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 fixed-size 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, 2000-2010 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 * Signal specifications:: Specifying signals
221 * Disambiguating names and IDs:: chgrp and chown owner and group syntax
222 * Random sources:: Sources of random data
223 * Target directory:: Target directory
224 * Trailing slashes:: Trailing slashes
225 * Traversing symlinks:: Traversing symlinks to directories
226 * Treating / specially:: Treating / specially
227 * Standards conformance:: Standards conformance
229 Output of entire files
231 * cat invocation:: Concatenate and write files
232 * tac invocation:: Concatenate and write files in reverse
233 * nl invocation:: Number lines and write files
234 * od invocation:: Write files in octal or other formats
235 * base64 invocation:: Transform data into printable data
237 Formatting file contents
239 * fmt invocation:: Reformat paragraph text
240 * pr invocation:: Paginate or columnate files for printing
241 * fold invocation:: Wrap input lines to fit in specified width
243 Output of parts of files
245 * head invocation:: Output the first part of files
246 * tail invocation:: Output the last part of files
247 * split invocation:: Split a file into fixed-size pieces
248 * csplit invocation:: Split a file into context-determined pieces
252 * wc invocation:: Print newline, word, and byte counts
253 * sum invocation:: Print checksum and block counts
254 * cksum invocation:: Print CRC checksum and byte counts
255 * md5sum invocation:: Print or check MD5 digests
256 * sha1sum invocation:: Print or check SHA-1 digests
257 * sha2 utilities:: Print or check SHA-2 digests
259 Operating on sorted files
261 * sort invocation:: Sort text files
262 * shuf invocation:: Shuffle text files
263 * uniq invocation:: Uniquify files
264 * comm invocation:: Compare two sorted files line by line
265 * ptx invocation:: Produce a permuted index of file contents
266 * tsort invocation:: Topological sort
268 @command{ptx}: Produce permuted indexes
270 * General options in ptx:: Options which affect general program behavior
271 * Charset selection in ptx:: Underlying character set considerations
272 * Input processing in ptx:: Input fields, contexts, and keyword selection
273 * Output formatting in ptx:: Types of output format, and sizing the fields
274 * Compatibility in ptx:: The @acronym{GNU} extensions to @command{ptx}
278 * cut invocation:: Print selected parts of lines
279 * paste invocation:: Merge lines of files
280 * join invocation:: Join lines on a common field
282 Operating on characters
284 * tr invocation:: Translate, squeeze, and/or delete characters
285 * expand invocation:: Convert tabs to spaces
286 * unexpand invocation:: Convert spaces to tabs
288 @command{tr}: Translate, squeeze, and/or delete characters
290 * Character sets:: Specifying sets of characters
291 * Translating:: Changing one set of characters to another
292 * Squeezing:: Squeezing repeats and deleting
296 * ls invocation:: List directory contents
297 * dir invocation:: Briefly list directory contents
298 * vdir invocation:: Verbosely list directory contents
299 * dircolors invocation:: Color setup for @command{ls}
301 @command{ls}: List directory contents
303 * Which files are listed:: Which files are listed
304 * What information is listed:: What information is listed
305 * Sorting the output:: Sorting the output
306 * Details about version sort:: More details about version sort
307 * General output formatting:: General output formatting
308 * Formatting the file names:: Formatting the file names
312 * cp invocation:: Copy files and directories
313 * dd invocation:: Convert and copy a file
314 * install invocation:: Copy files and set attributes
315 * mv invocation:: Move (rename) files
316 * rm invocation:: Remove files or directories
317 * shred invocation:: Remove files more securely
321 * link invocation:: Make a hard link via the link syscall
322 * ln invocation:: Make links between files
323 * mkdir invocation:: Make directories
324 * mkfifo invocation:: Make FIFOs (named pipes)
325 * mknod invocation:: Make block or character special files
326 * readlink invocation:: Print value of a symlink or canonical file name
327 * rmdir invocation:: Remove empty directories
328 * unlink invocation:: Remove files via unlink syscall
330 Changing file attributes
332 * chown invocation:: Change file owner and group
333 * chgrp invocation:: Change group ownership
334 * chmod invocation:: Change access permissions
335 * touch invocation:: Change file timestamps
339 * df invocation:: Report file system disk space usage
340 * du invocation:: Estimate file space usage
341 * stat invocation:: Report file or file system status
342 * sync invocation:: Synchronize data on disk with memory
343 * truncate invocation:: Shrink or extend the size of a file
347 * echo invocation:: Print a line of text
348 * printf invocation:: Format and print data
349 * yes invocation:: Print a string until interrupted
353 * false invocation:: Do nothing, unsuccessfully
354 * true invocation:: Do nothing, successfully
355 * test invocation:: Check file types and compare values
356 * expr invocation:: Evaluate expressions
358 @command{test}: Check file types and compare values
360 * File type tests:: File type tests
361 * Access permission tests:: Access permission tests
362 * File characteristic tests:: File characteristic tests
363 * String tests:: String tests
364 * Numeric tests:: Numeric tests
366 @command{expr}: Evaluate expression
368 * String expressions:: + : match substr index length
369 * Numeric expressions:: + - * / %
370 * Relations for expr:: | & < <= = == != >= >
371 * Examples of expr:: Examples of using @command{expr}
375 * tee invocation:: Redirect output to multiple files or processes
377 File name manipulation
379 * basename invocation:: Strip directory and suffix from a file name
380 * dirname invocation:: Strip last file name component
381 * pathchk invocation:: Check file name validity and portability
382 * mktemp invocation:: Create temporary file or directory
386 * pwd invocation:: Print working directory
387 * stty invocation:: Print or change terminal characteristics
388 * printenv invocation:: Print all or some environment variables
389 * tty invocation:: Print file name of terminal on standard input
391 @command{stty}: Print or change terminal characteristics
393 * Control:: Control settings
394 * Input:: Input settings
395 * Output:: Output settings
396 * Local:: Local settings
397 * Combination:: Combination settings
398 * Characters:: Special characters
399 * Special:: Special settings
403 * id invocation:: Print user identity
404 * logname invocation:: Print current login name
405 * whoami invocation:: Print effective user ID
406 * groups invocation:: Print group names a user is in
407 * users invocation:: Print login names of users currently logged in
408 * who invocation:: Print who is currently logged in
412 * arch invocation:: Print machine hardware name
413 * date invocation:: Print or set system date and time
414 * nproc invocation:: Print the number of processors
415 * uname invocation:: Print system information
416 * hostname invocation:: Print or set system name
417 * hostid invocation:: Print numeric host identifier
418 * uptime invocation:: Print system uptime and load
420 @command{date}: Print or set system date and time
422 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
423 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
424 * Literal conversion specifiers:: %[%nt]
425 * Padding and other flags:: Pad with zeros, spaces, etc.
426 * Setting the time:: Changing the system clock
427 * Options for date:: Instead of the current time
428 * Date input formats:: Specifying date strings
429 * Examples of date:: Examples
433 * chcon invocation:: Change SELinux context of file
434 * runcon invocation:: Run a command in specified SELinux context
436 Modified command invocation
438 * chroot invocation:: Run a command with a different root directory
439 * env invocation:: Run a command in a modified environment
440 * nice invocation:: Run a command with modified niceness
441 * nohup invocation:: Run a command immune to hangups
442 * stdbuf invocation:: Run a command with modified I/O buffering
443 * su invocation:: Run a command with substitute user and group ID
444 * timeout invocation:: Run a command with a time limit
448 * kill invocation:: Sending a signal to processes.
452 * sleep invocation:: Delay for a specified time
456 * factor invocation:: Print prime factors
457 * seq invocation:: Print numeric sequences
461 * Mode Structure:: Structure of file mode bits
462 * Symbolic Modes:: Mnemonic representation of file mode bits
463 * Numeric Modes:: File mode bits as octal numbers
464 * Directory Setuid and Setgid:: Set-user-ID and set-group-ID on directories
468 * General date syntax:: Common rules
469 * Calendar date items:: 19 Dec 1994
470 * Time of day items:: 9:20pm
471 * Time zone items:: @sc{est}, @sc{pdt}, @sc{gmt}
472 * Day of week items:: Monday and others
473 * Relative items in date strings:: next tuesday, 2 years ago
474 * Pure numbers in date strings:: 19931219, 1440
475 * Seconds since the Epoch:: @@1078100502
476 * Specifying time zone rules:: TZ="America/New_York", TZ="UTC0"
477 * Authors of parse_datetime:: Bellovin, Eggert, Salz, Berets, et al
479 Opening the software toolbox
481 * Toolbox introduction:: Toolbox introduction
482 * I/O redirection:: I/O redirection
483 * The who command:: The @command{who} command
484 * The cut command:: The @command{cut} command
485 * The sort command:: The @command{sort} command
486 * The uniq command:: The @command{uniq} command
487 * Putting the tools together:: Putting the tools together
491 * GNU Free Documentation License:: Copying and sharing this manual
498 @chapter Introduction
500 This manual is a work in progress: many sections make no attempt to explain
501 basic concepts in a way suitable for novices. Thus, if you are interested,
502 please get involved in improving this manual. The entire @sc{gnu} community
505 @cindex @acronym{POSIX}
506 The @sc{gnu} utilities documented here are mostly compatible with the
507 @acronym{POSIX} standard.
508 @cindex bugs, reporting
509 Please report bugs to @email{bug-coreutils@@gnu.org}. Remember
510 to include the version number, machine architecture, input files, and
511 any other information needed to reproduce the bug: your input, what you
512 expected, what you got, and why it is wrong. Diffs are welcome, but
513 please include a description of the problem as well, since this is
514 sometimes difficult to infer. @xref{Bugs, , , gcc, Using and Porting GNU CC}.
520 @cindex MacKenzie, D.
523 This manual was originally derived from the Unix man pages in the
524 distributions, which were written by David MacKenzie and updated by Jim
525 Meyering. What you are reading now is the authoritative documentation
526 for these utilities; the man pages are no longer being maintained. The
527 original @command{fmt} man page was written by Ross Paterson. Fran@,{c}ois
528 Pinard did the initial conversion to Texinfo format. Karl Berry did the
529 indexing, some reorganization, and editing of the results. Brian
530 Youmans of the Free Software Foundation office staff combined the
531 manuals for textutils, fileutils, and sh-utils to produce the present
532 omnibus manual. Richard Stallman contributed his usual invaluable
533 insights to the overall process.
536 @chapter Common options
540 @itemx @w{@kbd{--backup}[=@var{method}]}
543 @vindex VERSION_CONTROL
544 @cindex backups, making
545 @xref{Backup options}.
546 Make a backup of each file that would otherwise be overwritten or removed.
549 @macro optBackupSuffix
550 @item -S @var{suffix}
551 @itemx --suffix=@var{suffix}
554 Append @var{suffix} to each backup file made with @option{-b}.
555 @xref{Backup options}.
558 @macro optTargetDirectory
559 @item -t @var{directory}
560 @itemx @w{@kbd{--target-directory}=@var{directory}}
562 @opindex --target-directory
563 @cindex target directory
564 @cindex destination directory
565 Specify the destination @var{directory}.
566 @xref{Target directory}.
569 @macro optNoTargetDirectory
571 @itemx --no-target-directory
573 @opindex --no-target-directory
574 @cindex target directory
575 @cindex destination directory
576 Do not treat the last operand specially when it is a directory or a
577 symbolic link to a directory. @xref{Target directory}.
585 @cindex output @sc{nul}-byte-terminated lines
586 Output a zero byte (@acronym{ASCII} @sc{nul}) at the end of each line,
587 rather than a newline. This option enables other programs to parse the
588 output of @command{\cmd\} even when that output would contain data
589 with embedded newlines.
596 Append an SI-style abbreviation to each size, such as @samp{M} for
597 megabytes. Powers of 1000 are used, not 1024; @samp{M} stands for
598 1,000,000 bytes. This option is equivalent to
599 @option{--block-size=si}. Use the @option{-h} or
600 @option{--human-readable} option if
601 you prefer powers of 1024.
604 @macro optHumanReadable
606 @itemx --human-readable
608 @opindex --human-readable
609 @cindex human-readable output
610 Append a size letter to each size, such as @samp{M} for mebibytes.
611 Powers of 1024 are used, not 1000; @samp{M} stands for 1,048,576 bytes.
612 This option is equivalent to @option{--block-size=human-readable}.
613 Use the @option{--si} option if you prefer powers of 1000.
616 @macro optStripTrailingSlashes
617 @itemx @w{@kbd{--strip-trailing-slashes}}
618 @opindex --strip-trailing-slashes
619 @cindex stripping trailing slashes
620 Remove any trailing slashes from each @var{source} argument.
621 @xref{Trailing slashes}.
624 @macro mayConflictWithShellBuiltIn{cmd}
625 @cindex conflicts with shell built-ins
626 @cindex built-in shell commands, conflicts with
627 Due to shell aliases and built-in @command{\cmd\} command, using an
628 unadorned @command{\cmd\} interactively or in a script may get you
629 different functionality than that described here. Invoke it via
630 @command{env} (i.e., @code{env \cmd\ @dots{}}) to avoid interference
635 @macro multiplierSuffixes{varName}
636 @var{\varName\} may be, or may be an integer optionally followed by,
637 one of the following multiplicative suffixes:
639 @samp{b} => 512 ("blocks")
640 @samp{KB} => 1000 (KiloBytes)
641 @samp{K} => 1024 (KibiBytes)
642 @samp{MB} => 1000*1000 (MegaBytes)
643 @samp{M} => 1024*1024 (MebiBytes)
644 @samp{GB} => 1000*1000*1000 (GigaBytes)
645 @samp{G} => 1024*1024*1024 (GibiBytes)
647 and so on for @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}.
650 @c FIXME: same as above, but no ``blocks'' line.
651 @macro multiplierSuffixesNoBlocks{varName}
652 @var{\varName\} may be, or may be an integer optionally followed by,
653 one of the following multiplicative suffixes:
655 @samp{KB} => 1000 (KiloBytes)
656 @samp{K} => 1024 (KibiBytes)
657 @samp{MB} => 1000*1000 (MegaBytes)
658 @samp{M} => 1024*1024 (MebiBytes)
659 @samp{GB} => 1000*1000*1000 (GigaBytes)
660 @samp{G} => 1024*1024*1024 (GibiBytes)
662 and so on for @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}.
665 @cindex common options
667 Certain options are available in all of these programs. Rather than
668 writing identical descriptions for each of the programs, they are
669 described here. (In fact, every @sc{gnu} program accepts (or should accept)
672 @vindex POSIXLY_CORRECT
673 Normally options and operands can appear in any order, and programs act
674 as if all the options appear before any operands. For example,
675 @samp{sort -r passwd -t :} acts like @samp{sort -r -t : passwd}, since
676 @samp{:} is an option-argument of @option{-t}. However, if the
677 @env{POSIXLY_CORRECT} environment variable is set, options must appear
678 before operands, unless otherwise specified for a particular command.
680 A few programs can usefully have trailing operands with leading
681 @samp{-}. With such a program, options must precede operands even if
682 @env{POSIXLY_CORRECT} is not set, and this fact is noted in the
683 program description. For example, the @command{env} command's options
684 must appear before its operands, since in some cases the operands
685 specify a command that itself contains options.
687 Most programs that accept long options recognize unambiguous
688 abbreviations of those options. For example, @samp{rmdir
689 --ignore-fail-on-non-empty} can be invoked as @samp{rmdir
690 --ignore-fail} or even @samp{rmdir --i}. Ambiguous options, such as
691 @samp{ls --h}, are identified as such.
693 Some of these programs recognize the @option{--help} and @option{--version}
694 options only when one of them is the sole command line argument. For
695 these programs, abbreviations of the long options are not always recognized.
702 Print a usage message listing all available options, then exit successfully.
706 @cindex version number, finding
707 Print the version number, then exit successfully.
711 @cindex option delimiter
712 Delimit the option list. Later arguments, if any, are treated as
713 operands even if they begin with @samp{-}. For example, @samp{sort --
714 -r} reads from the file named @file{-r}.
718 @cindex standard input
719 @cindex standard output
720 A single @samp{-} operand is not really an option, though it looks like one. It
721 stands for standard input, or for standard output if that is clear from
722 the context. For example, @samp{sort -} reads from standard input,
723 and is equivalent to plain @samp{sort}, and @samp{tee -} writes an
724 extra copy of its input to standard output. Unless otherwise
725 specified, @samp{-} can appear as any operand that requires a file
729 * Exit status:: Indicating program success or failure.
730 * Backup options:: -b -S, in some programs.
731 * Block size:: BLOCK_SIZE and --block-size, in some programs.
732 * Signal specifications:: Specifying signals using the --signal option.
733 * Disambiguating names and IDs:: chgrp and chown owner and group syntax
734 * Random sources:: --random-source, in some programs.
735 * Target directory:: Specifying a target directory, in some programs.
736 * Trailing slashes:: --strip-trailing-slashes, in some programs.
737 * Traversing symlinks:: -H, -L, or -P, in some programs.
738 * Treating / specially:: --preserve-root and --no-preserve-root.
739 * Special built-in utilities:: @command{break}, @command{:}, @command{eval}, @dots{}
740 * Standards conformance:: Conformance to the @acronym{POSIX} standard.
748 An exit status of zero indicates success,
749 and a nonzero value indicates failure.
752 Nearly every command invocation yields an integral @dfn{exit status}
753 that can be used to change how other commands work.
754 For the vast majority of commands, an exit status of zero indicates
755 success. Failure is indicated by a nonzero value---typically
756 @samp{1}, though it may differ on unusual platforms as @acronym{POSIX}
757 requires only that it be nonzero.
759 However, some of the programs documented here do produce
760 other exit status values and a few associate different
761 meanings with the values @samp{0} and @samp{1}.
762 Here are some of the exceptions:
763 @command{chroot}, @command{env}, @command{expr}, @command{nice},
764 @command{nohup}, @command{printenv}, @command{sort}, @command{stdbuf},
765 @command{su}, @command{test}, @command{timeout}, @command{tty}.
769 @section Backup options
771 @cindex backup options
773 Some @sc{gnu} programs (at least @command{cp}, @command{install},
774 @command{ln}, and @command{mv}) optionally make backups of files
775 before writing new versions.
776 These options control the details of these backups. The options are also
777 briefly mentioned in the descriptions of the particular programs.
782 @itemx @w{@kbd{--backup}[=@var{method}]}
785 @vindex VERSION_CONTROL
786 @cindex backups, making
787 Make a backup of each file that would otherwise be overwritten or removed.
788 Without this option, the original versions are destroyed.
789 Use @var{method} to determine the type of backups to make.
790 When this option is used but @var{method} is not specified,
791 then the value of the @env{VERSION_CONTROL}
792 environment variable is used. And if @env{VERSION_CONTROL} is not set,
793 the default backup type is @samp{existing}.
795 Note that the short form of this option, @option{-b} does not accept any
796 argument. Using @option{-b} is equivalent to using @option{--backup=existing}.
798 @vindex version-control @r{Emacs variable}
799 This option corresponds to the Emacs variable @samp{version-control};
800 the values for @var{method} are the same as those used in Emacs.
801 This option also accepts more descriptive names.
802 The valid @var{method}s are (unique abbreviations are accepted):
807 @opindex none @r{backup method}
812 @opindex numbered @r{backup method}
813 Always make numbered backups.
817 @opindex existing @r{backup method}
818 Make numbered backups of files that already have them, simple backups
823 @opindex simple @r{backup method}
824 Always make simple backups. Please note @samp{never} is not to be
825 confused with @samp{none}.
829 @item -S @var{suffix}
830 @itemx --suffix=@var{suffix}
833 @cindex backup suffix
834 @vindex SIMPLE_BACKUP_SUFFIX
835 Append @var{suffix} to each backup file made with @option{-b}. If this
836 option is not specified, the value of the @env{SIMPLE_BACKUP_SUFFIX}
837 environment variable is used. And if @env{SIMPLE_BACKUP_SUFFIX} is not
838 set, the default is @samp{~}, just as in Emacs.
847 Some @sc{gnu} programs (at least @command{df}, @command{du}, and
848 @command{ls}) display sizes in ``blocks''. You can adjust the block size
849 and method of display to make sizes easier to read. The block size
850 used for display is independent of any file system block size.
851 Fractional block counts are rounded up to the nearest integer.
853 @opindex --block-size=@var{size}
856 @vindex DF_BLOCK_SIZE
857 @vindex DU_BLOCK_SIZE
858 @vindex LS_BLOCK_SIZE
859 @vindex POSIXLY_CORRECT@r{, and block size}
861 The default block size is chosen by examining the following environment
862 variables in turn; the first one that is set determines the block size.
867 This specifies the default block size for the @command{df} command.
868 Similarly, @env{DU_BLOCK_SIZE} specifies the default for @command{du} and
869 @env{LS_BLOCK_SIZE} for @command{ls}.
872 This specifies the default block size for all three commands, if the
873 above command-specific environment variables are not set.
876 This specifies the default block size for all values that are normally
877 printed as blocks, if neither @env{BLOCK_SIZE} nor the above
878 command-specific environment variables are set. Unlike the other
879 environment variables, @env{BLOCKSIZE} does not affect values that are
880 normally printed as byte counts, e.g., the file sizes contained in
883 @item POSIXLY_CORRECT
884 If neither @env{@var{command}_BLOCK_SIZE}, nor @env{BLOCK_SIZE}, nor
885 @env{BLOCKSIZE} is set, but this variable is set, the block size
890 If none of the above environment variables are set, the block size
891 currently defaults to 1024 bytes in most contexts, but this number may
892 change in the future. For @command{ls} file sizes, the block size
895 @cindex human-readable output
898 A block size specification can be a positive integer specifying the number
899 of bytes per block, or it can be @code{human-readable} or @code{si} to
900 select a human-readable format. Integers may be followed by suffixes
901 that are upward compatible with the
902 @uref{http://www.bipm.fr/enus/3_SI/si-prefixes.html, SI prefixes}
903 for decimal multiples and with the
904 @uref{http://physics.nist.gov/cuu/Units/binary.html, IEC 60027-2
905 prefixes for binary multiples}.
907 With human-readable formats, output sizes are followed by a size letter
908 such as @samp{M} for megabytes. @code{BLOCK_SIZE=human-readable} uses
909 powers of 1024; @samp{M} stands for 1,048,576 bytes.
910 @code{BLOCK_SIZE=si} is similar, but uses powers of 1000 and appends
911 @samp{B}; @samp{MB} stands for 1,000,000 bytes.
914 A block size specification preceded by @samp{'} causes output sizes to
915 be displayed with thousands separators. The @env{LC_NUMERIC} locale
916 specifies the thousands separator and grouping. For example, in an
917 American English locale, @samp{--block-size="'1kB"} would cause a size
918 of 1234000 bytes to be displayed as @samp{1,234}. In the default C
919 locale, there is no thousands separator so a leading @samp{'} has no
922 An integer block size can be followed by a suffix to specify a
923 multiple of that size. A bare size letter,
924 or one followed by @samp{iB}, specifies
925 a multiple using powers of 1024. A size letter followed by @samp{B}
926 specifies powers of 1000 instead. For example, @samp{1M} and
927 @samp{1MiB} are equivalent to @samp{1048576}, whereas @samp{1MB} is
928 equivalent to @samp{1000000}.
930 A plain suffix without a preceding integer acts as if @samp{1} were
931 prepended, except that it causes a size indication to be appended to
932 the output. For example, @samp{--block-size="kB"} displays 3000 as
935 The following suffixes are defined. Large sizes like @code{1Y}
936 may be rejected by your computer due to limitations of its arithmetic.
940 @cindex kilobyte, definition of
941 kilobyte: @math{10^3 = 1000}.
945 @cindex kibibyte, definition of
946 kibibyte: @math{2^{10} = 1024}. @samp{K} is special: the SI prefix is
947 @samp{k} and the IEC 60027-2 prefix is @samp{Ki}, but tradition and
948 @acronym{POSIX} use @samp{k} to mean @samp{KiB}.
950 @cindex megabyte, definition of
951 megabyte: @math{10^6 = 1,000,000}.
954 @cindex mebibyte, definition of
955 mebibyte: @math{2^{20} = 1,048,576}.
957 @cindex gigabyte, definition of
958 gigabyte: @math{10^9 = 1,000,000,000}.
961 @cindex gibibyte, definition of
962 gibibyte: @math{2^{30} = 1,073,741,824}.
964 @cindex terabyte, definition of
965 terabyte: @math{10^{12} = 1,000,000,000,000}.
968 @cindex tebibyte, definition of
969 tebibyte: @math{2^{40} = 1,099,511,627,776}.
971 @cindex petabyte, definition of
972 petabyte: @math{10^{15} = 1,000,000,000,000,000}.
975 @cindex pebibyte, definition of
976 pebibyte: @math{2^{50} = 1,125,899,906,842,624}.
978 @cindex exabyte, definition of
979 exabyte: @math{10^{18} = 1,000,000,000,000,000,000}.
982 @cindex exbibyte, definition of
983 exbibyte: @math{2^{60} = 1,152,921,504,606,846,976}.
985 @cindex zettabyte, definition of
986 zettabyte: @math{10^{21} = 1,000,000,000,000,000,000,000}
989 @math{2^{70} = 1,180,591,620,717,411,303,424}.
990 (@samp{Zi} is a @acronym{GNU} extension to IEC 60027-2.)
992 @cindex yottabyte, definition of
993 yottabyte: @math{10^{24} = 1,000,000,000,000,000,000,000,000}.
996 @math{2^{80} = 1,208,925,819,614,629,174,706,176}.
997 (@samp{Yi} is a @acronym{GNU} extension to IEC 60027-2.)
1002 @opindex --block-size
1003 @opindex --human-readable
1006 Block size defaults can be overridden by an explicit
1007 @option{--block-size=@var{size}} option. The @option{-k}
1008 option is equivalent to @option{--block-size=1K}, which
1009 is the default unless the @env{POSIXLY_CORRECT} environment variable is
1010 set. The @option{-h} or @option{--human-readable} option is equivalent to
1011 @option{--block-size=human-readable}. The @option{--si} option is
1012 equivalent to @option{--block-size=si}.
1014 @node Signal specifications
1015 @section Signal specifications
1016 @cindex signals, specifying
1018 A @var{signal} may be a signal name like @samp{HUP}, or a signal
1019 number like @samp{1}, or an exit status of a process terminated by the
1020 signal. A signal name can be given in canonical form or prefixed by
1021 @samp{SIG}. The case of the letters is ignored. The following signal names
1022 and numbers are supported on all @acronym{POSIX} compliant systems:
1028 2. Terminal interrupt.
1034 9. Kill (cannot be caught or ignored).
1042 Other supported signal names have system-dependent corresponding
1043 numbers. All systems conforming to @acronym{POSIX} 1003.1-2001 also
1044 support the following signals:
1048 Access to an undefined portion of a memory object.
1050 Child process terminated, stopped, or continued.
1052 Continue executing, if stopped.
1054 Erroneous arithmetic operation.
1056 Illegal Instruction.
1058 Write on a pipe with no one to read it.
1060 Invalid memory reference.
1062 Stop executing (cannot be caught or ignored).
1066 Background process attempting read.
1068 Background process attempting write.
1070 High bandwidth data is available at a socket.
1072 User-defined signal 1.
1074 User-defined signal 2.
1078 @acronym{POSIX} 1003.1-2001 systems that support the @acronym{XSI} extension
1079 also support the following signals:
1085 Profiling timer expired.
1089 Trace/breakpoint trap.
1091 Virtual timer expired.
1093 CPU time limit exceeded.
1095 File size limit exceeded.
1099 @acronym{POSIX} 1003.1-2001 systems that support the @acronym{XRT} extension
1100 also support at least eight real-time signals called @samp{RTMIN},
1101 @samp{RTMIN+1}, @dots{}, @samp{RTMAX-1}, @samp{RTMAX}.
1103 @node Disambiguating names and IDs
1104 @section chown and chgrp: Disambiguating user names and IDs
1105 @cindex user names, disambiguating
1106 @cindex user IDs, disambiguating
1107 @cindex group names, disambiguating
1108 @cindex group IDs, disambiguating
1109 @cindex disambiguating group names and IDs
1111 Since the @var{owner} and @var{group} arguments to @command{chown} and
1112 @command{chgrp} may be specified as names or numeric IDs, there is an
1114 What if a user or group @emph{name} is a string of digits?
1115 @footnote{Using a number as a user name is common in some environments.}
1116 Should the command interpret it as a user name or as an ID?
1117 @acronym{POSIX} requires that @command{chown} and @command{chgrp}
1118 first attempt to resolve the specified string as a name, and
1119 only once that fails, then try to interpret it as an ID.
1120 This is troublesome when you want to specify a numeric ID, say 42,
1121 and it must work even in a pathological situation where
1122 @samp{42} is a user name that maps to some other user ID, say 1000.
1123 Simply invoking @code{chown 42 F}, will set @file{F}s owner ID to
1124 1000---not what you intended.
1126 GNU @command{chown} and @command{chgrp} provide a way to work around this,
1127 that at the same time may result in a significant performance improvement
1128 by eliminating a database look-up.
1129 Simply precede each numeric user ID and/or group ID with a @samp{+},
1130 in order to force its interpretation as an integer:
1134 chgrp +$numeric_group_id another-file
1138 GNU @command{chown} and @command{chgrp}
1139 skip the name look-up process for each @samp{+}-prefixed string,
1140 because a string containing @samp{+} is never a valid user or group name.
1141 This syntax is accepted on most common Unix systems, but not on Solaris 10.
1143 @node Random sources
1144 @section Sources of random data
1146 @cindex random sources
1148 The @command{shuf}, @command{shred}, and @command{sort} commands
1149 sometimes need random data to do their work. For example, @samp{sort
1150 -R} must choose a hash function at random, and it needs random data to
1151 make this selection.
1153 By default these commands use an internal pseudorandom generator
1154 initialized by a small amount of entropy, but can be directed to use
1155 an external source with the @option{--random-source=@var{file}} option.
1156 An error is reported if @var{file} does not contain enough bytes.
1158 For example, the device file @file{/dev/urandom} could be used as the
1159 source of random data. Typically, this device gathers environmental
1160 noise from device drivers and other sources into an entropy pool, and
1161 uses the pool to generate random bits. If the pool is short of data,
1162 the device reuses the internal pool to produce more bits, using a
1163 cryptographically secure pseudorandom number generator. But be aware
1164 that this device is not designed for bulk random data generation
1165 and is relatively slow.
1167 @file{/dev/urandom} suffices for most practical uses, but applications
1168 requiring high-value or long-term protection of private data may
1169 require an alternate data source like @file{/dev/random} or
1170 @file{/dev/arandom}. The set of available sources depends on your
1173 To reproduce the results of an earlier invocation of a command, you
1174 can save some random data into a file and then use that file as the
1175 random source in earlier and later invocations of the command.
1177 @node Target directory
1178 @section Target directory
1180 @cindex target directory
1182 The @command{cp}, @command{install}, @command{ln}, and @command{mv}
1183 commands normally treat the last operand specially when it is a
1184 directory or a symbolic link to a directory. For example, @samp{cp
1185 source dest} is equivalent to @samp{cp source dest/source} if
1186 @file{dest} is a directory. Sometimes this behavior is not exactly
1187 what is wanted, so these commands support the following options to
1188 allow more fine-grained control:
1193 @itemx --no-target-directory
1194 @opindex --no-target-directory
1195 @cindex target directory
1196 @cindex destination directory
1197 Do not treat the last operand specially when it is a directory or a
1198 symbolic link to a directory. This can help avoid race conditions in
1199 programs that operate in a shared area. For example, when the command
1200 @samp{mv /tmp/source /tmp/dest} succeeds, there is no guarantee that
1201 @file{/tmp/source} was renamed to @file{/tmp/dest}: it could have been
1202 renamed to @file{/tmp/dest/source} instead, if some other process
1203 created @file{/tmp/dest} as a directory. However, if @file{mv
1204 -T /tmp/source /tmp/dest} succeeds, there is no
1205 question that @file{/tmp/source} was renamed to @file{/tmp/dest}.
1207 In the opposite situation, where you want the last operand to be
1208 treated as a directory and want a diagnostic otherwise, you can use
1209 the @option{--target-directory} (@option{-t}) option.
1211 @item -t @var{directory}
1212 @itemx @w{@kbd{--target-directory}=@var{directory}}
1213 @opindex --target-directory
1214 @cindex target directory
1215 @cindex destination directory
1216 Use @var{directory} as the directory component of each destination
1219 The interface for most programs is that after processing options and a
1220 finite (possibly zero) number of fixed-position arguments, the remaining
1221 argument list is either expected to be empty, or is a list of items
1222 (usually files) that will all be handled identically. The @command{xargs}
1223 program is designed to work well with this convention.
1225 The commands in the @command{mv}-family are unusual in that they take
1226 a variable number of arguments with a special case at the @emph{end}
1227 (namely, the target directory). This makes it nontrivial to perform some
1228 operations, e.g., ``move all files from here to ../d/'', because
1229 @code{mv * ../d/} might exhaust the argument space, and @code{ls | xargs ...}
1230 doesn't have a clean way to specify an extra final argument for each
1231 invocation of the subject command. (It can be done by going through a
1232 shell command, but that requires more human labor and brain power than
1235 The @w{@kbd{--target-directory}} (@option{-t}) option allows the @command{cp},
1236 @command{install}, @command{ln}, and @command{mv} programs to be used
1237 conveniently with @command{xargs}. For example, you can move the files
1238 from the current directory to a sibling directory, @code{d} like this:
1241 ls | xargs mv -t ../d --
1244 However, this doesn't move files whose names begin with @samp{.}.
1245 If you use the @sc{gnu} @command{find} program, you can move those
1246 files too, with this command:
1249 find . -mindepth 1 -maxdepth 1 \
1253 But both of the above approaches fail if there are no files in the
1254 current directory, or if any file has a name containing a blank or
1255 some other special characters.
1256 The following example removes those limitations and requires both
1257 @sc{gnu} @command{find} and @sc{gnu} @command{xargs}:
1260 find . -mindepth 1 -maxdepth 1 -print0 \
1261 | xargs --null --no-run-if-empty \
1268 The @option{--target-directory} (@option{-t}) and
1269 @option{--no-target-directory} (@option{-T})
1270 options cannot be combined.
1272 @node Trailing slashes
1273 @section Trailing slashes
1275 @cindex trailing slashes
1277 Some @sc{gnu} programs (at least @command{cp} and @command{mv}) allow you to
1278 remove any trailing slashes from each @var{source} argument before
1279 operating on it. The @w{@kbd{--strip-trailing-slashes}} option enables
1282 This is useful when a @var{source} argument may have a trailing slash and
1283 @c FIXME: mv's behavior in this case is system-dependent
1284 specify a symbolic link to a directory. This scenario is in fact rather
1285 common because some shells can automatically append a trailing slash when
1286 performing file name completion on such symbolic links. Without this
1287 option, @command{mv}, for example, (via the system's rename function) must
1288 interpret a trailing slash as a request to dereference the symbolic link
1289 and so must rename the indirectly referenced @emph{directory} and not
1290 the symbolic link. Although it may seem surprising that such behavior
1291 be the default, it is required by @acronym{POSIX} and is consistent with
1292 other parts of that standard.
1294 @node Traversing symlinks
1295 @section Traversing symlinks
1297 @cindex symbolic link to directory, controlling traversal of
1299 The following options modify how @command{chown} and @command{chgrp}
1300 @c FIXME: note that `du' has these options, too, but they have slightly
1301 @c different meaning.
1302 traverse a hierarchy when the @option{--recursive} (@option{-R})
1303 option is also specified.
1304 If more than one of the following options is specified, only the final
1306 These options specify whether processing a symbolic link to a directory
1307 entails operating on just the symbolic link or on all files in the
1308 hierarchy rooted at that directory.
1310 These options are independent of @option{--dereference} and
1311 @option{--no-dereference} (@option{-h}), which control whether to modify
1312 a symlink or its referent.
1319 @cindex symbolic link to directory, traverse each that is specified on the command line
1320 If @option{--recursive} (@option{-R}) is specified and
1321 a command line argument is a symbolic link to a directory, traverse it.
1328 @cindex symbolic link to directory, traverse each that is encountered
1329 In a recursive traversal, traverse every symbolic link to a directory
1330 that is encountered.
1337 @cindex symbolic link to directory, never traverse
1338 Do not traverse any symbolic links.
1339 This is the default if none of @option{-H}, @option{-L},
1340 or @option{-P} is specified.
1347 @node Treating / specially
1348 @section Treating @file{/} specially
1350 Certain commands can operate destructively on entire hierarchies.
1351 For example, if a user with appropriate privileges mistakenly runs
1352 @samp{rm -rf / tmp/junk}, that may remove
1353 all files on the entire system. Since there are so few
1354 legitimate uses for such a command,
1355 @sc{gnu} @command{rm} normally declines to operate on any directory
1356 that resolves to @file{/}. If you really want to try to remove all
1357 the files on your system, you can use the @option{--no-preserve-root}
1358 option, but the default behavior, specified by the
1359 @option{--preserve-option}, is safer for most purposes.
1361 The commands @command{chgrp}, @command{chmod} and @command{chown}
1362 can also operate destructively on entire hierarchies, so they too
1363 support these options. Although, unlike @command{rm}, they don't
1364 actually unlink files, these commands are arguably more dangerous
1365 when operating recursively on @file{/}, since they often work much
1366 more quickly, and hence damage more files before an alert user can
1367 interrupt them. Tradition and @acronym{POSIX} require these commands
1368 to operate recursively on @file{/}, so they default to
1369 @option{--no-preserve-root}, but using the @option{--preserve-root}
1370 option makes them safer for most purposes. For convenience you can
1371 specify @option{--preserve-root} in an alias or in a shell function.
1373 Note that the @option{--preserve-root} option also ensures
1374 that @command{chgrp} and @command{chown} do not modify @file{/}
1375 even when dereferencing a symlink pointing to @file{/}.
1377 @node Special built-in utilities
1378 @section Special built-in utilities
1380 Some programs like @command{nice} can invoke other programs; for
1381 example, the command @samp{nice cat file} invokes the program
1382 @command{cat} by executing the command @samp{cat file}. However,
1383 @dfn{special built-in utilities} like @command{exit} cannot be invoked
1384 this way. For example, the command @samp{nice exit} does not have a
1385 well-defined behavior: it may generate an error message instead of
1388 Here is a list of the special built-in utilities that are standardized
1389 by @acronym{POSIX} 1003.1-2004.
1392 @t{.@: : break continue eval exec exit export readonly
1393 return set shift times trap unset}
1396 For example, because @samp{.}, @samp{:}, and @samp{exec} are special,
1397 the commands @samp{nice . foo.sh}, @samp{nice :}, and @samp{nice exec
1398 pwd} do not work as you might expect.
1400 Many shells extend this list. For example, Bash has several extra
1401 special built-in utilities like @command{history}, and
1402 @command{suspend}, and with Bash the command @samp{nice suspend}
1403 generates an error message instead of suspending.
1405 @node Standards conformance
1406 @section Standards conformance
1408 @vindex POSIXLY_CORRECT
1409 In a few cases, the @sc{gnu} utilities' default behavior is
1410 incompatible with the @acronym{POSIX} standard. To suppress these
1411 incompatibilities, define the @env{POSIXLY_CORRECT} environment
1412 variable. Unless you are checking for @acronym{POSIX} conformance, you
1413 probably do not need to define @env{POSIXLY_CORRECT}.
1415 Newer versions of @acronym{POSIX} are occasionally incompatible with older
1416 versions. For example, older versions of @acronym{POSIX} required the
1417 command @samp{sort +1} to sort based on the second and succeeding
1418 fields in each input line, but starting with @acronym{POSIX} 1003.1-2001
1419 the same command is required to sort the file named @file{+1}, and you
1420 must instead use the command @samp{sort -k 2} to get the field-based
1423 @vindex _POSIX2_VERSION
1424 The @sc{gnu} utilities normally conform to the version of @acronym{POSIX}
1425 that is standard for your system. To cause them to conform to a
1426 different version of @acronym{POSIX}, define the @env{_POSIX2_VERSION}
1427 environment variable to a value of the form @var{yyyymm} specifying
1428 the year and month the standard was adopted. Two values are currently
1429 supported for @env{_POSIX2_VERSION}: @samp{199209} stands for
1430 @acronym{POSIX} 1003.2-1992, and @samp{200112} stands for @acronym{POSIX}
1431 1003.1-2001. For example, if you have a newer system but are running software
1432 that assumes an older version of @acronym{POSIX} and uses @samp{sort +1}
1433 or @samp{tail +10}, you can work around any compatibility problems by setting
1434 @samp{_POSIX2_VERSION=199209} in your environment.
1436 @node Output of entire files
1437 @chapter Output of entire files
1439 @cindex output of entire files
1440 @cindex entire files, output of
1442 These commands read and write entire files, possibly transforming them
1446 * cat invocation:: Concatenate and write files.
1447 * tac invocation:: Concatenate and write files in reverse.
1448 * nl invocation:: Number lines and write files.
1449 * od invocation:: Write files in octal or other formats.
1450 * base64 invocation:: Transform data into printable data.
1453 @node cat invocation
1454 @section @command{cat}: Concatenate and write files
1457 @cindex concatenate and write files
1458 @cindex copying files
1460 @command{cat} copies each @var{file} (@samp{-} means standard input), or
1461 standard input if none are given, to standard output. Synopsis:
1464 cat [@var{option}] [@var{file}]@dots{}
1467 The program accepts the following options. Also see @ref{Common options}.
1475 Equivalent to @option{-vET}.
1478 @itemx --number-nonblank
1480 @opindex --number-nonblank
1481 Number all nonempty output lines, starting with 1.
1485 Equivalent to @option{-vE}.
1490 @opindex --show-ends
1491 Display a @samp{$} after the end of each line.
1497 Number all output lines, starting with 1. This option is ignored
1498 if @option{-b} is in effect.
1501 @itemx --squeeze-blank
1503 @opindex --squeeze-blank
1504 @cindex squeezing empty lines
1505 Suppress repeated adjacent empty lines; output just one empty line
1510 Equivalent to @option{-vT}.
1515 @opindex --show-tabs
1516 Display TAB characters as @samp{^I}.
1520 Ignored; for @acronym{POSIX} compatibility.
1523 @itemx --show-nonprinting
1525 @opindex --show-nonprinting
1526 Display control characters except for LFD and TAB using
1527 @samp{^} notation and precede characters that have the high bit set with
1532 On systems like MS-DOS that distinguish between text and binary files,
1533 @command{cat} normally reads and writes in binary mode. However,
1534 @command{cat} reads in text mode if one of the options
1535 @option{-bensAE} is used or if @command{cat} is reading from standard
1536 input and standard input is a terminal. Similarly, @command{cat}
1537 writes in text mode if one of the options @option{-bensAE} is used or
1538 if standard output is a terminal.
1545 # Output f's contents, then standard input, then g's contents.
1548 # Copy standard input to standard output.
1553 @node tac invocation
1554 @section @command{tac}: Concatenate and write files in reverse
1557 @cindex reversing files
1559 @command{tac} copies each @var{file} (@samp{-} means standard input), or
1560 standard input if none are given, to standard output, reversing the
1561 records (lines by default) in each separately. Synopsis:
1564 tac [@var{option}]@dots{} [@var{file}]@dots{}
1567 @dfn{Records} are separated by instances of a string (newline by
1568 default). By default, this separator string is attached to the end of
1569 the record that it follows in the file.
1571 The program accepts the following options. Also see @ref{Common options}.
1579 The separator is attached to the beginning of the record that it
1580 precedes in the file.
1586 Treat the separator string as a regular expression. Users of @command{tac}
1587 on MS-DOS/MS-Windows should note that, since @command{tac} reads files in
1588 binary mode, each line of a text file might end with a CR/LF pair
1589 instead of the Unix-style LF.
1591 @item -s @var{separator}
1592 @itemx --separator=@var{separator}
1594 @opindex --separator
1595 Use @var{separator} as the record separator, instead of newline.
1603 @section @command{nl}: Number lines and write files
1606 @cindex numbering lines
1607 @cindex line numbering
1609 @command{nl} writes each @var{file} (@samp{-} means standard input), or
1610 standard input if none are given, to standard output, with line numbers
1611 added to some or all of the lines. Synopsis:
1614 nl [@var{option}]@dots{} [@var{file}]@dots{}
1617 @cindex logical pages, numbering on
1618 @command{nl} decomposes its input into (logical) pages; by default, the
1619 line number is reset to 1 at the top of each logical page. @command{nl}
1620 treats all of the input files as a single document; it does not reset
1621 line numbers or logical pages between files.
1623 @cindex headers, numbering
1624 @cindex body, numbering
1625 @cindex footers, numbering
1626 A logical page consists of three sections: header, body, and footer.
1627 Any of the sections can be empty. Each can be numbered in a different
1628 style from the others.
1630 The beginnings of the sections of logical pages are indicated in the
1631 input file by a line containing exactly one of these delimiter strings:
1642 The two characters from which these strings are made can be changed from
1643 @samp{\} and @samp{:} via options (see below), but the pattern and
1644 length of each string cannot be changed.
1646 A section delimiter is replaced by an empty line on output. Any text
1647 that comes before the first section delimiter string in the input file
1648 is considered to be part of a body section, so @command{nl} treats a
1649 file that contains no section delimiters as a single body section.
1651 The program accepts the following options. Also see @ref{Common options}.
1655 @item -b @var{style}
1656 @itemx --body-numbering=@var{style}
1658 @opindex --body-numbering
1659 Select the numbering style for lines in the body section of each
1660 logical page. When a line is not numbered, the current line number
1661 is not incremented, but the line number separator character is still
1662 prepended to the line. The styles are:
1668 number only nonempty lines (default for body),
1670 do not number lines (default for header and footer),
1672 number only lines that contain a match for the basic regular
1673 expression @var{bre}.
1674 @xref{Regular Expressions, , Regular Expressions, grep, The GNU Grep Manual}.
1678 @itemx --section-delimiter=@var{cd}
1680 @opindex --section-delimiter
1681 @cindex section delimiters of pages
1682 Set the section delimiter characters to @var{cd}; default is
1683 @samp{\:}. If only @var{c} is given, the second remains @samp{:}.
1684 (Remember to protect @samp{\} or other metacharacters from shell
1685 expansion with quotes or extra backslashes.)
1687 @item -f @var{style}
1688 @itemx --footer-numbering=@var{style}
1690 @opindex --footer-numbering
1691 Analogous to @option{--body-numbering}.
1693 @item -h @var{style}
1694 @itemx --header-numbering=@var{style}
1696 @opindex --header-numbering
1697 Analogous to @option{--body-numbering}.
1699 @item -i @var{number}
1700 @itemx --line-increment=@var{number}
1702 @opindex --line-increment
1703 Increment line numbers by @var{number} (default 1).
1705 @item -l @var{number}
1706 @itemx --join-blank-lines=@var{number}
1708 @opindex --join-blank-lines
1709 @cindex empty lines, numbering
1710 @cindex blank lines, numbering
1711 Consider @var{number} (default 1) consecutive empty lines to be one
1712 logical line for numbering, and only number the last one. Where fewer
1713 than @var{number} consecutive empty lines occur, do not number them.
1714 An empty line is one that contains no characters, not even spaces
1717 @item -n @var{format}
1718 @itemx --number-format=@var{format}
1720 @opindex --number-format
1721 Select the line numbering format (default is @code{rn}):
1725 @opindex ln @r{format for @command{nl}}
1726 left justified, no leading zeros;
1728 @opindex rn @r{format for @command{nl}}
1729 right justified, no leading zeros;
1731 @opindex rz @r{format for @command{nl}}
1732 right justified, leading zeros.
1736 @itemx --no-renumber
1738 @opindex --no-renumber
1739 Do not reset the line number at the start of a logical page.
1741 @item -s @var{string}
1742 @itemx --number-separator=@var{string}
1744 @opindex --number-separator
1745 Separate the line number from the text line in the output with
1746 @var{string} (default is the TAB character).
1748 @item -v @var{number}
1749 @itemx --starting-line-number=@var{number}
1751 @opindex --starting-line-number
1752 Set the initial line number on each logical page to @var{number} (default 1).
1754 @item -w @var{number}
1755 @itemx --number-width=@var{number}
1757 @opindex --number-width
1758 Use @var{number} characters for line numbers (default 6).
1766 @section @command{od}: Write files in octal or other formats
1769 @cindex octal dump of files
1770 @cindex hex dump of files
1771 @cindex ASCII dump of files
1772 @cindex file contents, dumping unambiguously
1774 @command{od} writes an unambiguous representation of each @var{file}
1775 (@samp{-} means standard input), or standard input if none are given.
1779 od [@var{option}]@dots{} [@var{file}]@dots{}
1780 od [-abcdfilosx]@dots{} [@var{file}] [[+]@var{offset}[.][b]]
1781 od [@var{option}]@dots{} --traditional [@var{file}] [[+]@var{offset}[.][b] [[+]@var{label}[.][b]]]
1784 Each line of output consists of the offset in the input, followed by
1785 groups of data from the file. By default, @command{od} prints the offset in
1786 octal, and each group of file data is a C @code{short int}'s worth of input
1787 printed as a single octal number.
1789 If @var{offset} is given, it specifies how many input bytes to skip
1790 before formatting and writing. By default, it is interpreted as an
1791 octal number, but the optional trailing decimal point causes it to be
1792 interpreted as decimal. If no decimal is specified and the offset
1793 begins with @samp{0x} or @samp{0X} it is interpreted as a hexadecimal
1794 number. If there is a trailing @samp{b}, the number of bytes skipped
1795 will be @var{offset} multiplied by 512.
1797 If a command is of both the first and second forms, the second form is
1798 assumed if the last operand begins with @samp{+} or (if there are two
1799 operands) a digit. For example, in @samp{od foo 10} and @samp{od +10}
1800 the @samp{10} is an offset, whereas in @samp{od 10} the @samp{10} is a
1803 The program accepts the following options. Also see @ref{Common options}.
1807 @item -A @var{radix}
1808 @itemx --address-radix=@var{radix}
1810 @opindex --address-radix
1811 @cindex radix for file offsets
1812 @cindex file offset radix
1813 Select the base in which file offsets are printed. @var{radix} can
1814 be one of the following:
1824 none (do not print offsets).
1827 The default is octal.
1829 @item -j @var{bytes}
1830 @itemx --skip-bytes=@var{bytes}
1832 @opindex --skip-bytes
1833 Skip @var{bytes} input bytes before formatting and writing. If
1834 @var{bytes} begins with @samp{0x} or @samp{0X}, it is interpreted in
1835 hexadecimal; otherwise, if it begins with @samp{0}, in octal; otherwise,
1837 @multiplierSuffixes{bytes}
1839 @item -N @var{bytes}
1840 @itemx --read-bytes=@var{bytes}
1842 @opindex --read-bytes
1843 Output at most @var{bytes} bytes of the input. Prefixes and suffixes on
1844 @code{bytes} are interpreted as for the @option{-j} option.
1846 @item -S @var{bytes}
1847 @itemx --strings[=@var{bytes}]
1850 @cindex string constants, outputting
1851 Instead of the normal output, output only @dfn{string constants}: at
1852 least @var{bytes} consecutive @acronym{ASCII} graphic characters,
1853 followed by a zero byte (@acronym{ASCII} @sc{nul}).
1854 Prefixes and suffixes on @code{bytes} are interpreted as for the
1857 If @var{n} is omitted with @option{--strings}, the default is 3.
1860 @itemx --format=@var{type}
1863 Select the format in which to output the file data. @var{type} is a
1864 string of one or more of the below type indicator characters. If you
1865 include more than one type indicator character in a single @var{type}
1866 string, or use this option more than once, @command{od} writes one copy
1867 of each output line using each of the data types that you specified,
1868 in the order that you specified.
1870 Adding a trailing ``z'' to any type specification appends a display
1871 of the @acronym{ASCII} character representation of the printable characters
1872 to the output line generated by the type specification.
1876 named character, ignoring high-order bit
1878 @acronym{ASCII} character or backslash escape,
1891 The type @code{a} outputs things like @samp{sp} for space, @samp{nl} for
1892 newline, and @samp{nul} for a zero byte. Only the least significant
1893 seven bits of each byte is used; the high-order bit is ignored.
1894 Type @code{c} outputs
1895 @samp{ }, @samp{\n}, and @code{\0}, respectively.
1898 Except for types @samp{a} and @samp{c}, you can specify the number
1899 of bytes to use in interpreting each number in the given data type
1900 by following the type indicator character with a decimal integer.
1901 Alternately, you can specify the size of one of the C compiler's
1902 built-in data types by following the type indicator character with
1903 one of the following characters. For integers (@samp{d}, @samp{o},
1904 @samp{u}, @samp{x}):
1917 For floating point (@code{f}):
1929 @itemx --output-duplicates
1931 @opindex --output-duplicates
1932 Output consecutive lines that are identical. By default, when two or
1933 more consecutive output lines would be identical, @command{od} outputs only
1934 the first line, and puts just an asterisk on the following line to
1935 indicate the elision.
1938 @itemx --width[=@var{n}]
1941 Dump @code{n} input bytes per output line. This must be a multiple of
1942 the least common multiple of the sizes associated with the specified
1945 If this option is not given at all, the default is 16. If @var{n} is
1946 omitted, the default is 32.
1950 The next several options are shorthands for format specifications.
1951 @sc{gnu} @command{od} accepts any combination of shorthands and format
1952 specification options. These options accumulate.
1958 Output as named characters. Equivalent to @samp{-t a}.
1962 Output as octal bytes. Equivalent to @samp{-t o1}.
1966 Output as @acronym{ASCII} characters or backslash escapes. Equivalent to
1971 Output as unsigned decimal two-byte units. Equivalent to @samp{-t u2}.
1975 Output as floats. Equivalent to @samp{-t fF}.
1979 Output as decimal ints. Equivalent to @samp{-t dI}.
1983 Output as decimal long ints. Equivalent to @samp{-t dL}.
1987 Output as octal two-byte units. Equivalent to @option{-t o2}.
1991 Output as decimal two-byte units. Equivalent to @option{-t d2}.
1995 Output as hexadecimal two-byte units. Equivalent to @samp{-t x2}.
1998 @opindex --traditional
1999 Recognize the non-option label argument that traditional @command{od}
2000 accepted. The following syntax:
2003 od --traditional [@var{file}] [[+]@var{offset}[.][b] [[+]@var{label}[.][b]]]
2007 can be used to specify at most one file and optional arguments
2008 specifying an offset and a pseudo-start address, @var{label}.
2009 The @var{label} argument is interpreted
2010 just like @var{offset}, but it specifies an initial pseudo-address. The
2011 pseudo-addresses are displayed in parentheses following any normal
2018 @node base64 invocation
2019 @section @command{base64}: Transform data into printable data
2022 @cindex base64 encoding
2024 @command{base64} transforms data read from a file, or standard input,
2025 into (or from) base64 encoded form. The base64 encoded form uses
2026 printable @acronym{ASCII} characters to represent binary data.
2030 base64 [@var{option}]@dots{} [@var{file}]
2031 base64 --decode [@var{option}]@dots{} [@var{file}]
2034 The base64 encoding expands data to roughly 133% of the original.
2035 The format conforms to
2036 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc4648.txt, RFC 4648}.
2038 The program accepts the following options. Also see @ref{Common options}.
2043 @itemx --wrap=@var{cols}
2047 @cindex column to wrap data after
2048 During encoding, wrap lines after @var{cols} characters. This must be
2051 The default is to wrap after 76 characters. Use the value 0 to
2052 disable line wrapping altogether.
2058 @cindex Decode base64 data
2059 @cindex Base64 decoding
2060 Change the mode of operation, from the default of encoding data, to
2061 decoding data. Input is expected to be base64 encoded data, and the
2062 output will be the original data.
2065 @itemx --ignore-garbage
2067 @opindex --ignore-garbage
2068 @cindex Ignore garbage in base64 stream
2069 When decoding, newlines are always accepted.
2070 During decoding, ignore unrecognized bytes,
2071 to permit distorted data to be decoded.
2078 @node Formatting file contents
2079 @chapter Formatting file contents
2081 @cindex formatting file contents
2083 These commands reformat the contents of files.
2086 * fmt invocation:: Reformat paragraph text.
2087 * pr invocation:: Paginate or columnate files for printing.
2088 * fold invocation:: Wrap input lines to fit in specified width.
2092 @node fmt invocation
2093 @section @command{fmt}: Reformat paragraph text
2096 @cindex reformatting paragraph text
2097 @cindex paragraphs, reformatting
2098 @cindex text, reformatting
2100 @command{fmt} fills and joins lines to produce output lines of (at most)
2101 a given number of characters (75 by default). Synopsis:
2104 fmt [@var{option}]@dots{} [@var{file}]@dots{}
2107 @command{fmt} reads from the specified @var{file} arguments (or standard
2108 input if none are given), and writes to standard output.
2110 By default, blank lines, spaces between words, and indentation are
2111 preserved in the output; successive input lines with different
2112 indentation are not joined; tabs are expanded on input and introduced on
2115 @cindex line-breaking
2116 @cindex sentences and line-breaking
2117 @cindex Knuth, Donald E.
2118 @cindex Plass, Michael F.
2119 @command{fmt} prefers breaking lines at the end of a sentence, and tries to
2120 avoid line breaks after the first word of a sentence or before the last
2121 word of a sentence. A @dfn{sentence break} is defined as either the end
2122 of a paragraph or a word ending in any of @samp{.?!}, followed by two
2123 spaces or end of line, ignoring any intervening parentheses or quotes.
2124 Like @TeX{}, @command{fmt} reads entire ``paragraphs'' before choosing line
2125 breaks; the algorithm is a variant of that given by Donald E. Knuth
2126 and Michael F. Plass in ``Breaking Paragraphs Into Lines'',
2127 @cite{Software---Practice & Experience} @b{11}, 11 (November 1981),
2130 The program accepts the following options. Also see @ref{Common options}.
2135 @itemx --crown-margin
2137 @opindex --crown-margin
2138 @cindex crown margin
2139 @dfn{Crown margin} mode: preserve the indentation of the first two
2140 lines within a paragraph, and align the left margin of each subsequent
2141 line with that of the second line.
2144 @itemx --tagged-paragraph
2146 @opindex --tagged-paragraph
2147 @cindex tagged paragraphs
2148 @dfn{Tagged paragraph} mode: like crown margin mode, except that if
2149 indentation of the first line of a paragraph is the same as the
2150 indentation of the second, the first line is treated as a one-line
2156 @opindex --split-only
2157 Split lines only. Do not join short lines to form longer ones. This
2158 prevents sample lines of code, and other such ``formatted'' text from
2159 being unduly combined.
2162 @itemx --uniform-spacing
2164 @opindex --uniform-spacing
2165 Uniform spacing. Reduce spacing between words to one space, and spacing
2166 between sentences to two spaces.
2169 @itemx -w @var{width}
2170 @itemx --width=@var{width}
2171 @opindex -@var{width}
2174 Fill output lines up to @var{width} characters (default 75). @command{fmt}
2175 initially tries to make lines about 7% shorter than this, to give it
2176 room to balance line lengths.
2178 @item -p @var{prefix}
2179 @itemx --prefix=@var{prefix}
2180 Only lines beginning with @var{prefix} (possibly preceded by whitespace)
2181 are subject to formatting. The prefix and any preceding whitespace are
2182 stripped for the formatting and then re-attached to each formatted output
2183 line. One use is to format certain kinds of program comments, while
2184 leaving the code unchanged.
2192 @section @command{pr}: Paginate or columnate files for printing
2195 @cindex printing, preparing files for
2196 @cindex multicolumn output, generating
2197 @cindex merging files in parallel
2199 @command{pr} writes each @var{file} (@samp{-} means standard input), or
2200 standard input if none are given, to standard output, paginating and
2201 optionally outputting in multicolumn format; optionally merges all
2202 @var{file}s, printing all in parallel, one per column. Synopsis:
2205 pr [@var{option}]@dots{} [@var{file}]@dots{}
2209 By default, a 5-line header is printed at each page: two blank lines;
2210 a line with the date, the file name, and the page count; and two more
2211 blank lines. A footer of five blank lines is also printed.
2212 The default @var{page_length} is 66
2213 lines. The default number of text lines is therefore 56.
2214 The text line of the header takes the form
2215 @samp{@var{date} @var{string} @var{page}}, with spaces inserted around
2216 @var{string} so that the line takes up the full @var{page_width}. Here,
2217 @var{date} is the date (see the @option{-D} or @option{--date-format}
2218 option for details), @var{string} is the centered header string, and
2219 @var{page} identifies the page number. The @env{LC_MESSAGES} locale
2220 category affects the spelling of @var{page}; in the default C locale, it
2221 is @samp{Page @var{number}} where @var{number} is the decimal page
2224 Form feeds in the input cause page breaks in the output. Multiple form
2225 feeds produce empty pages.
2227 Columns are of equal width, separated by an optional string (default
2228 is @samp{space}). For multicolumn output, lines will always be truncated to
2229 @var{page_width} (default 72), unless you use the @option{-J} option.
2231 column output no line truncation occurs by default. Use @option{-W} option to
2232 truncate lines in that case.
2234 The following changes were made in version 1.22i and apply to later
2235 versions of @command{pr}:
2236 @c FIXME: this whole section here sounds very awkward to me. I
2237 @c made a few small changes, but really it all needs to be redone. - Brian
2238 @c OK, I fixed another sentence or two, but some of it I just don't understand.
2243 Some small @var{letter options} (@option{-s}, @option{-w}) have been
2244 redefined for better @acronym{POSIX} compliance. The output of some further
2245 cases has been adapted to other Unix systems. These changes are not
2246 compatible with earlier versions of the program.
2249 Some @var{new capital letter} options (@option{-J}, @option{-S}, @option{-W})
2250 have been introduced to turn off unexpected interferences of small letter
2251 options. The @option{-N} option and the second argument @var{last_page}
2252 of @samp{+FIRST_PAGE} offer more flexibility. The detailed handling of
2253 form feeds set in the input files requires the @option{-T} option.
2256 Capital letter options override small letter ones.
2259 Some of the option-arguments (compare @option{-s}, @option{-e},
2260 @option{-i}, @option{-n}) cannot be specified as separate arguments from the
2261 preceding option letter (already stated in the @acronym{POSIX} specification).
2264 The program accepts the following options. Also see @ref{Common options}.
2268 @item +@var{first_page}[:@var{last_page}]
2269 @itemx --pages=@var{first_page}[:@var{last_page}]
2270 @c The two following @opindex lines evoke warnings because they contain `:'
2271 @c The `info' spec does not permit that. If we use those lines, we end
2272 @c up with truncated index entries that don't work.
2273 @c @opindex +@var{first_page}[:@var{last_page}]
2274 @c @opindex --pages=@var{first_page}[:@var{last_page}]
2275 @opindex +@var{page_range}
2276 @opindex --pages=@var{page_range}
2277 Begin printing with page @var{first_page} and stop with @var{last_page}.
2278 Missing @samp{:@var{last_page}} implies end of file. While estimating
2279 the number of skipped pages each form feed in the input file results
2280 in a new page. Page counting with and without @samp{+@var{first_page}}
2281 is identical. By default, counting starts with the first page of input
2282 file (not first page printed). Line numbering may be altered by @option{-N}
2286 @itemx --columns=@var{column}
2287 @opindex -@var{column}
2289 @cindex down columns
2290 With each single @var{file}, produce @var{column} columns of output
2291 (default is 1) and print columns down, unless @option{-a} is used. The
2292 column width is automatically decreased as @var{column} increases; unless
2293 you use the @option{-W/-w} option to increase @var{page_width} as well.
2294 This option might well cause some lines to be truncated. The number of
2295 lines in the columns on each page are balanced. The options @option{-e}
2296 and @option{-i} are on for multiple text-column output. Together with
2297 @option{-J} option column alignment and line truncation is turned off.
2298 Lines of full length are joined in a free field format and @option{-S}
2299 option may set field separators. @option{-@var{column}} may not be used
2300 with @option{-m} option.
2306 @cindex across columns
2307 With each single @var{file}, print columns across rather than down. The
2308 @option{-@var{column}} option must be given with @var{column} greater than one.
2309 If a line is too long to fit in a column, it is truncated.
2312 @itemx --show-control-chars
2314 @opindex --show-control-chars
2315 Print control characters using hat notation (e.g., @samp{^G}); print
2316 other nonprinting characters in octal backslash notation. By default,
2317 nonprinting characters are not changed.
2320 @itemx --double-space
2322 @opindex --double-space
2323 @cindex double spacing
2324 Double space the output.
2326 @item -D @var{format}
2327 @itemx --date-format=@var{format}
2328 @cindex time formats
2329 @cindex formatting times
2330 Format header dates using @var{format}, using the same conventions as
2331 for the command @samp{date +@var{format}}; @xref{date invocation}.
2332 Except for directives, which start with
2333 @samp{%}, characters in @var{format} are printed unchanged. You can use
2334 this option to specify an arbitrary string in place of the header date,
2335 e.g., @option{--date-format="Monday morning"}.
2337 @vindex POSIXLY_CORRECT
2339 The default date format is @samp{%Y-%m-%d %H:%M} (for example,
2340 @samp{2001-12-04 23:59});
2341 but if the @env{POSIXLY_CORRECT} environment variable is set
2342 and the @env{LC_TIME} locale category specifies the @acronym{POSIX}
2343 locale, the default is @samp{%b %e %H:%M %Y} (for example,
2344 @samp{Dec@ @ 4 23:59 2001}.
2347 Time stamps are listed according to the time zone rules specified by
2348 the @env{TZ} environment variable, or by the system default rules if
2349 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
2350 with @env{TZ}, libc, The GNU C Library Reference Manual}.
2352 @item -e[@var{in-tabchar}[@var{in-tabwidth}]]
2353 @itemx --expand-tabs[=@var{in-tabchar}[@var{in-tabwidth}]]
2355 @opindex --expand-tabs
2357 Expand @var{tab}s to spaces on input. Optional argument @var{in-tabchar} is
2358 the input tab character (default is the TAB character). Second optional
2359 argument @var{in-tabwidth} is the input tab character's width (default
2367 @opindex --form-feed
2368 Use a form feed instead of newlines to separate output pages. This does
2369 not alter the default page length of 66 lines.
2371 @item -h @var{header}
2372 @itemx --header=@var{header}
2375 Replace the file name in the header with the centered string @var{header}.
2376 When using the shell, @var{header} should be quoted and should be
2377 separated from @option{-h} by a space.
2379 @item -i[@var{out-tabchar}[@var{out-tabwidth}]]
2380 @itemx --output-tabs[=@var{out-tabchar}[@var{out-tabwidth}]]
2382 @opindex --output-tabs
2384 Replace spaces with @var{tab}s on output. Optional argument @var{out-tabchar}
2385 is the output tab character (default is the TAB character). Second optional
2386 argument @var{out-tabwidth} is the output tab character's width (default
2392 @opindex --join-lines
2393 Merge lines of full length. Used together with the column options
2394 @option{-@var{column}}, @option{-a -@var{column}} or @option{-m}. Turns off
2395 @option{-W/-w} line truncation;
2396 no column alignment used; may be used with
2397 @option{--sep-string[=@var{string}]}. @option{-J} has been introduced
2398 (together with @option{-W} and @option{--sep-string})
2399 to disentangle the old (@acronym{POSIX}-compliant) options @option{-w} and
2400 @option{-s} along with the three column options.
2403 @item -l @var{page_length}
2404 @itemx --length=@var{page_length}
2407 Set the page length to @var{page_length} (default 66) lines, including
2408 the lines of the header [and the footer]. If @var{page_length} is less
2409 than or equal to 10, the header and footer are omitted, as if the
2410 @option{-t} option had been given.
2416 Merge and print all @var{file}s in parallel, one in each column. If a
2417 line is too long to fit in a column, it is truncated, unless the @option{-J}
2418 option is used. @option{--sep-string[=@var{string}]} may be used.
2420 some @var{file}s (form feeds set) produce empty columns, still marked
2421 by @var{string}. The result is a continuous line numbering and column
2422 marking throughout the whole merged file. Completely empty merged pages
2423 show no separators or line numbers. The default header becomes
2424 @samp{@var{date} @var{page}} with spaces inserted in the middle; this
2425 may be used with the @option{-h} or @option{--header} option to fill up
2426 the middle blank part.
2428 @item -n[@var{number-separator}[@var{digits}]]
2429 @itemx --number-lines[=@var{number-separator}[@var{digits}]]
2431 @opindex --number-lines
2432 Provide @var{digits} digit line numbering (default for @var{digits} is
2433 5). With multicolumn output the number occupies the first @var{digits}
2434 column positions of each text column or only each line of @option{-m}
2435 output. With single column output the number precedes each line just as
2436 @option{-m} does. Default counting of the line numbers starts with the
2437 first line of the input file (not the first line printed, compare the
2438 @option{--page} option and @option{-N} option).
2439 Optional argument @var{number-separator} is the character appended to
2440 the line number to separate it from the text followed. The default
2441 separator is the TAB character. In a strict sense a TAB is always
2442 printed with single column output only. The TAB width varies
2443 with the TAB position, e.g., with the left @var{margin} specified
2444 by @option{-o} option. With multicolumn output priority is given to
2445 @samp{equal width of output columns} (a @acronym{POSIX} specification).
2446 The TAB width is fixed to the value of the first column and does
2447 not change with different values of left @var{margin}. That means a
2448 fixed number of spaces is always printed in the place of the
2449 @var{number-separator} TAB. The tabification depends upon the output
2452 @item -N @var{line_number}
2453 @itemx --first-line-number=@var{line_number}
2455 @opindex --first-line-number
2456 Start line counting with the number @var{line_number} at first line of
2457 first page printed (in most cases not the first line of the input file).
2459 @item -o @var{margin}
2460 @itemx --indent=@var{margin}
2463 @cindex indenting lines
2465 Indent each line with a margin @var{margin} spaces wide (default is zero).
2466 The total page width is the size of the margin plus the @var{page_width}
2467 set with the @option{-W/-w} option. A limited overflow may occur with
2468 numbered single column output (compare @option{-n} option).
2471 @itemx --no-file-warnings
2473 @opindex --no-file-warnings
2474 Do not print a warning message when an argument @var{file} cannot be
2475 opened. (The exit status will still be nonzero, however.)
2477 @item -s[@var{char}]
2478 @itemx --separator[=@var{char}]
2480 @opindex --separator
2481 Separate columns by a single character @var{char}. The default for
2482 @var{char} is the TAB character without @option{-w} and @samp{no
2483 character} with @option{-w}. Without @option{-s} the default separator
2484 @samp{space} is set. @option{-s[char]} turns off line truncation of all
2485 three column options (@option{-COLUMN}|@option{-a -COLUMN}|@option{-m}) unless
2486 @option{-w} is set. This is a @acronym{POSIX}-compliant formulation.
2489 @item -S@var{string}
2490 @itemx --sep-string[=@var{string}]
2492 @opindex --sep-string
2493 Use @var{string} to separate output columns. The @option{-S} option doesn't
2494 affect the @option{-W/-w} option, unlike the @option{-s} option which does. It
2495 does not affect line truncation or column alignment.
2496 Without @option{-S}, and with @option{-J}, @command{pr} uses the default output
2498 Without @option{-S} or @option{-J}, @command{pr} uses a @samp{space}
2499 (same as @option{-S"@w{ }"}). @option{--sep-string} with no
2500 @samp{=@var{string}} is equivalent to @option{--sep-string=""}.
2503 @itemx --omit-header
2505 @opindex --omit-header
2506 Do not print the usual header [and footer] on each page, and do not fill
2507 out the bottom of pages (with blank lines or a form feed). No page
2508 structure is produced, but form feeds set in the input files are retained.
2509 The predefined pagination is not changed. @option{-t} or @option{-T} may be
2510 useful together with other options; e.g.: @option{-t -e4}, expand TAB characters
2511 in the input file to 4 spaces but don't make any other changes. Use of
2512 @option{-t} overrides @option{-h}.
2515 @itemx --omit-pagination
2517 @opindex --omit-pagination
2518 Do not print header [and footer]. In addition eliminate all form feeds
2519 set in the input files.
2522 @itemx --show-nonprinting
2524 @opindex --show-nonprinting
2525 Print nonprinting characters in octal backslash notation.
2527 @item -w @var{page_width}
2528 @itemx --width=@var{page_width}
2531 Set page width to @var{page_width} characters for multiple text-column
2532 output only (default for @var{page_width} is 72). @option{-s[CHAR]} turns
2533 off the default page width and any line truncation and column alignment.
2534 Lines of full length are merged, regardless of the column options
2535 set. No @var{page_width} setting is possible with single column output.
2536 A @acronym{POSIX}-compliant formulation.
2538 @item -W @var{page_width}
2539 @itemx --page_width=@var{page_width}
2541 @opindex --page_width
2542 Set the page width to @var{page_width} characters. That's valid with and
2543 without a column option. Text lines are truncated, unless @option{-J}
2544 is used. Together with one of the three column options
2545 (@option{-@var{column}}, @option{-a -@var{column}} or @option{-m}) column
2546 alignment is always used. The separator options @option{-S} or @option{-s}
2547 don't affect the @option{-W} option. Default is 72 characters. Without
2548 @option{-W @var{page_width}} and without any of the column options NO line
2549 truncation is used (defined to keep downward compatibility and to meet
2550 most frequent tasks). That's equivalent to @option{-W 72 -J}. The header
2551 line is never truncated.
2558 @node fold invocation
2559 @section @command{fold}: Wrap input lines to fit in specified width
2562 @cindex wrapping long input lines
2563 @cindex folding long input lines
2565 @command{fold} writes each @var{file} (@option{-} means standard input), or
2566 standard input if none are given, to standard output, breaking long
2570 fold [@var{option}]@dots{} [@var{file}]@dots{}
2573 By default, @command{fold} breaks lines wider than 80 columns. The output
2574 is split into as many lines as necessary.
2576 @cindex screen columns
2577 @command{fold} counts screen columns by default; thus, a tab may count more
2578 than one column, backspace decreases the column count, and carriage
2579 return sets the column to zero.
2581 The program accepts the following options. Also see @ref{Common options}.
2589 Count bytes rather than columns, so that tabs, backspaces, and carriage
2590 returns are each counted as taking up one column, just like other
2597 Break at word boundaries: the line is broken after the last blank before
2598 the maximum line length. If the line contains no such blanks, the line
2599 is broken at the maximum line length as usual.
2601 @item -w @var{width}
2602 @itemx --width=@var{width}
2605 Use a maximum line length of @var{width} columns instead of 80.
2607 For compatibility @command{fold} supports an obsolete option syntax
2608 @option{-@var{width}}. New scripts should use @option{-w @var{width}}
2616 @node Output of parts of files
2617 @chapter Output of parts of files
2619 @cindex output of parts of files
2620 @cindex parts of files, output of
2622 These commands output pieces of the input.
2625 * head invocation:: Output the first part of files.
2626 * tail invocation:: Output the last part of files.
2627 * split invocation:: Split a file into fixed-size pieces.
2628 * csplit invocation:: Split a file into context-determined pieces.
2631 @node head invocation
2632 @section @command{head}: Output the first part of files
2635 @cindex initial part of files, outputting
2636 @cindex first part of files, outputting
2638 @command{head} prints the first part (10 lines by default) of each
2639 @var{file}; it reads from standard input if no files are given or
2640 when given a @var{file} of @option{-}. Synopsis:
2643 head [@var{option}]@dots{} [@var{file}]@dots{}
2646 If more than one @var{file} is specified, @command{head} prints a
2647 one-line header consisting of:
2650 ==> @var{file name} <==
2654 before the output for each @var{file}.
2656 The program accepts the following options. Also see @ref{Common options}.
2661 @itemx --bytes=@var{k}
2664 Print the first @var{k} bytes, instead of initial lines.
2665 However, if @var{k} starts with a @samp{-},
2666 print all but the last @var{k} bytes of each file.
2667 @multiplierSuffixes{k}
2670 @itemx --lines=@var{k}
2673 Output the first @var{k} lines.
2674 However, if @var{k} starts with a @samp{-},
2675 print all but the last @var{k} lines of each file.
2676 Size multiplier suffixes are the same as with the @option{-c} option.
2684 Never print file name headers.
2690 Always print file name headers.
2694 For compatibility @command{head} also supports an obsolete option syntax
2695 @option{-@var{count}@var{options}}, which is recognized only if it is
2696 specified first. @var{count} is a decimal number optionally followed
2697 by a size letter (@samp{b}, @samp{k}, @samp{m}) as in @option{-c}, or
2698 @samp{l} to mean count by lines, or other option letters (@samp{cqv}).
2699 Scripts intended for standard hosts should use @option{-c @var{count}}
2700 or @option{-n @var{count}} instead. If your script must also run on
2701 hosts that support only the obsolete syntax, it is usually simpler to
2702 avoid @command{head}, e.g., by using @samp{sed 5q} instead of
2708 @node tail invocation
2709 @section @command{tail}: Output the last part of files
2712 @cindex last part of files, outputting
2714 @command{tail} prints the last part (10 lines by default) of each
2715 @var{file}; it reads from standard input if no files are given or
2716 when given a @var{file} of @samp{-}. Synopsis:
2719 tail [@var{option}]@dots{} [@var{file}]@dots{}
2722 If more than one @var{file} is specified, @command{tail} prints a
2723 one-line header consisting of:
2726 ==> @var{file name} <==
2730 before the output for each @var{file}.
2732 @cindex BSD @command{tail}
2733 @sc{gnu} @command{tail} can output any amount of data (some other versions of
2734 @command{tail} cannot). It also has no @option{-r} option (print in
2735 reverse), since reversing a file is really a different job from printing
2736 the end of a file; BSD @command{tail} (which is the one with @option{-r}) can
2737 only reverse files that are at most as large as its buffer, which is
2738 typically 32 KiB@. A more reliable and versatile way to reverse files is
2739 the @sc{gnu} @command{tac} command.
2741 The program accepts the following options. Also see @ref{Common options}.
2746 @itemx --bytes=@var{k}
2749 Output the last @var{k} bytes, instead of final lines.
2750 However, if @var{k} starts with a @samp{+}, start printing with the
2751 @var{k}th byte from the start of each file, instead of from the end.
2752 @multiplierSuffixes{k}
2755 @itemx --follow[=@var{how}]
2758 @cindex growing files
2759 @vindex name @r{follow option}
2760 @vindex descriptor @r{follow option}
2761 Loop forever trying to read more characters at the end of the file,
2762 presumably because the file is growing.
2763 If more than one file is given, @command{tail} prints a header whenever it
2764 gets output from a different file, to indicate which file that output is
2767 There are two ways to specify how you'd like to track files with this option,
2768 but that difference is noticeable only when a followed file is removed or
2770 If you'd like to continue to track the end of a growing file even after
2771 it has been unlinked, use @option{--follow=descriptor}. This is the default
2772 behavior, but it is not useful if you're tracking a log file that may be
2773 rotated (removed or renamed, then reopened). In that case, use
2774 @option{--follow=name} to track the named file, perhaps by reopening it
2775 periodically to see if it has been removed and recreated by some other program.
2776 Note that the inotify-based implementation handles this case without
2777 the need for any periodic reopening.
2779 No matter which method you use, if the tracked file is determined to have
2780 shrunk, @command{tail} prints a message saying the file has been truncated
2781 and resumes tracking the end of the file from the newly-determined endpoint.
2783 When a file is removed, @command{tail}'s behavior depends on whether it is
2784 following the name or the descriptor. When following by name, tail can
2785 detect that a file has been removed and gives a message to that effect,
2786 and if @option{--retry} has been specified it will continue checking
2787 periodically to see if the file reappears.
2788 When following a descriptor, tail does not detect that the file has
2789 been unlinked or renamed and issues no message; even though the file
2790 may no longer be accessible via its original name, it may still be
2793 The option values @samp{descriptor} and @samp{name} may be specified only
2794 with the long form of the option, not with @option{-f}.
2796 The @option{-f} option is ignored if
2797 no @var{file} operand is specified and standard input is a FIFO or a pipe.
2798 Likewise, the @option{-f} option has no effect for any
2799 operand specified as @samp{-}, when standard input is a FIFO or a pipe.
2803 This option is the same as @option{--follow=name --retry}. That is, tail
2804 will attempt to reopen a file when it is removed. Should this fail, tail
2805 will keep trying until it becomes accessible again.
2809 This option is useful mainly when following by name (i.e., with
2810 @option{--follow=name}).
2811 Without this option, when tail encounters a file that doesn't
2812 exist or is otherwise inaccessible, it reports that fact and
2813 never checks it again.
2815 @itemx --sleep-interval=@var{number}
2816 @opindex --sleep-interval
2817 Change the number of seconds to wait between iterations (the default is 1.0).
2818 During one iteration, every specified file is checked to see if it has
2820 Historical implementations of @command{tail} have required that
2821 @var{number} be an integer. However, GNU @command{tail} accepts
2822 an arbitrary floating point number (using a period before any
2825 @itemx --pid=@var{pid}
2827 When following by name or by descriptor, you may specify the process ID,
2828 @var{pid}, of the sole writer of all @var{file} arguments. Then, shortly
2829 after that process terminates, tail will also terminate. This will
2830 work properly only if the writer and the tailing process are running on
2831 the same machine. For example, to save the output of a build in a file
2832 and to watch the file grow, if you invoke @command{make} and @command{tail}
2833 like this then the tail process will stop when your build completes.
2834 Without this option, you would have had to kill the @code{tail -f}
2838 $ make >& makerr & tail --pid=$! -f makerr
2841 If you specify a @var{pid} that is not in use or that does not correspond
2842 to the process that is writing to the tailed files, then @command{tail}
2843 may terminate long before any @var{file}s stop growing or it may not
2844 terminate until long after the real writer has terminated.
2845 Note that @option{--pid} cannot be supported on some systems; @command{tail}
2846 will print a warning if this is the case.
2848 @itemx --max-unchanged-stats=@var{n}
2849 @opindex --max-unchanged-stats
2850 When tailing a file by name, if there have been @var{n} (default
2851 n=@value{DEFAULT_MAX_N_UNCHANGED_STATS_BETWEEN_OPENS}) consecutive
2852 iterations for which the file has not changed, then
2853 @code{open}/@code{fstat} the file to determine if that file name is
2854 still associated with the same device/inode-number pair as before.
2855 When following a log file that is rotated, this is approximately the
2856 number of seconds between when tail prints the last pre-rotation lines
2857 and when it prints the lines that have accumulated in the new log file.
2858 This option is meaningful only when following by name.
2861 @itemx --lines=@var{k}
2864 Output the last @var{k} lines.
2865 However, if @var{k} starts with a @samp{+}, start printing with the
2866 @var{k}th line from the start of each file, instead of from the end.
2867 Size multiplier suffixes are the same as with the @option{-c} option.
2875 Never print file name headers.
2881 Always print file name headers.
2885 For compatibility @command{tail} also supports an obsolete usage
2886 @samp{tail -[@var{count}][bcl][f] [@var{file}]}, which is recognized
2887 only if it does not conflict with the usage described
2888 above. This obsolete form uses exactly one option and at most one
2889 file. In the option, @var{count} is an optional decimal number optionally
2890 followed by a size letter (@samp{b}, @samp{c}, @samp{l}) to mean count
2891 by 512-byte blocks, bytes, or lines, optionally followed by @samp{f}
2892 which has the same meaning as @option{-f}.
2894 @vindex _POSIX2_VERSION
2895 On older systems, the leading @samp{-} can be replaced by @samp{+} in
2896 the obsolete option syntax with the same meaning as in counts, and
2897 obsolete usage overrides normal usage when the two conflict.
2898 This obsolete behavior can be enabled or disabled with the
2899 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
2902 Scripts intended for use on standard hosts should avoid obsolete
2903 syntax and should use @option{-c @var{count}[b]}, @option{-n
2904 @var{count}}, and/or @option{-f} instead. If your script must also
2905 run on hosts that support only the obsolete syntax, you can often
2906 rewrite it to avoid problematic usages, e.g., by using @samp{sed -n
2907 '$p'} rather than @samp{tail -1}. If that's not possible, the script
2908 can use a test like @samp{if tail -c +1 </dev/null >/dev/null 2>&1;
2909 then @dots{}} to decide which syntax to use.
2911 Even if your script assumes the standard behavior, you should still
2912 beware usages whose behaviors differ depending on the @acronym{POSIX}
2913 version. For example, avoid @samp{tail - main.c}, since it might be
2914 interpreted as either @samp{tail main.c} or as @samp{tail -- -
2915 main.c}; avoid @samp{tail -c 4}, since it might mean either @samp{tail
2916 -c4} or @samp{tail -c 10 4}; and avoid @samp{tail +4}, since it might
2917 mean either @samp{tail ./+4} or @samp{tail -n +4}.
2922 @node split invocation
2923 @section @command{split}: Split a file into fixed-size pieces
2926 @cindex splitting a file into pieces
2927 @cindex pieces, splitting a file into
2929 @command{split} creates output files containing consecutive sections of
2930 @var{input} (standard input if none is given or @var{input} is
2931 @samp{-}). Synopsis:
2934 split [@var{option}] [@var{input} [@var{prefix}]]
2937 By default, @command{split} puts 1000 lines of @var{input} (or whatever is
2938 left over for the last section), into each output file.
2940 @cindex output file name prefix
2941 The output files' names consist of @var{prefix} (@samp{x} by default)
2942 followed by a group of characters (@samp{aa}, @samp{ab}, @dots{} by
2943 default), such that concatenating the output files in traditional
2944 sorted order by file name produces
2945 the original input file. If the output file names are exhausted,
2946 @command{split} reports an error without deleting the output files
2949 The program accepts the following options. Also see @ref{Common options}.
2953 @item -l @var{lines}
2954 @itemx --lines=@var{lines}
2957 Put @var{lines} lines of @var{input} into each output file.
2959 For compatibility @command{split} also supports an obsolete
2960 option syntax @option{-@var{lines}}. New scripts should use @option{-l
2961 @var{lines}} instead.
2964 @itemx --bytes=@var{size}
2967 Put @var{size} bytes of @var{input} into each output file.
2968 @multiplierSuffixes{size}
2971 @itemx --line-bytes=@var{size}
2973 @opindex --line-bytes
2974 Put into each output file as many complete lines of @var{input} as
2975 possible without exceeding @var{size} bytes. Individual lines longer than
2976 @var{size} bytes are broken into multiple files.
2977 @var{size} has the same format as for the @option{--bytes} option.
2979 @item -a @var{length}
2980 @itemx --suffix-length=@var{length}
2982 @opindex --suffix-length
2983 Use suffixes of length @var{length}. The default @var{length} is 2.
2986 @itemx --numeric-suffixes
2988 @opindex --numeric-suffixes
2989 Use digits in suffixes rather than lower-case letters.
2993 Write a diagnostic just before each output file is opened.
3000 @node csplit invocation
3001 @section @command{csplit}: Split a file into context-determined pieces
3004 @cindex context splitting
3005 @cindex splitting a file into pieces by context
3007 @command{csplit} creates zero or more output files containing sections of
3008 @var{input} (standard input if @var{input} is @samp{-}). Synopsis:
3011 csplit [@var{option}]@dots{} @var{input} @var{pattern}@dots{}
3014 The contents of the output files are determined by the @var{pattern}
3015 arguments, as detailed below. An error occurs if a @var{pattern}
3016 argument refers to a nonexistent line of the input file (e.g., if no
3017 remaining line matches a given regular expression). After every
3018 @var{pattern} has been matched, any remaining input is copied into one
3021 By default, @command{csplit} prints the number of bytes written to each
3022 output file after it has been created.
3024 The types of pattern arguments are:
3029 Create an output file containing the input up to but not including line
3030 @var{n} (a positive integer). If followed by a repeat count, also
3031 create an output file containing the next @var{n} lines of the input
3032 file once for each repeat.
3034 @item /@var{regexp}/[@var{offset}]
3035 Create an output file containing the current line up to (but not
3036 including) the next line of the input file that contains a match for
3037 @var{regexp}. The optional @var{offset} is an integer.
3038 If it is given, the input up to (but not including) the
3039 matching line plus or minus @var{offset} is put into the output file,
3040 and the line after that begins the next section of input.
3042 @item %@var{regexp}%[@var{offset}]
3043 Like the previous type, except that it does not create an output
3044 file, so that section of the input file is effectively ignored.
3046 @item @{@var{repeat-count}@}
3047 Repeat the previous pattern @var{repeat-count} additional
3048 times. The @var{repeat-count} can either be a positive integer or an
3049 asterisk, meaning repeat as many times as necessary until the input is
3054 The output files' names consist of a prefix (@samp{xx} by default)
3055 followed by a suffix. By default, the suffix is an ascending sequence
3056 of two-digit decimal numbers from @samp{00} to @samp{99}. In any case,
3057 concatenating the output files in sorted order by file name produces the
3058 original input file.
3060 By default, if @command{csplit} encounters an error or receives a hangup,
3061 interrupt, quit, or terminate signal, it removes any output files
3062 that it has created so far before it exits.
3064 The program accepts the following options. Also see @ref{Common options}.
3068 @item -f @var{prefix}
3069 @itemx --prefix=@var{prefix}
3072 @cindex output file name prefix
3073 Use @var{prefix} as the output file name prefix.
3075 @item -b @var{suffix}
3076 @itemx --suffix=@var{suffix}
3079 @cindex output file name suffix
3080 Use @var{suffix} as the output file name suffix. When this option is
3081 specified, the suffix string must include exactly one
3082 @code{printf(3)}-style conversion specification, possibly including
3083 format specification flags, a field width, a precision specifications,
3084 or all of these kinds of modifiers. The format letter must convert a
3085 binary unsigned integer argument to readable form. The format letters
3086 @samp{d} and @samp{i} are aliases for @samp{u}, and the
3087 @samp{u}, @samp{o}, @samp{x}, and @samp{X} conversions are allowed. The
3088 entire @var{suffix} is given (with the current output file number) to
3089 @code{sprintf(3)} to form the file name suffixes for each of the
3090 individual output files in turn. If this option is used, the
3091 @option{--digits} option is ignored.
3093 @item -n @var{digits}
3094 @itemx --digits=@var{digits}
3097 Use output file names containing numbers that are @var{digits} digits
3098 long instead of the default 2.
3103 @opindex --keep-files
3104 Do not remove output files when errors are encountered.
3107 @itemx --elide-empty-files
3109 @opindex --elide-empty-files
3110 Suppress the generation of zero-length output files. (In cases where
3111 the section delimiters of the input file are supposed to mark the first
3112 lines of each of the sections, the first output file will generally be a
3113 zero-length file unless you use this option.) The output file sequence
3114 numbers always run consecutively starting from 0, even when this option
3125 Do not print counts of output file sizes.
3131 Here is an example of its usage.
3132 First, create an empty directory for the exercise,
3139 Now, split the sequence of 1..14 on lines that end with 0 or 5:
3142 $ seq 14 | csplit - '/[05]$/' '@{*@}'
3148 Each number printed above is the size of an output
3149 file that csplit has just created.
3150 List the names of those output files:
3157 Use @command{head} to show their contents:
3182 @node Summarizing files
3183 @chapter Summarizing files
3185 @cindex summarizing files
3187 These commands generate just a few numbers representing entire
3191 * wc invocation:: Print newline, word, and byte counts.
3192 * sum invocation:: Print checksum and block counts.
3193 * cksum invocation:: Print CRC checksum and byte counts.
3194 * md5sum invocation:: Print or check MD5 digests.
3195 * sha1sum invocation:: Print or check SHA-1 digests.
3196 * sha2 utilities:: Print or check SHA-2 digests.
3201 @section @command{wc}: Print newline, word, and byte counts
3205 @cindex character count
3209 @command{wc} counts the number of bytes, characters, whitespace-separated
3210 words, and newlines in each given @var{file}, or standard input if none
3211 are given or for a @var{file} of @samp{-}. Synopsis:
3214 wc [@var{option}]@dots{} [@var{file}]@dots{}
3217 @cindex total counts
3218 @command{wc} prints one line of counts for each file, and if the file was
3219 given as an argument, it prints the file name following the counts. If
3220 more than one @var{file} is given, @command{wc} prints a final line
3221 containing the cumulative counts, with the file name @file{total}. The
3222 counts are printed in this order: newlines, words, characters, bytes,
3223 maximum line length.
3224 Each count is printed right-justified in a field with at least one
3225 space between fields so that the numbers and file names normally line
3226 up nicely in columns. The width of the count fields varies depending
3227 on the inputs, so you should not depend on a particular field width.
3228 However, as a @acronym{GNU} extension, if only one count is printed,
3229 it is guaranteed to be printed without leading spaces.
3231 By default, @command{wc} prints three counts: the newline, words, and byte
3232 counts. Options can specify that only certain counts be printed.
3233 Options do not undo others previously given, so
3240 prints both the byte counts and the word counts.
3242 With the @option{--max-line-length} option, @command{wc} prints the length
3243 of the longest line per file, and if there is more than one file it
3244 prints the maximum (not the sum) of those lengths. The line lengths here
3245 are measured in screen columns, according to the current locale and
3246 assuming tab positions in every 8th column.
3248 The program accepts the following options. Also see @ref{Common options}.
3256 Print only the byte counts.
3262 Print only the character counts.
3268 Print only the word counts.
3274 Print only the newline counts.
3277 @itemx --max-line-length
3279 @opindex --max-line-length
3280 Print only the maximum line lengths.
3282 @macro filesZeroFromOption{cmd,withTotalOption,subListOutput}
3283 @itemx --files0-from=@var{file}
3284 @opindex --files0-from=@var{file}
3285 @c This is commented out to avoid a texi2dvi failure.
3286 @c texi2dvi (GNU Texinfo 4.11) 1.104
3287 @c @cindex including files from @command{\cmd\}
3288 Disallow processing files named on the command line, and instead process
3289 those named in file @var{file}; each name being terminated by a zero byte
3290 (@acronym{ASCII} @sc{nul}).
3291 This is useful \withTotalOption\
3292 when the list of file names is so long that it may exceed a command line
3294 In such cases, running @command{\cmd\} via @command{xargs} is undesirable
3295 because it splits the list into pieces and makes @command{\cmd\} print
3296 \subListOutput\ for each sublist rather than for the entire list.
3297 One way to produce a list of @acronym{ASCII} @sc{nul} terminated file names is with @sc{gnu}
3298 @command{find}, using its @option{-print0} predicate.
3299 If @var{file} is @samp{-} then the @acronym{ASCII} @sc{nul} terminated file names
3300 are read from standard input.
3302 @filesZeroFromOption{wc,,a total}
3304 For example, to find the length of the longest line in any @file{.c} or
3305 @file{.h} file in the current hierarchy, do this:
3308 find . -name '*.[ch]' -print0 |
3309 wc -L --files0-from=- | tail -n1
3317 @node sum invocation
3318 @section @command{sum}: Print checksum and block counts
3321 @cindex 16-bit checksum
3322 @cindex checksum, 16-bit
3324 @command{sum} computes a 16-bit checksum for each given @var{file}, or
3325 standard input if none are given or for a @var{file} of @samp{-}. Synopsis:
3328 sum [@var{option}]@dots{} [@var{file}]@dots{}
3331 @command{sum} prints the checksum for each @var{file} followed by the
3332 number of blocks in the file (rounded up). If more than one @var{file}
3333 is given, file names are also printed (by default). (With the
3334 @option{--sysv} option, corresponding file names are printed when there is
3335 at least one file argument.)
3337 By default, @sc{gnu} @command{sum} computes checksums using an algorithm
3338 compatible with BSD @command{sum} and prints file sizes in units of
3341 The program accepts the following options. Also see @ref{Common options}.
3347 @cindex BSD @command{sum}
3348 Use the default (BSD compatible) algorithm. This option is included for
3349 compatibility with the System V @command{sum}. Unless @option{-s} was also
3350 given, it has no effect.
3356 @cindex System V @command{sum}
3357 Compute checksums using an algorithm compatible with System V
3358 @command{sum}'s default, and print file sizes in units of 512-byte blocks.
3362 @command{sum} is provided for compatibility; the @command{cksum} program (see
3363 next section) is preferable in new applications.
3368 @node cksum invocation
3369 @section @command{cksum}: Print CRC checksum and byte counts
3372 @cindex cyclic redundancy check
3373 @cindex CRC checksum
3375 @command{cksum} computes a cyclic redundancy check (CRC) checksum for each
3376 given @var{file}, or standard input if none are given or for a
3377 @var{file} of @samp{-}. Synopsis:
3380 cksum [@var{option}]@dots{} [@var{file}]@dots{}
3383 @command{cksum} prints the CRC checksum for each file along with the number
3384 of bytes in the file, and the file name unless no arguments were given.
3386 @command{cksum} is typically used to ensure that files
3387 transferred by unreliable means (e.g., netnews) have not been corrupted,
3388 by comparing the @command{cksum} output for the received files with the
3389 @command{cksum} output for the original files (typically given in the
3392 The CRC algorithm is specified by the @acronym{POSIX} standard. It is not
3393 compatible with the BSD or System V @command{sum} algorithms (see the
3394 previous section); it is more robust.
3396 The only options are @option{--help} and @option{--version}. @xref{Common
3402 @node md5sum invocation
3403 @section @command{md5sum}: Print or check MD5 digests
3407 @cindex 128-bit checksum
3408 @cindex checksum, 128-bit
3409 @cindex fingerprint, 128-bit
3410 @cindex message-digest, 128-bit
3412 @command{md5sum} computes a 128-bit checksum (or @dfn{fingerprint} or
3413 @dfn{message-digest}) for each specified @var{file}.
3415 Note: The MD5 digest is more reliable than a simple CRC (provided by
3416 the @command{cksum} command) for detecting accidental file corruption,
3417 as the chances of accidentally having two files with identical MD5
3418 are vanishingly small. However, it should not be considered secure
3419 against malicious tampering: although finding a file with a given MD5
3420 fingerprint is considered infeasible at the moment, it is known how
3421 to modify certain files, including digital certificates, so that they
3422 appear valid when signed with an MD5 digest.
3423 For more secure hashes, consider using SHA-2. @xref{sha2 utilities}.
3425 If a @var{file} is specified as @samp{-} or if no files are given
3426 @command{md5sum} computes the checksum for the standard input.
3427 @command{md5sum} can also determine whether a file and checksum are
3428 consistent. Synopsis:
3431 md5sum [@var{option}]@dots{} [@var{file}]@dots{}
3434 For each @var{file}, @samp{md5sum} outputs the MD5 checksum, a flag
3435 indicating a binary or text input file, and the file name.
3436 If @var{file} contains a backslash or newline, the
3437 line is started with a backslash, and each problematic character in
3438 the file name is escaped with a backslash, making the output
3439 unambiguous even in the presence of arbitrary file names.
3440 If @var{file} is omitted or specified as @samp{-}, standard input is read.
3442 The program accepts the following options. Also see @ref{Common options}.
3450 @cindex binary input files
3451 Treat each input file as binary, by reading it in binary mode and
3452 outputting a @samp{*} flag. This is the inverse of @option{--text}.
3453 On systems like @acronym{GNU} that do not distinguish between binary
3454 and text files, this option merely flags each input file as binary:
3455 the MD5 checksum is unaffected. This option is the default on systems
3456 like MS-DOS that distinguish between binary and text files, except
3457 for reading standard input when standard input is a terminal.
3461 Read file names and checksum information (not data) from each
3462 @var{file} (or from stdin if no @var{file} was specified) and report
3463 whether the checksums match the contents of the named files.
3464 The input to this mode of @command{md5sum} is usually the output of
3465 a prior, checksum-generating run of @samp{md5sum}.
3466 Each valid line of input consists of an MD5 checksum, a binary/text
3467 flag, and then a file name.
3468 Binary files are marked with @samp{*}, text with @samp{ }.
3469 For each such line, @command{md5sum} reads the named file and computes its
3470 MD5 checksum. Then, if the computed message digest does not match the
3471 one on the line with the file name, the file is noted as having
3472 failed the test. Otherwise, the file passes the test.
3473 By default, for each valid line, one line is written to standard
3474 output indicating whether the named file passed the test.
3475 After all checks have been performed, if there were any failures,
3476 a warning is issued to standard error.
3477 Use the @option{--status} option to inhibit that output.
3478 If any listed file cannot be opened or read, if any valid line has
3479 an MD5 checksum inconsistent with the associated file, or if no valid
3480 line is found, @command{md5sum} exits with nonzero status. Otherwise,
3481 it exits successfully.
3485 @cindex verifying MD5 checksums
3486 This option is useful only when verifying checksums.
3487 When verifying checksums, don't generate an 'OK' message per successfully
3488 checked file. Files that fail the verification are reported in the
3489 default one-line-per-file format. If there is any checksum mismatch,
3490 print a warning summarizing the failures to standard error.
3494 @cindex verifying MD5 checksums
3495 This option is useful only when verifying checksums.
3496 When verifying checksums, don't generate the default one-line-per-file
3497 diagnostic and don't output the warning summarizing any failures.
3498 Failures to open or read a file still evoke individual diagnostics to
3500 If all listed files are readable and are consistent with the associated
3501 MD5 checksums, exit successfully. Otherwise exit with a status code
3502 indicating there was a failure.
3508 @cindex text input files
3509 Treat each input file as text, by reading it in text mode and
3510 outputting a @samp{ } flag. This is the inverse of @option{--binary}.
3511 This option is the default on systems like @acronym{GNU} that do not
3512 distinguish between binary and text files. On other systems, it is
3513 the default for reading standard input when standard input is a
3520 @cindex verifying MD5 checksums
3521 When verifying checksums, warn about improperly formatted MD5 checksum lines.
3522 This option is useful only if all but a few lines in the checked input
3530 @node sha1sum invocation
3531 @section @command{sha1sum}: Print or check SHA-1 digests
3535 @cindex 160-bit checksum
3536 @cindex checksum, 160-bit
3537 @cindex fingerprint, 160-bit
3538 @cindex message-digest, 160-bit
3540 @command{sha1sum} computes a 160-bit checksum for each specified
3541 @var{file}. The usage and options of this command are precisely the
3542 same as for @command{md5sum}. @xref{md5sum invocation}.
3544 Note: The SHA-1 digest is more secure than MD5, and no collisions of
3545 it are known (different files having the same fingerprint). However,
3546 it is known that they can be produced with considerable, but not
3547 unreasonable, resources. For this reason, it is generally considered
3548 that SHA-1 should be gradually phased out in favor of the more secure
3549 SHA-2 hash algorithms. @xref{sha2 utilities}.
3552 @node sha2 utilities
3553 @section sha2 utilities: Print or check SHA-2 digests
3560 @cindex 224-bit checksum
3561 @cindex 256-bit checksum
3562 @cindex 384-bit checksum
3563 @cindex 512-bit checksum
3564 @cindex checksum, 224-bit
3565 @cindex checksum, 256-bit
3566 @cindex checksum, 384-bit
3567 @cindex checksum, 512-bit
3568 @cindex fingerprint, 224-bit
3569 @cindex fingerprint, 256-bit
3570 @cindex fingerprint, 384-bit
3571 @cindex fingerprint, 512-bit
3572 @cindex message-digest, 224-bit
3573 @cindex message-digest, 256-bit
3574 @cindex message-digest, 384-bit
3575 @cindex message-digest, 512-bit
3577 The commands @command{sha224sum}, @command{sha256sum},
3578 @command{sha384sum} and @command{sha512sum} compute checksums of
3579 various lengths (respectively 224, 256, 384 and 512 bits),
3580 collectively known as the SHA-2 hashes. The usage and options of
3581 these commands are precisely the same as for @command{md5sum}.
3582 @xref{md5sum invocation}.
3584 Note: The SHA384 and SHA512 digests are considerably slower to
3585 compute, especially on 32-bit computers, than SHA224 or SHA256.
3588 @node Operating on sorted files
3589 @chapter Operating on sorted files
3591 @cindex operating on sorted files
3592 @cindex sorted files, operations on
3594 These commands work with (or produce) sorted files.
3597 * sort invocation:: Sort text files.
3598 * shuf invocation:: Shuffle text files.
3599 * uniq invocation:: Uniquify files.
3600 * comm invocation:: Compare two sorted files line by line.
3601 * ptx invocation:: Produce a permuted index of file contents.
3602 * tsort invocation:: Topological sort.
3606 @node sort invocation
3607 @section @command{sort}: Sort text files
3610 @cindex sorting files
3612 @command{sort} sorts, merges, or compares all the lines from the given
3613 files, or standard input if none are given or for a @var{file} of
3614 @samp{-}. By default, @command{sort} writes the results to standard
3618 sort [@var{option}]@dots{} [@var{file}]@dots{}
3621 @command{sort} has three modes of operation: sort (the default), merge,
3622 and check for sortedness. The following options change the operation
3629 @itemx --check=diagnose-first
3632 @cindex checking for sortedness
3633 Check whether the given file is already sorted: if it is not all
3634 sorted, print a diagnostic containing the first out-of-order line and
3635 exit with a status of 1.
3636 Otherwise, exit successfully.
3637 At most one input file can be given.
3640 @itemx --check=quiet
3641 @itemx --check=silent
3644 @cindex checking for sortedness
3645 Exit successfully if the given file is already sorted, and
3646 exit with status 1 otherwise.
3647 At most one input file can be given.
3648 This is like @option{-c}, except it does not print a diagnostic.
3654 @cindex merging sorted files
3655 Merge the given files by sorting them as a group. Each input file must
3656 always be individually sorted. It always works to sort instead of
3657 merge; merging is provided because it is faster, in the case where it
3662 @cindex sort stability
3663 @cindex sort's last-resort comparison
3664 A pair of lines is compared as follows:
3665 @command{sort} compares each pair of fields, in the
3666 order specified on the command line, according to the associated
3667 ordering options, until a difference is found or no fields are left.
3668 If no key fields are specified, @command{sort} uses a default key of
3669 the entire line. Finally, as a last resort when all keys compare
3670 equal, @command{sort} compares entire lines as if no ordering options
3671 other than @option{--reverse} (@option{-r}) were specified. The
3672 @option{--stable} (@option{-s}) option disables this @dfn{last-resort
3673 comparison} so that lines in which all fields compare equal are left
3674 in their original relative order. The @option{--unique}
3675 (@option{-u}) option also disables the last-resort comparison.
3679 Unless otherwise specified, all comparisons use the character collating
3680 sequence specified by the @env{LC_COLLATE} locale.@footnote{If you
3681 use a non-@acronym{POSIX} locale (e.g., by setting @env{LC_ALL}
3682 to @samp{en_US}), then @command{sort} may produce output that is sorted
3683 differently than you're accustomed to. In that case, set the @env{LC_ALL}
3684 environment variable to @samp{C}. Note that setting only @env{LC_COLLATE}
3685 has two problems. First, it is ineffective if @env{LC_ALL} is also set.
3686 Second, it has undefined behavior if @env{LC_CTYPE} (or @env{LANG}, if
3687 @env{LC_CTYPE} is unset) is set to an incompatible value. For example,
3688 you get undefined behavior if @env{LC_CTYPE} is @code{ja_JP.PCK} but
3689 @env{LC_COLLATE} is @code{en_US.UTF-8}.}
3691 @sc{gnu} @command{sort} (as specified for all @sc{gnu} utilities) has no
3692 limit on input line length or restrictions on bytes allowed within lines.
3693 In addition, if the final byte of an input file is not a newline, @sc{gnu}
3694 @command{sort} silently supplies one. A line's trailing newline is not
3695 part of the line for comparison purposes.
3697 @cindex exit status of @command{sort}
3701 0 if no error occurred
3702 1 if invoked with @option{-c} or @option{-C} and the input is not sorted
3703 2 if an error occurred
3707 If the environment variable @env{TMPDIR} is set, @command{sort} uses its
3708 value as the directory for temporary files instead of @file{/tmp}. The
3709 @option{--temporary-directory} (@option{-T}) option in turn overrides
3710 the environment variable.
3712 The following options affect the ordering of output lines. They may be
3713 specified globally or as part of a specific key field. If no key
3714 fields are specified, global options apply to comparison of entire
3715 lines; otherwise the global options are inherited by key fields that do
3716 not specify any special options of their own. In pre-@acronym{POSIX}
3717 versions of @command{sort}, global options affect only later key fields,
3718 so portable shell scripts should specify global options first.
3723 @itemx --ignore-leading-blanks
3725 @opindex --ignore-leading-blanks
3726 @cindex blanks, ignoring leading
3728 Ignore leading blanks when finding sort keys in each line.
3729 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3730 can change this. Note blanks may be ignored by your locale's collating
3731 rules, but without this option they will be significant for character
3732 positions specified in keys with the @option{-k} option.
3735 @itemx --dictionary-order
3737 @opindex --dictionary-order
3738 @cindex dictionary order
3739 @cindex phone directory order
3740 @cindex telephone directory order
3742 Sort in @dfn{phone directory} order: ignore all characters except
3743 letters, digits and blanks when sorting.
3744 By default letters and digits are those of @acronym{ASCII} and a blank
3745 is a space or a tab, but the @env{LC_CTYPE} locale can change this.
3748 @itemx --ignore-case
3750 @opindex --ignore-case
3751 @cindex ignoring case
3752 @cindex case folding
3754 Fold lowercase characters into the equivalent uppercase characters when
3755 comparing so that, for example, @samp{b} and @samp{B} sort as equal.
3756 The @env{LC_CTYPE} locale determines character types.
3757 When used with @option{--unique} those lower case equivalent lines are
3758 thrown away. (There is currently no way to throw away the upper case
3759 equivalent instead. (Any @option{--reverse} given would only affect
3760 the final result, after the throwing away.))
3763 @itemx --general-numeric-sort
3764 @itemx --sort=general-numeric
3766 @opindex --general-numeric-sort
3768 @cindex general numeric sort
3770 Sort numerically, using the standard C function @code{strtold} to convert
3771 a prefix of each line to a long double-precision floating point number.
3772 This allows floating point numbers to be specified in scientific notation,
3773 like @code{1.0e-34} and @code{10e100}.
3774 The @env{LC_NUMERIC} locale determines the decimal-point character.
3775 Do not report overflow, underflow, or conversion errors.
3776 Use the following collating sequence:
3780 Lines that do not start with numbers (all considered to be equal).
3782 NaNs (``Not a Number'' values, in IEEE floating point arithmetic)
3783 in a consistent but machine-dependent order.
3787 Finite numbers in ascending numeric order (with @math{-0} and @math{+0} equal).
3792 Use this option only if there is no alternative; it is much slower than
3793 @option{--numeric-sort} (@option{-n}) and it can lose information when
3794 converting to floating point.
3797 @itemx --human-numeric-sort
3798 @itemx --sort=human-numeric
3800 @opindex --human-numeric-sort
3802 @cindex human numeric sort
3804 Sort numerically, first by numeric sign (negative, zero, or positive);
3805 then by @acronym{SI} suffix (either empty, or @samp{k} or @samp{K}, or
3806 one of @samp{MGTPEZY}, in that order; @pxref{Block size}); and finally
3807 by numeric value. For example, @samp{1023M} sorts before @samp{1G}
3808 because @samp{M} (mega) precedes @samp{G} (giga) as an @acronym{SI}
3809 suffix. This option sorts values that are consistently scaled to the
3810 nearest suffix, regardless of whether suffixes denote powers of 1000
3811 or 1024, and it therefore sorts the output of any single invocation of
3812 the @command{df}, @command{du}, or @command{ls} commands that are
3813 invoked with their @option{--human-readable} or @option{--si} options.
3814 The syntax for numbers is the same as for the @option{--numeric-sort}
3815 option; the @acronym{SI} suffix must immediately follow the number.
3818 @itemx --ignore-nonprinting
3820 @opindex --ignore-nonprinting
3821 @cindex nonprinting characters, ignoring
3822 @cindex unprintable characters, ignoring
3824 Ignore nonprinting characters.
3825 The @env{LC_CTYPE} locale determines character types.
3826 This option has no effect if the stronger @option{--dictionary-order}
3827 (@option{-d}) option is also given.
3833 @opindex --month-sort
3835 @cindex months, sorting by
3837 An initial string, consisting of any amount of blanks, followed
3838 by a month name abbreviation, is folded to UPPER case and
3839 compared in the order @samp{JAN} < @samp{FEB} < @dots{} < @samp{DEC}.
3840 Invalid names compare low to valid names. The @env{LC_TIME} locale
3841 category determines the month spellings.
3842 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3846 @itemx --numeric-sort
3847 @itemx --sort=numeric
3849 @opindex --numeric-sort
3851 @cindex numeric sort
3853 Sort numerically. The number begins each line and consists
3854 of optional blanks, an optional @samp{-} sign, and zero or more
3855 digits possibly separated by thousands separators, optionally followed
3856 by a decimal-point character and zero or more digits. An empty
3857 number is treated as @samp{0}. The @env{LC_NUMERIC}
3858 locale specifies the decimal-point character and thousands separator.
3859 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3862 Comparison is exact; there is no rounding error.
3864 Neither a leading @samp{+} nor exponential notation is recognized.
3865 To compare such strings numerically, use the
3866 @option{--general-numeric-sort} (@option{-g}) option.
3869 @itemx --version-sort
3871 @opindex --version-sort
3872 @cindex version number sort
3873 Sort by version name and number. It behaves like a standard sort,
3874 except that each sequence of decimal digits is treated numerically
3875 as an index/version number. (@xref{Details about version sort}.)
3881 @cindex reverse sorting
3882 Reverse the result of comparison, so that lines with greater key values
3883 appear earlier in the output instead of later.
3886 @itemx --random-sort
3887 @itemx --sort=random
3889 @opindex --random-sort
3892 Sort by hashing the input keys and then sorting the hash values.
3893 Choose the hash function at random, ensuring that it is free of
3894 collisions so that differing keys have differing hash values. This is
3895 like a random permutation of the inputs (@pxref{shuf invocation}),
3896 except that keys with the same value sort together.
3898 If multiple random sort fields are specified, the same random hash
3899 function is used for all fields. To use different random hash
3900 functions for different fields, you can invoke @command{sort} more
3903 The choice of hash function is affected by the
3904 @option{--random-source} option.
3912 @item --compress-program=@var{prog}
3913 Compress any temporary files with the program @var{prog}.
3915 With no arguments, @var{prog} must compress standard input to standard
3916 output, and when given the @option{-d} option it must decompress
3917 standard input to standard output.
3919 Terminate with an error if @var{prog} exits with nonzero status.
3921 White space and the backslash character should not appear in
3922 @var{prog}; they are reserved for future use.
3924 @filesZeroFromOption{sort,,sorted output}
3926 @item -k @var{pos1}[,@var{pos2}]
3927 @itemx --key=@var{pos1}[,@var{pos2}]
3931 Specify a sort field that consists of the part of the line between
3932 @var{pos1} and @var{pos2} (or the end of the line, if @var{pos2} is
3933 omitted), @emph{inclusive}.
3935 Each @var{pos} has the form @samp{@var{f}[.@var{c}][@var{opts}]},
3936 where @var{f} is the number of the field to use, and @var{c} is the number
3937 of the first character from the beginning of the field. Fields and character
3938 positions are numbered starting with 1; a character position of zero in
3939 @var{pos2} indicates the field's last character. If @samp{.@var{c}} is
3940 omitted from @var{pos1}, it defaults to 1 (the beginning of the field);
3941 if omitted from @var{pos2}, it defaults to 0 (the end of the field).
3942 @var{opts} are ordering options, allowing individual keys to be sorted
3943 according to different rules; see below for details. Keys can span
3946 Example: To sort on the second field, use @option{--key=2,2}
3947 (@option{-k 2,2}). See below for more notes on keys and more examples.
3948 See also the @option{--debug} option to help determine the part
3949 of the line being used in the sort.
3952 Highlight the portion of each line used for sorting.
3953 Also issue warnings about questionable usage to stderr.
3955 @item --batch-size=@var{nmerge}
3956 @opindex --batch-size
3957 @cindex number of inputs to merge, nmerge
3958 Merge at most @var{nmerge} inputs at once.
3960 When @command{sort} has to merge more than @var{nmerge} inputs,
3961 it merges them in groups of @var{nmerge}, saving the result in
3962 a temporary file, which is then used as an input in a subsequent merge.
3964 A large value of @var{nmerge} may improve merge performance and decrease
3965 temporary storage utilization at the expense of increased memory usage
3966 and I/0. Conversely a small value of @var{nmerge} may reduce memory
3967 requirements and I/0 at the expense of temporary storage consumption and
3970 The value of @var{nmerge} must be at least 2. The default value is
3971 currently 16, but this is implementation-dependent and may change in
3974 The value of @var{nmerge} may be bounded by a resource limit for open
3975 file descriptors. The commands @samp{ulimit -n} or @samp{getconf
3976 OPEN_MAX} may display limits for your systems; these limits may be
3977 modified further if your program already has some files open, or if
3978 the operating system has other limits on the number of open files. If
3979 the value of @var{nmerge} exceeds the resource limit, @command{sort}
3980 silently uses a smaller value.
3982 @item -o @var{output-file}
3983 @itemx --output=@var{output-file}
3986 @cindex overwriting of input, allowed
3987 Write output to @var{output-file} instead of standard output.
3988 Normally, @command{sort} reads all input before opening
3989 @var{output-file}, so you can safely sort a file in place by using
3990 commands like @code{sort -o F F} and @code{cat F | sort -o F}.
3991 However, @command{sort} with @option{--merge} (@option{-m}) can open
3992 the output file before reading all input, so a command like @code{cat
3993 F | sort -m -o F - G} is not safe as @command{sort} might start
3994 writing @file{F} before @command{cat} is done reading it.
3996 @vindex POSIXLY_CORRECT
3997 On newer systems, @option{-o} cannot appear after an input file if
3998 @env{POSIXLY_CORRECT} is set, e.g., @samp{sort F -o F}. Portable
3999 scripts should specify @option{-o @var{output-file}} before any input
4002 @item --random-source=@var{file}
4003 @opindex --random-source
4004 @cindex random source for sorting
4005 Use @var{file} as a source of random data used to determine which
4006 random hash function to use with the @option{-R} option. @xref{Random
4013 @cindex sort stability
4014 @cindex sort's last-resort comparison
4016 Make @command{sort} stable by disabling its last-resort comparison.
4017 This option has no effect if no fields or global ordering options
4018 other than @option{--reverse} (@option{-r}) are specified.
4021 @itemx --buffer-size=@var{size}
4023 @opindex --buffer-size
4024 @cindex size for main memory sorting
4025 Use a main-memory sort buffer of the given @var{size}. By default,
4026 @var{size} is in units of 1024 bytes. Appending @samp{%} causes
4027 @var{size} to be interpreted as a percentage of physical memory.
4028 Appending @samp{K} multiplies @var{size} by 1024 (the default),
4029 @samp{M} by 1,048,576, @samp{G} by 1,073,741,824, and so on for
4030 @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}. Appending
4031 @samp{b} causes @var{size} to be interpreted as a byte count, with no
4034 This option can improve the performance of @command{sort} by causing it
4035 to start with a larger or smaller sort buffer than the default.
4036 However, this option affects only the initial buffer size. The buffer
4037 grows beyond @var{size} if @command{sort} encounters input lines larger
4040 @item -t @var{separator}
4041 @itemx --field-separator=@var{separator}
4043 @opindex --field-separator
4044 @cindex field separator character
4045 Use character @var{separator} as the field separator when finding the
4046 sort keys in each line. By default, fields are separated by the empty
4047 string between a non-blank character and a blank character.
4048 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4051 That is, given the input line @w{@samp{ foo bar}}, @command{sort} breaks it
4052 into fields @w{@samp{ foo}} and @w{@samp{ bar}}. The field separator is
4053 not considered to be part of either the field preceding or the field
4054 following, so with @samp{sort @w{-t " "}} the same input line has
4055 three fields: an empty field, @samp{foo}, and @samp{bar}.
4056 However, fields that extend to the end of the line,
4057 as @option{-k 2}, or fields consisting of a range, as @option{-k 2,3},
4058 retain the field separators present between the endpoints of the range.
4060 To specify @acronym{ASCII} @sc{nul} as the field separator,
4061 use the two-character string @samp{\0}, e.g., @samp{sort -t '\0'}.
4063 @item -T @var{tempdir}
4064 @itemx --temporary-directory=@var{tempdir}
4066 @opindex --temporary-directory
4067 @cindex temporary directory
4069 Use directory @var{tempdir} to store temporary files, overriding the
4070 @env{TMPDIR} environment variable. If this option is given more than
4071 once, temporary files are stored in all the directories given. If you
4072 have a large sort or merge that is I/O-bound, you can often improve
4073 performance by using this option to specify directories on different
4074 disks and controllers.
4076 @item --parallel=@var{n}
4078 @cindex multithreaded sort
4079 Limit the number of sorts run in parallel to @var{n}. By default,
4080 @var{n} is set to the number of available processors, and values
4081 greater than that are reduced to that limit. Also see
4082 @ref{nproc invocation}.
4088 @cindex uniquifying output
4090 Normally, output only the first of a sequence of lines that compare
4091 equal. For the @option{--check} (@option{-c} or @option{-C}) option,
4092 check that no pair of consecutive lines compares equal.
4094 This option also disables the default last-resort comparison.
4096 The commands @code{sort -u} and @code{sort | uniq} are equivalent, but
4097 this equivalence does not extend to arbitrary @command{sort} options.
4098 For example, @code{sort -n -u} inspects only the value of the initial
4099 numeric string when checking for uniqueness, whereas @code{sort -n |
4100 uniq} inspects the entire line. @xref{uniq invocation}.
4102 @macro zeroTerminatedOption
4104 @itemx --zero-terminated
4106 @opindex --zero-terminated
4107 @cindex process zero-terminated items
4108 Delimit items with a zero byte rather than a newline (@acronym{ASCII} @sc{lf}).
4109 I.E. treat input as items separated by @acronym{ASCII} @sc{nul}
4110 and terminate output items with @acronym{ASCII} @sc{nul}.
4111 This option can be useful in conjunction with @samp{perl -0} or
4112 @samp{find -print0} and @samp{xargs -0} which do the same in order to
4113 reliably handle arbitrary file names (even those containing blanks
4114 or other special characters).
4116 @zeroTerminatedOption
4120 Historical (BSD and System V) implementations of @command{sort} have
4121 differed in their interpretation of some options, particularly
4122 @option{-b}, @option{-f}, and @option{-n}. @sc{gnu} sort follows the @acronym{POSIX}
4123 behavior, which is usually (but not always!) like the System V behavior.
4124 According to @acronym{POSIX}, @option{-n} no longer implies @option{-b}. For
4125 consistency, @option{-M} has been changed in the same way. This may
4126 affect the meaning of character positions in field specifications in
4127 obscure cases. The only fix is to add an explicit @option{-b}.
4129 A position in a sort field specified with @option{-k} may have any
4130 of the option letters @samp{MbdfghinRrV} appended to it, in which case no
4131 global ordering options are inherited by that particular field. The
4132 @option{-b} option may be independently attached to either or both of
4133 the start and end positions of a field specification, and if it is
4134 inherited from the global options it will be attached to both.
4135 If input lines can contain leading or adjacent blanks and @option{-t}
4136 is not used, then @option{-k} is typically combined with @option{-b} or
4137 an option that implicitly ignores leading blanks (@samp{Mghn}) as otherwise
4138 the varying numbers of leading blanks in fields can cause confusing results.
4140 If the start position in a sort field specifier falls after the end of
4141 the line or after the end field, the field is empty. If the @option{-b}
4142 option was specified, the @samp{.@var{c}} part of a field specification
4143 is counted from the first nonblank character of the field.
4145 @vindex _POSIX2_VERSION
4146 @vindex POSIXLY_CORRECT
4147 On older systems, @command{sort} supports an obsolete origin-zero
4148 syntax @samp{+@var{pos1} [-@var{pos2}]} for specifying sort keys.
4149 The obsolete sequence @samp{sort +@var{a}.@var{x} -@var{b}.@var{y}}
4150 is equivalent to @samp{sort -k @var{a+1}.@var{x+1},@var{b}} if @var{y}
4151 is @samp{0} or absent, otherwise it is equivalent to @samp{sort -k
4152 @var{a+1}.@var{x+1},@var{b+1}.@var{y}}.
4154 This obsolete behavior can be enabled or disabled with the
4155 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4156 conformance}); it can also be enabled when @env{POSIXLY_CORRECT} is
4157 not set by using the obsolete syntax with @samp{-@var{pos2}} present.
4159 Scripts intended for use on standard hosts should avoid obsolete
4160 syntax and should use @option{-k} instead. For example, avoid
4161 @samp{sort +2}, since it might be interpreted as either @samp{sort
4162 ./+2} or @samp{sort -k 3}. If your script must also run on hosts that
4163 support only the obsolete syntax, it can use a test like @samp{if sort
4164 -k 1 </dev/null >/dev/null 2>&1; then @dots{}} to decide which syntax
4167 Here are some examples to illustrate various combinations of options.
4172 Sort in descending (reverse) numeric order.
4179 Run no more that 4 sorts concurrently, using a buffer size of 10M.
4182 sort --parallel=4 -S 10M
4186 Sort alphabetically, omitting the first and second fields
4187 and the blanks at the start of the third field.
4188 This uses a single key composed of the characters beginning
4189 at the start of the first nonblank character in field three
4190 and extending to the end of each line.
4197 Sort numerically on the second field and resolve ties by sorting
4198 alphabetically on the third and fourth characters of field five.
4199 Use @samp{:} as the field delimiter.
4202 sort -t : -k 2,2n -k 5.3,5.4
4205 Note that if you had written @option{-k 2n} instead of @option{-k 2,2n}
4206 @command{sort} would have used all characters beginning in the second field
4207 and extending to the end of the line as the primary @emph{numeric}
4208 key. For the large majority of applications, treating keys spanning
4209 more than one field as numeric will not do what you expect.
4211 Also note that the @samp{n} modifier was applied to the field-end
4212 specifier for the first key. It would have been equivalent to
4213 specify @option{-k 2n,2} or @option{-k 2n,2n}. All modifiers except
4214 @samp{b} apply to the associated @emph{field}, regardless of whether
4215 the modifier character is attached to the field-start and/or the
4216 field-end part of the key specifier.
4219 Sort the password file on the fifth field and ignore any
4220 leading blanks. Sort lines with equal values in field five
4221 on the numeric user ID in field three. Fields are separated
4225 sort -t : -k 5b,5 -k 3,3n /etc/passwd
4226 sort -t : -n -k 5b,5 -k 3,3 /etc/passwd
4227 sort -t : -b -k 5,5 -k 3,3n /etc/passwd
4230 These three commands have equivalent effect. The first specifies that
4231 the first key's start position ignores leading blanks and the second
4232 key is sorted numerically. The other two commands rely on global
4233 options being inherited by sort keys that lack modifiers. The inheritance
4234 works in this case because @option{-k 5b,5b} and @option{-k 5b,5} are
4235 equivalent, as the location of a field-end lacking a @samp{.@var{c}}
4236 character position is not affected by whether initial blanks are
4240 Sort a set of log files, primarily by IPv4 address and secondarily by
4241 time stamp. If two lines' primary and secondary keys are identical,
4242 output the lines in the same order that they were input. The log
4243 files contain lines that look like this:
4246 4.150.156.3 - - [01/Apr/2004:06:31:51 +0000] message 1
4247 211.24.3.231 - - [24/Apr/2004:20:17:39 +0000] message 2
4250 Fields are separated by exactly one space. Sort IPv4 addresses
4251 lexicographically, e.g., 212.61.52.2 sorts before 212.129.233.201
4252 because 61 is less than 129.
4255 sort -s -t ' ' -k 4.9n -k 4.5M -k 4.2n -k 4.14,4.21 file*.log |
4256 sort -s -t '.' -k 1,1n -k 2,2n -k 3,3n -k 4,4n
4259 This example cannot be done with a single @command{sort} invocation,
4260 since IPv4 address components are separated by @samp{.} while dates
4261 come just after a space. So it is broken down into two invocations of
4262 @command{sort}: the first sorts by time stamp and the second by IPv4
4263 address. The time stamp is sorted by year, then month, then day, and
4264 finally by hour-minute-second field, using @option{-k} to isolate each
4265 field. Except for hour-minute-second there's no need to specify the
4266 end of each key field, since the @samp{n} and @samp{M} modifiers sort
4267 based on leading prefixes that cannot cross field boundaries. The
4268 IPv4 addresses are sorted lexicographically. The second sort uses
4269 @samp{-s} so that ties in the primary key are broken by the secondary
4270 key; the first sort uses @samp{-s} so that the combination of the two
4274 Generate a tags file in case-insensitive sorted order.
4277 find src -type f -print0 | sort -z -f | xargs -0 etags --append
4280 The use of @option{-print0}, @option{-z}, and @option{-0} in this case means
4281 that file names that contain blanks or other special characters are
4283 by the sort operation.
4285 @c This example is a bit contrived and needs more explanation.
4287 @c Sort records separated by an arbitrary string by using a pipe to convert
4288 @c each record delimiter string to @samp{\0}, then using sort's -z option,
4289 @c and converting each @samp{\0} back to the original record delimiter.
4292 @c printf 'c\n\nb\n\na\n'|perl -0pe 's/\n\n/\n\0/g'|sort -z|perl -0pe 's/\0/\n/g'
4296 Use the common @acronym{DSU, Decorate Sort Undecorate} idiom to
4297 sort lines according to their length.
4300 awk '@{print length, $0@}' /etc/passwd | sort -n | cut -f2- -d' '
4303 In general this technique can be used to sort data that the @command{sort}
4304 command does not support, or is inefficient at, sorting directly.
4307 Shuffle a list of directories, but preserve the order of files within
4308 each directory. For instance, one could use this to generate a music
4309 playlist in which albums are shuffled but the songs of each album are
4313 ls */* | sort -t / -k 1,1R -k 2,2
4319 @node shuf invocation
4320 @section @command{shuf}: Shuffling text
4323 @cindex shuffling files
4325 @command{shuf} shuffles its input by outputting a random permutation
4326 of its input lines. Each output permutation is equally likely.
4330 shuf [@var{option}]@dots{} [@var{file}]
4331 shuf -e [@var{option}]@dots{} [@var{arg}]@dots{}
4332 shuf -i @var{lo}-@var{hi} [@var{option}]@dots{}
4335 @command{shuf} has three modes of operation that affect where it
4336 obtains its input lines. By default, it reads lines from standard
4337 input. The following options change the operation mode:
4345 @cindex command-line operands to shuffle
4346 Treat each command-line operand as an input line.
4348 @item -i @var{lo}-@var{hi}
4349 @itemx --input-range=@var{lo}-@var{hi}
4351 @opindex --input-range
4352 @cindex input range to shuffle
4353 Act as if input came from a file containing the range of unsigned
4354 decimal integers @var{lo}@dots{}@var{hi}, one per line.
4358 @command{shuf}'s other options can affect its behavior in all
4363 @item -n @var{lines}
4364 @itemx --head-count=@var{count}
4366 @opindex --head-count
4367 @cindex head of output
4368 Output at most @var{count} lines. By default, all input lines are
4371 @item -o @var{output-file}
4372 @itemx --output=@var{output-file}
4375 @cindex overwriting of input, allowed
4376 Write output to @var{output-file} instead of standard output.
4377 @command{shuf} reads all input before opening
4378 @var{output-file}, so you can safely shuffle a file in place by using
4379 commands like @code{shuf -o F <F} and @code{cat F | shuf -o F}.
4381 @item --random-source=@var{file}
4382 @opindex --random-source
4383 @cindex random source for shuffling
4384 Use @var{file} as a source of random data used to determine which
4385 permutation to generate. @xref{Random sources}.
4387 @zeroTerminatedOption
4403 might produce the output
4413 Similarly, the command:
4416 shuf -e clubs hearts diamonds spades
4430 and the command @samp{shuf -i 1-4} might output:
4440 These examples all have four input lines, so @command{shuf} might
4441 produce any of the twenty-four possible permutations of the input. In
4442 general, if there are @var{n} input lines, there are @var{n}! (i.e.,
4443 @var{n} factorial, or @var{n} * (@var{n} - 1) * @dots{} * 1) possible
4444 output permutations.
4449 @node uniq invocation
4450 @section @command{uniq}: Uniquify files
4453 @cindex uniquify files
4455 @command{uniq} writes the unique lines in the given @file{input}, or
4456 standard input if nothing is given or for an @var{input} name of
4460 uniq [@var{option}]@dots{} [@var{input} [@var{output}]]
4463 By default, @command{uniq} prints its input lines, except that
4464 it discards all but the first of adjacent repeated lines, so that
4465 no output lines are repeated. Optionally, it can instead discard
4466 lines that are not repeated, or all repeated lines.
4468 The input need not be sorted, but repeated input lines are detected
4469 only if they are adjacent. If you want to discard non-adjacent
4470 duplicate lines, perhaps you want to use @code{sort -u}.
4471 @xref{sort invocation}.
4474 Comparisons honor the rules specified by the @env{LC_COLLATE}
4477 If no @var{output} file is specified, @command{uniq} writes to standard
4480 The program accepts the following options. Also see @ref{Common options}.
4485 @itemx --skip-fields=@var{n}
4487 @opindex --skip-fields
4488 Skip @var{n} fields on each line before checking for uniqueness. Use
4489 a null string for comparison if a line has fewer than @var{n} fields. Fields
4490 are sequences of non-space non-tab characters that are separated from
4491 each other by at least one space or tab.
4493 For compatibility @command{uniq} supports an obsolete option syntax
4494 @option{-@var{n}}. New scripts should use @option{-f @var{n}} instead.
4497 @itemx --skip-chars=@var{n}
4499 @opindex --skip-chars
4500 Skip @var{n} characters before checking for uniqueness. Use a null string
4501 for comparison if a line has fewer than @var{n} characters. If you use both
4502 the field and character skipping options, fields are skipped over first.
4504 @vindex _POSIX2_VERSION
4505 On older systems, @command{uniq} supports an obsolete option syntax
4507 This obsolete behavior can be enabled or disabled with the
4508 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4509 conformance}), but portable scripts should avoid commands whose
4510 behavior depends on this variable.
4511 For example, use @samp{uniq ./+10} or @samp{uniq -s 10} rather than
4512 the ambiguous @samp{uniq +10}.
4518 Print the number of times each line occurred along with the line.
4521 @itemx --ignore-case
4523 @opindex --ignore-case
4524 Ignore differences in case when comparing lines.
4530 @cindex repeated lines, outputting
4531 Discard lines that are not repeated. When used by itself, this option
4532 causes @command{uniq} to print the first copy of each repeated line,
4536 @itemx --all-repeated[=@var{delimit-method}]
4538 @opindex --all-repeated
4539 @cindex all repeated lines, outputting
4540 Do not discard the second and subsequent repeated input lines,
4541 but discard lines that are not repeated.
4542 This option is useful mainly in conjunction with other options e.g.,
4543 to ignore case or to compare only selected fields.
4544 The optional @var{delimit-method} tells how to delimit
4545 groups of repeated lines, and must be one of the following:
4550 Do not delimit groups of repeated lines.
4551 This is equivalent to @option{--all-repeated} (@option{-D}).
4554 Output a newline before each group of repeated lines.
4555 With @option{--zero-terminated} (@option{-z}), use a zero
4556 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4559 Separate groups of repeated lines with a single newline.
4560 With @option{--zero-terminated} (@option{-z}), use a zero
4561 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4562 This is the same as using @samp{prepend}, except that
4563 no delimiter is inserted before the first group, and hence
4564 may be better suited for output direct to users.
4567 Note that when groups are delimited and the input stream contains
4568 two or more consecutive blank lines, then the output is ambiguous.
4569 To avoid that, filter the input through @samp{tr -s '\n'} to replace
4570 each sequence of consecutive newlines with a single newline.
4572 This is a @sc{gnu} extension.
4573 @c FIXME: give an example showing *how* it's useful
4579 @cindex unique lines, outputting
4580 Discard the first repeated line. When used by itself, this option
4581 causes @command{uniq} to print unique lines, and nothing else.
4584 @itemx --check-chars=@var{n}
4586 @opindex --check-chars
4587 Compare at most @var{n} characters on each line (after skipping any specified
4588 fields and characters). By default the entire rest of the lines are
4591 @zeroTerminatedOption
4598 @node comm invocation
4599 @section @command{comm}: Compare two sorted files line by line
4602 @cindex line-by-line comparison
4603 @cindex comparing sorted files
4605 @command{comm} writes to standard output lines that are common, and lines
4606 that are unique, to two input files; a file name of @samp{-} means
4607 standard input. Synopsis:
4610 comm [@var{option}]@dots{} @var{file1} @var{file2}
4614 Before @command{comm} can be used, the input files must be sorted using the
4615 collating sequence specified by the @env{LC_COLLATE} locale.
4616 If an input file ends in a non-newline
4617 character, a newline is silently appended. The @command{sort} command with
4618 no options always outputs a file that is suitable input to @command{comm}.
4620 @cindex differing lines
4621 @cindex common lines
4622 With no options, @command{comm} produces three-column output. Column one
4623 contains lines unique to @var{file1}, column two contains lines unique
4624 to @var{file2}, and column three contains lines common to both files.
4625 Columns are separated by a single TAB character.
4626 @c FIXME: when there's an option to supply an alternative separator
4627 @c string, append `by default' to the above sentence.
4632 The options @option{-1}, @option{-2}, and @option{-3} suppress printing of
4633 the corresponding columns (and separators). Also see @ref{Common options}.
4635 Unlike some other comparison utilities, @command{comm} has an exit
4636 status that does not depend on the result of the comparison.
4637 Upon normal completion @command{comm} produces an exit code of zero.
4638 If there is an error it exits with nonzero status.
4640 @macro checkOrderOption{cmd}
4641 If the @option{--check-order} option is given, unsorted inputs will
4642 cause a fatal error message. If the option @option{--nocheck-order}
4643 is given, unsorted inputs will never cause an error message. If
4644 neither of these options is given, wrongly sorted inputs are diagnosed
4645 only if an input file is found to contain unpairable lines. If an
4646 input file is diagnosed as being unsorted, the @command{\cmd\} command
4647 will exit with a nonzero status (and the output should not be used).
4649 Forcing @command{\cmd\} to process wrongly sorted input files
4650 containing unpairable lines by specifying @option{--nocheck-order} is
4651 not guaranteed to produce any particular output. The output will
4652 probably not correspond with whatever you hoped it would be.
4654 @checkOrderOption{comm}
4659 Fail with an error message if either input file is wrongly ordered.
4661 @item --nocheck-order
4662 Do not check that both input files are in sorted order.
4666 @item --output-delimiter=@var{str}
4667 Print @var{str} between adjacent output columns,
4668 rather than the default of a single TAB character.
4670 The delimiter @var{str} may not be empty.
4674 @node ptx invocation
4675 @section @command{ptx}: Produce permuted indexes
4679 @command{ptx} reads a text file and essentially produces a permuted index, with
4680 each keyword in its context. The calling sketch is either one of:
4683 ptx [@var{option} @dots{}] [@var{file} @dots{}]
4684 ptx -G [@var{option} @dots{}] [@var{input} [@var{output}]]
4687 The @option{-G} (or its equivalent: @option{--traditional}) option disables
4688 all @sc{gnu} extensions and reverts to traditional mode, thus introducing some
4689 limitations and changing several of the program's default option values.
4690 When @option{-G} is not specified, @sc{gnu} extensions are always enabled.
4691 @sc{gnu} extensions to @command{ptx} are documented wherever appropriate in this
4692 document. For the full list, see @xref{Compatibility in ptx}.
4694 Individual options are explained in the following sections.
4696 When @sc{gnu} extensions are enabled, there may be zero, one or several
4697 @var{file}s after the options. If there is no @var{file}, the program
4698 reads the standard input. If there is one or several @var{file}s, they
4699 give the name of input files which are all read in turn, as if all the
4700 input files were concatenated. However, there is a full contextual
4701 break between each file and, when automatic referencing is requested,
4702 file names and line numbers refer to individual text input files. In
4703 all cases, the program outputs the permuted index to the standard
4706 When @sc{gnu} extensions are @emph{not} enabled, that is, when the program
4707 operates in traditional mode, there may be zero, one or two parameters
4708 besides the options. If there are no parameters, the program reads the
4709 standard input and outputs the permuted index to the standard output.
4710 If there is only one parameter, it names the text @var{input} to be read
4711 instead of the standard input. If two parameters are given, they give
4712 respectively the name of the @var{input} file to read and the name of
4713 the @var{output} file to produce. @emph{Be very careful} to note that,
4714 in this case, the contents of file given by the second parameter is
4715 destroyed. This behavior is dictated by System V @command{ptx}
4716 compatibility; @sc{gnu} Standards normally discourage output parameters not
4717 introduced by an option.
4719 Note that for @emph{any} file named as the value of an option or as an
4720 input text file, a single dash @kbd{-} may be used, in which case
4721 standard input is assumed. However, it would not make sense to use this
4722 convention more than once per program invocation.
4725 * General options in ptx:: Options which affect general program behavior.
4726 * Charset selection in ptx:: Underlying character set considerations.
4727 * Input processing in ptx:: Input fields, contexts, and keyword selection.
4728 * Output formatting in ptx:: Types of output format, and sizing the fields.
4729 * Compatibility in ptx::
4733 @node General options in ptx
4734 @subsection General options
4739 @itemx --traditional
4740 As already explained, this option disables all @sc{gnu} extensions to
4741 @command{ptx} and switches to traditional mode.
4744 Print a short help on standard output, then exit without further
4748 Print the program version on standard output, then exit without further
4756 @node Charset selection in ptx
4757 @subsection Charset selection
4759 @c FIXME: People don't necessarily know what an IBM-PC was these days.
4760 As it is set up now, the program assumes that the input file is coded
4761 using 8-bit @acronym{ISO} 8859-1 code, also known as Latin-1 character set,
4762 @emph{unless} it is compiled for MS-DOS, in which case it uses the
4763 character set of the IBM-PC@. (@sc{gnu} @command{ptx} is not known to work on
4764 smaller MS-DOS machines anymore.) Compared to 7-bit @acronym{ASCII}, the set
4765 of characters which are letters is different; this alters the behavior
4766 of regular expression matching. Thus, the default regular expression
4767 for a keyword allows foreign or diacriticized letters. Keyword sorting,
4768 however, is still crude; it obeys the underlying character set ordering
4774 @itemx --ignore-case
4775 Fold lower case letters to upper case for sorting.
4780 @node Input processing in ptx
4781 @subsection Word selection and input processing
4786 @itemx --break-file=@var{file}
4788 This option provides an alternative (to @option{-W}) method of describing
4789 which characters make up words. It introduces the name of a
4790 file which contains a list of characters which can@emph{not} be part of
4791 one word; this file is called the @dfn{Break file}. Any character which
4792 is not part of the Break file is a word constituent. If both options
4793 @option{-b} and @option{-W} are specified, then @option{-W} has precedence and
4794 @option{-b} is ignored.
4796 When @sc{gnu} extensions are enabled, the only way to avoid newline as a
4797 break character is to write all the break characters in the file with no
4798 newline at all, not even at the end of the file. When @sc{gnu} extensions
4799 are disabled, spaces, tabs and newlines are always considered as break
4800 characters even if not included in the Break file.
4803 @itemx --ignore-file=@var{file}
4805 The file associated with this option contains a list of words which will
4806 never be taken as keywords in concordance output. It is called the
4807 @dfn{Ignore file}. The file contains exactly one word in each line; the
4808 end of line separation of words is not subject to the value of the
4812 @itemx --only-file=@var{file}
4814 The file associated with this option contains a list of words which will
4815 be retained in concordance output; any word not mentioned in this file
4816 is ignored. The file is called the @dfn{Only file}. The file contains
4817 exactly one word in each line; the end of line separation of words is
4818 not subject to the value of the @option{-S} option.
4820 There is no default for the Only file. When both an Only file and an
4821 Ignore file are specified, a word is considered a keyword only
4822 if it is listed in the Only file and not in the Ignore file.
4827 On each input line, the leading sequence of non-white space characters will be
4828 taken to be a reference that has the purpose of identifying this input
4829 line in the resulting permuted index. For more information about reference
4830 production, see @xref{Output formatting in ptx}.
4831 Using this option changes the default value for option @option{-S}.
4833 Using this option, the program does not try very hard to remove
4834 references from contexts in output, but it succeeds in doing so
4835 @emph{when} the context ends exactly at the newline. If option
4836 @option{-r} is used with @option{-S} default value, or when @sc{gnu} extensions
4837 are disabled, this condition is always met and references are completely
4838 excluded from the output contexts.
4840 @item -S @var{regexp}
4841 @itemx --sentence-regexp=@var{regexp}
4843 This option selects which regular expression will describe the end of a
4844 line or the end of a sentence. In fact, this regular expression is not
4845 the only distinction between end of lines or end of sentences, and input
4846 line boundaries have no special significance outside this option. By
4847 default, when @sc{gnu} extensions are enabled and if @option{-r} option is not
4848 used, end of sentences are used. In this case, this @var{regex} is
4849 imported from @sc{gnu} Emacs:
4852 [.?!][]\"')@}]*\\($\\|\t\\| \\)[ \t\n]*
4855 Whenever @sc{gnu} extensions are disabled or if @option{-r} option is used, end
4856 of lines are used; in this case, the default @var{regexp} is just:
4862 Using an empty @var{regexp} is equivalent to completely disabling end of
4863 line or end of sentence recognition. In this case, the whole file is
4864 considered to be a single big line or sentence. The user might want to
4865 disallow all truncation flag generation as well, through option @option{-F
4866 ""}. @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
4869 When the keywords happen to be near the beginning of the input line or
4870 sentence, this often creates an unused area at the beginning of the
4871 output context line; when the keywords happen to be near the end of the
4872 input line or sentence, this often creates an unused area at the end of
4873 the output context line. The program tries to fill those unused areas
4874 by wrapping around context in them; the tail of the input line or
4875 sentence is used to fill the unused area on the left of the output line;
4876 the head of the input line or sentence is used to fill the unused area
4877 on the right of the output line.
4879 As a matter of convenience to the user, many usual backslashed escape
4880 sequences from the C language are recognized and converted to the
4881 corresponding characters by @command{ptx} itself.
4883 @item -W @var{regexp}
4884 @itemx --word-regexp=@var{regexp}
4886 This option selects which regular expression will describe each keyword.
4887 By default, if @sc{gnu} extensions are enabled, a word is a sequence of
4888 letters; the @var{regexp} used is @samp{\w+}. When @sc{gnu} extensions are
4889 disabled, a word is by default anything which ends with a space, a tab
4890 or a newline; the @var{regexp} used is @samp{[^ \t\n]+}.
4892 An empty @var{regexp} is equivalent to not using this option.
4893 @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
4896 As a matter of convenience to the user, many usual backslashed escape
4897 sequences, as found in the C language, are recognized and converted to
4898 the corresponding characters by @command{ptx} itself.
4903 @node Output formatting in ptx
4904 @subsection Output formatting
4906 Output format is mainly controlled by the @option{-O} and @option{-T} options
4907 described in the table below. When neither @option{-O} nor @option{-T} are
4908 selected, and if @sc{gnu} extensions are enabled, the program chooses an
4909 output format suitable for a dumb terminal. Each keyword occurrence is
4910 output to the center of one line, surrounded by its left and right
4911 contexts. Each field is properly justified, so the concordance output
4912 can be readily observed. As a special feature, if automatic
4913 references are selected by option @option{-A} and are output before the
4914 left context, that is, if option @option{-R} is @emph{not} selected, then
4915 a colon is added after the reference; this nicely interfaces with @sc{gnu}
4916 Emacs @code{next-error} processing. In this default output format, each
4917 white space character, like newline and tab, is merely changed to
4918 exactly one space, with no special attempt to compress consecutive
4919 spaces. This might change in the future. Except for those white space
4920 characters, every other character of the underlying set of 256
4921 characters is transmitted verbatim.
4923 Output format is further controlled by the following options.
4927 @item -g @var{number}
4928 @itemx --gap-size=@var{number}
4930 Select the size of the minimum white space gap between the fields on the
4933 @item -w @var{number}
4934 @itemx --width=@var{number}
4936 Select the maximum output width of each final line. If references are
4937 used, they are included or excluded from the maximum output width
4938 depending on the value of option @option{-R}. If this option is not
4939 selected, that is, when references are output before the left context,
4940 the maximum output width takes into account the maximum length of all
4941 references. If this option is selected, that is, when references are
4942 output after the right context, the maximum output width does not take
4943 into account the space taken by references, nor the gap that precedes
4947 @itemx --auto-reference
4949 Select automatic references. Each input line will have an automatic
4950 reference made up of the file name and the line ordinal, with a single
4951 colon between them. However, the file name will be empty when standard
4952 input is being read. If both @option{-A} and @option{-r} are selected, then
4953 the input reference is still read and skipped, but the automatic
4954 reference is used at output time, overriding the input reference.
4957 @itemx --right-side-refs
4959 In the default output format, when option @option{-R} is not used, any
4960 references produced by the effect of options @option{-r} or @option{-A} are
4961 placed to the far right of output lines, after the right context. With
4962 default output format, when the @option{-R} option is specified, references
4963 are rather placed at the beginning of each output line, before the left
4964 context. For any other output format, option @option{-R} is
4965 ignored, with one exception: with @option{-R} the width of references
4966 is @emph{not} taken into account in total output width given by @option{-w}.
4968 This option is automatically selected whenever @sc{gnu} extensions are
4971 @item -F @var{string}
4972 @itemx --flac-truncation=@var{string}
4974 This option will request that any truncation in the output be reported
4975 using the string @var{string}. Most output fields theoretically extend
4976 towards the beginning or the end of the current line, or current
4977 sentence, as selected with option @option{-S}. But there is a maximum
4978 allowed output line width, changeable through option @option{-w}, which is
4979 further divided into space for various output fields. When a field has
4980 to be truncated because it cannot extend beyond the beginning or the end of
4981 the current line to fit in, then a truncation occurs. By default,
4982 the string used is a single slash, as in @option{-F /}.
4984 @var{string} may have more than one character, as in @option{-F ...}.
4985 Also, in the particular case when @var{string} is empty (@option{-F ""}),
4986 truncation flagging is disabled, and no truncation marks are appended in
4989 As a matter of convenience to the user, many usual backslashed escape
4990 sequences, as found in the C language, are recognized and converted to
4991 the corresponding characters by @command{ptx} itself.
4993 @item -M @var{string}
4994 @itemx --macro-name=@var{string}
4996 Select another @var{string} to be used instead of @samp{xx}, while
4997 generating output suitable for @command{nroff}, @command{troff} or @TeX{}.
5000 @itemx --format=roff
5002 Choose an output format suitable for @command{nroff} or @command{troff}
5003 processing. Each output line will look like:
5006 .xx "@var{tail}" "@var{before}" "@var{keyword_and_after}" "@var{head}" "@var{ref}"
5009 so it will be possible to write a @samp{.xx} roff macro to take care of
5010 the output typesetting. This is the default output format when @sc{gnu}
5011 extensions are disabled. Option @option{-M} can be used to change
5012 @samp{xx} to another macro name.
5014 In this output format, each non-graphical character, like newline and
5015 tab, is merely changed to exactly one space, with no special attempt to
5016 compress consecutive spaces. Each quote character: @kbd{"} is doubled
5017 so it will be correctly processed by @command{nroff} or @command{troff}.
5022 Choose an output format suitable for @TeX{} processing. Each output
5023 line will look like:
5026 \xx @{@var{tail}@}@{@var{before}@}@{@var{keyword}@}@{@var{after}@}@{@var{head}@}@{@var{ref}@}
5030 so it will be possible to write a @code{\xx} definition to take care of
5031 the output typesetting. Note that when references are not being
5032 produced, that is, neither option @option{-A} nor option @option{-r} is
5033 selected, the last parameter of each @code{\xx} call is inhibited.
5034 Option @option{-M} can be used to change @samp{xx} to another macro
5037 In this output format, some special characters, like @kbd{$}, @kbd{%},
5038 @kbd{&}, @kbd{#} and @kbd{_} are automatically protected with a
5039 backslash. Curly brackets @kbd{@{}, @kbd{@}} are protected with a
5040 backslash and a pair of dollar signs (to force mathematical mode). The
5041 backslash itself produces the sequence @code{\backslash@{@}}.
5042 Circumflex and tilde diacritical marks produce the sequence @code{^\@{ @}} and
5043 @code{~\@{ @}} respectively. Other diacriticized characters of the
5044 underlying character set produce an appropriate @TeX{} sequence as far
5045 as possible. The other non-graphical characters, like newline and tab,
5046 and all other characters which are not part of @acronym{ASCII}, are merely
5047 changed to exactly one space, with no special attempt to compress
5048 consecutive spaces. Let me know how to improve this special character
5049 processing for @TeX{}.
5054 @node Compatibility in ptx
5055 @subsection The @sc{gnu} extensions to @command{ptx}
5057 This version of @command{ptx} contains a few features which do not exist in
5058 System V @command{ptx}. These extra features are suppressed by using the
5059 @option{-G} command line option, unless overridden by other command line
5060 options. Some @sc{gnu} extensions cannot be recovered by overriding, so the
5061 simple rule is to avoid @option{-G} if you care about @sc{gnu} extensions.
5062 Here are the differences between this program and System V @command{ptx}.
5067 This program can read many input files at once, it always writes the
5068 resulting concordance on standard output. On the other hand, System V
5069 @command{ptx} reads only one file and sends the result to standard output
5070 or, if a second @var{file} parameter is given on the command, to that
5073 Having output parameters not introduced by options is a dangerous
5074 practice which @sc{gnu} avoids as far as possible. So, for using @command{ptx}
5075 portably between @sc{gnu} and System V, you should always use it with a
5076 single input file, and always expect the result on standard output. You
5077 might also want to automatically configure in a @option{-G} option to
5078 @command{ptx} calls in products using @command{ptx}, if the configurator finds
5079 that the installed @command{ptx} accepts @option{-G}.
5082 The only options available in System V @command{ptx} are options @option{-b},
5083 @option{-f}, @option{-g}, @option{-i}, @option{-o}, @option{-r}, @option{-t} and
5084 @option{-w}. All other options are @sc{gnu} extensions and are not repeated in
5085 this enumeration. Moreover, some options have a slightly different
5086 meaning when @sc{gnu} extensions are enabled, as explained below.
5089 By default, concordance output is not formatted for @command{troff} or
5090 @command{nroff}. It is rather formatted for a dumb terminal. @command{troff}
5091 or @command{nroff} output may still be selected through option @option{-O}.
5094 Unless @option{-R} option is used, the maximum reference width is
5095 subtracted from the total output line width. With @sc{gnu} extensions
5096 disabled, width of references is not taken into account in the output
5097 line width computations.
5100 All 256 bytes, even @acronym{ASCII} @sc{nul} bytes, are always read and
5101 processed from input file with no adverse effect, even if @sc{gnu} extensions
5102 are disabled. However, System V @command{ptx} does not accept 8-bit characters,
5103 a few control characters are rejected, and the tilde @kbd{~} is also rejected.
5106 Input line length is only limited by available memory, even if @sc{gnu}
5107 extensions are disabled. However, System V @command{ptx} processes only
5108 the first 200 characters in each line.
5111 The break (non-word) characters default to be every character except all
5112 letters of the underlying character set, diacriticized or not. When @sc{gnu}
5113 extensions are disabled, the break characters default to space, tab and
5117 The program makes better use of output line width. If @sc{gnu} extensions
5118 are disabled, the program rather tries to imitate System V @command{ptx},
5119 but still, there are some slight disposition glitches this program does
5120 not completely reproduce.
5123 The user can specify both an Ignore file and an Only file. This is not
5124 allowed with System V @command{ptx}.
5129 @node tsort invocation
5130 @section @command{tsort}: Topological sort
5133 @cindex topological sort
5135 @command{tsort} performs a topological sort on the given @var{file}, or
5136 standard input if no input file is given or for a @var{file} of
5137 @samp{-}. For more details and some history, see @ref{tsort background}.
5141 tsort [@var{option}] [@var{file}]
5144 @command{tsort} reads its input as pairs of strings, separated by blanks,
5145 indicating a partial ordering. The output is a total ordering that
5146 corresponds to the given partial ordering.
5160 will produce the output
5171 Consider a more realistic example.
5172 You have a large set of functions all in one file, and they may all be
5173 declared static except one. Currently that one (say @code{main}) is the
5174 first function defined in the file, and the ones it calls directly follow
5175 it, followed by those they call, etc. Let's say that you are determined
5176 to take advantage of prototypes, so you have to choose between declaring
5177 all of those functions (which means duplicating a lot of information from
5178 the definitions) and rearranging the functions so that as many as possible
5179 are defined before they are used. One way to automate the latter process
5180 is to get a list for each function of the functions it calls directly.
5181 Many programs can generate such lists. They describe a call graph.
5182 Consider the following list, in which a given line indicates that the
5183 function on the left calls the one on the right directly.
5189 tail_file pretty_name
5190 tail_file write_header
5192 tail_forever recheck
5193 tail_forever pretty_name
5194 tail_forever write_header
5195 tail_forever dump_remainder
5198 tail_lines start_lines
5199 tail_lines dump_remainder
5200 tail_lines file_lines
5201 tail_lines pipe_lines
5203 tail_bytes start_bytes
5204 tail_bytes dump_remainder
5205 tail_bytes pipe_bytes
5206 file_lines dump_remainder
5210 then you can use @command{tsort} to produce an ordering of those
5211 functions that satisfies your requirement.
5214 example$ tsort call-graph | tac
5234 @command{tsort} detects any cycles in the input and writes the first cycle
5235 encountered to standard error.
5237 Note that for a given partial ordering, generally there is no unique
5238 total ordering. In the context of the call graph above, the function
5239 @code{parse_options} may be placed anywhere in the list as long as it
5240 precedes @code{main}.
5242 The only options are @option{--help} and @option{--version}. @xref{Common
5248 * tsort background:: Where tsort came from.
5251 @node tsort background
5252 @subsection @command{tsort}: Background
5254 @command{tsort} exists because very early versions of the Unix linker processed
5255 an archive file exactly once, and in order. As @command{ld} read each object
5256 in the archive, it decided whether it was needed in the program based on
5257 whether it defined any symbols which were undefined at that point in
5260 This meant that dependencies within the archive had to be handled
5261 specially. For example, @code{scanf} probably calls @code{read}. That means
5262 that in a single pass through an archive, it was important for @code{scanf.o}
5263 to appear before read.o, because otherwise a program which calls
5264 @code{scanf} but not @code{read} might end up with an unexpected unresolved
5265 reference to @code{read}.
5267 The way to address this problem was to first generate a set of
5268 dependencies of one object file on another. This was done by a shell
5269 script called @command{lorder}. The GNU tools don't provide a version of
5270 lorder, as far as I know, but you can still find it in BSD
5273 Then you ran @command{tsort} over the @command{lorder} output, and you used the
5274 resulting sort to define the order in which you added objects to the archive.
5276 This whole procedure has been obsolete since about 1980, because
5277 Unix archives now contain a symbol table (traditionally built by
5278 @command{ranlib}, now generally built by @command{ar} itself), and the Unix
5279 linker uses the symbol table to effectively make multiple passes over
5282 Anyhow, that's where tsort came from. To solve an old problem with
5283 the way the linker handled archive files, which has since been solved
5287 @node Operating on fields
5288 @chapter Operating on fields
5291 * cut invocation:: Print selected parts of lines.
5292 * paste invocation:: Merge lines of files.
5293 * join invocation:: Join lines on a common field.
5297 @node cut invocation
5298 @section @command{cut}: Print selected parts of lines
5301 @command{cut} writes to standard output selected parts of each line of each
5302 input file, or standard input if no files are given or for a file name of
5306 cut @var{option}@dots{} [@var{file}]@dots{}
5309 In the table which follows, the @var{byte-list}, @var{character-list},
5310 and @var{field-list} are one or more numbers or ranges (two numbers
5311 separated by a dash) separated by commas. Bytes, characters, and
5312 fields are numbered starting at 1. Incomplete ranges may be
5313 given: @option{-@var{m}} means @samp{1-@var{m}}; @samp{@var{n}-} means
5314 @samp{@var{n}} through end of line or last field. The list elements
5315 can be repeated, can overlap, and can be specified in any order; but
5316 the selected input is written in the same order that it is read, and
5317 is written exactly once.
5319 The program accepts the following options. Also see @ref{Common
5324 @item -b @var{byte-list}
5325 @itemx --bytes=@var{byte-list}
5328 Select for printing only the bytes in positions listed in
5329 @var{byte-list}. Tabs and backspaces are treated like any other
5330 character; they take up 1 byte. If an output delimiter is specified,
5331 (see the description of @option{--output-delimiter}), then output that
5332 string between ranges of selected bytes.
5334 @item -c @var{character-list}
5335 @itemx --characters=@var{character-list}
5337 @opindex --characters
5338 Select for printing only the characters in positions listed in
5339 @var{character-list}. The same as @option{-b} for now, but
5340 internationalization will change that. Tabs and backspaces are
5341 treated like any other character; they take up 1 character. If an
5342 output delimiter is specified, (see the description of
5343 @option{--output-delimiter}), then output that string between ranges
5346 @item -f @var{field-list}
5347 @itemx --fields=@var{field-list}
5350 Select for printing only the fields listed in @var{field-list}.
5351 Fields are separated by a TAB character by default. Also print any
5352 line that contains no delimiter character, unless the
5353 @option{--only-delimited} (@option{-s}) option is specified.
5354 Note @command{cut} does not support specifying runs of whitespace as a
5355 delimiter, so to achieve that common functionality one can pre-process
5356 with @command{tr} like:
5358 tr -s '[:blank:]' '\t' | cut -f@dots{}
5361 @item -d @var{input_delim_byte}
5362 @itemx --delimiter=@var{input_delim_byte}
5364 @opindex --delimiter
5365 With @option{-f}, use the first byte of @var{input_delim_byte} as
5366 the input fields separator (default is TAB).
5370 Do not split multi-byte characters (no-op for now).
5373 @itemx --only-delimited
5375 @opindex --only-delimited
5376 For @option{-f}, do not print lines that do not contain the field separator
5377 character. Normally, any line without a field separator is printed verbatim.
5379 @item --output-delimiter=@var{output_delim_string}
5380 @opindex --output-delimiter
5381 With @option{-f}, output fields are separated by @var{output_delim_string}.
5382 The default with @option{-f} is to use the input delimiter.
5383 When using @option{-b} or @option{-c} to select ranges of byte or
5384 character offsets (as opposed to ranges of fields),
5385 output @var{output_delim_string} between non-overlapping
5386 ranges of selected bytes.
5389 @opindex --complement
5390 This option is a @acronym{GNU} extension.
5391 Select for printing the complement of the bytes, characters or fields
5392 selected with the @option{-b}, @option{-c} or @option{-f} options.
5393 In other words, do @emph{not} print the bytes, characters or fields
5394 specified via those options. This option is useful when you have
5395 many fields and want to print all but a few of them.
5402 @node paste invocation
5403 @section @command{paste}: Merge lines of files
5406 @cindex merging files
5408 @command{paste} writes to standard output lines consisting of sequentially
5409 corresponding lines of each given file, separated by a TAB character.
5410 Standard input is used for a file name of @samp{-} or if no input files
5432 paste [@var{option}]@dots{} [@var{file}]@dots{}
5435 The program accepts the following options. Also see @ref{Common options}.
5443 Paste the lines of one file at a time rather than one line from each
5444 file. Using the above example data:
5447 $ paste -s num2 let3
5452 @item -d @var{delim-list}
5453 @itemx --delimiters=@var{delim-list}
5455 @opindex --delimiters
5456 Consecutively use the characters in @var{delim-list} instead of
5457 TAB to separate merged lines. When @var{delim-list} is
5458 exhausted, start again at its beginning. Using the above example data:
5461 $ paste -d '%_' num2 let3 num2
5472 @node join invocation
5473 @section @command{join}: Join lines on a common field
5476 @cindex common field, joining on
5478 @command{join} writes to standard output a line for each pair of input
5479 lines that have identical join fields. Synopsis:
5482 join [@var{option}]@dots{} @var{file1} @var{file2}
5485 Either @var{file1} or @var{file2} (but not both) can be @samp{-},
5486 meaning standard input. @var{file1} and @var{file2} should be
5487 sorted on the join fields.
5490 Normally, the sort order is that of the
5491 collating sequence specified by the @env{LC_COLLATE} locale. Unless
5492 the @option{-t} option is given, the sort comparison ignores blanks at
5493 the start of the join field, as in @code{sort -b}. If the
5494 @option{--ignore-case} option is given, the sort comparison ignores
5495 the case of characters in the join field, as in @code{sort -f}.
5497 The @command{sort} and @command{join} commands should use consistent
5498 locales and options if the output of @command{sort} is fed to
5499 @command{join}. You can use a command like @samp{sort -k 1b,1} to
5500 sort a file on its default join field, but if you select a non-default
5501 locale, join field, separator, or comparison options, then you should
5502 do so consistently between @command{join} and @command{sort}.
5503 If @samp{join -t ''} is specified then the whole line is considered which
5504 matches the default operation of sort.
5506 If the input has no unpairable lines, a @acronym{GNU} extension is
5507 available; the sort order can be any order that considers two fields
5508 to be equal if and only if the sort comparison described above
5509 considers them to be equal. For example:
5526 @checkOrderOption{join}
5530 @item the join field is the first field in each line;
5531 @item fields in the input are separated by one or more blanks, with leading
5532 blanks on the line ignored;
5533 @item fields in the output are separated by a space;
5534 @item each output line consists of the join field, the remaining
5535 fields from @var{file1}, then the remaining fields from @var{file2}.
5538 The program accepts the following options. Also see @ref{Common options}.
5542 @item -a @var{file-number}
5544 Print a line for each unpairable line in file @var{file-number} (either
5545 @samp{1} or @samp{2}), in addition to the normal output.
5548 Fail with an error message if either input file is wrongly ordered.
5550 @item --nocheck-order
5551 Do not check that both input files are in sorted order. This is the default.
5553 @item -e @var{string}
5555 Replace those output fields that are missing in the input with
5560 Treat the first line of each input file as a header line. The header lines will
5561 be joined and printed as the first output line. If @option{-o} is used to
5562 specify output format, the header line will be printed according to the
5563 specified format. The header lines will not be checked for ordering even if
5564 @option{--check-order} is specified. Also if the header lines from each file
5565 do not match, the heading fields from the first file will be used.
5568 @itemx --ignore-case
5570 @opindex --ignore-case
5571 Ignore differences in case when comparing keys.
5572 With this option, the lines of the input files must be ordered in the same way.
5573 Use @samp{sort -f} to produce this ordering.
5575 @item -1 @var{field}
5577 Join on field @var{field} (a positive integer) of file 1.
5579 @item -2 @var{field}
5581 Join on field @var{field} (a positive integer) of file 2.
5583 @item -j @var{field}
5584 Equivalent to @option{-1 @var{field} -2 @var{field}}.
5586 @item -o @var{field-list}
5587 Construct each output line according to the format in @var{field-list}.
5588 Each element in @var{field-list} is either the single character @samp{0} or
5589 has the form @var{m.n} where the file number, @var{m}, is @samp{1} or
5590 @samp{2} and @var{n} is a positive field number.
5592 A field specification of @samp{0} denotes the join field.
5593 In most cases, the functionality of the @samp{0} field spec
5594 may be reproduced using the explicit @var{m.n} that corresponds
5595 to the join field. However, when printing unpairable lines
5596 (using either of the @option{-a} or @option{-v} options), there is no way
5597 to specify the join field using @var{m.n} in @var{field-list}
5598 if there are unpairable lines in both files.
5599 To give @command{join} that functionality, @acronym{POSIX} invented the @samp{0}
5600 field specification notation.
5602 The elements in @var{field-list}
5603 are separated by commas or blanks.
5604 Blank separators typically need to be quoted for the shell. For
5605 example, the commands @samp{join -o 1.2,2.2} and @samp{join -o '1.2
5606 2.2'} are equivalent.
5608 All output lines---including those printed because of any -a or -v
5609 option---are subject to the specified @var{field-list}.
5612 Use character @var{char} as the input and output field separator.
5613 Treat as significant each occurrence of @var{char} in the input file.
5614 Use @samp{sort -t @var{char}}, without the @option{-b} option of
5615 @samp{sort}, to produce this ordering. If @samp{join -t ''} is specified,
5616 the whole line is considered, matching the default operation of sort.
5617 If @samp{-t '\0'} is specified then the @acronym{ASCII} @sc{nul}
5618 character is used to delimit the fields.
5620 @item -v @var{file-number}
5621 Print a line for each unpairable line in file @var{file-number}
5622 (either @samp{1} or @samp{2}), instead of the normal output.
5629 @node Operating on characters
5630 @chapter Operating on characters
5632 @cindex operating on characters
5634 This commands operate on individual characters.
5637 * tr invocation:: Translate, squeeze, and/or delete characters.
5638 * expand invocation:: Convert tabs to spaces.
5639 * unexpand invocation:: Convert spaces to tabs.
5644 @section @command{tr}: Translate, squeeze, and/or delete characters
5651 tr [@var{option}]@dots{} @var{set1} [@var{set2}]
5654 @command{tr} copies standard input to standard output, performing
5655 one of the following operations:
5659 translate, and optionally squeeze repeated characters in the result,
5661 squeeze repeated characters,
5665 delete characters, then squeeze repeated characters from the result.
5668 The @var{set1} and (if given) @var{set2} arguments define ordered
5669 sets of characters, referred to below as @var{set1} and @var{set2}. These
5670 sets are the characters of the input that @command{tr} operates on.
5671 The @option{--complement} (@option{-c}, @option{-C}) option replaces
5673 complement (all of the characters that are not in @var{set1}).
5675 Currently @command{tr} fully supports only single-byte characters.
5676 Eventually it will support multibyte characters; when it does, the
5677 @option{-C} option will cause it to complement the set of characters,
5678 whereas @option{-c} will cause it to complement the set of values.
5679 This distinction will matter only when some values are not characters,
5680 and this is possible only in locales using multibyte encodings when
5681 the input contains encoding errors.
5683 The program accepts the @option{--help} and @option{--version}
5684 options. @xref{Common options}. Options must precede operands.
5689 * Character sets:: Specifying sets of characters.
5690 * Translating:: Changing one set of characters to another.
5691 * Squeezing:: Squeezing repeats and deleting.
5695 @node Character sets
5696 @subsection Specifying sets of characters
5698 @cindex specifying sets of characters
5700 The format of the @var{set1} and @var{set2} arguments resembles
5701 the format of regular expressions; however, they are not regular
5702 expressions, only lists of characters. Most characters simply
5703 represent themselves in these strings, but the strings can contain
5704 the shorthands listed below, for convenience. Some of them can be
5705 used only in @var{set1} or @var{set2}, as noted below.
5709 @item Backslash escapes
5710 @cindex backslash escapes
5712 The following backslash escape sequences are recognized:
5730 The character with the value given by @var{ooo}, which is 1 to 3
5736 While a backslash followed by a character not listed above is
5737 interpreted as that character, the backslash also effectively
5738 removes any special significance, so it is useful to escape
5739 @samp{[}, @samp{]}, @samp{*}, and @samp{-}.
5744 The notation @samp{@var{m}-@var{n}} expands to all of the characters
5745 from @var{m} through @var{n}, in ascending order. @var{m} should
5746 collate before @var{n}; if it doesn't, an error results. As an example,
5747 @samp{0-9} is the same as @samp{0123456789}.
5749 @sc{gnu} @command{tr} does not support the System V syntax that uses square
5750 brackets to enclose ranges. Translations specified in that format
5751 sometimes work as expected, since the brackets are often transliterated
5752 to themselves. However, they should be avoided because they sometimes
5753 behave unexpectedly. For example, @samp{tr -d '[0-9]'} deletes brackets
5756 Many historically common and even accepted uses of ranges are not
5757 portable. For example, on @acronym{EBCDIC} hosts using the @samp{A-Z}
5758 range will not do what most would expect because @samp{A} through @samp{Z}
5759 are not contiguous as they are in @acronym{ASCII}.
5760 If you can rely on a @acronym{POSIX} compliant version of @command{tr}, then
5761 the best way to work around this is to use character classes (see below).
5762 Otherwise, it is most portable (and most ugly) to enumerate the members
5765 @item Repeated characters
5766 @cindex repeated characters
5768 The notation @samp{[@var{c}*@var{n}]} in @var{set2} expands to @var{n}
5769 copies of character @var{c}. Thus, @samp{[y*6]} is the same as
5770 @samp{yyyyyy}. The notation @samp{[@var{c}*]} in @var{string2} expands
5771 to as many copies of @var{c} as are needed to make @var{set2} as long as
5772 @var{set1}. If @var{n} begins with @samp{0}, it is interpreted in
5773 octal, otherwise in decimal.
5775 @item Character classes
5776 @cindex character classes
5778 The notation @samp{[:@var{class}:]} expands to all of the characters in
5779 the (predefined) class @var{class}. The characters expand in no
5780 particular order, except for the @code{upper} and @code{lower} classes,
5781 which expand in ascending order. When the @option{--delete} (@option{-d})
5782 and @option{--squeeze-repeats} (@option{-s}) options are both given, any
5783 character class can be used in @var{set2}. Otherwise, only the
5784 character classes @code{lower} and @code{upper} are accepted in
5785 @var{set2}, and then only if the corresponding character class
5786 (@code{upper} and @code{lower}, respectively) is specified in the same
5787 relative position in @var{set1}. Doing this specifies case conversion.
5788 The class names are given below; an error results when an invalid class
5800 Horizontal whitespace.
5809 Printable characters, not including space.
5815 Printable characters, including space.
5818 Punctuation characters.
5821 Horizontal or vertical whitespace.
5830 @item Equivalence classes
5831 @cindex equivalence classes
5833 The syntax @samp{[=@var{c}=]} expands to all of the characters that are
5834 equivalent to @var{c}, in no particular order. Equivalence classes are
5835 a relatively recent invention intended to support non-English alphabets.
5836 But there seems to be no standard way to define them or determine their
5837 contents. Therefore, they are not fully implemented in @sc{gnu} @command{tr};
5838 each character's equivalence class consists only of that character,
5839 which is of no particular use.
5845 @subsection Translating
5847 @cindex translating characters
5849 @command{tr} performs translation when @var{set1} and @var{set2} are
5850 both given and the @option{--delete} (@option{-d}) option is not given.
5851 @command{tr} translates each character of its input that is in @var{set1}
5852 to the corresponding character in @var{set2}. Characters not in
5853 @var{set1} are passed through unchanged. When a character appears more
5854 than once in @var{set1} and the corresponding characters in @var{set2}
5855 are not all the same, only the final one is used. For example, these
5856 two commands are equivalent:
5863 A common use of @command{tr} is to convert lowercase characters to
5864 uppercase. This can be done in many ways. Here are three of them:
5867 tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
5869 tr '[:lower:]' '[:upper:]'
5873 But note that using ranges like @code{a-z} above is not portable.
5875 When @command{tr} is performing translation, @var{set1} and @var{set2}
5876 typically have the same length. If @var{set1} is shorter than
5877 @var{set2}, the extra characters at the end of @var{set2} are ignored.
5879 On the other hand, making @var{set1} longer than @var{set2} is not
5880 portable; @acronym{POSIX} says that the result is undefined. In this situation,
5881 BSD @command{tr} pads @var{set2} to the length of @var{set1} by repeating
5882 the last character of @var{set2} as many times as necessary. System V
5883 @command{tr} truncates @var{set1} to the length of @var{set2}.
5885 By default, @sc{gnu} @command{tr} handles this case like BSD @command{tr}.
5886 When the @option{--truncate-set1} (@option{-t}) option is given,
5887 @sc{gnu} @command{tr} handles this case like the System V @command{tr}
5888 instead. This option is ignored for operations other than translation.
5890 Acting like System V @command{tr} in this case breaks the relatively common
5894 tr -cs A-Za-z0-9 '\012'
5898 because it converts only zero bytes (the first element in the
5899 complement of @var{set1}), rather than all non-alphanumerics, to
5903 By the way, the above idiom is not portable because it uses ranges, and
5904 it assumes that the octal code for newline is 012.
5905 Assuming a @acronym{POSIX} compliant @command{tr}, here is a better way to write it:
5908 tr -cs '[:alnum:]' '[\n*]'
5913 @subsection Squeezing repeats and deleting
5915 @cindex squeezing repeat characters
5916 @cindex deleting characters
5918 When given just the @option{--delete} (@option{-d}) option, @command{tr}
5919 removes any input characters that are in @var{set1}.
5921 When given just the @option{--squeeze-repeats} (@option{-s}) option,
5922 @command{tr} replaces each input sequence of a repeated character that
5923 is in @var{set1} with a single occurrence of that character.
5925 When given both @option{--delete} and @option{--squeeze-repeats}, @command{tr}
5926 first performs any deletions using @var{set1}, then squeezes repeats
5927 from any remaining characters using @var{set2}.
5929 The @option{--squeeze-repeats} option may also be used when translating,
5930 in which case @command{tr} first performs translation, then squeezes
5931 repeats from any remaining characters using @var{set2}.
5933 Here are some examples to illustrate various combinations of options:
5938 Remove all zero bytes:
5945 Put all words on lines by themselves. This converts all
5946 non-alphanumeric characters to newlines, then squeezes each string
5947 of repeated newlines into a single newline:
5950 tr -cs '[:alnum:]' '[\n*]'
5954 Convert each sequence of repeated newlines to a single newline:
5961 Find doubled occurrences of words in a document.
5962 @c Separate the following two "the"s, so typo checkers don't complain.
5963 For example, people often write ``the @w{}the'' with the repeated words
5964 separated by a newline. The Bourne shell script below works first
5965 by converting each sequence of punctuation and blank characters to a
5966 single newline. That puts each ``word'' on a line by itself.
5967 Next it maps all uppercase characters to lower case, and finally it
5968 runs @command{uniq} with the @option{-d} option to print out only the words
5974 | tr -s '[:punct:][:blank:]' '[\n*]' \
5975 | tr '[:upper:]' '[:lower:]' \
5980 Deleting a small set of characters is usually straightforward. For example,
5981 to remove all @samp{a}s, @samp{x}s, and @samp{M}s you would do this:
5987 However, when @samp{-} is one of those characters, it can be tricky because
5988 @samp{-} has special meanings. Performing the same task as above but also
5989 removing all @samp{-} characters, we might try @code{tr -d -axM}, but
5990 that would fail because @command{tr} would try to interpret @option{-a} as
5991 a command-line option. Alternatively, we could try putting the hyphen
5992 inside the string, @code{tr -d a-xM}, but that wouldn't work either because
5993 it would make @command{tr} interpret @code{a-x} as the range of characters
5994 @samp{a}@dots{}@samp{x} rather than the three.
5995 One way to solve the problem is to put the hyphen at the end of the list
6002 Or you can use @samp{--} to terminate option processing:
6008 More generally, use the character class notation @code{[=c=]}
6009 with @samp{-} (or any other character) in place of the @samp{c}:
6015 Note how single quotes are used in the above example to protect the
6016 square brackets from interpretation by a shell.
6021 @node expand invocation
6022 @section @command{expand}: Convert tabs to spaces
6025 @cindex tabs to spaces, converting
6026 @cindex converting tabs to spaces
6028 @command{expand} writes the contents of each given @var{file}, or standard
6029 input if none are given or for a @var{file} of @samp{-}, to standard
6030 output, with tab characters converted to the appropriate number of
6034 expand [@var{option}]@dots{} [@var{file}]@dots{}
6037 By default, @command{expand} converts all tabs to spaces. It preserves
6038 backspace characters in the output; they decrement the column count for
6039 tab calculations. The default action is equivalent to @option{-t 8} (set
6040 tabs every 8 columns).
6042 The program accepts the following options. Also see @ref{Common options}.
6046 @item -t @var{tab1}[,@var{tab2}]@dots{}
6047 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6050 @cindex tab stops, setting
6051 If only one tab stop is given, set the tabs @var{tab1} spaces apart
6052 (default is 8). Otherwise, set the tabs at columns @var{tab1},
6053 @var{tab2}, @dots{} (numbered from 0), and replace any tabs beyond the
6054 last tab stop given with single spaces. Tab stops can be separated by
6055 blanks as well as by commas.
6057 For compatibility, GNU @command{expand} also accepts the obsolete
6058 option syntax, @option{-@var{t1}[,@var{t2}]@dots{}}. New scripts
6059 should use @option{-t @var{t1}[,@var{t2}]@dots{}} instead.
6065 @cindex initial tabs, converting
6066 Only convert initial tabs (those that precede all non-space or non-tab
6067 characters) on each line to spaces.
6074 @node unexpand invocation
6075 @section @command{unexpand}: Convert spaces to tabs
6079 @command{unexpand} writes the contents of each given @var{file}, or
6080 standard input if none are given or for a @var{file} of @samp{-}, to
6081 standard output, converting blanks at the beginning of each line into
6082 as many tab characters as needed. In the default @acronym{POSIX}
6083 locale, a @dfn{blank} is a space or a tab; other locales may specify
6084 additional blank characters. Synopsis:
6087 unexpand [@var{option}]@dots{} [@var{file}]@dots{}
6090 By default, @command{unexpand} converts only initial blanks (those
6091 that precede all non-blank characters) on each line. It
6092 preserves backspace characters in the output; they decrement the column
6093 count for tab calculations. By default, tabs are set at every 8th
6096 The program accepts the following options. Also see @ref{Common options}.
6100 @item -t @var{tab1}[,@var{tab2}]@dots{}
6101 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6104 If only one tab stop is given, set the tabs @var{tab1} columns apart
6105 instead of the default 8. Otherwise, set the tabs at columns
6106 @var{tab1}, @var{tab2}, @dots{} (numbered from 0), and leave blanks
6107 beyond the tab stops given unchanged. Tab stops can be separated by
6108 blanks as well as by commas. This option implies the @option{-a} option.
6110 For compatibility, GNU @command{unexpand} supports the obsolete option syntax,
6111 @option{-@var{tab1}[,@var{tab2}]@dots{}}, where tab stops must be
6112 separated by commas. (Unlike @option{-t}, this obsolete option does
6113 not imply @option{-a}.) New scripts should use @option{--first-only -t
6114 @var{tab1}[,@var{tab2}]@dots{}} instead.
6120 Also convert all sequences of two or more blanks just before a tab stop,
6121 even if they occur after non-blank characters in a line.
6128 @node Directory listing
6129 @chapter Directory listing
6131 This chapter describes the @command{ls} command and its variants @command{dir}
6132 and @command{vdir}, which list information about files.
6135 * ls invocation:: List directory contents.
6136 * dir invocation:: Briefly ls.
6137 * vdir invocation:: Verbosely ls.
6138 * dircolors invocation:: Color setup for ls, etc.
6143 @section @command{ls}: List directory contents
6146 @cindex directory listing
6148 The @command{ls} program lists information about files (of any type,
6149 including directories). Options and file arguments can be intermixed
6150 arbitrarily, as usual.
6152 For non-option command-line arguments that are directories, by default
6153 @command{ls} lists the contents of directories, not recursively, and
6154 omitting files with names beginning with @samp{.}. For other non-option
6155 arguments, by default @command{ls} lists just the file name. If no
6156 non-option argument is specified, @command{ls} operates on the current
6157 directory, acting as if it had been invoked with a single argument of @samp{.}.
6160 By default, the output is sorted alphabetically, according to the locale
6161 settings in effect.@footnote{If you use a non-@acronym{POSIX}
6162 locale (e.g., by setting @env{LC_ALL} to @samp{en_US}), then @command{ls} may
6163 produce output that is sorted differently than you're accustomed to.
6164 In that case, set the @env{LC_ALL} environment variable to @samp{C}.}
6165 If standard output is
6166 a terminal, the output is in columns (sorted vertically) and control
6167 characters are output as question marks; otherwise, the output is listed
6168 one per line and control characters are output as-is.
6170 Because @command{ls} is such a fundamental program, it has accumulated many
6171 options over the years. They are described in the subsections below;
6172 within each section, options are listed alphabetically (ignoring case).
6173 The division of options into the subsections is not absolute, since some
6174 options affect more than one aspect of @command{ls}'s operation.
6176 @cindex exit status of @command{ls}
6181 1 minor problems (e.g., failure to access a file or directory not
6182 specified as a command line argument. This happens when listing a
6183 directory in which entries are actively being removed or renamed.)
6184 2 serious trouble (e.g., memory exhausted, invalid option, failure
6185 to access a file or directory specified as a command line argument
6186 or a directory loop)
6189 Also see @ref{Common options}.
6192 * Which files are listed::
6193 * What information is listed::
6194 * Sorting the output::
6195 * Details about version sort::
6196 * General output formatting::
6197 * Formatting file timestamps::
6198 * Formatting the file names::
6202 @node Which files are listed
6203 @subsection Which files are listed
6205 These options determine which files @command{ls} lists information for.
6206 By default, @command{ls} lists files and the contents of any
6207 directories on the command line, except that in directories it ignores
6208 files whose names start with @samp{.}.
6216 In directories, do not ignore file names that start with @samp{.}.
6221 @opindex --almost-all
6222 In directories, do not ignore all file names that start with @samp{.};
6223 ignore only @file{.} and @file{..}. The @option{--all} (@option{-a})
6224 option overrides this option.
6227 @itemx --ignore-backups
6229 @opindex --ignore-backups
6230 @cindex backup files, ignoring
6231 In directories, ignore files that end with @samp{~}. This option is
6232 equivalent to @samp{--ignore='*~' --ignore='.*~'}.
6237 @opindex --directory
6238 List just the names of directories, as with other types of files, rather
6239 than listing their contents.
6240 @c The following sentence is the same as the one for -F.
6241 Do not follow symbolic links listed on the
6242 command line unless the @option{--dereference-command-line} (@option{-H}),
6243 @option{--dereference} (@option{-L}), or
6244 @option{--dereference-command-line-symlink-to-dir} options are specified.
6247 @itemx --dereference-command-line
6249 @opindex --dereference-command-line
6250 @cindex symbolic links, dereferencing
6251 If a command line argument specifies a symbolic link, show information
6252 for the file the link references rather than for the link itself.
6254 @itemx --dereference-command-line-symlink-to-dir
6255 @opindex --dereference-command-line-symlink-to-dir
6256 @cindex symbolic links, dereferencing
6257 Do not dereference symbolic links, with one exception:
6258 if a command line argument specifies a symbolic link that refers to
6259 a directory, show information for that directory rather than for the
6261 This is the default behavior when no other dereferencing-related
6262 option has been specified (@option{--classify} (@option{-F}),
6263 @option{--directory} (@option{-d}),
6265 @option{--dereference} (@option{-L}), or
6266 @option{--dereference-command-line} (@option{-H})).
6268 @item --group-directories-first
6269 @opindex --group-directories-first
6270 Group all the directories before the files and then sort the
6271 directories and the files separately using the selected sort key
6272 (see --sort option).
6273 That is, this option specifies a primary sort key,
6274 and the --sort option specifies a secondary key.
6275 However, any use of @option{--sort=none}
6276 (@option{-U}) disables this option altogether.
6278 @item --hide=PATTERN
6279 @opindex --hide=@var{pattern}
6280 In directories, ignore files whose names match the shell pattern
6281 @var{pattern}, unless the @option{--all} (@option{-a}) or
6282 @option{--almost-all} (@option{-A}) is also given. This
6283 option acts like @option{--ignore=@var{pattern}} except that it has no
6284 effect if @option{--all} (@option{-a}) or @option{--almost-all}
6285 (@option{-A}) is also given.
6287 This option can be useful in shell aliases. For example, if
6288 @command{lx} is an alias for @samp{ls --hide='*~'} and @command{ly} is
6289 an alias for @samp{ls --ignore='*~'}, then the command @samp{lx -A}
6290 lists the file @file{README~} even though @samp{ly -A} would not.
6292 @item -I @var{pattern}
6293 @itemx --ignore=@var{pattern}
6295 @opindex --ignore=@var{pattern}
6296 In directories, ignore files whose names match the shell pattern
6297 (not regular expression) @var{pattern}. As
6298 in the shell, an initial @samp{.} in a file name does not match a
6299 wildcard at the start of @var{pattern}. Sometimes it is useful
6300 to give this option several times. For example,
6303 $ ls --ignore='.??*' --ignore='.[^.]' --ignore='#*'
6306 The first option ignores names of length 3 or more that start with @samp{.},
6307 the second ignores all two-character names that start with @samp{.}
6308 except @samp{..}, and the third ignores names that start with @samp{#}.
6311 @itemx --dereference
6313 @opindex --dereference
6314 @cindex symbolic links, dereferencing
6315 When showing file information for a symbolic link, show information
6316 for the file the link references rather than the link itself.
6317 However, even with this option, @command{ls} still prints the name
6318 of the link itself, not the name of the file that the link points to.
6323 @opindex --recursive
6324 @cindex recursive directory listing
6325 @cindex directory listing, recursive
6326 List the contents of all directories recursively.
6331 @node What information is listed
6332 @subsection What information is listed
6334 These options affect the information that @command{ls} displays. By
6335 default, only file names are shown.
6341 @cindex hurd, author, printing
6342 List each file's author when producing long format directory listings.
6343 In GNU/Hurd, file authors can differ from their owners, but in other
6344 operating systems the two are the same.
6350 @cindex dired Emacs mode support
6351 With the long listing (@option{-l}) format, print an additional line after
6355 //DIRED// @var{beg1} @var{end1} @var{beg2} @var{end2} @dots{}
6359 The @var{begn} and @var{endn} are unsigned integers that record the
6360 byte position of the beginning and end of each file name in the output.
6361 This makes it easy for Emacs to find the names, even when they contain
6362 unusual characters such as space or newline, without fancy searching.
6364 If directories are being listed recursively (@option{-R}), output a similar
6365 line with offsets for each subdirectory name:
6368 //SUBDIRED// @var{beg1} @var{end1} @dots{}
6371 Finally, output a line of the form:
6374 //DIRED-OPTIONS// --quoting-style=@var{word}
6378 where @var{word} is the quoting style (@pxref{Formatting the file names}).
6380 Here is an actual example:
6383 $ mkdir -p a/sub/deeper a/sub2
6385 $ touch a/sub/deeper/file
6386 $ ls -gloRF --dired a
6389 -rw-r--r-- 1 0 Jun 10 12:27 f1
6390 -rw-r--r-- 1 0 Jun 10 12:27 f2
6391 drwxr-xr-x 3 4096 Jun 10 12:27 sub/
6392 drwxr-xr-x 2 4096 Jun 10 12:27 sub2/
6396 drwxr-xr-x 2 4096 Jun 10 12:27 deeper/
6400 -rw-r--r-- 1 0 Jun 10 12:27 file
6404 //DIRED// 48 50 84 86 120 123 158 162 217 223 282 286
6405 //SUBDIRED// 2 3 167 172 228 240 290 296
6406 //DIRED-OPTIONS// --quoting-style=literal
6409 Note that the pairs of offsets on the @samp{//DIRED//} line above delimit
6410 these names: @file{f1}, @file{f2}, @file{sub}, @file{sub2}, @file{deeper},
6412 The offsets on the @samp{//SUBDIRED//} line delimit the following
6413 directory names: @file{a}, @file{a/sub}, @file{a/sub/deeper}, @file{a/sub2}.
6415 Here is an example of how to extract the fifth entry name, @samp{deeper},
6416 corresponding to the pair of offsets, 222 and 228:
6419 $ ls -gloRF --dired a > out
6420 $ dd bs=1 skip=222 count=6 < out 2>/dev/null; echo
6424 Note that although the listing above includes a trailing slash
6425 for the @samp{deeper} entry, the offsets select the name without
6426 the trailing slash. However, if you invoke @command{ls} with @option{--dired}
6427 along with an option like @option{--escape} (aka @option{-b}) and operate
6428 on a file whose name contains special characters, notice that the backslash
6433 $ ls -blog --dired 'a b'
6434 -rw-r--r-- 1 0 Jun 10 12:28 a\ b
6436 //DIRED-OPTIONS// --quoting-style=escape
6439 If you use a quoting style that adds quote marks
6440 (e.g., @option{--quoting-style=c}), then the offsets include the quote marks.
6441 So beware that the user may select the quoting style via the environment
6442 variable @env{QUOTING_STYLE}. Hence, applications using @option{--dired}
6443 should either specify an explicit @option{--quoting-style=literal} option
6444 (aka @option{-N} or @option{--literal}) on the command line, or else be
6445 prepared to parse the escaped names.
6448 @opindex --full-time
6449 Produce long format directory listings, and list times in full. It is
6450 equivalent to using @option{--format=long} with
6451 @option{--time-style=full-iso} (@pxref{Formatting file timestamps}).
6455 Produce long format directory listings, but don't display owner information.
6461 Inhibit display of group information in a long format directory listing.
6462 (This is the default in some non-@sc{gnu} versions of @command{ls}, so we
6463 provide this option for compatibility.)
6471 @cindex inode number, printing
6472 Print the inode number (also called the file serial number and index
6473 number) of each file to the left of the file name. (This number
6474 uniquely identifies each file within a particular file system.)
6477 @itemx --format=long
6478 @itemx --format=verbose
6481 @opindex long ls @r{format}
6482 @opindex verbose ls @r{format}
6483 In addition to the name of each file, print the file type, file mode bits,
6484 number of hard links, owner name, group name, size, and
6485 timestamp (@pxref{Formatting file timestamps}), normally
6486 the modification time. Print question marks for information that
6487 cannot be determined.
6489 Normally the size is printed as a byte count without punctuation, but
6490 this can be overridden (@pxref{Block size}). For example, @option{-h}
6491 prints an abbreviated, human-readable count, and
6492 @samp{--block-size="'1"} prints a byte count with the thousands
6493 separator of the current locale.
6495 For each directory that is listed, preface the files with a line
6496 @samp{total @var{blocks}}, where @var{blocks} is the total disk allocation
6497 for all files in that directory. The block size currently defaults to 1024
6498 bytes, but this can be overridden (@pxref{Block size}).
6499 The @var{blocks} computed counts each hard link separately;
6500 this is arguably a deficiency.
6502 The file type is one of the following characters:
6504 @c The commented-out entries are ones we're not sure about.
6512 character special file
6514 high performance (``contiguous data'') file
6518 door (Solaris 2.5 and up)
6520 @c semaphore, if this is a distinct file type
6524 @c multiplexed file (7th edition Unix; obsolete)
6526 off-line (``migrated'') file (Cray DMF)
6528 network special file (HP-UX)
6532 port (Solaris 10 and up)
6534 @c message queue, if this is a distinct file type
6538 @c shared memory object, if this is a distinct file type
6540 @c typed memory object, if this is a distinct file type
6542 @c whiteout (4.4BSD; not implemented)
6544 some other file type
6547 @cindex permissions, output by @command{ls}
6548 The file mode bits listed are similar to symbolic mode specifications
6549 (@pxref{Symbolic Modes}). But @command{ls} combines multiple bits into the
6550 third character of each set of permissions as follows:
6554 If the set-user-ID or set-group-ID bit and the corresponding executable bit
6558 If the set-user-ID or set-group-ID bit is set but the corresponding
6559 executable bit is not set.
6562 If the restricted deletion flag or sticky bit, and the
6563 other-executable bit, are both set. The restricted deletion flag is
6564 another name for the sticky bit. @xref{Mode Structure}.
6567 If the restricted deletion flag or sticky bit is set but the
6568 other-executable bit is not set.
6571 If the executable bit is set and none of the above apply.
6577 Following the file mode bits is a single character that specifies
6578 whether an alternate access method such as an access control list
6579 applies to the file. When the character following the file mode bits is a
6580 space, there is no alternate access method. When it is a printing
6581 character, then there is such a method.
6583 GNU @command{ls} uses a @samp{.} character to indicate a file
6584 with an SELinux security context, but no other alternate access method.
6586 A file with any other combination of alternate access methods
6587 is marked with a @samp{+} character.
6590 @itemx --numeric-uid-gid
6592 @opindex --numeric-uid-gid
6593 @cindex numeric uid and gid
6594 @cindex numeric user and group IDs
6595 Produce long format directory listings, but
6596 display numeric user and group IDs instead of the owner and group names.
6600 Produce long format directory listings, but don't display group information.
6601 It is equivalent to using @option{--format=long} with @option{--no-group} .
6607 @cindex disk allocation
6608 @cindex size of files, reporting
6609 Print the disk allocation of each file to the left of the file name.
6610 This is the amount of disk space used by the file, which is usually a
6611 bit more than the file's size, but it can be less if the file has holes.
6613 Normally the disk allocation is printed in units of
6614 1024 bytes, but this can be overridden (@pxref{Block size}).
6616 @cindex NFS mounts from BSD to HP-UX
6617 For files that are NFS-mounted from an HP-UX system to a BSD system,
6618 this option reports sizes that are half the correct values. On HP-UX
6619 systems, it reports sizes that are twice the correct values for files
6620 that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX;
6621 it also affects the HP-UX @command{ls} program.
6630 @cindex security context
6631 Display the SELinux security context or @samp{?} if none is found.
6632 When used with the @option{-l} option, print the security context
6633 to the left of the size column.
6638 @node Sorting the output
6639 @subsection Sorting the output
6641 @cindex sorting @command{ls} output
6642 These options change the order in which @command{ls} sorts the information
6643 it outputs. By default, sorting is done by character code
6644 (e.g., @acronym{ASCII} order).
6650 @itemx --time=status
6653 @opindex ctime@r{, printing or sorting by}
6654 @opindex status time@r{, printing or sorting by}
6655 @opindex use time@r{, printing or sorting files by}
6656 If the long listing format (e.g., @option{-l}, @option{-o}) is being used,
6657 print the status change time (the @samp{ctime} in the inode) instead of
6658 the modification time.
6659 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6660 or when not using a long listing format,
6661 sort according to the status change time.
6665 @cindex unsorted directory listing
6666 @cindex directory order, listing by
6667 Primarily, like @option{-U}---do not sort; list the files in whatever
6668 order they are stored in the directory. But also enable @option{-a} (list
6669 all files) and disable @option{-l}, @option{--color}, and @option{-s} (if they
6670 were specified before the @option{-f}).
6676 @cindex reverse sorting
6677 Reverse whatever the sorting method is---e.g., list files in reverse
6678 alphabetical order, youngest first, smallest first, or whatever.
6684 @opindex size of files@r{, sorting files by}
6685 Sort by file size, largest first.
6691 @opindex modification time@r{, sorting files by}
6692 Sort by modification time (the @samp{mtime} in the inode), newest first.
6696 @itemx --time=access
6700 @opindex use time@r{, printing or sorting files by}
6701 @opindex atime@r{, printing or sorting files by}
6702 @opindex access time@r{, printing or sorting files by}
6703 If the long listing format (e.g., @option{--format=long}) is being used,
6704 print the last access time (the @samp{atime} in the inode).
6705 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6706 or when not using a long listing format, sort according to the access time.
6712 @opindex none@r{, sorting option for @command{ls}}
6713 Do not sort; list the files in whatever order they are
6714 stored in the directory. (Do not do any of the other unrelated things
6715 that @option{-f} does.) This is especially useful when listing very large
6716 directories, since not doing any sorting can be noticeably faster.
6719 @itemx --sort=version
6722 @opindex version@r{, sorting option for @command{ls}}
6723 Sort by version name and number, lowest first. It behaves like a default
6724 sort, except that each sequence of decimal digits is treated numerically
6725 as an index/version number. (@xref{Details about version sort}.)
6728 @itemx --sort=extension
6731 @opindex extension@r{, sorting files by}
6732 Sort directory contents alphabetically by file extension (characters
6733 after the last @samp{.}); files with no extension are sorted first.
6738 @node Details about version sort
6739 @subsection Details about version sort
6741 Version sorting handles the fact that file names frequently include indices or
6742 version numbers. Standard sorting usually does not produce the order that one
6743 expects because comparisons are made on a character-by-character basis.
6744 Version sorting is especially useful when browsing directories that contain
6745 many files with indices/version numbers in their names:
6749 abc.zml-1.gz abc.zml-1.gz
6750 abc.zml-12.gz abc.zml-2.gz
6751 abc.zml-2.gz abc.zml-12.gz
6754 Version-sorted strings are compared such that if @var{ver1} and @var{ver2}
6755 are version numbers and @var{prefix} and @var{suffix} (@var{suffix} matching
6756 the regular expression @samp{(\.[A-Za-z~][A-Za-z0-9~]*)*}) are strings then
6757 @var{ver1} < @var{ver2} implies that the name composed of
6758 ``@var{prefix} @var{ver1} @var{suffix}'' sorts before
6759 ``@var{prefix} @var{ver2} @var{suffix}''.
6761 Note also that leading zeros of numeric parts are ignored:
6765 abc-1.007.tgz abc-1.01a.tgz
6766 abc-1.012b.tgz abc-1.007.tgz
6767 abc-1.01a.tgz abc-1.012b.tgz
6770 This functionality is implemented using gnulib's @code{filevercmp} function,
6771 which has some caveats worth noting.
6774 @item @env{LC_COLLATE} is ignored, which means @samp{ls -v} and @samp{sort -V}
6775 will sort non-numeric prefixes as if the @env{LC_COLLATE} locale category
6776 was set to @samp{C}.
6777 @item Some suffixes will not be matched by the regular
6778 expression mentioned above. Consequently these examples may
6779 not sort as you expect:
6787 abc-1.2.3.4.x86_64.rpm
6788 abc-1.2.3.x86_64.rpm
6792 @node General output formatting
6793 @subsection General output formatting
6795 These options affect the appearance of the overall output.
6800 @itemx --format=single-column
6803 @opindex single-column @r{output of files}
6804 List one file per line. This is the default for @command{ls} when standard
6805 output is not a terminal.
6808 @itemx --format=vertical
6811 @opindex vertical @r{sorted files in columns}
6812 List files in columns, sorted vertically. This is the default for
6813 @command{ls} if standard output is a terminal. It is always the default
6814 for the @command{dir} program.
6815 @sc{gnu} @command{ls} uses variable width columns to display as many files as
6816 possible in the fewest lines.
6818 @item --color [=@var{when}]
6820 @cindex color, distinguishing file types with
6821 Specify whether to use color for distinguishing file types. @var{when}
6822 may be omitted, or one of:
6825 @vindex none @r{color option}
6826 - Do not use color at all. This is the default.
6828 @vindex auto @r{color option}
6829 @cindex terminal, using color iff
6830 - Only use color if standard output is a terminal.
6832 @vindex always @r{color option}
6835 Specifying @option{--color} and no @var{when} is equivalent to
6836 @option{--color=always}.
6837 Piping a colorized listing through a pager like @command{more} or
6838 @command{less} usually produces unreadable results. However, using
6839 @code{more -f} does seem to work.
6843 @itemx --indicator-style=classify
6846 @opindex --indicator-style
6847 @cindex file type and executables, marking
6848 @cindex executables and file type, marking
6849 Append a character to each file name indicating the file type. Also,
6850 for regular files that are executable, append @samp{*}. The file type
6851 indicators are @samp{/} for directories, @samp{@@} for symbolic links,
6852 @samp{|} for FIFOs, @samp{=} for sockets, @samp{>} for doors,
6853 and nothing for regular files.
6854 @c The following sentence is the same as the one for -d.
6855 Do not follow symbolic links listed on the
6856 command line unless the @option{--dereference-command-line} (@option{-H}),
6857 @option{--dereference} (@option{-L}), or
6858 @option{--dereference-command-line-symlink-to-dir} options are specified.
6861 @itemx --indicator-style=file-type
6862 @opindex --file-type
6863 @opindex --indicator-style
6864 @cindex file type, marking
6865 Append a character to each file name indicating the file type. This is
6866 like @option{-F}, except that executables are not marked.
6868 @item --indicator-style=@var{word}
6869 @opindex --indicator-style
6870 Append a character indicator with style @var{word} to entry names,
6875 Do not append any character indicator; this is the default.
6877 Append @samp{/} for directories. This is the same as the @option{-p}
6880 Append @samp{/} for directories, @samp{@@} for symbolic links, @samp{|}
6881 for FIFOs, @samp{=} for sockets, and nothing for regular files. This is
6882 the same as the @option{--file-type} option.
6884 Append @samp{*} for executable regular files, otherwise behave as for
6885 @samp{file-type}. This is the same as the @option{-F} or
6886 @option{--classify} option.
6891 Print file sizes in 1024-byte blocks, overriding the default block
6892 size (@pxref{Block size}).
6893 This option is equivalent to @option{--block-size=1K}.
6896 @itemx --format=commas
6899 @opindex commas@r{, outputting between files}
6900 List files horizontally, with as many as will fit on each line,
6901 separated by @samp{, } (a comma and a space).
6904 @itemx --indicator-style=slash
6906 @opindex --indicator-style
6907 @cindex file type, marking
6908 Append a @samp{/} to directory names.
6911 @itemx --format=across
6912 @itemx --format=horizontal
6915 @opindex across@r{, listing files}
6916 @opindex horizontal@r{, listing files}
6917 List the files in columns, sorted horizontally.
6920 @itemx --tabsize=@var{cols}
6923 Assume that each tab stop is @var{cols} columns wide. The default is 8.
6924 @command{ls} uses tabs where possible in the output, for efficiency. If
6925 @var{cols} is zero, do not use tabs at all.
6927 @c FIXME: remove in 2009, if Apple Terminal has been fixed for long enough.
6928 Some terminal emulators (at least Apple Terminal 1.5 (133) from Mac OS X 10.4.8)
6929 do not properly align columns to the right of a TAB following a
6930 non-@acronym{ASCII} byte. If you use such a terminal emulator, use the
6931 @option{-T0} option or put @code{TABSIZE=0} in your environment to tell
6932 @command{ls} to align using spaces, not tabs.
6935 @itemx --width=@var{cols}
6939 Assume the screen is @var{cols} columns wide. The default is taken
6940 from the terminal settings if possible; otherwise the environment
6941 variable @env{COLUMNS} is used if it is set; otherwise the default
6947 @node Formatting file timestamps
6948 @subsection Formatting file timestamps
6950 By default, file timestamps are listed in abbreviated form, using
6951 a date like @samp{Mar 30@ @ 2002} for non-recent timestamps, and a
6952 date-without-year and time like @samp{Mar 30 23:45} for recent timestamps.
6953 This format can change depending on the current locale as detailed below.
6955 A timestamp is considered to be @dfn{recent} if it is less than six
6956 months old, and is not dated in the future. If a timestamp dated
6957 today is not listed in recent form, the timestamp is in the future,
6958 which means you probably have clock skew problems which may break
6959 programs like @command{make} that rely on file timestamps.
6962 Time stamps are listed according to the time zone rules specified by
6963 the @env{TZ} environment variable, or by the system default rules if
6964 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
6965 with @env{TZ}, libc, The GNU C Library Reference Manual}.
6967 The following option changes how file timestamps are printed.
6970 @item --time-style=@var{style}
6971 @opindex --time-style
6973 List timestamps in style @var{style}. The @var{style} should
6974 be one of the following:
6979 List timestamps using @var{format}, where @var{format} is interpreted
6980 like the format argument of @command{date} (@pxref{date invocation}).
6981 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
6982 @command{ls} to list timestamps like @samp{2002-03-30 23:45:56}. As
6983 with @command{date}, @var{format}'s interpretation is affected by the
6984 @env{LC_TIME} locale category.
6986 If @var{format} contains two format strings separated by a newline,
6987 the former is used for non-recent files and the latter for recent
6988 files; if you want output columns to line up, you may need to insert
6989 spaces in one of the two formats.
6992 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
6993 format with nanosecond precision, e.g., @samp{2002-03-30
6994 23:45:56.477817180 -0700}. This style is equivalent to
6995 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
6997 This is useful because the time output includes all the information that
6998 is available from the operating system. For example, this can help
6999 explain @command{make}'s behavior, since @acronym{GNU} @command{make}
7000 uses the full timestamp to determine whether a file is out of date.
7003 List @acronym{ISO} 8601 date and time in minutes, e.g.,
7004 @samp{2002-03-30 23:45}. These timestamps are shorter than
7005 @samp{full-iso} timestamps, and are usually good enough for everyday
7006 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
7009 List @acronym{ISO} 8601 dates for non-recent timestamps (e.g.,
7010 @samp{2002-03-30@ }), and @acronym{ISO} 8601 month, day, hour, and
7011 minute for recent timestamps (e.g., @samp{03-30 23:45}). These
7012 timestamps are uglier than @samp{long-iso} timestamps, but they carry
7013 nearly the same information in a smaller space and their brevity helps
7014 @command{ls} output fit within traditional 80-column output lines.
7015 The following two @command{ls} invocations are equivalent:
7020 ls -l --time-style="+%Y-%m-%d $newline%m-%d %H:%M"
7021 ls -l --time-style="iso"
7026 List timestamps in a locale-dependent form. For example, a Finnish
7027 locale might list non-recent timestamps like @samp{maalis 30@ @ 2002}
7028 and recent timestamps like @samp{maalis 30 23:45}. Locale-dependent
7029 timestamps typically consume more space than @samp{iso} timestamps and
7030 are harder for programs to parse because locale conventions vary so
7031 widely, but they are easier for many people to read.
7033 The @env{LC_TIME} locale category specifies the timestamp format. The
7034 default @acronym{POSIX} locale uses timestamps like @samp{Mar 30@
7035 @ 2002} and @samp{Mar 30 23:45}; in this locale, the following two
7036 @command{ls} invocations are equivalent:
7041 ls -l --time-style="+%b %e %Y$newline%b %e %H:%M"
7042 ls -l --time-style="locale"
7045 Other locales behave differently. For example, in a German locale,
7046 @option{--time-style="locale"} might be equivalent to
7047 @option{--time-style="+%e. %b %Y $newline%e. %b %H:%M"}
7048 and might generate timestamps like @samp{30. M@"ar 2002@ } and
7049 @samp{30. M@"ar 23:45}.
7051 @item posix-@var{style}
7053 List @acronym{POSIX}-locale timestamps if the @env{LC_TIME} locale
7054 category is @acronym{POSIX}, @var{style} timestamps otherwise. For
7055 example, the @samp{posix-long-iso} style lists
7056 timestamps like @samp{Mar 30@ @ 2002} and @samp{Mar 30 23:45} when in
7057 the @acronym{POSIX} locale, and like @samp{2002-03-30 23:45} otherwise.
7062 You can specify the default value of the @option{--time-style} option
7063 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
7064 the default style is @samp{locale}. @acronym{GNU} Emacs 21.3 and
7065 later use the @option{--dired} option and therefore can parse any date
7066 format, but if you are using Emacs 21.1 or 21.2 and specify a
7067 non-@acronym{POSIX} locale you may need to set
7068 @samp{TIME_STYLE="posix-long-iso"}.
7070 To avoid certain denial-of-service attacks, timestamps that would be
7071 longer than 1000 bytes may be treated as errors.
7074 @node Formatting the file names
7075 @subsection Formatting the file names
7077 These options change how file names themselves are printed.
7083 @itemx --quoting-style=escape
7086 @opindex --quoting-style
7087 @cindex backslash sequences for file names
7088 Quote nongraphic characters in file names using alphabetic and octal
7089 backslash sequences like those used in C.
7093 @itemx --quoting-style=literal
7096 @opindex --quoting-style
7097 Do not quote file names. However, with @command{ls} nongraphic
7098 characters are still printed as question marks if the output is a
7099 terminal and you do not specify the @option{--show-control-chars}
7103 @itemx --hide-control-chars
7105 @opindex --hide-control-chars
7106 Print question marks instead of nongraphic characters in file names.
7107 This is the default if the output is a terminal and the program is
7112 @itemx --quoting-style=c
7114 @opindex --quote-name
7115 @opindex --quoting-style
7116 Enclose file names in double quotes and quote nongraphic characters as
7119 @item --quoting-style=@var{word}
7120 @opindex --quoting-style
7121 @cindex quoting style
7122 Use style @var{word} to quote file names and other strings that may
7123 contain arbitrary characters. The @var{word} should
7124 be one of the following:
7128 Output strings as-is; this is the same as the @option{-N} or
7129 @option{--literal} option.
7131 Quote strings for the shell if they contain shell metacharacters or would
7132 cause ambiguous output.
7133 The quoting is suitable for @acronym{POSIX}-compatible shells like
7134 @command{bash}, but it does not always work for incompatible shells
7137 Quote strings for the shell, even if they would normally not require quoting.
7139 Quote strings as for C character string literals, including the
7140 surrounding double-quote characters; this is the same as the
7141 @option{-Q} or @option{--quote-name} option.
7143 Quote strings as for C character string literals, except omit the
7144 surrounding double-quote
7145 characters; this is the same as the @option{-b} or @option{--escape} option.
7147 Quote strings as for C character string literals, except use
7148 surrounding quotation marks appropriate for the
7151 @c Use @t instead of @samp to avoid duplicate quoting in some output styles.
7152 Quote strings as for C character string literals, except use
7153 surrounding quotation marks appropriate for the locale, and quote
7154 @t{`like this'} instead of @t{"like
7155 this"} in the default C locale. This looks nicer on many displays.
7158 You can specify the default value of the @option{--quoting-style} option
7159 with the environment variable @env{QUOTING_STYLE}. If that environment
7160 variable is not set, the default value is @samp{literal}, but this
7161 default may change to @samp{shell} in a future version of this package.
7163 @item --show-control-chars
7164 @opindex --show-control-chars
7165 Print nongraphic characters as-is in file names.
7166 This is the default unless the output is a terminal and the program is
7172 @node dir invocation
7173 @section @command{dir}: Briefly list directory contents
7176 @cindex directory listing, brief
7178 @command{dir} is equivalent to @code{ls -C
7179 -b}; that is, by default files are listed in columns, sorted vertically,
7180 and special characters are represented by backslash escape sequences.
7182 @xref{ls invocation, @command{ls}}.
7185 @node vdir invocation
7186 @section @command{vdir}: Verbosely list directory contents
7189 @cindex directory listing, verbose
7191 @command{vdir} is equivalent to @code{ls -l
7192 -b}; that is, by default files are listed in long format and special
7193 characters are represented by backslash escape sequences.
7195 @node dircolors invocation
7196 @section @command{dircolors}: Color setup for @command{ls}
7200 @cindex setup for color
7202 @command{dircolors} outputs a sequence of shell commands to set up the
7203 terminal for color output from @command{ls} (and @command{dir}, etc.).
7207 eval "`dircolors [@var{option}]@dots{} [@var{file}]`"
7210 If @var{file} is specified, @command{dircolors} reads it to determine which
7211 colors to use for which file types and extensions. Otherwise, a
7212 precompiled database is used. For details on the format of these files,
7213 run @samp{dircolors --print-database}.
7215 To make @command{dircolors} read a @file{~/.dircolors} file if it
7216 exists, you can put the following lines in your @file{~/.bashrc} (or
7217 adapt them to your favorite shell):
7221 test -r $d && eval "$(dircolors $d)"
7225 @vindex SHELL @r{environment variable, and color}
7226 The output is a shell command to set the @env{LS_COLORS} environment
7227 variable. You can specify the shell syntax to use on the command line,
7228 or @command{dircolors} will guess it from the value of the @env{SHELL}
7229 environment variable.
7231 The program accepts the following options. Also see @ref{Common options}.
7236 @itemx --bourne-shell
7239 @opindex --bourne-shell
7240 @cindex Bourne shell syntax for color setup
7241 @cindex @command{sh} syntax for color setup
7242 Output Bourne shell commands. This is the default if the @env{SHELL}
7243 environment variable is set and does not end with @samp{csh} or
7252 @cindex C shell syntax for color setup
7253 @cindex @command{csh} syntax for color setup
7254 Output C shell commands. This is the default if @code{SHELL} ends with
7255 @command{csh} or @command{tcsh}.
7258 @itemx --print-database
7260 @opindex --print-database
7261 @cindex color database, printing
7262 @cindex database for color setup, printing
7263 @cindex printing color database
7264 Print the (compiled-in) default color configuration database. This
7265 output is itself a valid configuration file, and is fairly descriptive
7266 of the possibilities.
7273 @node Basic operations
7274 @chapter Basic operations
7276 @cindex manipulating files
7278 This chapter describes the commands for basic file manipulation:
7279 copying, moving (renaming), and deleting (removing).
7282 * cp invocation:: Copy files.
7283 * dd invocation:: Convert and copy a file.
7284 * install invocation:: Copy files and set attributes.
7285 * mv invocation:: Move (rename) files.
7286 * rm invocation:: Remove files or directories.
7287 * shred invocation:: Remove files more securely.
7292 @section @command{cp}: Copy files and directories
7295 @cindex copying files and directories
7296 @cindex files, copying
7297 @cindex directories, copying
7299 @command{cp} copies files (or, optionally, directories). The copy is
7300 completely independent of the original. You can either copy one file to
7301 another, or copy arbitrarily many files to a destination directory.
7305 cp [@var{option}]@dots{} [-T] @var{source} @var{dest}
7306 cp [@var{option}]@dots{} @var{source}@dots{} @var{directory}
7307 cp [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
7312 If two file names are given, @command{cp} copies the first file to the
7316 If the @option{--target-directory} (@option{-t}) option is given, or
7317 failing that if the last file is a directory and the
7318 @option{--no-target-directory} (@option{-T}) option is not given,
7319 @command{cp} copies each @var{source} file to the specified directory,
7320 using the @var{source}s' names.
7323 Generally, files are written just as they are read. For exceptions,
7324 see the @option{--sparse} option below.
7326 By default, @command{cp} does not copy directories. However, the
7327 @option{-R}, @option{-a}, and @option{-r} options cause @command{cp} to
7328 copy recursively by descending into source directories and copying files
7329 to corresponding destination directories.
7331 When copying from a symbolic link, @command{cp} normally follows the
7332 link only when not copying
7333 recursively. This default can be overridden with the
7334 @option{--archive} (@option{-a}), @option{-d}, @option{--dereference}
7335 (@option{-L}), @option{--no-dereference} (@option{-P}), and
7336 @option{-H} options. If more than one of these options is specified,
7337 the last one silently overrides the others.
7339 When copying to a symbolic link, @command{cp} follows the
7340 link only when it refers to an existing regular file.
7341 However, when copying to a dangling symbolic link, @command{cp}
7342 refuses by default, and fails with a diagnostic, since the operation
7343 is inherently dangerous. This behavior is contrary to historical
7344 practice and to @acronym{POSIX}.
7345 Set @env{POSIXLY_CORRECT} to make @command{cp} attempt to create
7346 the target of a dangling destination symlink, in spite of the possible risk.
7347 Also, when an option like
7348 @option{--backup} or @option{--link} acts to rename or remove the
7349 destination before copying, @command{cp} renames or removes the
7350 symbolic link rather than the file it points to.
7352 By default, @command{cp} copies the contents of special files only
7353 when not copying recursively. This default can be overridden with the
7354 @option{--copy-contents} option.
7356 @cindex self-backups
7357 @cindex backups, making only
7358 @command{cp} generally refuses to copy a file onto itself, with the
7359 following exception: if @option{--force --backup} is specified with
7360 @var{source} and @var{dest} identical, and referring to a regular file,
7361 @command{cp} will make a backup file, either regular or numbered, as
7362 specified in the usual ways (@pxref{Backup options}). This is useful when
7363 you simply want to make a backup of an existing file before changing it.
7365 The program accepts the following options. Also see @ref{Common options}.
7372 Preserve as much as possible of the structure and attributes of the
7373 original files in the copy (but do not attempt to preserve internal
7374 directory structure; i.e., @samp{ls -U} may list the entries in a copied
7375 directory in a different order).
7376 Try to preserve SELinux security context and extended attributes (xattr),
7377 but ignore any failure to do that and print no corresponding diagnostic.
7378 Equivalent to @option{-dR --preserve=all} with the reduced diagnostics.
7380 @itemx --attributes-only
7381 @opindex --attributes-only
7382 Preserve the specified attributes of the original files in the copy,
7383 but do not copy any data. See the @option{--preserve} option for
7384 controlling which attributes to copy.
7387 @itemx @w{@kbd{--backup}[=@var{method}]}
7390 @vindex VERSION_CONTROL
7391 @cindex backups, making
7392 @xref{Backup options}.
7393 Make a backup of each file that would otherwise be overwritten or removed.
7394 As a special case, @command{cp} makes a backup of @var{source} when the force
7395 and backup options are given and @var{source} and @var{dest} are the same
7396 name for an existing, regular file. One useful application of this
7397 combination of options is this tiny Bourne shell script:
7401 # Usage: backup FILE...
7402 # Create a @sc{gnu}-style backup of each listed FILE.
7404 cp --backup --force -- "$i" "$i"
7408 @item --copy-contents
7409 @cindex directories, copying recursively
7410 @cindex copying directories recursively
7411 @cindex recursively copying directories
7412 @cindex non-directories, copying as special files
7413 If copying recursively, copy the contents of any special files (e.g.,
7414 FIFOs and device files) as if they were regular files. This means
7415 trying to read the data in each source file and writing it to the
7416 destination. It is usually a mistake to use this option, as it
7417 normally has undesirable effects on special files like FIFOs and the
7418 ones typically found in the @file{/dev} directory. In most cases,
7419 @code{cp -R --copy-contents} will hang indefinitely trying to read
7420 from FIFOs and special files like @file{/dev/console}, and it will
7421 fill up your destination disk if you use it to copy @file{/dev/zero}.
7422 This option has no effect unless copying recursively, and it does not
7423 affect the copying of symbolic links.
7427 @cindex symbolic links, copying
7428 @cindex hard links, preserving
7429 Copy symbolic links as symbolic links rather than copying the files that
7430 they point to, and preserve hard links between source files in the copies.
7431 Equivalent to @option{--no-dereference --preserve=links}.
7437 When copying without this option and an existing destination file cannot
7438 be opened for writing, the copy fails. However, with @option{--force}),
7439 when a destination file cannot be opened, @command{cp} then removes it and
7440 tries to open it again. Contrast this behavior with that enabled by
7441 @option{--link} and @option{--symbolic-link}, whereby the destination file
7442 is never opened but rather is removed unconditionally. Also see the
7443 description of @option{--remove-destination}.
7445 This option is independent of the @option{--interactive} or
7446 @option{-i} option: neither cancels the effect of the other.
7448 This option is redundant if the @option{--no-clobber} or @option{-n} option is
7453 If a command line argument specifies a symbolic link, then copy the
7454 file it points to rather than the symbolic link itself. However,
7455 copy (preserving its nature) any symbolic link that is encountered
7456 via recursive traversal.
7459 @itemx --interactive
7461 @opindex --interactive
7462 When copying a file other than a directory, prompt whether to
7463 overwrite an existing destination file. The @option{-i} option overrides
7464 a previous @option{-n} option.
7470 Make hard links instead of copies of non-directories.
7473 @itemx --dereference
7475 @opindex --dereference
7476 Follow symbolic links when copying from them.
7477 With this option, @command{cp} cannot create a symbolic link.
7478 For example, a symlink (to regular file) in the source tree will be copied to
7479 a regular file in the destination tree.
7484 @opindex --no-clobber
7485 Do not overwrite an existing file. The @option{-n} option overrides a previous
7486 @option{-i} option. This option is mutually exclusive with @option{-b} or
7487 @option{--backup} option.
7490 @itemx --no-dereference
7492 @opindex --no-dereference
7493 @cindex symbolic links, copying
7494 Copy symbolic links as symbolic links rather than copying the files that
7495 they point to. This option affects only symbolic links in the source;
7496 symbolic links in the destination are always followed if possible.
7499 @itemx @w{@kbd{--preserve}[=@var{attribute_list}]}
7502 @cindex file information, preserving, extended attributes, xattr
7503 Preserve the specified attributes of the original files.
7504 If specified, the @var{attribute_list} must be a comma-separated list
7505 of one or more of the following strings:
7509 Preserve the file mode bits and access control lists.
7511 Preserve the owner and group. On most modern systems,
7512 only users with appropriate privileges may change the owner of a file,
7514 may preserve the group ownership of a file only if they happen to be
7515 a member of the desired group.
7517 Preserve the times of last access and last modification, when possible.
7518 On older systems, it is not possible to preserve these attributes
7519 when the affected file is a symbolic link.
7520 However, many systems now provide the @code{utimensat} function,
7521 which makes it possible even for symbolic links.
7523 Preserve in the destination files
7524 any links between corresponding source files.
7525 Note that with @option{-L} or @option{-H}, this option can convert
7526 symbolic links to hard links. For example,
7528 $ mkdir c; : > a; ln -s a b; cp -aH a b c; ls -i1 c
7533 Note the inputs: @file{b} is a symlink to regular file @file{a},
7534 yet the files in destination directory, @file{c/}, are hard-linked.
7535 Since @option{-a} implies @option{--preserve=links}, and since @option{-H}
7536 tells @command{cp} to dereference command line arguments, it sees two files
7537 with the same inode number, and preserves the perceived hard link.
7539 Here is a similar example that exercises @command{cp}'s @option{-L} option:
7541 $ mkdir b c; (cd b; : > a; ln -s a b); cp -aL b c; ls -i1 c/b
7547 Preserve SELinux security context of the file, or fail with full diagnostics.
7549 Preserve extended attributes of the file, or fail with full diagnostics.
7550 If @command{cp} is built without xattr support, ignore this option.
7551 If SELinux context, ACLs or Capabilities are implemented using xattrs,
7552 they are preserved by this option as well.
7554 Preserve all file attributes.
7555 Equivalent to specifying all of the above, but with the difference
7556 that failure to preserve SELinux security context or extended attributes
7557 does not change @command{cp}'s exit status. In contrast to @option{-a},
7558 all but @samp{Operation not supported} warnings are output.
7561 Using @option{--preserve} with no @var{attribute_list} is equivalent
7562 to @option{--preserve=mode,ownership,timestamps}.
7564 In the absence of this option, each destination file is created with the
7565 mode bits of the corresponding source file, minus the bits set in the
7566 umask and minus the set-user-ID and set-group-ID bits.
7567 @xref{File permissions}.
7569 @itemx @w{@kbd{--no-preserve}=@var{attribute_list}}
7570 @cindex file information, preserving
7571 Do not preserve the specified attributes. The @var{attribute_list}
7572 has the same form as for @option{--preserve}.
7576 @cindex parent directories and @command{cp}
7577 Form the name of each destination file by appending to the target
7578 directory a slash and the specified name of the source file. The last
7579 argument given to @command{cp} must be the name of an existing directory.
7580 For example, the command:
7583 cp --parents a/b/c existing_dir
7587 copies the file @file{a/b/c} to @file{existing_dir/a/b/c}, creating
7588 any missing intermediate directories.
7595 @opindex --recursive
7596 @cindex directories, copying recursively
7597 @cindex copying directories recursively
7598 @cindex recursively copying directories
7599 @cindex non-directories, copying as special files
7600 Copy directories recursively. By default, do not follow symbolic
7601 links in the source; see the @option{--archive} (@option{-a}), @option{-d},
7602 @option{--dereference} (@option{-L}), @option{--no-dereference}
7603 (@option{-P}), and @option{-H} options. Special files are copied by
7604 creating a destination file of the same type as the source; see the
7605 @option{--copy-contents} option. It is not portable to use
7606 @option{-r} to copy symbolic links or special files. On some
7607 non-@sc{gnu} systems, @option{-r} implies the equivalent of
7608 @option{-L} and @option{--copy-contents} for historical reasons.
7609 Also, it is not portable to use @option{-R} to copy symbolic links
7610 unless you also specify @option{-P}, as @acronym{POSIX} allows
7611 implementations that dereference symbolic links by default.
7613 @item --reflink[=@var{when}]
7614 @opindex --reflink[=@var{when}]
7617 @cindex copy on write
7618 Perform a lightweight, copy-on-write (COW) copy, if supported by the
7619 file system. Once it has succeeded, beware that the source and destination
7620 files share the same disk data blocks as long as they remain unmodified.
7621 Thus, if a disk I/O error affects data blocks of one of the files,
7622 the other suffers the same fate.
7624 The @var{when} value can be one of the following:
7628 The default behavior: if the copy-on-write operation is not supported
7629 then report the failure for each file and exit with a failure status.
7632 If the copy-on-write operation is not supported then fall back
7633 to the standard copy behaviour.
7636 This option is overridden by the @option{--link}, @option{--symbolic-link}
7637 and @option{--attributes-only} options, thus allowing it to be used
7638 to configure the default data copying behavior for @command{cp}.
7639 For example, with the following alias, @command{cp} will use the
7640 minimum amount of space supported by the file system.
7643 alias cp='cp --reflink=auto --sparse=always'
7646 @item --remove-destination
7647 @opindex --remove-destination
7648 Remove each existing destination file before attempting to open it
7649 (contrast with @option{-f} above).
7651 @item --sparse=@var{when}
7652 @opindex --sparse=@var{when}
7653 @cindex sparse files, copying
7654 @cindex holes, copying files with
7655 @findex read @r{system call, and holes}
7656 A @dfn{sparse file} contains @dfn{holes}---a sequence of zero bytes that
7657 does not occupy any physical disk blocks; the @samp{read} system call
7658 reads these as zeros. This can both save considerable disk space and
7659 increase speed, since many binary files contain lots of consecutive zero
7660 bytes. By default, @command{cp} detects holes in input source files via a crude
7661 heuristic and makes the corresponding output file sparse as well.
7662 Only regular files may be sparse.
7664 The @var{when} value can be one of the following:
7668 The default behavior: if the input file is sparse, attempt to make
7669 the output file sparse, too. However, if an output file exists but
7670 refers to a non-regular file, then do not attempt to make it sparse.
7673 For each sufficiently long sequence of zero bytes in the input file,
7674 attempt to create a corresponding hole in the output file, even if the
7675 input file does not appear to be sparse.
7676 This is useful when the input file resides on a file system
7677 that does not support sparse files
7678 (for example, @samp{efs} file systems in SGI IRIX 5.3 and earlier),
7679 but the output file is on a type of file system that does support them.
7680 Holes may be created only in regular files, so if the destination file
7681 is of some other type, @command{cp} does not even try to make it sparse.
7684 Never make the output file sparse.
7685 This is useful in creating a file for use with the @command{mkswap} command,
7686 since such a file must not have any holes.
7689 @optStripTrailingSlashes
7692 @itemx --symbolic-link
7694 @opindex --symbolic-link
7695 @cindex symbolic links, copying with
7696 Make symbolic links instead of copies of non-directories. All source
7697 file names must be absolute (starting with @samp{/}) unless the
7698 destination files are in the current directory. This option merely
7699 results in an error message on systems that do not support symbolic links.
7705 @optNoTargetDirectory
7711 @cindex newer files, copying only
7712 Do not copy a non-directory that has an existing destination with the
7713 same or newer modification time. If time stamps are being preserved,
7714 the comparison is to the source time stamp truncated to the
7715 resolutions of the destination file system and of the system calls
7716 used to update time stamps; this avoids duplicate work if several
7717 @samp{cp -pu} commands are executed with the same source and
7724 Print the name of each file before copying it.
7727 @itemx --one-file-system
7729 @opindex --one-file-system
7730 @cindex file systems, omitting copying to different
7731 Skip subdirectories that are on different file systems from the one that
7732 the copy started on.
7733 However, mount point directories @emph{are} copied.
7741 @section @command{dd}: Convert and copy a file
7744 @cindex converting while copying a file
7746 @command{dd} copies a file (from standard input to standard output, by
7747 default) with a changeable I/O block size, while optionally performing
7748 conversions on it. Synopses:
7751 dd [@var{operand}]@dots{}
7755 The only options are @option{--help} and @option{--version}.
7756 @xref{Common options}. @command{dd} accepts the following operands.
7762 Read from @var{file} instead of standard input.
7766 Write to @var{file} instead of standard output. Unless
7767 @samp{conv=notrunc} is given, @command{dd} truncates @var{file} to zero
7768 bytes (or the size specified with @samp{seek=}).
7770 @item ibs=@var{bytes}
7772 @cindex block size of input
7773 @cindex input block size
7774 Set the input block size to @var{bytes}.
7775 This makes @command{dd} read @var{bytes} per block.
7776 The default is 512 bytes.
7778 @item obs=@var{bytes}
7780 @cindex block size of output
7781 @cindex output block size
7782 Set the output block size to @var{bytes}.
7783 This makes @command{dd} write @var{bytes} per block.
7784 The default is 512 bytes.
7786 @item bs=@var{bytes}
7789 Set both input and output block sizes to @var{bytes}.
7790 This makes @command{dd} read and write @var{bytes} per block,
7791 overriding any @samp{ibs} and @samp{obs} settings.
7792 In addition, if no data-transforming @option{conv} option is specified,
7793 each input block is copied to the output as a single block,
7794 without aggregating short reads.
7796 @item cbs=@var{bytes}
7798 @cindex block size of conversion
7799 @cindex conversion block size
7800 @cindex fixed-length records, converting to variable-length
7801 @cindex variable-length records, converting to fixed-length
7802 Set the conversion block size to @var{bytes}.
7803 When converting variable-length records to fixed-length ones
7804 (@option{conv=block}) or the reverse (@option{conv=unblock}),
7805 use @var{bytes} as the fixed record length.
7807 @item skip=@var{blocks}
7809 Skip @var{blocks} @samp{ibs}-byte blocks in the input file before copying.
7811 @item seek=@var{blocks}
7813 Skip @var{blocks} @samp{obs}-byte blocks in the output file before copying.
7815 @item count=@var{blocks}
7817 Copy @var{blocks} @samp{ibs}-byte blocks from the input file, instead
7818 of everything until the end of the file.
7822 Do not print the overall transfer rate and volume statistics
7823 that normally make up the third status line when @command{dd} exits.
7825 @item conv=@var{conversion}[,@var{conversion}]@dots{}
7827 Convert the file as specified by the @var{conversion} argument(s).
7828 (No spaces around any comma(s).)
7835 @opindex ascii@r{, converting to}
7836 Convert @acronym{EBCDIC} to @acronym{ASCII},
7837 using the conversion table specified by @acronym{POSIX}.
7838 This provides a 1:1 translation for all 256 bytes.
7841 @opindex ebcdic@r{, converting to}
7842 Convert @acronym{ASCII} to @acronym{EBCDIC}.
7843 This is the inverse of the @samp{ascii} conversion.
7846 @opindex alternate ebcdic@r{, converting to}
7847 Convert @acronym{ASCII} to alternate @acronym{EBCDIC},
7848 using the alternate conversion table specified by @acronym{POSIX}.
7849 This is not a 1:1 translation, but reflects common historical practice
7850 for @samp{~}, @samp{[}, and @samp{]}.
7852 The @samp{ascii}, @samp{ebcdic}, and @samp{ibm} conversions are
7856 @opindex block @r{(space-padding)}
7857 For each line in the input, output @samp{cbs} bytes, replacing the
7858 input newline with a space and padding with spaces as necessary.
7862 Remove any trailing spaces in each @samp{cbs}-sized input block,
7863 and append a newline.
7865 The @samp{block} and @samp{unblock} conversions are mutually exclusive.
7868 @opindex lcase@r{, converting to}
7869 Change uppercase letters to lowercase.
7872 @opindex ucase@r{, converting to}
7873 Change lowercase letters to uppercase.
7875 The @samp{lcase} and @samp{ucase} conversions are mutually exclusive.
7878 @opindex swab @r{(byte-swapping)}
7879 @cindex byte-swapping
7880 Swap every pair of input bytes. @sc{gnu} @command{dd}, unlike others, works
7881 when an odd number of bytes are read---the last byte is simply copied
7882 (since there is nothing to swap it with).
7886 @cindex read errors, ignoring
7887 Continue after read errors.
7891 @cindex creating output file, avoiding
7892 Do not create the output file; the output file must already exist.
7896 @cindex creating output file, requiring
7897 Fail if the output file already exists; @command{dd} must create the
7900 The @samp{excl} and @samp{nocreat} conversions are mutually exclusive.
7904 @cindex truncating output file, avoiding
7905 Do not truncate the output file.
7908 @opindex sync @r{(padding with @acronym{ASCII} @sc{nul}s)}
7909 Pad every input block to size of @samp{ibs} with trailing zero bytes.
7910 When used with @samp{block} or @samp{unblock}, pad with spaces instead of
7915 @cindex synchronized data writes, before finishing
7916 Synchronize output data just before finishing. This forces a physical
7917 write of output data.
7921 @cindex synchronized data and metadata writes, before finishing
7922 Synchronize output data and metadata just before finishing. This
7923 forces a physical write of output data and metadata.
7927 @item iflag=@var{flag}[,@var{flag}]@dots{}
7929 Access the input file using the flags specified by the @var{flag}
7930 argument(s). (No spaces around any comma(s).)
7932 @item oflag=@var{flag}[,@var{flag}]@dots{}
7934 Access the output file using the flags specified by the @var{flag}
7935 argument(s). (No spaces around any comma(s).)
7937 Here are the flags. Not every flag is supported on every operating
7944 @cindex appending to the output file
7945 Write in append mode, so that even if some other process is writing to
7946 this file, every @command{dd} write will append to the current
7947 contents of the file. This flag makes sense only for output.
7948 If you combine this flag with the @samp{of=@var{file}} operand,
7949 you should also specify @samp{conv=notrunc} unless you want the
7950 output file to be truncated before being appended to.
7954 @cindex concurrent I/O
7955 Use concurrent I/O mode for data. This mode performs direct I/O
7956 and drops the @acronym{POSIX} requirement to serialize all I/O to the same file.
7957 A file cannot be opened in CIO mode and with a standard open at the
7963 Use direct I/O for data, avoiding the buffer cache.
7964 Note that the kernel may impose restrictions on read or write buffer sizes.
7965 For example, with an ext4 destination file system and a linux-based kernel,
7966 using @samp{oflag=direct} will cause writes to fail with @code{EINVAL} if the
7967 output buffer size is not a multiple of 512.
7971 @cindex directory I/O
7973 Fail unless the file is a directory. Most operating systems do not
7974 allow I/O to a directory, so this flag has limited utility.
7978 @cindex synchronized data reads
7979 Use synchronized I/O for data. For the output file, this forces a
7980 physical write of output data on each write. For the input file,
7981 this flag can matter when reading from a remote file that has been
7982 written to synchronously by some other process. Metadata (e.g.,
7983 last-access and last-modified time) is not necessarily synchronized.
7987 @cindex synchronized data and metadata I/O
7988 Use synchronized I/O for both data and metadata.
7992 @cindex nonblocking I/O
7993 Use non-blocking I/O.
7998 Do not update the file's access time.
7999 Some older file systems silently ignore this flag, so it is a good
8000 idea to test it on your files before relying on it.
8004 @cindex controlling terminal
8005 Do not assign the file to be a controlling terminal for @command{dd}.
8006 This has no effect when the file is not a terminal.
8007 On many hosts (e.g., @acronym{GNU}/Linux hosts), this option has no effect
8012 @cindex symbolic links, following
8013 Do not follow symbolic links.
8018 Fail if the file has multiple hard links.
8023 Use binary I/O. This option has an effect only on nonstandard
8024 platforms that distinguish binary from text I/O.
8029 Use text I/O. Like @samp{binary}, this option has no effect on
8034 Accumulate full blocks from input. The @code{read} system call
8035 may return early if a full block is not available.
8036 When that happens, continue calling @code{read} to fill the remainder
8038 This flag can be used only with @code{iflag}.
8042 These flags are not supported on all systems, and @samp{dd} rejects
8043 attempts to use them when they are not supported. When reading from
8044 standard input or writing to standard output, the @samp{nofollow} and
8045 @samp{noctty} flags should not be specified, and the other flags
8046 (e.g., @samp{nonblock}) can affect how other processes behave with the
8047 affected file descriptors, even after @command{dd} exits.
8051 @cindex multipliers after numbers
8052 The numeric-valued strings above (@var{bytes} and @var{blocks}) can be
8053 followed by a multiplier: @samp{b}=512, @samp{c}=1,
8054 @samp{w}=2, @samp{x@var{m}}=@var{m}, or any of the
8055 standard block size suffixes like @samp{k}=1024 (@pxref{Block size}).
8057 Any block size you specify via @samp{bs=}, @samp{ibs=}, @samp{obs=}, @samp{cbs=}
8058 should not be too large---values larger than a few megabytes
8059 are generally wasteful or (as in the gigabyte..exabyte case) downright
8060 counterproductive or error-inducing.
8062 Use different @command{dd} invocations to use different block sizes for
8063 skipping and I/O@. For example, the following shell commands copy data
8064 in 512 KiB blocks between a disk and a tape, but do not save or restore a
8065 4 KiB label at the start of the disk:
8068 disk=/dev/rdsk/c0t1d0s2
8071 # Copy all but the label from disk to tape.
8072 (dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape
8074 # Copy from tape back to disk, but leave the disk label alone.
8075 (dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk
8078 Sending an @samp{INFO} signal to a running @command{dd}
8079 process makes it print I/O statistics to standard error
8080 and then resume copying. In the example below,
8081 @command{dd} is run in the background to copy 10 million blocks.
8082 The @command{kill} command makes it output intermediate I/O statistics,
8083 and when @command{dd} completes normally or is killed by the
8084 @code{SIGINT} signal, it outputs the final statistics.
8087 $ dd if=/dev/zero of=/dev/null count=10MB & pid=$!
8088 $ kill -s INFO $pid; wait $pid
8089 3385223+0 records in
8090 3385223+0 records out
8091 1733234176 bytes (1.7 GB) copied, 6.42173 seconds, 270 MB/s
8092 10000000+0 records in
8093 10000000+0 records out
8094 5120000000 bytes (5.1 GB) copied, 18.913 seconds, 271 MB/s
8097 @vindex POSIXLY_CORRECT
8098 On systems lacking the @samp{INFO} signal @command{dd} responds to the
8099 @samp{USR1} signal instead, unless the @env{POSIXLY_CORRECT}
8100 environment variable is set.
8105 @node install invocation
8106 @section @command{install}: Copy files and set attributes
8109 @cindex copying files and setting attributes
8111 @command{install} copies files while setting their file mode bits and, if
8112 possible, their owner and group. Synopses:
8115 install [@var{option}]@dots{} [-T] @var{source} @var{dest}
8116 install [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8117 install [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8118 install [@var{option}]@dots{} -d @var{directory}@dots{}
8123 If two file names are given, @command{install} copies the first file to the
8127 If the @option{--target-directory} (@option{-t}) option is given, or
8128 failing that if the last file is a directory and the
8129 @option{--no-target-directory} (@option{-T}) option is not given,
8130 @command{install} copies each @var{source} file to the specified
8131 directory, using the @var{source}s' names.
8134 If the @option{--directory} (@option{-d}) option is given,
8135 @command{install} creates each @var{directory} and any missing parent
8136 directories. Parent directories are created with mode
8137 @samp{u=rwx,go=rx} (755), regardless of the @option{-m} option or the
8138 current umask. @xref{Directory Setuid and Setgid}, for how the
8139 set-user-ID and set-group-ID bits of parent directories are inherited.
8142 @cindex Makefiles, installing programs in
8143 @command{install} is similar to @command{cp}, but allows you to control the
8144 attributes of destination files. It is typically used in Makefiles to
8145 copy programs into their destination directories. It refuses to copy
8146 files onto themselves.
8148 @cindex extended attributes, xattr
8149 @command{install} never preserves extended attributes (xattr).
8151 The program accepts the following options. Also see @ref{Common options}.
8161 Compare each pair of source and destination files, and if the destination has
8162 identical content and any specified owner, group, permissions, and possibly
8163 SELinux context, then do not modify the destination at all.
8167 Ignored; for compatibility with old Unix versions of @command{install}.
8171 Create any missing parent directories of @var{dest},
8172 then copy @var{source} to @var{dest}.
8173 This option is ignored if a destination directory is specified
8174 via @option{--target-directory=DIR}.
8179 @opindex --directory
8180 @cindex directories, creating with given attributes
8181 @cindex parent directories, creating missing
8182 @cindex leading directories, creating missing
8183 Create any missing parent directories, giving them the default
8184 attributes. Then create each given directory, setting their owner,
8185 group and mode as given on the command line or to the defaults.
8187 @item -g @var{group}
8188 @itemx --group=@var{group}
8191 @cindex group ownership of installed files, setting
8192 Set the group ownership of installed files or directories to
8193 @var{group}. The default is the process's current group. @var{group}
8194 may be either a group name or a numeric group ID.
8197 @itemx --mode=@var{mode}
8200 @cindex permissions of installed files, setting
8201 Set the file mode bits for the installed file or directory to @var{mode},
8202 which can be either an octal number, or a symbolic mode as in
8203 @command{chmod}, with @samp{a=} (no access allowed to anyone) as the
8204 point of departure (@pxref{File permissions}).
8205 The default mode is @samp{u=rwx,go=rx,a-s}---read, write, and
8206 execute for the owner, read and execute for group and other, and with
8207 set-user-ID and set-group-ID disabled.
8208 This default is not quite the same as @samp{755}, since it disables
8209 instead of preserving set-user-ID and set-group-ID on directories.
8210 @xref{Directory Setuid and Setgid}.
8212 @item -o @var{owner}
8213 @itemx --owner=@var{owner}
8216 @cindex ownership of installed files, setting
8217 @cindex appropriate privileges
8218 @vindex root @r{as default owner}
8219 If @command{install} has appropriate privileges (is run as root), set the
8220 ownership of installed files or directories to @var{owner}. The default
8221 is @code{root}. @var{owner} may be either a user name or a numeric user
8224 @item --preserve-context
8225 @opindex --preserve-context
8227 @cindex security context
8228 Preserve the SELinux security context of files and directories.
8229 Failure to preserve the context in all of the files or directories
8230 will result in an exit status of 1. If SELinux is disabled then
8231 print a warning and ignore the option.
8234 @itemx --preserve-timestamps
8236 @opindex --preserve-timestamps
8237 @cindex timestamps of installed files, preserving
8238 Set the time of last access and the time of last modification of each
8239 installed file to match those of each corresponding original file.
8240 When a file is installed without this option, its last access and
8241 last modification times are both set to the time of installation.
8242 This option is useful if you want to use the last modification times
8243 of installed files to keep track of when they were last built as opposed
8244 to when they were last installed.
8250 @cindex symbol table information, stripping
8251 @cindex stripping symbol table information
8252 Strip the symbol tables from installed binary executables.
8254 @itemx --strip-program=@var{program}
8255 @opindex --strip-program
8256 @cindex symbol table information, stripping, program
8257 Program used to strip binaries.
8263 @optNoTargetDirectory
8269 Print the name of each file before copying it.
8271 @item -Z @var{context}
8272 @itemx --context=@var{context}
8276 @cindex security context
8277 Set the default SELinux security context to be used for any
8278 created files and directories. If SELinux is disabled then
8279 print a warning and ignore the option.
8287 @section @command{mv}: Move (rename) files
8291 @command{mv} moves or renames files (or directories). Synopses:
8294 mv [@var{option}]@dots{} [-T] @var{source} @var{dest}
8295 mv [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8296 mv [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8301 If two file names are given, @command{mv} moves the first file to the
8305 If the @option{--target-directory} (@option{-t}) option is given, or
8306 failing that if the last file is a directory and the
8307 @option{--no-target-directory} (@option{-T}) option is not given,
8308 @command{mv} moves each @var{source} file to the specified
8309 directory, using the @var{source}s' names.
8312 @command{mv} can move any type of file from one file system to another.
8313 Prior to version @code{4.0} of the fileutils,
8314 @command{mv} could move only regular files between file systems.
8315 For example, now @command{mv} can move an entire directory hierarchy
8316 including special device files from one partition to another. It first
8317 uses some of the same code that's used by @code{cp -a} to copy the
8318 requested directories and files, then (assuming the copy succeeded)
8319 it removes the originals. If the copy fails, then the part that was
8320 copied to the destination partition is removed. If you were to copy
8321 three directories from one partition to another and the copy of the first
8322 directory succeeded, but the second didn't, the first would be left on
8323 the destination partition and the second and third would be left on the
8326 @cindex extended attributes, xattr
8327 @command{mv} always tries to copy extended attributes (xattr), which may
8328 include SELinux context, ACLs or Capabilities.
8329 Upon failure all but @samp{Operation not supported} warnings are output.
8331 @cindex prompting, and @command{mv}
8332 If a destination file exists but is normally unwritable, standard input
8333 is a terminal, and the @option{-f} or @option{--force} option is not given,
8334 @command{mv} prompts the user for whether to replace the file. (You might
8335 own the file, or have write permission on its directory.) If the
8336 response is not affirmative, the file is skipped.
8338 @emph{Warning}: Avoid specifying a source name with a trailing slash,
8339 when it might be a symlink to a directory.
8340 Otherwise, @command{mv} may do something very surprising, since
8341 its behavior depends on the underlying rename system call.
8342 On a system with a modern Linux-based kernel, it fails with @code{errno=ENOTDIR}.
8343 However, on other systems (at least FreeBSD 6.1 and Solaris 10) it silently
8344 renames not the symlink but rather the directory referenced by the symlink.
8345 @xref{Trailing slashes}.
8347 The program accepts the following options. Also see @ref{Common options}.
8357 @cindex prompts, omitting
8358 Do not prompt the user before removing a destination file.
8360 If you specify more than one of the @option{-i}, @option{-f}, @option{-n}
8361 options, only the final one takes effect.
8366 @itemx --interactive
8368 @opindex --interactive
8369 @cindex prompts, forcing
8370 Prompt whether to overwrite each existing destination file, regardless
8372 If the response is not affirmative, the file is skipped.
8378 @opindex --no-clobber
8379 @cindex prompts, omitting
8380 Do not overwrite an existing file.
8382 This option is mutually exclusive with @option{-b} or @option{--backup} option.
8388 @cindex newer files, moving only
8389 Do not move a non-directory that has an existing destination with the
8390 same or newer modification time.
8391 If the move is across file system boundaries, the comparison is to the
8392 source time stamp truncated to the resolutions of the destination file
8393 system and of the system calls used to update time stamps; this avoids
8394 duplicate work if several @samp{mv -u} commands are executed with the
8395 same source and destination.
8401 Print the name of each file before moving it.
8403 @optStripTrailingSlashes
8409 @optNoTargetDirectory
8417 @section @command{rm}: Remove files or directories
8420 @cindex removing files or directories
8422 @command{rm} removes each given @var{file}. By default, it does not remove
8423 directories. Synopsis:
8426 rm [@var{option}]@dots{} [@var{file}]@dots{}
8429 @cindex prompting, and @command{rm}
8430 If the @option{-I} or @option{--interactive=once} option is given,
8431 and there are more than three files or the @option{-r}, @option{-R},
8432 or @option{--recursive} are given, then @command{rm} prompts the user
8433 for whether to proceed with the entire operation. If the response is
8434 not affirmative, the entire command is aborted.
8436 Otherwise, if a file is unwritable, standard input is a terminal, and
8437 the @option{-f} or @option{--force} option is not given, or the
8438 @option{-i} or @option{--interactive=always} option @emph{is} given,
8439 @command{rm} prompts the user for whether to remove the file.
8440 If the response is not affirmative, the file is skipped.
8442 Any attempt to remove a file whose last file name component is
8443 @file{.} or @file{..} is rejected without any prompting.
8445 @emph{Warning}: If you use @command{rm} to remove a file, it is usually
8446 possible to recover the contents of that file. If you want more assurance
8447 that the contents are truly unrecoverable, consider using @command{shred}.
8449 The program accepts the following options. Also see @ref{Common options}.
8457 Ignore nonexistent files and never prompt the user.
8458 Ignore any previous @option{--interactive} (@option{-i}) option.
8462 Prompt whether to remove each file.
8463 If the response is not affirmative, the file is skipped.
8464 Ignore any previous @option{--force} (@option{-f}) option.
8465 Equivalent to @option{--interactive=always}.
8469 Prompt once whether to proceed with the command, if more than three
8470 files are named or if a recursive removal is requested. Ignore any
8471 previous @option{--force} (@option{-f}) option. Equivalent to
8472 @option{--interactive=once}.
8474 @itemx --interactive [=@var{when}]
8475 @opindex --interactive
8476 Specify when to issue an interactive prompt. @var{when} may be
8480 @vindex never @r{interactive option}
8481 - Do not prompt at all.
8483 @vindex once @r{interactive option}
8484 - Prompt once if more than three files are named or if a recursive
8485 removal is requested. Equivalent to @option{-I}.
8487 @vindex always @r{interactive option}
8488 - Prompt for every file being removed. Equivalent to @option{-i}.
8490 @option{--interactive} with no @var{when} is equivalent to
8491 @option{--interactive=always}.
8493 @itemx --one-file-system
8494 @opindex --one-file-system
8495 @cindex one file system, restricting @command{rm} to
8496 When removing a hierarchy recursively, skip any directory that is on a
8497 file system different from that of the corresponding command line argument.
8500 This option is useful when removing a build ``chroot'' hierarchy,
8501 which normally contains no valuable data. However, it is not uncommon
8502 to bind-mount @file{/home} into such a hierarchy, to make it easier to
8503 use one's start-up file. The catch is that it's easy to forget to
8504 unmount @file{/home}. Then, when you use @command{rm -rf} to remove
8505 your normally throw-away chroot, that command will remove everything
8506 under @file{/home}, too.
8507 Use the @option{--one-file-system} option, and it will
8508 warn about and skip directories on other file systems.
8509 Of course, this will not save your @file{/home} if it and your
8510 chroot happen to be on the same file system.
8512 @itemx --preserve-root
8513 @opindex --preserve-root
8514 @cindex root directory, disallow recursive destruction
8515 Fail upon any attempt to remove the root directory, @file{/},
8516 when used with the @option{--recursive} option.
8517 This is the default behavior.
8518 @xref{Treating / specially}.
8520 @itemx --no-preserve-root
8521 @opindex --no-preserve-root
8522 @cindex root directory, allow recursive destruction
8523 Do not treat @file{/} specially when removing recursively.
8524 This option is not recommended unless you really want to
8525 remove all the files on your computer.
8526 @xref{Treating / specially}.
8533 @opindex --recursive
8534 @cindex directories, removing (recursively)
8535 Remove the listed directories and their contents recursively.
8541 Print the name of each file before removing it.
8545 @cindex files beginning with @samp{-}, removing
8546 @cindex @samp{-}, removing files beginning with
8547 One common question is how to remove files whose names begin with a
8548 @samp{-}. @sc{gnu} @command{rm}, like every program that uses the @code{getopt}
8549 function to parse its arguments, lets you use the @samp{--} option to
8550 indicate that all following arguments are non-options. To remove a file
8551 called @file{-f} in the current directory, you could type either:
8564 @opindex - @r{and Unix @command{rm}}
8565 The Unix @command{rm} program's use of a single @samp{-} for this purpose
8566 predates the development of the getopt standard syntax.
8571 @node shred invocation
8572 @section @command{shred}: Remove files more securely
8575 @cindex data, erasing
8576 @cindex erasing data
8578 @command{shred} overwrites devices or files, to help prevent even
8579 very expensive hardware from recovering the data.
8581 Ordinarily when you remove a file (@pxref{rm invocation}), the data is
8582 not actually destroyed. Only the index listing where the file is
8583 stored is destroyed, and the storage is made available for reuse.
8584 There are undelete utilities that will attempt to reconstruct the index
8585 and can bring the file back if the parts were not reused.
8587 On a busy system with a nearly-full drive, space can get reused in a few
8588 seconds. But there is no way to know for sure. If you have sensitive
8589 data, you may want to be sure that recovery is not possible by actually
8590 overwriting the file with non-sensitive data.
8592 However, even after doing that, it is possible to take the disk back
8593 to a laboratory and use a lot of sensitive (and expensive) equipment
8594 to look for the faint ``echoes'' of the original data underneath the
8595 overwritten data. If the data has only been overwritten once, it's not
8598 The best way to remove something irretrievably is to destroy the media
8599 it's on with acid, melt it down, or the like. For cheap removable media
8600 like floppy disks, this is the preferred method. However, hard drives
8601 are expensive and hard to melt, so the @command{shred} utility tries
8602 to achieve a similar effect non-destructively.
8604 This uses many overwrite passes, with the data patterns chosen to
8605 maximize the damage they do to the old data. While this will work on
8606 floppies, the patterns are designed for best effect on hard drives.
8607 For more details, see the source code and Peter Gutmann's paper
8608 @uref{http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html,
8609 @cite{Secure Deletion of Data from Magnetic and Solid-State Memory}},
8610 from the proceedings of the Sixth @acronym{USENIX} Security Symposium (San Jose,
8611 California, July 22--25, 1996).
8613 @strong{Please note} that @command{shred} relies on a very important assumption:
8614 that the file system overwrites data in place. This is the traditional
8615 way to do things, but many modern file system designs do not satisfy this
8616 assumption. Exceptions include:
8621 Log-structured or journaled file systems, such as those supplied with
8622 AIX and Solaris, and JFS, ReiserFS, XFS, Ext3 (in @code{data=journal} mode),
8623 BFS, NTFS, etc.@: when they are configured to journal @emph{data}.
8626 File systems that write redundant data and carry on even if some writes
8627 fail, such as RAID-based file systems.
8630 File systems that make snapshots, such as Network Appliance's NFS server.
8633 File systems that cache in temporary locations, such as NFS version 3
8637 Compressed file systems.
8640 In the particular case of ext3 file systems, the above disclaimer applies (and
8641 @command{shred} is thus of limited effectiveness) only in @code{data=journal}
8642 mode, which journals file data in addition to just metadata. In both
8643 the @code{data=ordered} (default) and @code{data=writeback} modes,
8644 @command{shred} works as usual. Ext3 journaling modes can be changed
8645 by adding the @code{data=something} option to the mount options for a
8646 particular file system in the @file{/etc/fstab} file, as documented in
8647 the mount man page (man mount).
8649 If you are not sure how your file system operates, then you should assume
8650 that it does not overwrite data in place, which means that shred cannot
8651 reliably operate on regular files in your file system.
8653 Generally speaking, it is more reliable to shred a device than a file,
8654 since this bypasses the problem of file system design mentioned above.
8655 However, even shredding devices is not always completely reliable. For
8656 example, most disks map out bad sectors invisibly to the application; if
8657 the bad sectors contain sensitive data, @command{shred} won't be able to
8660 @command{shred} makes no attempt to detect or report this problem, just as
8661 it makes no attempt to do anything about backups. However, since it is
8662 more reliable to shred devices than files, @command{shred} by default does
8663 not truncate or remove the output file. This default is more suitable
8664 for devices, which typically cannot be truncated and should not be
8667 Finally, consider the risk of backups and mirrors.
8668 File system backups and remote mirrors may contain copies of the
8669 file that cannot be removed, and that will allow a shredded file
8670 to be recovered later. So if you keep any data you may later want
8671 to destroy using @command{shred}, be sure that it is not backed up or mirrored.
8674 shred [@var{option}]@dots{} @var{file}[@dots{}]
8677 The program accepts the following options. Also see @ref{Common options}.
8685 @cindex force deletion
8686 Override file permissions if necessary to allow overwriting.
8689 @itemx -n @var{number}
8690 @itemx --iterations=@var{number}
8691 @opindex -n @var{number}
8692 @opindex --iterations=@var{number}
8693 @cindex iterations, selecting the number of
8694 By default, @command{shred} uses @value{SHRED_DEFAULT_PASSES} passes of
8695 overwrite. You can reduce this to save time, or increase it if you think it's
8696 appropriate. After 25 passes all of the internal overwrite patterns will have
8697 been used at least once.
8699 @item --random-source=@var{file}
8700 @opindex --random-source
8701 @cindex random source for shredding
8702 Use @var{file} as a source of random data used to overwrite and to
8703 choose pass ordering. @xref{Random sources}.
8705 @item -s @var{bytes}
8706 @itemx --size=@var{bytes}
8707 @opindex -s @var{bytes}
8708 @opindex --size=@var{bytes}
8709 @cindex size of file to shred
8710 Shred the first @var{bytes} bytes of the file. The default is to shred
8711 the whole file. @var{bytes} can be followed by a size specification like
8712 @samp{K}, @samp{M}, or @samp{G} to specify a multiple. @xref{Block size}.
8718 @cindex removing files after shredding
8719 After shredding a file, truncate it (if possible) and then remove it.
8720 If a file has multiple links, only the named links will be removed.
8726 Display to standard error all status updates as sterilization proceeds.
8732 By default, @command{shred} rounds the size of a regular file up to the next
8733 multiple of the file system block size to fully erase the last block of the file.
8734 Use @option{--exact} to suppress that behavior.
8735 Thus, by default if you shred a 10-byte regular file on a system with 512-byte
8736 blocks, the resulting file will be 512 bytes long. With this option,
8737 shred does not increase the apparent size of the file.
8743 Normally, the last pass that @command{shred} writes is made up of
8744 random data. If this would be conspicuous on your hard drive (for
8745 example, because it looks like encrypted data), or you just think
8746 it's tidier, the @option{--zero} option adds an additional overwrite pass with
8747 all zero bits. This is in addition to the number of passes specified
8748 by the @option{--iterations} option.
8752 You might use the following command to erase all trace of the
8753 file system you'd created on the floppy disk in your first drive.
8754 That command takes about 20 minutes to erase a ``1.44MB'' (actually
8758 shred --verbose /dev/fd0
8761 Similarly, to erase all data on a selected partition of
8762 your hard disk, you could give a command like this:
8765 shred --verbose /dev/sda5
8768 A @var{file} of @samp{-} denotes standard output.
8769 The intended use of this is to shred a removed temporary file.
8776 echo "Hello, world" >&3
8781 However, the command @samp{shred - >file} does not shred the contents
8782 of @var{file}, since the shell truncates @var{file} before invoking
8783 @command{shred}. Use the command @samp{shred file} or (if using a
8784 Bourne-compatible shell) the command @samp{shred - 1<>file} instead.
8789 @node Special file types
8790 @chapter Special file types
8792 @cindex special file types
8793 @cindex file types, special
8795 This chapter describes commands which create special types of files (and
8796 @command{rmdir}, which removes directories, one special file type).
8798 @cindex special file types
8800 Although Unix-like operating systems have markedly fewer special file
8801 types than others, not @emph{everything} can be treated only as the
8802 undifferentiated byte stream of @dfn{normal files}. For example, when a
8803 file is created or removed, the system must record this information,
8804 which it does in a @dfn{directory}---a special type of file. Although
8805 you can read directories as normal files, if you're curious, in order
8806 for the system to do its job it must impose a structure, a certain
8807 order, on the bytes of the file. Thus it is a ``special'' type of file.
8809 Besides directories, other special file types include named pipes
8810 (FIFOs), symbolic links, sockets, and so-called @dfn{special files}.
8813 * link invocation:: Make a hard link via the link syscall
8814 * ln invocation:: Make links between files.
8815 * mkdir invocation:: Make directories.
8816 * mkfifo invocation:: Make FIFOs (named pipes).
8817 * mknod invocation:: Make block or character special files.
8818 * readlink invocation:: Print value of a symlink or canonical file name.
8819 * rmdir invocation:: Remove empty directories.
8820 * unlink invocation:: Remove files via the unlink syscall
8824 @node link invocation
8825 @section @command{link}: Make a hard link via the link syscall
8828 @cindex links, creating
8829 @cindex hard links, creating
8830 @cindex creating links (hard only)
8832 @command{link} creates a single hard link at a time.
8833 It is a minimalist interface to the system-provided
8834 @code{link} function. @xref{Hard Links, , , libc,
8835 The GNU C Library Reference Manual}.
8836 It avoids the bells and whistles of the more commonly-used
8837 @command{ln} command (@pxref{ln invocation}).
8841 link @var{filename} @var{linkname}
8844 @var{filename} must specify an existing file, and @var{linkname}
8845 must specify a nonexistent entry in an existing directory.
8846 @command{link} simply calls @code{link (@var{filename}, @var{linkname})}
8849 On a @acronym{GNU} system, this command acts like @samp{ln --directory
8850 --no-target-directory @var{filename} @var{linkname}}. However, the
8851 @option{--directory} and @option{--no-target-directory} options are
8852 not specified by @acronym{POSIX}, and the @command{link} command is
8853 more portable in practice.
8855 If @var{filename} is a symbolic link, it is unspecified whether
8856 @var{linkname} will be a hard link to the symbolic link or to the
8857 target of the symbolic link. Use @command{ln -P} or @command{ln -L}
8858 to specify which behavior is desired.
8864 @section @command{ln}: Make links between files
8867 @cindex links, creating
8868 @cindex hard links, creating
8869 @cindex symbolic (soft) links, creating
8870 @cindex creating links (hard or soft)
8872 @cindex file systems and hard links
8873 @command{ln} makes links between files. By default, it makes hard links;
8874 with the @option{-s} option, it makes symbolic (or @dfn{soft}) links.
8878 ln [@var{option}]@dots{} [-T] @var{target} @var{linkname}
8879 ln [@var{option}]@dots{} @var{target}
8880 ln [@var{option}]@dots{} @var{target}@dots{} @var{directory}
8881 ln [@var{option}]@dots{} -t @var{directory} @var{target}@dots{}
8887 If two file names are given, @command{ln} creates a link to the first
8888 file from the second.
8891 If one @var{target} is given, @command{ln} creates a link to that file
8892 in the current directory.
8895 If the @option{--target-directory} (@option{-t}) option is given, or
8896 failing that if the last file is a directory and the
8897 @option{--no-target-directory} (@option{-T}) option is not given,
8898 @command{ln} creates a link to each @var{target} file in the specified
8899 directory, using the @var{target}s' names.
8903 Normally @command{ln} does not remove existing files. Use the
8904 @option{--force} (@option{-f}) option to remove them unconditionally,
8905 the @option{--interactive} (@option{-i}) option to remove them
8906 conditionally, and the @option{--backup} (@option{-b}) option to
8909 @cindex hard link, defined
8910 @cindex inode, and hard links
8911 A @dfn{hard link} is another name for an existing file; the link and the
8912 original are indistinguishable. Technically speaking, they share the
8913 same inode, and the inode contains all the information about a
8914 file---indeed, it is not incorrect to say that the inode @emph{is} the
8915 file. Most systems prohibit making a hard link to
8916 a directory; on those where it is allowed, only the super-user can do
8917 so (and with caution, since creating a cycle will cause problems to many
8918 other utilities). Hard links cannot cross file system boundaries. (These
8919 restrictions are not mandated by @acronym{POSIX}, however.)
8921 @cindex dereferencing symbolic links
8922 @cindex symbolic link, defined
8923 @dfn{Symbolic links} (@dfn{symlinks} for short), on the other hand, are
8924 a special file type (which not all kernels support: System V release 3
8925 (and older) systems lack symlinks) in which the link file actually
8926 refers to a different file, by name. When most operations (opening,
8927 reading, writing, and so on) are passed the symbolic link file, the
8928 kernel automatically @dfn{dereferences} the link and operates on the
8929 target of the link. But some operations (e.g., removing) work on the
8930 link file itself, rather than on its target. The owner and group of a
8931 symlink are not significant to file access performed through
8932 the link, but do have implications on deleting a symbolic link from a
8933 directory with the restricted deletion bit set. On the GNU system,
8934 the mode of a symlink has no significance and cannot be changed, but
8935 on some BSD systems, the mode can be changed and will affect whether
8936 the symlink will be traversed in file name resolution. @xref{Symbolic Links,,,
8937 libc, The GNU C Library Reference Manual}.
8939 Symbolic links can contain arbitrary strings; a @dfn{dangling symlink}
8940 occurs when the string in the symlink does not resolve to a file.
8941 There are no restrictions against creating dangling symbolic links.
8942 There are trade-offs to using absolute or relative symlinks. An
8943 absolute symlink always points to the same file, even if the directory
8944 containing the link is moved. However, if the symlink is visible from
8945 more than one machine (such as on a networked file system), the file
8946 pointed to might not always be the same. A relative symbolic link is
8947 resolved in relation to the directory that contains the link, and is
8948 often useful in referring to files on the same device without regards
8949 to what name that device is mounted on when accessed via networked
8952 When creating a relative symlink in a different location than the
8953 current directory, the resolution of the symlink will be different
8954 than the resolution of the same string from the current directory.
8955 Therefore, many users prefer to first change directories to the
8956 location where the relative symlink will be created, so that
8957 tab-completion or other file resolution will find the same target as
8958 what will be placed in the symlink.
8960 The program accepts the following options. Also see @ref{Common options}.
8971 @opindex --directory
8972 @cindex hard links to directories
8973 Allow users with appropriate privileges to attempt to make hard links
8975 However, note that this will probably fail due to
8976 system restrictions, even for the super-user.
8982 Remove existing destination files.
8985 @itemx --interactive
8987 @opindex --interactive
8988 @cindex prompting, and @command{ln}
8989 Prompt whether to remove existing destination files.
8995 If @option{-s} is not in effect, and the source file is a symbolic
8996 link, create the hard link to the file referred to by the symbolic
8997 link, rather than the symbolic link itself.
9000 @itemx --no-dereference
9002 @opindex --no-dereference
9003 Do not treat the last operand specially when it is a symbolic link to
9004 a directory. Instead, treat it as if it were a normal file.
9006 When the destination is an actual directory (not a symlink to one),
9007 there is no ambiguity. The link is created in that directory.
9008 But when the specified destination is a symlink to a directory,
9009 there are two ways to treat the user's request. @command{ln} can
9010 treat the destination just as it would a normal directory and create
9011 the link in it. On the other hand, the destination can be viewed as a
9012 non-directory---as the symlink itself. In that case, @command{ln}
9013 must delete or backup that symlink before creating the new link.
9014 The default is to treat a destination that is a symlink to a directory
9015 just like a directory.
9017 This option is weaker than the @option{--no-target-directory}
9018 (@option{-T}) option, so it has no effect if both options are given.
9024 If @option{-s} is not in effect, and the source file is a symbolic
9025 link, create the hard link to the symbolic link itself. On platforms
9026 where this is not supported by the kernel, this option creates a
9027 symbolic link with identical contents; since symbolic link contents
9028 cannot be edited, any file name resolution performed through either
9029 link will be the same as if a hard link had been created.
9035 Make symbolic links instead of hard links. This option merely produces
9036 an error message on systems that do not support symbolic links.
9042 @optNoTargetDirectory
9048 Print the name of each file after linking it successfully.
9052 @cindex hard links to symbolic links
9053 @cindex symbolic links and @command{ln}
9054 If @option{-L} and @option{-P} are both given, the last one takes
9055 precedence. If @option{-s} is also given, @option{-L} and @option{-P}
9056 are silently ignored. If neither option is given, then this
9057 implementation defaults to @option{-P} if the system @code{link} supports
9058 hard links to symbolic links (such as the GNU system), and @option{-L}
9059 if @code{link} follows symbolic links (such as on BSD).
9068 # Create link ../a pointing to a in that directory.
9069 # Not really useful because it points to itself.
9074 # Change to the target before creating symlinks to avoid being confused.
9080 # Hard coded file names don't move well.
9081 ln -s $(pwd)/a /some/dir/
9085 # Relative file names survive directory moves and also
9086 # work across networked file systems.
9087 ln -s afile anotherfile
9088 ln -s ../adir/afile yetanotherfile
9092 @node mkdir invocation
9093 @section @command{mkdir}: Make directories
9096 @cindex directories, creating
9097 @cindex creating directories
9099 @command{mkdir} creates directories with the specified names. Synopsis:
9102 mkdir [@var{option}]@dots{} @var{name}@dots{}
9105 @command{mkdir} creates each directory @var{name} in the order given.
9106 It reports an error if @var{name} already exists, unless the
9107 @option{-p} option is given and @var{name} is a directory.
9109 The program accepts the following options. Also see @ref{Common options}.
9114 @itemx --mode=@var{mode}
9117 @cindex modes of created directories, setting
9118 Set the file permission bits of created directories to @var{mode},
9119 which uses the same syntax as
9120 in @command{chmod} and uses @samp{a=rwx} (read, write and execute allowed for
9121 everyone) for the point of the departure. @xref{File permissions}.
9123 Normally the directory has the desired file mode bits at the moment it
9124 is created. As a @acronym{GNU} extension, @var{mode} may also mention
9125 special mode bits, but in this case there may be a temporary window
9126 during which the directory exists but its special mode bits are
9127 incorrect. @xref{Directory Setuid and Setgid}, for how the
9128 set-user-ID and set-group-ID bits of directories are inherited unless
9129 overridden in this way.
9135 @cindex parent directories, creating
9136 Make any missing parent directories for each argument, setting their
9137 file permission bits to the umask modified by @samp{u+wx}. Ignore
9138 existing parent directories, and do not change their file permission
9141 To set the file permission bits of any newly-created parent
9142 directories to a value that includes @samp{u+wx}, you can set the
9143 umask before invoking @command{mkdir}. For example, if the shell
9144 command @samp{(umask u=rwx,go=rx; mkdir -p P/Q)} creates the parent
9145 @file{P} it sets the parent's permission bits to @samp{u=rwx,go=rx}.
9146 To set a parent's special mode bits as well, you can invoke
9147 @command{chmod} after @command{mkdir}. @xref{Directory Setuid and
9148 Setgid}, for how the set-user-ID and set-group-ID bits of
9149 newly-created parent directories are inherited.
9155 Print a message for each created directory. This is most useful with
9158 @item -Z @var{context}
9159 @itemx --context=@var{context}
9163 @cindex security context
9164 Set the default SELinux security context to be used for created directories.
9171 @node mkfifo invocation
9172 @section @command{mkfifo}: Make FIFOs (named pipes)
9175 @cindex FIFOs, creating
9176 @cindex named pipes, creating
9177 @cindex creating FIFOs (named pipes)
9179 @command{mkfifo} creates FIFOs (also called @dfn{named pipes}) with the
9180 specified names. Synopsis:
9183 mkfifo [@var{option}] @var{name}@dots{}
9186 A @dfn{FIFO} is a special file type that permits independent processes
9187 to communicate. One process opens the FIFO file for writing, and
9188 another for reading, after which data can flow as with the usual
9189 anonymous pipe in shells or elsewhere.
9191 The program accepts the following option. Also see @ref{Common options}.
9196 @itemx --mode=@var{mode}
9199 @cindex modes of created FIFOs, setting
9200 Set the mode of created FIFOs to @var{mode}, which is symbolic as in
9201 @command{chmod} and uses @samp{a=rw} (read and write allowed for everyone)
9202 for the point of departure. @var{mode} should specify only file
9203 permission bits. @xref{File permissions}.
9205 @item -Z @var{context}
9206 @itemx --context=@var{context}
9210 @cindex security context
9211 Set the default SELinux security context to be used for created FIFOs.
9218 @node mknod invocation
9219 @section @command{mknod}: Make block or character special files
9222 @cindex block special files, creating
9223 @cindex character special files, creating
9225 @command{mknod} creates a FIFO, character special file, or block special
9226 file with the specified name. Synopsis:
9229 mknod [@var{option}]@dots{} @var{name} @var{type} [@var{major} @var{minor}]
9232 @cindex special files
9233 @cindex block special files
9234 @cindex character special files
9235 Unlike the phrase ``special file type'' above, the term @dfn{special
9236 file} has a technical meaning on Unix: something that can generate or
9237 receive data. Usually this corresponds to a physical piece of hardware,
9238 e.g., a printer or a disk. (These files are typically created at
9239 system-configuration time.) The @command{mknod} command is what creates
9240 files of this type. Such devices can be read either a character at a
9241 time or a ``block'' (many characters) at a time, hence we say there are
9242 @dfn{block special} files and @dfn{character special} files.
9244 @c mknod is a shell built-in at least with OpenBSD's /bin/sh
9245 @mayConflictWithShellBuiltIn{mknod}
9247 The arguments after @var{name} specify the type of file to make:
9252 @opindex p @r{for FIFO file}
9256 @opindex b @r{for block special file}
9257 for a block special file
9260 @c Don't document the `u' option -- it's just a synonym for `c'.
9261 @c Do *any* versions of mknod still use it?
9263 @opindex c @r{for character special file}
9264 @c @opindex u @r{for character special file}
9265 for a character special file
9269 When making a block or character special file, the major and minor
9270 device numbers must be given after the file type.
9271 If a major or minor device number begins with @samp{0x} or @samp{0X},
9272 it is interpreted as hexadecimal; otherwise, if it begins with @samp{0},
9273 as octal; otherwise, as decimal.
9275 The program accepts the following option. Also see @ref{Common options}.
9280 @itemx --mode=@var{mode}
9283 Set the mode of created files to @var{mode}, which is symbolic as in
9284 @command{chmod} and uses @samp{a=rw} as the point of departure.
9285 @var{mode} should specify only file permission bits.
9286 @xref{File permissions}.
9288 @item -Z @var{context}
9289 @itemx --context=@var{context}
9293 @cindex security context
9294 Set the default SELinux security context to be used for created files.
9301 @node readlink invocation
9302 @section @command{readlink}: Print value of a symlink or canonical file name
9305 @cindex displaying value of a symbolic link
9306 @cindex canonical file name
9307 @cindex canonicalize a file name
9311 @command{readlink} may work in one of two supported modes:
9317 @command{readlink} outputs the value of the given symbolic link.
9318 If @command{readlink} is invoked with an argument other than the name
9319 of a symbolic link, it produces no output and exits with a nonzero exit code.
9321 @item Canonicalize mode
9323 @command{readlink} outputs the absolute name of the given file which contains
9324 no @file{.}, @file{..} components nor any repeated separators
9325 (@file{/}) or symbolic links.
9330 readlink [@var{option}] @var{file}
9333 By default, @command{readlink} operates in readlink mode.
9335 The program accepts the following options. Also see @ref{Common options}.
9340 @itemx --canonicalize
9342 @opindex --canonicalize
9343 Activate canonicalize mode.
9344 If any component of the file name except the last one is missing or unavailable,
9345 @command{readlink} produces no output and exits with a nonzero exit
9346 code. A trailing slash is ignored.
9349 @itemx --canonicalize-existing
9351 @opindex --canonicalize-existing
9352 Activate canonicalize mode.
9353 If any component is missing or unavailable, @command{readlink} produces
9354 no output and exits with a nonzero exit code. A trailing slash
9355 requires that the name resolve to a directory.
9358 @itemx --canonicalize-missing
9360 @opindex --canonicalize-missing
9361 Activate canonicalize mode.
9362 If any component is missing or unavailable, @command{readlink} treats it
9368 @opindex --no-newline
9369 Do not output the trailing newline.
9379 Suppress most error messages.
9385 Report error messages.
9389 The @command{readlink} utility first appeared in OpenBSD 2.1.
9391 There is a @command{realpath} command on some systems
9392 which operates like @command{readlink} in canonicalize mode.
9397 @node rmdir invocation
9398 @section @command{rmdir}: Remove empty directories
9401 @cindex removing empty directories
9402 @cindex directories, removing empty
9404 @command{rmdir} removes empty directories. Synopsis:
9407 rmdir [@var{option}]@dots{} @var{directory}@dots{}
9410 If any @var{directory} argument does not refer to an existing empty
9411 directory, it is an error.
9413 The program accepts the following options. Also see @ref{Common options}.
9417 @item --ignore-fail-on-non-empty
9418 @opindex --ignore-fail-on-non-empty
9419 @cindex directory deletion, ignoring failures
9420 Ignore each failure to remove a directory that is solely because
9421 the directory is non-empty.
9427 @cindex parent directories, removing
9428 Remove @var{directory}, then try to remove each component of @var{directory}.
9429 So, for example, @samp{rmdir -p a/b/c} is similar to @samp{rmdir a/b/c a/b a}.
9430 As such, it fails if any of those directories turns out not to be empty.
9431 Use the @option{--ignore-fail-on-non-empty} option to make it so such
9432 a failure does not evoke a diagnostic and does not cause @command{rmdir} to
9433 exit unsuccessfully.
9439 @cindex directory deletion, reporting
9440 Give a diagnostic for each successful removal.
9441 @var{directory} is removed.
9445 @xref{rm invocation}, for how to remove non-empty directories (recursively).
9450 @node unlink invocation
9451 @section @command{unlink}: Remove files via the unlink syscall
9454 @cindex removing files or directories (via the unlink syscall)
9456 @command{unlink} deletes a single specified file name.
9457 It is a minimalist interface to the system-provided
9458 @code{unlink} function. @xref{Deleting Files, , , libc,
9459 The GNU C Library Reference Manual}. Synopsis:
9460 It avoids the bells and whistles of the more commonly-used
9461 @command{rm} command (@pxref{rm invocation}).
9464 unlink @var{filename}
9467 On some systems @code{unlink} can be used to delete the name of a
9468 directory. On others, it can be used that way only by a privileged user.
9469 In the GNU system @code{unlink} can never delete the name of a directory.
9471 The @command{unlink} command honors the @option{--help} and
9472 @option{--version} options. To remove a file whose name begins with
9473 @samp{-}, prefix the name with @samp{./}, e.g., @samp{unlink ./--help}.
9478 @node Changing file attributes
9479 @chapter Changing file attributes
9481 @cindex changing file attributes
9482 @cindex file attributes, changing
9483 @cindex attributes, file
9485 A file is not merely its contents, a name, and a file type
9486 (@pxref{Special file types}). A file also has an owner (a user ID), a
9487 group (a group ID), permissions (what the owner can do with the file,
9488 what people in the group can do, and what everyone else can do), various
9489 timestamps, and other information. Collectively, we call these a file's
9492 These commands change file attributes.
9495 * chgrp invocation:: Change file groups.
9496 * chmod invocation:: Change access permissions.
9497 * chown invocation:: Change file owners and groups.
9498 * touch invocation:: Change file timestamps.
9502 @node chown invocation
9503 @section @command{chown}: Change file owner and group
9506 @cindex file ownership, changing
9507 @cindex group ownership, changing
9508 @cindex changing file ownership
9509 @cindex changing group ownership
9511 @command{chown} changes the user and/or group ownership of each given @var{file}
9512 to @var{new-owner} or to the user and group of an existing reference file.
9516 chown [@var{option}]@dots{} @{@var{new-owner} | --reference=@var{ref_file}@} @var{file}@dots{}
9519 If used, @var{new-owner} specifies the new owner and/or group as follows
9520 (with no embedded white space):
9523 [@var{owner}] [ : [@var{group}] ]
9530 If only an @var{owner} (a user name or numeric user ID) is given, that
9531 user is made the owner of each given file, and the files' group is not
9534 @item owner@samp{:}group
9535 If the @var{owner} is followed by a colon and a @var{group} (a
9536 group name or numeric group ID), with no spaces between them, the group
9537 ownership of the files is changed as well (to @var{group}).
9540 If a colon but no group name follows @var{owner}, that user is
9541 made the owner of the files and the group of the files is changed to
9542 @var{owner}'s login group.
9545 If the colon and following @var{group} are given, but the owner
9546 is omitted, only the group of the files is changed; in this case,
9547 @command{chown} performs the same function as @command{chgrp}.
9550 If only a colon is given, or if @var{new-owner} is empty, neither the
9551 owner nor the group is changed.
9555 If @var{owner} or @var{group} is intended to represent a numeric user
9556 or group ID, then you may specify it with a leading @samp{+}.
9557 @xref{Disambiguating names and IDs}.
9559 Some older scripts may still use @samp{.} in place of the @samp{:} separator.
9560 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) does not
9561 require support for that, but for backward compatibility @acronym{GNU}
9562 @command{chown} supports @samp{.} so long as no ambiguity results.
9563 New scripts should avoid the use of @samp{.} because it is not
9564 portable, and because it has undesirable results if the entire
9565 @var{owner@samp{.}group} happens to identify a user whose name
9568 The @command{chown} command sometimes clears the set-user-ID or
9569 set-group-ID permission bits. This behavior depends on the policy and
9570 functionality of the underlying @code{chown} system call, which may
9571 make system-dependent file mode modifications outside the control of
9572 the @command{chown} command. For example, the @command{chown} command
9573 might not affect those bits when invoked by a user with appropriate
9574 privileges, or when the
9575 bits signify some function other than executable permission (e.g.,
9577 When in doubt, check the underlying system behavior.
9579 The program accepts the following options. Also see @ref{Common options}.
9587 @cindex changed owners, verbosely describing
9588 Verbosely describe the action for each @var{file} whose ownership
9597 @cindex error messages, omitting
9598 Do not print error messages about files whose ownership cannot be
9601 @itemx @w{@kbd{--from}=@var{old-owner}}
9603 @cindex symbolic links, changing owner
9604 Change a @var{file}'s ownership only if it has current attributes specified
9605 by @var{old-owner}. @var{old-owner} has the same form as @var{new-owner}
9607 This option is useful primarily from a security standpoint in that
9608 it narrows considerably the window of potential abuse.
9609 For example, to reflect a user ID numbering change for one user's files
9610 without an option like this, @code{root} might run
9613 find / -owner OLDUSER -print0 | xargs -0 chown -h NEWUSER
9616 But that is dangerous because the interval between when the @command{find}
9617 tests the existing file's owner and when the @command{chown} is actually run
9619 One way to narrow the gap would be to invoke chown for each file
9623 find / -owner OLDUSER -exec chown -h NEWUSER @{@} \;
9626 But that is very slow if there are many affected files.
9627 With this option, it is safer (the gap is narrower still)
9628 though still not perfect:
9631 chown -h -R --from=OLDUSER NEWUSER /
9635 @opindex --dereference
9636 @cindex symbolic links, changing owner
9638 Do not act on symbolic links themselves but rather on what they point to.
9639 This is the default.
9642 @itemx --no-dereference
9644 @opindex --no-dereference
9645 @cindex symbolic links, changing owner
9647 Act on symbolic links themselves instead of what they point to.
9648 This mode relies on the @code{lchown} system call.
9649 On systems that do not provide the @code{lchown} system call,
9650 @command{chown} fails when a file specified on the command line
9652 By default, no diagnostic is issued for symbolic links encountered
9653 during a recursive traversal, but see @option{--verbose}.
9655 @itemx --preserve-root
9656 @opindex --preserve-root
9657 @cindex root directory, disallow recursive modification
9658 Fail upon any attempt to recursively change the root directory, @file{/}.
9659 Without @option{--recursive}, this option has no effect.
9660 @xref{Treating / specially}.
9662 @itemx --no-preserve-root
9663 @opindex --no-preserve-root
9664 @cindex root directory, allow recursive modification
9665 Cancel the effect of any preceding @option{--preserve-root} option.
9666 @xref{Treating / specially}.
9668 @item --reference=@var{ref_file}
9669 @opindex --reference
9670 Change the user and group of each @var{file} to be the same as those of
9671 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9672 user and group of the symbolic link, but rather those of the file it
9679 Output a diagnostic for every file processed.
9680 If a symbolic link is encountered during a recursive traversal
9681 on a system without the @code{lchown} system call, and @option{--no-dereference}
9682 is in effect, then issue a diagnostic saying neither the symbolic link nor
9683 its referent is being changed.
9688 @opindex --recursive
9689 @cindex recursively changing file ownership
9690 Recursively change ownership of directories and their contents.
9693 @xref{Traversing symlinks}.
9696 @xref{Traversing symlinks}.
9699 @xref{Traversing symlinks}.
9708 # Change the owner of /u to "root".
9711 # Likewise, but also change its group to "staff".
9714 # Change the owner of /u and subfiles to "root".
9719 @node chgrp invocation
9720 @section @command{chgrp}: Change group ownership
9723 @cindex group ownership, changing
9724 @cindex changing group ownership
9726 @command{chgrp} changes the group ownership of each given @var{file}
9727 to @var{group} (which can be either a group name or a numeric group ID)
9728 or to the group of an existing reference file. Synopsis:
9731 chgrp [@var{option}]@dots{} @{@var{group} | --reference=@var{ref_file}@} @var{file}@dots{}
9734 If @var{group} is intended to represent a
9735 numeric group ID, then you may specify it with a leading @samp{+}.
9736 @xref{Disambiguating names and IDs}.
9738 The program accepts the following options. Also see @ref{Common options}.
9746 @cindex changed files, verbosely describing
9747 Verbosely describe the action for each @var{file} whose group actually
9756 @cindex error messages, omitting
9757 Do not print error messages about files whose group cannot be
9761 @opindex --dereference
9762 @cindex symbolic links, changing owner
9764 Do not act on symbolic links themselves but rather on what they point to.
9765 This is the default.
9768 @itemx --no-dereference
9770 @opindex --no-dereference
9771 @cindex symbolic links, changing group
9773 Act on symbolic links themselves instead of what they point to.
9774 This mode relies on the @code{lchown} system call.
9775 On systems that do not provide the @code{lchown} system call,
9776 @command{chgrp} fails when a file specified on the command line
9778 By default, no diagnostic is issued for symbolic links encountered
9779 during a recursive traversal, but see @option{--verbose}.
9781 @itemx --preserve-root
9782 @opindex --preserve-root
9783 @cindex root directory, disallow recursive modification
9784 Fail upon any attempt to recursively change the root directory, @file{/}.
9785 Without @option{--recursive}, this option has no effect.
9786 @xref{Treating / specially}.
9788 @itemx --no-preserve-root
9789 @opindex --no-preserve-root
9790 @cindex root directory, allow recursive modification
9791 Cancel the effect of any preceding @option{--preserve-root} option.
9792 @xref{Treating / specially}.
9794 @item --reference=@var{ref_file}
9795 @opindex --reference
9796 Change the group of each @var{file} to be the same as that of
9797 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9798 group of the symbolic link, but rather that of the file it refers to.
9804 Output a diagnostic for every file processed.
9805 If a symbolic link is encountered during a recursive traversal
9806 on a system without the @code{lchown} system call, and @option{--no-dereference}
9807 is in effect, then issue a diagnostic saying neither the symbolic link nor
9808 its referent is being changed.
9813 @opindex --recursive
9814 @cindex recursively changing group ownership
9815 Recursively change the group ownership of directories and their contents.
9818 @xref{Traversing symlinks}.
9821 @xref{Traversing symlinks}.
9824 @xref{Traversing symlinks}.
9833 # Change the group of /u to "staff".
9836 # Change the group of /u and subfiles to "staff".
9841 @node chmod invocation
9842 @section @command{chmod}: Change access permissions
9845 @cindex changing access permissions
9846 @cindex access permissions, changing
9847 @cindex permissions, changing access
9849 @command{chmod} changes the access permissions of the named files. Synopsis:
9852 chmod [@var{option}]@dots{} @{@var{mode} | --reference=@var{ref_file}@} @var{file}@dots{}
9855 @cindex symbolic links, permissions of
9856 @command{chmod} never changes the permissions of symbolic links, since
9857 the @command{chmod} system call cannot change their permissions.
9858 This is not a problem since the permissions of symbolic links are
9859 never used. However, for each symbolic link listed on the command
9860 line, @command{chmod} changes the permissions of the pointed-to file.
9861 In contrast, @command{chmod} ignores symbolic links encountered during
9862 recursive directory traversals.
9864 A successful use of @command{chmod} clears the set-group-ID bit of a
9865 regular file if the file's group ID does not match the user's
9866 effective group ID or one of the user's supplementary group IDs,
9867 unless the user has appropriate privileges. Additional restrictions
9868 may cause the set-user-ID and set-group-ID bits of @var{mode} or
9869 @var{ref_file} to be ignored. This behavior depends on the policy and
9870 functionality of the underlying @code{chmod} system call. When in
9871 doubt, check the underlying system behavior.
9873 If used, @var{mode} specifies the new file mode bits.
9874 For details, see the section on @ref{File permissions}.
9875 If you really want @var{mode} to have a leading @samp{-}, you should
9876 use @option{--} first, e.g., @samp{chmod -- -w file}. Typically,
9877 though, @samp{chmod a-w file} is preferable, and @command{chmod -w
9878 file} (without the @option{--}) complains if it behaves differently
9879 from what @samp{chmod a-w file} would do.
9881 The program accepts the following options. Also see @ref{Common options}.
9889 Verbosely describe the action for each @var{file} whose permissions
9898 @cindex error messages, omitting
9899 Do not print error messages about files whose permissions cannot be
9902 @itemx --preserve-root
9903 @opindex --preserve-root
9904 @cindex root directory, disallow recursive modification
9905 Fail upon any attempt to recursively change the root directory, @file{/}.
9906 Without @option{--recursive}, this option has no effect.
9907 @xref{Treating / specially}.
9909 @itemx --no-preserve-root
9910 @opindex --no-preserve-root
9911 @cindex root directory, allow recursive modification
9912 Cancel the effect of any preceding @option{--preserve-root} option.
9913 @xref{Treating / specially}.
9919 Verbosely describe the action or non-action taken for every @var{file}.
9921 @item --reference=@var{ref_file}
9922 @opindex --reference
9923 Change the mode of each @var{file} to be the same as that of @var{ref_file}.
9924 @xref{File permissions}.
9925 If @var{ref_file} is a symbolic link, do not use the mode
9926 of the symbolic link, but rather that of the file it refers to.
9931 @opindex --recursive
9932 @cindex recursively changing access permissions
9933 Recursively change permissions of directories and their contents.
9940 @node touch invocation
9941 @section @command{touch}: Change file timestamps
9944 @cindex changing file timestamps
9945 @cindex file timestamps, changing
9946 @cindex timestamps, changing file
9948 @command{touch} changes the access and/or modification times of the
9949 specified files. Synopsis:
9952 touch [@var{option}]@dots{} @var{file}@dots{}
9955 @cindex empty files, creating
9956 Any @var{file} argument that does not exist is created empty, unless
9957 option @option{--no-create} (@option{-c}) or @option{--no-dereference}
9958 (@option{-h}) was in effect.
9960 A @var{file} argument string of @samp{-} is handled specially and
9961 causes @command{touch} to change the times of the file associated with
9964 @cindex permissions, for changing file timestamps
9965 If changing both the access and modification times to the current
9966 time, @command{touch} can change the timestamps for files that the user
9967 running it does not own but has write permission for. Otherwise, the
9968 user must own the files.
9970 Although @command{touch} provides options for changing two of the times---the
9971 times of last access and modification---of a file, there is actually
9972 a standard third one as well: the inode change time. This is often
9973 referred to as a file's @code{ctime}.
9974 The inode change time represents the time when the file's meta-information
9975 last changed. One common example of this is when the permissions of a
9976 file change. Changing the permissions doesn't access the file, so
9977 the atime doesn't change, nor does it modify the file, so the mtime
9978 doesn't change. Yet, something about the file itself has changed,
9979 and this must be noted somewhere. This is the job of the ctime field.
9980 This is necessary, so that, for example, a backup program can make a
9981 fresh copy of the file, including the new permissions value.
9982 Another operation that modifies a file's ctime without affecting
9983 the others is renaming. In any case, it is not possible, in normal
9984 operations, for a user to change the ctime field to a user-specified value.
9985 Some operating systems and file systems support a fourth time: the
9986 birth time, when the file was first created; by definition, this
9987 timestamp never changes.
9990 Time stamps assume the time zone rules specified by the @env{TZ}
9991 environment variable, or by the system default rules if @env{TZ} is
9992 not set. @xref{TZ Variable,, Specifying the Time Zone with @env{TZ},
9993 libc, The GNU C Library Reference Manual}.
9994 You can avoid ambiguities during
9995 daylight saving transitions by using @sc{utc} time stamps.
9997 The program accepts the following options. Also see @ref{Common options}.
10002 @itemx --time=atime
10003 @itemx --time=access
10007 @opindex atime@r{, changing}
10008 @opindex access @r{time, changing}
10009 @opindex use @r{time, changing}
10010 Change the access time only.
10015 @opindex --no-create
10016 Do not warn about or create files that do not exist.
10019 @itemx --date=@var{time}
10023 Use @var{time} instead of the current time. It can contain month names,
10024 time zones, @samp{am} and @samp{pm}, @samp{yesterday}, etc. For
10025 example, @option{--date="2004-02-27 14:19:13.489392193 +0530"}
10026 specifies the instant of time that is 489,392,193 nanoseconds after
10027 February 27, 2004 at 2:19:13 PM in a time zone that is 5 hours and 30
10028 minutes east of @acronym{UTC}. @xref{Date input formats}.
10029 File systems that do not support high-resolution time stamps
10030 silently ignore any excess precision here.
10034 @cindex BSD @command{touch} compatibility
10035 Ignored; for compatibility with BSD versions of @command{touch}.
10038 @itemx --no-dereference
10040 @opindex --no-dereference
10041 @cindex symbolic links, changing time
10043 Attempt to change the timestamps of a symbolic link, rather than what
10044 the link refers to. When using this option, empty files are not
10045 created, but option @option{-c} must also be used to avoid warning
10046 about files that do not exist. Not all systems support changing the
10047 timestamps of symlinks, since underlying system support for this
10048 action was not required until @acronym{POSIX} 2008. Also, on some
10049 systems, the mere act of examining a symbolic link changes the access
10050 time, such that only changes to the modification time will persist
10051 long enough to be observable. When coupled with option @option{-r}, a
10052 reference timestamp is taken from a symbolic link rather than the file
10056 @itemx --time=mtime
10057 @itemx --time=modify
10060 @opindex mtime@r{, changing}
10061 @opindex modify @r{time, changing}
10062 Change the modification time only.
10064 @item -r @var{file}
10065 @itemx --reference=@var{file}
10067 @opindex --reference
10068 Use the times of the reference @var{file} instead of the current time.
10069 If this option is combined with the @option{--date=@var{time}}
10070 (@option{-d @var{time}}) option, the reference @var{file}'s time is
10071 the origin for any relative @var{time}s given, but is otherwise ignored.
10072 For example, @samp{-r foo -d '-5 seconds'} specifies a time stamp
10073 equal to five seconds before the corresponding time stamp for @file{foo}.
10074 If @var{file} is a symbolic link, the reference timestamp is taken
10075 from the target of the symlink, unless @option{-h} was also in effect.
10077 @item -t [[@var{cc}]@var{yy}]@var{mmddhhmm}[.@var{ss}]
10078 Use the argument (optional four-digit or two-digit years, months,
10079 days, hours, minutes, optional seconds) instead of the current time.
10080 If the year is specified with only two digits, then @var{cc}
10081 is 20 for years in the range 0 @dots{} 68, and 19 for years in
10082 69 @dots{} 99. If no digits of the year are specified,
10083 the argument is interpreted as a date in the current year.
10084 Note that @var{ss} may be @samp{60}, to accommodate leap seconds.
10088 @vindex _POSIX2_VERSION
10089 On older systems, @command{touch} supports an obsolete syntax, as follows.
10090 If no timestamp is given with any of the @option{-d}, @option{-r}, or
10091 @option{-t} options, and if there are two or more @var{file}s and the
10092 first @var{file} is of the form @samp{@var{mmddhhmm}[@var{yy}]} and this
10093 would be a valid argument to the @option{-t} option (if the @var{yy}, if
10094 any, were moved to the front), and if the represented year
10095 is in the range 1969--1999, that argument is interpreted as the time
10096 for the other files instead of as a file name.
10097 This obsolete behavior can be enabled or disabled with the
10098 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
10099 conformance}), but portable scripts should avoid commands whose
10100 behavior depends on this variable.
10101 For example, use @samp{touch ./12312359 main.c} or @samp{touch -t
10102 12312359 main.c} rather than the ambiguous @samp{touch 12312359 main.c}.
10108 @chapter Disk usage
10112 No disk can hold an infinite amount of data. These commands report
10113 how much disk storage is in use or available, report other file and
10114 file status information, and write buffers to disk.
10117 * df invocation:: Report file system disk space usage.
10118 * du invocation:: Estimate file space usage.
10119 * stat invocation:: Report file or file system status.
10120 * sync invocation:: Synchronize memory and disk.
10121 * truncate invocation:: Shrink or extend the size of a file.
10125 @node df invocation
10126 @section @command{df}: Report file system disk space usage
10129 @cindex file system disk usage
10130 @cindex disk usage by file system
10132 @command{df} reports the amount of disk space used and available on
10133 file systems. Synopsis:
10136 df [@var{option}]@dots{} [@var{file}]@dots{}
10139 With no arguments, @command{df} reports the space used and available on all
10140 currently mounted file systems (of all types). Otherwise, @command{df}
10141 reports on the file system containing each argument @var{file}.
10143 Normally the disk space is printed in units of
10144 1024 bytes, but this can be overridden (@pxref{Block size}).
10145 Non-integer quantities are rounded up to the next higher unit.
10147 @cindex disk device file
10148 @cindex device file, disk
10149 If an argument @var{file} is a disk device file containing a mounted
10150 file system, @command{df} shows the space available on that file system
10151 rather than on the file system containing the device node (i.e., the root
10152 file system). @sc{gnu} @command{df} does not attempt to determine the disk usage
10153 on unmounted file systems, because on most kinds of systems doing so
10154 requires extremely nonportable intimate knowledge of file system
10157 The program accepts the following options. Also see @ref{Common options}.
10165 @cindex automounter file systems
10166 @cindex ignore file systems
10167 Include in the listing dummy file systems, which
10168 are omitted by default. Such file systems are typically special-purpose
10169 pseudo-file-systems, such as automounter entries.
10171 @item -B @var{size}
10172 @itemx --block-size=@var{size}
10174 @opindex --block-size
10175 @cindex file system sizes
10176 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10177 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10181 @cindex grand total of disk size, usage and available space
10182 Print a grand total of all arguments after all arguments have
10183 been processed. This can be used to find out the total disk size, usage
10184 and available space of all listed devices.
10190 Equivalent to @option{--si}.
10196 @cindex inode usage
10197 List inode usage information instead of block usage. An inode (short
10198 for index node) contains information about a file such as its owner,
10199 permissions, timestamps, and location on the disk.
10203 @cindex kibibytes for file system sizes
10204 Print sizes in 1024-byte blocks, overriding the default block size
10205 (@pxref{Block size}).
10206 This option is equivalent to @option{--block-size=1K}.
10212 @cindex file system types, limiting output to certain
10213 Limit the listing to local file systems. By default, remote file systems
10218 @cindex file system space, retrieving old data more quickly
10219 Do not invoke the @code{sync} system call before getting any usage data.
10220 This may make @command{df} run significantly faster on systems with many
10221 disks, but on some systems (notably SunOS) the results may be slightly
10222 out of date. This is the default.
10225 @itemx --portability
10227 @opindex --portability
10228 @cindex one-line output format
10229 @cindex @acronym{POSIX} output format
10230 @cindex portable output format
10231 @cindex output format, portable
10232 Use the @acronym{POSIX} output format. This is like the default format except
10237 The information about each file system is always printed on exactly
10238 one line; a mount device is never put on a line by itself. This means
10239 that if the mount device name is more than 20 characters long (e.g., for
10240 some network mounts), the columns are misaligned.
10243 The labels in the header output line are changed to conform to @acronym{POSIX}.
10246 The default block size and output format are unaffected by the
10247 @env{DF_BLOCK_SIZE}, @env{BLOCK_SIZE} and @env{BLOCKSIZE} environment
10248 variables. However, the default block size is still affected by
10249 @env{POSIXLY_CORRECT}: it is 512 if @env{POSIXLY_CORRECT} is set, 1024
10250 otherwise. @xref{Block size}.
10257 @cindex file system space, retrieving current data more slowly
10258 Invoke the @code{sync} system call before getting any usage data. On
10259 some systems (notably SunOS), doing this yields more up to date results,
10260 but in general this option makes @command{df} much slower, especially when
10261 there are many or very busy file systems.
10263 @item -t @var{fstype}
10264 @itemx --type=@var{fstype}
10267 @cindex file system types, limiting output to certain
10268 Limit the listing to file systems of type @var{fstype}. Multiple
10269 file system types can be specified by giving multiple @option{-t} options.
10270 By default, nothing is omitted.
10273 @itemx --print-type
10275 @opindex --print-type
10276 @cindex file system types, printing
10277 Print each file system's type. The types printed here are the same ones
10278 you can include or exclude with @option{-t} and @option{-x}. The particular
10279 types printed are whatever is supported by the system. Here are some of
10280 the common names (this list is certainly not exhaustive):
10285 @cindex @acronym{NFS} file system type
10286 An @acronym{NFS} file system, i.e., one mounted over a network from another
10287 machine. This is the one type name which seems to be used uniformly by
10290 @item 4.2@r{, }ufs@r{, }efs@dots{}
10291 @cindex Linux file system types
10292 @cindex local file system types
10293 @opindex 4.2 @r{file system type}
10294 @opindex ufs @r{file system type}
10295 @opindex efs @r{file system type}
10296 A file system on a locally-mounted hard disk. (The system might even
10297 support more than one type here; Linux does.)
10299 @item hsfs@r{, }cdfs
10300 @cindex CD-ROM file system type
10301 @cindex High Sierra file system
10302 @opindex hsfs @r{file system type}
10303 @opindex cdfs @r{file system type}
10304 A file system on a CD-ROM drive. HP-UX uses @samp{cdfs}, most other
10305 systems use @samp{hsfs} (@samp{hs} for ``High Sierra'').
10308 @cindex PC file system
10309 @cindex DOS file system
10310 @cindex MS-DOS file system
10311 @cindex diskette file system
10313 An MS-DOS file system, usually on a diskette.
10317 @item -x @var{fstype}
10318 @itemx --exclude-type=@var{fstype}
10320 @opindex --exclude-type
10321 Limit the listing to file systems not of type @var{fstype}.
10322 Multiple file system types can be eliminated by giving multiple
10323 @option{-x} options. By default, no file system types are omitted.
10326 Ignored; for compatibility with System V versions of @command{df}.
10331 Failure includes the case where no output is generated, so you can
10332 inspect the exit status of a command like @samp{df -t ext3 -t reiserfs
10333 @var{dir}} to test whether @var{dir} is on a file system of type
10334 @samp{ext3} or @samp{reiserfs}.
10337 @node du invocation
10338 @section @command{du}: Estimate file space usage
10341 @cindex file space usage
10342 @cindex disk usage for files
10344 @command{du} reports the amount of disk space used by the specified files
10345 and for each subdirectory (of directory arguments). Synopsis:
10348 du [@var{option}]@dots{} [@var{file}]@dots{}
10351 With no arguments, @command{du} reports the disk space for the current
10352 directory. Normally the disk space is printed in units of
10353 1024 bytes, but this can be overridden (@pxref{Block size}).
10354 Non-integer quantities are rounded up to the next higher unit.
10356 If two or more hard links point to the same file, only one of the hard
10357 links is counted. The @var{file} argument order affects which links
10358 are counted, and changing the argument order may change the numbers
10359 that @command{du} outputs.
10361 The program accepts the following options. Also see @ref{Common options}.
10369 Show counts for all files, not just directories.
10371 @itemx --apparent-size
10372 @opindex --apparent-size
10373 Print apparent sizes, rather than disk usage. The apparent size of a
10374 file is the number of bytes reported by @code{wc -c} on regular files,
10375 or more generally, @code{ls -l --block-size=1} or @code{stat --format=%s}.
10376 For example, a file containing the word @samp{zoo} with no newline would,
10377 of course, have an apparent size of 3. Such a small file may require
10378 anywhere from 0 to 16 KiB or more of disk space, depending on
10379 the type and configuration of the file system on which the file resides.
10380 However, a sparse file created with this command:
10383 dd bs=1 seek=2GiB if=/dev/null of=big
10387 has an apparent size of 2 GiB, yet on most modern
10388 systems, it actually uses almost no disk space.
10394 Equivalent to @code{--apparent-size --block-size=1}.
10396 @item -B @var{size}
10397 @itemx --block-size=@var{size}
10399 @opindex --block-size
10401 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10402 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10408 @cindex grand total of disk space
10409 Print a grand total of all arguments after all arguments have
10410 been processed. This can be used to find out the total disk usage of
10411 a given set of files or directories.
10414 @itemx --dereference-args
10416 @opindex --dereference-args
10417 Dereference symbolic links that are command line arguments.
10418 Does not affect other symbolic links. This is helpful for finding
10419 out the disk usage of directories, such as @file{/usr/tmp}, which
10420 are often symbolic links.
10422 @c --files0-from=FILE
10423 @filesZeroFromOption{du,, with the @option{--total} (@option{-c}) option}
10429 Equivalent to @option{--dereference-args} (@option{-D}).
10433 @cindex kibibytes for file sizes
10434 Print sizes in 1024-byte blocks, overriding the default block size
10435 (@pxref{Block size}).
10436 This option is equivalent to @option{--block-size=1K}.
10439 @itemx --count-links
10441 @opindex --count-links
10442 @cindex hard links, counting in @command{du}
10443 Count the size of all files, even if they have appeared already (as a
10447 @itemx --dereference
10449 @opindex --dereference
10450 @cindex symbolic links, dereferencing in @command{du}
10451 Dereference symbolic links (show the disk space used by the file
10452 or directory that the link points to instead of the space used by
10457 @cindex mebibytes for file sizes
10458 Print sizes in 1,048,576-byte blocks, overriding the default block size
10459 (@pxref{Block size}).
10460 This option is equivalent to @option{--block-size=1M}.
10463 @itemx --no-dereference
10465 @opindex --no-dereference
10466 @cindex symbolic links, dereferencing in @command{du}
10467 For each symbolic links encountered by @command{du},
10468 consider the disk space used by the symbolic link.
10470 @item -d @var{depth}
10471 @item --max-depth=@var{depth}
10472 @opindex -d @var{depth}
10473 @opindex --max-depth=@var{depth}
10474 @cindex limiting output of @command{du}
10475 Show the total for each directory (and file if --all) that is at
10476 most MAX_DEPTH levels down from the root of the hierarchy. The root
10477 is at level 0, so @code{du --max-depth=0} is equivalent to @code{du -s}.
10486 @opindex --summarize
10487 Display only a total for each argument.
10490 @itemx --separate-dirs
10492 @opindex --separate-dirs
10493 Normally, in the output of @command{du} (when not using @option{--summarize}),
10494 the size listed next to a directory name, @var{d}, represents the sum
10495 of sizes of all entries beneath @var{d} as well as the size of @var{d} itself.
10496 With @option{--separate-dirs}, the size reported for a directory name,
10497 @var{d}, is merely the @code{stat.st_size}-derived size of the directory
10502 @cindex last modified dates, displaying in @command{du}
10503 Show time of the most recent modification of any file in the directory,
10504 or any of its subdirectories.
10506 @itemx --time=ctime
10507 @itemx --time=status
10510 @opindex ctime@r{, show the most recent}
10511 @opindex status time@r{, show the most recent}
10512 @opindex use time@r{, show the most recent}
10513 Show the most recent status change time (the @samp{ctime} in the inode) of
10514 any file in the directory, instead of the modification time.
10516 @itemx --time=atime
10517 @itemx --time=access
10519 @opindex atime@r{, show the most recent}
10520 @opindex access time@r{, show the most recent}
10521 Show the most recent access time (the @samp{atime} in the inode) of
10522 any file in the directory, instead of the modification time.
10524 @item --time-style=@var{style}
10525 @opindex --time-style
10527 List timestamps in style @var{style}. This option has an effect only if
10528 the @option{--time} option is also specified. The @var{style} should
10529 be one of the following:
10532 @item +@var{format}
10534 List timestamps using @var{format}, where @var{format} is interpreted
10535 like the format argument of @command{date} (@pxref{date invocation}).
10536 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
10537 @command{du} to list timestamps like @samp{2002-03-30 23:45:56}. As
10538 with @command{date}, @var{format}'s interpretation is affected by the
10539 @env{LC_TIME} locale category.
10542 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
10543 format with nanosecond precision, e.g., @samp{2002-03-30
10544 23:45:56.477817180 -0700}. This style is equivalent to
10545 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
10548 List @acronym{ISO} 8601 date and time in minutes, e.g.,
10549 @samp{2002-03-30 23:45}. These timestamps are shorter than
10550 @samp{full-iso} timestamps, and are usually good enough for everyday
10551 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
10554 List @acronym{ISO} 8601 dates for timestamps, e.g., @samp{2002-03-30}.
10555 This style is equivalent to @samp{+%Y-%m-%d}.
10559 You can specify the default value of the @option{--time-style} option
10560 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
10561 the default style is @samp{long-iso}. For compatibility with @command{ls},
10562 if @env{TIME_STYLE} begins with @samp{+} and contains a newline,
10563 the newline and any later characters are ignored; if @env{TIME_STYLE}
10564 begins with @samp{posix-} the @samp{posix-} is ignored; and if
10565 @env{TIME_STYLE} is @samp{locale} it is ignored.
10568 @itemx --one-file-system
10570 @opindex --one-file-system
10571 @cindex one file system, restricting @command{du} to
10572 Skip directories that are on different file systems from the one that
10573 the argument being processed is on.
10575 @item --exclude=@var{pattern}
10576 @opindex --exclude=@var{pattern}
10577 @cindex excluding files from @command{du}
10578 When recursing, skip subdirectories or files matching @var{pattern}.
10579 For example, @code{du --exclude='*.o'} excludes files whose names
10582 @item -X @var{file}
10583 @itemx --exclude-from=@var{file}
10584 @opindex -X @var{file}
10585 @opindex --exclude-from=@var{file}
10586 @cindex excluding files from @command{du}
10587 Like @option{--exclude}, except take the patterns to exclude from @var{file},
10588 one per line. If @var{file} is @samp{-}, take the patterns from standard
10593 @cindex NFS mounts from BSD to HP-UX
10594 On BSD systems, @command{du} reports sizes that are half the correct
10595 values for files that are NFS-mounted from HP-UX systems. On HP-UX
10596 systems, it reports sizes that are twice the correct values for
10597 files that are NFS-mounted from BSD systems. This is due to a flaw
10598 in HP-UX; it also affects the HP-UX @command{du} program.
10603 @node stat invocation
10604 @section @command{stat}: Report file or file system status
10607 @cindex file status
10608 @cindex file system status
10610 @command{stat} displays information about the specified file(s). Synopsis:
10613 stat [@var{option}]@dots{} [@var{file}]@dots{}
10616 With no option, @command{stat} reports all information about the given files.
10617 But it also can be used to report the information of the file systems the
10618 given files are located on. If the files are links, @command{stat} can
10619 also give information about the files the links point to.
10621 @mayConflictWithShellBuiltIn{stat}
10626 @itemx --dereference
10628 @opindex --dereference
10629 @cindex symbolic links, dereferencing in @command{stat}
10630 Change how @command{stat} treats symbolic links.
10631 With this option, @command{stat} acts on the file referenced
10632 by each symbolic link argument.
10633 Without it, @command{stat} acts on any symbolic link argument directly.
10636 @itemx --file-system
10638 @opindex --file-system
10639 @cindex file systems
10640 Report information about the file systems where the given files are located
10641 instead of information about the files themselves.
10642 This option implies the @option{-L} option.
10645 @itemx --format=@var{format}
10647 @opindex --format=@var{format}
10648 @cindex output format
10649 Use @var{format} rather than the default format.
10650 @var{format} is automatically newline-terminated, so
10651 running a command like the following with two or more @var{file}
10652 operands produces a line of output for each operand:
10654 $ stat --format=%d:%i / /usr
10659 @itemx --printf=@var{format}
10660 @opindex --printf=@var{format}
10661 @cindex output format
10662 Use @var{format} rather than the default format.
10663 Like @option{--format}, but interpret backslash escapes,
10664 and do not output a mandatory trailing newline.
10665 If you want a newline, include @samp{\n} in the @var{format}.
10666 Here's how you would use @option{--printf} to print the device
10667 and inode numbers of @file{/} and @file{/usr}:
10669 $ stat --printf='%d:%i\n' / /usr
10678 @cindex terse output
10679 Print the information in terse form, suitable for parsing by other programs.
10683 The valid @var{format} directives for files with @option{--format} and
10684 @option{--printf} are:
10687 @item %a - Access rights in octal
10688 @item %A - Access rights in human readable form
10689 @item %b - Number of blocks allocated (see @samp{%B})
10690 @item %B - The size in bytes of each block reported by @samp{%b}
10691 @item %C - The SELinux security context of a file, if available
10692 @item %d - Device number in decimal
10693 @item %D - Device number in hex
10694 @item %f - Raw mode in hex
10695 @item %F - File type
10696 @item %g - Group ID of owner
10697 @item %G - Group name of owner
10698 @item %h - Number of hard links
10699 @item %i - Inode number
10700 @item %m - Mount point (See note below)
10701 @item %n - File name
10702 @item %N - Quoted file name with dereference if symbolic link
10703 @item %o - I/O block size
10704 @item %s - Total size, in bytes
10705 @item %t - Major device type in hex
10706 @item %T - Minor device type in hex
10707 @item %u - User ID of owner
10708 @item %U - User name of owner
10709 @item %w - Time of file birth, or @samp{-} if unknown
10710 @item %W - Time of file birth as seconds since Epoch, or @samp{0}
10711 @item %x - Time of last access
10712 @item %X - Time of last access as seconds since Epoch
10713 @item %y - Time of last modification
10714 @item %Y - Time of last modification as seconds since Epoch
10715 @item %z - Time of last change
10716 @item %Z - Time of last change as seconds since Epoch
10719 The @samp{%W}, @samp{%X}, @samp{%Y}, and @samp{%Z} formats accept a
10720 precision preceded by a period to specify the number of digits to
10721 print after the decimal point. For example, @samp{%.9X} outputs the
10722 last access time to nanosecond precision. If a period is given but no
10723 precision, @command{stat} uses the estimated precision of the file
10724 system. When discarding excess precision, time stamps are truncated
10725 toward minus infinity.
10729 $ stat -c '[%015Y]' /usr
10732 $ stat -c '[%15Y]' /usr
10734 $ stat -c '[%-15Y]' /usr
10737 $ stat -c '[%.3Y]' /usr
10739 $ stat -c '[%.Y]' /usr
10740 [1288929712.114951]
10743 The mount point printed by @samp{%m} is similar to that output
10744 by @command{df}, except that:
10747 stat does not dereference symlinks by default
10748 (unless @option{-L} is specified)
10750 stat does not search for specified device nodes in the
10751 file system list, instead operating on them directly
10754 stat outputs the alias for a bind mounted file, rather than
10755 the initial mount point of its backing device.
10756 One can recursively call stat until there is no change in output,
10757 to get the current base mount point
10760 When listing file system information (@option{--file-system} (@option{-f})),
10761 you must use a different set of @var{format} directives:
10764 @item %a - Free blocks available to non-super-user
10765 @item %b - Total data blocks in file system
10766 @item %c - Total file nodes in file system
10767 @item %d - Free file nodes in file system
10768 @item %f - Free blocks in file system
10769 @item %i - File System ID in hex
10770 @item %l - Maximum length of file names
10771 @item %n - File name
10772 @item %s - Block size (for faster transfers)
10773 @item %S - Fundamental block size (for block counts)
10774 @item %t - Type in hex
10775 @item %T - Type in human readable form
10779 Time stamps are listed according to the time zone rules specified by
10780 the @env{TZ} environment variable, or by the system default rules if
10781 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
10782 with @env{TZ}, libc, The GNU C Library Reference Manual}.
10787 @node sync invocation
10788 @section @command{sync}: Synchronize data on disk with memory
10791 @cindex synchronize disk and memory
10793 @cindex superblock, writing
10794 @cindex inodes, written buffered
10795 @command{sync} writes any data buffered in memory out to disk. This can
10796 include (but is not limited to) modified superblocks, modified inodes,
10797 and delayed reads and writes. This must be implemented by the kernel;
10798 The @command{sync} program does nothing but exercise the @code{sync} system
10801 @cindex crashes and corruption
10802 The kernel keeps data in memory to avoid doing (relatively slow) disk
10803 reads and writes. This improves performance, but if the computer
10804 crashes, data may be lost or the file system corrupted as a
10805 result. The @command{sync} command ensures everything in memory
10806 is written to disk.
10808 Any arguments are ignored, except for a lone @option{--help} or
10809 @option{--version} (@pxref{Common options}).
10814 @node truncate invocation
10815 @section @command{truncate}: Shrink or extend the size of a file
10818 @cindex truncating, file sizes
10820 @command{truncate} shrinks or extends the size of each @var{file} to the
10821 specified size. Synopsis:
10824 truncate @var{option}@dots{} @var{file}@dots{}
10827 @cindex files, creating
10828 Any @var{file} that does not exist is created.
10830 @cindex sparse files, creating
10831 @cindex holes, creating files with
10832 If a @var{file} is larger than the specified size, the extra data is lost.
10833 If a @var{file} is shorter, it is extended and the extended part (or hole)
10834 reads as zero bytes.
10836 The program accepts the following options. Also see @ref{Common options}.
10843 @opindex --no-create
10844 Do not create files that do not exist.
10849 @opindex --io-blocks
10850 Treat @var{size} as number of I/O blocks of the @var{file} rather than bytes.
10852 @item -r @var{rfile}
10853 @itemx --reference=@var{rfile}
10855 @opindex --reference
10856 Base the size of each @var{file} on the size of @var{rfile}.
10858 @item -s @var{size}
10859 @itemx --size=@var{size}
10862 Set or adjust the size of each @var{file} according to @var{size}.
10863 @multiplierSuffixesNoBlocks{size}
10865 @var{size} may also be prefixed by one of the following to adjust
10866 the size of each @var{file} based on their current size:
10868 @samp{+} => extend by
10869 @samp{-} => reduce by
10870 @samp{<} => at most
10871 @samp{>} => at least
10872 @samp{/} => round down to multiple of
10873 @samp{%} => round up to multiple of
10881 @node Printing text
10882 @chapter Printing text
10884 @cindex printing text, commands for
10885 @cindex commands for printing text
10887 This section describes commands that display text strings.
10890 * echo invocation:: Print a line of text.
10891 * printf invocation:: Format and print data.
10892 * yes invocation:: Print a string until interrupted.
10896 @node echo invocation
10897 @section @command{echo}: Print a line of text
10900 @cindex displaying text
10901 @cindex printing text
10902 @cindex text, displaying
10903 @cindex arbitrary text, displaying
10905 @command{echo} writes each given @var{string} to standard output, with a
10906 space between each and a newline after the last one. Synopsis:
10909 echo [@var{option}]@dots{} [@var{string}]@dots{}
10912 @mayConflictWithShellBuiltIn{echo}
10914 The program accepts the following options. Also see @ref{Common options}.
10915 Options must precede operands, and the normally-special argument
10916 @samp{--} has no special meaning and is treated like any other
10922 Do not output the trailing newline.
10926 @cindex backslash escapes
10927 Enable interpretation of the following backslash-escaped characters in
10936 produce no further output
10952 the eight-bit value that is the octal number @var{nnn}
10953 (zero to three octal digits)
10955 the eight-bit value that is the octal number @var{nnn}
10956 (one to three octal digits)
10958 the eight-bit value that is the hexadecimal number @var{hh}
10959 (one or two hexadecimal digits)
10964 @cindex backslash escapes
10965 Disable interpretation of backslash escapes in each @var{string}.
10966 This is the default. If @option{-e} and @option{-E} are both
10967 specified, the last one given takes effect.
10971 @vindex POSIXLY_CORRECT
10972 If the @env{POSIXLY_CORRECT} environment variable is set, then when
10973 @command{echo}'s first argument is not @option{-n} it outputs
10974 option-like arguments instead of treating them as options. For
10975 example, @code{echo -ne hello} outputs @samp{-ne hello} instead of
10976 plain @samp{hello}.
10978 @acronym{POSIX} does not require support for any options, and says
10979 that the behavior of @command{echo} is implementation-defined if any
10980 @var{string} contains a backslash or if the first argument is
10981 @option{-n}. Portable programs can use the @command{printf} command
10982 if they need to omit trailing newlines or output control characters or
10983 backslashes. @xref{printf invocation}.
10988 @node printf invocation
10989 @section @command{printf}: Format and print data
10992 @command{printf} does formatted printing of text. Synopsis:
10995 printf @var{format} [@var{argument}]@dots{}
10998 @command{printf} prints the @var{format} string, interpreting @samp{%}
10999 directives and @samp{\} escapes to format numeric and string arguments
11000 in a way that is mostly similar to the C @samp{printf} function.
11001 @xref{Output Conversion Syntax,, @command{printf} format directives,
11002 libc, The GNU C Library Reference Manual}, for details.
11003 The differences are listed below.
11005 @mayConflictWithShellBuiltIn{printf}
11010 The @var{format} argument is reused as necessary to convert all the
11011 given @var{argument}s. For example, the command @samp{printf %s a b}
11015 Missing @var{argument}s are treated as null strings or as zeros,
11016 depending on whether the context expects a string or a number. For
11017 example, the command @samp{printf %sx%d} prints @samp{x0}.
11021 An additional escape, @samp{\c}, causes @command{printf} to produce no
11022 further output. For example, the command @samp{printf 'A%sC\cD%sF' B
11023 E} prints @samp{ABC}.
11026 The hexadecimal escape sequence @samp{\x@var{hh}} has at most two
11027 digits, as opposed to C where it can have an unlimited number of
11028 digits. For example, the command @samp{printf '\x07e'} prints two
11029 bytes, whereas the C statement @samp{printf ("\x07e")} prints just
11034 @command{printf} has an additional directive, @samp{%b}, which prints its
11035 argument string with @samp{\} escapes interpreted in the same way as in
11036 the @var{format} string, except that octal escapes are of the form
11037 @samp{\0@var{ooo}} where @var{ooo} is 0 to 3 octal digits.
11038 If a precision is also given, it limits the number of bytes printed
11039 from the converted string.
11042 Numeric arguments must be single C constants, possibly with leading
11043 @samp{+} or @samp{-}. For example, @samp{printf %.4d -3} outputs
11047 @vindex POSIXLY_CORRECT
11048 If the leading character of a numeric argument is @samp{"} or @samp{'}
11049 then its value is the numeric value of the immediately following
11050 character. Any remaining characters are silently ignored if the
11051 @env{POSIXLY_CORRECT} environment variable is set; otherwise, a
11052 warning is printed. For example, @samp{printf "%d" "'a"} outputs
11053 @samp{97} on hosts that use the @acronym{ASCII} character set, since
11054 @samp{a} has the numeric value 97 in @acronym{ASCII}.
11059 A floating-point argument must use a period before any fractional
11060 digits, but is printed according to the @env{LC_NUMERIC} category of the
11061 current locale. For example, in a locale whose radix character is a
11062 comma, the command @samp{printf %g 3.14} outputs @samp{3,14} whereas
11063 the command @samp{printf %g 3,14} is an error.
11067 @command{printf} interprets @samp{\@var{ooo}} in @var{format} as an octal number
11068 (if @var{ooo} is 1 to 3 octal digits) specifying a character to print,
11069 and @samp{\x@var{hh}} as a hexadecimal number (if @var{hh} is 1 to 2 hex
11070 digits) specifying a character to print.
11075 @cindex ISO/IEC 10646
11077 @command{printf} interprets two character syntaxes introduced in
11078 @acronym{ISO} C 99:
11079 @samp{\u} for 16-bit Unicode (@acronym{ISO}/@acronym{IEC} 10646)
11080 characters, specified as
11081 four hexadecimal digits @var{hhhh}, and @samp{\U} for 32-bit Unicode
11082 characters, specified as eight hexadecimal digits @var{hhhhhhhh}.
11083 @command{printf} outputs the Unicode characters
11084 according to the @env{LC_CTYPE} locale. Unicode characters in the ranges
11085 U+0000...U+009F, U+D800...U+DFFF cannot be specified by this syntax, except
11086 for U+0024 ($), U+0040 (@@), and U+0060 (@`).
11088 The processing of @samp{\u} and @samp{\U} requires a full-featured
11089 @code{iconv} facility. It is activated on systems with glibc 2.2 (or newer),
11090 or when @code{libiconv} is installed prior to this package. Otherwise
11091 @samp{\u} and @samp{\U} will print as-is.
11093 The only options are a lone @option{--help} or
11094 @option{--version}. @xref{Common options}.
11095 Options must precede operands.
11097 The Unicode character syntaxes are useful for writing strings in a locale
11098 independent way. For example, a string containing the Euro currency symbol
11101 $ env printf '\u20AC 14.95'
11105 will be output correctly in all locales supporting the Euro symbol
11106 (@acronym{ISO}-8859-15, UTF-8, and others). Similarly, a Chinese string
11109 $ env printf '\u4e2d\u6587'
11113 will be output correctly in all Chinese locales (GB2312, BIG5, UTF-8, etc).
11115 Note that in these examples, the @command{printf} command has been
11116 invoked via @command{env} to ensure that we run the program found via
11117 your shell's search path, and not a shell alias or a built-in function.
11119 For larger strings, you don't need to look up the hexadecimal code
11120 values of each character one by one. @acronym{ASCII} characters mixed with \u
11121 escape sequences is also known as the JAVA source file encoding. You can
11122 use GNU recode 3.5c (or newer) to convert strings to this encoding. Here
11123 is how to convert a piece of text into a shell script which will output
11124 this text in a locale-independent way:
11127 $ LC_CTYPE=zh_CN.big5 /usr/local/bin/printf \
11128 '\u4e2d\u6587\n' > sample.txt
11129 $ recode BIG5..JAVA < sample.txt \
11130 | sed -e "s|^|/usr/local/bin/printf '|" -e "s|$|\\\\n'|" \
11137 @node yes invocation
11138 @section @command{yes}: Print a string until interrupted
11141 @cindex repeated output of a string
11143 @command{yes} prints the command line arguments, separated by spaces and
11144 followed by a newline, forever until it is killed. If no arguments are
11145 given, it prints @samp{y} followed by a newline forever until killed.
11147 Upon a write error, @command{yes} exits with status @samp{1}.
11149 The only options are a lone @option{--help} or @option{--version}.
11150 To output an argument that begins with
11151 @samp{-}, precede it with @option{--}, e.g., @samp{yes -- --help}.
11152 @xref{Common options}.
11156 @chapter Conditions
11159 @cindex commands for exit status
11160 @cindex exit status commands
11162 This section describes commands that are primarily useful for their exit
11163 status, rather than their output. Thus, they are often used as the
11164 condition of shell @code{if} statements, or as the last command in a
11168 * false invocation:: Do nothing, unsuccessfully.
11169 * true invocation:: Do nothing, successfully.
11170 * test invocation:: Check file types and compare values.
11171 * expr invocation:: Evaluate expressions.
11175 @node false invocation
11176 @section @command{false}: Do nothing, unsuccessfully
11179 @cindex do nothing, unsuccessfully
11180 @cindex failure exit status
11181 @cindex exit status of @command{false}
11183 @command{false} does nothing except return an exit status of 1, meaning
11184 @dfn{failure}. It can be used as a place holder in shell scripts
11185 where an unsuccessful command is needed.
11186 In most modern shells, @command{false} is a built-in command, so when
11187 you use @samp{false} in a script, you're probably using the built-in
11188 command, not the one documented here.
11190 @command{false} honors the @option{--help} and @option{--version} options.
11192 This version of @command{false} is implemented as a C program, and is thus
11193 more secure and faster than a shell script implementation, and may safely
11194 be used as a dummy shell for the purpose of disabling accounts.
11196 Note that @command{false} (unlike all other programs documented herein)
11197 exits unsuccessfully, even when invoked with
11198 @option{--help} or @option{--version}.
11200 Portable programs should not assume that the exit status of
11201 @command{false} is 1, as it is greater than 1 on some
11202 non-@acronym{GNU} hosts.
11205 @node true invocation
11206 @section @command{true}: Do nothing, successfully
11209 @cindex do nothing, successfully
11211 @cindex successful exit
11212 @cindex exit status of @command{true}
11214 @command{true} does nothing except return an exit status of 0, meaning
11215 @dfn{success}. It can be used as a place holder in shell scripts
11216 where a successful command is needed, although the shell built-in
11217 command @code{:} (colon) may do the same thing faster.
11218 In most modern shells, @command{true} is a built-in command, so when
11219 you use @samp{true} in a script, you're probably using the built-in
11220 command, not the one documented here.
11222 @command{true} honors the @option{--help} and @option{--version} options.
11224 Note, however, that it is possible to cause @command{true}
11225 to exit with nonzero status: with the @option{--help} or @option{--version}
11226 option, and with standard
11227 output already closed or redirected to a file that evokes an I/O error.
11228 For example, using a Bourne-compatible shell:
11231 $ ./true --version >&-
11232 ./true: write error: Bad file number
11233 $ ./true --version > /dev/full
11234 ./true: write error: No space left on device
11237 This version of @command{true} is implemented as a C program, and is thus
11238 more secure and faster than a shell script implementation, and may safely
11239 be used as a dummy shell for the purpose of disabling accounts.
11241 @node test invocation
11242 @section @command{test}: Check file types and compare values
11245 @cindex check file types
11246 @cindex compare values
11247 @cindex expression evaluation
11249 @command{test} returns a status of 0 (true) or 1 (false) depending on the
11250 evaluation of the conditional expression @var{expr}. Each part of the
11251 expression must be a separate argument.
11253 @command{test} has file status checks, string operators, and numeric
11254 comparison operators.
11256 @command{test} has an alternate form that uses opening and closing
11257 square brackets instead a leading @samp{test}. For example, instead
11258 of @samp{test -d /}, you can write @samp{[ -d / ]}. The square
11259 brackets must be separate arguments; for example, @samp{[-d /]} does
11260 not have the desired effect. Since @samp{test @var{expr}} and @samp{[
11261 @var{expr} ]} have the same meaning, only the former form is discussed
11267 test @var{expression}
11269 [ @var{expression} ]
11274 @mayConflictWithShellBuiltIn{test}
11276 If @var{expression} is omitted, @command{test} returns false.
11277 If @var{expression} is a single argument,
11278 @command{test} returns false if the argument is null and true otherwise. The argument
11279 can be any string, including strings like @samp{-d}, @samp{-1},
11280 @samp{--}, @samp{--help}, and @samp{--version} that most other
11281 programs would treat as options. To get help and version information,
11282 invoke the commands @samp{[ --help} and @samp{[ --version}, without
11283 the usual closing brackets. @xref{Common options}.
11285 @cindex exit status of @command{test}
11289 0 if the expression is true,
11290 1 if the expression is false,
11291 2 if an error occurred.
11295 * File type tests:: -[bcdfhLpSt]
11296 * Access permission tests:: -[gkruwxOG]
11297 * File characteristic tests:: -e -s -nt -ot -ef
11298 * String tests:: -z -n = !=
11299 * Numeric tests:: -eq -ne -lt -le -gt -ge
11300 * Connectives for test:: ! -a -o
11304 @node File type tests
11305 @subsection File type tests
11307 @cindex file type tests
11309 These options test for particular types of files. (Everything's a file,
11310 but not all files are the same!)
11314 @item -b @var{file}
11316 @cindex block special check
11317 True if @var{file} exists and is a block special device.
11319 @item -c @var{file}
11321 @cindex character special check
11322 True if @var{file} exists and is a character special device.
11324 @item -d @var{file}
11326 @cindex directory check
11327 True if @var{file} exists and is a directory.
11329 @item -f @var{file}
11331 @cindex regular file check
11332 True if @var{file} exists and is a regular file.
11334 @item -h @var{file}
11335 @itemx -L @var{file}
11338 @cindex symbolic link check
11339 True if @var{file} exists and is a symbolic link.
11340 Unlike all other file-related tests, this test does not dereference
11341 @var{file} if it is a symbolic link.
11343 @item -p @var{file}
11345 @cindex named pipe check
11346 True if @var{file} exists and is a named pipe.
11348 @item -S @var{file}
11350 @cindex socket check
11351 True if @var{file} exists and is a socket.
11355 @cindex terminal check
11356 True if @var{fd} is a file descriptor that is associated with a
11362 @node Access permission tests
11363 @subsection Access permission tests
11365 @cindex access permission tests
11366 @cindex permission tests
11368 These options test for particular access permissions.
11372 @item -g @var{file}
11374 @cindex set-group-ID check
11375 True if @var{file} exists and has its set-group-ID bit set.
11377 @item -k @var{file}
11379 @cindex sticky bit check
11380 True if @var{file} exists and has its @dfn{sticky} bit set.
11382 @item -r @var{file}
11384 @cindex readable file check
11385 True if @var{file} exists and read permission is granted.
11387 @item -u @var{file}
11389 @cindex set-user-ID check
11390 True if @var{file} exists and has its set-user-ID bit set.
11392 @item -w @var{file}
11394 @cindex writable file check
11395 True if @var{file} exists and write permission is granted.
11397 @item -x @var{file}
11399 @cindex executable file check
11400 True if @var{file} exists and execute permission is granted
11401 (or search permission, if it is a directory).
11403 @item -O @var{file}
11405 @cindex owned by effective user ID check
11406 True if @var{file} exists and is owned by the current effective user ID.
11408 @item -G @var{file}
11410 @cindex owned by effective group ID check
11411 True if @var{file} exists and is owned by the current effective group ID.
11415 @node File characteristic tests
11416 @subsection File characteristic tests
11418 @cindex file characteristic tests
11420 These options test other file characteristics.
11424 @item -e @var{file}
11426 @cindex existence-of-file check
11427 True if @var{file} exists.
11429 @item -s @var{file}
11431 @cindex nonempty file check
11432 True if @var{file} exists and has a size greater than zero.
11434 @item @var{file1} -nt @var{file2}
11436 @cindex newer-than file check
11437 True if @var{file1} is newer (according to modification date) than
11438 @var{file2}, or if @var{file1} exists and @var{file2} does not.
11440 @item @var{file1} -ot @var{file2}
11442 @cindex older-than file check
11443 True if @var{file1} is older (according to modification date) than
11444 @var{file2}, or if @var{file2} exists and @var{file1} does not.
11446 @item @var{file1} -ef @var{file2}
11448 @cindex same file check
11449 @cindex hard link check
11450 True if @var{file1} and @var{file2} have the same device and inode
11451 numbers, i.e., if they are hard links to each other.
11457 @subsection String tests
11459 @cindex string tests
11461 These options test string characteristics. You may need to quote
11462 @var{string} arguments for the shell. For example:
11468 The quotes here prevent the wrong arguments from being passed to
11469 @command{test} if @samp{$V} is empty or contains special characters.
11473 @item -z @var{string}
11475 @cindex zero-length string check
11476 True if the length of @var{string} is zero.
11478 @item -n @var{string}
11479 @itemx @var{string}
11481 @cindex nonzero-length string check
11482 True if the length of @var{string} is nonzero.
11484 @item @var{string1} = @var{string2}
11486 @cindex equal string check
11487 True if the strings are equal.
11489 @item @var{string1} != @var{string2}
11491 @cindex not-equal string check
11492 True if the strings are not equal.
11497 @node Numeric tests
11498 @subsection Numeric tests
11500 @cindex numeric tests
11501 @cindex arithmetic tests
11503 Numeric relational operators. The arguments must be entirely numeric
11504 (possibly negative), or the special expression @w{@code{-l @var{string}}},
11505 which evaluates to the length of @var{string}.
11509 @item @var{arg1} -eq @var{arg2}
11510 @itemx @var{arg1} -ne @var{arg2}
11511 @itemx @var{arg1} -lt @var{arg2}
11512 @itemx @var{arg1} -le @var{arg2}
11513 @itemx @var{arg1} -gt @var{arg2}
11514 @itemx @var{arg1} -ge @var{arg2}
11521 These arithmetic binary operators return true if @var{arg1} is equal,
11522 not-equal, less-than, less-than-or-equal, greater-than, or
11523 greater-than-or-equal than @var{arg2}, respectively.
11530 test -1 -gt -2 && echo yes
11532 test -l abc -gt 1 && echo yes
11535 @error{} test: integer expression expected before -eq
11539 @node Connectives for test
11540 @subsection Connectives for @command{test}
11542 @cindex logical connectives
11543 @cindex connectives, logical
11545 The usual logical connectives.
11551 True if @var{expr} is false.
11553 @item @var{expr1} -a @var{expr2}
11555 @cindex logical and operator
11556 @cindex and operator
11557 True if both @var{expr1} and @var{expr2} are true.
11559 @item @var{expr1} -o @var{expr2}
11561 @cindex logical or operator
11562 @cindex or operator
11563 True if either @var{expr1} or @var{expr2} is true.
11568 @node expr invocation
11569 @section @command{expr}: Evaluate expressions
11572 @cindex expression evaluation
11573 @cindex evaluation of expressions
11575 @command{expr} evaluates an expression and writes the result on standard
11576 output. Each token of the expression must be a separate argument.
11578 Operands are either integers or strings. Integers consist of one or
11579 more decimal digits, with an optional leading @samp{-}.
11580 @command{expr} converts
11581 anything appearing in an operand position to an integer or a string
11582 depending on the operation being applied to it.
11584 Strings are not quoted for @command{expr} itself, though you may need to
11585 quote them to protect characters with special meaning to the shell,
11586 e.g., spaces. However, regardless of whether it is quoted, a string
11587 operand should not be a parenthesis or any of @command{expr}'s
11588 operators like @code{+}, so you cannot safely pass an arbitrary string
11589 @code{$str} to expr merely by quoting it to the shell. One way to
11590 work around this is to use the @sc{gnu} extension @code{+},
11591 (e.g., @code{+ "$str" = foo}); a more portable way is to use
11592 @code{@w{" $str"}} and to adjust the rest of the expression to take
11593 the leading space into account (e.g., @code{@w{" $str" = " foo"}}).
11595 You should not pass a negative integer or a string with leading
11596 @samp{-} as @command{expr}'s first argument, as it might be
11597 misinterpreted as an option; this can be avoided by parenthesization.
11598 Also, portable scripts should not use a string operand that happens to
11599 take the form of an integer; this can be worked around by inserting
11600 leading spaces as mentioned above.
11602 @cindex parentheses for grouping
11603 Operators may be given as infix symbols or prefix keywords. Parentheses
11604 may be used for grouping in the usual manner. You must quote
11605 parentheses and many operators to avoid the shell evaluating them,
11608 When built with support for the GNU MP library, @command{expr} uses
11609 arbitrary-precision arithmetic; otherwise, it uses native arithmetic
11610 types and may fail due to arithmetic overflow.
11612 The only options are @option{--help} and @option{--version}. @xref{Common
11613 options}. Options must precede operands.
11615 @cindex exit status of @command{expr}
11619 0 if the expression is neither null nor 0,
11620 1 if the expression is null or 0,
11621 2 if the expression is invalid,
11622 3 if an internal error occurred (e.g., arithmetic overflow).
11626 * String expressions:: + : match substr index length
11627 * Numeric expressions:: + - * / %
11628 * Relations for expr:: | & < <= = == != >= >
11629 * Examples of expr:: Examples.
11633 @node String expressions
11634 @subsection String expressions
11636 @cindex string expressions
11637 @cindex expressions, string
11639 @command{expr} supports pattern matching and other string operators. These
11640 have higher precedence than both the numeric and relational operators (in
11641 the next sections).
11645 @item @var{string} : @var{regex}
11646 @cindex pattern matching
11647 @cindex regular expression matching
11648 @cindex matching patterns
11649 Perform pattern matching. The arguments are converted to strings and the
11650 second is considered to be a (basic, a la GNU @code{grep}) regular
11651 expression, with a @code{^} implicitly prepended. The first argument is
11652 then matched against this regular expression.
11654 If the match succeeds and @var{regex} uses @samp{\(} and @samp{\)}, the
11655 @code{:} expression returns the part of @var{string} that matched the
11656 subexpression; otherwise, it returns the number of characters matched.
11658 If the match fails, the @code{:} operator returns the null string if
11659 @samp{\(} and @samp{\)} are used in @var{regex}, otherwise 0.
11661 @kindex \( @r{regexp operator}
11662 Only the first @samp{\( @dots{} \)} pair is relevant to the return
11663 value; additional pairs are meaningful only for grouping the regular
11664 expression operators.
11666 @kindex \+ @r{regexp operator}
11667 @kindex \? @r{regexp operator}
11668 @kindex \| @r{regexp operator}
11669 In the regular expression, @code{\+}, @code{\?}, and @code{\|} are
11670 operators which respectively match one or more, zero or one, or separate
11671 alternatives. SunOS and other @command{expr}'s treat these as regular
11672 characters. (@acronym{POSIX} allows either behavior.)
11673 @xref{Top, , Regular Expression Library, regex, Regex}, for details of
11674 regular expression syntax. Some examples are in @ref{Examples of expr}.
11676 @item match @var{string} @var{regex}
11678 An alternative way to do pattern matching. This is the same as
11679 @w{@samp{@var{string} : @var{regex}}}.
11681 @item substr @var{string} @var{position} @var{length}
11683 Returns the substring of @var{string} beginning at @var{position}
11684 with length at most @var{length}. If either @var{position} or
11685 @var{length} is negative, zero, or non-numeric, returns the null string.
11687 @item index @var{string} @var{charset}
11689 Returns the first position in @var{string} where the first character in
11690 @var{charset} was found. If no character in @var{charset} is found in
11691 @var{string}, return 0.
11693 @item length @var{string}
11695 Returns the length of @var{string}.
11697 @item + @var{token}
11699 Interpret @var{token} as a string, even if it is a keyword like @var{match}
11700 or an operator like @code{/}.
11701 This makes it possible to test @code{expr length + "$x"} or
11702 @code{expr + "$x" : '.*/\(.\)'} and have it do the right thing even if
11703 the value of @var{$x} happens to be (for example) @code{/} or @code{index}.
11704 This operator is a @acronym{GNU} extension. Portable shell scripts should use
11705 @code{@w{" $token"} : @w{' \(.*\)'}} instead of @code{+ "$token"}.
11709 To make @command{expr} interpret keywords as strings, you must use the
11710 @code{quote} operator.
11713 @node Numeric expressions
11714 @subsection Numeric expressions
11716 @cindex numeric expressions
11717 @cindex expressions, numeric
11719 @command{expr} supports the usual numeric operators, in order of increasing
11720 precedence. These numeric operators have lower precedence than the
11721 string operators described in the previous section, and higher precedence
11722 than the connectives (next section).
11730 @cindex subtraction
11731 Addition and subtraction. Both arguments are converted to integers;
11732 an error occurs if this cannot be done.
11738 @cindex multiplication
11741 Multiplication, division, remainder. Both arguments are converted to
11742 integers; an error occurs if this cannot be done.
11747 @node Relations for expr
11748 @subsection Relations for @command{expr}
11750 @cindex connectives, logical
11751 @cindex logical connectives
11752 @cindex relations, numeric or string
11754 @command{expr} supports the usual logical connectives and relations. These
11755 have lower precedence than the string and numeric operators
11756 (previous sections). Here is the list, lowest-precedence operator first.
11762 @cindex logical or operator
11763 @cindex or operator
11764 Returns its first argument if that is neither null nor zero, otherwise
11765 its second argument if it is neither null nor zero, otherwise 0. It
11766 does not evaluate its second argument if its first argument is neither
11771 @cindex logical and operator
11772 @cindex and operator
11773 Return its first argument if neither argument is null or zero, otherwise
11774 0. It does not evaluate its second argument if its first argument is
11777 @item < <= = == != >= >
11784 @cindex comparison operators
11786 Compare the arguments and return 1 if the relation is true, 0 otherwise.
11787 @code{==} is a synonym for @code{=}. @command{expr} first tries to convert
11788 both arguments to integers and do a numeric comparison; if either
11789 conversion fails, it does a lexicographic comparison using the character
11790 collating sequence specified by the @env{LC_COLLATE} locale.
11795 @node Examples of expr
11796 @subsection Examples of using @command{expr}
11798 @cindex examples of @command{expr}
11799 Here are a few examples, including quoting for shell metacharacters.
11801 To add 1 to the shell variable @code{foo}, in Bourne-compatible shells:
11804 foo=`expr $foo + 1`
11807 To print the non-directory part of the file name stored in
11808 @code{$fname}, which need not contain a @code{/}:
11811 expr $fname : '.*/\(.*\)' '|' $fname
11814 An example showing that @code{\+} is an operator:
11822 expr abc : 'a\(.\)c'
11824 expr index abcdef cz
11827 @error{} expr: syntax error
11828 expr index + index a
11834 @chapter Redirection
11836 @cindex redirection
11837 @cindex commands for redirection
11839 Unix shells commonly provide several forms of @dfn{redirection}---ways
11840 to change the input source or output destination of a command. But one
11841 useful redirection is performed by a separate command, not by the shell;
11842 it's described here.
11845 * tee invocation:: Redirect output to multiple files or processes.
11849 @node tee invocation
11850 @section @command{tee}: Redirect output to multiple files or processes
11853 @cindex pipe fitting
11854 @cindex destinations, multiple output
11855 @cindex read from stdin and write to stdout and files
11857 The @command{tee} command copies standard input to standard output and also
11858 to any files given as arguments. This is useful when you want not only
11859 to send some data down a pipe, but also to save a copy. Synopsis:
11862 tee [@var{option}]@dots{} [@var{file}]@dots{}
11865 If a file being written to does not already exist, it is created. If a
11866 file being written to already exists, the data it previously contained
11867 is overwritten unless the @option{-a} option is used.
11869 A @var{file} of @samp{-} causes @command{tee} to send another copy of
11870 input to standard output, but this is typically not that useful as the
11871 copies are interleaved.
11873 The program accepts the following options. Also see @ref{Common options}.
11880 Append standard input to the given files rather than overwriting
11884 @itemx --ignore-interrupts
11886 @opindex --ignore-interrupts
11887 Ignore interrupt signals.
11891 The @command{tee} command is useful when you happen to be transferring a large
11892 amount of data and also want to summarize that data without reading
11893 it a second time. For example, when you are downloading a DVD image,
11894 you often want to verify its signature or checksum right away.
11895 The inefficient way to do it is simply:
11898 wget http://example.com/some.iso && sha1sum some.iso
11901 One problem with the above is that it makes you wait for the
11902 download to complete before starting the time-consuming SHA1 computation.
11903 Perhaps even more importantly, the above requires reading
11904 the DVD image a second time (the first was from the network).
11906 The efficient way to do it is to interleave the download
11907 and SHA1 computation. Then, you'll get the checksum for
11908 free, because the entire process parallelizes so well:
11911 # slightly contrived, to demonstrate process substitution
11912 wget -O - http://example.com/dvd.iso \
11913 | tee >(sha1sum > dvd.sha1) > dvd.iso
11916 That makes @command{tee} write not just to the expected output file,
11917 but also to a pipe running @command{sha1sum} and saving the final
11918 checksum in a file named @file{dvd.sha1}.
11920 Note, however, that this example relies on a feature of modern shells
11921 called @dfn{process substitution}
11922 (the @samp{>(command)} syntax, above;
11923 @xref{Process Substitution,,Process Substitution, bashref,
11924 The Bash Reference Manual}.),
11925 so it works with @command{zsh}, @command{bash}, and @command{ksh},
11926 but not with @command{/bin/sh}. So if you write code like this
11927 in a shell script, be sure to start the script with @samp{#!/bin/bash}.
11929 Since the above example writes to one file and one process,
11930 a more conventional and portable use of @command{tee} is even better:
11933 wget -O - http://example.com/dvd.iso \
11934 | tee dvd.iso | sha1sum > dvd.sha1
11937 You can extend this example to make @command{tee} write to two processes,
11938 computing MD5 and SHA1 checksums in parallel. In this case,
11939 process substitution is required:
11942 wget -O - http://example.com/dvd.iso \
11943 | tee >(sha1sum > dvd.sha1) \
11944 >(md5sum > dvd.md5) \
11948 This technique is also useful when you want to make a @emph{compressed}
11949 copy of the contents of a pipe.
11950 Consider a tool to graphically summarize disk usage data from @samp{du -ak}.
11951 For a large hierarchy, @samp{du -ak} can run for a long time,
11952 and can easily produce terabytes of data, so you won't want to
11953 rerun the command unnecessarily. Nor will you want to save
11954 the uncompressed output.
11956 Doing it the inefficient way, you can't even start the GUI
11957 until after you've compressed all of the @command{du} output:
11960 du -ak | gzip -9 > /tmp/du.gz
11961 gzip -d /tmp/du.gz | xdiskusage -a
11964 With @command{tee} and process substitution, you start the GUI
11965 right away and eliminate the decompression completely:
11968 du -ak | tee >(gzip -9 > /tmp/du.gz) | xdiskusage -a
11971 Finally, if you regularly create more than one type of
11972 compressed tarball at once, for example when @code{make dist} creates
11973 both @command{gzip}-compressed and @command{bzip2}-compressed tarballs,
11974 there may be a better way.
11975 Typical @command{automake}-generated @file{Makefile} rules create
11976 the two compressed tar archives with commands in sequence, like this
11977 (slightly simplified):
11980 tardir=your-pkg-M.N
11981 tar chof - "$tardir" | gzip -9 -c > your-pkg-M.N.tar.gz
11982 tar chof - "$tardir" | bzip2 -9 -c > your-pkg-M.N.tar.bz2
11985 However, if the hierarchy you are archiving and compressing is larger
11986 than a couple megabytes, and especially if you are using a multi-processor
11987 system with plenty of memory, then you can do much better by reading the
11988 directory contents only once and running the compression programs in parallel:
11991 tardir=your-pkg-M.N
11992 tar chof - "$tardir" \
11993 | tee >(gzip -9 -c > your-pkg-M.N.tar.gz) \
11994 | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12000 @node File name manipulation
12001 @chapter File name manipulation
12003 @cindex file name manipulation
12004 @cindex manipulation of file names
12005 @cindex commands for file name manipulation
12007 This section describes commands that manipulate file names.
12010 * basename invocation:: Strip directory and suffix from a file name.
12011 * dirname invocation:: Strip last file name component.
12012 * pathchk invocation:: Check file name validity and portability.
12013 * mktemp invocation:: Create temporary file or directory.
12017 @node basename invocation
12018 @section @command{basename}: Strip directory and suffix from a file name
12021 @cindex strip directory and suffix from file names
12022 @cindex directory, stripping from file names
12023 @cindex suffix, stripping from file names
12024 @cindex file names, stripping directory and suffix
12025 @cindex leading directory components, stripping
12027 @command{basename} removes any leading directory components from
12028 @var{name}. Synopsis:
12031 basename @var{name} [@var{suffix}]
12034 If @var{suffix} is specified and is identical to the end of @var{name},
12035 it is removed from @var{name} as well. Note that since trailing slashes
12036 are removed prior to suffix matching, @var{suffix} will do nothing if it
12037 contains slashes. @command{basename} prints the result on standard
12040 @c This test is used both here and in the section on dirname.
12041 @macro basenameAndDirname
12042 Together, @command{basename} and @command{dirname} are designed such
12043 that if @samp{ls "$name"} succeeds, then the command sequence @samp{cd
12044 "$(dirname "$name")"; ls "$(basename "$name")"} will, too. This works
12045 for everything except file names containing a trailing newline.
12047 @basenameAndDirname
12049 @acronym{POSIX} allows the implementation to define the results if
12050 @var{name} is empty or @samp{//}. In the former case, @acronym{GNU}
12051 @command{basename} returns the empty string. In the latter case, the
12052 result is @samp{//} on platforms where @var{//} is distinct from
12053 @var{/}, and @samp{/} on platforms where there is no difference.
12055 The only options are @option{--help} and @option{--version}. @xref{Common
12056 options}. Options must precede operands.
12064 basename /usr/bin/sort
12067 basename include/stdio.h .h
12071 @node dirname invocation
12072 @section @command{dirname}: Strip last file name component
12075 @cindex directory components, printing
12076 @cindex stripping non-directory suffix
12077 @cindex non-directory suffix, stripping
12079 @command{dirname} prints all but the final slash-delimited component of
12080 @var{name}. Slashes on either side of the final component are also
12081 removed. If the string contains no slash, @command{dirname} prints
12082 @samp{.} (meaning the current directory). Synopsis:
12088 @var{name} need not be a file name, but if it is, this operation
12089 effectively lists the directory that contains the final component,
12090 including the case when the final component is itself a directory.
12092 @basenameAndDirname
12094 @acronym{POSIX} allows the implementation to define the results if
12095 @var{name} is @samp{//}. With @acronym{GNU} @command{dirname}, the
12096 result is @samp{//} on platforms where @var{//} is distinct from
12097 @var{/}, and @samp{/} on platforms where there is no difference.
12099 The only options are @option{--help} and @option{--version}. @xref{Common
12107 # Output "/usr/bin".
12108 dirname /usr/bin/sort
12109 dirname /usr/bin//.//
12116 @node pathchk invocation
12117 @section @command{pathchk}: Check file name validity and portability
12120 @cindex file names, checking validity and portability
12121 @cindex valid file names, checking for
12122 @cindex portable file names, checking for
12124 @command{pathchk} checks validity and portability of file names. Synopsis:
12127 pathchk [@var{option}]@dots{} @var{name}@dots{}
12130 For each @var{name}, @command{pathchk} prints an error message if any of
12131 these conditions is true:
12135 One of the existing directories in @var{name} does not have search
12136 (execute) permission,
12138 The length of @var{name} is larger than the maximum supported by the
12141 The length of one component of @var{name} is longer than
12142 its file system's maximum.
12145 A nonexistent @var{name} is not an error, so long a file with that
12146 name could be created under the above conditions.
12148 The program accepts the following options. Also see @ref{Common options}.
12149 Options must precede operands.
12155 Instead of performing checks based on the underlying file system,
12156 print an error message if any of these conditions is true:
12160 A file name is empty.
12163 A file name contains a character outside the @acronym{POSIX} portable file
12164 name character set, namely, the ASCII letters and digits, @samp{.},
12165 @samp{_}, @samp{-}, and @samp{/}.
12168 The length of a file name or one of its components exceeds the
12169 @acronym{POSIX} minimum limits for portability.
12174 Print an error message if a file name is empty, or if it contains a component
12175 that begins with @samp{-}.
12177 @item --portability
12178 @opindex --portability
12179 Print an error message if a file name is not portable to all @acronym{POSIX}
12180 hosts. This option is equivalent to @samp{-p -P}.
12184 @cindex exit status of @command{pathchk}
12188 0 if all specified file names passed all checks,
12192 @node mktemp invocation
12193 @section @command{mktemp}: Create temporary file or directory
12196 @cindex file names, creating temporary
12197 @cindex directory, creating temporary
12198 @cindex temporary files and directories
12200 @command{mktemp} manages the creation of temporary files and
12201 directories. Synopsis:
12204 mktemp [@var{option}]@dots{} [@var{template}]
12207 Safely create a temporary file or directory based on @var{template},
12208 and print its name. If given, @var{template} must include at least
12209 three consecutive @samp{X}s in the last component. If omitted, the template
12210 @samp{tmp.XXXXXXXXXX} is used, and option @option{--tmpdir} is
12211 implied. The final run of @samp{X}s in the @var{template} will be replaced
12212 by alpha-numeric characters; thus, on a case-sensitive file system,
12213 and with a @var{template} including a run of @var{n} instances of @samp{X},
12214 there are @samp{62**@var{n}} potential file names.
12216 Older scripts used to create temporary files by simply joining the
12217 name of the program with the process id (@samp{$$}) as a suffix.
12218 However, that naming scheme is easily predictable, and suffers from a
12219 race condition where the attacker can create an appropriately named
12220 symbolic link, such that when the script then opens a handle to what
12221 it thought was an unused file, it is instead modifying an existing
12222 file. Using the same scheme to create a directory is slightly safer,
12223 since the @command{mkdir} will fail if the target already exists, but
12224 it is still inferior because it allows for denial of service attacks.
12225 Therefore, modern scripts should use the @command{mktemp} command to
12226 guarantee that the generated name will be unpredictable, and that
12227 knowledge of the temporary file name implies that the file was created
12228 by the current script and cannot be modified by other users.
12230 When creating a file, the resulting file has read and write
12231 permissions for the current user, but no permissions for the group or
12232 others; these permissions are reduced if the current umask is more
12235 Here are some examples (although note that if you repeat them, you
12236 will most likely get different file names):
12241 Create a temporary file in the current directory.
12248 Create a temporary file with a known suffix.
12250 $ mktemp --suffix=.txt file-XXXX
12252 $ mktemp file-XXXX-XXXX.txt
12257 Create a secure fifo relative to the user's choice of @env{TMPDIR},
12258 but falling back to the current directory rather than @file{/tmp}.
12259 Note that @command{mktemp} does not create fifos, but can create a
12260 secure directory in which the fifo can live. Exit the shell if the
12261 directory or fifo could not be created.
12263 $ dir=$(mktemp -p "$@{TMPDIR:-.@}" -d dir-XXXX) || exit 1
12265 $ mkfifo "$fifo" || @{ rmdir "$dir"; exit 1; @}
12269 Create and use a temporary file if possible, but ignore failure. The
12270 file will reside in the directory named by @env{TMPDIR}, if specified,
12271 or else in @file{/tmp}.
12273 $ file=$(mktemp -q) && @{
12274 > # Safe to use $file only within this block. Use quotes,
12275 > # since $TMPDIR, and thus $file, may contain whitespace.
12276 > echo ... > "$file"
12282 Act as a semi-random character generator (it is not fully random,
12283 since it is impacted by the contents of the current directory). To
12284 avoid security holes, do not use the resulting names to create a file.
12294 The program accepts the following options. Also see @ref{Common options}.
12301 @opindex --directory
12302 Create a directory rather than a file. The directory will have read,
12303 write, and search permissions for the current user, but no permissions
12304 for the group or others; these permissions are reduced if the current
12305 umask is more restrictive.
12311 Suppress diagnostics about failure to create a file or directory. The
12312 exit status will still reflect whether a file was created.
12318 Generate a temporary name that does not name an existing file, without
12319 changing the file system contents. Using the output of this command
12320 to create a new file is inherently unsafe, as there is a window of
12321 time between generating the name and using it where another process
12322 can create an object by the same name.
12325 @itemx --tmpdir[=@var{dir}]
12328 Treat @var{template} relative to the directory @var{dir}. If
12329 @var{dir} is not specified (only possible with the long option
12330 @option{--tmpdir}) or is the empty string, use the value of
12331 @env{TMPDIR} if available, otherwise use @samp{/tmp}. If this is
12332 specified, @var{template} must not be absolute. However,
12333 @var{template} can still contain slashes, although intermediate
12334 directories must already exist.
12336 @item --suffix=@var{suffix}
12338 Append @var{suffix} to the @var{template}. @var{suffix} must not
12339 contain slash. If @option{--suffix} is specified, @var{template} must
12340 end in @samp{X}; if it is not specified, then an appropriate
12341 @option{--suffix} is inferred by finding the last @samp{X} in
12342 @var{template}. This option exists for use with the default
12343 @var{template} and for the creation of a @var{suffix} that starts with
12348 Treat @var{template} as a single file relative to the value of
12349 @env{TMPDIR} if available, or to the directory specified by
12350 @option{-p}, otherwise to @samp{/tmp}. @var{template} must not
12351 contain slashes. This option is deprecated; the use of @option{-p}
12352 without @option{-t} offers better defaults (by favoring the command
12353 line over @env{TMPDIR}) and more flexibility (by allowing intermediate
12358 @cindex exit status of @command{mktemp}
12362 0 if the file was created,
12367 @node Working context
12368 @chapter Working context
12370 @cindex working context
12371 @cindex commands for printing the working context
12373 This section describes commands that display or alter the context in
12374 which you are working: the current directory, the terminal settings, and
12375 so forth. See also the user-related commands in the next section.
12378 * pwd invocation:: Print working directory.
12379 * stty invocation:: Print or change terminal characteristics.
12380 * printenv invocation:: Print environment variables.
12381 * tty invocation:: Print file name of terminal on standard input.
12385 @node pwd invocation
12386 @section @command{pwd}: Print working directory
12389 @cindex print name of current directory
12390 @cindex current working directory, printing
12391 @cindex working directory, printing
12394 @command{pwd} prints the name of the current directory. Synopsis:
12397 pwd [@var{option}]@dots{}
12400 The program accepts the following options. Also see @ref{Common options}.
12407 If the contents of the environment variable @env{PWD} provide an
12408 absolute name of the current directory with no @samp{.} or @samp{..}
12409 components, but possibly with symbolic links, then output those
12410 contents. Otherwise, fall back to default @option{-P} handling.
12415 @opindex --physical
12416 Print a fully resolved name for the current directory. That is, all
12417 components of the printed name will be actual directory names---none
12418 will be symbolic links.
12421 @cindex symbolic links and @command{pwd}
12422 If @option{-L} and @option{-P} are both given, the last one takes
12423 precedence. If neither option is given, then this implementation uses
12424 @option{-P} as the default unless the @env{POSIXLY_CORRECT}
12425 environment variable is set.
12427 @mayConflictWithShellBuiltIn{pwd}
12432 @node stty invocation
12433 @section @command{stty}: Print or change terminal characteristics
12436 @cindex change or print terminal settings
12437 @cindex terminal settings
12438 @cindex line settings of terminal
12440 @command{stty} prints or changes terminal characteristics, such as baud rate.
12444 stty [@var{option}] [@var{setting}]@dots{}
12445 stty [@var{option}]
12448 If given no line settings, @command{stty} prints the baud rate, line
12449 discipline number (on systems that support it), and line settings
12450 that have been changed from the values set by @samp{stty sane}.
12451 By default, mode reading and setting are performed on the tty line
12452 connected to standard input, although this can be modified by the
12453 @option{--file} option.
12455 @command{stty} accepts many non-option arguments that change aspects of
12456 the terminal line operation, as described below.
12458 The program accepts the following options. Also see @ref{Common options}.
12465 Print all current settings in human-readable form. This option may not
12466 be used in combination with any line settings.
12468 @item -F @var{device}
12469 @itemx --file=@var{device}
12472 Set the line opened by the file name specified in @var{device} instead of
12473 the tty line connected to standard input. This option is necessary
12474 because opening a @acronym{POSIX} tty requires use of the @code{O_NONDELAY} flag to
12475 prevent a @acronym{POSIX} tty from blocking until the carrier detect line is high if
12476 the @code{clocal} flag is not set. Hence, it is not always possible
12477 to allow the shell to open the device in the traditional manner.
12483 @cindex machine-readable @command{stty} output
12484 Print all current settings in a form that can be used as an argument to
12485 another @command{stty} command to restore the current settings. This option
12486 may not be used in combination with any line settings.
12490 Many settings can be turned off by preceding them with a @samp{-}.
12491 Such arguments are marked below with ``May be negated'' in their
12492 description. The descriptions themselves refer to the positive
12493 case, that is, when @emph{not} negated (unless stated otherwise,
12496 Some settings are not available on all @acronym{POSIX} systems, since they use
12497 extensions. Such arguments are marked below with ``Non-@acronym{POSIX}'' in their
12498 description. On non-@acronym{POSIX} systems, those or other settings also may not
12499 be available, but it's not feasible to document all the variations: just
12505 * Control:: Control settings
12506 * Input:: Input settings
12507 * Output:: Output settings
12508 * Local:: Local settings
12509 * Combination:: Combination settings
12510 * Characters:: Special characters
12511 * Special:: Special settings
12516 @subsection Control settings
12518 @cindex control settings
12524 @cindex two-way parity
12525 Generate parity bit in output and expect parity bit in input.
12531 @cindex even parity
12532 Set odd parity (even if negated). May be negated.
12539 @cindex character size
12540 @cindex eight-bit characters
12541 Set character size to 5, 6, 7, or 8 bits.
12546 Send a hangup signal when the last process closes the tty. May be
12552 Use two stop bits per character (one if negated). May be negated.
12556 Allow input to be received. May be negated.
12560 @cindex modem control
12561 Disable modem control signals. May be negated.
12565 @cindex hardware flow control
12566 @cindex flow control, hardware
12567 @cindex RTS/CTS flow control
12568 Enable RTS/CTS flow control. Non-@acronym{POSIX}. May be negated.
12573 @subsection Input settings
12575 @cindex input settings
12576 These settings control operations on data received from the terminal.
12581 @cindex breaks, ignoring
12582 Ignore break characters. May be negated.
12586 @cindex breaks, cause interrupts
12587 Make breaks cause an interrupt signal. May be negated.
12591 @cindex parity, ignoring
12592 Ignore characters with parity errors. May be negated.
12596 @cindex parity errors, marking
12597 Mark parity errors (with a 255-0-character sequence). May be negated.
12601 Enable input parity checking. May be negated.
12605 @cindex eight-bit input
12606 Clear high (8th) bit of input characters. May be negated.
12610 @cindex newline, translating to return
12611 Translate newline to carriage return. May be negated.
12615 @cindex return, ignoring
12616 Ignore carriage return. May be negated.
12620 @cindex return, translating to newline
12621 Translate carriage return to newline. May be negated.
12625 @cindex input encoding, UTF-8
12626 Assume input characters are UTF-8 encoded. May be negated.
12630 @kindex C-s/C-q flow control
12631 @cindex XON/XOFF flow control
12632 Enable XON/XOFF flow control (that is, @kbd{CTRL-S}/@kbd{CTRL-Q}). May
12639 @cindex software flow control
12640 @cindex flow control, software
12641 Enable sending of @code{stop} character when the system input buffer
12642 is almost full, and @code{start} character when it becomes almost
12643 empty again. May be negated.
12647 @cindex uppercase, translating to lowercase
12648 Translate uppercase characters to lowercase. Non-@acronym{POSIX}. May be
12649 negated. Note ilcuc is not implemented, as one would not be able to issue
12650 almost any (lowercase) Unix command, after invoking it.
12654 Allow any character to restart output (only the start character
12655 if negated). Non-@acronym{POSIX}. May be negated.
12659 @cindex beeping at input buffer full
12660 Enable beeping and not flushing input buffer if a character arrives
12661 when the input buffer is full. Non-@acronym{POSIX}. May be negated.
12666 @subsection Output settings
12668 @cindex output settings
12669 These settings control operations on data sent to the terminal.
12674 Postprocess output. May be negated.
12678 @cindex lowercase, translating to output
12679 Translate lowercase characters to uppercase. Non-@acronym{POSIX}. May be
12680 negated. (Note ouclc is not currently implemented.)
12684 @cindex return, translating to newline
12685 Translate carriage return to newline. Non-@acronym{POSIX}. May be negated.
12689 @cindex newline, translating to crlf
12690 Translate newline to carriage return-newline. Non-@acronym{POSIX}. May be
12695 Do not print carriage returns in the first column. Non-@acronym{POSIX}.
12700 Newline performs a carriage return. Non-@acronym{POSIX}. May be negated.
12704 @cindex pad instead of timing for delaying
12705 Use fill (padding) characters instead of timing for delays. Non-@acronym{POSIX}.
12710 @cindex pad character
12711 Use @acronym{ASCII} @sc{del} characters for fill instead of
12712 @acronym{ASCII} @sc{nul} characters. Non-@acronym{POSIX}.
12718 Newline delay style. Non-@acronym{POSIX}.
12725 Carriage return delay style. Non-@acronym{POSIX}.
12731 @opindex tab@var{n}
12732 Horizontal tab delay style. Non-@acronym{POSIX}.
12737 Backspace delay style. Non-@acronym{POSIX}.
12742 Vertical tab delay style. Non-@acronym{POSIX}.
12747 Form feed delay style. Non-@acronym{POSIX}.
12752 @subsection Local settings
12754 @cindex local settings
12759 Enable @code{interrupt}, @code{quit}, and @code{suspend} special
12760 characters. May be negated.
12764 Enable @code{erase}, @code{kill}, @code{werase}, and @code{rprnt}
12765 special characters. May be negated.
12769 Enable non-@acronym{POSIX} special characters. May be negated.
12773 Echo input characters. May be negated.
12779 Echo @code{erase} characters as backspace-space-backspace. May be
12784 @cindex newline echoing after @code{kill}
12785 Echo a newline after a @code{kill} character. May be negated.
12789 @cindex newline, echoing
12790 Echo newline even if not echoing other characters. May be negated.
12794 @cindex flushing, disabling
12795 Disable flushing after @code{interrupt} and @code{quit} special
12796 characters. May be negated.
12800 @cindex case translation
12801 Enable input and output of uppercase characters by preceding their
12802 lowercase equivalents with @samp{\}, when @code{icanon} is set.
12803 Non-@acronym{POSIX}. May be negated.
12807 @cindex background jobs, stopping at terminal write
12808 Stop background jobs that try to write to the terminal. Non-@acronym{POSIX}.
12815 Echo erased characters backward, between @samp{\} and @samp{/}.
12816 Non-@acronym{POSIX}. May be negated.
12822 @cindex control characters, using @samp{^@var{c}}
12823 @cindex hat notation for control characters
12824 Echo control characters in hat notation (@samp{^@var{c}}) instead
12825 of literally. Non-@acronym{POSIX}. May be negated.
12831 Echo the @code{kill} special character by erasing each character on
12832 the line as indicated by the @code{echoprt} and @code{echoe} settings,
12833 instead of by the @code{echoctl} and @code{echok} settings. Non-@acronym{POSIX}.
12839 @subsection Combination settings
12841 @cindex combination settings
12842 Combination settings:
12849 Same as @code{parenb -parodd cs7}. May be negated. If negated, same
12850 as @code{-parenb cs8}.
12854 Same as @code{parenb parodd cs7}. May be negated. If negated, same
12855 as @code{-parenb cs8}.
12859 Same as @code{-icrnl -onlcr}. May be negated. If negated, same as
12860 @code{icrnl -inlcr -igncr onlcr -ocrnl -onlret}.
12864 Reset the @code{erase} and @code{kill} special characters to their default
12871 @c This is too long to write inline.
12873 cread -ignbrk brkint -inlcr -igncr icrnl -ixoff
12874 -iuclc -ixany imaxbel opost -olcuc -ocrnl onlcr
12875 -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0
12876 ff0 isig icanon iexten echo echoe echok -echonl
12877 -noflsh -xcase -tostop -echoprt echoctl echoke
12881 and also sets all special characters to their default values.
12885 Same as @code{brkint ignpar istrip icrnl ixon opost isig icanon}, plus
12886 sets the @code{eof} and @code{eol} characters to their default values
12887 if they are the same as the @code{min} and @code{time} characters.
12888 May be negated. If negated, same as @code{raw}.
12895 -ignbrk -brkint -ignpar -parmrk -inpck -istrip
12896 -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany
12897 -imaxbel -opost -isig -icanon -xcase min 1 time 0
12901 May be negated. If negated, same as @code{cooked}.
12905 Same as @option{-icanon}. May be negated. If negated, same as
12910 @cindex eight-bit characters
12911 Same as @code{-parenb -istrip cs8}. May be negated. If negated,
12912 same as @code{parenb istrip cs7}.
12916 Same as @option{-parenb -istrip -opost cs8}. May be negated.
12917 If negated, same as @code{parenb istrip opost cs7}.
12921 Same as @option{-ixany}. Non-@acronym{POSIX}. May be negated.
12925 Same as @code{tab0}. Non-@acronym{POSIX}. May be negated. If negated, same
12932 Same as @code{xcase iuclc olcuc}. Non-@acronym{POSIX}. May be negated.
12933 (Used for terminals with uppercase characters only.)
12937 Same as @code{echoe echoctl echoke}.
12941 Same as @code{echoe echoctl echoke -ixany intr ^C erase ^? kill C-u}.
12946 @subsection Special characters
12948 @cindex special characters
12949 @cindex characters, special
12951 The special characters' default values vary from system to system.
12952 They are set with the syntax @samp{name value}, where the names are
12953 listed below and the value can be given either literally, in hat
12954 notation (@samp{^@var{c}}), or as an integer which may start with
12955 @samp{0x} to indicate hexadecimal, @samp{0} to indicate octal, or
12956 any other digit to indicate decimal.
12958 @cindex disabling special characters
12959 @kindex u@r{, and disabling special characters}
12960 For GNU stty, giving a value of @code{^-} or @code{undef} disables that
12961 special character. (This is incompatible with Ultrix @command{stty},
12962 which uses a value of @samp{u} to disable a special character. GNU
12963 @command{stty} treats a value @samp{u} like any other, namely to set that
12964 special character to @key{U}.)
12970 Send an interrupt signal.
12974 Send a quit signal.
12978 Erase the last character typed.
12982 Erase the current line.
12986 Send an end of file (terminate the input).
12994 Alternate character to end the line. Non-@acronym{POSIX}.
12998 Switch to a different shell layer. Non-@acronym{POSIX}.
13002 Restart the output after stopping it.
13010 Send a terminal stop signal.
13014 Send a terminal stop signal after flushing the input. Non-@acronym{POSIX}.
13018 Redraw the current line. Non-@acronym{POSIX}.
13022 Erase the last word typed. Non-@acronym{POSIX}.
13026 Enter the next character typed literally, even if it is a special
13027 character. Non-@acronym{POSIX}.
13032 @subsection Special settings
13034 @cindex special settings
13039 Set the minimum number of characters that will satisfy a read until
13040 the time value has expired, when @option{-icanon} is set.
13044 Set the number of tenths of a second before reads time out if the minimum
13045 number of characters have not been read, when @option{-icanon} is set.
13047 @item ispeed @var{n}
13049 Set the input speed to @var{n}.
13051 @item ospeed @var{n}
13053 Set the output speed to @var{n}.
13057 Tell the tty kernel driver that the terminal has @var{n} rows. Non-@acronym{POSIX}.
13060 @itemx columns @var{n}
13063 Tell the kernel that the terminal has @var{n} columns. Non-@acronym{POSIX}.
13069 Print the number of rows and columns that the kernel thinks the
13070 terminal has. (Systems that don't support rows and columns in the kernel
13071 typically use the environment variables @env{LINES} and @env{COLUMNS}
13072 instead; however, GNU @command{stty} does not know anything about them.)
13073 Non-@acronym{POSIX}.
13077 Use line discipline @var{n}. Non-@acronym{POSIX}.
13081 Print the terminal speed.
13084 @cindex baud rate, setting
13085 Set the input and output speeds to @var{n}. @var{n} can be one of: 0
13086 50 75 110 134 134.5 150 200 300 600 1200 1800 2400 4800 9600 19200
13087 38400 @code{exta} @code{extb}. @code{exta} is the same as 19200;
13088 @code{extb} is the same as 38400. Many systems, including GNU/Linux,
13089 support higher speeds. The @command{stty} command includes support
13106 4000000 where the system supports these.
13107 0 hangs up the line if @option{-clocal} is set.
13111 @node printenv invocation
13112 @section @command{printenv}: Print all or some environment variables
13115 @cindex printing all or some environment variables
13116 @cindex environment variables, printing
13118 @command{printenv} prints environment variable values. Synopsis:
13121 printenv [@var{option}] [@var{variable}]@dots{}
13124 If no @var{variable}s are specified, @command{printenv} prints the value of
13125 every environment variable. Otherwise, it prints the value of each
13126 @var{variable} that is set, and nothing for those that are not set.
13128 The program accepts the following option. Also see @ref{Common options}.
13136 @cindex exit status of @command{printenv}
13140 0 if all variables specified were found
13141 1 if at least one specified variable was not found
13142 2 if a write error occurred
13146 @node tty invocation
13147 @section @command{tty}: Print file name of terminal on standard input
13150 @cindex print terminal file name
13151 @cindex terminal file name, printing
13153 @command{tty} prints the file name of the terminal connected to its standard
13154 input. It prints @samp{not a tty} if standard input is not a terminal.
13158 tty [@var{option}]@dots{}
13161 The program accepts the following option. Also see @ref{Common options}.
13171 Print nothing; only return an exit status.
13175 @cindex exit status of @command{tty}
13179 0 if standard input is a terminal
13180 1 if standard input is not a terminal
13181 2 if given incorrect arguments
13182 3 if a write error occurs
13186 @node User information
13187 @chapter User information
13189 @cindex user information, commands for
13190 @cindex commands for printing user information
13192 This section describes commands that print user-related information:
13193 logins, groups, and so forth.
13196 * id invocation:: Print user identity.
13197 * logname invocation:: Print current login name.
13198 * whoami invocation:: Print effective user ID.
13199 * groups invocation:: Print group names a user is in.
13200 * users invocation:: Print login names of users currently logged in.
13201 * who invocation:: Print who is currently logged in.
13205 @node id invocation
13206 @section @command{id}: Print user identity
13209 @cindex real user and group IDs, printing
13210 @cindex effective user and group IDs, printing
13211 @cindex printing real and effective user and group IDs
13213 @command{id} prints information about the given user, or the process
13214 running it if no user is specified. Synopsis:
13217 id [@var{option}]@dots{} [@var{username}]
13220 @vindex POSIXLY_CORRECT
13221 By default, it prints the real user ID, real group ID, effective user ID
13222 if different from the real user ID, effective group ID if different from
13223 the real group ID, and supplemental group IDs.
13224 In addition, if SELinux
13225 is enabled and the @env{POSIXLY_CORRECT} environment variable is not set,
13226 then print @samp{context=@var{c}}, where @var{c} is the security context.
13228 Each of these numeric values is preceded by an identifying string and
13229 followed by the corresponding user or group name in parentheses.
13231 The options cause @command{id} to print only part of the above information.
13232 Also see @ref{Common options}.
13239 Print only the group ID.
13245 Print only the group ID and the supplementary groups.
13251 Print the user or group name instead of the ID number. Requires
13252 @option{-u}, @option{-g}, or @option{-G}.
13258 Print the real, instead of effective, user or group ID. Requires
13259 @option{-u}, @option{-g}, or @option{-G}.
13265 Print only the user ID.
13272 @cindex security context
13273 Print only the security context of the current user.
13274 If SELinux is disabled then print a warning and
13275 set the exit status to 1.
13281 @macro primaryAndSupplementaryGroups{cmd,arg}
13282 Primary and supplementary groups for a process are normally inherited
13283 from its parent and are usually unchanged since login. This means
13284 that if you change the group database after logging in, @command{\cmd\}
13285 will not reflect your changes within your existing login session.
13286 Running @command{\cmd\} with a \arg\ causes the user and group
13287 database to be consulted afresh, and so will give a different result.
13289 @primaryAndSupplementaryGroups{id,user argument}
13291 @node logname invocation
13292 @section @command{logname}: Print current login name
13295 @cindex printing user's login name
13296 @cindex login name, printing
13297 @cindex user name, printing
13300 @command{logname} prints the calling user's name, as found in a
13301 system-maintained file (often @file{/var/run/utmp} or
13302 @file{/etc/utmp}), and exits with a status of 0. If there is no entry
13303 for the calling process, @command{logname} prints
13304 an error message and exits with a status of 1.
13306 The only options are @option{--help} and @option{--version}. @xref{Common
13312 @node whoami invocation
13313 @section @command{whoami}: Print effective user ID
13316 @cindex effective user ID, printing
13317 @cindex printing the effective user ID
13319 @command{whoami} prints the user name associated with the current
13320 effective user ID. It is equivalent to the command @samp{id -un}.
13322 The only options are @option{--help} and @option{--version}. @xref{Common
13328 @node groups invocation
13329 @section @command{groups}: Print group names a user is in
13332 @cindex printing groups a user is in
13333 @cindex supplementary groups, printing
13335 @command{groups} prints the names of the primary and any supplementary
13336 groups for each given @var{username}, or the current process if no names
13337 are given. If more than one name is given, the name of each user is
13339 the list of that user's groups and the user name is separated from the
13340 group list by a colon. Synopsis:
13343 groups [@var{username}]@dots{}
13346 The group lists are equivalent to the output of the command @samp{id -Gn}.
13348 @primaryAndSupplementaryGroups{groups,list of users}
13350 The only options are @option{--help} and @option{--version}. @xref{Common
13356 @node users invocation
13357 @section @command{users}: Print login names of users currently logged in
13360 @cindex printing current usernames
13361 @cindex usernames, printing current
13363 @cindex login sessions, printing users with
13364 @command{users} prints on a single line a blank-separated list of user
13365 names of users currently logged in to the current host. Each user name
13366 corresponds to a login session, so if a user has more than one login
13367 session, that user's name will appear the same number of times in the
13376 With no @var{file} argument, @command{users} extracts its information from
13377 a system-maintained file (often @file{/var/run/utmp} or
13378 @file{/etc/utmp}). If a file argument is given, @command{users} uses
13379 that file instead. A common choice is @file{/var/log/wtmp}.
13381 The only options are @option{--help} and @option{--version}. @xref{Common
13387 @node who invocation
13388 @section @command{who}: Print who is currently logged in
13391 @cindex printing current user information
13392 @cindex information, about current users
13394 @command{who} prints information about users who are currently logged on.
13398 @command{who} [@var{option}] [@var{file}] [am i]
13401 @cindex terminal lines, currently used
13403 @cindex remote hostname
13404 If given no non-option arguments, @command{who} prints the following
13405 information for each user currently logged on: login name, terminal
13406 line, login time, and remote hostname or X display.
13410 If given one non-option argument, @command{who} uses that instead of
13411 a default system-maintained file (often @file{/var/run/utmp} or
13412 @file{/etc/utmp}) as the name of the file containing the record of
13413 users logged on. @file{/var/log/wtmp} is commonly given as an argument
13414 to @command{who} to look at who has previously logged on.
13418 If given two non-option arguments, @command{who} prints only the entry
13419 for the user running it (determined from its standard input), preceded
13420 by the hostname. Traditionally, the two arguments given are @samp{am
13421 i}, as in @samp{who am i}.
13424 Time stamps are listed according to the time zone rules specified by
13425 the @env{TZ} environment variable, or by the system default rules if
13426 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
13427 with @env{TZ}, libc, The GNU C Library Reference Manual}.
13429 The program accepts the following options. Also see @ref{Common options}.
13437 Same as @samp{-b -d --login -p -r -t -T -u}.
13443 Print the date and time of last system boot.
13449 Print information corresponding to dead processes.
13455 Print a line of column headings.
13461 List only the entries that correspond to processes via which the
13462 system is waiting for a user to login. The user name is always @samp{LOGIN}.
13466 Attempt to canonicalize hostnames found in utmp through a DNS lookup. This
13467 is not the default because it can cause significant delays on systems with
13468 automatic dial-up internet access.
13472 Same as @samp{who am i}.
13478 List active processes spawned by init.
13484 Print only the login names and the number of users logged on.
13485 Overrides all other options.
13490 @opindex --runlevel
13491 Print the current (and maybe previous) run-level of the init process.
13495 Ignored; for compatibility with other versions of @command{who}.
13501 Print last system clock change.
13506 After the login time, print the number of hours and minutes that the
13507 user has been idle. @samp{.} means the user was active in the last minute.
13508 @samp{old} means the user has been idle for more than 24 hours.
13519 @opindex --writable
13520 @cindex message status
13521 @pindex write@r{, allowed}
13522 After each login name print a character indicating the user's message status:
13525 @samp{+} allowing @code{write} messages
13526 @samp{-} disallowing @code{write} messages
13527 @samp{?} cannot find terminal device
13535 @node System context
13536 @chapter System context
13538 @cindex system context
13539 @cindex context, system
13540 @cindex commands for system context
13542 This section describes commands that print or change system-wide
13546 * date invocation:: Print or set system date and time.
13547 * arch invocation:: Print machine hardware name.
13548 * nproc invocation:: Print the number of processors.
13549 * uname invocation:: Print system information.
13550 * hostname invocation:: Print or set system name.
13551 * hostid invocation:: Print numeric host identifier.
13552 * uptime invocation:: Print system uptime and load.
13555 @node date invocation
13556 @section @command{date}: Print or set system date and time
13559 @cindex time, printing or setting
13560 @cindex printing the current time
13565 date [@var{option}]@dots{} [+@var{format}]
13566 date [-u|--utc|--universal] @c this avoids a newline in the output
13567 [ MMDDhhmm[[CC]YY][.ss] ]
13571 Invoking @command{date} with no @var{format} argument is equivalent to invoking
13572 it with a default format that depends on the @env{LC_TIME} locale category.
13573 In the default C locale, this format is @samp{'+%a %b %e %H:%M:%S %Z %Y'},
13574 so the output looks like @samp{Thu Mar @ 3 13:47:51 PST 2005}.
13577 Normally, @command{date} uses the time zone rules indicated by the
13578 @env{TZ} environment variable, or the system default rules if @env{TZ}
13579 is not set. @xref{TZ Variable,, Specifying the Time Zone with
13580 @env{TZ}, libc, The GNU C Library Reference Manual}.
13582 @findex strftime @r{and @command{date}}
13583 @cindex time formats
13584 @cindex formatting times
13585 If given an argument that starts with a @samp{+}, @command{date} prints the
13586 current date and time (or the date and time specified by the
13587 @option{--date} option, see below) in the format defined by that argument,
13588 which is similar to that of the @code{strftime} function. Except for
13589 conversion specifiers, which start with @samp{%}, characters in the
13590 format string are printed unchanged. The conversion specifiers are
13596 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
13597 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
13598 * Literal conversion specifiers:: %[%nt]
13599 * Padding and other flags:: Pad with zeros, spaces, etc.
13600 * Setting the time:: Changing the system clock.
13601 * Options for date:: Instead of the current time.
13603 * Date input formats:: Specifying date strings.
13605 * Examples of date:: Examples.
13608 @node Time conversion specifiers
13609 @subsection Time conversion specifiers
13611 @cindex time conversion specifiers
13612 @cindex conversion specifiers, time
13614 @command{date} conversion specifiers related to times.
13618 hour (@samp{00}@dots{}@samp{23})
13620 hour (@samp{01}@dots{}@samp{12})
13622 hour (@samp{ 0}@dots{}@samp{23}).
13623 This is a @acronym{GNU} extension.
13625 hour (@samp{ 1}@dots{}@samp{12}).
13626 This is a @acronym{GNU} extension.
13628 minute (@samp{00}@dots{}@samp{59})
13630 nanoseconds (@samp{000000000}@dots{}@samp{999999999}).
13631 This is a @acronym{GNU} extension.
13633 locale's equivalent of either @samp{AM} or @samp{PM};
13634 blank in many locales.
13635 Noon is treated as @samp{PM} and midnight as @samp{AM}.
13637 like @samp{%p}, except lower case.
13638 This is a @acronym{GNU} extension.
13640 locale's 12-hour clock time (e.g., @samp{11:11:04 PM})
13642 24-hour hour and minute. Same as @samp{%H:%M}.
13643 This is a @acronym{GNU} extension.
13645 @cindex epoch, seconds since
13646 @cindex seconds since the epoch
13647 @cindex beginning of time
13648 seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC.
13649 Leap seconds are not counted unless leap second support is available.
13650 @xref{%s-examples}, for examples.
13651 This is a @acronym{GNU} extension.
13653 second (@samp{00}@dots{}@samp{60}).
13654 This may be @samp{60} if leap seconds are supported.
13656 24-hour hour, minute, and second. Same as @samp{%H:%M:%S}.
13658 locale's time representation (e.g., @samp{23:13:48})
13660 @w{@acronym{RFC} 2822/@acronym{ISO} 8601} style numeric time zone
13661 (e.g., @samp{-0600} or @samp{+0530}), or nothing if no
13662 time zone is determinable. This value reflects the numeric time zone
13663 appropriate for the current time, using the time zone rules specified
13664 by the @env{TZ} environment variable.
13665 The time (and optionally, the time zone rules) can be overridden
13666 by the @option{--date} option.
13667 This is a @acronym{GNU} extension.
13669 @w{@acronym{RFC} 3339/@acronym{ISO} 8601} style numeric time zone with
13670 @samp{:} (e.g., @samp{-06:00} or @samp{+05:30}), or nothing if no time
13671 zone is determinable.
13672 This is a @acronym{GNU} extension.
13674 Numeric time zone to the nearest second with @samp{:} (e.g.,
13675 @samp{-06:00:00} or @samp{+05:30:00}), or nothing if no time zone is
13677 This is a @acronym{GNU} extension.
13679 Numeric time zone with @samp{:} using the minimum necessary precision
13680 (e.g., @samp{-06}, @samp{+05:30}, or @samp{-04:56:02}), or nothing if
13681 no time zone is determinable.
13682 This is a @acronym{GNU} extension.
13684 alphabetic time zone abbreviation (e.g., @samp{EDT}), or nothing if no
13685 time zone is determinable. See @samp{%z} for how it is determined.
13689 @node Date conversion specifiers
13690 @subsection Date conversion specifiers
13692 @cindex date conversion specifiers
13693 @cindex conversion specifiers, date
13695 @command{date} conversion specifiers related to dates.
13699 locale's abbreviated weekday name (e.g., @samp{Sun})
13701 locale's full weekday name, variable length (e.g., @samp{Sunday})
13703 locale's abbreviated month name (e.g., @samp{Jan})
13705 locale's full month name, variable length (e.g., @samp{January})
13707 locale's date and time (e.g., @samp{Thu Mar @ 3 23:05:25 2005})
13709 century. This is like @samp{%Y}, except the last two digits are omitted.
13710 For example, it is @samp{20} if @samp{%Y} is @samp{2000},
13711 and is @samp{-0} if @samp{%Y} is @samp{-001}.
13712 It is normally at least two characters, but it may be more.
13714 day of month (e.g., @samp{01})
13716 date; same as @samp{%m/%d/%y}
13718 day of month, space padded; same as @samp{%_d}
13720 full date in @acronym{ISO} 8601 format; same as @samp{%Y-%m-%d}.
13721 This is a good choice for a date format, as it is standard and
13722 is easy to sort in the usual case where years are in the range
13724 This is a @acronym{GNU} extension.
13726 year corresponding to the @acronym{ISO} week number, but without the century
13727 (range @samp{00} through @samp{99}). This has the same format and value
13728 as @samp{%y}, except that if the @acronym{ISO} week number (see
13730 to the previous or next year, that year is used instead.
13731 This is a @acronym{GNU} extension.
13733 year corresponding to the @acronym{ISO} week number. This has the
13734 same format and value as @samp{%Y}, except that if the @acronym{ISO}
13736 @samp{%V}) belongs to the previous or next year, that year is used
13738 It is normally useful only if @samp{%V} is also used;
13739 for example, the format @samp{%G-%m-%d} is probably a mistake,
13740 since it combines the ISO week number year with the conventional month and day.
13741 This is a @acronym{GNU} extension.
13745 day of year (@samp{001}@dots{}@samp{366})
13747 month (@samp{01}@dots{}@samp{12})
13749 day of week (@samp{1}@dots{}@samp{7}) with @samp{1} corresponding to Monday
13751 week number of year, with Sunday as the first day of the week
13752 (@samp{00}@dots{}@samp{53}).
13753 Days in a new year preceding the first Sunday are in week zero.
13755 @acronym{ISO} week number, that is, the
13756 week number of year, with Monday as the first day of the week
13757 (@samp{01}@dots{}@samp{53}).
13758 If the week containing January 1 has four or more days in
13759 the new year, then it is considered week 1; otherwise, it is week 53 of
13760 the previous year, and the next week is week 1. (See the @acronym{ISO} 8601
13763 day of week (@samp{0}@dots{}@samp{6}) with 0 corresponding to Sunday
13765 week number of year, with Monday as first day of week
13766 (@samp{00}@dots{}@samp{53}).
13767 Days in a new year preceding the first Monday are in week zero.
13769 locale's date representation (e.g., @samp{12/31/99})
13771 last two digits of year (@samp{00}@dots{}@samp{99})
13773 year. This is normally at least four characters, but it may be more.
13774 Year @samp{0000} precedes year @samp{0001}, and year @samp{-001}
13775 precedes year @samp{0000}.
13779 @node Literal conversion specifiers
13780 @subsection Literal conversion specifiers
13782 @cindex literal conversion specifiers
13783 @cindex conversion specifiers, literal
13785 @command{date} conversion specifiers that produce literal strings.
13797 @node Padding and other flags
13798 @subsection Padding and other flags
13800 @cindex numeric field padding
13801 @cindex padding of numeric fields
13802 @cindex fields, padding numeric
13804 Unless otherwise specified, @command{date} normally pads numeric fields
13805 with zeros, so that, for
13806 example, numeric months are always output as two digits.
13807 Seconds since the epoch are not padded, though,
13808 since there is no natural width for them.
13810 As a @acronym{GNU} extension, @command{date} recognizes any of the
13811 following optional flags after the @samp{%}:
13815 (hyphen) Do not pad the field; useful if the output is intended for
13818 (underscore) Pad with spaces; useful if you need a fixed
13819 number of characters in the output, but zeros are too distracting.
13821 (zero) Pad with zeros even if the conversion specifier
13822 would normally pad with spaces.
13824 Use upper case characters if possible.
13826 Use opposite case characters if possible.
13827 A field that is normally upper case becomes lower case, and vice versa.
13831 Here are some examples of padding:
13834 date +%d/%m -d "Feb 1"
13836 date +%-d/%-m -d "Feb 1"
13838 date +%_d/%_m -d "Feb 1"
13842 As a @acronym{GNU} extension, you can specify the field width
13843 (after any flag, if present) as a decimal number. If the natural size of the
13844 output of the field has less than the specified number of characters,
13845 the result is written right adjusted and padded to the given
13846 size. For example, @samp{%9B} prints the right adjusted month name in
13847 a field of width 9.
13849 An optional modifier can follow the optional flag and width
13850 specification. The modifiers are:
13854 Use the locale's alternate representation for date and time. This
13855 modifier applies to the @samp{%c}, @samp{%C}, @samp{%x}, @samp{%X},
13856 @samp{%y} and @samp{%Y} conversion specifiers. In a Japanese locale, for
13857 example, @samp{%Ex} might yield a date format based on the Japanese
13861 Use the locale's alternate numeric symbols for numbers. This modifier
13862 applies only to numeric conversion specifiers.
13865 If the format supports the modifier but no alternate representation
13866 is available, it is ignored.
13869 @node Setting the time
13870 @subsection Setting the time
13872 @cindex setting the time
13873 @cindex time setting
13874 @cindex appropriate privileges
13876 If given an argument that does not start with @samp{+}, @command{date} sets
13877 the system clock to the date and time specified by that argument (as
13878 described below). You must have appropriate privileges to set the
13879 system clock. The @option{--date} and @option{--set} options may not be
13880 used with such an argument. The @option{--universal} option may be used
13881 with such an argument to indicate that the specified date and time are
13882 relative to Coordinated Universal Time rather than to the local time
13885 The argument must consist entirely of digits, which have the following
13898 first two digits of year (optional)
13900 last two digits of year (optional)
13905 The @option{--set} option also sets the system clock; see the next section.
13908 @node Options for date
13909 @subsection Options for @command{date}
13911 @cindex @command{date} options
13912 @cindex options for @command{date}
13914 The program accepts the following options. Also see @ref{Common options}.
13918 @item -d @var{datestr}
13919 @itemx --date=@var{datestr}
13922 @cindex parsing date strings
13923 @cindex date strings, parsing
13924 @cindex arbitrary date strings, parsing
13927 @opindex next @var{day}
13928 @opindex last @var{day}
13929 Display the date and time specified in @var{datestr} instead of the
13930 current date and time. @var{datestr} can be in almost any common
13931 format. It can contain month names, time zones, @samp{am} and @samp{pm},
13932 @samp{yesterday}, etc. For example, @option{--date="2004-02-27
13933 14:19:13.489392193 +0530"} specifies the instant of time that is
13934 489,392,193 nanoseconds after February 27, 2004 at 2:19:13 PM in a
13935 time zone that is 5 hours and 30 minutes east of @acronym{UTC}.@*
13936 Note: input currently must be in locale independent format. E.g., the
13937 LC_TIME=C below is needed to print back the correct date in many locales:
13939 date -d "$(LC_TIME=C date)"
13941 @xref{Date input formats}.
13943 @item -f @var{datefile}
13944 @itemx --file=@var{datefile}
13947 Parse each line in @var{datefile} as with @option{-d} and display the
13948 resulting date and time. If @var{datefile} is @samp{-}, use standard
13949 input. This is useful when you have many dates to process, because the
13950 system overhead of starting up the @command{date} executable many times can
13953 @item -r @var{file}
13954 @itemx --reference=@var{file}
13956 @opindex --reference
13957 Display the date and time of the last modification of @var{file},
13958 instead of the current date and time.
13965 @opindex --rfc-2822
13966 Display the date and time using the format @samp{%a, %d %b %Y %H:%M:%S
13967 %z}, evaluated in the C locale so abbreviations are always in English.
13971 Fri, 09 Sep 2005 13:51:39 -0700
13974 This format conforms to
13975 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt, Internet
13976 @acronym{RFCs} 2822} and
13977 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc822.txt, 822}, the
13978 current and previous standards for Internet email.
13980 @item --rfc-3339=@var{timespec}
13981 @opindex --rfc-3339=@var{timespec}
13982 Display the date using a format specified by
13983 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt, Internet
13984 @acronym{RFC} 3339}. This is a subset of the @acronym{ISO} 8601
13985 format, except that it also permits applications to use a space rather
13986 than a @samp{T} to separate dates from times. Unlike the other
13987 standard formats, @acronym{RFC} 3339 format is always suitable as
13988 input for the @option{--date} (@option{-d}) and @option{--file}
13989 (@option{-f}) options, regardless of the current locale.
13991 The argument @var{timespec} specifies how much of the time to include.
13992 It can be one of the following:
13996 Print just the full-date, e.g., @samp{2005-09-14}.
13997 This is equivalent to the format @samp{%Y-%m-%d}.
14000 Print the full-date and full-time separated by a space, e.g.,
14001 @samp{2005-09-14 00:56:06+05:30}. The output ends with a numeric
14002 time-offset; here the @samp{+05:30} means that local time is five
14003 hours and thirty minutes east of @acronym{UTC}. This is equivalent to
14004 the format @samp{%Y-%m-%d %H:%M:%S%:z}.
14007 Like @samp{seconds}, but also print nanoseconds, e.g.,
14008 @samp{2005-09-14 00:56:06.998458565+05:30}.
14009 This is equivalent to the format @samp{%Y-%m-%d %H:%M:%S.%N%:z}.
14013 @item -s @var{datestr}
14014 @itemx --set=@var{datestr}
14017 Set the date and time to @var{datestr}. See @option{-d} above.
14024 @opindex --universal
14025 @cindex Coordinated Universal Time
14027 @cindex Greenwich Mean Time
14030 Use Coordinated Universal Time (@acronym{UTC}) by operating as if the
14031 @env{TZ} environment variable were set to the string @samp{UTC0}.
14033 Universal Time is often called ``Greenwich Mean Time'' (@sc{gmt}) for
14034 historical reasons.
14038 @node Examples of date
14039 @subsection Examples of @command{date}
14041 @cindex examples of @command{date}
14043 Here are a few examples. Also see the documentation for the @option{-d}
14044 option in the previous section.
14049 To print the date of the day before yesterday:
14052 date --date='2 days ago'
14056 To print the date of the day three months and one day hence:
14059 date --date='3 months 1 day'
14063 To print the day of year of Christmas in the current year:
14066 date --date='25 Dec' +%j
14070 To print the current full month name and the day of the month:
14076 But this may not be what you want because for the first nine days of
14077 the month, the @samp{%d} expands to a zero-padded two-digit field,
14078 for example @samp{date -d 1may '+%B %d'} will print @samp{May 01}.
14081 To print a date without the leading zero for one-digit days
14082 of the month, you can use the (@acronym{GNU} extension)
14083 @samp{-} flag to suppress
14084 the padding altogether:
14087 date -d 1may '+%B %-d
14091 To print the current date and time in the format required by many
14092 non-@acronym{GNU} versions of @command{date} when setting the system clock:
14095 date +%m%d%H%M%Y.%S
14099 To set the system clock forward by two minutes:
14102 date --set='+2 minutes'
14106 To print the date in @acronym{RFC} 2822 format,
14107 use @samp{date --rfc-2822}. Here is some example output:
14110 Fri, 09 Sep 2005 13:51:39 -0700
14113 @anchor{%s-examples}
14115 To convert a date string to the number of seconds since the epoch
14116 (which is 1970-01-01 00:00:00 UTC), use the @option{--date} option with
14117 the @samp{%s} format. That can be useful in sorting and/or graphing
14118 and/or comparing data by date. The following command outputs the
14119 number of the seconds since the epoch for the time two minutes after the
14123 date --date='1970-01-01 00:02:00 +0000' +%s
14127 If you do not specify time zone information in the date string,
14128 @command{date} uses your computer's idea of the time zone when
14129 interpreting the string. For example, if your computer's time zone is
14130 that of Cambridge, Massachusetts, which was then 5 hours (i.e., 18,000
14131 seconds) behind UTC:
14134 # local time zone used
14135 date --date='1970-01-01 00:02:00' +%s
14140 If you're sorting or graphing dated data, your raw date values may be
14141 represented as seconds since the epoch. But few people can look at
14142 the date @samp{946684800} and casually note ``Oh, that's the first second
14143 of the year 2000 in Greenwich, England.''
14146 date --date='2000-01-01 UTC' +%s
14150 An alternative is to use the @option{--utc} (@option{-u}) option.
14151 Then you may omit @samp{UTC} from the date string. Although this
14152 produces the same result for @samp{%s} and many other format sequences,
14153 with a time zone offset different from zero, it would give a different
14154 result for zone-dependent formats like @samp{%z}.
14157 date -u --date=2000-01-01 +%s
14161 To convert such an unwieldy number of seconds back to
14162 a more readable form, use a command like this:
14165 # local time zone used
14166 date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
14167 1999-12-31 19:00:00 -0500
14170 Or if you do not mind depending on the @samp{@@} feature present since
14171 coreutils 5.3.0, you could shorten this to:
14174 date -d @@946684800 +"%F %T %z"
14175 1999-12-31 19:00:00 -0500
14178 Often it is better to output UTC-relative date and time:
14181 date -u -d '1970-01-01 946684800 seconds' +"%Y-%m-%d %T %z"
14182 2000-01-01 00:00:00 +0000
14188 @node arch invocation
14189 @section @command{arch}: Print machine hardware name
14192 @cindex print machine hardware name
14193 @cindex system information, printing
14195 @command{arch} prints the machine hardware name,
14196 and is equivalent to @samp{uname -m}.
14200 arch [@var{option}]
14203 The program accepts the @ref{Common options} only.
14208 @node nproc invocation
14209 @section @command{nproc}: Print the number of available processors
14212 @cindex Print the number of processors
14213 @cindex system information, printing
14215 Print the number of processing units available to the current process,
14216 which may be less than the number of online processors.
14217 If this information is not accessible, then print the number of
14218 processors installed. If the @env{OMP_NUM_THREADS} environment variable is
14219 set, then it will determine the returned value. The result is guaranteed to be
14220 greater than zero. Synopsis:
14223 nproc [@var{option}]
14226 The program accepts the following options. Also see @ref{Common options}.
14232 Print the number of installed processors on the system, which may
14233 be greater than the number online or available to the current process.
14234 The @env{OMP_NUM_THREADS} environment variable is not honored in this case.
14236 @item --ignore=@var{number}
14238 If possible, exclude this @var{number} of processing units.
14245 @node uname invocation
14246 @section @command{uname}: Print system information
14249 @cindex print system information
14250 @cindex system information, printing
14252 @command{uname} prints information about the machine and operating system
14253 it is run on. If no options are given, @command{uname} acts as if the
14254 @option{-s} option were given. Synopsis:
14257 uname [@var{option}]@dots{}
14260 If multiple options or @option{-a} are given, the selected information is
14261 printed in this order:
14264 @var{kernel-name} @var{nodename} @var{kernel-release} @var{kernel-version}
14265 @var{machine} @var{processor} @var{hardware-platform} @var{operating-system}
14268 The information may contain internal spaces, so such output cannot be
14269 parsed reliably. In the following example, @var{release} is
14270 @samp{2.2.18ss.e820-bda652a #4 SMP Tue Jun 5 11:24:08 PDT 2001}:
14274 @result{} Linux dum 2.2.18 #4 SMP Tue Jun 5 11:24:08 PDT 2001 i686 unknown unknown GNU/Linux
14278 The program accepts the following options. Also see @ref{Common options}.
14286 Print all of the below information, except omit the processor type
14287 and the hardware platform name if they are unknown.
14290 @itemx --hardware-platform
14292 @opindex --hardware-platform
14293 @cindex implementation, hardware
14294 @cindex hardware platform
14295 @cindex platform, hardware
14296 Print the hardware platform name
14297 (sometimes called the hardware implementation).
14298 Print @samp{unknown} if the kernel does not make this information
14299 easily available, as is the case with Linux kernels.
14305 @cindex machine type
14306 @cindex hardware class
14307 @cindex hardware type
14308 Print the machine hardware name (sometimes called the hardware class
14314 @opindex --nodename
14317 @cindex network node name
14318 Print the network node hostname.
14323 @opindex --processor
14324 @cindex host processor type
14325 Print the processor type (sometimes called the instruction set
14326 architecture or ISA).
14327 Print @samp{unknown} if the kernel does not make this information
14328 easily available, as is the case with Linux kernels.
14331 @itemx --operating-system
14333 @opindex --operating-system
14334 @cindex operating system name
14335 Print the name of the operating system.
14338 @itemx --kernel-release
14340 @opindex --kernel-release
14341 @cindex kernel release
14342 @cindex release of kernel
14343 Print the kernel release.
14346 @itemx --kernel-name
14348 @opindex --kernel-name
14349 @cindex kernel name
14350 @cindex name of kernel
14351 Print the kernel name.
14352 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) calls this
14353 ``the implementation of the operating system'', because the
14354 @acronym{POSIX} specification itself has no notion of ``kernel''.
14355 The kernel name might be the same as the operating system name printed
14356 by the @option{-o} or @option{--operating-system} option, but it might
14357 differ. Some operating systems (e.g., FreeBSD, HP-UX) have the same
14358 name as their underlying kernels; others (e.g., GNU/Linux, Solaris)
14362 @itemx --kernel-version
14364 @opindex --kernel-version
14365 @cindex kernel version
14366 @cindex version of kernel
14367 Print the kernel version.
14374 @node hostname invocation
14375 @section @command{hostname}: Print or set system name
14378 @cindex setting the hostname
14379 @cindex printing the hostname
14380 @cindex system name, printing
14381 @cindex appropriate privileges
14383 With no arguments, @command{hostname} prints the name of the current host
14384 system. With one argument, it sets the current host name to the
14385 specified string. You must have appropriate privileges to set the host
14389 hostname [@var{name}]
14392 The only options are @option{--help} and @option{--version}. @xref{Common
14398 @node hostid invocation
14399 @section @command{hostid}: Print numeric host identifier
14402 @cindex printing the host identifier
14404 @command{hostid} prints the numeric identifier of the current host
14405 in hexadecimal. This command accepts no arguments.
14406 The only options are @option{--help} and @option{--version}.
14407 @xref{Common options}.
14409 For example, here's what it prints on one system I use:
14416 On that system, the 32-bit quantity happens to be closely
14417 related to the system's Internet address, but that isn't always
14422 @node uptime invocation
14423 @section @command{uptime}: Print system uptime and load
14426 @cindex printing the system uptime and load
14428 @command{uptime} prints the current time, the system's uptime, the
14429 number of logged-in users and the current load average.
14431 If an argument is specified, it is used as the file to be read
14432 to discover how many users are logged in. If no argument is
14433 specified, a system default is used (@command{uptime --help} indicates
14434 the default setting).
14436 The only options are @option{--help} and @option{--version}.
14437 @xref{Common options}.
14439 For example, here's what it prints right now on one system I use:
14443 14:07 up 3:35, 3 users, load average: 1.39, 1.15, 1.04
14446 The precise method of calculation of load average varies somewhat
14447 between systems. Some systems calculate it as the average number of
14448 runnable processes over the last 1, 5 and 15 minutes, but some systems
14449 also include processes in the uninterruptible sleep state (that is,
14450 those processes which are waiting for disk I/O). The Linux kernel
14451 includes uninterruptible processes.
14453 @node SELinux context
14454 @chapter SELinux context
14456 @cindex SELinux context
14457 @cindex SELinux, context
14458 @cindex commands for SELinux context
14460 This section describes commands for operations with SELinux
14464 * chcon invocation:: Change SELinux context of file
14465 * runcon invocation:: Run a command in specified SELinux context
14468 @node chcon invocation
14469 @section @command{chcon}: Change SELinux context of file
14472 @cindex changing security context
14473 @cindex change SELinux context
14475 @command{chcon} changes the SELinux security context of the selected files.
14479 chcon [@var{option}]@dots{} @var{context} @var{file}@dots{}
14480 chcon [@var{option}]@dots{} [-u @var{user}] [-r @var{role}] [-l @var{range}] [-t @var{type}] @var{file}@dots{}
14481 chcon [@var{option}]@dots{} --reference=@var{rfile} @var{file}@dots{}
14484 Change the SELinux security context of each @var{file} to @var{context}.
14485 With @option{--reference}, change the security context of each @var{file}
14486 to that of @var{rfile}.
14488 The program accepts the following options. Also see @ref{Common options}.
14493 @itemx --no-dereference
14495 @opindex --no-dereference
14496 @cindex no dereference
14497 Affect symbolic links instead of any referenced file.
14499 @item --reference=@var{rfile}
14500 @opindex --reference
14501 @cindex reference file
14502 Use @var{rfile}'s security context rather than specifying a @var{context} value.
14507 @opindex --recursive
14508 Operate on files and directories recursively.
14511 @xref{Traversing symlinks}.
14514 @xref{Traversing symlinks}.
14517 @xref{Traversing symlinks}.
14524 Output a diagnostic for every file processed.
14526 @item -u @var{user}
14527 @itemx --user=@var{user}
14530 Set user @var{user} in the target security context.
14532 @item -r @var{role}
14533 @itemx --role=@var{role}
14536 Set role @var{role} in the target security context.
14538 @item -t @var{type}
14539 @itemx --type=@var{type}
14542 Set type @var{type} in the target security context.
14544 @item -l @var{range}
14545 @itemx --range=@var{range}
14548 Set range @var{range} in the target security context.
14554 @node runcon invocation
14555 @section @command{runcon}: Run a command in specified SELinux context
14558 @cindex run with security context
14561 @command{runcon} runs file in specified SELinux security context.
14565 runcon @var{context} @var{command} [@var{args}]
14566 runcon [ -c ] [-u @var{user}] [-r @var{role}] [-t @var{type}] [-l @var{range}] @var{command} [@var{args}]
14569 Run @var{command} with completely-specified @var{context}, or with
14570 current or transitioned security context modified by one or more of @var{level},
14571 @var{role}, @var{type} and @var{user}.
14573 If none of @option{-c}, @option{-t}, @option{-u}, @option{-r}, or @option{-l}
14574 is specified, the first argument is used as the complete context.
14575 Any additional arguments after @var{command}
14576 are interpreted as arguments to the command.
14578 With neither @var{context} nor @var{command}, print the current security context.
14580 The program accepts the following options. Also see @ref{Common options}.
14588 Compute process transition context before modifying.
14590 @item -u @var{user}
14591 @itemx --user=@var{user}
14594 Set user @var{user} in the target security context.
14596 @item -r @var{role}
14597 @itemx --role=@var{role}
14600 Set role @var{role} in the target security context.
14602 @item -t @var{type}
14603 @itemx --type=@var{type}
14606 Set type @var{type} in the target security context.
14608 @item -l @var{range}
14609 @itemx --range=@var{range}
14612 Set range @var{range} in the target security context.
14616 @cindex exit status of @command{runcon}
14620 126 if @var{command} is found but cannot be invoked
14621 127 if @command{runcon} itself fails or if @var{command} cannot be found
14622 the exit status of @var{command} otherwise
14625 @node Modified command invocation
14626 @chapter Modified command invocation
14628 @cindex modified command invocation
14629 @cindex invocation of commands, modified
14630 @cindex commands for invoking other commands
14632 This section describes commands that run other commands in some context
14633 different than the current one: a modified environment, as a different
14637 * chroot invocation:: Modify the root directory.
14638 * env invocation:: Modify environment variables.
14639 * nice invocation:: Modify niceness.
14640 * nohup invocation:: Immunize to hangups.
14641 * stdbuf invocation:: Modify buffering of standard streams.
14642 * su invocation:: Modify user and group ID.
14643 * timeout invocation:: Run with time limit.
14647 @node chroot invocation
14648 @section @command{chroot}: Run a command with a different root directory
14651 @cindex running a program in a specified root directory
14652 @cindex root directory, running a program in a specified
14654 @command{chroot} runs a command with a specified root directory.
14655 On many systems, only the super-user can do this.@footnote{However,
14656 some systems (e.g., FreeBSD) can be configured to allow certain regular
14657 users to use the @code{chroot} system call, and hence to run this program.
14658 Also, on Cygwin, anyone can run the @command{chroot} command, because the
14659 underlying function is non-privileged due to lack of support in MS-Windows.}
14663 chroot @var{option} @var{newroot} [@var{command} [@var{args}]@dots{}]
14664 chroot @var{option}
14667 Ordinarily, file names are looked up starting at the root of the
14668 directory structure, i.e., @file{/}. @command{chroot} changes the root to
14669 the directory @var{newroot} (which must exist) and then runs
14670 @var{command} with optional @var{args}. If @var{command} is not
14671 specified, the default is the value of the @env{SHELL} environment
14672 variable or @command{/bin/sh} if not set, invoked with the @option{-i} option.
14673 @var{command} must not be a special built-in utility
14674 (@pxref{Special built-in utilities}).
14676 The program accepts the following options. Also see @ref{Common options}.
14677 Options must precede operands.
14681 @itemx --userspec=@var{user}[:@var{group}]
14682 @opindex --userspec
14683 By default, @var{command} is run with the same credentials
14684 as the invoking process.
14685 Use this option to run it as a different @var{user} and/or with a
14686 different primary @var{group}.
14688 @itemx --groups=@var{groups}
14690 Use this option to specify the supplementary @var{groups} to be
14691 used by the new process.
14692 The items in the list (names or numeric IDs) must be separated by commas.
14696 Here are a few tips to help avoid common problems in using chroot.
14697 To start with a simple example, make @var{command} refer to a statically
14698 linked binary. If you were to use a dynamically linked executable, then
14699 you'd have to arrange to have the shared libraries in the right place under
14700 your new root directory.
14702 For example, if you create a statically linked @command{ls} executable,
14703 and put it in @file{/tmp/empty}, you can run this command as root:
14706 $ chroot /tmp/empty /ls -Rl /
14709 Then you'll see output like this:
14714 -rwxr-xr-x 1 0 0 1041745 Aug 16 11:17 ls
14717 If you want to use a dynamically linked executable, say @command{bash},
14718 then first run @samp{ldd bash} to see what shared objects it needs.
14719 Then, in addition to copying the actual binary, also copy the listed
14720 files to the required positions under your intended new root directory.
14721 Finally, if the executable requires any other files (e.g., data, state,
14722 device files), copy them into place, too.
14724 @cindex exit status of @command{chroot}
14728 125 if @command{chroot} itself fails
14729 126 if @var{command} is found but cannot be invoked
14730 127 if @var{command} cannot be found
14731 the exit status of @var{command} otherwise
14735 @node env invocation
14736 @section @command{env}: Run a command in a modified environment
14739 @cindex environment, running a program in a modified
14740 @cindex modified environment, running a program in a
14741 @cindex running a program in a modified environment
14743 @command{env} runs a command with a modified environment. Synopses:
14746 env [@var{option}]@dots{} [@var{name}=@var{value}]@dots{} @c
14747 [@var{command} [@var{args}]@dots{}]
14751 Operands of the form @samp{@var{variable}=@var{value}} set
14752 the environment variable @var{variable} to value @var{value}.
14753 @var{value} may be empty (@samp{@var{variable}=}). Setting a variable
14754 to an empty value is different from unsetting it.
14755 These operands are evaluated left-to-right, so if two operands
14756 mention the same variable the earlier is ignored.
14758 Environment variable names can be empty, and can contain any
14759 characters other than @samp{=} and @acronym{ASCII} @sc{nul}.
14760 However, it is wise to limit yourself to names that
14761 consist solely of underscores, digits, and @acronym{ASCII} letters,
14762 and that begin with a non-digit, as applications like the shell do not
14763 work well with other names.
14766 The first operand that does not contain the character @samp{=}
14767 specifies the program to invoke; it is
14768 searched for according to the @env{PATH} environment variable. Any
14769 remaining arguments are passed as arguments to that program.
14770 The program should not be a special built-in utility
14771 (@pxref{Special built-in utilities}).
14773 Modifications to @env{PATH} take effect prior to searching for
14774 @var{command}. Use caution when reducing @env{PATH}; behavior is
14775 not portable when @env{PATH} is undefined or omits key directories
14776 such as @file{/bin}.
14778 In the rare case that a utility contains a @samp{=} in the name, the
14779 only way to disambiguate it from a variable assignment is to use an
14780 intermediate command for @var{command}, and pass the problematic
14781 program name via @var{args}. For example, if @file{./prog=} is an
14782 executable in the current @env{PATH}:
14785 env prog= true # runs 'true', with prog= in environment
14786 env ./prog= true # runs 'true', with ./prog= in environment
14787 env -- prog= true # runs 'true', with prog= in environment
14788 env sh -c '\prog= true' # runs 'prog=' with argument 'true'
14789 env sh -c 'exec "$@@"' sh prog= true # also runs 'prog='
14792 @cindex environment, printing
14794 If no command name is specified following the environment
14795 specifications, the resulting environment is printed. This is like
14796 specifying the @command{printenv} program.
14798 For some examples, suppose the environment passed to @command{env}
14799 contains @samp{LOGNAME=rms}, @samp{EDITOR=emacs}, and
14800 @samp{PATH=.:/gnubin:/hacks}:
14805 Output the current environment.
14807 $ env | LC_ALL=C sort
14810 PATH=.:/gnubin:/hacks
14814 Run @command{foo} with a reduced environment, preserving only the
14815 original @env{PATH} to avoid problems in locating @command{foo}.
14817 env - PATH="$PATH" foo
14821 Run @command{foo} with the environment containing @samp{LOGNAME=rms},
14822 @samp{EDITOR=emacs}, and @samp{PATH=.:/gnubin:/hacks}, and guarantees
14823 that @command{foo} was found in the file system rather than as a shell
14830 Run @command{nemacs} with the environment containing @samp{LOGNAME=foo},
14831 @samp{EDITOR=emacs}, @samp{PATH=.:/gnubin:/hacks}, and
14832 @samp{DISPLAY=gnu:0}.
14834 env DISPLAY=gnu:0 LOGNAME=foo nemacs
14838 Attempt to run the program @command{/energy/--} (as that is the only
14839 possible path search result); if the command exists, the environment
14840 will contain @samp{LOGNAME=rms} and @samp{PATH=/energy}, and the
14841 arguments will be @samp{e=mc2}, @samp{bar}, and @samp{baz}.
14843 env -u EDITOR PATH=/energy -- e=mc2 bar baz
14849 The program accepts the following options. Also see @ref{Common options}.
14850 Options must precede operands.
14856 @item -u @var{name}
14857 @itemx --unset=@var{name}
14860 Remove variable @var{name} from the environment, if it was in the
14865 @itemx --ignore-environment
14868 @opindex --ignore-environment
14869 Start with an empty environment, ignoring the inherited environment.
14873 @cindex exit status of @command{env}
14877 0 if no @var{command} is specified and the environment is output
14878 125 if @command{env} itself fails
14879 126 if @var{command} is found but cannot be invoked
14880 127 if @var{command} cannot be found
14881 the exit status of @var{command} otherwise
14885 @node nice invocation
14886 @section @command{nice}: Run a command with modified niceness
14890 @cindex scheduling, affecting
14891 @cindex appropriate privileges
14893 @command{nice} prints or modifies a process's @dfn{niceness},
14894 a parameter that affects whether the process is scheduled favorably.
14898 nice [@var{option}]@dots{} [@var{command} [@var{arg}]@dots{}]
14901 If no arguments are given, @command{nice} prints the current niceness.
14902 Otherwise, @command{nice} runs the given @var{command} with its
14903 niceness adjusted. By default, its niceness is incremented by 10.
14905 Niceness values range at least from @minus{}20 (process has high priority
14906 and gets more resources, thus slowing down other processes) through 19
14907 (process has lower priority and runs slowly itself, but has less impact
14908 on the speed of other running processes). Some systems
14909 may have a wider range of nicenesses; conversely, other systems may
14910 enforce more restrictive limits. An attempt to set the niceness
14911 outside the supported range is treated as an attempt to use the
14912 minimum or maximum supported value.
14914 A niceness should not be confused with a scheduling priority, which
14915 lets applications determine the order in which threads are scheduled
14916 to run. Unlike a priority, a niceness is merely advice to the
14917 scheduler, which the scheduler is free to ignore. Also, as a point of
14918 terminology, @acronym{POSIX} defines the behavior of @command{nice} in
14919 terms of a @dfn{nice value}, which is the nonnegative difference
14920 between a niceness and the minimum niceness. Though @command{nice}
14921 conforms to @acronym{POSIX}, its documentation and diagnostics use the
14922 term ``niceness'' for compatibility with historical practice.
14924 @var{command} must not be a special built-in utility (@pxref{Special
14925 built-in utilities}).
14927 @mayConflictWithShellBuiltIn{nice}
14929 The program accepts the following option. Also see @ref{Common options}.
14930 Options must precede operands.
14933 @item -n @var{adjustment}
14934 @itemx --adjustment=@var{adjustment}
14936 @opindex --adjustment
14937 Add @var{adjustment} instead of 10 to the command's niceness. If
14938 @var{adjustment} is negative and you lack appropriate privileges,
14939 @command{nice} issues a warning but otherwise acts as if you specified
14942 For compatibility @command{nice} also supports an obsolete
14943 option syntax @option{-@var{adjustment}}. New scripts should use
14944 @option{-n @var{adjustment}} instead.
14948 @cindex exit status of @command{nice}
14952 0 if no @var{command} is specified and the niceness is output
14953 125 if @command{nice} itself fails
14954 126 if @var{command} is found but cannot be invoked
14955 127 if @var{command} cannot be found
14956 the exit status of @var{command} otherwise
14959 It is sometimes useful to run a non-interactive program with reduced niceness.
14962 $ nice factor 4611686018427387903
14965 Since @command{nice} prints the current niceness,
14966 you can invoke it through itself to demonstrate how it works.
14968 The default behavior is to increase the niceness by @samp{10}:
14979 The @var{adjustment} is relative to the current niceness. In the
14980 next example, the first @command{nice} invocation runs the second one
14981 with niceness 10, and it in turn runs the final one with a niceness
14985 $ nice nice -n 3 nice
14989 Specifying a niceness larger than the supported range
14990 is the same as specifying the maximum supported value:
14993 $ nice -n 10000000000 nice
14997 Only a privileged user may run a process with lower niceness:
15001 nice: cannot set niceness: Permission denied
15003 $ sudo nice -n -1 nice
15008 @node nohup invocation
15009 @section @command{nohup}: Run a command immune to hangups
15012 @cindex hangups, immunity to
15013 @cindex immunity to hangups
15014 @cindex logging out and continuing to run
15017 @command{nohup} runs the given @var{command} with hangup signals ignored,
15018 so that the command can continue running in the background after you log
15022 nohup @var{command} [@var{arg}]@dots{}
15025 If standard input is a terminal, it is redirected from
15026 @file{/dev/null} so that terminal sessions do not mistakenly consider
15027 the terminal to be used by the command. This is a @acronym{GNU}
15028 extension; programs intended to be portable to non-@acronym{GNU} hosts
15029 should use @samp{nohup @var{command} [@var{arg}]@dots{} </dev/null}
15033 If standard output is a terminal, the command's standard output is appended
15034 to the file @file{nohup.out}; if that cannot be written to, it is appended
15035 to the file @file{$HOME/nohup.out}; and if that cannot be written to, the
15036 command is not run.
15037 Any @file{nohup.out} or @file{$HOME/nohup.out} file created by
15038 @command{nohup} is made readable and writable only to the user,
15039 regardless of the current umask settings.
15041 If standard error is a terminal, it is normally redirected to the same file
15042 descriptor as the (possibly-redirected) standard output.
15043 However, if standard output is closed, standard error terminal output
15044 is instead appended to the file @file{nohup.out} or
15045 @file{$HOME/nohup.out} as above.
15047 To capture the command's output to a file other than @file{nohup.out}
15048 you can redirect it. For example, to capture the output of
15052 nohup make > make.log
15055 @command{nohup} does not automatically put the command it runs in the
15056 background; you must do that explicitly, by ending the command line
15057 with an @samp{&}. Also, @command{nohup} does not alter the
15058 niceness of @var{command}; use @command{nice} for that,
15059 e.g., @samp{nohup nice @var{command}}.
15061 @var{command} must not be a special built-in utility (@pxref{Special
15062 built-in utilities}).
15064 The only options are @option{--help} and @option{--version}. @xref{Common
15065 options}. Options must precede operands.
15067 @cindex exit status of @command{nohup}
15071 125 if @command{nohup} itself fails, and @env{POSIXLY_CORRECT} is not set
15072 126 if @var{command} is found but cannot be invoked
15073 127 if @var{command} cannot be found
15074 the exit status of @var{command} otherwise
15077 If @env{POSIXLY_CORRECT} is set, internal failures give status 127
15081 @node stdbuf invocation
15082 @section @command{stdbuf}: Run a command with modified I/O stream buffering
15085 @cindex standard streams, buffering
15086 @cindex line buffered
15088 @command{stdbuf} allows one to modify the buffering operations of the
15089 three standard I/O streams associated with a program. Synopsis:
15092 stdbuf @var{option}@dots{} @var{command}
15095 Any additional @var{arg}s are passed as additional arguments to the
15098 The program accepts the following options. Also see @ref{Common options}.
15102 @item -i @var{mode}
15103 @itemx --input=@var{mode}
15106 Adjust the standard input stream buffering.
15108 @item -o @var{mode}
15109 @itemx --output=@var{mode}
15112 Adjust the standard output stream buffering.
15114 @item -e @var{mode}
15115 @itemx --error=@var{mode}
15118 Adjust the standard error stream buffering.
15122 The @var{mode} can be specified as follows:
15127 Set the stream to line buffered mode.
15128 In this mode data is coalesced until a newline is output or
15129 input is read from any stream attached to a terminal device.
15130 This option is invalid with standard input.
15133 Disable buffering of the selected stream.
15134 In this mode data is output immediately and only the
15135 amount of data requested is read from input.
15138 Specify the size of the buffer to use in fully buffered mode.
15139 @multiplierSuffixesNoBlocks{size}
15143 NOTE: If @var{command} adjusts the buffering of its standard streams
15144 (@command{tee} does for e.g.) then that will override corresponding settings
15145 changed by @command{stdbuf}. Also some filters (like @command{dd} and
15146 @command{cat} etc.) don't use streams for I/O, and are thus unaffected
15147 by @command{stdbuf} settings.
15149 @cindex exit status of @command{stdbuf}
15153 125 if @command{stdbuf} itself fails
15154 126 if @var{command} is found but cannot be invoked
15155 127 if @var{command} cannot be found
15156 the exit status of @var{command} otherwise
15160 @node su invocation
15161 @section @command{su}: Run a command with substitute user and group ID
15164 @cindex substitute user and group IDs
15165 @cindex user ID, switching
15166 @cindex super-user, becoming
15167 @cindex root, becoming
15169 @command{su} allows one user to temporarily become another user. It runs a
15170 command (often an interactive shell) with the real and effective user
15171 ID, group ID, and supplemental groups of a given @var{user}. Synopsis:
15174 su [@var{option}]@dots{} [@var{user} [@var{arg}]@dots{}]
15177 @cindex passwd entry, and @command{su} shell
15179 @flindex /etc/passwd
15180 If no @var{user} is given, the default is @code{root}, the super-user.
15181 The shell to use is taken from @var{user}'s @code{passwd} entry, or
15182 @file{/bin/sh} if none is specified there. If @var{user} has a
15183 password, @command{su} prompts for the password unless run by a user with
15184 effective user ID of zero (the super-user).
15190 @cindex login shell
15191 By default, @command{su} does not change the current directory.
15192 It sets the environment variables @env{HOME} and @env{SHELL}
15193 from the password entry for @var{user}, and if @var{user} is not
15194 the super-user, sets @env{USER} and @env{LOGNAME} to @var{user}.
15195 By default, the shell is not a login shell.
15197 Any additional @var{arg}s are passed as additional arguments to the
15200 @cindex @option{-su}
15201 GNU @command{su} does not treat @file{/bin/sh} or any other shells specially
15202 (e.g., by setting @code{argv[0]} to @option{-su}, passing @option{-c} only
15203 to certain shells, etc.).
15206 @command{su} can optionally be compiled to use @code{syslog} to report
15207 failed, and optionally successful, @command{su} attempts. (If the system
15208 supports @code{syslog}.) However, GNU @command{su} does not check if the
15209 user is a member of the @code{wheel} group; see below.
15211 The program accepts the following options. Also see @ref{Common options}.
15214 @item -c @var{command}
15215 @itemx --command=@var{command}
15218 Pass @var{command}, a single command line to run, to the shell with
15219 a @option{-c} option instead of starting an interactive shell.
15226 @cindex file name pattern expansion, disabled
15227 @cindex globbing, disabled
15228 Pass the @option{-f} option to the shell. This probably only makes sense
15229 if the shell run is @command{csh} or @command{tcsh}, for which the @option{-f}
15230 option prevents reading the startup file (@file{.cshrc}). With
15231 Bourne-like shells, the @option{-f} option disables file name pattern
15232 expansion (globbing), which is not likely to be useful.
15240 @c other variables already indexed above
15243 @cindex login shell, creating
15244 Make the shell a login shell. This means the following. Unset all
15245 environment variables except @env{TERM}, @env{HOME}, and @env{SHELL}
15246 (which are set as described above), and @env{USER} and @env{LOGNAME}
15247 (which are set, even for the super-user, as described above), and set
15248 @env{PATH} to a compiled-in default value. Change to @var{user}'s home
15249 directory. Prepend @samp{-} to the shell's name, intended to make it
15250 read its login startup file(s).
15254 @itemx --preserve-environment
15257 @opindex --preserve-environment
15258 @cindex environment, preserving
15259 @flindex /etc/shells
15260 @cindex restricted shell
15261 Do not change the environment variables @env{HOME}, @env{USER},
15262 @env{LOGNAME}, or @env{SHELL}. Run the shell given in the environment
15263 variable @env{SHELL} instead of the shell from @var{user}'s passwd
15264 entry, unless the user running @command{su} is not the super-user and
15265 @var{user}'s shell is restricted. A @dfn{restricted shell} is one that
15266 is not listed in the file @file{/etc/shells}, or in a compiled-in list
15267 if that file does not exist. Parts of what this option does can be
15268 overridden by @option{--login} and @option{--shell}.
15270 @item -s @var{shell}
15271 @itemx --shell=@var{shell}
15274 Run @var{shell} instead of the shell from @var{user}'s passwd entry,
15275 unless the user running @command{su} is not the super-user and @var{user}'s
15276 shell is restricted (see @option{-m} just above).
15280 @cindex exit status of @command{su}
15284 125 if @command{su} itself fails
15285 126 if subshell is found but cannot be invoked
15286 127 if subshell cannot be found
15287 the exit status of the subshell otherwise
15290 @cindex wheel group, not supported
15291 @cindex group wheel, not supported
15293 @subsection Why GNU @command{su} does not support the @samp{wheel} group
15295 (This section is by Richard Stallman.)
15299 Sometimes a few of the users try to hold total power over all the
15300 rest. For example, in 1984, a few users at the MIT AI lab decided to
15301 seize power by changing the operator password on the Twenex system and
15302 keeping it secret from everyone else. (I was able to thwart this coup
15303 and give power back to the users by patching the kernel, but I
15304 wouldn't know how to do that in Unix.)
15306 However, occasionally the rulers do tell someone. Under the usual
15307 @command{su} mechanism, once someone learns the root password who
15308 sympathizes with the ordinary users, he or she can tell the rest. The
15309 ``wheel group'' feature would make this impossible, and thus cement the
15310 power of the rulers.
15312 I'm on the side of the masses, not that of the rulers. If you are
15313 used to supporting the bosses and sysadmins in whatever they do, you
15314 might find this idea strange at first.
15317 @node timeout invocation
15318 @section @command{timeout}: Run a command with a time limit
15322 @cindex run commands with bounded time
15324 @command{timeout} runs the given @var{command} and kills it if it is
15325 still running after the specified time interval. Synopsis:
15328 timeout [@var{option}] @var{duration} @var{command} [@var{arg}]@dots{}
15331 @var{command} must not be a special built-in utility (@pxref{Special
15332 built-in utilities}).
15334 The program accepts the following options. Also see @ref{Common options}.
15335 Options must precede operands.
15338 @item -k @var{duration}
15339 @itemx --kill-after=@var{duration}
15341 @opindex --kill-after
15342 Ensure the monitored @var{command} is killed by also sending a @samp{KILL}
15343 signal, after the specified @var{duration}. Without this option, if the
15344 selected signal proves not to be fatal, @command{timeout} does not kill
15347 @item -s @var{signal}
15348 @itemx --signal=@var{signal}
15351 Send this @var{signal} to @var{command} on timeout, rather than the
15352 default @samp{TERM} signal. @var{signal} may be a name like @samp{HUP}
15353 or a number. Also see @xref{Signal specifications}.
15357 @var{duration} is an integer followed by an optional unit:
15359 @samp{s} for seconds (the default)
15360 @samp{m} for minutes
15364 A duration of 0 disables the associated timeout.
15366 @cindex exit status of @command{timeout}
15370 124 if @var{command} times out
15371 125 if @command{timeout} itself fails
15372 126 if @var{command} is found but cannot be invoked
15373 127 if @var{command} cannot be found
15374 the exit status of @var{command} otherwise
15378 @node Process control
15379 @chapter Process control
15381 @cindex processes, commands for controlling
15382 @cindex commands for controlling processes
15385 * kill invocation:: Sending a signal to processes.
15389 @node kill invocation
15390 @section @command{kill}: Send a signal to processes
15393 @cindex send a signal to processes
15395 The @command{kill} command sends a signal to processes, causing them
15396 to terminate or otherwise act upon receiving the signal in some way.
15397 Alternatively, it lists information about signals. Synopses:
15400 kill [-s @var{signal} | --signal @var{signal} | -@var{signal}] @var{pid}@dots{}
15401 kill [-l | --list | -t | --table] [@var{signal}]@dots{}
15404 @mayConflictWithShellBuiltIn{kill}
15406 The first form of the @command{kill} command sends a signal to all
15407 @var{pid} arguments. The default signal to send if none is specified
15408 is @samp{TERM}. The special signal number @samp{0} does not denote a
15409 valid signal, but can be used to test whether the @var{pid} arguments
15410 specify processes to which a signal could be sent.
15412 If @var{pid} is positive, the signal is sent to the process with the
15413 process ID @var{pid}. If @var{pid} is zero, the signal is sent to all
15414 processes in the process group of the current process. If @var{pid}
15415 is @minus{}1, the signal is sent to all processes for which the user has
15416 permission to send a signal. If @var{pid} is less than @minus{}1, the signal
15417 is sent to all processes in the process group that equals the absolute
15418 value of @var{pid}.
15420 If @var{pid} is not positive, a system-dependent set of system
15421 processes is excluded from the list of processes to which the signal
15424 If a negative @var{pid} argument is desired as the first one, it
15425 should be preceded by @option{--}. However, as a common extension to
15426 @acronym{POSIX}, @option{--} is not required with @samp{kill
15427 -@var{signal} -@var{pid}}. The following commands are equivalent:
15436 The first form of the @command{kill} command succeeds if every @var{pid}
15437 argument specifies at least one process that the signal was sent to.
15439 The second form of the @command{kill} command lists signal information.
15440 Either the @option{-l} or @option{--list} option, or the @option{-t}
15441 or @option{--table} option must be specified. Without any
15442 @var{signal} argument, all supported signals are listed. The output
15443 of @option{-l} or @option{--list} is a list of the signal names, one
15444 per line; if @var{signal} is already a name, the signal number is
15445 printed instead. The output of @option{-t} or @option{--table} is a
15446 table of signal numbers, names, and descriptions. This form of the
15447 @command{kill} command succeeds if all @var{signal} arguments are valid
15448 and if there is no output error.
15450 The @command{kill} command also supports the @option{--help} and
15451 @option{--version} options. @xref{Common options}.
15453 A @var{signal} may be a signal name like @samp{HUP}, or a signal
15454 number like @samp{1}, or an exit status of a process terminated by the
15455 signal. A signal name can be given in canonical form or prefixed by
15456 @samp{SIG}. The case of the letters is ignored, except for the
15457 @option{-@var{signal}} option which must use upper case to avoid
15458 ambiguity with lower case option letters. For a list of supported
15459 signal names and numbers see @xref{Signal specifications}.
15464 @cindex delaying commands
15465 @cindex commands for delaying
15467 @c Perhaps @command{wait} or other commands should be described here also?
15470 * sleep invocation:: Delay for a specified time.
15474 @node sleep invocation
15475 @section @command{sleep}: Delay for a specified time
15478 @cindex delay for a specified time
15480 @command{sleep} pauses for an amount of time specified by the sum of
15481 the values of the command line arguments.
15485 sleep @var{number}[smhd]@dots{}
15489 Each argument is a number followed by an optional unit; the default
15490 is seconds. The units are:
15503 Historical implementations of @command{sleep} have required that
15504 @var{number} be an integer, and only accepted a single argument
15505 without a suffix. However, GNU @command{sleep} accepts
15506 arbitrary floating point numbers (using a period before any fractional
15509 The only options are @option{--help} and @option{--version}. @xref{Common
15512 @c sleep is a shell built-in at least with Solaris 11's /bin/sh
15513 @mayConflictWithShellBuiltIn{sleep}
15518 @node Numeric operations
15519 @chapter Numeric operations
15521 @cindex numeric operations
15522 These programs do numerically-related operations.
15525 * factor invocation:: Show factors of numbers.
15526 * seq invocation:: Print sequences of numbers.
15530 @node factor invocation
15531 @section @command{factor}: Print prime factors
15534 @cindex prime factors
15536 @command{factor} prints prime factors. Synopses:
15539 factor [@var{number}]@dots{}
15540 factor @var{option}
15543 If no @var{number} is specified on the command line, @command{factor} reads
15544 numbers from standard input, delimited by newlines, tabs, or spaces.
15546 The @command{factor} command supports only a small number of options:
15550 Print a short help on standard output, then exit without further
15554 Print the program version on standard output, then exit without further
15558 Factoring the product of the eighth and ninth Mersenne primes
15559 takes about 30 milliseconds of CPU time on a 2.2 GHz Athlon.
15562 M8=`echo 2^31-1|bc` ; M9=`echo 2^61-1|bc`
15563 /usr/bin/time -f '%U' factor $(echo "$M8 * $M9" | bc)
15564 4951760154835678088235319297: 2147483647 2305843009213693951
15568 Similarly, factoring the eighth Fermat number @math{2^{256}+1} takes
15569 about 20 seconds on the same machine.
15571 Factoring large numbers is, in general, hard. The Pollard Rho
15572 algorithm used by @command{factor} is particularly effective for
15573 numbers with relatively small factors. If you wish to factor large
15574 numbers which do not have small factors (for example, numbers which
15575 are the product of two large primes), other methods are far better.
15577 If @command{factor} is built without using GNU MP, only
15578 single-precision arithmetic is available, and so large numbers
15579 (typically @math{2^{64}} and above) will not be supported. The single-precision
15580 code uses an algorithm which is designed for factoring smaller
15586 @node seq invocation
15587 @section @command{seq}: Print numeric sequences
15590 @cindex numeric sequences
15591 @cindex sequence of numbers
15593 @command{seq} prints a sequence of numbers to standard output. Synopses:
15596 seq [@var{option}]@dots{} @var{last}
15597 seq [@var{option}]@dots{} @var{first} @var{last}
15598 seq [@var{option}]@dots{} @var{first} @var{increment} @var{last}
15601 @command{seq} prints the numbers from @var{first} to @var{last} by
15602 @var{increment}. By default, each number is printed on a separate line.
15603 When @var{increment} is not specified, it defaults to @samp{1},
15604 even when @var{first} is larger than @var{last}.
15605 @var{first} also defaults to @samp{1}. So @code{seq 1} prints
15606 @samp{1}, but @code{seq 0} and @code{seq 10 5} produce no output.
15607 Floating-point numbers
15608 may be specified (using a period before any fractional digits).
15610 The program accepts the following options. Also see @ref{Common options}.
15611 Options must precede operands.
15614 @item -f @var{format}
15615 @itemx --format=@var{format}
15616 @opindex -f @var{format}
15617 @opindex --format=@var{format}
15618 @cindex formatting of numbers in @command{seq}
15619 Print all numbers using @var{format}.
15620 @var{format} must contain exactly one of the @samp{printf}-style
15621 floating point conversion specifications @samp{%a}, @samp{%e},
15622 @samp{%f}, @samp{%g}, @samp{%A}, @samp{%E}, @samp{%F}, @samp{%G}.
15623 The @samp{%} may be followed by zero or more flags taken from the set
15624 @samp{-+#0 '}, then an optional width containing one or more digits,
15625 then an optional precision consisting of a @samp{.} followed by zero
15626 or more digits. @var{format} may also contain any number of @samp{%%}
15627 conversion specifications. All conversion specifications have the
15628 same meaning as with @samp{printf}.
15630 The default format is derived from @var{first}, @var{step}, and
15631 @var{last}. If these all use a fixed point decimal representation,
15632 the default format is @samp{%.@var{p}f}, where @var{p} is the minimum
15633 precision that can represent the output numbers exactly. Otherwise,
15634 the default format is @samp{%g}.
15636 @item -s @var{string}
15637 @itemx --separator=@var{string}
15638 @cindex separator for numbers in @command{seq}
15639 Separate numbers with @var{string}; default is a newline.
15640 The output always terminates with a newline.
15643 @itemx --equal-width
15644 Print all numbers with the same width, by padding with leading zeros.
15645 @var{first}, @var{step}, and @var{last} should all use a fixed point
15646 decimal representation.
15647 (To have other kinds of padding, use @option{--format}).
15651 You can get finer-grained control over output with @option{-f}:
15654 $ seq -f '(%9.2E)' -9e5 1.1e6 1.3e6
15660 If you want hexadecimal integer output, you can use @command{printf}
15661 to perform the conversion:
15664 $ printf '%x\n' `seq 1048575 1024 1050623`
15670 For very long lists of numbers, use xargs to avoid
15671 system limitations on the length of an argument list:
15674 $ seq 1000000 | xargs printf '%x\n' | tail -n 3
15680 To generate octal output, use the printf @code{%o} format instead
15683 On most systems, seq can produce whole-number output for values up to
15684 at least @math{2^{53}}. Larger integers are approximated. The details
15685 differ depending on your floating-point implementation, but a common
15686 case is that @command{seq} works with integers through @math{2^{64}},
15687 and larger integers may not be numerically correct:
15690 $ seq 18446744073709551616 1 18446744073709551618
15691 18446744073709551616
15692 18446744073709551616
15693 18446744073709551618
15696 Be careful when using @command{seq} with outlandish values: otherwise
15697 you may see surprising results, as @command{seq} uses floating point
15698 internally. For example, on the x86 platform, where the internal
15699 representation uses a 64-bit fraction, the command:
15702 seq 1 0.0000000000000000001 1.0000000000000000009
15705 outputs 1.0000000000000000007 twice and skips 1.0000000000000000008.
15710 @node File permissions
15711 @chapter File permissions
15714 @include parse-datetime.texi
15718 @node Opening the software toolbox
15719 @chapter Opening the Software Toolbox
15721 An earlier version of this chapter appeared in
15722 @uref{http://www.linuxjournal.com/article.php?sid=2762, the
15723 @cite{What's GNU?} column of @cite{Linux Journal}, 2 (June, 1994)}.
15724 It was written by Arnold Robbins.
15727 * Toolbox introduction:: Toolbox introduction
15728 * I/O redirection:: I/O redirection
15729 * The who command:: The @command{who} command
15730 * The cut command:: The @command{cut} command
15731 * The sort command:: The @command{sort} command
15732 * The uniq command:: The @command{uniq} command
15733 * Putting the tools together:: Putting the tools together
15737 @node Toolbox introduction
15738 @unnumberedsec Toolbox Introduction
15740 This month's column is only peripherally related to the GNU Project, in
15741 that it describes a number of the GNU tools on your GNU/Linux system and how they
15742 might be used. What it's really about is the ``Software Tools'' philosophy
15743 of program development and usage.
15745 The software tools philosophy was an important and integral concept
15746 in the initial design and development of Unix (of which Linux and GNU are
15747 essentially clones). Unfortunately, in the modern day press of
15748 Internetworking and flashy GUIs, it seems to have fallen by the
15749 wayside. This is a shame, since it provides a powerful mental model
15750 for solving many kinds of problems.
15752 Many people carry a Swiss Army knife around in their pants pockets (or
15753 purse). A Swiss Army knife is a handy tool to have: it has several knife
15754 blades, a screwdriver, tweezers, toothpick, nail file, corkscrew, and perhaps
15755 a number of other things on it. For the everyday, small miscellaneous jobs
15756 where you need a simple, general purpose tool, it's just the thing.
15758 On the other hand, an experienced carpenter doesn't build a house using
15759 a Swiss Army knife. Instead, he has a toolbox chock full of specialized
15760 tools---a saw, a hammer, a screwdriver, a plane, and so on. And he knows
15761 exactly when and where to use each tool; you won't catch him hammering nails
15762 with the handle of his screwdriver.
15764 The Unix developers at Bell Labs were all professional programmers and trained
15765 computer scientists. They had found that while a one-size-fits-all program
15766 might appeal to a user because there's only one program to use, in practice
15771 difficult to write,
15774 difficult to maintain and
15778 difficult to extend to meet new situations.
15781 Instead, they felt that programs should be specialized tools. In short, each
15782 program ``should do one thing well.'' No more and no less. Such programs are
15783 simpler to design, write, and get right---they only do one thing.
15785 Furthermore, they found that with the right machinery for hooking programs
15786 together, that the whole was greater than the sum of the parts. By combining
15787 several special purpose programs, you could accomplish a specific task
15788 that none of the programs was designed for, and accomplish it much more
15789 quickly and easily than if you had to write a special purpose program.
15790 We will see some (classic) examples of this further on in the column.
15791 (An important additional point was that, if necessary, take a detour
15792 and build any software tools you may need first, if you don't already
15793 have something appropriate in the toolbox.)
15795 @node I/O redirection
15796 @unnumberedsec I/O Redirection
15798 Hopefully, you are familiar with the basics of I/O redirection in the
15799 shell, in particular the concepts of ``standard input,'' ``standard output,''
15800 and ``standard error''. Briefly, ``standard input'' is a data source, where
15801 data comes from. A program should not need to either know or care if the
15802 data source is a disk file, a keyboard, a magnetic tape, or even a punched
15803 card reader. Similarly, ``standard output'' is a data sink, where data goes
15804 to. The program should neither know nor care where this might be.
15805 Programs that only read their standard input, do something to the data,
15806 and then send it on, are called @dfn{filters}, by analogy to filters in a
15809 With the Unix shell, it's very easy to set up data pipelines:
15812 program_to_create_data | filter1 | ... | filterN > final.pretty.data
15815 We start out by creating the raw data; each filter applies some successive
15816 transformation to the data, until by the time it comes out of the pipeline,
15817 it is in the desired form.
15819 This is fine and good for standard input and standard output. Where does the
15820 standard error come in to play? Well, think about @command{filter1} in
15821 the pipeline above. What happens if it encounters an error in the data it
15822 sees? If it writes an error message to standard output, it will just
15823 disappear down the pipeline into @command{filter2}'s input, and the
15824 user will probably never see it. So programs need a place where they can send
15825 error messages so that the user will notice them. This is standard error,
15826 and it is usually connected to your console or window, even if you have
15827 redirected standard output of your program away from your screen.
15829 For filter programs to work together, the format of the data has to be
15830 agreed upon. The most straightforward and easiest format to use is simply
15831 lines of text. Unix data files are generally just streams of bytes, with
15832 lines delimited by the @acronym{ASCII} @sc{lf} (Line Feed) character,
15833 conventionally called a ``newline'' in the Unix literature. (This is
15834 @code{'\n'} if you're a C programmer.) This is the format used by all
15835 the traditional filtering programs. (Many earlier operating systems
15836 had elaborate facilities and special purpose programs for managing
15837 binary data. Unix has always shied away from such things, under the
15838 philosophy that it's easiest to simply be able to view and edit your
15839 data with a text editor.)
15841 OK, enough introduction. Let's take a look at some of the tools, and then
15842 we'll see how to hook them together in interesting ways. In the following
15843 discussion, we will only present those command line options that interest
15844 us. As you should always do, double check your system documentation
15845 for the full story.
15847 @node The who command
15848 @unnumberedsec The @command{who} Command
15850 The first program is the @command{who} command. By itself, it generates a
15851 list of the users who are currently logged in. Although I'm writing
15852 this on a single-user system, we'll pretend that several people are
15857 @print{} arnold console Jan 22 19:57
15858 @print{} miriam ttyp0 Jan 23 14:19(:0.0)
15859 @print{} bill ttyp1 Jan 21 09:32(:0.0)
15860 @print{} arnold ttyp2 Jan 23 20:48(:0.0)
15863 Here, the @samp{$} is the usual shell prompt, at which I typed @samp{who}.
15864 There are three people logged in, and I am logged in twice. On traditional
15865 Unix systems, user names are never more than eight characters long. This
15866 little bit of trivia will be useful later. The output of @command{who} is nice,
15867 but the data is not all that exciting.
15869 @node The cut command
15870 @unnumberedsec The @command{cut} Command
15872 The next program we'll look at is the @command{cut} command. This program
15873 cuts out columns or fields of input data. For example, we can tell it
15874 to print just the login name and full name from the @file{/etc/passwd}
15875 file. The @file{/etc/passwd} file has seven fields, separated by
15879 arnold:xyzzy:2076:10:Arnold D. Robbins:/home/arnold:/bin/bash
15882 To get the first and fifth fields, we would use @command{cut} like this:
15885 $ cut -d: -f1,5 /etc/passwd
15886 @print{} root:Operator
15888 @print{} arnold:Arnold D. Robbins
15889 @print{} miriam:Miriam A. Robbins
15893 With the @option{-c} option, @command{cut} will cut out specific characters
15894 (i.e., columns) in the input lines. This is useful for input data
15895 that has fixed width fields, and does not have a field separator. For
15896 example, list the Monday dates for the current month:
15898 @c Is using cal ok? Looked at gcal, but I don't like it.
15909 @node The sort command
15910 @unnumberedsec The @command{sort} Command
15912 Next we'll look at the @command{sort} command. This is one of the most
15913 powerful commands on a Unix-style system; one that you will often find
15914 yourself using when setting up fancy data plumbing.
15917 command reads and sorts each file named on the command line. It then
15918 merges the sorted data and writes it to standard output. It will read
15919 standard input if no files are given on the command line (thus
15920 making it into a filter). The sort is based on the character collating
15921 sequence or based on user-supplied ordering criteria.
15924 @node The uniq command
15925 @unnumberedsec The @command{uniq} Command
15927 Finally (at least for now), we'll look at the @command{uniq} program. When
15928 sorting data, you will often end up with duplicate lines, lines that
15929 are identical. Usually, all you need is one instance of each line.
15930 This is where @command{uniq} comes in. The @command{uniq} program reads its
15931 standard input. It prints only one
15932 copy of each repeated line. It does have several options. Later on,
15933 we'll use the @option{-c} option, which prints each unique line, preceded
15934 by a count of the number of times that line occurred in the input.
15937 @node Putting the tools together
15938 @unnumberedsec Putting the Tools Together
15940 Now, let's suppose this is a large ISP server system with dozens of users
15941 logged in. The management wants the system administrator to write a program that will
15942 generate a sorted list of logged in users. Furthermore, even if a user
15943 is logged in multiple times, his or her name should only show up in the
15946 The administrator could sit down with the system documentation and write a C
15947 program that did this. It would take perhaps a couple of hundred lines
15948 of code and about two hours to write it, test it, and debug it.
15949 However, knowing the software toolbox, the administrator can instead start out
15950 by generating just a list of logged on users:
15960 Next, sort the list:
15963 $ who | cut -c1-8 | sort
15970 Finally, run the sorted list through @command{uniq}, to weed out duplicates:
15973 $ who | cut -c1-8 | sort | uniq
15979 The @command{sort} command actually has a @option{-u} option that does what
15980 @command{uniq} does. However, @command{uniq} has other uses for which one
15981 cannot substitute @samp{sort -u}.
15983 The administrator puts this pipeline into a shell script, and makes it available for
15984 all the users on the system (@samp{#} is the system administrator,
15985 or @code{root}, prompt):
15988 # cat > /usr/local/bin/listusers
15989 who | cut -c1-8 | sort | uniq
15991 # chmod +x /usr/local/bin/listusers
15994 There are four major points to note here. First, with just four
15995 programs, on one command line, the administrator was able to save about two
15996 hours worth of work. Furthermore, the shell pipeline is just about as
15997 efficient as the C program would be, and it is much more efficient in
15998 terms of programmer time. People time is much more expensive than
15999 computer time, and in our modern ``there's never enough time to do
16000 everything'' society, saving two hours of programmer time is no mean
16003 Second, it is also important to emphasize that with the
16004 @emph{combination} of the tools, it is possible to do a special
16005 purpose job never imagined by the authors of the individual programs.
16007 Third, it is also valuable to build up your pipeline in stages, as we did here.
16008 This allows you to view the data at each stage in the pipeline, which helps
16009 you acquire the confidence that you are indeed using these tools correctly.
16011 Finally, by bundling the pipeline in a shell script, other users can use
16012 your command, without having to remember the fancy plumbing you set up for
16013 them. In terms of how you run them, shell scripts and compiled programs are
16016 After the previous warm-up exercise, we'll look at two additional, more
16017 complicated pipelines. For them, we need to introduce two more tools.
16019 The first is the @command{tr} command, which stands for ``transliterate.''
16020 The @command{tr} command works on a character-by-character basis, changing
16021 characters. Normally it is used for things like mapping upper case to
16025 $ echo ThIs ExAmPlE HaS MIXED case! | tr '[:upper:]' '[:lower:]'
16026 @print{} this example has mixed case!
16029 There are several options of interest:
16033 work on the complement of the listed characters, i.e.,
16034 operations apply to characters not in the given set
16037 delete characters in the first set from the output
16040 squeeze repeated characters in the output into just one character.
16043 We will be using all three options in a moment.
16045 The other command we'll look at is @command{comm}. The @command{comm}
16046 command takes two sorted input files as input data, and prints out the
16047 files' lines in three columns. The output columns are the data lines
16048 unique to the first file, the data lines unique to the second file, and
16049 the data lines that are common to both. The @option{-1}, @option{-2}, and
16050 @option{-3} command line options @emph{omit} the respective columns. (This is
16051 non-intuitive and takes a little getting used to.) For example:
16073 The file name @file{-} tells @command{comm} to read standard input
16074 instead of a regular file.
16076 Now we're ready to build a fancy pipeline. The first application is a word
16077 frequency counter. This helps an author determine if he or she is over-using
16080 The first step is to change the case of all the letters in our input file
16081 to one case. ``The'' and ``the'' are the same word when doing counting.
16084 $ tr '[:upper:]' '[:lower:]' < whats.gnu | ...
16087 The next step is to get rid of punctuation. Quoted words and unquoted words
16088 should be treated identically; it's easiest to just get the punctuation out of
16092 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | ...
16095 The second @command{tr} command operates on the complement of the listed
16096 characters, which are all the letters, the digits, the underscore, and
16097 the blank. The @samp{\n} represents the newline character; it has to
16098 be left alone. (The @acronym{ASCII} tab character should also be included for
16099 good measure in a production script.)
16101 At this point, we have data consisting of words separated by blank space.
16102 The words only contain alphanumeric characters (and the underscore). The
16103 next step is break the data apart so that we have one word per line. This
16104 makes the counting operation much easier, as we will see shortly.
16107 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16108 > tr -s ' ' '\n' | ...
16111 This command turns blanks into newlines. The @option{-s} option squeezes
16112 multiple newline characters in the output into just one. This helps us
16113 avoid blank lines. (The @samp{>} is the shell's ``secondary prompt.''
16114 This is what the shell prints when it notices you haven't finished
16115 typing in all of a command.)
16117 We now have data consisting of one word per line, no punctuation, all one
16118 case. We're ready to count each word:
16121 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16122 > tr -s ' ' '\n' | sort | uniq -c | ...
16125 At this point, the data might look something like this:
16138 The output is sorted by word, not by count! What we want is the most
16139 frequently used words first. Fortunately, this is easy to accomplish,
16140 with the help of two more @command{sort} options:
16144 do a numeric sort, not a textual one
16147 reverse the order of the sort
16150 The final pipeline looks like this:
16153 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16154 > tr -s ' ' '\n' | sort | uniq -c | sort -n -r
16163 Whew! That's a lot to digest. Yet, the same principles apply. With six
16164 commands, on two lines (really one long one split for convenience), we've
16165 created a program that does something interesting and useful, in much
16166 less time than we could have written a C program to do the same thing.
16168 A minor modification to the above pipeline can give us a simple spelling
16169 checker! To determine if you've spelled a word correctly, all you have to
16170 do is look it up in a dictionary. If it is not there, then chances are
16171 that your spelling is incorrect. So, we need a dictionary.
16172 The conventional location for a dictionary is @file{/usr/dict/words}.
16173 On my GNU/Linux system,@footnote{Redhat Linux 6.1, for the November 2000
16174 revision of this article.}
16175 this is a sorted, 45,402 word dictionary.
16177 Now, how to compare our file with the dictionary? As before, we generate
16178 a sorted list of words, one per line:
16181 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16182 > tr -s ' ' '\n' | sort -u | ...
16185 Now, all we need is a list of words that are @emph{not} in the
16186 dictionary. Here is where the @command{comm} command comes in.
16189 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16190 > tr -s ' ' '\n' | sort -u |
16191 > comm -23 - /usr/dict/words
16194 The @option{-2} and @option{-3} options eliminate lines that are only in the
16195 dictionary (the second file), and lines that are in both files. Lines
16196 only in the first file (standard input, our stream of words), are
16197 words that are not in the dictionary. These are likely candidates for
16198 spelling errors. This pipeline was the first cut at a production
16199 spelling checker on Unix.
16201 There are some other tools that deserve brief mention.
16205 search files for text that matches a regular expression
16208 count lines, words, characters
16211 a T-fitting for data pipes, copies data to files and to standard output
16214 the stream editor, an advanced tool
16217 a data manipulation language, another advanced tool
16220 The software tools philosophy also espoused the following bit of
16221 advice: ``Let someone else do the hard part.'' This means, take
16222 something that gives you most of what you need, and then massage it the
16223 rest of the way until it's in the form that you want.
16229 Each program should do one thing well. No more, no less.
16232 Combining programs with appropriate plumbing leads to results where
16233 the whole is greater than the sum of the parts. It also leads to novel
16234 uses of programs that the authors might never have imagined.
16237 Programs should never print extraneous header or trailer data, since these
16238 could get sent on down a pipeline. (A point we didn't mention earlier.)
16241 Let someone else do the hard part.
16244 Know your toolbox! Use each program appropriately. If you don't have an
16245 appropriate tool, build one.
16248 As of this writing, all the programs we've discussed are available via
16249 anonymous @command{ftp} from: @*
16250 @uref{ftp://gnudist.gnu.org/textutils/textutils-1.22.tar.gz}. (There may
16251 be more recent versions available now.)
16253 None of what I have presented in this column is new. The Software Tools
16254 philosophy was first introduced in the book @cite{Software Tools}, by
16255 Brian Kernighan and P.J. Plauger (Addison-Wesley, ISBN 0-201-03669-X).
16256 This book showed how to write and use software tools. It was written in
16257 1976, using a preprocessor for FORTRAN named @command{ratfor} (RATional
16258 FORtran). At the time, C was not as ubiquitous as it is now; FORTRAN
16259 was. The last chapter presented a @command{ratfor} to FORTRAN
16260 processor, written in @command{ratfor}. @command{ratfor} looks an awful
16261 lot like C; if you know C, you won't have any problem following the
16264 In 1981, the book was updated and made available as @cite{Software Tools
16265 in Pascal} (Addison-Wesley, ISBN 0-201-10342-7). Both books are
16266 still in print and are well worth
16267 reading if you're a programmer. They certainly made a major change in
16268 how I view programming.
16270 The programs in both books are available from
16271 @uref{http://cm.bell-labs.com/who/bwk, Brian Kernighan's home page}.
16272 For a number of years, there was an active
16273 Software Tools Users Group, whose members had ported the original
16274 @command{ratfor} programs to essentially every computer system with a
16275 FORTRAN compiler. The popularity of the group waned in the middle 1980s
16276 as Unix began to spread beyond universities.
16278 With the current proliferation of GNU code and other clones of Unix programs,
16279 these programs now receive little attention; modern C versions are
16280 much more efficient and do more than these programs do. Nevertheless, as
16281 exposition of good programming style, and evangelism for a still-valuable
16282 philosophy, these books are unparalleled, and I recommend them highly.
16284 Acknowledgment: I would like to express my gratitude to Brian Kernighan
16285 of Bell Labs, the original Software Toolsmith, for reviewing this column.
16287 @node GNU Free Documentation License
16288 @appendix GNU Free Documentation License
16292 @node Concept index
16299 @c Local variables:
16300 @c texinfo-column-for-description: 32