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{} 1994-1996, 2000-2012 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{ }"}). @option{--sep-string} with no
2532 @samp{=@var{string}} is equivalent to @option{--sep-string=""}.
2535 @itemx --omit-header
2537 @opindex --omit-header
2538 Do not print the usual header [and footer] on each page, and do not fill
2539 out the bottom of pages (with blank lines or a form feed). No page
2540 structure is produced, but form feeds set in the input files are retained.
2541 The predefined pagination is not changed. @option{-t} or @option{-T} may be
2542 useful together with other options; e.g.: @option{-t -e4}, expand TAB characters
2543 in the input file to 4 spaces but don't make any other changes. Use of
2544 @option{-t} overrides @option{-h}.
2547 @itemx --omit-pagination
2549 @opindex --omit-pagination
2550 Do not print header [and footer]. In addition eliminate all form feeds
2551 set in the input files.
2554 @itemx --show-nonprinting
2556 @opindex --show-nonprinting
2557 Print nonprinting characters in octal backslash notation.
2559 @item -w @var{page_width}
2560 @itemx --width=@var{page_width}
2563 Set page width to @var{page_width} characters for multiple text-column
2564 output only (default for @var{page_width} is 72). @option{-s[CHAR]} turns
2565 off the default page width and any line truncation and column alignment.
2566 Lines of full length are merged, regardless of the column options
2567 set. No @var{page_width} setting is possible with single column output.
2568 A @acronym{POSIX}-compliant formulation.
2570 @item -W @var{page_width}
2571 @itemx --page_width=@var{page_width}
2573 @opindex --page_width
2574 Set the page width to @var{page_width} characters. That's valid with and
2575 without a column option. Text lines are truncated, unless @option{-J}
2576 is used. Together with one of the three column options
2577 (@option{-@var{column}}, @option{-a -@var{column}} or @option{-m}) column
2578 alignment is always used. The separator options @option{-S} or @option{-s}
2579 don't affect the @option{-W} option. Default is 72 characters. Without
2580 @option{-W @var{page_width}} and without any of the column options NO line
2581 truncation is used (defined to keep downward compatibility and to meet
2582 most frequent tasks). That's equivalent to @option{-W 72 -J}. The header
2583 line is never truncated.
2590 @node fold invocation
2591 @section @command{fold}: Wrap input lines to fit in specified width
2594 @cindex wrapping long input lines
2595 @cindex folding long input lines
2597 @command{fold} writes each @var{file} (@option{-} means standard input), or
2598 standard input if none are given, to standard output, breaking long
2602 fold [@var{option}]@dots{} [@var{file}]@dots{}
2605 By default, @command{fold} breaks lines wider than 80 columns. The output
2606 is split into as many lines as necessary.
2608 @cindex screen columns
2609 @command{fold} counts screen columns by default; thus, a tab may count more
2610 than one column, backspace decreases the column count, and carriage
2611 return sets the column to zero.
2613 The program accepts the following options. Also see @ref{Common options}.
2621 Count bytes rather than columns, so that tabs, backspaces, and carriage
2622 returns are each counted as taking up one column, just like other
2629 Break at word boundaries: the line is broken after the last blank before
2630 the maximum line length. If the line contains no such blanks, the line
2631 is broken at the maximum line length as usual.
2633 @item -w @var{width}
2634 @itemx --width=@var{width}
2637 Use a maximum line length of @var{width} columns instead of 80.
2639 For compatibility @command{fold} supports an obsolete option syntax
2640 @option{-@var{width}}. New scripts should use @option{-w @var{width}}
2648 @node Output of parts of files
2649 @chapter Output of parts of files
2651 @cindex output of parts of files
2652 @cindex parts of files, output of
2654 These commands output pieces of the input.
2657 * head invocation:: Output the first part of files.
2658 * tail invocation:: Output the last part of files.
2659 * split invocation:: Split a file into pieces.
2660 * csplit invocation:: Split a file into context-determined pieces.
2663 @node head invocation
2664 @section @command{head}: Output the first part of files
2667 @cindex initial part of files, outputting
2668 @cindex first part of files, outputting
2670 @command{head} prints the first part (10 lines by default) of each
2671 @var{file}; it reads from standard input if no files are given or
2672 when given a @var{file} of @option{-}. Synopsis:
2675 head [@var{option}]@dots{} [@var{file}]@dots{}
2678 If more than one @var{file} is specified, @command{head} prints a
2679 one-line header consisting of:
2682 ==> @var{file name} <==
2686 before the output for each @var{file}.
2688 The program accepts the following options. Also see @ref{Common options}.
2693 @itemx --bytes=@var{k}
2696 Print the first @var{k} bytes, instead of initial lines.
2697 However, if @var{k} starts with a @samp{-},
2698 print all but the last @var{k} bytes of each file.
2699 @multiplierSuffixes{k}
2702 @itemx --lines=@var{k}
2705 Output the first @var{k} lines.
2706 However, if @var{k} starts with a @samp{-},
2707 print all but the last @var{k} lines of each file.
2708 Size multiplier suffixes are the same as with the @option{-c} option.
2716 Never print file name headers.
2722 Always print file name headers.
2726 For compatibility @command{head} also supports an obsolete option syntax
2727 @option{-@var{count}@var{options}}, which is recognized only if it is
2728 specified first. @var{count} is a decimal number optionally followed
2729 by a size letter (@samp{b}, @samp{k}, @samp{m}) as in @option{-c}, or
2730 @samp{l} to mean count by lines, or other option letters (@samp{cqv}).
2731 Scripts intended for standard hosts should use @option{-c @var{count}}
2732 or @option{-n @var{count}} instead. If your script must also run on
2733 hosts that support only the obsolete syntax, it is usually simpler to
2734 avoid @command{head}, e.g., by using @samp{sed 5q} instead of
2740 @node tail invocation
2741 @section @command{tail}: Output the last part of files
2744 @cindex last part of files, outputting
2746 @command{tail} prints the last part (10 lines by default) of each
2747 @var{file}; it reads from standard input if no files are given or
2748 when given a @var{file} of @samp{-}. Synopsis:
2751 tail [@var{option}]@dots{} [@var{file}]@dots{}
2754 If more than one @var{file} is specified, @command{tail} prints a
2755 one-line header consisting of:
2758 ==> @var{file name} <==
2762 before the output for each @var{file}.
2764 @cindex BSD @command{tail}
2765 @sc{gnu} @command{tail} can output any amount of data (some other versions of
2766 @command{tail} cannot). It also has no @option{-r} option (print in
2767 reverse), since reversing a file is really a different job from printing
2768 the end of a file; BSD @command{tail} (which is the one with @option{-r}) can
2769 only reverse files that are at most as large as its buffer, which is
2770 typically 32 KiB@. A more reliable and versatile way to reverse files is
2771 the @sc{gnu} @command{tac} command.
2773 The program accepts the following options. Also see @ref{Common options}.
2778 @itemx --bytes=@var{k}
2781 Output the last @var{k} bytes, instead of final lines.
2782 However, if @var{k} starts with a @samp{+}, start printing with the
2783 @var{k}th byte from the start of each file, instead of from the end.
2784 @multiplierSuffixes{k}
2787 @itemx --follow[=@var{how}]
2790 @cindex growing files
2791 @vindex name @r{follow option}
2792 @vindex descriptor @r{follow option}
2793 Loop forever trying to read more characters at the end of the file,
2794 presumably because the file is growing.
2795 If more than one file is given, @command{tail} prints a header whenever it
2796 gets output from a different file, to indicate which file that output is
2799 There are two ways to specify how you'd like to track files with this option,
2800 but that difference is noticeable only when a followed file is removed or
2802 If you'd like to continue to track the end of a growing file even after
2803 it has been unlinked, use @option{--follow=descriptor}. This is the default
2804 behavior, but it is not useful if you're tracking a log file that may be
2805 rotated (removed or renamed, then reopened). In that case, use
2806 @option{--follow=name} to track the named file, perhaps by reopening it
2807 periodically to see if it has been removed and recreated by some other program.
2808 Note that the inotify-based implementation handles this case without
2809 the need for any periodic reopening.
2811 No matter which method you use, if the tracked file is determined to have
2812 shrunk, @command{tail} prints a message saying the file has been truncated
2813 and resumes tracking the end of the file from the newly-determined endpoint.
2815 When a file is removed, @command{tail}'s behavior depends on whether it is
2816 following the name or the descriptor. When following by name, tail can
2817 detect that a file has been removed and gives a message to that effect,
2818 and if @option{--retry} has been specified it will continue checking
2819 periodically to see if the file reappears.
2820 When following a descriptor, tail does not detect that the file has
2821 been unlinked or renamed and issues no message; even though the file
2822 may no longer be accessible via its original name, it may still be
2825 The option values @samp{descriptor} and @samp{name} may be specified only
2826 with the long form of the option, not with @option{-f}.
2828 The @option{-f} option is ignored if
2829 no @var{file} operand is specified and standard input is a FIFO or a pipe.
2830 Likewise, the @option{-f} option has no effect for any
2831 operand specified as @samp{-}, when standard input is a FIFO or a pipe.
2833 With kernel inotify support, output is triggered by file changes
2834 and is generally very prompt.
2835 Otherwise, @command{tail} sleeps for one second between checks---
2836 use @option{--sleep-interval=@var{n}} to change that default---which can
2837 make the output appear slightly less responsive or bursty.
2838 When using tail without inotify support, you can make it more responsive
2839 by using a sub-second sleep interval, e.g., via an alias like this:
2842 alias tail='tail -s.1'
2847 This option is the same as @option{--follow=name --retry}. That is, tail
2848 will attempt to reopen a file when it is removed. Should this fail, tail
2849 will keep trying until it becomes accessible again.
2853 This option is useful mainly when following by name (i.e., with
2854 @option{--follow=name}).
2855 Without this option, when tail encounters a file that doesn't
2856 exist or is otherwise inaccessible, it reports that fact and
2857 never checks it again.
2859 @itemx --sleep-interval=@var{number}
2860 @opindex --sleep-interval
2861 Change the number of seconds to wait between iterations (the default is 1.0).
2862 During one iteration, every specified file is checked to see if it has
2864 Historical implementations of @command{tail} have required that
2865 @var{number} be an integer. However, GNU @command{tail} accepts
2866 an arbitrary floating point number. @xref{Floating point}.
2867 When @command{tail} uses inotify, this polling-related option
2868 is usually ignored. However, if you also specify @option{--pid=@var{p}},
2869 @command{tail} checks whether process @var{p} is alive at least
2870 every @var{number} seconds.
2872 @itemx --pid=@var{pid}
2874 When following by name or by descriptor, you may specify the process ID,
2875 @var{pid}, of the sole writer of all @var{file} arguments. Then, shortly
2876 after that process terminates, tail will also terminate. This will
2877 work properly only if the writer and the tailing process are running on
2878 the same machine. For example, to save the output of a build in a file
2879 and to watch the file grow, if you invoke @command{make} and @command{tail}
2880 like this then the tail process will stop when your build completes.
2881 Without this option, you would have had to kill the @code{tail -f}
2885 $ make >& makerr & tail --pid=$! -f makerr
2888 If you specify a @var{pid} that is not in use or that does not correspond
2889 to the process that is writing to the tailed files, then @command{tail}
2890 may terminate long before any @var{file}s stop growing or it may not
2891 terminate until long after the real writer has terminated.
2892 Note that @option{--pid} cannot be supported on some systems; @command{tail}
2893 will print a warning if this is the case.
2895 @itemx --max-unchanged-stats=@var{n}
2896 @opindex --max-unchanged-stats
2897 When tailing a file by name, if there have been @var{n} (default
2898 n=@value{DEFAULT_MAX_N_UNCHANGED_STATS_BETWEEN_OPENS}) consecutive
2899 iterations for which the file has not changed, then
2900 @code{open}/@code{fstat} the file to determine if that file name is
2901 still associated with the same device/inode-number pair as before.
2902 When following a log file that is rotated, this is approximately the
2903 number of seconds between when tail prints the last pre-rotation lines
2904 and when it prints the lines that have accumulated in the new log file.
2905 This option is meaningful only when polling (i.e., without inotify)
2906 and when following by name.
2909 @itemx --lines=@var{k}
2912 Output the last @var{k} lines.
2913 However, if @var{k} starts with a @samp{+}, start printing with the
2914 @var{k}th line from the start of each file, instead of from the end.
2915 Size multiplier suffixes are the same as with the @option{-c} option.
2923 Never print file name headers.
2929 Always print file name headers.
2933 For compatibility @command{tail} also supports an obsolete usage
2934 @samp{tail -[@var{count}][bcl][f] [@var{file}]}, which is recognized
2935 only if it does not conflict with the usage described
2936 above. This obsolete form uses exactly one option and at most one
2937 file. In the option, @var{count} is an optional decimal number optionally
2938 followed by a size letter (@samp{b}, @samp{c}, @samp{l}) to mean count
2939 by 512-byte blocks, bytes, or lines, optionally followed by @samp{f}
2940 which has the same meaning as @option{-f}.
2942 @vindex _POSIX2_VERSION
2943 On older systems, the leading @samp{-} can be replaced by @samp{+} in
2944 the obsolete option syntax with the same meaning as in counts, and
2945 obsolete usage overrides normal usage when the two conflict.
2946 This obsolete behavior can be enabled or disabled with the
2947 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
2950 Scripts intended for use on standard hosts should avoid obsolete
2951 syntax and should use @option{-c @var{count}[b]}, @option{-n
2952 @var{count}}, and/or @option{-f} instead. If your script must also
2953 run on hosts that support only the obsolete syntax, you can often
2954 rewrite it to avoid problematic usages, e.g., by using @samp{sed -n
2955 '$p'} rather than @samp{tail -1}. If that's not possible, the script
2956 can use a test like @samp{if tail -c +1 </dev/null >/dev/null 2>&1;
2957 then @dots{}} to decide which syntax to use.
2959 Even if your script assumes the standard behavior, you should still
2960 beware usages whose behaviors differ depending on the @acronym{POSIX}
2961 version. For example, avoid @samp{tail - main.c}, since it might be
2962 interpreted as either @samp{tail main.c} or as @samp{tail -- -
2963 main.c}; avoid @samp{tail -c 4}, since it might mean either @samp{tail
2964 -c4} or @samp{tail -c 10 4}; and avoid @samp{tail +4}, since it might
2965 mean either @samp{tail ./+4} or @samp{tail -n +4}.
2970 @node split invocation
2971 @section @command{split}: Split a file into pieces.
2974 @cindex splitting a file into pieces
2975 @cindex pieces, splitting a file into
2977 @command{split} creates output files containing consecutive or interleaved
2978 sections of @var{input} (standard input if none is given or @var{input}
2979 is @samp{-}). Synopsis:
2982 split [@var{option}] [@var{input} [@var{prefix}]]
2985 By default, @command{split} puts 1000 lines of @var{input} (or whatever is
2986 left over for the last section), into each output file.
2988 @cindex output file name prefix
2989 The output files' names consist of @var{prefix} (@samp{x} by default)
2990 followed by a group of characters (@samp{aa}, @samp{ab}, @dots{} by
2991 default), such that concatenating the output files in traditional
2992 sorted order by file name produces the original input file (except
2993 @option{-r}). If the output file names are exhausted, @command{split}
2994 reports an error without deleting the output files that it did create.
2996 The program accepts the following options. Also see @ref{Common options}.
3000 @item -l @var{lines}
3001 @itemx --lines=@var{lines}
3004 Put @var{lines} lines of @var{input} into each output file.
3006 For compatibility @command{split} also supports an obsolete
3007 option syntax @option{-@var{lines}}. New scripts should use
3008 @option{-l @var{lines}} instead.
3011 @itemx --bytes=@var{size}
3014 Put @var{size} bytes of @var{input} into each output file.
3015 @multiplierSuffixes{size}
3018 @itemx --line-bytes=@var{size}
3020 @opindex --line-bytes
3021 Put into each output file as many complete lines of @var{input} as
3022 possible without exceeding @var{size} bytes. Individual lines longer than
3023 @var{size} bytes are broken into multiple files.
3024 @var{size} has the same format as for the @option{--bytes} option.
3026 @itemx --filter=@var{command}
3028 With this option, rather than simply writing to each output file,
3029 write through a pipe to the specified shell @var{command} for each output file.
3030 @var{command} should use the $FILE environment variable, which is set
3031 to a different output file name for each invocation of the command.
3032 For example, imagine that you have a 1TiB compressed file
3033 that, if uncompressed, would be too large to reside on disk,
3034 yet you must split it into individually-compressed pieces
3035 of a more manageable size.
3036 To do that, you might run this command:
3039 xz -dc BIG.xz | split -b200G --filter='xz > $FILE.xz' - big-
3042 Assuming a 10:1 compression ratio, that would create about fifty 20GiB files
3043 with names @file{big-xaa.xz}, @file{big-xab.xz}, @file{big-xac.xz}, etc.
3045 @item -n @var{chunks}
3046 @itemx --number=@var{chunks}
3050 Split @var{input} to @var{chunks} output files where @var{chunks} may be:
3053 @var{n} generate @var{n} files based on current size of @var{input}
3054 @var{k}/@var{n} only output @var{k}th of @var{n} to stdout
3055 l/@var{n} generate @var{n} files without splitting lines
3056 l/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
3057 r/@var{n} like @samp{l} but use round robin distribution
3058 r/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
3061 Any excess bytes remaining after dividing the @var{input}
3062 into @var{n} chunks, are assigned to the last chunk.
3063 Any excess bytes appearing after the initial calculation are discarded
3064 (except when using @samp{r} mode).
3066 All @var{n} files are created even if there are fewer than @var{n} lines,
3067 or the @var{input} is truncated.
3069 For @samp{l} mode, chunks are approximately @var{input} size / @var{n}.
3070 The @var{input} is partitioned into @var{n} equal sized portions, with
3071 the last assigned any excess. If a line @emph{starts} within a partition
3072 it is written completely to the corresponding file. Since lines
3073 are not split even if they overlap a partition, the files written
3074 can be larger or smaller than the partition size, and even empty
3075 if a line is so long as to completely overlap the partition.
3077 For @samp{r} mode, the size of @var{input} is irrelevant,
3078 and so can be a pipe for example.
3080 @item -a @var{length}
3081 @itemx --suffix-length=@var{length}
3083 @opindex --suffix-length
3084 Use suffixes of length @var{length}. The default @var{length} is 2.
3087 @itemx --numeric-suffixes
3089 @opindex --numeric-suffixes
3090 Use digits in suffixes rather than lower-case letters.
3093 @itemx --elide-empty-files
3095 @opindex --elide-empty-files
3096 Suppress the generation of zero-length output files. This can happen
3097 with the @option{--number} option if a file is (truncated to be) shorter
3098 than the number requested, or if a line is so long as to completely
3099 span a chunk. The output file sequence numbers, always run consecutively
3100 even when this option is specified.
3105 @opindex --unbuffered
3106 Immediately copy input to output in @option{--number r/...} mode,
3107 which is a much slower mode of operation.
3111 Write a diagnostic just before each output file is opened.
3117 Here are a few examples to illustrate how the
3118 @option{--number} (@option{-n}) option works:
3120 Notice how, by default, one line may be split onto two or more:
3123 $ seq -w 6 10 > k; split -n3 k; head xa?
3136 Use the "l/" modifier to suppress that:
3139 $ seq -w 6 10 > k; split -nl/3 k; head xa?
3152 Use the "r/" modifier to distribute lines in a round-robin fashion:
3155 $ seq -w 6 10 > k; split -nr/3 k; head xa?
3168 You can also extract just the Kth chunk.
3169 This extracts and prints just the 7th "chunk" of 33:
3172 $ seq 100 > k; split -nl/7/33 k
3179 @node csplit invocation
3180 @section @command{csplit}: Split a file into context-determined pieces
3183 @cindex context splitting
3184 @cindex splitting a file into pieces by context
3186 @command{csplit} creates zero or more output files containing sections of
3187 @var{input} (standard input if @var{input} is @samp{-}). Synopsis:
3190 csplit [@var{option}]@dots{} @var{input} @var{pattern}@dots{}
3193 The contents of the output files are determined by the @var{pattern}
3194 arguments, as detailed below. An error occurs if a @var{pattern}
3195 argument refers to a nonexistent line of the input file (e.g., if no
3196 remaining line matches a given regular expression). After every
3197 @var{pattern} has been matched, any remaining input is copied into one
3200 By default, @command{csplit} prints the number of bytes written to each
3201 output file after it has been created.
3203 The types of pattern arguments are:
3208 Create an output file containing the input up to but not including line
3209 @var{n} (a positive integer). If followed by a repeat count, also
3210 create an output file containing the next @var{n} lines of the input
3211 file once for each repeat.
3213 @item /@var{regexp}/[@var{offset}]
3214 Create an output file containing the current line up to (but not
3215 including) the next line of the input file that contains a match for
3216 @var{regexp}. The optional @var{offset} is an integer.
3217 If it is given, the input up to (but not including) the
3218 matching line plus or minus @var{offset} is put into the output file,
3219 and the line after that begins the next section of input.
3221 @item %@var{regexp}%[@var{offset}]
3222 Like the previous type, except that it does not create an output
3223 file, so that section of the input file is effectively ignored.
3225 @item @{@var{repeat-count}@}
3226 Repeat the previous pattern @var{repeat-count} additional
3227 times. The @var{repeat-count} can either be a positive integer or an
3228 asterisk, meaning repeat as many times as necessary until the input is
3233 The output files' names consist of a prefix (@samp{xx} by default)
3234 followed by a suffix. By default, the suffix is an ascending sequence
3235 of two-digit decimal numbers from @samp{00} to @samp{99}. In any case,
3236 concatenating the output files in sorted order by file name produces the
3237 original input file.
3239 By default, if @command{csplit} encounters an error or receives a hangup,
3240 interrupt, quit, or terminate signal, it removes any output files
3241 that it has created so far before it exits.
3243 The program accepts the following options. Also see @ref{Common options}.
3247 @item -f @var{prefix}
3248 @itemx --prefix=@var{prefix}
3251 @cindex output file name prefix
3252 Use @var{prefix} as the output file name prefix.
3254 @item -b @var{suffix}
3255 @itemx --suffix=@var{suffix}
3258 @cindex output file name suffix
3259 Use @var{suffix} as the output file name suffix. When this option is
3260 specified, the suffix string must include exactly one
3261 @code{printf(3)}-style conversion specification, possibly including
3262 format specification flags, a field width, a precision specifications,
3263 or all of these kinds of modifiers. The format letter must convert a
3264 binary unsigned integer argument to readable form. The format letters
3265 @samp{d} and @samp{i} are aliases for @samp{u}, and the
3266 @samp{u}, @samp{o}, @samp{x}, and @samp{X} conversions are allowed. The
3267 entire @var{suffix} is given (with the current output file number) to
3268 @code{sprintf(3)} to form the file name suffixes for each of the
3269 individual output files in turn. If this option is used, the
3270 @option{--digits} option is ignored.
3272 @item -n @var{digits}
3273 @itemx --digits=@var{digits}
3276 Use output file names containing numbers that are @var{digits} digits
3277 long instead of the default 2.
3282 @opindex --keep-files
3283 Do not remove output files when errors are encountered.
3286 @itemx --elide-empty-files
3288 @opindex --elide-empty-files
3289 Suppress the generation of zero-length output files. (In cases where
3290 the section delimiters of the input file are supposed to mark the first
3291 lines of each of the sections, the first output file will generally be a
3292 zero-length file unless you use this option.) The output file sequence
3293 numbers always run consecutively starting from 0, even when this option
3304 Do not print counts of output file sizes.
3310 Here is an example of its usage.
3311 First, create an empty directory for the exercise,
3318 Now, split the sequence of 1..14 on lines that end with 0 or 5:
3321 $ seq 14 | csplit - '/[05]$/' '@{*@}'
3327 Each number printed above is the size of an output
3328 file that csplit has just created.
3329 List the names of those output files:
3336 Use @command{head} to show their contents:
3361 @node Summarizing files
3362 @chapter Summarizing files
3364 @cindex summarizing files
3366 These commands generate just a few numbers representing entire
3370 * wc invocation:: Print newline, word, and byte counts.
3371 * sum invocation:: Print checksum and block counts.
3372 * cksum invocation:: Print CRC checksum and byte counts.
3373 * md5sum invocation:: Print or check MD5 digests.
3374 * sha1sum invocation:: Print or check SHA-1 digests.
3375 * sha2 utilities:: Print or check SHA-2 digests.
3380 @section @command{wc}: Print newline, word, and byte counts
3384 @cindex character count
3388 @command{wc} counts the number of bytes, characters, whitespace-separated
3389 words, and newlines in each given @var{file}, or standard input if none
3390 are given or for a @var{file} of @samp{-}. Synopsis:
3393 wc [@var{option}]@dots{} [@var{file}]@dots{}
3396 @cindex total counts
3397 @command{wc} prints one line of counts for each file, and if the file was
3398 given as an argument, it prints the file name following the counts. If
3399 more than one @var{file} is given, @command{wc} prints a final line
3400 containing the cumulative counts, with the file name @file{total}. The
3401 counts are printed in this order: newlines, words, characters, bytes,
3402 maximum line length.
3403 Each count is printed right-justified in a field with at least one
3404 space between fields so that the numbers and file names normally line
3405 up nicely in columns. The width of the count fields varies depending
3406 on the inputs, so you should not depend on a particular field width.
3407 However, as a @acronym{GNU} extension, if only one count is printed,
3408 it is guaranteed to be printed without leading spaces.
3410 By default, @command{wc} prints three counts: the newline, words, and byte
3411 counts. Options can specify that only certain counts be printed.
3412 Options do not undo others previously given, so
3419 prints both the byte counts and the word counts.
3421 With the @option{--max-line-length} option, @command{wc} prints the length
3422 of the longest line per file, and if there is more than one file it
3423 prints the maximum (not the sum) of those lengths. The line lengths here
3424 are measured in screen columns, according to the current locale and
3425 assuming tab positions in every 8th column.
3427 The program accepts the following options. Also see @ref{Common options}.
3435 Print only the byte counts.
3441 Print only the character counts.
3447 Print only the word counts.
3453 Print only the newline counts.
3456 @itemx --max-line-length
3458 @opindex --max-line-length
3459 Print only the maximum line lengths.
3461 @macro filesZeroFromOption{cmd,withTotalOption,subListOutput}
3462 @itemx --files0-from=@var{file}
3463 @opindex --files0-from=@var{file}
3464 @c This is commented out to avoid a texi2dvi failure.
3465 @c texi2dvi (GNU Texinfo 4.11) 1.104
3466 @c @cindex including files from @command{\cmd\}
3467 Disallow processing files named on the command line, and instead process
3468 those named in file @var{file}; each name being terminated by a zero byte
3469 (@acronym{ASCII} @sc{nul}).
3470 This is useful \withTotalOption\
3471 when the list of file names is so long that it may exceed a command line
3473 In such cases, running @command{\cmd\} via @command{xargs} is undesirable
3474 because it splits the list into pieces and makes @command{\cmd\} print
3475 \subListOutput\ for each sublist rather than for the entire list.
3476 One way to produce a list of @acronym{ASCII} @sc{nul} terminated file
3477 names is with @sc{gnu}
3478 @command{find}, using its @option{-print0} predicate.
3479 If @var{file} is @samp{-} then the @acronym{ASCII} @sc{nul} terminated
3480 file names are read from standard input.
3482 @filesZeroFromOption{wc,,a total}
3484 For example, to find the length of the longest line in any @file{.c} or
3485 @file{.h} file in the current hierarchy, do this:
3488 find . -name '*.[ch]' -print0 |
3489 wc -L --files0-from=- | tail -n1
3497 @node sum invocation
3498 @section @command{sum}: Print checksum and block counts
3501 @cindex 16-bit checksum
3502 @cindex checksum, 16-bit
3504 @command{sum} computes a 16-bit checksum for each given @var{file}, or
3505 standard input if none are given or for a @var{file} of @samp{-}. Synopsis:
3508 sum [@var{option}]@dots{} [@var{file}]@dots{}
3511 @command{sum} prints the checksum for each @var{file} followed by the
3512 number of blocks in the file (rounded up). If more than one @var{file}
3513 is given, file names are also printed (by default). (With the
3514 @option{--sysv} option, corresponding file names are printed when there is
3515 at least one file argument.)
3517 By default, @sc{gnu} @command{sum} computes checksums using an algorithm
3518 compatible with BSD @command{sum} and prints file sizes in units of
3521 The program accepts the following options. Also see @ref{Common options}.
3527 @cindex BSD @command{sum}
3528 Use the default (BSD compatible) algorithm. This option is included for
3529 compatibility with the System V @command{sum}. Unless @option{-s} was also
3530 given, it has no effect.
3536 @cindex System V @command{sum}
3537 Compute checksums using an algorithm compatible with System V
3538 @command{sum}'s default, and print file sizes in units of 512-byte blocks.
3542 @command{sum} is provided for compatibility; the @command{cksum} program (see
3543 next section) is preferable in new applications.
3548 @node cksum invocation
3549 @section @command{cksum}: Print CRC checksum and byte counts
3552 @cindex cyclic redundancy check
3553 @cindex CRC checksum
3555 @command{cksum} computes a cyclic redundancy check (CRC) checksum for each
3556 given @var{file}, or standard input if none are given or for a
3557 @var{file} of @samp{-}. Synopsis:
3560 cksum [@var{option}]@dots{} [@var{file}]@dots{}
3563 @command{cksum} prints the CRC checksum for each file along with the number
3564 of bytes in the file, and the file name unless no arguments were given.
3566 @command{cksum} is typically used to ensure that files
3567 transferred by unreliable means (e.g., netnews) have not been corrupted,
3568 by comparing the @command{cksum} output for the received files with the
3569 @command{cksum} output for the original files (typically given in the
3572 The CRC algorithm is specified by the @acronym{POSIX} standard. It is not
3573 compatible with the BSD or System V @command{sum} algorithms (see the
3574 previous section); it is more robust.
3576 The only options are @option{--help} and @option{--version}. @xref{Common
3582 @node md5sum invocation
3583 @section @command{md5sum}: Print or check MD5 digests
3587 @cindex 128-bit checksum
3588 @cindex checksum, 128-bit
3589 @cindex fingerprint, 128-bit
3590 @cindex message-digest, 128-bit
3592 @command{md5sum} computes a 128-bit checksum (or @dfn{fingerprint} or
3593 @dfn{message-digest}) for each specified @var{file}.
3595 Note: The MD5 digest is more reliable than a simple CRC (provided by
3596 the @command{cksum} command) for detecting accidental file corruption,
3597 as the chances of accidentally having two files with identical MD5
3598 are vanishingly small. However, it should not be considered secure
3599 against malicious tampering: although finding a file with a given MD5
3600 fingerprint is considered infeasible at the moment, it is known how
3601 to modify certain files, including digital certificates, so that they
3602 appear valid when signed with an MD5 digest.
3603 For more secure hashes, consider using SHA-2. @xref{sha2 utilities}.
3605 If a @var{file} is specified as @samp{-} or if no files are given
3606 @command{md5sum} computes the checksum for the standard input.
3607 @command{md5sum} can also determine whether a file and checksum are
3608 consistent. Synopsis:
3611 md5sum [@var{option}]@dots{} [@var{file}]@dots{}
3614 For each @var{file}, @samp{md5sum} outputs the MD5 checksum, a flag
3615 indicating binary or text input mode, and the file name.
3616 If @var{file} contains a backslash or newline, the
3617 line is started with a backslash, and each problematic character in
3618 the file name is escaped with a backslash, making the output
3619 unambiguous even in the presence of arbitrary file names.
3620 If @var{file} is omitted or specified as @samp{-}, standard input is read.
3622 The program accepts the following options. Also see @ref{Common options}.
3630 @cindex binary input files
3631 Treat each input file as binary, by reading it in binary mode and
3632 outputting a @samp{*} flag. This is the inverse of @option{--text}.
3633 On systems like @acronym{GNU} that do not distinguish between binary
3634 and text files, this option merely flags each input mode as binary:
3635 the MD5 checksum is unaffected. This option is the default on systems
3636 like MS-DOS that distinguish between binary and text files, except
3637 for reading standard input when standard input is a terminal.
3641 Read file names and checksum information (not data) from each
3642 @var{file} (or from stdin if no @var{file} was specified) and report
3643 whether the checksums match the contents of the named files.
3644 The input to this mode of @command{md5sum} is usually the output of
3645 a prior, checksum-generating run of @samp{md5sum}.
3646 Each valid line of input consists of an MD5 checksum, a binary/text
3647 flag, and then a file name.
3648 Binary mode is indicated with @samp{*}, text with @samp{ } (space).
3649 For each such line, @command{md5sum} reads the named file and computes its
3650 MD5 checksum. Then, if the computed message digest does not match the
3651 one on the line with the file name, the file is noted as having
3652 failed the test. Otherwise, the file passes the test.
3653 By default, for each valid line, one line is written to standard
3654 output indicating whether the named file passed the test.
3655 After all checks have been performed, if there were any failures,
3656 a warning is issued to standard error.
3657 Use the @option{--status} option to inhibit that output.
3658 If any listed file cannot be opened or read, if any valid line has
3659 an MD5 checksum inconsistent with the associated file, or if no valid
3660 line is found, @command{md5sum} exits with nonzero status. Otherwise,
3661 it exits successfully.
3665 @cindex verifying MD5 checksums
3666 This option is useful only when verifying checksums.
3667 When verifying checksums, don't generate an 'OK' message per successfully
3668 checked file. Files that fail the verification are reported in the
3669 default one-line-per-file format. If there is any checksum mismatch,
3670 print a warning summarizing the failures to standard error.
3674 @cindex verifying MD5 checksums
3675 This option is useful only when verifying checksums.
3676 When verifying checksums, don't generate the default one-line-per-file
3677 diagnostic and don't output the warning summarizing any failures.
3678 Failures to open or read a file still evoke individual diagnostics to
3680 If all listed files are readable and are consistent with the associated
3681 MD5 checksums, exit successfully. Otherwise exit with a status code
3682 indicating there was a failure.
3688 @cindex text input files
3689 Treat each input file as text, by reading it in text mode and
3690 outputting a @samp{ } flag. This is the inverse of @option{--binary}.
3691 This option is the default on systems like @acronym{GNU} that do not
3692 distinguish between binary and text files. On other systems, it is
3693 the default for reading standard input when standard input is a
3700 @cindex verifying MD5 checksums
3701 When verifying checksums, warn about improperly formatted MD5 checksum lines.
3702 This option is useful only if all but a few lines in the checked input
3707 @cindex verifying MD5 checksums
3708 When verifying checksums,
3709 if one or more input line is invalid,
3710 exit nonzero after all warnings have been issued.
3717 @node sha1sum invocation
3718 @section @command{sha1sum}: Print or check SHA-1 digests
3722 @cindex 160-bit checksum
3723 @cindex checksum, 160-bit
3724 @cindex fingerprint, 160-bit
3725 @cindex message-digest, 160-bit
3727 @command{sha1sum} computes a 160-bit checksum for each specified
3728 @var{file}. The usage and options of this command are precisely the
3729 same as for @command{md5sum}. @xref{md5sum invocation}.
3731 Note: The SHA-1 digest is more secure than MD5, and no collisions of
3732 it are known (different files having the same fingerprint). However,
3733 it is known that they can be produced with considerable, but not
3734 unreasonable, resources. For this reason, it is generally considered
3735 that SHA-1 should be gradually phased out in favor of the more secure
3736 SHA-2 hash algorithms. @xref{sha2 utilities}.
3739 @node sha2 utilities
3740 @section sha2 utilities: Print or check SHA-2 digests
3747 @cindex 224-bit checksum
3748 @cindex 256-bit checksum
3749 @cindex 384-bit checksum
3750 @cindex 512-bit checksum
3751 @cindex checksum, 224-bit
3752 @cindex checksum, 256-bit
3753 @cindex checksum, 384-bit
3754 @cindex checksum, 512-bit
3755 @cindex fingerprint, 224-bit
3756 @cindex fingerprint, 256-bit
3757 @cindex fingerprint, 384-bit
3758 @cindex fingerprint, 512-bit
3759 @cindex message-digest, 224-bit
3760 @cindex message-digest, 256-bit
3761 @cindex message-digest, 384-bit
3762 @cindex message-digest, 512-bit
3764 The commands @command{sha224sum}, @command{sha256sum},
3765 @command{sha384sum} and @command{sha512sum} compute checksums of
3766 various lengths (respectively 224, 256, 384 and 512 bits),
3767 collectively known as the SHA-2 hashes. The usage and options of
3768 these commands are precisely the same as for @command{md5sum}.
3769 @xref{md5sum invocation}.
3771 Note: The SHA384 and SHA512 digests are considerably slower to
3772 compute, especially on 32-bit computers, than SHA224 or SHA256.
3775 @node Operating on sorted files
3776 @chapter Operating on sorted files
3778 @cindex operating on sorted files
3779 @cindex sorted files, operations on
3781 These commands work with (or produce) sorted files.
3784 * sort invocation:: Sort text files.
3785 * shuf invocation:: Shuffle text files.
3786 * uniq invocation:: Uniquify files.
3787 * comm invocation:: Compare two sorted files line by line.
3788 * ptx invocation:: Produce a permuted index of file contents.
3789 * tsort invocation:: Topological sort.
3793 @node sort invocation
3794 @section @command{sort}: Sort text files
3797 @cindex sorting files
3799 @command{sort} sorts, merges, or compares all the lines from the given
3800 files, or standard input if none are given or for a @var{file} of
3801 @samp{-}. By default, @command{sort} writes the results to standard
3805 sort [@var{option}]@dots{} [@var{file}]@dots{}
3808 @command{sort} has three modes of operation: sort (the default), merge,
3809 and check for sortedness. The following options change the operation
3816 @itemx --check=diagnose-first
3819 @cindex checking for sortedness
3820 Check whether the given file is already sorted: if it is not all
3821 sorted, print a diagnostic containing the first out-of-order line and
3822 exit with a status of 1.
3823 Otherwise, exit successfully.
3824 At most one input file can be given.
3827 @itemx --check=quiet
3828 @itemx --check=silent
3831 @cindex checking for sortedness
3832 Exit successfully if the given file is already sorted, and
3833 exit with status 1 otherwise.
3834 At most one input file can be given.
3835 This is like @option{-c}, except it does not print a diagnostic.
3841 @cindex merging sorted files
3842 Merge the given files by sorting them as a group. Each input file must
3843 always be individually sorted. It always works to sort instead of
3844 merge; merging is provided because it is faster, in the case where it
3849 @cindex sort stability
3850 @cindex sort's last-resort comparison
3851 A pair of lines is compared as follows:
3852 @command{sort} compares each pair of fields, in the
3853 order specified on the command line, according to the associated
3854 ordering options, until a difference is found or no fields are left.
3855 If no key fields are specified, @command{sort} uses a default key of
3856 the entire line. Finally, as a last resort when all keys compare
3857 equal, @command{sort} compares entire lines as if no ordering options
3858 other than @option{--reverse} (@option{-r}) were specified. The
3859 @option{--stable} (@option{-s}) option disables this @dfn{last-resort
3860 comparison} so that lines in which all fields compare equal are left
3861 in their original relative order. The @option{--unique}
3862 (@option{-u}) option also disables the last-resort comparison.
3866 Unless otherwise specified, all comparisons use the character collating
3867 sequence specified by the @env{LC_COLLATE} locale.@footnote{If you
3868 use a non-@acronym{POSIX} locale (e.g., by setting @env{LC_ALL}
3869 to @samp{en_US}), then @command{sort} may produce output that is sorted
3870 differently than you're accustomed to. In that case, set the @env{LC_ALL}
3871 environment variable to @samp{C}. Note that setting only @env{LC_COLLATE}
3872 has two problems. First, it is ineffective if @env{LC_ALL} is also set.
3873 Second, it has undefined behavior if @env{LC_CTYPE} (or @env{LANG}, if
3874 @env{LC_CTYPE} is unset) is set to an incompatible value. For example,
3875 you get undefined behavior if @env{LC_CTYPE} is @code{ja_JP.PCK} but
3876 @env{LC_COLLATE} is @code{en_US.UTF-8}.}
3878 @sc{gnu} @command{sort} (as specified for all @sc{gnu} utilities) has no
3879 limit on input line length or restrictions on bytes allowed within lines.
3880 In addition, if the final byte of an input file is not a newline, @sc{gnu}
3881 @command{sort} silently supplies one. A line's trailing newline is not
3882 part of the line for comparison purposes.
3884 @cindex exit status of @command{sort}
3888 0 if no error occurred
3889 1 if invoked with @option{-c} or @option{-C} and the input is not sorted
3890 2 if an error occurred
3894 If the environment variable @env{TMPDIR} is set, @command{sort} uses its
3895 value as the directory for temporary files instead of @file{/tmp}. The
3896 @option{--temporary-directory} (@option{-T}) option in turn overrides
3897 the environment variable.
3899 The following options affect the ordering of output lines. They may be
3900 specified globally or as part of a specific key field. If no key
3901 fields are specified, global options apply to comparison of entire
3902 lines; otherwise the global options are inherited by key fields that do
3903 not specify any special options of their own. In pre-@acronym{POSIX}
3904 versions of @command{sort}, global options affect only later key fields,
3905 so portable shell scripts should specify global options first.
3910 @itemx --ignore-leading-blanks
3912 @opindex --ignore-leading-blanks
3913 @cindex blanks, ignoring leading
3915 Ignore leading blanks when finding sort keys in each line.
3916 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3917 can change this. Note blanks may be ignored by your locale's collating
3918 rules, but without this option they will be significant for character
3919 positions specified in keys with the @option{-k} option.
3922 @itemx --dictionary-order
3924 @opindex --dictionary-order
3925 @cindex dictionary order
3926 @cindex phone directory order
3927 @cindex telephone directory order
3929 Sort in @dfn{phone directory} order: ignore all characters except
3930 letters, digits and blanks when sorting.
3931 By default letters and digits are those of @acronym{ASCII} and a blank
3932 is a space or a tab, but the @env{LC_CTYPE} locale can change this.
3935 @itemx --ignore-case
3937 @opindex --ignore-case
3938 @cindex ignoring case
3939 @cindex case folding
3941 Fold lowercase characters into the equivalent uppercase characters when
3942 comparing so that, for example, @samp{b} and @samp{B} sort as equal.
3943 The @env{LC_CTYPE} locale determines character types.
3944 When used with @option{--unique} those lower case equivalent lines are
3945 thrown away. (There is currently no way to throw away the upper case
3946 equivalent instead. (Any @option{--reverse} given would only affect
3947 the final result, after the throwing away.))
3950 @itemx --general-numeric-sort
3951 @itemx --sort=general-numeric
3953 @opindex --general-numeric-sort
3955 @cindex general numeric sort
3957 Sort numerically, converting a prefix of each line to a long
3958 double-precision floating point number. @xref{Floating point}.
3959 Do not report overflow, underflow, or conversion errors.
3960 Use the following collating sequence:
3964 Lines that do not start with numbers (all considered to be equal).
3966 NaNs (``Not a Number'' values, in IEEE floating point arithmetic)
3967 in a consistent but machine-dependent order.
3971 Finite numbers in ascending numeric order (with @math{-0} and @math{+0} equal).
3976 Use this option only if there is no alternative; it is much slower than
3977 @option{--numeric-sort} (@option{-n}) and it can lose information when
3978 converting to floating point.
3981 @itemx --human-numeric-sort
3982 @itemx --sort=human-numeric
3984 @opindex --human-numeric-sort
3986 @cindex human numeric sort
3988 Sort numerically, first by numeric sign (negative, zero, or positive);
3989 then by @acronym{SI} suffix (either empty, or @samp{k} or @samp{K}, or
3990 one of @samp{MGTPEZY}, in that order; @pxref{Block size}); and finally
3991 by numeric value. For example, @samp{1023M} sorts before @samp{1G}
3992 because @samp{M} (mega) precedes @samp{G} (giga) as an @acronym{SI}
3993 suffix. This option sorts values that are consistently scaled to the
3994 nearest suffix, regardless of whether suffixes denote powers of 1000
3995 or 1024, and it therefore sorts the output of any single invocation of
3996 the @command{df}, @command{du}, or @command{ls} commands that are
3997 invoked with their @option{--human-readable} or @option{--si} options.
3998 The syntax for numbers is the same as for the @option{--numeric-sort}
3999 option; the @acronym{SI} suffix must immediately follow the number.
4002 @itemx --ignore-nonprinting
4004 @opindex --ignore-nonprinting
4005 @cindex nonprinting characters, ignoring
4006 @cindex unprintable characters, ignoring
4008 Ignore nonprinting characters.
4009 The @env{LC_CTYPE} locale determines character types.
4010 This option has no effect if the stronger @option{--dictionary-order}
4011 (@option{-d}) option is also given.
4017 @opindex --month-sort
4019 @cindex months, sorting by
4021 An initial string, consisting of any amount of blanks, followed
4022 by a month name abbreviation, is folded to UPPER case and
4023 compared in the order @samp{JAN} < @samp{FEB} < @dots{} < @samp{DEC}.
4024 Invalid names compare low to valid names. The @env{LC_TIME} locale
4025 category determines the month spellings.
4026 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4030 @itemx --numeric-sort
4031 @itemx --sort=numeric
4033 @opindex --numeric-sort
4035 @cindex numeric sort
4037 Sort numerically. The number begins each line and consists
4038 of optional blanks, an optional @samp{-} sign, and zero or more
4039 digits possibly separated by thousands separators, optionally followed
4040 by a decimal-point character and zero or more digits. An empty
4041 number is treated as @samp{0}. The @env{LC_NUMERIC}
4042 locale specifies the decimal-point character and thousands separator.
4043 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4046 Comparison is exact; there is no rounding error.
4048 Neither a leading @samp{+} nor exponential notation is recognized.
4049 To compare such strings numerically, use the
4050 @option{--general-numeric-sort} (@option{-g}) option.
4053 @itemx --version-sort
4055 @opindex --version-sort
4056 @cindex version number sort
4057 Sort by version name and number. It behaves like a standard sort,
4058 except that each sequence of decimal digits is treated numerically
4059 as an index/version number. (@xref{Details about version sort}.)
4065 @cindex reverse sorting
4066 Reverse the result of comparison, so that lines with greater key values
4067 appear earlier in the output instead of later.
4070 @itemx --random-sort
4071 @itemx --sort=random
4073 @opindex --random-sort
4076 Sort by hashing the input keys and then sorting the hash values.
4077 Choose the hash function at random, ensuring that it is free of
4078 collisions so that differing keys have differing hash values. This is
4079 like a random permutation of the inputs (@pxref{shuf invocation}),
4080 except that keys with the same value sort together.
4082 If multiple random sort fields are specified, the same random hash
4083 function is used for all fields. To use different random hash
4084 functions for different fields, you can invoke @command{sort} more
4087 The choice of hash function is affected by the
4088 @option{--random-source} option.
4096 @item --compress-program=@var{prog}
4097 Compress any temporary files with the program @var{prog}.
4099 With no arguments, @var{prog} must compress standard input to standard
4100 output, and when given the @option{-d} option it must decompress
4101 standard input to standard output.
4103 Terminate with an error if @var{prog} exits with nonzero status.
4105 White space and the backslash character should not appear in
4106 @var{prog}; they are reserved for future use.
4108 @filesZeroFromOption{sort,,sorted output}
4110 @item -k @var{pos1}[,@var{pos2}]
4111 @itemx --key=@var{pos1}[,@var{pos2}]
4115 Specify a sort field that consists of the part of the line between
4116 @var{pos1} and @var{pos2} (or the end of the line, if @var{pos2} is
4117 omitted), @emph{inclusive}.
4119 Each @var{pos} has the form @samp{@var{f}[.@var{c}][@var{opts}]},
4120 where @var{f} is the number of the field to use, and @var{c} is the number
4121 of the first character from the beginning of the field. Fields and character
4122 positions are numbered starting with 1; a character position of zero in
4123 @var{pos2} indicates the field's last character. If @samp{.@var{c}} is
4124 omitted from @var{pos1}, it defaults to 1 (the beginning of the field);
4125 if omitted from @var{pos2}, it defaults to 0 (the end of the field).
4126 @var{opts} are ordering options, allowing individual keys to be sorted
4127 according to different rules; see below for details. Keys can span
4130 Example: To sort on the second field, use @option{--key=2,2}
4131 (@option{-k 2,2}). See below for more notes on keys and more examples.
4132 See also the @option{--debug} option to help determine the part
4133 of the line being used in the sort.
4136 Highlight the portion of each line used for sorting.
4137 Also issue warnings about questionable usage to stderr.
4139 @item --batch-size=@var{nmerge}
4140 @opindex --batch-size
4141 @cindex number of inputs to merge, nmerge
4142 Merge at most @var{nmerge} inputs at once.
4144 When @command{sort} has to merge more than @var{nmerge} inputs,
4145 it merges them in groups of @var{nmerge}, saving the result in
4146 a temporary file, which is then used as an input in a subsequent merge.
4148 A large value of @var{nmerge} may improve merge performance and decrease
4149 temporary storage utilization at the expense of increased memory usage
4150 and I/O. Conversely a small value of @var{nmerge} may reduce memory
4151 requirements and I/O at the expense of temporary storage consumption and
4154 The value of @var{nmerge} must be at least 2. The default value is
4155 currently 16, but this is implementation-dependent and may change in
4158 The value of @var{nmerge} may be bounded by a resource limit for open
4159 file descriptors. The commands @samp{ulimit -n} or @samp{getconf
4160 OPEN_MAX} may display limits for your systems; these limits may be
4161 modified further if your program already has some files open, or if
4162 the operating system has other limits on the number of open files. If
4163 the value of @var{nmerge} exceeds the resource limit, @command{sort}
4164 silently uses a smaller value.
4166 @item -o @var{output-file}
4167 @itemx --output=@var{output-file}
4170 @cindex overwriting of input, allowed
4171 Write output to @var{output-file} instead of standard output.
4172 Normally, @command{sort} reads all input before opening
4173 @var{output-file}, so you can safely sort a file in place by using
4174 commands like @code{sort -o F F} and @code{cat F | sort -o F}.
4175 However, @command{sort} with @option{--merge} (@option{-m}) can open
4176 the output file before reading all input, so a command like @code{cat
4177 F | sort -m -o F - G} is not safe as @command{sort} might start
4178 writing @file{F} before @command{cat} is done reading it.
4180 @vindex POSIXLY_CORRECT
4181 On newer systems, @option{-o} cannot appear after an input file if
4182 @env{POSIXLY_CORRECT} is set, e.g., @samp{sort F -o F}. Portable
4183 scripts should specify @option{-o @var{output-file}} before any input
4186 @item --random-source=@var{file}
4187 @opindex --random-source
4188 @cindex random source for sorting
4189 Use @var{file} as a source of random data used to determine which
4190 random hash function to use with the @option{-R} option. @xref{Random
4197 @cindex sort stability
4198 @cindex sort's last-resort comparison
4200 Make @command{sort} stable by disabling its last-resort comparison.
4201 This option has no effect if no fields or global ordering options
4202 other than @option{--reverse} (@option{-r}) are specified.
4205 @itemx --buffer-size=@var{size}
4207 @opindex --buffer-size
4208 @cindex size for main memory sorting
4209 Use a main-memory sort buffer of the given @var{size}. By default,
4210 @var{size} is in units of 1024 bytes. Appending @samp{%} causes
4211 @var{size} to be interpreted as a percentage of physical memory.
4212 Appending @samp{K} multiplies @var{size} by 1024 (the default),
4213 @samp{M} by 1,048,576, @samp{G} by 1,073,741,824, and so on for
4214 @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}. Appending
4215 @samp{b} causes @var{size} to be interpreted as a byte count, with no
4218 This option can improve the performance of @command{sort} by causing it
4219 to start with a larger or smaller sort buffer than the default.
4220 However, this option affects only the initial buffer size. The buffer
4221 grows beyond @var{size} if @command{sort} encounters input lines larger
4224 @item -t @var{separator}
4225 @itemx --field-separator=@var{separator}
4227 @opindex --field-separator
4228 @cindex field separator character
4229 Use character @var{separator} as the field separator when finding the
4230 sort keys in each line. By default, fields are separated by the empty
4231 string between a non-blank character and a blank character.
4232 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4235 That is, given the input line @w{@samp{ foo bar}}, @command{sort} breaks it
4236 into fields @w{@samp{ foo}} and @w{@samp{ bar}}. The field separator is
4237 not considered to be part of either the field preceding or the field
4238 following, so with @samp{sort @w{-t " "}} the same input line has
4239 three fields: an empty field, @samp{foo}, and @samp{bar}.
4240 However, fields that extend to the end of the line,
4241 as @option{-k 2}, or fields consisting of a range, as @option{-k 2,3},
4242 retain the field separators present between the endpoints of the range.
4244 To specify @acronym{ASCII} @sc{nul} as the field separator,
4245 use the two-character string @samp{\0}, e.g., @samp{sort -t '\0'}.
4247 @item -T @var{tempdir}
4248 @itemx --temporary-directory=@var{tempdir}
4250 @opindex --temporary-directory
4251 @cindex temporary directory
4253 Use directory @var{tempdir} to store temporary files, overriding the
4254 @env{TMPDIR} environment variable. If this option is given more than
4255 once, temporary files are stored in all the directories given. If you
4256 have a large sort or merge that is I/O-bound, you can often improve
4257 performance by using this option to specify directories on different
4258 disks and controllers.
4260 @item --parallel=@var{n}
4262 @cindex multithreaded sort
4263 Set the number of sorts run in parallel to @var{n}. By default,
4264 @var{n} is set to the number of available processors, but limited
4265 to 8, as there are diminishing performance gains after that.
4266 Note also that using @var{n} threads increases the memory usage by
4267 a factor of log @var{n}. Also see @ref{nproc invocation}.
4273 @cindex uniquifying output
4275 Normally, output only the first of a sequence of lines that compare
4276 equal. For the @option{--check} (@option{-c} or @option{-C}) option,
4277 check that no pair of consecutive lines compares equal.
4279 This option also disables the default last-resort comparison.
4281 The commands @code{sort -u} and @code{sort | uniq} are equivalent, but
4282 this equivalence does not extend to arbitrary @command{sort} options.
4283 For example, @code{sort -n -u} inspects only the value of the initial
4284 numeric string when checking for uniqueness, whereas @code{sort -n |
4285 uniq} inspects the entire line. @xref{uniq invocation}.
4287 @macro zeroTerminatedOption
4289 @itemx --zero-terminated
4291 @opindex --zero-terminated
4292 @cindex process zero-terminated items
4293 Delimit items with a zero byte rather than a newline (@acronym{ASCII} @sc{lf}).
4294 I.E. treat input as items separated by @acronym{ASCII} @sc{nul}
4295 and terminate output items with @acronym{ASCII} @sc{nul}.
4296 This option can be useful in conjunction with @samp{perl -0} or
4297 @samp{find -print0} and @samp{xargs -0} which do the same in order to
4298 reliably handle arbitrary file names (even those containing blanks
4299 or other special characters).
4301 @zeroTerminatedOption
4305 Historical (BSD and System V) implementations of @command{sort} have
4306 differed in their interpretation of some options, particularly
4307 @option{-b}, @option{-f}, and @option{-n}.
4308 @sc{gnu} sort follows the @acronym{POSIX}
4309 behavior, which is usually (but not always!) like the System V behavior.
4310 According to @acronym{POSIX}, @option{-n} no longer implies @option{-b}. For
4311 consistency, @option{-M} has been changed in the same way. This may
4312 affect the meaning of character positions in field specifications in
4313 obscure cases. The only fix is to add an explicit @option{-b}.
4315 A position in a sort field specified with @option{-k} may have any
4316 of the option letters @samp{MbdfghinRrV} appended to it, in which case no
4317 global ordering options are inherited by that particular field. The
4318 @option{-b} option may be independently attached to either or both of
4319 the start and end positions of a field specification, and if it is
4320 inherited from the global options it will be attached to both.
4321 If input lines can contain leading or adjacent blanks and @option{-t}
4322 is not used, then @option{-k} is typically combined with @option{-b} or
4323 an option that implicitly ignores leading blanks (@samp{Mghn}) as otherwise
4324 the varying numbers of leading blanks in fields can cause confusing results.
4326 If the start position in a sort field specifier falls after the end of
4327 the line or after the end field, the field is empty. If the @option{-b}
4328 option was specified, the @samp{.@var{c}} part of a field specification
4329 is counted from the first nonblank character of the field.
4331 @vindex _POSIX2_VERSION
4332 @vindex POSIXLY_CORRECT
4333 On older systems, @command{sort} supports an obsolete origin-zero
4334 syntax @samp{+@var{pos1} [-@var{pos2}]} for specifying sort keys.
4335 The obsolete sequence @samp{sort +@var{a}.@var{x} -@var{b}.@var{y}}
4336 is equivalent to @samp{sort -k @var{a+1}.@var{x+1},@var{b}} if @var{y}
4337 is @samp{0} or absent, otherwise it is equivalent to @samp{sort -k
4338 @var{a+1}.@var{x+1},@var{b+1}.@var{y}}.
4340 This obsolete behavior can be enabled or disabled with the
4341 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4342 conformance}); it can also be enabled when @env{POSIXLY_CORRECT} is
4343 not set by using the obsolete syntax with @samp{-@var{pos2}} present.
4345 Scripts intended for use on standard hosts should avoid obsolete
4346 syntax and should use @option{-k} instead. For example, avoid
4347 @samp{sort +2}, since it might be interpreted as either @samp{sort
4348 ./+2} or @samp{sort -k 3}. If your script must also run on hosts that
4349 support only the obsolete syntax, it can use a test like @samp{if sort
4350 -k 1 </dev/null >/dev/null 2>&1; then @dots{}} to decide which syntax
4353 Here are some examples to illustrate various combinations of options.
4358 Sort in descending (reverse) numeric order.
4365 Run no more than 4 sorts concurrently, using a buffer size of 10M.
4368 sort --parallel=4 -S 10M
4372 Sort alphabetically, omitting the first and second fields
4373 and the blanks at the start of the third field.
4374 This uses a single key composed of the characters beginning
4375 at the start of the first nonblank character in field three
4376 and extending to the end of each line.
4383 Sort numerically on the second field and resolve ties by sorting
4384 alphabetically on the third and fourth characters of field five.
4385 Use @samp{:} as the field delimiter.
4388 sort -t : -k 2,2n -k 5.3,5.4
4391 Note that if you had written @option{-k 2n} instead of @option{-k 2,2n}
4392 @command{sort} would have used all characters beginning in the second field
4393 and extending to the end of the line as the primary @emph{numeric}
4394 key. For the large majority of applications, treating keys spanning
4395 more than one field as numeric will not do what you expect.
4397 Also note that the @samp{n} modifier was applied to the field-end
4398 specifier for the first key. It would have been equivalent to
4399 specify @option{-k 2n,2} or @option{-k 2n,2n}. All modifiers except
4400 @samp{b} apply to the associated @emph{field}, regardless of whether
4401 the modifier character is attached to the field-start and/or the
4402 field-end part of the key specifier.
4405 Sort the password file on the fifth field and ignore any
4406 leading blanks. Sort lines with equal values in field five
4407 on the numeric user ID in field three. Fields are separated
4411 sort -t : -k 5b,5 -k 3,3n /etc/passwd
4412 sort -t : -n -k 5b,5 -k 3,3 /etc/passwd
4413 sort -t : -b -k 5,5 -k 3,3n /etc/passwd
4416 These three commands have equivalent effect. The first specifies that
4417 the first key's start position ignores leading blanks and the second
4418 key is sorted numerically. The other two commands rely on global
4419 options being inherited by sort keys that lack modifiers. The inheritance
4420 works in this case because @option{-k 5b,5b} and @option{-k 5b,5} are
4421 equivalent, as the location of a field-end lacking a @samp{.@var{c}}
4422 character position is not affected by whether initial blanks are
4426 Sort a set of log files, primarily by IPv4 address and secondarily by
4427 time stamp. If two lines' primary and secondary keys are identical,
4428 output the lines in the same order that they were input. The log
4429 files contain lines that look like this:
4432 4.150.156.3 - - [01/Apr/2004:06:31:51 +0000] message 1
4433 211.24.3.231 - - [24/Apr/2004:20:17:39 +0000] message 2
4436 Fields are separated by exactly one space. Sort IPv4 addresses
4437 lexicographically, e.g., 212.61.52.2 sorts before 212.129.233.201
4438 because 61 is less than 129.
4441 sort -s -t ' ' -k 4.9n -k 4.5M -k 4.2n -k 4.14,4.21 file*.log |
4442 sort -s -t '.' -k 1,1n -k 2,2n -k 3,3n -k 4,4n
4445 This example cannot be done with a single @command{sort} invocation,
4446 since IPv4 address components are separated by @samp{.} while dates
4447 come just after a space. So it is broken down into two invocations of
4448 @command{sort}: the first sorts by time stamp and the second by IPv4
4449 address. The time stamp is sorted by year, then month, then day, and
4450 finally by hour-minute-second field, using @option{-k} to isolate each
4451 field. Except for hour-minute-second there's no need to specify the
4452 end of each key field, since the @samp{n} and @samp{M} modifiers sort
4453 based on leading prefixes that cannot cross field boundaries. The
4454 IPv4 addresses are sorted lexicographically. The second sort uses
4455 @samp{-s} so that ties in the primary key are broken by the secondary
4456 key; the first sort uses @samp{-s} so that the combination of the two
4460 Generate a tags file in case-insensitive sorted order.
4463 find src -type f -print0 | sort -z -f | xargs -0 etags --append
4466 The use of @option{-print0}, @option{-z}, and @option{-0} in this case means
4467 that file names that contain blanks or other special characters are
4469 by the sort operation.
4471 @c This example is a bit contrived and needs more explanation.
4473 @c Sort records separated by an arbitrary string by using a pipe to convert
4474 @c each record delimiter string to @samp{\0}, then using sort's -z option,
4475 @c and converting each @samp{\0} back to the original record delimiter.
4478 @c printf 'c\n\nb\n\na\n' |
4479 @c perl -0pe 's/\n\n/\n\0/g' |
4481 @c perl -0pe 's/\0/\n/g'
4485 Use the common @acronym{DSU, Decorate Sort Undecorate} idiom to
4486 sort lines according to their length.
4489 awk '@{print length, $0@}' /etc/passwd | sort -n | cut -f2- -d' '
4492 In general this technique can be used to sort data that the @command{sort}
4493 command does not support, or is inefficient at, sorting directly.
4496 Shuffle a list of directories, but preserve the order of files within
4497 each directory. For instance, one could use this to generate a music
4498 playlist in which albums are shuffled but the songs of each album are
4502 ls */* | sort -t / -k 1,1R -k 2,2
4508 @node shuf invocation
4509 @section @command{shuf}: Shuffling text
4512 @cindex shuffling files
4514 @command{shuf} shuffles its input by outputting a random permutation
4515 of its input lines. Each output permutation is equally likely.
4519 shuf [@var{option}]@dots{} [@var{file}]
4520 shuf -e [@var{option}]@dots{} [@var{arg}]@dots{}
4521 shuf -i @var{lo}-@var{hi} [@var{option}]@dots{}
4524 @command{shuf} has three modes of operation that affect where it
4525 obtains its input lines. By default, it reads lines from standard
4526 input. The following options change the operation mode:
4534 @cindex command-line operands to shuffle
4535 Treat each command-line operand as an input line.
4537 @item -i @var{lo}-@var{hi}
4538 @itemx --input-range=@var{lo}-@var{hi}
4540 @opindex --input-range
4541 @cindex input range to shuffle
4542 Act as if input came from a file containing the range of unsigned
4543 decimal integers @var{lo}@dots{}@var{hi}, one per line.
4547 @command{shuf}'s other options can affect its behavior in all
4552 @item -n @var{lines}
4553 @itemx --head-count=@var{count}
4555 @opindex --head-count
4556 @cindex head of output
4557 Output at most @var{count} lines. By default, all input lines are
4560 @item -o @var{output-file}
4561 @itemx --output=@var{output-file}
4564 @cindex overwriting of input, allowed
4565 Write output to @var{output-file} instead of standard output.
4566 @command{shuf} reads all input before opening
4567 @var{output-file}, so you can safely shuffle a file in place by using
4568 commands like @code{shuf -o F <F} and @code{cat F | shuf -o F}.
4570 @item --random-source=@var{file}
4571 @opindex --random-source
4572 @cindex random source for shuffling
4573 Use @var{file} as a source of random data used to determine which
4574 permutation to generate. @xref{Random sources}.
4576 @zeroTerminatedOption
4592 might produce the output
4602 Similarly, the command:
4605 shuf -e clubs hearts diamonds spades
4619 and the command @samp{shuf -i 1-4} might output:
4629 These examples all have four input lines, so @command{shuf} might
4630 produce any of the twenty-four possible permutations of the input. In
4631 general, if there are @var{n} input lines, there are @var{n}! (i.e.,
4632 @var{n} factorial, or @var{n} * (@var{n} - 1) * @dots{} * 1) possible
4633 output permutations.
4638 @node uniq invocation
4639 @section @command{uniq}: Uniquify files
4642 @cindex uniquify files
4644 @command{uniq} writes the unique lines in the given @file{input}, or
4645 standard input if nothing is given or for an @var{input} name of
4649 uniq [@var{option}]@dots{} [@var{input} [@var{output}]]
4652 By default, @command{uniq} prints its input lines, except that
4653 it discards all but the first of adjacent repeated lines, so that
4654 no output lines are repeated. Optionally, it can instead discard
4655 lines that are not repeated, or all repeated lines.
4657 The input need not be sorted, but repeated input lines are detected
4658 only if they are adjacent. If you want to discard non-adjacent
4659 duplicate lines, perhaps you want to use @code{sort -u}.
4660 @xref{sort invocation}.
4663 Comparisons honor the rules specified by the @env{LC_COLLATE}
4666 If no @var{output} file is specified, @command{uniq} writes to standard
4669 The program accepts the following options. Also see @ref{Common options}.
4674 @itemx --skip-fields=@var{n}
4676 @opindex --skip-fields
4677 Skip @var{n} fields on each line before checking for uniqueness. Use
4678 a null string for comparison if a line has fewer than @var{n} fields. Fields
4679 are sequences of non-space non-tab characters that are separated from
4680 each other by at least one space or tab.
4682 For compatibility @command{uniq} supports an obsolete option syntax
4683 @option{-@var{n}}. New scripts should use @option{-f @var{n}} instead.
4686 @itemx --skip-chars=@var{n}
4688 @opindex --skip-chars
4689 Skip @var{n} characters before checking for uniqueness. Use a null string
4690 for comparison if a line has fewer than @var{n} characters. If you use both
4691 the field and character skipping options, fields are skipped over first.
4693 @vindex _POSIX2_VERSION
4694 On older systems, @command{uniq} supports an obsolete option syntax
4696 This obsolete behavior can be enabled or disabled with the
4697 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4698 conformance}), but portable scripts should avoid commands whose
4699 behavior depends on this variable.
4700 For example, use @samp{uniq ./+10} or @samp{uniq -s 10} rather than
4701 the ambiguous @samp{uniq +10}.
4707 Print the number of times each line occurred along with the line.
4710 @itemx --ignore-case
4712 @opindex --ignore-case
4713 Ignore differences in case when comparing lines.
4719 @cindex repeated lines, outputting
4720 Discard lines that are not repeated. When used by itself, this option
4721 causes @command{uniq} to print the first copy of each repeated line,
4725 @itemx --all-repeated[=@var{delimit-method}]
4727 @opindex --all-repeated
4728 @cindex all repeated lines, outputting
4729 Do not discard the second and subsequent repeated input lines,
4730 but discard lines that are not repeated.
4731 This option is useful mainly in conjunction with other options e.g.,
4732 to ignore case or to compare only selected fields.
4733 The optional @var{delimit-method} tells how to delimit
4734 groups of repeated lines, and must be one of the following:
4739 Do not delimit groups of repeated lines.
4740 This is equivalent to @option{--all-repeated} (@option{-D}).
4743 Output a newline before each group of repeated lines.
4744 With @option{--zero-terminated} (@option{-z}), use a zero
4745 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4748 Separate groups of repeated lines with a single newline.
4749 With @option{--zero-terminated} (@option{-z}), use a zero
4750 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4751 This is the same as using @samp{prepend}, except that
4752 no delimiter is inserted before the first group, and hence
4753 may be better suited for output direct to users.
4756 Note that when groups are delimited and the input stream contains
4757 two or more consecutive blank lines, then the output is ambiguous.
4758 To avoid that, filter the input through @samp{tr -s '\n'} to replace
4759 each sequence of consecutive newlines with a single newline.
4761 This is a @sc{gnu} extension.
4762 @c FIXME: give an example showing *how* it's useful
4768 @cindex unique lines, outputting
4769 Discard the first repeated line. When used by itself, this option
4770 causes @command{uniq} to print unique lines, and nothing else.
4773 @itemx --check-chars=@var{n}
4775 @opindex --check-chars
4776 Compare at most @var{n} characters on each line (after skipping any specified
4777 fields and characters). By default the entire rest of the lines are
4780 @zeroTerminatedOption
4787 @node comm invocation
4788 @section @command{comm}: Compare two sorted files line by line
4791 @cindex line-by-line comparison
4792 @cindex comparing sorted files
4794 @command{comm} writes to standard output lines that are common, and lines
4795 that are unique, to two input files; a file name of @samp{-} means
4796 standard input. Synopsis:
4799 comm [@var{option}]@dots{} @var{file1} @var{file2}
4803 Before @command{comm} can be used, the input files must be sorted using the
4804 collating sequence specified by the @env{LC_COLLATE} locale.
4805 If an input file ends in a non-newline
4806 character, a newline is silently appended. The @command{sort} command with
4807 no options always outputs a file that is suitable input to @command{comm}.
4809 @cindex differing lines
4810 @cindex common lines
4811 With no options, @command{comm} produces three-column output. Column one
4812 contains lines unique to @var{file1}, column two contains lines unique
4813 to @var{file2}, and column three contains lines common to both files.
4814 Columns are separated by a single TAB character.
4815 @c FIXME: when there's an option to supply an alternative separator
4816 @c string, append `by default' to the above sentence.
4821 The options @option{-1}, @option{-2}, and @option{-3} suppress printing of
4822 the corresponding columns (and separators). Also see @ref{Common options}.
4824 Unlike some other comparison utilities, @command{comm} has an exit
4825 status that does not depend on the result of the comparison.
4826 Upon normal completion @command{comm} produces an exit code of zero.
4827 If there is an error it exits with nonzero status.
4829 @macro checkOrderOption{cmd}
4830 If the @option{--check-order} option is given, unsorted inputs will
4831 cause a fatal error message. If the option @option{--nocheck-order}
4832 is given, unsorted inputs will never cause an error message. If neither
4833 of these options is given, wrongly sorted inputs are diagnosed
4834 only if an input file is found to contain unpairable
4836 lines, and when both input files are non empty.
4838 @ifclear JOIN_COMMAND
4841 If an input file is diagnosed as being unsorted, the @command{\cmd\}
4842 command will exit with a nonzero status (and the output should not be used).
4844 Forcing @command{\cmd\} to process wrongly sorted input files
4845 containing unpairable lines by specifying @option{--nocheck-order} is
4846 not guaranteed to produce any particular output. The output will
4847 probably not correspond with whatever you hoped it would be.
4849 @checkOrderOption{comm}
4854 Fail with an error message if either input file is wrongly ordered.
4856 @item --nocheck-order
4857 Do not check that both input files are in sorted order.
4861 @item --output-delimiter=@var{str}
4862 Print @var{str} between adjacent output columns,
4863 rather than the default of a single TAB character.
4865 The delimiter @var{str} may not be empty.
4869 @node ptx invocation
4870 @section @command{ptx}: Produce permuted indexes
4874 @command{ptx} reads a text file and essentially produces a permuted index, with
4875 each keyword in its context. The calling sketch is either one of:
4878 ptx [@var{option} @dots{}] [@var{file} @dots{}]
4879 ptx -G [@var{option} @dots{}] [@var{input} [@var{output}]]
4882 The @option{-G} (or its equivalent: @option{--traditional}) option disables
4883 all @sc{gnu} extensions and reverts to traditional mode, thus introducing some
4884 limitations and changing several of the program's default option values.
4885 When @option{-G} is not specified, @sc{gnu} extensions are always enabled.
4886 @sc{gnu} extensions to @command{ptx} are documented wherever appropriate in this
4887 document. For the full list, see @xref{Compatibility in ptx}.
4889 Individual options are explained in the following sections.
4891 When @sc{gnu} extensions are enabled, there may be zero, one or several
4892 @var{file}s after the options. If there is no @var{file}, the program
4893 reads the standard input. If there is one or several @var{file}s, they
4894 give the name of input files which are all read in turn, as if all the
4895 input files were concatenated. However, there is a full contextual
4896 break between each file and, when automatic referencing is requested,
4897 file names and line numbers refer to individual text input files. In
4898 all cases, the program outputs the permuted index to the standard
4901 When @sc{gnu} extensions are @emph{not} enabled, that is, when the program
4902 operates in traditional mode, there may be zero, one or two parameters
4903 besides the options. If there are no parameters, the program reads the
4904 standard input and outputs the permuted index to the standard output.
4905 If there is only one parameter, it names the text @var{input} to be read
4906 instead of the standard input. If two parameters are given, they give
4907 respectively the name of the @var{input} file to read and the name of
4908 the @var{output} file to produce. @emph{Be very careful} to note that,
4909 in this case, the contents of file given by the second parameter is
4910 destroyed. This behavior is dictated by System V @command{ptx}
4911 compatibility; @sc{gnu} Standards normally discourage output parameters not
4912 introduced by an option.
4914 Note that for @emph{any} file named as the value of an option or as an
4915 input text file, a single dash @kbd{-} may be used, in which case
4916 standard input is assumed. However, it would not make sense to use this
4917 convention more than once per program invocation.
4920 * General options in ptx:: Options which affect general program behavior.
4921 * Charset selection in ptx:: Underlying character set considerations.
4922 * Input processing in ptx:: Input fields, contexts, and keyword selection.
4923 * Output formatting in ptx:: Types of output format, and sizing the fields.
4924 * Compatibility in ptx::
4928 @node General options in ptx
4929 @subsection General options
4934 @itemx --traditional
4935 As already explained, this option disables all @sc{gnu} extensions to
4936 @command{ptx} and switches to traditional mode.
4939 Print a short help on standard output, then exit without further
4943 Print the program version on standard output, then exit without further
4951 @node Charset selection in ptx
4952 @subsection Charset selection
4954 @c FIXME: People don't necessarily know what an IBM-PC was these days.
4955 As it is set up now, the program assumes that the input file is coded
4956 using 8-bit @acronym{ISO} 8859-1 code, also known as Latin-1 character set,
4957 @emph{unless} it is compiled for MS-DOS, in which case it uses the
4958 character set of the IBM-PC@. (@sc{gnu} @command{ptx} is not known to work on
4959 smaller MS-DOS machines anymore.) Compared to 7-bit @acronym{ASCII}, the set
4960 of characters which are letters is different; this alters the behavior
4961 of regular expression matching. Thus, the default regular expression
4962 for a keyword allows foreign or diacriticized letters. Keyword sorting,
4963 however, is still crude; it obeys the underlying character set ordering
4969 @itemx --ignore-case
4970 Fold lower case letters to upper case for sorting.
4975 @node Input processing in ptx
4976 @subsection Word selection and input processing
4981 @itemx --break-file=@var{file}
4983 This option provides an alternative (to @option{-W}) method of describing
4984 which characters make up words. It introduces the name of a
4985 file which contains a list of characters which can@emph{not} be part of
4986 one word; this file is called the @dfn{Break file}. Any character which
4987 is not part of the Break file is a word constituent. If both options
4988 @option{-b} and @option{-W} are specified, then @option{-W} has precedence and
4989 @option{-b} is ignored.
4991 When @sc{gnu} extensions are enabled, the only way to avoid newline as a
4992 break character is to write all the break characters in the file with no
4993 newline at all, not even at the end of the file. When @sc{gnu} extensions
4994 are disabled, spaces, tabs and newlines are always considered as break
4995 characters even if not included in the Break file.
4998 @itemx --ignore-file=@var{file}
5000 The file associated with this option contains a list of words which will
5001 never be taken as keywords in concordance output. It is called the
5002 @dfn{Ignore file}. The file contains exactly one word in each line; the
5003 end of line separation of words is not subject to the value of the
5007 @itemx --only-file=@var{file}
5009 The file associated with this option contains a list of words which will
5010 be retained in concordance output; any word not mentioned in this file
5011 is ignored. The file is called the @dfn{Only file}. The file contains
5012 exactly one word in each line; the end of line separation of words is
5013 not subject to the value of the @option{-S} option.
5015 There is no default for the Only file. When both an Only file and an
5016 Ignore file are specified, a word is considered a keyword only
5017 if it is listed in the Only file and not in the Ignore file.
5022 On each input line, the leading sequence of non-white space characters will be
5023 taken to be a reference that has the purpose of identifying this input
5024 line in the resulting permuted index. For more information about reference
5025 production, see @xref{Output formatting in ptx}.
5026 Using this option changes the default value for option @option{-S}.
5028 Using this option, the program does not try very hard to remove
5029 references from contexts in output, but it succeeds in doing so
5030 @emph{when} the context ends exactly at the newline. If option
5031 @option{-r} is used with @option{-S} default value, or when @sc{gnu} extensions
5032 are disabled, this condition is always met and references are completely
5033 excluded from the output contexts.
5035 @item -S @var{regexp}
5036 @itemx --sentence-regexp=@var{regexp}
5038 This option selects which regular expression will describe the end of a
5039 line or the end of a sentence. In fact, this regular expression is not
5040 the only distinction between end of lines or end of sentences, and input
5041 line boundaries have no special significance outside this option. By
5042 default, when @sc{gnu} extensions are enabled and if @option{-r} option is not
5043 used, end of sentences are used. In this case, this @var{regex} is
5044 imported from @sc{gnu} Emacs:
5047 [.?!][]\"')@}]*\\($\\|\t\\| \\)[ \t\n]*
5050 Whenever @sc{gnu} extensions are disabled or if @option{-r} option is used, end
5051 of lines are used; in this case, the default @var{regexp} is just:
5057 Using an empty @var{regexp} is equivalent to completely disabling end of
5058 line or end of sentence recognition. In this case, the whole file is
5059 considered to be a single big line or sentence. The user might want to
5060 disallow all truncation flag generation as well, through option @option{-F
5061 ""}. @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
5064 When the keywords happen to be near the beginning of the input line or
5065 sentence, this often creates an unused area at the beginning of the
5066 output context line; when the keywords happen to be near the end of the
5067 input line or sentence, this often creates an unused area at the end of
5068 the output context line. The program tries to fill those unused areas
5069 by wrapping around context in them; the tail of the input line or
5070 sentence is used to fill the unused area on the left of the output line;
5071 the head of the input line or sentence is used to fill the unused area
5072 on the right of the output line.
5074 As a matter of convenience to the user, many usual backslashed escape
5075 sequences from the C language are recognized and converted to the
5076 corresponding characters by @command{ptx} itself.
5078 @item -W @var{regexp}
5079 @itemx --word-regexp=@var{regexp}
5081 This option selects which regular expression will describe each keyword.
5082 By default, if @sc{gnu} extensions are enabled, a word is a sequence of
5083 letters; the @var{regexp} used is @samp{\w+}. When @sc{gnu} extensions are
5084 disabled, a word is by default anything which ends with a space, a tab
5085 or a newline; the @var{regexp} used is @samp{[^ \t\n]+}.
5087 An empty @var{regexp} is equivalent to not using this option.
5088 @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
5091 As a matter of convenience to the user, many usual backslashed escape
5092 sequences, as found in the C language, are recognized and converted to
5093 the corresponding characters by @command{ptx} itself.
5098 @node Output formatting in ptx
5099 @subsection Output formatting
5101 Output format is mainly controlled by the @option{-O} and @option{-T} options
5102 described in the table below. When neither @option{-O} nor @option{-T} are
5103 selected, and if @sc{gnu} extensions are enabled, the program chooses an
5104 output format suitable for a dumb terminal. Each keyword occurrence is
5105 output to the center of one line, surrounded by its left and right
5106 contexts. Each field is properly justified, so the concordance output
5107 can be readily observed. As a special feature, if automatic
5108 references are selected by option @option{-A} and are output before the
5109 left context, that is, if option @option{-R} is @emph{not} selected, then
5110 a colon is added after the reference; this nicely interfaces with @sc{gnu}
5111 Emacs @code{next-error} processing. In this default output format, each
5112 white space character, like newline and tab, is merely changed to
5113 exactly one space, with no special attempt to compress consecutive
5114 spaces. This might change in the future. Except for those white space
5115 characters, every other character of the underlying set of 256
5116 characters is transmitted verbatim.
5118 Output format is further controlled by the following options.
5122 @item -g @var{number}
5123 @itemx --gap-size=@var{number}
5125 Select the size of the minimum white space gap between the fields on the
5128 @item -w @var{number}
5129 @itemx --width=@var{number}
5131 Select the maximum output width of each final line. If references are
5132 used, they are included or excluded from the maximum output width
5133 depending on the value of option @option{-R}. If this option is not
5134 selected, that is, when references are output before the left context,
5135 the maximum output width takes into account the maximum length of all
5136 references. If this option is selected, that is, when references are
5137 output after the right context, the maximum output width does not take
5138 into account the space taken by references, nor the gap that precedes
5142 @itemx --auto-reference
5144 Select automatic references. Each input line will have an automatic
5145 reference made up of the file name and the line ordinal, with a single
5146 colon between them. However, the file name will be empty when standard
5147 input is being read. If both @option{-A} and @option{-r} are selected, then
5148 the input reference is still read and skipped, but the automatic
5149 reference is used at output time, overriding the input reference.
5152 @itemx --right-side-refs
5154 In the default output format, when option @option{-R} is not used, any
5155 references produced by the effect of options @option{-r} or @option{-A} are
5156 placed to the far right of output lines, after the right context. With
5157 default output format, when the @option{-R} option is specified, references
5158 are rather placed at the beginning of each output line, before the left
5159 context. For any other output format, option @option{-R} is
5160 ignored, with one exception: with @option{-R} the width of references
5161 is @emph{not} taken into account in total output width given by @option{-w}.
5163 This option is automatically selected whenever @sc{gnu} extensions are
5166 @item -F @var{string}
5167 @itemx --flac-truncation=@var{string}
5169 This option will request that any truncation in the output be reported
5170 using the string @var{string}. Most output fields theoretically extend
5171 towards the beginning or the end of the current line, or current
5172 sentence, as selected with option @option{-S}. But there is a maximum
5173 allowed output line width, changeable through option @option{-w}, which is
5174 further divided into space for various output fields. When a field has
5175 to be truncated because it cannot extend beyond the beginning or the end of
5176 the current line to fit in, then a truncation occurs. By default,
5177 the string used is a single slash, as in @option{-F /}.
5179 @var{string} may have more than one character, as in @option{-F ...}.
5180 Also, in the particular case when @var{string} is empty (@option{-F ""}),
5181 truncation flagging is disabled, and no truncation marks are appended in
5184 As a matter of convenience to the user, many usual backslashed escape
5185 sequences, as found in the C language, are recognized and converted to
5186 the corresponding characters by @command{ptx} itself.
5188 @item -M @var{string}
5189 @itemx --macro-name=@var{string}
5191 Select another @var{string} to be used instead of @samp{xx}, while
5192 generating output suitable for @command{nroff}, @command{troff} or @TeX{}.
5195 @itemx --format=roff
5197 Choose an output format suitable for @command{nroff} or @command{troff}
5198 processing. Each output line will look like:
5201 .xx "@var{tail}" "@var{before}" "@var{keyword_and_after}"@c
5202 "@var{head}" "@var{ref}"
5205 so it will be possible to write a @samp{.xx} roff macro to take care of
5206 the output typesetting. This is the default output format when @sc{gnu}
5207 extensions are disabled. Option @option{-M} can be used to change
5208 @samp{xx} to another macro name.
5210 In this output format, each non-graphical character, like newline and
5211 tab, is merely changed to exactly one space, with no special attempt to
5212 compress consecutive spaces. Each quote character: @kbd{"} is doubled
5213 so it will be correctly processed by @command{nroff} or @command{troff}.
5218 Choose an output format suitable for @TeX{} processing. Each output
5219 line will look like:
5222 \xx @{@var{tail}@}@{@var{before}@}@{@var{keyword}@}@c
5223 @{@var{after}@}@{@var{head}@}@{@var{ref}@}
5227 so it will be possible to write a @code{\xx} definition to take care of
5228 the output typesetting. Note that when references are not being
5229 produced, that is, neither option @option{-A} nor option @option{-r} is
5230 selected, the last parameter of each @code{\xx} call is inhibited.
5231 Option @option{-M} can be used to change @samp{xx} to another macro
5234 In this output format, some special characters, like @kbd{$}, @kbd{%},
5235 @kbd{&}, @kbd{#} and @kbd{_} are automatically protected with a
5236 backslash. Curly brackets @kbd{@{}, @kbd{@}} are protected with a
5237 backslash and a pair of dollar signs (to force mathematical mode). The
5238 backslash itself produces the sequence @code{\backslash@{@}}.
5239 Circumflex and tilde diacritical marks produce the sequence @code{^\@{ @}} and
5240 @code{~\@{ @}} respectively. Other diacriticized characters of the
5241 underlying character set produce an appropriate @TeX{} sequence as far
5242 as possible. The other non-graphical characters, like newline and tab,
5243 and all other characters which are not part of @acronym{ASCII}, are merely
5244 changed to exactly one space, with no special attempt to compress
5245 consecutive spaces. Let me know how to improve this special character
5246 processing for @TeX{}.
5251 @node Compatibility in ptx
5252 @subsection The @sc{gnu} extensions to @command{ptx}
5254 This version of @command{ptx} contains a few features which do not exist in
5255 System V @command{ptx}. These extra features are suppressed by using the
5256 @option{-G} command line option, unless overridden by other command line
5257 options. Some @sc{gnu} extensions cannot be recovered by overriding, so the
5258 simple rule is to avoid @option{-G} if you care about @sc{gnu} extensions.
5259 Here are the differences between this program and System V @command{ptx}.
5264 This program can read many input files at once, it always writes the
5265 resulting concordance on standard output. On the other hand, System V
5266 @command{ptx} reads only one file and sends the result to standard output
5267 or, if a second @var{file} parameter is given on the command, to that
5270 Having output parameters not introduced by options is a dangerous
5271 practice which @sc{gnu} avoids as far as possible. So, for using @command{ptx}
5272 portably between @sc{gnu} and System V, you should always use it with a
5273 single input file, and always expect the result on standard output. You
5274 might also want to automatically configure in a @option{-G} option to
5275 @command{ptx} calls in products using @command{ptx}, if the configurator finds
5276 that the installed @command{ptx} accepts @option{-G}.
5279 The only options available in System V @command{ptx} are options @option{-b},
5280 @option{-f}, @option{-g}, @option{-i}, @option{-o}, @option{-r}, @option{-t} and
5281 @option{-w}. All other options are @sc{gnu} extensions and are not repeated in
5282 this enumeration. Moreover, some options have a slightly different
5283 meaning when @sc{gnu} extensions are enabled, as explained below.
5286 By default, concordance output is not formatted for @command{troff} or
5287 @command{nroff}. It is rather formatted for a dumb terminal. @command{troff}
5288 or @command{nroff} output may still be selected through option @option{-O}.
5291 Unless @option{-R} option is used, the maximum reference width is
5292 subtracted from the total output line width. With @sc{gnu} extensions
5293 disabled, width of references is not taken into account in the output
5294 line width computations.
5297 All 256 bytes, even @acronym{ASCII} @sc{nul} bytes, are always read and
5298 processed from input file with no adverse effect, even if @sc{gnu} extensions
5299 are disabled. However, System V @command{ptx} does not accept 8-bit characters,
5300 a few control characters are rejected, and the tilde @kbd{~} is also rejected.
5303 Input line length is only limited by available memory, even if @sc{gnu}
5304 extensions are disabled. However, System V @command{ptx} processes only
5305 the first 200 characters in each line.
5308 The break (non-word) characters default to be every character except all
5309 letters of the underlying character set, diacriticized or not. When @sc{gnu}
5310 extensions are disabled, the break characters default to space, tab and
5314 The program makes better use of output line width. If @sc{gnu} extensions
5315 are disabled, the program rather tries to imitate System V @command{ptx},
5316 but still, there are some slight disposition glitches this program does
5317 not completely reproduce.
5320 The user can specify both an Ignore file and an Only file. This is not
5321 allowed with System V @command{ptx}.
5326 @node tsort invocation
5327 @section @command{tsort}: Topological sort
5330 @cindex topological sort
5332 @command{tsort} performs a topological sort on the given @var{file}, or
5333 standard input if no input file is given or for a @var{file} of
5334 @samp{-}. For more details and some history, see @ref{tsort background}.
5338 tsort [@var{option}] [@var{file}]
5341 @command{tsort} reads its input as pairs of strings, separated by blanks,
5342 indicating a partial ordering. The output is a total ordering that
5343 corresponds to the given partial ordering.
5357 will produce the output
5368 Consider a more realistic example.
5369 You have a large set of functions all in one file, and they may all be
5370 declared static except one. Currently that one (say @code{main}) is the
5371 first function defined in the file, and the ones it calls directly follow
5372 it, followed by those they call, etc. Let's say that you are determined
5373 to take advantage of prototypes, so you have to choose between declaring
5374 all of those functions (which means duplicating a lot of information from
5375 the definitions) and rearranging the functions so that as many as possible
5376 are defined before they are used. One way to automate the latter process
5377 is to get a list for each function of the functions it calls directly.
5378 Many programs can generate such lists. They describe a call graph.
5379 Consider the following list, in which a given line indicates that the
5380 function on the left calls the one on the right directly.
5386 tail_file pretty_name
5387 tail_file write_header
5389 tail_forever recheck
5390 tail_forever pretty_name
5391 tail_forever write_header
5392 tail_forever dump_remainder
5395 tail_lines start_lines
5396 tail_lines dump_remainder
5397 tail_lines file_lines
5398 tail_lines pipe_lines
5400 tail_bytes start_bytes
5401 tail_bytes dump_remainder
5402 tail_bytes pipe_bytes
5403 file_lines dump_remainder
5407 then you can use @command{tsort} to produce an ordering of those
5408 functions that satisfies your requirement.
5411 example$ tsort call-graph | tac
5431 @command{tsort} detects any cycles in the input and writes the first cycle
5432 encountered to standard error.
5434 Note that for a given partial ordering, generally there is no unique
5435 total ordering. In the context of the call graph above, the function
5436 @code{parse_options} may be placed anywhere in the list as long as it
5437 precedes @code{main}.
5439 The only options are @option{--help} and @option{--version}. @xref{Common
5445 * tsort background:: Where tsort came from.
5448 @node tsort background
5449 @subsection @command{tsort}: Background
5451 @command{tsort} exists because very early versions of the Unix linker processed
5452 an archive file exactly once, and in order. As @command{ld} read each object
5453 in the archive, it decided whether it was needed in the program based on
5454 whether it defined any symbols which were undefined at that point in
5457 This meant that dependencies within the archive had to be handled
5458 specially. For example, @code{scanf} probably calls @code{read}. That means
5459 that in a single pass through an archive, it was important for @code{scanf.o}
5460 to appear before read.o, because otherwise a program which calls
5461 @code{scanf} but not @code{read} might end up with an unexpected unresolved
5462 reference to @code{read}.
5464 The way to address this problem was to first generate a set of
5465 dependencies of one object file on another. This was done by a shell
5466 script called @command{lorder}. The GNU tools don't provide a version of
5467 lorder, as far as I know, but you can still find it in BSD
5470 Then you ran @command{tsort} over the @command{lorder} output, and you used the
5471 resulting sort to define the order in which you added objects to the archive.
5473 This whole procedure has been obsolete since about 1980, because
5474 Unix archives now contain a symbol table (traditionally built by
5475 @command{ranlib}, now generally built by @command{ar} itself), and the Unix
5476 linker uses the symbol table to effectively make multiple passes over
5479 Anyhow, that's where tsort came from. To solve an old problem with
5480 the way the linker handled archive files, which has since been solved
5484 @node Operating on fields
5485 @chapter Operating on fields
5488 * cut invocation:: Print selected parts of lines.
5489 * paste invocation:: Merge lines of files.
5490 * join invocation:: Join lines on a common field.
5494 @node cut invocation
5495 @section @command{cut}: Print selected parts of lines
5498 @command{cut} writes to standard output selected parts of each line of each
5499 input file, or standard input if no files are given or for a file name of
5503 cut @var{option}@dots{} [@var{file}]@dots{}
5506 In the table which follows, the @var{byte-list}, @var{character-list},
5507 and @var{field-list} are one or more numbers or ranges (two numbers
5508 separated by a dash) separated by commas. Bytes, characters, and
5509 fields are numbered starting at 1. Incomplete ranges may be
5510 given: @option{-@var{m}} means @samp{1-@var{m}}; @samp{@var{n}-} means
5511 @samp{@var{n}} through end of line or last field. The list elements
5512 can be repeated, can overlap, and can be specified in any order; but
5513 the selected input is written in the same order that it is read, and
5514 is written exactly once.
5516 The program accepts the following options. Also see @ref{Common
5521 @item -b @var{byte-list}
5522 @itemx --bytes=@var{byte-list}
5525 Select for printing only the bytes in positions listed in
5526 @var{byte-list}. Tabs and backspaces are treated like any other
5527 character; they take up 1 byte. If an output delimiter is specified,
5528 (see the description of @option{--output-delimiter}), then output that
5529 string between ranges of selected bytes.
5531 @item -c @var{character-list}
5532 @itemx --characters=@var{character-list}
5534 @opindex --characters
5535 Select for printing only the characters in positions listed in
5536 @var{character-list}. The same as @option{-b} for now, but
5537 internationalization will change that. Tabs and backspaces are
5538 treated like any other character; they take up 1 character. If an
5539 output delimiter is specified, (see the description of
5540 @option{--output-delimiter}), then output that string between ranges
5543 @item -f @var{field-list}
5544 @itemx --fields=@var{field-list}
5547 Select for printing only the fields listed in @var{field-list}.
5548 Fields are separated by a TAB character by default. Also print any
5549 line that contains no delimiter character, unless the
5550 @option{--only-delimited} (@option{-s}) option is specified.
5552 Note @command{awk} supports more sophisticated field processing,
5553 and by default will use (and discard) runs of blank characters to
5554 separate fields, and ignore leading and trailing blanks.
5557 awk '{print $2}' # print the second field
5558 awk '{print $NF-1}' # print the penultimate field
5559 awk '{print $2,$1}' # reorder the first two fields
5563 In the unlikely event that @command{awk} is unavailable,
5564 one can use the @command{join} command, to process blank
5565 characters as @command{awk} does above.
5568 join -a1 -o 1.2 - /dev/null # print the second field
5569 join -a1 -o 1.2,1.1 - /dev/null # reorder the first two fields
5573 @item -d @var{input_delim_byte}
5574 @itemx --delimiter=@var{input_delim_byte}
5576 @opindex --delimiter
5577 With @option{-f}, use the first byte of @var{input_delim_byte} as
5578 the input fields separator (default is TAB).
5582 Do not split multi-byte characters (no-op for now).
5585 @itemx --only-delimited
5587 @opindex --only-delimited
5588 For @option{-f}, do not print lines that do not contain the field separator
5589 character. Normally, any line without a field separator is printed verbatim.
5591 @item --output-delimiter=@var{output_delim_string}
5592 @opindex --output-delimiter
5593 With @option{-f}, output fields are separated by @var{output_delim_string}.
5594 The default with @option{-f} is to use the input delimiter.
5595 When using @option{-b} or @option{-c} to select ranges of byte or
5596 character offsets (as opposed to ranges of fields),
5597 output @var{output_delim_string} between non-overlapping
5598 ranges of selected bytes.
5601 @opindex --complement
5602 This option is a @acronym{GNU} extension.
5603 Select for printing the complement of the bytes, characters or fields
5604 selected with the @option{-b}, @option{-c} or @option{-f} options.
5605 In other words, do @emph{not} print the bytes, characters or fields
5606 specified via those options. This option is useful when you have
5607 many fields and want to print all but a few of them.
5614 @node paste invocation
5615 @section @command{paste}: Merge lines of files
5618 @cindex merging files
5620 @command{paste} writes to standard output lines consisting of sequentially
5621 corresponding lines of each given file, separated by a TAB character.
5622 Standard input is used for a file name of @samp{-} or if no input files
5644 paste [@var{option}]@dots{} [@var{file}]@dots{}
5647 The program accepts the following options. Also see @ref{Common options}.
5655 Paste the lines of one file at a time rather than one line from each
5656 file. Using the above example data:
5659 $ paste -s num2 let3
5664 @item -d @var{delim-list}
5665 @itemx --delimiters=@var{delim-list}
5667 @opindex --delimiters
5668 Consecutively use the characters in @var{delim-list} instead of
5669 TAB to separate merged lines. When @var{delim-list} is
5670 exhausted, start again at its beginning. Using the above example data:
5673 $ paste -d '%_' num2 let3 num2
5684 @node join invocation
5685 @section @command{join}: Join lines on a common field
5688 @cindex common field, joining on
5690 @command{join} writes to standard output a line for each pair of input
5691 lines that have identical join fields. Synopsis:
5694 join [@var{option}]@dots{} @var{file1} @var{file2}
5697 Either @var{file1} or @var{file2} (but not both) can be @samp{-},
5698 meaning standard input. @var{file1} and @var{file2} should be
5699 sorted on the join fields.
5702 Normally, the sort order is that of the
5703 collating sequence specified by the @env{LC_COLLATE} locale. Unless
5704 the @option{-t} option is given, the sort comparison ignores blanks at
5705 the start of the join field, as in @code{sort -b}. If the
5706 @option{--ignore-case} option is given, the sort comparison ignores
5707 the case of characters in the join field, as in @code{sort -f}.
5709 The @command{sort} and @command{join} commands should use consistent
5710 locales and options if the output of @command{sort} is fed to
5711 @command{join}. You can use a command like @samp{sort -k 1b,1} to
5712 sort a file on its default join field, but if you select a non-default
5713 locale, join field, separator, or comparison options, then you should
5714 do so consistently between @command{join} and @command{sort}.
5715 If @samp{join -t ''} is specified then the whole line is considered which
5716 matches the default operation of sort.
5718 If the input has no unpairable lines, a @acronym{GNU} extension is
5719 available; the sort order can be any order that considers two fields
5720 to be equal if and only if the sort comparison described above
5721 considers them to be equal. For example:
5739 @checkOrderOption{join}
5744 @item the join field is the first field in each line;
5745 @item fields in the input are separated by one or more blanks, with leading
5746 blanks on the line ignored;
5747 @item fields in the output are separated by a space;
5748 @item each output line consists of the join field, the remaining
5749 fields from @var{file1}, then the remaining fields from @var{file2}.
5752 The program accepts the following options. Also see @ref{Common options}.
5756 @item -a @var{file-number}
5758 Print a line for each unpairable line in file @var{file-number} (either
5759 @samp{1} or @samp{2}), in addition to the normal output.
5762 Fail with an error message if either input file is wrongly ordered.
5764 @item --nocheck-order
5765 Do not check that both input files are in sorted order. This is the default.
5767 @item -e @var{string}
5769 Replace those output fields that are missing in the input with @var{string}.
5770 I.E. missing fields specified with the @option{-12jo} options.
5774 Treat the first line of each input file as a header line. The header lines will
5775 be joined and printed as the first output line. If @option{-o} is used to
5776 specify output format, the header line will be printed according to the
5777 specified format. The header lines will not be checked for ordering even if
5778 @option{--check-order} is specified. Also if the header lines from each file
5779 do not match, the heading fields from the first file will be used.
5782 @itemx --ignore-case
5784 @opindex --ignore-case
5785 Ignore differences in case when comparing keys.
5786 With this option, the lines of the input files must be ordered in the same way.
5787 Use @samp{sort -f} to produce this ordering.
5789 @item -1 @var{field}
5791 Join on field @var{field} (a positive integer) of file 1.
5793 @item -2 @var{field}
5795 Join on field @var{field} (a positive integer) of file 2.
5797 @item -j @var{field}
5798 Equivalent to @option{-1 @var{field} -2 @var{field}}.
5800 @item -o @var{field-list}
5802 If the keyword @samp{auto} is specified, infer the output format from
5803 the first line in each file. This is the same as the default output format
5804 but also ensures the same number of fields are output for each line.
5805 Missing fields are replaced with the @option{-e} option and extra fields
5808 Otherwise, construct each output line according to the format in
5809 @var{field-list}. Each element in @var{field-list} is either the single
5810 character @samp{0} or has the form @var{m.n} where the file number, @var{m},
5811 is @samp{1} or @samp{2} and @var{n} is a positive field number.
5813 A field specification of @samp{0} denotes the join field.
5814 In most cases, the functionality of the @samp{0} field spec
5815 may be reproduced using the explicit @var{m.n} that corresponds
5816 to the join field. However, when printing unpairable lines
5817 (using either of the @option{-a} or @option{-v} options), there is no way
5818 to specify the join field using @var{m.n} in @var{field-list}
5819 if there are unpairable lines in both files.
5820 To give @command{join} that functionality, @acronym{POSIX} invented the @samp{0}
5821 field specification notation.
5823 The elements in @var{field-list}
5824 are separated by commas or blanks.
5825 Blank separators typically need to be quoted for the shell. For
5826 example, the commands @samp{join -o 1.2,2.2} and @samp{join -o '1.2
5827 2.2'} are equivalent.
5829 All output lines---including those printed because of any -a or -v
5830 option---are subject to the specified @var{field-list}.
5833 Use character @var{char} as the input and output field separator.
5834 Treat as significant each occurrence of @var{char} in the input file.
5835 Use @samp{sort -t @var{char}}, without the @option{-b} option of
5836 @samp{sort}, to produce this ordering. If @samp{join -t ''} is specified,
5837 the whole line is considered, matching the default operation of sort.
5838 If @samp{-t '\0'} is specified then the @acronym{ASCII} @sc{nul}
5839 character is used to delimit the fields.
5841 @item -v @var{file-number}
5842 Print a line for each unpairable line in file @var{file-number}
5843 (either @samp{1} or @samp{2}), instead of the normal output.
5850 @node Operating on characters
5851 @chapter Operating on characters
5853 @cindex operating on characters
5855 This commands operate on individual characters.
5858 * tr invocation:: Translate, squeeze, and/or delete characters.
5859 * expand invocation:: Convert tabs to spaces.
5860 * unexpand invocation:: Convert spaces to tabs.
5865 @section @command{tr}: Translate, squeeze, and/or delete characters
5872 tr [@var{option}]@dots{} @var{set1} [@var{set2}]
5875 @command{tr} copies standard input to standard output, performing
5876 one of the following operations:
5880 translate, and optionally squeeze repeated characters in the result,
5882 squeeze repeated characters,
5886 delete characters, then squeeze repeated characters from the result.
5889 The @var{set1} and (if given) @var{set2} arguments define ordered
5890 sets of characters, referred to below as @var{set1} and @var{set2}. These
5891 sets are the characters of the input that @command{tr} operates on.
5892 The @option{--complement} (@option{-c}, @option{-C}) option replaces
5894 complement (all of the characters that are not in @var{set1}).
5896 Currently @command{tr} fully supports only single-byte characters.
5897 Eventually it will support multibyte characters; when it does, the
5898 @option{-C} option will cause it to complement the set of characters,
5899 whereas @option{-c} will cause it to complement the set of values.
5900 This distinction will matter only when some values are not characters,
5901 and this is possible only in locales using multibyte encodings when
5902 the input contains encoding errors.
5904 The program accepts the @option{--help} and @option{--version}
5905 options. @xref{Common options}. Options must precede operands.
5910 * Character sets:: Specifying sets of characters.
5911 * Translating:: Changing one set of characters to another.
5912 * Squeezing:: Squeezing repeats and deleting.
5916 @node Character sets
5917 @subsection Specifying sets of characters
5919 @cindex specifying sets of characters
5921 The format of the @var{set1} and @var{set2} arguments resembles
5922 the format of regular expressions; however, they are not regular
5923 expressions, only lists of characters. Most characters simply
5924 represent themselves in these strings, but the strings can contain
5925 the shorthands listed below, for convenience. Some of them can be
5926 used only in @var{set1} or @var{set2}, as noted below.
5930 @item Backslash escapes
5931 @cindex backslash escapes
5933 The following backslash escape sequences are recognized:
5951 The 8-bit character with the value given by @var{ooo}, which is 1 to 3
5952 octal digits. Note that @samp{\400} is interpreted as the two-byte
5953 sequence, @samp{\040} @samp{0}.
5958 While a backslash followed by a character not listed above is
5959 interpreted as that character, the backslash also effectively
5960 removes any special significance, so it is useful to escape
5961 @samp{[}, @samp{]}, @samp{*}, and @samp{-}.
5966 The notation @samp{@var{m}-@var{n}} expands to all of the characters
5967 from @var{m} through @var{n}, in ascending order. @var{m} should
5968 collate before @var{n}; if it doesn't, an error results. As an example,
5969 @samp{0-9} is the same as @samp{0123456789}.
5971 @sc{gnu} @command{tr} does not support the System V syntax that uses square
5972 brackets to enclose ranges. Translations specified in that format
5973 sometimes work as expected, since the brackets are often transliterated
5974 to themselves. However, they should be avoided because they sometimes
5975 behave unexpectedly. For example, @samp{tr -d '[0-9]'} deletes brackets
5978 Many historically common and even accepted uses of ranges are not
5979 portable. For example, on @acronym{EBCDIC} hosts using the @samp{A-Z}
5980 range will not do what most would expect because @samp{A} through @samp{Z}
5981 are not contiguous as they are in @acronym{ASCII}.
5982 If you can rely on a @acronym{POSIX} compliant version of @command{tr}, then
5983 the best way to work around this is to use character classes (see below).
5984 Otherwise, it is most portable (and most ugly) to enumerate the members
5987 @item Repeated characters
5988 @cindex repeated characters
5990 The notation @samp{[@var{c}*@var{n}]} in @var{set2} expands to @var{n}
5991 copies of character @var{c}. Thus, @samp{[y*6]} is the same as
5992 @samp{yyyyyy}. The notation @samp{[@var{c}*]} in @var{string2} expands
5993 to as many copies of @var{c} as are needed to make @var{set2} as long as
5994 @var{set1}. If @var{n} begins with @samp{0}, it is interpreted in
5995 octal, otherwise in decimal.
5997 @item Character classes
5998 @cindex character classes
6000 The notation @samp{[:@var{class}:]} expands to all of the characters in
6001 the (predefined) class @var{class}. The characters expand in no
6002 particular order, except for the @code{upper} and @code{lower} classes,
6003 which expand in ascending order. When the @option{--delete} (@option{-d})
6004 and @option{--squeeze-repeats} (@option{-s}) options are both given, any
6005 character class can be used in @var{set2}. Otherwise, only the
6006 character classes @code{lower} and @code{upper} are accepted in
6007 @var{set2}, and then only if the corresponding character class
6008 (@code{upper} and @code{lower}, respectively) is specified in the same
6009 relative position in @var{set1}. Doing this specifies case conversion.
6010 The class names are given below; an error results when an invalid class
6022 Horizontal whitespace.
6031 Printable characters, not including space.
6037 Printable characters, including space.
6040 Punctuation characters.
6043 Horizontal or vertical whitespace.
6052 @item Equivalence classes
6053 @cindex equivalence classes
6055 The syntax @samp{[=@var{c}=]} expands to all of the characters that are
6056 equivalent to @var{c}, in no particular order. Equivalence classes are
6057 a relatively recent invention intended to support non-English alphabets.
6058 But there seems to be no standard way to define them or determine their
6059 contents. Therefore, they are not fully implemented in @sc{gnu} @command{tr};
6060 each character's equivalence class consists only of that character,
6061 which is of no particular use.
6067 @subsection Translating
6069 @cindex translating characters
6071 @command{tr} performs translation when @var{set1} and @var{set2} are
6072 both given and the @option{--delete} (@option{-d}) option is not given.
6073 @command{tr} translates each character of its input that is in @var{set1}
6074 to the corresponding character in @var{set2}. Characters not in
6075 @var{set1} are passed through unchanged. When a character appears more
6076 than once in @var{set1} and the corresponding characters in @var{set2}
6077 are not all the same, only the final one is used. For example, these
6078 two commands are equivalent:
6085 A common use of @command{tr} is to convert lowercase characters to
6086 uppercase. This can be done in many ways. Here are three of them:
6089 tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
6091 tr '[:lower:]' '[:upper:]'
6095 But note that using ranges like @code{a-z} above is not portable.
6097 When @command{tr} is performing translation, @var{set1} and @var{set2}
6098 typically have the same length. If @var{set1} is shorter than
6099 @var{set2}, the extra characters at the end of @var{set2} are ignored.
6101 On the other hand, making @var{set1} longer than @var{set2} is not
6102 portable; @acronym{POSIX} says that the result is undefined. In this situation,
6103 BSD @command{tr} pads @var{set2} to the length of @var{set1} by repeating
6104 the last character of @var{set2} as many times as necessary. System V
6105 @command{tr} truncates @var{set1} to the length of @var{set2}.
6107 By default, @sc{gnu} @command{tr} handles this case like BSD @command{tr}.
6108 When the @option{--truncate-set1} (@option{-t}) option is given,
6109 @sc{gnu} @command{tr} handles this case like the System V @command{tr}
6110 instead. This option is ignored for operations other than translation.
6112 Acting like System V @command{tr} in this case breaks the relatively common
6116 tr -cs A-Za-z0-9 '\012'
6120 because it converts only zero bytes (the first element in the
6121 complement of @var{set1}), rather than all non-alphanumerics, to
6125 By the way, the above idiom is not portable because it uses ranges, and
6126 it assumes that the octal code for newline is 012.
6127 Assuming a @acronym{POSIX} compliant @command{tr}, here is a better
6131 tr -cs '[:alnum:]' '[\n*]'
6136 @subsection Squeezing repeats and deleting
6138 @cindex squeezing repeat characters
6139 @cindex deleting characters
6141 When given just the @option{--delete} (@option{-d}) option, @command{tr}
6142 removes any input characters that are in @var{set1}.
6144 When given just the @option{--squeeze-repeats} (@option{-s}) option,
6145 @command{tr} replaces each input sequence of a repeated character that
6146 is in @var{set1} with a single occurrence of that character.
6148 When given both @option{--delete} and @option{--squeeze-repeats}, @command{tr}
6149 first performs any deletions using @var{set1}, then squeezes repeats
6150 from any remaining characters using @var{set2}.
6152 The @option{--squeeze-repeats} option may also be used when translating,
6153 in which case @command{tr} first performs translation, then squeezes
6154 repeats from any remaining characters using @var{set2}.
6156 Here are some examples to illustrate various combinations of options:
6161 Remove all zero bytes:
6168 Put all words on lines by themselves. This converts all
6169 non-alphanumeric characters to newlines, then squeezes each string
6170 of repeated newlines into a single newline:
6173 tr -cs '[:alnum:]' '[\n*]'
6177 Convert each sequence of repeated newlines to a single newline:
6184 Find doubled occurrences of words in a document.
6185 @c Separate the following two "the"s, so typo checkers don't complain.
6186 For example, people often write ``the @w{}the'' with the repeated words
6187 separated by a newline. The Bourne shell script below works first
6188 by converting each sequence of punctuation and blank characters to a
6189 single newline. That puts each ``word'' on a line by itself.
6190 Next it maps all uppercase characters to lower case, and finally it
6191 runs @command{uniq} with the @option{-d} option to print out only the words
6197 | tr -s '[:punct:][:blank:]' '[\n*]' \
6198 | tr '[:upper:]' '[:lower:]' \
6203 Deleting a small set of characters is usually straightforward. For example,
6204 to remove all @samp{a}s, @samp{x}s, and @samp{M}s you would do this:
6210 However, when @samp{-} is one of those characters, it can be tricky because
6211 @samp{-} has special meanings. Performing the same task as above but also
6212 removing all @samp{-} characters, we might try @code{tr -d -axM}, but
6213 that would fail because @command{tr} would try to interpret @option{-a} as
6214 a command-line option. Alternatively, we could try putting the hyphen
6215 inside the string, @code{tr -d a-xM}, but that wouldn't work either because
6216 it would make @command{tr} interpret @code{a-x} as the range of characters
6217 @samp{a}@dots{}@samp{x} rather than the three.
6218 One way to solve the problem is to put the hyphen at the end of the list
6225 Or you can use @samp{--} to terminate option processing:
6231 More generally, use the character class notation @code{[=c=]}
6232 with @samp{-} (or any other character) in place of the @samp{c}:
6238 Note how single quotes are used in the above example to protect the
6239 square brackets from interpretation by a shell.
6244 @node expand invocation
6245 @section @command{expand}: Convert tabs to spaces
6248 @cindex tabs to spaces, converting
6249 @cindex converting tabs to spaces
6251 @command{expand} writes the contents of each given @var{file}, or standard
6252 input if none are given or for a @var{file} of @samp{-}, to standard
6253 output, with tab characters converted to the appropriate number of
6257 expand [@var{option}]@dots{} [@var{file}]@dots{}
6260 By default, @command{expand} converts all tabs to spaces. It preserves
6261 backspace characters in the output; they decrement the column count for
6262 tab calculations. The default action is equivalent to @option{-t 8} (set
6263 tabs every 8 columns).
6265 The program accepts the following options. Also see @ref{Common options}.
6269 @item -t @var{tab1}[,@var{tab2}]@dots{}
6270 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6273 @cindex tab stops, setting
6274 If only one tab stop is given, set the tabs @var{tab1} spaces apart
6275 (default is 8). Otherwise, set the tabs at columns @var{tab1},
6276 @var{tab2}, @dots{} (numbered from 0), and replace any tabs beyond the
6277 last tab stop given with single spaces. Tab stops can be separated by
6278 blanks as well as by commas.
6280 For compatibility, GNU @command{expand} also accepts the obsolete
6281 option syntax, @option{-@var{t1}[,@var{t2}]@dots{}}. New scripts
6282 should use @option{-t @var{t1}[,@var{t2}]@dots{}} instead.
6288 @cindex initial tabs, converting
6289 Only convert initial tabs (those that precede all non-space or non-tab
6290 characters) on each line to spaces.
6297 @node unexpand invocation
6298 @section @command{unexpand}: Convert spaces to tabs
6302 @command{unexpand} writes the contents of each given @var{file}, or
6303 standard input if none are given or for a @var{file} of @samp{-}, to
6304 standard output, converting blanks at the beginning of each line into
6305 as many tab characters as needed. In the default @acronym{POSIX}
6306 locale, a @dfn{blank} is a space or a tab; other locales may specify
6307 additional blank characters. Synopsis:
6310 unexpand [@var{option}]@dots{} [@var{file}]@dots{}
6313 By default, @command{unexpand} converts only initial blanks (those
6314 that precede all non-blank characters) on each line. It
6315 preserves backspace characters in the output; they decrement the column
6316 count for tab calculations. By default, tabs are set at every 8th
6319 The program accepts the following options. Also see @ref{Common options}.
6323 @item -t @var{tab1}[,@var{tab2}]@dots{}
6324 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6327 If only one tab stop is given, set the tabs @var{tab1} columns apart
6328 instead of the default 8. Otherwise, set the tabs at columns
6329 @var{tab1}, @var{tab2}, @dots{} (numbered from 0), and leave blanks
6330 beyond the tab stops given unchanged. Tab stops can be separated by
6331 blanks as well as by commas. This option implies the @option{-a} option.
6333 For compatibility, GNU @command{unexpand} supports the obsolete option syntax,
6334 @option{-@var{tab1}[,@var{tab2}]@dots{}}, where tab stops must be
6335 separated by commas. (Unlike @option{-t}, this obsolete option does
6336 not imply @option{-a}.) New scripts should use @option{--first-only -t
6337 @var{tab1}[,@var{tab2}]@dots{}} instead.
6343 Also convert all sequences of two or more blanks just before a tab stop,
6344 even if they occur after non-blank characters in a line.
6351 @node Directory listing
6352 @chapter Directory listing
6354 This chapter describes the @command{ls} command and its variants @command{dir}
6355 and @command{vdir}, which list information about files.
6358 * ls invocation:: List directory contents.
6359 * dir invocation:: Briefly ls.
6360 * vdir invocation:: Verbosely ls.
6361 * dircolors invocation:: Color setup for ls, etc.
6366 @section @command{ls}: List directory contents
6369 @cindex directory listing
6371 The @command{ls} program lists information about files (of any type,
6372 including directories). Options and file arguments can be intermixed
6373 arbitrarily, as usual.
6375 For non-option command-line arguments that are directories, by default
6376 @command{ls} lists the contents of directories, not recursively, and
6377 omitting files with names beginning with @samp{.}. For other non-option
6378 arguments, by default @command{ls} lists just the file name. If no
6379 non-option argument is specified, @command{ls} operates on the current
6380 directory, acting as if it had been invoked with a single argument of @samp{.}.
6383 By default, the output is sorted alphabetically, according to the locale
6384 settings in effect.@footnote{If you use a non-@acronym{POSIX}
6385 locale (e.g., by setting @env{LC_ALL} to @samp{en_US}), then @command{ls} may
6386 produce output that is sorted differently than you're accustomed to.
6387 In that case, set the @env{LC_ALL} environment variable to @samp{C}.}
6388 If standard output is
6389 a terminal, the output is in columns (sorted vertically) and control
6390 characters are output as question marks; otherwise, the output is listed
6391 one per line and control characters are output as-is.
6393 Because @command{ls} is such a fundamental program, it has accumulated many
6394 options over the years. They are described in the subsections below;
6395 within each section, options are listed alphabetically (ignoring case).
6396 The division of options into the subsections is not absolute, since some
6397 options affect more than one aspect of @command{ls}'s operation.
6399 @cindex exit status of @command{ls}
6404 1 minor problems (e.g., failure to access a file or directory not
6405 specified as a command line argument. This happens when listing a
6406 directory in which entries are actively being removed or renamed.)
6407 2 serious trouble (e.g., memory exhausted, invalid option, failure
6408 to access a file or directory specified as a command line argument
6409 or a directory loop)
6412 Also see @ref{Common options}.
6415 * Which files are listed::
6416 * What information is listed::
6417 * Sorting the output::
6418 * Details about version sort::
6419 * General output formatting::
6420 * Formatting file timestamps::
6421 * Formatting the file names::
6425 @node Which files are listed
6426 @subsection Which files are listed
6428 These options determine which files @command{ls} lists information for.
6429 By default, @command{ls} lists files and the contents of any
6430 directories on the command line, except that in directories it ignores
6431 files whose names start with @samp{.}.
6439 In directories, do not ignore file names that start with @samp{.}.
6444 @opindex --almost-all
6445 In directories, do not ignore all file names that start with @samp{.};
6446 ignore only @file{.} and @file{..}. The @option{--all} (@option{-a})
6447 option overrides this option.
6450 @itemx --ignore-backups
6452 @opindex --ignore-backups
6453 @cindex backup files, ignoring
6454 In directories, ignore files that end with @samp{~}. This option is
6455 equivalent to @samp{--ignore='*~' --ignore='.*~'}.
6460 @opindex --directory
6461 List just the names of directories, as with other types of files, rather
6462 than listing their contents.
6463 @c The following sentence is the same as the one for -F.
6464 Do not follow symbolic links listed on the
6465 command line unless the @option{--dereference-command-line} (@option{-H}),
6466 @option{--dereference} (@option{-L}), or
6467 @option{--dereference-command-line-symlink-to-dir} options are specified.
6470 @itemx --dereference-command-line
6472 @opindex --dereference-command-line
6473 @cindex symbolic links, dereferencing
6474 If a command line argument specifies a symbolic link, show information
6475 for the file the link references rather than for the link itself.
6477 @itemx --dereference-command-line-symlink-to-dir
6478 @opindex --dereference-command-line-symlink-to-dir
6479 @cindex symbolic links, dereferencing
6480 Do not dereference symbolic links, with one exception:
6481 if a command line argument specifies a symbolic link that refers to
6482 a directory, show information for that directory rather than for the
6484 This is the default behavior when no other dereferencing-related
6485 option has been specified (@option{--classify} (@option{-F}),
6486 @option{--directory} (@option{-d}),
6488 @option{--dereference} (@option{-L}), or
6489 @option{--dereference-command-line} (@option{-H})).
6491 @item --group-directories-first
6492 @opindex --group-directories-first
6493 Group all the directories before the files and then sort the
6494 directories and the files separately using the selected sort key
6495 (see --sort option).
6496 That is, this option specifies a primary sort key,
6497 and the --sort option specifies a secondary key.
6498 However, any use of @option{--sort=none}
6499 (@option{-U}) disables this option altogether.
6501 @item --hide=PATTERN
6502 @opindex --hide=@var{pattern}
6503 In directories, ignore files whose names match the shell pattern
6504 @var{pattern}, unless the @option{--all} (@option{-a}) or
6505 @option{--almost-all} (@option{-A}) is also given. This
6506 option acts like @option{--ignore=@var{pattern}} except that it has no
6507 effect if @option{--all} (@option{-a}) or @option{--almost-all}
6508 (@option{-A}) is also given.
6510 This option can be useful in shell aliases. For example, if
6511 @command{lx} is an alias for @samp{ls --hide='*~'} and @command{ly} is
6512 an alias for @samp{ls --ignore='*~'}, then the command @samp{lx -A}
6513 lists the file @file{README~} even though @samp{ly -A} would not.
6515 @item -I @var{pattern}
6516 @itemx --ignore=@var{pattern}
6518 @opindex --ignore=@var{pattern}
6519 In directories, ignore files whose names match the shell pattern
6520 (not regular expression) @var{pattern}. As
6521 in the shell, an initial @samp{.} in a file name does not match a
6522 wildcard at the start of @var{pattern}. Sometimes it is useful
6523 to give this option several times. For example,
6526 $ ls --ignore='.??*' --ignore='.[^.]' --ignore='#*'
6529 The first option ignores names of length 3 or more that start with @samp{.},
6530 the second ignores all two-character names that start with @samp{.}
6531 except @samp{..}, and the third ignores names that start with @samp{#}.
6534 @itemx --dereference
6536 @opindex --dereference
6537 @cindex symbolic links, dereferencing
6538 When showing file information for a symbolic link, show information
6539 for the file the link references rather than the link itself.
6540 However, even with this option, @command{ls} still prints the name
6541 of the link itself, not the name of the file that the link points to.
6546 @opindex --recursive
6547 @cindex recursive directory listing
6548 @cindex directory listing, recursive
6549 List the contents of all directories recursively.
6554 @node What information is listed
6555 @subsection What information is listed
6557 These options affect the information that @command{ls} displays. By
6558 default, only file names are shown.
6564 @cindex hurd, author, printing
6565 List each file's author when producing long format directory listings.
6566 In GNU/Hurd, file authors can differ from their owners, but in other
6567 operating systems the two are the same.
6573 @cindex dired Emacs mode support
6574 With the long listing (@option{-l}) format, print an additional line after
6578 //DIRED// @var{beg1} @var{end1} @var{beg2} @var{end2} @dots{}
6582 The @var{begn} and @var{endn} are unsigned integers that record the
6583 byte position of the beginning and end of each file name in the output.
6584 This makes it easy for Emacs to find the names, even when they contain
6585 unusual characters such as space or newline, without fancy searching.
6587 If directories are being listed recursively (@option{-R}), output a similar
6588 line with offsets for each subdirectory name:
6591 //SUBDIRED// @var{beg1} @var{end1} @dots{}
6594 Finally, output a line of the form:
6597 //DIRED-OPTIONS// --quoting-style=@var{word}
6601 where @var{word} is the quoting style (@pxref{Formatting the file names}).
6603 Here is an actual example:
6606 $ mkdir -p a/sub/deeper a/sub2
6608 $ touch a/sub/deeper/file
6609 $ ls -gloRF --dired a
6612 -rw-r--r-- 1 0 Jun 10 12:27 f1
6613 -rw-r--r-- 1 0 Jun 10 12:27 f2
6614 drwxr-xr-x 3 4096 Jun 10 12:27 sub/
6615 drwxr-xr-x 2 4096 Jun 10 12:27 sub2/
6619 drwxr-xr-x 2 4096 Jun 10 12:27 deeper/
6623 -rw-r--r-- 1 0 Jun 10 12:27 file
6627 //DIRED// 48 50 84 86 120 123 158 162 217 223 282 286
6628 //SUBDIRED// 2 3 167 172 228 240 290 296
6629 //DIRED-OPTIONS// --quoting-style=literal
6632 Note that the pairs of offsets on the @samp{//DIRED//} line above delimit
6633 these names: @file{f1}, @file{f2}, @file{sub}, @file{sub2}, @file{deeper},
6635 The offsets on the @samp{//SUBDIRED//} line delimit the following
6636 directory names: @file{a}, @file{a/sub}, @file{a/sub/deeper}, @file{a/sub2}.
6638 Here is an example of how to extract the fifth entry name, @samp{deeper},
6639 corresponding to the pair of offsets, 222 and 228:
6642 $ ls -gloRF --dired a > out
6643 $ dd bs=1 skip=222 count=6 < out 2>/dev/null; echo
6647 Note that although the listing above includes a trailing slash
6648 for the @samp{deeper} entry, the offsets select the name without
6649 the trailing slash. However, if you invoke @command{ls} with @option{--dired}
6650 along with an option like @option{--escape} (aka @option{-b}) and operate
6651 on a file whose name contains special characters, notice that the backslash
6656 $ ls -blog --dired 'a b'
6657 -rw-r--r-- 1 0 Jun 10 12:28 a\ b
6659 //DIRED-OPTIONS// --quoting-style=escape
6662 If you use a quoting style that adds quote marks
6663 (e.g., @option{--quoting-style=c}), then the offsets include the quote marks.
6664 So beware that the user may select the quoting style via the environment
6665 variable @env{QUOTING_STYLE}. Hence, applications using @option{--dired}
6666 should either specify an explicit @option{--quoting-style=literal} option
6667 (aka @option{-N} or @option{--literal}) on the command line, or else be
6668 prepared to parse the escaped names.
6671 @opindex --full-time
6672 Produce long format directory listings, and list times in full. It is
6673 equivalent to using @option{--format=long} with
6674 @option{--time-style=full-iso} (@pxref{Formatting file timestamps}).
6678 Produce long format directory listings, but don't display owner information.
6684 Inhibit display of group information in a long format directory listing.
6685 (This is the default in some non-@sc{gnu} versions of @command{ls}, so we
6686 provide this option for compatibility.)
6694 @cindex inode number, printing
6695 Print the inode number (also called the file serial number and index
6696 number) of each file to the left of the file name. (This number
6697 uniquely identifies each file within a particular file system.)
6700 @itemx --format=long
6701 @itemx --format=verbose
6704 @opindex long ls @r{format}
6705 @opindex verbose ls @r{format}
6706 In addition to the name of each file, print the file type, file mode bits,
6707 number of hard links, owner name, group name, size, and
6708 timestamp (@pxref{Formatting file timestamps}), normally
6709 the modification time. Print question marks for information that
6710 cannot be determined.
6712 Normally the size is printed as a byte count without punctuation, but
6713 this can be overridden (@pxref{Block size}). For example, @option{-h}
6714 prints an abbreviated, human-readable count, and
6715 @samp{--block-size="'1"} prints a byte count with the thousands
6716 separator of the current locale.
6718 For each directory that is listed, preface the files with a line
6719 @samp{total @var{blocks}}, where @var{blocks} is the total disk allocation
6720 for all files in that directory. The block size currently defaults to 1024
6721 bytes, but this can be overridden (@pxref{Block size}).
6722 The @var{blocks} computed counts each hard link separately;
6723 this is arguably a deficiency.
6725 The file type is one of the following characters:
6727 @c The commented-out entries are ones we're not sure about.
6735 character special file
6737 high performance (``contiguous data'') file
6741 door (Solaris 2.5 and up)
6743 @c semaphore, if this is a distinct file type
6747 @c multiplexed file (7th edition Unix; obsolete)
6749 off-line (``migrated'') file (Cray DMF)
6751 network special file (HP-UX)
6755 port (Solaris 10 and up)
6757 @c message queue, if this is a distinct file type
6761 @c shared memory object, if this is a distinct file type
6763 @c typed memory object, if this is a distinct file type
6765 @c whiteout (4.4BSD; not implemented)
6767 some other file type
6770 @cindex permissions, output by @command{ls}
6771 The file mode bits listed are similar to symbolic mode specifications
6772 (@pxref{Symbolic Modes}). But @command{ls} combines multiple bits into the
6773 third character of each set of permissions as follows:
6777 If the set-user-ID or set-group-ID bit and the corresponding executable bit
6781 If the set-user-ID or set-group-ID bit is set but the corresponding
6782 executable bit is not set.
6785 If the restricted deletion flag or sticky bit, and the
6786 other-executable bit, are both set. The restricted deletion flag is
6787 another name for the sticky bit. @xref{Mode Structure}.
6790 If the restricted deletion flag or sticky bit is set but the
6791 other-executable bit is not set.
6794 If the executable bit is set and none of the above apply.
6800 Following the file mode bits is a single character that specifies
6801 whether an alternate access method such as an access control list
6802 applies to the file. When the character following the file mode bits is a
6803 space, there is no alternate access method. When it is a printing
6804 character, then there is such a method.
6806 GNU @command{ls} uses a @samp{.} character to indicate a file
6807 with an SELinux security context, but no other alternate access method.
6809 A file with any other combination of alternate access methods
6810 is marked with a @samp{+} character.
6813 @itemx --numeric-uid-gid
6815 @opindex --numeric-uid-gid
6816 @cindex numeric uid and gid
6817 @cindex numeric user and group IDs
6818 Produce long format directory listings, but
6819 display numeric user and group IDs instead of the owner and group names.
6823 Produce long format directory listings, but don't display group information.
6824 It is equivalent to using @option{--format=long} with @option{--no-group} .
6830 @cindex disk allocation
6831 @cindex size of files, reporting
6832 Print the disk allocation of each file to the left of the file name.
6833 This is the amount of disk space used by the file, which is usually a
6834 bit more than the file's size, but it can be less if the file has holes.
6836 Normally the disk allocation is printed in units of
6837 1024 bytes, but this can be overridden (@pxref{Block size}).
6839 @cindex NFS mounts from BSD to HP-UX
6840 For files that are NFS-mounted from an HP-UX system to a BSD system,
6841 this option reports sizes that are half the correct values. On HP-UX
6842 systems, it reports sizes that are twice the correct values for files
6843 that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX;
6844 it also affects the HP-UX @command{ls} program.
6853 @cindex security context
6854 Display the SELinux security context or @samp{?} if none is found.
6855 When used with the @option{-l} option, print the security context
6856 to the left of the size column.
6861 @node Sorting the output
6862 @subsection Sorting the output
6864 @cindex sorting @command{ls} output
6865 These options change the order in which @command{ls} sorts the information
6866 it outputs. By default, sorting is done by character code
6867 (e.g., @acronym{ASCII} order).
6873 @itemx --time=status
6876 @opindex ctime@r{, printing or sorting by}
6877 @opindex status time@r{, printing or sorting by}
6878 @opindex use time@r{, printing or sorting files by}
6879 If the long listing format (e.g., @option{-l}, @option{-o}) is being used,
6880 print the status change time (the @samp{ctime} in the inode) instead of
6881 the modification time.
6882 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6883 or when not using a long listing format,
6884 sort according to the status change time.
6888 @cindex unsorted directory listing
6889 @cindex directory order, listing by
6890 Primarily, like @option{-U}---do not sort; list the files in whatever
6891 order they are stored in the directory. But also enable @option{-a} (list
6892 all files) and disable @option{-l}, @option{--color}, and @option{-s} (if they
6893 were specified before the @option{-f}).
6899 @cindex reverse sorting
6900 Reverse whatever the sorting method is---e.g., list files in reverse
6901 alphabetical order, youngest first, smallest first, or whatever.
6907 @opindex size of files@r{, sorting files by}
6908 Sort by file size, largest first.
6914 @opindex modification time@r{, sorting files by}
6915 Sort by modification time (the @samp{mtime} in the inode), newest first.
6919 @itemx --time=access
6923 @opindex use time@r{, printing or sorting files by}
6924 @opindex atime@r{, printing or sorting files by}
6925 @opindex access time@r{, printing or sorting files by}
6926 If the long listing format (e.g., @option{--format=long}) is being used,
6927 print the last access time (the @samp{atime} in the inode).
6928 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6929 or when not using a long listing format, sort according to the access time.
6935 @opindex none@r{, sorting option for @command{ls}}
6936 Do not sort; list the files in whatever order they are
6937 stored in the directory. (Do not do any of the other unrelated things
6938 that @option{-f} does.) This is especially useful when listing very large
6939 directories, since not doing any sorting can be noticeably faster.
6942 @itemx --sort=version
6945 @opindex version@r{, sorting option for @command{ls}}
6946 Sort by version name and number, lowest first. It behaves like a default
6947 sort, except that each sequence of decimal digits is treated numerically
6948 as an index/version number. (@xref{Details about version sort}.)
6951 @itemx --sort=extension
6954 @opindex extension@r{, sorting files by}
6955 Sort directory contents alphabetically by file extension (characters
6956 after the last @samp{.}); files with no extension are sorted first.
6961 @node Details about version sort
6962 @subsection Details about version sort
6964 Version sorting handles the fact that file names frequently include indices or
6965 version numbers. Standard sorting usually does not produce the order that one
6966 expects because comparisons are made on a character-by-character basis.
6967 Version sorting is especially useful when browsing directories that contain
6968 many files with indices/version numbers in their names:
6972 abc.zml-1.gz abc.zml-1.gz
6973 abc.zml-12.gz abc.zml-2.gz
6974 abc.zml-2.gz abc.zml-12.gz
6977 Version-sorted strings are compared such that if @var{ver1} and @var{ver2}
6978 are version numbers and @var{prefix} and @var{suffix} (@var{suffix} matching
6979 the regular expression @samp{(\.[A-Za-z~][A-Za-z0-9~]*)*}) are strings then
6980 @var{ver1} < @var{ver2} implies that the name composed of
6981 ``@var{prefix} @var{ver1} @var{suffix}'' sorts before
6982 ``@var{prefix} @var{ver2} @var{suffix}''.
6984 Note also that leading zeros of numeric parts are ignored:
6988 abc-1.007.tgz abc-1.01a.tgz
6989 abc-1.012b.tgz abc-1.007.tgz
6990 abc-1.01a.tgz abc-1.012b.tgz
6993 This functionality is implemented using gnulib's @code{filevercmp} function,
6994 which has some caveats worth noting.
6997 @item @env{LC_COLLATE} is ignored, which means @samp{ls -v} and @samp{sort -V}
6998 will sort non-numeric prefixes as if the @env{LC_COLLATE} locale category
6999 was set to @samp{C}.
7000 @item Some suffixes will not be matched by the regular
7001 expression mentioned above. Consequently these examples may
7002 not sort as you expect:
7010 abc-1.2.3.4.x86_64.rpm
7011 abc-1.2.3.x86_64.rpm
7015 @node General output formatting
7016 @subsection General output formatting
7018 These options affect the appearance of the overall output.
7023 @itemx --format=single-column
7026 @opindex single-column @r{output of files}
7027 List one file per line. This is the default for @command{ls} when standard
7028 output is not a terminal.
7031 @itemx --format=vertical
7034 @opindex vertical @r{sorted files in columns}
7035 List files in columns, sorted vertically. This is the default for
7036 @command{ls} if standard output is a terminal. It is always the default
7037 for the @command{dir} program.
7038 @sc{gnu} @command{ls} uses variable width columns to display as many files as
7039 possible in the fewest lines.
7041 @item --color [=@var{when}]
7043 @cindex color, distinguishing file types with
7044 Specify whether to use color for distinguishing file types. @var{when}
7045 may be omitted, or one of:
7048 @vindex none @r{color option}
7049 - Do not use color at all. This is the default.
7051 @vindex auto @r{color option}
7052 @cindex terminal, using color iff
7053 - Only use color if standard output is a terminal.
7055 @vindex always @r{color option}
7058 Specifying @option{--color} and no @var{when} is equivalent to
7059 @option{--color=always}.
7060 Piping a colorized listing through a pager like @command{more} or
7061 @command{less} usually produces unreadable results. However, using
7062 @code{more -f} does seem to work.
7065 @vindex SHELL @r{environment variable, and color}
7066 Note that using the @option{--color} option may incur a noticeable
7067 performance penalty when run in a directory with very many entries,
7068 because the default settings require that @command{ls} @code{stat} every
7069 single file it lists.
7070 However, if you would like most of the file-type coloring
7071 but can live without the other coloring options (e.g.,
7072 executable, orphan, sticky, other-writable, capability), use
7073 @command{dircolors} to set the @env{LS_COLORS} environment variable like this,
7075 eval $(dircolors -p | perl -pe \
7076 's/^((CAP|S[ET]|O[TR]|M|E)\w+).*/$1 00/' | dircolors -)
7078 and on a @code{dirent.d_type}-capable file system, @command{ls}
7079 will perform only one @code{stat} call per command line argument.
7083 @itemx --indicator-style=classify
7086 @opindex --indicator-style
7087 @cindex file type and executables, marking
7088 @cindex executables and file type, marking
7089 Append a character to each file name indicating the file type. Also,
7090 for regular files that are executable, append @samp{*}. The file type
7091 indicators are @samp{/} for directories, @samp{@@} for symbolic links,
7092 @samp{|} for FIFOs, @samp{=} for sockets, @samp{>} for doors,
7093 and nothing for regular files.
7094 @c The following sentence is the same as the one for -d.
7095 Do not follow symbolic links listed on the
7096 command line unless the @option{--dereference-command-line} (@option{-H}),
7097 @option{--dereference} (@option{-L}), or
7098 @option{--dereference-command-line-symlink-to-dir} options are specified.
7101 @itemx --indicator-style=file-type
7102 @opindex --file-type
7103 @opindex --indicator-style
7104 @cindex file type, marking
7105 Append a character to each file name indicating the file type. This is
7106 like @option{-F}, except that executables are not marked.
7108 @item --indicator-style=@var{word}
7109 @opindex --indicator-style
7110 Append a character indicator with style @var{word} to entry names,
7115 Do not append any character indicator; this is the default.
7117 Append @samp{/} for directories. This is the same as the @option{-p}
7120 Append @samp{/} for directories, @samp{@@} for symbolic links, @samp{|}
7121 for FIFOs, @samp{=} for sockets, and nothing for regular files. This is
7122 the same as the @option{--file-type} option.
7124 Append @samp{*} for executable regular files, otherwise behave as for
7125 @samp{file-type}. This is the same as the @option{-F} or
7126 @option{--classify} option.
7132 @opindex --kibibytes
7133 Set the default block size to its normal value of 1024 bytes,
7134 overriding any contrary specification in environment variables
7135 (@pxref{Block size}). This option is in turn overridden by the
7136 @option{--block-size}, @option{-h} or @option{--human-readable}, and
7137 @option{--si} options.
7139 The @option{-k} or @option{--kibibytes} option affects the
7140 per-directory block count written by the @option{-l} and similar
7141 options, and the size written by the @option{-s} or @option{--size}
7142 option. It does not affect the file size written by @option{-l}.
7145 @itemx --format=commas
7148 @opindex commas@r{, outputting between files}
7149 List files horizontally, with as many as will fit on each line,
7150 separated by @samp{, } (a comma and a space).
7153 @itemx --indicator-style=slash
7155 @opindex --indicator-style
7156 @cindex file type, marking
7157 Append a @samp{/} to directory names.
7160 @itemx --format=across
7161 @itemx --format=horizontal
7164 @opindex across@r{, listing files}
7165 @opindex horizontal@r{, listing files}
7166 List the files in columns, sorted horizontally.
7169 @itemx --tabsize=@var{cols}
7172 Assume that each tab stop is @var{cols} columns wide. The default is 8.
7173 @command{ls} uses tabs where possible in the output, for efficiency. If
7174 @var{cols} is zero, do not use tabs at all.
7176 @c FIXME: remove in 2009, if Apple Terminal has been fixed for long enough.
7177 Some terminal emulators (at least Apple Terminal 1.5 (133) from Mac OS X 10.4.8)
7178 do not properly align columns to the right of a TAB following a
7179 non-@acronym{ASCII} byte. If you use such a terminal emulator, use the
7180 @option{-T0} option or put @code{TABSIZE=0} in your environment to tell
7181 @command{ls} to align using spaces, not tabs.
7184 @itemx --width=@var{cols}
7188 Assume the screen is @var{cols} columns wide. The default is taken
7189 from the terminal settings if possible; otherwise the environment
7190 variable @env{COLUMNS} is used if it is set; otherwise the default
7196 @node Formatting file timestamps
7197 @subsection Formatting file timestamps
7199 By default, file timestamps are listed in abbreviated form, using
7200 a date like @samp{Mar 30@ @ 2002} for non-recent timestamps, and a
7201 date-without-year and time like @samp{Mar 30 23:45} for recent timestamps.
7202 This format can change depending on the current locale as detailed below.
7205 A timestamp is considered to be @dfn{recent} if it is less than six
7206 months old, and is not dated in the future. If a timestamp dated
7207 today is not listed in recent form, the timestamp is in the future,
7208 which means you probably have clock skew problems which may break
7209 programs like @command{make} that rely on file timestamps.
7212 Time stamps are listed according to the time zone rules specified by
7213 the @env{TZ} environment variable, or by the system default rules if
7214 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
7215 with @env{TZ}, libc, The GNU C Library Reference Manual}.
7217 The following option changes how file timestamps are printed.
7220 @item --time-style=@var{style}
7221 @opindex --time-style
7223 List timestamps in style @var{style}. The @var{style} should
7224 be one of the following:
7229 List timestamps using @var{format}, where @var{format} is interpreted
7230 like the format argument of @command{date} (@pxref{date invocation}).
7231 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
7232 @command{ls} to list timestamps like @samp{2002-03-30 23:45:56}. As
7233 with @command{date}, @var{format}'s interpretation is affected by the
7234 @env{LC_TIME} locale category.
7236 If @var{format} contains two format strings separated by a newline,
7237 the former is used for non-recent files and the latter for recent
7238 files; if you want output columns to line up, you may need to insert
7239 spaces in one of the two formats.
7242 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
7243 format with nanosecond precision, e.g., @samp{2002-03-30
7244 23:45:56.477817180 -0700}. This style is equivalent to
7245 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
7247 This is useful because the time output includes all the information that
7248 is available from the operating system. For example, this can help
7249 explain @command{make}'s behavior, since @acronym{GNU} @command{make}
7250 uses the full timestamp to determine whether a file is out of date.
7253 List @acronym{ISO} 8601 date and time in minutes, e.g.,
7254 @samp{2002-03-30 23:45}. These timestamps are shorter than
7255 @samp{full-iso} timestamps, and are usually good enough for everyday
7256 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
7259 List @acronym{ISO} 8601 dates for non-recent timestamps (e.g.,
7260 @samp{2002-03-30@ }), and @acronym{ISO} 8601 month, day, hour, and
7261 minute for recent timestamps (e.g., @samp{03-30 23:45}). These
7262 timestamps are uglier than @samp{long-iso} timestamps, but they carry
7263 nearly the same information in a smaller space and their brevity helps
7264 @command{ls} output fit within traditional 80-column output lines.
7265 The following two @command{ls} invocations are equivalent:
7270 ls -l --time-style="+%Y-%m-%d $newline%m-%d %H:%M"
7271 ls -l --time-style="iso"
7276 List timestamps in a locale-dependent form. For example, a Finnish
7277 locale might list non-recent timestamps like @samp{maalis 30@ @ 2002}
7278 and recent timestamps like @samp{maalis 30 23:45}. Locale-dependent
7279 timestamps typically consume more space than @samp{iso} timestamps and
7280 are harder for programs to parse because locale conventions vary so
7281 widely, but they are easier for many people to read.
7283 The @env{LC_TIME} locale category specifies the timestamp format. The
7284 default @acronym{POSIX} locale uses timestamps like @samp{Mar 30@
7285 @ 2002} and @samp{Mar 30 23:45}; in this locale, the following two
7286 @command{ls} invocations are equivalent:
7291 ls -l --time-style="+%b %e %Y$newline%b %e %H:%M"
7292 ls -l --time-style="locale"
7295 Other locales behave differently. For example, in a German locale,
7296 @option{--time-style="locale"} might be equivalent to
7297 @option{--time-style="+%e. %b %Y $newline%e. %b %H:%M"}
7298 and might generate timestamps like @samp{30. M@"ar 2002@ } and
7299 @samp{30. M@"ar 23:45}.
7301 @item posix-@var{style}
7303 List @acronym{POSIX}-locale timestamps if the @env{LC_TIME} locale
7304 category is @acronym{POSIX}, @var{style} timestamps otherwise. For
7305 example, the @samp{posix-long-iso} style lists
7306 timestamps like @samp{Mar 30@ @ 2002} and @samp{Mar 30 23:45} when in
7307 the @acronym{POSIX} locale, and like @samp{2002-03-30 23:45} otherwise.
7312 You can specify the default value of the @option{--time-style} option
7313 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
7314 the default style is @samp{locale}. @acronym{GNU} Emacs 21.3 and
7315 later use the @option{--dired} option and therefore can parse any date
7316 format, but if you are using Emacs 21.1 or 21.2 and specify a
7317 non-@acronym{POSIX} locale you may need to set
7318 @samp{TIME_STYLE="posix-long-iso"}.
7320 To avoid certain denial-of-service attacks, timestamps that would be
7321 longer than 1000 bytes may be treated as errors.
7324 @node Formatting the file names
7325 @subsection Formatting the file names
7327 These options change how file names themselves are printed.
7333 @itemx --quoting-style=escape
7336 @opindex --quoting-style
7337 @cindex backslash sequences for file names
7338 Quote nongraphic characters in file names using alphabetic and octal
7339 backslash sequences like those used in C.
7343 @itemx --quoting-style=literal
7346 @opindex --quoting-style
7347 Do not quote file names. However, with @command{ls} nongraphic
7348 characters are still printed as question marks if the output is a
7349 terminal and you do not specify the @option{--show-control-chars}
7353 @itemx --hide-control-chars
7355 @opindex --hide-control-chars
7356 Print question marks instead of nongraphic characters in file names.
7357 This is the default if the output is a terminal and the program is
7362 @itemx --quoting-style=c
7364 @opindex --quote-name
7365 @opindex --quoting-style
7366 Enclose file names in double quotes and quote nongraphic characters as
7369 @item --quoting-style=@var{word}
7370 @opindex --quoting-style
7371 @cindex quoting style
7372 Use style @var{word} to quote file names and other strings that may
7373 contain arbitrary characters. The @var{word} should
7374 be one of the following:
7378 Output strings as-is; this is the same as the @option{-N} or
7379 @option{--literal} option.
7381 Quote strings for the shell if they contain shell metacharacters or would
7382 cause ambiguous output.
7383 The quoting is suitable for @acronym{POSIX}-compatible shells like
7384 @command{bash}, but it does not always work for incompatible shells
7387 Quote strings for the shell, even if they would normally not require quoting.
7389 Quote strings as for C character string literals, including the
7390 surrounding double-quote characters; this is the same as the
7391 @option{-Q} or @option{--quote-name} option.
7393 Quote strings as for C character string literals, except omit the
7394 surrounding double-quote
7395 characters; this is the same as the @option{-b} or @option{--escape} option.
7397 Quote strings as for C character string literals, except use
7398 surrounding quotation marks appropriate for the
7401 @c Use @t instead of @samp to avoid duplicate quoting in some output styles.
7402 Quote strings as for C character string literals, except use
7403 surrounding quotation marks appropriate for the locale, and quote
7404 @t{`like this'} instead of @t{"like
7405 this"} in the default C locale. This looks nicer on many displays.
7408 You can specify the default value of the @option{--quoting-style} option
7409 with the environment variable @env{QUOTING_STYLE}. If that environment
7410 variable is not set, the default value is @samp{literal}, but this
7411 default may change to @samp{shell} in a future version of this package.
7413 @item --show-control-chars
7414 @opindex --show-control-chars
7415 Print nongraphic characters as-is in file names.
7416 This is the default unless the output is a terminal and the program is
7422 @node dir invocation
7423 @section @command{dir}: Briefly list directory contents
7426 @cindex directory listing, brief
7428 @command{dir} is equivalent to @code{ls -C
7429 -b}; that is, by default files are listed in columns, sorted vertically,
7430 and special characters are represented by backslash escape sequences.
7432 @xref{ls invocation, @command{ls}}.
7435 @node vdir invocation
7436 @section @command{vdir}: Verbosely list directory contents
7439 @cindex directory listing, verbose
7441 @command{vdir} is equivalent to @code{ls -l
7442 -b}; that is, by default files are listed in long format and special
7443 characters are represented by backslash escape sequences.
7445 @node dircolors invocation
7446 @section @command{dircolors}: Color setup for @command{ls}
7450 @cindex setup for color
7452 @command{dircolors} outputs a sequence of shell commands to set up the
7453 terminal for color output from @command{ls} (and @command{dir}, etc.).
7457 eval "`dircolors [@var{option}]@dots{} [@var{file}]`"
7460 If @var{file} is specified, @command{dircolors} reads it to determine which
7461 colors to use for which file types and extensions. Otherwise, a
7462 precompiled database is used. For details on the format of these files,
7463 run @samp{dircolors --print-database}.
7465 To make @command{dircolors} read a @file{~/.dircolors} file if it
7466 exists, you can put the following lines in your @file{~/.bashrc} (or
7467 adapt them to your favorite shell):
7471 test -r $d && eval "$(dircolors $d)"
7475 @vindex SHELL @r{environment variable, and color}
7476 The output is a shell command to set the @env{LS_COLORS} environment
7477 variable. You can specify the shell syntax to use on the command line,
7478 or @command{dircolors} will guess it from the value of the @env{SHELL}
7479 environment variable.
7481 The program accepts the following options. Also see @ref{Common options}.
7486 @itemx --bourne-shell
7489 @opindex --bourne-shell
7490 @cindex Bourne shell syntax for color setup
7491 @cindex @command{sh} syntax for color setup
7492 Output Bourne shell commands. This is the default if the @env{SHELL}
7493 environment variable is set and does not end with @samp{csh} or
7502 @cindex C shell syntax for color setup
7503 @cindex @command{csh} syntax for color setup
7504 Output C shell commands. This is the default if @code{SHELL} ends with
7505 @command{csh} or @command{tcsh}.
7508 @itemx --print-database
7510 @opindex --print-database
7511 @cindex color database, printing
7512 @cindex database for color setup, printing
7513 @cindex printing color database
7514 Print the (compiled-in) default color configuration database. This
7515 output is itself a valid configuration file, and is fairly descriptive
7516 of the possibilities.
7523 @node Basic operations
7524 @chapter Basic operations
7526 @cindex manipulating files
7528 This chapter describes the commands for basic file manipulation:
7529 copying, moving (renaming), and deleting (removing).
7532 * cp invocation:: Copy files.
7533 * dd invocation:: Convert and copy a file.
7534 * install invocation:: Copy files and set attributes.
7535 * mv invocation:: Move (rename) files.
7536 * rm invocation:: Remove files or directories.
7537 * shred invocation:: Remove files more securely.
7542 @section @command{cp}: Copy files and directories
7545 @cindex copying files and directories
7546 @cindex files, copying
7547 @cindex directories, copying
7549 @command{cp} copies files (or, optionally, directories). The copy is
7550 completely independent of the original. You can either copy one file to
7551 another, or copy arbitrarily many files to a destination directory.
7555 cp [@var{option}]@dots{} [-T] @var{source} @var{dest}
7556 cp [@var{option}]@dots{} @var{source}@dots{} @var{directory}
7557 cp [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
7562 If two file names are given, @command{cp} copies the first file to the
7566 If the @option{--target-directory} (@option{-t}) option is given, or
7567 failing that if the last file is a directory and the
7568 @option{--no-target-directory} (@option{-T}) option is not given,
7569 @command{cp} copies each @var{source} file to the specified directory,
7570 using the @var{source}s' names.
7573 Generally, files are written just as they are read. For exceptions,
7574 see the @option{--sparse} option below.
7576 By default, @command{cp} does not copy directories. However, the
7577 @option{-R}, @option{-a}, and @option{-r} options cause @command{cp} to
7578 copy recursively by descending into source directories and copying files
7579 to corresponding destination directories.
7581 When copying from a symbolic link, @command{cp} normally follows the
7582 link only when not copying
7583 recursively. This default can be overridden with the
7584 @option{--archive} (@option{-a}), @option{-d}, @option{--dereference}
7585 (@option{-L}), @option{--no-dereference} (@option{-P}), and
7586 @option{-H} options. If more than one of these options is specified,
7587 the last one silently overrides the others.
7589 When copying to a symbolic link, @command{cp} follows the
7590 link only when it refers to an existing regular file.
7591 However, when copying to a dangling symbolic link, @command{cp}
7592 refuses by default, and fails with a diagnostic, since the operation
7593 is inherently dangerous. This behavior is contrary to historical
7594 practice and to @acronym{POSIX}.
7595 Set @env{POSIXLY_CORRECT} to make @command{cp} attempt to create
7596 the target of a dangling destination symlink, in spite of the possible risk.
7597 Also, when an option like
7598 @option{--backup} or @option{--link} acts to rename or remove the
7599 destination before copying, @command{cp} renames or removes the
7600 symbolic link rather than the file it points to.
7602 By default, @command{cp} copies the contents of special files only
7603 when not copying recursively. This default can be overridden with the
7604 @option{--copy-contents} option.
7606 @cindex self-backups
7607 @cindex backups, making only
7608 @command{cp} generally refuses to copy a file onto itself, with the
7609 following exception: if @option{--force --backup} is specified with
7610 @var{source} and @var{dest} identical, and referring to a regular file,
7611 @command{cp} will make a backup file, either regular or numbered, as
7612 specified in the usual ways (@pxref{Backup options}). This is useful when
7613 you simply want to make a backup of an existing file before changing it.
7615 The program accepts the following options. Also see @ref{Common options}.
7622 Preserve as much as possible of the structure and attributes of the
7623 original files in the copy (but do not attempt to preserve internal
7624 directory structure; i.e., @samp{ls -U} may list the entries in a copied
7625 directory in a different order).
7626 Try to preserve SELinux security context and extended attributes (xattr),
7627 but ignore any failure to do that and print no corresponding diagnostic.
7628 Equivalent to @option{-dR --preserve=all} with the reduced diagnostics.
7630 @itemx --attributes-only
7631 @opindex --attributes-only
7632 Preserve the specified attributes of the original files in the copy,
7633 but do not copy any data. See the @option{--preserve} option for
7634 controlling which attributes to copy.
7637 @itemx @w{@kbd{--backup}[=@var{method}]}
7640 @vindex VERSION_CONTROL
7641 @cindex backups, making
7642 @xref{Backup options}.
7643 Make a backup of each file that would otherwise be overwritten or removed.
7644 As a special case, @command{cp} makes a backup of @var{source} when the force
7645 and backup options are given and @var{source} and @var{dest} are the same
7646 name for an existing, regular file. One useful application of this
7647 combination of options is this tiny Bourne shell script:
7651 # Usage: backup FILE...
7652 # Create a @sc{gnu}-style backup of each listed FILE.
7654 cp --backup --force -- "$i" "$i"
7658 @item --copy-contents
7659 @cindex directories, copying recursively
7660 @cindex copying directories recursively
7661 @cindex recursively copying directories
7662 @cindex non-directories, copying as special files
7663 If copying recursively, copy the contents of any special files (e.g.,
7664 FIFOs and device files) as if they were regular files. This means
7665 trying to read the data in each source file and writing it to the
7666 destination. It is usually a mistake to use this option, as it
7667 normally has undesirable effects on special files like FIFOs and the
7668 ones typically found in the @file{/dev} directory. In most cases,
7669 @code{cp -R --copy-contents} will hang indefinitely trying to read
7670 from FIFOs and special files like @file{/dev/console}, and it will
7671 fill up your destination disk if you use it to copy @file{/dev/zero}.
7672 This option has no effect unless copying recursively, and it does not
7673 affect the copying of symbolic links.
7677 @cindex symbolic links, copying
7678 @cindex hard links, preserving
7679 Copy symbolic links as symbolic links rather than copying the files that
7680 they point to, and preserve hard links between source files in the copies.
7681 Equivalent to @option{--no-dereference --preserve=links}.
7687 When copying without this option and an existing destination file cannot
7688 be opened for writing, the copy fails. However, with @option{--force}),
7689 when a destination file cannot be opened, @command{cp} then removes it and
7690 tries to open it again. Contrast this behavior with that enabled by
7691 @option{--link} and @option{--symbolic-link}, whereby the destination file
7692 is never opened but rather is removed unconditionally. Also see the
7693 description of @option{--remove-destination}.
7695 This option is independent of the @option{--interactive} or
7696 @option{-i} option: neither cancels the effect of the other.
7698 This option is redundant if the @option{--no-clobber} or @option{-n} option is
7703 If a command line argument specifies a symbolic link, then copy the
7704 file it points to rather than the symbolic link itself. However,
7705 copy (preserving its nature) any symbolic link that is encountered
7706 via recursive traversal.
7709 @itemx --interactive
7711 @opindex --interactive
7712 When copying a file other than a directory, prompt whether to
7713 overwrite an existing destination file. The @option{-i} option overrides
7714 a previous @option{-n} option.
7720 Make hard links instead of copies of non-directories.
7723 @itemx --dereference
7725 @opindex --dereference
7726 Follow symbolic links when copying from them.
7727 With this option, @command{cp} cannot create a symbolic link.
7728 For example, a symlink (to regular file) in the source tree will be copied to
7729 a regular file in the destination tree.
7734 @opindex --no-clobber
7735 Do not overwrite an existing file. The @option{-n} option overrides a previous
7736 @option{-i} option. This option is mutually exclusive with @option{-b} or
7737 @option{--backup} option.
7740 @itemx --no-dereference
7742 @opindex --no-dereference
7743 @cindex symbolic links, copying
7744 Copy symbolic links as symbolic links rather than copying the files that
7745 they point to. This option affects only symbolic links in the source;
7746 symbolic links in the destination are always followed if possible.
7749 @itemx @w{@kbd{--preserve}[=@var{attribute_list}]}
7752 @cindex file information, preserving, extended attributes, xattr
7753 Preserve the specified attributes of the original files.
7754 If specified, the @var{attribute_list} must be a comma-separated list
7755 of one or more of the following strings:
7759 Preserve the file mode bits and access control lists.
7761 Preserve the owner and group. On most modern systems,
7762 only users with appropriate privileges may change the owner of a file,
7764 may preserve the group ownership of a file only if they happen to be
7765 a member of the desired group.
7767 Preserve the times of last access and last modification, when possible.
7768 On older systems, it is not possible to preserve these attributes
7769 when the affected file is a symbolic link.
7770 However, many systems now provide the @code{utimensat} function,
7771 which makes it possible even for symbolic links.
7773 Preserve in the destination files
7774 any links between corresponding source files.
7775 Note that with @option{-L} or @option{-H}, this option can convert
7776 symbolic links to hard links. For example,
7778 $ mkdir c; : > a; ln -s a b; cp -aH a b c; ls -i1 c
7783 Note the inputs: @file{b} is a symlink to regular file @file{a},
7784 yet the files in destination directory, @file{c/}, are hard-linked.
7785 Since @option{-a} implies @option{--preserve=links}, and since @option{-H}
7786 tells @command{cp} to dereference command line arguments, it sees two files
7787 with the same inode number, and preserves the perceived hard link.
7789 Here is a similar example that exercises @command{cp}'s @option{-L} option:
7791 $ mkdir b c; (cd b; : > a; ln -s a b); cp -aL b c; ls -i1 c/b
7797 Preserve SELinux security context of the file, or fail with full diagnostics.
7799 Preserve extended attributes of the file, or fail with full diagnostics.
7800 If @command{cp} is built without xattr support, ignore this option.
7801 If SELinux context, ACLs or Capabilities are implemented using xattrs,
7802 they are preserved by this option as well.
7804 Preserve all file attributes.
7805 Equivalent to specifying all of the above, but with the difference
7806 that failure to preserve SELinux security context or extended attributes
7807 does not change @command{cp}'s exit status. In contrast to @option{-a},
7808 all but @samp{Operation not supported} warnings are output.
7811 Using @option{--preserve} with no @var{attribute_list} is equivalent
7812 to @option{--preserve=mode,ownership,timestamps}.
7814 In the absence of this option, each destination file is created with the
7815 mode bits of the corresponding source file, minus the bits set in the
7816 umask and minus the set-user-ID and set-group-ID bits.
7817 @xref{File permissions}.
7819 @itemx @w{@kbd{--no-preserve}=@var{attribute_list}}
7820 @cindex file information, preserving
7821 Do not preserve the specified attributes. The @var{attribute_list}
7822 has the same form as for @option{--preserve}.
7826 @cindex parent directories and @command{cp}
7827 Form the name of each destination file by appending to the target
7828 directory a slash and the specified name of the source file. The last
7829 argument given to @command{cp} must be the name of an existing directory.
7830 For example, the command:
7833 cp --parents a/b/c existing_dir
7837 copies the file @file{a/b/c} to @file{existing_dir/a/b/c}, creating
7838 any missing intermediate directories.
7845 @opindex --recursive
7846 @cindex directories, copying recursively
7847 @cindex copying directories recursively
7848 @cindex recursively copying directories
7849 @cindex non-directories, copying as special files
7850 Copy directories recursively. By default, do not follow symbolic
7851 links in the source; see the @option{--archive} (@option{-a}), @option{-d},
7852 @option{--dereference} (@option{-L}), @option{--no-dereference}
7853 (@option{-P}), and @option{-H} options. Special files are copied by
7854 creating a destination file of the same type as the source; see the
7855 @option{--copy-contents} option. It is not portable to use
7856 @option{-r} to copy symbolic links or special files. On some
7857 non-@sc{gnu} systems, @option{-r} implies the equivalent of
7858 @option{-L} and @option{--copy-contents} for historical reasons.
7859 Also, it is not portable to use @option{-R} to copy symbolic links
7860 unless you also specify @option{-P}, as @acronym{POSIX} allows
7861 implementations that dereference symbolic links by default.
7863 @item --reflink[=@var{when}]
7864 @opindex --reflink[=@var{when}]
7867 @cindex copy on write
7868 Perform a lightweight, copy-on-write (COW) copy, if supported by the
7869 file system. Once it has succeeded, beware that the source and destination
7870 files share the same disk data blocks as long as they remain unmodified.
7871 Thus, if a disk I/O error affects data blocks of one of the files,
7872 the other suffers the same fate.
7874 The @var{when} value can be one of the following:
7878 The default behavior: if the copy-on-write operation is not supported
7879 then report the failure for each file and exit with a failure status.
7882 If the copy-on-write operation is not supported then fall back
7883 to the standard copy behaviour.
7886 This option is overridden by the @option{--link}, @option{--symbolic-link}
7887 and @option{--attributes-only} options, thus allowing it to be used
7888 to configure the default data copying behavior for @command{cp}.
7889 For example, with the following alias, @command{cp} will use the
7890 minimum amount of space supported by the file system.
7893 alias cp='cp --reflink=auto --sparse=always'
7896 @item --remove-destination
7897 @opindex --remove-destination
7898 Remove each existing destination file before attempting to open it
7899 (contrast with @option{-f} above).
7901 @item --sparse=@var{when}
7902 @opindex --sparse=@var{when}
7903 @cindex sparse files, copying
7904 @cindex holes, copying files with
7905 @findex read @r{system call, and holes}
7906 A @dfn{sparse file} contains @dfn{holes}---a sequence of zero bytes that
7907 does not occupy any physical disk blocks; the @samp{read} system call
7908 reads these as zeros. This can both save considerable disk space and
7909 increase speed, since many binary files contain lots of consecutive zero
7910 bytes. By default, @command{cp} detects holes in input source files via a crude
7911 heuristic and makes the corresponding output file sparse as well.
7912 Only regular files may be sparse.
7914 The @var{when} value can be one of the following:
7918 The default behavior: if the input file is sparse, attempt to make
7919 the output file sparse, too. However, if an output file exists but
7920 refers to a non-regular file, then do not attempt to make it sparse.
7923 For each sufficiently long sequence of zero bytes in the input file,
7924 attempt to create a corresponding hole in the output file, even if the
7925 input file does not appear to be sparse.
7926 This is useful when the input file resides on a file system
7927 that does not support sparse files
7928 (for example, @samp{efs} file systems in SGI IRIX 5.3 and earlier),
7929 but the output file is on a type of file system that does support them.
7930 Holes may be created only in regular files, so if the destination file
7931 is of some other type, @command{cp} does not even try to make it sparse.
7934 Never make the output file sparse.
7935 This is useful in creating a file for use with the @command{mkswap} command,
7936 since such a file must not have any holes.
7939 @optStripTrailingSlashes
7942 @itemx --symbolic-link
7944 @opindex --symbolic-link
7945 @cindex symbolic links, copying with
7946 Make symbolic links instead of copies of non-directories. All source
7947 file names must be absolute (starting with @samp{/}) unless the
7948 destination files are in the current directory. This option merely
7949 results in an error message on systems that do not support symbolic links.
7955 @optNoTargetDirectory
7961 @cindex newer files, copying only
7962 Do not copy a non-directory that has an existing destination with the
7963 same or newer modification time. If time stamps are being preserved,
7964 the comparison is to the source time stamp truncated to the
7965 resolutions of the destination file system and of the system calls
7966 used to update time stamps; this avoids duplicate work if several
7967 @samp{cp -pu} commands are executed with the same source and destination.
7968 If @option{--preserve=links} is also specified (like with @samp{cp -au}
7969 for example), that will take precedence. Consequently, depending on the
7970 order that files are processed from the source, newer files in the destination
7971 may be replaced, to mirror hard links in the source.
7977 Print the name of each file before copying it.
7980 @itemx --one-file-system
7982 @opindex --one-file-system
7983 @cindex file systems, omitting copying to different
7984 Skip subdirectories that are on different file systems from the one that
7985 the copy started on.
7986 However, mount point directories @emph{are} copied.
7994 @section @command{dd}: Convert and copy a file
7997 @cindex converting while copying a file
7999 @command{dd} copies a file (from standard input to standard output, by
8000 default) with a changeable I/O block size, while optionally performing
8001 conversions on it. Synopses:
8004 dd [@var{operand}]@dots{}
8008 The only options are @option{--help} and @option{--version}.
8009 @xref{Common options}. @command{dd} accepts the following operands.
8015 Read from @var{file} instead of standard input.
8019 Write to @var{file} instead of standard output. Unless
8020 @samp{conv=notrunc} is given, @command{dd} truncates @var{file} to zero
8021 bytes (or the size specified with @samp{seek=}).
8023 @item ibs=@var{bytes}
8025 @cindex block size of input
8026 @cindex input block size
8027 Set the input block size to @var{bytes}.
8028 This makes @command{dd} read @var{bytes} per block.
8029 The default is 512 bytes.
8031 @item obs=@var{bytes}
8033 @cindex block size of output
8034 @cindex output block size
8035 Set the output block size to @var{bytes}.
8036 This makes @command{dd} write @var{bytes} per block.
8037 The default is 512 bytes.
8039 @item bs=@var{bytes}
8042 Set both input and output block sizes to @var{bytes}.
8043 This makes @command{dd} read and write @var{bytes} per block,
8044 overriding any @samp{ibs} and @samp{obs} settings.
8045 In addition, if no data-transforming @option{conv} option is specified,
8046 input is copied to the output as soon as it's read,
8047 even if it is smaller than the block size.
8049 @item cbs=@var{bytes}
8051 @cindex block size of conversion
8052 @cindex conversion block size
8053 @cindex fixed-length records, converting to variable-length
8054 @cindex variable-length records, converting to fixed-length
8055 Set the conversion block size to @var{bytes}.
8056 When converting variable-length records to fixed-length ones
8057 (@option{conv=block}) or the reverse (@option{conv=unblock}),
8058 use @var{bytes} as the fixed record length.
8060 @item skip=@var{blocks}
8062 Skip @var{blocks} @samp{ibs}-byte blocks in the input file before copying.
8064 @item seek=@var{blocks}
8066 Skip @var{blocks} @samp{obs}-byte blocks in the output file before copying.
8068 @item count=@var{blocks}
8070 Copy @var{blocks} @samp{ibs}-byte blocks from the input file, instead
8071 of everything until the end of the file.
8075 Do not print the overall transfer rate and volume statistics
8076 that normally make up the third status line when @command{dd} exits.
8078 @item conv=@var{conversion}[,@var{conversion}]@dots{}
8080 Convert the file as specified by the @var{conversion} argument(s).
8081 (No spaces around any comma(s).)
8088 @opindex ascii@r{, converting to}
8089 Convert @acronym{EBCDIC} to @acronym{ASCII},
8090 using the conversion table specified by @acronym{POSIX}.
8091 This provides a 1:1 translation for all 256 bytes.
8094 @opindex ebcdic@r{, converting to}
8095 Convert @acronym{ASCII} to @acronym{EBCDIC}.
8096 This is the inverse of the @samp{ascii} conversion.
8099 @opindex alternate ebcdic@r{, converting to}
8100 Convert @acronym{ASCII} to alternate @acronym{EBCDIC},
8101 using the alternate conversion table specified by @acronym{POSIX}.
8102 This is not a 1:1 translation, but reflects common historical practice
8103 for @samp{~}, @samp{[}, and @samp{]}.
8105 The @samp{ascii}, @samp{ebcdic}, and @samp{ibm} conversions are
8109 @opindex block @r{(space-padding)}
8110 For each line in the input, output @samp{cbs} bytes, replacing the
8111 input newline with a space and padding with spaces as necessary.
8115 Remove any trailing spaces in each @samp{cbs}-sized input block,
8116 and append a newline.
8118 The @samp{block} and @samp{unblock} conversions are mutually exclusive.
8121 @opindex lcase@r{, converting to}
8122 Change uppercase letters to lowercase.
8125 @opindex ucase@r{, converting to}
8126 Change lowercase letters to uppercase.
8128 The @samp{lcase} and @samp{ucase} conversions are mutually exclusive.
8131 @opindex swab @r{(byte-swapping)}
8132 @cindex byte-swapping
8133 Swap every pair of input bytes. @sc{gnu} @command{dd}, unlike others, works
8134 when an odd number of bytes are read---the last byte is simply copied
8135 (since there is nothing to swap it with).
8138 @opindex sync @r{(padding with @acronym{ASCII} @sc{nul}s)}
8139 Pad every input block to size of @samp{ibs} with trailing zero bytes.
8140 When used with @samp{block} or @samp{unblock}, pad with spaces instead of
8145 The following ``conversions'' are really file flags
8146 and don't affect internal processing:
8151 @cindex creating output file, requiring
8152 Fail if the output file already exists; @command{dd} must create the
8157 @cindex creating output file, avoiding
8158 Do not create the output file; the output file must already exist.
8160 The @samp{excl} and @samp{nocreat} conversions are mutually exclusive.
8164 @cindex truncating output file, avoiding
8165 Do not truncate the output file.
8169 @cindex read errors, ignoring
8170 Continue after read errors.
8174 @cindex synchronized data writes, before finishing
8175 Synchronize output data just before finishing. This forces a physical
8176 write of output data.
8180 @cindex synchronized data and metadata writes, before finishing
8181 Synchronize output data and metadata just before finishing. This
8182 forces a physical write of output data and metadata.
8186 @item iflag=@var{flag}[,@var{flag}]@dots{}
8188 Access the input file using the flags specified by the @var{flag}
8189 argument(s). (No spaces around any comma(s).)
8191 @item oflag=@var{flag}[,@var{flag}]@dots{}
8193 Access the output file using the flags specified by the @var{flag}
8194 argument(s). (No spaces around any comma(s).)
8196 Here are the flags. Not every flag is supported on every operating
8203 @cindex appending to the output file
8204 Write in append mode, so that even if some other process is writing to
8205 this file, every @command{dd} write will append to the current
8206 contents of the file. This flag makes sense only for output.
8207 If you combine this flag with the @samp{of=@var{file}} operand,
8208 you should also specify @samp{conv=notrunc} unless you want the
8209 output file to be truncated before being appended to.
8213 @cindex concurrent I/O
8214 Use concurrent I/O mode for data. This mode performs direct I/O
8215 and drops the @acronym{POSIX} requirement to serialize all I/O to the same file.
8216 A file cannot be opened in CIO mode and with a standard open at the
8222 Use direct I/O for data, avoiding the buffer cache.
8223 Note that the kernel may impose restrictions on read or write buffer sizes.
8224 For example, with an ext4 destination file system and a linux-based kernel,
8225 using @samp{oflag=direct} will cause writes to fail with @code{EINVAL} if the
8226 output buffer size is not a multiple of 512.
8230 @cindex directory I/O
8232 Fail unless the file is a directory. Most operating systems do not
8233 allow I/O to a directory, so this flag has limited utility.
8237 @cindex synchronized data reads
8238 Use synchronized I/O for data. For the output file, this forces a
8239 physical write of output data on each write. For the input file,
8240 this flag can matter when reading from a remote file that has been
8241 written to synchronously by some other process. Metadata (e.g.,
8242 last-access and last-modified time) is not necessarily synchronized.
8246 @cindex synchronized data and metadata I/O
8247 Use synchronized I/O for both data and metadata.
8251 @cindex discarding file cache
8252 Discard the data cache for a file.
8253 When count=0 all cache is discarded,
8254 otherwise the cache is dropped for the processed
8255 portion of the file. Also when count=0
8256 failure to discard the cache is diagnosed
8257 and reflected in the exit status.
8258 Here as some usage examples:
8261 # Advise to drop cache for whole file
8262 dd if=ifile iflag=nocache count=0
8264 # Ensure drop cache for the whole file
8265 dd of=ofile oflag=nocache conv=notrunc,fdatasync count=0
8267 # Drop cache for part of file
8268 dd if=ifile iflag=nocache skip=10 count=10 of=/dev/null
8270 # Stream data using just the read-ahead cache
8271 dd if=ifile of=ofile iflag=nocache oflag=nocache
8276 @cindex nonblocking I/O
8277 Use non-blocking I/O.
8282 Do not update the file's access time.
8283 Some older file systems silently ignore this flag, so it is a good
8284 idea to test it on your files before relying on it.
8288 @cindex controlling terminal
8289 Do not assign the file to be a controlling terminal for @command{dd}.
8290 This has no effect when the file is not a terminal.
8291 On many hosts (e.g., @acronym{GNU}/Linux hosts), this option has no effect
8296 @cindex symbolic links, following
8297 Do not follow symbolic links.
8302 Fail if the file has multiple hard links.
8307 Use binary I/O. This option has an effect only on nonstandard
8308 platforms that distinguish binary from text I/O.
8313 Use text I/O. Like @samp{binary}, this option has no effect on
8318 Accumulate full blocks from input. The @code{read} system call
8319 may return early if a full block is not available.
8320 When that happens, continue calling @code{read} to fill the remainder
8322 This flag can be used only with @code{iflag}.
8326 These flags are not supported on all systems, and @samp{dd} rejects
8327 attempts to use them when they are not supported. When reading from
8328 standard input or writing to standard output, the @samp{nofollow} and
8329 @samp{noctty} flags should not be specified, and the other flags
8330 (e.g., @samp{nonblock}) can affect how other processes behave with the
8331 affected file descriptors, even after @command{dd} exits.
8335 @cindex multipliers after numbers
8336 The numeric-valued strings above (@var{bytes} and @var{blocks}) can be
8337 followed by a multiplier: @samp{b}=512, @samp{c}=1,
8338 @samp{w}=2, @samp{x@var{m}}=@var{m}, or any of the
8339 standard block size suffixes like @samp{k}=1024 (@pxref{Block size}).
8341 Any block size you specify via @samp{bs=}, @samp{ibs=}, @samp{obs=}, @samp{cbs=}
8342 should not be too large---values larger than a few megabytes
8343 are generally wasteful or (as in the gigabyte..exabyte case) downright
8344 counterproductive or error-inducing.
8346 Use different @command{dd} invocations to use different block sizes for
8347 skipping and I/O@. For example, the following shell commands copy data
8348 in 512 KiB blocks between a disk and a tape, but do not save or restore a
8349 4 KiB label at the start of the disk:
8352 disk=/dev/rdsk/c0t1d0s2
8355 # Copy all but the label from disk to tape.
8356 (dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape
8358 # Copy from tape back to disk, but leave the disk label alone.
8359 (dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk
8362 Sending an @samp{INFO} signal to a running @command{dd}
8363 process makes it print I/O statistics to standard error
8364 and then resume copying. In the example below,
8365 @command{dd} is run in the background to copy 10 million blocks.
8366 The @command{kill} command makes it output intermediate I/O statistics,
8367 and when @command{dd} completes normally or is killed by the
8368 @code{SIGINT} signal, it outputs the final statistics.
8371 $ dd if=/dev/zero of=/dev/null count=10MB & pid=$!
8372 $ kill -s INFO $pid; wait $pid
8373 3385223+0 records in
8374 3385223+0 records out
8375 1733234176 bytes (1.7 GB) copied, 6.42173 seconds, 270 MB/s
8376 10000000+0 records in
8377 10000000+0 records out
8378 5120000000 bytes (5.1 GB) copied, 18.913 seconds, 271 MB/s
8381 @vindex POSIXLY_CORRECT
8382 On systems lacking the @samp{INFO} signal @command{dd} responds to the
8383 @samp{USR1} signal instead, unless the @env{POSIXLY_CORRECT}
8384 environment variable is set.
8389 @node install invocation
8390 @section @command{install}: Copy files and set attributes
8393 @cindex copying files and setting attributes
8395 @command{install} copies files while setting their file mode bits and, if
8396 possible, their owner and group. Synopses:
8399 install [@var{option}]@dots{} [-T] @var{source} @var{dest}
8400 install [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8401 install [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8402 install [@var{option}]@dots{} -d @var{directory}@dots{}
8407 If two file names are given, @command{install} copies the first file to the
8411 If the @option{--target-directory} (@option{-t}) option is given, or
8412 failing that if the last file is a directory and the
8413 @option{--no-target-directory} (@option{-T}) option is not given,
8414 @command{install} copies each @var{source} file to the specified
8415 directory, using the @var{source}s' names.
8418 If the @option{--directory} (@option{-d}) option is given,
8419 @command{install} creates each @var{directory} and any missing parent
8420 directories. Parent directories are created with mode
8421 @samp{u=rwx,go=rx} (755), regardless of the @option{-m} option or the
8422 current umask. @xref{Directory Setuid and Setgid}, for how the
8423 set-user-ID and set-group-ID bits of parent directories are inherited.
8426 @cindex Makefiles, installing programs in
8427 @command{install} is similar to @command{cp}, but allows you to control the
8428 attributes of destination files. It is typically used in Makefiles to
8429 copy programs into their destination directories. It refuses to copy
8430 files onto themselves.
8432 @cindex extended attributes, xattr
8433 @command{install} never preserves extended attributes (xattr).
8435 The program accepts the following options. Also see @ref{Common options}.
8445 Compare each pair of source and destination files, and if the destination has
8446 identical content and any specified owner, group, permissions, and possibly
8447 SELinux context, then do not modify the destination at all.
8451 Ignored; for compatibility with old Unix versions of @command{install}.
8455 Create any missing parent directories of @var{dest},
8456 then copy @var{source} to @var{dest}.
8457 This option is ignored if a destination directory is specified
8458 via @option{--target-directory=DIR}.
8463 @opindex --directory
8464 @cindex directories, creating with given attributes
8465 @cindex parent directories, creating missing
8466 @cindex leading directories, creating missing
8467 Create any missing parent directories, giving them the default
8468 attributes. Then create each given directory, setting their owner,
8469 group and mode as given on the command line or to the defaults.
8471 @item -g @var{group}
8472 @itemx --group=@var{group}
8475 @cindex group ownership of installed files, setting
8476 Set the group ownership of installed files or directories to
8477 @var{group}. The default is the process's current group. @var{group}
8478 may be either a group name or a numeric group ID.
8481 @itemx --mode=@var{mode}
8484 @cindex permissions of installed files, setting
8485 Set the file mode bits for the installed file or directory to @var{mode},
8486 which can be either an octal number, or a symbolic mode as in
8487 @command{chmod}, with @samp{a=} (no access allowed to anyone) as the
8488 point of departure (@pxref{File permissions}).
8489 The default mode is @samp{u=rwx,go=rx,a-s}---read, write, and
8490 execute for the owner, read and execute for group and other, and with
8491 set-user-ID and set-group-ID disabled.
8492 This default is not quite the same as @samp{755}, since it disables
8493 instead of preserving set-user-ID and set-group-ID on directories.
8494 @xref{Directory Setuid and Setgid}.
8496 @item -o @var{owner}
8497 @itemx --owner=@var{owner}
8500 @cindex ownership of installed files, setting
8501 @cindex appropriate privileges
8502 @vindex root @r{as default owner}
8503 If @command{install} has appropriate privileges (is run as root), set the
8504 ownership of installed files or directories to @var{owner}. The default
8505 is @code{root}. @var{owner} may be either a user name or a numeric user
8508 @item --preserve-context
8509 @opindex --preserve-context
8511 @cindex security context
8512 Preserve the SELinux security context of files and directories.
8513 Failure to preserve the context in all of the files or directories
8514 will result in an exit status of 1. If SELinux is disabled then
8515 print a warning and ignore the option.
8518 @itemx --preserve-timestamps
8520 @opindex --preserve-timestamps
8521 @cindex timestamps of installed files, preserving
8522 Set the time of last access and the time of last modification of each
8523 installed file to match those of each corresponding original file.
8524 When a file is installed without this option, its last access and
8525 last modification times are both set to the time of installation.
8526 This option is useful if you want to use the last modification times
8527 of installed files to keep track of when they were last built as opposed
8528 to when they were last installed.
8534 @cindex symbol table information, stripping
8535 @cindex stripping symbol table information
8536 Strip the symbol tables from installed binary executables.
8538 @itemx --strip-program=@var{program}
8539 @opindex --strip-program
8540 @cindex symbol table information, stripping, program
8541 Program used to strip binaries.
8547 @optNoTargetDirectory
8553 Print the name of each file before copying it.
8555 @item -Z @var{context}
8556 @itemx --context=@var{context}
8560 @cindex security context
8561 Set the default SELinux security context to be used for any
8562 created files and directories. If SELinux is disabled then
8563 print a warning and ignore the option.
8571 @section @command{mv}: Move (rename) files
8575 @command{mv} moves or renames files (or directories). Synopses:
8578 mv [@var{option}]@dots{} [-T] @var{source} @var{dest}
8579 mv [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8580 mv [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8585 If two file names are given, @command{mv} moves the first file to the
8589 If the @option{--target-directory} (@option{-t}) option is given, or
8590 failing that if the last file is a directory and the
8591 @option{--no-target-directory} (@option{-T}) option is not given,
8592 @command{mv} moves each @var{source} file to the specified
8593 directory, using the @var{source}s' names.
8596 @command{mv} can move any type of file from one file system to another.
8597 Prior to version @code{4.0} of the fileutils,
8598 @command{mv} could move only regular files between file systems.
8599 For example, now @command{mv} can move an entire directory hierarchy
8600 including special device files from one partition to another. It first
8601 uses some of the same code that's used by @code{cp -a} to copy the
8602 requested directories and files, then (assuming the copy succeeded)
8603 it removes the originals. If the copy fails, then the part that was
8604 copied to the destination partition is removed. If you were to copy
8605 three directories from one partition to another and the copy of the first
8606 directory succeeded, but the second didn't, the first would be left on
8607 the destination partition and the second and third would be left on the
8610 @cindex extended attributes, xattr
8611 @command{mv} always tries to copy extended attributes (xattr), which may
8612 include SELinux context, ACLs or Capabilities.
8613 Upon failure all but @samp{Operation not supported} warnings are output.
8615 @cindex prompting, and @command{mv}
8616 If a destination file exists but is normally unwritable, standard input
8617 is a terminal, and the @option{-f} or @option{--force} option is not given,
8618 @command{mv} prompts the user for whether to replace the file. (You might
8619 own the file, or have write permission on its directory.) If the
8620 response is not affirmative, the file is skipped.
8622 @emph{Warning}: Avoid specifying a source name with a trailing slash,
8623 when it might be a symlink to a directory.
8624 Otherwise, @command{mv} may do something very surprising, since
8625 its behavior depends on the underlying rename system call.
8626 On a system with a modern Linux-based kernel, it fails with
8627 @code{errno=ENOTDIR}.
8628 However, on other systems (at least FreeBSD 6.1 and Solaris 10) it silently
8629 renames not the symlink but rather the directory referenced by the symlink.
8630 @xref{Trailing slashes}.
8632 The program accepts the following options. Also see @ref{Common options}.
8642 @cindex prompts, omitting
8643 Do not prompt the user before removing a destination file.
8645 If you specify more than one of the @option{-i}, @option{-f}, @option{-n}
8646 options, only the final one takes effect.
8651 @itemx --interactive
8653 @opindex --interactive
8654 @cindex prompts, forcing
8655 Prompt whether to overwrite each existing destination file, regardless
8657 If the response is not affirmative, the file is skipped.
8663 @opindex --no-clobber
8664 @cindex prompts, omitting
8665 Do not overwrite an existing file.
8667 This option is mutually exclusive with @option{-b} or @option{--backup} option.
8673 @cindex newer files, moving only
8674 Do not move a non-directory that has an existing destination with the
8675 same or newer modification time.
8676 If the move is across file system boundaries, the comparison is to the
8677 source time stamp truncated to the resolutions of the destination file
8678 system and of the system calls used to update time stamps; this avoids
8679 duplicate work if several @samp{mv -u} commands are executed with the
8680 same source and destination.
8686 Print the name of each file before moving it.
8688 @optStripTrailingSlashes
8694 @optNoTargetDirectory
8702 @section @command{rm}: Remove files or directories
8705 @cindex removing files or directories
8707 @command{rm} removes each given @var{file}. By default, it does not remove
8708 directories. Synopsis:
8711 rm [@var{option}]@dots{} [@var{file}]@dots{}
8714 @cindex prompting, and @command{rm}
8715 If the @option{-I} or @option{--interactive=once} option is given,
8716 and there are more than three files or the @option{-r}, @option{-R},
8717 or @option{--recursive} are given, then @command{rm} prompts the user
8718 for whether to proceed with the entire operation. If the response is
8719 not affirmative, the entire command is aborted.
8721 Otherwise, if a file is unwritable, standard input is a terminal, and
8722 the @option{-f} or @option{--force} option is not given, or the
8723 @option{-i} or @option{--interactive=always} option @emph{is} given,
8724 @command{rm} prompts the user for whether to remove the file.
8725 If the response is not affirmative, the file is skipped.
8727 Any attempt to remove a file whose last file name component is
8728 @file{.} or @file{..} is rejected without any prompting.
8730 @emph{Warning}: If you use @command{rm} to remove a file, it is usually
8731 possible to recover the contents of that file. If you want more assurance
8732 that the contents are truly unrecoverable, consider using @command{shred}.
8734 The program accepts the following options. Also see @ref{Common options}.
8742 Ignore nonexistent files and never prompt the user.
8743 Ignore any previous @option{--interactive} (@option{-i}) option.
8747 Prompt whether to remove each file.
8748 If the response is not affirmative, the file is skipped.
8749 Ignore any previous @option{--force} (@option{-f}) option.
8750 Equivalent to @option{--interactive=always}.
8754 Prompt once whether to proceed with the command, if more than three
8755 files are named or if a recursive removal is requested. Ignore any
8756 previous @option{--force} (@option{-f}) option. Equivalent to
8757 @option{--interactive=once}.
8759 @itemx --interactive [=@var{when}]
8760 @opindex --interactive
8761 Specify when to issue an interactive prompt. @var{when} may be
8765 @vindex never @r{interactive option}
8766 - Do not prompt at all.
8768 @vindex once @r{interactive option}
8769 - Prompt once if more than three files are named or if a recursive
8770 removal is requested. Equivalent to @option{-I}.
8772 @vindex always @r{interactive option}
8773 - Prompt for every file being removed. Equivalent to @option{-i}.
8775 @option{--interactive} with no @var{when} is equivalent to
8776 @option{--interactive=always}.
8778 @itemx --one-file-system
8779 @opindex --one-file-system
8780 @cindex one file system, restricting @command{rm} to
8781 When removing a hierarchy recursively, skip any directory that is on a
8782 file system different from that of the corresponding command line argument.
8785 This option is useful when removing a build ``chroot'' hierarchy,
8786 which normally contains no valuable data. However, it is not uncommon
8787 to bind-mount @file{/home} into such a hierarchy, to make it easier to
8788 use one's start-up file. The catch is that it's easy to forget to
8789 unmount @file{/home}. Then, when you use @command{rm -rf} to remove
8790 your normally throw-away chroot, that command will remove everything
8791 under @file{/home}, too.
8792 Use the @option{--one-file-system} option, and it will
8793 warn about and skip directories on other file systems.
8794 Of course, this will not save your @file{/home} if it and your
8795 chroot happen to be on the same file system.
8797 @itemx --preserve-root
8798 @opindex --preserve-root
8799 @cindex root directory, disallow recursive destruction
8800 Fail upon any attempt to remove the root directory, @file{/},
8801 when used with the @option{--recursive} option.
8802 This is the default behavior.
8803 @xref{Treating / specially}.
8805 @itemx --no-preserve-root
8806 @opindex --no-preserve-root
8807 @cindex root directory, allow recursive destruction
8808 Do not treat @file{/} specially when removing recursively.
8809 This option is not recommended unless you really want to
8810 remove all the files on your computer.
8811 @xref{Treating / specially}.
8818 @opindex --recursive
8819 @cindex directories, removing (recursively)
8820 Remove the listed directories and their contents recursively.
8826 Print the name of each file before removing it.
8830 @cindex files beginning with @samp{-}, removing
8831 @cindex @samp{-}, removing files beginning with
8832 One common question is how to remove files whose names begin with a
8833 @samp{-}. @sc{gnu} @command{rm}, like every program that uses the @code{getopt}
8834 function to parse its arguments, lets you use the @samp{--} option to
8835 indicate that all following arguments are non-options. To remove a file
8836 called @file{-f} in the current directory, you could type either:
8849 @opindex - @r{and Unix @command{rm}}
8850 The Unix @command{rm} program's use of a single @samp{-} for this purpose
8851 predates the development of the getopt standard syntax.
8856 @node shred invocation
8857 @section @command{shred}: Remove files more securely
8860 @cindex data, erasing
8861 @cindex erasing data
8863 @command{shred} overwrites devices or files, to help prevent even
8864 very expensive hardware from recovering the data.
8866 Ordinarily when you remove a file (@pxref{rm invocation}), the data is
8867 not actually destroyed. Only the index listing where the file is
8868 stored is destroyed, and the storage is made available for reuse.
8869 There are undelete utilities that will attempt to reconstruct the index
8870 and can bring the file back if the parts were not reused.
8872 On a busy system with a nearly-full drive, space can get reused in a few
8873 seconds. But there is no way to know for sure. If you have sensitive
8874 data, you may want to be sure that recovery is not possible by actually
8875 overwriting the file with non-sensitive data.
8877 However, even after doing that, it is possible to take the disk back
8878 to a laboratory and use a lot of sensitive (and expensive) equipment
8879 to look for the faint ``echoes'' of the original data underneath the
8880 overwritten data. If the data has only been overwritten once, it's not
8883 The best way to remove something irretrievably is to destroy the media
8884 it's on with acid, melt it down, or the like. For cheap removable media
8885 like floppy disks, this is the preferred method. However, hard drives
8886 are expensive and hard to melt, so the @command{shred} utility tries
8887 to achieve a similar effect non-destructively.
8889 This uses many overwrite passes, with the data patterns chosen to
8890 maximize the damage they do to the old data. While this will work on
8891 floppies, the patterns are designed for best effect on hard drives.
8892 For more details, see the source code and Peter Gutmann's paper
8893 @uref{http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html,
8894 @cite{Secure Deletion of Data from Magnetic and Solid-State Memory}},
8895 from the proceedings of the Sixth @acronym{USENIX} Security Symposium (San Jose,
8896 California, July 22--25, 1996).
8898 @strong{Please note} that @command{shred} relies on a very important assumption:
8899 that the file system overwrites data in place. This is the traditional
8900 way to do things, but many modern file system designs do not satisfy this
8901 assumption. Exceptions include:
8906 Log-structured or journaled file systems, such as those supplied with
8907 AIX and Solaris, and JFS, ReiserFS, XFS, Ext3 (in @code{data=journal} mode),
8908 BFS, NTFS, etc.@: when they are configured to journal @emph{data}.
8911 File systems that write redundant data and carry on even if some writes
8912 fail, such as RAID-based file systems.
8915 File systems that make snapshots, such as Network Appliance's NFS server.
8918 File systems that cache in temporary locations, such as NFS version 3
8922 Compressed file systems.
8925 In the particular case of ext3 file systems, the above disclaimer applies (and
8926 @command{shred} is thus of limited effectiveness) only in @code{data=journal}
8927 mode, which journals file data in addition to just metadata. In both
8928 the @code{data=ordered} (default) and @code{data=writeback} modes,
8929 @command{shred} works as usual. Ext3 journaling modes can be changed
8930 by adding the @code{data=something} option to the mount options for a
8931 particular file system in the @file{/etc/fstab} file, as documented in
8932 the mount man page (man mount).
8934 If you are not sure how your file system operates, then you should assume
8935 that it does not overwrite data in place, which means that shred cannot
8936 reliably operate on regular files in your file system.
8938 Generally speaking, it is more reliable to shred a device than a file,
8939 since this bypasses the problem of file system design mentioned above.
8940 However, even shredding devices is not always completely reliable. For
8941 example, most disks map out bad sectors invisibly to the application; if
8942 the bad sectors contain sensitive data, @command{shred} won't be able to
8945 @command{shred} makes no attempt to detect or report this problem, just as
8946 it makes no attempt to do anything about backups. However, since it is
8947 more reliable to shred devices than files, @command{shred} by default does
8948 not truncate or remove the output file. This default is more suitable
8949 for devices, which typically cannot be truncated and should not be
8952 Finally, consider the risk of backups and mirrors.
8953 File system backups and remote mirrors may contain copies of the
8954 file that cannot be removed, and that will allow a shredded file
8955 to be recovered later. So if you keep any data you may later want
8956 to destroy using @command{shred}, be sure that it is not backed up or mirrored.
8959 shred [@var{option}]@dots{} @var{file}[@dots{}]
8962 The program accepts the following options. Also see @ref{Common options}.
8970 @cindex force deletion
8971 Override file permissions if necessary to allow overwriting.
8974 @itemx -n @var{number}
8975 @itemx --iterations=@var{number}
8976 @opindex -n @var{number}
8977 @opindex --iterations=@var{number}
8978 @cindex iterations, selecting the number of
8979 By default, @command{shred} uses @value{SHRED_DEFAULT_PASSES} passes of
8980 overwrite. You can reduce this to save time, or increase it if you think it's
8981 appropriate. After 25 passes all of the internal overwrite patterns will have
8982 been used at least once.
8984 @item --random-source=@var{file}
8985 @opindex --random-source
8986 @cindex random source for shredding
8987 Use @var{file} as a source of random data used to overwrite and to
8988 choose pass ordering. @xref{Random sources}.
8990 @item -s @var{bytes}
8991 @itemx --size=@var{bytes}
8992 @opindex -s @var{bytes}
8993 @opindex --size=@var{bytes}
8994 @cindex size of file to shred
8995 Shred the first @var{bytes} bytes of the file. The default is to shred
8996 the whole file. @var{bytes} can be followed by a size specification like
8997 @samp{K}, @samp{M}, or @samp{G} to specify a multiple. @xref{Block size}.
9003 @cindex removing files after shredding
9004 After shredding a file, truncate it (if possible) and then remove it.
9005 If a file has multiple links, only the named links will be removed.
9011 Display to standard error all status updates as sterilization proceeds.
9017 By default, @command{shred} rounds the size of a regular file up to the next
9018 multiple of the file system block size to fully erase the last block
9020 Use @option{--exact} to suppress that behavior.
9021 Thus, by default if you shred a 10-byte regular file on a system with 512-byte
9022 blocks, the resulting file will be 512 bytes long. With this option,
9023 shred does not increase the apparent size of the file.
9029 Normally, the last pass that @command{shred} writes is made up of
9030 random data. If this would be conspicuous on your hard drive (for
9031 example, because it looks like encrypted data), or you just think
9032 it's tidier, the @option{--zero} option adds an additional overwrite pass with
9033 all zero bits. This is in addition to the number of passes specified
9034 by the @option{--iterations} option.
9038 You might use the following command to erase all trace of the
9039 file system you'd created on the floppy disk in your first drive.
9040 That command takes about 20 minutes to erase a ``1.44MB'' (actually
9044 shred --verbose /dev/fd0
9047 Similarly, to erase all data on a selected partition of
9048 your hard disk, you could give a command like this:
9051 shred --verbose /dev/sda5
9054 On modern disks, a single pass should be adequate,
9055 and it will take one third the time of the default three-pass approach.
9058 # 1 pass, write pseudo-random data; 3x faster than the default
9059 shred --verbose -n1 /dev/sda5
9062 To be on the safe side, use at least one pass that overwrites using
9063 pseudo-random data. I.e., don't be tempted to use @samp{-n0 --zero},
9064 in case some disk controller optimizes the process of writing blocks
9065 of all zeros, and thereby does not clear all bytes in a block.
9066 Some SSDs may do just that.
9068 A @var{file} of @samp{-} denotes standard output.
9069 The intended use of this is to shred a removed temporary file.
9076 echo "Hello, world" >&3
9081 However, the command @samp{shred - >file} does not shred the contents
9082 of @var{file}, since the shell truncates @var{file} before invoking
9083 @command{shred}. Use the command @samp{shred file} or (if using a
9084 Bourne-compatible shell) the command @samp{shred - 1<>file} instead.
9089 @node Special file types
9090 @chapter Special file types
9092 @cindex special file types
9093 @cindex file types, special
9095 This chapter describes commands which create special types of files (and
9096 @command{rmdir}, which removes directories, one special file type).
9098 @cindex special file types
9100 Although Unix-like operating systems have markedly fewer special file
9101 types than others, not @emph{everything} can be treated only as the
9102 undifferentiated byte stream of @dfn{normal files}. For example, when a
9103 file is created or removed, the system must record this information,
9104 which it does in a @dfn{directory}---a special type of file. Although
9105 you can read directories as normal files, if you're curious, in order
9106 for the system to do its job it must impose a structure, a certain
9107 order, on the bytes of the file. Thus it is a ``special'' type of file.
9109 Besides directories, other special file types include named pipes
9110 (FIFOs), symbolic links, sockets, and so-called @dfn{special files}.
9113 * link invocation:: Make a hard link via the link syscall
9114 * ln invocation:: Make links between files.
9115 * mkdir invocation:: Make directories.
9116 * mkfifo invocation:: Make FIFOs (named pipes).
9117 * mknod invocation:: Make block or character special files.
9118 * readlink invocation:: Print value of a symlink or canonical file name.
9119 * rmdir invocation:: Remove empty directories.
9120 * unlink invocation:: Remove files via the unlink syscall
9124 @node link invocation
9125 @section @command{link}: Make a hard link via the link syscall
9128 @cindex links, creating
9129 @cindex hard links, creating
9130 @cindex creating links (hard only)
9132 @command{link} creates a single hard link at a time.
9133 It is a minimalist interface to the system-provided
9134 @code{link} function. @xref{Hard Links, , , libc,
9135 The GNU C Library Reference Manual}.
9136 It avoids the bells and whistles of the more commonly-used
9137 @command{ln} command (@pxref{ln invocation}).
9141 link @var{filename} @var{linkname}
9144 @var{filename} must specify an existing file, and @var{linkname}
9145 must specify a nonexistent entry in an existing directory.
9146 @command{link} simply calls @code{link (@var{filename}, @var{linkname})}
9149 On a @acronym{GNU} system, this command acts like @samp{ln --directory
9150 --no-target-directory @var{filename} @var{linkname}}. However, the
9151 @option{--directory} and @option{--no-target-directory} options are
9152 not specified by @acronym{POSIX}, and the @command{link} command is
9153 more portable in practice.
9155 If @var{filename} is a symbolic link, it is unspecified whether
9156 @var{linkname} will be a hard link to the symbolic link or to the
9157 target of the symbolic link. Use @command{ln -P} or @command{ln -L}
9158 to specify which behavior is desired.
9164 @section @command{ln}: Make links between files
9167 @cindex links, creating
9168 @cindex hard links, creating
9169 @cindex symbolic (soft) links, creating
9170 @cindex creating links (hard or soft)
9172 @cindex file systems and hard links
9173 @command{ln} makes links between files. By default, it makes hard links;
9174 with the @option{-s} option, it makes symbolic (or @dfn{soft}) links.
9178 ln [@var{option}]@dots{} [-T] @var{target} @var{linkname}
9179 ln [@var{option}]@dots{} @var{target}
9180 ln [@var{option}]@dots{} @var{target}@dots{} @var{directory}
9181 ln [@var{option}]@dots{} -t @var{directory} @var{target}@dots{}
9187 If two file names are given, @command{ln} creates a link to the first
9188 file from the second.
9191 If one @var{target} is given, @command{ln} creates a link to that file
9192 in the current directory.
9195 If the @option{--target-directory} (@option{-t}) option is given, or
9196 failing that if the last file is a directory and the
9197 @option{--no-target-directory} (@option{-T}) option is not given,
9198 @command{ln} creates a link to each @var{target} file in the specified
9199 directory, using the @var{target}s' names.
9203 Normally @command{ln} does not remove existing files. Use the
9204 @option{--force} (@option{-f}) option to remove them unconditionally,
9205 the @option{--interactive} (@option{-i}) option to remove them
9206 conditionally, and the @option{--backup} (@option{-b}) option to
9209 @cindex hard link, defined
9210 @cindex inode, and hard links
9211 A @dfn{hard link} is another name for an existing file; the link and the
9212 original are indistinguishable. Technically speaking, they share the
9213 same inode, and the inode contains all the information about a
9214 file---indeed, it is not incorrect to say that the inode @emph{is} the
9215 file. Most systems prohibit making a hard link to
9216 a directory; on those where it is allowed, only the super-user can do
9217 so (and with caution, since creating a cycle will cause problems to many
9218 other utilities). Hard links cannot cross file system boundaries. (These
9219 restrictions are not mandated by @acronym{POSIX}, however.)
9221 @cindex dereferencing symbolic links
9222 @cindex symbolic link, defined
9223 @dfn{Symbolic links} (@dfn{symlinks} for short), on the other hand, are
9224 a special file type (which not all kernels support: System V release 3
9225 (and older) systems lack symlinks) in which the link file actually
9226 refers to a different file, by name. When most operations (opening,
9227 reading, writing, and so on) are passed the symbolic link file, the
9228 kernel automatically @dfn{dereferences} the link and operates on the
9229 target of the link. But some operations (e.g., removing) work on the
9230 link file itself, rather than on its target. The owner and group of a
9231 symlink are not significant to file access performed through
9232 the link, but do have implications on deleting a symbolic link from a
9233 directory with the restricted deletion bit set. On the GNU system,
9234 the mode of a symlink has no significance and cannot be changed, but
9235 on some BSD systems, the mode can be changed and will affect whether
9236 the symlink will be traversed in file name resolution. @xref{Symbolic Links,,,
9237 libc, The GNU C Library Reference Manual}.
9239 Symbolic links can contain arbitrary strings; a @dfn{dangling symlink}
9240 occurs when the string in the symlink does not resolve to a file.
9241 There are no restrictions against creating dangling symbolic links.
9242 There are trade-offs to using absolute or relative symlinks. An
9243 absolute symlink always points to the same file, even if the directory
9244 containing the link is moved. However, if the symlink is visible from
9245 more than one machine (such as on a networked file system), the file
9246 pointed to might not always be the same. A relative symbolic link is
9247 resolved in relation to the directory that contains the link, and is
9248 often useful in referring to files on the same device without regards
9249 to what name that device is mounted on when accessed via networked
9252 When creating a relative symlink in a different location than the
9253 current directory, the resolution of the symlink will be different
9254 than the resolution of the same string from the current directory.
9255 Therefore, many users prefer to first change directories to the
9256 location where the relative symlink will be created, so that
9257 tab-completion or other file resolution will find the same target as
9258 what will be placed in the symlink.
9260 The program accepts the following options. Also see @ref{Common options}.
9271 @opindex --directory
9272 @cindex hard links to directories
9273 Allow users with appropriate privileges to attempt to make hard links
9275 However, note that this will probably fail due to
9276 system restrictions, even for the super-user.
9282 Remove existing destination files.
9285 @itemx --interactive
9287 @opindex --interactive
9288 @cindex prompting, and @command{ln}
9289 Prompt whether to remove existing destination files.
9295 If @option{-s} is not in effect, and the source file is a symbolic
9296 link, create the hard link to the file referred to by the symbolic
9297 link, rather than the symbolic link itself.
9300 @itemx --no-dereference
9302 @opindex --no-dereference
9303 Do not treat the last operand specially when it is a symbolic link to
9304 a directory. Instead, treat it as if it were a normal file.
9306 When the destination is an actual directory (not a symlink to one),
9307 there is no ambiguity. The link is created in that directory.
9308 But when the specified destination is a symlink to a directory,
9309 there are two ways to treat the user's request. @command{ln} can
9310 treat the destination just as it would a normal directory and create
9311 the link in it. On the other hand, the destination can be viewed as a
9312 non-directory---as the symlink itself. In that case, @command{ln}
9313 must delete or backup that symlink before creating the new link.
9314 The default is to treat a destination that is a symlink to a directory
9315 just like a directory.
9317 This option is weaker than the @option{--no-target-directory}
9318 (@option{-T}) option, so it has no effect if both options are given.
9324 If @option{-s} is not in effect, and the source file is a symbolic
9325 link, create the hard link to the symbolic link itself. On platforms
9326 where this is not supported by the kernel, this option creates a
9327 symbolic link with identical contents; since symbolic link contents
9328 cannot be edited, any file name resolution performed through either
9329 link will be the same as if a hard link had been created.
9335 Make symbolic links instead of hard links. This option merely produces
9336 an error message on systems that do not support symbolic links.
9342 @optNoTargetDirectory
9348 Print the name of each file after linking it successfully.
9352 @cindex hard links to symbolic links
9353 @cindex symbolic links and @command{ln}
9354 If @option{-L} and @option{-P} are both given, the last one takes
9355 precedence. If @option{-s} is also given, @option{-L} and @option{-P}
9356 are silently ignored. If neither option is given, then this
9357 implementation defaults to @option{-P} if the system @code{link} supports
9358 hard links to symbolic links (such as the GNU system), and @option{-L}
9359 if @code{link} follows symbolic links (such as on BSD).
9368 # Create link ../a pointing to a in that directory.
9369 # Not really useful because it points to itself.
9374 # Change to the target before creating symlinks to avoid being confused.
9380 # Hard coded file names don't move well.
9381 ln -s $(pwd)/a /some/dir/
9385 # Relative file names survive directory moves and also
9386 # work across networked file systems.
9387 ln -s afile anotherfile
9388 ln -s ../adir/afile yetanotherfile
9392 @node mkdir invocation
9393 @section @command{mkdir}: Make directories
9396 @cindex directories, creating
9397 @cindex creating directories
9399 @command{mkdir} creates directories with the specified names. Synopsis:
9402 mkdir [@var{option}]@dots{} @var{name}@dots{}
9405 @command{mkdir} creates each directory @var{name} in the order given.
9406 It reports an error if @var{name} already exists, unless the
9407 @option{-p} option is given and @var{name} is a directory.
9409 The program accepts the following options. Also see @ref{Common options}.
9414 @itemx --mode=@var{mode}
9417 @cindex modes of created directories, setting
9418 Set the file permission bits of created directories to @var{mode},
9419 which uses the same syntax as
9420 in @command{chmod} and uses @samp{a=rwx} (read, write and execute allowed for
9421 everyone) for the point of the departure. @xref{File permissions}.
9423 Normally the directory has the desired file mode bits at the moment it
9424 is created. As a @acronym{GNU} extension, @var{mode} may also mention
9425 special mode bits, but in this case there may be a temporary window
9426 during which the directory exists but its special mode bits are
9427 incorrect. @xref{Directory Setuid and Setgid}, for how the
9428 set-user-ID and set-group-ID bits of directories are inherited unless
9429 overridden in this way.
9435 @cindex parent directories, creating
9436 Make any missing parent directories for each argument, setting their
9437 file permission bits to the umask modified by @samp{u+wx}. Ignore
9438 existing parent directories, and do not change their file permission
9441 To set the file permission bits of any newly-created parent
9442 directories to a value that includes @samp{u+wx}, you can set the
9443 umask before invoking @command{mkdir}. For example, if the shell
9444 command @samp{(umask u=rwx,go=rx; mkdir -p P/Q)} creates the parent
9445 @file{P} it sets the parent's permission bits to @samp{u=rwx,go=rx}.
9446 To set a parent's special mode bits as well, you can invoke
9447 @command{chmod} after @command{mkdir}. @xref{Directory Setuid and
9448 Setgid}, for how the set-user-ID and set-group-ID bits of
9449 newly-created parent directories are inherited.
9455 Print a message for each created directory. This is most useful with
9458 @item -Z @var{context}
9459 @itemx --context=@var{context}
9463 @cindex security context
9464 Set the default SELinux security context to be used for created directories.
9471 @node mkfifo invocation
9472 @section @command{mkfifo}: Make FIFOs (named pipes)
9475 @cindex FIFOs, creating
9476 @cindex named pipes, creating
9477 @cindex creating FIFOs (named pipes)
9479 @command{mkfifo} creates FIFOs (also called @dfn{named pipes}) with the
9480 specified names. Synopsis:
9483 mkfifo [@var{option}] @var{name}@dots{}
9486 A @dfn{FIFO} is a special file type that permits independent processes
9487 to communicate. One process opens the FIFO file for writing, and
9488 another for reading, after which data can flow as with the usual
9489 anonymous pipe in shells or elsewhere.
9491 The program accepts the following option. Also see @ref{Common options}.
9496 @itemx --mode=@var{mode}
9499 @cindex modes of created FIFOs, setting
9500 Set the mode of created FIFOs to @var{mode}, which is symbolic as in
9501 @command{chmod} and uses @samp{a=rw} (read and write allowed for everyone)
9502 for the point of departure. @var{mode} should specify only file
9503 permission bits. @xref{File permissions}.
9505 @item -Z @var{context}
9506 @itemx --context=@var{context}
9510 @cindex security context
9511 Set the default SELinux security context to be used for created FIFOs.
9518 @node mknod invocation
9519 @section @command{mknod}: Make block or character special files
9522 @cindex block special files, creating
9523 @cindex character special files, creating
9525 @command{mknod} creates a FIFO, character special file, or block special
9526 file with the specified name. Synopsis:
9529 mknod [@var{option}]@dots{} @var{name} @var{type} [@var{major} @var{minor}]
9532 @cindex special files
9533 @cindex block special files
9534 @cindex character special files
9535 Unlike the phrase ``special file type'' above, the term @dfn{special
9536 file} has a technical meaning on Unix: something that can generate or
9537 receive data. Usually this corresponds to a physical piece of hardware,
9538 e.g., a printer or a disk. (These files are typically created at
9539 system-configuration time.) The @command{mknod} command is what creates
9540 files of this type. Such devices can be read either a character at a
9541 time or a ``block'' (many characters) at a time, hence we say there are
9542 @dfn{block special} files and @dfn{character special} files.
9544 @c mknod is a shell built-in at least with OpenBSD's /bin/sh
9545 @mayConflictWithShellBuiltIn{mknod}
9547 The arguments after @var{name} specify the type of file to make:
9552 @opindex p @r{for FIFO file}
9556 @opindex b @r{for block special file}
9557 for a block special file
9560 @c Don't document the `u' option -- it's just a synonym for `c'.
9561 @c Do *any* versions of mknod still use it?
9563 @opindex c @r{for character special file}
9564 @c @opindex u @r{for character special file}
9565 for a character special file
9569 When making a block or character special file, the major and minor
9570 device numbers must be given after the file type.
9571 If a major or minor device number begins with @samp{0x} or @samp{0X},
9572 it is interpreted as hexadecimal; otherwise, if it begins with @samp{0},
9573 as octal; otherwise, as decimal.
9575 The program accepts the following option. Also see @ref{Common options}.
9580 @itemx --mode=@var{mode}
9583 Set the mode of created files to @var{mode}, which is symbolic as in
9584 @command{chmod} and uses @samp{a=rw} as the point of departure.
9585 @var{mode} should specify only file permission bits.
9586 @xref{File permissions}.
9588 @item -Z @var{context}
9589 @itemx --context=@var{context}
9593 @cindex security context
9594 Set the default SELinux security context to be used for created files.
9601 @node readlink invocation
9602 @section @command{readlink}: Print value of a symlink or canonical file name
9605 @cindex displaying value of a symbolic link
9606 @cindex canonical file name
9607 @cindex canonicalize a file name
9610 @command{readlink} may work in one of two supported modes:
9616 @command{readlink} outputs the value of the given symbolic link.
9617 If @command{readlink} is invoked with an argument other than the name
9618 of a symbolic link, it produces no output and exits with a nonzero exit code.
9620 @item Canonicalize mode
9622 @command{readlink} outputs the absolute name of the given file which contains
9623 no @file{.}, @file{..} components nor any repeated separators
9624 (@file{/}) or symbolic links.
9629 readlink [@var{option}] @var{file}
9632 By default, @command{readlink} operates in readlink mode.
9634 The program accepts the following options. Also see @ref{Common options}.
9639 @itemx --canonicalize
9641 @opindex --canonicalize
9642 Activate canonicalize mode.
9643 If any component of the file name except the last one is missing or unavailable,
9644 @command{readlink} produces no output and exits with a nonzero exit
9645 code. A trailing slash is ignored.
9648 @itemx --canonicalize-existing
9650 @opindex --canonicalize-existing
9651 Activate canonicalize mode.
9652 If any component is missing or unavailable, @command{readlink} produces
9653 no output and exits with a nonzero exit code. A trailing slash
9654 requires that the name resolve to a directory.
9657 @itemx --canonicalize-missing
9659 @opindex --canonicalize-missing
9660 Activate canonicalize mode.
9661 If any component is missing or unavailable, @command{readlink} treats it
9667 @opindex --no-newline
9668 Do not output the trailing newline.
9678 Suppress most error messages.
9684 Report error messages.
9688 The @command{readlink} utility first appeared in OpenBSD 2.1.
9690 The @command{realpath} command without options, operates like
9691 @command{readlink} in canonicalize mode.
9696 @node rmdir invocation
9697 @section @command{rmdir}: Remove empty directories
9700 @cindex removing empty directories
9701 @cindex directories, removing empty
9703 @command{rmdir} removes empty directories. Synopsis:
9706 rmdir [@var{option}]@dots{} @var{directory}@dots{}
9709 If any @var{directory} argument does not refer to an existing empty
9710 directory, it is an error.
9712 The program accepts the following options. Also see @ref{Common options}.
9716 @item --ignore-fail-on-non-empty
9717 @opindex --ignore-fail-on-non-empty
9718 @cindex directory deletion, ignoring failures
9719 Ignore each failure to remove a directory that is solely because
9720 the directory is non-empty.
9726 @cindex parent directories, removing
9727 Remove @var{directory}, then try to remove each component of @var{directory}.
9728 So, for example, @samp{rmdir -p a/b/c} is similar to @samp{rmdir a/b/c a/b a}.
9729 As such, it fails if any of those directories turns out not to be empty.
9730 Use the @option{--ignore-fail-on-non-empty} option to make it so such
9731 a failure does not evoke a diagnostic and does not cause @command{rmdir} to
9732 exit unsuccessfully.
9738 @cindex directory deletion, reporting
9739 Give a diagnostic for each successful removal.
9740 @var{directory} is removed.
9744 @xref{rm invocation}, for how to remove non-empty directories (recursively).
9749 @node unlink invocation
9750 @section @command{unlink}: Remove files via the unlink syscall
9753 @cindex removing files or directories (via the unlink syscall)
9755 @command{unlink} deletes a single specified file name.
9756 It is a minimalist interface to the system-provided
9757 @code{unlink} function. @xref{Deleting Files, , , libc,
9758 The GNU C Library Reference Manual}. Synopsis:
9759 It avoids the bells and whistles of the more commonly-used
9760 @command{rm} command (@pxref{rm invocation}).
9763 unlink @var{filename}
9766 On some systems @code{unlink} can be used to delete the name of a
9767 directory. On others, it can be used that way only by a privileged user.
9768 In the GNU system @code{unlink} can never delete the name of a directory.
9770 The @command{unlink} command honors the @option{--help} and
9771 @option{--version} options. To remove a file whose name begins with
9772 @samp{-}, prefix the name with @samp{./}, e.g., @samp{unlink ./--help}.
9777 @node Changing file attributes
9778 @chapter Changing file attributes
9780 @cindex changing file attributes
9781 @cindex file attributes, changing
9782 @cindex attributes, file
9784 A file is not merely its contents, a name, and a file type
9785 (@pxref{Special file types}). A file also has an owner (a user ID), a
9786 group (a group ID), permissions (what the owner can do with the file,
9787 what people in the group can do, and what everyone else can do), various
9788 timestamps, and other information. Collectively, we call these a file's
9791 These commands change file attributes.
9794 * chgrp invocation:: Change file groups.
9795 * chmod invocation:: Change access permissions.
9796 * chown invocation:: Change file owners and groups.
9797 * touch invocation:: Change file timestamps.
9801 @node chown invocation
9802 @section @command{chown}: Change file owner and group
9805 @cindex file ownership, changing
9806 @cindex group ownership, changing
9807 @cindex changing file ownership
9808 @cindex changing group ownership
9810 @command{chown} changes the user and/or group ownership of each given @var{file}
9811 to @var{new-owner} or to the user and group of an existing reference file.
9815 chown [@var{option}]@dots{} @{@var{new-owner} | --reference=@var{ref_file}@}@c
9819 If used, @var{new-owner} specifies the new owner and/or group as follows
9820 (with no embedded white space):
9823 [@var{owner}] [ : [@var{group}] ]
9830 If only an @var{owner} (a user name or numeric user ID) is given, that
9831 user is made the owner of each given file, and the files' group is not
9834 @item owner@samp{:}group
9835 If the @var{owner} is followed by a colon and a @var{group} (a
9836 group name or numeric group ID), with no spaces between them, the group
9837 ownership of the files is changed as well (to @var{group}).
9840 If a colon but no group name follows @var{owner}, that user is
9841 made the owner of the files and the group of the files is changed to
9842 @var{owner}'s login group.
9845 If the colon and following @var{group} are given, but the owner
9846 is omitted, only the group of the files is changed; in this case,
9847 @command{chown} performs the same function as @command{chgrp}.
9850 If only a colon is given, or if @var{new-owner} is empty, neither the
9851 owner nor the group is changed.
9855 If @var{owner} or @var{group} is intended to represent a numeric user
9856 or group ID, then you may specify it with a leading @samp{+}.
9857 @xref{Disambiguating names and IDs}.
9859 Some older scripts may still use @samp{.} in place of the @samp{:} separator.
9860 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) does not
9861 require support for that, but for backward compatibility @acronym{GNU}
9862 @command{chown} supports @samp{.} so long as no ambiguity results.
9863 New scripts should avoid the use of @samp{.} because it is not
9864 portable, and because it has undesirable results if the entire
9865 @var{owner@samp{.}group} happens to identify a user whose name
9868 The @command{chown} command sometimes clears the set-user-ID or
9869 set-group-ID permission bits. This behavior depends on the policy and
9870 functionality of the underlying @code{chown} system call, which may
9871 make system-dependent file mode modifications outside the control of
9872 the @command{chown} command. For example, the @command{chown} command
9873 might not affect those bits when invoked by a user with appropriate
9874 privileges, or when the
9875 bits signify some function other than executable permission (e.g.,
9877 When in doubt, check the underlying system behavior.
9879 The program accepts the following options. Also see @ref{Common options}.
9887 @cindex changed owners, verbosely describing
9888 Verbosely describe the action for each @var{file} whose ownership
9897 @cindex error messages, omitting
9898 Do not print error messages about files whose ownership cannot be
9901 @itemx @w{@kbd{--from}=@var{old-owner}}
9903 @cindex symbolic links, changing owner
9904 Change a @var{file}'s ownership only if it has current attributes specified
9905 by @var{old-owner}. @var{old-owner} has the same form as @var{new-owner}
9907 This option is useful primarily from a security standpoint in that
9908 it narrows considerably the window of potential abuse.
9909 For example, to reflect a user ID numbering change for one user's files
9910 without an option like this, @code{root} might run
9913 find / -owner OLDUSER -print0 | xargs -0 chown -h NEWUSER
9916 But that is dangerous because the interval between when the @command{find}
9917 tests the existing file's owner and when the @command{chown} is actually run
9919 One way to narrow the gap would be to invoke chown for each file
9923 find / -owner OLDUSER -exec chown -h NEWUSER @{@} \;
9926 But that is very slow if there are many affected files.
9927 With this option, it is safer (the gap is narrower still)
9928 though still not perfect:
9931 chown -h -R --from=OLDUSER NEWUSER /
9935 @opindex --dereference
9936 @cindex symbolic links, changing owner
9938 Do not act on symbolic links themselves but rather on what they point to.
9939 This is the default.
9942 @itemx --no-dereference
9944 @opindex --no-dereference
9945 @cindex symbolic links, changing owner
9947 Act on symbolic links themselves instead of what they point to.
9948 This mode relies on the @code{lchown} system call.
9949 On systems that do not provide the @code{lchown} system call,
9950 @command{chown} fails when a file specified on the command line
9952 By default, no diagnostic is issued for symbolic links encountered
9953 during a recursive traversal, but see @option{--verbose}.
9955 @itemx --preserve-root
9956 @opindex --preserve-root
9957 @cindex root directory, disallow recursive modification
9958 Fail upon any attempt to recursively change the root directory, @file{/}.
9959 Without @option{--recursive}, this option has no effect.
9960 @xref{Treating / specially}.
9962 @itemx --no-preserve-root
9963 @opindex --no-preserve-root
9964 @cindex root directory, allow recursive modification
9965 Cancel the effect of any preceding @option{--preserve-root} option.
9966 @xref{Treating / specially}.
9968 @item --reference=@var{ref_file}
9969 @opindex --reference
9970 Change the user and group of each @var{file} to be the same as those of
9971 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9972 user and group of the symbolic link, but rather those of the file it
9979 Output a diagnostic for every file processed.
9980 If a symbolic link is encountered during a recursive traversal
9981 on a system without the @code{lchown} system call, and @option{--no-dereference}
9982 is in effect, then issue a diagnostic saying neither the symbolic link nor
9983 its referent is being changed.
9988 @opindex --recursive
9989 @cindex recursively changing file ownership
9990 Recursively change ownership of directories and their contents.
9993 @xref{Traversing symlinks}.
9996 @xref{Traversing symlinks}.
9999 @xref{Traversing symlinks}.
10008 # Change the owner of /u to "root".
10011 # Likewise, but also change its group to "staff".
10012 chown root:staff /u
10014 # Change the owner of /u and subfiles to "root".
10019 @node chgrp invocation
10020 @section @command{chgrp}: Change group ownership
10023 @cindex group ownership, changing
10024 @cindex changing group ownership
10026 @command{chgrp} changes the group ownership of each given @var{file}
10027 to @var{group} (which can be either a group name or a numeric group ID)
10028 or to the group of an existing reference file. Synopsis:
10031 chgrp [@var{option}]@dots{} @{@var{group} | --reference=@var{ref_file}@}@c
10035 If @var{group} is intended to represent a
10036 numeric group ID, then you may specify it with a leading @samp{+}.
10037 @xref{Disambiguating names and IDs}.
10039 The program accepts the following options. Also see @ref{Common options}.
10047 @cindex changed files, verbosely describing
10048 Verbosely describe the action for each @var{file} whose group actually
10057 @cindex error messages, omitting
10058 Do not print error messages about files whose group cannot be
10061 @item --dereference
10062 @opindex --dereference
10063 @cindex symbolic links, changing owner
10065 Do not act on symbolic links themselves but rather on what they point to.
10066 This is the default.
10069 @itemx --no-dereference
10071 @opindex --no-dereference
10072 @cindex symbolic links, changing group
10074 Act on symbolic links themselves instead of what they point to.
10075 This mode relies on the @code{lchown} system call.
10076 On systems that do not provide the @code{lchown} system call,
10077 @command{chgrp} fails when a file specified on the command line
10078 is a symbolic link.
10079 By default, no diagnostic is issued for symbolic links encountered
10080 during a recursive traversal, but see @option{--verbose}.
10082 @itemx --preserve-root
10083 @opindex --preserve-root
10084 @cindex root directory, disallow recursive modification
10085 Fail upon any attempt to recursively change the root directory, @file{/}.
10086 Without @option{--recursive}, this option has no effect.
10087 @xref{Treating / specially}.
10089 @itemx --no-preserve-root
10090 @opindex --no-preserve-root
10091 @cindex root directory, allow recursive modification
10092 Cancel the effect of any preceding @option{--preserve-root} option.
10093 @xref{Treating / specially}.
10095 @item --reference=@var{ref_file}
10096 @opindex --reference
10097 Change the group of each @var{file} to be the same as that of
10098 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
10099 group of the symbolic link, but rather that of the file it refers to.
10105 Output a diagnostic for every file processed.
10106 If a symbolic link is encountered during a recursive traversal
10107 on a system without the @code{lchown} system call, and @option{--no-dereference}
10108 is in effect, then issue a diagnostic saying neither the symbolic link nor
10109 its referent is being changed.
10114 @opindex --recursive
10115 @cindex recursively changing group ownership
10116 Recursively change the group ownership of directories and their contents.
10119 @xref{Traversing symlinks}.
10122 @xref{Traversing symlinks}.
10125 @xref{Traversing symlinks}.
10134 # Change the group of /u to "staff".
10137 # Change the group of /u and subfiles to "staff".
10142 @node chmod invocation
10143 @section @command{chmod}: Change access permissions
10146 @cindex changing access permissions
10147 @cindex access permissions, changing
10148 @cindex permissions, changing access
10150 @command{chmod} changes the access permissions of the named files. Synopsis:
10153 chmod [@var{option}]@dots{} @{@var{mode} | --reference=@var{ref_file}@}@c
10157 @cindex symbolic links, permissions of
10158 @command{chmod} never changes the permissions of symbolic links, since
10159 the @command{chmod} system call cannot change their permissions.
10160 This is not a problem since the permissions of symbolic links are
10161 never used. However, for each symbolic link listed on the command
10162 line, @command{chmod} changes the permissions of the pointed-to file.
10163 In contrast, @command{chmod} ignores symbolic links encountered during
10164 recursive directory traversals.
10166 A successful use of @command{chmod} clears the set-group-ID bit of a
10167 regular file if the file's group ID does not match the user's
10168 effective group ID or one of the user's supplementary group IDs,
10169 unless the user has appropriate privileges. Additional restrictions
10170 may cause the set-user-ID and set-group-ID bits of @var{mode} or
10171 @var{ref_file} to be ignored. This behavior depends on the policy and
10172 functionality of the underlying @code{chmod} system call. When in
10173 doubt, check the underlying system behavior.
10175 If used, @var{mode} specifies the new file mode bits.
10176 For details, see the section on @ref{File permissions}.
10177 If you really want @var{mode} to have a leading @samp{-}, you should
10178 use @option{--} first, e.g., @samp{chmod -- -w file}. Typically,
10179 though, @samp{chmod a-w file} is preferable, and @command{chmod -w
10180 file} (without the @option{--}) complains if it behaves differently
10181 from what @samp{chmod a-w file} would do.
10183 The program accepts the following options. Also see @ref{Common options}.
10191 Verbosely describe the action for each @var{file} whose permissions
10200 @cindex error messages, omitting
10201 Do not print error messages about files whose permissions cannot be
10204 @itemx --preserve-root
10205 @opindex --preserve-root
10206 @cindex root directory, disallow recursive modification
10207 Fail upon any attempt to recursively change the root directory, @file{/}.
10208 Without @option{--recursive}, this option has no effect.
10209 @xref{Treating / specially}.
10211 @itemx --no-preserve-root
10212 @opindex --no-preserve-root
10213 @cindex root directory, allow recursive modification
10214 Cancel the effect of any preceding @option{--preserve-root} option.
10215 @xref{Treating / specially}.
10221 Verbosely describe the action or non-action taken for every @var{file}.
10223 @item --reference=@var{ref_file}
10224 @opindex --reference
10225 Change the mode of each @var{file} to be the same as that of @var{ref_file}.
10226 @xref{File permissions}.
10227 If @var{ref_file} is a symbolic link, do not use the mode
10228 of the symbolic link, but rather that of the file it refers to.
10233 @opindex --recursive
10234 @cindex recursively changing access permissions
10235 Recursively change permissions of directories and their contents.
10242 @node touch invocation
10243 @section @command{touch}: Change file timestamps
10246 @cindex changing file timestamps
10247 @cindex file timestamps, changing
10248 @cindex timestamps, changing file
10250 @command{touch} changes the access and/or modification times of the
10251 specified files. Synopsis:
10254 touch [@var{option}]@dots{} @var{file}@dots{}
10257 @cindex empty files, creating
10258 Any @var{file} argument that does not exist is created empty, unless
10259 option @option{--no-create} (@option{-c}) or @option{--no-dereference}
10260 (@option{-h}) was in effect.
10262 A @var{file} argument string of @samp{-} is handled specially and
10263 causes @command{touch} to change the times of the file associated with
10267 By default, @command{touch} sets file timestamps to the current time.
10268 Because @command{touch} acts on its operands left to right, the
10269 resulting timestamps of earlier and later operands may disagree.
10270 Also, the determination of what time is ``current'' depends on the
10271 platform. Platforms with network file systems often use different
10272 clocks for the operating system and for file systems; because
10273 @command{touch} typically uses file systems' clocks by default, clock
10274 skew can cause the resulting file timestamps to appear to be in a
10275 program's ``future'' or ``past''.
10277 @cindex file timestamp resolution
10278 The @command{touch} command sets the file's timestamp to the greatest
10279 representable value that is not greater than the requested time. This
10280 can differ from the requested time for several reasons. First, the
10281 requested time may have a higher resolution than supported. Second, a
10282 file system may use different resolutions for different types of
10283 times. Third, file timestamps may use a different resolution than
10284 operating system timestamps. Fourth, the operating system primitives
10285 used to update timestamps may employ yet a different resolution. For
10286 example, in theory a file system might use 10-microsecond resolution
10287 for access time and 100-nanosecond resolution for modification time,
10288 and the operating system might use nanosecond resolution for the
10289 current time and microsecond resolution for the primitive that
10290 @command{touch} uses to set a file's timestamp to an arbitrary value.
10292 @cindex permissions, for changing file timestamps
10293 When setting file timestamps to the current time, @command{touch} can
10294 change the timestamps for files that the user does not own but has
10295 write permission for. Otherwise, the user must own the files. Some
10296 older systems have a further restriction: the user must own the files
10297 unless both the access and modification times are being set to the
10300 Although @command{touch} provides options for changing two of the times---the
10301 times of last access and modification---of a file, there is actually
10302 a standard third one as well: the inode change time. This is often
10303 referred to as a file's @code{ctime}.
10304 The inode change time represents the time when the file's meta-information
10305 last changed. One common example of this is when the permissions of a
10306 file change. Changing the permissions doesn't access the file, so
10307 the atime doesn't change, nor does it modify the file, so the mtime
10308 doesn't change. Yet, something about the file itself has changed,
10309 and this must be noted somewhere. This is the job of the ctime field.
10310 This is necessary, so that, for example, a backup program can make a
10311 fresh copy of the file, including the new permissions value.
10312 Another operation that modifies a file's ctime without affecting
10313 the others is renaming. In any case, it is not possible, in normal
10314 operations, for a user to change the ctime field to a user-specified value.
10315 Some operating systems and file systems support a fourth time: the
10316 birth time, when the file was first created; by definition, this
10317 timestamp never changes.
10320 Time stamps assume the time zone rules specified by the @env{TZ}
10321 environment variable, or by the system default rules if @env{TZ} is
10322 not set. @xref{TZ Variable,, Specifying the Time Zone with @env{TZ},
10323 libc, The GNU C Library Reference Manual}.
10324 You can avoid ambiguities during
10325 daylight saving transitions by using @sc{utc} time stamps.
10327 The program accepts the following options. Also see @ref{Common options}.
10332 @itemx --time=atime
10333 @itemx --time=access
10337 @opindex atime@r{, changing}
10338 @opindex access @r{time, changing}
10339 @opindex use @r{time, changing}
10340 Change the access time only.
10345 @opindex --no-create
10346 Do not warn about or create files that do not exist.
10349 @itemx --date=@var{time}
10353 Use @var{time} instead of the current time. It can contain month names,
10354 time zones, @samp{am} and @samp{pm}, @samp{yesterday}, etc. For
10355 example, @option{--date="2004-02-27 14:19:13.489392193 +0530"}
10356 specifies the instant of time that is 489,392,193 nanoseconds after
10357 February 27, 2004 at 2:19:13 PM in a time zone that is 5 hours and 30
10358 minutes east of @acronym{UTC}. @xref{Date input formats}.
10359 File systems that do not support high-resolution time stamps
10360 silently ignore any excess precision here.
10364 @cindex BSD @command{touch} compatibility
10365 Ignored; for compatibility with BSD versions of @command{touch}.
10368 @itemx --no-dereference
10370 @opindex --no-dereference
10371 @cindex symbolic links, changing time
10373 Attempt to change the timestamps of a symbolic link, rather than what
10374 the link refers to. When using this option, empty files are not
10375 created, but option @option{-c} must also be used to avoid warning
10376 about files that do not exist. Not all systems support changing the
10377 timestamps of symlinks, since underlying system support for this
10378 action was not required until @acronym{POSIX} 2008. Also, on some
10379 systems, the mere act of examining a symbolic link changes the access
10380 time, such that only changes to the modification time will persist
10381 long enough to be observable. When coupled with option @option{-r}, a
10382 reference timestamp is taken from a symbolic link rather than the file
10386 @itemx --time=mtime
10387 @itemx --time=modify
10390 @opindex mtime@r{, changing}
10391 @opindex modify @r{time, changing}
10392 Change the modification time only.
10394 @item -r @var{file}
10395 @itemx --reference=@var{file}
10397 @opindex --reference
10398 Use the times of the reference @var{file} instead of the current time.
10399 If this option is combined with the @option{--date=@var{time}}
10400 (@option{-d @var{time}}) option, the reference @var{file}'s time is
10401 the origin for any relative @var{time}s given, but is otherwise ignored.
10402 For example, @samp{-r foo -d '-5 seconds'} specifies a time stamp
10403 equal to five seconds before the corresponding time stamp for @file{foo}.
10404 If @var{file} is a symbolic link, the reference timestamp is taken
10405 from the target of the symlink, unless @option{-h} was also in effect.
10407 @item -t [[@var{cc}]@var{yy}]@var{mmddhhmm}[.@var{ss}]
10408 Use the argument (optional four-digit or two-digit years, months,
10409 days, hours, minutes, optional seconds) instead of the current time.
10410 If the year is specified with only two digits, then @var{cc}
10411 is 20 for years in the range 0 @dots{} 68, and 19 for years in
10412 69 @dots{} 99. If no digits of the year are specified,
10413 the argument is interpreted as a date in the current year.
10414 Note that @var{ss} may be @samp{60}, to accommodate leap seconds.
10418 @vindex _POSIX2_VERSION
10419 On older systems, @command{touch} supports an obsolete syntax, as follows.
10420 If no timestamp is given with any of the @option{-d}, @option{-r}, or
10421 @option{-t} options, and if there are two or more @var{file}s and the
10422 first @var{file} is of the form @samp{@var{mmddhhmm}[@var{yy}]} and this
10423 would be a valid argument to the @option{-t} option (if the @var{yy}, if
10424 any, were moved to the front), and if the represented year
10425 is in the range 1969--1999, that argument is interpreted as the time
10426 for the other files instead of as a file name.
10427 This obsolete behavior can be enabled or disabled with the
10428 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
10429 conformance}), but portable scripts should avoid commands whose
10430 behavior depends on this variable.
10431 For example, use @samp{touch ./12312359 main.c} or @samp{touch -t
10432 12312359 main.c} rather than the ambiguous @samp{touch 12312359 main.c}.
10438 @chapter Disk usage
10442 No disk can hold an infinite amount of data. These commands report
10443 how much disk storage is in use or available, report other file and
10444 file status information, and write buffers to disk.
10447 * df invocation:: Report file system disk space usage.
10448 * du invocation:: Estimate file space usage.
10449 * stat invocation:: Report file or file system status.
10450 * sync invocation:: Synchronize memory and disk.
10451 * truncate invocation:: Shrink or extend the size of a file.
10455 @node df invocation
10456 @section @command{df}: Report file system disk space usage
10459 @cindex file system disk usage
10460 @cindex disk usage by file system
10462 @command{df} reports the amount of disk space used and available on
10463 file systems. Synopsis:
10466 df [@var{option}]@dots{} [@var{file}]@dots{}
10469 With no arguments, @command{df} reports the space used and available on all
10470 currently mounted file systems (of all types). Otherwise, @command{df}
10471 reports on the file system containing each argument @var{file}.
10473 Normally the disk space is printed in units of
10474 1024 bytes, but this can be overridden (@pxref{Block size}).
10475 Non-integer quantities are rounded up to the next higher unit.
10477 @cindex disk device file
10478 @cindex device file, disk
10479 If an argument @var{file} is a disk device file containing a mounted
10480 file system, @command{df} shows the space available on that file system
10481 rather than on the file system containing the device node (i.e., the root
10482 file system). @sc{gnu} @command{df} does not attempt to determine the
10484 on unmounted file systems, because on most kinds of systems doing so
10485 requires extremely nonportable intimate knowledge of file system
10488 The program accepts the following options. Also see @ref{Common options}.
10496 @cindex automounter file systems
10497 @cindex ignore file systems
10498 Include in the listing dummy file systems, which
10499 are omitted by default. Such file systems are typically special-purpose
10500 pseudo-file-systems, such as automounter entries.
10502 @item -B @var{size}
10503 @itemx --block-size=@var{size}
10505 @opindex --block-size
10506 @cindex file system sizes
10507 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10508 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10512 @cindex grand total of disk size, usage and available space
10513 Print a grand total of all arguments after all arguments have
10514 been processed. This can be used to find out the total disk size, usage
10515 and available space of all listed devices.
10521 Equivalent to @option{--si}.
10527 @cindex inode usage
10528 List inode usage information instead of block usage. An inode (short
10529 for index node) contains information about a file such as its owner,
10530 permissions, timestamps, and location on the disk.
10534 @cindex kibibytes for file system sizes
10535 Print sizes in 1024-byte blocks, overriding the default block size
10536 (@pxref{Block size}).
10537 This option is equivalent to @option{--block-size=1K}.
10543 @cindex file system types, limiting output to certain
10544 Limit the listing to local file systems. By default, remote file systems
10549 @cindex file system space, retrieving old data more quickly
10550 Do not invoke the @code{sync} system call before getting any usage data.
10551 This may make @command{df} run significantly faster on systems with many
10552 disks, but on some systems (notably SunOS) the results may be slightly
10553 out of date. This is the default.
10556 @itemx --portability
10558 @opindex --portability
10559 @cindex one-line output format
10560 @cindex @acronym{POSIX} output format
10561 @cindex portable output format
10562 @cindex output format, portable
10563 Use the @acronym{POSIX} output format. This is like the default format except
10568 The information about each file system is always printed on exactly
10569 one line; a mount device is never put on a line by itself. This means
10570 that if the mount device name is more than 20 characters long (e.g., for
10571 some network mounts), the columns are misaligned.
10574 The labels in the header output line are changed to conform to @acronym{POSIX}.
10577 The default block size and output format are unaffected by the
10578 @env{DF_BLOCK_SIZE}, @env{BLOCK_SIZE} and @env{BLOCKSIZE} environment
10579 variables. However, the default block size is still affected by
10580 @env{POSIXLY_CORRECT}: it is 512 if @env{POSIXLY_CORRECT} is set, 1024
10581 otherwise. @xref{Block size}.
10588 @cindex file system space, retrieving current data more slowly
10589 Invoke the @code{sync} system call before getting any usage data. On
10590 some systems (notably SunOS), doing this yields more up to date results,
10591 but in general this option makes @command{df} much slower, especially when
10592 there are many or very busy file systems.
10594 @item -t @var{fstype}
10595 @itemx --type=@var{fstype}
10598 @cindex file system types, limiting output to certain
10599 Limit the listing to file systems of type @var{fstype}. Multiple
10600 file system types can be specified by giving multiple @option{-t} options.
10601 By default, nothing is omitted.
10604 @itemx --print-type
10606 @opindex --print-type
10607 @cindex file system types, printing
10608 Print each file system's type. The types printed here are the same ones
10609 you can include or exclude with @option{-t} and @option{-x}. The particular
10610 types printed are whatever is supported by the system. Here are some of
10611 the common names (this list is certainly not exhaustive):
10616 @cindex @acronym{NFS} file system type
10617 An @acronym{NFS} file system, i.e., one mounted over a network from another
10618 machine. This is the one type name which seems to be used uniformly by
10621 @item 4.2@r{, }ufs@r{, }efs@dots{}
10622 @cindex Linux file system types
10623 @cindex local file system types
10624 @opindex 4.2 @r{file system type}
10625 @opindex ufs @r{file system type}
10626 @opindex efs @r{file system type}
10627 A file system on a locally-mounted hard disk. (The system might even
10628 support more than one type here; Linux does.)
10630 @item hsfs@r{, }cdfs
10631 @cindex CD-ROM file system type
10632 @cindex High Sierra file system
10633 @opindex hsfs @r{file system type}
10634 @opindex cdfs @r{file system type}
10635 A file system on a CD-ROM drive. HP-UX uses @samp{cdfs}, most other
10636 systems use @samp{hsfs} (@samp{hs} for ``High Sierra'').
10639 @cindex PC file system
10640 @cindex DOS file system
10641 @cindex MS-DOS file system
10642 @cindex diskette file system
10644 An MS-DOS file system, usually on a diskette.
10648 @item -x @var{fstype}
10649 @itemx --exclude-type=@var{fstype}
10651 @opindex --exclude-type
10652 Limit the listing to file systems not of type @var{fstype}.
10653 Multiple file system types can be eliminated by giving multiple
10654 @option{-x} options. By default, no file system types are omitted.
10657 Ignored; for compatibility with System V versions of @command{df}.
10662 Failure includes the case where no output is generated, so you can
10663 inspect the exit status of a command like @samp{df -t ext3 -t reiserfs
10664 @var{dir}} to test whether @var{dir} is on a file system of type
10665 @samp{ext3} or @samp{reiserfs}.
10668 @node du invocation
10669 @section @command{du}: Estimate file space usage
10672 @cindex file space usage
10673 @cindex disk usage for files
10675 @command{du} reports the amount of disk space used by the specified files
10676 and for each subdirectory (of directory arguments). Synopsis:
10679 du [@var{option}]@dots{} [@var{file}]@dots{}
10682 With no arguments, @command{du} reports the disk space for the current
10683 directory. Normally the disk space is printed in units of
10684 1024 bytes, but this can be overridden (@pxref{Block size}).
10685 Non-integer quantities are rounded up to the next higher unit.
10687 If two or more hard links point to the same file, only one of the hard
10688 links is counted. The @var{file} argument order affects which links
10689 are counted, and changing the argument order may change the numbers
10690 that @command{du} outputs.
10692 The program accepts the following options. Also see @ref{Common options}.
10700 Show counts for all files, not just directories.
10702 @itemx --apparent-size
10703 @opindex --apparent-size
10704 Print apparent sizes, rather than disk usage. The apparent size of a
10705 file is the number of bytes reported by @code{wc -c} on regular files,
10706 or more generally, @code{ls -l --block-size=1} or @code{stat --format=%s}.
10707 For example, a file containing the word @samp{zoo} with no newline would,
10708 of course, have an apparent size of 3. Such a small file may require
10709 anywhere from 0 to 16 KiB or more of disk space, depending on
10710 the type and configuration of the file system on which the file resides.
10711 However, a sparse file created with this command:
10714 dd bs=1 seek=2GiB if=/dev/null of=big
10718 has an apparent size of 2 GiB, yet on most modern
10719 systems, it actually uses almost no disk space.
10725 Equivalent to @code{--apparent-size --block-size=1}.
10727 @item -B @var{size}
10728 @itemx --block-size=@var{size}
10730 @opindex --block-size
10732 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10733 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10739 @cindex grand total of disk space
10740 Print a grand total of all arguments after all arguments have
10741 been processed. This can be used to find out the total disk usage of
10742 a given set of files or directories.
10745 @itemx --dereference-args
10747 @opindex --dereference-args
10748 Dereference symbolic links that are command line arguments.
10749 Does not affect other symbolic links. This is helpful for finding
10750 out the disk usage of directories, such as @file{/usr/tmp}, which
10751 are often symbolic links.
10753 @c --files0-from=FILE
10754 @filesZeroFromOption{du,, with the @option{--total} (@option{-c}) option}
10760 Equivalent to @option{--dereference-args} (@option{-D}).
10764 @cindex kibibytes for file sizes
10765 Print sizes in 1024-byte blocks, overriding the default block size
10766 (@pxref{Block size}).
10767 This option is equivalent to @option{--block-size=1K}.
10770 @itemx --count-links
10772 @opindex --count-links
10773 @cindex hard links, counting in @command{du}
10774 Count the size of all files, even if they have appeared already (as a
10778 @itemx --dereference
10780 @opindex --dereference
10781 @cindex symbolic links, dereferencing in @command{du}
10782 Dereference symbolic links (show the disk space used by the file
10783 or directory that the link points to instead of the space used by
10788 @cindex mebibytes for file sizes
10789 Print sizes in 1,048,576-byte blocks, overriding the default block size
10790 (@pxref{Block size}).
10791 This option is equivalent to @option{--block-size=1M}.
10794 @itemx --no-dereference
10796 @opindex --no-dereference
10797 @cindex symbolic links, dereferencing in @command{du}
10798 For each symbolic links encountered by @command{du},
10799 consider the disk space used by the symbolic link.
10801 @item -d @var{depth}
10802 @item --max-depth=@var{depth}
10803 @opindex -d @var{depth}
10804 @opindex --max-depth=@var{depth}
10805 @cindex limiting output of @command{du}
10806 Show the total for each directory (and file if --all) that is at
10807 most MAX_DEPTH levels down from the root of the hierarchy. The root
10808 is at level 0, so @code{du --max-depth=0} is equivalent to @code{du -s}.
10817 @opindex --summarize
10818 Display only a total for each argument.
10821 @itemx --separate-dirs
10823 @opindex --separate-dirs
10824 Normally, in the output of @command{du} (when not using @option{--summarize}),
10825 the size listed next to a directory name, @var{d}, represents the sum
10826 of sizes of all entries beneath @var{d} as well as the size of @var{d} itself.
10827 With @option{--separate-dirs}, the size reported for a directory name,
10828 @var{d}, is merely the @code{stat.st_size}-derived size of the directory
10833 @cindex last modified dates, displaying in @command{du}
10834 Show time of the most recent modification of any file in the directory,
10835 or any of its subdirectories.
10837 @itemx --time=ctime
10838 @itemx --time=status
10841 @opindex ctime@r{, show the most recent}
10842 @opindex status time@r{, show the most recent}
10843 @opindex use time@r{, show the most recent}
10844 Show the most recent status change time (the @samp{ctime} in the inode) of
10845 any file in the directory, instead of the modification time.
10847 @itemx --time=atime
10848 @itemx --time=access
10850 @opindex atime@r{, show the most recent}
10851 @opindex access time@r{, show the most recent}
10852 Show the most recent access time (the @samp{atime} in the inode) of
10853 any file in the directory, instead of the modification time.
10855 @item --time-style=@var{style}
10856 @opindex --time-style
10858 List timestamps in style @var{style}. This option has an effect only if
10859 the @option{--time} option is also specified. The @var{style} should
10860 be one of the following:
10863 @item +@var{format}
10865 List timestamps using @var{format}, where @var{format} is interpreted
10866 like the format argument of @command{date} (@pxref{date invocation}).
10867 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
10868 @command{du} to list timestamps like @samp{2002-03-30 23:45:56}. As
10869 with @command{date}, @var{format}'s interpretation is affected by the
10870 @env{LC_TIME} locale category.
10873 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
10874 format with nanosecond precision, e.g., @samp{2002-03-30
10875 23:45:56.477817180 -0700}. This style is equivalent to
10876 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
10879 List @acronym{ISO} 8601 date and time in minutes, e.g.,
10880 @samp{2002-03-30 23:45}. These timestamps are shorter than
10881 @samp{full-iso} timestamps, and are usually good enough for everyday
10882 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
10885 List @acronym{ISO} 8601 dates for timestamps, e.g., @samp{2002-03-30}.
10886 This style is equivalent to @samp{+%Y-%m-%d}.
10890 You can specify the default value of the @option{--time-style} option
10891 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
10892 the default style is @samp{long-iso}. For compatibility with @command{ls},
10893 if @env{TIME_STYLE} begins with @samp{+} and contains a newline,
10894 the newline and any later characters are ignored; if @env{TIME_STYLE}
10895 begins with @samp{posix-} the @samp{posix-} is ignored; and if
10896 @env{TIME_STYLE} is @samp{locale} it is ignored.
10899 @itemx --one-file-system
10901 @opindex --one-file-system
10902 @cindex one file system, restricting @command{du} to
10903 Skip directories that are on different file systems from the one that
10904 the argument being processed is on.
10906 @item --exclude=@var{pattern}
10907 @opindex --exclude=@var{pattern}
10908 @cindex excluding files from @command{du}
10909 When recursing, skip subdirectories or files matching @var{pattern}.
10910 For example, @code{du --exclude='*.o'} excludes files whose names
10913 @item -X @var{file}
10914 @itemx --exclude-from=@var{file}
10915 @opindex -X @var{file}
10916 @opindex --exclude-from=@var{file}
10917 @cindex excluding files from @command{du}
10918 Like @option{--exclude}, except take the patterns to exclude from @var{file},
10919 one per line. If @var{file} is @samp{-}, take the patterns from standard
10924 @cindex NFS mounts from BSD to HP-UX
10925 On BSD systems, @command{du} reports sizes that are half the correct
10926 values for files that are NFS-mounted from HP-UX systems. On HP-UX
10927 systems, it reports sizes that are twice the correct values for
10928 files that are NFS-mounted from BSD systems. This is due to a flaw
10929 in HP-UX; it also affects the HP-UX @command{du} program.
10934 @node stat invocation
10935 @section @command{stat}: Report file or file system status
10938 @cindex file status
10939 @cindex file system status
10941 @command{stat} displays information about the specified file(s). Synopsis:
10944 stat [@var{option}]@dots{} [@var{file}]@dots{}
10947 With no option, @command{stat} reports all information about the given files.
10948 But it also can be used to report the information of the file systems the
10949 given files are located on. If the files are links, @command{stat} can
10950 also give information about the files the links point to.
10952 @mayConflictWithShellBuiltIn{stat}
10957 @itemx --dereference
10959 @opindex --dereference
10960 @cindex symbolic links, dereferencing in @command{stat}
10961 Change how @command{stat} treats symbolic links.
10962 With this option, @command{stat} acts on the file referenced
10963 by each symbolic link argument.
10964 Without it, @command{stat} acts on any symbolic link argument directly.
10967 @itemx --file-system
10969 @opindex --file-system
10970 @cindex file systems
10971 Report information about the file systems where the given files are located
10972 instead of information about the files themselves.
10973 This option implies the @option{-L} option.
10976 @itemx --format=@var{format}
10978 @opindex --format=@var{format}
10979 @cindex output format
10980 Use @var{format} rather than the default format.
10981 @var{format} is automatically newline-terminated, so
10982 running a command like the following with two or more @var{file}
10983 operands produces a line of output for each operand:
10985 $ stat --format=%d:%i / /usr
10990 @itemx --printf=@var{format}
10991 @opindex --printf=@var{format}
10992 @cindex output format
10993 Use @var{format} rather than the default format.
10994 Like @option{--format}, but interpret backslash escapes,
10995 and do not output a mandatory trailing newline.
10996 If you want a newline, include @samp{\n} in the @var{format}.
10997 Here's how you would use @option{--printf} to print the device
10998 and inode numbers of @file{/} and @file{/usr}:
11000 $ stat --printf='%d:%i\n' / /usr
11009 @cindex terse output
11010 Print the information in terse form, suitable for parsing by other programs.
11014 The valid @var{format} directives for files with @option{--format} and
11015 @option{--printf} are:
11018 @item %a - Access rights in octal
11019 @item %A - Access rights in human readable form
11020 @item %b - Number of blocks allocated (see @samp{%B})
11021 @item %B - The size in bytes of each block reported by @samp{%b}
11022 @item %C - The SELinux security context of a file, if available
11023 @item %d - Device number in decimal
11024 @item %D - Device number in hex
11025 @item %f - Raw mode in hex
11026 @item %F - File type
11027 @item %g - Group ID of owner
11028 @item %G - Group name of owner
11029 @item %h - Number of hard links
11030 @item %i - Inode number
11031 @item %m - Mount point (See note below)
11032 @item %n - File name
11033 @item %N - Quoted file name with dereference if symbolic link
11034 @item %o - I/O block size
11035 @item %s - Total size, in bytes
11036 @item %t - Major device type in hex
11037 @item %T - Minor device type in hex
11038 @item %u - User ID of owner
11039 @item %U - User name of owner
11040 @item %w - Time of file birth, or @samp{-} if unknown
11041 @item %W - Time of file birth as seconds since Epoch, or @samp{0}
11042 @item %x - Time of last access
11043 @item %X - Time of last access as seconds since Epoch
11044 @item %y - Time of last modification
11045 @item %Y - Time of last modification as seconds since Epoch
11046 @item %z - Time of last change
11047 @item %Z - Time of last change as seconds since Epoch
11050 The @samp{%W}, @samp{%X}, @samp{%Y}, and @samp{%Z} formats accept a
11051 precision preceded by a period to specify the number of digits to
11052 print after the decimal point. For example, @samp{%.3X} outputs the
11053 last access time to millisecond precision. If a period is given but no
11054 precision, @command{stat} uses 9 digits, so @samp{%.X} is equivalent to
11055 @samp{%.9X}. When discarding excess precision, time stamps are truncated
11056 toward minus infinity.
11060 $ stat -c '[%015Y]' /usr
11063 $ stat -c '[%15Y]' /usr
11065 $ stat -c '[%-15Y]' /usr
11068 $ stat -c '[%.3Y]' /usr
11070 $ stat -c '[%.Y]' /usr
11071 [1288929712.114951834]
11074 The mount point printed by @samp{%m} is similar to that output
11075 by @command{df}, except that:
11078 stat does not dereference symlinks by default
11079 (unless @option{-L} is specified)
11081 stat does not search for specified device nodes in the
11082 file system list, instead operating on them directly
11085 stat outputs the alias for a bind mounted file, rather than
11086 the initial mount point of its backing device.
11087 One can recursively call stat until there is no change in output,
11088 to get the current base mount point
11091 When listing file system information (@option{--file-system} (@option{-f})),
11092 you must use a different set of @var{format} directives:
11095 @item %a - Free blocks available to non-super-user
11096 @item %b - Total data blocks in file system
11097 @item %c - Total file nodes in file system
11098 @item %d - Free file nodes in file system
11099 @item %f - Free blocks in file system
11100 @item %i - File System ID in hex
11101 @item %l - Maximum length of file names
11102 @item %n - File name
11103 @item %s - Block size (for faster transfers)
11104 @item %S - Fundamental block size (for block counts)
11105 @item %t - Type in hex
11106 @item %T - Type in human readable form
11110 Time stamps are listed according to the time zone rules specified by
11111 the @env{TZ} environment variable, or by the system default rules if
11112 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
11113 with @env{TZ}, libc, The GNU C Library Reference Manual}.
11118 @node sync invocation
11119 @section @command{sync}: Synchronize data on disk with memory
11122 @cindex synchronize disk and memory
11124 @cindex superblock, writing
11125 @cindex inodes, written buffered
11126 @command{sync} writes any data buffered in memory out to disk. This can
11127 include (but is not limited to) modified superblocks, modified inodes,
11128 and delayed reads and writes. This must be implemented by the kernel;
11129 The @command{sync} program does nothing but exercise the @code{sync} system
11132 @cindex crashes and corruption
11133 The kernel keeps data in memory to avoid doing (relatively slow) disk
11134 reads and writes. This improves performance, but if the computer
11135 crashes, data may be lost or the file system corrupted as a
11136 result. The @command{sync} command ensures everything in memory
11137 is written to disk.
11139 Any arguments are ignored, except for a lone @option{--help} or
11140 @option{--version} (@pxref{Common options}).
11145 @node truncate invocation
11146 @section @command{truncate}: Shrink or extend the size of a file
11149 @cindex truncating, file sizes
11151 @command{truncate} shrinks or extends the size of each @var{file} to the
11152 specified size. Synopsis:
11155 truncate @var{option}@dots{} @var{file}@dots{}
11158 @cindex files, creating
11159 Any @var{file} that does not exist is created.
11161 @cindex sparse files, creating
11162 @cindex holes, creating files with
11163 If a @var{file} is larger than the specified size, the extra data is lost.
11164 If a @var{file} is shorter, it is extended and the extended part (or hole)
11165 reads as zero bytes.
11167 The program accepts the following options. Also see @ref{Common options}.
11174 @opindex --no-create
11175 Do not create files that do not exist.
11180 @opindex --io-blocks
11181 Treat @var{size} as number of I/O blocks of the @var{file} rather than bytes.
11183 @item -r @var{rfile}
11184 @itemx --reference=@var{rfile}
11186 @opindex --reference
11187 Base the size of each @var{file} on the size of @var{rfile}.
11189 @item -s @var{size}
11190 @itemx --size=@var{size}
11193 Set or adjust the size of each @var{file} according to @var{size}.
11194 @multiplierSuffixesNoBlocks{size}
11196 @var{size} may also be prefixed by one of the following to adjust
11197 the size of each @var{file} based on their current size:
11199 @samp{+} => extend by
11200 @samp{-} => reduce by
11201 @samp{<} => at most
11202 @samp{>} => at least
11203 @samp{/} => round down to multiple of
11204 @samp{%} => round up to multiple of
11212 @node Printing text
11213 @chapter Printing text
11215 @cindex printing text, commands for
11216 @cindex commands for printing text
11218 This section describes commands that display text strings.
11221 * echo invocation:: Print a line of text.
11222 * printf invocation:: Format and print data.
11223 * yes invocation:: Print a string until interrupted.
11227 @node echo invocation
11228 @section @command{echo}: Print a line of text
11231 @cindex displaying text
11232 @cindex printing text
11233 @cindex text, displaying
11234 @cindex arbitrary text, displaying
11236 @command{echo} writes each given @var{string} to standard output, with a
11237 space between each and a newline after the last one. Synopsis:
11240 echo [@var{option}]@dots{} [@var{string}]@dots{}
11243 @mayConflictWithShellBuiltIn{echo}
11245 The program accepts the following options. Also see @ref{Common options}.
11246 Options must precede operands, and the normally-special argument
11247 @samp{--} has no special meaning and is treated like any other
11253 Do not output the trailing newline.
11257 @cindex backslash escapes
11258 Enable interpretation of the following backslash-escaped characters in
11267 produce no further output
11283 the eight-bit value that is the octal number @var{nnn}
11284 (zero to three octal digits), if @var{nnn} is
11285 a nine-bit value, the ninth bit is ignored
11287 the eight-bit value that is the octal number @var{nnn}
11288 (one to three octal digits), if @var{nnn} is
11289 a nine-bit value, the ninth bit is ignored
11291 the eight-bit value that is the hexadecimal number @var{hh}
11292 (one or two hexadecimal digits)
11297 @cindex backslash escapes
11298 Disable interpretation of backslash escapes in each @var{string}.
11299 This is the default. If @option{-e} and @option{-E} are both
11300 specified, the last one given takes effect.
11304 @vindex POSIXLY_CORRECT
11305 If the @env{POSIXLY_CORRECT} environment variable is set, then when
11306 @command{echo}'s first argument is not @option{-n} it outputs
11307 option-like arguments instead of treating them as options. For
11308 example, @code{echo -ne hello} outputs @samp{-ne hello} instead of
11309 plain @samp{hello}.
11311 @acronym{POSIX} does not require support for any options, and says
11312 that the behavior of @command{echo} is implementation-defined if any
11313 @var{string} contains a backslash or if the first argument is
11314 @option{-n}. Portable programs can use the @command{printf} command
11315 if they need to omit trailing newlines or output control characters or
11316 backslashes. @xref{printf invocation}.
11321 @node printf invocation
11322 @section @command{printf}: Format and print data
11325 @command{printf} does formatted printing of text. Synopsis:
11328 printf @var{format} [@var{argument}]@dots{}
11331 @command{printf} prints the @var{format} string, interpreting @samp{%}
11332 directives and @samp{\} escapes to format numeric and string arguments
11333 in a way that is mostly similar to the C @samp{printf} function.
11334 @xref{Output Conversion Syntax,, @command{printf} format directives,
11335 libc, The GNU C Library Reference Manual}, for details.
11336 The differences are listed below.
11338 @mayConflictWithShellBuiltIn{printf}
11343 The @var{format} argument is reused as necessary to convert all the
11344 given @var{argument}s. For example, the command @samp{printf %s a b}
11348 Missing @var{argument}s are treated as null strings or as zeros,
11349 depending on whether the context expects a string or a number. For
11350 example, the command @samp{printf %sx%d} prints @samp{x0}.
11354 An additional escape, @samp{\c}, causes @command{printf} to produce no
11355 further output. For example, the command @samp{printf 'A%sC\cD%sF' B
11356 E} prints @samp{ABC}.
11359 The hexadecimal escape sequence @samp{\x@var{hh}} has at most two
11360 digits, as opposed to C where it can have an unlimited number of
11361 digits. For example, the command @samp{printf '\x07e'} prints two
11362 bytes, whereas the C statement @samp{printf ("\x07e")} prints just
11367 @command{printf} has an additional directive, @samp{%b}, which prints its
11368 argument string with @samp{\} escapes interpreted in the same way as in
11369 the @var{format} string, except that octal escapes are of the form
11370 @samp{\0@var{ooo}} where @var{ooo} is 0 to 3 octal digits. If
11371 @samp{\@var{ooo}} is nine-bit value, ignore the ninth bit.
11372 If a precision is also given, it limits the number of bytes printed
11373 from the converted string.
11376 Numeric arguments must be single C constants, possibly with leading
11377 @samp{+} or @samp{-}. For example, @samp{printf %.4d -3} outputs
11381 @vindex POSIXLY_CORRECT
11382 If the leading character of a numeric argument is @samp{"} or @samp{'}
11383 then its value is the numeric value of the immediately following
11384 character. Any remaining characters are silently ignored if the
11385 @env{POSIXLY_CORRECT} environment variable is set; otherwise, a
11386 warning is printed. For example, @samp{printf "%d" "'a"} outputs
11387 @samp{97} on hosts that use the @acronym{ASCII} character set, since
11388 @samp{a} has the numeric value 97 in @acronym{ASCII}.
11393 A floating-point argument must use a period before any fractional
11394 digits, but is printed according to the @env{LC_NUMERIC} category of the
11395 current locale. For example, in a locale whose radix character is a
11396 comma, the command @samp{printf %g 3.14} outputs @samp{3,14} whereas
11397 the command @samp{printf %g 3,14} is an error.
11398 @xref{Floating point}.
11402 @command{printf} interprets @samp{\@var{ooo}} in @var{format} as an octal number
11403 (if @var{ooo} is 1 to 3 octal digits) specifying a byte to print,
11404 and @samp{\x@var{hh}} as a hexadecimal number (if @var{hh} is 1 to 2 hex
11405 digits) specifying a character to print.
11406 Note however that when @samp{\@var{ooo}} specifies a number larger than 255,
11407 @command{printf} ignores the ninth bit.
11408 For example, @samp{printf '\400'} is equivalent to @samp{printf '\0'}.
11413 @cindex ISO/IEC 10646
11415 @command{printf} interprets two character syntaxes introduced in
11416 @acronym{ISO} C 99:
11417 @samp{\u} for 16-bit Unicode (@acronym{ISO}/@acronym{IEC} 10646)
11418 characters, specified as
11419 four hexadecimal digits @var{hhhh}, and @samp{\U} for 32-bit Unicode
11420 characters, specified as eight hexadecimal digits @var{hhhhhhhh}.
11421 @command{printf} outputs the Unicode characters
11422 according to the @env{LC_CTYPE} locale. Unicode characters in the ranges
11423 U+0000...U+009F, U+D800...U+DFFF cannot be specified by this syntax, except
11424 for U+0024 ($), U+0040 (@@), and U+0060 (@`).
11426 The processing of @samp{\u} and @samp{\U} requires a full-featured
11427 @code{iconv} facility. It is activated on systems with glibc 2.2 (or newer),
11428 or when @code{libiconv} is installed prior to this package. Otherwise
11429 @samp{\u} and @samp{\U} will print as-is.
11431 The only options are a lone @option{--help} or
11432 @option{--version}. @xref{Common options}.
11433 Options must precede operands.
11435 The Unicode character syntaxes are useful for writing strings in a locale
11436 independent way. For example, a string containing the Euro currency symbol
11439 $ env printf '\u20AC 14.95'
11443 will be output correctly in all locales supporting the Euro symbol
11444 (@acronym{ISO}-8859-15, UTF-8, and others). Similarly, a Chinese string
11447 $ env printf '\u4e2d\u6587'
11451 will be output correctly in all Chinese locales (GB2312, BIG5, UTF-8, etc).
11453 Note that in these examples, the @command{printf} command has been
11454 invoked via @command{env} to ensure that we run the program found via
11455 your shell's search path, and not a shell alias or a built-in function.
11457 For larger strings, you don't need to look up the hexadecimal code
11458 values of each character one by one. @acronym{ASCII} characters mixed with \u
11459 escape sequences is also known as the JAVA source file encoding. You can
11460 use GNU recode 3.5c (or newer) to convert strings to this encoding. Here
11461 is how to convert a piece of text into a shell script which will output
11462 this text in a locale-independent way:
11465 $ LC_CTYPE=zh_CN.big5 /usr/local/bin/printf \
11466 '\u4e2d\u6587\n' > sample.txt
11467 $ recode BIG5..JAVA < sample.txt \
11468 | sed -e "s|^|/usr/local/bin/printf '|" -e "s|$|\\\\n'|" \
11475 @node yes invocation
11476 @section @command{yes}: Print a string until interrupted
11479 @cindex repeated output of a string
11481 @command{yes} prints the command line arguments, separated by spaces and
11482 followed by a newline, forever until it is killed. If no arguments are
11483 given, it prints @samp{y} followed by a newline forever until killed.
11485 Upon a write error, @command{yes} exits with status @samp{1}.
11487 The only options are a lone @option{--help} or @option{--version}.
11488 To output an argument that begins with
11489 @samp{-}, precede it with @option{--}, e.g., @samp{yes -- --help}.
11490 @xref{Common options}.
11494 @chapter Conditions
11497 @cindex commands for exit status
11498 @cindex exit status commands
11500 This section describes commands that are primarily useful for their exit
11501 status, rather than their output. Thus, they are often used as the
11502 condition of shell @code{if} statements, or as the last command in a
11506 * false invocation:: Do nothing, unsuccessfully.
11507 * true invocation:: Do nothing, successfully.
11508 * test invocation:: Check file types and compare values.
11509 * expr invocation:: Evaluate expressions.
11513 @node false invocation
11514 @section @command{false}: Do nothing, unsuccessfully
11517 @cindex do nothing, unsuccessfully
11518 @cindex failure exit status
11519 @cindex exit status of @command{false}
11521 @command{false} does nothing except return an exit status of 1, meaning
11522 @dfn{failure}. It can be used as a place holder in shell scripts
11523 where an unsuccessful command is needed.
11524 In most modern shells, @command{false} is a built-in command, so when
11525 you use @samp{false} in a script, you're probably using the built-in
11526 command, not the one documented here.
11528 @command{false} honors the @option{--help} and @option{--version} options.
11530 This version of @command{false} is implemented as a C program, and is thus
11531 more secure and faster than a shell script implementation, and may safely
11532 be used as a dummy shell for the purpose of disabling accounts.
11534 Note that @command{false} (unlike all other programs documented herein)
11535 exits unsuccessfully, even when invoked with
11536 @option{--help} or @option{--version}.
11538 Portable programs should not assume that the exit status of
11539 @command{false} is 1, as it is greater than 1 on some
11540 non-@acronym{GNU} hosts.
11543 @node true invocation
11544 @section @command{true}: Do nothing, successfully
11547 @cindex do nothing, successfully
11549 @cindex successful exit
11550 @cindex exit status of @command{true}
11552 @command{true} does nothing except return an exit status of 0, meaning
11553 @dfn{success}. It can be used as a place holder in shell scripts
11554 where a successful command is needed, although the shell built-in
11555 command @code{:} (colon) may do the same thing faster.
11556 In most modern shells, @command{true} is a built-in command, so when
11557 you use @samp{true} in a script, you're probably using the built-in
11558 command, not the one documented here.
11560 @command{true} honors the @option{--help} and @option{--version} options.
11562 Note, however, that it is possible to cause @command{true}
11563 to exit with nonzero status: with the @option{--help} or @option{--version}
11564 option, and with standard
11565 output already closed or redirected to a file that evokes an I/O error.
11566 For example, using a Bourne-compatible shell:
11569 $ ./true --version >&-
11570 ./true: write error: Bad file number
11571 $ ./true --version > /dev/full
11572 ./true: write error: No space left on device
11575 This version of @command{true} is implemented as a C program, and is thus
11576 more secure and faster than a shell script implementation, and may safely
11577 be used as a dummy shell for the purpose of disabling accounts.
11579 @node test invocation
11580 @section @command{test}: Check file types and compare values
11583 @cindex check file types
11584 @cindex compare values
11585 @cindex expression evaluation
11587 @command{test} returns a status of 0 (true) or 1 (false) depending on the
11588 evaluation of the conditional expression @var{expr}. Each part of the
11589 expression must be a separate argument.
11591 @command{test} has file status checks, string operators, and numeric
11592 comparison operators.
11594 @command{test} has an alternate form that uses opening and closing
11595 square brackets instead a leading @samp{test}. For example, instead
11596 of @samp{test -d /}, you can write @samp{[ -d / ]}. The square
11597 brackets must be separate arguments; for example, @samp{[-d /]} does
11598 not have the desired effect. Since @samp{test @var{expr}} and @samp{[
11599 @var{expr} ]} have the same meaning, only the former form is discussed
11605 test @var{expression}
11607 [ @var{expression} ]
11612 @mayConflictWithShellBuiltIn{test}
11614 If @var{expression} is omitted, @command{test} returns false.
11615 If @var{expression} is a single argument,
11616 @command{test} returns false if the argument is null and true
11617 otherwise. The argument
11618 can be any string, including strings like @samp{-d}, @samp{-1},
11619 @samp{--}, @samp{--help}, and @samp{--version} that most other
11620 programs would treat as options. To get help and version information,
11621 invoke the commands @samp{[ --help} and @samp{[ --version}, without
11622 the usual closing brackets. @xref{Common options}.
11624 @cindex exit status of @command{test}
11628 0 if the expression is true,
11629 1 if the expression is false,
11630 2 if an error occurred.
11634 * File type tests:: -[bcdfhLpSt]
11635 * Access permission tests:: -[gkruwxOG]
11636 * File characteristic tests:: -e -s -nt -ot -ef
11637 * String tests:: -z -n = == !=
11638 * Numeric tests:: -eq -ne -lt -le -gt -ge
11639 * Connectives for test:: ! -a -o
11643 @node File type tests
11644 @subsection File type tests
11646 @cindex file type tests
11648 These options test for particular types of files. (Everything's a file,
11649 but not all files are the same!)
11653 @item -b @var{file}
11655 @cindex block special check
11656 True if @var{file} exists and is a block special device.
11658 @item -c @var{file}
11660 @cindex character special check
11661 True if @var{file} exists and is a character special device.
11663 @item -d @var{file}
11665 @cindex directory check
11666 True if @var{file} exists and is a directory.
11668 @item -f @var{file}
11670 @cindex regular file check
11671 True if @var{file} exists and is a regular file.
11673 @item -h @var{file}
11674 @itemx -L @var{file}
11677 @cindex symbolic link check
11678 True if @var{file} exists and is a symbolic link.
11679 Unlike all other file-related tests, this test does not dereference
11680 @var{file} if it is a symbolic link.
11682 @item -p @var{file}
11684 @cindex named pipe check
11685 True if @var{file} exists and is a named pipe.
11687 @item -S @var{file}
11689 @cindex socket check
11690 True if @var{file} exists and is a socket.
11694 @cindex terminal check
11695 True if @var{fd} is a file descriptor that is associated with a
11701 @node Access permission tests
11702 @subsection Access permission tests
11704 @cindex access permission tests
11705 @cindex permission tests
11707 These options test for particular access permissions.
11711 @item -g @var{file}
11713 @cindex set-group-ID check
11714 True if @var{file} exists and has its set-group-ID bit set.
11716 @item -k @var{file}
11718 @cindex sticky bit check
11719 True if @var{file} exists and has its @dfn{sticky} bit set.
11721 @item -r @var{file}
11723 @cindex readable file check
11724 True if @var{file} exists and read permission is granted.
11726 @item -u @var{file}
11728 @cindex set-user-ID check
11729 True if @var{file} exists and has its set-user-ID bit set.
11731 @item -w @var{file}
11733 @cindex writable file check
11734 True if @var{file} exists and write permission is granted.
11736 @item -x @var{file}
11738 @cindex executable file check
11739 True if @var{file} exists and execute permission is granted
11740 (or search permission, if it is a directory).
11742 @item -O @var{file}
11744 @cindex owned by effective user ID check
11745 True if @var{file} exists and is owned by the current effective user ID.
11747 @item -G @var{file}
11749 @cindex owned by effective group ID check
11750 True if @var{file} exists and is owned by the current effective group ID.
11754 @node File characteristic tests
11755 @subsection File characteristic tests
11757 @cindex file characteristic tests
11759 These options test other file characteristics.
11763 @item -e @var{file}
11765 @cindex existence-of-file check
11766 True if @var{file} exists.
11768 @item -s @var{file}
11770 @cindex nonempty file check
11771 True if @var{file} exists and has a size greater than zero.
11773 @item @var{file1} -nt @var{file2}
11775 @cindex newer-than file check
11776 True if @var{file1} is newer (according to modification date) than
11777 @var{file2}, or if @var{file1} exists and @var{file2} does not.
11779 @item @var{file1} -ot @var{file2}
11781 @cindex older-than file check
11782 True if @var{file1} is older (according to modification date) than
11783 @var{file2}, or if @var{file2} exists and @var{file1} does not.
11785 @item @var{file1} -ef @var{file2}
11787 @cindex same file check
11788 @cindex hard link check
11789 True if @var{file1} and @var{file2} have the same device and inode
11790 numbers, i.e., if they are hard links to each other.
11796 @subsection String tests
11798 @cindex string tests
11800 These options test string characteristics. You may need to quote
11801 @var{string} arguments for the shell. For example:
11807 The quotes here prevent the wrong arguments from being passed to
11808 @command{test} if @samp{$V} is empty or contains special characters.
11812 @item -z @var{string}
11814 @cindex zero-length string check
11815 True if the length of @var{string} is zero.
11817 @item -n @var{string}
11818 @itemx @var{string}
11820 @cindex nonzero-length string check
11821 True if the length of @var{string} is nonzero.
11823 @item @var{string1} = @var{string2}
11825 @cindex equal string check
11826 True if the strings are equal.
11828 @item @var{string1} == @var{string2}
11830 @cindex equal string check
11831 True if the strings are equal (synonym for =).
11833 @item @var{string1} != @var{string2}
11835 @cindex not-equal string check
11836 True if the strings are not equal.
11841 @node Numeric tests
11842 @subsection Numeric tests
11844 @cindex numeric tests
11845 @cindex arithmetic tests
11847 Numeric relational operators. The arguments must be entirely numeric
11848 (possibly negative), or the special expression @w{@code{-l @var{string}}},
11849 which evaluates to the length of @var{string}.
11853 @item @var{arg1} -eq @var{arg2}
11854 @itemx @var{arg1} -ne @var{arg2}
11855 @itemx @var{arg1} -lt @var{arg2}
11856 @itemx @var{arg1} -le @var{arg2}
11857 @itemx @var{arg1} -gt @var{arg2}
11858 @itemx @var{arg1} -ge @var{arg2}
11865 These arithmetic binary operators return true if @var{arg1} is equal,
11866 not-equal, less-than, less-than-or-equal, greater-than, or
11867 greater-than-or-equal than @var{arg2}, respectively.
11874 test -1 -gt -2 && echo yes
11876 test -l abc -gt 1 && echo yes
11879 @error{} test: integer expression expected before -eq
11883 @node Connectives for test
11884 @subsection Connectives for @command{test}
11886 @cindex logical connectives
11887 @cindex connectives, logical
11889 The usual logical connectives.
11895 True if @var{expr} is false.
11897 @item @var{expr1} -a @var{expr2}
11899 @cindex logical and operator
11900 @cindex and operator
11901 True if both @var{expr1} and @var{expr2} are true.
11903 @item @var{expr1} -o @var{expr2}
11905 @cindex logical or operator
11906 @cindex or operator
11907 True if either @var{expr1} or @var{expr2} is true.
11912 @node expr invocation
11913 @section @command{expr}: Evaluate expressions
11916 @cindex expression evaluation
11917 @cindex evaluation of expressions
11919 @command{expr} evaluates an expression and writes the result on standard
11920 output. Each token of the expression must be a separate argument.
11922 Operands are either integers or strings. Integers consist of one or
11923 more decimal digits, with an optional leading @samp{-}.
11924 @command{expr} converts
11925 anything appearing in an operand position to an integer or a string
11926 depending on the operation being applied to it.
11928 Strings are not quoted for @command{expr} itself, though you may need to
11929 quote them to protect characters with special meaning to the shell,
11930 e.g., spaces. However, regardless of whether it is quoted, a string
11931 operand should not be a parenthesis or any of @command{expr}'s
11932 operators like @code{+}, so you cannot safely pass an arbitrary string
11933 @code{$str} to expr merely by quoting it to the shell. One way to
11934 work around this is to use the @sc{gnu} extension @code{+},
11935 (e.g., @code{+ "$str" = foo}); a more portable way is to use
11936 @code{@w{" $str"}} and to adjust the rest of the expression to take
11937 the leading space into account (e.g., @code{@w{" $str" = " foo"}}).
11939 You should not pass a negative integer or a string with leading
11940 @samp{-} as @command{expr}'s first argument, as it might be
11941 misinterpreted as an option; this can be avoided by parenthesization.
11942 Also, portable scripts should not use a string operand that happens to
11943 take the form of an integer; this can be worked around by inserting
11944 leading spaces as mentioned above.
11946 @cindex parentheses for grouping
11947 Operators may be given as infix symbols or prefix keywords. Parentheses
11948 may be used for grouping in the usual manner. You must quote
11949 parentheses and many operators to avoid the shell evaluating them,
11952 When built with support for the GNU MP library, @command{expr} uses
11953 arbitrary-precision arithmetic; otherwise, it uses native arithmetic
11954 types and may fail due to arithmetic overflow.
11956 The only options are @option{--help} and @option{--version}. @xref{Common
11957 options}. Options must precede operands.
11959 @cindex exit status of @command{expr}
11963 0 if the expression is neither null nor 0,
11964 1 if the expression is null or 0,
11965 2 if the expression is invalid,
11966 3 if an internal error occurred (e.g., arithmetic overflow).
11970 * String expressions:: + : match substr index length
11971 * Numeric expressions:: + - * / %
11972 * Relations for expr:: | & < <= = == != >= >
11973 * Examples of expr:: Examples.
11977 @node String expressions
11978 @subsection String expressions
11980 @cindex string expressions
11981 @cindex expressions, string
11983 @command{expr} supports pattern matching and other string operators. These
11984 have higher precedence than both the numeric and relational operators (in
11985 the next sections).
11989 @item @var{string} : @var{regex}
11990 @cindex pattern matching
11991 @cindex regular expression matching
11992 @cindex matching patterns
11993 Perform pattern matching. The arguments are converted to strings and the
11994 second is considered to be a (basic, a la GNU @code{grep}) regular
11995 expression, with a @code{^} implicitly prepended. The first argument is
11996 then matched against this regular expression.
11998 If the match succeeds and @var{regex} uses @samp{\(} and @samp{\)}, the
11999 @code{:} expression returns the part of @var{string} that matched the
12000 subexpression; otherwise, it returns the number of characters matched.
12002 If the match fails, the @code{:} operator returns the null string if
12003 @samp{\(} and @samp{\)} are used in @var{regex}, otherwise 0.
12005 @kindex \( @r{regexp operator}
12006 Only the first @samp{\( @dots{} \)} pair is relevant to the return
12007 value; additional pairs are meaningful only for grouping the regular
12008 expression operators.
12010 @kindex \+ @r{regexp operator}
12011 @kindex \? @r{regexp operator}
12012 @kindex \| @r{regexp operator}
12013 In the regular expression, @code{\+}, @code{\?}, and @code{\|} are
12014 operators which respectively match one or more, zero or one, or separate
12015 alternatives. SunOS and other @command{expr}'s treat these as regular
12016 characters. (@acronym{POSIX} allows either behavior.)
12017 @xref{Top, , Regular Expression Library, regex, Regex}, for details of
12018 regular expression syntax. Some examples are in @ref{Examples of expr}.
12020 @item match @var{string} @var{regex}
12022 An alternative way to do pattern matching. This is the same as
12023 @w{@samp{@var{string} : @var{regex}}}.
12025 @item substr @var{string} @var{position} @var{length}
12027 Returns the substring of @var{string} beginning at @var{position}
12028 with length at most @var{length}. If either @var{position} or
12029 @var{length} is negative, zero, or non-numeric, returns the null string.
12031 @item index @var{string} @var{charset}
12033 Returns the first position in @var{string} where the first character in
12034 @var{charset} was found. If no character in @var{charset} is found in
12035 @var{string}, return 0.
12037 @item length @var{string}
12039 Returns the length of @var{string}.
12041 @item + @var{token}
12043 Interpret @var{token} as a string, even if it is a keyword like @var{match}
12044 or an operator like @code{/}.
12045 This makes it possible to test @code{expr length + "$x"} or
12046 @code{expr + "$x" : '.*/\(.\)'} and have it do the right thing even if
12047 the value of @var{$x} happens to be (for example) @code{/} or @code{index}.
12048 This operator is a @acronym{GNU} extension. Portable shell scripts should use
12049 @code{@w{" $token"} : @w{' \(.*\)'}} instead of @code{+ "$token"}.
12053 To make @command{expr} interpret keywords as strings, you must use the
12054 @code{quote} operator.
12057 @node Numeric expressions
12058 @subsection Numeric expressions
12060 @cindex numeric expressions
12061 @cindex expressions, numeric
12063 @command{expr} supports the usual numeric operators, in order of increasing
12064 precedence. These numeric operators have lower precedence than the
12065 string operators described in the previous section, and higher precedence
12066 than the connectives (next section).
12074 @cindex subtraction
12075 Addition and subtraction. Both arguments are converted to integers;
12076 an error occurs if this cannot be done.
12082 @cindex multiplication
12085 Multiplication, division, remainder. Both arguments are converted to
12086 integers; an error occurs if this cannot be done.
12091 @node Relations for expr
12092 @subsection Relations for @command{expr}
12094 @cindex connectives, logical
12095 @cindex logical connectives
12096 @cindex relations, numeric or string
12098 @command{expr} supports the usual logical connectives and relations. These
12099 have lower precedence than the string and numeric operators
12100 (previous sections). Here is the list, lowest-precedence operator first.
12106 @cindex logical or operator
12107 @cindex or operator
12108 Returns its first argument if that is neither null nor zero, otherwise
12109 its second argument if it is neither null nor zero, otherwise 0. It
12110 does not evaluate its second argument if its first argument is neither
12115 @cindex logical and operator
12116 @cindex and operator
12117 Return its first argument if neither argument is null or zero, otherwise
12118 0. It does not evaluate its second argument if its first argument is
12121 @item < <= = == != >= >
12128 @cindex comparison operators
12130 Compare the arguments and return 1 if the relation is true, 0 otherwise.
12131 @code{==} is a synonym for @code{=}. @command{expr} first tries to convert
12132 both arguments to integers and do a numeric comparison; if either
12133 conversion fails, it does a lexicographic comparison using the character
12134 collating sequence specified by the @env{LC_COLLATE} locale.
12139 @node Examples of expr
12140 @subsection Examples of using @command{expr}
12142 @cindex examples of @command{expr}
12143 Here are a few examples, including quoting for shell metacharacters.
12145 To add 1 to the shell variable @code{foo}, in Bourne-compatible shells:
12148 foo=`expr $foo + 1`
12151 To print the non-directory part of the file name stored in
12152 @code{$fname}, which need not contain a @code{/}:
12155 expr $fname : '.*/\(.*\)' '|' $fname
12158 An example showing that @code{\+} is an operator:
12166 expr abc : 'a\(.\)c'
12168 expr index abcdef cz
12171 @error{} expr: syntax error
12172 expr index + index a
12178 @chapter Redirection
12180 @cindex redirection
12181 @cindex commands for redirection
12183 Unix shells commonly provide several forms of @dfn{redirection}---ways
12184 to change the input source or output destination of a command. But one
12185 useful redirection is performed by a separate command, not by the shell;
12186 it's described here.
12189 * tee invocation:: Redirect output to multiple files or processes.
12193 @node tee invocation
12194 @section @command{tee}: Redirect output to multiple files or processes
12197 @cindex pipe fitting
12198 @cindex destinations, multiple output
12199 @cindex read from stdin and write to stdout and files
12201 The @command{tee} command copies standard input to standard output and also
12202 to any files given as arguments. This is useful when you want not only
12203 to send some data down a pipe, but also to save a copy. Synopsis:
12206 tee [@var{option}]@dots{} [@var{file}]@dots{}
12209 If a file being written to does not already exist, it is created. If a
12210 file being written to already exists, the data it previously contained
12211 is overwritten unless the @option{-a} option is used.
12213 A @var{file} of @samp{-} causes @command{tee} to send another copy of
12214 input to standard output, but this is typically not that useful as the
12215 copies are interleaved.
12217 The program accepts the following options. Also see @ref{Common options}.
12224 Append standard input to the given files rather than overwriting
12228 @itemx --ignore-interrupts
12230 @opindex --ignore-interrupts
12231 Ignore interrupt signals.
12235 The @command{tee} command is useful when you happen to be transferring a large
12236 amount of data and also want to summarize that data without reading
12237 it a second time. For example, when you are downloading a DVD image,
12238 you often want to verify its signature or checksum right away.
12239 The inefficient way to do it is simply:
12242 wget http://example.com/some.iso && sha1sum some.iso
12245 One problem with the above is that it makes you wait for the
12246 download to complete before starting the time-consuming SHA1 computation.
12247 Perhaps even more importantly, the above requires reading
12248 the DVD image a second time (the first was from the network).
12250 The efficient way to do it is to interleave the download
12251 and SHA1 computation. Then, you'll get the checksum for
12252 free, because the entire process parallelizes so well:
12255 # slightly contrived, to demonstrate process substitution
12256 wget -O - http://example.com/dvd.iso \
12257 | tee >(sha1sum > dvd.sha1) > dvd.iso
12260 That makes @command{tee} write not just to the expected output file,
12261 but also to a pipe running @command{sha1sum} and saving the final
12262 checksum in a file named @file{dvd.sha1}.
12264 Note, however, that this example relies on a feature of modern shells
12265 called @dfn{process substitution}
12266 (the @samp{>(command)} syntax, above;
12267 @xref{Process Substitution,,Process Substitution, bashref,
12268 The Bash Reference Manual}.),
12269 so it works with @command{zsh}, @command{bash}, and @command{ksh},
12270 but not with @command{/bin/sh}. So if you write code like this
12271 in a shell script, be sure to start the script with @samp{#!/bin/bash}.
12273 Since the above example writes to one file and one process,
12274 a more conventional and portable use of @command{tee} is even better:
12277 wget -O - http://example.com/dvd.iso \
12278 | tee dvd.iso | sha1sum > dvd.sha1
12281 You can extend this example to make @command{tee} write to two processes,
12282 computing MD5 and SHA1 checksums in parallel. In this case,
12283 process substitution is required:
12286 wget -O - http://example.com/dvd.iso \
12287 | tee >(sha1sum > dvd.sha1) \
12288 >(md5sum > dvd.md5) \
12292 This technique is also useful when you want to make a @emph{compressed}
12293 copy of the contents of a pipe.
12294 Consider a tool to graphically summarize disk usage data from @samp{du -ak}.
12295 For a large hierarchy, @samp{du -ak} can run for a long time,
12296 and can easily produce terabytes of data, so you won't want to
12297 rerun the command unnecessarily. Nor will you want to save
12298 the uncompressed output.
12300 Doing it the inefficient way, you can't even start the GUI
12301 until after you've compressed all of the @command{du} output:
12304 du -ak | gzip -9 > /tmp/du.gz
12305 gzip -d /tmp/du.gz | xdiskusage -a
12308 With @command{tee} and process substitution, you start the GUI
12309 right away and eliminate the decompression completely:
12312 du -ak | tee >(gzip -9 > /tmp/du.gz) | xdiskusage -a
12315 Finally, if you regularly create more than one type of
12316 compressed tarball at once, for example when @code{make dist} creates
12317 both @command{gzip}-compressed and @command{bzip2}-compressed tarballs,
12318 there may be a better way.
12319 Typical @command{automake}-generated @file{Makefile} rules create
12320 the two compressed tar archives with commands in sequence, like this
12321 (slightly simplified):
12324 tardir=your-pkg-M.N
12325 tar chof - "$tardir" | gzip -9 -c > your-pkg-M.N.tar.gz
12326 tar chof - "$tardir" | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12329 However, if the hierarchy you are archiving and compressing is larger
12330 than a couple megabytes, and especially if you are using a multi-processor
12331 system with plenty of memory, then you can do much better by reading the
12332 directory contents only once and running the compression programs in parallel:
12335 tardir=your-pkg-M.N
12336 tar chof - "$tardir" \
12337 | tee >(gzip -9 -c > your-pkg-M.N.tar.gz) \
12338 | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12344 @node File name manipulation
12345 @chapter File name manipulation
12347 @cindex file name manipulation
12348 @cindex manipulation of file names
12349 @cindex commands for file name manipulation
12351 This section describes commands that manipulate file names.
12354 * basename invocation:: Strip directory and suffix from a file name.
12355 * dirname invocation:: Strip last file name component.
12356 * pathchk invocation:: Check file name validity and portability.
12357 * mktemp invocation:: Create temporary file or directory.
12358 * realpath invocation:: Print resolved file names.
12362 @node basename invocation
12363 @section @command{basename}: Strip directory and suffix from a file name
12366 @cindex strip directory and suffix from file names
12367 @cindex directory, stripping from file names
12368 @cindex suffix, stripping from file names
12369 @cindex file names, stripping directory and suffix
12370 @cindex leading directory components, stripping
12372 @command{basename} removes any leading directory components from
12373 @var{name}. Synopsis:
12376 basename @var{name} [@var{suffix}]
12379 If @var{suffix} is specified and is identical to the end of @var{name},
12380 it is removed from @var{name} as well. Note that since trailing slashes
12381 are removed prior to suffix matching, @var{suffix} will do nothing if it
12382 contains slashes. @command{basename} prints the result on standard
12385 @c This test is used both here and in the section on dirname.
12386 @macro basenameAndDirname
12387 Together, @command{basename} and @command{dirname} are designed such
12388 that if @samp{ls "$name"} succeeds, then the command sequence @samp{cd
12389 "$(dirname "$name")"; ls "$(basename "$name")"} will, too. This works
12390 for everything except file names containing a trailing newline.
12392 @basenameAndDirname
12394 @acronym{POSIX} allows the implementation to define the results if
12395 @var{name} is empty or @samp{//}. In the former case, @acronym{GNU}
12396 @command{basename} returns the empty string. In the latter case, the
12397 result is @samp{//} on platforms where @var{//} is distinct from
12398 @var{/}, and @samp{/} on platforms where there is no difference.
12400 The only options are @option{--help} and @option{--version}. @xref{Common
12401 options}. Options must precede operands.
12409 basename /usr/bin/sort
12412 basename include/stdio.h .h
12416 @node dirname invocation
12417 @section @command{dirname}: Strip last file name component
12420 @cindex directory components, printing
12421 @cindex stripping non-directory suffix
12422 @cindex non-directory suffix, stripping
12424 @command{dirname} prints all but the final slash-delimited component of
12425 @var{name}. Slashes on either side of the final component are also
12426 removed. If the string contains no slash, @command{dirname} prints
12427 @samp{.} (meaning the current directory). Synopsis:
12433 @var{name} need not be a file name, but if it is, this operation
12434 effectively lists the directory that contains the final component,
12435 including the case when the final component is itself a directory.
12437 @basenameAndDirname
12439 @acronym{POSIX} allows the implementation to define the results if
12440 @var{name} is @samp{//}. With @acronym{GNU} @command{dirname}, the
12441 result is @samp{//} on platforms where @var{//} is distinct from
12442 @var{/}, and @samp{/} on platforms where there is no difference.
12444 The only options are @option{--help} and @option{--version}. @xref{Common
12452 # Output "/usr/bin".
12453 dirname /usr/bin/sort
12454 dirname /usr/bin//.//
12461 @node pathchk invocation
12462 @section @command{pathchk}: Check file name validity and portability
12465 @cindex file names, checking validity and portability
12466 @cindex valid file names, checking for
12467 @cindex portable file names, checking for
12469 @command{pathchk} checks validity and portability of file names. Synopsis:
12472 pathchk [@var{option}]@dots{} @var{name}@dots{}
12475 For each @var{name}, @command{pathchk} prints an error message if any of
12476 these conditions is true:
12480 One of the existing directories in @var{name} does not have search
12481 (execute) permission,
12483 The length of @var{name} is larger than the maximum supported by the
12486 The length of one component of @var{name} is longer than
12487 its file system's maximum.
12490 A nonexistent @var{name} is not an error, so long a file with that
12491 name could be created under the above conditions.
12493 The program accepts the following options. Also see @ref{Common options}.
12494 Options must precede operands.
12500 Instead of performing checks based on the underlying file system,
12501 print an error message if any of these conditions is true:
12505 A file name is empty.
12508 A file name contains a character outside the @acronym{POSIX} portable file
12509 name character set, namely, the ASCII letters and digits, @samp{.},
12510 @samp{_}, @samp{-}, and @samp{/}.
12513 The length of a file name or one of its components exceeds the
12514 @acronym{POSIX} minimum limits for portability.
12519 Print an error message if a file name is empty, or if it contains a component
12520 that begins with @samp{-}.
12522 @item --portability
12523 @opindex --portability
12524 Print an error message if a file name is not portable to all @acronym{POSIX}
12525 hosts. This option is equivalent to @samp{-p -P}.
12529 @cindex exit status of @command{pathchk}
12533 0 if all specified file names passed all checks,
12537 @node mktemp invocation
12538 @section @command{mktemp}: Create temporary file or directory
12541 @cindex file names, creating temporary
12542 @cindex directory, creating temporary
12543 @cindex temporary files and directories
12545 @command{mktemp} manages the creation of temporary files and
12546 directories. Synopsis:
12549 mktemp [@var{option}]@dots{} [@var{template}]
12552 Safely create a temporary file or directory based on @var{template},
12553 and print its name. If given, @var{template} must include at least
12554 three consecutive @samp{X}s in the last component. If omitted, the template
12555 @samp{tmp.XXXXXXXXXX} is used, and option @option{--tmpdir} is
12556 implied. The final run of @samp{X}s in the @var{template} will be replaced
12557 by alpha-numeric characters; thus, on a case-sensitive file system,
12558 and with a @var{template} including a run of @var{n} instances of @samp{X},
12559 there are @samp{62**@var{n}} potential file names.
12561 Older scripts used to create temporary files by simply joining the
12562 name of the program with the process id (@samp{$$}) as a suffix.
12563 However, that naming scheme is easily predictable, and suffers from a
12564 race condition where the attacker can create an appropriately named
12565 symbolic link, such that when the script then opens a handle to what
12566 it thought was an unused file, it is instead modifying an existing
12567 file. Using the same scheme to create a directory is slightly safer,
12568 since the @command{mkdir} will fail if the target already exists, but
12569 it is still inferior because it allows for denial of service attacks.
12570 Therefore, modern scripts should use the @command{mktemp} command to
12571 guarantee that the generated name will be unpredictable, and that
12572 knowledge of the temporary file name implies that the file was created
12573 by the current script and cannot be modified by other users.
12575 When creating a file, the resulting file has read and write
12576 permissions for the current user, but no permissions for the group or
12577 others; these permissions are reduced if the current umask is more
12580 Here are some examples (although note that if you repeat them, you
12581 will most likely get different file names):
12586 Create a temporary file in the current directory.
12593 Create a temporary file with a known suffix.
12595 $ mktemp --suffix=.txt file-XXXX
12597 $ mktemp file-XXXX-XXXX.txt
12602 Create a secure fifo relative to the user's choice of @env{TMPDIR},
12603 but falling back to the current directory rather than @file{/tmp}.
12604 Note that @command{mktemp} does not create fifos, but can create a
12605 secure directory in which the fifo can live. Exit the shell if the
12606 directory or fifo could not be created.
12608 $ dir=$(mktemp -p "$@{TMPDIR:-.@}" -d dir-XXXX) || exit 1
12610 $ mkfifo "$fifo" || @{ rmdir "$dir"; exit 1; @}
12614 Create and use a temporary file if possible, but ignore failure. The
12615 file will reside in the directory named by @env{TMPDIR}, if specified,
12616 or else in @file{/tmp}.
12618 $ file=$(mktemp -q) && @{
12619 > # Safe to use $file only within this block. Use quotes,
12620 > # since $TMPDIR, and thus $file, may contain whitespace.
12621 > echo ... > "$file"
12627 Act as a semi-random character generator (it is not fully random,
12628 since it is impacted by the contents of the current directory). To
12629 avoid security holes, do not use the resulting names to create a file.
12639 The program accepts the following options. Also see @ref{Common options}.
12646 @opindex --directory
12647 Create a directory rather than a file. The directory will have read,
12648 write, and search permissions for the current user, but no permissions
12649 for the group or others; these permissions are reduced if the current
12650 umask is more restrictive.
12656 Suppress diagnostics about failure to create a file or directory. The
12657 exit status will still reflect whether a file was created.
12663 Generate a temporary name that does not name an existing file, without
12664 changing the file system contents. Using the output of this command
12665 to create a new file is inherently unsafe, as there is a window of
12666 time between generating the name and using it where another process
12667 can create an object by the same name.
12670 @itemx --tmpdir[=@var{dir}]
12673 Treat @var{template} relative to the directory @var{dir}. If
12674 @var{dir} is not specified (only possible with the long option
12675 @option{--tmpdir}) or is the empty string, use the value of
12676 @env{TMPDIR} if available, otherwise use @samp{/tmp}. If this is
12677 specified, @var{template} must not be absolute. However,
12678 @var{template} can still contain slashes, although intermediate
12679 directories must already exist.
12681 @item --suffix=@var{suffix}
12683 Append @var{suffix} to the @var{template}. @var{suffix} must not
12684 contain slash. If @option{--suffix} is specified, @var{template} must
12685 end in @samp{X}; if it is not specified, then an appropriate
12686 @option{--suffix} is inferred by finding the last @samp{X} in
12687 @var{template}. This option exists for use with the default
12688 @var{template} and for the creation of a @var{suffix} that starts with
12693 Treat @var{template} as a single file relative to the value of
12694 @env{TMPDIR} if available, or to the directory specified by
12695 @option{-p}, otherwise to @samp{/tmp}. @var{template} must not
12696 contain slashes. This option is deprecated; the use of @option{-p}
12697 without @option{-t} offers better defaults (by favoring the command
12698 line over @env{TMPDIR}) and more flexibility (by allowing intermediate
12703 @cindex exit status of @command{mktemp}
12707 0 if the file was created,
12712 @node realpath invocation
12713 @section @command{realpath}: Print the resolved file name.
12716 @cindex file names, canonicalization
12717 @cindex symlinks, resolution
12718 @cindex canonical file name
12719 @cindex canonicalize a file name
12723 @command{realpath} expands all symbolic links and resolves references to
12724 @samp{/./}, @samp{/../} and extra @samp{/} characters. By default,
12725 all but the last component of the specified files must exist. Synopsis:
12728 realpath [@var{option}]@dots{} @var{file}@dots{}
12731 The program accepts the following options. Also see @ref{Common options}.
12736 @itemx --canonicalize-existing
12738 @opindex --canonicalize-existing
12739 Ensure that all components of the specified file names exist.
12740 If any component is missing or unavailable, @command{realpath} will output
12741 a diagnostic unless the @option{-q} option is specified, and exit with a
12742 nonzero exit code. A trailing slash requires that the name resolve to a
12746 @itemx --canonicalize-missing
12748 @opindex --canonicalize-missing
12749 If any component of a specified file name is missing or unavailable,
12750 treat it as a directory.
12756 Symbolic links are resolved in the specified file names,
12757 but they are resolved after any subsequent @samp{..} components are processed.
12762 @opindex --physical
12763 Symbolic links are resolved in the specified file names,
12764 and they are resolved before any subsequent @samp{..} components are processed.
12765 This is the default mode of operation.
12771 Suppress diagnostic messages for specified file names.
12775 @itemx --no-symlinks
12778 @opindex --no-symlinks
12779 Do not resolve symbolic links. Only resolve references to
12780 @samp{/./}, @samp{/../} and remove extra @samp{/} characters.
12781 When combined with the @option{-m} option, realpath operates
12782 only on the file name, and does not touch any actual file.
12788 Separate output items with @sc{nul} characters.
12790 @itemx --relative-to=@var{file}
12791 @opindex --relative-to
12793 Print the resolved file names relative to the specified file.
12794 Note this option honors the @option{-m} and @option{-e} options
12795 pertaining to file existence.
12797 @itemx --relative-base=@var{base}
12798 @opindex --relative-base
12799 This option is valid when used with @option{--relative-to}, and will restrict
12800 the output of @option{--relative-to} so that relative names are output,
12801 only when @var{file}s are descendants of @var{base}. Otherwise output the
12802 absolute file name. Note: this option honors the @option{-m} and @option{-e}
12803 options pertaining to file existence. For example:
12806 realpath --relative-to=/usr /tmp /usr/bin
12809 realpath --relative-base=/usr --relative-to=/usr /tmp /usr/bin
12816 @cindex exit status of @command{realpath}
12820 0 if all file names were printed without issue.
12825 @node Working context
12826 @chapter Working context
12828 @cindex working context
12829 @cindex commands for printing the working context
12831 This section describes commands that display or alter the context in
12832 which you are working: the current directory, the terminal settings, and
12833 so forth. See also the user-related commands in the next section.
12836 * pwd invocation:: Print working directory.
12837 * stty invocation:: Print or change terminal characteristics.
12838 * printenv invocation:: Print environment variables.
12839 * tty invocation:: Print file name of terminal on standard input.
12843 @node pwd invocation
12844 @section @command{pwd}: Print working directory
12847 @cindex print name of current directory
12848 @cindex current working directory, printing
12849 @cindex working directory, printing
12852 @command{pwd} prints the name of the current directory. Synopsis:
12855 pwd [@var{option}]@dots{}
12858 The program accepts the following options. Also see @ref{Common options}.
12865 If the contents of the environment variable @env{PWD} provide an
12866 absolute name of the current directory with no @samp{.} or @samp{..}
12867 components, but possibly with symbolic links, then output those
12868 contents. Otherwise, fall back to default @option{-P} handling.
12873 @opindex --physical
12874 Print a fully resolved name for the current directory. That is, all
12875 components of the printed name will be actual directory names---none
12876 will be symbolic links.
12879 @cindex symbolic links and @command{pwd}
12880 If @option{-L} and @option{-P} are both given, the last one takes
12881 precedence. If neither option is given, then this implementation uses
12882 @option{-P} as the default unless the @env{POSIXLY_CORRECT}
12883 environment variable is set.
12885 @mayConflictWithShellBuiltIn{pwd}
12890 @node stty invocation
12891 @section @command{stty}: Print or change terminal characteristics
12894 @cindex change or print terminal settings
12895 @cindex terminal settings
12896 @cindex line settings of terminal
12898 @command{stty} prints or changes terminal characteristics, such as baud rate.
12902 stty [@var{option}] [@var{setting}]@dots{}
12903 stty [@var{option}]
12906 If given no line settings, @command{stty} prints the baud rate, line
12907 discipline number (on systems that support it), and line settings
12908 that have been changed from the values set by @samp{stty sane}.
12909 By default, mode reading and setting are performed on the tty line
12910 connected to standard input, although this can be modified by the
12911 @option{--file} option.
12913 @command{stty} accepts many non-option arguments that change aspects of
12914 the terminal line operation, as described below.
12916 The program accepts the following options. Also see @ref{Common options}.
12923 Print all current settings in human-readable form. This option may not
12924 be used in combination with any line settings.
12926 @item -F @var{device}
12927 @itemx --file=@var{device}
12930 Set the line opened by the file name specified in @var{device} instead of
12931 the tty line connected to standard input. This option is necessary
12932 because opening a @acronym{POSIX} tty requires use of the
12933 @code{O_NONDELAY} flag to prevent a @acronym{POSIX} tty from blocking
12934 until the carrier detect line is high if
12935 the @code{clocal} flag is not set. Hence, it is not always possible
12936 to allow the shell to open the device in the traditional manner.
12942 @cindex machine-readable @command{stty} output
12943 Print all current settings in a form that can be used as an argument to
12944 another @command{stty} command to restore the current settings. This option
12945 may not be used in combination with any line settings.
12949 Many settings can be turned off by preceding them with a @samp{-}.
12950 Such arguments are marked below with ``May be negated'' in their
12951 description. The descriptions themselves refer to the positive
12952 case, that is, when @emph{not} negated (unless stated otherwise,
12955 Some settings are not available on all @acronym{POSIX} systems, since they use
12956 extensions. Such arguments are marked below with
12957 ``Non-@acronym{POSIX}'' in their description. On non-@acronym{POSIX}
12958 systems, those or other settings also may not
12959 be available, but it's not feasible to document all the variations: just
12965 * Control:: Control settings
12966 * Input:: Input settings
12967 * Output:: Output settings
12968 * Local:: Local settings
12969 * Combination:: Combination settings
12970 * Characters:: Special characters
12971 * Special:: Special settings
12976 @subsection Control settings
12978 @cindex control settings
12984 @cindex two-way parity
12985 Generate parity bit in output and expect parity bit in input.
12991 @cindex even parity
12992 Set odd parity (even if negated). May be negated.
12999 @cindex character size
13000 @cindex eight-bit characters
13001 Set character size to 5, 6, 7, or 8 bits.
13006 Send a hangup signal when the last process closes the tty. May be
13012 Use two stop bits per character (one if negated). May be negated.
13016 Allow input to be received. May be negated.
13020 @cindex modem control
13021 Disable modem control signals. May be negated.
13025 @cindex hardware flow control
13026 @cindex flow control, hardware
13027 @cindex RTS/CTS flow control
13028 Enable RTS/CTS flow control. Non-@acronym{POSIX}. May be negated.
13033 @subsection Input settings
13035 @cindex input settings
13036 These settings control operations on data received from the terminal.
13041 @cindex breaks, ignoring
13042 Ignore break characters. May be negated.
13046 @cindex breaks, cause interrupts
13047 Make breaks cause an interrupt signal. May be negated.
13051 @cindex parity, ignoring
13052 Ignore characters with parity errors. May be negated.
13056 @cindex parity errors, marking
13057 Mark parity errors (with a 255-0-character sequence). May be negated.
13061 Enable input parity checking. May be negated.
13065 @cindex eight-bit input
13066 Clear high (8th) bit of input characters. May be negated.
13070 @cindex newline, translating to return
13071 Translate newline to carriage return. May be negated.
13075 @cindex return, ignoring
13076 Ignore carriage return. May be negated.
13080 @cindex return, translating to newline
13081 Translate carriage return to newline. May be negated.
13085 @cindex input encoding, UTF-8
13086 Assume input characters are UTF-8 encoded. May be negated.
13090 @kindex C-s/C-q flow control
13091 @cindex XON/XOFF flow control
13092 Enable XON/XOFF flow control (that is, @kbd{CTRL-S}/@kbd{CTRL-Q}). May
13099 @cindex software flow control
13100 @cindex flow control, software
13101 Enable sending of @code{stop} character when the system input buffer
13102 is almost full, and @code{start} character when it becomes almost
13103 empty again. May be negated.
13107 @cindex uppercase, translating to lowercase
13108 Translate uppercase characters to lowercase. Non-@acronym{POSIX}. May be
13109 negated. Note ilcuc is not implemented, as one would not be able to issue
13110 almost any (lowercase) Unix command, after invoking it.
13114 Allow any character to restart output (only the start character
13115 if negated). Non-@acronym{POSIX}. May be negated.
13119 @cindex beeping at input buffer full
13120 Enable beeping and not flushing input buffer if a character arrives
13121 when the input buffer is full. Non-@acronym{POSIX}. May be negated.
13126 @subsection Output settings
13128 @cindex output settings
13129 These settings control operations on data sent to the terminal.
13134 Postprocess output. May be negated.
13138 @cindex lowercase, translating to output
13139 Translate lowercase characters to uppercase. Non-@acronym{POSIX}. May be
13140 negated. (Note ouclc is not currently implemented.)
13144 @cindex return, translating to newline
13145 Translate carriage return to newline. Non-@acronym{POSIX}. May be negated.
13149 @cindex newline, translating to crlf
13150 Translate newline to carriage return-newline. Non-@acronym{POSIX}. May be
13155 Do not print carriage returns in the first column. Non-@acronym{POSIX}.
13160 Newline performs a carriage return. Non-@acronym{POSIX}. May be negated.
13164 @cindex pad instead of timing for delaying
13165 Use fill (padding) characters instead of timing for delays.
13166 Non-@acronym{POSIX}.
13171 @cindex pad character
13172 Use @acronym{ASCII} @sc{del} characters for fill instead of
13173 @acronym{ASCII} @sc{nul} characters. Non-@acronym{POSIX}.
13179 Newline delay style. Non-@acronym{POSIX}.
13186 Carriage return delay style. Non-@acronym{POSIX}.
13192 @opindex tab@var{n}
13193 Horizontal tab delay style. Non-@acronym{POSIX}.
13198 Backspace delay style. Non-@acronym{POSIX}.
13203 Vertical tab delay style. Non-@acronym{POSIX}.
13208 Form feed delay style. Non-@acronym{POSIX}.
13213 @subsection Local settings
13215 @cindex local settings
13220 Enable @code{interrupt}, @code{quit}, and @code{suspend} special
13221 characters. May be negated.
13225 Enable @code{erase}, @code{kill}, @code{werase}, and @code{rprnt}
13226 special characters. May be negated.
13230 Enable non-@acronym{POSIX} special characters. May be negated.
13234 Echo input characters. May be negated.
13240 Echo @code{erase} characters as backspace-space-backspace. May be
13245 @cindex newline echoing after @code{kill}
13246 Echo a newline after a @code{kill} character. May be negated.
13250 @cindex newline, echoing
13251 Echo newline even if not echoing other characters. May be negated.
13255 @cindex flushing, disabling
13256 Disable flushing after @code{interrupt} and @code{quit} special
13257 characters. May be negated.
13261 @cindex case translation
13262 Enable input and output of uppercase characters by preceding their
13263 lowercase equivalents with @samp{\}, when @code{icanon} is set.
13264 Non-@acronym{POSIX}. May be negated.
13268 @cindex background jobs, stopping at terminal write
13269 Stop background jobs that try to write to the terminal. Non-@acronym{POSIX}.
13276 Echo erased characters backward, between @samp{\} and @samp{/}.
13277 Non-@acronym{POSIX}. May be negated.
13283 @cindex control characters, using @samp{^@var{c}}
13284 @cindex hat notation for control characters
13285 Echo control characters in hat notation (@samp{^@var{c}}) instead
13286 of literally. Non-@acronym{POSIX}. May be negated.
13292 Echo the @code{kill} special character by erasing each character on
13293 the line as indicated by the @code{echoprt} and @code{echoe} settings,
13294 instead of by the @code{echoctl} and @code{echok} settings.
13295 Non-@acronym{POSIX}.
13301 @subsection Combination settings
13303 @cindex combination settings
13304 Combination settings:
13311 Same as @code{parenb -parodd cs7}. May be negated. If negated, same
13312 as @code{-parenb cs8}.
13316 Same as @code{parenb parodd cs7}. May be negated. If negated, same
13317 as @code{-parenb cs8}.
13321 Same as @code{-icrnl -onlcr}. May be negated. If negated, same as
13322 @code{icrnl -inlcr -igncr onlcr -ocrnl -onlret}.
13326 Reset the @code{erase} and @code{kill} special characters to their default
13333 @c This is too long to write inline.
13335 cread -ignbrk brkint -inlcr -igncr icrnl -ixoff
13336 -iuclc -ixany imaxbel opost -olcuc -ocrnl onlcr
13337 -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0
13338 ff0 isig icanon iexten echo echoe echok -echonl
13339 -noflsh -xcase -tostop -echoprt echoctl echoke
13343 and also sets all special characters to their default values.
13347 Same as @code{brkint ignpar istrip icrnl ixon opost isig icanon}, plus
13348 sets the @code{eof} and @code{eol} characters to their default values
13349 if they are the same as the @code{min} and @code{time} characters.
13350 May be negated. If negated, same as @code{raw}.
13357 -ignbrk -brkint -ignpar -parmrk -inpck -istrip
13358 -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany
13359 -imaxbel -opost -isig -icanon -xcase min 1 time 0
13363 May be negated. If negated, same as @code{cooked}.
13367 Same as @option{-icanon}. May be negated. If negated, same as
13372 @cindex eight-bit characters
13373 Same as @code{-parenb -istrip cs8}. May be negated. If negated,
13374 same as @code{parenb istrip cs7}.
13378 Same as @option{-parenb -istrip -opost cs8}. May be negated.
13379 If negated, same as @code{parenb istrip opost cs7}.
13383 Same as @option{-ixany}. Non-@acronym{POSIX}. May be negated.
13387 Same as @code{tab0}. Non-@acronym{POSIX}. May be negated. If negated, same
13394 Same as @code{xcase iuclc olcuc}. Non-@acronym{POSIX}. May be negated.
13395 (Used for terminals with uppercase characters only.)
13399 Same as @code{echoe echoctl echoke}.
13403 Same as @code{echoe echoctl echoke -ixany intr ^C erase ^? kill C-u}.
13408 @subsection Special characters
13410 @cindex special characters
13411 @cindex characters, special
13413 The special characters' default values vary from system to system.
13414 They are set with the syntax @samp{name value}, where the names are
13415 listed below and the value can be given either literally, in hat
13416 notation (@samp{^@var{c}}), or as an integer which may start with
13417 @samp{0x} to indicate hexadecimal, @samp{0} to indicate octal, or
13418 any other digit to indicate decimal.
13420 @cindex disabling special characters
13421 @kindex u@r{, and disabling special characters}
13422 For GNU stty, giving a value of @code{^-} or @code{undef} disables that
13423 special character. (This is incompatible with Ultrix @command{stty},
13424 which uses a value of @samp{u} to disable a special character. GNU
13425 @command{stty} treats a value @samp{u} like any other, namely to set that
13426 special character to @key{U}.)
13432 Send an interrupt signal.
13436 Send a quit signal.
13440 Erase the last character typed.
13444 Erase the current line.
13448 Send an end of file (terminate the input).
13456 Alternate character to end the line. Non-@acronym{POSIX}.
13460 Switch to a different shell layer. Non-@acronym{POSIX}.
13464 Restart the output after stopping it.
13472 Send a terminal stop signal.
13476 Send a terminal stop signal after flushing the input. Non-@acronym{POSIX}.
13480 Redraw the current line. Non-@acronym{POSIX}.
13484 Erase the last word typed. Non-@acronym{POSIX}.
13488 Enter the next character typed literally, even if it is a special
13489 character. Non-@acronym{POSIX}.
13494 @subsection Special settings
13496 @cindex special settings
13501 Set the minimum number of characters that will satisfy a read until
13502 the time value has expired, when @option{-icanon} is set.
13506 Set the number of tenths of a second before reads time out if the minimum
13507 number of characters have not been read, when @option{-icanon} is set.
13509 @item ispeed @var{n}
13511 Set the input speed to @var{n}.
13513 @item ospeed @var{n}
13515 Set the output speed to @var{n}.
13519 Tell the tty kernel driver that the terminal has @var{n} rows.
13520 Non-@acronym{POSIX}.
13523 @itemx columns @var{n}
13526 Tell the kernel that the terminal has @var{n} columns. Non-@acronym{POSIX}.
13532 Print the number of rows and columns that the kernel thinks the
13533 terminal has. (Systems that don't support rows and columns in the kernel
13534 typically use the environment variables @env{LINES} and @env{COLUMNS}
13535 instead; however, GNU @command{stty} does not know anything about them.)
13536 Non-@acronym{POSIX}.
13540 Use line discipline @var{n}. Non-@acronym{POSIX}.
13544 Print the terminal speed.
13547 @cindex baud rate, setting
13548 Set the input and output speeds to @var{n}. @var{n} can be one of: 0
13549 50 75 110 134 134.5 150 200 300 600 1200 1800 2400 4800 9600 19200
13550 38400 @code{exta} @code{extb}. @code{exta} is the same as 19200;
13551 @code{extb} is the same as 38400. Many systems, including GNU/Linux,
13552 support higher speeds. The @command{stty} command includes support
13569 4000000 where the system supports these.
13570 0 hangs up the line if @option{-clocal} is set.
13574 @node printenv invocation
13575 @section @command{printenv}: Print all or some environment variables
13578 @cindex printing all or some environment variables
13579 @cindex environment variables, printing
13581 @command{printenv} prints environment variable values. Synopsis:
13584 printenv [@var{option}] [@var{variable}]@dots{}
13587 If no @var{variable}s are specified, @command{printenv} prints the value of
13588 every environment variable. Otherwise, it prints the value of each
13589 @var{variable} that is set, and nothing for those that are not set.
13591 The program accepts the following option. Also see @ref{Common options}.
13599 @cindex exit status of @command{printenv}
13603 0 if all variables specified were found
13604 1 if at least one specified variable was not found
13605 2 if a write error occurred
13609 @node tty invocation
13610 @section @command{tty}: Print file name of terminal on standard input
13613 @cindex print terminal file name
13614 @cindex terminal file name, printing
13616 @command{tty} prints the file name of the terminal connected to its standard
13617 input. It prints @samp{not a tty} if standard input is not a terminal.
13621 tty [@var{option}]@dots{}
13624 The program accepts the following option. Also see @ref{Common options}.
13634 Print nothing; only return an exit status.
13638 @cindex exit status of @command{tty}
13642 0 if standard input is a terminal
13643 1 if standard input is not a terminal
13644 2 if given incorrect arguments
13645 3 if a write error occurs
13649 @node User information
13650 @chapter User information
13652 @cindex user information, commands for
13653 @cindex commands for printing user information
13655 This section describes commands that print user-related information:
13656 logins, groups, and so forth.
13659 * id invocation:: Print user identity.
13660 * logname invocation:: Print current login name.
13661 * whoami invocation:: Print effective user ID.
13662 * groups invocation:: Print group names a user is in.
13663 * users invocation:: Print login names of users currently logged in.
13664 * who invocation:: Print who is currently logged in.
13668 @node id invocation
13669 @section @command{id}: Print user identity
13672 @cindex real user and group IDs, printing
13673 @cindex effective user and group IDs, printing
13674 @cindex printing real and effective user and group IDs
13676 @command{id} prints information about the given user, or the process
13677 running it if no user is specified. Synopsis:
13680 id [@var{option}]@dots{} [@var{username}]
13683 @vindex POSIXLY_CORRECT
13684 By default, it prints the real user ID, real group ID, effective user ID
13685 if different from the real user ID, effective group ID if different from
13686 the real group ID, and supplemental group IDs.
13687 In addition, if SELinux
13688 is enabled and the @env{POSIXLY_CORRECT} environment variable is not set,
13689 then print @samp{context=@var{c}}, where @var{c} is the security context.
13691 Each of these numeric values is preceded by an identifying string and
13692 followed by the corresponding user or group name in parentheses.
13694 The options cause @command{id} to print only part of the above information.
13695 Also see @ref{Common options}.
13702 Print only the group ID.
13708 Print only the group ID and the supplementary groups.
13714 Print the user or group name instead of the ID number. Requires
13715 @option{-u}, @option{-g}, or @option{-G}.
13721 Print the real, instead of effective, user or group ID. Requires
13722 @option{-u}, @option{-g}, or @option{-G}.
13728 Print only the user ID.
13735 @cindex security context
13736 Print only the security context of the current user.
13737 If SELinux is disabled then print a warning and
13738 set the exit status to 1.
13744 @macro primaryAndSupplementaryGroups{cmd,arg}
13745 Primary and supplementary groups for a process are normally inherited
13746 from its parent and are usually unchanged since login. This means
13747 that if you change the group database after logging in, @command{\cmd\}
13748 will not reflect your changes within your existing login session.
13749 Running @command{\cmd\} with a \arg\ causes the user and group
13750 database to be consulted afresh, and so will give a different result.
13752 @primaryAndSupplementaryGroups{id,user argument}
13754 @node logname invocation
13755 @section @command{logname}: Print current login name
13758 @cindex printing user's login name
13759 @cindex login name, printing
13760 @cindex user name, printing
13763 @command{logname} prints the calling user's name, as found in a
13764 system-maintained file (often @file{/var/run/utmp} or
13765 @file{/etc/utmp}), and exits with a status of 0. If there is no entry
13766 for the calling process, @command{logname} prints
13767 an error message and exits with a status of 1.
13769 The only options are @option{--help} and @option{--version}. @xref{Common
13775 @node whoami invocation
13776 @section @command{whoami}: Print effective user ID
13779 @cindex effective user ID, printing
13780 @cindex printing the effective user ID
13782 @command{whoami} prints the user name associated with the current
13783 effective user ID. It is equivalent to the command @samp{id -un}.
13785 The only options are @option{--help} and @option{--version}. @xref{Common
13791 @node groups invocation
13792 @section @command{groups}: Print group names a user is in
13795 @cindex printing groups a user is in
13796 @cindex supplementary groups, printing
13798 @command{groups} prints the names of the primary and any supplementary
13799 groups for each given @var{username}, or the current process if no names
13800 are given. If more than one name is given, the name of each user is
13802 the list of that user's groups and the user name is separated from the
13803 group list by a colon. Synopsis:
13806 groups [@var{username}]@dots{}
13809 The group lists are equivalent to the output of the command @samp{id -Gn}.
13811 @primaryAndSupplementaryGroups{groups,list of users}
13813 The only options are @option{--help} and @option{--version}. @xref{Common
13819 @node users invocation
13820 @section @command{users}: Print login names of users currently logged in
13823 @cindex printing current usernames
13824 @cindex usernames, printing current
13826 @cindex login sessions, printing users with
13827 @command{users} prints on a single line a blank-separated list of user
13828 names of users currently logged in to the current host. Each user name
13829 corresponds to a login session, so if a user has more than one login
13830 session, that user's name will appear the same number of times in the
13839 With no @var{file} argument, @command{users} extracts its information from
13840 a system-maintained file (often @file{/var/run/utmp} or
13841 @file{/etc/utmp}). If a file argument is given, @command{users} uses
13842 that file instead. A common choice is @file{/var/log/wtmp}.
13844 The only options are @option{--help} and @option{--version}. @xref{Common
13850 @node who invocation
13851 @section @command{who}: Print who is currently logged in
13854 @cindex printing current user information
13855 @cindex information, about current users
13857 @command{who} prints information about users who are currently logged on.
13861 @command{who} [@var{option}] [@var{file}] [am i]
13864 @cindex terminal lines, currently used
13866 @cindex remote hostname
13867 If given no non-option arguments, @command{who} prints the following
13868 information for each user currently logged on: login name, terminal
13869 line, login time, and remote hostname or X display.
13873 If given one non-option argument, @command{who} uses that instead of
13874 a default system-maintained file (often @file{/var/run/utmp} or
13875 @file{/etc/utmp}) as the name of the file containing the record of
13876 users logged on. @file{/var/log/wtmp} is commonly given as an argument
13877 to @command{who} to look at who has previously logged on.
13881 If given two non-option arguments, @command{who} prints only the entry
13882 for the user running it (determined from its standard input), preceded
13883 by the hostname. Traditionally, the two arguments given are @samp{am
13884 i}, as in @samp{who am i}.
13887 Time stamps are listed according to the time zone rules specified by
13888 the @env{TZ} environment variable, or by the system default rules if
13889 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
13890 with @env{TZ}, libc, The GNU C Library Reference Manual}.
13892 The program accepts the following options. Also see @ref{Common options}.
13900 Same as @samp{-b -d --login -p -r -t -T -u}.
13906 Print the date and time of last system boot.
13912 Print information corresponding to dead processes.
13918 Print a line of column headings.
13924 List only the entries that correspond to processes via which the
13925 system is waiting for a user to login. The user name is always @samp{LOGIN}.
13929 Attempt to canonicalize hostnames found in utmp through a DNS lookup. This
13930 is not the default because it can cause significant delays on systems with
13931 automatic dial-up internet access.
13935 Same as @samp{who am i}.
13941 List active processes spawned by init.
13947 Print only the login names and the number of users logged on.
13948 Overrides all other options.
13953 @opindex --runlevel
13954 Print the current (and maybe previous) run-level of the init process.
13958 Ignored; for compatibility with other versions of @command{who}.
13964 Print last system clock change.
13969 After the login time, print the number of hours and minutes that the
13970 user has been idle. @samp{.} means the user was active in the last minute.
13971 @samp{old} means the user has been idle for more than 24 hours.
13982 @opindex --writable
13983 @cindex message status
13984 @pindex write@r{, allowed}
13985 After each login name print a character indicating the user's message status:
13988 @samp{+} allowing @code{write} messages
13989 @samp{-} disallowing @code{write} messages
13990 @samp{?} cannot find terminal device
13998 @node System context
13999 @chapter System context
14001 @cindex system context
14002 @cindex context, system
14003 @cindex commands for system context
14005 This section describes commands that print or change system-wide
14009 * date invocation:: Print or set system date and time.
14010 * arch invocation:: Print machine hardware name.
14011 * nproc invocation:: Print the number of processors.
14012 * uname invocation:: Print system information.
14013 * hostname invocation:: Print or set system name.
14014 * hostid invocation:: Print numeric host identifier.
14015 * uptime invocation:: Print system uptime and load.
14018 @node date invocation
14019 @section @command{date}: Print or set system date and time
14022 @cindex time, printing or setting
14023 @cindex printing the current time
14028 date [@var{option}]@dots{} [+@var{format}]
14029 date [-u|--utc|--universal] @c this avoids a newline in the output
14030 [ MMDDhhmm[[CC]YY][.ss] ]
14034 Invoking @command{date} with no @var{format} argument is equivalent to invoking
14035 it with a default format that depends on the @env{LC_TIME} locale category.
14036 In the default C locale, this format is @samp{'+%a %b %e %H:%M:%S %Z %Y'},
14037 so the output looks like @samp{Thu Mar @ 3 13:47:51 PST 2005}.
14040 Normally, @command{date} uses the time zone rules indicated by the
14041 @env{TZ} environment variable, or the system default rules if @env{TZ}
14042 is not set. @xref{TZ Variable,, Specifying the Time Zone with
14043 @env{TZ}, libc, The GNU C Library Reference Manual}.
14045 @findex strftime @r{and @command{date}}
14046 @cindex time formats
14047 @cindex formatting times
14048 If given an argument that starts with a @samp{+}, @command{date} prints the
14049 current date and time (or the date and time specified by the
14050 @option{--date} option, see below) in the format defined by that argument,
14051 which is similar to that of the @code{strftime} function. Except for
14052 conversion specifiers, which start with @samp{%}, characters in the
14053 format string are printed unchanged. The conversion specifiers are
14059 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
14060 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
14061 * Literal conversion specifiers:: %[%nt]
14062 * Padding and other flags:: Pad with zeros, spaces, etc.
14063 * Setting the time:: Changing the system clock.
14064 * Options for date:: Instead of the current time.
14066 * Date input formats:: Specifying date strings.
14068 * Examples of date:: Examples.
14071 @node Time conversion specifiers
14072 @subsection Time conversion specifiers
14074 @cindex time conversion specifiers
14075 @cindex conversion specifiers, time
14077 @command{date} conversion specifiers related to times.
14081 hour (@samp{00}@dots{}@samp{23})
14083 hour (@samp{01}@dots{}@samp{12})
14085 hour, space padded (@samp{ 0}@dots{}@samp{23}); equivalent to @samp{%_H}.
14086 This is a @acronym{GNU} extension.
14088 hour, space padded (@samp{ 1}@dots{}@samp{12}); equivalent to @samp{%_I}.
14089 This is a @acronym{GNU} extension.
14091 minute (@samp{00}@dots{}@samp{59})
14093 nanoseconds (@samp{000000000}@dots{}@samp{999999999}).
14094 This is a @acronym{GNU} extension.
14096 locale's equivalent of either @samp{AM} or @samp{PM};
14097 blank in many locales.
14098 Noon is treated as @samp{PM} and midnight as @samp{AM}.
14100 like @samp{%p}, except lower case.
14101 This is a @acronym{GNU} extension.
14103 locale's 12-hour clock time (e.g., @samp{11:11:04 PM})
14105 24-hour hour and minute. Same as @samp{%H:%M}.
14107 @cindex epoch, seconds since
14108 @cindex seconds since the epoch
14109 @cindex beginning of time
14110 seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC.
14111 Leap seconds are not counted unless leap second support is available.
14112 @xref{%s-examples}, for examples.
14113 This is a @acronym{GNU} extension.
14115 second (@samp{00}@dots{}@samp{60}).
14116 This may be @samp{60} if leap seconds are supported.
14118 24-hour hour, minute, and second. Same as @samp{%H:%M:%S}.
14120 locale's time representation (e.g., @samp{23:13:48})
14122 @w{@acronym{RFC} 2822/@acronym{ISO} 8601} style numeric time zone
14123 (e.g., @samp{-0600} or @samp{+0530}), or nothing if no
14124 time zone is determinable. This value reflects the numeric time zone
14125 appropriate for the current time, using the time zone rules specified
14126 by the @env{TZ} environment variable.
14127 The time (and optionally, the time zone rules) can be overridden
14128 by the @option{--date} option.
14130 @w{@acronym{RFC} 3339/@acronym{ISO} 8601} style numeric time zone with
14131 @samp{:} (e.g., @samp{-06:00} or @samp{+05:30}), or nothing if no time
14132 zone is determinable.
14133 This is a @acronym{GNU} extension.
14135 Numeric time zone to the nearest second with @samp{:} (e.g.,
14136 @samp{-06:00:00} or @samp{+05:30:00}), or nothing if no time zone is
14138 This is a @acronym{GNU} extension.
14140 Numeric time zone with @samp{:} using the minimum necessary precision
14141 (e.g., @samp{-06}, @samp{+05:30}, or @samp{-04:56:02}), or nothing if
14142 no time zone is determinable.
14143 This is a @acronym{GNU} extension.
14145 alphabetic time zone abbreviation (e.g., @samp{EDT}), or nothing if no
14146 time zone is determinable. See @samp{%z} for how it is determined.
14150 @node Date conversion specifiers
14151 @subsection Date conversion specifiers
14153 @cindex date conversion specifiers
14154 @cindex conversion specifiers, date
14156 @command{date} conversion specifiers related to dates.
14160 locale's abbreviated weekday name (e.g., @samp{Sun})
14162 locale's full weekday name, variable length (e.g., @samp{Sunday})
14164 locale's abbreviated month name (e.g., @samp{Jan})
14166 locale's full month name, variable length (e.g., @samp{January})
14168 locale's date and time (e.g., @samp{Thu Mar @ 3 23:05:25 2005})
14170 century. This is like @samp{%Y}, except the last two digits are omitted.
14171 For example, it is @samp{20} if @samp{%Y} is @samp{2000},
14172 and is @samp{-0} if @samp{%Y} is @samp{-001}.
14173 It is normally at least two characters, but it may be more.
14175 day of month (e.g., @samp{01})
14177 date; same as @samp{%m/%d/%y}
14179 day of month, space padded; same as @samp{%_d}
14181 full date in @acronym{ISO} 8601 format; same as @samp{%Y-%m-%d}.
14182 This is a good choice for a date format, as it is standard and
14183 is easy to sort in the usual case where years are in the range
14186 year corresponding to the @acronym{ISO} week number, but without the century
14187 (range @samp{00} through @samp{99}). This has the same format and value
14188 as @samp{%y}, except that if the @acronym{ISO} week number (see
14190 to the previous or next year, that year is used instead.
14192 year corresponding to the @acronym{ISO} week number. This has the
14193 same format and value as @samp{%Y}, except that if the @acronym{ISO}
14195 @samp{%V}) belongs to the previous or next year, that year is used
14197 It is normally useful only if @samp{%V} is also used;
14198 for example, the format @samp{%G-%m-%d} is probably a mistake,
14199 since it combines the ISO week number year with the conventional month and day.
14203 day of year (@samp{001}@dots{}@samp{366})
14205 month (@samp{01}@dots{}@samp{12})
14207 day of week (@samp{1}@dots{}@samp{7}) with @samp{1} corresponding to Monday
14209 week number of year, with Sunday as the first day of the week
14210 (@samp{00}@dots{}@samp{53}).
14211 Days in a new year preceding the first Sunday are in week zero.
14213 @acronym{ISO} week number, that is, the
14214 week number of year, with Monday as the first day of the week
14215 (@samp{01}@dots{}@samp{53}).
14216 If the week containing January 1 has four or more days in
14217 the new year, then it is considered week 1; otherwise, it is week 53 of
14218 the previous year, and the next week is week 1. (See the @acronym{ISO} 8601
14221 day of week (@samp{0}@dots{}@samp{6}) with 0 corresponding to Sunday
14223 week number of year, with Monday as first day of week
14224 (@samp{00}@dots{}@samp{53}).
14225 Days in a new year preceding the first Monday are in week zero.
14227 locale's date representation (e.g., @samp{12/31/99})
14229 last two digits of year (@samp{00}@dots{}@samp{99})
14231 year. This is normally at least four characters, but it may be more.
14232 Year @samp{0000} precedes year @samp{0001}, and year @samp{-001}
14233 precedes year @samp{0000}.
14237 @node Literal conversion specifiers
14238 @subsection Literal conversion specifiers
14240 @cindex literal conversion specifiers
14241 @cindex conversion specifiers, literal
14243 @command{date} conversion specifiers that produce literal strings.
14255 @node Padding and other flags
14256 @subsection Padding and other flags
14258 @cindex numeric field padding
14259 @cindex padding of numeric fields
14260 @cindex fields, padding numeric
14262 Unless otherwise specified, @command{date} normally pads numeric fields
14263 with zeros, so that, for
14264 example, numeric months are always output as two digits.
14265 Seconds since the epoch are not padded, though,
14266 since there is no natural width for them.
14268 As a @acronym{GNU} extension, @command{date} recognizes any of the
14269 following optional flags after the @samp{%}:
14273 (hyphen) Do not pad the field; useful if the output is intended for
14276 (underscore) Pad with spaces; useful if you need a fixed
14277 number of characters in the output, but zeros are too distracting.
14279 (zero) Pad with zeros even if the conversion specifier
14280 would normally pad with spaces.
14282 Use upper case characters if possible.
14284 Use opposite case characters if possible.
14285 A field that is normally upper case becomes lower case, and vice versa.
14289 Here are some examples of padding:
14292 date +%d/%m -d "Feb 1"
14294 date +%-d/%-m -d "Feb 1"
14296 date +%_d/%_m -d "Feb 1"
14300 As a @acronym{GNU} extension, you can specify the field width
14301 (after any flag, if present) as a decimal number. If the natural size of the
14302 output of the field has less than the specified number of characters,
14303 the result is written right adjusted and padded to the given
14304 size. For example, @samp{%9B} prints the right adjusted month name in
14305 a field of width 9.
14307 An optional modifier can follow the optional flag and width
14308 specification. The modifiers are:
14312 Use the locale's alternate representation for date and time. This
14313 modifier applies to the @samp{%c}, @samp{%C}, @samp{%x}, @samp{%X},
14314 @samp{%y} and @samp{%Y} conversion specifiers. In a Japanese locale, for
14315 example, @samp{%Ex} might yield a date format based on the Japanese
14319 Use the locale's alternate numeric symbols for numbers. This modifier
14320 applies only to numeric conversion specifiers.
14323 If the format supports the modifier but no alternate representation
14324 is available, it is ignored.
14327 @node Setting the time
14328 @subsection Setting the time
14330 @cindex setting the time
14331 @cindex time setting
14332 @cindex appropriate privileges
14334 If given an argument that does not start with @samp{+}, @command{date} sets
14335 the system clock to the date and time specified by that argument (as
14336 described below). You must have appropriate privileges to set the
14337 system clock. Note for changes to persist across a reboot, the
14338 hardware clock may need to be updated from the system clock, which
14339 might not happen automatically on your system.
14341 The argument must consist entirely of digits, which have the following
14354 first two digits of year (optional)
14356 last two digits of year (optional)
14361 Note, the @option{--date} and @option{--set} options may not be used with an
14362 argument in the above format. The @option{--universal} option may be used
14363 with such an argument to indicate that the specified date and time are
14364 relative to Coordinated Universal Time rather than to the local time zone.
14367 @node Options for date
14368 @subsection Options for @command{date}
14370 @cindex @command{date} options
14371 @cindex options for @command{date}
14373 The program accepts the following options. Also see @ref{Common options}.
14377 @item -d @var{datestr}
14378 @itemx --date=@var{datestr}
14381 @cindex parsing date strings
14382 @cindex date strings, parsing
14383 @cindex arbitrary date strings, parsing
14386 @opindex next @var{day}
14387 @opindex last @var{day}
14388 Display the date and time specified in @var{datestr} instead of the
14389 current date and time. @var{datestr} can be in almost any common
14390 format. It can contain month names, time zones, @samp{am} and @samp{pm},
14391 @samp{yesterday}, etc. For example, @option{--date="2004-02-27
14392 14:19:13.489392193 +0530"} specifies the instant of time that is
14393 489,392,193 nanoseconds after February 27, 2004 at 2:19:13 PM in a
14394 time zone that is 5 hours and 30 minutes east of @acronym{UTC}.@*
14395 Note: input currently must be in locale independent format. E.g., the
14396 LC_TIME=C below is needed to print back the correct date in many locales:
14398 date -d "$(LC_TIME=C date)"
14400 @xref{Date input formats}.
14402 @item -f @var{datefile}
14403 @itemx --file=@var{datefile}
14406 Parse each line in @var{datefile} as with @option{-d} and display the
14407 resulting date and time. If @var{datefile} is @samp{-}, use standard
14408 input. This is useful when you have many dates to process, because the
14409 system overhead of starting up the @command{date} executable many times can
14412 @item -I[@var{timespec}]
14413 @itemx --iso-8601[=@var{timespec}]
14414 @opindex -I[@var{timespec}]
14415 @opindex --iso-8601[=@var{timespec}]
14416 Display the date using the @acronym{ISO} 8601 format, @samp{%Y-%m-%d}.
14418 The argument @var{timespec} specifies the number of additional
14419 terms of the time to include. It can be one of the following:
14422 Print just the date. This is the default if @var{timespec} is omitted.
14425 Append the hour of the day to the date.
14428 Append the hours and minutes.
14431 Append the hours, minutes and seconds.
14434 Append the hours, minutes, seconds and nanoseconds.
14437 If showing any time terms, then include the time zone using the format
14440 @item -r @var{file}
14441 @itemx --reference=@var{file}
14443 @opindex --reference
14444 Display the date and time of the last modification of @var{file},
14445 instead of the current date and time.
14452 @opindex --rfc-2822
14453 Display the date and time using the format @samp{%a, %d %b %Y %H:%M:%S
14454 %z}, evaluated in the C locale so abbreviations are always in English.
14458 Fri, 09 Sep 2005 13:51:39 -0700
14461 This format conforms to
14462 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt, Internet
14463 @acronym{RFCs} 2822} and
14464 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc822.txt, 822}, the
14465 current and previous standards for Internet email.
14467 @item --rfc-3339=@var{timespec}
14468 @opindex --rfc-3339=@var{timespec}
14469 Display the date using a format specified by
14470 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt, Internet
14471 @acronym{RFC} 3339}. This is a subset of the @acronym{ISO} 8601
14472 format, except that it also permits applications to use a space rather
14473 than a @samp{T} to separate dates from times. Unlike the other
14474 standard formats, @acronym{RFC} 3339 format is always suitable as
14475 input for the @option{--date} (@option{-d}) and @option{--file}
14476 (@option{-f}) options, regardless of the current locale.
14478 The argument @var{timespec} specifies how much of the time to include.
14479 It can be one of the following:
14483 Print just the full-date, e.g., @samp{2005-09-14}.
14484 This is equivalent to the format @samp{%Y-%m-%d}.
14487 Print the full-date and full-time separated by a space, e.g.,
14488 @samp{2005-09-14 00:56:06+05:30}. The output ends with a numeric
14489 time-offset; here the @samp{+05:30} means that local time is five
14490 hours and thirty minutes east of @acronym{UTC}. This is equivalent to
14491 the format @samp{%Y-%m-%d %H:%M:%S%:z}.
14494 Like @samp{seconds}, but also print nanoseconds, e.g.,
14495 @samp{2005-09-14 00:56:06.998458565+05:30}.
14496 This is equivalent to the format @samp{%Y-%m-%d %H:%M:%S.%N%:z}.
14500 @item -s @var{datestr}
14501 @itemx --set=@var{datestr}
14504 Set the date and time to @var{datestr}. See @option{-d} above.
14505 See also @ref{Setting the time}.
14512 @opindex --universal
14513 @cindex Coordinated Universal Time
14515 @cindex Greenwich Mean Time
14518 Use Coordinated Universal Time (@acronym{UTC}) by operating as if the
14519 @env{TZ} environment variable were set to the string @samp{UTC0}.
14521 Universal Time is often called ``Greenwich Mean Time'' (@sc{gmt}) for
14522 historical reasons.
14526 @node Examples of date
14527 @subsection Examples of @command{date}
14529 @cindex examples of @command{date}
14531 Here are a few examples. Also see the documentation for the @option{-d}
14532 option in the previous section.
14537 To print the date of the day before yesterday:
14540 date --date='2 days ago'
14544 To print the date of the day three months and one day hence:
14547 date --date='3 months 1 day'
14551 To print the day of year of Christmas in the current year:
14554 date --date='25 Dec' +%j
14558 To print the current full month name and the day of the month:
14564 But this may not be what you want because for the first nine days of
14565 the month, the @samp{%d} expands to a zero-padded two-digit field,
14566 for example @samp{date -d 1may '+%B %d'} will print @samp{May 01}.
14569 To print a date without the leading zero for one-digit days
14570 of the month, you can use the (@acronym{GNU} extension)
14571 @samp{-} flag to suppress
14572 the padding altogether:
14575 date -d 1may '+%B %-d
14579 To print the current date and time in the format required by many
14580 non-@acronym{GNU} versions of @command{date} when setting the system clock:
14583 date +%m%d%H%M%Y.%S
14587 To set the system clock forward by two minutes:
14590 date --set='+2 minutes'
14594 To print the date in @acronym{RFC} 2822 format,
14595 use @samp{date --rfc-2822}. Here is some example output:
14598 Fri, 09 Sep 2005 13:51:39 -0700
14601 @anchor{%s-examples}
14603 To convert a date string to the number of seconds since the epoch
14604 (which is 1970-01-01 00:00:00 UTC), use the @option{--date} option with
14605 the @samp{%s} format. That can be useful in sorting and/or graphing
14606 and/or comparing data by date. The following command outputs the
14607 number of the seconds since the epoch for the time two minutes after the
14611 date --date='1970-01-01 00:02:00 +0000' +%s
14615 If you do not specify time zone information in the date string,
14616 @command{date} uses your computer's idea of the time zone when
14617 interpreting the string. For example, if your computer's time zone is
14618 that of Cambridge, Massachusetts, which was then 5 hours (i.e., 18,000
14619 seconds) behind UTC:
14622 # local time zone used
14623 date --date='1970-01-01 00:02:00' +%s
14628 If you're sorting or graphing dated data, your raw date values may be
14629 represented as seconds since the epoch. But few people can look at
14630 the date @samp{946684800} and casually note ``Oh, that's the first second
14631 of the year 2000 in Greenwich, England.''
14634 date --date='2000-01-01 UTC' +%s
14638 An alternative is to use the @option{--utc} (@option{-u}) option.
14639 Then you may omit @samp{UTC} from the date string. Although this
14640 produces the same result for @samp{%s} and many other format sequences,
14641 with a time zone offset different from zero, it would give a different
14642 result for zone-dependent formats like @samp{%z}.
14645 date -u --date=2000-01-01 +%s
14649 To convert such an unwieldy number of seconds back to
14650 a more readable form, use a command like this:
14653 # local time zone used
14654 date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
14655 1999-12-31 19:00:00 -0500
14658 Or if you do not mind depending on the @samp{@@} feature present since
14659 coreutils 5.3.0, you could shorten this to:
14662 date -d @@946684800 +"%F %T %z"
14663 1999-12-31 19:00:00 -0500
14666 Often it is better to output UTC-relative date and time:
14669 date -u -d '1970-01-01 946684800 seconds' +"%Y-%m-%d %T %z"
14670 2000-01-01 00:00:00 +0000
14676 @node arch invocation
14677 @section @command{arch}: Print machine hardware name
14680 @cindex print machine hardware name
14681 @cindex system information, printing
14683 @command{arch} prints the machine hardware name,
14684 and is equivalent to @samp{uname -m}.
14688 arch [@var{option}]
14691 The program accepts the @ref{Common options} only.
14696 @node nproc invocation
14697 @section @command{nproc}: Print the number of available processors
14700 @cindex Print the number of processors
14701 @cindex system information, printing
14703 Print the number of processing units available to the current process,
14704 which may be less than the number of online processors.
14705 If this information is not accessible, then print the number of
14706 processors installed. If the @env{OMP_NUM_THREADS} environment variable is
14707 set, then it will determine the returned value. The result is guaranteed to be
14708 greater than zero. Synopsis:
14711 nproc [@var{option}]
14714 The program accepts the following options. Also see @ref{Common options}.
14720 Print the number of installed processors on the system, which may
14721 be greater than the number online or available to the current process.
14722 The @env{OMP_NUM_THREADS} environment variable is not honored in this case.
14724 @item --ignore=@var{number}
14726 If possible, exclude this @var{number} of processing units.
14733 @node uname invocation
14734 @section @command{uname}: Print system information
14737 @cindex print system information
14738 @cindex system information, printing
14740 @command{uname} prints information about the machine and operating system
14741 it is run on. If no options are given, @command{uname} acts as if the
14742 @option{-s} option were given. Synopsis:
14745 uname [@var{option}]@dots{}
14748 If multiple options or @option{-a} are given, the selected information is
14749 printed in this order:
14752 @var{kernel-name} @var{nodename} @var{kernel-release} @var{kernel-version}
14753 @var{machine} @var{processor} @var{hardware-platform} @var{operating-system}
14756 The information may contain internal spaces, so such output cannot be
14757 parsed reliably. In the following example, @var{release} is
14758 @samp{2.2.18ss.e820-bda652a #4 SMP Tue Jun 5 11:24:08 PDT 2001}:
14762 @result{} Linux dum 2.2.18 #4 SMP Tue Jun 5 11:24:08 PDT 2001 i686@c
14763 unknown unknown GNU/Linux
14767 The program accepts the following options. Also see @ref{Common options}.
14775 Print all of the below information, except omit the processor type
14776 and the hardware platform name if they are unknown.
14779 @itemx --hardware-platform
14781 @opindex --hardware-platform
14782 @cindex implementation, hardware
14783 @cindex hardware platform
14784 @cindex platform, hardware
14785 Print the hardware platform name
14786 (sometimes called the hardware implementation).
14787 Print @samp{unknown} if the kernel does not make this information
14788 easily available, as is the case with Linux kernels.
14794 @cindex machine type
14795 @cindex hardware class
14796 @cindex hardware type
14797 Print the machine hardware name (sometimes called the hardware class
14803 @opindex --nodename
14806 @cindex network node name
14807 Print the network node hostname.
14812 @opindex --processor
14813 @cindex host processor type
14814 Print the processor type (sometimes called the instruction set
14815 architecture or ISA).
14816 Print @samp{unknown} if the kernel does not make this information
14817 easily available, as is the case with Linux kernels.
14820 @itemx --operating-system
14822 @opindex --operating-system
14823 @cindex operating system name
14824 Print the name of the operating system.
14827 @itemx --kernel-release
14829 @opindex --kernel-release
14830 @cindex kernel release
14831 @cindex release of kernel
14832 Print the kernel release.
14835 @itemx --kernel-name
14837 @opindex --kernel-name
14838 @cindex kernel name
14839 @cindex name of kernel
14840 Print the kernel name.
14841 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) calls this
14842 ``the implementation of the operating system'', because the
14843 @acronym{POSIX} specification itself has no notion of ``kernel''.
14844 The kernel name might be the same as the operating system name printed
14845 by the @option{-o} or @option{--operating-system} option, but it might
14846 differ. Some operating systems (e.g., FreeBSD, HP-UX) have the same
14847 name as their underlying kernels; others (e.g., GNU/Linux, Solaris)
14851 @itemx --kernel-version
14853 @opindex --kernel-version
14854 @cindex kernel version
14855 @cindex version of kernel
14856 Print the kernel version.
14863 @node hostname invocation
14864 @section @command{hostname}: Print or set system name
14867 @cindex setting the hostname
14868 @cindex printing the hostname
14869 @cindex system name, printing
14870 @cindex appropriate privileges
14872 With no arguments, @command{hostname} prints the name of the current host
14873 system. With one argument, it sets the current host name to the
14874 specified string. You must have appropriate privileges to set the host
14878 hostname [@var{name}]
14881 The only options are @option{--help} and @option{--version}. @xref{Common
14887 @node hostid invocation
14888 @section @command{hostid}: Print numeric host identifier
14891 @cindex printing the host identifier
14893 @command{hostid} prints the numeric identifier of the current host
14894 in hexadecimal. This command accepts no arguments.
14895 The only options are @option{--help} and @option{--version}.
14896 @xref{Common options}.
14898 For example, here's what it prints on one system I use:
14905 On that system, the 32-bit quantity happens to be closely
14906 related to the system's Internet address, but that isn't always
14911 @node uptime invocation
14912 @section @command{uptime}: Print system uptime and load
14915 @cindex printing the system uptime and load
14917 @command{uptime} prints the current time, the system's uptime, the
14918 number of logged-in users and the current load average.
14920 If an argument is specified, it is used as the file to be read
14921 to discover how many users are logged in. If no argument is
14922 specified, a system default is used (@command{uptime --help} indicates
14923 the default setting).
14925 The only options are @option{--help} and @option{--version}.
14926 @xref{Common options}.
14928 For example, here's what it prints right now on one system I use:
14932 14:07 up 3:35, 3 users, load average: 1.39, 1.15, 1.04
14935 The precise method of calculation of load average varies somewhat
14936 between systems. Some systems calculate it as the average number of
14937 runnable processes over the last 1, 5 and 15 minutes, but some systems
14938 also include processes in the uninterruptible sleep state (that is,
14939 those processes which are waiting for disk I/O). The Linux kernel
14940 includes uninterruptible processes.
14942 @node SELinux context
14943 @chapter SELinux context
14945 @cindex SELinux context
14946 @cindex SELinux, context
14947 @cindex commands for SELinux context
14949 This section describes commands for operations with SELinux
14953 * chcon invocation:: Change SELinux context of file
14954 * runcon invocation:: Run a command in specified SELinux context
14957 @node chcon invocation
14958 @section @command{chcon}: Change SELinux context of file
14961 @cindex changing security context
14962 @cindex change SELinux context
14964 @command{chcon} changes the SELinux security context of the selected files.
14968 chcon [@var{option}]@dots{} @var{context} @var{file}@dots{}
14969 chcon [@var{option}]@dots{} [-u @var{user}] [-r @var{role}] [-l @var{range}]@c
14970 [-t @var{type}] @var{file}@dots{}
14971 chcon [@var{option}]@dots{} --reference=@var{rfile} @var{file}@dots{}
14974 Change the SELinux security context of each @var{file} to @var{context}.
14975 With @option{--reference}, change the security context of each @var{file}
14976 to that of @var{rfile}.
14978 The program accepts the following options. Also see @ref{Common options}.
14983 @itemx --no-dereference
14985 @opindex --no-dereference
14986 @cindex no dereference
14987 Affect symbolic links instead of any referenced file.
14989 @item --reference=@var{rfile}
14990 @opindex --reference
14991 @cindex reference file
14992 Use @var{rfile}'s security context rather than specifying a @var{context} value.
14997 @opindex --recursive
14998 Operate on files and directories recursively.
15001 @xref{Traversing symlinks}.
15004 @xref{Traversing symlinks}.
15007 @xref{Traversing symlinks}.
15014 Output a diagnostic for every file processed.
15016 @item -u @var{user}
15017 @itemx --user=@var{user}
15020 Set user @var{user} in the target security context.
15022 @item -r @var{role}
15023 @itemx --role=@var{role}
15026 Set role @var{role} in the target security context.
15028 @item -t @var{type}
15029 @itemx --type=@var{type}
15032 Set type @var{type} in the target security context.
15034 @item -l @var{range}
15035 @itemx --range=@var{range}
15038 Set range @var{range} in the target security context.
15044 @node runcon invocation
15045 @section @command{runcon}: Run a command in specified SELinux context
15048 @cindex run with security context
15051 @command{runcon} runs file in specified SELinux security context.
15055 runcon @var{context} @var{command} [@var{args}]
15056 runcon [ -c ] [-u @var{user}] [-r @var{role}] [-t @var{type}]@c
15057 [-l @var{range}] @var{command} [@var{args}]
15060 Run @var{command} with completely-specified @var{context}, or with
15061 current or transitioned security context modified by one or more of @var{level},
15062 @var{role}, @var{type} and @var{user}.
15064 If none of @option{-c}, @option{-t}, @option{-u}, @option{-r}, or @option{-l}
15065 is specified, the first argument is used as the complete context.
15066 Any additional arguments after @var{command}
15067 are interpreted as arguments to the command.
15069 With neither @var{context} nor @var{command}, print the current
15072 The program accepts the following options. Also see @ref{Common options}.
15080 Compute process transition context before modifying.
15082 @item -u @var{user}
15083 @itemx --user=@var{user}
15086 Set user @var{user} in the target security context.
15088 @item -r @var{role}
15089 @itemx --role=@var{role}
15092 Set role @var{role} in the target security context.
15094 @item -t @var{type}
15095 @itemx --type=@var{type}
15098 Set type @var{type} in the target security context.
15100 @item -l @var{range}
15101 @itemx --range=@var{range}
15104 Set range @var{range} in the target security context.
15108 @cindex exit status of @command{runcon}
15112 126 if @var{command} is found but cannot be invoked
15113 127 if @command{runcon} itself fails or if @var{command} cannot be found
15114 the exit status of @var{command} otherwise
15117 @node Modified command invocation
15118 @chapter Modified command invocation
15120 @cindex modified command invocation
15121 @cindex invocation of commands, modified
15122 @cindex commands for invoking other commands
15124 This section describes commands that run other commands in some context
15125 different than the current one: a modified environment, as a different
15129 * chroot invocation:: Modify the root directory.
15130 * env invocation:: Modify environment variables.
15131 * nice invocation:: Modify niceness.
15132 * nohup invocation:: Immunize to hangups.
15133 * stdbuf invocation:: Modify buffering of standard streams.
15134 * su invocation:: Modify user and group ID.
15135 * timeout invocation:: Run with time limit.
15139 @node chroot invocation
15140 @section @command{chroot}: Run a command with a different root directory
15143 @cindex running a program in a specified root directory
15144 @cindex root directory, running a program in a specified
15146 @command{chroot} runs a command with a specified root directory.
15147 On many systems, only the super-user can do this.@footnote{However,
15148 some systems (e.g., FreeBSD) can be configured to allow certain regular
15149 users to use the @code{chroot} system call, and hence to run this program.
15150 Also, on Cygwin, anyone can run the @command{chroot} command, because the
15151 underlying function is non-privileged due to lack of support in MS-Windows.}
15155 chroot @var{option} @var{newroot} [@var{command} [@var{args}]@dots{}]
15156 chroot @var{option}
15159 Ordinarily, file names are looked up starting at the root of the
15160 directory structure, i.e., @file{/}. @command{chroot} changes the root to
15161 the directory @var{newroot} (which must exist) and then runs
15162 @var{command} with optional @var{args}. If @var{command} is not
15163 specified, the default is the value of the @env{SHELL} environment
15164 variable or @command{/bin/sh} if not set, invoked with the @option{-i} option.
15165 @var{command} must not be a special built-in utility
15166 (@pxref{Special built-in utilities}).
15168 The program accepts the following options. Also see @ref{Common options}.
15169 Options must precede operands.
15173 @itemx --userspec=@var{user}[:@var{group}]
15174 @opindex --userspec
15175 By default, @var{command} is run with the same credentials
15176 as the invoking process.
15177 Use this option to run it as a different @var{user} and/or with a
15178 different primary @var{group}.
15180 @itemx --groups=@var{groups}
15182 Use this option to specify the supplementary @var{groups} to be
15183 used by the new process.
15184 The items in the list (names or numeric IDs) must be separated by commas.
15188 Here are a few tips to help avoid common problems in using chroot.
15189 To start with a simple example, make @var{command} refer to a statically
15190 linked binary. If you were to use a dynamically linked executable, then
15191 you'd have to arrange to have the shared libraries in the right place under
15192 your new root directory.
15194 For example, if you create a statically linked @command{ls} executable,
15195 and put it in @file{/tmp/empty}, you can run this command as root:
15198 $ chroot /tmp/empty /ls -Rl /
15201 Then you'll see output like this:
15206 -rwxr-xr-x 1 0 0 1041745 Aug 16 11:17 ls
15209 If you want to use a dynamically linked executable, say @command{bash},
15210 then first run @samp{ldd bash} to see what shared objects it needs.
15211 Then, in addition to copying the actual binary, also copy the listed
15212 files to the required positions under your intended new root directory.
15213 Finally, if the executable requires any other files (e.g., data, state,
15214 device files), copy them into place, too.
15216 @cindex exit status of @command{chroot}
15220 125 if @command{chroot} itself fails
15221 126 if @var{command} is found but cannot be invoked
15222 127 if @var{command} cannot be found
15223 the exit status of @var{command} otherwise
15227 @node env invocation
15228 @section @command{env}: Run a command in a modified environment
15231 @cindex environment, running a program in a modified
15232 @cindex modified environment, running a program in a
15233 @cindex running a program in a modified environment
15235 @command{env} runs a command with a modified environment. Synopses:
15238 env [@var{option}]@dots{} [@var{name}=@var{value}]@dots{} @c
15239 [@var{command} [@var{args}]@dots{}]
15243 Operands of the form @samp{@var{variable}=@var{value}} set
15244 the environment variable @var{variable} to value @var{value}.
15245 @var{value} may be empty (@samp{@var{variable}=}). Setting a variable
15246 to an empty value is different from unsetting it.
15247 These operands are evaluated left-to-right, so if two operands
15248 mention the same variable the earlier is ignored.
15250 Environment variable names can be empty, and can contain any
15251 characters other than @samp{=} and @acronym{ASCII} @sc{nul}.
15252 However, it is wise to limit yourself to names that
15253 consist solely of underscores, digits, and @acronym{ASCII} letters,
15254 and that begin with a non-digit, as applications like the shell do not
15255 work well with other names.
15258 The first operand that does not contain the character @samp{=}
15259 specifies the program to invoke; it is
15260 searched for according to the @env{PATH} environment variable. Any
15261 remaining arguments are passed as arguments to that program.
15262 The program should not be a special built-in utility
15263 (@pxref{Special built-in utilities}).
15265 Modifications to @env{PATH} take effect prior to searching for
15266 @var{command}. Use caution when reducing @env{PATH}; behavior is
15267 not portable when @env{PATH} is undefined or omits key directories
15268 such as @file{/bin}.
15270 In the rare case that a utility contains a @samp{=} in the name, the
15271 only way to disambiguate it from a variable assignment is to use an
15272 intermediate command for @var{command}, and pass the problematic
15273 program name via @var{args}. For example, if @file{./prog=} is an
15274 executable in the current @env{PATH}:
15277 env prog= true # runs 'true', with prog= in environment
15278 env ./prog= true # runs 'true', with ./prog= in environment
15279 env -- prog= true # runs 'true', with prog= in environment
15280 env sh -c '\prog= true' # runs 'prog=' with argument 'true'
15281 env sh -c 'exec "$@@"' sh prog= true # also runs 'prog='
15284 @cindex environment, printing
15286 If no command name is specified following the environment
15287 specifications, the resulting environment is printed. This is like
15288 specifying the @command{printenv} program.
15290 For some examples, suppose the environment passed to @command{env}
15291 contains @samp{LOGNAME=rms}, @samp{EDITOR=emacs}, and
15292 @samp{PATH=.:/gnubin:/hacks}:
15297 Output the current environment.
15299 $ env | LC_ALL=C sort
15302 PATH=.:/gnubin:/hacks
15306 Run @command{foo} with a reduced environment, preserving only the
15307 original @env{PATH} to avoid problems in locating @command{foo}.
15309 env - PATH="$PATH" foo
15313 Run @command{foo} with the environment containing @samp{LOGNAME=rms},
15314 @samp{EDITOR=emacs}, and @samp{PATH=.:/gnubin:/hacks}, and guarantees
15315 that @command{foo} was found in the file system rather than as a shell
15322 Run @command{nemacs} with the environment containing @samp{LOGNAME=foo},
15323 @samp{EDITOR=emacs}, @samp{PATH=.:/gnubin:/hacks}, and
15324 @samp{DISPLAY=gnu:0}.
15326 env DISPLAY=gnu:0 LOGNAME=foo nemacs
15330 Attempt to run the program @command{/energy/--} (as that is the only
15331 possible path search result); if the command exists, the environment
15332 will contain @samp{LOGNAME=rms} and @samp{PATH=/energy}, and the
15333 arguments will be @samp{e=mc2}, @samp{bar}, and @samp{baz}.
15335 env -u EDITOR PATH=/energy -- e=mc2 bar baz
15341 The program accepts the following options. Also see @ref{Common options}.
15342 Options must precede operands.
15348 @item -u @var{name}
15349 @itemx --unset=@var{name}
15352 Remove variable @var{name} from the environment, if it was in the
15357 @itemx --ignore-environment
15360 @opindex --ignore-environment
15361 Start with an empty environment, ignoring the inherited environment.
15365 @cindex exit status of @command{env}
15369 0 if no @var{command} is specified and the environment is output
15370 125 if @command{env} itself fails
15371 126 if @var{command} is found but cannot be invoked
15372 127 if @var{command} cannot be found
15373 the exit status of @var{command} otherwise
15377 @node nice invocation
15378 @section @command{nice}: Run a command with modified niceness
15382 @cindex scheduling, affecting
15383 @cindex appropriate privileges
15385 @command{nice} prints or modifies a process's @dfn{niceness},
15386 a parameter that affects whether the process is scheduled favorably.
15390 nice [@var{option}]@dots{} [@var{command} [@var{arg}]@dots{}]
15393 If no arguments are given, @command{nice} prints the current niceness.
15394 Otherwise, @command{nice} runs the given @var{command} with its
15395 niceness adjusted. By default, its niceness is incremented by 10.
15397 Niceness values range at least from @minus{}20 (process has high priority
15398 and gets more resources, thus slowing down other processes) through 19
15399 (process has lower priority and runs slowly itself, but has less impact
15400 on the speed of other running processes). Some systems
15401 may have a wider range of nicenesses; conversely, other systems may
15402 enforce more restrictive limits. An attempt to set the niceness
15403 outside the supported range is treated as an attempt to use the
15404 minimum or maximum supported value.
15406 A niceness should not be confused with a scheduling priority, which
15407 lets applications determine the order in which threads are scheduled
15408 to run. Unlike a priority, a niceness is merely advice to the
15409 scheduler, which the scheduler is free to ignore. Also, as a point of
15410 terminology, @acronym{POSIX} defines the behavior of @command{nice} in
15411 terms of a @dfn{nice value}, which is the nonnegative difference
15412 between a niceness and the minimum niceness. Though @command{nice}
15413 conforms to @acronym{POSIX}, its documentation and diagnostics use the
15414 term ``niceness'' for compatibility with historical practice.
15416 @var{command} must not be a special built-in utility (@pxref{Special
15417 built-in utilities}).
15419 @mayConflictWithShellBuiltIn{nice}
15421 The program accepts the following option. Also see @ref{Common options}.
15422 Options must precede operands.
15425 @item -n @var{adjustment}
15426 @itemx --adjustment=@var{adjustment}
15428 @opindex --adjustment
15429 Add @var{adjustment} instead of 10 to the command's niceness. If
15430 @var{adjustment} is negative and you lack appropriate privileges,
15431 @command{nice} issues a warning but otherwise acts as if you specified
15434 For compatibility @command{nice} also supports an obsolete
15435 option syntax @option{-@var{adjustment}}. New scripts should use
15436 @option{-n @var{adjustment}} instead.
15440 @cindex exit status of @command{nice}
15444 0 if no @var{command} is specified and the niceness is output
15445 125 if @command{nice} itself fails
15446 126 if @var{command} is found but cannot be invoked
15447 127 if @var{command} cannot be found
15448 the exit status of @var{command} otherwise
15451 It is sometimes useful to run a non-interactive program with reduced niceness.
15454 $ nice factor 4611686018427387903
15457 Since @command{nice} prints the current niceness,
15458 you can invoke it through itself to demonstrate how it works.
15460 The default behavior is to increase the niceness by @samp{10}:
15471 The @var{adjustment} is relative to the current niceness. In the
15472 next example, the first @command{nice} invocation runs the second one
15473 with niceness 10, and it in turn runs the final one with a niceness
15477 $ nice nice -n 3 nice
15481 Specifying a niceness larger than the supported range
15482 is the same as specifying the maximum supported value:
15485 $ nice -n 10000000000 nice
15489 Only a privileged user may run a process with lower niceness:
15493 nice: cannot set niceness: Permission denied
15495 $ sudo nice -n -1 nice
15500 @node nohup invocation
15501 @section @command{nohup}: Run a command immune to hangups
15504 @cindex hangups, immunity to
15505 @cindex immunity to hangups
15506 @cindex logging out and continuing to run
15509 @command{nohup} runs the given @var{command} with hangup signals ignored,
15510 so that the command can continue running in the background after you log
15514 nohup @var{command} [@var{arg}]@dots{}
15517 If standard input is a terminal, it is redirected from
15518 @file{/dev/null} so that terminal sessions do not mistakenly consider
15519 the terminal to be used by the command. This is a @acronym{GNU}
15520 extension; programs intended to be portable to non-@acronym{GNU} hosts
15521 should use @samp{nohup @var{command} [@var{arg}]@dots{} </dev/null}
15525 If standard output is a terminal, the command's standard output is appended
15526 to the file @file{nohup.out}; if that cannot be written to, it is appended
15527 to the file @file{$HOME/nohup.out}; and if that cannot be written to, the
15528 command is not run.
15529 Any @file{nohup.out} or @file{$HOME/nohup.out} file created by
15530 @command{nohup} is made readable and writable only to the user,
15531 regardless of the current umask settings.
15533 If standard error is a terminal, it is normally redirected to the same file
15534 descriptor as the (possibly-redirected) standard output.
15535 However, if standard output is closed, standard error terminal output
15536 is instead appended to the file @file{nohup.out} or
15537 @file{$HOME/nohup.out} as above.
15539 To capture the command's output to a file other than @file{nohup.out}
15540 you can redirect it. For example, to capture the output of
15544 nohup make > make.log
15547 @command{nohup} does not automatically put the command it runs in the
15548 background; you must do that explicitly, by ending the command line
15549 with an @samp{&}. Also, @command{nohup} does not alter the
15550 niceness of @var{command}; use @command{nice} for that,
15551 e.g., @samp{nohup nice @var{command}}.
15553 @var{command} must not be a special built-in utility (@pxref{Special
15554 built-in utilities}).
15556 The only options are @option{--help} and @option{--version}. @xref{Common
15557 options}. Options must precede operands.
15559 @cindex exit status of @command{nohup}
15563 125 if @command{nohup} itself fails, and @env{POSIXLY_CORRECT} is not set
15564 126 if @var{command} is found but cannot be invoked
15565 127 if @var{command} cannot be found
15566 the exit status of @var{command} otherwise
15569 If @env{POSIXLY_CORRECT} is set, internal failures give status 127
15573 @node stdbuf invocation
15574 @section @command{stdbuf}: Run a command with modified I/O stream buffering
15577 @cindex standard streams, buffering
15578 @cindex line buffered
15580 @command{stdbuf} allows one to modify the buffering operations of the
15581 three standard I/O streams associated with a program. Synopsis:
15584 stdbuf @var{option}@dots{} @var{command}
15587 @var{command} must start with the name of a program that
15590 uses the ISO C @code{FILE} streams for input/output (note the
15591 programs @command{dd} and @command{cat} don't do that),
15594 does not adjust the buffering of its standard streams (note the
15595 program @command{tee} is not in this category).
15598 Any additional @var{arg}s are passed as additional arguments to the
15601 The program accepts the following options. Also see @ref{Common options}.
15605 @item -i @var{mode}
15606 @itemx --input=@var{mode}
15609 Adjust the standard input stream buffering.
15611 @item -o @var{mode}
15612 @itemx --output=@var{mode}
15615 Adjust the standard output stream buffering.
15617 @item -e @var{mode}
15618 @itemx --error=@var{mode}
15621 Adjust the standard error stream buffering.
15625 The @var{mode} can be specified as follows:
15630 Set the stream to line buffered mode.
15631 In this mode data is coalesced until a newline is output or
15632 input is read from any stream attached to a terminal device.
15633 This option is invalid with standard input.
15636 Disable buffering of the selected stream.
15637 In this mode, data is output immediately and only the
15638 amount of data requested is read from input.
15639 Note the difference in function for input and output.
15640 Disabling buffering for input will not influence the responsiveness
15641 or blocking behavior of the stream input functions.
15642 For example @code{fread} will still block until @code{EOF} or error,
15643 even if the underlying @code{read} returns less data than requested.
15646 Specify the size of the buffer to use in fully buffered mode.
15647 @multiplierSuffixesNoBlocks{size}
15651 @cindex exit status of @command{stdbuf}
15655 125 if @command{stdbuf} itself fails
15656 126 if @var{command} is found but cannot be invoked
15657 127 if @var{command} cannot be found
15658 the exit status of @var{command} otherwise
15662 @node su invocation
15663 @section @command{su}: Run a command with substitute user and group ID
15666 @cindex substitute user and group IDs
15667 @cindex user ID, switching
15668 @cindex super-user, becoming
15669 @cindex root, becoming
15671 @command{su} allows one user to temporarily become another user. It runs a
15672 command (often an interactive shell) with the real and effective user
15673 ID, group ID, and supplemental groups of a given @var{user}. Synopsis:
15676 su [@var{option}]@dots{} [@var{user} [@var{arg}]@dots{}]
15679 @cindex passwd entry, and @command{su} shell
15681 @flindex /etc/passwd
15682 If no @var{user} is given, the default is @code{root}, the super-user.
15683 The shell to use is taken from @var{user}'s @code{passwd} entry, or
15684 @file{/bin/sh} if none is specified there. If @var{user} has a
15685 password, @command{su} prompts for the password unless run by a user with
15686 effective user ID of zero (the super-user).
15692 @cindex login shell
15693 By default, @command{su} does not change the current directory.
15694 It sets the environment variables @env{HOME} and @env{SHELL}
15695 from the password entry for @var{user}, and if @var{user} is not
15696 the super-user, sets @env{USER} and @env{LOGNAME} to @var{user}.
15697 By default, the shell is not a login shell.
15699 Any additional @var{arg}s are passed as additional arguments to the
15702 @cindex @option{-su}
15703 GNU @command{su} does not treat @file{/bin/sh} or any other shells specially
15704 (e.g., by setting @code{argv[0]} to @option{-su}, passing @option{-c} only
15705 to certain shells, etc.).
15708 @command{su} can optionally be compiled to use @code{syslog} to report
15709 failed, and optionally successful, @command{su} attempts. (If the system
15710 supports @code{syslog}.) However, GNU @command{su} does not check if the
15711 user is a member of the @code{wheel} group; see below.
15713 The program accepts the following options. Also see @ref{Common options}.
15716 @item -c @var{command}
15717 @itemx --command=@var{command}
15720 Pass @var{command}, a single command line to run, to the shell with
15721 a @option{-c} option instead of starting an interactive shell.
15728 @cindex file name pattern expansion, disabled
15729 @cindex globbing, disabled
15730 Pass the @option{-f} option to the shell. This probably only makes sense
15731 if the shell run is @command{csh} or @command{tcsh}, for which the @option{-f}
15732 option prevents reading the startup file (@file{.cshrc}). With
15733 Bourne-like shells, the @option{-f} option disables file name pattern
15734 expansion (globbing), which is not likely to be useful.
15742 @c other variables already indexed above
15745 @cindex login shell, creating
15746 Make the shell a login shell. This means the following. Unset all
15747 environment variables except @env{TERM}, @env{HOME}, and @env{SHELL}
15748 (which are set as described above), and @env{USER} and @env{LOGNAME}
15749 (which are set, even for the super-user, as described above), and set
15750 @env{PATH} to a compiled-in default value. Change to @var{user}'s home
15751 directory. Prepend @samp{-} to the shell's name, intended to make it
15752 read its login startup file(s).
15756 @itemx --preserve-environment
15759 @opindex --preserve-environment
15760 @cindex environment, preserving
15761 @flindex /etc/shells
15762 @cindex restricted shell
15763 Do not change the environment variables @env{HOME}, @env{USER},
15764 @env{LOGNAME}, or @env{SHELL}. Run the shell given in the environment
15765 variable @env{SHELL} instead of the shell from @var{user}'s passwd
15766 entry, unless the user running @command{su} is not the super-user and
15767 @var{user}'s shell is restricted. A @dfn{restricted shell} is one that
15768 is not listed in the file @file{/etc/shells}, or in a compiled-in list
15769 if that file does not exist. Parts of what this option does can be
15770 overridden by @option{--login} and @option{--shell}.
15772 @item -s @var{shell}
15773 @itemx --shell=@var{shell}
15776 Run @var{shell} instead of the shell from @var{user}'s passwd entry,
15777 unless the user running @command{su} is not the super-user and @var{user}'s
15778 shell is restricted (see @option{-m} just above).
15782 @cindex exit status of @command{su}
15786 125 if @command{su} itself fails
15787 126 if subshell is found but cannot be invoked
15788 127 if subshell cannot be found
15789 the exit status of the subshell otherwise
15792 @cindex wheel group, not supported
15793 @cindex group wheel, not supported
15795 @subsection Why GNU @command{su} does not support the @samp{wheel} group
15797 (This section is by Richard Stallman.)
15801 Sometimes a few of the users try to hold total power over all the
15802 rest. For example, in 1984, a few users at the MIT AI lab decided to
15803 seize power by changing the operator password on the Twenex system and
15804 keeping it secret from everyone else. (I was able to thwart this coup
15805 and give power back to the users by patching the kernel, but I
15806 wouldn't know how to do that in Unix.)
15808 However, occasionally the rulers do tell someone. Under the usual
15809 @command{su} mechanism, once someone learns the root password who
15810 sympathizes with the ordinary users, he or she can tell the rest. The
15811 ``wheel group'' feature would make this impossible, and thus cement the
15812 power of the rulers.
15814 I'm on the side of the masses, not that of the rulers. If you are
15815 used to supporting the bosses and sysadmins in whatever they do, you
15816 might find this idea strange at first.
15819 @node timeout invocation
15820 @section @command{timeout}: Run a command with a time limit
15824 @cindex run commands with bounded time
15826 @command{timeout} runs the given @var{command} and kills it if it is
15827 still running after the specified time interval. Synopsis:
15830 timeout [@var{option}] @var{duration} @var{command} [@var{arg}]@dots{}
15833 @var{command} must not be a special built-in utility (@pxref{Special
15834 built-in utilities}).
15836 The program accepts the following options. Also see @ref{Common options}.
15837 Options must precede operands.
15840 @itemx --foreground
15841 @opindex --foreground
15842 Don't create a separate background program group, so that
15843 the managed @var{command} can use the foreground TTY normally.
15844 This is needed to support timing out commands not started
15845 directly from an interactive shell, in two situations.
15848 @var{command} is interactive and needs to read from the terminal for example
15850 the user wants to support sending signals directly to @var{command}
15851 from the terminal (like Ctrl-C for example)
15854 Note in this mode of operation, any children of @var{command}
15855 will not be timed out.
15857 @item -k @var{duration}
15858 @itemx --kill-after=@var{duration}
15860 @opindex --kill-after
15861 Ensure the monitored @var{command} is killed by also sending a @samp{KILL}
15862 signal, after the specified @var{duration}. Without this option, if the
15863 selected signal proves not to be fatal, @command{timeout} does not kill
15866 @item -s @var{signal}
15867 @itemx --signal=@var{signal}
15870 Send this @var{signal} to @var{command} on timeout, rather than the
15871 default @samp{TERM} signal. @var{signal} may be a name like @samp{HUP}
15872 or a number. Also see @xref{Signal specifications}.
15876 @var{duration} is a floating point number followed by an optional unit:
15878 @samp{s} for seconds (the default)
15879 @samp{m} for minutes
15883 A duration of 0 disables the associated timeout.
15884 Note that the actual timeout duration is dependent on system conditions,
15885 which should be especially considered when specifying sub-second timeouts.
15887 @cindex exit status of @command{timeout}
15891 124 if @var{command} times out
15892 125 if @command{timeout} itself fails
15893 126 if @var{command} is found but cannot be invoked
15894 127 if @var{command} cannot be found
15895 the exit status of @var{command} otherwise
15899 @node Process control
15900 @chapter Process control
15902 @cindex processes, commands for controlling
15903 @cindex commands for controlling processes
15906 * kill invocation:: Sending a signal to processes.
15910 @node kill invocation
15911 @section @command{kill}: Send a signal to processes
15914 @cindex send a signal to processes
15916 The @command{kill} command sends a signal to processes, causing them
15917 to terminate or otherwise act upon receiving the signal in some way.
15918 Alternatively, it lists information about signals. Synopses:
15921 kill [-s @var{signal} | --signal @var{signal} | -@var{signal}] @var{pid}@dots{}
15922 kill [-l | --list | -t | --table] [@var{signal}]@dots{}
15925 @mayConflictWithShellBuiltIn{kill}
15927 The first form of the @command{kill} command sends a signal to all
15928 @var{pid} arguments. The default signal to send if none is specified
15929 is @samp{TERM}. The special signal number @samp{0} does not denote a
15930 valid signal, but can be used to test whether the @var{pid} arguments
15931 specify processes to which a signal could be sent.
15933 If @var{pid} is positive, the signal is sent to the process with the
15934 process ID @var{pid}. If @var{pid} is zero, the signal is sent to all
15935 processes in the process group of the current process. If @var{pid}
15936 is @minus{}1, the signal is sent to all processes for which the user has
15937 permission to send a signal. If @var{pid} is less than @minus{}1, the signal
15938 is sent to all processes in the process group that equals the absolute
15939 value of @var{pid}.
15941 If @var{pid} is not positive, a system-dependent set of system
15942 processes is excluded from the list of processes to which the signal
15945 If a negative @var{pid} argument is desired as the first one, it
15946 should be preceded by @option{--}. However, as a common extension to
15947 @acronym{POSIX}, @option{--} is not required with @samp{kill
15948 -@var{signal} -@var{pid}}. The following commands are equivalent:
15957 The first form of the @command{kill} command succeeds if every @var{pid}
15958 argument specifies at least one process that the signal was sent to.
15960 The second form of the @command{kill} command lists signal information.
15961 Either the @option{-l} or @option{--list} option, or the @option{-t}
15962 or @option{--table} option must be specified. Without any
15963 @var{signal} argument, all supported signals are listed. The output
15964 of @option{-l} or @option{--list} is a list of the signal names, one
15965 per line; if @var{signal} is already a name, the signal number is
15966 printed instead. The output of @option{-t} or @option{--table} is a
15967 table of signal numbers, names, and descriptions. This form of the
15968 @command{kill} command succeeds if all @var{signal} arguments are valid
15969 and if there is no output error.
15971 The @command{kill} command also supports the @option{--help} and
15972 @option{--version} options. @xref{Common options}.
15974 A @var{signal} may be a signal name like @samp{HUP}, or a signal
15975 number like @samp{1}, or an exit status of a process terminated by the
15976 signal. A signal name can be given in canonical form or prefixed by
15977 @samp{SIG}. The case of the letters is ignored, except for the
15978 @option{-@var{signal}} option which must use upper case to avoid
15979 ambiguity with lower case option letters. For a list of supported
15980 signal names and numbers see @xref{Signal specifications}.
15985 @cindex delaying commands
15986 @cindex commands for delaying
15988 @c Perhaps @command{wait} or other commands should be described here also?
15991 * sleep invocation:: Delay for a specified time.
15995 @node sleep invocation
15996 @section @command{sleep}: Delay for a specified time
15999 @cindex delay for a specified time
16001 @command{sleep} pauses for an amount of time specified by the sum of
16002 the values of the command line arguments.
16006 sleep @var{number}[smhd]@dots{}
16010 Each argument is a number followed by an optional unit; the default
16011 is seconds. The units are:
16024 Historical implementations of @command{sleep} have required that
16025 @var{number} be an integer, and only accepted a single argument
16026 without a suffix. However, GNU @command{sleep} accepts
16027 arbitrary floating point numbers. @xref{Floating point}.
16029 The only options are @option{--help} and @option{--version}. @xref{Common
16032 @c sleep is a shell built-in at least with Solaris 11's /bin/sh
16033 @mayConflictWithShellBuiltIn{sleep}
16038 @node Numeric operations
16039 @chapter Numeric operations
16041 @cindex numeric operations
16042 These programs do numerically-related operations.
16045 * factor invocation:: Show factors of numbers.
16046 * seq invocation:: Print sequences of numbers.
16050 @node factor invocation
16051 @section @command{factor}: Print prime factors
16054 @cindex prime factors
16056 @command{factor} prints prime factors. Synopses:
16059 factor [@var{number}]@dots{}
16060 factor @var{option}
16063 If no @var{number} is specified on the command line, @command{factor} reads
16064 numbers from standard input, delimited by newlines, tabs, or spaces.
16066 The @command{factor} command supports only a small number of options:
16070 Print a short help on standard output, then exit without further
16074 Print the program version on standard output, then exit without further
16078 Factoring the product of the eighth and ninth Mersenne primes
16079 takes about 30 milliseconds of CPU time on a 2.2 GHz Athlon.
16082 M8=$(echo 2^31-1|bc)
16083 M9=$(echo 2^61-1|bc)
16084 n=$(echo "$M8 * $M9" | bc)
16085 /usr/bin/time -f %U factor $n
16086 4951760154835678088235319297: 2147483647 2305843009213693951
16090 Similarly, factoring the eighth Fermat number @math{2^{256}+1} takes
16091 about 20 seconds on the same machine.
16093 Factoring large numbers is, in general, hard. The Pollard Rho
16094 algorithm used by @command{factor} is particularly effective for
16095 numbers with relatively small factors. If you wish to factor large
16096 numbers which do not have small factors (for example, numbers which
16097 are the product of two large primes), other methods are far better.
16099 If @command{factor} is built without using GNU MP, only
16100 single-precision arithmetic is available, and so large numbers
16101 (typically @math{2^{64}} and above) will not be supported. The single-precision
16102 code uses an algorithm which is designed for factoring smaller
16108 @node seq invocation
16109 @section @command{seq}: Print numeric sequences
16112 @cindex numeric sequences
16113 @cindex sequence of numbers
16115 @command{seq} prints a sequence of numbers to standard output. Synopses:
16118 seq [@var{option}]@dots{} @var{last}
16119 seq [@var{option}]@dots{} @var{first} @var{last}
16120 seq [@var{option}]@dots{} @var{first} @var{increment} @var{last}
16123 @command{seq} prints the numbers from @var{first} to @var{last} by
16124 @var{increment}. By default, each number is printed on a separate line.
16125 When @var{increment} is not specified, it defaults to @samp{1},
16126 even when @var{first} is larger than @var{last}.
16127 @var{first} also defaults to @samp{1}. So @code{seq 1} prints
16128 @samp{1}, but @code{seq 0} and @code{seq 10 5} produce no output.
16129 Floating-point numbers may be specified. @xref{Floating point}.
16131 The program accepts the following options. Also see @ref{Common options}.
16132 Options must precede operands.
16135 @item -f @var{format}
16136 @itemx --format=@var{format}
16137 @opindex -f @var{format}
16138 @opindex --format=@var{format}
16139 @cindex formatting of numbers in @command{seq}
16140 Print all numbers using @var{format}.
16141 @var{format} must contain exactly one of the @samp{printf}-style
16142 floating point conversion specifications @samp{%a}, @samp{%e},
16143 @samp{%f}, @samp{%g}, @samp{%A}, @samp{%E}, @samp{%F}, @samp{%G}.
16144 The @samp{%} may be followed by zero or more flags taken from the set
16145 @samp{-+#0 '}, then an optional width containing one or more digits,
16146 then an optional precision consisting of a @samp{.} followed by zero
16147 or more digits. @var{format} may also contain any number of @samp{%%}
16148 conversion specifications. All conversion specifications have the
16149 same meaning as with @samp{printf}.
16151 The default format is derived from @var{first}, @var{step}, and
16152 @var{last}. If these all use a fixed point decimal representation,
16153 the default format is @samp{%.@var{p}f}, where @var{p} is the minimum
16154 precision that can represent the output numbers exactly. Otherwise,
16155 the default format is @samp{%g}.
16157 @item -s @var{string}
16158 @itemx --separator=@var{string}
16159 @cindex separator for numbers in @command{seq}
16160 Separate numbers with @var{string}; default is a newline.
16161 The output always terminates with a newline.
16164 @itemx --equal-width
16165 Print all numbers with the same width, by padding with leading zeros.
16166 @var{first}, @var{step}, and @var{last} should all use a fixed point
16167 decimal representation.
16168 (To have other kinds of padding, use @option{--format}).
16172 You can get finer-grained control over output with @option{-f}:
16175 $ seq -f '(%9.2E)' -9e5 1.1e6 1.3e6
16181 If you want hexadecimal integer output, you can use @command{printf}
16182 to perform the conversion:
16185 $ printf '%x\n' `seq 1048575 1024 1050623`
16191 For very long lists of numbers, use xargs to avoid
16192 system limitations on the length of an argument list:
16195 $ seq 1000000 | xargs printf '%x\n' | tail -n 3
16201 To generate octal output, use the printf @code{%o} format instead
16204 On most systems, seq can produce whole-number output for values up to
16205 at least @math{2^{53}}. Larger integers are approximated. The details
16206 differ depending on your floating-point implementation.
16207 @xref{Floating point}. A common
16208 case is that @command{seq} works with integers through @math{2^{64}},
16209 and larger integers may not be numerically correct:
16212 $ seq 18446744073709551616 1 18446744073709551618
16213 18446744073709551616
16214 18446744073709551616
16215 18446744073709551618
16218 Be careful when using @command{seq} with outlandish values: otherwise
16219 you may see surprising results, as @command{seq} uses floating point
16220 internally. For example, on the x86 platform, where the internal
16221 representation uses a 64-bit fraction, the command:
16224 seq 1 0.0000000000000000001 1.0000000000000000009
16227 outputs 1.0000000000000000007 twice and skips 1.0000000000000000008.
16232 @node File permissions
16233 @chapter File permissions
16236 @include parse-datetime.texi
16240 @node Opening the software toolbox
16241 @chapter Opening the Software Toolbox
16243 An earlier version of this chapter appeared in
16244 @uref{http://www.linuxjournal.com/article.php?sid=2762, the
16245 @cite{What's GNU?} column of the June 1994 @cite{Linux Journal}}.
16246 It was written by Arnold Robbins.
16249 * Toolbox introduction:: Toolbox introduction
16250 * I/O redirection:: I/O redirection
16251 * The who command:: The @command{who} command
16252 * The cut command:: The @command{cut} command
16253 * The sort command:: The @command{sort} command
16254 * The uniq command:: The @command{uniq} command
16255 * Putting the tools together:: Putting the tools together
16259 @node Toolbox introduction
16260 @unnumberedsec Toolbox Introduction
16262 This month's column is only peripherally related to the GNU Project, in
16263 that it describes a number of the GNU tools on your GNU/Linux system
16265 might be used. What it's really about is the ``Software Tools'' philosophy
16266 of program development and usage.
16268 The software tools philosophy was an important and integral concept
16269 in the initial design and development of Unix (of which Linux and GNU are
16270 essentially clones). Unfortunately, in the modern day press of
16271 Internetworking and flashy GUIs, it seems to have fallen by the
16272 wayside. This is a shame, since it provides a powerful mental model
16273 for solving many kinds of problems.
16275 Many people carry a Swiss Army knife around in their pants pockets (or
16276 purse). A Swiss Army knife is a handy tool to have: it has several knife
16277 blades, a screwdriver, tweezers, toothpick, nail file, corkscrew, and perhaps
16278 a number of other things on it. For the everyday, small miscellaneous jobs
16279 where you need a simple, general purpose tool, it's just the thing.
16281 On the other hand, an experienced carpenter doesn't build a house using
16282 a Swiss Army knife. Instead, he has a toolbox chock full of specialized
16283 tools---a saw, a hammer, a screwdriver, a plane, and so on. And he knows
16284 exactly when and where to use each tool; you won't catch him hammering nails
16285 with the handle of his screwdriver.
16287 The Unix developers at Bell Labs were all professional programmers and trained
16288 computer scientists. They had found that while a one-size-fits-all program
16289 might appeal to a user because there's only one program to use, in practice
16294 difficult to write,
16297 difficult to maintain and
16301 difficult to extend to meet new situations.
16304 Instead, they felt that programs should be specialized tools. In short, each
16305 program ``should do one thing well.'' No more and no less. Such programs are
16306 simpler to design, write, and get right---they only do one thing.
16308 Furthermore, they found that with the right machinery for hooking programs
16309 together, that the whole was greater than the sum of the parts. By combining
16310 several special purpose programs, you could accomplish a specific task
16311 that none of the programs was designed for, and accomplish it much more
16312 quickly and easily than if you had to write a special purpose program.
16313 We will see some (classic) examples of this further on in the column.
16314 (An important additional point was that, if necessary, take a detour
16315 and build any software tools you may need first, if you don't already
16316 have something appropriate in the toolbox.)
16318 @node I/O redirection
16319 @unnumberedsec I/O Redirection
16321 Hopefully, you are familiar with the basics of I/O redirection in the
16322 shell, in particular the concepts of ``standard input,'' ``standard output,''
16323 and ``standard error''. Briefly, ``standard input'' is a data source, where
16324 data comes from. A program should not need to either know or care if the
16325 data source is a disk file, a keyboard, a magnetic tape, or even a punched
16326 card reader. Similarly, ``standard output'' is a data sink, where data goes
16327 to. The program should neither know nor care where this might be.
16328 Programs that only read their standard input, do something to the data,
16329 and then send it on, are called @dfn{filters}, by analogy to filters in a
16332 With the Unix shell, it's very easy to set up data pipelines:
16335 program_to_create_data | filter1 | ... | filterN > final.pretty.data
16338 We start out by creating the raw data; each filter applies some successive
16339 transformation to the data, until by the time it comes out of the pipeline,
16340 it is in the desired form.
16342 This is fine and good for standard input and standard output. Where does the
16343 standard error come in to play? Well, think about @command{filter1} in
16344 the pipeline above. What happens if it encounters an error in the data it
16345 sees? If it writes an error message to standard output, it will just
16346 disappear down the pipeline into @command{filter2}'s input, and the
16347 user will probably never see it. So programs need a place where they can send
16348 error messages so that the user will notice them. This is standard error,
16349 and it is usually connected to your console or window, even if you have
16350 redirected standard output of your program away from your screen.
16352 For filter programs to work together, the format of the data has to be
16353 agreed upon. The most straightforward and easiest format to use is simply
16354 lines of text. Unix data files are generally just streams of bytes, with
16355 lines delimited by the @acronym{ASCII} @sc{lf} (Line Feed) character,
16356 conventionally called a ``newline'' in the Unix literature. (This is
16357 @code{'\n'} if you're a C programmer.) This is the format used by all
16358 the traditional filtering programs. (Many earlier operating systems
16359 had elaborate facilities and special purpose programs for managing
16360 binary data. Unix has always shied away from such things, under the
16361 philosophy that it's easiest to simply be able to view and edit your
16362 data with a text editor.)
16364 OK, enough introduction. Let's take a look at some of the tools, and then
16365 we'll see how to hook them together in interesting ways. In the following
16366 discussion, we will only present those command line options that interest
16367 us. As you should always do, double check your system documentation
16368 for the full story.
16370 @node The who command
16371 @unnumberedsec The @command{who} Command
16373 The first program is the @command{who} command. By itself, it generates a
16374 list of the users who are currently logged in. Although I'm writing
16375 this on a single-user system, we'll pretend that several people are
16380 @print{} arnold console Jan 22 19:57
16381 @print{} miriam ttyp0 Jan 23 14:19(:0.0)
16382 @print{} bill ttyp1 Jan 21 09:32(:0.0)
16383 @print{} arnold ttyp2 Jan 23 20:48(:0.0)
16386 Here, the @samp{$} is the usual shell prompt, at which I typed @samp{who}.
16387 There are three people logged in, and I am logged in twice. On traditional
16388 Unix systems, user names are never more than eight characters long. This
16389 little bit of trivia will be useful later. The output of @command{who} is nice,
16390 but the data is not all that exciting.
16392 @node The cut command
16393 @unnumberedsec The @command{cut} Command
16395 The next program we'll look at is the @command{cut} command. This program
16396 cuts out columns or fields of input data. For example, we can tell it
16397 to print just the login name and full name from the @file{/etc/passwd}
16398 file. The @file{/etc/passwd} file has seven fields, separated by
16402 arnold:xyzzy:2076:10:Arnold D. Robbins:/home/arnold:/bin/bash
16405 To get the first and fifth fields, we would use @command{cut} like this:
16408 $ cut -d: -f1,5 /etc/passwd
16409 @print{} root:Operator
16411 @print{} arnold:Arnold D. Robbins
16412 @print{} miriam:Miriam A. Robbins
16416 With the @option{-c} option, @command{cut} will cut out specific characters
16417 (i.e., columns) in the input lines. This is useful for input data
16418 that has fixed width fields, and does not have a field separator. For
16419 example, list the Monday dates for the current month:
16421 @c Is using cal ok? Looked at gcal, but I don't like it.
16432 @node The sort command
16433 @unnumberedsec The @command{sort} Command
16435 Next we'll look at the @command{sort} command. This is one of the most
16436 powerful commands on a Unix-style system; one that you will often find
16437 yourself using when setting up fancy data plumbing.
16440 command reads and sorts each file named on the command line. It then
16441 merges the sorted data and writes it to standard output. It will read
16442 standard input if no files are given on the command line (thus
16443 making it into a filter). The sort is based on the character collating
16444 sequence or based on user-supplied ordering criteria.
16447 @node The uniq command
16448 @unnumberedsec The @command{uniq} Command
16450 Finally (at least for now), we'll look at the @command{uniq} program. When
16451 sorting data, you will often end up with duplicate lines, lines that
16452 are identical. Usually, all you need is one instance of each line.
16453 This is where @command{uniq} comes in. The @command{uniq} program reads its
16454 standard input. It prints only one
16455 copy of each repeated line. It does have several options. Later on,
16456 we'll use the @option{-c} option, which prints each unique line, preceded
16457 by a count of the number of times that line occurred in the input.
16460 @node Putting the tools together
16461 @unnumberedsec Putting the Tools Together
16463 Now, let's suppose this is a large ISP server system with dozens of users
16464 logged in. The management wants the system administrator to write a
16466 generate a sorted list of logged in users. Furthermore, even if a user
16467 is logged in multiple times, his or her name should only show up in the
16470 The administrator could sit down with the system documentation and write a C
16471 program that did this. It would take perhaps a couple of hundred lines
16472 of code and about two hours to write it, test it, and debug it.
16473 However, knowing the software toolbox, the administrator can instead start out
16474 by generating just a list of logged on users:
16484 Next, sort the list:
16487 $ who | cut -c1-8 | sort
16494 Finally, run the sorted list through @command{uniq}, to weed out duplicates:
16497 $ who | cut -c1-8 | sort | uniq
16503 The @command{sort} command actually has a @option{-u} option that does what
16504 @command{uniq} does. However, @command{uniq} has other uses for which one
16505 cannot substitute @samp{sort -u}.
16507 The administrator puts this pipeline into a shell script, and makes it
16509 all the users on the system (@samp{#} is the system administrator,
16510 or @code{root}, prompt):
16513 # cat > /usr/local/bin/listusers
16514 who | cut -c1-8 | sort | uniq
16516 # chmod +x /usr/local/bin/listusers
16519 There are four major points to note here. First, with just four
16520 programs, on one command line, the administrator was able to save about two
16521 hours worth of work. Furthermore, the shell pipeline is just about as
16522 efficient as the C program would be, and it is much more efficient in
16523 terms of programmer time. People time is much more expensive than
16524 computer time, and in our modern ``there's never enough time to do
16525 everything'' society, saving two hours of programmer time is no mean
16528 Second, it is also important to emphasize that with the
16529 @emph{combination} of the tools, it is possible to do a special
16530 purpose job never imagined by the authors of the individual programs.
16532 Third, it is also valuable to build up your pipeline in stages, as we did here.
16533 This allows you to view the data at each stage in the pipeline, which helps
16534 you acquire the confidence that you are indeed using these tools correctly.
16536 Finally, by bundling the pipeline in a shell script, other users can use
16537 your command, without having to remember the fancy plumbing you set up for
16538 them. In terms of how you run them, shell scripts and compiled programs are
16541 After the previous warm-up exercise, we'll look at two additional, more
16542 complicated pipelines. For them, we need to introduce two more tools.
16544 The first is the @command{tr} command, which stands for ``transliterate.''
16545 The @command{tr} command works on a character-by-character basis, changing
16546 characters. Normally it is used for things like mapping upper case to
16550 $ echo ThIs ExAmPlE HaS MIXED case! | tr '[:upper:]' '[:lower:]'
16551 @print{} this example has mixed case!
16554 There are several options of interest:
16558 work on the complement of the listed characters, i.e.,
16559 operations apply to characters not in the given set
16562 delete characters in the first set from the output
16565 squeeze repeated characters in the output into just one character.
16568 We will be using all three options in a moment.
16570 The other command we'll look at is @command{comm}. The @command{comm}
16571 command takes two sorted input files as input data, and prints out the
16572 files' lines in three columns. The output columns are the data lines
16573 unique to the first file, the data lines unique to the second file, and
16574 the data lines that are common to both. The @option{-1}, @option{-2}, and
16575 @option{-3} command line options @emph{omit} the respective columns. (This is
16576 non-intuitive and takes a little getting used to.) For example:
16598 The file name @file{-} tells @command{comm} to read standard input
16599 instead of a regular file.
16601 Now we're ready to build a fancy pipeline. The first application is a word
16602 frequency counter. This helps an author determine if he or she is over-using
16605 The first step is to change the case of all the letters in our input file
16606 to one case. ``The'' and ``the'' are the same word when doing counting.
16609 $ tr '[:upper:]' '[:lower:]' < whats.gnu | ...
16612 The next step is to get rid of punctuation. Quoted words and unquoted words
16613 should be treated identically; it's easiest to just get the punctuation out of
16617 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | ...
16620 The second @command{tr} command operates on the complement of the listed
16621 characters, which are all the letters, the digits, the underscore, and
16622 the blank. The @samp{\n} represents the newline character; it has to
16623 be left alone. (The @acronym{ASCII} tab character should also be included for
16624 good measure in a production script.)
16626 At this point, we have data consisting of words separated by blank space.
16627 The words only contain alphanumeric characters (and the underscore). The
16628 next step is break the data apart so that we have one word per line. This
16629 makes the counting operation much easier, as we will see shortly.
16632 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16633 > tr -s ' ' '\n' | ...
16636 This command turns blanks into newlines. The @option{-s} option squeezes
16637 multiple newline characters in the output into just one. This helps us
16638 avoid blank lines. (The @samp{>} is the shell's ``secondary prompt.''
16639 This is what the shell prints when it notices you haven't finished
16640 typing in all of a command.)
16642 We now have data consisting of one word per line, no punctuation, all one
16643 case. We're ready to count each word:
16646 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16647 > tr -s ' ' '\n' | sort | uniq -c | ...
16650 At this point, the data might look something like this:
16663 The output is sorted by word, not by count! What we want is the most
16664 frequently used words first. Fortunately, this is easy to accomplish,
16665 with the help of two more @command{sort} options:
16669 do a numeric sort, not a textual one
16672 reverse the order of the sort
16675 The final pipeline looks like this:
16678 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16679 > tr -s ' ' '\n' | sort | uniq -c | sort -n -r
16688 Whew! That's a lot to digest. Yet, the same principles apply. With six
16689 commands, on two lines (really one long one split for convenience), we've
16690 created a program that does something interesting and useful, in much
16691 less time than we could have written a C program to do the same thing.
16693 A minor modification to the above pipeline can give us a simple spelling
16694 checker! To determine if you've spelled a word correctly, all you have to
16695 do is look it up in a dictionary. If it is not there, then chances are
16696 that your spelling is incorrect. So, we need a dictionary.
16697 The conventional location for a dictionary is @file{/usr/dict/words}.
16698 On my GNU/Linux system,@footnote{Redhat Linux 6.1, for the November 2000
16699 revision of this article.}
16700 this is a sorted, 45,402 word dictionary.
16702 Now, how to compare our file with the dictionary? As before, we generate
16703 a sorted list of words, one per line:
16706 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16707 > tr -s ' ' '\n' | sort -u | ...
16710 Now, all we need is a list of words that are @emph{not} in the
16711 dictionary. Here is where the @command{comm} command comes in.
16714 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16715 > tr -s ' ' '\n' | sort -u |
16716 > comm -23 - /usr/dict/words
16719 The @option{-2} and @option{-3} options eliminate lines that are only in the
16720 dictionary (the second file), and lines that are in both files. Lines
16721 only in the first file (standard input, our stream of words), are
16722 words that are not in the dictionary. These are likely candidates for
16723 spelling errors. This pipeline was the first cut at a production
16724 spelling checker on Unix.
16726 There are some other tools that deserve brief mention.
16730 search files for text that matches a regular expression
16733 count lines, words, characters
16736 a T-fitting for data pipes, copies data to files and to standard output
16739 the stream editor, an advanced tool
16742 a data manipulation language, another advanced tool
16745 The software tools philosophy also espoused the following bit of
16746 advice: ``Let someone else do the hard part.'' This means, take
16747 something that gives you most of what you need, and then massage it the
16748 rest of the way until it's in the form that you want.
16754 Each program should do one thing well. No more, no less.
16757 Combining programs with appropriate plumbing leads to results where
16758 the whole is greater than the sum of the parts. It also leads to novel
16759 uses of programs that the authors might never have imagined.
16762 Programs should never print extraneous header or trailer data, since these
16763 could get sent on down a pipeline. (A point we didn't mention earlier.)
16766 Let someone else do the hard part.
16769 Know your toolbox! Use each program appropriately. If you don't have an
16770 appropriate tool, build one.
16773 As of this writing, all the programs we've discussed are available via
16774 anonymous @command{ftp} from: @*
16775 @uref{ftp://gnudist.gnu.org/textutils/textutils-1.22.tar.gz}. (There may
16776 be more recent versions available now.)
16778 None of what I have presented in this column is new. The Software Tools
16779 philosophy was first introduced in the book @cite{Software Tools}, by
16780 Brian Kernighan and P.J. Plauger (Addison-Wesley, ISBN 0-201-03669-X).
16781 This book showed how to write and use software tools. It was written in
16782 1976, using a preprocessor for FORTRAN named @command{ratfor} (RATional
16783 FORtran). At the time, C was not as ubiquitous as it is now; FORTRAN
16784 was. The last chapter presented a @command{ratfor} to FORTRAN
16785 processor, written in @command{ratfor}. @command{ratfor} looks an awful
16786 lot like C; if you know C, you won't have any problem following the
16789 In 1981, the book was updated and made available as @cite{Software Tools
16790 in Pascal} (Addison-Wesley, ISBN 0-201-10342-7). Both books are
16791 still in print and are well worth
16792 reading if you're a programmer. They certainly made a major change in
16793 how I view programming.
16795 The programs in both books are available from
16796 @uref{http://cm.bell-labs.com/who/bwk, Brian Kernighan's home page}.
16797 For a number of years, there was an active
16798 Software Tools Users Group, whose members had ported the original
16799 @command{ratfor} programs to essentially every computer system with a
16800 FORTRAN compiler. The popularity of the group waned in the middle 1980s
16801 as Unix began to spread beyond universities.
16803 With the current proliferation of GNU code and other clones of Unix programs,
16804 these programs now receive little attention; modern C versions are
16805 much more efficient and do more than these programs do. Nevertheless, as
16806 exposition of good programming style, and evangelism for a still-valuable
16807 philosophy, these books are unparalleled, and I recommend them highly.
16809 Acknowledgment: I would like to express my gratitude to Brian Kernighan
16810 of Bell Labs, the original Software Toolsmith, for reviewing this column.
16812 @node GNU Free Documentation License
16813 @appendix GNU Free Documentation License
16817 @node Concept index
16824 @c Local variables:
16825 @c texinfo-column-for-description: 32