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
4791 neither of these options is given, wrongly sorted inputs are diagnosed
4792 only if an input file is found to contain unpairable lines. If an
4793 input file is diagnosed as being unsorted, the @command{\cmd\} command
4794 will exit with a nonzero status (and the output should not be used).
4796 Forcing @command{\cmd\} to process wrongly sorted input files
4797 containing unpairable lines by specifying @option{--nocheck-order} is
4798 not guaranteed to produce any particular output. The output will
4799 probably not correspond with whatever you hoped it would be.
4801 @checkOrderOption{comm}
4806 Fail with an error message if either input file is wrongly ordered.
4808 @item --nocheck-order
4809 Do not check that both input files are in sorted order.
4813 @item --output-delimiter=@var{str}
4814 Print @var{str} between adjacent output columns,
4815 rather than the default of a single TAB character.
4817 The delimiter @var{str} may not be empty.
4821 @node ptx invocation
4822 @section @command{ptx}: Produce permuted indexes
4826 @command{ptx} reads a text file and essentially produces a permuted index, with
4827 each keyword in its context. The calling sketch is either one of:
4830 ptx [@var{option} @dots{}] [@var{file} @dots{}]
4831 ptx -G [@var{option} @dots{}] [@var{input} [@var{output}]]
4834 The @option{-G} (or its equivalent: @option{--traditional}) option disables
4835 all @sc{gnu} extensions and reverts to traditional mode, thus introducing some
4836 limitations and changing several of the program's default option values.
4837 When @option{-G} is not specified, @sc{gnu} extensions are always enabled.
4838 @sc{gnu} extensions to @command{ptx} are documented wherever appropriate in this
4839 document. For the full list, see @xref{Compatibility in ptx}.
4841 Individual options are explained in the following sections.
4843 When @sc{gnu} extensions are enabled, there may be zero, one or several
4844 @var{file}s after the options. If there is no @var{file}, the program
4845 reads the standard input. If there is one or several @var{file}s, they
4846 give the name of input files which are all read in turn, as if all the
4847 input files were concatenated. However, there is a full contextual
4848 break between each file and, when automatic referencing is requested,
4849 file names and line numbers refer to individual text input files. In
4850 all cases, the program outputs the permuted index to the standard
4853 When @sc{gnu} extensions are @emph{not} enabled, that is, when the program
4854 operates in traditional mode, there may be zero, one or two parameters
4855 besides the options. If there are no parameters, the program reads the
4856 standard input and outputs the permuted index to the standard output.
4857 If there is only one parameter, it names the text @var{input} to be read
4858 instead of the standard input. If two parameters are given, they give
4859 respectively the name of the @var{input} file to read and the name of
4860 the @var{output} file to produce. @emph{Be very careful} to note that,
4861 in this case, the contents of file given by the second parameter is
4862 destroyed. This behavior is dictated by System V @command{ptx}
4863 compatibility; @sc{gnu} Standards normally discourage output parameters not
4864 introduced by an option.
4866 Note that for @emph{any} file named as the value of an option or as an
4867 input text file, a single dash @kbd{-} may be used, in which case
4868 standard input is assumed. However, it would not make sense to use this
4869 convention more than once per program invocation.
4872 * General options in ptx:: Options which affect general program behavior.
4873 * Charset selection in ptx:: Underlying character set considerations.
4874 * Input processing in ptx:: Input fields, contexts, and keyword selection.
4875 * Output formatting in ptx:: Types of output format, and sizing the fields.
4876 * Compatibility in ptx::
4880 @node General options in ptx
4881 @subsection General options
4886 @itemx --traditional
4887 As already explained, this option disables all @sc{gnu} extensions to
4888 @command{ptx} and switches to traditional mode.
4891 Print a short help on standard output, then exit without further
4895 Print the program version on standard output, then exit without further
4903 @node Charset selection in ptx
4904 @subsection Charset selection
4906 @c FIXME: People don't necessarily know what an IBM-PC was these days.
4907 As it is set up now, the program assumes that the input file is coded
4908 using 8-bit @acronym{ISO} 8859-1 code, also known as Latin-1 character set,
4909 @emph{unless} it is compiled for MS-DOS, in which case it uses the
4910 character set of the IBM-PC@. (@sc{gnu} @command{ptx} is not known to work on
4911 smaller MS-DOS machines anymore.) Compared to 7-bit @acronym{ASCII}, the set
4912 of characters which are letters is different; this alters the behavior
4913 of regular expression matching. Thus, the default regular expression
4914 for a keyword allows foreign or diacriticized letters. Keyword sorting,
4915 however, is still crude; it obeys the underlying character set ordering
4921 @itemx --ignore-case
4922 Fold lower case letters to upper case for sorting.
4927 @node Input processing in ptx
4928 @subsection Word selection and input processing
4933 @itemx --break-file=@var{file}
4935 This option provides an alternative (to @option{-W}) method of describing
4936 which characters make up words. It introduces the name of a
4937 file which contains a list of characters which can@emph{not} be part of
4938 one word; this file is called the @dfn{Break file}. Any character which
4939 is not part of the Break file is a word constituent. If both options
4940 @option{-b} and @option{-W} are specified, then @option{-W} has precedence and
4941 @option{-b} is ignored.
4943 When @sc{gnu} extensions are enabled, the only way to avoid newline as a
4944 break character is to write all the break characters in the file with no
4945 newline at all, not even at the end of the file. When @sc{gnu} extensions
4946 are disabled, spaces, tabs and newlines are always considered as break
4947 characters even if not included in the Break file.
4950 @itemx --ignore-file=@var{file}
4952 The file associated with this option contains a list of words which will
4953 never be taken as keywords in concordance output. It is called the
4954 @dfn{Ignore file}. The file contains exactly one word in each line; the
4955 end of line separation of words is not subject to the value of the
4959 @itemx --only-file=@var{file}
4961 The file associated with this option contains a list of words which will
4962 be retained in concordance output; any word not mentioned in this file
4963 is ignored. The file is called the @dfn{Only file}. The file contains
4964 exactly one word in each line; the end of line separation of words is
4965 not subject to the value of the @option{-S} option.
4967 There is no default for the Only file. When both an Only file and an
4968 Ignore file are specified, a word is considered a keyword only
4969 if it is listed in the Only file and not in the Ignore file.
4974 On each input line, the leading sequence of non-white space characters will be
4975 taken to be a reference that has the purpose of identifying this input
4976 line in the resulting permuted index. For more information about reference
4977 production, see @xref{Output formatting in ptx}.
4978 Using this option changes the default value for option @option{-S}.
4980 Using this option, the program does not try very hard to remove
4981 references from contexts in output, but it succeeds in doing so
4982 @emph{when} the context ends exactly at the newline. If option
4983 @option{-r} is used with @option{-S} default value, or when @sc{gnu} extensions
4984 are disabled, this condition is always met and references are completely
4985 excluded from the output contexts.
4987 @item -S @var{regexp}
4988 @itemx --sentence-regexp=@var{regexp}
4990 This option selects which regular expression will describe the end of a
4991 line or the end of a sentence. In fact, this regular expression is not
4992 the only distinction between end of lines or end of sentences, and input
4993 line boundaries have no special significance outside this option. By
4994 default, when @sc{gnu} extensions are enabled and if @option{-r} option is not
4995 used, end of sentences are used. In this case, this @var{regex} is
4996 imported from @sc{gnu} Emacs:
4999 [.?!][]\"')@}]*\\($\\|\t\\| \\)[ \t\n]*
5002 Whenever @sc{gnu} extensions are disabled or if @option{-r} option is used, end
5003 of lines are used; in this case, the default @var{regexp} is just:
5009 Using an empty @var{regexp} is equivalent to completely disabling end of
5010 line or end of sentence recognition. In this case, the whole file is
5011 considered to be a single big line or sentence. The user might want to
5012 disallow all truncation flag generation as well, through option @option{-F
5013 ""}. @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
5016 When the keywords happen to be near the beginning of the input line or
5017 sentence, this often creates an unused area at the beginning of the
5018 output context line; when the keywords happen to be near the end of the
5019 input line or sentence, this often creates an unused area at the end of
5020 the output context line. The program tries to fill those unused areas
5021 by wrapping around context in them; the tail of the input line or
5022 sentence is used to fill the unused area on the left of the output line;
5023 the head of the input line or sentence is used to fill the unused area
5024 on the right of the output line.
5026 As a matter of convenience to the user, many usual backslashed escape
5027 sequences from the C language are recognized and converted to the
5028 corresponding characters by @command{ptx} itself.
5030 @item -W @var{regexp}
5031 @itemx --word-regexp=@var{regexp}
5033 This option selects which regular expression will describe each keyword.
5034 By default, if @sc{gnu} extensions are enabled, a word is a sequence of
5035 letters; the @var{regexp} used is @samp{\w+}. When @sc{gnu} extensions are
5036 disabled, a word is by default anything which ends with a space, a tab
5037 or a newline; the @var{regexp} used is @samp{[^ \t\n]+}.
5039 An empty @var{regexp} is equivalent to not using this option.
5040 @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
5043 As a matter of convenience to the user, many usual backslashed escape
5044 sequences, as found in the C language, are recognized and converted to
5045 the corresponding characters by @command{ptx} itself.
5050 @node Output formatting in ptx
5051 @subsection Output formatting
5053 Output format is mainly controlled by the @option{-O} and @option{-T} options
5054 described in the table below. When neither @option{-O} nor @option{-T} are
5055 selected, and if @sc{gnu} extensions are enabled, the program chooses an
5056 output format suitable for a dumb terminal. Each keyword occurrence is
5057 output to the center of one line, surrounded by its left and right
5058 contexts. Each field is properly justified, so the concordance output
5059 can be readily observed. As a special feature, if automatic
5060 references are selected by option @option{-A} and are output before the
5061 left context, that is, if option @option{-R} is @emph{not} selected, then
5062 a colon is added after the reference; this nicely interfaces with @sc{gnu}
5063 Emacs @code{next-error} processing. In this default output format, each
5064 white space character, like newline and tab, is merely changed to
5065 exactly one space, with no special attempt to compress consecutive
5066 spaces. This might change in the future. Except for those white space
5067 characters, every other character of the underlying set of 256
5068 characters is transmitted verbatim.
5070 Output format is further controlled by the following options.
5074 @item -g @var{number}
5075 @itemx --gap-size=@var{number}
5077 Select the size of the minimum white space gap between the fields on the
5080 @item -w @var{number}
5081 @itemx --width=@var{number}
5083 Select the maximum output width of each final line. If references are
5084 used, they are included or excluded from the maximum output width
5085 depending on the value of option @option{-R}. If this option is not
5086 selected, that is, when references are output before the left context,
5087 the maximum output width takes into account the maximum length of all
5088 references. If this option is selected, that is, when references are
5089 output after the right context, the maximum output width does not take
5090 into account the space taken by references, nor the gap that precedes
5094 @itemx --auto-reference
5096 Select automatic references. Each input line will have an automatic
5097 reference made up of the file name and the line ordinal, with a single
5098 colon between them. However, the file name will be empty when standard
5099 input is being read. If both @option{-A} and @option{-r} are selected, then
5100 the input reference is still read and skipped, but the automatic
5101 reference is used at output time, overriding the input reference.
5104 @itemx --right-side-refs
5106 In the default output format, when option @option{-R} is not used, any
5107 references produced by the effect of options @option{-r} or @option{-A} are
5108 placed to the far right of output lines, after the right context. With
5109 default output format, when the @option{-R} option is specified, references
5110 are rather placed at the beginning of each output line, before the left
5111 context. For any other output format, option @option{-R} is
5112 ignored, with one exception: with @option{-R} the width of references
5113 is @emph{not} taken into account in total output width given by @option{-w}.
5115 This option is automatically selected whenever @sc{gnu} extensions are
5118 @item -F @var{string}
5119 @itemx --flac-truncation=@var{string}
5121 This option will request that any truncation in the output be reported
5122 using the string @var{string}. Most output fields theoretically extend
5123 towards the beginning or the end of the current line, or current
5124 sentence, as selected with option @option{-S}. But there is a maximum
5125 allowed output line width, changeable through option @option{-w}, which is
5126 further divided into space for various output fields. When a field has
5127 to be truncated because it cannot extend beyond the beginning or the end of
5128 the current line to fit in, then a truncation occurs. By default,
5129 the string used is a single slash, as in @option{-F /}.
5131 @var{string} may have more than one character, as in @option{-F ...}.
5132 Also, in the particular case when @var{string} is empty (@option{-F ""}),
5133 truncation flagging is disabled, and no truncation marks are appended in
5136 As a matter of convenience to the user, many usual backslashed escape
5137 sequences, as found in the C language, are recognized and converted to
5138 the corresponding characters by @command{ptx} itself.
5140 @item -M @var{string}
5141 @itemx --macro-name=@var{string}
5143 Select another @var{string} to be used instead of @samp{xx}, while
5144 generating output suitable for @command{nroff}, @command{troff} or @TeX{}.
5147 @itemx --format=roff
5149 Choose an output format suitable for @command{nroff} or @command{troff}
5150 processing. Each output line will look like:
5153 .xx "@var{tail}" "@var{before}" "@var{keyword_and_after}"@c
5154 "@var{head}" "@var{ref}"
5157 so it will be possible to write a @samp{.xx} roff macro to take care of
5158 the output typesetting. This is the default output format when @sc{gnu}
5159 extensions are disabled. Option @option{-M} can be used to change
5160 @samp{xx} to another macro name.
5162 In this output format, each non-graphical character, like newline and
5163 tab, is merely changed to exactly one space, with no special attempt to
5164 compress consecutive spaces. Each quote character: @kbd{"} is doubled
5165 so it will be correctly processed by @command{nroff} or @command{troff}.
5170 Choose an output format suitable for @TeX{} processing. Each output
5171 line will look like:
5174 \xx @{@var{tail}@}@{@var{before}@}@{@var{keyword}@}@c
5175 @{@var{after}@}@{@var{head}@}@{@var{ref}@}
5179 so it will be possible to write a @code{\xx} definition to take care of
5180 the output typesetting. Note that when references are not being
5181 produced, that is, neither option @option{-A} nor option @option{-r} is
5182 selected, the last parameter of each @code{\xx} call is inhibited.
5183 Option @option{-M} can be used to change @samp{xx} to another macro
5186 In this output format, some special characters, like @kbd{$}, @kbd{%},
5187 @kbd{&}, @kbd{#} and @kbd{_} are automatically protected with a
5188 backslash. Curly brackets @kbd{@{}, @kbd{@}} are protected with a
5189 backslash and a pair of dollar signs (to force mathematical mode). The
5190 backslash itself produces the sequence @code{\backslash@{@}}.
5191 Circumflex and tilde diacritical marks produce the sequence @code{^\@{ @}} and
5192 @code{~\@{ @}} respectively. Other diacriticized characters of the
5193 underlying character set produce an appropriate @TeX{} sequence as far
5194 as possible. The other non-graphical characters, like newline and tab,
5195 and all other characters which are not part of @acronym{ASCII}, are merely
5196 changed to exactly one space, with no special attempt to compress
5197 consecutive spaces. Let me know how to improve this special character
5198 processing for @TeX{}.
5203 @node Compatibility in ptx
5204 @subsection The @sc{gnu} extensions to @command{ptx}
5206 This version of @command{ptx} contains a few features which do not exist in
5207 System V @command{ptx}. These extra features are suppressed by using the
5208 @option{-G} command line option, unless overridden by other command line
5209 options. Some @sc{gnu} extensions cannot be recovered by overriding, so the
5210 simple rule is to avoid @option{-G} if you care about @sc{gnu} extensions.
5211 Here are the differences between this program and System V @command{ptx}.
5216 This program can read many input files at once, it always writes the
5217 resulting concordance on standard output. On the other hand, System V
5218 @command{ptx} reads only one file and sends the result to standard output
5219 or, if a second @var{file} parameter is given on the command, to that
5222 Having output parameters not introduced by options is a dangerous
5223 practice which @sc{gnu} avoids as far as possible. So, for using @command{ptx}
5224 portably between @sc{gnu} and System V, you should always use it with a
5225 single input file, and always expect the result on standard output. You
5226 might also want to automatically configure in a @option{-G} option to
5227 @command{ptx} calls in products using @command{ptx}, if the configurator finds
5228 that the installed @command{ptx} accepts @option{-G}.
5231 The only options available in System V @command{ptx} are options @option{-b},
5232 @option{-f}, @option{-g}, @option{-i}, @option{-o}, @option{-r}, @option{-t} and
5233 @option{-w}. All other options are @sc{gnu} extensions and are not repeated in
5234 this enumeration. Moreover, some options have a slightly different
5235 meaning when @sc{gnu} extensions are enabled, as explained below.
5238 By default, concordance output is not formatted for @command{troff} or
5239 @command{nroff}. It is rather formatted for a dumb terminal. @command{troff}
5240 or @command{nroff} output may still be selected through option @option{-O}.
5243 Unless @option{-R} option is used, the maximum reference width is
5244 subtracted from the total output line width. With @sc{gnu} extensions
5245 disabled, width of references is not taken into account in the output
5246 line width computations.
5249 All 256 bytes, even @acronym{ASCII} @sc{nul} bytes, are always read and
5250 processed from input file with no adverse effect, even if @sc{gnu} extensions
5251 are disabled. However, System V @command{ptx} does not accept 8-bit characters,
5252 a few control characters are rejected, and the tilde @kbd{~} is also rejected.
5255 Input line length is only limited by available memory, even if @sc{gnu}
5256 extensions are disabled. However, System V @command{ptx} processes only
5257 the first 200 characters in each line.
5260 The break (non-word) characters default to be every character except all
5261 letters of the underlying character set, diacriticized or not. When @sc{gnu}
5262 extensions are disabled, the break characters default to space, tab and
5266 The program makes better use of output line width. If @sc{gnu} extensions
5267 are disabled, the program rather tries to imitate System V @command{ptx},
5268 but still, there are some slight disposition glitches this program does
5269 not completely reproduce.
5272 The user can specify both an Ignore file and an Only file. This is not
5273 allowed with System V @command{ptx}.
5278 @node tsort invocation
5279 @section @command{tsort}: Topological sort
5282 @cindex topological sort
5284 @command{tsort} performs a topological sort on the given @var{file}, or
5285 standard input if no input file is given or for a @var{file} of
5286 @samp{-}. For more details and some history, see @ref{tsort background}.
5290 tsort [@var{option}] [@var{file}]
5293 @command{tsort} reads its input as pairs of strings, separated by blanks,
5294 indicating a partial ordering. The output is a total ordering that
5295 corresponds to the given partial ordering.
5309 will produce the output
5320 Consider a more realistic example.
5321 You have a large set of functions all in one file, and they may all be
5322 declared static except one. Currently that one (say @code{main}) is the
5323 first function defined in the file, and the ones it calls directly follow
5324 it, followed by those they call, etc. Let's say that you are determined
5325 to take advantage of prototypes, so you have to choose between declaring
5326 all of those functions (which means duplicating a lot of information from
5327 the definitions) and rearranging the functions so that as many as possible
5328 are defined before they are used. One way to automate the latter process
5329 is to get a list for each function of the functions it calls directly.
5330 Many programs can generate such lists. They describe a call graph.
5331 Consider the following list, in which a given line indicates that the
5332 function on the left calls the one on the right directly.
5338 tail_file pretty_name
5339 tail_file write_header
5341 tail_forever recheck
5342 tail_forever pretty_name
5343 tail_forever write_header
5344 tail_forever dump_remainder
5347 tail_lines start_lines
5348 tail_lines dump_remainder
5349 tail_lines file_lines
5350 tail_lines pipe_lines
5352 tail_bytes start_bytes
5353 tail_bytes dump_remainder
5354 tail_bytes pipe_bytes
5355 file_lines dump_remainder
5359 then you can use @command{tsort} to produce an ordering of those
5360 functions that satisfies your requirement.
5363 example$ tsort call-graph | tac
5383 @command{tsort} detects any cycles in the input and writes the first cycle
5384 encountered to standard error.
5386 Note that for a given partial ordering, generally there is no unique
5387 total ordering. In the context of the call graph above, the function
5388 @code{parse_options} may be placed anywhere in the list as long as it
5389 precedes @code{main}.
5391 The only options are @option{--help} and @option{--version}. @xref{Common
5397 * tsort background:: Where tsort came from.
5400 @node tsort background
5401 @subsection @command{tsort}: Background
5403 @command{tsort} exists because very early versions of the Unix linker processed
5404 an archive file exactly once, and in order. As @command{ld} read each object
5405 in the archive, it decided whether it was needed in the program based on
5406 whether it defined any symbols which were undefined at that point in
5409 This meant that dependencies within the archive had to be handled
5410 specially. For example, @code{scanf} probably calls @code{read}. That means
5411 that in a single pass through an archive, it was important for @code{scanf.o}
5412 to appear before read.o, because otherwise a program which calls
5413 @code{scanf} but not @code{read} might end up with an unexpected unresolved
5414 reference to @code{read}.
5416 The way to address this problem was to first generate a set of
5417 dependencies of one object file on another. This was done by a shell
5418 script called @command{lorder}. The GNU tools don't provide a version of
5419 lorder, as far as I know, but you can still find it in BSD
5422 Then you ran @command{tsort} over the @command{lorder} output, and you used the
5423 resulting sort to define the order in which you added objects to the archive.
5425 This whole procedure has been obsolete since about 1980, because
5426 Unix archives now contain a symbol table (traditionally built by
5427 @command{ranlib}, now generally built by @command{ar} itself), and the Unix
5428 linker uses the symbol table to effectively make multiple passes over
5431 Anyhow, that's where tsort came from. To solve an old problem with
5432 the way the linker handled archive files, which has since been solved
5436 @node Operating on fields
5437 @chapter Operating on fields
5440 * cut invocation:: Print selected parts of lines.
5441 * paste invocation:: Merge lines of files.
5442 * join invocation:: Join lines on a common field.
5446 @node cut invocation
5447 @section @command{cut}: Print selected parts of lines
5450 @command{cut} writes to standard output selected parts of each line of each
5451 input file, or standard input if no files are given or for a file name of
5455 cut @var{option}@dots{} [@var{file}]@dots{}
5458 In the table which follows, the @var{byte-list}, @var{character-list},
5459 and @var{field-list} are one or more numbers or ranges (two numbers
5460 separated by a dash) separated by commas. Bytes, characters, and
5461 fields are numbered starting at 1. Incomplete ranges may be
5462 given: @option{-@var{m}} means @samp{1-@var{m}}; @samp{@var{n}-} means
5463 @samp{@var{n}} through end of line or last field. The list elements
5464 can be repeated, can overlap, and can be specified in any order; but
5465 the selected input is written in the same order that it is read, and
5466 is written exactly once.
5468 The program accepts the following options. Also see @ref{Common
5473 @item -b @var{byte-list}
5474 @itemx --bytes=@var{byte-list}
5477 Select for printing only the bytes in positions listed in
5478 @var{byte-list}. Tabs and backspaces are treated like any other
5479 character; they take up 1 byte. If an output delimiter is specified,
5480 (see the description of @option{--output-delimiter}), then output that
5481 string between ranges of selected bytes.
5483 @item -c @var{character-list}
5484 @itemx --characters=@var{character-list}
5486 @opindex --characters
5487 Select for printing only the characters in positions listed in
5488 @var{character-list}. The same as @option{-b} for now, but
5489 internationalization will change that. Tabs and backspaces are
5490 treated like any other character; they take up 1 character. If an
5491 output delimiter is specified, (see the description of
5492 @option{--output-delimiter}), then output that string between ranges
5495 @item -f @var{field-list}
5496 @itemx --fields=@var{field-list}
5499 Select for printing only the fields listed in @var{field-list}.
5500 Fields are separated by a TAB character by default. Also print any
5501 line that contains no delimiter character, unless the
5502 @option{--only-delimited} (@option{-s}) option is specified.
5503 Note @command{cut} does not support specifying runs of whitespace as a
5504 delimiter, so to achieve that common functionality one can pre-process
5505 with @command{tr} like:
5507 tr -s '[:blank:]' '\t' | cut -f@dots{}
5510 @item -d @var{input_delim_byte}
5511 @itemx --delimiter=@var{input_delim_byte}
5513 @opindex --delimiter
5514 With @option{-f}, use the first byte of @var{input_delim_byte} as
5515 the input fields separator (default is TAB).
5519 Do not split multi-byte characters (no-op for now).
5522 @itemx --only-delimited
5524 @opindex --only-delimited
5525 For @option{-f}, do not print lines that do not contain the field separator
5526 character. Normally, any line without a field separator is printed verbatim.
5528 @item --output-delimiter=@var{output_delim_string}
5529 @opindex --output-delimiter
5530 With @option{-f}, output fields are separated by @var{output_delim_string}.
5531 The default with @option{-f} is to use the input delimiter.
5532 When using @option{-b} or @option{-c} to select ranges of byte or
5533 character offsets (as opposed to ranges of fields),
5534 output @var{output_delim_string} between non-overlapping
5535 ranges of selected bytes.
5538 @opindex --complement
5539 This option is a @acronym{GNU} extension.
5540 Select for printing the complement of the bytes, characters or fields
5541 selected with the @option{-b}, @option{-c} or @option{-f} options.
5542 In other words, do @emph{not} print the bytes, characters or fields
5543 specified via those options. This option is useful when you have
5544 many fields and want to print all but a few of them.
5551 @node paste invocation
5552 @section @command{paste}: Merge lines of files
5555 @cindex merging files
5557 @command{paste} writes to standard output lines consisting of sequentially
5558 corresponding lines of each given file, separated by a TAB character.
5559 Standard input is used for a file name of @samp{-} or if no input files
5581 paste [@var{option}]@dots{} [@var{file}]@dots{}
5584 The program accepts the following options. Also see @ref{Common options}.
5592 Paste the lines of one file at a time rather than one line from each
5593 file. Using the above example data:
5596 $ paste -s num2 let3
5601 @item -d @var{delim-list}
5602 @itemx --delimiters=@var{delim-list}
5604 @opindex --delimiters
5605 Consecutively use the characters in @var{delim-list} instead of
5606 TAB to separate merged lines. When @var{delim-list} is
5607 exhausted, start again at its beginning. Using the above example data:
5610 $ paste -d '%_' num2 let3 num2
5621 @node join invocation
5622 @section @command{join}: Join lines on a common field
5625 @cindex common field, joining on
5627 @command{join} writes to standard output a line for each pair of input
5628 lines that have identical join fields. Synopsis:
5631 join [@var{option}]@dots{} @var{file1} @var{file2}
5634 Either @var{file1} or @var{file2} (but not both) can be @samp{-},
5635 meaning standard input. @var{file1} and @var{file2} should be
5636 sorted on the join fields.
5639 Normally, the sort order is that of the
5640 collating sequence specified by the @env{LC_COLLATE} locale. Unless
5641 the @option{-t} option is given, the sort comparison ignores blanks at
5642 the start of the join field, as in @code{sort -b}. If the
5643 @option{--ignore-case} option is given, the sort comparison ignores
5644 the case of characters in the join field, as in @code{sort -f}.
5646 The @command{sort} and @command{join} commands should use consistent
5647 locales and options if the output of @command{sort} is fed to
5648 @command{join}. You can use a command like @samp{sort -k 1b,1} to
5649 sort a file on its default join field, but if you select a non-default
5650 locale, join field, separator, or comparison options, then you should
5651 do so consistently between @command{join} and @command{sort}.
5652 If @samp{join -t ''} is specified then the whole line is considered which
5653 matches the default operation of sort.
5655 If the input has no unpairable lines, a @acronym{GNU} extension is
5656 available; the sort order can be any order that considers two fields
5657 to be equal if and only if the sort comparison described above
5658 considers them to be equal. For example:
5675 @checkOrderOption{join}
5679 @item the join field is the first field in each line;
5680 @item fields in the input are separated by one or more blanks, with leading
5681 blanks on the line ignored;
5682 @item fields in the output are separated by a space;
5683 @item each output line consists of the join field, the remaining
5684 fields from @var{file1}, then the remaining fields from @var{file2}.
5687 The program accepts the following options. Also see @ref{Common options}.
5691 @item -a @var{file-number}
5693 Print a line for each unpairable line in file @var{file-number} (either
5694 @samp{1} or @samp{2}), in addition to the normal output.
5697 Fail with an error message if either input file is wrongly ordered.
5699 @item --nocheck-order
5700 Do not check that both input files are in sorted order. This is the default.
5702 @item -e @var{string}
5704 Replace those output fields that are missing in the input with
5709 Treat the first line of each input file as a header line. The header lines will
5710 be joined and printed as the first output line. If @option{-o} is used to
5711 specify output format, the header line will be printed according to the
5712 specified format. The header lines will not be checked for ordering even if
5713 @option{--check-order} is specified. Also if the header lines from each file
5714 do not match, the heading fields from the first file will be used.
5717 @itemx --ignore-case
5719 @opindex --ignore-case
5720 Ignore differences in case when comparing keys.
5721 With this option, the lines of the input files must be ordered in the same way.
5722 Use @samp{sort -f} to produce this ordering.
5724 @item -1 @var{field}
5726 Join on field @var{field} (a positive integer) of file 1.
5728 @item -2 @var{field}
5730 Join on field @var{field} (a positive integer) of file 2.
5732 @item -j @var{field}
5733 Equivalent to @option{-1 @var{field} -2 @var{field}}.
5735 @item -o @var{field-list}
5736 Construct each output line according to the format in @var{field-list}.
5737 Each element in @var{field-list} is either the single character @samp{0} or
5738 has the form @var{m.n} where the file number, @var{m}, is @samp{1} or
5739 @samp{2} and @var{n} is a positive field number.
5741 A field specification of @samp{0} denotes the join field.
5742 In most cases, the functionality of the @samp{0} field spec
5743 may be reproduced using the explicit @var{m.n} that corresponds
5744 to the join field. However, when printing unpairable lines
5745 (using either of the @option{-a} or @option{-v} options), there is no way
5746 to specify the join field using @var{m.n} in @var{field-list}
5747 if there are unpairable lines in both files.
5748 To give @command{join} that functionality, @acronym{POSIX} invented the @samp{0}
5749 field specification notation.
5751 The elements in @var{field-list}
5752 are separated by commas or blanks.
5753 Blank separators typically need to be quoted for the shell. For
5754 example, the commands @samp{join -o 1.2,2.2} and @samp{join -o '1.2
5755 2.2'} are equivalent.
5757 All output lines---including those printed because of any -a or -v
5758 option---are subject to the specified @var{field-list}.
5761 Use character @var{char} as the input and output field separator.
5762 Treat as significant each occurrence of @var{char} in the input file.
5763 Use @samp{sort -t @var{char}}, without the @option{-b} option of
5764 @samp{sort}, to produce this ordering. If @samp{join -t ''} is specified,
5765 the whole line is considered, matching the default operation of sort.
5766 If @samp{-t '\0'} is specified then the @acronym{ASCII} @sc{nul}
5767 character is used to delimit the fields.
5769 @item -v @var{file-number}
5770 Print a line for each unpairable line in file @var{file-number}
5771 (either @samp{1} or @samp{2}), instead of the normal output.
5778 @node Operating on characters
5779 @chapter Operating on characters
5781 @cindex operating on characters
5783 This commands operate on individual characters.
5786 * tr invocation:: Translate, squeeze, and/or delete characters.
5787 * expand invocation:: Convert tabs to spaces.
5788 * unexpand invocation:: Convert spaces to tabs.
5793 @section @command{tr}: Translate, squeeze, and/or delete characters
5800 tr [@var{option}]@dots{} @var{set1} [@var{set2}]
5803 @command{tr} copies standard input to standard output, performing
5804 one of the following operations:
5808 translate, and optionally squeeze repeated characters in the result,
5810 squeeze repeated characters,
5814 delete characters, then squeeze repeated characters from the result.
5817 The @var{set1} and (if given) @var{set2} arguments define ordered
5818 sets of characters, referred to below as @var{set1} and @var{set2}. These
5819 sets are the characters of the input that @command{tr} operates on.
5820 The @option{--complement} (@option{-c}, @option{-C}) option replaces
5822 complement (all of the characters that are not in @var{set1}).
5824 Currently @command{tr} fully supports only single-byte characters.
5825 Eventually it will support multibyte characters; when it does, the
5826 @option{-C} option will cause it to complement the set of characters,
5827 whereas @option{-c} will cause it to complement the set of values.
5828 This distinction will matter only when some values are not characters,
5829 and this is possible only in locales using multibyte encodings when
5830 the input contains encoding errors.
5832 The program accepts the @option{--help} and @option{--version}
5833 options. @xref{Common options}. Options must precede operands.
5838 * Character sets:: Specifying sets of characters.
5839 * Translating:: Changing one set of characters to another.
5840 * Squeezing:: Squeezing repeats and deleting.
5844 @node Character sets
5845 @subsection Specifying sets of characters
5847 @cindex specifying sets of characters
5849 The format of the @var{set1} and @var{set2} arguments resembles
5850 the format of regular expressions; however, they are not regular
5851 expressions, only lists of characters. Most characters simply
5852 represent themselves in these strings, but the strings can contain
5853 the shorthands listed below, for convenience. Some of them can be
5854 used only in @var{set1} or @var{set2}, as noted below.
5858 @item Backslash escapes
5859 @cindex backslash escapes
5861 The following backslash escape sequences are recognized:
5879 The 8-bit character with the value given by @var{ooo}, which is 1 to 3
5880 octal digits. Note that @samp{\400} is interpreted as the two-byte
5881 sequence, @samp{\040} @samp{0}.
5886 While a backslash followed by a character not listed above is
5887 interpreted as that character, the backslash also effectively
5888 removes any special significance, so it is useful to escape
5889 @samp{[}, @samp{]}, @samp{*}, and @samp{-}.
5894 The notation @samp{@var{m}-@var{n}} expands to all of the characters
5895 from @var{m} through @var{n}, in ascending order. @var{m} should
5896 collate before @var{n}; if it doesn't, an error results. As an example,
5897 @samp{0-9} is the same as @samp{0123456789}.
5899 @sc{gnu} @command{tr} does not support the System V syntax that uses square
5900 brackets to enclose ranges. Translations specified in that format
5901 sometimes work as expected, since the brackets are often transliterated
5902 to themselves. However, they should be avoided because they sometimes
5903 behave unexpectedly. For example, @samp{tr -d '[0-9]'} deletes brackets
5906 Many historically common and even accepted uses of ranges are not
5907 portable. For example, on @acronym{EBCDIC} hosts using the @samp{A-Z}
5908 range will not do what most would expect because @samp{A} through @samp{Z}
5909 are not contiguous as they are in @acronym{ASCII}.
5910 If you can rely on a @acronym{POSIX} compliant version of @command{tr}, then
5911 the best way to work around this is to use character classes (see below).
5912 Otherwise, it is most portable (and most ugly) to enumerate the members
5915 @item Repeated characters
5916 @cindex repeated characters
5918 The notation @samp{[@var{c}*@var{n}]} in @var{set2} expands to @var{n}
5919 copies of character @var{c}. Thus, @samp{[y*6]} is the same as
5920 @samp{yyyyyy}. The notation @samp{[@var{c}*]} in @var{string2} expands
5921 to as many copies of @var{c} as are needed to make @var{set2} as long as
5922 @var{set1}. If @var{n} begins with @samp{0}, it is interpreted in
5923 octal, otherwise in decimal.
5925 @item Character classes
5926 @cindex character classes
5928 The notation @samp{[:@var{class}:]} expands to all of the characters in
5929 the (predefined) class @var{class}. The characters expand in no
5930 particular order, except for the @code{upper} and @code{lower} classes,
5931 which expand in ascending order. When the @option{--delete} (@option{-d})
5932 and @option{--squeeze-repeats} (@option{-s}) options are both given, any
5933 character class can be used in @var{set2}. Otherwise, only the
5934 character classes @code{lower} and @code{upper} are accepted in
5935 @var{set2}, and then only if the corresponding character class
5936 (@code{upper} and @code{lower}, respectively) is specified in the same
5937 relative position in @var{set1}. Doing this specifies case conversion.
5938 The class names are given below; an error results when an invalid class
5950 Horizontal whitespace.
5959 Printable characters, not including space.
5965 Printable characters, including space.
5968 Punctuation characters.
5971 Horizontal or vertical whitespace.
5980 @item Equivalence classes
5981 @cindex equivalence classes
5983 The syntax @samp{[=@var{c}=]} expands to all of the characters that are
5984 equivalent to @var{c}, in no particular order. Equivalence classes are
5985 a relatively recent invention intended to support non-English alphabets.
5986 But there seems to be no standard way to define them or determine their
5987 contents. Therefore, they are not fully implemented in @sc{gnu} @command{tr};
5988 each character's equivalence class consists only of that character,
5989 which is of no particular use.
5995 @subsection Translating
5997 @cindex translating characters
5999 @command{tr} performs translation when @var{set1} and @var{set2} are
6000 both given and the @option{--delete} (@option{-d}) option is not given.
6001 @command{tr} translates each character of its input that is in @var{set1}
6002 to the corresponding character in @var{set2}. Characters not in
6003 @var{set1} are passed through unchanged. When a character appears more
6004 than once in @var{set1} and the corresponding characters in @var{set2}
6005 are not all the same, only the final one is used. For example, these
6006 two commands are equivalent:
6013 A common use of @command{tr} is to convert lowercase characters to
6014 uppercase. This can be done in many ways. Here are three of them:
6017 tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
6019 tr '[:lower:]' '[:upper:]'
6023 But note that using ranges like @code{a-z} above is not portable.
6025 When @command{tr} is performing translation, @var{set1} and @var{set2}
6026 typically have the same length. If @var{set1} is shorter than
6027 @var{set2}, the extra characters at the end of @var{set2} are ignored.
6029 On the other hand, making @var{set1} longer than @var{set2} is not
6030 portable; @acronym{POSIX} says that the result is undefined. In this situation,
6031 BSD @command{tr} pads @var{set2} to the length of @var{set1} by repeating
6032 the last character of @var{set2} as many times as necessary. System V
6033 @command{tr} truncates @var{set1} to the length of @var{set2}.
6035 By default, @sc{gnu} @command{tr} handles this case like BSD @command{tr}.
6036 When the @option{--truncate-set1} (@option{-t}) option is given,
6037 @sc{gnu} @command{tr} handles this case like the System V @command{tr}
6038 instead. This option is ignored for operations other than translation.
6040 Acting like System V @command{tr} in this case breaks the relatively common
6044 tr -cs A-Za-z0-9 '\012'
6048 because it converts only zero bytes (the first element in the
6049 complement of @var{set1}), rather than all non-alphanumerics, to
6053 By the way, the above idiom is not portable because it uses ranges, and
6054 it assumes that the octal code for newline is 012.
6055 Assuming a @acronym{POSIX} compliant @command{tr}, here is a better
6059 tr -cs '[:alnum:]' '[\n*]'
6064 @subsection Squeezing repeats and deleting
6066 @cindex squeezing repeat characters
6067 @cindex deleting characters
6069 When given just the @option{--delete} (@option{-d}) option, @command{tr}
6070 removes any input characters that are in @var{set1}.
6072 When given just the @option{--squeeze-repeats} (@option{-s}) option,
6073 @command{tr} replaces each input sequence of a repeated character that
6074 is in @var{set1} with a single occurrence of that character.
6076 When given both @option{--delete} and @option{--squeeze-repeats}, @command{tr}
6077 first performs any deletions using @var{set1}, then squeezes repeats
6078 from any remaining characters using @var{set2}.
6080 The @option{--squeeze-repeats} option may also be used when translating,
6081 in which case @command{tr} first performs translation, then squeezes
6082 repeats from any remaining characters using @var{set2}.
6084 Here are some examples to illustrate various combinations of options:
6089 Remove all zero bytes:
6096 Put all words on lines by themselves. This converts all
6097 non-alphanumeric characters to newlines, then squeezes each string
6098 of repeated newlines into a single newline:
6101 tr -cs '[:alnum:]' '[\n*]'
6105 Convert each sequence of repeated newlines to a single newline:
6112 Find doubled occurrences of words in a document.
6113 @c Separate the following two "the"s, so typo checkers don't complain.
6114 For example, people often write ``the @w{}the'' with the repeated words
6115 separated by a newline. The Bourne shell script below works first
6116 by converting each sequence of punctuation and blank characters to a
6117 single newline. That puts each ``word'' on a line by itself.
6118 Next it maps all uppercase characters to lower case, and finally it
6119 runs @command{uniq} with the @option{-d} option to print out only the words
6125 | tr -s '[:punct:][:blank:]' '[\n*]' \
6126 | tr '[:upper:]' '[:lower:]' \
6131 Deleting a small set of characters is usually straightforward. For example,
6132 to remove all @samp{a}s, @samp{x}s, and @samp{M}s you would do this:
6138 However, when @samp{-} is one of those characters, it can be tricky because
6139 @samp{-} has special meanings. Performing the same task as above but also
6140 removing all @samp{-} characters, we might try @code{tr -d -axM}, but
6141 that would fail because @command{tr} would try to interpret @option{-a} as
6142 a command-line option. Alternatively, we could try putting the hyphen
6143 inside the string, @code{tr -d a-xM}, but that wouldn't work either because
6144 it would make @command{tr} interpret @code{a-x} as the range of characters
6145 @samp{a}@dots{}@samp{x} rather than the three.
6146 One way to solve the problem is to put the hyphen at the end of the list
6153 Or you can use @samp{--} to terminate option processing:
6159 More generally, use the character class notation @code{[=c=]}
6160 with @samp{-} (or any other character) in place of the @samp{c}:
6166 Note how single quotes are used in the above example to protect the
6167 square brackets from interpretation by a shell.
6172 @node expand invocation
6173 @section @command{expand}: Convert tabs to spaces
6176 @cindex tabs to spaces, converting
6177 @cindex converting tabs to spaces
6179 @command{expand} writes the contents of each given @var{file}, or standard
6180 input if none are given or for a @var{file} of @samp{-}, to standard
6181 output, with tab characters converted to the appropriate number of
6185 expand [@var{option}]@dots{} [@var{file}]@dots{}
6188 By default, @command{expand} converts all tabs to spaces. It preserves
6189 backspace characters in the output; they decrement the column count for
6190 tab calculations. The default action is equivalent to @option{-t 8} (set
6191 tabs every 8 columns).
6193 The program accepts the following options. Also see @ref{Common options}.
6197 @item -t @var{tab1}[,@var{tab2}]@dots{}
6198 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6201 @cindex tab stops, setting
6202 If only one tab stop is given, set the tabs @var{tab1} spaces apart
6203 (default is 8). Otherwise, set the tabs at columns @var{tab1},
6204 @var{tab2}, @dots{} (numbered from 0), and replace any tabs beyond the
6205 last tab stop given with single spaces. Tab stops can be separated by
6206 blanks as well as by commas.
6208 For compatibility, GNU @command{expand} also accepts the obsolete
6209 option syntax, @option{-@var{t1}[,@var{t2}]@dots{}}. New scripts
6210 should use @option{-t @var{t1}[,@var{t2}]@dots{}} instead.
6216 @cindex initial tabs, converting
6217 Only convert initial tabs (those that precede all non-space or non-tab
6218 characters) on each line to spaces.
6225 @node unexpand invocation
6226 @section @command{unexpand}: Convert spaces to tabs
6230 @command{unexpand} writes the contents of each given @var{file}, or
6231 standard input if none are given or for a @var{file} of @samp{-}, to
6232 standard output, converting blanks at the beginning of each line into
6233 as many tab characters as needed. In the default @acronym{POSIX}
6234 locale, a @dfn{blank} is a space or a tab; other locales may specify
6235 additional blank characters. Synopsis:
6238 unexpand [@var{option}]@dots{} [@var{file}]@dots{}
6241 By default, @command{unexpand} converts only initial blanks (those
6242 that precede all non-blank characters) on each line. It
6243 preserves backspace characters in the output; they decrement the column
6244 count for tab calculations. By default, tabs are set at every 8th
6247 The program accepts the following options. Also see @ref{Common options}.
6251 @item -t @var{tab1}[,@var{tab2}]@dots{}
6252 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6255 If only one tab stop is given, set the tabs @var{tab1} columns apart
6256 instead of the default 8. Otherwise, set the tabs at columns
6257 @var{tab1}, @var{tab2}, @dots{} (numbered from 0), and leave blanks
6258 beyond the tab stops given unchanged. Tab stops can be separated by
6259 blanks as well as by commas. This option implies the @option{-a} option.
6261 For compatibility, GNU @command{unexpand} supports the obsolete option syntax,
6262 @option{-@var{tab1}[,@var{tab2}]@dots{}}, where tab stops must be
6263 separated by commas. (Unlike @option{-t}, this obsolete option does
6264 not imply @option{-a}.) New scripts should use @option{--first-only -t
6265 @var{tab1}[,@var{tab2}]@dots{}} instead.
6271 Also convert all sequences of two or more blanks just before a tab stop,
6272 even if they occur after non-blank characters in a line.
6279 @node Directory listing
6280 @chapter Directory listing
6282 This chapter describes the @command{ls} command and its variants @command{dir}
6283 and @command{vdir}, which list information about files.
6286 * ls invocation:: List directory contents.
6287 * dir invocation:: Briefly ls.
6288 * vdir invocation:: Verbosely ls.
6289 * dircolors invocation:: Color setup for ls, etc.
6294 @section @command{ls}: List directory contents
6297 @cindex directory listing
6299 The @command{ls} program lists information about files (of any type,
6300 including directories). Options and file arguments can be intermixed
6301 arbitrarily, as usual.
6303 For non-option command-line arguments that are directories, by default
6304 @command{ls} lists the contents of directories, not recursively, and
6305 omitting files with names beginning with @samp{.}. For other non-option
6306 arguments, by default @command{ls} lists just the file name. If no
6307 non-option argument is specified, @command{ls} operates on the current
6308 directory, acting as if it had been invoked with a single argument of @samp{.}.
6311 By default, the output is sorted alphabetically, according to the locale
6312 settings in effect.@footnote{If you use a non-@acronym{POSIX}
6313 locale (e.g., by setting @env{LC_ALL} to @samp{en_US}), then @command{ls} may
6314 produce output that is sorted differently than you're accustomed to.
6315 In that case, set the @env{LC_ALL} environment variable to @samp{C}.}
6316 If standard output is
6317 a terminal, the output is in columns (sorted vertically) and control
6318 characters are output as question marks; otherwise, the output is listed
6319 one per line and control characters are output as-is.
6321 Because @command{ls} is such a fundamental program, it has accumulated many
6322 options over the years. They are described in the subsections below;
6323 within each section, options are listed alphabetically (ignoring case).
6324 The division of options into the subsections is not absolute, since some
6325 options affect more than one aspect of @command{ls}'s operation.
6327 @cindex exit status of @command{ls}
6332 1 minor problems (e.g., failure to access a file or directory not
6333 specified as a command line argument. This happens when listing a
6334 directory in which entries are actively being removed or renamed.)
6335 2 serious trouble (e.g., memory exhausted, invalid option, failure
6336 to access a file or directory specified as a command line argument
6337 or a directory loop)
6340 Also see @ref{Common options}.
6343 * Which files are listed::
6344 * What information is listed::
6345 * Sorting the output::
6346 * Details about version sort::
6347 * General output formatting::
6348 * Formatting file timestamps::
6349 * Formatting the file names::
6353 @node Which files are listed
6354 @subsection Which files are listed
6356 These options determine which files @command{ls} lists information for.
6357 By default, @command{ls} lists files and the contents of any
6358 directories on the command line, except that in directories it ignores
6359 files whose names start with @samp{.}.
6367 In directories, do not ignore file names that start with @samp{.}.
6372 @opindex --almost-all
6373 In directories, do not ignore all file names that start with @samp{.};
6374 ignore only @file{.} and @file{..}. The @option{--all} (@option{-a})
6375 option overrides this option.
6378 @itemx --ignore-backups
6380 @opindex --ignore-backups
6381 @cindex backup files, ignoring
6382 In directories, ignore files that end with @samp{~}. This option is
6383 equivalent to @samp{--ignore='*~' --ignore='.*~'}.
6388 @opindex --directory
6389 List just the names of directories, as with other types of files, rather
6390 than listing their contents.
6391 @c The following sentence is the same as the one for -F.
6392 Do not follow symbolic links listed on the
6393 command line unless the @option{--dereference-command-line} (@option{-H}),
6394 @option{--dereference} (@option{-L}), or
6395 @option{--dereference-command-line-symlink-to-dir} options are specified.
6398 @itemx --dereference-command-line
6400 @opindex --dereference-command-line
6401 @cindex symbolic links, dereferencing
6402 If a command line argument specifies a symbolic link, show information
6403 for the file the link references rather than for the link itself.
6405 @itemx --dereference-command-line-symlink-to-dir
6406 @opindex --dereference-command-line-symlink-to-dir
6407 @cindex symbolic links, dereferencing
6408 Do not dereference symbolic links, with one exception:
6409 if a command line argument specifies a symbolic link that refers to
6410 a directory, show information for that directory rather than for the
6412 This is the default behavior when no other dereferencing-related
6413 option has been specified (@option{--classify} (@option{-F}),
6414 @option{--directory} (@option{-d}),
6416 @option{--dereference} (@option{-L}), or
6417 @option{--dereference-command-line} (@option{-H})).
6419 @item --group-directories-first
6420 @opindex --group-directories-first
6421 Group all the directories before the files and then sort the
6422 directories and the files separately using the selected sort key
6423 (see --sort option).
6424 That is, this option specifies a primary sort key,
6425 and the --sort option specifies a secondary key.
6426 However, any use of @option{--sort=none}
6427 (@option{-U}) disables this option altogether.
6429 @item --hide=PATTERN
6430 @opindex --hide=@var{pattern}
6431 In directories, ignore files whose names match the shell pattern
6432 @var{pattern}, unless the @option{--all} (@option{-a}) or
6433 @option{--almost-all} (@option{-A}) is also given. This
6434 option acts like @option{--ignore=@var{pattern}} except that it has no
6435 effect if @option{--all} (@option{-a}) or @option{--almost-all}
6436 (@option{-A}) is also given.
6438 This option can be useful in shell aliases. For example, if
6439 @command{lx} is an alias for @samp{ls --hide='*~'} and @command{ly} is
6440 an alias for @samp{ls --ignore='*~'}, then the command @samp{lx -A}
6441 lists the file @file{README~} even though @samp{ly -A} would not.
6443 @item -I @var{pattern}
6444 @itemx --ignore=@var{pattern}
6446 @opindex --ignore=@var{pattern}
6447 In directories, ignore files whose names match the shell pattern
6448 (not regular expression) @var{pattern}. As
6449 in the shell, an initial @samp{.} in a file name does not match a
6450 wildcard at the start of @var{pattern}. Sometimes it is useful
6451 to give this option several times. For example,
6454 $ ls --ignore='.??*' --ignore='.[^.]' --ignore='#*'
6457 The first option ignores names of length 3 or more that start with @samp{.},
6458 the second ignores all two-character names that start with @samp{.}
6459 except @samp{..}, and the third ignores names that start with @samp{#}.
6462 @itemx --dereference
6464 @opindex --dereference
6465 @cindex symbolic links, dereferencing
6466 When showing file information for a symbolic link, show information
6467 for the file the link references rather than the link itself.
6468 However, even with this option, @command{ls} still prints the name
6469 of the link itself, not the name of the file that the link points to.
6474 @opindex --recursive
6475 @cindex recursive directory listing
6476 @cindex directory listing, recursive
6477 List the contents of all directories recursively.
6482 @node What information is listed
6483 @subsection What information is listed
6485 These options affect the information that @command{ls} displays. By
6486 default, only file names are shown.
6492 @cindex hurd, author, printing
6493 List each file's author when producing long format directory listings.
6494 In GNU/Hurd, file authors can differ from their owners, but in other
6495 operating systems the two are the same.
6501 @cindex dired Emacs mode support
6502 With the long listing (@option{-l}) format, print an additional line after
6506 //DIRED// @var{beg1} @var{end1} @var{beg2} @var{end2} @dots{}
6510 The @var{begn} and @var{endn} are unsigned integers that record the
6511 byte position of the beginning and end of each file name in the output.
6512 This makes it easy for Emacs to find the names, even when they contain
6513 unusual characters such as space or newline, without fancy searching.
6515 If directories are being listed recursively (@option{-R}), output a similar
6516 line with offsets for each subdirectory name:
6519 //SUBDIRED// @var{beg1} @var{end1} @dots{}
6522 Finally, output a line of the form:
6525 //DIRED-OPTIONS// --quoting-style=@var{word}
6529 where @var{word} is the quoting style (@pxref{Formatting the file names}).
6531 Here is an actual example:
6534 $ mkdir -p a/sub/deeper a/sub2
6536 $ touch a/sub/deeper/file
6537 $ ls -gloRF --dired a
6540 -rw-r--r-- 1 0 Jun 10 12:27 f1
6541 -rw-r--r-- 1 0 Jun 10 12:27 f2
6542 drwxr-xr-x 3 4096 Jun 10 12:27 sub/
6543 drwxr-xr-x 2 4096 Jun 10 12:27 sub2/
6547 drwxr-xr-x 2 4096 Jun 10 12:27 deeper/
6551 -rw-r--r-- 1 0 Jun 10 12:27 file
6555 //DIRED// 48 50 84 86 120 123 158 162 217 223 282 286
6556 //SUBDIRED// 2 3 167 172 228 240 290 296
6557 //DIRED-OPTIONS// --quoting-style=literal
6560 Note that the pairs of offsets on the @samp{//DIRED//} line above delimit
6561 these names: @file{f1}, @file{f2}, @file{sub}, @file{sub2}, @file{deeper},
6563 The offsets on the @samp{//SUBDIRED//} line delimit the following
6564 directory names: @file{a}, @file{a/sub}, @file{a/sub/deeper}, @file{a/sub2}.
6566 Here is an example of how to extract the fifth entry name, @samp{deeper},
6567 corresponding to the pair of offsets, 222 and 228:
6570 $ ls -gloRF --dired a > out
6571 $ dd bs=1 skip=222 count=6 < out 2>/dev/null; echo
6575 Note that although the listing above includes a trailing slash
6576 for the @samp{deeper} entry, the offsets select the name without
6577 the trailing slash. However, if you invoke @command{ls} with @option{--dired}
6578 along with an option like @option{--escape} (aka @option{-b}) and operate
6579 on a file whose name contains special characters, notice that the backslash
6584 $ ls -blog --dired 'a b'
6585 -rw-r--r-- 1 0 Jun 10 12:28 a\ b
6587 //DIRED-OPTIONS// --quoting-style=escape
6590 If you use a quoting style that adds quote marks
6591 (e.g., @option{--quoting-style=c}), then the offsets include the quote marks.
6592 So beware that the user may select the quoting style via the environment
6593 variable @env{QUOTING_STYLE}. Hence, applications using @option{--dired}
6594 should either specify an explicit @option{--quoting-style=literal} option
6595 (aka @option{-N} or @option{--literal}) on the command line, or else be
6596 prepared to parse the escaped names.
6599 @opindex --full-time
6600 Produce long format directory listings, and list times in full. It is
6601 equivalent to using @option{--format=long} with
6602 @option{--time-style=full-iso} (@pxref{Formatting file timestamps}).
6606 Produce long format directory listings, but don't display owner information.
6612 Inhibit display of group information in a long format directory listing.
6613 (This is the default in some non-@sc{gnu} versions of @command{ls}, so we
6614 provide this option for compatibility.)
6622 @cindex inode number, printing
6623 Print the inode number (also called the file serial number and index
6624 number) of each file to the left of the file name. (This number
6625 uniquely identifies each file within a particular file system.)
6628 @itemx --format=long
6629 @itemx --format=verbose
6632 @opindex long ls @r{format}
6633 @opindex verbose ls @r{format}
6634 In addition to the name of each file, print the file type, file mode bits,
6635 number of hard links, owner name, group name, size, and
6636 timestamp (@pxref{Formatting file timestamps}), normally
6637 the modification time. Print question marks for information that
6638 cannot be determined.
6640 Normally the size is printed as a byte count without punctuation, but
6641 this can be overridden (@pxref{Block size}). For example, @option{-h}
6642 prints an abbreviated, human-readable count, and
6643 @samp{--block-size="'1"} prints a byte count with the thousands
6644 separator of the current locale.
6646 For each directory that is listed, preface the files with a line
6647 @samp{total @var{blocks}}, where @var{blocks} is the total disk allocation
6648 for all files in that directory. The block size currently defaults to 1024
6649 bytes, but this can be overridden (@pxref{Block size}).
6650 The @var{blocks} computed counts each hard link separately;
6651 this is arguably a deficiency.
6653 The file type is one of the following characters:
6655 @c The commented-out entries are ones we're not sure about.
6663 character special file
6665 high performance (``contiguous data'') file
6669 door (Solaris 2.5 and up)
6671 @c semaphore, if this is a distinct file type
6675 @c multiplexed file (7th edition Unix; obsolete)
6677 off-line (``migrated'') file (Cray DMF)
6679 network special file (HP-UX)
6683 port (Solaris 10 and up)
6685 @c message queue, if this is a distinct file type
6689 @c shared memory object, if this is a distinct file type
6691 @c typed memory object, if this is a distinct file type
6693 @c whiteout (4.4BSD; not implemented)
6695 some other file type
6698 @cindex permissions, output by @command{ls}
6699 The file mode bits listed are similar to symbolic mode specifications
6700 (@pxref{Symbolic Modes}). But @command{ls} combines multiple bits into the
6701 third character of each set of permissions as follows:
6705 If the set-user-ID or set-group-ID bit and the corresponding executable bit
6709 If the set-user-ID or set-group-ID bit is set but the corresponding
6710 executable bit is not set.
6713 If the restricted deletion flag or sticky bit, and the
6714 other-executable bit, are both set. The restricted deletion flag is
6715 another name for the sticky bit. @xref{Mode Structure}.
6718 If the restricted deletion flag or sticky bit is set but the
6719 other-executable bit is not set.
6722 If the executable bit is set and none of the above apply.
6728 Following the file mode bits is a single character that specifies
6729 whether an alternate access method such as an access control list
6730 applies to the file. When the character following the file mode bits is a
6731 space, there is no alternate access method. When it is a printing
6732 character, then there is such a method.
6734 GNU @command{ls} uses a @samp{.} character to indicate a file
6735 with an SELinux security context, but no other alternate access method.
6737 A file with any other combination of alternate access methods
6738 is marked with a @samp{+} character.
6741 @itemx --numeric-uid-gid
6743 @opindex --numeric-uid-gid
6744 @cindex numeric uid and gid
6745 @cindex numeric user and group IDs
6746 Produce long format directory listings, but
6747 display numeric user and group IDs instead of the owner and group names.
6751 Produce long format directory listings, but don't display group information.
6752 It is equivalent to using @option{--format=long} with @option{--no-group} .
6758 @cindex disk allocation
6759 @cindex size of files, reporting
6760 Print the disk allocation of each file to the left of the file name.
6761 This is the amount of disk space used by the file, which is usually a
6762 bit more than the file's size, but it can be less if the file has holes.
6764 Normally the disk allocation is printed in units of
6765 1024 bytes, but this can be overridden (@pxref{Block size}).
6767 @cindex NFS mounts from BSD to HP-UX
6768 For files that are NFS-mounted from an HP-UX system to a BSD system,
6769 this option reports sizes that are half the correct values. On HP-UX
6770 systems, it reports sizes that are twice the correct values for files
6771 that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX;
6772 it also affects the HP-UX @command{ls} program.
6781 @cindex security context
6782 Display the SELinux security context or @samp{?} if none is found.
6783 When used with the @option{-l} option, print the security context
6784 to the left of the size column.
6789 @node Sorting the output
6790 @subsection Sorting the output
6792 @cindex sorting @command{ls} output
6793 These options change the order in which @command{ls} sorts the information
6794 it outputs. By default, sorting is done by character code
6795 (e.g., @acronym{ASCII} order).
6801 @itemx --time=status
6804 @opindex ctime@r{, printing or sorting by}
6805 @opindex status time@r{, printing or sorting by}
6806 @opindex use time@r{, printing or sorting files by}
6807 If the long listing format (e.g., @option{-l}, @option{-o}) is being used,
6808 print the status change time (the @samp{ctime} in the inode) instead of
6809 the modification time.
6810 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6811 or when not using a long listing format,
6812 sort according to the status change time.
6816 @cindex unsorted directory listing
6817 @cindex directory order, listing by
6818 Primarily, like @option{-U}---do not sort; list the files in whatever
6819 order they are stored in the directory. But also enable @option{-a} (list
6820 all files) and disable @option{-l}, @option{--color}, and @option{-s} (if they
6821 were specified before the @option{-f}).
6827 @cindex reverse sorting
6828 Reverse whatever the sorting method is---e.g., list files in reverse
6829 alphabetical order, youngest first, smallest first, or whatever.
6835 @opindex size of files@r{, sorting files by}
6836 Sort by file size, largest first.
6842 @opindex modification time@r{, sorting files by}
6843 Sort by modification time (the @samp{mtime} in the inode), newest first.
6847 @itemx --time=access
6851 @opindex use time@r{, printing or sorting files by}
6852 @opindex atime@r{, printing or sorting files by}
6853 @opindex access time@r{, printing or sorting files by}
6854 If the long listing format (e.g., @option{--format=long}) is being used,
6855 print the last access time (the @samp{atime} in the inode).
6856 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6857 or when not using a long listing format, sort according to the access time.
6863 @opindex none@r{, sorting option for @command{ls}}
6864 Do not sort; list the files in whatever order they are
6865 stored in the directory. (Do not do any of the other unrelated things
6866 that @option{-f} does.) This is especially useful when listing very large
6867 directories, since not doing any sorting can be noticeably faster.
6870 @itemx --sort=version
6873 @opindex version@r{, sorting option for @command{ls}}
6874 Sort by version name and number, lowest first. It behaves like a default
6875 sort, except that each sequence of decimal digits is treated numerically
6876 as an index/version number. (@xref{Details about version sort}.)
6879 @itemx --sort=extension
6882 @opindex extension@r{, sorting files by}
6883 Sort directory contents alphabetically by file extension (characters
6884 after the last @samp{.}); files with no extension are sorted first.
6889 @node Details about version sort
6890 @subsection Details about version sort
6892 Version sorting handles the fact that file names frequently include indices or
6893 version numbers. Standard sorting usually does not produce the order that one
6894 expects because comparisons are made on a character-by-character basis.
6895 Version sorting is especially useful when browsing directories that contain
6896 many files with indices/version numbers in their names:
6900 abc.zml-1.gz abc.zml-1.gz
6901 abc.zml-12.gz abc.zml-2.gz
6902 abc.zml-2.gz abc.zml-12.gz
6905 Version-sorted strings are compared such that if @var{ver1} and @var{ver2}
6906 are version numbers and @var{prefix} and @var{suffix} (@var{suffix} matching
6907 the regular expression @samp{(\.[A-Za-z~][A-Za-z0-9~]*)*}) are strings then
6908 @var{ver1} < @var{ver2} implies that the name composed of
6909 ``@var{prefix} @var{ver1} @var{suffix}'' sorts before
6910 ``@var{prefix} @var{ver2} @var{suffix}''.
6912 Note also that leading zeros of numeric parts are ignored:
6916 abc-1.007.tgz abc-1.01a.tgz
6917 abc-1.012b.tgz abc-1.007.tgz
6918 abc-1.01a.tgz abc-1.012b.tgz
6921 This functionality is implemented using gnulib's @code{filevercmp} function,
6922 which has some caveats worth noting.
6925 @item @env{LC_COLLATE} is ignored, which means @samp{ls -v} and @samp{sort -V}
6926 will sort non-numeric prefixes as if the @env{LC_COLLATE} locale category
6927 was set to @samp{C}.
6928 @item Some suffixes will not be matched by the regular
6929 expression mentioned above. Consequently these examples may
6930 not sort as you expect:
6938 abc-1.2.3.4.x86_64.rpm
6939 abc-1.2.3.x86_64.rpm
6943 @node General output formatting
6944 @subsection General output formatting
6946 These options affect the appearance of the overall output.
6951 @itemx --format=single-column
6954 @opindex single-column @r{output of files}
6955 List one file per line. This is the default for @command{ls} when standard
6956 output is not a terminal.
6959 @itemx --format=vertical
6962 @opindex vertical @r{sorted files in columns}
6963 List files in columns, sorted vertically. This is the default for
6964 @command{ls} if standard output is a terminal. It is always the default
6965 for the @command{dir} program.
6966 @sc{gnu} @command{ls} uses variable width columns to display as many files as
6967 possible in the fewest lines.
6969 @item --color [=@var{when}]
6971 @cindex color, distinguishing file types with
6972 Specify whether to use color for distinguishing file types. @var{when}
6973 may be omitted, or one of:
6976 @vindex none @r{color option}
6977 - Do not use color at all. This is the default.
6979 @vindex auto @r{color option}
6980 @cindex terminal, using color iff
6981 - Only use color if standard output is a terminal.
6983 @vindex always @r{color option}
6986 Specifying @option{--color} and no @var{when} is equivalent to
6987 @option{--color=always}.
6988 Piping a colorized listing through a pager like @command{more} or
6989 @command{less} usually produces unreadable results. However, using
6990 @code{more -f} does seem to work.
6994 @itemx --indicator-style=classify
6997 @opindex --indicator-style
6998 @cindex file type and executables, marking
6999 @cindex executables and file type, marking
7000 Append a character to each file name indicating the file type. Also,
7001 for regular files that are executable, append @samp{*}. The file type
7002 indicators are @samp{/} for directories, @samp{@@} for symbolic links,
7003 @samp{|} for FIFOs, @samp{=} for sockets, @samp{>} for doors,
7004 and nothing for regular files.
7005 @c The following sentence is the same as the one for -d.
7006 Do not follow symbolic links listed on the
7007 command line unless the @option{--dereference-command-line} (@option{-H}),
7008 @option{--dereference} (@option{-L}), or
7009 @option{--dereference-command-line-symlink-to-dir} options are specified.
7012 @itemx --indicator-style=file-type
7013 @opindex --file-type
7014 @opindex --indicator-style
7015 @cindex file type, marking
7016 Append a character to each file name indicating the file type. This is
7017 like @option{-F}, except that executables are not marked.
7019 @item --indicator-style=@var{word}
7020 @opindex --indicator-style
7021 Append a character indicator with style @var{word} to entry names,
7026 Do not append any character indicator; this is the default.
7028 Append @samp{/} for directories. This is the same as the @option{-p}
7031 Append @samp{/} for directories, @samp{@@} for symbolic links, @samp{|}
7032 for FIFOs, @samp{=} for sockets, and nothing for regular files. This is
7033 the same as the @option{--file-type} option.
7035 Append @samp{*} for executable regular files, otherwise behave as for
7036 @samp{file-type}. This is the same as the @option{-F} or
7037 @option{--classify} option.
7042 Print file sizes in 1024-byte blocks, overriding the default block
7043 size (@pxref{Block size}).
7044 This option is equivalent to @option{--block-size=1K}.
7047 @itemx --format=commas
7050 @opindex commas@r{, outputting between files}
7051 List files horizontally, with as many as will fit on each line,
7052 separated by @samp{, } (a comma and a space).
7055 @itemx --indicator-style=slash
7057 @opindex --indicator-style
7058 @cindex file type, marking
7059 Append a @samp{/} to directory names.
7062 @itemx --format=across
7063 @itemx --format=horizontal
7066 @opindex across@r{, listing files}
7067 @opindex horizontal@r{, listing files}
7068 List the files in columns, sorted horizontally.
7071 @itemx --tabsize=@var{cols}
7074 Assume that each tab stop is @var{cols} columns wide. The default is 8.
7075 @command{ls} uses tabs where possible in the output, for efficiency. If
7076 @var{cols} is zero, do not use tabs at all.
7078 @c FIXME: remove in 2009, if Apple Terminal has been fixed for long enough.
7079 Some terminal emulators (at least Apple Terminal 1.5 (133) from Mac OS X 10.4.8)
7080 do not properly align columns to the right of a TAB following a
7081 non-@acronym{ASCII} byte. If you use such a terminal emulator, use the
7082 @option{-T0} option or put @code{TABSIZE=0} in your environment to tell
7083 @command{ls} to align using spaces, not tabs.
7086 @itemx --width=@var{cols}
7090 Assume the screen is @var{cols} columns wide. The default is taken
7091 from the terminal settings if possible; otherwise the environment
7092 variable @env{COLUMNS} is used if it is set; otherwise the default
7098 @node Formatting file timestamps
7099 @subsection Formatting file timestamps
7101 By default, file timestamps are listed in abbreviated form, using
7102 a date like @samp{Mar 30@ @ 2002} for non-recent timestamps, and a
7103 date-without-year and time like @samp{Mar 30 23:45} for recent timestamps.
7104 This format can change depending on the current locale as detailed below.
7106 A timestamp is considered to be @dfn{recent} if it is less than six
7107 months old, and is not dated in the future. If a timestamp dated
7108 today is not listed in recent form, the timestamp is in the future,
7109 which means you probably have clock skew problems which may break
7110 programs like @command{make} that rely on file timestamps.
7113 Time stamps are listed according to the time zone rules specified by
7114 the @env{TZ} environment variable, or by the system default rules if
7115 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
7116 with @env{TZ}, libc, The GNU C Library Reference Manual}.
7118 The following option changes how file timestamps are printed.
7121 @item --time-style=@var{style}
7122 @opindex --time-style
7124 List timestamps in style @var{style}. The @var{style} should
7125 be one of the following:
7130 List timestamps using @var{format}, where @var{format} is interpreted
7131 like the format argument of @command{date} (@pxref{date invocation}).
7132 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
7133 @command{ls} to list timestamps like @samp{2002-03-30 23:45:56}. As
7134 with @command{date}, @var{format}'s interpretation is affected by the
7135 @env{LC_TIME} locale category.
7137 If @var{format} contains two format strings separated by a newline,
7138 the former is used for non-recent files and the latter for recent
7139 files; if you want output columns to line up, you may need to insert
7140 spaces in one of the two formats.
7143 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
7144 format with nanosecond precision, e.g., @samp{2002-03-30
7145 23:45:56.477817180 -0700}. This style is equivalent to
7146 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
7148 This is useful because the time output includes all the information that
7149 is available from the operating system. For example, this can help
7150 explain @command{make}'s behavior, since @acronym{GNU} @command{make}
7151 uses the full timestamp to determine whether a file is out of date.
7154 List @acronym{ISO} 8601 date and time in minutes, e.g.,
7155 @samp{2002-03-30 23:45}. These timestamps are shorter than
7156 @samp{full-iso} timestamps, and are usually good enough for everyday
7157 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
7160 List @acronym{ISO} 8601 dates for non-recent timestamps (e.g.,
7161 @samp{2002-03-30@ }), and @acronym{ISO} 8601 month, day, hour, and
7162 minute for recent timestamps (e.g., @samp{03-30 23:45}). These
7163 timestamps are uglier than @samp{long-iso} timestamps, but they carry
7164 nearly the same information in a smaller space and their brevity helps
7165 @command{ls} output fit within traditional 80-column output lines.
7166 The following two @command{ls} invocations are equivalent:
7171 ls -l --time-style="+%Y-%m-%d $newline%m-%d %H:%M"
7172 ls -l --time-style="iso"
7177 List timestamps in a locale-dependent form. For example, a Finnish
7178 locale might list non-recent timestamps like @samp{maalis 30@ @ 2002}
7179 and recent timestamps like @samp{maalis 30 23:45}. Locale-dependent
7180 timestamps typically consume more space than @samp{iso} timestamps and
7181 are harder for programs to parse because locale conventions vary so
7182 widely, but they are easier for many people to read.
7184 The @env{LC_TIME} locale category specifies the timestamp format. The
7185 default @acronym{POSIX} locale uses timestamps like @samp{Mar 30@
7186 @ 2002} and @samp{Mar 30 23:45}; in this locale, the following two
7187 @command{ls} invocations are equivalent:
7192 ls -l --time-style="+%b %e %Y$newline%b %e %H:%M"
7193 ls -l --time-style="locale"
7196 Other locales behave differently. For example, in a German locale,
7197 @option{--time-style="locale"} might be equivalent to
7198 @option{--time-style="+%e. %b %Y $newline%e. %b %H:%M"}
7199 and might generate timestamps like @samp{30. M@"ar 2002@ } and
7200 @samp{30. M@"ar 23:45}.
7202 @item posix-@var{style}
7204 List @acronym{POSIX}-locale timestamps if the @env{LC_TIME} locale
7205 category is @acronym{POSIX}, @var{style} timestamps otherwise. For
7206 example, the @samp{posix-long-iso} style lists
7207 timestamps like @samp{Mar 30@ @ 2002} and @samp{Mar 30 23:45} when in
7208 the @acronym{POSIX} locale, and like @samp{2002-03-30 23:45} otherwise.
7213 You can specify the default value of the @option{--time-style} option
7214 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
7215 the default style is @samp{locale}. @acronym{GNU} Emacs 21.3 and
7216 later use the @option{--dired} option and therefore can parse any date
7217 format, but if you are using Emacs 21.1 or 21.2 and specify a
7218 non-@acronym{POSIX} locale you may need to set
7219 @samp{TIME_STYLE="posix-long-iso"}.
7221 To avoid certain denial-of-service attacks, timestamps that would be
7222 longer than 1000 bytes may be treated as errors.
7225 @node Formatting the file names
7226 @subsection Formatting the file names
7228 These options change how file names themselves are printed.
7234 @itemx --quoting-style=escape
7237 @opindex --quoting-style
7238 @cindex backslash sequences for file names
7239 Quote nongraphic characters in file names using alphabetic and octal
7240 backslash sequences like those used in C.
7244 @itemx --quoting-style=literal
7247 @opindex --quoting-style
7248 Do not quote file names. However, with @command{ls} nongraphic
7249 characters are still printed as question marks if the output is a
7250 terminal and you do not specify the @option{--show-control-chars}
7254 @itemx --hide-control-chars
7256 @opindex --hide-control-chars
7257 Print question marks instead of nongraphic characters in file names.
7258 This is the default if the output is a terminal and the program is
7263 @itemx --quoting-style=c
7265 @opindex --quote-name
7266 @opindex --quoting-style
7267 Enclose file names in double quotes and quote nongraphic characters as
7270 @item --quoting-style=@var{word}
7271 @opindex --quoting-style
7272 @cindex quoting style
7273 Use style @var{word} to quote file names and other strings that may
7274 contain arbitrary characters. The @var{word} should
7275 be one of the following:
7279 Output strings as-is; this is the same as the @option{-N} or
7280 @option{--literal} option.
7282 Quote strings for the shell if they contain shell metacharacters or would
7283 cause ambiguous output.
7284 The quoting is suitable for @acronym{POSIX}-compatible shells like
7285 @command{bash}, but it does not always work for incompatible shells
7288 Quote strings for the shell, even if they would normally not require quoting.
7290 Quote strings as for C character string literals, including the
7291 surrounding double-quote characters; this is the same as the
7292 @option{-Q} or @option{--quote-name} option.
7294 Quote strings as for C character string literals, except omit the
7295 surrounding double-quote
7296 characters; this is the same as the @option{-b} or @option{--escape} option.
7298 Quote strings as for C character string literals, except use
7299 surrounding quotation marks appropriate for the
7302 @c Use @t instead of @samp to avoid duplicate quoting in some output styles.
7303 Quote strings as for C character string literals, except use
7304 surrounding quotation marks appropriate for the locale, and quote
7305 @t{`like this'} instead of @t{"like
7306 this"} in the default C locale. This looks nicer on many displays.
7309 You can specify the default value of the @option{--quoting-style} option
7310 with the environment variable @env{QUOTING_STYLE}. If that environment
7311 variable is not set, the default value is @samp{literal}, but this
7312 default may change to @samp{shell} in a future version of this package.
7314 @item --show-control-chars
7315 @opindex --show-control-chars
7316 Print nongraphic characters as-is in file names.
7317 This is the default unless the output is a terminal and the program is
7323 @node dir invocation
7324 @section @command{dir}: Briefly list directory contents
7327 @cindex directory listing, brief
7329 @command{dir} is equivalent to @code{ls -C
7330 -b}; that is, by default files are listed in columns, sorted vertically,
7331 and special characters are represented by backslash escape sequences.
7333 @xref{ls invocation, @command{ls}}.
7336 @node vdir invocation
7337 @section @command{vdir}: Verbosely list directory contents
7340 @cindex directory listing, verbose
7342 @command{vdir} is equivalent to @code{ls -l
7343 -b}; that is, by default files are listed in long format and special
7344 characters are represented by backslash escape sequences.
7346 @node dircolors invocation
7347 @section @command{dircolors}: Color setup for @command{ls}
7351 @cindex setup for color
7353 @command{dircolors} outputs a sequence of shell commands to set up the
7354 terminal for color output from @command{ls} (and @command{dir}, etc.).
7358 eval "`dircolors [@var{option}]@dots{} [@var{file}]`"
7361 If @var{file} is specified, @command{dircolors} reads it to determine which
7362 colors to use for which file types and extensions. Otherwise, a
7363 precompiled database is used. For details on the format of these files,
7364 run @samp{dircolors --print-database}.
7366 To make @command{dircolors} read a @file{~/.dircolors} file if it
7367 exists, you can put the following lines in your @file{~/.bashrc} (or
7368 adapt them to your favorite shell):
7372 test -r $d && eval "$(dircolors $d)"
7376 @vindex SHELL @r{environment variable, and color}
7377 The output is a shell command to set the @env{LS_COLORS} environment
7378 variable. You can specify the shell syntax to use on the command line,
7379 or @command{dircolors} will guess it from the value of the @env{SHELL}
7380 environment variable.
7382 The program accepts the following options. Also see @ref{Common options}.
7387 @itemx --bourne-shell
7390 @opindex --bourne-shell
7391 @cindex Bourne shell syntax for color setup
7392 @cindex @command{sh} syntax for color setup
7393 Output Bourne shell commands. This is the default if the @env{SHELL}
7394 environment variable is set and does not end with @samp{csh} or
7403 @cindex C shell syntax for color setup
7404 @cindex @command{csh} syntax for color setup
7405 Output C shell commands. This is the default if @code{SHELL} ends with
7406 @command{csh} or @command{tcsh}.
7409 @itemx --print-database
7411 @opindex --print-database
7412 @cindex color database, printing
7413 @cindex database for color setup, printing
7414 @cindex printing color database
7415 Print the (compiled-in) default color configuration database. This
7416 output is itself a valid configuration file, and is fairly descriptive
7417 of the possibilities.
7424 @node Basic operations
7425 @chapter Basic operations
7427 @cindex manipulating files
7429 This chapter describes the commands for basic file manipulation:
7430 copying, moving (renaming), and deleting (removing).
7433 * cp invocation:: Copy files.
7434 * dd invocation:: Convert and copy a file.
7435 * install invocation:: Copy files and set attributes.
7436 * mv invocation:: Move (rename) files.
7437 * rm invocation:: Remove files or directories.
7438 * shred invocation:: Remove files more securely.
7443 @section @command{cp}: Copy files and directories
7446 @cindex copying files and directories
7447 @cindex files, copying
7448 @cindex directories, copying
7450 @command{cp} copies files (or, optionally, directories). The copy is
7451 completely independent of the original. You can either copy one file to
7452 another, or copy arbitrarily many files to a destination directory.
7456 cp [@var{option}]@dots{} [-T] @var{source} @var{dest}
7457 cp [@var{option}]@dots{} @var{source}@dots{} @var{directory}
7458 cp [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
7463 If two file names are given, @command{cp} copies the first file to the
7467 If the @option{--target-directory} (@option{-t}) option is given, or
7468 failing that if the last file is a directory and the
7469 @option{--no-target-directory} (@option{-T}) option is not given,
7470 @command{cp} copies each @var{source} file to the specified directory,
7471 using the @var{source}s' names.
7474 Generally, files are written just as they are read. For exceptions,
7475 see the @option{--sparse} option below.
7477 By default, @command{cp} does not copy directories. However, the
7478 @option{-R}, @option{-a}, and @option{-r} options cause @command{cp} to
7479 copy recursively by descending into source directories and copying files
7480 to corresponding destination directories.
7482 When copying from a symbolic link, @command{cp} normally follows the
7483 link only when not copying
7484 recursively. This default can be overridden with the
7485 @option{--archive} (@option{-a}), @option{-d}, @option{--dereference}
7486 (@option{-L}), @option{--no-dereference} (@option{-P}), and
7487 @option{-H} options. If more than one of these options is specified,
7488 the last one silently overrides the others.
7490 When copying to a symbolic link, @command{cp} follows the
7491 link only when it refers to an existing regular file.
7492 However, when copying to a dangling symbolic link, @command{cp}
7493 refuses by default, and fails with a diagnostic, since the operation
7494 is inherently dangerous. This behavior is contrary to historical
7495 practice and to @acronym{POSIX}.
7496 Set @env{POSIXLY_CORRECT} to make @command{cp} attempt to create
7497 the target of a dangling destination symlink, in spite of the possible risk.
7498 Also, when an option like
7499 @option{--backup} or @option{--link} acts to rename or remove the
7500 destination before copying, @command{cp} renames or removes the
7501 symbolic link rather than the file it points to.
7503 By default, @command{cp} copies the contents of special files only
7504 when not copying recursively. This default can be overridden with the
7505 @option{--copy-contents} option.
7507 @cindex self-backups
7508 @cindex backups, making only
7509 @command{cp} generally refuses to copy a file onto itself, with the
7510 following exception: if @option{--force --backup} is specified with
7511 @var{source} and @var{dest} identical, and referring to a regular file,
7512 @command{cp} will make a backup file, either regular or numbered, as
7513 specified in the usual ways (@pxref{Backup options}). This is useful when
7514 you simply want to make a backup of an existing file before changing it.
7516 The program accepts the following options. Also see @ref{Common options}.
7523 Preserve as much as possible of the structure and attributes of the
7524 original files in the copy (but do not attempt to preserve internal
7525 directory structure; i.e., @samp{ls -U} may list the entries in a copied
7526 directory in a different order).
7527 Try to preserve SELinux security context and extended attributes (xattr),
7528 but ignore any failure to do that and print no corresponding diagnostic.
7529 Equivalent to @option{-dR --preserve=all} with the reduced diagnostics.
7531 @itemx --attributes-only
7532 @opindex --attributes-only
7533 Preserve the specified attributes of the original files in the copy,
7534 but do not copy any data. See the @option{--preserve} option for
7535 controlling which attributes to copy.
7538 @itemx @w{@kbd{--backup}[=@var{method}]}
7541 @vindex VERSION_CONTROL
7542 @cindex backups, making
7543 @xref{Backup options}.
7544 Make a backup of each file that would otherwise be overwritten or removed.
7545 As a special case, @command{cp} makes a backup of @var{source} when the force
7546 and backup options are given and @var{source} and @var{dest} are the same
7547 name for an existing, regular file. One useful application of this
7548 combination of options is this tiny Bourne shell script:
7552 # Usage: backup FILE...
7553 # Create a @sc{gnu}-style backup of each listed FILE.
7555 cp --backup --force -- "$i" "$i"
7559 @item --copy-contents
7560 @cindex directories, copying recursively
7561 @cindex copying directories recursively
7562 @cindex recursively copying directories
7563 @cindex non-directories, copying as special files
7564 If copying recursively, copy the contents of any special files (e.g.,
7565 FIFOs and device files) as if they were regular files. This means
7566 trying to read the data in each source file and writing it to the
7567 destination. It is usually a mistake to use this option, as it
7568 normally has undesirable effects on special files like FIFOs and the
7569 ones typically found in the @file{/dev} directory. In most cases,
7570 @code{cp -R --copy-contents} will hang indefinitely trying to read
7571 from FIFOs and special files like @file{/dev/console}, and it will
7572 fill up your destination disk if you use it to copy @file{/dev/zero}.
7573 This option has no effect unless copying recursively, and it does not
7574 affect the copying of symbolic links.
7578 @cindex symbolic links, copying
7579 @cindex hard links, preserving
7580 Copy symbolic links as symbolic links rather than copying the files that
7581 they point to, and preserve hard links between source files in the copies.
7582 Equivalent to @option{--no-dereference --preserve=links}.
7588 When copying without this option and an existing destination file cannot
7589 be opened for writing, the copy fails. However, with @option{--force}),
7590 when a destination file cannot be opened, @command{cp} then removes it and
7591 tries to open it again. Contrast this behavior with that enabled by
7592 @option{--link} and @option{--symbolic-link}, whereby the destination file
7593 is never opened but rather is removed unconditionally. Also see the
7594 description of @option{--remove-destination}.
7596 This option is independent of the @option{--interactive} or
7597 @option{-i} option: neither cancels the effect of the other.
7599 This option is redundant if the @option{--no-clobber} or @option{-n} option is
7604 If a command line argument specifies a symbolic link, then copy the
7605 file it points to rather than the symbolic link itself. However,
7606 copy (preserving its nature) any symbolic link that is encountered
7607 via recursive traversal.
7610 @itemx --interactive
7612 @opindex --interactive
7613 When copying a file other than a directory, prompt whether to
7614 overwrite an existing destination file. The @option{-i} option overrides
7615 a previous @option{-n} option.
7621 Make hard links instead of copies of non-directories.
7624 @itemx --dereference
7626 @opindex --dereference
7627 Follow symbolic links when copying from them.
7628 With this option, @command{cp} cannot create a symbolic link.
7629 For example, a symlink (to regular file) in the source tree will be copied to
7630 a regular file in the destination tree.
7635 @opindex --no-clobber
7636 Do not overwrite an existing file. The @option{-n} option overrides a previous
7637 @option{-i} option. This option is mutually exclusive with @option{-b} or
7638 @option{--backup} option.
7641 @itemx --no-dereference
7643 @opindex --no-dereference
7644 @cindex symbolic links, copying
7645 Copy symbolic links as symbolic links rather than copying the files that
7646 they point to. This option affects only symbolic links in the source;
7647 symbolic links in the destination are always followed if possible.
7650 @itemx @w{@kbd{--preserve}[=@var{attribute_list}]}
7653 @cindex file information, preserving, extended attributes, xattr
7654 Preserve the specified attributes of the original files.
7655 If specified, the @var{attribute_list} must be a comma-separated list
7656 of one or more of the following strings:
7660 Preserve the file mode bits and access control lists.
7662 Preserve the owner and group. On most modern systems,
7663 only users with appropriate privileges may change the owner of a file,
7665 may preserve the group ownership of a file only if they happen to be
7666 a member of the desired group.
7668 Preserve the times of last access and last modification, when possible.
7669 On older systems, it is not possible to preserve these attributes
7670 when the affected file is a symbolic link.
7671 However, many systems now provide the @code{utimensat} function,
7672 which makes it possible even for symbolic links.
7674 Preserve in the destination files
7675 any links between corresponding source files.
7676 Note that with @option{-L} or @option{-H}, this option can convert
7677 symbolic links to hard links. For example,
7679 $ mkdir c; : > a; ln -s a b; cp -aH a b c; ls -i1 c
7684 Note the inputs: @file{b} is a symlink to regular file @file{a},
7685 yet the files in destination directory, @file{c/}, are hard-linked.
7686 Since @option{-a} implies @option{--preserve=links}, and since @option{-H}
7687 tells @command{cp} to dereference command line arguments, it sees two files
7688 with the same inode number, and preserves the perceived hard link.
7690 Here is a similar example that exercises @command{cp}'s @option{-L} option:
7692 $ mkdir b c; (cd b; : > a; ln -s a b); cp -aL b c; ls -i1 c/b
7698 Preserve SELinux security context of the file, or fail with full diagnostics.
7700 Preserve extended attributes of the file, or fail with full diagnostics.
7701 If @command{cp} is built without xattr support, ignore this option.
7702 If SELinux context, ACLs or Capabilities are implemented using xattrs,
7703 they are preserved by this option as well.
7705 Preserve all file attributes.
7706 Equivalent to specifying all of the above, but with the difference
7707 that failure to preserve SELinux security context or extended attributes
7708 does not change @command{cp}'s exit status. In contrast to @option{-a},
7709 all but @samp{Operation not supported} warnings are output.
7712 Using @option{--preserve} with no @var{attribute_list} is equivalent
7713 to @option{--preserve=mode,ownership,timestamps}.
7715 In the absence of this option, each destination file is created with the
7716 mode bits of the corresponding source file, minus the bits set in the
7717 umask and minus the set-user-ID and set-group-ID bits.
7718 @xref{File permissions}.
7720 @itemx @w{@kbd{--no-preserve}=@var{attribute_list}}
7721 @cindex file information, preserving
7722 Do not preserve the specified attributes. The @var{attribute_list}
7723 has the same form as for @option{--preserve}.
7727 @cindex parent directories and @command{cp}
7728 Form the name of each destination file by appending to the target
7729 directory a slash and the specified name of the source file. The last
7730 argument given to @command{cp} must be the name of an existing directory.
7731 For example, the command:
7734 cp --parents a/b/c existing_dir
7738 copies the file @file{a/b/c} to @file{existing_dir/a/b/c}, creating
7739 any missing intermediate directories.
7746 @opindex --recursive
7747 @cindex directories, copying recursively
7748 @cindex copying directories recursively
7749 @cindex recursively copying directories
7750 @cindex non-directories, copying as special files
7751 Copy directories recursively. By default, do not follow symbolic
7752 links in the source; see the @option{--archive} (@option{-a}), @option{-d},
7753 @option{--dereference} (@option{-L}), @option{--no-dereference}
7754 (@option{-P}), and @option{-H} options. Special files are copied by
7755 creating a destination file of the same type as the source; see the
7756 @option{--copy-contents} option. It is not portable to use
7757 @option{-r} to copy symbolic links or special files. On some
7758 non-@sc{gnu} systems, @option{-r} implies the equivalent of
7759 @option{-L} and @option{--copy-contents} for historical reasons.
7760 Also, it is not portable to use @option{-R} to copy symbolic links
7761 unless you also specify @option{-P}, as @acronym{POSIX} allows
7762 implementations that dereference symbolic links by default.
7764 @item --reflink[=@var{when}]
7765 @opindex --reflink[=@var{when}]
7768 @cindex copy on write
7769 Perform a lightweight, copy-on-write (COW) copy, if supported by the
7770 file system. Once it has succeeded, beware that the source and destination
7771 files share the same disk data blocks as long as they remain unmodified.
7772 Thus, if a disk I/O error affects data blocks of one of the files,
7773 the other suffers the same fate.
7775 The @var{when} value can be one of the following:
7779 The default behavior: if the copy-on-write operation is not supported
7780 then report the failure for each file and exit with a failure status.
7783 If the copy-on-write operation is not supported then fall back
7784 to the standard copy behaviour.
7787 This option is overridden by the @option{--link}, @option{--symbolic-link}
7788 and @option{--attributes-only} options, thus allowing it to be used
7789 to configure the default data copying behavior for @command{cp}.
7790 For example, with the following alias, @command{cp} will use the
7791 minimum amount of space supported by the file system.
7794 alias cp='cp --reflink=auto --sparse=always'
7797 @item --remove-destination
7798 @opindex --remove-destination
7799 Remove each existing destination file before attempting to open it
7800 (contrast with @option{-f} above).
7802 @item --sparse=@var{when}
7803 @opindex --sparse=@var{when}
7804 @cindex sparse files, copying
7805 @cindex holes, copying files with
7806 @findex read @r{system call, and holes}
7807 A @dfn{sparse file} contains @dfn{holes}---a sequence of zero bytes that
7808 does not occupy any physical disk blocks; the @samp{read} system call
7809 reads these as zeros. This can both save considerable disk space and
7810 increase speed, since many binary files contain lots of consecutive zero
7811 bytes. By default, @command{cp} detects holes in input source files via a crude
7812 heuristic and makes the corresponding output file sparse as well.
7813 Only regular files may be sparse.
7815 The @var{when} value can be one of the following:
7819 The default behavior: if the input file is sparse, attempt to make
7820 the output file sparse, too. However, if an output file exists but
7821 refers to a non-regular file, then do not attempt to make it sparse.
7824 For each sufficiently long sequence of zero bytes in the input file,
7825 attempt to create a corresponding hole in the output file, even if the
7826 input file does not appear to be sparse.
7827 This is useful when the input file resides on a file system
7828 that does not support sparse files
7829 (for example, @samp{efs} file systems in SGI IRIX 5.3 and earlier),
7830 but the output file is on a type of file system that does support them.
7831 Holes may be created only in regular files, so if the destination file
7832 is of some other type, @command{cp} does not even try to make it sparse.
7835 Never make the output file sparse.
7836 This is useful in creating a file for use with the @command{mkswap} command,
7837 since such a file must not have any holes.
7840 @optStripTrailingSlashes
7843 @itemx --symbolic-link
7845 @opindex --symbolic-link
7846 @cindex symbolic links, copying with
7847 Make symbolic links instead of copies of non-directories. All source
7848 file names must be absolute (starting with @samp{/}) unless the
7849 destination files are in the current directory. This option merely
7850 results in an error message on systems that do not support symbolic links.
7856 @optNoTargetDirectory
7862 @cindex newer files, copying only
7863 Do not copy a non-directory that has an existing destination with the
7864 same or newer modification time. If time stamps are being preserved,
7865 the comparison is to the source time stamp truncated to the
7866 resolutions of the destination file system and of the system calls
7867 used to update time stamps; this avoids duplicate work if several
7868 @samp{cp -pu} commands are executed with the same source and
7875 Print the name of each file before copying it.
7878 @itemx --one-file-system
7880 @opindex --one-file-system
7881 @cindex file systems, omitting copying to different
7882 Skip subdirectories that are on different file systems from the one that
7883 the copy started on.
7884 However, mount point directories @emph{are} copied.
7892 @section @command{dd}: Convert and copy a file
7895 @cindex converting while copying a file
7897 @command{dd} copies a file (from standard input to standard output, by
7898 default) with a changeable I/O block size, while optionally performing
7899 conversions on it. Synopses:
7902 dd [@var{operand}]@dots{}
7906 The only options are @option{--help} and @option{--version}.
7907 @xref{Common options}. @command{dd} accepts the following operands.
7913 Read from @var{file} instead of standard input.
7917 Write to @var{file} instead of standard output. Unless
7918 @samp{conv=notrunc} is given, @command{dd} truncates @var{file} to zero
7919 bytes (or the size specified with @samp{seek=}).
7921 @item ibs=@var{bytes}
7923 @cindex block size of input
7924 @cindex input block size
7925 Set the input block size to @var{bytes}.
7926 This makes @command{dd} read @var{bytes} per block.
7927 The default is 512 bytes.
7929 @item obs=@var{bytes}
7931 @cindex block size of output
7932 @cindex output block size
7933 Set the output block size to @var{bytes}.
7934 This makes @command{dd} write @var{bytes} per block.
7935 The default is 512 bytes.
7937 @item bs=@var{bytes}
7940 Set both input and output block sizes to @var{bytes}.
7941 This makes @command{dd} read and write @var{bytes} per block,
7942 overriding any @samp{ibs} and @samp{obs} settings.
7943 In addition, if no data-transforming @option{conv} option is specified,
7944 each input block is copied to the output as a single block,
7945 without aggregating short reads.
7947 @item cbs=@var{bytes}
7949 @cindex block size of conversion
7950 @cindex conversion block size
7951 @cindex fixed-length records, converting to variable-length
7952 @cindex variable-length records, converting to fixed-length
7953 Set the conversion block size to @var{bytes}.
7954 When converting variable-length records to fixed-length ones
7955 (@option{conv=block}) or the reverse (@option{conv=unblock}),
7956 use @var{bytes} as the fixed record length.
7958 @item skip=@var{blocks}
7960 Skip @var{blocks} @samp{ibs}-byte blocks in the input file before copying.
7962 @item seek=@var{blocks}
7964 Skip @var{blocks} @samp{obs}-byte blocks in the output file before copying.
7966 @item count=@var{blocks}
7968 Copy @var{blocks} @samp{ibs}-byte blocks from the input file, instead
7969 of everything until the end of the file.
7973 Do not print the overall transfer rate and volume statistics
7974 that normally make up the third status line when @command{dd} exits.
7976 @item conv=@var{conversion}[,@var{conversion}]@dots{}
7978 Convert the file as specified by the @var{conversion} argument(s).
7979 (No spaces around any comma(s).)
7986 @opindex ascii@r{, converting to}
7987 Convert @acronym{EBCDIC} to @acronym{ASCII},
7988 using the conversion table specified by @acronym{POSIX}.
7989 This provides a 1:1 translation for all 256 bytes.
7992 @opindex ebcdic@r{, converting to}
7993 Convert @acronym{ASCII} to @acronym{EBCDIC}.
7994 This is the inverse of the @samp{ascii} conversion.
7997 @opindex alternate ebcdic@r{, converting to}
7998 Convert @acronym{ASCII} to alternate @acronym{EBCDIC},
7999 using the alternate conversion table specified by @acronym{POSIX}.
8000 This is not a 1:1 translation, but reflects common historical practice
8001 for @samp{~}, @samp{[}, and @samp{]}.
8003 The @samp{ascii}, @samp{ebcdic}, and @samp{ibm} conversions are
8007 @opindex block @r{(space-padding)}
8008 For each line in the input, output @samp{cbs} bytes, replacing the
8009 input newline with a space and padding with spaces as necessary.
8013 Remove any trailing spaces in each @samp{cbs}-sized input block,
8014 and append a newline.
8016 The @samp{block} and @samp{unblock} conversions are mutually exclusive.
8019 @opindex lcase@r{, converting to}
8020 Change uppercase letters to lowercase.
8023 @opindex ucase@r{, converting to}
8024 Change lowercase letters to uppercase.
8026 The @samp{lcase} and @samp{ucase} conversions are mutually exclusive.
8029 @opindex swab @r{(byte-swapping)}
8030 @cindex byte-swapping
8031 Swap every pair of input bytes. @sc{gnu} @command{dd}, unlike others, works
8032 when an odd number of bytes are read---the last byte is simply copied
8033 (since there is nothing to swap it with).
8037 @cindex read errors, ignoring
8038 Continue after read errors.
8042 @cindex creating output file, avoiding
8043 Do not create the output file; the output file must already exist.
8047 @cindex creating output file, requiring
8048 Fail if the output file already exists; @command{dd} must create the
8051 The @samp{excl} and @samp{nocreat} conversions are mutually exclusive.
8055 @cindex truncating output file, avoiding
8056 Do not truncate the output file.
8059 @opindex sync @r{(padding with @acronym{ASCII} @sc{nul}s)}
8060 Pad every input block to size of @samp{ibs} with trailing zero bytes.
8061 When used with @samp{block} or @samp{unblock}, pad with spaces instead of
8066 @cindex synchronized data writes, before finishing
8067 Synchronize output data just before finishing. This forces a physical
8068 write of output data.
8072 @cindex synchronized data and metadata writes, before finishing
8073 Synchronize output data and metadata just before finishing. This
8074 forces a physical write of output data and metadata.
8078 @item iflag=@var{flag}[,@var{flag}]@dots{}
8080 Access the input file using the flags specified by the @var{flag}
8081 argument(s). (No spaces around any comma(s).)
8083 @item oflag=@var{flag}[,@var{flag}]@dots{}
8085 Access the output file using the flags specified by the @var{flag}
8086 argument(s). (No spaces around any comma(s).)
8088 Here are the flags. Not every flag is supported on every operating
8095 @cindex appending to the output file
8096 Write in append mode, so that even if some other process is writing to
8097 this file, every @command{dd} write will append to the current
8098 contents of the file. This flag makes sense only for output.
8099 If you combine this flag with the @samp{of=@var{file}} operand,
8100 you should also specify @samp{conv=notrunc} unless you want the
8101 output file to be truncated before being appended to.
8105 @cindex concurrent I/O
8106 Use concurrent I/O mode for data. This mode performs direct I/O
8107 and drops the @acronym{POSIX} requirement to serialize all I/O to the same file.
8108 A file cannot be opened in CIO mode and with a standard open at the
8114 Use direct I/O for data, avoiding the buffer cache.
8115 Note that the kernel may impose restrictions on read or write buffer sizes.
8116 For example, with an ext4 destination file system and a linux-based kernel,
8117 using @samp{oflag=direct} will cause writes to fail with @code{EINVAL} if the
8118 output buffer size is not a multiple of 512.
8122 @cindex directory I/O
8124 Fail unless the file is a directory. Most operating systems do not
8125 allow I/O to a directory, so this flag has limited utility.
8129 @cindex synchronized data reads
8130 Use synchronized I/O for data. For the output file, this forces a
8131 physical write of output data on each write. For the input file,
8132 this flag can matter when reading from a remote file that has been
8133 written to synchronously by some other process. Metadata (e.g.,
8134 last-access and last-modified time) is not necessarily synchronized.
8138 @cindex synchronized data and metadata I/O
8139 Use synchronized I/O for both data and metadata.
8143 @cindex nonblocking I/O
8144 Use non-blocking I/O.
8149 Do not update the file's access time.
8150 Some older file systems silently ignore this flag, so it is a good
8151 idea to test it on your files before relying on it.
8155 @cindex controlling terminal
8156 Do not assign the file to be a controlling terminal for @command{dd}.
8157 This has no effect when the file is not a terminal.
8158 On many hosts (e.g., @acronym{GNU}/Linux hosts), this option has no effect
8163 @cindex symbolic links, following
8164 Do not follow symbolic links.
8169 Fail if the file has multiple hard links.
8174 Use binary I/O. This option has an effect only on nonstandard
8175 platforms that distinguish binary from text I/O.
8180 Use text I/O. Like @samp{binary}, this option has no effect on
8185 Accumulate full blocks from input. The @code{read} system call
8186 may return early if a full block is not available.
8187 When that happens, continue calling @code{read} to fill the remainder
8189 This flag can be used only with @code{iflag}.
8193 These flags are not supported on all systems, and @samp{dd} rejects
8194 attempts to use them when they are not supported. When reading from
8195 standard input or writing to standard output, the @samp{nofollow} and
8196 @samp{noctty} flags should not be specified, and the other flags
8197 (e.g., @samp{nonblock}) can affect how other processes behave with the
8198 affected file descriptors, even after @command{dd} exits.
8202 @cindex multipliers after numbers
8203 The numeric-valued strings above (@var{bytes} and @var{blocks}) can be
8204 followed by a multiplier: @samp{b}=512, @samp{c}=1,
8205 @samp{w}=2, @samp{x@var{m}}=@var{m}, or any of the
8206 standard block size suffixes like @samp{k}=1024 (@pxref{Block size}).
8208 Any block size you specify via @samp{bs=}, @samp{ibs=}, @samp{obs=}, @samp{cbs=}
8209 should not be too large---values larger than a few megabytes
8210 are generally wasteful or (as in the gigabyte..exabyte case) downright
8211 counterproductive or error-inducing.
8213 Use different @command{dd} invocations to use different block sizes for
8214 skipping and I/O@. For example, the following shell commands copy data
8215 in 512 KiB blocks between a disk and a tape, but do not save or restore a
8216 4 KiB label at the start of the disk:
8219 disk=/dev/rdsk/c0t1d0s2
8222 # Copy all but the label from disk to tape.
8223 (dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape
8225 # Copy from tape back to disk, but leave the disk label alone.
8226 (dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk
8229 Sending an @samp{INFO} signal to a running @command{dd}
8230 process makes it print I/O statistics to standard error
8231 and then resume copying. In the example below,
8232 @command{dd} is run in the background to copy 10 million blocks.
8233 The @command{kill} command makes it output intermediate I/O statistics,
8234 and when @command{dd} completes normally or is killed by the
8235 @code{SIGINT} signal, it outputs the final statistics.
8238 $ dd if=/dev/zero of=/dev/null count=10MB & pid=$!
8239 $ kill -s INFO $pid; wait $pid
8240 3385223+0 records in
8241 3385223+0 records out
8242 1733234176 bytes (1.7 GB) copied, 6.42173 seconds, 270 MB/s
8243 10000000+0 records in
8244 10000000+0 records out
8245 5120000000 bytes (5.1 GB) copied, 18.913 seconds, 271 MB/s
8248 @vindex POSIXLY_CORRECT
8249 On systems lacking the @samp{INFO} signal @command{dd} responds to the
8250 @samp{USR1} signal instead, unless the @env{POSIXLY_CORRECT}
8251 environment variable is set.
8256 @node install invocation
8257 @section @command{install}: Copy files and set attributes
8260 @cindex copying files and setting attributes
8262 @command{install} copies files while setting their file mode bits and, if
8263 possible, their owner and group. Synopses:
8266 install [@var{option}]@dots{} [-T] @var{source} @var{dest}
8267 install [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8268 install [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8269 install [@var{option}]@dots{} -d @var{directory}@dots{}
8274 If two file names are given, @command{install} copies the first file to the
8278 If the @option{--target-directory} (@option{-t}) option is given, or
8279 failing that if the last file is a directory and the
8280 @option{--no-target-directory} (@option{-T}) option is not given,
8281 @command{install} copies each @var{source} file to the specified
8282 directory, using the @var{source}s' names.
8285 If the @option{--directory} (@option{-d}) option is given,
8286 @command{install} creates each @var{directory} and any missing parent
8287 directories. Parent directories are created with mode
8288 @samp{u=rwx,go=rx} (755), regardless of the @option{-m} option or the
8289 current umask. @xref{Directory Setuid and Setgid}, for how the
8290 set-user-ID and set-group-ID bits of parent directories are inherited.
8293 @cindex Makefiles, installing programs in
8294 @command{install} is similar to @command{cp}, but allows you to control the
8295 attributes of destination files. It is typically used in Makefiles to
8296 copy programs into their destination directories. It refuses to copy
8297 files onto themselves.
8299 @cindex extended attributes, xattr
8300 @command{install} never preserves extended attributes (xattr).
8302 The program accepts the following options. Also see @ref{Common options}.
8312 Compare each pair of source and destination files, and if the destination has
8313 identical content and any specified owner, group, permissions, and possibly
8314 SELinux context, then do not modify the destination at all.
8318 Ignored; for compatibility with old Unix versions of @command{install}.
8322 Create any missing parent directories of @var{dest},
8323 then copy @var{source} to @var{dest}.
8324 This option is ignored if a destination directory is specified
8325 via @option{--target-directory=DIR}.
8330 @opindex --directory
8331 @cindex directories, creating with given attributes
8332 @cindex parent directories, creating missing
8333 @cindex leading directories, creating missing
8334 Create any missing parent directories, giving them the default
8335 attributes. Then create each given directory, setting their owner,
8336 group and mode as given on the command line or to the defaults.
8338 @item -g @var{group}
8339 @itemx --group=@var{group}
8342 @cindex group ownership of installed files, setting
8343 Set the group ownership of installed files or directories to
8344 @var{group}. The default is the process's current group. @var{group}
8345 may be either a group name or a numeric group ID.
8348 @itemx --mode=@var{mode}
8351 @cindex permissions of installed files, setting
8352 Set the file mode bits for the installed file or directory to @var{mode},
8353 which can be either an octal number, or a symbolic mode as in
8354 @command{chmod}, with @samp{a=} (no access allowed to anyone) as the
8355 point of departure (@pxref{File permissions}).
8356 The default mode is @samp{u=rwx,go=rx,a-s}---read, write, and
8357 execute for the owner, read and execute for group and other, and with
8358 set-user-ID and set-group-ID disabled.
8359 This default is not quite the same as @samp{755}, since it disables
8360 instead of preserving set-user-ID and set-group-ID on directories.
8361 @xref{Directory Setuid and Setgid}.
8363 @item -o @var{owner}
8364 @itemx --owner=@var{owner}
8367 @cindex ownership of installed files, setting
8368 @cindex appropriate privileges
8369 @vindex root @r{as default owner}
8370 If @command{install} has appropriate privileges (is run as root), set the
8371 ownership of installed files or directories to @var{owner}. The default
8372 is @code{root}. @var{owner} may be either a user name or a numeric user
8375 @item --preserve-context
8376 @opindex --preserve-context
8378 @cindex security context
8379 Preserve the SELinux security context of files and directories.
8380 Failure to preserve the context in all of the files or directories
8381 will result in an exit status of 1. If SELinux is disabled then
8382 print a warning and ignore the option.
8385 @itemx --preserve-timestamps
8387 @opindex --preserve-timestamps
8388 @cindex timestamps of installed files, preserving
8389 Set the time of last access and the time of last modification of each
8390 installed file to match those of each corresponding original file.
8391 When a file is installed without this option, its last access and
8392 last modification times are both set to the time of installation.
8393 This option is useful if you want to use the last modification times
8394 of installed files to keep track of when they were last built as opposed
8395 to when they were last installed.
8401 @cindex symbol table information, stripping
8402 @cindex stripping symbol table information
8403 Strip the symbol tables from installed binary executables.
8405 @itemx --strip-program=@var{program}
8406 @opindex --strip-program
8407 @cindex symbol table information, stripping, program
8408 Program used to strip binaries.
8414 @optNoTargetDirectory
8420 Print the name of each file before copying it.
8422 @item -Z @var{context}
8423 @itemx --context=@var{context}
8427 @cindex security context
8428 Set the default SELinux security context to be used for any
8429 created files and directories. If SELinux is disabled then
8430 print a warning and ignore the option.
8438 @section @command{mv}: Move (rename) files
8442 @command{mv} moves or renames files (or directories). Synopses:
8445 mv [@var{option}]@dots{} [-T] @var{source} @var{dest}
8446 mv [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8447 mv [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8452 If two file names are given, @command{mv} moves the first file to the
8456 If the @option{--target-directory} (@option{-t}) option is given, or
8457 failing that if the last file is a directory and the
8458 @option{--no-target-directory} (@option{-T}) option is not given,
8459 @command{mv} moves each @var{source} file to the specified
8460 directory, using the @var{source}s' names.
8463 @command{mv} can move any type of file from one file system to another.
8464 Prior to version @code{4.0} of the fileutils,
8465 @command{mv} could move only regular files between file systems.
8466 For example, now @command{mv} can move an entire directory hierarchy
8467 including special device files from one partition to another. It first
8468 uses some of the same code that's used by @code{cp -a} to copy the
8469 requested directories and files, then (assuming the copy succeeded)
8470 it removes the originals. If the copy fails, then the part that was
8471 copied to the destination partition is removed. If you were to copy
8472 three directories from one partition to another and the copy of the first
8473 directory succeeded, but the second didn't, the first would be left on
8474 the destination partition and the second and third would be left on the
8477 @cindex extended attributes, xattr
8478 @command{mv} always tries to copy extended attributes (xattr), which may
8479 include SELinux context, ACLs or Capabilities.
8480 Upon failure all but @samp{Operation not supported} warnings are output.
8482 @cindex prompting, and @command{mv}
8483 If a destination file exists but is normally unwritable, standard input
8484 is a terminal, and the @option{-f} or @option{--force} option is not given,
8485 @command{mv} prompts the user for whether to replace the file. (You might
8486 own the file, or have write permission on its directory.) If the
8487 response is not affirmative, the file is skipped.
8489 @emph{Warning}: Avoid specifying a source name with a trailing slash,
8490 when it might be a symlink to a directory.
8491 Otherwise, @command{mv} may do something very surprising, since
8492 its behavior depends on the underlying rename system call.
8493 On a system with a modern Linux-based kernel, it fails with
8494 @code{errno=ENOTDIR}.
8495 However, on other systems (at least FreeBSD 6.1 and Solaris 10) it silently
8496 renames not the symlink but rather the directory referenced by the symlink.
8497 @xref{Trailing slashes}.
8499 The program accepts the following options. Also see @ref{Common options}.
8509 @cindex prompts, omitting
8510 Do not prompt the user before removing a destination file.
8512 If you specify more than one of the @option{-i}, @option{-f}, @option{-n}
8513 options, only the final one takes effect.
8518 @itemx --interactive
8520 @opindex --interactive
8521 @cindex prompts, forcing
8522 Prompt whether to overwrite each existing destination file, regardless
8524 If the response is not affirmative, the file is skipped.
8530 @opindex --no-clobber
8531 @cindex prompts, omitting
8532 Do not overwrite an existing file.
8534 This option is mutually exclusive with @option{-b} or @option{--backup} option.
8540 @cindex newer files, moving only
8541 Do not move a non-directory that has an existing destination with the
8542 same or newer modification time.
8543 If the move is across file system boundaries, the comparison is to the
8544 source time stamp truncated to the resolutions of the destination file
8545 system and of the system calls used to update time stamps; this avoids
8546 duplicate work if several @samp{mv -u} commands are executed with the
8547 same source and destination.
8553 Print the name of each file before moving it.
8555 @optStripTrailingSlashes
8561 @optNoTargetDirectory
8569 @section @command{rm}: Remove files or directories
8572 @cindex removing files or directories
8574 @command{rm} removes each given @var{file}. By default, it does not remove
8575 directories. Synopsis:
8578 rm [@var{option}]@dots{} [@var{file}]@dots{}
8581 @cindex prompting, and @command{rm}
8582 If the @option{-I} or @option{--interactive=once} option is given,
8583 and there are more than three files or the @option{-r}, @option{-R},
8584 or @option{--recursive} are given, then @command{rm} prompts the user
8585 for whether to proceed with the entire operation. If the response is
8586 not affirmative, the entire command is aborted.
8588 Otherwise, if a file is unwritable, standard input is a terminal, and
8589 the @option{-f} or @option{--force} option is not given, or the
8590 @option{-i} or @option{--interactive=always} option @emph{is} given,
8591 @command{rm} prompts the user for whether to remove the file.
8592 If the response is not affirmative, the file is skipped.
8594 Any attempt to remove a file whose last file name component is
8595 @file{.} or @file{..} is rejected without any prompting.
8597 @emph{Warning}: If you use @command{rm} to remove a file, it is usually
8598 possible to recover the contents of that file. If you want more assurance
8599 that the contents are truly unrecoverable, consider using @command{shred}.
8601 The program accepts the following options. Also see @ref{Common options}.
8609 Ignore nonexistent files and never prompt the user.
8610 Ignore any previous @option{--interactive} (@option{-i}) option.
8614 Prompt whether to remove each file.
8615 If the response is not affirmative, the file is skipped.
8616 Ignore any previous @option{--force} (@option{-f}) option.
8617 Equivalent to @option{--interactive=always}.
8621 Prompt once whether to proceed with the command, if more than three
8622 files are named or if a recursive removal is requested. Ignore any
8623 previous @option{--force} (@option{-f}) option. Equivalent to
8624 @option{--interactive=once}.
8626 @itemx --interactive [=@var{when}]
8627 @opindex --interactive
8628 Specify when to issue an interactive prompt. @var{when} may be
8632 @vindex never @r{interactive option}
8633 - Do not prompt at all.
8635 @vindex once @r{interactive option}
8636 - Prompt once if more than three files are named or if a recursive
8637 removal is requested. Equivalent to @option{-I}.
8639 @vindex always @r{interactive option}
8640 - Prompt for every file being removed. Equivalent to @option{-i}.
8642 @option{--interactive} with no @var{when} is equivalent to
8643 @option{--interactive=always}.
8645 @itemx --one-file-system
8646 @opindex --one-file-system
8647 @cindex one file system, restricting @command{rm} to
8648 When removing a hierarchy recursively, skip any directory that is on a
8649 file system different from that of the corresponding command line argument.
8652 This option is useful when removing a build ``chroot'' hierarchy,
8653 which normally contains no valuable data. However, it is not uncommon
8654 to bind-mount @file{/home} into such a hierarchy, to make it easier to
8655 use one's start-up file. The catch is that it's easy to forget to
8656 unmount @file{/home}. Then, when you use @command{rm -rf} to remove
8657 your normally throw-away chroot, that command will remove everything
8658 under @file{/home}, too.
8659 Use the @option{--one-file-system} option, and it will
8660 warn about and skip directories on other file systems.
8661 Of course, this will not save your @file{/home} if it and your
8662 chroot happen to be on the same file system.
8664 @itemx --preserve-root
8665 @opindex --preserve-root
8666 @cindex root directory, disallow recursive destruction
8667 Fail upon any attempt to remove the root directory, @file{/},
8668 when used with the @option{--recursive} option.
8669 This is the default behavior.
8670 @xref{Treating / specially}.
8672 @itemx --no-preserve-root
8673 @opindex --no-preserve-root
8674 @cindex root directory, allow recursive destruction
8675 Do not treat @file{/} specially when removing recursively.
8676 This option is not recommended unless you really want to
8677 remove all the files on your computer.
8678 @xref{Treating / specially}.
8685 @opindex --recursive
8686 @cindex directories, removing (recursively)
8687 Remove the listed directories and their contents recursively.
8693 Print the name of each file before removing it.
8697 @cindex files beginning with @samp{-}, removing
8698 @cindex @samp{-}, removing files beginning with
8699 One common question is how to remove files whose names begin with a
8700 @samp{-}. @sc{gnu} @command{rm}, like every program that uses the @code{getopt}
8701 function to parse its arguments, lets you use the @samp{--} option to
8702 indicate that all following arguments are non-options. To remove a file
8703 called @file{-f} in the current directory, you could type either:
8716 @opindex - @r{and Unix @command{rm}}
8717 The Unix @command{rm} program's use of a single @samp{-} for this purpose
8718 predates the development of the getopt standard syntax.
8723 @node shred invocation
8724 @section @command{shred}: Remove files more securely
8727 @cindex data, erasing
8728 @cindex erasing data
8730 @command{shred} overwrites devices or files, to help prevent even
8731 very expensive hardware from recovering the data.
8733 Ordinarily when you remove a file (@pxref{rm invocation}), the data is
8734 not actually destroyed. Only the index listing where the file is
8735 stored is destroyed, and the storage is made available for reuse.
8736 There are undelete utilities that will attempt to reconstruct the index
8737 and can bring the file back if the parts were not reused.
8739 On a busy system with a nearly-full drive, space can get reused in a few
8740 seconds. But there is no way to know for sure. If you have sensitive
8741 data, you may want to be sure that recovery is not possible by actually
8742 overwriting the file with non-sensitive data.
8744 However, even after doing that, it is possible to take the disk back
8745 to a laboratory and use a lot of sensitive (and expensive) equipment
8746 to look for the faint ``echoes'' of the original data underneath the
8747 overwritten data. If the data has only been overwritten once, it's not
8750 The best way to remove something irretrievably is to destroy the media
8751 it's on with acid, melt it down, or the like. For cheap removable media
8752 like floppy disks, this is the preferred method. However, hard drives
8753 are expensive and hard to melt, so the @command{shred} utility tries
8754 to achieve a similar effect non-destructively.
8756 This uses many overwrite passes, with the data patterns chosen to
8757 maximize the damage they do to the old data. While this will work on
8758 floppies, the patterns are designed for best effect on hard drives.
8759 For more details, see the source code and Peter Gutmann's paper
8760 @uref{http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html,
8761 @cite{Secure Deletion of Data from Magnetic and Solid-State Memory}},
8762 from the proceedings of the Sixth @acronym{USENIX} Security Symposium (San Jose,
8763 California, July 22--25, 1996).
8765 @strong{Please note} that @command{shred} relies on a very important assumption:
8766 that the file system overwrites data in place. This is the traditional
8767 way to do things, but many modern file system designs do not satisfy this
8768 assumption. Exceptions include:
8773 Log-structured or journaled file systems, such as those supplied with
8774 AIX and Solaris, and JFS, ReiserFS, XFS, Ext3 (in @code{data=journal} mode),
8775 BFS, NTFS, etc.@: when they are configured to journal @emph{data}.
8778 File systems that write redundant data and carry on even if some writes
8779 fail, such as RAID-based file systems.
8782 File systems that make snapshots, such as Network Appliance's NFS server.
8785 File systems that cache in temporary locations, such as NFS version 3
8789 Compressed file systems.
8792 In the particular case of ext3 file systems, the above disclaimer applies (and
8793 @command{shred} is thus of limited effectiveness) only in @code{data=journal}
8794 mode, which journals file data in addition to just metadata. In both
8795 the @code{data=ordered} (default) and @code{data=writeback} modes,
8796 @command{shred} works as usual. Ext3 journaling modes can be changed
8797 by adding the @code{data=something} option to the mount options for a
8798 particular file system in the @file{/etc/fstab} file, as documented in
8799 the mount man page (man mount).
8801 If you are not sure how your file system operates, then you should assume
8802 that it does not overwrite data in place, which means that shred cannot
8803 reliably operate on regular files in your file system.
8805 Generally speaking, it is more reliable to shred a device than a file,
8806 since this bypasses the problem of file system design mentioned above.
8807 However, even shredding devices is not always completely reliable. For
8808 example, most disks map out bad sectors invisibly to the application; if
8809 the bad sectors contain sensitive data, @command{shred} won't be able to
8812 @command{shred} makes no attempt to detect or report this problem, just as
8813 it makes no attempt to do anything about backups. However, since it is
8814 more reliable to shred devices than files, @command{shred} by default does
8815 not truncate or remove the output file. This default is more suitable
8816 for devices, which typically cannot be truncated and should not be
8819 Finally, consider the risk of backups and mirrors.
8820 File system backups and remote mirrors may contain copies of the
8821 file that cannot be removed, and that will allow a shredded file
8822 to be recovered later. So if you keep any data you may later want
8823 to destroy using @command{shred}, be sure that it is not backed up or mirrored.
8826 shred [@var{option}]@dots{} @var{file}[@dots{}]
8829 The program accepts the following options. Also see @ref{Common options}.
8837 @cindex force deletion
8838 Override file permissions if necessary to allow overwriting.
8841 @itemx -n @var{number}
8842 @itemx --iterations=@var{number}
8843 @opindex -n @var{number}
8844 @opindex --iterations=@var{number}
8845 @cindex iterations, selecting the number of
8846 By default, @command{shred} uses @value{SHRED_DEFAULT_PASSES} passes of
8847 overwrite. You can reduce this to save time, or increase it if you think it's
8848 appropriate. After 25 passes all of the internal overwrite patterns will have
8849 been used at least once.
8851 @item --random-source=@var{file}
8852 @opindex --random-source
8853 @cindex random source for shredding
8854 Use @var{file} as a source of random data used to overwrite and to
8855 choose pass ordering. @xref{Random sources}.
8857 @item -s @var{bytes}
8858 @itemx --size=@var{bytes}
8859 @opindex -s @var{bytes}
8860 @opindex --size=@var{bytes}
8861 @cindex size of file to shred
8862 Shred the first @var{bytes} bytes of the file. The default is to shred
8863 the whole file. @var{bytes} can be followed by a size specification like
8864 @samp{K}, @samp{M}, or @samp{G} to specify a multiple. @xref{Block size}.
8870 @cindex removing files after shredding
8871 After shredding a file, truncate it (if possible) and then remove it.
8872 If a file has multiple links, only the named links will be removed.
8878 Display to standard error all status updates as sterilization proceeds.
8884 By default, @command{shred} rounds the size of a regular file up to the next
8885 multiple of the file system block size to fully erase the last block
8887 Use @option{--exact} to suppress that behavior.
8888 Thus, by default if you shred a 10-byte regular file on a system with 512-byte
8889 blocks, the resulting file will be 512 bytes long. With this option,
8890 shred does not increase the apparent size of the file.
8896 Normally, the last pass that @command{shred} writes is made up of
8897 random data. If this would be conspicuous on your hard drive (for
8898 example, because it looks like encrypted data), or you just think
8899 it's tidier, the @option{--zero} option adds an additional overwrite pass with
8900 all zero bits. This is in addition to the number of passes specified
8901 by the @option{--iterations} option.
8905 You might use the following command to erase all trace of the
8906 file system you'd created on the floppy disk in your first drive.
8907 That command takes about 20 minutes to erase a ``1.44MB'' (actually
8911 shred --verbose /dev/fd0
8914 Similarly, to erase all data on a selected partition of
8915 your hard disk, you could give a command like this:
8918 shred --verbose /dev/sda5
8921 On modern disks, a single pass should be adequate,
8922 and it will take one third the time of the default three-pass approach.
8925 # 1 pass, write pseudo-random data; 3x faster than the default
8926 shred --verbose -n1 /dev/sda5
8929 To be on the safe side, use at least one pass that overwrites using
8930 pseudo-random data. I.e., don't be tempted to use @samp{-n0 --zero},
8931 in case some disk controller optimizes the process of writing blocks
8932 of all zeros, and thereby does not clear all bytes in a block.
8933 Some SSDs may do just that.
8935 A @var{file} of @samp{-} denotes standard output.
8936 The intended use of this is to shred a removed temporary file.
8943 echo "Hello, world" >&3
8948 However, the command @samp{shred - >file} does not shred the contents
8949 of @var{file}, since the shell truncates @var{file} before invoking
8950 @command{shred}. Use the command @samp{shred file} or (if using a
8951 Bourne-compatible shell) the command @samp{shred - 1<>file} instead.
8956 @node Special file types
8957 @chapter Special file types
8959 @cindex special file types
8960 @cindex file types, special
8962 This chapter describes commands which create special types of files (and
8963 @command{rmdir}, which removes directories, one special file type).
8965 @cindex special file types
8967 Although Unix-like operating systems have markedly fewer special file
8968 types than others, not @emph{everything} can be treated only as the
8969 undifferentiated byte stream of @dfn{normal files}. For example, when a
8970 file is created or removed, the system must record this information,
8971 which it does in a @dfn{directory}---a special type of file. Although
8972 you can read directories as normal files, if you're curious, in order
8973 for the system to do its job it must impose a structure, a certain
8974 order, on the bytes of the file. Thus it is a ``special'' type of file.
8976 Besides directories, other special file types include named pipes
8977 (FIFOs), symbolic links, sockets, and so-called @dfn{special files}.
8980 * link invocation:: Make a hard link via the link syscall
8981 * ln invocation:: Make links between files.
8982 * mkdir invocation:: Make directories.
8983 * mkfifo invocation:: Make FIFOs (named pipes).
8984 * mknod invocation:: Make block or character special files.
8985 * readlink invocation:: Print value of a symlink or canonical file name.
8986 * rmdir invocation:: Remove empty directories.
8987 * unlink invocation:: Remove files via the unlink syscall
8991 @node link invocation
8992 @section @command{link}: Make a hard link via the link syscall
8995 @cindex links, creating
8996 @cindex hard links, creating
8997 @cindex creating links (hard only)
8999 @command{link} creates a single hard link at a time.
9000 It is a minimalist interface to the system-provided
9001 @code{link} function. @xref{Hard Links, , , libc,
9002 The GNU C Library Reference Manual}.
9003 It avoids the bells and whistles of the more commonly-used
9004 @command{ln} command (@pxref{ln invocation}).
9008 link @var{filename} @var{linkname}
9011 @var{filename} must specify an existing file, and @var{linkname}
9012 must specify a nonexistent entry in an existing directory.
9013 @command{link} simply calls @code{link (@var{filename}, @var{linkname})}
9016 On a @acronym{GNU} system, this command acts like @samp{ln --directory
9017 --no-target-directory @var{filename} @var{linkname}}. However, the
9018 @option{--directory} and @option{--no-target-directory} options are
9019 not specified by @acronym{POSIX}, and the @command{link} command is
9020 more portable in practice.
9022 If @var{filename} is a symbolic link, it is unspecified whether
9023 @var{linkname} will be a hard link to the symbolic link or to the
9024 target of the symbolic link. Use @command{ln -P} or @command{ln -L}
9025 to specify which behavior is desired.
9031 @section @command{ln}: Make links between files
9034 @cindex links, creating
9035 @cindex hard links, creating
9036 @cindex symbolic (soft) links, creating
9037 @cindex creating links (hard or soft)
9039 @cindex file systems and hard links
9040 @command{ln} makes links between files. By default, it makes hard links;
9041 with the @option{-s} option, it makes symbolic (or @dfn{soft}) links.
9045 ln [@var{option}]@dots{} [-T] @var{target} @var{linkname}
9046 ln [@var{option}]@dots{} @var{target}
9047 ln [@var{option}]@dots{} @var{target}@dots{} @var{directory}
9048 ln [@var{option}]@dots{} -t @var{directory} @var{target}@dots{}
9054 If two file names are given, @command{ln} creates a link to the first
9055 file from the second.
9058 If one @var{target} is given, @command{ln} creates a link to that file
9059 in the current directory.
9062 If the @option{--target-directory} (@option{-t}) option is given, or
9063 failing that if the last file is a directory and the
9064 @option{--no-target-directory} (@option{-T}) option is not given,
9065 @command{ln} creates a link to each @var{target} file in the specified
9066 directory, using the @var{target}s' names.
9070 Normally @command{ln} does not remove existing files. Use the
9071 @option{--force} (@option{-f}) option to remove them unconditionally,
9072 the @option{--interactive} (@option{-i}) option to remove them
9073 conditionally, and the @option{--backup} (@option{-b}) option to
9076 @cindex hard link, defined
9077 @cindex inode, and hard links
9078 A @dfn{hard link} is another name for an existing file; the link and the
9079 original are indistinguishable. Technically speaking, they share the
9080 same inode, and the inode contains all the information about a
9081 file---indeed, it is not incorrect to say that the inode @emph{is} the
9082 file. Most systems prohibit making a hard link to
9083 a directory; on those where it is allowed, only the super-user can do
9084 so (and with caution, since creating a cycle will cause problems to many
9085 other utilities). Hard links cannot cross file system boundaries. (These
9086 restrictions are not mandated by @acronym{POSIX}, however.)
9088 @cindex dereferencing symbolic links
9089 @cindex symbolic link, defined
9090 @dfn{Symbolic links} (@dfn{symlinks} for short), on the other hand, are
9091 a special file type (which not all kernels support: System V release 3
9092 (and older) systems lack symlinks) in which the link file actually
9093 refers to a different file, by name. When most operations (opening,
9094 reading, writing, and so on) are passed the symbolic link file, the
9095 kernel automatically @dfn{dereferences} the link and operates on the
9096 target of the link. But some operations (e.g., removing) work on the
9097 link file itself, rather than on its target. The owner and group of a
9098 symlink are not significant to file access performed through
9099 the link, but do have implications on deleting a symbolic link from a
9100 directory with the restricted deletion bit set. On the GNU system,
9101 the mode of a symlink has no significance and cannot be changed, but
9102 on some BSD systems, the mode can be changed and will affect whether
9103 the symlink will be traversed in file name resolution. @xref{Symbolic Links,,,
9104 libc, The GNU C Library Reference Manual}.
9106 Symbolic links can contain arbitrary strings; a @dfn{dangling symlink}
9107 occurs when the string in the symlink does not resolve to a file.
9108 There are no restrictions against creating dangling symbolic links.
9109 There are trade-offs to using absolute or relative symlinks. An
9110 absolute symlink always points to the same file, even if the directory
9111 containing the link is moved. However, if the symlink is visible from
9112 more than one machine (such as on a networked file system), the file
9113 pointed to might not always be the same. A relative symbolic link is
9114 resolved in relation to the directory that contains the link, and is
9115 often useful in referring to files on the same device without regards
9116 to what name that device is mounted on when accessed via networked
9119 When creating a relative symlink in a different location than the
9120 current directory, the resolution of the symlink will be different
9121 than the resolution of the same string from the current directory.
9122 Therefore, many users prefer to first change directories to the
9123 location where the relative symlink will be created, so that
9124 tab-completion or other file resolution will find the same target as
9125 what will be placed in the symlink.
9127 The program accepts the following options. Also see @ref{Common options}.
9138 @opindex --directory
9139 @cindex hard links to directories
9140 Allow users with appropriate privileges to attempt to make hard links
9142 However, note that this will probably fail due to
9143 system restrictions, even for the super-user.
9149 Remove existing destination files.
9152 @itemx --interactive
9154 @opindex --interactive
9155 @cindex prompting, and @command{ln}
9156 Prompt whether to remove existing destination files.
9162 If @option{-s} is not in effect, and the source file is a symbolic
9163 link, create the hard link to the file referred to by the symbolic
9164 link, rather than the symbolic link itself.
9167 @itemx --no-dereference
9169 @opindex --no-dereference
9170 Do not treat the last operand specially when it is a symbolic link to
9171 a directory. Instead, treat it as if it were a normal file.
9173 When the destination is an actual directory (not a symlink to one),
9174 there is no ambiguity. The link is created in that directory.
9175 But when the specified destination is a symlink to a directory,
9176 there are two ways to treat the user's request. @command{ln} can
9177 treat the destination just as it would a normal directory and create
9178 the link in it. On the other hand, the destination can be viewed as a
9179 non-directory---as the symlink itself. In that case, @command{ln}
9180 must delete or backup that symlink before creating the new link.
9181 The default is to treat a destination that is a symlink to a directory
9182 just like a directory.
9184 This option is weaker than the @option{--no-target-directory}
9185 (@option{-T}) option, so it has no effect if both options are given.
9191 If @option{-s} is not in effect, and the source file is a symbolic
9192 link, create the hard link to the symbolic link itself. On platforms
9193 where this is not supported by the kernel, this option creates a
9194 symbolic link with identical contents; since symbolic link contents
9195 cannot be edited, any file name resolution performed through either
9196 link will be the same as if a hard link had been created.
9202 Make symbolic links instead of hard links. This option merely produces
9203 an error message on systems that do not support symbolic links.
9209 @optNoTargetDirectory
9215 Print the name of each file after linking it successfully.
9219 @cindex hard links to symbolic links
9220 @cindex symbolic links and @command{ln}
9221 If @option{-L} and @option{-P} are both given, the last one takes
9222 precedence. If @option{-s} is also given, @option{-L} and @option{-P}
9223 are silently ignored. If neither option is given, then this
9224 implementation defaults to @option{-P} if the system @code{link} supports
9225 hard links to symbolic links (such as the GNU system), and @option{-L}
9226 if @code{link} follows symbolic links (such as on BSD).
9235 # Create link ../a pointing to a in that directory.
9236 # Not really useful because it points to itself.
9241 # Change to the target before creating symlinks to avoid being confused.
9247 # Hard coded file names don't move well.
9248 ln -s $(pwd)/a /some/dir/
9252 # Relative file names survive directory moves and also
9253 # work across networked file systems.
9254 ln -s afile anotherfile
9255 ln -s ../adir/afile yetanotherfile
9259 @node mkdir invocation
9260 @section @command{mkdir}: Make directories
9263 @cindex directories, creating
9264 @cindex creating directories
9266 @command{mkdir} creates directories with the specified names. Synopsis:
9269 mkdir [@var{option}]@dots{} @var{name}@dots{}
9272 @command{mkdir} creates each directory @var{name} in the order given.
9273 It reports an error if @var{name} already exists, unless the
9274 @option{-p} option is given and @var{name} is a directory.
9276 The program accepts the following options. Also see @ref{Common options}.
9281 @itemx --mode=@var{mode}
9284 @cindex modes of created directories, setting
9285 Set the file permission bits of created directories to @var{mode},
9286 which uses the same syntax as
9287 in @command{chmod} and uses @samp{a=rwx} (read, write and execute allowed for
9288 everyone) for the point of the departure. @xref{File permissions}.
9290 Normally the directory has the desired file mode bits at the moment it
9291 is created. As a @acronym{GNU} extension, @var{mode} may also mention
9292 special mode bits, but in this case there may be a temporary window
9293 during which the directory exists but its special mode bits are
9294 incorrect. @xref{Directory Setuid and Setgid}, for how the
9295 set-user-ID and set-group-ID bits of directories are inherited unless
9296 overridden in this way.
9302 @cindex parent directories, creating
9303 Make any missing parent directories for each argument, setting their
9304 file permission bits to the umask modified by @samp{u+wx}. Ignore
9305 existing parent directories, and do not change their file permission
9308 To set the file permission bits of any newly-created parent
9309 directories to a value that includes @samp{u+wx}, you can set the
9310 umask before invoking @command{mkdir}. For example, if the shell
9311 command @samp{(umask u=rwx,go=rx; mkdir -p P/Q)} creates the parent
9312 @file{P} it sets the parent's permission bits to @samp{u=rwx,go=rx}.
9313 To set a parent's special mode bits as well, you can invoke
9314 @command{chmod} after @command{mkdir}. @xref{Directory Setuid and
9315 Setgid}, for how the set-user-ID and set-group-ID bits of
9316 newly-created parent directories are inherited.
9322 Print a message for each created directory. This is most useful with
9325 @item -Z @var{context}
9326 @itemx --context=@var{context}
9330 @cindex security context
9331 Set the default SELinux security context to be used for created directories.
9338 @node mkfifo invocation
9339 @section @command{mkfifo}: Make FIFOs (named pipes)
9342 @cindex FIFOs, creating
9343 @cindex named pipes, creating
9344 @cindex creating FIFOs (named pipes)
9346 @command{mkfifo} creates FIFOs (also called @dfn{named pipes}) with the
9347 specified names. Synopsis:
9350 mkfifo [@var{option}] @var{name}@dots{}
9353 A @dfn{FIFO} is a special file type that permits independent processes
9354 to communicate. One process opens the FIFO file for writing, and
9355 another for reading, after which data can flow as with the usual
9356 anonymous pipe in shells or elsewhere.
9358 The program accepts the following option. Also see @ref{Common options}.
9363 @itemx --mode=@var{mode}
9366 @cindex modes of created FIFOs, setting
9367 Set the mode of created FIFOs to @var{mode}, which is symbolic as in
9368 @command{chmod} and uses @samp{a=rw} (read and write allowed for everyone)
9369 for the point of departure. @var{mode} should specify only file
9370 permission bits. @xref{File permissions}.
9372 @item -Z @var{context}
9373 @itemx --context=@var{context}
9377 @cindex security context
9378 Set the default SELinux security context to be used for created FIFOs.
9385 @node mknod invocation
9386 @section @command{mknod}: Make block or character special files
9389 @cindex block special files, creating
9390 @cindex character special files, creating
9392 @command{mknod} creates a FIFO, character special file, or block special
9393 file with the specified name. Synopsis:
9396 mknod [@var{option}]@dots{} @var{name} @var{type} [@var{major} @var{minor}]
9399 @cindex special files
9400 @cindex block special files
9401 @cindex character special files
9402 Unlike the phrase ``special file type'' above, the term @dfn{special
9403 file} has a technical meaning on Unix: something that can generate or
9404 receive data. Usually this corresponds to a physical piece of hardware,
9405 e.g., a printer or a disk. (These files are typically created at
9406 system-configuration time.) The @command{mknod} command is what creates
9407 files of this type. Such devices can be read either a character at a
9408 time or a ``block'' (many characters) at a time, hence we say there are
9409 @dfn{block special} files and @dfn{character special} files.
9411 @c mknod is a shell built-in at least with OpenBSD's /bin/sh
9412 @mayConflictWithShellBuiltIn{mknod}
9414 The arguments after @var{name} specify the type of file to make:
9419 @opindex p @r{for FIFO file}
9423 @opindex b @r{for block special file}
9424 for a block special file
9427 @c Don't document the `u' option -- it's just a synonym for `c'.
9428 @c Do *any* versions of mknod still use it?
9430 @opindex c @r{for character special file}
9431 @c @opindex u @r{for character special file}
9432 for a character special file
9436 When making a block or character special file, the major and minor
9437 device numbers must be given after the file type.
9438 If a major or minor device number begins with @samp{0x} or @samp{0X},
9439 it is interpreted as hexadecimal; otherwise, if it begins with @samp{0},
9440 as octal; otherwise, as decimal.
9442 The program accepts the following option. Also see @ref{Common options}.
9447 @itemx --mode=@var{mode}
9450 Set the mode of created files to @var{mode}, which is symbolic as in
9451 @command{chmod} and uses @samp{a=rw} as the point of departure.
9452 @var{mode} should specify only file permission bits.
9453 @xref{File permissions}.
9455 @item -Z @var{context}
9456 @itemx --context=@var{context}
9460 @cindex security context
9461 Set the default SELinux security context to be used for created files.
9468 @node readlink invocation
9469 @section @command{readlink}: Print value of a symlink or canonical file name
9472 @cindex displaying value of a symbolic link
9473 @cindex canonical file name
9474 @cindex canonicalize a file name
9478 @command{readlink} may work in one of two supported modes:
9484 @command{readlink} outputs the value of the given symbolic link.
9485 If @command{readlink} is invoked with an argument other than the name
9486 of a symbolic link, it produces no output and exits with a nonzero exit code.
9488 @item Canonicalize mode
9490 @command{readlink} outputs the absolute name of the given file which contains
9491 no @file{.}, @file{..} components nor any repeated separators
9492 (@file{/}) or symbolic links.
9497 readlink [@var{option}] @var{file}
9500 By default, @command{readlink} operates in readlink mode.
9502 The program accepts the following options. Also see @ref{Common options}.
9507 @itemx --canonicalize
9509 @opindex --canonicalize
9510 Activate canonicalize mode.
9511 If any component of the file name except the last one is missing or unavailable,
9512 @command{readlink} produces no output and exits with a nonzero exit
9513 code. A trailing slash is ignored.
9516 @itemx --canonicalize-existing
9518 @opindex --canonicalize-existing
9519 Activate canonicalize mode.
9520 If any component is missing or unavailable, @command{readlink} produces
9521 no output and exits with a nonzero exit code. A trailing slash
9522 requires that the name resolve to a directory.
9525 @itemx --canonicalize-missing
9527 @opindex --canonicalize-missing
9528 Activate canonicalize mode.
9529 If any component is missing or unavailable, @command{readlink} treats it
9535 @opindex --no-newline
9536 Do not output the trailing newline.
9546 Suppress most error messages.
9552 Report error messages.
9556 The @command{readlink} utility first appeared in OpenBSD 2.1.
9558 There is a @command{realpath} command on some systems
9559 which operates like @command{readlink} in canonicalize mode.
9564 @node rmdir invocation
9565 @section @command{rmdir}: Remove empty directories
9568 @cindex removing empty directories
9569 @cindex directories, removing empty
9571 @command{rmdir} removes empty directories. Synopsis:
9574 rmdir [@var{option}]@dots{} @var{directory}@dots{}
9577 If any @var{directory} argument does not refer to an existing empty
9578 directory, it is an error.
9580 The program accepts the following options. Also see @ref{Common options}.
9584 @item --ignore-fail-on-non-empty
9585 @opindex --ignore-fail-on-non-empty
9586 @cindex directory deletion, ignoring failures
9587 Ignore each failure to remove a directory that is solely because
9588 the directory is non-empty.
9594 @cindex parent directories, removing
9595 Remove @var{directory}, then try to remove each component of @var{directory}.
9596 So, for example, @samp{rmdir -p a/b/c} is similar to @samp{rmdir a/b/c a/b a}.
9597 As such, it fails if any of those directories turns out not to be empty.
9598 Use the @option{--ignore-fail-on-non-empty} option to make it so such
9599 a failure does not evoke a diagnostic and does not cause @command{rmdir} to
9600 exit unsuccessfully.
9606 @cindex directory deletion, reporting
9607 Give a diagnostic for each successful removal.
9608 @var{directory} is removed.
9612 @xref{rm invocation}, for how to remove non-empty directories (recursively).
9617 @node unlink invocation
9618 @section @command{unlink}: Remove files via the unlink syscall
9621 @cindex removing files or directories (via the unlink syscall)
9623 @command{unlink} deletes a single specified file name.
9624 It is a minimalist interface to the system-provided
9625 @code{unlink} function. @xref{Deleting Files, , , libc,
9626 The GNU C Library Reference Manual}. Synopsis:
9627 It avoids the bells and whistles of the more commonly-used
9628 @command{rm} command (@pxref{rm invocation}).
9631 unlink @var{filename}
9634 On some systems @code{unlink} can be used to delete the name of a
9635 directory. On others, it can be used that way only by a privileged user.
9636 In the GNU system @code{unlink} can never delete the name of a directory.
9638 The @command{unlink} command honors the @option{--help} and
9639 @option{--version} options. To remove a file whose name begins with
9640 @samp{-}, prefix the name with @samp{./}, e.g., @samp{unlink ./--help}.
9645 @node Changing file attributes
9646 @chapter Changing file attributes
9648 @cindex changing file attributes
9649 @cindex file attributes, changing
9650 @cindex attributes, file
9652 A file is not merely its contents, a name, and a file type
9653 (@pxref{Special file types}). A file also has an owner (a user ID), a
9654 group (a group ID), permissions (what the owner can do with the file,
9655 what people in the group can do, and what everyone else can do), various
9656 timestamps, and other information. Collectively, we call these a file's
9659 These commands change file attributes.
9662 * chgrp invocation:: Change file groups.
9663 * chmod invocation:: Change access permissions.
9664 * chown invocation:: Change file owners and groups.
9665 * touch invocation:: Change file timestamps.
9669 @node chown invocation
9670 @section @command{chown}: Change file owner and group
9673 @cindex file ownership, changing
9674 @cindex group ownership, changing
9675 @cindex changing file ownership
9676 @cindex changing group ownership
9678 @command{chown} changes the user and/or group ownership of each given @var{file}
9679 to @var{new-owner} or to the user and group of an existing reference file.
9683 chown [@var{option}]@dots{} @{@var{new-owner} | --reference=@var{ref_file}@}@c
9687 If used, @var{new-owner} specifies the new owner and/or group as follows
9688 (with no embedded white space):
9691 [@var{owner}] [ : [@var{group}] ]
9698 If only an @var{owner} (a user name or numeric user ID) is given, that
9699 user is made the owner of each given file, and the files' group is not
9702 @item owner@samp{:}group
9703 If the @var{owner} is followed by a colon and a @var{group} (a
9704 group name or numeric group ID), with no spaces between them, the group
9705 ownership of the files is changed as well (to @var{group}).
9708 If a colon but no group name follows @var{owner}, that user is
9709 made the owner of the files and the group of the files is changed to
9710 @var{owner}'s login group.
9713 If the colon and following @var{group} are given, but the owner
9714 is omitted, only the group of the files is changed; in this case,
9715 @command{chown} performs the same function as @command{chgrp}.
9718 If only a colon is given, or if @var{new-owner} is empty, neither the
9719 owner nor the group is changed.
9723 If @var{owner} or @var{group} is intended to represent a numeric user
9724 or group ID, then you may specify it with a leading @samp{+}.
9725 @xref{Disambiguating names and IDs}.
9727 Some older scripts may still use @samp{.} in place of the @samp{:} separator.
9728 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) does not
9729 require support for that, but for backward compatibility @acronym{GNU}
9730 @command{chown} supports @samp{.} so long as no ambiguity results.
9731 New scripts should avoid the use of @samp{.} because it is not
9732 portable, and because it has undesirable results if the entire
9733 @var{owner@samp{.}group} happens to identify a user whose name
9736 The @command{chown} command sometimes clears the set-user-ID or
9737 set-group-ID permission bits. This behavior depends on the policy and
9738 functionality of the underlying @code{chown} system call, which may
9739 make system-dependent file mode modifications outside the control of
9740 the @command{chown} command. For example, the @command{chown} command
9741 might not affect those bits when invoked by a user with appropriate
9742 privileges, or when the
9743 bits signify some function other than executable permission (e.g.,
9745 When in doubt, check the underlying system behavior.
9747 The program accepts the following options. Also see @ref{Common options}.
9755 @cindex changed owners, verbosely describing
9756 Verbosely describe the action for each @var{file} whose ownership
9765 @cindex error messages, omitting
9766 Do not print error messages about files whose ownership cannot be
9769 @itemx @w{@kbd{--from}=@var{old-owner}}
9771 @cindex symbolic links, changing owner
9772 Change a @var{file}'s ownership only if it has current attributes specified
9773 by @var{old-owner}. @var{old-owner} has the same form as @var{new-owner}
9775 This option is useful primarily from a security standpoint in that
9776 it narrows considerably the window of potential abuse.
9777 For example, to reflect a user ID numbering change for one user's files
9778 without an option like this, @code{root} might run
9781 find / -owner OLDUSER -print0 | xargs -0 chown -h NEWUSER
9784 But that is dangerous because the interval between when the @command{find}
9785 tests the existing file's owner and when the @command{chown} is actually run
9787 One way to narrow the gap would be to invoke chown for each file
9791 find / -owner OLDUSER -exec chown -h NEWUSER @{@} \;
9794 But that is very slow if there are many affected files.
9795 With this option, it is safer (the gap is narrower still)
9796 though still not perfect:
9799 chown -h -R --from=OLDUSER NEWUSER /
9803 @opindex --dereference
9804 @cindex symbolic links, changing owner
9806 Do not act on symbolic links themselves but rather on what they point to.
9807 This is the default.
9810 @itemx --no-dereference
9812 @opindex --no-dereference
9813 @cindex symbolic links, changing owner
9815 Act on symbolic links themselves instead of what they point to.
9816 This mode relies on the @code{lchown} system call.
9817 On systems that do not provide the @code{lchown} system call,
9818 @command{chown} fails when a file specified on the command line
9820 By default, no diagnostic is issued for symbolic links encountered
9821 during a recursive traversal, but see @option{--verbose}.
9823 @itemx --preserve-root
9824 @opindex --preserve-root
9825 @cindex root directory, disallow recursive modification
9826 Fail upon any attempt to recursively change the root directory, @file{/}.
9827 Without @option{--recursive}, this option has no effect.
9828 @xref{Treating / specially}.
9830 @itemx --no-preserve-root
9831 @opindex --no-preserve-root
9832 @cindex root directory, allow recursive modification
9833 Cancel the effect of any preceding @option{--preserve-root} option.
9834 @xref{Treating / specially}.
9836 @item --reference=@var{ref_file}
9837 @opindex --reference
9838 Change the user and group of each @var{file} to be the same as those of
9839 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9840 user and group of the symbolic link, but rather those of the file it
9847 Output a diagnostic for every file processed.
9848 If a symbolic link is encountered during a recursive traversal
9849 on a system without the @code{lchown} system call, and @option{--no-dereference}
9850 is in effect, then issue a diagnostic saying neither the symbolic link nor
9851 its referent is being changed.
9856 @opindex --recursive
9857 @cindex recursively changing file ownership
9858 Recursively change ownership of directories and their contents.
9861 @xref{Traversing symlinks}.
9864 @xref{Traversing symlinks}.
9867 @xref{Traversing symlinks}.
9876 # Change the owner of /u to "root".
9879 # Likewise, but also change its group to "staff".
9882 # Change the owner of /u and subfiles to "root".
9887 @node chgrp invocation
9888 @section @command{chgrp}: Change group ownership
9891 @cindex group ownership, changing
9892 @cindex changing group ownership
9894 @command{chgrp} changes the group ownership of each given @var{file}
9895 to @var{group} (which can be either a group name or a numeric group ID)
9896 or to the group of an existing reference file. Synopsis:
9899 chgrp [@var{option}]@dots{} @{@var{group} | --reference=@var{ref_file}@}@c
9903 If @var{group} is intended to represent a
9904 numeric group ID, then you may specify it with a leading @samp{+}.
9905 @xref{Disambiguating names and IDs}.
9907 The program accepts the following options. Also see @ref{Common options}.
9915 @cindex changed files, verbosely describing
9916 Verbosely describe the action for each @var{file} whose group actually
9925 @cindex error messages, omitting
9926 Do not print error messages about files whose group cannot be
9930 @opindex --dereference
9931 @cindex symbolic links, changing owner
9933 Do not act on symbolic links themselves but rather on what they point to.
9934 This is the default.
9937 @itemx --no-dereference
9939 @opindex --no-dereference
9940 @cindex symbolic links, changing group
9942 Act on symbolic links themselves instead of what they point to.
9943 This mode relies on the @code{lchown} system call.
9944 On systems that do not provide the @code{lchown} system call,
9945 @command{chgrp} fails when a file specified on the command line
9947 By default, no diagnostic is issued for symbolic links encountered
9948 during a recursive traversal, but see @option{--verbose}.
9950 @itemx --preserve-root
9951 @opindex --preserve-root
9952 @cindex root directory, disallow recursive modification
9953 Fail upon any attempt to recursively change the root directory, @file{/}.
9954 Without @option{--recursive}, this option has no effect.
9955 @xref{Treating / specially}.
9957 @itemx --no-preserve-root
9958 @opindex --no-preserve-root
9959 @cindex root directory, allow recursive modification
9960 Cancel the effect of any preceding @option{--preserve-root} option.
9961 @xref{Treating / specially}.
9963 @item --reference=@var{ref_file}
9964 @opindex --reference
9965 Change the group of each @var{file} to be the same as that of
9966 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9967 group of the symbolic link, but rather that of the file it refers to.
9973 Output a diagnostic for every file processed.
9974 If a symbolic link is encountered during a recursive traversal
9975 on a system without the @code{lchown} system call, and @option{--no-dereference}
9976 is in effect, then issue a diagnostic saying neither the symbolic link nor
9977 its referent is being changed.
9982 @opindex --recursive
9983 @cindex recursively changing group ownership
9984 Recursively change the group ownership of directories and their contents.
9987 @xref{Traversing symlinks}.
9990 @xref{Traversing symlinks}.
9993 @xref{Traversing symlinks}.
10002 # Change the group of /u to "staff".
10005 # Change the group of /u and subfiles to "staff".
10010 @node chmod invocation
10011 @section @command{chmod}: Change access permissions
10014 @cindex changing access permissions
10015 @cindex access permissions, changing
10016 @cindex permissions, changing access
10018 @command{chmod} changes the access permissions of the named files. Synopsis:
10021 chmod [@var{option}]@dots{} @{@var{mode} | --reference=@var{ref_file}@}@c
10025 @cindex symbolic links, permissions of
10026 @command{chmod} never changes the permissions of symbolic links, since
10027 the @command{chmod} system call cannot change their permissions.
10028 This is not a problem since the permissions of symbolic links are
10029 never used. However, for each symbolic link listed on the command
10030 line, @command{chmod} changes the permissions of the pointed-to file.
10031 In contrast, @command{chmod} ignores symbolic links encountered during
10032 recursive directory traversals.
10034 A successful use of @command{chmod} clears the set-group-ID bit of a
10035 regular file if the file's group ID does not match the user's
10036 effective group ID or one of the user's supplementary group IDs,
10037 unless the user has appropriate privileges. Additional restrictions
10038 may cause the set-user-ID and set-group-ID bits of @var{mode} or
10039 @var{ref_file} to be ignored. This behavior depends on the policy and
10040 functionality of the underlying @code{chmod} system call. When in
10041 doubt, check the underlying system behavior.
10043 If used, @var{mode} specifies the new file mode bits.
10044 For details, see the section on @ref{File permissions}.
10045 If you really want @var{mode} to have a leading @samp{-}, you should
10046 use @option{--} first, e.g., @samp{chmod -- -w file}. Typically,
10047 though, @samp{chmod a-w file} is preferable, and @command{chmod -w
10048 file} (without the @option{--}) complains if it behaves differently
10049 from what @samp{chmod a-w file} would do.
10051 The program accepts the following options. Also see @ref{Common options}.
10059 Verbosely describe the action for each @var{file} whose permissions
10068 @cindex error messages, omitting
10069 Do not print error messages about files whose permissions cannot be
10072 @itemx --preserve-root
10073 @opindex --preserve-root
10074 @cindex root directory, disallow recursive modification
10075 Fail upon any attempt to recursively change the root directory, @file{/}.
10076 Without @option{--recursive}, this option has no effect.
10077 @xref{Treating / specially}.
10079 @itemx --no-preserve-root
10080 @opindex --no-preserve-root
10081 @cindex root directory, allow recursive modification
10082 Cancel the effect of any preceding @option{--preserve-root} option.
10083 @xref{Treating / specially}.
10089 Verbosely describe the action or non-action taken for every @var{file}.
10091 @item --reference=@var{ref_file}
10092 @opindex --reference
10093 Change the mode of each @var{file} to be the same as that of @var{ref_file}.
10094 @xref{File permissions}.
10095 If @var{ref_file} is a symbolic link, do not use the mode
10096 of the symbolic link, but rather that of the file it refers to.
10101 @opindex --recursive
10102 @cindex recursively changing access permissions
10103 Recursively change permissions of directories and their contents.
10110 @node touch invocation
10111 @section @command{touch}: Change file timestamps
10114 @cindex changing file timestamps
10115 @cindex file timestamps, changing
10116 @cindex timestamps, changing file
10118 @command{touch} changes the access and/or modification times of the
10119 specified files. Synopsis:
10122 touch [@var{option}]@dots{} @var{file}@dots{}
10125 @cindex empty files, creating
10126 Any @var{file} argument that does not exist is created empty, unless
10127 option @option{--no-create} (@option{-c}) or @option{--no-dereference}
10128 (@option{-h}) was in effect.
10130 A @var{file} argument string of @samp{-} is handled specially and
10131 causes @command{touch} to change the times of the file associated with
10134 @cindex permissions, for changing file timestamps
10135 If changing both the access and modification times to the current
10136 time, @command{touch} can change the timestamps for files that the user
10137 running it does not own but has write permission for. Otherwise, the
10138 user must own the files.
10140 Although @command{touch} provides options for changing two of the times---the
10141 times of last access and modification---of a file, there is actually
10142 a standard third one as well: the inode change time. This is often
10143 referred to as a file's @code{ctime}.
10144 The inode change time represents the time when the file's meta-information
10145 last changed. One common example of this is when the permissions of a
10146 file change. Changing the permissions doesn't access the file, so
10147 the atime doesn't change, nor does it modify the file, so the mtime
10148 doesn't change. Yet, something about the file itself has changed,
10149 and this must be noted somewhere. This is the job of the ctime field.
10150 This is necessary, so that, for example, a backup program can make a
10151 fresh copy of the file, including the new permissions value.
10152 Another operation that modifies a file's ctime without affecting
10153 the others is renaming. In any case, it is not possible, in normal
10154 operations, for a user to change the ctime field to a user-specified value.
10155 Some operating systems and file systems support a fourth time: the
10156 birth time, when the file was first created; by definition, this
10157 timestamp never changes.
10160 Time stamps assume the time zone rules specified by the @env{TZ}
10161 environment variable, or by the system default rules if @env{TZ} is
10162 not set. @xref{TZ Variable,, Specifying the Time Zone with @env{TZ},
10163 libc, The GNU C Library Reference Manual}.
10164 You can avoid ambiguities during
10165 daylight saving transitions by using @sc{utc} time stamps.
10167 The program accepts the following options. Also see @ref{Common options}.
10172 @itemx --time=atime
10173 @itemx --time=access
10177 @opindex atime@r{, changing}
10178 @opindex access @r{time, changing}
10179 @opindex use @r{time, changing}
10180 Change the access time only.
10185 @opindex --no-create
10186 Do not warn about or create files that do not exist.
10189 @itemx --date=@var{time}
10193 Use @var{time} instead of the current time. It can contain month names,
10194 time zones, @samp{am} and @samp{pm}, @samp{yesterday}, etc. For
10195 example, @option{--date="2004-02-27 14:19:13.489392193 +0530"}
10196 specifies the instant of time that is 489,392,193 nanoseconds after
10197 February 27, 2004 at 2:19:13 PM in a time zone that is 5 hours and 30
10198 minutes east of @acronym{UTC}. @xref{Date input formats}.
10199 File systems that do not support high-resolution time stamps
10200 silently ignore any excess precision here.
10204 @cindex BSD @command{touch} compatibility
10205 Ignored; for compatibility with BSD versions of @command{touch}.
10208 @itemx --no-dereference
10210 @opindex --no-dereference
10211 @cindex symbolic links, changing time
10213 Attempt to change the timestamps of a symbolic link, rather than what
10214 the link refers to. When using this option, empty files are not
10215 created, but option @option{-c} must also be used to avoid warning
10216 about files that do not exist. Not all systems support changing the
10217 timestamps of symlinks, since underlying system support for this
10218 action was not required until @acronym{POSIX} 2008. Also, on some
10219 systems, the mere act of examining a symbolic link changes the access
10220 time, such that only changes to the modification time will persist
10221 long enough to be observable. When coupled with option @option{-r}, a
10222 reference timestamp is taken from a symbolic link rather than the file
10226 @itemx --time=mtime
10227 @itemx --time=modify
10230 @opindex mtime@r{, changing}
10231 @opindex modify @r{time, changing}
10232 Change the modification time only.
10234 @item -r @var{file}
10235 @itemx --reference=@var{file}
10237 @opindex --reference
10238 Use the times of the reference @var{file} instead of the current time.
10239 If this option is combined with the @option{--date=@var{time}}
10240 (@option{-d @var{time}}) option, the reference @var{file}'s time is
10241 the origin for any relative @var{time}s given, but is otherwise ignored.
10242 For example, @samp{-r foo -d '-5 seconds'} specifies a time stamp
10243 equal to five seconds before the corresponding time stamp for @file{foo}.
10244 If @var{file} is a symbolic link, the reference timestamp is taken
10245 from the target of the symlink, unless @option{-h} was also in effect.
10247 @item -t [[@var{cc}]@var{yy}]@var{mmddhhmm}[.@var{ss}]
10248 Use the argument (optional four-digit or two-digit years, months,
10249 days, hours, minutes, optional seconds) instead of the current time.
10250 If the year is specified with only two digits, then @var{cc}
10251 is 20 for years in the range 0 @dots{} 68, and 19 for years in
10252 69 @dots{} 99. If no digits of the year are specified,
10253 the argument is interpreted as a date in the current year.
10254 Note that @var{ss} may be @samp{60}, to accommodate leap seconds.
10258 @vindex _POSIX2_VERSION
10259 On older systems, @command{touch} supports an obsolete syntax, as follows.
10260 If no timestamp is given with any of the @option{-d}, @option{-r}, or
10261 @option{-t} options, and if there are two or more @var{file}s and the
10262 first @var{file} is of the form @samp{@var{mmddhhmm}[@var{yy}]} and this
10263 would be a valid argument to the @option{-t} option (if the @var{yy}, if
10264 any, were moved to the front), and if the represented year
10265 is in the range 1969--1999, that argument is interpreted as the time
10266 for the other files instead of as a file name.
10267 This obsolete behavior can be enabled or disabled with the
10268 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
10269 conformance}), but portable scripts should avoid commands whose
10270 behavior depends on this variable.
10271 For example, use @samp{touch ./12312359 main.c} or @samp{touch -t
10272 12312359 main.c} rather than the ambiguous @samp{touch 12312359 main.c}.
10278 @chapter Disk usage
10282 No disk can hold an infinite amount of data. These commands report
10283 how much disk storage is in use or available, report other file and
10284 file status information, and write buffers to disk.
10287 * df invocation:: Report file system disk space usage.
10288 * du invocation:: Estimate file space usage.
10289 * stat invocation:: Report file or file system status.
10290 * sync invocation:: Synchronize memory and disk.
10291 * truncate invocation:: Shrink or extend the size of a file.
10295 @node df invocation
10296 @section @command{df}: Report file system disk space usage
10299 @cindex file system disk usage
10300 @cindex disk usage by file system
10302 @command{df} reports the amount of disk space used and available on
10303 file systems. Synopsis:
10306 df [@var{option}]@dots{} [@var{file}]@dots{}
10309 With no arguments, @command{df} reports the space used and available on all
10310 currently mounted file systems (of all types). Otherwise, @command{df}
10311 reports on the file system containing each argument @var{file}.
10313 Normally the disk space is printed in units of
10314 1024 bytes, but this can be overridden (@pxref{Block size}).
10315 Non-integer quantities are rounded up to the next higher unit.
10317 @cindex disk device file
10318 @cindex device file, disk
10319 If an argument @var{file} is a disk device file containing a mounted
10320 file system, @command{df} shows the space available on that file system
10321 rather than on the file system containing the device node (i.e., the root
10322 file system). @sc{gnu} @command{df} does not attempt to determine the
10324 on unmounted file systems, because on most kinds of systems doing so
10325 requires extremely nonportable intimate knowledge of file system
10328 The program accepts the following options. Also see @ref{Common options}.
10336 @cindex automounter file systems
10337 @cindex ignore file systems
10338 Include in the listing dummy file systems, which
10339 are omitted by default. Such file systems are typically special-purpose
10340 pseudo-file-systems, such as automounter entries.
10342 @item -B @var{size}
10343 @itemx --block-size=@var{size}
10345 @opindex --block-size
10346 @cindex file system sizes
10347 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10348 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10352 @cindex grand total of disk size, usage and available space
10353 Print a grand total of all arguments after all arguments have
10354 been processed. This can be used to find out the total disk size, usage
10355 and available space of all listed devices.
10361 Equivalent to @option{--si}.
10367 @cindex inode usage
10368 List inode usage information instead of block usage. An inode (short
10369 for index node) contains information about a file such as its owner,
10370 permissions, timestamps, and location on the disk.
10374 @cindex kibibytes for file system sizes
10375 Print sizes in 1024-byte blocks, overriding the default block size
10376 (@pxref{Block size}).
10377 This option is equivalent to @option{--block-size=1K}.
10383 @cindex file system types, limiting output to certain
10384 Limit the listing to local file systems. By default, remote file systems
10389 @cindex file system space, retrieving old data more quickly
10390 Do not invoke the @code{sync} system call before getting any usage data.
10391 This may make @command{df} run significantly faster on systems with many
10392 disks, but on some systems (notably SunOS) the results may be slightly
10393 out of date. This is the default.
10396 @itemx --portability
10398 @opindex --portability
10399 @cindex one-line output format
10400 @cindex @acronym{POSIX} output format
10401 @cindex portable output format
10402 @cindex output format, portable
10403 Use the @acronym{POSIX} output format. This is like the default format except
10408 The information about each file system is always printed on exactly
10409 one line; a mount device is never put on a line by itself. This means
10410 that if the mount device name is more than 20 characters long (e.g., for
10411 some network mounts), the columns are misaligned.
10414 The labels in the header output line are changed to conform to @acronym{POSIX}.
10417 The default block size and output format are unaffected by the
10418 @env{DF_BLOCK_SIZE}, @env{BLOCK_SIZE} and @env{BLOCKSIZE} environment
10419 variables. However, the default block size is still affected by
10420 @env{POSIXLY_CORRECT}: it is 512 if @env{POSIXLY_CORRECT} is set, 1024
10421 otherwise. @xref{Block size}.
10428 @cindex file system space, retrieving current data more slowly
10429 Invoke the @code{sync} system call before getting any usage data. On
10430 some systems (notably SunOS), doing this yields more up to date results,
10431 but in general this option makes @command{df} much slower, especially when
10432 there are many or very busy file systems.
10434 @item -t @var{fstype}
10435 @itemx --type=@var{fstype}
10438 @cindex file system types, limiting output to certain
10439 Limit the listing to file systems of type @var{fstype}. Multiple
10440 file system types can be specified by giving multiple @option{-t} options.
10441 By default, nothing is omitted.
10444 @itemx --print-type
10446 @opindex --print-type
10447 @cindex file system types, printing
10448 Print each file system's type. The types printed here are the same ones
10449 you can include or exclude with @option{-t} and @option{-x}. The particular
10450 types printed are whatever is supported by the system. Here are some of
10451 the common names (this list is certainly not exhaustive):
10456 @cindex @acronym{NFS} file system type
10457 An @acronym{NFS} file system, i.e., one mounted over a network from another
10458 machine. This is the one type name which seems to be used uniformly by
10461 @item 4.2@r{, }ufs@r{, }efs@dots{}
10462 @cindex Linux file system types
10463 @cindex local file system types
10464 @opindex 4.2 @r{file system type}
10465 @opindex ufs @r{file system type}
10466 @opindex efs @r{file system type}
10467 A file system on a locally-mounted hard disk. (The system might even
10468 support more than one type here; Linux does.)
10470 @item hsfs@r{, }cdfs
10471 @cindex CD-ROM file system type
10472 @cindex High Sierra file system
10473 @opindex hsfs @r{file system type}
10474 @opindex cdfs @r{file system type}
10475 A file system on a CD-ROM drive. HP-UX uses @samp{cdfs}, most other
10476 systems use @samp{hsfs} (@samp{hs} for ``High Sierra'').
10479 @cindex PC file system
10480 @cindex DOS file system
10481 @cindex MS-DOS file system
10482 @cindex diskette file system
10484 An MS-DOS file system, usually on a diskette.
10488 @item -x @var{fstype}
10489 @itemx --exclude-type=@var{fstype}
10491 @opindex --exclude-type
10492 Limit the listing to file systems not of type @var{fstype}.
10493 Multiple file system types can be eliminated by giving multiple
10494 @option{-x} options. By default, no file system types are omitted.
10497 Ignored; for compatibility with System V versions of @command{df}.
10502 Failure includes the case where no output is generated, so you can
10503 inspect the exit status of a command like @samp{df -t ext3 -t reiserfs
10504 @var{dir}} to test whether @var{dir} is on a file system of type
10505 @samp{ext3} or @samp{reiserfs}.
10508 @node du invocation
10509 @section @command{du}: Estimate file space usage
10512 @cindex file space usage
10513 @cindex disk usage for files
10515 @command{du} reports the amount of disk space used by the specified files
10516 and for each subdirectory (of directory arguments). Synopsis:
10519 du [@var{option}]@dots{} [@var{file}]@dots{}
10522 With no arguments, @command{du} reports the disk space for the current
10523 directory. Normally the disk space is printed in units of
10524 1024 bytes, but this can be overridden (@pxref{Block size}).
10525 Non-integer quantities are rounded up to the next higher unit.
10527 If two or more hard links point to the same file, only one of the hard
10528 links is counted. The @var{file} argument order affects which links
10529 are counted, and changing the argument order may change the numbers
10530 that @command{du} outputs.
10532 The program accepts the following options. Also see @ref{Common options}.
10540 Show counts for all files, not just directories.
10542 @itemx --apparent-size
10543 @opindex --apparent-size
10544 Print apparent sizes, rather than disk usage. The apparent size of a
10545 file is the number of bytes reported by @code{wc -c} on regular files,
10546 or more generally, @code{ls -l --block-size=1} or @code{stat --format=%s}.
10547 For example, a file containing the word @samp{zoo} with no newline would,
10548 of course, have an apparent size of 3. Such a small file may require
10549 anywhere from 0 to 16 KiB or more of disk space, depending on
10550 the type and configuration of the file system on which the file resides.
10551 However, a sparse file created with this command:
10554 dd bs=1 seek=2GiB if=/dev/null of=big
10558 has an apparent size of 2 GiB, yet on most modern
10559 systems, it actually uses almost no disk space.
10565 Equivalent to @code{--apparent-size --block-size=1}.
10567 @item -B @var{size}
10568 @itemx --block-size=@var{size}
10570 @opindex --block-size
10572 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10573 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10579 @cindex grand total of disk space
10580 Print a grand total of all arguments after all arguments have
10581 been processed. This can be used to find out the total disk usage of
10582 a given set of files or directories.
10585 @itemx --dereference-args
10587 @opindex --dereference-args
10588 Dereference symbolic links that are command line arguments.
10589 Does not affect other symbolic links. This is helpful for finding
10590 out the disk usage of directories, such as @file{/usr/tmp}, which
10591 are often symbolic links.
10593 @c --files0-from=FILE
10594 @filesZeroFromOption{du,, with the @option{--total} (@option{-c}) option}
10600 Equivalent to @option{--dereference-args} (@option{-D}).
10604 @cindex kibibytes for file sizes
10605 Print sizes in 1024-byte blocks, overriding the default block size
10606 (@pxref{Block size}).
10607 This option is equivalent to @option{--block-size=1K}.
10610 @itemx --count-links
10612 @opindex --count-links
10613 @cindex hard links, counting in @command{du}
10614 Count the size of all files, even if they have appeared already (as a
10618 @itemx --dereference
10620 @opindex --dereference
10621 @cindex symbolic links, dereferencing in @command{du}
10622 Dereference symbolic links (show the disk space used by the file
10623 or directory that the link points to instead of the space used by
10628 @cindex mebibytes for file sizes
10629 Print sizes in 1,048,576-byte blocks, overriding the default block size
10630 (@pxref{Block size}).
10631 This option is equivalent to @option{--block-size=1M}.
10634 @itemx --no-dereference
10636 @opindex --no-dereference
10637 @cindex symbolic links, dereferencing in @command{du}
10638 For each symbolic links encountered by @command{du},
10639 consider the disk space used by the symbolic link.
10641 @item -d @var{depth}
10642 @item --max-depth=@var{depth}
10643 @opindex -d @var{depth}
10644 @opindex --max-depth=@var{depth}
10645 @cindex limiting output of @command{du}
10646 Show the total for each directory (and file if --all) that is at
10647 most MAX_DEPTH levels down from the root of the hierarchy. The root
10648 is at level 0, so @code{du --max-depth=0} is equivalent to @code{du -s}.
10657 @opindex --summarize
10658 Display only a total for each argument.
10661 @itemx --separate-dirs
10663 @opindex --separate-dirs
10664 Normally, in the output of @command{du} (when not using @option{--summarize}),
10665 the size listed next to a directory name, @var{d}, represents the sum
10666 of sizes of all entries beneath @var{d} as well as the size of @var{d} itself.
10667 With @option{--separate-dirs}, the size reported for a directory name,
10668 @var{d}, is merely the @code{stat.st_size}-derived size of the directory
10673 @cindex last modified dates, displaying in @command{du}
10674 Show time of the most recent modification of any file in the directory,
10675 or any of its subdirectories.
10677 @itemx --time=ctime
10678 @itemx --time=status
10681 @opindex ctime@r{, show the most recent}
10682 @opindex status time@r{, show the most recent}
10683 @opindex use time@r{, show the most recent}
10684 Show the most recent status change time (the @samp{ctime} in the inode) of
10685 any file in the directory, instead of the modification time.
10687 @itemx --time=atime
10688 @itemx --time=access
10690 @opindex atime@r{, show the most recent}
10691 @opindex access time@r{, show the most recent}
10692 Show the most recent access time (the @samp{atime} in the inode) of
10693 any file in the directory, instead of the modification time.
10695 @item --time-style=@var{style}
10696 @opindex --time-style
10698 List timestamps in style @var{style}. This option has an effect only if
10699 the @option{--time} option is also specified. The @var{style} should
10700 be one of the following:
10703 @item +@var{format}
10705 List timestamps using @var{format}, where @var{format} is interpreted
10706 like the format argument of @command{date} (@pxref{date invocation}).
10707 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
10708 @command{du} to list timestamps like @samp{2002-03-30 23:45:56}. As
10709 with @command{date}, @var{format}'s interpretation is affected by the
10710 @env{LC_TIME} locale category.
10713 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
10714 format with nanosecond precision, e.g., @samp{2002-03-30
10715 23:45:56.477817180 -0700}. This style is equivalent to
10716 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
10719 List @acronym{ISO} 8601 date and time in minutes, e.g.,
10720 @samp{2002-03-30 23:45}. These timestamps are shorter than
10721 @samp{full-iso} timestamps, and are usually good enough for everyday
10722 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
10725 List @acronym{ISO} 8601 dates for timestamps, e.g., @samp{2002-03-30}.
10726 This style is equivalent to @samp{+%Y-%m-%d}.
10730 You can specify the default value of the @option{--time-style} option
10731 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
10732 the default style is @samp{long-iso}. For compatibility with @command{ls},
10733 if @env{TIME_STYLE} begins with @samp{+} and contains a newline,
10734 the newline and any later characters are ignored; if @env{TIME_STYLE}
10735 begins with @samp{posix-} the @samp{posix-} is ignored; and if
10736 @env{TIME_STYLE} is @samp{locale} it is ignored.
10739 @itemx --one-file-system
10741 @opindex --one-file-system
10742 @cindex one file system, restricting @command{du} to
10743 Skip directories that are on different file systems from the one that
10744 the argument being processed is on.
10746 @item --exclude=@var{pattern}
10747 @opindex --exclude=@var{pattern}
10748 @cindex excluding files from @command{du}
10749 When recursing, skip subdirectories or files matching @var{pattern}.
10750 For example, @code{du --exclude='*.o'} excludes files whose names
10753 @item -X @var{file}
10754 @itemx --exclude-from=@var{file}
10755 @opindex -X @var{file}
10756 @opindex --exclude-from=@var{file}
10757 @cindex excluding files from @command{du}
10758 Like @option{--exclude}, except take the patterns to exclude from @var{file},
10759 one per line. If @var{file} is @samp{-}, take the patterns from standard
10764 @cindex NFS mounts from BSD to HP-UX
10765 On BSD systems, @command{du} reports sizes that are half the correct
10766 values for files that are NFS-mounted from HP-UX systems. On HP-UX
10767 systems, it reports sizes that are twice the correct values for
10768 files that are NFS-mounted from BSD systems. This is due to a flaw
10769 in HP-UX; it also affects the HP-UX @command{du} program.
10774 @node stat invocation
10775 @section @command{stat}: Report file or file system status
10778 @cindex file status
10779 @cindex file system status
10781 @command{stat} displays information about the specified file(s). Synopsis:
10784 stat [@var{option}]@dots{} [@var{file}]@dots{}
10787 With no option, @command{stat} reports all information about the given files.
10788 But it also can be used to report the information of the file systems the
10789 given files are located on. If the files are links, @command{stat} can
10790 also give information about the files the links point to.
10792 @mayConflictWithShellBuiltIn{stat}
10797 @itemx --dereference
10799 @opindex --dereference
10800 @cindex symbolic links, dereferencing in @command{stat}
10801 Change how @command{stat} treats symbolic links.
10802 With this option, @command{stat} acts on the file referenced
10803 by each symbolic link argument.
10804 Without it, @command{stat} acts on any symbolic link argument directly.
10807 @itemx --file-system
10809 @opindex --file-system
10810 @cindex file systems
10811 Report information about the file systems where the given files are located
10812 instead of information about the files themselves.
10813 This option implies the @option{-L} option.
10816 @itemx --format=@var{format}
10818 @opindex --format=@var{format}
10819 @cindex output format
10820 Use @var{format} rather than the default format.
10821 @var{format} is automatically newline-terminated, so
10822 running a command like the following with two or more @var{file}
10823 operands produces a line of output for each operand:
10825 $ stat --format=%d:%i / /usr
10830 @itemx --printf=@var{format}
10831 @opindex --printf=@var{format}
10832 @cindex output format
10833 Use @var{format} rather than the default format.
10834 Like @option{--format}, but interpret backslash escapes,
10835 and do not output a mandatory trailing newline.
10836 If you want a newline, include @samp{\n} in the @var{format}.
10837 Here's how you would use @option{--printf} to print the device
10838 and inode numbers of @file{/} and @file{/usr}:
10840 $ stat --printf='%d:%i\n' / /usr
10849 @cindex terse output
10850 Print the information in terse form, suitable for parsing by other programs.
10854 The valid @var{format} directives for files with @option{--format} and
10855 @option{--printf} are:
10858 @item %a - Access rights in octal
10859 @item %A - Access rights in human readable form
10860 @item %b - Number of blocks allocated (see @samp{%B})
10861 @item %B - The size in bytes of each block reported by @samp{%b}
10862 @item %C - The SELinux security context of a file, if available
10863 @item %d - Device number in decimal
10864 @item %D - Device number in hex
10865 @item %f - Raw mode in hex
10866 @item %F - File type
10867 @item %g - Group ID of owner
10868 @item %G - Group name of owner
10869 @item %h - Number of hard links
10870 @item %i - Inode number
10871 @item %m - Mount point (See note below)
10872 @item %n - File name
10873 @item %N - Quoted file name with dereference if symbolic link
10874 @item %o - I/O block size
10875 @item %s - Total size, in bytes
10876 @item %t - Major device type in hex
10877 @item %T - Minor device type in hex
10878 @item %u - User ID of owner
10879 @item %U - User name of owner
10880 @item %w - Time of file birth, or @samp{-} if unknown
10881 @item %W - Time of file birth as seconds since Epoch, or @samp{0}
10882 @item %x - Time of last access
10883 @item %X - Time of last access as seconds since Epoch
10884 @item %y - Time of last modification
10885 @item %Y - Time of last modification as seconds since Epoch
10886 @item %z - Time of last change
10887 @item %Z - Time of last change as seconds since Epoch
10890 The @samp{%W}, @samp{%X}, @samp{%Y}, and @samp{%Z} formats accept a
10891 precision preceded by a period to specify the number of digits to
10892 print after the decimal point. For example, @samp{%.3X} outputs the
10893 last access time to millisecond precision. If a period is given but no
10894 precision, @command{stat} uses 9 digits, so @samp{%.X} is equivalent to
10895 @samp{%.9X}. When discarding excess precision, time stamps are truncated
10896 toward minus infinity.
10900 $ stat -c '[%015Y]' /usr
10903 $ stat -c '[%15Y]' /usr
10905 $ stat -c '[%-15Y]' /usr
10908 $ stat -c '[%.3Y]' /usr
10910 $ stat -c '[%.Y]' /usr
10911 [1288929712.114951834]
10914 The mount point printed by @samp{%m} is similar to that output
10915 by @command{df}, except that:
10918 stat does not dereference symlinks by default
10919 (unless @option{-L} is specified)
10921 stat does not search for specified device nodes in the
10922 file system list, instead operating on them directly
10925 stat outputs the alias for a bind mounted file, rather than
10926 the initial mount point of its backing device.
10927 One can recursively call stat until there is no change in output,
10928 to get the current base mount point
10931 When listing file system information (@option{--file-system} (@option{-f})),
10932 you must use a different set of @var{format} directives:
10935 @item %a - Free blocks available to non-super-user
10936 @item %b - Total data blocks in file system
10937 @item %c - Total file nodes in file system
10938 @item %d - Free file nodes in file system
10939 @item %f - Free blocks in file system
10940 @item %i - File System ID in hex
10941 @item %l - Maximum length of file names
10942 @item %n - File name
10943 @item %s - Block size (for faster transfers)
10944 @item %S - Fundamental block size (for block counts)
10945 @item %t - Type in hex
10946 @item %T - Type in human readable form
10950 Time stamps are listed according to the time zone rules specified by
10951 the @env{TZ} environment variable, or by the system default rules if
10952 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
10953 with @env{TZ}, libc, The GNU C Library Reference Manual}.
10958 @node sync invocation
10959 @section @command{sync}: Synchronize data on disk with memory
10962 @cindex synchronize disk and memory
10964 @cindex superblock, writing
10965 @cindex inodes, written buffered
10966 @command{sync} writes any data buffered in memory out to disk. This can
10967 include (but is not limited to) modified superblocks, modified inodes,
10968 and delayed reads and writes. This must be implemented by the kernel;
10969 The @command{sync} program does nothing but exercise the @code{sync} system
10972 @cindex crashes and corruption
10973 The kernel keeps data in memory to avoid doing (relatively slow) disk
10974 reads and writes. This improves performance, but if the computer
10975 crashes, data may be lost or the file system corrupted as a
10976 result. The @command{sync} command ensures everything in memory
10977 is written to disk.
10979 Any arguments are ignored, except for a lone @option{--help} or
10980 @option{--version} (@pxref{Common options}).
10985 @node truncate invocation
10986 @section @command{truncate}: Shrink or extend the size of a file
10989 @cindex truncating, file sizes
10991 @command{truncate} shrinks or extends the size of each @var{file} to the
10992 specified size. Synopsis:
10995 truncate @var{option}@dots{} @var{file}@dots{}
10998 @cindex files, creating
10999 Any @var{file} that does not exist is created.
11001 @cindex sparse files, creating
11002 @cindex holes, creating files with
11003 If a @var{file} is larger than the specified size, the extra data is lost.
11004 If a @var{file} is shorter, it is extended and the extended part (or hole)
11005 reads as zero bytes.
11007 The program accepts the following options. Also see @ref{Common options}.
11014 @opindex --no-create
11015 Do not create files that do not exist.
11020 @opindex --io-blocks
11021 Treat @var{size} as number of I/O blocks of the @var{file} rather than bytes.
11023 @item -r @var{rfile}
11024 @itemx --reference=@var{rfile}
11026 @opindex --reference
11027 Base the size of each @var{file} on the size of @var{rfile}.
11029 @item -s @var{size}
11030 @itemx --size=@var{size}
11033 Set or adjust the size of each @var{file} according to @var{size}.
11034 @multiplierSuffixesNoBlocks{size}
11036 @var{size} may also be prefixed by one of the following to adjust
11037 the size of each @var{file} based on their current size:
11039 @samp{+} => extend by
11040 @samp{-} => reduce by
11041 @samp{<} => at most
11042 @samp{>} => at least
11043 @samp{/} => round down to multiple of
11044 @samp{%} => round up to multiple of
11052 @node Printing text
11053 @chapter Printing text
11055 @cindex printing text, commands for
11056 @cindex commands for printing text
11058 This section describes commands that display text strings.
11061 * echo invocation:: Print a line of text.
11062 * printf invocation:: Format and print data.
11063 * yes invocation:: Print a string until interrupted.
11067 @node echo invocation
11068 @section @command{echo}: Print a line of text
11071 @cindex displaying text
11072 @cindex printing text
11073 @cindex text, displaying
11074 @cindex arbitrary text, displaying
11076 @command{echo} writes each given @var{string} to standard output, with a
11077 space between each and a newline after the last one. Synopsis:
11080 echo [@var{option}]@dots{} [@var{string}]@dots{}
11083 @mayConflictWithShellBuiltIn{echo}
11085 The program accepts the following options. Also see @ref{Common options}.
11086 Options must precede operands, and the normally-special argument
11087 @samp{--} has no special meaning and is treated like any other
11093 Do not output the trailing newline.
11097 @cindex backslash escapes
11098 Enable interpretation of the following backslash-escaped characters in
11107 produce no further output
11123 the eight-bit value that is the octal number @var{nnn}
11124 (zero to three octal digits), if @var{nnn} is
11125 a nine-bit value, the ninth bit is ignored
11127 the eight-bit value that is the octal number @var{nnn}
11128 (one to three octal digits), if @var{nnn} is
11129 a nine-bit value, the ninth bit is ignored
11131 the eight-bit value that is the hexadecimal number @var{hh}
11132 (one or two hexadecimal digits)
11137 @cindex backslash escapes
11138 Disable interpretation of backslash escapes in each @var{string}.
11139 This is the default. If @option{-e} and @option{-E} are both
11140 specified, the last one given takes effect.
11144 @vindex POSIXLY_CORRECT
11145 If the @env{POSIXLY_CORRECT} environment variable is set, then when
11146 @command{echo}'s first argument is not @option{-n} it outputs
11147 option-like arguments instead of treating them as options. For
11148 example, @code{echo -ne hello} outputs @samp{-ne hello} instead of
11149 plain @samp{hello}.
11151 @acronym{POSIX} does not require support for any options, and says
11152 that the behavior of @command{echo} is implementation-defined if any
11153 @var{string} contains a backslash or if the first argument is
11154 @option{-n}. Portable programs can use the @command{printf} command
11155 if they need to omit trailing newlines or output control characters or
11156 backslashes. @xref{printf invocation}.
11161 @node printf invocation
11162 @section @command{printf}: Format and print data
11165 @command{printf} does formatted printing of text. Synopsis:
11168 printf @var{format} [@var{argument}]@dots{}
11171 @command{printf} prints the @var{format} string, interpreting @samp{%}
11172 directives and @samp{\} escapes to format numeric and string arguments
11173 in a way that is mostly similar to the C @samp{printf} function.
11174 @xref{Output Conversion Syntax,, @command{printf} format directives,
11175 libc, The GNU C Library Reference Manual}, for details.
11176 The differences are listed below.
11178 @mayConflictWithShellBuiltIn{printf}
11183 The @var{format} argument is reused as necessary to convert all the
11184 given @var{argument}s. For example, the command @samp{printf %s a b}
11188 Missing @var{argument}s are treated as null strings or as zeros,
11189 depending on whether the context expects a string or a number. For
11190 example, the command @samp{printf %sx%d} prints @samp{x0}.
11194 An additional escape, @samp{\c}, causes @command{printf} to produce no
11195 further output. For example, the command @samp{printf 'A%sC\cD%sF' B
11196 E} prints @samp{ABC}.
11199 The hexadecimal escape sequence @samp{\x@var{hh}} has at most two
11200 digits, as opposed to C where it can have an unlimited number of
11201 digits. For example, the command @samp{printf '\x07e'} prints two
11202 bytes, whereas the C statement @samp{printf ("\x07e")} prints just
11207 @command{printf} has an additional directive, @samp{%b}, which prints its
11208 argument string with @samp{\} escapes interpreted in the same way as in
11209 the @var{format} string, except that octal escapes are of the form
11210 @samp{\0@var{ooo}} where @var{ooo} is 0 to 3 octal digits. If
11211 @samp{\@var{ooo}} is nine-bit value, ignore the ninth bit.
11212 If a precision is also given, it limits the number of bytes printed
11213 from the converted string.
11216 Numeric arguments must be single C constants, possibly with leading
11217 @samp{+} or @samp{-}. For example, @samp{printf %.4d -3} outputs
11221 @vindex POSIXLY_CORRECT
11222 If the leading character of a numeric argument is @samp{"} or @samp{'}
11223 then its value is the numeric value of the immediately following
11224 character. Any remaining characters are silently ignored if the
11225 @env{POSIXLY_CORRECT} environment variable is set; otherwise, a
11226 warning is printed. For example, @samp{printf "%d" "'a"} outputs
11227 @samp{97} on hosts that use the @acronym{ASCII} character set, since
11228 @samp{a} has the numeric value 97 in @acronym{ASCII}.
11233 A floating-point argument must use a period before any fractional
11234 digits, but is printed according to the @env{LC_NUMERIC} category of the
11235 current locale. For example, in a locale whose radix character is a
11236 comma, the command @samp{printf %g 3.14} outputs @samp{3,14} whereas
11237 the command @samp{printf %g 3,14} is an error.
11238 @xref{Floating point}.
11242 @command{printf} interprets @samp{\@var{ooo}} in @var{format} as an octal number
11243 (if @var{ooo} is 1 to 3 octal digits) specifying a byte to print,
11244 and @samp{\x@var{hh}} as a hexadecimal number (if @var{hh} is 1 to 2 hex
11245 digits) specifying a character to print.
11246 Note however that when @samp{\@var{ooo}} specifies a number larger than 255,
11247 @command{printf} ignores the ninth bit.
11248 For example, @samp{printf '\400'} is equivalent to @samp{printf '\0'}.
11253 @cindex ISO/IEC 10646
11255 @command{printf} interprets two character syntaxes introduced in
11256 @acronym{ISO} C 99:
11257 @samp{\u} for 16-bit Unicode (@acronym{ISO}/@acronym{IEC} 10646)
11258 characters, specified as
11259 four hexadecimal digits @var{hhhh}, and @samp{\U} for 32-bit Unicode
11260 characters, specified as eight hexadecimal digits @var{hhhhhhhh}.
11261 @command{printf} outputs the Unicode characters
11262 according to the @env{LC_CTYPE} locale. Unicode characters in the ranges
11263 U+0000...U+009F, U+D800...U+DFFF cannot be specified by this syntax, except
11264 for U+0024 ($), U+0040 (@@), and U+0060 (@`).
11266 The processing of @samp{\u} and @samp{\U} requires a full-featured
11267 @code{iconv} facility. It is activated on systems with glibc 2.2 (or newer),
11268 or when @code{libiconv} is installed prior to this package. Otherwise
11269 @samp{\u} and @samp{\U} will print as-is.
11271 The only options are a lone @option{--help} or
11272 @option{--version}. @xref{Common options}.
11273 Options must precede operands.
11275 The Unicode character syntaxes are useful for writing strings in a locale
11276 independent way. For example, a string containing the Euro currency symbol
11279 $ env printf '\u20AC 14.95'
11283 will be output correctly in all locales supporting the Euro symbol
11284 (@acronym{ISO}-8859-15, UTF-8, and others). Similarly, a Chinese string
11287 $ env printf '\u4e2d\u6587'
11291 will be output correctly in all Chinese locales (GB2312, BIG5, UTF-8, etc).
11293 Note that in these examples, the @command{printf} command has been
11294 invoked via @command{env} to ensure that we run the program found via
11295 your shell's search path, and not a shell alias or a built-in function.
11297 For larger strings, you don't need to look up the hexadecimal code
11298 values of each character one by one. @acronym{ASCII} characters mixed with \u
11299 escape sequences is also known as the JAVA source file encoding. You can
11300 use GNU recode 3.5c (or newer) to convert strings to this encoding. Here
11301 is how to convert a piece of text into a shell script which will output
11302 this text in a locale-independent way:
11305 $ LC_CTYPE=zh_CN.big5 /usr/local/bin/printf \
11306 '\u4e2d\u6587\n' > sample.txt
11307 $ recode BIG5..JAVA < sample.txt \
11308 | sed -e "s|^|/usr/local/bin/printf '|" -e "s|$|\\\\n'|" \
11315 @node yes invocation
11316 @section @command{yes}: Print a string until interrupted
11319 @cindex repeated output of a string
11321 @command{yes} prints the command line arguments, separated by spaces and
11322 followed by a newline, forever until it is killed. If no arguments are
11323 given, it prints @samp{y} followed by a newline forever until killed.
11325 Upon a write error, @command{yes} exits with status @samp{1}.
11327 The only options are a lone @option{--help} or @option{--version}.
11328 To output an argument that begins with
11329 @samp{-}, precede it with @option{--}, e.g., @samp{yes -- --help}.
11330 @xref{Common options}.
11334 @chapter Conditions
11337 @cindex commands for exit status
11338 @cindex exit status commands
11340 This section describes commands that are primarily useful for their exit
11341 status, rather than their output. Thus, they are often used as the
11342 condition of shell @code{if} statements, or as the last command in a
11346 * false invocation:: Do nothing, unsuccessfully.
11347 * true invocation:: Do nothing, successfully.
11348 * test invocation:: Check file types and compare values.
11349 * expr invocation:: Evaluate expressions.
11353 @node false invocation
11354 @section @command{false}: Do nothing, unsuccessfully
11357 @cindex do nothing, unsuccessfully
11358 @cindex failure exit status
11359 @cindex exit status of @command{false}
11361 @command{false} does nothing except return an exit status of 1, meaning
11362 @dfn{failure}. It can be used as a place holder in shell scripts
11363 where an unsuccessful command is needed.
11364 In most modern shells, @command{false} is a built-in command, so when
11365 you use @samp{false} in a script, you're probably using the built-in
11366 command, not the one documented here.
11368 @command{false} honors the @option{--help} and @option{--version} options.
11370 This version of @command{false} is implemented as a C program, and is thus
11371 more secure and faster than a shell script implementation, and may safely
11372 be used as a dummy shell for the purpose of disabling accounts.
11374 Note that @command{false} (unlike all other programs documented herein)
11375 exits unsuccessfully, even when invoked with
11376 @option{--help} or @option{--version}.
11378 Portable programs should not assume that the exit status of
11379 @command{false} is 1, as it is greater than 1 on some
11380 non-@acronym{GNU} hosts.
11383 @node true invocation
11384 @section @command{true}: Do nothing, successfully
11387 @cindex do nothing, successfully
11389 @cindex successful exit
11390 @cindex exit status of @command{true}
11392 @command{true} does nothing except return an exit status of 0, meaning
11393 @dfn{success}. It can be used as a place holder in shell scripts
11394 where a successful command is needed, although the shell built-in
11395 command @code{:} (colon) may do the same thing faster.
11396 In most modern shells, @command{true} is a built-in command, so when
11397 you use @samp{true} in a script, you're probably using the built-in
11398 command, not the one documented here.
11400 @command{true} honors the @option{--help} and @option{--version} options.
11402 Note, however, that it is possible to cause @command{true}
11403 to exit with nonzero status: with the @option{--help} or @option{--version}
11404 option, and with standard
11405 output already closed or redirected to a file that evokes an I/O error.
11406 For example, using a Bourne-compatible shell:
11409 $ ./true --version >&-
11410 ./true: write error: Bad file number
11411 $ ./true --version > /dev/full
11412 ./true: write error: No space left on device
11415 This version of @command{true} is implemented as a C program, and is thus
11416 more secure and faster than a shell script implementation, and may safely
11417 be used as a dummy shell for the purpose of disabling accounts.
11419 @node test invocation
11420 @section @command{test}: Check file types and compare values
11423 @cindex check file types
11424 @cindex compare values
11425 @cindex expression evaluation
11427 @command{test} returns a status of 0 (true) or 1 (false) depending on the
11428 evaluation of the conditional expression @var{expr}. Each part of the
11429 expression must be a separate argument.
11431 @command{test} has file status checks, string operators, and numeric
11432 comparison operators.
11434 @command{test} has an alternate form that uses opening and closing
11435 square brackets instead a leading @samp{test}. For example, instead
11436 of @samp{test -d /}, you can write @samp{[ -d / ]}. The square
11437 brackets must be separate arguments; for example, @samp{[-d /]} does
11438 not have the desired effect. Since @samp{test @var{expr}} and @samp{[
11439 @var{expr} ]} have the same meaning, only the former form is discussed
11445 test @var{expression}
11447 [ @var{expression} ]
11452 @mayConflictWithShellBuiltIn{test}
11454 If @var{expression} is omitted, @command{test} returns false.
11455 If @var{expression} is a single argument,
11456 @command{test} returns false if the argument is null and true
11457 otherwise. The argument
11458 can be any string, including strings like @samp{-d}, @samp{-1},
11459 @samp{--}, @samp{--help}, and @samp{--version} that most other
11460 programs would treat as options. To get help and version information,
11461 invoke the commands @samp{[ --help} and @samp{[ --version}, without
11462 the usual closing brackets. @xref{Common options}.
11464 @cindex exit status of @command{test}
11468 0 if the expression is true,
11469 1 if the expression is false,
11470 2 if an error occurred.
11474 * File type tests:: -[bcdfhLpSt]
11475 * Access permission tests:: -[gkruwxOG]
11476 * File characteristic tests:: -e -s -nt -ot -ef
11477 * String tests:: -z -n = !=
11478 * Numeric tests:: -eq -ne -lt -le -gt -ge
11479 * Connectives for test:: ! -a -o
11483 @node File type tests
11484 @subsection File type tests
11486 @cindex file type tests
11488 These options test for particular types of files. (Everything's a file,
11489 but not all files are the same!)
11493 @item -b @var{file}
11495 @cindex block special check
11496 True if @var{file} exists and is a block special device.
11498 @item -c @var{file}
11500 @cindex character special check
11501 True if @var{file} exists and is a character special device.
11503 @item -d @var{file}
11505 @cindex directory check
11506 True if @var{file} exists and is a directory.
11508 @item -f @var{file}
11510 @cindex regular file check
11511 True if @var{file} exists and is a regular file.
11513 @item -h @var{file}
11514 @itemx -L @var{file}
11517 @cindex symbolic link check
11518 True if @var{file} exists and is a symbolic link.
11519 Unlike all other file-related tests, this test does not dereference
11520 @var{file} if it is a symbolic link.
11522 @item -p @var{file}
11524 @cindex named pipe check
11525 True if @var{file} exists and is a named pipe.
11527 @item -S @var{file}
11529 @cindex socket check
11530 True if @var{file} exists and is a socket.
11534 @cindex terminal check
11535 True if @var{fd} is a file descriptor that is associated with a
11541 @node Access permission tests
11542 @subsection Access permission tests
11544 @cindex access permission tests
11545 @cindex permission tests
11547 These options test for particular access permissions.
11551 @item -g @var{file}
11553 @cindex set-group-ID check
11554 True if @var{file} exists and has its set-group-ID bit set.
11556 @item -k @var{file}
11558 @cindex sticky bit check
11559 True if @var{file} exists and has its @dfn{sticky} bit set.
11561 @item -r @var{file}
11563 @cindex readable file check
11564 True if @var{file} exists and read permission is granted.
11566 @item -u @var{file}
11568 @cindex set-user-ID check
11569 True if @var{file} exists and has its set-user-ID bit set.
11571 @item -w @var{file}
11573 @cindex writable file check
11574 True if @var{file} exists and write permission is granted.
11576 @item -x @var{file}
11578 @cindex executable file check
11579 True if @var{file} exists and execute permission is granted
11580 (or search permission, if it is a directory).
11582 @item -O @var{file}
11584 @cindex owned by effective user ID check
11585 True if @var{file} exists and is owned by the current effective user ID.
11587 @item -G @var{file}
11589 @cindex owned by effective group ID check
11590 True if @var{file} exists and is owned by the current effective group ID.
11594 @node File characteristic tests
11595 @subsection File characteristic tests
11597 @cindex file characteristic tests
11599 These options test other file characteristics.
11603 @item -e @var{file}
11605 @cindex existence-of-file check
11606 True if @var{file} exists.
11608 @item -s @var{file}
11610 @cindex nonempty file check
11611 True if @var{file} exists and has a size greater than zero.
11613 @item @var{file1} -nt @var{file2}
11615 @cindex newer-than file check
11616 True if @var{file1} is newer (according to modification date) than
11617 @var{file2}, or if @var{file1} exists and @var{file2} does not.
11619 @item @var{file1} -ot @var{file2}
11621 @cindex older-than file check
11622 True if @var{file1} is older (according to modification date) than
11623 @var{file2}, or if @var{file2} exists and @var{file1} does not.
11625 @item @var{file1} -ef @var{file2}
11627 @cindex same file check
11628 @cindex hard link check
11629 True if @var{file1} and @var{file2} have the same device and inode
11630 numbers, i.e., if they are hard links to each other.
11636 @subsection String tests
11638 @cindex string tests
11640 These options test string characteristics. You may need to quote
11641 @var{string} arguments for the shell. For example:
11647 The quotes here prevent the wrong arguments from being passed to
11648 @command{test} if @samp{$V} is empty or contains special characters.
11652 @item -z @var{string}
11654 @cindex zero-length string check
11655 True if the length of @var{string} is zero.
11657 @item -n @var{string}
11658 @itemx @var{string}
11660 @cindex nonzero-length string check
11661 True if the length of @var{string} is nonzero.
11663 @item @var{string1} = @var{string2}
11665 @cindex equal string check
11666 True if the strings are equal.
11668 @item @var{string1} != @var{string2}
11670 @cindex not-equal string check
11671 True if the strings are not equal.
11676 @node Numeric tests
11677 @subsection Numeric tests
11679 @cindex numeric tests
11680 @cindex arithmetic tests
11682 Numeric relational operators. The arguments must be entirely numeric
11683 (possibly negative), or the special expression @w{@code{-l @var{string}}},
11684 which evaluates to the length of @var{string}.
11688 @item @var{arg1} -eq @var{arg2}
11689 @itemx @var{arg1} -ne @var{arg2}
11690 @itemx @var{arg1} -lt @var{arg2}
11691 @itemx @var{arg1} -le @var{arg2}
11692 @itemx @var{arg1} -gt @var{arg2}
11693 @itemx @var{arg1} -ge @var{arg2}
11700 These arithmetic binary operators return true if @var{arg1} is equal,
11701 not-equal, less-than, less-than-or-equal, greater-than, or
11702 greater-than-or-equal than @var{arg2}, respectively.
11709 test -1 -gt -2 && echo yes
11711 test -l abc -gt 1 && echo yes
11714 @error{} test: integer expression expected before -eq
11718 @node Connectives for test
11719 @subsection Connectives for @command{test}
11721 @cindex logical connectives
11722 @cindex connectives, logical
11724 The usual logical connectives.
11730 True if @var{expr} is false.
11732 @item @var{expr1} -a @var{expr2}
11734 @cindex logical and operator
11735 @cindex and operator
11736 True if both @var{expr1} and @var{expr2} are true.
11738 @item @var{expr1} -o @var{expr2}
11740 @cindex logical or operator
11741 @cindex or operator
11742 True if either @var{expr1} or @var{expr2} is true.
11747 @node expr invocation
11748 @section @command{expr}: Evaluate expressions
11751 @cindex expression evaluation
11752 @cindex evaluation of expressions
11754 @command{expr} evaluates an expression and writes the result on standard
11755 output. Each token of the expression must be a separate argument.
11757 Operands are either integers or strings. Integers consist of one or
11758 more decimal digits, with an optional leading @samp{-}.
11759 @command{expr} converts
11760 anything appearing in an operand position to an integer or a string
11761 depending on the operation being applied to it.
11763 Strings are not quoted for @command{expr} itself, though you may need to
11764 quote them to protect characters with special meaning to the shell,
11765 e.g., spaces. However, regardless of whether it is quoted, a string
11766 operand should not be a parenthesis or any of @command{expr}'s
11767 operators like @code{+}, so you cannot safely pass an arbitrary string
11768 @code{$str} to expr merely by quoting it to the shell. One way to
11769 work around this is to use the @sc{gnu} extension @code{+},
11770 (e.g., @code{+ "$str" = foo}); a more portable way is to use
11771 @code{@w{" $str"}} and to adjust the rest of the expression to take
11772 the leading space into account (e.g., @code{@w{" $str" = " foo"}}).
11774 You should not pass a negative integer or a string with leading
11775 @samp{-} as @command{expr}'s first argument, as it might be
11776 misinterpreted as an option; this can be avoided by parenthesization.
11777 Also, portable scripts should not use a string operand that happens to
11778 take the form of an integer; this can be worked around by inserting
11779 leading spaces as mentioned above.
11781 @cindex parentheses for grouping
11782 Operators may be given as infix symbols or prefix keywords. Parentheses
11783 may be used for grouping in the usual manner. You must quote
11784 parentheses and many operators to avoid the shell evaluating them,
11787 When built with support for the GNU MP library, @command{expr} uses
11788 arbitrary-precision arithmetic; otherwise, it uses native arithmetic
11789 types and may fail due to arithmetic overflow.
11791 The only options are @option{--help} and @option{--version}. @xref{Common
11792 options}. Options must precede operands.
11794 @cindex exit status of @command{expr}
11798 0 if the expression is neither null nor 0,
11799 1 if the expression is null or 0,
11800 2 if the expression is invalid,
11801 3 if an internal error occurred (e.g., arithmetic overflow).
11805 * String expressions:: + : match substr index length
11806 * Numeric expressions:: + - * / %
11807 * Relations for expr:: | & < <= = == != >= >
11808 * Examples of expr:: Examples.
11812 @node String expressions
11813 @subsection String expressions
11815 @cindex string expressions
11816 @cindex expressions, string
11818 @command{expr} supports pattern matching and other string operators. These
11819 have higher precedence than both the numeric and relational operators (in
11820 the next sections).
11824 @item @var{string} : @var{regex}
11825 @cindex pattern matching
11826 @cindex regular expression matching
11827 @cindex matching patterns
11828 Perform pattern matching. The arguments are converted to strings and the
11829 second is considered to be a (basic, a la GNU @code{grep}) regular
11830 expression, with a @code{^} implicitly prepended. The first argument is
11831 then matched against this regular expression.
11833 If the match succeeds and @var{regex} uses @samp{\(} and @samp{\)}, the
11834 @code{:} expression returns the part of @var{string} that matched the
11835 subexpression; otherwise, it returns the number of characters matched.
11837 If the match fails, the @code{:} operator returns the null string if
11838 @samp{\(} and @samp{\)} are used in @var{regex}, otherwise 0.
11840 @kindex \( @r{regexp operator}
11841 Only the first @samp{\( @dots{} \)} pair is relevant to the return
11842 value; additional pairs are meaningful only for grouping the regular
11843 expression operators.
11845 @kindex \+ @r{regexp operator}
11846 @kindex \? @r{regexp operator}
11847 @kindex \| @r{regexp operator}
11848 In the regular expression, @code{\+}, @code{\?}, and @code{\|} are
11849 operators which respectively match one or more, zero or one, or separate
11850 alternatives. SunOS and other @command{expr}'s treat these as regular
11851 characters. (@acronym{POSIX} allows either behavior.)
11852 @xref{Top, , Regular Expression Library, regex, Regex}, for details of
11853 regular expression syntax. Some examples are in @ref{Examples of expr}.
11855 @item match @var{string} @var{regex}
11857 An alternative way to do pattern matching. This is the same as
11858 @w{@samp{@var{string} : @var{regex}}}.
11860 @item substr @var{string} @var{position} @var{length}
11862 Returns the substring of @var{string} beginning at @var{position}
11863 with length at most @var{length}. If either @var{position} or
11864 @var{length} is negative, zero, or non-numeric, returns the null string.
11866 @item index @var{string} @var{charset}
11868 Returns the first position in @var{string} where the first character in
11869 @var{charset} was found. If no character in @var{charset} is found in
11870 @var{string}, return 0.
11872 @item length @var{string}
11874 Returns the length of @var{string}.
11876 @item + @var{token}
11878 Interpret @var{token} as a string, even if it is a keyword like @var{match}
11879 or an operator like @code{/}.
11880 This makes it possible to test @code{expr length + "$x"} or
11881 @code{expr + "$x" : '.*/\(.\)'} and have it do the right thing even if
11882 the value of @var{$x} happens to be (for example) @code{/} or @code{index}.
11883 This operator is a @acronym{GNU} extension. Portable shell scripts should use
11884 @code{@w{" $token"} : @w{' \(.*\)'}} instead of @code{+ "$token"}.
11888 To make @command{expr} interpret keywords as strings, you must use the
11889 @code{quote} operator.
11892 @node Numeric expressions
11893 @subsection Numeric expressions
11895 @cindex numeric expressions
11896 @cindex expressions, numeric
11898 @command{expr} supports the usual numeric operators, in order of increasing
11899 precedence. These numeric operators have lower precedence than the
11900 string operators described in the previous section, and higher precedence
11901 than the connectives (next section).
11909 @cindex subtraction
11910 Addition and subtraction. Both arguments are converted to integers;
11911 an error occurs if this cannot be done.
11917 @cindex multiplication
11920 Multiplication, division, remainder. Both arguments are converted to
11921 integers; an error occurs if this cannot be done.
11926 @node Relations for expr
11927 @subsection Relations for @command{expr}
11929 @cindex connectives, logical
11930 @cindex logical connectives
11931 @cindex relations, numeric or string
11933 @command{expr} supports the usual logical connectives and relations. These
11934 have lower precedence than the string and numeric operators
11935 (previous sections). Here is the list, lowest-precedence operator first.
11941 @cindex logical or operator
11942 @cindex or operator
11943 Returns its first argument if that is neither null nor zero, otherwise
11944 its second argument if it is neither null nor zero, otherwise 0. It
11945 does not evaluate its second argument if its first argument is neither
11950 @cindex logical and operator
11951 @cindex and operator
11952 Return its first argument if neither argument is null or zero, otherwise
11953 0. It does not evaluate its second argument if its first argument is
11956 @item < <= = == != >= >
11963 @cindex comparison operators
11965 Compare the arguments and return 1 if the relation is true, 0 otherwise.
11966 @code{==} is a synonym for @code{=}. @command{expr} first tries to convert
11967 both arguments to integers and do a numeric comparison; if either
11968 conversion fails, it does a lexicographic comparison using the character
11969 collating sequence specified by the @env{LC_COLLATE} locale.
11974 @node Examples of expr
11975 @subsection Examples of using @command{expr}
11977 @cindex examples of @command{expr}
11978 Here are a few examples, including quoting for shell metacharacters.
11980 To add 1 to the shell variable @code{foo}, in Bourne-compatible shells:
11983 foo=`expr $foo + 1`
11986 To print the non-directory part of the file name stored in
11987 @code{$fname}, which need not contain a @code{/}:
11990 expr $fname : '.*/\(.*\)' '|' $fname
11993 An example showing that @code{\+} is an operator:
12001 expr abc : 'a\(.\)c'
12003 expr index abcdef cz
12006 @error{} expr: syntax error
12007 expr index + index a
12013 @chapter Redirection
12015 @cindex redirection
12016 @cindex commands for redirection
12018 Unix shells commonly provide several forms of @dfn{redirection}---ways
12019 to change the input source or output destination of a command. But one
12020 useful redirection is performed by a separate command, not by the shell;
12021 it's described here.
12024 * tee invocation:: Redirect output to multiple files or processes.
12028 @node tee invocation
12029 @section @command{tee}: Redirect output to multiple files or processes
12032 @cindex pipe fitting
12033 @cindex destinations, multiple output
12034 @cindex read from stdin and write to stdout and files
12036 The @command{tee} command copies standard input to standard output and also
12037 to any files given as arguments. This is useful when you want not only
12038 to send some data down a pipe, but also to save a copy. Synopsis:
12041 tee [@var{option}]@dots{} [@var{file}]@dots{}
12044 If a file being written to does not already exist, it is created. If a
12045 file being written to already exists, the data it previously contained
12046 is overwritten unless the @option{-a} option is used.
12048 A @var{file} of @samp{-} causes @command{tee} to send another copy of
12049 input to standard output, but this is typically not that useful as the
12050 copies are interleaved.
12052 The program accepts the following options. Also see @ref{Common options}.
12059 Append standard input to the given files rather than overwriting
12063 @itemx --ignore-interrupts
12065 @opindex --ignore-interrupts
12066 Ignore interrupt signals.
12070 The @command{tee} command is useful when you happen to be transferring a large
12071 amount of data and also want to summarize that data without reading
12072 it a second time. For example, when you are downloading a DVD image,
12073 you often want to verify its signature or checksum right away.
12074 The inefficient way to do it is simply:
12077 wget http://example.com/some.iso && sha1sum some.iso
12080 One problem with the above is that it makes you wait for the
12081 download to complete before starting the time-consuming SHA1 computation.
12082 Perhaps even more importantly, the above requires reading
12083 the DVD image a second time (the first was from the network).
12085 The efficient way to do it is to interleave the download
12086 and SHA1 computation. Then, you'll get the checksum for
12087 free, because the entire process parallelizes so well:
12090 # slightly contrived, to demonstrate process substitution
12091 wget -O - http://example.com/dvd.iso \
12092 | tee >(sha1sum > dvd.sha1) > dvd.iso
12095 That makes @command{tee} write not just to the expected output file,
12096 but also to a pipe running @command{sha1sum} and saving the final
12097 checksum in a file named @file{dvd.sha1}.
12099 Note, however, that this example relies on a feature of modern shells
12100 called @dfn{process substitution}
12101 (the @samp{>(command)} syntax, above;
12102 @xref{Process Substitution,,Process Substitution, bashref,
12103 The Bash Reference Manual}.),
12104 so it works with @command{zsh}, @command{bash}, and @command{ksh},
12105 but not with @command{/bin/sh}. So if you write code like this
12106 in a shell script, be sure to start the script with @samp{#!/bin/bash}.
12108 Since the above example writes to one file and one process,
12109 a more conventional and portable use of @command{tee} is even better:
12112 wget -O - http://example.com/dvd.iso \
12113 | tee dvd.iso | sha1sum > dvd.sha1
12116 You can extend this example to make @command{tee} write to two processes,
12117 computing MD5 and SHA1 checksums in parallel. In this case,
12118 process substitution is required:
12121 wget -O - http://example.com/dvd.iso \
12122 | tee >(sha1sum > dvd.sha1) \
12123 >(md5sum > dvd.md5) \
12127 This technique is also useful when you want to make a @emph{compressed}
12128 copy of the contents of a pipe.
12129 Consider a tool to graphically summarize disk usage data from @samp{du -ak}.
12130 For a large hierarchy, @samp{du -ak} can run for a long time,
12131 and can easily produce terabytes of data, so you won't want to
12132 rerun the command unnecessarily. Nor will you want to save
12133 the uncompressed output.
12135 Doing it the inefficient way, you can't even start the GUI
12136 until after you've compressed all of the @command{du} output:
12139 du -ak | gzip -9 > /tmp/du.gz
12140 gzip -d /tmp/du.gz | xdiskusage -a
12143 With @command{tee} and process substitution, you start the GUI
12144 right away and eliminate the decompression completely:
12147 du -ak | tee >(gzip -9 > /tmp/du.gz) | xdiskusage -a
12150 Finally, if you regularly create more than one type of
12151 compressed tarball at once, for example when @code{make dist} creates
12152 both @command{gzip}-compressed and @command{bzip2}-compressed tarballs,
12153 there may be a better way.
12154 Typical @command{automake}-generated @file{Makefile} rules create
12155 the two compressed tar archives with commands in sequence, like this
12156 (slightly simplified):
12159 tardir=your-pkg-M.N
12160 tar chof - "$tardir" | gzip -9 -c > your-pkg-M.N.tar.gz
12161 tar chof - "$tardir" | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12164 However, if the hierarchy you are archiving and compressing is larger
12165 than a couple megabytes, and especially if you are using a multi-processor
12166 system with plenty of memory, then you can do much better by reading the
12167 directory contents only once and running the compression programs in parallel:
12170 tardir=your-pkg-M.N
12171 tar chof - "$tardir" \
12172 | tee >(gzip -9 -c > your-pkg-M.N.tar.gz) \
12173 | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12179 @node File name manipulation
12180 @chapter File name manipulation
12182 @cindex file name manipulation
12183 @cindex manipulation of file names
12184 @cindex commands for file name manipulation
12186 This section describes commands that manipulate file names.
12189 * basename invocation:: Strip directory and suffix from a file name.
12190 * dirname invocation:: Strip last file name component.
12191 * pathchk invocation:: Check file name validity and portability.
12192 * mktemp invocation:: Create temporary file or directory.
12196 @node basename invocation
12197 @section @command{basename}: Strip directory and suffix from a file name
12200 @cindex strip directory and suffix from file names
12201 @cindex directory, stripping from file names
12202 @cindex suffix, stripping from file names
12203 @cindex file names, stripping directory and suffix
12204 @cindex leading directory components, stripping
12206 @command{basename} removes any leading directory components from
12207 @var{name}. Synopsis:
12210 basename @var{name} [@var{suffix}]
12213 If @var{suffix} is specified and is identical to the end of @var{name},
12214 it is removed from @var{name} as well. Note that since trailing slashes
12215 are removed prior to suffix matching, @var{suffix} will do nothing if it
12216 contains slashes. @command{basename} prints the result on standard
12219 @c This test is used both here and in the section on dirname.
12220 @macro basenameAndDirname
12221 Together, @command{basename} and @command{dirname} are designed such
12222 that if @samp{ls "$name"} succeeds, then the command sequence @samp{cd
12223 "$(dirname "$name")"; ls "$(basename "$name")"} will, too. This works
12224 for everything except file names containing a trailing newline.
12226 @basenameAndDirname
12228 @acronym{POSIX} allows the implementation to define the results if
12229 @var{name} is empty or @samp{//}. In the former case, @acronym{GNU}
12230 @command{basename} returns the empty string. In the latter case, the
12231 result is @samp{//} on platforms where @var{//} is distinct from
12232 @var{/}, and @samp{/} on platforms where there is no difference.
12234 The only options are @option{--help} and @option{--version}. @xref{Common
12235 options}. Options must precede operands.
12243 basename /usr/bin/sort
12246 basename include/stdio.h .h
12250 @node dirname invocation
12251 @section @command{dirname}: Strip last file name component
12254 @cindex directory components, printing
12255 @cindex stripping non-directory suffix
12256 @cindex non-directory suffix, stripping
12258 @command{dirname} prints all but the final slash-delimited component of
12259 @var{name}. Slashes on either side of the final component are also
12260 removed. If the string contains no slash, @command{dirname} prints
12261 @samp{.} (meaning the current directory). Synopsis:
12267 @var{name} need not be a file name, but if it is, this operation
12268 effectively lists the directory that contains the final component,
12269 including the case when the final component is itself a directory.
12271 @basenameAndDirname
12273 @acronym{POSIX} allows the implementation to define the results if
12274 @var{name} is @samp{//}. With @acronym{GNU} @command{dirname}, the
12275 result is @samp{//} on platforms where @var{//} is distinct from
12276 @var{/}, and @samp{/} on platforms where there is no difference.
12278 The only options are @option{--help} and @option{--version}. @xref{Common
12286 # Output "/usr/bin".
12287 dirname /usr/bin/sort
12288 dirname /usr/bin//.//
12295 @node pathchk invocation
12296 @section @command{pathchk}: Check file name validity and portability
12299 @cindex file names, checking validity and portability
12300 @cindex valid file names, checking for
12301 @cindex portable file names, checking for
12303 @command{pathchk} checks validity and portability of file names. Synopsis:
12306 pathchk [@var{option}]@dots{} @var{name}@dots{}
12309 For each @var{name}, @command{pathchk} prints an error message if any of
12310 these conditions is true:
12314 One of the existing directories in @var{name} does not have search
12315 (execute) permission,
12317 The length of @var{name} is larger than the maximum supported by the
12320 The length of one component of @var{name} is longer than
12321 its file system's maximum.
12324 A nonexistent @var{name} is not an error, so long a file with that
12325 name could be created under the above conditions.
12327 The program accepts the following options. Also see @ref{Common options}.
12328 Options must precede operands.
12334 Instead of performing checks based on the underlying file system,
12335 print an error message if any of these conditions is true:
12339 A file name is empty.
12342 A file name contains a character outside the @acronym{POSIX} portable file
12343 name character set, namely, the ASCII letters and digits, @samp{.},
12344 @samp{_}, @samp{-}, and @samp{/}.
12347 The length of a file name or one of its components exceeds the
12348 @acronym{POSIX} minimum limits for portability.
12353 Print an error message if a file name is empty, or if it contains a component
12354 that begins with @samp{-}.
12356 @item --portability
12357 @opindex --portability
12358 Print an error message if a file name is not portable to all @acronym{POSIX}
12359 hosts. This option is equivalent to @samp{-p -P}.
12363 @cindex exit status of @command{pathchk}
12367 0 if all specified file names passed all checks,
12371 @node mktemp invocation
12372 @section @command{mktemp}: Create temporary file or directory
12375 @cindex file names, creating temporary
12376 @cindex directory, creating temporary
12377 @cindex temporary files and directories
12379 @command{mktemp} manages the creation of temporary files and
12380 directories. Synopsis:
12383 mktemp [@var{option}]@dots{} [@var{template}]
12386 Safely create a temporary file or directory based on @var{template},
12387 and print its name. If given, @var{template} must include at least
12388 three consecutive @samp{X}s in the last component. If omitted, the template
12389 @samp{tmp.XXXXXXXXXX} is used, and option @option{--tmpdir} is
12390 implied. The final run of @samp{X}s in the @var{template} will be replaced
12391 by alpha-numeric characters; thus, on a case-sensitive file system,
12392 and with a @var{template} including a run of @var{n} instances of @samp{X},
12393 there are @samp{62**@var{n}} potential file names.
12395 Older scripts used to create temporary files by simply joining the
12396 name of the program with the process id (@samp{$$}) as a suffix.
12397 However, that naming scheme is easily predictable, and suffers from a
12398 race condition where the attacker can create an appropriately named
12399 symbolic link, such that when the script then opens a handle to what
12400 it thought was an unused file, it is instead modifying an existing
12401 file. Using the same scheme to create a directory is slightly safer,
12402 since the @command{mkdir} will fail if the target already exists, but
12403 it is still inferior because it allows for denial of service attacks.
12404 Therefore, modern scripts should use the @command{mktemp} command to
12405 guarantee that the generated name will be unpredictable, and that
12406 knowledge of the temporary file name implies that the file was created
12407 by the current script and cannot be modified by other users.
12409 When creating a file, the resulting file has read and write
12410 permissions for the current user, but no permissions for the group or
12411 others; these permissions are reduced if the current umask is more
12414 Here are some examples (although note that if you repeat them, you
12415 will most likely get different file names):
12420 Create a temporary file in the current directory.
12427 Create a temporary file with a known suffix.
12429 $ mktemp --suffix=.txt file-XXXX
12431 $ mktemp file-XXXX-XXXX.txt
12436 Create a secure fifo relative to the user's choice of @env{TMPDIR},
12437 but falling back to the current directory rather than @file{/tmp}.
12438 Note that @command{mktemp} does not create fifos, but can create a
12439 secure directory in which the fifo can live. Exit the shell if the
12440 directory or fifo could not be created.
12442 $ dir=$(mktemp -p "$@{TMPDIR:-.@}" -d dir-XXXX) || exit 1
12444 $ mkfifo "$fifo" || @{ rmdir "$dir"; exit 1; @}
12448 Create and use a temporary file if possible, but ignore failure. The
12449 file will reside in the directory named by @env{TMPDIR}, if specified,
12450 or else in @file{/tmp}.
12452 $ file=$(mktemp -q) && @{
12453 > # Safe to use $file only within this block. Use quotes,
12454 > # since $TMPDIR, and thus $file, may contain whitespace.
12455 > echo ... > "$file"
12461 Act as a semi-random character generator (it is not fully random,
12462 since it is impacted by the contents of the current directory). To
12463 avoid security holes, do not use the resulting names to create a file.
12473 The program accepts the following options. Also see @ref{Common options}.
12480 @opindex --directory
12481 Create a directory rather than a file. The directory will have read,
12482 write, and search permissions for the current user, but no permissions
12483 for the group or others; these permissions are reduced if the current
12484 umask is more restrictive.
12490 Suppress diagnostics about failure to create a file or directory. The
12491 exit status will still reflect whether a file was created.
12497 Generate a temporary name that does not name an existing file, without
12498 changing the file system contents. Using the output of this command
12499 to create a new file is inherently unsafe, as there is a window of
12500 time between generating the name and using it where another process
12501 can create an object by the same name.
12504 @itemx --tmpdir[=@var{dir}]
12507 Treat @var{template} relative to the directory @var{dir}. If
12508 @var{dir} is not specified (only possible with the long option
12509 @option{--tmpdir}) or is the empty string, use the value of
12510 @env{TMPDIR} if available, otherwise use @samp{/tmp}. If this is
12511 specified, @var{template} must not be absolute. However,
12512 @var{template} can still contain slashes, although intermediate
12513 directories must already exist.
12515 @item --suffix=@var{suffix}
12517 Append @var{suffix} to the @var{template}. @var{suffix} must not
12518 contain slash. If @option{--suffix} is specified, @var{template} must
12519 end in @samp{X}; if it is not specified, then an appropriate
12520 @option{--suffix} is inferred by finding the last @samp{X} in
12521 @var{template}. This option exists for use with the default
12522 @var{template} and for the creation of a @var{suffix} that starts with
12527 Treat @var{template} as a single file relative to the value of
12528 @env{TMPDIR} if available, or to the directory specified by
12529 @option{-p}, otherwise to @samp{/tmp}. @var{template} must not
12530 contain slashes. This option is deprecated; the use of @option{-p}
12531 without @option{-t} offers better defaults (by favoring the command
12532 line over @env{TMPDIR}) and more flexibility (by allowing intermediate
12537 @cindex exit status of @command{mktemp}
12541 0 if the file was created,
12546 @node Working context
12547 @chapter Working context
12549 @cindex working context
12550 @cindex commands for printing the working context
12552 This section describes commands that display or alter the context in
12553 which you are working: the current directory, the terminal settings, and
12554 so forth. See also the user-related commands in the next section.
12557 * pwd invocation:: Print working directory.
12558 * stty invocation:: Print or change terminal characteristics.
12559 * printenv invocation:: Print environment variables.
12560 * tty invocation:: Print file name of terminal on standard input.
12564 @node pwd invocation
12565 @section @command{pwd}: Print working directory
12568 @cindex print name of current directory
12569 @cindex current working directory, printing
12570 @cindex working directory, printing
12573 @command{pwd} prints the name of the current directory. Synopsis:
12576 pwd [@var{option}]@dots{}
12579 The program accepts the following options. Also see @ref{Common options}.
12586 If the contents of the environment variable @env{PWD} provide an
12587 absolute name of the current directory with no @samp{.} or @samp{..}
12588 components, but possibly with symbolic links, then output those
12589 contents. Otherwise, fall back to default @option{-P} handling.
12594 @opindex --physical
12595 Print a fully resolved name for the current directory. That is, all
12596 components of the printed name will be actual directory names---none
12597 will be symbolic links.
12600 @cindex symbolic links and @command{pwd}
12601 If @option{-L} and @option{-P} are both given, the last one takes
12602 precedence. If neither option is given, then this implementation uses
12603 @option{-P} as the default unless the @env{POSIXLY_CORRECT}
12604 environment variable is set.
12606 @mayConflictWithShellBuiltIn{pwd}
12611 @node stty invocation
12612 @section @command{stty}: Print or change terminal characteristics
12615 @cindex change or print terminal settings
12616 @cindex terminal settings
12617 @cindex line settings of terminal
12619 @command{stty} prints or changes terminal characteristics, such as baud rate.
12623 stty [@var{option}] [@var{setting}]@dots{}
12624 stty [@var{option}]
12627 If given no line settings, @command{stty} prints the baud rate, line
12628 discipline number (on systems that support it), and line settings
12629 that have been changed from the values set by @samp{stty sane}.
12630 By default, mode reading and setting are performed on the tty line
12631 connected to standard input, although this can be modified by the
12632 @option{--file} option.
12634 @command{stty} accepts many non-option arguments that change aspects of
12635 the terminal line operation, as described below.
12637 The program accepts the following options. Also see @ref{Common options}.
12644 Print all current settings in human-readable form. This option may not
12645 be used in combination with any line settings.
12647 @item -F @var{device}
12648 @itemx --file=@var{device}
12651 Set the line opened by the file name specified in @var{device} instead of
12652 the tty line connected to standard input. This option is necessary
12653 because opening a @acronym{POSIX} tty requires use of the
12654 @code{O_NONDELAY} flag to prevent a @acronym{POSIX} tty from blocking
12655 until the carrier detect line is high if
12656 the @code{clocal} flag is not set. Hence, it is not always possible
12657 to allow the shell to open the device in the traditional manner.
12663 @cindex machine-readable @command{stty} output
12664 Print all current settings in a form that can be used as an argument to
12665 another @command{stty} command to restore the current settings. This option
12666 may not be used in combination with any line settings.
12670 Many settings can be turned off by preceding them with a @samp{-}.
12671 Such arguments are marked below with ``May be negated'' in their
12672 description. The descriptions themselves refer to the positive
12673 case, that is, when @emph{not} negated (unless stated otherwise,
12676 Some settings are not available on all @acronym{POSIX} systems, since they use
12677 extensions. Such arguments are marked below with
12678 ``Non-@acronym{POSIX}'' in their description. On non-@acronym{POSIX}
12679 systems, those or other settings also may not
12680 be available, but it's not feasible to document all the variations: just
12686 * Control:: Control settings
12687 * Input:: Input settings
12688 * Output:: Output settings
12689 * Local:: Local settings
12690 * Combination:: Combination settings
12691 * Characters:: Special characters
12692 * Special:: Special settings
12697 @subsection Control settings
12699 @cindex control settings
12705 @cindex two-way parity
12706 Generate parity bit in output and expect parity bit in input.
12712 @cindex even parity
12713 Set odd parity (even if negated). May be negated.
12720 @cindex character size
12721 @cindex eight-bit characters
12722 Set character size to 5, 6, 7, or 8 bits.
12727 Send a hangup signal when the last process closes the tty. May be
12733 Use two stop bits per character (one if negated). May be negated.
12737 Allow input to be received. May be negated.
12741 @cindex modem control
12742 Disable modem control signals. May be negated.
12746 @cindex hardware flow control
12747 @cindex flow control, hardware
12748 @cindex RTS/CTS flow control
12749 Enable RTS/CTS flow control. Non-@acronym{POSIX}. May be negated.
12754 @subsection Input settings
12756 @cindex input settings
12757 These settings control operations on data received from the terminal.
12762 @cindex breaks, ignoring
12763 Ignore break characters. May be negated.
12767 @cindex breaks, cause interrupts
12768 Make breaks cause an interrupt signal. May be negated.
12772 @cindex parity, ignoring
12773 Ignore characters with parity errors. May be negated.
12777 @cindex parity errors, marking
12778 Mark parity errors (with a 255-0-character sequence). May be negated.
12782 Enable input parity checking. May be negated.
12786 @cindex eight-bit input
12787 Clear high (8th) bit of input characters. May be negated.
12791 @cindex newline, translating to return
12792 Translate newline to carriage return. May be negated.
12796 @cindex return, ignoring
12797 Ignore carriage return. May be negated.
12801 @cindex return, translating to newline
12802 Translate carriage return to newline. May be negated.
12806 @cindex input encoding, UTF-8
12807 Assume input characters are UTF-8 encoded. May be negated.
12811 @kindex C-s/C-q flow control
12812 @cindex XON/XOFF flow control
12813 Enable XON/XOFF flow control (that is, @kbd{CTRL-S}/@kbd{CTRL-Q}). May
12820 @cindex software flow control
12821 @cindex flow control, software
12822 Enable sending of @code{stop} character when the system input buffer
12823 is almost full, and @code{start} character when it becomes almost
12824 empty again. May be negated.
12828 @cindex uppercase, translating to lowercase
12829 Translate uppercase characters to lowercase. Non-@acronym{POSIX}. May be
12830 negated. Note ilcuc is not implemented, as one would not be able to issue
12831 almost any (lowercase) Unix command, after invoking it.
12835 Allow any character to restart output (only the start character
12836 if negated). Non-@acronym{POSIX}. May be negated.
12840 @cindex beeping at input buffer full
12841 Enable beeping and not flushing input buffer if a character arrives
12842 when the input buffer is full. Non-@acronym{POSIX}. May be negated.
12847 @subsection Output settings
12849 @cindex output settings
12850 These settings control operations on data sent to the terminal.
12855 Postprocess output. May be negated.
12859 @cindex lowercase, translating to output
12860 Translate lowercase characters to uppercase. Non-@acronym{POSIX}. May be
12861 negated. (Note ouclc is not currently implemented.)
12865 @cindex return, translating to newline
12866 Translate carriage return to newline. Non-@acronym{POSIX}. May be negated.
12870 @cindex newline, translating to crlf
12871 Translate newline to carriage return-newline. Non-@acronym{POSIX}. May be
12876 Do not print carriage returns in the first column. Non-@acronym{POSIX}.
12881 Newline performs a carriage return. Non-@acronym{POSIX}. May be negated.
12885 @cindex pad instead of timing for delaying
12886 Use fill (padding) characters instead of timing for delays.
12887 Non-@acronym{POSIX}.
12892 @cindex pad character
12893 Use @acronym{ASCII} @sc{del} characters for fill instead of
12894 @acronym{ASCII} @sc{nul} characters. Non-@acronym{POSIX}.
12900 Newline delay style. Non-@acronym{POSIX}.
12907 Carriage return delay style. Non-@acronym{POSIX}.
12913 @opindex tab@var{n}
12914 Horizontal tab delay style. Non-@acronym{POSIX}.
12919 Backspace delay style. Non-@acronym{POSIX}.
12924 Vertical tab delay style. Non-@acronym{POSIX}.
12929 Form feed delay style. Non-@acronym{POSIX}.
12934 @subsection Local settings
12936 @cindex local settings
12941 Enable @code{interrupt}, @code{quit}, and @code{suspend} special
12942 characters. May be negated.
12946 Enable @code{erase}, @code{kill}, @code{werase}, and @code{rprnt}
12947 special characters. May be negated.
12951 Enable non-@acronym{POSIX} special characters. May be negated.
12955 Echo input characters. May be negated.
12961 Echo @code{erase} characters as backspace-space-backspace. May be
12966 @cindex newline echoing after @code{kill}
12967 Echo a newline after a @code{kill} character. May be negated.
12971 @cindex newline, echoing
12972 Echo newline even if not echoing other characters. May be negated.
12976 @cindex flushing, disabling
12977 Disable flushing after @code{interrupt} and @code{quit} special
12978 characters. May be negated.
12982 @cindex case translation
12983 Enable input and output of uppercase characters by preceding their
12984 lowercase equivalents with @samp{\}, when @code{icanon} is set.
12985 Non-@acronym{POSIX}. May be negated.
12989 @cindex background jobs, stopping at terminal write
12990 Stop background jobs that try to write to the terminal. Non-@acronym{POSIX}.
12997 Echo erased characters backward, between @samp{\} and @samp{/}.
12998 Non-@acronym{POSIX}. May be negated.
13004 @cindex control characters, using @samp{^@var{c}}
13005 @cindex hat notation for control characters
13006 Echo control characters in hat notation (@samp{^@var{c}}) instead
13007 of literally. Non-@acronym{POSIX}. May be negated.
13013 Echo the @code{kill} special character by erasing each character on
13014 the line as indicated by the @code{echoprt} and @code{echoe} settings,
13015 instead of by the @code{echoctl} and @code{echok} settings.
13016 Non-@acronym{POSIX}.
13022 @subsection Combination settings
13024 @cindex combination settings
13025 Combination settings:
13032 Same as @code{parenb -parodd cs7}. May be negated. If negated, same
13033 as @code{-parenb cs8}.
13037 Same as @code{parenb parodd cs7}. May be negated. If negated, same
13038 as @code{-parenb cs8}.
13042 Same as @code{-icrnl -onlcr}. May be negated. If negated, same as
13043 @code{icrnl -inlcr -igncr onlcr -ocrnl -onlret}.
13047 Reset the @code{erase} and @code{kill} special characters to their default
13054 @c This is too long to write inline.
13056 cread -ignbrk brkint -inlcr -igncr icrnl -ixoff
13057 -iuclc -ixany imaxbel opost -olcuc -ocrnl onlcr
13058 -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0
13059 ff0 isig icanon iexten echo echoe echok -echonl
13060 -noflsh -xcase -tostop -echoprt echoctl echoke
13064 and also sets all special characters to their default values.
13068 Same as @code{brkint ignpar istrip icrnl ixon opost isig icanon}, plus
13069 sets the @code{eof} and @code{eol} characters to their default values
13070 if they are the same as the @code{min} and @code{time} characters.
13071 May be negated. If negated, same as @code{raw}.
13078 -ignbrk -brkint -ignpar -parmrk -inpck -istrip
13079 -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany
13080 -imaxbel -opost -isig -icanon -xcase min 1 time 0
13084 May be negated. If negated, same as @code{cooked}.
13088 Same as @option{-icanon}. May be negated. If negated, same as
13093 @cindex eight-bit characters
13094 Same as @code{-parenb -istrip cs8}. May be negated. If negated,
13095 same as @code{parenb istrip cs7}.
13099 Same as @option{-parenb -istrip -opost cs8}. May be negated.
13100 If negated, same as @code{parenb istrip opost cs7}.
13104 Same as @option{-ixany}. Non-@acronym{POSIX}. May be negated.
13108 Same as @code{tab0}. Non-@acronym{POSIX}. May be negated. If negated, same
13115 Same as @code{xcase iuclc olcuc}. Non-@acronym{POSIX}. May be negated.
13116 (Used for terminals with uppercase characters only.)
13120 Same as @code{echoe echoctl echoke}.
13124 Same as @code{echoe echoctl echoke -ixany intr ^C erase ^? kill C-u}.
13129 @subsection Special characters
13131 @cindex special characters
13132 @cindex characters, special
13134 The special characters' default values vary from system to system.
13135 They are set with the syntax @samp{name value}, where the names are
13136 listed below and the value can be given either literally, in hat
13137 notation (@samp{^@var{c}}), or as an integer which may start with
13138 @samp{0x} to indicate hexadecimal, @samp{0} to indicate octal, or
13139 any other digit to indicate decimal.
13141 @cindex disabling special characters
13142 @kindex u@r{, and disabling special characters}
13143 For GNU stty, giving a value of @code{^-} or @code{undef} disables that
13144 special character. (This is incompatible with Ultrix @command{stty},
13145 which uses a value of @samp{u} to disable a special character. GNU
13146 @command{stty} treats a value @samp{u} like any other, namely to set that
13147 special character to @key{U}.)
13153 Send an interrupt signal.
13157 Send a quit signal.
13161 Erase the last character typed.
13165 Erase the current line.
13169 Send an end of file (terminate the input).
13177 Alternate character to end the line. Non-@acronym{POSIX}.
13181 Switch to a different shell layer. Non-@acronym{POSIX}.
13185 Restart the output after stopping it.
13193 Send a terminal stop signal.
13197 Send a terminal stop signal after flushing the input. Non-@acronym{POSIX}.
13201 Redraw the current line. Non-@acronym{POSIX}.
13205 Erase the last word typed. Non-@acronym{POSIX}.
13209 Enter the next character typed literally, even if it is a special
13210 character. Non-@acronym{POSIX}.
13215 @subsection Special settings
13217 @cindex special settings
13222 Set the minimum number of characters that will satisfy a read until
13223 the time value has expired, when @option{-icanon} is set.
13227 Set the number of tenths of a second before reads time out if the minimum
13228 number of characters have not been read, when @option{-icanon} is set.
13230 @item ispeed @var{n}
13232 Set the input speed to @var{n}.
13234 @item ospeed @var{n}
13236 Set the output speed to @var{n}.
13240 Tell the tty kernel driver that the terminal has @var{n} rows.
13241 Non-@acronym{POSIX}.
13244 @itemx columns @var{n}
13247 Tell the kernel that the terminal has @var{n} columns. Non-@acronym{POSIX}.
13253 Print the number of rows and columns that the kernel thinks the
13254 terminal has. (Systems that don't support rows and columns in the kernel
13255 typically use the environment variables @env{LINES} and @env{COLUMNS}
13256 instead; however, GNU @command{stty} does not know anything about them.)
13257 Non-@acronym{POSIX}.
13261 Use line discipline @var{n}. Non-@acronym{POSIX}.
13265 Print the terminal speed.
13268 @cindex baud rate, setting
13269 Set the input and output speeds to @var{n}. @var{n} can be one of: 0
13270 50 75 110 134 134.5 150 200 300 600 1200 1800 2400 4800 9600 19200
13271 38400 @code{exta} @code{extb}. @code{exta} is the same as 19200;
13272 @code{extb} is the same as 38400. Many systems, including GNU/Linux,
13273 support higher speeds. The @command{stty} command includes support
13290 4000000 where the system supports these.
13291 0 hangs up the line if @option{-clocal} is set.
13295 @node printenv invocation
13296 @section @command{printenv}: Print all or some environment variables
13299 @cindex printing all or some environment variables
13300 @cindex environment variables, printing
13302 @command{printenv} prints environment variable values. Synopsis:
13305 printenv [@var{option}] [@var{variable}]@dots{}
13308 If no @var{variable}s are specified, @command{printenv} prints the value of
13309 every environment variable. Otherwise, it prints the value of each
13310 @var{variable} that is set, and nothing for those that are not set.
13312 The program accepts the following option. Also see @ref{Common options}.
13320 @cindex exit status of @command{printenv}
13324 0 if all variables specified were found
13325 1 if at least one specified variable was not found
13326 2 if a write error occurred
13330 @node tty invocation
13331 @section @command{tty}: Print file name of terminal on standard input
13334 @cindex print terminal file name
13335 @cindex terminal file name, printing
13337 @command{tty} prints the file name of the terminal connected to its standard
13338 input. It prints @samp{not a tty} if standard input is not a terminal.
13342 tty [@var{option}]@dots{}
13345 The program accepts the following option. Also see @ref{Common options}.
13355 Print nothing; only return an exit status.
13359 @cindex exit status of @command{tty}
13363 0 if standard input is a terminal
13364 1 if standard input is not a terminal
13365 2 if given incorrect arguments
13366 3 if a write error occurs
13370 @node User information
13371 @chapter User information
13373 @cindex user information, commands for
13374 @cindex commands for printing user information
13376 This section describes commands that print user-related information:
13377 logins, groups, and so forth.
13380 * id invocation:: Print user identity.
13381 * logname invocation:: Print current login name.
13382 * whoami invocation:: Print effective user ID.
13383 * groups invocation:: Print group names a user is in.
13384 * users invocation:: Print login names of users currently logged in.
13385 * who invocation:: Print who is currently logged in.
13389 @node id invocation
13390 @section @command{id}: Print user identity
13393 @cindex real user and group IDs, printing
13394 @cindex effective user and group IDs, printing
13395 @cindex printing real and effective user and group IDs
13397 @command{id} prints information about the given user, or the process
13398 running it if no user is specified. Synopsis:
13401 id [@var{option}]@dots{} [@var{username}]
13404 @vindex POSIXLY_CORRECT
13405 By default, it prints the real user ID, real group ID, effective user ID
13406 if different from the real user ID, effective group ID if different from
13407 the real group ID, and supplemental group IDs.
13408 In addition, if SELinux
13409 is enabled and the @env{POSIXLY_CORRECT} environment variable is not set,
13410 then print @samp{context=@var{c}}, where @var{c} is the security context.
13412 Each of these numeric values is preceded by an identifying string and
13413 followed by the corresponding user or group name in parentheses.
13415 The options cause @command{id} to print only part of the above information.
13416 Also see @ref{Common options}.
13423 Print only the group ID.
13429 Print only the group ID and the supplementary groups.
13435 Print the user or group name instead of the ID number. Requires
13436 @option{-u}, @option{-g}, or @option{-G}.
13442 Print the real, instead of effective, user or group ID. Requires
13443 @option{-u}, @option{-g}, or @option{-G}.
13449 Print only the user ID.
13456 @cindex security context
13457 Print only the security context of the current user.
13458 If SELinux is disabled then print a warning and
13459 set the exit status to 1.
13465 @macro primaryAndSupplementaryGroups{cmd,arg}
13466 Primary and supplementary groups for a process are normally inherited
13467 from its parent and are usually unchanged since login. This means
13468 that if you change the group database after logging in, @command{\cmd\}
13469 will not reflect your changes within your existing login session.
13470 Running @command{\cmd\} with a \arg\ causes the user and group
13471 database to be consulted afresh, and so will give a different result.
13473 @primaryAndSupplementaryGroups{id,user argument}
13475 @node logname invocation
13476 @section @command{logname}: Print current login name
13479 @cindex printing user's login name
13480 @cindex login name, printing
13481 @cindex user name, printing
13484 @command{logname} prints the calling user's name, as found in a
13485 system-maintained file (often @file{/var/run/utmp} or
13486 @file{/etc/utmp}), and exits with a status of 0. If there is no entry
13487 for the calling process, @command{logname} prints
13488 an error message and exits with a status of 1.
13490 The only options are @option{--help} and @option{--version}. @xref{Common
13496 @node whoami invocation
13497 @section @command{whoami}: Print effective user ID
13500 @cindex effective user ID, printing
13501 @cindex printing the effective user ID
13503 @command{whoami} prints the user name associated with the current
13504 effective user ID. It is equivalent to the command @samp{id -un}.
13506 The only options are @option{--help} and @option{--version}. @xref{Common
13512 @node groups invocation
13513 @section @command{groups}: Print group names a user is in
13516 @cindex printing groups a user is in
13517 @cindex supplementary groups, printing
13519 @command{groups} prints the names of the primary and any supplementary
13520 groups for each given @var{username}, or the current process if no names
13521 are given. If more than one name is given, the name of each user is
13523 the list of that user's groups and the user name is separated from the
13524 group list by a colon. Synopsis:
13527 groups [@var{username}]@dots{}
13530 The group lists are equivalent to the output of the command @samp{id -Gn}.
13532 @primaryAndSupplementaryGroups{groups,list of users}
13534 The only options are @option{--help} and @option{--version}. @xref{Common
13540 @node users invocation
13541 @section @command{users}: Print login names of users currently logged in
13544 @cindex printing current usernames
13545 @cindex usernames, printing current
13547 @cindex login sessions, printing users with
13548 @command{users} prints on a single line a blank-separated list of user
13549 names of users currently logged in to the current host. Each user name
13550 corresponds to a login session, so if a user has more than one login
13551 session, that user's name will appear the same number of times in the
13560 With no @var{file} argument, @command{users} extracts its information from
13561 a system-maintained file (often @file{/var/run/utmp} or
13562 @file{/etc/utmp}). If a file argument is given, @command{users} uses
13563 that file instead. A common choice is @file{/var/log/wtmp}.
13565 The only options are @option{--help} and @option{--version}. @xref{Common
13571 @node who invocation
13572 @section @command{who}: Print who is currently logged in
13575 @cindex printing current user information
13576 @cindex information, about current users
13578 @command{who} prints information about users who are currently logged on.
13582 @command{who} [@var{option}] [@var{file}] [am i]
13585 @cindex terminal lines, currently used
13587 @cindex remote hostname
13588 If given no non-option arguments, @command{who} prints the following
13589 information for each user currently logged on: login name, terminal
13590 line, login time, and remote hostname or X display.
13594 If given one non-option argument, @command{who} uses that instead of
13595 a default system-maintained file (often @file{/var/run/utmp} or
13596 @file{/etc/utmp}) as the name of the file containing the record of
13597 users logged on. @file{/var/log/wtmp} is commonly given as an argument
13598 to @command{who} to look at who has previously logged on.
13602 If given two non-option arguments, @command{who} prints only the entry
13603 for the user running it (determined from its standard input), preceded
13604 by the hostname. Traditionally, the two arguments given are @samp{am
13605 i}, as in @samp{who am i}.
13608 Time stamps are listed according to the time zone rules specified by
13609 the @env{TZ} environment variable, or by the system default rules if
13610 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
13611 with @env{TZ}, libc, The GNU C Library Reference Manual}.
13613 The program accepts the following options. Also see @ref{Common options}.
13621 Same as @samp{-b -d --login -p -r -t -T -u}.
13627 Print the date and time of last system boot.
13633 Print information corresponding to dead processes.
13639 Print a line of column headings.
13645 List only the entries that correspond to processes via which the
13646 system is waiting for a user to login. The user name is always @samp{LOGIN}.
13650 Attempt to canonicalize hostnames found in utmp through a DNS lookup. This
13651 is not the default because it can cause significant delays on systems with
13652 automatic dial-up internet access.
13656 Same as @samp{who am i}.
13662 List active processes spawned by init.
13668 Print only the login names and the number of users logged on.
13669 Overrides all other options.
13674 @opindex --runlevel
13675 Print the current (and maybe previous) run-level of the init process.
13679 Ignored; for compatibility with other versions of @command{who}.
13685 Print last system clock change.
13690 After the login time, print the number of hours and minutes that the
13691 user has been idle. @samp{.} means the user was active in the last minute.
13692 @samp{old} means the user has been idle for more than 24 hours.
13703 @opindex --writable
13704 @cindex message status
13705 @pindex write@r{, allowed}
13706 After each login name print a character indicating the user's message status:
13709 @samp{+} allowing @code{write} messages
13710 @samp{-} disallowing @code{write} messages
13711 @samp{?} cannot find terminal device
13719 @node System context
13720 @chapter System context
13722 @cindex system context
13723 @cindex context, system
13724 @cindex commands for system context
13726 This section describes commands that print or change system-wide
13730 * date invocation:: Print or set system date and time.
13731 * arch invocation:: Print machine hardware name.
13732 * nproc invocation:: Print the number of processors.
13733 * uname invocation:: Print system information.
13734 * hostname invocation:: Print or set system name.
13735 * hostid invocation:: Print numeric host identifier.
13736 * uptime invocation:: Print system uptime and load.
13739 @node date invocation
13740 @section @command{date}: Print or set system date and time
13743 @cindex time, printing or setting
13744 @cindex printing the current time
13749 date [@var{option}]@dots{} [+@var{format}]
13750 date [-u|--utc|--universal] @c this avoids a newline in the output
13751 [ MMDDhhmm[[CC]YY][.ss] ]
13755 Invoking @command{date} with no @var{format} argument is equivalent to invoking
13756 it with a default format that depends on the @env{LC_TIME} locale category.
13757 In the default C locale, this format is @samp{'+%a %b %e %H:%M:%S %Z %Y'},
13758 so the output looks like @samp{Thu Mar @ 3 13:47:51 PST 2005}.
13761 Normally, @command{date} uses the time zone rules indicated by the
13762 @env{TZ} environment variable, or the system default rules if @env{TZ}
13763 is not set. @xref{TZ Variable,, Specifying the Time Zone with
13764 @env{TZ}, libc, The GNU C Library Reference Manual}.
13766 @findex strftime @r{and @command{date}}
13767 @cindex time formats
13768 @cindex formatting times
13769 If given an argument that starts with a @samp{+}, @command{date} prints the
13770 current date and time (or the date and time specified by the
13771 @option{--date} option, see below) in the format defined by that argument,
13772 which is similar to that of the @code{strftime} function. Except for
13773 conversion specifiers, which start with @samp{%}, characters in the
13774 format string are printed unchanged. The conversion specifiers are
13780 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
13781 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
13782 * Literal conversion specifiers:: %[%nt]
13783 * Padding and other flags:: Pad with zeros, spaces, etc.
13784 * Setting the time:: Changing the system clock.
13785 * Options for date:: Instead of the current time.
13787 * Date input formats:: Specifying date strings.
13789 * Examples of date:: Examples.
13792 @node Time conversion specifiers
13793 @subsection Time conversion specifiers
13795 @cindex time conversion specifiers
13796 @cindex conversion specifiers, time
13798 @command{date} conversion specifiers related to times.
13802 hour (@samp{00}@dots{}@samp{23})
13804 hour (@samp{01}@dots{}@samp{12})
13806 hour (@samp{ 0}@dots{}@samp{23}).
13807 This is a @acronym{GNU} extension.
13809 hour (@samp{ 1}@dots{}@samp{12}).
13810 This is a @acronym{GNU} extension.
13812 minute (@samp{00}@dots{}@samp{59})
13814 nanoseconds (@samp{000000000}@dots{}@samp{999999999}).
13815 This is a @acronym{GNU} extension.
13817 locale's equivalent of either @samp{AM} or @samp{PM};
13818 blank in many locales.
13819 Noon is treated as @samp{PM} and midnight as @samp{AM}.
13821 like @samp{%p}, except lower case.
13822 This is a @acronym{GNU} extension.
13824 locale's 12-hour clock time (e.g., @samp{11:11:04 PM})
13826 24-hour hour and minute. Same as @samp{%H:%M}.
13827 This is a @acronym{GNU} extension.
13829 @cindex epoch, seconds since
13830 @cindex seconds since the epoch
13831 @cindex beginning of time
13832 seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC.
13833 Leap seconds are not counted unless leap second support is available.
13834 @xref{%s-examples}, for examples.
13835 This is a @acronym{GNU} extension.
13837 second (@samp{00}@dots{}@samp{60}).
13838 This may be @samp{60} if leap seconds are supported.
13840 24-hour hour, minute, and second. Same as @samp{%H:%M:%S}.
13842 locale's time representation (e.g., @samp{23:13:48})
13844 @w{@acronym{RFC} 2822/@acronym{ISO} 8601} style numeric time zone
13845 (e.g., @samp{-0600} or @samp{+0530}), or nothing if no
13846 time zone is determinable. This value reflects the numeric time zone
13847 appropriate for the current time, using the time zone rules specified
13848 by the @env{TZ} environment variable.
13849 The time (and optionally, the time zone rules) can be overridden
13850 by the @option{--date} option.
13851 This is a @acronym{GNU} extension.
13853 @w{@acronym{RFC} 3339/@acronym{ISO} 8601} style numeric time zone with
13854 @samp{:} (e.g., @samp{-06:00} or @samp{+05:30}), or nothing if no time
13855 zone is determinable.
13856 This is a @acronym{GNU} extension.
13858 Numeric time zone to the nearest second with @samp{:} (e.g.,
13859 @samp{-06:00:00} or @samp{+05:30:00}), or nothing if no time zone is
13861 This is a @acronym{GNU} extension.
13863 Numeric time zone with @samp{:} using the minimum necessary precision
13864 (e.g., @samp{-06}, @samp{+05:30}, or @samp{-04:56:02}), or nothing if
13865 no time zone is determinable.
13866 This is a @acronym{GNU} extension.
13868 alphabetic time zone abbreviation (e.g., @samp{EDT}), or nothing if no
13869 time zone is determinable. See @samp{%z} for how it is determined.
13873 @node Date conversion specifiers
13874 @subsection Date conversion specifiers
13876 @cindex date conversion specifiers
13877 @cindex conversion specifiers, date
13879 @command{date} conversion specifiers related to dates.
13883 locale's abbreviated weekday name (e.g., @samp{Sun})
13885 locale's full weekday name, variable length (e.g., @samp{Sunday})
13887 locale's abbreviated month name (e.g., @samp{Jan})
13889 locale's full month name, variable length (e.g., @samp{January})
13891 locale's date and time (e.g., @samp{Thu Mar @ 3 23:05:25 2005})
13893 century. This is like @samp{%Y}, except the last two digits are omitted.
13894 For example, it is @samp{20} if @samp{%Y} is @samp{2000},
13895 and is @samp{-0} if @samp{%Y} is @samp{-001}.
13896 It is normally at least two characters, but it may be more.
13898 day of month (e.g., @samp{01})
13900 date; same as @samp{%m/%d/%y}
13902 day of month, space padded; same as @samp{%_d}
13904 full date in @acronym{ISO} 8601 format; same as @samp{%Y-%m-%d}.
13905 This is a good choice for a date format, as it is standard and
13906 is easy to sort in the usual case where years are in the range
13908 This is a @acronym{GNU} extension.
13910 year corresponding to the @acronym{ISO} week number, but without the century
13911 (range @samp{00} through @samp{99}). This has the same format and value
13912 as @samp{%y}, except that if the @acronym{ISO} week number (see
13914 to the previous or next year, that year is used instead.
13915 This is a @acronym{GNU} extension.
13917 year corresponding to the @acronym{ISO} week number. This has the
13918 same format and value as @samp{%Y}, except that if the @acronym{ISO}
13920 @samp{%V}) belongs to the previous or next year, that year is used
13922 It is normally useful only if @samp{%V} is also used;
13923 for example, the format @samp{%G-%m-%d} is probably a mistake,
13924 since it combines the ISO week number year with the conventional month and day.
13925 This is a @acronym{GNU} extension.
13929 day of year (@samp{001}@dots{}@samp{366})
13931 month (@samp{01}@dots{}@samp{12})
13933 day of week (@samp{1}@dots{}@samp{7}) with @samp{1} corresponding to Monday
13935 week number of year, with Sunday as the first day of the week
13936 (@samp{00}@dots{}@samp{53}).
13937 Days in a new year preceding the first Sunday are in week zero.
13939 @acronym{ISO} week number, that is, the
13940 week number of year, with Monday as the first day of the week
13941 (@samp{01}@dots{}@samp{53}).
13942 If the week containing January 1 has four or more days in
13943 the new year, then it is considered week 1; otherwise, it is week 53 of
13944 the previous year, and the next week is week 1. (See the @acronym{ISO} 8601
13947 day of week (@samp{0}@dots{}@samp{6}) with 0 corresponding to Sunday
13949 week number of year, with Monday as first day of week
13950 (@samp{00}@dots{}@samp{53}).
13951 Days in a new year preceding the first Monday are in week zero.
13953 locale's date representation (e.g., @samp{12/31/99})
13955 last two digits of year (@samp{00}@dots{}@samp{99})
13957 year. This is normally at least four characters, but it may be more.
13958 Year @samp{0000} precedes year @samp{0001}, and year @samp{-001}
13959 precedes year @samp{0000}.
13963 @node Literal conversion specifiers
13964 @subsection Literal conversion specifiers
13966 @cindex literal conversion specifiers
13967 @cindex conversion specifiers, literal
13969 @command{date} conversion specifiers that produce literal strings.
13981 @node Padding and other flags
13982 @subsection Padding and other flags
13984 @cindex numeric field padding
13985 @cindex padding of numeric fields
13986 @cindex fields, padding numeric
13988 Unless otherwise specified, @command{date} normally pads numeric fields
13989 with zeros, so that, for
13990 example, numeric months are always output as two digits.
13991 Seconds since the epoch are not padded, though,
13992 since there is no natural width for them.
13994 As a @acronym{GNU} extension, @command{date} recognizes any of the
13995 following optional flags after the @samp{%}:
13999 (hyphen) Do not pad the field; useful if the output is intended for
14002 (underscore) Pad with spaces; useful if you need a fixed
14003 number of characters in the output, but zeros are too distracting.
14005 (zero) Pad with zeros even if the conversion specifier
14006 would normally pad with spaces.
14008 Use upper case characters if possible.
14010 Use opposite case characters if possible.
14011 A field that is normally upper case becomes lower case, and vice versa.
14015 Here are some examples of padding:
14018 date +%d/%m -d "Feb 1"
14020 date +%-d/%-m -d "Feb 1"
14022 date +%_d/%_m -d "Feb 1"
14026 As a @acronym{GNU} extension, you can specify the field width
14027 (after any flag, if present) as a decimal number. If the natural size of the
14028 output of the field has less than the specified number of characters,
14029 the result is written right adjusted and padded to the given
14030 size. For example, @samp{%9B} prints the right adjusted month name in
14031 a field of width 9.
14033 An optional modifier can follow the optional flag and width
14034 specification. The modifiers are:
14038 Use the locale's alternate representation for date and time. This
14039 modifier applies to the @samp{%c}, @samp{%C}, @samp{%x}, @samp{%X},
14040 @samp{%y} and @samp{%Y} conversion specifiers. In a Japanese locale, for
14041 example, @samp{%Ex} might yield a date format based on the Japanese
14045 Use the locale's alternate numeric symbols for numbers. This modifier
14046 applies only to numeric conversion specifiers.
14049 If the format supports the modifier but no alternate representation
14050 is available, it is ignored.
14053 @node Setting the time
14054 @subsection Setting the time
14056 @cindex setting the time
14057 @cindex time setting
14058 @cindex appropriate privileges
14060 If given an argument that does not start with @samp{+}, @command{date} sets
14061 the system clock to the date and time specified by that argument (as
14062 described below). You must have appropriate privileges to set the
14063 system clock. The @option{--date} and @option{--set} options may not be
14064 used with such an argument. The @option{--universal} option may be used
14065 with such an argument to indicate that the specified date and time are
14066 relative to Coordinated Universal Time rather than to the local time
14069 The argument must consist entirely of digits, which have the following
14082 first two digits of year (optional)
14084 last two digits of year (optional)
14089 The @option{--set} option also sets the system clock; see the next section.
14092 @node Options for date
14093 @subsection Options for @command{date}
14095 @cindex @command{date} options
14096 @cindex options for @command{date}
14098 The program accepts the following options. Also see @ref{Common options}.
14102 @item -d @var{datestr}
14103 @itemx --date=@var{datestr}
14106 @cindex parsing date strings
14107 @cindex date strings, parsing
14108 @cindex arbitrary date strings, parsing
14111 @opindex next @var{day}
14112 @opindex last @var{day}
14113 Display the date and time specified in @var{datestr} instead of the
14114 current date and time. @var{datestr} can be in almost any common
14115 format. It can contain month names, time zones, @samp{am} and @samp{pm},
14116 @samp{yesterday}, etc. For example, @option{--date="2004-02-27
14117 14:19:13.489392193 +0530"} specifies the instant of time that is
14118 489,392,193 nanoseconds after February 27, 2004 at 2:19:13 PM in a
14119 time zone that is 5 hours and 30 minutes east of @acronym{UTC}.@*
14120 Note: input currently must be in locale independent format. E.g., the
14121 LC_TIME=C below is needed to print back the correct date in many locales:
14123 date -d "$(LC_TIME=C date)"
14125 @xref{Date input formats}.
14127 @item -f @var{datefile}
14128 @itemx --file=@var{datefile}
14131 Parse each line in @var{datefile} as with @option{-d} and display the
14132 resulting date and time. If @var{datefile} is @samp{-}, use standard
14133 input. This is useful when you have many dates to process, because the
14134 system overhead of starting up the @command{date} executable many times can
14137 @item -r @var{file}
14138 @itemx --reference=@var{file}
14140 @opindex --reference
14141 Display the date and time of the last modification of @var{file},
14142 instead of the current date and time.
14149 @opindex --rfc-2822
14150 Display the date and time using the format @samp{%a, %d %b %Y %H:%M:%S
14151 %z}, evaluated in the C locale so abbreviations are always in English.
14155 Fri, 09 Sep 2005 13:51:39 -0700
14158 This format conforms to
14159 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt, Internet
14160 @acronym{RFCs} 2822} and
14161 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc822.txt, 822}, the
14162 current and previous standards for Internet email.
14164 @item --rfc-3339=@var{timespec}
14165 @opindex --rfc-3339=@var{timespec}
14166 Display the date using a format specified by
14167 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt, Internet
14168 @acronym{RFC} 3339}. This is a subset of the @acronym{ISO} 8601
14169 format, except that it also permits applications to use a space rather
14170 than a @samp{T} to separate dates from times. Unlike the other
14171 standard formats, @acronym{RFC} 3339 format is always suitable as
14172 input for the @option{--date} (@option{-d}) and @option{--file}
14173 (@option{-f}) options, regardless of the current locale.
14175 The argument @var{timespec} specifies how much of the time to include.
14176 It can be one of the following:
14180 Print just the full-date, e.g., @samp{2005-09-14}.
14181 This is equivalent to the format @samp{%Y-%m-%d}.
14184 Print the full-date and full-time separated by a space, e.g.,
14185 @samp{2005-09-14 00:56:06+05:30}. The output ends with a numeric
14186 time-offset; here the @samp{+05:30} means that local time is five
14187 hours and thirty minutes east of @acronym{UTC}. This is equivalent to
14188 the format @samp{%Y-%m-%d %H:%M:%S%:z}.
14191 Like @samp{seconds}, but also print nanoseconds, e.g.,
14192 @samp{2005-09-14 00:56:06.998458565+05:30}.
14193 This is equivalent to the format @samp{%Y-%m-%d %H:%M:%S.%N%:z}.
14197 @item -s @var{datestr}
14198 @itemx --set=@var{datestr}
14201 Set the date and time to @var{datestr}. See @option{-d} above.
14208 @opindex --universal
14209 @cindex Coordinated Universal Time
14211 @cindex Greenwich Mean Time
14214 Use Coordinated Universal Time (@acronym{UTC}) by operating as if the
14215 @env{TZ} environment variable were set to the string @samp{UTC0}.
14217 Universal Time is often called ``Greenwich Mean Time'' (@sc{gmt}) for
14218 historical reasons.
14222 @node Examples of date
14223 @subsection Examples of @command{date}
14225 @cindex examples of @command{date}
14227 Here are a few examples. Also see the documentation for the @option{-d}
14228 option in the previous section.
14233 To print the date of the day before yesterday:
14236 date --date='2 days ago'
14240 To print the date of the day three months and one day hence:
14243 date --date='3 months 1 day'
14247 To print the day of year of Christmas in the current year:
14250 date --date='25 Dec' +%j
14254 To print the current full month name and the day of the month:
14260 But this may not be what you want because for the first nine days of
14261 the month, the @samp{%d} expands to a zero-padded two-digit field,
14262 for example @samp{date -d 1may '+%B %d'} will print @samp{May 01}.
14265 To print a date without the leading zero for one-digit days
14266 of the month, you can use the (@acronym{GNU} extension)
14267 @samp{-} flag to suppress
14268 the padding altogether:
14271 date -d 1may '+%B %-d
14275 To print the current date and time in the format required by many
14276 non-@acronym{GNU} versions of @command{date} when setting the system clock:
14279 date +%m%d%H%M%Y.%S
14283 To set the system clock forward by two minutes:
14286 date --set='+2 minutes'
14290 To print the date in @acronym{RFC} 2822 format,
14291 use @samp{date --rfc-2822}. Here is some example output:
14294 Fri, 09 Sep 2005 13:51:39 -0700
14297 @anchor{%s-examples}
14299 To convert a date string to the number of seconds since the epoch
14300 (which is 1970-01-01 00:00:00 UTC), use the @option{--date} option with
14301 the @samp{%s} format. That can be useful in sorting and/or graphing
14302 and/or comparing data by date. The following command outputs the
14303 number of the seconds since the epoch for the time two minutes after the
14307 date --date='1970-01-01 00:02:00 +0000' +%s
14311 If you do not specify time zone information in the date string,
14312 @command{date} uses your computer's idea of the time zone when
14313 interpreting the string. For example, if your computer's time zone is
14314 that of Cambridge, Massachusetts, which was then 5 hours (i.e., 18,000
14315 seconds) behind UTC:
14318 # local time zone used
14319 date --date='1970-01-01 00:02:00' +%s
14324 If you're sorting or graphing dated data, your raw date values may be
14325 represented as seconds since the epoch. But few people can look at
14326 the date @samp{946684800} and casually note ``Oh, that's the first second
14327 of the year 2000 in Greenwich, England.''
14330 date --date='2000-01-01 UTC' +%s
14334 An alternative is to use the @option{--utc} (@option{-u}) option.
14335 Then you may omit @samp{UTC} from the date string. Although this
14336 produces the same result for @samp{%s} and many other format sequences,
14337 with a time zone offset different from zero, it would give a different
14338 result for zone-dependent formats like @samp{%z}.
14341 date -u --date=2000-01-01 +%s
14345 To convert such an unwieldy number of seconds back to
14346 a more readable form, use a command like this:
14349 # local time zone used
14350 date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
14351 1999-12-31 19:00:00 -0500
14354 Or if you do not mind depending on the @samp{@@} feature present since
14355 coreutils 5.3.0, you could shorten this to:
14358 date -d @@946684800 +"%F %T %z"
14359 1999-12-31 19:00:00 -0500
14362 Often it is better to output UTC-relative date and time:
14365 date -u -d '1970-01-01 946684800 seconds' +"%Y-%m-%d %T %z"
14366 2000-01-01 00:00:00 +0000
14372 @node arch invocation
14373 @section @command{arch}: Print machine hardware name
14376 @cindex print machine hardware name
14377 @cindex system information, printing
14379 @command{arch} prints the machine hardware name,
14380 and is equivalent to @samp{uname -m}.
14384 arch [@var{option}]
14387 The program accepts the @ref{Common options} only.
14392 @node nproc invocation
14393 @section @command{nproc}: Print the number of available processors
14396 @cindex Print the number of processors
14397 @cindex system information, printing
14399 Print the number of processing units available to the current process,
14400 which may be less than the number of online processors.
14401 If this information is not accessible, then print the number of
14402 processors installed. If the @env{OMP_NUM_THREADS} environment variable is
14403 set, then it will determine the returned value. The result is guaranteed to be
14404 greater than zero. Synopsis:
14407 nproc [@var{option}]
14410 The program accepts the following options. Also see @ref{Common options}.
14416 Print the number of installed processors on the system, which may
14417 be greater than the number online or available to the current process.
14418 The @env{OMP_NUM_THREADS} environment variable is not honored in this case.
14420 @item --ignore=@var{number}
14422 If possible, exclude this @var{number} of processing units.
14429 @node uname invocation
14430 @section @command{uname}: Print system information
14433 @cindex print system information
14434 @cindex system information, printing
14436 @command{uname} prints information about the machine and operating system
14437 it is run on. If no options are given, @command{uname} acts as if the
14438 @option{-s} option were given. Synopsis:
14441 uname [@var{option}]@dots{}
14444 If multiple options or @option{-a} are given, the selected information is
14445 printed in this order:
14448 @var{kernel-name} @var{nodename} @var{kernel-release} @var{kernel-version}
14449 @var{machine} @var{processor} @var{hardware-platform} @var{operating-system}
14452 The information may contain internal spaces, so such output cannot be
14453 parsed reliably. In the following example, @var{release} is
14454 @samp{2.2.18ss.e820-bda652a #4 SMP Tue Jun 5 11:24:08 PDT 2001}:
14458 @result{} Linux dum 2.2.18 #4 SMP Tue Jun 5 11:24:08 PDT 2001 i686@c
14459 unknown unknown GNU/Linux
14463 The program accepts the following options. Also see @ref{Common options}.
14471 Print all of the below information, except omit the processor type
14472 and the hardware platform name if they are unknown.
14475 @itemx --hardware-platform
14477 @opindex --hardware-platform
14478 @cindex implementation, hardware
14479 @cindex hardware platform
14480 @cindex platform, hardware
14481 Print the hardware platform name
14482 (sometimes called the hardware implementation).
14483 Print @samp{unknown} if the kernel does not make this information
14484 easily available, as is the case with Linux kernels.
14490 @cindex machine type
14491 @cindex hardware class
14492 @cindex hardware type
14493 Print the machine hardware name (sometimes called the hardware class
14499 @opindex --nodename
14502 @cindex network node name
14503 Print the network node hostname.
14508 @opindex --processor
14509 @cindex host processor type
14510 Print the processor type (sometimes called the instruction set
14511 architecture or ISA).
14512 Print @samp{unknown} if the kernel does not make this information
14513 easily available, as is the case with Linux kernels.
14516 @itemx --operating-system
14518 @opindex --operating-system
14519 @cindex operating system name
14520 Print the name of the operating system.
14523 @itemx --kernel-release
14525 @opindex --kernel-release
14526 @cindex kernel release
14527 @cindex release of kernel
14528 Print the kernel release.
14531 @itemx --kernel-name
14533 @opindex --kernel-name
14534 @cindex kernel name
14535 @cindex name of kernel
14536 Print the kernel name.
14537 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) calls this
14538 ``the implementation of the operating system'', because the
14539 @acronym{POSIX} specification itself has no notion of ``kernel''.
14540 The kernel name might be the same as the operating system name printed
14541 by the @option{-o} or @option{--operating-system} option, but it might
14542 differ. Some operating systems (e.g., FreeBSD, HP-UX) have the same
14543 name as their underlying kernels; others (e.g., GNU/Linux, Solaris)
14547 @itemx --kernel-version
14549 @opindex --kernel-version
14550 @cindex kernel version
14551 @cindex version of kernel
14552 Print the kernel version.
14559 @node hostname invocation
14560 @section @command{hostname}: Print or set system name
14563 @cindex setting the hostname
14564 @cindex printing the hostname
14565 @cindex system name, printing
14566 @cindex appropriate privileges
14568 With no arguments, @command{hostname} prints the name of the current host
14569 system. With one argument, it sets the current host name to the
14570 specified string. You must have appropriate privileges to set the host
14574 hostname [@var{name}]
14577 The only options are @option{--help} and @option{--version}. @xref{Common
14583 @node hostid invocation
14584 @section @command{hostid}: Print numeric host identifier
14587 @cindex printing the host identifier
14589 @command{hostid} prints the numeric identifier of the current host
14590 in hexadecimal. This command accepts no arguments.
14591 The only options are @option{--help} and @option{--version}.
14592 @xref{Common options}.
14594 For example, here's what it prints on one system I use:
14601 On that system, the 32-bit quantity happens to be closely
14602 related to the system's Internet address, but that isn't always
14607 @node uptime invocation
14608 @section @command{uptime}: Print system uptime and load
14611 @cindex printing the system uptime and load
14613 @command{uptime} prints the current time, the system's uptime, the
14614 number of logged-in users and the current load average.
14616 If an argument is specified, it is used as the file to be read
14617 to discover how many users are logged in. If no argument is
14618 specified, a system default is used (@command{uptime --help} indicates
14619 the default setting).
14621 The only options are @option{--help} and @option{--version}.
14622 @xref{Common options}.
14624 For example, here's what it prints right now on one system I use:
14628 14:07 up 3:35, 3 users, load average: 1.39, 1.15, 1.04
14631 The precise method of calculation of load average varies somewhat
14632 between systems. Some systems calculate it as the average number of
14633 runnable processes over the last 1, 5 and 15 minutes, but some systems
14634 also include processes in the uninterruptible sleep state (that is,
14635 those processes which are waiting for disk I/O). The Linux kernel
14636 includes uninterruptible processes.
14638 @node SELinux context
14639 @chapter SELinux context
14641 @cindex SELinux context
14642 @cindex SELinux, context
14643 @cindex commands for SELinux context
14645 This section describes commands for operations with SELinux
14649 * chcon invocation:: Change SELinux context of file
14650 * runcon invocation:: Run a command in specified SELinux context
14653 @node chcon invocation
14654 @section @command{chcon}: Change SELinux context of file
14657 @cindex changing security context
14658 @cindex change SELinux context
14660 @command{chcon} changes the SELinux security context of the selected files.
14664 chcon [@var{option}]@dots{} @var{context} @var{file}@dots{}
14665 chcon [@var{option}]@dots{} [-u @var{user}] [-r @var{role}] [-l @var{range}]@c
14666 [-t @var{type}] @var{file}@dots{}
14667 chcon [@var{option}]@dots{} --reference=@var{rfile} @var{file}@dots{}
14670 Change the SELinux security context of each @var{file} to @var{context}.
14671 With @option{--reference}, change the security context of each @var{file}
14672 to that of @var{rfile}.
14674 The program accepts the following options. Also see @ref{Common options}.
14679 @itemx --no-dereference
14681 @opindex --no-dereference
14682 @cindex no dereference
14683 Affect symbolic links instead of any referenced file.
14685 @item --reference=@var{rfile}
14686 @opindex --reference
14687 @cindex reference file
14688 Use @var{rfile}'s security context rather than specifying a @var{context} value.
14693 @opindex --recursive
14694 Operate on files and directories recursively.
14697 @xref{Traversing symlinks}.
14700 @xref{Traversing symlinks}.
14703 @xref{Traversing symlinks}.
14710 Output a diagnostic for every file processed.
14712 @item -u @var{user}
14713 @itemx --user=@var{user}
14716 Set user @var{user} in the target security context.
14718 @item -r @var{role}
14719 @itemx --role=@var{role}
14722 Set role @var{role} in the target security context.
14724 @item -t @var{type}
14725 @itemx --type=@var{type}
14728 Set type @var{type} in the target security context.
14730 @item -l @var{range}
14731 @itemx --range=@var{range}
14734 Set range @var{range} in the target security context.
14740 @node runcon invocation
14741 @section @command{runcon}: Run a command in specified SELinux context
14744 @cindex run with security context
14747 @command{runcon} runs file in specified SELinux security context.
14751 runcon @var{context} @var{command} [@var{args}]
14752 runcon [ -c ] [-u @var{user}] [-r @var{role}] [-t @var{type}]@c
14753 [-l @var{range}] @var{command} [@var{args}]
14756 Run @var{command} with completely-specified @var{context}, or with
14757 current or transitioned security context modified by one or more of @var{level},
14758 @var{role}, @var{type} and @var{user}.
14760 If none of @option{-c}, @option{-t}, @option{-u}, @option{-r}, or @option{-l}
14761 is specified, the first argument is used as the complete context.
14762 Any additional arguments after @var{command}
14763 are interpreted as arguments to the command.
14765 With neither @var{context} nor @var{command}, print the current
14768 The program accepts the following options. Also see @ref{Common options}.
14776 Compute process transition context before modifying.
14778 @item -u @var{user}
14779 @itemx --user=@var{user}
14782 Set user @var{user} in the target security context.
14784 @item -r @var{role}
14785 @itemx --role=@var{role}
14788 Set role @var{role} in the target security context.
14790 @item -t @var{type}
14791 @itemx --type=@var{type}
14794 Set type @var{type} in the target security context.
14796 @item -l @var{range}
14797 @itemx --range=@var{range}
14800 Set range @var{range} in the target security context.
14804 @cindex exit status of @command{runcon}
14808 126 if @var{command} is found but cannot be invoked
14809 127 if @command{runcon} itself fails or if @var{command} cannot be found
14810 the exit status of @var{command} otherwise
14813 @node Modified command invocation
14814 @chapter Modified command invocation
14816 @cindex modified command invocation
14817 @cindex invocation of commands, modified
14818 @cindex commands for invoking other commands
14820 This section describes commands that run other commands in some context
14821 different than the current one: a modified environment, as a different
14825 * chroot invocation:: Modify the root directory.
14826 * env invocation:: Modify environment variables.
14827 * nice invocation:: Modify niceness.
14828 * nohup invocation:: Immunize to hangups.
14829 * stdbuf invocation:: Modify buffering of standard streams.
14830 * su invocation:: Modify user and group ID.
14831 * timeout invocation:: Run with time limit.
14835 @node chroot invocation
14836 @section @command{chroot}: Run a command with a different root directory
14839 @cindex running a program in a specified root directory
14840 @cindex root directory, running a program in a specified
14842 @command{chroot} runs a command with a specified root directory.
14843 On many systems, only the super-user can do this.@footnote{However,
14844 some systems (e.g., FreeBSD) can be configured to allow certain regular
14845 users to use the @code{chroot} system call, and hence to run this program.
14846 Also, on Cygwin, anyone can run the @command{chroot} command, because the
14847 underlying function is non-privileged due to lack of support in MS-Windows.}
14851 chroot @var{option} @var{newroot} [@var{command} [@var{args}]@dots{}]
14852 chroot @var{option}
14855 Ordinarily, file names are looked up starting at the root of the
14856 directory structure, i.e., @file{/}. @command{chroot} changes the root to
14857 the directory @var{newroot} (which must exist) and then runs
14858 @var{command} with optional @var{args}. If @var{command} is not
14859 specified, the default is the value of the @env{SHELL} environment
14860 variable or @command{/bin/sh} if not set, invoked with the @option{-i} option.
14861 @var{command} must not be a special built-in utility
14862 (@pxref{Special built-in utilities}).
14864 The program accepts the following options. Also see @ref{Common options}.
14865 Options must precede operands.
14869 @itemx --userspec=@var{user}[:@var{group}]
14870 @opindex --userspec
14871 By default, @var{command} is run with the same credentials
14872 as the invoking process.
14873 Use this option to run it as a different @var{user} and/or with a
14874 different primary @var{group}.
14876 @itemx --groups=@var{groups}
14878 Use this option to specify the supplementary @var{groups} to be
14879 used by the new process.
14880 The items in the list (names or numeric IDs) must be separated by commas.
14884 Here are a few tips to help avoid common problems in using chroot.
14885 To start with a simple example, make @var{command} refer to a statically
14886 linked binary. If you were to use a dynamically linked executable, then
14887 you'd have to arrange to have the shared libraries in the right place under
14888 your new root directory.
14890 For example, if you create a statically linked @command{ls} executable,
14891 and put it in @file{/tmp/empty}, you can run this command as root:
14894 $ chroot /tmp/empty /ls -Rl /
14897 Then you'll see output like this:
14902 -rwxr-xr-x 1 0 0 1041745 Aug 16 11:17 ls
14905 If you want to use a dynamically linked executable, say @command{bash},
14906 then first run @samp{ldd bash} to see what shared objects it needs.
14907 Then, in addition to copying the actual binary, also copy the listed
14908 files to the required positions under your intended new root directory.
14909 Finally, if the executable requires any other files (e.g., data, state,
14910 device files), copy them into place, too.
14912 @cindex exit status of @command{chroot}
14916 125 if @command{chroot} itself fails
14917 126 if @var{command} is found but cannot be invoked
14918 127 if @var{command} cannot be found
14919 the exit status of @var{command} otherwise
14923 @node env invocation
14924 @section @command{env}: Run a command in a modified environment
14927 @cindex environment, running a program in a modified
14928 @cindex modified environment, running a program in a
14929 @cindex running a program in a modified environment
14931 @command{env} runs a command with a modified environment. Synopses:
14934 env [@var{option}]@dots{} [@var{name}=@var{value}]@dots{} @c
14935 [@var{command} [@var{args}]@dots{}]
14939 Operands of the form @samp{@var{variable}=@var{value}} set
14940 the environment variable @var{variable} to value @var{value}.
14941 @var{value} may be empty (@samp{@var{variable}=}). Setting a variable
14942 to an empty value is different from unsetting it.
14943 These operands are evaluated left-to-right, so if two operands
14944 mention the same variable the earlier is ignored.
14946 Environment variable names can be empty, and can contain any
14947 characters other than @samp{=} and @acronym{ASCII} @sc{nul}.
14948 However, it is wise to limit yourself to names that
14949 consist solely of underscores, digits, and @acronym{ASCII} letters,
14950 and that begin with a non-digit, as applications like the shell do not
14951 work well with other names.
14954 The first operand that does not contain the character @samp{=}
14955 specifies the program to invoke; it is
14956 searched for according to the @env{PATH} environment variable. Any
14957 remaining arguments are passed as arguments to that program.
14958 The program should not be a special built-in utility
14959 (@pxref{Special built-in utilities}).
14961 Modifications to @env{PATH} take effect prior to searching for
14962 @var{command}. Use caution when reducing @env{PATH}; behavior is
14963 not portable when @env{PATH} is undefined or omits key directories
14964 such as @file{/bin}.
14966 In the rare case that a utility contains a @samp{=} in the name, the
14967 only way to disambiguate it from a variable assignment is to use an
14968 intermediate command for @var{command}, and pass the problematic
14969 program name via @var{args}. For example, if @file{./prog=} is an
14970 executable in the current @env{PATH}:
14973 env prog= true # runs 'true', with prog= in environment
14974 env ./prog= true # runs 'true', with ./prog= in environment
14975 env -- prog= true # runs 'true', with prog= in environment
14976 env sh -c '\prog= true' # runs 'prog=' with argument 'true'
14977 env sh -c 'exec "$@@"' sh prog= true # also runs 'prog='
14980 @cindex environment, printing
14982 If no command name is specified following the environment
14983 specifications, the resulting environment is printed. This is like
14984 specifying the @command{printenv} program.
14986 For some examples, suppose the environment passed to @command{env}
14987 contains @samp{LOGNAME=rms}, @samp{EDITOR=emacs}, and
14988 @samp{PATH=.:/gnubin:/hacks}:
14993 Output the current environment.
14995 $ env | LC_ALL=C sort
14998 PATH=.:/gnubin:/hacks
15002 Run @command{foo} with a reduced environment, preserving only the
15003 original @env{PATH} to avoid problems in locating @command{foo}.
15005 env - PATH="$PATH" foo
15009 Run @command{foo} with the environment containing @samp{LOGNAME=rms},
15010 @samp{EDITOR=emacs}, and @samp{PATH=.:/gnubin:/hacks}, and guarantees
15011 that @command{foo} was found in the file system rather than as a shell
15018 Run @command{nemacs} with the environment containing @samp{LOGNAME=foo},
15019 @samp{EDITOR=emacs}, @samp{PATH=.:/gnubin:/hacks}, and
15020 @samp{DISPLAY=gnu:0}.
15022 env DISPLAY=gnu:0 LOGNAME=foo nemacs
15026 Attempt to run the program @command{/energy/--} (as that is the only
15027 possible path search result); if the command exists, the environment
15028 will contain @samp{LOGNAME=rms} and @samp{PATH=/energy}, and the
15029 arguments will be @samp{e=mc2}, @samp{bar}, and @samp{baz}.
15031 env -u EDITOR PATH=/energy -- e=mc2 bar baz
15037 The program accepts the following options. Also see @ref{Common options}.
15038 Options must precede operands.
15044 @item -u @var{name}
15045 @itemx --unset=@var{name}
15048 Remove variable @var{name} from the environment, if it was in the
15053 @itemx --ignore-environment
15056 @opindex --ignore-environment
15057 Start with an empty environment, ignoring the inherited environment.
15061 @cindex exit status of @command{env}
15065 0 if no @var{command} is specified and the environment is output
15066 125 if @command{env} itself fails
15067 126 if @var{command} is found but cannot be invoked
15068 127 if @var{command} cannot be found
15069 the exit status of @var{command} otherwise
15073 @node nice invocation
15074 @section @command{nice}: Run a command with modified niceness
15078 @cindex scheduling, affecting
15079 @cindex appropriate privileges
15081 @command{nice} prints or modifies a process's @dfn{niceness},
15082 a parameter that affects whether the process is scheduled favorably.
15086 nice [@var{option}]@dots{} [@var{command} [@var{arg}]@dots{}]
15089 If no arguments are given, @command{nice} prints the current niceness.
15090 Otherwise, @command{nice} runs the given @var{command} with its
15091 niceness adjusted. By default, its niceness is incremented by 10.
15093 Niceness values range at least from @minus{}20 (process has high priority
15094 and gets more resources, thus slowing down other processes) through 19
15095 (process has lower priority and runs slowly itself, but has less impact
15096 on the speed of other running processes). Some systems
15097 may have a wider range of nicenesses; conversely, other systems may
15098 enforce more restrictive limits. An attempt to set the niceness
15099 outside the supported range is treated as an attempt to use the
15100 minimum or maximum supported value.
15102 A niceness should not be confused with a scheduling priority, which
15103 lets applications determine the order in which threads are scheduled
15104 to run. Unlike a priority, a niceness is merely advice to the
15105 scheduler, which the scheduler is free to ignore. Also, as a point of
15106 terminology, @acronym{POSIX} defines the behavior of @command{nice} in
15107 terms of a @dfn{nice value}, which is the nonnegative difference
15108 between a niceness and the minimum niceness. Though @command{nice}
15109 conforms to @acronym{POSIX}, its documentation and diagnostics use the
15110 term ``niceness'' for compatibility with historical practice.
15112 @var{command} must not be a special built-in utility (@pxref{Special
15113 built-in utilities}).
15115 @mayConflictWithShellBuiltIn{nice}
15117 The program accepts the following option. Also see @ref{Common options}.
15118 Options must precede operands.
15121 @item -n @var{adjustment}
15122 @itemx --adjustment=@var{adjustment}
15124 @opindex --adjustment
15125 Add @var{adjustment} instead of 10 to the command's niceness. If
15126 @var{adjustment} is negative and you lack appropriate privileges,
15127 @command{nice} issues a warning but otherwise acts as if you specified
15130 For compatibility @command{nice} also supports an obsolete
15131 option syntax @option{-@var{adjustment}}. New scripts should use
15132 @option{-n @var{adjustment}} instead.
15136 @cindex exit status of @command{nice}
15140 0 if no @var{command} is specified and the niceness is output
15141 125 if @command{nice} itself fails
15142 126 if @var{command} is found but cannot be invoked
15143 127 if @var{command} cannot be found
15144 the exit status of @var{command} otherwise
15147 It is sometimes useful to run a non-interactive program with reduced niceness.
15150 $ nice factor 4611686018427387903
15153 Since @command{nice} prints the current niceness,
15154 you can invoke it through itself to demonstrate how it works.
15156 The default behavior is to increase the niceness by @samp{10}:
15167 The @var{adjustment} is relative to the current niceness. In the
15168 next example, the first @command{nice} invocation runs the second one
15169 with niceness 10, and it in turn runs the final one with a niceness
15173 $ nice nice -n 3 nice
15177 Specifying a niceness larger than the supported range
15178 is the same as specifying the maximum supported value:
15181 $ nice -n 10000000000 nice
15185 Only a privileged user may run a process with lower niceness:
15189 nice: cannot set niceness: Permission denied
15191 $ sudo nice -n -1 nice
15196 @node nohup invocation
15197 @section @command{nohup}: Run a command immune to hangups
15200 @cindex hangups, immunity to
15201 @cindex immunity to hangups
15202 @cindex logging out and continuing to run
15205 @command{nohup} runs the given @var{command} with hangup signals ignored,
15206 so that the command can continue running in the background after you log
15210 nohup @var{command} [@var{arg}]@dots{}
15213 If standard input is a terminal, it is redirected from
15214 @file{/dev/null} so that terminal sessions do not mistakenly consider
15215 the terminal to be used by the command. This is a @acronym{GNU}
15216 extension; programs intended to be portable to non-@acronym{GNU} hosts
15217 should use @samp{nohup @var{command} [@var{arg}]@dots{} </dev/null}
15221 If standard output is a terminal, the command's standard output is appended
15222 to the file @file{nohup.out}; if that cannot be written to, it is appended
15223 to the file @file{$HOME/nohup.out}; and if that cannot be written to, the
15224 command is not run.
15225 Any @file{nohup.out} or @file{$HOME/nohup.out} file created by
15226 @command{nohup} is made readable and writable only to the user,
15227 regardless of the current umask settings.
15229 If standard error is a terminal, it is normally redirected to the same file
15230 descriptor as the (possibly-redirected) standard output.
15231 However, if standard output is closed, standard error terminal output
15232 is instead appended to the file @file{nohup.out} or
15233 @file{$HOME/nohup.out} as above.
15235 To capture the command's output to a file other than @file{nohup.out}
15236 you can redirect it. For example, to capture the output of
15240 nohup make > make.log
15243 @command{nohup} does not automatically put the command it runs in the
15244 background; you must do that explicitly, by ending the command line
15245 with an @samp{&}. Also, @command{nohup} does not alter the
15246 niceness of @var{command}; use @command{nice} for that,
15247 e.g., @samp{nohup nice @var{command}}.
15249 @var{command} must not be a special built-in utility (@pxref{Special
15250 built-in utilities}).
15252 The only options are @option{--help} and @option{--version}. @xref{Common
15253 options}. Options must precede operands.
15255 @cindex exit status of @command{nohup}
15259 125 if @command{nohup} itself fails, and @env{POSIXLY_CORRECT} is not set
15260 126 if @var{command} is found but cannot be invoked
15261 127 if @var{command} cannot be found
15262 the exit status of @var{command} otherwise
15265 If @env{POSIXLY_CORRECT} is set, internal failures give status 127
15269 @node stdbuf invocation
15270 @section @command{stdbuf}: Run a command with modified I/O stream buffering
15273 @cindex standard streams, buffering
15274 @cindex line buffered
15276 @command{stdbuf} allows one to modify the buffering operations of the
15277 three standard I/O streams associated with a program. Synopsis:
15280 stdbuf @var{option}@dots{} @var{command}
15283 Any additional @var{arg}s are passed as additional arguments to the
15286 The program accepts the following options. Also see @ref{Common options}.
15290 @item -i @var{mode}
15291 @itemx --input=@var{mode}
15294 Adjust the standard input stream buffering.
15296 @item -o @var{mode}
15297 @itemx --output=@var{mode}
15300 Adjust the standard output stream buffering.
15302 @item -e @var{mode}
15303 @itemx --error=@var{mode}
15306 Adjust the standard error stream buffering.
15310 The @var{mode} can be specified as follows:
15315 Set the stream to line buffered mode.
15316 In this mode data is coalesced until a newline is output or
15317 input is read from any stream attached to a terminal device.
15318 This option is invalid with standard input.
15321 Disable buffering of the selected stream.
15322 In this mode data is output immediately and only the
15323 amount of data requested is read from input.
15326 Specify the size of the buffer to use in fully buffered mode.
15327 @multiplierSuffixesNoBlocks{size}
15331 NOTE: If @var{command} adjusts the buffering of its standard streams
15332 (@command{tee} does for e.g.) then that will override corresponding settings
15333 changed by @command{stdbuf}. Also some filters (like @command{dd} and
15334 @command{cat} etc.) don't use streams for I/O, and are thus unaffected
15335 by @command{stdbuf} settings.
15337 @cindex exit status of @command{stdbuf}
15341 125 if @command{stdbuf} itself fails
15342 126 if @var{command} is found but cannot be invoked
15343 127 if @var{command} cannot be found
15344 the exit status of @var{command} otherwise
15348 @node su invocation
15349 @section @command{su}: Run a command with substitute user and group ID
15352 @cindex substitute user and group IDs
15353 @cindex user ID, switching
15354 @cindex super-user, becoming
15355 @cindex root, becoming
15357 @command{su} allows one user to temporarily become another user. It runs a
15358 command (often an interactive shell) with the real and effective user
15359 ID, group ID, and supplemental groups of a given @var{user}. Synopsis:
15362 su [@var{option}]@dots{} [@var{user} [@var{arg}]@dots{}]
15365 @cindex passwd entry, and @command{su} shell
15367 @flindex /etc/passwd
15368 If no @var{user} is given, the default is @code{root}, the super-user.
15369 The shell to use is taken from @var{user}'s @code{passwd} entry, or
15370 @file{/bin/sh} if none is specified there. If @var{user} has a
15371 password, @command{su} prompts for the password unless run by a user with
15372 effective user ID of zero (the super-user).
15378 @cindex login shell
15379 By default, @command{su} does not change the current directory.
15380 It sets the environment variables @env{HOME} and @env{SHELL}
15381 from the password entry for @var{user}, and if @var{user} is not
15382 the super-user, sets @env{USER} and @env{LOGNAME} to @var{user}.
15383 By default, the shell is not a login shell.
15385 Any additional @var{arg}s are passed as additional arguments to the
15388 @cindex @option{-su}
15389 GNU @command{su} does not treat @file{/bin/sh} or any other shells specially
15390 (e.g., by setting @code{argv[0]} to @option{-su}, passing @option{-c} only
15391 to certain shells, etc.).
15394 @command{su} can optionally be compiled to use @code{syslog} to report
15395 failed, and optionally successful, @command{su} attempts. (If the system
15396 supports @code{syslog}.) However, GNU @command{su} does not check if the
15397 user is a member of the @code{wheel} group; see below.
15399 The program accepts the following options. Also see @ref{Common options}.
15402 @item -c @var{command}
15403 @itemx --command=@var{command}
15406 Pass @var{command}, a single command line to run, to the shell with
15407 a @option{-c} option instead of starting an interactive shell.
15414 @cindex file name pattern expansion, disabled
15415 @cindex globbing, disabled
15416 Pass the @option{-f} option to the shell. This probably only makes sense
15417 if the shell run is @command{csh} or @command{tcsh}, for which the @option{-f}
15418 option prevents reading the startup file (@file{.cshrc}). With
15419 Bourne-like shells, the @option{-f} option disables file name pattern
15420 expansion (globbing), which is not likely to be useful.
15428 @c other variables already indexed above
15431 @cindex login shell, creating
15432 Make the shell a login shell. This means the following. Unset all
15433 environment variables except @env{TERM}, @env{HOME}, and @env{SHELL}
15434 (which are set as described above), and @env{USER} and @env{LOGNAME}
15435 (which are set, even for the super-user, as described above), and set
15436 @env{PATH} to a compiled-in default value. Change to @var{user}'s home
15437 directory. Prepend @samp{-} to the shell's name, intended to make it
15438 read its login startup file(s).
15442 @itemx --preserve-environment
15445 @opindex --preserve-environment
15446 @cindex environment, preserving
15447 @flindex /etc/shells
15448 @cindex restricted shell
15449 Do not change the environment variables @env{HOME}, @env{USER},
15450 @env{LOGNAME}, or @env{SHELL}. Run the shell given in the environment
15451 variable @env{SHELL} instead of the shell from @var{user}'s passwd
15452 entry, unless the user running @command{su} is not the super-user and
15453 @var{user}'s shell is restricted. A @dfn{restricted shell} is one that
15454 is not listed in the file @file{/etc/shells}, or in a compiled-in list
15455 if that file does not exist. Parts of what this option does can be
15456 overridden by @option{--login} and @option{--shell}.
15458 @item -s @var{shell}
15459 @itemx --shell=@var{shell}
15462 Run @var{shell} instead of the shell from @var{user}'s passwd entry,
15463 unless the user running @command{su} is not the super-user and @var{user}'s
15464 shell is restricted (see @option{-m} just above).
15468 @cindex exit status of @command{su}
15472 125 if @command{su} itself fails
15473 126 if subshell is found but cannot be invoked
15474 127 if subshell cannot be found
15475 the exit status of the subshell otherwise
15478 @cindex wheel group, not supported
15479 @cindex group wheel, not supported
15481 @subsection Why GNU @command{su} does not support the @samp{wheel} group
15483 (This section is by Richard Stallman.)
15487 Sometimes a few of the users try to hold total power over all the
15488 rest. For example, in 1984, a few users at the MIT AI lab decided to
15489 seize power by changing the operator password on the Twenex system and
15490 keeping it secret from everyone else. (I was able to thwart this coup
15491 and give power back to the users by patching the kernel, but I
15492 wouldn't know how to do that in Unix.)
15494 However, occasionally the rulers do tell someone. Under the usual
15495 @command{su} mechanism, once someone learns the root password who
15496 sympathizes with the ordinary users, he or she can tell the rest. The
15497 ``wheel group'' feature would make this impossible, and thus cement the
15498 power of the rulers.
15500 I'm on the side of the masses, not that of the rulers. If you are
15501 used to supporting the bosses and sysadmins in whatever they do, you
15502 might find this idea strange at first.
15505 @node timeout invocation
15506 @section @command{timeout}: Run a command with a time limit
15510 @cindex run commands with bounded time
15512 @command{timeout} runs the given @var{command} and kills it if it is
15513 still running after the specified time interval. Synopsis:
15516 timeout [@var{option}] @var{duration} @var{command} [@var{arg}]@dots{}
15519 @var{command} must not be a special built-in utility (@pxref{Special
15520 built-in utilities}).
15522 The program accepts the following options. Also see @ref{Common options}.
15523 Options must precede operands.
15526 @item -k @var{duration}
15527 @itemx --kill-after=@var{duration}
15529 @opindex --kill-after
15530 Ensure the monitored @var{command} is killed by also sending a @samp{KILL}
15531 signal, after the specified @var{duration}. Without this option, if the
15532 selected signal proves not to be fatal, @command{timeout} does not kill
15535 @item -s @var{signal}
15536 @itemx --signal=@var{signal}
15539 Send this @var{signal} to @var{command} on timeout, rather than the
15540 default @samp{TERM} signal. @var{signal} may be a name like @samp{HUP}
15541 or a number. Also see @xref{Signal specifications}.
15545 @var{duration} is an integer followed by an optional unit:
15547 @samp{s} for seconds (the default)
15548 @samp{m} for minutes
15552 A duration of 0 disables the associated timeout.
15554 @cindex exit status of @command{timeout}
15558 124 if @var{command} times out
15559 125 if @command{timeout} itself fails
15560 126 if @var{command} is found but cannot be invoked
15561 127 if @var{command} cannot be found
15562 the exit status of @var{command} otherwise
15566 @node Process control
15567 @chapter Process control
15569 @cindex processes, commands for controlling
15570 @cindex commands for controlling processes
15573 * kill invocation:: Sending a signal to processes.
15577 @node kill invocation
15578 @section @command{kill}: Send a signal to processes
15581 @cindex send a signal to processes
15583 The @command{kill} command sends a signal to processes, causing them
15584 to terminate or otherwise act upon receiving the signal in some way.
15585 Alternatively, it lists information about signals. Synopses:
15588 kill [-s @var{signal} | --signal @var{signal} | -@var{signal}] @var{pid}@dots{}
15589 kill [-l | --list | -t | --table] [@var{signal}]@dots{}
15592 @mayConflictWithShellBuiltIn{kill}
15594 The first form of the @command{kill} command sends a signal to all
15595 @var{pid} arguments. The default signal to send if none is specified
15596 is @samp{TERM}. The special signal number @samp{0} does not denote a
15597 valid signal, but can be used to test whether the @var{pid} arguments
15598 specify processes to which a signal could be sent.
15600 If @var{pid} is positive, the signal is sent to the process with the
15601 process ID @var{pid}. If @var{pid} is zero, the signal is sent to all
15602 processes in the process group of the current process. If @var{pid}
15603 is @minus{}1, the signal is sent to all processes for which the user has
15604 permission to send a signal. If @var{pid} is less than @minus{}1, the signal
15605 is sent to all processes in the process group that equals the absolute
15606 value of @var{pid}.
15608 If @var{pid} is not positive, a system-dependent set of system
15609 processes is excluded from the list of processes to which the signal
15612 If a negative @var{pid} argument is desired as the first one, it
15613 should be preceded by @option{--}. However, as a common extension to
15614 @acronym{POSIX}, @option{--} is not required with @samp{kill
15615 -@var{signal} -@var{pid}}. The following commands are equivalent:
15624 The first form of the @command{kill} command succeeds if every @var{pid}
15625 argument specifies at least one process that the signal was sent to.
15627 The second form of the @command{kill} command lists signal information.
15628 Either the @option{-l} or @option{--list} option, or the @option{-t}
15629 or @option{--table} option must be specified. Without any
15630 @var{signal} argument, all supported signals are listed. The output
15631 of @option{-l} or @option{--list} is a list of the signal names, one
15632 per line; if @var{signal} is already a name, the signal number is
15633 printed instead. The output of @option{-t} or @option{--table} is a
15634 table of signal numbers, names, and descriptions. This form of the
15635 @command{kill} command succeeds if all @var{signal} arguments are valid
15636 and if there is no output error.
15638 The @command{kill} command also supports the @option{--help} and
15639 @option{--version} options. @xref{Common options}.
15641 A @var{signal} may be a signal name like @samp{HUP}, or a signal
15642 number like @samp{1}, or an exit status of a process terminated by the
15643 signal. A signal name can be given in canonical form or prefixed by
15644 @samp{SIG}. The case of the letters is ignored, except for the
15645 @option{-@var{signal}} option which must use upper case to avoid
15646 ambiguity with lower case option letters. For a list of supported
15647 signal names and numbers see @xref{Signal specifications}.
15652 @cindex delaying commands
15653 @cindex commands for delaying
15655 @c Perhaps @command{wait} or other commands should be described here also?
15658 * sleep invocation:: Delay for a specified time.
15662 @node sleep invocation
15663 @section @command{sleep}: Delay for a specified time
15666 @cindex delay for a specified time
15668 @command{sleep} pauses for an amount of time specified by the sum of
15669 the values of the command line arguments.
15673 sleep @var{number}[smhd]@dots{}
15677 Each argument is a number followed by an optional unit; the default
15678 is seconds. The units are:
15691 Historical implementations of @command{sleep} have required that
15692 @var{number} be an integer, and only accepted a single argument
15693 without a suffix. However, GNU @command{sleep} accepts
15694 arbitrary floating point numbers. @xref{Floating point}.
15696 The only options are @option{--help} and @option{--version}. @xref{Common
15699 @c sleep is a shell built-in at least with Solaris 11's /bin/sh
15700 @mayConflictWithShellBuiltIn{sleep}
15705 @node Numeric operations
15706 @chapter Numeric operations
15708 @cindex numeric operations
15709 These programs do numerically-related operations.
15712 * factor invocation:: Show factors of numbers.
15713 * seq invocation:: Print sequences of numbers.
15717 @node factor invocation
15718 @section @command{factor}: Print prime factors
15721 @cindex prime factors
15723 @command{factor} prints prime factors. Synopses:
15726 factor [@var{number}]@dots{}
15727 factor @var{option}
15730 If no @var{number} is specified on the command line, @command{factor} reads
15731 numbers from standard input, delimited by newlines, tabs, or spaces.
15733 The @command{factor} command supports only a small number of options:
15737 Print a short help on standard output, then exit without further
15741 Print the program version on standard output, then exit without further
15745 Factoring the product of the eighth and ninth Mersenne primes
15746 takes about 30 milliseconds of CPU time on a 2.2 GHz Athlon.
15749 M8=`echo 2^31-1|bc` ; M9=`echo 2^61-1|bc`
15750 /usr/bin/time -f '%U' factor $(echo "$M8 * $M9" | bc)
15751 4951760154835678088235319297: 2147483647 2305843009213693951
15755 Similarly, factoring the eighth Fermat number @math{2^{256}+1} takes
15756 about 20 seconds on the same machine.
15758 Factoring large numbers is, in general, hard. The Pollard Rho
15759 algorithm used by @command{factor} is particularly effective for
15760 numbers with relatively small factors. If you wish to factor large
15761 numbers which do not have small factors (for example, numbers which
15762 are the product of two large primes), other methods are far better.
15764 If @command{factor} is built without using GNU MP, only
15765 single-precision arithmetic is available, and so large numbers
15766 (typically @math{2^{64}} and above) will not be supported. The single-precision
15767 code uses an algorithm which is designed for factoring smaller
15773 @node seq invocation
15774 @section @command{seq}: Print numeric sequences
15777 @cindex numeric sequences
15778 @cindex sequence of numbers
15780 @command{seq} prints a sequence of numbers to standard output. Synopses:
15783 seq [@var{option}]@dots{} @var{last}
15784 seq [@var{option}]@dots{} @var{first} @var{last}
15785 seq [@var{option}]@dots{} @var{first} @var{increment} @var{last}
15788 @command{seq} prints the numbers from @var{first} to @var{last} by
15789 @var{increment}. By default, each number is printed on a separate line.
15790 When @var{increment} is not specified, it defaults to @samp{1},
15791 even when @var{first} is larger than @var{last}.
15792 @var{first} also defaults to @samp{1}. So @code{seq 1} prints
15793 @samp{1}, but @code{seq 0} and @code{seq 10 5} produce no output.
15794 Floating-point numbers may be specified. @xref{Floating point}.
15796 The program accepts the following options. Also see @ref{Common options}.
15797 Options must precede operands.
15800 @item -f @var{format}
15801 @itemx --format=@var{format}
15802 @opindex -f @var{format}
15803 @opindex --format=@var{format}
15804 @cindex formatting of numbers in @command{seq}
15805 Print all numbers using @var{format}.
15806 @var{format} must contain exactly one of the @samp{printf}-style
15807 floating point conversion specifications @samp{%a}, @samp{%e},
15808 @samp{%f}, @samp{%g}, @samp{%A}, @samp{%E}, @samp{%F}, @samp{%G}.
15809 The @samp{%} may be followed by zero or more flags taken from the set
15810 @samp{-+#0 '}, then an optional width containing one or more digits,
15811 then an optional precision consisting of a @samp{.} followed by zero
15812 or more digits. @var{format} may also contain any number of @samp{%%}
15813 conversion specifications. All conversion specifications have the
15814 same meaning as with @samp{printf}.
15816 The default format is derived from @var{first}, @var{step}, and
15817 @var{last}. If these all use a fixed point decimal representation,
15818 the default format is @samp{%.@var{p}f}, where @var{p} is the minimum
15819 precision that can represent the output numbers exactly. Otherwise,
15820 the default format is @samp{%g}.
15822 @item -s @var{string}
15823 @itemx --separator=@var{string}
15824 @cindex separator for numbers in @command{seq}
15825 Separate numbers with @var{string}; default is a newline.
15826 The output always terminates with a newline.
15829 @itemx --equal-width
15830 Print all numbers with the same width, by padding with leading zeros.
15831 @var{first}, @var{step}, and @var{last} should all use a fixed point
15832 decimal representation.
15833 (To have other kinds of padding, use @option{--format}).
15837 You can get finer-grained control over output with @option{-f}:
15840 $ seq -f '(%9.2E)' -9e5 1.1e6 1.3e6
15846 If you want hexadecimal integer output, you can use @command{printf}
15847 to perform the conversion:
15850 $ printf '%x\n' `seq 1048575 1024 1050623`
15856 For very long lists of numbers, use xargs to avoid
15857 system limitations on the length of an argument list:
15860 $ seq 1000000 | xargs printf '%x\n' | tail -n 3
15866 To generate octal output, use the printf @code{%o} format instead
15869 On most systems, seq can produce whole-number output for values up to
15870 at least @math{2^{53}}. Larger integers are approximated. The details
15871 differ depending on your floating-point implementation.
15872 @xref{Floating point}. A common
15873 case is that @command{seq} works with integers through @math{2^{64}},
15874 and larger integers may not be numerically correct:
15877 $ seq 18446744073709551616 1 18446744073709551618
15878 18446744073709551616
15879 18446744073709551616
15880 18446744073709551618
15883 Be careful when using @command{seq} with outlandish values: otherwise
15884 you may see surprising results, as @command{seq} uses floating point
15885 internally. For example, on the x86 platform, where the internal
15886 representation uses a 64-bit fraction, the command:
15889 seq 1 0.0000000000000000001 1.0000000000000000009
15892 outputs 1.0000000000000000007 twice and skips 1.0000000000000000008.
15897 @node File permissions
15898 @chapter File permissions
15901 @include parse-datetime.texi
15905 @node Opening the software toolbox
15906 @chapter Opening the Software Toolbox
15908 An earlier version of this chapter appeared in
15909 @uref{http://www.linuxjournal.com/article.php?sid=2762, the
15910 @cite{What's GNU?} column of @cite{Linux Journal}, 2 (June, 1994)}.
15911 It was written by Arnold Robbins.
15914 * Toolbox introduction:: Toolbox introduction
15915 * I/O redirection:: I/O redirection
15916 * The who command:: The @command{who} command
15917 * The cut command:: The @command{cut} command
15918 * The sort command:: The @command{sort} command
15919 * The uniq command:: The @command{uniq} command
15920 * Putting the tools together:: Putting the tools together
15924 @node Toolbox introduction
15925 @unnumberedsec Toolbox Introduction
15927 This month's column is only peripherally related to the GNU Project, in
15928 that it describes a number of the GNU tools on your GNU/Linux system
15930 might be used. What it's really about is the ``Software Tools'' philosophy
15931 of program development and usage.
15933 The software tools philosophy was an important and integral concept
15934 in the initial design and development of Unix (of which Linux and GNU are
15935 essentially clones). Unfortunately, in the modern day press of
15936 Internetworking and flashy GUIs, it seems to have fallen by the
15937 wayside. This is a shame, since it provides a powerful mental model
15938 for solving many kinds of problems.
15940 Many people carry a Swiss Army knife around in their pants pockets (or
15941 purse). A Swiss Army knife is a handy tool to have: it has several knife
15942 blades, a screwdriver, tweezers, toothpick, nail file, corkscrew, and perhaps
15943 a number of other things on it. For the everyday, small miscellaneous jobs
15944 where you need a simple, general purpose tool, it's just the thing.
15946 On the other hand, an experienced carpenter doesn't build a house using
15947 a Swiss Army knife. Instead, he has a toolbox chock full of specialized
15948 tools---a saw, a hammer, a screwdriver, a plane, and so on. And he knows
15949 exactly when and where to use each tool; you won't catch him hammering nails
15950 with the handle of his screwdriver.
15952 The Unix developers at Bell Labs were all professional programmers and trained
15953 computer scientists. They had found that while a one-size-fits-all program
15954 might appeal to a user because there's only one program to use, in practice
15959 difficult to write,
15962 difficult to maintain and
15966 difficult to extend to meet new situations.
15969 Instead, they felt that programs should be specialized tools. In short, each
15970 program ``should do one thing well.'' No more and no less. Such programs are
15971 simpler to design, write, and get right---they only do one thing.
15973 Furthermore, they found that with the right machinery for hooking programs
15974 together, that the whole was greater than the sum of the parts. By combining
15975 several special purpose programs, you could accomplish a specific task
15976 that none of the programs was designed for, and accomplish it much more
15977 quickly and easily than if you had to write a special purpose program.
15978 We will see some (classic) examples of this further on in the column.
15979 (An important additional point was that, if necessary, take a detour
15980 and build any software tools you may need first, if you don't already
15981 have something appropriate in the toolbox.)
15983 @node I/O redirection
15984 @unnumberedsec I/O Redirection
15986 Hopefully, you are familiar with the basics of I/O redirection in the
15987 shell, in particular the concepts of ``standard input,'' ``standard output,''
15988 and ``standard error''. Briefly, ``standard input'' is a data source, where
15989 data comes from. A program should not need to either know or care if the
15990 data source is a disk file, a keyboard, a magnetic tape, or even a punched
15991 card reader. Similarly, ``standard output'' is a data sink, where data goes
15992 to. The program should neither know nor care where this might be.
15993 Programs that only read their standard input, do something to the data,
15994 and then send it on, are called @dfn{filters}, by analogy to filters in a
15997 With the Unix shell, it's very easy to set up data pipelines:
16000 program_to_create_data | filter1 | ... | filterN > final.pretty.data
16003 We start out by creating the raw data; each filter applies some successive
16004 transformation to the data, until by the time it comes out of the pipeline,
16005 it is in the desired form.
16007 This is fine and good for standard input and standard output. Where does the
16008 standard error come in to play? Well, think about @command{filter1} in
16009 the pipeline above. What happens if it encounters an error in the data it
16010 sees? If it writes an error message to standard output, it will just
16011 disappear down the pipeline into @command{filter2}'s input, and the
16012 user will probably never see it. So programs need a place where they can send
16013 error messages so that the user will notice them. This is standard error,
16014 and it is usually connected to your console or window, even if you have
16015 redirected standard output of your program away from your screen.
16017 For filter programs to work together, the format of the data has to be
16018 agreed upon. The most straightforward and easiest format to use is simply
16019 lines of text. Unix data files are generally just streams of bytes, with
16020 lines delimited by the @acronym{ASCII} @sc{lf} (Line Feed) character,
16021 conventionally called a ``newline'' in the Unix literature. (This is
16022 @code{'\n'} if you're a C programmer.) This is the format used by all
16023 the traditional filtering programs. (Many earlier operating systems
16024 had elaborate facilities and special purpose programs for managing
16025 binary data. Unix has always shied away from such things, under the
16026 philosophy that it's easiest to simply be able to view and edit your
16027 data with a text editor.)
16029 OK, enough introduction. Let's take a look at some of the tools, and then
16030 we'll see how to hook them together in interesting ways. In the following
16031 discussion, we will only present those command line options that interest
16032 us. As you should always do, double check your system documentation
16033 for the full story.
16035 @node The who command
16036 @unnumberedsec The @command{who} Command
16038 The first program is the @command{who} command. By itself, it generates a
16039 list of the users who are currently logged in. Although I'm writing
16040 this on a single-user system, we'll pretend that several people are
16045 @print{} arnold console Jan 22 19:57
16046 @print{} miriam ttyp0 Jan 23 14:19(:0.0)
16047 @print{} bill ttyp1 Jan 21 09:32(:0.0)
16048 @print{} arnold ttyp2 Jan 23 20:48(:0.0)
16051 Here, the @samp{$} is the usual shell prompt, at which I typed @samp{who}.
16052 There are three people logged in, and I am logged in twice. On traditional
16053 Unix systems, user names are never more than eight characters long. This
16054 little bit of trivia will be useful later. The output of @command{who} is nice,
16055 but the data is not all that exciting.
16057 @node The cut command
16058 @unnumberedsec The @command{cut} Command
16060 The next program we'll look at is the @command{cut} command. This program
16061 cuts out columns or fields of input data. For example, we can tell it
16062 to print just the login name and full name from the @file{/etc/passwd}
16063 file. The @file{/etc/passwd} file has seven fields, separated by
16067 arnold:xyzzy:2076:10:Arnold D. Robbins:/home/arnold:/bin/bash
16070 To get the first and fifth fields, we would use @command{cut} like this:
16073 $ cut -d: -f1,5 /etc/passwd
16074 @print{} root:Operator
16076 @print{} arnold:Arnold D. Robbins
16077 @print{} miriam:Miriam A. Robbins
16081 With the @option{-c} option, @command{cut} will cut out specific characters
16082 (i.e., columns) in the input lines. This is useful for input data
16083 that has fixed width fields, and does not have a field separator. For
16084 example, list the Monday dates for the current month:
16086 @c Is using cal ok? Looked at gcal, but I don't like it.
16097 @node The sort command
16098 @unnumberedsec The @command{sort} Command
16100 Next we'll look at the @command{sort} command. This is one of the most
16101 powerful commands on a Unix-style system; one that you will often find
16102 yourself using when setting up fancy data plumbing.
16105 command reads and sorts each file named on the command line. It then
16106 merges the sorted data and writes it to standard output. It will read
16107 standard input if no files are given on the command line (thus
16108 making it into a filter). The sort is based on the character collating
16109 sequence or based on user-supplied ordering criteria.
16112 @node The uniq command
16113 @unnumberedsec The @command{uniq} Command
16115 Finally (at least for now), we'll look at the @command{uniq} program. When
16116 sorting data, you will often end up with duplicate lines, lines that
16117 are identical. Usually, all you need is one instance of each line.
16118 This is where @command{uniq} comes in. The @command{uniq} program reads its
16119 standard input. It prints only one
16120 copy of each repeated line. It does have several options. Later on,
16121 we'll use the @option{-c} option, which prints each unique line, preceded
16122 by a count of the number of times that line occurred in the input.
16125 @node Putting the tools together
16126 @unnumberedsec Putting the Tools Together
16128 Now, let's suppose this is a large ISP server system with dozens of users
16129 logged in. The management wants the system administrator to write a
16131 generate a sorted list of logged in users. Furthermore, even if a user
16132 is logged in multiple times, his or her name should only show up in the
16135 The administrator could sit down with the system documentation and write a C
16136 program that did this. It would take perhaps a couple of hundred lines
16137 of code and about two hours to write it, test it, and debug it.
16138 However, knowing the software toolbox, the administrator can instead start out
16139 by generating just a list of logged on users:
16149 Next, sort the list:
16152 $ who | cut -c1-8 | sort
16159 Finally, run the sorted list through @command{uniq}, to weed out duplicates:
16162 $ who | cut -c1-8 | sort | uniq
16168 The @command{sort} command actually has a @option{-u} option that does what
16169 @command{uniq} does. However, @command{uniq} has other uses for which one
16170 cannot substitute @samp{sort -u}.
16172 The administrator puts this pipeline into a shell script, and makes it
16174 all the users on the system (@samp{#} is the system administrator,
16175 or @code{root}, prompt):
16178 # cat > /usr/local/bin/listusers
16179 who | cut -c1-8 | sort | uniq
16181 # chmod +x /usr/local/bin/listusers
16184 There are four major points to note here. First, with just four
16185 programs, on one command line, the administrator was able to save about two
16186 hours worth of work. Furthermore, the shell pipeline is just about as
16187 efficient as the C program would be, and it is much more efficient in
16188 terms of programmer time. People time is much more expensive than
16189 computer time, and in our modern ``there's never enough time to do
16190 everything'' society, saving two hours of programmer time is no mean
16193 Second, it is also important to emphasize that with the
16194 @emph{combination} of the tools, it is possible to do a special
16195 purpose job never imagined by the authors of the individual programs.
16197 Third, it is also valuable to build up your pipeline in stages, as we did here.
16198 This allows you to view the data at each stage in the pipeline, which helps
16199 you acquire the confidence that you are indeed using these tools correctly.
16201 Finally, by bundling the pipeline in a shell script, other users can use
16202 your command, without having to remember the fancy plumbing you set up for
16203 them. In terms of how you run them, shell scripts and compiled programs are
16206 After the previous warm-up exercise, we'll look at two additional, more
16207 complicated pipelines. For them, we need to introduce two more tools.
16209 The first is the @command{tr} command, which stands for ``transliterate.''
16210 The @command{tr} command works on a character-by-character basis, changing
16211 characters. Normally it is used for things like mapping upper case to
16215 $ echo ThIs ExAmPlE HaS MIXED case! | tr '[:upper:]' '[:lower:]'
16216 @print{} this example has mixed case!
16219 There are several options of interest:
16223 work on the complement of the listed characters, i.e.,
16224 operations apply to characters not in the given set
16227 delete characters in the first set from the output
16230 squeeze repeated characters in the output into just one character.
16233 We will be using all three options in a moment.
16235 The other command we'll look at is @command{comm}. The @command{comm}
16236 command takes two sorted input files as input data, and prints out the
16237 files' lines in three columns. The output columns are the data lines
16238 unique to the first file, the data lines unique to the second file, and
16239 the data lines that are common to both. The @option{-1}, @option{-2}, and
16240 @option{-3} command line options @emph{omit} the respective columns. (This is
16241 non-intuitive and takes a little getting used to.) For example:
16263 The file name @file{-} tells @command{comm} to read standard input
16264 instead of a regular file.
16266 Now we're ready to build a fancy pipeline. The first application is a word
16267 frequency counter. This helps an author determine if he or she is over-using
16270 The first step is to change the case of all the letters in our input file
16271 to one case. ``The'' and ``the'' are the same word when doing counting.
16274 $ tr '[:upper:]' '[:lower:]' < whats.gnu | ...
16277 The next step is to get rid of punctuation. Quoted words and unquoted words
16278 should be treated identically; it's easiest to just get the punctuation out of
16282 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | ...
16285 The second @command{tr} command operates on the complement of the listed
16286 characters, which are all the letters, the digits, the underscore, and
16287 the blank. The @samp{\n} represents the newline character; it has to
16288 be left alone. (The @acronym{ASCII} tab character should also be included for
16289 good measure in a production script.)
16291 At this point, we have data consisting of words separated by blank space.
16292 The words only contain alphanumeric characters (and the underscore). The
16293 next step is break the data apart so that we have one word per line. This
16294 makes the counting operation much easier, as we will see shortly.
16297 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16298 > tr -s ' ' '\n' | ...
16301 This command turns blanks into newlines. The @option{-s} option squeezes
16302 multiple newline characters in the output into just one. This helps us
16303 avoid blank lines. (The @samp{>} is the shell's ``secondary prompt.''
16304 This is what the shell prints when it notices you haven't finished
16305 typing in all of a command.)
16307 We now have data consisting of one word per line, no punctuation, all one
16308 case. We're ready to count each word:
16311 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16312 > tr -s ' ' '\n' | sort | uniq -c | ...
16315 At this point, the data might look something like this:
16328 The output is sorted by word, not by count! What we want is the most
16329 frequently used words first. Fortunately, this is easy to accomplish,
16330 with the help of two more @command{sort} options:
16334 do a numeric sort, not a textual one
16337 reverse the order of the sort
16340 The final pipeline looks like this:
16343 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16344 > tr -s ' ' '\n' | sort | uniq -c | sort -n -r
16353 Whew! That's a lot to digest. Yet, the same principles apply. With six
16354 commands, on two lines (really one long one split for convenience), we've
16355 created a program that does something interesting and useful, in much
16356 less time than we could have written a C program to do the same thing.
16358 A minor modification to the above pipeline can give us a simple spelling
16359 checker! To determine if you've spelled a word correctly, all you have to
16360 do is look it up in a dictionary. If it is not there, then chances are
16361 that your spelling is incorrect. So, we need a dictionary.
16362 The conventional location for a dictionary is @file{/usr/dict/words}.
16363 On my GNU/Linux system,@footnote{Redhat Linux 6.1, for the November 2000
16364 revision of this article.}
16365 this is a sorted, 45,402 word dictionary.
16367 Now, how to compare our file with the dictionary? As before, we generate
16368 a sorted list of words, one per line:
16371 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16372 > tr -s ' ' '\n' | sort -u | ...
16375 Now, all we need is a list of words that are @emph{not} in the
16376 dictionary. Here is where the @command{comm} command comes in.
16379 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16380 > tr -s ' ' '\n' | sort -u |
16381 > comm -23 - /usr/dict/words
16384 The @option{-2} and @option{-3} options eliminate lines that are only in the
16385 dictionary (the second file), and lines that are in both files. Lines
16386 only in the first file (standard input, our stream of words), are
16387 words that are not in the dictionary. These are likely candidates for
16388 spelling errors. This pipeline was the first cut at a production
16389 spelling checker on Unix.
16391 There are some other tools that deserve brief mention.
16395 search files for text that matches a regular expression
16398 count lines, words, characters
16401 a T-fitting for data pipes, copies data to files and to standard output
16404 the stream editor, an advanced tool
16407 a data manipulation language, another advanced tool
16410 The software tools philosophy also espoused the following bit of
16411 advice: ``Let someone else do the hard part.'' This means, take
16412 something that gives you most of what you need, and then massage it the
16413 rest of the way until it's in the form that you want.
16419 Each program should do one thing well. No more, no less.
16422 Combining programs with appropriate plumbing leads to results where
16423 the whole is greater than the sum of the parts. It also leads to novel
16424 uses of programs that the authors might never have imagined.
16427 Programs should never print extraneous header or trailer data, since these
16428 could get sent on down a pipeline. (A point we didn't mention earlier.)
16431 Let someone else do the hard part.
16434 Know your toolbox! Use each program appropriately. If you don't have an
16435 appropriate tool, build one.
16438 As of this writing, all the programs we've discussed are available via
16439 anonymous @command{ftp} from: @*
16440 @uref{ftp://gnudist.gnu.org/textutils/textutils-1.22.tar.gz}. (There may
16441 be more recent versions available now.)
16443 None of what I have presented in this column is new. The Software Tools
16444 philosophy was first introduced in the book @cite{Software Tools}, by
16445 Brian Kernighan and P.J. Plauger (Addison-Wesley, ISBN 0-201-03669-X).
16446 This book showed how to write and use software tools. It was written in
16447 1976, using a preprocessor for FORTRAN named @command{ratfor} (RATional
16448 FORtran). At the time, C was not as ubiquitous as it is now; FORTRAN
16449 was. The last chapter presented a @command{ratfor} to FORTRAN
16450 processor, written in @command{ratfor}. @command{ratfor} looks an awful
16451 lot like C; if you know C, you won't have any problem following the
16454 In 1981, the book was updated and made available as @cite{Software Tools
16455 in Pascal} (Addison-Wesley, ISBN 0-201-10342-7). Both books are
16456 still in print and are well worth
16457 reading if you're a programmer. They certainly made a major change in
16458 how I view programming.
16460 The programs in both books are available from
16461 @uref{http://cm.bell-labs.com/who/bwk, Brian Kernighan's home page}.
16462 For a number of years, there was an active
16463 Software Tools Users Group, whose members had ported the original
16464 @command{ratfor} programs to essentially every computer system with a
16465 FORTRAN compiler. The popularity of the group waned in the middle 1980s
16466 as Unix began to spread beyond universities.
16468 With the current proliferation of GNU code and other clones of Unix programs,
16469 these programs now receive little attention; modern C versions are
16470 much more efficient and do more than these programs do. Nevertheless, as
16471 exposition of good programming style, and evangelism for a still-valuable
16472 philosophy, these books are unparalleled, and I recommend them highly.
16474 Acknowledgment: I would like to express my gratitude to Brian Kernighan
16475 of Bell Labs, the original Software Toolsmith, for reviewing this column.
16477 @node GNU Free Documentation License
16478 @appendix GNU Free Documentation License
16482 @node Concept index
16489 @c Local variables:
16490 @c texinfo-column-for-description: 32
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