex files beginning with @samp{-}, removing
8811 @cindex @samp{-}, removing files beginning with
8812 One common question is how to remove files whose names begin with a
8813 @samp{-}. @sc{gnu} @command{rm}, like every program that uses the @code{getopt}
8814 function to parse its arguments, lets you use the @samp{--} option to
8815 indicate that all following arguments are non-options. To remove a file
8816 called @file{-f} in the current directory, you could type either:
8829 @opindex - @r{and Unix @command{rm}}
8830 The Unix @command{rm} program's use of a single @samp{-} for this purpose
8831 predates the development of the getopt standard syntax.
8836 @node shred invocation
8837 @section @command{shred}: Remove files more securely
8840 @cindex data, erasing
8841 @cindex erasing data
8843 @command{shred} overwrites devices or files, to help prevent even
8844 very expensive hardware from recovering the data.
8846 Ordinarily when you remove a file (@pxref{rm invocation}), the data is
8847 not actually destroyed. Only the index listing where the file is
8848 stored is destroyed, and the storage is made available for reuse.
8849 There are undelete utilities that will attempt to reconstruct the index
8850 and can bring the file back if the parts were not reused.
8852 On a busy system with a nearly-full drive, space can get reused in a few
8853 seconds. But there is no way to know for sure. If you have sensitive
8854 data, you may want to be sure that recovery is not possible by actually
8855 overwriting the file with non-sensitive data.
8857 However, even after doing that, it is possible to take the disk back
8858 to a laboratory and use a lot of sensitive (and expensive) equipment
8859 to look for the faint ``echoes'' of the original data underneath the
8860 overwritten data. If the data has only been overwritten once, it's not
8863 The best way to remove something irretrievably is to destroy the media
8864 it's on with acid, melt it down, or the like. For cheap removable media
8865 like floppy disks, this is the preferred method. However, hard drives
8866 are expensive and hard to melt, so the @command{shred} utility tries
8867 to achieve a similar effect non-destructively.
8869 This uses many overwrite passes, with the data patterns chosen to
8870 maximize the damage they do to the old data. While this will work on
8871 floppies, the patterns are designed for best effect on hard drives.
8872 For more details, see the source code and Peter Gutmann's paper
8873 @uref{http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html,
8874 @cite{Secure Deletion of Data from Magnetic and Solid-State Memory}},
8875 from the proceedings of the Sixth @acronym{USENIX} Security Symposium (San Jose,
8876 California, July 22--25, 1996).
8878 @strong{Please note} that @command{shred} relies on a very important assumption:
8879 that the file system overwrites data in place. This is the traditional
8880 way to do things, but many modern file system designs do not satisfy this
8881 assumption. Exceptions include:
8886 Log-structured or journaled file systems, such as those supplied with
8887 AIX and Solaris, and JFS, ReiserFS, XFS, Ext3 (in @code{data=journal} mode),
8888 BFS, NTFS, etc.@: when they are configured to journal @emph{data}.
8891 File systems that write redundant data and carry on even if some writes
8892 fail, such as RAID-based file systems.
8895 File systems that make snapshots, such as Network Appliance's NFS server.
8898 File systems that cache in temporary locations, such as NFS version 3
8902 Compressed file systems.
8905 In the particular case of ext3 file systems, the above disclaimer applies (and
8906 @command{shred} is thus of limited effectiveness) only in @code{data=journal}
8907 mode, which journals file data in addition to just metadata. In both
8908 the @code{data=ordered} (default) and @code{data=writeback} modes,
8909 @command{shred} works as usual. Ext3 journaling modes can be changed
8910 by adding the @code{data=something} option to the mount options for a
8911 particular file system in the @file{/etc/fstab} file, as documented in
8912 the mount man page (man mount).
8914 If you are not sure how your file system operates, then you should assume
8915 that it does not overwrite data in place, which means that shred cannot
8916 reliably operate on regular files in your file system.
8918 Generally speaking, it is more reliable to shred a device than a file,
8919 since this bypasses the problem of file system design mentioned above.
8920 However, even shredding devices is not always completely reliable. For
8921 example, most disks map out bad sectors invisibly to the application; if
8922 the bad sectors contain sensitive data, @command{shred} won't be able to
8925 @command{shred} makes no attempt to detect or report this problem, just as
8926 it makes no attempt to do anything about backups. However, since it is
8927 more reliable to shred devices than files, @command{shred} by default does
8928 not truncate or remove the output file. This default is more suitable
8929 for devices, which typically cannot be truncated and should not be
8932 Finally, consider the risk of backups and mirrors.
8933 File system backups and remote mirrors may contain copies of the
8934 file that cannot be removed, and that will allow a shredded file
8935 to be recovered later. So if you keep any data you may later want
8936 to destroy using @command{shred}, be sure that it is not backed up or mirrored.
8939 shred [@var{option}]@dots{} @var{file}[@dots{}]
8942 The program accepts the following options. Also see @ref{Common options}.
8950 @cindex force deletion
8951 Override file permissions if necessary to allow overwriting.
8954 @itemx -n @var{number}
8955 @itemx --iterations=@var{number}
8956 @opindex -n @var{number}
8957 @opindex --iterations=@var{number}
8958 @cindex iterations, selecting the number of
8959 By default, @command{shred} uses @value{SHRED_DEFAULT_PASSES} passes of
8960 overwrite. You can reduce this to save time, or increase it if you think it's
8961 appropriate. After 25 passes all of the internal overwrite patterns will have
8962 been used at least once.
8964 @item --random-source=@var{file}
8965 @opindex --random-source
8966 @cindex random source for shredding
8967 Use @var{file} as a source of random data used to overwrite and to
8968 choose pass ordering. @xref{Random sources}.
8970 @item -s @var{bytes}
8971 @itemx --size=@var{bytes}
8972 @opindex -s @var{bytes}
8973 @opindex --size=@var{bytes}
8974 @cindex size of file to shred
8975 Shred the first @var{bytes} bytes of the file. The default is to shred
8976 the whole file. @var{bytes} can be followed by a size specification like
8977 @samp{K}, @samp{M}, or @samp{G} to specify a multiple. @xref{Block size}.
8983 @cindex removing files after shredding
8984 After shredding a file, truncate it (if possible) and then remove it.
8985 If a file has multiple links, only the named links will be removed.
8991 Display to standard error all status updates as sterilization proceeds.
8997 By default, @command{shred} rounds the size of a regular file up to the next
8998 multiple of the file system block size to fully erase the last block
9000 Use @option{--exact} to suppress that behavior.
9001 Thus, by default if you shred a 10-byte regular file on a system with 512-byte
9002 blocks, the resulting file will be 512 bytes long. With this option,
9003 shred does not increase the apparent size of the file.
9009 Normally, the last pass that @command{shred} writes is made up of
9010 random data. If this would be conspicuous on your hard drive (for
9011 example, because it looks like encrypted data), or you just think
9012 it's tidier, the @option{--zero} option adds an additional overwrite pass with
9013 all zero bits. This is in addition to the number of passes specified
9014 by the @option{--iterations} option.
9018 You might use the following command to erase all trace of the
9019 file system you'd created on the floppy disk in your first drive.
9020 That command takes about 20 minutes to erase a ``1.44MB'' (actually
9024 shred --verbose /dev/fd0
9027 Similarly, to erase all data on a selected partition of
9028 your hard disk, you could give a command like this:
9031 shred --verbose /dev/sda5
9034 On modern disks, a single pass should be adequate,
9035 and it will take one third the time of the default three-pass approach.
9038 # 1 pass, write pseudo-random data; 3x faster than the default
9039 shred --verbose -n1 /dev/sda5
9042 To be on the safe side, use at least one pass that overwrites using
9043 pseudo-random data. I.e., don't be tempted to use @samp{-n0 --zero},
9044 in case some disk controller optimizes the process of writing blocks
9045 of all zeros, and thereby does not clear all bytes in a block.
9046 Some SSDs may do just that.
9048 A @var{file} of @samp{-} denotes standard output.
9049 The intended use of this is to shred a removed temporary file.
9056 echo "Hello, world" >&3
9061 However, the command @samp{shred - >file} does not shred the contents
9062 of @var{file}, since the shell truncates @var{file} before invoking
9063 @command{shred}. Use the command @samp{shred file} or (if using a
9064 Bourne-compatible shell) the command @samp{shred - 1<>file} instead.
9069 @node Special file types
9070 @chapter Special file types
9072 @cindex special file types
9073 @cindex file types, special
9075 This chapter describes commands which create special types of files (and
9076 @command{rmdir}, which removes directories, one special file type).
9078 @cindex special file types
9080 Although Unix-like operating systems have markedly fewer special file
9081 types than others, not @emph{everything} can be treated only as the
9082 undifferentiated byte stream of @dfn{normal files}. For example, when a
9083 file is created or removed, the system must record this information,
9084 which it does in a @dfn{directory}---a special type of file. Although
9085 you can read directories as normal files, if you're curious, in order
9086 for the system to do its job it must impose a structure, a certain
9087 order, on the bytes of the file. Thus it is a ``special'' type of file.
9089 Besides directories, other special file types include named pipes
9090 (FIFOs), symbolic links, sockets, and so-called @dfn{special files}.
9093 * link invocation:: Make a hard link via the link syscall
9094 * ln invocation:: Make links between files.
9095 * mkdir invocation:: Make directories.
9096 * mkfifo invocation:: Make FIFOs (named pipes).
9097 * mknod invocation:: Make block or character special files.
9098 * readlink invocation:: Print value of a symlink or canonical file name.
9099 * rmdir invocation:: Remove empty directories.
9100 * unlink invocation:: Remove files via the unlink syscall
9104 @node link invocation
9105 @section @command{link}: Make a hard link via the link syscall
9108 @cindex links, creating
9109 @cindex hard links, creating
9110 @cindex creating links (hard only)
9112 @command{link} creates a single hard link at a time.
9113 It is a minimalist interface to the system-provided
9114 @code{link} function. @xref{Hard Links, , , libc,
9115 The GNU C Library Reference Manual}.
9116 It avoids the bells and whistles of the more commonly-used
9117 @command{ln} command (@pxref{ln invocation}).
9121 link @var{filename} @var{linkname}
9124 @var{filename} must specify an existing file, and @var{linkname}
9125 must specify a nonexistent entry in an existing directory.
9126 @command{link} simply calls @code{link (@var{filename}, @var{linkname})}
9129 On a @acronym{GNU} system, this command acts like @samp{ln --directory
9130 --no-target-directory @var{filename} @var{linkname}}. However, the
9131 @option{--directory} and @option{--no-target-directory} options are
9132 not specified by @acronym{POSIX}, and the @command{link} command is
9133 more portable in practice.
9135 If @var{filename} is a symbolic link, it is unspecified whether
9136 @var{linkname} will be a hard link to the symbolic link or to the
9137 target of the symbolic link. Use @command{ln -P} or @command{ln -L}
9138 to specify which behavior is desired.
9144 @section @command{ln}: Make links between files
9147 @cindex links, creating
9148 @cindex hard links, creating
9149 @cindex symbolic (soft) links, creating
9150 @cindex creating links (hard or soft)
9152 @cindex file systems and hard links
9153 @command{ln} makes links between files. By default, it makes hard links;
9154 with the @option{-s} option, it makes symbolic (or @dfn{soft}) links.
9158 ln [@var{option}]@dots{} [-T] @var{target} @var{linkname}
9159 ln [@var{option}]@dots{} @var{target}
9160 ln [@var{option}]@dots{} @var{target}@dots{} @var{directory}
9161 ln [@var{option}]@dots{} -t @var{directory} @var{target}@dots{}
9167 If two file names are given, @command{ln} creates a link to the first
9168 file from the second.
9171 If one @var{target} is given, @command{ln} creates a link to that file
9172 in the current directory.
9175 If the @option{--target-directory} (@option{-t}) option is given, or
9176 failing that if the last file is a directory and the
9177 @option{--no-target-directory} (@option{-T}) option is not given,
9178 @command{ln} creates a link to each @var{target} file in the specified
9179 directory, using the @var{target}s' names.
9183 Normally @command{ln} does not remove existing files. Use the
9184 @option{--force} (@option{-f}) option to remove them unconditionally,
9185 the @option{--interactive} (@option{-i}) option to remove them
9186 conditionally, and the @option{--backup} (@option{-b}) option to
9189 @cindex hard link, defined
9190 @cindex inode, and hard links
9191 A @dfn{hard link} is another name for an existing file; the link and the
9192 original are indistinguishable. Technically speaking, they share the
9193 same inode, and the inode contains all the information about a
9194 file---indeed, it is not incorrect to say that the inode @emph{is} the
9195 file. Most systems prohibit making a hard link to
9196 a directory; on those where it is allowed, only the super-user can do
9197 so (and with caution, since creating a cycle will cause problems to many
9198 other utilities). Hard links cannot cross file system boundaries. (These
9199 restrictions are not mandated by @acronym{POSIX}, however.)
9201 @cindex dereferencing symbolic links
9202 @cindex symbolic link, defined
9203 @dfn{Symbolic links} (@dfn{symlinks} for short), on the other hand, are
9204 a special file type (which not all kernels support: System V release 3
9205 (and older) systems lack symlinks) in which the link file actually
9206 refers to a different file, by name. When most operations (opening,
9207 reading, writing, and so on) are passed the symbolic link file, the
9208 kernel automatically @dfn{dereferences} the link and operates on the
9209 target of the link. But some operations (e.g., removing) work on the
9210 link file itself, rather than on its target. The owner and group of a
9211 symlink are not significant to file access performed through
9212 the link, but do have implications on deleting a symbolic link from a
9213 directory with the restricted deletion bit set. On the GNU system,
9214 the mode of a symlink has no significance and cannot be changed, but
9215 on some BSD systems, the mode can be changed and will affect whether
9216 the symlink will be traversed in file name resolution. @xref{Symbolic Links,,,
9217 libc, The GNU C Library Reference Manual}.
9219 Symbolic links can contain arbitrary strings; a @dfn{dangling symlink}
9220 occurs when the string in the symlink does not resolve to a file.
9221 There are no restrictions against creating dangling symbolic links.
9222 There are trade-offs to using absolute or relative symlinks. An
9223 absolute symlink always points to the same file, even if the directory
9224 containing the link is moved. However, if the symlink is visible from
9225 more than one machine (such as on a networked file system), the file
9226 pointed to might not always be the same. A relative symbolic link is
9227 resolved in relation to the directory that contains the link, and is
9228 often useful in referring to files on the same device without regards
9229 to what name that device is mounted on when accessed via networked
9232 When creating a relative symlink in a different location than the
9233 current directory, the resolution of the symlink will be different
9234 than the resolution of the same string from the current directory.
9235 Therefore, many users prefer to first change directories to the
9236 location where the relative symlink will be created, so that
9237 tab-completion or other file resolution will find the same target as
9238 what will be placed in the symlink.
9240 The program accepts the following options. Also see @ref{Common options}.
9251 @opindex --directory
9252 @cindex hard links to directories
9253 Allow users with appropriate privileges to attempt to make hard links
9255 However, note that this will probably fail due to
9256 system restrictions, even for the super-user.
9262 Remove existing destination files.
9265 @itemx --interactive
9267 @opindex --interactive
9268 @cindex prompting, and @command{ln}
9269 Prompt whether to remove existing destination files.
9275 If @option{-s} is not in effect, and the source file is a symbolic
9276 link, create the hard link to the file referred to by the symbolic
9277 link, rather than the symbolic link itself.
9280 @itemx --no-dereference
9282 @opindex --no-dereference
9283 Do not treat the last operand specially when it is a symbolic link to
9284 a directory. Instead, treat it as if it were a normal file.
9286 When the destination is an actual directory (not a symlink to one),
9287 there is no ambiguity. The link is created in that directory.
9288 But when the specified destination is a symlink to a directory,
9289 there are two ways to treat the user's request. @command{ln} can
9290 treat the destination just as it would a normal directory and create
9291 the link in it. On the other hand, the destination can be viewed as a
9292 non-directory---as the symlink itself. In that case, @command{ln}
9293 must delete or backup that symlink before creating the new link.
9294 The default is to treat a destination that is a symlink to a directory
9295 just like a directory.
9297 This option is weaker than the @option{--no-target-directory}
9298 (@option{-T}) option, so it has no effect if both options are given.
9304 If @option{-s} is not in effect, and the source file is a symbolic
9305 link, create the hard link to the symbolic link itself. On platforms
9306 where this is not supported by the kernel, this option creates a
9307 symbolic link with identical contents; since symbolic link contents
9308 cannot be edited, any file name resolution performed through either
9309 link will be the same as if a hard link had been created.
9315 Make symbolic links instead of hard links. This option merely produces
9316 an error message on systems that do not support symbolic links.
9322 @optNoTargetDirectory
9328 Print the name of each file after linking it successfully.
9332 @cindex hard links to symbolic links
9333 @cindex symbolic links and @command{ln}
9334 If @option{-L} and @option{-P} are both given, the last one takes
9335 precedence. If @option{-s} is also given, @option{-L} and @option{-P}
9336 are silently ignored. If neither option is given, then this
9337 implementation defaults to @option{-P} if the system @code{link} supports
9338 hard links to symbolic links (such as the GNU system), and @option{-L}
9339 if @code{link} follows symbolic links (such as on BSD).
9348 # Create link ../a pointing to a in that directory.
9349 # Not really useful because it points to itself.
9354 # Change to the target before creating symlinks to avoid being confused.
9360 # Hard coded file names don't move well.
9361 ln -s $(pwd)/a /some/dir/
9365 # Relative file names survive directory moves and also
9366 # work across networked file systems.
9367 ln -s afile anotherfile
9368 ln -s ../adir/afile yetanotherfile
9372 @node mkdir invocation
9373 @section @command{mkdir}: Make directories
9376 @cindex directories, creating
9377 @cindex creating directories
9379 @command{mkdir} creates directories with the specified names. Synopsis:
9382 mkdir [@var{option}]@dots{} @var{name}@dots{}
9385 @command{mkdir} creates each directory @var{name} in the order given.
9386 It reports an error if @var{name} already exists, unless the
9387 @option{-p} option is given and @var{name} is a directory.
9389 The program accepts the following options. Also see @ref{Common options}.
9394 @itemx --mode=@var{mode}
9397 @cindex modes of created directories, setting
9398 Set the file permission bits of created directories to @var{mode},
9399 which uses the same syntax as
9400 in @command{chmod} and uses @samp{a=rwx} (read, write and execute allowed for
9401 everyone) for the point of the departure. @xref{File permissions}.
9403 Normally the directory has the desired file mode bits at the moment it
9404 is created. As a @acronym{GNU} extension, @var{mode} may also mention
9405 special mode bits, but in this case there may be a temporary window
9406 during which the directory exists but its special mode bits are
9407 incorrect. @xref{Directory Setuid and Setgid}, for how the
9408 set-user-ID and set-group-ID bits of directories are inherited unless
9409 overridden in this way.
9415 @cindex parent directories, creating
9416 Make any missing parent directories for each argument, setting their
9417 file permission bits to the umask modified by @samp{u+wx}. Ignore
9418 existing parent directories, and do not change their file permission
9421 To set the file permission bits of any newly-created parent
9422 directories to a value that includes @samp{u+wx}, you can set the
9423 umask before invoking @command{mkdir}. For example, if the shell
9424 command @samp{(umask u=rwx,go=rx; mkdir -p P/Q)} creates the parent
9425 @file{P} it sets the parent's permission bits to @samp{u=rwx,go=rx}.
9426 To set a parent's special mode bits as well, you can invoke
9427 @command{chmod} after @command{mkdir}. @xref{Directory Setuid and
9428 Setgid}, for how the set-user-ID and set-group-ID bits of
9429 newly-created parent directories are inherited.
9435 Print a message for each created directory. This is most useful with
9438 @item -Z @var{context}
9439 @itemx --context=@var{context}
9443 @cindex security context
9444 Set the default SELinux security context to be used for created directories.
9451 @node mkfifo invocation
9452 @section @command{mkfifo}: Make FIFOs (named pipes)
9455 @cindex FIFOs, creating
9456 @cindex named pipes, creating
9457 @cindex creating FIFOs (named pipes)
9459 @command{mkfifo} creates FIFOs (also called @dfn{named pipes}) with the
9460 specified names. Synopsis:
9463 mkfifo [@var{option}] @var{name}@dots{}
9466 A @dfn{FIFO} is a special file type that permits independent processes
9467 to communicate. One process opens the FIFO file for writing, and
9468 another for reading, after which data can flow as with the usual
9469 anonymous pipe in shells or elsewhere.
9471 The program accepts the following option. Also see @ref{Common options}.
9476 @itemx --mode=@var{mode}
9479 @cindex modes of created FIFOs, setting
9480 Set the mode of created FIFOs to @var{mode}, which is symbolic as in
9481 @command{chmod} and uses @samp{a=rw} (read and write allowed for everyone)
9482 for the point of departure. @var{mode} should specify only file
9483 permission bits. @xref{File permissions}.
9485 @item -Z @var{context}
9486 @itemx --context=@var{context}
9490 @cindex security context
9491 Set the default SELinux security context to be used for created FIFOs.
9498 @node mknod invocation
9499 @section @command{mknod}: Make block or character special files
9502 @cindex block special files, creating
9503 @cindex character special files, creating
9505 @command{mknod} creates a FIFO, character special file, or block special
9506 file with the specified name. Synopsis:
9509 mknod [@var{option}]@dots{} @var{name} @var{type} [@var{major} @var{minor}]
9512 @cindex special files
9513 @cindex block special files
9514 @cindex character special files
9515 Unlike the phrase ``special file type'' above, the term @dfn{special
9516 file} has a technical meaning on Unix: something that can generate or
9517 receive data. Usually this corresponds to a physical piece of hardware,
9518 e.g., a printer or a disk. (These files are typically created at
9519 system-configuration time.) The @command{mknod} command is what creates
9520 files of this type. Such devices can be read either a character at a
9521 time or a ``block'' (many characters) at a time, hence we say there are
9522 @dfn{block special} files and @dfn{character special} files.
9524 @c mknod is a shell built-in at least with OpenBSD's /bin/sh
9525 @mayConflictWithShellBuiltIn{mknod}
9527 The arguments after @var{name} specify the type of file to make:
9532 @opindex p @r{for FIFO file}
9536 @opindex b @r{for block special file}
9537 for a block special file
9540 @c Don't document the `u' option -- it's just a synonym for `c'.
9541 @c Do *any* versions of mknod still use it?
9543 @opindex c @r{for character special file}
9544 @c @opindex u @r{for character special file}
9545 for a character special file
9549 When making a block or character special file, the major and minor
9550 device numbers must be given after the file type.
9551 If a major or minor device number begins with @samp{0x} or @samp{0X},
9552 it is interpreted as hexadecimal; otherwise, if it begins with @samp{0},
9553 as octal; otherwise, as decimal.
9555 The program accepts the following option. Also see @ref{Common options}.
9560 @itemx --mode=@var{mode}
9563 Set the mode of created files to @var{mode}, which is symbolic as in
9564 @command{chmod} and uses @samp{a=rw} as the point of departure.
9565 @var{mode} should specify only file permission bits.
9566 @xref{File permissions}.
9568 @item -Z @var{context}
9569 @itemx --context=@var{context}
9573 @cindex security context
9574 Set the default SELinux security context to be used for created files.
9581 @node readlink invocation
9582 @section @command{readlink}: Print value of a symlink or canonical file name
9585 @cindex displaying value of a symbolic link
9586 @cindex canonical file name
9587 @cindex canonicalize a file name
9591 @command{readlink} may work in one of two supported modes:
9597 @command{readlink} outputs the value of the given symbolic link.
9598 If @command{readlink} is invoked with an argument other than the name
9599 of a symbolic link, it produces no output and exits with a nonzero exit code.
9601 @item Canonicalize mode
9603 @command{readlink} outputs the absolute name of the given file which contains
9604 no @file{.}, @file{..} components nor any repeated separators
9605 (@file{/}) or symbolic links.
9610 readlink [@var{option}] @var{file}
9613 By default, @command{readlink} operates in readlink mode.
9615 The program accepts the following options. Also see @ref{Common options}.
9620 @itemx --canonicalize
9622 @opindex --canonicalize
9623 Activate canonicalize mode.
9624 If any component of the file name except the last one is missing or unavailable,
9625 @command{readlink} produces no output and exits with a nonzero exit
9626 code. A trailing slash is ignored.
9629 @itemx --canonicalize-existing
9631 @opindex --canonicalize-existing
9632 Activate canonicalize mode.
9633 If any component is missing or unavailable, @command{readlink} produces
9634 no output and exits with a nonzero exit code. A trailing slash
9635 requires that the name resolve to a directory.
9638 @itemx --canonicalize-missing
9640 @opindex --canonicalize-missing
9641 Activate canonicalize mode.
9642 If any component is missing or unavailable, @command{readlink} treats it
9648 @opindex --no-newline
9649 Do not output the trailing newline.
9659 Suppress most error messages.
9665 Report error messages.
9669 The @command{readlink} utility first appeared in OpenBSD 2.1.
9671 There is a @command{realpath} command on some systems
9672 which operates like @command{readlink} in canonicalize mode.
9677 @node rmdir invocation
9678 @section @command{rmdir}: Remove empty directories
9681 @cindex removing empty directories
9682 @cindex directories, removing empty
9684 @command{rmdir} removes empty directories. Synopsis:
9687 rmdir [@var{option}]@dots{} @var{directory}@dots{}
9690 If any @var{directory} argument does not refer to an existing empty
9691 directory, it is an error.
9693 The program accepts the following options. Also see @ref{Common options}.
9697 @item --ignore-fail-on-non-empty
9698 @opindex --ignore-fail-on-non-empty
9699 @cindex directory deletion, ignoring failures
9700 Ignore each failure to remove a directory that is solely because
9701 the directory is non-empty.
9707 @cindex parent directories, removing
9708 Remove @var{directory}, then try to remove each component of @var{directory}.
9709 So, for example, @samp{rmdir -p a/b/c} is similar to @samp{rmdir a/b/c a/b a}.
9710 As such, it fails if any of those directories turns out not to be empty.
9711 Use the @option{--ignore-fail-on-non-empty} option to make it so such
9712 a failure does not evoke a diagnostic and does not cause @command{rmdir} to
9713 exit unsuccessfully.
9719 @cindex directory deletion, reporting
9720 Give a diagnostic for each successful removal.
9721 @var{directory} is removed.
9725 @xref{rm invocation}, for how to remove non-empty directories (recursively).
9730 @node unlink invocation
9731 @section @command{unlink}: Remove files via the unlink syscall
9734 @cindex removing files or directories (via the unlink syscall)
9736 @command{unlink} deletes a single specified file name.
9737 It is a minimalist interface to the system-provided
9738 @code{unlink} function. @xref{Deleting Files, , , libc,
9739 The GNU C Library Reference Manual}. Synopsis:
9740 It avoids the bells and whistles of the more commonly-used
9741 @command{rm} command (@pxref{rm invocation}).
9744 unlink @var{filename}
9747 On some systems @code{unlink} can be used to delete the name of a
9748 directory. On others, it can be used that way only by a privileged user.
9749 In the GNU system @code{unlink} can never delete the name of a directory.
9751 The @command{unlink} command honors the @option{--help} and
9752 @option{--version} options. To remove a file whose name begins with
9753 @samp{-}, prefix the name with @samp{./}, e.g., @samp{unlink ./--help}.
9758 @node Changing file attributes
9759 @chapter Changing file attributes
9761 @cindex changing file attributes
9762 @cindex file attributes, changing
9763 @cindex attributes, file
9765 A file is not merely its contents, a name, and a file type
9766 (@pxref{Special file types}). A file also has an owner (a user ID), a
9767 group (a group ID), permissions (what the owner can do with the file,
9768 what people in the group can do, and what everyone else can do), various
9769 timestamps, and other information. Collectively, we call these a file's
9772 These commands change file attributes.
9775 * chgrp invocation:: Change file groups.
9776 * chmod invocation:: Change access permissions.
9777 * chown invocation:: Change file owners and groups.
9778 * touch invocation:: Change file timestamps.
9782 @node chown invocation
9783 @section @command{chown}: Change file owner and group
9786 @cindex file ownership, changing
9787 @cindex group ownership, changing
9788 @cindex changing file ownership
9789 @cindex changing group ownership
9791 @command{chown} changes the user and/or group ownership of each given @var{file}
9792 to @var{new-owner} or to the user and group of an existing reference file.
9796 chown [@var{option}]@dots{} @{@var{new-owner} | --reference=@var{ref_file}@}@c
9800 If used, @var{new-owner} specifies the new owner and/or group as follows
9801 (with no embedded white space):
9804 [@var{owner}] [ : [@var{group}] ]
9811 If only an @var{owner} (a user name or numeric user ID) is given, that
9812 user is made the owner of each given file, and the files' group is not
9815 @item owner@samp{:}group
9816 If the @var{owner} is followed by a colon and a @var{group} (a
9817 group name or numeric group ID), with no spaces between them, the group
9818 ownership of the files is changed as well (to @var{group}).
9821 If a colon but no group name follows @var{owner}, that user is
9822 made the owner of the files and the group of the files is changed to
9823 @var{owner}'s login group.
9826 If the colon and following @var{group} are given, but the owner
9827 is omitted, only the group of the files is changed; in this case,
9828 @command{chown} performs the same function as @command{chgrp}.
9831 If only a colon is given, or if @var{new-owner} is empty, neither the
9832 owner nor the group is changed.
9836 If @var{owner} or @var{group} is intended to represent a numeric user
9837 or group ID, then you may specify it with a leading @samp{+}.
9838 @xref{Disambiguating names and IDs}.
9840 Some older scripts may still use @samp{.} in place of the @samp{:} separator.
9841 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) does not
9842 require support for that, but for backward compatibility @acronym{GNU}
9843 @command{chown} supports @samp{.} so long as no ambiguity results.
9844 New scripts should avoid the use of @samp{.} because it is not
9845 portable, and because it has undesirable results if the entire
9846 @var{owner@samp{.}group} happens to identify a user whose name
9849 The @command{chown} command sometimes clears the set-user-ID or
9850 set-group-ID permission bits. This behavior depends on the policy and
9851 functionality of the underlying @code{chown} system call, which may
9852 make system-dependent file mode modifications outside the control of
9853 the @command{chown} command. For example, the @command{chown} command
9854 might not affect those bits when invoked by a user with appropriate
9855 privileges, or when the
9856 bits signify some function other than executable permission (e.g.,
9858 When in doubt, check the underlying system behavior.
9860 The program accepts the following options. Also see @ref{Common options}.
9868 @cindex changed owners, verbosely describing
9869 Verbosely describe the action for each @var{file} whose ownership
9878 @cindex error messages, omitting
9879 Do not print error messages about files whose ownership cannot be
9882 @itemx @w{@kbd{--from}=@var{old-owner}}
9884 @cindex symbolic links, changing owner
9885 Change a @var{file}'s ownership only if it has current attributes specified
9886 by @var{old-owner}. @var{old-owner} has the same form as @var{new-owner}
9888 This option is useful primarily from a security standpoint in that
9889 it narrows considerably the window of potential abuse.
9890 For example, to reflect a user ID numbering change for one user's files
9891 without an option like this, @code{root} might run
9894 find / -owner OLDUSER -print0 | xargs -0 chown -h NEWUSER
9897 But that is dangerous because the interval between when the @command{find}
9898 tests the existing file's owner and when the @command{chown} is actually run
9900 One way to narrow the gap would be to invoke chown for each file
9904 find / -owner OLDUSER -exec chown -h NEWUSER @{@} \;
9907 But that is very slow if there are many affected files.
9908 With this option, it is safer (the gap is narrower still)
9909 though still not perfect:
9912 chown -h -R --from=OLDUSER NEWUSER /
9916 @opindex --dereference
9917 @cindex symbolic links, changing owner
9919 Do not act on symbolic links themselves but rather on what they point to.
9920 This is the default.
9923 @itemx --no-dereference
9925 @opindex --no-dereference
9926 @cindex symbolic links, changing owner
9928 Act on symbolic links themselves instead of what they point to.
9929 This mode relies on the @code{lchown} system call.
9930 On systems that do not provide the @code{lchown} system call,
9931 @command{chown} fails when a file specified on the command line
9933 By default, no diagnostic is issued for symbolic links encountered
9934 during a recursive traversal, but see @option{--verbose}.
9936 @itemx --preserve-root
9937 @opindex --preserve-root
9938 @cindex root directory, disallow recursive modification
9939 Fail upon any attempt to recursively change the root directory, @file{/}.
9940 Without @option{--recursive}, this option has no effect.
9941 @xref{Treating / specially}.
9943 @itemx --no-preserve-root
9944 @opindex --no-preserve-root
9945 @cindex root directory, allow recursive modification
9946 Cancel the effect of any preceding @option{--preserve-root} option.
9947 @xref{Treating / specially}.
9949 @item --reference=@var{ref_file}
9950 @opindex --reference
9951 Change the user and group of each @var{file} to be the same as those of
9952 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9953 user and group of the symbolic link, but rather those of the file it
9960 Output a diagnostic for every file processed.
9961 If a symbolic link is encountered during a recursive traversal
9962 on a system without the @code{lchown} system call, and @option{--no-dereference}
9963 is in effect, then issue a diagnostic saying neither the symbolic link nor
9964 its referent is being changed.
9969 @opindex --recursive
9970 @cindex recursively changing file ownership
9971 Recursively change ownership of directories and their contents.
9974 @xref{Traversing symlinks}.
9977 @xref{Traversing symlinks}.
9980 @xref{Traversing symlinks}.
9989 # Change the owner of /u to "root".
9992 # Likewise, but also change its group to "staff".
9995 # Change the owner of /u and subfiles to "root".
10000 @node chgrp invocation
10001 @section @command{chgrp}: Change group ownership
10004 @cindex group ownership, changing
10005 @cindex changing group ownership
10007 @command{chgrp} changes the group ownership of each given @var{file}
10008 to @var{group} (which can be either a group name or a numeric group ID)
10009 or to the group of an existing reference file. Synopsis:
10012 chgrp [@var{option}]@dots{} @{@var{group} | --reference=@var{ref_file}@}@c
10016 If @var{group} is intended to represent a
10017 numeric group ID, then you may specify it with a leading @samp{+}.
10018 @xref{Disambiguating names and IDs}.
10020 The program accepts the following options. Also see @ref{Common options}.
10028 @cindex changed files, verbosely describing
10029 Verbosely describe the action for each @var{file} whose group actually
10038 @cindex error messages, omitting
10039 Do not print error messages about files whose group cannot be
10042 @item --dereference
10043 @opindex --dereference
10044 @cindex symbolic links, changing owner
10046 Do not act on symbolic links themselves but rather on what they point to.
10047 This is the default.
10050 @itemx --no-dereference
10052 @opindex --no-dereference
10053 @cindex symbolic links, changing group
10055 Act on symbolic links themselves instead of what they point to.
10056 This mode relies on the @code{lchown} system call.
10057 On systems that do not provide the @code{lchown} system call,
10058 @command{chgrp} fails when a file specified on the command line
10059 is a symbolic link.
10060 By default, no diagnostic is issued for symbolic links encountered
10061 during a recursive traversal, but see @option{--verbose}.
10063 @itemx --preserve-root
10064 @opindex --preserve-root
10065 @cindex root directory, disallow recursive modification
10066 Fail upon any attempt to recursively change the root directory, @file{/}.
10067 Without @option{--recursive}, this option has no effect.
10068 @xref{Treating / specially}.
10070 @itemx --no-preserve-root
10071 @opindex --no-preserve-root
10072 @cindex root directory, allow recursive modification
10073 Cancel the effect of any preceding @option{--preserve-root} option.
10074 @xref{Treating / specially}.
10076 @item --reference=@var{ref_file}
10077 @opindex --reference
10078 Change the group of each @var{file} to be the same as that of
10079 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
10080 group of the symbolic link, but rather that of the file it refers to.
10086 Output a diagnostic for every file processed.
10087 If a symbolic link is encountered during a recursive traversal
10088 on a system without the @code{lchown} system call, and @option{--no-dereference}
10089 is in effect, then issue a diagnostic saying neither the symbolic link nor
10090 its referent is being changed.
10095 @opindex --recursive
10096 @cindex recursively changing group ownership
10097 Recursively change the group ownership of directories and their contents.
10100 @xref{Traversing symlinks}.
10103 @xref{Traversing symlinks}.
10106 @xref{Traversing symlinks}.
10115 # Change the group of /u to "staff".
10118 # Change the group of /u and subfiles to "staff".
10123 @node chmod invocation
10124 @section @command{chmod}: Change access permissions
10127 @cindex changing access permissions
10128 @cindex access permissions, changing
10129 @cindex permissions, changing access
10131 @command{chmod} changes the access permissions of the named files. Synopsis:
10134 chmod [@var{option}]@dots{} @{@var{mode} | --reference=@var{ref_file}@}@c
10138 @cindex symbolic links, permissions of
10139 @command{chmod} never changes the permissions of symbolic links, since
10140 the @command{chmod} system call cannot change their permissions.
10141 This is not a problem since the permissions of symbolic links are
10142 never used. However, for each symbolic link listed on the command
10143 line, @command{chmod} changes the permissions of the pointed-to file.
10144 In contrast, @command{chmod} ignores symbolic links encountered during
10145 recursive directory traversals.
10147 A successful use of @command{chmod} clears the set-group-ID bit of a
10148 regular file if the file's group ID does not match the user's
10149 effective group ID or one of the user's supplementary group IDs,
10150 unless the user has appropriate privileges. Additional restrictions
10151 may cause the set-user-ID and set-group-ID bits of @var{mode} or
10152 @var{ref_file} to be ignored. This behavior depends on the policy and
10153 functionality of the underlying @code{chmod} system call. When in
10154 doubt, check the underlying system behavior.
10156 If used, @var{mode} specifies the new file mode bits.
10157 For details, see the section on @ref{File permissions}.
10158 If you really want @var{mode} to have a leading @samp{-}, you should
10159 use @option{--} first, e.g., @samp{chmod -- -w file}. Typically,
10160 though, @samp{chmod a-w file} is preferable, and @command{chmod -w
10161 file} (without the @option{--}) complains if it behaves differently
10162 from what @samp{chmod a-w file} would do.
10164 The program accepts the following options. Also see @ref{Common options}.
10172 Verbosely describe the action for each @var{file} whose permissions
10181 @cindex error messages, omitting
10182 Do not print error messages about files whose permissions cannot be
10185 @itemx --preserve-root
10186 @opindex --preserve-root
10187 @cindex root directory, disallow recursive modification
10188 Fail upon any attempt to recursively change the root directory, @file{/}.
10189 Without @option{--recursive}, this option has no effect.
10190 @xref{Treating / specially}.
10192 @itemx --no-preserve-root
10193 @opindex --no-preserve-root
10194 @cindex root directory, allow recursive modification
10195 Cancel the effect of any preceding @option{--preserve-root} option.
10196 @xref{Treating / specially}.
10202 Verbosely describe the action or non-action taken for every @var{file}.
10204 @item --reference=@var{ref_file}
10205 @opindex --reference
10206 Change the mode of each @var{file} to be the same as that of @var{ref_file}.
10207 @xref{File permissions}.
10208 If @var{ref_file} is a symbolic link, do not use the mode
10209 of the symbolic link, but rather that of the file it refers to.
10214 @opindex --recursive
10215 @cindex recursively changing access permissions
10216 Recursively change permissions of directories and their contents.
10223 @node touch invocation
10224 @section @command{touch}: Change file timestamps
10227 @cindex changing file timestamps
10228 @cindex file timestamps, changing
10229 @cindex timestamps, changing file
10231 @command{touch} changes the access and/or modification times of the
10232 specified files. Synopsis:
10235 touch [@var{option}]@dots{} @var{file}@dots{}
10238 @cindex empty files, creating
10239 Any @var{file} argument that does not exist is created empty, unless
10240 option @option{--no-create} (@option{-c}) or @option{--no-dereference}
10241 (@option{-h}) was in effect.
10243 A @var{file} argument string of @samp{-} is handled specially and
10244 causes @command{touch} to change the times of the file associated with
10247 @cindex permissions, for changing file timestamps
10248 If changing both the access and modification times to the current
10249 time, @command{touch} can change the timestamps for files that the user
10250 running it does not own but has write permission for. Otherwise, the
10251 user must own the files.
10253 Although @command{touch} provides options for changing two of the times---the
10254 times of last access and modification---of a file, there is actually
10255 a standard third one as well: the inode change time. This is often
10256 referred to as a file's @code{ctime}.
10257 The inode change time represents the time when the file's meta-information
10258 last changed. One common example of this is when the permissions of a
10259 file change. Changing the permissions doesn't access the file, so
10260 the atime doesn't change, nor does it modify the file, so the mtime
10261 doesn't change. Yet, something about the file itself has changed,
10262 and this must be noted somewhere. This is the job of the ctime field.
10263 This is necessary, so that, for example, a backup program can make a
10264 fresh copy of the file, including the new permissions value.
10265 Another operation that modifies a file's ctime without affecting
10266 the others is renaming. In any case, it is not possible, in normal
10267 operations, for a user to change the ctime field to a user-specified value.
10268 Some operating systems and file systems support a fourth time: the
10269 birth time, when the file was first created; by definition, this
10270 timestamp never changes.
10273 Time stamps assume the time zone rules specified by the @env{TZ}
10274 environment variable, or by the system default rules if @env{TZ} is
10275 not set. @xref{TZ Variable,, Specifying the Time Zone with @env{TZ},
10276 libc, The GNU C Library Reference Manual}.
10277 You can avoid ambiguities during
10278 daylight saving transitions by using @sc{utc} time stamps.
10280 The program accepts the following options. Also see @ref{Common options}.
10285 @itemx --time=atime
10286 @itemx --time=access
10290 @opindex atime@r{, changing}
10291 @opindex access @r{time, changing}
10292 @opindex use @r{time, changing}
10293 Change the access time only.
10298 @opindex --no-create
10299 Do not warn about or create files that do not exist.
10302 @itemx --date=@var{time}
10306 Use @var{time} instead of the current time. It can contain month names,
10307 time zones, @samp{am} and @samp{pm}, @samp{yesterday}, etc. For
10308 example, @option{--date="2004-02-27 14:19:13.489392193 +0530"}
10309 specifies the instant of time that is 489,392,193 nanoseconds after
10310 February 27, 2004 at 2:19:13 PM in a time zone that is 5 hours and 30
10311 minutes east of @acronym{UTC}. @xref{Date input formats}.
10312 File systems that do not support high-resolution time stamps
10313 silently ignore any excess precision here.
10317 @cindex BSD @command{touch} compatibility
10318 Ignored; for compatibility with BSD versions of @command{touch}.
10321 @itemx --no-dereference
10323 @opindex --no-dereference
10324 @cindex symbolic links, changing time
10326 Attempt to change the timestamps of a symbolic link, rather than what
10327 the link refers to. When using this option, empty files are not
10328 created, but option @option{-c} must also be used to avoid warning
10329 about files that do not exist. Not all systems support changing the
10330 timestamps of symlinks, since underlying system support for this
10331 action was not required until @acronym{POSIX} 2008. Also, on some
10332 systems, the mere act of examining a symbolic link changes the access
10333 time, such that only changes to the modification time will persist
10334 long enough to be observable. When coupled with option @option{-r}, a
10335 reference timestamp is taken from a symbolic link rather than the file
10339 @itemx --time=mtime
10340 @itemx --time=modify
10343 @opindex mtime@r{, changing}
10344 @opindex modify @r{time, changing}
10345 Change the modification time only.
10347 @item -r @var{file}
10348 @itemx --reference=@var{file}
10350 @opindex --reference
10351 Use the times of the reference @var{file} instead of the current time.
10352 If this option is combined with the @option{--date=@var{time}}
10353 (@option{-d @var{time}}) option, the reference @var{file}'s time is
10354 the origin for any relative @var{time}s given, but is otherwise ignored.
10355 For example, @samp{-r foo -d '-5 seconds'} specifies a time stamp
10356 equal to five seconds before the corresponding time stamp for @file{foo}.
10357 If @var{file} is a symbolic link, the reference timestamp is taken
10358 from the target of the symlink, unless @option{-h} was also in effect.
10360 @item -t [[@var{cc}]@var{yy}]@var{mmddhhmm}[.@var{ss}]
10361 Use the argument (optional four-digit or two-digit years, months,
10362 days, hours, minutes, optional seconds) instead of the current time.
10363 If the year is specified with only two digits, then @var{cc}
10364 is 20 for years in the range 0 @dots{} 68, and 19 for years in
10365 69 @dots{} 99. If no digits of the year are specified,
10366 the argument is interpreted as a date in the current year.
10367 Note that @var{ss} may be @samp{60}, to accommodate leap seconds.
10371 @vindex _POSIX2_VERSION
10372 On older systems, @command{touch} supports an obsolete syntax, as follows.
10373 If no timestamp is given with any of the @option{-d}, @option{-r}, or
10374 @option{-t} options, and if there are two or more @var{file}s and the
10375 first @var{file} is of the form @samp{@var{mmddhhmm}[@var{yy}]} and this
10376 would be a valid argument to the @option{-t} option (if the @var{yy}, if
10377 any, were moved to the front), and if the represented year
10378 is in the range 1969--1999, that argument is interpreted as the time
10379 for the other files instead of as a file name.
10380 This obsolete behavior can be enabled or disabled with the
10381 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
10382 conformance}), but portable scripts should avoid commands whose
10383 behavior depends on this variable.
10384 For example, use @samp{touch ./12312359 main.c} or @samp{touch -t
10385 12312359 main.c} rather than the ambiguous @samp{touch 12312359 main.c}.
10391 @chapter Disk usage
10395 No disk can hold an infinite amount of data. These commands report
10396 how much disk storage is in use or available, report other file and
10397 file status information, and write buffers to disk.
10400 * df invocation:: Report file system disk space usage.
10401 * du invocation:: Estimate file space usage.
10402 * stat invocation:: Report file or file system status.
10403 * sync invocation:: Synchronize memory and disk.
10404 * truncate invocation:: Shrink or extend the size of a file.
10408 @node df invocation
10409 @section @command{df}: Report file system disk space usage
10412 @cindex file system disk usage
10413 @cindex disk usage by file system
10415 @command{df} reports the amount of disk space used and available on
10416 file systems. Synopsis:
10419 df [@var{option}]@dots{} [@var{file}]@dots{}
10422 With no arguments, @command{df} reports the space used and available on all
10423 currently mounted file systems (of all types). Otherwise, @command{df}
10424 reports on the file system containing each argument @var{file}.
10426 Normally the disk space is printed in units of
10427 1024 bytes, but this can be overridden (@pxref{Block size}).
10428 Non-integer quantities are rounded up to the next higher unit.
10430 @cindex disk device file
10431 @cindex device file, disk
10432 If an argument @var{file} is a disk device file containing a mounted
10433 file system, @command{df} shows the space available on that file system
10434 rather than on the file system containing the device node (i.e., the root
10435 file system). @sc{gnu} @command{df} does not attempt to determine the
10437 on unmounted file systems, because on most kinds of systems doing so
10438 requires extremely nonportable intimate knowledge of file system
10441 The program accepts the following options. Also see @ref{Common options}.
10449 @cindex automounter file systems
10450 @cindex ignore file systems
10451 Include in the listing dummy file systems, which
10452 are omitted by default. Such file systems are typically special-purpose
10453 pseudo-file-systems, such as automounter entries.
10455 @item -B @var{size}
10456 @itemx --block-size=@var{size}
10458 @opindex --block-size
10459 @cindex file system sizes
10460 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10461 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10465 @cindex grand total of disk size, usage and available space
10466 Print a grand total of all arguments after all arguments have
10467 been processed. This can be used to find out the total disk size, usage
10468 and available space of all listed devices.
10474 Equivalent to @option{--si}.
10480 @cindex inode usage
10481 List inode usage information instead of block usage. An inode (short
10482 for index node) contains information about a file such as its owner,
10483 permissions, timestamps, and location on the disk.
10487 @cindex kibibytes for file system sizes
10488 Print sizes in 1024-byte blocks, overriding the default block size
10489 (@pxref{Block size}).
10490 This option is equivalent to @option{--block-size=1K}.
10496 @cindex file system types, limiting output to certain
10497 Limit the listing to local file systems. By default, remote file systems
10502 @cindex file system space, retrieving old data more quickly
10503 Do not invoke the @code{sync} system call before getting any usage data.
10504 This may make @command{df} run significantly faster on systems with many
10505 disks, but on some systems (notably SunOS) the results may be slightly
10506 out of date. This is the default.
10509 @itemx --portability
10511 @opindex --portability
10512 @cindex one-line output format
10513 @cindex @acronym{POSIX} output format
10514 @cindex portable output format
10515 @cindex output format, portable
10516 Use the @acronym{POSIX} output format. This is like the default format except
10521 The information about each file system is always printed on exactly
10522 one line; a mount device is never put on a line by itself. This means
10523 that if the mount device name is more than 20 characters long (e.g., for
10524 some network mounts), the columns are misaligned.
10527 The labels in the header output line are changed to conform to @acronym{POSIX}.
10530 The default block size and output format are unaffected by the
10531 @env{DF_BLOCK_SIZE}, @env{BLOCK_SIZE} and @env{BLOCKSIZE} environment
10532 variables. However, the default block size is still affected by
10533 @env{POSIXLY_CORRECT}: it is 512 if @env{POSIXLY_CORRECT} is set, 1024
10534 otherwise. @xref{Block size}.
10541 @cindex file system space, retrieving current data more slowly
10542 Invoke the @code{sync} system call before getting any usage data. On
10543 some systems (notably SunOS), doing this yields more up to date results,
10544 but in general this option makes @command{df} much slower, especially when
10545 there are many or very busy file systems.
10547 @item -t @var{fstype}
10548 @itemx --type=@var{fstype}
10551 @cindex file system types, limiting output to certain
10552 Limit the listing to file systems of type @var{fstype}. Multiple
10553 file system types can be specified by giving multiple @option{-t} options.
10554 By default, nothing is omitted.
10557 @itemx --print-type
10559 @opindex --print-type
10560 @cindex file system types, printing
10561 Print each file system's type. The types printed here are the same ones
10562 you can include or exclude with @option{-t} and @option{-x}. The particular
10563 types printed are whatever is supported by the system. Here are some of
10564 the common names (this list is certainly not exhaustive):
10569 @cindex @acronym{NFS} file system type
10570 An @acronym{NFS} file system, i.e., one mounted over a network from another
10571 machine. This is the one type name which seems to be used uniformly by
10574 @item 4.2@r{, }ufs@r{, }efs@dots{}
10575 @cindex Linux file system types
10576 @cindex local file system types
10577 @opindex 4.2 @r{file system type}
10578 @opindex ufs @r{file system type}
10579 @opindex efs @r{file system type}
10580 A file system on a locally-mounted hard disk. (The system might even
10581 support more than one type here; Linux does.)
10583 @item hsfs@r{, }cdfs
10584 @cindex CD-ROM file system type
10585 @cindex High Sierra file system
10586 @opindex hsfs @r{file system type}
10587 @opindex cdfs @r{file system type}
10588 A file system on a CD-ROM drive. HP-UX uses @samp{cdfs}, most other
10589 systems use @samp{hsfs} (@samp{hs} for ``High Sierra'').
10592 @cindex PC file system
10593 @cindex DOS file system
10594 @cindex MS-DOS file system
10595 @cindex diskette file system
10597 An MS-DOS file system, usually on a diskette.
10601 @item -x @var{fstype}
10602 @itemx --exclude-type=@var{fstype}
10604 @opindex --exclude-type
10605 Limit the listing to file systems not of type @var{fstype}.
10606 Multiple file system types can be eliminated by giving multiple
10607 @option{-x} options. By default, no file system types are omitted.
10610 Ignored; for compatibility with System V versions of @command{df}.
10615 Failure includes the case where no output is generated, so you can
10616 inspect the exit status of a command like @samp{df -t ext3 -t reiserfs
10617 @var{dir}} to test whether @var{dir} is on a file system of type
10618 @samp{ext3} or @samp{reiserfs}.
10621 @node du invocation
10622 @section @command{du}: Estimate file space usage
10625 @cindex file space usage
10626 @cindex disk usage for files
10628 @command{du} reports the amount of disk space used by the specified files
10629 and for each subdirectory (of directory arguments). Synopsis:
10632 du [@var{option}]@dots{} [@var{file}]@dots{}
10635 With no arguments, @command{du} reports the disk space for the current
10636 directory. Normally the disk space is printed in units of
10637 1024 bytes, but this can be overridden (@pxref{Block size}).
10638 Non-integer quantities are rounded up to the next higher unit.
10640 If two or more hard links point to the same file, only one of the hard
10641 links is counted. The @var{file} argument order affects which links
10642 are counted, and changing the argument order may change the numbers
10643 that @command{du} outputs.
10645 The program accepts the following options. Also see @ref{Common options}.
10653 Show counts for all files, not just directories.
10655 @itemx --apparent-size
10656 @opindex --apparent-size
10657 Print apparent sizes, rather than disk usage. The apparent size of a
10658 file is the number of bytes reported by @code{wc -c} on regular files,
10659 or more generally, @code{ls -l --block-size=1} or @code{stat --format=%s}.
10660 For example, a file containing the word @samp{zoo} with no newline would,
10661 of course, have an apparent size of 3. Such a small file may require
10662 anywhere from 0 to 16 KiB or more of disk space, depending on
10663 the type and configuration of the file system on which the file resides.
10664 However, a sparse file created with this command:
10667 dd bs=1 seek=2GiB if=/dev/null of=big
10671 has an apparent size of 2 GiB, yet on most modern
10672 systems, it actually uses almost no disk space.
10678 Equivalent to @code{--apparent-size --block-size=1}.
10680 @item -B @var{size}
10681 @itemx --block-size=@var{size}
10683 @opindex --block-size
10685 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10686 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10692 @cindex grand total of disk space
10693 Print a grand total of all arguments after all arguments have
10694 been processed. This can be used to find out the total disk usage of
10695 a given set of files or directories.
10698 @itemx --dereference-args
10700 @opindex --dereference-args
10701 Dereference symbolic links that are command line arguments.
10702 Does not affect other symbolic links. This is helpful for finding
10703 out the disk usage of directories, such as @file{/usr/tmp}, which
10704 are often symbolic links.
10706 @c --files0-from=FILE
10707 @filesZeroFromOption{du,, with the @option{--total} (@option{-c}) option}
10713 Equivalent to @option{--dereference-args} (@option{-D}).
10717 @cindex kibibytes for file sizes
10718 Print sizes in 1024-byte blocks, overriding the default block size
10719 (@pxref{Block size}).
10720 This option is equivalent to @option{--block-size=1K}.
10723 @itemx --count-links
10725 @opindex --count-links
10726 @cindex hard links, counting in @command{du}
10727 Count the size of all files, even if they have appeared already (as a
10731 @itemx --dereference
10733 @opindex --dereference
10734 @cindex symbolic links, dereferencing in @command{du}
10735 Dereference symbolic links (show the disk space used by the file
10736 or directory that the link points to instead of the space used by
10741 @cindex mebibytes for file sizes
10742 Print sizes in 1,048,576-byte blocks, overriding the default block size
10743 (@pxref{Block size}).
10744 This option is equivalent to @option{--block-size=1M}.
10747 @itemx --no-dereference
10749 @opindex --no-dereference
10750 @cindex symbolic links, dereferencing in @command{du}
10751 For each symbolic links encountered by @command{du},
10752 consider the disk space used by the symbolic link.
10754 @item -d @var{depth}
10755 @item --max-depth=@var{depth}
10756 @opindex -d @var{depth}
10757 @opindex --max-depth=@var{depth}
10758 @cindex limiting output of @command{du}
10759 Show the total for each directory (and file if --all) that is at
10760 most MAX_DEPTH levels down from the root of the hierarchy. The root
10761 is at level 0, so @code{du --max-depth=0} is equivalent to @code{du -s}.
10770 @opindex --summarize
10771 Display only a total for each argument.
10774 @itemx --separate-dirs
10776 @opindex --separate-dirs
10777 Normally, in the output of @command{du} (when not using @option{--summarize}),
10778 the size listed next to a directory name, @var{d}, represents the sum
10779 of sizes of all entries beneath @var{d} as well as the size of @var{d} itself.
10780 With @option{--separate-dirs}, the size reported for a directory name,
10781 @var{d}, is merely the @code{stat.st_size}-derived size of the directory
10786 @cindex last modified dates, displaying in @command{du}
10787 Show time of the most recent modification of any file in the directory,
10788 or any of its subdirectories.
10790 @itemx --time=ctime
10791 @itemx --time=status
10794 @opindex ctime@r{, show the most recent}
10795 @opindex status time@r{, show the most recent}
10796 @opindex use time@r{, show the most recent}
10797 Show the most recent status change time (the @samp{ctime} in the inode) of
10798 any file in the directory, instead of the modification time.
10800 @itemx --time=atime
10801 @itemx --time=access
10803 @opindex atime@r{, show the most recent}
10804 @opindex access time@r{, show the most recent}
10805 Show the most recent access time (the @samp{atime} in the inode) of
10806 any file in the directory, instead of the modification time.
10808 @item --time-style=@var{style}
10809 @opindex --time-style
10811 List timestamps in style @var{style}. This option has an effect only if
10812 the @option{--time} option is also specified. The @var{style} should
10813 be one of the following:
10816 @item +@var{format}
10818 List timestamps using @var{format}, where @var{format} is interpreted
10819 like the format argument of @command{date} (@pxref{date invocation}).
10820 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
10821 @command{du} to list timestamps like @samp{2002-03-30 23:45:56}. As
10822 with @command{date}, @var{format}'s interpretation is affected by the
10823 @env{LC_TIME} locale category.
10826 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
10827 format with nanosecond precision, e.g., @samp{2002-03-30
10828 23:45:56.477817180 -0700}. This style is equivalent to
10829 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
10832 List @acronym{ISO} 8601 date and time in minutes, e.g.,
10833 @samp{2002-03-30 23:45}. These timestamps are shorter than
10834 @samp{full-iso} timestamps, and are usually good enough for everyday
10835 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
10838 List @acronym{ISO} 8601 dates for timestamps, e.g., @samp{2002-03-30}.
10839 This style is equivalent to @samp{+%Y-%m-%d}.
10843 You can specify the default value of the @option{--time-style} option
10844 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
10845 the default style is @samp{long-iso}. For compatibility with @command{ls},
10846 if @env{TIME_STYLE} begins with @samp{+} and contains a newline,
10847 the newline and any later characters are ignored; if @env{TIME_STYLE}
10848 begins with @samp{posix-} the @samp{posix-} is ignored; and if
10849 @env{TIME_STYLE} is @samp{locale} it is ignored.
10852 @itemx --one-file-system
10854 @opindex --one-file-system
10855 @cindex one file system, restricting @command{du} to
10856 Skip directories that are on different file systems from the one that
10857 the argument being processed is on.
10859 @item --exclude=@var{pattern}
10860 @opindex --exclude=@var{pattern}
10861 @cindex excluding files from @command{du}
10862 When recursing, skip subdirectories or files matching @var{pattern}.
10863 For example, @code{du --exclude='*.o'} excludes files whose names
10866 @item -X @var{file}
10867 @itemx --exclude-from=@var{file}
10868 @opindex -X @var{file}
10869 @opindex --exclude-from=@var{file}
10870 @cindex excluding files from @command{du}
10871 Like @option{--exclude}, except take the patterns to exclude from @var{file},
10872 one per line. If @var{file} is @samp{-}, take the patterns from standard
10877 @cindex NFS mounts from BSD to HP-UX
10878 On BSD systems, @command{du} reports sizes that are half the correct
10879 values for files that are NFS-mounted from HP-UX systems. On HP-UX
10880 systems, it reports sizes that are twice the correct values for
10881 files that are NFS-mounted from BSD systems. This is due to a flaw
10882 in HP-UX; it also affects the HP-UX @command{du} program.
10887 @node stat invocation
10888 @section @command{stat}: Report file or file system status
10891 @cindex file status
10892 @cindex file system status
10894 @command{stat} displays information about the specified file(s). Synopsis:
10897 stat [@var{option}]@dots{} [@var{file}]@dots{}
10900 With no option, @command{stat} reports all information about the given files.
10901 But it also can be used to report the information of the file systems the
10902 given files are located on. If the files are links, @command{stat} can
10903 also give information about the files the links point to.
10905 @mayConflictWithShellBuiltIn{stat}
10910 @itemx --dereference
10912 @opindex --dereference
10913 @cindex symbolic links, dereferencing in @command{stat}
10914 Change how @command{stat} treats symbolic links.
10915 With this option, @command{stat} acts on the file referenced
10916 by each symbolic link argument.
10917 Without it, @command{stat} acts on any symbolic link argument directly.
10920 @itemx --file-system
10922 @opindex --file-system
10923 @cindex file systems
10924 Report information about the file systems where the given files are located
10925 instead of information about the files themselves.
10926 This option implies the @option{-L} option.
10929 @itemx --format=@var{format}
10931 @opindex --format=@var{format}
10932 @cindex output format
10933 Use @var{format} rather than the default format.
10934 @var{format} is automatically newline-terminated, so
10935 running a command like the following with two or more @var{file}
10936 operands produces a line of output for each operand:
10938 $ stat --format=%d:%i / /usr
10943 @itemx --printf=@var{format}
10944 @opindex --printf=@var{format}
10945 @cindex output format
10946 Use @var{format} rather than the default format.
10947 Like @option{--format}, but interpret backslash escapes,
10948 and do not output a mandatory trailing newline.
10949 If you want a newline, include @samp{\n} in the @var{format}.
10950 Here's how you would use @option{--printf} to print the device
10951 and inode numbers of @file{/} and @file{/usr}:
10953 $ stat --printf='%d:%i\n' / /usr
10962 @cindex terse output
10963 Print the information in terse form, suitable for parsing by other programs.
10967 The valid @var{format} directives for files with @option{--format} and
10968 @option{--printf} are:
10971 @item %a - Access rights in octal
10972 @item %A - Access rights in human readable form
10973 @item %b - Number of blocks allocated (see @samp{%B})
10974 @item %B - The size in bytes of each block reported by @samp{%b}
10975 @item %C - The SELinux security context of a file, if available
10976 @item %d - Device number in decimal
10977 @item %D - Device number in hex
10978 @item %f - Raw mode in hex
10979 @item %F - File type
10980 @item %g - Group ID of owner
10981 @item %G - Group name of owner
10982 @item %h - Number of hard links
10983 @item %i - Inode number
10984 @item %m - Mount point (See note below)
10985 @item %n - File name
10986 @item %N - Quoted file name with dereference if symbolic link
10987 @item %o - I/O block size
10988 @item %s - Total size, in bytes
10989 @item %t - Major device type in hex
10990 @item %T - Minor device type in hex
10991 @item %u - User ID of owner
10992 @item %U - User name of owner
10993 @item %w - Time of file birth, or @samp{-} if unknown
10994 @item %W - Time of file birth as seconds since Epoch, or @samp{0}
10995 @item %x - Time of last access
10996 @item %X - Time of last access as seconds since Epoch
10997 @item %y - Time of last modification
10998 @item %Y - Time of last modification as seconds since Epoch
10999 @item %z - Time of last change
11000 @item %Z - Time of last change as seconds since Epoch
11003 The @samp{%W}, @samp{%X}, @samp{%Y}, and @samp{%Z} formats accept a
11004 precision preceded by a period to specify the number of digits to
11005 print after the decimal point. For example, @samp{%.3X} outputs the
11006 last access time to millisecond precision. If a period is given but no
11007 precision, @command{stat} uses 9 digits, so @samp{%.X} is equivalent to
11008 @samp{%.9X}. When discarding excess precision, time stamps are truncated
11009 toward minus infinity.
11013 $ stat -c '[%015Y]' /usr
11016 $ stat -c '[%15Y]' /usr
11018 $ stat -c '[%-15Y]' /usr
11021 $ stat -c '[%.3Y]' /usr
11023 $ stat -c '[%.Y]' /usr
11024 [1288929712.114951834]
11027 The mount point printed by @samp{%m} is similar to that output
11028 by @command{df}, except that:
11031 stat does not dereference symlinks by default
11032 (unless @option{-L} is specified)
11034 stat does not search for specified device nodes in the
11035 file system list, instead operating on them directly
11038 stat outputs the alias for a bind mounted file, rather than
11039 the initial mount point of its backing device.
11040 One can recursively call stat until there is no change in output,
11041 to get the current base mount point
11044 When listing file system information (@option{--file-system} (@option{-f})),
11045 you must use a different set of @var{format} directives:
11048 @item %a - Free blocks available to non-super-user
11049 @item %b - Total data blocks in file system
11050 @item %c - Total file nodes in file system
11051 @item %d - Free file nodes in file system
11052 @item %f - Free blocks in file system
11053 @item %i - File System ID in hex
11054 @item %l - Maximum length of file names
11055 @item %n - File name
11056 @item %s - Block size (for faster transfers)
11057 @item %S - Fundamental block size (for block counts)
11058 @item %t - Type in hex
11059 @item %T - Type in human readable form
11063 Time stamps are listed according to the time zone rules specified by
11064 the @env{TZ} environment variable, or by the system default rules if
11065 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
11066 with @env{TZ}, libc, The GNU C Library Reference Manual}.
11071 @node sync invocation
11072 @section @command{sync}: Synchronize data on disk with memory
11075 @cindex synchronize disk and memory
11077 @cindex superblock, writing
11078 @cindex inodes, written buffered
11079 @command{sync} writes any data buffered in memory out to disk. This can
11080 include (but is not limited to) modified superblocks, modified inodes,
11081 and delayed reads and writes. This must be implemented by the kernel;
11082 The @command{sync} program does nothing but exercise the @code{sync} system
11085 @cindex crashes and corruption
11086 The kernel keeps data in memory to avoid doing (relatively slow) disk
11087 reads and writes. This improves performance, but if the computer
11088 crashes, data may be lost or the file system corrupted as a
11089 result. The @command{sync} command ensures everything in memory
11090 is written to disk.
11092 Any arguments are ignored, except for a lone @option{--help} or
11093 @option{--version} (@pxref{Common options}).
11098 @node truncate invocation
11099 @section @command{truncate}: Shrink or extend the size of a file
11102 @cindex truncating, file sizes
11104 @command{truncate} shrinks or extends the size of each @var{file} to the
11105 specified size. Synopsis:
11108 truncate @var{option}@dots{} @var{file}@dots{}
11111 @cindex files, creating
11112 Any @var{file} that does not exist is created.
11114 @cindex sparse files, creating
11115 @cindex holes, creating files with
11116 If a @var{file} is larger than the specified size, the extra data is lost.
11117 If a @var{file} is shorter, it is extended and the extended part (or hole)
11118 reads as zero bytes.
11120 The program accepts the following options. Also see @ref{Common options}.
11127 @opindex --no-create
11128 Do not create files that do not exist.
11133 @opindex --io-blocks
11134 Treat @var{size} as number of I/O blocks of the @var{file} rather than bytes.
11136 @item -r @var{rfile}
11137 @itemx --reference=@var{rfile}
11139 @opindex --reference
11140 Base the size of each @var{file} on the size of @var{rfile}.
11142 @item -s @var{size}
11143 @itemx --size=@var{size}
11146 Set or adjust the size of each @var{file} according to @var{size}.
11147 @multiplierSuffixesNoBlocks{size}
11149 @var{size} may also be prefixed by one of the following to adjust
11150 the size of each @var{file} based on their current size:
11152 @samp{+} => extend by
11153 @samp{-} => reduce by
11154 @samp{<} => at most
11155 @samp{>} => at least
11156 @samp{/} => round down to multiple of
11157 @samp{%} => round up to multiple of
11165 @node Printing text
11166 @chapter Printing text
11168 @cindex printing text, commands for
11169 @cindex commands for printing text
11171 This section describes commands that display text strings.
11174 * echo invocation:: Print a line of text.
11175 * printf invocation:: Format and print data.
11176 * yes invocation:: Print a string until interrupted.
11180 @node echo invocation
11181 @section @command{echo}: Print a line of text
11184 @cindex displaying text
11185 @cindex printing text
11186 @cindex text, displaying
11187 @cindex arbitrary text, displaying
11189 @command{echo} writes each given @var{string} to standard output, with a
11190 space between each and a newline after the last one. Synopsis:
11193 echo [@var{option}]@dots{} [@var{string}]@dots{}
11196 @mayConflictWithShellBuiltIn{echo}
11198 The program accepts the following options. Also see @ref{Common options}.
11199 Options must precede operands, and the normally-special argument
11200 @samp{--} has no special meaning and is treated like any other
11206 Do not output the trailing newline.
11210 @cindex backslash escapes
11211 Enable interpretation of the following backslash-escaped characters in
11220 produce no further output
11236 the eight-bit value that is the octal number @var{nnn}
11237 (zero to three octal digits), if @var{nnn} is
11238 a nine-bit value, the ninth bit is ignored
11240 the eight-bit value that is the octal number @var{nnn}
11241 (one to three octal digits), if @var{nnn} is
11242 a nine-bit value, the ninth bit is ignored
11244 the eight-bit value that is the hexadecimal number @var{hh}
11245 (one or two hexadecimal digits)
11250 @cindex backslash escapes
11251 Disable interpretation of backslash escapes in each @var{string}.
11252 This is the default. If @option{-e} and @option{-E} are both
11253 specified, the last one given takes effect.
11257 @vindex POSIXLY_CORRECT
11258 If the @env{POSIXLY_CORRECT} environment variable is set, then when
11259 @command{echo}'s first argument is not @option{-n} it outputs
11260 option-like arguments instead of treating them as options. For
11261 example, @code{echo -ne hello} outputs @samp{-ne hello} instead of
11262 plain @samp{hello}.
11264 @acronym{POSIX} does not require support for any options, and says
11265 that the behavior of @command{echo} is implementation-defined if any
11266 @var{string} contains a backslash or if the first argument is
11267 @option{-n}. Portable programs can use the @command{printf} command
11268 if they need to omit trailing newlines or output control characters or
11269 backslashes. @xref{printf invocation}.
11274 @node printf invocation
11275 @section @command{printf}: Format and print data
11278 @command{printf} does formatted printing of text. Synopsis:
11281 printf @var{format} [@var{argument}]@dots{}
11284 @command{printf} prints the @var{format} string, interpreting @samp{%}
11285 directives and @samp{\} escapes to format numeric and string arguments
11286 in a way that is mostly similar to the C @samp{printf} function.
11287 @xref{Output Conversion Syntax,, @command{printf} format directives,
11288 libc, The GNU C Library Reference Manual}, for details.
11289 The differences are listed below.
11291 @mayConflictWithShellBuiltIn{printf}
11296 The @var{format} argument is reused as necessary to convert all the
11297 given @var{argument}s. For example, the command @samp{printf %s a b}
11301 Missing @var{argument}s are treated as null strings or as zeros,
11302 depending on whether the context expects a string or a number. For
11303 example, the command @samp{printf %sx%d} prints @samp{x0}.
11307 An additional escape, @samp{\c}, causes @command{printf} to produce no
11308 further output. For example, the command @samp{printf 'A%sC\cD%sF' B
11309 E} prints @samp{ABC}.
11312 The hexadecimal escape sequence @samp{\x@var{hh}} has at most two
11313 digits, as opposed to C where it can have an unlimited number of
11314 digits. For example, the command @samp{printf '\x07e'} prints two
11315 bytes, whereas the C statement @samp{printf ("\x07e")} prints just
11320 @command{printf} has an additional directive, @samp{%b}, which prints its
11321 argument string with @samp{\} escapes interpreted in the same way as in
11322 the @var{format} string, except that octal escapes are of the form
11323 @samp{\0@var{ooo}} where @var{ooo} is 0 to 3 octal digits. If
11324 @samp{\@var{ooo}} is nine-bit value, ignore the ninth bit.
11325 If a precision is also given, it limits the number of bytes printed
11326 from the converted string.
11329 Numeric arguments must be single C constants, possibly with leading
11330 @samp{+} or @samp{-}. For example, @samp{printf %.4d -3} outputs
11334 @vindex POSIXLY_CORRECT
11335 If the leading character of a numeric argument is @samp{"} or @samp{'}
11336 then its value is the numeric value of the immediately following
11337 character. Any remaining characters are silently ignored if the
11338 @env{POSIXLY_CORRECT} environment variable is set; otherwise, a
11339 warning is printed. For example, @samp{printf "%d" "'a"} outputs
11340 @samp{97} on hosts that use the @acronym{ASCII} character set, since
11341 @samp{a} has the numeric value 97 in @acronym{ASCII}.
11346 A floating-point argument must use a period before any fractional
11347 digits, but is printed according to the @env{LC_NUMERIC} category of the
11348 current locale. For example, in a locale whose radix character is a
11349 comma, the command @samp{printf %g 3.14} outputs @samp{3,14} whereas
11350 the command @samp{printf %g 3,14} is an error.
11351 @xref{Floating point}.
11355 @command{printf} interprets @samp{\@var{ooo}} in @var{format} as an octal number
11356 (if @var{ooo} is 1 to 3 octal digits) specifying a byte to print,
11357 and @samp{\x@var{hh}} as a hexadecimal number (if @var{hh} is 1 to 2 hex
11358 digits) specifying a character to print.
11359 Note however that when @samp{\@var{ooo}} specifies a number larger than 255,
11360 @command{printf} ignores the ninth bit.
11361 For example, @samp{printf '\400'} is equivalent to @samp{printf '\0'}.
11366 @cindex ISO/IEC 10646
11368 @command{printf} interprets two character syntaxes introduced in
11369 @acronym{ISO} C 99:
11370 @samp{\u} for 16-bit Unicode (@acronym{ISO}/@acronym{IEC} 10646)
11371 characters, specified as
11372 four hexadecimal digits @var{hhhh}, and @samp{\U} for 32-bit Unicode
11373 characters, specified as eight hexadecimal digits @var{hhhhhhhh}.
11374 @command{printf} outputs the Unicode characters
11375 according to the @env{LC_CTYPE} locale. Unicode characters in the ranges
11376 U+0000...U+009F, U+D800...U+DFFF cannot be specified by this syntax, except
11377 for U+0024 ($), U+0040 (@@), and U+0060 (@`).
11379 The processing of @samp{\u} and @samp{\U} requires a full-featured
11380 @code{iconv} facility. It is activated on systems with glibc 2.2 (or newer),
11381 or when @code{libiconv} is installed prior to this package. Otherwise
11382 @samp{\u} and @samp{\U} will print as-is.
11384 The only options are a lone @option{--help} or
11385 @option{--version}. @xref{Common options}.
11386 Options must precede operands.
11388 The Unicode character syntaxes are useful for writing strings in a locale
11389 independent way. For example, a string containing the Euro currency symbol
11392 $ env printf '\u20AC 14.95'
11396 will be output correctly in all locales supporting the Euro symbol
11397 (@acronym{ISO}-8859-15, UTF-8, and others). Similarly, a Chinese string
11400 $ env printf '\u4e2d\u6587'
11404 will be output correctly in all Chinese locales (GB2312, BIG5, UTF-8, etc).
11406 Note that in these examples, the @command{printf} command has been
11407 invoked via @command{env} to ensure that we run the program found via
11408 your shell's search path, and not a shell alias or a built-in function.
11410 For larger strings, you don't need to look up the hexadecimal code
11411 values of each character one by one. @acronym{ASCII} characters mixed with \u
11412 escape sequences is also known as the JAVA source file encoding. You can
11413 use GNU recode 3.5c (or newer) to convert strings to this encoding. Here
11414 is how to convert a piece of text into a shell script which will output
11415 this text in a locale-independent way:
11418 $ LC_CTYPE=zh_CN.big5 /usr/local/bin/printf \
11419 '\u4e2d\u6587\n' > sample.txt
11420 $ recode BIG5..JAVA < sample.txt \
11421 | sed -e "s|^|/usr/local/bin/printf '|" -e "s|$|\\\\n'|" \
11428 @node yes invocation
11429 @section @command{yes}: Print a string until interrupted
11432 @cindex repeated output of a string
11434 @command{yes} prints the command line arguments, separated by spaces and
11435 followed by a newline, forever until it is killed. If no arguments are
11436 given, it prints @samp{y} followed by a newline forever until killed.
11438 Upon a write error, @command{yes} exits with status @samp{1}.
11440 The only options are a lone @option{--help} or @option{--version}.
11441 To output an argument that begins with
11442 @samp{-}, precede it with @option{--}, e.g., @samp{yes -- --help}.
11443 @xref{Common options}.
11447 @chapter Conditions
11450 @cindex commands for exit status
11451 @cindex exit status commands
11453 This section describes commands that are primarily useful for their exit
11454 status, rather than their output. Thus, they are often used as the
11455 condition of shell @code{if} statements, or as the last command in a
11459 * false invocation:: Do nothing, unsuccessfully.
11460 * true invocation:: Do nothing, successfully.
11461 * test invocation:: Check file types and compare values.
11462 * expr invocation:: Evaluate expressions.
11466 @node false invocation
11467 @section @command{false}: Do nothing, unsuccessfully
11470 @cindex do nothing, unsuccessfully
11471 @cindex failure exit status
11472 @cindex exit status of @command{false}
11474 @command{false} does nothing except return an exit status of 1, meaning
11475 @dfn{failure}. It can be used as a place holder in shell scripts
11476 where an unsuccessful command is needed.
11477 In most modern shells, @command{false} is a built-in command, so when
11478 you use @samp{false} in a script, you're probably using the built-in
11479 command, not the one documented here.
11481 @command{false} honors the @option{--help} and @option{--version} options.
11483 This version of @command{false} is implemented as a C program, and is thus
11484 more secure and faster than a shell script implementation, and may safely
11485 be used as a dummy shell for the purpose of disabling accounts.
11487 Note that @command{false} (unlike all other programs documented herein)
11488 exits unsuccessfully, even when invoked with
11489 @option{--help} or @option{--version}.
11491 Portable programs should not assume that the exit status of
11492 @command{false} is 1, as it is greater than 1 on some
11493 non-@acronym{GNU} hosts.
11496 @node true invocation
11497 @section @command{true}: Do nothing, successfully
11500 @cindex do nothing, successfully
11502 @cindex successful exit
11503 @cindex exit status of @command{true}
11505 @command{true} does nothing except return an exit status of 0, meaning
11506 @dfn{success}. It can be used as a place holder in shell scripts
11507 where a successful command is needed, although the shell built-in
11508 command @code{:} (colon) may do the same thing faster.
11509 In most modern shells, @command{true} is a built-in command, so when
11510 you use @samp{true} in a script, you're probably using the built-in
11511 command, not the one documented here.
11513 @command{true} honors the @option{--help} and @option{--version} options.
11515 Note, however, that it is possible to cause @command{true}
11516 to exit with nonzero status: with the @option{--help} or @option{--version}
11517 option, and with standard
11518 output already closed or redirected to a file that evokes an I/O error.
11519 For example, using a Bourne-compatible shell:
11522 $ ./true --version >&-
11523 ./true: write error: Bad file number
11524 $ ./true --version > /dev/full
11525 ./true: write error: No space left on device
11528 This version of @command{true} is implemented as a C program, and is thus
11529 more secure and faster than a shell script implementation, and may safely
11530 be used as a dummy shell for the purpose of disabling accounts.
11532 @node test invocation
11533 @section @command{test}: Check file types and compare values
11536 @cindex check file types
11537 @cindex compare values
11538 @cindex expression evaluation
11540 @command{test} returns a status of 0 (true) or 1 (false) depending on the
11541 evaluation of the conditional expression @var{expr}. Each part of the
11542 expression must be a separate argument.
11544 @command{test} has file status checks, string operators, and numeric
11545 comparison operators.
11547 @command{test} has an alternate form that uses opening and closing
11548 square brackets instead a leading @samp{test}. For example, instead
11549 of @samp{test -d /}, you can write @samp{[ -d / ]}. The square
11550 brackets must be separate arguments; for example, @samp{[-d /]} does
11551 not have the desired effect. Since @samp{test @var{expr}} and @samp{[
11552 @var{expr} ]} have the same meaning, only the former form is discussed
11558 test @var{expression}
11560 [ @var{expression} ]
11565 @mayConflictWithShellBuiltIn{test}
11567 If @var{expression} is omitted, @command{test} returns false.
11568 If @var{expression} is a single argument,
11569 @command{test} returns false if the argument is null and true
11570 otherwise. The argument
11571 can be any string, including strings like @samp{-d}, @samp{-1},
11572 @samp{--}, @samp{--help}, and @samp{--version} that most other
11573 programs would treat as options. To get help and version information,
11574 invoke the commands @samp{[ --help} and @samp{[ --version}, without
11575 the usual closing brackets. @xref{Common options}.
11577 @cindex exit status of @command{test}
11581 0 if the expression is true,
11582 1 if the expression is false,
11583 2 if an error occurred.
11587 * File type tests:: -[bcdfhLpSt]
11588 * Access permission tests:: -[gkruwxOG]
11589 * File characteristic tests:: -e -s -nt -ot -ef
11590 * String tests:: -z -n = == !=
11591 * Numeric tests:: -eq -ne -lt -le -gt -ge
11592 * Connectives for test:: ! -a -o
11596 @node File type tests
11597 @subsection File type tests
11599 @cindex file type tests
11601 These options test for particular types of files. (Everything's a file,
11602 but not all files are the same!)
11606 @item -b @var{file}
11608 @cindex block special check
11609 True if @var{file} exists and is a block special device.
11611 @item -c @var{file}
11613 @cindex character special check
11614 True if @var{file} exists and is a character special device.
11616 @item -d @var{file}
11618 @cindex directory check
11619 True if @var{file} exists and is a directory.
11621 @item -f @var{file}
11623 @cindex regular file check
11624 True if @var{file} exists and is a regular file.
11626 @item -h @var{file}
11627 @itemx -L @var{file}
11630 @cindex symbolic link check
11631 True if @var{file} exists and is a symbolic link.
11632 Unlike all other file-related tests, this test does not dereference
11633 @var{file} if it is a symbolic link.
11635 @item -p @var{file}
11637 @cindex named pipe check
11638 True if @var{file} exists and is a named pipe.
11640 @item -S @var{file}
11642 @cindex socket check
11643 True if @var{file} exists and is a socket.
11647 @cindex terminal check
11648 True if @var{fd} is a file descriptor that is associated with a
11654 @node Access permission tests
11655 @subsection Access permission tests
11657 @cindex access permission tests
11658 @cindex permission tests
11660 These options test for particular access permissions.
11664 @item -g @var{file}
11666 @cindex set-group-ID check
11667 True if @var{file} exists and has its set-group-ID bit set.
11669 @item -k @var{file}
11671 @cindex sticky bit check
11672 True if @var{file} exists and has its @dfn{sticky} bit set.
11674 @item -r @var{file}
11676 @cindex readable file check
11677 True if @var{file} exists and read permission is granted.
11679 @item -u @var{file}
11681 @cindex set-user-ID check
11682 True if @var{file} exists and has its set-user-ID bit set.
11684 @item -w @var{file}
11686 @cindex writable file check
11687 True if @var{file} exists and write permission is granted.
11689 @item -x @var{file}
11691 @cindex executable file check
11692 True if @var{file} exists and execute permission is granted
11693 (or search permission, if it is a directory).
11695 @item -O @var{file}
11697 @cindex owned by effective user ID check
11698 True if @var{file} exists and is owned by the current effective user ID.
11700 @item -G @var{file}
11702 @cindex owned by effective group ID check
11703 True if @var{file} exists and is owned by the current effective group ID.
11707 @node File characteristic tests
11708 @subsection File characteristic tests
11710 @cindex file characteristic tests
11712 These options test other file characteristics.
11716 @item -e @var{file}
11718 @cindex existence-of-file check
11719 True if @var{file} exists.
11721 @item -s @var{file}
11723 @cindex nonempty file check
11724 True if @var{file} exists and has a size greater than zero.
11726 @item @var{file1} -nt @var{file2}
11728 @cindex newer-than file check
11729 True if @var{file1} is newer (according to modification date) than
11730 @var{file2}, or if @var{file1} exists and @var{file2} does not.
11732 @item @var{file1} -ot @var{file2}
11734 @cindex older-than file check
11735 True if @var{file1} is older (according to modification date) than
11736 @var{file2}, or if @var{file2} exists and @var{file1} does not.
11738 @item @var{file1} -ef @var{file2}
11740 @cindex same file check
11741 @cindex hard link check
11742 True if @var{file1} and @var{file2} have the same device and inode
11743 numbers, i.e., if they are hard links to each other.
11749 @subsection String tests
11751 @cindex string tests
11753 These options test string characteristics. You may need to quote
11754 @var{string} arguments for the shell. For example:
11760 The quotes here prevent the wrong arguments from being passed to
11761 @command{test} if @samp{$V} is empty or contains special characters.
11765 @item -z @var{string}
11767 @cindex zero-length string check
11768 True if the length of @var{string} is zero.
11770 @item -n @var{string}
11771 @itemx @var{string}
11773 @cindex nonzero-length string check
11774 True if the length of @var{string} is nonzero.
11776 @item @var{string1} = @var{string2}
11778 @cindex equal string check
11779 True if the strings are equal.
11781 @item @var{string1} == @var{string2}
11783 @cindex equal string check
11784 True if the strings are equal (synonym for =).
11786 @item @var{string1} != @var{string2}
11788 @cindex not-equal string check
11789 True if the strings are not equal.
11794 @node Numeric tests
11795 @subsection Numeric tests
11797 @cindex numeric tests
11798 @cindex arithmetic tests
11800 Numeric relational operators. The arguments must be entirely numeric
11801 (possibly negative), or the special expression @w{@code{-l @var{string}}},
11802 which evaluates to the length of @var{string}.
11806 @item @var{arg1} -eq @var{arg2}
11807 @itemx @var{arg1} -ne @var{arg2}
11808 @itemx @var{arg1} -lt @var{arg2}
11809 @itemx @var{arg1} -le @var{arg2}
11810 @itemx @var{arg1} -gt @var{arg2}
11811 @itemx @var{arg1} -ge @var{arg2}
11818 These arithmetic binary operators return true if @var{arg1} is equal,
11819 not-equal, less-than, less-than-or-equal, greater-than, or
11820 greater-than-or-equal than @var{arg2}, respectively.
11827 test -1 -gt -2 && echo yes
11829 test -l abc -gt 1 && echo yes
11832 @error{} test: integer expression expected before -eq
11836 @node Connectives for test
11837 @subsection Connectives for @command{test}
11839 @cindex logical connectives
11840 @cindex connectives, logical
11842 The usual logical connectives.
11848 True if @var{expr} is false.
11850 @item @var{expr1} -a @var{expr2}
11852 @cindex logical and operator
11853 @cindex and operator
11854 True if both @var{expr1} and @var{expr2} are true.
11856 @item @var{expr1} -o @var{expr2}
11858 @cindex logical or operator
11859 @cindex or operator
11860 True if either @var{expr1} or @var{expr2} is true.
11865 @node expr invocation
11866 @section @command{expr}: Evaluate expressions
11869 @cindex expression evaluation
11870 @cindex evaluation of expressions
11872 @command{expr} evaluates an expression and writes the result on standard
11873 output. Each token of the expression must be a separate argument.
11875 Operands are either integers or strings. Integers consist of one or
11876 more decimal digits, with an optional leading @samp{-}.
11877 @command{expr} converts
11878 anything appearing in an operand position to an integer or a string
11879 depending on the operation being applied to it.
11881 Strings are not quoted for @command{expr} itself, though you may need to
11882 quote them to protect characters with special meaning to the shell,
11883 e.g., spaces. However, regardless of whether it is quoted, a string
11884 operand should not be a parenthesis or any of @command{expr}'s
11885 operators like @code{+}, so you cannot safely pass an arbitrary string
11886 @code{$str} to expr merely by quoting it to the shell. One way to
11887 work around this is to use the @sc{gnu} extension @code{+},
11888 (e.g., @code{+ "$str" = foo}); a more portable way is to use
11889 @code{@w{" $str"}} and to adjust the rest of the expression to take
11890 the leading space into account (e.g., @code{@w{" $str" = " foo"}}).
11892 You should not pass a negative integer or a string with leading
11893 @samp{-} as @command{expr}'s first argument, as it might be
11894 misinterpreted as an option; this can be avoided by parenthesization.
11895 Also, portable scripts should not use a string operand that happens to
11896 take the form of an integer; this can be worked around by inserting
11897 leading spaces as mentioned above.
11899 @cindex parentheses for grouping
11900 Operators may be given as infix symbols or prefix keywords. Parentheses
11901 may be used for grouping in the usual manner. You must quote
11902 parentheses and many operators to avoid the shell evaluating them,
11905 When built with support for the GNU MP library, @command{expr} uses
11906 arbitrary-precision arithmetic; otherwise, it uses native arithmetic
11907 types and may fail due to arithmetic overflow.
11909 The only options are @option{--help} and @option{--version}. @xref{Common
11910 options}. Options must precede operands.
11912 @cindex exit status of @command{expr}
11916 0 if the expression is neither null nor 0,
11917 1 if the expression is null or 0,
11918 2 if the expression is invalid,
11919 3 if an internal error occurred (e.g., arithmetic overflow).
11923 * String expressions:: + : match substr index length
11924 * Numeric expressions:: + - * / %
11925 * Relations for expr:: | & < <= = == != >= >
11926 * Examples of expr:: Examples.
11930 @node String expressions
11931 @subsection String expressions
11933 @cindex string expressions
11934 @cindex expressions, string
11936 @command{expr} supports pattern matching and other string operators. These
11937 have higher precedence than both the numeric and relational operators (in
11938 the next sections).
11942 @item @var{string} : @var{regex}
11943 @cindex pattern matching
11944 @cindex regular expression matching
11945 @cindex matching patterns
11946 Perform pattern matching. The arguments are converted to strings and the
11947 second is considered to be a (basic, a la GNU @code{grep}) regular
11948 expression, with a @code{^} implicitly prepended. The first argument is
11949 then matched against this regular expression.
11951 If the match succeeds and @var{regex} uses @samp{\(} and @samp{\)}, the
11952 @code{:} expression returns the part of @var{string} that matched the
11953 subexpression; otherwise, it returns the number of characters matched.
11955 If the match fails, the @code{:} operator returns the null string if
11956 @samp{\(} and @samp{\)} are used in @var{regex}, otherwise 0.
11958 @kindex \( @r{regexp operator}
11959 Only the first @samp{\( @dots{} \)} pair is relevant to the return
11960 value; additional pairs are meaningful only for grouping the regular
11961 expression operators.
11963 @kindex \+ @r{regexp operator}
11964 @kindex \? @r{regexp operator}
11965 @kindex \| @r{regexp operator}
11966 In the regular expression, @code{\+}, @code{\?}, and @code{\|} are
11967 operators which respectively match one or more, zero or one, or separate
11968 alternatives. SunOS and other @command{expr}'s treat these as regular
11969 characters. (@acronym{POSIX} allows either behavior.)
11970 @xref{Top, , Regular Expression Library, regex, Regex}, for details of
11971 regular expression syntax. Some examples are in @ref{Examples of expr}.
11973 @item match @var{string} @var{regex}
11975 An alternative way to do pattern matching. This is the same as
11976 @w{@samp{@var{string} : @var{regex}}}.
11978 @item substr @var{string} @var{position} @var{length}
11980 Returns the substring of @var{string} beginning at @var{position}
11981 with length at most @var{length}. If either @var{position} or
11982 @var{length} is negative, zero, or non-numeric, returns the null string.
11984 @item index @var{string} @var{charset}
11986 Returns the first position in @var{string} where the first character in
11987 @var{charset} was found. If no character in @var{charset} is found in
11988 @var{string}, return 0.
11990 @item length @var{string}
11992 Returns the length of @var{string}.
11994 @item + @var{token}
11996 Interpret @var{token} as a string, even if it is a keyword like @var{match}
11997 or an operator like @code{/}.
11998 This makes it possible to test @code{expr length + "$x"} or
11999 @code{expr + "$x" : '.*/\(.\)'} and have it do the right thing even if
12000 the value of @var{$x} happens to be (for example) @code{/} or @code{index}.
12001 This operator is a @acronym{GNU} extension. Portable shell scripts should use
12002 @code{@w{" $token"} : @w{' \(.*\)'}} instead of @code{+ "$token"}.
12006 To make @command{expr} interpret keywords as strings, you must use the
12007 @code{quote} operator.
12010 @node Numeric expressions
12011 @subsection Numeric expressions
12013 @cindex numeric expressions
12014 @cindex expressions, numeric
12016 @command{expr} supports the usual numeric operators, in order of increasing
12017 precedence. These numeric operators have lower precedence than the
12018 string operators described in the previous section, and higher precedence
12019 than the connectives (next section).
12027 @cindex subtraction
12028 Addition and subtraction. Both arguments are converted to integers;
12029 an error occurs if this cannot be done.
12035 @cindex multiplication
12038 Multiplication, division, remainder. Both arguments are converted to
12039 integers; an error occurs if this cannot be done.
12044 @node Relations for expr
12045 @subsection Relations for @command{expr}
12047 @cindex connectives, logical
12048 @cindex logical connectives
12049 @cindex relations, numeric or string
12051 @command{expr} supports the usual logical connectives and relations. These
12052 have lower precedence than the string and numeric operators
12053 (previous sections). Here is the list, lowest-precedence operator first.
12059 @cindex logical or operator
12060 @cindex or operator
12061 Returns its first argument if that is neither null nor zero, otherwise
12062 its second argument if it is neither null nor zero, otherwise 0. It
12063 does not evaluate its second argument if its first argument is neither
12068 @cindex logical and operator
12069 @cindex and operator
12070 Return its first argument if neither argument is null or zero, otherwise
12071 0. It does not evaluate its second argument if its first argument is
12074 @item < <= = == != >= >
12081 @cindex comparison operators
12083 Compare the arguments and return 1 if the relation is true, 0 otherwise.
12084 @code{==} is a synonym for @code{=}. @command{expr} first tries to convert
12085 both arguments to integers and do a numeric comparison; if either
12086 conversion fails, it does a lexicographic comparison using the character
12087 collating sequence specified by the @env{LC_COLLATE} locale.
12092 @node Examples of expr
12093 @subsection Examples of using @command{expr}
12095 @cindex examples of @command{expr}
12096 Here are a few examples, including quoting for shell metacharacters.
12098 To add 1 to the shell variable @code{foo}, in Bourne-compatible shells:
12101 foo=`expr $foo + 1`
12104 To print the non-directory part of the file name stored in
12105 @code{$fname}, which need not contain a @code{/}:
12108 expr $fname : '.*/\(.*\)' '|' $fname
12111 An example showing that @code{\+} is an operator:
12119 expr abc : 'a\(.\)c'
12121 expr index abcdef cz
12124 @error{} expr: syntax error
12125 expr index + index a
12131 @chapter Redirection
12133 @cindex redirection
12134 @cindex commands for redirection
12136 Unix shells commonly provide several forms of @dfn{redirection}---ways
12137 to change the input source or output destination of a command. But one
12138 useful redirection is performed by a separate command, not by the shell;
12139 it's described here.
12142 * tee invocation:: Redirect output to multiple files or processes.
12146 @node tee invocation
12147 @section @command{tee}: Redirect output to multiple files or processes
12150 @cindex pipe fitting
12151 @cindex destinations, multiple output
12152 @cindex read from stdin and write to stdout and files
12154 The @command{tee} command copies standard input to standard output and also
12155 to any files given as arguments. This is useful when you want not only
12156 to send some data down a pipe, but also to save a copy. Synopsis:
12159 tee [@var{option}]@dots{} [@var{file}]@dots{}
12162 If a file being written to does not already exist, it is created. If a
12163 file being written to already exists, the data it previously contained
12164 is overwritten unless the @option{-a} option is used.
12166 A @var{file} of @samp{-} causes @command{tee} to send another copy of
12167 input to standard output, but this is typically not that useful as the
12168 copies are interleaved.
12170 The program accepts the following options. Also see @ref{Common options}.
12177 Append standard input to the given files rather than overwriting
12181 @itemx --ignore-interrupts
12183 @opindex --ignore-interrupts
12184 Ignore interrupt signals.
12188 The @command{tee} command is useful when you happen to be transferring a large
12189 amount of data and also want to summarize that data without reading
12190 it a second time. For example, when you are downloading a DVD image,
12191 you often want to verify its signature or checksum right away.
12192 The inefficient way to do it is simply:
12195 wget http://example.com/some.iso && sha1sum some.iso
12198 One problem with the above is that it makes you wait for the
12199 download to complete before starting the time-consuming SHA1 computation.
12200 Perhaps even more importantly, the above requires reading
12201 the DVD image a second time (the first was from the network).
12203 The efficient way to do it is to interleave the download
12204 and SHA1 computation. Then, you'll get the checksum for
12205 free, because the entire process parallelizes so well:
12208 # slightly contrived, to demonstrate process substitution
12209 wget -O - http://example.com/dvd.iso \
12210 | tee >(sha1sum > dvd.sha1) > dvd.iso
12213 That makes @command{tee} write not just to the expected output file,
12214 but also to a pipe running @command{sha1sum} and saving the final
12215 checksum in a file named @file{dvd.sha1}.
12217 Note, however, that this example relies on a feature of modern shells
12218 called @dfn{process substitution}
12219 (the @samp{>(command)} syntax, above;
12220 @xref{Process Substitution,,Process Substitution, bashref,
12221 The Bash Reference Manual}.),
12222 so it works with @command{zsh}, @command{bash}, and @command{ksh},
12223 but not with @command{/bin/sh}. So if you write code like this
12224 in a shell script, be sure to start the script with @samp{#!/bin/bash}.
12226 Since the above example writes to one file and one process,
12227 a more conventional and portable use of @command{tee} is even better:
12230 wget -O - http://example.com/dvd.iso \
12231 | tee dvd.iso | sha1sum > dvd.sha1
12234 You can extend this example to make @command{tee} write to two processes,
12235 computing MD5 and SHA1 checksums in parallel. In this case,
12236 process substitution is required:
12239 wget -O - http://example.com/dvd.iso \
12240 | tee >(sha1sum > dvd.sha1) \
12241 >(md5sum > dvd.md5) \
12245 This technique is also useful when you want to make a @emph{compressed}
12246 copy of the contents of a pipe.
12247 Consider a tool to graphically summarize disk usage data from @samp{du -ak}.
12248 For a large hierarchy, @samp{du -ak} can run for a long time,
12249 and can easily produce terabytes of data, so you won't want to
12250 rerun the command unnecessarily. Nor will you want to save
12251 the uncompressed output.
12253 Doing it the inefficient way, you can't even start the GUI
12254 until after you've compressed all of the @command{du} output:
12257 du -ak | gzip -9 > /tmp/du.gz
12258 gzip -d /tmp/du.gz | xdiskusage -a
12261 With @command{tee} and process substitution, you start the GUI
12262 right away and eliminate the decompression completely:
12265 du -ak | tee >(gzip -9 > /tmp/du.gz) | xdiskusage -a
12268 Finally, if you regularly create more than one type of
12269 compressed tarball at once, for example when @code{make dist} creates
12270 both @command{gzip}-compressed and @command{bzip2}-compressed tarballs,
12271 there may be a better way.
12272 Typical @command{automake}-generated @file{Makefile} rules create
12273 the two compressed tar archives with commands in sequence, like this
12274 (slightly simplified):
12277 tardir=your-pkg-M.N
12278 tar chof - "$tardir" | gzip -9 -c > your-pkg-M.N.tar.gz
12279 tar chof - "$tardir" | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12282 However, if the hierarchy you are archiving and compressing is larger
12283 than a couple megabytes, and especially if you are using a multi-processor
12284 system with plenty of memory, then you can do much better by reading the
12285 directory contents only once and running the compression programs in parallel:
12288 tardir=your-pkg-M.N
12289 tar chof - "$tardir" \
12290 | tee >(gzip -9 -c > your-pkg-M.N.tar.gz) \
12291 | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12297 @node File name manipulation
12298 @chapter File name manipulation
12300 @cindex file name manipulation
12301 @cindex manipulation of file names
12302 @cindex commands for file name manipulation
12304 This section describes commands that manipulate file names.
12307 * basename invocation:: Strip directory and suffix from a file name.
12308 * dirname invocation:: Strip last file name component.
12309 * pathchk invocation:: Check file name validity and portability.
12310 * mktemp invocation:: Create temporary file or directory.
12314 @node basename invocation
12315 @section @command{basename}: Strip directory and suffix from a file name
12318 @cindex strip directory and suffix from file names
12319 @cindex directory, stripping from file names
12320 @cindex suffix, stripping from file names
12321 @cindex file names, stripping directory and suffix
12322 @cindex leading directory components, stripping
12324 @command{basename} removes any leading directory components from
12325 @var{name}. Synopsis:
12328 basename @var{name} [@var{suffix}]
12331 If @var{suffix} is specified and is identical to the end of @var{name},
12332 it is removed from @var{name} as well. Note that since trailing slashes
12333 are removed prior to suffix matching, @var{suffix} will do nothing if it
12334 contains slashes. @command{basename} prints the result on standard
12337 @c This test is used both here and in the section on dirname.
12338 @macro basenameAndDirname
12339 Together, @command{basename} and @command{dirname} are designed such
12340 that if @samp{ls "$name"} succeeds, then the command sequence @samp{cd
12341 "$(dirname "$name")"; ls "$(basename "$name")"} will, too. This works
12342 for everything except file names containing a trailing newline.
12344 @basenameAndDirname
12346 @acronym{POSIX} allows the implementation to define the results if
12347 @var{name} is empty or @samp{//}. In the former case, @acronym{GNU}
12348 @command{basename} returns the empty string. In the latter case, the
12349 result is @samp{//} on platforms where @var{//} is distinct from
12350 @var{/}, and @samp{/} on platforms where there is no difference.
12352 The only options are @option{--help} and @option{--version}. @xref{Common
12353 options}. Options must precede operands.
12361 basename /usr/bin/sort
12364 basename include/stdio.h .h
12368 @node dirname invocation
12369 @section @command{dirname}: Strip last file name component
12372 @cindex directory components, printing
12373 @cindex stripping non-directory suffix
12374 @cindex non-directory suffix, stripping
12376 @command{dirname} prints all but the final slash-delimited component of
12377 @var{name}. Slashes on either side of the final component are also
12378 removed. If the string contains no slash, @command{dirname} prints
12379 @samp{.} (meaning the current directory). Synopsis:
12385 @var{name} need not be a file name, but if it is, this operation
12386 effectively lists the directory that contains the final component,
12387 including the case when the final component is itself a directory.
12389 @basenameAndDirname
12391 @acronym{POSIX} allows the implementation to define the results if
12392 @var{name} is @samp{//}. With @acronym{GNU} @command{dirname}, the
12393 result is @samp{//} on platforms where @var{//} is distinct from
12394 @var{/}, and @samp{/} on platforms where there is no difference.
12396 The only options are @option{--help} and @option{--version}. @xref{Common
12404 # Output "/usr/bin".
12405 dirname /usr/bin/sort
12406 dirname /usr/bin//.//
12413 @node pathchk invocation
12414 @section @command{pathchk}: Check file name validity and portability
12417 @cindex file names, checking validity and portability
12418 @cindex valid file names, checking for
12419 @cindex portable file names, checking for
12421 @command{pathchk} checks validity and portability of file names. Synopsis:
12424 pathchk [@var{option}]@dots{} @var{name}@dots{}
12427 For each @var{name}, @command{pathchk} prints an error message if any of
12428 these conditions is true:
12432 One of the existing directories in @var{name} does not have search
12433 (execute) permission,
12435 The length of @var{name} is larger than the maximum supported by the
12438 The length of one component of @var{name} is longer than
12439 its file system's maximum.
12442 A nonexistent @var{name} is not an error, so long a file with that
12443 name could be created under the above conditions.
12445 The program accepts the following options. Also see @ref{Common options}.
12446 Options must precede operands.
12452 Instead of performing checks based on the underlying file system,
12453 print an error message if any of these conditions is true:
12457 A file name is empty.
12460 A file name contains a character outside the @acronym{POSIX} portable file
12461 name character set, namely, the ASCII letters and digits, @samp{.},
12462 @samp{_}, @samp{-}, and @samp{/}.
12465 The length of a file name or one of its components exceeds the
12466 @acronym{POSIX} minimum limits for portability.
12471 Print an error message if a file name is empty, or if it contains a component
12472 that begins with @samp{-}.
12474 @item --portability
12475 @opindex --portability
12476 Print an error message if a file name is not portable to all @acronym{POSIX}
12477 hosts. This option is equivalent to @samp{-p -P}.
12481 @cindex exit status of @command{pathchk}
12485 0 if all specified file names passed all checks,
12489 @node mktemp invocation
12490 @section @command{mktemp}: Create temporary file or directory
12493 @cindex file names, creating temporary
12494 @cindex directory, creating temporary
12495 @cindex temporary files and directories
12497 @command{mktemp} manages the creation of temporary files and
12498 directories. Synopsis:
12501 mktemp [@var{option}]@dots{} [@var{template}]
12504 Safely create a temporary file or directory based on @var{template},
12505 and print its name. If given, @var{template} must include at least
12506 three consecutive @samp{X}s in the last component. If omitted, the template
12507 @samp{tmp.XXXXXXXXXX} is used, and option @option{--tmpdir} is
12508 implied. The final run of @samp{X}s in the @var{template} will be replaced
12509 by alpha-numeric characters; thus, on a case-sensitive file system,
12510 and with a @var{template} including a run of @var{n} instances of @samp{X},
12511 there are @samp{62**@var{n}} potential file names.
12513 Older scripts used to create temporary files by simply joining the
12514 name of the program with the process id (@samp{$$}) as a suffix.
12515 However, that naming scheme is easily predictable, and suffers from a
12516 race condition where the attacker can create an appropriately named
12517 symbolic link, such that when the script then opens a handle to what
12518 it thought was an unused file, it is instead modifying an existing
12519 file. Using the same scheme to create a directory is slightly safer,
12520 since the @command{mkdir} will fail if the target already exists, but
12521 it is still inferior because it allows for denial of service attacks.
12522 Therefore, modern scripts should use the @command{mktemp} command to
12523 guarantee that the generated name will be unpredictable, and that
12524 knowledge of the temporary file name implies that the file was created
12525 by the current script and cannot be modified by other users.
12527 When creating a file, the resulting file has read and write
12528 permissions for the current user, but no permissions for the group or
12529 others; these permissions are reduced if the current umask is more
12532 Here are some examples (although note that if you repeat them, you
12533 will most likely get different file names):
12538 Create a temporary file in the current directory.
12545 Create a temporary file with a known suffix.
12547 $ mktemp --suffix=.txt file-XXXX
12549 $ mktemp file-XXXX-XXXX.txt
12554 Create a secure fifo relative to the user's choice of @env{TMPDIR},
12555 but falling back to the current directory rather than @file{/tmp}.
12556 Note that @command{mktemp} does not create fifos, but can create a
12557 secure directory in which the fifo can live. Exit the shell if the
12558 directory or fifo could not be created.
12560 $ dir=$(mktemp -p "$@{TMPDIR:-.@}" -d dir-XXXX) || exit 1
12562 $ mkfifo "$fifo" || @{ rmdir "$dir"; exit 1; @}
12566 Create and use a temporary file if possible, but ignore failure. The
12567 file will reside in the directory named by @env{TMPDIR}, if specified,
12568 or else in @file{/tmp}.
12570 $ file=$(mktemp -q) && @{
12571 > # Safe to use $file only within this block. Use quotes,
12572 > # since $TMPDIR, and thus $file, may contain whitespace.
12573 > echo ... > "$file"
12579 Act as a semi-random character generator (it is not fully random,
12580 since it is impacted by the contents of the current directory). To
12581 avoid security holes, do not use the resulting names to create a file.
12591 The program accepts the following options. Also see @ref{Common options}.
12598 @opindex --directory
12599 Create a directory rather than a file. The directory will have read,
12600 write, and search permissions for the current user, but no permissions
12601 for the group or others; these permissions are reduced if the current
12602 umask is more restrictive.
12608 Suppress diagnostics about failure to create a file or directory. The
12609 exit status will still reflect whether a file was created.
12615 Generate a temporary name that does not name an existing file, without
12616 changing the file system contents. Using the output of this command
12617 to create a new file is inherently unsafe, as there is a window of
12618 time between generating the name and using it where another process
12619 can create an object by the same name.
12622 @itemx --tmpdir[=@var{dir}]
12625 Treat @var{template} relative to the directory @var{dir}. If
12626 @var{dir} is not specified (only possible with the long option
12627 @option{--tmpdir}) or is the empty string, use the value of
12628 @env{TMPDIR} if available, otherwise use @samp{/tmp}. If this is
12629 specified, @var{template} must not be absolute. However,
12630 @var{template} can still contain slashes, although intermediate
12631 directories must already exist.
12633 @item --suffix=@var{suffix}
12635 Append @var{suffix} to the @var{template}. @var{suffix} must not
12636 contain slash. If @option{--suffix} is specified, @var{template} must
12637 end in @samp{X}; if it is not specified, then an appropriate
12638 @option{--suffix} is inferred by finding the last @samp{X} in
12639 @var{template}. This option exists for use with the default
12640 @var{template} and for the creation of a @var{suffix} that starts with
12645 Treat @var{template} as a single file relative to the value of
12646 @env{TMPDIR} if available, or to the directory specified by
12647 @option{-p}, otherwise to @samp{/tmp}. @var{template} must not
12648 contain slashes. This option is deprecated; the use of @option{-p}
12649 without @option{-t} offers better defaults (by favoring the command
12650 line over @env{TMPDIR}) and more flexibility (by allowing intermediate
12655 @cindex exit status of @command{mktemp}
12659 0 if the file was created,
12664 @node Working context
12665 @chapter Working context
12667 @cindex working context
12668 @cindex commands for printing the working context
12670 This section describes commands that display or alter the context in
12671 which you are working: the current directory, the terminal settings, and
12672 so forth. See also the user-related commands in the next section.
12675 * pwd invocation:: Print working directory.
12676 * stty invocation:: Print or change terminal characteristics.
12677 * printenv invocation:: Print environment variables.
12678 * tty invocation:: Print file name of terminal on standard input.
12682 @node pwd invocation
12683 @section @command{pwd}: Print working directory
12686 @cindex print name of current directory
12687 @cindex current working directory, printing
12688 @cindex working directory, printing
12691 @command{pwd} prints the name of the current directory. Synopsis:
12694 pwd [@var{option}]@dots{}
12697 The program accepts the following options. Also see @ref{Common options}.
12704 If the contents of the environment variable @env{PWD} provide an
12705 absolute name of the current directory with no @samp{.} or @samp{..}
12706 components, but possibly with symbolic links, then output those
12707 contents. Otherwise, fall back to default @option{-P} handling.
12712 @opindex --physical
12713 Print a fully resolved name for the current directory. That is, all
12714 components of the printed name will be actual directory names---none
12715 will be symbolic links.
12718 @cindex symbolic links and @command{pwd}
12719 If @option{-L} and @option{-P} are both given, the last one takes
12720 precedence. If neither option is given, then this implementation uses
12721 @option{-P} as the default unless the @env{POSIXLY_CORRECT}
12722 environment variable is set.
12724 @mayConflictWithShellBuiltIn{pwd}
12729 @node stty invocation
12730 @section @command{stty}: Print or change terminal characteristics
12733 @cindex change or print terminal settings
12734 @cindex terminal settings
12735 @cindex line settings of terminal
12737 @command{stty} prints or changes terminal characteristics, such as baud rate.
12741 stty [@var{option}] [@var{setting}]@dots{}
12742 stty [@var{option}]
12745 If given no line settings, @command{stty} prints the baud rate, line
12746 discipline number (on systems that support it), and line settings
12747 that have been changed from the values set by @samp{stty sane}.
12748 By default, mode reading and setting are performed on the tty line
12749 connected to standard input, although this can be modified by the
12750 @option{--file} option.
12752 @command{stty} accepts many non-option arguments that change aspects of
12753 the terminal line operation, as described below.
12755 The program accepts the following options. Also see @ref{Common options}.
12762 Print all current settings in human-readable form. This option may not
12763 be used in combination with any line settings.
12765 @item -F @var{device}
12766 @itemx --file=@var{device}
12769 Set the line opened by the file name specified in @var{device} instead of
12770 the tty line connected to standard input. This option is necessary
12771 because opening a @acronym{POSIX} tty requires use of the
12772 @code{O_NONDELAY} flag to prevent a @acronym{POSIX} tty from blocking
12773 until the carrier detect line is high if
12774 the @code{clocal} flag is not set. Hence, it is not always possible
12775 to allow the shell to open the device in the traditional manner.
12781 @cindex machine-readable @command{stty} output
12782 Print all current settings in a form that can be used as an argument to
12783 another @command{stty} command to restore the current settings. This option
12784 may not be used in combination with any line settings.
12788 Many settings can be turned off by preceding them with a @samp{-}.
12789 Such arguments are marked below with ``May be negated'' in their
12790 description. The descriptions themselves refer to the positive
12791 case, that is, when @emph{not} negated (unless stated otherwise,
12794 Some settings are not available on all @acronym{POSIX} systems, since they use
12795 extensions. Such arguments are marked below with
12796 ``Non-@acronym{POSIX}'' in their description. On non-@acronym{POSIX}
12797 systems, those or other settings also may not
12798 be available, but it's not feasible to document all the variations: just
12804 * Control:: Control settings
12805 * Input:: Input settings
12806 * Output:: Output settings
12807 * Local:: Local settings
12808 * Combination:: Combination settings
12809 * Characters:: Special characters
12810 * Special:: Special settings
12815 @subsection Control settings
12817 @cindex control settings
12823 @cindex two-way parity
12824 Generate parity bit in output and expect parity bit in input.
12830 @cindex even parity
12831 Set odd parity (even if negated). May be negated.
12838 @cindex character size
12839 @cindex eight-bit characters
12840 Set character size to 5, 6, 7, or 8 bits.
12845 Send a hangup signal when the last process closes the tty. May be
12851 Use two stop bits per character (one if negated). May be negated.
12855 Allow input to be received. May be negated.
12859 @cindex modem control
12860 Disable modem control signals. May be negated.
12864 @cindex hardware flow control
12865 @cindex flow control, hardware
12866 @cindex RTS/CTS flow control
12867 Enable RTS/CTS flow control. Non-@acronym{POSIX}. May be negated.
12872 @subsection Input settings
12874 @cindex input settings
12875 These settings control operations on data received from the terminal.
12880 @cindex breaks, ignoring
12881 Ignore break characters. May be negated.
12885 @cindex breaks, cause interrupts
12886 Make breaks cause an interrupt signal. May be negated.
12890 @cindex parity, ignoring
12891 Ignore characters with parity errors. May be negated.
12895 @cindex parity errors, marking
12896 Mark parity errors (with a 255-0-character sequence). May be negated.
12900 Enable input parity checking. May be negated.
12904 @cindex eight-bit input
12905 Clear high (8th) bit of input characters. May be negated.
12909 @cindex newline, translating to return
12910 Translate newline to carriage return. May be negated.
12914 @cindex return, ignoring
12915 Ignore carriage return. May be negated.
12919 @cindex return, translating to newline
12920 Translate carriage return to newline. May be negated.
12924 @cindex input encoding, UTF-8
12925 Assume input characters are UTF-8 encoded. May be negated.
12929 @kindex C-s/C-q flow control
12930 @cindex XON/XOFF flow control
12931 Enable XON/XOFF flow control (that is, @kbd{CTRL-S}/@kbd{CTRL-Q}). May
12938 @cindex software flow control
12939 @cindex flow control, software
12940 Enable sending of @code{stop} character when the system input buffer
12941 is almost full, and @code{start} character when it becomes almost
12942 empty again. May be negated.
12946 @cindex uppercase, translating to lowercase
12947 Translate uppercase characters to lowercase. Non-@acronym{POSIX}. May be
12948 negated. Note ilcuc is not implemented, as one would not be able to issue
12949 almost any (lowercase) Unix command, after invoking it.
12953 Allow any character to restart output (only the start character
12954 if negated). Non-@acronym{POSIX}. May be negated.
12958 @cindex beeping at input buffer full
12959 Enable beeping and not flushing input buffer if a character arrives
12960 when the input buffer is full. Non-@acronym{POSIX}. May be negated.
12965 @subsection Output settings
12967 @cindex output settings
12968 These settings control operations on data sent to the terminal.
12973 Postprocess output. May be negated.
12977 @cindex lowercase, translating to output
12978 Translate lowercase characters to uppercase. Non-@acronym{POSIX}. May be
12979 negated. (Note ouclc is not currently implemented.)
12983 @cindex return, translating to newline
12984 Translate carriage return to newline. Non-@acronym{POSIX}. May be negated.
12988 @cindex newline, translating to crlf
12989 Translate newline to carriage return-newline. Non-@acronym{POSIX}. May be
12994 Do not print carriage returns in the first column. Non-@acronym{POSIX}.
12999 Newline performs a carriage return. Non-@acronym{POSIX}. May be negated.
13003 @cindex pad instead of timing for delaying
13004 Use fill (padding) characters instead of timing for delays.
13005 Non-@acronym{POSIX}.
13010 @cindex pad character
13011 Use @acronym{ASCII} @sc{del} characters for fill instead of
13012 @acronym{ASCII} @sc{nul} characters. Non-@acronym{POSIX}.
13018 Newline delay style. Non-@acronym{POSIX}.
13025 Carriage return delay style. Non-@acronym{POSIX}.
13031 @opindex tab@var{n}
13032 Horizontal tab delay style. Non-@acronym{POSIX}.
13037 Backspace delay style. Non-@acronym{POSIX}.
13042 Vertical tab delay style. Non-@acronym{POSIX}.
13047 Form feed delay style. Non-@acronym{POSIX}.
13052 @subsection Local settings
13054 @cindex local settings
13059 Enable @code{interrupt}, @code{quit}, and @code{suspend} special
13060 characters. May be negated.
13064 Enable @code{erase}, @code{kill}, @code{werase}, and @code{rprnt}
13065 special characters. May be negated.
13069 Enable non-@acronym{POSIX} special characters. May be negated.
13073 Echo input characters. May be negated.
13079 Echo @code{erase} characters as backspace-space-backspace. May be
13084 @cindex newline echoing after @code{kill}
13085 Echo a newline after a @code{kill} character. May be negated.
13089 @cindex newline, echoing
13090 Echo newline even if not echoing other characters. May be negated.
13094 @cindex flushing, disabling
13095 Disable flushing after @code{interrupt} and @code{quit} special
13096 characters. May be negated.
13100 @cindex case translation
13101 Enable input and output of uppercase characters by preceding their
13102 lowercase equivalents with @samp{\}, when @code{icanon} is set.
13103 Non-@acronym{POSIX}. May be negated.
13107 @cindex background jobs, stopping at terminal write
13108 Stop background jobs that try to write to the terminal. Non-@acronym{POSIX}.
13115 Echo erased characters backward, between @samp{\} and @samp{/}.
13116 Non-@acronym{POSIX}. May be negated.
13122 @cindex control characters, using @samp{^@var{c}}
13123 @cindex hat notation for control characters
13124 Echo control characters in hat notation (@samp{^@var{c}}) instead
13125 of literally. Non-@acronym{POSIX}. May be negated.
13131 Echo the @code{kill} special character by erasing each character on
13132 the line as indicated by the @code{echoprt} and @code{echoe} settings,
13133 instead of by the @code{echoctl} and @code{echok} settings.
13134 Non-@acronym{POSIX}.
13140 @subsection Combination settings
13142 @cindex combination settings
13143 Combination settings:
13150 Same as @code{parenb -parodd cs7}. May be negated. If negated, same
13151 as @code{-parenb cs8}.
13155 Same as @code{parenb parodd cs7}. May be negated. If negated, same
13156 as @code{-parenb cs8}.
13160 Same as @code{-icrnl -onlcr}. May be negated. If negated, same as
13161 @code{icrnl -inlcr -igncr onlcr -ocrnl -onlret}.
13165 Reset the @code{erase} and @code{kill} special characters to their default
13172 @c This is too long to write inline.
13174 cread -ignbrk brkint -inlcr -igncr icrnl -ixoff
13175 -iuclc -ixany imaxbel opost -olcuc -ocrnl onlcr
13176 -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0
13177 ff0 isig icanon iexten echo echoe echok -echonl
13178 -noflsh -xcase -tostop -echoprt echoctl echoke
13182 and also sets all special characters to their default values.
13186 Same as @code{brkint ignpar istrip icrnl ixon opost isig icanon}, plus
13187 sets the @code{eof} and @code{eol} characters to their default values
13188 if they are the same as the @code{min} and @code{time} characters.
13189 May be negated. If negated, same as @code{raw}.
13196 -ignbrk -brkint -ignpar -parmrk -inpck -istrip
13197 -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany
13198 -imaxbel -opost -isig -icanon -xcase min 1 time 0
13202 May be negated. If negated, same as @code{cooked}.
13206 Same as @option{-icanon}. May be negated. If negated, same as
13211 @cindex eight-bit characters
13212 Same as @code{-parenb -istrip cs8}. May be negated. If negated,
13213 same as @code{parenb istrip cs7}.
13217 Same as @option{-parenb -istrip -opost cs8}. May be negated.
13218 If negated, same as @code{parenb istrip opost cs7}.
13222 Same as @option{-ixany}. Non-@acronym{POSIX}. May be negated.
13226 Same as @code{tab0}. Non-@acronym{POSIX}. May be negated. If negated, same
13233 Same as @code{xcase iuclc olcuc}. Non-@acronym{POSIX}. May be negated.
13234 (Used for terminals with uppercase characters only.)
13238 Same as @code{echoe echoctl echoke}.
13242 Same as @code{echoe echoctl echoke -ixany intr ^C erase ^? kill C-u}.
13247 @subsection Special characters
13249 @cindex special characters
13250 @cindex characters, special
13252 The special characters' default values vary from system to system.
13253 They are set with the syntax @samp{name value}, where the names are
13254 listed below and the value can be given either literally, in hat
13255 notation (@samp{^@var{c}}), or as an integer which may start with
13256 @samp{0x} to indicate hexadecimal, @samp{0} to indicate octal, or
13257 any other digit to indicate decimal.
13259 @cindex disabling special characters
13260 @kindex u@r{, and disabling special characters}
13261 For GNU stty, giving a value of @code{^-} or @code{undef} disables that
13262 special character. (This is incompatible with Ultrix @command{stty},
13263 which uses a value of @samp{u} to disable a special character. GNU
13264 @command{stty} treats a value @samp{u} like any other, namely to set that
13265 special character to @key{U}.)
13271 Send an interrupt signal.
13275 Send a quit signal.
13279 Erase the last character typed.
13283 Erase the current line.
13287 Send an end of file (terminate the input).
13295 Alternate character to end the line. Non-@acronym{POSIX}.
13299 Switch to a different shell layer. Non-@acronym{POSIX}.
13303 Restart the output after stopping it.
13311 Send a terminal stop signal.
13315 Send a terminal stop signal after flushing the input. Non-@acronym{POSIX}.
13319 Redraw the current line. Non-@acronym{POSIX}.
13323 Erase the last word typed. Non-@acronym{POSIX}.
13327 Enter the next character typed literally, even if it is a special
13328 character. Non-@acronym{POSIX}.
13333 @subsection Special settings
13335 @cindex special settings
13340 Set the minimum number of characters that will satisfy a read until
13341 the time value has expired, when @option{-icanon} is set.
13345 Set the number of tenths of a second before reads time out if the minimum
13346 number of characters have not been read, when @option{-icanon} is set.
13348 @item ispeed @var{n}
13350 Set the input speed to @var{n}.
13352 @item ospeed @var{n}
13354 Set the output speed to @var{n}.
13358 Tell the tty kernel driver that the terminal has @var{n} rows.
13359 Non-@acronym{POSIX}.
13362 @itemx columns @var{n}
13365 Tell the kernel that the terminal has @var{n} columns. Non-@acronym{POSIX}.
13371 Print the number of rows and columns that the kernel thinks the
13372 terminal has. (Systems that don't support rows and columns in the kernel
13373 typically use the environment variables @env{LINES} and @env{COLUMNS}
13374 instead; however, GNU @command{stty} does not know anything about them.)
13375 Non-@acronym{POSIX}.
13379 Use line discipline @var{n}. Non-@acronym{POSIX}.
13383 Print the terminal speed.
13386 @cindex baud rate, setting
13387 Set the input and output speeds to @var{n}. @var{n} can be one of: 0
13388 50 75 110 134 134.5 150 200 300 600 1200 1800 2400 4800 9600 19200
13389 38400 @code{exta} @code{extb}. @code{exta} is the same as 19200;
13390 @code{extb} is the same as 38400. Many systems, including GNU/Linux,
13391 support higher speeds. The @command{stty} command includes support
13408 4000000 where the system supports these.
13409 0 hangs up the line if @option{-clocal} is set.
13413 @node printenv invocation
13414 @section @command{printenv}: Print all or some environment variables
13417 @cindex printing all or some environment variables
13418 @cindex environment variables, printing
13420 @command{printenv} prints environment variable values. Synopsis:
13423 printenv [@var{option}] [@var{variable}]@dots{}
13426 If no @var{variable}s are specified, @command{printenv} prints the value of
13427 every environment variable. Otherwise, it prints the value of each
13428 @var{variable} that is set, and nothing for those that are not set.
13430 The program accepts the following option. Also see @ref{Common options}.
13438 @cindex exit status of @command{printenv}
13442 0 if all variables specified were found
13443 1 if at least one specified variable was not found
13444 2 if a write error occurred
13448 @node tty invocation
13449 @section @command{tty}: Print file name of terminal on standard input
13452 @cindex print terminal file name
13453 @cindex terminal file name, printing
13455 @command{tty} prints the file name of the terminal connected to its standard
13456 input. It prints @samp{not a tty} if standard input is not a terminal.
13460 tty [@var{option}]@dots{}
13463 The program accepts the following option. Also see @ref{Common options}.
13473 Print nothing; only return an exit status.
13477 @cindex exit status of @command{tty}
13481 0 if standard input is a terminal
13482 1 if standard input is not a terminal
13483 2 if given incorrect arguments
13484 3 if a write error occurs
13488 @node User information
13489 @chapter User information
13491 @cindex user information, commands for
13492 @cindex commands for printing user information
13494 This section describes commands that print user-related information:
13495 logins, groups, and so forth.
13498 * id invocation:: Print user identity.
13499 * logname invocation:: Print current login name.
13500 * whoami invocation:: Print effective user ID.
13501 * groups invocation:: Print group names a user is in.
13502 * users invocation:: Print login names of users currently logged in.
13503 * who invocation:: Print who is currently logged in.
13507 @node id invocation
13508 @section @command{id}: Print user identity
13511 @cindex real user and group IDs, printing
13512 @cindex effective user and group IDs, printing
13513 @cindex printing real and effective user and group IDs
13515 @command{id} prints information about the given user, or the process
13516 running it if no user is specified. Synopsis:
13519 id [@var{option}]@dots{} [@var{username}]
13522 @vindex POSIXLY_CORRECT
13523 By default, it prints the real user ID, real group ID, effective user ID
13524 if different from the real user ID, effective group ID if different from
13525 the real group ID, and supplemental group IDs.
13526 In addition, if SELinux
13527 is enabled and the @env{POSIXLY_CORRECT} environment variable is not set,
13528 then print @samp{context=@var{c}}, where @var{c} is the security context.
13530 Each of these numeric values is preceded by an identifying string and
13531 followed by the corresponding user or group name in parentheses.
13533 The options cause @command{id} to print only part of the above information.
13534 Also see @ref{Common options}.
13541 Print only the group ID.
13547 Print only the group ID and the supplementary groups.
13553 Print the user or group name instead of the ID number. Requires
13554 @option{-u}, @option{-g}, or @option{-G}.
13560 Print the real, instead of effective, user or group ID. Requires
13561 @option{-u}, @option{-g}, or @option{-G}.
13567 Print only the user ID.
13574 @cindex security context
13575 Print only the security context of the current user.
13576 If SELinux is disabled then print a warning and
13577 set the exit status to 1.
13583 @macro primaryAndSupplementaryGroups{cmd,arg}
13584 Primary and supplementary groups for a process are normally inherited
13585 from its parent and are usually unchanged since login. This means
13586 that if you change the group database after logging in, @command{\cmd\}
13587 will not reflect your changes within your existing login session.
13588 Running @command{\cmd\} with a \arg\ causes the user and group
13589 database to be consulted afresh, and so will give a different result.
13591 @primaryAndSupplementaryGroups{id,user argument}
13593 @node logname invocation
13594 @section @command{logname}: Print current login name
13597 @cindex printing user's login name
13598 @cindex login name, printing
13599 @cindex user name, printing
13602 @command{logname} prints the calling user's name, as found in a
13603 system-maintained file (often @file{/var/run/utmp} or
13604 @file{/etc/utmp}), and exits with a status of 0. If there is no entry
13605 for the calling process, @command{logname} prints
13606 an error message and exits with a status of 1.
13608 The only options are @option{--help} and @option{--version}. @xref{Common
13614 @node whoami invocation
13615 @section @command{whoami}: Print effective user ID
13618 @cindex effective user ID, printing
13619 @cindex printing the effective user ID
13621 @command{whoami} prints the user name associated with the current
13622 effective user ID. It is equivalent to the command @samp{id -un}.
13624 The only options are @option{--help} and @option{--version}. @xref{Common
13630 @node groups invocation
13631 @section @command{groups}: Print group names a user is in
13634 @cindex printing groups a user is in
13635 @cindex supplementary groups, printing
13637 @command{groups} prints the names of the primary and any supplementary
13638 groups for each given @var{username}, or the current process if no names
13639 are given. If more than one name is given, the name of each user is
13641 the list of that user's groups and the user name is separated from the
13642 group list by a colon. Synopsis:
13645 groups [@var{username}]@dots{}
13648 The group lists are equivalent to the output of the command @samp{id -Gn}.
13650 @primaryAndSupplementaryGroups{groups,list of users}
13652 The only options are @option{--help} and @option{--version}. @xref{Common
13658 @node users invocation
13659 @section @command{users}: Print login names of users currently logged in
13662 @cindex printing current usernames
13663 @cindex usernames, printing current
13665 @cindex login sessions, printing users with
13666 @command{users} prints on a single line a blank-separated list of user
13667 names of users currently logged in to the current host. Each user name
13668 corresponds to a login session, so if a user has more than one login
13669 session, that user's name will appear the same number of times in the
13678 With no @var{file} argument, @command{users} extracts its information from
13679 a system-maintained file (often @file{/var/run/utmp} or
13680 @file{/etc/utmp}). If a file argument is given, @command{users} uses
13681 that file instead. A common choice is @file{/var/log/wtmp}.
13683 The only options are @option{--help} and @option{--version}. @xref{Common
13689 @node who invocation
13690 @section @command{who}: Print who is currently logged in
13693 @cindex printing current user information
13694 @cindex information, about current users
13696 @command{who} prints information about users who are currently logged on.
13700 @command{who} [@var{option}] [@var{file}] [am i]
13703 @cindex terminal lines, currently used
13705 @cindex remote hostname
13706 If given no non-option arguments, @command{who} prints the following
13707 information for each user currently logged on: login name, terminal
13708 line, login time, and remote hostname or X display.
13712 If given one non-option argument, @command{who} uses that instead of
13713 a default system-maintained file (often @file{/var/run/utmp} or
13714 @file{/etc/utmp}) as the name of the file containing the record of
13715 users logged on. @file{/var/log/wtmp} is commonly given as an argument
13716 to @command{who} to look at who has previously logged on.
13720 If given two non-option arguments, @command{who} prints only the entry
13721 for the user running it (determined from its standard input), preceded
13722 by the hostname. Traditionally, the two arguments given are @samp{am
13723 i}, as in @samp{who am i}.
13726 Time stamps are listed according to the time zone rules specified by
13727 the @env{TZ} environment variable, or by the system default rules if
13728 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
13729 with @env{TZ}, libc, The GNU C Library Reference Manual}.
13731 The program accepts the following options. Also see @ref{Common options}.
13739 Same as @samp{-b -d --login -p -r -t -T -u}.
13745 Print the date and time of last system boot.
13751 Print information corresponding to dead processes.
13757 Print a line of column headings.
13763 List only the entries that correspond to processes via which the
13764 system is waiting for a user to login. The user name is always @samp{LOGIN}.
13768 Attempt to canonicalize hostnames found in utmp through a DNS lookup. This
13769 is not the default because it can cause significant delays on systems with
13770 automatic dial-up internet access.
13774 Same as @samp{who am i}.
13780 List active processes spawned by init.
13786 Print only the login names and the number of users logged on.
13787 Overrides all other options.
13792 @opindex --runlevel
13793 Print the current (and maybe previous) run-level of the init process.
13797 Ignored; for compatibility with other versions of @command{who}.
13803 Print last system clock change.
13808 After the login time, print the number of hours and minutes that the
13809 user has been idle. @samp{.} means the user was active in the last minute.
13810 @samp{old} means the user has been idle for more than 24 hours.
13821 @opindex --writable
13822 @cindex message status
13823 @pindex write@r{, allowed}
13824 After each login name print a character indicating the user's message status:
13827 @samp{+} allowing @code{write} messages
13828 @samp{-} disallowing @code{write} messages
13829 @samp{?} cannot find terminal device
13837 @node System context
13838 @chapter System context
13840 @cindex system context
13841 @cindex context, system
13842 @cindex commands for system context
13844 This section describes commands that print or change system-wide
13848 * date invocation:: Print or set system date and time.
13849 * arch invocation:: Print machine hardware name.
13850 * nproc invocation:: Print the number of processors.
13851 * uname invocation:: Print system information.
13852 * hostname invocation:: Print or set system name.
13853 * hostid invocation:: Print numeric host identifier.
13854 * uptime invocation:: Print system uptime and load.
13857 @node date invocation
13858 @section @command{date}: Print or set system date and time
13861 @cindex time, printing or setting
13862 @cindex printing the current time
13867 date [@var{option}]@dots{} [+@var{format}]
13868 date [-u|--utc|--universal] @c this avoids a newline in the output
13869 [ MMDDhhmm[[CC]YY][.ss] ]
13873 Invoking @command{date} with no @var{format} argument is equivalent to invoking
13874 it with a default format that depends on the @env{LC_TIME} locale category.
13875 In the default C locale, this format is @samp{'+%a %b %e %H:%M:%S %Z %Y'},
13876 so the output looks like @samp{Thu Mar @ 3 13:47:51 PST 2005}.
13879 Normally, @command{date} uses the time zone rules indicated by the
13880 @env{TZ} environment variable, or the system default rules if @env{TZ}
13881 is not set. @xref{TZ Variable,, Specifying the Time Zone with
13882 @env{TZ}, libc, The GNU C Library Reference Manual}.
13884 @findex strftime @r{and @command{date}}
13885 @cindex time formats
13886 @cindex formatting times
13887 If given an argument that starts with a @samp{+}, @command{date} prints the
13888 current date and time (or the date and time specified by the
13889 @option{--date} option, see below) in the format defined by that argument,
13890 which is similar to that of the @code{strftime} function. Except for
13891 conversion specifiers, which start with @samp{%}, characters in the
13892 format string are printed unchanged. The conversion specifiers are
13898 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
13899 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
13900 * Literal conversion specifiers:: %[%nt]
13901 * Padding and other flags:: Pad with zeros, spaces, etc.
13902 * Setting the time:: Changing the system clock.
13903 * Options for date:: Instead of the current time.
13905 * Date input formats:: Specifying date strings.
13907 * Examples of date:: Examples.
13910 @node Time conversion specifiers
13911 @subsection Time conversion specifiers
13913 @cindex time conversion specifiers
13914 @cindex conversion specifiers, time
13916 @command{date} conversion specifiers related to times.
13920 hour (@samp{00}@dots{}@samp{23})
13922 hour (@samp{01}@dots{}@samp{12})
13924 hour (@samp{ 0}@dots{}@samp{23}).
13925 This is a @acronym{GNU} extension.
13927 hour (@samp{ 1}@dots{}@samp{12}).
13928 This is a @acronym{GNU} extension.
13930 minute (@samp{00}@dots{}@samp{59})
13932 nanoseconds (@samp{000000000}@dots{}@samp{999999999}).
13933 This is a @acronym{GNU} extension.
13935 locale's equivalent of either @samp{AM} or @samp{PM};
13936 blank in many locales.
13937 Noon is treated as @samp{PM} and midnight as @samp{AM}.
13939 like @samp{%p}, except lower case.
13940 This is a @acronym{GNU} extension.
13942 locale's 12-hour clock time (e.g., @samp{11:11:04 PM})
13944 24-hour hour and minute. Same as @samp{%H:%M}.
13945 This is a @acronym{GNU} extension.
13947 @cindex epoch, seconds since
13948 @cindex seconds since the epoch
13949 @cindex beginning of time
13950 seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC.
13951 Leap seconds are not counted unless leap second support is available.
13952 @xref{%s-examples}, for examples.
13953 This is a @acronym{GNU} extension.
13955 second (@samp{00}@dots{}@samp{60}).
13956 This may be @samp{60} if leap seconds are supported.
13958 24-hour hour, minute, and second. Same as @samp{%H:%M:%S}.
13960 locale's time representation (e.g., @samp{23:13:48})
13962 @w{@acronym{RFC} 2822/@acronym{ISO} 8601} style numeric time zone
13963 (e.g., @samp{-0600} or @samp{+0530}), or nothing if no
13964 time zone is determinable. This value reflects the numeric time zone
13965 appropriate for the current time, using the time zone rules specified
13966 by the @env{TZ} environment variable.
13967 The time (and optionally, the time zone rules) can be overridden
13968 by the @option{--date} option.
13969 This is a @acronym{GNU} extension.
13971 @w{@acronym{RFC} 3339/@acronym{ISO} 8601} style numeric time zone with
13972 @samp{:} (e.g., @samp{-06:00} or @samp{+05:30}), or nothing if no time
13973 zone is determinable.
13974 This is a @acronym{GNU} extension.
13976 Numeric time zone to the nearest second with @samp{:} (e.g.,
13977 @samp{-06:00:00} or @samp{+05:30:00}), or nothing if no time zone is
13979 This is a @acronym{GNU} extension.
13981 Numeric time zone with @samp{:} using the minimum necessary precision
13982 (e.g., @samp{-06}, @samp{+05:30}, or @samp{-04:56:02}), or nothing if
13983 no time zone is determinable.
13984 This is a @acronym{GNU} extension.
13986 alphabetic time zone abbreviation (e.g., @samp{EDT}), or nothing if no
13987 time zone is determinable. See @samp{%z} for how it is determined.
13991 @node Date conversion specifiers
13992 @subsection Date conversion specifiers
13994 @cindex date conversion specifiers
13995 @cindex conversion specifiers, date
13997 @command{date} conversion specifiers related to dates.
14001 locale's abbreviated weekday name (e.g., @samp{Sun})
14003 locale's full weekday name, variable length (e.g., @samp{Sunday})
14005 locale's abbreviated month name (e.g., @samp{Jan})
14007 locale's full month name, variable length (e.g., @samp{January})
14009 locale's date and time (e.g., @samp{Thu Mar @ 3 23:05:25 2005})
14011 century. This is like @samp{%Y}, except the last two digits are omitted.
14012 For example, it is @samp{20} if @samp{%Y} is @samp{2000},
14013 and is @samp{-0} if @samp{%Y} is @samp{-001}.
14014 It is normally at least two characters, but it may be more.
14016 day of month (e.g., @samp{01})
14018 date; same as @samp{%m/%d/%y}
14020 day of month, space padded; same as @samp{%_d}
14022 full date in @acronym{ISO} 8601 format; same as @samp{%Y-%m-%d}.
14023 This is a good choice for a date format, as it is standard and
14024 is easy to sort in the usual case where years are in the range
14026 This is a @acronym{GNU} extension.
14028 year corresponding to the @acronym{ISO} week number, but without the century
14029 (range @samp{00} through @samp{99}). This has the same format and value
14030 as @samp{%y}, except that if the @acronym{ISO} week number (see
14032 to the previous or next year, that year is used instead.
14033 This is a @acronym{GNU} extension.
14035 year corresponding to the @acronym{ISO} week number. This has the
14036 same format and value as @samp{%Y}, except that if the @acronym{ISO}
14038 @samp{%V}) belongs to the previous or next year, that year is used
14040 It is normally useful only if @samp{%V} is also used;
14041 for example, the format @samp{%G-%m-%d} is probably a mistake,
14042 since it combines the ISO week number year with the conventional month and day.
14043 This is a @acronym{GNU} extension.
14047 day of year (@samp{001}@dots{}@samp{366})
14049 month (@samp{01}@dots{}@samp{12})
14051 day of week (@samp{1}@dots{}@samp{7}) with @samp{1} corresponding to Monday
14053 week number of year, with Sunday as the first day of the week
14054 (@samp{00}@dots{}@samp{53}).
14055 Days in a new year preceding the first Sunday are in week zero.
14057 @acronym{ISO} week number, that is, the
14058 week number of year, with Monday as the first day of the week
14059 (@samp{01}@dots{}@samp{53}).
14060 If the week containing January 1 has four or more days in
14061 the new year, then it is considered week 1; otherwise, it is week 53 of
14062 the previous year, and the next week is week 1. (See the @acronym{ISO} 8601
14065 day of week (@samp{0}@dots{}@samp{6}) with 0 corresponding to Sunday
14067 week number of year, with Monday as first day of week
14068 (@samp{00}@dots{}@samp{53}).
14069 Days in a new year preceding the first Monday are in week zero.
14071 locale's date representation (e.g., @samp{12/31/99})
14073 last two digits of year (@samp{00}@dots{}@samp{99})
14075 year. This is normally at least four characters, but it may be more.
14076 Year @samp{0000} precedes year @samp{0001}, and year @samp{-001}
14077 precedes year @samp{0000}.
14081 @node Literal conversion specifiers
14082 @subsection Literal conversion specifiers
14084 @cindex literal conversion specifiers
14085 @cindex conversion specifiers, literal
14087 @command{date} conversion specifiers that produce literal strings.
14099 @node Padding and other flags
14100 @subsection Padding and other flags
14102 @cindex numeric field padding
14103 @cindex padding of numeric fields
14104 @cindex fields, padding numeric
14106 Unless otherwise specified, @command{date} normally pads numeric fields
14107 with zeros, so that, for
14108 example, numeric months are always output as two digits.
14109 Seconds since the epoch are not padded, though,
14110 since there is no natural width for them.
14112 As a @acronym{GNU} extension, @command{date} recognizes any of the
14113 following optional flags after the @samp{%}:
14117 (hyphen) Do not pad the field; useful if the output is intended for
14120 (underscore) Pad with spaces; useful if you need a fixed
14121 number of characters in the output, but zeros are too distracting.
14123 (zero) Pad with zeros even if the conversion specifier
14124 would normally pad with spaces.
14126 Use upper case characters if possible.
14128 Use opposite case characters if possible.
14129 A field that is normally upper case becomes lower case, and vice versa.
14133 Here are some examples of padding:
14136 date +%d/%m -d "Feb 1"
14138 date +%-d/%-m -d "Feb 1"
14140 date +%_d/%_m -d "Feb 1"
14144 As a @acronym{GNU} extension, you can specify the field width
14145 (after any flag, if present) as a decimal number. If the natural size of the
14146 output of the field has less than the specified number of characters,
14147 the result is written right adjusted and padded to the given
14148 size. For example, @samp{%9B} prints the right adjusted month name in
14149 a field of width 9.
14151 An optional modifier can follow the optional flag and width
14152 specification. The modifiers are:
14156 Use the locale's alternate representation for date and time. This
14157 modifier applies to the @samp{%c}, @samp{%C}, @samp{%x}, @samp{%X},
14158 @samp{%y} and @samp{%Y} conversion specifiers. In a Japanese locale, for
14159 example, @samp{%Ex} might yield a date format based on the Japanese
14163 Use the locale's alternate numeric symbols for numbers. This modifier
14164 applies only to numeric conversion specifiers.
14167 If the format supports the modifier but no alternate representation
14168 is available, it is ignored.
14171 @node Setting the time
14172 @subsection Setting the time
14174 @cindex setting the time
14175 @cindex time setting
14176 @cindex appropriate privileges
14178 If given an argument that does not start with @samp{+}, @command{date} sets
14179 the system clock to the date and time specified by that argument (as
14180 described below). You must have appropriate privileges to set the
14181 system clock. The @option{--date} and @option{--set} options may not be
14182 used with such an argument. The @option{--universal} option may be used
14183 with such an argument to indicate that the specified date and time are
14184 relative to Coordinated Universal Time rather than to the local time
14187 The argument must consist entirely of digits, which have the following
14200 first two digits of year (optional)
14202 last two digits of year (optional)
14207 The @option{--set} option also sets the system clock; see the next section.
14210 @node Options for date
14211 @subsection Options for @command{date}
14213 @cindex @command{date} options
14214 @cindex options for @command{date}
14216 The program accepts the following options. Also see @ref{Common options}.
14220 @item -d @var{datestr}
14221 @itemx --date=@var{datestr}
14224 @cindex parsing date strings
14225 @cindex date strings, parsing
14226 @cindex arbitrary date strings, parsing
14229 @opindex next @var{day}
14230 @opindex last @var{day}
14231 Display the date and time specified in @var{datestr} instead of the
14232 current date and time. @var{datestr} can be in almost any common
14233 format. It can contain month names, time zones, @samp{am} and @samp{pm},
14234 @samp{yesterday}, etc. For example, @option{--date="2004-02-27
14235 14:19:13.489392193 +0530"} specifies the instant of time that is
14236 489,392,193 nanoseconds after February 27, 2004 at 2:19:13 PM in a
14237 time zone that is 5 hours and 30 minutes east of @acronym{UTC}.@*
14238 Note: input currently must be in locale independent format. E.g., the
14239 LC_TIME=C below is needed to print back the correct date in many locales:
14241 date -d "$(LC_TIME=C date)"
14243 @xref{Date input formats}.
14245 @item -f @var{datefile}
14246 @itemx --file=@var{datefile}
14249 Parse each line in @var{datefile} as with @option{-d} and display the
14250 resulting date and time. If @var{datefile} is @samp{-}, use standard
14251 input. This is useful when you have many dates to process, because the
14252 system overhead of starting up the @command{date} executable many times can
14255 @item -r @var{file}
14256 @itemx --reference=@var{file}
14258 @opindex --reference
14259 Display the date and time of the last modification of @var{file},
14260 instead of the current date and time.
14267 @opindex --rfc-2822
14268 Display the date and time using the format @samp{%a, %d %b %Y %H:%M:%S
14269 %z}, evaluated in the C locale so abbreviations are always in English.
14273 Fri, 09 Sep 2005 13:51:39 -0700
14276 This format conforms to
14277 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt, Internet
14278 @acronym{RFCs} 2822} and
14279 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc822.txt, 822}, the
14280 current and previous standards for Internet email.
14282 @item --rfc-3339=@var{timespec}
14283 @opindex --rfc-3339=@var{timespec}
14284 Display the date using a format specified by
14285 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt, Internet
14286 @acronym{RFC} 3339}. This is a subset of the @acronym{ISO} 8601
14287 format, except that it also permits applications to use a space rather
14288 than a @samp{T} to separate dates from times. Unlike the other
14289 standard formats, @acronym{RFC} 3339 format is always suitable as
14290 input for the @option{--date} (@option{-d}) and @option{--file}
14291 (@option{-f}) options, regardless of the current locale.
14293 The argument @var{timespec} specifies how much of the time to include.
14294 It can be one of the following:
14298 Print just the full-date, e.g., @samp{2005-09-14}.
14299 This is equivalent to the format @samp{%Y-%m-%d}.
14302 Print the full-date and full-time separated by a space, e.g.,
14303 @samp{2005-09-14 00:56:06+05:30}. The output ends with a numeric
14304 time-offset; here the @samp{+05:30} means that local time is five
14305 hours and thirty minutes east of @acronym{UTC}. This is equivalent to
14306 the format @samp{%Y-%m-%d %H:%M:%S%:z}.
14309 Like @samp{seconds}, but also print nanoseconds, e.g.,
14310 @samp{2005-09-14 00:56:06.998458565+05:30}.
14311 This is equivalent to the format @samp{%Y-%m-%d %H:%M:%S.%N%:z}.
14315 @item -s @var{datestr}
14316 @itemx --set=@var{datestr}
14319 Set the date and time to @var{datestr}. See @option{-d} above.
14326 @opindex --universal
14327 @cindex Coordinated Universal Time
14329 @cindex Greenwich Mean Time
14332 Use Coordinated Universal Time (@acronym{UTC}) by operating as if the
14333 @env{TZ} environment variable were set to the string @samp{UTC0}.
14335 Universal Time is often called ``Greenwich Mean Time'' (@sc{gmt}) for
14336 historical reasons.
14340 @node Examples of date
14341 @subsection Examples of @command{date}
14343 @cindex examples of @command{date}
14345 Here are a few examples. Also see the documentation for the @option{-d}
14346 option in the previous section.
14351 To print the date of the day before yesterday:
14354 date --date='2 days ago'
14358 To print the date of the day three months and one day hence:
14361 date --date='3 months 1 day'
14365 To print the day of year of Christmas in the current year:
14368 date --date='25 Dec' +%j
14372 To print the current full month name and the day of the month:
14378 But this may not be what you want because for the first nine days of
14379 the month, the @samp{%d} expands to a zero-padded two-digit field,
14380 for example @samp{date -d 1may '+%B %d'} will print @samp{May 01}.
14383 To print a date without the leading zero for one-digit days
14384 of the month, you can use the (@acronym{GNU} extension)
14385 @samp{-} flag to suppress
14386 the padding altogether:
14389 date -d 1may '+%B %-d
14393 To print the current date and time in the format required by many
14394 non-@acronym{GNU} versions of @command{date} when setting the system clock:
14397 date +%m%d%H%M%Y.%S
14401 To set the system clock forward by two minutes:
14404 date --set='+2 minutes'
14408 To print the date in @acronym{RFC} 2822 format,
14409 use @samp{date --rfc-2822}. Here is some example output:
14412 Fri, 09 Sep 2005 13:51:39 -0700
14415 @anchor{%s-examples}
14417 To convert a date string to the number of seconds since the epoch
14418 (which is 1970-01-01 00:00:00 UTC), use the @option{--date} option with
14419 the @samp{%s} format. That can be useful in sorting and/or graphing
14420 and/or comparing data by date. The following command outputs the
14421 number of the seconds since the epoch for the time two minutes after the
14425 date --date='1970-01-01 00:02:00 +0000' +%s
14429 If you do not specify time zone information in the date string,
14430 @command{date} uses your computer's idea of the time zone when
14431 interpreting the string. For example, if your computer's time zone is
14432 that of Cambridge, Massachusetts, which was then 5 hours (i.e., 18,000
14433 seconds) behind UTC:
14436 # local time zone used
14437 date --date='1970-01-01 00:02:00' +%s
14442 If you're sorting or graphing dated data, your raw date values may be
14443 represented as seconds since the epoch. But few people can look at
14444 the date @samp{946684800} and casually note ``Oh, that's the first second
14445 of the year 2000 in Greenwich, England.''
14448 date --date='2000-01-01 UTC' +%s
14452 An alternative is to use the @option{--utc} (@option{-u}) option.
14453 Then you may omit @samp{UTC} from the date string. Although this
14454 produces the same result for @samp{%s} and many other format sequences,
14455 with a time zone offset different from zero, it would give a different
14456 result for zone-dependent formats like @samp{%z}.
14459 date -u --date=2000-01-01 +%s
14463 To convert such an unwieldy number of seconds back to
14464 a more readable form, use a command like this:
14467 # local time zone used
14468 date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
14469 1999-12-31 19:00:00 -0500
14472 Or if you do not mind depending on the @samp{@@} feature present since
14473 coreutils 5.3.0, you could shorten this to:
14476 date -d @@946684800 +"%F %T %z"
14477 1999-12-31 19:00:00 -0500
14480 Often it is better to output UTC-relative date and time:
14483 date -u -d '1970-01-01 946684800 seconds' +"%Y-%m-%d %T %z"
14484 2000-01-01 00:00:00 +0000
14490 @node arch invocation
14491 @section @command{arch}: Print machine hardware name
14494 @cindex print machine hardware name
14495 @cindex system information, printing
14497 @command{arch} prints the machine hardware name,
14498 and is equivalent to @samp{uname -m}.
14502 arch [@var{option}]
14505 The program accepts the @ref{Common options} only.
14510 @node nproc invocation
14511 @section @command{nproc}: Print the number of available processors
14514 @cindex Print the number of processors
14515 @cindex system information, printing
14517 Print the number of processing units available to the current process,
14518 which may be less than the number of online processors.
14519 If this information is not accessible, then print the number of
14520 processors installed. If the @env{OMP_NUM_THREADS} environment variable is
14521 set, then it will determine the returned value. The result is guaranteed to be
14522 greater than zero. Synopsis:
14525 nproc [@var{option}]
14528 The program accepts the following options. Also see @ref{Common options}.
14534 Print the number of installed processors on the system, which may
14535 be greater than the number online or available to the current process.
14536 The @env{OMP_NUM_THREADS} environment variable is not honored in this case.
14538 @item --ignore=@var{number}
14540 If possible, exclude this @var{number} of processing units.
14547 @node uname invocation
14548 @section @command{uname}: Print system information
14551 @cindex print system information
14552 @cindex system information, printing
14554 @command{uname} prints information about the machine and operating system
14555 it is run on. If no options are given, @command{uname} acts as if the
14556 @option{-s} option were given. Synopsis:
14559 uname [@var{option}]@dots{}
14562 If multiple options or @option{-a} are given, the selected information is
14563 printed in this order:
14566 @var{kernel-name} @var{nodename} @var{kernel-release} @var{kernel-version}
14567 @var{machine} @var{processor} @var{hardware-platform} @var{operating-system}
14570 The information may contain internal spaces, so such output cannot be
14571 parsed reliably. In the following example, @var{release} is
14572 @samp{2.2.18ss.e820-bda652a #4 SMP Tue Jun 5 11:24:08 PDT 2001}:
14576 @result{} Linux dum 2.2.18 #4 SMP Tue Jun 5 11:24:08 PDT 2001 i686@c
14577 unknown unknown GNU/Linux
14581 The program accepts the following options. Also see @ref{Common options}.
14589 Print all of the below information, except omit the processor type
14590 and the hardware platform name if they are unknown.
14593 @itemx --hardware-platform
14595 @opindex --hardware-platform
14596 @cindex implementation, hardware
14597 @cindex hardware platform
14598 @cindex platform, hardware
14599 Print the hardware platform name
14600 (sometimes called the hardware implementation).
14601 Print @samp{unknown} if the kernel does not make this information
14602 easily available, as is the case with Linux kernels.
14608 @cindex machine type
14609 @cindex hardware class
14610 @cindex hardware type
14611 Print the machine hardware name (sometimes called the hardware class
14617 @opindex --nodename
14620 @cindex network node name
14621 Print the network node hostname.
14626 @opindex --processor
14627 @cindex host processor type
14628 Print the processor type (sometimes called the instruction set
14629 architecture or ISA).
14630 Print @samp{unknown} if the kernel does not make this information
14631 easily available, as is the case with Linux kernels.
14634 @itemx --operating-system
14636 @opindex --operating-system
14637 @cindex operating system name
14638 Print the name of the operating system.
14641 @itemx --kernel-release
14643 @opindex --kernel-release
14644 @cindex kernel release
14645 @cindex release of kernel
14646 Print the kernel release.
14649 @itemx --kernel-name
14651 @opindex --kernel-name
14652 @cindex kernel name
14653 @cindex name of kernel
14654 Print the kernel name.
14655 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) calls this
14656 ``the implementation of the operating system'', because the
14657 @acronym{POSIX} specification itself has no notion of ``kernel''.
14658 The kernel name might be the same as the operating system name printed
14659 by the @option{-o} or @option{--operating-system} option, but it might
14660 differ. Some operating systems (e.g., FreeBSD, HP-UX) have the same
14661 name as their underlying kernels; others (e.g., GNU/Linux, Solaris)
14665 @itemx --kernel-version
14667 @opindex --kernel-version
14668 @cindex kernel version
14669 @cindex version of kernel
14670 Print the kernel version.
14677 @node hostname invocation
14678 @section @command{hostname}: Print or set system name
14681 @cindex setting the hostname
14682 @cindex printing the hostname
14683 @cindex system name, printing
14684 @cindex appropriate privileges
14686 With no arguments, @command{hostname} prints the name of the current host
14687 system. With one argument, it sets the current host name to the
14688 specified string. You must have appropriate privileges to set the host
14692 hostname [@var{name}]
14695 The only options are @option{--help} and @option{--version}. @xref{Common
14701 @node hostid invocation
14702 @section @command{hostid}: Print numeric host identifier
14705 @cindex printing the host identifier
14707 @command{hostid} prints the numeric identifier of the current host
14708 in hexadecimal. This command accepts no arguments.
14709 The only options are @option{--help} and @option{--version}.
14710 @xref{Common options}.
14712 For example, here's what it prints on one system I use:
14719 On that system, the 32-bit quantity happens to be closely
14720 related to the system's Internet address, but that isn't always
14725 @node uptime invocation
14726 @section @command{uptime}: Print system uptime and load
14729 @cindex printing the system uptime and load
14731 @command{uptime} prints the current time, the system's uptime, the
14732 number of logged-in users and the current load average.
14734 If an argument is specified, it is used as the file to be read
14735 to discover how many users are logged in. If no argument is
14736 specified, a system default is used (@command{uptime --help} indicates
14737 the default setting).
14739 The only options are @option{--help} and @option{--version}.
14740 @xref{Common options}.
14742 For example, here's what it prints right now on one system I use:
14746 14:07 up 3:35, 3 users, load average: 1.39, 1.15, 1.04
14749 The precise method of calculation of load average varies somewhat
14750 between systems. Some systems calculate it as the average number of
14751 runnable processes over the last 1, 5 and 15 minutes, but some systems
14752 also include processes in the uninterruptible sleep state (that is,
14753 those processes which are waiting for disk I/O). The Linux kernel
14754 includes uninterruptible processes.
14756 @node SELinux context
14757 @chapter SELinux context
14759 @cindex SELinux context
14760 @cindex SELinux, context
14761 @cindex commands for SELinux context
14763 This section describes commands for operations with SELinux
14767 * chcon invocation:: Change SELinux context of file
14768 * runcon invocation:: Run a command in specified SELinux context
14771 @node chcon invocation
14772 @section @command{chcon}: Change SELinux context of file
14775 @cindex changing security context
14776 @cindex change SELinux context
14778 @command{chcon} changes the SELinux security context of the selected files.
14782 chcon [@var{option}]@dots{} @var{context} @var{file}@dots{}
14783 chcon [@var{option}]@dots{} [-u @var{user}] [-r @var{role}] [-l @var{range}]@c
14784 [-t @var{type}] @var{file}@dots{}
14785 chcon [@var{option}]@dots{} --reference=@var{rfile} @var{file}@dots{}
14788 Change the SELinux security context of each @var{file} to @var{context}.
14789 With @option{--reference}, change the security context of each @var{file}
14790 to that of @var{rfile}.
14792 The program accepts the following options. Also see @ref{Common options}.
14797 @itemx --no-dereference
14799 @opindex --no-dereference
14800 @cindex no dereference
14801 Affect symbolic links instead of any referenced file.
14803 @item --reference=@var{rfile}
14804 @opindex --reference
14805 @cindex reference file
14806 Use @var{rfile}'s security context rather than specifying a @var{context} value.
14811 @opindex --recursive
14812 Operate on files and directories recursively.
14815 @xref{Traversing symlinks}.
14818 @xref{Traversing symlinks}.
14821 @xref{Traversing symlinks}.
14828 Output a diagnostic for every file processed.
14830 @item -u @var{user}
14831 @itemx --user=@var{user}
14834 Set user @var{user} in the target security context.
14836 @item -r @var{role}
14837 @itemx --role=@var{role}
14840 Set role @var{role} in the target security context.
14842 @item -t @var{type}
14843 @itemx --type=@var{type}
14846 Set type @var{type} in the target security context.
14848 @item -l @var{range}
14849 @itemx --range=@var{range}
14852 Set range @var{range} in the target security context.
14858 @node runcon invocation
14859 @section @command{runcon}: Run a command in specified SELinux context
14862 @cindex run with security context
14865 @command{runcon} runs file in specified SELinux security context.
14869 runcon @var{context} @var{command} [@var{args}]
14870 runcon [ -c ] [-u @var{user}] [-r @var{role}] [-t @var{type}]@c
14871 [-l @var{range}] @var{command} [@var{args}]
14874 Run @var{command} with completely-specified @var{context}, or with
14875 current or transitioned security context modified by one or more of @var{level},
14876 @var{role}, @var{type} and @var{user}.
14878 If none of @option{-c}, @option{-t}, @option{-u}, @option{-r}, or @option{-l}
14879 is specified, the first argument is used as the complete context.
14880 Any additional arguments after @var{command}
14881 are interpreted as arguments to the command.
14883 With neither @var{context} nor @var{command}, print the current
14886 The program accepts the following options. Also see @ref{Common options}.
14894 Compute process transition context before modifying.
14896 @item -u @var{user}
14897 @itemx --user=@var{user}
14900 Set user @var{user} in the target security context.
14902 @item -r @var{role}
14903 @itemx --role=@var{role}
14906 Set role @var{role} in the target security context.
14908 @item -t @var{type}
14909 @itemx --type=@var{type}
14912 Set type @var{type} in the target security context.
14914 @item -l @var{range}
14915 @itemx --range=@var{range}
14918 Set range @var{range} in the target security context.
14922 @cindex exit status of @command{runcon}
14926 126 if @var{command} is found but cannot be invoked
14927 127 if @command{runcon} itself fails or if @var{command} cannot be found
14928 the exit status of @var{command} otherwise
14931 @node Modified command invocation
14932 @chapter Modified command invocation
14934 @cindex modified command invocation
14935 @cindex invocation of commands, modified
14936 @cindex commands for invoking other commands
14938 This section describes commands that run other commands in some context
14939 different than the current one: a modified environment, as a different
14943 * chroot invocation:: Modify the root directory.
14944 * env invocation:: Modify environment variables.
14945 * nice invocation:: Modify niceness.
14946 * nohup invocation:: Immunize to hangups.
14947 * stdbuf invocation:: Modify buffering of standard streams.
14948 * su invocation:: Modify user and group ID.
14949 * timeout invocation:: Run with time limit.
14953 @node chroot invocation
14954 @section @command{chroot}: Run a command with a different root directory
14957 @cindex running a program in a specified root directory
14958 @cindex root directory, running a program in a specified
14960 @command{chroot} runs a command with a specified root directory.
14961 On many systems, only the super-user can do this.@footnote{However,
14962 some systems (e.g., FreeBSD) can be configured to allow certain regular
14963 users to use the @code{chroot} system call, and hence to run this program.
14964 Also, on Cygwin, anyone can run the @command{chroot} command, because the
14965 underlying function is non-privileged due to lack of support in MS-Windows.}
14969 chroot @var{option} @var{newroot} [@var{command} [@var{args}]@dots{}]
14970 chroot @var{option}
14973 Ordinarily, file names are looked up starting at the root of the
14974 directory structure, i.e., @file{/}. @command{chroot} changes the root to
14975 the directory @var{newroot} (which must exist) and then runs
14976 @var{command} with optional @var{args}. If @var{command} is not
14977 specified, the default is the value of the @env{SHELL} environment
14978 variable or @command{/bin/sh} if not set, invoked with the @option{-i} option.
14979 @var{command} must not be a special built-in utility
14980 (@pxref{Special built-in utilities}).
14982 The program accepts the following options. Also see @ref{Common options}.
14983 Options must precede operands.
14987 @itemx --userspec=@var{user}[:@var{group}]
14988 @opindex --userspec
14989 By default, @var{command} is run with the same credentials
14990 as the invoking process.
14991 Use this option to run it as a different @var{user} and/or with a
14992 different primary @var{group}.
14994 @itemx --groups=@var{groups}
14996 Use this option to specify the supplementary @var{groups} to be
14997 used by the new process.
14998 The items in the list (names or numeric IDs) must be separated by commas.
15002 Here are a few tips to help avoid common problems in using chroot.
15003 To start with a simple example, make @var{command} refer to a statically
15004 linked binary. If you were to use a dynamically linked executable, then
15005 you'd have to arrange to have the shared libraries in the right place under
15006 your new root directory.
15008 For example, if you create a statically linked @command{ls} executable,
15009 and put it in @file{/tmp/empty}, you can run this command as root:
15012 $ chroot /tmp/empty /ls -Rl /
15015 Then you'll see output like this:
15020 -rwxr-xr-x 1 0 0 1041745 Aug 16 11:17 ls
15023 If you want to use a dynamically linked executable, say @command{bash},
15024 then first run @samp{ldd bash} to see what shared objects it needs.
15025 Then, in addition to copying the actual binary, also copy the listed
15026 files to the required positions under your intended new root directory.
15027 Finally, if the executable requires any other files (e.g., data, state,
15028 device files), copy them into place, too.
15030 @cindex exit status of @command{chroot}
15034 125 if @command{chroot} itself fails
15035 126 if @var{command} is found but cannot be invoked
15036 127 if @var{command} cannot be found
15037 the exit status of @var{command} otherwise
15041 @node env invocation
15042 @section @command{env}: Run a command in a modified environment
15045 @cindex environment, running a program in a modified
15046 @cindex modified environment, running a program in a
15047 @cindex running a program in a modified environment
15049 @command{env} runs a command with a modified environment. Synopses:
15052 env [@var{option}]@dots{} [@var{name}=@var{value}]@dots{} @c
15053 [@var{command} [@var{args}]@dots{}]
15057 Operands of the form @samp{@var{variable}=@var{value}} set
15058 the environment variable @var{variable} to value @var{value}.
15059 @var{value} may be empty (@samp{@var{variable}=}). Setting a variable
15060 to an empty value is different from unsetting it.
15061 These operands are evaluated left-to-right, so if two operands
15062 mention the same variable the earlier is ignored.
15064 Environment variable names can be empty, and can contain any
15065 characters other than @samp{=} and @acronym{ASCII} @sc{nul}.
15066 However, it is wise to limit yourself to names that
15067 consist solely of underscores, digits, and @acronym{ASCII} letters,
15068 and that begin with a non-digit, as applications like the shell do not
15069 work well with other names.
15072 The first operand that does not contain the character @samp{=}
15073 specifies the program to invoke; it is
15074 searched for according to the @env{PATH} environment variable. Any
15075 remaining arguments are passed as arguments to that program.
15076 The program should not be a special built-in utility
15077 (@pxref{Special built-in utilities}).
15079 Modifications to @env{PATH} take effect prior to searching for
15080 @var{command}. Use caution when reducing @env{PATH}; behavior is
15081 not portable when @env{PATH} is undefined or omits key directories
15082 such as @file{/bin}.
15084 In the rare case that a utility contains a @samp{=} in the name, the
15085 only way to disambiguate it from a variable assignment is to use an
15086 intermediate command for @var{command}, and pass the problematic
15087 program name via @var{args}. For example, if @file{./prog=} is an
15088 executable in the current @env{PATH}:
15091 env prog= true # runs 'true', with prog= in environment
15092 env ./prog= true # runs 'true', with ./prog= in environment
15093 env -- prog= true # runs 'true', with prog= in environment
15094 env sh -c '\prog= true' # runs 'prog=' with argument 'true'
15095 env sh -c 'exec "$@@"' sh prog= true # also runs 'prog='
15098 @cindex environment, printing
15100 If no command name is specified following the environment
15101 specifications, the resulting environment is printed. This is like
15102 specifying the @command{printenv} program.
15104 For some examples, suppose the environment passed to @command{env}
15105 contains @samp{LOGNAME=rms}, @samp{EDITOR=emacs}, and
15106 @samp{PATH=.:/gnubin:/hacks}:
15111 Output the current environment.
15113 $ env | LC_ALL=C sort
15116 PATH=.:/gnubin:/hacks
15120 Run @command{foo} with a reduced environment, preserving only the
15121 original @env{PATH} to avoid problems in locating @command{foo}.
15123 env - PATH="$PATH" foo
15127 Run @command{foo} with the environment containing @samp{LOGNAME=rms},
15128 @samp{EDITOR=emacs}, and @samp{PATH=.:/gnubin:/hacks}, and guarantees
15129 that @command{foo} was found in the file system rather than as a shell
15136 Run @command{nemacs} with the environment containing @samp{LOGNAME=foo},
15137 @samp{EDITOR=emacs}, @samp{PATH=.:/gnubin:/hacks}, and
15138 @samp{DISPLAY=gnu:0}.
15140 env DISPLAY=gnu:0 LOGNAME=foo nemacs
15144 Attempt to run the program @command{/energy/--} (as that is the only
15145 possible path search result); if the command exists, the environment
15146 will contain @samp{LOGNAME=rms} and @samp{PATH=/energy}, and the
15147 arguments will be @samp{e=mc2}, @samp{bar}, and @samp{baz}.
15149 env -u EDITOR PATH=/energy -- e=mc2 bar baz
15155 The program accepts the following options. Also see @ref{Common options}.
15156 Options must precede operands.
15162 @item -u @var{name}
15163 @itemx --unset=@var{name}
15166 Remove variable @var{name} from the environment, if it was in the
15171 @itemx --ignore-environment
15174 @opindex --ignore-environment
15175 Start with an empty environment, ignoring the inherited environment.
15179 @cindex exit status of @command{env}
15183 0 if no @var{command} is specified and the environment is output
15184 125 if @command{env} itself fails
15185 126 if @var{command} is found but cannot be invoked
15186 127 if @var{command} cannot be found
15187 the exit status of @var{command} otherwise
15191 @node nice invocation
15192 @section @command{nice}: Run a command with modified niceness
15196 @cindex scheduling, affecting
15197 @cindex appropriate privileges
15199 @command{nice} prints or modifies a process's @dfn{niceness},
15200 a parameter that affects whether the process is scheduled favorably.
15204 nice [@var{option}]@dots{} [@var{command} [@var{arg}]@dots{}]
15207 If no arguments are given, @command{nice} prints the current niceness.
15208 Otherwise, @command{nice} runs the given @var{command} with its
15209 niceness adjusted. By default, its niceness is incremented by 10.
15211 Niceness values range at least from @minus{}20 (process has high priority
15212 and gets more resources, thus slowing down other processes) through 19
15213 (process has lower priority and runs slowly itself, but has less impact
15214 on the speed of other running processes). Some systems
15215 may have a wider range of nicenesses; conversely, other systems may
15216 enforce more restrictive limits. An attempt to set the niceness
15217 outside the supported range is treated as an attempt to use the
15218 minimum or maximum supported value.
15220 A niceness should not be confused with a scheduling priority, which
15221 lets applications determine the order in which threads are scheduled
15222 to run. Unlike a priority, a niceness is merely advice to the
15223 scheduler, which the scheduler is free to ignore. Also, as a point of
15224 terminology, @acronym{POSIX} defines the behavior of @command{nice} in
15225 terms of a @dfn{nice value}, which is the nonnegative difference
15226 between a niceness and the minimum niceness. Though @command{nice}
15227 conforms to @acronym{POSIX}, its documentation and diagnostics use the
15228 term ``niceness'' for compatibility with historical practice.
15230 @var{command} must not be a special built-in utility (@pxref{Special
15231 built-in utilities}).
15233 @mayConflictWithShellBuiltIn{nice}
15235 The program accepts the following option. Also see @ref{Common options}.
15236 Options must precede operands.
15239 @item -n @var{adjustment}
15240 @itemx --adjustment=@var{adjustment}
15242 @opindex --adjustment
15243 Add @var{adjustment} instead of 10 to the command's niceness. If
15244 @var{adjustment} is negative and you lack appropriate privileges,
15245 @command{nice} issues a warning but otherwise acts as if you specified
15248 For compatibility @command{nice} also supports an obsolete
15249 option syntax @option{-@var{adjustment}}. New scripts should use
15250 @option{-n @var{adjustment}} instead.
15254 @cindex exit status of @command{nice}
15258 0 if no @var{command} is specified and the niceness is output
15259 125 if @command{nice} itself fails
15260 126 if @var{command} is found but cannot be invoked
15261 127 if @var{command} cannot be found
15262 the exit status of @var{command} otherwise
15265 It is sometimes useful to run a non-interactive program with reduced niceness.
15268 $ nice factor 4611686018427387903
15271 Since @command{nice} prints the current niceness,
15272 you can invoke it through itself to demonstrate how it works.
15274 The default behavior is to increase the niceness by @samp{10}:
15285 The @var{adjustment} is relative to the current niceness. In the
15286 next example, the first @command{nice} invocation runs the second one
15287 with niceness 10, and it in turn runs the final one with a niceness
15291 $ nice nice -n 3 nice
15295 Specifying a niceness larger than the supported range
15296 is the same as specifying the maximum supported value:
15299 $ nice -n 10000000000 nice
15303 Only a privileged user may run a process with lower niceness:
15307 nice: cannot set niceness: Permission denied
15309 $ sudo nice -n -1 nice
15314 @node nohup invocation
15315 @section @command{nohup}: Run a command immune to hangups
15318 @cindex hangups, immunity to
15319 @cindex immunity to hangups
15320 @cindex logging out and continuing to run
15323 @command{nohup} runs the given @var{command} with hangup signals ignored,
15324 so that the command can continue running in the background after you log
15328 nohup @var{command} [@var{arg}]@dots{}
15331 If standard input is a terminal, it is redirected from
15332 @file{/dev/null} so that terminal sessions do not mistakenly consider
15333 the terminal to be used by the command. This is a @acronym{GNU}
15334 extension; programs intended to be portable to non-@acronym{GNU} hosts
15335 should use @samp{nohup @var{command} [@var{arg}]@dots{} </dev/null}
15339 If standard output is a terminal, the command's standard output is appended
15340 to the file @file{nohup.out}; if that cannot be written to, it is appended
15341 to the file @file{$HOME/nohup.out}; and if that cannot be written to, the
15342 command is not run.
15343 Any @file{nohup.out} or @file{$HOME/nohup.out} file created by
15344 @command{nohup} is made readable and writable only to the user,
15345 regardless of the current umask settings.
15347 If standard error is a terminal, it is normally redirected to the same file
15348 descriptor as the (possibly-redirected) standard output.
15349 However, if standard output is closed, standard error terminal output
15350 is instead appended to the file @file{nohup.out} or
15351 @file{$HOME/nohup.out} as above.
15353 To capture the command's output to a file other than @file{nohup.out}
15354 you can redirect it. For example, to capture the output of
15358 nohup make > make.log
15361 @command{nohup} does not automatically put the command it runs in the
15362 background; you must do that explicitly, by ending the command line
15363 with an @samp{&}. Also, @command{nohup} does not alter the
15364 niceness of @var{command}; use @command{nice} for that,
15365 e.g., @samp{nohup nice @var{command}}.
15367 @var{command} must not be a special built-in utility (@pxref{Special
15368 built-in utilities}).
15370 The only options are @option{--help} and @option{--version}. @xref{Common
15371 options}. Options must precede operands.
15373 @cindex exit status of @command{nohup}
15377 125 if @command{nohup} itself fails, and @env{POSIXLY_CORRECT} is not set
15378 126 if @var{command} is found but cannot be invoked
15379 127 if @var{command} cannot be found
15380 the exit status of @var{command} otherwise
15383 If @env{POSIXLY_CORRECT} is set, internal failures give status 127
15387 @node stdbuf invocation
15388 @section @command{stdbuf}: Run a command with modified I/O stream buffering
15391 @cindex standard streams, buffering
15392 @cindex line buffered
15394 @command{stdbuf} allows one to modify the buffering operations of the
15395 three standard I/O streams associated with a program. Synopsis:
15398 stdbuf @var{option}@dots{} @var{command}
15401 Any additional @var{arg}s are passed as additional arguments to the
15404 The program accepts the following options. Also see @ref{Common options}.
15408 @item -i @var{mode}
15409 @itemx --input=@var{mode}
15412 Adjust the standard input stream buffering.
15414 @item -o @var{mode}
15415 @itemx --output=@var{mode}
15418 Adjust the standard output stream buffering.
15420 @item -e @var{mode}
15421 @itemx --error=@var{mode}
15424 Adjust the standard error stream buffering.
15428 The @var{mode} can be specified as follows:
15433 Set the stream to line buffered mode.
15434 In this mode data is coalesced until a newline is output or
15435 input is read from any stream attached to a terminal device.
15436 This option is invalid with standard input.
15439 Disable buffering of the selected stream.
15440 In this mode data is output immediately and only the
15441 amount of data requested is read from input.
15444 Specify the size of the buffer to use in fully buffered mode.
15445 @multiplierSuffixesNoBlocks{size}
15449 NOTE: If @var{command} adjusts the buffering of its standard streams
15450 (@command{tee} does for e.g.) then that will override corresponding settings
15451 changed by @command{stdbuf}. Also some filters (like @command{dd} and
15452 @command{cat} etc.) don't use streams for I/O, and are thus unaffected
15453 by @command{stdbuf} settings.
15455 @cindex exit status of @command{stdbuf}
15459 125 if @command{stdbuf} itself fails
15460 126 if @var{command} is found but cannot be invoked
15461 127 if @var{command} cannot be found
15462 the exit status of @var{command} otherwise
15466 @node su invocation
15467 @section @command{su}: Run a command with substitute user and group ID
15470 @cindex substitute user and group IDs
15471 @cindex user ID, switching
15472 @cindex super-user, becoming
15473 @cindex root, becoming
15475 @command{su} allows one user to temporarily become another user. It runs a
15476 command (often an interactive shell) with the real and effective user
15477 ID, group ID, and supplemental groups of a given @var{user}. Synopsis:
15480 su [@var{option}]@dots{} [@var{user} [@var{arg}]@dots{}]
15483 @cindex passwd entry, and @command{su} shell
15485 @flindex /etc/passwd
15486 If no @var{user} is given, the default is @code{root}, the super-user.
15487 The shell to use is taken from @var{user}'s @code{passwd} entry, or
15488 @file{/bin/sh} if none is specified there. If @var{user} has a
15489 password, @command{su} prompts for the password unless run by a user with
15490 effective user ID of zero (the super-user).
15496 @cindex login shell
15497 By default, @command{su} does not change the current directory.
15498 It sets the environment variables @env{HOME} and @env{SHELL}
15499 from the password entry for @var{user}, and if @var{user} is not
15500 the super-user, sets @env{USER} and @env{LOGNAME} to @var{user}.
15501 By default, the shell is not a login shell.
15503 Any additional @var{arg}s are passed as additional arguments to the
15506 @cindex @option{-su}
15507 GNU @command{su} does not treat @file{/bin/sh} or any other shells specially
15508 (e.g., by setting @code{argv[0]} to @option{-su}, passing @option{-c} only
15509 to certain shells, etc.).
15512 @command{su} can optionally be compiled to use @code{syslog} to report
15513 failed, and optionally successful, @command{su} attempts. (If the system
15514 supports @code{syslog}.) However, GNU @command{su} does not check if the
15515 user is a member of the @code{wheel} group; see below.
15517 The program accepts the following options. Also see @ref{Common options}.
15520 @item -c @var{command}
15521 @itemx --command=@var{command}
15524 Pass @var{command}, a single command line to run, to the shell with
15525 a @option{-c} option instead of starting an interactive shell.
15532 @cindex file name pattern expansion, disabled
15533 @cindex globbing, disabled
15534 Pass the @option{-f} option to the shell. This probably only makes sense
15535 if the shell run is @command{csh} or @command{tcsh}, for which the @option{-f}
15536 option prevents reading the startup file (@file{.cshrc}). With
15537 Bourne-like shells, the @option{-f} option disables file name pattern
15538 expansion (globbing), which is not likely to be useful.
15546 @c other variables already indexed above
15549 @cindex login shell, creating
15550 Make the shell a login shell. This means the following. Unset all
15551 environment variables except @env{TERM}, @env{HOME}, and @env{SHELL}
15552 (which are set as described above), and @env{USER} and @env{LOGNAME}
15553 (which are set, even for the super-user, as described above), and set
15554 @env{PATH} to a compiled-in default value. Change to @var{user}'s home
15555 directory. Prepend @samp{-} to the shell's name, intended to make it
15556 read its login startup file(s).
15560 @itemx --preserve-environment
15563 @opindex --preserve-environment
15564 @cindex environment, preserving
15565 @flindex /etc/shells
15566 @cindex restricted shell
15567 Do not change the environment variables @env{HOME}, @env{USER},
15568 @env{LOGNAME}, or @env{SHELL}. Run the shell given in the environment
15569 variable @env{SHELL} instead of the shell from @var{user}'s passwd
15570 entry, unless the user running @command{su} is not the super-user and
15571 @var{user}'s shell is restricted. A @dfn{restricted shell} is one that
15572 is not listed in the file @file{/etc/shells}, or in a compiled-in list
15573 if that file does not exist. Parts of what this option does can be
15574 overridden by @option{--login} and @option{--shell}.
15576 @item -s @var{shell}
15577 @itemx --shell=@var{shell}
15580 Run @var{shell} instead of the shell from @var{user}'s passwd entry,
15581 unless the user running @command{su} is not the super-user and @var{user}'s
15582 shell is restricted (see @option{-m} just above).
15586 @cindex exit status of @command{su}
15590 125 if @command{su} itself fails
15591 126 if subshell is found but cannot be invoked
15592 127 if subshell cannot be found
15593 the exit status of the subshell otherwise
15596 @cindex wheel group, not supported
15597 @cindex group wheel, not supported
15599 @subsection Why GNU @command{su} does not support the @samp{wheel} group
15601 (This section is by Richard Stallman.)
15605 Sometimes a few of the users try to hold total power over all the
15606 rest. For example, in 1984, a few users at the MIT AI lab decided to
15607 seize power by changing the operator password on the Twenex system and
15608 keeping it secret from everyone else. (I was able to thwart this coup
15609 and give power back to the users by patching the kernel, but I
15610 wouldn't know how to do that in Unix.)
15612 However, occasionally the rulers do tell someone. Under the usual
15613 @command{su} mechanism, once someone learns the root password who
15614 sympathizes with the ordinary users, he or she can tell the rest. The
15615 ``wheel group'' feature would make this impossible, and thus cement the
15616 power of the rulers.
15618 I'm on the side of the masses, not that of the rulers. If you are
15619 used to supporting the bosses and sysadmins in whatever they do, you
15620 might find this idea strange at first.
15623 @node timeout invocation
15624 @section @command{timeout}: Run a command with a time limit
15628 @cindex run commands with bounded time
15630 @command{timeout} runs the given @var{command} and kills it if it is
15631 still running after the specified time interval. Synopsis:
15634 timeout [@var{option}] @var{duration} @var{command} [@var{arg}]@dots{}
15637 @var{command} must not be a special built-in utility (@pxref{Special
15638 built-in utilities}).
15640 The program accepts the following options. Also see @ref{Common options}.
15641 Options must precede operands.
15644 @item -k @var{duration}
15645 @itemx --kill-after=@var{duration}
15647 @opindex --kill-after
15648 Ensure the monitored @var{command} is killed by also sending a @samp{KILL}
15649 signal, after the specified @var{duration}. Without this option, if the
15650 selected signal proves not to be fatal, @command{timeout} does not kill
15653 @item -s @var{signal}
15654 @itemx --signal=@var{signal}
15657 Send this @var{signal} to @var{command} on timeout, rather than the
15658 default @samp{TERM} signal. @var{signal} may be a name like @samp{HUP}
15659 or a number. Also see @xref{Signal specifications}.
15663 @var{duration} is an integer followed by an optional unit:
15665 @samp{s} for seconds (the default)
15666 @samp{m} for minutes
15670 A duration of 0 disables the associated timeout.
15672 @cindex exit status of @command{timeout}
15676 124 if @var{command} times out
15677 125 if @command{timeout} itself fails
15678 126 if @var{command} is found but cannot be invoked
15679 127 if @var{command} cannot be found
15680 the exit status of @var{command} otherwise
15684 @node Process control
15685 @chapter Process control
15687 @cindex processes, commands for controlling
15688 @cindex commands for controlling processes
15691 * kill invocation:: Sending a signal to processes.
15695 @node kill invocation
15696 @section @command{kill}: Send a signal to processes
15699 @cindex send a signal to processes
15701 The @command{kill} command sends a signal to processes, causing them
15702 to terminate or otherwise act upon receiving the signal in some way.
15703 Alternatively, it lists information about signals. Synopses:
15706 kill [-s @var{signal} | --signal @var{signal} | -@var{signal}] @var{pid}@dots{}
15707 kill [-l | --list | -t | --table] [@var{signal}]@dots{}
15710 @mayConflictWithShellBuiltIn{kill}
15712 The first form of the @command{kill} command sends a signal to all
15713 @var{pid} arguments. The default signal to send if none is specified
15714 is @samp{TERM}. The special signal number @samp{0} does not denote a
15715 valid signal, but can be used to test whether the @var{pid} arguments
15716 specify processes to which a signal could be sent.
15718 If @var{pid} is positive, the signal is sent to the process with the
15719 process ID @var{pid}. If @var{pid} is zero, the signal is sent to all
15720 processes in the process group of the current process. If @var{pid}
15721 is @minus{}1, the signal is sent to all processes for which the user has
15722 permission to send a signal. If @var{pid} is less than @minus{}1, the signal
15723 is sent to all processes in the process group that equals the absolute
15724 value of @var{pid}.
15726 If @var{pid} is not positive, a system-dependent set of system
15727 processes is excluded from the list of processes to which the signal
15730 If a negative @var{pid} argument is desired as the first one, it
15731 should be preceded by @option{--}. However, as a common extension to
15732 @acronym{POSIX}, @option{--} is not required with @samp{kill
15733 -@var{signal} -@var{pid}}. The following commands are equivalent:
15742 The first form of the @command{kill} command succeeds if every @var{pid}
15743 argument specifies at least one process that the signal was sent to.
15745 The second form of the @command{kill} command lists signal information.
15746 Either the @option{-l} or @option{--list} option, or the @option{-t}
15747 or @option{--table} option must be specified. Without any
15748 @var{signal} argument, all supported signals are listed. The output
15749 of @option{-l} or @option{--list} is a list of the signal names, one
15750 per line; if @var{signal} is already a name, the signal number is
15751 printed instead. The output of @option{-t} or @option{--table} is a
15752 table of signal numbers, names, and descriptions. This form of the
15753 @command{kill} command succeeds if all @var{signal} arguments are valid
15754 and if there is no output error.
15756 The @command{kill} command also supports the @option{--help} and
15757 @option{--version} options. @xref{Common options}.
15759 A @var{signal} may be a signal name like @samp{HUP}, or a signal
15760 number like @samp{1}, or an exit status of a process terminated by the
15761 signal. A signal name can be given in canonical form or prefixed by
15762 @samp{SIG}. The case of the letters is ignored, except for the
15763 @option{-@var{signal}} option which must use upper case to avoid
15764 ambiguity with lower case option letters. For a list of supported
15765 signal names and numbers see @xref{Signal specifications}.
15770 @cindex delaying commands
15771 @cindex commands for delaying
15773 @c Perhaps @command{wait} or other commands should be described here also?
15776 * sleep invocation:: Delay for a specified time.
15780 @node sleep invocation
15781 @section @command{sleep}: Delay for a specified time
15784 @cindex delay for a specified time
15786 @command{sleep} pauses for an amount of time specified by the sum of
15787 the values of the command line arguments.
15791 sleep @var{number}[smhd]@dots{}
15795 Each argument is a number followed by an optional unit; the default
15796 is seconds. The units are:
15809 Historical implementations of @command{sleep} have required that
15810 @var{number} be an integer, and only accepted a single argument
15811 without a suffix. However, GNU @command{sleep} accepts
15812 arbitrary floating point numbers. @xref{Floating point}.
15814 The only options are @option{--help} and @option{--version}. @xref{Common
15817 @c sleep is a shell built-in at least with Solaris 11's /bin/sh
15818 @mayConflictWithShellBuiltIn{sleep}
15823 @node Numeric operations
15824 @chapter Numeric operations
15826 @cindex numeric operations
15827 These programs do numerically-related operations.
15830 * factor invocation:: Show factors of numbers.
15831 * seq invocation:: Print sequences of numbers.
15835 @node factor invocation
15836 @section @command{factor}: Print prime factors
15839 @cindex prime factors
15841 @command{factor} prints prime factors. Synopses:
15844 factor [@var{number}]@dots{}
15845 factor @var{option}
15848 If no @var{number} is specified on the command line, @command{factor} reads
15849 numbers from standard input, delimited by newlines, tabs, or spaces.
15851 The @command{factor} command supports only a small number of options:
15855 Print a short help on standard output, then exit without further
15859 Print the program version on standard output, then exit without further
15863 Factoring the product of the eighth and ninth Mersenne primes
15864 takes about 30 milliseconds of CPU time on a 2.2 GHz Athlon.
15867 M8=`echo 2^31-1|bc` ; M9=`echo 2^61-1|bc`
15868 /usr/bin/time -f '%U' factor $(echo "$M8 * $M9" | bc)
15869 4951760154835678088235319297: 2147483647 2305843009213693951
15873 Similarly, factoring the eighth Fermat number @math{2^{256}+1} takes
15874 about 20 seconds on the same machine.
15876 Factoring large numbers is, in general, hard. The Pollard Rho
15877 algorithm used by @command{factor} is particularly effective for
15878 numbers with relatively small factors. If you wish to factor large
15879 numbers which do not have small factors (for example, numbers which
15880 are the product of two large primes), other methods are far better.
15882 If @command{factor} is built without using GNU MP, only
15883 single-precision arithmetic is available, and so large numbers
15884 (typically @math{2^{64}} and above) will not be supported. The single-precision
15885 code uses an algorithm which is designed for factoring smaller
15891 @node seq invocation
15892 @section @command{seq}: Print numeric sequences
15895 @cindex numeric sequences
15896 @cindex sequence of numbers
15898 @command{seq} prints a sequence of numbers to standard output. Synopses:
15901 seq [@var{option}]@dots{} @var{last}
15902 seq [@var{option}]@dots{} @var{first} @var{last}
15903 seq [@var{option}]@dots{} @var{first} @var{increment} @var{last}
15906 @command{seq} prints the numbers from @var{first} to @var{last} by
15907 @var{increment}. By default, each number is printed on a separate line.
15908 When @var{increment} is not specified, it defaults to @samp{1},
15909 even when @var{first} is larger than @var{last}.
15910 @var{first} also defaults to @samp{1}. So @code{seq 1} prints
15911 @samp{1}, but @code{seq 0} and @code{seq 10 5} produce no output.
15912 Floating-point numbers may be specified. @xref{Floating point}.
15914 The program accepts the following options. Also see @ref{Common options}.
15915 Options must precede operands.
15918 @item -f @var{format}
15919 @itemx --format=@var{format}
15920 @opindex -f @var{format}
15921 @opindex --format=@var{format}
15922 @cindex formatting of numbers in @command{seq}
15923 Print all numbers using @var{format}.
15924 @var{format} must contain exactly one of the @samp{printf}-style
15925 floating point conversion specifications @samp{%a}, @samp{%e},
15926 @samp{%f}, @samp{%g}, @samp{%A}, @samp{%E}, @samp{%F}, @samp{%G}.
15927 The @samp{%} may be followed by zero or more flags taken from the set
15928 @samp{-+#0 '}, then an optional width containing one or more digits,
15929 then an optional precision consisting of a @samp{.} followed by zero
15930 or more digits. @var{format} may also contain any number of @samp{%%}
15931 conversion specifications. All conversion specifications have the
15932 same meaning as with @samp{printf}.
15934 The default format is derived from @var{first}, @var{step}, and
15935 @var{last}. If these all use a fixed point decimal representation,
15936 the default format is @samp{%.@var{p}f}, where @var{p} is the minimum
15937 precision that can represent the output numbers exactly. Otherwise,
15938 the default format is @samp{%g}.
15940 @item -s @var{string}
15941 @itemx --separator=@var{string}
15942 @cindex separator for numbers in @command{seq}
15943 Separate numbers with @var{string}; default is a newline.
15944 The output always terminates with a newline.
15947 @itemx --equal-width
15948 Print all numbers with the same width, by padding with leading zeros.
15949 @var{first}, @var{step}, and @var{last} should all use a fixed point
15950 decimal representation.
15951 (To have other kinds of padding, use @option{--format}).
15955 You can get finer-grained control over output with @option{-f}:
15958 $ seq -f '(%9.2E)' -9e5 1.1e6 1.3e6
15964 If you want hexadecimal integer output, you can use @command{printf}
15965 to perform the conversion:
15968 $ printf '%x\n' `seq 1048575 1024 1050623`
15974 For very long lists of numbers, use xargs to avoid
15975 system limitations on the length of an argument list:
15978 $ seq 1000000 | xargs printf '%x\n' | tail -n 3
15984 To generate octal output, use the printf @code{%o} format instead
15987 On most systems, seq can produce whole-number output for values up to
15988 at least @math{2^{53}}. Larger integers are approximated. The details
15989 differ depending on your floating-point implementation.
15990 @xref{Floating point}. A common
15991 case is that @command{seq} works with integers through @math{2^{64}},
15992 and larger integers may not be numerically correct:
15995 $ seq 18446744073709551616 1 18446744073709551618
15996 18446744073709551616
15997 18446744073709551616
15998 18446744073709551618
16001 Be careful when using @command{seq} with outlandish values: otherwise
16002 you may see surprising results, as @command{seq} uses floating point
16003 internally. For example, on the x86 platform, where the internal
16004 representation uses a 64-bit fraction, the command:
16007 seq 1 0.0000000000000000001 1.0000000000000000009
16010 outputs 1.0000000000000000007 twice and skips 1.0000000000000000008.
16015 @node File permissions
16016 @chapter File permissions
16019 @include parse-datetime.texi
16023 @node Opening the software toolbox
16024 @chapter Opening the Software Toolbox
16026 An earlier version of this chapter appeared in
16027 @uref{http://www.linuxjournal.com/article.php?sid=2762, the
16028 @cite{What's GNU?} column of @cite{Linux Journal}, 2 (June, 1994)}.
16029 It was written by Arnold Robbins.
16032 * Toolbox introduction:: Toolbox introduction
16033 * I/O redirection:: I/O redirection
16034 * The who command:: The @command{who} command
16035 * The cut command:: The @command{cut} command
16036 * The sort command:: The @command{sort} command
16037 * The uniq command:: The @command{uniq} command
16038 * Putting the tools together:: Putting the tools together
16042 @node Toolbox introduction
16043 @unnumberedsec Toolbox Introduction
16045 This month's column is only peripherally related to the GNU Project, in
16046 that it describes a number of the GNU tools on your GNU/Linux system
16048 might be used. What it's really about is the ``Software Tools'' philosophy
16049 of program development and usage.
16051 The software tools philosophy was an important and integral concept
16052 in the initial design and development of Unix (of which Linux and GNU are
16053 essentially clones). Unfortunately, in the modern day press of
16054 Internetworking and flashy GUIs, it seems to have fallen by the
16055 wayside. This is a shame, since it provides a powerful mental model
16056 for solving many kinds of problems.
16058 Many people carry a Swiss Army knife around in their pants pockets (or
16059 purse). A Swiss Army knife is a handy tool to have: it has several knife
16060 blades, a screwdriver, tweezers, toothpick, nail file, corkscrew, and perhaps
16061 a number of other things on it. For the everyday, small miscellaneous jobs
16062 where you need a simple, general purpose tool, it's just the thing.
16064 On the other hand, an experienced carpenter doesn't build a house using
16065 a Swiss Army knife. Instead, he has a toolbox chock full of specialized
16066 tools---a saw, a hammer, a screwdriver, a plane, and so on. And he knows
16067 exactly when and where to use each tool; you won't catch him hammering nails
16068 with the handle of his screwdriver.
16070 The Unix developers at Bell Labs were all professional programmers and trained
16071 computer scientists. They had found that while a one-size-fits-all program
16072 might appeal to a user because there's only one program to use, in practice
16077 difficult to write,
16080 difficult to maintain and
16084 difficult to extend to meet new situations.
16087 Instead, they felt that programs should be specialized tools. In short, each
16088 program ``should do one thing well.'' No more and no less. Such programs are
16089 simpler to design, write, and get right---they only do one thing.
16091 Furthermore, they found that with the right machinery for hooking programs
16092 together, that the whole was greater than the sum of the parts. By combining
16093 several special purpose programs, you could accomplish a specific task
16094 that none of the programs was designed for, and accomplish it much more
16095 quickly and easily than if you had to write a special purpose program.
16096 We will see some (classic) examples of this further on in the column.
16097 (An important additional point was that, if necessary, take a detour
16098 and build any software tools you may need first, if you don't already
16099 have something appropriate in the toolbox.)
16101 @node I/O redirection
16102 @unnumberedsec I/O Redirection
16104 Hopefully, you are familiar with the basics of I/O redirection in the
16105 shell, in particular the concepts of ``standard input,'' ``standard output,''
16106 and ``standard error''. Briefly, ``standard input'' is a data source, where
16107 data comes from. A program should not need to either know or care if the
16108 data source is a disk file, a keyboard, a magnetic tape, or even a punched
16109 card reader. Similarly, ``standard output'' is a data sink, where data goes
16110 to. The program should neither know nor care where this might be.
16111 Programs that only read their standard input, do something to the data,
16112 and then send it on, are called @dfn{filters}, by analogy to filters in a
16115 With the Unix shell, it's very easy to set up data pipelines:
16118 program_to_create_data | filter1 | ... | filterN > final.pretty.data
16121 We start out by creating the raw data; each filter applies some successive
16122 transformation to the data, until by the time it comes out of the pipeline,
16123 it is in the desired form.
16125 This is fine and good for standard input and standard output. Where does the
16126 standard error come in to play? Well, think about @command{filter1} in
16127 the pipeline above. What happens if it encounters an error in the data it
16128 sees? If it writes an error message to standard output, it will just
16129 disappear down the pipeline into @command{filter2}'s input, and the
16130 user will probably never see it. So programs need a place where they can send
16131 error messages so that the user will notice them. This is standard error,
16132 and it is usually connected to your console or window, even if you have
16133 redirected standard output of your program away from your screen.
16135 For filter programs to work together, the format of the data has to be
16136 agreed upon. The most straightforward and easiest format to use is simply
16137 lines of text. Unix data files are generally just streams of bytes, with
16138 lines delimited by the @acronym{ASCII} @sc{lf} (Line Feed) character,
16139 conventionally called a ``newline'' in the Unix literature. (This is
16140 @code{'\n'} if you're a C programmer.) This is the format used by all
16141 the traditional filtering programs. (Many earlier operating systems
16142 had elaborate facilities and special purpose programs for managing
16143 binary data. Unix has always shied away from such things, under the
16144 philosophy that it's easiest to simply be able to view and edit your
16145 data with a text editor.)
16147 OK, enough introduction. Let's take a look at some of the tools, and then
16148 we'll see how to hook them together in interesting ways. In the following
16149 discussion, we will only present those command line options that interest
16150 us. As you should always do, double check your system documentation
16151 for the full story.
16153 @node The who command
16154 @unnumberedsec The @command{who} Command
16156 The first program is the @command{who} command. By itself, it generates a
16157 list of the users who are currently logged in. Although I'm writing
16158 this on a single-user system, we'll pretend that several people are
16163 @print{} arnold console Jan 22 19:57
16164 @print{} miriam ttyp0 Jan 23 14:19(:0.0)
16165 @print{} bill ttyp1 Jan 21 09:32(:0.0)
16166 @print{} arnold ttyp2 Jan 23 20:48(:0.0)
16169 Here, the @samp{$} is the usual shell prompt, at which I typed @samp{who}.
16170 There are three people logged in, and I am logged in twice. On traditional
16171 Unix systems, user names are never more than eight characters long. This
16172 little bit of trivia will be useful later. The output of @command{who} is nice,
16173 but the data is not all that exciting.
16175 @node The cut command
16176 @unnumberedsec The @command{cut} Command
16178 The next program we'll look at is the @command{cut} command. This program
16179 cuts out columns or fields of input data. For example, we can tell it
16180 to print just the login name and full name from the @file{/etc/passwd}
16181 file. The @file{/etc/passwd} file has seven fields, separated by
16185 arnold:xyzzy:2076:10:Arnold D. Robbins:/home/arnold:/bin/bash
16188 To get the first and fifth fields, we would use @command{cut} like this:
16191 $ cut -d: -f1,5 /etc/passwd
16192 @print{} root:Operator
16194 @print{} arnold:Arnold D. Robbins
16195 @print{} miriam:Miriam A. Robbins
16199 With the @option{-c} option, @command{cut} will cut out specific characters
16200 (i.e., columns) in the input lines. This is useful for input data
16201 that has fixed width fields, and does not have a field separator. For
16202 example, list the Monday dates for the current month:
16204 @c Is using cal ok? Looked at gcal, but I don't like it.
16215 @node The sort command
16216 @unnumberedsec The @command{sort} Command
16218 Next we'll look at the @command{sort} command. This is one of the most
16219 powerful commands on a Unix-style system; one that you will often find
16220 yourself using when setting up fancy data plumbing.
16223 command reads and sorts each file named on the command line. It then
16224 merges the sorted data and writes it to standard output. It will read
16225 standard input if no files are given on the command line (thus
16226 making it into a filter). The sort is based on the character collating
16227 sequence or based on user-supplied ordering criteria.
16230 @node The uniq command
16231 @unnumberedsec The @command{uniq} Command
16233 Finally (at least for now), we'll look at the @command{uniq} program. When
16234 sorting data, you will often end up with duplicate lines, lines that
16235 are identical. Usually, all you need is one instance of each line.
16236 This is where @command{uniq} comes in. The @command{uniq} program reads its
16237 standard input. It prints only one
16238 copy of each repeated line. It does have several options. Later on,
16239 we'll use the @option{-c} option, which prints each unique line, preceded
16240 by a count of the number of times that line occurred in the input.
16243 @node Putting the tools together
16244 @unnumberedsec Putting the Tools Together
16246 Now, let's suppose this is a large ISP server system with dozens of users
16247 logged in. The management wants the system administrator to write a
16249 generate a sorted list of logged in users. Furthermore, even if a user
16250 is logged in multiple times, his or her name should only show up in the
16253 The administrator could sit down with the system documentation and write a C
16254 program that did this. It would take perhaps a couple of hundred lines
16255 of code and about two hours to write it, test it, and debug it.
16256 However, knowing the software toolbox, the administrator can instead start out
16257 by generating just a list of logged on users:
16267 Next, sort the list:
16270 $ who | cut -c1-8 | sort
16277 Finally, run the sorted list through @command{uniq}, to weed out duplicates:
16280 $ who | cut -c1-8 | sort | uniq
16286 The @command{sort} command actually has a @option{-u} option that does what
16287 @command{uniq} does. However, @command{uniq} has other uses for which one
16288 cannot substitute @samp{sort -u}.
16290 The administrator puts this pipeline into a shell script, and makes it
16292 all the users on the system (@samp{#} is the system administrator,
16293 or @code{root}, prompt):
16296 # cat > /usr/local/bin/listusers
16297 who | cut -c1-8 | sort | uniq
16299 # chmod +x /usr/local/bin/listusers
16302 There are four major points to note here. First, with just four
16303 programs, on one command line, the administrator was able to save about two
16304 hours worth of work. Furthermore, the shell pipeline is just about as
16305 efficient as the C program would be, and it is much more efficient in
16306 terms of programmer time. People time is much more expensive than
16307 computer time, and in our modern ``there's never enough time to do
16308 everything'' society, saving two hours of programmer time is no mean
16311 Second, it is also important to emphasize that with the
16312 @emph{combination} of the tools, it is possible to do a special
16313 purpose job never imagined by the authors of the individual programs.
16315 Third, it is also valuable to build up your pipeline in stages, as we did here.
16316 This allows you to view the data at each stage in the pipeline, which helps
16317 you acquire the confidence that you are indeed using these tools correctly.
16319 Finally, by bundling the pipeline in a shell script, other users can use
16320 your command, without having to remember the fancy plumbing you set up for
16321 them. In terms of how you run them, shell scripts and compiled programs are
16324 After the previous warm-up exercise, we'll look at two additional, more
16325 complicated pipelines. For them, we need to introduce two more tools.
16327 The first is the @command{tr} command, which stands for ``transliterate.''
16328 The @command{tr} command works on a character-by-character basis, changing
16329 characters. Normally it is used for things like mapping upper case to
16333 $ echo ThIs ExAmPlE HaS MIXED case! | tr '[:upper:]' '[:lower:]'
16334 @print{} this example has mixed case!
16337 There are several options of interest:
16341 work on the complement of the listed characters, i.e.,
16342 operations apply to characters not in the given set
16345 delete characters in the first set from the output
16348 squeeze repeated characters in the output into just one character.
16351 We will be using all three options in a moment.
16353 The other command we'll look at is @command{comm}. The @command{comm}
16354 command takes two sorted input files as input data, and prints out the
16355 files' lines in three columns. The output columns are the data lines
16356 unique to the first file, the data lines unique to the second file, and
16357 the data lines that are common to both. The @option{-1}, @option{-2}, and
16358 @option{-3} command line options @emph{omit} the respective columns. (This is
16359 non-intuitive and takes a little getting used to.) For example:
16381 The file name @file{-} tells @command{comm} to read standard input
16382 instead of a regular file.
16384 Now we're ready to build a fancy pipeline. The first application is a word
16385 frequency counter. This helps an author determine if he or she is over-using
16388 The first step is to change the case of all the letters in our input file
16389 to one case. ``The'' and ``the'' are the same word when doing counting.
16392 $ tr '[:upper:]' '[:lower:]' < whats.gnu | ...
16395 The next step is to get rid of punctuation. Quoted words and unquoted words
16396 should be treated identically; it's easiest to just get the punctuation out of
16400 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | ...
16403 The second @command{tr} command operates on the complement of the listed
16404 characters, which are all the letters, the digits, the underscore, and
16405 the blank. The @samp{\n} represents the newline character; it has to
16406 be left alone. (The @acronym{ASCII} tab character should also be included for
16407 good measure in a production script.)
16409 At this point, we have data consisting of words separated by blank space.
16410 The words only contain alphanumeric characters (and the underscore). The
16411 next step is break the data apart so that we have one word per line. This
16412 makes the counting operation much easier, as we will see shortly.
16415 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16416 > tr -s ' ' '\n' | ...
16419 This command turns blanks into newlines. The @option{-s} option squeezes
16420 multiple newline characters in the output into just one. This helps us
16421 avoid blank lines. (The @samp{>} is the shell's ``secondary prompt.''
16422 This is what the shell prints when it notices you haven't finished
16423 typing in all of a command.)
16425 We now have data consisting of one word per line, no punctuation, all one
16426 case. We're ready to count each word:
16429 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16430 > tr -s ' ' '\n' | sort | uniq -c | ...
16433 At this point, the data might look something like this:
16446 The output is sorted by word, not by count! What we want is the most
16447 frequently used words first. Fortunately, this is easy to accomplish,
16448 with the help of two more @command{sort} options:
16452 do a numeric sort, not a textual one
16455 reverse the order of the sort
16458 The final pipeline looks like this:
16461 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16462 > tr -s ' ' '\n' | sort | uniq -c | sort -n -r
16471 Whew! That's a lot to digest. Yet, the same principles apply. With six
16472 commands, on two lines (really one long one split for convenience), we've
16473 created a program that does something interesting and useful, in much
16474 less time than we could have written a C program to do the same thing.
16476 A minor modification to the above pipeline can give us a simple spelling
16477 checker! To determine if you've spelled a word correctly, all you have to
16478 do is look it up in a dictionary. If it is not there, then chances are
16479 that your spelling is incorrect. So, we need a dictionary.
16480 The conventional location for a dictionary is @file{/usr/dict/words}.
16481 On my GNU/Linux system,@footnote{Redhat Linux 6.1, for the November 2000
16482 revision of this article.}
16483 this is a sorted, 45,402 word dictionary.
16485 Now, how to compare our file with the dictionary? As before, we generate
16486 a sorted list of words, one per line:
16489 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16490 > tr -s ' ' '\n' | sort -u | ...
16493 Now, all we need is a list of words that are @emph{not} in the
16494 dictionary. Here is where the @command{comm} command comes in.
16497 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16498 > tr -s ' ' '\n' | sort -u |
16499 > comm -23 - /usr/dict/words
16502 The @option{-2} and @option{-3} options eliminate lines that are only in the
16503 dictionary (the second file), and lines that are in both files. Lines
16504 only in the first file (standard input, our stream of words), are
16505 words that are not in the dictionary. These are likely candidates for
16506 spelling errors. This pipeline was the first cut at a production
16507 spelling checker on Unix.
16509 There are some other tools that deserve brief mention.
16513 search files for text that matches a regular expression
16516 count lines, words, characters
16519 a T-fitting for data pipes, copies data to files and to standard output
16522 the stream editor, an advanced tool
16525 a data manipulation language, another advanced tool
16528 The software tools philosophy also espoused the following bit of
16529 advice: ``Let someone else do the hard part.'' This means, take
16530 something that gives you most of what you need, and then massage it the
16531 rest of the way until it's in the form that you want.
16537 Each program should do one thing well. No more, no less.
16540 Combining programs with appropriate plumbing leads to results where
16541 the whole is greater than the sum of the parts. It also leads to novel
16542 uses of programs that the authors might never have imagined.
16545 Programs should never print extraneous header or trailer data, since these
16546 could get sent on down a pipeline. (A point we didn't mention earlier.)
16549 Let someone else do the hard part.
16552 Know your toolbox! Use each program appropriately. If you don't have an
16553 appropriate tool, build one.
16556 As of this writing, all the programs we've discussed are available via
16557 anonymous @command{ftp} from: @*
16558 @uref{ftp://gnudist.gnu.org/textutils/textutils-1.22.tar.gz}. (There may
16559 be more recent versions available now.)
16561 None of what I have presented in this column is new. The Software Tools
16562 philosophy was first introduced in the book @cite{Software Tools}, by
16563 Brian Kernighan and P.J. Plauger (Addison-Wesley, ISBN 0-201-03669-X).
16564 This book showed how to write and use software tools. It was written in
16565 1976, using a preprocessor for FORTRAN named @command{ratfor} (RATional
16566 FORtran). At the time, C was not as ubiquitous as it is now; FORTRAN
16567 was. The last chapter presented a @command{ratfor} to FORTRAN
16568 processor, written in @command{ratfor}. @command{ratfor} looks an awful
16569 lot like C; if you know C, you won't have any problem following the
16572 In 1981, the book was updated and made available as @cite{Software Tools
16573 in Pascal} (Addison-Wesley, ISBN 0-201-10342-7). Both books are
16574 still in print and are well worth
16575 reading if you're a programmer. They certainly made a major change in
16576 how I view programming.
16578 The programs in both books are available from
16579 @uref{http://cm.bell-labs.com/who/bwk, Brian Kernighan's home page}.
16580 For a number of years, there was an active
16581 Software Tools Users Group, whose members had ported the original
16582 @command{ratfor} programs to essentially every computer system with a
16583 FORTRAN compiler. The popularity of the group waned in the middle 1980s
16584 as Unix began to spread beyond universities.
16586 With the current proliferation of GNU code and other clones of Unix programs,
16587 these programs now receive little attention; modern C versions are
16588 much more efficient and do more than these programs do. Nevertheless, as
16589 exposition of good programming style, and evangelism for a still-valuable
16590 philosophy, these books are unparalleled, and I recommend them highly.
16592 Acknowledgment: I would like to express my gratitude to Brian Kernighan
16593 of Bell Labs, the original Software Toolsmith, for reviewing this column.
16595 @node GNU Free Documentation License
16596 @appendix GNU Free Documentation License
16600 @node Concept index
16607 @c Local variables:
16608 @c texinfo-column-for-description: 32