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
2996 @option{-l @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 @itemx --filter=@var{command}
3016 With this option, rather than simply writing to each output file,
3017 write through a pipe to the specified shell @var{command} for each output file.
3018 @var{command} should use the $FILE environment variable, which is set
3019 to a different output file name for each invocation of the command.
3020 For example, imagine that you have a 1TiB compressed file
3021 that, if uncompressed, would be too large to reside on disk,
3022 yet you must split it into individually-compressed pieces
3023 of a more manageable size.
3024 To do that, you might run this command:
3027 xz -dc BIG.xz | split -b200G --filter='xz > $FILE.xz' - big-
3030 Assuming a 10:1 compression ratio, that would create about fifty 20GiB files
3031 with names @file{big-xaa.xz}, @file{big-xab.xz}, @file{big-xac.xz}, etc.
3033 @item -n @var{chunks}
3034 @itemx --number=@var{chunks}
3038 Split @var{input} to @var{chunks} output files where @var{chunks} may be:
3041 @var{n} generate @var{n} files based on current size of @var{input}
3042 @var{k}/@var{n} only output @var{k}th of @var{n} to stdout
3043 l/@var{n} generate @var{n} files without splitting lines
3044 l/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
3045 r/@var{n} like @samp{l} but use round robin distribution
3046 r/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
3049 Any excess bytes remaining after dividing the @var{input}
3050 into @var{n} chunks, are assigned to the last chunk.
3051 Any excess bytes appearing after the initial calculation are discarded
3052 (except when using @samp{r} mode).
3054 All @var{n} files are created even if there are fewer than @var{n} lines,
3055 or the @var{input} is truncated.
3057 For @samp{l} mode, chunks are approximately @var{input} size / @var{n}.
3058 The @var{input} is partitioned into @var{n} equal sized portions, with
3059 the last assigned any excess. If a line @emph{starts} within a partition
3060 it is written completely to the corresponding file. Since lines
3061 are not split even if they overlap a partition, the files written
3062 can be larger or smaller than the partition size, and even empty
3063 if a line is so long as to completely overlap the partition.
3065 For @samp{r} mode, the size of @var{input} is irrelevant,
3066 and so can be a pipe for example.
3068 @item -a @var{length}
3069 @itemx --suffix-length=@var{length}
3071 @opindex --suffix-length
3072 Use suffixes of length @var{length}. The default @var{length} is 2.
3075 @itemx --numeric-suffixes
3077 @opindex --numeric-suffixes
3078 Use digits in suffixes rather than lower-case letters.
3081 @itemx --elide-empty-files
3083 @opindex --elide-empty-files
3084 Suppress the generation of zero-length output files. This can happen
3085 with the @option{--number} option if a file is (truncated to be) shorter
3086 than the number requested, or if a line is so long as to completely
3087 span a chunk. The output file sequence numbers, always run consecutively
3088 even when this option is specified.
3093 @opindex --unbuffered
3094 Immediately copy input to output in @option{--number r/...} mode,
3095 which is a much slower mode of operation.
3099 Write a diagnostic just before each output file is opened.
3105 Here are a few examples to illustrate how the
3106 @option{--number} (@option{-n}) option works:
3108 Notice how, by default, one line may be split onto two or more:
3111 $ seq -w 6 10 > k; split -n3 k; head xa?
3124 Use the "l/" modifier to suppress that:
3127 $ seq -w 6 10 > k; split -nl/3 k; head xa?
3140 Use the "r/" modifier to distribute lines in a round-robin fashion:
3143 $ seq -w 6 10 > k; split -nr/3 k; head xa?
3156 You can also extract just the Kth chunk.
3157 This extracts and prints just the 7th "chunk" of 33:
3160 $ seq 100 > k; split -nl/7/33 k
3167 @node csplit invocation
3168 @section @command{csplit}: Split a file into context-determined pieces
3171 @cindex context splitting
3172 @cindex splitting a file into pieces by context
3174 @command{csplit} creates zero or more output files containing sections of
3175 @var{input} (standard input if @var{input} is @samp{-}). Synopsis:
3178 csplit [@var{option}]@dots{} @var{input} @var{pattern}@dots{}
3181 The contents of the output files are determined by the @var{pattern}
3182 arguments, as detailed below. An error occurs if a @var{pattern}
3183 argument refers to a nonexistent line of the input file (e.g., if no
3184 remaining line matches a given regular expression). After every
3185 @var{pattern} has been matched, any remaining input is copied into one
3188 By default, @command{csplit} prints the number of bytes written to each
3189 output file after it has been created.
3191 The types of pattern arguments are:
3196 Create an output file containing the input up to but not including line
3197 @var{n} (a positive integer). If followed by a repeat count, also
3198 create an output file containing the next @var{n} lines of the input
3199 file once for each repeat.
3201 @item /@var{regexp}/[@var{offset}]
3202 Create an output file containing the current line up to (but not
3203 including) the next line of the input file that contains a match for
3204 @var{regexp}. The optional @var{offset} is an integer.
3205 If it is given, the input up to (but not including) the
3206 matching line plus or minus @var{offset} is put into the output file,
3207 and the line after that begins the next section of input.
3209 @item %@var{regexp}%[@var{offset}]
3210 Like the previous type, except that it does not create an output
3211 file, so that section of the input file is effectively ignored.
3213 @item @{@var{repeat-count}@}
3214 Repeat the previous pattern @var{repeat-count} additional
3215 times. The @var{repeat-count} can either be a positive integer or an
3216 asterisk, meaning repeat as many times as necessary until the input is
3221 The output files' names consist of a prefix (@samp{xx} by default)
3222 followed by a suffix. By default, the suffix is an ascending sequence
3223 of two-digit decimal numbers from @samp{00} to @samp{99}. In any case,
3224 concatenating the output files in sorted order by file name produces the
3225 original input file.
3227 By default, if @command{csplit} encounters an error or receives a hangup,
3228 interrupt, quit, or terminate signal, it removes any output files
3229 that it has created so far before it exits.
3231 The program accepts the following options. Also see @ref{Common options}.
3235 @item -f @var{prefix}
3236 @itemx --prefix=@var{prefix}
3239 @cindex output file name prefix
3240 Use @var{prefix} as the output file name prefix.
3242 @item -b @var{suffix}
3243 @itemx --suffix=@var{suffix}
3246 @cindex output file name suffix
3247 Use @var{suffix} as the output file name suffix. When this option is
3248 specified, the suffix string must include exactly one
3249 @code{printf(3)}-style conversion specification, possibly including
3250 format specification flags, a field width, a precision specifications,
3251 or all of these kinds of modifiers. The format letter must convert a
3252 binary unsigned integer argument to readable form. The format letters
3253 @samp{d} and @samp{i} are aliases for @samp{u}, and the
3254 @samp{u}, @samp{o}, @samp{x}, and @samp{X} conversions are allowed. The
3255 entire @var{suffix} is given (with the current output file number) to
3256 @code{sprintf(3)} to form the file name suffixes for each of the
3257 individual output files in turn. If this option is used, the
3258 @option{--digits} option is ignored.
3260 @item -n @var{digits}
3261 @itemx --digits=@var{digits}
3264 Use output file names containing numbers that are @var{digits} digits
3265 long instead of the default 2.
3270 @opindex --keep-files
3271 Do not remove output files when errors are encountered.
3274 @itemx --elide-empty-files
3276 @opindex --elide-empty-files
3277 Suppress the generation of zero-length output files. (In cases where
3278 the section delimiters of the input file are supposed to mark the first
3279 lines of each of the sections, the first output file will generally be a
3280 zero-length file unless you use this option.) The output file sequence
3281 numbers always run consecutively starting from 0, even when this option
3292 Do not print counts of output file sizes.
3298 Here is an example of its usage.
3299 First, create an empty directory for the exercise,
3306 Now, split the sequence of 1..14 on lines that end with 0 or 5:
3309 $ seq 14 | csplit - '/[05]$/' '@{*@}'
3315 Each number printed above is the size of an output
3316 file that csplit has just created.
3317 List the names of those output files:
3324 Use @command{head} to show their contents:
3349 @node Summarizing files
3350 @chapter Summarizing files
3352 @cindex summarizing files
3354 These commands generate just a few numbers representing entire
3358 * wc invocation:: Print newline, word, and byte counts.
3359 * sum invocation:: Print checksum and block counts.
3360 * cksum invocation:: Print CRC checksum and byte counts.
3361 * md5sum invocation:: Print or check MD5 digests.
3362 * sha1sum invocation:: Print or check SHA-1 digests.
3363 * sha2 utilities:: Print or check SHA-2 digests.
3368 @section @command{wc}: Print newline, word, and byte counts
3372 @cindex character count
3376 @command{wc} counts the number of bytes, characters, whitespace-separated
3377 words, and newlines in each given @var{file}, or standard input if none
3378 are given or for a @var{file} of @samp{-}. Synopsis:
3381 wc [@var{option}]@dots{} [@var{file}]@dots{}
3384 @cindex total counts
3385 @command{wc} prints one line of counts for each file, and if the file was
3386 given as an argument, it prints the file name following the counts. If
3387 more than one @var{file} is given, @command{wc} prints a final line
3388 containing the cumulative counts, with the file name @file{total}. The
3389 counts are printed in this order: newlines, words, characters, bytes,
3390 maximum line length.
3391 Each count is printed right-justified in a field with at least one
3392 space between fields so that the numbers and file names normally line
3393 up nicely in columns. The width of the count fields varies depending
3394 on the inputs, so you should not depend on a particular field width.
3395 However, as a @acronym{GNU} extension, if only one count is printed,
3396 it is guaranteed to be printed without leading spaces.
3398 By default, @command{wc} prints three counts: the newline, words, and byte
3399 counts. Options can specify that only certain counts be printed.
3400 Options do not undo others previously given, so
3407 prints both the byte counts and the word counts.
3409 With the @option{--max-line-length} option, @command{wc} prints the length
3410 of the longest line per file, and if there is more than one file it
3411 prints the maximum (not the sum) of those lengths. The line lengths here
3412 are measured in screen columns, according to the current locale and
3413 assuming tab positions in every 8th column.
3415 The program accepts the following options. Also see @ref{Common options}.
3423 Print only the byte counts.
3429 Print only the character counts.
3435 Print only the word counts.
3441 Print only the newline counts.
3444 @itemx --max-line-length
3446 @opindex --max-line-length
3447 Print only the maximum line lengths.
3449 @macro filesZeroFromOption{cmd,withTotalOption,subListOutput}
3450 @itemx --files0-from=@var{file}
3451 @opindex --files0-from=@var{file}
3452 @c This is commented out to avoid a texi2dvi failure.
3453 @c texi2dvi (GNU Texinfo 4.11) 1.104
3454 @c @cindex including files from @command{\cmd\}
3455 Disallow processing files named on the command line, and instead process
3456 those named in file @var{file}; each name being terminated by a zero byte
3457 (@acronym{ASCII} @sc{nul}).
3458 This is useful \withTotalOption\
3459 when the list of file names is so long that it may exceed a command line
3461 In such cases, running @command{\cmd\} via @command{xargs} is undesirable
3462 because it splits the list into pieces and makes @command{\cmd\} print
3463 \subListOutput\ for each sublist rather than for the entire list.
3464 One way to produce a list of @acronym{ASCII} @sc{nul} terminated file
3465 names is with @sc{gnu}
3466 @command{find}, using its @option{-print0} predicate.
3467 If @var{file} is @samp{-} then the @acronym{ASCII} @sc{nul} terminated
3468 file names are read from standard input.
3470 @filesZeroFromOption{wc,,a total}
3472 For example, to find the length of the longest line in any @file{.c} or
3473 @file{.h} file in the current hierarchy, do this:
3476 find . -name '*.[ch]' -print0 |
3477 wc -L --files0-from=- | tail -n1
3485 @node sum invocation
3486 @section @command{sum}: Print checksum and block counts
3489 @cindex 16-bit checksum
3490 @cindex checksum, 16-bit
3492 @command{sum} computes a 16-bit checksum for each given @var{file}, or
3493 standard input if none are given or for a @var{file} of @samp{-}. Synopsis:
3496 sum [@var{option}]@dots{} [@var{file}]@dots{}
3499 @command{sum} prints the checksum for each @var{file} followed by the
3500 number of blocks in the file (rounded up). If more than one @var{file}
3501 is given, file names are also printed (by default). (With the
3502 @option{--sysv} option, corresponding file names are printed when there is
3503 at least one file argument.)
3505 By default, @sc{gnu} @command{sum} computes checksums using an algorithm
3506 compatible with BSD @command{sum} and prints file sizes in units of
3509 The program accepts the following options. Also see @ref{Common options}.
3515 @cindex BSD @command{sum}
3516 Use the default (BSD compatible) algorithm. This option is included for
3517 compatibility with the System V @command{sum}. Unless @option{-s} was also
3518 given, it has no effect.
3524 @cindex System V @command{sum}
3525 Compute checksums using an algorithm compatible with System V
3526 @command{sum}'s default, and print file sizes in units of 512-byte blocks.
3530 @command{sum} is provided for compatibility; the @command{cksum} program (see
3531 next section) is preferable in new applications.
3536 @node cksum invocation
3537 @section @command{cksum}: Print CRC checksum and byte counts
3540 @cindex cyclic redundancy check
3541 @cindex CRC checksum
3543 @command{cksum} computes a cyclic redundancy check (CRC) checksum for each
3544 given @var{file}, or standard input if none are given or for a
3545 @var{file} of @samp{-}. Synopsis:
3548 cksum [@var{option}]@dots{} [@var{file}]@dots{}
3551 @command{cksum} prints the CRC checksum for each file along with the number
3552 of bytes in the file, and the file name unless no arguments were given.
3554 @command{cksum} is typically used to ensure that files
3555 transferred by unreliable means (e.g., netnews) have not been corrupted,
3556 by comparing the @command{cksum} output for the received files with the
3557 @command{cksum} output for the original files (typically given in the
3560 The CRC algorithm is specified by the @acronym{POSIX} standard. It is not
3561 compatible with the BSD or System V @command{sum} algorithms (see the
3562 previous section); it is more robust.
3564 The only options are @option{--help} and @option{--version}. @xref{Common
3570 @node md5sum invocation
3571 @section @command{md5sum}: Print or check MD5 digests
3575 @cindex 128-bit checksum
3576 @cindex checksum, 128-bit
3577 @cindex fingerprint, 128-bit
3578 @cindex message-digest, 128-bit
3580 @command{md5sum} computes a 128-bit checksum (or @dfn{fingerprint} or
3581 @dfn{message-digest}) for each specified @var{file}.
3583 Note: The MD5 digest is more reliable than a simple CRC (provided by
3584 the @command{cksum} command) for detecting accidental file corruption,
3585 as the chances of accidentally having two files with identical MD5
3586 are vanishingly small. However, it should not be considered secure
3587 against malicious tampering: although finding a file with a given MD5
3588 fingerprint is considered infeasible at the moment, it is known how
3589 to modify certain files, including digital certificates, so that they
3590 appear valid when signed with an MD5 digest.
3591 For more secure hashes, consider using SHA-2. @xref{sha2 utilities}.
3593 If a @var{file} is specified as @samp{-} or if no files are given
3594 @command{md5sum} computes the checksum for the standard input.
3595 @command{md5sum} can also determine whether a file and checksum are
3596 consistent. Synopsis:
3599 md5sum [@var{option}]@dots{} [@var{file}]@dots{}
3602 For each @var{file}, @samp{md5sum} outputs the MD5 checksum, a flag
3603 indicating a binary or text input file, and the file name.
3604 If @var{file} contains a backslash or newline, the
3605 line is started with a backslash, and each problematic character in
3606 the file name is escaped with a backslash, making the output
3607 unambiguous even in the presence of arbitrary file names.
3608 If @var{file} is omitted or specified as @samp{-}, standard input is read.
3610 The program accepts the following options. Also see @ref{Common options}.
3618 @cindex binary input files
3619 Treat each input file as binary, by reading it in binary mode and
3620 outputting a @samp{*} flag. This is the inverse of @option{--text}.
3621 On systems like @acronym{GNU} that do not distinguish between binary
3622 and text files, this option merely flags each input file as binary:
3623 the MD5 checksum is unaffected. This option is the default on systems
3624 like MS-DOS that distinguish between binary and text files, except
3625 for reading standard input when standard input is a terminal.
3629 Read file names and checksum information (not data) from each
3630 @var{file} (or from stdin if no @var{file} was specified) and report
3631 whether the checksums match the contents of the named files.
3632 The input to this mode of @command{md5sum} is usually the output of
3633 a prior, checksum-generating run of @samp{md5sum}.
3634 Each valid line of input consists of an MD5 checksum, a binary/text
3635 flag, and then a file name.
3636 Binary files are marked with @samp{*}, text with @samp{ }.
3637 For each such line, @command{md5sum} reads the named file and computes its
3638 MD5 checksum. Then, if the computed message digest does not match the
3639 one on the line with the file name, the file is noted as having
3640 failed the test. Otherwise, the file passes the test.
3641 By default, for each valid line, one line is written to standard
3642 output indicating whether the named file passed the test.
3643 After all checks have been performed, if there were any failures,
3644 a warning is issued to standard error.
3645 Use the @option{--status} option to inhibit that output.
3646 If any listed file cannot be opened or read, if any valid line has
3647 an MD5 checksum inconsistent with the associated file, or if no valid
3648 line is found, @command{md5sum} exits with nonzero status. Otherwise,
3649 it exits successfully.
3653 @cindex verifying MD5 checksums
3654 This option is useful only when verifying checksums.
3655 When verifying checksums, don't generate an 'OK' message per successfully
3656 checked file. Files that fail the verification are reported in the
3657 default one-line-per-file format. If there is any checksum mismatch,
3658 print a warning summarizing the failures to standard error.
3662 @cindex verifying MD5 checksums
3663 This option is useful only when verifying checksums.
3664 When verifying checksums, don't generate the default one-line-per-file
3665 diagnostic and don't output the warning summarizing any failures.
3666 Failures to open or read a file still evoke individual diagnostics to
3668 If all listed files are readable and are consistent with the associated
3669 MD5 checksums, exit successfully. Otherwise exit with a status code
3670 indicating there was a failure.
3676 @cindex text input files
3677 Treat each input file as text, by reading it in text mode and
3678 outputting a @samp{ } flag. This is the inverse of @option{--binary}.
3679 This option is the default on systems like @acronym{GNU} that do not
3680 distinguish between binary and text files. On other systems, it is
3681 the default for reading standard input when standard input is a
3688 @cindex verifying MD5 checksums
3689 When verifying checksums, warn about improperly formatted MD5 checksum lines.
3690 This option is useful only if all but a few lines in the checked input
3698 @node sha1sum invocation
3699 @section @command{sha1sum}: Print or check SHA-1 digests
3703 @cindex 160-bit checksum
3704 @cindex checksum, 160-bit
3705 @cindex fingerprint, 160-bit
3706 @cindex message-digest, 160-bit
3708 @command{sha1sum} computes a 160-bit checksum for each specified
3709 @var{file}. The usage and options of this command are precisely the
3710 same as for @command{md5sum}. @xref{md5sum invocation}.
3712 Note: The SHA-1 digest is more secure than MD5, and no collisions of
3713 it are known (different files having the same fingerprint). However,
3714 it is known that they can be produced with considerable, but not
3715 unreasonable, resources. For this reason, it is generally considered
3716 that SHA-1 should be gradually phased out in favor of the more secure
3717 SHA-2 hash algorithms. @xref{sha2 utilities}.
3720 @node sha2 utilities
3721 @section sha2 utilities: Print or check SHA-2 digests
3728 @cindex 224-bit checksum
3729 @cindex 256-bit checksum
3730 @cindex 384-bit checksum
3731 @cindex 512-bit checksum
3732 @cindex checksum, 224-bit
3733 @cindex checksum, 256-bit
3734 @cindex checksum, 384-bit
3735 @cindex checksum, 512-bit
3736 @cindex fingerprint, 224-bit
3737 @cindex fingerprint, 256-bit
3738 @cindex fingerprint, 384-bit
3739 @cindex fingerprint, 512-bit
3740 @cindex message-digest, 224-bit
3741 @cindex message-digest, 256-bit
3742 @cindex message-digest, 384-bit
3743 @cindex message-digest, 512-bit
3745 The commands @command{sha224sum}, @command{sha256sum},
3746 @command{sha384sum} and @command{sha512sum} compute checksums of
3747 various lengths (respectively 224, 256, 384 and 512 bits),
3748 collectively known as the SHA-2 hashes. The usage and options of
3749 these commands are precisely the same as for @command{md5sum}.
3750 @xref{md5sum invocation}.
3752 Note: The SHA384 and SHA512 digests are considerably slower to
3753 compute, especially on 32-bit computers, than SHA224 or SHA256.
3756 @node Operating on sorted files
3757 @chapter Operating on sorted files
3759 @cindex operating on sorted files
3760 @cindex sorted files, operations on
3762 These commands work with (or produce) sorted files.
3765 * sort invocation:: Sort text files.
3766 * shuf invocation:: Shuffle text files.
3767 * uniq invocation:: Uniquify files.
3768 * comm invocation:: Compare two sorted files line by line.
3769 * ptx invocation:: Produce a permuted index of file contents.
3770 * tsort invocation:: Topological sort.
3774 @node sort invocation
3775 @section @command{sort}: Sort text files
3778 @cindex sorting files
3780 @command{sort} sorts, merges, or compares all the lines from the given
3781 files, or standard input if none are given or for a @var{file} of
3782 @samp{-}. By default, @command{sort} writes the results to standard
3786 sort [@var{option}]@dots{} [@var{file}]@dots{}
3789 @command{sort} has three modes of operation: sort (the default), merge,
3790 and check for sortedness. The following options change the operation
3797 @itemx --check=diagnose-first
3800 @cindex checking for sortedness
3801 Check whether the given file is already sorted: if it is not all
3802 sorted, print a diagnostic containing the first out-of-order line and
3803 exit with a status of 1.
3804 Otherwise, exit successfully.
3805 At most one input file can be given.
3808 @itemx --check=quiet
3809 @itemx --check=silent
3812 @cindex checking for sortedness
3813 Exit successfully if the given file is already sorted, and
3814 exit with status 1 otherwise.
3815 At most one input file can be given.
3816 This is like @option{-c}, except it does not print a diagnostic.
3822 @cindex merging sorted files
3823 Merge the given files by sorting them as a group. Each input file must
3824 always be individually sorted. It always works to sort instead of
3825 merge; merging is provided because it is faster, in the case where it
3830 @cindex sort stability
3831 @cindex sort's last-resort comparison
3832 A pair of lines is compared as follows:
3833 @command{sort} compares each pair of fields, in the
3834 order specified on the command line, according to the associated
3835 ordering options, until a difference is found or no fields are left.
3836 If no key fields are specified, @command{sort} uses a default key of
3837 the entire line. Finally, as a last resort when all keys compare
3838 equal, @command{sort} compares entire lines as if no ordering options
3839 other than @option{--reverse} (@option{-r}) were specified. The
3840 @option{--stable} (@option{-s}) option disables this @dfn{last-resort
3841 comparison} so that lines in which all fields compare equal are left
3842 in their original relative order. The @option{--unique}
3843 (@option{-u}) option also disables the last-resort comparison.
3847 Unless otherwise specified, all comparisons use the character collating
3848 sequence specified by the @env{LC_COLLATE} locale.@footnote{If you
3849 use a non-@acronym{POSIX} locale (e.g., by setting @env{LC_ALL}
3850 to @samp{en_US}), then @command{sort} may produce output that is sorted
3851 differently than you're accustomed to. In that case, set the @env{LC_ALL}
3852 environment variable to @samp{C}. Note that setting only @env{LC_COLLATE}
3853 has two problems. First, it is ineffective if @env{LC_ALL} is also set.
3854 Second, it has undefined behavior if @env{LC_CTYPE} (or @env{LANG}, if
3855 @env{LC_CTYPE} is unset) is set to an incompatible value. For example,
3856 you get undefined behavior if @env{LC_CTYPE} is @code{ja_JP.PCK} but
3857 @env{LC_COLLATE} is @code{en_US.UTF-8}.}
3859 @sc{gnu} @command{sort} (as specified for all @sc{gnu} utilities) has no
3860 limit on input line length or restrictions on bytes allowed within lines.
3861 In addition, if the final byte of an input file is not a newline, @sc{gnu}
3862 @command{sort} silently supplies one. A line's trailing newline is not
3863 part of the line for comparison purposes.
3865 @cindex exit status of @command{sort}
3869 0 if no error occurred
3870 1 if invoked with @option{-c} or @option{-C} and the input is not sorted
3871 2 if an error occurred
3875 If the environment variable @env{TMPDIR} is set, @command{sort} uses its
3876 value as the directory for temporary files instead of @file{/tmp}. The
3877 @option{--temporary-directory} (@option{-T}) option in turn overrides
3878 the environment variable.
3880 The following options affect the ordering of output lines. They may be
3881 specified globally or as part of a specific key field. If no key
3882 fields are specified, global options apply to comparison of entire
3883 lines; otherwise the global options are inherited by key fields that do
3884 not specify any special options of their own. In pre-@acronym{POSIX}
3885 versions of @command{sort}, global options affect only later key fields,
3886 so portable shell scripts should specify global options first.
3891 @itemx --ignore-leading-blanks
3893 @opindex --ignore-leading-blanks
3894 @cindex blanks, ignoring leading
3896 Ignore leading blanks when finding sort keys in each line.
3897 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3898 can change this. Note blanks may be ignored by your locale's collating
3899 rules, but without this option they will be significant for character
3900 positions specified in keys with the @option{-k} option.
3903 @itemx --dictionary-order
3905 @opindex --dictionary-order
3906 @cindex dictionary order
3907 @cindex phone directory order
3908 @cindex telephone directory order
3910 Sort in @dfn{phone directory} order: ignore all characters except
3911 letters, digits and blanks when sorting.
3912 By default letters and digits are those of @acronym{ASCII} and a blank
3913 is a space or a tab, but the @env{LC_CTYPE} locale can change this.
3916 @itemx --ignore-case
3918 @opindex --ignore-case
3919 @cindex ignoring case
3920 @cindex case folding
3922 Fold lowercase characters into the equivalent uppercase characters when
3923 comparing so that, for example, @samp{b} and @samp{B} sort as equal.
3924 The @env{LC_CTYPE} locale determines character types.
3925 When used with @option{--unique} those lower case equivalent lines are
3926 thrown away. (There is currently no way to throw away the upper case
3927 equivalent instead. (Any @option{--reverse} given would only affect
3928 the final result, after the throwing away.))
3931 @itemx --general-numeric-sort
3932 @itemx --sort=general-numeric
3934 @opindex --general-numeric-sort
3936 @cindex general numeric sort
3938 Sort numerically, converting a prefix of each line to a long
3939 double-precision floating point number. @xref{Floating point}.
3940 Do not report overflow, underflow, or conversion errors.
3941 Use the following collating sequence:
3945 Lines that do not start with numbers (all considered to be equal).
3947 NaNs (``Not a Number'' values, in IEEE floating point arithmetic)
3948 in a consistent but machine-dependent order.
3952 Finite numbers in ascending numeric order (with @math{-0} and @math{+0} equal).
3957 Use this option only if there is no alternative; it is much slower than
3958 @option{--numeric-sort} (@option{-n}) and it can lose information when
3959 converting to floating point.
3962 @itemx --human-numeric-sort
3963 @itemx --sort=human-numeric
3965 @opindex --human-numeric-sort
3967 @cindex human numeric sort
3969 Sort numerically, first by numeric sign (negative, zero, or positive);
3970 then by @acronym{SI} suffix (either empty, or @samp{k} or @samp{K}, or
3971 one of @samp{MGTPEZY}, in that order; @pxref{Block size}); and finally
3972 by numeric value. For example, @samp{1023M} sorts before @samp{1G}
3973 because @samp{M} (mega) precedes @samp{G} (giga) as an @acronym{SI}
3974 suffix. This option sorts values that are consistently scaled to the
3975 nearest suffix, regardless of whether suffixes denote powers of 1000
3976 or 1024, and it therefore sorts the output of any single invocation of
3977 the @command{df}, @command{du}, or @command{ls} commands that are
3978 invoked with their @option{--human-readable} or @option{--si} options.
3979 The syntax for numbers is the same as for the @option{--numeric-sort}
3980 option; the @acronym{SI} suffix must immediately follow the number.
3983 @itemx --ignore-nonprinting
3985 @opindex --ignore-nonprinting
3986 @cindex nonprinting characters, ignoring
3987 @cindex unprintable characters, ignoring
3989 Ignore nonprinting characters.
3990 The @env{LC_CTYPE} locale determines character types.
3991 This option has no effect if the stronger @option{--dictionary-order}
3992 (@option{-d}) option is also given.
3998 @opindex --month-sort
4000 @cindex months, sorting by
4002 An initial string, consisting of any amount of blanks, followed
4003 by a month name abbreviation, is folded to UPPER case and
4004 compared in the order @samp{JAN} < @samp{FEB} < @dots{} < @samp{DEC}.
4005 Invalid names compare low to valid names. The @env{LC_TIME} locale
4006 category determines the month spellings.
4007 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4011 @itemx --numeric-sort
4012 @itemx --sort=numeric
4014 @opindex --numeric-sort
4016 @cindex numeric sort
4018 Sort numerically. The number begins each line and consists
4019 of optional blanks, an optional @samp{-} sign, and zero or more
4020 digits possibly separated by thousands separators, optionally followed
4021 by a decimal-point character and zero or more digits. An empty
4022 number is treated as @samp{0}. The @env{LC_NUMERIC}
4023 locale specifies the decimal-point character and thousands separator.
4024 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4027 Comparison is exact; there is no rounding error.
4029 Neither a leading @samp{+} nor exponential notation is recognized.
4030 To compare such strings numerically, use the
4031 @option{--general-numeric-sort} (@option{-g}) option.
4034 @itemx --version-sort
4036 @opindex --version-sort
4037 @cindex version number sort
4038 Sort by version name and number. It behaves like a standard sort,
4039 except that each sequence of decimal digits is treated numerically
4040 as an index/version number. (@xref{Details about version sort}.)
4046 @cindex reverse sorting
4047 Reverse the result of comparison, so that lines with greater key values
4048 appear earlier in the output instead of later.
4051 @itemx --random-sort
4052 @itemx --sort=random
4054 @opindex --random-sort
4057 Sort by hashing the input keys and then sorting the hash values.
4058 Choose the hash function at random, ensuring that it is free of
4059 collisions so that differing keys have differing hash values. This is
4060 like a random permutation of the inputs (@pxref{shuf invocation}),
4061 except that keys with the same value sort together.
4063 If multiple random sort fields are specified, the same random hash
4064 function is used for all fields. To use different random hash
4065 functions for different fields, you can invoke @command{sort} more
4068 The choice of hash function is affected by the
4069 @option{--random-source} option.
4077 @item --compress-program=@var{prog}
4078 Compress any temporary files with the program @var{prog}.
4080 With no arguments, @var{prog} must compress standard input to standard
4081 output, and when given the @option{-d} option it must decompress
4082 standard input to standard output.
4084 Terminate with an error if @var{prog} exits with nonzero status.
4086 White space and the backslash character should not appear in
4087 @var{prog}; they are reserved for future use.
4089 @filesZeroFromOption{sort,,sorted output}
4091 @item -k @var{pos1}[,@var{pos2}]
4092 @itemx --key=@var{pos1}[,@var{pos2}]
4096 Specify a sort field that consists of the part of the line between
4097 @var{pos1} and @var{pos2} (or the end of the line, if @var{pos2} is
4098 omitted), @emph{inclusive}.
4100 Each @var{pos} has the form @samp{@var{f}[.@var{c}][@var{opts}]},
4101 where @var{f} is the number of the field to use, and @var{c} is the number
4102 of the first character from the beginning of the field. Fields and character
4103 positions are numbered starting with 1; a character position of zero in
4104 @var{pos2} indicates the field's last character. If @samp{.@var{c}} is
4105 omitted from @var{pos1}, it defaults to 1 (the beginning of the field);
4106 if omitted from @var{pos2}, it defaults to 0 (the end of the field).
4107 @var{opts} are ordering options, allowing individual keys to be sorted
4108 according to different rules; see below for details. Keys can span
4111 Example: To sort on the second field, use @option{--key=2,2}
4112 (@option{-k 2,2}). See below for more notes on keys and more examples.
4113 See also the @option{--debug} option to help determine the part
4114 of the line being used in the sort.
4117 Highlight the portion of each line used for sorting.
4118 Also issue warnings about questionable usage to stderr.
4120 @item --batch-size=@var{nmerge}
4121 @opindex --batch-size
4122 @cindex number of inputs to merge, nmerge
4123 Merge at most @var{nmerge} inputs at once.
4125 When @command{sort} has to merge more than @var{nmerge} inputs,
4126 it merges them in groups of @var{nmerge}, saving the result in
4127 a temporary file, which is then used as an input in a subsequent merge.
4129 A large value of @var{nmerge} may improve merge performance and decrease
4130 temporary storage utilization at the expense of increased memory usage
4131 and I/0. Conversely a small value of @var{nmerge} may reduce memory
4132 requirements and I/0 at the expense of temporary storage consumption and
4135 The value of @var{nmerge} must be at least 2. The default value is
4136 currently 16, but this is implementation-dependent and may change in
4139 The value of @var{nmerge} may be bounded by a resource limit for open
4140 file descriptors. The commands @samp{ulimit -n} or @samp{getconf
4141 OPEN_MAX} may display limits for your systems; these limits may be
4142 modified further if your program already has some files open, or if
4143 the operating system has other limits on the number of open files. If
4144 the value of @var{nmerge} exceeds the resource limit, @command{sort}
4145 silently uses a smaller value.
4147 @item -o @var{output-file}
4148 @itemx --output=@var{output-file}
4151 @cindex overwriting of input, allowed
4152 Write output to @var{output-file} instead of standard output.
4153 Normally, @command{sort} reads all input before opening
4154 @var{output-file}, so you can safely sort a file in place by using
4155 commands like @code{sort -o F F} and @code{cat F | sort -o F}.
4156 However, @command{sort} with @option{--merge} (@option{-m}) can open
4157 the output file before reading all input, so a command like @code{cat
4158 F | sort -m -o F - G} is not safe as @command{sort} might start
4159 writing @file{F} before @command{cat} is done reading it.
4161 @vindex POSIXLY_CORRECT
4162 On newer systems, @option{-o} cannot appear after an input file if
4163 @env{POSIXLY_CORRECT} is set, e.g., @samp{sort F -o F}. Portable
4164 scripts should specify @option{-o @var{output-file}} before any input
4167 @item --random-source=@var{file}
4168 @opindex --random-source
4169 @cindex random source for sorting
4170 Use @var{file} as a source of random data used to determine which
4171 random hash function to use with the @option{-R} option. @xref{Random
4178 @cindex sort stability
4179 @cindex sort's last-resort comparison
4181 Make @command{sort} stable by disabling its last-resort comparison.
4182 This option has no effect if no fields or global ordering options
4183 other than @option{--reverse} (@option{-r}) are specified.
4186 @itemx --buffer-size=@var{size}
4188 @opindex --buffer-size
4189 @cindex size for main memory sorting
4190 Use a main-memory sort buffer of the given @var{size}. By default,
4191 @var{size} is in units of 1024 bytes. Appending @samp{%} causes
4192 @var{size} to be interpreted as a percentage of physical memory.
4193 Appending @samp{K} multiplies @var{size} by 1024 (the default),
4194 @samp{M} by 1,048,576, @samp{G} by 1,073,741,824, and so on for
4195 @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}. Appending
4196 @samp{b} causes @var{size} to be interpreted as a byte count, with no
4199 This option can improve the performance of @command{sort} by causing it
4200 to start with a larger or smaller sort buffer than the default.
4201 However, this option affects only the initial buffer size. The buffer
4202 grows beyond @var{size} if @command{sort} encounters input lines larger
4205 @item -t @var{separator}
4206 @itemx --field-separator=@var{separator}
4208 @opindex --field-separator
4209 @cindex field separator character
4210 Use character @var{separator} as the field separator when finding the
4211 sort keys in each line. By default, fields are separated by the empty
4212 string between a non-blank character and a blank character.
4213 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4216 That is, given the input line @w{@samp{ foo bar}}, @command{sort} breaks it
4217 into fields @w{@samp{ foo}} and @w{@samp{ bar}}. The field separator is
4218 not considered to be part of either the field preceding or the field
4219 following, so with @samp{sort @w{-t " "}} the same input line has
4220 three fields: an empty field, @samp{foo}, and @samp{bar}.
4221 However, fields that extend to the end of the line,
4222 as @option{-k 2}, or fields consisting of a range, as @option{-k 2,3},
4223 retain the field separators present between the endpoints of the range.
4225 To specify @acronym{ASCII} @sc{nul} as the field separator,
4226 use the two-character string @samp{\0}, e.g., @samp{sort -t '\0'}.
4228 @item -T @var{tempdir}
4229 @itemx --temporary-directory=@var{tempdir}
4231 @opindex --temporary-directory
4232 @cindex temporary directory
4234 Use directory @var{tempdir} to store temporary files, overriding the
4235 @env{TMPDIR} environment variable. If this option is given more than
4236 once, temporary files are stored in all the directories given. If you
4237 have a large sort or merge that is I/O-bound, you can often improve
4238 performance by using this option to specify directories on different
4239 disks and controllers.
4241 @item --parallel=@var{n}
4243 @cindex multithreaded sort
4244 Set the number of sorts run in parallel to @var{n}. By default,
4245 @var{n} is set to the number of available processors, but limited
4246 to 8, as there are diminishing performance gains after that.
4247 Note also that using @var{n} threads increases the memory usage by
4248 a factor of log @var{n}. Also see @ref{nproc invocation}.
4254 @cindex uniquifying output
4256 Normally, output only the first of a sequence of lines that compare
4257 equal. For the @option{--check} (@option{-c} or @option{-C}) option,
4258 check that no pair of consecutive lines compares equal.
4260 This option also disables the default last-resort comparison.
4262 The commands @code{sort -u} and @code{sort | uniq} are equivalent, but
4263 this equivalence does not extend to arbitrary @command{sort} options.
4264 For example, @code{sort -n -u} inspects only the value of the initial
4265 numeric string when checking for uniqueness, whereas @code{sort -n |
4266 uniq} inspects the entire line. @xref{uniq invocation}.
4268 @macro zeroTerminatedOption
4270 @itemx --zero-terminated
4272 @opindex --zero-terminated
4273 @cindex process zero-terminated items
4274 Delimit items with a zero byte rather than a newline (@acronym{ASCII} @sc{lf}).
4275 I.E. treat input as items separated by @acronym{ASCII} @sc{nul}
4276 and terminate output items with @acronym{ASCII} @sc{nul}.
4277 This option can be useful in conjunction with @samp{perl -0} or
4278 @samp{find -print0} and @samp{xargs -0} which do the same in order to
4279 reliably handle arbitrary file names (even those containing blanks
4280 or other special characters).
4282 @zeroTerminatedOption
4286 Historical (BSD and System V) implementations of @command{sort} have
4287 differed in their interpretation of some options, particularly
4288 @option{-b}, @option{-f}, and @option{-n}.
4289 @sc{gnu} sort follows the @acronym{POSIX}
4290 behavior, which is usually (but not always!) like the System V behavior.
4291 According to @acronym{POSIX}, @option{-n} no longer implies @option{-b}. For
4292 consistency, @option{-M} has been changed in the same way. This may
4293 affect the meaning of character positions in field specifications in
4294 obscure cases. The only fix is to add an explicit @option{-b}.
4296 A position in a sort field specified with @option{-k} may have any
4297 of the option letters @samp{MbdfghinRrV} appended to it, in which case no
4298 global ordering options are inherited by that particular field. The
4299 @option{-b} option may be independently attached to either or both of
4300 the start and end positions of a field specification, and if it is
4301 inherited from the global options it will be attached to both.
4302 If input lines can contain leading or adjacent blanks and @option{-t}
4303 is not used, then @option{-k} is typically combined with @option{-b} or
4304 an option that implicitly ignores leading blanks (@samp{Mghn}) as otherwise
4305 the varying numbers of leading blanks in fields can cause confusing results.
4307 If the start position in a sort field specifier falls after the end of
4308 the line or after the end field, the field is empty. If the @option{-b}
4309 option was specified, the @samp{.@var{c}} part of a field specification
4310 is counted from the first nonblank character of the field.
4312 @vindex _POSIX2_VERSION
4313 @vindex POSIXLY_CORRECT
4314 On older systems, @command{sort} supports an obsolete origin-zero
4315 syntax @samp{+@var{pos1} [-@var{pos2}]} for specifying sort keys.
4316 The obsolete sequence @samp{sort +@var{a}.@var{x} -@var{b}.@var{y}}
4317 is equivalent to @samp{sort -k @var{a+1}.@var{x+1},@var{b}} if @var{y}
4318 is @samp{0} or absent, otherwise it is equivalent to @samp{sort -k
4319 @var{a+1}.@var{x+1},@var{b+1}.@var{y}}.
4321 This obsolete behavior can be enabled or disabled with the
4322 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4323 conformance}); it can also be enabled when @env{POSIXLY_CORRECT} is
4324 not set by using the obsolete syntax with @samp{-@var{pos2}} present.
4326 Scripts intended for use on standard hosts should avoid obsolete
4327 syntax and should use @option{-k} instead. For example, avoid
4328 @samp{sort +2}, since it might be interpreted as either @samp{sort
4329 ./+2} or @samp{sort -k 3}. If your script must also run on hosts that
4330 support only the obsolete syntax, it can use a test like @samp{if sort
4331 -k 1 </dev/null >/dev/null 2>&1; then @dots{}} to decide which syntax
4334 Here are some examples to illustrate various combinations of options.
4339 Sort in descending (reverse) numeric order.
4346 Run no more that 4 sorts concurrently, using a buffer size of 10M.
4349 sort --parallel=4 -S 10M
4353 Sort alphabetically, omitting the first and second fields
4354 and the blanks at the start of the third field.
4355 This uses a single key composed of the characters beginning
4356 at the start of the first nonblank character in field three
4357 and extending to the end of each line.
4364 Sort numerically on the second field and resolve ties by sorting
4365 alphabetically on the third and fourth characters of field five.
4366 Use @samp{:} as the field delimiter.
4369 sort -t : -k 2,2n -k 5.3,5.4
4372 Note that if you had written @option{-k 2n} instead of @option{-k 2,2n}
4373 @command{sort} would have used all characters beginning in the second field
4374 and extending to the end of the line as the primary @emph{numeric}
4375 key. For the large majority of applications, treating keys spanning
4376 more than one field as numeric will not do what you expect.
4378 Also note that the @samp{n} modifier was applied to the field-end
4379 specifier for the first key. It would have been equivalent to
4380 specify @option{-k 2n,2} or @option{-k 2n,2n}. All modifiers except
4381 @samp{b} apply to the associated @emph{field}, regardless of whether
4382 the modifier character is attached to the field-start and/or the
4383 field-end part of the key specifier.
4386 Sort the password file on the fifth field and ignore any
4387 leading blanks. Sort lines with equal values in field five
4388 on the numeric user ID in field three. Fields are separated
4392 sort -t : -k 5b,5 -k 3,3n /etc/passwd
4393 sort -t : -n -k 5b,5 -k 3,3 /etc/passwd
4394 sort -t : -b -k 5,5 -k 3,3n /etc/passwd
4397 These three commands have equivalent effect. The first specifies that
4398 the first key's start position ignores leading blanks and the second
4399 key is sorted numerically. The other two commands rely on global
4400 options being inherited by sort keys that lack modifiers. The inheritance
4401 works in this case because @option{-k 5b,5b} and @option{-k 5b,5} are
4402 equivalent, as the location of a field-end lacking a @samp{.@var{c}}
4403 character position is not affected by whether initial blanks are
4407 Sort a set of log files, primarily by IPv4 address and secondarily by
4408 time stamp. If two lines' primary and secondary keys are identical,
4409 output the lines in the same order that they were input. The log
4410 files contain lines that look like this:
4413 4.150.156.3 - - [01/Apr/2004:06:31:51 +0000] message 1
4414 211.24.3.231 - - [24/Apr/2004:20:17:39 +0000] message 2
4417 Fields are separated by exactly one space. Sort IPv4 addresses
4418 lexicographically, e.g., 212.61.52.2 sorts before 212.129.233.201
4419 because 61 is less than 129.
4422 sort -s -t ' ' -k 4.9n -k 4.5M -k 4.2n -k 4.14,4.21 file*.log |
4423 sort -s -t '.' -k 1,1n -k 2,2n -k 3,3n -k 4,4n
4426 This example cannot be done with a single @command{sort} invocation,
4427 since IPv4 address components are separated by @samp{.} while dates
4428 come just after a space. So it is broken down into two invocations of
4429 @command{sort}: the first sorts by time stamp and the second by IPv4
4430 address. The time stamp is sorted by year, then month, then day, and
4431 finally by hour-minute-second field, using @option{-k} to isolate each
4432 field. Except for hour-minute-second there's no need to specify the
4433 end of each key field, since the @samp{n} and @samp{M} modifiers sort
4434 based on leading prefixes that cannot cross field boundaries. The
4435 IPv4 addresses are sorted lexicographically. The second sort uses
4436 @samp{-s} so that ties in the primary key are broken by the secondary
4437 key; the first sort uses @samp{-s} so that the combination of the two
4441 Generate a tags file in case-insensitive sorted order.
4444 find src -type f -print0 | sort -z -f | xargs -0 etags --append
4447 The use of @option{-print0}, @option{-z}, and @option{-0} in this case means
4448 that file names that contain blanks or other special characters are
4450 by the sort operation.
4452 @c This example is a bit contrived and needs more explanation.
4454 @c Sort records separated by an arbitrary string by using a pipe to convert
4455 @c each record delimiter string to @samp{\0}, then using sort's -z option,
4456 @c and converting each @samp{\0} back to the original record delimiter.
4459 @c printf 'c\n\nb\n\na\n' |
4460 @c perl -0pe 's/\n\n/\n\0/g' |
4462 @c perl -0pe 's/\0/\n/g'
4466 Use the common @acronym{DSU, Decorate Sort Undecorate} idiom to
4467 sort lines according to their length.
4470 awk '@{print length, $0@}' /etc/passwd | sort -n | cut -f2- -d' '
4473 In general this technique can be used to sort data that the @command{sort}
4474 command does not support, or is inefficient at, sorting directly.
4477 Shuffle a list of directories, but preserve the order of files within
4478 each directory. For instance, one could use this to generate a music
4479 playlist in which albums are shuffled but the songs of each album are
4483 ls */* | sort -t / -k 1,1R -k 2,2
4489 @node shuf invocation
4490 @section @command{shuf}: Shuffling text
4493 @cindex shuffling files
4495 @command{shuf} shuffles its input by outputting a random permutation
4496 of its input lines. Each output permutation is equally likely.
4500 shuf [@var{option}]@dots{} [@var{file}]
4501 shuf -e [@var{option}]@dots{} [@var{arg}]@dots{}
4502 shuf -i @var{lo}-@var{hi} [@var{option}]@dots{}
4505 @command{shuf} has three modes of operation that affect where it
4506 obtains its input lines. By default, it reads lines from standard
4507 input. The following options change the operation mode:
4515 @cindex command-line operands to shuffle
4516 Treat each command-line operand as an input line.
4518 @item -i @var{lo}-@var{hi}
4519 @itemx --input-range=@var{lo}-@var{hi}
4521 @opindex --input-range
4522 @cindex input range to shuffle
4523 Act as if input came from a file containing the range of unsigned
4524 decimal integers @var{lo}@dots{}@var{hi}, one per line.
4528 @command{shuf}'s other options can affect its behavior in all
4533 @item -n @var{lines}
4534 @itemx --head-count=@var{count}
4536 @opindex --head-count
4537 @cindex head of output
4538 Output at most @var{count} lines. By default, all input lines are
4541 @item -o @var{output-file}
4542 @itemx --output=@var{output-file}
4545 @cindex overwriting of input, allowed
4546 Write output to @var{output-file} instead of standard output.
4547 @command{shuf} reads all input before opening
4548 @var{output-file}, so you can safely shuffle a file in place by using
4549 commands like @code{shuf -o F <F} and @code{cat F | shuf -o F}.
4551 @item --random-source=@var{file}
4552 @opindex --random-source
4553 @cindex random source for shuffling
4554 Use @var{file} as a source of random data used to determine which
4555 permutation to generate. @xref{Random sources}.
4557 @zeroTerminatedOption
4573 might produce the output
4583 Similarly, the command:
4586 shuf -e clubs hearts diamonds spades
4600 and the command @samp{shuf -i 1-4} might output:
4610 These examples all have four input lines, so @command{shuf} might
4611 produce any of the twenty-four possible permutations of the input. In
4612 general, if there are @var{n} input lines, there are @var{n}! (i.e.,
4613 @var{n} factorial, or @var{n} * (@var{n} - 1) * @dots{} * 1) possible
4614 output permutations.
4619 @node uniq invocation
4620 @section @command{uniq}: Uniquify files
4623 @cindex uniquify files
4625 @command{uniq} writes the unique lines in the given @file{input}, or
4626 standard input if nothing is given or for an @var{input} name of
4630 uniq [@var{option}]@dots{} [@var{input} [@var{output}]]
4633 By default, @command{uniq} prints its input lines, except that
4634 it discards all but the first of adjacent repeated lines, so that
4635 no output lines are repeated. Optionally, it can instead discard
4636 lines that are not repeated, or all repeated lines.
4638 The input need not be sorted, but repeated input lines are detected
4639 only if they are adjacent. If you want to discard non-adjacent
4640 duplicate lines, perhaps you want to use @code{sort -u}.
4641 @xref{sort invocation}.
4644 Comparisons honor the rules specified by the @env{LC_COLLATE}
4647 If no @var{output} file is specified, @command{uniq} writes to standard
4650 The program accepts the following options. Also see @ref{Common options}.
4655 @itemx --skip-fields=@var{n}
4657 @opindex --skip-fields
4658 Skip @var{n} fields on each line before checking for uniqueness. Use
4659 a null string for comparison if a line has fewer than @var{n} fields. Fields
4660 are sequences of non-space non-tab characters that are separated from
4661 each other by at least one space or tab.
4663 For compatibility @command{uniq} supports an obsolete option syntax
4664 @option{-@var{n}}. New scripts should use @option{-f @var{n}} instead.
4667 @itemx --skip-chars=@var{n}
4669 @opindex --skip-chars
4670 Skip @var{n} characters before checking for uniqueness. Use a null string
4671 for comparison if a line has fewer than @var{n} characters. If you use both
4672 the field and character skipping options, fields are skipped over first.
4674 @vindex _POSIX2_VERSION
4675 On older systems, @command{uniq} supports an obsolete option syntax
4677 This obsolete behavior can be enabled or disabled with the
4678 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4679 conformance}), but portable scripts should avoid commands whose
4680 behavior depends on this variable.
4681 For example, use @samp{uniq ./+10} or @samp{uniq -s 10} rather than
4682 the ambiguous @samp{uniq +10}.
4688 Print the number of times each line occurred along with the line.
4691 @itemx --ignore-case
4693 @opindex --ignore-case
4694 Ignore differences in case when comparing lines.
4700 @cindex repeated lines, outputting
4701 Discard lines that are not repeated. When used by itself, this option
4702 causes @command{uniq} to print the first copy of each repeated line,
4706 @itemx --all-repeated[=@var{delimit-method}]
4708 @opindex --all-repeated
4709 @cindex all repeated lines, outputting
4710 Do not discard the second and subsequent repeated input lines,
4711 but discard lines that are not repeated.
4712 This option is useful mainly in conjunction with other options e.g.,
4713 to ignore case or to compare only selected fields.
4714 The optional @var{delimit-method} tells how to delimit
4715 groups of repeated lines, and must be one of the following:
4720 Do not delimit groups of repeated lines.
4721 This is equivalent to @option{--all-repeated} (@option{-D}).
4724 Output a newline before each group of repeated lines.
4725 With @option{--zero-terminated} (@option{-z}), use a zero
4726 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4729 Separate groups of repeated lines with a single newline.
4730 With @option{--zero-terminated} (@option{-z}), use a zero
4731 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4732 This is the same as using @samp{prepend}, except that
4733 no delimiter is inserted before the first group, and hence
4734 may be better suited for output direct to users.
4737 Note that when groups are delimited and the input stream contains
4738 two or more consecutive blank lines, then the output is ambiguous.
4739 To avoid that, filter the input through @samp{tr -s '\n'} to replace
4740 each sequence of consecutive newlines with a single newline.
4742 This is a @sc{gnu} extension.
4743 @c FIXME: give an example showing *how* it's useful
4749 @cindex unique lines, outputting
4750 Discard the first repeated line. When used by itself, this option
4751 causes @command{uniq} to print unique lines, and nothing else.
4754 @itemx --check-chars=@var{n}
4756 @opindex --check-chars
4757 Compare at most @var{n} characters on each line (after skipping any specified
4758 fields and characters). By default the entire rest of the lines are
4761 @zeroTerminatedOption
4768 @node comm invocation
4769 @section @command{comm}: Compare two sorted files line by line
4772 @cindex line-by-line comparison
4773 @cindex comparing sorted files
4775 @command{comm} writes to standard output lines that are common, and lines
4776 that are unique, to two input files; a file name of @samp{-} means
4777 standard input. Synopsis:
4780 comm [@var{option}]@dots{} @var{file1} @var{file2}
4784 Before @command{comm} can be used, the input files must be sorted using the
4785 collating sequence specified by the @env{LC_COLLATE} locale.
4786 If an input file ends in a non-newline
4787 character, a newline is silently appended. The @command{sort} command with
4788 no options always outputs a file that is suitable input to @command{comm}.
4790 @cindex differing lines
4791 @cindex common lines
4792 With no options, @command{comm} produces three-column output. Column one
4793 contains lines unique to @var{file1}, column two contains lines unique
4794 to @var{file2}, and column three contains lines common to both files.
4795 Columns are separated by a single TAB character.
4796 @c FIXME: when there's an option to supply an alternative separator
4797 @c string, append `by default' to the above sentence.
4802 The options @option{-1}, @option{-2}, and @option{-3} suppress printing of
4803 the corresponding columns (and separators). Also see @ref{Common options}.
4805 Unlike some other comparison utilities, @command{comm} has an exit
4806 status that does not depend on the result of the comparison.
4807 Upon normal completion @command{comm} produces an exit code of zero.
4808 If there is an error it exits with nonzero status.
4810 @macro checkOrderOption{cmd}
4811 If the @option{--check-order} option is given, unsorted inputs will
4812 cause a fatal error message. If the option @option{--nocheck-order}
4813 is given, unsorted inputs will never cause an error message. If neither
4814 of these options is given, wrongly sorted inputs are diagnosed
4815 only if an input file is found to contain unpairable
4817 lines, and when both input files are non empty.
4819 @ifclear JOIN_COMMAND
4822 If an input file is diagnosed as being unsorted, the @command{\cmd\}
4823 command will exit with a nonzero status (and the output should not be used).
4825 Forcing @command{\cmd\} to process wrongly sorted input files
4826 containing unpairable lines by specifying @option{--nocheck-order} is
4827 not guaranteed to produce any particular output. The output will
4828 probably not correspond with whatever you hoped it would be.
4830 @checkOrderOption{comm}
4835 Fail with an error message if either input file is wrongly ordered.
4837 @item --nocheck-order
4838 Do not check that both input files are in sorted order.
4842 @item --output-delimiter=@var{str}
4843 Print @var{str} between adjacent output columns,
4844 rather than the default of a single TAB character.
4846 The delimiter @var{str} may not be empty.
4850 @node ptx invocation
4851 @section @command{ptx}: Produce permuted indexes
4855 @command{ptx} reads a text file and essentially produces a permuted index, with
4856 each keyword in its context. The calling sketch is either one of:
4859 ptx [@var{option} @dots{}] [@var{file} @dots{}]
4860 ptx -G [@var{option} @dots{}] [@var{input} [@var{output}]]
4863 The @option{-G} (or its equivalent: @option{--traditional}) option disables
4864 all @sc{gnu} extensions and reverts to traditional mode, thus introducing some
4865 limitations and changing several of the program's default option values.
4866 When @option{-G} is not specified, @sc{gnu} extensions are always enabled.
4867 @sc{gnu} extensions to @command{ptx} are documented wherever appropriate in this
4868 document. For the full list, see @xref{Compatibility in ptx}.
4870 Individual options are explained in the following sections.
4872 When @sc{gnu} extensions are enabled, there may be zero, one or several
4873 @var{file}s after the options. If there is no @var{file}, the program
4874 reads the standard input. If there is one or several @var{file}s, they
4875 give the name of input files which are all read in turn, as if all the
4876 input files were concatenated. However, there is a full contextual
4877 break between each file and, when automatic referencing is requested,
4878 file names and line numbers refer to individual text input files. In
4879 all cases, the program outputs the permuted index to the standard
4882 When @sc{gnu} extensions are @emph{not} enabled, that is, when the program
4883 operates in traditional mode, there may be zero, one or two parameters
4884 besides the options. If there are no parameters, the program reads the
4885 standard input and outputs the permuted index to the standard output.
4886 If there is only one parameter, it names the text @var{input} to be read
4887 instead of the standard input. If two parameters are given, they give
4888 respectively the name of the @var{input} file to read and the name of
4889 the @var{output} file to produce. @emph{Be very careful} to note that,
4890 in this case, the contents of file given by the second parameter is
4891 destroyed. This behavior is dictated by System V @command{ptx}
4892 compatibility; @sc{gnu} Standards normally discourage output parameters not
4893 introduced by an option.
4895 Note that for @emph{any} file named as the value of an option or as an
4896 input text file, a single dash @kbd{-} may be used, in which case
4897 standard input is assumed. However, it would not make sense to use this
4898 convention more than once per program invocation.
4901 * General options in ptx:: Options which affect general program behavior.
4902 * Charset selection in ptx:: Underlying character set considerations.
4903 * Input processing in ptx:: Input fields, contexts, and keyword selection.
4904 * Output formatting in ptx:: Types of output format, and sizing the fields.
4905 * Compatibility in ptx::
4909 @node General options in ptx
4910 @subsection General options
4915 @itemx --traditional
4916 As already explained, this option disables all @sc{gnu} extensions to
4917 @command{ptx} and switches to traditional mode.
4920 Print a short help on standard output, then exit without further
4924 Print the program version on standard output, then exit without further
4932 @node Charset selection in ptx
4933 @subsection Charset selection
4935 @c FIXME: People don't necessarily know what an IBM-PC was these days.
4936 As it is set up now, the program assumes that the input file is coded
4937 using 8-bit @acronym{ISO} 8859-1 code, also known as Latin-1 character set,
4938 @emph{unless} it is compiled for MS-DOS, in which case it uses the
4939 character set of the IBM-PC@. (@sc{gnu} @command{ptx} is not known to work on
4940 smaller MS-DOS machines anymore.) Compared to 7-bit @acronym{ASCII}, the set
4941 of characters which are letters is different; this alters the behavior
4942 of regular expression matching. Thus, the default regular expression
4943 for a keyword allows foreign or diacriticized letters. Keyword sorting,
4944 however, is still crude; it obeys the underlying character set ordering
4950 @itemx --ignore-case
4951 Fold lower case letters to upper case for sorting.
4956 @node Input processing in ptx
4957 @subsection Word selection and input processing
4962 @itemx --break-file=@var{file}
4964 This option provides an alternative (to @option{-W}) method of describing
4965 which characters make up words. It introduces the name of a
4966 file which contains a list of characters which can@emph{not} be part of
4967 one word; this file is called the @dfn{Break file}. Any character which
4968 is not part of the Break file is a word constituent. If both options
4969 @option{-b} and @option{-W} are specified, then @option{-W} has precedence and
4970 @option{-b} is ignored.
4972 When @sc{gnu} extensions are enabled, the only way to avoid newline as a
4973 break character is to write all the break characters in the file with no
4974 newline at all, not even at the end of the file. When @sc{gnu} extensions
4975 are disabled, spaces, tabs and newlines are always considered as break
4976 characters even if not included in the Break file.
4979 @itemx --ignore-file=@var{file}
4981 The file associated with this option contains a list of words which will
4982 never be taken as keywords in concordance output. It is called the
4983 @dfn{Ignore file}. The file contains exactly one word in each line; the
4984 end of line separation of words is not subject to the value of the
4988 @itemx --only-file=@var{file}
4990 The file associated with this option contains a list of words which will
4991 be retained in concordance output; any word not mentioned in this file
4992 is ignored. The file is called the @dfn{Only file}. The file contains
4993 exactly one word in each line; the end of line separation of words is
4994 not subject to the value of the @option{-S} option.
4996 There is no default for the Only file. When both an Only file and an
4997 Ignore file are specified, a word is considered a keyword only
4998 if it is listed in the Only file and not in the Ignore file.
5003 On each input line, the leading sequence of non-white space characters will be
5004 taken to be a reference that has the purpose of identifying this input
5005 line in the resulting permuted index. For more information about reference
5006 production, see @xref{Output formatting in ptx}.
5007 Using this option changes the default value for option @option{-S}.
5009 Using this option, the program does not try very hard to remove
5010 references from contexts in output, but it succeeds in doing so
5011 @emph{when} the context ends exactly at the newline. If option
5012 @option{-r} is used with @option{-S} default value, or when @sc{gnu} extensions
5013 are disabled, this condition is always met and references are completely
5014 excluded from the output contexts.
5016 @item -S @var{regexp}
5017 @itemx --sentence-regexp=@var{regexp}
5019 This option selects which regular expression will describe the end of a
5020 line or the end of a sentence. In fact, this regular expression is not
5021 the only distinction between end of lines or end of sentences, and input
5022 line boundaries have no special significance outside this option. By
5023 default, when @sc{gnu} extensions are enabled and if @option{-r} option is not
5024 used, end of sentences are used. In this case, this @var{regex} is
5025 imported from @sc{gnu} Emacs:
5028 [.?!][]\"')@}]*\\($\\|\t\\| \\)[ \t\n]*
5031 Whenever @sc{gnu} extensions are disabled or if @option{-r} option is used, end
5032 of lines are used; in this case, the default @var{regexp} is just:
5038 Using an empty @var{regexp} is equivalent to completely disabling end of
5039 line or end of sentence recognition. In this case, the whole file is
5040 considered to be a single big line or sentence. The user might want to
5041 disallow all truncation flag generation as well, through option @option{-F
5042 ""}. @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
5045 When the keywords happen to be near the beginning of the input line or
5046 sentence, this often creates an unused area at the beginning of the
5047 output context line; when the keywords happen to be near the end of the
5048 input line or sentence, this often creates an unused area at the end of
5049 the output context line. The program tries to fill those unused areas
5050 by wrapping around context in them; the tail of the input line or
5051 sentence is used to fill the unused area on the left of the output line;
5052 the head of the input line or sentence is used to fill the unused area
5053 on the right of the output line.
5055 As a matter of convenience to the user, many usual backslashed escape
5056 sequences from the C language are recognized and converted to the
5057 corresponding characters by @command{ptx} itself.
5059 @item -W @var{regexp}
5060 @itemx --word-regexp=@var{regexp}
5062 This option selects which regular expression will describe each keyword.
5063 By default, if @sc{gnu} extensions are enabled, a word is a sequence of
5064 letters; the @var{regexp} used is @samp{\w+}. When @sc{gnu} extensions are
5065 disabled, a word is by default anything which ends with a space, a tab
5066 or a newline; the @var{regexp} used is @samp{[^ \t\n]+}.
5068 An empty @var{regexp} is equivalent to not using this option.
5069 @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
5072 As a matter of convenience to the user, many usual backslashed escape
5073 sequences, as found in the C language, are recognized and converted to
5074 the corresponding characters by @command{ptx} itself.
5079 @node Output formatting in ptx
5080 @subsection Output formatting
5082 Output format is mainly controlled by the @option{-O} and @option{-T} options
5083 described in the table below. When neither @option{-O} nor @option{-T} are
5084 selected, and if @sc{gnu} extensions are enabled, the program chooses an
5085 output format suitable for a dumb terminal. Each keyword occurrence is
5086 output to the center of one line, surrounded by its left and right
5087 contexts. Each field is properly justified, so the concordance output
5088 can be readily observed. As a special feature, if automatic
5089 references are selected by option @option{-A} and are output before the
5090 left context, that is, if option @option{-R} is @emph{not} selected, then
5091 a colon is added after the reference; this nicely interfaces with @sc{gnu}
5092 Emacs @code{next-error} processing. In this default output format, each
5093 white space character, like newline and tab, is merely changed to
5094 exactly one space, with no special attempt to compress consecutive
5095 spaces. This might change in the future. Except for those white space
5096 characters, every other character of the underlying set of 256
5097 characters is transmitted verbatim.
5099 Output format is further controlled by the following options.
5103 @item -g @var{number}
5104 @itemx --gap-size=@var{number}
5106 Select the size of the minimum white space gap between the fields on the
5109 @item -w @var{number}
5110 @itemx --width=@var{number}
5112 Select the maximum output width of each final line. If references are
5113 used, they are included or excluded from the maximum output width
5114 depending on the value of option @option{-R}. If this option is not
5115 selected, that is, when references are output before the left context,
5116 the maximum output width takes into account the maximum length of all
5117 references. If this option is selected, that is, when references are
5118 output after the right context, the maximum output width does not take
5119 into account the space taken by references, nor the gap that precedes
5123 @itemx --auto-reference
5125 Select automatic references. Each input line will have an automatic
5126 reference made up of the file name and the line ordinal, with a single
5127 colon between them. However, the file name will be empty when standard
5128 input is being read. If both @option{-A} and @option{-r} are selected, then
5129 the input reference is still read and skipped, but the automatic
5130 reference is used at output time, overriding the input reference.
5133 @itemx --right-side-refs
5135 In the default output format, when option @option{-R} is not used, any
5136 references produced by the effect of options @option{-r} or @option{-A} are
5137 placed to the far right of output lines, after the right context. With
5138 default output format, when the @option{-R} option is specified, references
5139 are rather placed at the beginning of each output line, before the left
5140 context. For any other output format, option @option{-R} is
5141 ignored, with one exception: with @option{-R} the width of references
5142 is @emph{not} taken into account in total output width given by @option{-w}.
5144 This option is automatically selected whenever @sc{gnu} extensions are
5147 @item -F @var{string}
5148 @itemx --flac-truncation=@var{string}
5150 This option will request that any truncation in the output be reported
5151 using the string @var{string}. Most output fields theoretically extend
5152 towards the beginning or the end of the current line, or current
5153 sentence, as selected with option @option{-S}. But there is a maximum
5154 allowed output line width, changeable through option @option{-w}, which is
5155 further divided into space for various output fields. When a field has
5156 to be truncated because it cannot extend beyond the beginning or the end of
5157 the current line to fit in, then a truncation occurs. By default,
5158 the string used is a single slash, as in @option{-F /}.
5160 @var{string} may have more than one character, as in @option{-F ...}.
5161 Also, in the particular case when @var{string} is empty (@option{-F ""}),
5162 truncation flagging is disabled, and no truncation marks are appended in
5165 As a matter of convenience to the user, many usual backslashed escape
5166 sequences, as found in the C language, are recognized and converted to
5167 the corresponding characters by @command{ptx} itself.
5169 @item -M @var{string}
5170 @itemx --macro-name=@var{string}
5172 Select another @var{string} to be used instead of @samp{xx}, while
5173 generating output suitable for @command{nroff}, @command{troff} or @TeX{}.
5176 @itemx --format=roff
5178 Choose an output format suitable for @command{nroff} or @command{troff}
5179 processing. Each output line will look like:
5182 .xx "@var{tail}" "@var{before}" "@var{keyword_and_after}"@c
5183 "@var{head}" "@var{ref}"
5186 so it will be possible to write a @samp{.xx} roff macro to take care of
5187 the output typesetting. This is the default output format when @sc{gnu}
5188 extensions are disabled. Option @option{-M} can be used to change
5189 @samp{xx} to another macro name.
5191 In this output format, each non-graphical character, like newline and
5192 tab, is merely changed to exactly one space, with no special attempt to
5193 compress consecutive spaces. Each quote character: @kbd{"} is doubled
5194 so it will be correctly processed by @command{nroff} or @command{troff}.
5199 Choose an output format suitable for @TeX{} processing. Each output
5200 line will look like:
5203 \xx @{@var{tail}@}@{@var{before}@}@{@var{keyword}@}@c
5204 @{@var{after}@}@{@var{head}@}@{@var{ref}@}
5208 so it will be possible to write a @code{\xx} definition to take care of
5209 the output typesetting. Note that when references are not being
5210 produced, that is, neither option @option{-A} nor option @option{-r} is
5211 selected, the last parameter of each @code{\xx} call is inhibited.
5212 Option @option{-M} can be used to change @samp{xx} to another macro
5215 In this output format, some special characters, like @kbd{$}, @kbd{%},
5216 @kbd{&}, @kbd{#} and @kbd{_} are automatically protected with a
5217 backslash. Curly brackets @kbd{@{}, @kbd{@}} are protected with a
5218 backslash and a pair of dollar signs (to force mathematical mode). The
5219 backslash itself produces the sequence @code{\backslash@{@}}.
5220 Circumflex and tilde diacritical marks produce the sequence @code{^\@{ @}} and
5221 @code{~\@{ @}} respectively. Other diacriticized characters of the
5222 underlying character set produce an appropriate @TeX{} sequence as far
5223 as possible. The other non-graphical characters, like newline and tab,
5224 and all other characters which are not part of @acronym{ASCII}, are merely
5225 changed to exactly one space, with no special attempt to compress
5226 consecutive spaces. Let me know how to improve this special character
5227 processing for @TeX{}.
5232 @node Compatibility in ptx
5233 @subsection The @sc{gnu} extensions to @command{ptx}
5235 This version of @command{ptx} contains a few features which do not exist in
5236 System V @command{ptx}. These extra features are suppressed by using the
5237 @option{-G} command line option, unless overridden by other command line
5238 options. Some @sc{gnu} extensions cannot be recovered by overriding, so the
5239 simple rule is to avoid @option{-G} if you care about @sc{gnu} extensions.
5240 Here are the differences between this program and System V @command{ptx}.
5245 This program can read many input files at once, it always writes the
5246 resulting concordance on standard output. On the other hand, System V
5247 @command{ptx} reads only one file and sends the result to standard output
5248 or, if a second @var{file} parameter is given on the command, to that
5251 Having output parameters not introduced by options is a dangerous
5252 practice which @sc{gnu} avoids as far as possible. So, for using @command{ptx}
5253 portably between @sc{gnu} and System V, you should always use it with a
5254 single input file, and always expect the result on standard output. You
5255 might also want to automatically configure in a @option{-G} option to
5256 @command{ptx} calls in products using @command{ptx}, if the configurator finds
5257 that the installed @command{ptx} accepts @option{-G}.
5260 The only options available in System V @command{ptx} are options @option{-b},
5261 @option{-f}, @option{-g}, @option{-i}, @option{-o}, @option{-r}, @option{-t} and
5262 @option{-w}. All other options are @sc{gnu} extensions and are not repeated in
5263 this enumeration. Moreover, some options have a slightly different
5264 meaning when @sc{gnu} extensions are enabled, as explained below.
5267 By default, concordance output is not formatted for @command{troff} or
5268 @command{nroff}. It is rather formatted for a dumb terminal. @command{troff}
5269 or @command{nroff} output may still be selected through option @option{-O}.
5272 Unless @option{-R} option is used, the maximum reference width is
5273 subtracted from the total output line width. With @sc{gnu} extensions
5274 disabled, width of references is not taken into account in the output
5275 line width computations.
5278 All 256 bytes, even @acronym{ASCII} @sc{nul} bytes, are always read and
5279 processed from input file with no adverse effect, even if @sc{gnu} extensions
5280 are disabled. However, System V @command{ptx} does not accept 8-bit characters,
5281 a few control characters are rejected, and the tilde @kbd{~} is also rejected.
5284 Input line length is only limited by available memory, even if @sc{gnu}
5285 extensions are disabled. However, System V @command{ptx} processes only
5286 the first 200 characters in each line.
5289 The break (non-word) characters default to be every character except all
5290 letters of the underlying character set, diacriticized or not. When @sc{gnu}
5291 extensions are disabled, the break characters default to space, tab and
5295 The program makes better use of output line width. If @sc{gnu} extensions
5296 are disabled, the program rather tries to imitate System V @command{ptx},
5297 but still, there are some slight disposition glitches this program does
5298 not completely reproduce.
5301 The user can specify both an Ignore file and an Only file. This is not
5302 allowed with System V @command{ptx}.
5307 @node tsort invocation
5308 @section @command{tsort}: Topological sort
5311 @cindex topological sort
5313 @command{tsort} performs a topological sort on the given @var{file}, or
5314 standard input if no input file is given or for a @var{file} of
5315 @samp{-}. For more details and some history, see @ref{tsort background}.
5319 tsort [@var{option}] [@var{file}]
5322 @command{tsort} reads its input as pairs of strings, separated by blanks,
5323 indicating a partial ordering. The output is a total ordering that
5324 corresponds to the given partial ordering.
5338 will produce the output
5349 Consider a more realistic example.
5350 You have a large set of functions all in one file, and they may all be
5351 declared static except one. Currently that one (say @code{main}) is the
5352 first function defined in the file, and the ones it calls directly follow
5353 it, followed by those they call, etc. Let's say that you are determined
5354 to take advantage of prototypes, so you have to choose between declaring
5355 all of those functions (which means duplicating a lot of information from
5356 the definitions) and rearranging the functions so that as many as possible
5357 are defined before they are used. One way to automate the latter process
5358 is to get a list for each function of the functions it calls directly.
5359 Many programs can generate such lists. They describe a call graph.
5360 Consider the following list, in which a given line indicates that the
5361 function on the left calls the one on the right directly.
5367 tail_file pretty_name
5368 tail_file write_header
5370 tail_forever recheck
5371 tail_forever pretty_name
5372 tail_forever write_header
5373 tail_forever dump_remainder
5376 tail_lines start_lines
5377 tail_lines dump_remainder
5378 tail_lines file_lines
5379 tail_lines pipe_lines
5381 tail_bytes start_bytes
5382 tail_bytes dump_remainder
5383 tail_bytes pipe_bytes
5384 file_lines dump_remainder
5388 then you can use @command{tsort} to produce an ordering of those
5389 functions that satisfies your requirement.
5392 example$ tsort call-graph | tac
5412 @command{tsort} detects any cycles in the input and writes the first cycle
5413 encountered to standard error.
5415 Note that for a given partial ordering, generally there is no unique
5416 total ordering. In the context of the call graph above, the function
5417 @code{parse_options} may be placed anywhere in the list as long as it
5418 precedes @code{main}.
5420 The only options are @option{--help} and @option{--version}. @xref{Common
5426 * tsort background:: Where tsort came from.
5429 @node tsort background
5430 @subsection @command{tsort}: Background
5432 @command{tsort} exists because very early versions of the Unix linker processed
5433 an archive file exactly once, and in order. As @command{ld} read each object
5434 in the archive, it decided whether it was needed in the program based on
5435 whether it defined any symbols which were undefined at that point in
5438 This meant that dependencies within the archive had to be handled
5439 specially. For example, @code{scanf} probably calls @code{read}. That means
5440 that in a single pass through an archive, it was important for @code{scanf.o}
5441 to appear before read.o, because otherwise a program which calls
5442 @code{scanf} but not @code{read} might end up with an unexpected unresolved
5443 reference to @code{read}.
5445 The way to address this problem was to first generate a set of
5446 dependencies of one object file on another. This was done by a shell
5447 script called @command{lorder}. The GNU tools don't provide a version of
5448 lorder, as far as I know, but you can still find it in BSD
5451 Then you ran @command{tsort} over the @command{lorder} output, and you used the
5452 resulting sort to define the order in which you added objects to the archive.
5454 This whole procedure has been obsolete since about 1980, because
5455 Unix archives now contain a symbol table (traditionally built by
5456 @command{ranlib}, now generally built by @command{ar} itself), and the Unix
5457 linker uses the symbol table to effectively make multiple passes over
5460 Anyhow, that's where tsort came from. To solve an old problem with
5461 the way the linker handled archive files, which has since been solved
5465 @node Operating on fields
5466 @chapter Operating on fields
5469 * cut invocation:: Print selected parts of lines.
5470 * paste invocation:: Merge lines of files.
5471 * join invocation:: Join lines on a common field.
5475 @node cut invocation
5476 @section @command{cut}: Print selected parts of lines
5479 @command{cut} writes to standard output selected parts of each line of each
5480 input file, or standard input if no files are given or for a file name of
5484 cut @var{option}@dots{} [@var{file}]@dots{}
5487 In the table which follows, the @var{byte-list}, @var{character-list},
5488 and @var{field-list} are one or more numbers or ranges (two numbers
5489 separated by a dash) separated by commas. Bytes, characters, and
5490 fields are numbered starting at 1. Incomplete ranges may be
5491 given: @option{-@var{m}} means @samp{1-@var{m}}; @samp{@var{n}-} means
5492 @samp{@var{n}} through end of line or last field. The list elements
5493 can be repeated, can overlap, and can be specified in any order; but
5494 the selected input is written in the same order that it is read, and
5495 is written exactly once.
5497 The program accepts the following options. Also see @ref{Common
5502 @item -b @var{byte-list}
5503 @itemx --bytes=@var{byte-list}
5506 Select for printing only the bytes in positions listed in
5507 @var{byte-list}. Tabs and backspaces are treated like any other
5508 character; they take up 1 byte. If an output delimiter is specified,
5509 (see the description of @option{--output-delimiter}), then output that
5510 string between ranges of selected bytes.
5512 @item -c @var{character-list}
5513 @itemx --characters=@var{character-list}
5515 @opindex --characters
5516 Select for printing only the characters in positions listed in
5517 @var{character-list}. The same as @option{-b} for now, but
5518 internationalization will change that. Tabs and backspaces are
5519 treated like any other character; they take up 1 character. If an
5520 output delimiter is specified, (see the description of
5521 @option{--output-delimiter}), then output that string between ranges
5524 @item -f @var{field-list}
5525 @itemx --fields=@var{field-list}
5528 Select for printing only the fields listed in @var{field-list}.
5529 Fields are separated by a TAB character by default. Also print any
5530 line that contains no delimiter character, unless the
5531 @option{--only-delimited} (@option{-s}) option is specified.
5533 Note @command{awk} supports more sophisticated field processing,
5534 and by default will use (and discard) runs of blank characters to
5535 separate fields, and ignore leading and trailing blanks.
5538 awk '{print $2}' # print the second field
5539 awk '{print $NF-1}' # print the penultimate field
5540 awk '{print $2,$1}' # reorder the first two fields
5544 In the unlikely event that @command{awk} is unavailable,
5545 one can use the @command{join} command, to process blank
5546 characters as @command{awk} does above.
5549 join -a1 -o 1.2 - /dev/null # print the second field
5550 join -a1 -o 1.2,1.1 - /dev/null # reorder the first two fields
5554 @item -d @var{input_delim_byte}
5555 @itemx --delimiter=@var{input_delim_byte}
5557 @opindex --delimiter
5558 With @option{-f}, use the first byte of @var{input_delim_byte} as
5559 the input fields separator (default is TAB).
5563 Do not split multi-byte characters (no-op for now).
5566 @itemx --only-delimited
5568 @opindex --only-delimited
5569 For @option{-f}, do not print lines that do not contain the field separator
5570 character. Normally, any line without a field separator is printed verbatim.
5572 @item --output-delimiter=@var{output_delim_string}
5573 @opindex --output-delimiter
5574 With @option{-f}, output fields are separated by @var{output_delim_string}.
5575 The default with @option{-f} is to use the input delimiter.
5576 When using @option{-b} or @option{-c} to select ranges of byte or
5577 character offsets (as opposed to ranges of fields),
5578 output @var{output_delim_string} between non-overlapping
5579 ranges of selected bytes.
5582 @opindex --complement
5583 This option is a @acronym{GNU} extension.
5584 Select for printing the complement of the bytes, characters or fields
5585 selected with the @option{-b}, @option{-c} or @option{-f} options.
5586 In other words, do @emph{not} print the bytes, characters or fields
5587 specified via those options. This option is useful when you have
5588 many fields and want to print all but a few of them.
5595 @node paste invocation
5596 @section @command{paste}: Merge lines of files
5599 @cindex merging files
5601 @command{paste} writes to standard output lines consisting of sequentially
5602 corresponding lines of each given file, separated by a TAB character.
5603 Standard input is used for a file name of @samp{-} or if no input files
5625 paste [@var{option}]@dots{} [@var{file}]@dots{}
5628 The program accepts the following options. Also see @ref{Common options}.
5636 Paste the lines of one file at a time rather than one line from each
5637 file. Using the above example data:
5640 $ paste -s num2 let3
5645 @item -d @var{delim-list}
5646 @itemx --delimiters=@var{delim-list}
5648 @opindex --delimiters
5649 Consecutively use the characters in @var{delim-list} instead of
5650 TAB to separate merged lines. When @var{delim-list} is
5651 exhausted, start again at its beginning. Using the above example data:
5654 $ paste -d '%_' num2 let3 num2
5665 @node join invocation
5666 @section @command{join}: Join lines on a common field
5669 @cindex common field, joining on
5671 @command{join} writes to standard output a line for each pair of input
5672 lines that have identical join fields. Synopsis:
5675 join [@var{option}]@dots{} @var{file1} @var{file2}
5678 Either @var{file1} or @var{file2} (but not both) can be @samp{-},
5679 meaning standard input. @var{file1} and @var{file2} should be
5680 sorted on the join fields.
5683 Normally, the sort order is that of the
5684 collating sequence specified by the @env{LC_COLLATE} locale. Unless
5685 the @option{-t} option is given, the sort comparison ignores blanks at
5686 the start of the join field, as in @code{sort -b}. If the
5687 @option{--ignore-case} option is given, the sort comparison ignores
5688 the case of characters in the join field, as in @code{sort -f}.
5690 The @command{sort} and @command{join} commands should use consistent
5691 locales and options if the output of @command{sort} is fed to
5692 @command{join}. You can use a command like @samp{sort -k 1b,1} to
5693 sort a file on its default join field, but if you select a non-default
5694 locale, join field, separator, or comparison options, then you should
5695 do so consistently between @command{join} and @command{sort}.
5696 If @samp{join -t ''} is specified then the whole line is considered which
5697 matches the default operation of sort.
5699 If the input has no unpairable lines, a @acronym{GNU} extension is
5700 available; the sort order can be any order that considers two fields
5701 to be equal if and only if the sort comparison described above
5702 considers them to be equal. For example:
5720 @checkOrderOption{join}
5725 @item the join field is the first field in each line;
5726 @item fields in the input are separated by one or more blanks, with leading
5727 blanks on the line ignored;
5728 @item fields in the output are separated by a space;
5729 @item each output line consists of the join field, the remaining
5730 fields from @var{file1}, then the remaining fields from @var{file2}.
5733 The program accepts the following options. Also see @ref{Common options}.
5737 @item -a @var{file-number}
5739 Print a line for each unpairable line in file @var{file-number} (either
5740 @samp{1} or @samp{2}), in addition to the normal output.
5743 Fail with an error message if either input file is wrongly ordered.
5745 @item --nocheck-order
5746 Do not check that both input files are in sorted order. This is the default.
5748 @item -e @var{string}
5750 Replace those output fields that are missing in the input with @var{string}.
5751 I.E. missing fields specified with the @option{-12jo} options.
5755 Treat the first line of each input file as a header line. The header lines will
5756 be joined and printed as the first output line. If @option{-o} is used to
5757 specify output format, the header line will be printed according to the
5758 specified format. The header lines will not be checked for ordering even if
5759 @option{--check-order} is specified. Also if the header lines from each file
5760 do not match, the heading fields from the first file will be used.
5763 @itemx --ignore-case
5765 @opindex --ignore-case
5766 Ignore differences in case when comparing keys.
5767 With this option, the lines of the input files must be ordered in the same way.
5768 Use @samp{sort -f} to produce this ordering.
5770 @item -1 @var{field}
5772 Join on field @var{field} (a positive integer) of file 1.
5774 @item -2 @var{field}
5776 Join on field @var{field} (a positive integer) of file 2.
5778 @item -j @var{field}
5779 Equivalent to @option{-1 @var{field} -2 @var{field}}.
5781 @item -o @var{field-list}
5783 If the keyword @samp{auto} is specified, infer the output format from
5784 the first line in each file. This is the same as the default output format
5785 but also ensures the same number of fields are output for each line.
5786 Missing fields are replaced with the @option{-e} option and extra fields
5789 Otherwise, construct each output line according to the format in
5790 @var{field-list}. Each element in @var{field-list} is either the single
5791 character @samp{0} or has the form @var{m.n} where the file number, @var{m},
5792 is @samp{1} or @samp{2} and @var{n} is a positive field number.
5794 A field specification of @samp{0} denotes the join field.
5795 In most cases, the functionality of the @samp{0} field spec
5796 may be reproduced using the explicit @var{m.n} that corresponds
5797 to the join field. However, when printing unpairable lines
5798 (using either of the @option{-a} or @option{-v} options), there is no way
5799 to specify the join field using @var{m.n} in @var{field-list}
5800 if there are unpairable lines in both files.
5801 To give @command{join} that functionality, @acronym{POSIX} invented the @samp{0}
5802 field specification notation.
5804 The elements in @var{field-list}
5805 are separated by commas or blanks.
5806 Blank separators typically need to be quoted for the shell. For
5807 example, the commands @samp{join -o 1.2,2.2} and @samp{join -o '1.2
5808 2.2'} are equivalent.
5810 All output lines---including those printed because of any -a or -v
5811 option---are subject to the specified @var{field-list}.
5814 Use character @var{char} as the input and output field separator.
5815 Treat as significant each occurrence of @var{char} in the input file.
5816 Use @samp{sort -t @var{char}}, without the @option{-b} option of
5817 @samp{sort}, to produce this ordering. If @samp{join -t ''} is specified,
5818 the whole line is considered, matching the default operation of sort.
5819 If @samp{-t '\0'} is specified then the @acronym{ASCII} @sc{nul}
5820 character is used to delimit the fields.
5822 @item -v @var{file-number}
5823 Print a line for each unpairable line in file @var{file-number}
5824 (either @samp{1} or @samp{2}), instead of the normal output.
5831 @node Operating on characters
5832 @chapter Operating on characters
5834 @cindex operating on characters
5836 This commands operate on individual characters.
5839 * tr invocation:: Translate, squeeze, and/or delete characters.
5840 * expand invocation:: Convert tabs to spaces.
5841 * unexpand invocation:: Convert spaces to tabs.
5846 @section @command{tr}: Translate, squeeze, and/or delete characters
5853 tr [@var{option}]@dots{} @var{set1} [@var{set2}]
5856 @command{tr} copies standard input to standard output, performing
5857 one of the following operations:
5861 translate, and optionally squeeze repeated characters in the result,
5863 squeeze repeated characters,
5867 delete characters, then squeeze repeated characters from the result.
5870 The @var{set1} and (if given) @var{set2} arguments define ordered
5871 sets of characters, referred to below as @var{set1} and @var{set2}. These
5872 sets are the characters of the input that @command{tr} operates on.
5873 The @option{--complement} (@option{-c}, @option{-C}) option replaces
5875 complement (all of the characters that are not in @var{set1}).
5877 Currently @command{tr} fully supports only single-byte characters.
5878 Eventually it will support multibyte characters; when it does, the
5879 @option{-C} option will cause it to complement the set of characters,
5880 whereas @option{-c} will cause it to complement the set of values.
5881 This distinction will matter only when some values are not characters,
5882 and this is possible only in locales using multibyte encodings when
5883 the input contains encoding errors.
5885 The program accepts the @option{--help} and @option{--version}
5886 options. @xref{Common options}. Options must precede operands.
5891 * Character sets:: Specifying sets of characters.
5892 * Translating:: Changing one set of characters to another.
5893 * Squeezing:: Squeezing repeats and deleting.
5897 @node Character sets
5898 @subsection Specifying sets of characters
5900 @cindex specifying sets of characters
5902 The format of the @var{set1} and @var{set2} arguments resembles
5903 the format of regular expressions; however, they are not regular
5904 expressions, only lists of characters. Most characters simply
5905 represent themselves in these strings, but the strings can contain
5906 the shorthands listed below, for convenience. Some of them can be
5907 used only in @var{set1} or @var{set2}, as noted below.
5911 @item Backslash escapes
5912 @cindex backslash escapes
5914 The following backslash escape sequences are recognized:
5932 The 8-bit character with the value given by @var{ooo}, which is 1 to 3
5933 octal digits. Note that @samp{\400} is interpreted as the two-byte
5934 sequence, @samp{\040} @samp{0}.
5939 While a backslash followed by a character not listed above is
5940 interpreted as that character, the backslash also effectively
5941 removes any special significance, so it is useful to escape
5942 @samp{[}, @samp{]}, @samp{*}, and @samp{-}.
5947 The notation @samp{@var{m}-@var{n}} expands to all of the characters
5948 from @var{m} through @var{n}, in ascending order. @var{m} should
5949 collate before @var{n}; if it doesn't, an error results. As an example,
5950 @samp{0-9} is the same as @samp{0123456789}.
5952 @sc{gnu} @command{tr} does not support the System V syntax that uses square
5953 brackets to enclose ranges. Translations specified in that format
5954 sometimes work as expected, since the brackets are often transliterated
5955 to themselves. However, they should be avoided because they sometimes
5956 behave unexpectedly. For example, @samp{tr -d '[0-9]'} deletes brackets
5959 Many historically common and even accepted uses of ranges are not
5960 portable. For example, on @acronym{EBCDIC} hosts using the @samp{A-Z}
5961 range will not do what most would expect because @samp{A} through @samp{Z}
5962 are not contiguous as they are in @acronym{ASCII}.
5963 If you can rely on a @acronym{POSIX} compliant version of @command{tr}, then
5964 the best way to work around this is to use character classes (see below).
5965 Otherwise, it is most portable (and most ugly) to enumerate the members
5968 @item Repeated characters
5969 @cindex repeated characters
5971 The notation @samp{[@var{c}*@var{n}]} in @var{set2} expands to @var{n}
5972 copies of character @var{c}. Thus, @samp{[y*6]} is the same as
5973 @samp{yyyyyy}. The notation @samp{[@var{c}*]} in @var{string2} expands
5974 to as many copies of @var{c} as are needed to make @var{set2} as long as
5975 @var{set1}. If @var{n} begins with @samp{0}, it is interpreted in
5976 octal, otherwise in decimal.
5978 @item Character classes
5979 @cindex character classes
5981 The notation @samp{[:@var{class}:]} expands to all of the characters in
5982 the (predefined) class @var{class}. The characters expand in no
5983 particular order, except for the @code{upper} and @code{lower} classes,
5984 which expand in ascending order. When the @option{--delete} (@option{-d})
5985 and @option{--squeeze-repeats} (@option{-s}) options are both given, any
5986 character class can be used in @var{set2}. Otherwise, only the
5987 character classes @code{lower} and @code{upper} are accepted in
5988 @var{set2}, and then only if the corresponding character class
5989 (@code{upper} and @code{lower}, respectively) is specified in the same
5990 relative position in @var{set1}. Doing this specifies case conversion.
5991 The class names are given below; an error results when an invalid class
6003 Horizontal whitespace.
6012 Printable characters, not including space.
6018 Printable characters, including space.
6021 Punctuation characters.
6024 Horizontal or vertical whitespace.
6033 @item Equivalence classes
6034 @cindex equivalence classes
6036 The syntax @samp{[=@var{c}=]} expands to all of the characters that are
6037 equivalent to @var{c}, in no particular order. Equivalence classes are
6038 a relatively recent invention intended to support non-English alphabets.
6039 But there seems to be no standard way to define them or determine their
6040 contents. Therefore, they are not fully implemented in @sc{gnu} @command{tr};
6041 each character's equivalence class consists only of that character,
6042 which is of no particular use.
6048 @subsection Translating
6050 @cindex translating characters
6052 @command{tr} performs translation when @var{set1} and @var{set2} are
6053 both given and the @option{--delete} (@option{-d}) option is not given.
6054 @command{tr} translates each character of its input that is in @var{set1}
6055 to the corresponding character in @var{set2}. Characters not in
6056 @var{set1} are passed through unchanged. When a character appears more
6057 than once in @var{set1} and the corresponding characters in @var{set2}
6058 are not all the same, only the final one is used. For example, these
6059 two commands are equivalent:
6066 A common use of @command{tr} is to convert lowercase characters to
6067 uppercase. This can be done in many ways. Here are three of them:
6070 tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
6072 tr '[:lower:]' '[:upper:]'
6076 But note that using ranges like @code{a-z} above is not portable.
6078 When @command{tr} is performing translation, @var{set1} and @var{set2}
6079 typically have the same length. If @var{set1} is shorter than
6080 @var{set2}, the extra characters at the end of @var{set2} are ignored.
6082 On the other hand, making @var{set1} longer than @var{set2} is not
6083 portable; @acronym{POSIX} says that the result is undefined. In this situation,
6084 BSD @command{tr} pads @var{set2} to the length of @var{set1} by repeating
6085 the last character of @var{set2} as many times as necessary. System V
6086 @command{tr} truncates @var{set1} to the length of @var{set2}.
6088 By default, @sc{gnu} @command{tr} handles this case like BSD @command{tr}.
6089 When the @option{--truncate-set1} (@option{-t}) option is given,
6090 @sc{gnu} @command{tr} handles this case like the System V @command{tr}
6091 instead. This option is ignored for operations other than translation.
6093 Acting like System V @command{tr} in this case breaks the relatively common
6097 tr -cs A-Za-z0-9 '\012'
6101 because it converts only zero bytes (the first element in the
6102 complement of @var{set1}), rather than all non-alphanumerics, to
6106 By the way, the above idiom is not portable because it uses ranges, and
6107 it assumes that the octal code for newline is 012.
6108 Assuming a @acronym{POSIX} compliant @command{tr}, here is a better
6112 tr -cs '[:alnum:]' '[\n*]'
6117 @subsection Squeezing repeats and deleting
6119 @cindex squeezing repeat characters
6120 @cindex deleting characters
6122 When given just the @option{--delete} (@option{-d}) option, @command{tr}
6123 removes any input characters that are in @var{set1}.
6125 When given just the @option{--squeeze-repeats} (@option{-s}) option,
6126 @command{tr} replaces each input sequence of a repeated character that
6127 is in @var{set1} with a single occurrence of that character.
6129 When given both @option{--delete} and @option{--squeeze-repeats}, @command{tr}
6130 first performs any deletions using @var{set1}, then squeezes repeats
6131 from any remaining characters using @var{set2}.
6133 The @option{--squeeze-repeats} option may also be used when translating,
6134 in which case @command{tr} first performs translation, then squeezes
6135 repeats from any remaining characters using @var{set2}.
6137 Here are some examples to illustrate various combinations of options:
6142 Remove all zero bytes:
6149 Put all words on lines by themselves. This converts all
6150 non-alphanumeric characters to newlines, then squeezes each string
6151 of repeated newlines into a single newline:
6154 tr -cs '[:alnum:]' '[\n*]'
6158 Convert each sequence of repeated newlines to a single newline:
6165 Find doubled occurrences of words in a document.
6166 @c Separate the following two "the"s, so typo checkers don't complain.
6167 For example, people often write ``the @w{}the'' with the repeated words
6168 separated by a newline. The Bourne shell script below works first
6169 by converting each sequence of punctuation and blank characters to a
6170 single newline. That puts each ``word'' on a line by itself.
6171 Next it maps all uppercase characters to lower case, and finally it
6172 runs @command{uniq} with the @option{-d} option to print out only the words
6178 | tr -s '[:punct:][:blank:]' '[\n*]' \
6179 | tr '[:upper:]' '[:lower:]' \
6184 Deleting a small set of characters is usually straightforward. For example,
6185 to remove all @samp{a}s, @samp{x}s, and @samp{M}s you would do this:
6191 However, when @samp{-} is one of those characters, it can be tricky because
6192 @samp{-} has special meanings. Performing the same task as above but also
6193 removing all @samp{-} characters, we might try @code{tr -d -axM}, but
6194 that would fail because @command{tr} would try to interpret @option{-a} as
6195 a command-line option. Alternatively, we could try putting the hyphen
6196 inside the string, @code{tr -d a-xM}, but that wouldn't work either because
6197 it would make @command{tr} interpret @code{a-x} as the range of characters
6198 @samp{a}@dots{}@samp{x} rather than the three.
6199 One way to solve the problem is to put the hyphen at the end of the list
6206 Or you can use @samp{--} to terminate option processing:
6212 More generally, use the character class notation @code{[=c=]}
6213 with @samp{-} (or any other character) in place of the @samp{c}:
6219 Note how single quotes are used in the above example to protect the
6220 square brackets from interpretation by a shell.
6225 @node expand invocation
6226 @section @command{expand}: Convert tabs to spaces
6229 @cindex tabs to spaces, converting
6230 @cindex converting tabs to spaces
6232 @command{expand} writes the contents of each given @var{file}, or standard
6233 input if none are given or for a @var{file} of @samp{-}, to standard
6234 output, with tab characters converted to the appropriate number of
6238 expand [@var{option}]@dots{} [@var{file}]@dots{}
6241 By default, @command{expand} converts all tabs to spaces. It preserves
6242 backspace characters in the output; they decrement the column count for
6243 tab calculations. The default action is equivalent to @option{-t 8} (set
6244 tabs every 8 columns).
6246 The program accepts the following options. Also see @ref{Common options}.
6250 @item -t @var{tab1}[,@var{tab2}]@dots{}
6251 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6254 @cindex tab stops, setting
6255 If only one tab stop is given, set the tabs @var{tab1} spaces apart
6256 (default is 8). Otherwise, set the tabs at columns @var{tab1},
6257 @var{tab2}, @dots{} (numbered from 0), and replace any tabs beyond the
6258 last tab stop given with single spaces. Tab stops can be separated by
6259 blanks as well as by commas.
6261 For compatibility, GNU @command{expand} also accepts the obsolete
6262 option syntax, @option{-@var{t1}[,@var{t2}]@dots{}}. New scripts
6263 should use @option{-t @var{t1}[,@var{t2}]@dots{}} instead.
6269 @cindex initial tabs, converting
6270 Only convert initial tabs (those that precede all non-space or non-tab
6271 characters) on each line to spaces.
6278 @node unexpand invocation
6279 @section @command{unexpand}: Convert spaces to tabs
6283 @command{unexpand} writes the contents of each given @var{file}, or
6284 standard input if none are given or for a @var{file} of @samp{-}, to
6285 standard output, converting blanks at the beginning of each line into
6286 as many tab characters as needed. In the default @acronym{POSIX}
6287 locale, a @dfn{blank} is a space or a tab; other locales may specify
6288 additional blank characters. Synopsis:
6291 unexpand [@var{option}]@dots{} [@var{file}]@dots{}
6294 By default, @command{unexpand} converts only initial blanks (those
6295 that precede all non-blank characters) on each line. It
6296 preserves backspace characters in the output; they decrement the column
6297 count for tab calculations. By default, tabs are set at every 8th
6300 The program accepts the following options. Also see @ref{Common options}.
6304 @item -t @var{tab1}[,@var{tab2}]@dots{}
6305 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6308 If only one tab stop is given, set the tabs @var{tab1} columns apart
6309 instead of the default 8. Otherwise, set the tabs at columns
6310 @var{tab1}, @var{tab2}, @dots{} (numbered from 0), and leave blanks
6311 beyond the tab stops given unchanged. Tab stops can be separated by
6312 blanks as well as by commas. This option implies the @option{-a} option.
6314 For compatibility, GNU @command{unexpand} supports the obsolete option syntax,
6315 @option{-@var{tab1}[,@var{tab2}]@dots{}}, where tab stops must be
6316 separated by commas. (Unlike @option{-t}, this obsolete option does
6317 not imply @option{-a}.) New scripts should use @option{--first-only -t
6318 @var{tab1}[,@var{tab2}]@dots{}} instead.
6324 Also convert all sequences of two or more blanks just before a tab stop,
6325 even if they occur after non-blank characters in a line.
6332 @node Directory listing
6333 @chapter Directory listing
6335 This chapter describes the @command{ls} command and its variants @command{dir}
6336 and @command{vdir}, which list information about files.
6339 * ls invocation:: List directory contents.
6340 * dir invocation:: Briefly ls.
6341 * vdir invocation:: Verbosely ls.
6342 * dircolors invocation:: Color setup for ls, etc.
6347 @section @command{ls}: List directory contents
6350 @cindex directory listing
6352 The @command{ls} program lists information about files (of any type,
6353 including directories). Options and file arguments can be intermixed
6354 arbitrarily, as usual.
6356 For non-option command-line arguments that are directories, by default
6357 @command{ls} lists the contents of directories, not recursively, and
6358 omitting files with names beginning with @samp{.}. For other non-option
6359 arguments, by default @command{ls} lists just the file name. If no
6360 non-option argument is specified, @command{ls} operates on the current
6361 directory, acting as if it had been invoked with a single argument of @samp{.}.
6364 By default, the output is sorted alphabetically, according to the locale
6365 settings in effect.@footnote{If you use a non-@acronym{POSIX}
6366 locale (e.g., by setting @env{LC_ALL} to @samp{en_US}), then @command{ls} may
6367 produce output that is sorted differently than you're accustomed to.
6368 In that case, set the @env{LC_ALL} environment variable to @samp{C}.}
6369 If standard output is
6370 a terminal, the output is in columns (sorted vertically) and control
6371 characters are output as question marks; otherwise, the output is listed
6372 one per line and control characters are output as-is.
6374 Because @command{ls} is such a fundamental program, it has accumulated many
6375 options over the years. They are described in the subsections below;
6376 within each section, options are listed alphabetically (ignoring case).
6377 The division of options into the subsections is not absolute, since some
6378 options affect more than one aspect of @command{ls}'s operation.
6380 @cindex exit status of @command{ls}
6385 1 minor problems (e.g., failure to access a file or directory not
6386 specified as a command line argument. This happens when listing a
6387 directory in which entries are actively being removed or renamed.)
6388 2 serious trouble (e.g., memory exhausted, invalid option, failure
6389 to access a file or directory specified as a command line argument
6390 or a directory loop)
6393 Also see @ref{Common options}.
6396 * Which files are listed::
6397 * What information is listed::
6398 * Sorting the output::
6399 * Details about version sort::
6400 * General output formatting::
6401 * Formatting file timestamps::
6402 * Formatting the file names::
6406 @node Which files are listed
6407 @subsection Which files are listed
6409 These options determine which files @command{ls} lists information for.
6410 By default, @command{ls} lists files and the contents of any
6411 directories on the command line, except that in directories it ignores
6412 files whose names start with @samp{.}.
6420 In directories, do not ignore file names that start with @samp{.}.
6425 @opindex --almost-all
6426 In directories, do not ignore all file names that start with @samp{.};
6427 ignore only @file{.} and @file{..}. The @option{--all} (@option{-a})
6428 option overrides this option.
6431 @itemx --ignore-backups
6433 @opindex --ignore-backups
6434 @cindex backup files, ignoring
6435 In directories, ignore files that end with @samp{~}. This option is
6436 equivalent to @samp{--ignore='*~' --ignore='.*~'}.
6441 @opindex --directory
6442 List just the names of directories, as with other types of files, rather
6443 than listing their contents.
6444 @c The following sentence is the same as the one for -F.
6445 Do not follow symbolic links listed on the
6446 command line unless the @option{--dereference-command-line} (@option{-H}),
6447 @option{--dereference} (@option{-L}), or
6448 @option{--dereference-command-line-symlink-to-dir} options are specified.
6451 @itemx --dereference-command-line
6453 @opindex --dereference-command-line
6454 @cindex symbolic links, dereferencing
6455 If a command line argument specifies a symbolic link, show information
6456 for the file the link references rather than for the link itself.
6458 @itemx --dereference-command-line-symlink-to-dir
6459 @opindex --dereference-command-line-symlink-to-dir
6460 @cindex symbolic links, dereferencing
6461 Do not dereference symbolic links, with one exception:
6462 if a command line argument specifies a symbolic link that refers to
6463 a directory, show information for that directory rather than for the
6465 This is the default behavior when no other dereferencing-related
6466 option has been specified (@option{--classify} (@option{-F}),
6467 @option{--directory} (@option{-d}),
6469 @option{--dereference} (@option{-L}), or
6470 @option{--dereference-command-line} (@option{-H})).
6472 @item --group-directories-first
6473 @opindex --group-directories-first
6474 Group all the directories before the files and then sort the
6475 directories and the files separately using the selected sort key
6476 (see --sort option).
6477 That is, this option specifies a primary sort key,
6478 and the --sort option specifies a secondary key.
6479 However, any use of @option{--sort=none}
6480 (@option{-U}) disables this option altogether.
6482 @item --hide=PATTERN
6483 @opindex --hide=@var{pattern}
6484 In directories, ignore files whose names match the shell pattern
6485 @var{pattern}, unless the @option{--all} (@option{-a}) or
6486 @option{--almost-all} (@option{-A}) is also given. This
6487 option acts like @option{--ignore=@var{pattern}} except that it has no
6488 effect if @option{--all} (@option{-a}) or @option{--almost-all}
6489 (@option{-A}) is also given.
6491 This option can be useful in shell aliases. For example, if
6492 @command{lx} is an alias for @samp{ls --hide='*~'} and @command{ly} is
6493 an alias for @samp{ls --ignore='*~'}, then the command @samp{lx -A}
6494 lists the file @file{README~} even though @samp{ly -A} would not.
6496 @item -I @var{pattern}
6497 @itemx --ignore=@var{pattern}
6499 @opindex --ignore=@var{pattern}
6500 In directories, ignore files whose names match the shell pattern
6501 (not regular expression) @var{pattern}. As
6502 in the shell, an initial @samp{.} in a file name does not match a
6503 wildcard at the start of @var{pattern}. Sometimes it is useful
6504 to give this option several times. For example,
6507 $ ls --ignore='.??*' --ignore='.[^.]' --ignore='#*'
6510 The first option ignores names of length 3 or more that start with @samp{.},
6511 the second ignores all two-character names that start with @samp{.}
6512 except @samp{..}, and the third ignores names that start with @samp{#}.
6515 @itemx --dereference
6517 @opindex --dereference
6518 @cindex symbolic links, dereferencing
6519 When showing file information for a symbolic link, show information
6520 for the file the link references rather than the link itself.
6521 However, even with this option, @command{ls} still prints the name
6522 of the link itself, not the name of the file that the link points to.
6527 @opindex --recursive
6528 @cindex recursive directory listing
6529 @cindex directory listing, recursive
6530 List the contents of all directories recursively.
6535 @node What information is listed
6536 @subsection What information is listed
6538 These options affect the information that @command{ls} displays. By
6539 default, only file names are shown.
6545 @cindex hurd, author, printing
6546 List each file's author when producing long format directory listings.
6547 In GNU/Hurd, file authors can differ from their owners, but in other
6548 operating systems the two are the same.
6554 @cindex dired Emacs mode support
6555 With the long listing (@option{-l}) format, print an additional line after
6559 //DIRED// @var{beg1} @var{end1} @var{beg2} @var{end2} @dots{}
6563 The @var{begn} and @var{endn} are unsigned integers that record the
6564 byte position of the beginning and end of each file name in the output.
6565 This makes it easy for Emacs to find the names, even when they contain
6566 unusual characters such as space or newline, without fancy searching.
6568 If directories are being listed recursively (@option{-R}), output a similar
6569 line with offsets for each subdirectory name:
6572 //SUBDIRED// @var{beg1} @var{end1} @dots{}
6575 Finally, output a line of the form:
6578 //DIRED-OPTIONS// --quoting-style=@var{word}
6582 where @var{word} is the quoting style (@pxref{Formatting the file names}).
6584 Here is an actual example:
6587 $ mkdir -p a/sub/deeper a/sub2
6589 $ touch a/sub/deeper/file
6590 $ ls -gloRF --dired a
6593 -rw-r--r-- 1 0 Jun 10 12:27 f1
6594 -rw-r--r-- 1 0 Jun 10 12:27 f2
6595 drwxr-xr-x 3 4096 Jun 10 12:27 sub/
6596 drwxr-xr-x 2 4096 Jun 10 12:27 sub2/
6600 drwxr-xr-x 2 4096 Jun 10 12:27 deeper/
6604 -rw-r--r-- 1 0 Jun 10 12:27 file
6608 //DIRED// 48 50 84 86 120 123 158 162 217 223 282 286
6609 //SUBDIRED// 2 3 167 172 228 240 290 296
6610 //DIRED-OPTIONS// --quoting-style=literal
6613 Note that the pairs of offsets on the @samp{//DIRED//} line above delimit
6614 these names: @file{f1}, @file{f2}, @file{sub}, @file{sub2}, @file{deeper},
6616 The offsets on the @samp{//SUBDIRED//} line delimit the following
6617 directory names: @file{a}, @file{a/sub}, @file{a/sub/deeper}, @file{a/sub2}.
6619 Here is an example of how to extract the fifth entry name, @samp{deeper},
6620 corresponding to the pair of offsets, 222 and 228:
6623 $ ls -gloRF --dired a > out
6624 $ dd bs=1 skip=222 count=6 < out 2>/dev/null; echo
6628 Note that although the listing above includes a trailing slash
6629 for the @samp{deeper} entry, the offsets select the name without
6630 the trailing slash. However, if you invoke @command{ls} with @option{--dired}
6631 along with an option like @option{--escape} (aka @option{-b}) and operate
6632 on a file whose name contains special characters, notice that the backslash
6637 $ ls -blog --dired 'a b'
6638 -rw-r--r-- 1 0 Jun 10 12:28 a\ b
6640 //DIRED-OPTIONS// --quoting-style=escape
6643 If you use a quoting style that adds quote marks
6644 (e.g., @option{--quoting-style=c}), then the offsets include the quote marks.
6645 So beware that the user may select the quoting style via the environment
6646 variable @env{QUOTING_STYLE}. Hence, applications using @option{--dired}
6647 should either specify an explicit @option{--quoting-style=literal} option
6648 (aka @option{-N} or @option{--literal}) on the command line, or else be
6649 prepared to parse the escaped names.
6652 @opindex --full-time
6653 Produce long format directory listings, and list times in full. It is
6654 equivalent to using @option{--format=long} with
6655 @option{--time-style=full-iso} (@pxref{Formatting file timestamps}).
6659 Produce long format directory listings, but don't display owner information.
6665 Inhibit display of group information in a long format directory listing.
6666 (This is the default in some non-@sc{gnu} versions of @command{ls}, so we
6667 provide this option for compatibility.)
6675 @cindex inode number, printing
6676 Print the inode number (also called the file serial number and index
6677 number) of each file to the left of the file name. (This number
6678 uniquely identifies each file within a particular file system.)
6681 @itemx --format=long
6682 @itemx --format=verbose
6685 @opindex long ls @r{format}
6686 @opindex verbose ls @r{format}
6687 In addition to the name of each file, print the file type, file mode bits,
6688 number of hard links, owner name, group name, size, and
6689 timestamp (@pxref{Formatting file timestamps}), normally
6690 the modification time. Print question marks for information that
6691 cannot be determined.
6693 Normally the size is printed as a byte count without punctuation, but
6694 this can be overridden (@pxref{Block size}). For example, @option{-h}
6695 prints an abbreviated, human-readable count, and
6696 @samp{--block-size="'1"} prints a byte count with the thousands
6697 separator of the current locale.
6699 For each directory that is listed, preface the files with a line
6700 @samp{total @var{blocks}}, where @var{blocks} is the total disk allocation
6701 for all files in that directory. The block size currently defaults to 1024
6702 bytes, but this can be overridden (@pxref{Block size}).
6703 The @var{blocks} computed counts each hard link separately;
6704 this is arguably a deficiency.
6706 The file type is one of the following characters:
6708 @c The commented-out entries are ones we're not sure about.
6716 character special file
6718 high performance (``contiguous data'') file
6722 door (Solaris 2.5 and up)
6724 @c semaphore, if this is a distinct file type
6728 @c multiplexed file (7th edition Unix; obsolete)
6730 off-line (``migrated'') file (Cray DMF)
6732 network special file (HP-UX)
6736 port (Solaris 10 and up)
6738 @c message queue, if this is a distinct file type
6742 @c shared memory object, if this is a distinct file type
6744 @c typed memory object, if this is a distinct file type
6746 @c whiteout (4.4BSD; not implemented)
6748 some other file type
6751 @cindex permissions, output by @command{ls}
6752 The file mode bits listed are similar to symbolic mode specifications
6753 (@pxref{Symbolic Modes}). But @command{ls} combines multiple bits into the
6754 third character of each set of permissions as follows:
6758 If the set-user-ID or set-group-ID bit and the corresponding executable bit
6762 If the set-user-ID or set-group-ID bit is set but the corresponding
6763 executable bit is not set.
6766 If the restricted deletion flag or sticky bit, and the
6767 other-executable bit, are both set. The restricted deletion flag is
6768 another name for the sticky bit. @xref{Mode Structure}.
6771 If the restricted deletion flag or sticky bit is set but the
6772 other-executable bit is not set.
6775 If the executable bit is set and none of the above apply.
6781 Following the file mode bits is a single character that specifies
6782 whether an alternate access method such as an access control list
6783 applies to the file. When the character following the file mode bits is a
6784 space, there is no alternate access method. When it is a printing
6785 character, then there is such a method.
6787 GNU @command{ls} uses a @samp{.} character to indicate a file
6788 with an SELinux security context, but no other alternate access method.
6790 A file with any other combination of alternate access methods
6791 is marked with a @samp{+} character.
6794 @itemx --numeric-uid-gid
6796 @opindex --numeric-uid-gid
6797 @cindex numeric uid and gid
6798 @cindex numeric user and group IDs
6799 Produce long format directory listings, but
6800 display numeric user and group IDs instead of the owner and group names.
6804 Produce long format directory listings, but don't display group information.
6805 It is equivalent to using @option{--format=long} with @option{--no-group} .
6811 @cindex disk allocation
6812 @cindex size of files, reporting
6813 Print the disk allocation of each file to the left of the file name.
6814 This is the amount of disk space used by the file, which is usually a
6815 bit more than the file's size, but it can be less if the file has holes.
6817 Normally the disk allocation is printed in units of
6818 1024 bytes, but this can be overridden (@pxref{Block size}).
6820 @cindex NFS mounts from BSD to HP-UX
6821 For files that are NFS-mounted from an HP-UX system to a BSD system,
6822 this option reports sizes that are half the correct values. On HP-UX
6823 systems, it reports sizes that are twice the correct values for files
6824 that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX;
6825 it also affects the HP-UX @command{ls} program.
6834 @cindex security context
6835 Display the SELinux security context or @samp{?} if none is found.
6836 When used with the @option{-l} option, print the security context
6837 to the left of the size column.
6842 @node Sorting the output
6843 @subsection Sorting the output
6845 @cindex sorting @command{ls} output
6846 These options change the order in which @command{ls} sorts the information
6847 it outputs. By default, sorting is done by character code
6848 (e.g., @acronym{ASCII} order).
6854 @itemx --time=status
6857 @opindex ctime@r{, printing or sorting by}
6858 @opindex status time@r{, printing or sorting by}
6859 @opindex use time@r{, printing or sorting files by}
6860 If the long listing format (e.g., @option{-l}, @option{-o}) is being used,
6861 print the status change time (the @samp{ctime} in the inode) instead of
6862 the modification time.
6863 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6864 or when not using a long listing format,
6865 sort according to the status change time.
6869 @cindex unsorted directory listing
6870 @cindex directory order, listing by
6871 Primarily, like @option{-U}---do not sort; list the files in whatever
6872 order they are stored in the directory. But also enable @option{-a} (list
6873 all files) and disable @option{-l}, @option{--color}, and @option{-s} (if they
6874 were specified before the @option{-f}).
6880 @cindex reverse sorting
6881 Reverse whatever the sorting method is---e.g., list files in reverse
6882 alphabetical order, youngest first, smallest first, or whatever.
6888 @opindex size of files@r{, sorting files by}
6889 Sort by file size, largest first.
6895 @opindex modification time@r{, sorting files by}
6896 Sort by modification time (the @samp{mtime} in the inode), newest first.
6900 @itemx --time=access
6904 @opindex use time@r{, printing or sorting files by}
6905 @opindex atime@r{, printing or sorting files by}
6906 @opindex access time@r{, printing or sorting files by}
6907 If the long listing format (e.g., @option{--format=long}) is being used,
6908 print the last access time (the @samp{atime} in the inode).
6909 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6910 or when not using a long listing format, sort according to the access time.
6916 @opindex none@r{, sorting option for @command{ls}}
6917 Do not sort; list the files in whatever order they are
6918 stored in the directory. (Do not do any of the other unrelated things
6919 that @option{-f} does.) This is especially useful when listing very large
6920 directories, since not doing any sorting can be noticeably faster.
6923 @itemx --sort=version
6926 @opindex version@r{, sorting option for @command{ls}}
6927 Sort by version name and number, lowest first. It behaves like a default
6928 sort, except that each sequence of decimal digits is treated numerically
6929 as an index/version number. (@xref{Details about version sort}.)
6932 @itemx --sort=extension
6935 @opindex extension@r{, sorting files by}
6936 Sort directory contents alphabetically by file extension (characters
6937 after the last @samp{.}); files with no extension are sorted first.
6942 @node Details about version sort
6943 @subsection Details about version sort
6945 Version sorting handles the fact that file names frequently include indices or
6946 version numbers. Standard sorting usually does not produce the order that one
6947 expects because comparisons are made on a character-by-character basis.
6948 Version sorting is especially useful when browsing directories that contain
6949 many files with indices/version numbers in their names:
6953 abc.zml-1.gz abc.zml-1.gz
6954 abc.zml-12.gz abc.zml-2.gz
6955 abc.zml-2.gz abc.zml-12.gz
6958 Version-sorted strings are compared such that if @var{ver1} and @var{ver2}
6959 are version numbers and @var{prefix} and @var{suffix} (@var{suffix} matching
6960 the regular expression @samp{(\.[A-Za-z~][A-Za-z0-9~]*)*}) are strings then
6961 @var{ver1} < @var{ver2} implies that the name composed of
6962 ``@var{prefix} @var{ver1} @var{suffix}'' sorts before
6963 ``@var{prefix} @var{ver2} @var{suffix}''.
6965 Note also that leading zeros of numeric parts are ignored:
6969 abc-1.007.tgz abc-1.01a.tgz
6970 abc-1.012b.tgz abc-1.007.tgz
6971 abc-1.01a.tgz abc-1.012b.tgz
6974 This functionality is implemented using gnulib's @code{filevercmp} function,
6975 which has some caveats worth noting.
6978 @item @env{LC_COLLATE} is ignored, which means @samp{ls -v} and @samp{sort -V}
6979 will sort non-numeric prefixes as if the @env{LC_COLLATE} locale category
6980 was set to @samp{C}.
6981 @item Some suffixes will not be matched by the regular
6982 expression mentioned above. Consequently these examples may
6983 not sort as you expect:
6991 abc-1.2.3.4.x86_64.rpm
6992 abc-1.2.3.x86_64.rpm
6996 @node General output formatting
6997 @subsection General output formatting
6999 These options affect the appearance of the overall output.
7004 @itemx --format=single-column
7007 @opindex single-column @r{output of files}
7008 List one file per line. This is the default for @command{ls} when standard
7009 output is not a terminal.
7012 @itemx --format=vertical
7015 @opindex vertical @r{sorted files in columns}
7016 List files in columns, sorted vertically. This is the default for
7017 @command{ls} if standard output is a terminal. It is always the default
7018 for the @command{dir} program.
7019 @sc{gnu} @command{ls} uses variable width columns to display as many files as
7020 possible in the fewest lines.
7022 @item --color [=@var{when}]
7024 @cindex color, distinguishing file types with
7025 Specify whether to use color for distinguishing file types. @var{when}
7026 may be omitted, or one of:
7029 @vindex none @r{color option}
7030 - Do not use color at all. This is the default.
7032 @vindex auto @r{color option}
7033 @cindex terminal, using color iff
7034 - Only use color if standard output is a terminal.
7036 @vindex always @r{color option}
7039 Specifying @option{--color} and no @var{when} is equivalent to
7040 @option{--color=always}.
7041 Piping a colorized listing through a pager like @command{more} or
7042 @command{less} usually produces unreadable results. However, using
7043 @code{more -f} does seem to work.
7047 @itemx --indicator-style=classify
7050 @opindex --indicator-style
7051 @cindex file type and executables, marking
7052 @cindex executables and file type, marking
7053 Append a character to each file name indicating the file type. Also,
7054 for regular files that are executable, append @samp{*}. The file type
7055 indicators are @samp{/} for directories, @samp{@@} for symbolic links,
7056 @samp{|} for FIFOs, @samp{=} for sockets, @samp{>} for doors,
7057 and nothing for regular files.
7058 @c The following sentence is the same as the one for -d.
7059 Do not follow symbolic links listed on the
7060 command line unless the @option{--dereference-command-line} (@option{-H}),
7061 @option{--dereference} (@option{-L}), or
7062 @option{--dereference-command-line-symlink-to-dir} options are specified.
7065 @itemx --indicator-style=file-type
7066 @opindex --file-type
7067 @opindex --indicator-style
7068 @cindex file type, marking
7069 Append a character to each file name indicating the file type. This is
7070 like @option{-F}, except that executables are not marked.
7072 @item --indicator-style=@var{word}
7073 @opindex --indicator-style
7074 Append a character indicator with style @var{word} to entry names,
7079 Do not append any character indicator; this is the default.
7081 Append @samp{/} for directories. This is the same as the @option{-p}
7084 Append @samp{/} for directories, @samp{@@} for symbolic links, @samp{|}
7085 for FIFOs, @samp{=} for sockets, and nothing for regular files. This is
7086 the same as the @option{--file-type} option.
7088 Append @samp{*} for executable regular files, otherwise behave as for
7089 @samp{file-type}. This is the same as the @option{-F} or
7090 @option{--classify} option.
7095 Print file sizes in 1024-byte blocks, overriding the default block
7096 size (@pxref{Block size}).
7097 This option is equivalent to @option{--block-size=1K}.
7100 @itemx --format=commas
7103 @opindex commas@r{, outputting between files}
7104 List files horizontally, with as many as will fit on each line,
7105 separated by @samp{, } (a comma and a space).
7108 @itemx --indicator-style=slash
7110 @opindex --indicator-style
7111 @cindex file type, marking
7112 Append a @samp{/} to directory names.
7115 @itemx --format=across
7116 @itemx --format=horizontal
7119 @opindex across@r{, listing files}
7120 @opindex horizontal@r{, listing files}
7121 List the files in columns, sorted horizontally.
7124 @itemx --tabsize=@var{cols}
7127 Assume that each tab stop is @var{cols} columns wide. The default is 8.
7128 @command{ls} uses tabs where possible in the output, for efficiency. If
7129 @var{cols} is zero, do not use tabs at all.
7131 @c FIXME: remove in 2009, if Apple Terminal has been fixed for long enough.
7132 Some terminal emulators (at least Apple Terminal 1.5 (133) from Mac OS X 10.4.8)
7133 do not properly align columns to the right of a TAB following a
7134 non-@acronym{ASCII} byte. If you use such a terminal emulator, use the
7135 @option{-T0} option or put @code{TABSIZE=0} in your environment to tell
7136 @command{ls} to align using spaces, not tabs.
7139 @itemx --width=@var{cols}
7143 Assume the screen is @var{cols} columns wide. The default is taken
7144 from the terminal settings if possible; otherwise the environment
7145 variable @env{COLUMNS} is used if it is set; otherwise the default
7151 @node Formatting file timestamps
7152 @subsection Formatting file timestamps
7154 By default, file timestamps are listed in abbreviated form, using
7155 a date like @samp{Mar 30@ @ 2002} for non-recent timestamps, and a
7156 date-without-year and time like @samp{Mar 30 23:45} for recent timestamps.
7157 This format can change depending on the current locale as detailed below.
7159 A timestamp is considered to be @dfn{recent} if it is less than six
7160 months old, and is not dated in the future. If a timestamp dated
7161 today is not listed in recent form, the timestamp is in the future,
7162 which means you probably have clock skew problems which may break
7163 programs like @command{make} that rely on file timestamps.
7166 Time stamps are listed according to the time zone rules specified by
7167 the @env{TZ} environment variable, or by the system default rules if
7168 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
7169 with @env{TZ}, libc, The GNU C Library Reference Manual}.
7171 The following option changes how file timestamps are printed.
7174 @item --time-style=@var{style}
7175 @opindex --time-style
7177 List timestamps in style @var{style}. The @var{style} should
7178 be one of the following:
7183 List timestamps using @var{format}, where @var{format} is interpreted
7184 like the format argument of @command{date} (@pxref{date invocation}).
7185 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
7186 @command{ls} to list timestamps like @samp{2002-03-30 23:45:56}. As
7187 with @command{date}, @var{format}'s interpretation is affected by the
7188 @env{LC_TIME} locale category.
7190 If @var{format} contains two format strings separated by a newline,
7191 the former is used for non-recent files and the latter for recent
7192 files; if you want output columns to line up, you may need to insert
7193 spaces in one of the two formats.
7196 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
7197 format with nanosecond precision, e.g., @samp{2002-03-30
7198 23:45:56.477817180 -0700}. This style is equivalent to
7199 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
7201 This is useful because the time output includes all the information that
7202 is available from the operating system. For example, this can help
7203 explain @command{make}'s behavior, since @acronym{GNU} @command{make}
7204 uses the full timestamp to determine whether a file is out of date.
7207 List @acronym{ISO} 8601 date and time in minutes, e.g.,
7208 @samp{2002-03-30 23:45}. These timestamps are shorter than
7209 @samp{full-iso} timestamps, and are usually good enough for everyday
7210 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
7213 List @acronym{ISO} 8601 dates for non-recent timestamps (e.g.,
7214 @samp{2002-03-30@ }), and @acronym{ISO} 8601 month, day, hour, and
7215 minute for recent timestamps (e.g., @samp{03-30 23:45}). These
7216 timestamps are uglier than @samp{long-iso} timestamps, but they carry
7217 nearly the same information in a smaller space and their brevity helps
7218 @command{ls} output fit within traditional 80-column output lines.
7219 The following two @command{ls} invocations are equivalent:
7224 ls -l --time-style="+%Y-%m-%d $newline%m-%d %H:%M"
7225 ls -l --time-style="iso"
7230 List timestamps in a locale-dependent form. For example, a Finnish
7231 locale might list non-recent timestamps like @samp{maalis 30@ @ 2002}
7232 and recent timestamps like @samp{maalis 30 23:45}. Locale-dependent
7233 timestamps typically consume more space than @samp{iso} timestamps and
7234 are harder for programs to parse because locale conventions vary so
7235 widely, but they are easier for many people to read.
7237 The @env{LC_TIME} locale category specifies the timestamp format. The
7238 default @acronym{POSIX} locale uses timestamps like @samp{Mar 30@
7239 @ 2002} and @samp{Mar 30 23:45}; in this locale, the following two
7240 @command{ls} invocations are equivalent:
7245 ls -l --time-style="+%b %e %Y$newline%b %e %H:%M"
7246 ls -l --time-style="locale"
7249 Other locales behave differently. For example, in a German locale,
7250 @option{--time-style="locale"} might be equivalent to
7251 @option{--time-style="+%e. %b %Y $newline%e. %b %H:%M"}
7252 and might generate timestamps like @samp{30. M@"ar 2002@ } and
7253 @samp{30. M@"ar 23:45}.
7255 @item posix-@var{style}
7257 List @acronym{POSIX}-locale timestamps if the @env{LC_TIME} locale
7258 category is @acronym{POSIX}, @var{style} timestamps otherwise. For
7259 example, the @samp{posix-long-iso} style lists
7260 timestamps like @samp{Mar 30@ @ 2002} and @samp{Mar 30 23:45} when in
7261 the @acronym{POSIX} locale, and like @samp{2002-03-30 23:45} otherwise.
7266 You can specify the default value of the @option{--time-style} option
7267 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
7268 the default style is @samp{locale}. @acronym{GNU} Emacs 21.3 and
7269 later use the @option{--dired} option and therefore can parse any date
7270 format, but if you are using Emacs 21.1 or 21.2 and specify a
7271 non-@acronym{POSIX} locale you may need to set
7272 @samp{TIME_STYLE="posix-long-iso"}.
7274 To avoid certain denial-of-service attacks, timestamps that would be
7275 longer than 1000 bytes may be treated as errors.
7278 @node Formatting the file names
7279 @subsection Formatting the file names
7281 These options change how file names themselves are printed.
7287 @itemx --quoting-style=escape
7290 @opindex --quoting-style
7291 @cindex backslash sequences for file names
7292 Quote nongraphic characters in file names using alphabetic and octal
7293 backslash sequences like those used in C.
7297 @itemx --quoting-style=literal
7300 @opindex --quoting-style
7301 Do not quote file names. However, with @command{ls} nongraphic
7302 characters are still printed as question marks if the output is a
7303 terminal and you do not specify the @option{--show-control-chars}
7307 @itemx --hide-control-chars
7309 @opindex --hide-control-chars
7310 Print question marks instead of nongraphic characters in file names.
7311 This is the default if the output is a terminal and the program is
7316 @itemx --quoting-style=c
7318 @opindex --quote-name
7319 @opindex --quoting-style
7320 Enclose file names in double quotes and quote nongraphic characters as
7323 @item --quoting-style=@var{word}
7324 @opindex --quoting-style
7325 @cindex quoting style
7326 Use style @var{word} to quote file names and other strings that may
7327 contain arbitrary characters. The @var{word} should
7328 be one of the following:
7332 Output strings as-is; this is the same as the @option{-N} or
7333 @option{--literal} option.
7335 Quote strings for the shell if they contain shell metacharacters or would
7336 cause ambiguous output.
7337 The quoting is suitable for @acronym{POSIX}-compatible shells like
7338 @command{bash}, but it does not always work for incompatible shells
7341 Quote strings for the shell, even if they would normally not require quoting.
7343 Quote strings as for C character string literals, including the
7344 surrounding double-quote characters; this is the same as the
7345 @option{-Q} or @option{--quote-name} option.
7347 Quote strings as for C character string literals, except omit the
7348 surrounding double-quote
7349 characters; this is the same as the @option{-b} or @option{--escape} option.
7351 Quote strings as for C character string literals, except use
7352 surrounding quotation marks appropriate for the
7355 @c Use @t instead of @samp to avoid duplicate quoting in some output styles.
7356 Quote strings as for C character string literals, except use
7357 surrounding quotation marks appropriate for the locale, and quote
7358 @t{`like this'} instead of @t{"like
7359 this"} in the default C locale. This looks nicer on many displays.
7362 You can specify the default value of the @option{--quoting-style} option
7363 with the environment variable @env{QUOTING_STYLE}. If that environment
7364 variable is not set, the default value is @samp{literal}, but this
7365 default may change to @samp{shell} in a future version of this package.
7367 @item --show-control-chars
7368 @opindex --show-control-chars
7369 Print nongraphic characters as-is in file names.
7370 This is the default unless the output is a terminal and the program is
7376 @node dir invocation
7377 @section @command{dir}: Briefly list directory contents
7380 @cindex directory listing, brief
7382 @command{dir} is equivalent to @code{ls -C
7383 -b}; that is, by default files are listed in columns, sorted vertically,
7384 and special characters are represented by backslash escape sequences.
7386 @xref{ls invocation, @command{ls}}.
7389 @node vdir invocation
7390 @section @command{vdir}: Verbosely list directory contents
7393 @cindex directory listing, verbose
7395 @command{vdir} is equivalent to @code{ls -l
7396 -b}; that is, by default files are listed in long format and special
7397 characters are represented by backslash escape sequences.
7399 @node dircolors invocation
7400 @section @command{dircolors}: Color setup for @command{ls}
7404 @cindex setup for color
7406 @command{dircolors} outputs a sequence of shell commands to set up the
7407 terminal for color output from @command{ls} (and @command{dir}, etc.).
7411 eval "`dircolors [@var{option}]@dots{} [@var{file}]`"
7414 If @var{file} is specified, @command{dircolors} reads it to determine which
7415 colors to use for which file types and extensions. Otherwise, a
7416 precompiled database is used. For details on the format of these files,
7417 run @samp{dircolors --print-database}.
7419 To make @command{dircolors} read a @file{~/.dircolors} file if it
7420 exists, you can put the following lines in your @file{~/.bashrc} (or
7421 adapt them to your favorite shell):
7425 test -r $d && eval "$(dircolors $d)"
7429 @vindex SHELL @r{environment variable, and color}
7430 The output is a shell command to set the @env{LS_COLORS} environment
7431 variable. You can specify the shell syntax to use on the command line,
7432 or @command{dircolors} will guess it from the value of the @env{SHELL}
7433 environment variable.
7435 The program accepts the following options. Also see @ref{Common options}.
7440 @itemx --bourne-shell
7443 @opindex --bourne-shell
7444 @cindex Bourne shell syntax for color setup
7445 @cindex @command{sh} syntax for color setup
7446 Output Bourne shell commands. This is the default if the @env{SHELL}
7447 environment variable is set and does not end with @samp{csh} or
7456 @cindex C shell syntax for color setup
7457 @cindex @command{csh} syntax for color setup
7458 Output C shell commands. This is the default if @code{SHELL} ends with
7459 @command{csh} or @command{tcsh}.
7462 @itemx --print-database
7464 @opindex --print-database
7465 @cindex color database, printing
7466 @cindex database for color setup, printing
7467 @cindex printing color database
7468 Print the (compiled-in) default color configuration database. This
7469 output is itself a valid configuration file, and is fairly descriptive
7470 of the possibilities.
7477 @node Basic operations
7478 @chapter Basic operations
7480 @cindex manipulating files
7482 This chapter describes the commands for basic file manipulation:
7483 copying, moving (renaming), and deleting (removing).
7486 * cp invocation:: Copy files.
7487 * dd invocation:: Convert and copy a file.
7488 * install invocation:: Copy files and set attributes.
7489 * mv invocation:: Move (rename) files.
7490 * rm invocation:: Remove files or directories.
7491 * shred invocation:: Remove files more securely.
7496 @section @command{cp}: Copy files and directories
7499 @cindex copying files and directories
7500 @cindex files, copying
7501 @cindex directories, copying
7503 @command{cp} copies files (or, optionally, directories). The copy is
7504 completely independent of the original. You can either copy one file to
7505 another, or copy arbitrarily many files to a destination directory.
7509 cp [@var{option}]@dots{} [-T] @var{source} @var{dest}
7510 cp [@var{option}]@dots{} @var{source}@dots{} @var{directory}
7511 cp [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
7516 If two file names are given, @command{cp} copies the first file to the
7520 If the @option{--target-directory} (@option{-t}) option is given, or
7521 failing that if the last file is a directory and the
7522 @option{--no-target-directory} (@option{-T}) option is not given,
7523 @command{cp} copies each @var{source} file to the specified directory,
7524 using the @var{source}s' names.
7527 Generally, files are written just as they are read. For exceptions,
7528 see the @option{--sparse} option below.
7530 By default, @command{cp} does not copy directories. However, the
7531 @option{-R}, @option{-a}, and @option{-r} options cause @command{cp} to
7532 copy recursively by descending into source directories and copying files
7533 to corresponding destination directories.
7535 When copying from a symbolic link, @command{cp} normally follows the
7536 link only when not copying
7537 recursively. This default can be overridden with the
7538 @option{--archive} (@option{-a}), @option{-d}, @option{--dereference}
7539 (@option{-L}), @option{--no-dereference} (@option{-P}), and
7540 @option{-H} options. If more than one of these options is specified,
7541 the last one silently overrides the others.
7543 When copying to a symbolic link, @command{cp} follows the
7544 link only when it refers to an existing regular file.
7545 However, when copying to a dangling symbolic link, @command{cp}
7546 refuses by default, and fails with a diagnostic, since the operation
7547 is inherently dangerous. This behavior is contrary to historical
7548 practice and to @acronym{POSIX}.
7549 Set @env{POSIXLY_CORRECT} to make @command{cp} attempt to create
7550 the target of a dangling destination symlink, in spite of the possible risk.
7551 Also, when an option like
7552 @option{--backup} or @option{--link} acts to rename or remove the
7553 destination before copying, @command{cp} renames or removes the
7554 symbolic link rather than the file it points to.
7556 By default, @command{cp} copies the contents of special files only
7557 when not copying recursively. This default can be overridden with the
7558 @option{--copy-contents} option.
7560 @cindex self-backups
7561 @cindex backups, making only
7562 @command{cp} generally refuses to copy a file onto itself, with the
7563 following exception: if @option{--force --backup} is specified with
7564 @var{source} and @var{dest} identical, and referring to a regular file,
7565 @command{cp} will make a backup file, either regular or numbered, as
7566 specified in the usual ways (@pxref{Backup options}). This is useful when
7567 you simply want to make a backup of an existing file before changing it.
7569 The program accepts the following options. Also see @ref{Common options}.
7576 Preserve as much as possible of the structure and attributes of the
7577 original files in the copy (but do not attempt to preserve internal
7578 directory structure; i.e., @samp{ls -U} may list the entries in a copied
7579 directory in a different order).
7580 Try to preserve SELinux security context and extended attributes (xattr),
7581 but ignore any failure to do that and print no corresponding diagnostic.
7582 Equivalent to @option{-dR --preserve=all} with the reduced diagnostics.
7584 @itemx --attributes-only
7585 @opindex --attributes-only
7586 Preserve the specified attributes of the original files in the copy,
7587 but do not copy any data. See the @option{--preserve} option for
7588 controlling which attributes to copy.
7591 @itemx @w{@kbd{--backup}[=@var{method}]}
7594 @vindex VERSION_CONTROL
7595 @cindex backups, making
7596 @xref{Backup options}.
7597 Make a backup of each file that would otherwise be overwritten or removed.
7598 As a special case, @command{cp} makes a backup of @var{source} when the force
7599 and backup options are given and @var{source} and @var{dest} are the same
7600 name for an existing, regular file. One useful application of this
7601 combination of options is this tiny Bourne shell script:
7605 # Usage: backup FILE...
7606 # Create a @sc{gnu}-style backup of each listed FILE.
7608 cp --backup --force -- "$i" "$i"
7612 @item --copy-contents
7613 @cindex directories, copying recursively
7614 @cindex copying directories recursively
7615 @cindex recursively copying directories
7616 @cindex non-directories, copying as special files
7617 If copying recursively, copy the contents of any special files (e.g.,
7618 FIFOs and device files) as if they were regular files. This means
7619 trying to read the data in each source file and writing it to the
7620 destination. It is usually a mistake to use this option, as it
7621 normally has undesirable effects on special files like FIFOs and the
7622 ones typically found in the @file{/dev} directory. In most cases,
7623 @code{cp -R --copy-contents} will hang indefinitely trying to read
7624 from FIFOs and special files like @file{/dev/console}, and it will
7625 fill up your destination disk if you use it to copy @file{/dev/zero}.
7626 This option has no effect unless copying recursively, and it does not
7627 affect the copying of symbolic links.
7631 @cindex symbolic links, copying
7632 @cindex hard links, preserving
7633 Copy symbolic links as symbolic links rather than copying the files that
7634 they point to, and preserve hard links between source files in the copies.
7635 Equivalent to @option{--no-dereference --preserve=links}.
7641 When copying without this option and an existing destination file cannot
7642 be opened for writing, the copy fails. However, with @option{--force}),
7643 when a destination file cannot be opened, @command{cp} then removes it and
7644 tries to open it again. Contrast this behavior with that enabled by
7645 @option{--link} and @option{--symbolic-link}, whereby the destination file
7646 is never opened but rather is removed unconditionally. Also see the
7647 description of @option{--remove-destination}.
7649 This option is independent of the @option{--interactive} or
7650 @option{-i} option: neither cancels the effect of the other.
7652 This option is redundant if the @option{--no-clobber} or @option{-n} option is
7657 If a command line argument specifies a symbolic link, then copy the
7658 file it points to rather than the symbolic link itself. However,
7659 copy (preserving its nature) any symbolic link that is encountered
7660 via recursive traversal.
7663 @itemx --interactive
7665 @opindex --interactive
7666 When copying a file other than a directory, prompt whether to
7667 overwrite an existing destination file. The @option{-i} option overrides
7668 a previous @option{-n} option.
7674 Make hard links instead of copies of non-directories.
7677 @itemx --dereference
7679 @opindex --dereference
7680 Follow symbolic links when copying from them.
7681 With this option, @command{cp} cannot create a symbolic link.
7682 For example, a symlink (to regular file) in the source tree will be copied to
7683 a regular file in the destination tree.
7688 @opindex --no-clobber
7689 Do not overwrite an existing file. The @option{-n} option overrides a previous
7690 @option{-i} option. This option is mutually exclusive with @option{-b} or
7691 @option{--backup} option.
7694 @itemx --no-dereference
7696 @opindex --no-dereference
7697 @cindex symbolic links, copying
7698 Copy symbolic links as symbolic links rather than copying the files that
7699 they point to. This option affects only symbolic links in the source;
7700 symbolic links in the destination are always followed if possible.
7703 @itemx @w{@kbd{--preserve}[=@var{attribute_list}]}
7706 @cindex file information, preserving, extended attributes, xattr
7707 Preserve the specified attributes of the original files.
7708 If specified, the @var{attribute_list} must be a comma-separated list
7709 of one or more of the following strings:
7713 Preserve the file mode bits and access control lists.
7715 Preserve the owner and group. On most modern systems,
7716 only users with appropriate privileges may change the owner of a file,
7718 may preserve the group ownership of a file only if they happen to be
7719 a member of the desired group.
7721 Preserve the times of last access and last modification, when possible.
7722 On older systems, it is not possible to preserve these attributes
7723 when the affected file is a symbolic link.
7724 However, many systems now provide the @code{utimensat} function,
7725 which makes it possible even for symbolic links.
7727 Preserve in the destination files
7728 any links between corresponding source files.
7729 Note that with @option{-L} or @option{-H}, this option can convert
7730 symbolic links to hard links. For example,
7732 $ mkdir c; : > a; ln -s a b; cp -aH a b c; ls -i1 c
7737 Note the inputs: @file{b} is a symlink to regular file @file{a},
7738 yet the files in destination directory, @file{c/}, are hard-linked.
7739 Since @option{-a} implies @option{--preserve=links}, and since @option{-H}
7740 tells @command{cp} to dereference command line arguments, it sees two files
7741 with the same inode number, and preserves the perceived hard link.
7743 Here is a similar example that exercises @command{cp}'s @option{-L} option:
7745 $ mkdir b c; (cd b; : > a; ln -s a b); cp -aL b c; ls -i1 c/b
7751 Preserve SELinux security context of the file, or fail with full diagnostics.
7753 Preserve extended attributes of the file, or fail with full diagnostics.
7754 If @command{cp} is built without xattr support, ignore this option.
7755 If SELinux context, ACLs or Capabilities are implemented using xattrs,
7756 they are preserved by this option as well.
7758 Preserve all file attributes.
7759 Equivalent to specifying all of the above, but with the difference
7760 that failure to preserve SELinux security context or extended attributes
7761 does not change @command{cp}'s exit status. In contrast to @option{-a},
7762 all but @samp{Operation not supported} warnings are output.
7765 Using @option{--preserve} with no @var{attribute_list} is equivalent
7766 to @option{--preserve=mode,ownership,timestamps}.
7768 In the absence of this option, each destination file is created with the
7769 mode bits of the corresponding source file, minus the bits set in the
7770 umask and minus the set-user-ID and set-group-ID bits.
7771 @xref{File permissions}.
7773 @itemx @w{@kbd{--no-preserve}=@var{attribute_list}}
7774 @cindex file information, preserving
7775 Do not preserve the specified attributes. The @var{attribute_list}
7776 has the same form as for @option{--preserve}.
7780 @cindex parent directories and @command{cp}
7781 Form the name of each destination file by appending to the target
7782 directory a slash and the specified name of the source file. The last
7783 argument given to @command{cp} must be the name of an existing directory.
7784 For example, the command:
7787 cp --parents a/b/c existing_dir
7791 copies the file @file{a/b/c} to @file{existing_dir/a/b/c}, creating
7792 any missing intermediate directories.
7799 @opindex --recursive
7800 @cindex directories, copying recursively
7801 @cindex copying directories recursively
7802 @cindex recursively copying directories
7803 @cindex non-directories, copying as special files
7804 Copy directories recursively. By default, do not follow symbolic
7805 links in the source; see the @option{--archive} (@option{-a}), @option{-d},
7806 @option{--dereference} (@option{-L}), @option{--no-dereference}
7807 (@option{-P}), and @option{-H} options. Special files are copied by
7808 creating a destination file of the same type as the source; see the
7809 @option{--copy-contents} option. It is not portable to use
7810 @option{-r} to copy symbolic links or special files. On some
7811 non-@sc{gnu} systems, @option{-r} implies the equivalent of
7812 @option{-L} and @option{--copy-contents} for historical reasons.
7813 Also, it is not portable to use @option{-R} to copy symbolic links
7814 unless you also specify @option{-P}, as @acronym{POSIX} allows
7815 implementations that dereference symbolic links by default.
7817 @item --reflink[=@var{when}]
7818 @opindex --reflink[=@var{when}]
7821 @cindex copy on write
7822 Perform a lightweight, copy-on-write (COW) copy, if supported by the
7823 file system. Once it has succeeded, beware that the source and destination
7824 files share the same disk data blocks as long as they remain unmodified.
7825 Thus, if a disk I/O error affects data blocks of one of the files,
7826 the other suffers the same fate.
7828 The @var{when} value can be one of the following:
7832 The default behavior: if the copy-on-write operation is not supported
7833 then report the failure for each file and exit with a failure status.
7836 If the copy-on-write operation is not supported then fall back
7837 to the standard copy behaviour.
7840 This option is overridden by the @option{--link}, @option{--symbolic-link}
7841 and @option{--attributes-only} options, thus allowing it to be used
7842 to configure the default data copying behavior for @command{cp}.
7843 For example, with the following alias, @command{cp} will use the
7844 minimum amount of space supported by the file system.
7847 alias cp='cp --reflink=auto --sparse=always'
7850 @item --remove-destination
7851 @opindex --remove-destination
7852 Remove each existing destination file before attempting to open it
7853 (contrast with @option{-f} above).
7855 @item --sparse=@var{when}
7856 @opindex --sparse=@var{when}
7857 @cindex sparse files, copying
7858 @cindex holes, copying files with
7859 @findex read @r{system call, and holes}
7860 A @dfn{sparse file} contains @dfn{holes}---a sequence of zero bytes that
7861 does not occupy any physical disk blocks; the @samp{read} system call
7862 reads these as zeros. This can both save considerable disk space and
7863 increase speed, since many binary files contain lots of consecutive zero
7864 bytes. By default, @command{cp} detects holes in input source files via a crude
7865 heuristic and makes the corresponding output file sparse as well.
7866 Only regular files may be sparse.
7868 The @var{when} value can be one of the following:
7872 The default behavior: if the input file is sparse, attempt to make
7873 the output file sparse, too. However, if an output file exists but
7874 refers to a non-regular file, then do not attempt to make it sparse.
7877 For each sufficiently long sequence of zero bytes in the input file,
7878 attempt to create a corresponding hole in the output file, even if the
7879 input file does not appear to be sparse.
7880 This is useful when the input file resides on a file system
7881 that does not support sparse files
7882 (for example, @samp{efs} file systems in SGI IRIX 5.3 and earlier),
7883 but the output file is on a type of file system that does support them.
7884 Holes may be created only in regular files, so if the destination file
7885 is of some other type, @command{cp} does not even try to make it sparse.
7888 Never make the output file sparse.
7889 This is useful in creating a file for use with the @command{mkswap} command,
7890 since such a file must not have any holes.
7893 @optStripTrailingSlashes
7896 @itemx --symbolic-link
7898 @opindex --symbolic-link
7899 @cindex symbolic links, copying with
7900 Make symbolic links instead of copies of non-directories. All source
7901 file names must be absolute (starting with @samp{/}) unless the
7902 destination files are in the current directory. This option merely
7903 results in an error message on systems that do not support symbolic links.
7909 @optNoTargetDirectory
7915 @cindex newer files, copying only
7916 Do not copy a non-directory that has an existing destination with the
7917 same or newer modification time. If time stamps are being preserved,
7918 the comparison is to the source time stamp truncated to the
7919 resolutions of the destination file system and of the system calls
7920 used to update time stamps; this avoids duplicate work if several
7921 @samp{cp -pu} commands are executed with the same source and
7928 Print the name of each file before copying it.
7931 @itemx --one-file-system
7933 @opindex --one-file-system
7934 @cindex file systems, omitting copying to different
7935 Skip subdirectories that are on different file systems from the one that
7936 the copy started on.
7937 However, mount point directories @emph{are} copied.
7945 @section @command{dd}: Convert and copy a file
7948 @cindex converting while copying a file
7950 @command{dd} copies a file (from standard input to standard output, by
7951 default) with a changeable I/O block size, while optionally performing
7952 conversions on it. Synopses:
7955 dd [@var{operand}]@dots{}
7959 The only options are @option{--help} and @option{--version}.
7960 @xref{Common options}. @command{dd} accepts the following operands.
7966 Read from @var{file} instead of standard input.
7970 Write to @var{file} instead of standard output. Unless
7971 @samp{conv=notrunc} is given, @command{dd} truncates @var{file} to zero
7972 bytes (or the size specified with @samp{seek=}).
7974 @item ibs=@var{bytes}
7976 @cindex block size of input
7977 @cindex input block size
7978 Set the input block size to @var{bytes}.
7979 This makes @command{dd} read @var{bytes} per block.
7980 The default is 512 bytes.
7982 @item obs=@var{bytes}
7984 @cindex block size of output
7985 @cindex output block size
7986 Set the output block size to @var{bytes}.
7987 This makes @command{dd} write @var{bytes} per block.
7988 The default is 512 bytes.
7990 @item bs=@var{bytes}
7993 Set both input and output block sizes to @var{bytes}.
7994 This makes @command{dd} read and write @var{bytes} per block,
7995 overriding any @samp{ibs} and @samp{obs} settings.
7996 In addition, if no data-transforming @option{conv} option is specified,
7997 input is copied to the output as soon as it's read,
7998 even if it is smaller than the block size.
8000 @item cbs=@var{bytes}
8002 @cindex block size of conversion
8003 @cindex conversion block size
8004 @cindex fixed-length records, converting to variable-length
8005 @cindex variable-length records, converting to fixed-length
8006 Set the conversion block size to @var{bytes}.
8007 When converting variable-length records to fixed-length ones
8008 (@option{conv=block}) or the reverse (@option{conv=unblock}),
8009 use @var{bytes} as the fixed record length.
8011 @item skip=@var{blocks}
8013 Skip @var{blocks} @samp{ibs}-byte blocks in the input file before copying.
8015 @item seek=@var{blocks}
8017 Skip @var{blocks} @samp{obs}-byte blocks in the output file before copying.
8019 @item count=@var{blocks}
8021 Copy @var{blocks} @samp{ibs}-byte blocks from the input file, instead
8022 of everything until the end of the file.
8026 Do not print the overall transfer rate and volume statistics
8027 that normally make up the third status line when @command{dd} exits.
8029 @item conv=@var{conversion}[,@var{conversion}]@dots{}
8031 Convert the file as specified by the @var{conversion} argument(s).
8032 (No spaces around any comma(s).)
8039 @opindex ascii@r{, converting to}
8040 Convert @acronym{EBCDIC} to @acronym{ASCII},
8041 using the conversion table specified by @acronym{POSIX}.
8042 This provides a 1:1 translation for all 256 bytes.
8045 @opindex ebcdic@r{, converting to}
8046 Convert @acronym{ASCII} to @acronym{EBCDIC}.
8047 This is the inverse of the @samp{ascii} conversion.
8050 @opindex alternate ebcdic@r{, converting to}
8051 Convert @acronym{ASCII} to alternate @acronym{EBCDIC},
8052 using the alternate conversion table specified by @acronym{POSIX}.
8053 This is not a 1:1 translation, but reflects common historical practice
8054 for @samp{~}, @samp{[}, and @samp{]}.
8056 The @samp{ascii}, @samp{ebcdic}, and @samp{ibm} conversions are
8060 @opindex block @r{(space-padding)}
8061 For each line in the input, output @samp{cbs} bytes, replacing the
8062 input newline with a space and padding with spaces as necessary.
8066 Remove any trailing spaces in each @samp{cbs}-sized input block,
8067 and append a newline.
8069 The @samp{block} and @samp{unblock} conversions are mutually exclusive.
8072 @opindex lcase@r{, converting to}
8073 Change uppercase letters to lowercase.
8076 @opindex ucase@r{, converting to}
8077 Change lowercase letters to uppercase.
8079 The @samp{lcase} and @samp{ucase} conversions are mutually exclusive.
8082 @opindex swab @r{(byte-swapping)}
8083 @cindex byte-swapping
8084 Swap every pair of input bytes. @sc{gnu} @command{dd}, unlike others, works
8085 when an odd number of bytes are read---the last byte is simply copied
8086 (since there is nothing to swap it with).
8089 @opindex sync @r{(padding with @acronym{ASCII} @sc{nul}s)}
8090 Pad every input block to size of @samp{ibs} with trailing zero bytes.
8091 When used with @samp{block} or @samp{unblock}, pad with spaces instead of
8096 The following ``conversions'' are really file flags
8097 and don't affect internal processing:
8102 @cindex creating output file, requiring
8103 Fail if the output file already exists; @command{dd} must create the
8108 @cindex creating output file, avoiding
8109 Do not create the output file; the output file must already exist.
8111 The @samp{excl} and @samp{nocreat} conversions are mutually exclusive.
8115 @cindex truncating output file, avoiding
8116 Do not truncate the output file.
8120 @cindex read errors, ignoring
8121 Continue after read errors.
8125 @cindex synchronized data writes, before finishing
8126 Synchronize output data just before finishing. This forces a physical
8127 write of output data.
8131 @cindex synchronized data and metadata writes, before finishing
8132 Synchronize output data and metadata just before finishing. This
8133 forces a physical write of output data and metadata.
8137 @item iflag=@var{flag}[,@var{flag}]@dots{}
8139 Access the input file using the flags specified by the @var{flag}
8140 argument(s). (No spaces around any comma(s).)
8142 @item oflag=@var{flag}[,@var{flag}]@dots{}
8144 Access the output file using the flags specified by the @var{flag}
8145 argument(s). (No spaces around any comma(s).)
8147 Here are the flags. Not every flag is supported on every operating
8154 @cindex appending to the output file
8155 Write in append mode, so that even if some other process is writing to
8156 this file, every @command{dd} write will append to the current
8157 contents of the file. This flag makes sense only for output.
8158 If you combine this flag with the @samp{of=@var{file}} operand,
8159 you should also specify @samp{conv=notrunc} unless you want the
8160 output file to be truncated before being appended to.
8164 @cindex concurrent I/O
8165 Use concurrent I/O mode for data. This mode performs direct I/O
8166 and drops the @acronym{POSIX} requirement to serialize all I/O to the same file.
8167 A file cannot be opened in CIO mode and with a standard open at the
8173 Use direct I/O for data, avoiding the buffer cache.
8174 Note that the kernel may impose restrictions on read or write buffer sizes.
8175 For example, with an ext4 destination file system and a linux-based kernel,
8176 using @samp{oflag=direct} will cause writes to fail with @code{EINVAL} if the
8177 output buffer size is not a multiple of 512.
8181 @cindex directory I/O
8183 Fail unless the file is a directory. Most operating systems do not
8184 allow I/O to a directory, so this flag has limited utility.
8188 @cindex synchronized data reads
8189 Use synchronized I/O for data. For the output file, this forces a
8190 physical write of output data on each write. For the input file,
8191 this flag can matter when reading from a remote file that has been
8192 written to synchronously by some other process. Metadata (e.g.,
8193 last-access and last-modified time) is not necessarily synchronized.
8197 @cindex synchronized data and metadata I/O
8198 Use synchronized I/O for both data and metadata.
8202 @cindex discarding file cache
8203 Discard the data cache for a file.
8204 When count=0 all cache is discarded,
8205 otherwise the cache is dropped for the processed
8206 portion of the file. Also when count=0
8207 failure to discard the cache is diagnosed
8208 and reflected in the exit status.
8209 Here as some usage examples:
8212 # Advise to drop cache for whole file
8213 dd if=ifile iflag=nocache count=0
8215 # Ensure drop cache for the whole file
8216 dd of=ofile oflag=nocache conv=notrunc,fdatasync count=0
8218 # Drop cache for part of file
8219 dd if=ifile iflag=nocache skip=10 count=10 of=/dev/null
8221 # Stream data using just the read-ahead cache
8222 dd if=ifile of=ofile iflag=nocache oflag=nocache
8227 @cindex nonblocking I/O
8228 Use non-blocking I/O.
8233 Do not update the file's access time.
8234 Some older file systems silently ignore this flag, so it is a good
8235 idea to test it on your files before relying on it.
8239 @cindex controlling terminal
8240 Do not assign the file to be a controlling terminal for @command{dd}.
8241 This has no effect when the file is not a terminal.
8242 On many hosts (e.g., @acronym{GNU}/Linux hosts), this option has no effect
8247 @cindex symbolic links, following
8248 Do not follow symbolic links.
8253 Fail if the file has multiple hard links.
8258 Use binary I/O. This option has an effect only on nonstandard
8259 platforms that distinguish binary from text I/O.
8264 Use text I/O. Like @samp{binary}, this option has no effect on
8269 Accumulate full blocks from input. The @code{read} system call
8270 may return early if a full block is not available.
8271 When that happens, continue calling @code{read} to fill the remainder
8273 This flag can be used only with @code{iflag}.
8277 These flags are not supported on all systems, and @samp{dd} rejects
8278 attempts to use them when they are not supported. When reading from
8279 standard input or writing to standard output, the @samp{nofollow} and
8280 @samp{noctty} flags should not be specified, and the other flags
8281 (e.g., @samp{nonblock}) can affect how other processes behave with the
8282 affected file descriptors, even after @command{dd} exits.
8286 @cindex multipliers after numbers
8287 The numeric-valued strings above (@var{bytes} and @var{blocks}) can be
8288 followed by a multiplier: @samp{b}=512, @samp{c}=1,
8289 @samp{w}=2, @samp{x@var{m}}=@var{m}, or any of the
8290 standard block size suffixes like @samp{k}=1024 (@pxref{Block size}).
8292 Any block size you specify via @samp{bs=}, @samp{ibs=}, @samp{obs=}, @samp{cbs=}
8293 should not be too large---values larger than a few megabytes
8294 are generally wasteful or (as in the gigabyte..exabyte case) downright
8295 counterproductive or error-inducing.
8297 Use different @command{dd} invocations to use different block sizes for
8298 skipping and I/O@. For example, the following shell commands copy data
8299 in 512 KiB blocks between a disk and a tape, but do not save or restore a
8300 4 KiB label at the start of the disk:
8303 disk=/dev/rdsk/c0t1d0s2
8306 # Copy all but the label from disk to tape.
8307 (dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape
8309 # Copy from tape back to disk, but leave the disk label alone.
8310 (dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk
8313 Sending an @samp{INFO} signal to a running @command{dd}
8314 process makes it print I/O statistics to standard error
8315 and then resume copying. In the example below,
8316 @command{dd} is run in the background to copy 10 million blocks.
8317 The @command{kill} command makes it output intermediate I/O statistics,
8318 and when @command{dd} completes normally or is killed by the
8319 @code{SIGINT} signal, it outputs the final statistics.
8322 $ dd if=/dev/zero of=/dev/null count=10MB & pid=$!
8323 $ kill -s INFO $pid; wait $pid
8324 3385223+0 records in
8325 3385223+0 records out
8326 1733234176 bytes (1.7 GB) copied, 6.42173 seconds, 270 MB/s
8327 10000000+0 records in
8328 10000000+0 records out
8329 5120000000 bytes (5.1 GB) copied, 18.913 seconds, 271 MB/s
8332 @vindex POSIXLY_CORRECT
8333 On systems lacking the @samp{INFO} signal @command{dd} responds to the
8334 @samp{USR1} signal instead, unless the @env{POSIXLY_CORRECT}
8335 environment variable is set.
8340 @node install invocation
8341 @section @command{install}: Copy files and set attributes
8344 @cindex copying files and setting attributes
8346 @command{install} copies files while setting their file mode bits and, if
8347 possible, their owner and group. Synopses:
8350 install [@var{option}]@dots{} [-T] @var{source} @var{dest}
8351 install [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8352 install [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8353 install [@var{option}]@dots{} -d @var{directory}@dots{}
8358 If two file names are given, @command{install} copies the first file to the
8362 If the @option{--target-directory} (@option{-t}) option is given, or
8363 failing that if the last file is a directory and the
8364 @option{--no-target-directory} (@option{-T}) option is not given,
8365 @command{install} copies each @var{source} file to the specified
8366 directory, using the @var{source}s' names.
8369 If the @option{--directory} (@option{-d}) option is given,
8370 @command{install} creates each @var{directory} and any missing parent
8371 directories. Parent directories are created with mode
8372 @samp{u=rwx,go=rx} (755), regardless of the @option{-m} option or the
8373 current umask. @xref{Directory Setuid and Setgid}, for how the
8374 set-user-ID and set-group-ID bits of parent directories are inherited.
8377 @cindex Makefiles, installing programs in
8378 @command{install} is similar to @command{cp}, but allows you to control the
8379 attributes of destination files. It is typically used in Makefiles to
8380 copy programs into their destination directories. It refuses to copy
8381 files onto themselves.
8383 @cindex extended attributes, xattr
8384 @command{install} never preserves extended attributes (xattr).
8386 The program accepts the following options. Also see @ref{Common options}.
8396 Compare each pair of source and destination files, and if the destination has
8397 identical content and any specified owner, group, permissions, and possibly
8398 SELinux context, then do not modify the destination at all.
8402 Ignored; for compatibility with old Unix versions of @command{install}.
8406 Create any missing parent directories of @var{dest},
8407 then copy @var{source} to @var{dest}.
8408 This option is ignored if a destination directory is specified
8409 via @option{--target-directory=DIR}.
8414 @opindex --directory
8415 @cindex directories, creating with given attributes
8416 @cindex parent directories, creating missing
8417 @cindex leading directories, creating missing
8418 Create any missing parent directories, giving them the default
8419 attributes. Then create each given directory, setting their owner,
8420 group and mode as given on the command line or to the defaults.
8422 @item -g @var{group}
8423 @itemx --group=@var{group}
8426 @cindex group ownership of installed files, setting
8427 Set the group ownership of installed files or directories to
8428 @var{group}. The default is the process's current group. @var{group}
8429 may be either a group name or a numeric group ID.
8432 @itemx --mode=@var{mode}
8435 @cindex permissions of installed files, setting
8436 Set the file mode bits for the installed file or directory to @var{mode},
8437 which can be either an octal number, or a symbolic mode as in
8438 @command{chmod}, with @samp{a=} (no access allowed to anyone) as the
8439 point of departure (@pxref{File permissions}).
8440 The default mode is @samp{u=rwx,go=rx,a-s}---read, write, and
8441 execute for the owner, read and execute for group and other, and with
8442 set-user-ID and set-group-ID disabled.
8443 This default is not quite the same as @samp{755}, since it disables
8444 instead of preserving set-user-ID and set-group-ID on directories.
8445 @xref{Directory Setuid and Setgid}.
8447 @item -o @var{owner}
8448 @itemx --owner=@var{owner}
8451 @cindex ownership of installed files, setting
8452 @cindex appropriate privileges
8453 @vindex root @r{as default owner}
8454 If @command{install} has appropriate privileges (is run as root), set the
8455 ownership of installed files or directories to @var{owner}. The default
8456 is @code{root}. @var{owner} may be either a user name or a numeric user
8459 @item --preserve-context
8460 @opindex --preserve-context
8462 @cindex security context
8463 Preserve the SELinux security context of files and directories.
8464 Failure to preserve the context in all of the files or directories
8465 will result in an exit status of 1. If SELinux is disabled then
8466 print a warning and ignore the option.
8469 @itemx --preserve-timestamps
8471 @opindex --preserve-timestamps
8472 @cindex timestamps of installed files, preserving
8473 Set the time of last access and the time of last modification of each
8474 installed file to match those of each corresponding original file.
8475 When a file is installed without this option, its last access and
8476 last modification times are both set to the time of installation.
8477 This option is useful if you want to use the last modification times
8478 of installed files to keep track of when they were last built as opposed
8479 to when they were last installed.
8485 @cindex symbol table information, stripping
8486 @cindex stripping symbol table information
8487 Strip the symbol tables from installed binary executables.
8489 @itemx --strip-program=@var{program}
8490 @opindex --strip-program
8491 @cindex symbol table information, stripping, program
8492 Program used to strip binaries.
8498 @optNoTargetDirectory
8504 Print the name of each file before copying it.
8506 @item -Z @var{context}
8507 @itemx --context=@var{context}
8511 @cindex security context
8512 Set the default SELinux security context to be used for any
8513 created files and directories. If SELinux is disabled then
8514 print a warning and ignore the option.
8522 @section @command{mv}: Move (rename) files
8526 @command{mv} moves or renames files (or directories). Synopses:
8529 mv [@var{option}]@dots{} [-T] @var{source} @var{dest}
8530 mv [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8531 mv [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8536 If two file names are given, @command{mv} moves the first file to the
8540 If the @option{--target-directory} (@option{-t}) option is given, or
8541 failing that if the last file is a directory and the
8542 @option{--no-target-directory} (@option{-T}) option is not given,
8543 @command{mv} moves each @var{source} file to the specified
8544 directory, using the @var{source}s' names.
8547 @command{mv} can move any type of file from one file system to another.
8548 Prior to version @code{4.0} of the fileutils,
8549 @command{mv} could move only regular files between file systems.
8550 For example, now @command{mv} can move an entire directory hierarchy
8551 including special device files from one partition to another. It first
8552 uses some of the same code that's used by @code{cp -a} to copy the
8553 requested directories and files, then (assuming the copy succeeded)
8554 it removes the originals. If the copy fails, then the part that was
8555 copied to the destination partition is removed. If you were to copy
8556 three directories from one partition to another and the copy of the first
8557 directory succeeded, but the second didn't, the first would be left on
8558 the destination partition and the second and third would be left on the
8561 @cindex extended attributes, xattr
8562 @command{mv} always tries to copy extended attributes (xattr), which may
8563 include SELinux context, ACLs or Capabilities.
8564 Upon failure all but @samp{Operation not supported} warnings are output.
8566 @cindex prompting, and @command{mv}
8567 If a destination file exists but is normally unwritable, standard input
8568 is a terminal, and the @option{-f} or @option{--force} option is not given,
8569 @command{mv} prompts the user for whether to replace the file. (You might
8570 own the file, or have write permission on its directory.) If the
8571 response is not affirmative, the file is skipped.
8573 @emph{Warning}: Avoid specifying a source name with a trailing slash,
8574 when it might be a symlink to a directory.
8575 Otherwise, @command{mv} may do something very surprising, since
8576 its behavior depends on the underlying rename system call.
8577 On a system with a modern Linux-based kernel, it fails with
8578 @code{errno=ENOTDIR}.
8579 However, on other systems (at least FreeBSD 6.1 and Solaris 10) it silently
8580 renames not the symlink but rather the directory referenced by the symlink.
8581 @xref{Trailing slashes}.
8583 The program accepts the following options. Also see @ref{Common options}.
8593 @cindex prompts, omitting
8594 Do not prompt the user before removing a destination file.
8596 If you specify more than one of the @option{-i}, @option{-f}, @option{-n}
8597 options, only the final one takes effect.
8602 @itemx --interactive
8604 @opindex --interactive
8605 @cindex prompts, forcing
8606 Prompt whether to overwrite each existing destination file, regardless
8608 If the response is not affirmative, the file is skipped.
8614 @opindex --no-clobber
8615 @cindex prompts, omitting
8616 Do not overwrite an existing file.
8618 This option is mutually exclusive with @option{-b} or @option{--backup} option.
8624 @cindex newer files, moving only
8625 Do not move a non-directory that has an existing destination with the
8626 same or newer modification time.
8627 If the move is across file system boundaries, the comparison is to the
8628 source time stamp truncated to the resolutions of the destination file
8629 system and of the system calls used to update time stamps; this avoids
8630 duplicate work if several @samp{mv -u} commands are executed with the
8631 same source and destination.
8637 Print the name of each file before moving it.
8639 @optStripTrailingSlashes
8645 @optNoTargetDirectory
8653 @section @command{rm}: Remove files or directories
8656 @cindex removing files or directories
8658 @command{rm} removes each given @var{file}. By default, it does not remove
8659 directories. Synopsis:
8662 rm [@var{option}]@dots{} [@var{file}]@dots{}
8665 @cindex prompting, and @command{rm}
8666 If the @option{-I} or @option{--interactive=once} option is given,
8667 and there are more than three files or the @option{-r}, @option{-R},
8668 or @option{--recursive} are given, then @command{rm} prompts the user
8669 for whether to proceed with the entire operation. If the response is
8670 not affirmative, the entire command is aborted.
8672 Otherwise, if a file is unwritable, standard input is a terminal, and
8673 the @option{-f} or @option{--force} option is not given, or the
8674 @option{-i} or @option{--interactive=always} option @emph{is} given,
8675 @command{rm} prompts the user for whether to remove the file.
8676 If the response is not affirmative, the file is skipped.
8678 Any attempt to remove a file whose last file name component is
8679 @file{.} or @file{..} is rejected without any prompting.
8681 @emph{Warning}: If you use @command{rm} to remove a file, it is usually
8682 possible to recover the contents of that file. If you want more assurance
8683 that the contents are truly unrecoverable, consider using @command{shred}.
8685 The program accepts the following options. Also see @ref{Common options}.
8693 Ignore nonexistent files and never prompt the user.
8694 Ignore any previous @option{--interactive} (@option{-i}) option.
8698 Prompt whether to remove each file.
8699 If the response is not affirmative, the file is skipped.
8700 Ignore any previous @option{--force} (@option{-f}) option.
8701 Equivalent to @option{--interactive=always}.
8705 Prompt once whether to proceed with the command, if more than three
8706 files are named or if a recursive removal is requested. Ignore any
8707 previous @option{--force} (@option{-f}) option. Equivalent to
8708 @option{--interactive=once}.
8710 @itemx --interactive [=@var{when}]
8711 @opindex --interactive
8712 Specify when to issue an interactive prompt. @var{when} may be
8716 @vindex never @r{interactive option}
8717 - Do not prompt at all.
8719 @vindex once @r{interactive option}
8720 - Prompt once if more than three files are named or if a recursive
8721 removal is requested. Equivalent to @option{-I}.
8723 @vindex always @r{interactive option}
8724 - Prompt for every file being removed. Equivalent to @option{-i}.
8726 @option{--interactive} with no @var{when} is equivalent to
8727 @option{--interactive=always}.
8729 @itemx --one-file-system
8730 @opindex --one-file-system
8731 @cindex one file system, restricting @command{rm} to
8732 When removing a hierarchy recursively, skip any directory that is on a
8733 file system different from that of the corresponding command line argument.
8736 This option is useful when removing a build ``chroot'' hierarchy,
8737 which normally contains no valuable data. However, it is not uncommon
8738 to bind-mount @file{/home} into such a hierarchy, to make it easier to
8739 use one's start-up file. The catch is that it's easy to forget to
8740 unmount @file{/home}. Then, when you use @command{rm -rf} to remove
8741 your normally throw-away chroot, that command will remove everything
8742 under @file{/home}, too.
8743 Use the @option{--one-file-system} option, and it will
8744 warn about and skip directories on other file systems.
8745 Of course, this will not save your @file{/home} if it and your
8746 chroot happen to be on the same file system.
8748 @itemx --preserve-root
8749 @opindex --preserve-root
8750 @cindex root directory, disallow recursive destruction
8751 Fail upon any attempt to remove the root directory, @file{/},
8752 when used with the @option{--recursive} option.
8753 This is the default behavior.
8754 @xref{Treating / specially}.
8756 @itemx --no-preserve-root
8757 @opindex --no-preserve-root
8758 @cindex root directory, allow recursive destruction
8759 Do not treat @file{/} specially when removing recursively.
8760 This option is not recommended unless you really want to
8761 remove all the files on your computer.
8762 @xref{Treating / specially}.
8769 @opindex --recursive
8770 @cindex directories, removing (recursively)
8771 Remove the listed directories and their contents recursively.
8777 Print the name of each file before removing it.
8781 @cindex files beginning with @samp{-}, removing
8782 @cindex @samp{-}, removing files beginning with
8783 One common question is how to remove files whose names begin with a
8784 @samp{-}. @sc{gnu} @command{rm}, like every program that uses the @code{getopt}
8785 function to parse its arguments, lets you use the @samp{--} option to
8786 indicate that all following arguments are non-options. To remove a file
8787 called @file{-f} in the current directory, you could type either:
8800 @opindex - @r{and Unix @command{rm}}
8801 The Unix @command{rm} program's use of a single @samp{-} for this purpose
8802 predates the development of the getopt standard syntax.
8807 @node shred invocation
8808 @section @command{shred}: Remove files more securely
8811 @cindex data, erasing
8812 @cindex erasing data
8814 @command{shred} overwrites devices or files, to help prevent even
8815 very expensive hardware from recovering the data.
8817 Ordinarily when you remove a file (@pxref{rm invocation}), the data is
8818 not actually destroyed. Only the index listing where the file is
8819 stored is destroyed, and the storage is made available for reuse.
8820 There are undelete utilities that will attempt to reconstruct the index
8821 and can bring the file back if the parts were not reused.
8823 On a busy system with a nearly-full drive, space can get reused in a few
8824 seconds. But there is no way to know for sure. If you have sensitive
8825 data, you may want to be sure that recovery is not possible by actually
8826 overwriting the file with non-sensitive data.
8828 However, even after doing that, it is possible to take the disk back
8829 to a laboratory and use a lot of sensitive (and expensive) equipment
8830 to look for the faint ``echoes'' of the original data underneath the
8831 overwritten data. If the data has only been overwritten once, it's not
8834 The best way to remove something irretrievably is to destroy the media
8835 it's on with acid, melt it down, or the like. For cheap removable media
8836 like floppy disks, this is the preferred method. However, hard drives
8837 are expensive and hard to melt, so the @command{shred} utility tries
8838 to achieve a similar effect non-destructively.
8840 This uses many overwrite passes, with the data patterns chosen to
8841 maximize the damage they do to the old data. While this will work on
8842 floppies, the patterns are designed for best effect on hard drives.
8843 For more details, see the source code and Peter Gutmann's paper
8844 @uref{http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html,
8845 @cite{Secure Deletion of Data from Magnetic and Solid-State Memory}},
8846 from the proceedings of the Sixth @acronym{USENIX} Security Symposium (San Jose,
8847 California, July 22--25, 1996).
8849 @strong{Please note} that @command{shred} relies on a very important assumption:
8850 that the file system overwrites data in place. This is the traditional
8851 way to do things, but many modern file system designs do not satisfy this
8852 assumption. Exceptions include:
8857 Log-structured or journaled file systems, such as those supplied with
8858 AIX and Solaris, and JFS, ReiserFS, XFS, Ext3 (in @code{data=journal} mode),
8859 BFS, NTFS, etc.@: when they are configured to journal @emph{data}.
8862 File systems that write redundant data and carry on even if some writes
8863 fail, such as RAID-based file systems.
8866 File systems that make snapshots, such as Network Appliance's NFS server.
8869 File systems that cache in temporary locations, such as NFS version 3
8873 Compressed file systems.
8876 In the particular case of ext3 file systems, the above disclaimer applies (and
8877 @command{shred} is thus of limited effectiveness) only in @code{data=journal}
8878 mode, which journals file data in addition to just metadata. In both
8879 the @code{data=ordered} (default) and @code{data=writeback} modes,
8880 @command{shred} works as usual. Ext3 journaling modes can be changed
8881 by adding the @code{data=something} option to the mount options for a
8882 particular file system in the @file{/etc/fstab} file, as documented in
8883 the mount man page (man mount).
8885 If you are not sure how your file system operates, then you should assume
8886 that it does not overwrite data in place, which means that shred cannot
8887 reliably operate on regular files in your file system.
8889 Generally speaking, it is more reliable to shred a device than a file,
8890 since this bypasses the problem of file system design mentioned above.
8891 However, even shredding devices is not always completely reliable. For
8892 example, most disks map out bad sectors invisibly to the application; if
8893 the bad sectors contain sensitive data, @command{shred} won't be able to
8896 @command{shred} makes no attempt to detect or report this problem, just as
8897 it makes no attempt to do anything about backups. However, since it is
8898 more reliable to shred devices than files, @command{shred} by default does
8899 not truncate or remove the output file. This default is more suitable
8900 for devices, which typically cannot be truncated and should not be
8903 Finally, consider the risk of backups and mirrors.
8904 File system backups and remote mirrors may contain copies of the
8905 file that cannot be removed, and that will allow a shredded file
8906 to be recovered later. So if you keep any data you may later want
8907 to destroy using @command{shred}, be sure that it is not backed up or mirrored.
8910 shred [@var{option}]@dots{} @var{file}[@dots{}]
8913 The program accepts the following options. Also see @ref{Common options}.
8921 @cindex force deletion
8922 Override file permissions if necessary to allow overwriting.
8925 @itemx -n @var{number}
8926 @itemx --iterations=@var{number}
8927 @opindex -n @var{number}
8928 @opindex --iterations=@var{number}
8929 @cindex iterations, selecting the number of
8930 By default, @command{shred} uses @value{SHRED_DEFAULT_PASSES} passes of
8931 overwrite. You can reduce this to save time, or increase it if you think it's
8932 appropriate. After 25 passes all of the internal overwrite patterns will have
8933 been used at least once.
8935 @item --random-source=@var{file}
8936 @opindex --random-source
8937 @cindex random source for shredding
8938 Use @var{file} as a source of random data used to overwrite and to
8939 choose pass ordering. @xref{Random sources}.
8941 @item -s @var{bytes}
8942 @itemx --size=@var{bytes}
8943 @opindex -s @var{bytes}
8944 @opindex --size=@var{bytes}
8945 @cindex size of file to shred
8946 Shred the first @var{bytes} bytes of the file. The default is to shred
8947 the whole file. @var{bytes} can be followed by a size specification like
8948 @samp{K}, @samp{M}, or @samp{G} to specify a multiple. @xref{Block size}.
8954 @cindex removing files after shredding
8955 After shredding a file, truncate it (if possible) and then remove it.
8956 If a file has multiple links, only the named links will be removed.
8962 Display to standard error all status updates as sterilization proceeds.
8968 By default, @command{shred} rounds the size of a regular file up to the next
8969 multiple of the file system block size to fully erase the last block
8971 Use @option{--exact} to suppress that behavior.
8972 Thus, by default if you shred a 10-byte regular file on a system with 512-byte
8973 blocks, the resulting file will be 512 bytes long. With this option,
8974 shred does not increase the apparent size of the file.
8980 Normally, the last pass that @command{shred} writes is made up of
8981 random data. If this would be conspicuous on your hard drive (for
8982 example, because it looks like encrypted data), or you just think
8983 it's tidier, the @option{--zero} option adds an additional overwrite pass with
8984 all zero bits. This is in addition to the number of passes specified
8985 by the @option{--iterations} option.
8989 You might use the following command to erase all trace of the
8990 file system you'd created on the floppy disk in your first drive.
8991 That command takes about 20 minutes to erase a ``1.44MB'' (actually
8995 shred --verbose /dev/fd0
8998 Similarly, to erase all data on a selected partition of
8999 your hard disk, you could give a command like this:
9002 shred --verbose /dev/sda5
9005 On modern disks, a single pass should be adequate,
9006 and it will take one third the time of the default three-pass approach.
9009 # 1 pass, write pseudo-random data; 3x faster than the default
9010 shred --verbose -n1 /dev/sda5
9013 To be on the safe side, use at least one pass that overwrites using
9014 pseudo-random data. I.e., don't be tempted to use @samp{-n0 --zero},
9015 in case some disk controller optimizes the process of writing blocks
9016 of all zeros, and thereby does not clear all bytes in a block.
9017 Some SSDs may do just that.
9019 A @var{file} of @samp{-} denotes standard output.
9020 The intended use of this is to shred a removed temporary file.
9027 echo "Hello, world" >&3
9032 However, the command @samp{shred - >file} does not shred the contents
9033 of @var{file}, since the shell truncates @var{file} before invoking
9034 @command{shred}. Use the command @samp{shred file} or (if using a
9035 Bourne-compatible shell) the command @samp{shred - 1<>file} instead.
9040 @node Special file types
9041 @chapter Special file types
9043 @cindex special file types
9044 @cindex file types, special
9046 This chapter describes commands which create special types of files (and
9047 @command{rmdir}, which removes directories, one special file type).
9049 @cindex special file types
9051 Although Unix-like operating systems have markedly fewer special file
9052 types than others, not @emph{everything} can be treated only as the
9053 undifferentiated byte stream of @dfn{normal files}. For example, when a
9054 file is created or removed, the system must record this information,
9055 which it does in a @dfn{directory}---a special type of file. Although
9056 you can read directories as normal files, if you're curious, in order
9057 for the system to do its job it must impose a structure, a certain
9058 order, on the bytes of the file. Thus it is a ``special'' type of file.
9060 Besides directories, other special file types include named pipes
9061 (FIFOs), symbolic links, sockets, and so-called @dfn{special files}.
9064 * link invocation:: Make a hard link via the link syscall
9065 * ln invocation:: Make links between files.
9066 * mkdir invocation:: Make directories.
9067 * mkfifo invocation:: Make FIFOs (named pipes).
9068 * mknod invocation:: Make block or character special files.
9069 * readlink invocation:: Print value of a symlink or canonical file name.
9070 * rmdir invocation:: Remove empty directories.
9071 * unlink invocation:: Remove files via the unlink syscall
9075 @node link invocation
9076 @section @command{link}: Make a hard link via the link syscall
9079 @cindex links, creating
9080 @cindex hard links, creating
9081 @cindex creating links (hard only)
9083 @command{link} creates a single hard link at a time.
9084 It is a minimalist interface to the system-provided
9085 @code{link} function. @xref{Hard Links, , , libc,
9086 The GNU C Library Reference Manual}.
9087 It avoids the bells and whistles of the more commonly-used
9088 @command{ln} command (@pxref{ln invocation}).
9092 link @var{filename} @var{linkname}
9095 @var{filename} must specify an existing file, and @var{linkname}
9096 must specify a nonexistent entry in an existing directory.
9097 @command{link} simply calls @code{link (@var{filename}, @var{linkname})}
9100 On a @acronym{GNU} system, this command acts like @samp{ln --directory
9101 --no-target-directory @var{filename} @var{linkname}}. However, the
9102 @option{--directory} and @option{--no-target-directory} options are
9103 not specified by @acronym{POSIX}, and the @command{link} command is
9104 more portable in practice.
9106 If @var{filename} is a symbolic link, it is unspecified whether
9107 @var{linkname} will be a hard link to the symbolic link or to the
9108 target of the symbolic link. Use @command{ln -P} or @command{ln -L}
9109 to specify which behavior is desired.
9115 @section @command{ln}: Make links between files
9118 @cindex links, creating
9119 @cindex hard links, creating
9120 @cindex symbolic (soft) links, creating
9121 @cindex creating links (hard or soft)
9123 @cindex file systems and hard links
9124 @command{ln} makes links between files. By default, it makes hard links;
9125 with the @option{-s} option, it makes symbolic (or @dfn{soft}) links.
9129 ln [@var{option}]@dots{} [-T] @var{target} @var{linkname}
9130 ln [@var{option}]@dots{} @var{target}
9131 ln [@var{option}]@dots{} @var{target}@dots{} @var{directory}
9132 ln [@var{option}]@dots{} -t @var{directory} @var{target}@dots{}
9138 If two file names are given, @command{ln} creates a link to the first
9139 file from the second.
9142 If one @var{target} is given, @command{ln} creates a link to that file
9143 in the current directory.
9146 If the @option{--target-directory} (@option{-t}) option is given, or
9147 failing that if the last file is a directory and the
9148 @option{--no-target-directory} (@option{-T}) option is not given,
9149 @command{ln} creates a link to each @var{target} file in the specified
9150 directory, using the @var{target}s' names.
9154 Normally @command{ln} does not remove existing files. Use the
9155 @option{--force} (@option{-f}) option to remove them unconditionally,
9156 the @option{--interactive} (@option{-i}) option to remove them
9157 conditionally, and the @option{--backup} (@option{-b}) option to
9160 @cindex hard link, defined
9161 @cindex inode, and hard links
9162 A @dfn{hard link} is another name for an existing file; the link and the
9163 original are indistinguishable. Technically speaking, they share the
9164 same inode, and the inode contains all the information about a
9165 file---indeed, it is not incorrect to say that the inode @emph{is} the
9166 file. Most systems prohibit making a hard link to
9167 a directory; on those where it is allowed, only the super-user can do
9168 so (and with caution, since creating a cycle will cause problems to many
9169 other utilities). Hard links cannot cross file system boundaries. (These
9170 restrictions are not mandated by @acronym{POSIX}, however.)
9172 @cindex dereferencing symbolic links
9173 @cindex symbolic link, defined
9174 @dfn{Symbolic links} (@dfn{symlinks} for short), on the other hand, are
9175 a special file type (which not all kernels support: System V release 3
9176 (and older) systems lack symlinks) in which the link file actually
9177 refers to a different file, by name. When most operations (opening,
9178 reading, writing, and so on) are passed the symbolic link file, the
9179 kernel automatically @dfn{dereferences} the link and operates on the
9180 target of the link. But some operations (e.g., removing) work on the
9181 link file itself, rather than on its target. The owner and group of a
9182 symlink are not significant to file access performed through
9183 the link, but do have implications on deleting a symbolic link from a
9184 directory with the restricted deletion bit set. On the GNU system,
9185 the mode of a symlink has no significance and cannot be changed, but
9186 on some BSD systems, the mode can be changed and will affect whether
9187 the symlink will be traversed in file name resolution. @xref{Symbolic Links,,,
9188 libc, The GNU C Library Reference Manual}.
9190 Symbolic links can contain arbitrary strings; a @dfn{dangling symlink}
9191 occurs when the string in the symlink does not resolve to a file.
9192 There are no restrictions against creating dangling symbolic links.
9193 There are trade-offs to using absolute or relative symlinks. An
9194 absolute symlink always points to the same file, even if the directory
9195 containing the link is moved. However, if the symlink is visible from
9196 more than one machine (such as on a networked file system), the file
9197 pointed to might not always be the same. A relative symbolic link is
9198 resolved in relation to the directory that contains the link, and is
9199 often useful in referring to files on the same device without regards
9200 to what name that device is mounted on when accessed via networked
9203 When creating a relative symlink in a different location than the
9204 current directory, the resolution of the symlink will be different
9205 than the resolution of the same string from the current directory.
9206 Therefore, many users prefer to first change directories to the
9207 location where the relative symlink will be created, so that
9208 tab-completion or other file resolution will find the same target as
9209 what will be placed in the symlink.
9211 The program accepts the following options. Also see @ref{Common options}.
9222 @opindex --directory
9223 @cindex hard links to directories
9224 Allow users with appropriate privileges to attempt to make hard links
9226 However, note that this will probably fail due to
9227 system restrictions, even for the super-user.
9233 Remove existing destination files.
9236 @itemx --interactive
9238 @opindex --interactive
9239 @cindex prompting, and @command{ln}
9240 Prompt whether to remove existing destination files.
9246 If @option{-s} is not in effect, and the source file is a symbolic
9247 link, create the hard link to the file referred to by the symbolic
9248 link, rather than the symbolic link itself.
9251 @itemx --no-dereference
9253 @opindex --no-dereference
9254 Do not treat the last operand specially when it is a symbolic link to
9255 a directory. Instead, treat it as if it were a normal file.
9257 When the destination is an actual directory (not a symlink to one),
9258 there is no ambiguity. The link is created in that directory.
9259 But when the specified destination is a symlink to a directory,
9260 there are two ways to treat the user's request. @command{ln} can
9261 treat the destination just as it would a normal directory and create
9262 the link in it. On the other hand, the destination can be viewed as a
9263 non-directory---as the symlink itself. In that case, @command{ln}
9264 must delete or backup that symlink before creating the new link.
9265 The default is to treat a destination that is a symlink to a directory
9266 just like a directory.
9268 This option is weaker than the @option{--no-target-directory}
9269 (@option{-T}) option, so it has no effect if both options are given.
9275 If @option{-s} is not in effect, and the source file is a symbolic
9276 link, create the hard link to the symbolic link itself. On platforms
9277 where this is not supported by the kernel, this option creates a
9278 symbolic link with identical contents; since symbolic link contents
9279 cannot be edited, any file name resolution performed through either
9280 link will be the same as if a hard link had been created.
9286 Make symbolic links instead of hard links. This option merely produces
9287 an error message on systems that do not support symbolic links.
9293 @optNoTargetDirectory
9299 Print the name of each file after linking it successfully.
9303 @cindex hard links to symbolic links
9304 @cindex symbolic links and @command{ln}
9305 If @option{-L} and @option{-P} are both given, the last one takes
9306 precedence. If @option{-s} is also given, @option{-L} and @option{-P}
9307 are silently ignored. If neither option is given, then this
9308 implementation defaults to @option{-P} if the system @code{link} supports
9309 hard links to symbolic links (such as the GNU system), and @option{-L}
9310 if @code{link} follows symbolic links (such as on BSD).
9319 # Create link ../a pointing to a in that directory.
9320 # Not really useful because it points to itself.
9325 # Change to the target before creating symlinks to avoid being confused.
9331 # Hard coded file names don't move well.
9332 ln -s $(pwd)/a /some/dir/
9336 # Relative file names survive directory moves and also
9337 # work across networked file systems.
9338 ln -s afile anotherfile
9339 ln -s ../adir/afile yetanotherfile
9343 @node mkdir invocation
9344 @section @command{mkdir}: Make directories
9347 @cindex directories, creating
9348 @cindex creating directories
9350 @command{mkdir} creates directories with the specified names. Synopsis:
9353 mkdir [@var{option}]@dots{} @var{name}@dots{}
9356 @command{mkdir} creates each directory @var{name} in the order given.
9357 It reports an error if @var{name} already exists, unless the
9358 @option{-p} option is given and @var{name} is a directory.
9360 The program accepts the following options. Also see @ref{Common options}.
9365 @itemx --mode=@var{mode}
9368 @cindex modes of created directories, setting
9369 Set the file permission bits of created directories to @var{mode},
9370 which uses the same syntax as
9371 in @command{chmod} and uses @samp{a=rwx} (read, write and execute allowed for
9372 everyone) for the point of the departure. @xref{File permissions}.
9374 Normally the directory has the desired file mode bits at the moment it
9375 is created. As a @acronym{GNU} extension, @var{mode} may also mention
9376 special mode bits, but in this case there may be a temporary window
9377 during which the directory exists but its special mode bits are
9378 incorrect. @xref{Directory Setuid and Setgid}, for how the
9379 set-user-ID and set-group-ID bits of directories are inherited unless
9380 overridden in this way.
9386 @cindex parent directories, creating
9387 Make any missing parent directories for each argument, setting their
9388 file permission bits to the umask modified by @samp{u+wx}. Ignore
9389 existing parent directories, and do not change their file permission
9392 To set the file permission bits of any newly-created parent
9393 directories to a value that includes @samp{u+wx}, you can set the
9394 umask before invoking @command{mkdir}. For example, if the shell
9395 command @samp{(umask u=rwx,go=rx; mkdir -p P/Q)} creates the parent
9396 @file{P} it sets the parent's permission bits to @samp{u=rwx,go=rx}.
9397 To set a parent's special mode bits as well, you can invoke
9398 @command{chmod} after @command{mkdir}. @xref{Directory Setuid and
9399 Setgid}, for how the set-user-ID and set-group-ID bits of
9400 newly-created parent directories are inherited.
9406 Print a message for each created directory. This is most useful with
9409 @item -Z @var{context}
9410 @itemx --context=@var{context}
9414 @cindex security context
9415 Set the default SELinux security context to be used for created directories.
9422 @node mkfifo invocation
9423 @section @command{mkfifo}: Make FIFOs (named pipes)
9426 @cindex FIFOs, creating
9427 @cindex named pipes, creating
9428 @cindex creating FIFOs (named pipes)
9430 @command{mkfifo} creates FIFOs (also called @dfn{named pipes}) with the
9431 specified names. Synopsis:
9434 mkfifo [@var{option}] @var{name}@dots{}
9437 A @dfn{FIFO} is a special file type that permits independent processes
9438 to communicate. One process opens the FIFO file for writing, and
9439 another for reading, after which data can flow as with the usual
9440 anonymous pipe in shells or elsewhere.
9442 The program accepts the following option. Also see @ref{Common options}.
9447 @itemx --mode=@var{mode}
9450 @cindex modes of created FIFOs, setting
9451 Set the mode of created FIFOs to @var{mode}, which is symbolic as in
9452 @command{chmod} and uses @samp{a=rw} (read and write allowed for everyone)
9453 for the point of departure. @var{mode} should specify only file
9454 permission bits. @xref{File permissions}.
9456 @item -Z @var{context}
9457 @itemx --context=@var{context}
9461 @cindex security context
9462 Set the default SELinux security context to be used for created FIFOs.
9469 @node mknod invocation
9470 @section @command{mknod}: Make block or character special files
9473 @cindex block special files, creating
9474 @cindex character special files, creating
9476 @command{mknod} creates a FIFO, character special file, or block special
9477 file with the specified name. Synopsis:
9480 mknod [@var{option}]@dots{} @var{name} @var{type} [@var{major} @var{minor}]
9483 @cindex special files
9484 @cindex block special files
9485 @cindex character special files
9486 Unlike the phrase ``special file type'' above, the term @dfn{special
9487 file} has a technical meaning on Unix: something that can generate or
9488 receive data. Usually this corresponds to a physical piece of hardware,
9489 e.g., a printer or a disk. (These files are typically created at
9490 system-configuration time.) The @command{mknod} command is what creates
9491 files of this type. Such devices can be read either a character at a
9492 time or a ``block'' (many characters) at a time, hence we say there are
9493 @dfn{block special} files and @dfn{character special} files.
9495 @c mknod is a shell built-in at least with OpenBSD's /bin/sh
9496 @mayConflictWithShellBuiltIn{mknod}
9498 The arguments after @var{name} specify the type of file to make:
9503 @opindex p @r{for FIFO file}
9507 @opindex b @r{for block special file}
9508 for a block special file
9511 @c Don't document the `u' option -- it's just a synonym for `c'.
9512 @c Do *any* versions of mknod still use it?
9514 @opindex c @r{for character special file}
9515 @c @opindex u @r{for character special file}
9516 for a character special file
9520 When making a block or character special file, the major and minor
9521 device numbers must be given after the file type.
9522 If a major or minor device number begins with @samp{0x} or @samp{0X},
9523 it is interpreted as hexadecimal; otherwise, if it begins with @samp{0},
9524 as octal; otherwise, as decimal.
9526 The program accepts the following option. Also see @ref{Common options}.
9531 @itemx --mode=@var{mode}
9534 Set the mode of created files to @var{mode}, which is symbolic as in
9535 @command{chmod} and uses @samp{a=rw} as the point of departure.
9536 @var{mode} should specify only file permission bits.
9537 @xref{File permissions}.
9539 @item -Z @var{context}
9540 @itemx --context=@var{context}
9544 @cindex security context
9545 Set the default SELinux security context to be used for created files.
9552 @node readlink invocation
9553 @section @command{readlink}: Print value of a symlink or canonical file name
9556 @cindex displaying value of a symbolic link
9557 @cindex canonical file name
9558 @cindex canonicalize a file name
9562 @command{readlink} may work in one of two supported modes:
9568 @command{readlink} outputs the value of the given symbolic link.
9569 If @command{readlink} is invoked with an argument other than the name
9570 of a symbolic link, it produces no output and exits with a nonzero exit code.
9572 @item Canonicalize mode
9574 @command{readlink} outputs the absolute name of the given file which contains
9575 no @file{.}, @file{..} components nor any repeated separators
9576 (@file{/}) or symbolic links.
9581 readlink [@var{option}] @var{file}
9584 By default, @command{readlink} operates in readlink mode.
9586 The program accepts the following options. Also see @ref{Common options}.
9591 @itemx --canonicalize
9593 @opindex --canonicalize
9594 Activate canonicalize mode.
9595 If any component of the file name except the last one is missing or unavailable,
9596 @command{readlink} produces no output and exits with a nonzero exit
9597 code. A trailing slash is ignored.
9600 @itemx --canonicalize-existing
9602 @opindex --canonicalize-existing
9603 Activate canonicalize mode.
9604 If any component is missing or unavailable, @command{readlink} produces
9605 no output and exits with a nonzero exit code. A trailing slash
9606 requires that the name resolve to a directory.
9609 @itemx --canonicalize-missing
9611 @opindex --canonicalize-missing
9612 Activate canonicalize mode.
9613 If any component is missing or unavailable, @command{readlink} treats it
9619 @opindex --no-newline
9620 Do not output the trailing newline.
9630 Suppress most error messages.
9636 Report error messages.
9640 The @command{readlink} utility first appeared in OpenBSD 2.1.
9642 There is a @command{realpath} command on some systems
9643 which operates like @command{readlink} in canonicalize mode.
9648 @node rmdir invocation
9649 @section @command{rmdir}: Remove empty directories
9652 @cindex removing empty directories
9653 @cindex directories, removing empty
9655 @command{rmdir} removes empty directories. Synopsis:
9658 rmdir [@var{option}]@dots{} @var{directory}@dots{}
9661 If any @var{directory} argument does not refer to an existing empty
9662 directory, it is an error.
9664 The program accepts the following options. Also see @ref{Common options}.
9668 @item --ignore-fail-on-non-empty
9669 @opindex --ignore-fail-on-non-empty
9670 @cindex directory deletion, ignoring failures
9671 Ignore each failure to remove a directory that is solely because
9672 the directory is non-empty.
9678 @cindex parent directories, removing
9679 Remove @var{directory}, then try to remove each component of @var{directory}.
9680 So, for example, @samp{rmdir -p a/b/c} is similar to @samp{rmdir a/b/c a/b a}.
9681 As such, it fails if any of those directories turns out not to be empty.
9682 Use the @option{--ignore-fail-on-non-empty} option to make it so such
9683 a failure does not evoke a diagnostic and does not cause @command{rmdir} to
9684 exit unsuccessfully.
9690 @cindex directory deletion, reporting
9691 Give a diagnostic for each successful removal.
9692 @var{directory} is removed.
9696 @xref{rm invocation}, for how to remove non-empty directories (recursively).
9701 @node unlink invocation
9702 @section @command{unlink}: Remove files via the unlink syscall
9705 @cindex removing files or directories (via the unlink syscall)
9707 @command{unlink} deletes a single specified file name.
9708 It is a minimalist interface to the system-provided
9709 @code{unlink} function. @xref{Deleting Files, , , libc,
9710 The GNU C Library Reference Manual}. Synopsis:
9711 It avoids the bells and whistles of the more commonly-used
9712 @command{rm} command (@pxref{rm invocation}).
9715 unlink @var{filename}
9718 On some systems @code{unlink} can be used to delete the name of a
9719 directory. On others, it can be used that way only by a privileged user.
9720 In the GNU system @code{unlink} can never delete the name of a directory.
9722 The @command{unlink} command honors the @option{--help} and
9723 @option{--version} options. To remove a file whose name begins with
9724 @samp{-}, prefix the name with @samp{./}, e.g., @samp{unlink ./--help}.
9729 @node Changing file attributes
9730 @chapter Changing file attributes
9732 @cindex changing file attributes
9733 @cindex file attributes, changing
9734 @cindex attributes, file
9736 A file is not merely its contents, a name, and a file type
9737 (@pxref{Special file types}). A file also has an owner (a user ID), a
9738 group (a group ID), permissions (what the owner can do with the file,
9739 what people in the group can do, and what everyone else can do), various
9740 timestamps, and other information. Collectively, we call these a file's
9743 These commands change file attributes.
9746 * chgrp invocation:: Change file groups.
9747 * chmod invocation:: Change access permissions.
9748 * chown invocation:: Change file owners and groups.
9749 * touch invocation:: Change file timestamps.
9753 @node chown invocation
9754 @section @command{chown}: Change file owner and group
9757 @cindex file ownership, changing
9758 @cindex group ownership, changing
9759 @cindex changing file ownership
9760 @cindex changing group ownership
9762 @command{chown} changes the user and/or group ownership of each given @var{file}
9763 to @var{new-owner} or to the user and group of an existing reference file.
9767 chown [@var{option}]@dots{} @{@var{new-owner} | --reference=@var{ref_file}@}@c
9771 If used, @var{new-owner} specifies the new owner and/or group as follows
9772 (with no embedded white space):
9775 [@var{owner}] [ : [@var{group}] ]
9782 If only an @var{owner} (a user name or numeric user ID) is given, that
9783 user is made the owner of each given file, and the files' group is not
9786 @item owner@samp{:}group
9787 If the @var{owner} is followed by a colon and a @var{group} (a
9788 group name or numeric group ID), with no spaces between them, the group
9789 ownership of the files is changed as well (to @var{group}).
9792 If a colon but no group name follows @var{owner}, that user is
9793 made the owner of the files and the group of the files is changed to
9794 @var{owner}'s login group.
9797 If the colon and following @var{group} are given, but the owner
9798 is omitted, only the group of the files is changed; in this case,
9799 @command{chown} performs the same function as @command{chgrp}.
9802 If only a colon is given, or if @var{new-owner} is empty, neither the
9803 owner nor the group is changed.
9807 If @var{owner} or @var{group} is intended to represent a numeric user
9808 or group ID, then you may specify it with a leading @samp{+}.
9809 @xref{Disambiguating names and IDs}.
9811 Some older scripts may still use @samp{.} in place of the @samp{:} separator.
9812 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) does not
9813 require support for that, but for backward compatibility @acronym{GNU}
9814 @command{chown} supports @samp{.} so long as no ambiguity results.
9815 New scripts should avoid the use of @samp{.} because it is not
9816 portable, and because it has undesirable results if the entire
9817 @var{owner@samp{.}group} happens to identify a user whose name
9820 The @command{chown} command sometimes clears the set-user-ID or
9821 set-group-ID permission bits. This behavior depends on the policy and
9822 functionality of the underlying @code{chown} system call, which may
9823 make system-dependent file mode modifications outside the control of
9824 the @command{chown} command. For example, the @command{chown} command
9825 might not affect those bits when invoked by a user with appropriate
9826 privileges, or when the
9827 bits signify some function other than executable permission (e.g.,
9829 When in doubt, check the underlying system behavior.
9831 The program accepts the following options. Also see @ref{Common options}.
9839 @cindex changed owners, verbosely describing
9840 Verbosely describe the action for each @var{file} whose ownership
9849 @cindex error messages, omitting
9850 Do not print error messages about files whose ownership cannot be
9853 @itemx @w{@kbd{--from}=@var{old-owner}}
9855 @cindex symbolic links, changing owner
9856 Change a @var{file}'s ownership only if it has current attributes specified
9857 by @var{old-owner}. @var{old-owner} has the same form as @var{new-owner}
9859 This option is useful primarily from a security standpoint in that
9860 it narrows considerably the window of potential abuse.
9861 For example, to reflect a user ID numbering change for one user's files
9862 without an option like this, @code{root} might run
9865 find / -owner OLDUSER -print0 | xargs -0 chown -h NEWUSER
9868 But that is dangerous because the interval between when the @command{find}
9869 tests the existing file's owner and when the @command{chown} is actually run
9871 One way to narrow the gap would be to invoke chown for each file
9875 find / -owner OLDUSER -exec chown -h NEWUSER @{@} \;
9878 But that is very slow if there are many affected files.
9879 With this option, it is safer (the gap is narrower still)
9880 though still not perfect:
9883 chown -h -R --from=OLDUSER NEWUSER /
9887 @opindex --dereference
9888 @cindex symbolic links, changing owner
9890 Do not act on symbolic links themselves but rather on what they point to.
9891 This is the default.
9894 @itemx --no-dereference
9896 @opindex --no-dereference
9897 @cindex symbolic links, changing owner
9899 Act on symbolic links themselves instead of what they point to.
9900 This mode relies on the @code{lchown} system call.
9901 On systems that do not provide the @code{lchown} system call,
9902 @command{chown} fails when a file specified on the command line
9904 By default, no diagnostic is issued for symbolic links encountered
9905 during a recursive traversal, but see @option{--verbose}.
9907 @itemx --preserve-root
9908 @opindex --preserve-root
9909 @cindex root directory, disallow recursive modification
9910 Fail upon any attempt to recursively change the root directory, @file{/}.
9911 Without @option{--recursive}, this option has no effect.
9912 @xref{Treating / specially}.
9914 @itemx --no-preserve-root
9915 @opindex --no-preserve-root
9916 @cindex root directory, allow recursive modification
9917 Cancel the effect of any preceding @option{--preserve-root} option.
9918 @xref{Treating / specially}.
9920 @item --reference=@var{ref_file}
9921 @opindex --reference
9922 Change the user and group of each @var{file} to be the same as those of
9923 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9924 user and group of the symbolic link, but rather those of the file it
9931 Output a diagnostic for every file processed.
9932 If a symbolic link is encountered during a recursive traversal
9933 on a system without the @code{lchown} system call, and @option{--no-dereference}
9934 is in effect, then issue a diagnostic saying neither the symbolic link nor
9935 its referent is being changed.
9940 @opindex --recursive
9941 @cindex recursively changing file ownership
9942 Recursively change ownership of directories and their contents.
9945 @xref{Traversing symlinks}.
9948 @xref{Traversing symlinks}.
9951 @xref{Traversing symlinks}.
9960 # Change the owner of /u to "root".
9963 # Likewise, but also change its group to "staff".
9966 # Change the owner of /u and subfiles to "root".
9971 @node chgrp invocation
9972 @section @command{chgrp}: Change group ownership
9975 @cindex group ownership, changing
9976 @cindex changing group ownership
9978 @command{chgrp} changes the group ownership of each given @var{file}
9979 to @var{group} (which can be either a group name or a numeric group ID)
9980 or to the group of an existing reference file. Synopsis:
9983 chgrp [@var{option}]@dots{} @{@var{group} | --reference=@var{ref_file}@}@c
9987 If @var{group} is intended to represent a
9988 numeric group ID, then you may specify it with a leading @samp{+}.
9989 @xref{Disambiguating names and IDs}.
9991 The program accepts the following options. Also see @ref{Common options}.
9999 @cindex changed files, verbosely describing
10000 Verbosely describe the action for each @var{file} whose group actually
10009 @cindex error messages, omitting
10010 Do not print error messages about files whose group cannot be
10013 @item --dereference
10014 @opindex --dereference
10015 @cindex symbolic links, changing owner
10017 Do not act on symbolic links themselves but rather on what they point to.
10018 This is the default.
10021 @itemx --no-dereference
10023 @opindex --no-dereference
10024 @cindex symbolic links, changing group
10026 Act on symbolic links themselves instead of what they point to.
10027 This mode relies on the @code{lchown} system call.
10028 On systems that do not provide the @code{lchown} system call,
10029 @command{chgrp} fails when a file specified on the command line
10030 is a symbolic link.
10031 By default, no diagnostic is issued for symbolic links encountered
10032 during a recursive traversal, but see @option{--verbose}.
10034 @itemx --preserve-root
10035 @opindex --preserve-root
10036 @cindex root directory, disallow recursive modification
10037 Fail upon any attempt to recursively change the root directory, @file{/}.
10038 Without @option{--recursive}, this option has no effect.
10039 @xref{Treating / specially}.
10041 @itemx --no-preserve-root
10042 @opindex --no-preserve-root
10043 @cindex root directory, allow recursive modification
10044 Cancel the effect of any preceding @option{--preserve-root} option.
10045 @xref{Treating / specially}.
10047 @item --reference=@var{ref_file}
10048 @opindex --reference
10049 Change the group of each @var{file} to be the same as that of
10050 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
10051 group of the symbolic link, but rather that of the file it refers to.
10057 Output a diagnostic for every file processed.
10058 If a symbolic link is encountered during a recursive traversal
10059 on a system without the @code{lchown} system call, and @option{--no-dereference}
10060 is in effect, then issue a diagnostic saying neither the symbolic link nor
10061 its referent is being changed.
10066 @opindex --recursive
10067 @cindex recursively changing group ownership
10068 Recursively change the group ownership of directories and their contents.
10071 @xref{Traversing symlinks}.
10074 @xref{Traversing symlinks}.
10077 @xref{Traversing symlinks}.
10086 # Change the group of /u to "staff".
10089 # Change the group of /u and subfiles to "staff".
10094 @node chmod invocation
10095 @section @command{chmod}: Change access permissions
10098 @cindex changing access permissions
10099 @cindex access permissions, changing
10100 @cindex permissions, changing access
10102 @command{chmod} changes the access permissions of the named files. Synopsis:
10105 chmod [@var{option}]@dots{} @{@var{mode} | --reference=@var{ref_file}@}@c
10109 @cindex symbolic links, permissions of
10110 @command{chmod} never changes the permissions of symbolic links, since
10111 the @command{chmod} system call cannot change their permissions.
10112 This is not a problem since the permissions of symbolic links are
10113 never used. However, for each symbolic link listed on the command
10114 line, @command{chmod} changes the permissions of the pointed-to file.
10115 In contrast, @command{chmod} ignores symbolic links encountered during
10116 recursive directory traversals.
10118 A successful use of @command{chmod} clears the set-group-ID bit of a
10119 regular file if the file's group ID does not match the user's
10120 effective group ID or one of the user's supplementary group IDs,
10121 unless the user has appropriate privileges. Additional restrictions
10122 may cause the set-user-ID and set-group-ID bits of @var{mode} or
10123 @var{ref_file} to be ignored. This behavior depends on the policy and
10124 functionality of the underlying @code{chmod} system call. When in
10125 doubt, check the underlying system behavior.
10127 If used, @var{mode} specifies the new file mode bits.
10128 For details, see the section on @ref{File permissions}.
10129 If you really want @var{mode} to have a leading @samp{-}, you should
10130 use @option{--} first, e.g., @samp{chmod -- -w file}. Typically,
10131 though, @samp{chmod a-w file} is preferable, and @command{chmod -w
10132 file} (without the @option{--}) complains if it behaves differently
10133 from what @samp{chmod a-w file} would do.
10135 The program accepts the following options. Also see @ref{Common options}.
10143 Verbosely describe the action for each @var{file} whose permissions
10152 @cindex error messages, omitting
10153 Do not print error messages about files whose permissions cannot be
10156 @itemx --preserve-root
10157 @opindex --preserve-root
10158 @cindex root directory, disallow recursive modification
10159 Fail upon any attempt to recursively change the root directory, @file{/}.
10160 Without @option{--recursive}, this option has no effect.
10161 @xref{Treating / specially}.
10163 @itemx --no-preserve-root
10164 @opindex --no-preserve-root
10165 @cindex root directory, allow recursive modification
10166 Cancel the effect of any preceding @option{--preserve-root} option.
10167 @xref{Treating / specially}.
10173 Verbosely describe the action or non-action taken for every @var{file}.
10175 @item --reference=@var{ref_file}
10176 @opindex --reference
10177 Change the mode of each @var{file} to be the same as that of @var{ref_file}.
10178 @xref{File permissions}.
10179 If @var{ref_file} is a symbolic link, do not use the mode
10180 of the symbolic link, but rather that of the file it refers to.
10185 @opindex --recursive
10186 @cindex recursively changing access permissions
10187 Recursively change permissions of directories and their contents.
10194 @node touch invocation
10195 @section @command{touch}: Change file timestamps
10198 @cindex changing file timestamps
10199 @cindex file timestamps, changing
10200 @cindex timestamps, changing file
10202 @command{touch} changes the access and/or modification times of the
10203 specified files. Synopsis:
10206 touch [@var{option}]@dots{} @var{file}@dots{}
10209 @cindex empty files, creating
10210 Any @var{file} argument that does not exist is created empty, unless
10211 option @option{--no-create} (@option{-c}) or @option{--no-dereference}
10212 (@option{-h}) was in effect.
10214 A @var{file} argument string of @samp{-} is handled specially and
10215 causes @command{touch} to change the times of the file associated with
10218 @cindex permissions, for changing file timestamps
10219 If changing both the access and modification times to the current
10220 time, @command{touch} can change the timestamps for files that the user
10221 running it does not own but has write permission for. Otherwise, the
10222 user must own the files.
10224 Although @command{touch} provides options for changing two of the times---the
10225 times of last access and modification---of a file, there is actually
10226 a standard third one as well: the inode change time. This is often
10227 referred to as a file's @code{ctime}.
10228 The inode change time represents the time when the file's meta-information
10229 last changed. One common example of this is when the permissions of a
10230 file change. Changing the permissions doesn't access the file, so
10231 the atime doesn't change, nor does it modify the file, so the mtime
10232 doesn't change. Yet, something about the file itself has changed,
10233 and this must be noted somewhere. This is the job of the ctime field.
10234 This is necessary, so that, for example, a backup program can make a
10235 fresh copy of the file, including the new permissions value.
10236 Another operation that modifies a file's ctime without affecting
10237 the others is renaming. In any case, it is not possible, in normal
10238 operations, for a user to change the ctime field to a user-specified value.
10239 Some operating systems and file systems support a fourth time: the
10240 birth time, when the file was first created; by definition, this
10241 timestamp never changes.
10244 Time stamps assume the time zone rules specified by the @env{TZ}
10245 environment variable, or by the system default rules if @env{TZ} is
10246 not set. @xref{TZ Variable,, Specifying the Time Zone with @env{TZ},
10247 libc, The GNU C Library Reference Manual}.
10248 You can avoid ambiguities during
10249 daylight saving transitions by using @sc{utc} time stamps.
10251 The program accepts the following options. Also see @ref{Common options}.
10256 @itemx --time=atime
10257 @itemx --time=access
10261 @opindex atime@r{, changing}
10262 @opindex access @r{time, changing}
10263 @opindex use @r{time, changing}
10264 Change the access time only.
10269 @opindex --no-create
10270 Do not warn about or create files that do not exist.
10273 @itemx --date=@var{time}
10277 Use @var{time} instead of the current time. It can contain month names,
10278 time zones, @samp{am} and @samp{pm}, @samp{yesterday}, etc. For
10279 example, @option{--date="2004-02-27 14:19:13.489392193 +0530"}
10280 specifies the instant of time that is 489,392,193 nanoseconds after
10281 February 27, 2004 at 2:19:13 PM in a time zone that is 5 hours and 30
10282 minutes east of @acronym{UTC}. @xref{Date input formats}.
10283 File systems that do not support high-resolution time stamps
10284 silently ignore any excess precision here.
10288 @cindex BSD @command{touch} compatibility
10289 Ignored; for compatibility with BSD versions of @command{touch}.
10292 @itemx --no-dereference
10294 @opindex --no-dereference
10295 @cindex symbolic links, changing time
10297 Attempt to change the timestamps of a symbolic link, rather than what
10298 the link refers to. When using this option, empty files are not
10299 created, but option @option{-c} must also be used to avoid warning
10300 about files that do not exist. Not all systems support changing the
10301 timestamps of symlinks, since underlying system support for this
10302 action was not required until @acronym{POSIX} 2008. Also, on some
10303 systems, the mere act of examining a symbolic link changes the access
10304 time, such that only changes to the modification time will persist
10305 long enough to be observable. When coupled with option @option{-r}, a
10306 reference timestamp is taken from a symbolic link rather than the file
10310 @itemx --time=mtime
10311 @itemx --time=modify
10314 @opindex mtime@r{, changing}
10315 @opindex modify @r{time, changing}
10316 Change the modification time only.
10318 @item -r @var{file}
10319 @itemx --reference=@var{file}
10321 @opindex --reference
10322 Use the times of the reference @var{file} instead of the current time.
10323 If this option is combined with the @option{--date=@var{time}}
10324 (@option{-d @var{time}}) option, the reference @var{file}'s time is
10325 the origin for any relative @var{time}s given, but is otherwise ignored.
10326 For example, @samp{-r foo -d '-5 seconds'} specifies a time stamp
10327 equal to five seconds before the corresponding time stamp for @file{foo}.
10328 If @var{file} is a symbolic link, the reference timestamp is taken
10329 from the target of the symlink, unless @option{-h} was also in effect.
10331 @item -t [[@var{cc}]@var{yy}]@var{mmddhhmm}[.@var{ss}]
10332 Use the argument (optional four-digit or two-digit years, months,
10333 days, hours, minutes, optional seconds) instead of the current time.
10334 If the year is specified with only two digits, then @var{cc}
10335 is 20 for years in the range 0 @dots{} 68, and 19 for years in
10336 69 @dots{} 99. If no digits of the year are specified,
10337 the argument is interpreted as a date in the current year.
10338 Note that @var{ss} may be @samp{60}, to accommodate leap seconds.
10342 @vindex _POSIX2_VERSION
10343 On older systems, @command{touch} supports an obsolete syntax, as follows.
10344 If no timestamp is given with any of the @option{-d}, @option{-r}, or
10345 @option{-t} options, and if there are two or more @var{file}s and the
10346 first @var{file} is of the form @samp{@var{mmddhhmm}[@var{yy}]} and this
10347 would be a valid argument to the @option{-t} option (if the @var{yy}, if
10348 any, were moved to the front), and if the represented year
10349 is in the range 1969--1999, that argument is interpreted as the time
10350 for the other files instead of as a file name.
10351 This obsolete behavior can be enabled or disabled with the
10352 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
10353 conformance}), but portable scripts should avoid commands whose
10354 behavior depends on this variable.
10355 For example, use @samp{touch ./12312359 main.c} or @samp{touch -t
10356 12312359 main.c} rather than the ambiguous @samp{touch 12312359 main.c}.
10362 @chapter Disk usage
10366 No disk can hold an infinite amount of data. These commands report
10367 how much disk storage is in use or available, report other file and
10368 file status information, and write buffers to disk.
10371 * df invocation:: Report file system disk space usage.
10372 * du invocation:: Estimate file space usage.
10373 * stat invocation:: Report file or file system status.
10374 * sync invocation:: Synchronize memory and disk.
10375 * truncate invocation:: Shrink or extend the size of a file.
10379 @node df invocation
10380 @section @command{df}: Report file system disk space usage
10383 @cindex file system disk usage
10384 @cindex disk usage by file system
10386 @command{df} reports the amount of disk space used and available on
10387 file systems. Synopsis:
10390 df [@var{option}]@dots{} [@var{file}]@dots{}
10393 With no arguments, @command{df} reports the space used and available on all
10394 currently mounted file systems (of all types). Otherwise, @command{df}
10395 reports on the file system containing each argument @var{file}.
10397 Normally the disk space is printed in units of
10398 1024 bytes, but this can be overridden (@pxref{Block size}).
10399 Non-integer quantities are rounded up to the next higher unit.
10401 @cindex disk device file
10402 @cindex device file, disk
10403 If an argument @var{file} is a disk device file containing a mounted
10404 file system, @command{df} shows the space available on that file system
10405 rather than on the file system containing the device node (i.e., the root
10406 file system). @sc{gnu} @command{df} does not attempt to determine the
10408 on unmounted file systems, because on most kinds of systems doing so
10409 requires extremely nonportable intimate knowledge of file system
10412 The program accepts the following options. Also see @ref{Common options}.
10420 @cindex automounter file systems
10421 @cindex ignore file systems
10422 Include in the listing dummy file systems, which
10423 are omitted by default. Such file systems are typically special-purpose
10424 pseudo-file-systems, such as automounter entries.
10426 @item -B @var{size}
10427 @itemx --block-size=@var{size}
10429 @opindex --block-size
10430 @cindex file system sizes
10431 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10432 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10436 @cindex grand total of disk size, usage and available space
10437 Print a grand total of all arguments after all arguments have
10438 been processed. This can be used to find out the total disk size, usage
10439 and available space of all listed devices.
10445 Equivalent to @option{--si}.
10451 @cindex inode usage
10452 List inode usage information instead of block usage. An inode (short
10453 for index node) contains information about a file such as its owner,
10454 permissions, timestamps, and location on the disk.
10458 @cindex kibibytes for file system sizes
10459 Print sizes in 1024-byte blocks, overriding the default block size
10460 (@pxref{Block size}).
10461 This option is equivalent to @option{--block-size=1K}.
10467 @cindex file system types, limiting output to certain
10468 Limit the listing to local file systems. By default, remote file systems
10473 @cindex file system space, retrieving old data more quickly
10474 Do not invoke the @code{sync} system call before getting any usage data.
10475 This may make @command{df} run significantly faster on systems with many
10476 disks, but on some systems (notably SunOS) the results may be slightly
10477 out of date. This is the default.
10480 @itemx --portability
10482 @opindex --portability
10483 @cindex one-line output format
10484 @cindex @acronym{POSIX} output format
10485 @cindex portable output format
10486 @cindex output format, portable
10487 Use the @acronym{POSIX} output format. This is like the default format except
10492 The information about each file system is always printed on exactly
10493 one line; a mount device is never put on a line by itself. This means
10494 that if the mount device name is more than 20 characters long (e.g., for
10495 some network mounts), the columns are misaligned.
10498 The labels in the header output line are changed to conform to @acronym{POSIX}.
10501 The default block size and output format are unaffected by the
10502 @env{DF_BLOCK_SIZE}, @env{BLOCK_SIZE} and @env{BLOCKSIZE} environment
10503 variables. However, the default block size is still affected by
10504 @env{POSIXLY_CORRECT}: it is 512 if @env{POSIXLY_CORRECT} is set, 1024
10505 otherwise. @xref{Block size}.
10512 @cindex file system space, retrieving current data more slowly
10513 Invoke the @code{sync} system call before getting any usage data. On
10514 some systems (notably SunOS), doing this yields more up to date results,
10515 but in general this option makes @command{df} much slower, especially when
10516 there are many or very busy file systems.
10518 @item -t @var{fstype}
10519 @itemx --type=@var{fstype}
10522 @cindex file system types, limiting output to certain
10523 Limit the listing to file systems of type @var{fstype}. Multiple
10524 file system types can be specified by giving multiple @option{-t} options.
10525 By default, nothing is omitted.
10528 @itemx --print-type
10530 @opindex --print-type
10531 @cindex file system types, printing
10532 Print each file system's type. The types printed here are the same ones
10533 you can include or exclude with @option{-t} and @option{-x}. The particular
10534 types printed are whatever is supported by the system. Here are some of
10535 the common names (this list is certainly not exhaustive):
10540 @cindex @acronym{NFS} file system type
10541 An @acronym{NFS} file system, i.e., one mounted over a network from another
10542 machine. This is the one type name which seems to be used uniformly by
10545 @item 4.2@r{, }ufs@r{, }efs@dots{}
10546 @cindex Linux file system types
10547 @cindex local file system types
10548 @opindex 4.2 @r{file system type}
10549 @opindex ufs @r{file system type}
10550 @opindex efs @r{file system type}
10551 A file system on a locally-mounted hard disk. (The system might even
10552 support more than one type here; Linux does.)
10554 @item hsfs@r{, }cdfs
10555 @cindex CD-ROM file system type
10556 @cindex High Sierra file system
10557 @opindex hsfs @r{file system type}
10558 @opindex cdfs @r{file system type}
10559 A file system on a CD-ROM drive. HP-UX uses @samp{cdfs}, most other
10560 systems use @samp{hsfs} (@samp{hs} for ``High Sierra'').
10563 @cindex PC file system
10564 @cindex DOS file system
10565 @cindex MS-DOS file system
10566 @cindex diskette file system
10568 An MS-DOS file system, usually on a diskette.
10572 @item -x @var{fstype}
10573 @itemx --exclude-type=@var{fstype}
10575 @opindex --exclude-type
10576 Limit the listing to file systems not of type @var{fstype}.
10577 Multiple file system types can be eliminated by giving multiple
10578 @option{-x} options. By default, no file system types are omitted.
10581 Ignored; for compatibility with System V versions of @command{df}.
10586 Failure includes the case where no output is generated, so you can
10587 inspect the exit status of a command like @samp{df -t ext3 -t reiserfs
10588 @var{dir}} to test whether @var{dir} is on a file system of type
10589 @samp{ext3} or @samp{reiserfs}.
10592 @node du invocation
10593 @section @command{du}: Estimate file space usage
10596 @cindex file space usage
10597 @cindex disk usage for files
10599 @command{du} reports the amount of disk space used by the specified files
10600 and for each subdirectory (of directory arguments). Synopsis:
10603 du [@var{option}]@dots{} [@var{file}]@dots{}
10606 With no arguments, @command{du} reports the disk space for the current
10607 directory. Normally the disk space is printed in units of
10608 1024 bytes, but this can be overridden (@pxref{Block size}).
10609 Non-integer quantities are rounded up to the next higher unit.
10611 If two or more hard links point to the same file, only one of the hard
10612 links is counted. The @var{file} argument order affects which links
10613 are counted, and changing the argument order may change the numbers
10614 that @command{du} outputs.
10616 The program accepts the following options. Also see @ref{Common options}.
10624 Show counts for all files, not just directories.
10626 @itemx --apparent-size
10627 @opindex --apparent-size
10628 Print apparent sizes, rather than disk usage. The apparent size of a
10629 file is the number of bytes reported by @code{wc -c} on regular files,
10630 or more generally, @code{ls -l --block-size=1} or @code{stat --format=%s}.
10631 For example, a file containing the word @samp{zoo} with no newline would,
10632 of course, have an apparent size of 3. Such a small file may require
10633 anywhere from 0 to 16 KiB or more of disk space, depending on
10634 the type and configuration of the file system on which the file resides.
10635 However, a sparse file created with this command:
10638 dd bs=1 seek=2GiB if=/dev/null of=big
10642 has an apparent size of 2 GiB, yet on most modern
10643 systems, it actually uses almost no disk space.
10649 Equivalent to @code{--apparent-size --block-size=1}.
10651 @item -B @var{size}
10652 @itemx --block-size=@var{size}
10654 @opindex --block-size
10656 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10657 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10663 @cindex grand total of disk space
10664 Print a grand total of all arguments after all arguments have
10665 been processed. This can be used to find out the total disk usage of
10666 a given set of files or directories.
10669 @itemx --dereference-args
10671 @opindex --dereference-args
10672 Dereference symbolic links that are command line arguments.
10673 Does not affect other symbolic links. This is helpful for finding
10674 out the disk usage of directories, such as @file{/usr/tmp}, which
10675 are often symbolic links.
10677 @c --files0-from=FILE
10678 @filesZeroFromOption{du,, with the @option{--total} (@option{-c}) option}
10684 Equivalent to @option{--dereference-args} (@option{-D}).
10688 @cindex kibibytes for file sizes
10689 Print sizes in 1024-byte blocks, overriding the default block size
10690 (@pxref{Block size}).
10691 This option is equivalent to @option{--block-size=1K}.
10694 @itemx --count-links
10696 @opindex --count-links
10697 @cindex hard links, counting in @command{du}
10698 Count the size of all files, even if they have appeared already (as a
10702 @itemx --dereference
10704 @opindex --dereference
10705 @cindex symbolic links, dereferencing in @command{du}
10706 Dereference symbolic links (show the disk space used by the file
10707 or directory that the link points to instead of the space used by
10712 @cindex mebibytes for file sizes
10713 Print sizes in 1,048,576-byte blocks, overriding the default block size
10714 (@pxref{Block size}).
10715 This option is equivalent to @option{--block-size=1M}.
10718 @itemx --no-dereference
10720 @opindex --no-dereference
10721 @cindex symbolic links, dereferencing in @command{du}
10722 For each symbolic links encountered by @command{du},
10723 consider the disk space used by the symbolic link.
10725 @item -d @var{depth}
10726 @item --max-depth=@var{depth}
10727 @opindex -d @var{depth}
10728 @opindex --max-depth=@var{depth}
10729 @cindex limiting output of @command{du}
10730 Show the total for each directory (and file if --all) that is at
10731 most MAX_DEPTH levels down from the root of the hierarchy. The root
10732 is at level 0, so @code{du --max-depth=0} is equivalent to @code{du -s}.
10741 @opindex --summarize
10742 Display only a total for each argument.
10745 @itemx --separate-dirs
10747 @opindex --separate-dirs
10748 Normally, in the output of @command{du} (when not using @option{--summarize}),
10749 the size listed next to a directory name, @var{d}, represents the sum
10750 of sizes of all entries beneath @var{d} as well as the size of @var{d} itself.
10751 With @option{--separate-dirs}, the size reported for a directory name,
10752 @var{d}, is merely the @code{stat.st_size}-derived size of the directory
10757 @cindex last modified dates, displaying in @command{du}
10758 Show time of the most recent modification of any file in the directory,
10759 or any of its subdirectories.
10761 @itemx --time=ctime
10762 @itemx --time=status
10765 @opindex ctime@r{, show the most recent}
10766 @opindex status time@r{, show the most recent}
10767 @opindex use time@r{, show the most recent}
10768 Show the most recent status change time (the @samp{ctime} in the inode) of
10769 any file in the directory, instead of the modification time.
10771 @itemx --time=atime
10772 @itemx --time=access
10774 @opindex atime@r{, show the most recent}
10775 @opindex access time@r{, show the most recent}
10776 Show the most recent access time (the @samp{atime} in the inode) of
10777 any file in the directory, instead of the modification time.
10779 @item --time-style=@var{style}
10780 @opindex --time-style
10782 List timestamps in style @var{style}. This option has an effect only if
10783 the @option{--time} option is also specified. The @var{style} should
10784 be one of the following:
10787 @item +@var{format}
10789 List timestamps using @var{format}, where @var{format} is interpreted
10790 like the format argument of @command{date} (@pxref{date invocation}).
10791 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
10792 @command{du} to list timestamps like @samp{2002-03-30 23:45:56}. As
10793 with @command{date}, @var{format}'s interpretation is affected by the
10794 @env{LC_TIME} locale category.
10797 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
10798 format with nanosecond precision, e.g., @samp{2002-03-30
10799 23:45:56.477817180 -0700}. This style is equivalent to
10800 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
10803 List @acronym{ISO} 8601 date and time in minutes, e.g.,
10804 @samp{2002-03-30 23:45}. These timestamps are shorter than
10805 @samp{full-iso} timestamps, and are usually good enough for everyday
10806 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
10809 List @acronym{ISO} 8601 dates for timestamps, e.g., @samp{2002-03-30}.
10810 This style is equivalent to @samp{+%Y-%m-%d}.
10814 You can specify the default value of the @option{--time-style} option
10815 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
10816 the default style is @samp{long-iso}. For compatibility with @command{ls},
10817 if @env{TIME_STYLE} begins with @samp{+} and contains a newline,
10818 the newline and any later characters are ignored; if @env{TIME_STYLE}
10819 begins with @samp{posix-} the @samp{posix-} is ignored; and if
10820 @env{TIME_STYLE} is @samp{locale} it is ignored.
10823 @itemx --one-file-system
10825 @opindex --one-file-system
10826 @cindex one file system, restricting @command{du} to
10827 Skip directories that are on different file systems from the one that
10828 the argument being processed is on.
10830 @item --exclude=@var{pattern}
10831 @opindex --exclude=@var{pattern}
10832 @cindex excluding files from @command{du}
10833 When recursing, skip subdirectories or files matching @var{pattern}.
10834 For example, @code{du --exclude='*.o'} excludes files whose names
10837 @item -X @var{file}
10838 @itemx --exclude-from=@var{file}
10839 @opindex -X @var{file}
10840 @opindex --exclude-from=@var{file}
10841 @cindex excluding files from @command{du}
10842 Like @option{--exclude}, except take the patterns to exclude from @var{file},
10843 one per line. If @var{file} is @samp{-}, take the patterns from standard
10848 @cindex NFS mounts from BSD to HP-UX
10849 On BSD systems, @command{du} reports sizes that are half the correct
10850 values for files that are NFS-mounted from HP-UX systems. On HP-UX
10851 systems, it reports sizes that are twice the correct values for
10852 files that are NFS-mounted from BSD systems. This is due to a flaw
10853 in HP-UX; it also affects the HP-UX @command{du} program.
10858 @node stat invocation
10859 @section @command{stat}: Report file or file system status
10862 @cindex file status
10863 @cindex file system status
10865 @command{stat} displays information about the specified file(s). Synopsis:
10868 stat [@var{option}]@dots{} [@var{file}]@dots{}
10871 With no option, @command{stat} reports all information about the given files.
10872 But it also can be used to report the information of the file systems the
10873 given files are located on. If the files are links, @command{stat} can
10874 also give information about the files the links point to.
10876 @mayConflictWithShellBuiltIn{stat}
10881 @itemx --dereference
10883 @opindex --dereference
10884 @cindex symbolic links, dereferencing in @command{stat}
10885 Change how @command{stat} treats symbolic links.
10886 With this option, @command{stat} acts on the file referenced
10887 by each symbolic link argument.
10888 Without it, @command{stat} acts on any symbolic link argument directly.
10891 @itemx --file-system
10893 @opindex --file-system
10894 @cindex file systems
10895 Report information about the file systems where the given files are located
10896 instead of information about the files themselves.
10897 This option implies the @option{-L} option.
10900 @itemx --format=@var{format}
10902 @opindex --format=@var{format}
10903 @cindex output format
10904 Use @var{format} rather than the default format.
10905 @var{format} is automatically newline-terminated, so
10906 running a command like the following with two or more @var{file}
10907 operands produces a line of output for each operand:
10909 $ stat --format=%d:%i / /usr
10914 @itemx --printf=@var{format}
10915 @opindex --printf=@var{format}
10916 @cindex output format
10917 Use @var{format} rather than the default format.
10918 Like @option{--format}, but interpret backslash escapes,
10919 and do not output a mandatory trailing newline.
10920 If you want a newline, include @samp{\n} in the @var{format}.
10921 Here's how you would use @option{--printf} to print the device
10922 and inode numbers of @file{/} and @file{/usr}:
10924 $ stat --printf='%d:%i\n' / /usr
10933 @cindex terse output
10934 Print the information in terse form, suitable for parsing by other programs.
10938 The valid @var{format} directives for files with @option{--format} and
10939 @option{--printf} are:
10942 @item %a - Access rights in octal
10943 @item %A - Access rights in human readable form
10944 @item %b - Number of blocks allocated (see @samp{%B})
10945 @item %B - The size in bytes of each block reported by @samp{%b}
10946 @item %C - The SELinux security context of a file, if available
10947 @item %d - Device number in decimal
10948 @item %D - Device number in hex
10949 @item %f - Raw mode in hex
10950 @item %F - File type
10951 @item %g - Group ID of owner
10952 @item %G - Group name of owner
10953 @item %h - Number of hard links
10954 @item %i - Inode number
10955 @item %m - Mount point (See note below)
10956 @item %n - File name
10957 @item %N - Quoted file name with dereference if symbolic link
10958 @item %o - I/O block size
10959 @item %s - Total size, in bytes
10960 @item %t - Major device type in hex
10961 @item %T - Minor device type in hex
10962 @item %u - User ID of owner
10963 @item %U - User name of owner
10964 @item %w - Time of file birth, or @samp{-} if unknown
10965 @item %W - Time of file birth as seconds since Epoch, or @samp{0}
10966 @item %x - Time of last access
10967 @item %X - Time of last access as seconds since Epoch
10968 @item %y - Time of last modification
10969 @item %Y - Time of last modification as seconds since Epoch
10970 @item %z - Time of last change
10971 @item %Z - Time of last change as seconds since Epoch
10974 The @samp{%W}, @samp{%X}, @samp{%Y}, and @samp{%Z} formats accept a
10975 precision preceded by a period to specify the number of digits to
10976 print after the decimal point. For example, @samp{%.3X} outputs the
10977 last access time to millisecond precision. If a period is given but no
10978 precision, @command{stat} uses 9 digits, so @samp{%.X} is equivalent to
10979 @samp{%.9X}. When discarding excess precision, time stamps are truncated
10980 toward minus infinity.
10984 $ stat -c '[%015Y]' /usr
10987 $ stat -c '[%15Y]' /usr
10989 $ stat -c '[%-15Y]' /usr
10992 $ stat -c '[%.3Y]' /usr
10994 $ stat -c '[%.Y]' /usr
10995 [1288929712.114951834]
10998 The mount point printed by @samp{%m} is similar to that output
10999 by @command{df}, except that:
11002 stat does not dereference symlinks by default
11003 (unless @option{-L} is specified)
11005 stat does not search for specified device nodes in the
11006 file system list, instead operating on them directly
11009 stat outputs the alias for a bind mounted file, rather than
11010 the initial mount point of its backing device.
11011 One can recursively call stat until there is no change in output,
11012 to get the current base mount point
11015 When listing file system information (@option{--file-system} (@option{-f})),
11016 you must use a different set of @var{format} directives:
11019 @item %a - Free blocks available to non-super-user
11020 @item %b - Total data blocks in file system
11021 @item %c - Total file nodes in file system
11022 @item %d - Free file nodes in file system
11023 @item %f - Free blocks in file system
11024 @item %i - File System ID in hex
11025 @item %l - Maximum length of file names
11026 @item %n - File name
11027 @item %s - Block size (for faster transfers)
11028 @item %S - Fundamental block size (for block counts)
11029 @item %t - Type in hex
11030 @item %T - Type in human readable form
11034 Time stamps are listed according to the time zone rules specified by
11035 the @env{TZ} environment variable, or by the system default rules if
11036 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
11037 with @env{TZ}, libc, The GNU C Library Reference Manual}.
11042 @node sync invocation
11043 @section @command{sync}: Synchronize data on disk with memory
11046 @cindex synchronize disk and memory
11048 @cindex superblock, writing
11049 @cindex inodes, written buffered
11050 @command{sync} writes any data buffered in memory out to disk. This can
11051 include (but is not limited to) modified superblocks, modified inodes,
11052 and delayed reads and writes. This must be implemented by the kernel;
11053 The @command{sync} program does nothing but exercise the @code{sync} system
11056 @cindex crashes and corruption
11057 The kernel keeps data in memory to avoid doing (relatively slow) disk
11058 reads and writes. This improves performance, but if the computer
11059 crashes, data may be lost or the file system corrupted as a
11060 result. The @command{sync} command ensures everything in memory
11061 is written to disk.
11063 Any arguments are ignored, except for a lone @option{--help} or
11064 @option{--version} (@pxref{Common options}).
11069 @node truncate invocation
11070 @section @command{truncate}: Shrink or extend the size of a file
11073 @cindex truncating, file sizes
11075 @command{truncate} shrinks or extends the size of each @var{file} to the
11076 specified size. Synopsis:
11079 truncate @var{option}@dots{} @var{file}@dots{}
11082 @cindex files, creating
11083 Any @var{file} that does not exist is created.
11085 @cindex sparse files, creating
11086 @cindex holes, creating files with
11087 If a @var{file} is larger than the specified size, the extra data is lost.
11088 If a @var{file} is shorter, it is extended and the extended part (or hole)
11089 reads as zero bytes.
11091 The program accepts the following options. Also see @ref{Common options}.
11098 @opindex --no-create
11099 Do not create files that do not exist.
11104 @opindex --io-blocks
11105 Treat @var{size} as number of I/O blocks of the @var{file} rather than bytes.
11107 @item -r @var{rfile}
11108 @itemx --reference=@var{rfile}
11110 @opindex --reference
11111 Base the size of each @var{file} on the size of @var{rfile}.
11113 @item -s @var{size}
11114 @itemx --size=@var{size}
11117 Set or adjust the size of each @var{file} according to @var{size}.
11118 @multiplierSuffixesNoBlocks{size}
11120 @var{size} may also be prefixed by one of the following to adjust
11121 the size of each @var{file} based on their current size:
11123 @samp{+} => extend by
11124 @samp{-} => reduce by
11125 @samp{<} => at most
11126 @samp{>} => at least
11127 @samp{/} => round down to multiple of
11128 @samp{%} => round up to multiple of
11136 @node Printing text
11137 @chapter Printing text
11139 @cindex printing text, commands for
11140 @cindex commands for printing text
11142 This section describes commands that display text strings.
11145 * echo invocation:: Print a line of text.
11146 * printf invocation:: Format and print data.
11147 * yes invocation:: Print a string until interrupted.
11151 @node echo invocation
11152 @section @command{echo}: Print a line of text
11155 @cindex displaying text
11156 @cindex printing text
11157 @cindex text, displaying
11158 @cindex arbitrary text, displaying
11160 @command{echo} writes each given @var{string} to standard output, with a
11161 space between each and a newline after the last one. Synopsis:
11164 echo [@var{option}]@dots{} [@var{string}]@dots{}
11167 @mayConflictWithShellBuiltIn{echo}
11169 The program accepts the following options. Also see @ref{Common options}.
11170 Options must precede operands, and the normally-special argument
11171 @samp{--} has no special meaning and is treated like any other
11177 Do not output the trailing newline.
11181 @cindex backslash escapes
11182 Enable interpretation of the following backslash-escaped characters in
11191 produce no further output
11207 the eight-bit value that is the octal number @var{nnn}
11208 (zero to three octal digits), if @var{nnn} is
11209 a nine-bit value, the ninth bit is ignored
11211 the eight-bit value that is the octal number @var{nnn}
11212 (one to three octal digits), if @var{nnn} is
11213 a nine-bit value, the ninth bit is ignored
11215 the eight-bit value that is the hexadecimal number @var{hh}
11216 (one or two hexadecimal digits)
11221 @cindex backslash escapes
11222 Disable interpretation of backslash escapes in each @var{string}.
11223 This is the default. If @option{-e} and @option{-E} are both
11224 specified, the last one given takes effect.
11228 @vindex POSIXLY_CORRECT
11229 If the @env{POSIXLY_CORRECT} environment variable is set, then when
11230 @command{echo}'s first argument is not @option{-n} it outputs
11231 option-like arguments instead of treating them as options. For
11232 example, @code{echo -ne hello} outputs @samp{-ne hello} instead of
11233 plain @samp{hello}.
11235 @acronym{POSIX} does not require support for any options, and says
11236 that the behavior of @command{echo} is implementation-defined if any
11237 @var{string} contains a backslash or if the first argument is
11238 @option{-n}. Portable programs can use the @command{printf} command
11239 if they need to omit trailing newlines or output control characters or
11240 backslashes. @xref{printf invocation}.
11245 @node printf invocation
11246 @section @command{printf}: Format and print data
11249 @command{printf} does formatted printing of text. Synopsis:
11252 printf @var{format} [@var{argument}]@dots{}
11255 @command{printf} prints the @var{format} string, interpreting @samp{%}
11256 directives and @samp{\} escapes to format numeric and string arguments
11257 in a way that is mostly similar to the C @samp{printf} function.
11258 @xref{Output Conversion Syntax,, @command{printf} format directives,
11259 libc, The GNU C Library Reference Manual}, for details.
11260 The differences are listed below.
11262 @mayConflictWithShellBuiltIn{printf}
11267 The @var{format} argument is reused as necessary to convert all the
11268 given @var{argument}s. For example, the command @samp{printf %s a b}
11272 Missing @var{argument}s are treated as null strings or as zeros,
11273 depending on whether the context expects a string or a number. For
11274 example, the command @samp{printf %sx%d} prints @samp{x0}.
11278 An additional escape, @samp{\c}, causes @command{printf} to produce no
11279 further output. For example, the command @samp{printf 'A%sC\cD%sF' B
11280 E} prints @samp{ABC}.
11283 The hexadecimal escape sequence @samp{\x@var{hh}} has at most two
11284 digits, as opposed to C where it can have an unlimited number of
11285 digits. For example, the command @samp{printf '\x07e'} prints two
11286 bytes, whereas the C statement @samp{printf ("\x07e")} prints just
11291 @command{printf} has an additional directive, @samp{%b}, which prints its
11292 argument string with @samp{\} escapes interpreted in the same way as in
11293 the @var{format} string, except that octal escapes are of the form
11294 @samp{\0@var{ooo}} where @var{ooo} is 0 to 3 octal digits. If
11295 @samp{\@var{ooo}} is nine-bit value, ignore the ninth bit.
11296 If a precision is also given, it limits the number of bytes printed
11297 from the converted string.
11300 Numeric arguments must be single C constants, possibly with leading
11301 @samp{+} or @samp{-}. For example, @samp{printf %.4d -3} outputs
11305 @vindex POSIXLY_CORRECT
11306 If the leading character of a numeric argument is @samp{"} or @samp{'}
11307 then its value is the numeric value of the immediately following
11308 character. Any remaining characters are silently ignored if the
11309 @env{POSIXLY_CORRECT} environment variable is set; otherwise, a
11310 warning is printed. For example, @samp{printf "%d" "'a"} outputs
11311 @samp{97} on hosts that use the @acronym{ASCII} character set, since
11312 @samp{a} has the numeric value 97 in @acronym{ASCII}.
11317 A floating-point argument must use a period before any fractional
11318 digits, but is printed according to the @env{LC_NUMERIC} category of the
11319 current locale. For example, in a locale whose radix character is a
11320 comma, the command @samp{printf %g 3.14} outputs @samp{3,14} whereas
11321 the command @samp{printf %g 3,14} is an error.
11322 @xref{Floating point}.
11326 @command{printf} interprets @samp{\@var{ooo}} in @var{format} as an octal number
11327 (if @var{ooo} is 1 to 3 octal digits) specifying a byte to print,
11328 and @samp{\x@var{hh}} as a hexadecimal number (if @var{hh} is 1 to 2 hex
11329 digits) specifying a character to print.
11330 Note however that when @samp{\@var{ooo}} specifies a number larger than 255,
11331 @command{printf} ignores the ninth bit.
11332 For example, @samp{printf '\400'} is equivalent to @samp{printf '\0'}.
11337 @cindex ISO/IEC 10646
11339 @command{printf} interprets two character syntaxes introduced in
11340 @acronym{ISO} C 99:
11341 @samp{\u} for 16-bit Unicode (@acronym{ISO}/@acronym{IEC} 10646)
11342 characters, specified as
11343 four hexadecimal digits @var{hhhh}, and @samp{\U} for 32-bit Unicode
11344 characters, specified as eight hexadecimal digits @var{hhhhhhhh}.
11345 @command{printf} outputs the Unicode characters
11346 according to the @env{LC_CTYPE} locale. Unicode characters in the ranges
11347 U+0000...U+009F, U+D800...U+DFFF cannot be specified by this syntax, except
11348 for U+0024 ($), U+0040 (@@), and U+0060 (@`).
11350 The processing of @samp{\u} and @samp{\U} requires a full-featured
11351 @code{iconv} facility. It is activated on systems with glibc 2.2 (or newer),
11352 or when @code{libiconv} is installed prior to this package. Otherwise
11353 @samp{\u} and @samp{\U} will print as-is.
11355 The only options are a lone @option{--help} or
11356 @option{--version}. @xref{Common options}.
11357 Options must precede operands.
11359 The Unicode character syntaxes are useful for writing strings in a locale
11360 independent way. For example, a string containing the Euro currency symbol
11363 $ env printf '\u20AC 14.95'
11367 will be output correctly in all locales supporting the Euro symbol
11368 (@acronym{ISO}-8859-15, UTF-8, and others). Similarly, a Chinese string
11371 $ env printf '\u4e2d\u6587'
11375 will be output correctly in all Chinese locales (GB2312, BIG5, UTF-8, etc).
11377 Note that in these examples, the @command{printf} command has been
11378 invoked via @command{env} to ensure that we run the program found via
11379 your shell's search path, and not a shell alias or a built-in function.
11381 For larger strings, you don't need to look up the hexadecimal code
11382 values of each character one by one. @acronym{ASCII} characters mixed with \u
11383 escape sequences is also known as the JAVA source file encoding. You can
11384 use GNU recode 3.5c (or newer) to convert strings to this encoding. Here
11385 is how to convert a piece of text into a shell script which will output
11386 this text in a locale-independent way:
11389 $ LC_CTYPE=zh_CN.big5 /usr/local/bin/printf \
11390 '\u4e2d\u6587\n' > sample.txt
11391 $ recode BIG5..JAVA < sample.txt \
11392 | sed -e "s|^|/usr/local/bin/printf '|" -e "s|$|\\\\n'|" \
11399 @node yes invocation
11400 @section @command{yes}: Print a string until interrupted
11403 @cindex repeated output of a string
11405 @command{yes} prints the command line arguments, separated by spaces and
11406 followed by a newline, forever until it is killed. If no arguments are
11407 given, it prints @samp{y} followed by a newline forever until killed.
11409 Upon a write error, @command{yes} exits with status @samp{1}.
11411 The only options are a lone @option{--help} or @option{--version}.
11412 To output an argument that begins with
11413 @samp{-}, precede it with @option{--}, e.g., @samp{yes -- --help}.
11414 @xref{Common options}.
11418 @chapter Conditions
11421 @cindex commands for exit status
11422 @cindex exit status commands
11424 This section describes commands that are primarily useful for their exit
11425 status, rather than their output. Thus, they are often used as the
11426 condition of shell @code{if} statements, or as the last command in a
11430 * false invocation:: Do nothing, unsuccessfully.
11431 * true invocation:: Do nothing, successfully.
11432 * test invocation:: Check file types and compare values.
11433 * expr invocation:: Evaluate expressions.
11437 @node false invocation
11438 @section @command{false}: Do nothing, unsuccessfully
11441 @cindex do nothing, unsuccessfully
11442 @cindex failure exit status
11443 @cindex exit status of @command{false}
11445 @command{false} does nothing except return an exit status of 1, meaning
11446 @dfn{failure}. It can be used as a place holder in shell scripts
11447 where an unsuccessful command is needed.
11448 In most modern shells, @command{false} is a built-in command, so when
11449 you use @samp{false} in a script, you're probably using the built-in
11450 command, not the one documented here.
11452 @command{false} honors the @option{--help} and @option{--version} options.
11454 This version of @command{false} is implemented as a C program, and is thus
11455 more secure and faster than a shell script implementation, and may safely
11456 be used as a dummy shell for the purpose of disabling accounts.
11458 Note that @command{false} (unlike all other programs documented herein)
11459 exits unsuccessfully, even when invoked with
11460 @option{--help} or @option{--version}.
11462 Portable programs should not assume that the exit status of
11463 @command{false} is 1, as it is greater than 1 on some
11464 non-@acronym{GNU} hosts.
11467 @node true invocation
11468 @section @command{true}: Do nothing, successfully
11471 @cindex do nothing, successfully
11473 @cindex successful exit
11474 @cindex exit status of @command{true}
11476 @command{true} does nothing except return an exit status of 0, meaning
11477 @dfn{success}. It can be used as a place holder in shell scripts
11478 where a successful command is needed, although the shell built-in
11479 command @code{:} (colon) may do the same thing faster.
11480 In most modern shells, @command{true} is a built-in command, so when
11481 you use @samp{true} in a script, you're probably using the built-in
11482 command, not the one documented here.
11484 @command{true} honors the @option{--help} and @option{--version} options.
11486 Note, however, that it is possible to cause @command{true}
11487 to exit with nonzero status: with the @option{--help} or @option{--version}
11488 option, and with standard
11489 output already closed or redirected to a file that evokes an I/O error.
11490 For example, using a Bourne-compatible shell:
11493 $ ./true --version >&-
11494 ./true: write error: Bad file number
11495 $ ./true --version > /dev/full
11496 ./true: write error: No space left on device
11499 This version of @command{true} is implemented as a C program, and is thus
11500 more secure and faster than a shell script implementation, and may safely
11501 be used as a dummy shell for the purpose of disabling accounts.
11503 @node test invocation
11504 @section @command{test}: Check file types and compare values
11507 @cindex check file types
11508 @cindex compare values
11509 @cindex expression evaluation
11511 @command{test} returns a status of 0 (true) or 1 (false) depending on the
11512 evaluation of the conditional expression @var{expr}. Each part of the
11513 expression must be a separate argument.
11515 @command{test} has file status checks, string operators, and numeric
11516 comparison operators.
11518 @command{test} has an alternate form that uses opening and closing
11519 square brackets instead a leading @samp{test}. For example, instead
11520 of @samp{test -d /}, you can write @samp{[ -d / ]}. The square
11521 brackets must be separate arguments; for example, @samp{[-d /]} does
11522 not have the desired effect. Since @samp{test @var{expr}} and @samp{[
11523 @var{expr} ]} have the same meaning, only the former form is discussed
11529 test @var{expression}
11531 [ @var{expression} ]
11536 @mayConflictWithShellBuiltIn{test}
11538 If @var{expression} is omitted, @command{test} returns false.
11539 If @var{expression} is a single argument,
11540 @command{test} returns false if the argument is null and true
11541 otherwise. The argument
11542 can be any string, including strings like @samp{-d}, @samp{-1},
11543 @samp{--}, @samp{--help}, and @samp{--version} that most other
11544 programs would treat as options. To get help and version information,
11545 invoke the commands @samp{[ --help} and @samp{[ --version}, without
11546 the usual closing brackets. @xref{Common options}.
11548 @cindex exit status of @command{test}
11552 0 if the expression is true,
11553 1 if the expression is false,
11554 2 if an error occurred.
11558 * File type tests:: -[bcdfhLpSt]
11559 * Access permission tests:: -[gkruwxOG]
11560 * File characteristic tests:: -e -s -nt -ot -ef
11561 * String tests:: -z -n = == !=
11562 * Numeric tests:: -eq -ne -lt -le -gt -ge
11563 * Connectives for test:: ! -a -o
11567 @node File type tests
11568 @subsection File type tests
11570 @cindex file type tests
11572 These options test for particular types of files. (Everything's a file,
11573 but not all files are the same!)
11577 @item -b @var{file}
11579 @cindex block special check
11580 True if @var{file} exists and is a block special device.
11582 @item -c @var{file}
11584 @cindex character special check
11585 True if @var{file} exists and is a character special device.
11587 @item -d @var{file}
11589 @cindex directory check
11590 True if @var{file} exists and is a directory.
11592 @item -f @var{file}
11594 @cindex regular file check
11595 True if @var{file} exists and is a regular file.
11597 @item -h @var{file}
11598 @itemx -L @var{file}
11601 @cindex symbolic link check
11602 True if @var{file} exists and is a symbolic link.
11603 Unlike all other file-related tests, this test does not dereference
11604 @var{file} if it is a symbolic link.
11606 @item -p @var{file}
11608 @cindex named pipe check
11609 True if @var{file} exists and is a named pipe.
11611 @item -S @var{file}
11613 @cindex socket check
11614 True if @var{file} exists and is a socket.
11618 @cindex terminal check
11619 True if @var{fd} is a file descriptor that is associated with a
11625 @node Access permission tests
11626 @subsection Access permission tests
11628 @cindex access permission tests
11629 @cindex permission tests
11631 These options test for particular access permissions.
11635 @item -g @var{file}
11637 @cindex set-group-ID check
11638 True if @var{file} exists and has its set-group-ID bit set.
11640 @item -k @var{file}
11642 @cindex sticky bit check
11643 True if @var{file} exists and has its @dfn{sticky} bit set.
11645 @item -r @var{file}
11647 @cindex readable file check
11648 True if @var{file} exists and read permission is granted.
11650 @item -u @var{file}
11652 @cindex set-user-ID check
11653 True if @var{file} exists and has its set-user-ID bit set.
11655 @item -w @var{file}
11657 @cindex writable file check
11658 True if @var{file} exists and write permission is granted.
11660 @item -x @var{file}
11662 @cindex executable file check
11663 True if @var{file} exists and execute permission is granted
11664 (or search permission, if it is a directory).
11666 @item -O @var{file}
11668 @cindex owned by effective user ID check
11669 True if @var{file} exists and is owned by the current effective user ID.
11671 @item -G @var{file}
11673 @cindex owned by effective group ID check
11674 True if @var{file} exists and is owned by the current effective group ID.
11678 @node File characteristic tests
11679 @subsection File characteristic tests
11681 @cindex file characteristic tests
11683 These options test other file characteristics.
11687 @item -e @var{file}
11689 @cindex existence-of-file check
11690 True if @var{file} exists.
11692 @item -s @var{file}
11694 @cindex nonempty file check
11695 True if @var{file} exists and has a size greater than zero.
11697 @item @var{file1} -nt @var{file2}
11699 @cindex newer-than file check
11700 True if @var{file1} is newer (according to modification date) than
11701 @var{file2}, or if @var{file1} exists and @var{file2} does not.
11703 @item @var{file1} -ot @var{file2}
11705 @cindex older-than file check
11706 True if @var{file1} is older (according to modification date) than
11707 @var{file2}, or if @var{file2} exists and @var{file1} does not.
11709 @item @var{file1} -ef @var{file2}
11711 @cindex same file check
11712 @cindex hard link check
11713 True if @var{file1} and @var{file2} have the same device and inode
11714 numbers, i.e., if they are hard links to each other.
11720 @subsection String tests
11722 @cindex string tests
11724 These options test string characteristics. You may need to quote
11725 @var{string} arguments for the shell. For example:
11731 The quotes here prevent the wrong arguments from being passed to
11732 @command{test} if @samp{$V} is empty or contains special characters.
11736 @item -z @var{string}
11738 @cindex zero-length string check
11739 True if the length of @var{string} is zero.
11741 @item -n @var{string}
11742 @itemx @var{string}
11744 @cindex nonzero-length string check
11745 True if the length of @var{string} is nonzero.
11747 @item @var{string1} = @var{string2}
11749 @cindex equal string check
11750 True if the strings are equal.
11752 @item @var{string1} == @var{string2}
11754 @cindex equal string check
11755 True if the strings are equal (synonym for =).
11757 @item @var{string1} != @var{string2}
11759 @cindex not-equal string check
11760 True if the strings are not equal.
11765 @node Numeric tests
11766 @subsection Numeric tests
11768 @cindex numeric tests
11769 @cindex arithmetic tests
11771 Numeric relational operators. The arguments must be entirely numeric
11772 (possibly negative), or the special expression @w{@code{-l @var{string}}},
11773 which evaluates to the length of @var{string}.
11777 @item @var{arg1} -eq @var{arg2}
11778 @itemx @var{arg1} -ne @var{arg2}
11779 @itemx @var{arg1} -lt @var{arg2}
11780 @itemx @var{arg1} -le @var{arg2}
11781 @itemx @var{arg1} -gt @var{arg2}
11782 @itemx @var{arg1} -ge @var{arg2}
11789 These arithmetic binary operators return true if @var{arg1} is equal,
11790 not-equal, less-than, less-than-or-equal, greater-than, or
11791 greater-than-or-equal than @var{arg2}, respectively.
11798 test -1 -gt -2 && echo yes
11800 test -l abc -gt 1 && echo yes
11803 @error{} test: integer expression expected before -eq
11807 @node Connectives for test
11808 @subsection Connectives for @command{test}
11810 @cindex logical connectives
11811 @cindex connectives, logical
11813 The usual logical connectives.
11819 True if @var{expr} is false.
11821 @item @var{expr1} -a @var{expr2}
11823 @cindex logical and operator
11824 @cindex and operator
11825 True if both @var{expr1} and @var{expr2} are true.
11827 @item @var{expr1} -o @var{expr2}
11829 @cindex logical or operator
11830 @cindex or operator
11831 True if either @var{expr1} or @var{expr2} is true.
11836 @node expr invocation
11837 @section @command{expr}: Evaluate expressions
11840 @cindex expression evaluation
11841 @cindex evaluation of expressions
11843 @command{expr} evaluates an expression and writes the result on standard
11844 output. Each token of the expression must be a separate argument.
11846 Operands are either integers or strings. Integers consist of one or
11847 more decimal digits, with an optional leading @samp{-}.
11848 @command{expr} converts
11849 anything appearing in an operand position to an integer or a string
11850 depending on the operation being applied to it.
11852 Strings are not quoted for @command{expr} itself, though you may need to
11853 quote them to protect characters with special meaning to the shell,
11854 e.g., spaces. However, regardless of whether it is quoted, a string
11855 operand should not be a parenthesis or any of @command{expr}'s
11856 operators like @code{+}, so you cannot safely pass an arbitrary string
11857 @code{$str} to expr merely by quoting it to the shell. One way to
11858 work around this is to use the @sc{gnu} extension @code{+},
11859 (e.g., @code{+ "$str" = foo}); a more portable way is to use
11860 @code{@w{" $str"}} and to adjust the rest of the expression to take
11861 the leading space into account (e.g., @code{@w{" $str" = " foo"}}).
11863 You should not pass a negative integer or a string with leading
11864 @samp{-} as @command{expr}'s first argument, as it might be
11865 misinterpreted as an option; this can be avoided by parenthesization.
11866 Also, portable scripts should not use a string operand that happens to
11867 take the form of an integer; this can be worked around by inserting
11868 leading spaces as mentioned above.
11870 @cindex parentheses for grouping
11871 Operators may be given as infix symbols or prefix keywords. Parentheses
11872 may be used for grouping in the usual manner. You must quote
11873 parentheses and many operators to avoid the shell evaluating them,
11876 When built with support for the GNU MP library, @command{expr} uses
11877 arbitrary-precision arithmetic; otherwise, it uses native arithmetic
11878 types and may fail due to arithmetic overflow.
11880 The only options are @option{--help} and @option{--version}. @xref{Common
11881 options}. Options must precede operands.
11883 @cindex exit status of @command{expr}
11887 0 if the expression is neither null nor 0,
11888 1 if the expression is null or 0,
11889 2 if the expression is invalid,
11890 3 if an internal error occurred (e.g., arithmetic overflow).
11894 * String expressions:: + : match substr index length
11895 * Numeric expressions:: + - * / %
11896 * Relations for expr:: | & < <= = == != >= >
11897 * Examples of expr:: Examples.
11901 @node String expressions
11902 @subsection String expressions
11904 @cindex string expressions
11905 @cindex expressions, string
11907 @command{expr} supports pattern matching and other string operators. These
11908 have higher precedence than both the numeric and relational operators (in
11909 the next sections).
11913 @item @var{string} : @var{regex}
11914 @cindex pattern matching
11915 @cindex regular expression matching
11916 @cindex matching patterns
11917 Perform pattern matching. The arguments are converted to strings and the
11918 second is considered to be a (basic, a la GNU @code{grep}) regular
11919 expression, with a @code{^} implicitly prepended. The first argument is
11920 then matched against this regular expression.
11922 If the match succeeds and @var{regex} uses @samp{\(} and @samp{\)}, the
11923 @code{:} expression returns the part of @var{string} that matched the
11924 subexpression; otherwise, it returns the number of characters matched.
11926 If the match fails, the @code{:} operator returns the null string if
11927 @samp{\(} and @samp{\)} are used in @var{regex}, otherwise 0.
11929 @kindex \( @r{regexp operator}
11930 Only the first @samp{\( @dots{} \)} pair is relevant to the return
11931 value; additional pairs are meaningful only for grouping the regular
11932 expression operators.
11934 @kindex \+ @r{regexp operator}
11935 @kindex \? @r{regexp operator}
11936 @kindex \| @r{regexp operator}
11937 In the regular expression, @code{\+}, @code{\?}, and @code{\|} are
11938 operators which respectively match one or more, zero or one, or separate
11939 alternatives. SunOS and other @command{expr}'s treat these as regular
11940 characters. (@acronym{POSIX} allows either behavior.)
11941 @xref{Top, , Regular Expression Library, regex, Regex}, for details of
11942 regular expression syntax. Some examples are in @ref{Examples of expr}.
11944 @item match @var{string} @var{regex}
11946 An alternative way to do pattern matching. This is the same as
11947 @w{@samp{@var{string} : @var{regex}}}.
11949 @item substr @var{string} @var{position} @var{length}
11951 Returns the substring of @var{string} beginning at @var{position}
11952 with length at most @var{length}. If either @var{position} or
11953 @var{length} is negative, zero, or non-numeric, returns the null string.
11955 @item index @var{string} @var{charset}
11957 Returns the first position in @var{string} where the first character in
11958 @var{charset} was found. If no character in @var{charset} is found in
11959 @var{string}, return 0.
11961 @item length @var{string}
11963 Returns the length of @var{string}.
11965 @item + @var{token}
11967 Interpret @var{token} as a string, even if it is a keyword like @var{match}
11968 or an operator like @code{/}.
11969 This makes it possible to test @code{expr length + "$x"} or
11970 @code{expr + "$x" : '.*/\(.\)'} and have it do the right thing even if
11971 the value of @var{$x} happens to be (for example) @code{/} or @code{index}.
11972 This operator is a @acronym{GNU} extension. Portable shell scripts should use
11973 @code{@w{" $token"} : @w{' \(.*\)'}} instead of @code{+ "$token"}.
11977 To make @command{expr} interpret keywords as strings, you must use the
11978 @code{quote} operator.
11981 @node Numeric expressions
11982 @subsection Numeric expressions
11984 @cindex numeric expressions
11985 @cindex expressions, numeric
11987 @command{expr} supports the usual numeric operators, in order of increasing
11988 precedence. These numeric operators have lower precedence than the
11989 string operators described in the previous section, and higher precedence
11990 than the connectives (next section).
11998 @cindex subtraction
11999 Addition and subtraction. Both arguments are converted to integers;
12000 an error occurs if this cannot be done.
12006 @cindex multiplication
12009 Multiplication, division, remainder. Both arguments are converted to
12010 integers; an error occurs if this cannot be done.
12015 @node Relations for expr
12016 @subsection Relations for @command{expr}
12018 @cindex connectives, logical
12019 @cindex logical connectives
12020 @cindex relations, numeric or string
12022 @command{expr} supports the usual logical connectives and relations. These
12023 have lower precedence than the string and numeric operators
12024 (previous sections). Here is the list, lowest-precedence operator first.
12030 @cindex logical or operator
12031 @cindex or operator
12032 Returns its first argument if that is neither null nor zero, otherwise
12033 its second argument if it is neither null nor zero, otherwise 0. It
12034 does not evaluate its second argument if its first argument is neither
12039 @cindex logical and operator
12040 @cindex and operator
12041 Return its first argument if neither argument is null or zero, otherwise
12042 0. It does not evaluate its second argument if its first argument is
12045 @item < <= = == != >= >
12052 @cindex comparison operators
12054 Compare the arguments and return 1 if the relation is true, 0 otherwise.
12055 @code{==} is a synonym for @code{=}. @command{expr} first tries to convert
12056 both arguments to integers and do a numeric comparison; if either
12057 conversion fails, it does a lexicographic comparison using the character
12058 collating sequence specified by the @env{LC_COLLATE} locale.
12063 @node Examples of expr
12064 @subsection Examples of using @command{expr}
12066 @cindex examples of @command{expr}
12067 Here are a few examples, including quoting for shell metacharacters.
12069 To add 1 to the shell variable @code{foo}, in Bourne-compatible shells:
12072 foo=`expr $foo + 1`
12075 To print the non-directory part of the file name stored in
12076 @code{$fname}, which need not contain a @code{/}:
12079 expr $fname : '.*/\(.*\)' '|' $fname
12082 An example showing that @code{\+} is an operator:
12090 expr abc : 'a\(.\)c'
12092 expr index abcdef cz
12095 @error{} expr: syntax error
12096 expr index + index a
12102 @chapter Redirection
12104 @cindex redirection
12105 @cindex commands for redirection
12107 Unix shells commonly provide several forms of @dfn{redirection}---ways
12108 to change the input source or output destination of a command. But one
12109 useful redirection is performed by a separate command, not by the shell;
12110 it's described here.
12113 * tee invocation:: Redirect output to multiple files or processes.
12117 @node tee invocation
12118 @section @command{tee}: Redirect output to multiple files or processes
12121 @cindex pipe fitting
12122 @cindex destinations, multiple output
12123 @cindex read from stdin and write to stdout and files
12125 The @command{tee} command copies standard input to standard output and also
12126 to any files given as arguments. This is useful when you want not only
12127 to send some data down a pipe, but also to save a copy. Synopsis:
12130 tee [@var{option}]@dots{} [@var{file}]@dots{}
12133 If a file being written to does not already exist, it is created. If a
12134 file being written to already exists, the data it previously contained
12135 is overwritten unless the @option{-a} option is used.
12137 A @var{file} of @samp{-} causes @command{tee} to send another copy of
12138 input to standard output, but this is typically not that useful as the
12139 copies are interleaved.
12141 The program accepts the following options. Also see @ref{Common options}.
12148 Append standard input to the given files rather than overwriting
12152 @itemx --ignore-interrupts
12154 @opindex --ignore-interrupts
12155 Ignore interrupt signals.
12159 The @command{tee} command is useful when you happen to be transferring a large
12160 amount of data and also want to summarize that data without reading
12161 it a second time. For example, when you are downloading a DVD image,
12162 you often want to verify its signature or checksum right away.
12163 The inefficient way to do it is simply:
12166 wget http://example.com/some.iso && sha1sum some.iso
12169 One problem with the above is that it makes you wait for the
12170 download to complete before starting the time-consuming SHA1 computation.
12171 Perhaps even more importantly, the above requires reading
12172 the DVD image a second time (the first was from the network).
12174 The efficient way to do it is to interleave the download
12175 and SHA1 computation. Then, you'll get the checksum for
12176 free, because the entire process parallelizes so well:
12179 # slightly contrived, to demonstrate process substitution
12180 wget -O - http://example.com/dvd.iso \
12181 | tee >(sha1sum > dvd.sha1) > dvd.iso
12184 That makes @command{tee} write not just to the expected output file,
12185 but also to a pipe running @command{sha1sum} and saving the final
12186 checksum in a file named @file{dvd.sha1}.
12188 Note, however, that this example relies on a feature of modern shells
12189 called @dfn{process substitution}
12190 (the @samp{>(command)} syntax, above;
12191 @xref{Process Substitution,,Process Substitution, bashref,
12192 The Bash Reference Manual}.),
12193 so it works with @command{zsh}, @command{bash}, and @command{ksh},
12194 but not with @command{/bin/sh}. So if you write code like this
12195 in a shell script, be sure to start the script with @samp{#!/bin/bash}.
12197 Since the above example writes to one file and one process,
12198 a more conventional and portable use of @command{tee} is even better:
12201 wget -O - http://example.com/dvd.iso \
12202 | tee dvd.iso | sha1sum > dvd.sha1
12205 You can extend this example to make @command{tee} write to two processes,
12206 computing MD5 and SHA1 checksums in parallel. In this case,
12207 process substitution is required:
12210 wget -O - http://example.com/dvd.iso \
12211 | tee >(sha1sum > dvd.sha1) \
12212 >(md5sum > dvd.md5) \
12216 This technique is also useful when you want to make a @emph{compressed}
12217 copy of the contents of a pipe.
12218 Consider a tool to graphically summarize disk usage data from @samp{du -ak}.
12219 For a large hierarchy, @samp{du -ak} can run for a long time,
12220 and can easily produce terabytes of data, so you won't want to
12221 rerun the command unnecessarily. Nor will you want to save
12222 the uncompressed output.
12224 Doing it the inefficient way, you can't even start the GUI
12225 until after you've compressed all of the @command{du} output:
12228 du -ak | gzip -9 > /tmp/du.gz
12229 gzip -d /tmp/du.gz | xdiskusage -a
12232 With @command{tee} and process substitution, you start the GUI
12233 right away and eliminate the decompression completely:
12236 du -ak | tee >(gzip -9 > /tmp/du.gz) | xdiskusage -a
12239 Finally, if you regularly create more than one type of
12240 compressed tarball at once, for example when @code{make dist} creates
12241 both @command{gzip}-compressed and @command{bzip2}-compressed tarballs,
12242 there may be a better way.
12243 Typical @command{automake}-generated @file{Makefile} rules create
12244 the two compressed tar archives with commands in sequence, like this
12245 (slightly simplified):
12248 tardir=your-pkg-M.N
12249 tar chof - "$tardir" | gzip -9 -c > your-pkg-M.N.tar.gz
12250 tar chof - "$tardir" | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12253 However, if the hierarchy you are archiving and compressing is larger
12254 than a couple megabytes, and especially if you are using a multi-processor
12255 system with plenty of memory, then you can do much better by reading the
12256 directory contents only once and running the compression programs in parallel:
12259 tardir=your-pkg-M.N
12260 tar chof - "$tardir" \
12261 | tee >(gzip -9 -c > your-pkg-M.N.tar.gz) \
12262 | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12268 @node File name manipulation
12269 @chapter File name manipulation
12271 @cindex file name manipulation
12272 @cindex manipulation of file names
12273 @cindex commands for file name manipulation
12275 This section describes commands that manipulate file names.
12278 * basename invocation:: Strip directory and suffix from a file name.
12279 * dirname invocation:: Strip last file name component.
12280 * pathchk invocation:: Check file name validity and portability.
12281 * mktemp invocation:: Create temporary file or directory.
12285 @node basename invocation
12286 @section @command{basename}: Strip directory and suffix from a file name
12289 @cindex strip directory and suffix from file names
12290 @cindex directory, stripping from file names
12291 @cindex suffix, stripping from file names
12292 @cindex file names, stripping directory and suffix
12293 @cindex leading directory components, stripping
12295 @command{basename} removes any leading directory components from
12296 @var{name}. Synopsis:
12299 basename @var{name} [@var{suffix}]
12302 If @var{suffix} is specified and is identical to the end of @var{name},
12303 it is removed from @var{name} as well. Note that since trailing slashes
12304 are removed prior to suffix matching, @var{suffix} will do nothing if it
12305 contains slashes. @command{basename} prints the result on standard
12308 @c This test is used both here and in the section on dirname.
12309 @macro basenameAndDirname
12310 Together, @command{basename} and @command{dirname} are designed such
12311 that if @samp{ls "$name"} succeeds, then the command sequence @samp{cd
12312 "$(dirname "$name")"; ls "$(basename "$name")"} will, too. This works
12313 for everything except file names containing a trailing newline.
12315 @basenameAndDirname
12317 @acronym{POSIX} allows the implementation to define the results if
12318 @var{name} is empty or @samp{//}. In the former case, @acronym{GNU}
12319 @command{basename} returns the empty string. In the latter case, the
12320 result is @samp{//} on platforms where @var{//} is distinct from
12321 @var{/}, and @samp{/} on platforms where there is no difference.
12323 The only options are @option{--help} and @option{--version}. @xref{Common
12324 options}. Options must precede operands.
12332 basename /usr/bin/sort
12335 basename include/stdio.h .h
12339 @node dirname invocation
12340 @section @command{dirname}: Strip last file name component
12343 @cindex directory components, printing
12344 @cindex stripping non-directory suffix
12345 @cindex non-directory suffix, stripping
12347 @command{dirname} prints all but the final slash-delimited component of
12348 @var{name}. Slashes on either side of the final component are also
12349 removed. If the string contains no slash, @command{dirname} prints
12350 @samp{.} (meaning the current directory). Synopsis:
12356 @var{name} need not be a file name, but if it is, this operation
12357 effectively lists the directory that contains the final component,
12358 including the case when the final component is itself a directory.
12360 @basenameAndDirname
12362 @acronym{POSIX} allows the implementation to define the results if
12363 @var{name} is @samp{//}. With @acronym{GNU} @command{dirname}, the
12364 result is @samp{//} on platforms where @var{//} is distinct from
12365 @var{/}, and @samp{/} on platforms where there is no difference.
12367 The only options are @option{--help} and @option{--version}. @xref{Common
12375 # Output "/usr/bin".
12376 dirname /usr/bin/sort
12377 dirname /usr/bin//.//
12384 @node pathchk invocation
12385 @section @command{pathchk}: Check file name validity and portability
12388 @cindex file names, checking validity and portability
12389 @cindex valid file names, checking for
12390 @cindex portable file names, checking for
12392 @command{pathchk} checks validity and portability of file names. Synopsis:
12395 pathchk [@var{option}]@dots{} @var{name}@dots{}
12398 For each @var{name}, @command{pathchk} prints an error message if any of
12399 these conditions is true:
12403 One of the existing directories in @var{name} does not have search
12404 (execute) permission,
12406 The length of @var{name} is larger than the maximum supported by the
12409 The length of one component of @var{name} is longer than
12410 its file system's maximum.
12413 A nonexistent @var{name} is not an error, so long a file with that
12414 name could be created under the above conditions.
12416 The program accepts the following options. Also see @ref{Common options}.
12417 Options must precede operands.
12423 Instead of performing checks based on the underlying file system,
12424 print an error message if any of these conditions is true:
12428 A file name is empty.
12431 A file name contains a character outside the @acronym{POSIX} portable file
12432 name character set, namely, the ASCII letters and digits, @samp{.},
12433 @samp{_}, @samp{-}, and @samp{/}.
12436 The length of a file name or one of its components exceeds the
12437 @acronym{POSIX} minimum limits for portability.
12442 Print an error message if a file name is empty, or if it contains a component
12443 that begins with @samp{-}.
12445 @item --portability
12446 @opindex --portability
12447 Print an error message if a file name is not portable to all @acronym{POSIX}
12448 hosts. This option is equivalent to @samp{-p -P}.
12452 @cindex exit status of @command{pathchk}
12456 0 if all specified file names passed all checks,
12460 @node mktemp invocation
12461 @section @command{mktemp}: Create temporary file or directory
12464 @cindex file names, creating temporary
12465 @cindex directory, creating temporary
12466 @cindex temporary files and directories
12468 @command{mktemp} manages the creation of temporary files and
12469 directories. Synopsis:
12472 mktemp [@var{option}]@dots{} [@var{template}]
12475 Safely create a temporary file or directory based on @var{template},
12476 and print its name. If given, @var{template} must include at least
12477 three consecutive @samp{X}s in the last component. If omitted, the template
12478 @samp{tmp.XXXXXXXXXX} is used, and option @option{--tmpdir} is
12479 implied. The final run of @samp{X}s in the @var{template} will be replaced
12480 by alpha-numeric characters; thus, on a case-sensitive file system,
12481 and with a @var{template} including a run of @var{n} instances of @samp{X},
12482 there are @samp{62**@var{n}} potential file names.
12484 Older scripts used to create temporary files by simply joining the
12485 name of the program with the process id (@samp{$$}) as a suffix.
12486 However, that naming scheme is easily predictable, and suffers from a
12487 race condition where the attacker can create an appropriately named
12488 symbolic link, such that when the script then opens a handle to what
12489 it thought was an unused file, it is instead modifying an existing
12490 file. Using the same scheme to create a directory is slightly safer,
12491 since the @command{mkdir} will fail if the target already exists, but
12492 it is still inferior because it allows for denial of service attacks.
12493 Therefore, modern scripts should use the @command{mktemp} command to
12494 guarantee that the generated name will be unpredictable, and that
12495 knowledge of the temporary file name implies that the file was created
12496 by the current script and cannot be modified by other users.
12498 When creating a file, the resulting file has read and write
12499 permissions for the current user, but no permissions for the group or
12500 others; these permissions are reduced if the current umask is more
12503 Here are some examples (although note that if you repeat them, you
12504 will most likely get different file names):
12509 Create a temporary file in the current directory.
12516 Create a temporary file with a known suffix.
12518 $ mktemp --suffix=.txt file-XXXX
12520 $ mktemp file-XXXX-XXXX.txt
12525 Create a secure fifo relative to the user's choice of @env{TMPDIR},
12526 but falling back to the current directory rather than @file{/tmp}.
12527 Note that @command{mktemp} does not create fifos, but can create a
12528 secure directory in which the fifo can live. Exit the shell if the
12529 directory or fifo could not be created.
12531 $ dir=$(mktemp -p "$@{TMPDIR:-.@}" -d dir-XXXX) || exit 1
12533 $ mkfifo "$fifo" || @{ rmdir "$dir"; exit 1; @}
12537 Create and use a temporary file if possible, but ignore failure. The
12538 file will reside in the directory named by @env{TMPDIR}, if specified,
12539 or else in @file{/tmp}.
12541 $ file=$(mktemp -q) && @{
12542 > # Safe to use $file only within this block. Use quotes,
12543 > # since $TMPDIR, and thus $file, may contain whitespace.
12544 > echo ... > "$file"
12550 Act as a semi-random character generator (it is not fully random,
12551 since it is impacted by the contents of the current directory). To
12552 avoid security holes, do not use the resulting names to create a file.
12562 The program accepts the following options. Also see @ref{Common options}.
12569 @opindex --directory
12570 Create a directory rather than a file. The directory will have read,
12571 write, and search permissions for the current user, but no permissions
12572 for the group or others; these permissions are reduced if the current
12573 umask is more restrictive.
12579 Suppress diagnostics about failure to create a file or directory. The
12580 exit status will still reflect whether a file was created.
12586 Generate a temporary name that does not name an existing file, without
12587 changing the file system contents. Using the output of this command
12588 to create a new file is inherently unsafe, as there is a window of
12589 time between generating the name and using it where another process
12590 can create an object by the same name.
12593 @itemx --tmpdir[=@var{dir}]
12596 Treat @var{template} relative to the directory @var{dir}. If
12597 @var{dir} is not specified (only possible with the long option
12598 @option{--tmpdir}) or is the empty string, use the value of
12599 @env{TMPDIR} if available, otherwise use @samp{/tmp}. If this is
12600 specified, @var{template} must not be absolute. However,
12601 @var{template} can still contain slashes, although intermediate
12602 directories must already exist.
12604 @item --suffix=@var{suffix}
12606 Append @var{suffix} to the @var{template}. @var{suffix} must not
12607 contain slash. If @option{--suffix} is specified, @var{template} must
12608 end in @samp{X}; if it is not specified, then an appropriate
12609 @option{--suffix} is inferred by finding the last @samp{X} in
12610 @var{template}. This option exists for use with the default
12611 @var{template} and for the creation of a @var{suffix} that starts with
12616 Treat @var{template} as a single file relative to the value of
12617 @env{TMPDIR} if available, or to the directory specified by
12618 @option{-p}, otherwise to @samp{/tmp}. @var{template} must not
12619 contain slashes. This option is deprecated; the use of @option{-p}
12620 without @option{-t} offers better defaults (by favoring the command
12621 line over @env{TMPDIR}) and more flexibility (by allowing intermediate
12626 @cindex exit status of @command{mktemp}
12630 0 if the file was created,
12635 @node Working context
12636 @chapter Working context
12638 @cindex working context
12639 @cindex commands for printing the working context
12641 This section describes commands that display or alter the context in
12642 which you are working: the current directory, the terminal settings, and
12643 so forth. See also the user-related commands in the next section.
12646 * pwd invocation:: Print working directory.
12647 * stty invocation:: Print or change terminal characteristics.
12648 * printenv invocation:: Print environment variables.
12649 * tty invocation:: Print file name of terminal on standard input.
12653 @node pwd invocation
12654 @section @command{pwd}: Print working directory
12657 @cindex print name of current directory
12658 @cindex current working directory, printing
12659 @cindex working directory, printing
12662 @command{pwd} prints the name of the current directory. Synopsis:
12665 pwd [@var{option}]@dots{}
12668 The program accepts the following options. Also see @ref{Common options}.
12675 If the contents of the environment variable @env{PWD} provide an
12676 absolute name of the current directory with no @samp{.} or @samp{..}
12677 components, but possibly with symbolic links, then output those
12678 contents. Otherwise, fall back to default @option{-P} handling.
12683 @opindex --physical
12684 Print a fully resolved name for the current directory. That is, all
12685 components of the printed name will be actual directory names---none
12686 will be symbolic links.
12689 @cindex symbolic links and @command{pwd}
12690 If @option{-L} and @option{-P} are both given, the last one takes
12691 precedence. If neither option is given, then this implementation uses
12692 @option{-P} as the default unless the @env{POSIXLY_CORRECT}
12693 environment variable is set.
12695 @mayConflictWithShellBuiltIn{pwd}
12700 @node stty invocation
12701 @section @command{stty}: Print or change terminal characteristics
12704 @cindex change or print terminal settings
12705 @cindex terminal settings
12706 @cindex line settings of terminal
12708 @command{stty} prints or changes terminal characteristics, such as baud rate.
12712 stty [@var{option}] [@var{setting}]@dots{}
12713 stty [@var{option}]
12716 If given no line settings, @command{stty} prints the baud rate, line
12717 discipline number (on systems that support it), and line settings
12718 that have been changed from the values set by @samp{stty sane}.
12719 By default, mode reading and setting are performed on the tty line
12720 connected to standard input, although this can be modified by the
12721 @option{--file} option.
12723 @command{stty} accepts many non-option arguments that change aspects of
12724 the terminal line operation, as described below.
12726 The program accepts the following options. Also see @ref{Common options}.
12733 Print all current settings in human-readable form. This option may not
12734 be used in combination with any line settings.
12736 @item -F @var{device}
12737 @itemx --file=@var{device}
12740 Set the line opened by the file name specified in @var{device} instead of
12741 the tty line connected to standard input. This option is necessary
12742 because opening a @acronym{POSIX} tty requires use of the
12743 @code{O_NONDELAY} flag to prevent a @acronym{POSIX} tty from blocking
12744 until the carrier detect line is high if
12745 the @code{clocal} flag is not set. Hence, it is not always possible
12746 to allow the shell to open the device in the traditional manner.
12752 @cindex machine-readable @command{stty} output
12753 Print all current settings in a form that can be used as an argument to
12754 another @command{stty} command to restore the current settings. This option
12755 may not be used in combination with any line settings.
12759 Many settings can be turned off by preceding them with a @samp{-}.
12760 Such arguments are marked below with ``May be negated'' in their
12761 description. The descriptions themselves refer to the positive
12762 case, that is, when @emph{not} negated (unless stated otherwise,
12765 Some settings are not available on all @acronym{POSIX} systems, since they use
12766 extensions. Such arguments are marked below with
12767 ``Non-@acronym{POSIX}'' in their description. On non-@acronym{POSIX}
12768 systems, those or other settings also may not
12769 be available, but it's not feasible to document all the variations: just
12775 * Control:: Control settings
12776 * Input:: Input settings
12777 * Output:: Output settings
12778 * Local:: Local settings
12779 * Combination:: Combination settings
12780 * Characters:: Special characters
12781 * Special:: Special settings
12786 @subsection Control settings
12788 @cindex control settings
12794 @cindex two-way parity
12795 Generate parity bit in output and expect parity bit in input.
12801 @cindex even parity
12802 Set odd parity (even if negated). May be negated.
12809 @cindex character size
12810 @cindex eight-bit characters
12811 Set character size to 5, 6, 7, or 8 bits.
12816 Send a hangup signal when the last process closes the tty. May be
12822 Use two stop bits per character (one if negated). May be negated.
12826 Allow input to be received. May be negated.
12830 @cindex modem control
12831 Disable modem control signals. May be negated.
12835 @cindex hardware flow control
12836 @cindex flow control, hardware
12837 @cindex RTS/CTS flow control
12838 Enable RTS/CTS flow control. Non-@acronym{POSIX}. May be negated.
12843 @subsection Input settings
12845 @cindex input settings
12846 These settings control operations on data received from the terminal.
12851 @cindex breaks, ignoring
12852 Ignore break characters. May be negated.
12856 @cindex breaks, cause interrupts
12857 Make breaks cause an interrupt signal. May be negated.
12861 @cindex parity, ignoring
12862 Ignore characters with parity errors. May be negated.
12866 @cindex parity errors, marking
12867 Mark parity errors (with a 255-0-character sequence). May be negated.
12871 Enable input parity checking. May be negated.
12875 @cindex eight-bit input
12876 Clear high (8th) bit of input characters. May be negated.
12880 @cindex newline, translating to return
12881 Translate newline to carriage return. May be negated.
12885 @cindex return, ignoring
12886 Ignore carriage return. May be negated.
12890 @cindex return, translating to newline
12891 Translate carriage return to newline. May be negated.
12895 @cindex input encoding, UTF-8
12896 Assume input characters are UTF-8 encoded. May be negated.
12900 @kindex C-s/C-q flow control
12901 @cindex XON/XOFF flow control
12902 Enable XON/XOFF flow control (that is, @kbd{CTRL-S}/@kbd{CTRL-Q}). May
12909 @cindex software flow control
12910 @cindex flow control, software
12911 Enable sending of @code{stop} character when the system input buffer
12912 is almost full, and @code{start} character when it becomes almost
12913 empty again. May be negated.
12917 @cindex uppercase, translating to lowercase
12918 Translate uppercase characters to lowercase. Non-@acronym{POSIX}. May be
12919 negated. Note ilcuc is not implemented, as one would not be able to issue
12920 almost any (lowercase) Unix command, after invoking it.
12924 Allow any character to restart output (only the start character
12925 if negated). Non-@acronym{POSIX}. May be negated.
12929 @cindex beeping at input buffer full
12930 Enable beeping and not flushing input buffer if a character arrives
12931 when the input buffer is full. Non-@acronym{POSIX}. May be negated.
12936 @subsection Output settings
12938 @cindex output settings
12939 These settings control operations on data sent to the terminal.
12944 Postprocess output. May be negated.
12948 @cindex lowercase, translating to output
12949 Translate lowercase characters to uppercase. Non-@acronym{POSIX}. May be
12950 negated. (Note ouclc is not currently implemented.)
12954 @cindex return, translating to newline
12955 Translate carriage return to newline. Non-@acronym{POSIX}. May be negated.
12959 @cindex newline, translating to crlf
12960 Translate newline to carriage return-newline. Non-@acronym{POSIX}. May be
12965 Do not print carriage returns in the first column. Non-@acronym{POSIX}.
12970 Newline performs a carriage return. Non-@acronym{POSIX}. May be negated.
12974 @cindex pad instead of timing for delaying
12975 Use fill (padding) characters instead of timing for delays.
12976 Non-@acronym{POSIX}.
12981 @cindex pad character
12982 Use @acronym{ASCII} @sc{del} characters for fill instead of
12983 @acronym{ASCII} @sc{nul} characters. Non-@acronym{POSIX}.
12989 Newline delay style. Non-@acronym{POSIX}.
12996 Carriage return delay style. Non-@acronym{POSIX}.
13002 @opindex tab@var{n}
13003 Horizontal tab delay style. Non-@acronym{POSIX}.
13008 Backspace delay style. Non-@acronym{POSIX}.
13013 Vertical tab delay style. Non-@acronym{POSIX}.
13018 Form feed delay style. Non-@acronym{POSIX}.
13023 @subsection Local settings
13025 @cindex local settings
13030 Enable @code{interrupt}, @code{quit}, and @code{suspend} special
13031 characters. May be negated.
13035 Enable @code{erase}, @code{kill}, @code{werase}, and @code{rprnt}
13036 special characters. May be negated.
13040 Enable non-@acronym{POSIX} special characters. May be negated.
13044 Echo input characters. May be negated.
13050 Echo @code{erase} characters as backspace-space-backspace. May be
13055 @cindex newline echoing after @code{kill}
13056 Echo a newline after a @code{kill} character. May be negated.
13060 @cindex newline, echoing
13061 Echo newline even if not echoing other characters. May be negated.
13065 @cindex flushing, disabling
13066 Disable flushing after @code{interrupt} and @code{quit} special
13067 characters. May be negated.
13071 @cindex case translation
13072 Enable input and output of uppercase characters by preceding their
13073 lowercase equivalents with @samp{\}, when @code{icanon} is set.
13074 Non-@acronym{POSIX}. May be negated.
13078 @cindex background jobs, stopping at terminal write
13079 Stop background jobs that try to write to the terminal. Non-@acronym{POSIX}.
13086 Echo erased characters backward, between @samp{\} and @samp{/}.
13087 Non-@acronym{POSIX}. May be negated.
13093 @cindex control characters, using @samp{^@var{c}}
13094 @cindex hat notation for control characters
13095 Echo control characters in hat notation (@samp{^@var{c}}) instead
13096 of literally. Non-@acronym{POSIX}. May be negated.
13102 Echo the @code{kill} special character by erasing each character on
13103 the line as indicated by the @code{echoprt} and @code{echoe} settings,
13104 instead of by the @code{echoctl} and @code{echok} settings.
13105 Non-@acronym{POSIX}.
13111 @subsection Combination settings
13113 @cindex combination settings
13114 Combination settings:
13121 Same as @code{parenb -parodd cs7}. May be negated. If negated, same
13122 as @code{-parenb cs8}.
13126 Same as @code{parenb parodd cs7}. May be negated. If negated, same
13127 as @code{-parenb cs8}.
13131 Same as @code{-icrnl -onlcr}. May be negated. If negated, same as
13132 @code{icrnl -inlcr -igncr onlcr -ocrnl -onlret}.
13136 Reset the @code{erase} and @code{kill} special characters to their default
13143 @c This is too long to write inline.
13145 cread -ignbrk brkint -inlcr -igncr icrnl -ixoff
13146 -iuclc -ixany imaxbel opost -olcuc -ocrnl onlcr
13147 -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0
13148 ff0 isig icanon iexten echo echoe echok -echonl
13149 -noflsh -xcase -tostop -echoprt echoctl echoke
13153 and also sets all special characters to their default values.
13157 Same as @code{brkint ignpar istrip icrnl ixon opost isig icanon}, plus
13158 sets the @code{eof} and @code{eol} characters to their default values
13159 if they are the same as the @code{min} and @code{time} characters.
13160 May be negated. If negated, same as @code{raw}.
13167 -ignbrk -brkint -ignpar -parmrk -inpck -istrip
13168 -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany
13169 -imaxbel -opost -isig -icanon -xcase min 1 time 0
13173 May be negated. If negated, same as @code{cooked}.
13177 Same as @option{-icanon}. May be negated. If negated, same as
13182 @cindex eight-bit characters
13183 Same as @code{-parenb -istrip cs8}. May be negated. If negated,
13184 same as @code{parenb istrip cs7}.
13188 Same as @option{-parenb -istrip -opost cs8}. May be negated.
13189 If negated, same as @code{parenb istrip opost cs7}.
13193 Same as @option{-ixany}. Non-@acronym{POSIX}. May be negated.
13197 Same as @code{tab0}. Non-@acronym{POSIX}. May be negated. If negated, same
13204 Same as @code{xcase iuclc olcuc}. Non-@acronym{POSIX}. May be negated.
13205 (Used for terminals with uppercase characters only.)
13209 Same as @code{echoe echoctl echoke}.
13213 Same as @code{echoe echoctl echoke -ixany intr ^C erase ^? kill C-u}.
13218 @subsection Special characters
13220 @cindex special characters
13221 @cindex characters, special
13223 The special characters' default values vary from system to system.
13224 They are set with the syntax @samp{name value}, where the names are
13225 listed below and the value can be given either literally, in hat
13226 notation (@samp{^@var{c}}), or as an integer which may start with
13227 @samp{0x} to indicate hexadecimal, @samp{0} to indicate octal, or
13228 any other digit to indicate decimal.
13230 @cindex disabling special characters
13231 @kindex u@r{, and disabling special characters}
13232 For GNU stty, giving a value of @code{^-} or @code{undef} disables that
13233 special character. (This is incompatible with Ultrix @command{stty},
13234 which uses a value of @samp{u} to disable a special character. GNU
13235 @command{stty} treats a value @samp{u} like any other, namely to set that
13236 special character to @key{U}.)
13242 Send an interrupt signal.
13246 Send a quit signal.
13250 Erase the last character typed.
13254 Erase the current line.
13258 Send an end of file (terminate the input).
13266 Alternate character to end the line. Non-@acronym{POSIX}.
13270 Switch to a different shell layer. Non-@acronym{POSIX}.
13274 Restart the output after stopping it.
13282 Send a terminal stop signal.
13286 Send a terminal stop signal after flushing the input. Non-@acronym{POSIX}.
13290 Redraw the current line. Non-@acronym{POSIX}.
13294 Erase the last word typed. Non-@acronym{POSIX}.
13298 Enter the next character typed literally, even if it is a special
13299 character. Non-@acronym{POSIX}.
13304 @subsection Special settings
13306 @cindex special settings
13311 Set the minimum number of characters that will satisfy a read until
13312 the time value has expired, when @option{-icanon} is set.
13316 Set the number of tenths of a second before reads time out if the minimum
13317 number of characters have not been read, when @option{-icanon} is set.
13319 @item ispeed @var{n}
13321 Set the input speed to @var{n}.
13323 @item ospeed @var{n}
13325 Set the output speed to @var{n}.
13329 Tell the tty kernel driver that the terminal has @var{n} rows.
13330 Non-@acronym{POSIX}.
13333 @itemx columns @var{n}
13336 Tell the kernel that the terminal has @var{n} columns. Non-@acronym{POSIX}.
13342 Print the number of rows and columns that the kernel thinks the
13343 terminal has. (Systems that don't support rows and columns in the kernel
13344 typically use the environment variables @env{LINES} and @env{COLUMNS}
13345 instead; however, GNU @command{stty} does not know anything about them.)
13346 Non-@acronym{POSIX}.
13350 Use line discipline @var{n}. Non-@acronym{POSIX}.
13354 Print the terminal speed.
13357 @cindex baud rate, setting
13358 Set the input and output speeds to @var{n}. @var{n} can be one of: 0
13359 50 75 110 134 134.5 150 200 300 600 1200 1800 2400 4800 9600 19200
13360 38400 @code{exta} @code{extb}. @code{exta} is the same as 19200;
13361 @code{extb} is the same as 38400. Many systems, including GNU/Linux,
13362 support higher speeds. The @command{stty} command includes support
13379 4000000 where the system supports these.
13380 0 hangs up the line if @option{-clocal} is set.
13384 @node printenv invocation
13385 @section @command{printenv}: Print all or some environment variables
13388 @cindex printing all or some environment variables
13389 @cindex environment variables, printing
13391 @command{printenv} prints environment variable values. Synopsis:
13394 printenv [@var{option}] [@var{variable}]@dots{}
13397 If no @var{variable}s are specified, @command{printenv} prints the value of
13398 every environment variable. Otherwise, it prints the value of each
13399 @var{variable} that is set, and nothing for those that are not set.
13401 The program accepts the following option. Also see @ref{Common options}.
13409 @cindex exit status of @command{printenv}
13413 0 if all variables specified were found
13414 1 if at least one specified variable was not found
13415 2 if a write error occurred
13419 @node tty invocation
13420 @section @command{tty}: Print file name of terminal on standard input
13423 @cindex print terminal file name
13424 @cindex terminal file name, printing
13426 @command{tty} prints the file name of the terminal connected to its standard
13427 input. It prints @samp{not a tty} if standard input is not a terminal.
13431 tty [@var{option}]@dots{}
13434 The program accepts the following option. Also see @ref{Common options}.
13444 Print nothing; only return an exit status.
13448 @cindex exit status of @command{tty}
13452 0 if standard input is a terminal
13453 1 if standard input is not a terminal
13454 2 if given incorrect arguments
13455 3 if a write error occurs
13459 @node User information
13460 @chapter User information
13462 @cindex user information, commands for
13463 @cindex commands for printing user information
13465 This section describes commands that print user-related information:
13466 logins, groups, and so forth.
13469 * id invocation:: Print user identity.
13470 * logname invocation:: Print current login name.
13471 * whoami invocation:: Print effective user ID.
13472 * groups invocation:: Print group names a user is in.
13473 * users invocation:: Print login names of users currently logged in.
13474 * who invocation:: Print who is currently logged in.
13478 @node id invocation
13479 @section @command{id}: Print user identity
13482 @cindex real user and group IDs, printing
13483 @cindex effective user and group IDs, printing
13484 @cindex printing real and effective user and group IDs
13486 @command{id} prints information about the given user, or the process
13487 running it if no user is specified. Synopsis:
13490 id [@var{option}]@dots{} [@var{username}]
13493 @vindex POSIXLY_CORRECT
13494 By default, it prints the real user ID, real group ID, effective user ID
13495 if different from the real user ID, effective group ID if different from
13496 the real group ID, and supplemental group IDs.
13497 In addition, if SELinux
13498 is enabled and the @env{POSIXLY_CORRECT} environment variable is not set,
13499 then print @samp{context=@var{c}}, where @var{c} is the security context.
13501 Each of these numeric values is preceded by an identifying string and
13502 followed by the corresponding user or group name in parentheses.
13504 The options cause @command{id} to print only part of the above information.
13505 Also see @ref{Common options}.
13512 Print only the group ID.
13518 Print only the group ID and the supplementary groups.
13524 Print the user or group name instead of the ID number. Requires
13525 @option{-u}, @option{-g}, or @option{-G}.
13531 Print the real, instead of effective, user or group ID. Requires
13532 @option{-u}, @option{-g}, or @option{-G}.
13538 Print only the user ID.
13545 @cindex security context
13546 Print only the security context of the current user.
13547 If SELinux is disabled then print a warning and
13548 set the exit status to 1.
13554 @macro primaryAndSupplementaryGroups{cmd,arg}
13555 Primary and supplementary groups for a process are normally inherited
13556 from its parent and are usually unchanged since login. This means
13557 that if you change the group database after logging in, @command{\cmd\}
13558 will not reflect your changes within your existing login session.
13559 Running @command{\cmd\} with a \arg\ causes the user and group
13560 database to be consulted afresh, and so will give a different result.
13562 @primaryAndSupplementaryGroups{id,user argument}
13564 @node logname invocation
13565 @section @command{logname}: Print current login name
13568 @cindex printing user's login name
13569 @cindex login name, printing
13570 @cindex user name, printing
13573 @command{logname} prints the calling user's name, as found in a
13574 system-maintained file (often @file{/var/run/utmp} or
13575 @file{/etc/utmp}), and exits with a status of 0. If there is no entry
13576 for the calling process, @command{logname} prints
13577 an error message and exits with a status of 1.
13579 The only options are @option{--help} and @option{--version}. @xref{Common
13585 @node whoami invocation
13586 @section @command{whoami}: Print effective user ID
13589 @cindex effective user ID, printing
13590 @cindex printing the effective user ID
13592 @command{whoami} prints the user name associated with the current
13593 effective user ID. It is equivalent to the command @samp{id -un}.
13595 The only options are @option{--help} and @option{--version}. @xref{Common
13601 @node groups invocation
13602 @section @command{groups}: Print group names a user is in
13605 @cindex printing groups a user is in
13606 @cindex supplementary groups, printing
13608 @command{groups} prints the names of the primary and any supplementary
13609 groups for each given @var{username}, or the current process if no names
13610 are given. If more than one name is given, the name of each user is
13612 the list of that user's groups and the user name is separated from the
13613 group list by a colon. Synopsis:
13616 groups [@var{username}]@dots{}
13619 The group lists are equivalent to the output of the command @samp{id -Gn}.
13621 @primaryAndSupplementaryGroups{groups,list of users}
13623 The only options are @option{--help} and @option{--version}. @xref{Common
13629 @node users invocation
13630 @section @command{users}: Print login names of users currently logged in
13633 @cindex printing current usernames
13634 @cindex usernames, printing current
13636 @cindex login sessions, printing users with
13637 @command{users} prints on a single line a blank-separated list of user
13638 names of users currently logged in to the current host. Each user name
13639 corresponds to a login session, so if a user has more than one login
13640 session, that user's name will appear the same number of times in the
13649 With no @var{file} argument, @command{users} extracts its information from
13650 a system-maintained file (often @file{/var/run/utmp} or
13651 @file{/etc/utmp}). If a file argument is given, @command{users} uses
13652 that file instead. A common choice is @file{/var/log/wtmp}.
13654 The only options are @option{--help} and @option{--version}. @xref{Common
13660 @node who invocation
13661 @section @command{who}: Print who is currently logged in
13664 @cindex printing current user information
13665 @cindex information, about current users
13667 @command{who} prints information about users who are currently logged on.
13671 @command{who} [@var{option}] [@var{file}] [am i]
13674 @cindex terminal lines, currently used
13676 @cindex remote hostname
13677 If given no non-option arguments, @command{who} prints the following
13678 information for each user currently logged on: login name, terminal
13679 line, login time, and remote hostname or X display.
13683 If given one non-option argument, @command{who} uses that instead of
13684 a default system-maintained file (often @file{/var/run/utmp} or
13685 @file{/etc/utmp}) as the name of the file containing the record of
13686 users logged on. @file{/var/log/wtmp} is commonly given as an argument
13687 to @command{who} to look at who has previously logged on.
13691 If given two non-option arguments, @command{who} prints only the entry
13692 for the user running it (determined from its standard input), preceded
13693 by the hostname. Traditionally, the two arguments given are @samp{am
13694 i}, as in @samp{who am i}.
13697 Time stamps are listed according to the time zone rules specified by
13698 the @env{TZ} environment variable, or by the system default rules if
13699 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
13700 with @env{TZ}, libc, The GNU C Library Reference Manual}.
13702 The program accepts the following options. Also see @ref{Common options}.
13710 Same as @samp{-b -d --login -p -r -t -T -u}.
13716 Print the date and time of last system boot.
13722 Print information corresponding to dead processes.
13728 Print a line of column headings.
13734 List only the entries that correspond to processes via which the
13735 system is waiting for a user to login. The user name is always @samp{LOGIN}.
13739 Attempt to canonicalize hostnames found in utmp through a DNS lookup. This
13740 is not the default because it can cause significant delays on systems with
13741 automatic dial-up internet access.
13745 Same as @samp{who am i}.
13751 List active processes spawned by init.
13757 Print only the login names and the number of users logged on.
13758 Overrides all other options.
13763 @opindex --runlevel
13764 Print the current (and maybe previous) run-level of the init process.
13768 Ignored; for compatibility with other versions of @command{who}.
13774 Print last system clock change.
13779 After the login time, print the number of hours and minutes that the
13780 user has been idle. @samp{.} means the user was active in the last minute.
13781 @samp{old} means the user has been idle for more than 24 hours.
13792 @opindex --writable
13793 @cindex message status
13794 @pindex write@r{, allowed}
13795 After each login name print a character indicating the user's message status:
13798 @samp{+} allowing @code{write} messages
13799 @samp{-} disallowing @code{write} messages
13800 @samp{?} cannot find terminal device
13808 @node System context
13809 @chapter System context
13811 @cindex system context
13812 @cindex context, system
13813 @cindex commands for system context
13815 This section describes commands that print or change system-wide
13819 * date invocation:: Print or set system date and time.
13820 * arch invocation:: Print machine hardware name.
13821 * nproc invocation:: Print the number of processors.
13822 * uname invocation:: Print system information.
13823 * hostname invocation:: Print or set system name.
13824 * hostid invocation:: Print numeric host identifier.
13825 * uptime invocation:: Print system uptime and load.
13828 @node date invocation
13829 @section @command{date}: Print or set system date and time
13832 @cindex time, printing or setting
13833 @cindex printing the current time
13838 date [@var{option}]@dots{} [+@var{format}]
13839 date [-u|--utc|--universal] @c this avoids a newline in the output
13840 [ MMDDhhmm[[CC]YY][.ss] ]
13844 Invoking @command{date} with no @var{format} argument is equivalent to invoking
13845 it with a default format that depends on the @env{LC_TIME} locale category.
13846 In the default C locale, this format is @samp{'+%a %b %e %H:%M:%S %Z %Y'},
13847 so the output looks like @samp{Thu Mar @ 3 13:47:51 PST 2005}.
13850 Normally, @command{date} uses the time zone rules indicated by the
13851 @env{TZ} environment variable, or the system default rules if @env{TZ}
13852 is not set. @xref{TZ Variable,, Specifying the Time Zone with
13853 @env{TZ}, libc, The GNU C Library Reference Manual}.
13855 @findex strftime @r{and @command{date}}
13856 @cindex time formats
13857 @cindex formatting times
13858 If given an argument that starts with a @samp{+}, @command{date} prints the
13859 current date and time (or the date and time specified by the
13860 @option{--date} option, see below) in the format defined by that argument,
13861 which is similar to that of the @code{strftime} function. Except for
13862 conversion specifiers, which start with @samp{%}, characters in the
13863 format string are printed unchanged. The conversion specifiers are
13869 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
13870 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
13871 * Literal conversion specifiers:: %[%nt]
13872 * Padding and other flags:: Pad with zeros, spaces, etc.
13873 * Setting the time:: Changing the system clock.
13874 * Options for date:: Instead of the current time.
13876 * Date input formats:: Specifying date strings.
13878 * Examples of date:: Examples.
13881 @node Time conversion specifiers
13882 @subsection Time conversion specifiers
13884 @cindex time conversion specifiers
13885 @cindex conversion specifiers, time
13887 @command{date} conversion specifiers related to times.
13891 hour (@samp{00}@dots{}@samp{23})
13893 hour (@samp{01}@dots{}@samp{12})
13895 hour (@samp{ 0}@dots{}@samp{23}).
13896 This is a @acronym{GNU} extension.
13898 hour (@samp{ 1}@dots{}@samp{12}).
13899 This is a @acronym{GNU} extension.
13901 minute (@samp{00}@dots{}@samp{59})
13903 nanoseconds (@samp{000000000}@dots{}@samp{999999999}).
13904 This is a @acronym{GNU} extension.
13906 locale's equivalent of either @samp{AM} or @samp{PM};
13907 blank in many locales.
13908 Noon is treated as @samp{PM} and midnight as @samp{AM}.
13910 like @samp{%p}, except lower case.
13911 This is a @acronym{GNU} extension.
13913 locale's 12-hour clock time (e.g., @samp{11:11:04 PM})
13915 24-hour hour and minute. Same as @samp{%H:%M}.
13916 This is a @acronym{GNU} extension.
13918 @cindex epoch, seconds since
13919 @cindex seconds since the epoch
13920 @cindex beginning of time
13921 seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC.
13922 Leap seconds are not counted unless leap second support is available.
13923 @xref{%s-examples}, for examples.
13924 This is a @acronym{GNU} extension.
13926 second (@samp{00}@dots{}@samp{60}).
13927 This may be @samp{60} if leap seconds are supported.
13929 24-hour hour, minute, and second. Same as @samp{%H:%M:%S}.
13931 locale's time representation (e.g., @samp{23:13:48})
13933 @w{@acronym{RFC} 2822/@acronym{ISO} 8601} style numeric time zone
13934 (e.g., @samp{-0600} or @samp{+0530}), or nothing if no
13935 time zone is determinable. This value reflects the numeric time zone
13936 appropriate for the current time, using the time zone rules specified
13937 by the @env{TZ} environment variable.
13938 The time (and optionally, the time zone rules) can be overridden
13939 by the @option{--date} option.
13940 This is a @acronym{GNU} extension.
13942 @w{@acronym{RFC} 3339/@acronym{ISO} 8601} style numeric time zone with
13943 @samp{:} (e.g., @samp{-06:00} or @samp{+05:30}), or nothing if no time
13944 zone is determinable.
13945 This is a @acronym{GNU} extension.
13947 Numeric time zone to the nearest second with @samp{:} (e.g.,
13948 @samp{-06:00:00} or @samp{+05:30:00}), or nothing if no time zone is
13950 This is a @acronym{GNU} extension.
13952 Numeric time zone with @samp{:} using the minimum necessary precision
13953 (e.g., @samp{-06}, @samp{+05:30}, or @samp{-04:56:02}), or nothing if
13954 no time zone is determinable.
13955 This is a @acronym{GNU} extension.
13957 alphabetic time zone abbreviation (e.g., @samp{EDT}), or nothing if no
13958 time zone is determinable. See @samp{%z} for how it is determined.
13962 @node Date conversion specifiers
13963 @subsection Date conversion specifiers
13965 @cindex date conversion specifiers
13966 @cindex conversion specifiers, date
13968 @command{date} conversion specifiers related to dates.
13972 locale's abbreviated weekday name (e.g., @samp{Sun})
13974 locale's full weekday name, variable length (e.g., @samp{Sunday})
13976 locale's abbreviated month name (e.g., @samp{Jan})
13978 locale's full month name, variable length (e.g., @samp{January})
13980 locale's date and time (e.g., @samp{Thu Mar @ 3 23:05:25 2005})
13982 century. This is like @samp{%Y}, except the last two digits are omitted.
13983 For example, it is @samp{20} if @samp{%Y} is @samp{2000},
13984 and is @samp{-0} if @samp{%Y} is @samp{-001}.
13985 It is normally at least two characters, but it may be more.
13987 day of month (e.g., @samp{01})
13989 date; same as @samp{%m/%d/%y}
13991 day of month, space padded; same as @samp{%_d}
13993 full date in @acronym{ISO} 8601 format; same as @samp{%Y-%m-%d}.
13994 This is a good choice for a date format, as it is standard and
13995 is easy to sort in the usual case where years are in the range
13997 This is a @acronym{GNU} extension.
13999 year corresponding to the @acronym{ISO} week number, but without the century
14000 (range @samp{00} through @samp{99}). This has the same format and value
14001 as @samp{%y}, except that if the @acronym{ISO} week number (see
14003 to the previous or next year, that year is used instead.
14004 This is a @acronym{GNU} extension.
14006 year corresponding to the @acronym{ISO} week number. This has the
14007 same format and value as @samp{%Y}, except that if the @acronym{ISO}
14009 @samp{%V}) belongs to the previous or next year, that year is used
14011 It is normally useful only if @samp{%V} is also used;
14012 for example, the format @samp{%G-%m-%d} is probably a mistake,
14013 since it combines the ISO week number year with the conventional month and day.
14014 This is a @acronym{GNU} extension.
14018 day of year (@samp{001}@dots{}@samp{366})
14020 month (@samp{01}@dots{}@samp{12})
14022 day of week (@samp{1}@dots{}@samp{7}) with @samp{1} corresponding to Monday
14024 week number of year, with Sunday as the first day of the week
14025 (@samp{00}@dots{}@samp{53}).
14026 Days in a new year preceding the first Sunday are in week zero.
14028 @acronym{ISO} week number, that is, the
14029 week number of year, with Monday as the first day of the week
14030 (@samp{01}@dots{}@samp{53}).
14031 If the week containing January 1 has four or more days in
14032 the new year, then it is considered week 1; otherwise, it is week 53 of
14033 the previous year, and the next week is week 1. (See the @acronym{ISO} 8601
14036 day of week (@samp{0}@dots{}@samp{6}) with 0 corresponding to Sunday
14038 week number of year, with Monday as first day of week
14039 (@samp{00}@dots{}@samp{53}).
14040 Days in a new year preceding the first Monday are in week zero.
14042 locale's date representation (e.g., @samp{12/31/99})
14044 last two digits of year (@samp{00}@dots{}@samp{99})
14046 year. This is normally at least four characters, but it may be more.
14047 Year @samp{0000} precedes year @samp{0001}, and year @samp{-001}
14048 precedes year @samp{0000}.
14052 @node Literal conversion specifiers
14053 @subsection Literal conversion specifiers
14055 @cindex literal conversion specifiers
14056 @cindex conversion specifiers, literal
14058 @command{date} conversion specifiers that produce literal strings.
14070 @node Padding and other flags
14071 @subsection Padding and other flags
14073 @cindex numeric field padding
14074 @cindex padding of numeric fields
14075 @cindex fields, padding numeric
14077 Unless otherwise specified, @command{date} normally pads numeric fields
14078 with zeros, so that, for
14079 example, numeric months are always output as two digits.
14080 Seconds since the epoch are not padded, though,
14081 since there is no natural width for them.
14083 As a @acronym{GNU} extension, @command{date} recognizes any of the
14084 following optional flags after the @samp{%}:
14088 (hyphen) Do not pad the field; useful if the output is intended for
14091 (underscore) Pad with spaces; useful if you need a fixed
14092 number of characters in the output, but zeros are too distracting.
14094 (zero) Pad with zeros even if the conversion specifier
14095 would normally pad with spaces.
14097 Use upper case characters if possible.
14099 Use opposite case characters if possible.
14100 A field that is normally upper case becomes lower case, and vice versa.
14104 Here are some examples of padding:
14107 date +%d/%m -d "Feb 1"
14109 date +%-d/%-m -d "Feb 1"
14111 date +%_d/%_m -d "Feb 1"
14115 As a @acronym{GNU} extension, you can specify the field width
14116 (after any flag, if present) as a decimal number. If the natural size of the
14117 output of the field has less than the specified number of characters,
14118 the result is written right adjusted and padded to the given
14119 size. For example, @samp{%9B} prints the right adjusted month name in
14120 a field of width 9.
14122 An optional modifier can follow the optional flag and width
14123 specification. The modifiers are:
14127 Use the locale's alternate representation for date and time. This
14128 modifier applies to the @samp{%c}, @samp{%C}, @samp{%x}, @samp{%X},
14129 @samp{%y} and @samp{%Y} conversion specifiers. In a Japanese locale, for
14130 example, @samp{%Ex} might yield a date format based on the Japanese
14134 Use the locale's alternate numeric symbols for numbers. This modifier
14135 applies only to numeric conversion specifiers.
14138 If the format supports the modifier but no alternate representation
14139 is available, it is ignored.
14142 @node Setting the time
14143 @subsection Setting the time
14145 @cindex setting the time
14146 @cindex time setting
14147 @cindex appropriate privileges
14149 If given an argument that does not start with @samp{+}, @command{date} sets
14150 the system clock to the date and time specified by that argument (as
14151 described below). You must have appropriate privileges to set the
14152 system clock. The @option{--date} and @option{--set} options may not be
14153 used with such an argument. The @option{--universal} option may be used
14154 with such an argument to indicate that the specified date and time are
14155 relative to Coordinated Universal Time rather than to the local time
14158 The argument must consist entirely of digits, which have the following
14171 first two digits of year (optional)
14173 last two digits of year (optional)
14178 The @option{--set} option also sets the system clock; see the next section.
14181 @node Options for date
14182 @subsection Options for @command{date}
14184 @cindex @command{date} options
14185 @cindex options for @command{date}
14187 The program accepts the following options. Also see @ref{Common options}.
14191 @item -d @var{datestr}
14192 @itemx --date=@var{datestr}
14195 @cindex parsing date strings
14196 @cindex date strings, parsing
14197 @cindex arbitrary date strings, parsing
14200 @opindex next @var{day}
14201 @opindex last @var{day}
14202 Display the date and time specified in @var{datestr} instead of the
14203 current date and time. @var{datestr} can be in almost any common
14204 format. It can contain month names, time zones, @samp{am} and @samp{pm},
14205 @samp{yesterday}, etc. For example, @option{--date="2004-02-27
14206 14:19:13.489392193 +0530"} specifies the instant of time that is
14207 489,392,193 nanoseconds after February 27, 2004 at 2:19:13 PM in a
14208 time zone that is 5 hours and 30 minutes east of @acronym{UTC}.@*
14209 Note: input currently must be in locale independent format. E.g., the
14210 LC_TIME=C below is needed to print back the correct date in many locales:
14212 date -d "$(LC_TIME=C date)"
14214 @xref{Date input formats}.
14216 @item -f @var{datefile}
14217 @itemx --file=@var{datefile}
14220 Parse each line in @var{datefile} as with @option{-d} and display the
14221 resulting date and time. If @var{datefile} is @samp{-}, use standard
14222 input. This is useful when you have many dates to process, because the
14223 system overhead of starting up the @command{date} executable many times can
14226 @item -r @var{file}
14227 @itemx --reference=@var{file}
14229 @opindex --reference
14230 Display the date and time of the last modification of @var{file},
14231 instead of the current date and time.
14238 @opindex --rfc-2822
14239 Display the date and time using the format @samp{%a, %d %b %Y %H:%M:%S
14240 %z}, evaluated in the C locale so abbreviations are always in English.
14244 Fri, 09 Sep 2005 13:51:39 -0700
14247 This format conforms to
14248 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt, Internet
14249 @acronym{RFCs} 2822} and
14250 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc822.txt, 822}, the
14251 current and previous standards for Internet email.
14253 @item --rfc-3339=@var{timespec}
14254 @opindex --rfc-3339=@var{timespec}
14255 Display the date using a format specified by
14256 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt, Internet
14257 @acronym{RFC} 3339}. This is a subset of the @acronym{ISO} 8601
14258 format, except that it also permits applications to use a space rather
14259 than a @samp{T} to separate dates from times. Unlike the other
14260 standard formats, @acronym{RFC} 3339 format is always suitable as
14261 input for the @option{--date} (@option{-d}) and @option{--file}
14262 (@option{-f}) options, regardless of the current locale.
14264 The argument @var{timespec} specifies how much of the time to include.
14265 It can be one of the following:
14269 Print just the full-date, e.g., @samp{2005-09-14}.
14270 This is equivalent to the format @samp{%Y-%m-%d}.
14273 Print the full-date and full-time separated by a space, e.g.,
14274 @samp{2005-09-14 00:56:06+05:30}. The output ends with a numeric
14275 time-offset; here the @samp{+05:30} means that local time is five
14276 hours and thirty minutes east of @acronym{UTC}. This is equivalent to
14277 the format @samp{%Y-%m-%d %H:%M:%S%:z}.
14280 Like @samp{seconds}, but also print nanoseconds, e.g.,
14281 @samp{2005-09-14 00:56:06.998458565+05:30}.
14282 This is equivalent to the format @samp{%Y-%m-%d %H:%M:%S.%N%:z}.
14286 @item -s @var{datestr}
14287 @itemx --set=@var{datestr}
14290 Set the date and time to @var{datestr}. See @option{-d} above.
14297 @opindex --universal
14298 @cindex Coordinated Universal Time
14300 @cindex Greenwich Mean Time
14303 Use Coordinated Universal Time (@acronym{UTC}) by operating as if the
14304 @env{TZ} environment variable were set to the string @samp{UTC0}.
14306 Universal Time is often called ``Greenwich Mean Time'' (@sc{gmt}) for
14307 historical reasons.
14311 @node Examples of date
14312 @subsection Examples of @command{date}
14314 @cindex examples of @command{date}
14316 Here are a few examples. Also see the documentation for the @option{-d}
14317 option in the previous section.
14322 To print the date of the day before yesterday:
14325 date --date='2 days ago'
14329 To print the date of the day three months and one day hence:
14332 date --date='3 months 1 day'
14336 To print the day of year of Christmas in the current year:
14339 date --date='25 Dec' +%j
14343 To print the current full month name and the day of the month:
14349 But this may not be what you want because for the first nine days of
14350 the month, the @samp{%d} expands to a zero-padded two-digit field,
14351 for example @samp{date -d 1may '+%B %d'} will print @samp{May 01}.
14354 To print a date without the leading zero for one-digit days
14355 of the month, you can use the (@acronym{GNU} extension)
14356 @samp{-} flag to suppress
14357 the padding altogether:
14360 date -d 1may '+%B %-d
14364 To print the current date and time in the format required by many
14365 non-@acronym{GNU} versions of @command{date} when setting the system clock:
14368 date +%m%d%H%M%Y.%S
14372 To set the system clock forward by two minutes:
14375 date --set='+2 minutes'
14379 To print the date in @acronym{RFC} 2822 format,
14380 use @samp{date --rfc-2822}. Here is some example output:
14383 Fri, 09 Sep 2005 13:51:39 -0700
14386 @anchor{%s-examples}
14388 To convert a date string to the number of seconds since the epoch
14389 (which is 1970-01-01 00:00:00 UTC), use the @option{--date} option with
14390 the @samp{%s} format. That can be useful in sorting and/or graphing
14391 and/or comparing data by date. The following command outputs the
14392 number of the seconds since the epoch for the time two minutes after the
14396 date --date='1970-01-01 00:02:00 +0000' +%s
14400 If you do not specify time zone information in the date string,
14401 @command{date} uses your computer's idea of the time zone when
14402 interpreting the string. For example, if your computer's time zone is
14403 that of Cambridge, Massachusetts, which was then 5 hours (i.e., 18,000
14404 seconds) behind UTC:
14407 # local time zone used
14408 date --date='1970-01-01 00:02:00' +%s
14413 If you're sorting or graphing dated data, your raw date values may be
14414 represented as seconds since the epoch. But few people can look at
14415 the date @samp{946684800} and casually note ``Oh, that's the first second
14416 of the year 2000 in Greenwich, England.''
14419 date --date='2000-01-01 UTC' +%s
14423 An alternative is to use the @option{--utc} (@option{-u}) option.
14424 Then you may omit @samp{UTC} from the date string. Although this
14425 produces the same result for @samp{%s} and many other format sequences,
14426 with a time zone offset different from zero, it would give a different
14427 result for zone-dependent formats like @samp{%z}.
14430 date -u --date=2000-01-01 +%s
14434 To convert such an unwieldy number of seconds back to
14435 a more readable form, use a command like this:
14438 # local time zone used
14439 date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
14440 1999-12-31 19:00:00 -0500
14443 Or if you do not mind depending on the @samp{@@} feature present since
14444 coreutils 5.3.0, you could shorten this to:
14447 date -d @@946684800 +"%F %T %z"
14448 1999-12-31 19:00:00 -0500
14451 Often it is better to output UTC-relative date and time:
14454 date -u -d '1970-01-01 946684800 seconds' +"%Y-%m-%d %T %z"
14455 2000-01-01 00:00:00 +0000
14461 @node arch invocation
14462 @section @command{arch}: Print machine hardware name
14465 @cindex print machine hardware name
14466 @cindex system information, printing
14468 @command{arch} prints the machine hardware name,
14469 and is equivalent to @samp{uname -m}.
14473 arch [@var{option}]
14476 The program accepts the @ref{Common options} only.
14481 @node nproc invocation
14482 @section @command{nproc}: Print the number of available processors
14485 @cindex Print the number of processors
14486 @cindex system information, printing
14488 Print the number of processing units available to the current process,
14489 which may be less than the number of online processors.
14490 If this information is not accessible, then print the number of
14491 processors installed. If the @env{OMP_NUM_THREADS} environment variable is
14492 set, then it will determine the returned value. The result is guaranteed to be
14493 greater than zero. Synopsis:
14496 nproc [@var{option}]
14499 The program accepts the following options. Also see @ref{Common options}.
14505 Print the number of installed processors on the system, which may
14506 be greater than the number online or available to the current process.
14507 The @env{OMP_NUM_THREADS} environment variable is not honored in this case.
14509 @item --ignore=@var{number}
14511 If possible, exclude this @var{number} of processing units.
14518 @node uname invocation
14519 @section @command{uname}: Print system information
14522 @cindex print system information
14523 @cindex system information, printing
14525 @command{uname} prints information about the machine and operating system
14526 it is run on. If no options are given, @command{uname} acts as if the
14527 @option{-s} option were given. Synopsis:
14530 uname [@var{option}]@dots{}
14533 If multiple options or @option{-a} are given, the selected information is
14534 printed in this order:
14537 @var{kernel-name} @var{nodename} @var{kernel-release} @var{kernel-version}
14538 @var{machine} @var{processor} @var{hardware-platform} @var{operating-system}
14541 The information may contain internal spaces, so such output cannot be
14542 parsed reliably. In the following example, @var{release} is
14543 @samp{2.2.18ss.e820-bda652a #4 SMP Tue Jun 5 11:24:08 PDT 2001}:
14547 @result{} Linux dum 2.2.18 #4 SMP Tue Jun 5 11:24:08 PDT 2001 i686@c
14548 unknown unknown GNU/Linux
14552 The program accepts the following options. Also see @ref{Common options}.
14560 Print all of the below information, except omit the processor type
14561 and the hardware platform name if they are unknown.
14564 @itemx --hardware-platform
14566 @opindex --hardware-platform
14567 @cindex implementation, hardware
14568 @cindex hardware platform
14569 @cindex platform, hardware
14570 Print the hardware platform name
14571 (sometimes called the hardware implementation).
14572 Print @samp{unknown} if the kernel does not make this information
14573 easily available, as is the case with Linux kernels.
14579 @cindex machine type
14580 @cindex hardware class
14581 @cindex hardware type
14582 Print the machine hardware name (sometimes called the hardware class
14588 @opindex --nodename
14591 @cindex network node name
14592 Print the network node hostname.
14597 @opindex --processor
14598 @cindex host processor type
14599 Print the processor type (sometimes called the instruction set
14600 architecture or ISA).
14601 Print @samp{unknown} if the kernel does not make this information
14602 easily available, as is the case with Linux kernels.
14605 @itemx --operating-system
14607 @opindex --operating-system
14608 @cindex operating system name
14609 Print the name of the operating system.
14612 @itemx --kernel-release
14614 @opindex --kernel-release
14615 @cindex kernel release
14616 @cindex release of kernel
14617 Print the kernel release.
14620 @itemx --kernel-name
14622 @opindex --kernel-name
14623 @cindex kernel name
14624 @cindex name of kernel
14625 Print the kernel name.
14626 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) calls this
14627 ``the implementation of the operating system'', because the
14628 @acronym{POSIX} specification itself has no notion of ``kernel''.
14629 The kernel name might be the same as the operating system name printed
14630 by the @option{-o} or @option{--operating-system} option, but it might
14631 differ. Some operating systems (e.g., FreeBSD, HP-UX) have the same
14632 name as their underlying kernels; others (e.g., GNU/Linux, Solaris)
14636 @itemx --kernel-version
14638 @opindex --kernel-version
14639 @cindex kernel version
14640 @cindex version of kernel
14641 Print the kernel version.
14648 @node hostname invocation
14649 @section @command{hostname}: Print or set system name
14652 @cindex setting the hostname
14653 @cindex printing the hostname
14654 @cindex system name, printing
14655 @cindex appropriate privileges
14657 With no arguments, @command{hostname} prints the name of the current host
14658 system. With one argument, it sets the current host name to the
14659 specified string. You must have appropriate privileges to set the host
14663 hostname [@var{name}]
14666 The only options are @option{--help} and @option{--version}. @xref{Common
14672 @node hostid invocation
14673 @section @command{hostid}: Print numeric host identifier
14676 @cindex printing the host identifier
14678 @command{hostid} prints the numeric identifier of the current host
14679 in hexadecimal. This command accepts no arguments.
14680 The only options are @option{--help} and @option{--version}.
14681 @xref{Common options}.
14683 For example, here's what it prints on one system I use:
14690 On that system, the 32-bit quantity happens to be closely
14691 related to the system's Internet address, but that isn't always
14696 @node uptime invocation
14697 @section @command{uptime}: Print system uptime and load
14700 @cindex printing the system uptime and load
14702 @command{uptime} prints the current time, the system's uptime, the
14703 number of logged-in users and the current load average.
14705 If an argument is specified, it is used as the file to be read
14706 to discover how many users are logged in. If no argument is
14707 specified, a system default is used (@command{uptime --help} indicates
14708 the default setting).
14710 The only options are @option{--help} and @option{--version}.
14711 @xref{Common options}.
14713 For example, here's what it prints right now on one system I use:
14717 14:07 up 3:35, 3 users, load average: 1.39, 1.15, 1.04
14720 The precise method of calculation of load average varies somewhat
14721 between systems. Some systems calculate it as the average number of
14722 runnable processes over the last 1, 5 and 15 minutes, but some systems
14723 also include processes in the uninterruptible sleep state (that is,
14724 those processes which are waiting for disk I/O). The Linux kernel
14725 includes uninterruptible processes.
14727 @node SELinux context
14728 @chapter SELinux context
14730 @cindex SELinux context
14731 @cindex SELinux, context
14732 @cindex commands for SELinux context
14734 This section describes commands for operations with SELinux
14738 * chcon invocation:: Change SELinux context of file
14739 * runcon invocation:: Run a command in specified SELinux context
14742 @node chcon invocation
14743 @section @command{chcon}: Change SELinux context of file
14746 @cindex changing security context
14747 @cindex change SELinux context
14749 @command{chcon} changes the SELinux security context of the selected files.
14753 chcon [@var{option}]@dots{} @var{context} @var{file}@dots{}
14754 chcon [@var{option}]@dots{} [-u @var{user}] [-r @var{role}] [-l @var{range}]@c
14755 [-t @var{type}] @var{file}@dots{}
14756 chcon [@var{option}]@dots{} --reference=@var{rfile} @var{file}@dots{}
14759 Change the SELinux security context of each @var{file} to @var{context}.
14760 With @option{--reference}, change the security context of each @var{file}
14761 to that of @var{rfile}.
14763 The program accepts the following options. Also see @ref{Common options}.
14768 @itemx --no-dereference
14770 @opindex --no-dereference
14771 @cindex no dereference
14772 Affect symbolic links instead of any referenced file.
14774 @item --reference=@var{rfile}
14775 @opindex --reference
14776 @cindex reference file
14777 Use @var{rfile}'s security context rather than specifying a @var{context} value.
14782 @opindex --recursive
14783 Operate on files and directories recursively.
14786 @xref{Traversing symlinks}.
14789 @xref{Traversing symlinks}.
14792 @xref{Traversing symlinks}.
14799 Output a diagnostic for every file processed.
14801 @item -u @var{user}
14802 @itemx --user=@var{user}
14805 Set user @var{user} in the target security context.
14807 @item -r @var{role}
14808 @itemx --role=@var{role}
14811 Set role @var{role} in the target security context.
14813 @item -t @var{type}
14814 @itemx --type=@var{type}
14817 Set type @var{type} in the target security context.
14819 @item -l @var{range}
14820 @itemx --range=@var{range}
14823 Set range @var{range} in the target security context.
14829 @node runcon invocation
14830 @section @command{runcon}: Run a command in specified SELinux context
14833 @cindex run with security context
14836 @command{runcon} runs file in specified SELinux security context.
14840 runcon @var{context} @var{command} [@var{args}]
14841 runcon [ -c ] [-u @var{user}] [-r @var{role}] [-t @var{type}]@c
14842 [-l @var{range}] @var{command} [@var{args}]
14845 Run @var{command} with completely-specified @var{context}, or with
14846 current or transitioned security context modified by one or more of @var{level},
14847 @var{role}, @var{type} and @var{user}.
14849 If none of @option{-c}, @option{-t}, @option{-u}, @option{-r}, or @option{-l}
14850 is specified, the first argument is used as the complete context.
14851 Any additional arguments after @var{command}
14852 are interpreted as arguments to the command.
14854 With neither @var{context} nor @var{command}, print the current
14857 The program accepts the following options. Also see @ref{Common options}.
14865 Compute process transition context before modifying.
14867 @item -u @var{user}
14868 @itemx --user=@var{user}
14871 Set user @var{user} in the target security context.
14873 @item -r @var{role}
14874 @itemx --role=@var{role}
14877 Set role @var{role} in the target security context.
14879 @item -t @var{type}
14880 @itemx --type=@var{type}
14883 Set type @var{type} in the target security context.
14885 @item -l @var{range}
14886 @itemx --range=@var{range}
14889 Set range @var{range} in the target security context.
14893 @cindex exit status of @command{runcon}
14897 126 if @var{command} is found but cannot be invoked
14898 127 if @command{runcon} itself fails or if @var{command} cannot be found
14899 the exit status of @var{command} otherwise
14902 @node Modified command invocation
14903 @chapter Modified command invocation
14905 @cindex modified command invocation
14906 @cindex invocation of commands, modified
14907 @cindex commands for invoking other commands
14909 This section describes commands that run other commands in some context
14910 different than the current one: a modified environment, as a different
14914 * chroot invocation:: Modify the root directory.
14915 * env invocation:: Modify environment variables.
14916 * nice invocation:: Modify niceness.
14917 * nohup invocation:: Immunize to hangups.
14918 * stdbuf invocation:: Modify buffering of standard streams.
14919 * su invocation:: Modify user and group ID.
14920 * timeout invocation:: Run with time limit.
14924 @node chroot invocation
14925 @section @command{chroot}: Run a command with a different root directory
14928 @cindex running a program in a specified root directory
14929 @cindex root directory, running a program in a specified
14931 @command{chroot} runs a command with a specified root directory.
14932 On many systems, only the super-user can do this.@footnote{However,
14933 some systems (e.g., FreeBSD) can be configured to allow certain regular
14934 users to use the @code{chroot} system call, and hence to run this program.
14935 Also, on Cygwin, anyone can run the @command{chroot} command, because the
14936 underlying function is non-privileged due to lack of support in MS-Windows.}
14940 chroot @var{option} @var{newroot} [@var{command} [@var{args}]@dots{}]
14941 chroot @var{option}
14944 Ordinarily, file names are looked up starting at the root of the
14945 directory structure, i.e., @file{/}. @command{chroot} changes the root to
14946 the directory @var{newroot} (which must exist) and then runs
14947 @var{command} with optional @var{args}. If @var{command} is not
14948 specified, the default is the value of the @env{SHELL} environment
14949 variable or @command{/bin/sh} if not set, invoked with the @option{-i} option.
14950 @var{command} must not be a special built-in utility
14951 (@pxref{Special built-in utilities}).
14953 The program accepts the following options. Also see @ref{Common options}.
14954 Options must precede operands.
14958 @itemx --userspec=@var{user}[:@var{group}]
14959 @opindex --userspec
14960 By default, @var{command} is run with the same credentials
14961 as the invoking process.
14962 Use this option to run it as a different @var{user} and/or with a
14963 different primary @var{group}.
14965 @itemx --groups=@var{groups}
14967 Use this option to specify the supplementary @var{groups} to be
14968 used by the new process.
14969 The items in the list (names or numeric IDs) must be separated by commas.
14973 Here are a few tips to help avoid common problems in using chroot.
14974 To start with a simple example, make @var{command} refer to a statically
14975 linked binary. If you were to use a dynamically linked executable, then
14976 you'd have to arrange to have the shared libraries in the right place under
14977 your new root directory.
14979 For example, if you create a statically linked @command{ls} executable,
14980 and put it in @file{/tmp/empty}, you can run this command as root:
14983 $ chroot /tmp/empty /ls -Rl /
14986 Then you'll see output like this:
14991 -rwxr-xr-x 1 0 0 1041745 Aug 16 11:17 ls
14994 If you want to use a dynamically linked executable, say @command{bash},
14995 then first run @samp{ldd bash} to see what shared objects it needs.
14996 Then, in addition to copying the actual binary, also copy the listed
14997 files to the required positions under your intended new root directory.
14998 Finally, if the executable requires any other files (e.g., data, state,
14999 device files), copy them into place, too.
15001 @cindex exit status of @command{chroot}
15005 125 if @command{chroot} itself fails
15006 126 if @var{command} is found but cannot be invoked
15007 127 if @var{command} cannot be found
15008 the exit status of @var{command} otherwise
15012 @node env invocation
15013 @section @command{env}: Run a command in a modified environment
15016 @cindex environment, running a program in a modified
15017 @cindex modified environment, running a program in a
15018 @cindex running a program in a modified environment
15020 @command{env} runs a command with a modified environment. Synopses:
15023 env [@var{option}]@dots{} [@var{name}=@var{value}]@dots{} @c
15024 [@var{command} [@var{args}]@dots{}]
15028 Operands of the form @samp{@var{variable}=@var{value}} set
15029 the environment variable @var{variable} to value @var{value}.
15030 @var{value} may be empty (@samp{@var{variable}=}). Setting a variable
15031 to an empty value is different from unsetting it.
15032 These operands are evaluated left-to-right, so if two operands
15033 mention the same variable the earlier is ignored.
15035 Environment variable names can be empty, and can contain any
15036 characters other than @samp{=} and @acronym{ASCII} @sc{nul}.
15037 However, it is wise to limit yourself to names that
15038 consist solely of underscores, digits, and @acronym{ASCII} letters,
15039 and that begin with a non-digit, as applications like the shell do not
15040 work well with other names.
15043 The first operand that does not contain the character @samp{=}
15044 specifies the program to invoke; it is
15045 searched for according to the @env{PATH} environment variable. Any
15046 remaining arguments are passed as arguments to that program.
15047 The program should not be a special built-in utility
15048 (@pxref{Special built-in utilities}).
15050 Modifications to @env{PATH} take effect prior to searching for
15051 @var{command}. Use caution when reducing @env{PATH}; behavior is
15052 not portable when @env{PATH} is undefined or omits key directories
15053 such as @file{/bin}.
15055 In the rare case that a utility contains a @samp{=} in the name, the
15056 only way to disambiguate it from a variable assignment is to use an
15057 intermediate command for @var{command}, and pass the problematic
15058 program name via @var{args}. For example, if @file{./prog=} is an
15059 executable in the current @env{PATH}:
15062 env prog= true # runs 'true', with prog= in environment
15063 env ./prog= true # runs 'true', with ./prog= in environment
15064 env -- prog= true # runs 'true', with prog= in environment
15065 env sh -c '\prog= true' # runs 'prog=' with argument 'true'
15066 env sh -c 'exec "$@@"' sh prog= true # also runs 'prog='
15069 @cindex environment, printing
15071 If no command name is specified following the environment
15072 specifications, the resulting environment is printed. This is like
15073 specifying the @command{printenv} program.
15075 For some examples, suppose the environment passed to @command{env}
15076 contains @samp{LOGNAME=rms}, @samp{EDITOR=emacs}, and
15077 @samp{PATH=.:/gnubin:/hacks}:
15082 Output the current environment.
15084 $ env | LC_ALL=C sort
15087 PATH=.:/gnubin:/hacks
15091 Run @command{foo} with a reduced environment, preserving only the
15092 original @env{PATH} to avoid problems in locating @command{foo}.
15094 env - PATH="$PATH" foo
15098 Run @command{foo} with the environment containing @samp{LOGNAME=rms},
15099 @samp{EDITOR=emacs}, and @samp{PATH=.:/gnubin:/hacks}, and guarantees
15100 that @command{foo} was found in the file system rather than as a shell
15107 Run @command{nemacs} with the environment containing @samp{LOGNAME=foo},
15108 @samp{EDITOR=emacs}, @samp{PATH=.:/gnubin:/hacks}, and
15109 @samp{DISPLAY=gnu:0}.
15111 env DISPLAY=gnu:0 LOGNAME=foo nemacs
15115 Attempt to run the program @command{/energy/--} (as that is the only
15116 possible path search result); if the command exists, the environment
15117 will contain @samp{LOGNAME=rms} and @samp{PATH=/energy}, and the
15118 arguments will be @samp{e=mc2}, @samp{bar}, and @samp{baz}.
15120 env -u EDITOR PATH=/energy -- e=mc2 bar baz
15126 The program accepts the following options. Also see @ref{Common options}.
15127 Options must precede operands.
15133 @item -u @var{name}
15134 @itemx --unset=@var{name}
15137 Remove variable @var{name} from the environment, if it was in the
15142 @itemx --ignore-environment
15145 @opindex --ignore-environment
15146 Start with an empty environment, ignoring the inherited environment.
15150 @cindex exit status of @command{env}
15154 0 if no @var{command} is specified and the environment is output
15155 125 if @command{env} itself fails
15156 126 if @var{command} is found but cannot be invoked
15157 127 if @var{command} cannot be found
15158 the exit status of @var{command} otherwise
15162 @node nice invocation
15163 @section @command{nice}: Run a command with modified niceness
15167 @cindex scheduling, affecting
15168 @cindex appropriate privileges
15170 @command{nice} prints or modifies a process's @dfn{niceness},
15171 a parameter that affects whether the process is scheduled favorably.
15175 nice [@var{option}]@dots{} [@var{command} [@var{arg}]@dots{}]
15178 If no arguments are given, @command{nice} prints the current niceness.
15179 Otherwise, @command{nice} runs the given @var{command} with its
15180 niceness adjusted. By default, its niceness is incremented by 10.
15182 Niceness values range at least from @minus{}20 (process has high priority
15183 and gets more resources, thus slowing down other processes) through 19
15184 (process has lower priority and runs slowly itself, but has less impact
15185 on the speed of other running processes). Some systems
15186 may have a wider range of nicenesses; conversely, other systems may
15187 enforce more restrictive limits. An attempt to set the niceness
15188 outside the supported range is treated as an attempt to use the
15189 minimum or maximum supported value.
15191 A niceness should not be confused with a scheduling priority, which
15192 lets applications determine the order in which threads are scheduled
15193 to run. Unlike a priority, a niceness is merely advice to the
15194 scheduler, which the scheduler is free to ignore. Also, as a point of
15195 terminology, @acronym{POSIX} defines the behavior of @command{nice} in
15196 terms of a @dfn{nice value}, which is the nonnegative difference
15197 between a niceness and the minimum niceness. Though @command{nice}
15198 conforms to @acronym{POSIX}, its documentation and diagnostics use the
15199 term ``niceness'' for compatibility with historical practice.
15201 @var{command} must not be a special built-in utility (@pxref{Special
15202 built-in utilities}).
15204 @mayConflictWithShellBuiltIn{nice}
15206 The program accepts the following option. Also see @ref{Common options}.
15207 Options must precede operands.
15210 @item -n @var{adjustment}
15211 @itemx --adjustment=@var{adjustment}
15213 @opindex --adjustment
15214 Add @var{adjustment} instead of 10 to the command's niceness. If
15215 @var{adjustment} is negative and you lack appropriate privileges,
15216 @command{nice} issues a warning but otherwise acts as if you specified
15219 For compatibility @command{nice} also supports an obsolete
15220 option syntax @option{-@var{adjustment}}. New scripts should use
15221 @option{-n @var{adjustment}} instead.
15225 @cindex exit status of @command{nice}
15229 0 if no @var{command} is specified and the niceness is output
15230 125 if @command{nice} itself fails
15231 126 if @var{command} is found but cannot be invoked
15232 127 if @var{command} cannot be found
15233 the exit status of @var{command} otherwise
15236 It is sometimes useful to run a non-interactive program with reduced niceness.
15239 $ nice factor 4611686018427387903
15242 Since @command{nice} prints the current niceness,
15243 you can invoke it through itself to demonstrate how it works.
15245 The default behavior is to increase the niceness by @samp{10}:
15256 The @var{adjustment} is relative to the current niceness. In the
15257 next example, the first @command{nice} invocation runs the second one
15258 with niceness 10, and it in turn runs the final one with a niceness
15262 $ nice nice -n 3 nice
15266 Specifying a niceness larger than the supported range
15267 is the same as specifying the maximum supported value:
15270 $ nice -n 10000000000 nice
15274 Only a privileged user may run a process with lower niceness:
15278 nice: cannot set niceness: Permission denied
15280 $ sudo nice -n -1 nice
15285 @node nohup invocation
15286 @section @command{nohup}: Run a command immune to hangups
15289 @cindex hangups, immunity to
15290 @cindex immunity to hangups
15291 @cindex logging out and continuing to run
15294 @command{nohup} runs the given @var{command} with hangup signals ignored,
15295 so that the command can continue running in the background after you log
15299 nohup @var{command} [@var{arg}]@dots{}
15302 If standard input is a terminal, it is redirected from
15303 @file{/dev/null} so that terminal sessions do not mistakenly consider
15304 the terminal to be used by the command. This is a @acronym{GNU}
15305 extension; programs intended to be portable to non-@acronym{GNU} hosts
15306 should use @samp{nohup @var{command} [@var{arg}]@dots{} </dev/null}
15310 If standard output is a terminal, the command's standard output is appended
15311 to the file @file{nohup.out}; if that cannot be written to, it is appended
15312 to the file @file{$HOME/nohup.out}; and if that cannot be written to, the
15313 command is not run.
15314 Any @file{nohup.out} or @file{$HOME/nohup.out} file created by
15315 @command{nohup} is made readable and writable only to the user,
15316 regardless of the current umask settings.
15318 If standard error is a terminal, it is normally redirected to the same file
15319 descriptor as the (possibly-redirected) standard output.
15320 However, if standard output is closed, standard error terminal output
15321 is instead appended to the file @file{nohup.out} or
15322 @file{$HOME/nohup.out} as above.
15324 To capture the command's output to a file other than @file{nohup.out}
15325 you can redirect it. For example, to capture the output of
15329 nohup make > make.log
15332 @command{nohup} does not automatically put the command it runs in the
15333 background; you must do that explicitly, by ending the command line
15334 with an @samp{&}. Also, @command{nohup} does not alter the
15335 niceness of @var{command}; use @command{nice} for that,
15336 e.g., @samp{nohup nice @var{command}}.
15338 @var{command} must not be a special built-in utility (@pxref{Special
15339 built-in utilities}).
15341 The only options are @option{--help} and @option{--version}. @xref{Common
15342 options}. Options must precede operands.
15344 @cindex exit status of @command{nohup}
15348 125 if @command{nohup} itself fails, and @env{POSIXLY_CORRECT} is not set
15349 126 if @var{command} is found but cannot be invoked
15350 127 if @var{command} cannot be found
15351 the exit status of @var{command} otherwise
15354 If @env{POSIXLY_CORRECT} is set, internal failures give status 127
15358 @node stdbuf invocation
15359 @section @command{stdbuf}: Run a command with modified I/O stream buffering
15362 @cindex standard streams, buffering
15363 @cindex line buffered
15365 @command{stdbuf} allows one to modify the buffering operations of the
15366 three standard I/O streams associated with a program. Synopsis:
15369 stdbuf @var{option}@dots{} @var{command}
15372 Any additional @var{arg}s are passed as additional arguments to the
15375 The program accepts the following options. Also see @ref{Common options}.
15379 @item -i @var{mode}
15380 @itemx --input=@var{mode}
15383 Adjust the standard input stream buffering.
15385 @item -o @var{mode}
15386 @itemx --output=@var{mode}
15389 Adjust the standard output stream buffering.
15391 @item -e @var{mode}
15392 @itemx --error=@var{mode}
15395 Adjust the standard error stream buffering.
15399 The @var{mode} can be specified as follows:
15404 Set the stream to line buffered mode.
15405 In this mode data is coalesced until a newline is output or
15406 input is read from any stream attached to a terminal device.
15407 This option is invalid with standard input.
15410 Disable buffering of the selected stream.
15411 In this mode data is output immediately and only the
15412 amount of data requested is read from input.
15415 Specify the size of the buffer to use in fully buffered mode.
15416 @multiplierSuffixesNoBlocks{size}
15420 NOTE: If @var{command} adjusts the buffering of its standard streams
15421 (@command{tee} does for e.g.) then that will override corresponding settings
15422 changed by @command{stdbuf}. Also some filters (like @command{dd} and
15423 @command{cat} etc.) don't use streams for I/O, and are thus unaffected
15424 by @command{stdbuf} settings.
15426 @cindex exit status of @command{stdbuf}
15430 125 if @command{stdbuf} itself fails
15431 126 if @var{command} is found but cannot be invoked
15432 127 if @var{command} cannot be found
15433 the exit status of @var{command} otherwise
15437 @node su invocation
15438 @section @command{su}: Run a command with substitute user and group ID
15441 @cindex substitute user and group IDs
15442 @cindex user ID, switching
15443 @cindex super-user, becoming
15444 @cindex root, becoming
15446 @command{su} allows one user to temporarily become another user. It runs a
15447 command (often an interactive shell) with the real and effective user
15448 ID, group ID, and supplemental groups of a given @var{user}. Synopsis:
15451 su [@var{option}]@dots{} [@var{user} [@var{arg}]@dots{}]
15454 @cindex passwd entry, and @command{su} shell
15456 @flindex /etc/passwd
15457 If no @var{user} is given, the default is @code{root}, the super-user.
15458 The shell to use is taken from @var{user}'s @code{passwd} entry, or
15459 @file{/bin/sh} if none is specified there. If @var{user} has a
15460 password, @command{su} prompts for the password unless run by a user with
15461 effective user ID of zero (the super-user).
15467 @cindex login shell
15468 By default, @command{su} does not change the current directory.
15469 It sets the environment variables @env{HOME} and @env{SHELL}
15470 from the password entry for @var{user}, and if @var{user} is not
15471 the super-user, sets @env{USER} and @env{LOGNAME} to @var{user}.
15472 By default, the shell is not a login shell.
15474 Any additional @var{arg}s are passed as additional arguments to the
15477 @cindex @option{-su}
15478 GNU @command{su} does not treat @file{/bin/sh} or any other shells specially
15479 (e.g., by setting @code{argv[0]} to @option{-su}, passing @option{-c} only
15480 to certain shells, etc.).
15483 @command{su} can optionally be compiled to use @code{syslog} to report
15484 failed, and optionally successful, @command{su} attempts. (If the system
15485 supports @code{syslog}.) However, GNU @command{su} does not check if the
15486 user is a member of the @code{wheel} group; see below.
15488 The program accepts the following options. Also see @ref{Common options}.
15491 @item -c @var{command}
15492 @itemx --command=@var{command}
15495 Pass @var{command}, a single command line to run, to the shell with
15496 a @option{-c} option instead of starting an interactive shell.
15503 @cindex file name pattern expansion, disabled
15504 @cindex globbing, disabled
15505 Pass the @option{-f} option to the shell. This probably only makes sense
15506 if the shell run is @command{csh} or @command{tcsh}, for which the @option{-f}
15507 option prevents reading the startup file (@file{.cshrc}). With
15508 Bourne-like shells, the @option{-f} option disables file name pattern
15509 expansion (globbing), which is not likely to be useful.
15517 @c other variables already indexed above
15520 @cindex login shell, creating
15521 Make the shell a login shell. This means the following. Unset all
15522 environment variables except @env{TERM}, @env{HOME}, and @env{SHELL}
15523 (which are set as described above), and @env{USER} and @env{LOGNAME}
15524 (which are set, even for the super-user, as described above), and set
15525 @env{PATH} to a compiled-in default value. Change to @var{user}'s home
15526 directory. Prepend @samp{-} to the shell's name, intended to make it
15527 read its login startup file(s).
15531 @itemx --preserve-environment
15534 @opindex --preserve-environment
15535 @cindex environment, preserving
15536 @flindex /etc/shells
15537 @cindex restricted shell
15538 Do not change the environment variables @env{HOME}, @env{USER},
15539 @env{LOGNAME}, or @env{SHELL}. Run the shell given in the environment
15540 variable @env{SHELL} instead of the shell from @var{user}'s passwd
15541 entry, unless the user running @command{su} is not the super-user and
15542 @var{user}'s shell is restricted. A @dfn{restricted shell} is one that
15543 is not listed in the file @file{/etc/shells}, or in a compiled-in list
15544 if that file does not exist. Parts of what this option does can be
15545 overridden by @option{--login} and @option{--shell}.
15547 @item -s @var{shell}
15548 @itemx --shell=@var{shell}
15551 Run @var{shell} instead of the shell from @var{user}'s passwd entry,
15552 unless the user running @command{su} is not the super-user and @var{user}'s
15553 shell is restricted (see @option{-m} just above).
15557 @cindex exit status of @command{su}
15561 125 if @command{su} itself fails
15562 126 if subshell is found but cannot be invoked
15563 127 if subshell cannot be found
15564 the exit status of the subshell otherwise
15567 @cindex wheel group, not supported
15568 @cindex group wheel, not supported
15570 @subsection Why GNU @command{su} does not support the @samp{wheel} group
15572 (This section is by Richard Stallman.)
15576 Sometimes a few of the users try to hold total power over all the
15577 rest. For example, in 1984, a few users at the MIT AI lab decided to
15578 seize power by changing the operator password on the Twenex system and
15579 keeping it secret from everyone else. (I was able to thwart this coup
15580 and give power back to the users by patching the kernel, but I
15581 wouldn't know how to do that in Unix.)
15583 However, occasionally the rulers do tell someone. Under the usual
15584 @command{su} mechanism, once someone learns the root password who
15585 sympathizes with the ordinary users, he or she can tell the rest. The
15586 ``wheel group'' feature would make this impossible, and thus cement the
15587 power of the rulers.
15589 I'm on the side of the masses, not that of the rulers. If you are
15590 used to supporting the bosses and sysadmins in whatever they do, you
15591 might find this idea strange at first.
15594 @node timeout invocation
15595 @section @command{timeout}: Run a command with a time limit
15599 @cindex run commands with bounded time
15601 @command{timeout} runs the given @var{command} and kills it if it is
15602 still running after the specified time interval. Synopsis:
15605 timeout [@var{option}] @var{duration} @var{command} [@var{arg}]@dots{}
15608 @var{command} must not be a special built-in utility (@pxref{Special
15609 built-in utilities}).
15611 The program accepts the following options. Also see @ref{Common options}.
15612 Options must precede operands.
15615 @item -k @var{duration}
15616 @itemx --kill-after=@var{duration}
15618 @opindex --kill-after
15619 Ensure the monitored @var{command} is killed by also sending a @samp{KILL}
15620 signal, after the specified @var{duration}. Without this option, if the
15621 selected signal proves not to be fatal, @command{timeout} does not kill
15624 @item -s @var{signal}
15625 @itemx --signal=@var{signal}
15628 Send this @var{signal} to @var{command} on timeout, rather than the
15629 default @samp{TERM} signal. @var{signal} may be a name like @samp{HUP}
15630 or a number. Also see @xref{Signal specifications}.
15634 @var{duration} is an integer followed by an optional unit:
15636 @samp{s} for seconds (the default)
15637 @samp{m} for minutes
15641 A duration of 0 disables the associated timeout.
15643 @cindex exit status of @command{timeout}
15647 124 if @var{command} times out
15648 125 if @command{timeout} itself fails
15649 126 if @var{command} is found but cannot be invoked
15650 127 if @var{command} cannot be found
15651 the exit status of @var{command} otherwise
15655 @node Process control
15656 @chapter Process control
15658 @cindex processes, commands for controlling
15659 @cindex commands for controlling processes
15662 * kill invocation:: Sending a signal to processes.
15666 @node kill invocation
15667 @section @command{kill}: Send a signal to processes
15670 @cindex send a signal to processes
15672 The @command{kill} command sends a signal to processes, causing them
15673 to terminate or otherwise act upon receiving the signal in some way.
15674 Alternatively, it lists information about signals. Synopses:
15677 kill [-s @var{signal} | --signal @var{signal} | -@var{signal}] @var{pid}@dots{}
15678 kill [-l | --list | -t | --table] [@var{signal}]@dots{}
15681 @mayConflictWithShellBuiltIn{kill}
15683 The first form of the @command{kill} command sends a signal to all
15684 @var{pid} arguments. The default signal to send if none is specified
15685 is @samp{TERM}. The special signal number @samp{0} does not denote a
15686 valid signal, but can be used to test whether the @var{pid} arguments
15687 specify processes to which a signal could be sent.
15689 If @var{pid} is positive, the signal is sent to the process with the
15690 process ID @var{pid}. If @var{pid} is zero, the signal is sent to all
15691 processes in the process group of the current process. If @var{pid}
15692 is @minus{}1, the signal is sent to all processes for which the user has
15693 permission to send a signal. If @var{pid} is less than @minus{}1, the signal
15694 is sent to all processes in the process group that equals the absolute
15695 value of @var{pid}.
15697 If @var{pid} is not positive, a system-dependent set of system
15698 processes is excluded from the list of processes to which the signal
15701 If a negative @var{pid} argument is desired as the first one, it
15702 should be preceded by @option{--}. However, as a common extension to
15703 @acronym{POSIX}, @option{--} is not required with @samp{kill
15704 -@var{signal} -@var{pid}}. The following commands are equivalent:
15713 The first form of the @command{kill} command succeeds if every @var{pid}
15714 argument specifies at least one process that the signal was sent to.
15716 The second form of the @command{kill} command lists signal information.
15717 Either the @option{-l} or @option{--list} option, or the @option{-t}
15718 or @option{--table} option must be specified. Without any
15719 @var{signal} argument, all supported signals are listed. The output
15720 of @option{-l} or @option{--list} is a list of the signal names, one
15721 per line; if @var{signal} is already a name, the signal number is
15722 printed instead. The output of @option{-t} or @option{--table} is a
15723 table of signal numbers, names, and descriptions. This form of the
15724 @command{kill} command succeeds if all @var{signal} arguments are valid
15725 and if there is no output error.
15727 The @command{kill} command also supports the @option{--help} and
15728 @option{--version} options. @xref{Common options}.
15730 A @var{signal} may be a signal name like @samp{HUP}, or a signal
15731 number like @samp{1}, or an exit status of a process terminated by the
15732 signal. A signal name can be given in canonical form or prefixed by
15733 @samp{SIG}. The case of the letters is ignored, except for the
15734 @option{-@var{signal}} option which must use upper case to avoid
15735 ambiguity with lower case option letters. For a list of supported
15736 signal names and numbers see @xref{Signal specifications}.
15741 @cindex delaying commands
15742 @cindex commands for delaying
15744 @c Perhaps @command{wait} or other commands should be described here also?
15747 * sleep invocation:: Delay for a specified time.
15751 @node sleep invocation
15752 @section @command{sleep}: Delay for a specified time
15755 @cindex delay for a specified time
15757 @command{sleep} pauses for an amount of time specified by the sum of
15758 the values of the command line arguments.
15762 sleep @var{number}[smhd]@dots{}
15766 Each argument is a number followed by an optional unit; the default
15767 is seconds. The units are:
15780 Historical implementations of @command{sleep} have required that
15781 @var{number} be an integer, and only accepted a single argument
15782 without a suffix. However, GNU @command{sleep} accepts
15783 arbitrary floating point numbers. @xref{Floating point}.
15785 The only options are @option{--help} and @option{--version}. @xref{Common
15788 @c sleep is a shell built-in at least with Solaris 11's /bin/sh
15789 @mayConflictWithShellBuiltIn{sleep}
15794 @node Numeric operations
15795 @chapter Numeric operations
15797 @cindex numeric operations
15798 These programs do numerically-related operations.
15801 * factor invocation:: Show factors of numbers.
15802 * seq invocation:: Print sequences of numbers.
15806 @node factor invocation
15807 @section @command{factor}: Print prime factors
15810 @cindex prime factors
15812 @command{factor} prints prime factors. Synopses:
15815 factor [@var{number}]@dots{}
15816 factor @var{option}
15819 If no @var{number} is specified on the command line, @command{factor} reads
15820 numbers from standard input, delimited by newlines, tabs, or spaces.
15822 The @command{factor} command supports only a small number of options:
15826 Print a short help on standard output, then exit without further
15830 Print the program version on standard output, then exit without further
15834 Factoring the product of the eighth and ninth Mersenne primes
15835 takes about 30 milliseconds of CPU time on a 2.2 GHz Athlon.
15838 M8=`echo 2^31-1|bc` ; M9=`echo 2^61-1|bc`
15839 /usr/bin/time -f '%U' factor $(echo "$M8 * $M9" | bc)
15840 4951760154835678088235319297: 2147483647 2305843009213693951
15844 Similarly, factoring the eighth Fermat number @math{2^{256}+1} takes
15845 about 20 seconds on the same machine.
15847 Factoring large numbers is, in general, hard. The Pollard Rho
15848 algorithm used by @command{factor} is particularly effective for
15849 numbers with relatively small factors. If you wish to factor large
15850 numbers which do not have small factors (for example, numbers which
15851 are the product of two large primes), other methods are far better.
15853 If @command{factor} is built without using GNU MP, only
15854 single-precision arithmetic is available, and so large numbers
15855 (typically @math{2^{64}} and above) will not be supported. The single-precision
15856 code uses an algorithm which is designed for factoring smaller
15862 @node seq invocation
15863 @section @command{seq}: Print numeric sequences
15866 @cindex numeric sequences
15867 @cindex sequence of numbers
15869 @command{seq} prints a sequence of numbers to standard output. Synopses:
15872 seq [@var{option}]@dots{} @var{last}
15873 seq [@var{option}]@dots{} @var{first} @var{last}
15874 seq [@var{option}]@dots{} @var{first} @var{increment} @var{last}
15877 @command{seq} prints the numbers from @var{first} to @var{last} by
15878 @var{increment}. By default, each number is printed on a separate line.
15879 When @var{increment} is not specified, it defaults to @samp{1},
15880 even when @var{first} is larger than @var{last}.
15881 @var{first} also defaults to @samp{1}. So @code{seq 1} prints
15882 @samp{1}, but @code{seq 0} and @code{seq 10 5} produce no output.
15883 Floating-point numbers may be specified. @xref{Floating point}.
15885 The program accepts the following options. Also see @ref{Common options}.
15886 Options must precede operands.
15889 @item -f @var{format}
15890 @itemx --format=@var{format}
15891 @opindex -f @var{format}
15892 @opindex --format=@var{format}
15893 @cindex formatting of numbers in @command{seq}
15894 Print all numbers using @var{format}.
15895 @var{format} must contain exactly one of the @samp{printf}-style
15896 floating point conversion specifications @samp{%a}, @samp{%e},
15897 @samp{%f}, @samp{%g}, @samp{%A}, @samp{%E}, @samp{%F}, @samp{%G}.
15898 The @samp{%} may be followed by zero or more flags taken from the set
15899 @samp{-+#0 '}, then an optional width containing one or more digits,
15900 then an optional precision consisting of a @samp{.} followed by zero
15901 or more digits. @var{format} may also contain any number of @samp{%%}
15902 conversion specifications. All conversion specifications have the
15903 same meaning as with @samp{printf}.
15905 The default format is derived from @var{first}, @var{step}, and
15906 @var{last}. If these all use a fixed point decimal representation,
15907 the default format is @samp{%.@var{p}f}, where @var{p} is the minimum
15908 precision that can represent the output numbers exactly. Otherwise,
15909 the default format is @samp{%g}.
15911 @item -s @var{string}
15912 @itemx --separator=@var{string}
15913 @cindex separator for numbers in @command{seq}
15914 Separate numbers with @var{string}; default is a newline.
15915 The output always terminates with a newline.
15918 @itemx --equal-width
15919 Print all numbers with the same width, by padding with leading zeros.
15920 @var{first}, @var{step}, and @var{last} should all use a fixed point
15921 decimal representation.
15922 (To have other kinds of padding, use @option{--format}).
15926 You can get finer-grained control over output with @option{-f}:
15929 $ seq -f '(%9.2E)' -9e5 1.1e6 1.3e6
15935 If you want hexadecimal integer output, you can use @command{printf}
15936 to perform the conversion:
15939 $ printf '%x\n' `seq 1048575 1024 1050623`
15945 For very long lists of numbers, use xargs to avoid
15946 system limitations on the length of an argument list:
15949 $ seq 1000000 | xargs printf '%x\n' | tail -n 3
15955 To generate octal output, use the printf @code{%o} format instead
15958 On most systems, seq can produce whole-number output for values up to
15959 at least @math{2^{53}}. Larger integers are approximated. The details
15960 differ depending on your floating-point implementation.
15961 @xref{Floating point}. A common
15962 case is that @command{seq} works with integers through @math{2^{64}},
15963 and larger integers may not be numerically correct:
15966 $ seq 18446744073709551616 1 18446744073709551618
15967 18446744073709551616
15968 18446744073709551616
15969 18446744073709551618
15972 Be careful when using @command{seq} with outlandish values: otherwise
15973 you may see surprising results, as @command{seq} uses floating point
15974 internally. For example, on the x86 platform, where the internal
15975 representation uses a 64-bit fraction, the command:
15978 seq 1 0.0000000000000000001 1.0000000000000000009
15981 outputs 1.0000000000000000007 twice and skips 1.0000000000000000008.
15986 @node File permissions
15987 @chapter File permissions
15990 @include parse-datetime.texi
15994 @node Opening the software toolbox
15995 @chapter Opening the Software Toolbox
15997 An earlier version of this chapter appeared in
15998 @uref{http://www.linuxjournal.com/article.php?sid=2762, the
15999 @cite{What's GNU?} column of @cite{Linux Journal}, 2 (June, 1994)}.
16000 It was written by Arnold Robbins.
16003 * Toolbox introduction:: Toolbox introduction
16004 * I/O redirection:: I/O redirection
16005 * The who command:: The @command{who} command
16006 * The cut command:: The @command{cut} command
16007 * The sort command:: The @command{sort} command
16008 * The uniq command:: The @command{uniq} command
16009 * Putting the tools together:: Putting the tools together
16013 @node Toolbox introduction
16014 @unnumberedsec Toolbox Introduction
16016 This month's column is only peripherally related to the GNU Project, in
16017 that it describes a number of the GNU tools on your GNU/Linux system
16019 might be used. What it's really about is the ``Software Tools'' philosophy
16020 of program development and usage.
16022 The software tools philosophy was an important and integral concept
16023 in the initial design and development of Unix (of which Linux and GNU are
16024 essentially clones). Unfortunately, in the modern day press of
16025 Internetworking and flashy GUIs, it seems to have fallen by the
16026 wayside. This is a shame, since it provides a powerful mental model
16027 for solving many kinds of problems.
16029 Many people carry a Swiss Army knife around in their pants pockets (or
16030 purse). A Swiss Army knife is a handy tool to have: it has several knife
16031 blades, a screwdriver, tweezers, toothpick, nail file, corkscrew, and perhaps
16032 a number of other things on it. For the everyday, small miscellaneous jobs
16033 where you need a simple, general purpose tool, it's just the thing.
16035 On the other hand, an experienced carpenter doesn't build a house using
16036 a Swiss Army knife. Instead, he has a toolbox chock full of specialized
16037 tools---a saw, a hammer, a screwdriver, a plane, and so on. And he knows
16038 exactly when and where to use each tool; you won't catch him hammering nails
16039 with the handle of his screwdriver.
16041 The Unix developers at Bell Labs were all professional programmers and trained
16042 computer scientists. They had found that while a one-size-fits-all program
16043 might appeal to a user because there's only one program to use, in practice
16048 difficult to write,
16051 difficult to maintain and
16055 difficult to extend to meet new situations.
16058 Instead, they felt that programs should be specialized tools. In short, each
16059 program ``should do one thing well.'' No more and no less. Such programs are
16060 simpler to design, write, and get right---they only do one thing.
16062 Furthermore, they found that with the right machinery for hooking programs
16063 together, that the whole was greater than the sum of the parts. By combining
16064 several special purpose programs, you could accomplish a specific task
16065 that none of the programs was designed for, and accomplish it much more
16066 quickly and easily than if you had to write a special purpose program.
16067 We will see some (classic) examples of this further on in the column.
16068 (An important additional point was that, if necessary, take a detour
16069 and build any software tools you may need first, if you don't already
16070 have something appropriate in the toolbox.)
16072 @node I/O redirection
16073 @unnumberedsec I/O Redirection
16075 Hopefully, you are familiar with the basics of I/O redirection in the
16076 shell, in particular the concepts of ``standard input,'' ``standard output,''
16077 and ``standard error''. Briefly, ``standard input'' is a data source, where
16078 data comes from. A program should not need to either know or care if the
16079 data source is a disk file, a keyboard, a magnetic tape, or even a punched
16080 card reader. Similarly, ``standard output'' is a data sink, where data goes
16081 to. The program should neither know nor care where this might be.
16082 Programs that only read their standard input, do something to the data,
16083 and then send it on, are called @dfn{filters}, by analogy to filters in a
16086 With the Unix shell, it's very easy to set up data pipelines:
16089 program_to_create_data | filter1 | ... | filterN > final.pretty.data
16092 We start out by creating the raw data; each filter applies some successive
16093 transformation to the data, until by the time it comes out of the pipeline,
16094 it is in the desired form.
16096 This is fine and good for standard input and standard output. Where does the
16097 standard error come in to play? Well, think about @command{filter1} in
16098 the pipeline above. What happens if it encounters an error in the data it
16099 sees? If it writes an error message to standard output, it will just
16100 disappear down the pipeline into @command{filter2}'s input, and the
16101 user will probably never see it. So programs need a place where they can send
16102 error messages so that the user will notice them. This is standard error,
16103 and it is usually connected to your console or window, even if you have
16104 redirected standard output of your program away from your screen.
16106 For filter programs to work together, the format of the data has to be
16107 agreed upon. The most straightforward and easiest format to use is simply
16108 lines of text. Unix data files are generally just streams of bytes, with
16109 lines delimited by the @acronym{ASCII} @sc{lf} (Line Feed) character,
16110 conventionally called a ``newline'' in the Unix literature. (This is
16111 @code{'\n'} if you're a C programmer.) This is the format used by all
16112 the traditional filtering programs. (Many earlier operating systems
16113 had elaborate facilities and special purpose programs for managing
16114 binary data. Unix has always shied away from such things, under the
16115 philosophy that it's easiest to simply be able to view and edit your
16116 data with a text editor.)
16118 OK, enough introduction. Let's take a look at some of the tools, and then
16119 we'll see how to hook them together in interesting ways. In the following
16120 discussion, we will only present those command line options that interest
16121 us. As you should always do, double check your system documentation
16122 for the full story.
16124 @node The who command
16125 @unnumberedsec The @command{who} Command
16127 The first program is the @command{who} command. By itself, it generates a
16128 list of the users who are currently logged in. Although I'm writing
16129 this on a single-user system, we'll pretend that several people are
16134 @print{} arnold console Jan 22 19:57
16135 @print{} miriam ttyp0 Jan 23 14:19(:0.0)
16136 @print{} bill ttyp1 Jan 21 09:32(:0.0)
16137 @print{} arnold ttyp2 Jan 23 20:48(:0.0)
16140 Here, the @samp{$} is the usual shell prompt, at which I typed @samp{who}.
16141 There are three people logged in, and I am logged in twice. On traditional
16142 Unix systems, user names are never more than eight characters long. This
16143 little bit of trivia will be useful later. The output of @command{who} is nice,
16144 but the data is not all that exciting.
16146 @node The cut command
16147 @unnumberedsec The @command{cut} Command
16149 The next program we'll look at is the @command{cut} command. This program
16150 cuts out columns or fields of input data. For example, we can tell it
16151 to print just the login name and full name from the @file{/etc/passwd}
16152 file. The @file{/etc/passwd} file has seven fields, separated by
16156 arnold:xyzzy:2076:10:Arnold D. Robbins:/home/arnold:/bin/bash
16159 To get the first and fifth fields, we would use @command{cut} like this:
16162 $ cut -d: -f1,5 /etc/passwd
16163 @print{} root:Operator
16165 @print{} arnold:Arnold D. Robbins
16166 @print{} miriam:Miriam A. Robbins
16170 With the @option{-c} option, @command{cut} will cut out specific characters
16171 (i.e., columns) in the input lines. This is useful for input data
16172 that has fixed width fields, and does not have a field separator. For
16173 example, list the Monday dates for the current month:
16175 @c Is using cal ok? Looked at gcal, but I don't like it.
16186 @node The sort command
16187 @unnumberedsec The @command{sort} Command
16189 Next we'll look at the @command{sort} command. This is one of the most
16190 powerful commands on a Unix-style system; one that you will often find
16191 yourself using when setting up fancy data plumbing.
16194 command reads and sorts each file named on the command line. It then
16195 merges the sorted data and writes it to standard output. It will read
16196 standard input if no files are given on the command line (thus
16197 making it into a filter). The sort is based on the character collating
16198 sequence or based on user-supplied ordering criteria.
16201 @node The uniq command
16202 @unnumberedsec The @command{uniq} Command
16204 Finally (at least for now), we'll look at the @command{uniq} program. When
16205 sorting data, you will often end up with duplicate lines, lines that
16206 are identical. Usually, all you need is one instance of each line.
16207 This is where @command{uniq} comes in. The @command{uniq} program reads its
16208 standard input. It prints only one
16209 copy of each repeated line. It does have several options. Later on,
16210 we'll use the @option{-c} option, which prints each unique line, preceded
16211 by a count of the number of times that line occurred in the input.
16214 @node Putting the tools together
16215 @unnumberedsec Putting the Tools Together
16217 Now, let's suppose this is a large ISP server system with dozens of users
16218 logged in. The management wants the system administrator to write a
16220 generate a sorted list of logged in users. Furthermore, even if a user
16221 is logged in multiple times, his or her name should only show up in the
16224 The administrator could sit down with the system documentation and write a C
16225 program that did this. It would take perhaps a couple of hundred lines
16226 of code and about two hours to write it, test it, and debug it.
16227 However, knowing the software toolbox, the administrator can instead start out
16228 by generating just a list of logged on users:
16238 Next, sort the list:
16241 $ who | cut -c1-8 | sort
16248 Finally, run the sorted list through @command{uniq}, to weed out duplicates:
16251 $ who | cut -c1-8 | sort | uniq
16257 The @command{sort} command actually has a @option{-u} option that does what
16258 @command{uniq} does. However, @command{uniq} has other uses for which one
16259 cannot substitute @samp{sort -u}.
16261 The administrator puts this pipeline into a shell script, and makes it
16263 all the users on the system (@samp{#} is the system administrator,
16264 or @code{root}, prompt):
16267 # cat > /usr/local/bin/listusers
16268 who | cut -c1-8 | sort | uniq
16270 # chmod +x /usr/local/bin/listusers
16273 There are four major points to note here. First, with just four
16274 programs, on one command line, the administrator was able to save about two
16275 hours worth of work. Furthermore, the shell pipeline is just about as
16276 efficient as the C program would be, and it is much more efficient in
16277 terms of programmer time. People time is much more expensive than
16278 computer time, and in our modern ``there's never enough time to do
16279 everything'' society, saving two hours of programmer time is no mean
16282 Second, it is also important to emphasize that with the
16283 @emph{combination} of the tools, it is possible to do a special
16284 purpose job never imagined by the authors of the individual programs.
16286 Third, it is also valuable to build up your pipeline in stages, as we did here.
16287 This allows you to view the data at each stage in the pipeline, which helps
16288 you acquire the confidence that you are indeed using these tools correctly.
16290 Finally, by bundling the pipeline in a shell script, other users can use
16291 your command, without having to remember the fancy plumbing you set up for
16292 them. In terms of how you run them, shell scripts and compiled programs are
16295 After the previous warm-up exercise, we'll look at two additional, more
16296 complicated pipelines. For them, we need to introduce two more tools.
16298 The first is the @command{tr} command, which stands for ``transliterate.''
16299 The @command{tr} command works on a character-by-character basis, changing
16300 characters. Normally it is used for things like mapping upper case to
16304 $ echo ThIs ExAmPlE HaS MIXED case! | tr '[:upper:]' '[:lower:]'
16305 @print{} this example has mixed case!
16308 There are several options of interest:
16312 work on the complement of the listed characters, i.e.,
16313 operations apply to characters not in the given set
16316 delete characters in the first set from the output
16319 squeeze repeated characters in the output into just one character.
16322 We will be using all three options in a moment.
16324 The other command we'll look at is @command{comm}. The @command{comm}
16325 command takes two sorted input files as input data, and prints out the
16326 files' lines in three columns. The output columns are the data lines
16327 unique to the first file, the data lines unique to the second file, and
16328 the data lines that are common to both. The @option{-1}, @option{-2}, and
16329 @option{-3} command line options @emph{omit} the respective columns. (This is
16330 non-intuitive and takes a little getting used to.) For example:
16352 The file name @file{-} tells @command{comm} to read standard input
16353 instead of a regular file.
16355 Now we're ready to build a fancy pipeline. The first application is a word
16356 frequency counter. This helps an author determine if he or she is over-using
16359 The first step is to change the case of all the letters in our input file
16360 to one case. ``The'' and ``the'' are the same word when doing counting.
16363 $ tr '[:upper:]' '[:lower:]' < whats.gnu | ...
16366 The next step is to get rid of punctuation. Quoted words and unquoted words
16367 should be treated identically; it's easiest to just get the punctuation out of
16371 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | ...
16374 The second @command{tr} command operates on the complement of the listed
16375 characters, which are all the letters, the digits, the underscore, and
16376 the blank. The @samp{\n} represents the newline character; it has to
16377 be left alone. (The @acronym{ASCII} tab character should also be included for
16378 good measure in a production script.)
16380 At this point, we have data consisting of words separated by blank space.
16381 The words only contain alphanumeric characters (and the underscore). The
16382 next step is break the data apart so that we have one word per line. This
16383 makes the counting operation much easier, as we will see shortly.
16386 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16387 > tr -s ' ' '\n' | ...
16390 This command turns blanks into newlines. The @option{-s} option squeezes
16391 multiple newline characters in the output into just one. This helps us
16392 avoid blank lines. (The @samp{>} is the shell's ``secondary prompt.''
16393 This is what the shell prints when it notices you haven't finished
16394 typing in all of a command.)
16396 We now have data consisting of one word per line, no punctuation, all one
16397 case. We're ready to count each word:
16400 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16401 > tr -s ' ' '\n' | sort | uniq -c | ...
16404 At this point, the data might look something like this:
16417 The output is sorted by word, not by count! What we want is the most
16418 frequently used words first. Fortunately, this is easy to accomplish,
16419 with the help of two more @command{sort} options:
16423 do a numeric sort, not a textual one
16426 reverse the order of the sort
16429 The final pipeline looks like this:
16432 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16433 > tr -s ' ' '\n' | sort | uniq -c | sort -n -r
16442 Whew! That's a lot to digest. Yet, the same principles apply. With six
16443 commands, on two lines (really one long one split for convenience), we've
16444 created a program that does something interesting and useful, in much
16445 less time than we could have written a C program to do the same thing.
16447 A minor modification to the above pipeline can give us a simple spelling
16448 checker! To determine if you've spelled a word correctly, all you have to
16449 do is look it up in a dictionary. If it is not there, then chances are
16450 that your spelling is incorrect. So, we need a dictionary.
16451 The conventional location for a dictionary is @file{/usr/dict/words}.
16452 On my GNU/Linux system,@footnote{Redhat Linux 6.1, for the November 2000
16453 revision of this article.}
16454 this is a sorted, 45,402 word dictionary.
16456 Now, how to compare our file with the dictionary? As before, we generate
16457 a sorted list of words, one per line:
16460 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16461 > tr -s ' ' '\n' | sort -u | ...
16464 Now, all we need is a list of words that are @emph{not} in the
16465 dictionary. Here is where the @command{comm} command comes in.
16468 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16469 > tr -s ' ' '\n' | sort -u |
16470 > comm -23 - /usr/dict/words
16473 The @option{-2} and @option{-3} options eliminate lines that are only in the
16474 dictionary (the second file), and lines that are in both files. Lines
16475 only in the first file (standard input, our stream of words), are
16476 words that are not in the dictionary. These are likely candidates for
16477 spelling errors. This pipeline was the first cut at a production
16478 spelling checker on Unix.
16480 There are some other tools that deserve brief mention.
16484 search files for text that matches a regular expression
16487 count lines, words, characters
16490 a T-fitting for data pipes, copies data to files and to standard output
16493 the stream editor, an advanced tool
16496 a data manipulation language, another advanced tool
16499 The software tools philosophy also espoused the following bit of
16500 advice: ``Let someone else do the hard part.'' This means, take
16501 something that gives you most of what you need, and then massage it the
16502 rest of the way until it's in the form that you want.
16508 Each program should do one thing well. No more, no less.
16511 Combining programs with appropriate plumbing leads to results where
16512 the whole is greater than the sum of the parts. It also leads to novel
16513 uses of programs that the authors might never have imagined.
16516 Programs should never print extraneous header or trailer data, since these
16517 could get sent on down a pipeline. (A point we didn't mention earlier.)
16520 Let someone else do the hard part.
16523 Know your toolbox! Use each program appropriately. If you don't have an
16524 appropriate tool, build one.
16527 As of this writing, all the programs we've discussed are available via
16528 anonymous @command{ftp} from: @*
16529 @uref{ftp://gnudist.gnu.org/textutils/textutils-1.22.tar.gz}. (There may
16530 be more recent versions available now.)
16532 None of what I have presented in this column is new. The Software Tools
16533 philosophy was first introduced in the book @cite{Software Tools}, by
16534 Brian Kernighan and P.J. Plauger (Addison-Wesley, ISBN 0-201-03669-X).
16535 This book showed how to write and use software tools. It was written in
16536 1976, using a preprocessor for FORTRAN named @command{ratfor} (RATional
16537 FORtran). At the time, C was not as ubiquitous as it is now; FORTRAN
16538 was. The last chapter presented a @command{ratfor} to FORTRAN
16539 processor, written in @command{ratfor}. @command{ratfor} looks an awful
16540 lot like C; if you know C, you won't have any problem following the
16543 In 1981, the book was updated and made available as @cite{Software Tools
16544 in Pascal} (Addison-Wesley, ISBN 0-201-10342-7). Both books are
16545 still in print and are well worth
16546 reading if you're a programmer. They certainly made a major change in
16547 how I view programming.
16549 The programs in both books are available from
16550 @uref{http://cm.bell-labs.com/who/bwk, Brian Kernighan's home page}.
16551 For a number of years, there was an active
16552 Software Tools Users Group, whose members had ported the original
16553 @command{ratfor} programs to essentially every computer system with a
16554 FORTRAN compiler. The popularity of the group waned in the middle 1980s
16555 as Unix began to spread beyond universities.
16557 With the current proliferation of GNU code and other clones of Unix programs,
16558 these programs now receive little attention; modern C versions are
16559 much more efficient and do more than these programs do. Nevertheless, as
16560 exposition of good programming style, and evangelism for a still-valuable
16561 philosophy, these books are unparalleled, and I recommend them highly.
16563 Acknowledgment: I would like to express my gratitude to Brian Kernighan
16564 of Bell Labs, the original Software Toolsmith, for reviewing this column.
16566 @node GNU Free Documentation License
16567 @appendix GNU Free Documentation License
16571 @node Concept index
16578 @c Local variables:
16579 @c texinfo-column-for-description: 32