3 @setfilename coreutils.info
4 @settitle @sc{gnu} Coreutils
9 @include constants.texi
11 @c Define new indices.
15 @c Put everything in one index (arbitrarily chosen to be the concept index).
25 * Coreutils: (coreutils). Core GNU (file, text, shell) utilities.
26 * Common options: (coreutils)Common options. Common options.
27 * File permissions: (coreutils)File permissions. Access modes.
28 * Date input formats: (coreutils)Date input formats.
31 @c FIXME: the following need documentation
32 @c * [: (coreutils)[ invocation. File/string tests.
33 @c * pinky: (coreutils)pinky invocation. FIXME.
35 @dircategory Individual utilities
37 * arch: (coreutils)arch invocation. Print machine hardware name.
38 * base64: (coreutils)base64 invocation. Base64 encode/decode data.
39 * basename: (coreutils)basename invocation. Strip directory and suffix.
40 * cat: (coreutils)cat invocation. Concatenate and write files.
41 * chcon: (coreutils)chcon invocation. Change SELinux CTX of files.
42 * chgrp: (coreutils)chgrp invocation. Change file groups.
43 * chmod: (coreutils)chmod invocation. Change file permissions.
44 * chown: (coreutils)chown invocation. Change file owners/groups.
45 * chroot: (coreutils)chroot invocation. Specify the root directory.
46 * cksum: (coreutils)cksum invocation. Print POSIX CRC checksum.
47 * comm: (coreutils)comm invocation. Compare sorted files by line.
48 * cp: (coreutils)cp invocation. Copy files.
49 * csplit: (coreutils)csplit invocation. Split by context.
50 * cut: (coreutils)cut invocation. Print selected parts of lines.
51 * date: (coreutils)date invocation. Print/set system date and time.
52 * dd: (coreutils)dd invocation. Copy and convert a file.
53 * df: (coreutils)df invocation. Report file system disk usage.
54 * dir: (coreutils)dir invocation. List directories briefly.
55 * dircolors: (coreutils)dircolors invocation. Color setup for ls.
56 * dirname: (coreutils)dirname invocation. Strip last file name component.
57 * du: (coreutils)du invocation. Report on disk usage.
58 * echo: (coreutils)echo invocation. Print a line of text.
59 * env: (coreutils)env invocation. Modify the environment.
60 * expand: (coreutils)expand invocation. Convert tabs to spaces.
61 * expr: (coreutils)expr invocation. Evaluate expressions.
62 * factor: (coreutils)factor invocation. Print prime factors
63 * false: (coreutils)false invocation. Do nothing, unsuccessfully.
64 * fmt: (coreutils)fmt invocation. Reformat paragraph text.
65 * fold: (coreutils)fold invocation. Wrap long input lines.
66 * groups: (coreutils)groups invocation. Print group names a user is in.
67 * head: (coreutils)head invocation. Output the first part of files.
68 * hostid: (coreutils)hostid invocation. Print numeric host identifier.
69 * hostname: (coreutils)hostname invocation. Print or set system name.
70 * id: (coreutils)id invocation. Print user identity.
71 * install: (coreutils)install invocation. Copy and change attributes.
72 * join: (coreutils)join invocation. Join lines on a common field.
73 * kill: (coreutils)kill invocation. Send a signal to processes.
74 * link: (coreutils)link invocation. Make hard links between files.
75 * ln: (coreutils)ln invocation. Make links between files.
76 * logname: (coreutils)logname invocation. Print current login name.
77 * ls: (coreutils)ls invocation. List directory contents.
78 * md5sum: (coreutils)md5sum invocation. Print or check MD5 digests.
79 * mkdir: (coreutils)mkdir invocation. Create directories.
80 * mkfifo: (coreutils)mkfifo invocation. Create FIFOs (named pipes).
81 * mknod: (coreutils)mknod invocation. Create special files.
82 * mktemp: (coreutils)mktemp invocation. Create temporary files.
83 * mv: (coreutils)mv invocation. Rename files.
84 * nice: (coreutils)nice invocation. Modify niceness.
85 * nl: (coreutils)nl invocation. Number lines and write files.
86 * nohup: (coreutils)nohup invocation. Immunize to hangups.
87 * nproc: (coreutils)nproc invocation. Print the number of processors.
88 * od: (coreutils)od invocation. Dump files in octal, etc.
89 * paste: (coreutils)paste invocation. Merge lines of files.
90 * pathchk: (coreutils)pathchk invocation. Check file name portability.
91 * pr: (coreutils)pr invocation. Paginate or columnate files.
92 * printenv: (coreutils)printenv invocation. Print environment variables.
93 * printf: (coreutils)printf invocation. Format and print data.
94 * ptx: (coreutils)ptx invocation. Produce permuted indexes.
95 * pwd: (coreutils)pwd invocation. Print working directory.
96 * readlink: (coreutils)readlink invocation. Print referent of a symlink.
97 * rm: (coreutils)rm invocation. Remove files.
98 * rmdir: (coreutils)rmdir invocation. Remove empty directories.
99 * runcon: (coreutils)runcon invocation. Run in specified SELinux CTX.
100 * seq: (coreutils)seq invocation. Print numeric sequences
101 * sha1sum: (coreutils)sha1sum invocation. Print or check SHA-1 digests.
102 * sha2: (coreutils)sha2 utilities. Print or check SHA-2 digests.
103 * shred: (coreutils)shred invocation. Remove files more securely.
104 * shuf: (coreutils)shuf invocation. Shuffling text files.
105 * sleep: (coreutils)sleep invocation. Delay for a specified time.
106 * sort: (coreutils)sort invocation. Sort text files.
107 * split: (coreutils)split invocation. Split into pieces.
108 * stat: (coreutils)stat invocation. Report file(system) status.
109 * stdbuf: (coreutils)stdbuf invocation. Modify stdio buffering.
110 * stty: (coreutils)stty invocation. Print/change terminal settings.
111 * su: (coreutils)su invocation. Modify user and group ID.
112 * sum: (coreutils)sum invocation. Print traditional checksum.
113 * sync: (coreutils)sync invocation. Synchronize memory and disk.
114 * tac: (coreutils)tac invocation. Reverse files.
115 * tail: (coreutils)tail invocation. Output the last part of files.
116 * tee: (coreutils)tee invocation. Redirect to multiple files.
117 * test: (coreutils)test invocation. File/string tests.
118 * timeout: (coreutils)timeout invocation. Run with time limit.
119 * touch: (coreutils)touch invocation. Change file timestamps.
120 * tr: (coreutils)tr invocation. Translate characters.
121 * true: (coreutils)true invocation. Do nothing, successfully.
122 * truncate: (coreutils)truncate invocation. Shrink/extend size of a file.
123 * tsort: (coreutils)tsort invocation. Topological sort.
124 * tty: (coreutils)tty invocation. Print terminal name.
125 * uname: (coreutils)uname invocation. Print system information.
126 * unexpand: (coreutils)unexpand invocation. Convert spaces to tabs.
127 * uniq: (coreutils)uniq invocation. Uniquify files.
128 * unlink: (coreutils)unlink invocation. Removal via unlink(2).
129 * uptime: (coreutils)uptime invocation. Print uptime and load.
130 * users: (coreutils)users invocation. Print current user names.
131 * vdir: (coreutils)vdir invocation. List directories verbosely.
132 * wc: (coreutils)wc invocation. Line, word, and byte counts.
133 * who: (coreutils)who invocation. Print who is logged in.
134 * whoami: (coreutils)whoami invocation. Print effective user ID.
135 * yes: (coreutils)yes invocation. Print a string indefinitely.
139 This manual documents version @value{VERSION} of the @sc{gnu} core
140 utilities, including the standard programs for text and file manipulation.
142 Copyright @copyright{} 1994-1996, 2000-2011 Free Software Foundation, Inc.
145 Permission is granted to copy, distribute and/or modify this document
146 under the terms of the GNU Free Documentation License, Version 1.3 or
147 any later version published by the Free Software Foundation; with no
148 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
149 Texts. A copy of the license is included in the section entitled ``GNU
150 Free Documentation License''.
155 @title @sc{gnu} @code{Coreutils}
156 @subtitle Core GNU utilities
157 @subtitle for version @value{VERSION}, @value{UPDATED}
158 @author David MacKenzie et al.
161 @vskip 0pt plus 1filll
174 @cindex core utilities
175 @cindex text utilities
176 @cindex shell utilities
177 @cindex file utilities
180 * Introduction:: Caveats, overview, and authors
181 * Common options:: Common options
182 * Output of entire files:: cat tac nl od base64
183 * Formatting file contents:: fmt pr fold
184 * Output of parts of files:: head tail split csplit
185 * Summarizing files:: wc sum cksum md5sum sha1sum sha2
186 * Operating on sorted files:: sort shuf uniq comm ptx tsort
187 * Operating on fields:: cut paste join
188 * Operating on characters:: tr expand unexpand
189 * Directory listing:: ls dir vdir dircolors
190 * Basic operations:: cp dd install mv rm shred
191 * Special file types:: mkdir rmdir unlink mkfifo mknod ln link readlink
192 * Changing file attributes:: chgrp chmod chown touch
193 * Disk usage:: df du stat sync truncate
194 * Printing text:: echo printf yes
195 * Conditions:: false true test expr
197 * File name manipulation:: dirname basename pathchk mktemp
198 * Working context:: pwd stty printenv tty
199 * User information:: id logname whoami groups users who
200 * System context:: date arch nproc uname hostname hostid uptime
201 * SELinux context:: chcon runcon
202 * Modified command invocation:: chroot env nice nohup stdbuf su timeout
203 * Process control:: kill
205 * Numeric operations:: factor seq
206 * File permissions:: Access modes
207 * Date input formats:: Specifying date strings
208 * Opening the software toolbox:: The software tools philosophy
209 * GNU Free Documentation License:: Copying and sharing this manual
210 * Concept index:: General index
213 --- The Detailed Node Listing ---
217 * Exit status:: Indicating program success or failure
218 * Backup options:: Backup options
219 * Block size:: Block size
220 * Floating point:: Floating point number representation
221 * Signal specifications:: Specifying signals
222 * Disambiguating names and IDs:: chgrp and chown owner and group syntax
223 * Random sources:: Sources of random data
224 * Target directory:: Target directory
225 * Trailing slashes:: Trailing slashes
226 * Traversing symlinks:: Traversing symlinks to directories
227 * Treating / specially:: Treating / specially
228 * Standards conformance:: Standards conformance
230 Output of entire files
232 * cat invocation:: Concatenate and write files
233 * tac invocation:: Concatenate and write files in reverse
234 * nl invocation:: Number lines and write files
235 * od invocation:: Write files in octal or other formats
236 * base64 invocation:: Transform data into printable data
238 Formatting file contents
240 * fmt invocation:: Reformat paragraph text
241 * pr invocation:: Paginate or columnate files for printing
242 * fold invocation:: Wrap input lines to fit in specified width
244 Output of parts of files
246 * head invocation:: Output the first part of files
247 * tail invocation:: Output the last part of files
248 * split invocation:: Split a file into fixed-size pieces
249 * csplit invocation:: Split a file into context-determined pieces
253 * wc invocation:: Print newline, word, and byte counts
254 * sum invocation:: Print checksum and block counts
255 * cksum invocation:: Print CRC checksum and byte counts
256 * md5sum invocation:: Print or check MD5 digests
257 * sha1sum invocation:: Print or check SHA-1 digests
258 * sha2 utilities:: Print or check SHA-2 digests
260 Operating on sorted files
262 * sort invocation:: Sort text files
263 * shuf invocation:: Shuffle text files
264 * uniq invocation:: Uniquify files
265 * comm invocation:: Compare two sorted files line by line
266 * ptx invocation:: Produce a permuted index of file contents
267 * tsort invocation:: Topological sort
269 @command{ptx}: Produce permuted indexes
271 * General options in ptx:: Options which affect general program behavior
272 * Charset selection in ptx:: Underlying character set considerations
273 * Input processing in ptx:: Input fields, contexts, and keyword selection
274 * Output formatting in ptx:: Types of output format, and sizing the fields
275 * Compatibility in ptx:: The @acronym{GNU} extensions to @command{ptx}
279 * cut invocation:: Print selected parts of lines
280 * paste invocation:: Merge lines of files
281 * join invocation:: Join lines on a common field
283 Operating on characters
285 * tr invocation:: Translate, squeeze, and/or delete characters
286 * expand invocation:: Convert tabs to spaces
287 * unexpand invocation:: Convert spaces to tabs
289 @command{tr}: Translate, squeeze, and/or delete characters
291 * Character sets:: Specifying sets of characters
292 * Translating:: Changing one set of characters to another
293 * Squeezing:: Squeezing repeats and deleting
297 * ls invocation:: List directory contents
298 * dir invocation:: Briefly list directory contents
299 * vdir invocation:: Verbosely list directory contents
300 * dircolors invocation:: Color setup for @command{ls}
302 @command{ls}: List directory contents
304 * Which files are listed:: Which files are listed
305 * What information is listed:: What information is listed
306 * Sorting the output:: Sorting the output
307 * Details about version sort:: More details about version sort
308 * General output formatting:: General output formatting
309 * Formatting the file names:: Formatting the file names
313 * cp invocation:: Copy files and directories
314 * dd invocation:: Convert and copy a file
315 * install invocation:: Copy files and set attributes
316 * mv invocation:: Move (rename) files
317 * rm invocation:: Remove files or directories
318 * shred invocation:: Remove files more securely
322 * link invocation:: Make a hard link via the link syscall
323 * ln invocation:: Make links between files
324 * mkdir invocation:: Make directories
325 * mkfifo invocation:: Make FIFOs (named pipes)
326 * mknod invocation:: Make block or character special files
327 * readlink invocation:: Print value of a symlink or canonical file name
328 * rmdir invocation:: Remove empty directories
329 * unlink invocation:: Remove files via unlink syscall
331 Changing file attributes
333 * chown invocation:: Change file owner and group
334 * chgrp invocation:: Change group ownership
335 * chmod invocation:: Change access permissions
336 * touch invocation:: Change file timestamps
340 * df invocation:: Report file system disk space usage
341 * du invocation:: Estimate file space usage
342 * stat invocation:: Report file or file system status
343 * sync invocation:: Synchronize data on disk with memory
344 * truncate invocation:: Shrink or extend the size of a file
348 * echo invocation:: Print a line of text
349 * printf invocation:: Format and print data
350 * yes invocation:: Print a string until interrupted
354 * false invocation:: Do nothing, unsuccessfully
355 * true invocation:: Do nothing, successfully
356 * test invocation:: Check file types and compare values
357 * expr invocation:: Evaluate expressions
359 @command{test}: Check file types and compare values
361 * File type tests:: File type tests
362 * Access permission tests:: Access permission tests
363 * File characteristic tests:: File characteristic tests
364 * String tests:: String tests
365 * Numeric tests:: Numeric tests
367 @command{expr}: Evaluate expression
369 * String expressions:: + : match substr index length
370 * Numeric expressions:: + - * / %
371 * Relations for expr:: | & < <= = == != >= >
372 * Examples of expr:: Examples of using @command{expr}
376 * tee invocation:: Redirect output to multiple files or processes
378 File name manipulation
380 * basename invocation:: Strip directory and suffix from a file name
381 * dirname invocation:: Strip last file name component
382 * pathchk invocation:: Check file name validity and portability
383 * mktemp invocation:: Create temporary file or directory
387 * pwd invocation:: Print working directory
388 * stty invocation:: Print or change terminal characteristics
389 * printenv invocation:: Print all or some environment variables
390 * tty invocation:: Print file name of terminal on standard input
392 @command{stty}: Print or change terminal characteristics
394 * Control:: Control settings
395 * Input:: Input settings
396 * Output:: Output settings
397 * Local:: Local settings
398 * Combination:: Combination settings
399 * Characters:: Special characters
400 * Special:: Special settings
404 * id invocation:: Print user identity
405 * logname invocation:: Print current login name
406 * whoami invocation:: Print effective user ID
407 * groups invocation:: Print group names a user is in
408 * users invocation:: Print login names of users currently logged in
409 * who invocation:: Print who is currently logged in
413 * arch invocation:: Print machine hardware name
414 * date invocation:: Print or set system date and time
415 * nproc invocation:: Print the number of processors
416 * uname invocation:: Print system information
417 * hostname invocation:: Print or set system name
418 * hostid invocation:: Print numeric host identifier
419 * uptime invocation:: Print system uptime and load
421 @command{date}: Print or set system date and time
423 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
424 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
425 * Literal conversion specifiers:: %[%nt]
426 * Padding and other flags:: Pad with zeros, spaces, etc.
427 * Setting the time:: Changing the system clock
428 * Options for date:: Instead of the current time
429 * Date input formats:: Specifying date strings
430 * Examples of date:: Examples
434 * chcon invocation:: Change SELinux context of file
435 * runcon invocation:: Run a command in specified SELinux context
437 Modified command invocation
439 * chroot invocation:: Run a command with a different root directory
440 * env invocation:: Run a command in a modified environment
441 * nice invocation:: Run a command with modified niceness
442 * nohup invocation:: Run a command immune to hangups
443 * stdbuf invocation:: Run a command with modified I/O buffering
444 * su invocation:: Run a command with substitute user and group ID
445 * timeout invocation:: Run a command with a time limit
449 * kill invocation:: Sending a signal to processes.
453 * sleep invocation:: Delay for a specified time
457 * factor invocation:: Print prime factors
458 * seq invocation:: Print numeric sequences
462 * Mode Structure:: Structure of file mode bits
463 * Symbolic Modes:: Mnemonic representation of file mode bits
464 * Numeric Modes:: File mode bits as octal numbers
465 * Directory Setuid and Setgid:: Set-user-ID and set-group-ID on directories
469 * General date syntax:: Common rules
470 * Calendar date items:: 19 Dec 1994
471 * Time of day items:: 9:20pm
472 * Time zone items:: @sc{est}, @sc{pdt}, @sc{gmt}
473 * Day of week items:: Monday and others
474 * Relative items in date strings:: next tuesday, 2 years ago
475 * Pure numbers in date strings:: 19931219, 1440
476 * Seconds since the Epoch:: @@1078100502
477 * Specifying time zone rules:: TZ="America/New_York", TZ="UTC0"
478 * Authors of parse_datetime:: Bellovin, Eggert, Salz, Berets, et al
480 Opening the software toolbox
482 * Toolbox introduction:: Toolbox introduction
483 * I/O redirection:: I/O redirection
484 * The who command:: The @command{who} command
485 * The cut command:: The @command{cut} command
486 * The sort command:: The @command{sort} command
487 * The uniq command:: The @command{uniq} command
488 * Putting the tools together:: Putting the tools together
492 * GNU Free Documentation License:: Copying and sharing this manual
499 @chapter Introduction
501 This manual is a work in progress: many sections make no attempt to explain
502 basic concepts in a way suitable for novices. Thus, if you are interested,
503 please get involved in improving this manual. The entire @sc{gnu} community
506 @cindex @acronym{POSIX}
507 The @sc{gnu} utilities documented here are mostly compatible with the
508 @acronym{POSIX} standard.
509 @cindex bugs, reporting
510 Please report bugs to @email{bug-coreutils@@gnu.org}. Remember
511 to include the version number, machine architecture, input files, and
512 any other information needed to reproduce the bug: your input, what you
513 expected, what you got, and why it is wrong. Diffs are welcome, but
514 please include a description of the problem as well, since this is
515 sometimes difficult to infer. @xref{Bugs, , , gcc, Using and Porting GNU CC}.
521 @cindex MacKenzie, D.
524 This manual was originally derived from the Unix man pages in the
525 distributions, which were written by David MacKenzie and updated by Jim
526 Meyering. What you are reading now is the authoritative documentation
527 for these utilities; the man pages are no longer being maintained. The
528 original @command{fmt} man page was written by Ross Paterson. Fran@,{c}ois
529 Pinard did the initial conversion to Texinfo format. Karl Berry did the
530 indexing, some reorganization, and editing of the results. Brian
531 Youmans of the Free Software Foundation office staff combined the
532 manuals for textutils, fileutils, and sh-utils to produce the present
533 omnibus manual. Richard Stallman contributed his usual invaluable
534 insights to the overall process.
537 @chapter Common options
541 @itemx @w{@kbd{--backup}[=@var{method}]}
544 @vindex VERSION_CONTROL
545 @cindex backups, making
546 @xref{Backup options}.
547 Make a backup of each file that would otherwise be overwritten or removed.
550 @macro optBackupSuffix
551 @item -S @var{suffix}
552 @itemx --suffix=@var{suffix}
555 Append @var{suffix} to each backup file made with @option{-b}.
556 @xref{Backup options}.
559 @macro optTargetDirectory
560 @item -t @var{directory}
561 @itemx @w{@kbd{--target-directory}=@var{directory}}
563 @opindex --target-directory
564 @cindex target directory
565 @cindex destination directory
566 Specify the destination @var{directory}.
567 @xref{Target directory}.
570 @macro optNoTargetDirectory
572 @itemx --no-target-directory
574 @opindex --no-target-directory
575 @cindex target directory
576 @cindex destination directory
577 Do not treat the last operand specially when it is a directory or a
578 symbolic link to a directory. @xref{Target directory}.
586 @cindex output @sc{nul}-byte-terminated lines
587 Output a zero byte (@acronym{ASCII} @sc{nul}) at the end of each line,
588 rather than a newline. This option enables other programs to parse the
589 output of @command{\cmd\} even when that output would contain data
590 with embedded newlines.
597 Append an SI-style abbreviation to each size, such as @samp{M} for
598 megabytes. Powers of 1000 are used, not 1024; @samp{M} stands for
599 1,000,000 bytes. This option is equivalent to
600 @option{--block-size=si}. Use the @option{-h} or
601 @option{--human-readable} option if
602 you prefer powers of 1024.
605 @macro optHumanReadable
607 @itemx --human-readable
609 @opindex --human-readable
610 @cindex human-readable output
611 Append a size letter to each size, such as @samp{M} for mebibytes.
612 Powers of 1024 are used, not 1000; @samp{M} stands for 1,048,576 bytes.
613 This option is equivalent to @option{--block-size=human-readable}.
614 Use the @option{--si} option if you prefer powers of 1000.
617 @macro optStripTrailingSlashes
618 @itemx @w{@kbd{--strip-trailing-slashes}}
619 @opindex --strip-trailing-slashes
620 @cindex stripping trailing slashes
621 Remove any trailing slashes from each @var{source} argument.
622 @xref{Trailing slashes}.
625 @macro mayConflictWithShellBuiltIn{cmd}
626 @cindex conflicts with shell built-ins
627 @cindex built-in shell commands, conflicts with
628 Due to shell aliases and built-in @command{\cmd\} functions, using an
629 unadorned @command{\cmd\} interactively or in a script may get you
630 different functionality than that described here. Invoke it via
631 @command{env} (i.e., @code{env \cmd\ @dots{}}) to avoid interference
636 @macro multiplierSuffixes{varName}
637 @var{\varName\} may be, or may be an integer optionally followed by,
638 one of the following multiplicative suffixes:
640 @samp{b} => 512 ("blocks")
641 @samp{KB} => 1000 (KiloBytes)
642 @samp{K} => 1024 (KibiBytes)
643 @samp{MB} => 1000*1000 (MegaBytes)
644 @samp{M} => 1024*1024 (MebiBytes)
645 @samp{GB} => 1000*1000*1000 (GigaBytes)
646 @samp{G} => 1024*1024*1024 (GibiBytes)
648 and so on for @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}.
651 @c FIXME: same as above, but no ``blocks'' line.
652 @macro multiplierSuffixesNoBlocks{varName}
653 @var{\varName\} may be, or may be an integer optionally followed by,
654 one of the following multiplicative suffixes:
656 @samp{KB} => 1000 (KiloBytes)
657 @samp{K} => 1024 (KibiBytes)
658 @samp{MB} => 1000*1000 (MegaBytes)
659 @samp{M} => 1024*1024 (MebiBytes)
660 @samp{GB} => 1000*1000*1000 (GigaBytes)
661 @samp{G} => 1024*1024*1024 (GibiBytes)
663 and so on for @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}.
666 @cindex common options
668 Certain options are available in all of these programs. Rather than
669 writing identical descriptions for each of the programs, they are
670 described here. (In fact, every @sc{gnu} program accepts (or should accept)
673 @vindex POSIXLY_CORRECT
674 Normally options and operands can appear in any order, and programs act
675 as if all the options appear before any operands. For example,
676 @samp{sort -r passwd -t :} acts like @samp{sort -r -t : passwd}, since
677 @samp{:} is an option-argument of @option{-t}. However, if the
678 @env{POSIXLY_CORRECT} environment variable is set, options must appear
679 before operands, unless otherwise specified for a particular command.
681 A few programs can usefully have trailing operands with leading
682 @samp{-}. With such a program, options must precede operands even if
683 @env{POSIXLY_CORRECT} is not set, and this fact is noted in the
684 program description. For example, the @command{env} command's options
685 must appear before its operands, since in some cases the operands
686 specify a command that itself contains options.
688 Most programs that accept long options recognize unambiguous
689 abbreviations of those options. For example, @samp{rmdir
690 --ignore-fail-on-non-empty} can be invoked as @samp{rmdir
691 --ignore-fail} or even @samp{rmdir --i}. Ambiguous options, such as
692 @samp{ls --h}, are identified as such.
694 Some of these programs recognize the @option{--help} and @option{--version}
695 options only when one of them is the sole command line argument. For
696 these programs, abbreviations of the long options are not always recognized.
703 Print a usage message listing all available options, then exit successfully.
707 @cindex version number, finding
708 Print the version number, then exit successfully.
712 @cindex option delimiter
713 Delimit the option list. Later arguments, if any, are treated as
714 operands even if they begin with @samp{-}. For example, @samp{sort --
715 -r} reads from the file named @file{-r}.
719 @cindex standard input
720 @cindex standard output
721 A single @samp{-} operand is not really an option, though it looks like one. It
722 stands for standard input, or for standard output if that is clear from
723 the context. For example, @samp{sort -} reads from standard input,
724 and is equivalent to plain @samp{sort}, and @samp{tee -} writes an
725 extra copy of its input to standard output. Unless otherwise
726 specified, @samp{-} can appear as any operand that requires a file
730 * Exit status:: Indicating program success or failure.
731 * Backup options:: -b -S, in some programs.
732 * Block size:: BLOCK_SIZE and --block-size, in some programs.
733 * Floating point:: Floating point number representation.
734 * Signal specifications:: Specifying signals using the --signal option.
735 * Disambiguating names and IDs:: chgrp and chown owner and group syntax
736 * Random sources:: --random-source, in some programs.
737 * Target directory:: Specifying a target directory, in some programs.
738 * Trailing slashes:: --strip-trailing-slashes, in some programs.
739 * Traversing symlinks:: -H, -L, or -P, in some programs.
740 * Treating / specially:: --preserve-root and --no-preserve-root.
741 * Special built-in utilities:: @command{break}, @command{:}, @dots{}
742 * Standards conformance:: Conformance to the @acronym{POSIX} standard.
750 An exit status of zero indicates success,
751 and a nonzero value indicates failure.
754 Nearly every command invocation yields an integral @dfn{exit status}
755 that can be used to change how other commands work.
756 For the vast majority of commands, an exit status of zero indicates
757 success. Failure is indicated by a nonzero value---typically
758 @samp{1}, though it may differ on unusual platforms as @acronym{POSIX}
759 requires only that it be nonzero.
761 However, some of the programs documented here do produce
762 other exit status values and a few associate different
763 meanings with the values @samp{0} and @samp{1}.
764 Here are some of the exceptions:
765 @command{chroot}, @command{env}, @command{expr}, @command{nice},
766 @command{nohup}, @command{printenv}, @command{sort}, @command{stdbuf},
767 @command{su}, @command{test}, @command{timeout}, @command{tty}.
771 @section Backup options
773 @cindex backup options
775 Some @sc{gnu} programs (at least @command{cp}, @command{install},
776 @command{ln}, and @command{mv}) optionally make backups of files
777 before writing new versions.
778 These options control the details of these backups. The options are also
779 briefly mentioned in the descriptions of the particular programs.
784 @itemx @w{@kbd{--backup}[=@var{method}]}
787 @vindex VERSION_CONTROL
788 @cindex backups, making
789 Make a backup of each file that would otherwise be overwritten or removed.
790 Without this option, the original versions are destroyed.
791 Use @var{method} to determine the type of backups to make.
792 When this option is used but @var{method} is not specified,
793 then the value of the @env{VERSION_CONTROL}
794 environment variable is used. And if @env{VERSION_CONTROL} is not set,
795 the default backup type is @samp{existing}.
797 Note that the short form of this option, @option{-b} does not accept any
798 argument. Using @option{-b} is equivalent to using @option{--backup=existing}.
800 @vindex version-control @r{Emacs variable}
801 This option corresponds to the Emacs variable @samp{version-control};
802 the values for @var{method} are the same as those used in Emacs.
803 This option also accepts more descriptive names.
804 The valid @var{method}s are (unique abbreviations are accepted):
809 @opindex none @r{backup method}
814 @opindex numbered @r{backup method}
815 Always make numbered backups.
819 @opindex existing @r{backup method}
820 Make numbered backups of files that already have them, simple backups
825 @opindex simple @r{backup method}
826 Always make simple backups. Please note @samp{never} is not to be
827 confused with @samp{none}.
831 @item -S @var{suffix}
832 @itemx --suffix=@var{suffix}
835 @cindex backup suffix
836 @vindex SIMPLE_BACKUP_SUFFIX
837 Append @var{suffix} to each backup file made with @option{-b}. If this
838 option is not specified, the value of the @env{SIMPLE_BACKUP_SUFFIX}
839 environment variable is used. And if @env{SIMPLE_BACKUP_SUFFIX} is not
840 set, the default is @samp{~}, just as in Emacs.
849 Some @sc{gnu} programs (at least @command{df}, @command{du}, and
850 @command{ls}) display sizes in ``blocks''. You can adjust the block size
851 and method of display to make sizes easier to read. The block size
852 used for display is independent of any file system block size.
853 Fractional block counts are rounded up to the nearest integer.
855 @opindex --block-size=@var{size}
858 @vindex DF_BLOCK_SIZE
859 @vindex DU_BLOCK_SIZE
860 @vindex LS_BLOCK_SIZE
861 @vindex POSIXLY_CORRECT@r{, and block size}
863 The default block size is chosen by examining the following environment
864 variables in turn; the first one that is set determines the block size.
869 This specifies the default block size for the @command{df} command.
870 Similarly, @env{DU_BLOCK_SIZE} specifies the default for @command{du} and
871 @env{LS_BLOCK_SIZE} for @command{ls}.
874 This specifies the default block size for all three commands, if the
875 above command-specific environment variables are not set.
878 This specifies the default block size for all values that are normally
879 printed as blocks, if neither @env{BLOCK_SIZE} nor the above
880 command-specific environment variables are set. Unlike the other
881 environment variables, @env{BLOCKSIZE} does not affect values that are
882 normally printed as byte counts, e.g., the file sizes contained in
885 @item POSIXLY_CORRECT
886 If neither @env{@var{command}_BLOCK_SIZE}, nor @env{BLOCK_SIZE}, nor
887 @env{BLOCKSIZE} is set, but this variable is set, the block size
892 If none of the above environment variables are set, the block size
893 currently defaults to 1024 bytes in most contexts, but this number may
894 change in the future. For @command{ls} file sizes, the block size
897 @cindex human-readable output
900 A block size specification can be a positive integer specifying the number
901 of bytes per block, or it can be @code{human-readable} or @code{si} to
902 select a human-readable format. Integers may be followed by suffixes
903 that are upward compatible with the
904 @uref{http://www.bipm.fr/enus/3_SI/si-prefixes.html, SI prefixes}
905 for decimal multiples and with the
906 @uref{http://physics.nist.gov/cuu/Units/binary.html, IEC 60027-2
907 prefixes for binary multiples}.
909 With human-readable formats, output sizes are followed by a size letter
910 such as @samp{M} for megabytes. @code{BLOCK_SIZE=human-readable} uses
911 powers of 1024; @samp{M} stands for 1,048,576 bytes.
912 @code{BLOCK_SIZE=si} is similar, but uses powers of 1000 and appends
913 @samp{B}; @samp{MB} stands for 1,000,000 bytes.
916 A block size specification preceded by @samp{'} causes output sizes to
917 be displayed with thousands separators. The @env{LC_NUMERIC} locale
918 specifies the thousands separator and grouping. For example, in an
919 American English locale, @samp{--block-size="'1kB"} would cause a size
920 of 1234000 bytes to be displayed as @samp{1,234}. In the default C
921 locale, there is no thousands separator so a leading @samp{'} has no
924 An integer block size can be followed by a suffix to specify a
925 multiple of that size. A bare size letter,
926 or one followed by @samp{iB}, specifies
927 a multiple using powers of 1024. A size letter followed by @samp{B}
928 specifies powers of 1000 instead. For example, @samp{1M} and
929 @samp{1MiB} are equivalent to @samp{1048576}, whereas @samp{1MB} is
930 equivalent to @samp{1000000}.
932 A plain suffix without a preceding integer acts as if @samp{1} were
933 prepended, except that it causes a size indication to be appended to
934 the output. For example, @samp{--block-size="kB"} displays 3000 as
937 The following suffixes are defined. Large sizes like @code{1Y}
938 may be rejected by your computer due to limitations of its arithmetic.
942 @cindex kilobyte, definition of
943 kilobyte: @math{10^3 = 1000}.
947 @cindex kibibyte, definition of
948 kibibyte: @math{2^{10} = 1024}. @samp{K} is special: the SI prefix is
949 @samp{k} and the IEC 60027-2 prefix is @samp{Ki}, but tradition and
950 @acronym{POSIX} use @samp{k} to mean @samp{KiB}.
952 @cindex megabyte, definition of
953 megabyte: @math{10^6 = 1,000,000}.
956 @cindex mebibyte, definition of
957 mebibyte: @math{2^{20} = 1,048,576}.
959 @cindex gigabyte, definition of
960 gigabyte: @math{10^9 = 1,000,000,000}.
963 @cindex gibibyte, definition of
964 gibibyte: @math{2^{30} = 1,073,741,824}.
966 @cindex terabyte, definition of
967 terabyte: @math{10^{12} = 1,000,000,000,000}.
970 @cindex tebibyte, definition of
971 tebibyte: @math{2^{40} = 1,099,511,627,776}.
973 @cindex petabyte, definition of
974 petabyte: @math{10^{15} = 1,000,000,000,000,000}.
977 @cindex pebibyte, definition of
978 pebibyte: @math{2^{50} = 1,125,899,906,842,624}.
980 @cindex exabyte, definition of
981 exabyte: @math{10^{18} = 1,000,000,000,000,000,000}.
984 @cindex exbibyte, definition of
985 exbibyte: @math{2^{60} = 1,152,921,504,606,846,976}.
987 @cindex zettabyte, definition of
988 zettabyte: @math{10^{21} = 1,000,000,000,000,000,000,000}
991 @math{2^{70} = 1,180,591,620,717,411,303,424}.
992 (@samp{Zi} is a @acronym{GNU} extension to IEC 60027-2.)
994 @cindex yottabyte, definition of
995 yottabyte: @math{10^{24} = 1,000,000,000,000,000,000,000,000}.
998 @math{2^{80} = 1,208,925,819,614,629,174,706,176}.
999 (@samp{Yi} is a @acronym{GNU} extension to IEC 60027-2.)
1004 @opindex --block-size
1005 @opindex --human-readable
1008 Block size defaults can be overridden by an explicit
1009 @option{--block-size=@var{size}} option. The @option{-k}
1010 option is equivalent to @option{--block-size=1K}, which
1011 is the default unless the @env{POSIXLY_CORRECT} environment variable is
1012 set. The @option{-h} or @option{--human-readable} option is equivalent to
1013 @option{--block-size=human-readable}. The @option{--si} option is
1014 equivalent to @option{--block-size=si}.
1016 @node Floating point
1017 @section Floating point numbers
1018 @cindex floating point
1019 @cindex IEEE floating point
1021 Commands that accept or produce floating point numbers employ the
1022 floating point representation of the underlying system, and suffer
1023 from rounding error, overflow, and similar floating-point issues.
1024 Almost all modern systems use IEEE-754 floating point, and it is
1025 typically portable to assume IEEE-754 behavior these days. IEEE-754
1026 has positive and negative infinity, distinguishes positive from
1027 negative zero, and uses special values called NaNs to represent
1028 invalid computations such as dividing zero by itself. For more
1029 information, please see David Goldberg's paper
1030 @uref{http://@/www.validlab.com/@/goldberg/@/paper.pdf, What Every
1031 Computer Scientist Should Know About Floating-Point Arithmetic}.
1034 Commands that accept floating point numbers as options, operands or
1035 input use the standard C functions @code{strtod} and @code{strtold} to
1036 convert from text to floating point numbers. These floating point
1037 numbers therefore can use scientific notation like @code{1.0e-34} and
1038 @code{-10e100}. Modern C implementations also accept hexadecimal
1039 floating point numbers such as @code{-0x.ep-3}, which stands for
1040 @minus{}14/16 times @math{2^-3}, which equals @minus{}0.109375. The
1041 @env{LC_NUMERIC} locale determines the decimal-point character.
1042 @xref{Parsing of Floats,,, libc, The GNU C Library Reference Manual}.
1044 @node Signal specifications
1045 @section Signal specifications
1046 @cindex signals, specifying
1048 A @var{signal} may be a signal name like @samp{HUP}, or a signal
1049 number like @samp{1}, or an exit status of a process terminated by the
1050 signal. A signal name can be given in canonical form or prefixed by
1051 @samp{SIG}. The case of the letters is ignored. The following signal names
1052 and numbers are supported on all @acronym{POSIX} compliant systems:
1058 2. Terminal interrupt.
1064 9. Kill (cannot be caught or ignored).
1072 Other supported signal names have system-dependent corresponding
1073 numbers. All systems conforming to @acronym{POSIX} 1003.1-2001 also
1074 support the following signals:
1078 Access to an undefined portion of a memory object.
1080 Child process terminated, stopped, or continued.
1082 Continue executing, if stopped.
1084 Erroneous arithmetic operation.
1086 Illegal Instruction.
1088 Write on a pipe with no one to read it.
1090 Invalid memory reference.
1092 Stop executing (cannot be caught or ignored).
1096 Background process attempting read.
1098 Background process attempting write.
1100 High bandwidth data is available at a socket.
1102 User-defined signal 1.
1104 User-defined signal 2.
1108 @acronym{POSIX} 1003.1-2001 systems that support the @acronym{XSI} extension
1109 also support the following signals:
1115 Profiling timer expired.
1119 Trace/breakpoint trap.
1121 Virtual timer expired.
1123 CPU time limit exceeded.
1125 File size limit exceeded.
1129 @acronym{POSIX} 1003.1-2001 systems that support the @acronym{XRT} extension
1130 also support at least eight real-time signals called @samp{RTMIN},
1131 @samp{RTMIN+1}, @dots{}, @samp{RTMAX-1}, @samp{RTMAX}.
1133 @node Disambiguating names and IDs
1134 @section chown and chgrp: Disambiguating user names and IDs
1135 @cindex user names, disambiguating
1136 @cindex user IDs, disambiguating
1137 @cindex group names, disambiguating
1138 @cindex group IDs, disambiguating
1139 @cindex disambiguating group names and IDs
1141 Since the @var{owner} and @var{group} arguments to @command{chown} and
1142 @command{chgrp} may be specified as names or numeric IDs, there is an
1144 What if a user or group @emph{name} is a string of digits?
1145 @footnote{Using a number as a user name is common in some environments.}
1146 Should the command interpret it as a user name or as an ID?
1147 @acronym{POSIX} requires that @command{chown} and @command{chgrp}
1148 first attempt to resolve the specified string as a name, and
1149 only once that fails, then try to interpret it as an ID.
1150 This is troublesome when you want to specify a numeric ID, say 42,
1151 and it must work even in a pathological situation where
1152 @samp{42} is a user name that maps to some other user ID, say 1000.
1153 Simply invoking @code{chown 42 F}, will set @file{F}s owner ID to
1154 1000---not what you intended.
1156 GNU @command{chown} and @command{chgrp} provide a way to work around this,
1157 that at the same time may result in a significant performance improvement
1158 by eliminating a database look-up.
1159 Simply precede each numeric user ID and/or group ID with a @samp{+},
1160 in order to force its interpretation as an integer:
1164 chgrp +$numeric_group_id another-file
1168 GNU @command{chown} and @command{chgrp}
1169 skip the name look-up process for each @samp{+}-prefixed string,
1170 because a string containing @samp{+} is never a valid user or group name.
1171 This syntax is accepted on most common Unix systems, but not on Solaris 10.
1173 @node Random sources
1174 @section Sources of random data
1176 @cindex random sources
1178 The @command{shuf}, @command{shred}, and @command{sort} commands
1179 sometimes need random data to do their work. For example, @samp{sort
1180 -R} must choose a hash function at random, and it needs random data to
1181 make this selection.
1183 By default these commands use an internal pseudorandom generator
1184 initialized by a small amount of entropy, but can be directed to use
1185 an external source with the @option{--random-source=@var{file}} option.
1186 An error is reported if @var{file} does not contain enough bytes.
1188 For example, the device file @file{/dev/urandom} could be used as the
1189 source of random data. Typically, this device gathers environmental
1190 noise from device drivers and other sources into an entropy pool, and
1191 uses the pool to generate random bits. If the pool is short of data,
1192 the device reuses the internal pool to produce more bits, using a
1193 cryptographically secure pseudorandom number generator. But be aware
1194 that this device is not designed for bulk random data generation
1195 and is relatively slow.
1197 @file{/dev/urandom} suffices for most practical uses, but applications
1198 requiring high-value or long-term protection of private data may
1199 require an alternate data source like @file{/dev/random} or
1200 @file{/dev/arandom}. The set of available sources depends on your
1203 To reproduce the results of an earlier invocation of a command, you
1204 can save some random data into a file and then use that file as the
1205 random source in earlier and later invocations of the command.
1207 @node Target directory
1208 @section Target directory
1210 @cindex target directory
1212 The @command{cp}, @command{install}, @command{ln}, and @command{mv}
1213 commands normally treat the last operand specially when it is a
1214 directory or a symbolic link to a directory. For example, @samp{cp
1215 source dest} is equivalent to @samp{cp source dest/source} if
1216 @file{dest} is a directory. Sometimes this behavior is not exactly
1217 what is wanted, so these commands support the following options to
1218 allow more fine-grained control:
1223 @itemx --no-target-directory
1224 @opindex --no-target-directory
1225 @cindex target directory
1226 @cindex destination directory
1227 Do not treat the last operand specially when it is a directory or a
1228 symbolic link to a directory. This can help avoid race conditions in
1229 programs that operate in a shared area. For example, when the command
1230 @samp{mv /tmp/source /tmp/dest} succeeds, there is no guarantee that
1231 @file{/tmp/source} was renamed to @file{/tmp/dest}: it could have been
1232 renamed to @file{/tmp/dest/source} instead, if some other process
1233 created @file{/tmp/dest} as a directory. However, if @file{mv
1234 -T /tmp/source /tmp/dest} succeeds, there is no
1235 question that @file{/tmp/source} was renamed to @file{/tmp/dest}.
1237 In the opposite situation, where you want the last operand to be
1238 treated as a directory and want a diagnostic otherwise, you can use
1239 the @option{--target-directory} (@option{-t}) option.
1241 @item -t @var{directory}
1242 @itemx @w{@kbd{--target-directory}=@var{directory}}
1243 @opindex --target-directory
1244 @cindex target directory
1245 @cindex destination directory
1246 Use @var{directory} as the directory component of each destination
1249 The interface for most programs is that after processing options and a
1250 finite (possibly zero) number of fixed-position arguments, the remaining
1251 argument list is either expected to be empty, or is a list of items
1252 (usually files) that will all be handled identically. The @command{xargs}
1253 program is designed to work well with this convention.
1255 The commands in the @command{mv}-family are unusual in that they take
1256 a variable number of arguments with a special case at the @emph{end}
1257 (namely, the target directory). This makes it nontrivial to perform some
1258 operations, e.g., ``move all files from here to ../d/'', because
1259 @code{mv * ../d/} might exhaust the argument space, and @code{ls | xargs ...}
1260 doesn't have a clean way to specify an extra final argument for each
1261 invocation of the subject command. (It can be done by going through a
1262 shell command, but that requires more human labor and brain power than
1265 The @w{@kbd{--target-directory}} (@option{-t}) option allows the @command{cp},
1266 @command{install}, @command{ln}, and @command{mv} programs to be used
1267 conveniently with @command{xargs}. For example, you can move the files
1268 from the current directory to a sibling directory, @code{d} like this:
1271 ls | xargs mv -t ../d --
1274 However, this doesn't move files whose names begin with @samp{.}.
1275 If you use the @sc{gnu} @command{find} program, you can move those
1276 files too, with this command:
1279 find . -mindepth 1 -maxdepth 1 \
1283 But both of the above approaches fail if there are no files in the
1284 current directory, or if any file has a name containing a blank or
1285 some other special characters.
1286 The following example removes those limitations and requires both
1287 @sc{gnu} @command{find} and @sc{gnu} @command{xargs}:
1290 find . -mindepth 1 -maxdepth 1 -print0 \
1291 | xargs --null --no-run-if-empty \
1298 The @option{--target-directory} (@option{-t}) and
1299 @option{--no-target-directory} (@option{-T})
1300 options cannot be combined.
1302 @node Trailing slashes
1303 @section Trailing slashes
1305 @cindex trailing slashes
1307 Some @sc{gnu} programs (at least @command{cp} and @command{mv}) allow you to
1308 remove any trailing slashes from each @var{source} argument before
1309 operating on it. The @w{@kbd{--strip-trailing-slashes}} option enables
1312 This is useful when a @var{source} argument may have a trailing slash and
1313 @c FIXME: mv's behavior in this case is system-dependent
1314 specify a symbolic link to a directory. This scenario is in fact rather
1315 common because some shells can automatically append a trailing slash when
1316 performing file name completion on such symbolic links. Without this
1317 option, @command{mv}, for example, (via the system's rename function) must
1318 interpret a trailing slash as a request to dereference the symbolic link
1319 and so must rename the indirectly referenced @emph{directory} and not
1320 the symbolic link. Although it may seem surprising that such behavior
1321 be the default, it is required by @acronym{POSIX} and is consistent with
1322 other parts of that standard.
1324 @node Traversing symlinks
1325 @section Traversing symlinks
1327 @cindex symbolic link to directory, controlling traversal of
1329 The following options modify how @command{chown} and @command{chgrp}
1330 @c FIXME: note that `du' has these options, too, but they have slightly
1331 @c different meaning.
1332 traverse a hierarchy when the @option{--recursive} (@option{-R})
1333 option is also specified.
1334 If more than one of the following options is specified, only the final
1336 These options specify whether processing a symbolic link to a directory
1337 entails operating on just the symbolic link or on all files in the
1338 hierarchy rooted at that directory.
1340 These options are independent of @option{--dereference} and
1341 @option{--no-dereference} (@option{-h}), which control whether to modify
1342 a symlink or its referent.
1349 @cindex symbolic link to directory, traverse if on the command line
1350 If @option{--recursive} (@option{-R}) is specified and
1351 a command line argument is a symbolic link to a directory, traverse it.
1358 @cindex symbolic link to directory, traverse each that is encountered
1359 In a recursive traversal, traverse every symbolic link to a directory
1360 that is encountered.
1367 @cindex symbolic link to directory, never traverse
1368 Do not traverse any symbolic links.
1369 This is the default if none of @option{-H}, @option{-L},
1370 or @option{-P} is specified.
1377 @node Treating / specially
1378 @section Treating @file{/} specially
1380 Certain commands can operate destructively on entire hierarchies.
1381 For example, if a user with appropriate privileges mistakenly runs
1382 @samp{rm -rf / tmp/junk}, that may remove
1383 all files on the entire system. Since there are so few
1384 legitimate uses for such a command,
1385 @sc{gnu} @command{rm} normally declines to operate on any directory
1386 that resolves to @file{/}. If you really want to try to remove all
1387 the files on your system, you can use the @option{--no-preserve-root}
1388 option, but the default behavior, specified by the
1389 @option{--preserve-option}, is safer for most purposes.
1391 The commands @command{chgrp}, @command{chmod} and @command{chown}
1392 can also operate destructively on entire hierarchies, so they too
1393 support these options. Although, unlike @command{rm}, they don't
1394 actually unlink files, these commands are arguably more dangerous
1395 when operating recursively on @file{/}, since they often work much
1396 more quickly, and hence damage more files before an alert user can
1397 interrupt them. Tradition and @acronym{POSIX} require these commands
1398 to operate recursively on @file{/}, so they default to
1399 @option{--no-preserve-root}, but using the @option{--preserve-root}
1400 option makes them safer for most purposes. For convenience you can
1401 specify @option{--preserve-root} in an alias or in a shell function.
1403 Note that the @option{--preserve-root} option also ensures
1404 that @command{chgrp} and @command{chown} do not modify @file{/}
1405 even when dereferencing a symlink pointing to @file{/}.
1407 @node Special built-in utilities
1408 @section Special built-in utilities
1410 Some programs like @command{nice} can invoke other programs; for
1411 example, the command @samp{nice cat file} invokes the program
1412 @command{cat} by executing the command @samp{cat file}. However,
1413 @dfn{special built-in utilities} like @command{exit} cannot be invoked
1414 this way. For example, the command @samp{nice exit} does not have a
1415 well-defined behavior: it may generate an error message instead of
1418 Here is a list of the special built-in utilities that are standardized
1419 by @acronym{POSIX} 1003.1-2004.
1422 @t{.@: : break continue eval exec exit export readonly
1423 return set shift times trap unset}
1426 For example, because @samp{.}, @samp{:}, and @samp{exec} are special,
1427 the commands @samp{nice . foo.sh}, @samp{nice :}, and @samp{nice exec
1428 pwd} do not work as you might expect.
1430 Many shells extend this list. For example, Bash has several extra
1431 special built-in utilities like @command{history}, and
1432 @command{suspend}, and with Bash the command @samp{nice suspend}
1433 generates an error message instead of suspending.
1435 @node Standards conformance
1436 @section Standards conformance
1438 @vindex POSIXLY_CORRECT
1439 In a few cases, the @sc{gnu} utilities' default behavior is
1440 incompatible with the @acronym{POSIX} standard. To suppress these
1441 incompatibilities, define the @env{POSIXLY_CORRECT} environment
1442 variable. Unless you are checking for @acronym{POSIX} conformance, you
1443 probably do not need to define @env{POSIXLY_CORRECT}.
1445 Newer versions of @acronym{POSIX} are occasionally incompatible with older
1446 versions. For example, older versions of @acronym{POSIX} required the
1447 command @samp{sort +1} to sort based on the second and succeeding
1448 fields in each input line, but starting with @acronym{POSIX} 1003.1-2001
1449 the same command is required to sort the file named @file{+1}, and you
1450 must instead use the command @samp{sort -k 2} to get the field-based
1453 @vindex _POSIX2_VERSION
1454 The @sc{gnu} utilities normally conform to the version of @acronym{POSIX}
1455 that is standard for your system. To cause them to conform to a
1456 different version of @acronym{POSIX}, define the @env{_POSIX2_VERSION}
1457 environment variable to a value of the form @var{yyyymm} specifying
1458 the year and month the standard was adopted. Two values are currently
1459 supported for @env{_POSIX2_VERSION}: @samp{199209} stands for
1460 @acronym{POSIX} 1003.2-1992, and @samp{200112} stands for @acronym{POSIX}
1461 1003.1-2001. For example, if you have a newer system but are running software
1462 that assumes an older version of @acronym{POSIX} and uses @samp{sort +1}
1463 or @samp{tail +10}, you can work around any compatibility problems by setting
1464 @samp{_POSIX2_VERSION=199209} in your environment.
1466 @node Output of entire files
1467 @chapter Output of entire files
1469 @cindex output of entire files
1470 @cindex entire files, output of
1472 These commands read and write entire files, possibly transforming them
1476 * cat invocation:: Concatenate and write files.
1477 * tac invocation:: Concatenate and write files in reverse.
1478 * nl invocation:: Number lines and write files.
1479 * od invocation:: Write files in octal or other formats.
1480 * base64 invocation:: Transform data into printable data.
1483 @node cat invocation
1484 @section @command{cat}: Concatenate and write files
1487 @cindex concatenate and write files
1488 @cindex copying files
1490 @command{cat} copies each @var{file} (@samp{-} means standard input), or
1491 standard input if none are given, to standard output. Synopsis:
1494 cat [@var{option}] [@var{file}]@dots{}
1497 The program accepts the following options. Also see @ref{Common options}.
1505 Equivalent to @option{-vET}.
1508 @itemx --number-nonblank
1510 @opindex --number-nonblank
1511 Number all nonempty output lines, starting with 1.
1515 Equivalent to @option{-vE}.
1520 @opindex --show-ends
1521 Display a @samp{$} after the end of each line.
1527 Number all output lines, starting with 1. This option is ignored
1528 if @option{-b} is in effect.
1531 @itemx --squeeze-blank
1533 @opindex --squeeze-blank
1534 @cindex squeezing empty lines
1535 Suppress repeated adjacent empty lines; output just one empty line
1540 Equivalent to @option{-vT}.
1545 @opindex --show-tabs
1546 Display TAB characters as @samp{^I}.
1550 Ignored; for @acronym{POSIX} compatibility.
1553 @itemx --show-nonprinting
1555 @opindex --show-nonprinting
1556 Display control characters except for LFD and TAB using
1557 @samp{^} notation and precede characters that have the high bit set with
1562 On systems like MS-DOS that distinguish between text and binary files,
1563 @command{cat} normally reads and writes in binary mode. However,
1564 @command{cat} reads in text mode if one of the options
1565 @option{-bensAE} is used or if @command{cat} is reading from standard
1566 input and standard input is a terminal. Similarly, @command{cat}
1567 writes in text mode if one of the options @option{-bensAE} is used or
1568 if standard output is a terminal.
1575 # Output f's contents, then standard input, then g's contents.
1578 # Copy standard input to standard output.
1583 @node tac invocation
1584 @section @command{tac}: Concatenate and write files in reverse
1587 @cindex reversing files
1589 @command{tac} copies each @var{file} (@samp{-} means standard input), or
1590 standard input if none are given, to standard output, reversing the
1591 records (lines by default) in each separately. Synopsis:
1594 tac [@var{option}]@dots{} [@var{file}]@dots{}
1597 @dfn{Records} are separated by instances of a string (newline by
1598 default). By default, this separator string is attached to the end of
1599 the record that it follows in the file.
1601 The program accepts the following options. Also see @ref{Common options}.
1609 The separator is attached to the beginning of the record that it
1610 precedes in the file.
1616 Treat the separator string as a regular expression. Users of @command{tac}
1617 on MS-DOS/MS-Windows should note that, since @command{tac} reads files in
1618 binary mode, each line of a text file might end with a CR/LF pair
1619 instead of the Unix-style LF.
1621 @item -s @var{separator}
1622 @itemx --separator=@var{separator}
1624 @opindex --separator
1625 Use @var{separator} as the record separator, instead of newline.
1633 @section @command{nl}: Number lines and write files
1636 @cindex numbering lines
1637 @cindex line numbering
1639 @command{nl} writes each @var{file} (@samp{-} means standard input), or
1640 standard input if none are given, to standard output, with line numbers
1641 added to some or all of the lines. Synopsis:
1644 nl [@var{option}]@dots{} [@var{file}]@dots{}
1647 @cindex logical pages, numbering on
1648 @command{nl} decomposes its input into (logical) pages; by default, the
1649 line number is reset to 1 at the top of each logical page. @command{nl}
1650 treats all of the input files as a single document; it does not reset
1651 line numbers or logical pages between files.
1653 @cindex headers, numbering
1654 @cindex body, numbering
1655 @cindex footers, numbering
1656 A logical page consists of three sections: header, body, and footer.
1657 Any of the sections can be empty. Each can be numbered in a different
1658 style from the others.
1660 The beginnings of the sections of logical pages are indicated in the
1661 input file by a line containing exactly one of these delimiter strings:
1672 The two characters from which these strings are made can be changed from
1673 @samp{\} and @samp{:} via options (see below), but the pattern and
1674 length of each string cannot be changed.
1676 A section delimiter is replaced by an empty line on output. Any text
1677 that comes before the first section delimiter string in the input file
1678 is considered to be part of a body section, so @command{nl} treats a
1679 file that contains no section delimiters as a single body section.
1681 The program accepts the following options. Also see @ref{Common options}.
1685 @item -b @var{style}
1686 @itemx --body-numbering=@var{style}
1688 @opindex --body-numbering
1689 Select the numbering style for lines in the body section of each
1690 logical page. When a line is not numbered, the current line number
1691 is not incremented, but the line number separator character is still
1692 prepended to the line. The styles are:
1698 number only nonempty lines (default for body),
1700 do not number lines (default for header and footer),
1702 number only lines that contain a match for the basic regular
1703 expression @var{bre}.
1704 @xref{Regular Expressions, , Regular Expressions, grep, The GNU Grep Manual}.
1708 @itemx --section-delimiter=@var{cd}
1710 @opindex --section-delimiter
1711 @cindex section delimiters of pages
1712 Set the section delimiter characters to @var{cd}; default is
1713 @samp{\:}. If only @var{c} is given, the second remains @samp{:}.
1714 (Remember to protect @samp{\} or other metacharacters from shell
1715 expansion with quotes or extra backslashes.)
1717 @item -f @var{style}
1718 @itemx --footer-numbering=@var{style}
1720 @opindex --footer-numbering
1721 Analogous to @option{--body-numbering}.
1723 @item -h @var{style}
1724 @itemx --header-numbering=@var{style}
1726 @opindex --header-numbering
1727 Analogous to @option{--body-numbering}.
1729 @item -i @var{number}
1730 @itemx --line-increment=@var{number}
1732 @opindex --line-increment
1733 Increment line numbers by @var{number} (default 1).
1735 @item -l @var{number}
1736 @itemx --join-blank-lines=@var{number}
1738 @opindex --join-blank-lines
1739 @cindex empty lines, numbering
1740 @cindex blank lines, numbering
1741 Consider @var{number} (default 1) consecutive empty lines to be one
1742 logical line for numbering, and only number the last one. Where fewer
1743 than @var{number} consecutive empty lines occur, do not number them.
1744 An empty line is one that contains no characters, not even spaces
1747 @item -n @var{format}
1748 @itemx --number-format=@var{format}
1750 @opindex --number-format
1751 Select the line numbering format (default is @code{rn}):
1755 @opindex ln @r{format for @command{nl}}
1756 left justified, no leading zeros;
1758 @opindex rn @r{format for @command{nl}}
1759 right justified, no leading zeros;
1761 @opindex rz @r{format for @command{nl}}
1762 right justified, leading zeros.
1766 @itemx --no-renumber
1768 @opindex --no-renumber
1769 Do not reset the line number at the start of a logical page.
1771 @item -s @var{string}
1772 @itemx --number-separator=@var{string}
1774 @opindex --number-separator
1775 Separate the line number from the text line in the output with
1776 @var{string} (default is the TAB character).
1778 @item -v @var{number}
1779 @itemx --starting-line-number=@var{number}
1781 @opindex --starting-line-number
1782 Set the initial line number on each logical page to @var{number} (default 1).
1784 @item -w @var{number}
1785 @itemx --number-width=@var{number}
1787 @opindex --number-width
1788 Use @var{number} characters for line numbers (default 6).
1796 @section @command{od}: Write files in octal or other formats
1799 @cindex octal dump of files
1800 @cindex hex dump of files
1801 @cindex ASCII dump of files
1802 @cindex file contents, dumping unambiguously
1804 @command{od} writes an unambiguous representation of each @var{file}
1805 (@samp{-} means standard input), or standard input if none are given.
1809 od [@var{option}]@dots{} [@var{file}]@dots{}
1810 od [-abcdfilosx]@dots{} [@var{file}] [[+]@var{offset}[.][b]]
1811 od [@var{option}]@dots{} --traditional [@var{file}]@c
1812 [[+]@var{offset}[.][b] [[+]@var{label}[.][b]]]
1815 Each line of output consists of the offset in the input, followed by
1816 groups of data from the file. By default, @command{od} prints the offset in
1817 octal, and each group of file data is a C @code{short int}'s worth of input
1818 printed as a single octal number.
1820 If @var{offset} is given, it specifies how many input bytes to skip
1821 before formatting and writing. By default, it is interpreted as an
1822 octal number, but the optional trailing decimal point causes it to be
1823 interpreted as decimal. If no decimal is specified and the offset
1824 begins with @samp{0x} or @samp{0X} it is interpreted as a hexadecimal
1825 number. If there is a trailing @samp{b}, the number of bytes skipped
1826 will be @var{offset} multiplied by 512.
1828 If a command is of both the first and second forms, the second form is
1829 assumed if the last operand begins with @samp{+} or (if there are two
1830 operands) a digit. For example, in @samp{od foo 10} and @samp{od +10}
1831 the @samp{10} is an offset, whereas in @samp{od 10} the @samp{10} is a
1834 The program accepts the following options. Also see @ref{Common options}.
1838 @item -A @var{radix}
1839 @itemx --address-radix=@var{radix}
1841 @opindex --address-radix
1842 @cindex radix for file offsets
1843 @cindex file offset radix
1844 Select the base in which file offsets are printed. @var{radix} can
1845 be one of the following:
1855 none (do not print offsets).
1858 The default is octal.
1860 @item -j @var{bytes}
1861 @itemx --skip-bytes=@var{bytes}
1863 @opindex --skip-bytes
1864 Skip @var{bytes} input bytes before formatting and writing. If
1865 @var{bytes} begins with @samp{0x} or @samp{0X}, it is interpreted in
1866 hexadecimal; otherwise, if it begins with @samp{0}, in octal; otherwise,
1868 @multiplierSuffixes{bytes}
1870 @item -N @var{bytes}
1871 @itemx --read-bytes=@var{bytes}
1873 @opindex --read-bytes
1874 Output at most @var{bytes} bytes of the input. Prefixes and suffixes on
1875 @code{bytes} are interpreted as for the @option{-j} option.
1877 @item -S @var{bytes}
1878 @itemx --strings[=@var{bytes}]
1881 @cindex string constants, outputting
1882 Instead of the normal output, output only @dfn{string constants}: at
1883 least @var{bytes} consecutive @acronym{ASCII} graphic characters,
1884 followed by a zero byte (@acronym{ASCII} @sc{nul}).
1885 Prefixes and suffixes on @code{bytes} are interpreted as for the
1888 If @var{n} is omitted with @option{--strings}, the default is 3.
1891 @itemx --format=@var{type}
1894 Select the format in which to output the file data. @var{type} is a
1895 string of one or more of the below type indicator characters. If you
1896 include more than one type indicator character in a single @var{type}
1897 string, or use this option more than once, @command{od} writes one copy
1898 of each output line using each of the data types that you specified,
1899 in the order that you specified.
1901 Adding a trailing ``z'' to any type specification appends a display
1902 of the @acronym{ASCII} character representation of the printable characters
1903 to the output line generated by the type specification.
1907 named character, ignoring high-order bit
1909 @acronym{ASCII} character or backslash escape,
1913 floating point (@pxref{Floating point})
1922 The type @code{a} outputs things like @samp{sp} for space, @samp{nl} for
1923 newline, and @samp{nul} for a zero byte. Only the least significant
1924 seven bits of each byte is used; the high-order bit is ignored.
1925 Type @code{c} outputs
1926 @samp{ }, @samp{\n}, and @code{\0}, respectively.
1929 Except for types @samp{a} and @samp{c}, you can specify the number
1930 of bytes to use in interpreting each number in the given data type
1931 by following the type indicator character with a decimal integer.
1932 Alternately, you can specify the size of one of the C compiler's
1933 built-in data types by following the type indicator character with
1934 one of the following characters. For integers (@samp{d}, @samp{o},
1935 @samp{u}, @samp{x}):
1948 For floating point (@code{f}):
1960 @itemx --output-duplicates
1962 @opindex --output-duplicates
1963 Output consecutive lines that are identical. By default, when two or
1964 more consecutive output lines would be identical, @command{od} outputs only
1965 the first line, and puts just an asterisk on the following line to
1966 indicate the elision.
1969 @itemx --width[=@var{n}]
1972 Dump @code{n} input bytes per output line. This must be a multiple of
1973 the least common multiple of the sizes associated with the specified
1976 If this option is not given at all, the default is 16. If @var{n} is
1977 omitted, the default is 32.
1981 The next several options are shorthands for format specifications.
1982 @sc{gnu} @command{od} accepts any combination of shorthands and format
1983 specification options. These options accumulate.
1989 Output as named characters. Equivalent to @samp{-t a}.
1993 Output as octal bytes. Equivalent to @samp{-t o1}.
1997 Output as @acronym{ASCII} characters or backslash escapes. Equivalent to
2002 Output as unsigned decimal two-byte units. Equivalent to @samp{-t u2}.
2006 Output as floats. Equivalent to @samp{-t fF}.
2010 Output as decimal ints. Equivalent to @samp{-t dI}.
2014 Output as decimal long ints. Equivalent to @samp{-t dL}.
2018 Output as octal two-byte units. Equivalent to @option{-t o2}.
2022 Output as decimal two-byte units. Equivalent to @option{-t d2}.
2026 Output as hexadecimal two-byte units. Equivalent to @samp{-t x2}.
2029 @opindex --traditional
2030 Recognize the non-option label argument that traditional @command{od}
2031 accepted. The following syntax:
2034 od --traditional [@var{file}] [[+]@var{offset}[.][b] [[+]@var{label}[.][b]]]
2038 can be used to specify at most one file and optional arguments
2039 specifying an offset and a pseudo-start address, @var{label}.
2040 The @var{label} argument is interpreted
2041 just like @var{offset}, but it specifies an initial pseudo-address. The
2042 pseudo-addresses are displayed in parentheses following any normal
2049 @node base64 invocation
2050 @section @command{base64}: Transform data into printable data
2053 @cindex base64 encoding
2055 @command{base64} transforms data read from a file, or standard input,
2056 into (or from) base64 encoded form. The base64 encoded form uses
2057 printable @acronym{ASCII} characters to represent binary data.
2061 base64 [@var{option}]@dots{} [@var{file}]
2062 base64 --decode [@var{option}]@dots{} [@var{file}]
2065 The base64 encoding expands data to roughly 133% of the original.
2066 The format conforms to
2067 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc4648.txt, RFC 4648}.
2069 The program accepts the following options. Also see @ref{Common options}.
2074 @itemx --wrap=@var{cols}
2078 @cindex column to wrap data after
2079 During encoding, wrap lines after @var{cols} characters. This must be
2082 The default is to wrap after 76 characters. Use the value 0 to
2083 disable line wrapping altogether.
2089 @cindex Decode base64 data
2090 @cindex Base64 decoding
2091 Change the mode of operation, from the default of encoding data, to
2092 decoding data. Input is expected to be base64 encoded data, and the
2093 output will be the original data.
2096 @itemx --ignore-garbage
2098 @opindex --ignore-garbage
2099 @cindex Ignore garbage in base64 stream
2100 When decoding, newlines are always accepted.
2101 During decoding, ignore unrecognized bytes,
2102 to permit distorted data to be decoded.
2109 @node Formatting file contents
2110 @chapter Formatting file contents
2112 @cindex formatting file contents
2114 These commands reformat the contents of files.
2117 * fmt invocation:: Reformat paragraph text.
2118 * pr invocation:: Paginate or columnate files for printing.
2119 * fold invocation:: Wrap input lines to fit in specified width.
2123 @node fmt invocation
2124 @section @command{fmt}: Reformat paragraph text
2127 @cindex reformatting paragraph text
2128 @cindex paragraphs, reformatting
2129 @cindex text, reformatting
2131 @command{fmt} fills and joins lines to produce output lines of (at most)
2132 a given number of characters (75 by default). Synopsis:
2135 fmt [@var{option}]@dots{} [@var{file}]@dots{}
2138 @command{fmt} reads from the specified @var{file} arguments (or standard
2139 input if none are given), and writes to standard output.
2141 By default, blank lines, spaces between words, and indentation are
2142 preserved in the output; successive input lines with different
2143 indentation are not joined; tabs are expanded on input and introduced on
2146 @cindex line-breaking
2147 @cindex sentences and line-breaking
2148 @cindex Knuth, Donald E.
2149 @cindex Plass, Michael F.
2150 @command{fmt} prefers breaking lines at the end of a sentence, and tries to
2151 avoid line breaks after the first word of a sentence or before the last
2152 word of a sentence. A @dfn{sentence break} is defined as either the end
2153 of a paragraph or a word ending in any of @samp{.?!}, followed by two
2154 spaces or end of line, ignoring any intervening parentheses or quotes.
2155 Like @TeX{}, @command{fmt} reads entire ``paragraphs'' before choosing line
2156 breaks; the algorithm is a variant of that given by Donald E. Knuth
2157 and Michael F. Plass in ``Breaking Paragraphs Into Lines'',
2158 @cite{Software---Practice & Experience} @b{11}, 11 (November 1981),
2161 The program accepts the following options. Also see @ref{Common options}.
2166 @itemx --crown-margin
2168 @opindex --crown-margin
2169 @cindex crown margin
2170 @dfn{Crown margin} mode: preserve the indentation of the first two
2171 lines within a paragraph, and align the left margin of each subsequent
2172 line with that of the second line.
2175 @itemx --tagged-paragraph
2177 @opindex --tagged-paragraph
2178 @cindex tagged paragraphs
2179 @dfn{Tagged paragraph} mode: like crown margin mode, except that if
2180 indentation of the first line of a paragraph is the same as the
2181 indentation of the second, the first line is treated as a one-line
2187 @opindex --split-only
2188 Split lines only. Do not join short lines to form longer ones. This
2189 prevents sample lines of code, and other such ``formatted'' text from
2190 being unduly combined.
2193 @itemx --uniform-spacing
2195 @opindex --uniform-spacing
2196 Uniform spacing. Reduce spacing between words to one space, and spacing
2197 between sentences to two spaces.
2200 @itemx -w @var{width}
2201 @itemx --width=@var{width}
2202 @opindex -@var{width}
2205 Fill output lines up to @var{width} characters (default 75). @command{fmt}
2206 initially tries to make lines about 7% shorter than this, to give it
2207 room to balance line lengths.
2209 @item -p @var{prefix}
2210 @itemx --prefix=@var{prefix}
2211 Only lines beginning with @var{prefix} (possibly preceded by whitespace)
2212 are subject to formatting. The prefix and any preceding whitespace are
2213 stripped for the formatting and then re-attached to each formatted output
2214 line. One use is to format certain kinds of program comments, while
2215 leaving the code unchanged.
2223 @section @command{pr}: Paginate or columnate files for printing
2226 @cindex printing, preparing files for
2227 @cindex multicolumn output, generating
2228 @cindex merging files in parallel
2230 @command{pr} writes each @var{file} (@samp{-} means standard input), or
2231 standard input if none are given, to standard output, paginating and
2232 optionally outputting in multicolumn format; optionally merges all
2233 @var{file}s, printing all in parallel, one per column. Synopsis:
2236 pr [@var{option}]@dots{} [@var{file}]@dots{}
2240 By default, a 5-line header is printed at each page: two blank lines;
2241 a line with the date, the file name, and the page count; and two more
2242 blank lines. A footer of five blank lines is also printed.
2243 The default @var{page_length} is 66
2244 lines. The default number of text lines is therefore 56.
2245 The text line of the header takes the form
2246 @samp{@var{date} @var{string} @var{page}}, with spaces inserted around
2247 @var{string} so that the line takes up the full @var{page_width}. Here,
2248 @var{date} is the date (see the @option{-D} or @option{--date-format}
2249 option for details), @var{string} is the centered header string, and
2250 @var{page} identifies the page number. The @env{LC_MESSAGES} locale
2251 category affects the spelling of @var{page}; in the default C locale, it
2252 is @samp{Page @var{number}} where @var{number} is the decimal page
2255 Form feeds in the input cause page breaks in the output. Multiple form
2256 feeds produce empty pages.
2258 Columns are of equal width, separated by an optional string (default
2259 is @samp{space}). For multicolumn output, lines will always be truncated to
2260 @var{page_width} (default 72), unless you use the @option{-J} option.
2262 column output no line truncation occurs by default. Use @option{-W} option to
2263 truncate lines in that case.
2265 The following changes were made in version 1.22i and apply to later
2266 versions of @command{pr}:
2267 @c FIXME: this whole section here sounds very awkward to me. I
2268 @c made a few small changes, but really it all needs to be redone. - Brian
2269 @c OK, I fixed another sentence or two, but some of it I just don't understand.
2274 Some small @var{letter options} (@option{-s}, @option{-w}) have been
2275 redefined for better @acronym{POSIX} compliance. The output of some further
2276 cases has been adapted to other Unix systems. These changes are not
2277 compatible with earlier versions of the program.
2280 Some @var{new capital letter} options (@option{-J}, @option{-S}, @option{-W})
2281 have been introduced to turn off unexpected interferences of small letter
2282 options. The @option{-N} option and the second argument @var{last_page}
2283 of @samp{+FIRST_PAGE} offer more flexibility. The detailed handling of
2284 form feeds set in the input files requires the @option{-T} option.
2287 Capital letter options override small letter ones.
2290 Some of the option-arguments (compare @option{-s}, @option{-e},
2291 @option{-i}, @option{-n}) cannot be specified as separate arguments from the
2292 preceding option letter (already stated in the @acronym{POSIX} specification).
2295 The program accepts the following options. Also see @ref{Common options}.
2299 @item +@var{first_page}[:@var{last_page}]
2300 @itemx --pages=@var{first_page}[:@var{last_page}]
2301 @c The two following @opindex lines evoke warnings because they contain `:'
2302 @c The `info' spec does not permit that. If we use those lines, we end
2303 @c up with truncated index entries that don't work.
2304 @c @opindex +@var{first_page}[:@var{last_page}]
2305 @c @opindex --pages=@var{first_page}[:@var{last_page}]
2306 @opindex +@var{page_range}
2307 @opindex --pages=@var{page_range}
2308 Begin printing with page @var{first_page} and stop with @var{last_page}.
2309 Missing @samp{:@var{last_page}} implies end of file. While estimating
2310 the number of skipped pages each form feed in the input file results
2311 in a new page. Page counting with and without @samp{+@var{first_page}}
2312 is identical. By default, counting starts with the first page of input
2313 file (not first page printed). Line numbering may be altered by @option{-N}
2317 @itemx --columns=@var{column}
2318 @opindex -@var{column}
2320 @cindex down columns
2321 With each single @var{file}, produce @var{column} columns of output
2322 (default is 1) and print columns down, unless @option{-a} is used. The
2323 column width is automatically decreased as @var{column} increases; unless
2324 you use the @option{-W/-w} option to increase @var{page_width} as well.
2325 This option might well cause some lines to be truncated. The number of
2326 lines in the columns on each page are balanced. The options @option{-e}
2327 and @option{-i} are on for multiple text-column output. Together with
2328 @option{-J} option column alignment and line truncation is turned off.
2329 Lines of full length are joined in a free field format and @option{-S}
2330 option may set field separators. @option{-@var{column}} may not be used
2331 with @option{-m} option.
2337 @cindex across columns
2338 With each single @var{file}, print columns across rather than down. The
2339 @option{-@var{column}} option must be given with @var{column} greater than one.
2340 If a line is too long to fit in a column, it is truncated.
2343 @itemx --show-control-chars
2345 @opindex --show-control-chars
2346 Print control characters using hat notation (e.g., @samp{^G}); print
2347 other nonprinting characters in octal backslash notation. By default,
2348 nonprinting characters are not changed.
2351 @itemx --double-space
2353 @opindex --double-space
2354 @cindex double spacing
2355 Double space the output.
2357 @item -D @var{format}
2358 @itemx --date-format=@var{format}
2359 @cindex time formats
2360 @cindex formatting times
2361 Format header dates using @var{format}, using the same conventions as
2362 for the command @samp{date +@var{format}}; @xref{date invocation}.
2363 Except for directives, which start with
2364 @samp{%}, characters in @var{format} are printed unchanged. You can use
2365 this option to specify an arbitrary string in place of the header date,
2366 e.g., @option{--date-format="Monday morning"}.
2368 @vindex POSIXLY_CORRECT
2370 The default date format is @samp{%Y-%m-%d %H:%M} (for example,
2371 @samp{2001-12-04 23:59});
2372 but if the @env{POSIXLY_CORRECT} environment variable is set
2373 and the @env{LC_TIME} locale category specifies the @acronym{POSIX}
2374 locale, the default is @samp{%b %e %H:%M %Y} (for example,
2375 @samp{Dec@ @ 4 23:59 2001}.
2378 Time stamps are listed according to the time zone rules specified by
2379 the @env{TZ} environment variable, or by the system default rules if
2380 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
2381 with @env{TZ}, libc, The GNU C Library Reference Manual}.
2383 @item -e[@var{in-tabchar}[@var{in-tabwidth}]]
2384 @itemx --expand-tabs[=@var{in-tabchar}[@var{in-tabwidth}]]
2386 @opindex --expand-tabs
2388 Expand @var{tab}s to spaces on input. Optional argument @var{in-tabchar} is
2389 the input tab character (default is the TAB character). Second optional
2390 argument @var{in-tabwidth} is the input tab character's width (default
2398 @opindex --form-feed
2399 Use a form feed instead of newlines to separate output pages. This does
2400 not alter the default page length of 66 lines.
2402 @item -h @var{header}
2403 @itemx --header=@var{header}
2406 Replace the file name in the header with the centered string @var{header}.
2407 When using the shell, @var{header} should be quoted and should be
2408 separated from @option{-h} by a space.
2410 @item -i[@var{out-tabchar}[@var{out-tabwidth}]]
2411 @itemx --output-tabs[=@var{out-tabchar}[@var{out-tabwidth}]]
2413 @opindex --output-tabs
2415 Replace spaces with @var{tab}s on output. Optional argument @var{out-tabchar}
2416 is the output tab character (default is the TAB character). Second optional
2417 argument @var{out-tabwidth} is the output tab character's width (default
2423 @opindex --join-lines
2424 Merge lines of full length. Used together with the column options
2425 @option{-@var{column}}, @option{-a -@var{column}} or @option{-m}. Turns off
2426 @option{-W/-w} line truncation;
2427 no column alignment used; may be used with
2428 @option{--sep-string[=@var{string}]}. @option{-J} has been introduced
2429 (together with @option{-W} and @option{--sep-string})
2430 to disentangle the old (@acronym{POSIX}-compliant) options @option{-w} and
2431 @option{-s} along with the three column options.
2434 @item -l @var{page_length}
2435 @itemx --length=@var{page_length}
2438 Set the page length to @var{page_length} (default 66) lines, including
2439 the lines of the header [and the footer]. If @var{page_length} is less
2440 than or equal to 10, the header and footer are omitted, as if the
2441 @option{-t} option had been given.
2447 Merge and print all @var{file}s in parallel, one in each column. If a
2448 line is too long to fit in a column, it is truncated, unless the @option{-J}
2449 option is used. @option{--sep-string[=@var{string}]} may be used.
2451 some @var{file}s (form feeds set) produce empty columns, still marked
2452 by @var{string}. The result is a continuous line numbering and column
2453 marking throughout the whole merged file. Completely empty merged pages
2454 show no separators or line numbers. The default header becomes
2455 @samp{@var{date} @var{page}} with spaces inserted in the middle; this
2456 may be used with the @option{-h} or @option{--header} option to fill up
2457 the middle blank part.
2459 @item -n[@var{number-separator}[@var{digits}]]
2460 @itemx --number-lines[=@var{number-separator}[@var{digits}]]
2462 @opindex --number-lines
2463 Provide @var{digits} digit line numbering (default for @var{digits} is
2464 5). With multicolumn output the number occupies the first @var{digits}
2465 column positions of each text column or only each line of @option{-m}
2466 output. With single column output the number precedes each line just as
2467 @option{-m} does. Default counting of the line numbers starts with the
2468 first line of the input file (not the first line printed, compare the
2469 @option{--page} option and @option{-N} option).
2470 Optional argument @var{number-separator} is the character appended to
2471 the line number to separate it from the text followed. The default
2472 separator is the TAB character. In a strict sense a TAB is always
2473 printed with single column output only. The TAB width varies
2474 with the TAB position, e.g., with the left @var{margin} specified
2475 by @option{-o} option. With multicolumn output priority is given to
2476 @samp{equal width of output columns} (a @acronym{POSIX} specification).
2477 The TAB width is fixed to the value of the first column and does
2478 not change with different values of left @var{margin}. That means a
2479 fixed number of spaces is always printed in the place of the
2480 @var{number-separator} TAB. The tabification depends upon the output
2483 @item -N @var{line_number}
2484 @itemx --first-line-number=@var{line_number}
2486 @opindex --first-line-number
2487 Start line counting with the number @var{line_number} at first line of
2488 first page printed (in most cases not the first line of the input file).
2490 @item -o @var{margin}
2491 @itemx --indent=@var{margin}
2494 @cindex indenting lines
2496 Indent each line with a margin @var{margin} spaces wide (default is zero).
2497 The total page width is the size of the margin plus the @var{page_width}
2498 set with the @option{-W/-w} option. A limited overflow may occur with
2499 numbered single column output (compare @option{-n} option).
2502 @itemx --no-file-warnings
2504 @opindex --no-file-warnings
2505 Do not print a warning message when an argument @var{file} cannot be
2506 opened. (The exit status will still be nonzero, however.)
2508 @item -s[@var{char}]
2509 @itemx --separator[=@var{char}]
2511 @opindex --separator
2512 Separate columns by a single character @var{char}. The default for
2513 @var{char} is the TAB character without @option{-w} and @samp{no
2514 character} with @option{-w}. Without @option{-s} the default separator
2515 @samp{space} is set. @option{-s[char]} turns off line truncation of all
2516 three column options (@option{-COLUMN}|@option{-a -COLUMN}|@option{-m}) unless
2517 @option{-w} is set. This is a @acronym{POSIX}-compliant formulation.
2520 @item -S@var{string}
2521 @itemx --sep-string[=@var{string}]
2523 @opindex --sep-string
2524 Use @var{string} to separate output columns. The @option{-S} option doesn't
2525 affect the @option{-W/-w} option, unlike the @option{-s} option which does. It
2526 does not affect line truncation or column alignment.
2527 Without @option{-S}, and with @option{-J}, @command{pr} uses the default output
2529 Without @option{-S} or @option{-J}, @command{pr} uses a @samp{space}
2530 (same as @option{-S"@w{ }"}). @option{--sep-string} with no
2531 @samp{=@var{string}} is equivalent to @option{--sep-string=""}.
2534 @itemx --omit-header
2536 @opindex --omit-header
2537 Do not print the usual header [and footer] on each page, and do not fill
2538 out the bottom of pages (with blank lines or a form feed). No page
2539 structure is produced, but form feeds set in the input files are retained.
2540 The predefined pagination is not changed. @option{-t} or @option{-T} may be
2541 useful together with other options; e.g.: @option{-t -e4}, expand TAB characters
2542 in the input file to 4 spaces but don't make any other changes. Use of
2543 @option{-t} overrides @option{-h}.
2546 @itemx --omit-pagination
2548 @opindex --omit-pagination
2549 Do not print header [and footer]. In addition eliminate all form feeds
2550 set in the input files.
2553 @itemx --show-nonprinting
2555 @opindex --show-nonprinting
2556 Print nonprinting characters in octal backslash notation.
2558 @item -w @var{page_width}
2559 @itemx --width=@var{page_width}
2562 Set page width to @var{page_width} characters for multiple text-column
2563 output only (default for @var{page_width} is 72). @option{-s[CHAR]} turns
2564 off the default page width and any line truncation and column alignment.
2565 Lines of full length are merged, regardless of the column options
2566 set. No @var{page_width} setting is possible with single column output.
2567 A @acronym{POSIX}-compliant formulation.
2569 @item -W @var{page_width}
2570 @itemx --page_width=@var{page_width}
2572 @opindex --page_width
2573 Set the page width to @var{page_width} characters. That's valid with and
2574 without a column option. Text lines are truncated, unless @option{-J}
2575 is used. Together with one of the three column options
2576 (@option{-@var{column}}, @option{-a -@var{column}} or @option{-m}) column
2577 alignment is always used. The separator options @option{-S} or @option{-s}
2578 don't affect the @option{-W} option. Default is 72 characters. Without
2579 @option{-W @var{page_width}} and without any of the column options NO line
2580 truncation is used (defined to keep downward compatibility and to meet
2581 most frequent tasks). That's equivalent to @option{-W 72 -J}. The header
2582 line is never truncated.
2589 @node fold invocation
2590 @section @command{fold}: Wrap input lines to fit in specified width
2593 @cindex wrapping long input lines
2594 @cindex folding long input lines
2596 @command{fold} writes each @var{file} (@option{-} means standard input), or
2597 standard input if none are given, to standard output, breaking long
2601 fold [@var{option}]@dots{} [@var{file}]@dots{}
2604 By default, @command{fold} breaks lines wider than 80 columns. The output
2605 is split into as many lines as necessary.
2607 @cindex screen columns
2608 @command{fold} counts screen columns by default; thus, a tab may count more
2609 than one column, backspace decreases the column count, and carriage
2610 return sets the column to zero.
2612 The program accepts the following options. Also see @ref{Common options}.
2620 Count bytes rather than columns, so that tabs, backspaces, and carriage
2621 returns are each counted as taking up one column, just like other
2628 Break at word boundaries: the line is broken after the last blank before
2629 the maximum line length. If the line contains no such blanks, the line
2630 is broken at the maximum line length as usual.
2632 @item -w @var{width}
2633 @itemx --width=@var{width}
2636 Use a maximum line length of @var{width} columns instead of 80.
2638 For compatibility @command{fold} supports an obsolete option syntax
2639 @option{-@var{width}}. New scripts should use @option{-w @var{width}}
2647 @node Output of parts of files
2648 @chapter Output of parts of files
2650 @cindex output of parts of files
2651 @cindex parts of files, output of
2653 These commands output pieces of the input.
2656 * head invocation:: Output the first part of files.
2657 * tail invocation:: Output the last part of files.
2658 * split invocation:: Split a file into pieces.
2659 * csplit invocation:: Split a file into context-determined pieces.
2662 @node head invocation
2663 @section @command{head}: Output the first part of files
2666 @cindex initial part of files, outputting
2667 @cindex first part of files, outputting
2669 @command{head} prints the first part (10 lines by default) of each
2670 @var{file}; it reads from standard input if no files are given or
2671 when given a @var{file} of @option{-}. Synopsis:
2674 head [@var{option}]@dots{} [@var{file}]@dots{}
2677 If more than one @var{file} is specified, @command{head} prints a
2678 one-line header consisting of:
2681 ==> @var{file name} <==
2685 before the output for each @var{file}.
2687 The program accepts the following options. Also see @ref{Common options}.
2692 @itemx --bytes=@var{k}
2695 Print the first @var{k} bytes, instead of initial lines.
2696 However, if @var{k} starts with a @samp{-},
2697 print all but the last @var{k} bytes of each file.
2698 @multiplierSuffixes{k}
2701 @itemx --lines=@var{k}
2704 Output the first @var{k} lines.
2705 However, if @var{k} starts with a @samp{-},
2706 print all but the last @var{k} lines of each file.
2707 Size multiplier suffixes are the same as with the @option{-c} option.
2715 Never print file name headers.
2721 Always print file name headers.
2725 For compatibility @command{head} also supports an obsolete option syntax
2726 @option{-@var{count}@var{options}}, which is recognized only if it is
2727 specified first. @var{count} is a decimal number optionally followed
2728 by a size letter (@samp{b}, @samp{k}, @samp{m}) as in @option{-c}, or
2729 @samp{l} to mean count by lines, or other option letters (@samp{cqv}).
2730 Scripts intended for standard hosts should use @option{-c @var{count}}
2731 or @option{-n @var{count}} instead. If your script must also run on
2732 hosts that support only the obsolete syntax, it is usually simpler to
2733 avoid @command{head}, e.g., by using @samp{sed 5q} instead of
2739 @node tail invocation
2740 @section @command{tail}: Output the last part of files
2743 @cindex last part of files, outputting
2745 @command{tail} prints the last part (10 lines by default) of each
2746 @var{file}; it reads from standard input if no files are given or
2747 when given a @var{file} of @samp{-}. Synopsis:
2750 tail [@var{option}]@dots{} [@var{file}]@dots{}
2753 If more than one @var{file} is specified, @command{tail} prints a
2754 one-line header consisting of:
2757 ==> @var{file name} <==
2761 before the output for each @var{file}.
2763 @cindex BSD @command{tail}
2764 @sc{gnu} @command{tail} can output any amount of data (some other versions of
2765 @command{tail} cannot). It also has no @option{-r} option (print in
2766 reverse), since reversing a file is really a different job from printing
2767 the end of a file; BSD @command{tail} (which is the one with @option{-r}) can
2768 only reverse files that are at most as large as its buffer, which is
2769 typically 32 KiB@. A more reliable and versatile way to reverse files is
2770 the @sc{gnu} @command{tac} command.
2772 The program accepts the following options. Also see @ref{Common options}.
2777 @itemx --bytes=@var{k}
2780 Output the last @var{k} bytes, instead of final lines.
2781 However, if @var{k} starts with a @samp{+}, start printing with the
2782 @var{k}th byte from the start of each file, instead of from the end.
2783 @multiplierSuffixes{k}
2786 @itemx --follow[=@var{how}]
2789 @cindex growing files
2790 @vindex name @r{follow option}
2791 @vindex descriptor @r{follow option}
2792 Loop forever trying to read more characters at the end of the file,
2793 presumably because the file is growing.
2794 If more than one file is given, @command{tail} prints a header whenever it
2795 gets output from a different file, to indicate which file that output is
2798 There are two ways to specify how you'd like to track files with this option,
2799 but that difference is noticeable only when a followed file is removed or
2801 If you'd like to continue to track the end of a growing file even after
2802 it has been unlinked, use @option{--follow=descriptor}. This is the default
2803 behavior, but it is not useful if you're tracking a log file that may be
2804 rotated (removed or renamed, then reopened). In that case, use
2805 @option{--follow=name} to track the named file, perhaps by reopening it
2806 periodically to see if it has been removed and recreated by some other program.
2807 Note that the inotify-based implementation handles this case without
2808 the need for any periodic reopening.
2810 No matter which method you use, if the tracked file is determined to have
2811 shrunk, @command{tail} prints a message saying the file has been truncated
2812 and resumes tracking the end of the file from the newly-determined endpoint.
2814 When a file is removed, @command{tail}'s behavior depends on whether it is
2815 following the name or the descriptor. When following by name, tail can
2816 detect that a file has been removed and gives a message to that effect,
2817 and if @option{--retry} has been specified it will continue checking
2818 periodically to see if the file reappears.
2819 When following a descriptor, tail does not detect that the file has
2820 been unlinked or renamed and issues no message; even though the file
2821 may no longer be accessible via its original name, it may still be
2824 The option values @samp{descriptor} and @samp{name} may be specified only
2825 with the long form of the option, not with @option{-f}.
2827 The @option{-f} option is ignored if
2828 no @var{file} operand is specified and standard input is a FIFO or a pipe.
2829 Likewise, the @option{-f} option has no effect for any
2830 operand specified as @samp{-}, when standard input is a FIFO or a pipe.
2834 This option is the same as @option{--follow=name --retry}. That is, tail
2835 will attempt to reopen a file when it is removed. Should this fail, tail
2836 will keep trying until it becomes accessible again.
2840 This option is useful mainly when following by name (i.e., with
2841 @option{--follow=name}).
2842 Without this option, when tail encounters a file that doesn't
2843 exist or is otherwise inaccessible, it reports that fact and
2844 never checks it again.
2846 @itemx --sleep-interval=@var{number}
2847 @opindex --sleep-interval
2848 Change the number of seconds to wait between iterations (the default is 1.0).
2849 During one iteration, every specified file is checked to see if it has
2851 Historical implementations of @command{tail} have required that
2852 @var{number} be an integer. However, GNU @command{tail} accepts
2853 an arbitrary floating point number. @xref{Floating point}.
2854 When @command{tail} uses inotify, this polling-related option is ignored.
2856 @itemx --pid=@var{pid}
2858 When following by name or by descriptor, you may specify the process ID,
2859 @var{pid}, of the sole writer of all @var{file} arguments. Then, shortly
2860 after that process terminates, tail will also terminate. This will
2861 work properly only if the writer and the tailing process are running on
2862 the same machine. For example, to save the output of a build in a file
2863 and to watch the file grow, if you invoke @command{make} and @command{tail}
2864 like this then the tail process will stop when your build completes.
2865 Without this option, you would have had to kill the @code{tail -f}
2869 $ make >& makerr & tail --pid=$! -f makerr
2872 If you specify a @var{pid} that is not in use or that does not correspond
2873 to the process that is writing to the tailed files, then @command{tail}
2874 may terminate long before any @var{file}s stop growing or it may not
2875 terminate until long after the real writer has terminated.
2876 Note that @option{--pid} cannot be supported on some systems; @command{tail}
2877 will print a warning if this is the case.
2879 @itemx --max-unchanged-stats=@var{n}
2880 @opindex --max-unchanged-stats
2881 When tailing a file by name, if there have been @var{n} (default
2882 n=@value{DEFAULT_MAX_N_UNCHANGED_STATS_BETWEEN_OPENS}) consecutive
2883 iterations for which the file has not changed, then
2884 @code{open}/@code{fstat} the file to determine if that file name is
2885 still associated with the same device/inode-number pair as before.
2886 When following a log file that is rotated, this is approximately the
2887 number of seconds between when tail prints the last pre-rotation lines
2888 and when it prints the lines that have accumulated in the new log file.
2889 This option is meaningful only when polling (i.e., without inotify)
2890 and when following by name.
2893 @itemx --lines=@var{k}
2896 Output the last @var{k} lines.
2897 However, if @var{k} starts with a @samp{+}, start printing with the
2898 @var{k}th line from the start of each file, instead of from the end.
2899 Size multiplier suffixes are the same as with the @option{-c} option.
2907 Never print file name headers.
2913 Always print file name headers.
2917 For compatibility @command{tail} also supports an obsolete usage
2918 @samp{tail -[@var{count}][bcl][f] [@var{file}]}, which is recognized
2919 only if it does not conflict with the usage described
2920 above. This obsolete form uses exactly one option and at most one
2921 file. In the option, @var{count} is an optional decimal number optionally
2922 followed by a size letter (@samp{b}, @samp{c}, @samp{l}) to mean count
2923 by 512-byte blocks, bytes, or lines, optionally followed by @samp{f}
2924 which has the same meaning as @option{-f}.
2926 @vindex _POSIX2_VERSION
2927 On older systems, the leading @samp{-} can be replaced by @samp{+} in
2928 the obsolete option syntax with the same meaning as in counts, and
2929 obsolete usage overrides normal usage when the two conflict.
2930 This obsolete behavior can be enabled or disabled with the
2931 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
2934 Scripts intended for use on standard hosts should avoid obsolete
2935 syntax and should use @option{-c @var{count}[b]}, @option{-n
2936 @var{count}}, and/or @option{-f} instead. If your script must also
2937 run on hosts that support only the obsolete syntax, you can often
2938 rewrite it to avoid problematic usages, e.g., by using @samp{sed -n
2939 '$p'} rather than @samp{tail -1}. If that's not possible, the script
2940 can use a test like @samp{if tail -c +1 </dev/null >/dev/null 2>&1;
2941 then @dots{}} to decide which syntax to use.
2943 Even if your script assumes the standard behavior, you should still
2944 beware usages whose behaviors differ depending on the @acronym{POSIX}
2945 version. For example, avoid @samp{tail - main.c}, since it might be
2946 interpreted as either @samp{tail main.c} or as @samp{tail -- -
2947 main.c}; avoid @samp{tail -c 4}, since it might mean either @samp{tail
2948 -c4} or @samp{tail -c 10 4}; and avoid @samp{tail +4}, since it might
2949 mean either @samp{tail ./+4} or @samp{tail -n +4}.
2954 @node split invocation
2955 @section @command{split}: Split a file into pieces.
2958 @cindex splitting a file into pieces
2959 @cindex pieces, splitting a file into
2961 @command{split} creates output files containing consecutive or interleaved
2962 sections of @var{input} (standard input if none is given or @var{input}
2963 is @samp{-}). Synopsis:
2966 split [@var{option}] [@var{input} [@var{prefix}]]
2969 By default, @command{split} puts 1000 lines of @var{input} (or whatever is
2970 left over for the last section), into each output file.
2972 @cindex output file name prefix
2973 The output files' names consist of @var{prefix} (@samp{x} by default)
2974 followed by a group of characters (@samp{aa}, @samp{ab}, @dots{} by
2975 default), such that concatenating the output files in traditional
2976 sorted order by file name produces the original input file (except
2977 @option{-r}). If the output file names are exhausted, @command{split}
2978 reports an error without deleting the output files that it did create.
2980 The program accepts the following options. Also see @ref{Common options}.
2984 @item -l @var{lines}
2985 @itemx --lines=@var{lines}
2988 Put @var{lines} lines of @var{input} into each output file.
2990 For compatibility @command{split} also supports an obsolete
2991 option syntax @option{-@var{lines}}. New scripts should use @option{-l
2992 @var{lines}} instead.
2995 @itemx --bytes=@var{size}
2998 Put @var{size} bytes of @var{input} into each output file.
2999 @multiplierSuffixes{size}
3002 @itemx --line-bytes=@var{size}
3004 @opindex --line-bytes
3005 Put into each output file as many complete lines of @var{input} as
3006 possible without exceeding @var{size} bytes. Individual lines longer than
3007 @var{size} bytes are broken into multiple files.
3008 @var{size} has the same format as for the @option{--bytes} option.
3010 @item -n @var{chunks}
3011 @itemx --number=@var{chunks}
3015 Split @var{input} to @var{chunks} output files where @var{chunks} may be:
3018 @var{n} generate @var{n} files based on current size of @var{input}
3019 @var{k}/@var{n} only output @var{k}th of @var{n} to stdout
3020 l/@var{n} generate @var{n} files without splitting lines
3021 l/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
3022 r/@var{n} like @samp{l} but use round robin distribution
3023 r/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
3026 Any excess bytes remaining after dividing the @var{input}
3027 into @var{n} chunks, are assigned to the last chunk.
3028 Any excess bytes appearing after the initial calculation are discarded
3029 (except when using @samp{r} mode).
3031 All @var{n} files are created even if there are fewer than @var{n} lines,
3032 or the @var{input} is truncated.
3034 For @samp{l} mode, chunks are approximately @var{input} size / @var{n}.
3035 The @var{input} is partitioned into @var{n} equal sized portions, with
3036 the last assigned any excess. If a line @emph{starts} within a partition
3037 it is written completely to the corresponding file. Since lines
3038 are not split even if they overlap a partition, the files written
3039 can be larger or smaller than the partition size, and even empty
3040 if a line is so long as to completely overlap the partition.
3042 For @samp{r} mode, the size of @var{input} is irrelevant,
3043 and so can be a pipe for example.
3045 @item -a @var{length}
3046 @itemx --suffix-length=@var{length}
3048 @opindex --suffix-length
3049 Use suffixes of length @var{length}. The default @var{length} is 2.
3052 @itemx --numeric-suffixes
3054 @opindex --numeric-suffixes
3055 Use digits in suffixes rather than lower-case letters.
3058 @itemx --elide-empty-files
3060 @opindex --elide-empty-files
3061 Suppress the generation of zero-length output files. This can happen
3062 with the @option{--number} option if a file is (truncated to be) shorter
3063 than the number requested, or if a line is so long as to completely
3064 span a chunk. The output file sequence numbers, always run consecutively
3065 even when this option is specified.
3070 @opindex --unbuffered
3071 Immediately copy input to output in @option{--number r/...} mode,
3072 which is a much slower mode of operation.
3076 Write a diagnostic just before each output file is opened.
3082 Here are a few examples to illustrate how the
3083 @option{--number} (@option{-n}) option works:
3085 Notice how, by default, one line may be split onto two or more:
3088 $ seq -w 6 10 > k; split -n3 k; head xa?
3101 Use the "l/" modifier to suppress that:
3104 $ seq -w 6 10 > k; split -nl/3 k; head xa?
3117 Use the "r/" modifier to distribute lines in a round-robin fashion:
3120 $ seq -w 6 10 > k; split -nr/3 k; head xa?
3133 You can also extract just the Kth chunk.
3134 This extracts and prints just the 7th "chunk" of 33:
3137 $ seq 100 > k; split -nl/7/33 k
3144 @node csplit invocation
3145 @section @command{csplit}: Split a file into context-determined pieces
3148 @cindex context splitting
3149 @cindex splitting a file into pieces by context
3151 @command{csplit} creates zero or more output files containing sections of
3152 @var{input} (standard input if @var{input} is @samp{-}). Synopsis:
3155 csplit [@var{option}]@dots{} @var{input} @var{pattern}@dots{}
3158 The contents of the output files are determined by the @var{pattern}
3159 arguments, as detailed below. An error occurs if a @var{pattern}
3160 argument refers to a nonexistent line of the input file (e.g., if no
3161 remaining line matches a given regular expression). After every
3162 @var{pattern} has been matched, any remaining input is copied into one
3165 By default, @command{csplit} prints the number of bytes written to each
3166 output file after it has been created.
3168 The types of pattern arguments are:
3173 Create an output file containing the input up to but not including line
3174 @var{n} (a positive integer). If followed by a repeat count, also
3175 create an output file containing the next @var{n} lines of the input
3176 file once for each repeat.
3178 @item /@var{regexp}/[@var{offset}]
3179 Create an output file containing the current line up to (but not
3180 including) the next line of the input file that contains a match for
3181 @var{regexp}. The optional @var{offset} is an integer.
3182 If it is given, the input up to (but not including) the
3183 matching line plus or minus @var{offset} is put into the output file,
3184 and the line after that begins the next section of input.
3186 @item %@var{regexp}%[@var{offset}]
3187 Like the previous type, except that it does not create an output
3188 file, so that section of the input file is effectively ignored.
3190 @item @{@var{repeat-count}@}
3191 Repeat the previous pattern @var{repeat-count} additional
3192 times. The @var{repeat-count} can either be a positive integer or an
3193 asterisk, meaning repeat as many times as necessary until the input is
3198 The output files' names consist of a prefix (@samp{xx} by default)
3199 followed by a suffix. By default, the suffix is an ascending sequence
3200 of two-digit decimal numbers from @samp{00} to @samp{99}. In any case,
3201 concatenating the output files in sorted order by file name produces the
3202 original input file.
3204 By default, if @command{csplit} encounters an error or receives a hangup,
3205 interrupt, quit, or terminate signal, it removes any output files
3206 that it has created so far before it exits.
3208 The program accepts the following options. Also see @ref{Common options}.
3212 @item -f @var{prefix}
3213 @itemx --prefix=@var{prefix}
3216 @cindex output file name prefix
3217 Use @var{prefix} as the output file name prefix.
3219 @item -b @var{suffix}
3220 @itemx --suffix=@var{suffix}
3223 @cindex output file name suffix
3224 Use @var{suffix} as the output file name suffix. When this option is
3225 specified, the suffix string must include exactly one
3226 @code{printf(3)}-style conversion specification, possibly including
3227 format specification flags, a field width, a precision specifications,
3228 or all of these kinds of modifiers. The format letter must convert a
3229 binary unsigned integer argument to readable form. The format letters
3230 @samp{d} and @samp{i} are aliases for @samp{u}, and the
3231 @samp{u}, @samp{o}, @samp{x}, and @samp{X} conversions are allowed. The
3232 entire @var{suffix} is given (with the current output file number) to
3233 @code{sprintf(3)} to form the file name suffixes for each of the
3234 individual output files in turn. If this option is used, the
3235 @option{--digits} option is ignored.
3237 @item -n @var{digits}
3238 @itemx --digits=@var{digits}
3241 Use output file names containing numbers that are @var{digits} digits
3242 long instead of the default 2.
3247 @opindex --keep-files
3248 Do not remove output files when errors are encountered.
3251 @itemx --elide-empty-files
3253 @opindex --elide-empty-files
3254 Suppress the generation of zero-length output files. (In cases where
3255 the section delimiters of the input file are supposed to mark the first
3256 lines of each of the sections, the first output file will generally be a
3257 zero-length file unless you use this option.) The output file sequence
3258 numbers always run consecutively starting from 0, even when this option
3269 Do not print counts of output file sizes.
3275 Here is an example of its usage.
3276 First, create an empty directory for the exercise,
3283 Now, split the sequence of 1..14 on lines that end with 0 or 5:
3286 $ seq 14 | csplit - '/[05]$/' '@{*@}'
3292 Each number printed above is the size of an output
3293 file that csplit has just created.
3294 List the names of those output files:
3301 Use @command{head} to show their contents:
3326 @node Summarizing files
3327 @chapter Summarizing files
3329 @cindex summarizing files
3331 These commands generate just a few numbers representing entire
3335 * wc invocation:: Print newline, word, and byte counts.
3336 * sum invocation:: Print checksum and block counts.
3337 * cksum invocation:: Print CRC checksum and byte counts.
3338 * md5sum invocation:: Print or check MD5 digests.
3339 * sha1sum invocation:: Print or check SHA-1 digests.
3340 * sha2 utilities:: Print or check SHA-2 digests.
3345 @section @command{wc}: Print newline, word, and byte counts
3349 @cindex character count
3353 @command{wc} counts the number of bytes, characters, whitespace-separated
3354 words, and newlines in each given @var{file}, or standard input if none
3355 are given or for a @var{file} of @samp{-}. Synopsis:
3358 wc [@var{option}]@dots{} [@var{file}]@dots{}
3361 @cindex total counts
3362 @command{wc} prints one line of counts for each file, and if the file was
3363 given as an argument, it prints the file name following the counts. If
3364 more than one @var{file} is given, @command{wc} prints a final line
3365 containing the cumulative counts, with the file name @file{total}. The
3366 counts are printed in this order: newlines, words, characters, bytes,
3367 maximum line length.
3368 Each count is printed right-justified in a field with at least one
3369 space between fields so that the numbers and file names normally line
3370 up nicely in columns. The width of the count fields varies depending
3371 on the inputs, so you should not depend on a particular field width.
3372 However, as a @acronym{GNU} extension, if only one count is printed,
3373 it is guaranteed to be printed without leading spaces.
3375 By default, @command{wc} prints three counts: the newline, words, and byte
3376 counts. Options can specify that only certain counts be printed.
3377 Options do not undo others previously given, so
3384 prints both the byte counts and the word counts.
3386 With the @option{--max-line-length} option, @command{wc} prints the length
3387 of the longest line per file, and if there is more than one file it
3388 prints the maximum (not the sum) of those lengths. The line lengths here
3389 are measured in screen columns, according to the current locale and
3390 assuming tab positions in every 8th column.
3392 The program accepts the following options. Also see @ref{Common options}.
3400 Print only the byte counts.
3406 Print only the character counts.
3412 Print only the word counts.
3418 Print only the newline counts.
3421 @itemx --max-line-length
3423 @opindex --max-line-length
3424 Print only the maximum line lengths.
3426 @macro filesZeroFromOption{cmd,withTotalOption,subListOutput}
3427 @itemx --files0-from=@var{file}
3428 @opindex --files0-from=@var{file}
3429 @c This is commented out to avoid a texi2dvi failure.
3430 @c texi2dvi (GNU Texinfo 4.11) 1.104
3431 @c @cindex including files from @command{\cmd\}
3432 Disallow processing files named on the command line, and instead process
3433 those named in file @var{file}; each name being terminated by a zero byte
3434 (@acronym{ASCII} @sc{nul}).
3435 This is useful \withTotalOption\
3436 when the list of file names is so long that it may exceed a command line
3438 In such cases, running @command{\cmd\} via @command{xargs} is undesirable
3439 because it splits the list into pieces and makes @command{\cmd\} print
3440 \subListOutput\ for each sublist rather than for the entire list.
3441 One way to produce a list of @acronym{ASCII} @sc{nul} terminated file
3442 names is with @sc{gnu}
3443 @command{find}, using its @option{-print0} predicate.
3444 If @var{file} is @samp{-} then the @acronym{ASCII} @sc{nul} terminated
3445 file names are read from standard input.
3447 @filesZeroFromOption{wc,,a total}
3449 For example, to find the length of the longest line in any @file{.c} or
3450 @file{.h} file in the current hierarchy, do this:
3453 find . -name '*.[ch]' -print0 |
3454 wc -L --files0-from=- | tail -n1
3462 @node sum invocation
3463 @section @command{sum}: Print checksum and block counts
3466 @cindex 16-bit checksum
3467 @cindex checksum, 16-bit
3469 @command{sum} computes a 16-bit checksum for each given @var{file}, or
3470 standard input if none are given or for a @var{file} of @samp{-}. Synopsis:
3473 sum [@var{option}]@dots{} [@var{file}]@dots{}
3476 @command{sum} prints the checksum for each @var{file} followed by the
3477 number of blocks in the file (rounded up). If more than one @var{file}
3478 is given, file names are also printed (by default). (With the
3479 @option{--sysv} option, corresponding file names are printed when there is
3480 at least one file argument.)
3482 By default, @sc{gnu} @command{sum} computes checksums using an algorithm
3483 compatible with BSD @command{sum} and prints file sizes in units of
3486 The program accepts the following options. Also see @ref{Common options}.
3492 @cindex BSD @command{sum}
3493 Use the default (BSD compatible) algorithm. This option is included for
3494 compatibility with the System V @command{sum}. Unless @option{-s} was also
3495 given, it has no effect.
3501 @cindex System V @command{sum}
3502 Compute checksums using an algorithm compatible with System V
3503 @command{sum}'s default, and print file sizes in units of 512-byte blocks.
3507 @command{sum} is provided for compatibility; the @command{cksum} program (see
3508 next section) is preferable in new applications.
3513 @node cksum invocation
3514 @section @command{cksum}: Print CRC checksum and byte counts
3517 @cindex cyclic redundancy check
3518 @cindex CRC checksum
3520 @command{cksum} computes a cyclic redundancy check (CRC) checksum for each
3521 given @var{file}, or standard input if none are given or for a
3522 @var{file} of @samp{-}. Synopsis:
3525 cksum [@var{option}]@dots{} [@var{file}]@dots{}
3528 @command{cksum} prints the CRC checksum for each file along with the number
3529 of bytes in the file, and the file name unless no arguments were given.
3531 @command{cksum} is typically used to ensure that files
3532 transferred by unreliable means (e.g., netnews) have not been corrupted,
3533 by comparing the @command{cksum} output for the received files with the
3534 @command{cksum} output for the original files (typically given in the
3537 The CRC algorithm is specified by the @acronym{POSIX} standard. It is not
3538 compatible with the BSD or System V @command{sum} algorithms (see the
3539 previous section); it is more robust.
3541 The only options are @option{--help} and @option{--version}. @xref{Common
3547 @node md5sum invocation
3548 @section @command{md5sum}: Print or check MD5 digests
3552 @cindex 128-bit checksum
3553 @cindex checksum, 128-bit
3554 @cindex fingerprint, 128-bit
3555 @cindex message-digest, 128-bit
3557 @command{md5sum} computes a 128-bit checksum (or @dfn{fingerprint} or
3558 @dfn{message-digest}) for each specified @var{file}.
3560 Note: The MD5 digest is more reliable than a simple CRC (provided by
3561 the @command{cksum} command) for detecting accidental file corruption,
3562 as the chances of accidentally having two files with identical MD5
3563 are vanishingly small. However, it should not be considered secure
3564 against malicious tampering: although finding a file with a given MD5
3565 fingerprint is considered infeasible at the moment, it is known how
3566 to modify certain files, including digital certificates, so that they
3567 appear valid when signed with an MD5 digest.
3568 For more secure hashes, consider using SHA-2. @xref{sha2 utilities}.
3570 If a @var{file} is specified as @samp{-} or if no files are given
3571 @command{md5sum} computes the checksum for the standard input.
3572 @command{md5sum} can also determine whether a file and checksum are
3573 consistent. Synopsis:
3576 md5sum [@var{option}]@dots{} [@var{file}]@dots{}
3579 For each @var{file}, @samp{md5sum} outputs the MD5 checksum, a flag
3580 indicating a binary or text input file, and the file name.
3581 If @var{file} contains a backslash or newline, the
3582 line is started with a backslash, and each problematic character in
3583 the file name is escaped with a backslash, making the output
3584 unambiguous even in the presence of arbitrary file names.
3585 If @var{file} is omitted or specified as @samp{-}, standard input is read.
3587 The program accepts the following options. Also see @ref{Common options}.
3595 @cindex binary input files
3596 Treat each input file as binary, by reading it in binary mode and
3597 outputting a @samp{*} flag. This is the inverse of @option{--text}.
3598 On systems like @acronym{GNU} that do not distinguish between binary
3599 and text files, this option merely flags each input file as binary:
3600 the MD5 checksum is unaffected. This option is the default on systems
3601 like MS-DOS that distinguish between binary and text files, except
3602 for reading standard input when standard input is a terminal.
3606 Read file names and checksum information (not data) from each
3607 @var{file} (or from stdin if no @var{file} was specified) and report
3608 whether the checksums match the contents of the named files.
3609 The input to this mode of @command{md5sum} is usually the output of
3610 a prior, checksum-generating run of @samp{md5sum}.
3611 Each valid line of input consists of an MD5 checksum, a binary/text
3612 flag, and then a file name.
3613 Binary files are marked with @samp{*}, text with @samp{ }.
3614 For each such line, @command{md5sum} reads the named file and computes its
3615 MD5 checksum. Then, if the computed message digest does not match the
3616 one on the line with the file name, the file is noted as having
3617 failed the test. Otherwise, the file passes the test.
3618 By default, for each valid line, one line is written to standard
3619 output indicating whether the named file passed the test.
3620 After all checks have been performed, if there were any failures,
3621 a warning is issued to standard error.
3622 Use the @option{--status} option to inhibit that output.
3623 If any listed file cannot be opened or read, if any valid line has
3624 an MD5 checksum inconsistent with the associated file, or if no valid
3625 line is found, @command{md5sum} exits with nonzero status. Otherwise,
3626 it exits successfully.
3630 @cindex verifying MD5 checksums
3631 This option is useful only when verifying checksums.
3632 When verifying checksums, don't generate an 'OK' message per successfully
3633 checked file. Files that fail the verification are reported in the
3634 default one-line-per-file format. If there is any checksum mismatch,
3635 print a warning summarizing the failures to standard error.
3639 @cindex verifying MD5 checksums
3640 This option is useful only when verifying checksums.
3641 When verifying checksums, don't generate the default one-line-per-file
3642 diagnostic and don't output the warning summarizing any failures.
3643 Failures to open or read a file still evoke individual diagnostics to
3645 If all listed files are readable and are consistent with the associated
3646 MD5 checksums, exit successfully. Otherwise exit with a status code
3647 indicating there was a failure.
3653 @cindex text input files
3654 Treat each input file as text, by reading it in text mode and
3655 outputting a @samp{ } flag. This is the inverse of @option{--binary}.
3656 This option is the default on systems like @acronym{GNU} that do not
3657 distinguish between binary and text files. On other systems, it is
3658 the default for reading standard input when standard input is a
3665 @cindex verifying MD5 checksums
3666 When verifying checksums, warn about improperly formatted MD5 checksum lines.
3667 This option is useful only if all but a few lines in the checked input
3675 @node sha1sum invocation
3676 @section @command{sha1sum}: Print or check SHA-1 digests
3680 @cindex 160-bit checksum
3681 @cindex checksum, 160-bit
3682 @cindex fingerprint, 160-bit
3683 @cindex message-digest, 160-bit
3685 @command{sha1sum} computes a 160-bit checksum for each specified
3686 @var{file}. The usage and options of this command are precisely the
3687 same as for @command{md5sum}. @xref{md5sum invocation}.
3689 Note: The SHA-1 digest is more secure than MD5, and no collisions of
3690 it are known (different files having the same fingerprint). However,
3691 it is known that they can be produced with considerable, but not
3692 unreasonable, resources. For this reason, it is generally considered
3693 that SHA-1 should be gradually phased out in favor of the more secure
3694 SHA-2 hash algorithms. @xref{sha2 utilities}.
3697 @node sha2 utilities
3698 @section sha2 utilities: Print or check SHA-2 digests
3705 @cindex 224-bit checksum
3706 @cindex 256-bit checksum
3707 @cindex 384-bit checksum
3708 @cindex 512-bit checksum
3709 @cindex checksum, 224-bit
3710 @cindex checksum, 256-bit
3711 @cindex checksum, 384-bit
3712 @cindex checksum, 512-bit
3713 @cindex fingerprint, 224-bit
3714 @cindex fingerprint, 256-bit
3715 @cindex fingerprint, 384-bit
3716 @cindex fingerprint, 512-bit
3717 @cindex message-digest, 224-bit
3718 @cindex message-digest, 256-bit
3719 @cindex message-digest, 384-bit
3720 @cindex message-digest, 512-bit
3722 The commands @command{sha224sum}, @command{sha256sum},
3723 @command{sha384sum} and @command{sha512sum} compute checksums of
3724 various lengths (respectively 224, 256, 384 and 512 bits),
3725 collectively known as the SHA-2 hashes. The usage and options of
3726 these commands are precisely the same as for @command{md5sum}.
3727 @xref{md5sum invocation}.
3729 Note: The SHA384 and SHA512 digests are considerably slower to
3730 compute, especially on 32-bit computers, than SHA224 or SHA256.
3733 @node Operating on sorted files
3734 @chapter Operating on sorted files
3736 @cindex operating on sorted files
3737 @cindex sorted files, operations on
3739 These commands work with (or produce) sorted files.
3742 * sort invocation:: Sort text files.
3743 * shuf invocation:: Shuffle text files.
3744 * uniq invocation:: Uniquify files.
3745 * comm invocation:: Compare two sorted files line by line.
3746 * ptx invocation:: Produce a permuted index of file contents.
3747 * tsort invocation:: Topological sort.
3751 @node sort invocation
3752 @section @command{sort}: Sort text files
3755 @cindex sorting files
3757 @command{sort} sorts, merges, or compares all the lines from the given
3758 files, or standard input if none are given or for a @var{file} of
3759 @samp{-}. By default, @command{sort} writes the results to standard
3763 sort [@var{option}]@dots{} [@var{file}]@dots{}
3766 @command{sort} has three modes of operation: sort (the default), merge,
3767 and check for sortedness. The following options change the operation
3774 @itemx --check=diagnose-first
3777 @cindex checking for sortedness
3778 Check whether the given file is already sorted: if it is not all
3779 sorted, print a diagnostic containing the first out-of-order line and
3780 exit with a status of 1.
3781 Otherwise, exit successfully.
3782 At most one input file can be given.
3785 @itemx --check=quiet
3786 @itemx --check=silent
3789 @cindex checking for sortedness
3790 Exit successfully if the given file is already sorted, and
3791 exit with status 1 otherwise.
3792 At most one input file can be given.
3793 This is like @option{-c}, except it does not print a diagnostic.
3799 @cindex merging sorted files
3800 Merge the given files by sorting them as a group. Each input file must
3801 always be individually sorted. It always works to sort instead of
3802 merge; merging is provided because it is faster, in the case where it
3807 @cindex sort stability
3808 @cindex sort's last-resort comparison
3809 A pair of lines is compared as follows:
3810 @command{sort} compares each pair of fields, in the
3811 order specified on the command line, according to the associated
3812 ordering options, until a difference is found or no fields are left.
3813 If no key fields are specified, @command{sort} uses a default key of
3814 the entire line. Finally, as a last resort when all keys compare
3815 equal, @command{sort} compares entire lines as if no ordering options
3816 other than @option{--reverse} (@option{-r}) were specified. The
3817 @option{--stable} (@option{-s}) option disables this @dfn{last-resort
3818 comparison} so that lines in which all fields compare equal are left
3819 in their original relative order. The @option{--unique}
3820 (@option{-u}) option also disables the last-resort comparison.
3824 Unless otherwise specified, all comparisons use the character collating
3825 sequence specified by the @env{LC_COLLATE} locale.@footnote{If you
3826 use a non-@acronym{POSIX} locale (e.g., by setting @env{LC_ALL}
3827 to @samp{en_US}), then @command{sort} may produce output that is sorted
3828 differently than you're accustomed to. In that case, set the @env{LC_ALL}
3829 environment variable to @samp{C}. Note that setting only @env{LC_COLLATE}
3830 has two problems. First, it is ineffective if @env{LC_ALL} is also set.
3831 Second, it has undefined behavior if @env{LC_CTYPE} (or @env{LANG}, if
3832 @env{LC_CTYPE} is unset) is set to an incompatible value. For example,
3833 you get undefined behavior if @env{LC_CTYPE} is @code{ja_JP.PCK} but
3834 @env{LC_COLLATE} is @code{en_US.UTF-8}.}
3836 @sc{gnu} @command{sort} (as specified for all @sc{gnu} utilities) has no
3837 limit on input line length or restrictions on bytes allowed within lines.
3838 In addition, if the final byte of an input file is not a newline, @sc{gnu}
3839 @command{sort} silently supplies one. A line's trailing newline is not
3840 part of the line for comparison purposes.
3842 @cindex exit status of @command{sort}
3846 0 if no error occurred
3847 1 if invoked with @option{-c} or @option{-C} and the input is not sorted
3848 2 if an error occurred
3852 If the environment variable @env{TMPDIR} is set, @command{sort} uses its
3853 value as the directory for temporary files instead of @file{/tmp}. The
3854 @option{--temporary-directory} (@option{-T}) option in turn overrides
3855 the environment variable.
3857 The following options affect the ordering of output lines. They may be
3858 specified globally or as part of a specific key field. If no key
3859 fields are specified, global options apply to comparison of entire
3860 lines; otherwise the global options are inherited by key fields that do
3861 not specify any special options of their own. In pre-@acronym{POSIX}
3862 versions of @command{sort}, global options affect only later key fields,
3863 so portable shell scripts should specify global options first.
3868 @itemx --ignore-leading-blanks
3870 @opindex --ignore-leading-blanks
3871 @cindex blanks, ignoring leading
3873 Ignore leading blanks when finding sort keys in each line.
3874 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3875 can change this. Note blanks may be ignored by your locale's collating
3876 rules, but without this option they will be significant for character
3877 positions specified in keys with the @option{-k} option.
3880 @itemx --dictionary-order
3882 @opindex --dictionary-order
3883 @cindex dictionary order
3884 @cindex phone directory order
3885 @cindex telephone directory order
3887 Sort in @dfn{phone directory} order: ignore all characters except
3888 letters, digits and blanks when sorting.
3889 By default letters and digits are those of @acronym{ASCII} and a blank
3890 is a space or a tab, but the @env{LC_CTYPE} locale can change this.
3893 @itemx --ignore-case
3895 @opindex --ignore-case
3896 @cindex ignoring case
3897 @cindex case folding
3899 Fold lowercase characters into the equivalent uppercase characters when
3900 comparing so that, for example, @samp{b} and @samp{B} sort as equal.
3901 The @env{LC_CTYPE} locale determines character types.
3902 When used with @option{--unique} those lower case equivalent lines are
3903 thrown away. (There is currently no way to throw away the upper case
3904 equivalent instead. (Any @option{--reverse} given would only affect
3905 the final result, after the throwing away.))
3908 @itemx --general-numeric-sort
3909 @itemx --sort=general-numeric
3911 @opindex --general-numeric-sort
3913 @cindex general numeric sort
3915 Sort numerically, converting a prefix of each line to a long
3916 double-precision floating point number. @xref{Floating point}.
3917 Do not report overflow, underflow, or conversion errors.
3918 Use the following collating sequence:
3922 Lines that do not start with numbers (all considered to be equal).
3924 NaNs (``Not a Number'' values, in IEEE floating point arithmetic)
3925 in a consistent but machine-dependent order.
3929 Finite numbers in ascending numeric order (with @math{-0} and @math{+0} equal).
3934 Use this option only if there is no alternative; it is much slower than
3935 @option{--numeric-sort} (@option{-n}) and it can lose information when
3936 converting to floating point.
3939 @itemx --human-numeric-sort
3940 @itemx --sort=human-numeric
3942 @opindex --human-numeric-sort
3944 @cindex human numeric sort
3946 Sort numerically, first by numeric sign (negative, zero, or positive);
3947 then by @acronym{SI} suffix (either empty, or @samp{k} or @samp{K}, or
3948 one of @samp{MGTPEZY}, in that order; @pxref{Block size}); and finally
3949 by numeric value. For example, @samp{1023M} sorts before @samp{1G}
3950 because @samp{M} (mega) precedes @samp{G} (giga) as an @acronym{SI}
3951 suffix. This option sorts values that are consistently scaled to the
3952 nearest suffix, regardless of whether suffixes denote powers of 1000
3953 or 1024, and it therefore sorts the output of any single invocation of
3954 the @command{df}, @command{du}, or @command{ls} commands that are
3955 invoked with their @option{--human-readable} or @option{--si} options.
3956 The syntax for numbers is the same as for the @option{--numeric-sort}
3957 option; the @acronym{SI} suffix must immediately follow the number.
3960 @itemx --ignore-nonprinting
3962 @opindex --ignore-nonprinting
3963 @cindex nonprinting characters, ignoring
3964 @cindex unprintable characters, ignoring
3966 Ignore nonprinting characters.
3967 The @env{LC_CTYPE} locale determines character types.
3968 This option has no effect if the stronger @option{--dictionary-order}
3969 (@option{-d}) option is also given.
3975 @opindex --month-sort
3977 @cindex months, sorting by
3979 An initial string, consisting of any amount of blanks, followed
3980 by a month name abbreviation, is folded to UPPER case and
3981 compared in the order @samp{JAN} < @samp{FEB} < @dots{} < @samp{DEC}.
3982 Invalid names compare low to valid names. The @env{LC_TIME} locale
3983 category determines the month spellings.
3984 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3988 @itemx --numeric-sort
3989 @itemx --sort=numeric
3991 @opindex --numeric-sort
3993 @cindex numeric sort
3995 Sort numerically. The number begins each line and consists
3996 of optional blanks, an optional @samp{-} sign, and zero or more
3997 digits possibly separated by thousands separators, optionally followed
3998 by a decimal-point character and zero or more digits. An empty
3999 number is treated as @samp{0}. The @env{LC_NUMERIC}
4000 locale specifies the decimal-point character and thousands separator.
4001 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4004 Comparison is exact; there is no rounding error.
4006 Neither a leading @samp{+} nor exponential notation is recognized.
4007 To compare such strings numerically, use the
4008 @option{--general-numeric-sort} (@option{-g}) option.
4011 @itemx --version-sort
4013 @opindex --version-sort
4014 @cindex version number sort
4015 Sort by version name and number. It behaves like a standard sort,
4016 except that each sequence of decimal digits is treated numerically
4017 as an index/version number. (@xref{Details about version sort}.)
4023 @cindex reverse sorting
4024 Reverse the result of comparison, so that lines with greater key values
4025 appear earlier in the output instead of later.
4028 @itemx --random-sort
4029 @itemx --sort=random
4031 @opindex --random-sort
4034 Sort by hashing the input keys and then sorting the hash values.
4035 Choose the hash function at random, ensuring that it is free of
4036 collisions so that differing keys have differing hash values. This is
4037 like a random permutation of the inputs (@pxref{shuf invocation}),
4038 except that keys with the same value sort together.
4040 If multiple random sort fields are specified, the same random hash
4041 function is used for all fields. To use different random hash
4042 functions for different fields, you can invoke @command{sort} more
4045 The choice of hash function is affected by the
4046 @option{--random-source} option.
4054 @item --compress-program=@var{prog}
4055 Compress any temporary files with the program @var{prog}.
4057 With no arguments, @var{prog} must compress standard input to standard
4058 output, and when given the @option{-d} option it must decompress
4059 standard input to standard output.
4061 Terminate with an error if @var{prog} exits with nonzero status.
4063 White space and the backslash character should not appear in
4064 @var{prog}; they are reserved for future use.
4066 @filesZeroFromOption{sort,,sorted output}
4068 @item -k @var{pos1}[,@var{pos2}]
4069 @itemx --key=@var{pos1}[,@var{pos2}]
4073 Specify a sort field that consists of the part of the line between
4074 @var{pos1} and @var{pos2} (or the end of the line, if @var{pos2} is
4075 omitted), @emph{inclusive}.
4077 Each @var{pos} has the form @samp{@var{f}[.@var{c}][@var{opts}]},
4078 where @var{f} is the number of the field to use, and @var{c} is the number
4079 of the first character from the beginning of the field. Fields and character
4080 positions are numbered starting with 1; a character position of zero in
4081 @var{pos2} indicates the field's last character. If @samp{.@var{c}} is
4082 omitted from @var{pos1}, it defaults to 1 (the beginning of the field);
4083 if omitted from @var{pos2}, it defaults to 0 (the end of the field).
4084 @var{opts} are ordering options, allowing individual keys to be sorted
4085 according to different rules; see below for details. Keys can span
4088 Example: To sort on the second field, use @option{--key=2,2}
4089 (@option{-k 2,2}). See below for more notes on keys and more examples.
4090 See also the @option{--debug} option to help determine the part
4091 of the line being used in the sort.
4094 Highlight the portion of each line used for sorting.
4095 Also issue warnings about questionable usage to stderr.
4097 @item --batch-size=@var{nmerge}
4098 @opindex --batch-size
4099 @cindex number of inputs to merge, nmerge
4100 Merge at most @var{nmerge} inputs at once.
4102 When @command{sort} has to merge more than @var{nmerge} inputs,
4103 it merges them in groups of @var{nmerge}, saving the result in
4104 a temporary file, which is then used as an input in a subsequent merge.
4106 A large value of @var{nmerge} may improve merge performance and decrease
4107 temporary storage utilization at the expense of increased memory usage
4108 and I/0. Conversely a small value of @var{nmerge} may reduce memory
4109 requirements and I/0 at the expense of temporary storage consumption and
4112 The value of @var{nmerge} must be at least 2. The default value is
4113 currently 16, but this is implementation-dependent and may change in
4116 The value of @var{nmerge} may be bounded by a resource limit for open
4117 file descriptors. The commands @samp{ulimit -n} or @samp{getconf
4118 OPEN_MAX} may display limits for your systems; these limits may be
4119 modified further if your program already has some files open, or if
4120 the operating system has other limits on the number of open files. If
4121 the value of @var{nmerge} exceeds the resource limit, @command{sort}
4122 silently uses a smaller value.
4124 @item -o @var{output-file}
4125 @itemx --output=@var{output-file}
4128 @cindex overwriting of input, allowed
4129 Write output to @var{output-file} instead of standard output.
4130 Normally, @command{sort} reads all input before opening
4131 @var{output-file}, so you can safely sort a file in place by using
4132 commands like @code{sort -o F F} and @code{cat F | sort -o F}.
4133 However, @command{sort} with @option{--merge} (@option{-m}) can open
4134 the output file before reading all input, so a command like @code{cat
4135 F | sort -m -o F - G} is not safe as @command{sort} might start
4136 writing @file{F} before @command{cat} is done reading it.
4138 @vindex POSIXLY_CORRECT
4139 On newer systems, @option{-o} cannot appear after an input file if
4140 @env{POSIXLY_CORRECT} is set, e.g., @samp{sort F -o F}. Portable
4141 scripts should specify @option{-o @var{output-file}} before any input
4144 @item --random-source=@var{file}
4145 @opindex --random-source
4146 @cindex random source for sorting
4147 Use @var{file} as a source of random data used to determine which
4148 random hash function to use with the @option{-R} option. @xref{Random
4155 @cindex sort stability
4156 @cindex sort's last-resort comparison
4158 Make @command{sort} stable by disabling its last-resort comparison.
4159 This option has no effect if no fields or global ordering options
4160 other than @option{--reverse} (@option{-r}) are specified.
4163 @itemx --buffer-size=@var{size}
4165 @opindex --buffer-size
4166 @cindex size for main memory sorting
4167 Use a main-memory sort buffer of the given @var{size}. By default,
4168 @var{size} is in units of 1024 bytes. Appending @samp{%} causes
4169 @var{size} to be interpreted as a percentage of physical memory.
4170 Appending @samp{K} multiplies @var{size} by 1024 (the default),
4171 @samp{M} by 1,048,576, @samp{G} by 1,073,741,824, and so on for
4172 @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}. Appending
4173 @samp{b} causes @var{size} to be interpreted as a byte count, with no
4176 This option can improve the performance of @command{sort} by causing it
4177 to start with a larger or smaller sort buffer than the default.
4178 However, this option affects only the initial buffer size. The buffer
4179 grows beyond @var{size} if @command{sort} encounters input lines larger
4182 @item -t @var{separator}
4183 @itemx --field-separator=@var{separator}
4185 @opindex --field-separator
4186 @cindex field separator character
4187 Use character @var{separator} as the field separator when finding the
4188 sort keys in each line. By default, fields are separated by the empty
4189 string between a non-blank character and a blank character.
4190 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4193 That is, given the input line @w{@samp{ foo bar}}, @command{sort} breaks it
4194 into fields @w{@samp{ foo}} and @w{@samp{ bar}}. The field separator is
4195 not considered to be part of either the field preceding or the field
4196 following, so with @samp{sort @w{-t " "}} the same input line has
4197 three fields: an empty field, @samp{foo}, and @samp{bar}.
4198 However, fields that extend to the end of the line,
4199 as @option{-k 2}, or fields consisting of a range, as @option{-k 2,3},
4200 retain the field separators present between the endpoints of the range.
4202 To specify @acronym{ASCII} @sc{nul} as the field separator,
4203 use the two-character string @samp{\0}, e.g., @samp{sort -t '\0'}.
4205 @item -T @var{tempdir}
4206 @itemx --temporary-directory=@var{tempdir}
4208 @opindex --temporary-directory
4209 @cindex temporary directory
4211 Use directory @var{tempdir} to store temporary files, overriding the
4212 @env{TMPDIR} environment variable. If this option is given more than
4213 once, temporary files are stored in all the directories given. If you
4214 have a large sort or merge that is I/O-bound, you can often improve
4215 performance by using this option to specify directories on different
4216 disks and controllers.
4218 @item --parallel=@var{n}
4220 @cindex multithreaded sort
4221 Set the number of sorts run in parallel to @var{n}. By default,
4222 @var{n} is set to the number of available processors, but limited
4223 to 8, as there are diminishing performance gains after that.
4224 Note also that using @var{n} threads increases the memory usage by
4225 a factor of log @var{n}. Also see @ref{nproc invocation}.
4231 @cindex uniquifying output
4233 Normally, output only the first of a sequence of lines that compare
4234 equal. For the @option{--check} (@option{-c} or @option{-C}) option,
4235 check that no pair of consecutive lines compares equal.
4237 This option also disables the default last-resort comparison.
4239 The commands @code{sort -u} and @code{sort | uniq} are equivalent, but
4240 this equivalence does not extend to arbitrary @command{sort} options.
4241 For example, @code{sort -n -u} inspects only the value of the initial
4242 numeric string when checking for uniqueness, whereas @code{sort -n |
4243 uniq} inspects the entire line. @xref{uniq invocation}.
4245 @macro zeroTerminatedOption
4247 @itemx --zero-terminated
4249 @opindex --zero-terminated
4250 @cindex process zero-terminated items
4251 Delimit items with a zero byte rather than a newline (@acronym{ASCII} @sc{lf}).
4252 I.E. treat input as items separated by @acronym{ASCII} @sc{nul}
4253 and terminate output items with @acronym{ASCII} @sc{nul}.
4254 This option can be useful in conjunction with @samp{perl -0} or
4255 @samp{find -print0} and @samp{xargs -0} which do the same in order to
4256 reliably handle arbitrary file names (even those containing blanks
4257 or other special characters).
4259 @zeroTerminatedOption
4263 Historical (BSD and System V) implementations of @command{sort} have
4264 differed in their interpretation of some options, particularly
4265 @option{-b}, @option{-f}, and @option{-n}.
4266 @sc{gnu} sort follows the @acronym{POSIX}
4267 behavior, which is usually (but not always!) like the System V behavior.
4268 According to @acronym{POSIX}, @option{-n} no longer implies @option{-b}. For
4269 consistency, @option{-M} has been changed in the same way. This may
4270 affect the meaning of character positions in field specifications in
4271 obscure cases. The only fix is to add an explicit @option{-b}.
4273 A position in a sort field specified with @option{-k} may have any
4274 of the option letters @samp{MbdfghinRrV} appended to it, in which case no
4275 global ordering options are inherited by that particular field. The
4276 @option{-b} option may be independently attached to either or both of
4277 the start and end positions of a field specification, and if it is
4278 inherited from the global options it will be attached to both.
4279 If input lines can contain leading or adjacent blanks and @option{-t}
4280 is not used, then @option{-k} is typically combined with @option{-b} or
4281 an option that implicitly ignores leading blanks (@samp{Mghn}) as otherwise
4282 the varying numbers of leading blanks in fields can cause confusing results.
4284 If the start position in a sort field specifier falls after the end of
4285 the line or after the end field, the field is empty. If the @option{-b}
4286 option was specified, the @samp{.@var{c}} part of a field specification
4287 is counted from the first nonblank character of the field.
4289 @vindex _POSIX2_VERSION
4290 @vindex POSIXLY_CORRECT
4291 On older systems, @command{sort} supports an obsolete origin-zero
4292 syntax @samp{+@var{pos1} [-@var{pos2}]} for specifying sort keys.
4293 The obsolete sequence @samp{sort +@var{a}.@var{x} -@var{b}.@var{y}}
4294 is equivalent to @samp{sort -k @var{a+1}.@var{x+1},@var{b}} if @var{y}
4295 is @samp{0} or absent, otherwise it is equivalent to @samp{sort -k
4296 @var{a+1}.@var{x+1},@var{b+1}.@var{y}}.
4298 This obsolete behavior can be enabled or disabled with the
4299 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4300 conformance}); it can also be enabled when @env{POSIXLY_CORRECT} is
4301 not set by using the obsolete syntax with @samp{-@var{pos2}} present.
4303 Scripts intended for use on standard hosts should avoid obsolete
4304 syntax and should use @option{-k} instead. For example, avoid
4305 @samp{sort +2}, since it might be interpreted as either @samp{sort
4306 ./+2} or @samp{sort -k 3}. If your script must also run on hosts that
4307 support only the obsolete syntax, it can use a test like @samp{if sort
4308 -k 1 </dev/null >/dev/null 2>&1; then @dots{}} to decide which syntax
4311 Here are some examples to illustrate various combinations of options.
4316 Sort in descending (reverse) numeric order.
4323 Run no more that 4 sorts concurrently, using a buffer size of 10M.
4326 sort --parallel=4 -S 10M
4330 Sort alphabetically, omitting the first and second fields
4331 and the blanks at the start of the third field.
4332 This uses a single key composed of the characters beginning
4333 at the start of the first nonblank character in field three
4334 and extending to the end of each line.
4341 Sort numerically on the second field and resolve ties by sorting
4342 alphabetically on the third and fourth characters of field five.
4343 Use @samp{:} as the field delimiter.
4346 sort -t : -k 2,2n -k 5.3,5.4
4349 Note that if you had written @option{-k 2n} instead of @option{-k 2,2n}
4350 @command{sort} would have used all characters beginning in the second field
4351 and extending to the end of the line as the primary @emph{numeric}
4352 key. For the large majority of applications, treating keys spanning
4353 more than one field as numeric will not do what you expect.
4355 Also note that the @samp{n} modifier was applied to the field-end
4356 specifier for the first key. It would have been equivalent to
4357 specify @option{-k 2n,2} or @option{-k 2n,2n}. All modifiers except
4358 @samp{b} apply to the associated @emph{field}, regardless of whether
4359 the modifier character is attached to the field-start and/or the
4360 field-end part of the key specifier.
4363 Sort the password file on the fifth field and ignore any
4364 leading blanks. Sort lines with equal values in field five
4365 on the numeric user ID in field three. Fields are separated
4369 sort -t : -k 5b,5 -k 3,3n /etc/passwd
4370 sort -t : -n -k 5b,5 -k 3,3 /etc/passwd
4371 sort -t : -b -k 5,5 -k 3,3n /etc/passwd
4374 These three commands have equivalent effect. The first specifies that
4375 the first key's start position ignores leading blanks and the second
4376 key is sorted numerically. The other two commands rely on global
4377 options being inherited by sort keys that lack modifiers. The inheritance
4378 works in this case because @option{-k 5b,5b} and @option{-k 5b,5} are
4379 equivalent, as the location of a field-end lacking a @samp{.@var{c}}
4380 character position is not affected by whether initial blanks are
4384 Sort a set of log files, primarily by IPv4 address and secondarily by
4385 time stamp. If two lines' primary and secondary keys are identical,
4386 output the lines in the same order that they were input. The log
4387 files contain lines that look like this:
4390 4.150.156.3 - - [01/Apr/2004:06:31:51 +0000] message 1
4391 211.24.3.231 - - [24/Apr/2004:20:17:39 +0000] message 2
4394 Fields are separated by exactly one space. Sort IPv4 addresses
4395 lexicographically, e.g., 212.61.52.2 sorts before 212.129.233.201
4396 because 61 is less than 129.
4399 sort -s -t ' ' -k 4.9n -k 4.5M -k 4.2n -k 4.14,4.21 file*.log |
4400 sort -s -t '.' -k 1,1n -k 2,2n -k 3,3n -k 4,4n
4403 This example cannot be done with a single @command{sort} invocation,
4404 since IPv4 address components are separated by @samp{.} while dates
4405 come just after a space. So it is broken down into two invocations of
4406 @command{sort}: the first sorts by time stamp and the second by IPv4
4407 address. The time stamp is sorted by year, then month, then day, and
4408 finally by hour-minute-second field, using @option{-k} to isolate each
4409 field. Except for hour-minute-second there's no need to specify the
4410 end of each key field, since the @samp{n} and @samp{M} modifiers sort
4411 based on leading prefixes that cannot cross field boundaries. The
4412 IPv4 addresses are sorted lexicographically. The second sort uses
4413 @samp{-s} so that ties in the primary key are broken by the secondary
4414 key; the first sort uses @samp{-s} so that the combination of the two
4418 Generate a tags file in case-insensitive sorted order.
4421 find src -type f -print0 | sort -z -f | xargs -0 etags --append
4424 The use of @option{-print0}, @option{-z}, and @option{-0} in this case means
4425 that file names that contain blanks or other special characters are
4427 by the sort operation.
4429 @c This example is a bit contrived and needs more explanation.
4431 @c Sort records separated by an arbitrary string by using a pipe to convert
4432 @c each record delimiter string to @samp{\0}, then using sort's -z option,
4433 @c and converting each @samp{\0} back to the original record delimiter.
4436 @c printf 'c\n\nb\n\na\n' |
4437 @c perl -0pe 's/\n\n/\n\0/g' |
4439 @c perl -0pe 's/\0/\n/g'
4443 Use the common @acronym{DSU, Decorate Sort Undecorate} idiom to
4444 sort lines according to their length.
4447 awk '@{print length, $0@}' /etc/passwd | sort -n | cut -f2- -d' '
4450 In general this technique can be used to sort data that the @command{sort}
4451 command does not support, or is inefficient at, sorting directly.
4454 Shuffle a list of directories, but preserve the order of files within
4455 each directory. For instance, one could use this to generate a music
4456 playlist in which albums are shuffled but the songs of each album are
4460 ls */* | sort -t / -k 1,1R -k 2,2
4466 @node shuf invocation
4467 @section @command{shuf}: Shuffling text
4470 @cindex shuffling files
4472 @command{shuf} shuffles its input by outputting a random permutation
4473 of its input lines. Each output permutation is equally likely.
4477 shuf [@var{option}]@dots{} [@var{file}]
4478 shuf -e [@var{option}]@dots{} [@var{arg}]@dots{}
4479 shuf -i @var{lo}-@var{hi} [@var{option}]@dots{}
4482 @command{shuf} has three modes of operation that affect where it
4483 obtains its input lines. By default, it reads lines from standard
4484 input. The following options change the operation mode:
4492 @cindex command-line operands to shuffle
4493 Treat each command-line operand as an input line.
4495 @item -i @var{lo}-@var{hi}
4496 @itemx --input-range=@var{lo}-@var{hi}
4498 @opindex --input-range
4499 @cindex input range to shuffle
4500 Act as if input came from a file containing the range of unsigned
4501 decimal integers @var{lo}@dots{}@var{hi}, one per line.
4505 @command{shuf}'s other options can affect its behavior in all
4510 @item -n @var{lines}
4511 @itemx --head-count=@var{count}
4513 @opindex --head-count
4514 @cindex head of output
4515 Output at most @var{count} lines. By default, all input lines are
4518 @item -o @var{output-file}
4519 @itemx --output=@var{output-file}
4522 @cindex overwriting of input, allowed
4523 Write output to @var{output-file} instead of standard output.
4524 @command{shuf} reads all input before opening
4525 @var{output-file}, so you can safely shuffle a file in place by using
4526 commands like @code{shuf -o F <F} and @code{cat F | shuf -o F}.
4528 @item --random-source=@var{file}
4529 @opindex --random-source
4530 @cindex random source for shuffling
4531 Use @var{file} as a source of random data used to determine which
4532 permutation to generate. @xref{Random sources}.
4534 @zeroTerminatedOption
4550 might produce the output
4560 Similarly, the command:
4563 shuf -e clubs hearts diamonds spades
4577 and the command @samp{shuf -i 1-4} might output:
4587 These examples all have four input lines, so @command{shuf} might
4588 produce any of the twenty-four possible permutations of the input. In
4589 general, if there are @var{n} input lines, there are @var{n}! (i.e.,
4590 @var{n} factorial, or @var{n} * (@var{n} - 1) * @dots{} * 1) possible
4591 output permutations.
4596 @node uniq invocation
4597 @section @command{uniq}: Uniquify files
4600 @cindex uniquify files
4602 @command{uniq} writes the unique lines in the given @file{input}, or
4603 standard input if nothing is given or for an @var{input} name of
4607 uniq [@var{option}]@dots{} [@var{input} [@var{output}]]
4610 By default, @command{uniq} prints its input lines, except that
4611 it discards all but the first of adjacent repeated lines, so that
4612 no output lines are repeated. Optionally, it can instead discard
4613 lines that are not repeated, or all repeated lines.
4615 The input need not be sorted, but repeated input lines are detected
4616 only if they are adjacent. If you want to discard non-adjacent
4617 duplicate lines, perhaps you want to use @code{sort -u}.
4618 @xref{sort invocation}.
4621 Comparisons honor the rules specified by the @env{LC_COLLATE}
4624 If no @var{output} file is specified, @command{uniq} writes to standard
4627 The program accepts the following options. Also see @ref{Common options}.
4632 @itemx --skip-fields=@var{n}
4634 @opindex --skip-fields
4635 Skip @var{n} fields on each line before checking for uniqueness. Use
4636 a null string for comparison if a line has fewer than @var{n} fields. Fields
4637 are sequences of non-space non-tab characters that are separated from
4638 each other by at least one space or tab.
4640 For compatibility @command{uniq} supports an obsolete option syntax
4641 @option{-@var{n}}. New scripts should use @option{-f @var{n}} instead.
4644 @itemx --skip-chars=@var{n}
4646 @opindex --skip-chars
4647 Skip @var{n} characters before checking for uniqueness. Use a null string
4648 for comparison if a line has fewer than @var{n} characters. If you use both
4649 the field and character skipping options, fields are skipped over first.
4651 @vindex _POSIX2_VERSION
4652 On older systems, @command{uniq} supports an obsolete option syntax
4654 This obsolete behavior can be enabled or disabled with the
4655 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4656 conformance}), but portable scripts should avoid commands whose
4657 behavior depends on this variable.
4658 For example, use @samp{uniq ./+10} or @samp{uniq -s 10} rather than
4659 the ambiguous @samp{uniq +10}.
4665 Print the number of times each line occurred along with the line.
4668 @itemx --ignore-case
4670 @opindex --ignore-case
4671 Ignore differences in case when comparing lines.
4677 @cindex repeated lines, outputting
4678 Discard lines that are not repeated. When used by itself, this option
4679 causes @command{uniq} to print the first copy of each repeated line,
4683 @itemx --all-repeated[=@var{delimit-method}]
4685 @opindex --all-repeated
4686 @cindex all repeated lines, outputting
4687 Do not discard the second and subsequent repeated input lines,
4688 but discard lines that are not repeated.
4689 This option is useful mainly in conjunction with other options e.g.,
4690 to ignore case or to compare only selected fields.
4691 The optional @var{delimit-method} tells how to delimit
4692 groups of repeated lines, and must be one of the following:
4697 Do not delimit groups of repeated lines.
4698 This is equivalent to @option{--all-repeated} (@option{-D}).
4701 Output a newline before each group of repeated lines.
4702 With @option{--zero-terminated} (@option{-z}), use a zero
4703 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4706 Separate groups of repeated lines with a single newline.
4707 With @option{--zero-terminated} (@option{-z}), use a zero
4708 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4709 This is the same as using @samp{prepend}, except that
4710 no delimiter is inserted before the first group, and hence
4711 may be better suited for output direct to users.
4714 Note that when groups are delimited and the input stream contains
4715 two or more consecutive blank lines, then the output is ambiguous.
4716 To avoid that, filter the input through @samp{tr -s '\n'} to replace
4717 each sequence of consecutive newlines with a single newline.
4719 This is a @sc{gnu} extension.
4720 @c FIXME: give an example showing *how* it's useful
4726 @cindex unique lines, outputting
4727 Discard the first repeated line. When used by itself, this option
4728 causes @command{uniq} to print unique lines, and nothing else.
4731 @itemx --check-chars=@var{n}
4733 @opindex --check-chars
4734 Compare at most @var{n} characters on each line (after skipping any specified
4735 fields and characters). By default the entire rest of the lines are
4738 @zeroTerminatedOption
4745 @node comm invocation
4746 @section @command{comm}: Compare two sorted files line by line
4749 @cindex line-by-line comparison
4750 @cindex comparing sorted files
4752 @command{comm} writes to standard output lines that are common, and lines
4753 that are unique, to two input files; a file name of @samp{-} means
4754 standard input. Synopsis:
4757 comm [@var{option}]@dots{} @var{file1} @var{file2}
4761 Before @command{comm} can be used, the input files must be sorted using the
4762 collating sequence specified by the @env{LC_COLLATE} locale.
4763 If an input file ends in a non-newline
4764 character, a newline is silently appended. The @command{sort} command with
4765 no options always outputs a file that is suitable input to @command{comm}.
4767 @cindex differing lines
4768 @cindex common lines
4769 With no options, @command{comm} produces three-column output. Column one
4770 contains lines unique to @var{file1}, column two contains lines unique
4771 to @var{file2}, and column three contains lines common to both files.
4772 Columns are separated by a single TAB character.
4773 @c FIXME: when there's an option to supply an alternative separator
4774 @c string, append `by default' to the above sentence.
4779 The options @option{-1}, @option{-2}, and @option{-3} suppress printing of
4780 the corresponding columns (and separators). Also see @ref{Common options}.
4782 Unlike some other comparison utilities, @command{comm} has an exit
4783 status that does not depend on the result of the comparison.
4784 Upon normal completion @command{comm} produces an exit code of zero.
4785 If there is an error it exits with nonzero status.
4787 @macro checkOrderOption{cmd}
4788 If the @option{--check-order} option is given, unsorted inputs will
4789 cause a fatal error message. If the option @option{--nocheck-order}
4790 is given, unsorted inputs will never cause an error message. If neither
4791 of these options is given, wrongly sorted inputs are diagnosed
4792 only if an input file is found to contain unpairable
4794 lines, and when both input files are non empty.
4796 @ifclear JOIN_COMMAND
4799 If an input file is diagnosed as being unsorted, the @command{\cmd\}
4800 command will exit with a nonzero status (and the output should not be used).
4802 Forcing @command{\cmd\} to process wrongly sorted input files
4803 containing unpairable lines by specifying @option{--nocheck-order} is
4804 not guaranteed to produce any particular output. The output will
4805 probably not correspond with whatever you hoped it would be.
4807 @checkOrderOption{comm}
4812 Fail with an error message if either input file is wrongly ordered.
4814 @item --nocheck-order
4815 Do not check that both input files are in sorted order.
4819 @item --output-delimiter=@var{str}
4820 Print @var{str} between adjacent output columns,
4821 rather than the default of a single TAB character.
4823 The delimiter @var{str} may not be empty.
4827 @node ptx invocation
4828 @section @command{ptx}: Produce permuted indexes
4832 @command{ptx} reads a text file and essentially produces a permuted index, with
4833 each keyword in its context. The calling sketch is either one of:
4836 ptx [@var{option} @dots{}] [@var{file} @dots{}]
4837 ptx -G [@var{option} @dots{}] [@var{input} [@var{output}]]
4840 The @option{-G} (or its equivalent: @option{--traditional}) option disables
4841 all @sc{gnu} extensions and reverts to traditional mode, thus introducing some
4842 limitations and changing several of the program's default option values.
4843 When @option{-G} is not specified, @sc{gnu} extensions are always enabled.
4844 @sc{gnu} extensions to @command{ptx} are documented wherever appropriate in this
4845 document. For the full list, see @xref{Compatibility in ptx}.
4847 Individual options are explained in the following sections.
4849 When @sc{gnu} extensions are enabled, there may be zero, one or several
4850 @var{file}s after the options. If there is no @var{file}, the program
4851 reads the standard input. If there is one or several @var{file}s, they
4852 give the name of input files which are all read in turn, as if all the
4853 input files were concatenated. However, there is a full contextual
4854 break between each file and, when automatic referencing is requested,
4855 file names and line numbers refer to individual text input files. In
4856 all cases, the program outputs the permuted index to the standard
4859 When @sc{gnu} extensions are @emph{not} enabled, that is, when the program
4860 operates in traditional mode, there may be zero, one or two parameters
4861 besides the options. If there are no parameters, the program reads the
4862 standard input and outputs the permuted index to the standard output.
4863 If there is only one parameter, it names the text @var{input} to be read
4864 instead of the standard input. If two parameters are given, they give
4865 respectively the name of the @var{input} file to read and the name of
4866 the @var{output} file to produce. @emph{Be very careful} to note that,
4867 in this case, the contents of file given by the second parameter is
4868 destroyed. This behavior is dictated by System V @command{ptx}
4869 compatibility; @sc{gnu} Standards normally discourage output parameters not
4870 introduced by an option.
4872 Note that for @emph{any} file named as the value of an option or as an
4873 input text file, a single dash @kbd{-} may be used, in which case
4874 standard input is assumed. However, it would not make sense to use this
4875 convention more than once per program invocation.
4878 * General options in ptx:: Options which affect general program behavior.
4879 * Charset selection in ptx:: Underlying character set considerations.
4880 * Input processing in ptx:: Input fields, contexts, and keyword selection.
4881 * Output formatting in ptx:: Types of output format, and sizing the fields.
4882 * Compatibility in ptx::
4886 @node General options in ptx
4887 @subsection General options
4892 @itemx --traditional
4893 As already explained, this option disables all @sc{gnu} extensions to
4894 @command{ptx} and switches to traditional mode.
4897 Print a short help on standard output, then exit without further
4901 Print the program version on standard output, then exit without further
4909 @node Charset selection in ptx
4910 @subsection Charset selection
4912 @c FIXME: People don't necessarily know what an IBM-PC was these days.
4913 As it is set up now, the program assumes that the input file is coded
4914 using 8-bit @acronym{ISO} 8859-1 code, also known as Latin-1 character set,
4915 @emph{unless} it is compiled for MS-DOS, in which case it uses the
4916 character set of the IBM-PC@. (@sc{gnu} @command{ptx} is not known to work on
4917 smaller MS-DOS machines anymore.) Compared to 7-bit @acronym{ASCII}, the set
4918 of characters which are letters is different; this alters the behavior
4919 of regular expression matching. Thus, the default regular expression
4920 for a keyword allows foreign or diacriticized letters. Keyword sorting,
4921 however, is still crude; it obeys the underlying character set ordering
4927 @itemx --ignore-case
4928 Fold lower case letters to upper case for sorting.
4933 @node Input processing in ptx
4934 @subsection Word selection and input processing
4939 @itemx --break-file=@var{file}
4941 This option provides an alternative (to @option{-W}) method of describing
4942 which characters make up words. It introduces the name of a
4943 file which contains a list of characters which can@emph{not} be part of
4944 one word; this file is called the @dfn{Break file}. Any character which
4945 is not part of the Break file is a word constituent. If both options
4946 @option{-b} and @option{-W} are specified, then @option{-W} has precedence and
4947 @option{-b} is ignored.
4949 When @sc{gnu} extensions are enabled, the only way to avoid newline as a
4950 break character is to write all the break characters in the file with no
4951 newline at all, not even at the end of the file. When @sc{gnu} extensions
4952 are disabled, spaces, tabs and newlines are always considered as break
4953 characters even if not included in the Break file.
4956 @itemx --ignore-file=@var{file}
4958 The file associated with this option contains a list of words which will
4959 never be taken as keywords in concordance output. It is called the
4960 @dfn{Ignore file}. The file contains exactly one word in each line; the
4961 end of line separation of words is not subject to the value of the
4965 @itemx --only-file=@var{file}
4967 The file associated with this option contains a list of words which will
4968 be retained in concordance output; any word not mentioned in this file
4969 is ignored. The file is called the @dfn{Only file}. The file contains
4970 exactly one word in each line; the end of line separation of words is
4971 not subject to the value of the @option{-S} option.
4973 There is no default for the Only file. When both an Only file and an
4974 Ignore file are specified, a word is considered a keyword only
4975 if it is listed in the Only file and not in the Ignore file.
4980 On each input line, the leading sequence of non-white space characters will be
4981 taken to be a reference that has the purpose of identifying this input
4982 line in the resulting permuted index. For more information about reference
4983 production, see @xref{Output formatting in ptx}.
4984 Using this option changes the default value for option @option{-S}.
4986 Using this option, the program does not try very hard to remove
4987 references from contexts in output, but it succeeds in doing so
4988 @emph{when} the context ends exactly at the newline. If option
4989 @option{-r} is used with @option{-S} default value, or when @sc{gnu} extensions
4990 are disabled, this condition is always met and references are completely
4991 excluded from the output contexts.
4993 @item -S @var{regexp}
4994 @itemx --sentence-regexp=@var{regexp}
4996 This option selects which regular expression will describe the end of a
4997 line or the end of a sentence. In fact, this regular expression is not
4998 the only distinction between end of lines or end of sentences, and input
4999 line boundaries have no special significance outside this option. By
5000 default, when @sc{gnu} extensions are enabled and if @option{-r} option is not
5001 used, end of sentences are used. In this case, this @var{regex} is
5002 imported from @sc{gnu} Emacs:
5005 [.?!][]\"')@}]*\\($\\|\t\\| \\)[ \t\n]*
5008 Whenever @sc{gnu} extensions are disabled or if @option{-r} option is used, end
5009 of lines are used; in this case, the default @var{regexp} is just:
5015 Using an empty @var{regexp} is equivalent to completely disabling end of
5016 line or end of sentence recognition. In this case, the whole file is
5017 considered to be a single big line or sentence. The user might want to
5018 disallow all truncation flag generation as well, through option @option{-F
5019 ""}. @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
5022 When the keywords happen to be near the beginning of the input line or
5023 sentence, this often creates an unused area at the beginning of the
5024 output context line; when the keywords happen to be near the end of the
5025 input line or sentence, this often creates an unused area at the end of
5026 the output context line. The program tries to fill those unused areas
5027 by wrapping around context in them; the tail of the input line or
5028 sentence is used to fill the unused area on the left of the output line;
5029 the head of the input line or sentence is used to fill the unused area
5030 on the right of the output line.
5032 As a matter of convenience to the user, many usual backslashed escape
5033 sequences from the C language are recognized and converted to the
5034 corresponding characters by @command{ptx} itself.
5036 @item -W @var{regexp}
5037 @itemx --word-regexp=@var{regexp}
5039 This option selects which regular expression will describe each keyword.
5040 By default, if @sc{gnu} extensions are enabled, a word is a sequence of
5041 letters; the @var{regexp} used is @samp{\w+}. When @sc{gnu} extensions are
5042 disabled, a word is by default anything which ends with a space, a tab
5043 or a newline; the @var{regexp} used is @samp{[^ \t\n]+}.
5045 An empty @var{regexp} is equivalent to not using this option.
5046 @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
5049 As a matter of convenience to the user, many usual backslashed escape
5050 sequences, as found in the C language, are recognized and converted to
5051 the corresponding characters by @command{ptx} itself.
5056 @node Output formatting in ptx
5057 @subsection Output formatting
5059 Output format is mainly controlled by the @option{-O} and @option{-T} options
5060 described in the table below. When neither @option{-O} nor @option{-T} are
5061 selected, and if @sc{gnu} extensions are enabled, the program chooses an
5062 output format suitable for a dumb terminal. Each keyword occurrence is
5063 output to the center of one line, surrounded by its left and right
5064 contexts. Each field is properly justified, so the concordance output
5065 can be readily observed. As a special feature, if automatic
5066 references are selected by option @option{-A} and are output before the
5067 left context, that is, if option @option{-R} is @emph{not} selected, then
5068 a colon is added after the reference; this nicely interfaces with @sc{gnu}
5069 Emacs @code{next-error} processing. In this default output format, each
5070 white space character, like newline and tab, is merely changed to
5071 exactly one space, with no special attempt to compress consecutive
5072 spaces. This might change in the future. Except for those white space
5073 characters, every other character of the underlying set of 256
5074 characters is transmitted verbatim.
5076 Output format is further controlled by the following options.
5080 @item -g @var{number}
5081 @itemx --gap-size=@var{number}
5083 Select the size of the minimum white space gap between the fields on the
5086 @item -w @var{number}
5087 @itemx --width=@var{number}
5089 Select the maximum output width of each final line. If references are
5090 used, they are included or excluded from the maximum output width
5091 depending on the value of option @option{-R}. If this option is not
5092 selected, that is, when references are output before the left context,
5093 the maximum output width takes into account the maximum length of all
5094 references. If this option is selected, that is, when references are
5095 output after the right context, the maximum output width does not take
5096 into account the space taken by references, nor the gap that precedes
5100 @itemx --auto-reference
5102 Select automatic references. Each input line will have an automatic
5103 reference made up of the file name and the line ordinal, with a single
5104 colon between them. However, the file name will be empty when standard
5105 input is being read. If both @option{-A} and @option{-r} are selected, then
5106 the input reference is still read and skipped, but the automatic
5107 reference is used at output time, overriding the input reference.
5110 @itemx --right-side-refs
5112 In the default output format, when option @option{-R} is not used, any
5113 references produced by the effect of options @option{-r} or @option{-A} are
5114 placed to the far right of output lines, after the right context. With
5115 default output format, when the @option{-R} option is specified, references
5116 are rather placed at the beginning of each output line, before the left
5117 context. For any other output format, option @option{-R} is
5118 ignored, with one exception: with @option{-R} the width of references
5119 is @emph{not} taken into account in total output width given by @option{-w}.
5121 This option is automatically selected whenever @sc{gnu} extensions are
5124 @item -F @var{string}
5125 @itemx --flac-truncation=@var{string}
5127 This option will request that any truncation in the output be reported
5128 using the string @var{string}. Most output fields theoretically extend
5129 towards the beginning or the end of the current line, or current
5130 sentence, as selected with option @option{-S}. But there is a maximum
5131 allowed output line width, changeable through option @option{-w}, which is
5132 further divided into space for various output fields. When a field has
5133 to be truncated because it cannot extend beyond the beginning or the end of
5134 the current line to fit in, then a truncation occurs. By default,
5135 the string used is a single slash, as in @option{-F /}.
5137 @var{string} may have more than one character, as in @option{-F ...}.
5138 Also, in the particular case when @var{string} is empty (@option{-F ""}),
5139 truncation flagging is disabled, and no truncation marks are appended in
5142 As a matter of convenience to the user, many usual backslashed escape
5143 sequences, as found in the C language, are recognized and converted to
5144 the corresponding characters by @command{ptx} itself.
5146 @item -M @var{string}
5147 @itemx --macro-name=@var{string}
5149 Select another @var{string} to be used instead of @samp{xx}, while
5150 generating output suitable for @command{nroff}, @command{troff} or @TeX{}.
5153 @itemx --format=roff
5155 Choose an output format suitable for @command{nroff} or @command{troff}
5156 processing. Each output line will look like:
5159 .xx "@var{tail}" "@var{before}" "@var{keyword_and_after}"@c
5160 "@var{head}" "@var{ref}"
5163 so it will be possible to write a @samp{.xx} roff macro to take care of
5164 the output typesetting. This is the default output format when @sc{gnu}
5165 extensions are disabled. Option @option{-M} can be used to change
5166 @samp{xx} to another macro name.
5168 In this output format, each non-graphical character, like newline and
5169 tab, is merely changed to exactly one space, with no special attempt to
5170 compress consecutive spaces. Each quote character: @kbd{"} is doubled
5171 so it will be correctly processed by @command{nroff} or @command{troff}.
5176 Choose an output format suitable for @TeX{} processing. Each output
5177 line will look like:
5180 \xx @{@var{tail}@}@{@var{before}@}@{@var{keyword}@}@c
5181 @{@var{after}@}@{@var{head}@}@{@var{ref}@}
5185 so it will be possible to write a @code{\xx} definition to take care of
5186 the output typesetting. Note that when references are not being
5187 produced, that is, neither option @option{-A} nor option @option{-r} is
5188 selected, the last parameter of each @code{\xx} call is inhibited.
5189 Option @option{-M} can be used to change @samp{xx} to another macro
5192 In this output format, some special characters, like @kbd{$}, @kbd{%},
5193 @kbd{&}, @kbd{#} and @kbd{_} are automatically protected with a
5194 backslash. Curly brackets @kbd{@{}, @kbd{@}} are protected with a
5195 backslash and a pair of dollar signs (to force mathematical mode). The
5196 backslash itself produces the sequence @code{\backslash@{@}}.
5197 Circumflex and tilde diacritical marks produce the sequence @code{^\@{ @}} and
5198 @code{~\@{ @}} respectively. Other diacriticized characters of the
5199 underlying character set produce an appropriate @TeX{} sequence as far
5200 as possible. The other non-graphical characters, like newline and tab,
5201 and all other characters which are not part of @acronym{ASCII}, are merely
5202 changed to exactly one space, with no special attempt to compress
5203 consecutive spaces. Let me know how to improve this special character
5204 processing for @TeX{}.
5209 @node Compatibility in ptx
5210 @subsection The @sc{gnu} extensions to @command{ptx}
5212 This version of @command{ptx} contains a few features which do not exist in
5213 System V @command{ptx}. These extra features are suppressed by using the
5214 @option{-G} command line option, unless overridden by other command line
5215 options. Some @sc{gnu} extensions cannot be recovered by overriding, so the
5216 simple rule is to avoid @option{-G} if you care about @sc{gnu} extensions.
5217 Here are the differences between this program and System V @command{ptx}.
5222 This program can read many input files at once, it always writes the
5223 resulting concordance on standard output. On the other hand, System V
5224 @command{ptx} reads only one file and sends the result to standard output
5225 or, if a second @var{file} parameter is given on the command, to that
5228 Having output parameters not introduced by options is a dangerous
5229 practice which @sc{gnu} avoids as far as possible. So, for using @command{ptx}
5230 portably between @sc{gnu} and System V, you should always use it with a
5231 single input file, and always expect the result on standard output. You
5232 might also want to automatically configure in a @option{-G} option to
5233 @command{ptx} calls in products using @command{ptx}, if the configurator finds
5234 that the installed @command{ptx} accepts @option{-G}.
5237 The only options available in System V @command{ptx} are options @option{-b},
5238 @option{-f}, @option{-g}, @option{-i}, @option{-o}, @option{-r}, @option{-t} and
5239 @option{-w}. All other options are @sc{gnu} extensions and are not repeated in
5240 this enumeration. Moreover, some options have a slightly different
5241 meaning when @sc{gnu} extensions are enabled, as explained below.
5244 By default, concordance output is not formatted for @command{troff} or
5245 @command{nroff}. It is rather formatted for a dumb terminal. @command{troff}
5246 or @command{nroff} output may still be selected through option @option{-O}.
5249 Unless @option{-R} option is used, the maximum reference width is
5250 subtracted from the total output line width. With @sc{gnu} extensions
5251 disabled, width of references is not taken into account in the output
5252 line width computations.
5255 All 256 bytes, even @acronym{ASCII} @sc{nul} bytes, are always read and
5256 processed from input file with no adverse effect, even if @sc{gnu} extensions
5257 are disabled. However, System V @command{ptx} does not accept 8-bit characters,
5258 a few control characters are rejected, and the tilde @kbd{~} is also rejected.
5261 Input line length is only limited by available memory, even if @sc{gnu}
5262 extensions are disabled. However, System V @command{ptx} processes only
5263 the first 200 characters in each line.
5266 The break (non-word) characters default to be every character except all
5267 letters of the underlying character set, diacriticized or not. When @sc{gnu}
5268 extensions are disabled, the break characters default to space, tab and
5272 The program makes better use of output line width. If @sc{gnu} extensions
5273 are disabled, the program rather tries to imitate System V @command{ptx},
5274 but still, there are some slight disposition glitches this program does
5275 not completely reproduce.
5278 The user can specify both an Ignore file and an Only file. This is not
5279 allowed with System V @command{ptx}.
5284 @node tsort invocation
5285 @section @command{tsort}: Topological sort
5288 @cindex topological sort
5290 @command{tsort} performs a topological sort on the given @var{file}, or
5291 standard input if no input file is given or for a @var{file} of
5292 @samp{-}. For more details and some history, see @ref{tsort background}.
5296 tsort [@var{option}] [@var{file}]
5299 @command{tsort} reads its input as pairs of strings, separated by blanks,
5300 indicating a partial ordering. The output is a total ordering that
5301 corresponds to the given partial ordering.
5315 will produce the output
5326 Consider a more realistic example.
5327 You have a large set of functions all in one file, and they may all be
5328 declared static except one. Currently that one (say @code{main}) is the
5329 first function defined in the file, and the ones it calls directly follow
5330 it, followed by those they call, etc. Let's say that you are determined
5331 to take advantage of prototypes, so you have to choose between declaring
5332 all of those functions (which means duplicating a lot of information from
5333 the definitions) and rearranging the functions so that as many as possible
5334 are defined before they are used. One way to automate the latter process
5335 is to get a list for each function of the functions it calls directly.
5336 Many programs can generate such lists. They describe a call graph.
5337 Consider the following list, in which a given line indicates that the
5338 function on the left calls the one on the right directly.
5344 tail_file pretty_name
5345 tail_file write_header
5347 tail_forever recheck
5348 tail_forever pretty_name
5349 tail_forever write_header
5350 tail_forever dump_remainder
5353 tail_lines start_lines
5354 tail_lines dump_remainder
5355 tail_lines file_lines
5356 tail_lines pipe_lines
5358 tail_bytes start_bytes
5359 tail_bytes dump_remainder
5360 tail_bytes pipe_bytes
5361 file_lines dump_remainder
5365 then you can use @command{tsort} to produce an ordering of those
5366 functions that satisfies your requirement.
5369 example$ tsort call-graph | tac
5389 @command{tsort} detects any cycles in the input and writes the first cycle
5390 encountered to standard error.
5392 Note that for a given partial ordering, generally there is no unique
5393 total ordering. In the context of the call graph above, the function
5394 @code{parse_options} may be placed anywhere in the list as long as it
5395 precedes @code{main}.
5397 The only options are @option{--help} and @option{--version}. @xref{Common
5403 * tsort background:: Where tsort came from.
5406 @node tsort background
5407 @subsection @command{tsort}: Background
5409 @command{tsort} exists because very early versions of the Unix linker processed
5410 an archive file exactly once, and in order. As @command{ld} read each object
5411 in the archive, it decided whether it was needed in the program based on
5412 whether it defined any symbols which were undefined at that point in
5415 This meant that dependencies within the archive had to be handled
5416 specially. For example, @code{scanf} probably calls @code{read}. That means
5417 that in a single pass through an archive, it was important for @code{scanf.o}
5418 to appear before read.o, because otherwise a program which calls
5419 @code{scanf} but not @code{read} might end up with an unexpected unresolved
5420 reference to @code{read}.
5422 The way to address this problem was to first generate a set of
5423 dependencies of one object file on another. This was done by a shell
5424 script called @command{lorder}. The GNU tools don't provide a version of
5425 lorder, as far as I know, but you can still find it in BSD
5428 Then you ran @command{tsort} over the @command{lorder} output, and you used the
5429 resulting sort to define the order in which you added objects to the archive.
5431 This whole procedure has been obsolete since about 1980, because
5432 Unix archives now contain a symbol table (traditionally built by
5433 @command{ranlib}, now generally built by @command{ar} itself), and the Unix
5434 linker uses the symbol table to effectively make multiple passes over
5437 Anyhow, that's where tsort came from. To solve an old problem with
5438 the way the linker handled archive files, which has since been solved
5442 @node Operating on fields
5443 @chapter Operating on fields
5446 * cut invocation:: Print selected parts of lines.
5447 * paste invocation:: Merge lines of files.
5448 * join invocation:: Join lines on a common field.
5452 @node cut invocation
5453 @section @command{cut}: Print selected parts of lines
5456 @command{cut} writes to standard output selected parts of each line of each
5457 input file, or standard input if no files are given or for a file name of
5461 cut @var{option}@dots{} [@var{file}]@dots{}
5464 In the table which follows, the @var{byte-list}, @var{character-list},
5465 and @var{field-list} are one or more numbers or ranges (two numbers
5466 separated by a dash) separated by commas. Bytes, characters, and
5467 fields are numbered starting at 1. Incomplete ranges may be
5468 given: @option{-@var{m}} means @samp{1-@var{m}}; @samp{@var{n}-} means
5469 @samp{@var{n}} through end of line or last field. The list elements
5470 can be repeated, can overlap, and can be specified in any order; but
5471 the selected input is written in the same order that it is read, and
5472 is written exactly once.
5474 The program accepts the following options. Also see @ref{Common
5479 @item -b @var{byte-list}
5480 @itemx --bytes=@var{byte-list}
5483 Select for printing only the bytes in positions listed in
5484 @var{byte-list}. Tabs and backspaces are treated like any other
5485 character; they take up 1 byte. If an output delimiter is specified,
5486 (see the description of @option{--output-delimiter}), then output that
5487 string between ranges of selected bytes.
5489 @item -c @var{character-list}
5490 @itemx --characters=@var{character-list}
5492 @opindex --characters
5493 Select for printing only the characters in positions listed in
5494 @var{character-list}. The same as @option{-b} for now, but
5495 internationalization will change that. Tabs and backspaces are
5496 treated like any other character; they take up 1 character. If an
5497 output delimiter is specified, (see the description of
5498 @option{--output-delimiter}), then output that string between ranges
5501 @item -f @var{field-list}
5502 @itemx --fields=@var{field-list}
5505 Select for printing only the fields listed in @var{field-list}.
5506 Fields are separated by a TAB character by default. Also print any
5507 line that contains no delimiter character, unless the
5508 @option{--only-delimited} (@option{-s}) option is specified.
5510 Note @command{awk} supports more sophisticated field processing,
5511 and by default will use (and discard) runs of blank characters to
5512 separate fields, and ignore leading and trailing blanks.
5515 awk '{print $2}' # print the second field
5516 awk '{print $NF-1}' # print the penultimate field
5517 awk '{print $2,$1}' # reorder the first two fields
5521 In the unlikely event that @command{awk} is unavailable,
5522 one can use the @command{join} command, to process blank
5523 characters as @command{awk} does above.
5526 join -a1 -o 1.2 - /dev/null # print the second field
5527 join -a1 -o 1.2,1.1 - /dev/null # reorder the first two fields
5531 @item -d @var{input_delim_byte}
5532 @itemx --delimiter=@var{input_delim_byte}
5534 @opindex --delimiter
5535 With @option{-f}, use the first byte of @var{input_delim_byte} as
5536 the input fields separator (default is TAB).
5540 Do not split multi-byte characters (no-op for now).
5543 @itemx --only-delimited
5545 @opindex --only-delimited
5546 For @option{-f}, do not print lines that do not contain the field separator
5547 character. Normally, any line without a field separator is printed verbatim.
5549 @item --output-delimiter=@var{output_delim_string}
5550 @opindex --output-delimiter
5551 With @option{-f}, output fields are separated by @var{output_delim_string}.
5552 The default with @option{-f} is to use the input delimiter.
5553 When using @option{-b} or @option{-c} to select ranges of byte or
5554 character offsets (as opposed to ranges of fields),
5555 output @var{output_delim_string} between non-overlapping
5556 ranges of selected bytes.
5559 @opindex --complement
5560 This option is a @acronym{GNU} extension.
5561 Select for printing the complement of the bytes, characters or fields
5562 selected with the @option{-b}, @option{-c} or @option{-f} options.
5563 In other words, do @emph{not} print the bytes, characters or fields
5564 specified via those options. This option is useful when you have
5565 many fields and want to print all but a few of them.
5572 @node paste invocation
5573 @section @command{paste}: Merge lines of files
5576 @cindex merging files
5578 @command{paste} writes to standard output lines consisting of sequentially
5579 corresponding lines of each given file, separated by a TAB character.
5580 Standard input is used for a file name of @samp{-} or if no input files
5602 paste [@var{option}]@dots{} [@var{file}]@dots{}
5605 The program accepts the following options. Also see @ref{Common options}.
5613 Paste the lines of one file at a time rather than one line from each
5614 file. Using the above example data:
5617 $ paste -s num2 let3
5622 @item -d @var{delim-list}
5623 @itemx --delimiters=@var{delim-list}
5625 @opindex --delimiters
5626 Consecutively use the characters in @var{delim-list} instead of
5627 TAB to separate merged lines. When @var{delim-list} is
5628 exhausted, start again at its beginning. Using the above example data:
5631 $ paste -d '%_' num2 let3 num2
5642 @node join invocation
5643 @section @command{join}: Join lines on a common field
5646 @cindex common field, joining on
5648 @command{join} writes to standard output a line for each pair of input
5649 lines that have identical join fields. Synopsis:
5652 join [@var{option}]@dots{} @var{file1} @var{file2}
5655 Either @var{file1} or @var{file2} (but not both) can be @samp{-},
5656 meaning standard input. @var{file1} and @var{file2} should be
5657 sorted on the join fields.
5660 Normally, the sort order is that of the
5661 collating sequence specified by the @env{LC_COLLATE} locale. Unless
5662 the @option{-t} option is given, the sort comparison ignores blanks at
5663 the start of the join field, as in @code{sort -b}. If the
5664 @option{--ignore-case} option is given, the sort comparison ignores
5665 the case of characters in the join field, as in @code{sort -f}.
5667 The @command{sort} and @command{join} commands should use consistent
5668 locales and options if the output of @command{sort} is fed to
5669 @command{join}. You can use a command like @samp{sort -k 1b,1} to
5670 sort a file on its default join field, but if you select a non-default
5671 locale, join field, separator, or comparison options, then you should
5672 do so consistently between @command{join} and @command{sort}.
5673 If @samp{join -t ''} is specified then the whole line is considered which
5674 matches the default operation of sort.
5676 If the input has no unpairable lines, a @acronym{GNU} extension is
5677 available; the sort order can be any order that considers two fields
5678 to be equal if and only if the sort comparison described above
5679 considers them to be equal. For example:
5697 @checkOrderOption{join}
5702 @item the join field is the first field in each line;
5703 @item fields in the input are separated by one or more blanks, with leading
5704 blanks on the line ignored;
5705 @item fields in the output are separated by a space;
5706 @item each output line consists of the join field, the remaining
5707 fields from @var{file1}, then the remaining fields from @var{file2}.
5710 The program accepts the following options. Also see @ref{Common options}.
5714 @item -a @var{file-number}
5716 Print a line for each unpairable line in file @var{file-number} (either
5717 @samp{1} or @samp{2}), in addition to the normal output.
5720 Fail with an error message if either input file is wrongly ordered.
5722 @item --nocheck-order
5723 Do not check that both input files are in sorted order. This is the default.
5725 @item -e @var{string}
5727 Replace those output fields that are missing in the input with @var{string}.
5728 I.E. missing fields specified with the @option{-12jo} options.
5732 Treat the first line of each input file as a header line. The header lines will
5733 be joined and printed as the first output line. If @option{-o} is used to
5734 specify output format, the header line will be printed according to the
5735 specified format. The header lines will not be checked for ordering even if
5736 @option{--check-order} is specified. Also if the header lines from each file
5737 do not match, the heading fields from the first file will be used.
5740 @itemx --ignore-case
5742 @opindex --ignore-case
5743 Ignore differences in case when comparing keys.
5744 With this option, the lines of the input files must be ordered in the same way.
5745 Use @samp{sort -f} to produce this ordering.
5747 @item -1 @var{field}
5749 Join on field @var{field} (a positive integer) of file 1.
5751 @item -2 @var{field}
5753 Join on field @var{field} (a positive integer) of file 2.
5755 @item -j @var{field}
5756 Equivalent to @option{-1 @var{field} -2 @var{field}}.
5758 @item -o @var{field-list}
5760 If the keyword @samp{auto} is specified, infer the output format from
5761 the first line in each file. This is the same as the default output format
5762 but also ensures the same number of fields are output for each line.
5763 Missing fields are replaced with the @option{-e} option and extra fields
5766 Otherwise, construct each output line according to the format in
5767 @var{field-list}. Each element in @var{field-list} is either the single
5768 character @samp{0} or has the form @var{m.n} where the file number, @var{m},
5769 is @samp{1} or @samp{2} and @var{n} is a positive field number.
5771 A field specification of @samp{0} denotes the join field.
5772 In most cases, the functionality of the @samp{0} field spec
5773 may be reproduced using the explicit @var{m.n} that corresponds
5774 to the join field. However, when printing unpairable lines
5775 (using either of the @option{-a} or @option{-v} options), there is no way
5776 to specify the join field using @var{m.n} in @var{field-list}
5777 if there are unpairable lines in both files.
5778 To give @command{join} that functionality, @acronym{POSIX} invented the @samp{0}
5779 field specification notation.
5781 The elements in @var{field-list}
5782 are separated by commas or blanks.
5783 Blank separators typically need to be quoted for the shell. For
5784 example, the commands @samp{join -o 1.2,2.2} and @samp{join -o '1.2
5785 2.2'} are equivalent.
5787 All output lines---including those printed because of any -a or -v
5788 option---are subject to the specified @var{field-list}.
5791 Use character @var{char} as the input and output field separator.
5792 Treat as significant each occurrence of @var{char} in the input file.
5793 Use @samp{sort -t @var{char}}, without the @option{-b} option of
5794 @samp{sort}, to produce this ordering. If @samp{join -t ''} is specified,
5795 the whole line is considered, matching the default operation of sort.
5796 If @samp{-t '\0'} is specified then the @acronym{ASCII} @sc{nul}
5797 character is used to delimit the fields.
5799 @item -v @var{file-number}
5800 Print a line for each unpairable line in file @var{file-number}
5801 (either @samp{1} or @samp{2}), instead of the normal output.
5808 @node Operating on characters
5809 @chapter Operating on characters
5811 @cindex operating on characters
5813 This commands operate on individual characters.
5816 * tr invocation:: Translate, squeeze, and/or delete characters.
5817 * expand invocation:: Convert tabs to spaces.
5818 * unexpand invocation:: Convert spaces to tabs.
5823 @section @command{tr}: Translate, squeeze, and/or delete characters
5830 tr [@var{option}]@dots{} @var{set1} [@var{set2}]
5833 @command{tr} copies standard input to standard output, performing
5834 one of the following operations:
5838 translate, and optionally squeeze repeated characters in the result,
5840 squeeze repeated characters,
5844 delete characters, then squeeze repeated characters from the result.
5847 The @var{set1} and (if given) @var{set2} arguments define ordered
5848 sets of characters, referred to below as @var{set1} and @var{set2}. These
5849 sets are the characters of the input that @command{tr} operates on.
5850 The @option{--complement} (@option{-c}, @option{-C}) option replaces
5852 complement (all of the characters that are not in @var{set1}).
5854 Currently @command{tr} fully supports only single-byte characters.
5855 Eventually it will support multibyte characters; when it does, the
5856 @option{-C} option will cause it to complement the set of characters,
5857 whereas @option{-c} will cause it to complement the set of values.
5858 This distinction will matter only when some values are not characters,
5859 and this is possible only in locales using multibyte encodings when
5860 the input contains encoding errors.
5862 The program accepts the @option{--help} and @option{--version}
5863 options. @xref{Common options}. Options must precede operands.
5868 * Character sets:: Specifying sets of characters.
5869 * Translating:: Changing one set of characters to another.
5870 * Squeezing:: Squeezing repeats and deleting.
5874 @node Character sets
5875 @subsection Specifying sets of characters
5877 @cindex specifying sets of characters
5879 The format of the @var{set1} and @var{set2} arguments resembles
5880 the format of regular expressions; however, they are not regular
5881 expressions, only lists of characters. Most characters simply
5882 represent themselves in these strings, but the strings can contain
5883 the shorthands listed below, for convenience. Some of them can be
5884 used only in @var{set1} or @var{set2}, as noted below.
5888 @item Backslash escapes
5889 @cindex backslash escapes
5891 The following backslash escape sequences are recognized:
5909 The 8-bit character with the value given by @var{ooo}, which is 1 to 3
5910 octal digits. Note that @samp{\400} is interpreted as the two-byte
5911 sequence, @samp{\040} @samp{0}.
5916 While a backslash followed by a character not listed above is
5917 interpreted as that character, the backslash also effectively
5918 removes any special significance, so it is useful to escape
5919 @samp{[}, @samp{]}, @samp{*}, and @samp{-}.
5924 The notation @samp{@var{m}-@var{n}} expands to all of the characters
5925 from @var{m} through @var{n}, in ascending order. @var{m} should
5926 collate before @var{n}; if it doesn't, an error results. As an example,
5927 @samp{0-9} is the same as @samp{0123456789}.
5929 @sc{gnu} @command{tr} does not support the System V syntax that uses square
5930 brackets to enclose ranges. Translations specified in that format
5931 sometimes work as expected, since the brackets are often transliterated
5932 to themselves. However, they should be avoided because they sometimes
5933 behave unexpectedly. For example, @samp{tr -d '[0-9]'} deletes brackets
5936 Many historically common and even accepted uses of ranges are not
5937 portable. For example, on @acronym{EBCDIC} hosts using the @samp{A-Z}
5938 range will not do what most would expect because @samp{A} through @samp{Z}
5939 are not contiguous as they are in @acronym{ASCII}.
5940 If you can rely on a @acronym{POSIX} compliant version of @command{tr}, then
5941 the best way to work around this is to use character classes (see below).
5942 Otherwise, it is most portable (and most ugly) to enumerate the members
5945 @item Repeated characters
5946 @cindex repeated characters
5948 The notation @samp{[@var{c}*@var{n}]} in @var{set2} expands to @var{n}
5949 copies of character @var{c}. Thus, @samp{[y*6]} is the same as
5950 @samp{yyyyyy}. The notation @samp{[@var{c}*]} in @var{string2} expands
5951 to as many copies of @var{c} as are needed to make @var{set2} as long as
5952 @var{set1}. If @var{n} begins with @samp{0}, it is interpreted in
5953 octal, otherwise in decimal.
5955 @item Character classes
5956 @cindex character classes
5958 The notation @samp{[:@var{class}:]} expands to all of the characters in
5959 the (predefined) class @var{class}. The characters expand in no
5960 particular order, except for the @code{upper} and @code{lower} classes,
5961 which expand in ascending order. When the @option{--delete} (@option{-d})
5962 and @option{--squeeze-repeats} (@option{-s}) options are both given, any
5963 character class can be used in @var{set2}. Otherwise, only the
5964 character classes @code{lower} and @code{upper} are accepted in
5965 @var{set2}, and then only if the corresponding character class
5966 (@code{upper} and @code{lower}, respectively) is specified in the same
5967 relative position in @var{set1}. Doing this specifies case conversion.
5968 The class names are given below; an error results when an invalid class
5980 Horizontal whitespace.
5989 Printable characters, not including space.
5995 Printable characters, including space.
5998 Punctuation characters.
6001 Horizontal or vertical whitespace.
6010 @item Equivalence classes
6011 @cindex equivalence classes
6013 The syntax @samp{[=@var{c}=]} expands to all of the characters that are
6014 equivalent to @var{c}, in no particular order. Equivalence classes are
6015 a relatively recent invention intended to support non-English alphabets.
6016 But there seems to be no standard way to define them or determine their
6017 contents. Therefore, they are not fully implemented in @sc{gnu} @command{tr};
6018 each character's equivalence class consists only of that character,
6019 which is of no particular use.
6025 @subsection Translating
6027 @cindex translating characters
6029 @command{tr} performs translation when @var{set1} and @var{set2} are
6030 both given and the @option{--delete} (@option{-d}) option is not given.
6031 @command{tr} translates each character of its input that is in @var{set1}
6032 to the corresponding character in @var{set2}. Characters not in
6033 @var{set1} are passed through unchanged. When a character appears more
6034 than once in @var{set1} and the corresponding characters in @var{set2}
6035 are not all the same, only the final one is used. For example, these
6036 two commands are equivalent:
6043 A common use of @command{tr} is to convert lowercase characters to
6044 uppercase. This can be done in many ways. Here are three of them:
6047 tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
6049 tr '[:lower:]' '[:upper:]'
6053 But note that using ranges like @code{a-z} above is not portable.
6055 When @command{tr} is performing translation, @var{set1} and @var{set2}
6056 typically have the same length. If @var{set1} is shorter than
6057 @var{set2}, the extra characters at the end of @var{set2} are ignored.
6059 On the other hand, making @var{set1} longer than @var{set2} is not
6060 portable; @acronym{POSIX} says that the result is undefined. In this situation,
6061 BSD @command{tr} pads @var{set2} to the length of @var{set1} by repeating
6062 the last character of @var{set2} as many times as necessary. System V
6063 @command{tr} truncates @var{set1} to the length of @var{set2}.
6065 By default, @sc{gnu} @command{tr} handles this case like BSD @command{tr}.
6066 When the @option{--truncate-set1} (@option{-t}) option is given,
6067 @sc{gnu} @command{tr} handles this case like the System V @command{tr}
6068 instead. This option is ignored for operations other than translation.
6070 Acting like System V @command{tr} in this case breaks the relatively common
6074 tr -cs A-Za-z0-9 '\012'
6078 because it converts only zero bytes (the first element in the
6079 complement of @var{set1}), rather than all non-alphanumerics, to
6083 By the way, the above idiom is not portable because it uses ranges, and
6084 it assumes that the octal code for newline is 012.
6085 Assuming a @acronym{POSIX} compliant @command{tr}, here is a better
6089 tr -cs '[:alnum:]' '[\n*]'
6094 @subsection Squeezing repeats and deleting
6096 @cindex squeezing repeat characters
6097 @cindex deleting characters
6099 When given just the @option{--delete} (@option{-d}) option, @command{tr}
6100 removes any input characters that are in @var{set1}.
6102 When given just the @option{--squeeze-repeats} (@option{-s}) option,
6103 @command{tr} replaces each input sequence of a repeated character that
6104 is in @var{set1} with a single occurrence of that character.
6106 When given both @option{--delete} and @option{--squeeze-repeats}, @command{tr}
6107 first performs any deletions using @var{set1}, then squeezes repeats
6108 from any remaining characters using @var{set2}.
6110 The @option{--squeeze-repeats} option may also be used when translating,
6111 in which case @command{tr} first performs translation, then squeezes
6112 repeats from any remaining characters using @var{set2}.
6114 Here are some examples to illustrate various combinations of options:
6119 Remove all zero bytes:
6126 Put all words on lines by themselves. This converts all
6127 non-alphanumeric characters to newlines, then squeezes each string
6128 of repeated newlines into a single newline:
6131 tr -cs '[:alnum:]' '[\n*]'
6135 Convert each sequence of repeated newlines to a single newline:
6142 Find doubled occurrences of words in a document.
6143 @c Separate the following two "the"s, so typo checkers don't complain.
6144 For example, people often write ``the @w{}the'' with the repeated words
6145 separated by a newline. The Bourne shell script below works first
6146 by converting each sequence of punctuation and blank characters to a
6147 single newline. That puts each ``word'' on a line by itself.
6148 Next it maps all uppercase characters to lower case, and finally it
6149 runs @command{uniq} with the @option{-d} option to print out only the words
6155 | tr -s '[:punct:][:blank:]' '[\n*]' \
6156 | tr '[:upper:]' '[:lower:]' \
6161 Deleting a small set of characters is usually straightforward. For example,
6162 to remove all @samp{a}s, @samp{x}s, and @samp{M}s you would do this:
6168 However, when @samp{-} is one of those characters, it can be tricky because
6169 @samp{-} has special meanings. Performing the same task as above but also
6170 removing all @samp{-} characters, we might try @code{tr -d -axM}, but
6171 that would fail because @command{tr} would try to interpret @option{-a} as
6172 a command-line option. Alternatively, we could try putting the hyphen
6173 inside the string, @code{tr -d a-xM}, but that wouldn't work either because
6174 it would make @command{tr} interpret @code{a-x} as the range of characters
6175 @samp{a}@dots{}@samp{x} rather than the three.
6176 One way to solve the problem is to put the hyphen at the end of the list
6183 Or you can use @samp{--} to terminate option processing:
6189 More generally, use the character class notation @code{[=c=]}
6190 with @samp{-} (or any other character) in place of the @samp{c}:
6196 Note how single quotes are used in the above example to protect the
6197 square brackets from interpretation by a shell.
6202 @node expand invocation
6203 @section @command{expand}: Convert tabs to spaces
6206 @cindex tabs to spaces, converting
6207 @cindex converting tabs to spaces
6209 @command{expand} writes the contents of each given @var{file}, or standard
6210 input if none are given or for a @var{file} of @samp{-}, to standard
6211 output, with tab characters converted to the appropriate number of
6215 expand [@var{option}]@dots{} [@var{file}]@dots{}
6218 By default, @command{expand} converts all tabs to spaces. It preserves
6219 backspace characters in the output; they decrement the column count for
6220 tab calculations. The default action is equivalent to @option{-t 8} (set
6221 tabs every 8 columns).
6223 The program accepts the following options. Also see @ref{Common options}.
6227 @item -t @var{tab1}[,@var{tab2}]@dots{}
6228 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6231 @cindex tab stops, setting
6232 If only one tab stop is given, set the tabs @var{tab1} spaces apart
6233 (default is 8). Otherwise, set the tabs at columns @var{tab1},
6234 @var{tab2}, @dots{} (numbered from 0), and replace any tabs beyond the
6235 last tab stop given with single spaces. Tab stops can be separated by
6236 blanks as well as by commas.
6238 For compatibility, GNU @command{expand} also accepts the obsolete
6239 option syntax, @option{-@var{t1}[,@var{t2}]@dots{}}. New scripts
6240 should use @option{-t @var{t1}[,@var{t2}]@dots{}} instead.
6246 @cindex initial tabs, converting
6247 Only convert initial tabs (those that precede all non-space or non-tab
6248 characters) on each line to spaces.
6255 @node unexpand invocation
6256 @section @command{unexpand}: Convert spaces to tabs
6260 @command{unexpand} writes the contents of each given @var{file}, or
6261 standard input if none are given or for a @var{file} of @samp{-}, to
6262 standard output, converting blanks at the beginning of each line into
6263 as many tab characters as needed. In the default @acronym{POSIX}
6264 locale, a @dfn{blank} is a space or a tab; other locales may specify
6265 additional blank characters. Synopsis:
6268 unexpand [@var{option}]@dots{} [@var{file}]@dots{}
6271 By default, @command{unexpand} converts only initial blanks (those
6272 that precede all non-blank characters) on each line. It
6273 preserves backspace characters in the output; they decrement the column
6274 count for tab calculations. By default, tabs are set at every 8th
6277 The program accepts the following options. Also see @ref{Common options}.
6281 @item -t @var{tab1}[,@var{tab2}]@dots{}
6282 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6285 If only one tab stop is given, set the tabs @var{tab1} columns apart
6286 instead of the default 8. Otherwise, set the tabs at columns
6287 @var{tab1}, @var{tab2}, @dots{} (numbered from 0), and leave blanks
6288 beyond the tab stops given unchanged. Tab stops can be separated by
6289 blanks as well as by commas. This option implies the @option{-a} option.
6291 For compatibility, GNU @command{unexpand} supports the obsolete option syntax,
6292 @option{-@var{tab1}[,@var{tab2}]@dots{}}, where tab stops must be
6293 separated by commas. (Unlike @option{-t}, this obsolete option does
6294 not imply @option{-a}.) New scripts should use @option{--first-only -t
6295 @var{tab1}[,@var{tab2}]@dots{}} instead.
6301 Also convert all sequences of two or more blanks just before a tab stop,
6302 even if they occur after non-blank characters in a line.
6309 @node Directory listing
6310 @chapter Directory listing
6312 This chapter describes the @command{ls} command and its variants @command{dir}
6313 and @command{vdir}, which list information about files.
6316 * ls invocation:: List directory contents.
6317 * dir invocation:: Briefly ls.
6318 * vdir invocation:: Verbosely ls.
6319 * dircolors invocation:: Color setup for ls, etc.
6324 @section @command{ls}: List directory contents
6327 @cindex directory listing
6329 The @command{ls} program lists information about files (of any type,
6330 including directories). Options and file arguments can be intermixed
6331 arbitrarily, as usual.
6333 For non-option command-line arguments that are directories, by default
6334 @command{ls} lists the contents of directories, not recursively, and
6335 omitting files with names beginning with @samp{.}. For other non-option
6336 arguments, by default @command{ls} lists just the file name. If no
6337 non-option argument is specified, @command{ls} operates on the current
6338 directory, acting as if it had been invoked with a single argument of @samp{.}.
6341 By default, the output is sorted alphabetically, according to the locale
6342 settings in effect.@footnote{If you use a non-@acronym{POSIX}
6343 locale (e.g., by setting @env{LC_ALL} to @samp{en_US}), then @command{ls} may
6344 produce output that is sorted differently than you're accustomed to.
6345 In that case, set the @env{LC_ALL} environment variable to @samp{C}.}
6346 If standard output is
6347 a terminal, the output is in columns (sorted vertically) and control
6348 characters are output as question marks; otherwise, the output is listed
6349 one per line and control characters are output as-is.
6351 Because @command{ls} is such a fundamental program, it has accumulated many
6352 options over the years. They are described in the subsections below;
6353 within each section, options are listed alphabetically (ignoring case).
6354 The division of options into the subsections is not absolute, since some
6355 options affect more than one aspect of @command{ls}'s operation.
6357 @cindex exit status of @command{ls}
6362 1 minor problems (e.g., failure to access a file or directory not
6363 specified as a command line argument. This happens when listing a
6364 directory in which entries are actively being removed or renamed.)
6365 2 serious trouble (e.g., memory exhausted, invalid option, failure
6366 to access a file or directory specified as a command line argument
6367 or a directory loop)
6370 Also see @ref{Common options}.
6373 * Which files are listed::
6374 * What information is listed::
6375 * Sorting the output::
6376 * Details about version sort::
6377 * General output formatting::
6378 * Formatting file timestamps::
6379 * Formatting the file names::
6383 @node Which files are listed
6384 @subsection Which files are listed
6386 These options determine which files @command{ls} lists information for.
6387 By default, @command{ls} lists files and the contents of any
6388 directories on the command line, except that in directories it ignores
6389 files whose names start with @samp{.}.
6397 In directories, do not ignore file names that start with @samp{.}.
6402 @opindex --almost-all
6403 In directories, do not ignore all file names that start with @samp{.};
6404 ignore only @file{.} and @file{..}. The @option{--all} (@option{-a})
6405 option overrides this option.
6408 @itemx --ignore-backups
6410 @opindex --ignore-backups
6411 @cindex backup files, ignoring
6412 In directories, ignore files that end with @samp{~}. This option is
6413 equivalent to @samp{--ignore='*~' --ignore='.*~'}.
6418 @opindex --directory
6419 List just the names of directories, as with other types of files, rather
6420 than listing their contents.
6421 @c The following sentence is the same as the one for -F.
6422 Do not follow symbolic links listed on the
6423 command line unless the @option{--dereference-command-line} (@option{-H}),
6424 @option{--dereference} (@option{-L}), or
6425 @option{--dereference-command-line-symlink-to-dir} options are specified.
6428 @itemx --dereference-command-line
6430 @opindex --dereference-command-line
6431 @cindex symbolic links, dereferencing
6432 If a command line argument specifies a symbolic link, show information
6433 for the file the link references rather than for the link itself.
6435 @itemx --dereference-command-line-symlink-to-dir
6436 @opindex --dereference-command-line-symlink-to-dir
6437 @cindex symbolic links, dereferencing
6438 Do not dereference symbolic links, with one exception:
6439 if a command line argument specifies a symbolic link that refers to
6440 a directory, show information for that directory rather than for the
6442 This is the default behavior when no other dereferencing-related
6443 option has been specified (@option{--classify} (@option{-F}),
6444 @option{--directory} (@option{-d}),
6446 @option{--dereference} (@option{-L}), or
6447 @option{--dereference-command-line} (@option{-H})).
6449 @item --group-directories-first
6450 @opindex --group-directories-first
6451 Group all the directories before the files and then sort the
6452 directories and the files separately using the selected sort key
6453 (see --sort option).
6454 That is, this option specifies a primary sort key,
6455 and the --sort option specifies a secondary key.
6456 However, any use of @option{--sort=none}
6457 (@option{-U}) disables this option altogether.
6459 @item --hide=PATTERN
6460 @opindex --hide=@var{pattern}
6461 In directories, ignore files whose names match the shell pattern
6462 @var{pattern}, unless the @option{--all} (@option{-a}) or
6463 @option{--almost-all} (@option{-A}) is also given. This
6464 option acts like @option{--ignore=@var{pattern}} except that it has no
6465 effect if @option{--all} (@option{-a}) or @option{--almost-all}
6466 (@option{-A}) is also given.
6468 This option can be useful in shell aliases. For example, if
6469 @command{lx} is an alias for @samp{ls --hide='*~'} and @command{ly} is
6470 an alias for @samp{ls --ignore='*~'}, then the command @samp{lx -A}
6471 lists the file @file{README~} even though @samp{ly -A} would not.
6473 @item -I @var{pattern}
6474 @itemx --ignore=@var{pattern}
6476 @opindex --ignore=@var{pattern}
6477 In directories, ignore files whose names match the shell pattern
6478 (not regular expression) @var{pattern}. As
6479 in the shell, an initial @samp{.} in a file name does not match a
6480 wildcard at the start of @var{pattern}. Sometimes it is useful
6481 to give this option several times. For example,
6484 $ ls --ignore='.??*' --ignore='.[^.]' --ignore='#*'
6487 The first option ignores names of length 3 or more that start with @samp{.},
6488 the second ignores all two-character names that start with @samp{.}
6489 except @samp{..}, and the third ignores names that start with @samp{#}.
6492 @itemx --dereference
6494 @opindex --dereference
6495 @cindex symbolic links, dereferencing
6496 When showing file information for a symbolic link, show information
6497 for the file the link references rather than the link itself.
6498 However, even with this option, @command{ls} still prints the name
6499 of the link itself, not the name of the file that the link points to.
6504 @opindex --recursive
6505 @cindex recursive directory listing
6506 @cindex directory listing, recursive
6507 List the contents of all directories recursively.
6512 @node What information is listed
6513 @subsection What information is listed
6515 These options affect the information that @command{ls} displays. By
6516 default, only file names are shown.
6522 @cindex hurd, author, printing
6523 List each file's author when producing long format directory listings.
6524 In GNU/Hurd, file authors can differ from their owners, but in other
6525 operating systems the two are the same.
6531 @cindex dired Emacs mode support
6532 With the long listing (@option{-l}) format, print an additional line after
6536 //DIRED// @var{beg1} @var{end1} @var{beg2} @var{end2} @dots{}
6540 The @var{begn} and @var{endn} are unsigned integers that record the
6541 byte position of the beginning and end of each file name in the output.
6542 This makes it easy for Emacs to find the names, even when they contain
6543 unusual characters such as space or newline, without fancy searching.
6545 If directories are being listed recursively (@option{-R}), output a similar
6546 line with offsets for each subdirectory name:
6549 //SUBDIRED// @var{beg1} @var{end1} @dots{}
6552 Finally, output a line of the form:
6555 //DIRED-OPTIONS// --quoting-style=@var{word}
6559 where @var{word} is the quoting style (@pxref{Formatting the file names}).
6561 Here is an actual example:
6564 $ mkdir -p a/sub/deeper a/sub2
6566 $ touch a/sub/deeper/file
6567 $ ls -gloRF --dired a
6570 -rw-r--r-- 1 0 Jun 10 12:27 f1
6571 -rw-r--r-- 1 0 Jun 10 12:27 f2
6572 drwxr-xr-x 3 4096 Jun 10 12:27 sub/
6573 drwxr-xr-x 2 4096 Jun 10 12:27 sub2/
6577 drwxr-xr-x 2 4096 Jun 10 12:27 deeper/
6581 -rw-r--r-- 1 0 Jun 10 12:27 file
6585 //DIRED// 48 50 84 86 120 123 158 162 217 223 282 286
6586 //SUBDIRED// 2 3 167 172 228 240 290 296
6587 //DIRED-OPTIONS// --quoting-style=literal
6590 Note that the pairs of offsets on the @samp{//DIRED//} line above delimit
6591 these names: @file{f1}, @file{f2}, @file{sub}, @file{sub2}, @file{deeper},
6593 The offsets on the @samp{//SUBDIRED//} line delimit the following
6594 directory names: @file{a}, @file{a/sub}, @file{a/sub/deeper}, @file{a/sub2}.
6596 Here is an example of how to extract the fifth entry name, @samp{deeper},
6597 corresponding to the pair of offsets, 222 and 228:
6600 $ ls -gloRF --dired a > out
6601 $ dd bs=1 skip=222 count=6 < out 2>/dev/null; echo
6605 Note that although the listing above includes a trailing slash
6606 for the @samp{deeper} entry, the offsets select the name without
6607 the trailing slash. However, if you invoke @command{ls} with @option{--dired}
6608 along with an option like @option{--escape} (aka @option{-b}) and operate
6609 on a file whose name contains special characters, notice that the backslash
6614 $ ls -blog --dired 'a b'
6615 -rw-r--r-- 1 0 Jun 10 12:28 a\ b
6617 //DIRED-OPTIONS// --quoting-style=escape
6620 If you use a quoting style that adds quote marks
6621 (e.g., @option{--quoting-style=c}), then the offsets include the quote marks.
6622 So beware that the user may select the quoting style via the environment
6623 variable @env{QUOTING_STYLE}. Hence, applications using @option{--dired}
6624 should either specify an explicit @option{--quoting-style=literal} option
6625 (aka @option{-N} or @option{--literal}) on the command line, or else be
6626 prepared to parse the escaped names.
6629 @opindex --full-time
6630 Produce long format directory listings, and list times in full. It is
6631 equivalent to using @option{--format=long} with
6632 @option{--time-style=full-iso} (@pxref{Formatting file timestamps}).
6636 Produce long format directory listings, but don't display owner information.
6642 Inhibit display of group information in a long format directory listing.
6643 (This is the default in some non-@sc{gnu} versions of @command{ls}, so we
6644 provide this option for compatibility.)
6652 @cindex inode number, printing
6653 Print the inode number (also called the file serial number and index
6654 number) of each file to the left of the file name. (This number
6655 uniquely identifies each file within a particular file system.)
6658 @itemx --format=long
6659 @itemx --format=verbose
6662 @opindex long ls @r{format}
6663 @opindex verbose ls @r{format}
6664 In addition to the name of each file, print the file type, file mode bits,
6665 number of hard links, owner name, group name, size, and
6666 timestamp (@pxref{Formatting file timestamps}), normally
6667 the modification time. Print question marks for information that
6668 cannot be determined.
6670 Normally the size is printed as a byte count without punctuation, but
6671 this can be overridden (@pxref{Block size}). For example, @option{-h}
6672 prints an abbreviated, human-readable count, and
6673 @samp{--block-size="'1"} prints a byte count with the thousands
6674 separator of the current locale.
6676 For each directory that is listed, preface the files with a line
6677 @samp{total @var{blocks}}, where @var{blocks} is the total disk allocation
6678 for all files in that directory. The block size currently defaults to 1024
6679 bytes, but this can be overridden (@pxref{Block size}).
6680 The @var{blocks} computed counts each hard link separately;
6681 this is arguably a deficiency.
6683 The file type is one of the following characters:
6685 @c The commented-out entries are ones we're not sure about.
6693 character special file
6695 high performance (``contiguous data'') file
6699 door (Solaris 2.5 and up)
6701 @c semaphore, if this is a distinct file type
6705 @c multiplexed file (7th edition Unix; obsolete)
6707 off-line (``migrated'') file (Cray DMF)
6709 network special file (HP-UX)
6713 port (Solaris 10 and up)
6715 @c message queue, if this is a distinct file type
6719 @c shared memory object, if this is a distinct file type
6721 @c typed memory object, if this is a distinct file type
6723 @c whiteout (4.4BSD; not implemented)
6725 some other file type
6728 @cindex permissions, output by @command{ls}
6729 The file mode bits listed are similar to symbolic mode specifications
6730 (@pxref{Symbolic Modes}). But @command{ls} combines multiple bits into the
6731 third character of each set of permissions as follows:
6735 If the set-user-ID or set-group-ID bit and the corresponding executable bit
6739 If the set-user-ID or set-group-ID bit is set but the corresponding
6740 executable bit is not set.
6743 If the restricted deletion flag or sticky bit, and the
6744 other-executable bit, are both set. The restricted deletion flag is
6745 another name for the sticky bit. @xref{Mode Structure}.
6748 If the restricted deletion flag or sticky bit is set but the
6749 other-executable bit is not set.
6752 If the executable bit is set and none of the above apply.
6758 Following the file mode bits is a single character that specifies
6759 whether an alternate access method such as an access control list
6760 applies to the file. When the character following the file mode bits is a
6761 space, there is no alternate access method. When it is a printing
6762 character, then there is such a method.
6764 GNU @command{ls} uses a @samp{.} character to indicate a file
6765 with an SELinux security context, but no other alternate access method.
6767 A file with any other combination of alternate access methods
6768 is marked with a @samp{+} character.
6771 @itemx --numeric-uid-gid
6773 @opindex --numeric-uid-gid
6774 @cindex numeric uid and gid
6775 @cindex numeric user and group IDs
6776 Produce long format directory listings, but
6777 display numeric user and group IDs instead of the owner and group names.
6781 Produce long format directory listings, but don't display group information.
6782 It is equivalent to using @option{--format=long} with @option{--no-group} .
6788 @cindex disk allocation
6789 @cindex size of files, reporting
6790 Print the disk allocation of each file to the left of the file name.
6791 This is the amount of disk space used by the file, which is usually a
6792 bit more than the file's size, but it can be less if the file has holes.
6794 Normally the disk allocation is printed in units of
6795 1024 bytes, but this can be overridden (@pxref{Block size}).
6797 @cindex NFS mounts from BSD to HP-UX
6798 For files that are NFS-mounted from an HP-UX system to a BSD system,
6799 this option reports sizes that are half the correct values. On HP-UX
6800 systems, it reports sizes that are twice the correct values for files
6801 that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX;
6802 it also affects the HP-UX @command{ls} program.
6811 @cindex security context
6812 Display the SELinux security context or @samp{?} if none is found.
6813 When used with the @option{-l} option, print the security context
6814 to the left of the size column.
6819 @node Sorting the output
6820 @subsection Sorting the output
6822 @cindex sorting @command{ls} output
6823 These options change the order in which @command{ls} sorts the information
6824 it outputs. By default, sorting is done by character code
6825 (e.g., @acronym{ASCII} order).
6831 @itemx --time=status
6834 @opindex ctime@r{, printing or sorting by}
6835 @opindex status time@r{, printing or sorting by}
6836 @opindex use time@r{, printing or sorting files by}
6837 If the long listing format (e.g., @option{-l}, @option{-o}) is being used,
6838 print the status change time (the @samp{ctime} in the inode) instead of
6839 the modification time.
6840 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6841 or when not using a long listing format,
6842 sort according to the status change time.
6846 @cindex unsorted directory listing
6847 @cindex directory order, listing by
6848 Primarily, like @option{-U}---do not sort; list the files in whatever
6849 order they are stored in the directory. But also enable @option{-a} (list
6850 all files) and disable @option{-l}, @option{--color}, and @option{-s} (if they
6851 were specified before the @option{-f}).
6857 @cindex reverse sorting
6858 Reverse whatever the sorting method is---e.g., list files in reverse
6859 alphabetical order, youngest first, smallest first, or whatever.
6865 @opindex size of files@r{, sorting files by}
6866 Sort by file size, largest first.
6872 @opindex modification time@r{, sorting files by}
6873 Sort by modification time (the @samp{mtime} in the inode), newest first.
6877 @itemx --time=access
6881 @opindex use time@r{, printing or sorting files by}
6882 @opindex atime@r{, printing or sorting files by}
6883 @opindex access time@r{, printing or sorting files by}
6884 If the long listing format (e.g., @option{--format=long}) is being used,
6885 print the last access time (the @samp{atime} in the inode).
6886 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6887 or when not using a long listing format, sort according to the access time.
6893 @opindex none@r{, sorting option for @command{ls}}
6894 Do not sort; list the files in whatever order they are
6895 stored in the directory. (Do not do any of the other unrelated things
6896 that @option{-f} does.) This is especially useful when listing very large
6897 directories, since not doing any sorting can be noticeably faster.
6900 @itemx --sort=version
6903 @opindex version@r{, sorting option for @command{ls}}
6904 Sort by version name and number, lowest first. It behaves like a default
6905 sort, except that each sequence of decimal digits is treated numerically
6906 as an index/version number. (@xref{Details about version sort}.)
6909 @itemx --sort=extension
6912 @opindex extension@r{, sorting files by}
6913 Sort directory contents alphabetically by file extension (characters
6914 after the last @samp{.}); files with no extension are sorted first.
6919 @node Details about version sort
6920 @subsection Details about version sort
6922 Version sorting handles the fact that file names frequently include indices or
6923 version numbers. Standard sorting usually does not produce the order that one
6924 expects because comparisons are made on a character-by-character basis.
6925 Version sorting is especially useful when browsing directories that contain
6926 many files with indices/version numbers in their names:
6930 abc.zml-1.gz abc.zml-1.gz
6931 abc.zml-12.gz abc.zml-2.gz
6932 abc.zml-2.gz abc.zml-12.gz
6935 Version-sorted strings are compared such that if @var{ver1} and @var{ver2}
6936 are version numbers and @var{prefix} and @var{suffix} (@var{suffix} matching
6937 the regular expression @samp{(\.[A-Za-z~][A-Za-z0-9~]*)*}) are strings then
6938 @var{ver1} < @var{ver2} implies that the name composed of
6939 ``@var{prefix} @var{ver1} @var{suffix}'' sorts before
6940 ``@var{prefix} @var{ver2} @var{suffix}''.
6942 Note also that leading zeros of numeric parts are ignored:
6946 abc-1.007.tgz abc-1.01a.tgz
6947 abc-1.012b.tgz abc-1.007.tgz
6948 abc-1.01a.tgz abc-1.012b.tgz
6951 This functionality is implemented using gnulib's @code{filevercmp} function,
6952 which has some caveats worth noting.
6955 @item @env{LC_COLLATE} is ignored, which means @samp{ls -v} and @samp{sort -V}
6956 will sort non-numeric prefixes as if the @env{LC_COLLATE} locale category
6957 was set to @samp{C}.
6958 @item Some suffixes will not be matched by the regular
6959 expression mentioned above. Consequently these examples may
6960 not sort as you expect:
6968 abc-1.2.3.4.x86_64.rpm
6969 abc-1.2.3.x86_64.rpm
6973 @node General output formatting
6974 @subsection General output formatting
6976 These options affect the appearance of the overall output.
6981 @itemx --format=single-column
6984 @opindex single-column @r{output of files}
6985 List one file per line. This is the default for @command{ls} when standard
6986 output is not a terminal.
6989 @itemx --format=vertical
6992 @opindex vertical @r{sorted files in columns}
6993 List files in columns, sorted vertically. This is the default for
6994 @command{ls} if standard output is a terminal. It is always the default
6995 for the @command{dir} program.
6996 @sc{gnu} @command{ls} uses variable width columns to display as many files as
6997 possible in the fewest lines.
6999 @item --color [=@var{when}]
7001 @cindex color, distinguishing file types with
7002 Specify whether to use color for distinguishing file types. @var{when}
7003 may be omitted, or one of:
7006 @vindex none @r{color option}
7007 - Do not use color at all. This is the default.
7009 @vindex auto @r{color option}
7010 @cindex terminal, using color iff
7011 - Only use color if standard output is a terminal.
7013 @vindex always @r{color option}
7016 Specifying @option{--color} and no @var{when} is equivalent to
7017 @option{--color=always}.
7018 Piping a colorized listing through a pager like @command{more} or
7019 @command{less} usually produces unreadable results. However, using
7020 @code{more -f} does seem to work.
7024 @itemx --indicator-style=classify
7027 @opindex --indicator-style
7028 @cindex file type and executables, marking
7029 @cindex executables and file type, marking
7030 Append a character to each file name indicating the file type. Also,
7031 for regular files that are executable, append @samp{*}. The file type
7032 indicators are @samp{/} for directories, @samp{@@} for symbolic links,
7033 @samp{|} for FIFOs, @samp{=} for sockets, @samp{>} for doors,
7034 and nothing for regular files.
7035 @c The following sentence is the same as the one for -d.
7036 Do not follow symbolic links listed on the
7037 command line unless the @option{--dereference-command-line} (@option{-H}),
7038 @option{--dereference} (@option{-L}), or
7039 @option{--dereference-command-line-symlink-to-dir} options are specified.
7042 @itemx --indicator-style=file-type
7043 @opindex --file-type
7044 @opindex --indicator-style
7045 @cindex file type, marking
7046 Append a character to each file name indicating the file type. This is
7047 like @option{-F}, except that executables are not marked.
7049 @item --indicator-style=@var{word}
7050 @opindex --indicator-style
7051 Append a character indicator with style @var{word} to entry names,
7056 Do not append any character indicator; this is the default.
7058 Append @samp{/} for directories. This is the same as the @option{-p}
7061 Append @samp{/} for directories, @samp{@@} for symbolic links, @samp{|}
7062 for FIFOs, @samp{=} for sockets, and nothing for regular files. This is
7063 the same as the @option{--file-type} option.
7065 Append @samp{*} for executable regular files, otherwise behave as for
7066 @samp{file-type}. This is the same as the @option{-F} or
7067 @option{--classify} option.
7072 Print file sizes in 1024-byte blocks, overriding the default block
7073 size (@pxref{Block size}).
7074 This option is equivalent to @option{--block-size=1K}.
7077 @itemx --format=commas
7080 @opindex commas@r{, outputting between files}
7081 List files horizontally, with as many as will fit on each line,
7082 separated by @samp{, } (a comma and a space).
7085 @itemx --indicator-style=slash
7087 @opindex --indicator-style
7088 @cindex file type, marking
7089 Append a @samp{/} to directory names.
7092 @itemx --format=across
7093 @itemx --format=horizontal
7096 @opindex across@r{, listing files}
7097 @opindex horizontal@r{, listing files}
7098 List the files in columns, sorted horizontally.
7101 @itemx --tabsize=@var{cols}
7104 Assume that each tab stop is @var{cols} columns wide. The default is 8.
7105 @command{ls} uses tabs where possible in the output, for efficiency. If
7106 @var{cols} is zero, do not use tabs at all.
7108 @c FIXME: remove in 2009, if Apple Terminal has been fixed for long enough.
7109 Some terminal emulators (at least Apple Terminal 1.5 (133) from Mac OS X 10.4.8)
7110 do not properly align columns to the right of a TAB following a
7111 non-@acronym{ASCII} byte. If you use such a terminal emulator, use the
7112 @option{-T0} option or put @code{TABSIZE=0} in your environment to tell
7113 @command{ls} to align using spaces, not tabs.
7116 @itemx --width=@var{cols}
7120 Assume the screen is @var{cols} columns wide. The default is taken
7121 from the terminal settings if possible; otherwise the environment
7122 variable @env{COLUMNS} is used if it is set; otherwise the default
7128 @node Formatting file timestamps
7129 @subsection Formatting file timestamps
7131 By default, file timestamps are listed in abbreviated form, using
7132 a date like @samp{Mar 30@ @ 2002} for non-recent timestamps, and a
7133 date-without-year and time like @samp{Mar 30 23:45} for recent timestamps.
7134 This format can change depending on the current locale as detailed below.
7136 A timestamp is considered to be @dfn{recent} if it is less than six
7137 months old, and is not dated in the future. If a timestamp dated
7138 today is not listed in recent form, the timestamp is in the future,
7139 which means you probably have clock skew problems which may break
7140 programs like @command{make} that rely on file timestamps.
7143 Time stamps are listed according to the time zone rules specified by
7144 the @env{TZ} environment variable, or by the system default rules if
7145 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
7146 with @env{TZ}, libc, The GNU C Library Reference Manual}.
7148 The following option changes how file timestamps are printed.
7151 @item --time-style=@var{style}
7152 @opindex --time-style
7154 List timestamps in style @var{style}. The @var{style} should
7155 be one of the following:
7160 List timestamps using @var{format}, where @var{format} is interpreted
7161 like the format argument of @command{date} (@pxref{date invocation}).
7162 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
7163 @command{ls} to list timestamps like @samp{2002-03-30 23:45:56}. As
7164 with @command{date}, @var{format}'s interpretation is affected by the
7165 @env{LC_TIME} locale category.
7167 If @var{format} contains two format strings separated by a newline,
7168 the former is used for non-recent files and the latter for recent
7169 files; if you want output columns to line up, you may need to insert
7170 spaces in one of the two formats.
7173 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
7174 format with nanosecond precision, e.g., @samp{2002-03-30
7175 23:45:56.477817180 -0700}. This style is equivalent to
7176 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
7178 This is useful because the time output includes all the information that
7179 is available from the operating system. For example, this can help
7180 explain @command{make}'s behavior, since @acronym{GNU} @command{make}
7181 uses the full timestamp to determine whether a file is out of date.
7184 List @acronym{ISO} 8601 date and time in minutes, e.g.,
7185 @samp{2002-03-30 23:45}. These timestamps are shorter than
7186 @samp{full-iso} timestamps, and are usually good enough for everyday
7187 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
7190 List @acronym{ISO} 8601 dates for non-recent timestamps (e.g.,
7191 @samp{2002-03-30@ }), and @acronym{ISO} 8601 month, day, hour, and
7192 minute for recent timestamps (e.g., @samp{03-30 23:45}). These
7193 timestamps are uglier than @samp{long-iso} timestamps, but they carry
7194 nearly the same information in a smaller space and their brevity helps
7195 @command{ls} output fit within traditional 80-column output lines.
7196 The following two @command{ls} invocations are equivalent:
7201 ls -l --time-style="+%Y-%m-%d $newline%m-%d %H:%M"
7202 ls -l --time-style="iso"
7207 List timestamps in a locale-dependent form. For example, a Finnish
7208 locale might list non-recent timestamps like @samp{maalis 30@ @ 2002}
7209 and recent timestamps like @samp{maalis 30 23:45}. Locale-dependent
7210 timestamps typically consume more space than @samp{iso} timestamps and
7211 are harder for programs to parse because locale conventions vary so
7212 widely, but they are easier for many people to read.
7214 The @env{LC_TIME} locale category specifies the timestamp format. The
7215 default @acronym{POSIX} locale uses timestamps like @samp{Mar 30@
7216 @ 2002} and @samp{Mar 30 23:45}; in this locale, the following two
7217 @command{ls} invocations are equivalent:
7222 ls -l --time-style="+%b %e %Y$newline%b %e %H:%M"
7223 ls -l --time-style="locale"
7226 Other locales behave differently. For example, in a German locale,
7227 @option{--time-style="locale"} might be equivalent to
7228 @option{--time-style="+%e. %b %Y $newline%e. %b %H:%M"}
7229 and might generate timestamps like @samp{30. M@"ar 2002@ } and
7230 @samp{30. M@"ar 23:45}.
7232 @item posix-@var{style}
7234 List @acronym{POSIX}-locale timestamps if the @env{LC_TIME} locale
7235 category is @acronym{POSIX}, @var{style} timestamps otherwise. For
7236 example, the @samp{posix-long-iso} style lists
7237 timestamps like @samp{Mar 30@ @ 2002} and @samp{Mar 30 23:45} when in
7238 the @acronym{POSIX} locale, and like @samp{2002-03-30 23:45} otherwise.
7243 You can specify the default value of the @option{--time-style} option
7244 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
7245 the default style is @samp{locale}. @acronym{GNU} Emacs 21.3 and
7246 later use the @option{--dired} option and therefore can parse any date
7247 format, but if you are using Emacs 21.1 or 21.2 and specify a
7248 non-@acronym{POSIX} locale you may need to set
7249 @samp{TIME_STYLE="posix-long-iso"}.
7251 To avoid certain denial-of-service attacks, timestamps that would be
7252 longer than 1000 bytes may be treated as errors.
7255 @node Formatting the file names
7256 @subsection Formatting the file names
7258 These options change how file names themselves are printed.
7264 @itemx --quoting-style=escape
7267 @opindex --quoting-style
7268 @cindex backslash sequences for file names
7269 Quote nongraphic characters in file names using alphabetic and octal
7270 backslash sequences like those used in C.
7274 @itemx --quoting-style=literal
7277 @opindex --quoting-style
7278 Do not quote file names. However, with @command{ls} nongraphic
7279 characters are still printed as question marks if the output is a
7280 terminal and you do not specify the @option{--show-control-chars}
7284 @itemx --hide-control-chars
7286 @opindex --hide-control-chars
7287 Print question marks instead of nongraphic characters in file names.
7288 This is the default if the output is a terminal and the program is
7293 @itemx --quoting-style=c
7295 @opindex --quote-name
7296 @opindex --quoting-style
7297 Enclose file names in double quotes and quote nongraphic characters as
7300 @item --quoting-style=@var{word}
7301 @opindex --quoting-style
7302 @cindex quoting style
7303 Use style @var{word} to quote file names and other strings that may
7304 contain arbitrary characters. The @var{word} should
7305 be one of the following:
7309 Output strings as-is; this is the same as the @option{-N} or
7310 @option{--literal} option.
7312 Quote strings for the shell if they contain shell metacharacters or would
7313 cause ambiguous output.
7314 The quoting is suitable for @acronym{POSIX}-compatible shells like
7315 @command{bash}, but it does not always work for incompatible shells
7318 Quote strings for the shell, even if they would normally not require quoting.
7320 Quote strings as for C character string literals, including the
7321 surrounding double-quote characters; this is the same as the
7322 @option{-Q} or @option{--quote-name} option.
7324 Quote strings as for C character string literals, except omit the
7325 surrounding double-quote
7326 characters; this is the same as the @option{-b} or @option{--escape} option.
7328 Quote strings as for C character string literals, except use
7329 surrounding quotation marks appropriate for the
7332 @c Use @t instead of @samp to avoid duplicate quoting in some output styles.
7333 Quote strings as for C character string literals, except use
7334 surrounding quotation marks appropriate for the locale, and quote
7335 @t{`like this'} instead of @t{"like
7336 this"} in the default C locale. This looks nicer on many displays.
7339 You can specify the default value of the @option{--quoting-style} option
7340 with the environment variable @env{QUOTING_STYLE}. If that environment
7341 variable is not set, the default value is @samp{literal}, but this
7342 default may change to @samp{shell} in a future version of this package.
7344 @item --show-control-chars
7345 @opindex --show-control-chars
7346 Print nongraphic characters as-is in file names.
7347 This is the default unless the output is a terminal and the program is
7353 @node dir invocation
7354 @section @command{dir}: Briefly list directory contents
7357 @cindex directory listing, brief
7359 @command{dir} is equivalent to @code{ls -C
7360 -b}; that is, by default files are listed in columns, sorted vertically,
7361 and special characters are represented by backslash escape sequences.
7363 @xref{ls invocation, @command{ls}}.
7366 @node vdir invocation
7367 @section @command{vdir}: Verbosely list directory contents
7370 @cindex directory listing, verbose
7372 @command{vdir} is equivalent to @code{ls -l
7373 -b}; that is, by default files are listed in long format and special
7374 characters are represented by backslash escape sequences.
7376 @node dircolors invocation
7377 @section @command{dircolors}: Color setup for @command{ls}
7381 @cindex setup for color
7383 @command{dircolors} outputs a sequence of shell commands to set up the
7384 terminal for color output from @command{ls} (and @command{dir}, etc.).
7388 eval "`dircolors [@var{option}]@dots{} [@var{file}]`"
7391 If @var{file} is specified, @command{dircolors} reads it to determine which
7392 colors to use for which file types and extensions. Otherwise, a
7393 precompiled database is used. For details on the format of these files,
7394 run @samp{dircolors --print-database}.
7396 To make @command{dircolors} read a @file{~/.dircolors} file if it
7397 exists, you can put the following lines in your @file{~/.bashrc} (or
7398 adapt them to your favorite shell):
7402 test -r $d && eval "$(dircolors $d)"
7406 @vindex SHELL @r{environment variable, and color}
7407 The output is a shell command to set the @env{LS_COLORS} environment
7408 variable. You can specify the shell syntax to use on the command line,
7409 or @command{dircolors} will guess it from the value of the @env{SHELL}
7410 environment variable.
7412 The program accepts the following options. Also see @ref{Common options}.
7417 @itemx --bourne-shell
7420 @opindex --bourne-shell
7421 @cindex Bourne shell syntax for color setup
7422 @cindex @command{sh} syntax for color setup
7423 Output Bourne shell commands. This is the default if the @env{SHELL}
7424 environment variable is set and does not end with @samp{csh} or
7433 @cindex C shell syntax for color setup
7434 @cindex @command{csh} syntax for color setup
7435 Output C shell commands. This is the default if @code{SHELL} ends with
7436 @command{csh} or @command{tcsh}.
7439 @itemx --print-database
7441 @opindex --print-database
7442 @cindex color database, printing
7443 @cindex database for color setup, printing
7444 @cindex printing color database
7445 Print the (compiled-in) default color configuration database. This
7446 output is itself a valid configuration file, and is fairly descriptive
7447 of the possibilities.
7454 @node Basic operations
7455 @chapter Basic operations
7457 @cindex manipulating files
7459 This chapter describes the commands for basic file manipulation:
7460 copying, moving (renaming), and deleting (removing).
7463 * cp invocation:: Copy files.
7464 * dd invocation:: Convert and copy a file.
7465 * install invocation:: Copy files and set attributes.
7466 * mv invocation:: Move (rename) files.
7467 * rm invocation:: Remove files or directories.
7468 * shred invocation:: Remove files more securely.
7473 @section @command{cp}: Copy files and directories
7476 @cindex copying files and directories
7477 @cindex files, copying
7478 @cindex directories, copying
7480 @command{cp} copies files (or, optionally, directories). The copy is
7481 completely independent of the original. You can either copy one file to
7482 another, or copy arbitrarily many files to a destination directory.
7486 cp [@var{option}]@dots{} [-T] @var{source} @var{dest}
7487 cp [@var{option}]@dots{} @var{source}@dots{} @var{directory}
7488 cp [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
7493 If two file names are given, @command{cp} copies the first file to the
7497 If the @option{--target-directory} (@option{-t}) option is given, or
7498 failing that if the last file is a directory and the
7499 @option{--no-target-directory} (@option{-T}) option is not given,
7500 @command{cp} copies each @var{source} file to the specified directory,
7501 using the @var{source}s' names.
7504 Generally, files are written just as they are read. For exceptions,
7505 see the @option{--sparse} option below.
7507 By default, @command{cp} does not copy directories. However, the
7508 @option{-R}, @option{-a}, and @option{-r} options cause @command{cp} to
7509 copy recursively by descending into source directories and copying files
7510 to corresponding destination directories.
7512 When copying from a symbolic link, @command{cp} normally follows the
7513 link only when not copying
7514 recursively. This default can be overridden with the
7515 @option{--archive} (@option{-a}), @option{-d}, @option{--dereference}
7516 (@option{-L}), @option{--no-dereference} (@option{-P}), and
7517 @option{-H} options. If more than one of these options is specified,
7518 the last one silently overrides the others.
7520 When copying to a symbolic link, @command{cp} follows the
7521 link only when it refers to an existing regular file.
7522 However, when copying to a dangling symbolic link, @command{cp}
7523 refuses by default, and fails with a diagnostic, since the operation
7524 is inherently dangerous. This behavior is contrary to historical
7525 practice and to @acronym{POSIX}.
7526 Set @env{POSIXLY_CORRECT} to make @command{cp} attempt to create
7527 the target of a dangling destination symlink, in spite of the possible risk.
7528 Also, when an option like
7529 @option{--backup} or @option{--link} acts to rename or remove the
7530 destination before copying, @command{cp} renames or removes the
7531 symbolic link rather than the file it points to.
7533 By default, @command{cp} copies the contents of special files only
7534 when not copying recursively. This default can be overridden with the
7535 @option{--copy-contents} option.
7537 @cindex self-backups
7538 @cindex backups, making only
7539 @command{cp} generally refuses to copy a file onto itself, with the
7540 following exception: if @option{--force --backup} is specified with
7541 @var{source} and @var{dest} identical, and referring to a regular file,
7542 @command{cp} will make a backup file, either regular or numbered, as
7543 specified in the usual ways (@pxref{Backup options}). This is useful when
7544 you simply want to make a backup of an existing file before changing it.
7546 The program accepts the following options. Also see @ref{Common options}.
7553 Preserve as much as possible of the structure and attributes of the
7554 original files in the copy (but do not attempt to preserve internal
7555 directory structure; i.e., @samp{ls -U} may list the entries in a copied
7556 directory in a different order).
7557 Try to preserve SELinux security context and extended attributes (xattr),
7558 but ignore any failure to do that and print no corresponding diagnostic.
7559 Equivalent to @option{-dR --preserve=all} with the reduced diagnostics.
7561 @itemx --attributes-only
7562 @opindex --attributes-only
7563 Preserve the specified attributes of the original files in the copy,
7564 but do not copy any data. See the @option{--preserve} option for
7565 controlling which attributes to copy.
7568 @itemx @w{@kbd{--backup}[=@var{method}]}
7571 @vindex VERSION_CONTROL
7572 @cindex backups, making
7573 @xref{Backup options}.
7574 Make a backup of each file that would otherwise be overwritten or removed.
7575 As a special case, @command{cp} makes a backup of @var{source} when the force
7576 and backup options are given and @var{source} and @var{dest} are the same
7577 name for an existing, regular file. One useful application of this
7578 combination of options is this tiny Bourne shell script:
7582 # Usage: backup FILE...
7583 # Create a @sc{gnu}-style backup of each listed FILE.
7585 cp --backup --force -- "$i" "$i"
7589 @item --copy-contents
7590 @cindex directories, copying recursively
7591 @cindex copying directories recursively
7592 @cindex recursively copying directories
7593 @cindex non-directories, copying as special files
7594 If copying recursively, copy the contents of any special files (e.g.,
7595 FIFOs and device files) as if they were regular files. This means
7596 trying to read the data in each source file and writing it to the
7597 destination. It is usually a mistake to use this option, as it
7598 normally has undesirable effects on special files like FIFOs and the
7599 ones typically found in the @file{/dev} directory. In most cases,
7600 @code{cp -R --copy-contents} will hang indefinitely trying to read
7601 from FIFOs and special files like @file{/dev/console}, and it will
7602 fill up your destination disk if you use it to copy @file{/dev/zero}.
7603 This option has no effect unless copying recursively, and it does not
7604 affect the copying of symbolic links.
7608 @cindex symbolic links, copying
7609 @cindex hard links, preserving
7610 Copy symbolic links as symbolic links rather than copying the files that
7611 they point to, and preserve hard links between source files in the copies.
7612 Equivalent to @option{--no-dereference --preserve=links}.
7618 When copying without this option and an existing destination file cannot
7619 be opened for writing, the copy fails. However, with @option{--force}),
7620 when a destination file cannot be opened, @command{cp} then removes it and
7621 tries to open it again. Contrast this behavior with that enabled by
7622 @option{--link} and @option{--symbolic-link}, whereby the destination file
7623 is never opened but rather is removed unconditionally. Also see the
7624 description of @option{--remove-destination}.
7626 This option is independent of the @option{--interactive} or
7627 @option{-i} option: neither cancels the effect of the other.
7629 This option is redundant if the @option{--no-clobber} or @option{-n} option is
7634 If a command line argument specifies a symbolic link, then copy the
7635 file it points to rather than the symbolic link itself. However,
7636 copy (preserving its nature) any symbolic link that is encountered
7637 via recursive traversal.
7640 @itemx --interactive
7642 @opindex --interactive
7643 When copying a file other than a directory, prompt whether to
7644 overwrite an existing destination file. The @option{-i} option overrides
7645 a previous @option{-n} option.
7651 Make hard links instead of copies of non-directories.
7654 @itemx --dereference
7656 @opindex --dereference
7657 Follow symbolic links when copying from them.
7658 With this option, @command{cp} cannot create a symbolic link.
7659 For example, a symlink (to regular file) in the source tree will be copied to
7660 a regular file in the destination tree.
7665 @opindex --no-clobber
7666 Do not overwrite an existing file. The @option{-n} option overrides a previous
7667 @option{-i} option. This option is mutually exclusive with @option{-b} or
7668 @option{--backup} option.
7671 @itemx --no-dereference
7673 @opindex --no-dereference
7674 @cindex symbolic links, copying
7675 Copy symbolic links as symbolic links rather than copying the files that
7676 they point to. This option affects only symbolic links in the source;
7677 symbolic links in the destination are always followed if possible.
7680 @itemx @w{@kbd{--preserve}[=@var{attribute_list}]}
7683 @cindex file information, preserving, extended attributes, xattr
7684 Preserve the specified attributes of the original files.
7685 If specified, the @var{attribute_list} must be a comma-separated list
7686 of one or more of the following strings:
7690 Preserve the file mode bits and access control lists.
7692 Preserve the owner and group. On most modern systems,
7693 only users with appropriate privileges may change the owner of a file,
7695 may preserve the group ownership of a file only if they happen to be
7696 a member of the desired group.
7698 Preserve the times of last access and last modification, when possible.
7699 On older systems, it is not possible to preserve these attributes
7700 when the affected file is a symbolic link.
7701 However, many systems now provide the @code{utimensat} function,
7702 which makes it possible even for symbolic links.
7704 Preserve in the destination files
7705 any links between corresponding source files.
7706 Note that with @option{-L} or @option{-H}, this option can convert
7707 symbolic links to hard links. For example,
7709 $ mkdir c; : > a; ln -s a b; cp -aH a b c; ls -i1 c
7714 Note the inputs: @file{b} is a symlink to regular file @file{a},
7715 yet the files in destination directory, @file{c/}, are hard-linked.
7716 Since @option{-a} implies @option{--preserve=links}, and since @option{-H}
7717 tells @command{cp} to dereference command line arguments, it sees two files
7718 with the same inode number, and preserves the perceived hard link.
7720 Here is a similar example that exercises @command{cp}'s @option{-L} option:
7722 $ mkdir b c; (cd b; : > a; ln -s a b); cp -aL b c; ls -i1 c/b
7728 Preserve SELinux security context of the file, or fail with full diagnostics.
7730 Preserve extended attributes of the file, or fail with full diagnostics.
7731 If @command{cp} is built without xattr support, ignore this option.
7732 If SELinux context, ACLs or Capabilities are implemented using xattrs,
7733 they are preserved by this option as well.
7735 Preserve all file attributes.
7736 Equivalent to specifying all of the above, but with the difference
7737 that failure to preserve SELinux security context or extended attributes
7738 does not change @command{cp}'s exit status. In contrast to @option{-a},
7739 all but @samp{Operation not supported} warnings are output.
7742 Using @option{--preserve} with no @var{attribute_list} is equivalent
7743 to @option{--preserve=mode,ownership,timestamps}.
7745 In the absence of this option, each destination file is created with the
7746 mode bits of the corresponding source file, minus the bits set in the
7747 umask and minus the set-user-ID and set-group-ID bits.
7748 @xref{File permissions}.
7750 @itemx @w{@kbd{--no-preserve}=@var{attribute_list}}
7751 @cindex file information, preserving
7752 Do not preserve the specified attributes. The @var{attribute_list}
7753 has the same form as for @option{--preserve}.
7757 @cindex parent directories and @command{cp}
7758 Form the name of each destination file by appending to the target
7759 directory a slash and the specified name of the source file. The last
7760 argument given to @command{cp} must be the name of an existing directory.
7761 For example, the command:
7764 cp --parents a/b/c existing_dir
7768 copies the file @file{a/b/c} to @file{existing_dir/a/b/c}, creating
7769 any missing intermediate directories.
7776 @opindex --recursive
7777 @cindex directories, copying recursively
7778 @cindex copying directories recursively
7779 @cindex recursively copying directories
7780 @cindex non-directories, copying as special files
7781 Copy directories recursively. By default, do not follow symbolic
7782 links in the source; see the @option{--archive} (@option{-a}), @option{-d},
7783 @option{--dereference} (@option{-L}), @option{--no-dereference}
7784 (@option{-P}), and @option{-H} options. Special files are copied by
7785 creating a destination file of the same type as the source; see the
7786 @option{--copy-contents} option. It is not portable to use
7787 @option{-r} to copy symbolic links or special files. On some
7788 non-@sc{gnu} systems, @option{-r} implies the equivalent of
7789 @option{-L} and @option{--copy-contents} for historical reasons.
7790 Also, it is not portable to use @option{-R} to copy symbolic links
7791 unless you also specify @option{-P}, as @acronym{POSIX} allows
7792 implementations that dereference symbolic links by default.
7794 @item --reflink[=@var{when}]
7795 @opindex --reflink[=@var{when}]
7798 @cindex copy on write
7799 Perform a lightweight, copy-on-write (COW) copy, if supported by the
7800 file system. Once it has succeeded, beware that the source and destination
7801 files share the same disk data blocks as long as they remain unmodified.
7802 Thus, if a disk I/O error affects data blocks of one of the files,
7803 the other suffers the same fate.
7805 The @var{when} value can be one of the following:
7809 The default behavior: if the copy-on-write operation is not supported
7810 then report the failure for each file and exit with a failure status.
7813 If the copy-on-write operation is not supported then fall back
7814 to the standard copy behaviour.
7817 This option is overridden by the @option{--link}, @option{--symbolic-link}
7818 and @option{--attributes-only} options, thus allowing it to be used
7819 to configure the default data copying behavior for @command{cp}.
7820 For example, with the following alias, @command{cp} will use the
7821 minimum amount of space supported by the file system.
7824 alias cp='cp --reflink=auto --sparse=always'
7827 @item --remove-destination
7828 @opindex --remove-destination
7829 Remove each existing destination file before attempting to open it
7830 (contrast with @option{-f} above).
7832 @item --sparse=@var{when}
7833 @opindex --sparse=@var{when}
7834 @cindex sparse files, copying
7835 @cindex holes, copying files with
7836 @findex read @r{system call, and holes}
7837 A @dfn{sparse file} contains @dfn{holes}---a sequence of zero bytes that
7838 does not occupy any physical disk blocks; the @samp{read} system call
7839 reads these as zeros. This can both save considerable disk space and
7840 increase speed, since many binary files contain lots of consecutive zero
7841 bytes. By default, @command{cp} detects holes in input source files via a crude
7842 heuristic and makes the corresponding output file sparse as well.
7843 Only regular files may be sparse.
7845 The @var{when} value can be one of the following:
7849 The default behavior: if the input file is sparse, attempt to make
7850 the output file sparse, too. However, if an output file exists but
7851 refers to a non-regular file, then do not attempt to make it sparse.
7854 For each sufficiently long sequence of zero bytes in the input file,
7855 attempt to create a corresponding hole in the output file, even if the
7856 input file does not appear to be sparse.
7857 This is useful when the input file resides on a file system
7858 that does not support sparse files
7859 (for example, @samp{efs} file systems in SGI IRIX 5.3 and earlier),
7860 but the output file is on a type of file system that does support them.
7861 Holes may be created only in regular files, so if the destination file
7862 is of some other type, @command{cp} does not even try to make it sparse.
7865 Never make the output file sparse.
7866 This is useful in creating a file for use with the @command{mkswap} command,
7867 since such a file must not have any holes.
7870 @optStripTrailingSlashes
7873 @itemx --symbolic-link
7875 @opindex --symbolic-link
7876 @cindex symbolic links, copying with
7877 Make symbolic links instead of copies of non-directories. All source
7878 file names must be absolute (starting with @samp{/}) unless the
7879 destination files are in the current directory. This option merely
7880 results in an error message on systems that do not support symbolic links.
7886 @optNoTargetDirectory
7892 @cindex newer files, copying only
7893 Do not copy a non-directory that has an existing destination with the
7894 same or newer modification time. If time stamps are being preserved,
7895 the comparison is to the source time stamp truncated to the
7896 resolutions of the destination file system and of the system calls
7897 used to update time stamps; this avoids duplicate work if several
7898 @samp{cp -pu} commands are executed with the same source and
7905 Print the name of each file before copying it.
7908 @itemx --one-file-system
7910 @opindex --one-file-system
7911 @cindex file systems, omitting copying to different
7912 Skip subdirectories that are on different file systems from the one that
7913 the copy started on.
7914 However, mount point directories @emph{are} copied.
7922 @section @command{dd}: Convert and copy a file
7925 @cindex converting while copying a file
7927 @command{dd} copies a file (from standard input to standard output, by
7928 default) with a changeable I/O block size, while optionally performing
7929 conversions on it. Synopses:
7932 dd [@var{operand}]@dots{}
7936 The only options are @option{--help} and @option{--version}.
7937 @xref{Common options}. @command{dd} accepts the following operands.
7943 Read from @var{file} instead of standard input.
7947 Write to @var{file} instead of standard output. Unless
7948 @samp{conv=notrunc} is given, @command{dd} truncates @var{file} to zero
7949 bytes (or the size specified with @samp{seek=}).
7951 @item ibs=@var{bytes}
7953 @cindex block size of input
7954 @cindex input block size
7955 Set the input block size to @var{bytes}.
7956 This makes @command{dd} read @var{bytes} per block.
7957 The default is 512 bytes.
7959 @item obs=@var{bytes}
7961 @cindex block size of output
7962 @cindex output block size
7963 Set the output block size to @var{bytes}.
7964 This makes @command{dd} write @var{bytes} per block.
7965 The default is 512 bytes.
7967 @item bs=@var{bytes}
7970 Set both input and output block sizes to @var{bytes}.
7971 This makes @command{dd} read and write @var{bytes} per block,
7972 overriding any @samp{ibs} and @samp{obs} settings.
7973 In addition, if no data-transforming @option{conv} option is specified,
7974 input is copied to the output as soon as it's read,
7975 even if it is smaller than the block size.
7977 @item cbs=@var{bytes}
7979 @cindex block size of conversion
7980 @cindex conversion block size
7981 @cindex fixed-length records, converting to variable-length
7982 @cindex variable-length records, converting to fixed-length
7983 Set the conversion block size to @var{bytes}.
7984 When converting variable-length records to fixed-length ones
7985 (@option{conv=block}) or the reverse (@option{conv=unblock}),
7986 use @var{bytes} as the fixed record length.
7988 @item skip=@var{blocks}
7990 Skip @var{blocks} @samp{ibs}-byte blocks in the input file before copying.
7992 @item seek=@var{blocks}
7994 Skip @var{blocks} @samp{obs}-byte blocks in the output file before copying.
7996 @item count=@var{blocks}
7998 Copy @var{blocks} @samp{ibs}-byte blocks from the input file, instead
7999 of everything until the end of the file.
8003 Do not print the overall transfer rate and volume statistics
8004 that normally make up the third status line when @command{dd} exits.
8006 @item conv=@var{conversion}[,@var{conversion}]@dots{}
8008 Convert the file as specified by the @var{conversion} argument(s).
8009 (No spaces around any comma(s).)
8016 @opindex ascii@r{, converting to}
8017 Convert @acronym{EBCDIC} to @acronym{ASCII},
8018 using the conversion table specified by @acronym{POSIX}.
8019 This provides a 1:1 translation for all 256 bytes.
8022 @opindex ebcdic@r{, converting to}
8023 Convert @acronym{ASCII} to @acronym{EBCDIC}.
8024 This is the inverse of the @samp{ascii} conversion.
8027 @opindex alternate ebcdic@r{, converting to}
8028 Convert @acronym{ASCII} to alternate @acronym{EBCDIC},
8029 using the alternate conversion table specified by @acronym{POSIX}.
8030 This is not a 1:1 translation, but reflects common historical practice
8031 for @samp{~}, @samp{[}, and @samp{]}.
8033 The @samp{ascii}, @samp{ebcdic}, and @samp{ibm} conversions are
8037 @opindex block @r{(space-padding)}
8038 For each line in the input, output @samp{cbs} bytes, replacing the
8039 input newline with a space and padding with spaces as necessary.
8043 Remove any trailing spaces in each @samp{cbs}-sized input block,
8044 and append a newline.
8046 The @samp{block} and @samp{unblock} conversions are mutually exclusive.
8049 @opindex lcase@r{, converting to}
8050 Change uppercase letters to lowercase.
8053 @opindex ucase@r{, converting to}
8054 Change lowercase letters to uppercase.
8056 The @samp{lcase} and @samp{ucase} conversions are mutually exclusive.
8059 @opindex swab @r{(byte-swapping)}
8060 @cindex byte-swapping
8061 Swap every pair of input bytes. @sc{gnu} @command{dd}, unlike others, works
8062 when an odd number of bytes are read---the last byte is simply copied
8063 (since there is nothing to swap it with).
8066 @opindex sync @r{(padding with @acronym{ASCII} @sc{nul}s)}
8067 Pad every input block to size of @samp{ibs} with trailing zero bytes.
8068 When used with @samp{block} or @samp{unblock}, pad with spaces instead of
8073 The following ''conversions'' are really file flags
8074 and don't effect and internal processing:
8079 @cindex creating output file, requiring
8080 Fail if the output file already exists; @command{dd} must create the
8085 @cindex creating output file, avoiding
8086 Do not create the output file; the output file must already exist.
8088 The @samp{excl} and @samp{nocreat} conversions are mutually exclusive.
8092 @cindex truncating output file, avoiding
8093 Do not truncate the output file.
8097 @cindex read errors, ignoring
8098 Continue after read errors.
8102 @cindex synchronized data writes, before finishing
8103 Synchronize output data just before finishing. This forces a physical
8104 write of output data.
8108 @cindex synchronized data and metadata writes, before finishing
8109 Synchronize output data and metadata just before finishing. This
8110 forces a physical write of output data and metadata.
8114 @item iflag=@var{flag}[,@var{flag}]@dots{}
8116 Access the input file using the flags specified by the @var{flag}
8117 argument(s). (No spaces around any comma(s).)
8119 @item oflag=@var{flag}[,@var{flag}]@dots{}
8121 Access the output file using the flags specified by the @var{flag}
8122 argument(s). (No spaces around any comma(s).)
8124 Here are the flags. Not every flag is supported on every operating
8131 @cindex appending to the output file
8132 Write in append mode, so that even if some other process is writing to
8133 this file, every @command{dd} write will append to the current
8134 contents of the file. This flag makes sense only for output.
8135 If you combine this flag with the @samp{of=@var{file}} operand,
8136 you should also specify @samp{conv=notrunc} unless you want the
8137 output file to be truncated before being appended to.
8141 @cindex concurrent I/O
8142 Use concurrent I/O mode for data. This mode performs direct I/O
8143 and drops the @acronym{POSIX} requirement to serialize all I/O to the same file.
8144 A file cannot be opened in CIO mode and with a standard open at the
8150 Use direct I/O for data, avoiding the buffer cache.
8151 Note that the kernel may impose restrictions on read or write buffer sizes.
8152 For example, with an ext4 destination file system and a linux-based kernel,
8153 using @samp{oflag=direct} will cause writes to fail with @code{EINVAL} if the
8154 output buffer size is not a multiple of 512.
8158 @cindex directory I/O
8160 Fail unless the file is a directory. Most operating systems do not
8161 allow I/O to a directory, so this flag has limited utility.
8165 @cindex synchronized data reads
8166 Use synchronized I/O for data. For the output file, this forces a
8167 physical write of output data on each write. For the input file,
8168 this flag can matter when reading from a remote file that has been
8169 written to synchronously by some other process. Metadata (e.g.,
8170 last-access and last-modified time) is not necessarily synchronized.
8174 @cindex synchronized data and metadata I/O
8175 Use synchronized I/O for both data and metadata.
8179 @cindex nonblocking I/O
8180 Use non-blocking I/O.
8185 Do not update the file's access time.
8186 Some older file systems silently ignore this flag, so it is a good
8187 idea to test it on your files before relying on it.
8191 @cindex controlling terminal
8192 Do not assign the file to be a controlling terminal for @command{dd}.
8193 This has no effect when the file is not a terminal.
8194 On many hosts (e.g., @acronym{GNU}/Linux hosts), this option has no effect
8199 @cindex symbolic links, following
8200 Do not follow symbolic links.
8205 Fail if the file has multiple hard links.
8210 Use binary I/O. This option has an effect only on nonstandard
8211 platforms that distinguish binary from text I/O.
8216 Use text I/O. Like @samp{binary}, this option has no effect on
8221 Accumulate full blocks from input. The @code{read} system call
8222 may return early if a full block is not available.
8223 When that happens, continue calling @code{read} to fill the remainder
8225 This flag can be used only with @code{iflag}.
8229 These flags are not supported on all systems, and @samp{dd} rejects
8230 attempts to use them when they are not supported. When reading from
8231 standard input or writing to standard output, the @samp{nofollow} and
8232 @samp{noctty} flags should not be specified, and the other flags
8233 (e.g., @samp{nonblock}) can affect how other processes behave with the
8234 affected file descriptors, even after @command{dd} exits.
8238 @cindex multipliers after numbers
8239 The numeric-valued strings above (@var{bytes} and @var{blocks}) can be
8240 followed by a multiplier: @samp{b}=512, @samp{c}=1,
8241 @samp{w}=2, @samp{x@var{m}}=@var{m}, or any of the
8242 standard block size suffixes like @samp{k}=1024 (@pxref{Block size}).
8244 Any block size you specify via @samp{bs=}, @samp{ibs=}, @samp{obs=}, @samp{cbs=}
8245 should not be too large---values larger than a few megabytes
8246 are generally wasteful or (as in the gigabyte..exabyte case) downright
8247 counterproductive or error-inducing.
8249 Use different @command{dd} invocations to use different block sizes for
8250 skipping and I/O@. For example, the following shell commands copy data
8251 in 512 KiB blocks between a disk and a tape, but do not save or restore a
8252 4 KiB label at the start of the disk:
8255 disk=/dev/rdsk/c0t1d0s2
8258 # Copy all but the label from disk to tape.
8259 (dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape
8261 # Copy from tape back to disk, but leave the disk label alone.
8262 (dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk
8265 Sending an @samp{INFO} signal to a running @command{dd}
8266 process makes it print I/O statistics to standard error
8267 and then resume copying. In the example below,
8268 @command{dd} is run in the background to copy 10 million blocks.
8269 The @command{kill} command makes it output intermediate I/O statistics,
8270 and when @command{dd} completes normally or is killed by the
8271 @code{SIGINT} signal, it outputs the final statistics.
8274 $ dd if=/dev/zero of=/dev/null count=10MB & pid=$!
8275 $ kill -s INFO $pid; wait $pid
8276 3385223+0 records in
8277 3385223+0 records out
8278 1733234176 bytes (1.7 GB) copied, 6.42173 seconds, 270 MB/s
8279 10000000+0 records in
8280 10000000+0 records out
8281 5120000000 bytes (5.1 GB) copied, 18.913 seconds, 271 MB/s
8284 @vindex POSIXLY_CORRECT
8285 On systems lacking the @samp{INFO} signal @command{dd} responds to the
8286 @samp{USR1} signal instead, unless the @env{POSIXLY_CORRECT}
8287 environment variable is set.
8292 @node install invocation
8293 @section @command{install}: Copy files and set attributes
8296 @cindex copying files and setting attributes
8298 @command{install} copies files while setting their file mode bits and, if
8299 possible, their owner and group. Synopses:
8302 install [@var{option}]@dots{} [-T] @var{source} @var{dest}
8303 install [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8304 install [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8305 install [@var{option}]@dots{} -d @var{directory}@dots{}
8310 If two file names are given, @command{install} copies the first file to the
8314 If the @option{--target-directory} (@option{-t}) option is given, or
8315 failing that if the last file is a directory and the
8316 @option{--no-target-directory} (@option{-T}) option is not given,
8317 @command{install} copies each @var{source} file to the specified
8318 directory, using the @var{source}s' names.
8321 If the @option{--directory} (@option{-d}) option is given,
8322 @command{install} creates each @var{directory} and any missing parent
8323 directories. Parent directories are created with mode
8324 @samp{u=rwx,go=rx} (755), regardless of the @option{-m} option or the
8325 current umask. @xref{Directory Setuid and Setgid}, for how the
8326 set-user-ID and set-group-ID bits of parent directories are inherited.
8329 @cindex Makefiles, installing programs in
8330 @command{install} is similar to @command{cp}, but allows you to control the
8331 attributes of destination files. It is typically used in Makefiles to
8332 copy programs into their destination directories. It refuses to copy
8333 files onto themselves.
8335 @cindex extended attributes, xattr
8336 @command{install} never preserves extended attributes (xattr).
8338 The program accepts the following options. Also see @ref{Common options}.
8348 Compare each pair of source and destination files, and if the destination has
8349 identical content and any specified owner, group, permissions, and possibly
8350 SELinux context, then do not modify the destination at all.
8354 Ignored; for compatibility with old Unix versions of @command{install}.
8358 Create any missing parent directories of @var{dest},
8359 then copy @var{source} to @var{dest}.
8360 This option is ignored if a destination directory is specified
8361 via @option{--target-directory=DIR}.
8366 @opindex --directory
8367 @cindex directories, creating with given attributes
8368 @cindex parent directories, creating missing
8369 @cindex leading directories, creating missing
8370 Create any missing parent directories, giving them the default
8371 attributes. Then create each given directory, setting their owner,
8372 group and mode as given on the command line or to the defaults.
8374 @item -g @var{group}
8375 @itemx --group=@var{group}
8378 @cindex group ownership of installed files, setting
8379 Set the group ownership of installed files or directories to
8380 @var{group}. The default is the process's current group. @var{group}
8381 may be either a group name or a numeric group ID.
8384 @itemx --mode=@var{mode}
8387 @cindex permissions of installed files, setting
8388 Set the file mode bits for the installed file or directory to @var{mode},
8389 which can be either an octal number, or a symbolic mode as in
8390 @command{chmod}, with @samp{a=} (no access allowed to anyone) as the
8391 point of departure (@pxref{File permissions}).
8392 The default mode is @samp{u=rwx,go=rx,a-s}---read, write, and
8393 execute for the owner, read and execute for group and other, and with
8394 set-user-ID and set-group-ID disabled.
8395 This default is not quite the same as @samp{755}, since it disables
8396 instead of preserving set-user-ID and set-group-ID on directories.
8397 @xref{Directory Setuid and Setgid}.
8399 @item -o @var{owner}
8400 @itemx --owner=@var{owner}
8403 @cindex ownership of installed files, setting
8404 @cindex appropriate privileges
8405 @vindex root @r{as default owner}
8406 If @command{install} has appropriate privileges (is run as root), set the
8407 ownership of installed files or directories to @var{owner}. The default
8408 is @code{root}. @var{owner} may be either a user name or a numeric user
8411 @item --preserve-context
8412 @opindex --preserve-context
8414 @cindex security context
8415 Preserve the SELinux security context of files and directories.
8416 Failure to preserve the context in all of the files or directories
8417 will result in an exit status of 1. If SELinux is disabled then
8418 print a warning and ignore the option.
8421 @itemx --preserve-timestamps
8423 @opindex --preserve-timestamps
8424 @cindex timestamps of installed files, preserving
8425 Set the time of last access and the time of last modification of each
8426 installed file to match those of each corresponding original file.
8427 When a file is installed without this option, its last access and
8428 last modification times are both set to the time of installation.
8429 This option is useful if you want to use the last modification times
8430 of installed files to keep track of when they were last built as opposed
8431 to when they were last installed.
8437 @cindex symbol table information, stripping
8438 @cindex stripping symbol table information
8439 Strip the symbol tables from installed binary executables.
8441 @itemx --strip-program=@var{program}
8442 @opindex --strip-program
8443 @cindex symbol table information, stripping, program
8444 Program used to strip binaries.
8450 @optNoTargetDirectory
8456 Print the name of each file before copying it.
8458 @item -Z @var{context}
8459 @itemx --context=@var{context}
8463 @cindex security context
8464 Set the default SELinux security context to be used for any
8465 created files and directories. If SELinux is disabled then
8466 print a warning and ignore the option.
8474 @section @command{mv}: Move (rename) files
8478 @command{mv} moves or renames files (or directories). Synopses:
8481 mv [@var{option}]@dots{} [-T] @var{source} @var{dest}
8482 mv [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8483 mv [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8488 If two file names are given, @command{mv} moves the first file to the
8492 If the @option{--target-directory} (@option{-t}) option is given, or
8493 failing that if the last file is a directory and the
8494 @option{--no-target-directory} (@option{-T}) option is not given,
8495 @command{mv} moves each @var{source} file to the specified
8496 directory, using the @var{source}s' names.
8499 @command{mv} can move any type of file from one file system to another.
8500 Prior to version @code{4.0} of the fileutils,
8501 @command{mv} could move only regular files between file systems.
8502 For example, now @command{mv} can move an entire directory hierarchy
8503 including special device files from one partition to another. It first
8504 uses some of the same code that's used by @code{cp -a} to copy the
8505 requested directories and files, then (assuming the copy succeeded)
8506 it removes the originals. If the copy fails, then the part that was
8507 copied to the destination partition is removed. If you were to copy
8508 three directories from one partition to another and the copy of the first
8509 directory succeeded, but the second didn't, the first would be left on
8510 the destination partition and the second and third would be left on the
8513 @cindex extended attributes, xattr
8514 @command{mv} always tries to copy extended attributes (xattr), which may
8515 include SELinux context, ACLs or Capabilities.
8516 Upon failure all but @samp{Operation not supported} warnings are output.
8518 @cindex prompting, and @command{mv}
8519 If a destination file exists but is normally unwritable, standard input
8520 is a terminal, and the @option{-f} or @option{--force} option is not given,
8521 @command{mv} prompts the user for whether to replace the file. (You might
8522 own the file, or have write permission on its directory.) If the
8523 response is not affirmative, the file is skipped.
8525 @emph{Warning}: Avoid specifying a source name with a trailing slash,
8526 when it might be a symlink to a directory.
8527 Otherwise, @command{mv} may do something very surprising, since
8528 its behavior depends on the underlying rename system call.
8529 On a system with a modern Linux-based kernel, it fails with
8530 @code{errno=ENOTDIR}.
8531 However, on other systems (at least FreeBSD 6.1 and Solaris 10) it silently
8532 renames not the symlink but rather the directory referenced by the symlink.
8533 @xref{Trailing slashes}.
8535 The program accepts the following options. Also see @ref{Common options}.
8545 @cindex prompts, omitting
8546 Do not prompt the user before removing a destination file.
8548 If you specify more than one of the @option{-i}, @option{-f}, @option{-n}
8549 options, only the final one takes effect.
8554 @itemx --interactive
8556 @opindex --interactive
8557 @cindex prompts, forcing
8558 Prompt whether to overwrite each existing destination file, regardless
8560 If the response is not affirmative, the file is skipped.
8566 @opindex --no-clobber
8567 @cindex prompts, omitting
8568 Do not overwrite an existing file.
8570 This option is mutually exclusive with @option{-b} or @option{--backup} option.
8576 @cindex newer files, moving only
8577 Do not move a non-directory that has an existing destination with the
8578 same or newer modification time.
8579 If the move is across file system boundaries, the comparison is to the
8580 source time stamp truncated to the resolutions of the destination file
8581 system and of the system calls used to update time stamps; this avoids
8582 duplicate work if several @samp{mv -u} commands are executed with the
8583 same source and destination.
8589 Print the name of each file before moving it.
8591 @optStripTrailingSlashes
8597 @optNoTargetDirectory
8605 @section @command{rm}: Remove files or directories
8608 @cindex removing files or directories
8610 @command{rm} removes each given @var{file}. By default, it does not remove
8611 directories. Synopsis:
8614 rm [@var{option}]@dots{} [@var{file}]@dots{}
8617 @cindex prompting, and @command{rm}
8618 If the @option{-I} or @option{--interactive=once} option is given,
8619 and there are more than three files or the @option{-r}, @option{-R},
8620 or @option{--recursive} are given, then @command{rm} prompts the user
8621 for whether to proceed with the entire operation. If the response is
8622 not affirmative, the entire command is aborted.
8624 Otherwise, if a file is unwritable, standard input is a terminal, and
8625 the @option{-f} or @option{--force} option is not given, or the
8626 @option{-i} or @option{--interactive=always} option @emph{is} given,
8627 @command{rm} prompts the user for whether to remove the file.
8628 If the response is not affirmative, the file is skipped.
8630 Any attempt to remove a file whose last file name component is
8631 @file{.} or @file{..} is rejected without any prompting.
8633 @emph{Warning}: If you use @command{rm} to remove a file, it is usually
8634 possible to recover the contents of that file. If you want more assurance
8635 that the contents are truly unrecoverable, consider using @command{shred}.
8637 The program accepts the following options. Also see @ref{Common options}.
8645 Ignore nonexistent files and never prompt the user.
8646 Ignore any previous @option{--interactive} (@option{-i}) option.
8650 Prompt whether to remove each file.
8651 If the response is not affirmative, the file is skipped.
8652 Ignore any previous @option{--force} (@option{-f}) option.
8653 Equivalent to @option{--interactive=always}.
8657 Prompt once whether to proceed with the command, if more than three
8658 files are named or if a recursive removal is requested. Ignore any
8659 previous @option{--force} (@option{-f}) option. Equivalent to
8660 @option{--interactive=once}.
8662 @itemx --interactive [=@var{when}]
8663 @opindex --interactive
8664 Specify when to issue an interactive prompt. @var{when} may be
8668 @vindex never @r{interactive option}
8669 - Do not prompt at all.
8671 @vindex once @r{interactive option}
8672 - Prompt once if more than three files are named or if a recursive
8673 removal is requested. Equivalent to @option{-I}.
8675 @vindex always @r{interactive option}
8676 - Prompt for every file being removed. Equivalent to @option{-i}.
8678 @option{--interactive} with no @var{when} is equivalent to
8679 @option{--interactive=always}.
8681 @itemx --one-file-system
8682 @opindex --one-file-system
8683 @cindex one file system, restricting @command{rm} to
8684 When removing a hierarchy recursively, skip any directory that is on a
8685 file system different from that of the corresponding command line argument.
8688 This option is useful when removing a build ``chroot'' hierarchy,
8689 which normally contains no valuable data. However, it is not uncommon
8690 to bind-mount @file{/home} into such a hierarchy, to make it easier to
8691 use one's start-up file. The catch is that it's easy to forget to
8692 unmount @file{/home}. Then, when you use @command{rm -rf} to remove
8693 your normally throw-away chroot, that command will remove everything
8694 under @file{/home}, too.
8695 Use the @option{--one-file-system} option, and it will
8696 warn about and skip directories on other file systems.
8697 Of course, this will not save your @file{/home} if it and your
8698 chroot happen to be on the same file system.
8700 @itemx --preserve-root
8701 @opindex --preserve-root
8702 @cindex root directory, disallow recursive destruction
8703 Fail upon any attempt to remove the root directory, @file{/},
8704 when used with the @option{--recursive} option.
8705 This is the default behavior.
8706 @xref{Treating / specially}.
8708 @itemx --no-preserve-root
8709 @opindex --no-preserve-root
8710 @cindex root directory, allow recursive destruction
8711 Do not treat @file{/} specially when removing recursively.
8712 This option is not recommended unless you really want to
8713 remove all the files on your computer.
8714 @xref{Treating / specially}.
8721 @opindex --recursive
8722 @cindex directories, removing (recursively)
8723 Remove the listed directories and their contents recursively.
8729 Print the name of each file before removing it.
8733 @cindex files beginning with @samp{-}, removing
8734 @cindex @samp{-}, removing files beginning with
8735 One common question is how to remove files whose names begin with a
8736 @samp{-}. @sc{gnu} @command{rm}, like every program that uses the @code{getopt}
8737 function to parse its arguments, lets you use the @samp{--} option to
8738 indicate that all following arguments are non-options. To remove a file
8739 called @file{-f} in the current directory, you could type either:
8752 @opindex - @r{and Unix @command{rm}}
8753 The Unix @command{rm} program's use of a single @samp{-} for this purpose
8754 predates the development of the getopt standard syntax.
8759 @node shred invocation
8760 @section @command{shred}: Remove files more securely
8763 @cindex data, erasing
8764 @cindex erasing data
8766 @command{shred} overwrites devices or files, to help prevent even
8767 very expensive hardware from recovering the data.
8769 Ordinarily when you remove a file (@pxref{rm invocation}), the data is
8770 not actually destroyed. Only the index listing where the file is
8771 stored is destroyed, and the storage is made available for reuse.
8772 There are undelete utilities that will attempt to reconstruct the index
8773 and can bring the file back if the parts were not reused.
8775 On a busy system with a nearly-full drive, space can get reused in a few
8776 seconds. But there is no way to know for sure. If you have sensitive
8777 data, you may want to be sure that recovery is not possible by actually
8778 overwriting the file with non-sensitive data.
8780 However, even after doing that, it is possible to take the disk back
8781 to a laboratory and use a lot of sensitive (and expensive) equipment
8782 to look for the faint ``echoes'' of the original data underneath the
8783 overwritten data. If the data has only been overwritten once, it's not
8786 The best way to remove something irretrievably is to destroy the media
8787 it's on with acid, melt it down, or the like. For cheap removable media
8788 like floppy disks, this is the preferred method. However, hard drives
8789 are expensive and hard to melt, so the @command{shred} utility tries
8790 to achieve a similar effect non-destructively.
8792 This uses many overwrite passes, with the data patterns chosen to
8793 maximize the damage they do to the old data. While this will work on
8794 floppies, the patterns are designed for best effect on hard drives.
8795 For more details, see the source code and Peter Gutmann's paper
8796 @uref{http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html,
8797 @cite{Secure Deletion of Data from Magnetic and Solid-State Memory}},
8798 from the proceedings of the Sixth @acronym{USENIX} Security Symposium (San Jose,
8799 California, July 22--25, 1996).
8801 @strong{Please note} that @command{shred} relies on a very important assumption:
8802 that the file system overwrites data in place. This is the traditional
8803 way to do things, but many modern file system designs do not satisfy this
8804 assumption. Exceptions include:
8809 Log-structured or journaled file systems, such as those supplied with
8810 AIX and Solaris, and JFS, ReiserFS, XFS, Ext3 (in @code{data=journal} mode),
8811 BFS, NTFS, etc.@: when they are configured to journal @emph{data}.
8814 File systems that write redundant data and carry on even if some writes
8815 fail, such as RAID-based file systems.
8818 File systems that make snapshots, such as Network Appliance's NFS server.
8821 File systems that cache in temporary locations, such as NFS version 3
8825 Compressed file systems.
8828 In the particular case of ext3 file systems, the above disclaimer applies (and
8829 @command{shred} is thus of limited effectiveness) only in @code{data=journal}
8830 mode, which journals file data in addition to just metadata. In both
8831 the @code{data=ordered} (default) and @code{data=writeback} modes,
8832 @command{shred} works as usual. Ext3 journaling modes can be changed
8833 by adding the @code{data=something} option to the mount options for a
8834 particular file system in the @file{/etc/fstab} file, as documented in
8835 the mount man page (man mount).
8837 If you are not sure how your file system operates, then you should assume
8838 that it does not overwrite data in place, which means that shred cannot
8839 reliably operate on regular files in your file system.
8841 Generally speaking, it is more reliable to shred a device than a file,
8842 since this bypasses the problem of file system design mentioned above.
8843 However, even shredding devices is not always completely reliable. For
8844 example, most disks map out bad sectors invisibly to the application; if
8845 the bad sectors contain sensitive data, @command{shred} won't be able to
8848 @command{shred} makes no attempt to detect or report this problem, just as
8849 it makes no attempt to do anything about backups. However, since it is
8850 more reliable to shred devices than files, @command{shred} by default does
8851 not truncate or remove the output file. This default is more suitable
8852 for devices, which typically cannot be truncated and should not be
8855 Finally, consider the risk of backups and mirrors.
8856 File system backups and remote mirrors may contain copies of the
8857 file that cannot be removed, and that will allow a shredded file
8858 to be recovered later. So if you keep any data you may later want
8859 to destroy using @command{shred}, be sure that it is not backed up or mirrored.
8862 shred [@var{option}]@dots{} @var{file}[@dots{}]
8865 The program accepts the following options. Also see @ref{Common options}.
8873 @cindex force deletion
8874 Override file permissions if necessary to allow overwriting.
8877 @itemx -n @var{number}
8878 @itemx --iterations=@var{number}
8879 @opindex -n @var{number}
8880 @opindex --iterations=@var{number}
8881 @cindex iterations, selecting the number of
8882 By default, @command{shred} uses @value{SHRED_DEFAULT_PASSES} passes of
8883 overwrite. You can reduce this to save time, or increase it if you think it's
8884 appropriate. After 25 passes all of the internal overwrite patterns will have
8885 been used at least once.
8887 @item --random-source=@var{file}
8888 @opindex --random-source
8889 @cindex random source for shredding
8890 Use @var{file} as a source of random data used to overwrite and to
8891 choose pass ordering. @xref{Random sources}.
8893 @item -s @var{bytes}
8894 @itemx --size=@var{bytes}
8895 @opindex -s @var{bytes}
8896 @opindex --size=@var{bytes}
8897 @cindex size of file to shred
8898 Shred the first @var{bytes} bytes of the file. The default is to shred
8899 the whole file. @var{bytes} can be followed by a size specification like
8900 @samp{K}, @samp{M}, or @samp{G} to specify a multiple. @xref{Block size}.
8906 @cindex removing files after shredding
8907 After shredding a file, truncate it (if possible) and then remove it.
8908 If a file has multiple links, only the named links will be removed.
8914 Display to standard error all status updates as sterilization proceeds.
8920 By default, @command{shred} rounds the size of a regular file up to the next
8921 multiple of the file system block size to fully erase the last block
8923 Use @option{--exact} to suppress that behavior.
8924 Thus, by default if you shred a 10-byte regular file on a system with 512-byte
8925 blocks, the resulting file will be 512 bytes long. With this option,
8926 shred does not increase the apparent size of the file.
8932 Normally, the last pass that @command{shred} writes is made up of
8933 random data. If this would be conspicuous on your hard drive (for
8934 example, because it looks like encrypted data), or you just think
8935 it's tidier, the @option{--zero} option adds an additional overwrite pass with
8936 all zero bits. This is in addition to the number of passes specified
8937 by the @option{--iterations} option.
8941 You might use the following command to erase all trace of the
8942 file system you'd created on the floppy disk in your first drive.
8943 That command takes about 20 minutes to erase a ``1.44MB'' (actually
8947 shred --verbose /dev/fd0
8950 Similarly, to erase all data on a selected partition of
8951 your hard disk, you could give a command like this:
8954 shred --verbose /dev/sda5
8957 On modern disks, a single pass should be adequate,
8958 and it will take one third the time of the default three-pass approach.
8961 # 1 pass, write pseudo-random data; 3x faster than the default
8962 shred --verbose -n1 /dev/sda5
8965 To be on the safe side, use at least one pass that overwrites using
8966 pseudo-random data. I.e., don't be tempted to use @samp{-n0 --zero},
8967 in case some disk controller optimizes the process of writing blocks
8968 of all zeros, and thereby does not clear all bytes in a block.
8969 Some SSDs may do just that.
8971 A @var{file} of @samp{-} denotes standard output.
8972 The intended use of this is to shred a removed temporary file.
8979 echo "Hello, world" >&3
8984 However, the command @samp{shred - >file} does not shred the contents
8985 of @var{file}, since the shell truncates @var{file} before invoking
8986 @command{shred}. Use the command @samp{shred file} or (if using a
8987 Bourne-compatible shell) the command @samp{shred - 1<>file} instead.
8992 @node Special file types
8993 @chapter Special file types
8995 @cindex special file types
8996 @cindex file types, special
8998 This chapter describes commands which create special types of files (and
8999 @command{rmdir}, which removes directories, one special file type).
9001 @cindex special file types
9003 Although Unix-like operating systems have markedly fewer special file
9004 types than others, not @emph{everything} can be treated only as the
9005 undifferentiated byte stream of @dfn{normal files}. For example, when a
9006 file is created or removed, the system must record this information,
9007 which it does in a @dfn{directory}---a special type of file. Although
9008 you can read directories as normal files, if you're curious, in order
9009 for the system to do its job it must impose a structure, a certain
9010 order, on the bytes of the file. Thus it is a ``special'' type of file.
9012 Besides directories, other special file types include named pipes
9013 (FIFOs), symbolic links, sockets, and so-called @dfn{special files}.
9016 * link invocation:: Make a hard link via the link syscall
9017 * ln invocation:: Make links between files.
9018 * mkdir invocation:: Make directories.
9019 * mkfifo invocation:: Make FIFOs (named pipes).
9020 * mknod invocation:: Make block or character special files.
9021 * readlink invocation:: Print value of a symlink or canonical file name.
9022 * rmdir invocation:: Remove empty directories.
9023 * unlink invocation:: Remove files via the unlink syscall
9027 @node link invocation
9028 @section @command{link}: Make a hard link via the link syscall
9031 @cindex links, creating
9032 @cindex hard links, creating
9033 @cindex creating links (hard only)
9035 @command{link} creates a single hard link at a time.
9036 It is a minimalist interface to the system-provided
9037 @code{link} function. @xref{Hard Links, , , libc,
9038 The GNU C Library Reference Manual}.
9039 It avoids the bells and whistles of the more commonly-used
9040 @command{ln} command (@pxref{ln invocation}).
9044 link @var{filename} @var{linkname}
9047 @var{filename} must specify an existing file, and @var{linkname}
9048 must specify a nonexistent entry in an existing directory.
9049 @command{link} simply calls @code{link (@var{filename}, @var{linkname})}
9052 On a @acronym{GNU} system, this command acts like @samp{ln --directory
9053 --no-target-directory @var{filename} @var{linkname}}. However, the
9054 @option{--directory} and @option{--no-target-directory} options are
9055 not specified by @acronym{POSIX}, and the @command{link} command is
9056 more portable in practice.
9058 If @var{filename} is a symbolic link, it is unspecified whether
9059 @var{linkname} will be a hard link to the symbolic link or to the
9060 target of the symbolic link. Use @command{ln -P} or @command{ln -L}
9061 to specify which behavior is desired.
9067 @section @command{ln}: Make links between files
9070 @cindex links, creating
9071 @cindex hard links, creating
9072 @cindex symbolic (soft) links, creating
9073 @cindex creating links (hard or soft)
9075 @cindex file systems and hard links
9076 @command{ln} makes links between files. By default, it makes hard links;
9077 with the @option{-s} option, it makes symbolic (or @dfn{soft}) links.
9081 ln [@var{option}]@dots{} [-T] @var{target} @var{linkname}
9082 ln [@var{option}]@dots{} @var{target}
9083 ln [@var{option}]@dots{} @var{target}@dots{} @var{directory}
9084 ln [@var{option}]@dots{} -t @var{directory} @var{target}@dots{}
9090 If two file names are given, @command{ln} creates a link to the first
9091 file from the second.
9094 If one @var{target} is given, @command{ln} creates a link to that file
9095 in the current directory.
9098 If the @option{--target-directory} (@option{-t}) option is given, or
9099 failing that if the last file is a directory and the
9100 @option{--no-target-directory} (@option{-T}) option is not given,
9101 @command{ln} creates a link to each @var{target} file in the specified
9102 directory, using the @var{target}s' names.
9106 Normally @command{ln} does not remove existing files. Use the
9107 @option{--force} (@option{-f}) option to remove them unconditionally,
9108 the @option{--interactive} (@option{-i}) option to remove them
9109 conditionally, and the @option{--backup} (@option{-b}) option to
9112 @cindex hard link, defined
9113 @cindex inode, and hard links
9114 A @dfn{hard link} is another name for an existing file; the link and the
9115 original are indistinguishable. Technically speaking, they share the
9116 same inode, and the inode contains all the information about a
9117 file---indeed, it is not incorrect to say that the inode @emph{is} the
9118 file. Most systems prohibit making a hard link to
9119 a directory; on those where it is allowed, only the super-user can do
9120 so (and with caution, since creating a cycle will cause problems to many
9121 other utilities). Hard links cannot cross file system boundaries. (These
9122 restrictions are not mandated by @acronym{POSIX}, however.)
9124 @cindex dereferencing symbolic links
9125 @cindex symbolic link, defined
9126 @dfn{Symbolic links} (@dfn{symlinks} for short), on the other hand, are
9127 a special file type (which not all kernels support: System V release 3
9128 (and older) systems lack symlinks) in which the link file actually
9129 refers to a different file, by name. When most operations (opening,
9130 reading, writing, and so on) are passed the symbolic link file, the
9131 kernel automatically @dfn{dereferences} the link and operates on the
9132 target of the link. But some operations (e.g., removing) work on the
9133 link file itself, rather than on its target. The owner and group of a
9134 symlink are not significant to file access performed through
9135 the link, but do have implications on deleting a symbolic link from a
9136 directory with the restricted deletion bit set. On the GNU system,
9137 the mode of a symlink has no significance and cannot be changed, but
9138 on some BSD systems, the mode can be changed and will affect whether
9139 the symlink will be traversed in file name resolution. @xref{Symbolic Links,,,
9140 libc, The GNU C Library Reference Manual}.
9142 Symbolic links can contain arbitrary strings; a @dfn{dangling symlink}
9143 occurs when the string in the symlink does not resolve to a file.
9144 There are no restrictions against creating dangling symbolic links.
9145 There are trade-offs to using absolute or relative symlinks. An
9146 absolute symlink always points to the same file, even if the directory
9147 containing the link is moved. However, if the symlink is visible from
9148 more than one machine (such as on a networked file system), the file
9149 pointed to might not always be the same. A relative symbolic link is
9150 resolved in relation to the directory that contains the link, and is
9151 often useful in referring to files on the same device without regards
9152 to what name that device is mounted on when accessed via networked
9155 When creating a relative symlink in a different location than the
9156 current directory, the resolution of the symlink will be different
9157 than the resolution of the same string from the current directory.
9158 Therefore, many users prefer to first change directories to the
9159 location where the relative symlink will be created, so that
9160 tab-completion or other file resolution will find the same target as
9161 what will be placed in the symlink.
9163 The program accepts the following options. Also see @ref{Common options}.
9174 @opindex --directory
9175 @cindex hard links to directories
9176 Allow users with appropriate privileges to attempt to make hard links
9178 However, note that this will probably fail due to
9179 system restrictions, even for the super-user.
9185 Remove existing destination files.
9188 @itemx --interactive
9190 @opindex --interactive
9191 @cindex prompting, and @command{ln}
9192 Prompt whether to remove existing destination files.
9198 If @option{-s} is not in effect, and the source file is a symbolic
9199 link, create the hard link to the file referred to by the symbolic
9200 link, rather than the symbolic link itself.
9203 @itemx --no-dereference
9205 @opindex --no-dereference
9206 Do not treat the last operand specially when it is a symbolic link to
9207 a directory. Instead, treat it as if it were a normal file.
9209 When the destination is an actual directory (not a symlink to one),
9210 there is no ambiguity. The link is created in that directory.
9211 But when the specified destination is a symlink to a directory,
9212 there are two ways to treat the user's request. @command{ln} can
9213 treat the destination just as it would a normal directory and create
9214 the link in it. On the other hand, the destination can be viewed as a
9215 non-directory---as the symlink itself. In that case, @command{ln}
9216 must delete or backup that symlink before creating the new link.
9217 The default is to treat a destination that is a symlink to a directory
9218 just like a directory.
9220 This option is weaker than the @option{--no-target-directory}
9221 (@option{-T}) option, so it has no effect if both options are given.
9227 If @option{-s} is not in effect, and the source file is a symbolic
9228 link, create the hard link to the symbolic link itself. On platforms
9229 where this is not supported by the kernel, this option creates a
9230 symbolic link with identical contents; since symbolic link contents
9231 cannot be edited, any file name resolution performed through either
9232 link will be the same as if a hard link had been created.
9238 Make symbolic links instead of hard links. This option merely produces
9239 an error message on systems that do not support symbolic links.
9245 @optNoTargetDirectory
9251 Print the name of each file after linking it successfully.
9255 @cindex hard links to symbolic links
9256 @cindex symbolic links and @command{ln}
9257 If @option{-L} and @option{-P} are both given, the last one takes
9258 precedence. If @option{-s} is also given, @option{-L} and @option{-P}
9259 are silently ignored. If neither option is given, then this
9260 implementation defaults to @option{-P} if the system @code{link} supports
9261 hard links to symbolic links (such as the GNU system), and @option{-L}
9262 if @code{link} follows symbolic links (such as on BSD).
9271 # Create link ../a pointing to a in that directory.
9272 # Not really useful because it points to itself.
9277 # Change to the target before creating symlinks to avoid being confused.
9283 # Hard coded file names don't move well.
9284 ln -s $(pwd)/a /some/dir/
9288 # Relative file names survive directory moves and also
9289 # work across networked file systems.
9290 ln -s afile anotherfile
9291 ln -s ../adir/afile yetanotherfile
9295 @node mkdir invocation
9296 @section @command{mkdir}: Make directories
9299 @cindex directories, creating
9300 @cindex creating directories
9302 @command{mkdir} creates directories with the specified names. Synopsis:
9305 mkdir [@var{option}]@dots{} @var{name}@dots{}
9308 @command{mkdir} creates each directory @var{name} in the order given.
9309 It reports an error if @var{name} already exists, unless the
9310 @option{-p} option is given and @var{name} is a directory.
9312 The program accepts the following options. Also see @ref{Common options}.
9317 @itemx --mode=@var{mode}
9320 @cindex modes of created directories, setting
9321 Set the file permission bits of created directories to @var{mode},
9322 which uses the same syntax as
9323 in @command{chmod} and uses @samp{a=rwx} (read, write and execute allowed for
9324 everyone) for the point of the departure. @xref{File permissions}.
9326 Normally the directory has the desired file mode bits at the moment it
9327 is created. As a @acronym{GNU} extension, @var{mode} may also mention
9328 special mode bits, but in this case there may be a temporary window
9329 during which the directory exists but its special mode bits are
9330 incorrect. @xref{Directory Setuid and Setgid}, for how the
9331 set-user-ID and set-group-ID bits of directories are inherited unless
9332 overridden in this way.
9338 @cindex parent directories, creating
9339 Make any missing parent directories for each argument, setting their
9340 file permission bits to the umask modified by @samp{u+wx}. Ignore
9341 existing parent directories, and do not change their file permission
9344 To set the file permission bits of any newly-created parent
9345 directories to a value that includes @samp{u+wx}, you can set the
9346 umask before invoking @command{mkdir}. For example, if the shell
9347 command @samp{(umask u=rwx,go=rx; mkdir -p P/Q)} creates the parent
9348 @file{P} it sets the parent's permission bits to @samp{u=rwx,go=rx}.
9349 To set a parent's special mode bits as well, you can invoke
9350 @command{chmod} after @command{mkdir}. @xref{Directory Setuid and
9351 Setgid}, for how the set-user-ID and set-group-ID bits of
9352 newly-created parent directories are inherited.
9358 Print a message for each created directory. This is most useful with
9361 @item -Z @var{context}
9362 @itemx --context=@var{context}
9366 @cindex security context
9367 Set the default SELinux security context to be used for created directories.
9374 @node mkfifo invocation
9375 @section @command{mkfifo}: Make FIFOs (named pipes)
9378 @cindex FIFOs, creating
9379 @cindex named pipes, creating
9380 @cindex creating FIFOs (named pipes)
9382 @command{mkfifo} creates FIFOs (also called @dfn{named pipes}) with the
9383 specified names. Synopsis:
9386 mkfifo [@var{option}] @var{name}@dots{}
9389 A @dfn{FIFO} is a special file type that permits independent processes
9390 to communicate. One process opens the FIFO file for writing, and
9391 another for reading, after which data can flow as with the usual
9392 anonymous pipe in shells or elsewhere.
9394 The program accepts the following option. Also see @ref{Common options}.
9399 @itemx --mode=@var{mode}
9402 @cindex modes of created FIFOs, setting
9403 Set the mode of created FIFOs to @var{mode}, which is symbolic as in
9404 @command{chmod} and uses @samp{a=rw} (read and write allowed for everyone)
9405 for the point of departure. @var{mode} should specify only file
9406 permission bits. @xref{File permissions}.
9408 @item -Z @var{context}
9409 @itemx --context=@var{context}
9413 @cindex security context
9414 Set the default SELinux security context to be used for created FIFOs.
9421 @node mknod invocation
9422 @section @command{mknod}: Make block or character special files
9425 @cindex block special files, creating
9426 @cindex character special files, creating
9428 @command{mknod} creates a FIFO, character special file, or block special
9429 file with the specified name. Synopsis:
9432 mknod [@var{option}]@dots{} @var{name} @var{type} [@var{major} @var{minor}]
9435 @cindex special files
9436 @cindex block special files
9437 @cindex character special files
9438 Unlike the phrase ``special file type'' above, the term @dfn{special
9439 file} has a technical meaning on Unix: something that can generate or
9440 receive data. Usually this corresponds to a physical piece of hardware,
9441 e.g., a printer or a disk. (These files are typically created at
9442 system-configuration time.) The @command{mknod} command is what creates
9443 files of this type. Such devices can be read either a character at a
9444 time or a ``block'' (many characters) at a time, hence we say there are
9445 @dfn{block special} files and @dfn{character special} files.
9447 @c mknod is a shell built-in at least with OpenBSD's /bin/sh
9448 @mayConflictWithShellBuiltIn{mknod}
9450 The arguments after @var{name} specify the type of file to make:
9455 @opindex p @r{for FIFO file}
9459 @opindex b @r{for block special file}
9460 for a block special file
9463 @c Don't document the `u' option -- it's just a synonym for `c'.
9464 @c Do *any* versions of mknod still use it?
9466 @opindex c @r{for character special file}
9467 @c @opindex u @r{for character special file}
9468 for a character special file
9472 When making a block or character special file, the major and minor
9473 device numbers must be given after the file type.
9474 If a major or minor device number begins with @samp{0x} or @samp{0X},
9475 it is interpreted as hexadecimal; otherwise, if it begins with @samp{0},
9476 as octal; otherwise, as decimal.
9478 The program accepts the following option. Also see @ref{Common options}.
9483 @itemx --mode=@var{mode}
9486 Set the mode of created files to @var{mode}, which is symbolic as in
9487 @command{chmod} and uses @samp{a=rw} as the point of departure.
9488 @var{mode} should specify only file permission bits.
9489 @xref{File permissions}.
9491 @item -Z @var{context}
9492 @itemx --context=@var{context}
9496 @cindex security context
9497 Set the default SELinux security context to be used for created files.
9504 @node readlink invocation
9505 @section @command{readlink}: Print value of a symlink or canonical file name
9508 @cindex displaying value of a symbolic link
9509 @cindex canonical file name
9510 @cindex canonicalize a file name
9514 @command{readlink} may work in one of two supported modes:
9520 @command{readlink} outputs the value of the given symbolic link.
9521 If @command{readlink} is invoked with an argument other than the name
9522 of a symbolic link, it produces no output and exits with a nonzero exit code.
9524 @item Canonicalize mode
9526 @command{readlink} outputs the absolute name of the given file which contains
9527 no @file{.}, @file{..} components nor any repeated separators
9528 (@file{/}) or symbolic links.
9533 readlink [@var{option}] @var{file}
9536 By default, @command{readlink} operates in readlink mode.
9538 The program accepts the following options. Also see @ref{Common options}.
9543 @itemx --canonicalize
9545 @opindex --canonicalize
9546 Activate canonicalize mode.
9547 If any component of the file name except the last one is missing or unavailable,
9548 @command{readlink} produces no output and exits with a nonzero exit
9549 code. A trailing slash is ignored.
9552 @itemx --canonicalize-existing
9554 @opindex --canonicalize-existing
9555 Activate canonicalize mode.
9556 If any component is missing or unavailable, @command{readlink} produces
9557 no output and exits with a nonzero exit code. A trailing slash
9558 requires that the name resolve to a directory.
9561 @itemx --canonicalize-missing
9563 @opindex --canonicalize-missing
9564 Activate canonicalize mode.
9565 If any component is missing or unavailable, @command{readlink} treats it
9571 @opindex --no-newline
9572 Do not output the trailing newline.
9582 Suppress most error messages.
9588 Report error messages.
9592 The @command{readlink} utility first appeared in OpenBSD 2.1.
9594 There is a @command{realpath} command on some systems
9595 which operates like @command{readlink} in canonicalize mode.
9600 @node rmdir invocation
9601 @section @command{rmdir}: Remove empty directories
9604 @cindex removing empty directories
9605 @cindex directories, removing empty
9607 @command{rmdir} removes empty directories. Synopsis:
9610 rmdir [@var{option}]@dots{} @var{directory}@dots{}
9613 If any @var{directory} argument does not refer to an existing empty
9614 directory, it is an error.
9616 The program accepts the following options. Also see @ref{Common options}.
9620 @item --ignore-fail-on-non-empty
9621 @opindex --ignore-fail-on-non-empty
9622 @cindex directory deletion, ignoring failures
9623 Ignore each failure to remove a directory that is solely because
9624 the directory is non-empty.
9630 @cindex parent directories, removing
9631 Remove @var{directory}, then try to remove each component of @var{directory}.
9632 So, for example, @samp{rmdir -p a/b/c} is similar to @samp{rmdir a/b/c a/b a}.
9633 As such, it fails if any of those directories turns out not to be empty.
9634 Use the @option{--ignore-fail-on-non-empty} option to make it so such
9635 a failure does not evoke a diagnostic and does not cause @command{rmdir} to
9636 exit unsuccessfully.
9642 @cindex directory deletion, reporting
9643 Give a diagnostic for each successful removal.
9644 @var{directory} is removed.
9648 @xref{rm invocation}, for how to remove non-empty directories (recursively).
9653 @node unlink invocation
9654 @section @command{unlink}: Remove files via the unlink syscall
9657 @cindex removing files or directories (via the unlink syscall)
9659 @command{unlink} deletes a single specified file name.
9660 It is a minimalist interface to the system-provided
9661 @code{unlink} function. @xref{Deleting Files, , , libc,
9662 The GNU C Library Reference Manual}. Synopsis:
9663 It avoids the bells and whistles of the more commonly-used
9664 @command{rm} command (@pxref{rm invocation}).
9667 unlink @var{filename}
9670 On some systems @code{unlink} can be used to delete the name of a
9671 directory. On others, it can be used that way only by a privileged user.
9672 In the GNU system @code{unlink} can never delete the name of a directory.
9674 The @command{unlink} command honors the @option{--help} and
9675 @option{--version} options. To remove a file whose name begins with
9676 @samp{-}, prefix the name with @samp{./}, e.g., @samp{unlink ./--help}.
9681 @node Changing file attributes
9682 @chapter Changing file attributes
9684 @cindex changing file attributes
9685 @cindex file attributes, changing
9686 @cindex attributes, file
9688 A file is not merely its contents, a name, and a file type
9689 (@pxref{Special file types}). A file also has an owner (a user ID), a
9690 group (a group ID), permissions (what the owner can do with the file,
9691 what people in the group can do, and what everyone else can do), various
9692 timestamps, and other information. Collectively, we call these a file's
9695 These commands change file attributes.
9698 * chgrp invocation:: Change file groups.
9699 * chmod invocation:: Change access permissions.
9700 * chown invocation:: Change file owners and groups.
9701 * touch invocation:: Change file timestamps.
9705 @node chown invocation
9706 @section @command{chown}: Change file owner and group
9709 @cindex file ownership, changing
9710 @cindex group ownership, changing
9711 @cindex changing file ownership
9712 @cindex changing group ownership
9714 @command{chown} changes the user and/or group ownership of each given @var{file}
9715 to @var{new-owner} or to the user and group of an existing reference file.
9719 chown [@var{option}]@dots{} @{@var{new-owner} | --reference=@var{ref_file}@}@c
9723 If used, @var{new-owner} specifies the new owner and/or group as follows
9724 (with no embedded white space):
9727 [@var{owner}] [ : [@var{group}] ]
9734 If only an @var{owner} (a user name or numeric user ID) is given, that
9735 user is made the owner of each given file, and the files' group is not
9738 @item owner@samp{:}group
9739 If the @var{owner} is followed by a colon and a @var{group} (a
9740 group name or numeric group ID), with no spaces between them, the group
9741 ownership of the files is changed as well (to @var{group}).
9744 If a colon but no group name follows @var{owner}, that user is
9745 made the owner of the files and the group of the files is changed to
9746 @var{owner}'s login group.
9749 If the colon and following @var{group} are given, but the owner
9750 is omitted, only the group of the files is changed; in this case,
9751 @command{chown} performs the same function as @command{chgrp}.
9754 If only a colon is given, or if @var{new-owner} is empty, neither the
9755 owner nor the group is changed.
9759 If @var{owner} or @var{group} is intended to represent a numeric user
9760 or group ID, then you may specify it with a leading @samp{+}.
9761 @xref{Disambiguating names and IDs}.
9763 Some older scripts may still use @samp{.} in place of the @samp{:} separator.
9764 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) does not
9765 require support for that, but for backward compatibility @acronym{GNU}
9766 @command{chown} supports @samp{.} so long as no ambiguity results.
9767 New scripts should avoid the use of @samp{.} because it is not
9768 portable, and because it has undesirable results if the entire
9769 @var{owner@samp{.}group} happens to identify a user whose name
9772 The @command{chown} command sometimes clears the set-user-ID or
9773 set-group-ID permission bits. This behavior depends on the policy and
9774 functionality of the underlying @code{chown} system call, which may
9775 make system-dependent file mode modifications outside the control of
9776 the @command{chown} command. For example, the @command{chown} command
9777 might not affect those bits when invoked by a user with appropriate
9778 privileges, or when the
9779 bits signify some function other than executable permission (e.g.,
9781 When in doubt, check the underlying system behavior.
9783 The program accepts the following options. Also see @ref{Common options}.
9791 @cindex changed owners, verbosely describing
9792 Verbosely describe the action for each @var{file} whose ownership
9801 @cindex error messages, omitting
9802 Do not print error messages about files whose ownership cannot be
9805 @itemx @w{@kbd{--from}=@var{old-owner}}
9807 @cindex symbolic links, changing owner
9808 Change a @var{file}'s ownership only if it has current attributes specified
9809 by @var{old-owner}. @var{old-owner} has the same form as @var{new-owner}
9811 This option is useful primarily from a security standpoint in that
9812 it narrows considerably the window of potential abuse.
9813 For example, to reflect a user ID numbering change for one user's files
9814 without an option like this, @code{root} might run
9817 find / -owner OLDUSER -print0 | xargs -0 chown -h NEWUSER
9820 But that is dangerous because the interval between when the @command{find}
9821 tests the existing file's owner and when the @command{chown} is actually run
9823 One way to narrow the gap would be to invoke chown for each file
9827 find / -owner OLDUSER -exec chown -h NEWUSER @{@} \;
9830 But that is very slow if there are many affected files.
9831 With this option, it is safer (the gap is narrower still)
9832 though still not perfect:
9835 chown -h -R --from=OLDUSER NEWUSER /
9839 @opindex --dereference
9840 @cindex symbolic links, changing owner
9842 Do not act on symbolic links themselves but rather on what they point to.
9843 This is the default.
9846 @itemx --no-dereference
9848 @opindex --no-dereference
9849 @cindex symbolic links, changing owner
9851 Act on symbolic links themselves instead of what they point to.
9852 This mode relies on the @code{lchown} system call.
9853 On systems that do not provide the @code{lchown} system call,
9854 @command{chown} fails when a file specified on the command line
9856 By default, no diagnostic is issued for symbolic links encountered
9857 during a recursive traversal, but see @option{--verbose}.
9859 @itemx --preserve-root
9860 @opindex --preserve-root
9861 @cindex root directory, disallow recursive modification
9862 Fail upon any attempt to recursively change the root directory, @file{/}.
9863 Without @option{--recursive}, this option has no effect.
9864 @xref{Treating / specially}.
9866 @itemx --no-preserve-root
9867 @opindex --no-preserve-root
9868 @cindex root directory, allow recursive modification
9869 Cancel the effect of any preceding @option{--preserve-root} option.
9870 @xref{Treating / specially}.
9872 @item --reference=@var{ref_file}
9873 @opindex --reference
9874 Change the user and group of each @var{file} to be the same as those of
9875 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9876 user and group of the symbolic link, but rather those of the file it
9883 Output a diagnostic for every file processed.
9884 If a symbolic link is encountered during a recursive traversal
9885 on a system without the @code{lchown} system call, and @option{--no-dereference}
9886 is in effect, then issue a diagnostic saying neither the symbolic link nor
9887 its referent is being changed.
9892 @opindex --recursive
9893 @cindex recursively changing file ownership
9894 Recursively change ownership of directories and their contents.
9897 @xref{Traversing symlinks}.
9900 @xref{Traversing symlinks}.
9903 @xref{Traversing symlinks}.
9912 # Change the owner of /u to "root".
9915 # Likewise, but also change its group to "staff".
9918 # Change the owner of /u and subfiles to "root".
9923 @node chgrp invocation
9924 @section @command{chgrp}: Change group ownership
9927 @cindex group ownership, changing
9928 @cindex changing group ownership
9930 @command{chgrp} changes the group ownership of each given @var{file}
9931 to @var{group} (which can be either a group name or a numeric group ID)
9932 or to the group of an existing reference file. Synopsis:
9935 chgrp [@var{option}]@dots{} @{@var{group} | --reference=@var{ref_file}@}@c
9939 If @var{group} is intended to represent a
9940 numeric group ID, then you may specify it with a leading @samp{+}.
9941 @xref{Disambiguating names and IDs}.
9943 The program accepts the following options. Also see @ref{Common options}.
9951 @cindex changed files, verbosely describing
9952 Verbosely describe the action for each @var{file} whose group actually
9961 @cindex error messages, omitting
9962 Do not print error messages about files whose group cannot be
9966 @opindex --dereference
9967 @cindex symbolic links, changing owner
9969 Do not act on symbolic links themselves but rather on what they point to.
9970 This is the default.
9973 @itemx --no-dereference
9975 @opindex --no-dereference
9976 @cindex symbolic links, changing group
9978 Act on symbolic links themselves instead of what they point to.
9979 This mode relies on the @code{lchown} system call.
9980 On systems that do not provide the @code{lchown} system call,
9981 @command{chgrp} fails when a file specified on the command line
9983 By default, no diagnostic is issued for symbolic links encountered
9984 during a recursive traversal, but see @option{--verbose}.
9986 @itemx --preserve-root
9987 @opindex --preserve-root
9988 @cindex root directory, disallow recursive modification
9989 Fail upon any attempt to recursively change the root directory, @file{/}.
9990 Without @option{--recursive}, this option has no effect.
9991 @xref{Treating / specially}.
9993 @itemx --no-preserve-root
9994 @opindex --no-preserve-root
9995 @cindex root directory, allow recursive modification
9996 Cancel the effect of any preceding @option{--preserve-root} option.
9997 @xref{Treating / specially}.
9999 @item --reference=@var{ref_file}
10000 @opindex --reference
10001 Change the group of each @var{file} to be the same as that of
10002 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
10003 group of the symbolic link, but rather that of the file it refers to.
10009 Output a diagnostic for every file processed.
10010 If a symbolic link is encountered during a recursive traversal
10011 on a system without the @code{lchown} system call, and @option{--no-dereference}
10012 is in effect, then issue a diagnostic saying neither the symbolic link nor
10013 its referent is being changed.
10018 @opindex --recursive
10019 @cindex recursively changing group ownership
10020 Recursively change the group ownership of directories and their contents.
10023 @xref{Traversing symlinks}.
10026 @xref{Traversing symlinks}.
10029 @xref{Traversing symlinks}.
10038 # Change the group of /u to "staff".
10041 # Change the group of /u and subfiles to "staff".
10046 @node chmod invocation
10047 @section @command{chmod}: Change access permissions
10050 @cindex changing access permissions
10051 @cindex access permissions, changing
10052 @cindex permissions, changing access
10054 @command{chmod} changes the access permissions of the named files. Synopsis:
10057 chmod [@var{option}]@dots{} @{@var{mode} | --reference=@var{ref_file}@}@c
10061 @cindex symbolic links, permissions of
10062 @command{chmod} never changes the permissions of symbolic links, since
10063 the @command{chmod} system call cannot change their permissions.
10064 This is not a problem since the permissions of symbolic links are
10065 never used. However, for each symbolic link listed on the command
10066 line, @command{chmod} changes the permissions of the pointed-to file.
10067 In contrast, @command{chmod} ignores symbolic links encountered during
10068 recursive directory traversals.
10070 A successful use of @command{chmod} clears the set-group-ID bit of a
10071 regular file if the file's group ID does not match the user's
10072 effective group ID or one of the user's supplementary group IDs,
10073 unless the user has appropriate privileges. Additional restrictions
10074 may cause the set-user-ID and set-group-ID bits of @var{mode} or
10075 @var{ref_file} to be ignored. This behavior depends on the policy and
10076 functionality of the underlying @code{chmod} system call. When in
10077 doubt, check the underlying system behavior.
10079 If used, @var{mode} specifies the new file mode bits.
10080 For details, see the section on @ref{File permissions}.
10081 If you really want @var{mode} to have a leading @samp{-}, you should
10082 use @option{--} first, e.g., @samp{chmod -- -w file}. Typically,
10083 though, @samp{chmod a-w file} is preferable, and @command{chmod -w
10084 file} (without the @option{--}) complains if it behaves differently
10085 from what @samp{chmod a-w file} would do.
10087 The program accepts the following options. Also see @ref{Common options}.
10095 Verbosely describe the action for each @var{file} whose permissions
10104 @cindex error messages, omitting
10105 Do not print error messages about files whose permissions cannot be
10108 @itemx --preserve-root
10109 @opindex --preserve-root
10110 @cindex root directory, disallow recursive modification
10111 Fail upon any attempt to recursively change the root directory, @file{/}.
10112 Without @option{--recursive}, this option has no effect.
10113 @xref{Treating / specially}.
10115 @itemx --no-preserve-root
10116 @opindex --no-preserve-root
10117 @cindex root directory, allow recursive modification
10118 Cancel the effect of any preceding @option{--preserve-root} option.
10119 @xref{Treating / specially}.
10125 Verbosely describe the action or non-action taken for every @var{file}.
10127 @item --reference=@var{ref_file}
10128 @opindex --reference
10129 Change the mode of each @var{file} to be the same as that of @var{ref_file}.
10130 @xref{File permissions}.
10131 If @var{ref_file} is a symbolic link, do not use the mode
10132 of the symbolic link, but rather that of the file it refers to.
10137 @opindex --recursive
10138 @cindex recursively changing access permissions
10139 Recursively change permissions of directories and their contents.
10146 @node touch invocation
10147 @section @command{touch}: Change file timestamps
10150 @cindex changing file timestamps
10151 @cindex file timestamps, changing
10152 @cindex timestamps, changing file
10154 @command{touch} changes the access and/or modification times of the
10155 specified files. Synopsis:
10158 touch [@var{option}]@dots{} @var{file}@dots{}
10161 @cindex empty files, creating
10162 Any @var{file} argument that does not exist is created empty, unless
10163 option @option{--no-create} (@option{-c}) or @option{--no-dereference}
10164 (@option{-h}) was in effect.
10166 A @var{file} argument string of @samp{-} is handled specially and
10167 causes @command{touch} to change the times of the file associated with
10170 @cindex permissions, for changing file timestamps
10171 If changing both the access and modification times to the current
10172 time, @command{touch} can change the timestamps for files that the user
10173 running it does not own but has write permission for. Otherwise, the
10174 user must own the files.
10176 Although @command{touch} provides options for changing two of the times---the
10177 times of last access and modification---of a file, there is actually
10178 a standard third one as well: the inode change time. This is often
10179 referred to as a file's @code{ctime}.
10180 The inode change time represents the time when the file's meta-information
10181 last changed. One common example of this is when the permissions of a
10182 file change. Changing the permissions doesn't access the file, so
10183 the atime doesn't change, nor does it modify the file, so the mtime
10184 doesn't change. Yet, something about the file itself has changed,
10185 and this must be noted somewhere. This is the job of the ctime field.
10186 This is necessary, so that, for example, a backup program can make a
10187 fresh copy of the file, including the new permissions value.
10188 Another operation that modifies a file's ctime without affecting
10189 the others is renaming. In any case, it is not possible, in normal
10190 operations, for a user to change the ctime field to a user-specified value.
10191 Some operating systems and file systems support a fourth time: the
10192 birth time, when the file was first created; by definition, this
10193 timestamp never changes.
10196 Time stamps assume the time zone rules specified by the @env{TZ}
10197 environment variable, or by the system default rules if @env{TZ} is
10198 not set. @xref{TZ Variable,, Specifying the Time Zone with @env{TZ},
10199 libc, The GNU C Library Reference Manual}.
10200 You can avoid ambiguities during
10201 daylight saving transitions by using @sc{utc} time stamps.
10203 The program accepts the following options. Also see @ref{Common options}.
10208 @itemx --time=atime
10209 @itemx --time=access
10213 @opindex atime@r{, changing}
10214 @opindex access @r{time, changing}
10215 @opindex use @r{time, changing}
10216 Change the access time only.
10221 @opindex --no-create
10222 Do not warn about or create files that do not exist.
10225 @itemx --date=@var{time}
10229 Use @var{time} instead of the current time. It can contain month names,
10230 time zones, @samp{am} and @samp{pm}, @samp{yesterday}, etc. For
10231 example, @option{--date="2004-02-27 14:19:13.489392193 +0530"}
10232 specifies the instant of time that is 489,392,193 nanoseconds after
10233 February 27, 2004 at 2:19:13 PM in a time zone that is 5 hours and 30
10234 minutes east of @acronym{UTC}. @xref{Date input formats}.
10235 File systems that do not support high-resolution time stamps
10236 silently ignore any excess precision here.
10240 @cindex BSD @command{touch} compatibility
10241 Ignored; for compatibility with BSD versions of @command{touch}.
10244 @itemx --no-dereference
10246 @opindex --no-dereference
10247 @cindex symbolic links, changing time
10249 Attempt to change the timestamps of a symbolic link, rather than what
10250 the link refers to. When using this option, empty files are not
10251 created, but option @option{-c} must also be used to avoid warning
10252 about files that do not exist. Not all systems support changing the
10253 timestamps of symlinks, since underlying system support for this
10254 action was not required until @acronym{POSIX} 2008. Also, on some
10255 systems, the mere act of examining a symbolic link changes the access
10256 time, such that only changes to the modification time will persist
10257 long enough to be observable. When coupled with option @option{-r}, a
10258 reference timestamp is taken from a symbolic link rather than the file
10262 @itemx --time=mtime
10263 @itemx --time=modify
10266 @opindex mtime@r{, changing}
10267 @opindex modify @r{time, changing}
10268 Change the modification time only.
10270 @item -r @var{file}
10271 @itemx --reference=@var{file}
10273 @opindex --reference
10274 Use the times of the reference @var{file} instead of the current time.
10275 If this option is combined with the @option{--date=@var{time}}
10276 (@option{-d @var{time}}) option, the reference @var{file}'s time is
10277 the origin for any relative @var{time}s given, but is otherwise ignored.
10278 For example, @samp{-r foo -d '-5 seconds'} specifies a time stamp
10279 equal to five seconds before the corresponding time stamp for @file{foo}.
10280 If @var{file} is a symbolic link, the reference timestamp is taken
10281 from the target of the symlink, unless @option{-h} was also in effect.
10283 @item -t [[@var{cc}]@var{yy}]@var{mmddhhmm}[.@var{ss}]
10284 Use the argument (optional four-digit or two-digit years, months,
10285 days, hours, minutes, optional seconds) instead of the current time.
10286 If the year is specified with only two digits, then @var{cc}
10287 is 20 for years in the range 0 @dots{} 68, and 19 for years in
10288 69 @dots{} 99. If no digits of the year are specified,
10289 the argument is interpreted as a date in the current year.
10290 Note that @var{ss} may be @samp{60}, to accommodate leap seconds.
10294 @vindex _POSIX2_VERSION
10295 On older systems, @command{touch} supports an obsolete syntax, as follows.
10296 If no timestamp is given with any of the @option{-d}, @option{-r}, or
10297 @option{-t} options, and if there are two or more @var{file}s and the
10298 first @var{file} is of the form @samp{@var{mmddhhmm}[@var{yy}]} and this
10299 would be a valid argument to the @option{-t} option (if the @var{yy}, if
10300 any, were moved to the front), and if the represented year
10301 is in the range 1969--1999, that argument is interpreted as the time
10302 for the other files instead of as a file name.
10303 This obsolete behavior can be enabled or disabled with the
10304 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
10305 conformance}), but portable scripts should avoid commands whose
10306 behavior depends on this variable.
10307 For example, use @samp{touch ./12312359 main.c} or @samp{touch -t
10308 12312359 main.c} rather than the ambiguous @samp{touch 12312359 main.c}.
10314 @chapter Disk usage
10318 No disk can hold an infinite amount of data. These commands report
10319 how much disk storage is in use or available, report other file and
10320 file status information, and write buffers to disk.
10323 * df invocation:: Report file system disk space usage.
10324 * du invocation:: Estimate file space usage.
10325 * stat invocation:: Report file or file system status.
10326 * sync invocation:: Synchronize memory and disk.
10327 * truncate invocation:: Shrink or extend the size of a file.
10331 @node df invocation
10332 @section @command{df}: Report file system disk space usage
10335 @cindex file system disk usage
10336 @cindex disk usage by file system
10338 @command{df} reports the amount of disk space used and available on
10339 file systems. Synopsis:
10342 df [@var{option}]@dots{} [@var{file}]@dots{}
10345 With no arguments, @command{df} reports the space used and available on all
10346 currently mounted file systems (of all types). Otherwise, @command{df}
10347 reports on the file system containing each argument @var{file}.
10349 Normally the disk space is printed in units of
10350 1024 bytes, but this can be overridden (@pxref{Block size}).
10351 Non-integer quantities are rounded up to the next higher unit.
10353 @cindex disk device file
10354 @cindex device file, disk
10355 If an argument @var{file} is a disk device file containing a mounted
10356 file system, @command{df} shows the space available on that file system
10357 rather than on the file system containing the device node (i.e., the root
10358 file system). @sc{gnu} @command{df} does not attempt to determine the
10360 on unmounted file systems, because on most kinds of systems doing so
10361 requires extremely nonportable intimate knowledge of file system
10364 The program accepts the following options. Also see @ref{Common options}.
10372 @cindex automounter file systems
10373 @cindex ignore file systems
10374 Include in the listing dummy file systems, which
10375 are omitted by default. Such file systems are typically special-purpose
10376 pseudo-file-systems, such as automounter entries.
10378 @item -B @var{size}
10379 @itemx --block-size=@var{size}
10381 @opindex --block-size
10382 @cindex file system sizes
10383 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10384 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10388 @cindex grand total of disk size, usage and available space
10389 Print a grand total of all arguments after all arguments have
10390 been processed. This can be used to find out the total disk size, usage
10391 and available space of all listed devices.
10397 Equivalent to @option{--si}.
10403 @cindex inode usage
10404 List inode usage information instead of block usage. An inode (short
10405 for index node) contains information about a file such as its owner,
10406 permissions, timestamps, and location on the disk.
10410 @cindex kibibytes for file system sizes
10411 Print sizes in 1024-byte blocks, overriding the default block size
10412 (@pxref{Block size}).
10413 This option is equivalent to @option{--block-size=1K}.
10419 @cindex file system types, limiting output to certain
10420 Limit the listing to local file systems. By default, remote file systems
10425 @cindex file system space, retrieving old data more quickly
10426 Do not invoke the @code{sync} system call before getting any usage data.
10427 This may make @command{df} run significantly faster on systems with many
10428 disks, but on some systems (notably SunOS) the results may be slightly
10429 out of date. This is the default.
10432 @itemx --portability
10434 @opindex --portability
10435 @cindex one-line output format
10436 @cindex @acronym{POSIX} output format
10437 @cindex portable output format
10438 @cindex output format, portable
10439 Use the @acronym{POSIX} output format. This is like the default format except
10444 The information about each file system is always printed on exactly
10445 one line; a mount device is never put on a line by itself. This means
10446 that if the mount device name is more than 20 characters long (e.g., for
10447 some network mounts), the columns are misaligned.
10450 The labels in the header output line are changed to conform to @acronym{POSIX}.
10453 The default block size and output format are unaffected by the
10454 @env{DF_BLOCK_SIZE}, @env{BLOCK_SIZE} and @env{BLOCKSIZE} environment
10455 variables. However, the default block size is still affected by
10456 @env{POSIXLY_CORRECT}: it is 512 if @env{POSIXLY_CORRECT} is set, 1024
10457 otherwise. @xref{Block size}.
10464 @cindex file system space, retrieving current data more slowly
10465 Invoke the @code{sync} system call before getting any usage data. On
10466 some systems (notably SunOS), doing this yields more up to date results,
10467 but in general this option makes @command{df} much slower, especially when
10468 there are many or very busy file systems.
10470 @item -t @var{fstype}
10471 @itemx --type=@var{fstype}
10474 @cindex file system types, limiting output to certain
10475 Limit the listing to file systems of type @var{fstype}. Multiple
10476 file system types can be specified by giving multiple @option{-t} options.
10477 By default, nothing is omitted.
10480 @itemx --print-type
10482 @opindex --print-type
10483 @cindex file system types, printing
10484 Print each file system's type. The types printed here are the same ones
10485 you can include or exclude with @option{-t} and @option{-x}. The particular
10486 types printed are whatever is supported by the system. Here are some of
10487 the common names (this list is certainly not exhaustive):
10492 @cindex @acronym{NFS} file system type
10493 An @acronym{NFS} file system, i.e., one mounted over a network from another
10494 machine. This is the one type name which seems to be used uniformly by
10497 @item 4.2@r{, }ufs@r{, }efs@dots{}
10498 @cindex Linux file system types
10499 @cindex local file system types
10500 @opindex 4.2 @r{file system type}
10501 @opindex ufs @r{file system type}
10502 @opindex efs @r{file system type}
10503 A file system on a locally-mounted hard disk. (The system might even
10504 support more than one type here; Linux does.)
10506 @item hsfs@r{, }cdfs
10507 @cindex CD-ROM file system type
10508 @cindex High Sierra file system
10509 @opindex hsfs @r{file system type}
10510 @opindex cdfs @r{file system type}
10511 A file system on a CD-ROM drive. HP-UX uses @samp{cdfs}, most other
10512 systems use @samp{hsfs} (@samp{hs} for ``High Sierra'').
10515 @cindex PC file system
10516 @cindex DOS file system
10517 @cindex MS-DOS file system
10518 @cindex diskette file system
10520 An MS-DOS file system, usually on a diskette.
10524 @item -x @var{fstype}
10525 @itemx --exclude-type=@var{fstype}
10527 @opindex --exclude-type
10528 Limit the listing to file systems not of type @var{fstype}.
10529 Multiple file system types can be eliminated by giving multiple
10530 @option{-x} options. By default, no file system types are omitted.
10533 Ignored; for compatibility with System V versions of @command{df}.
10538 Failure includes the case where no output is generated, so you can
10539 inspect the exit status of a command like @samp{df -t ext3 -t reiserfs
10540 @var{dir}} to test whether @var{dir} is on a file system of type
10541 @samp{ext3} or @samp{reiserfs}.
10544 @node du invocation
10545 @section @command{du}: Estimate file space usage
10548 @cindex file space usage
10549 @cindex disk usage for files
10551 @command{du} reports the amount of disk space used by the specified files
10552 and for each subdirectory (of directory arguments). Synopsis:
10555 du [@var{option}]@dots{} [@var{file}]@dots{}
10558 With no arguments, @command{du} reports the disk space for the current
10559 directory. Normally the disk space is printed in units of
10560 1024 bytes, but this can be overridden (@pxref{Block size}).
10561 Non-integer quantities are rounded up to the next higher unit.
10563 If two or more hard links point to the same file, only one of the hard
10564 links is counted. The @var{file} argument order affects which links
10565 are counted, and changing the argument order may change the numbers
10566 that @command{du} outputs.
10568 The program accepts the following options. Also see @ref{Common options}.
10576 Show counts for all files, not just directories.
10578 @itemx --apparent-size
10579 @opindex --apparent-size
10580 Print apparent sizes, rather than disk usage. The apparent size of a
10581 file is the number of bytes reported by @code{wc -c} on regular files,
10582 or more generally, @code{ls -l --block-size=1} or @code{stat --format=%s}.
10583 For example, a file containing the word @samp{zoo} with no newline would,
10584 of course, have an apparent size of 3. Such a small file may require
10585 anywhere from 0 to 16 KiB or more of disk space, depending on
10586 the type and configuration of the file system on which the file resides.
10587 However, a sparse file created with this command:
10590 dd bs=1 seek=2GiB if=/dev/null of=big
10594 has an apparent size of 2 GiB, yet on most modern
10595 systems, it actually uses almost no disk space.
10601 Equivalent to @code{--apparent-size --block-size=1}.
10603 @item -B @var{size}
10604 @itemx --block-size=@var{size}
10606 @opindex --block-size
10608 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10609 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10615 @cindex grand total of disk space
10616 Print a grand total of all arguments after all arguments have
10617 been processed. This can be used to find out the total disk usage of
10618 a given set of files or directories.
10621 @itemx --dereference-args
10623 @opindex --dereference-args
10624 Dereference symbolic links that are command line arguments.
10625 Does not affect other symbolic links. This is helpful for finding
10626 out the disk usage of directories, such as @file{/usr/tmp}, which
10627 are often symbolic links.
10629 @c --files0-from=FILE
10630 @filesZeroFromOption{du,, with the @option{--total} (@option{-c}) option}
10636 Equivalent to @option{--dereference-args} (@option{-D}).
10640 @cindex kibibytes for file sizes
10641 Print sizes in 1024-byte blocks, overriding the default block size
10642 (@pxref{Block size}).
10643 This option is equivalent to @option{--block-size=1K}.
10646 @itemx --count-links
10648 @opindex --count-links
10649 @cindex hard links, counting in @command{du}
10650 Count the size of all files, even if they have appeared already (as a
10654 @itemx --dereference
10656 @opindex --dereference
10657 @cindex symbolic links, dereferencing in @command{du}
10658 Dereference symbolic links (show the disk space used by the file
10659 or directory that the link points to instead of the space used by
10664 @cindex mebibytes for file sizes
10665 Print sizes in 1,048,576-byte blocks, overriding the default block size
10666 (@pxref{Block size}).
10667 This option is equivalent to @option{--block-size=1M}.
10670 @itemx --no-dereference
10672 @opindex --no-dereference
10673 @cindex symbolic links, dereferencing in @command{du}
10674 For each symbolic links encountered by @command{du},
10675 consider the disk space used by the symbolic link.
10677 @item -d @var{depth}
10678 @item --max-depth=@var{depth}
10679 @opindex -d @var{depth}
10680 @opindex --max-depth=@var{depth}
10681 @cindex limiting output of @command{du}
10682 Show the total for each directory (and file if --all) that is at
10683 most MAX_DEPTH levels down from the root of the hierarchy. The root
10684 is at level 0, so @code{du --max-depth=0} is equivalent to @code{du -s}.
10693 @opindex --summarize
10694 Display only a total for each argument.
10697 @itemx --separate-dirs
10699 @opindex --separate-dirs
10700 Normally, in the output of @command{du} (when not using @option{--summarize}),
10701 the size listed next to a directory name, @var{d}, represents the sum
10702 of sizes of all entries beneath @var{d} as well as the size of @var{d} itself.
10703 With @option{--separate-dirs}, the size reported for a directory name,
10704 @var{d}, is merely the @code{stat.st_size}-derived size of the directory
10709 @cindex last modified dates, displaying in @command{du}
10710 Show time of the most recent modification of any file in the directory,
10711 or any of its subdirectories.
10713 @itemx --time=ctime
10714 @itemx --time=status
10717 @opindex ctime@r{, show the most recent}
10718 @opindex status time@r{, show the most recent}
10719 @opindex use time@r{, show the most recent}
10720 Show the most recent status change time (the @samp{ctime} in the inode) of
10721 any file in the directory, instead of the modification time.
10723 @itemx --time=atime
10724 @itemx --time=access
10726 @opindex atime@r{, show the most recent}
10727 @opindex access time@r{, show the most recent}
10728 Show the most recent access time (the @samp{atime} in the inode) of
10729 any file in the directory, instead of the modification time.
10731 @item --time-style=@var{style}
10732 @opindex --time-style
10734 List timestamps in style @var{style}. This option has an effect only if
10735 the @option{--time} option is also specified. The @var{style} should
10736 be one of the following:
10739 @item +@var{format}
10741 List timestamps using @var{format}, where @var{format} is interpreted
10742 like the format argument of @command{date} (@pxref{date invocation}).
10743 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
10744 @command{du} to list timestamps like @samp{2002-03-30 23:45:56}. As
10745 with @command{date}, @var{format}'s interpretation is affected by the
10746 @env{LC_TIME} locale category.
10749 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
10750 format with nanosecond precision, e.g., @samp{2002-03-30
10751 23:45:56.477817180 -0700}. This style is equivalent to
10752 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
10755 List @acronym{ISO} 8601 date and time in minutes, e.g.,
10756 @samp{2002-03-30 23:45}. These timestamps are shorter than
10757 @samp{full-iso} timestamps, and are usually good enough for everyday
10758 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
10761 List @acronym{ISO} 8601 dates for timestamps, e.g., @samp{2002-03-30}.
10762 This style is equivalent to @samp{+%Y-%m-%d}.
10766 You can specify the default value of the @option{--time-style} option
10767 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
10768 the default style is @samp{long-iso}. For compatibility with @command{ls},
10769 if @env{TIME_STYLE} begins with @samp{+} and contains a newline,
10770 the newline and any later characters are ignored; if @env{TIME_STYLE}
10771 begins with @samp{posix-} the @samp{posix-} is ignored; and if
10772 @env{TIME_STYLE} is @samp{locale} it is ignored.
10775 @itemx --one-file-system
10777 @opindex --one-file-system
10778 @cindex one file system, restricting @command{du} to
10779 Skip directories that are on different file systems from the one that
10780 the argument being processed is on.
10782 @item --exclude=@var{pattern}
10783 @opindex --exclude=@var{pattern}
10784 @cindex excluding files from @command{du}
10785 When recursing, skip subdirectories or files matching @var{pattern}.
10786 For example, @code{du --exclude='*.o'} excludes files whose names
10789 @item -X @var{file}
10790 @itemx --exclude-from=@var{file}
10791 @opindex -X @var{file}
10792 @opindex --exclude-from=@var{file}
10793 @cindex excluding files from @command{du}
10794 Like @option{--exclude}, except take the patterns to exclude from @var{file},
10795 one per line. If @var{file} is @samp{-}, take the patterns from standard
10800 @cindex NFS mounts from BSD to HP-UX
10801 On BSD systems, @command{du} reports sizes that are half the correct
10802 values for files that are NFS-mounted from HP-UX systems. On HP-UX
10803 systems, it reports sizes that are twice the correct values for
10804 files that are NFS-mounted from BSD systems. This is due to a flaw
10805 in HP-UX; it also affects the HP-UX @command{du} program.
10810 @node stat invocation
10811 @section @command{stat}: Report file or file system status
10814 @cindex file status
10815 @cindex file system status
10817 @command{stat} displays information about the specified file(s). Synopsis:
10820 stat [@var{option}]@dots{} [@var{file}]@dots{}
10823 With no option, @command{stat} reports all information about the given files.
10824 But it also can be used to report the information of the file systems the
10825 given files are located on. If the files are links, @command{stat} can
10826 also give information about the files the links point to.
10828 @mayConflictWithShellBuiltIn{stat}
10833 @itemx --dereference
10835 @opindex --dereference
10836 @cindex symbolic links, dereferencing in @command{stat}
10837 Change how @command{stat} treats symbolic links.
10838 With this option, @command{stat} acts on the file referenced
10839 by each symbolic link argument.
10840 Without it, @command{stat} acts on any symbolic link argument directly.
10843 @itemx --file-system
10845 @opindex --file-system
10846 @cindex file systems
10847 Report information about the file systems where the given files are located
10848 instead of information about the files themselves.
10849 This option implies the @option{-L} option.
10852 @itemx --format=@var{format}
10854 @opindex --format=@var{format}
10855 @cindex output format
10856 Use @var{format} rather than the default format.
10857 @var{format} is automatically newline-terminated, so
10858 running a command like the following with two or more @var{file}
10859 operands produces a line of output for each operand:
10861 $ stat --format=%d:%i / /usr
10866 @itemx --printf=@var{format}
10867 @opindex --printf=@var{format}
10868 @cindex output format
10869 Use @var{format} rather than the default format.
10870 Like @option{--format}, but interpret backslash escapes,
10871 and do not output a mandatory trailing newline.
10872 If you want a newline, include @samp{\n} in the @var{format}.
10873 Here's how you would use @option{--printf} to print the device
10874 and inode numbers of @file{/} and @file{/usr}:
10876 $ stat --printf='%d:%i\n' / /usr
10885 @cindex terse output
10886 Print the information in terse form, suitable for parsing by other programs.
10890 The valid @var{format} directives for files with @option{--format} and
10891 @option{--printf} are:
10894 @item %a - Access rights in octal
10895 @item %A - Access rights in human readable form
10896 @item %b - Number of blocks allocated (see @samp{%B})
10897 @item %B - The size in bytes of each block reported by @samp{%b}
10898 @item %C - The SELinux security context of a file, if available
10899 @item %d - Device number in decimal
10900 @item %D - Device number in hex
10901 @item %f - Raw mode in hex
10902 @item %F - File type
10903 @item %g - Group ID of owner
10904 @item %G - Group name of owner
10905 @item %h - Number of hard links
10906 @item %i - Inode number
10907 @item %m - Mount point (See note below)
10908 @item %n - File name
10909 @item %N - Quoted file name with dereference if symbolic link
10910 @item %o - I/O block size
10911 @item %s - Total size, in bytes
10912 @item %t - Major device type in hex
10913 @item %T - Minor device type in hex
10914 @item %u - User ID of owner
10915 @item %U - User name of owner
10916 @item %w - Time of file birth, or @samp{-} if unknown
10917 @item %W - Time of file birth as seconds since Epoch, or @samp{0}
10918 @item %x - Time of last access
10919 @item %X - Time of last access as seconds since Epoch
10920 @item %y - Time of last modification
10921 @item %Y - Time of last modification as seconds since Epoch
10922 @item %z - Time of last change
10923 @item %Z - Time of last change as seconds since Epoch
10926 The @samp{%W}, @samp{%X}, @samp{%Y}, and @samp{%Z} formats accept a
10927 precision preceded by a period to specify the number of digits to
10928 print after the decimal point. For example, @samp{%.3X} outputs the
10929 last access time to millisecond precision. If a period is given but no
10930 precision, @command{stat} uses 9 digits, so @samp{%.X} is equivalent to
10931 @samp{%.9X}. When discarding excess precision, time stamps are truncated
10932 toward minus infinity.
10936 $ stat -c '[%015Y]' /usr
10939 $ stat -c '[%15Y]' /usr
10941 $ stat -c '[%-15Y]' /usr
10944 $ stat -c '[%.3Y]' /usr
10946 $ stat -c '[%.Y]' /usr
10947 [1288929712.114951834]
10950 The mount point printed by @samp{%m} is similar to that output
10951 by @command{df}, except that:
10954 stat does not dereference symlinks by default
10955 (unless @option{-L} is specified)
10957 stat does not search for specified device nodes in the
10958 file system list, instead operating on them directly
10961 stat outputs the alias for a bind mounted file, rather than
10962 the initial mount point of its backing device.
10963 One can recursively call stat until there is no change in output,
10964 to get the current base mount point
10967 When listing file system information (@option{--file-system} (@option{-f})),
10968 you must use a different set of @var{format} directives:
10971 @item %a - Free blocks available to non-super-user
10972 @item %b - Total data blocks in file system
10973 @item %c - Total file nodes in file system
10974 @item %d - Free file nodes in file system
10975 @item %f - Free blocks in file system
10976 @item %i - File System ID in hex
10977 @item %l - Maximum length of file names
10978 @item %n - File name
10979 @item %s - Block size (for faster transfers)
10980 @item %S - Fundamental block size (for block counts)
10981 @item %t - Type in hex
10982 @item %T - Type in human readable form
10986 Time stamps are listed according to the time zone rules specified by
10987 the @env{TZ} environment variable, or by the system default rules if
10988 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
10989 with @env{TZ}, libc, The GNU C Library Reference Manual}.
10994 @node sync invocation
10995 @section @command{sync}: Synchronize data on disk with memory
10998 @cindex synchronize disk and memory
11000 @cindex superblock, writing
11001 @cindex inodes, written buffered
11002 @command{sync} writes any data buffered in memory out to disk. This can
11003 include (but is not limited to) modified superblocks, modified inodes,
11004 and delayed reads and writes. This must be implemented by the kernel;
11005 The @command{sync} program does nothing but exercise the @code{sync} system
11008 @cindex crashes and corruption
11009 The kernel keeps data in memory to avoid doing (relatively slow) disk
11010 reads and writes. This improves performance, but if the computer
11011 crashes, data may be lost or the file system corrupted as a
11012 result. The @command{sync} command ensures everything in memory
11013 is written to disk.
11015 Any arguments are ignored, except for a lone @option{--help} or
11016 @option{--version} (@pxref{Common options}).
11021 @node truncate invocation
11022 @section @command{truncate}: Shrink or extend the size of a file
11025 @cindex truncating, file sizes
11027 @command{truncate} shrinks or extends the size of each @var{file} to the
11028 specified size. Synopsis:
11031 truncate @var{option}@dots{} @var{file}@dots{}
11034 @cindex files, creating
11035 Any @var{file} that does not exist is created.
11037 @cindex sparse files, creating
11038 @cindex holes, creating files with
11039 If a @var{file} is larger than the specified size, the extra data is lost.
11040 If a @var{file} is shorter, it is extended and the extended part (or hole)
11041 reads as zero bytes.
11043 The program accepts the following options. Also see @ref{Common options}.
11050 @opindex --no-create
11051 Do not create files that do not exist.
11056 @opindex --io-blocks
11057 Treat @var{size} as number of I/O blocks of the @var{file} rather than bytes.
11059 @item -r @var{rfile}
11060 @itemx --reference=@var{rfile}
11062 @opindex --reference
11063 Base the size of each @var{file} on the size of @var{rfile}.
11065 @item -s @var{size}
11066 @itemx --size=@var{size}
11069 Set or adjust the size of each @var{file} according to @var{size}.
11070 @multiplierSuffixesNoBlocks{size}
11072 @var{size} may also be prefixed by one of the following to adjust
11073 the size of each @var{file} based on their current size:
11075 @samp{+} => extend by
11076 @samp{-} => reduce by
11077 @samp{<} => at most
11078 @samp{>} => at least
11079 @samp{/} => round down to multiple of
11080 @samp{%} => round up to multiple of
11088 @node Printing text
11089 @chapter Printing text
11091 @cindex printing text, commands for
11092 @cindex commands for printing text
11094 This section describes commands that display text strings.
11097 * echo invocation:: Print a line of text.
11098 * printf invocation:: Format and print data.
11099 * yes invocation:: Print a string until interrupted.
11103 @node echo invocation
11104 @section @command{echo}: Print a line of text
11107 @cindex displaying text
11108 @cindex printing text
11109 @cindex text, displaying
11110 @cindex arbitrary text, displaying
11112 @command{echo} writes each given @var{string} to standard output, with a
11113 space between each and a newline after the last one. Synopsis:
11116 echo [@var{option}]@dots{} [@var{string}]@dots{}
11119 @mayConflictWithShellBuiltIn{echo}
11121 The program accepts the following options. Also see @ref{Common options}.
11122 Options must precede operands, and the normally-special argument
11123 @samp{--} has no special meaning and is treated like any other
11129 Do not output the trailing newline.
11133 @cindex backslash escapes
11134 Enable interpretation of the following backslash-escaped characters in
11143 produce no further output
11159 the eight-bit value that is the octal number @var{nnn}
11160 (zero to three octal digits), if @var{nnn} is
11161 a nine-bit value, the ninth bit is ignored
11163 the eight-bit value that is the octal number @var{nnn}
11164 (one to three octal digits), if @var{nnn} is
11165 a nine-bit value, the ninth bit is ignored
11167 the eight-bit value that is the hexadecimal number @var{hh}
11168 (one or two hexadecimal digits)
11173 @cindex backslash escapes
11174 Disable interpretation of backslash escapes in each @var{string}.
11175 This is the default. If @option{-e} and @option{-E} are both
11176 specified, the last one given takes effect.
11180 @vindex POSIXLY_CORRECT
11181 If the @env{POSIXLY_CORRECT} environment variable is set, then when
11182 @command{echo}'s first argument is not @option{-n} it outputs
11183 option-like arguments instead of treating them as options. For
11184 example, @code{echo -ne hello} outputs @samp{-ne hello} instead of
11185 plain @samp{hello}.
11187 @acronym{POSIX} does not require support for any options, and says
11188 that the behavior of @command{echo} is implementation-defined if any
11189 @var{string} contains a backslash or if the first argument is
11190 @option{-n}. Portable programs can use the @command{printf} command
11191 if they need to omit trailing newlines or output control characters or
11192 backslashes. @xref{printf invocation}.
11197 @node printf invocation
11198 @section @command{printf}: Format and print data
11201 @command{printf} does formatted printing of text. Synopsis:
11204 printf @var{format} [@var{argument}]@dots{}
11207 @command{printf} prints the @var{format} string, interpreting @samp{%}
11208 directives and @samp{\} escapes to format numeric and string arguments
11209 in a way that is mostly similar to the C @samp{printf} function.
11210 @xref{Output Conversion Syntax,, @command{printf} format directives,
11211 libc, The GNU C Library Reference Manual}, for details.
11212 The differences are listed below.
11214 @mayConflictWithShellBuiltIn{printf}
11219 The @var{format} argument is reused as necessary to convert all the
11220 given @var{argument}s. For example, the command @samp{printf %s a b}
11224 Missing @var{argument}s are treated as null strings or as zeros,
11225 depending on whether the context expects a string or a number. For
11226 example, the command @samp{printf %sx%d} prints @samp{x0}.
11230 An additional escape, @samp{\c}, causes @command{printf} to produce no
11231 further output. For example, the command @samp{printf 'A%sC\cD%sF' B
11232 E} prints @samp{ABC}.
11235 The hexadecimal escape sequence @samp{\x@var{hh}} has at most two
11236 digits, as opposed to C where it can have an unlimited number of
11237 digits. For example, the command @samp{printf '\x07e'} prints two
11238 bytes, whereas the C statement @samp{printf ("\x07e")} prints just
11243 @command{printf} has an additional directive, @samp{%b}, which prints its
11244 argument string with @samp{\} escapes interpreted in the same way as in
11245 the @var{format} string, except that octal escapes are of the form
11246 @samp{\0@var{ooo}} where @var{ooo} is 0 to 3 octal digits. If
11247 @samp{\@var{ooo}} is nine-bit value, ignore the ninth bit.
11248 If a precision is also given, it limits the number of bytes printed
11249 from the converted string.
11252 Numeric arguments must be single C constants, possibly with leading
11253 @samp{+} or @samp{-}. For example, @samp{printf %.4d -3} outputs
11257 @vindex POSIXLY_CORRECT
11258 If the leading character of a numeric argument is @samp{"} or @samp{'}
11259 then its value is the numeric value of the immediately following
11260 character. Any remaining characters are silently ignored if the
11261 @env{POSIXLY_CORRECT} environment variable is set; otherwise, a
11262 warning is printed. For example, @samp{printf "%d" "'a"} outputs
11263 @samp{97} on hosts that use the @acronym{ASCII} character set, since
11264 @samp{a} has the numeric value 97 in @acronym{ASCII}.
11269 A floating-point argument must use a period before any fractional
11270 digits, but is printed according to the @env{LC_NUMERIC} category of the
11271 current locale. For example, in a locale whose radix character is a
11272 comma, the command @samp{printf %g 3.14} outputs @samp{3,14} whereas
11273 the command @samp{printf %g 3,14} is an error.
11274 @xref{Floating point}.
11278 @command{printf} interprets @samp{\@var{ooo}} in @var{format} as an octal number
11279 (if @var{ooo} is 1 to 3 octal digits) specifying a byte to print,
11280 and @samp{\x@var{hh}} as a hexadecimal number (if @var{hh} is 1 to 2 hex
11281 digits) specifying a character to print.
11282 Note however that when @samp{\@var{ooo}} specifies a number larger than 255,
11283 @command{printf} ignores the ninth bit.
11284 For example, @samp{printf '\400'} is equivalent to @samp{printf '\0'}.
11289 @cindex ISO/IEC 10646
11291 @command{printf} interprets two character syntaxes introduced in
11292 @acronym{ISO} C 99:
11293 @samp{\u} for 16-bit Unicode (@acronym{ISO}/@acronym{IEC} 10646)
11294 characters, specified as
11295 four hexadecimal digits @var{hhhh}, and @samp{\U} for 32-bit Unicode
11296 characters, specified as eight hexadecimal digits @var{hhhhhhhh}.
11297 @command{printf} outputs the Unicode characters
11298 according to the @env{LC_CTYPE} locale. Unicode characters in the ranges
11299 U+0000...U+009F, U+D800...U+DFFF cannot be specified by this syntax, except
11300 for U+0024 ($), U+0040 (@@), and U+0060 (@`).
11302 The processing of @samp{\u} and @samp{\U} requires a full-featured
11303 @code{iconv} facility. It is activated on systems with glibc 2.2 (or newer),
11304 or when @code{libiconv} is installed prior to this package. Otherwise
11305 @samp{\u} and @samp{\U} will print as-is.
11307 The only options are a lone @option{--help} or
11308 @option{--version}. @xref{Common options}.
11309 Options must precede operands.
11311 The Unicode character syntaxes are useful for writing strings in a locale
11312 independent way. For example, a string containing the Euro currency symbol
11315 $ env printf '\u20AC 14.95'
11319 will be output correctly in all locales supporting the Euro symbol
11320 (@acronym{ISO}-8859-15, UTF-8, and others). Similarly, a Chinese string
11323 $ env printf '\u4e2d\u6587'
11327 will be output correctly in all Chinese locales (GB2312, BIG5, UTF-8, etc).
11329 Note that in these examples, the @command{printf} command has been
11330 invoked via @command{env} to ensure that we run the program found via
11331 your shell's search path, and not a shell alias or a built-in function.
11333 For larger strings, you don't need to look up the hexadecimal code
11334 values of each character one by one. @acronym{ASCII} characters mixed with \u
11335 escape sequences is also known as the JAVA source file encoding. You can
11336 use GNU recode 3.5c (or newer) to convert strings to this encoding. Here
11337 is how to convert a piece of text into a shell script which will output
11338 this text in a locale-independent way:
11341 $ LC_CTYPE=zh_CN.big5 /usr/local/bin/printf \
11342 '\u4e2d\u6587\n' > sample.txt
11343 $ recode BIG5..JAVA < sample.txt \
11344 | sed -e "s|^|/usr/local/bin/printf '|" -e "s|$|\\\\n'|" \
11351 @node yes invocation
11352 @section @command{yes}: Print a string until interrupted
11355 @cindex repeated output of a string
11357 @command{yes} prints the command line arguments, separated by spaces and
11358 followed by a newline, forever until it is killed. If no arguments are
11359 given, it prints @samp{y} followed by a newline forever until killed.
11361 Upon a write error, @command{yes} exits with status @samp{1}.
11363 The only options are a lone @option{--help} or @option{--version}.
11364 To output an argument that begins with
11365 @samp{-}, precede it with @option{--}, e.g., @samp{yes -- --help}.
11366 @xref{Common options}.
11370 @chapter Conditions
11373 @cindex commands for exit status
11374 @cindex exit status commands
11376 This section describes commands that are primarily useful for their exit
11377 status, rather than their output. Thus, they are often used as the
11378 condition of shell @code{if} statements, or as the last command in a
11382 * false invocation:: Do nothing, unsuccessfully.
11383 * true invocation:: Do nothing, successfully.
11384 * test invocation:: Check file types and compare values.
11385 * expr invocation:: Evaluate expressions.
11389 @node false invocation
11390 @section @command{false}: Do nothing, unsuccessfully
11393 @cindex do nothing, unsuccessfully
11394 @cindex failure exit status
11395 @cindex exit status of @command{false}
11397 @command{false} does nothing except return an exit status of 1, meaning
11398 @dfn{failure}. It can be used as a place holder in shell scripts
11399 where an unsuccessful command is needed.
11400 In most modern shells, @command{false} is a built-in command, so when
11401 you use @samp{false} in a script, you're probably using the built-in
11402 command, not the one documented here.
11404 @command{false} honors the @option{--help} and @option{--version} options.
11406 This version of @command{false} is implemented as a C program, and is thus
11407 more secure and faster than a shell script implementation, and may safely
11408 be used as a dummy shell for the purpose of disabling accounts.
11410 Note that @command{false} (unlike all other programs documented herein)
11411 exits unsuccessfully, even when invoked with
11412 @option{--help} or @option{--version}.
11414 Portable programs should not assume that the exit status of
11415 @command{false} is 1, as it is greater than 1 on some
11416 non-@acronym{GNU} hosts.
11419 @node true invocation
11420 @section @command{true}: Do nothing, successfully
11423 @cindex do nothing, successfully
11425 @cindex successful exit
11426 @cindex exit status of @command{true}
11428 @command{true} does nothing except return an exit status of 0, meaning
11429 @dfn{success}. It can be used as a place holder in shell scripts
11430 where a successful command is needed, although the shell built-in
11431 command @code{:} (colon) may do the same thing faster.
11432 In most modern shells, @command{true} is a built-in command, so when
11433 you use @samp{true} in a script, you're probably using the built-in
11434 command, not the one documented here.
11436 @command{true} honors the @option{--help} and @option{--version} options.
11438 Note, however, that it is possible to cause @command{true}
11439 to exit with nonzero status: with the @option{--help} or @option{--version}
11440 option, and with standard
11441 output already closed or redirected to a file that evokes an I/O error.
11442 For example, using a Bourne-compatible shell:
11445 $ ./true --version >&-
11446 ./true: write error: Bad file number
11447 $ ./true --version > /dev/full
11448 ./true: write error: No space left on device
11451 This version of @command{true} is implemented as a C program, and is thus
11452 more secure and faster than a shell script implementation, and may safely
11453 be used as a dummy shell for the purpose of disabling accounts.
11455 @node test invocation
11456 @section @command{test}: Check file types and compare values
11459 @cindex check file types
11460 @cindex compare values
11461 @cindex expression evaluation
11463 @command{test} returns a status of 0 (true) or 1 (false) depending on the
11464 evaluation of the conditional expression @var{expr}. Each part of the
11465 expression must be a separate argument.
11467 @command{test} has file status checks, string operators, and numeric
11468 comparison operators.
11470 @command{test} has an alternate form that uses opening and closing
11471 square brackets instead a leading @samp{test}. For example, instead
11472 of @samp{test -d /}, you can write @samp{[ -d / ]}. The square
11473 brackets must be separate arguments; for example, @samp{[-d /]} does
11474 not have the desired effect. Since @samp{test @var{expr}} and @samp{[
11475 @var{expr} ]} have the same meaning, only the former form is discussed
11481 test @var{expression}
11483 [ @var{expression} ]
11488 @mayConflictWithShellBuiltIn{test}
11490 If @var{expression} is omitted, @command{test} returns false.
11491 If @var{expression} is a single argument,
11492 @command{test} returns false if the argument is null and true
11493 otherwise. The argument
11494 can be any string, including strings like @samp{-d}, @samp{-1},
11495 @samp{--}, @samp{--help}, and @samp{--version} that most other
11496 programs would treat as options. To get help and version information,
11497 invoke the commands @samp{[ --help} and @samp{[ --version}, without
11498 the usual closing brackets. @xref{Common options}.
11500 @cindex exit status of @command{test}
11504 0 if the expression is true,
11505 1 if the expression is false,
11506 2 if an error occurred.
11510 * File type tests:: -[bcdfhLpSt]
11511 * Access permission tests:: -[gkruwxOG]
11512 * File characteristic tests:: -e -s -nt -ot -ef
11513 * String tests:: -z -n = !=
11514 * Numeric tests:: -eq -ne -lt -le -gt -ge
11515 * Connectives for test:: ! -a -o
11519 @node File type tests
11520 @subsection File type tests
11522 @cindex file type tests
11524 These options test for particular types of files. (Everything's a file,
11525 but not all files are the same!)
11529 @item -b @var{file}
11531 @cindex block special check
11532 True if @var{file} exists and is a block special device.
11534 @item -c @var{file}
11536 @cindex character special check
11537 True if @var{file} exists and is a character special device.
11539 @item -d @var{file}
11541 @cindex directory check
11542 True if @var{file} exists and is a directory.
11544 @item -f @var{file}
11546 @cindex regular file check
11547 True if @var{file} exists and is a regular file.
11549 @item -h @var{file}
11550 @itemx -L @var{file}
11553 @cindex symbolic link check
11554 True if @var{file} exists and is a symbolic link.
11555 Unlike all other file-related tests, this test does not dereference
11556 @var{file} if it is a symbolic link.
11558 @item -p @var{file}
11560 @cindex named pipe check
11561 True if @var{file} exists and is a named pipe.
11563 @item -S @var{file}
11565 @cindex socket check
11566 True if @var{file} exists and is a socket.
11570 @cindex terminal check
11571 True if @var{fd} is a file descriptor that is associated with a
11577 @node Access permission tests
11578 @subsection Access permission tests
11580 @cindex access permission tests
11581 @cindex permission tests
11583 These options test for particular access permissions.
11587 @item -g @var{file}
11589 @cindex set-group-ID check
11590 True if @var{file} exists and has its set-group-ID bit set.
11592 @item -k @var{file}
11594 @cindex sticky bit check
11595 True if @var{file} exists and has its @dfn{sticky} bit set.
11597 @item -r @var{file}
11599 @cindex readable file check
11600 True if @var{file} exists and read permission is granted.
11602 @item -u @var{file}
11604 @cindex set-user-ID check
11605 True if @var{file} exists and has its set-user-ID bit set.
11607 @item -w @var{file}
11609 @cindex writable file check
11610 True if @var{file} exists and write permission is granted.
11612 @item -x @var{file}
11614 @cindex executable file check
11615 True if @var{file} exists and execute permission is granted
11616 (or search permission, if it is a directory).
11618 @item -O @var{file}
11620 @cindex owned by effective user ID check
11621 True if @var{file} exists and is owned by the current effective user ID.
11623 @item -G @var{file}
11625 @cindex owned by effective group ID check
11626 True if @var{file} exists and is owned by the current effective group ID.
11630 @node File characteristic tests
11631 @subsection File characteristic tests
11633 @cindex file characteristic tests
11635 These options test other file characteristics.
11639 @item -e @var{file}
11641 @cindex existence-of-file check
11642 True if @var{file} exists.
11644 @item -s @var{file}
11646 @cindex nonempty file check
11647 True if @var{file} exists and has a size greater than zero.
11649 @item @var{file1} -nt @var{file2}
11651 @cindex newer-than file check
11652 True if @var{file1} is newer (according to modification date) than
11653 @var{file2}, or if @var{file1} exists and @var{file2} does not.
11655 @item @var{file1} -ot @var{file2}
11657 @cindex older-than file check
11658 True if @var{file1} is older (according to modification date) than
11659 @var{file2}, or if @var{file2} exists and @var{file1} does not.
11661 @item @var{file1} -ef @var{file2}
11663 @cindex same file check
11664 @cindex hard link check
11665 True if @var{file1} and @var{file2} have the same device and inode
11666 numbers, i.e., if they are hard links to each other.
11672 @subsection String tests
11674 @cindex string tests
11676 These options test string characteristics. You may need to quote
11677 @var{string} arguments for the shell. For example:
11683 The quotes here prevent the wrong arguments from being passed to
11684 @command{test} if @samp{$V} is empty or contains special characters.
11688 @item -z @var{string}
11690 @cindex zero-length string check
11691 True if the length of @var{string} is zero.
11693 @item -n @var{string}
11694 @itemx @var{string}
11696 @cindex nonzero-length string check
11697 True if the length of @var{string} is nonzero.
11699 @item @var{string1} = @var{string2}
11701 @cindex equal string check
11702 True if the strings are equal.
11704 @item @var{string1} != @var{string2}
11706 @cindex not-equal string check
11707 True if the strings are not equal.
11712 @node Numeric tests
11713 @subsection Numeric tests
11715 @cindex numeric tests
11716 @cindex arithmetic tests
11718 Numeric relational operators. The arguments must be entirely numeric
11719 (possibly negative), or the special expression @w{@code{-l @var{string}}},
11720 which evaluates to the length of @var{string}.
11724 @item @var{arg1} -eq @var{arg2}
11725 @itemx @var{arg1} -ne @var{arg2}
11726 @itemx @var{arg1} -lt @var{arg2}
11727 @itemx @var{arg1} -le @var{arg2}
11728 @itemx @var{arg1} -gt @var{arg2}
11729 @itemx @var{arg1} -ge @var{arg2}
11736 These arithmetic binary operators return true if @var{arg1} is equal,
11737 not-equal, less-than, less-than-or-equal, greater-than, or
11738 greater-than-or-equal than @var{arg2}, respectively.
11745 test -1 -gt -2 && echo yes
11747 test -l abc -gt 1 && echo yes
11750 @error{} test: integer expression expected before -eq
11754 @node Connectives for test
11755 @subsection Connectives for @command{test}
11757 @cindex logical connectives
11758 @cindex connectives, logical
11760 The usual logical connectives.
11766 True if @var{expr} is false.
11768 @item @var{expr1} -a @var{expr2}
11770 @cindex logical and operator
11771 @cindex and operator
11772 True if both @var{expr1} and @var{expr2} are true.
11774 @item @var{expr1} -o @var{expr2}
11776 @cindex logical or operator
11777 @cindex or operator
11778 True if either @var{expr1} or @var{expr2} is true.
11783 @node expr invocation
11784 @section @command{expr}: Evaluate expressions
11787 @cindex expression evaluation
11788 @cindex evaluation of expressions
11790 @command{expr} evaluates an expression and writes the result on standard
11791 output. Each token of the expression must be a separate argument.
11793 Operands are either integers or strings. Integers consist of one or
11794 more decimal digits, with an optional leading @samp{-}.
11795 @command{expr} converts
11796 anything appearing in an operand position to an integer or a string
11797 depending on the operation being applied to it.
11799 Strings are not quoted for @command{expr} itself, though you may need to
11800 quote them to protect characters with special meaning to the shell,
11801 e.g., spaces. However, regardless of whether it is quoted, a string
11802 operand should not be a parenthesis or any of @command{expr}'s
11803 operators like @code{+}, so you cannot safely pass an arbitrary string
11804 @code{$str} to expr merely by quoting it to the shell. One way to
11805 work around this is to use the @sc{gnu} extension @code{+},
11806 (e.g., @code{+ "$str" = foo}); a more portable way is to use
11807 @code{@w{" $str"}} and to adjust the rest of the expression to take
11808 the leading space into account (e.g., @code{@w{" $str" = " foo"}}).
11810 You should not pass a negative integer or a string with leading
11811 @samp{-} as @command{expr}'s first argument, as it might be
11812 misinterpreted as an option; this can be avoided by parenthesization.
11813 Also, portable scripts should not use a string operand that happens to
11814 take the form of an integer; this can be worked around by inserting
11815 leading spaces as mentioned above.
11817 @cindex parentheses for grouping
11818 Operators may be given as infix symbols or prefix keywords. Parentheses
11819 may be used for grouping in the usual manner. You must quote
11820 parentheses and many operators to avoid the shell evaluating them,
11823 When built with support for the GNU MP library, @command{expr} uses
11824 arbitrary-precision arithmetic; otherwise, it uses native arithmetic
11825 types and may fail due to arithmetic overflow.
11827 The only options are @option{--help} and @option{--version}. @xref{Common
11828 options}. Options must precede operands.
11830 @cindex exit status of @command{expr}
11834 0 if the expression is neither null nor 0,
11835 1 if the expression is null or 0,
11836 2 if the expression is invalid,
11837 3 if an internal error occurred (e.g., arithmetic overflow).
11841 * String expressions:: + : match substr index length
11842 * Numeric expressions:: + - * / %
11843 * Relations for expr:: | & < <= = == != >= >
11844 * Examples of expr:: Examples.
11848 @node String expressions
11849 @subsection String expressions
11851 @cindex string expressions
11852 @cindex expressions, string
11854 @command{expr} supports pattern matching and other string operators. These
11855 have higher precedence than both the numeric and relational operators (in
11856 the next sections).
11860 @item @var{string} : @var{regex}
11861 @cindex pattern matching
11862 @cindex regular expression matching
11863 @cindex matching patterns
11864 Perform pattern matching. The arguments are converted to strings and the
11865 second is considered to be a (basic, a la GNU @code{grep}) regular
11866 expression, with a @code{^} implicitly prepended. The first argument is
11867 then matched against this regular expression.
11869 If the match succeeds and @var{regex} uses @samp{\(} and @samp{\)}, the
11870 @code{:} expression returns the part of @var{string} that matched the
11871 subexpression; otherwise, it returns the number of characters matched.
11873 If the match fails, the @code{:} operator returns the null string if
11874 @samp{\(} and @samp{\)} are used in @var{regex}, otherwise 0.
11876 @kindex \( @r{regexp operator}
11877 Only the first @samp{\( @dots{} \)} pair is relevant to the return
11878 value; additional pairs are meaningful only for grouping the regular
11879 expression operators.
11881 @kindex \+ @r{regexp operator}
11882 @kindex \? @r{regexp operator}
11883 @kindex \| @r{regexp operator}
11884 In the regular expression, @code{\+}, @code{\?}, and @code{\|} are
11885 operators which respectively match one or more, zero or one, or separate
11886 alternatives. SunOS and other @command{expr}'s treat these as regular
11887 characters. (@acronym{POSIX} allows either behavior.)
11888 @xref{Top, , Regular Expression Library, regex, Regex}, for details of
11889 regular expression syntax. Some examples are in @ref{Examples of expr}.
11891 @item match @var{string} @var{regex}
11893 An alternative way to do pattern matching. This is the same as
11894 @w{@samp{@var{string} : @var{regex}}}.
11896 @item substr @var{string} @var{position} @var{length}
11898 Returns the substring of @var{string} beginning at @var{position}
11899 with length at most @var{length}. If either @var{position} or
11900 @var{length} is negative, zero, or non-numeric, returns the null string.
11902 @item index @var{string} @var{charset}
11904 Returns the first position in @var{string} where the first character in
11905 @var{charset} was found. If no character in @var{charset} is found in
11906 @var{string}, return 0.
11908 @item length @var{string}
11910 Returns the length of @var{string}.
11912 @item + @var{token}
11914 Interpret @var{token} as a string, even if it is a keyword like @var{match}
11915 or an operator like @code{/}.
11916 This makes it possible to test @code{expr length + "$x"} or
11917 @code{expr + "$x" : '.*/\(.\)'} and have it do the right thing even if
11918 the value of @var{$x} happens to be (for example) @code{/} or @code{index}.
11919 This operator is a @acronym{GNU} extension. Portable shell scripts should use
11920 @code{@w{" $token"} : @w{' \(.*\)'}} instead of @code{+ "$token"}.
11924 To make @command{expr} interpret keywords as strings, you must use the
11925 @code{quote} operator.
11928 @node Numeric expressions
11929 @subsection Numeric expressions
11931 @cindex numeric expressions
11932 @cindex expressions, numeric
11934 @command{expr} supports the usual numeric operators, in order of increasing
11935 precedence. These numeric operators have lower precedence than the
11936 string operators described in the previous section, and higher precedence
11937 than the connectives (next section).
11945 @cindex subtraction
11946 Addition and subtraction. Both arguments are converted to integers;
11947 an error occurs if this cannot be done.
11953 @cindex multiplication
11956 Multiplication, division, remainder. Both arguments are converted to
11957 integers; an error occurs if this cannot be done.
11962 @node Relations for expr
11963 @subsection Relations for @command{expr}
11965 @cindex connectives, logical
11966 @cindex logical connectives
11967 @cindex relations, numeric or string
11969 @command{expr} supports the usual logical connectives and relations. These
11970 have lower precedence than the string and numeric operators
11971 (previous sections). Here is the list, lowest-precedence operator first.
11977 @cindex logical or operator
11978 @cindex or operator
11979 Returns its first argument if that is neither null nor zero, otherwise
11980 its second argument if it is neither null nor zero, otherwise 0. It
11981 does not evaluate its second argument if its first argument is neither
11986 @cindex logical and operator
11987 @cindex and operator
11988 Return its first argument if neither argument is null or zero, otherwise
11989 0. It does not evaluate its second argument if its first argument is
11992 @item < <= = == != >= >
11999 @cindex comparison operators
12001 Compare the arguments and return 1 if the relation is true, 0 otherwise.
12002 @code{==} is a synonym for @code{=}. @command{expr} first tries to convert
12003 both arguments to integers and do a numeric comparison; if either
12004 conversion fails, it does a lexicographic comparison using the character
12005 collating sequence specified by the @env{LC_COLLATE} locale.
12010 @node Examples of expr
12011 @subsection Examples of using @command{expr}
12013 @cindex examples of @command{expr}
12014 Here are a few examples, including quoting for shell metacharacters.
12016 To add 1 to the shell variable @code{foo}, in Bourne-compatible shells:
12019 foo=`expr $foo + 1`
12022 To print the non-directory part of the file name stored in
12023 @code{$fname}, which need not contain a @code{/}:
12026 expr $fname : '.*/\(.*\)' '|' $fname
12029 An example showing that @code{\+} is an operator:
12037 expr abc : 'a\(.\)c'
12039 expr index abcdef cz
12042 @error{} expr: syntax error
12043 expr index + index a
12049 @chapter Redirection
12051 @cindex redirection
12052 @cindex commands for redirection
12054 Unix shells commonly provide several forms of @dfn{redirection}---ways
12055 to change the input source or output destination of a command. But one
12056 useful redirection is performed by a separate command, not by the shell;
12057 it's described here.
12060 * tee invocation:: Redirect output to multiple files or processes.
12064 @node tee invocation
12065 @section @command{tee}: Redirect output to multiple files or processes
12068 @cindex pipe fitting
12069 @cindex destinations, multiple output
12070 @cindex read from stdin and write to stdout and files
12072 The @command{tee} command copies standard input to standard output and also
12073 to any files given as arguments. This is useful when you want not only
12074 to send some data down a pipe, but also to save a copy. Synopsis:
12077 tee [@var{option}]@dots{} [@var{file}]@dots{}
12080 If a file being written to does not already exist, it is created. If a
12081 file being written to already exists, the data it previously contained
12082 is overwritten unless the @option{-a} option is used.
12084 A @var{file} of @samp{-} causes @command{tee} to send another copy of
12085 input to standard output, but this is typically not that useful as the
12086 copies are interleaved.
12088 The program accepts the following options. Also see @ref{Common options}.
12095 Append standard input to the given files rather than overwriting
12099 @itemx --ignore-interrupts
12101 @opindex --ignore-interrupts
12102 Ignore interrupt signals.
12106 The @command{tee} command is useful when you happen to be transferring a large
12107 amount of data and also want to summarize that data without reading
12108 it a second time. For example, when you are downloading a DVD image,
12109 you often want to verify its signature or checksum right away.
12110 The inefficient way to do it is simply:
12113 wget http://example.com/some.iso && sha1sum some.iso
12116 One problem with the above is that it makes you wait for the
12117 download to complete before starting the time-consuming SHA1 computation.
12118 Perhaps even more importantly, the above requires reading
12119 the DVD image a second time (the first was from the network).
12121 The efficient way to do it is to interleave the download
12122 and SHA1 computation. Then, you'll get the checksum for
12123 free, because the entire process parallelizes so well:
12126 # slightly contrived, to demonstrate process substitution
12127 wget -O - http://example.com/dvd.iso \
12128 | tee >(sha1sum > dvd.sha1) > dvd.iso
12131 That makes @command{tee} write not just to the expected output file,
12132 but also to a pipe running @command{sha1sum} and saving the final
12133 checksum in a file named @file{dvd.sha1}.
12135 Note, however, that this example relies on a feature of modern shells
12136 called @dfn{process substitution}
12137 (the @samp{>(command)} syntax, above;
12138 @xref{Process Substitution,,Process Substitution, bashref,
12139 The Bash Reference Manual}.),
12140 so it works with @command{zsh}, @command{bash}, and @command{ksh},
12141 but not with @command{/bin/sh}. So if you write code like this
12142 in a shell script, be sure to start the script with @samp{#!/bin/bash}.
12144 Since the above example writes to one file and one process,
12145 a more conventional and portable use of @command{tee} is even better:
12148 wget -O - http://example.com/dvd.iso \
12149 | tee dvd.iso | sha1sum > dvd.sha1
12152 You can extend this example to make @command{tee} write to two processes,
12153 computing MD5 and SHA1 checksums in parallel. In this case,
12154 process substitution is required:
12157 wget -O - http://example.com/dvd.iso \
12158 | tee >(sha1sum > dvd.sha1) \
12159 >(md5sum > dvd.md5) \
12163 This technique is also useful when you want to make a @emph{compressed}
12164 copy of the contents of a pipe.
12165 Consider a tool to graphically summarize disk usage data from @samp{du -ak}.
12166 For a large hierarchy, @samp{du -ak} can run for a long time,
12167 and can easily produce terabytes of data, so you won't want to
12168 rerun the command unnecessarily. Nor will you want to save
12169 the uncompressed output.
12171 Doing it the inefficient way, you can't even start the GUI
12172 until after you've compressed all of the @command{du} output:
12175 du -ak | gzip -9 > /tmp/du.gz
12176 gzip -d /tmp/du.gz | xdiskusage -a
12179 With @command{tee} and process substitution, you start the GUI
12180 right away and eliminate the decompression completely:
12183 du -ak | tee >(gzip -9 > /tmp/du.gz) | xdiskusage -a
12186 Finally, if you regularly create more than one type of
12187 compressed tarball at once, for example when @code{make dist} creates
12188 both @command{gzip}-compressed and @command{bzip2}-compressed tarballs,
12189 there may be a better way.
12190 Typical @command{automake}-generated @file{Makefile} rules create
12191 the two compressed tar archives with commands in sequence, like this
12192 (slightly simplified):
12195 tardir=your-pkg-M.N
12196 tar chof - "$tardir" | gzip -9 -c > your-pkg-M.N.tar.gz
12197 tar chof - "$tardir" | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12200 However, if the hierarchy you are archiving and compressing is larger
12201 than a couple megabytes, and especially if you are using a multi-processor
12202 system with plenty of memory, then you can do much better by reading the
12203 directory contents only once and running the compression programs in parallel:
12206 tardir=your-pkg-M.N
12207 tar chof - "$tardir" \
12208 | tee >(gzip -9 -c > your-pkg-M.N.tar.gz) \
12209 | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12215 @node File name manipulation
12216 @chapter File name manipulation
12218 @cindex file name manipulation
12219 @cindex manipulation of file names
12220 @cindex commands for file name manipulation
12222 This section describes commands that manipulate file names.
12225 * basename invocation:: Strip directory and suffix from a file name.
12226 * dirname invocation:: Strip last file name component.
12227 * pathchk invocation:: Check file name validity and portability.
12228 * mktemp invocation:: Create temporary file or directory.
12232 @node basename invocation
12233 @section @command{basename}: Strip directory and suffix from a file name
12236 @cindex strip directory and suffix from file names
12237 @cindex directory, stripping from file names
12238 @cindex suffix, stripping from file names
12239 @cindex file names, stripping directory and suffix
12240 @cindex leading directory components, stripping
12242 @command{basename} removes any leading directory components from
12243 @var{name}. Synopsis:
12246 basename @var{name} [@var{suffix}]
12249 If @var{suffix} is specified and is identical to the end of @var{name},
12250 it is removed from @var{name} as well. Note that since trailing slashes
12251 are removed prior to suffix matching, @var{suffix} will do nothing if it
12252 contains slashes. @command{basename} prints the result on standard
12255 @c This test is used both here and in the section on dirname.
12256 @macro basenameAndDirname
12257 Together, @command{basename} and @command{dirname} are designed such
12258 that if @samp{ls "$name"} succeeds, then the command sequence @samp{cd
12259 "$(dirname "$name")"; ls "$(basename "$name")"} will, too. This works
12260 for everything except file names containing a trailing newline.
12262 @basenameAndDirname
12264 @acronym{POSIX} allows the implementation to define the results if
12265 @var{name} is empty or @samp{//}. In the former case, @acronym{GNU}
12266 @command{basename} returns the empty string. In the latter case, the
12267 result is @samp{//} on platforms where @var{//} is distinct from
12268 @var{/}, and @samp{/} on platforms where there is no difference.
12270 The only options are @option{--help} and @option{--version}. @xref{Common
12271 options}. Options must precede operands.
12279 basename /usr/bin/sort
12282 basename include/stdio.h .h
12286 @node dirname invocation
12287 @section @command{dirname}: Strip last file name component
12290 @cindex directory components, printing
12291 @cindex stripping non-directory suffix
12292 @cindex non-directory suffix, stripping
12294 @command{dirname} prints all but the final slash-delimited component of
12295 @var{name}. Slashes on either side of the final component are also
12296 removed. If the string contains no slash, @command{dirname} prints
12297 @samp{.} (meaning the current directory). Synopsis:
12303 @var{name} need not be a file name, but if it is, this operation
12304 effectively lists the directory that contains the final component,
12305 including the case when the final component is itself a directory.
12307 @basenameAndDirname
12309 @acronym{POSIX} allows the implementation to define the results if
12310 @var{name} is @samp{//}. With @acronym{GNU} @command{dirname}, the
12311 result is @samp{//} on platforms where @var{//} is distinct from
12312 @var{/}, and @samp{/} on platforms where there is no difference.
12314 The only options are @option{--help} and @option{--version}. @xref{Common
12322 # Output "/usr/bin".
12323 dirname /usr/bin/sort
12324 dirname /usr/bin//.//
12331 @node pathchk invocation
12332 @section @command{pathchk}: Check file name validity and portability
12335 @cindex file names, checking validity and portability
12336 @cindex valid file names, checking for
12337 @cindex portable file names, checking for
12339 @command{pathchk} checks validity and portability of file names. Synopsis:
12342 pathchk [@var{option}]@dots{} @var{name}@dots{}
12345 For each @var{name}, @command{pathchk} prints an error message if any of
12346 these conditions is true:
12350 One of the existing directories in @var{name} does not have search
12351 (execute) permission,
12353 The length of @var{name} is larger than the maximum supported by the
12356 The length of one component of @var{name} is longer than
12357 its file system's maximum.
12360 A nonexistent @var{name} is not an error, so long a file with that
12361 name could be created under the above conditions.
12363 The program accepts the following options. Also see @ref{Common options}.
12364 Options must precede operands.
12370 Instead of performing checks based on the underlying file system,
12371 print an error message if any of these conditions is true:
12375 A file name is empty.
12378 A file name contains a character outside the @acronym{POSIX} portable file
12379 name character set, namely, the ASCII letters and digits, @samp{.},
12380 @samp{_}, @samp{-}, and @samp{/}.
12383 The length of a file name or one of its components exceeds the
12384 @acronym{POSIX} minimum limits for portability.
12389 Print an error message if a file name is empty, or if it contains a component
12390 that begins with @samp{-}.
12392 @item --portability
12393 @opindex --portability
12394 Print an error message if a file name is not portable to all @acronym{POSIX}
12395 hosts. This option is equivalent to @samp{-p -P}.
12399 @cindex exit status of @command{pathchk}
12403 0 if all specified file names passed all checks,
12407 @node mktemp invocation
12408 @section @command{mktemp}: Create temporary file or directory
12411 @cindex file names, creating temporary
12412 @cindex directory, creating temporary
12413 @cindex temporary files and directories
12415 @command{mktemp} manages the creation of temporary files and
12416 directories. Synopsis:
12419 mktemp [@var{option}]@dots{} [@var{template}]
12422 Safely create a temporary file or directory based on @var{template},
12423 and print its name. If given, @var{template} must include at least
12424 three consecutive @samp{X}s in the last component. If omitted, the template
12425 @samp{tmp.XXXXXXXXXX} is used, and option @option{--tmpdir} is
12426 implied. The final run of @samp{X}s in the @var{template} will be replaced
12427 by alpha-numeric characters; thus, on a case-sensitive file system,
12428 and with a @var{template} including a run of @var{n} instances of @samp{X},
12429 there are @samp{62**@var{n}} potential file names.
12431 Older scripts used to create temporary files by simply joining the
12432 name of the program with the process id (@samp{$$}) as a suffix.
12433 However, that naming scheme is easily predictable, and suffers from a
12434 race condition where the attacker can create an appropriately named
12435 symbolic link, such that when the script then opens a handle to what
12436 it thought was an unused file, it is instead modifying an existing
12437 file. Using the same scheme to create a directory is slightly safer,
12438 since the @command{mkdir} will fail if the target already exists, but
12439 it is still inferior because it allows for denial of service attacks.
12440 Therefore, modern scripts should use the @command{mktemp} command to
12441 guarantee that the generated name will be unpredictable, and that
12442 knowledge of the temporary file name implies that the file was created
12443 by the current script and cannot be modified by other users.
12445 When creating a file, the resulting file has read and write
12446 permissions for the current user, but no permissions for the group or
12447 others; these permissions are reduced if the current umask is more
12450 Here are some examples (although note that if you repeat them, you
12451 will most likely get different file names):
12456 Create a temporary file in the current directory.
12463 Create a temporary file with a known suffix.
12465 $ mktemp --suffix=.txt file-XXXX
12467 $ mktemp file-XXXX-XXXX.txt
12472 Create a secure fifo relative to the user's choice of @env{TMPDIR},
12473 but falling back to the current directory rather than @file{/tmp}.
12474 Note that @command{mktemp} does not create fifos, but can create a
12475 secure directory in which the fifo can live. Exit the shell if the
12476 directory or fifo could not be created.
12478 $ dir=$(mktemp -p "$@{TMPDIR:-.@}" -d dir-XXXX) || exit 1
12480 $ mkfifo "$fifo" || @{ rmdir "$dir"; exit 1; @}
12484 Create and use a temporary file if possible, but ignore failure. The
12485 file will reside in the directory named by @env{TMPDIR}, if specified,
12486 or else in @file{/tmp}.
12488 $ file=$(mktemp -q) && @{
12489 > # Safe to use $file only within this block. Use quotes,
12490 > # since $TMPDIR, and thus $file, may contain whitespace.
12491 > echo ... > "$file"
12497 Act as a semi-random character generator (it is not fully random,
12498 since it is impacted by the contents of the current directory). To
12499 avoid security holes, do not use the resulting names to create a file.
12509 The program accepts the following options. Also see @ref{Common options}.
12516 @opindex --directory
12517 Create a directory rather than a file. The directory will have read,
12518 write, and search permissions for the current user, but no permissions
12519 for the group or others; these permissions are reduced if the current
12520 umask is more restrictive.
12526 Suppress diagnostics about failure to create a file or directory. The
12527 exit status will still reflect whether a file was created.
12533 Generate a temporary name that does not name an existing file, without
12534 changing the file system contents. Using the output of this command
12535 to create a new file is inherently unsafe, as there is a window of
12536 time between generating the name and using it where another process
12537 can create an object by the same name.
12540 @itemx --tmpdir[=@var{dir}]
12543 Treat @var{template} relative to the directory @var{dir}. If
12544 @var{dir} is not specified (only possible with the long option
12545 @option{--tmpdir}) or is the empty string, use the value of
12546 @env{TMPDIR} if available, otherwise use @samp{/tmp}. If this is
12547 specified, @var{template} must not be absolute. However,
12548 @var{template} can still contain slashes, although intermediate
12549 directories must already exist.
12551 @item --suffix=@var{suffix}
12553 Append @var{suffix} to the @var{template}. @var{suffix} must not
12554 contain slash. If @option{--suffix} is specified, @var{template} must
12555 end in @samp{X}; if it is not specified, then an appropriate
12556 @option{--suffix} is inferred by finding the last @samp{X} in
12557 @var{template}. This option exists for use with the default
12558 @var{template} and for the creation of a @var{suffix} that starts with
12563 Treat @var{template} as a single file relative to the value of
12564 @env{TMPDIR} if available, or to the directory specified by
12565 @option{-p}, otherwise to @samp{/tmp}. @var{template} must not
12566 contain slashes. This option is deprecated; the use of @option{-p}
12567 without @option{-t} offers better defaults (by favoring the command
12568 line over @env{TMPDIR}) and more flexibility (by allowing intermediate
12573 @cindex exit status of @command{mktemp}
12577 0 if the file was created,
12582 @node Working context
12583 @chapter Working context
12585 @cindex working context
12586 @cindex commands for printing the working context
12588 This section describes commands that display or alter the context in
12589 which you are working: the current directory, the terminal settings, and
12590 so forth. See also the user-related commands in the next section.
12593 * pwd invocation:: Print working directory.
12594 * stty invocation:: Print or change terminal characteristics.
12595 * printenv invocation:: Print environment variables.
12596 * tty invocation:: Print file name of terminal on standard input.
12600 @node pwd invocation
12601 @section @command{pwd}: Print working directory
12604 @cindex print name of current directory
12605 @cindex current working directory, printing
12606 @cindex working directory, printing
12609 @command{pwd} prints the name of the current directory. Synopsis:
12612 pwd [@var{option}]@dots{}
12615 The program accepts the following options. Also see @ref{Common options}.
12622 If the contents of the environment variable @env{PWD} provide an
12623 absolute name of the current directory with no @samp{.} or @samp{..}
12624 components, but possibly with symbolic links, then output those
12625 contents. Otherwise, fall back to default @option{-P} handling.
12630 @opindex --physical
12631 Print a fully resolved name for the current directory. That is, all
12632 components of the printed name will be actual directory names---none
12633 will be symbolic links.
12636 @cindex symbolic links and @command{pwd}
12637 If @option{-L} and @option{-P} are both given, the last one takes
12638 precedence. If neither option is given, then this implementation uses
12639 @option{-P} as the default unless the @env{POSIXLY_CORRECT}
12640 environment variable is set.
12642 @mayConflictWithShellBuiltIn{pwd}
12647 @node stty invocation
12648 @section @command{stty}: Print or change terminal characteristics
12651 @cindex change or print terminal settings
12652 @cindex terminal settings
12653 @cindex line settings of terminal
12655 @command{stty} prints or changes terminal characteristics, such as baud rate.
12659 stty [@var{option}] [@var{setting}]@dots{}
12660 stty [@var{option}]
12663 If given no line settings, @command{stty} prints the baud rate, line
12664 discipline number (on systems that support it), and line settings
12665 that have been changed from the values set by @samp{stty sane}.
12666 By default, mode reading and setting are performed on the tty line
12667 connected to standard input, although this can be modified by the
12668 @option{--file} option.
12670 @command{stty} accepts many non-option arguments that change aspects of
12671 the terminal line operation, as described below.
12673 The program accepts the following options. Also see @ref{Common options}.
12680 Print all current settings in human-readable form. This option may not
12681 be used in combination with any line settings.
12683 @item -F @var{device}
12684 @itemx --file=@var{device}
12687 Set the line opened by the file name specified in @var{device} instead of
12688 the tty line connected to standard input. This option is necessary
12689 because opening a @acronym{POSIX} tty requires use of the
12690 @code{O_NONDELAY} flag to prevent a @acronym{POSIX} tty from blocking
12691 until the carrier detect line is high if
12692 the @code{clocal} flag is not set. Hence, it is not always possible
12693 to allow the shell to open the device in the traditional manner.
12699 @cindex machine-readable @command{stty} output
12700 Print all current settings in a form that can be used as an argument to
12701 another @command{stty} command to restore the current settings. This option
12702 may not be used in combination with any line settings.
12706 Many settings can be turned off by preceding them with a @samp{-}.
12707 Such arguments are marked below with ``May be negated'' in their
12708 description. The descriptions themselves refer to the positive
12709 case, that is, when @emph{not} negated (unless stated otherwise,
12712 Some settings are not available on all @acronym{POSIX} systems, since they use
12713 extensions. Such arguments are marked below with
12714 ``Non-@acronym{POSIX}'' in their description. On non-@acronym{POSIX}
12715 systems, those or other settings also may not
12716 be available, but it's not feasible to document all the variations: just
12722 * Control:: Control settings
12723 * Input:: Input settings
12724 * Output:: Output settings
12725 * Local:: Local settings
12726 * Combination:: Combination settings
12727 * Characters:: Special characters
12728 * Special:: Special settings
12733 @subsection Control settings
12735 @cindex control settings
12741 @cindex two-way parity
12742 Generate parity bit in output and expect parity bit in input.
12748 @cindex even parity
12749 Set odd parity (even if negated). May be negated.
12756 @cindex character size
12757 @cindex eight-bit characters
12758 Set character size to 5, 6, 7, or 8 bits.
12763 Send a hangup signal when the last process closes the tty. May be
12769 Use two stop bits per character (one if negated). May be negated.
12773 Allow input to be received. May be negated.
12777 @cindex modem control
12778 Disable modem control signals. May be negated.
12782 @cindex hardware flow control
12783 @cindex flow control, hardware
12784 @cindex RTS/CTS flow control
12785 Enable RTS/CTS flow control. Non-@acronym{POSIX}. May be negated.
12790 @subsection Input settings
12792 @cindex input settings
12793 These settings control operations on data received from the terminal.
12798 @cindex breaks, ignoring
12799 Ignore break characters. May be negated.
12803 @cindex breaks, cause interrupts
12804 Make breaks cause an interrupt signal. May be negated.
12808 @cindex parity, ignoring
12809 Ignore characters with parity errors. May be negated.
12813 @cindex parity errors, marking
12814 Mark parity errors (with a 255-0-character sequence). May be negated.
12818 Enable input parity checking. May be negated.
12822 @cindex eight-bit input
12823 Clear high (8th) bit of input characters. May be negated.
12827 @cindex newline, translating to return
12828 Translate newline to carriage return. May be negated.
12832 @cindex return, ignoring
12833 Ignore carriage return. May be negated.
12837 @cindex return, translating to newline
12838 Translate carriage return to newline. May be negated.
12842 @cindex input encoding, UTF-8
12843 Assume input characters are UTF-8 encoded. May be negated.
12847 @kindex C-s/C-q flow control
12848 @cindex XON/XOFF flow control
12849 Enable XON/XOFF flow control (that is, @kbd{CTRL-S}/@kbd{CTRL-Q}). May
12856 @cindex software flow control
12857 @cindex flow control, software
12858 Enable sending of @code{stop} character when the system input buffer
12859 is almost full, and @code{start} character when it becomes almost
12860 empty again. May be negated.
12864 @cindex uppercase, translating to lowercase
12865 Translate uppercase characters to lowercase. Non-@acronym{POSIX}. May be
12866 negated. Note ilcuc is not implemented, as one would not be able to issue
12867 almost any (lowercase) Unix command, after invoking it.
12871 Allow any character to restart output (only the start character
12872 if negated). Non-@acronym{POSIX}. May be negated.
12876 @cindex beeping at input buffer full
12877 Enable beeping and not flushing input buffer if a character arrives
12878 when the input buffer is full. Non-@acronym{POSIX}. May be negated.
12883 @subsection Output settings
12885 @cindex output settings
12886 These settings control operations on data sent to the terminal.
12891 Postprocess output. May be negated.
12895 @cindex lowercase, translating to output
12896 Translate lowercase characters to uppercase. Non-@acronym{POSIX}. May be
12897 negated. (Note ouclc is not currently implemented.)
12901 @cindex return, translating to newline
12902 Translate carriage return to newline. Non-@acronym{POSIX}. May be negated.
12906 @cindex newline, translating to crlf
12907 Translate newline to carriage return-newline. Non-@acronym{POSIX}. May be
12912 Do not print carriage returns in the first column. Non-@acronym{POSIX}.
12917 Newline performs a carriage return. Non-@acronym{POSIX}. May be negated.
12921 @cindex pad instead of timing for delaying
12922 Use fill (padding) characters instead of timing for delays.
12923 Non-@acronym{POSIX}.
12928 @cindex pad character
12929 Use @acronym{ASCII} @sc{del} characters for fill instead of
12930 @acronym{ASCII} @sc{nul} characters. Non-@acronym{POSIX}.
12936 Newline delay style. Non-@acronym{POSIX}.
12943 Carriage return delay style. Non-@acronym{POSIX}.
12949 @opindex tab@var{n}
12950 Horizontal tab delay style. Non-@acronym{POSIX}.
12955 Backspace delay style. Non-@acronym{POSIX}.
12960 Vertical tab delay style. Non-@acronym{POSIX}.
12965 Form feed delay style. Non-@acronym{POSIX}.
12970 @subsection Local settings
12972 @cindex local settings
12977 Enable @code{interrupt}, @code{quit}, and @code{suspend} special
12978 characters. May be negated.
12982 Enable @code{erase}, @code{kill}, @code{werase}, and @code{rprnt}
12983 special characters. May be negated.
12987 Enable non-@acronym{POSIX} special characters. May be negated.
12991 Echo input characters. May be negated.
12997 Echo @code{erase} characters as backspace-space-backspace. May be
13002 @cindex newline echoing after @code{kill}
13003 Echo a newline after a @code{kill} character. May be negated.
13007 @cindex newline, echoing
13008 Echo newline even if not echoing other characters. May be negated.
13012 @cindex flushing, disabling
13013 Disable flushing after @code{interrupt} and @code{quit} special
13014 characters. May be negated.
13018 @cindex case translation
13019 Enable input and output of uppercase characters by preceding their
13020 lowercase equivalents with @samp{\}, when @code{icanon} is set.
13021 Non-@acronym{POSIX}. May be negated.
13025 @cindex background jobs, stopping at terminal write
13026 Stop background jobs that try to write to the terminal. Non-@acronym{POSIX}.
13033 Echo erased characters backward, between @samp{\} and @samp{/}.
13034 Non-@acronym{POSIX}. May be negated.
13040 @cindex control characters, using @samp{^@var{c}}
13041 @cindex hat notation for control characters
13042 Echo control characters in hat notation (@samp{^@var{c}}) instead
13043 of literally. Non-@acronym{POSIX}. May be negated.
13049 Echo the @code{kill} special character by erasing each character on
13050 the line as indicated by the @code{echoprt} and @code{echoe} settings,
13051 instead of by the @code{echoctl} and @code{echok} settings.
13052 Non-@acronym{POSIX}.
13058 @subsection Combination settings
13060 @cindex combination settings
13061 Combination settings:
13068 Same as @code{parenb -parodd cs7}. May be negated. If negated, same
13069 as @code{-parenb cs8}.
13073 Same as @code{parenb parodd cs7}. May be negated. If negated, same
13074 as @code{-parenb cs8}.
13078 Same as @code{-icrnl -onlcr}. May be negated. If negated, same as
13079 @code{icrnl -inlcr -igncr onlcr -ocrnl -onlret}.
13083 Reset the @code{erase} and @code{kill} special characters to their default
13090 @c This is too long to write inline.
13092 cread -ignbrk brkint -inlcr -igncr icrnl -ixoff
13093 -iuclc -ixany imaxbel opost -olcuc -ocrnl onlcr
13094 -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0
13095 ff0 isig icanon iexten echo echoe echok -echonl
13096 -noflsh -xcase -tostop -echoprt echoctl echoke
13100 and also sets all special characters to their default values.
13104 Same as @code{brkint ignpar istrip icrnl ixon opost isig icanon}, plus
13105 sets the @code{eof} and @code{eol} characters to their default values
13106 if they are the same as the @code{min} and @code{time} characters.
13107 May be negated. If negated, same as @code{raw}.
13114 -ignbrk -brkint -ignpar -parmrk -inpck -istrip
13115 -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany
13116 -imaxbel -opost -isig -icanon -xcase min 1 time 0
13120 May be negated. If negated, same as @code{cooked}.
13124 Same as @option{-icanon}. May be negated. If negated, same as
13129 @cindex eight-bit characters
13130 Same as @code{-parenb -istrip cs8}. May be negated. If negated,
13131 same as @code{parenb istrip cs7}.
13135 Same as @option{-parenb -istrip -opost cs8}. May be negated.
13136 If negated, same as @code{parenb istrip opost cs7}.
13140 Same as @option{-ixany}. Non-@acronym{POSIX}. May be negated.
13144 Same as @code{tab0}. Non-@acronym{POSIX}. May be negated. If negated, same
13151 Same as @code{xcase iuclc olcuc}. Non-@acronym{POSIX}. May be negated.
13152 (Used for terminals with uppercase characters only.)
13156 Same as @code{echoe echoctl echoke}.
13160 Same as @code{echoe echoctl echoke -ixany intr ^C erase ^? kill C-u}.
13165 @subsection Special characters
13167 @cindex special characters
13168 @cindex characters, special
13170 The special characters' default values vary from system to system.
13171 They are set with the syntax @samp{name value}, where the names are
13172 listed below and the value can be given either literally, in hat
13173 notation (@samp{^@var{c}}), or as an integer which may start with
13174 @samp{0x} to indicate hexadecimal, @samp{0} to indicate octal, or
13175 any other digit to indicate decimal.
13177 @cindex disabling special characters
13178 @kindex u@r{, and disabling special characters}
13179 For GNU stty, giving a value of @code{^-} or @code{undef} disables that
13180 special character. (This is incompatible with Ultrix @command{stty},
13181 which uses a value of @samp{u} to disable a special character. GNU
13182 @command{stty} treats a value @samp{u} like any other, namely to set that
13183 special character to @key{U}.)
13189 Send an interrupt signal.
13193 Send a quit signal.
13197 Erase the last character typed.
13201 Erase the current line.
13205 Send an end of file (terminate the input).
13213 Alternate character to end the line. Non-@acronym{POSIX}.
13217 Switch to a different shell layer. Non-@acronym{POSIX}.
13221 Restart the output after stopping it.
13229 Send a terminal stop signal.
13233 Send a terminal stop signal after flushing the input. Non-@acronym{POSIX}.
13237 Redraw the current line. Non-@acronym{POSIX}.
13241 Erase the last word typed. Non-@acronym{POSIX}.
13245 Enter the next character typed literally, even if it is a special
13246 character. Non-@acronym{POSIX}.
13251 @subsection Special settings
13253 @cindex special settings
13258 Set the minimum number of characters that will satisfy a read until
13259 the time value has expired, when @option{-icanon} is set.
13263 Set the number of tenths of a second before reads time out if the minimum
13264 number of characters have not been read, when @option{-icanon} is set.
13266 @item ispeed @var{n}
13268 Set the input speed to @var{n}.
13270 @item ospeed @var{n}
13272 Set the output speed to @var{n}.
13276 Tell the tty kernel driver that the terminal has @var{n} rows.
13277 Non-@acronym{POSIX}.
13280 @itemx columns @var{n}
13283 Tell the kernel that the terminal has @var{n} columns. Non-@acronym{POSIX}.
13289 Print the number of rows and columns that the kernel thinks the
13290 terminal has. (Systems that don't support rows and columns in the kernel
13291 typically use the environment variables @env{LINES} and @env{COLUMNS}
13292 instead; however, GNU @command{stty} does not know anything about them.)
13293 Non-@acronym{POSIX}.
13297 Use line discipline @var{n}. Non-@acronym{POSIX}.
13301 Print the terminal speed.
13304 @cindex baud rate, setting
13305 Set the input and output speeds to @var{n}. @var{n} can be one of: 0
13306 50 75 110 134 134.5 150 200 300 600 1200 1800 2400 4800 9600 19200
13307 38400 @code{exta} @code{extb}. @code{exta} is the same as 19200;
13308 @code{extb} is the same as 38400. Many systems, including GNU/Linux,
13309 support higher speeds. The @command{stty} command includes support
13326 4000000 where the system supports these.
13327 0 hangs up the line if @option{-clocal} is set.
13331 @node printenv invocation
13332 @section @command{printenv}: Print all or some environment variables
13335 @cindex printing all or some environment variables
13336 @cindex environment variables, printing
13338 @command{printenv} prints environment variable values. Synopsis:
13341 printenv [@var{option}] [@var{variable}]@dots{}
13344 If no @var{variable}s are specified, @command{printenv} prints the value of
13345 every environment variable. Otherwise, it prints the value of each
13346 @var{variable} that is set, and nothing for those that are not set.
13348 The program accepts the following option. Also see @ref{Common options}.
13356 @cindex exit status of @command{printenv}
13360 0 if all variables specified were found
13361 1 if at least one specified variable was not found
13362 2 if a write error occurred
13366 @node tty invocation
13367 @section @command{tty}: Print file name of terminal on standard input
13370 @cindex print terminal file name
13371 @cindex terminal file name, printing
13373 @command{tty} prints the file name of the terminal connected to its standard
13374 input. It prints @samp{not a tty} if standard input is not a terminal.
13378 tty [@var{option}]@dots{}
13381 The program accepts the following option. Also see @ref{Common options}.
13391 Print nothing; only return an exit status.
13395 @cindex exit status of @command{tty}
13399 0 if standard input is a terminal
13400 1 if standard input is not a terminal
13401 2 if given incorrect arguments
13402 3 if a write error occurs
13406 @node User information
13407 @chapter User information
13409 @cindex user information, commands for
13410 @cindex commands for printing user information
13412 This section describes commands that print user-related information:
13413 logins, groups, and so forth.
13416 * id invocation:: Print user identity.
13417 * logname invocation:: Print current login name.
13418 * whoami invocation:: Print effective user ID.
13419 * groups invocation:: Print group names a user is in.
13420 * users invocation:: Print login names of users currently logged in.
13421 * who invocation:: Print who is currently logged in.
13425 @node id invocation
13426 @section @command{id}: Print user identity
13429 @cindex real user and group IDs, printing
13430 @cindex effective user and group IDs, printing
13431 @cindex printing real and effective user and group IDs
13433 @command{id} prints information about the given user, or the process
13434 running it if no user is specified. Synopsis:
13437 id [@var{option}]@dots{} [@var{username}]
13440 @vindex POSIXLY_CORRECT
13441 By default, it prints the real user ID, real group ID, effective user ID
13442 if different from the real user ID, effective group ID if different from
13443 the real group ID, and supplemental group IDs.
13444 In addition, if SELinux
13445 is enabled and the @env{POSIXLY_CORRECT} environment variable is not set,
13446 then print @samp{context=@var{c}}, where @var{c} is the security context.
13448 Each of these numeric values is preceded by an identifying string and
13449 followed by the corresponding user or group name in parentheses.
13451 The options cause @command{id} to print only part of the above information.
13452 Also see @ref{Common options}.
13459 Print only the group ID.
13465 Print only the group ID and the supplementary groups.
13471 Print the user or group name instead of the ID number. Requires
13472 @option{-u}, @option{-g}, or @option{-G}.
13478 Print the real, instead of effective, user or group ID. Requires
13479 @option{-u}, @option{-g}, or @option{-G}.
13485 Print only the user ID.
13492 @cindex security context
13493 Print only the security context of the current user.
13494 If SELinux is disabled then print a warning and
13495 set the exit status to 1.
13501 @macro primaryAndSupplementaryGroups{cmd,arg}
13502 Primary and supplementary groups for a process are normally inherited
13503 from its parent and are usually unchanged since login. This means
13504 that if you change the group database after logging in, @command{\cmd\}
13505 will not reflect your changes within your existing login session.
13506 Running @command{\cmd\} with a \arg\ causes the user and group
13507 database to be consulted afresh, and so will give a different result.
13509 @primaryAndSupplementaryGroups{id,user argument}
13511 @node logname invocation
13512 @section @command{logname}: Print current login name
13515 @cindex printing user's login name
13516 @cindex login name, printing
13517 @cindex user name, printing
13520 @command{logname} prints the calling user's name, as found in a
13521 system-maintained file (often @file{/var/run/utmp} or
13522 @file{/etc/utmp}), and exits with a status of 0. If there is no entry
13523 for the calling process, @command{logname} prints
13524 an error message and exits with a status of 1.
13526 The only options are @option{--help} and @option{--version}. @xref{Common
13532 @node whoami invocation
13533 @section @command{whoami}: Print effective user ID
13536 @cindex effective user ID, printing
13537 @cindex printing the effective user ID
13539 @command{whoami} prints the user name associated with the current
13540 effective user ID. It is equivalent to the command @samp{id -un}.
13542 The only options are @option{--help} and @option{--version}. @xref{Common
13548 @node groups invocation
13549 @section @command{groups}: Print group names a user is in
13552 @cindex printing groups a user is in
13553 @cindex supplementary groups, printing
13555 @command{groups} prints the names of the primary and any supplementary
13556 groups for each given @var{username}, or the current process if no names
13557 are given. If more than one name is given, the name of each user is
13559 the list of that user's groups and the user name is separated from the
13560 group list by a colon. Synopsis:
13563 groups [@var{username}]@dots{}
13566 The group lists are equivalent to the output of the command @samp{id -Gn}.
13568 @primaryAndSupplementaryGroups{groups,list of users}
13570 The only options are @option{--help} and @option{--version}. @xref{Common
13576 @node users invocation
13577 @section @command{users}: Print login names of users currently logged in
13580 @cindex printing current usernames
13581 @cindex usernames, printing current
13583 @cindex login sessions, printing users with
13584 @command{users} prints on a single line a blank-separated list of user
13585 names of users currently logged in to the current host. Each user name
13586 corresponds to a login session, so if a user has more than one login
13587 session, that user's name will appear the same number of times in the
13596 With no @var{file} argument, @command{users} extracts its information from
13597 a system-maintained file (often @file{/var/run/utmp} or
13598 @file{/etc/utmp}). If a file argument is given, @command{users} uses
13599 that file instead. A common choice is @file{/var/log/wtmp}.
13601 The only options are @option{--help} and @option{--version}. @xref{Common
13607 @node who invocation
13608 @section @command{who}: Print who is currently logged in
13611 @cindex printing current user information
13612 @cindex information, about current users
13614 @command{who} prints information about users who are currently logged on.
13618 @command{who} [@var{option}] [@var{file}] [am i]
13621 @cindex terminal lines, currently used
13623 @cindex remote hostname
13624 If given no non-option arguments, @command{who} prints the following
13625 information for each user currently logged on: login name, terminal
13626 line, login time, and remote hostname or X display.
13630 If given one non-option argument, @command{who} uses that instead of
13631 a default system-maintained file (often @file{/var/run/utmp} or
13632 @file{/etc/utmp}) as the name of the file containing the record of
13633 users logged on. @file{/var/log/wtmp} is commonly given as an argument
13634 to @command{who} to look at who has previously logged on.
13638 If given two non-option arguments, @command{who} prints only the entry
13639 for the user running it (determined from its standard input), preceded
13640 by the hostname. Traditionally, the two arguments given are @samp{am
13641 i}, as in @samp{who am i}.
13644 Time stamps are listed according to the time zone rules specified by
13645 the @env{TZ} environment variable, or by the system default rules if
13646 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
13647 with @env{TZ}, libc, The GNU C Library Reference Manual}.
13649 The program accepts the following options. Also see @ref{Common options}.
13657 Same as @samp{-b -d --login -p -r -t -T -u}.
13663 Print the date and time of last system boot.
13669 Print information corresponding to dead processes.
13675 Print a line of column headings.
13681 List only the entries that correspond to processes via which the
13682 system is waiting for a user to login. The user name is always @samp{LOGIN}.
13686 Attempt to canonicalize hostnames found in utmp through a DNS lookup. This
13687 is not the default because it can cause significant delays on systems with
13688 automatic dial-up internet access.
13692 Same as @samp{who am i}.
13698 List active processes spawned by init.
13704 Print only the login names and the number of users logged on.
13705 Overrides all other options.
13710 @opindex --runlevel
13711 Print the current (and maybe previous) run-level of the init process.
13715 Ignored; for compatibility with other versions of @command{who}.
13721 Print last system clock change.
13726 After the login time, print the number of hours and minutes that the
13727 user has been idle. @samp{.} means the user was active in the last minute.
13728 @samp{old} means the user has been idle for more than 24 hours.
13739 @opindex --writable
13740 @cindex message status
13741 @pindex write@r{, allowed}
13742 After each login name print a character indicating the user's message status:
13745 @samp{+} allowing @code{write} messages
13746 @samp{-} disallowing @code{write} messages
13747 @samp{?} cannot find terminal device
13755 @node System context
13756 @chapter System context
13758 @cindex system context
13759 @cindex context, system
13760 @cindex commands for system context
13762 This section describes commands that print or change system-wide
13766 * date invocation:: Print or set system date and time.
13767 * arch invocation:: Print machine hardware name.
13768 * nproc invocation:: Print the number of processors.
13769 * uname invocation:: Print system information.
13770 * hostname invocation:: Print or set system name.
13771 * hostid invocation:: Print numeric host identifier.
13772 * uptime invocation:: Print system uptime and load.
13775 @node date invocation
13776 @section @command{date}: Print or set system date and time
13779 @cindex time, printing or setting
13780 @cindex printing the current time
13785 date [@var{option}]@dots{} [+@var{format}]
13786 date [-u|--utc|--universal] @c this avoids a newline in the output
13787 [ MMDDhhmm[[CC]YY][.ss] ]
13791 Invoking @command{date} with no @var{format} argument is equivalent to invoking
13792 it with a default format that depends on the @env{LC_TIME} locale category.
13793 In the default C locale, this format is @samp{'+%a %b %e %H:%M:%S %Z %Y'},
13794 so the output looks like @samp{Thu Mar @ 3 13:47:51 PST 2005}.
13797 Normally, @command{date} uses the time zone rules indicated by the
13798 @env{TZ} environment variable, or the system default rules if @env{TZ}
13799 is not set. @xref{TZ Variable,, Specifying the Time Zone with
13800 @env{TZ}, libc, The GNU C Library Reference Manual}.
13802 @findex strftime @r{and @command{date}}
13803 @cindex time formats
13804 @cindex formatting times
13805 If given an argument that starts with a @samp{+}, @command{date} prints the
13806 current date and time (or the date and time specified by the
13807 @option{--date} option, see below) in the format defined by that argument,
13808 which is similar to that of the @code{strftime} function. Except for
13809 conversion specifiers, which start with @samp{%}, characters in the
13810 format string are printed unchanged. The conversion specifiers are
13816 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
13817 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
13818 * Literal conversion specifiers:: %[%nt]
13819 * Padding and other flags:: Pad with zeros, spaces, etc.
13820 * Setting the time:: Changing the system clock.
13821 * Options for date:: Instead of the current time.
13823 * Date input formats:: Specifying date strings.
13825 * Examples of date:: Examples.
13828 @node Time conversion specifiers
13829 @subsection Time conversion specifiers
13831 @cindex time conversion specifiers
13832 @cindex conversion specifiers, time
13834 @command{date} conversion specifiers related to times.
13838 hour (@samp{00}@dots{}@samp{23})
13840 hour (@samp{01}@dots{}@samp{12})
13842 hour (@samp{ 0}@dots{}@samp{23}).
13843 This is a @acronym{GNU} extension.
13845 hour (@samp{ 1}@dots{}@samp{12}).
13846 This is a @acronym{GNU} extension.
13848 minute (@samp{00}@dots{}@samp{59})
13850 nanoseconds (@samp{000000000}@dots{}@samp{999999999}).
13851 This is a @acronym{GNU} extension.
13853 locale's equivalent of either @samp{AM} or @samp{PM};
13854 blank in many locales.
13855 Noon is treated as @samp{PM} and midnight as @samp{AM}.
13857 like @samp{%p}, except lower case.
13858 This is a @acronym{GNU} extension.
13860 locale's 12-hour clock time (e.g., @samp{11:11:04 PM})
13862 24-hour hour and minute. Same as @samp{%H:%M}.
13863 This is a @acronym{GNU} extension.
13865 @cindex epoch, seconds since
13866 @cindex seconds since the epoch
13867 @cindex beginning of time
13868 seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC.
13869 Leap seconds are not counted unless leap second support is available.
13870 @xref{%s-examples}, for examples.
13871 This is a @acronym{GNU} extension.
13873 second (@samp{00}@dots{}@samp{60}).
13874 This may be @samp{60} if leap seconds are supported.
13876 24-hour hour, minute, and second. Same as @samp{%H:%M:%S}.
13878 locale's time representation (e.g., @samp{23:13:48})
13880 @w{@acronym{RFC} 2822/@acronym{ISO} 8601} style numeric time zone
13881 (e.g., @samp{-0600} or @samp{+0530}), or nothing if no
13882 time zone is determinable. This value reflects the numeric time zone
13883 appropriate for the current time, using the time zone rules specified
13884 by the @env{TZ} environment variable.
13885 The time (and optionally, the time zone rules) can be overridden
13886 by the @option{--date} option.
13887 This is a @acronym{GNU} extension.
13889 @w{@acronym{RFC} 3339/@acronym{ISO} 8601} style numeric time zone with
13890 @samp{:} (e.g., @samp{-06:00} or @samp{+05:30}), or nothing if no time
13891 zone is determinable.
13892 This is a @acronym{GNU} extension.
13894 Numeric time zone to the nearest second with @samp{:} (e.g.,
13895 @samp{-06:00:00} or @samp{+05:30:00}), or nothing if no time zone is
13897 This is a @acronym{GNU} extension.
13899 Numeric time zone with @samp{:} using the minimum necessary precision
13900 (e.g., @samp{-06}, @samp{+05:30}, or @samp{-04:56:02}), or nothing if
13901 no time zone is determinable.
13902 This is a @acronym{GNU} extension.
13904 alphabetic time zone abbreviation (e.g., @samp{EDT}), or nothing if no
13905 time zone is determinable. See @samp{%z} for how it is determined.
13909 @node Date conversion specifiers
13910 @subsection Date conversion specifiers
13912 @cindex date conversion specifiers
13913 @cindex conversion specifiers, date
13915 @command{date} conversion specifiers related to dates.
13919 locale's abbreviated weekday name (e.g., @samp{Sun})
13921 locale's full weekday name, variable length (e.g., @samp{Sunday})
13923 locale's abbreviated month name (e.g., @samp{Jan})
13925 locale's full month name, variable length (e.g., @samp{January})
13927 locale's date and time (e.g., @samp{Thu Mar @ 3 23:05:25 2005})
13929 century. This is like @samp{%Y}, except the last two digits are omitted.
13930 For example, it is @samp{20} if @samp{%Y} is @samp{2000},
13931 and is @samp{-0} if @samp{%Y} is @samp{-001}.
13932 It is normally at least two characters, but it may be more.
13934 day of month (e.g., @samp{01})
13936 date; same as @samp{%m/%d/%y}
13938 day of month, space padded; same as @samp{%_d}
13940 full date in @acronym{ISO} 8601 format; same as @samp{%Y-%m-%d}.
13941 This is a good choice for a date format, as it is standard and
13942 is easy to sort in the usual case where years are in the range
13944 This is a @acronym{GNU} extension.
13946 year corresponding to the @acronym{ISO} week number, but without the century
13947 (range @samp{00} through @samp{99}). This has the same format and value
13948 as @samp{%y}, except that if the @acronym{ISO} week number (see
13950 to the previous or next year, that year is used instead.
13951 This is a @acronym{GNU} extension.
13953 year corresponding to the @acronym{ISO} week number. This has the
13954 same format and value as @samp{%Y}, except that if the @acronym{ISO}
13956 @samp{%V}) belongs to the previous or next year, that year is used
13958 It is normally useful only if @samp{%V} is also used;
13959 for example, the format @samp{%G-%m-%d} is probably a mistake,
13960 since it combines the ISO week number year with the conventional month and day.
13961 This is a @acronym{GNU} extension.
13965 day of year (@samp{001}@dots{}@samp{366})
13967 month (@samp{01}@dots{}@samp{12})
13969 day of week (@samp{1}@dots{}@samp{7}) with @samp{1} corresponding to Monday
13971 week number of year, with Sunday as the first day of the week
13972 (@samp{00}@dots{}@samp{53}).
13973 Days in a new year preceding the first Sunday are in week zero.
13975 @acronym{ISO} week number, that is, the
13976 week number of year, with Monday as the first day of the week
13977 (@samp{01}@dots{}@samp{53}).
13978 If the week containing January 1 has four or more days in
13979 the new year, then it is considered week 1; otherwise, it is week 53 of
13980 the previous year, and the next week is week 1. (See the @acronym{ISO} 8601
13983 day of week (@samp{0}@dots{}@samp{6}) with 0 corresponding to Sunday
13985 week number of year, with Monday as first day of week
13986 (@samp{00}@dots{}@samp{53}).
13987 Days in a new year preceding the first Monday are in week zero.
13989 locale's date representation (e.g., @samp{12/31/99})
13991 last two digits of year (@samp{00}@dots{}@samp{99})
13993 year. This is normally at least four characters, but it may be more.
13994 Year @samp{0000} precedes year @samp{0001}, and year @samp{-001}
13995 precedes year @samp{0000}.
13999 @node Literal conversion specifiers
14000 @subsection Literal conversion specifiers
14002 @cindex literal conversion specifiers
14003 @cindex conversion specifiers, literal
14005 @command{date} conversion specifiers that produce literal strings.
14017 @node Padding and other flags
14018 @subsection Padding and other flags
14020 @cindex numeric field padding
14021 @cindex padding of numeric fields
14022 @cindex fields, padding numeric
14024 Unless otherwise specified, @command{date} normally pads numeric fields
14025 with zeros, so that, for
14026 example, numeric months are always output as two digits.
14027 Seconds since the epoch are not padded, though,
14028 since there is no natural width for them.
14030 As a @acronym{GNU} extension, @command{date} recognizes any of the
14031 following optional flags after the @samp{%}:
14035 (hyphen) Do not pad the field; useful if the output is intended for
14038 (underscore) Pad with spaces; useful if you need a fixed
14039 number of characters in the output, but zeros are too distracting.
14041 (zero) Pad with zeros even if the conversion specifier
14042 would normally pad with spaces.
14044 Use upper case characters if possible.
14046 Use opposite case characters if possible.
14047 A field that is normally upper case becomes lower case, and vice versa.
14051 Here are some examples of padding:
14054 date +%d/%m -d "Feb 1"
14056 date +%-d/%-m -d "Feb 1"
14058 date +%_d/%_m -d "Feb 1"
14062 As a @acronym{GNU} extension, you can specify the field width
14063 (after any flag, if present) as a decimal number. If the natural size of the
14064 output of the field has less than the specified number of characters,
14065 the result is written right adjusted and padded to the given
14066 size. For example, @samp{%9B} prints the right adjusted month name in
14067 a field of width 9.
14069 An optional modifier can follow the optional flag and width
14070 specification. The modifiers are:
14074 Use the locale's alternate representation for date and time. This
14075 modifier applies to the @samp{%c}, @samp{%C}, @samp{%x}, @samp{%X},
14076 @samp{%y} and @samp{%Y} conversion specifiers. In a Japanese locale, for
14077 example, @samp{%Ex} might yield a date format based on the Japanese
14081 Use the locale's alternate numeric symbols for numbers. This modifier
14082 applies only to numeric conversion specifiers.
14085 If the format supports the modifier but no alternate representation
14086 is available, it is ignored.
14089 @node Setting the time
14090 @subsection Setting the time
14092 @cindex setting the time
14093 @cindex time setting
14094 @cindex appropriate privileges
14096 If given an argument that does not start with @samp{+}, @command{date} sets
14097 the system clock to the date and time specified by that argument (as
14098 described below). You must have appropriate privileges to set the
14099 system clock. The @option{--date} and @option{--set} options may not be
14100 used with such an argument. The @option{--universal} option may be used
14101 with such an argument to indicate that the specified date and time are
14102 relative to Coordinated Universal Time rather than to the local time
14105 The argument must consist entirely of digits, which have the following
14118 first two digits of year (optional)
14120 last two digits of year (optional)
14125 The @option{--set} option also sets the system clock; see the next section.
14128 @node Options for date
14129 @subsection Options for @command{date}
14131 @cindex @command{date} options
14132 @cindex options for @command{date}
14134 The program accepts the following options. Also see @ref{Common options}.
14138 @item -d @var{datestr}
14139 @itemx --date=@var{datestr}
14142 @cindex parsing date strings
14143 @cindex date strings, parsing
14144 @cindex arbitrary date strings, parsing
14147 @opindex next @var{day}
14148 @opindex last @var{day}
14149 Display the date and time specified in @var{datestr} instead of the
14150 current date and time. @var{datestr} can be in almost any common
14151 format. It can contain month names, time zones, @samp{am} and @samp{pm},
14152 @samp{yesterday}, etc. For example, @option{--date="2004-02-27
14153 14:19:13.489392193 +0530"} specifies the instant of time that is
14154 489,392,193 nanoseconds after February 27, 2004 at 2:19:13 PM in a
14155 time zone that is 5 hours and 30 minutes east of @acronym{UTC}.@*
14156 Note: input currently must be in locale independent format. E.g., the
14157 LC_TIME=C below is needed to print back the correct date in many locales:
14159 date -d "$(LC_TIME=C date)"
14161 @xref{Date input formats}.
14163 @item -f @var{datefile}
14164 @itemx --file=@var{datefile}
14167 Parse each line in @var{datefile} as with @option{-d} and display the
14168 resulting date and time. If @var{datefile} is @samp{-}, use standard
14169 input. This is useful when you have many dates to process, because the
14170 system overhead of starting up the @command{date} executable many times can
14173 @item -r @var{file}
14174 @itemx --reference=@var{file}
14176 @opindex --reference
14177 Display the date and time of the last modification of @var{file},
14178 instead of the current date and time.
14185 @opindex --rfc-2822
14186 Display the date and time using the format @samp{%a, %d %b %Y %H:%M:%S
14187 %z}, evaluated in the C locale so abbreviations are always in English.
14191 Fri, 09 Sep 2005 13:51:39 -0700
14194 This format conforms to
14195 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt, Internet
14196 @acronym{RFCs} 2822} and
14197 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc822.txt, 822}, the
14198 current and previous standards for Internet email.
14200 @item --rfc-3339=@var{timespec}
14201 @opindex --rfc-3339=@var{timespec}
14202 Display the date using a format specified by
14203 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt, Internet
14204 @acronym{RFC} 3339}. This is a subset of the @acronym{ISO} 8601
14205 format, except that it also permits applications to use a space rather
14206 than a @samp{T} to separate dates from times. Unlike the other
14207 standard formats, @acronym{RFC} 3339 format is always suitable as
14208 input for the @option{--date} (@option{-d}) and @option{--file}
14209 (@option{-f}) options, regardless of the current locale.
14211 The argument @var{timespec} specifies how much of the time to include.
14212 It can be one of the following:
14216 Print just the full-date, e.g., @samp{2005-09-14}.
14217 This is equivalent to the format @samp{%Y-%m-%d}.
14220 Print the full-date and full-time separated by a space, e.g.,
14221 @samp{2005-09-14 00:56:06+05:30}. The output ends with a numeric
14222 time-offset; here the @samp{+05:30} means that local time is five
14223 hours and thirty minutes east of @acronym{UTC}. This is equivalent to
14224 the format @samp{%Y-%m-%d %H:%M:%S%:z}.
14227 Like @samp{seconds}, but also print nanoseconds, e.g.,
14228 @samp{2005-09-14 00:56:06.998458565+05:30}.
14229 This is equivalent to the format @samp{%Y-%m-%d %H:%M:%S.%N%:z}.
14233 @item -s @var{datestr}
14234 @itemx --set=@var{datestr}
14237 Set the date and time to @var{datestr}. See @option{-d} above.
14244 @opindex --universal
14245 @cindex Coordinated Universal Time
14247 @cindex Greenwich Mean Time
14250 Use Coordinated Universal Time (@acronym{UTC}) by operating as if the
14251 @env{TZ} environment variable were set to the string @samp{UTC0}.
14253 Universal Time is often called ``Greenwich Mean Time'' (@sc{gmt}) for
14254 historical reasons.
14258 @node Examples of date
14259 @subsection Examples of @command{date}
14261 @cindex examples of @command{date}
14263 Here are a few examples. Also see the documentation for the @option{-d}
14264 option in the previous section.
14269 To print the date of the day before yesterday:
14272 date --date='2 days ago'
14276 To print the date of the day three months and one day hence:
14279 date --date='3 months 1 day'
14283 To print the day of year of Christmas in the current year:
14286 date --date='25 Dec' +%j
14290 To print the current full month name and the day of the month:
14296 But this may not be what you want because for the first nine days of
14297 the month, the @samp{%d} expands to a zero-padded two-digit field,
14298 for example @samp{date -d 1may '+%B %d'} will print @samp{May 01}.
14301 To print a date without the leading zero for one-digit days
14302 of the month, you can use the (@acronym{GNU} extension)
14303 @samp{-} flag to suppress
14304 the padding altogether:
14307 date -d 1may '+%B %-d
14311 To print the current date and time in the format required by many
14312 non-@acronym{GNU} versions of @command{date} when setting the system clock:
14315 date +%m%d%H%M%Y.%S
14319 To set the system clock forward by two minutes:
14322 date --set='+2 minutes'
14326 To print the date in @acronym{RFC} 2822 format,
14327 use @samp{date --rfc-2822}. Here is some example output:
14330 Fri, 09 Sep 2005 13:51:39 -0700
14333 @anchor{%s-examples}
14335 To convert a date string to the number of seconds since the epoch
14336 (which is 1970-01-01 00:00:00 UTC), use the @option{--date} option with
14337 the @samp{%s} format. That can be useful in sorting and/or graphing
14338 and/or comparing data by date. The following command outputs the
14339 number of the seconds since the epoch for the time two minutes after the
14343 date --date='1970-01-01 00:02:00 +0000' +%s
14347 If you do not specify time zone information in the date string,
14348 @command{date} uses your computer's idea of the time zone when
14349 interpreting the string. For example, if your computer's time zone is
14350 that of Cambridge, Massachusetts, which was then 5 hours (i.e., 18,000
14351 seconds) behind UTC:
14354 # local time zone used
14355 date --date='1970-01-01 00:02:00' +%s
14360 If you're sorting or graphing dated data, your raw date values may be
14361 represented as seconds since the epoch. But few people can look at
14362 the date @samp{946684800} and casually note ``Oh, that's the first second
14363 of the year 2000 in Greenwich, England.''
14366 date --date='2000-01-01 UTC' +%s
14370 An alternative is to use the @option{--utc} (@option{-u}) option.
14371 Then you may omit @samp{UTC} from the date string. Although this
14372 produces the same result for @samp{%s} and many other format sequences,
14373 with a time zone offset different from zero, it would give a different
14374 result for zone-dependent formats like @samp{%z}.
14377 date -u --date=2000-01-01 +%s
14381 To convert such an unwieldy number of seconds back to
14382 a more readable form, use a command like this:
14385 # local time zone used
14386 date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
14387 1999-12-31 19:00:00 -0500
14390 Or if you do not mind depending on the @samp{@@} feature present since
14391 coreutils 5.3.0, you could shorten this to:
14394 date -d @@946684800 +"%F %T %z"
14395 1999-12-31 19:00:00 -0500
14398 Often it is better to output UTC-relative date and time:
14401 date -u -d '1970-01-01 946684800 seconds' +"%Y-%m-%d %T %z"
14402 2000-01-01 00:00:00 +0000
14408 @node arch invocation
14409 @section @command{arch}: Print machine hardware name
14412 @cindex print machine hardware name
14413 @cindex system information, printing
14415 @command{arch} prints the machine hardware name,
14416 and is equivalent to @samp{uname -m}.
14420 arch [@var{option}]
14423 The program accepts the @ref{Common options} only.
14428 @node nproc invocation
14429 @section @command{nproc}: Print the number of available processors
14432 @cindex Print the number of processors
14433 @cindex system information, printing
14435 Print the number of processing units available to the current process,
14436 which may be less than the number of online processors.
14437 If this information is not accessible, then print the number of
14438 processors installed. If the @env{OMP_NUM_THREADS} environment variable is
14439 set, then it will determine the returned value. The result is guaranteed to be
14440 greater than zero. Synopsis:
14443 nproc [@var{option}]
14446 The program accepts the following options. Also see @ref{Common options}.
14452 Print the number of installed processors on the system, which may
14453 be greater than the number online or available to the current process.
14454 The @env{OMP_NUM_THREADS} environment variable is not honored in this case.
14456 @item --ignore=@var{number}
14458 If possible, exclude this @var{number} of processing units.
14465 @node uname invocation
14466 @section @command{uname}: Print system information
14469 @cindex print system information
14470 @cindex system information, printing
14472 @command{uname} prints information about the machine and operating system
14473 it is run on. If no options are given, @command{uname} acts as if the
14474 @option{-s} option were given. Synopsis:
14477 uname [@var{option}]@dots{}
14480 If multiple options or @option{-a} are given, the selected information is
14481 printed in this order:
14484 @var{kernel-name} @var{nodename} @var{kernel-release} @var{kernel-version}
14485 @var{machine} @var{processor} @var{hardware-platform} @var{operating-system}
14488 The information may contain internal spaces, so such output cannot be
14489 parsed reliably. In the following example, @var{release} is
14490 @samp{2.2.18ss.e820-bda652a #4 SMP Tue Jun 5 11:24:08 PDT 2001}:
14494 @result{} Linux dum 2.2.18 #4 SMP Tue Jun 5 11:24:08 PDT 2001 i686@c
14495 unknown unknown GNU/Linux
14499 The program accepts the following options. Also see @ref{Common options}.
14507 Print all of the below information, except omit the processor type
14508 and the hardware platform name if they are unknown.
14511 @itemx --hardware-platform
14513 @opindex --hardware-platform
14514 @cindex implementation, hardware
14515 @cindex hardware platform
14516 @cindex platform, hardware
14517 Print the hardware platform name
14518 (sometimes called the hardware implementation).
14519 Print @samp{unknown} if the kernel does not make this information
14520 easily available, as is the case with Linux kernels.
14526 @cindex machine type
14527 @cindex hardware class
14528 @cindex hardware type
14529 Print the machine hardware name (sometimes called the hardware class
14535 @opindex --nodename
14538 @cindex network node name
14539 Print the network node hostname.
14544 @opindex --processor
14545 @cindex host processor type
14546 Print the processor type (sometimes called the instruction set
14547 architecture or ISA).
14548 Print @samp{unknown} if the kernel does not make this information
14549 easily available, as is the case with Linux kernels.
14552 @itemx --operating-system
14554 @opindex --operating-system
14555 @cindex operating system name
14556 Print the name of the operating system.
14559 @itemx --kernel-release
14561 @opindex --kernel-release
14562 @cindex kernel release
14563 @cindex release of kernel
14564 Print the kernel release.
14567 @itemx --kernel-name
14569 @opindex --kernel-name
14570 @cindex kernel name
14571 @cindex name of kernel
14572 Print the kernel name.
14573 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) calls this
14574 ``the implementation of the operating system'', because the
14575 @acronym{POSIX} specification itself has no notion of ``kernel''.
14576 The kernel name might be the same as the operating system name printed
14577 by the @option{-o} or @option{--operating-system} option, but it might
14578 differ. Some operating systems (e.g., FreeBSD, HP-UX) have the same
14579 name as their underlying kernels; others (e.g., GNU/Linux, Solaris)
14583 @itemx --kernel-version
14585 @opindex --kernel-version
14586 @cindex kernel version
14587 @cindex version of kernel
14588 Print the kernel version.
14595 @node hostname invocation
14596 @section @command{hostname}: Print or set system name
14599 @cindex setting the hostname
14600 @cindex printing the hostname
14601 @cindex system name, printing
14602 @cindex appropriate privileges
14604 With no arguments, @command{hostname} prints the name of the current host
14605 system. With one argument, it sets the current host name to the
14606 specified string. You must have appropriate privileges to set the host
14610 hostname [@var{name}]
14613 The only options are @option{--help} and @option{--version}. @xref{Common
14619 @node hostid invocation
14620 @section @command{hostid}: Print numeric host identifier
14623 @cindex printing the host identifier
14625 @command{hostid} prints the numeric identifier of the current host
14626 in hexadecimal. This command accepts no arguments.
14627 The only options are @option{--help} and @option{--version}.
14628 @xref{Common options}.
14630 For example, here's what it prints on one system I use:
14637 On that system, the 32-bit quantity happens to be closely
14638 related to the system's Internet address, but that isn't always
14643 @node uptime invocation
14644 @section @command{uptime}: Print system uptime and load
14647 @cindex printing the system uptime and load
14649 @command{uptime} prints the current time, the system's uptime, the
14650 number of logged-in users and the current load average.
14652 If an argument is specified, it is used as the file to be read
14653 to discover how many users are logged in. If no argument is
14654 specified, a system default is used (@command{uptime --help} indicates
14655 the default setting).
14657 The only options are @option{--help} and @option{--version}.
14658 @xref{Common options}.
14660 For example, here's what it prints right now on one system I use:
14664 14:07 up 3:35, 3 users, load average: 1.39, 1.15, 1.04
14667 The precise method of calculation of load average varies somewhat
14668 between systems. Some systems calculate it as the average number of
14669 runnable processes over the last 1, 5 and 15 minutes, but some systems
14670 also include processes in the uninterruptible sleep state (that is,
14671 those processes which are waiting for disk I/O). The Linux kernel
14672 includes uninterruptible processes.
14674 @node SELinux context
14675 @chapter SELinux context
14677 @cindex SELinux context
14678 @cindex SELinux, context
14679 @cindex commands for SELinux context
14681 This section describes commands for operations with SELinux
14685 * chcon invocation:: Change SELinux context of file
14686 * runcon invocation:: Run a command in specified SELinux context
14689 @node chcon invocation
14690 @section @command{chcon}: Change SELinux context of file
14693 @cindex changing security context
14694 @cindex change SELinux context
14696 @command{chcon} changes the SELinux security context of the selected files.
14700 chcon [@var{option}]@dots{} @var{context} @var{file}@dots{}
14701 chcon [@var{option}]@dots{} [-u @var{user}] [-r @var{role}] [-l @var{range}]@c
14702 [-t @var{type}] @var{file}@dots{}
14703 chcon [@var{option}]@dots{} --reference=@var{rfile} @var{file}@dots{}
14706 Change the SELinux security context of each @var{file} to @var{context}.
14707 With @option{--reference}, change the security context of each @var{file}
14708 to that of @var{rfile}.
14710 The program accepts the following options. Also see @ref{Common options}.
14715 @itemx --no-dereference
14717 @opindex --no-dereference
14718 @cindex no dereference
14719 Affect symbolic links instead of any referenced file.
14721 @item --reference=@var{rfile}
14722 @opindex --reference
14723 @cindex reference file
14724 Use @var{rfile}'s security context rather than specifying a @var{context} value.
14729 @opindex --recursive
14730 Operate on files and directories recursively.
14733 @xref{Traversing symlinks}.
14736 @xref{Traversing symlinks}.
14739 @xref{Traversing symlinks}.
14746 Output a diagnostic for every file processed.
14748 @item -u @var{user}
14749 @itemx --user=@var{user}
14752 Set user @var{user} in the target security context.
14754 @item -r @var{role}
14755 @itemx --role=@var{role}
14758 Set role @var{role} in the target security context.
14760 @item -t @var{type}
14761 @itemx --type=@var{type}
14764 Set type @var{type} in the target security context.
14766 @item -l @var{range}
14767 @itemx --range=@var{range}
14770 Set range @var{range} in the target security context.
14776 @node runcon invocation
14777 @section @command{runcon}: Run a command in specified SELinux context
14780 @cindex run with security context
14783 @command{runcon} runs file in specified SELinux security context.
14787 runcon @var{context} @var{command} [@var{args}]
14788 runcon [ -c ] [-u @var{user}] [-r @var{role}] [-t @var{type}]@c
14789 [-l @var{range}] @var{command} [@var{args}]
14792 Run @var{command} with completely-specified @var{context}, or with
14793 current or transitioned security context modified by one or more of @var{level},
14794 @var{role}, @var{type} and @var{user}.
14796 If none of @option{-c}, @option{-t}, @option{-u}, @option{-r}, or @option{-l}
14797 is specified, the first argument is used as the complete context.
14798 Any additional arguments after @var{command}
14799 are interpreted as arguments to the command.
14801 With neither @var{context} nor @var{command}, print the current
14804 The program accepts the following options. Also see @ref{Common options}.
14812 Compute process transition context before modifying.
14814 @item -u @var{user}
14815 @itemx --user=@var{user}
14818 Set user @var{user} in the target security context.
14820 @item -r @var{role}
14821 @itemx --role=@var{role}
14824 Set role @var{role} in the target security context.
14826 @item -t @var{type}
14827 @itemx --type=@var{type}
14830 Set type @var{type} in the target security context.
14832 @item -l @var{range}
14833 @itemx --range=@var{range}
14836 Set range @var{range} in the target security context.
14840 @cindex exit status of @command{runcon}
14844 126 if @var{command} is found but cannot be invoked
14845 127 if @command{runcon} itself fails or if @var{command} cannot be found
14846 the exit status of @var{command} otherwise
14849 @node Modified command invocation
14850 @chapter Modified command invocation
14852 @cindex modified command invocation
14853 @cindex invocation of commands, modified
14854 @cindex commands for invoking other commands
14856 This section describes commands that run other commands in some context
14857 different than the current one: a modified environment, as a different
14861 * chroot invocation:: Modify the root directory.
14862 * env invocation:: Modify environment variables.
14863 * nice invocation:: Modify niceness.
14864 * nohup invocation:: Immunize to hangups.
14865 * stdbuf invocation:: Modify buffering of standard streams.
14866 * su invocation:: Modify user and group ID.
14867 * timeout invocation:: Run with time limit.
14871 @node chroot invocation
14872 @section @command{chroot}: Run a command with a different root directory
14875 @cindex running a program in a specified root directory
14876 @cindex root directory, running a program in a specified
14878 @command{chroot} runs a command with a specified root directory.
14879 On many systems, only the super-user can do this.@footnote{However,
14880 some systems (e.g., FreeBSD) can be configured to allow certain regular
14881 users to use the @code{chroot} system call, and hence to run this program.
14882 Also, on Cygwin, anyone can run the @command{chroot} command, because the
14883 underlying function is non-privileged due to lack of support in MS-Windows.}
14887 chroot @var{option} @var{newroot} [@var{command} [@var{args}]@dots{}]
14888 chroot @var{option}
14891 Ordinarily, file names are looked up starting at the root of the
14892 directory structure, i.e., @file{/}. @command{chroot} changes the root to
14893 the directory @var{newroot} (which must exist) and then runs
14894 @var{command} with optional @var{args}. If @var{command} is not
14895 specified, the default is the value of the @env{SHELL} environment
14896 variable or @command{/bin/sh} if not set, invoked with the @option{-i} option.
14897 @var{command} must not be a special built-in utility
14898 (@pxref{Special built-in utilities}).
14900 The program accepts the following options. Also see @ref{Common options}.
14901 Options must precede operands.
14905 @itemx --userspec=@var{user}[:@var{group}]
14906 @opindex --userspec
14907 By default, @var{command} is run with the same credentials
14908 as the invoking process.
14909 Use this option to run it as a different @var{user} and/or with a
14910 different primary @var{group}.
14912 @itemx --groups=@var{groups}
14914 Use this option to specify the supplementary @var{groups} to be
14915 used by the new process.
14916 The items in the list (names or numeric IDs) must be separated by commas.
14920 Here are a few tips to help avoid common problems in using chroot.
14921 To start with a simple example, make @var{command} refer to a statically
14922 linked binary. If you were to use a dynamically linked executable, then
14923 you'd have to arrange to have the shared libraries in the right place under
14924 your new root directory.
14926 For example, if you create a statically linked @command{ls} executable,
14927 and put it in @file{/tmp/empty}, you can run this command as root:
14930 $ chroot /tmp/empty /ls -Rl /
14933 Then you'll see output like this:
14938 -rwxr-xr-x 1 0 0 1041745 Aug 16 11:17 ls
14941 If you want to use a dynamically linked executable, say @command{bash},
14942 then first run @samp{ldd bash} to see what shared objects it needs.
14943 Then, in addition to copying the actual binary, also copy the listed
14944 files to the required positions under your intended new root directory.
14945 Finally, if the executable requires any other files (e.g., data, state,
14946 device files), copy them into place, too.
14948 @cindex exit status of @command{chroot}
14952 125 if @command{chroot} itself fails
14953 126 if @var{command} is found but cannot be invoked
14954 127 if @var{command} cannot be found
14955 the exit status of @var{command} otherwise
14959 @node env invocation
14960 @section @command{env}: Run a command in a modified environment
14963 @cindex environment, running a program in a modified
14964 @cindex modified environment, running a program in a
14965 @cindex running a program in a modified environment
14967 @command{env} runs a command with a modified environment. Synopses:
14970 env [@var{option}]@dots{} [@var{name}=@var{value}]@dots{} @c
14971 [@var{command} [@var{args}]@dots{}]
14975 Operands of the form @samp{@var{variable}=@var{value}} set
14976 the environment variable @var{variable} to value @var{value}.
14977 @var{value} may be empty (@samp{@var{variable}=}). Setting a variable
14978 to an empty value is different from unsetting it.
14979 These operands are evaluated left-to-right, so if two operands
14980 mention the same variable the earlier is ignored.
14982 Environment variable names can be empty, and can contain any
14983 characters other than @samp{=} and @acronym{ASCII} @sc{nul}.
14984 However, it is wise to limit yourself to names that
14985 consist solely of underscores, digits, and @acronym{ASCII} letters,
14986 and that begin with a non-digit, as applications like the shell do not
14987 work well with other names.
14990 The first operand that does not contain the character @samp{=}
14991 specifies the program to invoke; it is
14992 searched for according to the @env{PATH} environment variable. Any
14993 remaining arguments are passed as arguments to that program.
14994 The program should not be a special built-in utility
14995 (@pxref{Special built-in utilities}).
14997 Modifications to @env{PATH} take effect prior to searching for
14998 @var{command}. Use caution when reducing @env{PATH}; behavior is
14999 not portable when @env{PATH} is undefined or omits key directories
15000 such as @file{/bin}.
15002 In the rare case that a utility contains a @samp{=} in the name, the
15003 only way to disambiguate it from a variable assignment is to use an
15004 intermediate command for @var{command}, and pass the problematic
15005 program name via @var{args}. For example, if @file{./prog=} is an
15006 executable in the current @env{PATH}:
15009 env prog= true # runs 'true', with prog= in environment
15010 env ./prog= true # runs 'true', with ./prog= in environment
15011 env -- prog= true # runs 'true', with prog= in environment
15012 env sh -c '\prog= true' # runs 'prog=' with argument 'true'
15013 env sh -c 'exec "$@@"' sh prog= true # also runs 'prog='
15016 @cindex environment, printing
15018 If no command name is specified following the environment
15019 specifications, the resulting environment is printed. This is like
15020 specifying the @command{printenv} program.
15022 For some examples, suppose the environment passed to @command{env}
15023 contains @samp{LOGNAME=rms}, @samp{EDITOR=emacs}, and
15024 @samp{PATH=.:/gnubin:/hacks}:
15029 Output the current environment.
15031 $ env | LC_ALL=C sort
15034 PATH=.:/gnubin:/hacks
15038 Run @command{foo} with a reduced environment, preserving only the
15039 original @env{PATH} to avoid problems in locating @command{foo}.
15041 env - PATH="$PATH" foo
15045 Run @command{foo} with the environment containing @samp{LOGNAME=rms},
15046 @samp{EDITOR=emacs}, and @samp{PATH=.:/gnubin:/hacks}, and guarantees
15047 that @command{foo} was found in the file system rather than as a shell
15054 Run @command{nemacs} with the environment containing @samp{LOGNAME=foo},
15055 @samp{EDITOR=emacs}, @samp{PATH=.:/gnubin:/hacks}, and
15056 @samp{DISPLAY=gnu:0}.
15058 env DISPLAY=gnu:0 LOGNAME=foo nemacs
15062 Attempt to run the program @command{/energy/--} (as that is the only
15063 possible path search result); if the command exists, the environment
15064 will contain @samp{LOGNAME=rms} and @samp{PATH=/energy}, and the
15065 arguments will be @samp{e=mc2}, @samp{bar}, and @samp{baz}.
15067 env -u EDITOR PATH=/energy -- e=mc2 bar baz
15073 The program accepts the following options. Also see @ref{Common options}.
15074 Options must precede operands.
15080 @item -u @var{name}
15081 @itemx --unset=@var{name}
15084 Remove variable @var{name} from the environment, if it was in the
15089 @itemx --ignore-environment
15092 @opindex --ignore-environment
15093 Start with an empty environment, ignoring the inherited environment.
15097 @cindex exit status of @command{env}
15101 0 if no @var{command} is specified and the environment is output
15102 125 if @command{env} itself fails
15103 126 if @var{command} is found but cannot be invoked
15104 127 if @var{command} cannot be found
15105 the exit status of @var{command} otherwise
15109 @node nice invocation
15110 @section @command{nice}: Run a command with modified niceness
15114 @cindex scheduling, affecting
15115 @cindex appropriate privileges
15117 @command{nice} prints or modifies a process's @dfn{niceness},
15118 a parameter that affects whether the process is scheduled favorably.
15122 nice [@var{option}]@dots{} [@var{command} [@var{arg}]@dots{}]
15125 If no arguments are given, @command{nice} prints the current niceness.
15126 Otherwise, @command{nice} runs the given @var{command} with its
15127 niceness adjusted. By default, its niceness is incremented by 10.
15129 Niceness values range at least from @minus{}20 (process has high priority
15130 and gets more resources, thus slowing down other processes) through 19
15131 (process has lower priority and runs slowly itself, but has less impact
15132 on the speed of other running processes). Some systems
15133 may have a wider range of nicenesses; conversely, other systems may
15134 enforce more restrictive limits. An attempt to set the niceness
15135 outside the supported range is treated as an attempt to use the
15136 minimum or maximum supported value.
15138 A niceness should not be confused with a scheduling priority, which
15139 lets applications determine the order in which threads are scheduled
15140 to run. Unlike a priority, a niceness is merely advice to the
15141 scheduler, which the scheduler is free to ignore. Also, as a point of
15142 terminology, @acronym{POSIX} defines the behavior of @command{nice} in
15143 terms of a @dfn{nice value}, which is the nonnegative difference
15144 between a niceness and the minimum niceness. Though @command{nice}
15145 conforms to @acronym{POSIX}, its documentation and diagnostics use the
15146 term ``niceness'' for compatibility with historical practice.
15148 @var{command} must not be a special built-in utility (@pxref{Special
15149 built-in utilities}).
15151 @mayConflictWithShellBuiltIn{nice}
15153 The program accepts the following option. Also see @ref{Common options}.
15154 Options must precede operands.
15157 @item -n @var{adjustment}
15158 @itemx --adjustment=@var{adjustment}
15160 @opindex --adjustment
15161 Add @var{adjustment} instead of 10 to the command's niceness. If
15162 @var{adjustment} is negative and you lack appropriate privileges,
15163 @command{nice} issues a warning but otherwise acts as if you specified
15166 For compatibility @command{nice} also supports an obsolete
15167 option syntax @option{-@var{adjustment}}. New scripts should use
15168 @option{-n @var{adjustment}} instead.
15172 @cindex exit status of @command{nice}
15176 0 if no @var{command} is specified and the niceness is output
15177 125 if @command{nice} itself fails
15178 126 if @var{command} is found but cannot be invoked
15179 127 if @var{command} cannot be found
15180 the exit status of @var{command} otherwise
15183 It is sometimes useful to run a non-interactive program with reduced niceness.
15186 $ nice factor 4611686018427387903
15189 Since @command{nice} prints the current niceness,
15190 you can invoke it through itself to demonstrate how it works.
15192 The default behavior is to increase the niceness by @samp{10}:
15203 The @var{adjustment} is relative to the current niceness. In the
15204 next example, the first @command{nice} invocation runs the second one
15205 with niceness 10, and it in turn runs the final one with a niceness
15209 $ nice nice -n 3 nice
15213 Specifying a niceness larger than the supported range
15214 is the same as specifying the maximum supported value:
15217 $ nice -n 10000000000 nice
15221 Only a privileged user may run a process with lower niceness:
15225 nice: cannot set niceness: Permission denied
15227 $ sudo nice -n -1 nice
15232 @node nohup invocation
15233 @section @command{nohup}: Run a command immune to hangups
15236 @cindex hangups, immunity to
15237 @cindex immunity to hangups
15238 @cindex logging out and continuing to run
15241 @command{nohup} runs the given @var{command} with hangup signals ignored,
15242 so that the command can continue running in the background after you log
15246 nohup @var{command} [@var{arg}]@dots{}
15249 If standard input is a terminal, it is redirected from
15250 @file{/dev/null} so that terminal sessions do not mistakenly consider
15251 the terminal to be used by the command. This is a @acronym{GNU}
15252 extension; programs intended to be portable to non-@acronym{GNU} hosts
15253 should use @samp{nohup @var{command} [@var{arg}]@dots{} </dev/null}
15257 If standard output is a terminal, the command's standard output is appended
15258 to the file @file{nohup.out}; if that cannot be written to, it is appended
15259 to the file @file{$HOME/nohup.out}; and if that cannot be written to, the
15260 command is not run.
15261 Any @file{nohup.out} or @file{$HOME/nohup.out} file created by
15262 @command{nohup} is made readable and writable only to the user,
15263 regardless of the current umask settings.
15265 If standard error is a terminal, it is normally redirected to the same file
15266 descriptor as the (possibly-redirected) standard output.
15267 However, if standard output is closed, standard error terminal output
15268 is instead appended to the file @file{nohup.out} or
15269 @file{$HOME/nohup.out} as above.
15271 To capture the command's output to a file other than @file{nohup.out}
15272 you can redirect it. For example, to capture the output of
15276 nohup make > make.log
15279 @command{nohup} does not automatically put the command it runs in the
15280 background; you must do that explicitly, by ending the command line
15281 with an @samp{&}. Also, @command{nohup} does not alter the
15282 niceness of @var{command}; use @command{nice} for that,
15283 e.g., @samp{nohup nice @var{command}}.
15285 @var{command} must not be a special built-in utility (@pxref{Special
15286 built-in utilities}).
15288 The only options are @option{--help} and @option{--version}. @xref{Common
15289 options}. Options must precede operands.
15291 @cindex exit status of @command{nohup}
15295 125 if @command{nohup} itself fails, and @env{POSIXLY_CORRECT} is not set
15296 126 if @var{command} is found but cannot be invoked
15297 127 if @var{command} cannot be found
15298 the exit status of @var{command} otherwise
15301 If @env{POSIXLY_CORRECT} is set, internal failures give status 127
15305 @node stdbuf invocation
15306 @section @command{stdbuf}: Run a command with modified I/O stream buffering
15309 @cindex standard streams, buffering
15310 @cindex line buffered
15312 @command{stdbuf} allows one to modify the buffering operations of the
15313 three standard I/O streams associated with a program. Synopsis:
15316 stdbuf @var{option}@dots{} @var{command}
15319 Any additional @var{arg}s are passed as additional arguments to the
15322 The program accepts the following options. Also see @ref{Common options}.
15326 @item -i @var{mode}
15327 @itemx --input=@var{mode}
15330 Adjust the standard input stream buffering.
15332 @item -o @var{mode}
15333 @itemx --output=@var{mode}
15336 Adjust the standard output stream buffering.
15338 @item -e @var{mode}
15339 @itemx --error=@var{mode}
15342 Adjust the standard error stream buffering.
15346 The @var{mode} can be specified as follows:
15351 Set the stream to line buffered mode.
15352 In this mode data is coalesced until a newline is output or
15353 input is read from any stream attached to a terminal device.
15354 This option is invalid with standard input.
15357 Disable buffering of the selected stream.
15358 In this mode data is output immediately and only the
15359 amount of data requested is read from input.
15362 Specify the size of the buffer to use in fully buffered mode.
15363 @multiplierSuffixesNoBlocks{size}
15367 NOTE: If @var{command} adjusts the buffering of its standard streams
15368 (@command{tee} does for e.g.) then that will override corresponding settings
15369 changed by @command{stdbuf}. Also some filters (like @command{dd} and
15370 @command{cat} etc.) don't use streams for I/O, and are thus unaffected
15371 by @command{stdbuf} settings.
15373 @cindex exit status of @command{stdbuf}
15377 125 if @command{stdbuf} itself fails
15378 126 if @var{command} is found but cannot be invoked
15379 127 if @var{command} cannot be found
15380 the exit status of @var{command} otherwise
15384 @node su invocation
15385 @section @command{su}: Run a command with substitute user and group ID
15388 @cindex substitute user and group IDs
15389 @cindex user ID, switching
15390 @cindex super-user, becoming
15391 @cindex root, becoming
15393 @command{su} allows one user to temporarily become another user. It runs a
15394 command (often an interactive shell) with the real and effective user
15395 ID, group ID, and supplemental groups of a given @var{user}. Synopsis:
15398 su [@var{option}]@dots{} [@var{user} [@var{arg}]@dots{}]
15401 @cindex passwd entry, and @command{su} shell
15403 @flindex /etc/passwd
15404 If no @var{user} is given, the default is @code{root}, the super-user.
15405 The shell to use is taken from @var{user}'s @code{passwd} entry, or
15406 @file{/bin/sh} if none is specified there. If @var{user} has a
15407 password, @command{su} prompts for the password unless run by a user with
15408 effective user ID of zero (the super-user).
15414 @cindex login shell
15415 By default, @command{su} does not change the current directory.
15416 It sets the environment variables @env{HOME} and @env{SHELL}
15417 from the password entry for @var{user}, and if @var{user} is not
15418 the super-user, sets @env{USER} and @env{LOGNAME} to @var{user}.
15419 By default, the shell is not a login shell.
15421 Any additional @var{arg}s are passed as additional arguments to the
15424 @cindex @option{-su}
15425 GNU @command{su} does not treat @file{/bin/sh} or any other shells specially
15426 (e.g., by setting @code{argv[0]} to @option{-su}, passing @option{-c} only
15427 to certain shells, etc.).
15430 @command{su} can optionally be compiled to use @code{syslog} to report
15431 failed, and optionally successful, @command{su} attempts. (If the system
15432 supports @code{syslog}.) However, GNU @command{su} does not check if the
15433 user is a member of the @code{wheel} group; see below.
15435 The program accepts the following options. Also see @ref{Common options}.
15438 @item -c @var{command}
15439 @itemx --command=@var{command}
15442 Pass @var{command}, a single command line to run, to the shell with
15443 a @option{-c} option instead of starting an interactive shell.
15450 @cindex file name pattern expansion, disabled
15451 @cindex globbing, disabled
15452 Pass the @option{-f} option to the shell. This probably only makes sense
15453 if the shell run is @command{csh} or @command{tcsh}, for which the @option{-f}
15454 option prevents reading the startup file (@file{.cshrc}). With
15455 Bourne-like shells, the @option{-f} option disables file name pattern
15456 expansion (globbing), which is not likely to be useful.
15464 @c other variables already indexed above
15467 @cindex login shell, creating
15468 Make the shell a login shell. This means the following. Unset all
15469 environment variables except @env{TERM}, @env{HOME}, and @env{SHELL}
15470 (which are set as described above), and @env{USER} and @env{LOGNAME}
15471 (which are set, even for the super-user, as described above), and set
15472 @env{PATH} to a compiled-in default value. Change to @var{user}'s home
15473 directory. Prepend @samp{-} to the shell's name, intended to make it
15474 read its login startup file(s).
15478 @itemx --preserve-environment
15481 @opindex --preserve-environment
15482 @cindex environment, preserving
15483 @flindex /etc/shells
15484 @cindex restricted shell
15485 Do not change the environment variables @env{HOME}, @env{USER},
15486 @env{LOGNAME}, or @env{SHELL}. Run the shell given in the environment
15487 variable @env{SHELL} instead of the shell from @var{user}'s passwd
15488 entry, unless the user running @command{su} is not the super-user and
15489 @var{user}'s shell is restricted. A @dfn{restricted shell} is one that
15490 is not listed in the file @file{/etc/shells}, or in a compiled-in list
15491 if that file does not exist. Parts of what this option does can be
15492 overridden by @option{--login} and @option{--shell}.
15494 @item -s @var{shell}
15495 @itemx --shell=@var{shell}
15498 Run @var{shell} instead of the shell from @var{user}'s passwd entry,
15499 unless the user running @command{su} is not the super-user and @var{user}'s
15500 shell is restricted (see @option{-m} just above).
15504 @cindex exit status of @command{su}
15508 125 if @command{su} itself fails
15509 126 if subshell is found but cannot be invoked
15510 127 if subshell cannot be found
15511 the exit status of the subshell otherwise
15514 @cindex wheel group, not supported
15515 @cindex group wheel, not supported
15517 @subsection Why GNU @command{su} does not support the @samp{wheel} group
15519 (This section is by Richard Stallman.)
15523 Sometimes a few of the users try to hold total power over all the
15524 rest. For example, in 1984, a few users at the MIT AI lab decided to
15525 seize power by changing the operator password on the Twenex system and
15526 keeping it secret from everyone else. (I was able to thwart this coup
15527 and give power back to the users by patching the kernel, but I
15528 wouldn't know how to do that in Unix.)
15530 However, occasionally the rulers do tell someone. Under the usual
15531 @command{su} mechanism, once someone learns the root password who
15532 sympathizes with the ordinary users, he or she can tell the rest. The
15533 ``wheel group'' feature would make this impossible, and thus cement the
15534 power of the rulers.
15536 I'm on the side of the masses, not that of the rulers. If you are
15537 used to supporting the bosses and sysadmins in whatever they do, you
15538 might find this idea strange at first.
15541 @node timeout invocation
15542 @section @command{timeout}: Run a command with a time limit
15546 @cindex run commands with bounded time
15548 @command{timeout} runs the given @var{command} and kills it if it is
15549 still running after the specified time interval. Synopsis:
15552 timeout [@var{option}] @var{duration} @var{command} [@var{arg}]@dots{}
15555 @var{command} must not be a special built-in utility (@pxref{Special
15556 built-in utilities}).
15558 The program accepts the following options. Also see @ref{Common options}.
15559 Options must precede operands.
15562 @item -k @var{duration}
15563 @itemx --kill-after=@var{duration}
15565 @opindex --kill-after
15566 Ensure the monitored @var{command} is killed by also sending a @samp{KILL}
15567 signal, after the specified @var{duration}. Without this option, if the
15568 selected signal proves not to be fatal, @command{timeout} does not kill
15571 @item -s @var{signal}
15572 @itemx --signal=@var{signal}
15575 Send this @var{signal} to @var{command} on timeout, rather than the
15576 default @samp{TERM} signal. @var{signal} may be a name like @samp{HUP}
15577 or a number. Also see @xref{Signal specifications}.
15581 @var{duration} is an integer followed by an optional unit:
15583 @samp{s} for seconds (the default)
15584 @samp{m} for minutes
15588 A duration of 0 disables the associated timeout.
15590 @cindex exit status of @command{timeout}
15594 124 if @var{command} times out
15595 125 if @command{timeout} itself fails
15596 126 if @var{command} is found but cannot be invoked
15597 127 if @var{command} cannot be found
15598 the exit status of @var{command} otherwise
15602 @node Process control
15603 @chapter Process control
15605 @cindex processes, commands for controlling
15606 @cindex commands for controlling processes
15609 * kill invocation:: Sending a signal to processes.
15613 @node kill invocation
15614 @section @command{kill}: Send a signal to processes
15617 @cindex send a signal to processes
15619 The @command{kill} command sends a signal to processes, causing them
15620 to terminate or otherwise act upon receiving the signal in some way.
15621 Alternatively, it lists information about signals. Synopses:
15624 kill [-s @var{signal} | --signal @var{signal} | -@var{signal}] @var{pid}@dots{}
15625 kill [-l | --list | -t | --table] [@var{signal}]@dots{}
15628 @mayConflictWithShellBuiltIn{kill}
15630 The first form of the @command{kill} command sends a signal to all
15631 @var{pid} arguments. The default signal to send if none is specified
15632 is @samp{TERM}. The special signal number @samp{0} does not denote a
15633 valid signal, but can be used to test whether the @var{pid} arguments
15634 specify processes to which a signal could be sent.
15636 If @var{pid} is positive, the signal is sent to the process with the
15637 process ID @var{pid}. If @var{pid} is zero, the signal is sent to all
15638 processes in the process group of the current process. If @var{pid}
15639 is @minus{}1, the signal is sent to all processes for which the user has
15640 permission to send a signal. If @var{pid} is less than @minus{}1, the signal
15641 is sent to all processes in the process group that equals the absolute
15642 value of @var{pid}.
15644 If @var{pid} is not positive, a system-dependent set of system
15645 processes is excluded from the list of processes to which the signal
15648 If a negative @var{pid} argument is desired as the first one, it
15649 should be preceded by @option{--}. However, as a common extension to
15650 @acronym{POSIX}, @option{--} is not required with @samp{kill
15651 -@var{signal} -@var{pid}}. The following commands are equivalent:
15660 The first form of the @command{kill} command succeeds if every @var{pid}
15661 argument specifies at least one process that the signal was sent to.
15663 The second form of the @command{kill} command lists signal information.
15664 Either the @option{-l} or @option{--list} option, or the @option{-t}
15665 or @option{--table} option must be specified. Without any
15666 @var{signal} argument, all supported signals are listed. The output
15667 of @option{-l} or @option{--list} is a list of the signal names, one
15668 per line; if @var{signal} is already a name, the signal number is
15669 printed instead. The output of @option{-t} or @option{--table} is a
15670 table of signal numbers, names, and descriptions. This form of the
15671 @command{kill} command succeeds if all @var{signal} arguments are valid
15672 and if there is no output error.
15674 The @command{kill} command also supports the @option{--help} and
15675 @option{--version} options. @xref{Common options}.
15677 A @var{signal} may be a signal name like @samp{HUP}, or a signal
15678 number like @samp{1}, or an exit status of a process terminated by the
15679 signal. A signal name can be given in canonical form or prefixed by
15680 @samp{SIG}. The case of the letters is ignored, except for the
15681 @option{-@var{signal}} option which must use upper case to avoid
15682 ambiguity with lower case option letters. For a list of supported
15683 signal names and numbers see @xref{Signal specifications}.
15688 @cindex delaying commands
15689 @cindex commands for delaying
15691 @c Perhaps @command{wait} or other commands should be described here also?
15694 * sleep invocation:: Delay for a specified time.
15698 @node sleep invocation
15699 @section @command{sleep}: Delay for a specified time
15702 @cindex delay for a specified time
15704 @command{sleep} pauses for an amount of time specified by the sum of
15705 the values of the command line arguments.
15709 sleep @var{number}[smhd]@dots{}
15713 Each argument is a number followed by an optional unit; the default
15714 is seconds. The units are:
15727 Historical implementations of @command{sleep} have required that
15728 @var{number} be an integer, and only accepted a single argument
15729 without a suffix. However, GNU @command{sleep} accepts
15730 arbitrary floating point numbers. @xref{Floating point}.
15732 The only options are @option{--help} and @option{--version}. @xref{Common
15735 @c sleep is a shell built-in at least with Solaris 11's /bin/sh
15736 @mayConflictWithShellBuiltIn{sleep}
15741 @node Numeric operations
15742 @chapter Numeric operations
15744 @cindex numeric operations
15745 These programs do numerically-related operations.
15748 * factor invocation:: Show factors of numbers.
15749 * seq invocation:: Print sequences of numbers.
15753 @node factor invocation
15754 @section @command{factor}: Print prime factors
15757 @cindex prime factors
15759 @command{factor} prints prime factors. Synopses:
15762 factor [@var{number}]@dots{}
15763 factor @var{option}
15766 If no @var{number} is specified on the command line, @command{factor} reads
15767 numbers from standard input, delimited by newlines, tabs, or spaces.
15769 The @command{factor} command supports only a small number of options:
15773 Print a short help on standard output, then exit without further
15777 Print the program version on standard output, then exit without further
15781 Factoring the product of the eighth and ninth Mersenne primes
15782 takes about 30 milliseconds of CPU time on a 2.2 GHz Athlon.
15785 M8=`echo 2^31-1|bc` ; M9=`echo 2^61-1|bc`
15786 /usr/bin/time -f '%U' factor $(echo "$M8 * $M9" | bc)
15787 4951760154835678088235319297: 2147483647 2305843009213693951
15791 Similarly, factoring the eighth Fermat number @math{2^{256}+1} takes
15792 about 20 seconds on the same machine.
15794 Factoring large numbers is, in general, hard. The Pollard Rho
15795 algorithm used by @command{factor} is particularly effective for
15796 numbers with relatively small factors. If you wish to factor large
15797 numbers which do not have small factors (for example, numbers which
15798 are the product of two large primes), other methods are far better.
15800 If @command{factor} is built without using GNU MP, only
15801 single-precision arithmetic is available, and so large numbers
15802 (typically @math{2^{64}} and above) will not be supported. The single-precision
15803 code uses an algorithm which is designed for factoring smaller
15809 @node seq invocation
15810 @section @command{seq}: Print numeric sequences
15813 @cindex numeric sequences
15814 @cindex sequence of numbers
15816 @command{seq} prints a sequence of numbers to standard output. Synopses:
15819 seq [@var{option}]@dots{} @var{last}
15820 seq [@var{option}]@dots{} @var{first} @var{last}
15821 seq [@var{option}]@dots{} @var{first} @var{increment} @var{last}
15824 @command{seq} prints the numbers from @var{first} to @var{last} by
15825 @var{increment}. By default, each number is printed on a separate line.
15826 When @var{increment} is not specified, it defaults to @samp{1},
15827 even when @var{first} is larger than @var{last}.
15828 @var{first} also defaults to @samp{1}. So @code{seq 1} prints
15829 @samp{1}, but @code{seq 0} and @code{seq 10 5} produce no output.
15830 Floating-point numbers may be specified. @xref{Floating point}.
15832 The program accepts the following options. Also see @ref{Common options}.
15833 Options must precede operands.
15836 @item -f @var{format}
15837 @itemx --format=@var{format}
15838 @opindex -f @var{format}
15839 @opindex --format=@var{format}
15840 @cindex formatting of numbers in @command{seq}
15841 Print all numbers using @var{format}.
15842 @var{format} must contain exactly one of the @samp{printf}-style
15843 floating point conversion specifications @samp{%a}, @samp{%e},
15844 @samp{%f}, @samp{%g}, @samp{%A}, @samp{%E}, @samp{%F}, @samp{%G}.
15845 The @samp{%} may be followed by zero or more flags taken from the set
15846 @samp{-+#0 '}, then an optional width containing one or more digits,
15847 then an optional precision consisting of a @samp{.} followed by zero
15848 or more digits. @var{format} may also contain any number of @samp{%%}
15849 conversion specifications. All conversion specifications have the
15850 same meaning as with @samp{printf}.
15852 The default format is derived from @var{first}, @var{step}, and
15853 @var{last}. If these all use a fixed point decimal representation,
15854 the default format is @samp{%.@var{p}f}, where @var{p} is the minimum
15855 precision that can represent the output numbers exactly. Otherwise,
15856 the default format is @samp{%g}.
15858 @item -s @var{string}
15859 @itemx --separator=@var{string}
15860 @cindex separator for numbers in @command{seq}
15861 Separate numbers with @var{string}; default is a newline.
15862 The output always terminates with a newline.
15865 @itemx --equal-width
15866 Print all numbers with the same width, by padding with leading zeros.
15867 @var{first}, @var{step}, and @var{last} should all use a fixed point
15868 decimal representation.
15869 (To have other kinds of padding, use @option{--format}).
15873 You can get finer-grained control over output with @option{-f}:
15876 $ seq -f '(%9.2E)' -9e5 1.1e6 1.3e6
15882 If you want hexadecimal integer output, you can use @command{printf}
15883 to perform the conversion:
15886 $ printf '%x\n' `seq 1048575 1024 1050623`
15892 For very long lists of numbers, use xargs to avoid
15893 system limitations on the length of an argument list:
15896 $ seq 1000000 | xargs printf '%x\n' | tail -n 3
15902 To generate octal output, use the printf @code{%o} format instead
15905 On most systems, seq can produce whole-number output for values up to
15906 at least @math{2^{53}}. Larger integers are approximated. The details
15907 differ depending on your floating-point implementation.
15908 @xref{Floating point}. A common
15909 case is that @command{seq} works with integers through @math{2^{64}},
15910 and larger integers may not be numerically correct:
15913 $ seq 18446744073709551616 1 18446744073709551618
15914 18446744073709551616
15915 18446744073709551616
15916 18446744073709551618
15919 Be careful when using @command{seq} with outlandish values: otherwise
15920 you may see surprising results, as @command{seq} uses floating point
15921 internally. For example, on the x86 platform, where the internal
15922 representation uses a 64-bit fraction, the command:
15925 seq 1 0.0000000000000000001 1.0000000000000000009
15928 outputs 1.0000000000000000007 twice and skips 1.0000000000000000008.
15933 @node File permissions
15934 @chapter File permissions
15937 @include parse-datetime.texi
15941 @node Opening the software toolbox
15942 @chapter Opening the Software Toolbox
15944 An earlier version of this chapter appeared in
15945 @uref{http://www.linuxjournal.com/article.php?sid=2762, the
15946 @cite{What's GNU?} column of @cite{Linux Journal}, 2 (June, 1994)}.
15947 It was written by Arnold Robbins.
15950 * Toolbox introduction:: Toolbox introduction
15951 * I/O redirection:: I/O redirection
15952 * The who command:: The @command{who} command
15953 * The cut command:: The @command{cut} command
15954 * The sort command:: The @command{sort} command
15955 * The uniq command:: The @command{uniq} command
15956 * Putting the tools together:: Putting the tools together
15960 @node Toolbox introduction
15961 @unnumberedsec Toolbox Introduction
15963 This month's column is only peripherally related to the GNU Project, in
15964 that it describes a number of the GNU tools on your GNU/Linux system
15966 might be used. What it's really about is the ``Software Tools'' philosophy
15967 of program development and usage.
15969 The software tools philosophy was an important and integral concept
15970 in the initial design and development of Unix (of which Linux and GNU are
15971 essentially clones). Unfortunately, in the modern day press of
15972 Internetworking and flashy GUIs, it seems to have fallen by the
15973 wayside. This is a shame, since it provides a powerful mental model
15974 for solving many kinds of problems.
15976 Many people carry a Swiss Army knife around in their pants pockets (or
15977 purse). A Swiss Army knife is a handy tool to have: it has several knife
15978 blades, a screwdriver, tweezers, toothpick, nail file, corkscrew, and perhaps
15979 a number of other things on it. For the everyday, small miscellaneous jobs
15980 where you need a simple, general purpose tool, it's just the thing.
15982 On the other hand, an experienced carpenter doesn't build a house using
15983 a Swiss Army knife. Instead, he has a toolbox chock full of specialized
15984 tools---a saw, a hammer, a screwdriver, a plane, and so on. And he knows
15985 exactly when and where to use each tool; you won't catch him hammering nails
15986 with the handle of his screwdriver.
15988 The Unix developers at Bell Labs were all professional programmers and trained
15989 computer scientists. They had found that while a one-size-fits-all program
15990 might appeal to a user because there's only one program to use, in practice
15995 difficult to write,
15998 difficult to maintain and
16002 difficult to extend to meet new situations.
16005 Instead, they felt that programs should be specialized tools. In short, each
16006 program ``should do one thing well.'' No more and no less. Such programs are
16007 simpler to design, write, and get right---they only do one thing.
16009 Furthermore, they found that with the right machinery for hooking programs
16010 together, that the whole was greater than the sum of the parts. By combining
16011 several special purpose programs, you could accomplish a specific task
16012 that none of the programs was designed for, and accomplish it much more
16013 quickly and easily than if you had to write a special purpose program.
16014 We will see some (classic) examples of this further on in the column.
16015 (An important additional point was that, if necessary, take a detour
16016 and build any software tools you may need first, if you don't already
16017 have something appropriate in the toolbox.)
16019 @node I/O redirection
16020 @unnumberedsec I/O Redirection
16022 Hopefully, you are familiar with the basics of I/O redirection in the
16023 shell, in particular the concepts of ``standard input,'' ``standard output,''
16024 and ``standard error''. Briefly, ``standard input'' is a data source, where
16025 data comes from. A program should not need to either know or care if the
16026 data source is a disk file, a keyboard, a magnetic tape, or even a punched
16027 card reader. Similarly, ``standard output'' is a data sink, where data goes
16028 to. The program should neither know nor care where this might be.
16029 Programs that only read their standard input, do something to the data,
16030 and then send it on, are called @dfn{filters}, by analogy to filters in a
16033 With the Unix shell, it's very easy to set up data pipelines:
16036 program_to_create_data | filter1 | ... | filterN > final.pretty.data
16039 We start out by creating the raw data; each filter applies some successive
16040 transformation to the data, until by the time it comes out of the pipeline,
16041 it is in the desired form.
16043 This is fine and good for standard input and standard output. Where does the
16044 standard error come in to play? Well, think about @command{filter1} in
16045 the pipeline above. What happens if it encounters an error in the data it
16046 sees? If it writes an error message to standard output, it will just
16047 disappear down the pipeline into @command{filter2}'s input, and the
16048 user will probably never see it. So programs need a place where they can send
16049 error messages so that the user will notice them. This is standard error,
16050 and it is usually connected to your console or window, even if you have
16051 redirected standard output of your program away from your screen.
16053 For filter programs to work together, the format of the data has to be
16054 agreed upon. The most straightforward and easiest format to use is simply
16055 lines of text. Unix data files are generally just streams of bytes, with
16056 lines delimited by the @acronym{ASCII} @sc{lf} (Line Feed) character,
16057 conventionally called a ``newline'' in the Unix literature. (This is
16058 @code{'\n'} if you're a C programmer.) This is the format used by all
16059 the traditional filtering programs. (Many earlier operating systems
16060 had elaborate facilities and special purpose programs for managing
16061 binary data. Unix has always shied away from such things, under the
16062 philosophy that it's easiest to simply be able to view and edit your
16063 data with a text editor.)
16065 OK, enough introduction. Let's take a look at some of the tools, and then
16066 we'll see how to hook them together in interesting ways. In the following
16067 discussion, we will only present those command line options that interest
16068 us. As you should always do, double check your system documentation
16069 for the full story.
16071 @node The who command
16072 @unnumberedsec The @command{who} Command
16074 The first program is the @command{who} command. By itself, it generates a
16075 list of the users who are currently logged in. Although I'm writing
16076 this on a single-user system, we'll pretend that several people are
16081 @print{} arnold console Jan 22 19:57
16082 @print{} miriam ttyp0 Jan 23 14:19(:0.0)
16083 @print{} bill ttyp1 Jan 21 09:32(:0.0)
16084 @print{} arnold ttyp2 Jan 23 20:48(:0.0)
16087 Here, the @samp{$} is the usual shell prompt, at which I typed @samp{who}.
16088 There are three people logged in, and I am logged in twice. On traditional
16089 Unix systems, user names are never more than eight characters long. This
16090 little bit of trivia will be useful later. The output of @command{who} is nice,
16091 but the data is not all that exciting.
16093 @node The cut command
16094 @unnumberedsec The @command{cut} Command
16096 The next program we'll look at is the @command{cut} command. This program
16097 cuts out columns or fields of input data. For example, we can tell it
16098 to print just the login name and full name from the @file{/etc/passwd}
16099 file. The @file{/etc/passwd} file has seven fields, separated by
16103 arnold:xyzzy:2076:10:Arnold D. Robbins:/home/arnold:/bin/bash
16106 To get the first and fifth fields, we would use @command{cut} like this:
16109 $ cut -d: -f1,5 /etc/passwd
16110 @print{} root:Operator
16112 @print{} arnold:Arnold D. Robbins
16113 @print{} miriam:Miriam A. Robbins
16117 With the @option{-c} option, @command{cut} will cut out specific characters
16118 (i.e., columns) in the input lines. This is useful for input data
16119 that has fixed width fields, and does not have a field separator. For
16120 example, list the Monday dates for the current month:
16122 @c Is using cal ok? Looked at gcal, but I don't like it.
16133 @node The sort command
16134 @unnumberedsec The @command{sort} Command
16136 Next we'll look at the @command{sort} command. This is one of the most
16137 powerful commands on a Unix-style system; one that you will often find
16138 yourself using when setting up fancy data plumbing.
16141 command reads and sorts each file named on the command line. It then
16142 merges the sorted data and writes it to standard output. It will read
16143 standard input if no files are given on the command line (thus
16144 making it into a filter). The sort is based on the character collating
16145 sequence or based on user-supplied ordering criteria.
16148 @node The uniq command
16149 @unnumberedsec The @command{uniq} Command
16151 Finally (at least for now), we'll look at the @command{uniq} program. When
16152 sorting data, you will often end up with duplicate lines, lines that
16153 are identical. Usually, all you need is one instance of each line.
16154 This is where @command{uniq} comes in. The @command{uniq} program reads its
16155 standard input. It prints only one
16156 copy of each repeated line. It does have several options. Later on,
16157 we'll use the @option{-c} option, which prints each unique line, preceded
16158 by a count of the number of times that line occurred in the input.
16161 @node Putting the tools together
16162 @unnumberedsec Putting the Tools Together
16164 Now, let's suppose this is a large ISP server system with dozens of users
16165 logged in. The management wants the system administrator to write a
16167 generate a sorted list of logged in users. Furthermore, even if a user
16168 is logged in multiple times, his or her name should only show up in the
16171 The administrator could sit down with the system documentation and write a C
16172 program that did this. It would take perhaps a couple of hundred lines
16173 of code and about two hours to write it, test it, and debug it.
16174 However, knowing the software toolbox, the administrator can instead start out
16175 by generating just a list of logged on users:
16185 Next, sort the list:
16188 $ who | cut -c1-8 | sort
16195 Finally, run the sorted list through @command{uniq}, to weed out duplicates:
16198 $ who | cut -c1-8 | sort | uniq
16204 The @command{sort} command actually has a @option{-u} option that does what
16205 @command{uniq} does. However, @command{uniq} has other uses for which one
16206 cannot substitute @samp{sort -u}.
16208 The administrator puts this pipeline into a shell script, and makes it
16210 all the users on the system (@samp{#} is the system administrator,
16211 or @code{root}, prompt):
16214 # cat > /usr/local/bin/listusers
16215 who | cut -c1-8 | sort | uniq
16217 # chmod +x /usr/local/bin/listusers
16220 There are four major points to note here. First, with just four
16221 programs, on one command line, the administrator was able to save about two
16222 hours worth of work. Furthermore, the shell pipeline is just about as
16223 efficient as the C program would be, and it is much more efficient in
16224 terms of programmer time. People time is much more expensive than
16225 computer time, and in our modern ``there's never enough time to do
16226 everything'' society, saving two hours of programmer time is no mean
16229 Second, it is also important to emphasize that with the
16230 @emph{combination} of the tools, it is possible to do a special
16231 purpose job never imagined by the authors of the individual programs.
16233 Third, it is also valuable to build up your pipeline in stages, as we did here.
16234 This allows you to view the data at each stage in the pipeline, which helps
16235 you acquire the confidence that you are indeed using these tools correctly.
16237 Finally, by bundling the pipeline in a shell script, other users can use
16238 your command, without having to remember the fancy plumbing you set up for
16239 them. In terms of how you run them, shell scripts and compiled programs are
16242 After the previous warm-up exercise, we'll look at two additional, more
16243 complicated pipelines. For them, we need to introduce two more tools.
16245 The first is the @command{tr} command, which stands for ``transliterate.''
16246 The @command{tr} command works on a character-by-character basis, changing
16247 characters. Normally it is used for things like mapping upper case to
16251 $ echo ThIs ExAmPlE HaS MIXED case! | tr '[:upper:]' '[:lower:]'
16252 @print{} this example has mixed case!
16255 There are several options of interest:
16259 work on the complement of the listed characters, i.e.,
16260 operations apply to characters not in the given set
16263 delete characters in the first set from the output
16266 squeeze repeated characters in the output into just one character.
16269 We will be using all three options in a moment.
16271 The other command we'll look at is @command{comm}. The @command{comm}
16272 command takes two sorted input files as input data, and prints out the
16273 files' lines in three columns. The output columns are the data lines
16274 unique to the first file, the data lines unique to the second file, and
16275 the data lines that are common to both. The @option{-1}, @option{-2}, and
16276 @option{-3} command line options @emph{omit} the respective columns. (This is
16277 non-intuitive and takes a little getting used to.) For example:
16299 The file name @file{-} tells @command{comm} to read standard input
16300 instead of a regular file.
16302 Now we're ready to build a fancy pipeline. The first application is a word
16303 frequency counter. This helps an author determine if he or she is over-using
16306 The first step is to change the case of all the letters in our input file
16307 to one case. ``The'' and ``the'' are the same word when doing counting.
16310 $ tr '[:upper:]' '[:lower:]' < whats.gnu | ...
16313 The next step is to get rid of punctuation. Quoted words and unquoted words
16314 should be treated identically; it's easiest to just get the punctuation out of
16318 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | ...
16321 The second @command{tr} command operates on the complement of the listed
16322 characters, which are all the letters, the digits, the underscore, and
16323 the blank. The @samp{\n} represents the newline character; it has to
16324 be left alone. (The @acronym{ASCII} tab character should also be included for
16325 good measure in a production script.)
16327 At this point, we have data consisting of words separated by blank space.
16328 The words only contain alphanumeric characters (and the underscore). The
16329 next step is break the data apart so that we have one word per line. This
16330 makes the counting operation much easier, as we will see shortly.
16333 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16334 > tr -s ' ' '\n' | ...
16337 This command turns blanks into newlines. The @option{-s} option squeezes
16338 multiple newline characters in the output into just one. This helps us
16339 avoid blank lines. (The @samp{>} is the shell's ``secondary prompt.''
16340 This is what the shell prints when it notices you haven't finished
16341 typing in all of a command.)
16343 We now have data consisting of one word per line, no punctuation, all one
16344 case. We're ready to count each word:
16347 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16348 > tr -s ' ' '\n' | sort | uniq -c | ...
16351 At this point, the data might look something like this:
16364 The output is sorted by word, not by count! What we want is the most
16365 frequently used words first. Fortunately, this is easy to accomplish,
16366 with the help of two more @command{sort} options:
16370 do a numeric sort, not a textual one
16373 reverse the order of the sort
16376 The final pipeline looks like this:
16379 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16380 > tr -s ' ' '\n' | sort | uniq -c | sort -n -r
16389 Whew! That's a lot to digest. Yet, the same principles apply. With six
16390 commands, on two lines (really one long one split for convenience), we've
16391 created a program that does something interesting and useful, in much
16392 less time than we could have written a C program to do the same thing.
16394 A minor modification to the above pipeline can give us a simple spelling
16395 checker! To determine if you've spelled a word correctly, all you have to
16396 do is look it up in a dictionary. If it is not there, then chances are
16397 that your spelling is incorrect. So, we need a dictionary.
16398 The conventional location for a dictionary is @file{/usr/dict/words}.
16399 On my GNU/Linux system,@footnote{Redhat Linux 6.1, for the November 2000
16400 revision of this article.}
16401 this is a sorted, 45,402 word dictionary.
16403 Now, how to compare our file with the dictionary? As before, we generate
16404 a sorted list of words, one per line:
16407 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16408 > tr -s ' ' '\n' | sort -u | ...
16411 Now, all we need is a list of words that are @emph{not} in the
16412 dictionary. Here is where the @command{comm} command comes in.
16415 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16416 > tr -s ' ' '\n' | sort -u |
16417 > comm -23 - /usr/dict/words
16420 The @option{-2} and @option{-3} options eliminate lines that are only in the
16421 dictionary (the second file), and lines that are in both files. Lines
16422 only in the first file (standard input, our stream of words), are
16423 words that are not in the dictionary. These are likely candidates for
16424 spelling errors. This pipeline was the first cut at a production
16425 spelling checker on Unix.
16427 There are some other tools that deserve brief mention.
16431 search files for text that matches a regular expression
16434 count lines, words, characters
16437 a T-fitting for data pipes, copies data to files and to standard output
16440 the stream editor, an advanced tool
16443 a data manipulation language, another advanced tool
16446 The software tools philosophy also espoused the following bit of
16447 advice: ``Let someone else do the hard part.'' This means, take
16448 something that gives you most of what you need, and then massage it the
16449 rest of the way until it's in the form that you want.
16455 Each program should do one thing well. No more, no less.
16458 Combining programs with appropriate plumbing leads to results where
16459 the whole is greater than the sum of the parts. It also leads to novel
16460 uses of programs that the authors might never have imagined.
16463 Programs should never print extraneous header or trailer data, since these
16464 could get sent on down a pipeline. (A point we didn't mention earlier.)
16467 Let someone else do the hard part.
16470 Know your toolbox! Use each program appropriately. If you don't have an
16471 appropriate tool, build one.
16474 As of this writing, all the programs we've discussed are available via
16475 anonymous @command{ftp} from: @*
16476 @uref{ftp://gnudist.gnu.org/textutils/textutils-1.22.tar.gz}. (There may
16477 be more recent versions available now.)
16479 None of what I have presented in this column is new. The Software Tools
16480 philosophy was first introduced in the book @cite{Software Tools}, by
16481 Brian Kernighan and P.J. Plauger (Addison-Wesley, ISBN 0-201-03669-X).
16482 This book showed how to write and use software tools. It was written in
16483 1976, using a preprocessor for FORTRAN named @command{ratfor} (RATional
16484 FORtran). At the time, C was not as ubiquitous as it is now; FORTRAN
16485 was. The last chapter presented a @command{ratfor} to FORTRAN
16486 processor, written in @command{ratfor}. @command{ratfor} looks an awful
16487 lot like C; if you know C, you won't have any problem following the
16490 In 1981, the book was updated and made available as @cite{Software Tools
16491 in Pascal} (Addison-Wesley, ISBN 0-201-10342-7). Both books are
16492 still in print and are well worth
16493 reading if you're a programmer. They certainly made a major change in
16494 how I view programming.
16496 The programs in both books are available from
16497 @uref{http://cm.bell-labs.com/who/bwk, Brian Kernighan's home page}.
16498 For a number of years, there was an active
16499 Software Tools Users Group, whose members had ported the original
16500 @command{ratfor} programs to essentially every computer system with a
16501 FORTRAN compiler. The popularity of the group waned in the middle 1980s
16502 as Unix began to spread beyond universities.
16504 With the current proliferation of GNU code and other clones of Unix programs,
16505 these programs now receive little attention; modern C versions are
16506 much more efficient and do more than these programs do. Nevertheless, as
16507 exposition of good programming style, and evangelism for a still-valuable
16508 philosophy, these books are unparalleled, and I recommend them highly.
16510 Acknowledgment: I would like to express my gratitude to Brian Kernighan
16511 of Bell Labs, the original Software Toolsmith, for reviewing this column.
16513 @node GNU Free Documentation License
16514 @appendix GNU Free Documentation License
16518 @node Concept index
16525 @c Local variables:
16526 @c texinfo-column-for-description: 32