3 @setfilename coreutils.info
4 @settitle @sc{gnu} Coreutils
9 @include constants.texi
11 @c Define new indices.
15 @c Put everything in one index (arbitrarily chosen to be the concept index).
25 * Coreutils: (coreutils). Core GNU (file, text, shell) utilities.
26 * Common options: (coreutils)Common options. Common options.
27 * File permissions: (coreutils)File permissions. Access modes.
28 * Date input formats: (coreutils)Date input formats.
31 @c FIXME: the following need documentation
32 @c * [: (coreutils)[ invocation. File/string tests.
33 @c * pinky: (coreutils)pinky invocation. FIXME.
35 @dircategory Individual utilities
37 * arch: (coreutils)arch invocation. Print machine hardware name.
38 * base64: (coreutils)base64 invocation. Base64 encode/decode data.
39 * basename: (coreutils)basename invocation. Strip directory and suffix.
40 * cat: (coreutils)cat invocation. Concatenate and write files.
41 * chcon: (coreutils)chcon invocation. Change SELinux CTX of files.
42 * chgrp: (coreutils)chgrp invocation. Change file groups.
43 * chmod: (coreutils)chmod invocation. Change file permissions.
44 * chown: (coreutils)chown invocation. Change file owners/groups.
45 * chroot: (coreutils)chroot invocation. Specify the root directory.
46 * cksum: (coreutils)cksum invocation. Print POSIX CRC checksum.
47 * comm: (coreutils)comm invocation. Compare sorted files by line.
48 * cp: (coreutils)cp invocation. Copy files.
49 * csplit: (coreutils)csplit invocation. Split by context.
50 * cut: (coreutils)cut invocation. Print selected parts of lines.
51 * date: (coreutils)date invocation. Print/set system date and time.
52 * dd: (coreutils)dd invocation. Copy and convert a file.
53 * df: (coreutils)df invocation. Report file system disk usage.
54 * dir: (coreutils)dir invocation. List directories briefly.
55 * dircolors: (coreutils)dircolors invocation. Color setup for ls.
56 * dirname: (coreutils)dirname invocation. Strip last file name component.
57 * du: (coreutils)du invocation. Report on disk usage.
58 * echo: (coreutils)echo invocation. Print a line of text.
59 * env: (coreutils)env invocation. Modify the environment.
60 * expand: (coreutils)expand invocation. Convert tabs to spaces.
61 * expr: (coreutils)expr invocation. Evaluate expressions.
62 * factor: (coreutils)factor invocation. Print prime factors
63 * false: (coreutils)false invocation. Do nothing, unsuccessfully.
64 * fmt: (coreutils)fmt invocation. Reformat paragraph text.
65 * fold: (coreutils)fold invocation. Wrap long input lines.
66 * groups: (coreutils)groups invocation. Print group names a user is in.
67 * head: (coreutils)head invocation. Output the first part of files.
68 * hostid: (coreutils)hostid invocation. Print numeric host identifier.
69 * hostname: (coreutils)hostname invocation. Print or set system name.
70 * id: (coreutils)id invocation. Print user identity.
71 * install: (coreutils)install invocation. Copy and change attributes.
72 * join: (coreutils)join invocation. Join lines on a common field.
73 * kill: (coreutils)kill invocation. Send a signal to processes.
74 * link: (coreutils)link invocation. Make hard links between files.
75 * ln: (coreutils)ln invocation. Make links between files.
76 * logname: (coreutils)logname invocation. Print current login name.
77 * ls: (coreutils)ls invocation. List directory contents.
78 * md5sum: (coreutils)md5sum invocation. Print or check MD5 digests.
79 * mkdir: (coreutils)mkdir invocation. Create directories.
80 * mkfifo: (coreutils)mkfifo invocation. Create FIFOs (named pipes).
81 * mknod: (coreutils)mknod invocation. Create special files.
82 * mktemp: (coreutils)mktemp invocation. Create temporary files.
83 * mv: (coreutils)mv invocation. Rename files.
84 * nice: (coreutils)nice invocation. Modify niceness.
85 * nl: (coreutils)nl invocation. Number lines and write files.
86 * nohup: (coreutils)nohup invocation. Immunize to hangups.
87 * nproc: (coreutils)nproc invocation. Print the number of processors.
88 * od: (coreutils)od invocation. Dump files in octal, etc.
89 * paste: (coreutils)paste invocation. Merge lines of files.
90 * pathchk: (coreutils)pathchk invocation. Check file name portability.
91 * pr: (coreutils)pr invocation. Paginate or columnate files.
92 * printenv: (coreutils)printenv invocation. Print environment variables.
93 * printf: (coreutils)printf invocation. Format and print data.
94 * ptx: (coreutils)ptx invocation. Produce permuted indexes.
95 * pwd: (coreutils)pwd invocation. Print working directory.
96 * readlink: (coreutils)readlink invocation. Print referent of a symlink.
97 * realpath: (coreutils)readpath invocation. Print resolved file names.
98 * rm: (coreutils)rm invocation. Remove files.
99 * rmdir: (coreutils)rmdir invocation. Remove empty directories.
100 * runcon: (coreutils)runcon invocation. Run in specified SELinux CTX.
101 * seq: (coreutils)seq invocation. Print numeric sequences
102 * sha1sum: (coreutils)sha1sum invocation. Print or check SHA-1 digests.
103 * sha2: (coreutils)sha2 utilities. Print or check SHA-2 digests.
104 * shred: (coreutils)shred invocation. Remove files more securely.
105 * shuf: (coreutils)shuf invocation. Shuffling text files.
106 * sleep: (coreutils)sleep invocation. Delay for a specified time.
107 * sort: (coreutils)sort invocation. Sort text files.
108 * split: (coreutils)split invocation. Split into pieces.
109 * stat: (coreutils)stat invocation. Report file(system) status.
110 * stdbuf: (coreutils)stdbuf invocation. Modify stdio buffering.
111 * stty: (coreutils)stty invocation. Print/change terminal settings.
112 * su: (coreutils)su invocation. Modify user and group ID.
113 * sum: (coreutils)sum invocation. Print traditional checksum.
114 * sync: (coreutils)sync invocation. Synchronize memory and disk.
115 * tac: (coreutils)tac invocation. Reverse files.
116 * tail: (coreutils)tail invocation. Output the last part of files.
117 * tee: (coreutils)tee invocation. Redirect to multiple files.
118 * test: (coreutils)test invocation. File/string tests.
119 * timeout: (coreutils)timeout invocation. Run with time limit.
120 * touch: (coreutils)touch invocation. Change file timestamps.
121 * tr: (coreutils)tr invocation. Translate characters.
122 * true: (coreutils)true invocation. Do nothing, successfully.
123 * truncate: (coreutils)truncate invocation. Shrink/extend size of a file.
124 * tsort: (coreutils)tsort invocation. Topological sort.
125 * tty: (coreutils)tty invocation. Print terminal name.
126 * uname: (coreutils)uname invocation. Print system information.
127 * unexpand: (coreutils)unexpand invocation. Convert spaces to tabs.
128 * uniq: (coreutils)uniq invocation. Uniquify files.
129 * unlink: (coreutils)unlink invocation. Removal via unlink(2).
130 * uptime: (coreutils)uptime invocation. Print uptime and load.
131 * users: (coreutils)users invocation. Print current user names.
132 * vdir: (coreutils)vdir invocation. List directories verbosely.
133 * wc: (coreutils)wc invocation. Line, word, and byte counts.
134 * who: (coreutils)who invocation. Print who is logged in.
135 * whoami: (coreutils)whoami invocation. Print effective user ID.
136 * yes: (coreutils)yes invocation. Print a string indefinitely.
140 This manual documents version @value{VERSION} of the @sc{gnu} core
141 utilities, including the standard programs for text and file manipulation.
143 Copyright @copyright{} 1994-2012 Free Software Foundation, Inc.
146 Permission is granted to copy, distribute and/or modify this document
147 under the terms of the GNU Free Documentation License, Version 1.3 or
148 any later version published by the Free Software Foundation; with no
149 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
150 Texts. A copy of the license is included in the section entitled ``GNU
151 Free Documentation License''.
156 @title @sc{gnu} @code{Coreutils}
157 @subtitle Core GNU utilities
158 @subtitle for version @value{VERSION}, @value{UPDATED}
159 @author David MacKenzie et al.
162 @vskip 0pt plus 1filll
175 @cindex core utilities
176 @cindex text utilities
177 @cindex shell utilities
178 @cindex file utilities
181 * Introduction:: Caveats, overview, and authors
182 * Common options:: Common options
183 * Output of entire files:: cat tac nl od base64
184 * Formatting file contents:: fmt pr fold
185 * Output of parts of files:: head tail split csplit
186 * Summarizing files:: wc sum cksum md5sum sha1sum sha2
187 * Operating on sorted files:: sort shuf uniq comm ptx tsort
188 * Operating on fields:: cut paste join
189 * Operating on characters:: tr expand unexpand
190 * Directory listing:: ls dir vdir dircolors
191 * Basic operations:: cp dd install mv rm shred
192 * Special file types:: mkdir rmdir unlink mkfifo mknod ln link readlink
193 * Changing file attributes:: chgrp chmod chown touch
194 * Disk usage:: df du stat sync truncate
195 * Printing text:: echo printf yes
196 * Conditions:: false true test expr
198 * File name manipulation:: dirname basename pathchk mktemp realpath
199 * Working context:: pwd stty printenv tty
200 * User information:: id logname whoami groups users who
201 * System context:: date arch nproc uname hostname hostid uptime
202 * SELinux context:: chcon runcon
203 * Modified command invocation:: chroot env nice nohup stdbuf su timeout
204 * Process control:: kill
206 * Numeric operations:: factor seq
207 * File permissions:: Access modes
208 * Date input formats:: Specifying date strings
209 * Opening the software toolbox:: The software tools philosophy
210 * GNU Free Documentation License:: Copying and sharing this manual
211 * Concept index:: General index
214 --- The Detailed Node Listing ---
218 * Exit status:: Indicating program success or failure
219 * Backup options:: Backup options
220 * Block size:: Block size
221 * Floating point:: Floating point number representation
222 * Signal specifications:: Specifying signals
223 * Disambiguating names and IDs:: chgrp and chown owner and group syntax
224 * Random sources:: Sources of random data
225 * Target directory:: Target directory
226 * Trailing slashes:: Trailing slashes
227 * Traversing symlinks:: Traversing symlinks to directories
228 * Treating / specially:: Treating / specially
229 * Standards conformance:: Standards conformance
231 Output of entire files
233 * cat invocation:: Concatenate and write files
234 * tac invocation:: Concatenate and write files in reverse
235 * nl invocation:: Number lines and write files
236 * od invocation:: Write files in octal or other formats
237 * base64 invocation:: Transform data into printable data
239 Formatting file contents
241 * fmt invocation:: Reformat paragraph text
242 * pr invocation:: Paginate or columnate files for printing
243 * fold invocation:: Wrap input lines to fit in specified width
245 Output of parts of files
247 * head invocation:: Output the first part of files
248 * tail invocation:: Output the last part of files
249 * split invocation:: Split a file into fixed-size pieces
250 * csplit invocation:: Split a file into context-determined pieces
254 * wc invocation:: Print newline, word, and byte counts
255 * sum invocation:: Print checksum and block counts
256 * cksum invocation:: Print CRC checksum and byte counts
257 * md5sum invocation:: Print or check MD5 digests
258 * sha1sum invocation:: Print or check SHA-1 digests
259 * sha2 utilities:: Print or check SHA-2 digests
261 Operating on sorted files
263 * sort invocation:: Sort text files
264 * shuf invocation:: Shuffle text files
265 * uniq invocation:: Uniquify files
266 * comm invocation:: Compare two sorted files line by line
267 * ptx invocation:: Produce a permuted index of file contents
268 * tsort invocation:: Topological sort
270 @command{ptx}: Produce permuted indexes
272 * General options in ptx:: Options which affect general program behavior
273 * Charset selection in ptx:: Underlying character set considerations
274 * Input processing in ptx:: Input fields, contexts, and keyword selection
275 * Output formatting in ptx:: Types of output format, and sizing the fields
276 * Compatibility in ptx:: The @acronym{GNU} extensions to @command{ptx}
280 * cut invocation:: Print selected parts of lines
281 * paste invocation:: Merge lines of files
282 * join invocation:: Join lines on a common field
284 Operating on characters
286 * tr invocation:: Translate, squeeze, and/or delete characters
287 * expand invocation:: Convert tabs to spaces
288 * unexpand invocation:: Convert spaces to tabs
290 @command{tr}: Translate, squeeze, and/or delete characters
292 * Character sets:: Specifying sets of characters
293 * Translating:: Changing one set of characters to another
294 * Squeezing:: Squeezing repeats and deleting
298 * ls invocation:: List directory contents
299 * dir invocation:: Briefly list directory contents
300 * vdir invocation:: Verbosely list directory contents
301 * dircolors invocation:: Color setup for @command{ls}
303 @command{ls}: List directory contents
305 * Which files are listed:: Which files are listed
306 * What information is listed:: What information is listed
307 * Sorting the output:: Sorting the output
308 * Details about version sort:: More details about version sort
309 * General output formatting:: General output formatting
310 * Formatting the file names:: Formatting the file names
314 * cp invocation:: Copy files and directories
315 * dd invocation:: Convert and copy a file
316 * install invocation:: Copy files and set attributes
317 * mv invocation:: Move (rename) files
318 * rm invocation:: Remove files or directories
319 * shred invocation:: Remove files more securely
323 * link invocation:: Make a hard link via the link syscall
324 * ln invocation:: Make links between files
325 * mkdir invocation:: Make directories
326 * mkfifo invocation:: Make FIFOs (named pipes)
327 * mknod invocation:: Make block or character special files
328 * readlink invocation:: Print value of a symlink or canonical file name
329 * rmdir invocation:: Remove empty directories
330 * unlink invocation:: Remove files via unlink syscall
332 Changing file attributes
334 * chown invocation:: Change file owner and group
335 * chgrp invocation:: Change group ownership
336 * chmod invocation:: Change access permissions
337 * touch invocation:: Change file timestamps
341 * df invocation:: Report file system disk space usage
342 * du invocation:: Estimate file space usage
343 * stat invocation:: Report file or file system status
344 * sync invocation:: Synchronize data on disk with memory
345 * truncate invocation:: Shrink or extend the size of a file
349 * echo invocation:: Print a line of text
350 * printf invocation:: Format and print data
351 * yes invocation:: Print a string until interrupted
355 * false invocation:: Do nothing, unsuccessfully
356 * true invocation:: Do nothing, successfully
357 * test invocation:: Check file types and compare values
358 * expr invocation:: Evaluate expressions
360 @command{test}: Check file types and compare values
362 * File type tests:: File type tests
363 * Access permission tests:: Access permission tests
364 * File characteristic tests:: File characteristic tests
365 * String tests:: String tests
366 * Numeric tests:: Numeric tests
368 @command{expr}: Evaluate expression
370 * String expressions:: + : match substr index length
371 * Numeric expressions:: + - * / %
372 * Relations for expr:: | & < <= = == != >= >
373 * Examples of expr:: Examples of using @command{expr}
377 * tee invocation:: Redirect output to multiple files or processes
379 File name manipulation
381 * basename invocation:: Strip directory and suffix from a file name
382 * dirname invocation:: Strip last file name component
383 * pathchk invocation:: Check file name validity and portability
384 * mktemp invocation:: Create temporary file or directory
385 * realpath invocation:: Print resolved file names
389 * pwd invocation:: Print working directory
390 * stty invocation:: Print or change terminal characteristics
391 * printenv invocation:: Print all or some environment variables
392 * tty invocation:: Print file name of terminal on standard input
394 @command{stty}: Print or change terminal characteristics
396 * Control:: Control settings
397 * Input:: Input settings
398 * Output:: Output settings
399 * Local:: Local settings
400 * Combination:: Combination settings
401 * Characters:: Special characters
402 * Special:: Special settings
406 * id invocation:: Print user identity
407 * logname invocation:: Print current login name
408 * whoami invocation:: Print effective user ID
409 * groups invocation:: Print group names a user is in
410 * users invocation:: Print login names of users currently logged in
411 * who invocation:: Print who is currently logged in
415 * arch invocation:: Print machine hardware name
416 * date invocation:: Print or set system date and time
417 * nproc invocation:: Print the number of processors
418 * uname invocation:: Print system information
419 * hostname invocation:: Print or set system name
420 * hostid invocation:: Print numeric host identifier
421 * uptime invocation:: Print system uptime and load
423 @command{date}: Print or set system date and time
425 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
426 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
427 * Literal conversion specifiers:: %[%nt]
428 * Padding and other flags:: Pad with zeros, spaces, etc.
429 * Setting the time:: Changing the system clock
430 * Options for date:: Instead of the current time
431 * Date input formats:: Specifying date strings
432 * Examples of date:: Examples
436 * chcon invocation:: Change SELinux context of file
437 * runcon invocation:: Run a command in specified SELinux context
439 Modified command invocation
441 * chroot invocation:: Run a command with a different root directory
442 * env invocation:: Run a command in a modified environment
443 * nice invocation:: Run a command with modified niceness
444 * nohup invocation:: Run a command immune to hangups
445 * stdbuf invocation:: Run a command with modified I/O buffering
446 * su invocation:: Run a command with substitute user and group ID
447 * timeout invocation:: Run a command with a time limit
451 * kill invocation:: Sending a signal to processes.
455 * sleep invocation:: Delay for a specified time
459 * factor invocation:: Print prime factors
460 * seq invocation:: Print numeric sequences
464 * Mode Structure:: Structure of file mode bits
465 * Symbolic Modes:: Mnemonic representation of file mode bits
466 * Numeric Modes:: File mode bits as octal numbers
467 * Directory Setuid and Setgid:: Set-user-ID and set-group-ID on directories
471 * General date syntax:: Common rules
472 * Calendar date items:: 19 Dec 1994
473 * Time of day items:: 9:20pm
474 * Time zone items:: @sc{est}, @sc{pdt}, @sc{gmt}
475 * Day of week items:: Monday and others
476 * Relative items in date strings:: next tuesday, 2 years ago
477 * Pure numbers in date strings:: 19931219, 1440
478 * Seconds since the Epoch:: @@1078100502
479 * Specifying time zone rules:: TZ="America/New_York", TZ="UTC0"
480 * Authors of parse_datetime:: Bellovin, Eggert, Salz, Berets, et al
482 Opening the software toolbox
484 * Toolbox introduction:: Toolbox introduction
485 * I/O redirection:: I/O redirection
486 * The who command:: The @command{who} command
487 * The cut command:: The @command{cut} command
488 * The sort command:: The @command{sort} command
489 * The uniq command:: The @command{uniq} command
490 * Putting the tools together:: Putting the tools together
494 * GNU Free Documentation License:: Copying and sharing this manual
501 @chapter Introduction
503 This manual is a work in progress: many sections make no attempt to explain
504 basic concepts in a way suitable for novices. Thus, if you are interested,
505 please get involved in improving this manual. The entire @sc{gnu} community
508 @cindex @acronym{POSIX}
509 The @sc{gnu} utilities documented here are mostly compatible with the
510 @acronym{POSIX} standard.
511 @cindex bugs, reporting
512 Please report bugs to @email{bug-coreutils@@gnu.org}. Remember
513 to include the version number, machine architecture, input files, and
514 any other information needed to reproduce the bug: your input, what you
515 expected, what you got, and why it is wrong. Diffs are welcome, but
516 please include a description of the problem as well, since this is
517 sometimes difficult to infer. @xref{Bugs, , , gcc, Using and Porting GNU CC}.
523 @cindex MacKenzie, D.
526 This manual was originally derived from the Unix man pages in the
527 distributions, which were written by David MacKenzie and updated by Jim
528 Meyering. What you are reading now is the authoritative documentation
529 for these utilities; the man pages are no longer being maintained. The
530 original @command{fmt} man page was written by Ross Paterson. Fran@,{c}ois
531 Pinard did the initial conversion to Texinfo format. Karl Berry did the
532 indexing, some reorganization, and editing of the results. Brian
533 Youmans of the Free Software Foundation office staff combined the
534 manuals for textutils, fileutils, and sh-utils to produce the present
535 omnibus manual. Richard Stallman contributed his usual invaluable
536 insights to the overall process.
539 @chapter Common options
543 @itemx @w{@kbd{--backup}[=@var{method}]}
546 @vindex VERSION_CONTROL
547 @cindex backups, making
548 @xref{Backup options}.
549 Make a backup of each file that would otherwise be overwritten or removed.
552 @macro optBackupSuffix
553 @item -S @var{suffix}
554 @itemx --suffix=@var{suffix}
557 Append @var{suffix} to each backup file made with @option{-b}.
558 @xref{Backup options}.
561 @macro optTargetDirectory
562 @item -t @var{directory}
563 @itemx @w{@kbd{--target-directory}=@var{directory}}
565 @opindex --target-directory
566 @cindex target directory
567 @cindex destination directory
568 Specify the destination @var{directory}.
569 @xref{Target directory}.
572 @macro optNoTargetDirectory
574 @itemx --no-target-directory
576 @opindex --no-target-directory
577 @cindex target directory
578 @cindex destination directory
579 Do not treat the last operand specially when it is a directory or a
580 symbolic link to a directory. @xref{Target directory}.
588 @cindex output @sc{nul}-byte-terminated lines
589 Output a zero byte (@acronym{ASCII} @sc{nul}) at the end of each line,
590 rather than a newline. This option enables other programs to parse the
591 output of @command{\cmd\} even when that output would contain data
592 with embedded newlines.
599 Append an SI-style abbreviation to each size, such as @samp{M} for
600 megabytes. Powers of 1000 are used, not 1024; @samp{M} stands for
601 1,000,000 bytes. This option is equivalent to
602 @option{--block-size=si}. Use the @option{-h} or
603 @option{--human-readable} option if
604 you prefer powers of 1024.
607 @macro optHumanReadable
609 @itemx --human-readable
611 @opindex --human-readable
612 @cindex human-readable output
613 Append a size letter to each size, such as @samp{M} for mebibytes.
614 Powers of 1024 are used, not 1000; @samp{M} stands for 1,048,576 bytes.
615 This option is equivalent to @option{--block-size=human-readable}.
616 Use the @option{--si} option if you prefer powers of 1000.
619 @macro optStripTrailingSlashes
620 @itemx @w{@kbd{--strip-trailing-slashes}}
621 @opindex --strip-trailing-slashes
622 @cindex stripping trailing slashes
623 Remove any trailing slashes from each @var{source} argument.
624 @xref{Trailing slashes}.
627 @macro mayConflictWithShellBuiltIn{cmd}
628 @cindex conflicts with shell built-ins
629 @cindex built-in shell commands, conflicts with
630 Due to shell aliases and built-in @command{\cmd\} functions, using an
631 unadorned @command{\cmd\} interactively or in a script may get you
632 different functionality than that described here. Invoke it via
633 @command{env} (i.e., @code{env \cmd\ @dots{}}) to avoid interference
638 @macro multiplierSuffixes{varName}
639 @var{\varName\} may be, or may be an integer optionally followed by,
640 one of the following multiplicative suffixes:
642 @samp{b} => 512 ("blocks")
643 @samp{KB} => 1000 (KiloBytes)
644 @samp{K} => 1024 (KibiBytes)
645 @samp{MB} => 1000*1000 (MegaBytes)
646 @samp{M} => 1024*1024 (MebiBytes)
647 @samp{GB} => 1000*1000*1000 (GigaBytes)
648 @samp{G} => 1024*1024*1024 (GibiBytes)
650 and so on for @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}.
653 @c FIXME: same as above, but no ``blocks'' line.
654 @macro multiplierSuffixesNoBlocks{varName}
655 @var{\varName\} may be, or may be an integer optionally followed by,
656 one of the following multiplicative suffixes:
658 @samp{KB} => 1000 (KiloBytes)
659 @samp{K} => 1024 (KibiBytes)
660 @samp{MB} => 1000*1000 (MegaBytes)
661 @samp{M} => 1024*1024 (MebiBytes)
662 @samp{GB} => 1000*1000*1000 (GigaBytes)
663 @samp{G} => 1024*1024*1024 (GibiBytes)
665 and so on for @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}.
668 @cindex common options
670 Certain options are available in all of these programs. Rather than
671 writing identical descriptions for each of the programs, they are
672 described here. (In fact, every @sc{gnu} program accepts (or should accept)
675 @vindex POSIXLY_CORRECT
676 Normally options and operands can appear in any order, and programs act
677 as if all the options appear before any operands. For example,
678 @samp{sort -r passwd -t :} acts like @samp{sort -r -t : passwd}, since
679 @samp{:} is an option-argument of @option{-t}. However, if the
680 @env{POSIXLY_CORRECT} environment variable is set, options must appear
681 before operands, unless otherwise specified for a particular command.
683 A few programs can usefully have trailing operands with leading
684 @samp{-}. With such a program, options must precede operands even if
685 @env{POSIXLY_CORRECT} is not set, and this fact is noted in the
686 program description. For example, the @command{env} command's options
687 must appear before its operands, since in some cases the operands
688 specify a command that itself contains options.
690 Most programs that accept long options recognize unambiguous
691 abbreviations of those options. For example, @samp{rmdir
692 --ignore-fail-on-non-empty} can be invoked as @samp{rmdir
693 --ignore-fail} or even @samp{rmdir --i}. Ambiguous options, such as
694 @samp{ls --h}, are identified as such.
696 Some of these programs recognize the @option{--help} and @option{--version}
697 options only when one of them is the sole command line argument. For
698 these programs, abbreviations of the long options are not always recognized.
705 Print a usage message listing all available options, then exit successfully.
709 @cindex version number, finding
710 Print the version number, then exit successfully.
714 @cindex option delimiter
715 Delimit the option list. Later arguments, if any, are treated as
716 operands even if they begin with @samp{-}. For example, @samp{sort --
717 -r} reads from the file named @file{-r}.
721 @cindex standard input
722 @cindex standard output
723 A single @samp{-} operand is not really an option, though it looks like one. It
724 stands for standard input, or for standard output if that is clear from
725 the context. For example, @samp{sort -} reads from standard input,
726 and is equivalent to plain @samp{sort}, and @samp{tee -} writes an
727 extra copy of its input to standard output. Unless otherwise
728 specified, @samp{-} can appear as any operand that requires a file
732 * Exit status:: Indicating program success or failure.
733 * Backup options:: -b -S, in some programs.
734 * Block size:: BLOCK_SIZE and --block-size, in some programs.
735 * Floating point:: Floating point number representation.
736 * Signal specifications:: Specifying signals using the --signal option.
737 * Disambiguating names and IDs:: chgrp and chown owner and group syntax
738 * Random sources:: --random-source, in some programs.
739 * Target directory:: Specifying a target directory, in some programs.
740 * Trailing slashes:: --strip-trailing-slashes, in some programs.
741 * Traversing symlinks:: -H, -L, or -P, in some programs.
742 * Treating / specially:: --preserve-root and --no-preserve-root.
743 * Special built-in utilities:: @command{break}, @command{:}, @dots{}
744 * Standards conformance:: Conformance to the @acronym{POSIX} standard.
752 An exit status of zero indicates success,
753 and a nonzero value indicates failure.
756 Nearly every command invocation yields an integral @dfn{exit status}
757 that can be used to change how other commands work.
758 For the vast majority of commands, an exit status of zero indicates
759 success. Failure is indicated by a nonzero value---typically
760 @samp{1}, though it may differ on unusual platforms as @acronym{POSIX}
761 requires only that it be nonzero.
763 However, some of the programs documented here do produce
764 other exit status values and a few associate different
765 meanings with the values @samp{0} and @samp{1}.
766 Here are some of the exceptions:
767 @command{chroot}, @command{env}, @command{expr}, @command{nice},
768 @command{nohup}, @command{printenv}, @command{sort}, @command{stdbuf},
769 @command{su}, @command{test}, @command{timeout}, @command{tty}.
773 @section Backup options
775 @cindex backup options
777 Some @sc{gnu} programs (at least @command{cp}, @command{install},
778 @command{ln}, and @command{mv}) optionally make backups of files
779 before writing new versions.
780 These options control the details of these backups. The options are also
781 briefly mentioned in the descriptions of the particular programs.
786 @itemx @w{@kbd{--backup}[=@var{method}]}
789 @vindex VERSION_CONTROL
790 @cindex backups, making
791 Make a backup of each file that would otherwise be overwritten or removed.
792 Without this option, the original versions are destroyed.
793 Use @var{method} to determine the type of backups to make.
794 When this option is used but @var{method} is not specified,
795 then the value of the @env{VERSION_CONTROL}
796 environment variable is used. And if @env{VERSION_CONTROL} is not set,
797 the default backup type is @samp{existing}.
799 Note that the short form of this option, @option{-b} does not accept any
800 argument. Using @option{-b} is equivalent to using @option{--backup=existing}.
802 @vindex version-control @r{Emacs variable}
803 This option corresponds to the Emacs variable @samp{version-control};
804 the values for @var{method} are the same as those used in Emacs.
805 This option also accepts more descriptive names.
806 The valid @var{method}s are (unique abbreviations are accepted):
811 @opindex none @r{backup method}
816 @opindex numbered @r{backup method}
817 Always make numbered backups.
821 @opindex existing @r{backup method}
822 Make numbered backups of files that already have them, simple backups
827 @opindex simple @r{backup method}
828 Always make simple backups. Please note @samp{never} is not to be
829 confused with @samp{none}.
833 @item -S @var{suffix}
834 @itemx --suffix=@var{suffix}
837 @cindex backup suffix
838 @vindex SIMPLE_BACKUP_SUFFIX
839 Append @var{suffix} to each backup file made with @option{-b}. If this
840 option is not specified, the value of the @env{SIMPLE_BACKUP_SUFFIX}
841 environment variable is used. And if @env{SIMPLE_BACKUP_SUFFIX} is not
842 set, the default is @samp{~}, just as in Emacs.
851 Some @sc{gnu} programs (at least @command{df}, @command{du}, and
852 @command{ls}) display sizes in ``blocks''. You can adjust the block size
853 and method of display to make sizes easier to read. The block size
854 used for display is independent of any file system block size.
855 Fractional block counts are rounded up to the nearest integer.
857 @opindex --block-size=@var{size}
860 @vindex DF_BLOCK_SIZE
861 @vindex DU_BLOCK_SIZE
862 @vindex LS_BLOCK_SIZE
863 @vindex POSIXLY_CORRECT@r{, and block size}
865 The default block size is chosen by examining the following environment
866 variables in turn; the first one that is set determines the block size.
871 This specifies the default block size for the @command{df} command.
872 Similarly, @env{DU_BLOCK_SIZE} specifies the default for @command{du} and
873 @env{LS_BLOCK_SIZE} for @command{ls}.
876 This specifies the default block size for all three commands, if the
877 above command-specific environment variables are not set.
880 This specifies the default block size for all values that are normally
881 printed as blocks, if neither @env{BLOCK_SIZE} nor the above
882 command-specific environment variables are set. Unlike the other
883 environment variables, @env{BLOCKSIZE} does not affect values that are
884 normally printed as byte counts, e.g., the file sizes contained in
887 @item POSIXLY_CORRECT
888 If neither @env{@var{command}_BLOCK_SIZE}, nor @env{BLOCK_SIZE}, nor
889 @env{BLOCKSIZE} is set, but this variable is set, the block size
894 If none of the above environment variables are set, the block size
895 currently defaults to 1024 bytes in most contexts, but this number may
896 change in the future. For @command{ls} file sizes, the block size
899 @cindex human-readable output
902 A block size specification can be a positive integer specifying the number
903 of bytes per block, or it can be @code{human-readable} or @code{si} to
904 select a human-readable format. Integers may be followed by suffixes
905 that are upward compatible with the
906 @uref{http://www.bipm.org/en/si/si_brochure/chapter3/prefixes.html, SI prefixes}
907 for decimal multiples and with the
908 @uref{http://physics.nist.gov/cuu/Units/binary.html, ISO/IEC 80000-13
909 (formerly IEC 60027-2) prefixes} for binary multiples.
911 With human-readable formats, output sizes are followed by a size letter
912 such as @samp{M} for megabytes. @code{BLOCK_SIZE=human-readable} uses
913 powers of 1024; @samp{M} stands for 1,048,576 bytes.
914 @code{BLOCK_SIZE=si} is similar, but uses powers of 1000 and appends
915 @samp{B}; @samp{MB} stands for 1,000,000 bytes.
918 A block size specification preceded by @samp{'} causes output sizes to
919 be displayed with thousands separators. The @env{LC_NUMERIC} locale
920 specifies the thousands separator and grouping. For example, in an
921 American English locale, @samp{--block-size="'1kB"} would cause a size
922 of 1234000 bytes to be displayed as @samp{1,234}. In the default C
923 locale, there is no thousands separator so a leading @samp{'} has no
926 An integer block size can be followed by a suffix to specify a
927 multiple of that size. A bare size letter,
928 or one followed by @samp{iB}, specifies
929 a multiple using powers of 1024. A size letter followed by @samp{B}
930 specifies powers of 1000 instead. For example, @samp{1M} and
931 @samp{1MiB} are equivalent to @samp{1048576}, whereas @samp{1MB} is
932 equivalent to @samp{1000000}.
934 A plain suffix without a preceding integer acts as if @samp{1} were
935 prepended, except that it causes a size indication to be appended to
936 the output. For example, @samp{--block-size="kB"} displays 3000 as
939 The following suffixes are defined. Large sizes like @code{1Y}
940 may be rejected by your computer due to limitations of its arithmetic.
944 @cindex kilobyte, definition of
945 kilobyte: @math{10^3 = 1000}.
949 @cindex kibibyte, definition of
950 kibibyte: @math{2^{10} = 1024}. @samp{K} is special: the SI prefix is
951 @samp{k} and the ISO/IEC 80000-13 prefix is @samp{Ki}, but tradition and
952 @acronym{POSIX} use @samp{k} to mean @samp{KiB}.
954 @cindex megabyte, definition of
955 megabyte: @math{10^6 = 1,000,000}.
958 @cindex mebibyte, definition of
959 mebibyte: @math{2^{20} = 1,048,576}.
961 @cindex gigabyte, definition of
962 gigabyte: @math{10^9 = 1,000,000,000}.
965 @cindex gibibyte, definition of
966 gibibyte: @math{2^{30} = 1,073,741,824}.
968 @cindex terabyte, definition of
969 terabyte: @math{10^{12} = 1,000,000,000,000}.
972 @cindex tebibyte, definition of
973 tebibyte: @math{2^{40} = 1,099,511,627,776}.
975 @cindex petabyte, definition of
976 petabyte: @math{10^{15} = 1,000,000,000,000,000}.
979 @cindex pebibyte, definition of
980 pebibyte: @math{2^{50} = 1,125,899,906,842,624}.
982 @cindex exabyte, definition of
983 exabyte: @math{10^{18} = 1,000,000,000,000,000,000}.
986 @cindex exbibyte, definition of
987 exbibyte: @math{2^{60} = 1,152,921,504,606,846,976}.
989 @cindex zettabyte, definition of
990 zettabyte: @math{10^{21} = 1,000,000,000,000,000,000,000}
993 @math{2^{70} = 1,180,591,620,717,411,303,424}.
995 @cindex yottabyte, definition of
996 yottabyte: @math{10^{24} = 1,000,000,000,000,000,000,000,000}.
999 @math{2^{80} = 1,208,925,819,614,629,174,706,176}.
1004 @opindex --block-size
1005 @opindex --human-readable
1008 Block size defaults can be overridden by an explicit
1009 @option{--block-size=@var{size}} option. The @option{-k}
1010 option is equivalent to @option{--block-size=1K}, which
1011 is the default unless the @env{POSIXLY_CORRECT} environment variable is
1012 set. The @option{-h} or @option{--human-readable} option is equivalent to
1013 @option{--block-size=human-readable}. The @option{--si} option is
1014 equivalent to @option{--block-size=si}.
1016 @node Floating point
1017 @section Floating point numbers
1018 @cindex floating point
1019 @cindex IEEE floating point
1021 Commands that accept or produce floating point numbers employ the
1022 floating point representation of the underlying system, and suffer
1023 from rounding error, overflow, and similar floating-point issues.
1024 Almost all modern systems use IEEE-754 floating point, and it is
1025 typically portable to assume IEEE-754 behavior these days. IEEE-754
1026 has positive and negative infinity, distinguishes positive from
1027 negative zero, and uses special values called NaNs to represent
1028 invalid computations such as dividing zero by itself. For more
1029 information, please see David Goldberg's paper
1030 @uref{http://@/www.validlab.com/@/goldberg/@/paper.pdf, What Every
1031 Computer Scientist Should Know About Floating-Point Arithmetic}.
1034 Commands that accept floating point numbers as options, operands or
1035 input use the standard C functions @code{strtod} and @code{strtold} to
1036 convert from text to floating point numbers. These floating point
1037 numbers therefore can use scientific notation like @code{1.0e-34} and
1038 @code{-10e100}. Modern C implementations also accept hexadecimal
1039 floating point numbers such as @code{-0x.ep-3}, which stands for
1040 @minus{}14/16 times @math{2^-3}, which equals @minus{}0.109375. The
1041 @env{LC_NUMERIC} locale determines the decimal-point character.
1042 @xref{Parsing of Floats,,, libc, The GNU C Library Reference Manual}.
1044 @node Signal specifications
1045 @section Signal specifications
1046 @cindex signals, specifying
1048 A @var{signal} may be a signal name like @samp{HUP}, or a signal
1049 number like @samp{1}, or an exit status of a process terminated by the
1050 signal. A signal name can be given in canonical form or prefixed by
1051 @samp{SIG}. The case of the letters is ignored. The following signal names
1052 and numbers are supported on all @acronym{POSIX} compliant systems:
1058 2. Terminal interrupt.
1064 9. Kill (cannot be caught or ignored).
1072 Other supported signal names have system-dependent corresponding
1073 numbers. All systems conforming to @acronym{POSIX} 1003.1-2001 also
1074 support the following signals:
1078 Access to an undefined portion of a memory object.
1080 Child process terminated, stopped, or continued.
1082 Continue executing, if stopped.
1084 Erroneous arithmetic operation.
1086 Illegal Instruction.
1088 Write on a pipe with no one to read it.
1090 Invalid memory reference.
1092 Stop executing (cannot be caught or ignored).
1096 Background process attempting read.
1098 Background process attempting write.
1100 High bandwidth data is available at a socket.
1102 User-defined signal 1.
1104 User-defined signal 2.
1108 @acronym{POSIX} 1003.1-2001 systems that support the @acronym{XSI} extension
1109 also support the following signals:
1115 Profiling timer expired.
1119 Trace/breakpoint trap.
1121 Virtual timer expired.
1123 CPU time limit exceeded.
1125 File size limit exceeded.
1129 @acronym{POSIX} 1003.1-2001 systems that support the @acronym{XRT} extension
1130 also support at least eight real-time signals called @samp{RTMIN},
1131 @samp{RTMIN+1}, @dots{}, @samp{RTMAX-1}, @samp{RTMAX}.
1133 @node Disambiguating names and IDs
1134 @section chown and chgrp: Disambiguating user names and IDs
1135 @cindex user names, disambiguating
1136 @cindex user IDs, disambiguating
1137 @cindex group names, disambiguating
1138 @cindex group IDs, disambiguating
1139 @cindex disambiguating group names and IDs
1141 Since the @var{owner} and @var{group} arguments to @command{chown} and
1142 @command{chgrp} may be specified as names or numeric IDs, there is an
1144 What if a user or group @emph{name} is a string of digits?
1145 @footnote{Using a number as a user name is common in some environments.}
1146 Should the command interpret it as a user name or as an ID?
1147 @acronym{POSIX} requires that @command{chown} and @command{chgrp}
1148 first attempt to resolve the specified string as a name, and
1149 only once that fails, then try to interpret it as an ID.
1150 This is troublesome when you want to specify a numeric ID, say 42,
1151 and it must work even in a pathological situation where
1152 @samp{42} is a user name that maps to some other user ID, say 1000.
1153 Simply invoking @code{chown 42 F}, will set @file{F}s owner ID to
1154 1000---not what you intended.
1156 GNU @command{chown} and @command{chgrp} provide a way to work around this,
1157 that at the same time may result in a significant performance improvement
1158 by eliminating a database look-up.
1159 Simply precede each numeric user ID and/or group ID with a @samp{+},
1160 in order to force its interpretation as an integer:
1164 chgrp +$numeric_group_id another-file
1168 GNU @command{chown} and @command{chgrp}
1169 skip the name look-up process for each @samp{+}-prefixed string,
1170 because a string containing @samp{+} is never a valid user or group name.
1171 This syntax is accepted on most common Unix systems, but not on Solaris 10.
1173 @node Random sources
1174 @section Sources of random data
1176 @cindex random sources
1178 The @command{shuf}, @command{shred}, and @command{sort} commands
1179 sometimes need random data to do their work. For example, @samp{sort
1180 -R} must choose a hash function at random, and it needs random data to
1181 make this selection.
1183 By default these commands use an internal pseudorandom generator
1184 initialized by a small amount of entropy, but can be directed to use
1185 an external source with the @option{--random-source=@var{file}} option.
1186 An error is reported if @var{file} does not contain enough bytes.
1188 For example, the device file @file{/dev/urandom} could be used as the
1189 source of random data. Typically, this device gathers environmental
1190 noise from device drivers and other sources into an entropy pool, and
1191 uses the pool to generate random bits. If the pool is short of data,
1192 the device reuses the internal pool to produce more bits, using a
1193 cryptographically secure pseudorandom number generator. But be aware
1194 that this device is not designed for bulk random data generation
1195 and is relatively slow.
1197 @file{/dev/urandom} suffices for most practical uses, but applications
1198 requiring high-value or long-term protection of private data may
1199 require an alternate data source like @file{/dev/random} or
1200 @file{/dev/arandom}. The set of available sources depends on your
1203 To reproduce the results of an earlier invocation of a command, you
1204 can save some random data into a file and then use that file as the
1205 random source in earlier and later invocations of the command.
1207 @node Target directory
1208 @section Target directory
1210 @cindex target directory
1212 The @command{cp}, @command{install}, @command{ln}, and @command{mv}
1213 commands normally treat the last operand specially when it is a
1214 directory or a symbolic link to a directory. For example, @samp{cp
1215 source dest} is equivalent to @samp{cp source dest/source} if
1216 @file{dest} is a directory. Sometimes this behavior is not exactly
1217 what is wanted, so these commands support the following options to
1218 allow more fine-grained control:
1223 @itemx --no-target-directory
1224 @opindex --no-target-directory
1225 @cindex target directory
1226 @cindex destination directory
1227 Do not treat the last operand specially when it is a directory or a
1228 symbolic link to a directory. This can help avoid race conditions in
1229 programs that operate in a shared area. For example, when the command
1230 @samp{mv /tmp/source /tmp/dest} succeeds, there is no guarantee that
1231 @file{/tmp/source} was renamed to @file{/tmp/dest}: it could have been
1232 renamed to @file{/tmp/dest/source} instead, if some other process
1233 created @file{/tmp/dest} as a directory. However, if @file{mv
1234 -T /tmp/source /tmp/dest} succeeds, there is no
1235 question that @file{/tmp/source} was renamed to @file{/tmp/dest}.
1237 In the opposite situation, where you want the last operand to be
1238 treated as a directory and want a diagnostic otherwise, you can use
1239 the @option{--target-directory} (@option{-t}) option.
1241 @item -t @var{directory}
1242 @itemx @w{@kbd{--target-directory}=@var{directory}}
1243 @opindex --target-directory
1244 @cindex target directory
1245 @cindex destination directory
1246 Use @var{directory} as the directory component of each destination
1249 The interface for most programs is that after processing options and a
1250 finite (possibly zero) number of fixed-position arguments, the remaining
1251 argument list is either expected to be empty, or is a list of items
1252 (usually files) that will all be handled identically. The @command{xargs}
1253 program is designed to work well with this convention.
1255 The commands in the @command{mv}-family are unusual in that they take
1256 a variable number of arguments with a special case at the @emph{end}
1257 (namely, the target directory). This makes it nontrivial to perform some
1258 operations, e.g., ``move all files from here to ../d/'', because
1259 @code{mv * ../d/} might exhaust the argument space, and @code{ls | xargs ...}
1260 doesn't have a clean way to specify an extra final argument for each
1261 invocation of the subject command. (It can be done by going through a
1262 shell command, but that requires more human labor and brain power than
1265 The @w{@kbd{--target-directory}} (@option{-t}) option allows the @command{cp},
1266 @command{install}, @command{ln}, and @command{mv} programs to be used
1267 conveniently with @command{xargs}. For example, you can move the files
1268 from the current directory to a sibling directory, @code{d} like this:
1271 ls | xargs mv -t ../d --
1274 However, this doesn't move files whose names begin with @samp{.}.
1275 If you use the @sc{gnu} @command{find} program, you can move those
1276 files too, with this command:
1279 find . -mindepth 1 -maxdepth 1 \
1283 But both of the above approaches fail if there are no files in the
1284 current directory, or if any file has a name containing a blank or
1285 some other special characters.
1286 The following example removes those limitations and requires both
1287 @sc{gnu} @command{find} and @sc{gnu} @command{xargs}:
1290 find . -mindepth 1 -maxdepth 1 -print0 \
1291 | xargs --null --no-run-if-empty \
1298 The @option{--target-directory} (@option{-t}) and
1299 @option{--no-target-directory} (@option{-T})
1300 options cannot be combined.
1302 @node Trailing slashes
1303 @section Trailing slashes
1305 @cindex trailing slashes
1307 Some @sc{gnu} programs (at least @command{cp} and @command{mv}) allow you to
1308 remove any trailing slashes from each @var{source} argument before
1309 operating on it. The @w{@kbd{--strip-trailing-slashes}} option enables
1312 This is useful when a @var{source} argument may have a trailing slash and
1313 @c FIXME: mv's behavior in this case is system-dependent
1314 specify a symbolic link to a directory. This scenario is in fact rather
1315 common because some shells can automatically append a trailing slash when
1316 performing file name completion on such symbolic links. Without this
1317 option, @command{mv}, for example, (via the system's rename function) must
1318 interpret a trailing slash as a request to dereference the symbolic link
1319 and so must rename the indirectly referenced @emph{directory} and not
1320 the symbolic link. Although it may seem surprising that such behavior
1321 be the default, it is required by @acronym{POSIX} and is consistent with
1322 other parts of that standard.
1324 @node Traversing symlinks
1325 @section Traversing symlinks
1327 @cindex symbolic link to directory, controlling traversal of
1329 The following options modify how @command{chown} and @command{chgrp}
1330 @c FIXME: note that 'du' has these options, too, but they have slightly
1331 @c different meaning.
1332 traverse a hierarchy when the @option{--recursive} (@option{-R})
1333 option is also specified.
1334 If more than one of the following options is specified, only the final
1336 These options specify whether processing a symbolic link to a directory
1337 entails operating on just the symbolic link or on all files in the
1338 hierarchy rooted at that directory.
1340 These options are independent of @option{--dereference} and
1341 @option{--no-dereference} (@option{-h}), which control whether to modify
1342 a symlink or its referent.
1349 @cindex symbolic link to directory, traverse if on the command line
1350 If @option{--recursive} (@option{-R}) is specified and
1351 a command line argument is a symbolic link to a directory, traverse it.
1358 @cindex symbolic link to directory, traverse each that is encountered
1359 In a recursive traversal, traverse every symbolic link to a directory
1360 that is encountered.
1367 @cindex symbolic link to directory, never traverse
1368 Do not traverse any symbolic links.
1369 This is the default if none of @option{-H}, @option{-L},
1370 or @option{-P} is specified.
1377 @node Treating / specially
1378 @section Treating @file{/} specially
1380 Certain commands can operate destructively on entire hierarchies.
1381 For example, if a user with appropriate privileges mistakenly runs
1382 @samp{rm -rf / tmp/junk}, that may remove
1383 all files on the entire system. Since there are so few
1384 legitimate uses for such a command,
1385 @sc{gnu} @command{rm} normally declines to operate on any directory
1386 that resolves to @file{/}. If you really want to try to remove all
1387 the files on your system, you can use the @option{--no-preserve-root}
1388 option, but the default behavior, specified by the
1389 @option{--preserve-option}, is safer for most purposes.
1391 The commands @command{chgrp}, @command{chmod} and @command{chown}
1392 can also operate destructively on entire hierarchies, so they too
1393 support these options. Although, unlike @command{rm}, they don't
1394 actually unlink files, these commands are arguably more dangerous
1395 when operating recursively on @file{/}, since they often work much
1396 more quickly, and hence damage more files before an alert user can
1397 interrupt them. Tradition and @acronym{POSIX} require these commands
1398 to operate recursively on @file{/}, so they default to
1399 @option{--no-preserve-root}, but using the @option{--preserve-root}
1400 option makes them safer for most purposes. For convenience you can
1401 specify @option{--preserve-root} in an alias or in a shell function.
1403 Note that the @option{--preserve-root} option also ensures
1404 that @command{chgrp} and @command{chown} do not modify @file{/}
1405 even when dereferencing a symlink pointing to @file{/}.
1407 @node Special built-in utilities
1408 @section Special built-in utilities
1410 Some programs like @command{nice} can invoke other programs; for
1411 example, the command @samp{nice cat file} invokes the program
1412 @command{cat} by executing the command @samp{cat file}. However,
1413 @dfn{special built-in utilities} like @command{exit} cannot be invoked
1414 this way. For example, the command @samp{nice exit} does not have a
1415 well-defined behavior: it may generate an error message instead of
1418 Here is a list of the special built-in utilities that are standardized
1419 by @acronym{POSIX} 1003.1-2004.
1422 @t{.@: : break continue eval exec exit export readonly
1423 return set shift times trap unset}
1426 For example, because @samp{.}, @samp{:}, and @samp{exec} are special,
1427 the commands @samp{nice . foo.sh}, @samp{nice :}, and @samp{nice exec
1428 pwd} do not work as you might expect.
1430 Many shells extend this list. For example, Bash has several extra
1431 special built-in utilities like @command{history}, and
1432 @command{suspend}, and with Bash the command @samp{nice suspend}
1433 generates an error message instead of suspending.
1435 @node Standards conformance
1436 @section Standards conformance
1438 @vindex POSIXLY_CORRECT
1439 In a few cases, the @sc{gnu} utilities' default behavior is
1440 incompatible with the @acronym{POSIX} standard. To suppress these
1441 incompatibilities, define the @env{POSIXLY_CORRECT} environment
1442 variable. Unless you are checking for @acronym{POSIX} conformance, you
1443 probably do not need to define @env{POSIXLY_CORRECT}.
1445 Newer versions of @acronym{POSIX} are occasionally incompatible with older
1446 versions. For example, older versions of @acronym{POSIX} required the
1447 command @samp{sort +1} to sort based on the second and succeeding
1448 fields in each input line, but starting with @acronym{POSIX} 1003.1-2001
1449 the same command is required to sort the file named @file{+1}, and you
1450 must instead use the command @samp{sort -k 2} to get the field-based
1453 @vindex _POSIX2_VERSION
1454 The @sc{gnu} utilities normally conform to the version of @acronym{POSIX}
1455 that is standard for your system. To cause them to conform to a
1456 different version of @acronym{POSIX}, define the @env{_POSIX2_VERSION}
1457 environment variable to a value of the form @var{yyyymm} specifying
1458 the year and month the standard was adopted. Three values are currently
1459 supported for @env{_POSIX2_VERSION}: @samp{199209} stands for
1460 @acronym{POSIX} 1003.2-1992, @samp{200112} stands for @acronym{POSIX}
1461 1003.1-2001, and @samp{200809} stands for @acronym{POSIX} 1003.1-2008.
1462 For example, if you have a newer system but are running software
1463 that assumes an older version of @acronym{POSIX} and uses @samp{sort +1}
1464 or @samp{tail +10}, you can work around any compatibility problems by setting
1465 @samp{_POSIX2_VERSION=199209} in your environment.
1467 @node Output of entire files
1468 @chapter Output of entire files
1470 @cindex output of entire files
1471 @cindex entire files, output of
1473 These commands read and write entire files, possibly transforming them
1477 * cat invocation:: Concatenate and write files.
1478 * tac invocation:: Concatenate and write files in reverse.
1479 * nl invocation:: Number lines and write files.
1480 * od invocation:: Write files in octal or other formats.
1481 * base64 invocation:: Transform data into printable data.
1484 @node cat invocation
1485 @section @command{cat}: Concatenate and write files
1488 @cindex concatenate and write files
1489 @cindex copying files
1491 @command{cat} copies each @var{file} (@samp{-} means standard input), or
1492 standard input if none are given, to standard output. Synopsis:
1495 cat [@var{option}] [@var{file}]@dots{}
1498 The program accepts the following options. Also see @ref{Common options}.
1506 Equivalent to @option{-vET}.
1509 @itemx --number-nonblank
1511 @opindex --number-nonblank
1512 Number all nonempty output lines, starting with 1.
1516 Equivalent to @option{-vE}.
1521 @opindex --show-ends
1522 Display a @samp{$} after the end of each line.
1528 Number all output lines, starting with 1. This option is ignored
1529 if @option{-b} is in effect.
1532 @itemx --squeeze-blank
1534 @opindex --squeeze-blank
1535 @cindex squeezing empty lines
1536 Suppress repeated adjacent empty lines; output just one empty line
1541 Equivalent to @option{-vT}.
1546 @opindex --show-tabs
1547 Display TAB characters as @samp{^I}.
1551 Ignored; for @acronym{POSIX} compatibility.
1554 @itemx --show-nonprinting
1556 @opindex --show-nonprinting
1557 Display control characters except for LFD and TAB using
1558 @samp{^} notation and precede characters that have the high bit set with
1563 On systems like MS-DOS that distinguish between text and binary files,
1564 @command{cat} normally reads and writes in binary mode. However,
1565 @command{cat} reads in text mode if one of the options
1566 @option{-bensAE} is used or if @command{cat} is reading from standard
1567 input and standard input is a terminal. Similarly, @command{cat}
1568 writes in text mode if one of the options @option{-bensAE} is used or
1569 if standard output is a terminal.
1576 # Output f's contents, then standard input, then g's contents.
1579 # Copy standard input to standard output.
1584 @node tac invocation
1585 @section @command{tac}: Concatenate and write files in reverse
1588 @cindex reversing files
1590 @command{tac} copies each @var{file} (@samp{-} means standard input), or
1591 standard input if none are given, to standard output, reversing the
1592 records (lines by default) in each separately. Synopsis:
1595 tac [@var{option}]@dots{} [@var{file}]@dots{}
1598 @dfn{Records} are separated by instances of a string (newline by
1599 default). By default, this separator string is attached to the end of
1600 the record that it follows in the file.
1602 The program accepts the following options. Also see @ref{Common options}.
1610 The separator is attached to the beginning of the record that it
1611 precedes in the file.
1617 Treat the separator string as a regular expression. Users of @command{tac}
1618 on MS-DOS/MS-Windows should note that, since @command{tac} reads files in
1619 binary mode, each line of a text file might end with a CR/LF pair
1620 instead of the Unix-style LF.
1622 @item -s @var{separator}
1623 @itemx --separator=@var{separator}
1625 @opindex --separator
1626 Use @var{separator} as the record separator, instead of newline.
1634 @section @command{nl}: Number lines and write files
1637 @cindex numbering lines
1638 @cindex line numbering
1640 @command{nl} writes each @var{file} (@samp{-} means standard input), or
1641 standard input if none are given, to standard output, with line numbers
1642 added to some or all of the lines. Synopsis:
1645 nl [@var{option}]@dots{} [@var{file}]@dots{}
1648 @cindex logical pages, numbering on
1649 @command{nl} decomposes its input into (logical) pages; by default, the
1650 line number is reset to 1 at the top of each logical page. @command{nl}
1651 treats all of the input files as a single document; it does not reset
1652 line numbers or logical pages between files.
1654 @cindex headers, numbering
1655 @cindex body, numbering
1656 @cindex footers, numbering
1657 A logical page consists of three sections: header, body, and footer.
1658 Any of the sections can be empty. Each can be numbered in a different
1659 style from the others.
1661 The beginnings of the sections of logical pages are indicated in the
1662 input file by a line containing exactly one of these delimiter strings:
1673 The two characters from which these strings are made can be changed from
1674 @samp{\} and @samp{:} via options (see below), but the pattern and
1675 length of each string cannot be changed.
1677 A section delimiter is replaced by an empty line on output. Any text
1678 that comes before the first section delimiter string in the input file
1679 is considered to be part of a body section, so @command{nl} treats a
1680 file that contains no section delimiters as a single body section.
1682 The program accepts the following options. Also see @ref{Common options}.
1686 @item -b @var{style}
1687 @itemx --body-numbering=@var{style}
1689 @opindex --body-numbering
1690 Select the numbering style for lines in the body section of each
1691 logical page. When a line is not numbered, the current line number
1692 is not incremented, but the line number separator character is still
1693 prepended to the line. The styles are:
1699 number only nonempty lines (default for body),
1701 do not number lines (default for header and footer),
1703 number only lines that contain a match for the basic regular
1704 expression @var{bre}.
1705 @xref{Regular Expressions, , Regular Expressions, grep, The GNU Grep Manual}.
1709 @itemx --section-delimiter=@var{cd}
1711 @opindex --section-delimiter
1712 @cindex section delimiters of pages
1713 Set the section delimiter characters to @var{cd}; default is
1714 @samp{\:}. If only @var{c} is given, the second remains @samp{:}.
1715 (Remember to protect @samp{\} or other metacharacters from shell
1716 expansion with quotes or extra backslashes.)
1718 @item -f @var{style}
1719 @itemx --footer-numbering=@var{style}
1721 @opindex --footer-numbering
1722 Analogous to @option{--body-numbering}.
1724 @item -h @var{style}
1725 @itemx --header-numbering=@var{style}
1727 @opindex --header-numbering
1728 Analogous to @option{--body-numbering}.
1730 @item -i @var{number}
1731 @itemx --line-increment=@var{number}
1733 @opindex --line-increment
1734 Increment line numbers by @var{number} (default 1).
1736 @item -l @var{number}
1737 @itemx --join-blank-lines=@var{number}
1739 @opindex --join-blank-lines
1740 @cindex empty lines, numbering
1741 @cindex blank lines, numbering
1742 Consider @var{number} (default 1) consecutive empty lines to be one
1743 logical line for numbering, and only number the last one. Where fewer
1744 than @var{number} consecutive empty lines occur, do not number them.
1745 An empty line is one that contains no characters, not even spaces
1748 @item -n @var{format}
1749 @itemx --number-format=@var{format}
1751 @opindex --number-format
1752 Select the line numbering format (default is @code{rn}):
1756 @opindex ln @r{format for @command{nl}}
1757 left justified, no leading zeros;
1759 @opindex rn @r{format for @command{nl}}
1760 right justified, no leading zeros;
1762 @opindex rz @r{format for @command{nl}}
1763 right justified, leading zeros.
1767 @itemx --no-renumber
1769 @opindex --no-renumber
1770 Do not reset the line number at the start of a logical page.
1772 @item -s @var{string}
1773 @itemx --number-separator=@var{string}
1775 @opindex --number-separator
1776 Separate the line number from the text line in the output with
1777 @var{string} (default is the TAB character).
1779 @item -v @var{number}
1780 @itemx --starting-line-number=@var{number}
1782 @opindex --starting-line-number
1783 Set the initial line number on each logical page to @var{number} (default 1).
1785 @item -w @var{number}
1786 @itemx --number-width=@var{number}
1788 @opindex --number-width
1789 Use @var{number} characters for line numbers (default 6).
1797 @section @command{od}: Write files in octal or other formats
1800 @cindex octal dump of files
1801 @cindex hex dump of files
1802 @cindex ASCII dump of files
1803 @cindex file contents, dumping unambiguously
1805 @command{od} writes an unambiguous representation of each @var{file}
1806 (@samp{-} means standard input), or standard input if none are given.
1810 od [@var{option}]@dots{} [@var{file}]@dots{}
1811 od [-abcdfilosx]@dots{} [@var{file}] [[+]@var{offset}[.][b]]
1812 od [@var{option}]@dots{} --traditional [@var{file}]@c
1813 [[+]@var{offset}[.][b] [[+]@var{label}[.][b]]]
1816 Each line of output consists of the offset in the input, followed by
1817 groups of data from the file. By default, @command{od} prints the offset in
1818 octal, and each group of file data is a C @code{short int}'s worth of input
1819 printed as a single octal number.
1821 If @var{offset} is given, it specifies how many input bytes to skip
1822 before formatting and writing. By default, it is interpreted as an
1823 octal number, but the optional trailing decimal point causes it to be
1824 interpreted as decimal. If no decimal is specified and the offset
1825 begins with @samp{0x} or @samp{0X} it is interpreted as a hexadecimal
1826 number. If there is a trailing @samp{b}, the number of bytes skipped
1827 will be @var{offset} multiplied by 512.
1829 If a command is of both the first and second forms, the second form is
1830 assumed if the last operand begins with @samp{+} or (if there are two
1831 operands) a digit. For example, in @samp{od foo 10} and @samp{od +10}
1832 the @samp{10} is an offset, whereas in @samp{od 10} the @samp{10} is a
1835 The program accepts the following options. Also see @ref{Common options}.
1839 @item -A @var{radix}
1840 @itemx --address-radix=@var{radix}
1842 @opindex --address-radix
1843 @cindex radix for file offsets
1844 @cindex file offset radix
1845 Select the base in which file offsets are printed. @var{radix} can
1846 be one of the following:
1856 none (do not print offsets).
1859 The default is octal.
1861 @item -j @var{bytes}
1862 @itemx --skip-bytes=@var{bytes}
1864 @opindex --skip-bytes
1865 Skip @var{bytes} input bytes before formatting and writing. If
1866 @var{bytes} begins with @samp{0x} or @samp{0X}, it is interpreted in
1867 hexadecimal; otherwise, if it begins with @samp{0}, in octal; otherwise,
1869 @multiplierSuffixes{bytes}
1871 @item -N @var{bytes}
1872 @itemx --read-bytes=@var{bytes}
1874 @opindex --read-bytes
1875 Output at most @var{bytes} bytes of the input. Prefixes and suffixes on
1876 @code{bytes} are interpreted as for the @option{-j} option.
1878 @item -S @var{bytes}
1879 @itemx --strings[=@var{bytes}]
1882 @cindex string constants, outputting
1883 Instead of the normal output, output only @dfn{string constants}: at
1884 least @var{bytes} consecutive @acronym{ASCII} graphic characters,
1885 followed by a zero byte (@acronym{ASCII} @sc{nul}).
1886 Prefixes and suffixes on @var{bytes} are interpreted as for the
1889 If @var{n} is omitted with @option{--strings}, the default is 3.
1892 @itemx --format=@var{type}
1895 Select the format in which to output the file data. @var{type} is a
1896 string of one or more of the below type indicator characters. If you
1897 include more than one type indicator character in a single @var{type}
1898 string, or use this option more than once, @command{od} writes one copy
1899 of each output line using each of the data types that you specified,
1900 in the order that you specified.
1902 Adding a trailing ``z'' to any type specification appends a display
1903 of the @acronym{ASCII} character representation of the printable characters
1904 to the output line generated by the type specification.
1908 named character, ignoring high-order bit
1910 @acronym{ASCII} character or backslash escape,
1914 floating point (@pxref{Floating point})
1923 The type @code{a} outputs things like @samp{sp} for space, @samp{nl} for
1924 newline, and @samp{nul} for a zero byte. Only the least significant
1925 seven bits of each byte is used; the high-order bit is ignored.
1926 Type @code{c} outputs
1927 @samp{ }, @samp{\n}, and @code{\0}, respectively.
1930 Except for types @samp{a} and @samp{c}, you can specify the number
1931 of bytes to use in interpreting each number in the given data type
1932 by following the type indicator character with a decimal integer.
1933 Alternately, you can specify the size of one of the C compiler's
1934 built-in data types by following the type indicator character with
1935 one of the following characters. For integers (@samp{d}, @samp{o},
1936 @samp{u}, @samp{x}):
1949 For floating point (@code{f}):
1961 @itemx --output-duplicates
1963 @opindex --output-duplicates
1964 Output consecutive lines that are identical. By default, when two or
1965 more consecutive output lines would be identical, @command{od} outputs only
1966 the first line, and puts just an asterisk on the following line to
1967 indicate the elision.
1970 @itemx --width[=@var{n}]
1973 Dump @code{n} input bytes per output line. This must be a multiple of
1974 the least common multiple of the sizes associated with the specified
1977 If this option is not given at all, the default is 16. If @var{n} is
1978 omitted, the default is 32.
1982 The next several options are shorthands for format specifications.
1983 @sc{gnu} @command{od} accepts any combination of shorthands and format
1984 specification options. These options accumulate.
1990 Output as named characters. Equivalent to @samp{-t a}.
1994 Output as octal bytes. Equivalent to @samp{-t o1}.
1998 Output as @acronym{ASCII} characters or backslash escapes. Equivalent to
2003 Output as unsigned decimal two-byte units. Equivalent to @samp{-t u2}.
2007 Output as floats. Equivalent to @samp{-t fF}.
2011 Output as decimal ints. Equivalent to @samp{-t dI}.
2015 Output as decimal long ints. Equivalent to @samp{-t dL}.
2019 Output as octal two-byte units. Equivalent to @option{-t o2}.
2023 Output as decimal two-byte units. Equivalent to @option{-t d2}.
2027 Output as hexadecimal two-byte units. Equivalent to @samp{-t x2}.
2030 @opindex --traditional
2031 Recognize the non-option label argument that traditional @command{od}
2032 accepted. The following syntax:
2035 od --traditional [@var{file}] [[+]@var{offset}[.][b] [[+]@var{label}[.][b]]]
2039 can be used to specify at most one file and optional arguments
2040 specifying an offset and a pseudo-start address, @var{label}.
2041 The @var{label} argument is interpreted
2042 just like @var{offset}, but it specifies an initial pseudo-address. The
2043 pseudo-addresses are displayed in parentheses following any normal
2050 @node base64 invocation
2051 @section @command{base64}: Transform data into printable data
2054 @cindex base64 encoding
2056 @command{base64} transforms data read from a file, or standard input,
2057 into (or from) base64 encoded form. The base64 encoded form uses
2058 printable @acronym{ASCII} characters to represent binary data.
2062 base64 [@var{option}]@dots{} [@var{file}]
2063 base64 --decode [@var{option}]@dots{} [@var{file}]
2066 The base64 encoding expands data to roughly 133% of the original.
2067 The format conforms to
2068 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc4648.txt, RFC 4648}.
2070 The program accepts the following options. Also see @ref{Common options}.
2075 @itemx --wrap=@var{cols}
2079 @cindex column to wrap data after
2080 During encoding, wrap lines after @var{cols} characters. This must be
2083 The default is to wrap after 76 characters. Use the value 0 to
2084 disable line wrapping altogether.
2090 @cindex Decode base64 data
2091 @cindex Base64 decoding
2092 Change the mode of operation, from the default of encoding data, to
2093 decoding data. Input is expected to be base64 encoded data, and the
2094 output will be the original data.
2097 @itemx --ignore-garbage
2099 @opindex --ignore-garbage
2100 @cindex Ignore garbage in base64 stream
2101 When decoding, newlines are always accepted.
2102 During decoding, ignore unrecognized bytes,
2103 to permit distorted data to be decoded.
2110 @node Formatting file contents
2111 @chapter Formatting file contents
2113 @cindex formatting file contents
2115 These commands reformat the contents of files.
2118 * fmt invocation:: Reformat paragraph text.
2119 * pr invocation:: Paginate or columnate files for printing.
2120 * fold invocation:: Wrap input lines to fit in specified width.
2124 @node fmt invocation
2125 @section @command{fmt}: Reformat paragraph text
2128 @cindex reformatting paragraph text
2129 @cindex paragraphs, reformatting
2130 @cindex text, reformatting
2132 @command{fmt} fills and joins lines to produce output lines of (at most)
2133 a given number of characters (75 by default). Synopsis:
2136 fmt [@var{option}]@dots{} [@var{file}]@dots{}
2139 @command{fmt} reads from the specified @var{file} arguments (or standard
2140 input if none are given), and writes to standard output.
2142 By default, blank lines, spaces between words, and indentation are
2143 preserved in the output; successive input lines with different
2144 indentation are not joined; tabs are expanded on input and introduced on
2147 @cindex line-breaking
2148 @cindex sentences and line-breaking
2149 @cindex Knuth, Donald E.
2150 @cindex Plass, Michael F.
2151 @command{fmt} prefers breaking lines at the end of a sentence, and tries to
2152 avoid line breaks after the first word of a sentence or before the last
2153 word of a sentence. A @dfn{sentence break} is defined as either the end
2154 of a paragraph or a word ending in any of @samp{.?!}, followed by two
2155 spaces or end of line, ignoring any intervening parentheses or quotes.
2156 Like @TeX{}, @command{fmt} reads entire ``paragraphs'' before choosing line
2157 breaks; the algorithm is a variant of that given by Donald E. Knuth
2158 and Michael F. Plass in ``Breaking Paragraphs Into Lines'',
2159 @cite{Software---Practice & Experience} @b{11}, 11 (November 1981),
2162 The program accepts the following options. Also see @ref{Common options}.
2167 @itemx --crown-margin
2169 @opindex --crown-margin
2170 @cindex crown margin
2171 @dfn{Crown margin} mode: preserve the indentation of the first two
2172 lines within a paragraph, and align the left margin of each subsequent
2173 line with that of the second line.
2176 @itemx --tagged-paragraph
2178 @opindex --tagged-paragraph
2179 @cindex tagged paragraphs
2180 @dfn{Tagged paragraph} mode: like crown margin mode, except that if
2181 indentation of the first line of a paragraph is the same as the
2182 indentation of the second, the first line is treated as a one-line
2188 @opindex --split-only
2189 Split lines only. Do not join short lines to form longer ones. This
2190 prevents sample lines of code, and other such ``formatted'' text from
2191 being unduly combined.
2194 @itemx --uniform-spacing
2196 @opindex --uniform-spacing
2197 Uniform spacing. Reduce spacing between words to one space, and spacing
2198 between sentences to two spaces.
2201 @itemx -w @var{width}
2202 @itemx --width=@var{width}
2203 @opindex -@var{width}
2206 Fill output lines up to @var{width} characters (default 75). @command{fmt}
2207 initially tries to make lines about 7% shorter than this, to give it
2208 room to balance line lengths.
2210 @item -p @var{prefix}
2211 @itemx --prefix=@var{prefix}
2212 Only lines beginning with @var{prefix} (possibly preceded by whitespace)
2213 are subject to formatting. The prefix and any preceding whitespace are
2214 stripped for the formatting and then re-attached to each formatted output
2215 line. One use is to format certain kinds of program comments, while
2216 leaving the code unchanged.
2224 @section @command{pr}: Paginate or columnate files for printing
2227 @cindex printing, preparing files for
2228 @cindex multicolumn output, generating
2229 @cindex merging files in parallel
2231 @command{pr} writes each @var{file} (@samp{-} means standard input), or
2232 standard input if none are given, to standard output, paginating and
2233 optionally outputting in multicolumn format; optionally merges all
2234 @var{file}s, printing all in parallel, one per column. Synopsis:
2237 pr [@var{option}]@dots{} [@var{file}]@dots{}
2241 By default, a 5-line header is printed at each page: two blank lines;
2242 a line with the date, the file name, and the page count; and two more
2243 blank lines. A footer of five blank lines is also printed.
2244 The default @var{page_length} is 66
2245 lines. The default number of text lines is therefore 56.
2246 The text line of the header takes the form
2247 @samp{@var{date} @var{string} @var{page}}, with spaces inserted around
2248 @var{string} so that the line takes up the full @var{page_width}. Here,
2249 @var{date} is the date (see the @option{-D} or @option{--date-format}
2250 option for details), @var{string} is the centered header string, and
2251 @var{page} identifies the page number. The @env{LC_MESSAGES} locale
2252 category affects the spelling of @var{page}; in the default C locale, it
2253 is @samp{Page @var{number}} where @var{number} is the decimal page
2256 Form feeds in the input cause page breaks in the output. Multiple form
2257 feeds produce empty pages.
2259 Columns are of equal width, separated by an optional string (default
2260 is @samp{space}). For multicolumn output, lines will always be truncated to
2261 @var{page_width} (default 72), unless you use the @option{-J} option.
2263 column output no line truncation occurs by default. Use @option{-W} option to
2264 truncate lines in that case.
2266 The following changes were made in version 1.22i and apply to later
2267 versions of @command{pr}:
2268 @c FIXME: this whole section here sounds very awkward to me. I
2269 @c made a few small changes, but really it all needs to be redone. - Brian
2270 @c OK, I fixed another sentence or two, but some of it I just don't understand.
2275 Some small @var{letter options} (@option{-s}, @option{-w}) have been
2276 redefined for better @acronym{POSIX} compliance. The output of some further
2277 cases has been adapted to other Unix systems. These changes are not
2278 compatible with earlier versions of the program.
2281 Some @var{new capital letter} options (@option{-J}, @option{-S}, @option{-W})
2282 have been introduced to turn off unexpected interferences of small letter
2283 options. The @option{-N} option and the second argument @var{last_page}
2284 of @samp{+FIRST_PAGE} offer more flexibility. The detailed handling of
2285 form feeds set in the input files requires the @option{-T} option.
2288 Capital letter options override small letter ones.
2291 Some of the option-arguments (compare @option{-s}, @option{-e},
2292 @option{-i}, @option{-n}) cannot be specified as separate arguments from the
2293 preceding option letter (already stated in the @acronym{POSIX} specification).
2296 The program accepts the following options. Also see @ref{Common options}.
2300 @item +@var{first_page}[:@var{last_page}]
2301 @itemx --pages=@var{first_page}[:@var{last_page}]
2302 @c The two following @opindex lines evoke warnings because they contain ':'
2303 @c The 'info' spec does not permit that. If we use those lines, we end
2304 @c up with truncated index entries that don't work.
2305 @c @opindex +@var{first_page}[:@var{last_page}]
2306 @c @opindex --pages=@var{first_page}[:@var{last_page}]
2307 @opindex +@var{page_range}
2308 @opindex --pages=@var{page_range}
2309 Begin printing with page @var{first_page} and stop with @var{last_page}.
2310 Missing @samp{:@var{last_page}} implies end of file. While estimating
2311 the number of skipped pages each form feed in the input file results
2312 in a new page. Page counting with and without @samp{+@var{first_page}}
2313 is identical. By default, counting starts with the first page of input
2314 file (not first page printed). Line numbering may be altered by @option{-N}
2318 @itemx --columns=@var{column}
2319 @opindex -@var{column}
2321 @cindex down columns
2322 With each single @var{file}, produce @var{column} columns of output
2323 (default is 1) and print columns down, unless @option{-a} is used. The
2324 column width is automatically decreased as @var{column} increases; unless
2325 you use the @option{-W/-w} option to increase @var{page_width} as well.
2326 This option might well cause some lines to be truncated. The number of
2327 lines in the columns on each page are balanced. The options @option{-e}
2328 and @option{-i} are on for multiple text-column output. Together with
2329 @option{-J} option column alignment and line truncation is turned off.
2330 Lines of full length are joined in a free field format and @option{-S}
2331 option may set field separators. @option{-@var{column}} may not be used
2332 with @option{-m} option.
2338 @cindex across columns
2339 With each single @var{file}, print columns across rather than down. The
2340 @option{-@var{column}} option must be given with @var{column} greater than one.
2341 If a line is too long to fit in a column, it is truncated.
2344 @itemx --show-control-chars
2346 @opindex --show-control-chars
2347 Print control characters using hat notation (e.g., @samp{^G}); print
2348 other nonprinting characters in octal backslash notation. By default,
2349 nonprinting characters are not changed.
2352 @itemx --double-space
2354 @opindex --double-space
2355 @cindex double spacing
2356 Double space the output.
2358 @item -D @var{format}
2359 @itemx --date-format=@var{format}
2360 @cindex time formats
2361 @cindex formatting times
2362 Format header dates using @var{format}, using the same conventions as
2363 for the command @samp{date +@var{format}}; @xref{date invocation}.
2364 Except for directives, which start with
2365 @samp{%}, characters in @var{format} are printed unchanged. You can use
2366 this option to specify an arbitrary string in place of the header date,
2367 e.g., @option{--date-format="Monday morning"}.
2369 @vindex POSIXLY_CORRECT
2371 The default date format is @samp{%Y-%m-%d %H:%M} (for example,
2372 @samp{2001-12-04 23:59});
2373 but if the @env{POSIXLY_CORRECT} environment variable is set
2374 and the @env{LC_TIME} locale category specifies the @acronym{POSIX}
2375 locale, the default is @samp{%b %e %H:%M %Y} (for example,
2376 @samp{Dec@ @ 4 23:59 2001}.
2379 Time stamps are listed according to the time zone rules specified by
2380 the @env{TZ} environment variable, or by the system default rules if
2381 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
2382 with @env{TZ}, libc, The GNU C Library Reference Manual}.
2384 @item -e[@var{in-tabchar}[@var{in-tabwidth}]]
2385 @itemx --expand-tabs[=@var{in-tabchar}[@var{in-tabwidth}]]
2387 @opindex --expand-tabs
2389 Expand @var{tab}s to spaces on input. Optional argument @var{in-tabchar} is
2390 the input tab character (default is the TAB character). Second optional
2391 argument @var{in-tabwidth} is the input tab character's width (default
2399 @opindex --form-feed
2400 Use a form feed instead of newlines to separate output pages. This does
2401 not alter the default page length of 66 lines.
2403 @item -h @var{header}
2404 @itemx --header=@var{header}
2407 Replace the file name in the header with the centered string @var{header}.
2408 When using the shell, @var{header} should be quoted and should be
2409 separated from @option{-h} by a space.
2411 @item -i[@var{out-tabchar}[@var{out-tabwidth}]]
2412 @itemx --output-tabs[=@var{out-tabchar}[@var{out-tabwidth}]]
2414 @opindex --output-tabs
2416 Replace spaces with @var{tab}s on output. Optional argument @var{out-tabchar}
2417 is the output tab character (default is the TAB character). Second optional
2418 argument @var{out-tabwidth} is the output tab character's width (default
2424 @opindex --join-lines
2425 Merge lines of full length. Used together with the column options
2426 @option{-@var{column}}, @option{-a -@var{column}} or @option{-m}. Turns off
2427 @option{-W/-w} line truncation;
2428 no column alignment used; may be used with
2429 @option{--sep-string[=@var{string}]}. @option{-J} has been introduced
2430 (together with @option{-W} and @option{--sep-string})
2431 to disentangle the old (@acronym{POSIX}-compliant) options @option{-w} and
2432 @option{-s} along with the three column options.
2435 @item -l @var{page_length}
2436 @itemx --length=@var{page_length}
2439 Set the page length to @var{page_length} (default 66) lines, including
2440 the lines of the header [and the footer]. If @var{page_length} is less
2441 than or equal to 10, the header and footer are omitted, as if the
2442 @option{-t} option had been given.
2448 Merge and print all @var{file}s in parallel, one in each column. If a
2449 line is too long to fit in a column, it is truncated, unless the @option{-J}
2450 option is used. @option{--sep-string[=@var{string}]} may be used.
2452 some @var{file}s (form feeds set) produce empty columns, still marked
2453 by @var{string}. The result is a continuous line numbering and column
2454 marking throughout the whole merged file. Completely empty merged pages
2455 show no separators or line numbers. The default header becomes
2456 @samp{@var{date} @var{page}} with spaces inserted in the middle; this
2457 may be used with the @option{-h} or @option{--header} option to fill up
2458 the middle blank part.
2460 @item -n[@var{number-separator}[@var{digits}]]
2461 @itemx --number-lines[=@var{number-separator}[@var{digits}]]
2463 @opindex --number-lines
2464 Provide @var{digits} digit line numbering (default for @var{digits} is
2465 5). With multicolumn output the number occupies the first @var{digits}
2466 column positions of each text column or only each line of @option{-m}
2467 output. With single column output the number precedes each line just as
2468 @option{-m} does. Default counting of the line numbers starts with the
2469 first line of the input file (not the first line printed, compare the
2470 @option{--page} option and @option{-N} option).
2471 Optional argument @var{number-separator} is the character appended to
2472 the line number to separate it from the text followed. The default
2473 separator is the TAB character. In a strict sense a TAB is always
2474 printed with single column output only. The TAB width varies
2475 with the TAB position, e.g., with the left @var{margin} specified
2476 by @option{-o} option. With multicolumn output priority is given to
2477 @samp{equal width of output columns} (a @acronym{POSIX} specification).
2478 The TAB width is fixed to the value of the first column and does
2479 not change with different values of left @var{margin}. That means a
2480 fixed number of spaces is always printed in the place of the
2481 @var{number-separator} TAB. The tabification depends upon the output
2484 @item -N @var{line_number}
2485 @itemx --first-line-number=@var{line_number}
2487 @opindex --first-line-number
2488 Start line counting with the number @var{line_number} at first line of
2489 first page printed (in most cases not the first line of the input file).
2491 @item -o @var{margin}
2492 @itemx --indent=@var{margin}
2495 @cindex indenting lines
2497 Indent each line with a margin @var{margin} spaces wide (default is zero).
2498 The total page width is the size of the margin plus the @var{page_width}
2499 set with the @option{-W/-w} option. A limited overflow may occur with
2500 numbered single column output (compare @option{-n} option).
2503 @itemx --no-file-warnings
2505 @opindex --no-file-warnings
2506 Do not print a warning message when an argument @var{file} cannot be
2507 opened. (The exit status will still be nonzero, however.)
2509 @item -s[@var{char}]
2510 @itemx --separator[=@var{char}]
2512 @opindex --separator
2513 Separate columns by a single character @var{char}. The default for
2514 @var{char} is the TAB character without @option{-w} and @samp{no
2515 character} with @option{-w}. Without @option{-s} the default separator
2516 @samp{space} is set. @option{-s[char]} turns off line truncation of all
2517 three column options (@option{-COLUMN}|@option{-a -COLUMN}|@option{-m}) unless
2518 @option{-w} is set. This is a @acronym{POSIX}-compliant formulation.
2521 @item -S[@var{string}]
2522 @itemx --sep-string[=@var{string}]
2524 @opindex --sep-string
2525 Use @var{string} to separate output columns. The @option{-S} option doesn't
2526 affect the @option{-W/-w} option, unlike the @option{-s} option which does. It
2527 does not affect line truncation or column alignment.
2528 Without @option{-S}, and with @option{-J}, @command{pr} uses the default output
2530 Without @option{-S} or @option{-J}, @command{pr} uses a @samp{space}
2531 (same as @option{-S"@w{ }"}).
2532 If no @samp{@var{string}} argument is specified, @samp{""} is assumed.
2535 @itemx --omit-header
2537 @opindex --omit-header
2538 Do not print the usual header [and footer] on each page, and do not fill
2539 out the bottom of pages (with blank lines or a form feed). No page
2540 structure is produced, but form feeds set in the input files are retained.
2541 The predefined pagination is not changed. @option{-t} or @option{-T} may be
2542 useful together with other options; e.g.: @option{-t -e4}, expand TAB characters
2543 in the input file to 4 spaces but don't make any other changes. Use of
2544 @option{-t} overrides @option{-h}.
2547 @itemx --omit-pagination
2549 @opindex --omit-pagination
2550 Do not print header [and footer]. In addition eliminate all form feeds
2551 set in the input files.
2554 @itemx --show-nonprinting
2556 @opindex --show-nonprinting
2557 Print nonprinting characters in octal backslash notation.
2559 @item -w @var{page_width}
2560 @itemx --width=@var{page_width}
2563 Set page width to @var{page_width} characters for multiple text-column
2564 output only (default for @var{page_width} is 72). @option{-s[CHAR]} turns
2565 off the default page width and any line truncation and column alignment.
2566 Lines of full length are merged, regardless of the column options
2567 set. No @var{page_width} setting is possible with single column output.
2568 A @acronym{POSIX}-compliant formulation.
2570 @item -W @var{page_width}
2571 @itemx --page_width=@var{page_width}
2573 @opindex --page_width
2574 Set the page width to @var{page_width} characters. That's valid with and
2575 without a column option. Text lines are truncated, unless @option{-J}
2576 is used. Together with one of the three column options
2577 (@option{-@var{column}}, @option{-a -@var{column}} or @option{-m}) column
2578 alignment is always used. The separator options @option{-S} or @option{-s}
2579 don't affect the @option{-W} option. Default is 72 characters. Without
2580 @option{-W @var{page_width}} and without any of the column options NO line
2581 truncation is used (defined to keep downward compatibility and to meet
2582 most frequent tasks). That's equivalent to @option{-W 72 -J}. The header
2583 line is never truncated.
2590 @node fold invocation
2591 @section @command{fold}: Wrap input lines to fit in specified width
2594 @cindex wrapping long input lines
2595 @cindex folding long input lines
2597 @command{fold} writes each @var{file} (@option{-} means standard input), or
2598 standard input if none are given, to standard output, breaking long
2602 fold [@var{option}]@dots{} [@var{file}]@dots{}
2605 By default, @command{fold} breaks lines wider than 80 columns. The output
2606 is split into as many lines as necessary.
2608 @cindex screen columns
2609 @command{fold} counts screen columns by default; thus, a tab may count more
2610 than one column, backspace decreases the column count, and carriage
2611 return sets the column to zero.
2613 The program accepts the following options. Also see @ref{Common options}.
2621 Count bytes rather than columns, so that tabs, backspaces, and carriage
2622 returns are each counted as taking up one column, just like other
2629 Break at word boundaries: the line is broken after the last blank before
2630 the maximum line length. If the line contains no such blanks, the line
2631 is broken at the maximum line length as usual.
2633 @item -w @var{width}
2634 @itemx --width=@var{width}
2637 Use a maximum line length of @var{width} columns instead of 80.
2639 For compatibility @command{fold} supports an obsolete option syntax
2640 @option{-@var{width}}. New scripts should use @option{-w @var{width}}
2648 @node Output of parts of files
2649 @chapter Output of parts of files
2651 @cindex output of parts of files
2652 @cindex parts of files, output of
2654 These commands output pieces of the input.
2657 * head invocation:: Output the first part of files.
2658 * tail invocation:: Output the last part of files.
2659 * split invocation:: Split a file into pieces.
2660 * csplit invocation:: Split a file into context-determined pieces.
2663 @node head invocation
2664 @section @command{head}: Output the first part of files
2667 @cindex initial part of files, outputting
2668 @cindex first part of files, outputting
2670 @command{head} prints the first part (10 lines by default) of each
2671 @var{file}; it reads from standard input if no files are given or
2672 when given a @var{file} of @option{-}. Synopsis:
2675 head [@var{option}]@dots{} [@var{file}]@dots{}
2678 If more than one @var{file} is specified, @command{head} prints a
2679 one-line header consisting of:
2682 ==> @var{file name} <==
2686 before the output for each @var{file}.
2688 The program accepts the following options. Also see @ref{Common options}.
2693 @itemx --bytes=@var{k}
2696 Print the first @var{k} bytes, instead of initial lines.
2697 However, if @var{k} starts with a @samp{-},
2698 print all but the last @var{k} bytes of each file.
2699 @multiplierSuffixes{k}
2702 @itemx --lines=@var{k}
2705 Output the first @var{k} lines.
2706 However, if @var{k} starts with a @samp{-},
2707 print all but the last @var{k} lines of each file.
2708 Size multiplier suffixes are the same as with the @option{-c} option.
2716 Never print file name headers.
2722 Always print file name headers.
2726 For compatibility @command{head} also supports an obsolete option syntax
2727 @option{-@var{count}@var{options}}, which is recognized only if it is
2728 specified first. @var{count} is a decimal number optionally followed
2729 by a size letter (@samp{b}, @samp{k}, @samp{m}) as in @option{-c}, or
2730 @samp{l} to mean count by lines, or other option letters (@samp{cqv}).
2731 Scripts intended for standard hosts should use @option{-c @var{count}}
2732 or @option{-n @var{count}} instead. If your script must also run on
2733 hosts that support only the obsolete syntax, it is usually simpler to
2734 avoid @command{head}, e.g., by using @samp{sed 5q} instead of
2740 @node tail invocation
2741 @section @command{tail}: Output the last part of files
2744 @cindex last part of files, outputting
2746 @command{tail} prints the last part (10 lines by default) of each
2747 @var{file}; it reads from standard input if no files are given or
2748 when given a @var{file} of @samp{-}. Synopsis:
2751 tail [@var{option}]@dots{} [@var{file}]@dots{}
2754 If more than one @var{file} is specified, @command{tail} prints a
2755 one-line header consisting of:
2758 ==> @var{file name} <==
2762 before the output for each @var{file}.
2764 @cindex BSD @command{tail}
2765 @sc{gnu} @command{tail} can output any amount of data (some other versions of
2766 @command{tail} cannot). It also has no @option{-r} option (print in
2767 reverse), since reversing a file is really a different job from printing
2768 the end of a file; BSD @command{tail} (which is the one with @option{-r}) can
2769 only reverse files that are at most as large as its buffer, which is
2770 typically 32 KiB@. A more reliable and versatile way to reverse files is
2771 the @sc{gnu} @command{tac} command.
2773 The program accepts the following options. Also see @ref{Common options}.
2778 @itemx --bytes=@var{k}
2781 Output the last @var{k} bytes, instead of final lines.
2782 However, if @var{k} starts with a @samp{+}, start printing with the
2783 @var{k}th byte from the start of each file, instead of from the end.
2784 @multiplierSuffixes{k}
2787 @itemx --follow[=@var{how}]
2790 @cindex growing files
2791 @vindex name @r{follow option}
2792 @vindex descriptor @r{follow option}
2793 Loop forever trying to read more characters at the end of the file,
2794 presumably because the file is growing.
2795 If more than one file is given, @command{tail} prints a header whenever it
2796 gets output from a different file, to indicate which file that output is
2799 There are two ways to specify how you'd like to track files with this option,
2800 but that difference is noticeable only when a followed file is removed or
2802 If you'd like to continue to track the end of a growing file even after
2803 it has been unlinked, use @option{--follow=descriptor}. This is the default
2804 behavior, but it is not useful if you're tracking a log file that may be
2805 rotated (removed or renamed, then reopened). In that case, use
2806 @option{--follow=name} to track the named file, perhaps by reopening it
2807 periodically to see if it has been removed and recreated by some other program.
2808 Note that the inotify-based implementation handles this case without
2809 the need for any periodic reopening.
2811 No matter which method you use, if the tracked file is determined to have
2812 shrunk, @command{tail} prints a message saying the file has been truncated
2813 and resumes tracking the end of the file from the newly-determined endpoint.
2815 When a file is removed, @command{tail}'s behavior depends on whether it is
2816 following the name or the descriptor. When following by name, tail can
2817 detect that a file has been removed and gives a message to that effect,
2818 and if @option{--retry} has been specified it will continue checking
2819 periodically to see if the file reappears.
2820 When following a descriptor, tail does not detect that the file has
2821 been unlinked or renamed and issues no message; even though the file
2822 may no longer be accessible via its original name, it may still be
2825 The option values @samp{descriptor} and @samp{name} may be specified only
2826 with the long form of the option, not with @option{-f}.
2828 The @option{-f} option is ignored if
2829 no @var{file} operand is specified and standard input is a FIFO or a pipe.
2830 Likewise, the @option{-f} option has no effect for any
2831 operand specified as @samp{-}, when standard input is a FIFO or a pipe.
2833 With kernel inotify support, output is triggered by file changes
2834 and is generally very prompt.
2835 Otherwise, @command{tail} sleeps for one second between checks---
2836 use @option{--sleep-interval=@var{n}} to change that default---which can
2837 make the output appear slightly less responsive or bursty.
2838 When using tail without inotify support, you can make it more responsive
2839 by using a sub-second sleep interval, e.g., via an alias like this:
2842 alias tail='tail -s.1'
2847 This option is the same as @option{--follow=name --retry}. That is, tail
2848 will attempt to reopen a file when it is removed. Should this fail, tail
2849 will keep trying until it becomes accessible again.
2853 This option is useful mainly when following by name (i.e., with
2854 @option{--follow=name}).
2855 Without this option, when tail encounters a file that doesn't
2856 exist or is otherwise inaccessible, it reports that fact and
2857 never checks it again.
2859 @itemx --sleep-interval=@var{number}
2860 @opindex --sleep-interval
2861 Change the number of seconds to wait between iterations (the default is 1.0).
2862 During one iteration, every specified file is checked to see if it has
2864 Historical implementations of @command{tail} have required that
2865 @var{number} be an integer. However, GNU @command{tail} accepts
2866 an arbitrary floating point number. @xref{Floating point}.
2867 When @command{tail} uses inotify, this polling-related option
2868 is usually ignored. However, if you also specify @option{--pid=@var{p}},
2869 @command{tail} checks whether process @var{p} is alive at least
2870 every @var{number} seconds.
2872 @itemx --pid=@var{pid}
2874 When following by name or by descriptor, you may specify the process ID,
2875 @var{pid}, of the sole writer of all @var{file} arguments. Then, shortly
2876 after that process terminates, tail will also terminate. This will
2877 work properly only if the writer and the tailing process are running on
2878 the same machine. For example, to save the output of a build in a file
2879 and to watch the file grow, if you invoke @command{make} and @command{tail}
2880 like this then the tail process will stop when your build completes.
2881 Without this option, you would have had to kill the @code{tail -f}
2885 $ make >& makerr & tail --pid=$! -f makerr
2888 If you specify a @var{pid} that is not in use or that does not correspond
2889 to the process that is writing to the tailed files, then @command{tail}
2890 may terminate long before any @var{file}s stop growing or it may not
2891 terminate until long after the real writer has terminated.
2892 Note that @option{--pid} cannot be supported on some systems; @command{tail}
2893 will print a warning if this is the case.
2895 @itemx --max-unchanged-stats=@var{n}
2896 @opindex --max-unchanged-stats
2897 When tailing a file by name, if there have been @var{n} (default
2898 n=@value{DEFAULT_MAX_N_UNCHANGED_STATS_BETWEEN_OPENS}) consecutive
2899 iterations for which the file has not changed, then
2900 @code{open}/@code{fstat} the file to determine if that file name is
2901 still associated with the same device/inode-number pair as before.
2902 When following a log file that is rotated, this is approximately the
2903 number of seconds between when tail prints the last pre-rotation lines
2904 and when it prints the lines that have accumulated in the new log file.
2905 This option is meaningful only when polling (i.e., without inotify)
2906 and when following by name.
2909 @itemx --lines=@var{k}
2912 Output the last @var{k} lines.
2913 However, if @var{k} starts with a @samp{+}, start printing with the
2914 @var{k}th line from the start of each file, instead of from the end.
2915 Size multiplier suffixes are the same as with the @option{-c} option.
2923 Never print file name headers.
2929 Always print file name headers.
2933 For compatibility @command{tail} also supports an obsolete usage
2934 @samp{tail -[@var{count}][bcl][f] [@var{file}]}, which is recognized
2935 only if it does not conflict with the usage described
2936 above. This obsolete form uses exactly one option and at most one
2937 file. In the option, @var{count} is an optional decimal number optionally
2938 followed by a size letter (@samp{b}, @samp{c}, @samp{l}) to mean count
2939 by 512-byte blocks, bytes, or lines, optionally followed by @samp{f}
2940 which has the same meaning as @option{-f}.
2942 @vindex _POSIX2_VERSION
2943 On older systems, the leading @samp{-} can be replaced by @samp{+} in
2944 the obsolete option syntax with the same meaning as in counts, and
2945 obsolete usage overrides normal usage when the two conflict.
2946 This obsolete behavior can be enabled or disabled with the
2947 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
2950 Scripts intended for use on standard hosts should avoid obsolete
2951 syntax and should use @option{-c @var{count}[b]}, @option{-n
2952 @var{count}}, and/or @option{-f} instead. If your script must also
2953 run on hosts that support only the obsolete syntax, you can often
2954 rewrite it to avoid problematic usages, e.g., by using @samp{sed -n
2955 '$p'} rather than @samp{tail -1}. If that's not possible, the script
2956 can use a test like @samp{if tail -c +1 </dev/null >/dev/null 2>&1;
2957 then @dots{}} to decide which syntax to use.
2959 Even if your script assumes the standard behavior, you should still
2960 beware usages whose behaviors differ depending on the @acronym{POSIX}
2961 version. For example, avoid @samp{tail - main.c}, since it might be
2962 interpreted as either @samp{tail main.c} or as @samp{tail -- -
2963 main.c}; avoid @samp{tail -c 4}, since it might mean either @samp{tail
2964 -c4} or @samp{tail -c 10 4}; and avoid @samp{tail +4}, since it might
2965 mean either @samp{tail ./+4} or @samp{tail -n +4}.
2970 @node split invocation
2971 @section @command{split}: Split a file into pieces.
2974 @cindex splitting a file into pieces
2975 @cindex pieces, splitting a file into
2977 @command{split} creates output files containing consecutive or interleaved
2978 sections of @var{input} (standard input if none is given or @var{input}
2979 is @samp{-}). Synopsis:
2982 split [@var{option}] [@var{input} [@var{prefix}]]
2985 By default, @command{split} puts 1000 lines of @var{input} (or whatever is
2986 left over for the last section), into each output file.
2988 @cindex output file name prefix
2989 The output files' names consist of @var{prefix} (@samp{x} by default)
2990 followed by a group of characters (@samp{aa}, @samp{ab}, @dots{} by
2991 default), such that concatenating the output files in traditional
2992 sorted order by file name produces the original input file (except
2993 @option{-nr/@var{n}}). If the output file names are exhausted,
2994 @command{split} reports an error without deleting the output files
2997 The program accepts the following options. Also see @ref{Common options}.
3001 @item -l @var{lines}
3002 @itemx --lines=@var{lines}
3005 Put @var{lines} lines of @var{input} into each output file.
3007 For compatibility @command{split} also supports an obsolete
3008 option syntax @option{-@var{lines}}. New scripts should use
3009 @option{-l @var{lines}} instead.
3012 @itemx --bytes=@var{size}
3015 Put @var{size} bytes of @var{input} into each output file.
3016 @multiplierSuffixes{size}
3019 @itemx --line-bytes=@var{size}
3021 @opindex --line-bytes
3022 Put into each output file as many complete lines of @var{input} as
3023 possible without exceeding @var{size} bytes. Individual lines longer than
3024 @var{size} bytes are broken into multiple files.
3025 @var{size} has the same format as for the @option{--bytes} option.
3027 @itemx --filter=@var{command}
3029 With this option, rather than simply writing to each output file,
3030 write through a pipe to the specified shell @var{command} for each output file.
3031 @var{command} should use the $FILE environment variable, which is set
3032 to a different output file name for each invocation of the command.
3033 For example, imagine that you have a 1TiB compressed file
3034 that, if uncompressed, would be too large to reside on disk,
3035 yet you must split it into individually-compressed pieces
3036 of a more manageable size.
3037 To do that, you might run this command:
3040 xz -dc BIG.xz | split -b200G --filter='xz > $FILE.xz' - big-
3043 Assuming a 10:1 compression ratio, that would create about fifty 20GiB files
3044 with names @file{big-aa.xz}, @file{big-ab.xz}, @file{big-ac.xz}, etc.
3046 @item -n @var{chunks}
3047 @itemx --number=@var{chunks}
3051 Split @var{input} to @var{chunks} output files where @var{chunks} may be:
3054 @var{n} generate @var{n} files based on current size of @var{input}
3055 @var{k}/@var{n} only output @var{k}th of @var{n} to stdout
3056 l/@var{n} generate @var{n} files without splitting lines
3057 l/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
3058 r/@var{n} like @samp{l} but use round robin distribution
3059 r/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
3062 Any excess bytes remaining after dividing the @var{input}
3063 into @var{n} chunks, are assigned to the last chunk.
3064 Any excess bytes appearing after the initial calculation are discarded
3065 (except when using @samp{r} mode).
3067 All @var{n} files are created even if there are fewer than @var{n} lines,
3068 or the @var{input} is truncated.
3070 For @samp{l} mode, chunks are approximately @var{input} size / @var{n}.
3071 The @var{input} is partitioned into @var{n} equal sized portions, with
3072 the last assigned any excess. If a line @emph{starts} within a partition
3073 it is written completely to the corresponding file. Since lines
3074 are not split even if they overlap a partition, the files written
3075 can be larger or smaller than the partition size, and even empty
3076 if a line is so long as to completely overlap the partition.
3078 For @samp{r} mode, the size of @var{input} is irrelevant,
3079 and so can be a pipe for example.
3081 @item -a @var{length}
3082 @itemx --suffix-length=@var{length}
3084 @opindex --suffix-length
3085 Use suffixes of length @var{length}. The default @var{length} is 2.
3088 @itemx --numeric-suffixes[=@var{from}]
3090 @opindex --numeric-suffixes
3091 Use digits in suffixes rather than lower-case letters. The numerical
3092 suffix counts from @var{from} if specified, 0 otherwise.
3094 @itemx --additional-suffix=@var{suffix}
3095 @opindex --additional-suffix
3096 Append an additional @var{suffix} to output file names. @var{suffix}
3097 must not contain slash.
3100 @itemx --elide-empty-files
3102 @opindex --elide-empty-files
3103 Suppress the generation of zero-length output files. This can happen
3104 with the @option{--number} option if a file is (truncated to be) shorter
3105 than the number requested, or if a line is so long as to completely
3106 span a chunk. The output file sequence numbers, always run consecutively
3107 even when this option is specified.
3112 @opindex --unbuffered
3113 Immediately copy input to output in @option{--number r/...} mode,
3114 which is a much slower mode of operation.
3118 Write a diagnostic just before each output file is opened.
3124 Here are a few examples to illustrate how the
3125 @option{--number} (@option{-n}) option works:
3127 Notice how, by default, one line may be split onto two or more:
3130 $ seq -w 6 10 > k; split -n3 k; head xa?
3143 Use the "l/" modifier to suppress that:
3146 $ seq -w 6 10 > k; split -nl/3 k; head xa?
3159 Use the "r/" modifier to distribute lines in a round-robin fashion:
3162 $ seq -w 6 10 > k; split -nr/3 k; head xa?
3175 You can also extract just the Kth chunk.
3176 This extracts and prints just the 7th "chunk" of 33:
3179 $ seq 100 > k; split -nl/7/33 k
3186 @node csplit invocation
3187 @section @command{csplit}: Split a file into context-determined pieces
3190 @cindex context splitting
3191 @cindex splitting a file into pieces by context
3193 @command{csplit} creates zero or more output files containing sections of
3194 @var{input} (standard input if @var{input} is @samp{-}). Synopsis:
3197 csplit [@var{option}]@dots{} @var{input} @var{pattern}@dots{}
3200 The contents of the output files are determined by the @var{pattern}
3201 arguments, as detailed below. An error occurs if a @var{pattern}
3202 argument refers to a nonexistent line of the input file (e.g., if no
3203 remaining line matches a given regular expression). After every
3204 @var{pattern} has been matched, any remaining input is copied into one
3207 By default, @command{csplit} prints the number of bytes written to each
3208 output file after it has been created.
3210 The types of pattern arguments are:
3215 Create an output file containing the input up to but not including line
3216 @var{n} (a positive integer). If followed by a repeat count, also
3217 create an output file containing the next @var{n} lines of the input
3218 file once for each repeat.
3220 @item /@var{regexp}/[@var{offset}]
3221 Create an output file containing the current line up to (but not
3222 including) the next line of the input file that contains a match for
3223 @var{regexp}. The optional @var{offset} is an integer.
3224 If it is given, the input up to (but not including) the
3225 matching line plus or minus @var{offset} is put into the output file,
3226 and the line after that begins the next section of input.
3228 @item %@var{regexp}%[@var{offset}]
3229 Like the previous type, except that it does not create an output
3230 file, so that section of the input file is effectively ignored.
3232 @item @{@var{repeat-count}@}
3233 Repeat the previous pattern @var{repeat-count} additional
3234 times. The @var{repeat-count} can either be a positive integer or an
3235 asterisk, meaning repeat as many times as necessary until the input is
3240 The output files' names consist of a prefix (@samp{xx} by default)
3241 followed by a suffix. By default, the suffix is an ascending sequence
3242 of two-digit decimal numbers from @samp{00} to @samp{99}. In any case,
3243 concatenating the output files in sorted order by file name produces the
3244 original input file.
3246 By default, if @command{csplit} encounters an error or receives a hangup,
3247 interrupt, quit, or terminate signal, it removes any output files
3248 that it has created so far before it exits.
3250 The program accepts the following options. Also see @ref{Common options}.
3254 @item -f @var{prefix}
3255 @itemx --prefix=@var{prefix}
3258 @cindex output file name prefix
3259 Use @var{prefix} as the output file name prefix.
3261 @item -b @var{suffix}
3262 @itemx --suffix=@var{suffix}
3265 @cindex output file name suffix
3266 Use @var{suffix} as the output file name suffix. When this option is
3267 specified, the suffix string must include exactly one
3268 @code{printf(3)}-style conversion specification, possibly including
3269 format specification flags, a field width, a precision specifications,
3270 or all of these kinds of modifiers. The format letter must convert a
3271 binary unsigned integer argument to readable form. The format letters
3272 @samp{d} and @samp{i} are aliases for @samp{u}, and the
3273 @samp{u}, @samp{o}, @samp{x}, and @samp{X} conversions are allowed. The
3274 entire @var{suffix} is given (with the current output file number) to
3275 @code{sprintf(3)} to form the file name suffixes for each of the
3276 individual output files in turn. If this option is used, the
3277 @option{--digits} option is ignored.
3279 @item -n @var{digits}
3280 @itemx --digits=@var{digits}
3283 Use output file names containing numbers that are @var{digits} digits
3284 long instead of the default 2.
3289 @opindex --keep-files
3290 Do not remove output files when errors are encountered.
3293 @itemx --elide-empty-files
3295 @opindex --elide-empty-files
3296 Suppress the generation of zero-length output files. (In cases where
3297 the section delimiters of the input file are supposed to mark the first
3298 lines of each of the sections, the first output file will generally be a
3299 zero-length file unless you use this option.) The output file sequence
3300 numbers always run consecutively starting from 0, even when this option
3311 Do not print counts of output file sizes.
3317 Here is an example of its usage.
3318 First, create an empty directory for the exercise,
3325 Now, split the sequence of 1..14 on lines that end with 0 or 5:
3328 $ seq 14 | csplit - '/[05]$/' '@{*@}'
3334 Each number printed above is the size of an output
3335 file that csplit has just created.
3336 List the names of those output files:
3343 Use @command{head} to show their contents:
3368 @node Summarizing files
3369 @chapter Summarizing files
3371 @cindex summarizing files
3373 These commands generate just a few numbers representing entire
3377 * wc invocation:: Print newline, word, and byte counts.
3378 * sum invocation:: Print checksum and block counts.
3379 * cksum invocation:: Print CRC checksum and byte counts.
3380 * md5sum invocation:: Print or check MD5 digests.
3381 * sha1sum invocation:: Print or check SHA-1 digests.
3382 * sha2 utilities:: Print or check SHA-2 digests.
3387 @section @command{wc}: Print newline, word, and byte counts
3391 @cindex character count
3395 @command{wc} counts the number of bytes, characters, whitespace-separated
3396 words, and newlines in each given @var{file}, or standard input if none
3397 are given or for a @var{file} of @samp{-}. Synopsis:
3400 wc [@var{option}]@dots{} [@var{file}]@dots{}
3403 @cindex total counts
3404 @command{wc} prints one line of counts for each file, and if the file was
3405 given as an argument, it prints the file name following the counts. If
3406 more than one @var{file} is given, @command{wc} prints a final line
3407 containing the cumulative counts, with the file name @file{total}. The
3408 counts are printed in this order: newlines, words, characters, bytes,
3409 maximum line length.
3410 Each count is printed right-justified in a field with at least one
3411 space between fields so that the numbers and file names normally line
3412 up nicely in columns. The width of the count fields varies depending
3413 on the inputs, so you should not depend on a particular field width.
3414 However, as a @acronym{GNU} extension, if only one count is printed,
3415 it is guaranteed to be printed without leading spaces.
3417 By default, @command{wc} prints three counts: the newline, words, and byte
3418 counts. Options can specify that only certain counts be printed.
3419 Options do not undo others previously given, so
3426 prints both the byte counts and the word counts.
3428 With the @option{--max-line-length} option, @command{wc} prints the length
3429 of the longest line per file, and if there is more than one file it
3430 prints the maximum (not the sum) of those lengths. The line lengths here
3431 are measured in screen columns, according to the current locale and
3432 assuming tab positions in every 8th column.
3434 The program accepts the following options. Also see @ref{Common options}.
3442 Print only the byte counts.
3448 Print only the character counts.
3454 Print only the word counts.
3460 Print only the newline counts.
3463 @itemx --max-line-length
3465 @opindex --max-line-length
3466 Print only the maximum line lengths.
3468 @macro filesZeroFromOption{cmd,withTotalOption,subListOutput}
3469 @itemx --files0-from=@var{file}
3470 @opindex --files0-from=@var{file}
3471 @c This is commented out to avoid a texi2dvi failure.
3472 @c texi2dvi (GNU Texinfo 4.11) 1.104
3473 @c @cindex including files from @command{\cmd\}
3474 Disallow processing files named on the command line, and instead process
3475 those named in file @var{file}; each name being terminated by a zero byte
3476 (@acronym{ASCII} @sc{nul}).
3477 This is useful \withTotalOption\
3478 when the list of file names is so long that it may exceed a command line
3480 In such cases, running @command{\cmd\} via @command{xargs} is undesirable
3481 because it splits the list into pieces and makes @command{\cmd\} print
3482 \subListOutput\ for each sublist rather than for the entire list.
3483 One way to produce a list of @acronym{ASCII} @sc{nul} terminated file
3484 names is with @sc{gnu}
3485 @command{find}, using its @option{-print0} predicate.
3486 If @var{file} is @samp{-} then the @acronym{ASCII} @sc{nul} terminated
3487 file names are read from standard input.
3489 @filesZeroFromOption{wc,,a total}
3491 For example, to find the length of the longest line in any @file{.c} or
3492 @file{.h} file in the current hierarchy, do this:
3495 find . -name '*.[ch]' -print0 |
3496 wc -L --files0-from=- | tail -n1
3504 @node sum invocation
3505 @section @command{sum}: Print checksum and block counts
3508 @cindex 16-bit checksum
3509 @cindex checksum, 16-bit
3511 @command{sum} computes a 16-bit checksum for each given @var{file}, or
3512 standard input if none are given or for a @var{file} of @samp{-}. Synopsis:
3515 sum [@var{option}]@dots{} [@var{file}]@dots{}
3518 @command{sum} prints the checksum for each @var{file} followed by the
3519 number of blocks in the file (rounded up). If more than one @var{file}
3520 is given, file names are also printed (by default). (With the
3521 @option{--sysv} option, corresponding file names are printed when there is
3522 at least one file argument.)
3524 By default, @sc{gnu} @command{sum} computes checksums using an algorithm
3525 compatible with BSD @command{sum} and prints file sizes in units of
3528 The program accepts the following options. Also see @ref{Common options}.
3534 @cindex BSD @command{sum}
3535 Use the default (BSD compatible) algorithm. This option is included for
3536 compatibility with the System V @command{sum}. Unless @option{-s} was also
3537 given, it has no effect.
3543 @cindex System V @command{sum}
3544 Compute checksums using an algorithm compatible with System V
3545 @command{sum}'s default, and print file sizes in units of 512-byte blocks.
3549 @command{sum} is provided for compatibility; the @command{cksum} program (see
3550 next section) is preferable in new applications.
3555 @node cksum invocation
3556 @section @command{cksum}: Print CRC checksum and byte counts
3559 @cindex cyclic redundancy check
3560 @cindex CRC checksum
3562 @command{cksum} computes a cyclic redundancy check (CRC) checksum for each
3563 given @var{file}, or standard input if none are given or for a
3564 @var{file} of @samp{-}. Synopsis:
3567 cksum [@var{option}]@dots{} [@var{file}]@dots{}
3570 @command{cksum} prints the CRC checksum for each file along with the number
3571 of bytes in the file, and the file name unless no arguments were given.
3573 @command{cksum} is typically used to ensure that files
3574 transferred by unreliable means (e.g., netnews) have not been corrupted,
3575 by comparing the @command{cksum} output for the received files with the
3576 @command{cksum} output for the original files (typically given in the
3579 The CRC algorithm is specified by the @acronym{POSIX} standard. It is not
3580 compatible with the BSD or System V @command{sum} algorithms (see the
3581 previous section); it is more robust.
3583 The only options are @option{--help} and @option{--version}. @xref{Common
3589 @node md5sum invocation
3590 @section @command{md5sum}: Print or check MD5 digests
3594 @cindex 128-bit checksum
3595 @cindex checksum, 128-bit
3596 @cindex fingerprint, 128-bit
3597 @cindex message-digest, 128-bit
3599 @command{md5sum} computes a 128-bit checksum (or @dfn{fingerprint} or
3600 @dfn{message-digest}) for each specified @var{file}.
3602 Note: The MD5 digest is more reliable than a simple CRC (provided by
3603 the @command{cksum} command) for detecting accidental file corruption,
3604 as the chances of accidentally having two files with identical MD5
3605 are vanishingly small. However, it should not be considered secure
3606 against malicious tampering: although finding a file with a given MD5
3607 fingerprint is considered infeasible at the moment, it is known how
3608 to modify certain files, including digital certificates, so that they
3609 appear valid when signed with an MD5 digest.
3610 For more secure hashes, consider using SHA-2. @xref{sha2 utilities}.
3612 If a @var{file} is specified as @samp{-} or if no files are given
3613 @command{md5sum} computes the checksum for the standard input.
3614 @command{md5sum} can also determine whether a file and checksum are
3615 consistent. Synopsis:
3618 md5sum [@var{option}]@dots{} [@var{file}]@dots{}
3621 For each @var{file}, @samp{md5sum} outputs the MD5 checksum, a flag
3622 indicating binary or text input mode, and the file name.
3623 If @var{file} contains a backslash or newline, the
3624 line is started with a backslash, and each problematic character in
3625 the file name is escaped with a backslash, making the output
3626 unambiguous even in the presence of arbitrary file names.
3627 If @var{file} is omitted or specified as @samp{-}, standard input is read.
3629 The program accepts the following options. Also see @ref{Common options}.
3637 @cindex binary input files
3638 Treat each input file as binary, by reading it in binary mode and
3639 outputting a @samp{*} flag. This is the inverse of @option{--text}.
3640 On systems like @acronym{GNU} that do not distinguish between binary
3641 and text files, this option merely flags each input mode as binary:
3642 the MD5 checksum is unaffected. This option is the default on systems
3643 like MS-DOS that distinguish between binary and text files, except
3644 for reading standard input when standard input is a terminal.
3648 Read file names and checksum information (not data) from each
3649 @var{file} (or from stdin if no @var{file} was specified) and report
3650 whether the checksums match the contents of the named files.
3651 The input to this mode of @command{md5sum} is usually the output of
3652 a prior, checksum-generating run of @samp{md5sum}.
3653 Each valid line of input consists of an MD5 checksum, a binary/text
3654 flag, and then a file name.
3655 Binary mode is indicated with @samp{*}, text with @samp{ } (space).
3656 For each such line, @command{md5sum} reads the named file and computes its
3657 MD5 checksum. Then, if the computed message digest does not match the
3658 one on the line with the file name, the file is noted as having
3659 failed the test. Otherwise, the file passes the test.
3660 By default, for each valid line, one line is written to standard
3661 output indicating whether the named file passed the test.
3662 After all checks have been performed, if there were any failures,
3663 a warning is issued to standard error.
3664 Use the @option{--status} option to inhibit that output.
3665 If any listed file cannot be opened or read, if any valid line has
3666 an MD5 checksum inconsistent with the associated file, or if no valid
3667 line is found, @command{md5sum} exits with nonzero status. Otherwise,
3668 it exits successfully.
3672 @cindex verifying MD5 checksums
3673 This option is useful only when verifying checksums.
3674 When verifying checksums, don't generate an 'OK' message per successfully
3675 checked file. Files that fail the verification are reported in the
3676 default one-line-per-file format. If there is any checksum mismatch,
3677 print a warning summarizing the failures to standard error.
3681 @cindex verifying MD5 checksums
3682 This option is useful only when verifying checksums.
3683 When verifying checksums, don't generate the default one-line-per-file
3684 diagnostic and don't output the warning summarizing any failures.
3685 Failures to open or read a file still evoke individual diagnostics to
3687 If all listed files are readable and are consistent with the associated
3688 MD5 checksums, exit successfully. Otherwise exit with a status code
3689 indicating there was a failure.
3695 @cindex text input files
3696 Treat each input file as text, by reading it in text mode and
3697 outputting a @samp{ } flag. This is the inverse of @option{--binary}.
3698 This option is the default on systems like @acronym{GNU} that do not
3699 distinguish between binary and text files. On other systems, it is
3700 the default for reading standard input when standard input is a
3707 @cindex verifying MD5 checksums
3708 When verifying checksums, warn about improperly formatted MD5 checksum lines.
3709 This option is useful only if all but a few lines in the checked input
3714 @cindex verifying MD5 checksums
3715 When verifying checksums,
3716 if one or more input line is invalid,
3717 exit nonzero after all warnings have been issued.
3724 @node sha1sum invocation
3725 @section @command{sha1sum}: Print or check SHA-1 digests
3729 @cindex 160-bit checksum
3730 @cindex checksum, 160-bit
3731 @cindex fingerprint, 160-bit
3732 @cindex message-digest, 160-bit
3734 @command{sha1sum} computes a 160-bit checksum for each specified
3735 @var{file}. The usage and options of this command are precisely the
3736 same as for @command{md5sum}. @xref{md5sum invocation}.
3738 Note: The SHA-1 digest is more secure than MD5, and no collisions of
3739 it are known (different files having the same fingerprint). However,
3740 it is known that they can be produced with considerable, but not
3741 unreasonable, resources. For this reason, it is generally considered
3742 that SHA-1 should be gradually phased out in favor of the more secure
3743 SHA-2 hash algorithms. @xref{sha2 utilities}.
3746 @node sha2 utilities
3747 @section sha2 utilities: Print or check SHA-2 digests
3754 @cindex 224-bit checksum
3755 @cindex 256-bit checksum
3756 @cindex 384-bit checksum
3757 @cindex 512-bit checksum
3758 @cindex checksum, 224-bit
3759 @cindex checksum, 256-bit
3760 @cindex checksum, 384-bit
3761 @cindex checksum, 512-bit
3762 @cindex fingerprint, 224-bit
3763 @cindex fingerprint, 256-bit
3764 @cindex fingerprint, 384-bit
3765 @cindex fingerprint, 512-bit
3766 @cindex message-digest, 224-bit
3767 @cindex message-digest, 256-bit
3768 @cindex message-digest, 384-bit
3769 @cindex message-digest, 512-bit
3771 The commands @command{sha224sum}, @command{sha256sum},
3772 @command{sha384sum} and @command{sha512sum} compute checksums of
3773 various lengths (respectively 224, 256, 384 and 512 bits),
3774 collectively known as the SHA-2 hashes. The usage and options of
3775 these commands are precisely the same as for @command{md5sum}.
3776 @xref{md5sum invocation}.
3778 Note: The SHA384 and SHA512 digests are considerably slower to
3779 compute, especially on 32-bit computers, than SHA224 or SHA256.
3782 @node Operating on sorted files
3783 @chapter Operating on sorted files
3785 @cindex operating on sorted files
3786 @cindex sorted files, operations on
3788 These commands work with (or produce) sorted files.
3791 * sort invocation:: Sort text files.
3792 * shuf invocation:: Shuffle text files.
3793 * uniq invocation:: Uniquify files.
3794 * comm invocation:: Compare two sorted files line by line.
3795 * ptx invocation:: Produce a permuted index of file contents.
3796 * tsort invocation:: Topological sort.
3800 @node sort invocation
3801 @section @command{sort}: Sort text files
3804 @cindex sorting files
3806 @command{sort} sorts, merges, or compares all the lines from the given
3807 files, or standard input if none are given or for a @var{file} of
3808 @samp{-}. By default, @command{sort} writes the results to standard
3812 sort [@var{option}]@dots{} [@var{file}]@dots{}
3815 @command{sort} has three modes of operation: sort (the default), merge,
3816 and check for sortedness. The following options change the operation
3823 @itemx --check=diagnose-first
3826 @cindex checking for sortedness
3827 Check whether the given file is already sorted: if it is not all
3828 sorted, print a diagnostic containing the first out-of-order line and
3829 exit with a status of 1.
3830 Otherwise, exit successfully.
3831 At most one input file can be given.
3834 @itemx --check=quiet
3835 @itemx --check=silent
3838 @cindex checking for sortedness
3839 Exit successfully if the given file is already sorted, and
3840 exit with status 1 otherwise.
3841 At most one input file can be given.
3842 This is like @option{-c}, except it does not print a diagnostic.
3848 @cindex merging sorted files
3849 Merge the given files by sorting them as a group. Each input file must
3850 always be individually sorted. It always works to sort instead of
3851 merge; merging is provided because it is faster, in the case where it
3856 @cindex sort stability
3857 @cindex sort's last-resort comparison
3858 A pair of lines is compared as follows:
3859 @command{sort} compares each pair of fields, in the
3860 order specified on the command line, according to the associated
3861 ordering options, until a difference is found or no fields are left.
3862 If no key fields are specified, @command{sort} uses a default key of
3863 the entire line. Finally, as a last resort when all keys compare
3864 equal, @command{sort} compares entire lines as if no ordering options
3865 other than @option{--reverse} (@option{-r}) were specified. The
3866 @option{--stable} (@option{-s}) option disables this @dfn{last-resort
3867 comparison} so that lines in which all fields compare equal are left
3868 in their original relative order. The @option{--unique}
3869 (@option{-u}) option also disables the last-resort comparison.
3873 Unless otherwise specified, all comparisons use the character collating
3874 sequence specified by the @env{LC_COLLATE} locale.@footnote{If you
3875 use a non-@acronym{POSIX} locale (e.g., by setting @env{LC_ALL}
3876 to @samp{en_US}), then @command{sort} may produce output that is sorted
3877 differently than you're accustomed to. In that case, set the @env{LC_ALL}
3878 environment variable to @samp{C}. Note that setting only @env{LC_COLLATE}
3879 has two problems. First, it is ineffective if @env{LC_ALL} is also set.
3880 Second, it has undefined behavior if @env{LC_CTYPE} (or @env{LANG}, if
3881 @env{LC_CTYPE} is unset) is set to an incompatible value. For example,
3882 you get undefined behavior if @env{LC_CTYPE} is @code{ja_JP.PCK} but
3883 @env{LC_COLLATE} is @code{en_US.UTF-8}.}
3885 @sc{gnu} @command{sort} (as specified for all @sc{gnu} utilities) has no
3886 limit on input line length or restrictions on bytes allowed within lines.
3887 In addition, if the final byte of an input file is not a newline, @sc{gnu}
3888 @command{sort} silently supplies one. A line's trailing newline is not
3889 part of the line for comparison purposes.
3891 @cindex exit status of @command{sort}
3895 0 if no error occurred
3896 1 if invoked with @option{-c} or @option{-C} and the input is not sorted
3897 2 if an error occurred
3901 If the environment variable @env{TMPDIR} is set, @command{sort} uses its
3902 value as the directory for temporary files instead of @file{/tmp}. The
3903 @option{--temporary-directory} (@option{-T}) option in turn overrides
3904 the environment variable.
3906 The following options affect the ordering of output lines. They may be
3907 specified globally or as part of a specific key field. If no key
3908 fields are specified, global options apply to comparison of entire
3909 lines; otherwise the global options are inherited by key fields that do
3910 not specify any special options of their own. In pre-@acronym{POSIX}
3911 versions of @command{sort}, global options affect only later key fields,
3912 so portable shell scripts should specify global options first.
3917 @itemx --ignore-leading-blanks
3919 @opindex --ignore-leading-blanks
3920 @cindex blanks, ignoring leading
3922 Ignore leading blanks when finding sort keys in each line.
3923 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3924 can change this. Note blanks may be ignored by your locale's collating
3925 rules, but without this option they will be significant for character
3926 positions specified in keys with the @option{-k} option.
3929 @itemx --dictionary-order
3931 @opindex --dictionary-order
3932 @cindex dictionary order
3933 @cindex phone directory order
3934 @cindex telephone directory order
3936 Sort in @dfn{phone directory} order: ignore all characters except
3937 letters, digits and blanks when sorting.
3938 By default letters and digits are those of @acronym{ASCII} and a blank
3939 is a space or a tab, but the @env{LC_CTYPE} locale can change this.
3942 @itemx --ignore-case
3944 @opindex --ignore-case
3945 @cindex ignoring case
3946 @cindex case folding
3948 Fold lowercase characters into the equivalent uppercase characters when
3949 comparing so that, for example, @samp{b} and @samp{B} sort as equal.
3950 The @env{LC_CTYPE} locale determines character types.
3951 When used with @option{--unique} those lower case equivalent lines are
3952 thrown away. (There is currently no way to throw away the upper case
3953 equivalent instead. (Any @option{--reverse} given would only affect
3954 the final result, after the throwing away.))
3957 @itemx --general-numeric-sort
3958 @itemx --sort=general-numeric
3960 @opindex --general-numeric-sort
3962 @cindex general numeric sort
3964 Sort numerically, converting a prefix of each line to a long
3965 double-precision floating point number. @xref{Floating point}.
3966 Do not report overflow, underflow, or conversion errors.
3967 Use the following collating sequence:
3971 Lines that do not start with numbers (all considered to be equal).
3973 NaNs (``Not a Number'' values, in IEEE floating point arithmetic)
3974 in a consistent but machine-dependent order.
3978 Finite numbers in ascending numeric order (with @math{-0} and @math{+0} equal).
3983 Use this option only if there is no alternative; it is much slower than
3984 @option{--numeric-sort} (@option{-n}) and it can lose information when
3985 converting to floating point.
3988 @itemx --human-numeric-sort
3989 @itemx --sort=human-numeric
3991 @opindex --human-numeric-sort
3993 @cindex human numeric sort
3995 Sort numerically, first by numeric sign (negative, zero, or positive);
3996 then by @acronym{SI} suffix (either empty, or @samp{k} or @samp{K}, or
3997 one of @samp{MGTPEZY}, in that order; @pxref{Block size}); and finally
3998 by numeric value. For example, @samp{1023M} sorts before @samp{1G}
3999 because @samp{M} (mega) precedes @samp{G} (giga) as an @acronym{SI}
4000 suffix. This option sorts values that are consistently scaled to the
4001 nearest suffix, regardless of whether suffixes denote powers of 1000
4002 or 1024, and it therefore sorts the output of any single invocation of
4003 the @command{df}, @command{du}, or @command{ls} commands that are
4004 invoked with their @option{--human-readable} or @option{--si} options.
4005 The syntax for numbers is the same as for the @option{--numeric-sort}
4006 option; the @acronym{SI} suffix must immediately follow the number.
4009 @itemx --ignore-nonprinting
4011 @opindex --ignore-nonprinting
4012 @cindex nonprinting characters, ignoring
4013 @cindex unprintable characters, ignoring
4015 Ignore nonprinting characters.
4016 The @env{LC_CTYPE} locale determines character types.
4017 This option has no effect if the stronger @option{--dictionary-order}
4018 (@option{-d}) option is also given.
4024 @opindex --month-sort
4026 @cindex months, sorting by
4028 An initial string, consisting of any amount of blanks, followed
4029 by a month name abbreviation, is folded to UPPER case and
4030 compared in the order @samp{JAN} < @samp{FEB} < @dots{} < @samp{DEC}.
4031 Invalid names compare low to valid names. The @env{LC_TIME} locale
4032 category determines the month spellings.
4033 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4037 @itemx --numeric-sort
4038 @itemx --sort=numeric
4040 @opindex --numeric-sort
4042 @cindex numeric sort
4044 Sort numerically. The number begins each line and consists
4045 of optional blanks, an optional @samp{-} sign, and zero or more
4046 digits possibly separated by thousands separators, optionally followed
4047 by a decimal-point character and zero or more digits. An empty
4048 number is treated as @samp{0}. The @env{LC_NUMERIC}
4049 locale specifies the decimal-point character and thousands separator.
4050 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4053 Comparison is exact; there is no rounding error.
4055 Neither a leading @samp{+} nor exponential notation is recognized.
4056 To compare such strings numerically, use the
4057 @option{--general-numeric-sort} (@option{-g}) option.
4060 @itemx --version-sort
4062 @opindex --version-sort
4063 @cindex version number sort
4064 Sort by version name and number. It behaves like a standard sort,
4065 except that each sequence of decimal digits is treated numerically
4066 as an index/version number. (@xref{Details about version sort}.)
4072 @cindex reverse sorting
4073 Reverse the result of comparison, so that lines with greater key values
4074 appear earlier in the output instead of later.
4077 @itemx --random-sort
4078 @itemx --sort=random
4080 @opindex --random-sort
4083 Sort by hashing the input keys and then sorting the hash values.
4084 Choose the hash function at random, ensuring that it is free of
4085 collisions so that differing keys have differing hash values. This is
4086 like a random permutation of the inputs (@pxref{shuf invocation}),
4087 except that keys with the same value sort together.
4089 If multiple random sort fields are specified, the same random hash
4090 function is used for all fields. To use different random hash
4091 functions for different fields, you can invoke @command{sort} more
4094 The choice of hash function is affected by the
4095 @option{--random-source} option.
4103 @item --compress-program=@var{prog}
4104 Compress any temporary files with the program @var{prog}.
4106 With no arguments, @var{prog} must compress standard input to standard
4107 output, and when given the @option{-d} option it must decompress
4108 standard input to standard output.
4110 Terminate with an error if @var{prog} exits with nonzero status.
4112 White space and the backslash character should not appear in
4113 @var{prog}; they are reserved for future use.
4115 @filesZeroFromOption{sort,,sorted output}
4117 @item -k @var{pos1}[,@var{pos2}]
4118 @itemx --key=@var{pos1}[,@var{pos2}]
4122 Specify a sort field that consists of the part of the line between
4123 @var{pos1} and @var{pos2} (or the end of the line, if @var{pos2} is
4124 omitted), @emph{inclusive}.
4126 Each @var{pos} has the form @samp{@var{f}[.@var{c}][@var{opts}]},
4127 where @var{f} is the number of the field to use, and @var{c} is the number
4128 of the first character from the beginning of the field. Fields and character
4129 positions are numbered starting with 1; a character position of zero in
4130 @var{pos2} indicates the field's last character. If @samp{.@var{c}} is
4131 omitted from @var{pos1}, it defaults to 1 (the beginning of the field);
4132 if omitted from @var{pos2}, it defaults to 0 (the end of the field).
4133 @var{opts} are ordering options, allowing individual keys to be sorted
4134 according to different rules; see below for details. Keys can span
4137 Example: To sort on the second field, use @option{--key=2,2}
4138 (@option{-k 2,2}). See below for more notes on keys and more examples.
4139 See also the @option{--debug} option to help determine the part
4140 of the line being used in the sort.
4143 Highlight the portion of each line used for sorting.
4144 Also issue warnings about questionable usage to stderr.
4146 @item --batch-size=@var{nmerge}
4147 @opindex --batch-size
4148 @cindex number of inputs to merge, nmerge
4149 Merge at most @var{nmerge} inputs at once.
4151 When @command{sort} has to merge more than @var{nmerge} inputs,
4152 it merges them in groups of @var{nmerge}, saving the result in
4153 a temporary file, which is then used as an input in a subsequent merge.
4155 A large value of @var{nmerge} may improve merge performance and decrease
4156 temporary storage utilization at the expense of increased memory usage
4157 and I/O. Conversely a small value of @var{nmerge} may reduce memory
4158 requirements and I/O at the expense of temporary storage consumption and
4161 The value of @var{nmerge} must be at least 2. The default value is
4162 currently 16, but this is implementation-dependent and may change in
4165 The value of @var{nmerge} may be bounded by a resource limit for open
4166 file descriptors. The commands @samp{ulimit -n} or @samp{getconf
4167 OPEN_MAX} may display limits for your systems; these limits may be
4168 modified further if your program already has some files open, or if
4169 the operating system has other limits on the number of open files. If
4170 the value of @var{nmerge} exceeds the resource limit, @command{sort}
4171 silently uses a smaller value.
4173 @item -o @var{output-file}
4174 @itemx --output=@var{output-file}
4177 @cindex overwriting of input, allowed
4178 Write output to @var{output-file} instead of standard output.
4179 Normally, @command{sort} reads all input before opening
4180 @var{output-file}, so you can safely sort a file in place by using
4181 commands like @code{sort -o F F} and @code{cat F | sort -o F}.
4182 However, @command{sort} with @option{--merge} (@option{-m}) can open
4183 the output file before reading all input, so a command like @code{cat
4184 F | sort -m -o F - G} is not safe as @command{sort} might start
4185 writing @file{F} before @command{cat} is done reading it.
4187 @vindex POSIXLY_CORRECT
4188 On newer systems, @option{-o} cannot appear after an input file if
4189 @env{POSIXLY_CORRECT} is set, e.g., @samp{sort F -o F}. Portable
4190 scripts should specify @option{-o @var{output-file}} before any input
4193 @item --random-source=@var{file}
4194 @opindex --random-source
4195 @cindex random source for sorting
4196 Use @var{file} as a source of random data used to determine which
4197 random hash function to use with the @option{-R} option. @xref{Random
4204 @cindex sort stability
4205 @cindex sort's last-resort comparison
4207 Make @command{sort} stable by disabling its last-resort comparison.
4208 This option has no effect if no fields or global ordering options
4209 other than @option{--reverse} (@option{-r}) are specified.
4212 @itemx --buffer-size=@var{size}
4214 @opindex --buffer-size
4215 @cindex size for main memory sorting
4216 Use a main-memory sort buffer of the given @var{size}. By default,
4217 @var{size} is in units of 1024 bytes. Appending @samp{%} causes
4218 @var{size} to be interpreted as a percentage of physical memory.
4219 Appending @samp{K} multiplies @var{size} by 1024 (the default),
4220 @samp{M} by 1,048,576, @samp{G} by 1,073,741,824, and so on for
4221 @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}. Appending
4222 @samp{b} causes @var{size} to be interpreted as a byte count, with no
4225 This option can improve the performance of @command{sort} by causing it
4226 to start with a larger or smaller sort buffer than the default.
4227 However, this option affects only the initial buffer size. The buffer
4228 grows beyond @var{size} if @command{sort} encounters input lines larger
4231 @item -t @var{separator}
4232 @itemx --field-separator=@var{separator}
4234 @opindex --field-separator
4235 @cindex field separator character
4236 Use character @var{separator} as the field separator when finding the
4237 sort keys in each line. By default, fields are separated by the empty
4238 string between a non-blank character and a blank character.
4239 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4242 That is, given the input line @w{@samp{ foo bar}}, @command{sort} breaks it
4243 into fields @w{@samp{ foo}} and @w{@samp{ bar}}. The field separator is
4244 not considered to be part of either the field preceding or the field
4245 following, so with @samp{sort @w{-t " "}} the same input line has
4246 three fields: an empty field, @samp{foo}, and @samp{bar}.
4247 However, fields that extend to the end of the line,
4248 as @option{-k 2}, or fields consisting of a range, as @option{-k 2,3},
4249 retain the field separators present between the endpoints of the range.
4251 To specify @acronym{ASCII} @sc{nul} as the field separator,
4252 use the two-character string @samp{\0}, e.g., @samp{sort -t '\0'}.
4254 @item -T @var{tempdir}
4255 @itemx --temporary-directory=@var{tempdir}
4257 @opindex --temporary-directory
4258 @cindex temporary directory
4260 Use directory @var{tempdir} to store temporary files, overriding the
4261 @env{TMPDIR} environment variable. If this option is given more than
4262 once, temporary files are stored in all the directories given. If you
4263 have a large sort or merge that is I/O-bound, you can often improve
4264 performance by using this option to specify directories on different
4265 disks and controllers.
4267 @item --parallel=@var{n}
4269 @cindex multithreaded sort
4270 Set the number of sorts run in parallel to @var{n}. By default,
4271 @var{n} is set to the number of available processors, but limited
4272 to 8, as there are diminishing performance gains after that.
4273 Note also that using @var{n} threads increases the memory usage by
4274 a factor of log @var{n}. Also see @ref{nproc invocation}.
4280 @cindex uniquifying output
4282 Normally, output only the first of a sequence of lines that compare
4283 equal. For the @option{--check} (@option{-c} or @option{-C}) option,
4284 check that no pair of consecutive lines compares equal.
4286 This option also disables the default last-resort comparison.
4288 The commands @code{sort -u} and @code{sort | uniq} are equivalent, but
4289 this equivalence does not extend to arbitrary @command{sort} options.
4290 For example, @code{sort -n -u} inspects only the value of the initial
4291 numeric string when checking for uniqueness, whereas @code{sort -n |
4292 uniq} inspects the entire line. @xref{uniq invocation}.
4294 @macro zeroTerminatedOption
4296 @itemx --zero-terminated
4298 @opindex --zero-terminated
4299 @cindex process zero-terminated items
4300 Delimit items with a zero byte rather than a newline (@acronym{ASCII} @sc{lf}).
4301 I.E. treat input as items separated by @acronym{ASCII} @sc{nul}
4302 and terminate output items with @acronym{ASCII} @sc{nul}.
4303 This option can be useful in conjunction with @samp{perl -0} or
4304 @samp{find -print0} and @samp{xargs -0} which do the same in order to
4305 reliably handle arbitrary file names (even those containing blanks
4306 or other special characters).
4308 @zeroTerminatedOption
4312 Historical (BSD and System V) implementations of @command{sort} have
4313 differed in their interpretation of some options, particularly
4314 @option{-b}, @option{-f}, and @option{-n}.
4315 @sc{gnu} sort follows the @acronym{POSIX}
4316 behavior, which is usually (but not always!) like the System V behavior.
4317 According to @acronym{POSIX}, @option{-n} no longer implies @option{-b}. For
4318 consistency, @option{-M} has been changed in the same way. This may
4319 affect the meaning of character positions in field specifications in
4320 obscure cases. The only fix is to add an explicit @option{-b}.
4322 A position in a sort field specified with @option{-k} may have any
4323 of the option letters @samp{MbdfghinRrV} appended to it, in which case no
4324 global ordering options are inherited by that particular field. The
4325 @option{-b} option may be independently attached to either or both of
4326 the start and end positions of a field specification, and if it is
4327 inherited from the global options it will be attached to both.
4328 If input lines can contain leading or adjacent blanks and @option{-t}
4329 is not used, then @option{-k} is typically combined with @option{-b} or
4330 an option that implicitly ignores leading blanks (@samp{Mghn}) as otherwise
4331 the varying numbers of leading blanks in fields can cause confusing results.
4333 If the start position in a sort field specifier falls after the end of
4334 the line or after the end field, the field is empty. If the @option{-b}
4335 option was specified, the @samp{.@var{c}} part of a field specification
4336 is counted from the first nonblank character of the field.
4338 @vindex _POSIX2_VERSION
4339 @vindex POSIXLY_CORRECT
4340 On older systems, @command{sort} supports an obsolete origin-zero
4341 syntax @samp{+@var{pos1} [-@var{pos2}]} for specifying sort keys.
4342 The obsolete sequence @samp{sort +@var{a}.@var{x} -@var{b}.@var{y}}
4343 is equivalent to @samp{sort -k @var{a+1}.@var{x+1},@var{b}} if @var{y}
4344 is @samp{0} or absent, otherwise it is equivalent to @samp{sort -k
4345 @var{a+1}.@var{x+1},@var{b+1}.@var{y}}.
4347 This obsolete behavior can be enabled or disabled with the
4348 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4349 conformance}); it can also be enabled when @env{POSIXLY_CORRECT} is
4350 not set by using the obsolete syntax with @samp{-@var{pos2}} present.
4352 Scripts intended for use on standard hosts should avoid obsolete
4353 syntax and should use @option{-k} instead. For example, avoid
4354 @samp{sort +2}, since it might be interpreted as either @samp{sort
4355 ./+2} or @samp{sort -k 3}. If your script must also run on hosts that
4356 support only the obsolete syntax, it can use a test like @samp{if sort
4357 -k 1 </dev/null >/dev/null 2>&1; then @dots{}} to decide which syntax
4360 Here are some examples to illustrate various combinations of options.
4365 Sort in descending (reverse) numeric order.
4372 Run no more than 4 sorts concurrently, using a buffer size of 10M.
4375 sort --parallel=4 -S 10M
4379 Sort alphabetically, omitting the first and second fields
4380 and the blanks at the start of the third field.
4381 This uses a single key composed of the characters beginning
4382 at the start of the first nonblank character in field three
4383 and extending to the end of each line.
4390 Sort numerically on the second field and resolve ties by sorting
4391 alphabetically on the third and fourth characters of field five.
4392 Use @samp{:} as the field delimiter.
4395 sort -t : -k 2,2n -k 5.3,5.4
4398 Note that if you had written @option{-k 2n} instead of @option{-k 2,2n}
4399 @command{sort} would have used all characters beginning in the second field
4400 and extending to the end of the line as the primary @emph{numeric}
4401 key. For the large majority of applications, treating keys spanning
4402 more than one field as numeric will not do what you expect.
4404 Also note that the @samp{n} modifier was applied to the field-end
4405 specifier for the first key. It would have been equivalent to
4406 specify @option{-k 2n,2} or @option{-k 2n,2n}. All modifiers except
4407 @samp{b} apply to the associated @emph{field}, regardless of whether
4408 the modifier character is attached to the field-start and/or the
4409 field-end part of the key specifier.
4412 Sort the password file on the fifth field and ignore any
4413 leading blanks. Sort lines with equal values in field five
4414 on the numeric user ID in field three. Fields are separated
4418 sort -t : -k 5b,5 -k 3,3n /etc/passwd
4419 sort -t : -n -k 5b,5 -k 3,3 /etc/passwd
4420 sort -t : -b -k 5,5 -k 3,3n /etc/passwd
4423 These three commands have equivalent effect. The first specifies that
4424 the first key's start position ignores leading blanks and the second
4425 key is sorted numerically. The other two commands rely on global
4426 options being inherited by sort keys that lack modifiers. The inheritance
4427 works in this case because @option{-k 5b,5b} and @option{-k 5b,5} are
4428 equivalent, as the location of a field-end lacking a @samp{.@var{c}}
4429 character position is not affected by whether initial blanks are
4433 Sort a set of log files, primarily by IPv4 address and secondarily by
4434 time stamp. If two lines' primary and secondary keys are identical,
4435 output the lines in the same order that they were input. The log
4436 files contain lines that look like this:
4439 4.150.156.3 - - [01/Apr/2004:06:31:51 +0000] message 1
4440 211.24.3.231 - - [24/Apr/2004:20:17:39 +0000] message 2
4443 Fields are separated by exactly one space. Sort IPv4 addresses
4444 lexicographically, e.g., 212.61.52.2 sorts before 212.129.233.201
4445 because 61 is less than 129.
4448 sort -s -t ' ' -k 4.9n -k 4.5M -k 4.2n -k 4.14,4.21 file*.log |
4449 sort -s -t '.' -k 1,1n -k 2,2n -k 3,3n -k 4,4n
4452 This example cannot be done with a single @command{sort} invocation,
4453 since IPv4 address components are separated by @samp{.} while dates
4454 come just after a space. So it is broken down into two invocations of
4455 @command{sort}: the first sorts by time stamp and the second by IPv4
4456 address. The time stamp is sorted by year, then month, then day, and
4457 finally by hour-minute-second field, using @option{-k} to isolate each
4458 field. Except for hour-minute-second there's no need to specify the
4459 end of each key field, since the @samp{n} and @samp{M} modifiers sort
4460 based on leading prefixes that cannot cross field boundaries. The
4461 IPv4 addresses are sorted lexicographically. The second sort uses
4462 @samp{-s} so that ties in the primary key are broken by the secondary
4463 key; the first sort uses @samp{-s} so that the combination of the two
4467 Generate a tags file in case-insensitive sorted order.
4470 find src -type f -print0 | sort -z -f | xargs -0 etags --append
4473 The use of @option{-print0}, @option{-z}, and @option{-0} in this case means
4474 that file names that contain blanks or other special characters are
4476 by the sort operation.
4478 @c This example is a bit contrived and needs more explanation.
4480 @c Sort records separated by an arbitrary string by using a pipe to convert
4481 @c each record delimiter string to @samp{\0}, then using sort's -z option,
4482 @c and converting each @samp{\0} back to the original record delimiter.
4485 @c printf 'c\n\nb\n\na\n' |
4486 @c perl -0pe 's/\n\n/\n\0/g' |
4488 @c perl -0pe 's/\0/\n/g'
4492 Use the common @acronym{DSU, Decorate Sort Undecorate} idiom to
4493 sort lines according to their length.
4496 awk '@{print length, $0@}' /etc/passwd | sort -n | cut -f2- -d' '
4499 In general this technique can be used to sort data that the @command{sort}
4500 command does not support, or is inefficient at, sorting directly.
4503 Shuffle a list of directories, but preserve the order of files within
4504 each directory. For instance, one could use this to generate a music
4505 playlist in which albums are shuffled but the songs of each album are
4509 ls */* | sort -t / -k 1,1R -k 2,2
4515 @node shuf invocation
4516 @section @command{shuf}: Shuffling text
4519 @cindex shuffling files
4521 @command{shuf} shuffles its input by outputting a random permutation
4522 of its input lines. Each output permutation is equally likely.
4526 shuf [@var{option}]@dots{} [@var{file}]
4527 shuf -e [@var{option}]@dots{} [@var{arg}]@dots{}
4528 shuf -i @var{lo}-@var{hi} [@var{option}]@dots{}
4531 @command{shuf} has three modes of operation that affect where it
4532 obtains its input lines. By default, it reads lines from standard
4533 input. The following options change the operation mode:
4541 @cindex command-line operands to shuffle
4542 Treat each command-line operand as an input line.
4544 @item -i @var{lo}-@var{hi}
4545 @itemx --input-range=@var{lo}-@var{hi}
4547 @opindex --input-range
4548 @cindex input range to shuffle
4549 Act as if input came from a file containing the range of unsigned
4550 decimal integers @var{lo}@dots{}@var{hi}, one per line.
4554 @command{shuf}'s other options can affect its behavior in all
4559 @item -n @var{lines}
4560 @itemx --head-count=@var{count}
4562 @opindex --head-count
4563 @cindex head of output
4564 Output at most @var{count} lines. By default, all input lines are
4567 @item -o @var{output-file}
4568 @itemx --output=@var{output-file}
4571 @cindex overwriting of input, allowed
4572 Write output to @var{output-file} instead of standard output.
4573 @command{shuf} reads all input before opening
4574 @var{output-file}, so you can safely shuffle a file in place by using
4575 commands like @code{shuf -o F <F} and @code{cat F | shuf -o F}.
4577 @item --random-source=@var{file}
4578 @opindex --random-source
4579 @cindex random source for shuffling
4580 Use @var{file} as a source of random data used to determine which
4581 permutation to generate. @xref{Random sources}.
4583 @zeroTerminatedOption
4599 might produce the output
4609 Similarly, the command:
4612 shuf -e clubs hearts diamonds spades
4626 and the command @samp{shuf -i 1-4} might output:
4636 These examples all have four input lines, so @command{shuf} might
4637 produce any of the twenty-four possible permutations of the input. In
4638 general, if there are @var{n} input lines, there are @var{n}! (i.e.,
4639 @var{n} factorial, or @var{n} * (@var{n} - 1) * @dots{} * 1) possible
4640 output permutations.
4645 @node uniq invocation
4646 @section @command{uniq}: Uniquify files
4649 @cindex uniquify files
4651 @command{uniq} writes the unique lines in the given @file{input}, or
4652 standard input if nothing is given or for an @var{input} name of
4656 uniq [@var{option}]@dots{} [@var{input} [@var{output}]]
4659 By default, @command{uniq} prints its input lines, except that
4660 it discards all but the first of adjacent repeated lines, so that
4661 no output lines are repeated. Optionally, it can instead discard
4662 lines that are not repeated, or all repeated lines.
4664 The input need not be sorted, but repeated input lines are detected
4665 only if they are adjacent. If you want to discard non-adjacent
4666 duplicate lines, perhaps you want to use @code{sort -u}.
4667 @xref{sort invocation}.
4670 Comparisons honor the rules specified by the @env{LC_COLLATE}
4673 If no @var{output} file is specified, @command{uniq} writes to standard
4676 The program accepts the following options. Also see @ref{Common options}.
4681 @itemx --skip-fields=@var{n}
4683 @opindex --skip-fields
4684 Skip @var{n} fields on each line before checking for uniqueness. Use
4685 a null string for comparison if a line has fewer than @var{n} fields. Fields
4686 are sequences of non-space non-tab characters that are separated from
4687 each other by at least one space or tab.
4689 For compatibility @command{uniq} supports an obsolete option syntax
4690 @option{-@var{n}}. New scripts should use @option{-f @var{n}} instead.
4693 @itemx --skip-chars=@var{n}
4695 @opindex --skip-chars
4696 Skip @var{n} characters before checking for uniqueness. Use a null string
4697 for comparison if a line has fewer than @var{n} characters. If you use both
4698 the field and character skipping options, fields are skipped over first.
4700 @vindex _POSIX2_VERSION
4701 On older systems, @command{uniq} supports an obsolete option syntax
4703 This obsolete behavior can be enabled or disabled with the
4704 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4705 conformance}), but portable scripts should avoid commands whose
4706 behavior depends on this variable.
4707 For example, use @samp{uniq ./+10} or @samp{uniq -s 10} rather than
4708 the ambiguous @samp{uniq +10}.
4714 Print the number of times each line occurred along with the line.
4717 @itemx --ignore-case
4719 @opindex --ignore-case
4720 Ignore differences in case when comparing lines.
4726 @cindex repeated lines, outputting
4727 Discard lines that are not repeated. When used by itself, this option
4728 causes @command{uniq} to print the first copy of each repeated line,
4732 @itemx --all-repeated[=@var{delimit-method}]
4734 @opindex --all-repeated
4735 @cindex all repeated lines, outputting
4736 Do not discard the second and subsequent repeated input lines,
4737 but discard lines that are not repeated.
4738 This option is useful mainly in conjunction with other options e.g.,
4739 to ignore case or to compare only selected fields.
4740 The optional @var{delimit-method} tells how to delimit
4741 groups of repeated lines, and must be one of the following:
4746 Do not delimit groups of repeated lines.
4747 This is equivalent to @option{--all-repeated} (@option{-D}).
4750 Output a newline before each group of repeated lines.
4751 With @option{--zero-terminated} (@option{-z}), use a zero
4752 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4755 Separate groups of repeated lines with a single newline.
4756 With @option{--zero-terminated} (@option{-z}), use a zero
4757 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4758 This is the same as using @samp{prepend}, except that
4759 no delimiter is inserted before the first group, and hence
4760 may be better suited for output direct to users.
4763 Note that when groups are delimited and the input stream contains
4764 two or more consecutive blank lines, then the output is ambiguous.
4765 To avoid that, filter the input through @samp{tr -s '\n'} to replace
4766 each sequence of consecutive newlines with a single newline.
4768 This is a @sc{gnu} extension.
4769 @c FIXME: give an example showing *how* it's useful
4775 @cindex unique lines, outputting
4776 Discard the first repeated line. When used by itself, this option
4777 causes @command{uniq} to print unique lines, and nothing else.
4780 @itemx --check-chars=@var{n}
4782 @opindex --check-chars
4783 Compare at most @var{n} characters on each line (after skipping any specified
4784 fields and characters). By default the entire rest of the lines are
4787 @zeroTerminatedOption
4794 @node comm invocation
4795 @section @command{comm}: Compare two sorted files line by line
4798 @cindex line-by-line comparison
4799 @cindex comparing sorted files
4801 @command{comm} writes to standard output lines that are common, and lines
4802 that are unique, to two input files; a file name of @samp{-} means
4803 standard input. Synopsis:
4806 comm [@var{option}]@dots{} @var{file1} @var{file2}
4810 Before @command{comm} can be used, the input files must be sorted using the
4811 collating sequence specified by the @env{LC_COLLATE} locale.
4812 If an input file ends in a non-newline
4813 character, a newline is silently appended. The @command{sort} command with
4814 no options always outputs a file that is suitable input to @command{comm}.
4816 @cindex differing lines
4817 @cindex common lines
4818 With no options, @command{comm} produces three-column output. Column one
4819 contains lines unique to @var{file1}, column two contains lines unique
4820 to @var{file2}, and column three contains lines common to both files.
4821 Columns are separated by a single TAB character.
4822 @c FIXME: when there's an option to supply an alternative separator
4823 @c string, append "by default" to the above sentence.
4828 The options @option{-1}, @option{-2}, and @option{-3} suppress printing of
4829 the corresponding columns (and separators). Also see @ref{Common options}.
4831 Unlike some other comparison utilities, @command{comm} has an exit
4832 status that does not depend on the result of the comparison.
4833 Upon normal completion @command{comm} produces an exit code of zero.
4834 If there is an error it exits with nonzero status.
4836 @macro checkOrderOption{cmd}
4837 If the @option{--check-order} option is given, unsorted inputs will
4838 cause a fatal error message. If the option @option{--nocheck-order}
4839 is given, unsorted inputs will never cause an error message. If neither
4840 of these options is given, wrongly sorted inputs are diagnosed
4841 only if an input file is found to contain unpairable
4843 lines, and when both input files are non empty.
4845 @ifclear JOIN_COMMAND
4848 If an input file is diagnosed as being unsorted, the @command{\cmd\}
4849 command will exit with a nonzero status (and the output should not be used).
4851 Forcing @command{\cmd\} to process wrongly sorted input files
4852 containing unpairable lines by specifying @option{--nocheck-order} is
4853 not guaranteed to produce any particular output. The output will
4854 probably not correspond with whatever you hoped it would be.
4856 @checkOrderOption{comm}
4861 Fail with an error message if either input file is wrongly ordered.
4863 @item --nocheck-order
4864 Do not check that both input files are in sorted order.
4868 @item --output-delimiter=@var{str}
4869 Print @var{str} between adjacent output columns,
4870 rather than the default of a single TAB character.
4872 The delimiter @var{str} may not be empty.
4876 @node ptx invocation
4877 @section @command{ptx}: Produce permuted indexes
4881 @command{ptx} reads a text file and essentially produces a permuted index, with
4882 each keyword in its context. The calling sketch is either one of:
4885 ptx [@var{option} @dots{}] [@var{file} @dots{}]
4886 ptx -G [@var{option} @dots{}] [@var{input} [@var{output}]]
4889 The @option{-G} (or its equivalent: @option{--traditional}) option disables
4890 all @sc{gnu} extensions and reverts to traditional mode, thus introducing some
4891 limitations and changing several of the program's default option values.
4892 When @option{-G} is not specified, @sc{gnu} extensions are always enabled.
4893 @sc{gnu} extensions to @command{ptx} are documented wherever appropriate in this
4894 document. For the full list, see @xref{Compatibility in ptx}.
4896 Individual options are explained in the following sections.
4898 When @sc{gnu} extensions are enabled, there may be zero, one or several
4899 @var{file}s after the options. If there is no @var{file}, the program
4900 reads the standard input. If there is one or several @var{file}s, they
4901 give the name of input files which are all read in turn, as if all the
4902 input files were concatenated. However, there is a full contextual
4903 break between each file and, when automatic referencing is requested,
4904 file names and line numbers refer to individual text input files. In
4905 all cases, the program outputs the permuted index to the standard
4908 When @sc{gnu} extensions are @emph{not} enabled, that is, when the program
4909 operates in traditional mode, there may be zero, one or two parameters
4910 besides the options. If there are no parameters, the program reads the
4911 standard input and outputs the permuted index to the standard output.
4912 If there is only one parameter, it names the text @var{input} to be read
4913 instead of the standard input. If two parameters are given, they give
4914 respectively the name of the @var{input} file to read and the name of
4915 the @var{output} file to produce. @emph{Be very careful} to note that,
4916 in this case, the contents of file given by the second parameter is
4917 destroyed. This behavior is dictated by System V @command{ptx}
4918 compatibility; @sc{gnu} Standards normally discourage output parameters not
4919 introduced by an option.
4921 Note that for @emph{any} file named as the value of an option or as an
4922 input text file, a single dash @kbd{-} may be used, in which case
4923 standard input is assumed. However, it would not make sense to use this
4924 convention more than once per program invocation.
4927 * General options in ptx:: Options which affect general program behavior.
4928 * Charset selection in ptx:: Underlying character set considerations.
4929 * Input processing in ptx:: Input fields, contexts, and keyword selection.
4930 * Output formatting in ptx:: Types of output format, and sizing the fields.
4931 * Compatibility in ptx::
4935 @node General options in ptx
4936 @subsection General options
4941 @itemx --traditional
4942 As already explained, this option disables all @sc{gnu} extensions to
4943 @command{ptx} and switches to traditional mode.
4946 Print a short help on standard output, then exit without further
4950 Print the program version on standard output, then exit without further
4958 @node Charset selection in ptx
4959 @subsection Charset selection
4961 @c FIXME: People don't necessarily know what an IBM-PC was these days.
4962 As it is set up now, the program assumes that the input file is coded
4963 using 8-bit @acronym{ISO} 8859-1 code, also known as Latin-1 character set,
4964 @emph{unless} it is compiled for MS-DOS, in which case it uses the
4965 character set of the IBM-PC@. (@sc{gnu} @command{ptx} is not known to work on
4966 smaller MS-DOS machines anymore.) Compared to 7-bit @acronym{ASCII}, the set
4967 of characters which are letters is different; this alters the behavior
4968 of regular expression matching. Thus, the default regular expression
4969 for a keyword allows foreign or diacriticized letters. Keyword sorting,
4970 however, is still crude; it obeys the underlying character set ordering
4976 @itemx --ignore-case
4977 Fold lower case letters to upper case for sorting.
4982 @node Input processing in ptx
4983 @subsection Word selection and input processing
4988 @itemx --break-file=@var{file}
4990 This option provides an alternative (to @option{-W}) method of describing
4991 which characters make up words. It introduces the name of a
4992 file which contains a list of characters which can@emph{not} be part of
4993 one word; this file is called the @dfn{Break file}. Any character which
4994 is not part of the Break file is a word constituent. If both options
4995 @option{-b} and @option{-W} are specified, then @option{-W} has precedence and
4996 @option{-b} is ignored.
4998 When @sc{gnu} extensions are enabled, the only way to avoid newline as a
4999 break character is to write all the break characters in the file with no
5000 newline at all, not even at the end of the file. When @sc{gnu} extensions
5001 are disabled, spaces, tabs and newlines are always considered as break
5002 characters even if not included in the Break file.
5005 @itemx --ignore-file=@var{file}
5007 The file associated with this option contains a list of words which will
5008 never be taken as keywords in concordance output. It is called the
5009 @dfn{Ignore file}. The file contains exactly one word in each line; the
5010 end of line separation of words is not subject to the value of the
5014 @itemx --only-file=@var{file}
5016 The file associated with this option contains a list of words which will
5017 be retained in concordance output; any word not mentioned in this file
5018 is ignored. The file is called the @dfn{Only file}. The file contains
5019 exactly one word in each line; the end of line separation of words is
5020 not subject to the value of the @option{-S} option.
5022 There is no default for the Only file. When both an Only file and an
5023 Ignore file are specified, a word is considered a keyword only
5024 if it is listed in the Only file and not in the Ignore file.
5029 On each input line, the leading sequence of non-white space characters will be
5030 taken to be a reference that has the purpose of identifying this input
5031 line in the resulting permuted index. For more information about reference
5032 production, see @xref{Output formatting in ptx}.
5033 Using this option changes the default value for option @option{-S}.
5035 Using this option, the program does not try very hard to remove
5036 references from contexts in output, but it succeeds in doing so
5037 @emph{when} the context ends exactly at the newline. If option
5038 @option{-r} is used with @option{-S} default value, or when @sc{gnu} extensions
5039 are disabled, this condition is always met and references are completely
5040 excluded from the output contexts.
5042 @item -S @var{regexp}
5043 @itemx --sentence-regexp=@var{regexp}
5045 This option selects which regular expression will describe the end of a
5046 line or the end of a sentence. In fact, this regular expression is not
5047 the only distinction between end of lines or end of sentences, and input
5048 line boundaries have no special significance outside this option. By
5049 default, when @sc{gnu} extensions are enabled and if @option{-r} option is not
5050 used, end of sentences are used. In this case, this @var{regex} is
5051 imported from @sc{gnu} Emacs:
5054 [.?!][]\"')@}]*\\($\\|\t\\| \\)[ \t\n]*
5057 Whenever @sc{gnu} extensions are disabled or if @option{-r} option is used, end
5058 of lines are used; in this case, the default @var{regexp} is just:
5064 Using an empty @var{regexp} is equivalent to completely disabling end of
5065 line or end of sentence recognition. In this case, the whole file is
5066 considered to be a single big line or sentence. The user might want to
5067 disallow all truncation flag generation as well, through option @option{-F
5068 ""}. @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
5071 When the keywords happen to be near the beginning of the input line or
5072 sentence, this often creates an unused area at the beginning of the
5073 output context line; when the keywords happen to be near the end of the
5074 input line or sentence, this often creates an unused area at the end of
5075 the output context line. The program tries to fill those unused areas
5076 by wrapping around context in them; the tail of the input line or
5077 sentence is used to fill the unused area on the left of the output line;
5078 the head of the input line or sentence is used to fill the unused area
5079 on the right of the output line.
5081 As a matter of convenience to the user, many usual backslashed escape
5082 sequences from the C language are recognized and converted to the
5083 corresponding characters by @command{ptx} itself.
5085 @item -W @var{regexp}
5086 @itemx --word-regexp=@var{regexp}
5088 This option selects which regular expression will describe each keyword.
5089 By default, if @sc{gnu} extensions are enabled, a word is a sequence of
5090 letters; the @var{regexp} used is @samp{\w+}. When @sc{gnu} extensions are
5091 disabled, a word is by default anything which ends with a space, a tab
5092 or a newline; the @var{regexp} used is @samp{[^ \t\n]+}.
5094 An empty @var{regexp} is equivalent to not using this option.
5095 @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
5098 As a matter of convenience to the user, many usual backslashed escape
5099 sequences, as found in the C language, are recognized and converted to
5100 the corresponding characters by @command{ptx} itself.
5105 @node Output formatting in ptx
5106 @subsection Output formatting
5108 Output format is mainly controlled by the @option{-O} and @option{-T} options
5109 described in the table below. When neither @option{-O} nor @option{-T} are
5110 selected, and if @sc{gnu} extensions are enabled, the program chooses an
5111 output format suitable for a dumb terminal. Each keyword occurrence is
5112 output to the center of one line, surrounded by its left and right
5113 contexts. Each field is properly justified, so the concordance output
5114 can be readily observed. As a special feature, if automatic
5115 references are selected by option @option{-A} and are output before the
5116 left context, that is, if option @option{-R} is @emph{not} selected, then
5117 a colon is added after the reference; this nicely interfaces with @sc{gnu}
5118 Emacs @code{next-error} processing. In this default output format, each
5119 white space character, like newline and tab, is merely changed to
5120 exactly one space, with no special attempt to compress consecutive
5121 spaces. This might change in the future. Except for those white space
5122 characters, every other character of the underlying set of 256
5123 characters is transmitted verbatim.
5125 Output format is further controlled by the following options.
5129 @item -g @var{number}
5130 @itemx --gap-size=@var{number}
5132 Select the size of the minimum white space gap between the fields on the
5135 @item -w @var{number}
5136 @itemx --width=@var{number}
5138 Select the maximum output width of each final line. If references are
5139 used, they are included or excluded from the maximum output width
5140 depending on the value of option @option{-R}. If this option is not
5141 selected, that is, when references are output before the left context,
5142 the maximum output width takes into account the maximum length of all
5143 references. If this option is selected, that is, when references are
5144 output after the right context, the maximum output width does not take
5145 into account the space taken by references, nor the gap that precedes
5149 @itemx --auto-reference
5151 Select automatic references. Each input line will have an automatic
5152 reference made up of the file name and the line ordinal, with a single
5153 colon between them. However, the file name will be empty when standard
5154 input is being read. If both @option{-A} and @option{-r} are selected, then
5155 the input reference is still read and skipped, but the automatic
5156 reference is used at output time, overriding the input reference.
5159 @itemx --right-side-refs
5161 In the default output format, when option @option{-R} is not used, any
5162 references produced by the effect of options @option{-r} or @option{-A} are
5163 placed to the far right of output lines, after the right context. With
5164 default output format, when the @option{-R} option is specified, references
5165 are rather placed at the beginning of each output line, before the left
5166 context. For any other output format, option @option{-R} is
5167 ignored, with one exception: with @option{-R} the width of references
5168 is @emph{not} taken into account in total output width given by @option{-w}.
5170 This option is automatically selected whenever @sc{gnu} extensions are
5173 @item -F @var{string}
5174 @itemx --flac-truncation=@var{string}
5176 This option will request that any truncation in the output be reported
5177 using the string @var{string}. Most output fields theoretically extend
5178 towards the beginning or the end of the current line, or current
5179 sentence, as selected with option @option{-S}. But there is a maximum
5180 allowed output line width, changeable through option @option{-w}, which is
5181 further divided into space for various output fields. When a field has
5182 to be truncated because it cannot extend beyond the beginning or the end of
5183 the current line to fit in, then a truncation occurs. By default,
5184 the string used is a single slash, as in @option{-F /}.
5186 @var{string} may have more than one character, as in @option{-F ...}.
5187 Also, in the particular case when @var{string} is empty (@option{-F ""}),
5188 truncation flagging is disabled, and no truncation marks are appended in
5191 As a matter of convenience to the user, many usual backslashed escape
5192 sequences, as found in the C language, are recognized and converted to
5193 the corresponding characters by @command{ptx} itself.
5195 @item -M @var{string}
5196 @itemx --macro-name=@var{string}
5198 Select another @var{string} to be used instead of @samp{xx}, while
5199 generating output suitable for @command{nroff}, @command{troff} or @TeX{}.
5202 @itemx --format=roff
5204 Choose an output format suitable for @command{nroff} or @command{troff}
5205 processing. Each output line will look like:
5208 .xx "@var{tail}" "@var{before}" "@var{keyword_and_after}"@c
5209 "@var{head}" "@var{ref}"
5212 so it will be possible to write a @samp{.xx} roff macro to take care of
5213 the output typesetting. This is the default output format when @sc{gnu}
5214 extensions are disabled. Option @option{-M} can be used to change
5215 @samp{xx} to another macro name.
5217 In this output format, each non-graphical character, like newline and
5218 tab, is merely changed to exactly one space, with no special attempt to
5219 compress consecutive spaces. Each quote character: @kbd{"} is doubled
5220 so it will be correctly processed by @command{nroff} or @command{troff}.
5225 Choose an output format suitable for @TeX{} processing. Each output
5226 line will look like:
5229 \xx @{@var{tail}@}@{@var{before}@}@{@var{keyword}@}@c
5230 @{@var{after}@}@{@var{head}@}@{@var{ref}@}
5234 so it will be possible to write a @code{\xx} definition to take care of
5235 the output typesetting. Note that when references are not being
5236 produced, that is, neither option @option{-A} nor option @option{-r} is
5237 selected, the last parameter of each @code{\xx} call is inhibited.
5238 Option @option{-M} can be used to change @samp{xx} to another macro
5241 In this output format, some special characters, like @kbd{$}, @kbd{%},
5242 @kbd{&}, @kbd{#} and @kbd{_} are automatically protected with a
5243 backslash. Curly brackets @kbd{@{}, @kbd{@}} are protected with a
5244 backslash and a pair of dollar signs (to force mathematical mode). The
5245 backslash itself produces the sequence @code{\backslash@{@}}.
5246 Circumflex and tilde diacritical marks produce the sequence @code{^\@{ @}} and
5247 @code{~\@{ @}} respectively. Other diacriticized characters of the
5248 underlying character set produce an appropriate @TeX{} sequence as far
5249 as possible. The other non-graphical characters, like newline and tab,
5250 and all other characters which are not part of @acronym{ASCII}, are merely
5251 changed to exactly one space, with no special attempt to compress
5252 consecutive spaces. Let me know how to improve this special character
5253 processing for @TeX{}.
5258 @node Compatibility in ptx
5259 @subsection The @sc{gnu} extensions to @command{ptx}
5261 This version of @command{ptx} contains a few features which do not exist in
5262 System V @command{ptx}. These extra features are suppressed by using the
5263 @option{-G} command line option, unless overridden by other command line
5264 options. Some @sc{gnu} extensions cannot be recovered by overriding, so the
5265 simple rule is to avoid @option{-G} if you care about @sc{gnu} extensions.
5266 Here are the differences between this program and System V @command{ptx}.
5271 This program can read many input files at once, it always writes the
5272 resulting concordance on standard output. On the other hand, System V
5273 @command{ptx} reads only one file and sends the result to standard output
5274 or, if a second @var{file} parameter is given on the command, to that
5277 Having output parameters not introduced by options is a dangerous
5278 practice which @sc{gnu} avoids as far as possible. So, for using @command{ptx}
5279 portably between @sc{gnu} and System V, you should always use it with a
5280 single input file, and always expect the result on standard output. You
5281 might also want to automatically configure in a @option{-G} option to
5282 @command{ptx} calls in products using @command{ptx}, if the configurator finds
5283 that the installed @command{ptx} accepts @option{-G}.
5286 The only options available in System V @command{ptx} are options @option{-b},
5287 @option{-f}, @option{-g}, @option{-i}, @option{-o}, @option{-r}, @option{-t} and
5288 @option{-w}. All other options are @sc{gnu} extensions and are not repeated in
5289 this enumeration. Moreover, some options have a slightly different
5290 meaning when @sc{gnu} extensions are enabled, as explained below.
5293 By default, concordance output is not formatted for @command{troff} or
5294 @command{nroff}. It is rather formatted for a dumb terminal. @command{troff}
5295 or @command{nroff} output may still be selected through option @option{-O}.
5298 Unless @option{-R} option is used, the maximum reference width is
5299 subtracted from the total output line width. With @sc{gnu} extensions
5300 disabled, width of references is not taken into account in the output
5301 line width computations.
5304 All 256 bytes, even @acronym{ASCII} @sc{nul} bytes, are always read and
5305 processed from input file with no adverse effect, even if @sc{gnu} extensions
5306 are disabled. However, System V @command{ptx} does not accept 8-bit characters,
5307 a few control characters are rejected, and the tilde @kbd{~} is also rejected.
5310 Input line length is only limited by available memory, even if @sc{gnu}
5311 extensions are disabled. However, System V @command{ptx} processes only
5312 the first 200 characters in each line.
5315 The break (non-word) characters default to be every character except all
5316 letters of the underlying character set, diacriticized or not. When @sc{gnu}
5317 extensions are disabled, the break characters default to space, tab and
5321 The program makes better use of output line width. If @sc{gnu} extensions
5322 are disabled, the program rather tries to imitate System V @command{ptx},
5323 but still, there are some slight disposition glitches this program does
5324 not completely reproduce.
5327 The user can specify both an Ignore file and an Only file. This is not
5328 allowed with System V @command{ptx}.
5333 @node tsort invocation
5334 @section @command{tsort}: Topological sort
5337 @cindex topological sort
5339 @command{tsort} performs a topological sort on the given @var{file}, or
5340 standard input if no input file is given or for a @var{file} of
5341 @samp{-}. For more details and some history, see @ref{tsort background}.
5345 tsort [@var{option}] [@var{file}]
5348 @command{tsort} reads its input as pairs of strings, separated by blanks,
5349 indicating a partial ordering. The output is a total ordering that
5350 corresponds to the given partial ordering.
5364 will produce the output
5375 Consider a more realistic example.
5376 You have a large set of functions all in one file, and they may all be
5377 declared static except one. Currently that one (say @code{main}) is the
5378 first function defined in the file, and the ones it calls directly follow
5379 it, followed by those they call, etc. Let's say that you are determined
5380 to take advantage of prototypes, so you have to choose between declaring
5381 all of those functions (which means duplicating a lot of information from
5382 the definitions) and rearranging the functions so that as many as possible
5383 are defined before they are used. One way to automate the latter process
5384 is to get a list for each function of the functions it calls directly.
5385 Many programs can generate such lists. They describe a call graph.
5386 Consider the following list, in which a given line indicates that the
5387 function on the left calls the one on the right directly.
5393 tail_file pretty_name
5394 tail_file write_header
5396 tail_forever recheck
5397 tail_forever pretty_name
5398 tail_forever write_header
5399 tail_forever dump_remainder
5402 tail_lines start_lines
5403 tail_lines dump_remainder
5404 tail_lines file_lines
5405 tail_lines pipe_lines
5407 tail_bytes start_bytes
5408 tail_bytes dump_remainder
5409 tail_bytes pipe_bytes
5410 file_lines dump_remainder
5414 then you can use @command{tsort} to produce an ordering of those
5415 functions that satisfies your requirement.
5418 example$ tsort call-graph | tac
5438 @command{tsort} detects any cycles in the input and writes the first cycle
5439 encountered to standard error.
5441 Note that for a given partial ordering, generally there is no unique
5442 total ordering. In the context of the call graph above, the function
5443 @code{parse_options} may be placed anywhere in the list as long as it
5444 precedes @code{main}.
5446 The only options are @option{--help} and @option{--version}. @xref{Common
5452 * tsort background:: Where tsort came from.
5455 @node tsort background
5456 @subsection @command{tsort}: Background
5458 @command{tsort} exists because very early versions of the Unix linker processed
5459 an archive file exactly once, and in order. As @command{ld} read each object
5460 in the archive, it decided whether it was needed in the program based on
5461 whether it defined any symbols which were undefined at that point in
5464 This meant that dependencies within the archive had to be handled
5465 specially. For example, @code{scanf} probably calls @code{read}. That means
5466 that in a single pass through an archive, it was important for @code{scanf.o}
5467 to appear before read.o, because otherwise a program which calls
5468 @code{scanf} but not @code{read} might end up with an unexpected unresolved
5469 reference to @code{read}.
5471 The way to address this problem was to first generate a set of
5472 dependencies of one object file on another. This was done by a shell
5473 script called @command{lorder}. The GNU tools don't provide a version of
5474 lorder, as far as I know, but you can still find it in BSD
5477 Then you ran @command{tsort} over the @command{lorder} output, and you used the
5478 resulting sort to define the order in which you added objects to the archive.
5480 This whole procedure has been obsolete since about 1980, because
5481 Unix archives now contain a symbol table (traditionally built by
5482 @command{ranlib}, now generally built by @command{ar} itself), and the Unix
5483 linker uses the symbol table to effectively make multiple passes over
5486 Anyhow, that's where tsort came from. To solve an old problem with
5487 the way the linker handled archive files, which has since been solved
5491 @node Operating on fields
5492 @chapter Operating on fields
5495 * cut invocation:: Print selected parts of lines.
5496 * paste invocation:: Merge lines of files.
5497 * join invocation:: Join lines on a common field.
5501 @node cut invocation
5502 @section @command{cut}: Print selected parts of lines
5505 @command{cut} writes to standard output selected parts of each line of each
5506 input file, or standard input if no files are given or for a file name of
5510 cut @var{option}@dots{} [@var{file}]@dots{}
5513 In the table which follows, the @var{byte-list}, @var{character-list},
5514 and @var{field-list} are one or more numbers or ranges (two numbers
5515 separated by a dash) separated by commas. Bytes, characters, and
5516 fields are numbered starting at 1. Incomplete ranges may be
5517 given: @option{-@var{m}} means @samp{1-@var{m}}; @samp{@var{n}-} means
5518 @samp{@var{n}} through end of line or last field. The list elements
5519 can be repeated, can overlap, and can be specified in any order; but
5520 the selected input is written in the same order that it is read, and
5521 is written exactly once.
5523 The program accepts the following options. Also see @ref{Common
5528 @item -b @var{byte-list}
5529 @itemx --bytes=@var{byte-list}
5532 Select for printing only the bytes in positions listed in
5533 @var{byte-list}. Tabs and backspaces are treated like any other
5534 character; they take up 1 byte. If an output delimiter is specified,
5535 (see the description of @option{--output-delimiter}), then output that
5536 string between ranges of selected bytes.
5538 @item -c @var{character-list}
5539 @itemx --characters=@var{character-list}
5541 @opindex --characters
5542 Select for printing only the characters in positions listed in
5543 @var{character-list}. The same as @option{-b} for now, but
5544 internationalization will change that. Tabs and backspaces are
5545 treated like any other character; they take up 1 character. If an
5546 output delimiter is specified, (see the description of
5547 @option{--output-delimiter}), then output that string between ranges
5550 @item -f @var{field-list}
5551 @itemx --fields=@var{field-list}
5554 Select for printing only the fields listed in @var{field-list}.
5555 Fields are separated by a TAB character by default. Also print any
5556 line that contains no delimiter character, unless the
5557 @option{--only-delimited} (@option{-s}) option is specified.
5559 Note @command{awk} supports more sophisticated field processing,
5560 and by default will use (and discard) runs of blank characters to
5561 separate fields, and ignore leading and trailing blanks.
5564 awk '{print $2}' # print the second field
5565 awk '{print $NF-1}' # print the penultimate field
5566 awk '{print $2,$1}' # reorder the first two fields
5570 In the unlikely event that @command{awk} is unavailable,
5571 one can use the @command{join} command, to process blank
5572 characters as @command{awk} does above.
5575 join -a1 -o 1.2 - /dev/null # print the second field
5576 join -a1 -o 1.2,1.1 - /dev/null # reorder the first two fields
5580 @item -d @var{input_delim_byte}
5581 @itemx --delimiter=@var{input_delim_byte}
5583 @opindex --delimiter
5584 With @option{-f}, use the first byte of @var{input_delim_byte} as
5585 the input fields separator (default is TAB).
5589 Do not split multi-byte characters (no-op for now).
5592 @itemx --only-delimited
5594 @opindex --only-delimited
5595 For @option{-f}, do not print lines that do not contain the field separator
5596 character. Normally, any line without a field separator is printed verbatim.
5598 @item --output-delimiter=@var{output_delim_string}
5599 @opindex --output-delimiter
5600 With @option{-f}, output fields are separated by @var{output_delim_string}.
5601 The default with @option{-f} is to use the input delimiter.
5602 When using @option{-b} or @option{-c} to select ranges of byte or
5603 character offsets (as opposed to ranges of fields),
5604 output @var{output_delim_string} between non-overlapping
5605 ranges of selected bytes.
5608 @opindex --complement
5609 This option is a @acronym{GNU} extension.
5610 Select for printing the complement of the bytes, characters or fields
5611 selected with the @option{-b}, @option{-c} or @option{-f} options.
5612 In other words, do @emph{not} print the bytes, characters or fields
5613 specified via those options. This option is useful when you have
5614 many fields and want to print all but a few of them.
5621 @node paste invocation
5622 @section @command{paste}: Merge lines of files
5625 @cindex merging files
5627 @command{paste} writes to standard output lines consisting of sequentially
5628 corresponding lines of each given file, separated by a TAB character.
5629 Standard input is used for a file name of @samp{-} or if no input files
5651 paste [@var{option}]@dots{} [@var{file}]@dots{}
5654 The program accepts the following options. Also see @ref{Common options}.
5662 Paste the lines of one file at a time rather than one line from each
5663 file. Using the above example data:
5666 $ paste -s num2 let3
5671 @item -d @var{delim-list}
5672 @itemx --delimiters=@var{delim-list}
5674 @opindex --delimiters
5675 Consecutively use the characters in @var{delim-list} instead of
5676 TAB to separate merged lines. When @var{delim-list} is
5677 exhausted, start again at its beginning. Using the above example data:
5680 $ paste -d '%_' num2 let3 num2
5691 @node join invocation
5692 @section @command{join}: Join lines on a common field
5695 @cindex common field, joining on
5697 @command{join} writes to standard output a line for each pair of input
5698 lines that have identical join fields. Synopsis:
5701 join [@var{option}]@dots{} @var{file1} @var{file2}
5704 Either @var{file1} or @var{file2} (but not both) can be @samp{-},
5705 meaning standard input. @var{file1} and @var{file2} should be
5706 sorted on the join fields.
5709 Normally, the sort order is that of the
5710 collating sequence specified by the @env{LC_COLLATE} locale. Unless
5711 the @option{-t} option is given, the sort comparison ignores blanks at
5712 the start of the join field, as in @code{sort -b}. If the
5713 @option{--ignore-case} option is given, the sort comparison ignores
5714 the case of characters in the join field, as in @code{sort -f}.
5716 The @command{sort} and @command{join} commands should use consistent
5717 locales and options if the output of @command{sort} is fed to
5718 @command{join}. You can use a command like @samp{sort -k 1b,1} to
5719 sort a file on its default join field, but if you select a non-default
5720 locale, join field, separator, or comparison options, then you should
5721 do so consistently between @command{join} and @command{sort}.
5722 If @samp{join -t ''} is specified then the whole line is considered which
5723 matches the default operation of sort.
5725 If the input has no unpairable lines, a @acronym{GNU} extension is
5726 available; the sort order can be any order that considers two fields
5727 to be equal if and only if the sort comparison described above
5728 considers them to be equal. For example:
5746 @checkOrderOption{join}
5751 @item the join field is the first field in each line;
5752 @item fields in the input are separated by one or more blanks, with leading
5753 blanks on the line ignored;
5754 @item fields in the output are separated by a space;
5755 @item each output line consists of the join field, the remaining
5756 fields from @var{file1}, then the remaining fields from @var{file2}.
5759 The program accepts the following options. Also see @ref{Common options}.
5763 @item -a @var{file-number}
5765 Print a line for each unpairable line in file @var{file-number} (either
5766 @samp{1} or @samp{2}), in addition to the normal output.
5769 Fail with an error message if either input file is wrongly ordered.
5771 @item --nocheck-order
5772 Do not check that both input files are in sorted order. This is the default.
5774 @item -e @var{string}
5776 Replace those output fields that are missing in the input with @var{string}.
5777 I.E. missing fields specified with the @option{-12jo} options.
5781 Treat the first line of each input file as a header line. The header lines will
5782 be joined and printed as the first output line. If @option{-o} is used to
5783 specify output format, the header line will be printed according to the
5784 specified format. The header lines will not be checked for ordering even if
5785 @option{--check-order} is specified. Also if the header lines from each file
5786 do not match, the heading fields from the first file will be used.
5789 @itemx --ignore-case
5791 @opindex --ignore-case
5792 Ignore differences in case when comparing keys.
5793 With this option, the lines of the input files must be ordered in the same way.
5794 Use @samp{sort -f} to produce this ordering.
5796 @item -1 @var{field}
5798 Join on field @var{field} (a positive integer) of file 1.
5800 @item -2 @var{field}
5802 Join on field @var{field} (a positive integer) of file 2.
5804 @item -j @var{field}
5805 Equivalent to @option{-1 @var{field} -2 @var{field}}.
5807 @item -o @var{field-list}
5809 If the keyword @samp{auto} is specified, infer the output format from
5810 the first line in each file. This is the same as the default output format
5811 but also ensures the same number of fields are output for each line.
5812 Missing fields are replaced with the @option{-e} option and extra fields
5815 Otherwise, construct each output line according to the format in
5816 @var{field-list}. Each element in @var{field-list} is either the single
5817 character @samp{0} or has the form @var{m.n} where the file number, @var{m},
5818 is @samp{1} or @samp{2} and @var{n} is a positive field number.
5820 A field specification of @samp{0} denotes the join field.
5821 In most cases, the functionality of the @samp{0} field spec
5822 may be reproduced using the explicit @var{m.n} that corresponds
5823 to the join field. However, when printing unpairable lines
5824 (using either of the @option{-a} or @option{-v} options), there is no way
5825 to specify the join field using @var{m.n} in @var{field-list}
5826 if there are unpairable lines in both files.
5827 To give @command{join} that functionality, @acronym{POSIX} invented the @samp{0}
5828 field specification notation.
5830 The elements in @var{field-list}
5831 are separated by commas or blanks.
5832 Blank separators typically need to be quoted for the shell. For
5833 example, the commands @samp{join -o 1.2,2.2} and @samp{join -o '1.2
5834 2.2'} are equivalent.
5836 All output lines---including those printed because of any -a or -v
5837 option---are subject to the specified @var{field-list}.
5840 Use character @var{char} as the input and output field separator.
5841 Treat as significant each occurrence of @var{char} in the input file.
5842 Use @samp{sort -t @var{char}}, without the @option{-b} option of
5843 @samp{sort}, to produce this ordering. If @samp{join -t ''} is specified,
5844 the whole line is considered, matching the default operation of sort.
5845 If @samp{-t '\0'} is specified then the @acronym{ASCII} @sc{nul}
5846 character is used to delimit the fields.
5848 @item -v @var{file-number}
5849 Print a line for each unpairable line in file @var{file-number}
5850 (either @samp{1} or @samp{2}), instead of the normal output.
5857 @node Operating on characters
5858 @chapter Operating on characters
5860 @cindex operating on characters
5862 This commands operate on individual characters.
5865 * tr invocation:: Translate, squeeze, and/or delete characters.
5866 * expand invocation:: Convert tabs to spaces.
5867 * unexpand invocation:: Convert spaces to tabs.
5872 @section @command{tr}: Translate, squeeze, and/or delete characters
5879 tr [@var{option}]@dots{} @var{set1} [@var{set2}]
5882 @command{tr} copies standard input to standard output, performing
5883 one of the following operations:
5887 translate, and optionally squeeze repeated characters in the result,
5889 squeeze repeated characters,
5893 delete characters, then squeeze repeated characters from the result.
5896 The @var{set1} and (if given) @var{set2} arguments define ordered
5897 sets of characters, referred to below as @var{set1} and @var{set2}. These
5898 sets are the characters of the input that @command{tr} operates on.
5899 The @option{--complement} (@option{-c}, @option{-C}) option replaces
5901 complement (all of the characters that are not in @var{set1}).
5903 Currently @command{tr} fully supports only single-byte characters.
5904 Eventually it will support multibyte characters; when it does, the
5905 @option{-C} option will cause it to complement the set of characters,
5906 whereas @option{-c} will cause it to complement the set of values.
5907 This distinction will matter only when some values are not characters,
5908 and this is possible only in locales using multibyte encodings when
5909 the input contains encoding errors.
5911 The program accepts the @option{--help} and @option{--version}
5912 options. @xref{Common options}. Options must precede operands.
5917 * Character sets:: Specifying sets of characters.
5918 * Translating:: Changing one set of characters to another.
5919 * Squeezing:: Squeezing repeats and deleting.
5923 @node Character sets
5924 @subsection Specifying sets of characters
5926 @cindex specifying sets of characters
5928 The format of the @var{set1} and @var{set2} arguments resembles
5929 the format of regular expressions; however, they are not regular
5930 expressions, only lists of characters. Most characters simply
5931 represent themselves in these strings, but the strings can contain
5932 the shorthands listed below, for convenience. Some of them can be
5933 used only in @var{set1} or @var{set2}, as noted below.
5937 @item Backslash escapes
5938 @cindex backslash escapes
5940 The following backslash escape sequences are recognized:
5958 The 8-bit character with the value given by @var{ooo}, which is 1 to 3
5959 octal digits. Note that @samp{\400} is interpreted as the two-byte
5960 sequence, @samp{\040} @samp{0}.
5965 While a backslash followed by a character not listed above is
5966 interpreted as that character, the backslash also effectively
5967 removes any special significance, so it is useful to escape
5968 @samp{[}, @samp{]}, @samp{*}, and @samp{-}.
5973 The notation @samp{@var{m}-@var{n}} expands to all of the characters
5974 from @var{m} through @var{n}, in ascending order. @var{m} should
5975 collate before @var{n}; if it doesn't, an error results. As an example,
5976 @samp{0-9} is the same as @samp{0123456789}.
5978 @sc{gnu} @command{tr} does not support the System V syntax that uses square
5979 brackets to enclose ranges. Translations specified in that format
5980 sometimes work as expected, since the brackets are often transliterated
5981 to themselves. However, they should be avoided because they sometimes
5982 behave unexpectedly. For example, @samp{tr -d '[0-9]'} deletes brackets
5985 Many historically common and even accepted uses of ranges are not
5986 portable. For example, on @acronym{EBCDIC} hosts using the @samp{A-Z}
5987 range will not do what most would expect because @samp{A} through @samp{Z}
5988 are not contiguous as they are in @acronym{ASCII}.
5989 If you can rely on a @acronym{POSIX} compliant version of @command{tr}, then
5990 the best way to work around this is to use character classes (see below).
5991 Otherwise, it is most portable (and most ugly) to enumerate the members
5994 @item Repeated characters
5995 @cindex repeated characters
5997 The notation @samp{[@var{c}*@var{n}]} in @var{set2} expands to @var{n}
5998 copies of character @var{c}. Thus, @samp{[y*6]} is the same as
5999 @samp{yyyyyy}. The notation @samp{[@var{c}*]} in @var{string2} expands
6000 to as many copies of @var{c} as are needed to make @var{set2} as long as
6001 @var{set1}. If @var{n} begins with @samp{0}, it is interpreted in
6002 octal, otherwise in decimal.
6004 @item Character classes
6005 @cindex character classes
6007 The notation @samp{[:@var{class}:]} expands to all of the characters in
6008 the (predefined) class @var{class}. The characters expand in no
6009 particular order, except for the @code{upper} and @code{lower} classes,
6010 which expand in ascending order. When the @option{--delete} (@option{-d})
6011 and @option{--squeeze-repeats} (@option{-s}) options are both given, any
6012 character class can be used in @var{set2}. Otherwise, only the
6013 character classes @code{lower} and @code{upper} are accepted in
6014 @var{set2}, and then only if the corresponding character class
6015 (@code{upper} and @code{lower}, respectively) is specified in the same
6016 relative position in @var{set1}. Doing this specifies case conversion.
6017 The class names are given below; an error results when an invalid class
6029 Horizontal whitespace.
6038 Printable characters, not including space.
6044 Printable characters, including space.
6047 Punctuation characters.
6050 Horizontal or vertical whitespace.
6059 @item Equivalence classes
6060 @cindex equivalence classes
6062 The syntax @samp{[=@var{c}=]} expands to all of the characters that are
6063 equivalent to @var{c}, in no particular order. Equivalence classes are
6064 a relatively recent invention intended to support non-English alphabets.
6065 But there seems to be no standard way to define them or determine their
6066 contents. Therefore, they are not fully implemented in @sc{gnu} @command{tr};
6067 each character's equivalence class consists only of that character,
6068 which is of no particular use.
6074 @subsection Translating
6076 @cindex translating characters
6078 @command{tr} performs translation when @var{set1} and @var{set2} are
6079 both given and the @option{--delete} (@option{-d}) option is not given.
6080 @command{tr} translates each character of its input that is in @var{set1}
6081 to the corresponding character in @var{set2}. Characters not in
6082 @var{set1} are passed through unchanged. When a character appears more
6083 than once in @var{set1} and the corresponding characters in @var{set2}
6084 are not all the same, only the final one is used. For example, these
6085 two commands are equivalent:
6092 A common use of @command{tr} is to convert lowercase characters to
6093 uppercase. This can be done in many ways. Here are three of them:
6096 tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
6098 tr '[:lower:]' '[:upper:]'
6102 But note that using ranges like @code{a-z} above is not portable.
6104 When @command{tr} is performing translation, @var{set1} and @var{set2}
6105 typically have the same length. If @var{set1} is shorter than
6106 @var{set2}, the extra characters at the end of @var{set2} are ignored.
6108 On the other hand, making @var{set1} longer than @var{set2} is not
6109 portable; @acronym{POSIX} says that the result is undefined. In this situation,
6110 BSD @command{tr} pads @var{set2} to the length of @var{set1} by repeating
6111 the last character of @var{set2} as many times as necessary. System V
6112 @command{tr} truncates @var{set1} to the length of @var{set2}.
6114 By default, @sc{gnu} @command{tr} handles this case like BSD @command{tr}.
6115 When the @option{--truncate-set1} (@option{-t}) option is given,
6116 @sc{gnu} @command{tr} handles this case like the System V @command{tr}
6117 instead. This option is ignored for operations other than translation.
6119 Acting like System V @command{tr} in this case breaks the relatively common
6123 tr -cs A-Za-z0-9 '\012'
6127 because it converts only zero bytes (the first element in the
6128 complement of @var{set1}), rather than all non-alphanumerics, to
6132 By the way, the above idiom is not portable because it uses ranges, and
6133 it assumes that the octal code for newline is 012.
6134 Assuming a @acronym{POSIX} compliant @command{tr}, here is a better
6138 tr -cs '[:alnum:]' '[\n*]'
6143 @subsection Squeezing repeats and deleting
6145 @cindex squeezing repeat characters
6146 @cindex deleting characters
6148 When given just the @option{--delete} (@option{-d}) option, @command{tr}
6149 removes any input characters that are in @var{set1}.
6151 When given just the @option{--squeeze-repeats} (@option{-s}) option,
6152 @command{tr} replaces each input sequence of a repeated character that
6153 is in @var{set1} with a single occurrence of that character.
6155 When given both @option{--delete} and @option{--squeeze-repeats}, @command{tr}
6156 first performs any deletions using @var{set1}, then squeezes repeats
6157 from any remaining characters using @var{set2}.
6159 The @option{--squeeze-repeats} option may also be used when translating,
6160 in which case @command{tr} first performs translation, then squeezes
6161 repeats from any remaining characters using @var{set2}.
6163 Here are some examples to illustrate various combinations of options:
6168 Remove all zero bytes:
6175 Put all words on lines by themselves. This converts all
6176 non-alphanumeric characters to newlines, then squeezes each string
6177 of repeated newlines into a single newline:
6180 tr -cs '[:alnum:]' '[\n*]'
6184 Convert each sequence of repeated newlines to a single newline:
6191 Find doubled occurrences of words in a document.
6192 @c Separate the following two "the"s, so typo checkers don't complain.
6193 For example, people often write ``the @w{}the'' with the repeated words
6194 separated by a newline. The Bourne shell script below works first
6195 by converting each sequence of punctuation and blank characters to a
6196 single newline. That puts each ``word'' on a line by itself.
6197 Next it maps all uppercase characters to lower case, and finally it
6198 runs @command{uniq} with the @option{-d} option to print out only the words
6204 | tr -s '[:punct:][:blank:]' '[\n*]' \
6205 | tr '[:upper:]' '[:lower:]' \
6210 Deleting a small set of characters is usually straightforward. For example,
6211 to remove all @samp{a}s, @samp{x}s, and @samp{M}s you would do this:
6217 However, when @samp{-} is one of those characters, it can be tricky because
6218 @samp{-} has special meanings. Performing the same task as above but also
6219 removing all @samp{-} characters, we might try @code{tr -d -axM}, but
6220 that would fail because @command{tr} would try to interpret @option{-a} as
6221 a command-line option. Alternatively, we could try putting the hyphen
6222 inside the string, @code{tr -d a-xM}, but that wouldn't work either because
6223 it would make @command{tr} interpret @code{a-x} as the range of characters
6224 @samp{a}@dots{}@samp{x} rather than the three.
6225 One way to solve the problem is to put the hyphen at the end of the list
6232 Or you can use @samp{--} to terminate option processing:
6238 More generally, use the character class notation @code{[=c=]}
6239 with @samp{-} (or any other character) in place of the @samp{c}:
6245 Note how single quotes are used in the above example to protect the
6246 square brackets from interpretation by a shell.
6251 @node expand invocation
6252 @section @command{expand}: Convert tabs to spaces
6255 @cindex tabs to spaces, converting
6256 @cindex converting tabs to spaces
6258 @command{expand} writes the contents of each given @var{file}, or standard
6259 input if none are given or for a @var{file} of @samp{-}, to standard
6260 output, with tab characters converted to the appropriate number of
6264 expand [@var{option}]@dots{} [@var{file}]@dots{}
6267 By default, @command{expand} converts all tabs to spaces. It preserves
6268 backspace characters in the output; they decrement the column count for
6269 tab calculations. The default action is equivalent to @option{-t 8} (set
6270 tabs every 8 columns).
6272 The program accepts the following options. Also see @ref{Common options}.
6276 @item -t @var{tab1}[,@var{tab2}]@dots{}
6277 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6280 @cindex tab stops, setting
6281 If only one tab stop is given, set the tabs @var{tab1} spaces apart
6282 (default is 8). Otherwise, set the tabs at columns @var{tab1},
6283 @var{tab2}, @dots{} (numbered from 0), and replace any tabs beyond the
6284 last tab stop given with single spaces. Tab stops can be separated by
6285 blanks as well as by commas.
6287 For compatibility, GNU @command{expand} also accepts the obsolete
6288 option syntax, @option{-@var{t1}[,@var{t2}]@dots{}}. New scripts
6289 should use @option{-t @var{t1}[,@var{t2}]@dots{}} instead.
6295 @cindex initial tabs, converting
6296 Only convert initial tabs (those that precede all non-space or non-tab
6297 characters) on each line to spaces.
6304 @node unexpand invocation
6305 @section @command{unexpand}: Convert spaces to tabs
6309 @command{unexpand} writes the contents of each given @var{file}, or
6310 standard input if none are given or for a @var{file} of @samp{-}, to
6311 standard output, converting blanks at the beginning of each line into
6312 as many tab characters as needed. In the default @acronym{POSIX}
6313 locale, a @dfn{blank} is a space or a tab; other locales may specify
6314 additional blank characters. Synopsis:
6317 unexpand [@var{option}]@dots{} [@var{file}]@dots{}
6320 By default, @command{unexpand} converts only initial blanks (those
6321 that precede all non-blank characters) on each line. It
6322 preserves backspace characters in the output; they decrement the column
6323 count for tab calculations. By default, tabs are set at every 8th
6326 The program accepts the following options. Also see @ref{Common options}.
6330 @item -t @var{tab1}[,@var{tab2}]@dots{}
6331 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6334 If only one tab stop is given, set the tabs @var{tab1} columns apart
6335 instead of the default 8. Otherwise, set the tabs at columns
6336 @var{tab1}, @var{tab2}, @dots{} (numbered from 0), and leave blanks
6337 beyond the tab stops given unchanged. Tab stops can be separated by
6338 blanks as well as by commas. This option implies the @option{-a} option.
6340 For compatibility, GNU @command{unexpand} supports the obsolete option syntax,
6341 @option{-@var{tab1}[,@var{tab2}]@dots{}}, where tab stops must be
6342 separated by commas. (Unlike @option{-t}, this obsolete option does
6343 not imply @option{-a}.) New scripts should use @option{--first-only -t
6344 @var{tab1}[,@var{tab2}]@dots{}} instead.
6350 Also convert all sequences of two or more blanks just before a tab stop,
6351 even if they occur after non-blank characters in a line.
6358 @node Directory listing
6359 @chapter Directory listing
6361 This chapter describes the @command{ls} command and its variants @command{dir}
6362 and @command{vdir}, which list information about files.
6365 * ls invocation:: List directory contents.
6366 * dir invocation:: Briefly ls.
6367 * vdir invocation:: Verbosely ls.
6368 * dircolors invocation:: Color setup for ls, etc.
6373 @section @command{ls}: List directory contents
6376 @cindex directory listing
6378 The @command{ls} program lists information about files (of any type,
6379 including directories). Options and file arguments can be intermixed
6380 arbitrarily, as usual.
6382 For non-option command-line arguments that are directories, by default
6383 @command{ls} lists the contents of directories, not recursively, and
6384 omitting files with names beginning with @samp{.}. For other non-option
6385 arguments, by default @command{ls} lists just the file name. If no
6386 non-option argument is specified, @command{ls} operates on the current
6387 directory, acting as if it had been invoked with a single argument of @samp{.}.
6390 By default, the output is sorted alphabetically, according to the locale
6391 settings in effect.@footnote{If you use a non-@acronym{POSIX}
6392 locale (e.g., by setting @env{LC_ALL} to @samp{en_US}), then @command{ls} may
6393 produce output that is sorted differently than you're accustomed to.
6394 In that case, set the @env{LC_ALL} environment variable to @samp{C}.}
6395 If standard output is
6396 a terminal, the output is in columns (sorted vertically) and control
6397 characters are output as question marks; otherwise, the output is listed
6398 one per line and control characters are output as-is.
6400 Because @command{ls} is such a fundamental program, it has accumulated many
6401 options over the years. They are described in the subsections below;
6402 within each section, options are listed alphabetically (ignoring case).
6403 The division of options into the subsections is not absolute, since some
6404 options affect more than one aspect of @command{ls}'s operation.
6406 @cindex exit status of @command{ls}
6411 1 minor problems (e.g., failure to access a file or directory not
6412 specified as a command line argument. This happens when listing a
6413 directory in which entries are actively being removed or renamed.)
6414 2 serious trouble (e.g., memory exhausted, invalid option, failure
6415 to access a file or directory specified as a command line argument
6416 or a directory loop)
6419 Also see @ref{Common options}.
6422 * Which files are listed::
6423 * What information is listed::
6424 * Sorting the output::
6425 * Details about version sort::
6426 * General output formatting::
6427 * Formatting file timestamps::
6428 * Formatting the file names::
6432 @node Which files are listed
6433 @subsection Which files are listed
6435 These options determine which files @command{ls} lists information for.
6436 By default, @command{ls} lists files and the contents of any
6437 directories on the command line, except that in directories it ignores
6438 files whose names start with @samp{.}.
6446 In directories, do not ignore file names that start with @samp{.}.
6451 @opindex --almost-all
6452 In directories, do not ignore all file names that start with @samp{.};
6453 ignore only @file{.} and @file{..}. The @option{--all} (@option{-a})
6454 option overrides this option.
6457 @itemx --ignore-backups
6459 @opindex --ignore-backups
6460 @cindex backup files, ignoring
6461 In directories, ignore files that end with @samp{~}. This option is
6462 equivalent to @samp{--ignore='*~' --ignore='.*~'}.
6467 @opindex --directory
6468 List just the names of directories, as with other types of files, rather
6469 than listing their contents.
6470 @c The following sentence is the same as the one for -F.
6471 Do not follow symbolic links listed on the
6472 command line unless the @option{--dereference-command-line} (@option{-H}),
6473 @option{--dereference} (@option{-L}), or
6474 @option{--dereference-command-line-symlink-to-dir} options are specified.
6477 @itemx --dereference-command-line
6479 @opindex --dereference-command-line
6480 @cindex symbolic links, dereferencing
6481 If a command line argument specifies a symbolic link, show information
6482 for the file the link references rather than for the link itself.
6484 @itemx --dereference-command-line-symlink-to-dir
6485 @opindex --dereference-command-line-symlink-to-dir
6486 @cindex symbolic links, dereferencing
6487 Do not dereference symbolic links, with one exception:
6488 if a command line argument specifies a symbolic link that refers to
6489 a directory, show information for that directory rather than for the
6491 This is the default behavior when no other dereferencing-related
6492 option has been specified (@option{--classify} (@option{-F}),
6493 @option{--directory} (@option{-d}),
6495 @option{--dereference} (@option{-L}), or
6496 @option{--dereference-command-line} (@option{-H})).
6498 @item --group-directories-first
6499 @opindex --group-directories-first
6500 Group all the directories before the files and then sort the
6501 directories and the files separately using the selected sort key
6502 (see --sort option).
6503 That is, this option specifies a primary sort key,
6504 and the --sort option specifies a secondary key.
6505 However, any use of @option{--sort=none}
6506 (@option{-U}) disables this option altogether.
6508 @item --hide=PATTERN
6509 @opindex --hide=@var{pattern}
6510 In directories, ignore files whose names match the shell pattern
6511 @var{pattern}, unless the @option{--all} (@option{-a}) or
6512 @option{--almost-all} (@option{-A}) is also given. This
6513 option acts like @option{--ignore=@var{pattern}} except that it has no
6514 effect if @option{--all} (@option{-a}) or @option{--almost-all}
6515 (@option{-A}) is also given.
6517 This option can be useful in shell aliases. For example, if
6518 @command{lx} is an alias for @samp{ls --hide='*~'} and @command{ly} is
6519 an alias for @samp{ls --ignore='*~'}, then the command @samp{lx -A}
6520 lists the file @file{README~} even though @samp{ly -A} would not.
6522 @item -I @var{pattern}
6523 @itemx --ignore=@var{pattern}
6525 @opindex --ignore=@var{pattern}
6526 In directories, ignore files whose names match the shell pattern
6527 (not regular expression) @var{pattern}. As
6528 in the shell, an initial @samp{.} in a file name does not match a
6529 wildcard at the start of @var{pattern}. Sometimes it is useful
6530 to give this option several times. For example,
6533 $ ls --ignore='.??*' --ignore='.[^.]' --ignore='#*'
6536 The first option ignores names of length 3 or more that start with @samp{.},
6537 the second ignores all two-character names that start with @samp{.}
6538 except @samp{..}, and the third ignores names that start with @samp{#}.
6541 @itemx --dereference
6543 @opindex --dereference
6544 @cindex symbolic links, dereferencing
6545 When showing file information for a symbolic link, show information
6546 for the file the link references rather than the link itself.
6547 However, even with this option, @command{ls} still prints the name
6548 of the link itself, not the name of the file that the link points to.
6553 @opindex --recursive
6554 @cindex recursive directory listing
6555 @cindex directory listing, recursive
6556 List the contents of all directories recursively.
6561 @node What information is listed
6562 @subsection What information is listed
6564 These options affect the information that @command{ls} displays. By
6565 default, only file names are shown.
6571 @cindex hurd, author, printing
6572 List each file's author when producing long format directory listings.
6573 In GNU/Hurd, file authors can differ from their owners, but in other
6574 operating systems the two are the same.
6580 @cindex dired Emacs mode support
6581 With the long listing (@option{-l}) format, print an additional line after
6585 //DIRED// @var{beg1} @var{end1} @var{beg2} @var{end2} @dots{}
6589 The @var{begn} and @var{endn} are unsigned integers that record the
6590 byte position of the beginning and end of each file name in the output.
6591 This makes it easy for Emacs to find the names, even when they contain
6592 unusual characters such as space or newline, without fancy searching.
6594 If directories are being listed recursively (@option{-R}), output a similar
6595 line with offsets for each subdirectory name:
6598 //SUBDIRED// @var{beg1} @var{end1} @dots{}
6601 Finally, output a line of the form:
6604 //DIRED-OPTIONS// --quoting-style=@var{word}
6608 where @var{word} is the quoting style (@pxref{Formatting the file names}).
6610 Here is an actual example:
6613 $ mkdir -p a/sub/deeper a/sub2
6615 $ touch a/sub/deeper/file
6616 $ ls -gloRF --dired a
6619 -rw-r--r-- 1 0 Jun 10 12:27 f1
6620 -rw-r--r-- 1 0 Jun 10 12:27 f2
6621 drwxr-xr-x 3 4096 Jun 10 12:27 sub/
6622 drwxr-xr-x 2 4096 Jun 10 12:27 sub2/
6626 drwxr-xr-x 2 4096 Jun 10 12:27 deeper/
6630 -rw-r--r-- 1 0 Jun 10 12:27 file
6634 //DIRED// 48 50 84 86 120 123 158 162 217 223 282 286
6635 //SUBDIRED// 2 3 167 172 228 240 290 296
6636 //DIRED-OPTIONS// --quoting-style=literal
6639 Note that the pairs of offsets on the @samp{//DIRED//} line above delimit
6640 these names: @file{f1}, @file{f2}, @file{sub}, @file{sub2}, @file{deeper},
6642 The offsets on the @samp{//SUBDIRED//} line delimit the following
6643 directory names: @file{a}, @file{a/sub}, @file{a/sub/deeper}, @file{a/sub2}.
6645 Here is an example of how to extract the fifth entry name, @samp{deeper},
6646 corresponding to the pair of offsets, 222 and 228:
6649 $ ls -gloRF --dired a > out
6650 $ dd bs=1 skip=222 count=6 < out 2>/dev/null; echo
6654 Note that although the listing above includes a trailing slash
6655 for the @samp{deeper} entry, the offsets select the name without
6656 the trailing slash. However, if you invoke @command{ls} with @option{--dired}
6657 along with an option like @option{--escape} (aka @option{-b}) and operate
6658 on a file whose name contains special characters, notice that the backslash
6663 $ ls -blog --dired 'a b'
6664 -rw-r--r-- 1 0 Jun 10 12:28 a\ b
6666 //DIRED-OPTIONS// --quoting-style=escape
6669 If you use a quoting style that adds quote marks
6670 (e.g., @option{--quoting-style=c}), then the offsets include the quote marks.
6671 So beware that the user may select the quoting style via the environment
6672 variable @env{QUOTING_STYLE}. Hence, applications using @option{--dired}
6673 should either specify an explicit @option{--quoting-style=literal} option
6674 (aka @option{-N} or @option{--literal}) on the command line, or else be
6675 prepared to parse the escaped names.
6678 @opindex --full-time
6679 Produce long format directory listings, and list times in full. It is
6680 equivalent to using @option{--format=long} with
6681 @option{--time-style=full-iso} (@pxref{Formatting file timestamps}).
6685 Produce long format directory listings, but don't display owner information.
6691 Inhibit display of group information in a long format directory listing.
6692 (This is the default in some non-@sc{gnu} versions of @command{ls}, so we
6693 provide this option for compatibility.)
6701 @cindex inode number, printing
6702 Print the inode number (also called the file serial number and index
6703 number) of each file to the left of the file name. (This number
6704 uniquely identifies each file within a particular file system.)
6707 @itemx --format=long
6708 @itemx --format=verbose
6711 @opindex long ls @r{format}
6712 @opindex verbose ls @r{format}
6713 In addition to the name of each file, print the file type, file mode bits,
6714 number of hard links, owner name, group name, size, and
6715 timestamp (@pxref{Formatting file timestamps}), normally
6716 the modification time. Print question marks for information that
6717 cannot be determined.
6719 Normally the size is printed as a byte count without punctuation, but
6720 this can be overridden (@pxref{Block size}). For example, @option{-h}
6721 prints an abbreviated, human-readable count, and
6722 @samp{--block-size="'1"} prints a byte count with the thousands
6723 separator of the current locale.
6725 For each directory that is listed, preface the files with a line
6726 @samp{total @var{blocks}}, where @var{blocks} is the total disk allocation
6727 for all files in that directory. The block size currently defaults to 1024
6728 bytes, but this can be overridden (@pxref{Block size}).
6729 The @var{blocks} computed counts each hard link separately;
6730 this is arguably a deficiency.
6732 The file type is one of the following characters:
6734 @c The commented-out entries are ones we're not sure about.
6742 character special file
6744 high performance (``contiguous data'') file
6748 door (Solaris 2.5 and up)
6750 @c semaphore, if this is a distinct file type
6754 @c multiplexed file (7th edition Unix; obsolete)
6756 off-line (``migrated'') file (Cray DMF)
6758 network special file (HP-UX)
6762 port (Solaris 10 and up)
6764 @c message queue, if this is a distinct file type
6768 @c shared memory object, if this is a distinct file type
6770 @c typed memory object, if this is a distinct file type
6772 @c whiteout (4.4BSD; not implemented)
6774 some other file type
6777 @cindex permissions, output by @command{ls}
6778 The file mode bits listed are similar to symbolic mode specifications
6779 (@pxref{Symbolic Modes}). But @command{ls} combines multiple bits into the
6780 third character of each set of permissions as follows:
6784 If the set-user-ID or set-group-ID bit and the corresponding executable bit
6788 If the set-user-ID or set-group-ID bit is set but the corresponding
6789 executable bit is not set.
6792 If the restricted deletion flag or sticky bit, and the
6793 other-executable bit, are both set. The restricted deletion flag is
6794 another name for the sticky bit. @xref{Mode Structure}.
6797 If the restricted deletion flag or sticky bit is set but the
6798 other-executable bit is not set.
6801 If the executable bit is set and none of the above apply.
6807 Following the file mode bits is a single character that specifies
6808 whether an alternate access method such as an access control list
6809 applies to the file. When the character following the file mode bits is a
6810 space, there is no alternate access method. When it is a printing
6811 character, then there is such a method.
6813 GNU @command{ls} uses a @samp{.} character to indicate a file
6814 with an SELinux security context, but no other alternate access method.
6816 A file with any other combination of alternate access methods
6817 is marked with a @samp{+} character.
6820 @itemx --numeric-uid-gid
6822 @opindex --numeric-uid-gid
6823 @cindex numeric uid and gid
6824 @cindex numeric user and group IDs
6825 Produce long format directory listings, but
6826 display numeric user and group IDs instead of the owner and group names.
6830 Produce long format directory listings, but don't display group information.
6831 It is equivalent to using @option{--format=long} with @option{--no-group} .
6837 @cindex disk allocation
6838 @cindex size of files, reporting
6839 Print the disk allocation of each file to the left of the file name.
6840 This is the amount of disk space used by the file, which is usually a
6841 bit more than the file's size, but it can be less if the file has holes.
6843 Normally the disk allocation is printed in units of
6844 1024 bytes, but this can be overridden (@pxref{Block size}).
6846 @cindex NFS mounts from BSD to HP-UX
6847 For files that are NFS-mounted from an HP-UX system to a BSD system,
6848 this option reports sizes that are half the correct values. On HP-UX
6849 systems, it reports sizes that are twice the correct values for files
6850 that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX;
6851 it also affects the HP-UX @command{ls} program.
6860 @cindex security context
6861 Display the SELinux security context or @samp{?} if none is found.
6862 When used with the @option{-l} option, print the security context
6863 to the left of the size column.
6868 @node Sorting the output
6869 @subsection Sorting the output
6871 @cindex sorting @command{ls} output
6872 These options change the order in which @command{ls} sorts the information
6873 it outputs. By default, sorting is done by character code
6874 (e.g., @acronym{ASCII} order).
6880 @itemx --time=status
6883 @opindex ctime@r{, printing or sorting by}
6884 @opindex status time@r{, printing or sorting by}
6885 @opindex use time@r{, printing or sorting files by}
6886 If the long listing format (e.g., @option{-l}, @option{-o}) is being used,
6887 print the status change time (the @samp{ctime} in the inode) instead of
6888 the modification time.
6889 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6890 or when not using a long listing format,
6891 sort according to the status change time.
6895 @cindex unsorted directory listing
6896 @cindex directory order, listing by
6897 Primarily, like @option{-U}---do not sort; list the files in whatever
6898 order they are stored in the directory. But also enable @option{-a} (list
6899 all files) and disable @option{-l}, @option{--color}, and @option{-s} (if they
6900 were specified before the @option{-f}).
6906 @cindex reverse sorting
6907 Reverse whatever the sorting method is---e.g., list files in reverse
6908 alphabetical order, youngest first, smallest first, or whatever.
6914 @opindex size of files@r{, sorting files by}
6915 Sort by file size, largest first.
6921 @opindex modification time@r{, sorting files by}
6922 Sort by modification time (the @samp{mtime} in the inode), newest first.
6926 @itemx --time=access
6930 @opindex use time@r{, printing or sorting files by}
6931 @opindex atime@r{, printing or sorting files by}
6932 @opindex access time@r{, printing or sorting files by}
6933 If the long listing format (e.g., @option{--format=long}) is being used,
6934 print the last access time (the @samp{atime} in the inode).
6935 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6936 or when not using a long listing format, sort according to the access time.
6942 @opindex none@r{, sorting option for @command{ls}}
6943 Do not sort; list the files in whatever order they are
6944 stored in the directory. (Do not do any of the other unrelated things
6945 that @option{-f} does.) This is especially useful when listing very large
6946 directories, since not doing any sorting can be noticeably faster.
6949 @itemx --sort=version
6952 @opindex version@r{, sorting option for @command{ls}}
6953 Sort by version name and number, lowest first. It behaves like a default
6954 sort, except that each sequence of decimal digits is treated numerically
6955 as an index/version number. (@xref{Details about version sort}.)
6958 @itemx --sort=extension
6961 @opindex extension@r{, sorting files by}
6962 Sort directory contents alphabetically by file extension (characters
6963 after the last @samp{.}); files with no extension are sorted first.
6968 @node Details about version sort
6969 @subsection Details about version sort
6971 Version sorting handles the fact that file names frequently include indices or
6972 version numbers. Standard sorting usually does not produce the order that one
6973 expects because comparisons are made on a character-by-character basis.
6974 Version sorting is especially useful when browsing directories that contain
6975 many files with indices/version numbers in their names:
6979 abc.zml-1.gz abc.zml-1.gz
6980 abc.zml-12.gz abc.zml-2.gz
6981 abc.zml-2.gz abc.zml-12.gz
6984 Version-sorted strings are compared such that if @var{ver1} and @var{ver2}
6985 are version numbers and @var{prefix} and @var{suffix} (@var{suffix} matching
6986 the regular expression @samp{(\.[A-Za-z~][A-Za-z0-9~]*)*}) are strings then
6987 @var{ver1} < @var{ver2} implies that the name composed of
6988 ``@var{prefix} @var{ver1} @var{suffix}'' sorts before
6989 ``@var{prefix} @var{ver2} @var{suffix}''.
6991 Note also that leading zeros of numeric parts are ignored:
6995 abc-1.007.tgz abc-1.01a.tgz
6996 abc-1.012b.tgz abc-1.007.tgz
6997 abc-1.01a.tgz abc-1.012b.tgz
7000 This functionality is implemented using gnulib's @code{filevercmp} function,
7001 which has some caveats worth noting.
7004 @item @env{LC_COLLATE} is ignored, which means @samp{ls -v} and @samp{sort -V}
7005 will sort non-numeric prefixes as if the @env{LC_COLLATE} locale category
7006 was set to @samp{C}.
7007 @item Some suffixes will not be matched by the regular
7008 expression mentioned above. Consequently these examples may
7009 not sort as you expect:
7017 abc-1.2.3.4.x86_64.rpm
7018 abc-1.2.3.x86_64.rpm
7022 @node General output formatting
7023 @subsection General output formatting
7025 These options affect the appearance of the overall output.
7030 @itemx --format=single-column
7033 @opindex single-column @r{output of files}
7034 List one file per line. This is the default for @command{ls} when standard
7035 output is not a terminal.
7038 @itemx --format=vertical
7041 @opindex vertical @r{sorted files in columns}
7042 List files in columns, sorted vertically. This is the default for
7043 @command{ls} if standard output is a terminal. It is always the default
7044 for the @command{dir} program.
7045 @sc{gnu} @command{ls} uses variable width columns to display as many files as
7046 possible in the fewest lines.
7048 @item --color [=@var{when}]
7050 @cindex color, distinguishing file types with
7051 Specify whether to use color for distinguishing file types. @var{when}
7052 may be omitted, or one of:
7055 @vindex none @r{color option}
7056 - Do not use color at all. This is the default.
7058 @vindex auto @r{color option}
7059 @cindex terminal, using color iff
7060 - Only use color if standard output is a terminal.
7062 @vindex always @r{color option}
7065 Specifying @option{--color} and no @var{when} is equivalent to
7066 @option{--color=always}.
7067 Piping a colorized listing through a pager like @command{more} or
7068 @command{less} usually produces unreadable results. However, using
7069 @code{more -f} does seem to work.
7072 @vindex SHELL @r{environment variable, and color}
7073 Note that using the @option{--color} option may incur a noticeable
7074 performance penalty when run in a directory with very many entries,
7075 because the default settings require that @command{ls} @code{stat} every
7076 single file it lists.
7077 However, if you would like most of the file-type coloring
7078 but can live without the other coloring options (e.g.,
7079 executable, orphan, sticky, other-writable, capability), use
7080 @command{dircolors} to set the @env{LS_COLORS} environment variable like this,
7082 eval $(dircolors -p | perl -pe \
7083 's/^((CAP|S[ET]|O[TR]|M|E)\w+).*/$1 00/' | dircolors -)
7085 and on a @code{dirent.d_type}-capable file system, @command{ls}
7086 will perform only one @code{stat} call per command line argument.
7090 @itemx --indicator-style=classify
7093 @opindex --indicator-style
7094 @cindex file type and executables, marking
7095 @cindex executables and file type, marking
7096 Append a character to each file name indicating the file type. Also,
7097 for regular files that are executable, append @samp{*}. The file type
7098 indicators are @samp{/} for directories, @samp{@@} for symbolic links,
7099 @samp{|} for FIFOs, @samp{=} for sockets, @samp{>} for doors,
7100 and nothing for regular files.
7101 @c The following sentence is the same as the one for -d.
7102 Do not follow symbolic links listed on the
7103 command line unless the @option{--dereference-command-line} (@option{-H}),
7104 @option{--dereference} (@option{-L}), or
7105 @option{--dereference-command-line-symlink-to-dir} options are specified.
7108 @itemx --indicator-style=file-type
7109 @opindex --file-type
7110 @opindex --indicator-style
7111 @cindex file type, marking
7112 Append a character to each file name indicating the file type. This is
7113 like @option{-F}, except that executables are not marked.
7115 @item --indicator-style=@var{word}
7116 @opindex --indicator-style
7117 Append a character indicator with style @var{word} to entry names,
7122 Do not append any character indicator; this is the default.
7124 Append @samp{/} for directories. This is the same as the @option{-p}
7127 Append @samp{/} for directories, @samp{@@} for symbolic links, @samp{|}
7128 for FIFOs, @samp{=} for sockets, and nothing for regular files. This is
7129 the same as the @option{--file-type} option.
7131 Append @samp{*} for executable regular files, otherwise behave as for
7132 @samp{file-type}. This is the same as the @option{-F} or
7133 @option{--classify} option.
7139 @opindex --kibibytes
7140 Set the default block size to its normal value of 1024 bytes,
7141 overriding any contrary specification in environment variables
7142 (@pxref{Block size}). This option is in turn overridden by the
7143 @option{--block-size}, @option{-h} or @option{--human-readable}, and
7144 @option{--si} options.
7146 The @option{-k} or @option{--kibibytes} option affects the
7147 per-directory block count written by the @option{-l} and similar
7148 options, and the size written by the @option{-s} or @option{--size}
7149 option. It does not affect the file size written by @option{-l}.
7152 @itemx --format=commas
7155 @opindex commas@r{, outputting between files}
7156 List files horizontally, with as many as will fit on each line,
7157 separated by @samp{, } (a comma and a space).
7160 @itemx --indicator-style=slash
7162 @opindex --indicator-style
7163 @cindex file type, marking
7164 Append a @samp{/} to directory names.
7167 @itemx --format=across
7168 @itemx --format=horizontal
7171 @opindex across@r{, listing files}
7172 @opindex horizontal@r{, listing files}
7173 List the files in columns, sorted horizontally.
7176 @itemx --tabsize=@var{cols}
7179 Assume that each tab stop is @var{cols} columns wide. The default is 8.
7180 @command{ls} uses tabs where possible in the output, for efficiency. If
7181 @var{cols} is zero, do not use tabs at all.
7183 @c FIXME: remove in 2009, if Apple Terminal has been fixed for long enough.
7184 Some terminal emulators (at least Apple Terminal 1.5 (133) from Mac OS X 10.4.8)
7185 do not properly align columns to the right of a TAB following a
7186 non-@acronym{ASCII} byte. If you use such a terminal emulator, use the
7187 @option{-T0} option or put @code{TABSIZE=0} in your environment to tell
7188 @command{ls} to align using spaces, not tabs.
7191 @itemx --width=@var{cols}
7195 Assume the screen is @var{cols} columns wide. The default is taken
7196 from the terminal settings if possible; otherwise the environment
7197 variable @env{COLUMNS} is used if it is set; otherwise the default
7203 @node Formatting file timestamps
7204 @subsection Formatting file timestamps
7206 By default, file timestamps are listed in abbreviated form, using
7207 a date like @samp{Mar 30@ @ 2002} for non-recent timestamps, and a
7208 date-without-year and time like @samp{Mar 30 23:45} for recent timestamps.
7209 This format can change depending on the current locale as detailed below.
7212 A timestamp is considered to be @dfn{recent} if it is less than six
7213 months old, and is not dated in the future. If a timestamp dated
7214 today is not listed in recent form, the timestamp is in the future,
7215 which means you probably have clock skew problems which may break
7216 programs like @command{make} that rely on file timestamps.
7219 Time stamps are listed according to the time zone rules specified by
7220 the @env{TZ} environment variable, or by the system default rules if
7221 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
7222 with @env{TZ}, libc, The GNU C Library Reference Manual}.
7224 The following option changes how file timestamps are printed.
7227 @item --time-style=@var{style}
7228 @opindex --time-style
7230 List timestamps in style @var{style}. The @var{style} should
7231 be one of the following:
7236 List timestamps using @var{format}, where @var{format} is interpreted
7237 like the format argument of @command{date} (@pxref{date invocation}).
7238 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
7239 @command{ls} to list timestamps like @samp{2002-03-30 23:45:56}. As
7240 with @command{date}, @var{format}'s interpretation is affected by the
7241 @env{LC_TIME} locale category.
7243 If @var{format} contains two format strings separated by a newline,
7244 the former is used for non-recent files and the latter for recent
7245 files; if you want output columns to line up, you may need to insert
7246 spaces in one of the two formats.
7249 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
7250 format with nanosecond precision, e.g., @samp{2002-03-30
7251 23:45:56.477817180 -0700}. This style is equivalent to
7252 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
7254 This is useful because the time output includes all the information that
7255 is available from the operating system. For example, this can help
7256 explain @command{make}'s behavior, since @acronym{GNU} @command{make}
7257 uses the full timestamp to determine whether a file is out of date.
7260 List @acronym{ISO} 8601 date and time in minutes, e.g.,
7261 @samp{2002-03-30 23:45}. These timestamps are shorter than
7262 @samp{full-iso} timestamps, and are usually good enough for everyday
7263 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
7266 List @acronym{ISO} 8601 dates for non-recent timestamps (e.g.,
7267 @samp{2002-03-30@ }), and @acronym{ISO} 8601 month, day, hour, and
7268 minute for recent timestamps (e.g., @samp{03-30 23:45}). These
7269 timestamps are uglier than @samp{long-iso} timestamps, but they carry
7270 nearly the same information in a smaller space and their brevity helps
7271 @command{ls} output fit within traditional 80-column output lines.
7272 The following two @command{ls} invocations are equivalent:
7277 ls -l --time-style="+%Y-%m-%d $newline%m-%d %H:%M"
7278 ls -l --time-style="iso"
7283 List timestamps in a locale-dependent form. For example, a Finnish
7284 locale might list non-recent timestamps like @samp{maalis 30@ @ 2002}
7285 and recent timestamps like @samp{maalis 30 23:45}. Locale-dependent
7286 timestamps typically consume more space than @samp{iso} timestamps and
7287 are harder for programs to parse because locale conventions vary so
7288 widely, but they are easier for many people to read.
7290 The @env{LC_TIME} locale category specifies the timestamp format. The
7291 default @acronym{POSIX} locale uses timestamps like @samp{Mar 30@
7292 @ 2002} and @samp{Mar 30 23:45}; in this locale, the following two
7293 @command{ls} invocations are equivalent:
7298 ls -l --time-style="+%b %e %Y$newline%b %e %H:%M"
7299 ls -l --time-style="locale"
7302 Other locales behave differently. For example, in a German locale,
7303 @option{--time-style="locale"} might be equivalent to
7304 @option{--time-style="+%e. %b %Y $newline%e. %b %H:%M"}
7305 and might generate timestamps like @samp{30. M@"ar 2002@ } and
7306 @samp{30. M@"ar 23:45}.
7308 @item posix-@var{style}
7310 List @acronym{POSIX}-locale timestamps if the @env{LC_TIME} locale
7311 category is @acronym{POSIX}, @var{style} timestamps otherwise. For
7312 example, the @samp{posix-long-iso} style lists
7313 timestamps like @samp{Mar 30@ @ 2002} and @samp{Mar 30 23:45} when in
7314 the @acronym{POSIX} locale, and like @samp{2002-03-30 23:45} otherwise.
7319 You can specify the default value of the @option{--time-style} option
7320 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
7321 the default style is @samp{locale}. @acronym{GNU} Emacs 21.3 and
7322 later use the @option{--dired} option and therefore can parse any date
7323 format, but if you are using Emacs 21.1 or 21.2 and specify a
7324 non-@acronym{POSIX} locale you may need to set
7325 @samp{TIME_STYLE="posix-long-iso"}.
7327 To avoid certain denial-of-service attacks, timestamps that would be
7328 longer than 1000 bytes may be treated as errors.
7331 @node Formatting the file names
7332 @subsection Formatting the file names
7334 These options change how file names themselves are printed.
7340 @itemx --quoting-style=escape
7343 @opindex --quoting-style
7344 @cindex backslash sequences for file names
7345 Quote nongraphic characters in file names using alphabetic and octal
7346 backslash sequences like those used in C.
7350 @itemx --quoting-style=literal
7353 @opindex --quoting-style
7354 Do not quote file names. However, with @command{ls} nongraphic
7355 characters are still printed as question marks if the output is a
7356 terminal and you do not specify the @option{--show-control-chars}
7360 @itemx --hide-control-chars
7362 @opindex --hide-control-chars
7363 Print question marks instead of nongraphic characters in file names.
7364 This is the default if the output is a terminal and the program is
7369 @itemx --quoting-style=c
7371 @opindex --quote-name
7372 @opindex --quoting-style
7373 Enclose file names in double quotes and quote nongraphic characters as
7376 @item --quoting-style=@var{word}
7377 @opindex --quoting-style
7378 @cindex quoting style
7379 Use style @var{word} to quote file names and other strings that may
7380 contain arbitrary characters. The @var{word} should
7381 be one of the following:
7385 Output strings as-is; this is the same as the @option{-N} or
7386 @option{--literal} option.
7388 Quote strings for the shell if they contain shell metacharacters or would
7389 cause ambiguous output.
7390 The quoting is suitable for @acronym{POSIX}-compatible shells like
7391 @command{bash}, but it does not always work for incompatible shells
7394 Quote strings for the shell, even if they would normally not require quoting.
7396 Quote strings as for C character string literals, including the
7397 surrounding double-quote characters; this is the same as the
7398 @option{-Q} or @option{--quote-name} option.
7400 Quote strings as for C character string literals, except omit the
7401 surrounding double-quote
7402 characters; this is the same as the @option{-b} or @option{--escape} option.
7404 Quote strings as for C character string literals, except use
7405 surrounding quotation marks appropriate for the
7408 @c Use @t instead of @samp to avoid duplicate quoting in some output styles.
7409 Quote strings as for C character string literals, except use
7410 surrounding quotation marks appropriate for the locale, and quote
7411 @t{'like this'} instead of @t{"like
7412 this"} in the default C locale. This looks nicer on many displays.
7415 You can specify the default value of the @option{--quoting-style} option
7416 with the environment variable @env{QUOTING_STYLE}. If that environment
7417 variable is not set, the default value is @samp{literal}, but this
7418 default may change to @samp{shell} in a future version of this package.
7420 @item --show-control-chars
7421 @opindex --show-control-chars
7422 Print nongraphic characters as-is in file names.
7423 This is the default unless the output is a terminal and the program is
7429 @node dir invocation
7430 @section @command{dir}: Briefly list directory contents
7433 @cindex directory listing, brief
7435 @command{dir} is equivalent to @code{ls -C
7436 -b}; that is, by default files are listed in columns, sorted vertically,
7437 and special characters are represented by backslash escape sequences.
7439 @xref{ls invocation, @command{ls}}.
7442 @node vdir invocation
7443 @section @command{vdir}: Verbosely list directory contents
7446 @cindex directory listing, verbose
7448 @command{vdir} is equivalent to @code{ls -l
7449 -b}; that is, by default files are listed in long format and special
7450 characters are represented by backslash escape sequences.
7452 @node dircolors invocation
7453 @section @command{dircolors}: Color setup for @command{ls}
7457 @cindex setup for color
7459 @command{dircolors} outputs a sequence of shell commands to set up the
7460 terminal for color output from @command{ls} (and @command{dir}, etc.).
7464 eval "`dircolors [@var{option}]@dots{} [@var{file}]`"
7467 If @var{file} is specified, @command{dircolors} reads it to determine which
7468 colors to use for which file types and extensions. Otherwise, a
7469 precompiled database is used. For details on the format of these files,
7470 run @samp{dircolors --print-database}.
7472 To make @command{dircolors} read a @file{~/.dircolors} file if it
7473 exists, you can put the following lines in your @file{~/.bashrc} (or
7474 adapt them to your favorite shell):
7478 test -r $d && eval "$(dircolors $d)"
7482 @vindex SHELL @r{environment variable, and color}
7483 The output is a shell command to set the @env{LS_COLORS} environment
7484 variable. You can specify the shell syntax to use on the command line,
7485 or @command{dircolors} will guess it from the value of the @env{SHELL}
7486 environment variable.
7488 The program accepts the following options. Also see @ref{Common options}.
7493 @itemx --bourne-shell
7496 @opindex --bourne-shell
7497 @cindex Bourne shell syntax for color setup
7498 @cindex @command{sh} syntax for color setup
7499 Output Bourne shell commands. This is the default if the @env{SHELL}
7500 environment variable is set and does not end with @samp{csh} or
7509 @cindex C shell syntax for color setup
7510 @cindex @command{csh} syntax for color setup
7511 Output C shell commands. This is the default if @code{SHELL} ends with
7512 @command{csh} or @command{tcsh}.
7515 @itemx --print-database
7517 @opindex --print-database
7518 @cindex color database, printing
7519 @cindex database for color setup, printing
7520 @cindex printing color database
7521 Print the (compiled-in) default color configuration database. This
7522 output is itself a valid configuration file, and is fairly descriptive
7523 of the possibilities.
7530 @node Basic operations
7531 @chapter Basic operations
7533 @cindex manipulating files
7535 This chapter describes the commands for basic file manipulation:
7536 copying, moving (renaming), and deleting (removing).
7539 * cp invocation:: Copy files.
7540 * dd invocation:: Convert and copy a file.
7541 * install invocation:: Copy files and set attributes.
7542 * mv invocation:: Move (rename) files.
7543 * rm invocation:: Remove files or directories.
7544 * shred invocation:: Remove files more securely.
7549 @section @command{cp}: Copy files and directories
7552 @cindex copying files and directories
7553 @cindex files, copying
7554 @cindex directories, copying
7556 @command{cp} copies files (or, optionally, directories). The copy is
7557 completely independent of the original. You can either copy one file to
7558 another, or copy arbitrarily many files to a destination directory.
7562 cp [@var{option}]@dots{} [-T] @var{source} @var{dest}
7563 cp [@var{option}]@dots{} @var{source}@dots{} @var{directory}
7564 cp [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
7569 If two file names are given, @command{cp} copies the first file to the
7573 If the @option{--target-directory} (@option{-t}) option is given, or
7574 failing that if the last file is a directory and the
7575 @option{--no-target-directory} (@option{-T}) option is not given,
7576 @command{cp} copies each @var{source} file to the specified directory,
7577 using the @var{source}s' names.
7580 Generally, files are written just as they are read. For exceptions,
7581 see the @option{--sparse} option below.
7583 By default, @command{cp} does not copy directories. However, the
7584 @option{-R}, @option{-a}, and @option{-r} options cause @command{cp} to
7585 copy recursively by descending into source directories and copying files
7586 to corresponding destination directories.
7588 When copying from a symbolic link, @command{cp} normally follows the
7589 link only when not copying
7590 recursively. This default can be overridden with the
7591 @option{--archive} (@option{-a}), @option{-d}, @option{--dereference}
7592 (@option{-L}), @option{--no-dereference} (@option{-P}), and
7593 @option{-H} options. If more than one of these options is specified,
7594 the last one silently overrides the others.
7596 When copying to a symbolic link, @command{cp} follows the
7597 link only when it refers to an existing regular file.
7598 However, when copying to a dangling symbolic link, @command{cp}
7599 refuses by default, and fails with a diagnostic, since the operation
7600 is inherently dangerous. This behavior is contrary to historical
7601 practice and to @acronym{POSIX}.
7602 Set @env{POSIXLY_CORRECT} to make @command{cp} attempt to create
7603 the target of a dangling destination symlink, in spite of the possible risk.
7604 Also, when an option like
7605 @option{--backup} or @option{--link} acts to rename or remove the
7606 destination before copying, @command{cp} renames or removes the
7607 symbolic link rather than the file it points to.
7609 By default, @command{cp} copies the contents of special files only
7610 when not copying recursively. This default can be overridden with the
7611 @option{--copy-contents} option.
7613 @cindex self-backups
7614 @cindex backups, making only
7615 @command{cp} generally refuses to copy a file onto itself, with the
7616 following exception: if @option{--force --backup} is specified with
7617 @var{source} and @var{dest} identical, and referring to a regular file,
7618 @command{cp} will make a backup file, either regular or numbered, as
7619 specified in the usual ways (@pxref{Backup options}). This is useful when
7620 you simply want to make a backup of an existing file before changing it.
7622 The program accepts the following options. Also see @ref{Common options}.
7629 Preserve as much as possible of the structure and attributes of the
7630 original files in the copy (but do not attempt to preserve internal
7631 directory structure; i.e., @samp{ls -U} may list the entries in a copied
7632 directory in a different order).
7633 Try to preserve SELinux security context and extended attributes (xattr),
7634 but ignore any failure to do that and print no corresponding diagnostic.
7635 Equivalent to @option{-dR --preserve=all} with the reduced diagnostics.
7637 @itemx --attributes-only
7638 @opindex --attributes-only
7639 Preserve the specified attributes of the original files in the copy,
7640 but do not copy any data. See the @option{--preserve} option for
7641 controlling which attributes to copy.
7644 @itemx @w{@kbd{--backup}[=@var{method}]}
7647 @vindex VERSION_CONTROL
7648 @cindex backups, making
7649 @xref{Backup options}.
7650 Make a backup of each file that would otherwise be overwritten or removed.
7651 As a special case, @command{cp} makes a backup of @var{source} when the force
7652 and backup options are given and @var{source} and @var{dest} are the same
7653 name for an existing, regular file. One useful application of this
7654 combination of options is this tiny Bourne shell script:
7658 # Usage: backup FILE...
7659 # Create a @sc{gnu}-style backup of each listed FILE.
7661 cp --backup --force -- "$i" "$i"
7665 @item --copy-contents
7666 @cindex directories, copying recursively
7667 @cindex copying directories recursively
7668 @cindex recursively copying directories
7669 @cindex non-directories, copying as special files
7670 If copying recursively, copy the contents of any special files (e.g.,
7671 FIFOs and device files) as if they were regular files. This means
7672 trying to read the data in each source file and writing it to the
7673 destination. It is usually a mistake to use this option, as it
7674 normally has undesirable effects on special files like FIFOs and the
7675 ones typically found in the @file{/dev} directory. In most cases,
7676 @code{cp -R --copy-contents} will hang indefinitely trying to read
7677 from FIFOs and special files like @file{/dev/console}, and it will
7678 fill up your destination disk if you use it to copy @file{/dev/zero}.
7679 This option has no effect unless copying recursively, and it does not
7680 affect the copying of symbolic links.
7684 @cindex symbolic links, copying
7685 @cindex hard links, preserving
7686 Copy symbolic links as symbolic links rather than copying the files that
7687 they point to, and preserve hard links between source files in the copies.
7688 Equivalent to @option{--no-dereference --preserve=links}.
7694 When copying without this option and an existing destination file cannot
7695 be opened for writing, the copy fails. However, with @option{--force}),
7696 when a destination file cannot be opened, @command{cp} then removes it and
7697 tries to open it again. Contrast this behavior with that enabled by
7698 @option{--link} and @option{--symbolic-link}, whereby the destination file
7699 is never opened but rather is removed unconditionally. Also see the
7700 description of @option{--remove-destination}.
7702 This option is independent of the @option{--interactive} or
7703 @option{-i} option: neither cancels the effect of the other.
7705 This option is redundant if the @option{--no-clobber} or @option{-n} option is
7710 If a command line argument specifies a symbolic link, then copy the
7711 file it points to rather than the symbolic link itself. However,
7712 copy (preserving its nature) any symbolic link that is encountered
7713 via recursive traversal.
7716 @itemx --interactive
7718 @opindex --interactive
7719 When copying a file other than a directory, prompt whether to
7720 overwrite an existing destination file. The @option{-i} option overrides
7721 a previous @option{-n} option.
7727 Make hard links instead of copies of non-directories.
7730 @itemx --dereference
7732 @opindex --dereference
7733 Follow symbolic links when copying from them.
7734 With this option, @command{cp} cannot create a symbolic link.
7735 For example, a symlink (to regular file) in the source tree will be copied to
7736 a regular file in the destination tree.
7741 @opindex --no-clobber
7742 Do not overwrite an existing file. The @option{-n} option overrides a previous
7743 @option{-i} option. This option is mutually exclusive with @option{-b} or
7744 @option{--backup} option.
7747 @itemx --no-dereference
7749 @opindex --no-dereference
7750 @cindex symbolic links, copying
7751 Copy symbolic links as symbolic links rather than copying the files that
7752 they point to. This option affects only symbolic links in the source;
7753 symbolic links in the destination are always followed if possible.
7756 @itemx @w{@kbd{--preserve}[=@var{attribute_list}]}
7759 @cindex file information, preserving, extended attributes, xattr
7760 Preserve the specified attributes of the original files.
7761 If specified, the @var{attribute_list} must be a comma-separated list
7762 of one or more of the following strings:
7766 Preserve the file mode bits and access control lists.
7768 Preserve the owner and group. On most modern systems,
7769 only users with appropriate privileges may change the owner of a file,
7771 may preserve the group ownership of a file only if they happen to be
7772 a member of the desired group.
7774 Preserve the times of last access and last modification, when possible.
7775 On older systems, it is not possible to preserve these attributes
7776 when the affected file is a symbolic link.
7777 However, many systems now provide the @code{utimensat} function,
7778 which makes it possible even for symbolic links.
7780 Preserve in the destination files
7781 any links between corresponding source files.
7782 Note that with @option{-L} or @option{-H}, this option can convert
7783 symbolic links to hard links. For example,
7785 $ mkdir c; : > a; ln -s a b; cp -aH a b c; ls -i1 c
7790 Note the inputs: @file{b} is a symlink to regular file @file{a},
7791 yet the files in destination directory, @file{c/}, are hard-linked.
7792 Since @option{-a} implies @option{--preserve=links}, and since @option{-H}
7793 tells @command{cp} to dereference command line arguments, it sees two files
7794 with the same inode number, and preserves the perceived hard link.
7796 Here is a similar example that exercises @command{cp}'s @option{-L} option:
7798 $ mkdir b c; (cd b; : > a; ln -s a b); cp -aL b c; ls -i1 c/b
7804 Preserve SELinux security context of the file, or fail with full diagnostics.
7806 Preserve extended attributes of the file, or fail with full diagnostics.
7807 If @command{cp} is built without xattr support, ignore this option.
7808 If SELinux context, ACLs or Capabilities are implemented using xattrs,
7809 they are preserved by this option as well.
7811 Preserve all file attributes.
7812 Equivalent to specifying all of the above, but with the difference
7813 that failure to preserve SELinux security context or extended attributes
7814 does not change @command{cp}'s exit status. In contrast to @option{-a},
7815 all but @samp{Operation not supported} warnings are output.
7818 Using @option{--preserve} with no @var{attribute_list} is equivalent
7819 to @option{--preserve=mode,ownership,timestamps}.
7821 In the absence of this option, each destination file is created with the
7822 mode bits of the corresponding source file, minus the bits set in the
7823 umask and minus the set-user-ID and set-group-ID bits.
7824 @xref{File permissions}.
7826 @itemx @w{@kbd{--no-preserve}=@var{attribute_list}}
7827 @cindex file information, preserving
7828 Do not preserve the specified attributes. The @var{attribute_list}
7829 has the same form as for @option{--preserve}.
7833 @cindex parent directories and @command{cp}
7834 Form the name of each destination file by appending to the target
7835 directory a slash and the specified name of the source file. The last
7836 argument given to @command{cp} must be the name of an existing directory.
7837 For example, the command:
7840 cp --parents a/b/c existing_dir
7844 copies the file @file{a/b/c} to @file{existing_dir/a/b/c}, creating
7845 any missing intermediate directories.
7852 @opindex --recursive
7853 @cindex directories, copying recursively
7854 @cindex copying directories recursively
7855 @cindex recursively copying directories
7856 @cindex non-directories, copying as special files
7857 Copy directories recursively. By default, do not follow symbolic
7858 links in the source; see the @option{--archive} (@option{-a}), @option{-d},
7859 @option{--dereference} (@option{-L}), @option{--no-dereference}
7860 (@option{-P}), and @option{-H} options. Special files are copied by
7861 creating a destination file of the same type as the source; see the
7862 @option{--copy-contents} option. It is not portable to use
7863 @option{-r} to copy symbolic links or special files. On some
7864 non-@sc{gnu} systems, @option{-r} implies the equivalent of
7865 @option{-L} and @option{--copy-contents} for historical reasons.
7866 Also, it is not portable to use @option{-R} to copy symbolic links
7867 unless you also specify @option{-P}, as @acronym{POSIX} allows
7868 implementations that dereference symbolic links by default.
7870 @item --reflink[=@var{when}]
7871 @opindex --reflink[=@var{when}]
7874 @cindex copy on write
7875 Perform a lightweight, copy-on-write (COW) copy, if supported by the
7876 file system. Once it has succeeded, beware that the source and destination
7877 files share the same disk data blocks as long as they remain unmodified.
7878 Thus, if a disk I/O error affects data blocks of one of the files,
7879 the other suffers the same fate.
7881 The @var{when} value can be one of the following:
7885 The default behavior: if the copy-on-write operation is not supported
7886 then report the failure for each file and exit with a failure status.
7889 If the copy-on-write operation is not supported then fall back
7890 to the standard copy behaviour.
7893 This option is overridden by the @option{--link}, @option{--symbolic-link}
7894 and @option{--attributes-only} options, thus allowing it to be used
7895 to configure the default data copying behavior for @command{cp}.
7896 For example, with the following alias, @command{cp} will use the
7897 minimum amount of space supported by the file system.
7900 alias cp='cp --reflink=auto --sparse=always'
7903 @item --remove-destination
7904 @opindex --remove-destination
7905 Remove each existing destination file before attempting to open it
7906 (contrast with @option{-f} above).
7908 @item --sparse=@var{when}
7909 @opindex --sparse=@var{when}
7910 @cindex sparse files, copying
7911 @cindex holes, copying files with
7912 @findex read @r{system call, and holes}
7913 A @dfn{sparse file} contains @dfn{holes}---a sequence of zero bytes that
7914 does not occupy any physical disk blocks; the @samp{read} system call
7915 reads these as zeros. This can both save considerable disk space and
7916 increase speed, since many binary files contain lots of consecutive zero
7917 bytes. By default, @command{cp} detects holes in input source files via a crude
7918 heuristic and makes the corresponding output file sparse as well.
7919 Only regular files may be sparse.
7921 The @var{when} value can be one of the following:
7925 The default behavior: if the input file is sparse, attempt to make
7926 the output file sparse, too. However, if an output file exists but
7927 refers to a non-regular file, then do not attempt to make it sparse.
7930 For each sufficiently long sequence of zero bytes in the input file,
7931 attempt to create a corresponding hole in the output file, even if the
7932 input file does not appear to be sparse.
7933 This is useful when the input file resides on a file system
7934 that does not support sparse files
7935 (for example, @samp{efs} file systems in SGI IRIX 5.3 and earlier),
7936 but the output file is on a type of file system that does support them.
7937 Holes may be created only in regular files, so if the destination file
7938 is of some other type, @command{cp} does not even try to make it sparse.
7941 Never make the output file sparse.
7942 This is useful in creating a file for use with the @command{mkswap} command,
7943 since such a file must not have any holes.
7946 @optStripTrailingSlashes
7949 @itemx --symbolic-link
7951 @opindex --symbolic-link
7952 @cindex symbolic links, copying with
7953 Make symbolic links instead of copies of non-directories. All source
7954 file names must be absolute (starting with @samp{/}) unless the
7955 destination files are in the current directory. This option merely
7956 results in an error message on systems that do not support symbolic links.
7962 @optNoTargetDirectory
7968 @cindex newer files, copying only
7969 Do not copy a non-directory that has an existing destination with the
7970 same or newer modification time. If time stamps are being preserved,
7971 the comparison is to the source time stamp truncated to the
7972 resolutions of the destination file system and of the system calls
7973 used to update time stamps; this avoids duplicate work if several
7974 @samp{cp -pu} commands are executed with the same source and destination.
7975 If @option{--preserve=links} is also specified (like with @samp{cp -au}
7976 for example), that will take precedence. Consequently, depending on the
7977 order that files are processed from the source, newer files in the destination
7978 may be replaced, to mirror hard links in the source.
7984 Print the name of each file before copying it.
7987 @itemx --one-file-system
7989 @opindex --one-file-system
7990 @cindex file systems, omitting copying to different
7991 Skip subdirectories that are on different file systems from the one that
7992 the copy started on.
7993 However, mount point directories @emph{are} copied.
8001 @section @command{dd}: Convert and copy a file
8004 @cindex converting while copying a file
8006 @command{dd} copies a file (from standard input to standard output, by
8007 default) with a changeable I/O block size, while optionally performing
8008 conversions on it. Synopses:
8011 dd [@var{operand}]@dots{}
8015 The only options are @option{--help} and @option{--version}.
8016 @xref{Common options}. @command{dd} accepts the following operands.
8022 Read from @var{file} instead of standard input.
8026 Write to @var{file} instead of standard output. Unless
8027 @samp{conv=notrunc} is given, @command{dd} truncates @var{file} to zero
8028 bytes (or the size specified with @samp{seek=}).
8030 @item ibs=@var{bytes}
8032 @cindex block size of input
8033 @cindex input block size
8034 Set the input block size to @var{bytes}.
8035 This makes @command{dd} read @var{bytes} per block.
8036 The default is 512 bytes.
8038 @item obs=@var{bytes}
8040 @cindex block size of output
8041 @cindex output block size
8042 Set the output block size to @var{bytes}.
8043 This makes @command{dd} write @var{bytes} per block.
8044 The default is 512 bytes.
8046 @item bs=@var{bytes}
8049 Set both input and output block sizes to @var{bytes}.
8050 This makes @command{dd} read and write @var{bytes} per block,
8051 overriding any @samp{ibs} and @samp{obs} settings.
8052 In addition, if no data-transforming @option{conv} option is specified,
8053 input is copied to the output as soon as it's read,
8054 even if it is smaller than the block size.
8056 @item cbs=@var{bytes}
8058 @cindex block size of conversion
8059 @cindex conversion block size
8060 @cindex fixed-length records, converting to variable-length
8061 @cindex variable-length records, converting to fixed-length
8062 Set the conversion block size to @var{bytes}.
8063 When converting variable-length records to fixed-length ones
8064 (@option{conv=block}) or the reverse (@option{conv=unblock}),
8065 use @var{bytes} as the fixed record length.
8069 Skip @var{n} @samp{ibs}-byte blocks in the input file before copying.
8070 If @samp{iflag=skip_bytes} is specified, @var{n} is interpreted
8071 as a byte count rather than a block count.
8075 Skip @var{n} @samp{obs}-byte blocks in the output file before copying.
8076 if @samp{oflag=seek_bytes} is specified, @var{n} is interpreted
8077 as a byte count rather than a block count.
8081 Copy @var{n} @samp{ibs}-byte blocks from the input file, instead
8082 of everything until the end of the file.
8083 if @samp{iflag=count_bytes} is specified, @var{n} is interpreted
8084 as a byte count rather than a block count.
8088 Do not print the overall transfer rate and volume statistics
8089 that normally make up the third status line when @command{dd} exits.
8091 @item conv=@var{conversion}[,@var{conversion}]@dots{}
8093 Convert the file as specified by the @var{conversion} argument(s).
8094 (No spaces around any comma(s).)
8101 @opindex ascii@r{, converting to}
8102 Convert @acronym{EBCDIC} to @acronym{ASCII},
8103 using the conversion table specified by @acronym{POSIX}.
8104 This provides a 1:1 translation for all 256 bytes.
8107 @opindex ebcdic@r{, converting to}
8108 Convert @acronym{ASCII} to @acronym{EBCDIC}.
8109 This is the inverse of the @samp{ascii} conversion.
8112 @opindex alternate ebcdic@r{, converting to}
8113 Convert @acronym{ASCII} to alternate @acronym{EBCDIC},
8114 using the alternate conversion table specified by @acronym{POSIX}.
8115 This is not a 1:1 translation, but reflects common historical practice
8116 for @samp{~}, @samp{[}, and @samp{]}.
8118 The @samp{ascii}, @samp{ebcdic}, and @samp{ibm} conversions are
8122 @opindex block @r{(space-padding)}
8123 For each line in the input, output @samp{cbs} bytes, replacing the
8124 input newline with a space and padding with spaces as necessary.
8128 Remove any trailing spaces in each @samp{cbs}-sized input block,
8129 and append a newline.
8131 The @samp{block} and @samp{unblock} conversions are mutually exclusive.
8134 @opindex lcase@r{, converting to}
8135 Change uppercase letters to lowercase.
8138 @opindex ucase@r{, converting to}
8139 Change lowercase letters to uppercase.
8141 The @samp{lcase} and @samp{ucase} conversions are mutually exclusive.
8144 @opindex swab @r{(byte-swapping)}
8145 @cindex byte-swapping
8146 Swap every pair of input bytes. @sc{gnu} @command{dd}, unlike others, works
8147 when an odd number of bytes are read---the last byte is simply copied
8148 (since there is nothing to swap it with).
8151 @opindex sync @r{(padding with @acronym{ASCII} @sc{nul}s)}
8152 Pad every input block to size of @samp{ibs} with trailing zero bytes.
8153 When used with @samp{block} or @samp{unblock}, pad with spaces instead of
8158 The following ``conversions'' are really file flags
8159 and don't affect internal processing:
8164 @cindex creating output file, requiring
8165 Fail if the output file already exists; @command{dd} must create the
8170 @cindex creating output file, avoiding
8171 Do not create the output file; the output file must already exist.
8173 The @samp{excl} and @samp{nocreat} conversions are mutually exclusive.
8177 @cindex truncating output file, avoiding
8178 Do not truncate the output file.
8182 @cindex read errors, ignoring
8183 Continue after read errors.
8187 @cindex synchronized data writes, before finishing
8188 Synchronize output data just before finishing. This forces a physical
8189 write of output data.
8193 @cindex synchronized data and metadata writes, before finishing
8194 Synchronize output data and metadata just before finishing. This
8195 forces a physical write of output data and metadata.
8199 @item iflag=@var{flag}[,@var{flag}]@dots{}
8201 Access the input file using the flags specified by the @var{flag}
8202 argument(s). (No spaces around any comma(s).)
8204 @item oflag=@var{flag}[,@var{flag}]@dots{}
8206 Access the output file using the flags specified by the @var{flag}
8207 argument(s). (No spaces around any comma(s).)
8209 Here are the flags. Not every flag is supported on every operating
8216 @cindex appending to the output file
8217 Write in append mode, so that even if some other process is writing to
8218 this file, every @command{dd} write will append to the current
8219 contents of the file. This flag makes sense only for output.
8220 If you combine this flag with the @samp{of=@var{file}} operand,
8221 you should also specify @samp{conv=notrunc} unless you want the
8222 output file to be truncated before being appended to.
8226 @cindex concurrent I/O
8227 Use concurrent I/O mode for data. This mode performs direct I/O
8228 and drops the @acronym{POSIX} requirement to serialize all I/O to the same file.
8229 A file cannot be opened in CIO mode and with a standard open at the
8235 Use direct I/O for data, avoiding the buffer cache.
8236 Note that the kernel may impose restrictions on read or write buffer sizes.
8237 For example, with an ext4 destination file system and a linux-based kernel,
8238 using @samp{oflag=direct} will cause writes to fail with @code{EINVAL} if the
8239 output buffer size is not a multiple of 512.
8243 @cindex directory I/O
8245 Fail unless the file is a directory. Most operating systems do not
8246 allow I/O to a directory, so this flag has limited utility.
8250 @cindex synchronized data reads
8251 Use synchronized I/O for data. For the output file, this forces a
8252 physical write of output data on each write. For the input file,
8253 this flag can matter when reading from a remote file that has been
8254 written to synchronously by some other process. Metadata (e.g.,
8255 last-access and last-modified time) is not necessarily synchronized.
8259 @cindex synchronized data and metadata I/O
8260 Use synchronized I/O for both data and metadata.
8264 @cindex discarding file cache
8265 Discard the data cache for a file.
8266 When count=0 all cache is discarded,
8267 otherwise the cache is dropped for the processed
8268 portion of the file. Also when count=0
8269 failure to discard the cache is diagnosed
8270 and reflected in the exit status.
8271 Here as some usage examples:
8274 # Advise to drop cache for whole file
8275 dd if=ifile iflag=nocache count=0
8277 # Ensure drop cache for the whole file
8278 dd of=ofile oflag=nocache conv=notrunc,fdatasync count=0
8280 # Drop cache for part of file
8281 dd if=ifile iflag=nocache skip=10 count=10 of=/dev/null
8283 # Stream data using just the read-ahead cache
8284 dd if=ifile of=ofile iflag=nocache oflag=nocache
8289 @cindex nonblocking I/O
8290 Use non-blocking I/O.
8295 Do not update the file's access time.
8296 Some older file systems silently ignore this flag, so it is a good
8297 idea to test it on your files before relying on it.
8301 @cindex controlling terminal
8302 Do not assign the file to be a controlling terminal for @command{dd}.
8303 This has no effect when the file is not a terminal.
8304 On many hosts (e.g., @acronym{GNU}/Linux hosts), this option has no effect
8309 @cindex symbolic links, following
8310 Do not follow symbolic links.
8315 Fail if the file has multiple hard links.
8320 Use binary I/O. This option has an effect only on nonstandard
8321 platforms that distinguish binary from text I/O.
8326 Use text I/O. Like @samp{binary}, this option has no effect on
8331 Accumulate full blocks from input. The @code{read} system call
8332 may return early if a full block is not available.
8333 When that happens, continue calling @code{read} to fill the remainder
8335 This flag can be used only with @code{iflag}.
8338 @opindex count_bytes
8339 Interpret the @samp{count=} operand as a byte count,
8340 rather than a block count, which allows specifying
8341 a length that is not a multiple of the I/O block size.
8342 This flag can be used only with @code{iflag}.
8346 Interpret the @samp{skip=} operand as a byte count,
8347 rather than a block count, which allows specifying
8348 an offset that is not a multiple of the I/O block size.
8349 This flag can be used only with @code{iflag}.
8353 Interpret the @samp{seek=} operand as a byte count,
8354 rather than a block count, which allows specifying
8355 an offset that is not a multiple of the I/O block size.
8356 This flag can be used only with @code{oflag}.
8360 These flags are not supported on all systems, and @samp{dd} rejects
8361 attempts to use them when they are not supported. When reading from
8362 standard input or writing to standard output, the @samp{nofollow} and
8363 @samp{noctty} flags should not be specified, and the other flags
8364 (e.g., @samp{nonblock}) can affect how other processes behave with the
8365 affected file descriptors, even after @command{dd} exits.
8369 @cindex multipliers after numbers
8370 The numeric-valued strings above (@var{bytes} and @var{blocks}) can be
8371 followed by a multiplier: @samp{b}=512, @samp{c}=1,
8372 @samp{w}=2, @samp{x@var{m}}=@var{m}, or any of the
8373 standard block size suffixes like @samp{k}=1024 (@pxref{Block size}).
8375 Any block size you specify via @samp{bs=}, @samp{ibs=}, @samp{obs=}, @samp{cbs=}
8376 should not be too large---values larger than a few megabytes
8377 are generally wasteful or (as in the gigabyte..exabyte case) downright
8378 counterproductive or error-inducing.
8380 To process data that is at an offset or size that is not a
8381 multiple of the I/O@ block size, you can use the @samp{skip_bytes},
8382 @samp{seek_bytes} and @samp{count_bytes} flags. Alternatively
8383 the traditional method of separate @command{dd} invocations can be used.
8384 For example, the following shell commands copy data
8385 in 512 KiB blocks between a disk and a tape, but do not save
8386 or restore a 4 KiB label at the start of the disk:
8389 disk=/dev/rdsk/c0t1d0s2
8392 # Copy all but the label from disk to tape.
8393 (dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape
8395 # Copy from tape back to disk, but leave the disk label alone.
8396 (dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk
8399 Sending an @samp{INFO} signal to a running @command{dd}
8400 process makes it print I/O statistics to standard error
8401 and then resume copying. In the example below,
8402 @command{dd} is run in the background to copy 10 million blocks.
8403 The @command{kill} command makes it output intermediate I/O statistics,
8404 and when @command{dd} completes normally or is killed by the
8405 @code{SIGINT} signal, it outputs the final statistics.
8408 $ dd if=/dev/zero of=/dev/null count=10MB & pid=$!
8409 $ kill -s INFO $pid; wait $pid
8410 3385223+0 records in
8411 3385223+0 records out
8412 1733234176 bytes (1.7 GB) copied, 6.42173 seconds, 270 MB/s
8413 10000000+0 records in
8414 10000000+0 records out
8415 5120000000 bytes (5.1 GB) copied, 18.913 seconds, 271 MB/s
8418 @vindex POSIXLY_CORRECT
8419 On systems lacking the @samp{INFO} signal @command{dd} responds to the
8420 @samp{USR1} signal instead, unless the @env{POSIXLY_CORRECT}
8421 environment variable is set.
8426 @node install invocation
8427 @section @command{install}: Copy files and set attributes
8430 @cindex copying files and setting attributes
8432 @command{install} copies files while setting their file mode bits and, if
8433 possible, their owner and group. Synopses:
8436 install [@var{option}]@dots{} [-T] @var{source} @var{dest}
8437 install [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8438 install [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8439 install [@var{option}]@dots{} -d @var{directory}@dots{}
8444 If two file names are given, @command{install} copies the first file to the
8448 If the @option{--target-directory} (@option{-t}) option is given, or
8449 failing that if the last file is a directory and the
8450 @option{--no-target-directory} (@option{-T}) option is not given,
8451 @command{install} copies each @var{source} file to the specified
8452 directory, using the @var{source}s' names.
8455 If the @option{--directory} (@option{-d}) option is given,
8456 @command{install} creates each @var{directory} and any missing parent
8457 directories. Parent directories are created with mode
8458 @samp{u=rwx,go=rx} (755), regardless of the @option{-m} option or the
8459 current umask. @xref{Directory Setuid and Setgid}, for how the
8460 set-user-ID and set-group-ID bits of parent directories are inherited.
8463 @cindex Makefiles, installing programs in
8464 @command{install} is similar to @command{cp}, but allows you to control the
8465 attributes of destination files. It is typically used in Makefiles to
8466 copy programs into their destination directories. It refuses to copy
8467 files onto themselves.
8469 @cindex extended attributes, xattr
8470 @command{install} never preserves extended attributes (xattr).
8472 The program accepts the following options. Also see @ref{Common options}.
8482 Compare each pair of source and destination files, and if the destination has
8483 identical content and any specified owner, group, permissions, and possibly
8484 SELinux context, then do not modify the destination at all.
8488 Ignored; for compatibility with old Unix versions of @command{install}.
8492 Create any missing parent directories of @var{dest},
8493 then copy @var{source} to @var{dest}.
8494 This option is ignored if a destination directory is specified
8495 via @option{--target-directory=DIR}.
8500 @opindex --directory
8501 @cindex directories, creating with given attributes
8502 @cindex parent directories, creating missing
8503 @cindex leading directories, creating missing
8504 Create any missing parent directories, giving them the default
8505 attributes. Then create each given directory, setting their owner,
8506 group and mode as given on the command line or to the defaults.
8508 @item -g @var{group}
8509 @itemx --group=@var{group}
8512 @cindex group ownership of installed files, setting
8513 Set the group ownership of installed files or directories to
8514 @var{group}. The default is the process's current group. @var{group}
8515 may be either a group name or a numeric group ID.
8518 @itemx --mode=@var{mode}
8521 @cindex permissions of installed files, setting
8522 Set the file mode bits for the installed file or directory to @var{mode},
8523 which can be either an octal number, or a symbolic mode as in
8524 @command{chmod}, with @samp{a=} (no access allowed to anyone) as the
8525 point of departure (@pxref{File permissions}).
8526 The default mode is @samp{u=rwx,go=rx,a-s}---read, write, and
8527 execute for the owner, read and execute for group and other, and with
8528 set-user-ID and set-group-ID disabled.
8529 This default is not quite the same as @samp{755}, since it disables
8530 instead of preserving set-user-ID and set-group-ID on directories.
8531 @xref{Directory Setuid and Setgid}.
8533 @item -o @var{owner}
8534 @itemx --owner=@var{owner}
8537 @cindex ownership of installed files, setting
8538 @cindex appropriate privileges
8539 @vindex root @r{as default owner}
8540 If @command{install} has appropriate privileges (is run as root), set the
8541 ownership of installed files or directories to @var{owner}. The default
8542 is @code{root}. @var{owner} may be either a user name or a numeric user
8545 @item --preserve-context
8546 @opindex --preserve-context
8548 @cindex security context
8549 Preserve the SELinux security context of files and directories.
8550 Failure to preserve the context in all of the files or directories
8551 will result in an exit status of 1. If SELinux is disabled then
8552 print a warning and ignore the option.
8555 @itemx --preserve-timestamps
8557 @opindex --preserve-timestamps
8558 @cindex timestamps of installed files, preserving
8559 Set the time of last access and the time of last modification of each
8560 installed file to match those of each corresponding original file.
8561 When a file is installed without this option, its last access and
8562 last modification times are both set to the time of installation.
8563 This option is useful if you want to use the last modification times
8564 of installed files to keep track of when they were last built as opposed
8565 to when they were last installed.
8571 @cindex symbol table information, stripping
8572 @cindex stripping symbol table information
8573 Strip the symbol tables from installed binary executables.
8575 @itemx --strip-program=@var{program}
8576 @opindex --strip-program
8577 @cindex symbol table information, stripping, program
8578 Program used to strip binaries.
8584 @optNoTargetDirectory
8590 Print the name of each file before copying it.
8592 @item -Z @var{context}
8593 @itemx --context=@var{context}
8597 @cindex security context
8598 Set the default SELinux security context to be used for any
8599 created files and directories. If SELinux is disabled then
8600 print a warning and ignore the option.
8608 @section @command{mv}: Move (rename) files
8612 @command{mv} moves or renames files (or directories). Synopses:
8615 mv [@var{option}]@dots{} [-T] @var{source} @var{dest}
8616 mv [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8617 mv [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8622 If two file names are given, @command{mv} moves the first file to the
8626 If the @option{--target-directory} (@option{-t}) option is given, or
8627 failing that if the last file is a directory and the
8628 @option{--no-target-directory} (@option{-T}) option is not given,
8629 @command{mv} moves each @var{source} file to the specified
8630 directory, using the @var{source}s' names.
8633 @command{mv} can move any type of file from one file system to another.
8634 Prior to version @code{4.0} of the fileutils,
8635 @command{mv} could move only regular files between file systems.
8636 For example, now @command{mv} can move an entire directory hierarchy
8637 including special device files from one partition to another. It first
8638 uses some of the same code that's used by @code{cp -a} to copy the
8639 requested directories and files, then (assuming the copy succeeded)
8640 it removes the originals. If the copy fails, then the part that was
8641 copied to the destination partition is removed. If you were to copy
8642 three directories from one partition to another and the copy of the first
8643 directory succeeded, but the second didn't, the first would be left on
8644 the destination partition and the second and third would be left on the
8647 @cindex extended attributes, xattr
8648 @command{mv} always tries to copy extended attributes (xattr), which may
8649 include SELinux context, ACLs or Capabilities.
8650 Upon failure all but @samp{Operation not supported} warnings are output.
8652 @cindex prompting, and @command{mv}
8653 If a destination file exists but is normally unwritable, standard input
8654 is a terminal, and the @option{-f} or @option{--force} option is not given,
8655 @command{mv} prompts the user for whether to replace the file. (You might
8656 own the file, or have write permission on its directory.) If the
8657 response is not affirmative, the file is skipped.
8659 @emph{Warning}: Avoid specifying a source name with a trailing slash,
8660 when it might be a symlink to a directory.
8661 Otherwise, @command{mv} may do something very surprising, since
8662 its behavior depends on the underlying rename system call.
8663 On a system with a modern Linux-based kernel, it fails with
8664 @code{errno=ENOTDIR}.
8665 However, on other systems (at least FreeBSD 6.1 and Solaris 10) it silently
8666 renames not the symlink but rather the directory referenced by the symlink.
8667 @xref{Trailing slashes}.
8669 The program accepts the following options. Also see @ref{Common options}.
8679 @cindex prompts, omitting
8680 Do not prompt the user before removing a destination file.
8682 If you specify more than one of the @option{-i}, @option{-f}, @option{-n}
8683 options, only the final one takes effect.
8688 @itemx --interactive
8690 @opindex --interactive
8691 @cindex prompts, forcing
8692 Prompt whether to overwrite each existing destination file, regardless
8694 If the response is not affirmative, the file is skipped.
8700 @opindex --no-clobber
8701 @cindex prompts, omitting
8702 Do not overwrite an existing file.
8704 This option is mutually exclusive with @option{-b} or @option{--backup} option.
8710 @cindex newer files, moving only
8711 Do not move a non-directory that has an existing destination with the
8712 same or newer modification time.
8713 If the move is across file system boundaries, the comparison is to the
8714 source time stamp truncated to the resolutions of the destination file
8715 system and of the system calls used to update time stamps; this avoids
8716 duplicate work if several @samp{mv -u} commands are executed with the
8717 same source and destination.
8723 Print the name of each file before moving it.
8725 @optStripTrailingSlashes
8731 @optNoTargetDirectory
8739 @section @command{rm}: Remove files or directories
8742 @cindex removing files or directories
8744 @command{rm} removes each given @var{file}. By default, it does not remove
8745 directories. Synopsis:
8748 rm [@var{option}]@dots{} [@var{file}]@dots{}
8751 @cindex prompting, and @command{rm}
8752 If the @option{-I} or @option{--interactive=once} option is given,
8753 and there are more than three files or the @option{-r}, @option{-R},
8754 or @option{--recursive} are given, then @command{rm} prompts the user
8755 for whether to proceed with the entire operation. If the response is
8756 not affirmative, the entire command is aborted.
8758 Otherwise, if a file is unwritable, standard input is a terminal, and
8759 the @option{-f} or @option{--force} option is not given, or the
8760 @option{-i} or @option{--interactive=always} option @emph{is} given,
8761 @command{rm} prompts the user for whether to remove the file.
8762 If the response is not affirmative, the file is skipped.
8764 Any attempt to remove a file whose last file name component is
8765 @file{.} or @file{..} is rejected without any prompting.
8767 @emph{Warning}: If you use @command{rm} to remove a file, it is usually
8768 possible to recover the contents of that file. If you want more assurance
8769 that the contents are truly unrecoverable, consider using @command{shred}.
8771 The program accepts the following options. Also see @ref{Common options}.
8779 Ignore nonexistent files and missing operands, and never prompt the user.
8780 Ignore any previous @option{--interactive} (@option{-i}) option.
8784 Prompt whether to remove each file.
8785 If the response is not affirmative, the file is skipped.
8786 Ignore any previous @option{--force} (@option{-f}) option.
8787 Equivalent to @option{--interactive=always}.
8791 Prompt once whether to proceed with the command, if more than three
8792 files are named or if a recursive removal is requested. Ignore any
8793 previous @option{--force} (@option{-f}) option. Equivalent to
8794 @option{--interactive=once}.
8796 @itemx --interactive [=@var{when}]
8797 @opindex --interactive
8798 Specify when to issue an interactive prompt. @var{when} may be
8802 @vindex never @r{interactive option}
8803 - Do not prompt at all.
8805 @vindex once @r{interactive option}
8806 - Prompt once if more than three files are named or if a recursive
8807 removal is requested. Equivalent to @option{-I}.
8809 @vindex always @r{interactive option}
8810 - Prompt for every file being removed. Equivalent to @option{-i}.
8812 @option{--interactive} with no @var{when} is equivalent to
8813 @option{--interactive=always}.
8815 @itemx --one-file-system
8816 @opindex --one-file-system
8817 @cindex one file system, restricting @command{rm} to
8818 When removing a hierarchy recursively, skip any directory that is on a
8819 file system different from that of the corresponding command line argument.
8822 This option is useful when removing a build ``chroot'' hierarchy,
8823 which normally contains no valuable data. However, it is not uncommon
8824 to bind-mount @file{/home} into such a hierarchy, to make it easier to
8825 use one's start-up file. The catch is that it's easy to forget to
8826 unmount @file{/home}. Then, when you use @command{rm -rf} to remove
8827 your normally throw-away chroot, that command will remove everything
8828 under @file{/home}, too.
8829 Use the @option{--one-file-system} option, and it will
8830 warn about and skip directories on other file systems.
8831 Of course, this will not save your @file{/home} if it and your
8832 chroot happen to be on the same file system.
8834 @itemx --preserve-root
8835 @opindex --preserve-root
8836 @cindex root directory, disallow recursive destruction
8837 Fail upon any attempt to remove the root directory, @file{/},
8838 when used with the @option{--recursive} option.
8839 This is the default behavior.
8840 @xref{Treating / specially}.
8842 @itemx --no-preserve-root
8843 @opindex --no-preserve-root
8844 @cindex root directory, allow recursive destruction
8845 Do not treat @file{/} specially when removing recursively.
8846 This option is not recommended unless you really want to
8847 remove all the files on your computer.
8848 @xref{Treating / specially}.
8855 @opindex --recursive
8856 @cindex directories, removing (recursively)
8857 Remove the listed directories and their contents recursively.
8863 Print the name of each file before removing it.
8867 @cindex files beginning with @samp{-}, removing
8868 @cindex @samp{-}, removing files beginning with
8869 One common question is how to remove files whose names begin with a
8870 @samp{-}. @sc{gnu} @command{rm}, like every program that uses the @code{getopt}
8871 function to parse its arguments, lets you use the @samp{--} option to
8872 indicate that all following arguments are non-options. To remove a file
8873 called @file{-f} in the current directory, you could type either:
8886 @opindex - @r{and Unix @command{rm}}
8887 The Unix @command{rm} program's use of a single @samp{-} for this purpose
8888 predates the development of the getopt standard syntax.
8893 @node shred invocation
8894 @section @command{shred}: Remove files more securely
8897 @cindex data, erasing
8898 @cindex erasing data
8900 @command{shred} overwrites devices or files, to help prevent even
8901 very expensive hardware from recovering the data.
8903 Ordinarily when you remove a file (@pxref{rm invocation}), the data is
8904 not actually destroyed. Only the index listing where the file is
8905 stored is destroyed, and the storage is made available for reuse.
8906 There are undelete utilities that will attempt to reconstruct the index
8907 and can bring the file back if the parts were not reused.
8909 On a busy system with a nearly-full drive, space can get reused in a few
8910 seconds. But there is no way to know for sure. If you have sensitive
8911 data, you may want to be sure that recovery is not possible by actually
8912 overwriting the file with non-sensitive data.
8914 However, even after doing that, it is possible to take the disk back
8915 to a laboratory and use a lot of sensitive (and expensive) equipment
8916 to look for the faint ``echoes'' of the original data underneath the
8917 overwritten data. If the data has only been overwritten once, it's not
8920 The best way to remove something irretrievably is to destroy the media
8921 it's on with acid, melt it down, or the like. For cheap removable media
8922 like floppy disks, this is the preferred method. However, hard drives
8923 are expensive and hard to melt, so the @command{shred} utility tries
8924 to achieve a similar effect non-destructively.
8926 This uses many overwrite passes, with the data patterns chosen to
8927 maximize the damage they do to the old data. While this will work on
8928 floppies, the patterns are designed for best effect on hard drives.
8929 For more details, see the source code and Peter Gutmann's paper
8930 @uref{http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html,
8931 @cite{Secure Deletion of Data from Magnetic and Solid-State Memory}},
8932 from the proceedings of the Sixth @acronym{USENIX} Security Symposium (San Jose,
8933 California, July 22--25, 1996).
8935 @strong{Please note} that @command{shred} relies on a very important assumption:
8936 that the file system overwrites data in place. This is the traditional
8937 way to do things, but many modern file system designs do not satisfy this
8938 assumption. Exceptions include:
8943 Log-structured or journaled file systems, such as those supplied with
8944 AIX and Solaris, and JFS, ReiserFS, XFS, Ext3 (in @code{data=journal} mode),
8945 BFS, NTFS, etc.@: when they are configured to journal @emph{data}.
8948 File systems that write redundant data and carry on even if some writes
8949 fail, such as RAID-based file systems.
8952 File systems that make snapshots, such as Network Appliance's NFS server.
8955 File systems that cache in temporary locations, such as NFS version 3
8959 Compressed file systems.
8962 In the particular case of ext3 file systems, the above disclaimer applies (and
8963 @command{shred} is thus of limited effectiveness) only in @code{data=journal}
8964 mode, which journals file data in addition to just metadata. In both
8965 the @code{data=ordered} (default) and @code{data=writeback} modes,
8966 @command{shred} works as usual. Ext3 journaling modes can be changed
8967 by adding the @code{data=something} option to the mount options for a
8968 particular file system in the @file{/etc/fstab} file, as documented in
8969 the mount man page (man mount).
8971 If you are not sure how your file system operates, then you should assume
8972 that it does not overwrite data in place, which means that shred cannot
8973 reliably operate on regular files in your file system.
8975 Generally speaking, it is more reliable to shred a device than a file,
8976 since this bypasses the problem of file system design mentioned above.
8977 However, even shredding devices is not always completely reliable. For
8978 example, most disks map out bad sectors invisibly to the application; if
8979 the bad sectors contain sensitive data, @command{shred} won't be able to
8982 @command{shred} makes no attempt to detect or report this problem, just as
8983 it makes no attempt to do anything about backups. However, since it is
8984 more reliable to shred devices than files, @command{shred} by default does
8985 not truncate or remove the output file. This default is more suitable
8986 for devices, which typically cannot be truncated and should not be
8989 Finally, consider the risk of backups and mirrors.
8990 File system backups and remote mirrors may contain copies of the
8991 file that cannot be removed, and that will allow a shredded file
8992 to be recovered later. So if you keep any data you may later want
8993 to destroy using @command{shred}, be sure that it is not backed up or mirrored.
8996 shred [@var{option}]@dots{} @var{file}[@dots{}]
8999 The program accepts the following options. Also see @ref{Common options}.
9007 @cindex force deletion
9008 Override file permissions if necessary to allow overwriting.
9011 @itemx -n @var{number}
9012 @itemx --iterations=@var{number}
9013 @opindex -n @var{number}
9014 @opindex --iterations=@var{number}
9015 @cindex iterations, selecting the number of
9016 By default, @command{shred} uses @value{SHRED_DEFAULT_PASSES} passes of
9017 overwrite. You can reduce this to save time, or increase it if you think it's
9018 appropriate. After 25 passes all of the internal overwrite patterns will have
9019 been used at least once.
9021 @item --random-source=@var{file}
9022 @opindex --random-source
9023 @cindex random source for shredding
9024 Use @var{file} as a source of random data used to overwrite and to
9025 choose pass ordering. @xref{Random sources}.
9027 @item -s @var{bytes}
9028 @itemx --size=@var{bytes}
9029 @opindex -s @var{bytes}
9030 @opindex --size=@var{bytes}
9031 @cindex size of file to shred
9032 Shred the first @var{bytes} bytes of the file. The default is to shred
9033 the whole file. @var{bytes} can be followed by a size specification like
9034 @samp{K}, @samp{M}, or @samp{G} to specify a multiple. @xref{Block size}.
9040 @cindex removing files after shredding
9041 After shredding a file, truncate it (if possible) and then remove it.
9042 If a file has multiple links, only the named links will be removed.
9048 Display to standard error all status updates as sterilization proceeds.
9054 By default, @command{shred} rounds the size of a regular file up to the next
9055 multiple of the file system block size to fully erase the last block
9057 Use @option{--exact} to suppress that behavior.
9058 Thus, by default if you shred a 10-byte regular file on a system with 512-byte
9059 blocks, the resulting file will be 512 bytes long. With this option,
9060 shred does not increase the apparent size of the file.
9066 Normally, the last pass that @command{shred} writes is made up of
9067 random data. If this would be conspicuous on your hard drive (for
9068 example, because it looks like encrypted data), or you just think
9069 it's tidier, the @option{--zero} option adds an additional overwrite pass with
9070 all zero bits. This is in addition to the number of passes specified
9071 by the @option{--iterations} option.
9075 You might use the following command to erase all trace of the
9076 file system you'd created on the floppy disk in your first drive.
9077 That command takes about 20 minutes to erase a ``1.44MB'' (actually
9081 shred --verbose /dev/fd0
9084 Similarly, to erase all data on a selected partition of
9085 your hard disk, you could give a command like this:
9088 shred --verbose /dev/sda5
9091 On modern disks, a single pass should be adequate,
9092 and it will take one third the time of the default three-pass approach.
9095 # 1 pass, write pseudo-random data; 3x faster than the default
9096 shred --verbose -n1 /dev/sda5
9099 To be on the safe side, use at least one pass that overwrites using
9100 pseudo-random data. I.e., don't be tempted to use @samp{-n0 --zero},
9101 in case some disk controller optimizes the process of writing blocks
9102 of all zeros, and thereby does not clear all bytes in a block.
9103 Some SSDs may do just that.
9105 A @var{file} of @samp{-} denotes standard output.
9106 The intended use of this is to shred a removed temporary file.
9113 echo "Hello, world" >&3
9118 However, the command @samp{shred - >file} does not shred the contents
9119 of @var{file}, since the shell truncates @var{file} before invoking
9120 @command{shred}. Use the command @samp{shred file} or (if using a
9121 Bourne-compatible shell) the command @samp{shred - 1<>file} instead.
9126 @node Special file types
9127 @chapter Special file types
9129 @cindex special file types
9130 @cindex file types, special
9132 This chapter describes commands which create special types of files (and
9133 @command{rmdir}, which removes directories, one special file type).
9135 @cindex special file types
9137 Although Unix-like operating systems have markedly fewer special file
9138 types than others, not @emph{everything} can be treated only as the
9139 undifferentiated byte stream of @dfn{normal files}. For example, when a
9140 file is created or removed, the system must record this information,
9141 which it does in a @dfn{directory}---a special type of file. Although
9142 you can read directories as normal files, if you're curious, in order
9143 for the system to do its job it must impose a structure, a certain
9144 order, on the bytes of the file. Thus it is a ``special'' type of file.
9146 Besides directories, other special file types include named pipes
9147 (FIFOs), symbolic links, sockets, and so-called @dfn{special files}.
9150 * link invocation:: Make a hard link via the link syscall
9151 * ln invocation:: Make links between files.
9152 * mkdir invocation:: Make directories.
9153 * mkfifo invocation:: Make FIFOs (named pipes).
9154 * mknod invocation:: Make block or character special files.
9155 * readlink invocation:: Print value of a symlink or canonical file name.
9156 * rmdir invocation:: Remove empty directories.
9157 * unlink invocation:: Remove files via the unlink syscall
9161 @node link invocation
9162 @section @command{link}: Make a hard link via the link syscall
9165 @cindex links, creating
9166 @cindex hard links, creating
9167 @cindex creating links (hard only)
9169 @command{link} creates a single hard link at a time.
9170 It is a minimalist interface to the system-provided
9171 @code{link} function. @xref{Hard Links, , , libc,
9172 The GNU C Library Reference Manual}.
9173 It avoids the bells and whistles of the more commonly-used
9174 @command{ln} command (@pxref{ln invocation}).
9178 link @var{filename} @var{linkname}
9181 @var{filename} must specify an existing file, and @var{linkname}
9182 must specify a nonexistent entry in an existing directory.
9183 @command{link} simply calls @code{link (@var{filename}, @var{linkname})}
9186 On a @acronym{GNU} system, this command acts like @samp{ln --directory
9187 --no-target-directory @var{filename} @var{linkname}}. However, the
9188 @option{--directory} and @option{--no-target-directory} options are
9189 not specified by @acronym{POSIX}, and the @command{link} command is
9190 more portable in practice.
9192 If @var{filename} is a symbolic link, it is unspecified whether
9193 @var{linkname} will be a hard link to the symbolic link or to the
9194 target of the symbolic link. Use @command{ln -P} or @command{ln -L}
9195 to specify which behavior is desired.
9201 @section @command{ln}: Make links between files
9204 @cindex links, creating
9205 @cindex hard links, creating
9206 @cindex symbolic (soft) links, creating
9207 @cindex creating links (hard or soft)
9209 @cindex file systems and hard links
9210 @command{ln} makes links between files. By default, it makes hard links;
9211 with the @option{-s} option, it makes symbolic (or @dfn{soft}) links.
9215 ln [@var{option}]@dots{} [-T] @var{target} @var{linkname}
9216 ln [@var{option}]@dots{} @var{target}
9217 ln [@var{option}]@dots{} @var{target}@dots{} @var{directory}
9218 ln [@var{option}]@dots{} -t @var{directory} @var{target}@dots{}
9224 If two file names are given, @command{ln} creates a link to the first
9225 file from the second.
9228 If one @var{target} is given, @command{ln} creates a link to that file
9229 in the current directory.
9232 If the @option{--target-directory} (@option{-t}) option is given, or
9233 failing that if the last file is a directory and the
9234 @option{--no-target-directory} (@option{-T}) option is not given,
9235 @command{ln} creates a link to each @var{target} file in the specified
9236 directory, using the @var{target}s' names.
9240 Normally @command{ln} does not remove existing files. Use the
9241 @option{--force} (@option{-f}) option to remove them unconditionally,
9242 the @option{--interactive} (@option{-i}) option to remove them
9243 conditionally, and the @option{--backup} (@option{-b}) option to
9246 @cindex hard link, defined
9247 @cindex inode, and hard links
9248 A @dfn{hard link} is another name for an existing file; the link and the
9249 original are indistinguishable. Technically speaking, they share the
9250 same inode, and the inode contains all the information about a
9251 file---indeed, it is not incorrect to say that the inode @emph{is} the
9252 file. Most systems prohibit making a hard link to
9253 a directory; on those where it is allowed, only the super-user can do
9254 so (and with caution, since creating a cycle will cause problems to many
9255 other utilities). Hard links cannot cross file system boundaries. (These
9256 restrictions are not mandated by @acronym{POSIX}, however.)
9258 @cindex dereferencing symbolic links
9259 @cindex symbolic link, defined
9260 @dfn{Symbolic links} (@dfn{symlinks} for short), on the other hand, are
9261 a special file type (which not all kernels support: System V release 3
9262 (and older) systems lack symlinks) in which the link file actually
9263 refers to a different file, by name. When most operations (opening,
9264 reading, writing, and so on) are passed the symbolic link file, the
9265 kernel automatically @dfn{dereferences} the link and operates on the
9266 target of the link. But some operations (e.g., removing) work on the
9267 link file itself, rather than on its target. The owner and group of a
9268 symlink are not significant to file access performed through
9269 the link, but do have implications on deleting a symbolic link from a
9270 directory with the restricted deletion bit set. On the GNU system,
9271 the mode of a symlink has no significance and cannot be changed, but
9272 on some BSD systems, the mode can be changed and will affect whether
9273 the symlink will be traversed in file name resolution. @xref{Symbolic Links,,,
9274 libc, The GNU C Library Reference Manual}.
9276 Symbolic links can contain arbitrary strings; a @dfn{dangling symlink}
9277 occurs when the string in the symlink does not resolve to a file.
9278 There are no restrictions against creating dangling symbolic links.
9279 There are trade-offs to using absolute or relative symlinks. An
9280 absolute symlink always points to the same file, even if the directory
9281 containing the link is moved. However, if the symlink is visible from
9282 more than one machine (such as on a networked file system), the file
9283 pointed to might not always be the same. A relative symbolic link is
9284 resolved in relation to the directory that contains the link, and is
9285 often useful in referring to files on the same device without regards
9286 to what name that device is mounted on when accessed via networked
9289 When creating a relative symlink in a different location than the
9290 current directory, the resolution of the symlink will be different
9291 than the resolution of the same string from the current directory.
9292 Therefore, many users prefer to first change directories to the
9293 location where the relative symlink will be created, so that
9294 tab-completion or other file resolution will find the same target as
9295 what will be placed in the symlink.
9297 The program accepts the following options. Also see @ref{Common options}.
9308 @opindex --directory
9309 @cindex hard links to directories
9310 Allow users with appropriate privileges to attempt to make hard links
9312 However, note that this will probably fail due to
9313 system restrictions, even for the super-user.
9319 Remove existing destination files.
9322 @itemx --interactive
9324 @opindex --interactive
9325 @cindex prompting, and @command{ln}
9326 Prompt whether to remove existing destination files.
9332 If @option{-s} is not in effect, and the source file is a symbolic
9333 link, create the hard link to the file referred to by the symbolic
9334 link, rather than the symbolic link itself.
9337 @itemx --no-dereference
9339 @opindex --no-dereference
9340 Do not treat the last operand specially when it is a symbolic link to
9341 a directory. Instead, treat it as if it were a normal file.
9343 When the destination is an actual directory (not a symlink to one),
9344 there is no ambiguity. The link is created in that directory.
9345 But when the specified destination is a symlink to a directory,
9346 there are two ways to treat the user's request. @command{ln} can
9347 treat the destination just as it would a normal directory and create
9348 the link in it. On the other hand, the destination can be viewed as a
9349 non-directory---as the symlink itself. In that case, @command{ln}
9350 must delete or backup that symlink before creating the new link.
9351 The default is to treat a destination that is a symlink to a directory
9352 just like a directory.
9354 This option is weaker than the @option{--no-target-directory}
9355 (@option{-T}) option, so it has no effect if both options are given.
9361 If @option{-s} is not in effect, and the source file is a symbolic
9362 link, create the hard link to the symbolic link itself. On platforms
9363 where this is not supported by the kernel, this option creates a
9364 symbolic link with identical contents; since symbolic link contents
9365 cannot be edited, any file name resolution performed through either
9366 link will be the same as if a hard link had been created.
9372 Make symbolic links instead of hard links. This option merely produces
9373 an error message on systems that do not support symbolic links.
9379 @optNoTargetDirectory
9385 Print the name of each file after linking it successfully.
9389 @cindex hard links to symbolic links
9390 @cindex symbolic links and @command{ln}
9391 If @option{-L} and @option{-P} are both given, the last one takes
9392 precedence. If @option{-s} is also given, @option{-L} and @option{-P}
9393 are silently ignored. If neither option is given, then this
9394 implementation defaults to @option{-P} if the system @code{link} supports
9395 hard links to symbolic links (such as the GNU system), and @option{-L}
9396 if @code{link} follows symbolic links (such as on BSD).
9405 # Create link ../a pointing to a in that directory.
9406 # Not really useful because it points to itself.
9411 # Change to the target before creating symlinks to avoid being confused.
9417 # Hard coded file names don't move well.
9418 ln -s $(pwd)/a /some/dir/
9422 # Relative file names survive directory moves and also
9423 # work across networked file systems.
9424 ln -s afile anotherfile
9425 ln -s ../adir/afile yetanotherfile
9429 @node mkdir invocation
9430 @section @command{mkdir}: Make directories
9433 @cindex directories, creating
9434 @cindex creating directories
9436 @command{mkdir} creates directories with the specified names. Synopsis:
9439 mkdir [@var{option}]@dots{} @var{name}@dots{}
9442 @command{mkdir} creates each directory @var{name} in the order given.
9443 It reports an error if @var{name} already exists, unless the
9444 @option{-p} option is given and @var{name} is a directory.
9446 The program accepts the following options. Also see @ref{Common options}.
9451 @itemx --mode=@var{mode}
9454 @cindex modes of created directories, setting
9455 Set the file permission bits of created directories to @var{mode},
9456 which uses the same syntax as
9457 in @command{chmod} and uses @samp{a=rwx} (read, write and execute allowed for
9458 everyone) for the point of the departure. @xref{File permissions}.
9460 Normally the directory has the desired file mode bits at the moment it
9461 is created. As a @acronym{GNU} extension, @var{mode} may also mention
9462 special mode bits, but in this case there may be a temporary window
9463 during which the directory exists but its special mode bits are
9464 incorrect. @xref{Directory Setuid and Setgid}, for how the
9465 set-user-ID and set-group-ID bits of directories are inherited unless
9466 overridden in this way.
9472 @cindex parent directories, creating
9473 Make any missing parent directories for each argument, setting their
9474 file permission bits to the umask modified by @samp{u+wx}. Ignore
9475 existing parent directories, and do not change their file permission
9478 To set the file permission bits of any newly-created parent
9479 directories to a value that includes @samp{u+wx}, you can set the
9480 umask before invoking @command{mkdir}. For example, if the shell
9481 command @samp{(umask u=rwx,go=rx; mkdir -p P/Q)} creates the parent
9482 @file{P} it sets the parent's permission bits to @samp{u=rwx,go=rx}.
9483 To set a parent's special mode bits as well, you can invoke
9484 @command{chmod} after @command{mkdir}. @xref{Directory Setuid and
9485 Setgid}, for how the set-user-ID and set-group-ID bits of
9486 newly-created parent directories are inherited.
9492 Print a message for each created directory. This is most useful with
9495 @item -Z @var{context}
9496 @itemx --context=@var{context}
9500 @cindex security context
9501 Set the default SELinux security context to be used for created directories.
9508 @node mkfifo invocation
9509 @section @command{mkfifo}: Make FIFOs (named pipes)
9512 @cindex FIFOs, creating
9513 @cindex named pipes, creating
9514 @cindex creating FIFOs (named pipes)
9516 @command{mkfifo} creates FIFOs (also called @dfn{named pipes}) with the
9517 specified names. Synopsis:
9520 mkfifo [@var{option}] @var{name}@dots{}
9523 A @dfn{FIFO} is a special file type that permits independent processes
9524 to communicate. One process opens the FIFO file for writing, and
9525 another for reading, after which data can flow as with the usual
9526 anonymous pipe in shells or elsewhere.
9528 The program accepts the following option. Also see @ref{Common options}.
9533 @itemx --mode=@var{mode}
9536 @cindex modes of created FIFOs, setting
9537 Set the mode of created FIFOs to @var{mode}, which is symbolic as in
9538 @command{chmod} and uses @samp{a=rw} (read and write allowed for everyone)
9539 for the point of departure. @var{mode} should specify only file
9540 permission bits. @xref{File permissions}.
9542 @item -Z @var{context}
9543 @itemx --context=@var{context}
9547 @cindex security context
9548 Set the default SELinux security context to be used for created FIFOs.
9555 @node mknod invocation
9556 @section @command{mknod}: Make block or character special files
9559 @cindex block special files, creating
9560 @cindex character special files, creating
9562 @command{mknod} creates a FIFO, character special file, or block special
9563 file with the specified name. Synopsis:
9566 mknod [@var{option}]@dots{} @var{name} @var{type} [@var{major} @var{minor}]
9569 @cindex special files
9570 @cindex block special files
9571 @cindex character special files
9572 Unlike the phrase ``special file type'' above, the term @dfn{special
9573 file} has a technical meaning on Unix: something that can generate or
9574 receive data. Usually this corresponds to a physical piece of hardware,
9575 e.g., a printer or a disk. (These files are typically created at
9576 system-configuration time.) The @command{mknod} command is what creates
9577 files of this type. Such devices can be read either a character at a
9578 time or a ``block'' (many characters) at a time, hence we say there are
9579 @dfn{block special} files and @dfn{character special} files.
9581 @c mknod is a shell built-in at least with OpenBSD's /bin/sh
9582 @mayConflictWithShellBuiltIn{mknod}
9584 The arguments after @var{name} specify the type of file to make:
9589 @opindex p @r{for FIFO file}
9593 @opindex b @r{for block special file}
9594 for a block special file
9597 @c Don't document the 'u' option -- it's just a synonym for 'c'.
9598 @c Do *any* versions of mknod still use it?
9600 @opindex c @r{for character special file}
9601 @c @opindex u @r{for character special file}
9602 for a character special file
9606 When making a block or character special file, the major and minor
9607 device numbers must be given after the file type.
9608 If a major or minor device number begins with @samp{0x} or @samp{0X},
9609 it is interpreted as hexadecimal; otherwise, if it begins with @samp{0},
9610 as octal; otherwise, as decimal.
9612 The program accepts the following option. Also see @ref{Common options}.
9617 @itemx --mode=@var{mode}
9620 Set the mode of created files to @var{mode}, which is symbolic as in
9621 @command{chmod} and uses @samp{a=rw} as the point of departure.
9622 @var{mode} should specify only file permission bits.
9623 @xref{File permissions}.
9625 @item -Z @var{context}
9626 @itemx --context=@var{context}
9630 @cindex security context
9631 Set the default SELinux security context to be used for created files.
9638 @node readlink invocation
9639 @section @command{readlink}: Print value of a symlink or canonical file name
9642 @cindex displaying value of a symbolic link
9643 @cindex canonical file name
9644 @cindex canonicalize a file name
9647 @command{readlink} may work in one of two supported modes:
9653 @command{readlink} outputs the value of the given symbolic link.
9654 If @command{readlink} is invoked with an argument other than the name
9655 of a symbolic link, it produces no output and exits with a nonzero exit code.
9657 @item Canonicalize mode
9659 @command{readlink} outputs the absolute name of the given file which contains
9660 no @file{.}, @file{..} components nor any repeated separators
9661 (@file{/}) or symbolic links.
9666 readlink [@var{option}] @var{file}
9669 By default, @command{readlink} operates in readlink mode.
9671 The program accepts the following options. Also see @ref{Common options}.
9676 @itemx --canonicalize
9678 @opindex --canonicalize
9679 Activate canonicalize mode.
9680 If any component of the file name except the last one is missing or unavailable,
9681 @command{readlink} produces no output and exits with a nonzero exit
9682 code. A trailing slash is ignored.
9685 @itemx --canonicalize-existing
9687 @opindex --canonicalize-existing
9688 Activate canonicalize mode.
9689 If any component is missing or unavailable, @command{readlink} produces
9690 no output and exits with a nonzero exit code. A trailing slash
9691 requires that the name resolve to a directory.
9694 @itemx --canonicalize-missing
9696 @opindex --canonicalize-missing
9697 Activate canonicalize mode.
9698 If any component is missing or unavailable, @command{readlink} treats it
9704 @opindex --no-newline
9705 Do not output the trailing newline.
9715 Suppress most error messages.
9721 Report error messages.
9725 The @command{readlink} utility first appeared in OpenBSD 2.1.
9727 The @command{realpath} command without options, operates like
9728 @command{readlink} in canonicalize mode.
9733 @node rmdir invocation
9734 @section @command{rmdir}: Remove empty directories
9737 @cindex removing empty directories
9738 @cindex directories, removing empty
9740 @command{rmdir} removes empty directories. Synopsis:
9743 rmdir [@var{option}]@dots{} @var{directory}@dots{}
9746 If any @var{directory} argument does not refer to an existing empty
9747 directory, it is an error.
9749 The program accepts the following options. Also see @ref{Common options}.
9753 @item --ignore-fail-on-non-empty
9754 @opindex --ignore-fail-on-non-empty
9755 @cindex directory deletion, ignoring failures
9756 Ignore each failure to remove a directory that is solely because
9757 the directory is non-empty.
9763 @cindex parent directories, removing
9764 Remove @var{directory}, then try to remove each component of @var{directory}.
9765 So, for example, @samp{rmdir -p a/b/c} is similar to @samp{rmdir a/b/c a/b a}.
9766 As such, it fails if any of those directories turns out not to be empty.
9767 Use the @option{--ignore-fail-on-non-empty} option to make it so such
9768 a failure does not evoke a diagnostic and does not cause @command{rmdir} to
9769 exit unsuccessfully.
9775 @cindex directory deletion, reporting
9776 Give a diagnostic for each successful removal.
9777 @var{directory} is removed.
9781 @xref{rm invocation}, for how to remove non-empty directories (recursively).
9786 @node unlink invocation
9787 @section @command{unlink}: Remove files via the unlink syscall
9790 @cindex removing files or directories (via the unlink syscall)
9792 @command{unlink} deletes a single specified file name.
9793 It is a minimalist interface to the system-provided
9794 @code{unlink} function. @xref{Deleting Files, , , libc,
9795 The GNU C Library Reference Manual}. Synopsis:
9796 It avoids the bells and whistles of the more commonly-used
9797 @command{rm} command (@pxref{rm invocation}).
9800 unlink @var{filename}
9803 On some systems @code{unlink} can be used to delete the name of a
9804 directory. On others, it can be used that way only by a privileged user.
9805 In the GNU system @code{unlink} can never delete the name of a directory.
9807 The @command{unlink} command honors the @option{--help} and
9808 @option{--version} options. To remove a file whose name begins with
9809 @samp{-}, prefix the name with @samp{./}, e.g., @samp{unlink ./--help}.
9814 @node Changing file attributes
9815 @chapter Changing file attributes
9817 @cindex changing file attributes
9818 @cindex file attributes, changing
9819 @cindex attributes, file
9821 A file is not merely its contents, a name, and a file type
9822 (@pxref{Special file types}). A file also has an owner (a user ID), a
9823 group (a group ID), permissions (what the owner can do with the file,
9824 what people in the group can do, and what everyone else can do), various
9825 timestamps, and other information. Collectively, we call these a file's
9828 These commands change file attributes.
9831 * chgrp invocation:: Change file groups.
9832 * chmod invocation:: Change access permissions.
9833 * chown invocation:: Change file owners and groups.
9834 * touch invocation:: Change file timestamps.
9838 @node chown invocation
9839 @section @command{chown}: Change file owner and group
9842 @cindex file ownership, changing
9843 @cindex group ownership, changing
9844 @cindex changing file ownership
9845 @cindex changing group ownership
9847 @command{chown} changes the user and/or group ownership of each given @var{file}
9848 to @var{new-owner} or to the user and group of an existing reference file.
9852 chown [@var{option}]@dots{} @{@var{new-owner} | --reference=@var{ref_file}@}@c
9856 If used, @var{new-owner} specifies the new owner and/or group as follows
9857 (with no embedded white space):
9860 [@var{owner}] [ : [@var{group}] ]
9867 If only an @var{owner} (a user name or numeric user ID) is given, that
9868 user is made the owner of each given file, and the files' group is not
9871 @item owner@samp{:}group
9872 If the @var{owner} is followed by a colon and a @var{group} (a
9873 group name or numeric group ID), with no spaces between them, the group
9874 ownership of the files is changed as well (to @var{group}).
9877 If a colon but no group name follows @var{owner}, that user is
9878 made the owner of the files and the group of the files is changed to
9879 @var{owner}'s login group.
9882 If the colon and following @var{group} are given, but the owner
9883 is omitted, only the group of the files is changed; in this case,
9884 @command{chown} performs the same function as @command{chgrp}.
9887 If only a colon is given, or if @var{new-owner} is empty, neither the
9888 owner nor the group is changed.
9892 If @var{owner} or @var{group} is intended to represent a numeric user
9893 or group ID, then you may specify it with a leading @samp{+}.
9894 @xref{Disambiguating names and IDs}.
9896 Some older scripts may still use @samp{.} in place of the @samp{:} separator.
9897 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) does not
9898 require support for that, but for backward compatibility @acronym{GNU}
9899 @command{chown} supports @samp{.} so long as no ambiguity results.
9900 New scripts should avoid the use of @samp{.} because it is not
9901 portable, and because it has undesirable results if the entire
9902 @var{owner@samp{.}group} happens to identify a user whose name
9905 The @command{chown} command sometimes clears the set-user-ID or
9906 set-group-ID permission bits. This behavior depends on the policy and
9907 functionality of the underlying @code{chown} system call, which may
9908 make system-dependent file mode modifications outside the control of
9909 the @command{chown} command. For example, the @command{chown} command
9910 might not affect those bits when invoked by a user with appropriate
9911 privileges, or when the
9912 bits signify some function other than executable permission (e.g.,
9914 When in doubt, check the underlying system behavior.
9916 The program accepts the following options. Also see @ref{Common options}.
9924 @cindex changed owners, verbosely describing
9925 Verbosely describe the action for each @var{file} whose ownership
9934 @cindex error messages, omitting
9935 Do not print error messages about files whose ownership cannot be
9938 @itemx @w{@kbd{--from}=@var{old-owner}}
9940 @cindex symbolic links, changing owner
9941 Change a @var{file}'s ownership only if it has current attributes specified
9942 by @var{old-owner}. @var{old-owner} has the same form as @var{new-owner}
9944 This option is useful primarily from a security standpoint in that
9945 it narrows considerably the window of potential abuse.
9946 For example, to reflect a user ID numbering change for one user's files
9947 without an option like this, @code{root} might run
9950 find / -owner OLDUSER -print0 | xargs -0 chown -h NEWUSER
9953 But that is dangerous because the interval between when the @command{find}
9954 tests the existing file's owner and when the @command{chown} is actually run
9956 One way to narrow the gap would be to invoke chown for each file
9960 find / -owner OLDUSER -exec chown -h NEWUSER @{@} \;
9963 But that is very slow if there are many affected files.
9964 With this option, it is safer (the gap is narrower still)
9965 though still not perfect:
9968 chown -h -R --from=OLDUSER NEWUSER /
9972 @opindex --dereference
9973 @cindex symbolic links, changing owner
9975 Do not act on symbolic links themselves but rather on what they point to.
9976 This is the default.
9979 @itemx --no-dereference
9981 @opindex --no-dereference
9982 @cindex symbolic links, changing owner
9984 Act on symbolic links themselves instead of what they point to.
9985 This mode relies on the @code{lchown} system call.
9986 On systems that do not provide the @code{lchown} system call,
9987 @command{chown} fails when a file specified on the command line
9989 By default, no diagnostic is issued for symbolic links encountered
9990 during a recursive traversal, but see @option{--verbose}.
9992 @itemx --preserve-root
9993 @opindex --preserve-root
9994 @cindex root directory, disallow recursive modification
9995 Fail upon any attempt to recursively change the root directory, @file{/}.
9996 Without @option{--recursive}, this option has no effect.
9997 @xref{Treating / specially}.
9999 @itemx --no-preserve-root
10000 @opindex --no-preserve-root
10001 @cindex root directory, allow recursive modification
10002 Cancel the effect of any preceding @option{--preserve-root} option.
10003 @xref{Treating / specially}.
10005 @item --reference=@var{ref_file}
10006 @opindex --reference
10007 Change the user and group of each @var{file} to be the same as those of
10008 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
10009 user and group of the symbolic link, but rather those of the file it
10016 Output a diagnostic for every file processed.
10017 If a symbolic link is encountered during a recursive traversal
10018 on a system without the @code{lchown} system call, and @option{--no-dereference}
10019 is in effect, then issue a diagnostic saying neither the symbolic link nor
10020 its referent is being changed.
10025 @opindex --recursive
10026 @cindex recursively changing file ownership
10027 Recursively change ownership of directories and their contents.
10030 @xref{Traversing symlinks}.
10033 @xref{Traversing symlinks}.
10036 @xref{Traversing symlinks}.
10045 # Change the owner of /u to "root".
10048 # Likewise, but also change its group to "staff".
10049 chown root:staff /u
10051 # Change the owner of /u and subfiles to "root".
10056 @node chgrp invocation
10057 @section @command{chgrp}: Change group ownership
10060 @cindex group ownership, changing
10061 @cindex changing group ownership
10063 @command{chgrp} changes the group ownership of each given @var{file}
10064 to @var{group} (which can be either a group name or a numeric group ID)
10065 or to the group of an existing reference file. Synopsis:
10068 chgrp [@var{option}]@dots{} @{@var{group} | --reference=@var{ref_file}@}@c
10072 If @var{group} is intended to represent a
10073 numeric group ID, then you may specify it with a leading @samp{+}.
10074 @xref{Disambiguating names and IDs}.
10076 The program accepts the following options. Also see @ref{Common options}.
10084 @cindex changed files, verbosely describing
10085 Verbosely describe the action for each @var{file} whose group actually
10094 @cindex error messages, omitting
10095 Do not print error messages about files whose group cannot be
10098 @item --dereference
10099 @opindex --dereference
10100 @cindex symbolic links, changing owner
10102 Do not act on symbolic links themselves but rather on what they point to.
10103 This is the default.
10106 @itemx --no-dereference
10108 @opindex --no-dereference
10109 @cindex symbolic links, changing group
10111 Act on symbolic links themselves instead of what they point to.
10112 This mode relies on the @code{lchown} system call.
10113 On systems that do not provide the @code{lchown} system call,
10114 @command{chgrp} fails when a file specified on the command line
10115 is a symbolic link.
10116 By default, no diagnostic is issued for symbolic links encountered
10117 during a recursive traversal, but see @option{--verbose}.
10119 @itemx --preserve-root
10120 @opindex --preserve-root
10121 @cindex root directory, disallow recursive modification
10122 Fail upon any attempt to recursively change the root directory, @file{/}.
10123 Without @option{--recursive}, this option has no effect.
10124 @xref{Treating / specially}.
10126 @itemx --no-preserve-root
10127 @opindex --no-preserve-root
10128 @cindex root directory, allow recursive modification
10129 Cancel the effect of any preceding @option{--preserve-root} option.
10130 @xref{Treating / specially}.
10132 @item --reference=@var{ref_file}
10133 @opindex --reference
10134 Change the group of each @var{file} to be the same as that of
10135 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
10136 group of the symbolic link, but rather that of the file it refers to.
10142 Output a diagnostic for every file processed.
10143 If a symbolic link is encountered during a recursive traversal
10144 on a system without the @code{lchown} system call, and @option{--no-dereference}
10145 is in effect, then issue a diagnostic saying neither the symbolic link nor
10146 its referent is being changed.
10151 @opindex --recursive
10152 @cindex recursively changing group ownership
10153 Recursively change the group ownership of directories and their contents.
10156 @xref{Traversing symlinks}.
10159 @xref{Traversing symlinks}.
10162 @xref{Traversing symlinks}.
10171 # Change the group of /u to "staff".
10174 # Change the group of /u and subfiles to "staff".
10179 @node chmod invocation
10180 @section @command{chmod}: Change access permissions
10183 @cindex changing access permissions
10184 @cindex access permissions, changing
10185 @cindex permissions, changing access
10187 @command{chmod} changes the access permissions of the named files. Synopsis:
10190 chmod [@var{option}]@dots{} @{@var{mode} | --reference=@var{ref_file}@}@c
10194 @cindex symbolic links, permissions of
10195 @command{chmod} never changes the permissions of symbolic links, since
10196 the @command{chmod} system call cannot change their permissions.
10197 This is not a problem since the permissions of symbolic links are
10198 never used. However, for each symbolic link listed on the command
10199 line, @command{chmod} changes the permissions of the pointed-to file.
10200 In contrast, @command{chmod} ignores symbolic links encountered during
10201 recursive directory traversals.
10203 A successful use of @command{chmod} clears the set-group-ID bit of a
10204 regular file if the file's group ID does not match the user's
10205 effective group ID or one of the user's supplementary group IDs,
10206 unless the user has appropriate privileges. Additional restrictions
10207 may cause the set-user-ID and set-group-ID bits of @var{mode} or
10208 @var{ref_file} to be ignored. This behavior depends on the policy and
10209 functionality of the underlying @code{chmod} system call. When in
10210 doubt, check the underlying system behavior.
10212 If used, @var{mode} specifies the new file mode bits.
10213 For details, see the section on @ref{File permissions}.
10214 If you really want @var{mode} to have a leading @samp{-}, you should
10215 use @option{--} first, e.g., @samp{chmod -- -w file}. Typically,
10216 though, @samp{chmod a-w file} is preferable, and @command{chmod -w
10217 file} (without the @option{--}) complains if it behaves differently
10218 from what @samp{chmod a-w file} would do.
10220 The program accepts the following options. Also see @ref{Common options}.
10228 Verbosely describe the action for each @var{file} whose permissions
10237 @cindex error messages, omitting
10238 Do not print error messages about files whose permissions cannot be
10241 @itemx --preserve-root
10242 @opindex --preserve-root
10243 @cindex root directory, disallow recursive modification
10244 Fail upon any attempt to recursively change the root directory, @file{/}.
10245 Without @option{--recursive}, this option has no effect.
10246 @xref{Treating / specially}.
10248 @itemx --no-preserve-root
10249 @opindex --no-preserve-root
10250 @cindex root directory, allow recursive modification
10251 Cancel the effect of any preceding @option{--preserve-root} option.
10252 @xref{Treating / specially}.
10258 Verbosely describe the action or non-action taken for every @var{file}.
10260 @item --reference=@var{ref_file}
10261 @opindex --reference
10262 Change the mode of each @var{file} to be the same as that of @var{ref_file}.
10263 @xref{File permissions}.
10264 If @var{ref_file} is a symbolic link, do not use the mode
10265 of the symbolic link, but rather that of the file it refers to.
10270 @opindex --recursive
10271 @cindex recursively changing access permissions
10272 Recursively change permissions of directories and their contents.
10279 @node touch invocation
10280 @section @command{touch}: Change file timestamps
10283 @cindex changing file timestamps
10284 @cindex file timestamps, changing
10285 @cindex timestamps, changing file
10287 @command{touch} changes the access and/or modification times of the
10288 specified files. Synopsis:
10291 touch [@var{option}]@dots{} @var{file}@dots{}
10294 @cindex empty files, creating
10295 Any @var{file} argument that does not exist is created empty, unless
10296 option @option{--no-create} (@option{-c}) or @option{--no-dereference}
10297 (@option{-h}) was in effect.
10299 A @var{file} argument string of @samp{-} is handled specially and
10300 causes @command{touch} to change the times of the file associated with
10304 By default, @command{touch} sets file timestamps to the current time.
10305 Because @command{touch} acts on its operands left to right, the
10306 resulting timestamps of earlier and later operands may disagree.
10307 Also, the determination of what time is ``current'' depends on the
10308 platform. Platforms with network file systems often use different
10309 clocks for the operating system and for file systems; because
10310 @command{touch} typically uses file systems' clocks by default, clock
10311 skew can cause the resulting file timestamps to appear to be in a
10312 program's ``future'' or ``past''.
10314 @cindex file timestamp resolution
10315 The @command{touch} command sets the file's timestamp to the greatest
10316 representable value that is not greater than the requested time. This
10317 can differ from the requested time for several reasons. First, the
10318 requested time may have a higher resolution than supported. Second, a
10319 file system may use different resolutions for different types of
10320 times. Third, file timestamps may use a different resolution than
10321 operating system timestamps. Fourth, the operating system primitives
10322 used to update timestamps may employ yet a different resolution. For
10323 example, in theory a file system might use 10-microsecond resolution
10324 for access time and 100-nanosecond resolution for modification time,
10325 and the operating system might use nanosecond resolution for the
10326 current time and microsecond resolution for the primitive that
10327 @command{touch} uses to set a file's timestamp to an arbitrary value.
10329 @cindex permissions, for changing file timestamps
10330 When setting file timestamps to the current time, @command{touch} can
10331 change the timestamps for files that the user does not own but has
10332 write permission for. Otherwise, the user must own the files. Some
10333 older systems have a further restriction: the user must own the files
10334 unless both the access and modification times are being set to the
10337 Although @command{touch} provides options for changing two of the times---the
10338 times of last access and modification---of a file, there is actually
10339 a standard third one as well: the inode change time. This is often
10340 referred to as a file's @code{ctime}.
10341 The inode change time represents the time when the file's meta-information
10342 last changed. One common example of this is when the permissions of a
10343 file change. Changing the permissions doesn't access the file, so
10344 the atime doesn't change, nor does it modify the file, so the mtime
10345 doesn't change. Yet, something about the file itself has changed,
10346 and this must be noted somewhere. This is the job of the ctime field.
10347 This is necessary, so that, for example, a backup program can make a
10348 fresh copy of the file, including the new permissions value.
10349 Another operation that modifies a file's ctime without affecting
10350 the others is renaming. In any case, it is not possible, in normal
10351 operations, for a user to change the ctime field to a user-specified value.
10352 Some operating systems and file systems support a fourth time: the
10353 birth time, when the file was first created; by definition, this
10354 timestamp never changes.
10357 Time stamps assume the time zone rules specified by the @env{TZ}
10358 environment variable, or by the system default rules if @env{TZ} is
10359 not set. @xref{TZ Variable,, Specifying the Time Zone with @env{TZ},
10360 libc, The GNU C Library Reference Manual}.
10361 You can avoid ambiguities during
10362 daylight saving transitions by using @sc{utc} time stamps.
10364 The program accepts the following options. Also see @ref{Common options}.
10369 @itemx --time=atime
10370 @itemx --time=access
10374 @opindex atime@r{, changing}
10375 @opindex access @r{time, changing}
10376 @opindex use @r{time, changing}
10377 Change the access time only.
10382 @opindex --no-create
10383 Do not warn about or create files that do not exist.
10386 @itemx --date=@var{time}
10390 Use @var{time} instead of the current time. It can contain month names,
10391 time zones, @samp{am} and @samp{pm}, @samp{yesterday}, etc. For
10392 example, @option{--date="2004-02-27 14:19:13.489392193 +0530"}
10393 specifies the instant of time that is 489,392,193 nanoseconds after
10394 February 27, 2004 at 2:19:13 PM in a time zone that is 5 hours and 30
10395 minutes east of @acronym{UTC}. @xref{Date input formats}.
10396 File systems that do not support high-resolution time stamps
10397 silently ignore any excess precision here.
10401 @cindex BSD @command{touch} compatibility
10402 Ignored; for compatibility with BSD versions of @command{touch}.
10405 @itemx --no-dereference
10407 @opindex --no-dereference
10408 @cindex symbolic links, changing time
10410 Attempt to change the timestamps of a symbolic link, rather than what
10411 the link refers to. When using this option, empty files are not
10412 created, but option @option{-c} must also be used to avoid warning
10413 about files that do not exist. Not all systems support changing the
10414 timestamps of symlinks, since underlying system support for this
10415 action was not required until @acronym{POSIX} 2008. Also, on some
10416 systems, the mere act of examining a symbolic link changes the access
10417 time, such that only changes to the modification time will persist
10418 long enough to be observable. When coupled with option @option{-r}, a
10419 reference timestamp is taken from a symbolic link rather than the file
10423 @itemx --time=mtime
10424 @itemx --time=modify
10427 @opindex mtime@r{, changing}
10428 @opindex modify @r{time, changing}
10429 Change the modification time only.
10431 @item -r @var{file}
10432 @itemx --reference=@var{file}
10434 @opindex --reference
10435 Use the times of the reference @var{file} instead of the current time.
10436 If this option is combined with the @option{--date=@var{time}}
10437 (@option{-d @var{time}}) option, the reference @var{file}'s time is
10438 the origin for any relative @var{time}s given, but is otherwise ignored.
10439 For example, @samp{-r foo -d '-5 seconds'} specifies a time stamp
10440 equal to five seconds before the corresponding time stamp for @file{foo}.
10441 If @var{file} is a symbolic link, the reference timestamp is taken
10442 from the target of the symlink, unless @option{-h} was also in effect.
10444 @item -t [[@var{cc}]@var{yy}]@var{mmddhhmm}[.@var{ss}]
10445 Use the argument (optional four-digit or two-digit years, months,
10446 days, hours, minutes, optional seconds) instead of the current time.
10447 If the year is specified with only two digits, then @var{cc}
10448 is 20 for years in the range 0 @dots{} 68, and 19 for years in
10449 69 @dots{} 99. If no digits of the year are specified,
10450 the argument is interpreted as a date in the current year.
10451 Note that @var{ss} may be @samp{60}, to accommodate leap seconds.
10455 @vindex _POSIX2_VERSION
10456 On older systems, @command{touch} supports an obsolete syntax, as follows.
10457 If no timestamp is given with any of the @option{-d}, @option{-r}, or
10458 @option{-t} options, and if there are two or more @var{file}s and the
10459 first @var{file} is of the form @samp{@var{mmddhhmm}[@var{yy}]} and this
10460 would be a valid argument to the @option{-t} option (if the @var{yy}, if
10461 any, were moved to the front), and if the represented year
10462 is in the range 1969--1999, that argument is interpreted as the time
10463 for the other files instead of as a file name.
10464 This obsolete behavior can be enabled or disabled with the
10465 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
10466 conformance}), but portable scripts should avoid commands whose
10467 behavior depends on this variable.
10468 For example, use @samp{touch ./12312359 main.c} or @samp{touch -t
10469 12312359 main.c} rather than the ambiguous @samp{touch 12312359 main.c}.
10475 @chapter Disk usage
10479 No disk can hold an infinite amount of data. These commands report
10480 how much disk storage is in use or available, report other file and
10481 file status information, and write buffers to disk.
10484 * df invocation:: Report file system disk space usage.
10485 * du invocation:: Estimate file space usage.
10486 * stat invocation:: Report file or file system status.
10487 * sync invocation:: Synchronize memory and disk.
10488 * truncate invocation:: Shrink or extend the size of a file.
10492 @node df invocation
10493 @section @command{df}: Report file system disk space usage
10496 @cindex file system disk usage
10497 @cindex disk usage by file system
10499 @command{df} reports the amount of disk space used and available on
10500 file systems. Synopsis:
10503 df [@var{option}]@dots{} [@var{file}]@dots{}
10506 With no arguments, @command{df} reports the space used and available on all
10507 currently mounted file systems (of all types). Otherwise, @command{df}
10508 reports on the file system containing each argument @var{file}.
10510 Normally the disk space is printed in units of
10511 1024 bytes, but this can be overridden (@pxref{Block size}).
10512 Non-integer quantities are rounded up to the next higher unit.
10514 @cindex disk device file
10515 @cindex device file, disk
10516 If an argument @var{file} is a disk device file containing a mounted
10517 file system, @command{df} shows the space available on that file system
10518 rather than on the file system containing the device node (i.e., the root
10519 file system). @sc{gnu} @command{df} does not attempt to determine the
10521 on unmounted file systems, because on most kinds of systems doing so
10522 requires extremely nonportable intimate knowledge of file system
10525 The program accepts the following options. Also see @ref{Common options}.
10533 @cindex automounter file systems
10534 @cindex ignore file systems
10535 Include in the listing dummy file systems, which
10536 are omitted by default. Such file systems are typically special-purpose
10537 pseudo-file-systems, such as automounter entries.
10539 @item -B @var{size}
10540 @itemx --block-size=@var{size}
10542 @opindex --block-size
10543 @cindex file system sizes
10544 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10545 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10549 @cindex grand total of disk size, usage and available space
10550 Print a grand total of all arguments after all arguments have
10551 been processed. This can be used to find out the total disk size, usage
10552 and available space of all listed devices.
10558 Equivalent to @option{--si}.
10564 @cindex inode usage
10565 List inode usage information instead of block usage. An inode (short
10566 for index node) contains information about a file such as its owner,
10567 permissions, timestamps, and location on the disk.
10571 @cindex kibibytes for file system sizes
10572 Print sizes in 1024-byte blocks, overriding the default block size
10573 (@pxref{Block size}).
10574 This option is equivalent to @option{--block-size=1K}.
10580 @cindex file system types, limiting output to certain
10581 Limit the listing to local file systems. By default, remote file systems
10586 @cindex file system space, retrieving old data more quickly
10587 Do not invoke the @code{sync} system call before getting any usage data.
10588 This may make @command{df} run significantly faster on systems with many
10589 disks, but on some systems (notably SunOS) the results may be slightly
10590 out of date. This is the default.
10593 @itemx --portability
10595 @opindex --portability
10596 @cindex one-line output format
10597 @cindex @acronym{POSIX} output format
10598 @cindex portable output format
10599 @cindex output format, portable
10600 Use the @acronym{POSIX} output format. This is like the default format except
10605 The information about each file system is always printed on exactly
10606 one line; a mount device is never put on a line by itself. This means
10607 that if the mount device name is more than 20 characters long (e.g., for
10608 some network mounts), the columns are misaligned.
10611 The labels in the header output line are changed to conform to @acronym{POSIX}.
10614 The default block size and output format are unaffected by the
10615 @env{DF_BLOCK_SIZE}, @env{BLOCK_SIZE} and @env{BLOCKSIZE} environment
10616 variables. However, the default block size is still affected by
10617 @env{POSIXLY_CORRECT}: it is 512 if @env{POSIXLY_CORRECT} is set, 1024
10618 otherwise. @xref{Block size}.
10625 @cindex file system space, retrieving current data more slowly
10626 Invoke the @code{sync} system call before getting any usage data. On
10627 some systems (notably SunOS), doing this yields more up to date results,
10628 but in general this option makes @command{df} much slower, especially when
10629 there are many or very busy file systems.
10631 @item -t @var{fstype}
10632 @itemx --type=@var{fstype}
10635 @cindex file system types, limiting output to certain
10636 Limit the listing to file systems of type @var{fstype}. Multiple
10637 file system types can be specified by giving multiple @option{-t} options.
10638 By default, nothing is omitted.
10641 @itemx --print-type
10643 @opindex --print-type
10644 @cindex file system types, printing
10645 Print each file system's type. The types printed here are the same ones
10646 you can include or exclude with @option{-t} and @option{-x}. The particular
10647 types printed are whatever is supported by the system. Here are some of
10648 the common names (this list is certainly not exhaustive):
10653 @cindex @acronym{NFS} file system type
10654 An @acronym{NFS} file system, i.e., one mounted over a network from another
10655 machine. This is the one type name which seems to be used uniformly by
10658 @item 4.2@r{, }ufs@r{, }efs@dots{}
10659 @cindex Linux file system types
10660 @cindex local file system types
10661 @opindex 4.2 @r{file system type}
10662 @opindex ufs @r{file system type}
10663 @opindex efs @r{file system type}
10664 A file system on a locally-mounted hard disk. (The system might even
10665 support more than one type here; Linux does.)
10667 @item hsfs@r{, }cdfs
10668 @cindex CD-ROM file system type
10669 @cindex High Sierra file system
10670 @opindex hsfs @r{file system type}
10671 @opindex cdfs @r{file system type}
10672 A file system on a CD-ROM drive. HP-UX uses @samp{cdfs}, most other
10673 systems use @samp{hsfs} (@samp{hs} for ``High Sierra'').
10676 @cindex PC file system
10677 @cindex DOS file system
10678 @cindex MS-DOS file system
10679 @cindex diskette file system
10681 An MS-DOS file system, usually on a diskette.
10685 @item -x @var{fstype}
10686 @itemx --exclude-type=@var{fstype}
10688 @opindex --exclude-type
10689 Limit the listing to file systems not of type @var{fstype}.
10690 Multiple file system types can be eliminated by giving multiple
10691 @option{-x} options. By default, no file system types are omitted.
10694 Ignored; for compatibility with System V versions of @command{df}.
10699 Failure includes the case where no output is generated, so you can
10700 inspect the exit status of a command like @samp{df -t ext3 -t reiserfs
10701 @var{dir}} to test whether @var{dir} is on a file system of type
10702 @samp{ext3} or @samp{reiserfs}.
10705 @node du invocation
10706 @section @command{du}: Estimate file space usage
10709 @cindex file space usage
10710 @cindex disk usage for files
10712 @command{du} reports the amount of disk space used by the specified files
10713 and for each subdirectory (of directory arguments). Synopsis:
10716 du [@var{option}]@dots{} [@var{file}]@dots{}
10719 With no arguments, @command{du} reports the disk space for the current
10720 directory. Normally the disk space is printed in units of
10721 1024 bytes, but this can be overridden (@pxref{Block size}).
10722 Non-integer quantities are rounded up to the next higher unit.
10724 If two or more hard links point to the same file, only one of the hard
10725 links is counted. The @var{file} argument order affects which links
10726 are counted, and changing the argument order may change the numbers
10727 that @command{du} outputs.
10729 The program accepts the following options. Also see @ref{Common options}.
10737 Show counts for all files, not just directories.
10739 @itemx --apparent-size
10740 @opindex --apparent-size
10741 Print apparent sizes, rather than disk usage. The apparent size of a
10742 file is the number of bytes reported by @code{wc -c} on regular files,
10743 or more generally, @code{ls -l --block-size=1} or @code{stat --format=%s}.
10744 For example, a file containing the word @samp{zoo} with no newline would,
10745 of course, have an apparent size of 3. Such a small file may require
10746 anywhere from 0 to 16 KiB or more of disk space, depending on
10747 the type and configuration of the file system on which the file resides.
10748 However, a sparse file created with this command:
10751 dd bs=1 seek=2GiB if=/dev/null of=big
10755 has an apparent size of 2 GiB, yet on most modern
10756 systems, it actually uses almost no disk space.
10762 Equivalent to @code{--apparent-size --block-size=1}.
10764 @item -B @var{size}
10765 @itemx --block-size=@var{size}
10767 @opindex --block-size
10769 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10770 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10776 @cindex grand total of disk space
10777 Print a grand total of all arguments after all arguments have
10778 been processed. This can be used to find out the total disk usage of
10779 a given set of files or directories.
10782 @itemx --dereference-args
10784 @opindex --dereference-args
10785 Dereference symbolic links that are command line arguments.
10786 Does not affect other symbolic links. This is helpful for finding
10787 out the disk usage of directories, such as @file{/usr/tmp}, which
10788 are often symbolic links.
10790 @c --files0-from=FILE
10791 @filesZeroFromOption{du,, with the @option{--total} (@option{-c}) option}
10797 Equivalent to @option{--dereference-args} (@option{-D}).
10801 @cindex kibibytes for file sizes
10802 Print sizes in 1024-byte blocks, overriding the default block size
10803 (@pxref{Block size}).
10804 This option is equivalent to @option{--block-size=1K}.
10807 @itemx --count-links
10809 @opindex --count-links
10810 @cindex hard links, counting in @command{du}
10811 Count the size of all files, even if they have appeared already (as a
10815 @itemx --dereference
10817 @opindex --dereference
10818 @cindex symbolic links, dereferencing in @command{du}
10819 Dereference symbolic links (show the disk space used by the file
10820 or directory that the link points to instead of the space used by
10825 @cindex mebibytes for file sizes
10826 Print sizes in 1,048,576-byte blocks, overriding the default block size
10827 (@pxref{Block size}).
10828 This option is equivalent to @option{--block-size=1M}.
10831 @itemx --no-dereference
10833 @opindex --no-dereference
10834 @cindex symbolic links, dereferencing in @command{du}
10835 For each symbolic links encountered by @command{du},
10836 consider the disk space used by the symbolic link.
10838 @item -d @var{depth}
10839 @item --max-depth=@var{depth}
10840 @opindex -d @var{depth}
10841 @opindex --max-depth=@var{depth}
10842 @cindex limiting output of @command{du}
10843 Show the total for each directory (and file if --all) that is at
10844 most MAX_DEPTH levels down from the root of the hierarchy. The root
10845 is at level 0, so @code{du --max-depth=0} is equivalent to @code{du -s}.
10854 @opindex --summarize
10855 Display only a total for each argument.
10858 @itemx --separate-dirs
10860 @opindex --separate-dirs
10861 Normally, in the output of @command{du} (when not using @option{--summarize}),
10862 the size listed next to a directory name, @var{d}, represents the sum
10863 of sizes of all entries beneath @var{d} as well as the size of @var{d} itself.
10864 With @option{--separate-dirs}, the size reported for a directory name,
10865 @var{d}, is merely the @code{stat.st_size}-derived size of the directory
10870 @cindex last modified dates, displaying in @command{du}
10871 Show time of the most recent modification of any file in the directory,
10872 or any of its subdirectories.
10874 @itemx --time=ctime
10875 @itemx --time=status
10878 @opindex ctime@r{, show the most recent}
10879 @opindex status time@r{, show the most recent}
10880 @opindex use time@r{, show the most recent}
10881 Show the most recent status change time (the @samp{ctime} in the inode) of
10882 any file in the directory, instead of the modification time.
10884 @itemx --time=atime
10885 @itemx --time=access
10887 @opindex atime@r{, show the most recent}
10888 @opindex access time@r{, show the most recent}
10889 Show the most recent access time (the @samp{atime} in the inode) of
10890 any file in the directory, instead of the modification time.
10892 @item --time-style=@var{style}
10893 @opindex --time-style
10895 List timestamps in style @var{style}. This option has an effect only if
10896 the @option{--time} option is also specified. The @var{style} should
10897 be one of the following:
10900 @item +@var{format}
10902 List timestamps using @var{format}, where @var{format} is interpreted
10903 like the format argument of @command{date} (@pxref{date invocation}).
10904 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
10905 @command{du} to list timestamps like @samp{2002-03-30 23:45:56}. As
10906 with @command{date}, @var{format}'s interpretation is affected by the
10907 @env{LC_TIME} locale category.
10910 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
10911 format with nanosecond precision, e.g., @samp{2002-03-30
10912 23:45:56.477817180 -0700}. This style is equivalent to
10913 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
10916 List @acronym{ISO} 8601 date and time in minutes, e.g.,
10917 @samp{2002-03-30 23:45}. These timestamps are shorter than
10918 @samp{full-iso} timestamps, and are usually good enough for everyday
10919 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
10922 List @acronym{ISO} 8601 dates for timestamps, e.g., @samp{2002-03-30}.
10923 This style is equivalent to @samp{+%Y-%m-%d}.
10927 You can specify the default value of the @option{--time-style} option
10928 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
10929 the default style is @samp{long-iso}. For compatibility with @command{ls},
10930 if @env{TIME_STYLE} begins with @samp{+} and contains a newline,
10931 the newline and any later characters are ignored; if @env{TIME_STYLE}
10932 begins with @samp{posix-} the @samp{posix-} is ignored; and if
10933 @env{TIME_STYLE} is @samp{locale} it is ignored.
10936 @itemx --one-file-system
10938 @opindex --one-file-system
10939 @cindex one file system, restricting @command{du} to
10940 Skip directories that are on different file systems from the one that
10941 the argument being processed is on.
10943 @item --exclude=@var{pattern}
10944 @opindex --exclude=@var{pattern}
10945 @cindex excluding files from @command{du}
10946 When recursing, skip subdirectories or files matching @var{pattern}.
10947 For example, @code{du --exclude='*.o'} excludes files whose names
10950 @item -X @var{file}
10951 @itemx --exclude-from=@var{file}
10952 @opindex -X @var{file}
10953 @opindex --exclude-from=@var{file}
10954 @cindex excluding files from @command{du}
10955 Like @option{--exclude}, except take the patterns to exclude from @var{file},
10956 one per line. If @var{file} is @samp{-}, take the patterns from standard
10961 @cindex NFS mounts from BSD to HP-UX
10962 On BSD systems, @command{du} reports sizes that are half the correct
10963 values for files that are NFS-mounted from HP-UX systems. On HP-UX
10964 systems, it reports sizes that are twice the correct values for
10965 files that are NFS-mounted from BSD systems. This is due to a flaw
10966 in HP-UX; it also affects the HP-UX @command{du} program.
10971 @node stat invocation
10972 @section @command{stat}: Report file or file system status
10975 @cindex file status
10976 @cindex file system status
10978 @command{stat} displays information about the specified file(s). Synopsis:
10981 stat [@var{option}]@dots{} [@var{file}]@dots{}
10984 With no option, @command{stat} reports all information about the given files.
10985 But it also can be used to report the information of the file systems the
10986 given files are located on. If the files are links, @command{stat} can
10987 also give information about the files the links point to.
10989 @mayConflictWithShellBuiltIn{stat}
10994 @itemx --dereference
10996 @opindex --dereference
10997 @cindex symbolic links, dereferencing in @command{stat}
10998 Change how @command{stat} treats symbolic links.
10999 With this option, @command{stat} acts on the file referenced
11000 by each symbolic link argument.
11001 Without it, @command{stat} acts on any symbolic link argument directly.
11004 @itemx --file-system
11006 @opindex --file-system
11007 @cindex file systems
11008 Report information about the file systems where the given files are located
11009 instead of information about the files themselves.
11010 This option implies the @option{-L} option.
11013 @itemx --format=@var{format}
11015 @opindex --format=@var{format}
11016 @cindex output format
11017 Use @var{format} rather than the default format.
11018 @var{format} is automatically newline-terminated, so
11019 running a command like the following with two or more @var{file}
11020 operands produces a line of output for each operand:
11022 $ stat --format=%d:%i / /usr
11027 @itemx --printf=@var{format}
11028 @opindex --printf=@var{format}
11029 @cindex output format
11030 Use @var{format} rather than the default format.
11031 Like @option{--format}, but interpret backslash escapes,
11032 and do not output a mandatory trailing newline.
11033 If you want a newline, include @samp{\n} in the @var{format}.
11034 Here's how you would use @option{--printf} to print the device
11035 and inode numbers of @file{/} and @file{/usr}:
11037 $ stat --printf='%d:%i\n' / /usr
11046 @cindex terse output
11047 Print the information in terse form, suitable for parsing by other programs.
11051 The valid @var{format} directives for files with @option{--format} and
11052 @option{--printf} are:
11055 @item %a - Access rights in octal
11056 @item %A - Access rights in human readable form
11057 @item %b - Number of blocks allocated (see @samp{%B})
11058 @item %B - The size in bytes of each block reported by @samp{%b}
11059 @item %C - The SELinux security context of a file, if available
11060 @item %d - Device number in decimal
11061 @item %D - Device number in hex
11062 @item %f - Raw mode in hex
11063 @item %F - File type
11064 @item %g - Group ID of owner
11065 @item %G - Group name of owner
11066 @item %h - Number of hard links
11067 @item %i - Inode number
11068 @item %m - Mount point (See note below)
11069 @item %n - File name
11070 @item %N - Quoted file name with dereference if symbolic link
11071 @item %o - Optimal I/O transfer size hint
11072 @item %s - Total size, in bytes
11073 @item %t - Major device type in hex
11074 @item %T - Minor device type in hex
11075 @item %u - User ID of owner
11076 @item %U - User name of owner
11077 @item %w - Time of file birth, or @samp{-} if unknown
11078 @item %W - Time of file birth as seconds since Epoch, or @samp{0}
11079 @item %x - Time of last access
11080 @item %X - Time of last access as seconds since Epoch
11081 @item %y - Time of last modification
11082 @item %Y - Time of last modification as seconds since Epoch
11083 @item %z - Time of last change
11084 @item %Z - Time of last change as seconds since Epoch
11087 The @samp{%W}, @samp{%X}, @samp{%Y}, and @samp{%Z} formats accept a
11088 precision preceded by a period to specify the number of digits to
11089 print after the decimal point. For example, @samp{%.3X} outputs the
11090 last access time to millisecond precision. If a period is given but no
11091 precision, @command{stat} uses 9 digits, so @samp{%.X} is equivalent to
11092 @samp{%.9X}. When discarding excess precision, time stamps are truncated
11093 toward minus infinity.
11097 $ stat -c '[%015Y]' /usr
11100 $ stat -c '[%15Y]' /usr
11102 $ stat -c '[%-15Y]' /usr
11105 $ stat -c '[%.3Y]' /usr
11107 $ stat -c '[%.Y]' /usr
11108 [1288929712.114951834]
11111 The mount point printed by @samp{%m} is similar to that output
11112 by @command{df}, except that:
11115 stat does not dereference symlinks by default
11116 (unless @option{-L} is specified)
11118 stat does not search for specified device nodes in the
11119 file system list, instead operating on them directly
11122 stat outputs the alias for a bind mounted file, rather than
11123 the initial mount point of its backing device.
11124 One can recursively call stat until there is no change in output,
11125 to get the current base mount point
11128 When listing file system information (@option{--file-system} (@option{-f})),
11129 you must use a different set of @var{format} directives:
11132 @item %a - Free blocks available to non-super-user
11133 @item %b - Total data blocks in file system
11134 @item %c - Total file nodes in file system
11135 @item %d - Free file nodes in file system
11136 @item %f - Free blocks in file system
11137 @item %i - File System ID in hex
11138 @item %l - Maximum length of file names
11139 @item %n - File name
11140 @item %s - Block size (for faster transfers)
11141 @item %S - Fundamental block size (for block counts)
11142 @item %t - Type in hex
11143 @item %T - Type in human readable form
11147 Time stamps are listed according to the time zone rules specified by
11148 the @env{TZ} environment variable, or by the system default rules if
11149 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
11150 with @env{TZ}, libc, The GNU C Library Reference Manual}.
11155 @node sync invocation
11156 @section @command{sync}: Synchronize data on disk with memory
11159 @cindex synchronize disk and memory
11161 @cindex superblock, writing
11162 @cindex inodes, written buffered
11163 @command{sync} writes any data buffered in memory out to disk. This can
11164 include (but is not limited to) modified superblocks, modified inodes,
11165 and delayed reads and writes. This must be implemented by the kernel;
11166 The @command{sync} program does nothing but exercise the @code{sync} system
11169 @cindex crashes and corruption
11170 The kernel keeps data in memory to avoid doing (relatively slow) disk
11171 reads and writes. This improves performance, but if the computer
11172 crashes, data may be lost or the file system corrupted as a
11173 result. The @command{sync} command ensures everything in memory
11174 is written to disk.
11176 Any arguments are ignored, except for a lone @option{--help} or
11177 @option{--version} (@pxref{Common options}).
11182 @node truncate invocation
11183 @section @command{truncate}: Shrink or extend the size of a file
11186 @cindex truncating, file sizes
11188 @command{truncate} shrinks or extends the size of each @var{file} to the
11189 specified size. Synopsis:
11192 truncate @var{option}@dots{} @var{file}@dots{}
11195 @cindex files, creating
11196 Any @var{file} that does not exist is created.
11198 @cindex sparse files, creating
11199 @cindex holes, creating files with
11200 If a @var{file} is larger than the specified size, the extra data is lost.
11201 If a @var{file} is shorter, it is extended and the extended part (or hole)
11202 reads as zero bytes.
11204 The program accepts the following options. Also see @ref{Common options}.
11211 @opindex --no-create
11212 Do not create files that do not exist.
11217 @opindex --io-blocks
11218 Treat @var{size} as number of I/O blocks of the @var{file} rather than bytes.
11220 @item -r @var{rfile}
11221 @itemx --reference=@var{rfile}
11223 @opindex --reference
11224 Base the size of each @var{file} on the size of @var{rfile}.
11226 @item -s @var{size}
11227 @itemx --size=@var{size}
11230 Set or adjust the size of each @var{file} according to @var{size}.
11231 @multiplierSuffixesNoBlocks{size}
11233 @var{size} may also be prefixed by one of the following to adjust
11234 the size of each @var{file} based on their current size:
11236 @samp{+} => extend by
11237 @samp{-} => reduce by
11238 @samp{<} => at most
11239 @samp{>} => at least
11240 @samp{/} => round down to multiple of
11241 @samp{%} => round up to multiple of
11249 @node Printing text
11250 @chapter Printing text
11252 @cindex printing text, commands for
11253 @cindex commands for printing text
11255 This section describes commands that display text strings.
11258 * echo invocation:: Print a line of text.
11259 * printf invocation:: Format and print data.
11260 * yes invocation:: Print a string until interrupted.
11264 @node echo invocation
11265 @section @command{echo}: Print a line of text
11268 @cindex displaying text
11269 @cindex printing text
11270 @cindex text, displaying
11271 @cindex arbitrary text, displaying
11273 @command{echo} writes each given @var{string} to standard output, with a
11274 space between each and a newline after the last one. Synopsis:
11277 echo [@var{option}]@dots{} [@var{string}]@dots{}
11280 @mayConflictWithShellBuiltIn{echo}
11282 The program accepts the following options. Also see @ref{Common options}.
11283 Options must precede operands, and the normally-special argument
11284 @samp{--} has no special meaning and is treated like any other
11290 Do not output the trailing newline.
11294 @cindex backslash escapes
11295 Enable interpretation of the following backslash-escaped characters in
11304 produce no further output
11320 the eight-bit value that is the octal number @var{nnn}
11321 (zero to three octal digits), if @var{nnn} is
11322 a nine-bit value, the ninth bit is ignored
11324 the eight-bit value that is the octal number @var{nnn}
11325 (one to three octal digits), if @var{nnn} is
11326 a nine-bit value, the ninth bit is ignored
11328 the eight-bit value that is the hexadecimal number @var{hh}
11329 (one or two hexadecimal digits)
11334 @cindex backslash escapes
11335 Disable interpretation of backslash escapes in each @var{string}.
11336 This is the default. If @option{-e} and @option{-E} are both
11337 specified, the last one given takes effect.
11341 @vindex POSIXLY_CORRECT
11342 If the @env{POSIXLY_CORRECT} environment variable is set, then when
11343 @command{echo}'s first argument is not @option{-n} it outputs
11344 option-like arguments instead of treating them as options. For
11345 example, @code{echo -ne hello} outputs @samp{-ne hello} instead of
11346 plain @samp{hello}.
11348 @acronym{POSIX} does not require support for any options, and says
11349 that the behavior of @command{echo} is implementation-defined if any
11350 @var{string} contains a backslash or if the first argument is
11351 @option{-n}. Portable programs can use the @command{printf} command
11352 if they need to omit trailing newlines or output control characters or
11353 backslashes. @xref{printf invocation}.
11358 @node printf invocation
11359 @section @command{printf}: Format and print data
11362 @command{printf} does formatted printing of text. Synopsis:
11365 printf @var{format} [@var{argument}]@dots{}
11368 @command{printf} prints the @var{format} string, interpreting @samp{%}
11369 directives and @samp{\} escapes to format numeric and string arguments
11370 in a way that is mostly similar to the C @samp{printf} function.
11371 @xref{Output Conversion Syntax,, @command{printf} format directives,
11372 libc, The GNU C Library Reference Manual}, for details.
11373 The differences are listed below.
11375 @mayConflictWithShellBuiltIn{printf}
11380 The @var{format} argument is reused as necessary to convert all the
11381 given @var{argument}s. For example, the command @samp{printf %s a b}
11385 Missing @var{argument}s are treated as null strings or as zeros,
11386 depending on whether the context expects a string or a number. For
11387 example, the command @samp{printf %sx%d} prints @samp{x0}.
11391 An additional escape, @samp{\c}, causes @command{printf} to produce no
11392 further output. For example, the command @samp{printf 'A%sC\cD%sF' B
11393 E} prints @samp{ABC}.
11396 The hexadecimal escape sequence @samp{\x@var{hh}} has at most two
11397 digits, as opposed to C where it can have an unlimited number of
11398 digits. For example, the command @samp{printf '\x07e'} prints two
11399 bytes, whereas the C statement @samp{printf ("\x07e")} prints just
11404 @command{printf} has an additional directive, @samp{%b}, which prints its
11405 argument string with @samp{\} escapes interpreted in the same way as in
11406 the @var{format} string, except that octal escapes are of the form
11407 @samp{\0@var{ooo}} where @var{ooo} is 0 to 3 octal digits. If
11408 @samp{\@var{ooo}} is nine-bit value, ignore the ninth bit.
11409 If a precision is also given, it limits the number of bytes printed
11410 from the converted string.
11413 Numeric arguments must be single C constants, possibly with leading
11414 @samp{+} or @samp{-}. For example, @samp{printf %.4d -3} outputs
11418 @vindex POSIXLY_CORRECT
11419 If the leading character of a numeric argument is @samp{"} or @samp{'}
11420 then its value is the numeric value of the immediately following
11421 character. Any remaining characters are silently ignored if the
11422 @env{POSIXLY_CORRECT} environment variable is set; otherwise, a
11423 warning is printed. For example, @samp{printf "%d" "'a"} outputs
11424 @samp{97} on hosts that use the @acronym{ASCII} character set, since
11425 @samp{a} has the numeric value 97 in @acronym{ASCII}.
11430 A floating-point argument must use a period before any fractional
11431 digits, but is printed according to the @env{LC_NUMERIC} category of the
11432 current locale. For example, in a locale whose radix character is a
11433 comma, the command @samp{printf %g 3.14} outputs @samp{3,14} whereas
11434 the command @samp{printf %g 3,14} is an error.
11435 @xref{Floating point}.
11439 @command{printf} interprets @samp{\@var{ooo}} in @var{format} as an octal number
11440 (if @var{ooo} is 1 to 3 octal digits) specifying a byte to print,
11441 and @samp{\x@var{hh}} as a hexadecimal number (if @var{hh} is 1 to 2 hex
11442 digits) specifying a character to print.
11443 Note however that when @samp{\@var{ooo}} specifies a number larger than 255,
11444 @command{printf} ignores the ninth bit.
11445 For example, @samp{printf '\400'} is equivalent to @samp{printf '\0'}.
11450 @cindex ISO/IEC 10646
11452 @command{printf} interprets two character syntaxes introduced in
11453 @acronym{ISO} C 99:
11454 @samp{\u} for 16-bit Unicode (@acronym{ISO}/@acronym{IEC} 10646)
11455 characters, specified as
11456 four hexadecimal digits @var{hhhh}, and @samp{\U} for 32-bit Unicode
11457 characters, specified as eight hexadecimal digits @var{hhhhhhhh}.
11458 @command{printf} outputs the Unicode characters
11459 according to the @env{LC_CTYPE} locale. Unicode characters in the ranges
11460 U+0000...U+009F, U+D800...U+DFFF cannot be specified by this syntax, except
11461 for U+0024 ($), U+0040 (@@), and U+0060 (@`).
11463 The processing of @samp{\u} and @samp{\U} requires a full-featured
11464 @code{iconv} facility. It is activated on systems with glibc 2.2 (or newer),
11465 or when @code{libiconv} is installed prior to this package. Otherwise
11466 @samp{\u} and @samp{\U} will print as-is.
11468 The only options are a lone @option{--help} or
11469 @option{--version}. @xref{Common options}.
11470 Options must precede operands.
11472 The Unicode character syntaxes are useful for writing strings in a locale
11473 independent way. For example, a string containing the Euro currency symbol
11476 $ env printf '\u20AC 14.95'
11480 will be output correctly in all locales supporting the Euro symbol
11481 (@acronym{ISO}-8859-15, UTF-8, and others). Similarly, a Chinese string
11484 $ env printf '\u4e2d\u6587'
11488 will be output correctly in all Chinese locales (GB2312, BIG5, UTF-8, etc).
11490 Note that in these examples, the @command{printf} command has been
11491 invoked via @command{env} to ensure that we run the program found via
11492 your shell's search path, and not a shell alias or a built-in function.
11494 For larger strings, you don't need to look up the hexadecimal code
11495 values of each character one by one. @acronym{ASCII} characters mixed with \u
11496 escape sequences is also known as the JAVA source file encoding. You can
11497 use GNU recode 3.5c (or newer) to convert strings to this encoding. Here
11498 is how to convert a piece of text into a shell script which will output
11499 this text in a locale-independent way:
11502 $ LC_CTYPE=zh_CN.big5 /usr/local/bin/printf \
11503 '\u4e2d\u6587\n' > sample.txt
11504 $ recode BIG5..JAVA < sample.txt \
11505 | sed -e "s|^|/usr/local/bin/printf '|" -e "s|$|\\\\n'|" \
11512 @node yes invocation
11513 @section @command{yes}: Print a string until interrupted
11516 @cindex repeated output of a string
11518 @command{yes} prints the command line arguments, separated by spaces and
11519 followed by a newline, forever until it is killed. If no arguments are
11520 given, it prints @samp{y} followed by a newline forever until killed.
11522 Upon a write error, @command{yes} exits with status @samp{1}.
11524 The only options are a lone @option{--help} or @option{--version}.
11525 To output an argument that begins with
11526 @samp{-}, precede it with @option{--}, e.g., @samp{yes -- --help}.
11527 @xref{Common options}.
11531 @chapter Conditions
11534 @cindex commands for exit status
11535 @cindex exit status commands
11537 This section describes commands that are primarily useful for their exit
11538 status, rather than their output. Thus, they are often used as the
11539 condition of shell @code{if} statements, or as the last command in a
11543 * false invocation:: Do nothing, unsuccessfully.
11544 * true invocation:: Do nothing, successfully.
11545 * test invocation:: Check file types and compare values.
11546 * expr invocation:: Evaluate expressions.
11550 @node false invocation
11551 @section @command{false}: Do nothing, unsuccessfully
11554 @cindex do nothing, unsuccessfully
11555 @cindex failure exit status
11556 @cindex exit status of @command{false}
11558 @command{false} does nothing except return an exit status of 1, meaning
11559 @dfn{failure}. It can be used as a place holder in shell scripts
11560 where an unsuccessful command is needed.
11561 In most modern shells, @command{false} is a built-in command, so when
11562 you use @samp{false} in a script, you're probably using the built-in
11563 command, not the one documented here.
11565 @command{false} honors the @option{--help} and @option{--version} options.
11567 This version of @command{false} is implemented as a C program, and is thus
11568 more secure and faster than a shell script implementation, and may safely
11569 be used as a dummy shell for the purpose of disabling accounts.
11571 Note that @command{false} (unlike all other programs documented herein)
11572 exits unsuccessfully, even when invoked with
11573 @option{--help} or @option{--version}.
11575 Portable programs should not assume that the exit status of
11576 @command{false} is 1, as it is greater than 1 on some
11577 non-@acronym{GNU} hosts.
11580 @node true invocation
11581 @section @command{true}: Do nothing, successfully
11584 @cindex do nothing, successfully
11586 @cindex successful exit
11587 @cindex exit status of @command{true}
11589 @command{true} does nothing except return an exit status of 0, meaning
11590 @dfn{success}. It can be used as a place holder in shell scripts
11591 where a successful command is needed, although the shell built-in
11592 command @code{:} (colon) may do the same thing faster.
11593 In most modern shells, @command{true} is a built-in command, so when
11594 you use @samp{true} in a script, you're probably using the built-in
11595 command, not the one documented here.
11597 @command{true} honors the @option{--help} and @option{--version} options.
11599 Note, however, that it is possible to cause @command{true}
11600 to exit with nonzero status: with the @option{--help} or @option{--version}
11601 option, and with standard
11602 output already closed or redirected to a file that evokes an I/O error.
11603 For example, using a Bourne-compatible shell:
11606 $ ./true --version >&-
11607 ./true: write error: Bad file number
11608 $ ./true --version > /dev/full
11609 ./true: write error: No space left on device
11612 This version of @command{true} is implemented as a C program, and is thus
11613 more secure and faster than a shell script implementation, and may safely
11614 be used as a dummy shell for the purpose of disabling accounts.
11616 @node test invocation
11617 @section @command{test}: Check file types and compare values
11620 @cindex check file types
11621 @cindex compare values
11622 @cindex expression evaluation
11624 @command{test} returns a status of 0 (true) or 1 (false) depending on the
11625 evaluation of the conditional expression @var{expr}. Each part of the
11626 expression must be a separate argument.
11628 @command{test} has file status checks, string operators, and numeric
11629 comparison operators.
11631 @command{test} has an alternate form that uses opening and closing
11632 square brackets instead a leading @samp{test}. For example, instead
11633 of @samp{test -d /}, you can write @samp{[ -d / ]}. The square
11634 brackets must be separate arguments; for example, @samp{[-d /]} does
11635 not have the desired effect. Since @samp{test @var{expr}} and @samp{[
11636 @var{expr} ]} have the same meaning, only the former form is discussed
11642 test @var{expression}
11644 [ @var{expression} ]
11649 @mayConflictWithShellBuiltIn{test}
11651 If @var{expression} is omitted, @command{test} returns false.
11652 If @var{expression} is a single argument,
11653 @command{test} returns false if the argument is null and true
11654 otherwise. The argument
11655 can be any string, including strings like @samp{-d}, @samp{-1},
11656 @samp{--}, @samp{--help}, and @samp{--version} that most other
11657 programs would treat as options. To get help and version information,
11658 invoke the commands @samp{[ --help} and @samp{[ --version}, without
11659 the usual closing brackets. @xref{Common options}.
11661 @cindex exit status of @command{test}
11665 0 if the expression is true,
11666 1 if the expression is false,
11667 2 if an error occurred.
11671 * File type tests:: -[bcdfhLpSt]
11672 * Access permission tests:: -[gkruwxOG]
11673 * File characteristic tests:: -e -s -nt -ot -ef
11674 * String tests:: -z -n = == !=
11675 * Numeric tests:: -eq -ne -lt -le -gt -ge
11676 * Connectives for test:: ! -a -o
11680 @node File type tests
11681 @subsection File type tests
11683 @cindex file type tests
11685 These options test for particular types of files. (Everything's a file,
11686 but not all files are the same!)
11690 @item -b @var{file}
11692 @cindex block special check
11693 True if @var{file} exists and is a block special device.
11695 @item -c @var{file}
11697 @cindex character special check
11698 True if @var{file} exists and is a character special device.
11700 @item -d @var{file}
11702 @cindex directory check
11703 True if @var{file} exists and is a directory.
11705 @item -f @var{file}
11707 @cindex regular file check
11708 True if @var{file} exists and is a regular file.
11710 @item -h @var{file}
11711 @itemx -L @var{file}
11714 @cindex symbolic link check
11715 True if @var{file} exists and is a symbolic link.
11716 Unlike all other file-related tests, this test does not dereference
11717 @var{file} if it is a symbolic link.
11719 @item -p @var{file}
11721 @cindex named pipe check
11722 True if @var{file} exists and is a named pipe.
11724 @item -S @var{file}
11726 @cindex socket check
11727 True if @var{file} exists and is a socket.
11731 @cindex terminal check
11732 True if @var{fd} is a file descriptor that is associated with a
11738 @node Access permission tests
11739 @subsection Access permission tests
11741 @cindex access permission tests
11742 @cindex permission tests
11744 These options test for particular access permissions.
11748 @item -g @var{file}
11750 @cindex set-group-ID check
11751 True if @var{file} exists and has its set-group-ID bit set.
11753 @item -k @var{file}
11755 @cindex sticky bit check
11756 True if @var{file} exists and has its @dfn{sticky} bit set.
11758 @item -r @var{file}
11760 @cindex readable file check
11761 True if @var{file} exists and read permission is granted.
11763 @item -u @var{file}
11765 @cindex set-user-ID check
11766 True if @var{file} exists and has its set-user-ID bit set.
11768 @item -w @var{file}
11770 @cindex writable file check
11771 True if @var{file} exists and write permission is granted.
11773 @item -x @var{file}
11775 @cindex executable file check
11776 True if @var{file} exists and execute permission is granted
11777 (or search permission, if it is a directory).
11779 @item -O @var{file}
11781 @cindex owned by effective user ID check
11782 True if @var{file} exists and is owned by the current effective user ID.
11784 @item -G @var{file}
11786 @cindex owned by effective group ID check
11787 True if @var{file} exists and is owned by the current effective group ID.
11791 @node File characteristic tests
11792 @subsection File characteristic tests
11794 @cindex file characteristic tests
11796 These options test other file characteristics.
11800 @item -e @var{file}
11802 @cindex existence-of-file check
11803 True if @var{file} exists.
11805 @item -s @var{file}
11807 @cindex nonempty file check
11808 True if @var{file} exists and has a size greater than zero.
11810 @item @var{file1} -nt @var{file2}
11812 @cindex newer-than file check
11813 True if @var{file1} is newer (according to modification date) than
11814 @var{file2}, or if @var{file1} exists and @var{file2} does not.
11816 @item @var{file1} -ot @var{file2}
11818 @cindex older-than file check
11819 True if @var{file1} is older (according to modification date) than
11820 @var{file2}, or if @var{file2} exists and @var{file1} does not.
11822 @item @var{file1} -ef @var{file2}
11824 @cindex same file check
11825 @cindex hard link check
11826 True if @var{file1} and @var{file2} have the same device and inode
11827 numbers, i.e., if they are hard links to each other.
11833 @subsection String tests
11835 @cindex string tests
11837 These options test string characteristics. You may need to quote
11838 @var{string} arguments for the shell. For example:
11844 The quotes here prevent the wrong arguments from being passed to
11845 @command{test} if @samp{$V} is empty or contains special characters.
11849 @item -z @var{string}
11851 @cindex zero-length string check
11852 True if the length of @var{string} is zero.
11854 @item -n @var{string}
11855 @itemx @var{string}
11857 @cindex nonzero-length string check
11858 True if the length of @var{string} is nonzero.
11860 @item @var{string1} = @var{string2}
11862 @cindex equal string check
11863 True if the strings are equal.
11865 @item @var{string1} == @var{string2}
11867 @cindex equal string check
11868 True if the strings are equal (synonym for =).
11870 @item @var{string1} != @var{string2}
11872 @cindex not-equal string check
11873 True if the strings are not equal.
11878 @node Numeric tests
11879 @subsection Numeric tests
11881 @cindex numeric tests
11882 @cindex arithmetic tests
11884 Numeric relational operators. The arguments must be entirely numeric
11885 (possibly negative), or the special expression @w{@code{-l @var{string}}},
11886 which evaluates to the length of @var{string}.
11890 @item @var{arg1} -eq @var{arg2}
11891 @itemx @var{arg1} -ne @var{arg2}
11892 @itemx @var{arg1} -lt @var{arg2}
11893 @itemx @var{arg1} -le @var{arg2}
11894 @itemx @var{arg1} -gt @var{arg2}
11895 @itemx @var{arg1} -ge @var{arg2}
11902 These arithmetic binary operators return true if @var{arg1} is equal,
11903 not-equal, less-than, less-than-or-equal, greater-than, or
11904 greater-than-or-equal than @var{arg2}, respectively.
11911 test -1 -gt -2 && echo yes
11913 test -l abc -gt 1 && echo yes
11916 @error{} test: integer expression expected before -eq
11920 @node Connectives for test
11921 @subsection Connectives for @command{test}
11923 @cindex logical connectives
11924 @cindex connectives, logical
11926 The usual logical connectives.
11932 True if @var{expr} is false.
11934 @item @var{expr1} -a @var{expr2}
11936 @cindex logical and operator
11937 @cindex and operator
11938 True if both @var{expr1} and @var{expr2} are true.
11940 @item @var{expr1} -o @var{expr2}
11942 @cindex logical or operator
11943 @cindex or operator
11944 True if either @var{expr1} or @var{expr2} is true.
11949 @node expr invocation
11950 @section @command{expr}: Evaluate expressions
11953 @cindex expression evaluation
11954 @cindex evaluation of expressions
11956 @command{expr} evaluates an expression and writes the result on standard
11957 output. Each token of the expression must be a separate argument.
11959 Operands are either integers or strings. Integers consist of one or
11960 more decimal digits, with an optional leading @samp{-}.
11961 @command{expr} converts
11962 anything appearing in an operand position to an integer or a string
11963 depending on the operation being applied to it.
11965 Strings are not quoted for @command{expr} itself, though you may need to
11966 quote them to protect characters with special meaning to the shell,
11967 e.g., spaces. However, regardless of whether it is quoted, a string
11968 operand should not be a parenthesis or any of @command{expr}'s
11969 operators like @code{+}, so you cannot safely pass an arbitrary string
11970 @code{$str} to expr merely by quoting it to the shell. One way to
11971 work around this is to use the @sc{gnu} extension @code{+},
11972 (e.g., @code{+ "$str" = foo}); a more portable way is to use
11973 @code{@w{" $str"}} and to adjust the rest of the expression to take
11974 the leading space into account (e.g., @code{@w{" $str" = " foo"}}).
11976 You should not pass a negative integer or a string with leading
11977 @samp{-} as @command{expr}'s first argument, as it might be
11978 misinterpreted as an option; this can be avoided by parenthesization.
11979 Also, portable scripts should not use a string operand that happens to
11980 take the form of an integer; this can be worked around by inserting
11981 leading spaces as mentioned above.
11983 @cindex parentheses for grouping
11984 Operators may be given as infix symbols or prefix keywords. Parentheses
11985 may be used for grouping in the usual manner. You must quote
11986 parentheses and many operators to avoid the shell evaluating them,
11989 When built with support for the GNU MP library, @command{expr} uses
11990 arbitrary-precision arithmetic; otherwise, it uses native arithmetic
11991 types and may fail due to arithmetic overflow.
11993 The only options are @option{--help} and @option{--version}. @xref{Common
11994 options}. Options must precede operands.
11996 @cindex exit status of @command{expr}
12000 0 if the expression is neither null nor 0,
12001 1 if the expression is null or 0,
12002 2 if the expression is invalid,
12003 3 if an internal error occurred (e.g., arithmetic overflow).
12007 * String expressions:: + : match substr index length
12008 * Numeric expressions:: + - * / %
12009 * Relations for expr:: | & < <= = == != >= >
12010 * Examples of expr:: Examples.
12014 @node String expressions
12015 @subsection String expressions
12017 @cindex string expressions
12018 @cindex expressions, string
12020 @command{expr} supports pattern matching and other string operators. These
12021 have higher precedence than both the numeric and relational operators (in
12022 the next sections).
12026 @item @var{string} : @var{regex}
12027 @cindex pattern matching
12028 @cindex regular expression matching
12029 @cindex matching patterns
12030 Perform pattern matching. The arguments are converted to strings and the
12031 second is considered to be a (basic, a la GNU @code{grep}) regular
12032 expression, with a @code{^} implicitly prepended. The first argument is
12033 then matched against this regular expression.
12035 If the match succeeds and @var{regex} uses @samp{\(} and @samp{\)}, the
12036 @code{:} expression returns the part of @var{string} that matched the
12037 subexpression; otherwise, it returns the number of characters matched.
12039 If the match fails, the @code{:} operator returns the null string if
12040 @samp{\(} and @samp{\)} are used in @var{regex}, otherwise 0.
12042 @kindex \( @r{regexp operator}
12043 Only the first @samp{\( @dots{} \)} pair is relevant to the return
12044 value; additional pairs are meaningful only for grouping the regular
12045 expression operators.
12047 @kindex \+ @r{regexp operator}
12048 @kindex \? @r{regexp operator}
12049 @kindex \| @r{regexp operator}
12050 In the regular expression, @code{\+}, @code{\?}, and @code{\|} are
12051 operators which respectively match one or more, zero or one, or separate
12052 alternatives. SunOS and other @command{expr}'s treat these as regular
12053 characters. (@acronym{POSIX} allows either behavior.)
12054 @xref{Top, , Regular Expression Library, regex, Regex}, for details of
12055 regular expression syntax. Some examples are in @ref{Examples of expr}.
12057 @item match @var{string} @var{regex}
12059 An alternative way to do pattern matching. This is the same as
12060 @w{@samp{@var{string} : @var{regex}}}.
12062 @item substr @var{string} @var{position} @var{length}
12064 Returns the substring of @var{string} beginning at @var{position}
12065 with length at most @var{length}. If either @var{position} or
12066 @var{length} is negative, zero, or non-numeric, returns the null string.
12068 @item index @var{string} @var{charset}
12070 Returns the first position in @var{string} where the first character in
12071 @var{charset} was found. If no character in @var{charset} is found in
12072 @var{string}, return 0.
12074 @item length @var{string}
12076 Returns the length of @var{string}.
12078 @item + @var{token}
12080 Interpret @var{token} as a string, even if it is a keyword like @var{match}
12081 or an operator like @code{/}.
12082 This makes it possible to test @code{expr length + "$x"} or
12083 @code{expr + "$x" : '.*/\(.\)'} and have it do the right thing even if
12084 the value of @var{$x} happens to be (for example) @code{/} or @code{index}.
12085 This operator is a @acronym{GNU} extension. Portable shell scripts should use
12086 @code{@w{" $token"} : @w{' \(.*\)'}} instead of @code{+ "$token"}.
12090 To make @command{expr} interpret keywords as strings, you must use the
12091 @code{quote} operator.
12094 @node Numeric expressions
12095 @subsection Numeric expressions
12097 @cindex numeric expressions
12098 @cindex expressions, numeric
12100 @command{expr} supports the usual numeric operators, in order of increasing
12101 precedence. These numeric operators have lower precedence than the
12102 string operators described in the previous section, and higher precedence
12103 than the connectives (next section).
12111 @cindex subtraction
12112 Addition and subtraction. Both arguments are converted to integers;
12113 an error occurs if this cannot be done.
12119 @cindex multiplication
12122 Multiplication, division, remainder. Both arguments are converted to
12123 integers; an error occurs if this cannot be done.
12128 @node Relations for expr
12129 @subsection Relations for @command{expr}
12131 @cindex connectives, logical
12132 @cindex logical connectives
12133 @cindex relations, numeric or string
12135 @command{expr} supports the usual logical connectives and relations. These
12136 have lower precedence than the string and numeric operators
12137 (previous sections). Here is the list, lowest-precedence operator first.
12143 @cindex logical or operator
12144 @cindex or operator
12145 Returns its first argument if that is neither null nor zero, otherwise
12146 its second argument if it is neither null nor zero, otherwise 0. It
12147 does not evaluate its second argument if its first argument is neither
12152 @cindex logical and operator
12153 @cindex and operator
12154 Return its first argument if neither argument is null or zero, otherwise
12155 0. It does not evaluate its second argument if its first argument is
12158 @item < <= = == != >= >
12165 @cindex comparison operators
12167 Compare the arguments and return 1 if the relation is true, 0 otherwise.
12168 @code{==} is a synonym for @code{=}. @command{expr} first tries to convert
12169 both arguments to integers and do a numeric comparison; if either
12170 conversion fails, it does a lexicographic comparison using the character
12171 collating sequence specified by the @env{LC_COLLATE} locale.
12176 @node Examples of expr
12177 @subsection Examples of using @command{expr}
12179 @cindex examples of @command{expr}
12180 Here are a few examples, including quoting for shell metacharacters.
12182 To add 1 to the shell variable @code{foo}, in Bourne-compatible shells:
12185 foo=`expr $foo + 1`
12188 To print the non-directory part of the file name stored in
12189 @code{$fname}, which need not contain a @code{/}:
12192 expr $fname : '.*/\(.*\)' '|' $fname
12195 An example showing that @code{\+} is an operator:
12203 expr abc : 'a\(.\)c'
12205 expr index abcdef cz
12208 @error{} expr: syntax error
12209 expr index + index a
12215 @chapter Redirection
12217 @cindex redirection
12218 @cindex commands for redirection
12220 Unix shells commonly provide several forms of @dfn{redirection}---ways
12221 to change the input source or output destination of a command. But one
12222 useful redirection is performed by a separate command, not by the shell;
12223 it's described here.
12226 * tee invocation:: Redirect output to multiple files or processes.
12230 @node tee invocation
12231 @section @command{tee}: Redirect output to multiple files or processes
12234 @cindex pipe fitting
12235 @cindex destinations, multiple output
12236 @cindex read from stdin and write to stdout and files
12238 The @command{tee} command copies standard input to standard output and also
12239 to any files given as arguments. This is useful when you want not only
12240 to send some data down a pipe, but also to save a copy. Synopsis:
12243 tee [@var{option}]@dots{} [@var{file}]@dots{}
12246 If a file being written to does not already exist, it is created. If a
12247 file being written to already exists, the data it previously contained
12248 is overwritten unless the @option{-a} option is used.
12250 A @var{file} of @samp{-} causes @command{tee} to send another copy of
12251 input to standard output, but this is typically not that useful as the
12252 copies are interleaved.
12254 The program accepts the following options. Also see @ref{Common options}.
12261 Append standard input to the given files rather than overwriting
12265 @itemx --ignore-interrupts
12267 @opindex --ignore-interrupts
12268 Ignore interrupt signals.
12272 The @command{tee} command is useful when you happen to be transferring a large
12273 amount of data and also want to summarize that data without reading
12274 it a second time. For example, when you are downloading a DVD image,
12275 you often want to verify its signature or checksum right away.
12276 The inefficient way to do it is simply:
12279 wget http://example.com/some.iso && sha1sum some.iso
12282 One problem with the above is that it makes you wait for the
12283 download to complete before starting the time-consuming SHA1 computation.
12284 Perhaps even more importantly, the above requires reading
12285 the DVD image a second time (the first was from the network).
12287 The efficient way to do it is to interleave the download
12288 and SHA1 computation. Then, you'll get the checksum for
12289 free, because the entire process parallelizes so well:
12292 # slightly contrived, to demonstrate process substitution
12293 wget -O - http://example.com/dvd.iso \
12294 | tee >(sha1sum > dvd.sha1) > dvd.iso
12297 That makes @command{tee} write not just to the expected output file,
12298 but also to a pipe running @command{sha1sum} and saving the final
12299 checksum in a file named @file{dvd.sha1}.
12301 Note, however, that this example relies on a feature of modern shells
12302 called @dfn{process substitution}
12303 (the @samp{>(command)} syntax, above;
12304 @xref{Process Substitution,,Process Substitution, bashref,
12305 The Bash Reference Manual}.),
12306 so it works with @command{zsh}, @command{bash}, and @command{ksh},
12307 but not with @command{/bin/sh}. So if you write code like this
12308 in a shell script, be sure to start the script with @samp{#!/bin/bash}.
12310 Since the above example writes to one file and one process,
12311 a more conventional and portable use of @command{tee} is even better:
12314 wget -O - http://example.com/dvd.iso \
12315 | tee dvd.iso | sha1sum > dvd.sha1
12318 You can extend this example to make @command{tee} write to two processes,
12319 computing MD5 and SHA1 checksums in parallel. In this case,
12320 process substitution is required:
12323 wget -O - http://example.com/dvd.iso \
12324 | tee >(sha1sum > dvd.sha1) \
12325 >(md5sum > dvd.md5) \
12329 This technique is also useful when you want to make a @emph{compressed}
12330 copy of the contents of a pipe.
12331 Consider a tool to graphically summarize disk usage data from @samp{du -ak}.
12332 For a large hierarchy, @samp{du -ak} can run for a long time,
12333 and can easily produce terabytes of data, so you won't want to
12334 rerun the command unnecessarily. Nor will you want to save
12335 the uncompressed output.
12337 Doing it the inefficient way, you can't even start the GUI
12338 until after you've compressed all of the @command{du} output:
12341 du -ak | gzip -9 > /tmp/du.gz
12342 gzip -d /tmp/du.gz | xdiskusage -a
12345 With @command{tee} and process substitution, you start the GUI
12346 right away and eliminate the decompression completely:
12349 du -ak | tee >(gzip -9 > /tmp/du.gz) | xdiskusage -a
12352 Finally, if you regularly create more than one type of
12353 compressed tarball at once, for example when @code{make dist} creates
12354 both @command{gzip}-compressed and @command{bzip2}-compressed tarballs,
12355 there may be a better way.
12356 Typical @command{automake}-generated @file{Makefile} rules create
12357 the two compressed tar archives with commands in sequence, like this
12358 (slightly simplified):
12361 tardir=your-pkg-M.N
12362 tar chof - "$tardir" | gzip -9 -c > your-pkg-M.N.tar.gz
12363 tar chof - "$tardir" | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12366 However, if the hierarchy you are archiving and compressing is larger
12367 than a couple megabytes, and especially if you are using a multi-processor
12368 system with plenty of memory, then you can do much better by reading the
12369 directory contents only once and running the compression programs in parallel:
12372 tardir=your-pkg-M.N
12373 tar chof - "$tardir" \
12374 | tee >(gzip -9 -c > your-pkg-M.N.tar.gz) \
12375 | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12381 @node File name manipulation
12382 @chapter File name manipulation
12384 @cindex file name manipulation
12385 @cindex manipulation of file names
12386 @cindex commands for file name manipulation
12388 This section describes commands that manipulate file names.
12391 * basename invocation:: Strip directory and suffix from a file name.
12392 * dirname invocation:: Strip last file name component.
12393 * pathchk invocation:: Check file name validity and portability.
12394 * mktemp invocation:: Create temporary file or directory.
12395 * realpath invocation:: Print resolved file names.
12399 @node basename invocation
12400 @section @command{basename}: Strip directory and suffix from a file name
12403 @cindex strip directory and suffix from file names
12404 @cindex directory, stripping from file names
12405 @cindex suffix, stripping from file names
12406 @cindex file names, stripping directory and suffix
12407 @cindex leading directory components, stripping
12409 @command{basename} removes any leading directory components from
12410 @var{name}. Synopsis:
12413 basename @var{name} [@var{suffix}]
12416 If @var{suffix} is specified and is identical to the end of @var{name},
12417 it is removed from @var{name} as well. Note that since trailing slashes
12418 are removed prior to suffix matching, @var{suffix} will do nothing if it
12419 contains slashes. @command{basename} prints the result on standard
12422 @c This test is used both here and in the section on dirname.
12423 @macro basenameAndDirname
12424 Together, @command{basename} and @command{dirname} are designed such
12425 that if @samp{ls "$name"} succeeds, then the command sequence @samp{cd
12426 "$(dirname "$name")"; ls "$(basename "$name")"} will, too. This works
12427 for everything except file names containing a trailing newline.
12429 @basenameAndDirname
12431 @acronym{POSIX} allows the implementation to define the results if
12432 @var{name} is empty or @samp{//}. In the former case, @acronym{GNU}
12433 @command{basename} returns the empty string. In the latter case, the
12434 result is @samp{//} on platforms where @var{//} is distinct from
12435 @var{/}, and @samp{/} on platforms where there is no difference.
12437 The only options are @option{--help} and @option{--version}. @xref{Common
12438 options}. Options must precede operands.
12446 basename /usr/bin/sort
12449 basename include/stdio.h .h
12453 @node dirname invocation
12454 @section @command{dirname}: Strip last file name component
12457 @cindex directory components, printing
12458 @cindex stripping non-directory suffix
12459 @cindex non-directory suffix, stripping
12461 @command{dirname} prints all but the final slash-delimited component of
12462 @var{name}. Slashes on either side of the final component are also
12463 removed. If the string contains no slash, @command{dirname} prints
12464 @samp{.} (meaning the current directory). Synopsis:
12470 @var{name} need not be a file name, but if it is, this operation
12471 effectively lists the directory that contains the final component,
12472 including the case when the final component is itself a directory.
12474 @basenameAndDirname
12476 @acronym{POSIX} allows the implementation to define the results if
12477 @var{name} is @samp{//}. With @acronym{GNU} @command{dirname}, the
12478 result is @samp{//} on platforms where @var{//} is distinct from
12479 @var{/}, and @samp{/} on platforms where there is no difference.
12481 The only options are @option{--help} and @option{--version}. @xref{Common
12489 # Output "/usr/bin".
12490 dirname /usr/bin/sort
12491 dirname /usr/bin//.//
12498 @node pathchk invocation
12499 @section @command{pathchk}: Check file name validity and portability
12502 @cindex file names, checking validity and portability
12503 @cindex valid file names, checking for
12504 @cindex portable file names, checking for
12506 @command{pathchk} checks validity and portability of file names. Synopsis:
12509 pathchk [@var{option}]@dots{} @var{name}@dots{}
12512 For each @var{name}, @command{pathchk} prints an error message if any of
12513 these conditions is true:
12517 One of the existing directories in @var{name} does not have search
12518 (execute) permission,
12520 The length of @var{name} is larger than the maximum supported by the
12523 The length of one component of @var{name} is longer than
12524 its file system's maximum.
12527 A nonexistent @var{name} is not an error, so long a file with that
12528 name could be created under the above conditions.
12530 The program accepts the following options. Also see @ref{Common options}.
12531 Options must precede operands.
12537 Instead of performing checks based on the underlying file system,
12538 print an error message if any of these conditions is true:
12542 A file name is empty.
12545 A file name contains a character outside the @acronym{POSIX} portable file
12546 name character set, namely, the ASCII letters and digits, @samp{.},
12547 @samp{_}, @samp{-}, and @samp{/}.
12550 The length of a file name or one of its components exceeds the
12551 @acronym{POSIX} minimum limits for portability.
12556 Print an error message if a file name is empty, or if it contains a component
12557 that begins with @samp{-}.
12559 @item --portability
12560 @opindex --portability
12561 Print an error message if a file name is not portable to all @acronym{POSIX}
12562 hosts. This option is equivalent to @samp{-p -P}.
12566 @cindex exit status of @command{pathchk}
12570 0 if all specified file names passed all checks,
12574 @node mktemp invocation
12575 @section @command{mktemp}: Create temporary file or directory
12578 @cindex file names, creating temporary
12579 @cindex directory, creating temporary
12580 @cindex temporary files and directories
12582 @command{mktemp} manages the creation of temporary files and
12583 directories. Synopsis:
12586 mktemp [@var{option}]@dots{} [@var{template}]
12589 Safely create a temporary file or directory based on @var{template},
12590 and print its name. If given, @var{template} must include at least
12591 three consecutive @samp{X}s in the last component. If omitted, the template
12592 @samp{tmp.XXXXXXXXXX} is used, and option @option{--tmpdir} is
12593 implied. The final run of @samp{X}s in the @var{template} will be replaced
12594 by alpha-numeric characters; thus, on a case-sensitive file system,
12595 and with a @var{template} including a run of @var{n} instances of @samp{X},
12596 there are @samp{62**@var{n}} potential file names.
12598 Older scripts used to create temporary files by simply joining the
12599 name of the program with the process id (@samp{$$}) as a suffix.
12600 However, that naming scheme is easily predictable, and suffers from a
12601 race condition where the attacker can create an appropriately named
12602 symbolic link, such that when the script then opens a handle to what
12603 it thought was an unused file, it is instead modifying an existing
12604 file. Using the same scheme to create a directory is slightly safer,
12605 since the @command{mkdir} will fail if the target already exists, but
12606 it is still inferior because it allows for denial of service attacks.
12607 Therefore, modern scripts should use the @command{mktemp} command to
12608 guarantee that the generated name will be unpredictable, and that
12609 knowledge of the temporary file name implies that the file was created
12610 by the current script and cannot be modified by other users.
12612 When creating a file, the resulting file has read and write
12613 permissions for the current user, but no permissions for the group or
12614 others; these permissions are reduced if the current umask is more
12617 Here are some examples (although note that if you repeat them, you
12618 will most likely get different file names):
12623 Create a temporary file in the current directory.
12630 Create a temporary file with a known suffix.
12632 $ mktemp --suffix=.txt file-XXXX
12634 $ mktemp file-XXXX-XXXX.txt
12639 Create a secure fifo relative to the user's choice of @env{TMPDIR},
12640 but falling back to the current directory rather than @file{/tmp}.
12641 Note that @command{mktemp} does not create fifos, but can create a
12642 secure directory in which the fifo can live. Exit the shell if the
12643 directory or fifo could not be created.
12645 $ dir=$(mktemp -p "$@{TMPDIR:-.@}" -d dir-XXXX) || exit 1
12647 $ mkfifo "$fifo" || @{ rmdir "$dir"; exit 1; @}
12651 Create and use a temporary file if possible, but ignore failure. The
12652 file will reside in the directory named by @env{TMPDIR}, if specified,
12653 or else in @file{/tmp}.
12655 $ file=$(mktemp -q) && @{
12656 > # Safe to use $file only within this block. Use quotes,
12657 > # since $TMPDIR, and thus $file, may contain whitespace.
12658 > echo ... > "$file"
12664 Act as a semi-random character generator (it is not fully random,
12665 since it is impacted by the contents of the current directory). To
12666 avoid security holes, do not use the resulting names to create a file.
12676 The program accepts the following options. Also see @ref{Common options}.
12683 @opindex --directory
12684 Create a directory rather than a file. The directory will have read,
12685 write, and search permissions for the current user, but no permissions
12686 for the group or others; these permissions are reduced if the current
12687 umask is more restrictive.
12693 Suppress diagnostics about failure to create a file or directory. The
12694 exit status will still reflect whether a file was created.
12700 Generate a temporary name that does not name an existing file, without
12701 changing the file system contents. Using the output of this command
12702 to create a new file is inherently unsafe, as there is a window of
12703 time between generating the name and using it where another process
12704 can create an object by the same name.
12707 @itemx --tmpdir[=@var{dir}]
12710 Treat @var{template} relative to the directory @var{dir}. If
12711 @var{dir} is not specified (only possible with the long option
12712 @option{--tmpdir}) or is the empty string, use the value of
12713 @env{TMPDIR} if available, otherwise use @samp{/tmp}. If this is
12714 specified, @var{template} must not be absolute. However,
12715 @var{template} can still contain slashes, although intermediate
12716 directories must already exist.
12718 @item --suffix=@var{suffix}
12720 Append @var{suffix} to the @var{template}. @var{suffix} must not
12721 contain slash. If @option{--suffix} is specified, @var{template} must
12722 end in @samp{X}; if it is not specified, then an appropriate
12723 @option{--suffix} is inferred by finding the last @samp{X} in
12724 @var{template}. This option exists for use with the default
12725 @var{template} and for the creation of a @var{suffix} that starts with
12730 Treat @var{template} as a single file relative to the value of
12731 @env{TMPDIR} if available, or to the directory specified by
12732 @option{-p}, otherwise to @samp{/tmp}. @var{template} must not
12733 contain slashes. This option is deprecated; the use of @option{-p}
12734 without @option{-t} offers better defaults (by favoring the command
12735 line over @env{TMPDIR}) and more flexibility (by allowing intermediate
12740 @cindex exit status of @command{mktemp}
12744 0 if the file was created,
12749 @node realpath invocation
12750 @section @command{realpath}: Print the resolved file name.
12753 @cindex file names, canonicalization
12754 @cindex symlinks, resolution
12755 @cindex canonical file name
12756 @cindex canonicalize a file name
12760 @command{realpath} expands all symbolic links and resolves references to
12761 @samp{/./}, @samp{/../} and extra @samp{/} characters. By default,
12762 all but the last component of the specified files must exist. Synopsis:
12765 realpath [@var{option}]@dots{} @var{file}@dots{}
12768 The program accepts the following options. Also see @ref{Common options}.
12773 @itemx --canonicalize-existing
12775 @opindex --canonicalize-existing
12776 Ensure that all components of the specified file names exist.
12777 If any component is missing or unavailable, @command{realpath} will output
12778 a diagnostic unless the @option{-q} option is specified, and exit with a
12779 nonzero exit code. A trailing slash requires that the name resolve to a
12783 @itemx --canonicalize-missing
12785 @opindex --canonicalize-missing
12786 If any component of a specified file name is missing or unavailable,
12787 treat it as a directory.
12793 Symbolic links are resolved in the specified file names,
12794 but they are resolved after any subsequent @samp{..} components are processed.
12799 @opindex --physical
12800 Symbolic links are resolved in the specified file names,
12801 and they are resolved before any subsequent @samp{..} components are processed.
12802 This is the default mode of operation.
12808 Suppress diagnostic messages for specified file names.
12812 @itemx --no-symlinks
12815 @opindex --no-symlinks
12816 Do not resolve symbolic links. Only resolve references to
12817 @samp{/./}, @samp{/../} and remove extra @samp{/} characters.
12818 When combined with the @option{-m} option, realpath operates
12819 only on the file name, and does not touch any actual file.
12825 Separate output items with @sc{nul} characters.
12827 @itemx --relative-to=@var{file}
12828 @opindex --relative-to
12830 Print the resolved file names relative to the specified file.
12831 Note this option honors the @option{-m} and @option{-e} options
12832 pertaining to file existence.
12834 @itemx --relative-base=@var{base}
12835 @opindex --relative-base
12836 This option is valid when used with @option{--relative-to}, and will restrict
12837 the output of @option{--relative-to} so that relative names are output,
12838 only when @var{file}s are descendants of @var{base}. Otherwise output the
12839 absolute file name. Note: this option honors the @option{-m} and @option{-e}
12840 options pertaining to file existence. For example:
12843 realpath --relative-to=/usr /tmp /usr/bin
12846 realpath --relative-base=/usr --relative-to=/usr /tmp /usr/bin
12853 @cindex exit status of @command{realpath}
12857 0 if all file names were printed without issue.
12862 @node Working context
12863 @chapter Working context
12865 @cindex working context
12866 @cindex commands for printing the working context
12868 This section describes commands that display or alter the context in
12869 which you are working: the current directory, the terminal settings, and
12870 so forth. See also the user-related commands in the next section.
12873 * pwd invocation:: Print working directory.
12874 * stty invocation:: Print or change terminal characteristics.
12875 * printenv invocation:: Print environment variables.
12876 * tty invocation:: Print file name of terminal on standard input.
12880 @node pwd invocation
12881 @section @command{pwd}: Print working directory
12884 @cindex print name of current directory
12885 @cindex current working directory, printing
12886 @cindex working directory, printing
12889 @command{pwd} prints the name of the current directory. Synopsis:
12892 pwd [@var{option}]@dots{}
12895 The program accepts the following options. Also see @ref{Common options}.
12902 If the contents of the environment variable @env{PWD} provide an
12903 absolute name of the current directory with no @samp{.} or @samp{..}
12904 components, but possibly with symbolic links, then output those
12905 contents. Otherwise, fall back to default @option{-P} handling.
12910 @opindex --physical
12911 Print a fully resolved name for the current directory. That is, all
12912 components of the printed name will be actual directory names---none
12913 will be symbolic links.
12916 @cindex symbolic links and @command{pwd}
12917 If @option{-L} and @option{-P} are both given, the last one takes
12918 precedence. If neither option is given, then this implementation uses
12919 @option{-P} as the default unless the @env{POSIXLY_CORRECT}
12920 environment variable is set.
12922 @mayConflictWithShellBuiltIn{pwd}
12927 @node stty invocation
12928 @section @command{stty}: Print or change terminal characteristics
12931 @cindex change or print terminal settings
12932 @cindex terminal settings
12933 @cindex line settings of terminal
12935 @command{stty} prints or changes terminal characteristics, such as baud rate.
12939 stty [@var{option}] [@var{setting}]@dots{}
12940 stty [@var{option}]
12943 If given no line settings, @command{stty} prints the baud rate, line
12944 discipline number (on systems that support it), and line settings
12945 that have been changed from the values set by @samp{stty sane}.
12946 By default, mode reading and setting are performed on the tty line
12947 connected to standard input, although this can be modified by the
12948 @option{--file} option.
12950 @command{stty} accepts many non-option arguments that change aspects of
12951 the terminal line operation, as described below.
12953 The program accepts the following options. Also see @ref{Common options}.
12960 Print all current settings in human-readable form. This option may not
12961 be used in combination with any line settings.
12963 @item -F @var{device}
12964 @itemx --file=@var{device}
12967 Set the line opened by the file name specified in @var{device} instead of
12968 the tty line connected to standard input. This option is necessary
12969 because opening a @acronym{POSIX} tty requires use of the
12970 @code{O_NONDELAY} flag to prevent a @acronym{POSIX} tty from blocking
12971 until the carrier detect line is high if
12972 the @code{clocal} flag is not set. Hence, it is not always possible
12973 to allow the shell to open the device in the traditional manner.
12979 @cindex machine-readable @command{stty} output
12980 Print all current settings in a form that can be used as an argument to
12981 another @command{stty} command to restore the current settings. This option
12982 may not be used in combination with any line settings.
12986 Many settings can be turned off by preceding them with a @samp{-}.
12987 Such arguments are marked below with ``May be negated'' in their
12988 description. The descriptions themselves refer to the positive
12989 case, that is, when @emph{not} negated (unless stated otherwise,
12992 Some settings are not available on all @acronym{POSIX} systems, since they use
12993 extensions. Such arguments are marked below with
12994 ``Non-@acronym{POSIX}'' in their description. On non-@acronym{POSIX}
12995 systems, those or other settings also may not
12996 be available, but it's not feasible to document all the variations: just
13002 * Control:: Control settings
13003 * Input:: Input settings
13004 * Output:: Output settings
13005 * Local:: Local settings
13006 * Combination:: Combination settings
13007 * Characters:: Special characters
13008 * Special:: Special settings
13013 @subsection Control settings
13015 @cindex control settings
13021 @cindex two-way parity
13022 Generate parity bit in output and expect parity bit in input.
13028 @cindex even parity
13029 Set odd parity (even if negated). May be negated.
13036 @cindex character size
13037 @cindex eight-bit characters
13038 Set character size to 5, 6, 7, or 8 bits.
13043 Send a hangup signal when the last process closes the tty. May be
13049 Use two stop bits per character (one if negated). May be negated.
13053 Allow input to be received. May be negated.
13057 @cindex modem control
13058 Disable modem control signals. May be negated.
13062 @cindex hardware flow control
13063 @cindex flow control, hardware
13064 @cindex RTS/CTS flow control
13065 Enable RTS/CTS flow control. Non-@acronym{POSIX}. May be negated.
13070 @subsection Input settings
13072 @cindex input settings
13073 These settings control operations on data received from the terminal.
13078 @cindex breaks, ignoring
13079 Ignore break characters. May be negated.
13083 @cindex breaks, cause interrupts
13084 Make breaks cause an interrupt signal. May be negated.
13088 @cindex parity, ignoring
13089 Ignore characters with parity errors. May be negated.
13093 @cindex parity errors, marking
13094 Mark parity errors (with a 255-0-character sequence). May be negated.
13098 Enable input parity checking. May be negated.
13102 @cindex eight-bit input
13103 Clear high (8th) bit of input characters. May be negated.
13107 @cindex newline, translating to return
13108 Translate newline to carriage return. May be negated.
13112 @cindex return, ignoring
13113 Ignore carriage return. May be negated.
13117 @cindex return, translating to newline
13118 Translate carriage return to newline. May be negated.
13122 @cindex input encoding, UTF-8
13123 Assume input characters are UTF-8 encoded. May be negated.
13127 @kindex C-s/C-q flow control
13128 @cindex XON/XOFF flow control
13129 Enable XON/XOFF flow control (that is, @kbd{CTRL-S}/@kbd{CTRL-Q}). May
13136 @cindex software flow control
13137 @cindex flow control, software
13138 Enable sending of @code{stop} character when the system input buffer
13139 is almost full, and @code{start} character when it becomes almost
13140 empty again. May be negated.
13144 @cindex uppercase, translating to lowercase
13145 Translate uppercase characters to lowercase. Non-@acronym{POSIX}. May be
13146 negated. Note ilcuc is not implemented, as one would not be able to issue
13147 almost any (lowercase) Unix command, after invoking it.
13151 Allow any character to restart output (only the start character
13152 if negated). Non-@acronym{POSIX}. May be negated.
13156 @cindex beeping at input buffer full
13157 Enable beeping and not flushing input buffer if a character arrives
13158 when the input buffer is full. Non-@acronym{POSIX}. May be negated.
13163 @subsection Output settings
13165 @cindex output settings
13166 These settings control operations on data sent to the terminal.
13171 Postprocess output. May be negated.
13175 @cindex lowercase, translating to output
13176 Translate lowercase characters to uppercase. Non-@acronym{POSIX}. May be
13177 negated. (Note ouclc is not currently implemented.)
13181 @cindex return, translating to newline
13182 Translate carriage return to newline. Non-@acronym{POSIX}. May be negated.
13186 @cindex newline, translating to crlf
13187 Translate newline to carriage return-newline. Non-@acronym{POSIX}. May be
13192 Do not print carriage returns in the first column. Non-@acronym{POSIX}.
13197 Newline performs a carriage return. Non-@acronym{POSIX}. May be negated.
13201 @cindex pad instead of timing for delaying
13202 Use fill (padding) characters instead of timing for delays.
13203 Non-@acronym{POSIX}.
13208 @cindex pad character
13209 Use @acronym{ASCII} @sc{del} characters for fill instead of
13210 @acronym{ASCII} @sc{nul} characters. Non-@acronym{POSIX}.
13216 Newline delay style. Non-@acronym{POSIX}.
13223 Carriage return delay style. Non-@acronym{POSIX}.
13229 @opindex tab@var{n}
13230 Horizontal tab delay style. Non-@acronym{POSIX}.
13235 Backspace delay style. Non-@acronym{POSIX}.
13240 Vertical tab delay style. Non-@acronym{POSIX}.
13245 Form feed delay style. Non-@acronym{POSIX}.
13250 @subsection Local settings
13252 @cindex local settings
13257 Enable @code{interrupt}, @code{quit}, and @code{suspend} special
13258 characters. May be negated.
13262 Enable @code{erase}, @code{kill}, @code{werase}, and @code{rprnt}
13263 special characters. May be negated.
13267 Enable non-@acronym{POSIX} special characters. May be negated.
13271 Echo input characters. May be negated.
13277 Echo @code{erase} characters as backspace-space-backspace. May be
13282 @cindex newline echoing after @code{kill}
13283 Echo a newline after a @code{kill} character. May be negated.
13287 @cindex newline, echoing
13288 Echo newline even if not echoing other characters. May be negated.
13292 @cindex flushing, disabling
13293 Disable flushing after @code{interrupt} and @code{quit} special
13294 characters. May be negated.
13298 @cindex case translation
13299 Enable input and output of uppercase characters by preceding their
13300 lowercase equivalents with @samp{\}, when @code{icanon} is set.
13301 Non-@acronym{POSIX}. May be negated.
13305 @cindex background jobs, stopping at terminal write
13306 Stop background jobs that try to write to the terminal. Non-@acronym{POSIX}.
13313 Echo erased characters backward, between @samp{\} and @samp{/}.
13314 Non-@acronym{POSIX}. May be negated.
13320 @cindex control characters, using @samp{^@var{c}}
13321 @cindex hat notation for control characters
13322 Echo control characters in hat notation (@samp{^@var{c}}) instead
13323 of literally. Non-@acronym{POSIX}. May be negated.
13329 Echo the @code{kill} special character by erasing each character on
13330 the line as indicated by the @code{echoprt} and @code{echoe} settings,
13331 instead of by the @code{echoctl} and @code{echok} settings.
13332 Non-@acronym{POSIX}.
13338 @subsection Combination settings
13340 @cindex combination settings
13341 Combination settings:
13348 Same as @code{parenb -parodd cs7}. May be negated. If negated, same
13349 as @code{-parenb cs8}.
13353 Same as @code{parenb parodd cs7}. May be negated. If negated, same
13354 as @code{-parenb cs8}.
13358 Same as @code{-icrnl -onlcr}. May be negated. If negated, same as
13359 @code{icrnl -inlcr -igncr onlcr -ocrnl -onlret}.
13363 Reset the @code{erase} and @code{kill} special characters to their default
13370 @c This is too long to write inline.
13372 cread -ignbrk brkint -inlcr -igncr icrnl -ixoff
13373 -iuclc -ixany imaxbel opost -olcuc -ocrnl onlcr
13374 -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0
13375 ff0 isig icanon iexten echo echoe echok -echonl
13376 -noflsh -xcase -tostop -echoprt echoctl echoke
13380 and also sets all special characters to their default values.
13384 Same as @code{brkint ignpar istrip icrnl ixon opost isig icanon}, plus
13385 sets the @code{eof} and @code{eol} characters to their default values
13386 if they are the same as the @code{min} and @code{time} characters.
13387 May be negated. If negated, same as @code{raw}.
13394 -ignbrk -brkint -ignpar -parmrk -inpck -istrip
13395 -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany
13396 -imaxbel -opost -isig -icanon -xcase min 1 time 0
13400 May be negated. If negated, same as @code{cooked}.
13404 Same as @option{-icanon}. May be negated. If negated, same as
13409 @cindex eight-bit characters
13410 Same as @code{-parenb -istrip cs8}. May be negated. If negated,
13411 same as @code{parenb istrip cs7}.
13415 Same as @option{-parenb -istrip -opost cs8}. May be negated.
13416 If negated, same as @code{parenb istrip opost cs7}.
13420 Same as @option{-ixany}. Non-@acronym{POSIX}. May be negated.
13424 Same as @code{tab0}. Non-@acronym{POSIX}. May be negated. If negated, same
13431 Same as @code{xcase iuclc olcuc}. Non-@acronym{POSIX}. May be negated.
13432 (Used for terminals with uppercase characters only.)
13436 Same as @code{echoe echoctl echoke}.
13440 Same as @code{echoe echoctl echoke -ixany intr ^C erase ^? kill C-u}.
13445 @subsection Special characters
13447 @cindex special characters
13448 @cindex characters, special
13450 The special characters' default values vary from system to system.
13451 They are set with the syntax @samp{name value}, where the names are
13452 listed below and the value can be given either literally, in hat
13453 notation (@samp{^@var{c}}), or as an integer which may start with
13454 @samp{0x} to indicate hexadecimal, @samp{0} to indicate octal, or
13455 any other digit to indicate decimal.
13457 @cindex disabling special characters
13458 @kindex u@r{, and disabling special characters}
13459 For GNU stty, giving a value of @code{^-} or @code{undef} disables that
13460 special character. (This is incompatible with Ultrix @command{stty},
13461 which uses a value of @samp{u} to disable a special character. GNU
13462 @command{stty} treats a value @samp{u} like any other, namely to set that
13463 special character to @key{U}.)
13469 Send an interrupt signal.
13473 Send a quit signal.
13477 Erase the last character typed.
13481 Erase the current line.
13485 Send an end of file (terminate the input).
13493 Alternate character to end the line. Non-@acronym{POSIX}.
13497 Switch to a different shell layer. Non-@acronym{POSIX}.
13501 Restart the output after stopping it.
13509 Send a terminal stop signal.
13513 Send a terminal stop signal after flushing the input. Non-@acronym{POSIX}.
13517 Redraw the current line. Non-@acronym{POSIX}.
13521 Erase the last word typed. Non-@acronym{POSIX}.
13525 Enter the next character typed literally, even if it is a special
13526 character. Non-@acronym{POSIX}.
13531 @subsection Special settings
13533 @cindex special settings
13538 Set the minimum number of characters that will satisfy a read until
13539 the time value has expired, when @option{-icanon} is set.
13543 Set the number of tenths of a second before reads time out if the minimum
13544 number of characters have not been read, when @option{-icanon} is set.
13546 @item ispeed @var{n}
13548 Set the input speed to @var{n}.
13550 @item ospeed @var{n}
13552 Set the output speed to @var{n}.
13556 Tell the tty kernel driver that the terminal has @var{n} rows.
13557 Non-@acronym{POSIX}.
13560 @itemx columns @var{n}
13563 Tell the kernel that the terminal has @var{n} columns. Non-@acronym{POSIX}.
13569 Print the number of rows and columns that the kernel thinks the
13570 terminal has. (Systems that don't support rows and columns in the kernel
13571 typically use the environment variables @env{LINES} and @env{COLUMNS}
13572 instead; however, GNU @command{stty} does not know anything about them.)
13573 Non-@acronym{POSIX}.
13577 Use line discipline @var{n}. Non-@acronym{POSIX}.
13581 Print the terminal speed.
13584 @cindex baud rate, setting
13585 Set the input and output speeds to @var{n}. @var{n} can be one of: 0
13586 50 75 110 134 134.5 150 200 300 600 1200 1800 2400 4800 9600 19200
13587 38400 @code{exta} @code{extb}. @code{exta} is the same as 19200;
13588 @code{extb} is the same as 38400. Many systems, including GNU/Linux,
13589 support higher speeds. The @command{stty} command includes support
13606 4000000 where the system supports these.
13607 0 hangs up the line if @option{-clocal} is set.
13611 @node printenv invocation
13612 @section @command{printenv}: Print all or some environment variables
13615 @cindex printing all or some environment variables
13616 @cindex environment variables, printing
13618 @command{printenv} prints environment variable values. Synopsis:
13621 printenv [@var{option}] [@var{variable}]@dots{}
13624 If no @var{variable}s are specified, @command{printenv} prints the value of
13625 every environment variable. Otherwise, it prints the value of each
13626 @var{variable} that is set, and nothing for those that are not set.
13628 The program accepts the following option. Also see @ref{Common options}.
13636 @cindex exit status of @command{printenv}
13640 0 if all variables specified were found
13641 1 if at least one specified variable was not found
13642 2 if a write error occurred
13646 @node tty invocation
13647 @section @command{tty}: Print file name of terminal on standard input
13650 @cindex print terminal file name
13651 @cindex terminal file name, printing
13653 @command{tty} prints the file name of the terminal connected to its standard
13654 input. It prints @samp{not a tty} if standard input is not a terminal.
13658 tty [@var{option}]@dots{}
13661 The program accepts the following option. Also see @ref{Common options}.
13671 Print nothing; only return an exit status.
13675 @cindex exit status of @command{tty}
13679 0 if standard input is a terminal
13680 1 if standard input is not a terminal
13681 2 if given incorrect arguments
13682 3 if a write error occurs
13686 @node User information
13687 @chapter User information
13689 @cindex user information, commands for
13690 @cindex commands for printing user information
13692 This section describes commands that print user-related information:
13693 logins, groups, and so forth.
13696 * id invocation:: Print user identity.
13697 * logname invocation:: Print current login name.
13698 * whoami invocation:: Print effective user ID.
13699 * groups invocation:: Print group names a user is in.
13700 * users invocation:: Print login names of users currently logged in.
13701 * who invocation:: Print who is currently logged in.
13705 @node id invocation
13706 @section @command{id}: Print user identity
13709 @cindex real user and group IDs, printing
13710 @cindex effective user and group IDs, printing
13711 @cindex printing real and effective user and group IDs
13713 @command{id} prints information about the given user, or the process
13714 running it if no user is specified. Synopsis:
13717 id [@var{option}]@dots{} [@var{username}]
13720 @vindex POSIXLY_CORRECT
13721 By default, it prints the real user ID, real group ID, effective user ID
13722 if different from the real user ID, effective group ID if different from
13723 the real group ID, and supplemental group IDs.
13724 In addition, if SELinux
13725 is enabled and the @env{POSIXLY_CORRECT} environment variable is not set,
13726 then print @samp{context=@var{c}}, where @var{c} is the security context.
13728 Each of these numeric values is preceded by an identifying string and
13729 followed by the corresponding user or group name in parentheses.
13731 The options cause @command{id} to print only part of the above information.
13732 Also see @ref{Common options}.
13739 Print only the group ID.
13745 Print only the group ID and the supplementary groups.
13751 Print the user or group name instead of the ID number. Requires
13752 @option{-u}, @option{-g}, or @option{-G}.
13758 Print the real, instead of effective, user or group ID. Requires
13759 @option{-u}, @option{-g}, or @option{-G}.
13765 Print only the user ID.
13772 @cindex security context
13773 Print only the security context of the current user.
13774 If SELinux is disabled then print a warning and
13775 set the exit status to 1.
13781 @macro primaryAndSupplementaryGroups{cmd,arg}
13782 Primary and supplementary groups for a process are normally inherited
13783 from its parent and are usually unchanged since login. This means
13784 that if you change the group database after logging in, @command{\cmd\}
13785 will not reflect your changes within your existing login session.
13786 Running @command{\cmd\} with a \arg\ causes the user and group
13787 database to be consulted afresh, and so will give a different result.
13789 @primaryAndSupplementaryGroups{id,user argument}
13791 @node logname invocation
13792 @section @command{logname}: Print current login name
13795 @cindex printing user's login name
13796 @cindex login name, printing
13797 @cindex user name, printing
13800 @command{logname} prints the calling user's name, as found in a
13801 system-maintained file (often @file{/var/run/utmp} or
13802 @file{/etc/utmp}), and exits with a status of 0. If there is no entry
13803 for the calling process, @command{logname} prints
13804 an error message and exits with a status of 1.
13806 The only options are @option{--help} and @option{--version}. @xref{Common
13812 @node whoami invocation
13813 @section @command{whoami}: Print effective user ID
13816 @cindex effective user ID, printing
13817 @cindex printing the effective user ID
13819 @command{whoami} prints the user name associated with the current
13820 effective user ID. It is equivalent to the command @samp{id -un}.
13822 The only options are @option{--help} and @option{--version}. @xref{Common
13828 @node groups invocation
13829 @section @command{groups}: Print group names a user is in
13832 @cindex printing groups a user is in
13833 @cindex supplementary groups, printing
13835 @command{groups} prints the names of the primary and any supplementary
13836 groups for each given @var{username}, or the current process if no names
13837 are given. If more than one name is given, the name of each user is
13839 the list of that user's groups and the user name is separated from the
13840 group list by a colon. Synopsis:
13843 groups [@var{username}]@dots{}
13846 The group lists are equivalent to the output of the command @samp{id -Gn}.
13848 @primaryAndSupplementaryGroups{groups,list of users}
13850 The only options are @option{--help} and @option{--version}. @xref{Common
13856 @node users invocation
13857 @section @command{users}: Print login names of users currently logged in
13860 @cindex printing current usernames
13861 @cindex usernames, printing current
13863 @cindex login sessions, printing users with
13864 @command{users} prints on a single line a blank-separated list of user
13865 names of users currently logged in to the current host. Each user name
13866 corresponds to a login session, so if a user has more than one login
13867 session, that user's name will appear the same number of times in the
13876 With no @var{file} argument, @command{users} extracts its information from
13877 a system-maintained file (often @file{/var/run/utmp} or
13878 @file{/etc/utmp}). If a file argument is given, @command{users} uses
13879 that file instead. A common choice is @file{/var/log/wtmp}.
13881 The only options are @option{--help} and @option{--version}. @xref{Common
13887 @node who invocation
13888 @section @command{who}: Print who is currently logged in
13891 @cindex printing current user information
13892 @cindex information, about current users
13894 @command{who} prints information about users who are currently logged on.
13898 @command{who} [@var{option}] [@var{file}] [am i]
13901 @cindex terminal lines, currently used
13903 @cindex remote hostname
13904 If given no non-option arguments, @command{who} prints the following
13905 information for each user currently logged on: login name, terminal
13906 line, login time, and remote hostname or X display.
13910 If given one non-option argument, @command{who} uses that instead of
13911 a default system-maintained file (often @file{/var/run/utmp} or
13912 @file{/etc/utmp}) as the name of the file containing the record of
13913 users logged on. @file{/var/log/wtmp} is commonly given as an argument
13914 to @command{who} to look at who has previously logged on.
13918 If given two non-option arguments, @command{who} prints only the entry
13919 for the user running it (determined from its standard input), preceded
13920 by the hostname. Traditionally, the two arguments given are @samp{am
13921 i}, as in @samp{who am i}.
13924 Time stamps are listed according to the time zone rules specified by
13925 the @env{TZ} environment variable, or by the system default rules if
13926 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
13927 with @env{TZ}, libc, The GNU C Library Reference Manual}.
13929 The program accepts the following options. Also see @ref{Common options}.
13937 Same as @samp{-b -d --login -p -r -t -T -u}.
13943 Print the date and time of last system boot.
13949 Print information corresponding to dead processes.
13955 Print a line of column headings.
13961 List only the entries that correspond to processes via which the
13962 system is waiting for a user to login. The user name is always @samp{LOGIN}.
13966 Attempt to canonicalize hostnames found in utmp through a DNS lookup. This
13967 is not the default because it can cause significant delays on systems with
13968 automatic dial-up internet access.
13972 Same as @samp{who am i}.
13978 List active processes spawned by init.
13984 Print only the login names and the number of users logged on.
13985 Overrides all other options.
13990 @opindex --runlevel
13991 Print the current (and maybe previous) run-level of the init process.
13995 Ignored; for compatibility with other versions of @command{who}.
14001 Print last system clock change.
14006 After the login time, print the number of hours and minutes that the
14007 user has been idle. @samp{.} means the user was active in the last minute.
14008 @samp{old} means the user has been idle for more than 24 hours.
14019 @opindex --writable
14020 @cindex message status
14021 @pindex write@r{, allowed}
14022 After each login name print a character indicating the user's message status:
14025 @samp{+} allowing @code{write} messages
14026 @samp{-} disallowing @code{write} messages
14027 @samp{?} cannot find terminal device
14035 @node System context
14036 @chapter System context
14038 @cindex system context
14039 @cindex context, system
14040 @cindex commands for system context
14042 This section describes commands that print or change system-wide
14046 * date invocation:: Print or set system date and time.
14047 * arch invocation:: Print machine hardware name.
14048 * nproc invocation:: Print the number of processors.
14049 * uname invocation:: Print system information.
14050 * hostname invocation:: Print or set system name.
14051 * hostid invocation:: Print numeric host identifier.
14052 * uptime invocation:: Print system uptime and load.
14055 @node date invocation
14056 @section @command{date}: Print or set system date and time
14059 @cindex time, printing or setting
14060 @cindex printing the current time
14065 date [@var{option}]@dots{} [+@var{format}]
14066 date [-u|--utc|--universal] @c this avoids a newline in the output
14067 [ MMDDhhmm[[CC]YY][.ss] ]
14071 Invoking @command{date} with no @var{format} argument is equivalent to invoking
14072 it with a default format that depends on the @env{LC_TIME} locale category.
14073 In the default C locale, this format is @samp{'+%a %b %e %H:%M:%S %Z %Y'},
14074 so the output looks like @samp{Thu Mar @ 3 13:47:51 PST 2005}.
14077 Normally, @command{date} uses the time zone rules indicated by the
14078 @env{TZ} environment variable, or the system default rules if @env{TZ}
14079 is not set. @xref{TZ Variable,, Specifying the Time Zone with
14080 @env{TZ}, libc, The GNU C Library Reference Manual}.
14082 @findex strftime @r{and @command{date}}
14083 @cindex time formats
14084 @cindex formatting times
14085 If given an argument that starts with a @samp{+}, @command{date} prints the
14086 current date and time (or the date and time specified by the
14087 @option{--date} option, see below) in the format defined by that argument,
14088 which is similar to that of the @code{strftime} function. Except for
14089 conversion specifiers, which start with @samp{%}, characters in the
14090 format string are printed unchanged. The conversion specifiers are
14096 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
14097 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
14098 * Literal conversion specifiers:: %[%nt]
14099 * Padding and other flags:: Pad with zeros, spaces, etc.
14100 * Setting the time:: Changing the system clock.
14101 * Options for date:: Instead of the current time.
14103 * Date input formats:: Specifying date strings.
14105 * Examples of date:: Examples.
14108 @node Time conversion specifiers
14109 @subsection Time conversion specifiers
14111 @cindex time conversion specifiers
14112 @cindex conversion specifiers, time
14114 @command{date} conversion specifiers related to times.
14118 hour (@samp{00}@dots{}@samp{23})
14120 hour (@samp{01}@dots{}@samp{12})
14122 hour, space padded (@samp{ 0}@dots{}@samp{23}); equivalent to @samp{%_H}.
14123 This is a @acronym{GNU} extension.
14125 hour, space padded (@samp{ 1}@dots{}@samp{12}); equivalent to @samp{%_I}.
14126 This is a @acronym{GNU} extension.
14128 minute (@samp{00}@dots{}@samp{59})
14130 nanoseconds (@samp{000000000}@dots{}@samp{999999999}).
14131 This is a @acronym{GNU} extension.
14133 locale's equivalent of either @samp{AM} or @samp{PM};
14134 blank in many locales.
14135 Noon is treated as @samp{PM} and midnight as @samp{AM}.
14137 like @samp{%p}, except lower case.
14138 This is a @acronym{GNU} extension.
14140 locale's 12-hour clock time (e.g., @samp{11:11:04 PM})
14142 24-hour hour and minute. Same as @samp{%H:%M}.
14144 @cindex epoch, seconds since
14145 @cindex seconds since the epoch
14146 @cindex beginning of time
14147 seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC.
14148 Leap seconds are not counted unless leap second support is available.
14149 @xref{%s-examples}, for examples.
14150 This is a @acronym{GNU} extension.
14152 second (@samp{00}@dots{}@samp{60}).
14153 This may be @samp{60} if leap seconds are supported.
14155 24-hour hour, minute, and second. Same as @samp{%H:%M:%S}.
14157 locale's time representation (e.g., @samp{23:13:48})
14159 @w{@acronym{RFC} 2822/@acronym{ISO} 8601} style numeric time zone
14160 (e.g., @samp{-0600} or @samp{+0530}), or nothing if no
14161 time zone is determinable. This value reflects the numeric time zone
14162 appropriate for the current time, using the time zone rules specified
14163 by the @env{TZ} environment variable.
14164 The time (and optionally, the time zone rules) can be overridden
14165 by the @option{--date} option.
14167 @w{@acronym{RFC} 3339/@acronym{ISO} 8601} style numeric time zone with
14168 @samp{:} (e.g., @samp{-06:00} or @samp{+05:30}), or nothing if no time
14169 zone is determinable.
14170 This is a @acronym{GNU} extension.
14172 Numeric time zone to the nearest second with @samp{:} (e.g.,
14173 @samp{-06:00:00} or @samp{+05:30:00}), or nothing if no time zone is
14175 This is a @acronym{GNU} extension.
14177 Numeric time zone with @samp{:} using the minimum necessary precision
14178 (e.g., @samp{-06}, @samp{+05:30}, or @samp{-04:56:02}), or nothing if
14179 no time zone is determinable.
14180 This is a @acronym{GNU} extension.
14182 alphabetic time zone abbreviation (e.g., @samp{EDT}), or nothing if no
14183 time zone is determinable. See @samp{%z} for how it is determined.
14187 @node Date conversion specifiers
14188 @subsection Date conversion specifiers
14190 @cindex date conversion specifiers
14191 @cindex conversion specifiers, date
14193 @command{date} conversion specifiers related to dates.
14197 locale's abbreviated weekday name (e.g., @samp{Sun})
14199 locale's full weekday name, variable length (e.g., @samp{Sunday})
14201 locale's abbreviated month name (e.g., @samp{Jan})
14203 locale's full month name, variable length (e.g., @samp{January})
14205 locale's date and time (e.g., @samp{Thu Mar @ 3 23:05:25 2005})
14207 century. This is like @samp{%Y}, except the last two digits are omitted.
14208 For example, it is @samp{20} if @samp{%Y} is @samp{2000},
14209 and is @samp{-0} if @samp{%Y} is @samp{-001}.
14210 It is normally at least two characters, but it may be more.
14212 day of month (e.g., @samp{01})
14214 date; same as @samp{%m/%d/%y}
14216 day of month, space padded; same as @samp{%_d}
14218 full date in @acronym{ISO} 8601 format; same as @samp{%Y-%m-%d}.
14219 This is a good choice for a date format, as it is standard and
14220 is easy to sort in the usual case where years are in the range
14223 year corresponding to the @acronym{ISO} week number, but without the century
14224 (range @samp{00} through @samp{99}). This has the same format and value
14225 as @samp{%y}, except that if the @acronym{ISO} week number (see
14227 to the previous or next year, that year is used instead.
14229 year corresponding to the @acronym{ISO} week number. This has the
14230 same format and value as @samp{%Y}, except that if the @acronym{ISO}
14232 @samp{%V}) belongs to the previous or next year, that year is used
14234 It is normally useful only if @samp{%V} is also used;
14235 for example, the format @samp{%G-%m-%d} is probably a mistake,
14236 since it combines the ISO week number year with the conventional month and day.
14240 day of year (@samp{001}@dots{}@samp{366})
14242 month (@samp{01}@dots{}@samp{12})
14244 day of week (@samp{1}@dots{}@samp{7}) with @samp{1} corresponding to Monday
14246 week number of year, with Sunday as the first day of the week
14247 (@samp{00}@dots{}@samp{53}).
14248 Days in a new year preceding the first Sunday are in week zero.
14250 @acronym{ISO} week number, that is, the
14251 week number of year, with Monday as the first day of the week
14252 (@samp{01}@dots{}@samp{53}).
14253 If the week containing January 1 has four or more days in
14254 the new year, then it is considered week 1; otherwise, it is week 53 of
14255 the previous year, and the next week is week 1. (See the @acronym{ISO} 8601
14258 day of week (@samp{0}@dots{}@samp{6}) with 0 corresponding to Sunday
14260 week number of year, with Monday as first day of week
14261 (@samp{00}@dots{}@samp{53}).
14262 Days in a new year preceding the first Monday are in week zero.
14264 locale's date representation (e.g., @samp{12/31/99})
14266 last two digits of year (@samp{00}@dots{}@samp{99})
14268 year. This is normally at least four characters, but it may be more.
14269 Year @samp{0000} precedes year @samp{0001}, and year @samp{-001}
14270 precedes year @samp{0000}.
14274 @node Literal conversion specifiers
14275 @subsection Literal conversion specifiers
14277 @cindex literal conversion specifiers
14278 @cindex conversion specifiers, literal
14280 @command{date} conversion specifiers that produce literal strings.
14292 @node Padding and other flags
14293 @subsection Padding and other flags
14295 @cindex numeric field padding
14296 @cindex padding of numeric fields
14297 @cindex fields, padding numeric
14299 Unless otherwise specified, @command{date} normally pads numeric fields
14300 with zeros, so that, for
14301 example, numeric months are always output as two digits.
14302 Seconds since the epoch are not padded, though,
14303 since there is no natural width for them.
14305 As a @acronym{GNU} extension, @command{date} recognizes any of the
14306 following optional flags after the @samp{%}:
14310 (hyphen) Do not pad the field; useful if the output is intended for
14313 (underscore) Pad with spaces; useful if you need a fixed
14314 number of characters in the output, but zeros are too distracting.
14316 (zero) Pad with zeros even if the conversion specifier
14317 would normally pad with spaces.
14319 Use upper case characters if possible.
14321 Use opposite case characters if possible.
14322 A field that is normally upper case becomes lower case, and vice versa.
14326 Here are some examples of padding:
14329 date +%d/%m -d "Feb 1"
14331 date +%-d/%-m -d "Feb 1"
14333 date +%_d/%_m -d "Feb 1"
14337 As a @acronym{GNU} extension, you can specify the field width
14338 (after any flag, if present) as a decimal number. If the natural size of the
14339 output of the field has less than the specified number of characters,
14340 the result is written right adjusted and padded to the given
14341 size. For example, @samp{%9B} prints the right adjusted month name in
14342 a field of width 9.
14344 An optional modifier can follow the optional flag and width
14345 specification. The modifiers are:
14349 Use the locale's alternate representation for date and time. This
14350 modifier applies to the @samp{%c}, @samp{%C}, @samp{%x}, @samp{%X},
14351 @samp{%y} and @samp{%Y} conversion specifiers. In a Japanese locale, for
14352 example, @samp{%Ex} might yield a date format based on the Japanese
14356 Use the locale's alternate numeric symbols for numbers. This modifier
14357 applies only to numeric conversion specifiers.
14360 If the format supports the modifier but no alternate representation
14361 is available, it is ignored.
14364 @node Setting the time
14365 @subsection Setting the time
14367 @cindex setting the time
14368 @cindex time setting
14369 @cindex appropriate privileges
14371 If given an argument that does not start with @samp{+}, @command{date} sets
14372 the system clock to the date and time specified by that argument (as
14373 described below). You must have appropriate privileges to set the
14374 system clock. Note for changes to persist across a reboot, the
14375 hardware clock may need to be updated from the system clock, which
14376 might not happen automatically on your system.
14378 The argument must consist entirely of digits, which have the following
14391 first two digits of year (optional)
14393 last two digits of year (optional)
14398 Note, the @option{--date} and @option{--set} options may not be used with an
14399 argument in the above format. The @option{--universal} option may be used
14400 with such an argument to indicate that the specified date and time are
14401 relative to Coordinated Universal Time rather than to the local time zone.
14404 @node Options for date
14405 @subsection Options for @command{date}
14407 @cindex @command{date} options
14408 @cindex options for @command{date}
14410 The program accepts the following options. Also see @ref{Common options}.
14414 @item -d @var{datestr}
14415 @itemx --date=@var{datestr}
14418 @cindex parsing date strings
14419 @cindex date strings, parsing
14420 @cindex arbitrary date strings, parsing
14423 @opindex next @var{day}
14424 @opindex last @var{day}
14425 Display the date and time specified in @var{datestr} instead of the
14426 current date and time. @var{datestr} can be in almost any common
14427 format. It can contain month names, time zones, @samp{am} and @samp{pm},
14428 @samp{yesterday}, etc. For example, @option{--date="2004-02-27
14429 14:19:13.489392193 +0530"} specifies the instant of time that is
14430 489,392,193 nanoseconds after February 27, 2004 at 2:19:13 PM in a
14431 time zone that is 5 hours and 30 minutes east of @acronym{UTC}.@*
14432 Note: input currently must be in locale independent format. E.g., the
14433 LC_TIME=C below is needed to print back the correct date in many locales:
14435 date -d "$(LC_TIME=C date)"
14437 @xref{Date input formats}.
14439 @item -f @var{datefile}
14440 @itemx --file=@var{datefile}
14443 Parse each line in @var{datefile} as with @option{-d} and display the
14444 resulting date and time. If @var{datefile} is @samp{-}, use standard
14445 input. This is useful when you have many dates to process, because the
14446 system overhead of starting up the @command{date} executable many times can
14449 @item -I[@var{timespec}]
14450 @itemx --iso-8601[=@var{timespec}]
14451 @opindex -I[@var{timespec}]
14452 @opindex --iso-8601[=@var{timespec}]
14453 Display the date using the @acronym{ISO} 8601 format, @samp{%Y-%m-%d}.
14455 The argument @var{timespec} specifies the number of additional
14456 terms of the time to include. It can be one of the following:
14459 Print just the date. This is the default if @var{timespec} is omitted.
14462 Append the hour of the day to the date.
14465 Append the hours and minutes.
14468 Append the hours, minutes and seconds.
14471 Append the hours, minutes, seconds and nanoseconds.
14474 If showing any time terms, then include the time zone using the format
14477 @item -r @var{file}
14478 @itemx --reference=@var{file}
14480 @opindex --reference
14481 Display the date and time of the last modification of @var{file},
14482 instead of the current date and time.
14489 @opindex --rfc-2822
14490 Display the date and time using the format @samp{%a, %d %b %Y %H:%M:%S
14491 %z}, evaluated in the C locale so abbreviations are always in English.
14495 Fri, 09 Sep 2005 13:51:39 -0700
14498 This format conforms to
14499 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt, Internet
14500 @acronym{RFCs} 2822} and
14501 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc822.txt, 822}, the
14502 current and previous standards for Internet email.
14504 @item --rfc-3339=@var{timespec}
14505 @opindex --rfc-3339=@var{timespec}
14506 Display the date using a format specified by
14507 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt, Internet
14508 @acronym{RFC} 3339}. This is a subset of the @acronym{ISO} 8601
14509 format, except that it also permits applications to use a space rather
14510 than a @samp{T} to separate dates from times. Unlike the other
14511 standard formats, @acronym{RFC} 3339 format is always suitable as
14512 input for the @option{--date} (@option{-d}) and @option{--file}
14513 (@option{-f}) options, regardless of the current locale.
14515 The argument @var{timespec} specifies how much of the time to include.
14516 It can be one of the following:
14520 Print just the full-date, e.g., @samp{2005-09-14}.
14521 This is equivalent to the format @samp{%Y-%m-%d}.
14524 Print the full-date and full-time separated by a space, e.g.,
14525 @samp{2005-09-14 00:56:06+05:30}. The output ends with a numeric
14526 time-offset; here the @samp{+05:30} means that local time is five
14527 hours and thirty minutes east of @acronym{UTC}. This is equivalent to
14528 the format @samp{%Y-%m-%d %H:%M:%S%:z}.
14531 Like @samp{seconds}, but also print nanoseconds, e.g.,
14532 @samp{2005-09-14 00:56:06.998458565+05:30}.
14533 This is equivalent to the format @samp{%Y-%m-%d %H:%M:%S.%N%:z}.
14537 @item -s @var{datestr}
14538 @itemx --set=@var{datestr}
14541 Set the date and time to @var{datestr}. See @option{-d} above.
14542 See also @ref{Setting the time}.
14549 @opindex --universal
14550 @cindex Coordinated Universal Time
14552 @cindex Greenwich Mean Time
14555 Use Coordinated Universal Time (@acronym{UTC}) by operating as if the
14556 @env{TZ} environment variable were set to the string @samp{UTC0}.
14558 Universal Time is often called ``Greenwich Mean Time'' (@sc{gmt}) for
14559 historical reasons.
14563 @node Examples of date
14564 @subsection Examples of @command{date}
14566 @cindex examples of @command{date}
14568 Here are a few examples. Also see the documentation for the @option{-d}
14569 option in the previous section.
14574 To print the date of the day before yesterday:
14577 date --date='2 days ago'
14581 To print the date of the day three months and one day hence:
14584 date --date='3 months 1 day'
14588 To print the day of year of Christmas in the current year:
14591 date --date='25 Dec' +%j
14595 To print the current full month name and the day of the month:
14601 But this may not be what you want because for the first nine days of
14602 the month, the @samp{%d} expands to a zero-padded two-digit field,
14603 for example @samp{date -d 1may '+%B %d'} will print @samp{May 01}.
14606 To print a date without the leading zero for one-digit days
14607 of the month, you can use the (@acronym{GNU} extension)
14608 @samp{-} flag to suppress
14609 the padding altogether:
14612 date -d 1may '+%B %-d
14616 To print the current date and time in the format required by many
14617 non-@acronym{GNU} versions of @command{date} when setting the system clock:
14620 date +%m%d%H%M%Y.%S
14624 To set the system clock forward by two minutes:
14627 date --set='+2 minutes'
14631 To print the date in @acronym{RFC} 2822 format,
14632 use @samp{date --rfc-2822}. Here is some example output:
14635 Fri, 09 Sep 2005 13:51:39 -0700
14638 @anchor{%s-examples}
14640 To convert a date string to the number of seconds since the epoch
14641 (which is 1970-01-01 00:00:00 UTC), use the @option{--date} option with
14642 the @samp{%s} format. That can be useful in sorting and/or graphing
14643 and/or comparing data by date. The following command outputs the
14644 number of the seconds since the epoch for the time two minutes after the
14648 date --date='1970-01-01 00:02:00 +0000' +%s
14652 If you do not specify time zone information in the date string,
14653 @command{date} uses your computer's idea of the time zone when
14654 interpreting the string. For example, if your computer's time zone is
14655 that of Cambridge, Massachusetts, which was then 5 hours (i.e., 18,000
14656 seconds) behind UTC:
14659 # local time zone used
14660 date --date='1970-01-01 00:02:00' +%s
14665 If you're sorting or graphing dated data, your raw date values may be
14666 represented as seconds since the epoch. But few people can look at
14667 the date @samp{946684800} and casually note ``Oh, that's the first second
14668 of the year 2000 in Greenwich, England.''
14671 date --date='2000-01-01 UTC' +%s
14675 An alternative is to use the @option{--utc} (@option{-u}) option.
14676 Then you may omit @samp{UTC} from the date string. Although this
14677 produces the same result for @samp{%s} and many other format sequences,
14678 with a time zone offset different from zero, it would give a different
14679 result for zone-dependent formats like @samp{%z}.
14682 date -u --date=2000-01-01 +%s
14686 To convert such an unwieldy number of seconds back to
14687 a more readable form, use a command like this:
14690 # local time zone used
14691 date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
14692 1999-12-31 19:00:00 -0500
14695 Or if you do not mind depending on the @samp{@@} feature present since
14696 coreutils 5.3.0, you could shorten this to:
14699 date -d @@946684800 +"%F %T %z"
14700 1999-12-31 19:00:00 -0500
14703 Often it is better to output UTC-relative date and time:
14706 date -u -d '1970-01-01 946684800 seconds' +"%Y-%m-%d %T %z"
14707 2000-01-01 00:00:00 +0000
14713 @node arch invocation
14714 @section @command{arch}: Print machine hardware name
14717 @cindex print machine hardware name
14718 @cindex system information, printing
14720 @command{arch} prints the machine hardware name,
14721 and is equivalent to @samp{uname -m}.
14725 arch [@var{option}]
14728 The program accepts the @ref{Common options} only.
14733 @node nproc invocation
14734 @section @command{nproc}: Print the number of available processors
14737 @cindex Print the number of processors
14738 @cindex system information, printing
14740 Print the number of processing units available to the current process,
14741 which may be less than the number of online processors.
14742 If this information is not accessible, then print the number of
14743 processors installed. If the @env{OMP_NUM_THREADS} environment variable is
14744 set, then it will determine the returned value. The result is guaranteed to be
14745 greater than zero. Synopsis:
14748 nproc [@var{option}]
14751 The program accepts the following options. Also see @ref{Common options}.
14757 Print the number of installed processors on the system, which may
14758 be greater than the number online or available to the current process.
14759 The @env{OMP_NUM_THREADS} environment variable is not honored in this case.
14761 @item --ignore=@var{number}
14763 If possible, exclude this @var{number} of processing units.
14770 @node uname invocation
14771 @section @command{uname}: Print system information
14774 @cindex print system information
14775 @cindex system information, printing
14777 @command{uname} prints information about the machine and operating system
14778 it is run on. If no options are given, @command{uname} acts as if the
14779 @option{-s} option were given. Synopsis:
14782 uname [@var{option}]@dots{}
14785 If multiple options or @option{-a} are given, the selected information is
14786 printed in this order:
14789 @var{kernel-name} @var{nodename} @var{kernel-release} @var{kernel-version}
14790 @var{machine} @var{processor} @var{hardware-platform} @var{operating-system}
14793 The information may contain internal spaces, so such output cannot be
14794 parsed reliably. In the following example, @var{release} is
14795 @samp{2.2.18ss.e820-bda652a #4 SMP Tue Jun 5 11:24:08 PDT 2001}:
14799 @result{} Linux dum 2.2.18 #4 SMP Tue Jun 5 11:24:08 PDT 2001 i686@c
14800 unknown unknown GNU/Linux
14804 The program accepts the following options. Also see @ref{Common options}.
14812 Print all of the below information, except omit the processor type
14813 and the hardware platform name if they are unknown.
14816 @itemx --hardware-platform
14818 @opindex --hardware-platform
14819 @cindex implementation, hardware
14820 @cindex hardware platform
14821 @cindex platform, hardware
14822 Print the hardware platform name
14823 (sometimes called the hardware implementation).
14824 Print @samp{unknown} if the kernel does not make this information
14825 easily available, as is the case with Linux kernels.
14831 @cindex machine type
14832 @cindex hardware class
14833 @cindex hardware type
14834 Print the machine hardware name (sometimes called the hardware class
14840 @opindex --nodename
14843 @cindex network node name
14844 Print the network node hostname.
14849 @opindex --processor
14850 @cindex host processor type
14851 Print the processor type (sometimes called the instruction set
14852 architecture or ISA).
14853 Print @samp{unknown} if the kernel does not make this information
14854 easily available, as is the case with Linux kernels.
14857 @itemx --operating-system
14859 @opindex --operating-system
14860 @cindex operating system name
14861 Print the name of the operating system.
14864 @itemx --kernel-release
14866 @opindex --kernel-release
14867 @cindex kernel release
14868 @cindex release of kernel
14869 Print the kernel release.
14872 @itemx --kernel-name
14874 @opindex --kernel-name
14875 @cindex kernel name
14876 @cindex name of kernel
14877 Print the kernel name.
14878 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) calls this
14879 ``the implementation of the operating system'', because the
14880 @acronym{POSIX} specification itself has no notion of ``kernel''.
14881 The kernel name might be the same as the operating system name printed
14882 by the @option{-o} or @option{--operating-system} option, but it might
14883 differ. Some operating systems (e.g., FreeBSD, HP-UX) have the same
14884 name as their underlying kernels; others (e.g., GNU/Linux, Solaris)
14888 @itemx --kernel-version
14890 @opindex --kernel-version
14891 @cindex kernel version
14892 @cindex version of kernel
14893 Print the kernel version.
14900 @node hostname invocation
14901 @section @command{hostname}: Print or set system name
14904 @cindex setting the hostname
14905 @cindex printing the hostname
14906 @cindex system name, printing
14907 @cindex appropriate privileges
14909 With no arguments, @command{hostname} prints the name of the current host
14910 system. With one argument, it sets the current host name to the
14911 specified string. You must have appropriate privileges to set the host
14915 hostname [@var{name}]
14918 The only options are @option{--help} and @option{--version}. @xref{Common
14924 @node hostid invocation
14925 @section @command{hostid}: Print numeric host identifier
14928 @cindex printing the host identifier
14930 @command{hostid} prints the numeric identifier of the current host
14931 in hexadecimal. This command accepts no arguments.
14932 The only options are @option{--help} and @option{--version}.
14933 @xref{Common options}.
14935 For example, here's what it prints on one system I use:
14942 On that system, the 32-bit quantity happens to be closely
14943 related to the system's Internet address, but that isn't always
14948 @node uptime invocation
14949 @section @command{uptime}: Print system uptime and load
14952 @cindex printing the system uptime and load
14954 @command{uptime} prints the current time, the system's uptime, the
14955 number of logged-in users and the current load average.
14957 If an argument is specified, it is used as the file to be read
14958 to discover how many users are logged in. If no argument is
14959 specified, a system default is used (@command{uptime --help} indicates
14960 the default setting).
14962 The only options are @option{--help} and @option{--version}.
14963 @xref{Common options}.
14965 For example, here's what it prints right now on one system I use:
14969 14:07 up 3:35, 3 users, load average: 1.39, 1.15, 1.04
14972 The precise method of calculation of load average varies somewhat
14973 between systems. Some systems calculate it as the average number of
14974 runnable processes over the last 1, 5 and 15 minutes, but some systems
14975 also include processes in the uninterruptible sleep state (that is,
14976 those processes which are waiting for disk I/O). The Linux kernel
14977 includes uninterruptible processes.
14979 @node SELinux context
14980 @chapter SELinux context
14982 @cindex SELinux context
14983 @cindex SELinux, context
14984 @cindex commands for SELinux context
14986 This section describes commands for operations with SELinux
14990 * chcon invocation:: Change SELinux context of file
14991 * runcon invocation:: Run a command in specified SELinux context
14994 @node chcon invocation
14995 @section @command{chcon}: Change SELinux context of file
14998 @cindex changing security context
14999 @cindex change SELinux context
15001 @command{chcon} changes the SELinux security context of the selected files.
15005 chcon [@var{option}]@dots{} @var{context} @var{file}@dots{}
15006 chcon [@var{option}]@dots{} [-u @var{user}] [-r @var{role}] [-l @var{range}]@c
15007 [-t @var{type}] @var{file}@dots{}
15008 chcon [@var{option}]@dots{} --reference=@var{rfile} @var{file}@dots{}
15011 Change the SELinux security context of each @var{file} to @var{context}.
15012 With @option{--reference}, change the security context of each @var{file}
15013 to that of @var{rfile}.
15015 The program accepts the following options. Also see @ref{Common options}.
15020 @itemx --no-dereference
15022 @opindex --no-dereference
15023 @cindex no dereference
15024 Affect symbolic links instead of any referenced file.
15026 @item --reference=@var{rfile}
15027 @opindex --reference
15028 @cindex reference file
15029 Use @var{rfile}'s security context rather than specifying a @var{context} value.
15034 @opindex --recursive
15035 Operate on files and directories recursively.
15038 @xref{Traversing symlinks}.
15041 @xref{Traversing symlinks}.
15044 @xref{Traversing symlinks}.
15051 Output a diagnostic for every file processed.
15053 @item -u @var{user}
15054 @itemx --user=@var{user}
15057 Set user @var{user} in the target security context.
15059 @item -r @var{role}
15060 @itemx --role=@var{role}
15063 Set role @var{role} in the target security context.
15065 @item -t @var{type}
15066 @itemx --type=@var{type}
15069 Set type @var{type} in the target security context.
15071 @item -l @var{range}
15072 @itemx --range=@var{range}
15075 Set range @var{range} in the target security context.
15081 @node runcon invocation
15082 @section @command{runcon}: Run a command in specified SELinux context
15085 @cindex run with security context
15088 @command{runcon} runs file in specified SELinux security context.
15092 runcon @var{context} @var{command} [@var{args}]
15093 runcon [ -c ] [-u @var{user}] [-r @var{role}] [-t @var{type}]@c
15094 [-l @var{range}] @var{command} [@var{args}]
15097 Run @var{command} with completely-specified @var{context}, or with
15098 current or transitioned security context modified by one or more of @var{level},
15099 @var{role}, @var{type} and @var{user}.
15101 If none of @option{-c}, @option{-t}, @option{-u}, @option{-r}, or @option{-l}
15102 is specified, the first argument is used as the complete context.
15103 Any additional arguments after @var{command}
15104 are interpreted as arguments to the command.
15106 With neither @var{context} nor @var{command}, print the current
15109 The program accepts the following options. Also see @ref{Common options}.
15117 Compute process transition context before modifying.
15119 @item -u @var{user}
15120 @itemx --user=@var{user}
15123 Set user @var{user} in the target security context.
15125 @item -r @var{role}
15126 @itemx --role=@var{role}
15129 Set role @var{role} in the target security context.
15131 @item -t @var{type}
15132 @itemx --type=@var{type}
15135 Set type @var{type} in the target security context.
15137 @item -l @var{range}
15138 @itemx --range=@var{range}
15141 Set range @var{range} in the target security context.
15145 @cindex exit status of @command{runcon}
15149 126 if @var{command} is found but cannot be invoked
15150 127 if @command{runcon} itself fails or if @var{command} cannot be found
15151 the exit status of @var{command} otherwise
15154 @node Modified command invocation
15155 @chapter Modified command invocation
15157 @cindex modified command invocation
15158 @cindex invocation of commands, modified
15159 @cindex commands for invoking other commands
15161 This section describes commands that run other commands in some context
15162 different than the current one: a modified environment, as a different
15166 * chroot invocation:: Modify the root directory.
15167 * env invocation:: Modify environment variables.
15168 * nice invocation:: Modify niceness.
15169 * nohup invocation:: Immunize to hangups.
15170 * stdbuf invocation:: Modify buffering of standard streams.
15171 * su invocation:: Modify user and group ID.
15172 * timeout invocation:: Run with time limit.
15176 @node chroot invocation
15177 @section @command{chroot}: Run a command with a different root directory
15180 @cindex running a program in a specified root directory
15181 @cindex root directory, running a program in a specified
15183 @command{chroot} runs a command with a specified root directory.
15184 On many systems, only the super-user can do this.@footnote{However,
15185 some systems (e.g., FreeBSD) can be configured to allow certain regular
15186 users to use the @code{chroot} system call, and hence to run this program.
15187 Also, on Cygwin, anyone can run the @command{chroot} command, because the
15188 underlying function is non-privileged due to lack of support in MS-Windows.}
15192 chroot @var{option} @var{newroot} [@var{command} [@var{args}]@dots{}]
15193 chroot @var{option}
15196 Ordinarily, file names are looked up starting at the root of the
15197 directory structure, i.e., @file{/}. @command{chroot} changes the root to
15198 the directory @var{newroot} (which must exist) and then runs
15199 @var{command} with optional @var{args}. If @var{command} is not
15200 specified, the default is the value of the @env{SHELL} environment
15201 variable or @command{/bin/sh} if not set, invoked with the @option{-i} option.
15202 @var{command} must not be a special built-in utility
15203 (@pxref{Special built-in utilities}).
15205 The program accepts the following options. Also see @ref{Common options}.
15206 Options must precede operands.
15210 @itemx --userspec=@var{user}[:@var{group}]
15211 @opindex --userspec
15212 By default, @var{command} is run with the same credentials
15213 as the invoking process.
15214 Use this option to run it as a different @var{user} and/or with a
15215 different primary @var{group}.
15217 @itemx --groups=@var{groups}
15219 Use this option to specify the supplementary @var{groups} to be
15220 used by the new process.
15221 The items in the list (names or numeric IDs) must be separated by commas.
15225 Here are a few tips to help avoid common problems in using chroot.
15226 To start with a simple example, make @var{command} refer to a statically
15227 linked binary. If you were to use a dynamically linked executable, then
15228 you'd have to arrange to have the shared libraries in the right place under
15229 your new root directory.
15231 For example, if you create a statically linked @command{ls} executable,
15232 and put it in @file{/tmp/empty}, you can run this command as root:
15235 $ chroot /tmp/empty /ls -Rl /
15238 Then you'll see output like this:
15243 -rwxr-xr-x 1 0 0 1041745 Aug 16 11:17 ls
15246 If you want to use a dynamically linked executable, say @command{bash},
15247 then first run @samp{ldd bash} to see what shared objects it needs.
15248 Then, in addition to copying the actual binary, also copy the listed
15249 files to the required positions under your intended new root directory.
15250 Finally, if the executable requires any other files (e.g., data, state,
15251 device files), copy them into place, too.
15253 @cindex exit status of @command{chroot}
15257 125 if @command{chroot} itself fails
15258 126 if @var{command} is found but cannot be invoked
15259 127 if @var{command} cannot be found
15260 the exit status of @var{command} otherwise
15264 @node env invocation
15265 @section @command{env}: Run a command in a modified environment
15268 @cindex environment, running a program in a modified
15269 @cindex modified environment, running a program in a
15270 @cindex running a program in a modified environment
15272 @command{env} runs a command with a modified environment. Synopses:
15275 env [@var{option}]@dots{} [@var{name}=@var{value}]@dots{} @c
15276 [@var{command} [@var{args}]@dots{}]
15280 Operands of the form @samp{@var{variable}=@var{value}} set
15281 the environment variable @var{variable} to value @var{value}.
15282 @var{value} may be empty (@samp{@var{variable}=}). Setting a variable
15283 to an empty value is different from unsetting it.
15284 These operands are evaluated left-to-right, so if two operands
15285 mention the same variable the earlier is ignored.
15287 Environment variable names can be empty, and can contain any
15288 characters other than @samp{=} and @acronym{ASCII} @sc{nul}.
15289 However, it is wise to limit yourself to names that
15290 consist solely of underscores, digits, and @acronym{ASCII} letters,
15291 and that begin with a non-digit, as applications like the shell do not
15292 work well with other names.
15295 The first operand that does not contain the character @samp{=}
15296 specifies the program to invoke; it is
15297 searched for according to the @env{PATH} environment variable. Any
15298 remaining arguments are passed as arguments to that program.
15299 The program should not be a special built-in utility
15300 (@pxref{Special built-in utilities}).
15302 Modifications to @env{PATH} take effect prior to searching for
15303 @var{command}. Use caution when reducing @env{PATH}; behavior is
15304 not portable when @env{PATH} is undefined or omits key directories
15305 such as @file{/bin}.
15307 In the rare case that a utility contains a @samp{=} in the name, the
15308 only way to disambiguate it from a variable assignment is to use an
15309 intermediate command for @var{command}, and pass the problematic
15310 program name via @var{args}. For example, if @file{./prog=} is an
15311 executable in the current @env{PATH}:
15314 env prog= true # runs 'true', with prog= in environment
15315 env ./prog= true # runs 'true', with ./prog= in environment
15316 env -- prog= true # runs 'true', with prog= in environment
15317 env sh -c '\prog= true' # runs 'prog=' with argument 'true'
15318 env sh -c 'exec "$@@"' sh prog= true # also runs 'prog='
15321 @cindex environment, printing
15323 If no command name is specified following the environment
15324 specifications, the resulting environment is printed. This is like
15325 specifying the @command{printenv} program.
15327 For some examples, suppose the environment passed to @command{env}
15328 contains @samp{LOGNAME=rms}, @samp{EDITOR=emacs}, and
15329 @samp{PATH=.:/gnubin:/hacks}:
15334 Output the current environment.
15336 $ env | LC_ALL=C sort
15339 PATH=.:/gnubin:/hacks
15343 Run @command{foo} with a reduced environment, preserving only the
15344 original @env{PATH} to avoid problems in locating @command{foo}.
15346 env - PATH="$PATH" foo
15350 Run @command{foo} with the environment containing @samp{LOGNAME=rms},
15351 @samp{EDITOR=emacs}, and @samp{PATH=.:/gnubin:/hacks}, and guarantees
15352 that @command{foo} was found in the file system rather than as a shell
15359 Run @command{nemacs} with the environment containing @samp{LOGNAME=foo},
15360 @samp{EDITOR=emacs}, @samp{PATH=.:/gnubin:/hacks}, and
15361 @samp{DISPLAY=gnu:0}.
15363 env DISPLAY=gnu:0 LOGNAME=foo nemacs
15367 Attempt to run the program @command{/energy/--} (as that is the only
15368 possible path search result); if the command exists, the environment
15369 will contain @samp{LOGNAME=rms} and @samp{PATH=/energy}, and the
15370 arguments will be @samp{e=mc2}, @samp{bar}, and @samp{baz}.
15372 env -u EDITOR PATH=/energy -- e=mc2 bar baz
15378 The program accepts the following options. Also see @ref{Common options}.
15379 Options must precede operands.
15385 @item -u @var{name}
15386 @itemx --unset=@var{name}
15389 Remove variable @var{name} from the environment, if it was in the
15394 @itemx --ignore-environment
15397 @opindex --ignore-environment
15398 Start with an empty environment, ignoring the inherited environment.
15402 @cindex exit status of @command{env}
15406 0 if no @var{command} is specified and the environment is output
15407 125 if @command{env} itself fails
15408 126 if @var{command} is found but cannot be invoked
15409 127 if @var{command} cannot be found
15410 the exit status of @var{command} otherwise
15414 @node nice invocation
15415 @section @command{nice}: Run a command with modified niceness
15419 @cindex scheduling, affecting
15420 @cindex appropriate privileges
15422 @command{nice} prints or modifies a process's @dfn{niceness},
15423 a parameter that affects whether the process is scheduled favorably.
15427 nice [@var{option}]@dots{} [@var{command} [@var{arg}]@dots{}]
15430 If no arguments are given, @command{nice} prints the current niceness.
15431 Otherwise, @command{nice} runs the given @var{command} with its
15432 niceness adjusted. By default, its niceness is incremented by 10.
15434 Niceness values range at least from @minus{}20 (process has high priority
15435 and gets more resources, thus slowing down other processes) through 19
15436 (process has lower priority and runs slowly itself, but has less impact
15437 on the speed of other running processes). Some systems
15438 may have a wider range of nicenesses; conversely, other systems may
15439 enforce more restrictive limits. An attempt to set the niceness
15440 outside the supported range is treated as an attempt to use the
15441 minimum or maximum supported value.
15443 A niceness should not be confused with a scheduling priority, which
15444 lets applications determine the order in which threads are scheduled
15445 to run. Unlike a priority, a niceness is merely advice to the
15446 scheduler, which the scheduler is free to ignore. Also, as a point of
15447 terminology, @acronym{POSIX} defines the behavior of @command{nice} in
15448 terms of a @dfn{nice value}, which is the nonnegative difference
15449 between a niceness and the minimum niceness. Though @command{nice}
15450 conforms to @acronym{POSIX}, its documentation and diagnostics use the
15451 term ``niceness'' for compatibility with historical practice.
15453 @var{command} must not be a special built-in utility (@pxref{Special
15454 built-in utilities}).
15456 @mayConflictWithShellBuiltIn{nice}
15458 The program accepts the following option. Also see @ref{Common options}.
15459 Options must precede operands.
15462 @item -n @var{adjustment}
15463 @itemx --adjustment=@var{adjustment}
15465 @opindex --adjustment
15466 Add @var{adjustment} instead of 10 to the command's niceness. If
15467 @var{adjustment} is negative and you lack appropriate privileges,
15468 @command{nice} issues a warning but otherwise acts as if you specified
15471 For compatibility @command{nice} also supports an obsolete
15472 option syntax @option{-@var{adjustment}}. New scripts should use
15473 @option{-n @var{adjustment}} instead.
15477 @cindex exit status of @command{nice}
15481 0 if no @var{command} is specified and the niceness is output
15482 125 if @command{nice} itself fails
15483 126 if @var{command} is found but cannot be invoked
15484 127 if @var{command} cannot be found
15485 the exit status of @var{command} otherwise
15488 It is sometimes useful to run a non-interactive program with reduced niceness.
15491 $ nice factor 4611686018427387903
15494 Since @command{nice} prints the current niceness,
15495 you can invoke it through itself to demonstrate how it works.
15497 The default behavior is to increase the niceness by @samp{10}:
15508 The @var{adjustment} is relative to the current niceness. In the
15509 next example, the first @command{nice} invocation runs the second one
15510 with niceness 10, and it in turn runs the final one with a niceness
15514 $ nice nice -n 3 nice
15518 Specifying a niceness larger than the supported range
15519 is the same as specifying the maximum supported value:
15522 $ nice -n 10000000000 nice
15526 Only a privileged user may run a process with lower niceness:
15530 nice: cannot set niceness: Permission denied
15532 $ sudo nice -n -1 nice
15537 @node nohup invocation
15538 @section @command{nohup}: Run a command immune to hangups
15541 @cindex hangups, immunity to
15542 @cindex immunity to hangups
15543 @cindex logging out and continuing to run
15546 @command{nohup} runs the given @var{command} with hangup signals ignored,
15547 so that the command can continue running in the background after you log
15551 nohup @var{command} [@var{arg}]@dots{}
15554 If standard input is a terminal, it is redirected from
15555 @file{/dev/null} so that terminal sessions do not mistakenly consider
15556 the terminal to be used by the command. This is a @acronym{GNU}
15557 extension; programs intended to be portable to non-@acronym{GNU} hosts
15558 should use @samp{nohup @var{command} [@var{arg}]@dots{} </dev/null}
15562 If standard output is a terminal, the command's standard output is appended
15563 to the file @file{nohup.out}; if that cannot be written to, it is appended
15564 to the file @file{$HOME/nohup.out}; and if that cannot be written to, the
15565 command is not run.
15566 Any @file{nohup.out} or @file{$HOME/nohup.out} file created by
15567 @command{nohup} is made readable and writable only to the user,
15568 regardless of the current umask settings.
15570 If standard error is a terminal, it is normally redirected to the same file
15571 descriptor as the (possibly-redirected) standard output.
15572 However, if standard output is closed, standard error terminal output
15573 is instead appended to the file @file{nohup.out} or
15574 @file{$HOME/nohup.out} as above.
15576 To capture the command's output to a file other than @file{nohup.out}
15577 you can redirect it. For example, to capture the output of
15581 nohup make > make.log
15584 @command{nohup} does not automatically put the command it runs in the
15585 background; you must do that explicitly, by ending the command line
15586 with an @samp{&}. Also, @command{nohup} does not alter the
15587 niceness of @var{command}; use @command{nice} for that,
15588 e.g., @samp{nohup nice @var{command}}.
15590 @var{command} must not be a special built-in utility (@pxref{Special
15591 built-in utilities}).
15593 The only options are @option{--help} and @option{--version}. @xref{Common
15594 options}. Options must precede operands.
15596 @cindex exit status of @command{nohup}
15600 125 if @command{nohup} itself fails, and @env{POSIXLY_CORRECT} is not set
15601 126 if @var{command} is found but cannot be invoked
15602 127 if @var{command} cannot be found
15603 the exit status of @var{command} otherwise
15606 If @env{POSIXLY_CORRECT} is set, internal failures give status 127
15610 @node stdbuf invocation
15611 @section @command{stdbuf}: Run a command with modified I/O stream buffering
15614 @cindex standard streams, buffering
15615 @cindex line buffered
15617 @command{stdbuf} allows one to modify the buffering operations of the
15618 three standard I/O streams associated with a program. Synopsis:
15621 stdbuf @var{option}@dots{} @var{command}
15624 @var{command} must start with the name of a program that
15627 uses the ISO C @code{FILE} streams for input/output (note the
15628 programs @command{dd} and @command{cat} don't do that),
15631 does not adjust the buffering of its standard streams (note the
15632 program @command{tee} is not in this category).
15635 Any additional @var{arg}s are passed as additional arguments to the
15638 The program accepts the following options. Also see @ref{Common options}.
15642 @item -i @var{mode}
15643 @itemx --input=@var{mode}
15646 Adjust the standard input stream buffering.
15648 @item -o @var{mode}
15649 @itemx --output=@var{mode}
15652 Adjust the standard output stream buffering.
15654 @item -e @var{mode}
15655 @itemx --error=@var{mode}
15658 Adjust the standard error stream buffering.
15662 The @var{mode} can be specified as follows:
15667 Set the stream to line buffered mode.
15668 In this mode data is coalesced until a newline is output or
15669 input is read from any stream attached to a terminal device.
15670 This option is invalid with standard input.
15673 Disable buffering of the selected stream.
15674 In this mode, data is output immediately and only the
15675 amount of data requested is read from input.
15676 Note the difference in function for input and output.
15677 Disabling buffering for input will not influence the responsiveness
15678 or blocking behavior of the stream input functions.
15679 For example @code{fread} will still block until @code{EOF} or error,
15680 even if the underlying @code{read} returns less data than requested.
15683 Specify the size of the buffer to use in fully buffered mode.
15684 @multiplierSuffixesNoBlocks{size}
15688 @cindex exit status of @command{stdbuf}
15692 125 if @command{stdbuf} itself fails
15693 126 if @var{command} is found but cannot be invoked
15694 127 if @var{command} cannot be found
15695 the exit status of @var{command} otherwise
15699 @node su invocation
15700 @section @command{su}: Run a command with substitute user and group ID
15703 @cindex substitute user and group IDs
15704 @cindex user ID, switching
15705 @cindex super-user, becoming
15706 @cindex root, becoming
15708 @command{su} allows one user to temporarily become another user. It runs a
15709 command (often an interactive shell) with the real and effective user
15710 ID, group ID, and supplemental groups of a given @var{user}. Synopsis:
15713 su [@var{option}]@dots{} [@var{user} [@var{arg}]@dots{}]
15716 @cindex passwd entry, and @command{su} shell
15718 @flindex /etc/passwd
15719 If no @var{user} is given, the default is @code{root}, the super-user.
15720 The shell to use is taken from @var{user}'s @code{passwd} entry, or
15721 @file{/bin/sh} if none is specified there. If @var{user} has a
15722 password, @command{su} prompts for the password unless run by a user with
15723 effective user ID of zero (the super-user).
15729 @cindex login shell
15730 By default, @command{su} does not change the current directory.
15731 It sets the environment variables @env{HOME} and @env{SHELL}
15732 from the password entry for @var{user}, and if @var{user} is not
15733 the super-user, sets @env{USER} and @env{LOGNAME} to @var{user}.
15734 By default, the shell is not a login shell.
15736 Any additional @var{arg}s are passed as additional arguments to the
15739 @cindex @option{-su}
15740 GNU @command{su} does not treat @file{/bin/sh} or any other shells specially
15741 (e.g., by setting @code{argv[0]} to @option{-su}, passing @option{-c} only
15742 to certain shells, etc.).
15745 @command{su} can optionally be compiled to use @code{syslog} to report
15746 failed, and optionally successful, @command{su} attempts. (If the system
15747 supports @code{syslog}.) However, GNU @command{su} does not check if the
15748 user is a member of the @code{wheel} group; see below.
15750 The program accepts the following options. Also see @ref{Common options}.
15753 @item -c @var{command}
15754 @itemx --command=@var{command}
15757 Pass @var{command}, a single command line to run, to the shell with
15758 a @option{-c} option instead of starting an interactive shell.
15765 @cindex file name pattern expansion, disabled
15766 @cindex globbing, disabled
15767 Pass the @option{-f} option to the shell. This probably only makes sense
15768 if the shell run is @command{csh} or @command{tcsh}, for which the @option{-f}
15769 option prevents reading the startup file (@file{.cshrc}). With
15770 Bourne-like shells, the @option{-f} option disables file name pattern
15771 expansion (globbing), which is not likely to be useful.
15779 @c other variables already indexed above
15782 @cindex login shell, creating
15783 Make the shell a login shell. This means the following. Unset all
15784 environment variables except @env{TERM}, @env{HOME}, and @env{SHELL}
15785 (which are set as described above), and @env{USER} and @env{LOGNAME}
15786 (which are set, even for the super-user, as described above), and set
15787 @env{PATH} to a compiled-in default value. Change to @var{user}'s home
15788 directory. Prepend @samp{-} to the shell's name, intended to make it
15789 read its login startup file(s).
15793 @itemx --preserve-environment
15796 @opindex --preserve-environment
15797 @cindex environment, preserving
15798 @flindex /etc/shells
15799 @cindex restricted shell
15800 Do not change the environment variables @env{HOME}, @env{USER},
15801 @env{LOGNAME}, or @env{SHELL}. Run the shell given in the environment
15802 variable @env{SHELL} instead of the shell from @var{user}'s passwd
15803 entry, unless the user running @command{su} is not the super-user and
15804 @var{user}'s shell is restricted. A @dfn{restricted shell} is one that
15805 is not listed in the file @file{/etc/shells}, or in a compiled-in list
15806 if that file does not exist. Parts of what this option does can be
15807 overridden by @option{--login} and @option{--shell}.
15809 @item -s @var{shell}
15810 @itemx --shell=@var{shell}
15813 Run @var{shell} instead of the shell from @var{user}'s passwd entry,
15814 unless the user running @command{su} is not the super-user and @var{user}'s
15815 shell is restricted (see @option{-m} just above).
15819 @cindex exit status of @command{su}
15823 125 if @command{su} itself fails
15824 126 if subshell is found but cannot be invoked
15825 127 if subshell cannot be found
15826 the exit status of the subshell otherwise
15829 @cindex wheel group, not supported
15830 @cindex group wheel, not supported
15832 @subsection Why GNU @command{su} does not support the @samp{wheel} group
15834 (This section is by Richard Stallman.)
15838 Sometimes a few of the users try to hold total power over all the
15839 rest. For example, in 1984, a few users at the MIT AI lab decided to
15840 seize power by changing the operator password on the Twenex system and
15841 keeping it secret from everyone else. (I was able to thwart this coup
15842 and give power back to the users by patching the kernel, but I
15843 wouldn't know how to do that in Unix.)
15845 However, occasionally the rulers do tell someone. Under the usual
15846 @command{su} mechanism, once someone learns the root password who
15847 sympathizes with the ordinary users, he or she can tell the rest. The
15848 ``wheel group'' feature would make this impossible, and thus cement the
15849 power of the rulers.
15851 I'm on the side of the masses, not that of the rulers. If you are
15852 used to supporting the bosses and sysadmins in whatever they do, you
15853 might find this idea strange at first.
15856 @node timeout invocation
15857 @section @command{timeout}: Run a command with a time limit
15861 @cindex run commands with bounded time
15863 @command{timeout} runs the given @var{command} and kills it if it is
15864 still running after the specified time interval. Synopsis:
15867 timeout [@var{option}] @var{duration} @var{command} [@var{arg}]@dots{}
15870 @var{command} must not be a special built-in utility (@pxref{Special
15871 built-in utilities}).
15873 The program accepts the following options. Also see @ref{Common options}.
15874 Options must precede operands.
15877 @itemx --foreground
15878 @opindex --foreground
15879 Don't create a separate background program group, so that
15880 the managed @var{command} can use the foreground TTY normally.
15881 This is needed to support timing out commands not started
15882 directly from an interactive shell, in two situations.
15885 @var{command} is interactive and needs to read from the terminal for example
15887 the user wants to support sending signals directly to @var{command}
15888 from the terminal (like Ctrl-C for example)
15891 Note in this mode of operation, any children of @var{command}
15892 will not be timed out.
15894 @item -k @var{duration}
15895 @itemx --kill-after=@var{duration}
15897 @opindex --kill-after
15898 Ensure the monitored @var{command} is killed by also sending a @samp{KILL}
15899 signal, after the specified @var{duration}. Without this option, if the
15900 selected signal proves not to be fatal, @command{timeout} does not kill
15903 @item -s @var{signal}
15904 @itemx --signal=@var{signal}
15907 Send this @var{signal} to @var{command} on timeout, rather than the
15908 default @samp{TERM} signal. @var{signal} may be a name like @samp{HUP}
15909 or a number. Also see @xref{Signal specifications}.
15913 @var{duration} is a floating point number followed by an optional unit:
15915 @samp{s} for seconds (the default)
15916 @samp{m} for minutes
15920 A duration of 0 disables the associated timeout.
15921 Note that the actual timeout duration is dependent on system conditions,
15922 which should be especially considered when specifying sub-second timeouts.
15924 @cindex exit status of @command{timeout}
15928 124 if @var{command} times out
15929 125 if @command{timeout} itself fails
15930 126 if @var{command} is found but cannot be invoked
15931 127 if @var{command} cannot be found
15932 the exit status of @var{command} otherwise
15936 @node Process control
15937 @chapter Process control
15939 @cindex processes, commands for controlling
15940 @cindex commands for controlling processes
15943 * kill invocation:: Sending a signal to processes.
15947 @node kill invocation
15948 @section @command{kill}: Send a signal to processes
15951 @cindex send a signal to processes
15953 The @command{kill} command sends a signal to processes, causing them
15954 to terminate or otherwise act upon receiving the signal in some way.
15955 Alternatively, it lists information about signals. Synopses:
15958 kill [-s @var{signal} | --signal @var{signal} | -@var{signal}] @var{pid}@dots{}
15959 kill [-l | --list | -t | --table] [@var{signal}]@dots{}
15962 @mayConflictWithShellBuiltIn{kill}
15964 The first form of the @command{kill} command sends a signal to all
15965 @var{pid} arguments. The default signal to send if none is specified
15966 is @samp{TERM}. The special signal number @samp{0} does not denote a
15967 valid signal, but can be used to test whether the @var{pid} arguments
15968 specify processes to which a signal could be sent.
15970 If @var{pid} is positive, the signal is sent to the process with the
15971 process ID @var{pid}. If @var{pid} is zero, the signal is sent to all
15972 processes in the process group of the current process. If @var{pid}
15973 is @minus{}1, the signal is sent to all processes for which the user has
15974 permission to send a signal. If @var{pid} is less than @minus{}1, the signal
15975 is sent to all processes in the process group that equals the absolute
15976 value of @var{pid}.
15978 If @var{pid} is not positive, a system-dependent set of system
15979 processes is excluded from the list of processes to which the signal
15982 If a negative @var{pid} argument is desired as the first one, it
15983 should be preceded by @option{--}. However, as a common extension to
15984 @acronym{POSIX}, @option{--} is not required with @samp{kill
15985 -@var{signal} -@var{pid}}. The following commands are equivalent:
15994 The first form of the @command{kill} command succeeds if every @var{pid}
15995 argument specifies at least one process that the signal was sent to.
15997 The second form of the @command{kill} command lists signal information.
15998 Either the @option{-l} or @option{--list} option, or the @option{-t}
15999 or @option{--table} option must be specified. Without any
16000 @var{signal} argument, all supported signals are listed. The output
16001 of @option{-l} or @option{--list} is a list of the signal names, one
16002 per line; if @var{signal} is already a name, the signal number is
16003 printed instead. The output of @option{-t} or @option{--table} is a
16004 table of signal numbers, names, and descriptions. This form of the
16005 @command{kill} command succeeds if all @var{signal} arguments are valid
16006 and if there is no output error.
16008 The @command{kill} command also supports the @option{--help} and
16009 @option{--version} options. @xref{Common options}.
16011 A @var{signal} may be a signal name like @samp{HUP}, or a signal
16012 number like @samp{1}, or an exit status of a process terminated by the
16013 signal. A signal name can be given in canonical form or prefixed by
16014 @samp{SIG}. The case of the letters is ignored, except for the
16015 @option{-@var{signal}} option which must use upper case to avoid
16016 ambiguity with lower case option letters. For a list of supported
16017 signal names and numbers see @xref{Signal specifications}.
16022 @cindex delaying commands
16023 @cindex commands for delaying
16025 @c Perhaps @command{wait} or other commands should be described here also?
16028 * sleep invocation:: Delay for a specified time.
16032 @node sleep invocation
16033 @section @command{sleep}: Delay for a specified time
16036 @cindex delay for a specified time
16038 @command{sleep} pauses for an amount of time specified by the sum of
16039 the values of the command line arguments.
16043 sleep @var{number}[smhd]@dots{}
16047 Each argument is a number followed by an optional unit; the default
16048 is seconds. The units are:
16061 Historical implementations of @command{sleep} have required that
16062 @var{number} be an integer, and only accepted a single argument
16063 without a suffix. However, GNU @command{sleep} accepts
16064 arbitrary floating point numbers. @xref{Floating point}.
16066 The only options are @option{--help} and @option{--version}. @xref{Common
16069 @c sleep is a shell built-in at least with Solaris 11's /bin/sh
16070 @mayConflictWithShellBuiltIn{sleep}
16075 @node Numeric operations
16076 @chapter Numeric operations
16078 @cindex numeric operations
16079 These programs do numerically-related operations.
16082 * factor invocation:: Show factors of numbers.
16083 * seq invocation:: Print sequences of numbers.
16087 @node factor invocation
16088 @section @command{factor}: Print prime factors
16091 @cindex prime factors
16093 @command{factor} prints prime factors. Synopses:
16096 factor [@var{number}]@dots{}
16097 factor @var{option}
16100 If no @var{number} is specified on the command line, @command{factor} reads
16101 numbers from standard input, delimited by newlines, tabs, or spaces.
16103 The @command{factor} command supports only a small number of options:
16107 Print a short help on standard output, then exit without further
16111 Print the program version on standard output, then exit without further
16115 Factoring the product of the eighth and ninth Mersenne primes
16116 takes about 30 milliseconds of CPU time on a 2.2 GHz Athlon.
16119 M8=$(echo 2^31-1|bc)
16120 M9=$(echo 2^61-1|bc)
16121 n=$(echo "$M8 * $M9" | bc)
16122 /usr/bin/time -f %U factor $n
16123 4951760154835678088235319297: 2147483647 2305843009213693951
16127 Similarly, factoring the eighth Fermat number @math{2^{256}+1} takes
16128 about 20 seconds on the same machine.
16130 Factoring large numbers is, in general, hard. The Pollard Rho
16131 algorithm used by @command{factor} is particularly effective for
16132 numbers with relatively small factors. If you wish to factor large
16133 numbers which do not have small factors (for example, numbers which
16134 are the product of two large primes), other methods are far better.
16136 If @command{factor} is built without using GNU MP, only
16137 single-precision arithmetic is available, and so large numbers
16138 (typically @math{2^{64}} and above) will not be supported. The single-precision
16139 code uses an algorithm which is designed for factoring smaller
16145 @node seq invocation
16146 @section @command{seq}: Print numeric sequences
16149 @cindex numeric sequences
16150 @cindex sequence of numbers
16152 @command{seq} prints a sequence of numbers to standard output. Synopses:
16155 seq [@var{option}]@dots{} @var{last}
16156 seq [@var{option}]@dots{} @var{first} @var{last}
16157 seq [@var{option}]@dots{} @var{first} @var{increment} @var{last}
16160 @command{seq} prints the numbers from @var{first} to @var{last} by
16161 @var{increment}. By default, each number is printed on a separate line.
16162 When @var{increment} is not specified, it defaults to @samp{1},
16163 even when @var{first} is larger than @var{last}.
16164 @var{first} also defaults to @samp{1}. So @code{seq 1} prints
16165 @samp{1}, but @code{seq 0} and @code{seq 10 5} produce no output.
16166 Floating-point numbers may be specified. @xref{Floating point}.
16168 The program accepts the following options. Also see @ref{Common options}.
16169 Options must precede operands.
16172 @item -f @var{format}
16173 @itemx --format=@var{format}
16174 @opindex -f @var{format}
16175 @opindex --format=@var{format}
16176 @cindex formatting of numbers in @command{seq}
16177 Print all numbers using @var{format}.
16178 @var{format} must contain exactly one of the @samp{printf}-style
16179 floating point conversion specifications @samp{%a}, @samp{%e},
16180 @samp{%f}, @samp{%g}, @samp{%A}, @samp{%E}, @samp{%F}, @samp{%G}.
16181 The @samp{%} may be followed by zero or more flags taken from the set
16182 @samp{-+#0 '}, then an optional width containing one or more digits,
16183 then an optional precision consisting of a @samp{.} followed by zero
16184 or more digits. @var{format} may also contain any number of @samp{%%}
16185 conversion specifications. All conversion specifications have the
16186 same meaning as with @samp{printf}.
16188 The default format is derived from @var{first}, @var{step}, and
16189 @var{last}. If these all use a fixed point decimal representation,
16190 the default format is @samp{%.@var{p}f}, where @var{p} is the minimum
16191 precision that can represent the output numbers exactly. Otherwise,
16192 the default format is @samp{%g}.
16194 @item -s @var{string}
16195 @itemx --separator=@var{string}
16196 @cindex separator for numbers in @command{seq}
16197 Separate numbers with @var{string}; default is a newline.
16198 The output always terminates with a newline.
16201 @itemx --equal-width
16202 Print all numbers with the same width, by padding with leading zeros.
16203 @var{first}, @var{step}, and @var{last} should all use a fixed point
16204 decimal representation.
16205 (To have other kinds of padding, use @option{--format}).
16209 You can get finer-grained control over output with @option{-f}:
16212 $ seq -f '(%9.2E)' -9e5 1.1e6 1.3e6
16218 If you want hexadecimal integer output, you can use @command{printf}
16219 to perform the conversion:
16222 $ printf '%x\n' `seq 1048575 1024 1050623`
16228 For very long lists of numbers, use xargs to avoid
16229 system limitations on the length of an argument list:
16232 $ seq 1000000 | xargs printf '%x\n' | tail -n 3
16238 To generate octal output, use the printf @code{%o} format instead
16241 On most systems, seq can produce whole-number output for values up to
16242 at least @math{2^{53}}. Larger integers are approximated. The details
16243 differ depending on your floating-point implementation.
16244 @xref{Floating point}. A common
16245 case is that @command{seq} works with integers through @math{2^{64}},
16246 and larger integers may not be numerically correct:
16249 $ seq 18446744073709551616 1 18446744073709551618
16250 18446744073709551616
16251 18446744073709551616
16252 18446744073709551618
16255 Be careful when using @command{seq} with outlandish values: otherwise
16256 you may see surprising results, as @command{seq} uses floating point
16257 internally. For example, on the x86 platform, where the internal
16258 representation uses a 64-bit fraction, the command:
16261 seq 1 0.0000000000000000001 1.0000000000000000009
16264 outputs 1.0000000000000000007 twice and skips 1.0000000000000000008.
16269 @node File permissions
16270 @chapter File permissions
16273 @include parse-datetime.texi
16277 @node Opening the software toolbox
16278 @chapter Opening the Software Toolbox
16280 An earlier version of this chapter appeared in
16281 @uref{http://www.linuxjournal.com/article.php?sid=2762, the
16282 @cite{What's GNU?} column of the June 1994 @cite{Linux Journal}}.
16283 It was written by Arnold Robbins.
16286 * Toolbox introduction:: Toolbox introduction
16287 * I/O redirection:: I/O redirection
16288 * The who command:: The @command{who} command
16289 * The cut command:: The @command{cut} command
16290 * The sort command:: The @command{sort} command
16291 * The uniq command:: The @command{uniq} command
16292 * Putting the tools together:: Putting the tools together
16296 @node Toolbox introduction
16297 @unnumberedsec Toolbox Introduction
16299 This month's column is only peripherally related to the GNU Project, in
16300 that it describes a number of the GNU tools on your GNU/Linux system
16302 might be used. What it's really about is the ``Software Tools'' philosophy
16303 of program development and usage.
16305 The software tools philosophy was an important and integral concept
16306 in the initial design and development of Unix (of which Linux and GNU are
16307 essentially clones). Unfortunately, in the modern day press of
16308 Internetworking and flashy GUIs, it seems to have fallen by the
16309 wayside. This is a shame, since it provides a powerful mental model
16310 for solving many kinds of problems.
16312 Many people carry a Swiss Army knife around in their pants pockets (or
16313 purse). A Swiss Army knife is a handy tool to have: it has several knife
16314 blades, a screwdriver, tweezers, toothpick, nail file, corkscrew, and perhaps
16315 a number of other things on it. For the everyday, small miscellaneous jobs
16316 where you need a simple, general purpose tool, it's just the thing.
16318 On the other hand, an experienced carpenter doesn't build a house using
16319 a Swiss Army knife. Instead, he has a toolbox chock full of specialized
16320 tools---a saw, a hammer, a screwdriver, a plane, and so on. And he knows
16321 exactly when and where to use each tool; you won't catch him hammering nails
16322 with the handle of his screwdriver.
16324 The Unix developers at Bell Labs were all professional programmers and trained
16325 computer scientists. They had found that while a one-size-fits-all program
16326 might appeal to a user because there's only one program to use, in practice
16331 difficult to write,
16334 difficult to maintain and
16338 difficult to extend to meet new situations.
16341 Instead, they felt that programs should be specialized tools. In short, each
16342 program ``should do one thing well.'' No more and no less. Such programs are
16343 simpler to design, write, and get right---they only do one thing.
16345 Furthermore, they found that with the right machinery for hooking programs
16346 together, that the whole was greater than the sum of the parts. By combining
16347 several special purpose programs, you could accomplish a specific task
16348 that none of the programs was designed for, and accomplish it much more
16349 quickly and easily than if you had to write a special purpose program.
16350 We will see some (classic) examples of this further on in the column.
16351 (An important additional point was that, if necessary, take a detour
16352 and build any software tools you may need first, if you don't already
16353 have something appropriate in the toolbox.)
16355 @node I/O redirection
16356 @unnumberedsec I/O Redirection
16358 Hopefully, you are familiar with the basics of I/O redirection in the
16359 shell, in particular the concepts of ``standard input,'' ``standard output,''
16360 and ``standard error''. Briefly, ``standard input'' is a data source, where
16361 data comes from. A program should not need to either know or care if the
16362 data source is a disk file, a keyboard, a magnetic tape, or even a punched
16363 card reader. Similarly, ``standard output'' is a data sink, where data goes
16364 to. The program should neither know nor care where this might be.
16365 Programs that only read their standard input, do something to the data,
16366 and then send it on, are called @dfn{filters}, by analogy to filters in a
16369 With the Unix shell, it's very easy to set up data pipelines:
16372 program_to_create_data | filter1 | ... | filterN > final.pretty.data
16375 We start out by creating the raw data; each filter applies some successive
16376 transformation to the data, until by the time it comes out of the pipeline,
16377 it is in the desired form.
16379 This is fine and good for standard input and standard output. Where does the
16380 standard error come in to play? Well, think about @command{filter1} in
16381 the pipeline above. What happens if it encounters an error in the data it
16382 sees? If it writes an error message to standard output, it will just
16383 disappear down the pipeline into @command{filter2}'s input, and the
16384 user will probably never see it. So programs need a place where they can send
16385 error messages so that the user will notice them. This is standard error,
16386 and it is usually connected to your console or window, even if you have
16387 redirected standard output of your program away from your screen.
16389 For filter programs to work together, the format of the data has to be
16390 agreed upon. The most straightforward and easiest format to use is simply
16391 lines of text. Unix data files are generally just streams of bytes, with
16392 lines delimited by the @acronym{ASCII} @sc{lf} (Line Feed) character,
16393 conventionally called a ``newline'' in the Unix literature. (This is
16394 @code{'\n'} if you're a C programmer.) This is the format used by all
16395 the traditional filtering programs. (Many earlier operating systems
16396 had elaborate facilities and special purpose programs for managing
16397 binary data. Unix has always shied away from such things, under the
16398 philosophy that it's easiest to simply be able to view and edit your
16399 data with a text editor.)
16401 OK, enough introduction. Let's take a look at some of the tools, and then
16402 we'll see how to hook them together in interesting ways. In the following
16403 discussion, we will only present those command line options that interest
16404 us. As you should always do, double check your system documentation
16405 for the full story.
16407 @node The who command
16408 @unnumberedsec The @command{who} Command
16410 The first program is the @command{who} command. By itself, it generates a
16411 list of the users who are currently logged in. Although I'm writing
16412 this on a single-user system, we'll pretend that several people are
16417 @print{} arnold console Jan 22 19:57
16418 @print{} miriam ttyp0 Jan 23 14:19(:0.0)
16419 @print{} bill ttyp1 Jan 21 09:32(:0.0)
16420 @print{} arnold ttyp2 Jan 23 20:48(:0.0)
16423 Here, the @samp{$} is the usual shell prompt, at which I typed @samp{who}.
16424 There are three people logged in, and I am logged in twice. On traditional
16425 Unix systems, user names are never more than eight characters long. This
16426 little bit of trivia will be useful later. The output of @command{who} is nice,
16427 but the data is not all that exciting.
16429 @node The cut command
16430 @unnumberedsec The @command{cut} Command
16432 The next program we'll look at is the @command{cut} command. This program
16433 cuts out columns or fields of input data. For example, we can tell it
16434 to print just the login name and full name from the @file{/etc/passwd}
16435 file. The @file{/etc/passwd} file has seven fields, separated by
16439 arnold:xyzzy:2076:10:Arnold D. Robbins:/home/arnold:/bin/bash
16442 To get the first and fifth fields, we would use @command{cut} like this:
16445 $ cut -d: -f1,5 /etc/passwd
16446 @print{} root:Operator
16448 @print{} arnold:Arnold D. Robbins
16449 @print{} miriam:Miriam A. Robbins
16453 With the @option{-c} option, @command{cut} will cut out specific characters
16454 (i.e., columns) in the input lines. This is useful for input data
16455 that has fixed width fields, and does not have a field separator. For
16456 example, list the Monday dates for the current month:
16458 @c Is using cal ok? Looked at gcal, but I don't like it.
16469 @node The sort command
16470 @unnumberedsec The @command{sort} Command
16472 Next we'll look at the @command{sort} command. This is one of the most
16473 powerful commands on a Unix-style system; one that you will often find
16474 yourself using when setting up fancy data plumbing.
16477 command reads and sorts each file named on the command line. It then
16478 merges the sorted data and writes it to standard output. It will read
16479 standard input if no files are given on the command line (thus
16480 making it into a filter). The sort is based on the character collating
16481 sequence or based on user-supplied ordering criteria.
16484 @node The uniq command
16485 @unnumberedsec The @command{uniq} Command
16487 Finally (at least for now), we'll look at the @command{uniq} program. When
16488 sorting data, you will often end up with duplicate lines, lines that
16489 are identical. Usually, all you need is one instance of each line.
16490 This is where @command{uniq} comes in. The @command{uniq} program reads its
16491 standard input. It prints only one
16492 copy of each repeated line. It does have several options. Later on,
16493 we'll use the @option{-c} option, which prints each unique line, preceded
16494 by a count of the number of times that line occurred in the input.
16497 @node Putting the tools together
16498 @unnumberedsec Putting the Tools Together
16500 Now, let's suppose this is a large ISP server system with dozens of users
16501 logged in. The management wants the system administrator to write a
16503 generate a sorted list of logged in users. Furthermore, even if a user
16504 is logged in multiple times, his or her name should only show up in the
16507 The administrator could sit down with the system documentation and write a C
16508 program that did this. It would take perhaps a couple of hundred lines
16509 of code and about two hours to write it, test it, and debug it.
16510 However, knowing the software toolbox, the administrator can instead start out
16511 by generating just a list of logged on users:
16521 Next, sort the list:
16524 $ who | cut -c1-8 | sort
16531 Finally, run the sorted list through @command{uniq}, to weed out duplicates:
16534 $ who | cut -c1-8 | sort | uniq
16540 The @command{sort} command actually has a @option{-u} option that does what
16541 @command{uniq} does. However, @command{uniq} has other uses for which one
16542 cannot substitute @samp{sort -u}.
16544 The administrator puts this pipeline into a shell script, and makes it
16546 all the users on the system (@samp{#} is the system administrator,
16547 or @code{root}, prompt):
16550 # cat > /usr/local/bin/listusers
16551 who | cut -c1-8 | sort | uniq
16553 # chmod +x /usr/local/bin/listusers
16556 There are four major points to note here. First, with just four
16557 programs, on one command line, the administrator was able to save about two
16558 hours worth of work. Furthermore, the shell pipeline is just about as
16559 efficient as the C program would be, and it is much more efficient in
16560 terms of programmer time. People time is much more expensive than
16561 computer time, and in our modern ``there's never enough time to do
16562 everything'' society, saving two hours of programmer time is no mean
16565 Second, it is also important to emphasize that with the
16566 @emph{combination} of the tools, it is possible to do a special
16567 purpose job never imagined by the authors of the individual programs.
16569 Third, it is also valuable to build up your pipeline in stages, as we did here.
16570 This allows you to view the data at each stage in the pipeline, which helps
16571 you acquire the confidence that you are indeed using these tools correctly.
16573 Finally, by bundling the pipeline in a shell script, other users can use
16574 your command, without having to remember the fancy plumbing you set up for
16575 them. In terms of how you run them, shell scripts and compiled programs are
16578 After the previous warm-up exercise, we'll look at two additional, more
16579 complicated pipelines. For them, we need to introduce two more tools.
16581 The first is the @command{tr} command, which stands for ``transliterate.''
16582 The @command{tr} command works on a character-by-character basis, changing
16583 characters. Normally it is used for things like mapping upper case to
16587 $ echo ThIs ExAmPlE HaS MIXED case! | tr '[:upper:]' '[:lower:]'
16588 @print{} this example has mixed case!
16591 There are several options of interest:
16595 work on the complement of the listed characters, i.e.,
16596 operations apply to characters not in the given set
16599 delete characters in the first set from the output
16602 squeeze repeated characters in the output into just one character.
16605 We will be using all three options in a moment.
16607 The other command we'll look at is @command{comm}. The @command{comm}
16608 command takes two sorted input files as input data, and prints out the
16609 files' lines in three columns. The output columns are the data lines
16610 unique to the first file, the data lines unique to the second file, and
16611 the data lines that are common to both. The @option{-1}, @option{-2}, and
16612 @option{-3} command line options @emph{omit} the respective columns. (This is
16613 non-intuitive and takes a little getting used to.) For example:
16635 The file name @file{-} tells @command{comm} to read standard input
16636 instead of a regular file.
16638 Now we're ready to build a fancy pipeline. The first application is a word
16639 frequency counter. This helps an author determine if he or she is over-using
16642 The first step is to change the case of all the letters in our input file
16643 to one case. ``The'' and ``the'' are the same word when doing counting.
16646 $ tr '[:upper:]' '[:lower:]' < whats.gnu | ...
16649 The next step is to get rid of punctuation. Quoted words and unquoted words
16650 should be treated identically; it's easiest to just get the punctuation out of
16654 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | ...
16657 The second @command{tr} command operates on the complement of the listed
16658 characters, which are all the letters, the digits, the underscore, and
16659 the blank. The @samp{\n} represents the newline character; it has to
16660 be left alone. (The @acronym{ASCII} tab character should also be included for
16661 good measure in a production script.)
16663 At this point, we have data consisting of words separated by blank space.
16664 The words only contain alphanumeric characters (and the underscore). The
16665 next step is break the data apart so that we have one word per line. This
16666 makes the counting operation much easier, as we will see shortly.
16669 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16670 > tr -s ' ' '\n' | ...
16673 This command turns blanks into newlines. The @option{-s} option squeezes
16674 multiple newline characters in the output into just one. This helps us
16675 avoid blank lines. (The @samp{>} is the shell's ``secondary prompt.''
16676 This is what the shell prints when it notices you haven't finished
16677 typing in all of a command.)
16679 We now have data consisting of one word per line, no punctuation, all one
16680 case. We're ready to count each word:
16683 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16684 > tr -s ' ' '\n' | sort | uniq -c | ...
16687 At this point, the data might look something like this:
16700 The output is sorted by word, not by count! What we want is the most
16701 frequently used words first. Fortunately, this is easy to accomplish,
16702 with the help of two more @command{sort} options:
16706 do a numeric sort, not a textual one
16709 reverse the order of the sort
16712 The final pipeline looks like this:
16715 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16716 > tr -s ' ' '\n' | sort | uniq -c | sort -n -r
16725 Whew! That's a lot to digest. Yet, the same principles apply. With six
16726 commands, on two lines (really one long one split for convenience), we've
16727 created a program that does something interesting and useful, in much
16728 less time than we could have written a C program to do the same thing.
16730 A minor modification to the above pipeline can give us a simple spelling
16731 checker! To determine if you've spelled a word correctly, all you have to
16732 do is look it up in a dictionary. If it is not there, then chances are
16733 that your spelling is incorrect. So, we need a dictionary.
16734 The conventional location for a dictionary is @file{/usr/dict/words}.
16735 On my GNU/Linux system,@footnote{Redhat Linux 6.1, for the November 2000
16736 revision of this article.}
16737 this is a sorted, 45,402 word dictionary.
16739 Now, how to compare our file with the dictionary? As before, we generate
16740 a sorted list of words, one per line:
16743 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16744 > tr -s ' ' '\n' | sort -u | ...
16747 Now, all we need is a list of words that are @emph{not} in the
16748 dictionary. Here is where the @command{comm} command comes in.
16751 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16752 > tr -s ' ' '\n' | sort -u |
16753 > comm -23 - /usr/dict/words
16756 The @option{-2} and @option{-3} options eliminate lines that are only in the
16757 dictionary (the second file), and lines that are in both files. Lines
16758 only in the first file (standard input, our stream of words), are
16759 words that are not in the dictionary. These are likely candidates for
16760 spelling errors. This pipeline was the first cut at a production
16761 spelling checker on Unix.
16763 There are some other tools that deserve brief mention.
16767 search files for text that matches a regular expression
16770 count lines, words, characters
16773 a T-fitting for data pipes, copies data to files and to standard output
16776 the stream editor, an advanced tool
16779 a data manipulation language, another advanced tool
16782 The software tools philosophy also espoused the following bit of
16783 advice: ``Let someone else do the hard part.'' This means, take
16784 something that gives you most of what you need, and then massage it the
16785 rest of the way until it's in the form that you want.
16791 Each program should do one thing well. No more, no less.
16794 Combining programs with appropriate plumbing leads to results where
16795 the whole is greater than the sum of the parts. It also leads to novel
16796 uses of programs that the authors might never have imagined.
16799 Programs should never print extraneous header or trailer data, since these
16800 could get sent on down a pipeline. (A point we didn't mention earlier.)
16803 Let someone else do the hard part.
16806 Know your toolbox! Use each program appropriately. If you don't have an
16807 appropriate tool, build one.
16810 As of this writing, all the programs we've discussed are available via
16811 anonymous @command{ftp} from: @*
16812 @uref{ftp://gnudist.gnu.org/textutils/textutils-1.22.tar.gz}. (There may
16813 be more recent versions available now.)
16815 None of what I have presented in this column is new. The Software Tools
16816 philosophy was first introduced in the book @cite{Software Tools}, by
16817 Brian Kernighan and P.J. Plauger (Addison-Wesley, ISBN 0-201-03669-X).
16818 This book showed how to write and use software tools. It was written in
16819 1976, using a preprocessor for FORTRAN named @command{ratfor} (RATional
16820 FORtran). At the time, C was not as ubiquitous as it is now; FORTRAN
16821 was. The last chapter presented a @command{ratfor} to FORTRAN
16822 processor, written in @command{ratfor}. @command{ratfor} looks an awful
16823 lot like C; if you know C, you won't have any problem following the
16826 In 1981, the book was updated and made available as @cite{Software Tools
16827 in Pascal} (Addison-Wesley, ISBN 0-201-10342-7). Both books are
16828 still in print and are well worth
16829 reading if you're a programmer. They certainly made a major change in
16830 how I view programming.
16832 The programs in both books are available from
16833 @uref{http://cm.bell-labs.com/who/bwk, Brian Kernighan's home page}.
16834 For a number of years, there was an active
16835 Software Tools Users Group, whose members had ported the original
16836 @command{ratfor} programs to essentially every computer system with a
16837 FORTRAN compiler. The popularity of the group waned in the middle 1980s
16838 as Unix began to spread beyond universities.
16840 With the current proliferation of GNU code and other clones of Unix programs,
16841 these programs now receive little attention; modern C versions are
16842 much more efficient and do more than these programs do. Nevertheless, as
16843 exposition of good programming style, and evangelism for a still-valuable
16844 philosophy, these books are unparalleled, and I recommend them highly.
16846 Acknowledgment: I would like to express my gratitude to Brian Kernighan
16847 of Bell Labs, the original Software Toolsmith, for reviewing this column.
16849 @node GNU Free Documentation License
16850 @appendix GNU Free Documentation License
16854 @node Concept index
16861 @c Local variables:
16862 @c texinfo-column-for-description: 32