3 @setfilename coreutils.info
4 @settitle @sc{gnu} Coreutils
9 @include constants.texi
11 @c Define new indices.
15 @c Put everything in one index (arbitrarily chosen to be the concept index).
25 * Coreutils: (coreutils). Core GNU (file, text, shell) utilities.
26 * Common options: (coreutils)Common options. Common options.
27 * File permissions: (coreutils)File permissions. Access modes.
28 * Date input formats: (coreutils)Date input formats.
31 @c FIXME: the following need documentation
32 @c * [: (coreutils)[ invocation. File/string tests.
33 @c * pinky: (coreutils)pinky invocation. FIXME.
35 @dircategory Individual utilities
37 * arch: (coreutils)arch invocation. Print machine hardware name.
38 * base64: (coreutils)base64 invocation. Base64 encode/decode data.
39 * basename: (coreutils)basename invocation. Strip directory and suffix.
40 * cat: (coreutils)cat invocation. Concatenate and write files.
41 * chcon: (coreutils)chcon invocation. Change SELinux CTX of files.
42 * chgrp: (coreutils)chgrp invocation. Change file groups.
43 * chmod: (coreutils)chmod invocation. Change file permissions.
44 * chown: (coreutils)chown invocation. Change file owners/groups.
45 * chroot: (coreutils)chroot invocation. Specify the root directory.
46 * cksum: (coreutils)cksum invocation. Print POSIX CRC checksum.
47 * comm: (coreutils)comm invocation. Compare sorted files by line.
48 * cp: (coreutils)cp invocation. Copy files.
49 * csplit: (coreutils)csplit invocation. Split by context.
50 * cut: (coreutils)cut invocation. Print selected parts of lines.
51 * date: (coreutils)date invocation. Print/set system date and time.
52 * dd: (coreutils)dd invocation. Copy and convert a file.
53 * df: (coreutils)df invocation. Report file system disk usage.
54 * dir: (coreutils)dir invocation. List directories briefly.
55 * dircolors: (coreutils)dircolors invocation. Color setup for ls.
56 * dirname: (coreutils)dirname invocation. Strip last file name component.
57 * du: (coreutils)du invocation. Report on disk usage.
58 * echo: (coreutils)echo invocation. Print a line of text.
59 * env: (coreutils)env invocation. Modify the environment.
60 * expand: (coreutils)expand invocation. Convert tabs to spaces.
61 * expr: (coreutils)expr invocation. Evaluate expressions.
62 * factor: (coreutils)factor invocation. Print prime factors
63 * false: (coreutils)false invocation. Do nothing, unsuccessfully.
64 * fmt: (coreutils)fmt invocation. Reformat paragraph text.
65 * fold: (coreutils)fold invocation. Wrap long input lines.
66 * groups: (coreutils)groups invocation. Print group names a user is in.
67 * head: (coreutils)head invocation. Output the first part of files.
68 * hostid: (coreutils)hostid invocation. Print numeric host identifier.
69 * hostname: (coreutils)hostname invocation. Print or set system name.
70 * id: (coreutils)id invocation. Print user identity.
71 * install: (coreutils)install invocation. Copy and change attributes.
72 * join: (coreutils)join invocation. Join lines on a common field.
73 * kill: (coreutils)kill invocation. Send a signal to processes.
74 * link: (coreutils)link invocation. Make hard links between files.
75 * ln: (coreutils)ln invocation. Make links between files.
76 * logname: (coreutils)logname invocation. Print current login name.
77 * ls: (coreutils)ls invocation. List directory contents.
78 * md5sum: (coreutils)md5sum invocation. Print or check MD5 digests.
79 * mkdir: (coreutils)mkdir invocation. Create directories.
80 * mkfifo: (coreutils)mkfifo invocation. Create FIFOs (named pipes).
81 * mknod: (coreutils)mknod invocation. Create special files.
82 * mktemp: (coreutils)mktemp invocation. Create temporary files.
83 * mv: (coreutils)mv invocation. Rename files.
84 * nice: (coreutils)nice invocation. Modify niceness.
85 * nl: (coreutils)nl invocation. Number lines and write files.
86 * nohup: (coreutils)nohup invocation. Immunize to hangups.
87 * nproc: (coreutils)nproc invocation. Print the number of processors.
88 * od: (coreutils)od invocation. Dump files in octal, etc.
89 * paste: (coreutils)paste invocation. Merge lines of files.
90 * pathchk: (coreutils)pathchk invocation. Check file name portability.
91 * pr: (coreutils)pr invocation. Paginate or columnate files.
92 * printenv: (coreutils)printenv invocation. Print environment variables.
93 * printf: (coreutils)printf invocation. Format and print data.
94 * ptx: (coreutils)ptx invocation. Produce permuted indexes.
95 * pwd: (coreutils)pwd invocation. Print working directory.
96 * readlink: (coreutils)readlink invocation. Print referent of a symlink.
97 * rm: (coreutils)rm invocation. Remove files.
98 * rmdir: (coreutils)rmdir invocation. Remove empty directories.
99 * runcon: (coreutils)runcon invocation. Run in specified SELinux CTX.
100 * seq: (coreutils)seq invocation. Print numeric sequences
101 * sha1sum: (coreutils)sha1sum invocation. Print or check SHA-1 digests.
102 * sha2: (coreutils)sha2 utilities. Print or check SHA-2 digests.
103 * shred: (coreutils)shred invocation. Remove files more securely.
104 * shuf: (coreutils)shuf invocation. Shuffling text files.
105 * sleep: (coreutils)sleep invocation. Delay for a specified time.
106 * sort: (coreutils)sort invocation. Sort text files.
107 * split: (coreutils)split invocation. Split into pieces.
108 * stat: (coreutils)stat invocation. Report file(system) status.
109 * stdbuf: (coreutils)stdbuf invocation. Modify stdio buffering.
110 * stty: (coreutils)stty invocation. Print/change terminal settings.
111 * su: (coreutils)su invocation. Modify user and group ID.
112 * sum: (coreutils)sum invocation. Print traditional checksum.
113 * sync: (coreutils)sync invocation. Synchronize memory and disk.
114 * tac: (coreutils)tac invocation. Reverse files.
115 * tail: (coreutils)tail invocation. Output the last part of files.
116 * tee: (coreutils)tee invocation. Redirect to multiple files.
117 * test: (coreutils)test invocation. File/string tests.
118 * timeout: (coreutils)timeout invocation. Run with time limit.
119 * touch: (coreutils)touch invocation. Change file timestamps.
120 * tr: (coreutils)tr invocation. Translate characters.
121 * true: (coreutils)true invocation. Do nothing, successfully.
122 * truncate: (coreutils)truncate invocation. Shrink/extend size of a file.
123 * tsort: (coreutils)tsort invocation. Topological sort.
124 * tty: (coreutils)tty invocation. Print terminal name.
125 * uname: (coreutils)uname invocation. Print system information.
126 * unexpand: (coreutils)unexpand invocation. Convert spaces to tabs.
127 * uniq: (coreutils)uniq invocation. Uniquify files.
128 * unlink: (coreutils)unlink invocation. Removal via unlink(2).
129 * uptime: (coreutils)uptime invocation. Print uptime and load.
130 * users: (coreutils)users invocation. Print current user names.
131 * vdir: (coreutils)vdir invocation. List directories verbosely.
132 * wc: (coreutils)wc invocation. Line, word, and byte counts.
133 * who: (coreutils)who invocation. Print who is logged in.
134 * whoami: (coreutils)whoami invocation. Print effective user ID.
135 * yes: (coreutils)yes invocation. Print a string indefinitely.
139 This manual documents version @value{VERSION} of the @sc{gnu} core
140 utilities, including the standard programs for text and file manipulation.
142 Copyright @copyright{} 1994-1996, 2000-2011 Free Software Foundation, Inc.
145 Permission is granted to copy, distribute and/or modify this document
146 under the terms of the GNU Free Documentation License, Version 1.3 or
147 any later version published by the Free Software Foundation; with no
148 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
149 Texts. A copy of the license is included in the section entitled ``GNU
150 Free Documentation License''.
155 @title @sc{gnu} @code{Coreutils}
156 @subtitle Core GNU utilities
157 @subtitle for version @value{VERSION}, @value{UPDATED}
158 @author David MacKenzie et al.
161 @vskip 0pt plus 1filll
174 @cindex core utilities
175 @cindex text utilities
176 @cindex shell utilities
177 @cindex file utilities
180 * Introduction:: Caveats, overview, and authors
181 * Common options:: Common options
182 * Output of entire files:: cat tac nl od base64
183 * Formatting file contents:: fmt pr fold
184 * Output of parts of files:: head tail split csplit
185 * Summarizing files:: wc sum cksum md5sum sha1sum sha2
186 * Operating on sorted files:: sort shuf uniq comm ptx tsort
187 * Operating on fields:: cut paste join
188 * Operating on characters:: tr expand unexpand
189 * Directory listing:: ls dir vdir dircolors
190 * Basic operations:: cp dd install mv rm shred
191 * Special file types:: mkdir rmdir unlink mkfifo mknod ln link readlink
192 * Changing file attributes:: chgrp chmod chown touch
193 * Disk usage:: df du stat sync truncate
194 * Printing text:: echo printf yes
195 * Conditions:: false true test expr
197 * File name manipulation:: dirname basename pathchk mktemp
198 * Working context:: pwd stty printenv tty
199 * User information:: id logname whoami groups users who
200 * System context:: date arch nproc uname hostname hostid uptime
201 * SELinux context:: chcon runcon
202 * Modified command invocation:: chroot env nice nohup stdbuf su timeout
203 * Process control:: kill
205 * Numeric operations:: factor seq
206 * File permissions:: Access modes
207 * Date input formats:: Specifying date strings
208 * Opening the software toolbox:: The software tools philosophy
209 * GNU Free Documentation License:: Copying and sharing this manual
210 * Concept index:: General index
213 --- The Detailed Node Listing ---
217 * Exit status:: Indicating program success or failure
218 * Backup options:: Backup options
219 * Block size:: Block size
220 * Floating point:: Floating point number representation
221 * Signal specifications:: Specifying signals
222 * Disambiguating names and IDs:: chgrp and chown owner and group syntax
223 * Random sources:: Sources of random data
224 * Target directory:: Target directory
225 * Trailing slashes:: Trailing slashes
226 * Traversing symlinks:: Traversing symlinks to directories
227 * Treating / specially:: Treating / specially
228 * Standards conformance:: Standards conformance
230 Output of entire files
232 * cat invocation:: Concatenate and write files
233 * tac invocation:: Concatenate and write files in reverse
234 * nl invocation:: Number lines and write files
235 * od invocation:: Write files in octal or other formats
236 * base64 invocation:: Transform data into printable data
238 Formatting file contents
240 * fmt invocation:: Reformat paragraph text
241 * pr invocation:: Paginate or columnate files for printing
242 * fold invocation:: Wrap input lines to fit in specified width
244 Output of parts of files
246 * head invocation:: Output the first part of files
247 * tail invocation:: Output the last part of files
248 * split invocation:: Split a file into fixed-size pieces
249 * csplit invocation:: Split a file into context-determined pieces
253 * wc invocation:: Print newline, word, and byte counts
254 * sum invocation:: Print checksum and block counts
255 * cksum invocation:: Print CRC checksum and byte counts
256 * md5sum invocation:: Print or check MD5 digests
257 * sha1sum invocation:: Print or check SHA-1 digests
258 * sha2 utilities:: Print or check SHA-2 digests
260 Operating on sorted files
262 * sort invocation:: Sort text files
263 * shuf invocation:: Shuffle text files
264 * uniq invocation:: Uniquify files
265 * comm invocation:: Compare two sorted files line by line
266 * ptx invocation:: Produce a permuted index of file contents
267 * tsort invocation:: Topological sort
269 @command{ptx}: Produce permuted indexes
271 * General options in ptx:: Options which affect general program behavior
272 * Charset selection in ptx:: Underlying character set considerations
273 * Input processing in ptx:: Input fields, contexts, and keyword selection
274 * Output formatting in ptx:: Types of output format, and sizing the fields
275 * Compatibility in ptx:: The @acronym{GNU} extensions to @command{ptx}
279 * cut invocation:: Print selected parts of lines
280 * paste invocation:: Merge lines of files
281 * join invocation:: Join lines on a common field
283 Operating on characters
285 * tr invocation:: Translate, squeeze, and/or delete characters
286 * expand invocation:: Convert tabs to spaces
287 * unexpand invocation:: Convert spaces to tabs
289 @command{tr}: Translate, squeeze, and/or delete characters
291 * Character sets:: Specifying sets of characters
292 * Translating:: Changing one set of characters to another
293 * Squeezing:: Squeezing repeats and deleting
297 * ls invocation:: List directory contents
298 * dir invocation:: Briefly list directory contents
299 * vdir invocation:: Verbosely list directory contents
300 * dircolors invocation:: Color setup for @command{ls}
302 @command{ls}: List directory contents
304 * Which files are listed:: Which files are listed
305 * What information is listed:: What information is listed
306 * Sorting the output:: Sorting the output
307 * Details about version sort:: More details about version sort
308 * General output formatting:: General output formatting
309 * Formatting the file names:: Formatting the file names
313 * cp invocation:: Copy files and directories
314 * dd invocation:: Convert and copy a file
315 * install invocation:: Copy files and set attributes
316 * mv invocation:: Move (rename) files
317 * rm invocation:: Remove files or directories
318 * shred invocation:: Remove files more securely
322 * link invocation:: Make a hard link via the link syscall
323 * ln invocation:: Make links between files
324 * mkdir invocation:: Make directories
325 * mkfifo invocation:: Make FIFOs (named pipes)
326 * mknod invocation:: Make block or character special files
327 * readlink invocation:: Print value of a symlink or canonical file name
328 * rmdir invocation:: Remove empty directories
329 * unlink invocation:: Remove files via unlink syscall
331 Changing file attributes
333 * chown invocation:: Change file owner and group
334 * chgrp invocation:: Change group ownership
335 * chmod invocation:: Change access permissions
336 * touch invocation:: Change file timestamps
340 * df invocation:: Report file system disk space usage
341 * du invocation:: Estimate file space usage
342 * stat invocation:: Report file or file system status
343 * sync invocation:: Synchronize data on disk with memory
344 * truncate invocation:: Shrink or extend the size of a file
348 * echo invocation:: Print a line of text
349 * printf invocation:: Format and print data
350 * yes invocation:: Print a string until interrupted
354 * false invocation:: Do nothing, unsuccessfully
355 * true invocation:: Do nothing, successfully
356 * test invocation:: Check file types and compare values
357 * expr invocation:: Evaluate expressions
359 @command{test}: Check file types and compare values
361 * File type tests:: File type tests
362 * Access permission tests:: Access permission tests
363 * File characteristic tests:: File characteristic tests
364 * String tests:: String tests
365 * Numeric tests:: Numeric tests
367 @command{expr}: Evaluate expression
369 * String expressions:: + : match substr index length
370 * Numeric expressions:: + - * / %
371 * Relations for expr:: | & < <= = == != >= >
372 * Examples of expr:: Examples of using @command{expr}
376 * tee invocation:: Redirect output to multiple files or processes
378 File name manipulation
380 * basename invocation:: Strip directory and suffix from a file name
381 * dirname invocation:: Strip last file name component
382 * pathchk invocation:: Check file name validity and portability
383 * mktemp invocation:: Create temporary file or directory
387 * pwd invocation:: Print working directory
388 * stty invocation:: Print or change terminal characteristics
389 * printenv invocation:: Print all or some environment variables
390 * tty invocation:: Print file name of terminal on standard input
392 @command{stty}: Print or change terminal characteristics
394 * Control:: Control settings
395 * Input:: Input settings
396 * Output:: Output settings
397 * Local:: Local settings
398 * Combination:: Combination settings
399 * Characters:: Special characters
400 * Special:: Special settings
404 * id invocation:: Print user identity
405 * logname invocation:: Print current login name
406 * whoami invocation:: Print effective user ID
407 * groups invocation:: Print group names a user is in
408 * users invocation:: Print login names of users currently logged in
409 * who invocation:: Print who is currently logged in
413 * arch invocation:: Print machine hardware name
414 * date invocation:: Print or set system date and time
415 * nproc invocation:: Print the number of processors
416 * uname invocation:: Print system information
417 * hostname invocation:: Print or set system name
418 * hostid invocation:: Print numeric host identifier
419 * uptime invocation:: Print system uptime and load
421 @command{date}: Print or set system date and time
423 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
424 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
425 * Literal conversion specifiers:: %[%nt]
426 * Padding and other flags:: Pad with zeros, spaces, etc.
427 * Setting the time:: Changing the system clock
428 * Options for date:: Instead of the current time
429 * Date input formats:: Specifying date strings
430 * Examples of date:: Examples
434 * chcon invocation:: Change SELinux context of file
435 * runcon invocation:: Run a command in specified SELinux context
437 Modified command invocation
439 * chroot invocation:: Run a command with a different root directory
440 * env invocation:: Run a command in a modified environment
441 * nice invocation:: Run a command with modified niceness
442 * nohup invocation:: Run a command immune to hangups
443 * stdbuf invocation:: Run a command with modified I/O buffering
444 * su invocation:: Run a command with substitute user and group ID
445 * timeout invocation:: Run a command with a time limit
449 * kill invocation:: Sending a signal to processes.
453 * sleep invocation:: Delay for a specified time
457 * factor invocation:: Print prime factors
458 * seq invocation:: Print numeric sequences
462 * Mode Structure:: Structure of file mode bits
463 * Symbolic Modes:: Mnemonic representation of file mode bits
464 * Numeric Modes:: File mode bits as octal numbers
465 * Directory Setuid and Setgid:: Set-user-ID and set-group-ID on directories
469 * General date syntax:: Common rules
470 * Calendar date items:: 19 Dec 1994
471 * Time of day items:: 9:20pm
472 * Time zone items:: @sc{est}, @sc{pdt}, @sc{gmt}
473 * Day of week items:: Monday and others
474 * Relative items in date strings:: next tuesday, 2 years ago
475 * Pure numbers in date strings:: 19931219, 1440
476 * Seconds since the Epoch:: @@1078100502
477 * Specifying time zone rules:: TZ="America/New_York", TZ="UTC0"
478 * Authors of parse_datetime:: Bellovin, Eggert, Salz, Berets, et al
480 Opening the software toolbox
482 * Toolbox introduction:: Toolbox introduction
483 * I/O redirection:: I/O redirection
484 * The who command:: The @command{who} command
485 * The cut command:: The @command{cut} command
486 * The sort command:: The @command{sort} command
487 * The uniq command:: The @command{uniq} command
488 * Putting the tools together:: Putting the tools together
492 * GNU Free Documentation License:: Copying and sharing this manual
499 @chapter Introduction
501 This manual is a work in progress: many sections make no attempt to explain
502 basic concepts in a way suitable for novices. Thus, if you are interested,
503 please get involved in improving this manual. The entire @sc{gnu} community
506 @cindex @acronym{POSIX}
507 The @sc{gnu} utilities documented here are mostly compatible with the
508 @acronym{POSIX} standard.
509 @cindex bugs, reporting
510 Please report bugs to @email{bug-coreutils@@gnu.org}. Remember
511 to include the version number, machine architecture, input files, and
512 any other information needed to reproduce the bug: your input, what you
513 expected, what you got, and why it is wrong. Diffs are welcome, but
514 please include a description of the problem as well, since this is
515 sometimes difficult to infer. @xref{Bugs, , , gcc, Using and Porting GNU CC}.
521 @cindex MacKenzie, D.
524 This manual was originally derived from the Unix man pages in the
525 distributions, which were written by David MacKenzie and updated by Jim
526 Meyering. What you are reading now is the authoritative documentation
527 for these utilities; the man pages are no longer being maintained. The
528 original @command{fmt} man page was written by Ross Paterson. Fran@,{c}ois
529 Pinard did the initial conversion to Texinfo format. Karl Berry did the
530 indexing, some reorganization, and editing of the results. Brian
531 Youmans of the Free Software Foundation office staff combined the
532 manuals for textutils, fileutils, and sh-utils to produce the present
533 omnibus manual. Richard Stallman contributed his usual invaluable
534 insights to the overall process.
537 @chapter Common options
541 @itemx @w{@kbd{--backup}[=@var{method}]}
544 @vindex VERSION_CONTROL
545 @cindex backups, making
546 @xref{Backup options}.
547 Make a backup of each file that would otherwise be overwritten or removed.
550 @macro optBackupSuffix
551 @item -S @var{suffix}
552 @itemx --suffix=@var{suffix}
555 Append @var{suffix} to each backup file made with @option{-b}.
556 @xref{Backup options}.
559 @macro optTargetDirectory
560 @item -t @var{directory}
561 @itemx @w{@kbd{--target-directory}=@var{directory}}
563 @opindex --target-directory
564 @cindex target directory
565 @cindex destination directory
566 Specify the destination @var{directory}.
567 @xref{Target directory}.
570 @macro optNoTargetDirectory
572 @itemx --no-target-directory
574 @opindex --no-target-directory
575 @cindex target directory
576 @cindex destination directory
577 Do not treat the last operand specially when it is a directory or a
578 symbolic link to a directory. @xref{Target directory}.
586 @cindex output @sc{nul}-byte-terminated lines
587 Output a zero byte (@acronym{ASCII} @sc{nul}) at the end of each line,
588 rather than a newline. This option enables other programs to parse the
589 output of @command{\cmd\} even when that output would contain data
590 with embedded newlines.
597 Append an SI-style abbreviation to each size, such as @samp{M} for
598 megabytes. Powers of 1000 are used, not 1024; @samp{M} stands for
599 1,000,000 bytes. This option is equivalent to
600 @option{--block-size=si}. Use the @option{-h} or
601 @option{--human-readable} option if
602 you prefer powers of 1024.
605 @macro optHumanReadable
607 @itemx --human-readable
609 @opindex --human-readable
610 @cindex human-readable output
611 Append a size letter to each size, such as @samp{M} for mebibytes.
612 Powers of 1024 are used, not 1000; @samp{M} stands for 1,048,576 bytes.
613 This option is equivalent to @option{--block-size=human-readable}.
614 Use the @option{--si} option if you prefer powers of 1000.
617 @macro optStripTrailingSlashes
618 @itemx @w{@kbd{--strip-trailing-slashes}}
619 @opindex --strip-trailing-slashes
620 @cindex stripping trailing slashes
621 Remove any trailing slashes from each @var{source} argument.
622 @xref{Trailing slashes}.
625 @macro mayConflictWithShellBuiltIn{cmd}
626 @cindex conflicts with shell built-ins
627 @cindex built-in shell commands, conflicts with
628 Due to shell aliases and built-in @command{\cmd\} functions, using an
629 unadorned @command{\cmd\} interactively or in a script may get you
630 different functionality than that described here. Invoke it via
631 @command{env} (i.e., @code{env \cmd\ @dots{}}) to avoid interference
636 @macro multiplierSuffixes{varName}
637 @var{\varName\} may be, or may be an integer optionally followed by,
638 one of the following multiplicative suffixes:
640 @samp{b} => 512 ("blocks")
641 @samp{KB} => 1000 (KiloBytes)
642 @samp{K} => 1024 (KibiBytes)
643 @samp{MB} => 1000*1000 (MegaBytes)
644 @samp{M} => 1024*1024 (MebiBytes)
645 @samp{GB} => 1000*1000*1000 (GigaBytes)
646 @samp{G} => 1024*1024*1024 (GibiBytes)
648 and so on for @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}.
651 @c FIXME: same as above, but no ``blocks'' line.
652 @macro multiplierSuffixesNoBlocks{varName}
653 @var{\varName\} may be, or may be an integer optionally followed by,
654 one of the following multiplicative suffixes:
656 @samp{KB} => 1000 (KiloBytes)
657 @samp{K} => 1024 (KibiBytes)
658 @samp{MB} => 1000*1000 (MegaBytes)
659 @samp{M} => 1024*1024 (MebiBytes)
660 @samp{GB} => 1000*1000*1000 (GigaBytes)
661 @samp{G} => 1024*1024*1024 (GibiBytes)
663 and so on for @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}.
666 @cindex common options
668 Certain options are available in all of these programs. Rather than
669 writing identical descriptions for each of the programs, they are
670 described here. (In fact, every @sc{gnu} program accepts (or should accept)
673 @vindex POSIXLY_CORRECT
674 Normally options and operands can appear in any order, and programs act
675 as if all the options appear before any operands. For example,
676 @samp{sort -r passwd -t :} acts like @samp{sort -r -t : passwd}, since
677 @samp{:} is an option-argument of @option{-t}. However, if the
678 @env{POSIXLY_CORRECT} environment variable is set, options must appear
679 before operands, unless otherwise specified for a particular command.
681 A few programs can usefully have trailing operands with leading
682 @samp{-}. With such a program, options must precede operands even if
683 @env{POSIXLY_CORRECT} is not set, and this fact is noted in the
684 program description. For example, the @command{env} command's options
685 must appear before its operands, since in some cases the operands
686 specify a command that itself contains options.
688 Most programs that accept long options recognize unambiguous
689 abbreviations of those options. For example, @samp{rmdir
690 --ignore-fail-on-non-empty} can be invoked as @samp{rmdir
691 --ignore-fail} or even @samp{rmdir --i}. Ambiguous options, such as
692 @samp{ls --h}, are identified as such.
694 Some of these programs recognize the @option{--help} and @option{--version}
695 options only when one of them is the sole command line argument. For
696 these programs, abbreviations of the long options are not always recognized.
703 Print a usage message listing all available options, then exit successfully.
707 @cindex version number, finding
708 Print the version number, then exit successfully.
712 @cindex option delimiter
713 Delimit the option list. Later arguments, if any, are treated as
714 operands even if they begin with @samp{-}. For example, @samp{sort --
715 -r} reads from the file named @file{-r}.
719 @cindex standard input
720 @cindex standard output
721 A single @samp{-} operand is not really an option, though it looks like one. It
722 stands for standard input, or for standard output if that is clear from
723 the context. For example, @samp{sort -} reads from standard input,
724 and is equivalent to plain @samp{sort}, and @samp{tee -} writes an
725 extra copy of its input to standard output. Unless otherwise
726 specified, @samp{-} can appear as any operand that requires a file
730 * Exit status:: Indicating program success or failure.
731 * Backup options:: -b -S, in some programs.
732 * Block size:: BLOCK_SIZE and --block-size, in some programs.
733 * Floating point:: Floating point number representation.
734 * Signal specifications:: Specifying signals using the --signal option.
735 * Disambiguating names and IDs:: chgrp and chown owner and group syntax
736 * Random sources:: --random-source, in some programs.
737 * Target directory:: Specifying a target directory, in some programs.
738 * Trailing slashes:: --strip-trailing-slashes, in some programs.
739 * Traversing symlinks:: -H, -L, or -P, in some programs.
740 * Treating / specially:: --preserve-root and --no-preserve-root.
741 * Special built-in utilities:: @command{break}, @command{:}, @dots{}
742 * Standards conformance:: Conformance to the @acronym{POSIX} standard.
750 An exit status of zero indicates success,
751 and a nonzero value indicates failure.
754 Nearly every command invocation yields an integral @dfn{exit status}
755 that can be used to change how other commands work.
756 For the vast majority of commands, an exit status of zero indicates
757 success. Failure is indicated by a nonzero value---typically
758 @samp{1}, though it may differ on unusual platforms as @acronym{POSIX}
759 requires only that it be nonzero.
761 However, some of the programs documented here do produce
762 other exit status values and a few associate different
763 meanings with the values @samp{0} and @samp{1}.
764 Here are some of the exceptions:
765 @command{chroot}, @command{env}, @command{expr}, @command{nice},
766 @command{nohup}, @command{printenv}, @command{sort}, @command{stdbuf},
767 @command{su}, @command{test}, @command{timeout}, @command{tty}.
771 @section Backup options
773 @cindex backup options
775 Some @sc{gnu} programs (at least @command{cp}, @command{install},
776 @command{ln}, and @command{mv}) optionally make backups of files
777 before writing new versions.
778 These options control the details of these backups. The options are also
779 briefly mentioned in the descriptions of the particular programs.
784 @itemx @w{@kbd{--backup}[=@var{method}]}
787 @vindex VERSION_CONTROL
788 @cindex backups, making
789 Make a backup of each file that would otherwise be overwritten or removed.
790 Without this option, the original versions are destroyed.
791 Use @var{method} to determine the type of backups to make.
792 When this option is used but @var{method} is not specified,
793 then the value of the @env{VERSION_CONTROL}
794 environment variable is used. And if @env{VERSION_CONTROL} is not set,
795 the default backup type is @samp{existing}.
797 Note that the short form of this option, @option{-b} does not accept any
798 argument. Using @option{-b} is equivalent to using @option{--backup=existing}.
800 @vindex version-control @r{Emacs variable}
801 This option corresponds to the Emacs variable @samp{version-control};
802 the values for @var{method} are the same as those used in Emacs.
803 This option also accepts more descriptive names.
804 The valid @var{method}s are (unique abbreviations are accepted):
809 @opindex none @r{backup method}
814 @opindex numbered @r{backup method}
815 Always make numbered backups.
819 @opindex existing @r{backup method}
820 Make numbered backups of files that already have them, simple backups
825 @opindex simple @r{backup method}
826 Always make simple backups. Please note @samp{never} is not to be
827 confused with @samp{none}.
831 @item -S @var{suffix}
832 @itemx --suffix=@var{suffix}
835 @cindex backup suffix
836 @vindex SIMPLE_BACKUP_SUFFIX
837 Append @var{suffix} to each backup file made with @option{-b}. If this
838 option is not specified, the value of the @env{SIMPLE_BACKUP_SUFFIX}
839 environment variable is used. And if @env{SIMPLE_BACKUP_SUFFIX} is not
840 set, the default is @samp{~}, just as in Emacs.
849 Some @sc{gnu} programs (at least @command{df}, @command{du}, and
850 @command{ls}) display sizes in ``blocks''. You can adjust the block size
851 and method of display to make sizes easier to read. The block size
852 used for display is independent of any file system block size.
853 Fractional block counts are rounded up to the nearest integer.
855 @opindex --block-size=@var{size}
858 @vindex DF_BLOCK_SIZE
859 @vindex DU_BLOCK_SIZE
860 @vindex LS_BLOCK_SIZE
861 @vindex POSIXLY_CORRECT@r{, and block size}
863 The default block size is chosen by examining the following environment
864 variables in turn; the first one that is set determines the block size.
869 This specifies the default block size for the @command{df} command.
870 Similarly, @env{DU_BLOCK_SIZE} specifies the default for @command{du} and
871 @env{LS_BLOCK_SIZE} for @command{ls}.
874 This specifies the default block size for all three commands, if the
875 above command-specific environment variables are not set.
878 This specifies the default block size for all values that are normally
879 printed as blocks, if neither @env{BLOCK_SIZE} nor the above
880 command-specific environment variables are set. Unlike the other
881 environment variables, @env{BLOCKSIZE} does not affect values that are
882 normally printed as byte counts, e.g., the file sizes contained in
885 @item POSIXLY_CORRECT
886 If neither @env{@var{command}_BLOCK_SIZE}, nor @env{BLOCK_SIZE}, nor
887 @env{BLOCKSIZE} is set, but this variable is set, the block size
892 If none of the above environment variables are set, the block size
893 currently defaults to 1024 bytes in most contexts, but this number may
894 change in the future. For @command{ls} file sizes, the block size
897 @cindex human-readable output
900 A block size specification can be a positive integer specifying the number
901 of bytes per block, or it can be @code{human-readable} or @code{si} to
902 select a human-readable format. Integers may be followed by suffixes
903 that are upward compatible with the
904 @uref{http://www.bipm.fr/enus/3_SI/si-prefixes.html, SI prefixes}
905 for decimal multiples and with the
906 @uref{http://physics.nist.gov/cuu/Units/binary.html, IEC 60027-2
907 prefixes for binary multiples}.
909 With human-readable formats, output sizes are followed by a size letter
910 such as @samp{M} for megabytes. @code{BLOCK_SIZE=human-readable} uses
911 powers of 1024; @samp{M} stands for 1,048,576 bytes.
912 @code{BLOCK_SIZE=si} is similar, but uses powers of 1000 and appends
913 @samp{B}; @samp{MB} stands for 1,000,000 bytes.
916 A block size specification preceded by @samp{'} causes output sizes to
917 be displayed with thousands separators. The @env{LC_NUMERIC} locale
918 specifies the thousands separator and grouping. For example, in an
919 American English locale, @samp{--block-size="'1kB"} would cause a size
920 of 1234000 bytes to be displayed as @samp{1,234}. In the default C
921 locale, there is no thousands separator so a leading @samp{'} has no
924 An integer block size can be followed by a suffix to specify a
925 multiple of that size. A bare size letter,
926 or one followed by @samp{iB}, specifies
927 a multiple using powers of 1024. A size letter followed by @samp{B}
928 specifies powers of 1000 instead. For example, @samp{1M} and
929 @samp{1MiB} are equivalent to @samp{1048576}, whereas @samp{1MB} is
930 equivalent to @samp{1000000}.
932 A plain suffix without a preceding integer acts as if @samp{1} were
933 prepended, except that it causes a size indication to be appended to
934 the output. For example, @samp{--block-size="kB"} displays 3000 as
937 The following suffixes are defined. Large sizes like @code{1Y}
938 may be rejected by your computer due to limitations of its arithmetic.
942 @cindex kilobyte, definition of
943 kilobyte: @math{10^3 = 1000}.
947 @cindex kibibyte, definition of
948 kibibyte: @math{2^{10} = 1024}. @samp{K} is special: the SI prefix is
949 @samp{k} and the IEC 60027-2 prefix is @samp{Ki}, but tradition and
950 @acronym{POSIX} use @samp{k} to mean @samp{KiB}.
952 @cindex megabyte, definition of
953 megabyte: @math{10^6 = 1,000,000}.
956 @cindex mebibyte, definition of
957 mebibyte: @math{2^{20} = 1,048,576}.
959 @cindex gigabyte, definition of
960 gigabyte: @math{10^9 = 1,000,000,000}.
963 @cindex gibibyte, definition of
964 gibibyte: @math{2^{30} = 1,073,741,824}.
966 @cindex terabyte, definition of
967 terabyte: @math{10^{12} = 1,000,000,000,000}.
970 @cindex tebibyte, definition of
971 tebibyte: @math{2^{40} = 1,099,511,627,776}.
973 @cindex petabyte, definition of
974 petabyte: @math{10^{15} = 1,000,000,000,000,000}.
977 @cindex pebibyte, definition of
978 pebibyte: @math{2^{50} = 1,125,899,906,842,624}.
980 @cindex exabyte, definition of
981 exabyte: @math{10^{18} = 1,000,000,000,000,000,000}.
984 @cindex exbibyte, definition of
985 exbibyte: @math{2^{60} = 1,152,921,504,606,846,976}.
987 @cindex zettabyte, definition of
988 zettabyte: @math{10^{21} = 1,000,000,000,000,000,000,000}
991 @math{2^{70} = 1,180,591,620,717,411,303,424}.
992 (@samp{Zi} is a @acronym{GNU} extension to IEC 60027-2.)
994 @cindex yottabyte, definition of
995 yottabyte: @math{10^{24} = 1,000,000,000,000,000,000,000,000}.
998 @math{2^{80} = 1,208,925,819,614,629,174,706,176}.
999 (@samp{Yi} is a @acronym{GNU} extension to IEC 60027-2.)
1004 @opindex --block-size
1005 @opindex --human-readable
1008 Block size defaults can be overridden by an explicit
1009 @option{--block-size=@var{size}} option. The @option{-k}
1010 option is equivalent to @option{--block-size=1K}, which
1011 is the default unless the @env{POSIXLY_CORRECT} environment variable is
1012 set. The @option{-h} or @option{--human-readable} option is equivalent to
1013 @option{--block-size=human-readable}. The @option{--si} option is
1014 equivalent to @option{--block-size=si}.
1016 @node Floating point
1017 @section Floating point numbers
1018 @cindex floating point
1019 @cindex IEEE floating point
1021 Commands that accept or produce floating point numbers employ the
1022 floating point representation of the underlying system, and suffer
1023 from rounding error, overflow, and similar floating-point issues.
1024 Almost all modern systems use IEEE-754 floating point, and it is
1025 typically portable to assume IEEE-754 behavior these days. IEEE-754
1026 has positive and negative infinity, distinguishes positive from
1027 negative zero, and uses special values called NaNs to represent
1028 invalid computations such as dividing zero by itself. For more
1029 information, please see David Goldberg's paper
1030 @uref{http://@/www.validlab.com/@/goldberg/@/paper.pdf, What Every
1031 Computer Scientist Should Know About Floating-Point Arithmetic}.
1034 Commands that accept floating point numbers as options, operands or
1035 input use the standard C functions @code{strtod} and @code{strtold} to
1036 convert from text to floating point numbers. These floating point
1037 numbers therefore can use scientific notation like @code{1.0e-34} and
1038 @code{-10e100}. Modern C implementations also accept hexadecimal
1039 floating point numbers such as @code{-0x.ep-3}, which stands for
1040 @minus{}14/16 times @math{2^-3}, which equals @minus{}0.109375. The
1041 @env{LC_NUMERIC} locale determines the decimal-point character.
1042 @xref{Parsing of Floats,,, libc, The GNU C Library Reference Manual}.
1044 @node Signal specifications
1045 @section Signal specifications
1046 @cindex signals, specifying
1048 A @var{signal} may be a signal name like @samp{HUP}, or a signal
1049 number like @samp{1}, or an exit status of a process terminated by the
1050 signal. A signal name can be given in canonical form or prefixed by
1051 @samp{SIG}. The case of the letters is ignored. The following signal names
1052 and numbers are supported on all @acronym{POSIX} compliant systems:
1058 2. Terminal interrupt.
1064 9. Kill (cannot be caught or ignored).
1072 Other supported signal names have system-dependent corresponding
1073 numbers. All systems conforming to @acronym{POSIX} 1003.1-2001 also
1074 support the following signals:
1078 Access to an undefined portion of a memory object.
1080 Child process terminated, stopped, or continued.
1082 Continue executing, if stopped.
1084 Erroneous arithmetic operation.
1086 Illegal Instruction.
1088 Write on a pipe with no one to read it.
1090 Invalid memory reference.
1092 Stop executing (cannot be caught or ignored).
1096 Background process attempting read.
1098 Background process attempting write.
1100 High bandwidth data is available at a socket.
1102 User-defined signal 1.
1104 User-defined signal 2.
1108 @acronym{POSIX} 1003.1-2001 systems that support the @acronym{XSI} extension
1109 also support the following signals:
1115 Profiling timer expired.
1119 Trace/breakpoint trap.
1121 Virtual timer expired.
1123 CPU time limit exceeded.
1125 File size limit exceeded.
1129 @acronym{POSIX} 1003.1-2001 systems that support the @acronym{XRT} extension
1130 also support at least eight real-time signals called @samp{RTMIN},
1131 @samp{RTMIN+1}, @dots{}, @samp{RTMAX-1}, @samp{RTMAX}.
1133 @node Disambiguating names and IDs
1134 @section chown and chgrp: Disambiguating user names and IDs
1135 @cindex user names, disambiguating
1136 @cindex user IDs, disambiguating
1137 @cindex group names, disambiguating
1138 @cindex group IDs, disambiguating
1139 @cindex disambiguating group names and IDs
1141 Since the @var{owner} and @var{group} arguments to @command{chown} and
1142 @command{chgrp} may be specified as names or numeric IDs, there is an
1144 What if a user or group @emph{name} is a string of digits?
1145 @footnote{Using a number as a user name is common in some environments.}
1146 Should the command interpret it as a user name or as an ID?
1147 @acronym{POSIX} requires that @command{chown} and @command{chgrp}
1148 first attempt to resolve the specified string as a name, and
1149 only once that fails, then try to interpret it as an ID.
1150 This is troublesome when you want to specify a numeric ID, say 42,
1151 and it must work even in a pathological situation where
1152 @samp{42} is a user name that maps to some other user ID, say 1000.
1153 Simply invoking @code{chown 42 F}, will set @file{F}s owner ID to
1154 1000---not what you intended.
1156 GNU @command{chown} and @command{chgrp} provide a way to work around this,
1157 that at the same time may result in a significant performance improvement
1158 by eliminating a database look-up.
1159 Simply precede each numeric user ID and/or group ID with a @samp{+},
1160 in order to force its interpretation as an integer:
1164 chgrp +$numeric_group_id another-file
1168 GNU @command{chown} and @command{chgrp}
1169 skip the name look-up process for each @samp{+}-prefixed string,
1170 because a string containing @samp{+} is never a valid user or group name.
1171 This syntax is accepted on most common Unix systems, but not on Solaris 10.
1173 @node Random sources
1174 @section Sources of random data
1176 @cindex random sources
1178 The @command{shuf}, @command{shred}, and @command{sort} commands
1179 sometimes need random data to do their work. For example, @samp{sort
1180 -R} must choose a hash function at random, and it needs random data to
1181 make this selection.
1183 By default these commands use an internal pseudorandom generator
1184 initialized by a small amount of entropy, but can be directed to use
1185 an external source with the @option{--random-source=@var{file}} option.
1186 An error is reported if @var{file} does not contain enough bytes.
1188 For example, the device file @file{/dev/urandom} could be used as the
1189 source of random data. Typically, this device gathers environmental
1190 noise from device drivers and other sources into an entropy pool, and
1191 uses the pool to generate random bits. If the pool is short of data,
1192 the device reuses the internal pool to produce more bits, using a
1193 cryptographically secure pseudorandom number generator. But be aware
1194 that this device is not designed for bulk random data generation
1195 and is relatively slow.
1197 @file{/dev/urandom} suffices for most practical uses, but applications
1198 requiring high-value or long-term protection of private data may
1199 require an alternate data source like @file{/dev/random} or
1200 @file{/dev/arandom}. The set of available sources depends on your
1203 To reproduce the results of an earlier invocation of a command, you
1204 can save some random data into a file and then use that file as the
1205 random source in earlier and later invocations of the command.
1207 @node Target directory
1208 @section Target directory
1210 @cindex target directory
1212 The @command{cp}, @command{install}, @command{ln}, and @command{mv}
1213 commands normally treat the last operand specially when it is a
1214 directory or a symbolic link to a directory. For example, @samp{cp
1215 source dest} is equivalent to @samp{cp source dest/source} if
1216 @file{dest} is a directory. Sometimes this behavior is not exactly
1217 what is wanted, so these commands support the following options to
1218 allow more fine-grained control:
1223 @itemx --no-target-directory
1224 @opindex --no-target-directory
1225 @cindex target directory
1226 @cindex destination directory
1227 Do not treat the last operand specially when it is a directory or a
1228 symbolic link to a directory. This can help avoid race conditions in
1229 programs that operate in a shared area. For example, when the command
1230 @samp{mv /tmp/source /tmp/dest} succeeds, there is no guarantee that
1231 @file{/tmp/source} was renamed to @file{/tmp/dest}: it could have been
1232 renamed to @file{/tmp/dest/source} instead, if some other process
1233 created @file{/tmp/dest} as a directory. However, if @file{mv
1234 -T /tmp/source /tmp/dest} succeeds, there is no
1235 question that @file{/tmp/source} was renamed to @file{/tmp/dest}.
1237 In the opposite situation, where you want the last operand to be
1238 treated as a directory and want a diagnostic otherwise, you can use
1239 the @option{--target-directory} (@option{-t}) option.
1241 @item -t @var{directory}
1242 @itemx @w{@kbd{--target-directory}=@var{directory}}
1243 @opindex --target-directory
1244 @cindex target directory
1245 @cindex destination directory
1246 Use @var{directory} as the directory component of each destination
1249 The interface for most programs is that after processing options and a
1250 finite (possibly zero) number of fixed-position arguments, the remaining
1251 argument list is either expected to be empty, or is a list of items
1252 (usually files) that will all be handled identically. The @command{xargs}
1253 program is designed to work well with this convention.
1255 The commands in the @command{mv}-family are unusual in that they take
1256 a variable number of arguments with a special case at the @emph{end}
1257 (namely, the target directory). This makes it nontrivial to perform some
1258 operations, e.g., ``move all files from here to ../d/'', because
1259 @code{mv * ../d/} might exhaust the argument space, and @code{ls | xargs ...}
1260 doesn't have a clean way to specify an extra final argument for each
1261 invocation of the subject command. (It can be done by going through a
1262 shell command, but that requires more human labor and brain power than
1265 The @w{@kbd{--target-directory}} (@option{-t}) option allows the @command{cp},
1266 @command{install}, @command{ln}, and @command{mv} programs to be used
1267 conveniently with @command{xargs}. For example, you can move the files
1268 from the current directory to a sibling directory, @code{d} like this:
1271 ls | xargs mv -t ../d --
1274 However, this doesn't move files whose names begin with @samp{.}.
1275 If you use the @sc{gnu} @command{find} program, you can move those
1276 files too, with this command:
1279 find . -mindepth 1 -maxdepth 1 \
1283 But both of the above approaches fail if there are no files in the
1284 current directory, or if any file has a name containing a blank or
1285 some other special characters.
1286 The following example removes those limitations and requires both
1287 @sc{gnu} @command{find} and @sc{gnu} @command{xargs}:
1290 find . -mindepth 1 -maxdepth 1 -print0 \
1291 | xargs --null --no-run-if-empty \
1298 The @option{--target-directory} (@option{-t}) and
1299 @option{--no-target-directory} (@option{-T})
1300 options cannot be combined.
1302 @node Trailing slashes
1303 @section Trailing slashes
1305 @cindex trailing slashes
1307 Some @sc{gnu} programs (at least @command{cp} and @command{mv}) allow you to
1308 remove any trailing slashes from each @var{source} argument before
1309 operating on it. The @w{@kbd{--strip-trailing-slashes}} option enables
1312 This is useful when a @var{source} argument may have a trailing slash and
1313 @c FIXME: mv's behavior in this case is system-dependent
1314 specify a symbolic link to a directory. This scenario is in fact rather
1315 common because some shells can automatically append a trailing slash when
1316 performing file name completion on such symbolic links. Without this
1317 option, @command{mv}, for example, (via the system's rename function) must
1318 interpret a trailing slash as a request to dereference the symbolic link
1319 and so must rename the indirectly referenced @emph{directory} and not
1320 the symbolic link. Although it may seem surprising that such behavior
1321 be the default, it is required by @acronym{POSIX} and is consistent with
1322 other parts of that standard.
1324 @node Traversing symlinks
1325 @section Traversing symlinks
1327 @cindex symbolic link to directory, controlling traversal of
1329 The following options modify how @command{chown} and @command{chgrp}
1330 @c FIXME: note that `du' has these options, too, but they have slightly
1331 @c different meaning.
1332 traverse a hierarchy when the @option{--recursive} (@option{-R})
1333 option is also specified.
1334 If more than one of the following options is specified, only the final
1336 These options specify whether processing a symbolic link to a directory
1337 entails operating on just the symbolic link or on all files in the
1338 hierarchy rooted at that directory.
1340 These options are independent of @option{--dereference} and
1341 @option{--no-dereference} (@option{-h}), which control whether to modify
1342 a symlink or its referent.
1349 @cindex symbolic link to directory, traverse if on the command line
1350 If @option{--recursive} (@option{-R}) is specified and
1351 a command line argument is a symbolic link to a directory, traverse it.
1358 @cindex symbolic link to directory, traverse each that is encountered
1359 In a recursive traversal, traverse every symbolic link to a directory
1360 that is encountered.
1367 @cindex symbolic link to directory, never traverse
1368 Do not traverse any symbolic links.
1369 This is the default if none of @option{-H}, @option{-L},
1370 or @option{-P} is specified.
1377 @node Treating / specially
1378 @section Treating @file{/} specially
1380 Certain commands can operate destructively on entire hierarchies.
1381 For example, if a user with appropriate privileges mistakenly runs
1382 @samp{rm -rf / tmp/junk}, that may remove
1383 all files on the entire system. Since there are so few
1384 legitimate uses for such a command,
1385 @sc{gnu} @command{rm} normally declines to operate on any directory
1386 that resolves to @file{/}. If you really want to try to remove all
1387 the files on your system, you can use the @option{--no-preserve-root}
1388 option, but the default behavior, specified by the
1389 @option{--preserve-option}, is safer for most purposes.
1391 The commands @command{chgrp}, @command{chmod} and @command{chown}
1392 can also operate destructively on entire hierarchies, so they too
1393 support these options. Although, unlike @command{rm}, they don't
1394 actually unlink files, these commands are arguably more dangerous
1395 when operating recursively on @file{/}, since they often work much
1396 more quickly, and hence damage more files before an alert user can
1397 interrupt them. Tradition and @acronym{POSIX} require these commands
1398 to operate recursively on @file{/}, so they default to
1399 @option{--no-preserve-root}, but using the @option{--preserve-root}
1400 option makes them safer for most purposes. For convenience you can
1401 specify @option{--preserve-root} in an alias or in a shell function.
1403 Note that the @option{--preserve-root} option also ensures
1404 that @command{chgrp} and @command{chown} do not modify @file{/}
1405 even when dereferencing a symlink pointing to @file{/}.
1407 @node Special built-in utilities
1408 @section Special built-in utilities
1410 Some programs like @command{nice} can invoke other programs; for
1411 example, the command @samp{nice cat file} invokes the program
1412 @command{cat} by executing the command @samp{cat file}. However,
1413 @dfn{special built-in utilities} like @command{exit} cannot be invoked
1414 this way. For example, the command @samp{nice exit} does not have a
1415 well-defined behavior: it may generate an error message instead of
1418 Here is a list of the special built-in utilities that are standardized
1419 by @acronym{POSIX} 1003.1-2004.
1422 @t{.@: : break continue eval exec exit export readonly
1423 return set shift times trap unset}
1426 For example, because @samp{.}, @samp{:}, and @samp{exec} are special,
1427 the commands @samp{nice . foo.sh}, @samp{nice :}, and @samp{nice exec
1428 pwd} do not work as you might expect.
1430 Many shells extend this list. For example, Bash has several extra
1431 special built-in utilities like @command{history}, and
1432 @command{suspend}, and with Bash the command @samp{nice suspend}
1433 generates an error message instead of suspending.
1435 @node Standards conformance
1436 @section Standards conformance
1438 @vindex POSIXLY_CORRECT
1439 In a few cases, the @sc{gnu} utilities' default behavior is
1440 incompatible with the @acronym{POSIX} standard. To suppress these
1441 incompatibilities, define the @env{POSIXLY_CORRECT} environment
1442 variable. Unless you are checking for @acronym{POSIX} conformance, you
1443 probably do not need to define @env{POSIXLY_CORRECT}.
1445 Newer versions of @acronym{POSIX} are occasionally incompatible with older
1446 versions. For example, older versions of @acronym{POSIX} required the
1447 command @samp{sort +1} to sort based on the second and succeeding
1448 fields in each input line, but starting with @acronym{POSIX} 1003.1-2001
1449 the same command is required to sort the file named @file{+1}, and you
1450 must instead use the command @samp{sort -k 2} to get the field-based
1453 @vindex _POSIX2_VERSION
1454 The @sc{gnu} utilities normally conform to the version of @acronym{POSIX}
1455 that is standard for your system. To cause them to conform to a
1456 different version of @acronym{POSIX}, define the @env{_POSIX2_VERSION}
1457 environment variable to a value of the form @var{yyyymm} specifying
1458 the year and month the standard was adopted. Three values are currently
1459 supported for @env{_POSIX2_VERSION}: @samp{199209} stands for
1460 @acronym{POSIX} 1003.2-1992, @samp{200112} stands for @acronym{POSIX}
1461 1003.1-2001, and @samp{200809} stands for @acronym{POSIX} 1003.1-2008.
1462 For example, if you have a newer system but are running software
1463 that assumes an older version of @acronym{POSIX} and uses @samp{sort +1}
1464 or @samp{tail +10}, you can work around any compatibility problems by setting
1465 @samp{_POSIX2_VERSION=199209} in your environment.
1467 @node Output of entire files
1468 @chapter Output of entire files
1470 @cindex output of entire files
1471 @cindex entire files, output of
1473 These commands read and write entire files, possibly transforming them
1477 * cat invocation:: Concatenate and write files.
1478 * tac invocation:: Concatenate and write files in reverse.
1479 * nl invocation:: Number lines and write files.
1480 * od invocation:: Write files in octal or other formats.
1481 * base64 invocation:: Transform data into printable data.
1484 @node cat invocation
1485 @section @command{cat}: Concatenate and write files
1488 @cindex concatenate and write files
1489 @cindex copying files
1491 @command{cat} copies each @var{file} (@samp{-} means standard input), or
1492 standard input if none are given, to standard output. Synopsis:
1495 cat [@var{option}] [@var{file}]@dots{}
1498 The program accepts the following options. Also see @ref{Common options}.
1506 Equivalent to @option{-vET}.
1509 @itemx --number-nonblank
1511 @opindex --number-nonblank
1512 Number all nonempty output lines, starting with 1.
1516 Equivalent to @option{-vE}.
1521 @opindex --show-ends
1522 Display a @samp{$} after the end of each line.
1528 Number all output lines, starting with 1. This option is ignored
1529 if @option{-b} is in effect.
1532 @itemx --squeeze-blank
1534 @opindex --squeeze-blank
1535 @cindex squeezing empty lines
1536 Suppress repeated adjacent empty lines; output just one empty line
1541 Equivalent to @option{-vT}.
1546 @opindex --show-tabs
1547 Display TAB characters as @samp{^I}.
1551 Ignored; for @acronym{POSIX} compatibility.
1554 @itemx --show-nonprinting
1556 @opindex --show-nonprinting
1557 Display control characters except for LFD and TAB using
1558 @samp{^} notation and precede characters that have the high bit set with
1563 On systems like MS-DOS that distinguish between text and binary files,
1564 @command{cat} normally reads and writes in binary mode. However,
1565 @command{cat} reads in text mode if one of the options
1566 @option{-bensAE} is used or if @command{cat} is reading from standard
1567 input and standard input is a terminal. Similarly, @command{cat}
1568 writes in text mode if one of the options @option{-bensAE} is used or
1569 if standard output is a terminal.
1576 # Output f's contents, then standard input, then g's contents.
1579 # Copy standard input to standard output.
1584 @node tac invocation
1585 @section @command{tac}: Concatenate and write files in reverse
1588 @cindex reversing files
1590 @command{tac} copies each @var{file} (@samp{-} means standard input), or
1591 standard input if none are given, to standard output, reversing the
1592 records (lines by default) in each separately. Synopsis:
1595 tac [@var{option}]@dots{} [@var{file}]@dots{}
1598 @dfn{Records} are separated by instances of a string (newline by
1599 default). By default, this separator string is attached to the end of
1600 the record that it follows in the file.
1602 The program accepts the following options. Also see @ref{Common options}.
1610 The separator is attached to the beginning of the record that it
1611 precedes in the file.
1617 Treat the separator string as a regular expression. Users of @command{tac}
1618 on MS-DOS/MS-Windows should note that, since @command{tac} reads files in
1619 binary mode, each line of a text file might end with a CR/LF pair
1620 instead of the Unix-style LF.
1622 @item -s @var{separator}
1623 @itemx --separator=@var{separator}
1625 @opindex --separator
1626 Use @var{separator} as the record separator, instead of newline.
1634 @section @command{nl}: Number lines and write files
1637 @cindex numbering lines
1638 @cindex line numbering
1640 @command{nl} writes each @var{file} (@samp{-} means standard input), or
1641 standard input if none are given, to standard output, with line numbers
1642 added to some or all of the lines. Synopsis:
1645 nl [@var{option}]@dots{} [@var{file}]@dots{}
1648 @cindex logical pages, numbering on
1649 @command{nl} decomposes its input into (logical) pages; by default, the
1650 line number is reset to 1 at the top of each logical page. @command{nl}
1651 treats all of the input files as a single document; it does not reset
1652 line numbers or logical pages between files.
1654 @cindex headers, numbering
1655 @cindex body, numbering
1656 @cindex footers, numbering
1657 A logical page consists of three sections: header, body, and footer.
1658 Any of the sections can be empty. Each can be numbered in a different
1659 style from the others.
1661 The beginnings of the sections of logical pages are indicated in the
1662 input file by a line containing exactly one of these delimiter strings:
1673 The two characters from which these strings are made can be changed from
1674 @samp{\} and @samp{:} via options (see below), but the pattern and
1675 length of each string cannot be changed.
1677 A section delimiter is replaced by an empty line on output. Any text
1678 that comes before the first section delimiter string in the input file
1679 is considered to be part of a body section, so @command{nl} treats a
1680 file that contains no section delimiters as a single body section.
1682 The program accepts the following options. Also see @ref{Common options}.
1686 @item -b @var{style}
1687 @itemx --body-numbering=@var{style}
1689 @opindex --body-numbering
1690 Select the numbering style for lines in the body section of each
1691 logical page. When a line is not numbered, the current line number
1692 is not incremented, but the line number separator character is still
1693 prepended to the line. The styles are:
1699 number only nonempty lines (default for body),
1701 do not number lines (default for header and footer),
1703 number only lines that contain a match for the basic regular
1704 expression @var{bre}.
1705 @xref{Regular Expressions, , Regular Expressions, grep, The GNU Grep Manual}.
1709 @itemx --section-delimiter=@var{cd}
1711 @opindex --section-delimiter
1712 @cindex section delimiters of pages
1713 Set the section delimiter characters to @var{cd}; default is
1714 @samp{\:}. If only @var{c} is given, the second remains @samp{:}.
1715 (Remember to protect @samp{\} or other metacharacters from shell
1716 expansion with quotes or extra backslashes.)
1718 @item -f @var{style}
1719 @itemx --footer-numbering=@var{style}
1721 @opindex --footer-numbering
1722 Analogous to @option{--body-numbering}.
1724 @item -h @var{style}
1725 @itemx --header-numbering=@var{style}
1727 @opindex --header-numbering
1728 Analogous to @option{--body-numbering}.
1730 @item -i @var{number}
1731 @itemx --line-increment=@var{number}
1733 @opindex --line-increment
1734 Increment line numbers by @var{number} (default 1).
1736 @item -l @var{number}
1737 @itemx --join-blank-lines=@var{number}
1739 @opindex --join-blank-lines
1740 @cindex empty lines, numbering
1741 @cindex blank lines, numbering
1742 Consider @var{number} (default 1) consecutive empty lines to be one
1743 logical line for numbering, and only number the last one. Where fewer
1744 than @var{number} consecutive empty lines occur, do not number them.
1745 An empty line is one that contains no characters, not even spaces
1748 @item -n @var{format}
1749 @itemx --number-format=@var{format}
1751 @opindex --number-format
1752 Select the line numbering format (default is @code{rn}):
1756 @opindex ln @r{format for @command{nl}}
1757 left justified, no leading zeros;
1759 @opindex rn @r{format for @command{nl}}
1760 right justified, no leading zeros;
1762 @opindex rz @r{format for @command{nl}}
1763 right justified, leading zeros.
1767 @itemx --no-renumber
1769 @opindex --no-renumber
1770 Do not reset the line number at the start of a logical page.
1772 @item -s @var{string}
1773 @itemx --number-separator=@var{string}
1775 @opindex --number-separator
1776 Separate the line number from the text line in the output with
1777 @var{string} (default is the TAB character).
1779 @item -v @var{number}
1780 @itemx --starting-line-number=@var{number}
1782 @opindex --starting-line-number
1783 Set the initial line number on each logical page to @var{number} (default 1).
1785 @item -w @var{number}
1786 @itemx --number-width=@var{number}
1788 @opindex --number-width
1789 Use @var{number} characters for line numbers (default 6).
1797 @section @command{od}: Write files in octal or other formats
1800 @cindex octal dump of files
1801 @cindex hex dump of files
1802 @cindex ASCII dump of files
1803 @cindex file contents, dumping unambiguously
1805 @command{od} writes an unambiguous representation of each @var{file}
1806 (@samp{-} means standard input), or standard input if none are given.
1810 od [@var{option}]@dots{} [@var{file}]@dots{}
1811 od [-abcdfilosx]@dots{} [@var{file}] [[+]@var{offset}[.][b]]
1812 od [@var{option}]@dots{} --traditional [@var{file}]@c
1813 [[+]@var{offset}[.][b] [[+]@var{label}[.][b]]]
1816 Each line of output consists of the offset in the input, followed by
1817 groups of data from the file. By default, @command{od} prints the offset in
1818 octal, and each group of file data is a C @code{short int}'s worth of input
1819 printed as a single octal number.
1821 If @var{offset} is given, it specifies how many input bytes to skip
1822 before formatting and writing. By default, it is interpreted as an
1823 octal number, but the optional trailing decimal point causes it to be
1824 interpreted as decimal. If no decimal is specified and the offset
1825 begins with @samp{0x} or @samp{0X} it is interpreted as a hexadecimal
1826 number. If there is a trailing @samp{b}, the number of bytes skipped
1827 will be @var{offset} multiplied by 512.
1829 If a command is of both the first and second forms, the second form is
1830 assumed if the last operand begins with @samp{+} or (if there are two
1831 operands) a digit. For example, in @samp{od foo 10} and @samp{od +10}
1832 the @samp{10} is an offset, whereas in @samp{od 10} the @samp{10} is a
1835 The program accepts the following options. Also see @ref{Common options}.
1839 @item -A @var{radix}
1840 @itemx --address-radix=@var{radix}
1842 @opindex --address-radix
1843 @cindex radix for file offsets
1844 @cindex file offset radix
1845 Select the base in which file offsets are printed. @var{radix} can
1846 be one of the following:
1856 none (do not print offsets).
1859 The default is octal.
1861 @item -j @var{bytes}
1862 @itemx --skip-bytes=@var{bytes}
1864 @opindex --skip-bytes
1865 Skip @var{bytes} input bytes before formatting and writing. If
1866 @var{bytes} begins with @samp{0x} or @samp{0X}, it is interpreted in
1867 hexadecimal; otherwise, if it begins with @samp{0}, in octal; otherwise,
1869 @multiplierSuffixes{bytes}
1871 @item -N @var{bytes}
1872 @itemx --read-bytes=@var{bytes}
1874 @opindex --read-bytes
1875 Output at most @var{bytes} bytes of the input. Prefixes and suffixes on
1876 @code{bytes} are interpreted as for the @option{-j} option.
1878 @item -S @var{bytes}
1879 @itemx --strings[=@var{bytes}]
1882 @cindex string constants, outputting
1883 Instead of the normal output, output only @dfn{string constants}: at
1884 least @var{bytes} consecutive @acronym{ASCII} graphic characters,
1885 followed by a zero byte (@acronym{ASCII} @sc{nul}).
1886 Prefixes and suffixes on @code{bytes} are interpreted as for the
1889 If @var{n} is omitted with @option{--strings}, the default is 3.
1892 @itemx --format=@var{type}
1895 Select the format in which to output the file data. @var{type} is a
1896 string of one or more of the below type indicator characters. If you
1897 include more than one type indicator character in a single @var{type}
1898 string, or use this option more than once, @command{od} writes one copy
1899 of each output line using each of the data types that you specified,
1900 in the order that you specified.
1902 Adding a trailing ``z'' to any type specification appends a display
1903 of the @acronym{ASCII} character representation of the printable characters
1904 to the output line generated by the type specification.
1908 named character, ignoring high-order bit
1910 @acronym{ASCII} character or backslash escape,
1914 floating point (@pxref{Floating point})
1923 The type @code{a} outputs things like @samp{sp} for space, @samp{nl} for
1924 newline, and @samp{nul} for a zero byte. Only the least significant
1925 seven bits of each byte is used; the high-order bit is ignored.
1926 Type @code{c} outputs
1927 @samp{ }, @samp{\n}, and @code{\0}, respectively.
1930 Except for types @samp{a} and @samp{c}, you can specify the number
1931 of bytes to use in interpreting each number in the given data type
1932 by following the type indicator character with a decimal integer.
1933 Alternately, you can specify the size of one of the C compiler's
1934 built-in data types by following the type indicator character with
1935 one of the following characters. For integers (@samp{d}, @samp{o},
1936 @samp{u}, @samp{x}):
1949 For floating point (@code{f}):
1961 @itemx --output-duplicates
1963 @opindex --output-duplicates
1964 Output consecutive lines that are identical. By default, when two or
1965 more consecutive output lines would be identical, @command{od} outputs only
1966 the first line, and puts just an asterisk on the following line to
1967 indicate the elision.
1970 @itemx --width[=@var{n}]
1973 Dump @code{n} input bytes per output line. This must be a multiple of
1974 the least common multiple of the sizes associated with the specified
1977 If this option is not given at all, the default is 16. If @var{n} is
1978 omitted, the default is 32.
1982 The next several options are shorthands for format specifications.
1983 @sc{gnu} @command{od} accepts any combination of shorthands and format
1984 specification options. These options accumulate.
1990 Output as named characters. Equivalent to @samp{-t a}.
1994 Output as octal bytes. Equivalent to @samp{-t o1}.
1998 Output as @acronym{ASCII} characters or backslash escapes. Equivalent to
2003 Output as unsigned decimal two-byte units. Equivalent to @samp{-t u2}.
2007 Output as floats. Equivalent to @samp{-t fF}.
2011 Output as decimal ints. Equivalent to @samp{-t dI}.
2015 Output as decimal long ints. Equivalent to @samp{-t dL}.
2019 Output as octal two-byte units. Equivalent to @option{-t o2}.
2023 Output as decimal two-byte units. Equivalent to @option{-t d2}.
2027 Output as hexadecimal two-byte units. Equivalent to @samp{-t x2}.
2030 @opindex --traditional
2031 Recognize the non-option label argument that traditional @command{od}
2032 accepted. The following syntax:
2035 od --traditional [@var{file}] [[+]@var{offset}[.][b] [[+]@var{label}[.][b]]]
2039 can be used to specify at most one file and optional arguments
2040 specifying an offset and a pseudo-start address, @var{label}.
2041 The @var{label} argument is interpreted
2042 just like @var{offset}, but it specifies an initial pseudo-address. The
2043 pseudo-addresses are displayed in parentheses following any normal
2050 @node base64 invocation
2051 @section @command{base64}: Transform data into printable data
2054 @cindex base64 encoding
2056 @command{base64} transforms data read from a file, or standard input,
2057 into (or from) base64 encoded form. The base64 encoded form uses
2058 printable @acronym{ASCII} characters to represent binary data.
2062 base64 [@var{option}]@dots{} [@var{file}]
2063 base64 --decode [@var{option}]@dots{} [@var{file}]
2066 The base64 encoding expands data to roughly 133% of the original.
2067 The format conforms to
2068 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc4648.txt, RFC 4648}.
2070 The program accepts the following options. Also see @ref{Common options}.
2075 @itemx --wrap=@var{cols}
2079 @cindex column to wrap data after
2080 During encoding, wrap lines after @var{cols} characters. This must be
2083 The default is to wrap after 76 characters. Use the value 0 to
2084 disable line wrapping altogether.
2090 @cindex Decode base64 data
2091 @cindex Base64 decoding
2092 Change the mode of operation, from the default of encoding data, to
2093 decoding data. Input is expected to be base64 encoded data, and the
2094 output will be the original data.
2097 @itemx --ignore-garbage
2099 @opindex --ignore-garbage
2100 @cindex Ignore garbage in base64 stream
2101 When decoding, newlines are always accepted.
2102 During decoding, ignore unrecognized bytes,
2103 to permit distorted data to be decoded.
2110 @node Formatting file contents
2111 @chapter Formatting file contents
2113 @cindex formatting file contents
2115 These commands reformat the contents of files.
2118 * fmt invocation:: Reformat paragraph text.
2119 * pr invocation:: Paginate or columnate files for printing.
2120 * fold invocation:: Wrap input lines to fit in specified width.
2124 @node fmt invocation
2125 @section @command{fmt}: Reformat paragraph text
2128 @cindex reformatting paragraph text
2129 @cindex paragraphs, reformatting
2130 @cindex text, reformatting
2132 @command{fmt} fills and joins lines to produce output lines of (at most)
2133 a given number of characters (75 by default). Synopsis:
2136 fmt [@var{option}]@dots{} [@var{file}]@dots{}
2139 @command{fmt} reads from the specified @var{file} arguments (or standard
2140 input if none are given), and writes to standard output.
2142 By default, blank lines, spaces between words, and indentation are
2143 preserved in the output; successive input lines with different
2144 indentation are not joined; tabs are expanded on input and introduced on
2147 @cindex line-breaking
2148 @cindex sentences and line-breaking
2149 @cindex Knuth, Donald E.
2150 @cindex Plass, Michael F.
2151 @command{fmt} prefers breaking lines at the end of a sentence, and tries to
2152 avoid line breaks after the first word of a sentence or before the last
2153 word of a sentence. A @dfn{sentence break} is defined as either the end
2154 of a paragraph or a word ending in any of @samp{.?!}, followed by two
2155 spaces or end of line, ignoring any intervening parentheses or quotes.
2156 Like @TeX{}, @command{fmt} reads entire ``paragraphs'' before choosing line
2157 breaks; the algorithm is a variant of that given by Donald E. Knuth
2158 and Michael F. Plass in ``Breaking Paragraphs Into Lines'',
2159 @cite{Software---Practice & Experience} @b{11}, 11 (November 1981),
2162 The program accepts the following options. Also see @ref{Common options}.
2167 @itemx --crown-margin
2169 @opindex --crown-margin
2170 @cindex crown margin
2171 @dfn{Crown margin} mode: preserve the indentation of the first two
2172 lines within a paragraph, and align the left margin of each subsequent
2173 line with that of the second line.
2176 @itemx --tagged-paragraph
2178 @opindex --tagged-paragraph
2179 @cindex tagged paragraphs
2180 @dfn{Tagged paragraph} mode: like crown margin mode, except that if
2181 indentation of the first line of a paragraph is the same as the
2182 indentation of the second, the first line is treated as a one-line
2188 @opindex --split-only
2189 Split lines only. Do not join short lines to form longer ones. This
2190 prevents sample lines of code, and other such ``formatted'' text from
2191 being unduly combined.
2194 @itemx --uniform-spacing
2196 @opindex --uniform-spacing
2197 Uniform spacing. Reduce spacing between words to one space, and spacing
2198 between sentences to two spaces.
2201 @itemx -w @var{width}
2202 @itemx --width=@var{width}
2203 @opindex -@var{width}
2206 Fill output lines up to @var{width} characters (default 75). @command{fmt}
2207 initially tries to make lines about 7% shorter than this, to give it
2208 room to balance line lengths.
2210 @item -p @var{prefix}
2211 @itemx --prefix=@var{prefix}
2212 Only lines beginning with @var{prefix} (possibly preceded by whitespace)
2213 are subject to formatting. The prefix and any preceding whitespace are
2214 stripped for the formatting and then re-attached to each formatted output
2215 line. One use is to format certain kinds of program comments, while
2216 leaving the code unchanged.
2224 @section @command{pr}: Paginate or columnate files for printing
2227 @cindex printing, preparing files for
2228 @cindex multicolumn output, generating
2229 @cindex merging files in parallel
2231 @command{pr} writes each @var{file} (@samp{-} means standard input), or
2232 standard input if none are given, to standard output, paginating and
2233 optionally outputting in multicolumn format; optionally merges all
2234 @var{file}s, printing all in parallel, one per column. Synopsis:
2237 pr [@var{option}]@dots{} [@var{file}]@dots{}
2241 By default, a 5-line header is printed at each page: two blank lines;
2242 a line with the date, the file name, and the page count; and two more
2243 blank lines. A footer of five blank lines is also printed.
2244 The default @var{page_length} is 66
2245 lines. The default number of text lines is therefore 56.
2246 The text line of the header takes the form
2247 @samp{@var{date} @var{string} @var{page}}, with spaces inserted around
2248 @var{string} so that the line takes up the full @var{page_width}. Here,
2249 @var{date} is the date (see the @option{-D} or @option{--date-format}
2250 option for details), @var{string} is the centered header string, and
2251 @var{page} identifies the page number. The @env{LC_MESSAGES} locale
2252 category affects the spelling of @var{page}; in the default C locale, it
2253 is @samp{Page @var{number}} where @var{number} is the decimal page
2256 Form feeds in the input cause page breaks in the output. Multiple form
2257 feeds produce empty pages.
2259 Columns are of equal width, separated by an optional string (default
2260 is @samp{space}). For multicolumn output, lines will always be truncated to
2261 @var{page_width} (default 72), unless you use the @option{-J} option.
2263 column output no line truncation occurs by default. Use @option{-W} option to
2264 truncate lines in that case.
2266 The following changes were made in version 1.22i and apply to later
2267 versions of @command{pr}:
2268 @c FIXME: this whole section here sounds very awkward to me. I
2269 @c made a few small changes, but really it all needs to be redone. - Brian
2270 @c OK, I fixed another sentence or two, but some of it I just don't understand.
2275 Some small @var{letter options} (@option{-s}, @option{-w}) have been
2276 redefined for better @acronym{POSIX} compliance. The output of some further
2277 cases has been adapted to other Unix systems. These changes are not
2278 compatible with earlier versions of the program.
2281 Some @var{new capital letter} options (@option{-J}, @option{-S}, @option{-W})
2282 have been introduced to turn off unexpected interferences of small letter
2283 options. The @option{-N} option and the second argument @var{last_page}
2284 of @samp{+FIRST_PAGE} offer more flexibility. The detailed handling of
2285 form feeds set in the input files requires the @option{-T} option.
2288 Capital letter options override small letter ones.
2291 Some of the option-arguments (compare @option{-s}, @option{-e},
2292 @option{-i}, @option{-n}) cannot be specified as separate arguments from the
2293 preceding option letter (already stated in the @acronym{POSIX} specification).
2296 The program accepts the following options. Also see @ref{Common options}.
2300 @item +@var{first_page}[:@var{last_page}]
2301 @itemx --pages=@var{first_page}[:@var{last_page}]
2302 @c The two following @opindex lines evoke warnings because they contain `:'
2303 @c The `info' spec does not permit that. If we use those lines, we end
2304 @c up with truncated index entries that don't work.
2305 @c @opindex +@var{first_page}[:@var{last_page}]
2306 @c @opindex --pages=@var{first_page}[:@var{last_page}]
2307 @opindex +@var{page_range}
2308 @opindex --pages=@var{page_range}
2309 Begin printing with page @var{first_page} and stop with @var{last_page}.
2310 Missing @samp{:@var{last_page}} implies end of file. While estimating
2311 the number of skipped pages each form feed in the input file results
2312 in a new page. Page counting with and without @samp{+@var{first_page}}
2313 is identical. By default, counting starts with the first page of input
2314 file (not first page printed). Line numbering may be altered by @option{-N}
2318 @itemx --columns=@var{column}
2319 @opindex -@var{column}
2321 @cindex down columns
2322 With each single @var{file}, produce @var{column} columns of output
2323 (default is 1) and print columns down, unless @option{-a} is used. The
2324 column width is automatically decreased as @var{column} increases; unless
2325 you use the @option{-W/-w} option to increase @var{page_width} as well.
2326 This option might well cause some lines to be truncated. The number of
2327 lines in the columns on each page are balanced. The options @option{-e}
2328 and @option{-i} are on for multiple text-column output. Together with
2329 @option{-J} option column alignment and line truncation is turned off.
2330 Lines of full length are joined in a free field format and @option{-S}
2331 option may set field separators. @option{-@var{column}} may not be used
2332 with @option{-m} option.
2338 @cindex across columns
2339 With each single @var{file}, print columns across rather than down. The
2340 @option{-@var{column}} option must be given with @var{column} greater than one.
2341 If a line is too long to fit in a column, it is truncated.
2344 @itemx --show-control-chars
2346 @opindex --show-control-chars
2347 Print control characters using hat notation (e.g., @samp{^G}); print
2348 other nonprinting characters in octal backslash notation. By default,
2349 nonprinting characters are not changed.
2352 @itemx --double-space
2354 @opindex --double-space
2355 @cindex double spacing
2356 Double space the output.
2358 @item -D @var{format}
2359 @itemx --date-format=@var{format}
2360 @cindex time formats
2361 @cindex formatting times
2362 Format header dates using @var{format}, using the same conventions as
2363 for the command @samp{date +@var{format}}; @xref{date invocation}.
2364 Except for directives, which start with
2365 @samp{%}, characters in @var{format} are printed unchanged. You can use
2366 this option to specify an arbitrary string in place of the header date,
2367 e.g., @option{--date-format="Monday morning"}.
2369 @vindex POSIXLY_CORRECT
2371 The default date format is @samp{%Y-%m-%d %H:%M} (for example,
2372 @samp{2001-12-04 23:59});
2373 but if the @env{POSIXLY_CORRECT} environment variable is set
2374 and the @env{LC_TIME} locale category specifies the @acronym{POSIX}
2375 locale, the default is @samp{%b %e %H:%M %Y} (for example,
2376 @samp{Dec@ @ 4 23:59 2001}.
2379 Time stamps are listed according to the time zone rules specified by
2380 the @env{TZ} environment variable, or by the system default rules if
2381 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
2382 with @env{TZ}, libc, The GNU C Library Reference Manual}.
2384 @item -e[@var{in-tabchar}[@var{in-tabwidth}]]
2385 @itemx --expand-tabs[=@var{in-tabchar}[@var{in-tabwidth}]]
2387 @opindex --expand-tabs
2389 Expand @var{tab}s to spaces on input. Optional argument @var{in-tabchar} is
2390 the input tab character (default is the TAB character). Second optional
2391 argument @var{in-tabwidth} is the input tab character's width (default
2399 @opindex --form-feed
2400 Use a form feed instead of newlines to separate output pages. This does
2401 not alter the default page length of 66 lines.
2403 @item -h @var{header}
2404 @itemx --header=@var{header}
2407 Replace the file name in the header with the centered string @var{header}.
2408 When using the shell, @var{header} should be quoted and should be
2409 separated from @option{-h} by a space.
2411 @item -i[@var{out-tabchar}[@var{out-tabwidth}]]
2412 @itemx --output-tabs[=@var{out-tabchar}[@var{out-tabwidth}]]
2414 @opindex --output-tabs
2416 Replace spaces with @var{tab}s on output. Optional argument @var{out-tabchar}
2417 is the output tab character (default is the TAB character). Second optional
2418 argument @var{out-tabwidth} is the output tab character's width (default
2424 @opindex --join-lines
2425 Merge lines of full length. Used together with the column options
2426 @option{-@var{column}}, @option{-a -@var{column}} or @option{-m}. Turns off
2427 @option{-W/-w} line truncation;
2428 no column alignment used; may be used with
2429 @option{--sep-string[=@var{string}]}. @option{-J} has been introduced
2430 (together with @option{-W} and @option{--sep-string})
2431 to disentangle the old (@acronym{POSIX}-compliant) options @option{-w} and
2432 @option{-s} along with the three column options.
2435 @item -l @var{page_length}
2436 @itemx --length=@var{page_length}
2439 Set the page length to @var{page_length} (default 66) lines, including
2440 the lines of the header [and the footer]. If @var{page_length} is less
2441 than or equal to 10, the header and footer are omitted, as if the
2442 @option{-t} option had been given.
2448 Merge and print all @var{file}s in parallel, one in each column. If a
2449 line is too long to fit in a column, it is truncated, unless the @option{-J}
2450 option is used. @option{--sep-string[=@var{string}]} may be used.
2452 some @var{file}s (form feeds set) produce empty columns, still marked
2453 by @var{string}. The result is a continuous line numbering and column
2454 marking throughout the whole merged file. Completely empty merged pages
2455 show no separators or line numbers. The default header becomes
2456 @samp{@var{date} @var{page}} with spaces inserted in the middle; this
2457 may be used with the @option{-h} or @option{--header} option to fill up
2458 the middle blank part.
2460 @item -n[@var{number-separator}[@var{digits}]]
2461 @itemx --number-lines[=@var{number-separator}[@var{digits}]]
2463 @opindex --number-lines
2464 Provide @var{digits} digit line numbering (default for @var{digits} is
2465 5). With multicolumn output the number occupies the first @var{digits}
2466 column positions of each text column or only each line of @option{-m}
2467 output. With single column output the number precedes each line just as
2468 @option{-m} does. Default counting of the line numbers starts with the
2469 first line of the input file (not the first line printed, compare the
2470 @option{--page} option and @option{-N} option).
2471 Optional argument @var{number-separator} is the character appended to
2472 the line number to separate it from the text followed. The default
2473 separator is the TAB character. In a strict sense a TAB is always
2474 printed with single column output only. The TAB width varies
2475 with the TAB position, e.g., with the left @var{margin} specified
2476 by @option{-o} option. With multicolumn output priority is given to
2477 @samp{equal width of output columns} (a @acronym{POSIX} specification).
2478 The TAB width is fixed to the value of the first column and does
2479 not change with different values of left @var{margin}. That means a
2480 fixed number of spaces is always printed in the place of the
2481 @var{number-separator} TAB. The tabification depends upon the output
2484 @item -N @var{line_number}
2485 @itemx --first-line-number=@var{line_number}
2487 @opindex --first-line-number
2488 Start line counting with the number @var{line_number} at first line of
2489 first page printed (in most cases not the first line of the input file).
2491 @item -o @var{margin}
2492 @itemx --indent=@var{margin}
2495 @cindex indenting lines
2497 Indent each line with a margin @var{margin} spaces wide (default is zero).
2498 The total page width is the size of the margin plus the @var{page_width}
2499 set with the @option{-W/-w} option. A limited overflow may occur with
2500 numbered single column output (compare @option{-n} option).
2503 @itemx --no-file-warnings
2505 @opindex --no-file-warnings
2506 Do not print a warning message when an argument @var{file} cannot be
2507 opened. (The exit status will still be nonzero, however.)
2509 @item -s[@var{char}]
2510 @itemx --separator[=@var{char}]
2512 @opindex --separator
2513 Separate columns by a single character @var{char}. The default for
2514 @var{char} is the TAB character without @option{-w} and @samp{no
2515 character} with @option{-w}. Without @option{-s} the default separator
2516 @samp{space} is set. @option{-s[char]} turns off line truncation of all
2517 three column options (@option{-COLUMN}|@option{-a -COLUMN}|@option{-m}) unless
2518 @option{-w} is set. This is a @acronym{POSIX}-compliant formulation.
2521 @item -S@var{string}
2522 @itemx --sep-string[=@var{string}]
2524 @opindex --sep-string
2525 Use @var{string} to separate output columns. The @option{-S} option doesn't
2526 affect the @option{-W/-w} option, unlike the @option{-s} option which does. It
2527 does not affect line truncation or column alignment.
2528 Without @option{-S}, and with @option{-J}, @command{pr} uses the default output
2530 Without @option{-S} or @option{-J}, @command{pr} uses a @samp{space}
2531 (same as @option{-S"@w{ }"}). @option{--sep-string} with no
2532 @samp{=@var{string}} is equivalent to @option{--sep-string=""}.
2535 @itemx --omit-header
2537 @opindex --omit-header
2538 Do not print the usual header [and footer] on each page, and do not fill
2539 out the bottom of pages (with blank lines or a form feed). No page
2540 structure is produced, but form feeds set in the input files are retained.
2541 The predefined pagination is not changed. @option{-t} or @option{-T} may be
2542 useful together with other options; e.g.: @option{-t -e4}, expand TAB characters
2543 in the input file to 4 spaces but don't make any other changes. Use of
2544 @option{-t} overrides @option{-h}.
2547 @itemx --omit-pagination
2549 @opindex --omit-pagination
2550 Do not print header [and footer]. In addition eliminate all form feeds
2551 set in the input files.
2554 @itemx --show-nonprinting
2556 @opindex --show-nonprinting
2557 Print nonprinting characters in octal backslash notation.
2559 @item -w @var{page_width}
2560 @itemx --width=@var{page_width}
2563 Set page width to @var{page_width} characters for multiple text-column
2564 output only (default for @var{page_width} is 72). @option{-s[CHAR]} turns
2565 off the default page width and any line truncation and column alignment.
2566 Lines of full length are merged, regardless of the column options
2567 set. No @var{page_width} setting is possible with single column output.
2568 A @acronym{POSIX}-compliant formulation.
2570 @item -W @var{page_width}
2571 @itemx --page_width=@var{page_width}
2573 @opindex --page_width
2574 Set the page width to @var{page_width} characters. That's valid with and
2575 without a column option. Text lines are truncated, unless @option{-J}
2576 is used. Together with one of the three column options
2577 (@option{-@var{column}}, @option{-a -@var{column}} or @option{-m}) column
2578 alignment is always used. The separator options @option{-S} or @option{-s}
2579 don't affect the @option{-W} option. Default is 72 characters. Without
2580 @option{-W @var{page_width}} and without any of the column options NO line
2581 truncation is used (defined to keep downward compatibility and to meet
2582 most frequent tasks). That's equivalent to @option{-W 72 -J}. The header
2583 line is never truncated.
2590 @node fold invocation
2591 @section @command{fold}: Wrap input lines to fit in specified width
2594 @cindex wrapping long input lines
2595 @cindex folding long input lines
2597 @command{fold} writes each @var{file} (@option{-} means standard input), or
2598 standard input if none are given, to standard output, breaking long
2602 fold [@var{option}]@dots{} [@var{file}]@dots{}
2605 By default, @command{fold} breaks lines wider than 80 columns. The output
2606 is split into as many lines as necessary.
2608 @cindex screen columns
2609 @command{fold} counts screen columns by default; thus, a tab may count more
2610 than one column, backspace decreases the column count, and carriage
2611 return sets the column to zero.
2613 The program accepts the following options. Also see @ref{Common options}.
2621 Count bytes rather than columns, so that tabs, backspaces, and carriage
2622 returns are each counted as taking up one column, just like other
2629 Break at word boundaries: the line is broken after the last blank before
2630 the maximum line length. If the line contains no such blanks, the line
2631 is broken at the maximum line length as usual.
2633 @item -w @var{width}
2634 @itemx --width=@var{width}
2637 Use a maximum line length of @var{width} columns instead of 80.
2639 For compatibility @command{fold} supports an obsolete option syntax
2640 @option{-@var{width}}. New scripts should use @option{-w @var{width}}
2648 @node Output of parts of files
2649 @chapter Output of parts of files
2651 @cindex output of parts of files
2652 @cindex parts of files, output of
2654 These commands output pieces of the input.
2657 * head invocation:: Output the first part of files.
2658 * tail invocation:: Output the last part of files.
2659 * split invocation:: Split a file into pieces.
2660 * csplit invocation:: Split a file into context-determined pieces.
2663 @node head invocation
2664 @section @command{head}: Output the first part of files
2667 @cindex initial part of files, outputting
2668 @cindex first part of files, outputting
2670 @command{head} prints the first part (10 lines by default) of each
2671 @var{file}; it reads from standard input if no files are given or
2672 when given a @var{file} of @option{-}. Synopsis:
2675 head [@var{option}]@dots{} [@var{file}]@dots{}
2678 If more than one @var{file} is specified, @command{head} prints a
2679 one-line header consisting of:
2682 ==> @var{file name} <==
2686 before the output for each @var{file}.
2688 The program accepts the following options. Also see @ref{Common options}.
2693 @itemx --bytes=@var{k}
2696 Print the first @var{k} bytes, instead of initial lines.
2697 However, if @var{k} starts with a @samp{-},
2698 print all but the last @var{k} bytes of each file.
2699 @multiplierSuffixes{k}
2702 @itemx --lines=@var{k}
2705 Output the first @var{k} lines.
2706 However, if @var{k} starts with a @samp{-},
2707 print all but the last @var{k} lines of each file.
2708 Size multiplier suffixes are the same as with the @option{-c} option.
2716 Never print file name headers.
2722 Always print file name headers.
2726 For compatibility @command{head} also supports an obsolete option syntax
2727 @option{-@var{count}@var{options}}, which is recognized only if it is
2728 specified first. @var{count} is a decimal number optionally followed
2729 by a size letter (@samp{b}, @samp{k}, @samp{m}) as in @option{-c}, or
2730 @samp{l} to mean count by lines, or other option letters (@samp{cqv}).
2731 Scripts intended for standard hosts should use @option{-c @var{count}}
2732 or @option{-n @var{count}} instead. If your script must also run on
2733 hosts that support only the obsolete syntax, it is usually simpler to
2734 avoid @command{head}, e.g., by using @samp{sed 5q} instead of
2740 @node tail invocation
2741 @section @command{tail}: Output the last part of files
2744 @cindex last part of files, outputting
2746 @command{tail} prints the last part (10 lines by default) of each
2747 @var{file}; it reads from standard input if no files are given or
2748 when given a @var{file} of @samp{-}. Synopsis:
2751 tail [@var{option}]@dots{} [@var{file}]@dots{}
2754 If more than one @var{file} is specified, @command{tail} prints a
2755 one-line header consisting of:
2758 ==> @var{file name} <==
2762 before the output for each @var{file}.
2764 @cindex BSD @command{tail}
2765 @sc{gnu} @command{tail} can output any amount of data (some other versions of
2766 @command{tail} cannot). It also has no @option{-r} option (print in
2767 reverse), since reversing a file is really a different job from printing
2768 the end of a file; BSD @command{tail} (which is the one with @option{-r}) can
2769 only reverse files that are at most as large as its buffer, which is
2770 typically 32 KiB@. A more reliable and versatile way to reverse files is
2771 the @sc{gnu} @command{tac} command.
2773 The program accepts the following options. Also see @ref{Common options}.
2778 @itemx --bytes=@var{k}
2781 Output the last @var{k} bytes, instead of final lines.
2782 However, if @var{k} starts with a @samp{+}, start printing with the
2783 @var{k}th byte from the start of each file, instead of from the end.
2784 @multiplierSuffixes{k}
2787 @itemx --follow[=@var{how}]
2790 @cindex growing files
2791 @vindex name @r{follow option}
2792 @vindex descriptor @r{follow option}
2793 Loop forever trying to read more characters at the end of the file,
2794 presumably because the file is growing.
2795 If more than one file is given, @command{tail} prints a header whenever it
2796 gets output from a different file, to indicate which file that output is
2799 There are two ways to specify how you'd like to track files with this option,
2800 but that difference is noticeable only when a followed file is removed or
2802 If you'd like to continue to track the end of a growing file even after
2803 it has been unlinked, use @option{--follow=descriptor}. This is the default
2804 behavior, but it is not useful if you're tracking a log file that may be
2805 rotated (removed or renamed, then reopened). In that case, use
2806 @option{--follow=name} to track the named file, perhaps by reopening it
2807 periodically to see if it has been removed and recreated by some other program.
2808 Note that the inotify-based implementation handles this case without
2809 the need for any periodic reopening.
2811 No matter which method you use, if the tracked file is determined to have
2812 shrunk, @command{tail} prints a message saying the file has been truncated
2813 and resumes tracking the end of the file from the newly-determined endpoint.
2815 When a file is removed, @command{tail}'s behavior depends on whether it is
2816 following the name or the descriptor. When following by name, tail can
2817 detect that a file has been removed and gives a message to that effect,
2818 and if @option{--retry} has been specified it will continue checking
2819 periodically to see if the file reappears.
2820 When following a descriptor, tail does not detect that the file has
2821 been unlinked or renamed and issues no message; even though the file
2822 may no longer be accessible via its original name, it may still be
2825 The option values @samp{descriptor} and @samp{name} may be specified only
2826 with the long form of the option, not with @option{-f}.
2828 The @option{-f} option is ignored if
2829 no @var{file} operand is specified and standard input is a FIFO or a pipe.
2830 Likewise, the @option{-f} option has no effect for any
2831 operand specified as @samp{-}, when standard input is a FIFO or a pipe.
2835 This option is the same as @option{--follow=name --retry}. That is, tail
2836 will attempt to reopen a file when it is removed. Should this fail, tail
2837 will keep trying until it becomes accessible again.
2841 This option is useful mainly when following by name (i.e., with
2842 @option{--follow=name}).
2843 Without this option, when tail encounters a file that doesn't
2844 exist or is otherwise inaccessible, it reports that fact and
2845 never checks it again.
2847 @itemx --sleep-interval=@var{number}
2848 @opindex --sleep-interval
2849 Change the number of seconds to wait between iterations (the default is 1.0).
2850 During one iteration, every specified file is checked to see if it has
2852 Historical implementations of @command{tail} have required that
2853 @var{number} be an integer. However, GNU @command{tail} accepts
2854 an arbitrary floating point number. @xref{Floating point}.
2855 When @command{tail} uses inotify, this polling-related option
2856 is usually ignored. However, if you also specify @option{--pid=@var{p}},
2857 @command{tail} checks whether process @var{p} is alive at least
2858 every @var{number} seconds.
2860 @itemx --pid=@var{pid}
2862 When following by name or by descriptor, you may specify the process ID,
2863 @var{pid}, of the sole writer of all @var{file} arguments. Then, shortly
2864 after that process terminates, tail will also terminate. This will
2865 work properly only if the writer and the tailing process are running on
2866 the same machine. For example, to save the output of a build in a file
2867 and to watch the file grow, if you invoke @command{make} and @command{tail}
2868 like this then the tail process will stop when your build completes.
2869 Without this option, you would have had to kill the @code{tail -f}
2873 $ make >& makerr & tail --pid=$! -f makerr
2876 If you specify a @var{pid} that is not in use or that does not correspond
2877 to the process that is writing to the tailed files, then @command{tail}
2878 may terminate long before any @var{file}s stop growing or it may not
2879 terminate until long after the real writer has terminated.
2880 Note that @option{--pid} cannot be supported on some systems; @command{tail}
2881 will print a warning if this is the case.
2883 @itemx --max-unchanged-stats=@var{n}
2884 @opindex --max-unchanged-stats
2885 When tailing a file by name, if there have been @var{n} (default
2886 n=@value{DEFAULT_MAX_N_UNCHANGED_STATS_BETWEEN_OPENS}) consecutive
2887 iterations for which the file has not changed, then
2888 @code{open}/@code{fstat} the file to determine if that file name is
2889 still associated with the same device/inode-number pair as before.
2890 When following a log file that is rotated, this is approximately the
2891 number of seconds between when tail prints the last pre-rotation lines
2892 and when it prints the lines that have accumulated in the new log file.
2893 This option is meaningful only when polling (i.e., without inotify)
2894 and when following by name.
2897 @itemx --lines=@var{k}
2900 Output the last @var{k} lines.
2901 However, if @var{k} starts with a @samp{+}, start printing with the
2902 @var{k}th line from the start of each file, instead of from the end.
2903 Size multiplier suffixes are the same as with the @option{-c} option.
2911 Never print file name headers.
2917 Always print file name headers.
2921 For compatibility @command{tail} also supports an obsolete usage
2922 @samp{tail -[@var{count}][bcl][f] [@var{file}]}, which is recognized
2923 only if it does not conflict with the usage described
2924 above. This obsolete form uses exactly one option and at most one
2925 file. In the option, @var{count} is an optional decimal number optionally
2926 followed by a size letter (@samp{b}, @samp{c}, @samp{l}) to mean count
2927 by 512-byte blocks, bytes, or lines, optionally followed by @samp{f}
2928 which has the same meaning as @option{-f}.
2930 @vindex _POSIX2_VERSION
2931 On older systems, the leading @samp{-} can be replaced by @samp{+} in
2932 the obsolete option syntax with the same meaning as in counts, and
2933 obsolete usage overrides normal usage when the two conflict.
2934 This obsolete behavior can be enabled or disabled with the
2935 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
2938 Scripts intended for use on standard hosts should avoid obsolete
2939 syntax and should use @option{-c @var{count}[b]}, @option{-n
2940 @var{count}}, and/or @option{-f} instead. If your script must also
2941 run on hosts that support only the obsolete syntax, you can often
2942 rewrite it to avoid problematic usages, e.g., by using @samp{sed -n
2943 '$p'} rather than @samp{tail -1}. If that's not possible, the script
2944 can use a test like @samp{if tail -c +1 </dev/null >/dev/null 2>&1;
2945 then @dots{}} to decide which syntax to use.
2947 Even if your script assumes the standard behavior, you should still
2948 beware usages whose behaviors differ depending on the @acronym{POSIX}
2949 version. For example, avoid @samp{tail - main.c}, since it might be
2950 interpreted as either @samp{tail main.c} or as @samp{tail -- -
2951 main.c}; avoid @samp{tail -c 4}, since it might mean either @samp{tail
2952 -c4} or @samp{tail -c 10 4}; and avoid @samp{tail +4}, since it might
2953 mean either @samp{tail ./+4} or @samp{tail -n +4}.
2958 @node split invocation
2959 @section @command{split}: Split a file into pieces.
2962 @cindex splitting a file into pieces
2963 @cindex pieces, splitting a file into
2965 @command{split} creates output files containing consecutive or interleaved
2966 sections of @var{input} (standard input if none is given or @var{input}
2967 is @samp{-}). Synopsis:
2970 split [@var{option}] [@var{input} [@var{prefix}]]
2973 By default, @command{split} puts 1000 lines of @var{input} (or whatever is
2974 left over for the last section), into each output file.
2976 @cindex output file name prefix
2977 The output files' names consist of @var{prefix} (@samp{x} by default)
2978 followed by a group of characters (@samp{aa}, @samp{ab}, @dots{} by
2979 default), such that concatenating the output files in traditional
2980 sorted order by file name produces the original input file (except
2981 @option{-r}). If the output file names are exhausted, @command{split}
2982 reports an error without deleting the output files that it did create.
2984 The program accepts the following options. Also see @ref{Common options}.
2988 @item -l @var{lines}
2989 @itemx --lines=@var{lines}
2992 Put @var{lines} lines of @var{input} into each output file.
2994 For compatibility @command{split} also supports an obsolete
2995 option syntax @option{-@var{lines}}. New scripts should use @option{-l
2996 @var{lines}} instead.
2999 @itemx --bytes=@var{size}
3002 Put @var{size} bytes of @var{input} into each output file.
3003 @multiplierSuffixes{size}
3006 @itemx --line-bytes=@var{size}
3008 @opindex --line-bytes
3009 Put into each output file as many complete lines of @var{input} as
3010 possible without exceeding @var{size} bytes. Individual lines longer than
3011 @var{size} bytes are broken into multiple files.
3012 @var{size} has the same format as for the @option{--bytes} option.
3014 @item -n @var{chunks}
3015 @itemx --number=@var{chunks}
3019 Split @var{input} to @var{chunks} output files where @var{chunks} may be:
3022 @var{n} generate @var{n} files based on current size of @var{input}
3023 @var{k}/@var{n} only output @var{k}th of @var{n} to stdout
3024 l/@var{n} generate @var{n} files without splitting lines
3025 l/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
3026 r/@var{n} like @samp{l} but use round robin distribution
3027 r/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
3030 Any excess bytes remaining after dividing the @var{input}
3031 into @var{n} chunks, are assigned to the last chunk.
3032 Any excess bytes appearing after the initial calculation are discarded
3033 (except when using @samp{r} mode).
3035 All @var{n} files are created even if there are fewer than @var{n} lines,
3036 or the @var{input} is truncated.
3038 For @samp{l} mode, chunks are approximately @var{input} size / @var{n}.
3039 The @var{input} is partitioned into @var{n} equal sized portions, with
3040 the last assigned any excess. If a line @emph{starts} within a partition
3041 it is written completely to the corresponding file. Since lines
3042 are not split even if they overlap a partition, the files written
3043 can be larger or smaller than the partition size, and even empty
3044 if a line is so long as to completely overlap the partition.
3046 For @samp{r} mode, the size of @var{input} is irrelevant,
3047 and so can be a pipe for example.
3049 @item -a @var{length}
3050 @itemx --suffix-length=@var{length}
3052 @opindex --suffix-length
3053 Use suffixes of length @var{length}. The default @var{length} is 2.
3056 @itemx --numeric-suffixes
3058 @opindex --numeric-suffixes
3059 Use digits in suffixes rather than lower-case letters.
3062 @itemx --elide-empty-files
3064 @opindex --elide-empty-files
3065 Suppress the generation of zero-length output files. This can happen
3066 with the @option{--number} option if a file is (truncated to be) shorter
3067 than the number requested, or if a line is so long as to completely
3068 span a chunk. The output file sequence numbers, always run consecutively
3069 even when this option is specified.
3074 @opindex --unbuffered
3075 Immediately copy input to output in @option{--number r/...} mode,
3076 which is a much slower mode of operation.
3080 Write a diagnostic just before each output file is opened.
3086 Here are a few examples to illustrate how the
3087 @option{--number} (@option{-n}) option works:
3089 Notice how, by default, one line may be split onto two or more:
3092 $ seq -w 6 10 > k; split -n3 k; head xa?
3105 Use the "l/" modifier to suppress that:
3108 $ seq -w 6 10 > k; split -nl/3 k; head xa?
3121 Use the "r/" modifier to distribute lines in a round-robin fashion:
3124 $ seq -w 6 10 > k; split -nr/3 k; head xa?
3137 You can also extract just the Kth chunk.
3138 This extracts and prints just the 7th "chunk" of 33:
3141 $ seq 100 > k; split -nl/7/33 k
3148 @node csplit invocation
3149 @section @command{csplit}: Split a file into context-determined pieces
3152 @cindex context splitting
3153 @cindex splitting a file into pieces by context
3155 @command{csplit} creates zero or more output files containing sections of
3156 @var{input} (standard input if @var{input} is @samp{-}). Synopsis:
3159 csplit [@var{option}]@dots{} @var{input} @var{pattern}@dots{}
3162 The contents of the output files are determined by the @var{pattern}
3163 arguments, as detailed below. An error occurs if a @var{pattern}
3164 argument refers to a nonexistent line of the input file (e.g., if no
3165 remaining line matches a given regular expression). After every
3166 @var{pattern} has been matched, any remaining input is copied into one
3169 By default, @command{csplit} prints the number of bytes written to each
3170 output file after it has been created.
3172 The types of pattern arguments are:
3177 Create an output file containing the input up to but not including line
3178 @var{n} (a positive integer). If followed by a repeat count, also
3179 create an output file containing the next @var{n} lines of the input
3180 file once for each repeat.
3182 @item /@var{regexp}/[@var{offset}]
3183 Create an output file containing the current line up to (but not
3184 including) the next line of the input file that contains a match for
3185 @var{regexp}. The optional @var{offset} is an integer.
3186 If it is given, the input up to (but not including) the
3187 matching line plus or minus @var{offset} is put into the output file,
3188 and the line after that begins the next section of input.
3190 @item %@var{regexp}%[@var{offset}]
3191 Like the previous type, except that it does not create an output
3192 file, so that section of the input file is effectively ignored.
3194 @item @{@var{repeat-count}@}
3195 Repeat the previous pattern @var{repeat-count} additional
3196 times. The @var{repeat-count} can either be a positive integer or an
3197 asterisk, meaning repeat as many times as necessary until the input is
3202 The output files' names consist of a prefix (@samp{xx} by default)
3203 followed by a suffix. By default, the suffix is an ascending sequence
3204 of two-digit decimal numbers from @samp{00} to @samp{99}. In any case,
3205 concatenating the output files in sorted order by file name produces the
3206 original input file.
3208 By default, if @command{csplit} encounters an error or receives a hangup,
3209 interrupt, quit, or terminate signal, it removes any output files
3210 that it has created so far before it exits.
3212 The program accepts the following options. Also see @ref{Common options}.
3216 @item -f @var{prefix}
3217 @itemx --prefix=@var{prefix}
3220 @cindex output file name prefix
3221 Use @var{prefix} as the output file name prefix.
3223 @item -b @var{suffix}
3224 @itemx --suffix=@var{suffix}
3227 @cindex output file name suffix
3228 Use @var{suffix} as the output file name suffix. When this option is
3229 specified, the suffix string must include exactly one
3230 @code{printf(3)}-style conversion specification, possibly including
3231 format specification flags, a field width, a precision specifications,
3232 or all of these kinds of modifiers. The format letter must convert a
3233 binary unsigned integer argument to readable form. The format letters
3234 @samp{d} and @samp{i} are aliases for @samp{u}, and the
3235 @samp{u}, @samp{o}, @samp{x}, and @samp{X} conversions are allowed. The
3236 entire @var{suffix} is given (with the current output file number) to
3237 @code{sprintf(3)} to form the file name suffixes for each of the
3238 individual output files in turn. If this option is used, the
3239 @option{--digits} option is ignored.
3241 @item -n @var{digits}
3242 @itemx --digits=@var{digits}
3245 Use output file names containing numbers that are @var{digits} digits
3246 long instead of the default 2.
3251 @opindex --keep-files
3252 Do not remove output files when errors are encountered.
3255 @itemx --elide-empty-files
3257 @opindex --elide-empty-files
3258 Suppress the generation of zero-length output files. (In cases where
3259 the section delimiters of the input file are supposed to mark the first
3260 lines of each of the sections, the first output file will generally be a
3261 zero-length file unless you use this option.) The output file sequence
3262 numbers always run consecutively starting from 0, even when this option
3273 Do not print counts of output file sizes.
3279 Here is an example of its usage.
3280 First, create an empty directory for the exercise,
3287 Now, split the sequence of 1..14 on lines that end with 0 or 5:
3290 $ seq 14 | csplit - '/[05]$/' '@{*@}'
3296 Each number printed above is the size of an output
3297 file that csplit has just created.
3298 List the names of those output files:
3305 Use @command{head} to show their contents:
3330 @node Summarizing files
3331 @chapter Summarizing files
3333 @cindex summarizing files
3335 These commands generate just a few numbers representing entire
3339 * wc invocation:: Print newline, word, and byte counts.
3340 * sum invocation:: Print checksum and block counts.
3341 * cksum invocation:: Print CRC checksum and byte counts.
3342 * md5sum invocation:: Print or check MD5 digests.
3343 * sha1sum invocation:: Print or check SHA-1 digests.
3344 * sha2 utilities:: Print or check SHA-2 digests.
3349 @section @command{wc}: Print newline, word, and byte counts
3353 @cindex character count
3357 @command{wc} counts the number of bytes, characters, whitespace-separated
3358 words, and newlines in each given @var{file}, or standard input if none
3359 are given or for a @var{file} of @samp{-}. Synopsis:
3362 wc [@var{option}]@dots{} [@var{file}]@dots{}
3365 @cindex total counts
3366 @command{wc} prints one line of counts for each file, and if the file was
3367 given as an argument, it prints the file name following the counts. If
3368 more than one @var{file} is given, @command{wc} prints a final line
3369 containing the cumulative counts, with the file name @file{total}. The
3370 counts are printed in this order: newlines, words, characters, bytes,
3371 maximum line length.
3372 Each count is printed right-justified in a field with at least one
3373 space between fields so that the numbers and file names normally line
3374 up nicely in columns. The width of the count fields varies depending
3375 on the inputs, so you should not depend on a particular field width.
3376 However, as a @acronym{GNU} extension, if only one count is printed,
3377 it is guaranteed to be printed without leading spaces.
3379 By default, @command{wc} prints three counts: the newline, words, and byte
3380 counts. Options can specify that only certain counts be printed.
3381 Options do not undo others previously given, so
3388 prints both the byte counts and the word counts.
3390 With the @option{--max-line-length} option, @command{wc} prints the length
3391 of the longest line per file, and if there is more than one file it
3392 prints the maximum (not the sum) of those lengths. The line lengths here
3393 are measured in screen columns, according to the current locale and
3394 assuming tab positions in every 8th column.
3396 The program accepts the following options. Also see @ref{Common options}.
3404 Print only the byte counts.
3410 Print only the character counts.
3416 Print only the word counts.
3422 Print only the newline counts.
3425 @itemx --max-line-length
3427 @opindex --max-line-length
3428 Print only the maximum line lengths.
3430 @macro filesZeroFromOption{cmd,withTotalOption,subListOutput}
3431 @itemx --files0-from=@var{file}
3432 @opindex --files0-from=@var{file}
3433 @c This is commented out to avoid a texi2dvi failure.
3434 @c texi2dvi (GNU Texinfo 4.11) 1.104
3435 @c @cindex including files from @command{\cmd\}
3436 Disallow processing files named on the command line, and instead process
3437 those named in file @var{file}; each name being terminated by a zero byte
3438 (@acronym{ASCII} @sc{nul}).
3439 This is useful \withTotalOption\
3440 when the list of file names is so long that it may exceed a command line
3442 In such cases, running @command{\cmd\} via @command{xargs} is undesirable
3443 because it splits the list into pieces and makes @command{\cmd\} print
3444 \subListOutput\ for each sublist rather than for the entire list.
3445 One way to produce a list of @acronym{ASCII} @sc{nul} terminated file
3446 names is with @sc{gnu}
3447 @command{find}, using its @option{-print0} predicate.
3448 If @var{file} is @samp{-} then the @acronym{ASCII} @sc{nul} terminated
3449 file names are read from standard input.
3451 @filesZeroFromOption{wc,,a total}
3453 For example, to find the length of the longest line in any @file{.c} or
3454 @file{.h} file in the current hierarchy, do this:
3457 find . -name '*.[ch]' -print0 |
3458 wc -L --files0-from=- | tail -n1
3466 @node sum invocation
3467 @section @command{sum}: Print checksum and block counts
3470 @cindex 16-bit checksum
3471 @cindex checksum, 16-bit
3473 @command{sum} computes a 16-bit checksum for each given @var{file}, or
3474 standard input if none are given or for a @var{file} of @samp{-}. Synopsis:
3477 sum [@var{option}]@dots{} [@var{file}]@dots{}
3480 @command{sum} prints the checksum for each @var{file} followed by the
3481 number of blocks in the file (rounded up). If more than one @var{file}
3482 is given, file names are also printed (by default). (With the
3483 @option{--sysv} option, corresponding file names are printed when there is
3484 at least one file argument.)
3486 By default, @sc{gnu} @command{sum} computes checksums using an algorithm
3487 compatible with BSD @command{sum} and prints file sizes in units of
3490 The program accepts the following options. Also see @ref{Common options}.
3496 @cindex BSD @command{sum}
3497 Use the default (BSD compatible) algorithm. This option is included for
3498 compatibility with the System V @command{sum}. Unless @option{-s} was also
3499 given, it has no effect.
3505 @cindex System V @command{sum}
3506 Compute checksums using an algorithm compatible with System V
3507 @command{sum}'s default, and print file sizes in units of 512-byte blocks.
3511 @command{sum} is provided for compatibility; the @command{cksum} program (see
3512 next section) is preferable in new applications.
3517 @node cksum invocation
3518 @section @command{cksum}: Print CRC checksum and byte counts
3521 @cindex cyclic redundancy check
3522 @cindex CRC checksum
3524 @command{cksum} computes a cyclic redundancy check (CRC) checksum for each
3525 given @var{file}, or standard input if none are given or for a
3526 @var{file} of @samp{-}. Synopsis:
3529 cksum [@var{option}]@dots{} [@var{file}]@dots{}
3532 @command{cksum} prints the CRC checksum for each file along with the number
3533 of bytes in the file, and the file name unless no arguments were given.
3535 @command{cksum} is typically used to ensure that files
3536 transferred by unreliable means (e.g., netnews) have not been corrupted,
3537 by comparing the @command{cksum} output for the received files with the
3538 @command{cksum} output for the original files (typically given in the
3541 The CRC algorithm is specified by the @acronym{POSIX} standard. It is not
3542 compatible with the BSD or System V @command{sum} algorithms (see the
3543 previous section); it is more robust.
3545 The only options are @option{--help} and @option{--version}. @xref{Common
3551 @node md5sum invocation
3552 @section @command{md5sum}: Print or check MD5 digests
3556 @cindex 128-bit checksum
3557 @cindex checksum, 128-bit
3558 @cindex fingerprint, 128-bit
3559 @cindex message-digest, 128-bit
3561 @command{md5sum} computes a 128-bit checksum (or @dfn{fingerprint} or
3562 @dfn{message-digest}) for each specified @var{file}.
3564 Note: The MD5 digest is more reliable than a simple CRC (provided by
3565 the @command{cksum} command) for detecting accidental file corruption,
3566 as the chances of accidentally having two files with identical MD5
3567 are vanishingly small. However, it should not be considered secure
3568 against malicious tampering: although finding a file with a given MD5
3569 fingerprint is considered infeasible at the moment, it is known how
3570 to modify certain files, including digital certificates, so that they
3571 appear valid when signed with an MD5 digest.
3572 For more secure hashes, consider using SHA-2. @xref{sha2 utilities}.
3574 If a @var{file} is specified as @samp{-} or if no files are given
3575 @command{md5sum} computes the checksum for the standard input.
3576 @command{md5sum} can also determine whether a file and checksum are
3577 consistent. Synopsis:
3580 md5sum [@var{option}]@dots{} [@var{file}]@dots{}
3583 For each @var{file}, @samp{md5sum} outputs the MD5 checksum, a flag
3584 indicating a binary or text input file, and the file name.
3585 If @var{file} contains a backslash or newline, the
3586 line is started with a backslash, and each problematic character in
3587 the file name is escaped with a backslash, making the output
3588 unambiguous even in the presence of arbitrary file names.
3589 If @var{file} is omitted or specified as @samp{-}, standard input is read.
3591 The program accepts the following options. Also see @ref{Common options}.
3599 @cindex binary input files
3600 Treat each input file as binary, by reading it in binary mode and
3601 outputting a @samp{*} flag. This is the inverse of @option{--text}.
3602 On systems like @acronym{GNU} that do not distinguish between binary
3603 and text files, this option merely flags each input file as binary:
3604 the MD5 checksum is unaffected. This option is the default on systems
3605 like MS-DOS that distinguish between binary and text files, except
3606 for reading standard input when standard input is a terminal.
3610 Read file names and checksum information (not data) from each
3611 @var{file} (or from stdin if no @var{file} was specified) and report
3612 whether the checksums match the contents of the named files.
3613 The input to this mode of @command{md5sum} is usually the output of
3614 a prior, checksum-generating run of @samp{md5sum}.
3615 Each valid line of input consists of an MD5 checksum, a binary/text
3616 flag, and then a file name.
3617 Binary files are marked with @samp{*}, text with @samp{ }.
3618 For each such line, @command{md5sum} reads the named file and computes its
3619 MD5 checksum. Then, if the computed message digest does not match the
3620 one on the line with the file name, the file is noted as having
3621 failed the test. Otherwise, the file passes the test.
3622 By default, for each valid line, one line is written to standard
3623 output indicating whether the named file passed the test.
3624 After all checks have been performed, if there were any failures,
3625 a warning is issued to standard error.
3626 Use the @option{--status} option to inhibit that output.
3627 If any listed file cannot be opened or read, if any valid line has
3628 an MD5 checksum inconsistent with the associated file, or if no valid
3629 line is found, @command{md5sum} exits with nonzero status. Otherwise,
3630 it exits successfully.
3634 @cindex verifying MD5 checksums
3635 This option is useful only when verifying checksums.
3636 When verifying checksums, don't generate an 'OK' message per successfully
3637 checked file. Files that fail the verification are reported in the
3638 default one-line-per-file format. If there is any checksum mismatch,
3639 print a warning summarizing the failures to standard error.
3643 @cindex verifying MD5 checksums
3644 This option is useful only when verifying checksums.
3645 When verifying checksums, don't generate the default one-line-per-file
3646 diagnostic and don't output the warning summarizing any failures.
3647 Failures to open or read a file still evoke individual diagnostics to
3649 If all listed files are readable and are consistent with the associated
3650 MD5 checksums, exit successfully. Otherwise exit with a status code
3651 indicating there was a failure.
3657 @cindex text input files
3658 Treat each input file as text, by reading it in text mode and
3659 outputting a @samp{ } flag. This is the inverse of @option{--binary}.
3660 This option is the default on systems like @acronym{GNU} that do not
3661 distinguish between binary and text files. On other systems, it is
3662 the default for reading standard input when standard input is a
3669 @cindex verifying MD5 checksums
3670 When verifying checksums, warn about improperly formatted MD5 checksum lines.
3671 This option is useful only if all but a few lines in the checked input
3679 @node sha1sum invocation
3680 @section @command{sha1sum}: Print or check SHA-1 digests
3684 @cindex 160-bit checksum
3685 @cindex checksum, 160-bit
3686 @cindex fingerprint, 160-bit
3687 @cindex message-digest, 160-bit
3689 @command{sha1sum} computes a 160-bit checksum for each specified
3690 @var{file}. The usage and options of this command are precisely the
3691 same as for @command{md5sum}. @xref{md5sum invocation}.
3693 Note: The SHA-1 digest is more secure than MD5, and no collisions of
3694 it are known (different files having the same fingerprint). However,
3695 it is known that they can be produced with considerable, but not
3696 unreasonable, resources. For this reason, it is generally considered
3697 that SHA-1 should be gradually phased out in favor of the more secure
3698 SHA-2 hash algorithms. @xref{sha2 utilities}.
3701 @node sha2 utilities
3702 @section sha2 utilities: Print or check SHA-2 digests
3709 @cindex 224-bit checksum
3710 @cindex 256-bit checksum
3711 @cindex 384-bit checksum
3712 @cindex 512-bit checksum
3713 @cindex checksum, 224-bit
3714 @cindex checksum, 256-bit
3715 @cindex checksum, 384-bit
3716 @cindex checksum, 512-bit
3717 @cindex fingerprint, 224-bit
3718 @cindex fingerprint, 256-bit
3719 @cindex fingerprint, 384-bit
3720 @cindex fingerprint, 512-bit
3721 @cindex message-digest, 224-bit
3722 @cindex message-digest, 256-bit
3723 @cindex message-digest, 384-bit
3724 @cindex message-digest, 512-bit
3726 The commands @command{sha224sum}, @command{sha256sum},
3727 @command{sha384sum} and @command{sha512sum} compute checksums of
3728 various lengths (respectively 224, 256, 384 and 512 bits),
3729 collectively known as the SHA-2 hashes. The usage and options of
3730 these commands are precisely the same as for @command{md5sum}.
3731 @xref{md5sum invocation}.
3733 Note: The SHA384 and SHA512 digests are considerably slower to
3734 compute, especially on 32-bit computers, than SHA224 or SHA256.
3737 @node Operating on sorted files
3738 @chapter Operating on sorted files
3740 @cindex operating on sorted files
3741 @cindex sorted files, operations on
3743 These commands work with (or produce) sorted files.
3746 * sort invocation:: Sort text files.
3747 * shuf invocation:: Shuffle text files.
3748 * uniq invocation:: Uniquify files.
3749 * comm invocation:: Compare two sorted files line by line.
3750 * ptx invocation:: Produce a permuted index of file contents.
3751 * tsort invocation:: Topological sort.
3755 @node sort invocation
3756 @section @command{sort}: Sort text files
3759 @cindex sorting files
3761 @command{sort} sorts, merges, or compares all the lines from the given
3762 files, or standard input if none are given or for a @var{file} of
3763 @samp{-}. By default, @command{sort} writes the results to standard
3767 sort [@var{option}]@dots{} [@var{file}]@dots{}
3770 @command{sort} has three modes of operation: sort (the default), merge,
3771 and check for sortedness. The following options change the operation
3778 @itemx --check=diagnose-first
3781 @cindex checking for sortedness
3782 Check whether the given file is already sorted: if it is not all
3783 sorted, print a diagnostic containing the first out-of-order line and
3784 exit with a status of 1.
3785 Otherwise, exit successfully.
3786 At most one input file can be given.
3789 @itemx --check=quiet
3790 @itemx --check=silent
3793 @cindex checking for sortedness
3794 Exit successfully if the given file is already sorted, and
3795 exit with status 1 otherwise.
3796 At most one input file can be given.
3797 This is like @option{-c}, except it does not print a diagnostic.
3803 @cindex merging sorted files
3804 Merge the given files by sorting them as a group. Each input file must
3805 always be individually sorted. It always works to sort instead of
3806 merge; merging is provided because it is faster, in the case where it
3811 @cindex sort stability
3812 @cindex sort's last-resort comparison
3813 A pair of lines is compared as follows:
3814 @command{sort} compares each pair of fields, in the
3815 order specified on the command line, according to the associated
3816 ordering options, until a difference is found or no fields are left.
3817 If no key fields are specified, @command{sort} uses a default key of
3818 the entire line. Finally, as a last resort when all keys compare
3819 equal, @command{sort} compares entire lines as if no ordering options
3820 other than @option{--reverse} (@option{-r}) were specified. The
3821 @option{--stable} (@option{-s}) option disables this @dfn{last-resort
3822 comparison} so that lines in which all fields compare equal are left
3823 in their original relative order. The @option{--unique}
3824 (@option{-u}) option also disables the last-resort comparison.
3828 Unless otherwise specified, all comparisons use the character collating
3829 sequence specified by the @env{LC_COLLATE} locale.@footnote{If you
3830 use a non-@acronym{POSIX} locale (e.g., by setting @env{LC_ALL}
3831 to @samp{en_US}), then @command{sort} may produce output that is sorted
3832 differently than you're accustomed to. In that case, set the @env{LC_ALL}
3833 environment variable to @samp{C}. Note that setting only @env{LC_COLLATE}
3834 has two problems. First, it is ineffective if @env{LC_ALL} is also set.
3835 Second, it has undefined behavior if @env{LC_CTYPE} (or @env{LANG}, if
3836 @env{LC_CTYPE} is unset) is set to an incompatible value. For example,
3837 you get undefined behavior if @env{LC_CTYPE} is @code{ja_JP.PCK} but
3838 @env{LC_COLLATE} is @code{en_US.UTF-8}.}
3840 @sc{gnu} @command{sort} (as specified for all @sc{gnu} utilities) has no
3841 limit on input line length or restrictions on bytes allowed within lines.
3842 In addition, if the final byte of an input file is not a newline, @sc{gnu}
3843 @command{sort} silently supplies one. A line's trailing newline is not
3844 part of the line for comparison purposes.
3846 @cindex exit status of @command{sort}
3850 0 if no error occurred
3851 1 if invoked with @option{-c} or @option{-C} and the input is not sorted
3852 2 if an error occurred
3856 If the environment variable @env{TMPDIR} is set, @command{sort} uses its
3857 value as the directory for temporary files instead of @file{/tmp}. The
3858 @option{--temporary-directory} (@option{-T}) option in turn overrides
3859 the environment variable.
3861 The following options affect the ordering of output lines. They may be
3862 specified globally or as part of a specific key field. If no key
3863 fields are specified, global options apply to comparison of entire
3864 lines; otherwise the global options are inherited by key fields that do
3865 not specify any special options of their own. In pre-@acronym{POSIX}
3866 versions of @command{sort}, global options affect only later key fields,
3867 so portable shell scripts should specify global options first.
3872 @itemx --ignore-leading-blanks
3874 @opindex --ignore-leading-blanks
3875 @cindex blanks, ignoring leading
3877 Ignore leading blanks when finding sort keys in each line.
3878 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3879 can change this. Note blanks may be ignored by your locale's collating
3880 rules, but without this option they will be significant for character
3881 positions specified in keys with the @option{-k} option.
3884 @itemx --dictionary-order
3886 @opindex --dictionary-order
3887 @cindex dictionary order
3888 @cindex phone directory order
3889 @cindex telephone directory order
3891 Sort in @dfn{phone directory} order: ignore all characters except
3892 letters, digits and blanks when sorting.
3893 By default letters and digits are those of @acronym{ASCII} and a blank
3894 is a space or a tab, but the @env{LC_CTYPE} locale can change this.
3897 @itemx --ignore-case
3899 @opindex --ignore-case
3900 @cindex ignoring case
3901 @cindex case folding
3903 Fold lowercase characters into the equivalent uppercase characters when
3904 comparing so that, for example, @samp{b} and @samp{B} sort as equal.
3905 The @env{LC_CTYPE} locale determines character types.
3906 When used with @option{--unique} those lower case equivalent lines are
3907 thrown away. (There is currently no way to throw away the upper case
3908 equivalent instead. (Any @option{--reverse} given would only affect
3909 the final result, after the throwing away.))
3912 @itemx --general-numeric-sort
3913 @itemx --sort=general-numeric
3915 @opindex --general-numeric-sort
3917 @cindex general numeric sort
3919 Sort numerically, converting a prefix of each line to a long
3920 double-precision floating point number. @xref{Floating point}.
3921 Do not report overflow, underflow, or conversion errors.
3922 Use the following collating sequence:
3926 Lines that do not start with numbers (all considered to be equal).
3928 NaNs (``Not a Number'' values, in IEEE floating point arithmetic)
3929 in a consistent but machine-dependent order.
3933 Finite numbers in ascending numeric order (with @math{-0} and @math{+0} equal).
3938 Use this option only if there is no alternative; it is much slower than
3939 @option{--numeric-sort} (@option{-n}) and it can lose information when
3940 converting to floating point.
3943 @itemx --human-numeric-sort
3944 @itemx --sort=human-numeric
3946 @opindex --human-numeric-sort
3948 @cindex human numeric sort
3950 Sort numerically, first by numeric sign (negative, zero, or positive);
3951 then by @acronym{SI} suffix (either empty, or @samp{k} or @samp{K}, or
3952 one of @samp{MGTPEZY}, in that order; @pxref{Block size}); and finally
3953 by numeric value. For example, @samp{1023M} sorts before @samp{1G}
3954 because @samp{M} (mega) precedes @samp{G} (giga) as an @acronym{SI}
3955 suffix. This option sorts values that are consistently scaled to the
3956 nearest suffix, regardless of whether suffixes denote powers of 1000
3957 or 1024, and it therefore sorts the output of any single invocation of
3958 the @command{df}, @command{du}, or @command{ls} commands that are
3959 invoked with their @option{--human-readable} or @option{--si} options.
3960 The syntax for numbers is the same as for the @option{--numeric-sort}
3961 option; the @acronym{SI} suffix must immediately follow the number.
3964 @itemx --ignore-nonprinting
3966 @opindex --ignore-nonprinting
3967 @cindex nonprinting characters, ignoring
3968 @cindex unprintable characters, ignoring
3970 Ignore nonprinting characters.
3971 The @env{LC_CTYPE} locale determines character types.
3972 This option has no effect if the stronger @option{--dictionary-order}
3973 (@option{-d}) option is also given.
3979 @opindex --month-sort
3981 @cindex months, sorting by
3983 An initial string, consisting of any amount of blanks, followed
3984 by a month name abbreviation, is folded to UPPER case and
3985 compared in the order @samp{JAN} < @samp{FEB} < @dots{} < @samp{DEC}.
3986 Invalid names compare low to valid names. The @env{LC_TIME} locale
3987 category determines the month spellings.
3988 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3992 @itemx --numeric-sort
3993 @itemx --sort=numeric
3995 @opindex --numeric-sort
3997 @cindex numeric sort
3999 Sort numerically. The number begins each line and consists
4000 of optional blanks, an optional @samp{-} sign, and zero or more
4001 digits possibly separated by thousands separators, optionally followed
4002 by a decimal-point character and zero or more digits. An empty
4003 number is treated as @samp{0}. The @env{LC_NUMERIC}
4004 locale specifies the decimal-point character and thousands separator.
4005 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4008 Comparison is exact; there is no rounding error.
4010 Neither a leading @samp{+} nor exponential notation is recognized.
4011 To compare such strings numerically, use the
4012 @option{--general-numeric-sort} (@option{-g}) option.
4015 @itemx --version-sort
4017 @opindex --version-sort
4018 @cindex version number sort
4019 Sort by version name and number. It behaves like a standard sort,
4020 except that each sequence of decimal digits is treated numerically
4021 as an index/version number. (@xref{Details about version sort}.)
4027 @cindex reverse sorting
4028 Reverse the result of comparison, so that lines with greater key values
4029 appear earlier in the output instead of later.
4032 @itemx --random-sort
4033 @itemx --sort=random
4035 @opindex --random-sort
4038 Sort by hashing the input keys and then sorting the hash values.
4039 Choose the hash function at random, ensuring that it is free of
4040 collisions so that differing keys have differing hash values. This is
4041 like a random permutation of the inputs (@pxref{shuf invocation}),
4042 except that keys with the same value sort together.
4044 If multiple random sort fields are specified, the same random hash
4045 function is used for all fields. To use different random hash
4046 functions for different fields, you can invoke @command{sort} more
4049 The choice of hash function is affected by the
4050 @option{--random-source} option.
4058 @item --compress-program=@var{prog}
4059 Compress any temporary files with the program @var{prog}.
4061 With no arguments, @var{prog} must compress standard input to standard
4062 output, and when given the @option{-d} option it must decompress
4063 standard input to standard output.
4065 Terminate with an error if @var{prog} exits with nonzero status.
4067 White space and the backslash character should not appear in
4068 @var{prog}; they are reserved for future use.
4070 @filesZeroFromOption{sort,,sorted output}
4072 @item -k @var{pos1}[,@var{pos2}]
4073 @itemx --key=@var{pos1}[,@var{pos2}]
4077 Specify a sort field that consists of the part of the line between
4078 @var{pos1} and @var{pos2} (or the end of the line, if @var{pos2} is
4079 omitted), @emph{inclusive}.
4081 Each @var{pos} has the form @samp{@var{f}[.@var{c}][@var{opts}]},
4082 where @var{f} is the number of the field to use, and @var{c} is the number
4083 of the first character from the beginning of the field. Fields and character
4084 positions are numbered starting with 1; a character position of zero in
4085 @var{pos2} indicates the field's last character. If @samp{.@var{c}} is
4086 omitted from @var{pos1}, it defaults to 1 (the beginning of the field);
4087 if omitted from @var{pos2}, it defaults to 0 (the end of the field).
4088 @var{opts} are ordering options, allowing individual keys to be sorted
4089 according to different rules; see below for details. Keys can span
4092 Example: To sort on the second field, use @option{--key=2,2}
4093 (@option{-k 2,2}). See below for more notes on keys and more examples.
4094 See also the @option{--debug} option to help determine the part
4095 of the line being used in the sort.
4098 Highlight the portion of each line used for sorting.
4099 Also issue warnings about questionable usage to stderr.
4101 @item --batch-size=@var{nmerge}
4102 @opindex --batch-size
4103 @cindex number of inputs to merge, nmerge
4104 Merge at most @var{nmerge} inputs at once.
4106 When @command{sort} has to merge more than @var{nmerge} inputs,
4107 it merges them in groups of @var{nmerge}, saving the result in
4108 a temporary file, which is then used as an input in a subsequent merge.
4110 A large value of @var{nmerge} may improve merge performance and decrease
4111 temporary storage utilization at the expense of increased memory usage
4112 and I/0. Conversely a small value of @var{nmerge} may reduce memory
4113 requirements and I/0 at the expense of temporary storage consumption and
4116 The value of @var{nmerge} must be at least 2. The default value is
4117 currently 16, but this is implementation-dependent and may change in
4120 The value of @var{nmerge} may be bounded by a resource limit for open
4121 file descriptors. The commands @samp{ulimit -n} or @samp{getconf
4122 OPEN_MAX} may display limits for your systems; these limits may be
4123 modified further if your program already has some files open, or if
4124 the operating system has other limits on the number of open files. If
4125 the value of @var{nmerge} exceeds the resource limit, @command{sort}
4126 silently uses a smaller value.
4128 @item -o @var{output-file}
4129 @itemx --output=@var{output-file}
4132 @cindex overwriting of input, allowed
4133 Write output to @var{output-file} instead of standard output.
4134 Normally, @command{sort} reads all input before opening
4135 @var{output-file}, so you can safely sort a file in place by using
4136 commands like @code{sort -o F F} and @code{cat F | sort -o F}.
4137 However, @command{sort} with @option{--merge} (@option{-m}) can open
4138 the output file before reading all input, so a command like @code{cat
4139 F | sort -m -o F - G} is not safe as @command{sort} might start
4140 writing @file{F} before @command{cat} is done reading it.
4142 @vindex POSIXLY_CORRECT
4143 On newer systems, @option{-o} cannot appear after an input file if
4144 @env{POSIXLY_CORRECT} is set, e.g., @samp{sort F -o F}. Portable
4145 scripts should specify @option{-o @var{output-file}} before any input
4148 @item --random-source=@var{file}
4149 @opindex --random-source
4150 @cindex random source for sorting
4151 Use @var{file} as a source of random data used to determine which
4152 random hash function to use with the @option{-R} option. @xref{Random
4159 @cindex sort stability
4160 @cindex sort's last-resort comparison
4162 Make @command{sort} stable by disabling its last-resort comparison.
4163 This option has no effect if no fields or global ordering options
4164 other than @option{--reverse} (@option{-r}) are specified.
4167 @itemx --buffer-size=@var{size}
4169 @opindex --buffer-size
4170 @cindex size for main memory sorting
4171 Use a main-memory sort buffer of the given @var{size}. By default,
4172 @var{size} is in units of 1024 bytes. Appending @samp{%} causes
4173 @var{size} to be interpreted as a percentage of physical memory.
4174 Appending @samp{K} multiplies @var{size} by 1024 (the default),
4175 @samp{M} by 1,048,576, @samp{G} by 1,073,741,824, and so on for
4176 @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}. Appending
4177 @samp{b} causes @var{size} to be interpreted as a byte count, with no
4180 This option can improve the performance of @command{sort} by causing it
4181 to start with a larger or smaller sort buffer than the default.
4182 However, this option affects only the initial buffer size. The buffer
4183 grows beyond @var{size} if @command{sort} encounters input lines larger
4186 @item -t @var{separator}
4187 @itemx --field-separator=@var{separator}
4189 @opindex --field-separator
4190 @cindex field separator character
4191 Use character @var{separator} as the field separator when finding the
4192 sort keys in each line. By default, fields are separated by the empty
4193 string between a non-blank character and a blank character.
4194 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4197 That is, given the input line @w{@samp{ foo bar}}, @command{sort} breaks it
4198 into fields @w{@samp{ foo}} and @w{@samp{ bar}}. The field separator is
4199 not considered to be part of either the field preceding or the field
4200 following, so with @samp{sort @w{-t " "}} the same input line has
4201 three fields: an empty field, @samp{foo}, and @samp{bar}.
4202 However, fields that extend to the end of the line,
4203 as @option{-k 2}, or fields consisting of a range, as @option{-k 2,3},
4204 retain the field separators present between the endpoints of the range.
4206 To specify @acronym{ASCII} @sc{nul} as the field separator,
4207 use the two-character string @samp{\0}, e.g., @samp{sort -t '\0'}.
4209 @item -T @var{tempdir}
4210 @itemx --temporary-directory=@var{tempdir}
4212 @opindex --temporary-directory
4213 @cindex temporary directory
4215 Use directory @var{tempdir} to store temporary files, overriding the
4216 @env{TMPDIR} environment variable. If this option is given more than
4217 once, temporary files are stored in all the directories given. If you
4218 have a large sort or merge that is I/O-bound, you can often improve
4219 performance by using this option to specify directories on different
4220 disks and controllers.
4222 @item --parallel=@var{n}
4224 @cindex multithreaded sort
4225 Set the number of sorts run in parallel to @var{n}. By default,
4226 @var{n} is set to the number of available processors, but limited
4227 to 8, as there are diminishing performance gains after that.
4228 Note also that using @var{n} threads increases the memory usage by
4229 a factor of log @var{n}. Also see @ref{nproc invocation}.
4235 @cindex uniquifying output
4237 Normally, output only the first of a sequence of lines that compare
4238 equal. For the @option{--check} (@option{-c} or @option{-C}) option,
4239 check that no pair of consecutive lines compares equal.
4241 This option also disables the default last-resort comparison.
4243 The commands @code{sort -u} and @code{sort | uniq} are equivalent, but
4244 this equivalence does not extend to arbitrary @command{sort} options.
4245 For example, @code{sort -n -u} inspects only the value of the initial
4246 numeric string when checking for uniqueness, whereas @code{sort -n |
4247 uniq} inspects the entire line. @xref{uniq invocation}.
4249 @macro zeroTerminatedOption
4251 @itemx --zero-terminated
4253 @opindex --zero-terminated
4254 @cindex process zero-terminated items
4255 Delimit items with a zero byte rather than a newline (@acronym{ASCII} @sc{lf}).
4256 I.E. treat input as items separated by @acronym{ASCII} @sc{nul}
4257 and terminate output items with @acronym{ASCII} @sc{nul}.
4258 This option can be useful in conjunction with @samp{perl -0} or
4259 @samp{find -print0} and @samp{xargs -0} which do the same in order to
4260 reliably handle arbitrary file names (even those containing blanks
4261 or other special characters).
4263 @zeroTerminatedOption
4267 Historical (BSD and System V) implementations of @command{sort} have
4268 differed in their interpretation of some options, particularly
4269 @option{-b}, @option{-f}, and @option{-n}.
4270 @sc{gnu} sort follows the @acronym{POSIX}
4271 behavior, which is usually (but not always!) like the System V behavior.
4272 According to @acronym{POSIX}, @option{-n} no longer implies @option{-b}. For
4273 consistency, @option{-M} has been changed in the same way. This may
4274 affect the meaning of character positions in field specifications in
4275 obscure cases. The only fix is to add an explicit @option{-b}.
4277 A position in a sort field specified with @option{-k} may have any
4278 of the option letters @samp{MbdfghinRrV} appended to it, in which case no
4279 global ordering options are inherited by that particular field. The
4280 @option{-b} option may be independently attached to either or both of
4281 the start and end positions of a field specification, and if it is
4282 inherited from the global options it will be attached to both.
4283 If input lines can contain leading or adjacent blanks and @option{-t}
4284 is not used, then @option{-k} is typically combined with @option{-b} or
4285 an option that implicitly ignores leading blanks (@samp{Mghn}) as otherwise
4286 the varying numbers of leading blanks in fields can cause confusing results.
4288 If the start position in a sort field specifier falls after the end of
4289 the line or after the end field, the field is empty. If the @option{-b}
4290 option was specified, the @samp{.@var{c}} part of a field specification
4291 is counted from the first nonblank character of the field.
4293 @vindex _POSIX2_VERSION
4294 @vindex POSIXLY_CORRECT
4295 On older systems, @command{sort} supports an obsolete origin-zero
4296 syntax @samp{+@var{pos1} [-@var{pos2}]} for specifying sort keys.
4297 The obsolete sequence @samp{sort +@var{a}.@var{x} -@var{b}.@var{y}}
4298 is equivalent to @samp{sort -k @var{a+1}.@var{x+1},@var{b}} if @var{y}
4299 is @samp{0} or absent, otherwise it is equivalent to @samp{sort -k
4300 @var{a+1}.@var{x+1},@var{b+1}.@var{y}}.
4302 This obsolete behavior can be enabled or disabled with the
4303 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4304 conformance}); it can also be enabled when @env{POSIXLY_CORRECT} is
4305 not set by using the obsolete syntax with @samp{-@var{pos2}} present.
4307 Scripts intended for use on standard hosts should avoid obsolete
4308 syntax and should use @option{-k} instead. For example, avoid
4309 @samp{sort +2}, since it might be interpreted as either @samp{sort
4310 ./+2} or @samp{sort -k 3}. If your script must also run on hosts that
4311 support only the obsolete syntax, it can use a test like @samp{if sort
4312 -k 1 </dev/null >/dev/null 2>&1; then @dots{}} to decide which syntax
4315 Here are some examples to illustrate various combinations of options.
4320 Sort in descending (reverse) numeric order.
4327 Run no more that 4 sorts concurrently, using a buffer size of 10M.
4330 sort --parallel=4 -S 10M
4334 Sort alphabetically, omitting the first and second fields
4335 and the blanks at the start of the third field.
4336 This uses a single key composed of the characters beginning
4337 at the start of the first nonblank character in field three
4338 and extending to the end of each line.
4345 Sort numerically on the second field and resolve ties by sorting
4346 alphabetically on the third and fourth characters of field five.
4347 Use @samp{:} as the field delimiter.
4350 sort -t : -k 2,2n -k 5.3,5.4
4353 Note that if you had written @option{-k 2n} instead of @option{-k 2,2n}
4354 @command{sort} would have used all characters beginning in the second field
4355 and extending to the end of the line as the primary @emph{numeric}
4356 key. For the large majority of applications, treating keys spanning
4357 more than one field as numeric will not do what you expect.
4359 Also note that the @samp{n} modifier was applied to the field-end
4360 specifier for the first key. It would have been equivalent to
4361 specify @option{-k 2n,2} or @option{-k 2n,2n}. All modifiers except
4362 @samp{b} apply to the associated @emph{field}, regardless of whether
4363 the modifier character is attached to the field-start and/or the
4364 field-end part of the key specifier.
4367 Sort the password file on the fifth field and ignore any
4368 leading blanks. Sort lines with equal values in field five
4369 on the numeric user ID in field three. Fields are separated
4373 sort -t : -k 5b,5 -k 3,3n /etc/passwd
4374 sort -t : -n -k 5b,5 -k 3,3 /etc/passwd
4375 sort -t : -b -k 5,5 -k 3,3n /etc/passwd
4378 These three commands have equivalent effect. The first specifies that
4379 the first key's start position ignores leading blanks and the second
4380 key is sorted numerically. The other two commands rely on global
4381 options being inherited by sort keys that lack modifiers. The inheritance
4382 works in this case because @option{-k 5b,5b} and @option{-k 5b,5} are
4383 equivalent, as the location of a field-end lacking a @samp{.@var{c}}
4384 character position is not affected by whether initial blanks are
4388 Sort a set of log files, primarily by IPv4 address and secondarily by
4389 time stamp. If two lines' primary and secondary keys are identical,
4390 output the lines in the same order that they were input. The log
4391 files contain lines that look like this:
4394 4.150.156.3 - - [01/Apr/2004:06:31:51 +0000] message 1
4395 211.24.3.231 - - [24/Apr/2004:20:17:39 +0000] message 2
4398 Fields are separated by exactly one space. Sort IPv4 addresses
4399 lexicographically, e.g., 212.61.52.2 sorts before 212.129.233.201
4400 because 61 is less than 129.
4403 sort -s -t ' ' -k 4.9n -k 4.5M -k 4.2n -k 4.14,4.21 file*.log |
4404 sort -s -t '.' -k 1,1n -k 2,2n -k 3,3n -k 4,4n
4407 This example cannot be done with a single @command{sort} invocation,
4408 since IPv4 address components are separated by @samp{.} while dates
4409 come just after a space. So it is broken down into two invocations of
4410 @command{sort}: the first sorts by time stamp and the second by IPv4
4411 address. The time stamp is sorted by year, then month, then day, and
4412 finally by hour-minute-second field, using @option{-k} to isolate each
4413 field. Except for hour-minute-second there's no need to specify the
4414 end of each key field, since the @samp{n} and @samp{M} modifiers sort
4415 based on leading prefixes that cannot cross field boundaries. The
4416 IPv4 addresses are sorted lexicographically. The second sort uses
4417 @samp{-s} so that ties in the primary key are broken by the secondary
4418 key; the first sort uses @samp{-s} so that the combination of the two
4422 Generate a tags file in case-insensitive sorted order.
4425 find src -type f -print0 | sort -z -f | xargs -0 etags --append
4428 The use of @option{-print0}, @option{-z}, and @option{-0} in this case means
4429 that file names that contain blanks or other special characters are
4431 by the sort operation.
4433 @c This example is a bit contrived and needs more explanation.
4435 @c Sort records separated by an arbitrary string by using a pipe to convert
4436 @c each record delimiter string to @samp{\0}, then using sort's -z option,
4437 @c and converting each @samp{\0} back to the original record delimiter.
4440 @c printf 'c\n\nb\n\na\n' |
4441 @c perl -0pe 's/\n\n/\n\0/g' |
4443 @c perl -0pe 's/\0/\n/g'
4447 Use the common @acronym{DSU, Decorate Sort Undecorate} idiom to
4448 sort lines according to their length.
4451 awk '@{print length, $0@}' /etc/passwd | sort -n | cut -f2- -d' '
4454 In general this technique can be used to sort data that the @command{sort}
4455 command does not support, or is inefficient at, sorting directly.
4458 Shuffle a list of directories, but preserve the order of files within
4459 each directory. For instance, one could use this to generate a music
4460 playlist in which albums are shuffled but the songs of each album are
4464 ls */* | sort -t / -k 1,1R -k 2,2
4470 @node shuf invocation
4471 @section @command{shuf}: Shuffling text
4474 @cindex shuffling files
4476 @command{shuf} shuffles its input by outputting a random permutation
4477 of its input lines. Each output permutation is equally likely.
4481 shuf [@var{option}]@dots{} [@var{file}]
4482 shuf -e [@var{option}]@dots{} [@var{arg}]@dots{}
4483 shuf -i @var{lo}-@var{hi} [@var{option}]@dots{}
4486 @command{shuf} has three modes of operation that affect where it
4487 obtains its input lines. By default, it reads lines from standard
4488 input. The following options change the operation mode:
4496 @cindex command-line operands to shuffle
4497 Treat each command-line operand as an input line.
4499 @item -i @var{lo}-@var{hi}
4500 @itemx --input-range=@var{lo}-@var{hi}
4502 @opindex --input-range
4503 @cindex input range to shuffle
4504 Act as if input came from a file containing the range of unsigned
4505 decimal integers @var{lo}@dots{}@var{hi}, one per line.
4509 @command{shuf}'s other options can affect its behavior in all
4514 @item -n @var{lines}
4515 @itemx --head-count=@var{count}
4517 @opindex --head-count
4518 @cindex head of output
4519 Output at most @var{count} lines. By default, all input lines are
4522 @item -o @var{output-file}
4523 @itemx --output=@var{output-file}
4526 @cindex overwriting of input, allowed
4527 Write output to @var{output-file} instead of standard output.
4528 @command{shuf} reads all input before opening
4529 @var{output-file}, so you can safely shuffle a file in place by using
4530 commands like @code{shuf -o F <F} and @code{cat F | shuf -o F}.
4532 @item --random-source=@var{file}
4533 @opindex --random-source
4534 @cindex random source for shuffling
4535 Use @var{file} as a source of random data used to determine which
4536 permutation to generate. @xref{Random sources}.
4538 @zeroTerminatedOption
4554 might produce the output
4564 Similarly, the command:
4567 shuf -e clubs hearts diamonds spades
4581 and the command @samp{shuf -i 1-4} might output:
4591 These examples all have four input lines, so @command{shuf} might
4592 produce any of the twenty-four possible permutations of the input. In
4593 general, if there are @var{n} input lines, there are @var{n}! (i.e.,
4594 @var{n} factorial, or @var{n} * (@var{n} - 1) * @dots{} * 1) possible
4595 output permutations.
4600 @node uniq invocation
4601 @section @command{uniq}: Uniquify files
4604 @cindex uniquify files
4606 @command{uniq} writes the unique lines in the given @file{input}, or
4607 standard input if nothing is given or for an @var{input} name of
4611 uniq [@var{option}]@dots{} [@var{input} [@var{output}]]
4614 By default, @command{uniq} prints its input lines, except that
4615 it discards all but the first of adjacent repeated lines, so that
4616 no output lines are repeated. Optionally, it can instead discard
4617 lines that are not repeated, or all repeated lines.
4619 The input need not be sorted, but repeated input lines are detected
4620 only if they are adjacent. If you want to discard non-adjacent
4621 duplicate lines, perhaps you want to use @code{sort -u}.
4622 @xref{sort invocation}.
4625 Comparisons honor the rules specified by the @env{LC_COLLATE}
4628 If no @var{output} file is specified, @command{uniq} writes to standard
4631 The program accepts the following options. Also see @ref{Common options}.
4636 @itemx --skip-fields=@var{n}
4638 @opindex --skip-fields
4639 Skip @var{n} fields on each line before checking for uniqueness. Use
4640 a null string for comparison if a line has fewer than @var{n} fields. Fields
4641 are sequences of non-space non-tab characters that are separated from
4642 each other by at least one space or tab.
4644 For compatibility @command{uniq} supports an obsolete option syntax
4645 @option{-@var{n}}. New scripts should use @option{-f @var{n}} instead.
4648 @itemx --skip-chars=@var{n}
4650 @opindex --skip-chars
4651 Skip @var{n} characters before checking for uniqueness. Use a null string
4652 for comparison if a line has fewer than @var{n} characters. If you use both
4653 the field and character skipping options, fields are skipped over first.
4655 @vindex _POSIX2_VERSION
4656 On older systems, @command{uniq} supports an obsolete option syntax
4658 This obsolete behavior can be enabled or disabled with the
4659 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4660 conformance}), but portable scripts should avoid commands whose
4661 behavior depends on this variable.
4662 For example, use @samp{uniq ./+10} or @samp{uniq -s 10} rather than
4663 the ambiguous @samp{uniq +10}.
4669 Print the number of times each line occurred along with the line.
4672 @itemx --ignore-case
4674 @opindex --ignore-case
4675 Ignore differences in case when comparing lines.
4681 @cindex repeated lines, outputting
4682 Discard lines that are not repeated. When used by itself, this option
4683 causes @command{uniq} to print the first copy of each repeated line,
4687 @itemx --all-repeated[=@var{delimit-method}]
4689 @opindex --all-repeated
4690 @cindex all repeated lines, outputting
4691 Do not discard the second and subsequent repeated input lines,
4692 but discard lines that are not repeated.
4693 This option is useful mainly in conjunction with other options e.g.,
4694 to ignore case or to compare only selected fields.
4695 The optional @var{delimit-method} tells how to delimit
4696 groups of repeated lines, and must be one of the following:
4701 Do not delimit groups of repeated lines.
4702 This is equivalent to @option{--all-repeated} (@option{-D}).
4705 Output a newline before each group of repeated lines.
4706 With @option{--zero-terminated} (@option{-z}), use a zero
4707 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4710 Separate groups of repeated lines with a single newline.
4711 With @option{--zero-terminated} (@option{-z}), use a zero
4712 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4713 This is the same as using @samp{prepend}, except that
4714 no delimiter is inserted before the first group, and hence
4715 may be better suited for output direct to users.
4718 Note that when groups are delimited and the input stream contains
4719 two or more consecutive blank lines, then the output is ambiguous.
4720 To avoid that, filter the input through @samp{tr -s '\n'} to replace
4721 each sequence of consecutive newlines with a single newline.
4723 This is a @sc{gnu} extension.
4724 @c FIXME: give an example showing *how* it's useful
4730 @cindex unique lines, outputting
4731 Discard the first repeated line. When used by itself, this option
4732 causes @command{uniq} to print unique lines, and nothing else.
4735 @itemx --check-chars=@var{n}
4737 @opindex --check-chars
4738 Compare at most @var{n} characters on each line (after skipping any specified
4739 fields and characters). By default the entire rest of the lines are
4742 @zeroTerminatedOption
4749 @node comm invocation
4750 @section @command{comm}: Compare two sorted files line by line
4753 @cindex line-by-line comparison
4754 @cindex comparing sorted files
4756 @command{comm} writes to standard output lines that are common, and lines
4757 that are unique, to two input files; a file name of @samp{-} means
4758 standard input. Synopsis:
4761 comm [@var{option}]@dots{} @var{file1} @var{file2}
4765 Before @command{comm} can be used, the input files must be sorted using the
4766 collating sequence specified by the @env{LC_COLLATE} locale.
4767 If an input file ends in a non-newline
4768 character, a newline is silently appended. The @command{sort} command with
4769 no options always outputs a file that is suitable input to @command{comm}.
4771 @cindex differing lines
4772 @cindex common lines
4773 With no options, @command{comm} produces three-column output. Column one
4774 contains lines unique to @var{file1}, column two contains lines unique
4775 to @var{file2}, and column three contains lines common to both files.
4776 Columns are separated by a single TAB character.
4777 @c FIXME: when there's an option to supply an alternative separator
4778 @c string, append `by default' to the above sentence.
4783 The options @option{-1}, @option{-2}, and @option{-3} suppress printing of
4784 the corresponding columns (and separators). Also see @ref{Common options}.
4786 Unlike some other comparison utilities, @command{comm} has an exit
4787 status that does not depend on the result of the comparison.
4788 Upon normal completion @command{comm} produces an exit code of zero.
4789 If there is an error it exits with nonzero status.
4791 @macro checkOrderOption{cmd}
4792 If the @option{--check-order} option is given, unsorted inputs will
4793 cause a fatal error message. If the option @option{--nocheck-order}
4794 is given, unsorted inputs will never cause an error message. If neither
4795 of these options is given, wrongly sorted inputs are diagnosed
4796 only if an input file is found to contain unpairable
4798 lines, and when both input files are non empty.
4800 @ifclear JOIN_COMMAND
4803 If an input file is diagnosed as being unsorted, the @command{\cmd\}
4804 command will exit with a nonzero status (and the output should not be used).
4806 Forcing @command{\cmd\} to process wrongly sorted input files
4807 containing unpairable lines by specifying @option{--nocheck-order} is
4808 not guaranteed to produce any particular output. The output will
4809 probably not correspond with whatever you hoped it would be.
4811 @checkOrderOption{comm}
4816 Fail with an error message if either input file is wrongly ordered.
4818 @item --nocheck-order
4819 Do not check that both input files are in sorted order.
4823 @item --output-delimiter=@var{str}
4824 Print @var{str} between adjacent output columns,
4825 rather than the default of a single TAB character.
4827 The delimiter @var{str} may not be empty.
4831 @node ptx invocation
4832 @section @command{ptx}: Produce permuted indexes
4836 @command{ptx} reads a text file and essentially produces a permuted index, with
4837 each keyword in its context. The calling sketch is either one of:
4840 ptx [@var{option} @dots{}] [@var{file} @dots{}]
4841 ptx -G [@var{option} @dots{}] [@var{input} [@var{output}]]
4844 The @option{-G} (or its equivalent: @option{--traditional}) option disables
4845 all @sc{gnu} extensions and reverts to traditional mode, thus introducing some
4846 limitations and changing several of the program's default option values.
4847 When @option{-G} is not specified, @sc{gnu} extensions are always enabled.
4848 @sc{gnu} extensions to @command{ptx} are documented wherever appropriate in this
4849 document. For the full list, see @xref{Compatibility in ptx}.
4851 Individual options are explained in the following sections.
4853 When @sc{gnu} extensions are enabled, there may be zero, one or several
4854 @var{file}s after the options. If there is no @var{file}, the program
4855 reads the standard input. If there is one or several @var{file}s, they
4856 give the name of input files which are all read in turn, as if all the
4857 input files were concatenated. However, there is a full contextual
4858 break between each file and, when automatic referencing is requested,
4859 file names and line numbers refer to individual text input files. In
4860 all cases, the program outputs the permuted index to the standard
4863 When @sc{gnu} extensions are @emph{not} enabled, that is, when the program
4864 operates in traditional mode, there may be zero, one or two parameters
4865 besides the options. If there are no parameters, the program reads the
4866 standard input and outputs the permuted index to the standard output.
4867 If there is only one parameter, it names the text @var{input} to be read
4868 instead of the standard input. If two parameters are given, they give
4869 respectively the name of the @var{input} file to read and the name of
4870 the @var{output} file to produce. @emph{Be very careful} to note that,
4871 in this case, the contents of file given by the second parameter is
4872 destroyed. This behavior is dictated by System V @command{ptx}
4873 compatibility; @sc{gnu} Standards normally discourage output parameters not
4874 introduced by an option.
4876 Note that for @emph{any} file named as the value of an option or as an
4877 input text file, a single dash @kbd{-} may be used, in which case
4878 standard input is assumed. However, it would not make sense to use this
4879 convention more than once per program invocation.
4882 * General options in ptx:: Options which affect general program behavior.
4883 * Charset selection in ptx:: Underlying character set considerations.
4884 * Input processing in ptx:: Input fields, contexts, and keyword selection.
4885 * Output formatting in ptx:: Types of output format, and sizing the fields.
4886 * Compatibility in ptx::
4890 @node General options in ptx
4891 @subsection General options
4896 @itemx --traditional
4897 As already explained, this option disables all @sc{gnu} extensions to
4898 @command{ptx} and switches to traditional mode.
4901 Print a short help on standard output, then exit without further
4905 Print the program version on standard output, then exit without further
4913 @node Charset selection in ptx
4914 @subsection Charset selection
4916 @c FIXME: People don't necessarily know what an IBM-PC was these days.
4917 As it is set up now, the program assumes that the input file is coded
4918 using 8-bit @acronym{ISO} 8859-1 code, also known as Latin-1 character set,
4919 @emph{unless} it is compiled for MS-DOS, in which case it uses the
4920 character set of the IBM-PC@. (@sc{gnu} @command{ptx} is not known to work on
4921 smaller MS-DOS machines anymore.) Compared to 7-bit @acronym{ASCII}, the set
4922 of characters which are letters is different; this alters the behavior
4923 of regular expression matching. Thus, the default regular expression
4924 for a keyword allows foreign or diacriticized letters. Keyword sorting,
4925 however, is still crude; it obeys the underlying character set ordering
4931 @itemx --ignore-case
4932 Fold lower case letters to upper case for sorting.
4937 @node Input processing in ptx
4938 @subsection Word selection and input processing
4943 @itemx --break-file=@var{file}
4945 This option provides an alternative (to @option{-W}) method of describing
4946 which characters make up words. It introduces the name of a
4947 file which contains a list of characters which can@emph{not} be part of
4948 one word; this file is called the @dfn{Break file}. Any character which
4949 is not part of the Break file is a word constituent. If both options
4950 @option{-b} and @option{-W} are specified, then @option{-W} has precedence and
4951 @option{-b} is ignored.
4953 When @sc{gnu} extensions are enabled, the only way to avoid newline as a
4954 break character is to write all the break characters in the file with no
4955 newline at all, not even at the end of the file. When @sc{gnu} extensions
4956 are disabled, spaces, tabs and newlines are always considered as break
4957 characters even if not included in the Break file.
4960 @itemx --ignore-file=@var{file}
4962 The file associated with this option contains a list of words which will
4963 never be taken as keywords in concordance output. It is called the
4964 @dfn{Ignore file}. The file contains exactly one word in each line; the
4965 end of line separation of words is not subject to the value of the
4969 @itemx --only-file=@var{file}
4971 The file associated with this option contains a list of words which will
4972 be retained in concordance output; any word not mentioned in this file
4973 is ignored. The file is called the @dfn{Only file}. The file contains
4974 exactly one word in each line; the end of line separation of words is
4975 not subject to the value of the @option{-S} option.
4977 There is no default for the Only file. When both an Only file and an
4978 Ignore file are specified, a word is considered a keyword only
4979 if it is listed in the Only file and not in the Ignore file.
4984 On each input line, the leading sequence of non-white space characters will be
4985 taken to be a reference that has the purpose of identifying this input
4986 line in the resulting permuted index. For more information about reference
4987 production, see @xref{Output formatting in ptx}.
4988 Using this option changes the default value for option @option{-S}.
4990 Using this option, the program does not try very hard to remove
4991 references from contexts in output, but it succeeds in doing so
4992 @emph{when} the context ends exactly at the newline. If option
4993 @option{-r} is used with @option{-S} default value, or when @sc{gnu} extensions
4994 are disabled, this condition is always met and references are completely
4995 excluded from the output contexts.
4997 @item -S @var{regexp}
4998 @itemx --sentence-regexp=@var{regexp}
5000 This option selects which regular expression will describe the end of a
5001 line or the end of a sentence. In fact, this regular expression is not
5002 the only distinction between end of lines or end of sentences, and input
5003 line boundaries have no special significance outside this option. By
5004 default, when @sc{gnu} extensions are enabled and if @option{-r} option is not
5005 used, end of sentences are used. In this case, this @var{regex} is
5006 imported from @sc{gnu} Emacs:
5009 [.?!][]\"')@}]*\\($\\|\t\\| \\)[ \t\n]*
5012 Whenever @sc{gnu} extensions are disabled or if @option{-r} option is used, end
5013 of lines are used; in this case, the default @var{regexp} is just:
5019 Using an empty @var{regexp} is equivalent to completely disabling end of
5020 line or end of sentence recognition. In this case, the whole file is
5021 considered to be a single big line or sentence. The user might want to
5022 disallow all truncation flag generation as well, through option @option{-F
5023 ""}. @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
5026 When the keywords happen to be near the beginning of the input line or
5027 sentence, this often creates an unused area at the beginning of the
5028 output context line; when the keywords happen to be near the end of the
5029 input line or sentence, this often creates an unused area at the end of
5030 the output context line. The program tries to fill those unused areas
5031 by wrapping around context in them; the tail of the input line or
5032 sentence is used to fill the unused area on the left of the output line;
5033 the head of the input line or sentence is used to fill the unused area
5034 on the right of the output line.
5036 As a matter of convenience to the user, many usual backslashed escape
5037 sequences from the C language are recognized and converted to the
5038 corresponding characters by @command{ptx} itself.
5040 @item -W @var{regexp}
5041 @itemx --word-regexp=@var{regexp}
5043 This option selects which regular expression will describe each keyword.
5044 By default, if @sc{gnu} extensions are enabled, a word is a sequence of
5045 letters; the @var{regexp} used is @samp{\w+}. When @sc{gnu} extensions are
5046 disabled, a word is by default anything which ends with a space, a tab
5047 or a newline; the @var{regexp} used is @samp{[^ \t\n]+}.
5049 An empty @var{regexp} is equivalent to not using this option.
5050 @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
5053 As a matter of convenience to the user, many usual backslashed escape
5054 sequences, as found in the C language, are recognized and converted to
5055 the corresponding characters by @command{ptx} itself.
5060 @node Output formatting in ptx
5061 @subsection Output formatting
5063 Output format is mainly controlled by the @option{-O} and @option{-T} options
5064 described in the table below. When neither @option{-O} nor @option{-T} are
5065 selected, and if @sc{gnu} extensions are enabled, the program chooses an
5066 output format suitable for a dumb terminal. Each keyword occurrence is
5067 output to the center of one line, surrounded by its left and right
5068 contexts. Each field is properly justified, so the concordance output
5069 can be readily observed. As a special feature, if automatic
5070 references are selected by option @option{-A} and are output before the
5071 left context, that is, if option @option{-R} is @emph{not} selected, then
5072 a colon is added after the reference; this nicely interfaces with @sc{gnu}
5073 Emacs @code{next-error} processing. In this default output format, each
5074 white space character, like newline and tab, is merely changed to
5075 exactly one space, with no special attempt to compress consecutive
5076 spaces. This might change in the future. Except for those white space
5077 characters, every other character of the underlying set of 256
5078 characters is transmitted verbatim.
5080 Output format is further controlled by the following options.
5084 @item -g @var{number}
5085 @itemx --gap-size=@var{number}
5087 Select the size of the minimum white space gap between the fields on the
5090 @item -w @var{number}
5091 @itemx --width=@var{number}
5093 Select the maximum output width of each final line. If references are
5094 used, they are included or excluded from the maximum output width
5095 depending on the value of option @option{-R}. If this option is not
5096 selected, that is, when references are output before the left context,
5097 the maximum output width takes into account the maximum length of all
5098 references. If this option is selected, that is, when references are
5099 output after the right context, the maximum output width does not take
5100 into account the space taken by references, nor the gap that precedes
5104 @itemx --auto-reference
5106 Select automatic references. Each input line will have an automatic
5107 reference made up of the file name and the line ordinal, with a single
5108 colon between them. However, the file name will be empty when standard
5109 input is being read. If both @option{-A} and @option{-r} are selected, then
5110 the input reference is still read and skipped, but the automatic
5111 reference is used at output time, overriding the input reference.
5114 @itemx --right-side-refs
5116 In the default output format, when option @option{-R} is not used, any
5117 references produced by the effect of options @option{-r} or @option{-A} are
5118 placed to the far right of output lines, after the right context. With
5119 default output format, when the @option{-R} option is specified, references
5120 are rather placed at the beginning of each output line, before the left
5121 context. For any other output format, option @option{-R} is
5122 ignored, with one exception: with @option{-R} the width of references
5123 is @emph{not} taken into account in total output width given by @option{-w}.
5125 This option is automatically selected whenever @sc{gnu} extensions are
5128 @item -F @var{string}
5129 @itemx --flac-truncation=@var{string}
5131 This option will request that any truncation in the output be reported
5132 using the string @var{string}. Most output fields theoretically extend
5133 towards the beginning or the end of the current line, or current
5134 sentence, as selected with option @option{-S}. But there is a maximum
5135 allowed output line width, changeable through option @option{-w}, which is
5136 further divided into space for various output fields. When a field has
5137 to be truncated because it cannot extend beyond the beginning or the end of
5138 the current line to fit in, then a truncation occurs. By default,
5139 the string used is a single slash, as in @option{-F /}.
5141 @var{string} may have more than one character, as in @option{-F ...}.
5142 Also, in the particular case when @var{string} is empty (@option{-F ""}),
5143 truncation flagging is disabled, and no truncation marks are appended in
5146 As a matter of convenience to the user, many usual backslashed escape
5147 sequences, as found in the C language, are recognized and converted to
5148 the corresponding characters by @command{ptx} itself.
5150 @item -M @var{string}
5151 @itemx --macro-name=@var{string}
5153 Select another @var{string} to be used instead of @samp{xx}, while
5154 generating output suitable for @command{nroff}, @command{troff} or @TeX{}.
5157 @itemx --format=roff
5159 Choose an output format suitable for @command{nroff} or @command{troff}
5160 processing. Each output line will look like:
5163 .xx "@var{tail}" "@var{before}" "@var{keyword_and_after}"@c
5164 "@var{head}" "@var{ref}"
5167 so it will be possible to write a @samp{.xx} roff macro to take care of
5168 the output typesetting. This is the default output format when @sc{gnu}
5169 extensions are disabled. Option @option{-M} can be used to change
5170 @samp{xx} to another macro name.
5172 In this output format, each non-graphical character, like newline and
5173 tab, is merely changed to exactly one space, with no special attempt to
5174 compress consecutive spaces. Each quote character: @kbd{"} is doubled
5175 so it will be correctly processed by @command{nroff} or @command{troff}.
5180 Choose an output format suitable for @TeX{} processing. Each output
5181 line will look like:
5184 \xx @{@var{tail}@}@{@var{before}@}@{@var{keyword}@}@c
5185 @{@var{after}@}@{@var{head}@}@{@var{ref}@}
5189 so it will be possible to write a @code{\xx} definition to take care of
5190 the output typesetting. Note that when references are not being
5191 produced, that is, neither option @option{-A} nor option @option{-r} is
5192 selected, the last parameter of each @code{\xx} call is inhibited.
5193 Option @option{-M} can be used to change @samp{xx} to another macro
5196 In this output format, some special characters, like @kbd{$}, @kbd{%},
5197 @kbd{&}, @kbd{#} and @kbd{_} are automatically protected with a
5198 backslash. Curly brackets @kbd{@{}, @kbd{@}} are protected with a
5199 backslash and a pair of dollar signs (to force mathematical mode). The
5200 backslash itself produces the sequence @code{\backslash@{@}}.
5201 Circumflex and tilde diacritical marks produce the sequence @code{^\@{ @}} and
5202 @code{~\@{ @}} respectively. Other diacriticized characters of the
5203 underlying character set produce an appropriate @TeX{} sequence as far
5204 as possible. The other non-graphical characters, like newline and tab,
5205 and all other characters which are not part of @acronym{ASCII}, are merely
5206 changed to exactly one space, with no special attempt to compress
5207 consecutive spaces. Let me know how to improve this special character
5208 processing for @TeX{}.
5213 @node Compatibility in ptx
5214 @subsection The @sc{gnu} extensions to @command{ptx}
5216 This version of @command{ptx} contains a few features which do not exist in
5217 System V @command{ptx}. These extra features are suppressed by using the
5218 @option{-G} command line option, unless overridden by other command line
5219 options. Some @sc{gnu} extensions cannot be recovered by overriding, so the
5220 simple rule is to avoid @option{-G} if you care about @sc{gnu} extensions.
5221 Here are the differences between this program and System V @command{ptx}.
5226 This program can read many input files at once, it always writes the
5227 resulting concordance on standard output. On the other hand, System V
5228 @command{ptx} reads only one file and sends the result to standard output
5229 or, if a second @var{file} parameter is given on the command, to that
5232 Having output parameters not introduced by options is a dangerous
5233 practice which @sc{gnu} avoids as far as possible. So, for using @command{ptx}
5234 portably between @sc{gnu} and System V, you should always use it with a
5235 single input file, and always expect the result on standard output. You
5236 might also want to automatically configure in a @option{-G} option to
5237 @command{ptx} calls in products using @command{ptx}, if the configurator finds
5238 that the installed @command{ptx} accepts @option{-G}.
5241 The only options available in System V @command{ptx} are options @option{-b},
5242 @option{-f}, @option{-g}, @option{-i}, @option{-o}, @option{-r}, @option{-t} and
5243 @option{-w}. All other options are @sc{gnu} extensions and are not repeated in
5244 this enumeration. Moreover, some options have a slightly different
5245 meaning when @sc{gnu} extensions are enabled, as explained below.
5248 By default, concordance output is not formatted for @command{troff} or
5249 @command{nroff}. It is rather formatted for a dumb terminal. @command{troff}
5250 or @command{nroff} output may still be selected through option @option{-O}.
5253 Unless @option{-R} option is used, the maximum reference width is
5254 subtracted from the total output line width. With @sc{gnu} extensions
5255 disabled, width of references is not taken into account in the output
5256 line width computations.
5259 All 256 bytes, even @acronym{ASCII} @sc{nul} bytes, are always read and
5260 processed from input file with no adverse effect, even if @sc{gnu} extensions
5261 are disabled. However, System V @command{ptx} does not accept 8-bit characters,
5262 a few control characters are rejected, and the tilde @kbd{~} is also rejected.
5265 Input line length is only limited by available memory, even if @sc{gnu}
5266 extensions are disabled. However, System V @command{ptx} processes only
5267 the first 200 characters in each line.
5270 The break (non-word) characters default to be every character except all
5271 letters of the underlying character set, diacriticized or not. When @sc{gnu}
5272 extensions are disabled, the break characters default to space, tab and
5276 The program makes better use of output line width. If @sc{gnu} extensions
5277 are disabled, the program rather tries to imitate System V @command{ptx},
5278 but still, there are some slight disposition glitches this program does
5279 not completely reproduce.
5282 The user can specify both an Ignore file and an Only file. This is not
5283 allowed with System V @command{ptx}.
5288 @node tsort invocation
5289 @section @command{tsort}: Topological sort
5292 @cindex topological sort
5294 @command{tsort} performs a topological sort on the given @var{file}, or
5295 standard input if no input file is given or for a @var{file} of
5296 @samp{-}. For more details and some history, see @ref{tsort background}.
5300 tsort [@var{option}] [@var{file}]
5303 @command{tsort} reads its input as pairs of strings, separated by blanks,
5304 indicating a partial ordering. The output is a total ordering that
5305 corresponds to the given partial ordering.
5319 will produce the output
5330 Consider a more realistic example.
5331 You have a large set of functions all in one file, and they may all be
5332 declared static except one. Currently that one (say @code{main}) is the
5333 first function defined in the file, and the ones it calls directly follow
5334 it, followed by those they call, etc. Let's say that you are determined
5335 to take advantage of prototypes, so you have to choose between declaring
5336 all of those functions (which means duplicating a lot of information from
5337 the definitions) and rearranging the functions so that as many as possible
5338 are defined before they are used. One way to automate the latter process
5339 is to get a list for each function of the functions it calls directly.
5340 Many programs can generate such lists. They describe a call graph.
5341 Consider the following list, in which a given line indicates that the
5342 function on the left calls the one on the right directly.
5348 tail_file pretty_name
5349 tail_file write_header
5351 tail_forever recheck
5352 tail_forever pretty_name
5353 tail_forever write_header
5354 tail_forever dump_remainder
5357 tail_lines start_lines
5358 tail_lines dump_remainder
5359 tail_lines file_lines
5360 tail_lines pipe_lines
5362 tail_bytes start_bytes
5363 tail_bytes dump_remainder
5364 tail_bytes pipe_bytes
5365 file_lines dump_remainder
5369 then you can use @command{tsort} to produce an ordering of those
5370 functions that satisfies your requirement.
5373 example$ tsort call-graph | tac
5393 @command{tsort} detects any cycles in the input and writes the first cycle
5394 encountered to standard error.
5396 Note that for a given partial ordering, generally there is no unique
5397 total ordering. In the context of the call graph above, the function
5398 @code{parse_options} may be placed anywhere in the list as long as it
5399 precedes @code{main}.
5401 The only options are @option{--help} and @option{--version}. @xref{Common
5407 * tsort background:: Where tsort came from.
5410 @node tsort background
5411 @subsection @command{tsort}: Background
5413 @command{tsort} exists because very early versions of the Unix linker processed
5414 an archive file exactly once, and in order. As @command{ld} read each object
5415 in the archive, it decided whether it was needed in the program based on
5416 whether it defined any symbols which were undefined at that point in
5419 This meant that dependencies within the archive had to be handled
5420 specially. For example, @code{scanf} probably calls @code{read}. That means
5421 that in a single pass through an archive, it was important for @code{scanf.o}
5422 to appear before read.o, because otherwise a program which calls
5423 @code{scanf} but not @code{read} might end up with an unexpected unresolved
5424 reference to @code{read}.
5426 The way to address this problem was to first generate a set of
5427 dependencies of one object file on another. This was done by a shell
5428 script called @command{lorder}. The GNU tools don't provide a version of
5429 lorder, as far as I know, but you can still find it in BSD
5432 Then you ran @command{tsort} over the @command{lorder} output, and you used the
5433 resulting sort to define the order in which you added objects to the archive.
5435 This whole procedure has been obsolete since about 1980, because
5436 Unix archives now contain a symbol table (traditionally built by
5437 @command{ranlib}, now generally built by @command{ar} itself), and the Unix
5438 linker uses the symbol table to effectively make multiple passes over
5441 Anyhow, that's where tsort came from. To solve an old problem with
5442 the way the linker handled archive files, which has since been solved
5446 @node Operating on fields
5447 @chapter Operating on fields
5450 * cut invocation:: Print selected parts of lines.
5451 * paste invocation:: Merge lines of files.
5452 * join invocation:: Join lines on a common field.
5456 @node cut invocation
5457 @section @command{cut}: Print selected parts of lines
5460 @command{cut} writes to standard output selected parts of each line of each
5461 input file, or standard input if no files are given or for a file name of
5465 cut @var{option}@dots{} [@var{file}]@dots{}
5468 In the table which follows, the @var{byte-list}, @var{character-list},
5469 and @var{field-list} are one or more numbers or ranges (two numbers
5470 separated by a dash) separated by commas. Bytes, characters, and
5471 fields are numbered starting at 1. Incomplete ranges may be
5472 given: @option{-@var{m}} means @samp{1-@var{m}}; @samp{@var{n}-} means
5473 @samp{@var{n}} through end of line or last field. The list elements
5474 can be repeated, can overlap, and can be specified in any order; but
5475 the selected input is written in the same order that it is read, and
5476 is written exactly once.
5478 The program accepts the following options. Also see @ref{Common
5483 @item -b @var{byte-list}
5484 @itemx --bytes=@var{byte-list}
5487 Select for printing only the bytes in positions listed in
5488 @var{byte-list}. Tabs and backspaces are treated like any other
5489 character; they take up 1 byte. If an output delimiter is specified,
5490 (see the description of @option{--output-delimiter}), then output that
5491 string between ranges of selected bytes.
5493 @item -c @var{character-list}
5494 @itemx --characters=@var{character-list}
5496 @opindex --characters
5497 Select for printing only the characters in positions listed in
5498 @var{character-list}. The same as @option{-b} for now, but
5499 internationalization will change that. Tabs and backspaces are
5500 treated like any other character; they take up 1 character. If an
5501 output delimiter is specified, (see the description of
5502 @option{--output-delimiter}), then output that string between ranges
5505 @item -f @var{field-list}
5506 @itemx --fields=@var{field-list}
5509 Select for printing only the fields listed in @var{field-list}.
5510 Fields are separated by a TAB character by default. Also print any
5511 line that contains no delimiter character, unless the
5512 @option{--only-delimited} (@option{-s}) option is specified.
5514 Note @command{awk} supports more sophisticated field processing,
5515 and by default will use (and discard) runs of blank characters to
5516 separate fields, and ignore leading and trailing blanks.
5519 awk '{print $2}' # print the second field
5520 awk '{print $NF-1}' # print the penultimate field
5521 awk '{print $2,$1}' # reorder the first two fields
5525 In the unlikely event that @command{awk} is unavailable,
5526 one can use the @command{join} command, to process blank
5527 characters as @command{awk} does above.
5530 join -a1 -o 1.2 - /dev/null # print the second field
5531 join -a1 -o 1.2,1.1 - /dev/null # reorder the first two fields
5535 @item -d @var{input_delim_byte}
5536 @itemx --delimiter=@var{input_delim_byte}
5538 @opindex --delimiter
5539 With @option{-f}, use the first byte of @var{input_delim_byte} as
5540 the input fields separator (default is TAB).
5544 Do not split multi-byte characters (no-op for now).
5547 @itemx --only-delimited
5549 @opindex --only-delimited
5550 For @option{-f}, do not print lines that do not contain the field separator
5551 character. Normally, any line without a field separator is printed verbatim.
5553 @item --output-delimiter=@var{output_delim_string}
5554 @opindex --output-delimiter
5555 With @option{-f}, output fields are separated by @var{output_delim_string}.
5556 The default with @option{-f} is to use the input delimiter.
5557 When using @option{-b} or @option{-c} to select ranges of byte or
5558 character offsets (as opposed to ranges of fields),
5559 output @var{output_delim_string} between non-overlapping
5560 ranges of selected bytes.
5563 @opindex --complement
5564 This option is a @acronym{GNU} extension.
5565 Select for printing the complement of the bytes, characters or fields
5566 selected with the @option{-b}, @option{-c} or @option{-f} options.
5567 In other words, do @emph{not} print the bytes, characters or fields
5568 specified via those options. This option is useful when you have
5569 many fields and want to print all but a few of them.
5576 @node paste invocation
5577 @section @command{paste}: Merge lines of files
5580 @cindex merging files
5582 @command{paste} writes to standard output lines consisting of sequentially
5583 corresponding lines of each given file, separated by a TAB character.
5584 Standard input is used for a file name of @samp{-} or if no input files
5606 paste [@var{option}]@dots{} [@var{file}]@dots{}
5609 The program accepts the following options. Also see @ref{Common options}.
5617 Paste the lines of one file at a time rather than one line from each
5618 file. Using the above example data:
5621 $ paste -s num2 let3
5626 @item -d @var{delim-list}
5627 @itemx --delimiters=@var{delim-list}
5629 @opindex --delimiters
5630 Consecutively use the characters in @var{delim-list} instead of
5631 TAB to separate merged lines. When @var{delim-list} is
5632 exhausted, start again at its beginning. Using the above example data:
5635 $ paste -d '%_' num2 let3 num2
5646 @node join invocation
5647 @section @command{join}: Join lines on a common field
5650 @cindex common field, joining on
5652 @command{join} writes to standard output a line for each pair of input
5653 lines that have identical join fields. Synopsis:
5656 join [@var{option}]@dots{} @var{file1} @var{file2}
5659 Either @var{file1} or @var{file2} (but not both) can be @samp{-},
5660 meaning standard input. @var{file1} and @var{file2} should be
5661 sorted on the join fields.
5664 Normally, the sort order is that of the
5665 collating sequence specified by the @env{LC_COLLATE} locale. Unless
5666 the @option{-t} option is given, the sort comparison ignores blanks at
5667 the start of the join field, as in @code{sort -b}. If the
5668 @option{--ignore-case} option is given, the sort comparison ignores
5669 the case of characters in the join field, as in @code{sort -f}.
5671 The @command{sort} and @command{join} commands should use consistent
5672 locales and options if the output of @command{sort} is fed to
5673 @command{join}. You can use a command like @samp{sort -k 1b,1} to
5674 sort a file on its default join field, but if you select a non-default
5675 locale, join field, separator, or comparison options, then you should
5676 do so consistently between @command{join} and @command{sort}.
5677 If @samp{join -t ''} is specified then the whole line is considered which
5678 matches the default operation of sort.
5680 If the input has no unpairable lines, a @acronym{GNU} extension is
5681 available; the sort order can be any order that considers two fields
5682 to be equal if and only if the sort comparison described above
5683 considers them to be equal. For example:
5701 @checkOrderOption{join}
5706 @item the join field is the first field in each line;
5707 @item fields in the input are separated by one or more blanks, with leading
5708 blanks on the line ignored;
5709 @item fields in the output are separated by a space;
5710 @item each output line consists of the join field, the remaining
5711 fields from @var{file1}, then the remaining fields from @var{file2}.
5714 The program accepts the following options. Also see @ref{Common options}.
5718 @item -a @var{file-number}
5720 Print a line for each unpairable line in file @var{file-number} (either
5721 @samp{1} or @samp{2}), in addition to the normal output.
5724 Fail with an error message if either input file is wrongly ordered.
5726 @item --nocheck-order
5727 Do not check that both input files are in sorted order. This is the default.
5729 @item -e @var{string}
5731 Replace those output fields that are missing in the input with @var{string}.
5732 I.E. missing fields specified with the @option{-12jo} options.
5736 Treat the first line of each input file as a header line. The header lines will
5737 be joined and printed as the first output line. If @option{-o} is used to
5738 specify output format, the header line will be printed according to the
5739 specified format. The header lines will not be checked for ordering even if
5740 @option{--check-order} is specified. Also if the header lines from each file
5741 do not match, the heading fields from the first file will be used.
5744 @itemx --ignore-case
5746 @opindex --ignore-case
5747 Ignore differences in case when comparing keys.
5748 With this option, the lines of the input files must be ordered in the same way.
5749 Use @samp{sort -f} to produce this ordering.
5751 @item -1 @var{field}
5753 Join on field @var{field} (a positive integer) of file 1.
5755 @item -2 @var{field}
5757 Join on field @var{field} (a positive integer) of file 2.
5759 @item -j @var{field}
5760 Equivalent to @option{-1 @var{field} -2 @var{field}}.
5762 @item -o @var{field-list}
5764 If the keyword @samp{auto} is specified, infer the output format from
5765 the first line in each file. This is the same as the default output format
5766 but also ensures the same number of fields are output for each line.
5767 Missing fields are replaced with the @option{-e} option and extra fields
5770 Otherwise, construct each output line according to the format in
5771 @var{field-list}. Each element in @var{field-list} is either the single
5772 character @samp{0} or has the form @var{m.n} where the file number, @var{m},
5773 is @samp{1} or @samp{2} and @var{n} is a positive field number.
5775 A field specification of @samp{0} denotes the join field.
5776 In most cases, the functionality of the @samp{0} field spec
5777 may be reproduced using the explicit @var{m.n} that corresponds
5778 to the join field. However, when printing unpairable lines
5779 (using either of the @option{-a} or @option{-v} options), there is no way
5780 to specify the join field using @var{m.n} in @var{field-list}
5781 if there are unpairable lines in both files.
5782 To give @command{join} that functionality, @acronym{POSIX} invented the @samp{0}
5783 field specification notation.
5785 The elements in @var{field-list}
5786 are separated by commas or blanks.
5787 Blank separators typically need to be quoted for the shell. For
5788 example, the commands @samp{join -o 1.2,2.2} and @samp{join -o '1.2
5789 2.2'} are equivalent.
5791 All output lines---including those printed because of any -a or -v
5792 option---are subject to the specified @var{field-list}.
5795 Use character @var{char} as the input and output field separator.
5796 Treat as significant each occurrence of @var{char} in the input file.
5797 Use @samp{sort -t @var{char}}, without the @option{-b} option of
5798 @samp{sort}, to produce this ordering. If @samp{join -t ''} is specified,
5799 the whole line is considered, matching the default operation of sort.
5800 If @samp{-t '\0'} is specified then the @acronym{ASCII} @sc{nul}
5801 character is used to delimit the fields.
5803 @item -v @var{file-number}
5804 Print a line for each unpairable line in file @var{file-number}
5805 (either @samp{1} or @samp{2}), instead of the normal output.
5812 @node Operating on characters
5813 @chapter Operating on characters
5815 @cindex operating on characters
5817 This commands operate on individual characters.
5820 * tr invocation:: Translate, squeeze, and/or delete characters.
5821 * expand invocation:: Convert tabs to spaces.
5822 * unexpand invocation:: Convert spaces to tabs.
5827 @section @command{tr}: Translate, squeeze, and/or delete characters
5834 tr [@var{option}]@dots{} @var{set1} [@var{set2}]
5837 @command{tr} copies standard input to standard output, performing
5838 one of the following operations:
5842 translate, and optionally squeeze repeated characters in the result,
5844 squeeze repeated characters,
5848 delete characters, then squeeze repeated characters from the result.
5851 The @var{set1} and (if given) @var{set2} arguments define ordered
5852 sets of characters, referred to below as @var{set1} and @var{set2}. These
5853 sets are the characters of the input that @command{tr} operates on.
5854 The @option{--complement} (@option{-c}, @option{-C}) option replaces
5856 complement (all of the characters that are not in @var{set1}).
5858 Currently @command{tr} fully supports only single-byte characters.
5859 Eventually it will support multibyte characters; when it does, the
5860 @option{-C} option will cause it to complement the set of characters,
5861 whereas @option{-c} will cause it to complement the set of values.
5862 This distinction will matter only when some values are not characters,
5863 and this is possible only in locales using multibyte encodings when
5864 the input contains encoding errors.
5866 The program accepts the @option{--help} and @option{--version}
5867 options. @xref{Common options}. Options must precede operands.
5872 * Character sets:: Specifying sets of characters.
5873 * Translating:: Changing one set of characters to another.
5874 * Squeezing:: Squeezing repeats and deleting.
5878 @node Character sets
5879 @subsection Specifying sets of characters
5881 @cindex specifying sets of characters
5883 The format of the @var{set1} and @var{set2} arguments resembles
5884 the format of regular expressions; however, they are not regular
5885 expressions, only lists of characters. Most characters simply
5886 represent themselves in these strings, but the strings can contain
5887 the shorthands listed below, for convenience. Some of them can be
5888 used only in @var{set1} or @var{set2}, as noted below.
5892 @item Backslash escapes
5893 @cindex backslash escapes
5895 The following backslash escape sequences are recognized:
5913 The 8-bit character with the value given by @var{ooo}, which is 1 to 3
5914 octal digits. Note that @samp{\400} is interpreted as the two-byte
5915 sequence, @samp{\040} @samp{0}.
5920 While a backslash followed by a character not listed above is
5921 interpreted as that character, the backslash also effectively
5922 removes any special significance, so it is useful to escape
5923 @samp{[}, @samp{]}, @samp{*}, and @samp{-}.
5928 The notation @samp{@var{m}-@var{n}} expands to all of the characters
5929 from @var{m} through @var{n}, in ascending order. @var{m} should
5930 collate before @var{n}; if it doesn't, an error results. As an example,
5931 @samp{0-9} is the same as @samp{0123456789}.
5933 @sc{gnu} @command{tr} does not support the System V syntax that uses square
5934 brackets to enclose ranges. Translations specified in that format
5935 sometimes work as expected, since the brackets are often transliterated
5936 to themselves. However, they should be avoided because they sometimes
5937 behave unexpectedly. For example, @samp{tr -d '[0-9]'} deletes brackets
5940 Many historically common and even accepted uses of ranges are not
5941 portable. For example, on @acronym{EBCDIC} hosts using the @samp{A-Z}
5942 range will not do what most would expect because @samp{A} through @samp{Z}
5943 are not contiguous as they are in @acronym{ASCII}.
5944 If you can rely on a @acronym{POSIX} compliant version of @command{tr}, then
5945 the best way to work around this is to use character classes (see below).
5946 Otherwise, it is most portable (and most ugly) to enumerate the members
5949 @item Repeated characters
5950 @cindex repeated characters
5952 The notation @samp{[@var{c}*@var{n}]} in @var{set2} expands to @var{n}
5953 copies of character @var{c}. Thus, @samp{[y*6]} is the same as
5954 @samp{yyyyyy}. The notation @samp{[@var{c}*]} in @var{string2} expands
5955 to as many copies of @var{c} as are needed to make @var{set2} as long as
5956 @var{set1}. If @var{n} begins with @samp{0}, it is interpreted in
5957 octal, otherwise in decimal.
5959 @item Character classes
5960 @cindex character classes
5962 The notation @samp{[:@var{class}:]} expands to all of the characters in
5963 the (predefined) class @var{class}. The characters expand in no
5964 particular order, except for the @code{upper} and @code{lower} classes,
5965 which expand in ascending order. When the @option{--delete} (@option{-d})
5966 and @option{--squeeze-repeats} (@option{-s}) options are both given, any
5967 character class can be used in @var{set2}. Otherwise, only the
5968 character classes @code{lower} and @code{upper} are accepted in
5969 @var{set2}, and then only if the corresponding character class
5970 (@code{upper} and @code{lower}, respectively) is specified in the same
5971 relative position in @var{set1}. Doing this specifies case conversion.
5972 The class names are given below; an error results when an invalid class
5984 Horizontal whitespace.
5993 Printable characters, not including space.
5999 Printable characters, including space.
6002 Punctuation characters.
6005 Horizontal or vertical whitespace.
6014 @item Equivalence classes
6015 @cindex equivalence classes
6017 The syntax @samp{[=@var{c}=]} expands to all of the characters that are
6018 equivalent to @var{c}, in no particular order. Equivalence classes are
6019 a relatively recent invention intended to support non-English alphabets.
6020 But there seems to be no standard way to define them or determine their
6021 contents. Therefore, they are not fully implemented in @sc{gnu} @command{tr};
6022 each character's equivalence class consists only of that character,
6023 which is of no particular use.
6029 @subsection Translating
6031 @cindex translating characters
6033 @command{tr} performs translation when @var{set1} and @var{set2} are
6034 both given and the @option{--delete} (@option{-d}) option is not given.
6035 @command{tr} translates each character of its input that is in @var{set1}
6036 to the corresponding character in @var{set2}. Characters not in
6037 @var{set1} are passed through unchanged. When a character appears more
6038 than once in @var{set1} and the corresponding characters in @var{set2}
6039 are not all the same, only the final one is used. For example, these
6040 two commands are equivalent:
6047 A common use of @command{tr} is to convert lowercase characters to
6048 uppercase. This can be done in many ways. Here are three of them:
6051 tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
6053 tr '[:lower:]' '[:upper:]'
6057 But note that using ranges like @code{a-z} above is not portable.
6059 When @command{tr} is performing translation, @var{set1} and @var{set2}
6060 typically have the same length. If @var{set1} is shorter than
6061 @var{set2}, the extra characters at the end of @var{set2} are ignored.
6063 On the other hand, making @var{set1} longer than @var{set2} is not
6064 portable; @acronym{POSIX} says that the result is undefined. In this situation,
6065 BSD @command{tr} pads @var{set2} to the length of @var{set1} by repeating
6066 the last character of @var{set2} as many times as necessary. System V
6067 @command{tr} truncates @var{set1} to the length of @var{set2}.
6069 By default, @sc{gnu} @command{tr} handles this case like BSD @command{tr}.
6070 When the @option{--truncate-set1} (@option{-t}) option is given,
6071 @sc{gnu} @command{tr} handles this case like the System V @command{tr}
6072 instead. This option is ignored for operations other than translation.
6074 Acting like System V @command{tr} in this case breaks the relatively common
6078 tr -cs A-Za-z0-9 '\012'
6082 because it converts only zero bytes (the first element in the
6083 complement of @var{set1}), rather than all non-alphanumerics, to
6087 By the way, the above idiom is not portable because it uses ranges, and
6088 it assumes that the octal code for newline is 012.
6089 Assuming a @acronym{POSIX} compliant @command{tr}, here is a better
6093 tr -cs '[:alnum:]' '[\n*]'
6098 @subsection Squeezing repeats and deleting
6100 @cindex squeezing repeat characters
6101 @cindex deleting characters
6103 When given just the @option{--delete} (@option{-d}) option, @command{tr}
6104 removes any input characters that are in @var{set1}.
6106 When given just the @option{--squeeze-repeats} (@option{-s}) option,
6107 @command{tr} replaces each input sequence of a repeated character that
6108 is in @var{set1} with a single occurrence of that character.
6110 When given both @option{--delete} and @option{--squeeze-repeats}, @command{tr}
6111 first performs any deletions using @var{set1}, then squeezes repeats
6112 from any remaining characters using @var{set2}.
6114 The @option{--squeeze-repeats} option may also be used when translating,
6115 in which case @command{tr} first performs translation, then squeezes
6116 repeats from any remaining characters using @var{set2}.
6118 Here are some examples to illustrate various combinations of options:
6123 Remove all zero bytes:
6130 Put all words on lines by themselves. This converts all
6131 non-alphanumeric characters to newlines, then squeezes each string
6132 of repeated newlines into a single newline:
6135 tr -cs '[:alnum:]' '[\n*]'
6139 Convert each sequence of repeated newlines to a single newline:
6146 Find doubled occurrences of words in a document.
6147 @c Separate the following two "the"s, so typo checkers don't complain.
6148 For example, people often write ``the @w{}the'' with the repeated words
6149 separated by a newline. The Bourne shell script below works first
6150 by converting each sequence of punctuation and blank characters to a
6151 single newline. That puts each ``word'' on a line by itself.
6152 Next it maps all uppercase characters to lower case, and finally it
6153 runs @command{uniq} with the @option{-d} option to print out only the words
6159 | tr -s '[:punct:][:blank:]' '[\n*]' \
6160 | tr '[:upper:]' '[:lower:]' \
6165 Deleting a small set of characters is usually straightforward. For example,
6166 to remove all @samp{a}s, @samp{x}s, and @samp{M}s you would do this:
6172 However, when @samp{-} is one of those characters, it can be tricky because
6173 @samp{-} has special meanings. Performing the same task as above but also
6174 removing all @samp{-} characters, we might try @code{tr -d -axM}, but
6175 that would fail because @command{tr} would try to interpret @option{-a} as
6176 a command-line option. Alternatively, we could try putting the hyphen
6177 inside the string, @code{tr -d a-xM}, but that wouldn't work either because
6178 it would make @command{tr} interpret @code{a-x} as the range of characters
6179 @samp{a}@dots{}@samp{x} rather than the three.
6180 One way to solve the problem is to put the hyphen at the end of the list
6187 Or you can use @samp{--} to terminate option processing:
6193 More generally, use the character class notation @code{[=c=]}
6194 with @samp{-} (or any other character) in place of the @samp{c}:
6200 Note how single quotes are used in the above example to protect the
6201 square brackets from interpretation by a shell.
6206 @node expand invocation
6207 @section @command{expand}: Convert tabs to spaces
6210 @cindex tabs to spaces, converting
6211 @cindex converting tabs to spaces
6213 @command{expand} writes the contents of each given @var{file}, or standard
6214 input if none are given or for a @var{file} of @samp{-}, to standard
6215 output, with tab characters converted to the appropriate number of
6219 expand [@var{option}]@dots{} [@var{file}]@dots{}
6222 By default, @command{expand} converts all tabs to spaces. It preserves
6223 backspace characters in the output; they decrement the column count for
6224 tab calculations. The default action is equivalent to @option{-t 8} (set
6225 tabs every 8 columns).
6227 The program accepts the following options. Also see @ref{Common options}.
6231 @item -t @var{tab1}[,@var{tab2}]@dots{}
6232 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6235 @cindex tab stops, setting
6236 If only one tab stop is given, set the tabs @var{tab1} spaces apart
6237 (default is 8). Otherwise, set the tabs at columns @var{tab1},
6238 @var{tab2}, @dots{} (numbered from 0), and replace any tabs beyond the
6239 last tab stop given with single spaces. Tab stops can be separated by
6240 blanks as well as by commas.
6242 For compatibility, GNU @command{expand} also accepts the obsolete
6243 option syntax, @option{-@var{t1}[,@var{t2}]@dots{}}. New scripts
6244 should use @option{-t @var{t1}[,@var{t2}]@dots{}} instead.
6250 @cindex initial tabs, converting
6251 Only convert initial tabs (those that precede all non-space or non-tab
6252 characters) on each line to spaces.
6259 @node unexpand invocation
6260 @section @command{unexpand}: Convert spaces to tabs
6264 @command{unexpand} writes the contents of each given @var{file}, or
6265 standard input if none are given or for a @var{file} of @samp{-}, to
6266 standard output, converting blanks at the beginning of each line into
6267 as many tab characters as needed. In the default @acronym{POSIX}
6268 locale, a @dfn{blank} is a space or a tab; other locales may specify
6269 additional blank characters. Synopsis:
6272 unexpand [@var{option}]@dots{} [@var{file}]@dots{}
6275 By default, @command{unexpand} converts only initial blanks (those
6276 that precede all non-blank characters) on each line. It
6277 preserves backspace characters in the output; they decrement the column
6278 count for tab calculations. By default, tabs are set at every 8th
6281 The program accepts the following options. Also see @ref{Common options}.
6285 @item -t @var{tab1}[,@var{tab2}]@dots{}
6286 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6289 If only one tab stop is given, set the tabs @var{tab1} columns apart
6290 instead of the default 8. Otherwise, set the tabs at columns
6291 @var{tab1}, @var{tab2}, @dots{} (numbered from 0), and leave blanks
6292 beyond the tab stops given unchanged. Tab stops can be separated by
6293 blanks as well as by commas. This option implies the @option{-a} option.
6295 For compatibility, GNU @command{unexpand} supports the obsolete option syntax,
6296 @option{-@var{tab1}[,@var{tab2}]@dots{}}, where tab stops must be
6297 separated by commas. (Unlike @option{-t}, this obsolete option does
6298 not imply @option{-a}.) New scripts should use @option{--first-only -t
6299 @var{tab1}[,@var{tab2}]@dots{}} instead.
6305 Also convert all sequences of two or more blanks just before a tab stop,
6306 even if they occur after non-blank characters in a line.
6313 @node Directory listing
6314 @chapter Directory listing
6316 This chapter describes the @command{ls} command and its variants @command{dir}
6317 and @command{vdir}, which list information about files.
6320 * ls invocation:: List directory contents.
6321 * dir invocation:: Briefly ls.
6322 * vdir invocation:: Verbosely ls.
6323 * dircolors invocation:: Color setup for ls, etc.
6328 @section @command{ls}: List directory contents
6331 @cindex directory listing
6333 The @command{ls} program lists information about files (of any type,
6334 including directories). Options and file arguments can be intermixed
6335 arbitrarily, as usual.
6337 For non-option command-line arguments that are directories, by default
6338 @command{ls} lists the contents of directories, not recursively, and
6339 omitting files with names beginning with @samp{.}. For other non-option
6340 arguments, by default @command{ls} lists just the file name. If no
6341 non-option argument is specified, @command{ls} operates on the current
6342 directory, acting as if it had been invoked with a single argument of @samp{.}.
6345 By default, the output is sorted alphabetically, according to the locale
6346 settings in effect.@footnote{If you use a non-@acronym{POSIX}
6347 locale (e.g., by setting @env{LC_ALL} to @samp{en_US}), then @command{ls} may
6348 produce output that is sorted differently than you're accustomed to.
6349 In that case, set the @env{LC_ALL} environment variable to @samp{C}.}
6350 If standard output is
6351 a terminal, the output is in columns (sorted vertically) and control
6352 characters are output as question marks; otherwise, the output is listed
6353 one per line and control characters are output as-is.
6355 Because @command{ls} is such a fundamental program, it has accumulated many
6356 options over the years. They are described in the subsections below;
6357 within each section, options are listed alphabetically (ignoring case).
6358 The division of options into the subsections is not absolute, since some
6359 options affect more than one aspect of @command{ls}'s operation.
6361 @cindex exit status of @command{ls}
6366 1 minor problems (e.g., failure to access a file or directory not
6367 specified as a command line argument. This happens when listing a
6368 directory in which entries are actively being removed or renamed.)
6369 2 serious trouble (e.g., memory exhausted, invalid option, failure
6370 to access a file or directory specified as a command line argument
6371 or a directory loop)
6374 Also see @ref{Common options}.
6377 * Which files are listed::
6378 * What information is listed::
6379 * Sorting the output::
6380 * Details about version sort::
6381 * General output formatting::
6382 * Formatting file timestamps::
6383 * Formatting the file names::
6387 @node Which files are listed
6388 @subsection Which files are listed
6390 These options determine which files @command{ls} lists information for.
6391 By default, @command{ls} lists files and the contents of any
6392 directories on the command line, except that in directories it ignores
6393 files whose names start with @samp{.}.
6401 In directories, do not ignore file names that start with @samp{.}.
6406 @opindex --almost-all
6407 In directories, do not ignore all file names that start with @samp{.};
6408 ignore only @file{.} and @file{..}. The @option{--all} (@option{-a})
6409 option overrides this option.
6412 @itemx --ignore-backups
6414 @opindex --ignore-backups
6415 @cindex backup files, ignoring
6416 In directories, ignore files that end with @samp{~}. This option is
6417 equivalent to @samp{--ignore='*~' --ignore='.*~'}.
6422 @opindex --directory
6423 List just the names of directories, as with other types of files, rather
6424 than listing their contents.
6425 @c The following sentence is the same as the one for -F.
6426 Do not follow symbolic links listed on the
6427 command line unless the @option{--dereference-command-line} (@option{-H}),
6428 @option{--dereference} (@option{-L}), or
6429 @option{--dereference-command-line-symlink-to-dir} options are specified.
6432 @itemx --dereference-command-line
6434 @opindex --dereference-command-line
6435 @cindex symbolic links, dereferencing
6436 If a command line argument specifies a symbolic link, show information
6437 for the file the link references rather than for the link itself.
6439 @itemx --dereference-command-line-symlink-to-dir
6440 @opindex --dereference-command-line-symlink-to-dir
6441 @cindex symbolic links, dereferencing
6442 Do not dereference symbolic links, with one exception:
6443 if a command line argument specifies a symbolic link that refers to
6444 a directory, show information for that directory rather than for the
6446 This is the default behavior when no other dereferencing-related
6447 option has been specified (@option{--classify} (@option{-F}),
6448 @option{--directory} (@option{-d}),
6450 @option{--dereference} (@option{-L}), or
6451 @option{--dereference-command-line} (@option{-H})).
6453 @item --group-directories-first
6454 @opindex --group-directories-first
6455 Group all the directories before the files and then sort the
6456 directories and the files separately using the selected sort key
6457 (see --sort option).
6458 That is, this option specifies a primary sort key,
6459 and the --sort option specifies a secondary key.
6460 However, any use of @option{--sort=none}
6461 (@option{-U}) disables this option altogether.
6463 @item --hide=PATTERN
6464 @opindex --hide=@var{pattern}
6465 In directories, ignore files whose names match the shell pattern
6466 @var{pattern}, unless the @option{--all} (@option{-a}) or
6467 @option{--almost-all} (@option{-A}) is also given. This
6468 option acts like @option{--ignore=@var{pattern}} except that it has no
6469 effect if @option{--all} (@option{-a}) or @option{--almost-all}
6470 (@option{-A}) is also given.
6472 This option can be useful in shell aliases. For example, if
6473 @command{lx} is an alias for @samp{ls --hide='*~'} and @command{ly} is
6474 an alias for @samp{ls --ignore='*~'}, then the command @samp{lx -A}
6475 lists the file @file{README~} even though @samp{ly -A} would not.
6477 @item -I @var{pattern}
6478 @itemx --ignore=@var{pattern}
6480 @opindex --ignore=@var{pattern}
6481 In directories, ignore files whose names match the shell pattern
6482 (not regular expression) @var{pattern}. As
6483 in the shell, an initial @samp{.} in a file name does not match a
6484 wildcard at the start of @var{pattern}. Sometimes it is useful
6485 to give this option several times. For example,
6488 $ ls --ignore='.??*' --ignore='.[^.]' --ignore='#*'
6491 The first option ignores names of length 3 or more that start with @samp{.},
6492 the second ignores all two-character names that start with @samp{.}
6493 except @samp{..}, and the third ignores names that start with @samp{#}.
6496 @itemx --dereference
6498 @opindex --dereference
6499 @cindex symbolic links, dereferencing
6500 When showing file information for a symbolic link, show information
6501 for the file the link references rather than the link itself.
6502 However, even with this option, @command{ls} still prints the name
6503 of the link itself, not the name of the file that the link points to.
6508 @opindex --recursive
6509 @cindex recursive directory listing
6510 @cindex directory listing, recursive
6511 List the contents of all directories recursively.
6516 @node What information is listed
6517 @subsection What information is listed
6519 These options affect the information that @command{ls} displays. By
6520 default, only file names are shown.
6526 @cindex hurd, author, printing
6527 List each file's author when producing long format directory listings.
6528 In GNU/Hurd, file authors can differ from their owners, but in other
6529 operating systems the two are the same.
6535 @cindex dired Emacs mode support
6536 With the long listing (@option{-l}) format, print an additional line after
6540 //DIRED// @var{beg1} @var{end1} @var{beg2} @var{end2} @dots{}
6544 The @var{begn} and @var{endn} are unsigned integers that record the
6545 byte position of the beginning and end of each file name in the output.
6546 This makes it easy for Emacs to find the names, even when they contain
6547 unusual characters such as space or newline, without fancy searching.
6549 If directories are being listed recursively (@option{-R}), output a similar
6550 line with offsets for each subdirectory name:
6553 //SUBDIRED// @var{beg1} @var{end1} @dots{}
6556 Finally, output a line of the form:
6559 //DIRED-OPTIONS// --quoting-style=@var{word}
6563 where @var{word} is the quoting style (@pxref{Formatting the file names}).
6565 Here is an actual example:
6568 $ mkdir -p a/sub/deeper a/sub2
6570 $ touch a/sub/deeper/file
6571 $ ls -gloRF --dired a
6574 -rw-r--r-- 1 0 Jun 10 12:27 f1
6575 -rw-r--r-- 1 0 Jun 10 12:27 f2
6576 drwxr-xr-x 3 4096 Jun 10 12:27 sub/
6577 drwxr-xr-x 2 4096 Jun 10 12:27 sub2/
6581 drwxr-xr-x 2 4096 Jun 10 12:27 deeper/
6585 -rw-r--r-- 1 0 Jun 10 12:27 file
6589 //DIRED// 48 50 84 86 120 123 158 162 217 223 282 286
6590 //SUBDIRED// 2 3 167 172 228 240 290 296
6591 //DIRED-OPTIONS// --quoting-style=literal
6594 Note that the pairs of offsets on the @samp{//DIRED//} line above delimit
6595 these names: @file{f1}, @file{f2}, @file{sub}, @file{sub2}, @file{deeper},
6597 The offsets on the @samp{//SUBDIRED//} line delimit the following
6598 directory names: @file{a}, @file{a/sub}, @file{a/sub/deeper}, @file{a/sub2}.
6600 Here is an example of how to extract the fifth entry name, @samp{deeper},
6601 corresponding to the pair of offsets, 222 and 228:
6604 $ ls -gloRF --dired a > out
6605 $ dd bs=1 skip=222 count=6 < out 2>/dev/null; echo
6609 Note that although the listing above includes a trailing slash
6610 for the @samp{deeper} entry, the offsets select the name without
6611 the trailing slash. However, if you invoke @command{ls} with @option{--dired}
6612 along with an option like @option{--escape} (aka @option{-b}) and operate
6613 on a file whose name contains special characters, notice that the backslash
6618 $ ls -blog --dired 'a b'
6619 -rw-r--r-- 1 0 Jun 10 12:28 a\ b
6621 //DIRED-OPTIONS// --quoting-style=escape
6624 If you use a quoting style that adds quote marks
6625 (e.g., @option{--quoting-style=c}), then the offsets include the quote marks.
6626 So beware that the user may select the quoting style via the environment
6627 variable @env{QUOTING_STYLE}. Hence, applications using @option{--dired}
6628 should either specify an explicit @option{--quoting-style=literal} option
6629 (aka @option{-N} or @option{--literal}) on the command line, or else be
6630 prepared to parse the escaped names.
6633 @opindex --full-time
6634 Produce long format directory listings, and list times in full. It is
6635 equivalent to using @option{--format=long} with
6636 @option{--time-style=full-iso} (@pxref{Formatting file timestamps}).
6640 Produce long format directory listings, but don't display owner information.
6646 Inhibit display of group information in a long format directory listing.
6647 (This is the default in some non-@sc{gnu} versions of @command{ls}, so we
6648 provide this option for compatibility.)
6656 @cindex inode number, printing
6657 Print the inode number (also called the file serial number and index
6658 number) of each file to the left of the file name. (This number
6659 uniquely identifies each file within a particular file system.)
6662 @itemx --format=long
6663 @itemx --format=verbose
6666 @opindex long ls @r{format}
6667 @opindex verbose ls @r{format}
6668 In addition to the name of each file, print the file type, file mode bits,
6669 number of hard links, owner name, group name, size, and
6670 timestamp (@pxref{Formatting file timestamps}), normally
6671 the modification time. Print question marks for information that
6672 cannot be determined.
6674 Normally the size is printed as a byte count without punctuation, but
6675 this can be overridden (@pxref{Block size}). For example, @option{-h}
6676 prints an abbreviated, human-readable count, and
6677 @samp{--block-size="'1"} prints a byte count with the thousands
6678 separator of the current locale.
6680 For each directory that is listed, preface the files with a line
6681 @samp{total @var{blocks}}, where @var{blocks} is the total disk allocation
6682 for all files in that directory. The block size currently defaults to 1024
6683 bytes, but this can be overridden (@pxref{Block size}).
6684 The @var{blocks} computed counts each hard link separately;
6685 this is arguably a deficiency.
6687 The file type is one of the following characters:
6689 @c The commented-out entries are ones we're not sure about.
6697 character special file
6699 high performance (``contiguous data'') file
6703 door (Solaris 2.5 and up)
6705 @c semaphore, if this is a distinct file type
6709 @c multiplexed file (7th edition Unix; obsolete)
6711 off-line (``migrated'') file (Cray DMF)
6713 network special file (HP-UX)
6717 port (Solaris 10 and up)
6719 @c message queue, if this is a distinct file type
6723 @c shared memory object, if this is a distinct file type
6725 @c typed memory object, if this is a distinct file type
6727 @c whiteout (4.4BSD; not implemented)
6729 some other file type
6732 @cindex permissions, output by @command{ls}
6733 The file mode bits listed are similar to symbolic mode specifications
6734 (@pxref{Symbolic Modes}). But @command{ls} combines multiple bits into the
6735 third character of each set of permissions as follows:
6739 If the set-user-ID or set-group-ID bit and the corresponding executable bit
6743 If the set-user-ID or set-group-ID bit is set but the corresponding
6744 executable bit is not set.
6747 If the restricted deletion flag or sticky bit, and the
6748 other-executable bit, are both set. The restricted deletion flag is
6749 another name for the sticky bit. @xref{Mode Structure}.
6752 If the restricted deletion flag or sticky bit is set but the
6753 other-executable bit is not set.
6756 If the executable bit is set and none of the above apply.
6762 Following the file mode bits is a single character that specifies
6763 whether an alternate access method such as an access control list
6764 applies to the file. When the character following the file mode bits is a
6765 space, there is no alternate access method. When it is a printing
6766 character, then there is such a method.
6768 GNU @command{ls} uses a @samp{.} character to indicate a file
6769 with an SELinux security context, but no other alternate access method.
6771 A file with any other combination of alternate access methods
6772 is marked with a @samp{+} character.
6775 @itemx --numeric-uid-gid
6777 @opindex --numeric-uid-gid
6778 @cindex numeric uid and gid
6779 @cindex numeric user and group IDs
6780 Produce long format directory listings, but
6781 display numeric user and group IDs instead of the owner and group names.
6785 Produce long format directory listings, but don't display group information.
6786 It is equivalent to using @option{--format=long} with @option{--no-group} .
6792 @cindex disk allocation
6793 @cindex size of files, reporting
6794 Print the disk allocation of each file to the left of the file name.
6795 This is the amount of disk space used by the file, which is usually a
6796 bit more than the file's size, but it can be less if the file has holes.
6798 Normally the disk allocation is printed in units of
6799 1024 bytes, but this can be overridden (@pxref{Block size}).
6801 @cindex NFS mounts from BSD to HP-UX
6802 For files that are NFS-mounted from an HP-UX system to a BSD system,
6803 this option reports sizes that are half the correct values. On HP-UX
6804 systems, it reports sizes that are twice the correct values for files
6805 that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX;
6806 it also affects the HP-UX @command{ls} program.
6815 @cindex security context
6816 Display the SELinux security context or @samp{?} if none is found.
6817 When used with the @option{-l} option, print the security context
6818 to the left of the size column.
6823 @node Sorting the output
6824 @subsection Sorting the output
6826 @cindex sorting @command{ls} output
6827 These options change the order in which @command{ls} sorts the information
6828 it outputs. By default, sorting is done by character code
6829 (e.g., @acronym{ASCII} order).
6835 @itemx --time=status
6838 @opindex ctime@r{, printing or sorting by}
6839 @opindex status time@r{, printing or sorting by}
6840 @opindex use time@r{, printing or sorting files by}
6841 If the long listing format (e.g., @option{-l}, @option{-o}) is being used,
6842 print the status change time (the @samp{ctime} in the inode) instead of
6843 the modification time.
6844 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6845 or when not using a long listing format,
6846 sort according to the status change time.
6850 @cindex unsorted directory listing
6851 @cindex directory order, listing by
6852 Primarily, like @option{-U}---do not sort; list the files in whatever
6853 order they are stored in the directory. But also enable @option{-a} (list
6854 all files) and disable @option{-l}, @option{--color}, and @option{-s} (if they
6855 were specified before the @option{-f}).
6861 @cindex reverse sorting
6862 Reverse whatever the sorting method is---e.g., list files in reverse
6863 alphabetical order, youngest first, smallest first, or whatever.
6869 @opindex size of files@r{, sorting files by}
6870 Sort by file size, largest first.
6876 @opindex modification time@r{, sorting files by}
6877 Sort by modification time (the @samp{mtime} in the inode), newest first.
6881 @itemx --time=access
6885 @opindex use time@r{, printing or sorting files by}
6886 @opindex atime@r{, printing or sorting files by}
6887 @opindex access time@r{, printing or sorting files by}
6888 If the long listing format (e.g., @option{--format=long}) is being used,
6889 print the last access time (the @samp{atime} in the inode).
6890 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6891 or when not using a long listing format, sort according to the access time.
6897 @opindex none@r{, sorting option for @command{ls}}
6898 Do not sort; list the files in whatever order they are
6899 stored in the directory. (Do not do any of the other unrelated things
6900 that @option{-f} does.) This is especially useful when listing very large
6901 directories, since not doing any sorting can be noticeably faster.
6904 @itemx --sort=version
6907 @opindex version@r{, sorting option for @command{ls}}
6908 Sort by version name and number, lowest first. It behaves like a default
6909 sort, except that each sequence of decimal digits is treated numerically
6910 as an index/version number. (@xref{Details about version sort}.)
6913 @itemx --sort=extension
6916 @opindex extension@r{, sorting files by}
6917 Sort directory contents alphabetically by file extension (characters
6918 after the last @samp{.}); files with no extension are sorted first.
6923 @node Details about version sort
6924 @subsection Details about version sort
6926 Version sorting handles the fact that file names frequently include indices or
6927 version numbers. Standard sorting usually does not produce the order that one
6928 expects because comparisons are made on a character-by-character basis.
6929 Version sorting is especially useful when browsing directories that contain
6930 many files with indices/version numbers in their names:
6934 abc.zml-1.gz abc.zml-1.gz
6935 abc.zml-12.gz abc.zml-2.gz
6936 abc.zml-2.gz abc.zml-12.gz
6939 Version-sorted strings are compared such that if @var{ver1} and @var{ver2}
6940 are version numbers and @var{prefix} and @var{suffix} (@var{suffix} matching
6941 the regular expression @samp{(\.[A-Za-z~][A-Za-z0-9~]*)*}) are strings then
6942 @var{ver1} < @var{ver2} implies that the name composed of
6943 ``@var{prefix} @var{ver1} @var{suffix}'' sorts before
6944 ``@var{prefix} @var{ver2} @var{suffix}''.
6946 Note also that leading zeros of numeric parts are ignored:
6950 abc-1.007.tgz abc-1.01a.tgz
6951 abc-1.012b.tgz abc-1.007.tgz
6952 abc-1.01a.tgz abc-1.012b.tgz
6955 This functionality is implemented using gnulib's @code{filevercmp} function,
6956 which has some caveats worth noting.
6959 @item @env{LC_COLLATE} is ignored, which means @samp{ls -v} and @samp{sort -V}
6960 will sort non-numeric prefixes as if the @env{LC_COLLATE} locale category
6961 was set to @samp{C}.
6962 @item Some suffixes will not be matched by the regular
6963 expression mentioned above. Consequently these examples may
6964 not sort as you expect:
6972 abc-1.2.3.4.x86_64.rpm
6973 abc-1.2.3.x86_64.rpm
6977 @node General output formatting
6978 @subsection General output formatting
6980 These options affect the appearance of the overall output.
6985 @itemx --format=single-column
6988 @opindex single-column @r{output of files}
6989 List one file per line. This is the default for @command{ls} when standard
6990 output is not a terminal.
6993 @itemx --format=vertical
6996 @opindex vertical @r{sorted files in columns}
6997 List files in columns, sorted vertically. This is the default for
6998 @command{ls} if standard output is a terminal. It is always the default
6999 for the @command{dir} program.
7000 @sc{gnu} @command{ls} uses variable width columns to display as many files as
7001 possible in the fewest lines.
7003 @item --color [=@var{when}]
7005 @cindex color, distinguishing file types with
7006 Specify whether to use color for distinguishing file types. @var{when}
7007 may be omitted, or one of:
7010 @vindex none @r{color option}
7011 - Do not use color at all. This is the default.
7013 @vindex auto @r{color option}
7014 @cindex terminal, using color iff
7015 - Only use color if standard output is a terminal.
7017 @vindex always @r{color option}
7020 Specifying @option{--color} and no @var{when} is equivalent to
7021 @option{--color=always}.
7022 Piping a colorized listing through a pager like @command{more} or
7023 @command{less} usually produces unreadable results. However, using
7024 @code{more -f} does seem to work.
7028 @itemx --indicator-style=classify
7031 @opindex --indicator-style
7032 @cindex file type and executables, marking
7033 @cindex executables and file type, marking
7034 Append a character to each file name indicating the file type. Also,
7035 for regular files that are executable, append @samp{*}. The file type
7036 indicators are @samp{/} for directories, @samp{@@} for symbolic links,
7037 @samp{|} for FIFOs, @samp{=} for sockets, @samp{>} for doors,
7038 and nothing for regular files.
7039 @c The following sentence is the same as the one for -d.
7040 Do not follow symbolic links listed on the
7041 command line unless the @option{--dereference-command-line} (@option{-H}),
7042 @option{--dereference} (@option{-L}), or
7043 @option{--dereference-command-line-symlink-to-dir} options are specified.
7046 @itemx --indicator-style=file-type
7047 @opindex --file-type
7048 @opindex --indicator-style
7049 @cindex file type, marking
7050 Append a character to each file name indicating the file type. This is
7051 like @option{-F}, except that executables are not marked.
7053 @item --indicator-style=@var{word}
7054 @opindex --indicator-style
7055 Append a character indicator with style @var{word} to entry names,
7060 Do not append any character indicator; this is the default.
7062 Append @samp{/} for directories. This is the same as the @option{-p}
7065 Append @samp{/} for directories, @samp{@@} for symbolic links, @samp{|}
7066 for FIFOs, @samp{=} for sockets, and nothing for regular files. This is
7067 the same as the @option{--file-type} option.
7069 Append @samp{*} for executable regular files, otherwise behave as for
7070 @samp{file-type}. This is the same as the @option{-F} or
7071 @option{--classify} option.
7076 Print file sizes in 1024-byte blocks, overriding the default block
7077 size (@pxref{Block size}).
7078 This option is equivalent to @option{--block-size=1K}.
7081 @itemx --format=commas
7084 @opindex commas@r{, outputting between files}
7085 List files horizontally, with as many as will fit on each line,
7086 separated by @samp{, } (a comma and a space).
7089 @itemx --indicator-style=slash
7091 @opindex --indicator-style
7092 @cindex file type, marking
7093 Append a @samp{/} to directory names.
7096 @itemx --format=across
7097 @itemx --format=horizontal
7100 @opindex across@r{, listing files}
7101 @opindex horizontal@r{, listing files}
7102 List the files in columns, sorted horizontally.
7105 @itemx --tabsize=@var{cols}
7108 Assume that each tab stop is @var{cols} columns wide. The default is 8.
7109 @command{ls} uses tabs where possible in the output, for efficiency. If
7110 @var{cols} is zero, do not use tabs at all.
7112 @c FIXME: remove in 2009, if Apple Terminal has been fixed for long enough.
7113 Some terminal emulators (at least Apple Terminal 1.5 (133) from Mac OS X 10.4.8)
7114 do not properly align columns to the right of a TAB following a
7115 non-@acronym{ASCII} byte. If you use such a terminal emulator, use the
7116 @option{-T0} option or put @code{TABSIZE=0} in your environment to tell
7117 @command{ls} to align using spaces, not tabs.
7120 @itemx --width=@var{cols}
7124 Assume the screen is @var{cols} columns wide. The default is taken
7125 from the terminal settings if possible; otherwise the environment
7126 variable @env{COLUMNS} is used if it is set; otherwise the default
7132 @node Formatting file timestamps
7133 @subsection Formatting file timestamps
7135 By default, file timestamps are listed in abbreviated form, using
7136 a date like @samp{Mar 30@ @ 2002} for non-recent timestamps, and a
7137 date-without-year and time like @samp{Mar 30 23:45} for recent timestamps.
7138 This format can change depending on the current locale as detailed below.
7140 A timestamp is considered to be @dfn{recent} if it is less than six
7141 months old, and is not dated in the future. If a timestamp dated
7142 today is not listed in recent form, the timestamp is in the future,
7143 which means you probably have clock skew problems which may break
7144 programs like @command{make} that rely on file timestamps.
7147 Time stamps are listed according to the time zone rules specified by
7148 the @env{TZ} environment variable, or by the system default rules if
7149 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
7150 with @env{TZ}, libc, The GNU C Library Reference Manual}.
7152 The following option changes how file timestamps are printed.
7155 @item --time-style=@var{style}
7156 @opindex --time-style
7158 List timestamps in style @var{style}. The @var{style} should
7159 be one of the following:
7164 List timestamps using @var{format}, where @var{format} is interpreted
7165 like the format argument of @command{date} (@pxref{date invocation}).
7166 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
7167 @command{ls} to list timestamps like @samp{2002-03-30 23:45:56}. As
7168 with @command{date}, @var{format}'s interpretation is affected by the
7169 @env{LC_TIME} locale category.
7171 If @var{format} contains two format strings separated by a newline,
7172 the former is used for non-recent files and the latter for recent
7173 files; if you want output columns to line up, you may need to insert
7174 spaces in one of the two formats.
7177 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
7178 format with nanosecond precision, e.g., @samp{2002-03-30
7179 23:45:56.477817180 -0700}. This style is equivalent to
7180 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
7182 This is useful because the time output includes all the information that
7183 is available from the operating system. For example, this can help
7184 explain @command{make}'s behavior, since @acronym{GNU} @command{make}
7185 uses the full timestamp to determine whether a file is out of date.
7188 List @acronym{ISO} 8601 date and time in minutes, e.g.,
7189 @samp{2002-03-30 23:45}. These timestamps are shorter than
7190 @samp{full-iso} timestamps, and are usually good enough for everyday
7191 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
7194 List @acronym{ISO} 8601 dates for non-recent timestamps (e.g.,
7195 @samp{2002-03-30@ }), and @acronym{ISO} 8601 month, day, hour, and
7196 minute for recent timestamps (e.g., @samp{03-30 23:45}). These
7197 timestamps are uglier than @samp{long-iso} timestamps, but they carry
7198 nearly the same information in a smaller space and their brevity helps
7199 @command{ls} output fit within traditional 80-column output lines.
7200 The following two @command{ls} invocations are equivalent:
7205 ls -l --time-style="+%Y-%m-%d $newline%m-%d %H:%M"
7206 ls -l --time-style="iso"
7211 List timestamps in a locale-dependent form. For example, a Finnish
7212 locale might list non-recent timestamps like @samp{maalis 30@ @ 2002}
7213 and recent timestamps like @samp{maalis 30 23:45}. Locale-dependent
7214 timestamps typically consume more space than @samp{iso} timestamps and
7215 are harder for programs to parse because locale conventions vary so
7216 widely, but they are easier for many people to read.
7218 The @env{LC_TIME} locale category specifies the timestamp format. The
7219 default @acronym{POSIX} locale uses timestamps like @samp{Mar 30@
7220 @ 2002} and @samp{Mar 30 23:45}; in this locale, the following two
7221 @command{ls} invocations are equivalent:
7226 ls -l --time-style="+%b %e %Y$newline%b %e %H:%M"
7227 ls -l --time-style="locale"
7230 Other locales behave differently. For example, in a German locale,
7231 @option{--time-style="locale"} might be equivalent to
7232 @option{--time-style="+%e. %b %Y $newline%e. %b %H:%M"}
7233 and might generate timestamps like @samp{30. M@"ar 2002@ } and
7234 @samp{30. M@"ar 23:45}.
7236 @item posix-@var{style}
7238 List @acronym{POSIX}-locale timestamps if the @env{LC_TIME} locale
7239 category is @acronym{POSIX}, @var{style} timestamps otherwise. For
7240 example, the @samp{posix-long-iso} style lists
7241 timestamps like @samp{Mar 30@ @ 2002} and @samp{Mar 30 23:45} when in
7242 the @acronym{POSIX} locale, and like @samp{2002-03-30 23:45} otherwise.
7247 You can specify the default value of the @option{--time-style} option
7248 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
7249 the default style is @samp{locale}. @acronym{GNU} Emacs 21.3 and
7250 later use the @option{--dired} option and therefore can parse any date
7251 format, but if you are using Emacs 21.1 or 21.2 and specify a
7252 non-@acronym{POSIX} locale you may need to set
7253 @samp{TIME_STYLE="posix-long-iso"}.
7255 To avoid certain denial-of-service attacks, timestamps that would be
7256 longer than 1000 bytes may be treated as errors.
7259 @node Formatting the file names
7260 @subsection Formatting the file names
7262 These options change how file names themselves are printed.
7268 @itemx --quoting-style=escape
7271 @opindex --quoting-style
7272 @cindex backslash sequences for file names
7273 Quote nongraphic characters in file names using alphabetic and octal
7274 backslash sequences like those used in C.
7278 @itemx --quoting-style=literal
7281 @opindex --quoting-style
7282 Do not quote file names. However, with @command{ls} nongraphic
7283 characters are still printed as question marks if the output is a
7284 terminal and you do not specify the @option{--show-control-chars}
7288 @itemx --hide-control-chars
7290 @opindex --hide-control-chars
7291 Print question marks instead of nongraphic characters in file names.
7292 This is the default if the output is a terminal and the program is
7297 @itemx --quoting-style=c
7299 @opindex --quote-name
7300 @opindex --quoting-style
7301 Enclose file names in double quotes and quote nongraphic characters as
7304 @item --quoting-style=@var{word}
7305 @opindex --quoting-style
7306 @cindex quoting style
7307 Use style @var{word} to quote file names and other strings that may
7308 contain arbitrary characters. The @var{word} should
7309 be one of the following:
7313 Output strings as-is; this is the same as the @option{-N} or
7314 @option{--literal} option.
7316 Quote strings for the shell if they contain shell metacharacters or would
7317 cause ambiguous output.
7318 The quoting is suitable for @acronym{POSIX}-compatible shells like
7319 @command{bash}, but it does not always work for incompatible shells
7322 Quote strings for the shell, even if they would normally not require quoting.
7324 Quote strings as for C character string literals, including the
7325 surrounding double-quote characters; this is the same as the
7326 @option{-Q} or @option{--quote-name} option.
7328 Quote strings as for C character string literals, except omit the
7329 surrounding double-quote
7330 characters; this is the same as the @option{-b} or @option{--escape} option.
7332 Quote strings as for C character string literals, except use
7333 surrounding quotation marks appropriate for the
7336 @c Use @t instead of @samp to avoid duplicate quoting in some output styles.
7337 Quote strings as for C character string literals, except use
7338 surrounding quotation marks appropriate for the locale, and quote
7339 @t{`like this'} instead of @t{"like
7340 this"} in the default C locale. This looks nicer on many displays.
7343 You can specify the default value of the @option{--quoting-style} option
7344 with the environment variable @env{QUOTING_STYLE}. If that environment
7345 variable is not set, the default value is @samp{literal}, but this
7346 default may change to @samp{shell} in a future version of this package.
7348 @item --show-control-chars
7349 @opindex --show-control-chars
7350 Print nongraphic characters as-is in file names.
7351 This is the default unless the output is a terminal and the program is
7357 @node dir invocation
7358 @section @command{dir}: Briefly list directory contents
7361 @cindex directory listing, brief
7363 @command{dir} is equivalent to @code{ls -C
7364 -b}; that is, by default files are listed in columns, sorted vertically,
7365 and special characters are represented by backslash escape sequences.
7367 @xref{ls invocation, @command{ls}}.
7370 @node vdir invocation
7371 @section @command{vdir}: Verbosely list directory contents
7374 @cindex directory listing, verbose
7376 @command{vdir} is equivalent to @code{ls -l
7377 -b}; that is, by default files are listed in long format and special
7378 characters are represented by backslash escape sequences.
7380 @node dircolors invocation
7381 @section @command{dircolors}: Color setup for @command{ls}
7385 @cindex setup for color
7387 @command{dircolors} outputs a sequence of shell commands to set up the
7388 terminal for color output from @command{ls} (and @command{dir}, etc.).
7392 eval "`dircolors [@var{option}]@dots{} [@var{file}]`"
7395 If @var{file} is specified, @command{dircolors} reads it to determine which
7396 colors to use for which file types and extensions. Otherwise, a
7397 precompiled database is used. For details on the format of these files,
7398 run @samp{dircolors --print-database}.
7400 To make @command{dircolors} read a @file{~/.dircolors} file if it
7401 exists, you can put the following lines in your @file{~/.bashrc} (or
7402 adapt them to your favorite shell):
7406 test -r $d && eval "$(dircolors $d)"
7410 @vindex SHELL @r{environment variable, and color}
7411 The output is a shell command to set the @env{LS_COLORS} environment
7412 variable. You can specify the shell syntax to use on the command line,
7413 or @command{dircolors} will guess it from the value of the @env{SHELL}
7414 environment variable.
7416 The program accepts the following options. Also see @ref{Common options}.
7421 @itemx --bourne-shell
7424 @opindex --bourne-shell
7425 @cindex Bourne shell syntax for color setup
7426 @cindex @command{sh} syntax for color setup
7427 Output Bourne shell commands. This is the default if the @env{SHELL}
7428 environment variable is set and does not end with @samp{csh} or
7437 @cindex C shell syntax for color setup
7438 @cindex @command{csh} syntax for color setup
7439 Output C shell commands. This is the default if @code{SHELL} ends with
7440 @command{csh} or @command{tcsh}.
7443 @itemx --print-database
7445 @opindex --print-database
7446 @cindex color database, printing
7447 @cindex database for color setup, printing
7448 @cindex printing color database
7449 Print the (compiled-in) default color configuration database. This
7450 output is itself a valid configuration file, and is fairly descriptive
7451 of the possibilities.
7458 @node Basic operations
7459 @chapter Basic operations
7461 @cindex manipulating files
7463 This chapter describes the commands for basic file manipulation:
7464 copying, moving (renaming), and deleting (removing).
7467 * cp invocation:: Copy files.
7468 * dd invocation:: Convert and copy a file.
7469 * install invocation:: Copy files and set attributes.
7470 * mv invocation:: Move (rename) files.
7471 * rm invocation:: Remove files or directories.
7472 * shred invocation:: Remove files more securely.
7477 @section @command{cp}: Copy files and directories
7480 @cindex copying files and directories
7481 @cindex files, copying
7482 @cindex directories, copying
7484 @command{cp} copies files (or, optionally, directories). The copy is
7485 completely independent of the original. You can either copy one file to
7486 another, or copy arbitrarily many files to a destination directory.
7490 cp [@var{option}]@dots{} [-T] @var{source} @var{dest}
7491 cp [@var{option}]@dots{} @var{source}@dots{} @var{directory}
7492 cp [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
7497 If two file names are given, @command{cp} copies the first file to the
7501 If the @option{--target-directory} (@option{-t}) option is given, or
7502 failing that if the last file is a directory and the
7503 @option{--no-target-directory} (@option{-T}) option is not given,
7504 @command{cp} copies each @var{source} file to the specified directory,
7505 using the @var{source}s' names.
7508 Generally, files are written just as they are read. For exceptions,
7509 see the @option{--sparse} option below.
7511 By default, @command{cp} does not copy directories. However, the
7512 @option{-R}, @option{-a}, and @option{-r} options cause @command{cp} to
7513 copy recursively by descending into source directories and copying files
7514 to corresponding destination directories.
7516 When copying from a symbolic link, @command{cp} normally follows the
7517 link only when not copying
7518 recursively. This default can be overridden with the
7519 @option{--archive} (@option{-a}), @option{-d}, @option{--dereference}
7520 (@option{-L}), @option{--no-dereference} (@option{-P}), and
7521 @option{-H} options. If more than one of these options is specified,
7522 the last one silently overrides the others.
7524 When copying to a symbolic link, @command{cp} follows the
7525 link only when it refers to an existing regular file.
7526 However, when copying to a dangling symbolic link, @command{cp}
7527 refuses by default, and fails with a diagnostic, since the operation
7528 is inherently dangerous. This behavior is contrary to historical
7529 practice and to @acronym{POSIX}.
7530 Set @env{POSIXLY_CORRECT} to make @command{cp} attempt to create
7531 the target of a dangling destination symlink, in spite of the possible risk.
7532 Also, when an option like
7533 @option{--backup} or @option{--link} acts to rename or remove the
7534 destination before copying, @command{cp} renames or removes the
7535 symbolic link rather than the file it points to.
7537 By default, @command{cp} copies the contents of special files only
7538 when not copying recursively. This default can be overridden with the
7539 @option{--copy-contents} option.
7541 @cindex self-backups
7542 @cindex backups, making only
7543 @command{cp} generally refuses to copy a file onto itself, with the
7544 following exception: if @option{--force --backup} is specified with
7545 @var{source} and @var{dest} identical, and referring to a regular file,
7546 @command{cp} will make a backup file, either regular or numbered, as
7547 specified in the usual ways (@pxref{Backup options}). This is useful when
7548 you simply want to make a backup of an existing file before changing it.
7550 The program accepts the following options. Also see @ref{Common options}.
7557 Preserve as much as possible of the structure and attributes of the
7558 original files in the copy (but do not attempt to preserve internal
7559 directory structure; i.e., @samp{ls -U} may list the entries in a copied
7560 directory in a different order).
7561 Try to preserve SELinux security context and extended attributes (xattr),
7562 but ignore any failure to do that and print no corresponding diagnostic.
7563 Equivalent to @option{-dR --preserve=all} with the reduced diagnostics.
7565 @itemx --attributes-only
7566 @opindex --attributes-only
7567 Preserve the specified attributes of the original files in the copy,
7568 but do not copy any data. See the @option{--preserve} option for
7569 controlling which attributes to copy.
7572 @itemx @w{@kbd{--backup}[=@var{method}]}
7575 @vindex VERSION_CONTROL
7576 @cindex backups, making
7577 @xref{Backup options}.
7578 Make a backup of each file that would otherwise be overwritten or removed.
7579 As a special case, @command{cp} makes a backup of @var{source} when the force
7580 and backup options are given and @var{source} and @var{dest} are the same
7581 name for an existing, regular file. One useful application of this
7582 combination of options is this tiny Bourne shell script:
7586 # Usage: backup FILE...
7587 # Create a @sc{gnu}-style backup of each listed FILE.
7589 cp --backup --force -- "$i" "$i"
7593 @item --copy-contents
7594 @cindex directories, copying recursively
7595 @cindex copying directories recursively
7596 @cindex recursively copying directories
7597 @cindex non-directories, copying as special files
7598 If copying recursively, copy the contents of any special files (e.g.,
7599 FIFOs and device files) as if they were regular files. This means
7600 trying to read the data in each source file and writing it to the
7601 destination. It is usually a mistake to use this option, as it
7602 normally has undesirable effects on special files like FIFOs and the
7603 ones typically found in the @file{/dev} directory. In most cases,
7604 @code{cp -R --copy-contents} will hang indefinitely trying to read
7605 from FIFOs and special files like @file{/dev/console}, and it will
7606 fill up your destination disk if you use it to copy @file{/dev/zero}.
7607 This option has no effect unless copying recursively, and it does not
7608 affect the copying of symbolic links.
7612 @cindex symbolic links, copying
7613 @cindex hard links, preserving
7614 Copy symbolic links as symbolic links rather than copying the files that
7615 they point to, and preserve hard links between source files in the copies.
7616 Equivalent to @option{--no-dereference --preserve=links}.
7622 When copying without this option and an existing destination file cannot
7623 be opened for writing, the copy fails. However, with @option{--force}),
7624 when a destination file cannot be opened, @command{cp} then removes it and
7625 tries to open it again. Contrast this behavior with that enabled by
7626 @option{--link} and @option{--symbolic-link}, whereby the destination file
7627 is never opened but rather is removed unconditionally. Also see the
7628 description of @option{--remove-destination}.
7630 This option is independent of the @option{--interactive} or
7631 @option{-i} option: neither cancels the effect of the other.
7633 This option is redundant if the @option{--no-clobber} or @option{-n} option is
7638 If a command line argument specifies a symbolic link, then copy the
7639 file it points to rather than the symbolic link itself. However,
7640 copy (preserving its nature) any symbolic link that is encountered
7641 via recursive traversal.
7644 @itemx --interactive
7646 @opindex --interactive
7647 When copying a file other than a directory, prompt whether to
7648 overwrite an existing destination file. The @option{-i} option overrides
7649 a previous @option{-n} option.
7655 Make hard links instead of copies of non-directories.
7658 @itemx --dereference
7660 @opindex --dereference
7661 Follow symbolic links when copying from them.
7662 With this option, @command{cp} cannot create a symbolic link.
7663 For example, a symlink (to regular file) in the source tree will be copied to
7664 a regular file in the destination tree.
7669 @opindex --no-clobber
7670 Do not overwrite an existing file. The @option{-n} option overrides a previous
7671 @option{-i} option. This option is mutually exclusive with @option{-b} or
7672 @option{--backup} option.
7675 @itemx --no-dereference
7677 @opindex --no-dereference
7678 @cindex symbolic links, copying
7679 Copy symbolic links as symbolic links rather than copying the files that
7680 they point to. This option affects only symbolic links in the source;
7681 symbolic links in the destination are always followed if possible.
7684 @itemx @w{@kbd{--preserve}[=@var{attribute_list}]}
7687 @cindex file information, preserving, extended attributes, xattr
7688 Preserve the specified attributes of the original files.
7689 If specified, the @var{attribute_list} must be a comma-separated list
7690 of one or more of the following strings:
7694 Preserve the file mode bits and access control lists.
7696 Preserve the owner and group. On most modern systems,
7697 only users with appropriate privileges may change the owner of a file,
7699 may preserve the group ownership of a file only if they happen to be
7700 a member of the desired group.
7702 Preserve the times of last access and last modification, when possible.
7703 On older systems, it is not possible to preserve these attributes
7704 when the affected file is a symbolic link.
7705 However, many systems now provide the @code{utimensat} function,
7706 which makes it possible even for symbolic links.
7708 Preserve in the destination files
7709 any links between corresponding source files.
7710 Note that with @option{-L} or @option{-H}, this option can convert
7711 symbolic links to hard links. For example,
7713 $ mkdir c; : > a; ln -s a b; cp -aH a b c; ls -i1 c
7718 Note the inputs: @file{b} is a symlink to regular file @file{a},
7719 yet the files in destination directory, @file{c/}, are hard-linked.
7720 Since @option{-a} implies @option{--preserve=links}, and since @option{-H}
7721 tells @command{cp} to dereference command line arguments, it sees two files
7722 with the same inode number, and preserves the perceived hard link.
7724 Here is a similar example that exercises @command{cp}'s @option{-L} option:
7726 $ mkdir b c; (cd b; : > a; ln -s a b); cp -aL b c; ls -i1 c/b
7732 Preserve SELinux security context of the file, or fail with full diagnostics.
7734 Preserve extended attributes of the file, or fail with full diagnostics.
7735 If @command{cp} is built without xattr support, ignore this option.
7736 If SELinux context, ACLs or Capabilities are implemented using xattrs,
7737 they are preserved by this option as well.
7739 Preserve all file attributes.
7740 Equivalent to specifying all of the above, but with the difference
7741 that failure to preserve SELinux security context or extended attributes
7742 does not change @command{cp}'s exit status. In contrast to @option{-a},
7743 all but @samp{Operation not supported} warnings are output.
7746 Using @option{--preserve} with no @var{attribute_list} is equivalent
7747 to @option{--preserve=mode,ownership,timestamps}.
7749 In the absence of this option, each destination file is created with the
7750 mode bits of the corresponding source file, minus the bits set in the
7751 umask and minus the set-user-ID and set-group-ID bits.
7752 @xref{File permissions}.
7754 @itemx @w{@kbd{--no-preserve}=@var{attribute_list}}
7755 @cindex file information, preserving
7756 Do not preserve the specified attributes. The @var{attribute_list}
7757 has the same form as for @option{--preserve}.
7761 @cindex parent directories and @command{cp}
7762 Form the name of each destination file by appending to the target
7763 directory a slash and the specified name of the source file. The last
7764 argument given to @command{cp} must be the name of an existing directory.
7765 For example, the command:
7768 cp --parents a/b/c existing_dir
7772 copies the file @file{a/b/c} to @file{existing_dir/a/b/c}, creating
7773 any missing intermediate directories.
7780 @opindex --recursive
7781 @cindex directories, copying recursively
7782 @cindex copying directories recursively
7783 @cindex recursively copying directories
7784 @cindex non-directories, copying as special files
7785 Copy directories recursively. By default, do not follow symbolic
7786 links in the source; see the @option{--archive} (@option{-a}), @option{-d},
7787 @option{--dereference} (@option{-L}), @option{--no-dereference}
7788 (@option{-P}), and @option{-H} options. Special files are copied by
7789 creating a destination file of the same type as the source; see the
7790 @option{--copy-contents} option. It is not portable to use
7791 @option{-r} to copy symbolic links or special files. On some
7792 non-@sc{gnu} systems, @option{-r} implies the equivalent of
7793 @option{-L} and @option{--copy-contents} for historical reasons.
7794 Also, it is not portable to use @option{-R} to copy symbolic links
7795 unless you also specify @option{-P}, as @acronym{POSIX} allows
7796 implementations that dereference symbolic links by default.
7798 @item --reflink[=@var{when}]
7799 @opindex --reflink[=@var{when}]
7802 @cindex copy on write
7803 Perform a lightweight, copy-on-write (COW) copy, if supported by the
7804 file system. Once it has succeeded, beware that the source and destination
7805 files share the same disk data blocks as long as they remain unmodified.
7806 Thus, if a disk I/O error affects data blocks of one of the files,
7807 the other suffers the same fate.
7809 The @var{when} value can be one of the following:
7813 The default behavior: if the copy-on-write operation is not supported
7814 then report the failure for each file and exit with a failure status.
7817 If the copy-on-write operation is not supported then fall back
7818 to the standard copy behaviour.
7821 This option is overridden by the @option{--link}, @option{--symbolic-link}
7822 and @option{--attributes-only} options, thus allowing it to be used
7823 to configure the default data copying behavior for @command{cp}.
7824 For example, with the following alias, @command{cp} will use the
7825 minimum amount of space supported by the file system.
7828 alias cp='cp --reflink=auto --sparse=always'
7831 @item --remove-destination
7832 @opindex --remove-destination
7833 Remove each existing destination file before attempting to open it
7834 (contrast with @option{-f} above).
7836 @item --sparse=@var{when}
7837 @opindex --sparse=@var{when}
7838 @cindex sparse files, copying
7839 @cindex holes, copying files with
7840 @findex read @r{system call, and holes}
7841 A @dfn{sparse file} contains @dfn{holes}---a sequence of zero bytes that
7842 does not occupy any physical disk blocks; the @samp{read} system call
7843 reads these as zeros. This can both save considerable disk space and
7844 increase speed, since many binary files contain lots of consecutive zero
7845 bytes. By default, @command{cp} detects holes in input source files via a crude
7846 heuristic and makes the corresponding output file sparse as well.
7847 Only regular files may be sparse.
7849 The @var{when} value can be one of the following:
7853 The default behavior: if the input file is sparse, attempt to make
7854 the output file sparse, too. However, if an output file exists but
7855 refers to a non-regular file, then do not attempt to make it sparse.
7858 For each sufficiently long sequence of zero bytes in the input file,
7859 attempt to create a corresponding hole in the output file, even if the
7860 input file does not appear to be sparse.
7861 This is useful when the input file resides on a file system
7862 that does not support sparse files
7863 (for example, @samp{efs} file systems in SGI IRIX 5.3 and earlier),
7864 but the output file is on a type of file system that does support them.
7865 Holes may be created only in regular files, so if the destination file
7866 is of some other type, @command{cp} does not even try to make it sparse.
7869 Never make the output file sparse.
7870 This is useful in creating a file for use with the @command{mkswap} command,
7871 since such a file must not have any holes.
7874 @optStripTrailingSlashes
7877 @itemx --symbolic-link
7879 @opindex --symbolic-link
7880 @cindex symbolic links, copying with
7881 Make symbolic links instead of copies of non-directories. All source
7882 file names must be absolute (starting with @samp{/}) unless the
7883 destination files are in the current directory. This option merely
7884 results in an error message on systems that do not support symbolic links.
7890 @optNoTargetDirectory
7896 @cindex newer files, copying only
7897 Do not copy a non-directory that has an existing destination with the
7898 same or newer modification time. If time stamps are being preserved,
7899 the comparison is to the source time stamp truncated to the
7900 resolutions of the destination file system and of the system calls
7901 used to update time stamps; this avoids duplicate work if several
7902 @samp{cp -pu} commands are executed with the same source and
7909 Print the name of each file before copying it.
7912 @itemx --one-file-system
7914 @opindex --one-file-system
7915 @cindex file systems, omitting copying to different
7916 Skip subdirectories that are on different file systems from the one that
7917 the copy started on.
7918 However, mount point directories @emph{are} copied.
7926 @section @command{dd}: Convert and copy a file
7929 @cindex converting while copying a file
7931 @command{dd} copies a file (from standard input to standard output, by
7932 default) with a changeable I/O block size, while optionally performing
7933 conversions on it. Synopses:
7936 dd [@var{operand}]@dots{}
7940 The only options are @option{--help} and @option{--version}.
7941 @xref{Common options}. @command{dd} accepts the following operands.
7947 Read from @var{file} instead of standard input.
7951 Write to @var{file} instead of standard output. Unless
7952 @samp{conv=notrunc} is given, @command{dd} truncates @var{file} to zero
7953 bytes (or the size specified with @samp{seek=}).
7955 @item ibs=@var{bytes}
7957 @cindex block size of input
7958 @cindex input block size
7959 Set the input block size to @var{bytes}.
7960 This makes @command{dd} read @var{bytes} per block.
7961 The default is 512 bytes.
7963 @item obs=@var{bytes}
7965 @cindex block size of output
7966 @cindex output block size
7967 Set the output block size to @var{bytes}.
7968 This makes @command{dd} write @var{bytes} per block.
7969 The default is 512 bytes.
7971 @item bs=@var{bytes}
7974 Set both input and output block sizes to @var{bytes}.
7975 This makes @command{dd} read and write @var{bytes} per block,
7976 overriding any @samp{ibs} and @samp{obs} settings.
7977 In addition, if no data-transforming @option{conv} option is specified,
7978 input is copied to the output as soon as it's read,
7979 even if it is smaller than the block size.
7981 @item cbs=@var{bytes}
7983 @cindex block size of conversion
7984 @cindex conversion block size
7985 @cindex fixed-length records, converting to variable-length
7986 @cindex variable-length records, converting to fixed-length
7987 Set the conversion block size to @var{bytes}.
7988 When converting variable-length records to fixed-length ones
7989 (@option{conv=block}) or the reverse (@option{conv=unblock}),
7990 use @var{bytes} as the fixed record length.
7992 @item skip=@var{blocks}
7994 Skip @var{blocks} @samp{ibs}-byte blocks in the input file before copying.
7996 @item seek=@var{blocks}
7998 Skip @var{blocks} @samp{obs}-byte blocks in the output file before copying.
8000 @item count=@var{blocks}
8002 Copy @var{blocks} @samp{ibs}-byte blocks from the input file, instead
8003 of everything until the end of the file.
8007 Do not print the overall transfer rate and volume statistics
8008 that normally make up the third status line when @command{dd} exits.
8010 @item conv=@var{conversion}[,@var{conversion}]@dots{}
8012 Convert the file as specified by the @var{conversion} argument(s).
8013 (No spaces around any comma(s).)
8020 @opindex ascii@r{, converting to}
8021 Convert @acronym{EBCDIC} to @acronym{ASCII},
8022 using the conversion table specified by @acronym{POSIX}.
8023 This provides a 1:1 translation for all 256 bytes.
8026 @opindex ebcdic@r{, converting to}
8027 Convert @acronym{ASCII} to @acronym{EBCDIC}.
8028 This is the inverse of the @samp{ascii} conversion.
8031 @opindex alternate ebcdic@r{, converting to}
8032 Convert @acronym{ASCII} to alternate @acronym{EBCDIC},
8033 using the alternate conversion table specified by @acronym{POSIX}.
8034 This is not a 1:1 translation, but reflects common historical practice
8035 for @samp{~}, @samp{[}, and @samp{]}.
8037 The @samp{ascii}, @samp{ebcdic}, and @samp{ibm} conversions are
8041 @opindex block @r{(space-padding)}
8042 For each line in the input, output @samp{cbs} bytes, replacing the
8043 input newline with a space and padding with spaces as necessary.
8047 Remove any trailing spaces in each @samp{cbs}-sized input block,
8048 and append a newline.
8050 The @samp{block} and @samp{unblock} conversions are mutually exclusive.
8053 @opindex lcase@r{, converting to}
8054 Change uppercase letters to lowercase.
8057 @opindex ucase@r{, converting to}
8058 Change lowercase letters to uppercase.
8060 The @samp{lcase} and @samp{ucase} conversions are mutually exclusive.
8063 @opindex swab @r{(byte-swapping)}
8064 @cindex byte-swapping
8065 Swap every pair of input bytes. @sc{gnu} @command{dd}, unlike others, works
8066 when an odd number of bytes are read---the last byte is simply copied
8067 (since there is nothing to swap it with).
8070 @opindex sync @r{(padding with @acronym{ASCII} @sc{nul}s)}
8071 Pad every input block to size of @samp{ibs} with trailing zero bytes.
8072 When used with @samp{block} or @samp{unblock}, pad with spaces instead of
8077 The following ``conversions'' are really file flags
8078 and don't affect internal processing:
8083 @cindex creating output file, requiring
8084 Fail if the output file already exists; @command{dd} must create the
8089 @cindex creating output file, avoiding
8090 Do not create the output file; the output file must already exist.
8092 The @samp{excl} and @samp{nocreat} conversions are mutually exclusive.
8096 @cindex truncating output file, avoiding
8097 Do not truncate the output file.
8101 @cindex read errors, ignoring
8102 Continue after read errors.
8106 @cindex synchronized data writes, before finishing
8107 Synchronize output data just before finishing. This forces a physical
8108 write of output data.
8112 @cindex synchronized data and metadata writes, before finishing
8113 Synchronize output data and metadata just before finishing. This
8114 forces a physical write of output data and metadata.
8118 @item iflag=@var{flag}[,@var{flag}]@dots{}
8120 Access the input file using the flags specified by the @var{flag}
8121 argument(s). (No spaces around any comma(s).)
8123 @item oflag=@var{flag}[,@var{flag}]@dots{}
8125 Access the output file using the flags specified by the @var{flag}
8126 argument(s). (No spaces around any comma(s).)
8128 Here are the flags. Not every flag is supported on every operating
8135 @cindex appending to the output file
8136 Write in append mode, so that even if some other process is writing to
8137 this file, every @command{dd} write will append to the current
8138 contents of the file. This flag makes sense only for output.
8139 If you combine this flag with the @samp{of=@var{file}} operand,
8140 you should also specify @samp{conv=notrunc} unless you want the
8141 output file to be truncated before being appended to.
8145 @cindex concurrent I/O
8146 Use concurrent I/O mode for data. This mode performs direct I/O
8147 and drops the @acronym{POSIX} requirement to serialize all I/O to the same file.
8148 A file cannot be opened in CIO mode and with a standard open at the
8154 Use direct I/O for data, avoiding the buffer cache.
8155 Note that the kernel may impose restrictions on read or write buffer sizes.
8156 For example, with an ext4 destination file system and a linux-based kernel,
8157 using @samp{oflag=direct} will cause writes to fail with @code{EINVAL} if the
8158 output buffer size is not a multiple of 512.
8162 @cindex directory I/O
8164 Fail unless the file is a directory. Most operating systems do not
8165 allow I/O to a directory, so this flag has limited utility.
8169 @cindex synchronized data reads
8170 Use synchronized I/O for data. For the output file, this forces a
8171 physical write of output data on each write. For the input file,
8172 this flag can matter when reading from a remote file that has been
8173 written to synchronously by some other process. Metadata (e.g.,
8174 last-access and last-modified time) is not necessarily synchronized.
8178 @cindex synchronized data and metadata I/O
8179 Use synchronized I/O for both data and metadata.
8183 @cindex discarding file cache
8184 Discard the data cache for a file.
8185 When count=0 all cache is discarded,
8186 otherwise the cache is dropped for the processed
8187 portion of the file. Also when count=0
8188 failure to discard the cache is diagnosed
8189 and reflected in the exit status.
8190 Here as some usage examples:
8193 # Advise to drop cache for whole file
8194 dd if=ifile iflag=nocache count=0
8196 # Ensure drop cache for the whole file
8197 dd of=ofile oflag=nocache conv=notrunc,fdatasync count=0
8199 # Drop cache for part of file
8200 dd if=ifile iflag=nocache skip=10 count=10 of=/dev/null
8202 # Stream data using just the read-ahead cache
8203 dd if=ifile of=ofile iflag=nocache oflag=nocache
8208 @cindex nonblocking I/O
8209 Use non-blocking I/O.
8214 Do not update the file's access time.
8215 Some older file systems silently ignore this flag, so it is a good
8216 idea to test it on your files before relying on it.
8220 @cindex controlling terminal
8221 Do not assign the file to be a controlling terminal for @command{dd}.
8222 This has no effect when the file is not a terminal.
8223 On many hosts (e.g., @acronym{GNU}/Linux hosts), this option has no effect
8228 @cindex symbolic links, following
8229 Do not follow symbolic links.
8234 Fail if the file has multiple hard links.
8239 Use binary I/O. This option has an effect only on nonstandard
8240 platforms that distinguish binary from text I/O.
8245 Use text I/O. Like @samp{binary}, this option has no effect on
8250 Accumulate full blocks from input. The @code{read} system call
8251 may return early if a full block is not available.
8252 When that happens, continue calling @code{read} to fill the remainder
8254 This flag can be used only with @code{iflag}.
8258 These flags are not supported on all systems, and @samp{dd} rejects
8259 attempts to use them when they are not supported. When reading from
8260 standard input or writing to standard output, the @samp{nofollow} and
8261 @samp{noctty} flags should not be specified, and the other flags
8262 (e.g., @samp{nonblock}) can affect how other processes behave with the
8263 affected file descriptors, even after @command{dd} exits.
8267 @cindex multipliers after numbers
8268 The numeric-valued strings above (@var{bytes} and @var{blocks}) can be
8269 followed by a multiplier: @samp{b}=512, @samp{c}=1,
8270 @samp{w}=2, @samp{x@var{m}}=@var{m}, or any of the
8271 standard block size suffixes like @samp{k}=1024 (@pxref{Block size}).
8273 Any block size you specify via @samp{bs=}, @samp{ibs=}, @samp{obs=}, @samp{cbs=}
8274 should not be too large---values larger than a few megabytes
8275 are generally wasteful or (as in the gigabyte..exabyte case) downright
8276 counterproductive or error-inducing.
8278 Use different @command{dd} invocations to use different block sizes for
8279 skipping and I/O@. For example, the following shell commands copy data
8280 in 512 KiB blocks between a disk and a tape, but do not save or restore a
8281 4 KiB label at the start of the disk:
8284 disk=/dev/rdsk/c0t1d0s2
8287 # Copy all but the label from disk to tape.
8288 (dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape
8290 # Copy from tape back to disk, but leave the disk label alone.
8291 (dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk
8294 Sending an @samp{INFO} signal to a running @command{dd}
8295 process makes it print I/O statistics to standard error
8296 and then resume copying. In the example below,
8297 @command{dd} is run in the background to copy 10 million blocks.
8298 The @command{kill} command makes it output intermediate I/O statistics,
8299 and when @command{dd} completes normally or is killed by the
8300 @code{SIGINT} signal, it outputs the final statistics.
8303 $ dd if=/dev/zero of=/dev/null count=10MB & pid=$!
8304 $ kill -s INFO $pid; wait $pid
8305 3385223+0 records in
8306 3385223+0 records out
8307 1733234176 bytes (1.7 GB) copied, 6.42173 seconds, 270 MB/s
8308 10000000+0 records in
8309 10000000+0 records out
8310 5120000000 bytes (5.1 GB) copied, 18.913 seconds, 271 MB/s
8313 @vindex POSIXLY_CORRECT
8314 On systems lacking the @samp{INFO} signal @command{dd} responds to the
8315 @samp{USR1} signal instead, unless the @env{POSIXLY_CORRECT}
8316 environment variable is set.
8321 @node install invocation
8322 @section @command{install}: Copy files and set attributes
8325 @cindex copying files and setting attributes
8327 @command{install} copies files while setting their file mode bits and, if
8328 possible, their owner and group. Synopses:
8331 install [@var{option}]@dots{} [-T] @var{source} @var{dest}
8332 install [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8333 install [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8334 install [@var{option}]@dots{} -d @var{directory}@dots{}
8339 If two file names are given, @command{install} copies the first file to the
8343 If the @option{--target-directory} (@option{-t}) option is given, or
8344 failing that if the last file is a directory and the
8345 @option{--no-target-directory} (@option{-T}) option is not given,
8346 @command{install} copies each @var{source} file to the specified
8347 directory, using the @var{source}s' names.
8350 If the @option{--directory} (@option{-d}) option is given,
8351 @command{install} creates each @var{directory} and any missing parent
8352 directories. Parent directories are created with mode
8353 @samp{u=rwx,go=rx} (755), regardless of the @option{-m} option or the
8354 current umask. @xref{Directory Setuid and Setgid}, for how the
8355 set-user-ID and set-group-ID bits of parent directories are inherited.
8358 @cindex Makefiles, installing programs in
8359 @command{install} is similar to @command{cp}, but allows you to control the
8360 attributes of destination files. It is typically used in Makefiles to
8361 copy programs into their destination directories. It refuses to copy
8362 files onto themselves.
8364 @cindex extended attributes, xattr
8365 @command{install} never preserves extended attributes (xattr).
8367 The program accepts the following options. Also see @ref{Common options}.
8377 Compare each pair of source and destination files, and if the destination has
8378 identical content and any specified owner, group, permissions, and possibly
8379 SELinux context, then do not modify the destination at all.
8383 Ignored; for compatibility with old Unix versions of @command{install}.
8387 Create any missing parent directories of @var{dest},
8388 then copy @var{source} to @var{dest}.
8389 This option is ignored if a destination directory is specified
8390 via @option{--target-directory=DIR}.
8395 @opindex --directory
8396 @cindex directories, creating with given attributes
8397 @cindex parent directories, creating missing
8398 @cindex leading directories, creating missing
8399 Create any missing parent directories, giving them the default
8400 attributes. Then create each given directory, setting their owner,
8401 group and mode as given on the command line or to the defaults.
8403 @item -g @var{group}
8404 @itemx --group=@var{group}
8407 @cindex group ownership of installed files, setting
8408 Set the group ownership of installed files or directories to
8409 @var{group}. The default is the process's current group. @var{group}
8410 may be either a group name or a numeric group ID.
8413 @itemx --mode=@var{mode}
8416 @cindex permissions of installed files, setting
8417 Set the file mode bits for the installed file or directory to @var{mode},
8418 which can be either an octal number, or a symbolic mode as in
8419 @command{chmod}, with @samp{a=} (no access allowed to anyone) as the
8420 point of departure (@pxref{File permissions}).
8421 The default mode is @samp{u=rwx,go=rx,a-s}---read, write, and
8422 execute for the owner, read and execute for group and other, and with
8423 set-user-ID and set-group-ID disabled.
8424 This default is not quite the same as @samp{755}, since it disables
8425 instead of preserving set-user-ID and set-group-ID on directories.
8426 @xref{Directory Setuid and Setgid}.
8428 @item -o @var{owner}
8429 @itemx --owner=@var{owner}
8432 @cindex ownership of installed files, setting
8433 @cindex appropriate privileges
8434 @vindex root @r{as default owner}
8435 If @command{install} has appropriate privileges (is run as root), set the
8436 ownership of installed files or directories to @var{owner}. The default
8437 is @code{root}. @var{owner} may be either a user name or a numeric user
8440 @item --preserve-context
8441 @opindex --preserve-context
8443 @cindex security context
8444 Preserve the SELinux security context of files and directories.
8445 Failure to preserve the context in all of the files or directories
8446 will result in an exit status of 1. If SELinux is disabled then
8447 print a warning and ignore the option.
8450 @itemx --preserve-timestamps
8452 @opindex --preserve-timestamps
8453 @cindex timestamps of installed files, preserving
8454 Set the time of last access and the time of last modification of each
8455 installed file to match those of each corresponding original file.
8456 When a file is installed without this option, its last access and
8457 last modification times are both set to the time of installation.
8458 This option is useful if you want to use the last modification times
8459 of installed files to keep track of when they were last built as opposed
8460 to when they were last installed.
8466 @cindex symbol table information, stripping
8467 @cindex stripping symbol table information
8468 Strip the symbol tables from installed binary executables.
8470 @itemx --strip-program=@var{program}
8471 @opindex --strip-program
8472 @cindex symbol table information, stripping, program
8473 Program used to strip binaries.
8479 @optNoTargetDirectory
8485 Print the name of each file before copying it.
8487 @item -Z @var{context}
8488 @itemx --context=@var{context}
8492 @cindex security context
8493 Set the default SELinux security context to be used for any
8494 created files and directories. If SELinux is disabled then
8495 print a warning and ignore the option.
8503 @section @command{mv}: Move (rename) files
8507 @command{mv} moves or renames files (or directories). Synopses:
8510 mv [@var{option}]@dots{} [-T] @var{source} @var{dest}
8511 mv [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8512 mv [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8517 If two file names are given, @command{mv} moves the first file to the
8521 If the @option{--target-directory} (@option{-t}) option is given, or
8522 failing that if the last file is a directory and the
8523 @option{--no-target-directory} (@option{-T}) option is not given,
8524 @command{mv} moves each @var{source} file to the specified
8525 directory, using the @var{source}s' names.
8528 @command{mv} can move any type of file from one file system to another.
8529 Prior to version @code{4.0} of the fileutils,
8530 @command{mv} could move only regular files between file systems.
8531 For example, now @command{mv} can move an entire directory hierarchy
8532 including special device files from one partition to another. It first
8533 uses some of the same code that's used by @code{cp -a} to copy the
8534 requested directories and files, then (assuming the copy succeeded)
8535 it removes the originals. If the copy fails, then the part that was
8536 copied to the destination partition is removed. If you were to copy
8537 three directories from one partition to another and the copy of the first
8538 directory succeeded, but the second didn't, the first would be left on
8539 the destination partition and the second and third would be left on the
8542 @cindex extended attributes, xattr
8543 @command{mv} always tries to copy extended attributes (xattr), which may
8544 include SELinux context, ACLs or Capabilities.
8545 Upon failure all but @samp{Operation not supported} warnings are output.
8547 @cindex prompting, and @command{mv}
8548 If a destination file exists but is normally unwritable, standard input
8549 is a terminal, and the @option{-f} or @option{--force} option is not given,
8550 @command{mv} prompts the user for whether to replace the file. (You might
8551 own the file, or have write permission on its directory.) If the
8552 response is not affirmative, the file is skipped.
8554 @emph{Warning}: Avoid specifying a source name with a trailing slash,
8555 when it might be a symlink to a directory.
8556 Otherwise, @command{mv} may do something very surprising, since
8557 its behavior depends on the underlying rename system call.
8558 On a system with a modern Linux-based kernel, it fails with
8559 @code{errno=ENOTDIR}.
8560 However, on other systems (at least FreeBSD 6.1 and Solaris 10) it silently
8561 renames not the symlink but rather the directory referenced by the symlink.
8562 @xref{Trailing slashes}.
8564 The program accepts the following options. Also see @ref{Common options}.
8574 @cindex prompts, omitting
8575 Do not prompt the user before removing a destination file.
8577 If you specify more than one of the @option{-i}, @option{-f}, @option{-n}
8578 options, only the final one takes effect.
8583 @itemx --interactive
8585 @opindex --interactive
8586 @cindex prompts, forcing
8587 Prompt whether to overwrite each existing destination file, regardless
8589 If the response is not affirmative, the file is skipped.
8595 @opindex --no-clobber
8596 @cindex prompts, omitting
8597 Do not overwrite an existing file.
8599 This option is mutually exclusive with @option{-b} or @option{--backup} option.
8605 @cindex newer files, moving only
8606 Do not move a non-directory that has an existing destination with the
8607 same or newer modification time.
8608 If the move is across file system boundaries, the comparison is to the
8609 source time stamp truncated to the resolutions of the destination file
8610 system and of the system calls used to update time stamps; this avoids
8611 duplicate work if several @samp{mv -u} commands are executed with the
8612 same source and destination.
8618 Print the name of each file before moving it.
8620 @optStripTrailingSlashes
8626 @optNoTargetDirectory
8634 @section @command{rm}: Remove files or directories
8637 @cindex removing files or directories
8639 @command{rm} removes each given @var{file}. By default, it does not remove
8640 directories. Synopsis:
8643 rm [@var{option}]@dots{} [@var{file}]@dots{}
8646 @cindex prompting, and @command{rm}
8647 If the @option{-I} or @option{--interactive=once} option is given,
8648 and there are more than three files or the @option{-r}, @option{-R},
8649 or @option{--recursive} are given, then @command{rm} prompts the user
8650 for whether to proceed with the entire operation. If the response is
8651 not affirmative, the entire command is aborted.
8653 Otherwise, if a file is unwritable, standard input is a terminal, and
8654 the @option{-f} or @option{--force} option is not given, or the
8655 @option{-i} or @option{--interactive=always} option @emph{is} given,
8656 @command{rm} prompts the user for whether to remove the file.
8657 If the response is not affirmative, the file is skipped.
8659 Any attempt to remove a file whose last file name component is
8660 @file{.} or @file{..} is rejected without any prompting.
8662 @emph{Warning}: If you use @command{rm} to remove a file, it is usually
8663 possible to recover the contents of that file. If you want more assurance
8664 that the contents are truly unrecoverable, consider using @command{shred}.
8666 The program accepts the following options. Also see @ref{Common options}.
8674 Ignore nonexistent files and never prompt the user.
8675 Ignore any previous @option{--interactive} (@option{-i}) option.
8679 Prompt whether to remove each file.
8680 If the response is not affirmative, the file is skipped.
8681 Ignore any previous @option{--force} (@option{-f}) option.
8682 Equivalent to @option{--interactive=always}.
8686 Prompt once whether to proceed with the command, if more than three
8687 files are named or if a recursive removal is requested. Ignore any
8688 previous @option{--force} (@option{-f}) option. Equivalent to
8689 @option{--interactive=once}.
8691 @itemx --interactive [=@var{when}]
8692 @opindex --interactive
8693 Specify when to issue an interactive prompt. @var{when} may be
8697 @vindex never @r{interactive option}
8698 - Do not prompt at all.
8700 @vindex once @r{interactive option}
8701 - Prompt once if more than three files are named or if a recursive
8702 removal is requested. Equivalent to @option{-I}.
8704 @vindex always @r{interactive option}
8705 - Prompt for every file being removed. Equivalent to @option{-i}.
8707 @option{--interactive} with no @var{when} is equivalent to
8708 @option{--interactive=always}.
8710 @itemx --one-file-system
8711 @opindex --one-file-system
8712 @cindex one file system, restricting @command{rm} to
8713 When removing a hierarchy recursively, skip any directory that is on a
8714 file system different from that of the corresponding command line argument.
8717 This option is useful when removing a build ``chroot'' hierarchy,
8718 which normally contains no valuable data. However, it is not uncommon
8719 to bind-mount @file{/home} into such a hierarchy, to make it easier to
8720 use one's start-up file. The catch is that it's easy to forget to
8721 unmount @file{/home}. Then, when you use @command{rm -rf} to remove
8722 your normally throw-away chroot, that command will remove everything
8723 under @file{/home}, too.
8724 Use the @option{--one-file-system} option, and it will
8725 warn about and skip directories on other file systems.
8726 Of course, this will not save your @file{/home} if it and your
8727 chroot happen to be on the same file system.
8729 @itemx --preserve-root
8730 @opindex --preserve-root
8731 @cindex root directory, disallow recursive destruction
8732 Fail upon any attempt to remove the root directory, @file{/},
8733 when used with the @option{--recursive} option.
8734 This is the default behavior.
8735 @xref{Treating / specially}.
8737 @itemx --no-preserve-root
8738 @opindex --no-preserve-root
8739 @cindex root directory, allow recursive destruction
8740 Do not treat @file{/} specially when removing recursively.
8741 This option is not recommended unless you really want to
8742 remove all the files on your computer.
8743 @xref{Treating / specially}.
8750 @opindex --recursive
8751 @cindex directories, removing (recursively)
8752 Remove the listed directories and their contents recursively.
8758 Print the name of each file before removing it.
8762 @cindex files beginning with @samp{-}, removing
8763 @cindex @samp{-}, removing files beginning with
8764 One common question is how to remove files whose names begin with a
8765 @samp{-}. @sc{gnu} @command{rm}, like every program that uses the @code{getopt}
8766 function to parse its arguments, lets you use the @samp{--} option to
8767 indicate that all following arguments are non-options. To remove a file
8768 called @file{-f} in the current directory, you could type either:
8781 @opindex - @r{and Unix @command{rm}}
8782 The Unix @command{rm} program's use of a single @samp{-} for this purpose
8783 predates the development of the getopt standard syntax.
8788 @node shred invocation
8789 @section @command{shred}: Remove files more securely
8792 @cindex data, erasing
8793 @cindex erasing data
8795 @command{shred} overwrites devices or files, to help prevent even
8796 very expensive hardware from recovering the data.
8798 Ordinarily when you remove a file (@pxref{rm invocation}), the data is
8799 not actually destroyed. Only the index listing where the file is
8800 stored is destroyed, and the storage is made available for reuse.
8801 There are undelete utilities that will attempt to reconstruct the index
8802 and can bring the file back if the parts were not reused.
8804 On a busy system with a nearly-full drive, space can get reused in a few
8805 seconds. But there is no way to know for sure. If you have sensitive
8806 data, you may want to be sure that recovery is not possible by actually
8807 overwriting the file with non-sensitive data.
8809 However, even after doing that, it is possible to take the disk back
8810 to a laboratory and use a lot of sensitive (and expensive) equipment
8811 to look for the faint ``echoes'' of the original data underneath the
8812 overwritten data. If the data has only been overwritten once, it's not
8815 The best way to remove something irretrievably is to destroy the media
8816 it's on with acid, melt it down, or the like. For cheap removable media
8817 like floppy disks, this is the preferred method. However, hard drives
8818 are expensive and hard to melt, so the @command{shred} utility tries
8819 to achieve a similar effect non-destructively.
8821 This uses many overwrite passes, with the data patterns chosen to
8822 maximize the damage they do to the old data. While this will work on
8823 floppies, the patterns are designed for best effect on hard drives.
8824 For more details, see the source code and Peter Gutmann's paper
8825 @uref{http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html,
8826 @cite{Secure Deletion of Data from Magnetic and Solid-State Memory}},
8827 from the proceedings of the Sixth @acronym{USENIX} Security Symposium (San Jose,
8828 California, July 22--25, 1996).
8830 @strong{Please note} that @command{shred} relies on a very important assumption:
8831 that the file system overwrites data in place. This is the traditional
8832 way to do things, but many modern file system designs do not satisfy this
8833 assumption. Exceptions include:
8838 Log-structured or journaled file systems, such as those supplied with
8839 AIX and Solaris, and JFS, ReiserFS, XFS, Ext3 (in @code{data=journal} mode),
8840 BFS, NTFS, etc.@: when they are configured to journal @emph{data}.
8843 File systems that write redundant data and carry on even if some writes
8844 fail, such as RAID-based file systems.
8847 File systems that make snapshots, such as Network Appliance's NFS server.
8850 File systems that cache in temporary locations, such as NFS version 3
8854 Compressed file systems.
8857 In the particular case of ext3 file systems, the above disclaimer applies (and
8858 @command{shred} is thus of limited effectiveness) only in @code{data=journal}
8859 mode, which journals file data in addition to just metadata. In both
8860 the @code{data=ordered} (default) and @code{data=writeback} modes,
8861 @command{shred} works as usual. Ext3 journaling modes can be changed
8862 by adding the @code{data=something} option to the mount options for a
8863 particular file system in the @file{/etc/fstab} file, as documented in
8864 the mount man page (man mount).
8866 If you are not sure how your file system operates, then you should assume
8867 that it does not overwrite data in place, which means that shred cannot
8868 reliably operate on regular files in your file system.
8870 Generally speaking, it is more reliable to shred a device than a file,
8871 since this bypasses the problem of file system design mentioned above.
8872 However, even shredding devices is not always completely reliable. For
8873 example, most disks map out bad sectors invisibly to the application; if
8874 the bad sectors contain sensitive data, @command{shred} won't be able to
8877 @command{shred} makes no attempt to detect or report this problem, just as
8878 it makes no attempt to do anything about backups. However, since it is
8879 more reliable to shred devices than files, @command{shred} by default does
8880 not truncate or remove the output file. This default is more suitable
8881 for devices, which typically cannot be truncated and should not be
8884 Finally, consider the risk of backups and mirrors.
8885 File system backups and remote mirrors may contain copies of the
8886 file that cannot be removed, and that will allow a shredded file
8887 to be recovered later. So if you keep any data you may later want
8888 to destroy using @command{shred}, be sure that it is not backed up or mirrored.
8891 shred [@var{option}]@dots{} @var{file}[@dots{}]
8894 The program accepts the following options. Also see @ref{Common options}.
8902 @cindex force deletion
8903 Override file permissions if necessary to allow overwriting.
8906 @itemx -n @var{number}
8907 @itemx --iterations=@var{number}
8908 @opindex -n @var{number}
8909 @opindex --iterations=@var{number}
8910 @cindex iterations, selecting the number of
8911 By default, @command{shred} uses @value{SHRED_DEFAULT_PASSES} passes of
8912 overwrite. You can reduce this to save time, or increase it if you think it's
8913 appropriate. After 25 passes all of the internal overwrite patterns will have
8914 been used at least once.
8916 @item --random-source=@var{file}
8917 @opindex --random-source
8918 @cindex random source for shredding
8919 Use @var{file} as a source of random data used to overwrite and to
8920 choose pass ordering. @xref{Random sources}.
8922 @item -s @var{bytes}
8923 @itemx --size=@var{bytes}
8924 @opindex -s @var{bytes}
8925 @opindex --size=@var{bytes}
8926 @cindex size of file to shred
8927 Shred the first @var{bytes} bytes of the file. The default is to shred
8928 the whole file. @var{bytes} can be followed by a size specification like
8929 @samp{K}, @samp{M}, or @samp{G} to specify a multiple. @xref{Block size}.
8935 @cindex removing files after shredding
8936 After shredding a file, truncate it (if possible) and then remove it.
8937 If a file has multiple links, only the named links will be removed.
8943 Display to standard error all status updates as sterilization proceeds.
8949 By default, @command{shred} rounds the size of a regular file up to the next
8950 multiple of the file system block size to fully erase the last block
8952 Use @option{--exact} to suppress that behavior.
8953 Thus, by default if you shred a 10-byte regular file on a system with 512-byte
8954 blocks, the resulting file will be 512 bytes long. With this option,
8955 shred does not increase the apparent size of the file.
8961 Normally, the last pass that @command{shred} writes is made up of
8962 random data. If this would be conspicuous on your hard drive (for
8963 example, because it looks like encrypted data), or you just think
8964 it's tidier, the @option{--zero} option adds an additional overwrite pass with
8965 all zero bits. This is in addition to the number of passes specified
8966 by the @option{--iterations} option.
8970 You might use the following command to erase all trace of the
8971 file system you'd created on the floppy disk in your first drive.
8972 That command takes about 20 minutes to erase a ``1.44MB'' (actually
8976 shred --verbose /dev/fd0
8979 Similarly, to erase all data on a selected partition of
8980 your hard disk, you could give a command like this:
8983 shred --verbose /dev/sda5
8986 On modern disks, a single pass should be adequate,
8987 and it will take one third the time of the default three-pass approach.
8990 # 1 pass, write pseudo-random data; 3x faster than the default
8991 shred --verbose -n1 /dev/sda5
8994 To be on the safe side, use at least one pass that overwrites using
8995 pseudo-random data. I.e., don't be tempted to use @samp{-n0 --zero},
8996 in case some disk controller optimizes the process of writing blocks
8997 of all zeros, and thereby does not clear all bytes in a block.
8998 Some SSDs may do just that.
9000 A @var{file} of @samp{-} denotes standard output.
9001 The intended use of this is to shred a removed temporary file.
9008 echo "Hello, world" >&3
9013 However, the command @samp{shred - >file} does not shred the contents
9014 of @var{file}, since the shell truncates @var{file} before invoking
9015 @command{shred}. Use the command @samp{shred file} or (if using a
9016 Bourne-compatible shell) the command @samp{shred - 1<>file} instead.
9021 @node Special file types
9022 @chapter Special file types
9024 @cindex special file types
9025 @cindex file types, special
9027 This chapter describes commands which create special types of files (and
9028 @command{rmdir}, which removes directories, one special file type).
9030 @cindex special file types
9032 Although Unix-like operating systems have markedly fewer special file
9033 types than others, not @emph{everything} can be treated only as the
9034 undifferentiated byte stream of @dfn{normal files}. For example, when a
9035 file is created or removed, the system must record this information,
9036 which it does in a @dfn{directory}---a special type of file. Although
9037 you can read directories as normal files, if you're curious, in order
9038 for the system to do its job it must impose a structure, a certain
9039 order, on the bytes of the file. Thus it is a ``special'' type of file.
9041 Besides directories, other special file types include named pipes
9042 (FIFOs), symbolic links, sockets, and so-called @dfn{special files}.
9045 * link invocation:: Make a hard link via the link syscall
9046 * ln invocation:: Make links between files.
9047 * mkdir invocation:: Make directories.
9048 * mkfifo invocation:: Make FIFOs (named pipes).
9049 * mknod invocation:: Make block or character special files.
9050 * readlink invocation:: Print value of a symlink or canonical file name.
9051 * rmdir invocation:: Remove empty directories.
9052 * unlink invocation:: Remove files via the unlink syscall
9056 @node link invocation
9057 @section @command{link}: Make a hard link via the link syscall
9060 @cindex links, creating
9061 @cindex hard links, creating
9062 @cindex creating links (hard only)
9064 @command{link} creates a single hard link at a time.
9065 It is a minimalist interface to the system-provided
9066 @code{link} function. @xref{Hard Links, , , libc,
9067 The GNU C Library Reference Manual}.
9068 It avoids the bells and whistles of the more commonly-used
9069 @command{ln} command (@pxref{ln invocation}).
9073 link @var{filename} @var{linkname}
9076 @var{filename} must specify an existing file, and @var{linkname}
9077 must specify a nonexistent entry in an existing directory.
9078 @command{link} simply calls @code{link (@var{filename}, @var{linkname})}
9081 On a @acronym{GNU} system, this command acts like @samp{ln --directory
9082 --no-target-directory @var{filename} @var{linkname}}. However, the
9083 @option{--directory} and @option{--no-target-directory} options are
9084 not specified by @acronym{POSIX}, and the @command{link} command is
9085 more portable in practice.
9087 If @var{filename} is a symbolic link, it is unspecified whether
9088 @var{linkname} will be a hard link to the symbolic link or to the
9089 target of the symbolic link. Use @command{ln -P} or @command{ln -L}
9090 to specify which behavior is desired.
9096 @section @command{ln}: Make links between files
9099 @cindex links, creating
9100 @cindex hard links, creating
9101 @cindex symbolic (soft) links, creating
9102 @cindex creating links (hard or soft)
9104 @cindex file systems and hard links
9105 @command{ln} makes links between files. By default, it makes hard links;
9106 with the @option{-s} option, it makes symbolic (or @dfn{soft}) links.
9110 ln [@var{option}]@dots{} [-T] @var{target} @var{linkname}
9111 ln [@var{option}]@dots{} @var{target}
9112 ln [@var{option}]@dots{} @var{target}@dots{} @var{directory}
9113 ln [@var{option}]@dots{} -t @var{directory} @var{target}@dots{}
9119 If two file names are given, @command{ln} creates a link to the first
9120 file from the second.
9123 If one @var{target} is given, @command{ln} creates a link to that file
9124 in the current directory.
9127 If the @option{--target-directory} (@option{-t}) option is given, or
9128 failing that if the last file is a directory and the
9129 @option{--no-target-directory} (@option{-T}) option is not given,
9130 @command{ln} creates a link to each @var{target} file in the specified
9131 directory, using the @var{target}s' names.
9135 Normally @command{ln} does not remove existing files. Use the
9136 @option{--force} (@option{-f}) option to remove them unconditionally,
9137 the @option{--interactive} (@option{-i}) option to remove them
9138 conditionally, and the @option{--backup} (@option{-b}) option to
9141 @cindex hard link, defined
9142 @cindex inode, and hard links
9143 A @dfn{hard link} is another name for an existing file; the link and the
9144 original are indistinguishable. Technically speaking, they share the
9145 same inode, and the inode contains all the information about a
9146 file---indeed, it is not incorrect to say that the inode @emph{is} the
9147 file. Most systems prohibit making a hard link to
9148 a directory; on those where it is allowed, only the super-user can do
9149 so (and with caution, since creating a cycle will cause problems to many
9150 other utilities). Hard links cannot cross file system boundaries. (These
9151 restrictions are not mandated by @acronym{POSIX}, however.)
9153 @cindex dereferencing symbolic links
9154 @cindex symbolic link, defined
9155 @dfn{Symbolic links} (@dfn{symlinks} for short), on the other hand, are
9156 a special file type (which not all kernels support: System V release 3
9157 (and older) systems lack symlinks) in which the link file actually
9158 refers to a different file, by name. When most operations (opening,
9159 reading, writing, and so on) are passed the symbolic link file, the
9160 kernel automatically @dfn{dereferences} the link and operates on the
9161 target of the link. But some operations (e.g., removing) work on the
9162 link file itself, rather than on its target. The owner and group of a
9163 symlink are not significant to file access performed through
9164 the link, but do have implications on deleting a symbolic link from a
9165 directory with the restricted deletion bit set. On the GNU system,
9166 the mode of a symlink has no significance and cannot be changed, but
9167 on some BSD systems, the mode can be changed and will affect whether
9168 the symlink will be traversed in file name resolution. @xref{Symbolic Links,,,
9169 libc, The GNU C Library Reference Manual}.
9171 Symbolic links can contain arbitrary strings; a @dfn{dangling symlink}
9172 occurs when the string in the symlink does not resolve to a file.
9173 There are no restrictions against creating dangling symbolic links.
9174 There are trade-offs to using absolute or relative symlinks. An
9175 absolute symlink always points to the same file, even if the directory
9176 containing the link is moved. However, if the symlink is visible from
9177 more than one machine (such as on a networked file system), the file
9178 pointed to might not always be the same. A relative symbolic link is
9179 resolved in relation to the directory that contains the link, and is
9180 often useful in referring to files on the same device without regards
9181 to what name that device is mounted on when accessed via networked
9184 When creating a relative symlink in a different location than the
9185 current directory, the resolution of the symlink will be different
9186 than the resolution of the same string from the current directory.
9187 Therefore, many users prefer to first change directories to the
9188 location where the relative symlink will be created, so that
9189 tab-completion or other file resolution will find the same target as
9190 what will be placed in the symlink.
9192 The program accepts the following options. Also see @ref{Common options}.
9203 @opindex --directory
9204 @cindex hard links to directories
9205 Allow users with appropriate privileges to attempt to make hard links
9207 However, note that this will probably fail due to
9208 system restrictions, even for the super-user.
9214 Remove existing destination files.
9217 @itemx --interactive
9219 @opindex --interactive
9220 @cindex prompting, and @command{ln}
9221 Prompt whether to remove existing destination files.
9227 If @option{-s} is not in effect, and the source file is a symbolic
9228 link, create the hard link to the file referred to by the symbolic
9229 link, rather than the symbolic link itself.
9232 @itemx --no-dereference
9234 @opindex --no-dereference
9235 Do not treat the last operand specially when it is a symbolic link to
9236 a directory. Instead, treat it as if it were a normal file.
9238 When the destination is an actual directory (not a symlink to one),
9239 there is no ambiguity. The link is created in that directory.
9240 But when the specified destination is a symlink to a directory,
9241 there are two ways to treat the user's request. @command{ln} can
9242 treat the destination just as it would a normal directory and create
9243 the link in it. On the other hand, the destination can be viewed as a
9244 non-directory---as the symlink itself. In that case, @command{ln}
9245 must delete or backup that symlink before creating the new link.
9246 The default is to treat a destination that is a symlink to a directory
9247 just like a directory.
9249 This option is weaker than the @option{--no-target-directory}
9250 (@option{-T}) option, so it has no effect if both options are given.
9256 If @option{-s} is not in effect, and the source file is a symbolic
9257 link, create the hard link to the symbolic link itself. On platforms
9258 where this is not supported by the kernel, this option creates a
9259 symbolic link with identical contents; since symbolic link contents
9260 cannot be edited, any file name resolution performed through either
9261 link will be the same as if a hard link had been created.
9267 Make symbolic links instead of hard links. This option merely produces
9268 an error message on systems that do not support symbolic links.
9274 @optNoTargetDirectory
9280 Print the name of each file after linking it successfully.
9284 @cindex hard links to symbolic links
9285 @cindex symbolic links and @command{ln}
9286 If @option{-L} and @option{-P} are both given, the last one takes
9287 precedence. If @option{-s} is also given, @option{-L} and @option{-P}
9288 are silently ignored. If neither option is given, then this
9289 implementation defaults to @option{-P} if the system @code{link} supports
9290 hard links to symbolic links (such as the GNU system), and @option{-L}
9291 if @code{link} follows symbolic links (such as on BSD).
9300 # Create link ../a pointing to a in that directory.
9301 # Not really useful because it points to itself.
9306 # Change to the target before creating symlinks to avoid being confused.
9312 # Hard coded file names don't move well.
9313 ln -s $(pwd)/a /some/dir/
9317 # Relative file names survive directory moves and also
9318 # work across networked file systems.
9319 ln -s afile anotherfile
9320 ln -s ../adir/afile yetanotherfile
9324 @node mkdir invocation
9325 @section @command{mkdir}: Make directories
9328 @cindex directories, creating
9329 @cindex creating directories
9331 @command{mkdir} creates directories with the specified names. Synopsis:
9334 mkdir [@var{option}]@dots{} @var{name}@dots{}
9337 @command{mkdir} creates each directory @var{name} in the order given.
9338 It reports an error if @var{name} already exists, unless the
9339 @option{-p} option is given and @var{name} is a directory.
9341 The program accepts the following options. Also see @ref{Common options}.
9346 @itemx --mode=@var{mode}
9349 @cindex modes of created directories, setting
9350 Set the file permission bits of created directories to @var{mode},
9351 which uses the same syntax as
9352 in @command{chmod} and uses @samp{a=rwx} (read, write and execute allowed for
9353 everyone) for the point of the departure. @xref{File permissions}.
9355 Normally the directory has the desired file mode bits at the moment it
9356 is created. As a @acronym{GNU} extension, @var{mode} may also mention
9357 special mode bits, but in this case there may be a temporary window
9358 during which the directory exists but its special mode bits are
9359 incorrect. @xref{Directory Setuid and Setgid}, for how the
9360 set-user-ID and set-group-ID bits of directories are inherited unless
9361 overridden in this way.
9367 @cindex parent directories, creating
9368 Make any missing parent directories for each argument, setting their
9369 file permission bits to the umask modified by @samp{u+wx}. Ignore
9370 existing parent directories, and do not change their file permission
9373 To set the file permission bits of any newly-created parent
9374 directories to a value that includes @samp{u+wx}, you can set the
9375 umask before invoking @command{mkdir}. For example, if the shell
9376 command @samp{(umask u=rwx,go=rx; mkdir -p P/Q)} creates the parent
9377 @file{P} it sets the parent's permission bits to @samp{u=rwx,go=rx}.
9378 To set a parent's special mode bits as well, you can invoke
9379 @command{chmod} after @command{mkdir}. @xref{Directory Setuid and
9380 Setgid}, for how the set-user-ID and set-group-ID bits of
9381 newly-created parent directories are inherited.
9387 Print a message for each created directory. This is most useful with
9390 @item -Z @var{context}
9391 @itemx --context=@var{context}
9395 @cindex security context
9396 Set the default SELinux security context to be used for created directories.
9403 @node mkfifo invocation
9404 @section @command{mkfifo}: Make FIFOs (named pipes)
9407 @cindex FIFOs, creating
9408 @cindex named pipes, creating
9409 @cindex creating FIFOs (named pipes)
9411 @command{mkfifo} creates FIFOs (also called @dfn{named pipes}) with the
9412 specified names. Synopsis:
9415 mkfifo [@var{option}] @var{name}@dots{}
9418 A @dfn{FIFO} is a special file type that permits independent processes
9419 to communicate. One process opens the FIFO file for writing, and
9420 another for reading, after which data can flow as with the usual
9421 anonymous pipe in shells or elsewhere.
9423 The program accepts the following option. Also see @ref{Common options}.
9428 @itemx --mode=@var{mode}
9431 @cindex modes of created FIFOs, setting
9432 Set the mode of created FIFOs to @var{mode}, which is symbolic as in
9433 @command{chmod} and uses @samp{a=rw} (read and write allowed for everyone)
9434 for the point of departure. @var{mode} should specify only file
9435 permission bits. @xref{File permissions}.
9437 @item -Z @var{context}
9438 @itemx --context=@var{context}
9442 @cindex security context
9443 Set the default SELinux security context to be used for created FIFOs.
9450 @node mknod invocation
9451 @section @command{mknod}: Make block or character special files
9454 @cindex block special files, creating
9455 @cindex character special files, creating
9457 @command{mknod} creates a FIFO, character special file, or block special
9458 file with the specified name. Synopsis:
9461 mknod [@var{option}]@dots{} @var{name} @var{type} [@var{major} @var{minor}]
9464 @cindex special files
9465 @cindex block special files
9466 @cindex character special files
9467 Unlike the phrase ``special file type'' above, the term @dfn{special
9468 file} has a technical meaning on Unix: something that can generate or
9469 receive data. Usually this corresponds to a physical piece of hardware,
9470 e.g., a printer or a disk. (These files are typically created at
9471 system-configuration time.) The @command{mknod} command is what creates
9472 files of this type. Such devices can be read either a character at a
9473 time or a ``block'' (many characters) at a time, hence we say there are
9474 @dfn{block special} files and @dfn{character special} files.
9476 @c mknod is a shell built-in at least with OpenBSD's /bin/sh
9477 @mayConflictWithShellBuiltIn{mknod}
9479 The arguments after @var{name} specify the type of file to make:
9484 @opindex p @r{for FIFO file}
9488 @opindex b @r{for block special file}
9489 for a block special file
9492 @c Don't document the `u' option -- it's just a synonym for `c'.
9493 @c Do *any* versions of mknod still use it?
9495 @opindex c @r{for character special file}
9496 @c @opindex u @r{for character special file}
9497 for a character special file
9501 When making a block or character special file, the major and minor
9502 device numbers must be given after the file type.
9503 If a major or minor device number begins with @samp{0x} or @samp{0X},
9504 it is interpreted as hexadecimal; otherwise, if it begins with @samp{0},
9505 as octal; otherwise, as decimal.
9507 The program accepts the following option. Also see @ref{Common options}.
9512 @itemx --mode=@var{mode}
9515 Set the mode of created files to @var{mode}, which is symbolic as in
9516 @command{chmod} and uses @samp{a=rw} as the point of departure.
9517 @var{mode} should specify only file permission bits.
9518 @xref{File permissions}.
9520 @item -Z @var{context}
9521 @itemx --context=@var{context}
9525 @cindex security context
9526 Set the default SELinux security context to be used for created files.
9533 @node readlink invocation
9534 @section @command{readlink}: Print value of a symlink or canonical file name
9537 @cindex displaying value of a symbolic link
9538 @cindex canonical file name
9539 @cindex canonicalize a file name
9543 @command{readlink} may work in one of two supported modes:
9549 @command{readlink} outputs the value of the given symbolic link.
9550 If @command{readlink} is invoked with an argument other than the name
9551 of a symbolic link, it produces no output and exits with a nonzero exit code.
9553 @item Canonicalize mode
9555 @command{readlink} outputs the absolute name of the given file which contains
9556 no @file{.}, @file{..} components nor any repeated separators
9557 (@file{/}) or symbolic links.
9562 readlink [@var{option}] @var{file}
9565 By default, @command{readlink} operates in readlink mode.
9567 The program accepts the following options. Also see @ref{Common options}.
9572 @itemx --canonicalize
9574 @opindex --canonicalize
9575 Activate canonicalize mode.
9576 If any component of the file name except the last one is missing or unavailable,
9577 @command{readlink} produces no output and exits with a nonzero exit
9578 code. A trailing slash is ignored.
9581 @itemx --canonicalize-existing
9583 @opindex --canonicalize-existing
9584 Activate canonicalize mode.
9585 If any component is missing or unavailable, @command{readlink} produces
9586 no output and exits with a nonzero exit code. A trailing slash
9587 requires that the name resolve to a directory.
9590 @itemx --canonicalize-missing
9592 @opindex --canonicalize-missing
9593 Activate canonicalize mode.
9594 If any component is missing or unavailable, @command{readlink} treats it
9600 @opindex --no-newline
9601 Do not output the trailing newline.
9611 Suppress most error messages.
9617 Report error messages.
9621 The @command{readlink} utility first appeared in OpenBSD 2.1.
9623 There is a @command{realpath} command on some systems
9624 which operates like @command{readlink} in canonicalize mode.
9629 @node rmdir invocation
9630 @section @command{rmdir}: Remove empty directories
9633 @cindex removing empty directories
9634 @cindex directories, removing empty
9636 @command{rmdir} removes empty directories. Synopsis:
9639 rmdir [@var{option}]@dots{} @var{directory}@dots{}
9642 If any @var{directory} argument does not refer to an existing empty
9643 directory, it is an error.
9645 The program accepts the following options. Also see @ref{Common options}.
9649 @item --ignore-fail-on-non-empty
9650 @opindex --ignore-fail-on-non-empty
9651 @cindex directory deletion, ignoring failures
9652 Ignore each failure to remove a directory that is solely because
9653 the directory is non-empty.
9659 @cindex parent directories, removing
9660 Remove @var{directory}, then try to remove each component of @var{directory}.
9661 So, for example, @samp{rmdir -p a/b/c} is similar to @samp{rmdir a/b/c a/b a}.
9662 As such, it fails if any of those directories turns out not to be empty.
9663 Use the @option{--ignore-fail-on-non-empty} option to make it so such
9664 a failure does not evoke a diagnostic and does not cause @command{rmdir} to
9665 exit unsuccessfully.
9671 @cindex directory deletion, reporting
9672 Give a diagnostic for each successful removal.
9673 @var{directory} is removed.
9677 @xref{rm invocation}, for how to remove non-empty directories (recursively).
9682 @node unlink invocation
9683 @section @command{unlink}: Remove files via the unlink syscall
9686 @cindex removing files or directories (via the unlink syscall)
9688 @command{unlink} deletes a single specified file name.
9689 It is a minimalist interface to the system-provided
9690 @code{unlink} function. @xref{Deleting Files, , , libc,
9691 The GNU C Library Reference Manual}. Synopsis:
9692 It avoids the bells and whistles of the more commonly-used
9693 @command{rm} command (@pxref{rm invocation}).
9696 unlink @var{filename}
9699 On some systems @code{unlink} can be used to delete the name of a
9700 directory. On others, it can be used that way only by a privileged user.
9701 In the GNU system @code{unlink} can never delete the name of a directory.
9703 The @command{unlink} command honors the @option{--help} and
9704 @option{--version} options. To remove a file whose name begins with
9705 @samp{-}, prefix the name with @samp{./}, e.g., @samp{unlink ./--help}.
9710 @node Changing file attributes
9711 @chapter Changing file attributes
9713 @cindex changing file attributes
9714 @cindex file attributes, changing
9715 @cindex attributes, file
9717 A file is not merely its contents, a name, and a file type
9718 (@pxref{Special file types}). A file also has an owner (a user ID), a
9719 group (a group ID), permissions (what the owner can do with the file,
9720 what people in the group can do, and what everyone else can do), various
9721 timestamps, and other information. Collectively, we call these a file's
9724 These commands change file attributes.
9727 * chgrp invocation:: Change file groups.
9728 * chmod invocation:: Change access permissions.
9729 * chown invocation:: Change file owners and groups.
9730 * touch invocation:: Change file timestamps.
9734 @node chown invocation
9735 @section @command{chown}: Change file owner and group
9738 @cindex file ownership, changing
9739 @cindex group ownership, changing
9740 @cindex changing file ownership
9741 @cindex changing group ownership
9743 @command{chown} changes the user and/or group ownership of each given @var{file}
9744 to @var{new-owner} or to the user and group of an existing reference file.
9748 chown [@var{option}]@dots{} @{@var{new-owner} | --reference=@var{ref_file}@}@c
9752 If used, @var{new-owner} specifies the new owner and/or group as follows
9753 (with no embedded white space):
9756 [@var{owner}] [ : [@var{group}] ]
9763 If only an @var{owner} (a user name or numeric user ID) is given, that
9764 user is made the owner of each given file, and the files' group is not
9767 @item owner@samp{:}group
9768 If the @var{owner} is followed by a colon and a @var{group} (a
9769 group name or numeric group ID), with no spaces between them, the group
9770 ownership of the files is changed as well (to @var{group}).
9773 If a colon but no group name follows @var{owner}, that user is
9774 made the owner of the files and the group of the files is changed to
9775 @var{owner}'s login group.
9778 If the colon and following @var{group} are given, but the owner
9779 is omitted, only the group of the files is changed; in this case,
9780 @command{chown} performs the same function as @command{chgrp}.
9783 If only a colon is given, or if @var{new-owner} is empty, neither the
9784 owner nor the group is changed.
9788 If @var{owner} or @var{group} is intended to represent a numeric user
9789 or group ID, then you may specify it with a leading @samp{+}.
9790 @xref{Disambiguating names and IDs}.
9792 Some older scripts may still use @samp{.} in place of the @samp{:} separator.
9793 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) does not
9794 require support for that, but for backward compatibility @acronym{GNU}
9795 @command{chown} supports @samp{.} so long as no ambiguity results.
9796 New scripts should avoid the use of @samp{.} because it is not
9797 portable, and because it has undesirable results if the entire
9798 @var{owner@samp{.}group} happens to identify a user whose name
9801 The @command{chown} command sometimes clears the set-user-ID or
9802 set-group-ID permission bits. This behavior depends on the policy and
9803 functionality of the underlying @code{chown} system call, which may
9804 make system-dependent file mode modifications outside the control of
9805 the @command{chown} command. For example, the @command{chown} command
9806 might not affect those bits when invoked by a user with appropriate
9807 privileges, or when the
9808 bits signify some function other than executable permission (e.g.,
9810 When in doubt, check the underlying system behavior.
9812 The program accepts the following options. Also see @ref{Common options}.
9820 @cindex changed owners, verbosely describing
9821 Verbosely describe the action for each @var{file} whose ownership
9830 @cindex error messages, omitting
9831 Do not print error messages about files whose ownership cannot be
9834 @itemx @w{@kbd{--from}=@var{old-owner}}
9836 @cindex symbolic links, changing owner
9837 Change a @var{file}'s ownership only if it has current attributes specified
9838 by @var{old-owner}. @var{old-owner} has the same form as @var{new-owner}
9840 This option is useful primarily from a security standpoint in that
9841 it narrows considerably the window of potential abuse.
9842 For example, to reflect a user ID numbering change for one user's files
9843 without an option like this, @code{root} might run
9846 find / -owner OLDUSER -print0 | xargs -0 chown -h NEWUSER
9849 But that is dangerous because the interval between when the @command{find}
9850 tests the existing file's owner and when the @command{chown} is actually run
9852 One way to narrow the gap would be to invoke chown for each file
9856 find / -owner OLDUSER -exec chown -h NEWUSER @{@} \;
9859 But that is very slow if there are many affected files.
9860 With this option, it is safer (the gap is narrower still)
9861 though still not perfect:
9864 chown -h -R --from=OLDUSER NEWUSER /
9868 @opindex --dereference
9869 @cindex symbolic links, changing owner
9871 Do not act on symbolic links themselves but rather on what they point to.
9872 This is the default.
9875 @itemx --no-dereference
9877 @opindex --no-dereference
9878 @cindex symbolic links, changing owner
9880 Act on symbolic links themselves instead of what they point to.
9881 This mode relies on the @code{lchown} system call.
9882 On systems that do not provide the @code{lchown} system call,
9883 @command{chown} fails when a file specified on the command line
9885 By default, no diagnostic is issued for symbolic links encountered
9886 during a recursive traversal, but see @option{--verbose}.
9888 @itemx --preserve-root
9889 @opindex --preserve-root
9890 @cindex root directory, disallow recursive modification
9891 Fail upon any attempt to recursively change the root directory, @file{/}.
9892 Without @option{--recursive}, this option has no effect.
9893 @xref{Treating / specially}.
9895 @itemx --no-preserve-root
9896 @opindex --no-preserve-root
9897 @cindex root directory, allow recursive modification
9898 Cancel the effect of any preceding @option{--preserve-root} option.
9899 @xref{Treating / specially}.
9901 @item --reference=@var{ref_file}
9902 @opindex --reference
9903 Change the user and group of each @var{file} to be the same as those of
9904 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9905 user and group of the symbolic link, but rather those of the file it
9912 Output a diagnostic for every file processed.
9913 If a symbolic link is encountered during a recursive traversal
9914 on a system without the @code{lchown} system call, and @option{--no-dereference}
9915 is in effect, then issue a diagnostic saying neither the symbolic link nor
9916 its referent is being changed.
9921 @opindex --recursive
9922 @cindex recursively changing file ownership
9923 Recursively change ownership of directories and their contents.
9926 @xref{Traversing symlinks}.
9929 @xref{Traversing symlinks}.
9932 @xref{Traversing symlinks}.
9941 # Change the owner of /u to "root".
9944 # Likewise, but also change its group to "staff".
9947 # Change the owner of /u and subfiles to "root".
9952 @node chgrp invocation
9953 @section @command{chgrp}: Change group ownership
9956 @cindex group ownership, changing
9957 @cindex changing group ownership
9959 @command{chgrp} changes the group ownership of each given @var{file}
9960 to @var{group} (which can be either a group name or a numeric group ID)
9961 or to the group of an existing reference file. Synopsis:
9964 chgrp [@var{option}]@dots{} @{@var{group} | --reference=@var{ref_file}@}@c
9968 If @var{group} is intended to represent a
9969 numeric group ID, then you may specify it with a leading @samp{+}.
9970 @xref{Disambiguating names and IDs}.
9972 The program accepts the following options. Also see @ref{Common options}.
9980 @cindex changed files, verbosely describing
9981 Verbosely describe the action for each @var{file} whose group actually
9990 @cindex error messages, omitting
9991 Do not print error messages about files whose group cannot be
9995 @opindex --dereference
9996 @cindex symbolic links, changing owner
9998 Do not act on symbolic links themselves but rather on what they point to.
9999 This is the default.
10002 @itemx --no-dereference
10004 @opindex --no-dereference
10005 @cindex symbolic links, changing group
10007 Act on symbolic links themselves instead of what they point to.
10008 This mode relies on the @code{lchown} system call.
10009 On systems that do not provide the @code{lchown} system call,
10010 @command{chgrp} fails when a file specified on the command line
10011 is a symbolic link.
10012 By default, no diagnostic is issued for symbolic links encountered
10013 during a recursive traversal, but see @option{--verbose}.
10015 @itemx --preserve-root
10016 @opindex --preserve-root
10017 @cindex root directory, disallow recursive modification
10018 Fail upon any attempt to recursively change the root directory, @file{/}.
10019 Without @option{--recursive}, this option has no effect.
10020 @xref{Treating / specially}.
10022 @itemx --no-preserve-root
10023 @opindex --no-preserve-root
10024 @cindex root directory, allow recursive modification
10025 Cancel the effect of any preceding @option{--preserve-root} option.
10026 @xref{Treating / specially}.
10028 @item --reference=@var{ref_file}
10029 @opindex --reference
10030 Change the group of each @var{file} to be the same as that of
10031 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
10032 group of the symbolic link, but rather that of the file it refers to.
10038 Output a diagnostic for every file processed.
10039 If a symbolic link is encountered during a recursive traversal
10040 on a system without the @code{lchown} system call, and @option{--no-dereference}
10041 is in effect, then issue a diagnostic saying neither the symbolic link nor
10042 its referent is being changed.
10047 @opindex --recursive
10048 @cindex recursively changing group ownership
10049 Recursively change the group ownership of directories and their contents.
10052 @xref{Traversing symlinks}.
10055 @xref{Traversing symlinks}.
10058 @xref{Traversing symlinks}.
10067 # Change the group of /u to "staff".
10070 # Change the group of /u and subfiles to "staff".
10075 @node chmod invocation
10076 @section @command{chmod}: Change access permissions
10079 @cindex changing access permissions
10080 @cindex access permissions, changing
10081 @cindex permissions, changing access
10083 @command{chmod} changes the access permissions of the named files. Synopsis:
10086 chmod [@var{option}]@dots{} @{@var{mode} | --reference=@var{ref_file}@}@c
10090 @cindex symbolic links, permissions of
10091 @command{chmod} never changes the permissions of symbolic links, since
10092 the @command{chmod} system call cannot change their permissions.
10093 This is not a problem since the permissions of symbolic links are
10094 never used. However, for each symbolic link listed on the command
10095 line, @command{chmod} changes the permissions of the pointed-to file.
10096 In contrast, @command{chmod} ignores symbolic links encountered during
10097 recursive directory traversals.
10099 A successful use of @command{chmod} clears the set-group-ID bit of a
10100 regular file if the file's group ID does not match the user's
10101 effective group ID or one of the user's supplementary group IDs,
10102 unless the user has appropriate privileges. Additional restrictions
10103 may cause the set-user-ID and set-group-ID bits of @var{mode} or
10104 @var{ref_file} to be ignored. This behavior depends on the policy and
10105 functionality of the underlying @code{chmod} system call. When in
10106 doubt, check the underlying system behavior.
10108 If used, @var{mode} specifies the new file mode bits.
10109 For details, see the section on @ref{File permissions}.
10110 If you really want @var{mode} to have a leading @samp{-}, you should
10111 use @option{--} first, e.g., @samp{chmod -- -w file}. Typically,
10112 though, @samp{chmod a-w file} is preferable, and @command{chmod -w
10113 file} (without the @option{--}) complains if it behaves differently
10114 from what @samp{chmod a-w file} would do.
10116 The program accepts the following options. Also see @ref{Common options}.
10124 Verbosely describe the action for each @var{file} whose permissions
10133 @cindex error messages, omitting
10134 Do not print error messages about files whose permissions cannot be
10137 @itemx --preserve-root
10138 @opindex --preserve-root
10139 @cindex root directory, disallow recursive modification
10140 Fail upon any attempt to recursively change the root directory, @file{/}.
10141 Without @option{--recursive}, this option has no effect.
10142 @xref{Treating / specially}.
10144 @itemx --no-preserve-root
10145 @opindex --no-preserve-root
10146 @cindex root directory, allow recursive modification
10147 Cancel the effect of any preceding @option{--preserve-root} option.
10148 @xref{Treating / specially}.
10154 Verbosely describe the action or non-action taken for every @var{file}.
10156 @item --reference=@var{ref_file}
10157 @opindex --reference
10158 Change the mode of each @var{file} to be the same as that of @var{ref_file}.
10159 @xref{File permissions}.
10160 If @var{ref_file} is a symbolic link, do not use the mode
10161 of the symbolic link, but rather that of the file it refers to.
10166 @opindex --recursive
10167 @cindex recursively changing access permissions
10168 Recursively change permissions of directories and their contents.
10175 @node touch invocation
10176 @section @command{touch}: Change file timestamps
10179 @cindex changing file timestamps
10180 @cindex file timestamps, changing
10181 @cindex timestamps, changing file
10183 @command{touch} changes the access and/or modification times of the
10184 specified files. Synopsis:
10187 touch [@var{option}]@dots{} @var{file}@dots{}
10190 @cindex empty files, creating
10191 Any @var{file} argument that does not exist is created empty, unless
10192 option @option{--no-create} (@option{-c}) or @option{--no-dereference}
10193 (@option{-h}) was in effect.
10195 A @var{file} argument string of @samp{-} is handled specially and
10196 causes @command{touch} to change the times of the file associated with
10199 @cindex permissions, for changing file timestamps
10200 If changing both the access and modification times to the current
10201 time, @command{touch} can change the timestamps for files that the user
10202 running it does not own but has write permission for. Otherwise, the
10203 user must own the files.
10205 Although @command{touch} provides options for changing two of the times---the
10206 times of last access and modification---of a file, there is actually
10207 a standard third one as well: the inode change time. This is often
10208 referred to as a file's @code{ctime}.
10209 The inode change time represents the time when the file's meta-information
10210 last changed. One common example of this is when the permissions of a
10211 file change. Changing the permissions doesn't access the file, so
10212 the atime doesn't change, nor does it modify the file, so the mtime
10213 doesn't change. Yet, something about the file itself has changed,
10214 and this must be noted somewhere. This is the job of the ctime field.
10215 This is necessary, so that, for example, a backup program can make a
10216 fresh copy of the file, including the new permissions value.
10217 Another operation that modifies a file's ctime without affecting
10218 the others is renaming. In any case, it is not possible, in normal
10219 operations, for a user to change the ctime field to a user-specified value.
10220 Some operating systems and file systems support a fourth time: the
10221 birth time, when the file was first created; by definition, this
10222 timestamp never changes.
10225 Time stamps assume the time zone rules specified by the @env{TZ}
10226 environment variable, or by the system default rules if @env{TZ} is
10227 not set. @xref{TZ Variable,, Specifying the Time Zone with @env{TZ},
10228 libc, The GNU C Library Reference Manual}.
10229 You can avoid ambiguities during
10230 daylight saving transitions by using @sc{utc} time stamps.
10232 The program accepts the following options. Also see @ref{Common options}.
10237 @itemx --time=atime
10238 @itemx --time=access
10242 @opindex atime@r{, changing}
10243 @opindex access @r{time, changing}
10244 @opindex use @r{time, changing}
10245 Change the access time only.
10250 @opindex --no-create
10251 Do not warn about or create files that do not exist.
10254 @itemx --date=@var{time}
10258 Use @var{time} instead of the current time. It can contain month names,
10259 time zones, @samp{am} and @samp{pm}, @samp{yesterday}, etc. For
10260 example, @option{--date="2004-02-27 14:19:13.489392193 +0530"}
10261 specifies the instant of time that is 489,392,193 nanoseconds after
10262 February 27, 2004 at 2:19:13 PM in a time zone that is 5 hours and 30
10263 minutes east of @acronym{UTC}. @xref{Date input formats}.
10264 File systems that do not support high-resolution time stamps
10265 silently ignore any excess precision here.
10269 @cindex BSD @command{touch} compatibility
10270 Ignored; for compatibility with BSD versions of @command{touch}.
10273 @itemx --no-dereference
10275 @opindex --no-dereference
10276 @cindex symbolic links, changing time
10278 Attempt to change the timestamps of a symbolic link, rather than what
10279 the link refers to. When using this option, empty files are not
10280 created, but option @option{-c} must also be used to avoid warning
10281 about files that do not exist. Not all systems support changing the
10282 timestamps of symlinks, since underlying system support for this
10283 action was not required until @acronym{POSIX} 2008. Also, on some
10284 systems, the mere act of examining a symbolic link changes the access
10285 time, such that only changes to the modification time will persist
10286 long enough to be observable. When coupled with option @option{-r}, a
10287 reference timestamp is taken from a symbolic link rather than the file
10291 @itemx --time=mtime
10292 @itemx --time=modify
10295 @opindex mtime@r{, changing}
10296 @opindex modify @r{time, changing}
10297 Change the modification time only.
10299 @item -r @var{file}
10300 @itemx --reference=@var{file}
10302 @opindex --reference
10303 Use the times of the reference @var{file} instead of the current time.
10304 If this option is combined with the @option{--date=@var{time}}
10305 (@option{-d @var{time}}) option, the reference @var{file}'s time is
10306 the origin for any relative @var{time}s given, but is otherwise ignored.
10307 For example, @samp{-r foo -d '-5 seconds'} specifies a time stamp
10308 equal to five seconds before the corresponding time stamp for @file{foo}.
10309 If @var{file} is a symbolic link, the reference timestamp is taken
10310 from the target of the symlink, unless @option{-h} was also in effect.
10312 @item -t [[@var{cc}]@var{yy}]@var{mmddhhmm}[.@var{ss}]
10313 Use the argument (optional four-digit or two-digit years, months,
10314 days, hours, minutes, optional seconds) instead of the current time.
10315 If the year is specified with only two digits, then @var{cc}
10316 is 20 for years in the range 0 @dots{} 68, and 19 for years in
10317 69 @dots{} 99. If no digits of the year are specified,
10318 the argument is interpreted as a date in the current year.
10319 Note that @var{ss} may be @samp{60}, to accommodate leap seconds.
10323 @vindex _POSIX2_VERSION
10324 On older systems, @command{touch} supports an obsolete syntax, as follows.
10325 If no timestamp is given with any of the @option{-d}, @option{-r}, or
10326 @option{-t} options, and if there are two or more @var{file}s and the
10327 first @var{file} is of the form @samp{@var{mmddhhmm}[@var{yy}]} and this
10328 would be a valid argument to the @option{-t} option (if the @var{yy}, if
10329 any, were moved to the front), and if the represented year
10330 is in the range 1969--1999, that argument is interpreted as the time
10331 for the other files instead of as a file name.
10332 This obsolete behavior can be enabled or disabled with the
10333 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
10334 conformance}), but portable scripts should avoid commands whose
10335 behavior depends on this variable.
10336 For example, use @samp{touch ./12312359 main.c} or @samp{touch -t
10337 12312359 main.c} rather than the ambiguous @samp{touch 12312359 main.c}.
10343 @chapter Disk usage
10347 No disk can hold an infinite amount of data. These commands report
10348 how much disk storage is in use or available, report other file and
10349 file status information, and write buffers to disk.
10352 * df invocation:: Report file system disk space usage.
10353 * du invocation:: Estimate file space usage.
10354 * stat invocation:: Report file or file system status.
10355 * sync invocation:: Synchronize memory and disk.
10356 * truncate invocation:: Shrink or extend the size of a file.
10360 @node df invocation
10361 @section @command{df}: Report file system disk space usage
10364 @cindex file system disk usage
10365 @cindex disk usage by file system
10367 @command{df} reports the amount of disk space used and available on
10368 file systems. Synopsis:
10371 df [@var{option}]@dots{} [@var{file}]@dots{}
10374 With no arguments, @command{df} reports the space used and available on all
10375 currently mounted file systems (of all types). Otherwise, @command{df}
10376 reports on the file system containing each argument @var{file}.
10378 Normally the disk space is printed in units of
10379 1024 bytes, but this can be overridden (@pxref{Block size}).
10380 Non-integer quantities are rounded up to the next higher unit.
10382 @cindex disk device file
10383 @cindex device file, disk
10384 If an argument @var{file} is a disk device file containing a mounted
10385 file system, @command{df} shows the space available on that file system
10386 rather than on the file system containing the device node (i.e., the root
10387 file system). @sc{gnu} @command{df} does not attempt to determine the
10389 on unmounted file systems, because on most kinds of systems doing so
10390 requires extremely nonportable intimate knowledge of file system
10393 The program accepts the following options. Also see @ref{Common options}.
10401 @cindex automounter file systems
10402 @cindex ignore file systems
10403 Include in the listing dummy file systems, which
10404 are omitted by default. Such file systems are typically special-purpose
10405 pseudo-file-systems, such as automounter entries.
10407 @item -B @var{size}
10408 @itemx --block-size=@var{size}
10410 @opindex --block-size
10411 @cindex file system sizes
10412 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10413 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10417 @cindex grand total of disk size, usage and available space
10418 Print a grand total of all arguments after all arguments have
10419 been processed. This can be used to find out the total disk size, usage
10420 and available space of all listed devices.
10426 Equivalent to @option{--si}.
10432 @cindex inode usage
10433 List inode usage information instead of block usage. An inode (short
10434 for index node) contains information about a file such as its owner,
10435 permissions, timestamps, and location on the disk.
10439 @cindex kibibytes for file system sizes
10440 Print sizes in 1024-byte blocks, overriding the default block size
10441 (@pxref{Block size}).
10442 This option is equivalent to @option{--block-size=1K}.
10448 @cindex file system types, limiting output to certain
10449 Limit the listing to local file systems. By default, remote file systems
10454 @cindex file system space, retrieving old data more quickly
10455 Do not invoke the @code{sync} system call before getting any usage data.
10456 This may make @command{df} run significantly faster on systems with many
10457 disks, but on some systems (notably SunOS) the results may be slightly
10458 out of date. This is the default.
10461 @itemx --portability
10463 @opindex --portability
10464 @cindex one-line output format
10465 @cindex @acronym{POSIX} output format
10466 @cindex portable output format
10467 @cindex output format, portable
10468 Use the @acronym{POSIX} output format. This is like the default format except
10473 The information about each file system is always printed on exactly
10474 one line; a mount device is never put on a line by itself. This means
10475 that if the mount device name is more than 20 characters long (e.g., for
10476 some network mounts), the columns are misaligned.
10479 The labels in the header output line are changed to conform to @acronym{POSIX}.
10482 The default block size and output format are unaffected by the
10483 @env{DF_BLOCK_SIZE}, @env{BLOCK_SIZE} and @env{BLOCKSIZE} environment
10484 variables. However, the default block size is still affected by
10485 @env{POSIXLY_CORRECT}: it is 512 if @env{POSIXLY_CORRECT} is set, 1024
10486 otherwise. @xref{Block size}.
10493 @cindex file system space, retrieving current data more slowly
10494 Invoke the @code{sync} system call before getting any usage data. On
10495 some systems (notably SunOS), doing this yields more up to date results,
10496 but in general this option makes @command{df} much slower, especially when
10497 there are many or very busy file systems.
10499 @item -t @var{fstype}
10500 @itemx --type=@var{fstype}
10503 @cindex file system types, limiting output to certain
10504 Limit the listing to file systems of type @var{fstype}. Multiple
10505 file system types can be specified by giving multiple @option{-t} options.
10506 By default, nothing is omitted.
10509 @itemx --print-type
10511 @opindex --print-type
10512 @cindex file system types, printing
10513 Print each file system's type. The types printed here are the same ones
10514 you can include or exclude with @option{-t} and @option{-x}. The particular
10515 types printed are whatever is supported by the system. Here are some of
10516 the common names (this list is certainly not exhaustive):
10521 @cindex @acronym{NFS} file system type
10522 An @acronym{NFS} file system, i.e., one mounted over a network from another
10523 machine. This is the one type name which seems to be used uniformly by
10526 @item 4.2@r{, }ufs@r{, }efs@dots{}
10527 @cindex Linux file system types
10528 @cindex local file system types
10529 @opindex 4.2 @r{file system type}
10530 @opindex ufs @r{file system type}
10531 @opindex efs @r{file system type}
10532 A file system on a locally-mounted hard disk. (The system might even
10533 support more than one type here; Linux does.)
10535 @item hsfs@r{, }cdfs
10536 @cindex CD-ROM file system type
10537 @cindex High Sierra file system
10538 @opindex hsfs @r{file system type}
10539 @opindex cdfs @r{file system type}
10540 A file system on a CD-ROM drive. HP-UX uses @samp{cdfs}, most other
10541 systems use @samp{hsfs} (@samp{hs} for ``High Sierra'').
10544 @cindex PC file system
10545 @cindex DOS file system
10546 @cindex MS-DOS file system
10547 @cindex diskette file system
10549 An MS-DOS file system, usually on a diskette.
10553 @item -x @var{fstype}
10554 @itemx --exclude-type=@var{fstype}
10556 @opindex --exclude-type
10557 Limit the listing to file systems not of type @var{fstype}.
10558 Multiple file system types can be eliminated by giving multiple
10559 @option{-x} options. By default, no file system types are omitted.
10562 Ignored; for compatibility with System V versions of @command{df}.
10567 Failure includes the case where no output is generated, so you can
10568 inspect the exit status of a command like @samp{df -t ext3 -t reiserfs
10569 @var{dir}} to test whether @var{dir} is on a file system of type
10570 @samp{ext3} or @samp{reiserfs}.
10573 @node du invocation
10574 @section @command{du}: Estimate file space usage
10577 @cindex file space usage
10578 @cindex disk usage for files
10580 @command{du} reports the amount of disk space used by the specified files
10581 and for each subdirectory (of directory arguments). Synopsis:
10584 du [@var{option}]@dots{} [@var{file}]@dots{}
10587 With no arguments, @command{du} reports the disk space for the current
10588 directory. Normally the disk space is printed in units of
10589 1024 bytes, but this can be overridden (@pxref{Block size}).
10590 Non-integer quantities are rounded up to the next higher unit.
10592 If two or more hard links point to the same file, only one of the hard
10593 links is counted. The @var{file} argument order affects which links
10594 are counted, and changing the argument order may change the numbers
10595 that @command{du} outputs.
10597 The program accepts the following options. Also see @ref{Common options}.
10605 Show counts for all files, not just directories.
10607 @itemx --apparent-size
10608 @opindex --apparent-size
10609 Print apparent sizes, rather than disk usage. The apparent size of a
10610 file is the number of bytes reported by @code{wc -c} on regular files,
10611 or more generally, @code{ls -l --block-size=1} or @code{stat --format=%s}.
10612 For example, a file containing the word @samp{zoo} with no newline would,
10613 of course, have an apparent size of 3. Such a small file may require
10614 anywhere from 0 to 16 KiB or more of disk space, depending on
10615 the type and configuration of the file system on which the file resides.
10616 However, a sparse file created with this command:
10619 dd bs=1 seek=2GiB if=/dev/null of=big
10623 has an apparent size of 2 GiB, yet on most modern
10624 systems, it actually uses almost no disk space.
10630 Equivalent to @code{--apparent-size --block-size=1}.
10632 @item -B @var{size}
10633 @itemx --block-size=@var{size}
10635 @opindex --block-size
10637 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10638 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10644 @cindex grand total of disk space
10645 Print a grand total of all arguments after all arguments have
10646 been processed. This can be used to find out the total disk usage of
10647 a given set of files or directories.
10650 @itemx --dereference-args
10652 @opindex --dereference-args
10653 Dereference symbolic links that are command line arguments.
10654 Does not affect other symbolic links. This is helpful for finding
10655 out the disk usage of directories, such as @file{/usr/tmp}, which
10656 are often symbolic links.
10658 @c --files0-from=FILE
10659 @filesZeroFromOption{du,, with the @option{--total} (@option{-c}) option}
10665 Equivalent to @option{--dereference-args} (@option{-D}).
10669 @cindex kibibytes for file sizes
10670 Print sizes in 1024-byte blocks, overriding the default block size
10671 (@pxref{Block size}).
10672 This option is equivalent to @option{--block-size=1K}.
10675 @itemx --count-links
10677 @opindex --count-links
10678 @cindex hard links, counting in @command{du}
10679 Count the size of all files, even if they have appeared already (as a
10683 @itemx --dereference
10685 @opindex --dereference
10686 @cindex symbolic links, dereferencing in @command{du}
10687 Dereference symbolic links (show the disk space used by the file
10688 or directory that the link points to instead of the space used by
10693 @cindex mebibytes for file sizes
10694 Print sizes in 1,048,576-byte blocks, overriding the default block size
10695 (@pxref{Block size}).
10696 This option is equivalent to @option{--block-size=1M}.
10699 @itemx --no-dereference
10701 @opindex --no-dereference
10702 @cindex symbolic links, dereferencing in @command{du}
10703 For each symbolic links encountered by @command{du},
10704 consider the disk space used by the symbolic link.
10706 @item -d @var{depth}
10707 @item --max-depth=@var{depth}
10708 @opindex -d @var{depth}
10709 @opindex --max-depth=@var{depth}
10710 @cindex limiting output of @command{du}
10711 Show the total for each directory (and file if --all) that is at
10712 most MAX_DEPTH levels down from the root of the hierarchy. The root
10713 is at level 0, so @code{du --max-depth=0} is equivalent to @code{du -s}.
10722 @opindex --summarize
10723 Display only a total for each argument.
10726 @itemx --separate-dirs
10728 @opindex --separate-dirs
10729 Normally, in the output of @command{du} (when not using @option{--summarize}),
10730 the size listed next to a directory name, @var{d}, represents the sum
10731 of sizes of all entries beneath @var{d} as well as the size of @var{d} itself.
10732 With @option{--separate-dirs}, the size reported for a directory name,
10733 @var{d}, is merely the @code{stat.st_size}-derived size of the directory
10738 @cindex last modified dates, displaying in @command{du}
10739 Show time of the most recent modification of any file in the directory,
10740 or any of its subdirectories.
10742 @itemx --time=ctime
10743 @itemx --time=status
10746 @opindex ctime@r{, show the most recent}
10747 @opindex status time@r{, show the most recent}
10748 @opindex use time@r{, show the most recent}
10749 Show the most recent status change time (the @samp{ctime} in the inode) of
10750 any file in the directory, instead of the modification time.
10752 @itemx --time=atime
10753 @itemx --time=access
10755 @opindex atime@r{, show the most recent}
10756 @opindex access time@r{, show the most recent}
10757 Show the most recent access time (the @samp{atime} in the inode) of
10758 any file in the directory, instead of the modification time.
10760 @item --time-style=@var{style}
10761 @opindex --time-style
10763 List timestamps in style @var{style}. This option has an effect only if
10764 the @option{--time} option is also specified. The @var{style} should
10765 be one of the following:
10768 @item +@var{format}
10770 List timestamps using @var{format}, where @var{format} is interpreted
10771 like the format argument of @command{date} (@pxref{date invocation}).
10772 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
10773 @command{du} to list timestamps like @samp{2002-03-30 23:45:56}. As
10774 with @command{date}, @var{format}'s interpretation is affected by the
10775 @env{LC_TIME} locale category.
10778 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
10779 format with nanosecond precision, e.g., @samp{2002-03-30
10780 23:45:56.477817180 -0700}. This style is equivalent to
10781 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
10784 List @acronym{ISO} 8601 date and time in minutes, e.g.,
10785 @samp{2002-03-30 23:45}. These timestamps are shorter than
10786 @samp{full-iso} timestamps, and are usually good enough for everyday
10787 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
10790 List @acronym{ISO} 8601 dates for timestamps, e.g., @samp{2002-03-30}.
10791 This style is equivalent to @samp{+%Y-%m-%d}.
10795 You can specify the default value of the @option{--time-style} option
10796 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
10797 the default style is @samp{long-iso}. For compatibility with @command{ls},
10798 if @env{TIME_STYLE} begins with @samp{+} and contains a newline,
10799 the newline and any later characters are ignored; if @env{TIME_STYLE}
10800 begins with @samp{posix-} the @samp{posix-} is ignored; and if
10801 @env{TIME_STYLE} is @samp{locale} it is ignored.
10804 @itemx --one-file-system
10806 @opindex --one-file-system
10807 @cindex one file system, restricting @command{du} to
10808 Skip directories that are on different file systems from the one that
10809 the argument being processed is on.
10811 @item --exclude=@var{pattern}
10812 @opindex --exclude=@var{pattern}
10813 @cindex excluding files from @command{du}
10814 When recursing, skip subdirectories or files matching @var{pattern}.
10815 For example, @code{du --exclude='*.o'} excludes files whose names
10818 @item -X @var{file}
10819 @itemx --exclude-from=@var{file}
10820 @opindex -X @var{file}
10821 @opindex --exclude-from=@var{file}
10822 @cindex excluding files from @command{du}
10823 Like @option{--exclude}, except take the patterns to exclude from @var{file},
10824 one per line. If @var{file} is @samp{-}, take the patterns from standard
10829 @cindex NFS mounts from BSD to HP-UX
10830 On BSD systems, @command{du} reports sizes that are half the correct
10831 values for files that are NFS-mounted from HP-UX systems. On HP-UX
10832 systems, it reports sizes that are twice the correct values for
10833 files that are NFS-mounted from BSD systems. This is due to a flaw
10834 in HP-UX; it also affects the HP-UX @command{du} program.
10839 @node stat invocation
10840 @section @command{stat}: Report file or file system status
10843 @cindex file status
10844 @cindex file system status
10846 @command{stat} displays information about the specified file(s). Synopsis:
10849 stat [@var{option}]@dots{} [@var{file}]@dots{}
10852 With no option, @command{stat} reports all information about the given files.
10853 But it also can be used to report the information of the file systems the
10854 given files are located on. If the files are links, @command{stat} can
10855 also give information about the files the links point to.
10857 @mayConflictWithShellBuiltIn{stat}
10862 @itemx --dereference
10864 @opindex --dereference
10865 @cindex symbolic links, dereferencing in @command{stat}
10866 Change how @command{stat} treats symbolic links.
10867 With this option, @command{stat} acts on the file referenced
10868 by each symbolic link argument.
10869 Without it, @command{stat} acts on any symbolic link argument directly.
10872 @itemx --file-system
10874 @opindex --file-system
10875 @cindex file systems
10876 Report information about the file systems where the given files are located
10877 instead of information about the files themselves.
10878 This option implies the @option{-L} option.
10881 @itemx --format=@var{format}
10883 @opindex --format=@var{format}
10884 @cindex output format
10885 Use @var{format} rather than the default format.
10886 @var{format} is automatically newline-terminated, so
10887 running a command like the following with two or more @var{file}
10888 operands produces a line of output for each operand:
10890 $ stat --format=%d:%i / /usr
10895 @itemx --printf=@var{format}
10896 @opindex --printf=@var{format}
10897 @cindex output format
10898 Use @var{format} rather than the default format.
10899 Like @option{--format}, but interpret backslash escapes,
10900 and do not output a mandatory trailing newline.
10901 If you want a newline, include @samp{\n} in the @var{format}.
10902 Here's how you would use @option{--printf} to print the device
10903 and inode numbers of @file{/} and @file{/usr}:
10905 $ stat --printf='%d:%i\n' / /usr
10914 @cindex terse output
10915 Print the information in terse form, suitable for parsing by other programs.
10919 The valid @var{format} directives for files with @option{--format} and
10920 @option{--printf} are:
10923 @item %a - Access rights in octal
10924 @item %A - Access rights in human readable form
10925 @item %b - Number of blocks allocated (see @samp{%B})
10926 @item %B - The size in bytes of each block reported by @samp{%b}
10927 @item %C - The SELinux security context of a file, if available
10928 @item %d - Device number in decimal
10929 @item %D - Device number in hex
10930 @item %f - Raw mode in hex
10931 @item %F - File type
10932 @item %g - Group ID of owner
10933 @item %G - Group name of owner
10934 @item %h - Number of hard links
10935 @item %i - Inode number
10936 @item %m - Mount point (See note below)
10937 @item %n - File name
10938 @item %N - Quoted file name with dereference if symbolic link
10939 @item %o - I/O block size
10940 @item %s - Total size, in bytes
10941 @item %t - Major device type in hex
10942 @item %T - Minor device type in hex
10943 @item %u - User ID of owner
10944 @item %U - User name of owner
10945 @item %w - Time of file birth, or @samp{-} if unknown
10946 @item %W - Time of file birth as seconds since Epoch, or @samp{0}
10947 @item %x - Time of last access
10948 @item %X - Time of last access as seconds since Epoch
10949 @item %y - Time of last modification
10950 @item %Y - Time of last modification as seconds since Epoch
10951 @item %z - Time of last change
10952 @item %Z - Time of last change as seconds since Epoch
10955 The @samp{%W}, @samp{%X}, @samp{%Y}, and @samp{%Z} formats accept a
10956 precision preceded by a period to specify the number of digits to
10957 print after the decimal point. For example, @samp{%.3X} outputs the
10958 last access time to millisecond precision. If a period is given but no
10959 precision, @command{stat} uses 9 digits, so @samp{%.X} is equivalent to
10960 @samp{%.9X}. When discarding excess precision, time stamps are truncated
10961 toward minus infinity.
10965 $ stat -c '[%015Y]' /usr
10968 $ stat -c '[%15Y]' /usr
10970 $ stat -c '[%-15Y]' /usr
10973 $ stat -c '[%.3Y]' /usr
10975 $ stat -c '[%.Y]' /usr
10976 [1288929712.114951834]
10979 The mount point printed by @samp{%m} is similar to that output
10980 by @command{df}, except that:
10983 stat does not dereference symlinks by default
10984 (unless @option{-L} is specified)
10986 stat does not search for specified device nodes in the
10987 file system list, instead operating on them directly
10990 stat outputs the alias for a bind mounted file, rather than
10991 the initial mount point of its backing device.
10992 One can recursively call stat until there is no change in output,
10993 to get the current base mount point
10996 When listing file system information (@option{--file-system} (@option{-f})),
10997 you must use a different set of @var{format} directives:
11000 @item %a - Free blocks available to non-super-user
11001 @item %b - Total data blocks in file system
11002 @item %c - Total file nodes in file system
11003 @item %d - Free file nodes in file system
11004 @item %f - Free blocks in file system
11005 @item %i - File System ID in hex
11006 @item %l - Maximum length of file names
11007 @item %n - File name
11008 @item %s - Block size (for faster transfers)
11009 @item %S - Fundamental block size (for block counts)
11010 @item %t - Type in hex
11011 @item %T - Type in human readable form
11015 Time stamps are listed according to the time zone rules specified by
11016 the @env{TZ} environment variable, or by the system default rules if
11017 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
11018 with @env{TZ}, libc, The GNU C Library Reference Manual}.
11023 @node sync invocation
11024 @section @command{sync}: Synchronize data on disk with memory
11027 @cindex synchronize disk and memory
11029 @cindex superblock, writing
11030 @cindex inodes, written buffered
11031 @command{sync} writes any data buffered in memory out to disk. This can
11032 include (but is not limited to) modified superblocks, modified inodes,
11033 and delayed reads and writes. This must be implemented by the kernel;
11034 The @command{sync} program does nothing but exercise the @code{sync} system
11037 @cindex crashes and corruption
11038 The kernel keeps data in memory to avoid doing (relatively slow) disk
11039 reads and writes. This improves performance, but if the computer
11040 crashes, data may be lost or the file system corrupted as a
11041 result. The @command{sync} command ensures everything in memory
11042 is written to disk.
11044 Any arguments are ignored, except for a lone @option{--help} or
11045 @option{--version} (@pxref{Common options}).
11050 @node truncate invocation
11051 @section @command{truncate}: Shrink or extend the size of a file
11054 @cindex truncating, file sizes
11056 @command{truncate} shrinks or extends the size of each @var{file} to the
11057 specified size. Synopsis:
11060 truncate @var{option}@dots{} @var{file}@dots{}
11063 @cindex files, creating
11064 Any @var{file} that does not exist is created.
11066 @cindex sparse files, creating
11067 @cindex holes, creating files with
11068 If a @var{file} is larger than the specified size, the extra data is lost.
11069 If a @var{file} is shorter, it is extended and the extended part (or hole)
11070 reads as zero bytes.
11072 The program accepts the following options. Also see @ref{Common options}.
11079 @opindex --no-create
11080 Do not create files that do not exist.
11085 @opindex --io-blocks
11086 Treat @var{size} as number of I/O blocks of the @var{file} rather than bytes.
11088 @item -r @var{rfile}
11089 @itemx --reference=@var{rfile}
11091 @opindex --reference
11092 Base the size of each @var{file} on the size of @var{rfile}.
11094 @item -s @var{size}
11095 @itemx --size=@var{size}
11098 Set or adjust the size of each @var{file} according to @var{size}.
11099 @multiplierSuffixesNoBlocks{size}
11101 @var{size} may also be prefixed by one of the following to adjust
11102 the size of each @var{file} based on their current size:
11104 @samp{+} => extend by
11105 @samp{-} => reduce by
11106 @samp{<} => at most
11107 @samp{>} => at least
11108 @samp{/} => round down to multiple of
11109 @samp{%} => round up to multiple of
11117 @node Printing text
11118 @chapter Printing text
11120 @cindex printing text, commands for
11121 @cindex commands for printing text
11123 This section describes commands that display text strings.
11126 * echo invocation:: Print a line of text.
11127 * printf invocation:: Format and print data.
11128 * yes invocation:: Print a string until interrupted.
11132 @node echo invocation
11133 @section @command{echo}: Print a line of text
11136 @cindex displaying text
11137 @cindex printing text
11138 @cindex text, displaying
11139 @cindex arbitrary text, displaying
11141 @command{echo} writes each given @var{string} to standard output, with a
11142 space between each and a newline after the last one. Synopsis:
11145 echo [@var{option}]@dots{} [@var{string}]@dots{}
11148 @mayConflictWithShellBuiltIn{echo}
11150 The program accepts the following options. Also see @ref{Common options}.
11151 Options must precede operands, and the normally-special argument
11152 @samp{--} has no special meaning and is treated like any other
11158 Do not output the trailing newline.
11162 @cindex backslash escapes
11163 Enable interpretation of the following backslash-escaped characters in
11172 produce no further output
11188 the eight-bit value that is the octal number @var{nnn}
11189 (zero to three octal digits), if @var{nnn} is
11190 a nine-bit value, the ninth bit is ignored
11192 the eight-bit value that is the octal number @var{nnn}
11193 (one to three octal digits), if @var{nnn} is
11194 a nine-bit value, the ninth bit is ignored
11196 the eight-bit value that is the hexadecimal number @var{hh}
11197 (one or two hexadecimal digits)
11202 @cindex backslash escapes
11203 Disable interpretation of backslash escapes in each @var{string}.
11204 This is the default. If @option{-e} and @option{-E} are both
11205 specified, the last one given takes effect.
11209 @vindex POSIXLY_CORRECT
11210 If the @env{POSIXLY_CORRECT} environment variable is set, then when
11211 @command{echo}'s first argument is not @option{-n} it outputs
11212 option-like arguments instead of treating them as options. For
11213 example, @code{echo -ne hello} outputs @samp{-ne hello} instead of
11214 plain @samp{hello}.
11216 @acronym{POSIX} does not require support for any options, and says
11217 that the behavior of @command{echo} is implementation-defined if any
11218 @var{string} contains a backslash or if the first argument is
11219 @option{-n}. Portable programs can use the @command{printf} command
11220 if they need to omit trailing newlines or output control characters or
11221 backslashes. @xref{printf invocation}.
11226 @node printf invocation
11227 @section @command{printf}: Format and print data
11230 @command{printf} does formatted printing of text. Synopsis:
11233 printf @var{format} [@var{argument}]@dots{}
11236 @command{printf} prints the @var{format} string, interpreting @samp{%}
11237 directives and @samp{\} escapes to format numeric and string arguments
11238 in a way that is mostly similar to the C @samp{printf} function.
11239 @xref{Output Conversion Syntax,, @command{printf} format directives,
11240 libc, The GNU C Library Reference Manual}, for details.
11241 The differences are listed below.
11243 @mayConflictWithShellBuiltIn{printf}
11248 The @var{format} argument is reused as necessary to convert all the
11249 given @var{argument}s. For example, the command @samp{printf %s a b}
11253 Missing @var{argument}s are treated as null strings or as zeros,
11254 depending on whether the context expects a string or a number. For
11255 example, the command @samp{printf %sx%d} prints @samp{x0}.
11259 An additional escape, @samp{\c}, causes @command{printf} to produce no
11260 further output. For example, the command @samp{printf 'A%sC\cD%sF' B
11261 E} prints @samp{ABC}.
11264 The hexadecimal escape sequence @samp{\x@var{hh}} has at most two
11265 digits, as opposed to C where it can have an unlimited number of
11266 digits. For example, the command @samp{printf '\x07e'} prints two
11267 bytes, whereas the C statement @samp{printf ("\x07e")} prints just
11272 @command{printf} has an additional directive, @samp{%b}, which prints its
11273 argument string with @samp{\} escapes interpreted in the same way as in
11274 the @var{format} string, except that octal escapes are of the form
11275 @samp{\0@var{ooo}} where @var{ooo} is 0 to 3 octal digits. If
11276 @samp{\@var{ooo}} is nine-bit value, ignore the ninth bit.
11277 If a precision is also given, it limits the number of bytes printed
11278 from the converted string.
11281 Numeric arguments must be single C constants, possibly with leading
11282 @samp{+} or @samp{-}. For example, @samp{printf %.4d -3} outputs
11286 @vindex POSIXLY_CORRECT
11287 If the leading character of a numeric argument is @samp{"} or @samp{'}
11288 then its value is the numeric value of the immediately following
11289 character. Any remaining characters are silently ignored if the
11290 @env{POSIXLY_CORRECT} environment variable is set; otherwise, a
11291 warning is printed. For example, @samp{printf "%d" "'a"} outputs
11292 @samp{97} on hosts that use the @acronym{ASCII} character set, since
11293 @samp{a} has the numeric value 97 in @acronym{ASCII}.
11298 A floating-point argument must use a period before any fractional
11299 digits, but is printed according to the @env{LC_NUMERIC} category of the
11300 current locale. For example, in a locale whose radix character is a
11301 comma, the command @samp{printf %g 3.14} outputs @samp{3,14} whereas
11302 the command @samp{printf %g 3,14} is an error.
11303 @xref{Floating point}.
11307 @command{printf} interprets @samp{\@var{ooo}} in @var{format} as an octal number
11308 (if @var{ooo} is 1 to 3 octal digits) specifying a byte to print,
11309 and @samp{\x@var{hh}} as a hexadecimal number (if @var{hh} is 1 to 2 hex
11310 digits) specifying a character to print.
11311 Note however that when @samp{\@var{ooo}} specifies a number larger than 255,
11312 @command{printf} ignores the ninth bit.
11313 For example, @samp{printf '\400'} is equivalent to @samp{printf '\0'}.
11318 @cindex ISO/IEC 10646
11320 @command{printf} interprets two character syntaxes introduced in
11321 @acronym{ISO} C 99:
11322 @samp{\u} for 16-bit Unicode (@acronym{ISO}/@acronym{IEC} 10646)
11323 characters, specified as
11324 four hexadecimal digits @var{hhhh}, and @samp{\U} for 32-bit Unicode
11325 characters, specified as eight hexadecimal digits @var{hhhhhhhh}.
11326 @command{printf} outputs the Unicode characters
11327 according to the @env{LC_CTYPE} locale. Unicode characters in the ranges
11328 U+0000...U+009F, U+D800...U+DFFF cannot be specified by this syntax, except
11329 for U+0024 ($), U+0040 (@@), and U+0060 (@`).
11331 The processing of @samp{\u} and @samp{\U} requires a full-featured
11332 @code{iconv} facility. It is activated on systems with glibc 2.2 (or newer),
11333 or when @code{libiconv} is installed prior to this package. Otherwise
11334 @samp{\u} and @samp{\U} will print as-is.
11336 The only options are a lone @option{--help} or
11337 @option{--version}. @xref{Common options}.
11338 Options must precede operands.
11340 The Unicode character syntaxes are useful for writing strings in a locale
11341 independent way. For example, a string containing the Euro currency symbol
11344 $ env printf '\u20AC 14.95'
11348 will be output correctly in all locales supporting the Euro symbol
11349 (@acronym{ISO}-8859-15, UTF-8, and others). Similarly, a Chinese string
11352 $ env printf '\u4e2d\u6587'
11356 will be output correctly in all Chinese locales (GB2312, BIG5, UTF-8, etc).
11358 Note that in these examples, the @command{printf} command has been
11359 invoked via @command{env} to ensure that we run the program found via
11360 your shell's search path, and not a shell alias or a built-in function.
11362 For larger strings, you don't need to look up the hexadecimal code
11363 values of each character one by one. @acronym{ASCII} characters mixed with \u
11364 escape sequences is also known as the JAVA source file encoding. You can
11365 use GNU recode 3.5c (or newer) to convert strings to this encoding. Here
11366 is how to convert a piece of text into a shell script which will output
11367 this text in a locale-independent way:
11370 $ LC_CTYPE=zh_CN.big5 /usr/local/bin/printf \
11371 '\u4e2d\u6587\n' > sample.txt
11372 $ recode BIG5..JAVA < sample.txt \
11373 | sed -e "s|^|/usr/local/bin/printf '|" -e "s|$|\\\\n'|" \
11380 @node yes invocation
11381 @section @command{yes}: Print a string until interrupted
11384 @cindex repeated output of a string
11386 @command{yes} prints the command line arguments, separated by spaces and
11387 followed by a newline, forever until it is killed. If no arguments are
11388 given, it prints @samp{y} followed by a newline forever until killed.
11390 Upon a write error, @command{yes} exits with status @samp{1}.
11392 The only options are a lone @option{--help} or @option{--version}.
11393 To output an argument that begins with
11394 @samp{-}, precede it with @option{--}, e.g., @samp{yes -- --help}.
11395 @xref{Common options}.
11399 @chapter Conditions
11402 @cindex commands for exit status
11403 @cindex exit status commands
11405 This section describes commands that are primarily useful for their exit
11406 status, rather than their output. Thus, they are often used as the
11407 condition of shell @code{if} statements, or as the last command in a
11411 * false invocation:: Do nothing, unsuccessfully.
11412 * true invocation:: Do nothing, successfully.
11413 * test invocation:: Check file types and compare values.
11414 * expr invocation:: Evaluate expressions.
11418 @node false invocation
11419 @section @command{false}: Do nothing, unsuccessfully
11422 @cindex do nothing, unsuccessfully
11423 @cindex failure exit status
11424 @cindex exit status of @command{false}
11426 @command{false} does nothing except return an exit status of 1, meaning
11427 @dfn{failure}. It can be used as a place holder in shell scripts
11428 where an unsuccessful command is needed.
11429 In most modern shells, @command{false} is a built-in command, so when
11430 you use @samp{false} in a script, you're probably using the built-in
11431 command, not the one documented here.
11433 @command{false} honors the @option{--help} and @option{--version} options.
11435 This version of @command{false} is implemented as a C program, and is thus
11436 more secure and faster than a shell script implementation, and may safely
11437 be used as a dummy shell for the purpose of disabling accounts.
11439 Note that @command{false} (unlike all other programs documented herein)
11440 exits unsuccessfully, even when invoked with
11441 @option{--help} or @option{--version}.
11443 Portable programs should not assume that the exit status of
11444 @command{false} is 1, as it is greater than 1 on some
11445 non-@acronym{GNU} hosts.
11448 @node true invocation
11449 @section @command{true}: Do nothing, successfully
11452 @cindex do nothing, successfully
11454 @cindex successful exit
11455 @cindex exit status of @command{true}
11457 @command{true} does nothing except return an exit status of 0, meaning
11458 @dfn{success}. It can be used as a place holder in shell scripts
11459 where a successful command is needed, although the shell built-in
11460 command @code{:} (colon) may do the same thing faster.
11461 In most modern shells, @command{true} is a built-in command, so when
11462 you use @samp{true} in a script, you're probably using the built-in
11463 command, not the one documented here.
11465 @command{true} honors the @option{--help} and @option{--version} options.
11467 Note, however, that it is possible to cause @command{true}
11468 to exit with nonzero status: with the @option{--help} or @option{--version}
11469 option, and with standard
11470 output already closed or redirected to a file that evokes an I/O error.
11471 For example, using a Bourne-compatible shell:
11474 $ ./true --version >&-
11475 ./true: write error: Bad file number
11476 $ ./true --version > /dev/full
11477 ./true: write error: No space left on device
11480 This version of @command{true} is implemented as a C program, and is thus
11481 more secure and faster than a shell script implementation, and may safely
11482 be used as a dummy shell for the purpose of disabling accounts.
11484 @node test invocation
11485 @section @command{test}: Check file types and compare values
11488 @cindex check file types
11489 @cindex compare values
11490 @cindex expression evaluation
11492 @command{test} returns a status of 0 (true) or 1 (false) depending on the
11493 evaluation of the conditional expression @var{expr}. Each part of the
11494 expression must be a separate argument.
11496 @command{test} has file status checks, string operators, and numeric
11497 comparison operators.
11499 @command{test} has an alternate form that uses opening and closing
11500 square brackets instead a leading @samp{test}. For example, instead
11501 of @samp{test -d /}, you can write @samp{[ -d / ]}. The square
11502 brackets must be separate arguments; for example, @samp{[-d /]} does
11503 not have the desired effect. Since @samp{test @var{expr}} and @samp{[
11504 @var{expr} ]} have the same meaning, only the former form is discussed
11510 test @var{expression}
11512 [ @var{expression} ]
11517 @mayConflictWithShellBuiltIn{test}
11519 If @var{expression} is omitted, @command{test} returns false.
11520 If @var{expression} is a single argument,
11521 @command{test} returns false if the argument is null and true
11522 otherwise. The argument
11523 can be any string, including strings like @samp{-d}, @samp{-1},
11524 @samp{--}, @samp{--help}, and @samp{--version} that most other
11525 programs would treat as options. To get help and version information,
11526 invoke the commands @samp{[ --help} and @samp{[ --version}, without
11527 the usual closing brackets. @xref{Common options}.
11529 @cindex exit status of @command{test}
11533 0 if the expression is true,
11534 1 if the expression is false,
11535 2 if an error occurred.
11539 * File type tests:: -[bcdfhLpSt]
11540 * Access permission tests:: -[gkruwxOG]
11541 * File characteristic tests:: -e -s -nt -ot -ef
11542 * String tests:: -z -n = == !=
11543 * Numeric tests:: -eq -ne -lt -le -gt -ge
11544 * Connectives for test:: ! -a -o
11548 @node File type tests
11549 @subsection File type tests
11551 @cindex file type tests
11553 These options test for particular types of files. (Everything's a file,
11554 but not all files are the same!)
11558 @item -b @var{file}
11560 @cindex block special check
11561 True if @var{file} exists and is a block special device.
11563 @item -c @var{file}
11565 @cindex character special check
11566 True if @var{file} exists and is a character special device.
11568 @item -d @var{file}
11570 @cindex directory check
11571 True if @var{file} exists and is a directory.
11573 @item -f @var{file}
11575 @cindex regular file check
11576 True if @var{file} exists and is a regular file.
11578 @item -h @var{file}
11579 @itemx -L @var{file}
11582 @cindex symbolic link check
11583 True if @var{file} exists and is a symbolic link.
11584 Unlike all other file-related tests, this test does not dereference
11585 @var{file} if it is a symbolic link.
11587 @item -p @var{file}
11589 @cindex named pipe check
11590 True if @var{file} exists and is a named pipe.
11592 @item -S @var{file}
11594 @cindex socket check
11595 True if @var{file} exists and is a socket.
11599 @cindex terminal check
11600 True if @var{fd} is a file descriptor that is associated with a
11606 @node Access permission tests
11607 @subsection Access permission tests
11609 @cindex access permission tests
11610 @cindex permission tests
11612 These options test for particular access permissions.
11616 @item -g @var{file}
11618 @cindex set-group-ID check
11619 True if @var{file} exists and has its set-group-ID bit set.
11621 @item -k @var{file}
11623 @cindex sticky bit check
11624 True if @var{file} exists and has its @dfn{sticky} bit set.
11626 @item -r @var{file}
11628 @cindex readable file check
11629 True if @var{file} exists and read permission is granted.
11631 @item -u @var{file}
11633 @cindex set-user-ID check
11634 True if @var{file} exists and has its set-user-ID bit set.
11636 @item -w @var{file}
11638 @cindex writable file check
11639 True if @var{file} exists and write permission is granted.
11641 @item -x @var{file}
11643 @cindex executable file check
11644 True if @var{file} exists and execute permission is granted
11645 (or search permission, if it is a directory).
11647 @item -O @var{file}
11649 @cindex owned by effective user ID check
11650 True if @var{file} exists and is owned by the current effective user ID.
11652 @item -G @var{file}
11654 @cindex owned by effective group ID check
11655 True if @var{file} exists and is owned by the current effective group ID.
11659 @node File characteristic tests
11660 @subsection File characteristic tests
11662 @cindex file characteristic tests
11664 These options test other file characteristics.
11668 @item -e @var{file}
11670 @cindex existence-of-file check
11671 True if @var{file} exists.
11673 @item -s @var{file}
11675 @cindex nonempty file check
11676 True if @var{file} exists and has a size greater than zero.
11678 @item @var{file1} -nt @var{file2}
11680 @cindex newer-than file check
11681 True if @var{file1} is newer (according to modification date) than
11682 @var{file2}, or if @var{file1} exists and @var{file2} does not.
11684 @item @var{file1} -ot @var{file2}
11686 @cindex older-than file check
11687 True if @var{file1} is older (according to modification date) than
11688 @var{file2}, or if @var{file2} exists and @var{file1} does not.
11690 @item @var{file1} -ef @var{file2}
11692 @cindex same file check
11693 @cindex hard link check
11694 True if @var{file1} and @var{file2} have the same device and inode
11695 numbers, i.e., if they are hard links to each other.
11701 @subsection String tests
11703 @cindex string tests
11705 These options test string characteristics. You may need to quote
11706 @var{string} arguments for the shell. For example:
11712 The quotes here prevent the wrong arguments from being passed to
11713 @command{test} if @samp{$V} is empty or contains special characters.
11717 @item -z @var{string}
11719 @cindex zero-length string check
11720 True if the length of @var{string} is zero.
11722 @item -n @var{string}
11723 @itemx @var{string}
11725 @cindex nonzero-length string check
11726 True if the length of @var{string} is nonzero.
11728 @item @var{string1} = @var{string2}
11730 @cindex equal string check
11731 True if the strings are equal.
11733 @item @var{string1} == @var{string2}
11735 @cindex equal string check
11736 True if the strings are equal (synonym for =).
11738 @item @var{string1} != @var{string2}
11740 @cindex not-equal string check
11741 True if the strings are not equal.
11746 @node Numeric tests
11747 @subsection Numeric tests
11749 @cindex numeric tests
11750 @cindex arithmetic tests
11752 Numeric relational operators. The arguments must be entirely numeric
11753 (possibly negative), or the special expression @w{@code{-l @var{string}}},
11754 which evaluates to the length of @var{string}.
11758 @item @var{arg1} -eq @var{arg2}
11759 @itemx @var{arg1} -ne @var{arg2}
11760 @itemx @var{arg1} -lt @var{arg2}
11761 @itemx @var{arg1} -le @var{arg2}
11762 @itemx @var{arg1} -gt @var{arg2}
11763 @itemx @var{arg1} -ge @var{arg2}
11770 These arithmetic binary operators return true if @var{arg1} is equal,
11771 not-equal, less-than, less-than-or-equal, greater-than, or
11772 greater-than-or-equal than @var{arg2}, respectively.
11779 test -1 -gt -2 && echo yes
11781 test -l abc -gt 1 && echo yes
11784 @error{} test: integer expression expected before -eq
11788 @node Connectives for test
11789 @subsection Connectives for @command{test}
11791 @cindex logical connectives
11792 @cindex connectives, logical
11794 The usual logical connectives.
11800 True if @var{expr} is false.
11802 @item @var{expr1} -a @var{expr2}
11804 @cindex logical and operator
11805 @cindex and operator
11806 True if both @var{expr1} and @var{expr2} are true.
11808 @item @var{expr1} -o @var{expr2}
11810 @cindex logical or operator
11811 @cindex or operator
11812 True if either @var{expr1} or @var{expr2} is true.
11817 @node expr invocation
11818 @section @command{expr}: Evaluate expressions
11821 @cindex expression evaluation
11822 @cindex evaluation of expressions
11824 @command{expr} evaluates an expression and writes the result on standard
11825 output. Each token of the expression must be a separate argument.
11827 Operands are either integers or strings. Integers consist of one or
11828 more decimal digits, with an optional leading @samp{-}.
11829 @command{expr} converts
11830 anything appearing in an operand position to an integer or a string
11831 depending on the operation being applied to it.
11833 Strings are not quoted for @command{expr} itself, though you may need to
11834 quote them to protect characters with special meaning to the shell,
11835 e.g., spaces. However, regardless of whether it is quoted, a string
11836 operand should not be a parenthesis or any of @command{expr}'s
11837 operators like @code{+}, so you cannot safely pass an arbitrary string
11838 @code{$str} to expr merely by quoting it to the shell. One way to
11839 work around this is to use the @sc{gnu} extension @code{+},
11840 (e.g., @code{+ "$str" = foo}); a more portable way is to use
11841 @code{@w{" $str"}} and to adjust the rest of the expression to take
11842 the leading space into account (e.g., @code{@w{" $str" = " foo"}}).
11844 You should not pass a negative integer or a string with leading
11845 @samp{-} as @command{expr}'s first argument, as it might be
11846 misinterpreted as an option; this can be avoided by parenthesization.
11847 Also, portable scripts should not use a string operand that happens to
11848 take the form of an integer; this can be worked around by inserting
11849 leading spaces as mentioned above.
11851 @cindex parentheses for grouping
11852 Operators may be given as infix symbols or prefix keywords. Parentheses
11853 may be used for grouping in the usual manner. You must quote
11854 parentheses and many operators to avoid the shell evaluating them,
11857 When built with support for the GNU MP library, @command{expr} uses
11858 arbitrary-precision arithmetic; otherwise, it uses native arithmetic
11859 types and may fail due to arithmetic overflow.
11861 The only options are @option{--help} and @option{--version}. @xref{Common
11862 options}. Options must precede operands.
11864 @cindex exit status of @command{expr}
11868 0 if the expression is neither null nor 0,
11869 1 if the expression is null or 0,
11870 2 if the expression is invalid,
11871 3 if an internal error occurred (e.g., arithmetic overflow).
11875 * String expressions:: + : match substr index length
11876 * Numeric expressions:: + - * / %
11877 * Relations for expr:: | & < <= = == != >= >
11878 * Examples of expr:: Examples.
11882 @node String expressions
11883 @subsection String expressions
11885 @cindex string expressions
11886 @cindex expressions, string
11888 @command{expr} supports pattern matching and other string operators. These
11889 have higher precedence than both the numeric and relational operators (in
11890 the next sections).
11894 @item @var{string} : @var{regex}
11895 @cindex pattern matching
11896 @cindex regular expression matching
11897 @cindex matching patterns
11898 Perform pattern matching. The arguments are converted to strings and the
11899 second is considered to be a (basic, a la GNU @code{grep}) regular
11900 expression, with a @code{^} implicitly prepended. The first argument is
11901 then matched against this regular expression.
11903 If the match succeeds and @var{regex} uses @samp{\(} and @samp{\)}, the
11904 @code{:} expression returns the part of @var{string} that matched the
11905 subexpression; otherwise, it returns the number of characters matched.
11907 If the match fails, the @code{:} operator returns the null string if
11908 @samp{\(} and @samp{\)} are used in @var{regex}, otherwise 0.
11910 @kindex \( @r{regexp operator}
11911 Only the first @samp{\( @dots{} \)} pair is relevant to the return
11912 value; additional pairs are meaningful only for grouping the regular
11913 expression operators.
11915 @kindex \+ @r{regexp operator}
11916 @kindex \? @r{regexp operator}
11917 @kindex \| @r{regexp operator}
11918 In the regular expression, @code{\+}, @code{\?}, and @code{\|} are
11919 operators which respectively match one or more, zero or one, or separate
11920 alternatives. SunOS and other @command{expr}'s treat these as regular
11921 characters. (@acronym{POSIX} allows either behavior.)
11922 @xref{Top, , Regular Expression Library, regex, Regex}, for details of
11923 regular expression syntax. Some examples are in @ref{Examples of expr}.
11925 @item match @var{string} @var{regex}
11927 An alternative way to do pattern matching. This is the same as
11928 @w{@samp{@var{string} : @var{regex}}}.
11930 @item substr @var{string} @var{position} @var{length}
11932 Returns the substring of @var{string} beginning at @var{position}
11933 with length at most @var{length}. If either @var{position} or
11934 @var{length} is negative, zero, or non-numeric, returns the null string.
11936 @item index @var{string} @var{charset}
11938 Returns the first position in @var{string} where the first character in
11939 @var{charset} was found. If no character in @var{charset} is found in
11940 @var{string}, return 0.
11942 @item length @var{string}
11944 Returns the length of @var{string}.
11946 @item + @var{token}
11948 Interpret @var{token} as a string, even if it is a keyword like @var{match}
11949 or an operator like @code{/}.
11950 This makes it possible to test @code{expr length + "$x"} or
11951 @code{expr + "$x" : '.*/\(.\)'} and have it do the right thing even if
11952 the value of @var{$x} happens to be (for example) @code{/} or @code{index}.
11953 This operator is a @acronym{GNU} extension. Portable shell scripts should use
11954 @code{@w{" $token"} : @w{' \(.*\)'}} instead of @code{+ "$token"}.
11958 To make @command{expr} interpret keywords as strings, you must use the
11959 @code{quote} operator.
11962 @node Numeric expressions
11963 @subsection Numeric expressions
11965 @cindex numeric expressions
11966 @cindex expressions, numeric
11968 @command{expr} supports the usual numeric operators, in order of increasing
11969 precedence. These numeric operators have lower precedence than the
11970 string operators described in the previous section, and higher precedence
11971 than the connectives (next section).
11979 @cindex subtraction
11980 Addition and subtraction. Both arguments are converted to integers;
11981 an error occurs if this cannot be done.
11987 @cindex multiplication
11990 Multiplication, division, remainder. Both arguments are converted to
11991 integers; an error occurs if this cannot be done.
11996 @node Relations for expr
11997 @subsection Relations for @command{expr}
11999 @cindex connectives, logical
12000 @cindex logical connectives
12001 @cindex relations, numeric or string
12003 @command{expr} supports the usual logical connectives and relations. These
12004 have lower precedence than the string and numeric operators
12005 (previous sections). Here is the list, lowest-precedence operator first.
12011 @cindex logical or operator
12012 @cindex or operator
12013 Returns its first argument if that is neither null nor zero, otherwise
12014 its second argument if it is neither null nor zero, otherwise 0. It
12015 does not evaluate its second argument if its first argument is neither
12020 @cindex logical and operator
12021 @cindex and operator
12022 Return its first argument if neither argument is null or zero, otherwise
12023 0. It does not evaluate its second argument if its first argument is
12026 @item < <= = == != >= >
12033 @cindex comparison operators
12035 Compare the arguments and return 1 if the relation is true, 0 otherwise.
12036 @code{==} is a synonym for @code{=}. @command{expr} first tries to convert
12037 both arguments to integers and do a numeric comparison; if either
12038 conversion fails, it does a lexicographic comparison using the character
12039 collating sequence specified by the @env{LC_COLLATE} locale.
12044 @node Examples of expr
12045 @subsection Examples of using @command{expr}
12047 @cindex examples of @command{expr}
12048 Here are a few examples, including quoting for shell metacharacters.
12050 To add 1 to the shell variable @code{foo}, in Bourne-compatible shells:
12053 foo=`expr $foo + 1`
12056 To print the non-directory part of the file name stored in
12057 @code{$fname}, which need not contain a @code{/}:
12060 expr $fname : '.*/\(.*\)' '|' $fname
12063 An example showing that @code{\+} is an operator:
12071 expr abc : 'a\(.\)c'
12073 expr index abcdef cz
12076 @error{} expr: syntax error
12077 expr index + index a
12083 @chapter Redirection
12085 @cindex redirection
12086 @cindex commands for redirection
12088 Unix shells commonly provide several forms of @dfn{redirection}---ways
12089 to change the input source or output destination of a command. But one
12090 useful redirection is performed by a separate command, not by the shell;
12091 it's described here.
12094 * tee invocation:: Redirect output to multiple files or processes.
12098 @node tee invocation
12099 @section @command{tee}: Redirect output to multiple files or processes
12102 @cindex pipe fitting
12103 @cindex destinations, multiple output
12104 @cindex read from stdin and write to stdout and files
12106 The @command{tee} command copies standard input to standard output and also
12107 to any files given as arguments. This is useful when you want not only
12108 to send some data down a pipe, but also to save a copy. Synopsis:
12111 tee [@var{option}]@dots{} [@var{file}]@dots{}
12114 If a file being written to does not already exist, it is created. If a
12115 file being written to already exists, the data it previously contained
12116 is overwritten unless the @option{-a} option is used.
12118 A @var{file} of @samp{-} causes @command{tee} to send another copy of
12119 input to standard output, but this is typically not that useful as the
12120 copies are interleaved.
12122 The program accepts the following options. Also see @ref{Common options}.
12129 Append standard input to the given files rather than overwriting
12133 @itemx --ignore-interrupts
12135 @opindex --ignore-interrupts
12136 Ignore interrupt signals.
12140 The @command{tee} command is useful when you happen to be transferring a large
12141 amount of data and also want to summarize that data without reading
12142 it a second time. For example, when you are downloading a DVD image,
12143 you often want to verify its signature or checksum right away.
12144 The inefficient way to do it is simply:
12147 wget http://example.com/some.iso && sha1sum some.iso
12150 One problem with the above is that it makes you wait for the
12151 download to complete before starting the time-consuming SHA1 computation.
12152 Perhaps even more importantly, the above requires reading
12153 the DVD image a second time (the first was from the network).
12155 The efficient way to do it is to interleave the download
12156 and SHA1 computation. Then, you'll get the checksum for
12157 free, because the entire process parallelizes so well:
12160 # slightly contrived, to demonstrate process substitution
12161 wget -O - http://example.com/dvd.iso \
12162 | tee >(sha1sum > dvd.sha1) > dvd.iso
12165 That makes @command{tee} write not just to the expected output file,
12166 but also to a pipe running @command{sha1sum} and saving the final
12167 checksum in a file named @file{dvd.sha1}.
12169 Note, however, that this example relies on a feature of modern shells
12170 called @dfn{process substitution}
12171 (the @samp{>(command)} syntax, above;
12172 @xref{Process Substitution,,Process Substitution, bashref,
12173 The Bash Reference Manual}.),
12174 so it works with @command{zsh}, @command{bash}, and @command{ksh},
12175 but not with @command{/bin/sh}. So if you write code like this
12176 in a shell script, be sure to start the script with @samp{#!/bin/bash}.
12178 Since the above example writes to one file and one process,
12179 a more conventional and portable use of @command{tee} is even better:
12182 wget -O - http://example.com/dvd.iso \
12183 | tee dvd.iso | sha1sum > dvd.sha1
12186 You can extend this example to make @command{tee} write to two processes,
12187 computing MD5 and SHA1 checksums in parallel. In this case,
12188 process substitution is required:
12191 wget -O - http://example.com/dvd.iso \
12192 | tee >(sha1sum > dvd.sha1) \
12193 >(md5sum > dvd.md5) \
12197 This technique is also useful when you want to make a @emph{compressed}
12198 copy of the contents of a pipe.
12199 Consider a tool to graphically summarize disk usage data from @samp{du -ak}.
12200 For a large hierarchy, @samp{du -ak} can run for a long time,
12201 and can easily produce terabytes of data, so you won't want to
12202 rerun the command unnecessarily. Nor will you want to save
12203 the uncompressed output.
12205 Doing it the inefficient way, you can't even start the GUI
12206 until after you've compressed all of the @command{du} output:
12209 du -ak | gzip -9 > /tmp/du.gz
12210 gzip -d /tmp/du.gz | xdiskusage -a
12213 With @command{tee} and process substitution, you start the GUI
12214 right away and eliminate the decompression completely:
12217 du -ak | tee >(gzip -9 > /tmp/du.gz) | xdiskusage -a
12220 Finally, if you regularly create more than one type of
12221 compressed tarball at once, for example when @code{make dist} creates
12222 both @command{gzip}-compressed and @command{bzip2}-compressed tarballs,
12223 there may be a better way.
12224 Typical @command{automake}-generated @file{Makefile} rules create
12225 the two compressed tar archives with commands in sequence, like this
12226 (slightly simplified):
12229 tardir=your-pkg-M.N
12230 tar chof - "$tardir" | gzip -9 -c > your-pkg-M.N.tar.gz
12231 tar chof - "$tardir" | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12234 However, if the hierarchy you are archiving and compressing is larger
12235 than a couple megabytes, and especially if you are using a multi-processor
12236 system with plenty of memory, then you can do much better by reading the
12237 directory contents only once and running the compression programs in parallel:
12240 tardir=your-pkg-M.N
12241 tar chof - "$tardir" \
12242 | tee >(gzip -9 -c > your-pkg-M.N.tar.gz) \
12243 | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12249 @node File name manipulation
12250 @chapter File name manipulation
12252 @cindex file name manipulation
12253 @cindex manipulation of file names
12254 @cindex commands for file name manipulation
12256 This section describes commands that manipulate file names.
12259 * basename invocation:: Strip directory and suffix from a file name.
12260 * dirname invocation:: Strip last file name component.
12261 * pathchk invocation:: Check file name validity and portability.
12262 * mktemp invocation:: Create temporary file or directory.
12266 @node basename invocation
12267 @section @command{basename}: Strip directory and suffix from a file name
12270 @cindex strip directory and suffix from file names
12271 @cindex directory, stripping from file names
12272 @cindex suffix, stripping from file names
12273 @cindex file names, stripping directory and suffix
12274 @cindex leading directory components, stripping
12276 @command{basename} removes any leading directory components from
12277 @var{name}. Synopsis:
12280 basename @var{name} [@var{suffix}]
12283 If @var{suffix} is specified and is identical to the end of @var{name},
12284 it is removed from @var{name} as well. Note that since trailing slashes
12285 are removed prior to suffix matching, @var{suffix} will do nothing if it
12286 contains slashes. @command{basename} prints the result on standard
12289 @c This test is used both here and in the section on dirname.
12290 @macro basenameAndDirname
12291 Together, @command{basename} and @command{dirname} are designed such
12292 that if @samp{ls "$name"} succeeds, then the command sequence @samp{cd
12293 "$(dirname "$name")"; ls "$(basename "$name")"} will, too. This works
12294 for everything except file names containing a trailing newline.
12296 @basenameAndDirname
12298 @acronym{POSIX} allows the implementation to define the results if
12299 @var{name} is empty or @samp{//}. In the former case, @acronym{GNU}
12300 @command{basename} returns the empty string. In the latter case, the
12301 result is @samp{//} on platforms where @var{//} is distinct from
12302 @var{/}, and @samp{/} on platforms where there is no difference.
12304 The only options are @option{--help} and @option{--version}. @xref{Common
12305 options}. Options must precede operands.
12313 basename /usr/bin/sort
12316 basename include/stdio.h .h
12320 @node dirname invocation
12321 @section @command{dirname}: Strip last file name component
12324 @cindex directory components, printing
12325 @cindex stripping non-directory suffix
12326 @cindex non-directory suffix, stripping
12328 @command{dirname} prints all but the final slash-delimited component of
12329 @var{name}. Slashes on either side of the final component are also
12330 removed. If the string contains no slash, @command{dirname} prints
12331 @samp{.} (meaning the current directory). Synopsis:
12337 @var{name} need not be a file name, but if it is, this operation
12338 effectively lists the directory that contains the final component,
12339 including the case when the final component is itself a directory.
12341 @basenameAndDirname
12343 @acronym{POSIX} allows the implementation to define the results if
12344 @var{name} is @samp{//}. With @acronym{GNU} @command{dirname}, the
12345 result is @samp{//} on platforms where @var{//} is distinct from
12346 @var{/}, and @samp{/} on platforms where there is no difference.
12348 The only options are @option{--help} and @option{--version}. @xref{Common
12356 # Output "/usr/bin".
12357 dirname /usr/bin/sort
12358 dirname /usr/bin//.//
12365 @node pathchk invocation
12366 @section @command{pathchk}: Check file name validity and portability
12369 @cindex file names, checking validity and portability
12370 @cindex valid file names, checking for
12371 @cindex portable file names, checking for
12373 @command{pathchk} checks validity and portability of file names. Synopsis:
12376 pathchk [@var{option}]@dots{} @var{name}@dots{}
12379 For each @var{name}, @command{pathchk} prints an error message if any of
12380 these conditions is true:
12384 One of the existing directories in @var{name} does not have search
12385 (execute) permission,
12387 The length of @var{name} is larger than the maximum supported by the
12390 The length of one component of @var{name} is longer than
12391 its file system's maximum.
12394 A nonexistent @var{name} is not an error, so long a file with that
12395 name could be created under the above conditions.
12397 The program accepts the following options. Also see @ref{Common options}.
12398 Options must precede operands.
12404 Instead of performing checks based on the underlying file system,
12405 print an error message if any of these conditions is true:
12409 A file name is empty.
12412 A file name contains a character outside the @acronym{POSIX} portable file
12413 name character set, namely, the ASCII letters and digits, @samp{.},
12414 @samp{_}, @samp{-}, and @samp{/}.
12417 The length of a file name or one of its components exceeds the
12418 @acronym{POSIX} minimum limits for portability.
12423 Print an error message if a file name is empty, or if it contains a component
12424 that begins with @samp{-}.
12426 @item --portability
12427 @opindex --portability
12428 Print an error message if a file name is not portable to all @acronym{POSIX}
12429 hosts. This option is equivalent to @samp{-p -P}.
12433 @cindex exit status of @command{pathchk}
12437 0 if all specified file names passed all checks,
12441 @node mktemp invocation
12442 @section @command{mktemp}: Create temporary file or directory
12445 @cindex file names, creating temporary
12446 @cindex directory, creating temporary
12447 @cindex temporary files and directories
12449 @command{mktemp} manages the creation of temporary files and
12450 directories. Synopsis:
12453 mktemp [@var{option}]@dots{} [@var{template}]
12456 Safely create a temporary file or directory based on @var{template},
12457 and print its name. If given, @var{template} must include at least
12458 three consecutive @samp{X}s in the last component. If omitted, the template
12459 @samp{tmp.XXXXXXXXXX} is used, and option @option{--tmpdir} is
12460 implied. The final run of @samp{X}s in the @var{template} will be replaced
12461 by alpha-numeric characters; thus, on a case-sensitive file system,
12462 and with a @var{template} including a run of @var{n} instances of @samp{X},
12463 there are @samp{62**@var{n}} potential file names.
12465 Older scripts used to create temporary files by simply joining the
12466 name of the program with the process id (@samp{$$}) as a suffix.
12467 However, that naming scheme is easily predictable, and suffers from a
12468 race condition where the attacker can create an appropriately named
12469 symbolic link, such that when the script then opens a handle to what
12470 it thought was an unused file, it is instead modifying an existing
12471 file. Using the same scheme to create a directory is slightly safer,
12472 since the @command{mkdir} will fail if the target already exists, but
12473 it is still inferior because it allows for denial of service attacks.
12474 Therefore, modern scripts should use the @command{mktemp} command to
12475 guarantee that the generated name will be unpredictable, and that
12476 knowledge of the temporary file name implies that the file was created
12477 by the current script and cannot be modified by other users.
12479 When creating a file, the resulting file has read and write
12480 permissions for the current user, but no permissions for the group or
12481 others; these permissions are reduced if the current umask is more
12484 Here are some examples (although note that if you repeat them, you
12485 will most likely get different file names):
12490 Create a temporary file in the current directory.
12497 Create a temporary file with a known suffix.
12499 $ mktemp --suffix=.txt file-XXXX
12501 $ mktemp file-XXXX-XXXX.txt
12506 Create a secure fifo relative to the user's choice of @env{TMPDIR},
12507 but falling back to the current directory rather than @file{/tmp}.
12508 Note that @command{mktemp} does not create fifos, but can create a
12509 secure directory in which the fifo can live. Exit the shell if the
12510 directory or fifo could not be created.
12512 $ dir=$(mktemp -p "$@{TMPDIR:-.@}" -d dir-XXXX) || exit 1
12514 $ mkfifo "$fifo" || @{ rmdir "$dir"; exit 1; @}
12518 Create and use a temporary file if possible, but ignore failure. The
12519 file will reside in the directory named by @env{TMPDIR}, if specified,
12520 or else in @file{/tmp}.
12522 $ file=$(mktemp -q) && @{
12523 > # Safe to use $file only within this block. Use quotes,
12524 > # since $TMPDIR, and thus $file, may contain whitespace.
12525 > echo ... > "$file"
12531 Act as a semi-random character generator (it is not fully random,
12532 since it is impacted by the contents of the current directory). To
12533 avoid security holes, do not use the resulting names to create a file.
12543 The program accepts the following options. Also see @ref{Common options}.
12550 @opindex --directory
12551 Create a directory rather than a file. The directory will have read,
12552 write, and search permissions for the current user, but no permissions
12553 for the group or others; these permissions are reduced if the current
12554 umask is more restrictive.
12560 Suppress diagnostics about failure to create a file or directory. The
12561 exit status will still reflect whether a file was created.
12567 Generate a temporary name that does not name an existing file, without
12568 changing the file system contents. Using the output of this command
12569 to create a new file is inherently unsafe, as there is a window of
12570 time between generating the name and using it where another process
12571 can create an object by the same name.
12574 @itemx --tmpdir[=@var{dir}]
12577 Treat @var{template} relative to the directory @var{dir}. If
12578 @var{dir} is not specified (only possible with the long option
12579 @option{--tmpdir}) or is the empty string, use the value of
12580 @env{TMPDIR} if available, otherwise use @samp{/tmp}. If this is
12581 specified, @var{template} must not be absolute. However,
12582 @var{template} can still contain slashes, although intermediate
12583 directories must already exist.
12585 @item --suffix=@var{suffix}
12587 Append @var{suffix} to the @var{template}. @var{suffix} must not
12588 contain slash. If @option{--suffix} is specified, @var{template} must
12589 end in @samp{X}; if it is not specified, then an appropriate
12590 @option{--suffix} is inferred by finding the last @samp{X} in
12591 @var{template}. This option exists for use with the default
12592 @var{template} and for the creation of a @var{suffix} that starts with
12597 Treat @var{template} as a single file relative to the value of
12598 @env{TMPDIR} if available, or to the directory specified by
12599 @option{-p}, otherwise to @samp{/tmp}. @var{template} must not
12600 contain slashes. This option is deprecated; the use of @option{-p}
12601 without @option{-t} offers better defaults (by favoring the command
12602 line over @env{TMPDIR}) and more flexibility (by allowing intermediate
12607 @cindex exit status of @command{mktemp}
12611 0 if the file was created,
12616 @node Working context
12617 @chapter Working context
12619 @cindex working context
12620 @cindex commands for printing the working context
12622 This section describes commands that display or alter the context in
12623 which you are working: the current directory, the terminal settings, and
12624 so forth. See also the user-related commands in the next section.
12627 * pwd invocation:: Print working directory.
12628 * stty invocation:: Print or change terminal characteristics.
12629 * printenv invocation:: Print environment variables.
12630 * tty invocation:: Print file name of terminal on standard input.
12634 @node pwd invocation
12635 @section @command{pwd}: Print working directory
12638 @cindex print name of current directory
12639 @cindex current working directory, printing
12640 @cindex working directory, printing
12643 @command{pwd} prints the name of the current directory. Synopsis:
12646 pwd [@var{option}]@dots{}
12649 The program accepts the following options. Also see @ref{Common options}.
12656 If the contents of the environment variable @env{PWD} provide an
12657 absolute name of the current directory with no @samp{.} or @samp{..}
12658 components, but possibly with symbolic links, then output those
12659 contents. Otherwise, fall back to default @option{-P} handling.
12664 @opindex --physical
12665 Print a fully resolved name for the current directory. That is, all
12666 components of the printed name will be actual directory names---none
12667 will be symbolic links.
12670 @cindex symbolic links and @command{pwd}
12671 If @option{-L} and @option{-P} are both given, the last one takes
12672 precedence. If neither option is given, then this implementation uses
12673 @option{-P} as the default unless the @env{POSIXLY_CORRECT}
12674 environment variable is set.
12676 @mayConflictWithShellBuiltIn{pwd}
12681 @node stty invocation
12682 @section @command{stty}: Print or change terminal characteristics
12685 @cindex change or print terminal settings
12686 @cindex terminal settings
12687 @cindex line settings of terminal
12689 @command{stty} prints or changes terminal characteristics, such as baud rate.
12693 stty [@var{option}] [@var{setting}]@dots{}
12694 stty [@var{option}]
12697 If given no line settings, @command{stty} prints the baud rate, line
12698 discipline number (on systems that support it), and line settings
12699 that have been changed from the values set by @samp{stty sane}.
12700 By default, mode reading and setting are performed on the tty line
12701 connected to standard input, although this can be modified by the
12702 @option{--file} option.
12704 @command{stty} accepts many non-option arguments that change aspects of
12705 the terminal line operation, as described below.
12707 The program accepts the following options. Also see @ref{Common options}.
12714 Print all current settings in human-readable form. This option may not
12715 be used in combination with any line settings.
12717 @item -F @var{device}
12718 @itemx --file=@var{device}
12721 Set the line opened by the file name specified in @var{device} instead of
12722 the tty line connected to standard input. This option is necessary
12723 because opening a @acronym{POSIX} tty requires use of the
12724 @code{O_NONDELAY} flag to prevent a @acronym{POSIX} tty from blocking
12725 until the carrier detect line is high if
12726 the @code{clocal} flag is not set. Hence, it is not always possible
12727 to allow the shell to open the device in the traditional manner.
12733 @cindex machine-readable @command{stty} output
12734 Print all current settings in a form that can be used as an argument to
12735 another @command{stty} command to restore the current settings. This option
12736 may not be used in combination with any line settings.
12740 Many settings can be turned off by preceding them with a @samp{-}.
12741 Such arguments are marked below with ``May be negated'' in their
12742 description. The descriptions themselves refer to the positive
12743 case, that is, when @emph{not} negated (unless stated otherwise,
12746 Some settings are not available on all @acronym{POSIX} systems, since they use
12747 extensions. Such arguments are marked below with
12748 ``Non-@acronym{POSIX}'' in their description. On non-@acronym{POSIX}
12749 systems, those or other settings also may not
12750 be available, but it's not feasible to document all the variations: just
12756 * Control:: Control settings
12757 * Input:: Input settings
12758 * Output:: Output settings
12759 * Local:: Local settings
12760 * Combination:: Combination settings
12761 * Characters:: Special characters
12762 * Special:: Special settings
12767 @subsection Control settings
12769 @cindex control settings
12775 @cindex two-way parity
12776 Generate parity bit in output and expect parity bit in input.
12782 @cindex even parity
12783 Set odd parity (even if negated). May be negated.
12790 @cindex character size
12791 @cindex eight-bit characters
12792 Set character size to 5, 6, 7, or 8 bits.
12797 Send a hangup signal when the last process closes the tty. May be
12803 Use two stop bits per character (one if negated). May be negated.
12807 Allow input to be received. May be negated.
12811 @cindex modem control
12812 Disable modem control signals. May be negated.
12816 @cindex hardware flow control
12817 @cindex flow control, hardware
12818 @cindex RTS/CTS flow control
12819 Enable RTS/CTS flow control. Non-@acronym{POSIX}. May be negated.
12824 @subsection Input settings
12826 @cindex input settings
12827 These settings control operations on data received from the terminal.
12832 @cindex breaks, ignoring
12833 Ignore break characters. May be negated.
12837 @cindex breaks, cause interrupts
12838 Make breaks cause an interrupt signal. May be negated.
12842 @cindex parity, ignoring
12843 Ignore characters with parity errors. May be negated.
12847 @cindex parity errors, marking
12848 Mark parity errors (with a 255-0-character sequence). May be negated.
12852 Enable input parity checking. May be negated.
12856 @cindex eight-bit input
12857 Clear high (8th) bit of input characters. May be negated.
12861 @cindex newline, translating to return
12862 Translate newline to carriage return. May be negated.
12866 @cindex return, ignoring
12867 Ignore carriage return. May be negated.
12871 @cindex return, translating to newline
12872 Translate carriage return to newline. May be negated.
12876 @cindex input encoding, UTF-8
12877 Assume input characters are UTF-8 encoded. May be negated.
12881 @kindex C-s/C-q flow control
12882 @cindex XON/XOFF flow control
12883 Enable XON/XOFF flow control (that is, @kbd{CTRL-S}/@kbd{CTRL-Q}). May
12890 @cindex software flow control
12891 @cindex flow control, software
12892 Enable sending of @code{stop} character when the system input buffer
12893 is almost full, and @code{start} character when it becomes almost
12894 empty again. May be negated.
12898 @cindex uppercase, translating to lowercase
12899 Translate uppercase characters to lowercase. Non-@acronym{POSIX}. May be
12900 negated. Note ilcuc is not implemented, as one would not be able to issue
12901 almost any (lowercase) Unix command, after invoking it.
12905 Allow any character to restart output (only the start character
12906 if negated). Non-@acronym{POSIX}. May be negated.
12910 @cindex beeping at input buffer full
12911 Enable beeping and not flushing input buffer if a character arrives
12912 when the input buffer is full. Non-@acronym{POSIX}. May be negated.
12917 @subsection Output settings
12919 @cindex output settings
12920 These settings control operations on data sent to the terminal.
12925 Postprocess output. May be negated.
12929 @cindex lowercase, translating to output
12930 Translate lowercase characters to uppercase. Non-@acronym{POSIX}. May be
12931 negated. (Note ouclc is not currently implemented.)
12935 @cindex return, translating to newline
12936 Translate carriage return to newline. Non-@acronym{POSIX}. May be negated.
12940 @cindex newline, translating to crlf
12941 Translate newline to carriage return-newline. Non-@acronym{POSIX}. May be
12946 Do not print carriage returns in the first column. Non-@acronym{POSIX}.
12951 Newline performs a carriage return. Non-@acronym{POSIX}. May be negated.
12955 @cindex pad instead of timing for delaying
12956 Use fill (padding) characters instead of timing for delays.
12957 Non-@acronym{POSIX}.
12962 @cindex pad character
12963 Use @acronym{ASCII} @sc{del} characters for fill instead of
12964 @acronym{ASCII} @sc{nul} characters. Non-@acronym{POSIX}.
12970 Newline delay style. Non-@acronym{POSIX}.
12977 Carriage return delay style. Non-@acronym{POSIX}.
12983 @opindex tab@var{n}
12984 Horizontal tab delay style. Non-@acronym{POSIX}.
12989 Backspace delay style. Non-@acronym{POSIX}.
12994 Vertical tab delay style. Non-@acronym{POSIX}.
12999 Form feed delay style. Non-@acronym{POSIX}.
13004 @subsection Local settings
13006 @cindex local settings
13011 Enable @code{interrupt}, @code{quit}, and @code{suspend} special
13012 characters. May be negated.
13016 Enable @code{erase}, @code{kill}, @code{werase}, and @code{rprnt}
13017 special characters. May be negated.
13021 Enable non-@acronym{POSIX} special characters. May be negated.
13025 Echo input characters. May be negated.
13031 Echo @code{erase} characters as backspace-space-backspace. May be
13036 @cindex newline echoing after @code{kill}
13037 Echo a newline after a @code{kill} character. May be negated.
13041 @cindex newline, echoing
13042 Echo newline even if not echoing other characters. May be negated.
13046 @cindex flushing, disabling
13047 Disable flushing after @code{interrupt} and @code{quit} special
13048 characters. May be negated.
13052 @cindex case translation
13053 Enable input and output of uppercase characters by preceding their
13054 lowercase equivalents with @samp{\}, when @code{icanon} is set.
13055 Non-@acronym{POSIX}. May be negated.
13059 @cindex background jobs, stopping at terminal write
13060 Stop background jobs that try to write to the terminal. Non-@acronym{POSIX}.
13067 Echo erased characters backward, between @samp{\} and @samp{/}.
13068 Non-@acronym{POSIX}. May be negated.
13074 @cindex control characters, using @samp{^@var{c}}
13075 @cindex hat notation for control characters
13076 Echo control characters in hat notation (@samp{^@var{c}}) instead
13077 of literally. Non-@acronym{POSIX}. May be negated.
13083 Echo the @code{kill} special character by erasing each character on
13084 the line as indicated by the @code{echoprt} and @code{echoe} settings,
13085 instead of by the @code{echoctl} and @code{echok} settings.
13086 Non-@acronym{POSIX}.
13092 @subsection Combination settings
13094 @cindex combination settings
13095 Combination settings:
13102 Same as @code{parenb -parodd cs7}. May be negated. If negated, same
13103 as @code{-parenb cs8}.
13107 Same as @code{parenb parodd cs7}. May be negated. If negated, same
13108 as @code{-parenb cs8}.
13112 Same as @code{-icrnl -onlcr}. May be negated. If negated, same as
13113 @code{icrnl -inlcr -igncr onlcr -ocrnl -onlret}.
13117 Reset the @code{erase} and @code{kill} special characters to their default
13124 @c This is too long to write inline.
13126 cread -ignbrk brkint -inlcr -igncr icrnl -ixoff
13127 -iuclc -ixany imaxbel opost -olcuc -ocrnl onlcr
13128 -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0
13129 ff0 isig icanon iexten echo echoe echok -echonl
13130 -noflsh -xcase -tostop -echoprt echoctl echoke
13134 and also sets all special characters to their default values.
13138 Same as @code{brkint ignpar istrip icrnl ixon opost isig icanon}, plus
13139 sets the @code{eof} and @code{eol} characters to their default values
13140 if they are the same as the @code{min} and @code{time} characters.
13141 May be negated. If negated, same as @code{raw}.
13148 -ignbrk -brkint -ignpar -parmrk -inpck -istrip
13149 -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany
13150 -imaxbel -opost -isig -icanon -xcase min 1 time 0
13154 May be negated. If negated, same as @code{cooked}.
13158 Same as @option{-icanon}. May be negated. If negated, same as
13163 @cindex eight-bit characters
13164 Same as @code{-parenb -istrip cs8}. May be negated. If negated,
13165 same as @code{parenb istrip cs7}.
13169 Same as @option{-parenb -istrip -opost cs8}. May be negated.
13170 If negated, same as @code{parenb istrip opost cs7}.
13174 Same as @option{-ixany}. Non-@acronym{POSIX}. May be negated.
13178 Same as @code{tab0}. Non-@acronym{POSIX}. May be negated. If negated, same
13185 Same as @code{xcase iuclc olcuc}. Non-@acronym{POSIX}. May be negated.
13186 (Used for terminals with uppercase characters only.)
13190 Same as @code{echoe echoctl echoke}.
13194 Same as @code{echoe echoctl echoke -ixany intr ^C erase ^? kill C-u}.
13199 @subsection Special characters
13201 @cindex special characters
13202 @cindex characters, special
13204 The special characters' default values vary from system to system.
13205 They are set with the syntax @samp{name value}, where the names are
13206 listed below and the value can be given either literally, in hat
13207 notation (@samp{^@var{c}}), or as an integer which may start with
13208 @samp{0x} to indicate hexadecimal, @samp{0} to indicate octal, or
13209 any other digit to indicate decimal.
13211 @cindex disabling special characters
13212 @kindex u@r{, and disabling special characters}
13213 For GNU stty, giving a value of @code{^-} or @code{undef} disables that
13214 special character. (This is incompatible with Ultrix @command{stty},
13215 which uses a value of @samp{u} to disable a special character. GNU
13216 @command{stty} treats a value @samp{u} like any other, namely to set that
13217 special character to @key{U}.)
13223 Send an interrupt signal.
13227 Send a quit signal.
13231 Erase the last character typed.
13235 Erase the current line.
13239 Send an end of file (terminate the input).
13247 Alternate character to end the line. Non-@acronym{POSIX}.
13251 Switch to a different shell layer. Non-@acronym{POSIX}.
13255 Restart the output after stopping it.
13263 Send a terminal stop signal.
13267 Send a terminal stop signal after flushing the input. Non-@acronym{POSIX}.
13271 Redraw the current line. Non-@acronym{POSIX}.
13275 Erase the last word typed. Non-@acronym{POSIX}.
13279 Enter the next character typed literally, even if it is a special
13280 character. Non-@acronym{POSIX}.
13285 @subsection Special settings
13287 @cindex special settings
13292 Set the minimum number of characters that will satisfy a read until
13293 the time value has expired, when @option{-icanon} is set.
13297 Set the number of tenths of a second before reads time out if the minimum
13298 number of characters have not been read, when @option{-icanon} is set.
13300 @item ispeed @var{n}
13302 Set the input speed to @var{n}.
13304 @item ospeed @var{n}
13306 Set the output speed to @var{n}.
13310 Tell the tty kernel driver that the terminal has @var{n} rows.
13311 Non-@acronym{POSIX}.
13314 @itemx columns @var{n}
13317 Tell the kernel that the terminal has @var{n} columns. Non-@acronym{POSIX}.
13323 Print the number of rows and columns that the kernel thinks the
13324 terminal has. (Systems that don't support rows and columns in the kernel
13325 typically use the environment variables @env{LINES} and @env{COLUMNS}
13326 instead; however, GNU @command{stty} does not know anything about them.)
13327 Non-@acronym{POSIX}.
13331 Use line discipline @var{n}. Non-@acronym{POSIX}.
13335 Print the terminal speed.
13338 @cindex baud rate, setting
13339 Set the input and output speeds to @var{n}. @var{n} can be one of: 0
13340 50 75 110 134 134.5 150 200 300 600 1200 1800 2400 4800 9600 19200
13341 38400 @code{exta} @code{extb}. @code{exta} is the same as 19200;
13342 @code{extb} is the same as 38400. Many systems, including GNU/Linux,
13343 support higher speeds. The @command{stty} command includes support
13360 4000000 where the system supports these.
13361 0 hangs up the line if @option{-clocal} is set.
13365 @node printenv invocation
13366 @section @command{printenv}: Print all or some environment variables
13369 @cindex printing all or some environment variables
13370 @cindex environment variables, printing
13372 @command{printenv} prints environment variable values. Synopsis:
13375 printenv [@var{option}] [@var{variable}]@dots{}
13378 If no @var{variable}s are specified, @command{printenv} prints the value of
13379 every environment variable. Otherwise, it prints the value of each
13380 @var{variable} that is set, and nothing for those that are not set.
13382 The program accepts the following option. Also see @ref{Common options}.
13390 @cindex exit status of @command{printenv}
13394 0 if all variables specified were found
13395 1 if at least one specified variable was not found
13396 2 if a write error occurred
13400 @node tty invocation
13401 @section @command{tty}: Print file name of terminal on standard input
13404 @cindex print terminal file name
13405 @cindex terminal file name, printing
13407 @command{tty} prints the file name of the terminal connected to its standard
13408 input. It prints @samp{not a tty} if standard input is not a terminal.
13412 tty [@var{option}]@dots{}
13415 The program accepts the following option. Also see @ref{Common options}.
13425 Print nothing; only return an exit status.
13429 @cindex exit status of @command{tty}
13433 0 if standard input is a terminal
13434 1 if standard input is not a terminal
13435 2 if given incorrect arguments
13436 3 if a write error occurs
13440 @node User information
13441 @chapter User information
13443 @cindex user information, commands for
13444 @cindex commands for printing user information
13446 This section describes commands that print user-related information:
13447 logins, groups, and so forth.
13450 * id invocation:: Print user identity.
13451 * logname invocation:: Print current login name.
13452 * whoami invocation:: Print effective user ID.
13453 * groups invocation:: Print group names a user is in.
13454 * users invocation:: Print login names of users currently logged in.
13455 * who invocation:: Print who is currently logged in.
13459 @node id invocation
13460 @section @command{id}: Print user identity
13463 @cindex real user and group IDs, printing
13464 @cindex effective user and group IDs, printing
13465 @cindex printing real and effective user and group IDs
13467 @command{id} prints information about the given user, or the process
13468 running it if no user is specified. Synopsis:
13471 id [@var{option}]@dots{} [@var{username}]
13474 @vindex POSIXLY_CORRECT
13475 By default, it prints the real user ID, real group ID, effective user ID
13476 if different from the real user ID, effective group ID if different from
13477 the real group ID, and supplemental group IDs.
13478 In addition, if SELinux
13479 is enabled and the @env{POSIXLY_CORRECT} environment variable is not set,
13480 then print @samp{context=@var{c}}, where @var{c} is the security context.
13482 Each of these numeric values is preceded by an identifying string and
13483 followed by the corresponding user or group name in parentheses.
13485 The options cause @command{id} to print only part of the above information.
13486 Also see @ref{Common options}.
13493 Print only the group ID.
13499 Print only the group ID and the supplementary groups.
13505 Print the user or group name instead of the ID number. Requires
13506 @option{-u}, @option{-g}, or @option{-G}.
13512 Print the real, instead of effective, user or group ID. Requires
13513 @option{-u}, @option{-g}, or @option{-G}.
13519 Print only the user ID.
13526 @cindex security context
13527 Print only the security context of the current user.
13528 If SELinux is disabled then print a warning and
13529 set the exit status to 1.
13535 @macro primaryAndSupplementaryGroups{cmd,arg}
13536 Primary and supplementary groups for a process are normally inherited
13537 from its parent and are usually unchanged since login. This means
13538 that if you change the group database after logging in, @command{\cmd\}
13539 will not reflect your changes within your existing login session.
13540 Running @command{\cmd\} with a \arg\ causes the user and group
13541 database to be consulted afresh, and so will give a different result.
13543 @primaryAndSupplementaryGroups{id,user argument}
13545 @node logname invocation
13546 @section @command{logname}: Print current login name
13549 @cindex printing user's login name
13550 @cindex login name, printing
13551 @cindex user name, printing
13554 @command{logname} prints the calling user's name, as found in a
13555 system-maintained file (often @file{/var/run/utmp} or
13556 @file{/etc/utmp}), and exits with a status of 0. If there is no entry
13557 for the calling process, @command{logname} prints
13558 an error message and exits with a status of 1.
13560 The only options are @option{--help} and @option{--version}. @xref{Common
13566 @node whoami invocation
13567 @section @command{whoami}: Print effective user ID
13570 @cindex effective user ID, printing
13571 @cindex printing the effective user ID
13573 @command{whoami} prints the user name associated with the current
13574 effective user ID. It is equivalent to the command @samp{id -un}.
13576 The only options are @option{--help} and @option{--version}. @xref{Common
13582 @node groups invocation
13583 @section @command{groups}: Print group names a user is in
13586 @cindex printing groups a user is in
13587 @cindex supplementary groups, printing
13589 @command{groups} prints the names of the primary and any supplementary
13590 groups for each given @var{username}, or the current process if no names
13591 are given. If more than one name is given, the name of each user is
13593 the list of that user's groups and the user name is separated from the
13594 group list by a colon. Synopsis:
13597 groups [@var{username}]@dots{}
13600 The group lists are equivalent to the output of the command @samp{id -Gn}.
13602 @primaryAndSupplementaryGroups{groups,list of users}
13604 The only options are @option{--help} and @option{--version}. @xref{Common
13610 @node users invocation
13611 @section @command{users}: Print login names of users currently logged in
13614 @cindex printing current usernames
13615 @cindex usernames, printing current
13617 @cindex login sessions, printing users with
13618 @command{users} prints on a single line a blank-separated list of user
13619 names of users currently logged in to the current host. Each user name
13620 corresponds to a login session, so if a user has more than one login
13621 session, that user's name will appear the same number of times in the
13630 With no @var{file} argument, @command{users} extracts its information from
13631 a system-maintained file (often @file{/var/run/utmp} or
13632 @file{/etc/utmp}). If a file argument is given, @command{users} uses
13633 that file instead. A common choice is @file{/var/log/wtmp}.
13635 The only options are @option{--help} and @option{--version}. @xref{Common
13641 @node who invocation
13642 @section @command{who}: Print who is currently logged in
13645 @cindex printing current user information
13646 @cindex information, about current users
13648 @command{who} prints information about users who are currently logged on.
13652 @command{who} [@var{option}] [@var{file}] [am i]
13655 @cindex terminal lines, currently used
13657 @cindex remote hostname
13658 If given no non-option arguments, @command{who} prints the following
13659 information for each user currently logged on: login name, terminal
13660 line, login time, and remote hostname or X display.
13664 If given one non-option argument, @command{who} uses that instead of
13665 a default system-maintained file (often @file{/var/run/utmp} or
13666 @file{/etc/utmp}) as the name of the file containing the record of
13667 users logged on. @file{/var/log/wtmp} is commonly given as an argument
13668 to @command{who} to look at who has previously logged on.
13672 If given two non-option arguments, @command{who} prints only the entry
13673 for the user running it (determined from its standard input), preceded
13674 by the hostname. Traditionally, the two arguments given are @samp{am
13675 i}, as in @samp{who am i}.
13678 Time stamps are listed according to the time zone rules specified by
13679 the @env{TZ} environment variable, or by the system default rules if
13680 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
13681 with @env{TZ}, libc, The GNU C Library Reference Manual}.
13683 The program accepts the following options. Also see @ref{Common options}.
13691 Same as @samp{-b -d --login -p -r -t -T -u}.
13697 Print the date and time of last system boot.
13703 Print information corresponding to dead processes.
13709 Print a line of column headings.
13715 List only the entries that correspond to processes via which the
13716 system is waiting for a user to login. The user name is always @samp{LOGIN}.
13720 Attempt to canonicalize hostnames found in utmp through a DNS lookup. This
13721 is not the default because it can cause significant delays on systems with
13722 automatic dial-up internet access.
13726 Same as @samp{who am i}.
13732 List active processes spawned by init.
13738 Print only the login names and the number of users logged on.
13739 Overrides all other options.
13744 @opindex --runlevel
13745 Print the current (and maybe previous) run-level of the init process.
13749 Ignored; for compatibility with other versions of @command{who}.
13755 Print last system clock change.
13760 After the login time, print the number of hours and minutes that the
13761 user has been idle. @samp{.} means the user was active in the last minute.
13762 @samp{old} means the user has been idle for more than 24 hours.
13773 @opindex --writable
13774 @cindex message status
13775 @pindex write@r{, allowed}
13776 After each login name print a character indicating the user's message status:
13779 @samp{+} allowing @code{write} messages
13780 @samp{-} disallowing @code{write} messages
13781 @samp{?} cannot find terminal device
13789 @node System context
13790 @chapter System context
13792 @cindex system context
13793 @cindex context, system
13794 @cindex commands for system context
13796 This section describes commands that print or change system-wide
13800 * date invocation:: Print or set system date and time.
13801 * arch invocation:: Print machine hardware name.
13802 * nproc invocation:: Print the number of processors.
13803 * uname invocation:: Print system information.
13804 * hostname invocation:: Print or set system name.
13805 * hostid invocation:: Print numeric host identifier.
13806 * uptime invocation:: Print system uptime and load.
13809 @node date invocation
13810 @section @command{date}: Print or set system date and time
13813 @cindex time, printing or setting
13814 @cindex printing the current time
13819 date [@var{option}]@dots{} [+@var{format}]
13820 date [-u|--utc|--universal] @c this avoids a newline in the output
13821 [ MMDDhhmm[[CC]YY][.ss] ]
13825 Invoking @command{date} with no @var{format} argument is equivalent to invoking
13826 it with a default format that depends on the @env{LC_TIME} locale category.
13827 In the default C locale, this format is @samp{'+%a %b %e %H:%M:%S %Z %Y'},
13828 so the output looks like @samp{Thu Mar @ 3 13:47:51 PST 2005}.
13831 Normally, @command{date} uses the time zone rules indicated by the
13832 @env{TZ} environment variable, or the system default rules if @env{TZ}
13833 is not set. @xref{TZ Variable,, Specifying the Time Zone with
13834 @env{TZ}, libc, The GNU C Library Reference Manual}.
13836 @findex strftime @r{and @command{date}}
13837 @cindex time formats
13838 @cindex formatting times
13839 If given an argument that starts with a @samp{+}, @command{date} prints the
13840 current date and time (or the date and time specified by the
13841 @option{--date} option, see below) in the format defined by that argument,
13842 which is similar to that of the @code{strftime} function. Except for
13843 conversion specifiers, which start with @samp{%}, characters in the
13844 format string are printed unchanged. The conversion specifiers are
13850 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
13851 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
13852 * Literal conversion specifiers:: %[%nt]
13853 * Padding and other flags:: Pad with zeros, spaces, etc.
13854 * Setting the time:: Changing the system clock.
13855 * Options for date:: Instead of the current time.
13857 * Date input formats:: Specifying date strings.
13859 * Examples of date:: Examples.
13862 @node Time conversion specifiers
13863 @subsection Time conversion specifiers
13865 @cindex time conversion specifiers
13866 @cindex conversion specifiers, time
13868 @command{date} conversion specifiers related to times.
13872 hour (@samp{00}@dots{}@samp{23})
13874 hour (@samp{01}@dots{}@samp{12})
13876 hour (@samp{ 0}@dots{}@samp{23}).
13877 This is a @acronym{GNU} extension.
13879 hour (@samp{ 1}@dots{}@samp{12}).
13880 This is a @acronym{GNU} extension.
13882 minute (@samp{00}@dots{}@samp{59})
13884 nanoseconds (@samp{000000000}@dots{}@samp{999999999}).
13885 This is a @acronym{GNU} extension.
13887 locale's equivalent of either @samp{AM} or @samp{PM};
13888 blank in many locales.
13889 Noon is treated as @samp{PM} and midnight as @samp{AM}.
13891 like @samp{%p}, except lower case.
13892 This is a @acronym{GNU} extension.
13894 locale's 12-hour clock time (e.g., @samp{11:11:04 PM})
13896 24-hour hour and minute. Same as @samp{%H:%M}.
13897 This is a @acronym{GNU} extension.
13899 @cindex epoch, seconds since
13900 @cindex seconds since the epoch
13901 @cindex beginning of time
13902 seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC.
13903 Leap seconds are not counted unless leap second support is available.
13904 @xref{%s-examples}, for examples.
13905 This is a @acronym{GNU} extension.
13907 second (@samp{00}@dots{}@samp{60}).
13908 This may be @samp{60} if leap seconds are supported.
13910 24-hour hour, minute, and second. Same as @samp{%H:%M:%S}.
13912 locale's time representation (e.g., @samp{23:13:48})
13914 @w{@acronym{RFC} 2822/@acronym{ISO} 8601} style numeric time zone
13915 (e.g., @samp{-0600} or @samp{+0530}), or nothing if no
13916 time zone is determinable. This value reflects the numeric time zone
13917 appropriate for the current time, using the time zone rules specified
13918 by the @env{TZ} environment variable.
13919 The time (and optionally, the time zone rules) can be overridden
13920 by the @option{--date} option.
13921 This is a @acronym{GNU} extension.
13923 @w{@acronym{RFC} 3339/@acronym{ISO} 8601} style numeric time zone with
13924 @samp{:} (e.g., @samp{-06:00} or @samp{+05:30}), or nothing if no time
13925 zone is determinable.
13926 This is a @acronym{GNU} extension.
13928 Numeric time zone to the nearest second with @samp{:} (e.g.,
13929 @samp{-06:00:00} or @samp{+05:30:00}), or nothing if no time zone is
13931 This is a @acronym{GNU} extension.
13933 Numeric time zone with @samp{:} using the minimum necessary precision
13934 (e.g., @samp{-06}, @samp{+05:30}, or @samp{-04:56:02}), or nothing if
13935 no time zone is determinable.
13936 This is a @acronym{GNU} extension.
13938 alphabetic time zone abbreviation (e.g., @samp{EDT}), or nothing if no
13939 time zone is determinable. See @samp{%z} for how it is determined.
13943 @node Date conversion specifiers
13944 @subsection Date conversion specifiers
13946 @cindex date conversion specifiers
13947 @cindex conversion specifiers, date
13949 @command{date} conversion specifiers related to dates.
13953 locale's abbreviated weekday name (e.g., @samp{Sun})
13955 locale's full weekday name, variable length (e.g., @samp{Sunday})
13957 locale's abbreviated month name (e.g., @samp{Jan})
13959 locale's full month name, variable length (e.g., @samp{January})
13961 locale's date and time (e.g., @samp{Thu Mar @ 3 23:05:25 2005})
13963 century. This is like @samp{%Y}, except the last two digits are omitted.
13964 For example, it is @samp{20} if @samp{%Y} is @samp{2000},
13965 and is @samp{-0} if @samp{%Y} is @samp{-001}.
13966 It is normally at least two characters, but it may be more.
13968 day of month (e.g., @samp{01})
13970 date; same as @samp{%m/%d/%y}
13972 day of month, space padded; same as @samp{%_d}
13974 full date in @acronym{ISO} 8601 format; same as @samp{%Y-%m-%d}.
13975 This is a good choice for a date format, as it is standard and
13976 is easy to sort in the usual case where years are in the range
13978 This is a @acronym{GNU} extension.
13980 year corresponding to the @acronym{ISO} week number, but without the century
13981 (range @samp{00} through @samp{99}). This has the same format and value
13982 as @samp{%y}, except that if the @acronym{ISO} week number (see
13984 to the previous or next year, that year is used instead.
13985 This is a @acronym{GNU} extension.
13987 year corresponding to the @acronym{ISO} week number. This has the
13988 same format and value as @samp{%Y}, except that if the @acronym{ISO}
13990 @samp{%V}) belongs to the previous or next year, that year is used
13992 It is normally useful only if @samp{%V} is also used;
13993 for example, the format @samp{%G-%m-%d} is probably a mistake,
13994 since it combines the ISO week number year with the conventional month and day.
13995 This is a @acronym{GNU} extension.
13999 day of year (@samp{001}@dots{}@samp{366})
14001 month (@samp{01}@dots{}@samp{12})
14003 day of week (@samp{1}@dots{}@samp{7}) with @samp{1} corresponding to Monday
14005 week number of year, with Sunday as the first day of the week
14006 (@samp{00}@dots{}@samp{53}).
14007 Days in a new year preceding the first Sunday are in week zero.
14009 @acronym{ISO} week number, that is, the
14010 week number of year, with Monday as the first day of the week
14011 (@samp{01}@dots{}@samp{53}).
14012 If the week containing January 1 has four or more days in
14013 the new year, then it is considered week 1; otherwise, it is week 53 of
14014 the previous year, and the next week is week 1. (See the @acronym{ISO} 8601
14017 day of week (@samp{0}@dots{}@samp{6}) with 0 corresponding to Sunday
14019 week number of year, with Monday as first day of week
14020 (@samp{00}@dots{}@samp{53}).
14021 Days in a new year preceding the first Monday are in week zero.
14023 locale's date representation (e.g., @samp{12/31/99})
14025 last two digits of year (@samp{00}@dots{}@samp{99})
14027 year. This is normally at least four characters, but it may be more.
14028 Year @samp{0000} precedes year @samp{0001}, and year @samp{-001}
14029 precedes year @samp{0000}.
14033 @node Literal conversion specifiers
14034 @subsection Literal conversion specifiers
14036 @cindex literal conversion specifiers
14037 @cindex conversion specifiers, literal
14039 @command{date} conversion specifiers that produce literal strings.
14051 @node Padding and other flags
14052 @subsection Padding and other flags
14054 @cindex numeric field padding
14055 @cindex padding of numeric fields
14056 @cindex fields, padding numeric
14058 Unless otherwise specified, @command{date} normally pads numeric fields
14059 with zeros, so that, for
14060 example, numeric months are always output as two digits.
14061 Seconds since the epoch are not padded, though,
14062 since there is no natural width for them.
14064 As a @acronym{GNU} extension, @command{date} recognizes any of the
14065 following optional flags after the @samp{%}:
14069 (hyphen) Do not pad the field; useful if the output is intended for
14072 (underscore) Pad with spaces; useful if you need a fixed
14073 number of characters in the output, but zeros are too distracting.
14075 (zero) Pad with zeros even if the conversion specifier
14076 would normally pad with spaces.
14078 Use upper case characters if possible.
14080 Use opposite case characters if possible.
14081 A field that is normally upper case becomes lower case, and vice versa.
14085 Here are some examples of padding:
14088 date +%d/%m -d "Feb 1"
14090 date +%-d/%-m -d "Feb 1"
14092 date +%_d/%_m -d "Feb 1"
14096 As a @acronym{GNU} extension, you can specify the field width
14097 (after any flag, if present) as a decimal number. If the natural size of the
14098 output of the field has less than the specified number of characters,
14099 the result is written right adjusted and padded to the given
14100 size. For example, @samp{%9B} prints the right adjusted month name in
14101 a field of width 9.
14103 An optional modifier can follow the optional flag and width
14104 specification. The modifiers are:
14108 Use the locale's alternate representation for date and time. This
14109 modifier applies to the @samp{%c}, @samp{%C}, @samp{%x}, @samp{%X},
14110 @samp{%y} and @samp{%Y} conversion specifiers. In a Japanese locale, for
14111 example, @samp{%Ex} might yield a date format based on the Japanese
14115 Use the locale's alternate numeric symbols for numbers. This modifier
14116 applies only to numeric conversion specifiers.
14119 If the format supports the modifier but no alternate representation
14120 is available, it is ignored.
14123 @node Setting the time
14124 @subsection Setting the time
14126 @cindex setting the time
14127 @cindex time setting
14128 @cindex appropriate privileges
14130 If given an argument that does not start with @samp{+}, @command{date} sets
14131 the system clock to the date and time specified by that argument (as
14132 described below). You must have appropriate privileges to set the
14133 system clock. The @option{--date} and @option{--set} options may not be
14134 used with such an argument. The @option{--universal} option may be used
14135 with such an argument to indicate that the specified date and time are
14136 relative to Coordinated Universal Time rather than to the local time
14139 The argument must consist entirely of digits, which have the following
14152 first two digits of year (optional)
14154 last two digits of year (optional)
14159 The @option{--set} option also sets the system clock; see the next section.
14162 @node Options for date
14163 @subsection Options for @command{date}
14165 @cindex @command{date} options
14166 @cindex options for @command{date}
14168 The program accepts the following options. Also see @ref{Common options}.
14172 @item -d @var{datestr}
14173 @itemx --date=@var{datestr}
14176 @cindex parsing date strings
14177 @cindex date strings, parsing
14178 @cindex arbitrary date strings, parsing
14181 @opindex next @var{day}
14182 @opindex last @var{day}
14183 Display the date and time specified in @var{datestr} instead of the
14184 current date and time. @var{datestr} can be in almost any common
14185 format. It can contain month names, time zones, @samp{am} and @samp{pm},
14186 @samp{yesterday}, etc. For example, @option{--date="2004-02-27
14187 14:19:13.489392193 +0530"} specifies the instant of time that is
14188 489,392,193 nanoseconds after February 27, 2004 at 2:19:13 PM in a
14189 time zone that is 5 hours and 30 minutes east of @acronym{UTC}.@*
14190 Note: input currently must be in locale independent format. E.g., the
14191 LC_TIME=C below is needed to print back the correct date in many locales:
14193 date -d "$(LC_TIME=C date)"
14195 @xref{Date input formats}.
14197 @item -f @var{datefile}
14198 @itemx --file=@var{datefile}
14201 Parse each line in @var{datefile} as with @option{-d} and display the
14202 resulting date and time. If @var{datefile} is @samp{-}, use standard
14203 input. This is useful when you have many dates to process, because the
14204 system overhead of starting up the @command{date} executable many times can
14207 @item -r @var{file}
14208 @itemx --reference=@var{file}
14210 @opindex --reference
14211 Display the date and time of the last modification of @var{file},
14212 instead of the current date and time.
14219 @opindex --rfc-2822
14220 Display the date and time using the format @samp{%a, %d %b %Y %H:%M:%S
14221 %z}, evaluated in the C locale so abbreviations are always in English.
14225 Fri, 09 Sep 2005 13:51:39 -0700
14228 This format conforms to
14229 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt, Internet
14230 @acronym{RFCs} 2822} and
14231 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc822.txt, 822}, the
14232 current and previous standards for Internet email.
14234 @item --rfc-3339=@var{timespec}
14235 @opindex --rfc-3339=@var{timespec}
14236 Display the date using a format specified by
14237 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt, Internet
14238 @acronym{RFC} 3339}. This is a subset of the @acronym{ISO} 8601
14239 format, except that it also permits applications to use a space rather
14240 than a @samp{T} to separate dates from times. Unlike the other
14241 standard formats, @acronym{RFC} 3339 format is always suitable as
14242 input for the @option{--date} (@option{-d}) and @option{--file}
14243 (@option{-f}) options, regardless of the current locale.
14245 The argument @var{timespec} specifies how much of the time to include.
14246 It can be one of the following:
14250 Print just the full-date, e.g., @samp{2005-09-14}.
14251 This is equivalent to the format @samp{%Y-%m-%d}.
14254 Print the full-date and full-time separated by a space, e.g.,
14255 @samp{2005-09-14 00:56:06+05:30}. The output ends with a numeric
14256 time-offset; here the @samp{+05:30} means that local time is five
14257 hours and thirty minutes east of @acronym{UTC}. This is equivalent to
14258 the format @samp{%Y-%m-%d %H:%M:%S%:z}.
14261 Like @samp{seconds}, but also print nanoseconds, e.g.,
14262 @samp{2005-09-14 00:56:06.998458565+05:30}.
14263 This is equivalent to the format @samp{%Y-%m-%d %H:%M:%S.%N%:z}.
14267 @item -s @var{datestr}
14268 @itemx --set=@var{datestr}
14271 Set the date and time to @var{datestr}. See @option{-d} above.
14278 @opindex --universal
14279 @cindex Coordinated Universal Time
14281 @cindex Greenwich Mean Time
14284 Use Coordinated Universal Time (@acronym{UTC}) by operating as if the
14285 @env{TZ} environment variable were set to the string @samp{UTC0}.
14287 Universal Time is often called ``Greenwich Mean Time'' (@sc{gmt}) for
14288 historical reasons.
14292 @node Examples of date
14293 @subsection Examples of @command{date}
14295 @cindex examples of @command{date}
14297 Here are a few examples. Also see the documentation for the @option{-d}
14298 option in the previous section.
14303 To print the date of the day before yesterday:
14306 date --date='2 days ago'
14310 To print the date of the day three months and one day hence:
14313 date --date='3 months 1 day'
14317 To print the day of year of Christmas in the current year:
14320 date --date='25 Dec' +%j
14324 To print the current full month name and the day of the month:
14330 But this may not be what you want because for the first nine days of
14331 the month, the @samp{%d} expands to a zero-padded two-digit field,
14332 for example @samp{date -d 1may '+%B %d'} will print @samp{May 01}.
14335 To print a date without the leading zero for one-digit days
14336 of the month, you can use the (@acronym{GNU} extension)
14337 @samp{-} flag to suppress
14338 the padding altogether:
14341 date -d 1may '+%B %-d
14345 To print the current date and time in the format required by many
14346 non-@acronym{GNU} versions of @command{date} when setting the system clock:
14349 date +%m%d%H%M%Y.%S
14353 To set the system clock forward by two minutes:
14356 date --set='+2 minutes'
14360 To print the date in @acronym{RFC} 2822 format,
14361 use @samp{date --rfc-2822}. Here is some example output:
14364 Fri, 09 Sep 2005 13:51:39 -0700
14367 @anchor{%s-examples}
14369 To convert a date string to the number of seconds since the epoch
14370 (which is 1970-01-01 00:00:00 UTC), use the @option{--date} option with
14371 the @samp{%s} format. That can be useful in sorting and/or graphing
14372 and/or comparing data by date. The following command outputs the
14373 number of the seconds since the epoch for the time two minutes after the
14377 date --date='1970-01-01 00:02:00 +0000' +%s
14381 If you do not specify time zone information in the date string,
14382 @command{date} uses your computer's idea of the time zone when
14383 interpreting the string. For example, if your computer's time zone is
14384 that of Cambridge, Massachusetts, which was then 5 hours (i.e., 18,000
14385 seconds) behind UTC:
14388 # local time zone used
14389 date --date='1970-01-01 00:02:00' +%s
14394 If you're sorting or graphing dated data, your raw date values may be
14395 represented as seconds since the epoch. But few people can look at
14396 the date @samp{946684800} and casually note ``Oh, that's the first second
14397 of the year 2000 in Greenwich, England.''
14400 date --date='2000-01-01 UTC' +%s
14404 An alternative is to use the @option{--utc} (@option{-u}) option.
14405 Then you may omit @samp{UTC} from the date string. Although this
14406 produces the same result for @samp{%s} and many other format sequences,
14407 with a time zone offset different from zero, it would give a different
14408 result for zone-dependent formats like @samp{%z}.
14411 date -u --date=2000-01-01 +%s
14415 To convert such an unwieldy number of seconds back to
14416 a more readable form, use a command like this:
14419 # local time zone used
14420 date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
14421 1999-12-31 19:00:00 -0500
14424 Or if you do not mind depending on the @samp{@@} feature present since
14425 coreutils 5.3.0, you could shorten this to:
14428 date -d @@946684800 +"%F %T %z"
14429 1999-12-31 19:00:00 -0500
14432 Often it is better to output UTC-relative date and time:
14435 date -u -d '1970-01-01 946684800 seconds' +"%Y-%m-%d %T %z"
14436 2000-01-01 00:00:00 +0000
14442 @node arch invocation
14443 @section @command{arch}: Print machine hardware name
14446 @cindex print machine hardware name
14447 @cindex system information, printing
14449 @command{arch} prints the machine hardware name,
14450 and is equivalent to @samp{uname -m}.
14454 arch [@var{option}]
14457 The program accepts the @ref{Common options} only.
14462 @node nproc invocation
14463 @section @command{nproc}: Print the number of available processors
14466 @cindex Print the number of processors
14467 @cindex system information, printing
14469 Print the number of processing units available to the current process,
14470 which may be less than the number of online processors.
14471 If this information is not accessible, then print the number of
14472 processors installed. If the @env{OMP_NUM_THREADS} environment variable is
14473 set, then it will determine the returned value. The result is guaranteed to be
14474 greater than zero. Synopsis:
14477 nproc [@var{option}]
14480 The program accepts the following options. Also see @ref{Common options}.
14486 Print the number of installed processors on the system, which may
14487 be greater than the number online or available to the current process.
14488 The @env{OMP_NUM_THREADS} environment variable is not honored in this case.
14490 @item --ignore=@var{number}
14492 If possible, exclude this @var{number} of processing units.
14499 @node uname invocation
14500 @section @command{uname}: Print system information
14503 @cindex print system information
14504 @cindex system information, printing
14506 @command{uname} prints information about the machine and operating system
14507 it is run on. If no options are given, @command{uname} acts as if the
14508 @option{-s} option were given. Synopsis:
14511 uname [@var{option}]@dots{}
14514 If multiple options or @option{-a} are given, the selected information is
14515 printed in this order:
14518 @var{kernel-name} @var{nodename} @var{kernel-release} @var{kernel-version}
14519 @var{machine} @var{processor} @var{hardware-platform} @var{operating-system}
14522 The information may contain internal spaces, so such output cannot be
14523 parsed reliably. In the following example, @var{release} is
14524 @samp{2.2.18ss.e820-bda652a #4 SMP Tue Jun 5 11:24:08 PDT 2001}:
14528 @result{} Linux dum 2.2.18 #4 SMP Tue Jun 5 11:24:08 PDT 2001 i686@c
14529 unknown unknown GNU/Linux
14533 The program accepts the following options. Also see @ref{Common options}.
14541 Print all of the below information, except omit the processor type
14542 and the hardware platform name if they are unknown.
14545 @itemx --hardware-platform
14547 @opindex --hardware-platform
14548 @cindex implementation, hardware
14549 @cindex hardware platform
14550 @cindex platform, hardware
14551 Print the hardware platform name
14552 (sometimes called the hardware implementation).
14553 Print @samp{unknown} if the kernel does not make this information
14554 easily available, as is the case with Linux kernels.
14560 @cindex machine type
14561 @cindex hardware class
14562 @cindex hardware type
14563 Print the machine hardware name (sometimes called the hardware class
14569 @opindex --nodename
14572 @cindex network node name
14573 Print the network node hostname.
14578 @opindex --processor
14579 @cindex host processor type
14580 Print the processor type (sometimes called the instruction set
14581 architecture or ISA).
14582 Print @samp{unknown} if the kernel does not make this information
14583 easily available, as is the case with Linux kernels.
14586 @itemx --operating-system
14588 @opindex --operating-system
14589 @cindex operating system name
14590 Print the name of the operating system.
14593 @itemx --kernel-release
14595 @opindex --kernel-release
14596 @cindex kernel release
14597 @cindex release of kernel
14598 Print the kernel release.
14601 @itemx --kernel-name
14603 @opindex --kernel-name
14604 @cindex kernel name
14605 @cindex name of kernel
14606 Print the kernel name.
14607 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) calls this
14608 ``the implementation of the operating system'', because the
14609 @acronym{POSIX} specification itself has no notion of ``kernel''.
14610 The kernel name might be the same as the operating system name printed
14611 by the @option{-o} or @option{--operating-system} option, but it might
14612 differ. Some operating systems (e.g., FreeBSD, HP-UX) have the same
14613 name as their underlying kernels; others (e.g., GNU/Linux, Solaris)
14617 @itemx --kernel-version
14619 @opindex --kernel-version
14620 @cindex kernel version
14621 @cindex version of kernel
14622 Print the kernel version.
14629 @node hostname invocation
14630 @section @command{hostname}: Print or set system name
14633 @cindex setting the hostname
14634 @cindex printing the hostname
14635 @cindex system name, printing
14636 @cindex appropriate privileges
14638 With no arguments, @command{hostname} prints the name of the current host
14639 system. With one argument, it sets the current host name to the
14640 specified string. You must have appropriate privileges to set the host
14644 hostname [@var{name}]
14647 The only options are @option{--help} and @option{--version}. @xref{Common
14653 @node hostid invocation
14654 @section @command{hostid}: Print numeric host identifier
14657 @cindex printing the host identifier
14659 @command{hostid} prints the numeric identifier of the current host
14660 in hexadecimal. This command accepts no arguments.
14661 The only options are @option{--help} and @option{--version}.
14662 @xref{Common options}.
14664 For example, here's what it prints on one system I use:
14671 On that system, the 32-bit quantity happens to be closely
14672 related to the system's Internet address, but that isn't always
14677 @node uptime invocation
14678 @section @command{uptime}: Print system uptime and load
14681 @cindex printing the system uptime and load
14683 @command{uptime} prints the current time, the system's uptime, the
14684 number of logged-in users and the current load average.
14686 If an argument is specified, it is used as the file to be read
14687 to discover how many users are logged in. If no argument is
14688 specified, a system default is used (@command{uptime --help} indicates
14689 the default setting).
14691 The only options are @option{--help} and @option{--version}.
14692 @xref{Common options}.
14694 For example, here's what it prints right now on one system I use:
14698 14:07 up 3:35, 3 users, load average: 1.39, 1.15, 1.04
14701 The precise method of calculation of load average varies somewhat
14702 between systems. Some systems calculate it as the average number of
14703 runnable processes over the last 1, 5 and 15 minutes, but some systems
14704 also include processes in the uninterruptible sleep state (that is,
14705 those processes which are waiting for disk I/O). The Linux kernel
14706 includes uninterruptible processes.
14708 @node SELinux context
14709 @chapter SELinux context
14711 @cindex SELinux context
14712 @cindex SELinux, context
14713 @cindex commands for SELinux context
14715 This section describes commands for operations with SELinux
14719 * chcon invocation:: Change SELinux context of file
14720 * runcon invocation:: Run a command in specified SELinux context
14723 @node chcon invocation
14724 @section @command{chcon}: Change SELinux context of file
14727 @cindex changing security context
14728 @cindex change SELinux context
14730 @command{chcon} changes the SELinux security context of the selected files.
14734 chcon [@var{option}]@dots{} @var{context} @var{file}@dots{}
14735 chcon [@var{option}]@dots{} [-u @var{user}] [-r @var{role}] [-l @var{range}]@c
14736 [-t @var{type}] @var{file}@dots{}
14737 chcon [@var{option}]@dots{} --reference=@var{rfile} @var{file}@dots{}
14740 Change the SELinux security context of each @var{file} to @var{context}.
14741 With @option{--reference}, change the security context of each @var{file}
14742 to that of @var{rfile}.
14744 The program accepts the following options. Also see @ref{Common options}.
14749 @itemx --no-dereference
14751 @opindex --no-dereference
14752 @cindex no dereference
14753 Affect symbolic links instead of any referenced file.
14755 @item --reference=@var{rfile}
14756 @opindex --reference
14757 @cindex reference file
14758 Use @var{rfile}'s security context rather than specifying a @var{context} value.
14763 @opindex --recursive
14764 Operate on files and directories recursively.
14767 @xref{Traversing symlinks}.
14770 @xref{Traversing symlinks}.
14773 @xref{Traversing symlinks}.
14780 Output a diagnostic for every file processed.
14782 @item -u @var{user}
14783 @itemx --user=@var{user}
14786 Set user @var{user} in the target security context.
14788 @item -r @var{role}
14789 @itemx --role=@var{role}
14792 Set role @var{role} in the target security context.
14794 @item -t @var{type}
14795 @itemx --type=@var{type}
14798 Set type @var{type} in the target security context.
14800 @item -l @var{range}
14801 @itemx --range=@var{range}
14804 Set range @var{range} in the target security context.
14810 @node runcon invocation
14811 @section @command{runcon}: Run a command in specified SELinux context
14814 @cindex run with security context
14817 @command{runcon} runs file in specified SELinux security context.
14821 runcon @var{context} @var{command} [@var{args}]
14822 runcon [ -c ] [-u @var{user}] [-r @var{role}] [-t @var{type}]@c
14823 [-l @var{range}] @var{command} [@var{args}]
14826 Run @var{command} with completely-specified @var{context}, or with
14827 current or transitioned security context modified by one or more of @var{level},
14828 @var{role}, @var{type} and @var{user}.
14830 If none of @option{-c}, @option{-t}, @option{-u}, @option{-r}, or @option{-l}
14831 is specified, the first argument is used as the complete context.
14832 Any additional arguments after @var{command}
14833 are interpreted as arguments to the command.
14835 With neither @var{context} nor @var{command}, print the current
14838 The program accepts the following options. Also see @ref{Common options}.
14846 Compute process transition context before modifying.
14848 @item -u @var{user}
14849 @itemx --user=@var{user}
14852 Set user @var{user} in the target security context.
14854 @item -r @var{role}
14855 @itemx --role=@var{role}
14858 Set role @var{role} in the target security context.
14860 @item -t @var{type}
14861 @itemx --type=@var{type}
14864 Set type @var{type} in the target security context.
14866 @item -l @var{range}
14867 @itemx --range=@var{range}
14870 Set range @var{range} in the target security context.
14874 @cindex exit status of @command{runcon}
14878 126 if @var{command} is found but cannot be invoked
14879 127 if @command{runcon} itself fails or if @var{command} cannot be found
14880 the exit status of @var{command} otherwise
14883 @node Modified command invocation
14884 @chapter Modified command invocation
14886 @cindex modified command invocation
14887 @cindex invocation of commands, modified
14888 @cindex commands for invoking other commands
14890 This section describes commands that run other commands in some context
14891 different than the current one: a modified environment, as a different
14895 * chroot invocation:: Modify the root directory.
14896 * env invocation:: Modify environment variables.
14897 * nice invocation:: Modify niceness.
14898 * nohup invocation:: Immunize to hangups.
14899 * stdbuf invocation:: Modify buffering of standard streams.
14900 * su invocation:: Modify user and group ID.
14901 * timeout invocation:: Run with time limit.
14905 @node chroot invocation
14906 @section @command{chroot}: Run a command with a different root directory
14909 @cindex running a program in a specified root directory
14910 @cindex root directory, running a program in a specified
14912 @command{chroot} runs a command with a specified root directory.
14913 On many systems, only the super-user can do this.@footnote{However,
14914 some systems (e.g., FreeBSD) can be configured to allow certain regular
14915 users to use the @code{chroot} system call, and hence to run this program.
14916 Also, on Cygwin, anyone can run the @command{chroot} command, because the
14917 underlying function is non-privileged due to lack of support in MS-Windows.}
14921 chroot @var{option} @var{newroot} [@var{command} [@var{args}]@dots{}]
14922 chroot @var{option}
14925 Ordinarily, file names are looked up starting at the root of the
14926 directory structure, i.e., @file{/}. @command{chroot} changes the root to
14927 the directory @var{newroot} (which must exist) and then runs
14928 @var{command} with optional @var{args}. If @var{command} is not
14929 specified, the default is the value of the @env{SHELL} environment
14930 variable or @command{/bin/sh} if not set, invoked with the @option{-i} option.
14931 @var{command} must not be a special built-in utility
14932 (@pxref{Special built-in utilities}).
14934 The program accepts the following options. Also see @ref{Common options}.
14935 Options must precede operands.
14939 @itemx --userspec=@var{user}[:@var{group}]
14940 @opindex --userspec
14941 By default, @var{command} is run with the same credentials
14942 as the invoking process.
14943 Use this option to run it as a different @var{user} and/or with a
14944 different primary @var{group}.
14946 @itemx --groups=@var{groups}
14948 Use this option to specify the supplementary @var{groups} to be
14949 used by the new process.
14950 The items in the list (names or numeric IDs) must be separated by commas.
14954 Here are a few tips to help avoid common problems in using chroot.
14955 To start with a simple example, make @var{command} refer to a statically
14956 linked binary. If you were to use a dynamically linked executable, then
14957 you'd have to arrange to have the shared libraries in the right place under
14958 your new root directory.
14960 For example, if you create a statically linked @command{ls} executable,
14961 and put it in @file{/tmp/empty}, you can run this command as root:
14964 $ chroot /tmp/empty /ls -Rl /
14967 Then you'll see output like this:
14972 -rwxr-xr-x 1 0 0 1041745 Aug 16 11:17 ls
14975 If you want to use a dynamically linked executable, say @command{bash},
14976 then first run @samp{ldd bash} to see what shared objects it needs.
14977 Then, in addition to copying the actual binary, also copy the listed
14978 files to the required positions under your intended new root directory.
14979 Finally, if the executable requires any other files (e.g., data, state,
14980 device files), copy them into place, too.
14982 @cindex exit status of @command{chroot}
14986 125 if @command{chroot} itself fails
14987 126 if @var{command} is found but cannot be invoked
14988 127 if @var{command} cannot be found
14989 the exit status of @var{command} otherwise
14993 @node env invocation
14994 @section @command{env}: Run a command in a modified environment
14997 @cindex environment, running a program in a modified
14998 @cindex modified environment, running a program in a
14999 @cindex running a program in a modified environment
15001 @command{env} runs a command with a modified environment. Synopses:
15004 env [@var{option}]@dots{} [@var{name}=@var{value}]@dots{} @c
15005 [@var{command} [@var{args}]@dots{}]
15009 Operands of the form @samp{@var{variable}=@var{value}} set
15010 the environment variable @var{variable} to value @var{value}.
15011 @var{value} may be empty (@samp{@var{variable}=}). Setting a variable
15012 to an empty value is different from unsetting it.
15013 These operands are evaluated left-to-right, so if two operands
15014 mention the same variable the earlier is ignored.
15016 Environment variable names can be empty, and can contain any
15017 characters other than @samp{=} and @acronym{ASCII} @sc{nul}.
15018 However, it is wise to limit yourself to names that
15019 consist solely of underscores, digits, and @acronym{ASCII} letters,
15020 and that begin with a non-digit, as applications like the shell do not
15021 work well with other names.
15024 The first operand that does not contain the character @samp{=}
15025 specifies the program to invoke; it is
15026 searched for according to the @env{PATH} environment variable. Any
15027 remaining arguments are passed as arguments to that program.
15028 The program should not be a special built-in utility
15029 (@pxref{Special built-in utilities}).
15031 Modifications to @env{PATH} take effect prior to searching for
15032 @var{command}. Use caution when reducing @env{PATH}; behavior is
15033 not portable when @env{PATH} is undefined or omits key directories
15034 such as @file{/bin}.
15036 In the rare case that a utility contains a @samp{=} in the name, the
15037 only way to disambiguate it from a variable assignment is to use an
15038 intermediate command for @var{command}, and pass the problematic
15039 program name via @var{args}. For example, if @file{./prog=} is an
15040 executable in the current @env{PATH}:
15043 env prog= true # runs 'true', with prog= in environment
15044 env ./prog= true # runs 'true', with ./prog= in environment
15045 env -- prog= true # runs 'true', with prog= in environment
15046 env sh -c '\prog= true' # runs 'prog=' with argument 'true'
15047 env sh -c 'exec "$@@"' sh prog= true # also runs 'prog='
15050 @cindex environment, printing
15052 If no command name is specified following the environment
15053 specifications, the resulting environment is printed. This is like
15054 specifying the @command{printenv} program.
15056 For some examples, suppose the environment passed to @command{env}
15057 contains @samp{LOGNAME=rms}, @samp{EDITOR=emacs}, and
15058 @samp{PATH=.:/gnubin:/hacks}:
15063 Output the current environment.
15065 $ env | LC_ALL=C sort
15068 PATH=.:/gnubin:/hacks
15072 Run @command{foo} with a reduced environment, preserving only the
15073 original @env{PATH} to avoid problems in locating @command{foo}.
15075 env - PATH="$PATH" foo
15079 Run @command{foo} with the environment containing @samp{LOGNAME=rms},
15080 @samp{EDITOR=emacs}, and @samp{PATH=.:/gnubin:/hacks}, and guarantees
15081 that @command{foo} was found in the file system rather than as a shell
15088 Run @command{nemacs} with the environment containing @samp{LOGNAME=foo},
15089 @samp{EDITOR=emacs}, @samp{PATH=.:/gnubin:/hacks}, and
15090 @samp{DISPLAY=gnu:0}.
15092 env DISPLAY=gnu:0 LOGNAME=foo nemacs
15096 Attempt to run the program @command{/energy/--} (as that is the only
15097 possible path search result); if the command exists, the environment
15098 will contain @samp{LOGNAME=rms} and @samp{PATH=/energy}, and the
15099 arguments will be @samp{e=mc2}, @samp{bar}, and @samp{baz}.
15101 env -u EDITOR PATH=/energy -- e=mc2 bar baz
15107 The program accepts the following options. Also see @ref{Common options}.
15108 Options must precede operands.
15114 @item -u @var{name}
15115 @itemx --unset=@var{name}
15118 Remove variable @var{name} from the environment, if it was in the
15123 @itemx --ignore-environment
15126 @opindex --ignore-environment
15127 Start with an empty environment, ignoring the inherited environment.
15131 @cindex exit status of @command{env}
15135 0 if no @var{command} is specified and the environment is output
15136 125 if @command{env} itself fails
15137 126 if @var{command} is found but cannot be invoked
15138 127 if @var{command} cannot be found
15139 the exit status of @var{command} otherwise
15143 @node nice invocation
15144 @section @command{nice}: Run a command with modified niceness
15148 @cindex scheduling, affecting
15149 @cindex appropriate privileges
15151 @command{nice} prints or modifies a process's @dfn{niceness},
15152 a parameter that affects whether the process is scheduled favorably.
15156 nice [@var{option}]@dots{} [@var{command} [@var{arg}]@dots{}]
15159 If no arguments are given, @command{nice} prints the current niceness.
15160 Otherwise, @command{nice} runs the given @var{command} with its
15161 niceness adjusted. By default, its niceness is incremented by 10.
15163 Niceness values range at least from @minus{}20 (process has high priority
15164 and gets more resources, thus slowing down other processes) through 19
15165 (process has lower priority and runs slowly itself, but has less impact
15166 on the speed of other running processes). Some systems
15167 may have a wider range of nicenesses; conversely, other systems may
15168 enforce more restrictive limits. An attempt to set the niceness
15169 outside the supported range is treated as an attempt to use the
15170 minimum or maximum supported value.
15172 A niceness should not be confused with a scheduling priority, which
15173 lets applications determine the order in which threads are scheduled
15174 to run. Unlike a priority, a niceness is merely advice to the
15175 scheduler, which the scheduler is free to ignore. Also, as a point of
15176 terminology, @acronym{POSIX} defines the behavior of @command{nice} in
15177 terms of a @dfn{nice value}, which is the nonnegative difference
15178 between a niceness and the minimum niceness. Though @command{nice}
15179 conforms to @acronym{POSIX}, its documentation and diagnostics use the
15180 term ``niceness'' for compatibility with historical practice.
15182 @var{command} must not be a special built-in utility (@pxref{Special
15183 built-in utilities}).
15185 @mayConflictWithShellBuiltIn{nice}
15187 The program accepts the following option. Also see @ref{Common options}.
15188 Options must precede operands.
15191 @item -n @var{adjustment}
15192 @itemx --adjustment=@var{adjustment}
15194 @opindex --adjustment
15195 Add @var{adjustment} instead of 10 to the command's niceness. If
15196 @var{adjustment} is negative and you lack appropriate privileges,
15197 @command{nice} issues a warning but otherwise acts as if you specified
15200 For compatibility @command{nice} also supports an obsolete
15201 option syntax @option{-@var{adjustment}}. New scripts should use
15202 @option{-n @var{adjustment}} instead.
15206 @cindex exit status of @command{nice}
15210 0 if no @var{command} is specified and the niceness is output
15211 125 if @command{nice} itself fails
15212 126 if @var{command} is found but cannot be invoked
15213 127 if @var{command} cannot be found
15214 the exit status of @var{command} otherwise
15217 It is sometimes useful to run a non-interactive program with reduced niceness.
15220 $ nice factor 4611686018427387903
15223 Since @command{nice} prints the current niceness,
15224 you can invoke it through itself to demonstrate how it works.
15226 The default behavior is to increase the niceness by @samp{10}:
15237 The @var{adjustment} is relative to the current niceness. In the
15238 next example, the first @command{nice} invocation runs the second one
15239 with niceness 10, and it in turn runs the final one with a niceness
15243 $ nice nice -n 3 nice
15247 Specifying a niceness larger than the supported range
15248 is the same as specifying the maximum supported value:
15251 $ nice -n 10000000000 nice
15255 Only a privileged user may run a process with lower niceness:
15259 nice: cannot set niceness: Permission denied
15261 $ sudo nice -n -1 nice
15266 @node nohup invocation
15267 @section @command{nohup}: Run a command immune to hangups
15270 @cindex hangups, immunity to
15271 @cindex immunity to hangups
15272 @cindex logging out and continuing to run
15275 @command{nohup} runs the given @var{command} with hangup signals ignored,
15276 so that the command can continue running in the background after you log
15280 nohup @var{command} [@var{arg}]@dots{}
15283 If standard input is a terminal, it is redirected from
15284 @file{/dev/null} so that terminal sessions do not mistakenly consider
15285 the terminal to be used by the command. This is a @acronym{GNU}
15286 extension; programs intended to be portable to non-@acronym{GNU} hosts
15287 should use @samp{nohup @var{command} [@var{arg}]@dots{} </dev/null}
15291 If standard output is a terminal, the command's standard output is appended
15292 to the file @file{nohup.out}; if that cannot be written to, it is appended
15293 to the file @file{$HOME/nohup.out}; and if that cannot be written to, the
15294 command is not run.
15295 Any @file{nohup.out} or @file{$HOME/nohup.out} file created by
15296 @command{nohup} is made readable and writable only to the user,
15297 regardless of the current umask settings.
15299 If standard error is a terminal, it is normally redirected to the same file
15300 descriptor as the (possibly-redirected) standard output.
15301 However, if standard output is closed, standard error terminal output
15302 is instead appended to the file @file{nohup.out} or
15303 @file{$HOME/nohup.out} as above.
15305 To capture the command's output to a file other than @file{nohup.out}
15306 you can redirect it. For example, to capture the output of
15310 nohup make > make.log
15313 @command{nohup} does not automatically put the command it runs in the
15314 background; you must do that explicitly, by ending the command line
15315 with an @samp{&}. Also, @command{nohup} does not alter the
15316 niceness of @var{command}; use @command{nice} for that,
15317 e.g., @samp{nohup nice @var{command}}.
15319 @var{command} must not be a special built-in utility (@pxref{Special
15320 built-in utilities}).
15322 The only options are @option{--help} and @option{--version}. @xref{Common
15323 options}. Options must precede operands.
15325 @cindex exit status of @command{nohup}
15329 125 if @command{nohup} itself fails, and @env{POSIXLY_CORRECT} is not set
15330 126 if @var{command} is found but cannot be invoked
15331 127 if @var{command} cannot be found
15332 the exit status of @var{command} otherwise
15335 If @env{POSIXLY_CORRECT} is set, internal failures give status 127
15339 @node stdbuf invocation
15340 @section @command{stdbuf}: Run a command with modified I/O stream buffering
15343 @cindex standard streams, buffering
15344 @cindex line buffered
15346 @command{stdbuf} allows one to modify the buffering operations of the
15347 three standard I/O streams associated with a program. Synopsis:
15350 stdbuf @var{option}@dots{} @var{command}
15353 Any additional @var{arg}s are passed as additional arguments to the
15356 The program accepts the following options. Also see @ref{Common options}.
15360 @item -i @var{mode}
15361 @itemx --input=@var{mode}
15364 Adjust the standard input stream buffering.
15366 @item -o @var{mode}
15367 @itemx --output=@var{mode}
15370 Adjust the standard output stream buffering.
15372 @item -e @var{mode}
15373 @itemx --error=@var{mode}
15376 Adjust the standard error stream buffering.
15380 The @var{mode} can be specified as follows:
15385 Set the stream to line buffered mode.
15386 In this mode data is coalesced until a newline is output or
15387 input is read from any stream attached to a terminal device.
15388 This option is invalid with standard input.
15391 Disable buffering of the selected stream.
15392 In this mode data is output immediately and only the
15393 amount of data requested is read from input.
15396 Specify the size of the buffer to use in fully buffered mode.
15397 @multiplierSuffixesNoBlocks{size}
15401 NOTE: If @var{command} adjusts the buffering of its standard streams
15402 (@command{tee} does for e.g.) then that will override corresponding settings
15403 changed by @command{stdbuf}. Also some filters (like @command{dd} and
15404 @command{cat} etc.) don't use streams for I/O, and are thus unaffected
15405 by @command{stdbuf} settings.
15407 @cindex exit status of @command{stdbuf}
15411 125 if @command{stdbuf} itself fails
15412 126 if @var{command} is found but cannot be invoked
15413 127 if @var{command} cannot be found
15414 the exit status of @var{command} otherwise
15418 @node su invocation
15419 @section @command{su}: Run a command with substitute user and group ID
15422 @cindex substitute user and group IDs
15423 @cindex user ID, switching
15424 @cindex super-user, becoming
15425 @cindex root, becoming
15427 @command{su} allows one user to temporarily become another user. It runs a
15428 command (often an interactive shell) with the real and effective user
15429 ID, group ID, and supplemental groups of a given @var{user}. Synopsis:
15432 su [@var{option}]@dots{} [@var{user} [@var{arg}]@dots{}]
15435 @cindex passwd entry, and @command{su} shell
15437 @flindex /etc/passwd
15438 If no @var{user} is given, the default is @code{root}, the super-user.
15439 The shell to use is taken from @var{user}'s @code{passwd} entry, or
15440 @file{/bin/sh} if none is specified there. If @var{user} has a
15441 password, @command{su} prompts for the password unless run by a user with
15442 effective user ID of zero (the super-user).
15448 @cindex login shell
15449 By default, @command{su} does not change the current directory.
15450 It sets the environment variables @env{HOME} and @env{SHELL}
15451 from the password entry for @var{user}, and if @var{user} is not
15452 the super-user, sets @env{USER} and @env{LOGNAME} to @var{user}.
15453 By default, the shell is not a login shell.
15455 Any additional @var{arg}s are passed as additional arguments to the
15458 @cindex @option{-su}
15459 GNU @command{su} does not treat @file{/bin/sh} or any other shells specially
15460 (e.g., by setting @code{argv[0]} to @option{-su}, passing @option{-c} only
15461 to certain shells, etc.).
15464 @command{su} can optionally be compiled to use @code{syslog} to report
15465 failed, and optionally successful, @command{su} attempts. (If the system
15466 supports @code{syslog}.) However, GNU @command{su} does not check if the
15467 user is a member of the @code{wheel} group; see below.
15469 The program accepts the following options. Also see @ref{Common options}.
15472 @item -c @var{command}
15473 @itemx --command=@var{command}
15476 Pass @var{command}, a single command line to run, to the shell with
15477 a @option{-c} option instead of starting an interactive shell.
15484 @cindex file name pattern expansion, disabled
15485 @cindex globbing, disabled
15486 Pass the @option{-f} option to the shell. This probably only makes sense
15487 if the shell run is @command{csh} or @command{tcsh}, for which the @option{-f}
15488 option prevents reading the startup file (@file{.cshrc}). With
15489 Bourne-like shells, the @option{-f} option disables file name pattern
15490 expansion (globbing), which is not likely to be useful.
15498 @c other variables already indexed above
15501 @cindex login shell, creating
15502 Make the shell a login shell. This means the following. Unset all
15503 environment variables except @env{TERM}, @env{HOME}, and @env{SHELL}
15504 (which are set as described above), and @env{USER} and @env{LOGNAME}
15505 (which are set, even for the super-user, as described above), and set
15506 @env{PATH} to a compiled-in default value. Change to @var{user}'s home
15507 directory. Prepend @samp{-} to the shell's name, intended to make it
15508 read its login startup file(s).
15512 @itemx --preserve-environment
15515 @opindex --preserve-environment
15516 @cindex environment, preserving
15517 @flindex /etc/shells
15518 @cindex restricted shell
15519 Do not change the environment variables @env{HOME}, @env{USER},
15520 @env{LOGNAME}, or @env{SHELL}. Run the shell given in the environment
15521 variable @env{SHELL} instead of the shell from @var{user}'s passwd
15522 entry, unless the user running @command{su} is not the super-user and
15523 @var{user}'s shell is restricted. A @dfn{restricted shell} is one that
15524 is not listed in the file @file{/etc/shells}, or in a compiled-in list
15525 if that file does not exist. Parts of what this option does can be
15526 overridden by @option{--login} and @option{--shell}.
15528 @item -s @var{shell}
15529 @itemx --shell=@var{shell}
15532 Run @var{shell} instead of the shell from @var{user}'s passwd entry,
15533 unless the user running @command{su} is not the super-user and @var{user}'s
15534 shell is restricted (see @option{-m} just above).
15538 @cindex exit status of @command{su}
15542 125 if @command{su} itself fails
15543 126 if subshell is found but cannot be invoked
15544 127 if subshell cannot be found
15545 the exit status of the subshell otherwise
15548 @cindex wheel group, not supported
15549 @cindex group wheel, not supported
15551 @subsection Why GNU @command{su} does not support the @samp{wheel} group
15553 (This section is by Richard Stallman.)
15557 Sometimes a few of the users try to hold total power over all the
15558 rest. For example, in 1984, a few users at the MIT AI lab decided to
15559 seize power by changing the operator password on the Twenex system and
15560 keeping it secret from everyone else. (I was able to thwart this coup
15561 and give power back to the users by patching the kernel, but I
15562 wouldn't know how to do that in Unix.)
15564 However, occasionally the rulers do tell someone. Under the usual
15565 @command{su} mechanism, once someone learns the root password who
15566 sympathizes with the ordinary users, he or she can tell the rest. The
15567 ``wheel group'' feature would make this impossible, and thus cement the
15568 power of the rulers.
15570 I'm on the side of the masses, not that of the rulers. If you are
15571 used to supporting the bosses and sysadmins in whatever they do, you
15572 might find this idea strange at first.
15575 @node timeout invocation
15576 @section @command{timeout}: Run a command with a time limit
15580 @cindex run commands with bounded time
15582 @command{timeout} runs the given @var{command} and kills it if it is
15583 still running after the specified time interval. Synopsis:
15586 timeout [@var{option}] @var{duration} @var{command} [@var{arg}]@dots{}
15589 @var{command} must not be a special built-in utility (@pxref{Special
15590 built-in utilities}).
15592 The program accepts the following options. Also see @ref{Common options}.
15593 Options must precede operands.
15596 @item -k @var{duration}
15597 @itemx --kill-after=@var{duration}
15599 @opindex --kill-after
15600 Ensure the monitored @var{command} is killed by also sending a @samp{KILL}
15601 signal, after the specified @var{duration}. Without this option, if the
15602 selected signal proves not to be fatal, @command{timeout} does not kill
15605 @item -s @var{signal}
15606 @itemx --signal=@var{signal}
15609 Send this @var{signal} to @var{command} on timeout, rather than the
15610 default @samp{TERM} signal. @var{signal} may be a name like @samp{HUP}
15611 or a number. Also see @xref{Signal specifications}.
15615 @var{duration} is an integer followed by an optional unit:
15617 @samp{s} for seconds (the default)
15618 @samp{m} for minutes
15622 A duration of 0 disables the associated timeout.
15624 @cindex exit status of @command{timeout}
15628 124 if @var{command} times out
15629 125 if @command{timeout} itself fails
15630 126 if @var{command} is found but cannot be invoked
15631 127 if @var{command} cannot be found
15632 the exit status of @var{command} otherwise
15636 @node Process control
15637 @chapter Process control
15639 @cindex processes, commands for controlling
15640 @cindex commands for controlling processes
15643 * kill invocation:: Sending a signal to processes.
15647 @node kill invocation
15648 @section @command{kill}: Send a signal to processes
15651 @cindex send a signal to processes
15653 The @command{kill} command sends a signal to processes, causing them
15654 to terminate or otherwise act upon receiving the signal in some way.
15655 Alternatively, it lists information about signals. Synopses:
15658 kill [-s @var{signal} | --signal @var{signal} | -@var{signal}] @var{pid}@dots{}
15659 kill [-l | --list | -t | --table] [@var{signal}]@dots{}
15662 @mayConflictWithShellBuiltIn{kill}
15664 The first form of the @command{kill} command sends a signal to all
15665 @var{pid} arguments. The default signal to send if none is specified
15666 is @samp{TERM}. The special signal number @samp{0} does not denote a
15667 valid signal, but can be used to test whether the @var{pid} arguments
15668 specify processes to which a signal could be sent.
15670 If @var{pid} is positive, the signal is sent to the process with the
15671 process ID @var{pid}. If @var{pid} is zero, the signal is sent to all
15672 processes in the process group of the current process. If @var{pid}
15673 is @minus{}1, the signal is sent to all processes for which the user has
15674 permission to send a signal. If @var{pid} is less than @minus{}1, the signal
15675 is sent to all processes in the process group that equals the absolute
15676 value of @var{pid}.
15678 If @var{pid} is not positive, a system-dependent set of system
15679 processes is excluded from the list of processes to which the signal
15682 If a negative @var{pid} argument is desired as the first one, it
15683 should be preceded by @option{--}. However, as a common extension to
15684 @acronym{POSIX}, @option{--} is not required with @samp{kill
15685 -@var{signal} -@var{pid}}. The following commands are equivalent:
15694 The first form of the @command{kill} command succeeds if every @var{pid}
15695 argument specifies at least one process that the signal was sent to.
15697 The second form of the @command{kill} command lists signal information.
15698 Either the @option{-l} or @option{--list} option, or the @option{-t}
15699 or @option{--table} option must be specified. Without any
15700 @var{signal} argument, all supported signals are listed. The output
15701 of @option{-l} or @option{--list} is a list of the signal names, one
15702 per line; if @var{signal} is already a name, the signal number is
15703 printed instead. The output of @option{-t} or @option{--table} is a
15704 table of signal numbers, names, and descriptions. This form of the
15705 @command{kill} command succeeds if all @var{signal} arguments are valid
15706 and if there is no output error.
15708 The @command{kill} command also supports the @option{--help} and
15709 @option{--version} options. @xref{Common options}.
15711 A @var{signal} may be a signal name like @samp{HUP}, or a signal
15712 number like @samp{1}, or an exit status of a process terminated by the
15713 signal. A signal name can be given in canonical form or prefixed by
15714 @samp{SIG}. The case of the letters is ignored, except for the
15715 @option{-@var{signal}} option which must use upper case to avoid
15716 ambiguity with lower case option letters. For a list of supported
15717 signal names and numbers see @xref{Signal specifications}.
15722 @cindex delaying commands
15723 @cindex commands for delaying
15725 @c Perhaps @command{wait} or other commands should be described here also?
15728 * sleep invocation:: Delay for a specified time.
15732 @node sleep invocation
15733 @section @command{sleep}: Delay for a specified time
15736 @cindex delay for a specified time
15738 @command{sleep} pauses for an amount of time specified by the sum of
15739 the values of the command line arguments.
15743 sleep @var{number}[smhd]@dots{}
15747 Each argument is a number followed by an optional unit; the default
15748 is seconds. The units are:
15761 Historical implementations of @command{sleep} have required that
15762 @var{number} be an integer, and only accepted a single argument
15763 without a suffix. However, GNU @command{sleep} accepts
15764 arbitrary floating point numbers. @xref{Floating point}.
15766 The only options are @option{--help} and @option{--version}. @xref{Common
15769 @c sleep is a shell built-in at least with Solaris 11's /bin/sh
15770 @mayConflictWithShellBuiltIn{sleep}
15775 @node Numeric operations
15776 @chapter Numeric operations
15778 @cindex numeric operations
15779 These programs do numerically-related operations.
15782 * factor invocation:: Show factors of numbers.
15783 * seq invocation:: Print sequences of numbers.
15787 @node factor invocation
15788 @section @command{factor}: Print prime factors
15791 @cindex prime factors
15793 @command{factor} prints prime factors. Synopses:
15796 factor [@var{number}]@dots{}
15797 factor @var{option}
15800 If no @var{number} is specified on the command line, @command{factor} reads
15801 numbers from standard input, delimited by newlines, tabs, or spaces.
15803 The @command{factor} command supports only a small number of options:
15807 Print a short help on standard output, then exit without further
15811 Print the program version on standard output, then exit without further
15815 Factoring the product of the eighth and ninth Mersenne primes
15816 takes about 30 milliseconds of CPU time on a 2.2 GHz Athlon.
15819 M8=`echo 2^31-1|bc` ; M9=`echo 2^61-1|bc`
15820 /usr/bin/time -f '%U' factor $(echo "$M8 * $M9" | bc)
15821 4951760154835678088235319297: 2147483647 2305843009213693951
15825 Similarly, factoring the eighth Fermat number @math{2^{256}+1} takes
15826 about 20 seconds on the same machine.
15828 Factoring large numbers is, in general, hard. The Pollard Rho
15829 algorithm used by @command{factor} is particularly effective for
15830 numbers with relatively small factors. If you wish to factor large
15831 numbers which do not have small factors (for example, numbers which
15832 are the product of two large primes), other methods are far better.
15834 If @command{factor} is built without using GNU MP, only
15835 single-precision arithmetic is available, and so large numbers
15836 (typically @math{2^{64}} and above) will not be supported. The single-precision
15837 code uses an algorithm which is designed for factoring smaller
15843 @node seq invocation
15844 @section @command{seq}: Print numeric sequences
15847 @cindex numeric sequences
15848 @cindex sequence of numbers
15850 @command{seq} prints a sequence of numbers to standard output. Synopses:
15853 seq [@var{option}]@dots{} @var{last}
15854 seq [@var{option}]@dots{} @var{first} @var{last}
15855 seq [@var{option}]@dots{} @var{first} @var{increment} @var{last}
15858 @command{seq} prints the numbers from @var{first} to @var{last} by
15859 @var{increment}. By default, each number is printed on a separate line.
15860 When @var{increment} is not specified, it defaults to @samp{1},
15861 even when @var{first} is larger than @var{last}.
15862 @var{first} also defaults to @samp{1}. So @code{seq 1} prints
15863 @samp{1}, but @code{seq 0} and @code{seq 10 5} produce no output.
15864 Floating-point numbers may be specified. @xref{Floating point}.
15866 The program accepts the following options. Also see @ref{Common options}.
15867 Options must precede operands.
15870 @item -f @var{format}
15871 @itemx --format=@var{format}
15872 @opindex -f @var{format}
15873 @opindex --format=@var{format}
15874 @cindex formatting of numbers in @command{seq}
15875 Print all numbers using @var{format}.
15876 @var{format} must contain exactly one of the @samp{printf}-style
15877 floating point conversion specifications @samp{%a}, @samp{%e},
15878 @samp{%f}, @samp{%g}, @samp{%A}, @samp{%E}, @samp{%F}, @samp{%G}.
15879 The @samp{%} may be followed by zero or more flags taken from the set
15880 @samp{-+#0 '}, then an optional width containing one or more digits,
15881 then an optional precision consisting of a @samp{.} followed by zero
15882 or more digits. @var{format} may also contain any number of @samp{%%}
15883 conversion specifications. All conversion specifications have the
15884 same meaning as with @samp{printf}.
15886 The default format is derived from @var{first}, @var{step}, and
15887 @var{last}. If these all use a fixed point decimal representation,
15888 the default format is @samp{%.@var{p}f}, where @var{p} is the minimum
15889 precision that can represent the output numbers exactly. Otherwise,
15890 the default format is @samp{%g}.
15892 @item -s @var{string}
15893 @itemx --separator=@var{string}
15894 @cindex separator for numbers in @command{seq}
15895 Separate numbers with @var{string}; default is a newline.
15896 The output always terminates with a newline.
15899 @itemx --equal-width
15900 Print all numbers with the same width, by padding with leading zeros.
15901 @var{first}, @var{step}, and @var{last} should all use a fixed point
15902 decimal representation.
15903 (To have other kinds of padding, use @option{--format}).
15907 You can get finer-grained control over output with @option{-f}:
15910 $ seq -f '(%9.2E)' -9e5 1.1e6 1.3e6
15916 If you want hexadecimal integer output, you can use @command{printf}
15917 to perform the conversion:
15920 $ printf '%x\n' `seq 1048575 1024 1050623`
15926 For very long lists of numbers, use xargs to avoid
15927 system limitations on the length of an argument list:
15930 $ seq 1000000 | xargs printf '%x\n' | tail -n 3
15936 To generate octal output, use the printf @code{%o} format instead
15939 On most systems, seq can produce whole-number output for values up to
15940 at least @math{2^{53}}. Larger integers are approximated. The details
15941 differ depending on your floating-point implementation.
15942 @xref{Floating point}. A common
15943 case is that @command{seq} works with integers through @math{2^{64}},
15944 and larger integers may not be numerically correct:
15947 $ seq 18446744073709551616 1 18446744073709551618
15948 18446744073709551616
15949 18446744073709551616
15950 18446744073709551618
15953 Be careful when using @command{seq} with outlandish values: otherwise
15954 you may see surprising results, as @command{seq} uses floating point
15955 internally. For example, on the x86 platform, where the internal
15956 representation uses a 64-bit fraction, the command:
15959 seq 1 0.0000000000000000001 1.0000000000000000009
15962 outputs 1.0000000000000000007 twice and skips 1.0000000000000000008.
15967 @node File permissions
15968 @chapter File permissions
15971 @include parse-datetime.texi
15975 @node Opening the software toolbox
15976 @chapter Opening the Software Toolbox
15978 An earlier version of this chapter appeared in
15979 @uref{http://www.linuxjournal.com/article.php?sid=2762, the
15980 @cite{What's GNU?} column of @cite{Linux Journal}, 2 (June, 1994)}.
15981 It was written by Arnold Robbins.
15984 * Toolbox introduction:: Toolbox introduction
15985 * I/O redirection:: I/O redirection
15986 * The who command:: The @command{who} command
15987 * The cut command:: The @command{cut} command
15988 * The sort command:: The @command{sort} command
15989 * The uniq command:: The @command{uniq} command
15990 * Putting the tools together:: Putting the tools together
15994 @node Toolbox introduction
15995 @unnumberedsec Toolbox Introduction
15997 This month's column is only peripherally related to the GNU Project, in
15998 that it describes a number of the GNU tools on your GNU/Linux system
16000 might be used. What it's really about is the ``Software Tools'' philosophy
16001 of program development and usage.
16003 The software tools philosophy was an important and integral concept
16004 in the initial design and development of Unix (of which Linux and GNU are
16005 essentially clones). Unfortunately, in the modern day press of
16006 Internetworking and flashy GUIs, it seems to have fallen by the
16007 wayside. This is a shame, since it provides a powerful mental model
16008 for solving many kinds of problems.
16010 Many people carry a Swiss Army knife around in their pants pockets (or
16011 purse). A Swiss Army knife is a handy tool to have: it has several knife
16012 blades, a screwdriver, tweezers, toothpick, nail file, corkscrew, and perhaps
16013 a number of other things on it. For the everyday, small miscellaneous jobs
16014 where you need a simple, general purpose tool, it's just the thing.
16016 On the other hand, an experienced carpenter doesn't build a house using
16017 a Swiss Army knife. Instead, he has a toolbox chock full of specialized
16018 tools---a saw, a hammer, a screwdriver, a plane, and so on. And he knows
16019 exactly when and where to use each tool; you won't catch him hammering nails
16020 with the handle of his screwdriver.
16022 The Unix developers at Bell Labs were all professional programmers and trained
16023 computer scientists. They had found that while a one-size-fits-all program
16024 might appeal to a user because there's only one program to use, in practice
16029 difficult to write,
16032 difficult to maintain and
16036 difficult to extend to meet new situations.
16039 Instead, they felt that programs should be specialized tools. In short, each
16040 program ``should do one thing well.'' No more and no less. Such programs are
16041 simpler to design, write, and get right---they only do one thing.
16043 Furthermore, they found that with the right machinery for hooking programs
16044 together, that the whole was greater than the sum of the parts. By combining
16045 several special purpose programs, you could accomplish a specific task
16046 that none of the programs was designed for, and accomplish it much more
16047 quickly and easily than if you had to write a special purpose program.
16048 We will see some (classic) examples of this further on in the column.
16049 (An important additional point was that, if necessary, take a detour
16050 and build any software tools you may need first, if you don't already
16051 have something appropriate in the toolbox.)
16053 @node I/O redirection
16054 @unnumberedsec I/O Redirection
16056 Hopefully, you are familiar with the basics of I/O redirection in the
16057 shell, in particular the concepts of ``standard input,'' ``standard output,''
16058 and ``standard error''. Briefly, ``standard input'' is a data source, where
16059 data comes from. A program should not need to either know or care if the
16060 data source is a disk file, a keyboard, a magnetic tape, or even a punched
16061 card reader. Similarly, ``standard output'' is a data sink, where data goes
16062 to. The program should neither know nor care where this might be.
16063 Programs that only read their standard input, do something to the data,
16064 and then send it on, are called @dfn{filters}, by analogy to filters in a
16067 With the Unix shell, it's very easy to set up data pipelines:
16070 program_to_create_data | filter1 | ... | filterN > final.pretty.data
16073 We start out by creating the raw data; each filter applies some successive
16074 transformation to the data, until by the time it comes out of the pipeline,
16075 it is in the desired form.
16077 This is fine and good for standard input and standard output. Where does the
16078 standard error come in to play? Well, think about @command{filter1} in
16079 the pipeline above. What happens if it encounters an error in the data it
16080 sees? If it writes an error message to standard output, it will just
16081 disappear down the pipeline into @command{filter2}'s input, and the
16082 user will probably never see it. So programs need a place where they can send
16083 error messages so that the user will notice them. This is standard error,
16084 and it is usually connected to your console or window, even if you have
16085 redirected standard output of your program away from your screen.
16087 For filter programs to work together, the format of the data has to be
16088 agreed upon. The most straightforward and easiest format to use is simply
16089 lines of text. Unix data files are generally just streams of bytes, with
16090 lines delimited by the @acronym{ASCII} @sc{lf} (Line Feed) character,
16091 conventionally called a ``newline'' in the Unix literature. (This is
16092 @code{'\n'} if you're a C programmer.) This is the format used by all
16093 the traditional filtering programs. (Many earlier operating systems
16094 had elaborate facilities and special purpose programs for managing
16095 binary data. Unix has always shied away from such things, under the
16096 philosophy that it's easiest to simply be able to view and edit your
16097 data with a text editor.)
16099 OK, enough introduction. Let's take a look at some of the tools, and then
16100 we'll see how to hook them together in interesting ways. In the following
16101 discussion, we will only present those command line options that interest
16102 us. As you should always do, double check your system documentation
16103 for the full story.
16105 @node The who command
16106 @unnumberedsec The @command{who} Command
16108 The first program is the @command{who} command. By itself, it generates a
16109 list of the users who are currently logged in. Although I'm writing
16110 this on a single-user system, we'll pretend that several people are
16115 @print{} arnold console Jan 22 19:57
16116 @print{} miriam ttyp0 Jan 23 14:19(:0.0)
16117 @print{} bill ttyp1 Jan 21 09:32(:0.0)
16118 @print{} arnold ttyp2 Jan 23 20:48(:0.0)
16121 Here, the @samp{$} is the usual shell prompt, at which I typed @samp{who}.
16122 There are three people logged in, and I am logged in twice. On traditional
16123 Unix systems, user names are never more than eight characters long. This
16124 little bit of trivia will be useful later. The output of @command{who} is nice,
16125 but the data is not all that exciting.
16127 @node The cut command
16128 @unnumberedsec The @command{cut} Command
16130 The next program we'll look at is the @command{cut} command. This program
16131 cuts out columns or fields of input data. For example, we can tell it
16132 to print just the login name and full name from the @file{/etc/passwd}
16133 file. The @file{/etc/passwd} file has seven fields, separated by
16137 arnold:xyzzy:2076:10:Arnold D. Robbins:/home/arnold:/bin/bash
16140 To get the first and fifth fields, we would use @command{cut} like this:
16143 $ cut -d: -f1,5 /etc/passwd
16144 @print{} root:Operator
16146 @print{} arnold:Arnold D. Robbins
16147 @print{} miriam:Miriam A. Robbins
16151 With the @option{-c} option, @command{cut} will cut out specific characters
16152 (i.e., columns) in the input lines. This is useful for input data
16153 that has fixed width fields, and does not have a field separator. For
16154 example, list the Monday dates for the current month:
16156 @c Is using cal ok? Looked at gcal, but I don't like it.
16167 @node The sort command
16168 @unnumberedsec The @command{sort} Command
16170 Next we'll look at the @command{sort} command. This is one of the most
16171 powerful commands on a Unix-style system; one that you will often find
16172 yourself using when setting up fancy data plumbing.
16175 command reads and sorts each file named on the command line. It then
16176 merges the sorted data and writes it to standard output. It will read
16177 standard input if no files are given on the command line (thus
16178 making it into a filter). The sort is based on the character collating
16179 sequence or based on user-supplied ordering criteria.
16182 @node The uniq command
16183 @unnumberedsec The @command{uniq} Command
16185 Finally (at least for now), we'll look at the @command{uniq} program. When
16186 sorting data, you will often end up with duplicate lines, lines that
16187 are identical. Usually, all you need is one instance of each line.
16188 This is where @command{uniq} comes in. The @command{uniq} program reads its
16189 standard input. It prints only one
16190 copy of each repeated line. It does have several options. Later on,
16191 we'll use the @option{-c} option, which prints each unique line, preceded
16192 by a count of the number of times that line occurred in the input.
16195 @node Putting the tools together
16196 @unnumberedsec Putting the Tools Together
16198 Now, let's suppose this is a large ISP server system with dozens of users
16199 logged in. The management wants the system administrator to write a
16201 generate a sorted list of logged in users. Furthermore, even if a user
16202 is logged in multiple times, his or her name should only show up in the
16205 The administrator could sit down with the system documentation and write a C
16206 program that did this. It would take perhaps a couple of hundred lines
16207 of code and about two hours to write it, test it, and debug it.
16208 However, knowing the software toolbox, the administrator can instead start out
16209 by generating just a list of logged on users:
16219 Next, sort the list:
16222 $ who | cut -c1-8 | sort
16229 Finally, run the sorted list through @command{uniq}, to weed out duplicates:
16232 $ who | cut -c1-8 | sort | uniq
16238 The @command{sort} command actually has a @option{-u} option that does what
16239 @command{uniq} does. However, @command{uniq} has other uses for which one
16240 cannot substitute @samp{sort -u}.
16242 The administrator puts this pipeline into a shell script, and makes it
16244 all the users on the system (@samp{#} is the system administrator,
16245 or @code{root}, prompt):
16248 # cat > /usr/local/bin/listusers
16249 who | cut -c1-8 | sort | uniq
16251 # chmod +x /usr/local/bin/listusers
16254 There are four major points to note here. First, with just four
16255 programs, on one command line, the administrator was able to save about two
16256 hours worth of work. Furthermore, the shell pipeline is just about as
16257 efficient as the C program would be, and it is much more efficient in
16258 terms of programmer time. People time is much more expensive than
16259 computer time, and in our modern ``there's never enough time to do
16260 everything'' society, saving two hours of programmer time is no mean
16263 Second, it is also important to emphasize that with the
16264 @emph{combination} of the tools, it is possible to do a special
16265 purpose job never imagined by the authors of the individual programs.
16267 Third, it is also valuable to build up your pipeline in stages, as we did here.
16268 This allows you to view the data at each stage in the pipeline, which helps
16269 you acquire the confidence that you are indeed using these tools correctly.
16271 Finally, by bundling the pipeline in a shell script, other users can use
16272 your command, without having to remember the fancy plumbing you set up for
16273 them. In terms of how you run them, shell scripts and compiled programs are
16276 After the previous warm-up exercise, we'll look at two additional, more
16277 complicated pipelines. For them, we need to introduce two more tools.
16279 The first is the @command{tr} command, which stands for ``transliterate.''
16280 The @command{tr} command works on a character-by-character basis, changing
16281 characters. Normally it is used for things like mapping upper case to
16285 $ echo ThIs ExAmPlE HaS MIXED case! | tr '[:upper:]' '[:lower:]'
16286 @print{} this example has mixed case!
16289 There are several options of interest:
16293 work on the complement of the listed characters, i.e.,
16294 operations apply to characters not in the given set
16297 delete characters in the first set from the output
16300 squeeze repeated characters in the output into just one character.
16303 We will be using all three options in a moment.
16305 The other command we'll look at is @command{comm}. The @command{comm}
16306 command takes two sorted input files as input data, and prints out the
16307 files' lines in three columns. The output columns are the data lines
16308 unique to the first file, the data lines unique to the second file, and
16309 the data lines that are common to both. The @option{-1}, @option{-2}, and
16310 @option{-3} command line options @emph{omit} the respective columns. (This is
16311 non-intuitive and takes a little getting used to.) For example:
16333 The file name @file{-} tells @command{comm} to read standard input
16334 instead of a regular file.
16336 Now we're ready to build a fancy pipeline. The first application is a word
16337 frequency counter. This helps an author determine if he or she is over-using
16340 The first step is to change the case of all the letters in our input file
16341 to one case. ``The'' and ``the'' are the same word when doing counting.
16344 $ tr '[:upper:]' '[:lower:]' < whats.gnu | ...
16347 The next step is to get rid of punctuation. Quoted words and unquoted words
16348 should be treated identically; it's easiest to just get the punctuation out of
16352 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | ...
16355 The second @command{tr} command operates on the complement of the listed
16356 characters, which are all the letters, the digits, the underscore, and
16357 the blank. The @samp{\n} represents the newline character; it has to
16358 be left alone. (The @acronym{ASCII} tab character should also be included for
16359 good measure in a production script.)
16361 At this point, we have data consisting of words separated by blank space.
16362 The words only contain alphanumeric characters (and the underscore). The
16363 next step is break the data apart so that we have one word per line. This
16364 makes the counting operation much easier, as we will see shortly.
16367 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16368 > tr -s ' ' '\n' | ...
16371 This command turns blanks into newlines. The @option{-s} option squeezes
16372 multiple newline characters in the output into just one. This helps us
16373 avoid blank lines. (The @samp{>} is the shell's ``secondary prompt.''
16374 This is what the shell prints when it notices you haven't finished
16375 typing in all of a command.)
16377 We now have data consisting of one word per line, no punctuation, all one
16378 case. We're ready to count each word:
16381 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16382 > tr -s ' ' '\n' | sort | uniq -c | ...
16385 At this point, the data might look something like this:
16398 The output is sorted by word, not by count! What we want is the most
16399 frequently used words first. Fortunately, this is easy to accomplish,
16400 with the help of two more @command{sort} options:
16404 do a numeric sort, not a textual one
16407 reverse the order of the sort
16410 The final pipeline looks like this:
16413 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16414 > tr -s ' ' '\n' | sort | uniq -c | sort -n -r
16423 Whew! That's a lot to digest. Yet, the same principles apply. With six
16424 commands, on two lines (really one long one split for convenience), we've
16425 created a program that does something interesting and useful, in much
16426 less time than we could have written a C program to do the same thing.
16428 A minor modification to the above pipeline can give us a simple spelling
16429 checker! To determine if you've spelled a word correctly, all you have to
16430 do is look it up in a dictionary. If it is not there, then chances are
16431 that your spelling is incorrect. So, we need a dictionary.
16432 The conventional location for a dictionary is @file{/usr/dict/words}.
16433 On my GNU/Linux system,@footnote{Redhat Linux 6.1, for the November 2000
16434 revision of this article.}
16435 this is a sorted, 45,402 word dictionary.
16437 Now, how to compare our file with the dictionary? As before, we generate
16438 a sorted list of words, one per line:
16441 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16442 > tr -s ' ' '\n' | sort -u | ...
16445 Now, all we need is a list of words that are @emph{not} in the
16446 dictionary. Here is where the @command{comm} command comes in.
16449 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16450 > tr -s ' ' '\n' | sort -u |
16451 > comm -23 - /usr/dict/words
16454 The @option{-2} and @option{-3} options eliminate lines that are only in the
16455 dictionary (the second file), and lines that are in both files. Lines
16456 only in the first file (standard input, our stream of words), are
16457 words that are not in the dictionary. These are likely candidates for
16458 spelling errors. This pipeline was the first cut at a production
16459 spelling checker on Unix.
16461 There are some other tools that deserve brief mention.
16465 search files for text that matches a regular expression
16468 count lines, words, characters
16471 a T-fitting for data pipes, copies data to files and to standard output
16474 the stream editor, an advanced tool
16477 a data manipulation language, another advanced tool
16480 The software tools philosophy also espoused the following bit of
16481 advice: ``Let someone else do the hard part.'' This means, take
16482 something that gives you most of what you need, and then massage it the
16483 rest of the way until it's in the form that you want.
16489 Each program should do one thing well. No more, no less.
16492 Combining programs with appropriate plumbing leads to results where
16493 the whole is greater than the sum of the parts. It also leads to novel
16494 uses of programs that the authors might never have imagined.
16497 Programs should never print extraneous header or trailer data, since these
16498 could get sent on down a pipeline. (A point we didn't mention earlier.)
16501 Let someone else do the hard part.
16504 Know your toolbox! Use each program appropriately. If you don't have an
16505 appropriate tool, build one.
16508 As of this writing, all the programs we've discussed are available via
16509 anonymous @command{ftp} from: @*
16510 @uref{ftp://gnudist.gnu.org/textutils/textutils-1.22.tar.gz}. (There may
16511 be more recent versions available now.)
16513 None of what I have presented in this column is new. The Software Tools
16514 philosophy was first introduced in the book @cite{Software Tools}, by
16515 Brian Kernighan and P.J. Plauger (Addison-Wesley, ISBN 0-201-03669-X).
16516 This book showed how to write and use software tools. It was written in
16517 1976, using a preprocessor for FORTRAN named @command{ratfor} (RATional
16518 FORtran). At the time, C was not as ubiquitous as it is now; FORTRAN
16519 was. The last chapter presented a @command{ratfor} to FORTRAN
16520 processor, written in @command{ratfor}. @command{ratfor} looks an awful
16521 lot like C; if you know C, you won't have any problem following the
16524 In 1981, the book was updated and made available as @cite{Software Tools
16525 in Pascal} (Addison-Wesley, ISBN 0-201-10342-7). Both books are
16526 still in print and are well worth
16527 reading if you're a programmer. They certainly made a major change in
16528 how I view programming.
16530 The programs in both books are available from
16531 @uref{http://cm.bell-labs.com/who/bwk, Brian Kernighan's home page}.
16532 For a number of years, there was an active
16533 Software Tools Users Group, whose members had ported the original
16534 @command{ratfor} programs to essentially every computer system with a
16535 FORTRAN compiler. The popularity of the group waned in the middle 1980s
16536 as Unix began to spread beyond universities.
16538 With the current proliferation of GNU code and other clones of Unix programs,
16539 these programs now receive little attention; modern C versions are
16540 much more efficient and do more than these programs do. Nevertheless, as
16541 exposition of good programming style, and evangelism for a still-valuable
16542 philosophy, these books are unparalleled, and I recommend them highly.
16544 Acknowledgment: I would like to express my gratitude to Brian Kernighan
16545 of Bell Labs, the original Software Toolsmith, for reviewing this column.
16547 @node GNU Free Documentation License
16548 @appendix GNU Free Documentation License
16552 @node Concept index
16559 @c Local variables:
16560 @c texinfo-column-for-description: 32