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 integer argument to readable form; thus, only @samp{d}, @samp{i},
3086 @samp{u}, @samp{o}, @samp{x}, and @samp{X} conversions are allowed. The
3087 entire @var{suffix} is given (with the current output file number) to
3088 @code{sprintf(3)} to form the file name suffixes for each of the
3089 individual output files in turn. If this option is used, the
3090 @option{--digits} option is ignored.
3092 @item -n @var{digits}
3093 @itemx --digits=@var{digits}
3096 Use output file names containing numbers that are @var{digits} digits
3097 long instead of the default 2.
3102 @opindex --keep-files
3103 Do not remove output files when errors are encountered.
3106 @itemx --elide-empty-files
3108 @opindex --elide-empty-files
3109 Suppress the generation of zero-length output files. (In cases where
3110 the section delimiters of the input file are supposed to mark the first
3111 lines of each of the sections, the first output file will generally be a
3112 zero-length file unless you use this option.) The output file sequence
3113 numbers always run consecutively starting from 0, even when this option
3124 Do not print counts of output file sizes.
3130 Here is an example of its usage.
3131 First, create an empty directory for the exercise,
3138 Now, split the sequence of 1..14 on lines that end with 0 or 5:
3141 $ seq 14 | csplit - '/[05]$/' '@{*@}'
3147 Each number printed above is the size of an output
3148 file that csplit has just created.
3149 List the names of those output files:
3156 Use @command{head} to show their contents:
3181 @node Summarizing files
3182 @chapter Summarizing files
3184 @cindex summarizing files
3186 These commands generate just a few numbers representing entire
3190 * wc invocation:: Print newline, word, and byte counts.
3191 * sum invocation:: Print checksum and block counts.
3192 * cksum invocation:: Print CRC checksum and byte counts.
3193 * md5sum invocation:: Print or check MD5 digests.
3194 * sha1sum invocation:: Print or check SHA-1 digests.
3195 * sha2 utilities:: Print or check SHA-2 digests.
3200 @section @command{wc}: Print newline, word, and byte counts
3204 @cindex character count
3208 @command{wc} counts the number of bytes, characters, whitespace-separated
3209 words, and newlines in each given @var{file}, or standard input if none
3210 are given or for a @var{file} of @samp{-}. Synopsis:
3213 wc [@var{option}]@dots{} [@var{file}]@dots{}
3216 @cindex total counts
3217 @command{wc} prints one line of counts for each file, and if the file was
3218 given as an argument, it prints the file name following the counts. If
3219 more than one @var{file} is given, @command{wc} prints a final line
3220 containing the cumulative counts, with the file name @file{total}. The
3221 counts are printed in this order: newlines, words, characters, bytes,
3222 maximum line length.
3223 Each count is printed right-justified in a field with at least one
3224 space between fields so that the numbers and file names normally line
3225 up nicely in columns. The width of the count fields varies depending
3226 on the inputs, so you should not depend on a particular field width.
3227 However, as a @acronym{GNU} extension, if only one count is printed,
3228 it is guaranteed to be printed without leading spaces.
3230 By default, @command{wc} prints three counts: the newline, words, and byte
3231 counts. Options can specify that only certain counts be printed.
3232 Options do not undo others previously given, so
3239 prints both the byte counts and the word counts.
3241 With the @option{--max-line-length} option, @command{wc} prints the length
3242 of the longest line per file, and if there is more than one file it
3243 prints the maximum (not the sum) of those lengths. The line lengths here
3244 are measured in screen columns, according to the current locale and
3245 assuming tab positions in every 8th column.
3247 The program accepts the following options. Also see @ref{Common options}.
3255 Print only the byte counts.
3261 Print only the character counts.
3267 Print only the word counts.
3273 Print only the newline counts.
3276 @itemx --max-line-length
3278 @opindex --max-line-length
3279 Print only the maximum line lengths.
3281 @macro filesZeroFromOption{cmd,withTotalOption,subListOutput}
3282 @itemx --files0-from=@var{file}
3283 @opindex --files0-from=@var{file}
3284 @c This is commented out to avoid a texi2dvi failure.
3285 @c texi2dvi (GNU Texinfo 4.11) 1.104
3286 @c @cindex including files from @command{\cmd\}
3287 Disallow processing files named on the command line, and instead process
3288 those named in file @var{file}; each name being terminated by a zero byte
3289 (@acronym{ASCII} @sc{nul}).
3290 This is useful \withTotalOption\
3291 when the list of file names is so long that it may exceed a command line
3293 In such cases, running @command{\cmd\} via @command{xargs} is undesirable
3294 because it splits the list into pieces and makes @command{\cmd\} print
3295 \subListOutput\ for each sublist rather than for the entire list.
3296 One way to produce a list of @acronym{ASCII} @sc{nul} terminated file names is with @sc{gnu}
3297 @command{find}, using its @option{-print0} predicate.
3298 If @var{file} is @samp{-} then the @acronym{ASCII} @sc{nul} terminated file names
3299 are read from standard input.
3301 @filesZeroFromOption{wc,,a total}
3303 For example, to find the length of the longest line in any @file{.c} or
3304 @file{.h} file in the current hierarchy, do this:
3307 find . -name '*.[ch]' -print0 |
3308 wc -L --files0-from=- | tail -n1
3316 @node sum invocation
3317 @section @command{sum}: Print checksum and block counts
3320 @cindex 16-bit checksum
3321 @cindex checksum, 16-bit
3323 @command{sum} computes a 16-bit checksum for each given @var{file}, or
3324 standard input if none are given or for a @var{file} of @samp{-}. Synopsis:
3327 sum [@var{option}]@dots{} [@var{file}]@dots{}
3330 @command{sum} prints the checksum for each @var{file} followed by the
3331 number of blocks in the file (rounded up). If more than one @var{file}
3332 is given, file names are also printed (by default). (With the
3333 @option{--sysv} option, corresponding file names are printed when there is
3334 at least one file argument.)
3336 By default, @sc{gnu} @command{sum} computes checksums using an algorithm
3337 compatible with BSD @command{sum} and prints file sizes in units of
3340 The program accepts the following options. Also see @ref{Common options}.
3346 @cindex BSD @command{sum}
3347 Use the default (BSD compatible) algorithm. This option is included for
3348 compatibility with the System V @command{sum}. Unless @option{-s} was also
3349 given, it has no effect.
3355 @cindex System V @command{sum}
3356 Compute checksums using an algorithm compatible with System V
3357 @command{sum}'s default, and print file sizes in units of 512-byte blocks.
3361 @command{sum} is provided for compatibility; the @command{cksum} program (see
3362 next section) is preferable in new applications.
3367 @node cksum invocation
3368 @section @command{cksum}: Print CRC checksum and byte counts
3371 @cindex cyclic redundancy check
3372 @cindex CRC checksum
3374 @command{cksum} computes a cyclic redundancy check (CRC) checksum for each
3375 given @var{file}, or standard input if none are given or for a
3376 @var{file} of @samp{-}. Synopsis:
3379 cksum [@var{option}]@dots{} [@var{file}]@dots{}
3382 @command{cksum} prints the CRC checksum for each file along with the number
3383 of bytes in the file, and the file name unless no arguments were given.
3385 @command{cksum} is typically used to ensure that files
3386 transferred by unreliable means (e.g., netnews) have not been corrupted,
3387 by comparing the @command{cksum} output for the received files with the
3388 @command{cksum} output for the original files (typically given in the
3391 The CRC algorithm is specified by the @acronym{POSIX} standard. It is not
3392 compatible with the BSD or System V @command{sum} algorithms (see the
3393 previous section); it is more robust.
3395 The only options are @option{--help} and @option{--version}. @xref{Common
3401 @node md5sum invocation
3402 @section @command{md5sum}: Print or check MD5 digests
3406 @cindex 128-bit checksum
3407 @cindex checksum, 128-bit
3408 @cindex fingerprint, 128-bit
3409 @cindex message-digest, 128-bit
3411 @command{md5sum} computes a 128-bit checksum (or @dfn{fingerprint} or
3412 @dfn{message-digest}) for each specified @var{file}.
3414 Note: The MD5 digest is more reliable than a simple CRC (provided by
3415 the @command{cksum} command) for detecting accidental file corruption,
3416 as the chances of accidentally having two files with identical MD5
3417 are vanishingly small. However, it should not be considered secure
3418 against malicious tampering: although finding a file with a given MD5
3419 fingerprint is considered infeasible at the moment, it is known how
3420 to modify certain files, including digital certificates, so that they
3421 appear valid when signed with an MD5 digest.
3422 For more secure hashes, consider using SHA-2. @xref{sha2 utilities}.
3424 If a @var{file} is specified as @samp{-} or if no files are given
3425 @command{md5sum} computes the checksum for the standard input.
3426 @command{md5sum} can also determine whether a file and checksum are
3427 consistent. Synopsis:
3430 md5sum [@var{option}]@dots{} [@var{file}]@dots{}
3433 For each @var{file}, @samp{md5sum} outputs the MD5 checksum, a flag
3434 indicating a binary or text input file, and the file name.
3435 If @var{file} contains a backslash or newline, the
3436 line is started with a backslash, and each problematic character in
3437 the file name is escaped with a backslash, making the output
3438 unambiguous even in the presence of arbitrary file names.
3439 If @var{file} is omitted or specified as @samp{-}, standard input is read.
3441 The program accepts the following options. Also see @ref{Common options}.
3449 @cindex binary input files
3450 Treat each input file as binary, by reading it in binary mode and
3451 outputting a @samp{*} flag. This is the inverse of @option{--text}.
3452 On systems like @acronym{GNU} that do not distinguish between binary
3453 and text files, this option merely flags each input file as binary:
3454 the MD5 checksum is unaffected. This option is the default on systems
3455 like MS-DOS that distinguish between binary and text files, except
3456 for reading standard input when standard input is a terminal.
3460 Read file names and checksum information (not data) from each
3461 @var{file} (or from stdin if no @var{file} was specified) and report
3462 whether the checksums match the contents of the named files.
3463 The input to this mode of @command{md5sum} is usually the output of
3464 a prior, checksum-generating run of @samp{md5sum}.
3465 Each valid line of input consists of an MD5 checksum, a binary/text
3466 flag, and then a file name.
3467 Binary files are marked with @samp{*}, text with @samp{ }.
3468 For each such line, @command{md5sum} reads the named file and computes its
3469 MD5 checksum. Then, if the computed message digest does not match the
3470 one on the line with the file name, the file is noted as having
3471 failed the test. Otherwise, the file passes the test.
3472 By default, for each valid line, one line is written to standard
3473 output indicating whether the named file passed the test.
3474 After all checks have been performed, if there were any failures,
3475 a warning is issued to standard error.
3476 Use the @option{--status} option to inhibit that output.
3477 If any listed file cannot be opened or read, if any valid line has
3478 an MD5 checksum inconsistent with the associated file, or if no valid
3479 line is found, @command{md5sum} exits with nonzero status. Otherwise,
3480 it exits successfully.
3484 @cindex verifying MD5 checksums
3485 This option is useful only when verifying checksums.
3486 When verifying checksums, don't generate an 'OK' message per successfully
3487 checked file. Files that fail the verification are reported in the
3488 default one-line-per-file format. If there is any checksum mismatch,
3489 print a warning summarizing the failures to standard error.
3493 @cindex verifying MD5 checksums
3494 This option is useful only when verifying checksums.
3495 When verifying checksums, don't generate the default one-line-per-file
3496 diagnostic and don't output the warning summarizing any failures.
3497 Failures to open or read a file still evoke individual diagnostics to
3499 If all listed files are readable and are consistent with the associated
3500 MD5 checksums, exit successfully. Otherwise exit with a status code
3501 indicating there was a failure.
3507 @cindex text input files
3508 Treat each input file as text, by reading it in text mode and
3509 outputting a @samp{ } flag. This is the inverse of @option{--binary}.
3510 This option is the default on systems like @acronym{GNU} that do not
3511 distinguish between binary and text files. On other systems, it is
3512 the default for reading standard input when standard input is a
3519 @cindex verifying MD5 checksums
3520 When verifying checksums, warn about improperly formatted MD5 checksum lines.
3521 This option is useful only if all but a few lines in the checked input
3529 @node sha1sum invocation
3530 @section @command{sha1sum}: Print or check SHA-1 digests
3534 @cindex 160-bit checksum
3535 @cindex checksum, 160-bit
3536 @cindex fingerprint, 160-bit
3537 @cindex message-digest, 160-bit
3539 @command{sha1sum} computes a 160-bit checksum for each specified
3540 @var{file}. The usage and options of this command are precisely the
3541 same as for @command{md5sum}. @xref{md5sum invocation}.
3543 Note: The SHA-1 digest is more secure than MD5, and no collisions of
3544 it are known (different files having the same fingerprint). However,
3545 it is known that they can be produced with considerable, but not
3546 unreasonable, resources. For this reason, it is generally considered
3547 that SHA-1 should be gradually phased out in favor of the more secure
3548 SHA-2 hash algorithms. @xref{sha2 utilities}.
3551 @node sha2 utilities
3552 @section sha2 utilities: Print or check SHA-2 digests
3559 @cindex 224-bit checksum
3560 @cindex 256-bit checksum
3561 @cindex 384-bit checksum
3562 @cindex 512-bit checksum
3563 @cindex checksum, 224-bit
3564 @cindex checksum, 256-bit
3565 @cindex checksum, 384-bit
3566 @cindex checksum, 512-bit
3567 @cindex fingerprint, 224-bit
3568 @cindex fingerprint, 256-bit
3569 @cindex fingerprint, 384-bit
3570 @cindex fingerprint, 512-bit
3571 @cindex message-digest, 224-bit
3572 @cindex message-digest, 256-bit
3573 @cindex message-digest, 384-bit
3574 @cindex message-digest, 512-bit
3576 The commands @command{sha224sum}, @command{sha256sum},
3577 @command{sha384sum} and @command{sha512sum} compute checksums of
3578 various lengths (respectively 224, 256, 384 and 512 bits),
3579 collectively known as the SHA-2 hashes. The usage and options of
3580 these commands are precisely the same as for @command{md5sum}.
3581 @xref{md5sum invocation}.
3583 Note: The SHA384 and SHA512 digests are considerably slower to
3584 compute, especially on 32-bit computers, than SHA224 or SHA256.
3587 @node Operating on sorted files
3588 @chapter Operating on sorted files
3590 @cindex operating on sorted files
3591 @cindex sorted files, operations on
3593 These commands work with (or produce) sorted files.
3596 * sort invocation:: Sort text files.
3597 * shuf invocation:: Shuffle text files.
3598 * uniq invocation:: Uniquify files.
3599 * comm invocation:: Compare two sorted files line by line.
3600 * ptx invocation:: Produce a permuted index of file contents.
3601 * tsort invocation:: Topological sort.
3605 @node sort invocation
3606 @section @command{sort}: Sort text files
3609 @cindex sorting files
3611 @command{sort} sorts, merges, or compares all the lines from the given
3612 files, or standard input if none are given or for a @var{file} of
3613 @samp{-}. By default, @command{sort} writes the results to standard
3617 sort [@var{option}]@dots{} [@var{file}]@dots{}
3620 @command{sort} has three modes of operation: sort (the default), merge,
3621 and check for sortedness. The following options change the operation
3628 @itemx --check=diagnose-first
3631 @cindex checking for sortedness
3632 Check whether the given file is already sorted: if it is not all
3633 sorted, print a diagnostic containing the first out-of-order line and
3634 exit with a status of 1.
3635 Otherwise, exit successfully.
3636 At most one input file can be given.
3639 @itemx --check=quiet
3640 @itemx --check=silent
3643 @cindex checking for sortedness
3644 Exit successfully if the given file is already sorted, and
3645 exit with status 1 otherwise.
3646 At most one input file can be given.
3647 This is like @option{-c}, except it does not print a diagnostic.
3653 @cindex merging sorted files
3654 Merge the given files by sorting them as a group. Each input file must
3655 always be individually sorted. It always works to sort instead of
3656 merge; merging is provided because it is faster, in the case where it
3661 @cindex sort stability
3662 @cindex sort's last-resort comparison
3663 A pair of lines is compared as follows:
3664 @command{sort} compares each pair of fields, in the
3665 order specified on the command line, according to the associated
3666 ordering options, until a difference is found or no fields are left.
3667 If no key fields are specified, @command{sort} uses a default key of
3668 the entire line. Finally, as a last resort when all keys compare
3669 equal, @command{sort} compares entire lines as if no ordering options
3670 other than @option{--reverse} (@option{-r}) were specified. The
3671 @option{--stable} (@option{-s}) option disables this @dfn{last-resort
3672 comparison} so that lines in which all fields compare equal are left
3673 in their original relative order. The @option{--unique}
3674 (@option{-u}) option also disables the last-resort comparison.
3678 Unless otherwise specified, all comparisons use the character collating
3679 sequence specified by the @env{LC_COLLATE} locale.@footnote{If you
3680 use a non-@acronym{POSIX} locale (e.g., by setting @env{LC_ALL}
3681 to @samp{en_US}), then @command{sort} may produce output that is sorted
3682 differently than you're accustomed to. In that case, set the @env{LC_ALL}
3683 environment variable to @samp{C}. Note that setting only @env{LC_COLLATE}
3684 has two problems. First, it is ineffective if @env{LC_ALL} is also set.
3685 Second, it has undefined behavior if @env{LC_CTYPE} (or @env{LANG}, if
3686 @env{LC_CTYPE} is unset) is set to an incompatible value. For example,
3687 you get undefined behavior if @env{LC_CTYPE} is @code{ja_JP.PCK} but
3688 @env{LC_COLLATE} is @code{en_US.UTF-8}.}
3690 @sc{gnu} @command{sort} (as specified for all @sc{gnu} utilities) has no
3691 limit on input line length or restrictions on bytes allowed within lines.
3692 In addition, if the final byte of an input file is not a newline, @sc{gnu}
3693 @command{sort} silently supplies one. A line's trailing newline is not
3694 part of the line for comparison purposes.
3696 @cindex exit status of @command{sort}
3700 0 if no error occurred
3701 1 if invoked with @option{-c} or @option{-C} and the input is not sorted
3702 2 if an error occurred
3706 If the environment variable @env{TMPDIR} is set, @command{sort} uses its
3707 value as the directory for temporary files instead of @file{/tmp}. The
3708 @option{--temporary-directory} (@option{-T}) option in turn overrides
3709 the environment variable.
3711 The following options affect the ordering of output lines. They may be
3712 specified globally or as part of a specific key field. If no key
3713 fields are specified, global options apply to comparison of entire
3714 lines; otherwise the global options are inherited by key fields that do
3715 not specify any special options of their own. In pre-@acronym{POSIX}
3716 versions of @command{sort}, global options affect only later key fields,
3717 so portable shell scripts should specify global options first.
3722 @itemx --ignore-leading-blanks
3724 @opindex --ignore-leading-blanks
3725 @cindex blanks, ignoring leading
3727 Ignore leading blanks when finding sort keys in each line.
3728 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3729 can change this. Note blanks may be ignored by your locale's collating
3730 rules, but without this option they will be significant for character
3731 positions specified in keys with the @option{-k} option.
3734 @itemx --dictionary-order
3736 @opindex --dictionary-order
3737 @cindex dictionary order
3738 @cindex phone directory order
3739 @cindex telephone directory order
3741 Sort in @dfn{phone directory} order: ignore all characters except
3742 letters, digits and blanks when sorting.
3743 By default letters and digits are those of @acronym{ASCII} and a blank
3744 is a space or a tab, but the @env{LC_CTYPE} locale can change this.
3747 @itemx --ignore-case
3749 @opindex --ignore-case
3750 @cindex ignoring case
3751 @cindex case folding
3753 Fold lowercase characters into the equivalent uppercase characters when
3754 comparing so that, for example, @samp{b} and @samp{B} sort as equal.
3755 The @env{LC_CTYPE} locale determines character types.
3756 When used with @option{--unique} those lower case equivalent lines are
3757 thrown away. (There is currently no way to throw away the upper case
3758 equivalent instead. (Any @option{--reverse} given would only affect
3759 the final result, after the throwing away.))
3762 @itemx --general-numeric-sort
3763 @itemx --sort=general-numeric
3765 @opindex --general-numeric-sort
3767 @cindex general numeric sort
3769 Sort numerically, using the standard C function @code{strtold} to convert
3770 a prefix of each line to a long double-precision floating point number.
3771 This allows floating point numbers to be specified in scientific notation,
3772 like @code{1.0e-34} and @code{10e100}.
3773 The @env{LC_NUMERIC} locale determines the decimal-point character.
3774 Do not report overflow, underflow, or conversion errors.
3775 Use the following collating sequence:
3779 Lines that do not start with numbers (all considered to be equal).
3781 NaNs (``Not a Number'' values, in IEEE floating point arithmetic)
3782 in a consistent but machine-dependent order.
3786 Finite numbers in ascending numeric order (with @math{-0} and @math{+0} equal).
3791 Use this option only if there is no alternative; it is much slower than
3792 @option{--numeric-sort} (@option{-n}) and it can lose information when
3793 converting to floating point.
3796 @itemx --human-numeric-sort
3797 @itemx --sort=human-numeric
3799 @opindex --human-numeric-sort
3801 @cindex human numeric sort
3803 Sort numerically, first by numeric sign (negative, zero, or positive);
3804 then by @acronym{SI} suffix (either empty, or @samp{k} or @samp{K}, or
3805 one of @samp{MGTPEZY}, in that order; @pxref{Block size}); and finally
3806 by numeric value. For example, @samp{1023M} sorts before @samp{1G}
3807 because @samp{M} (mega) precedes @samp{G} (giga) as an @acronym{SI}
3808 suffix. This option sorts values that are consistently scaled to the
3809 nearest suffix, regardless of whether suffixes denote powers of 1000
3810 or 1024, and it therefore sorts the output of any single invocation of
3811 the @command{df}, @command{du}, or @command{ls} commands that are
3812 invoked with their @option{--human-readable} or @option{--si} options.
3813 The syntax for numbers is the same as for the @option{--numeric-sort}
3814 option; the @acronym{SI} suffix must immediately follow the number.
3817 @itemx --ignore-nonprinting
3819 @opindex --ignore-nonprinting
3820 @cindex nonprinting characters, ignoring
3821 @cindex unprintable characters, ignoring
3823 Ignore nonprinting characters.
3824 The @env{LC_CTYPE} locale determines character types.
3825 This option has no effect if the stronger @option{--dictionary-order}
3826 (@option{-d}) option is also given.
3832 @opindex --month-sort
3834 @cindex months, sorting by
3836 An initial string, consisting of any amount of blanks, followed
3837 by a month name abbreviation, is folded to UPPER case and
3838 compared in the order @samp{JAN} < @samp{FEB} < @dots{} < @samp{DEC}.
3839 Invalid names compare low to valid names. The @env{LC_TIME} locale
3840 category determines the month spellings.
3841 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3845 @itemx --numeric-sort
3846 @itemx --sort=numeric
3848 @opindex --numeric-sort
3850 @cindex numeric sort
3852 Sort numerically. The number begins each line and consists
3853 of optional blanks, an optional @samp{-} sign, and zero or more
3854 digits possibly separated by thousands separators, optionally followed
3855 by a decimal-point character and zero or more digits. An empty
3856 number is treated as @samp{0}. The @env{LC_NUMERIC}
3857 locale specifies the decimal-point character and thousands separator.
3858 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3861 Comparison is exact; there is no rounding error.
3863 Neither a leading @samp{+} nor exponential notation is recognized.
3864 To compare such strings numerically, use the
3865 @option{--general-numeric-sort} (@option{-g}) option.
3868 @itemx --version-sort
3870 @opindex --version-sort
3871 @cindex version number sort
3872 Sort by version name and number. It behaves like a standard sort,
3873 except that each sequence of decimal digits is treated numerically
3874 as an index/version number. (@xref{Details about version sort}.)
3880 @cindex reverse sorting
3881 Reverse the result of comparison, so that lines with greater key values
3882 appear earlier in the output instead of later.
3885 @itemx --random-sort
3886 @itemx --sort=random
3888 @opindex --random-sort
3891 Sort by hashing the input keys and then sorting the hash values.
3892 Choose the hash function at random, ensuring that it is free of
3893 collisions so that differing keys have differing hash values. This is
3894 like a random permutation of the inputs (@pxref{shuf invocation}),
3895 except that keys with the same value sort together.
3897 If multiple random sort fields are specified, the same random hash
3898 function is used for all fields. To use different random hash
3899 functions for different fields, you can invoke @command{sort} more
3902 The choice of hash function is affected by the
3903 @option{--random-source} option.
3911 @item --compress-program=@var{prog}
3912 Compress any temporary files with the program @var{prog}.
3914 With no arguments, @var{prog} must compress standard input to standard
3915 output, and when given the @option{-d} option it must decompress
3916 standard input to standard output.
3918 Terminate with an error if @var{prog} exits with nonzero status.
3920 White space and the backslash character should not appear in
3921 @var{prog}; they are reserved for future use.
3923 @filesZeroFromOption{sort,,sorted output}
3925 @item -k @var{pos1}[,@var{pos2}]
3926 @itemx --key=@var{pos1}[,@var{pos2}]
3930 Specify a sort field that consists of the part of the line between
3931 @var{pos1} and @var{pos2} (or the end of the line, if @var{pos2} is
3932 omitted), @emph{inclusive}.
3934 Each @var{pos} has the form @samp{@var{f}[.@var{c}][@var{opts}]},
3935 where @var{f} is the number of the field to use, and @var{c} is the number
3936 of the first character from the beginning of the field. Fields and character
3937 positions are numbered starting with 1; a character position of zero in
3938 @var{pos2} indicates the field's last character. If @samp{.@var{c}} is
3939 omitted from @var{pos1}, it defaults to 1 (the beginning of the field);
3940 if omitted from @var{pos2}, it defaults to 0 (the end of the field).
3941 @var{opts} are ordering options, allowing individual keys to be sorted
3942 according to different rules; see below for details. Keys can span
3945 Example: To sort on the second field, use @option{--key=2,2}
3946 (@option{-k 2,2}). See below for more notes on keys and more examples.
3947 See also the @option{--debug} option to help determine the part
3948 of the line being used in the sort.
3951 Highlight the portion of each line used for sorting.
3952 Also issue warnings about questionable usage to stderr.
3954 @item --batch-size=@var{nmerge}
3955 @opindex --batch-size
3956 @cindex number of inputs to merge, nmerge
3957 Merge at most @var{nmerge} inputs at once.
3959 When @command{sort} has to merge more than @var{nmerge} inputs,
3960 it merges them in groups of @var{nmerge}, saving the result in
3961 a temporary file, which is then used as an input in a subsequent merge.
3963 A large value of @var{nmerge} may improve merge performance and decrease
3964 temporary storage utilization at the expense of increased memory usage
3965 and I/0. Conversely a small value of @var{nmerge} may reduce memory
3966 requirements and I/0 at the expense of temporary storage consumption and
3969 The value of @var{nmerge} must be at least 2. The default value is
3970 currently 16, but this is implementation-dependent and may change in
3973 The value of @var{nmerge} may be bounded by a resource limit for open
3974 file descriptors. The commands @samp{ulimit -n} or @samp{getconf
3975 OPEN_MAX} may display limits for your systems; these limits may be
3976 modified further if your program already has some files open, or if
3977 the operating system has other limits on the number of open files. If
3978 the value of @var{nmerge} exceeds the resource limit, @command{sort}
3979 silently uses a smaller value.
3981 @item -o @var{output-file}
3982 @itemx --output=@var{output-file}
3985 @cindex overwriting of input, allowed
3986 Write output to @var{output-file} instead of standard output.
3987 Normally, @command{sort} reads all input before opening
3988 @var{output-file}, so you can safely sort a file in place by using
3989 commands like @code{sort -o F F} and @code{cat F | sort -o F}.
3990 However, @command{sort} with @option{--merge} (@option{-m}) can open
3991 the output file before reading all input, so a command like @code{cat
3992 F | sort -m -o F - G} is not safe as @command{sort} might start
3993 writing @file{F} before @command{cat} is done reading it.
3995 @vindex POSIXLY_CORRECT
3996 On newer systems, @option{-o} cannot appear after an input file if
3997 @env{POSIXLY_CORRECT} is set, e.g., @samp{sort F -o F}. Portable
3998 scripts should specify @option{-o @var{output-file}} before any input
4001 @item --random-source=@var{file}
4002 @opindex --random-source
4003 @cindex random source for sorting
4004 Use @var{file} as a source of random data used to determine which
4005 random hash function to use with the @option{-R} option. @xref{Random
4012 @cindex sort stability
4013 @cindex sort's last-resort comparison
4015 Make @command{sort} stable by disabling its last-resort comparison.
4016 This option has no effect if no fields or global ordering options
4017 other than @option{--reverse} (@option{-r}) are specified.
4020 @itemx --buffer-size=@var{size}
4022 @opindex --buffer-size
4023 @cindex size for main memory sorting
4024 Use a main-memory sort buffer of the given @var{size}. By default,
4025 @var{size} is in units of 1024 bytes. Appending @samp{%} causes
4026 @var{size} to be interpreted as a percentage of physical memory.
4027 Appending @samp{K} multiplies @var{size} by 1024 (the default),
4028 @samp{M} by 1,048,576, @samp{G} by 1,073,741,824, and so on for
4029 @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}. Appending
4030 @samp{b} causes @var{size} to be interpreted as a byte count, with no
4033 This option can improve the performance of @command{sort} by causing it
4034 to start with a larger or smaller sort buffer than the default.
4035 However, this option affects only the initial buffer size. The buffer
4036 grows beyond @var{size} if @command{sort} encounters input lines larger
4039 @item -t @var{separator}
4040 @itemx --field-separator=@var{separator}
4042 @opindex --field-separator
4043 @cindex field separator character
4044 Use character @var{separator} as the field separator when finding the
4045 sort keys in each line. By default, fields are separated by the empty
4046 string between a non-blank character and a blank character.
4047 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4050 That is, given the input line @w{@samp{ foo bar}}, @command{sort} breaks it
4051 into fields @w{@samp{ foo}} and @w{@samp{ bar}}. The field separator is
4052 not considered to be part of either the field preceding or the field
4053 following, so with @samp{sort @w{-t " "}} the same input line has
4054 three fields: an empty field, @samp{foo}, and @samp{bar}.
4055 However, fields that extend to the end of the line,
4056 as @option{-k 2}, or fields consisting of a range, as @option{-k 2,3},
4057 retain the field separators present between the endpoints of the range.
4059 To specify @acronym{ASCII} @sc{nul} as the field separator,
4060 use the two-character string @samp{\0}, e.g., @samp{sort -t '\0'}.
4062 @item -T @var{tempdir}
4063 @itemx --temporary-directory=@var{tempdir}
4065 @opindex --temporary-directory
4066 @cindex temporary directory
4068 Use directory @var{tempdir} to store temporary files, overriding the
4069 @env{TMPDIR} environment variable. If this option is given more than
4070 once, temporary files are stored in all the directories given. If you
4071 have a large sort or merge that is I/O-bound, you can often improve
4072 performance by using this option to specify directories on different
4073 disks and controllers.
4075 @item --parallel=@var{n}
4077 @cindex multithreaded sort
4078 Limit the number of sorts run in parallel to @var{n}. By default,
4079 @var{n} is set to the number of available processors, and values
4080 greater than that are reduced to that limit. Also see
4081 @ref{nproc invocation}.
4087 @cindex uniquifying output
4089 Normally, output only the first of a sequence of lines that compare
4090 equal. For the @option{--check} (@option{-c} or @option{-C}) option,
4091 check that no pair of consecutive lines compares equal.
4093 This option also disables the default last-resort comparison.
4095 The commands @code{sort -u} and @code{sort | uniq} are equivalent, but
4096 this equivalence does not extend to arbitrary @command{sort} options.
4097 For example, @code{sort -n -u} inspects only the value of the initial
4098 numeric string when checking for uniqueness, whereas @code{sort -n |
4099 uniq} inspects the entire line. @xref{uniq invocation}.
4101 @macro zeroTerminatedOption
4103 @itemx --zero-terminated
4105 @opindex --zero-terminated
4106 @cindex process zero-terminated items
4107 Delimit items with a zero byte rather than a newline (@acronym{ASCII} @sc{lf}).
4108 I.E. treat input as items separated by @acronym{ASCII} @sc{nul}
4109 and terminate output items with @acronym{ASCII} @sc{nul}.
4110 This option can be useful in conjunction with @samp{perl -0} or
4111 @samp{find -print0} and @samp{xargs -0} which do the same in order to
4112 reliably handle arbitrary file names (even those containing blanks
4113 or other special characters).
4115 @zeroTerminatedOption
4119 Historical (BSD and System V) implementations of @command{sort} have
4120 differed in their interpretation of some options, particularly
4121 @option{-b}, @option{-f}, and @option{-n}. @sc{gnu} sort follows the @acronym{POSIX}
4122 behavior, which is usually (but not always!) like the System V behavior.
4123 According to @acronym{POSIX}, @option{-n} no longer implies @option{-b}. For
4124 consistency, @option{-M} has been changed in the same way. This may
4125 affect the meaning of character positions in field specifications in
4126 obscure cases. The only fix is to add an explicit @option{-b}.
4128 A position in a sort field specified with @option{-k} may have any
4129 of the option letters @samp{MbdfghinRrV} appended to it, in which case no
4130 global ordering options are inherited by that particular field. The
4131 @option{-b} option may be independently attached to either or both of
4132 the start and end positions of a field specification, and if it is
4133 inherited from the global options it will be attached to both.
4134 If input lines can contain leading or adjacent blanks and @option{-t}
4135 is not used, then @option{-k} is typically combined with @option{-b} or
4136 an option that implicitly ignores leading blanks (@samp{Mghn}) as otherwise
4137 the varying numbers of leading blanks in fields can cause confusing results.
4139 If the start position in a sort field specifier falls after the end of
4140 the line or after the end field, the field is empty. If the @option{-b}
4141 option was specified, the @samp{.@var{c}} part of a field specification
4142 is counted from the first nonblank character of the field.
4144 @vindex _POSIX2_VERSION
4145 @vindex POSIXLY_CORRECT
4146 On older systems, @command{sort} supports an obsolete origin-zero
4147 syntax @samp{+@var{pos1} [-@var{pos2}]} for specifying sort keys.
4148 The obsolete sequence @samp{sort +@var{a}.@var{x} -@var{b}.@var{y}}
4149 is equivalent to @samp{sort -k @var{a+1}.@var{x+1},@var{b}} if @var{y}
4150 is @samp{0} or absent, otherwise it is equivalent to @samp{sort -k
4151 @var{a+1}.@var{x+1},@var{b+1}.@var{y}}.
4153 This obsolete behavior can be enabled or disabled with the
4154 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4155 conformance}); it can also be enabled when @env{POSIXLY_CORRECT} is
4156 not set by using the obsolete syntax with @samp{-@var{pos2}} present.
4158 Scripts intended for use on standard hosts should avoid obsolete
4159 syntax and should use @option{-k} instead. For example, avoid
4160 @samp{sort +2}, since it might be interpreted as either @samp{sort
4161 ./+2} or @samp{sort -k 3}. If your script must also run on hosts that
4162 support only the obsolete syntax, it can use a test like @samp{if sort
4163 -k 1 </dev/null >/dev/null 2>&1; then @dots{}} to decide which syntax
4166 Here are some examples to illustrate various combinations of options.
4171 Sort in descending (reverse) numeric order.
4178 Run no more that 4 sorts concurrently, using a buffer size of 10M.
4181 sort --parallel=4 -S 10M
4185 Sort alphabetically, omitting the first and second fields
4186 and the blanks at the start of the third field.
4187 This uses a single key composed of the characters beginning
4188 at the start of the first nonblank character in field three
4189 and extending to the end of each line.
4196 Sort numerically on the second field and resolve ties by sorting
4197 alphabetically on the third and fourth characters of field five.
4198 Use @samp{:} as the field delimiter.
4201 sort -t : -k 2,2n -k 5.3,5.4
4204 Note that if you had written @option{-k 2n} instead of @option{-k 2,2n}
4205 @command{sort} would have used all characters beginning in the second field
4206 and extending to the end of the line as the primary @emph{numeric}
4207 key. For the large majority of applications, treating keys spanning
4208 more than one field as numeric will not do what you expect.
4210 Also note that the @samp{n} modifier was applied to the field-end
4211 specifier for the first key. It would have been equivalent to
4212 specify @option{-k 2n,2} or @option{-k 2n,2n}. All modifiers except
4213 @samp{b} apply to the associated @emph{field}, regardless of whether
4214 the modifier character is attached to the field-start and/or the
4215 field-end part of the key specifier.
4218 Sort the password file on the fifth field and ignore any
4219 leading blanks. Sort lines with equal values in field five
4220 on the numeric user ID in field three. Fields are separated
4224 sort -t : -k 5b,5 -k 3,3n /etc/passwd
4225 sort -t : -n -k 5b,5 -k 3,3 /etc/passwd
4226 sort -t : -b -k 5,5 -k 3,3n /etc/passwd
4229 These three commands have equivalent effect. The first specifies that
4230 the first key's start position ignores leading blanks and the second
4231 key is sorted numerically. The other two commands rely on global
4232 options being inherited by sort keys that lack modifiers. The inheritance
4233 works in this case because @option{-k 5b,5b} and @option{-k 5b,5} are
4234 equivalent, as the location of a field-end lacking a @samp{.@var{c}}
4235 character position is not affected by whether initial blanks are
4239 Sort a set of log files, primarily by IPv4 address and secondarily by
4240 time stamp. If two lines' primary and secondary keys are identical,
4241 output the lines in the same order that they were input. The log
4242 files contain lines that look like this:
4245 4.150.156.3 - - [01/Apr/2004:06:31:51 +0000] message 1
4246 211.24.3.231 - - [24/Apr/2004:20:17:39 +0000] message 2
4249 Fields are separated by exactly one space. Sort IPv4 addresses
4250 lexicographically, e.g., 212.61.52.2 sorts before 212.129.233.201
4251 because 61 is less than 129.
4254 sort -s -t ' ' -k 4.9n -k 4.5M -k 4.2n -k 4.14,4.21 file*.log |
4255 sort -s -t '.' -k 1,1n -k 2,2n -k 3,3n -k 4,4n
4258 This example cannot be done with a single @command{sort} invocation,
4259 since IPv4 address components are separated by @samp{.} while dates
4260 come just after a space. So it is broken down into two invocations of
4261 @command{sort}: the first sorts by time stamp and the second by IPv4
4262 address. The time stamp is sorted by year, then month, then day, and
4263 finally by hour-minute-second field, using @option{-k} to isolate each
4264 field. Except for hour-minute-second there's no need to specify the
4265 end of each key field, since the @samp{n} and @samp{M} modifiers sort
4266 based on leading prefixes that cannot cross field boundaries. The
4267 IPv4 addresses are sorted lexicographically. The second sort uses
4268 @samp{-s} so that ties in the primary key are broken by the secondary
4269 key; the first sort uses @samp{-s} so that the combination of the two
4273 Generate a tags file in case-insensitive sorted order.
4276 find src -type f -print0 | sort -z -f | xargs -0 etags --append
4279 The use of @option{-print0}, @option{-z}, and @option{-0} in this case means
4280 that file names that contain blanks or other special characters are
4282 by the sort operation.
4284 @c This example is a bit contrived and needs more explanation.
4286 @c Sort records separated by an arbitrary string by using a pipe to convert
4287 @c each record delimiter string to @samp{\0}, then using sort's -z option,
4288 @c and converting each @samp{\0} back to the original record delimiter.
4291 @c printf 'c\n\nb\n\na\n'|perl -0pe 's/\n\n/\n\0/g'|sort -z|perl -0pe 's/\0/\n/g'
4295 Use the common @acronym{DSU, Decorate Sort Undecorate} idiom to
4296 sort lines according to their length.
4299 awk '@{print length, $0@}' /etc/passwd | sort -n | cut -f2- -d' '
4302 In general this technique can be used to sort data that the @command{sort}
4303 command does not support, or is inefficient at, sorting directly.
4306 Shuffle a list of directories, but preserve the order of files within
4307 each directory. For instance, one could use this to generate a music
4308 playlist in which albums are shuffled but the songs of each album are
4312 ls */* | sort -t / -k 1,1R -k 2,2
4318 @node shuf invocation
4319 @section @command{shuf}: Shuffling text
4322 @cindex shuffling files
4324 @command{shuf} shuffles its input by outputting a random permutation
4325 of its input lines. Each output permutation is equally likely.
4329 shuf [@var{option}]@dots{} [@var{file}]
4330 shuf -e [@var{option}]@dots{} [@var{arg}]@dots{}
4331 shuf -i @var{lo}-@var{hi} [@var{option}]@dots{}
4334 @command{shuf} has three modes of operation that affect where it
4335 obtains its input lines. By default, it reads lines from standard
4336 input. The following options change the operation mode:
4344 @cindex command-line operands to shuffle
4345 Treat each command-line operand as an input line.
4347 @item -i @var{lo}-@var{hi}
4348 @itemx --input-range=@var{lo}-@var{hi}
4350 @opindex --input-range
4351 @cindex input range to shuffle
4352 Act as if input came from a file containing the range of unsigned
4353 decimal integers @var{lo}@dots{}@var{hi}, one per line.
4357 @command{shuf}'s other options can affect its behavior in all
4362 @item -n @var{lines}
4363 @itemx --head-count=@var{count}
4365 @opindex --head-count
4366 @cindex head of output
4367 Output at most @var{count} lines. By default, all input lines are
4370 @item -o @var{output-file}
4371 @itemx --output=@var{output-file}
4374 @cindex overwriting of input, allowed
4375 Write output to @var{output-file} instead of standard output.
4376 @command{shuf} reads all input before opening
4377 @var{output-file}, so you can safely shuffle a file in place by using
4378 commands like @code{shuf -o F <F} and @code{cat F | shuf -o F}.
4380 @item --random-source=@var{file}
4381 @opindex --random-source
4382 @cindex random source for shuffling
4383 Use @var{file} as a source of random data used to determine which
4384 permutation to generate. @xref{Random sources}.
4386 @zeroTerminatedOption
4402 might produce the output
4412 Similarly, the command:
4415 shuf -e clubs hearts diamonds spades
4429 and the command @samp{shuf -i 1-4} might output:
4439 These examples all have four input lines, so @command{shuf} might
4440 produce any of the twenty-four possible permutations of the input. In
4441 general, if there are @var{n} input lines, there are @var{n}! (i.e.,
4442 @var{n} factorial, or @var{n} * (@var{n} - 1) * @dots{} * 1) possible
4443 output permutations.
4448 @node uniq invocation
4449 @section @command{uniq}: Uniquify files
4452 @cindex uniquify files
4454 @command{uniq} writes the unique lines in the given @file{input}, or
4455 standard input if nothing is given or for an @var{input} name of
4459 uniq [@var{option}]@dots{} [@var{input} [@var{output}]]
4462 By default, @command{uniq} prints its input lines, except that
4463 it discards all but the first of adjacent repeated lines, so that
4464 no output lines are repeated. Optionally, it can instead discard
4465 lines that are not repeated, or all repeated lines.
4467 The input need not be sorted, but repeated input lines are detected
4468 only if they are adjacent. If you want to discard non-adjacent
4469 duplicate lines, perhaps you want to use @code{sort -u}.
4470 @xref{sort invocation}.
4473 Comparisons honor the rules specified by the @env{LC_COLLATE}
4476 If no @var{output} file is specified, @command{uniq} writes to standard
4479 The program accepts the following options. Also see @ref{Common options}.
4484 @itemx --skip-fields=@var{n}
4486 @opindex --skip-fields
4487 Skip @var{n} fields on each line before checking for uniqueness. Use
4488 a null string for comparison if a line has fewer than @var{n} fields. Fields
4489 are sequences of non-space non-tab characters that are separated from
4490 each other by at least one space or tab.
4492 For compatibility @command{uniq} supports an obsolete option syntax
4493 @option{-@var{n}}. New scripts should use @option{-f @var{n}} instead.
4496 @itemx --skip-chars=@var{n}
4498 @opindex --skip-chars
4499 Skip @var{n} characters before checking for uniqueness. Use a null string
4500 for comparison if a line has fewer than @var{n} characters. If you use both
4501 the field and character skipping options, fields are skipped over first.
4503 @vindex _POSIX2_VERSION
4504 On older systems, @command{uniq} supports an obsolete option syntax
4506 This obsolete behavior can be enabled or disabled with the
4507 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4508 conformance}), but portable scripts should avoid commands whose
4509 behavior depends on this variable.
4510 For example, use @samp{uniq ./+10} or @samp{uniq -s 10} rather than
4511 the ambiguous @samp{uniq +10}.
4517 Print the number of times each line occurred along with the line.
4520 @itemx --ignore-case
4522 @opindex --ignore-case
4523 Ignore differences in case when comparing lines.
4529 @cindex repeated lines, outputting
4530 Discard lines that are not repeated. When used by itself, this option
4531 causes @command{uniq} to print the first copy of each repeated line,
4535 @itemx --all-repeated[=@var{delimit-method}]
4537 @opindex --all-repeated
4538 @cindex all repeated lines, outputting
4539 Do not discard the second and subsequent repeated input lines,
4540 but discard lines that are not repeated.
4541 This option is useful mainly in conjunction with other options e.g.,
4542 to ignore case or to compare only selected fields.
4543 The optional @var{delimit-method} tells how to delimit
4544 groups of repeated lines, and must be one of the following:
4549 Do not delimit groups of repeated lines.
4550 This is equivalent to @option{--all-repeated} (@option{-D}).
4553 Output a newline before each group of repeated lines.
4554 With @option{--zero-terminated} (@option{-z}), use a zero
4555 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4558 Separate groups of repeated lines with a single newline.
4559 With @option{--zero-terminated} (@option{-z}), use a zero
4560 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4561 This is the same as using @samp{prepend}, except that
4562 no delimiter is inserted before the first group, and hence
4563 may be better suited for output direct to users.
4566 Note that when groups are delimited and the input stream contains
4567 two or more consecutive blank lines, then the output is ambiguous.
4568 To avoid that, filter the input through @samp{tr -s '\n'} to replace
4569 each sequence of consecutive newlines with a single newline.
4571 This is a @sc{gnu} extension.
4572 @c FIXME: give an example showing *how* it's useful
4578 @cindex unique lines, outputting
4579 Discard the first repeated line. When used by itself, this option
4580 causes @command{uniq} to print unique lines, and nothing else.
4583 @itemx --check-chars=@var{n}
4585 @opindex --check-chars
4586 Compare at most @var{n} characters on each line (after skipping any specified
4587 fields and characters). By default the entire rest of the lines are
4590 @zeroTerminatedOption
4597 @node comm invocation
4598 @section @command{comm}: Compare two sorted files line by line
4601 @cindex line-by-line comparison
4602 @cindex comparing sorted files
4604 @command{comm} writes to standard output lines that are common, and lines
4605 that are unique, to two input files; a file name of @samp{-} means
4606 standard input. Synopsis:
4609 comm [@var{option}]@dots{} @var{file1} @var{file2}
4613 Before @command{comm} can be used, the input files must be sorted using the
4614 collating sequence specified by the @env{LC_COLLATE} locale.
4615 If an input file ends in a non-newline
4616 character, a newline is silently appended. The @command{sort} command with
4617 no options always outputs a file that is suitable input to @command{comm}.
4619 @cindex differing lines
4620 @cindex common lines
4621 With no options, @command{comm} produces three-column output. Column one
4622 contains lines unique to @var{file1}, column two contains lines unique
4623 to @var{file2}, and column three contains lines common to both files.
4624 Columns are separated by a single TAB character.
4625 @c FIXME: when there's an option to supply an alternative separator
4626 @c string, append `by default' to the above sentence.
4631 The options @option{-1}, @option{-2}, and @option{-3} suppress printing of
4632 the corresponding columns (and separators). Also see @ref{Common options}.
4634 Unlike some other comparison utilities, @command{comm} has an exit
4635 status that does not depend on the result of the comparison.
4636 Upon normal completion @command{comm} produces an exit code of zero.
4637 If there is an error it exits with nonzero status.
4639 @macro checkOrderOption{cmd}
4640 If the @option{--check-order} option is given, unsorted inputs will
4641 cause a fatal error message. If the option @option{--nocheck-order}
4642 is given, unsorted inputs will never cause an error message. If
4643 neither of these options is given, wrongly sorted inputs are diagnosed
4644 only if an input file is found to contain unpairable lines. If an
4645 input file is diagnosed as being unsorted, the @command{\cmd\} command
4646 will exit with a nonzero status (and the output should not be used).
4648 Forcing @command{\cmd\} to process wrongly sorted input files
4649 containing unpairable lines by specifying @option{--nocheck-order} is
4650 not guaranteed to produce any particular output. The output will
4651 probably not correspond with whatever you hoped it would be.
4653 @checkOrderOption{comm}
4658 Fail with an error message if either input file is wrongly ordered.
4660 @item --nocheck-order
4661 Do not check that both input files are in sorted order.
4665 @item --output-delimiter=@var{str}
4666 Print @var{str} between adjacent output columns,
4667 rather than the default of a single TAB character.
4669 The delimiter @var{str} may not be empty.
4673 @node ptx invocation
4674 @section @command{ptx}: Produce permuted indexes
4678 @command{ptx} reads a text file and essentially produces a permuted index, with
4679 each keyword in its context. The calling sketch is either one of:
4682 ptx [@var{option} @dots{}] [@var{file} @dots{}]
4683 ptx -G [@var{option} @dots{}] [@var{input} [@var{output}]]
4686 The @option{-G} (or its equivalent: @option{--traditional}) option disables
4687 all @sc{gnu} extensions and reverts to traditional mode, thus introducing some
4688 limitations and changing several of the program's default option values.
4689 When @option{-G} is not specified, @sc{gnu} extensions are always enabled.
4690 @sc{gnu} extensions to @command{ptx} are documented wherever appropriate in this
4691 document. For the full list, see @xref{Compatibility in ptx}.
4693 Individual options are explained in the following sections.
4695 When @sc{gnu} extensions are enabled, there may be zero, one or several
4696 @var{file}s after the options. If there is no @var{file}, the program
4697 reads the standard input. If there is one or several @var{file}s, they
4698 give the name of input files which are all read in turn, as if all the
4699 input files were concatenated. However, there is a full contextual
4700 break between each file and, when automatic referencing is requested,
4701 file names and line numbers refer to individual text input files. In
4702 all cases, the program outputs the permuted index to the standard
4705 When @sc{gnu} extensions are @emph{not} enabled, that is, when the program
4706 operates in traditional mode, there may be zero, one or two parameters
4707 besides the options. If there are no parameters, the program reads the
4708 standard input and outputs the permuted index to the standard output.
4709 If there is only one parameter, it names the text @var{input} to be read
4710 instead of the standard input. If two parameters are given, they give
4711 respectively the name of the @var{input} file to read and the name of
4712 the @var{output} file to produce. @emph{Be very careful} to note that,
4713 in this case, the contents of file given by the second parameter is
4714 destroyed. This behavior is dictated by System V @command{ptx}
4715 compatibility; @sc{gnu} Standards normally discourage output parameters not
4716 introduced by an option.
4718 Note that for @emph{any} file named as the value of an option or as an
4719 input text file, a single dash @kbd{-} may be used, in which case
4720 standard input is assumed. However, it would not make sense to use this
4721 convention more than once per program invocation.
4724 * General options in ptx:: Options which affect general program behavior.
4725 * Charset selection in ptx:: Underlying character set considerations.
4726 * Input processing in ptx:: Input fields, contexts, and keyword selection.
4727 * Output formatting in ptx:: Types of output format, and sizing the fields.
4728 * Compatibility in ptx::
4732 @node General options in ptx
4733 @subsection General options
4738 @itemx --traditional
4739 As already explained, this option disables all @sc{gnu} extensions to
4740 @command{ptx} and switches to traditional mode.
4743 Print a short help on standard output, then exit without further
4747 Print the program version on standard output, then exit without further
4755 @node Charset selection in ptx
4756 @subsection Charset selection
4758 @c FIXME: People don't necessarily know what an IBM-PC was these days.
4759 As it is set up now, the program assumes that the input file is coded
4760 using 8-bit @acronym{ISO} 8859-1 code, also known as Latin-1 character set,
4761 @emph{unless} it is compiled for MS-DOS, in which case it uses the
4762 character set of the IBM-PC@. (@sc{gnu} @command{ptx} is not known to work on
4763 smaller MS-DOS machines anymore.) Compared to 7-bit @acronym{ASCII}, the set
4764 of characters which are letters is different; this alters the behavior
4765 of regular expression matching. Thus, the default regular expression
4766 for a keyword allows foreign or diacriticized letters. Keyword sorting,
4767 however, is still crude; it obeys the underlying character set ordering
4773 @itemx --ignore-case
4774 Fold lower case letters to upper case for sorting.
4779 @node Input processing in ptx
4780 @subsection Word selection and input processing
4785 @itemx --break-file=@var{file}
4787 This option provides an alternative (to @option{-W}) method of describing
4788 which characters make up words. It introduces the name of a
4789 file which contains a list of characters which can@emph{not} be part of
4790 one word; this file is called the @dfn{Break file}. Any character which
4791 is not part of the Break file is a word constituent. If both options
4792 @option{-b} and @option{-W} are specified, then @option{-W} has precedence and
4793 @option{-b} is ignored.
4795 When @sc{gnu} extensions are enabled, the only way to avoid newline as a
4796 break character is to write all the break characters in the file with no
4797 newline at all, not even at the end of the file. When @sc{gnu} extensions
4798 are disabled, spaces, tabs and newlines are always considered as break
4799 characters even if not included in the Break file.
4802 @itemx --ignore-file=@var{file}
4804 The file associated with this option contains a list of words which will
4805 never be taken as keywords in concordance output. It is called the
4806 @dfn{Ignore file}. The file contains exactly one word in each line; the
4807 end of line separation of words is not subject to the value of the
4811 @itemx --only-file=@var{file}
4813 The file associated with this option contains a list of words which will
4814 be retained in concordance output; any word not mentioned in this file
4815 is ignored. The file is called the @dfn{Only file}. The file contains
4816 exactly one word in each line; the end of line separation of words is
4817 not subject to the value of the @option{-S} option.
4819 There is no default for the Only file. When both an Only file and an
4820 Ignore file are specified, a word is considered a keyword only
4821 if it is listed in the Only file and not in the Ignore file.
4826 On each input line, the leading sequence of non-white space characters will be
4827 taken to be a reference that has the purpose of identifying this input
4828 line in the resulting permuted index. For more information about reference
4829 production, see @xref{Output formatting in ptx}.
4830 Using this option changes the default value for option @option{-S}.
4832 Using this option, the program does not try very hard to remove
4833 references from contexts in output, but it succeeds in doing so
4834 @emph{when} the context ends exactly at the newline. If option
4835 @option{-r} is used with @option{-S} default value, or when @sc{gnu} extensions
4836 are disabled, this condition is always met and references are completely
4837 excluded from the output contexts.
4839 @item -S @var{regexp}
4840 @itemx --sentence-regexp=@var{regexp}
4842 This option selects which regular expression will describe the end of a
4843 line or the end of a sentence. In fact, this regular expression is not
4844 the only distinction between end of lines or end of sentences, and input
4845 line boundaries have no special significance outside this option. By
4846 default, when @sc{gnu} extensions are enabled and if @option{-r} option is not
4847 used, end of sentences are used. In this case, this @var{regex} is
4848 imported from @sc{gnu} Emacs:
4851 [.?!][]\"')@}]*\\($\\|\t\\| \\)[ \t\n]*
4854 Whenever @sc{gnu} extensions are disabled or if @option{-r} option is used, end
4855 of lines are used; in this case, the default @var{regexp} is just:
4861 Using an empty @var{regexp} is equivalent to completely disabling end of
4862 line or end of sentence recognition. In this case, the whole file is
4863 considered to be a single big line or sentence. The user might want to
4864 disallow all truncation flag generation as well, through option @option{-F
4865 ""}. @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
4868 When the keywords happen to be near the beginning of the input line or
4869 sentence, this often creates an unused area at the beginning of the
4870 output context line; when the keywords happen to be near the end of the
4871 input line or sentence, this often creates an unused area at the end of
4872 the output context line. The program tries to fill those unused areas
4873 by wrapping around context in them; the tail of the input line or
4874 sentence is used to fill the unused area on the left of the output line;
4875 the head of the input line or sentence is used to fill the unused area
4876 on the right of the output line.
4878 As a matter of convenience to the user, many usual backslashed escape
4879 sequences from the C language are recognized and converted to the
4880 corresponding characters by @command{ptx} itself.
4882 @item -W @var{regexp}
4883 @itemx --word-regexp=@var{regexp}
4885 This option selects which regular expression will describe each keyword.
4886 By default, if @sc{gnu} extensions are enabled, a word is a sequence of
4887 letters; the @var{regexp} used is @samp{\w+}. When @sc{gnu} extensions are
4888 disabled, a word is by default anything which ends with a space, a tab
4889 or a newline; the @var{regexp} used is @samp{[^ \t\n]+}.
4891 An empty @var{regexp} is equivalent to not using this option.
4892 @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
4895 As a matter of convenience to the user, many usual backslashed escape
4896 sequences, as found in the C language, are recognized and converted to
4897 the corresponding characters by @command{ptx} itself.
4902 @node Output formatting in ptx
4903 @subsection Output formatting
4905 Output format is mainly controlled by the @option{-O} and @option{-T} options
4906 described in the table below. When neither @option{-O} nor @option{-T} are
4907 selected, and if @sc{gnu} extensions are enabled, the program chooses an
4908 output format suitable for a dumb terminal. Each keyword occurrence is
4909 output to the center of one line, surrounded by its left and right
4910 contexts. Each field is properly justified, so the concordance output
4911 can be readily observed. As a special feature, if automatic
4912 references are selected by option @option{-A} and are output before the
4913 left context, that is, if option @option{-R} is @emph{not} selected, then
4914 a colon is added after the reference; this nicely interfaces with @sc{gnu}
4915 Emacs @code{next-error} processing. In this default output format, each
4916 white space character, like newline and tab, is merely changed to
4917 exactly one space, with no special attempt to compress consecutive
4918 spaces. This might change in the future. Except for those white space
4919 characters, every other character of the underlying set of 256
4920 characters is transmitted verbatim.
4922 Output format is further controlled by the following options.
4926 @item -g @var{number}
4927 @itemx --gap-size=@var{number}
4929 Select the size of the minimum white space gap between the fields on the
4932 @item -w @var{number}
4933 @itemx --width=@var{number}
4935 Select the maximum output width of each final line. If references are
4936 used, they are included or excluded from the maximum output width
4937 depending on the value of option @option{-R}. If this option is not
4938 selected, that is, when references are output before the left context,
4939 the maximum output width takes into account the maximum length of all
4940 references. If this option is selected, that is, when references are
4941 output after the right context, the maximum output width does not take
4942 into account the space taken by references, nor the gap that precedes
4946 @itemx --auto-reference
4948 Select automatic references. Each input line will have an automatic
4949 reference made up of the file name and the line ordinal, with a single
4950 colon between them. However, the file name will be empty when standard
4951 input is being read. If both @option{-A} and @option{-r} are selected, then
4952 the input reference is still read and skipped, but the automatic
4953 reference is used at output time, overriding the input reference.
4956 @itemx --right-side-refs
4958 In the default output format, when option @option{-R} is not used, any
4959 references produced by the effect of options @option{-r} or @option{-A} are
4960 placed to the far right of output lines, after the right context. With
4961 default output format, when the @option{-R} option is specified, references
4962 are rather placed at the beginning of each output line, before the left
4963 context. For any other output format, option @option{-R} is
4964 ignored, with one exception: with @option{-R} the width of references
4965 is @emph{not} taken into account in total output width given by @option{-w}.
4967 This option is automatically selected whenever @sc{gnu} extensions are
4970 @item -F @var{string}
4971 @itemx --flac-truncation=@var{string}
4973 This option will request that any truncation in the output be reported
4974 using the string @var{string}. Most output fields theoretically extend
4975 towards the beginning or the end of the current line, or current
4976 sentence, as selected with option @option{-S}. But there is a maximum
4977 allowed output line width, changeable through option @option{-w}, which is
4978 further divided into space for various output fields. When a field has
4979 to be truncated because it cannot extend beyond the beginning or the end of
4980 the current line to fit in, then a truncation occurs. By default,
4981 the string used is a single slash, as in @option{-F /}.
4983 @var{string} may have more than one character, as in @option{-F ...}.
4984 Also, in the particular case when @var{string} is empty (@option{-F ""}),
4985 truncation flagging is disabled, and no truncation marks are appended in
4988 As a matter of convenience to the user, many usual backslashed escape
4989 sequences, as found in the C language, are recognized and converted to
4990 the corresponding characters by @command{ptx} itself.
4992 @item -M @var{string}
4993 @itemx --macro-name=@var{string}
4995 Select another @var{string} to be used instead of @samp{xx}, while
4996 generating output suitable for @command{nroff}, @command{troff} or @TeX{}.
4999 @itemx --format=roff
5001 Choose an output format suitable for @command{nroff} or @command{troff}
5002 processing. Each output line will look like:
5005 .xx "@var{tail}" "@var{before}" "@var{keyword_and_after}" "@var{head}" "@var{ref}"
5008 so it will be possible to write a @samp{.xx} roff macro to take care of
5009 the output typesetting. This is the default output format when @sc{gnu}
5010 extensions are disabled. Option @option{-M} can be used to change
5011 @samp{xx} to another macro name.
5013 In this output format, each non-graphical character, like newline and
5014 tab, is merely changed to exactly one space, with no special attempt to
5015 compress consecutive spaces. Each quote character: @kbd{"} is doubled
5016 so it will be correctly processed by @command{nroff} or @command{troff}.
5021 Choose an output format suitable for @TeX{} processing. Each output
5022 line will look like:
5025 \xx @{@var{tail}@}@{@var{before}@}@{@var{keyword}@}@{@var{after}@}@{@var{head}@}@{@var{ref}@}
5029 so it will be possible to write a @code{\xx} definition to take care of
5030 the output typesetting. Note that when references are not being
5031 produced, that is, neither option @option{-A} nor option @option{-r} is
5032 selected, the last parameter of each @code{\xx} call is inhibited.
5033 Option @option{-M} can be used to change @samp{xx} to another macro
5036 In this output format, some special characters, like @kbd{$}, @kbd{%},
5037 @kbd{&}, @kbd{#} and @kbd{_} are automatically protected with a
5038 backslash. Curly brackets @kbd{@{}, @kbd{@}} are protected with a
5039 backslash and a pair of dollar signs (to force mathematical mode). The
5040 backslash itself produces the sequence @code{\backslash@{@}}.
5041 Circumflex and tilde diacritical marks produce the sequence @code{^\@{ @}} and
5042 @code{~\@{ @}} respectively. Other diacriticized characters of the
5043 underlying character set produce an appropriate @TeX{} sequence as far
5044 as possible. The other non-graphical characters, like newline and tab,
5045 and all other characters which are not part of @acronym{ASCII}, are merely
5046 changed to exactly one space, with no special attempt to compress
5047 consecutive spaces. Let me know how to improve this special character
5048 processing for @TeX{}.
5053 @node Compatibility in ptx
5054 @subsection The @sc{gnu} extensions to @command{ptx}
5056 This version of @command{ptx} contains a few features which do not exist in
5057 System V @command{ptx}. These extra features are suppressed by using the
5058 @option{-G} command line option, unless overridden by other command line
5059 options. Some @sc{gnu} extensions cannot be recovered by overriding, so the
5060 simple rule is to avoid @option{-G} if you care about @sc{gnu} extensions.
5061 Here are the differences between this program and System V @command{ptx}.
5066 This program can read many input files at once, it always writes the
5067 resulting concordance on standard output. On the other hand, System V
5068 @command{ptx} reads only one file and sends the result to standard output
5069 or, if a second @var{file} parameter is given on the command, to that
5072 Having output parameters not introduced by options is a dangerous
5073 practice which @sc{gnu} avoids as far as possible. So, for using @command{ptx}
5074 portably between @sc{gnu} and System V, you should always use it with a
5075 single input file, and always expect the result on standard output. You
5076 might also want to automatically configure in a @option{-G} option to
5077 @command{ptx} calls in products using @command{ptx}, if the configurator finds
5078 that the installed @command{ptx} accepts @option{-G}.
5081 The only options available in System V @command{ptx} are options @option{-b},
5082 @option{-f}, @option{-g}, @option{-i}, @option{-o}, @option{-r}, @option{-t} and
5083 @option{-w}. All other options are @sc{gnu} extensions and are not repeated in
5084 this enumeration. Moreover, some options have a slightly different
5085 meaning when @sc{gnu} extensions are enabled, as explained below.
5088 By default, concordance output is not formatted for @command{troff} or
5089 @command{nroff}. It is rather formatted for a dumb terminal. @command{troff}
5090 or @command{nroff} output may still be selected through option @option{-O}.
5093 Unless @option{-R} option is used, the maximum reference width is
5094 subtracted from the total output line width. With @sc{gnu} extensions
5095 disabled, width of references is not taken into account in the output
5096 line width computations.
5099 All 256 bytes, even @acronym{ASCII} @sc{nul} bytes, are always read and
5100 processed from input file with no adverse effect, even if @sc{gnu} extensions
5101 are disabled. However, System V @command{ptx} does not accept 8-bit characters,
5102 a few control characters are rejected, and the tilde @kbd{~} is also rejected.
5105 Input line length is only limited by available memory, even if @sc{gnu}
5106 extensions are disabled. However, System V @command{ptx} processes only
5107 the first 200 characters in each line.
5110 The break (non-word) characters default to be every character except all
5111 letters of the underlying character set, diacriticized or not. When @sc{gnu}
5112 extensions are disabled, the break characters default to space, tab and
5116 The program makes better use of output line width. If @sc{gnu} extensions
5117 are disabled, the program rather tries to imitate System V @command{ptx},
5118 but still, there are some slight disposition glitches this program does
5119 not completely reproduce.
5122 The user can specify both an Ignore file and an Only file. This is not
5123 allowed with System V @command{ptx}.
5128 @node tsort invocation
5129 @section @command{tsort}: Topological sort
5132 @cindex topological sort
5134 @command{tsort} performs a topological sort on the given @var{file}, or
5135 standard input if no input file is given or for a @var{file} of
5136 @samp{-}. For more details and some history, see @ref{tsort background}.
5140 tsort [@var{option}] [@var{file}]
5143 @command{tsort} reads its input as pairs of strings, separated by blanks,
5144 indicating a partial ordering. The output is a total ordering that
5145 corresponds to the given partial ordering.
5159 will produce the output
5170 Consider a more realistic example.
5171 You have a large set of functions all in one file, and they may all be
5172 declared static except one. Currently that one (say @code{main}) is the
5173 first function defined in the file, and the ones it calls directly follow
5174 it, followed by those they call, etc. Let's say that you are determined
5175 to take advantage of prototypes, so you have to choose between declaring
5176 all of those functions (which means duplicating a lot of information from
5177 the definitions) and rearranging the functions so that as many as possible
5178 are defined before they are used. One way to automate the latter process
5179 is to get a list for each function of the functions it calls directly.
5180 Many programs can generate such lists. They describe a call graph.
5181 Consider the following list, in which a given line indicates that the
5182 function on the left calls the one on the right directly.
5188 tail_file pretty_name
5189 tail_file write_header
5191 tail_forever recheck
5192 tail_forever pretty_name
5193 tail_forever write_header
5194 tail_forever dump_remainder
5197 tail_lines start_lines
5198 tail_lines dump_remainder
5199 tail_lines file_lines
5200 tail_lines pipe_lines
5202 tail_bytes start_bytes
5203 tail_bytes dump_remainder
5204 tail_bytes pipe_bytes
5205 file_lines dump_remainder
5209 then you can use @command{tsort} to produce an ordering of those
5210 functions that satisfies your requirement.
5213 example$ tsort call-graph | tac
5233 @command{tsort} detects any cycles in the input and writes the first cycle
5234 encountered to standard error.
5236 Note that for a given partial ordering, generally there is no unique
5237 total ordering. In the context of the call graph above, the function
5238 @code{parse_options} may be placed anywhere in the list as long as it
5239 precedes @code{main}.
5241 The only options are @option{--help} and @option{--version}. @xref{Common
5247 * tsort background:: Where tsort came from.
5250 @node tsort background
5251 @subsection @command{tsort}: Background
5253 @command{tsort} exists because very early versions of the Unix linker processed
5254 an archive file exactly once, and in order. As @command{ld} read each object
5255 in the archive, it decided whether it was needed in the program based on
5256 whether it defined any symbols which were undefined at that point in
5259 This meant that dependencies within the archive had to be handled
5260 specially. For example, @code{scanf} probably calls @code{read}. That means
5261 that in a single pass through an archive, it was important for @code{scanf.o}
5262 to appear before read.o, because otherwise a program which calls
5263 @code{scanf} but not @code{read} might end up with an unexpected unresolved
5264 reference to @code{read}.
5266 The way to address this problem was to first generate a set of
5267 dependencies of one object file on another. This was done by a shell
5268 script called @command{lorder}. The GNU tools don't provide a version of
5269 lorder, as far as I know, but you can still find it in BSD
5272 Then you ran @command{tsort} over the @command{lorder} output, and you used the
5273 resulting sort to define the order in which you added objects to the archive.
5275 This whole procedure has been obsolete since about 1980, because
5276 Unix archives now contain a symbol table (traditionally built by
5277 @command{ranlib}, now generally built by @command{ar} itself), and the Unix
5278 linker uses the symbol table to effectively make multiple passes over
5281 Anyhow, that's where tsort came from. To solve an old problem with
5282 the way the linker handled archive files, which has since been solved
5286 @node Operating on fields
5287 @chapter Operating on fields
5290 * cut invocation:: Print selected parts of lines.
5291 * paste invocation:: Merge lines of files.
5292 * join invocation:: Join lines on a common field.
5296 @node cut invocation
5297 @section @command{cut}: Print selected parts of lines
5300 @command{cut} writes to standard output selected parts of each line of each
5301 input file, or standard input if no files are given or for a file name of
5305 cut @var{option}@dots{} [@var{file}]@dots{}
5308 In the table which follows, the @var{byte-list}, @var{character-list},
5309 and @var{field-list} are one or more numbers or ranges (two numbers
5310 separated by a dash) separated by commas. Bytes, characters, and
5311 fields are numbered starting at 1. Incomplete ranges may be
5312 given: @option{-@var{m}} means @samp{1-@var{m}}; @samp{@var{n}-} means
5313 @samp{@var{n}} through end of line or last field. The list elements
5314 can be repeated, can overlap, and can be specified in any order; but
5315 the selected input is written in the same order that it is read, and
5316 is written exactly once.
5318 The program accepts the following options. Also see @ref{Common
5323 @item -b @var{byte-list}
5324 @itemx --bytes=@var{byte-list}
5327 Select for printing only the bytes in positions listed in
5328 @var{byte-list}. Tabs and backspaces are treated like any other
5329 character; they take up 1 byte. If an output delimiter is specified,
5330 (see the description of @option{--output-delimiter}), then output that
5331 string between ranges of selected bytes.
5333 @item -c @var{character-list}
5334 @itemx --characters=@var{character-list}
5336 @opindex --characters
5337 Select for printing only the characters in positions listed in
5338 @var{character-list}. The same as @option{-b} for now, but
5339 internationalization will change that. Tabs and backspaces are
5340 treated like any other character; they take up 1 character. If an
5341 output delimiter is specified, (see the description of
5342 @option{--output-delimiter}), then output that string between ranges
5345 @item -f @var{field-list}
5346 @itemx --fields=@var{field-list}
5349 Select for printing only the fields listed in @var{field-list}.
5350 Fields are separated by a TAB character by default. Also print any
5351 line that contains no delimiter character, unless the
5352 @option{--only-delimited} (@option{-s}) option is specified.
5353 Note @command{cut} does not support specifying runs of whitespace as a
5354 delimiter, so to achieve that common functionality one can pre-process
5355 with @command{tr} like:
5357 tr -s '[:blank:]' '\t' | cut -f@dots{}
5360 @item -d @var{input_delim_byte}
5361 @itemx --delimiter=@var{input_delim_byte}
5363 @opindex --delimiter
5364 With @option{-f}, use the first byte of @var{input_delim_byte} as
5365 the input fields separator (default is TAB).
5369 Do not split multi-byte characters (no-op for now).
5372 @itemx --only-delimited
5374 @opindex --only-delimited
5375 For @option{-f}, do not print lines that do not contain the field separator
5376 character. Normally, any line without a field separator is printed verbatim.
5378 @item --output-delimiter=@var{output_delim_string}
5379 @opindex --output-delimiter
5380 With @option{-f}, output fields are separated by @var{output_delim_string}.
5381 The default with @option{-f} is to use the input delimiter.
5382 When using @option{-b} or @option{-c} to select ranges of byte or
5383 character offsets (as opposed to ranges of fields),
5384 output @var{output_delim_string} between non-overlapping
5385 ranges of selected bytes.
5388 @opindex --complement
5389 This option is a @acronym{GNU} extension.
5390 Select for printing the complement of the bytes, characters or fields
5391 selected with the @option{-b}, @option{-c} or @option{-f} options.
5392 In other words, do @emph{not} print the bytes, characters or fields
5393 specified via those options. This option is useful when you have
5394 many fields and want to print all but a few of them.
5401 @node paste invocation
5402 @section @command{paste}: Merge lines of files
5405 @cindex merging files
5407 @command{paste} writes to standard output lines consisting of sequentially
5408 corresponding lines of each given file, separated by a TAB character.
5409 Standard input is used for a file name of @samp{-} or if no input files
5431 paste [@var{option}]@dots{} [@var{file}]@dots{}
5434 The program accepts the following options. Also see @ref{Common options}.
5442 Paste the lines of one file at a time rather than one line from each
5443 file. Using the above example data:
5446 $ paste -s num2 let3
5451 @item -d @var{delim-list}
5452 @itemx --delimiters=@var{delim-list}
5454 @opindex --delimiters
5455 Consecutively use the characters in @var{delim-list} instead of
5456 TAB to separate merged lines. When @var{delim-list} is
5457 exhausted, start again at its beginning. Using the above example data:
5460 $ paste -d '%_' num2 let3 num2
5471 @node join invocation
5472 @section @command{join}: Join lines on a common field
5475 @cindex common field, joining on
5477 @command{join} writes to standard output a line for each pair of input
5478 lines that have identical join fields. Synopsis:
5481 join [@var{option}]@dots{} @var{file1} @var{file2}
5484 Either @var{file1} or @var{file2} (but not both) can be @samp{-},
5485 meaning standard input. @var{file1} and @var{file2} should be
5486 sorted on the join fields.
5489 Normally, the sort order is that of the
5490 collating sequence specified by the @env{LC_COLLATE} locale. Unless
5491 the @option{-t} option is given, the sort comparison ignores blanks at
5492 the start of the join field, as in @code{sort -b}. If the
5493 @option{--ignore-case} option is given, the sort comparison ignores
5494 the case of characters in the join field, as in @code{sort -f}.
5496 The @command{sort} and @command{join} commands should use consistent
5497 locales and options if the output of @command{sort} is fed to
5498 @command{join}. You can use a command like @samp{sort -k 1b,1} to
5499 sort a file on its default join field, but if you select a non-default
5500 locale, join field, separator, or comparison options, then you should
5501 do so consistently between @command{join} and @command{sort}.
5502 If @samp{join -t ''} is specified then the whole line is considered which
5503 matches the default operation of sort.
5505 If the input has no unpairable lines, a @acronym{GNU} extension is
5506 available; the sort order can be any order that considers two fields
5507 to be equal if and only if the sort comparison described above
5508 considers them to be equal. For example:
5525 @checkOrderOption{join}
5529 @item the join field is the first field in each line;
5530 @item fields in the input are separated by one or more blanks, with leading
5531 blanks on the line ignored;
5532 @item fields in the output are separated by a space;
5533 @item each output line consists of the join field, the remaining
5534 fields from @var{file1}, then the remaining fields from @var{file2}.
5537 The program accepts the following options. Also see @ref{Common options}.
5541 @item -a @var{file-number}
5543 Print a line for each unpairable line in file @var{file-number} (either
5544 @samp{1} or @samp{2}), in addition to the normal output.
5547 Fail with an error message if either input file is wrongly ordered.
5549 @item --nocheck-order
5550 Do not check that both input files are in sorted order. This is the default.
5552 @item -e @var{string}
5554 Replace those output fields that are missing in the input with
5559 Treat the first line of each input file as a header line. The header lines will
5560 be joined and printed as the first output line. If @option{-o} is used to
5561 specify output format, the header line will be printed according to the
5562 specified format. The header lines will not be checked for ordering even if
5563 @option{--check-order} is specified. Also if the header lines from each file
5564 do not match, the heading fields from the first file will be used.
5567 @itemx --ignore-case
5569 @opindex --ignore-case
5570 Ignore differences in case when comparing keys.
5571 With this option, the lines of the input files must be ordered in the same way.
5572 Use @samp{sort -f} to produce this ordering.
5574 @item -1 @var{field}
5576 Join on field @var{field} (a positive integer) of file 1.
5578 @item -2 @var{field}
5580 Join on field @var{field} (a positive integer) of file 2.
5582 @item -j @var{field}
5583 Equivalent to @option{-1 @var{field} -2 @var{field}}.
5585 @item -o @var{field-list}
5586 Construct each output line according to the format in @var{field-list}.
5587 Each element in @var{field-list} is either the single character @samp{0} or
5588 has the form @var{m.n} where the file number, @var{m}, is @samp{1} or
5589 @samp{2} and @var{n} is a positive field number.
5591 A field specification of @samp{0} denotes the join field.
5592 In most cases, the functionality of the @samp{0} field spec
5593 may be reproduced using the explicit @var{m.n} that corresponds
5594 to the join field. However, when printing unpairable lines
5595 (using either of the @option{-a} or @option{-v} options), there is no way
5596 to specify the join field using @var{m.n} in @var{field-list}
5597 if there are unpairable lines in both files.
5598 To give @command{join} that functionality, @acronym{POSIX} invented the @samp{0}
5599 field specification notation.
5601 The elements in @var{field-list}
5602 are separated by commas or blanks.
5603 Blank separators typically need to be quoted for the shell. For
5604 example, the commands @samp{join -o 1.2,2.2} and @samp{join -o '1.2
5605 2.2'} are equivalent.
5607 All output lines---including those printed because of any -a or -v
5608 option---are subject to the specified @var{field-list}.
5611 Use character @var{char} as the input and output field separator.
5612 Treat as significant each occurrence of @var{char} in the input file.
5613 Use @samp{sort -t @var{char}}, without the @option{-b} option of
5614 @samp{sort}, to produce this ordering. If @samp{join -t ''} is specified,
5615 the whole line is considered, matching the default operation of sort.
5616 If @samp{-t '\0'} is specified then the @acronym{ASCII} @sc{nul}
5617 character is used to delimit the fields.
5619 @item -v @var{file-number}
5620 Print a line for each unpairable line in file @var{file-number}
5621 (either @samp{1} or @samp{2}), instead of the normal output.
5628 @node Operating on characters
5629 @chapter Operating on characters
5631 @cindex operating on characters
5633 This commands operate on individual characters.
5636 * tr invocation:: Translate, squeeze, and/or delete characters.
5637 * expand invocation:: Convert tabs to spaces.
5638 * unexpand invocation:: Convert spaces to tabs.
5643 @section @command{tr}: Translate, squeeze, and/or delete characters
5650 tr [@var{option}]@dots{} @var{set1} [@var{set2}]
5653 @command{tr} copies standard input to standard output, performing
5654 one of the following operations:
5658 translate, and optionally squeeze repeated characters in the result,
5660 squeeze repeated characters,
5664 delete characters, then squeeze repeated characters from the result.
5667 The @var{set1} and (if given) @var{set2} arguments define ordered
5668 sets of characters, referred to below as @var{set1} and @var{set2}. These
5669 sets are the characters of the input that @command{tr} operates on.
5670 The @option{--complement} (@option{-c}, @option{-C}) option replaces
5672 complement (all of the characters that are not in @var{set1}).
5674 Currently @command{tr} fully supports only single-byte characters.
5675 Eventually it will support multibyte characters; when it does, the
5676 @option{-C} option will cause it to complement the set of characters,
5677 whereas @option{-c} will cause it to complement the set of values.
5678 This distinction will matter only when some values are not characters,
5679 and this is possible only in locales using multibyte encodings when
5680 the input contains encoding errors.
5682 The program accepts the @option{--help} and @option{--version}
5683 options. @xref{Common options}. Options must precede operands.
5688 * Character sets:: Specifying sets of characters.
5689 * Translating:: Changing one set of characters to another.
5690 * Squeezing:: Squeezing repeats and deleting.
5694 @node Character sets
5695 @subsection Specifying sets of characters
5697 @cindex specifying sets of characters
5699 The format of the @var{set1} and @var{set2} arguments resembles
5700 the format of regular expressions; however, they are not regular
5701 expressions, only lists of characters. Most characters simply
5702 represent themselves in these strings, but the strings can contain
5703 the shorthands listed below, for convenience. Some of them can be
5704 used only in @var{set1} or @var{set2}, as noted below.
5708 @item Backslash escapes
5709 @cindex backslash escapes
5711 The following backslash escape sequences are recognized:
5729 The character with the value given by @var{ooo}, which is 1 to 3
5735 While a backslash followed by a character not listed above is
5736 interpreted as that character, the backslash also effectively
5737 removes any special significance, so it is useful to escape
5738 @samp{[}, @samp{]}, @samp{*}, and @samp{-}.
5743 The notation @samp{@var{m}-@var{n}} expands to all of the characters
5744 from @var{m} through @var{n}, in ascending order. @var{m} should
5745 collate before @var{n}; if it doesn't, an error results. As an example,
5746 @samp{0-9} is the same as @samp{0123456789}.
5748 @sc{gnu} @command{tr} does not support the System V syntax that uses square
5749 brackets to enclose ranges. Translations specified in that format
5750 sometimes work as expected, since the brackets are often transliterated
5751 to themselves. However, they should be avoided because they sometimes
5752 behave unexpectedly. For example, @samp{tr -d '[0-9]'} deletes brackets
5755 Many historically common and even accepted uses of ranges are not
5756 portable. For example, on @acronym{EBCDIC} hosts using the @samp{A-Z}
5757 range will not do what most would expect because @samp{A} through @samp{Z}
5758 are not contiguous as they are in @acronym{ASCII}.
5759 If you can rely on a @acronym{POSIX} compliant version of @command{tr}, then
5760 the best way to work around this is to use character classes (see below).
5761 Otherwise, it is most portable (and most ugly) to enumerate the members
5764 @item Repeated characters
5765 @cindex repeated characters
5767 The notation @samp{[@var{c}*@var{n}]} in @var{set2} expands to @var{n}
5768 copies of character @var{c}. Thus, @samp{[y*6]} is the same as
5769 @samp{yyyyyy}. The notation @samp{[@var{c}*]} in @var{string2} expands
5770 to as many copies of @var{c} as are needed to make @var{set2} as long as
5771 @var{set1}. If @var{n} begins with @samp{0}, it is interpreted in
5772 octal, otherwise in decimal.
5774 @item Character classes
5775 @cindex character classes
5777 The notation @samp{[:@var{class}:]} expands to all of the characters in
5778 the (predefined) class @var{class}. The characters expand in no
5779 particular order, except for the @code{upper} and @code{lower} classes,
5780 which expand in ascending order. When the @option{--delete} (@option{-d})
5781 and @option{--squeeze-repeats} (@option{-s}) options are both given, any
5782 character class can be used in @var{set2}. Otherwise, only the
5783 character classes @code{lower} and @code{upper} are accepted in
5784 @var{set2}, and then only if the corresponding character class
5785 (@code{upper} and @code{lower}, respectively) is specified in the same
5786 relative position in @var{set1}. Doing this specifies case conversion.
5787 The class names are given below; an error results when an invalid class
5799 Horizontal whitespace.
5808 Printable characters, not including space.
5814 Printable characters, including space.
5817 Punctuation characters.
5820 Horizontal or vertical whitespace.
5829 @item Equivalence classes
5830 @cindex equivalence classes
5832 The syntax @samp{[=@var{c}=]} expands to all of the characters that are
5833 equivalent to @var{c}, in no particular order. Equivalence classes are
5834 a relatively recent invention intended to support non-English alphabets.
5835 But there seems to be no standard way to define them or determine their
5836 contents. Therefore, they are not fully implemented in @sc{gnu} @command{tr};
5837 each character's equivalence class consists only of that character,
5838 which is of no particular use.
5844 @subsection Translating
5846 @cindex translating characters
5848 @command{tr} performs translation when @var{set1} and @var{set2} are
5849 both given and the @option{--delete} (@option{-d}) option is not given.
5850 @command{tr} translates each character of its input that is in @var{set1}
5851 to the corresponding character in @var{set2}. Characters not in
5852 @var{set1} are passed through unchanged. When a character appears more
5853 than once in @var{set1} and the corresponding characters in @var{set2}
5854 are not all the same, only the final one is used. For example, these
5855 two commands are equivalent:
5862 A common use of @command{tr} is to convert lowercase characters to
5863 uppercase. This can be done in many ways. Here are three of them:
5866 tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
5868 tr '[:lower:]' '[:upper:]'
5872 But note that using ranges like @code{a-z} above is not portable.
5874 When @command{tr} is performing translation, @var{set1} and @var{set2}
5875 typically have the same length. If @var{set1} is shorter than
5876 @var{set2}, the extra characters at the end of @var{set2} are ignored.
5878 On the other hand, making @var{set1} longer than @var{set2} is not
5879 portable; @acronym{POSIX} says that the result is undefined. In this situation,
5880 BSD @command{tr} pads @var{set2} to the length of @var{set1} by repeating
5881 the last character of @var{set2} as many times as necessary. System V
5882 @command{tr} truncates @var{set1} to the length of @var{set2}.
5884 By default, @sc{gnu} @command{tr} handles this case like BSD @command{tr}.
5885 When the @option{--truncate-set1} (@option{-t}) option is given,
5886 @sc{gnu} @command{tr} handles this case like the System V @command{tr}
5887 instead. This option is ignored for operations other than translation.
5889 Acting like System V @command{tr} in this case breaks the relatively common
5893 tr -cs A-Za-z0-9 '\012'
5897 because it converts only zero bytes (the first element in the
5898 complement of @var{set1}), rather than all non-alphanumerics, to
5902 By the way, the above idiom is not portable because it uses ranges, and
5903 it assumes that the octal code for newline is 012.
5904 Assuming a @acronym{POSIX} compliant @command{tr}, here is a better way to write it:
5907 tr -cs '[:alnum:]' '[\n*]'
5912 @subsection Squeezing repeats and deleting
5914 @cindex squeezing repeat characters
5915 @cindex deleting characters
5917 When given just the @option{--delete} (@option{-d}) option, @command{tr}
5918 removes any input characters that are in @var{set1}.
5920 When given just the @option{--squeeze-repeats} (@option{-s}) option,
5921 @command{tr} replaces each input sequence of a repeated character that
5922 is in @var{set1} with a single occurrence of that character.
5924 When given both @option{--delete} and @option{--squeeze-repeats}, @command{tr}
5925 first performs any deletions using @var{set1}, then squeezes repeats
5926 from any remaining characters using @var{set2}.
5928 The @option{--squeeze-repeats} option may also be used when translating,
5929 in which case @command{tr} first performs translation, then squeezes
5930 repeats from any remaining characters using @var{set2}.
5932 Here are some examples to illustrate various combinations of options:
5937 Remove all zero bytes:
5944 Put all words on lines by themselves. This converts all
5945 non-alphanumeric characters to newlines, then squeezes each string
5946 of repeated newlines into a single newline:
5949 tr -cs '[:alnum:]' '[\n*]'
5953 Convert each sequence of repeated newlines to a single newline:
5960 Find doubled occurrences of words in a document.
5961 @c Separate the following two "the"s, so typo checkers don't complain.
5962 For example, people often write ``the @w{}the'' with the repeated words
5963 separated by a newline. The Bourne shell script below works first
5964 by converting each sequence of punctuation and blank characters to a
5965 single newline. That puts each ``word'' on a line by itself.
5966 Next it maps all uppercase characters to lower case, and finally it
5967 runs @command{uniq} with the @option{-d} option to print out only the words
5973 | tr -s '[:punct:][:blank:]' '[\n*]' \
5974 | tr '[:upper:]' '[:lower:]' \
5979 Deleting a small set of characters is usually straightforward. For example,
5980 to remove all @samp{a}s, @samp{x}s, and @samp{M}s you would do this:
5986 However, when @samp{-} is one of those characters, it can be tricky because
5987 @samp{-} has special meanings. Performing the same task as above but also
5988 removing all @samp{-} characters, we might try @code{tr -d -axM}, but
5989 that would fail because @command{tr} would try to interpret @option{-a} as
5990 a command-line option. Alternatively, we could try putting the hyphen
5991 inside the string, @code{tr -d a-xM}, but that wouldn't work either because
5992 it would make @command{tr} interpret @code{a-x} as the range of characters
5993 @samp{a}@dots{}@samp{x} rather than the three.
5994 One way to solve the problem is to put the hyphen at the end of the list
6001 Or you can use @samp{--} to terminate option processing:
6007 More generally, use the character class notation @code{[=c=]}
6008 with @samp{-} (or any other character) in place of the @samp{c}:
6014 Note how single quotes are used in the above example to protect the
6015 square brackets from interpretation by a shell.
6020 @node expand invocation
6021 @section @command{expand}: Convert tabs to spaces
6024 @cindex tabs to spaces, converting
6025 @cindex converting tabs to spaces
6027 @command{expand} writes the contents of each given @var{file}, or standard
6028 input if none are given or for a @var{file} of @samp{-}, to standard
6029 output, with tab characters converted to the appropriate number of
6033 expand [@var{option}]@dots{} [@var{file}]@dots{}
6036 By default, @command{expand} converts all tabs to spaces. It preserves
6037 backspace characters in the output; they decrement the column count for
6038 tab calculations. The default action is equivalent to @option{-t 8} (set
6039 tabs every 8 columns).
6041 The program accepts the following options. Also see @ref{Common options}.
6045 @item -t @var{tab1}[,@var{tab2}]@dots{}
6046 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6049 @cindex tab stops, setting
6050 If only one tab stop is given, set the tabs @var{tab1} spaces apart
6051 (default is 8). Otherwise, set the tabs at columns @var{tab1},
6052 @var{tab2}, @dots{} (numbered from 0), and replace any tabs beyond the
6053 last tab stop given with single spaces. Tab stops can be separated by
6054 blanks as well as by commas.
6056 For compatibility, GNU @command{expand} also accepts the obsolete
6057 option syntax, @option{-@var{t1}[,@var{t2}]@dots{}}. New scripts
6058 should use @option{-t @var{t1}[,@var{t2}]@dots{}} instead.
6064 @cindex initial tabs, converting
6065 Only convert initial tabs (those that precede all non-space or non-tab
6066 characters) on each line to spaces.
6073 @node unexpand invocation
6074 @section @command{unexpand}: Convert spaces to tabs
6078 @command{unexpand} writes the contents of each given @var{file}, or
6079 standard input if none are given or for a @var{file} of @samp{-}, to
6080 standard output, converting blanks at the beginning of each line into
6081 as many tab characters as needed. In the default @acronym{POSIX}
6082 locale, a @dfn{blank} is a space or a tab; other locales may specify
6083 additional blank characters. Synopsis:
6086 unexpand [@var{option}]@dots{} [@var{file}]@dots{}
6089 By default, @command{unexpand} converts only initial blanks (those
6090 that precede all non-blank characters) on each line. It
6091 preserves backspace characters in the output; they decrement the column
6092 count for tab calculations. By default, tabs are set at every 8th
6095 The program accepts the following options. Also see @ref{Common options}.
6099 @item -t @var{tab1}[,@var{tab2}]@dots{}
6100 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6103 If only one tab stop is given, set the tabs @var{tab1} columns apart
6104 instead of the default 8. Otherwise, set the tabs at columns
6105 @var{tab1}, @var{tab2}, @dots{} (numbered from 0), and leave blanks
6106 beyond the tab stops given unchanged. Tab stops can be separated by
6107 blanks as well as by commas. This option implies the @option{-a} option.
6109 For compatibility, GNU @command{unexpand} supports the obsolete option syntax,
6110 @option{-@var{tab1}[,@var{tab2}]@dots{}}, where tab stops must be
6111 separated by commas. (Unlike @option{-t}, this obsolete option does
6112 not imply @option{-a}.) New scripts should use @option{--first-only -t
6113 @var{tab1}[,@var{tab2}]@dots{}} instead.
6119 Also convert all sequences of two or more blanks just before a tab stop,
6120 even if they occur after non-blank characters in a line.
6127 @node Directory listing
6128 @chapter Directory listing
6130 This chapter describes the @command{ls} command and its variants @command{dir}
6131 and @command{vdir}, which list information about files.
6134 * ls invocation:: List directory contents.
6135 * dir invocation:: Briefly ls.
6136 * vdir invocation:: Verbosely ls.
6137 * dircolors invocation:: Color setup for ls, etc.
6142 @section @command{ls}: List directory contents
6145 @cindex directory listing
6147 The @command{ls} program lists information about files (of any type,
6148 including directories). Options and file arguments can be intermixed
6149 arbitrarily, as usual.
6151 For non-option command-line arguments that are directories, by default
6152 @command{ls} lists the contents of directories, not recursively, and
6153 omitting files with names beginning with @samp{.}. For other non-option
6154 arguments, by default @command{ls} lists just the file name. If no
6155 non-option argument is specified, @command{ls} operates on the current
6156 directory, acting as if it had been invoked with a single argument of @samp{.}.
6159 By default, the output is sorted alphabetically, according to the locale
6160 settings in effect.@footnote{If you use a non-@acronym{POSIX}
6161 locale (e.g., by setting @env{LC_ALL} to @samp{en_US}), then @command{ls} may
6162 produce output that is sorted differently than you're accustomed to.
6163 In that case, set the @env{LC_ALL} environment variable to @samp{C}.}
6164 If standard output is
6165 a terminal, the output is in columns (sorted vertically) and control
6166 characters are output as question marks; otherwise, the output is listed
6167 one per line and control characters are output as-is.
6169 Because @command{ls} is such a fundamental program, it has accumulated many
6170 options over the years. They are described in the subsections below;
6171 within each section, options are listed alphabetically (ignoring case).
6172 The division of options into the subsections is not absolute, since some
6173 options affect more than one aspect of @command{ls}'s operation.
6175 @cindex exit status of @command{ls}
6180 1 minor problems (e.g., failure to access a file or directory not
6181 specified as a command line argument. This happens when listing a
6182 directory in which entries are actively being removed or renamed.)
6183 2 serious trouble (e.g., memory exhausted, invalid option, failure
6184 to access a file or directory specified as a command line argument
6185 or a directory loop)
6188 Also see @ref{Common options}.
6191 * Which files are listed::
6192 * What information is listed::
6193 * Sorting the output::
6194 * Details about version sort::
6195 * General output formatting::
6196 * Formatting file timestamps::
6197 * Formatting the file names::
6201 @node Which files are listed
6202 @subsection Which files are listed
6204 These options determine which files @command{ls} lists information for.
6205 By default, @command{ls} lists files and the contents of any
6206 directories on the command line, except that in directories it ignores
6207 files whose names start with @samp{.}.
6215 In directories, do not ignore file names that start with @samp{.}.
6220 @opindex --almost-all
6221 In directories, do not ignore all file names that start with @samp{.};
6222 ignore only @file{.} and @file{..}. The @option{--all} (@option{-a})
6223 option overrides this option.
6226 @itemx --ignore-backups
6228 @opindex --ignore-backups
6229 @cindex backup files, ignoring
6230 In directories, ignore files that end with @samp{~}. This option is
6231 equivalent to @samp{--ignore='*~' --ignore='.*~'}.
6236 @opindex --directory
6237 List just the names of directories, as with other types of files, rather
6238 than listing their contents.
6239 @c The following sentence is the same as the one for -F.
6240 Do not follow symbolic links listed on the
6241 command line unless the @option{--dereference-command-line} (@option{-H}),
6242 @option{--dereference} (@option{-L}), or
6243 @option{--dereference-command-line-symlink-to-dir} options are specified.
6246 @itemx --dereference-command-line
6248 @opindex --dereference-command-line
6249 @cindex symbolic links, dereferencing
6250 If a command line argument specifies a symbolic link, show information
6251 for the file the link references rather than for the link itself.
6253 @itemx --dereference-command-line-symlink-to-dir
6254 @opindex --dereference-command-line-symlink-to-dir
6255 @cindex symbolic links, dereferencing
6256 Do not dereference symbolic links, with one exception:
6257 if a command line argument specifies a symbolic link that refers to
6258 a directory, show information for that directory rather than for the
6260 This is the default behavior when no other dereferencing-related
6261 option has been specified (@option{--classify} (@option{-F}),
6262 @option{--directory} (@option{-d}),
6264 @option{--dereference} (@option{-L}), or
6265 @option{--dereference-command-line} (@option{-H})).
6267 @item --group-directories-first
6268 @opindex --group-directories-first
6269 Group all the directories before the files and then sort the
6270 directories and the files separately using the selected sort key
6271 (see --sort option).
6272 That is, this option specifies a primary sort key,
6273 and the --sort option specifies a secondary key.
6274 However, any use of @option{--sort=none}
6275 (@option{-U}) disables this option altogether.
6277 @item --hide=PATTERN
6278 @opindex --hide=@var{pattern}
6279 In directories, ignore files whose names match the shell pattern
6280 @var{pattern}, unless the @option{--all} (@option{-a}) or
6281 @option{--almost-all} (@option{-A}) is also given. This
6282 option acts like @option{--ignore=@var{pattern}} except that it has no
6283 effect if @option{--all} (@option{-a}) or @option{--almost-all}
6284 (@option{-A}) is also given.
6286 This option can be useful in shell aliases. For example, if
6287 @command{lx} is an alias for @samp{ls --hide='*~'} and @command{ly} is
6288 an alias for @samp{ls --ignore='*~'}, then the command @samp{lx -A}
6289 lists the file @file{README~} even though @samp{ly -A} would not.
6291 @item -I @var{pattern}
6292 @itemx --ignore=@var{pattern}
6294 @opindex --ignore=@var{pattern}
6295 In directories, ignore files whose names match the shell pattern
6296 (not regular expression) @var{pattern}. As
6297 in the shell, an initial @samp{.} in a file name does not match a
6298 wildcard at the start of @var{pattern}. Sometimes it is useful
6299 to give this option several times. For example,
6302 $ ls --ignore='.??*' --ignore='.[^.]' --ignore='#*'
6305 The first option ignores names of length 3 or more that start with @samp{.},
6306 the second ignores all two-character names that start with @samp{.}
6307 except @samp{..}, and the third ignores names that start with @samp{#}.
6310 @itemx --dereference
6312 @opindex --dereference
6313 @cindex symbolic links, dereferencing
6314 When showing file information for a symbolic link, show information
6315 for the file the link references rather than the link itself.
6316 However, even with this option, @command{ls} still prints the name
6317 of the link itself, not the name of the file that the link points to.
6322 @opindex --recursive
6323 @cindex recursive directory listing
6324 @cindex directory listing, recursive
6325 List the contents of all directories recursively.
6330 @node What information is listed
6331 @subsection What information is listed
6333 These options affect the information that @command{ls} displays. By
6334 default, only file names are shown.
6340 @cindex hurd, author, printing
6341 List each file's author when producing long format directory listings.
6342 In GNU/Hurd, file authors can differ from their owners, but in other
6343 operating systems the two are the same.
6349 @cindex dired Emacs mode support
6350 With the long listing (@option{-l}) format, print an additional line after
6354 //DIRED// @var{beg1} @var{end1} @var{beg2} @var{end2} @dots{}
6358 The @var{begn} and @var{endn} are unsigned integers that record the
6359 byte position of the beginning and end of each file name in the output.
6360 This makes it easy for Emacs to find the names, even when they contain
6361 unusual characters such as space or newline, without fancy searching.
6363 If directories are being listed recursively (@option{-R}), output a similar
6364 line with offsets for each subdirectory name:
6367 //SUBDIRED// @var{beg1} @var{end1} @dots{}
6370 Finally, output a line of the form:
6373 //DIRED-OPTIONS// --quoting-style=@var{word}
6377 where @var{word} is the quoting style (@pxref{Formatting the file names}).
6379 Here is an actual example:
6382 $ mkdir -p a/sub/deeper a/sub2
6384 $ touch a/sub/deeper/file
6385 $ ls -gloRF --dired a
6388 -rw-r--r-- 1 0 Jun 10 12:27 f1
6389 -rw-r--r-- 1 0 Jun 10 12:27 f2
6390 drwxr-xr-x 3 4096 Jun 10 12:27 sub/
6391 drwxr-xr-x 2 4096 Jun 10 12:27 sub2/
6395 drwxr-xr-x 2 4096 Jun 10 12:27 deeper/
6399 -rw-r--r-- 1 0 Jun 10 12:27 file
6403 //DIRED// 48 50 84 86 120 123 158 162 217 223 282 286
6404 //SUBDIRED// 2 3 167 172 228 240 290 296
6405 //DIRED-OPTIONS// --quoting-style=literal
6408 Note that the pairs of offsets on the @samp{//DIRED//} line above delimit
6409 these names: @file{f1}, @file{f2}, @file{sub}, @file{sub2}, @file{deeper},
6411 The offsets on the @samp{//SUBDIRED//} line delimit the following
6412 directory names: @file{a}, @file{a/sub}, @file{a/sub/deeper}, @file{a/sub2}.
6414 Here is an example of how to extract the fifth entry name, @samp{deeper},
6415 corresponding to the pair of offsets, 222 and 228:
6418 $ ls -gloRF --dired a > out
6419 $ dd bs=1 skip=222 count=6 < out 2>/dev/null; echo
6423 Note that although the listing above includes a trailing slash
6424 for the @samp{deeper} entry, the offsets select the name without
6425 the trailing slash. However, if you invoke @command{ls} with @option{--dired}
6426 along with an option like @option{--escape} (aka @option{-b}) and operate
6427 on a file whose name contains special characters, notice that the backslash
6432 $ ls -blog --dired 'a b'
6433 -rw-r--r-- 1 0 Jun 10 12:28 a\ b
6435 //DIRED-OPTIONS// --quoting-style=escape
6438 If you use a quoting style that adds quote marks
6439 (e.g., @option{--quoting-style=c}), then the offsets include the quote marks.
6440 So beware that the user may select the quoting style via the environment
6441 variable @env{QUOTING_STYLE}. Hence, applications using @option{--dired}
6442 should either specify an explicit @option{--quoting-style=literal} option
6443 (aka @option{-N} or @option{--literal}) on the command line, or else be
6444 prepared to parse the escaped names.
6447 @opindex --full-time
6448 Produce long format directory listings, and list times in full. It is
6449 equivalent to using @option{--format=long} with
6450 @option{--time-style=full-iso} (@pxref{Formatting file timestamps}).
6454 Produce long format directory listings, but don't display owner information.
6460 Inhibit display of group information in a long format directory listing.
6461 (This is the default in some non-@sc{gnu} versions of @command{ls}, so we
6462 provide this option for compatibility.)
6470 @cindex inode number, printing
6471 Print the inode number (also called the file serial number and index
6472 number) of each file to the left of the file name. (This number
6473 uniquely identifies each file within a particular file system.)
6476 @itemx --format=long
6477 @itemx --format=verbose
6480 @opindex long ls @r{format}
6481 @opindex verbose ls @r{format}
6482 In addition to the name of each file, print the file type, file mode bits,
6483 number of hard links, owner name, group name, size, and
6484 timestamp (@pxref{Formatting file timestamps}), normally
6485 the modification time. Print question marks for information that
6486 cannot be determined.
6488 Normally the size is printed as a byte count without punctuation, but
6489 this can be overridden (@pxref{Block size}). For example, @option{-h}
6490 prints an abbreviated, human-readable count, and
6491 @samp{--block-size="'1"} prints a byte count with the thousands
6492 separator of the current locale.
6494 For each directory that is listed, preface the files with a line
6495 @samp{total @var{blocks}}, where @var{blocks} is the total disk allocation
6496 for all files in that directory. The block size currently defaults to 1024
6497 bytes, but this can be overridden (@pxref{Block size}).
6498 The @var{blocks} computed counts each hard link separately;
6499 this is arguably a deficiency.
6501 The file type is one of the following characters:
6503 @c The commented-out entries are ones we're not sure about.
6511 character special file
6513 high performance (``contiguous data'') file
6517 door (Solaris 2.5 and up)
6519 @c semaphore, if this is a distinct file type
6523 @c multiplexed file (7th edition Unix; obsolete)
6525 off-line (``migrated'') file (Cray DMF)
6527 network special file (HP-UX)
6531 port (Solaris 10 and up)
6533 @c message queue, if this is a distinct file type
6537 @c shared memory object, if this is a distinct file type
6539 @c typed memory object, if this is a distinct file type
6541 @c whiteout (4.4BSD; not implemented)
6543 some other file type
6546 @cindex permissions, output by @command{ls}
6547 The file mode bits listed are similar to symbolic mode specifications
6548 (@pxref{Symbolic Modes}). But @command{ls} combines multiple bits into the
6549 third character of each set of permissions as follows:
6553 If the set-user-ID or set-group-ID bit and the corresponding executable bit
6557 If the set-user-ID or set-group-ID bit is set but the corresponding
6558 executable bit is not set.
6561 If the restricted deletion flag or sticky bit, and the
6562 other-executable bit, are both set. The restricted deletion flag is
6563 another name for the sticky bit. @xref{Mode Structure}.
6566 If the restricted deletion flag or sticky bit is set but the
6567 other-executable bit is not set.
6570 If the executable bit is set and none of the above apply.
6576 Following the file mode bits is a single character that specifies
6577 whether an alternate access method such as an access control list
6578 applies to the file. When the character following the file mode bits is a
6579 space, there is no alternate access method. When it is a printing
6580 character, then there is such a method.
6582 GNU @command{ls} uses a @samp{.} character to indicate a file
6583 with an SELinux security context, but no other alternate access method.
6585 A file with any other combination of alternate access methods
6586 is marked with a @samp{+} character.
6589 @itemx --numeric-uid-gid
6591 @opindex --numeric-uid-gid
6592 @cindex numeric uid and gid
6593 @cindex numeric user and group IDs
6594 Produce long format directory listings, but
6595 display numeric user and group IDs instead of the owner and group names.
6599 Produce long format directory listings, but don't display group information.
6600 It is equivalent to using @option{--format=long} with @option{--no-group} .
6606 @cindex disk allocation
6607 @cindex size of files, reporting
6608 Print the disk allocation of each file to the left of the file name.
6609 This is the amount of disk space used by the file, which is usually a
6610 bit more than the file's size, but it can be less if the file has holes.
6612 Normally the disk allocation is printed in units of
6613 1024 bytes, but this can be overridden (@pxref{Block size}).
6615 @cindex NFS mounts from BSD to HP-UX
6616 For files that are NFS-mounted from an HP-UX system to a BSD system,
6617 this option reports sizes that are half the correct values. On HP-UX
6618 systems, it reports sizes that are twice the correct values for files
6619 that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX;
6620 it also affects the HP-UX @command{ls} program.
6629 @cindex security context
6630 Display the SELinux security context or @samp{?} if none is found.
6631 When used with the @option{-l} option, print the security context
6632 to the left of the size column.
6637 @node Sorting the output
6638 @subsection Sorting the output
6640 @cindex sorting @command{ls} output
6641 These options change the order in which @command{ls} sorts the information
6642 it outputs. By default, sorting is done by character code
6643 (e.g., @acronym{ASCII} order).
6649 @itemx --time=status
6652 @opindex ctime@r{, printing or sorting by}
6653 @opindex status time@r{, printing or sorting by}
6654 @opindex use time@r{, printing or sorting files by}
6655 If the long listing format (e.g., @option{-l}, @option{-o}) is being used,
6656 print the status change time (the @samp{ctime} in the inode) instead of
6657 the modification time.
6658 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6659 or when not using a long listing format,
6660 sort according to the status change time.
6664 @cindex unsorted directory listing
6665 @cindex directory order, listing by
6666 Primarily, like @option{-U}---do not sort; list the files in whatever
6667 order they are stored in the directory. But also enable @option{-a} (list
6668 all files) and disable @option{-l}, @option{--color}, and @option{-s} (if they
6669 were specified before the @option{-f}).
6675 @cindex reverse sorting
6676 Reverse whatever the sorting method is---e.g., list files in reverse
6677 alphabetical order, youngest first, smallest first, or whatever.
6683 @opindex size of files@r{, sorting files by}
6684 Sort by file size, largest first.
6690 @opindex modification time@r{, sorting files by}
6691 Sort by modification time (the @samp{mtime} in the inode), newest first.
6695 @itemx --time=access
6699 @opindex use time@r{, printing or sorting files by}
6700 @opindex atime@r{, printing or sorting files by}
6701 @opindex access time@r{, printing or sorting files by}
6702 If the long listing format (e.g., @option{--format=long}) is being used,
6703 print the last access time (the @samp{atime} in the inode).
6704 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6705 or when not using a long listing format, sort according to the access time.
6711 @opindex none@r{, sorting option for @command{ls}}
6712 Do not sort; list the files in whatever order they are
6713 stored in the directory. (Do not do any of the other unrelated things
6714 that @option{-f} does.) This is especially useful when listing very large
6715 directories, since not doing any sorting can be noticeably faster.
6718 @itemx --sort=version
6721 @opindex version@r{, sorting option for @command{ls}}
6722 Sort by version name and number, lowest first. It behaves like a default
6723 sort, except that each sequence of decimal digits is treated numerically
6724 as an index/version number. (@xref{Details about version sort}.)
6727 @itemx --sort=extension
6730 @opindex extension@r{, sorting files by}
6731 Sort directory contents alphabetically by file extension (characters
6732 after the last @samp{.}); files with no extension are sorted first.
6737 @node Details about version sort
6738 @subsection Details about version sort
6740 Version sorting handles the fact that file names frequently include indices or
6741 version numbers. Standard sorting usually does not produce the order that one
6742 expects because comparisons are made on a character-by-character basis.
6743 Version sorting is especially useful when browsing directories that contain
6744 many files with indices/version numbers in their names:
6748 abc.zml-1.gz abc.zml-1.gz
6749 abc.zml-12.gz abc.zml-2.gz
6750 abc.zml-2.gz abc.zml-12.gz
6753 Version-sorted strings are compared such that if @var{ver1} and @var{ver2}
6754 are version numbers and @var{prefix} and @var{suffix} (@var{suffix} matching
6755 the regular expression @samp{(\.[A-Za-z~][A-Za-z0-9~]*)*}) are strings then
6756 @var{ver1} < @var{ver2} implies that the name composed of
6757 ``@var{prefix} @var{ver1} @var{suffix}'' sorts before
6758 ``@var{prefix} @var{ver2} @var{suffix}''.
6760 Note also that leading zeros of numeric parts are ignored:
6764 abc-1.007.tgz abc-1.01a.tgz
6765 abc-1.012b.tgz abc-1.007.tgz
6766 abc-1.01a.tgz abc-1.012b.tgz
6769 This functionality is implemented using gnulib's @code{filevercmp} function,
6770 which has some caveats worth noting.
6773 @item @env{LC_COLLATE} is ignored, which means @samp{ls -v} and @samp{sort -V}
6774 will sort non-numeric prefixes as if the @env{LC_COLLATE} locale category
6775 was set to @samp{C}.
6776 @item Some suffixes will not be matched by the regular
6777 expression mentioned above. Consequently these examples may
6778 not sort as you expect:
6786 abc-1.2.3.4.x86_64.rpm
6787 abc-1.2.3.x86_64.rpm
6791 @node General output formatting
6792 @subsection General output formatting
6794 These options affect the appearance of the overall output.
6799 @itemx --format=single-column
6802 @opindex single-column @r{output of files}
6803 List one file per line. This is the default for @command{ls} when standard
6804 output is not a terminal.
6807 @itemx --format=vertical
6810 @opindex vertical @r{sorted files in columns}
6811 List files in columns, sorted vertically. This is the default for
6812 @command{ls} if standard output is a terminal. It is always the default
6813 for the @command{dir} program.
6814 @sc{gnu} @command{ls} uses variable width columns to display as many files as
6815 possible in the fewest lines.
6817 @item --color [=@var{when}]
6819 @cindex color, distinguishing file types with
6820 Specify whether to use color for distinguishing file types. @var{when}
6821 may be omitted, or one of:
6824 @vindex none @r{color option}
6825 - Do not use color at all. This is the default.
6827 @vindex auto @r{color option}
6828 @cindex terminal, using color iff
6829 - Only use color if standard output is a terminal.
6831 @vindex always @r{color option}
6834 Specifying @option{--color} and no @var{when} is equivalent to
6835 @option{--color=always}.
6836 Piping a colorized listing through a pager like @command{more} or
6837 @command{less} usually produces unreadable results. However, using
6838 @code{more -f} does seem to work.
6842 @itemx --indicator-style=classify
6845 @opindex --indicator-style
6846 @cindex file type and executables, marking
6847 @cindex executables and file type, marking
6848 Append a character to each file name indicating the file type. Also,
6849 for regular files that are executable, append @samp{*}. The file type
6850 indicators are @samp{/} for directories, @samp{@@} for symbolic links,
6851 @samp{|} for FIFOs, @samp{=} for sockets, @samp{>} for doors,
6852 and nothing for regular files.
6853 @c The following sentence is the same as the one for -d.
6854 Do not follow symbolic links listed on the
6855 command line unless the @option{--dereference-command-line} (@option{-H}),
6856 @option{--dereference} (@option{-L}), or
6857 @option{--dereference-command-line-symlink-to-dir} options are specified.
6860 @itemx --indicator-style=file-type
6861 @opindex --file-type
6862 @opindex --indicator-style
6863 @cindex file type, marking
6864 Append a character to each file name indicating the file type. This is
6865 like @option{-F}, except that executables are not marked.
6867 @item --indicator-style=@var{word}
6868 @opindex --indicator-style
6869 Append a character indicator with style @var{word} to entry names,
6874 Do not append any character indicator; this is the default.
6876 Append @samp{/} for directories. This is the same as the @option{-p}
6879 Append @samp{/} for directories, @samp{@@} for symbolic links, @samp{|}
6880 for FIFOs, @samp{=} for sockets, and nothing for regular files. This is
6881 the same as the @option{--file-type} option.
6883 Append @samp{*} for executable regular files, otherwise behave as for
6884 @samp{file-type}. This is the same as the @option{-F} or
6885 @option{--classify} option.
6890 Print file sizes in 1024-byte blocks, overriding the default block
6891 size (@pxref{Block size}).
6892 This option is equivalent to @option{--block-size=1K}.
6895 @itemx --format=commas
6898 @opindex commas@r{, outputting between files}
6899 List files horizontally, with as many as will fit on each line,
6900 separated by @samp{, } (a comma and a space).
6903 @itemx --indicator-style=slash
6905 @opindex --indicator-style
6906 @cindex file type, marking
6907 Append a @samp{/} to directory names.
6910 @itemx --format=across
6911 @itemx --format=horizontal
6914 @opindex across@r{, listing files}
6915 @opindex horizontal@r{, listing files}
6916 List the files in columns, sorted horizontally.
6919 @itemx --tabsize=@var{cols}
6922 Assume that each tab stop is @var{cols} columns wide. The default is 8.
6923 @command{ls} uses tabs where possible in the output, for efficiency. If
6924 @var{cols} is zero, do not use tabs at all.
6926 @c FIXME: remove in 2009, if Apple Terminal has been fixed for long enough.
6927 Some terminal emulators (at least Apple Terminal 1.5 (133) from Mac OS X 10.4.8)
6928 do not properly align columns to the right of a TAB following a
6929 non-@acronym{ASCII} byte. If you use such a terminal emulator, use the
6930 @option{-T0} option or put @code{TABSIZE=0} in your environment to tell
6931 @command{ls} to align using spaces, not tabs.
6934 @itemx --width=@var{cols}
6938 Assume the screen is @var{cols} columns wide. The default is taken
6939 from the terminal settings if possible; otherwise the environment
6940 variable @env{COLUMNS} is used if it is set; otherwise the default
6946 @node Formatting file timestamps
6947 @subsection Formatting file timestamps
6949 By default, file timestamps are listed in abbreviated form, using
6950 a date like @samp{Mar 30@ @ 2002} for non-recent timestamps, and a
6951 date-without-year and time like @samp{Mar 30 23:45} for recent timestamps.
6952 This format can change depending on the current locale as detailed below.
6954 A timestamp is considered to be @dfn{recent} if it is less than six
6955 months old, and is not dated in the future. If a timestamp dated
6956 today is not listed in recent form, the timestamp is in the future,
6957 which means you probably have clock skew problems which may break
6958 programs like @command{make} that rely on file timestamps.
6961 Time stamps are listed according to the time zone rules specified by
6962 the @env{TZ} environment variable, or by the system default rules if
6963 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
6964 with @env{TZ}, libc, The GNU C Library Reference Manual}.
6966 The following option changes how file timestamps are printed.
6969 @item --time-style=@var{style}
6970 @opindex --time-style
6972 List timestamps in style @var{style}. The @var{style} should
6973 be one of the following:
6978 List timestamps using @var{format}, where @var{format} is interpreted
6979 like the format argument of @command{date} (@pxref{date invocation}).
6980 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
6981 @command{ls} to list timestamps like @samp{2002-03-30 23:45:56}. As
6982 with @command{date}, @var{format}'s interpretation is affected by the
6983 @env{LC_TIME} locale category.
6985 If @var{format} contains two format strings separated by a newline,
6986 the former is used for non-recent files and the latter for recent
6987 files; if you want output columns to line up, you may need to insert
6988 spaces in one of the two formats.
6991 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
6992 format with nanosecond precision, e.g., @samp{2002-03-30
6993 23:45:56.477817180 -0700}. This style is equivalent to
6994 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
6996 This is useful because the time output includes all the information that
6997 is available from the operating system. For example, this can help
6998 explain @command{make}'s behavior, since @acronym{GNU} @command{make}
6999 uses the full timestamp to determine whether a file is out of date.
7002 List @acronym{ISO} 8601 date and time in minutes, e.g.,
7003 @samp{2002-03-30 23:45}. These timestamps are shorter than
7004 @samp{full-iso} timestamps, and are usually good enough for everyday
7005 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
7008 List @acronym{ISO} 8601 dates for non-recent timestamps (e.g.,
7009 @samp{2002-03-30@ }), and @acronym{ISO} 8601 month, day, hour, and
7010 minute for recent timestamps (e.g., @samp{03-30 23:45}). These
7011 timestamps are uglier than @samp{long-iso} timestamps, but they carry
7012 nearly the same information in a smaller space and their brevity helps
7013 @command{ls} output fit within traditional 80-column output lines.
7014 The following two @command{ls} invocations are equivalent:
7019 ls -l --time-style="+%Y-%m-%d $newline%m-%d %H:%M"
7020 ls -l --time-style="iso"
7025 List timestamps in a locale-dependent form. For example, a Finnish
7026 locale might list non-recent timestamps like @samp{maalis 30@ @ 2002}
7027 and recent timestamps like @samp{maalis 30 23:45}. Locale-dependent
7028 timestamps typically consume more space than @samp{iso} timestamps and
7029 are harder for programs to parse because locale conventions vary so
7030 widely, but they are easier for many people to read.
7032 The @env{LC_TIME} locale category specifies the timestamp format. The
7033 default @acronym{POSIX} locale uses timestamps like @samp{Mar 30@
7034 @ 2002} and @samp{Mar 30 23:45}; in this locale, the following two
7035 @command{ls} invocations are equivalent:
7040 ls -l --time-style="+%b %e %Y$newline%b %e %H:%M"
7041 ls -l --time-style="locale"
7044 Other locales behave differently. For example, in a German locale,
7045 @option{--time-style="locale"} might be equivalent to
7046 @option{--time-style="+%e. %b %Y $newline%e. %b %H:%M"}
7047 and might generate timestamps like @samp{30. M@"ar 2002@ } and
7048 @samp{30. M@"ar 23:45}.
7050 @item posix-@var{style}
7052 List @acronym{POSIX}-locale timestamps if the @env{LC_TIME} locale
7053 category is @acronym{POSIX}, @var{style} timestamps otherwise. For
7054 example, the @samp{posix-long-iso} style lists
7055 timestamps like @samp{Mar 30@ @ 2002} and @samp{Mar 30 23:45} when in
7056 the @acronym{POSIX} locale, and like @samp{2002-03-30 23:45} otherwise.
7061 You can specify the default value of the @option{--time-style} option
7062 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
7063 the default style is @samp{locale}. @acronym{GNU} Emacs 21.3 and
7064 later use the @option{--dired} option and therefore can parse any date
7065 format, but if you are using Emacs 21.1 or 21.2 and specify a
7066 non-@acronym{POSIX} locale you may need to set
7067 @samp{TIME_STYLE="posix-long-iso"}.
7069 To avoid certain denial-of-service attacks, timestamps that would be
7070 longer than 1000 bytes may be treated as errors.
7073 @node Formatting the file names
7074 @subsection Formatting the file names
7076 These options change how file names themselves are printed.
7082 @itemx --quoting-style=escape
7085 @opindex --quoting-style
7086 @cindex backslash sequences for file names
7087 Quote nongraphic characters in file names using alphabetic and octal
7088 backslash sequences like those used in C.
7092 @itemx --quoting-style=literal
7095 @opindex --quoting-style
7096 Do not quote file names. However, with @command{ls} nongraphic
7097 characters are still printed as question marks if the output is a
7098 terminal and you do not specify the @option{--show-control-chars}
7102 @itemx --hide-control-chars
7104 @opindex --hide-control-chars
7105 Print question marks instead of nongraphic characters in file names.
7106 This is the default if the output is a terminal and the program is
7111 @itemx --quoting-style=c
7113 @opindex --quote-name
7114 @opindex --quoting-style
7115 Enclose file names in double quotes and quote nongraphic characters as
7118 @item --quoting-style=@var{word}
7119 @opindex --quoting-style
7120 @cindex quoting style
7121 Use style @var{word} to quote file names and other strings that may
7122 contain arbitrary characters. The @var{word} should
7123 be one of the following:
7127 Output strings as-is; this is the same as the @option{-N} or
7128 @option{--literal} option.
7130 Quote strings for the shell if they contain shell metacharacters or would
7131 cause ambiguous output.
7132 The quoting is suitable for @acronym{POSIX}-compatible shells like
7133 @command{bash}, but it does not always work for incompatible shells
7136 Quote strings for the shell, even if they would normally not require quoting.
7138 Quote strings as for C character string literals, including the
7139 surrounding double-quote characters; this is the same as the
7140 @option{-Q} or @option{--quote-name} option.
7142 Quote strings as for C character string literals, except omit the
7143 surrounding double-quote
7144 characters; this is the same as the @option{-b} or @option{--escape} option.
7146 Quote strings as for C character string literals, except use
7147 surrounding quotation marks appropriate for the
7150 @c Use @t instead of @samp to avoid duplicate quoting in some output styles.
7151 Quote strings as for C character string literals, except use
7152 surrounding quotation marks appropriate for the locale, and quote
7153 @t{`like this'} instead of @t{"like
7154 this"} in the default C locale. This looks nicer on many displays.
7157 You can specify the default value of the @option{--quoting-style} option
7158 with the environment variable @env{QUOTING_STYLE}. If that environment
7159 variable is not set, the default value is @samp{literal}, but this
7160 default may change to @samp{shell} in a future version of this package.
7162 @item --show-control-chars
7163 @opindex --show-control-chars
7164 Print nongraphic characters as-is in file names.
7165 This is the default unless the output is a terminal and the program is
7171 @node dir invocation
7172 @section @command{dir}: Briefly list directory contents
7175 @cindex directory listing, brief
7177 @command{dir} is equivalent to @code{ls -C
7178 -b}; that is, by default files are listed in columns, sorted vertically,
7179 and special characters are represented by backslash escape sequences.
7181 @xref{ls invocation, @command{ls}}.
7184 @node vdir invocation
7185 @section @command{vdir}: Verbosely list directory contents
7188 @cindex directory listing, verbose
7190 @command{vdir} is equivalent to @code{ls -l
7191 -b}; that is, by default files are listed in long format and special
7192 characters are represented by backslash escape sequences.
7194 @node dircolors invocation
7195 @section @command{dircolors}: Color setup for @command{ls}
7199 @cindex setup for color
7201 @command{dircolors} outputs a sequence of shell commands to set up the
7202 terminal for color output from @command{ls} (and @command{dir}, etc.).
7206 eval "`dircolors [@var{option}]@dots{} [@var{file}]`"
7209 If @var{file} is specified, @command{dircolors} reads it to determine which
7210 colors to use for which file types and extensions. Otherwise, a
7211 precompiled database is used. For details on the format of these files,
7212 run @samp{dircolors --print-database}.
7214 To make @command{dircolors} read a @file{~/.dircolors} file if it
7215 exists, you can put the following lines in your @file{~/.bashrc} (or
7216 adapt them to your favorite shell):
7220 test -r $d && eval "$(dircolors $d)"
7224 @vindex SHELL @r{environment variable, and color}
7225 The output is a shell command to set the @env{LS_COLORS} environment
7226 variable. You can specify the shell syntax to use on the command line,
7227 or @command{dircolors} will guess it from the value of the @env{SHELL}
7228 environment variable.
7230 The program accepts the following options. Also see @ref{Common options}.
7235 @itemx --bourne-shell
7238 @opindex --bourne-shell
7239 @cindex Bourne shell syntax for color setup
7240 @cindex @command{sh} syntax for color setup
7241 Output Bourne shell commands. This is the default if the @env{SHELL}
7242 environment variable is set and does not end with @samp{csh} or
7251 @cindex C shell syntax for color setup
7252 @cindex @command{csh} syntax for color setup
7253 Output C shell commands. This is the default if @code{SHELL} ends with
7254 @command{csh} or @command{tcsh}.
7257 @itemx --print-database
7259 @opindex --print-database
7260 @cindex color database, printing
7261 @cindex database for color setup, printing
7262 @cindex printing color database
7263 Print the (compiled-in) default color configuration database. This
7264 output is itself a valid configuration file, and is fairly descriptive
7265 of the possibilities.
7272 @node Basic operations
7273 @chapter Basic operations
7275 @cindex manipulating files
7277 This chapter describes the commands for basic file manipulation:
7278 copying, moving (renaming), and deleting (removing).
7281 * cp invocation:: Copy files.
7282 * dd invocation:: Convert and copy a file.
7283 * install invocation:: Copy files and set attributes.
7284 * mv invocation:: Move (rename) files.
7285 * rm invocation:: Remove files or directories.
7286 * shred invocation:: Remove files more securely.
7291 @section @command{cp}: Copy files and directories
7294 @cindex copying files and directories
7295 @cindex files, copying
7296 @cindex directories, copying
7298 @command{cp} copies files (or, optionally, directories). The copy is
7299 completely independent of the original. You can either copy one file to
7300 another, or copy arbitrarily many files to a destination directory.
7304 cp [@var{option}]@dots{} [-T] @var{source} @var{dest}
7305 cp [@var{option}]@dots{} @var{source}@dots{} @var{directory}
7306 cp [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
7311 If two file names are given, @command{cp} copies the first file to the
7315 If the @option{--target-directory} (@option{-t}) option is given, or
7316 failing that if the last file is a directory and the
7317 @option{--no-target-directory} (@option{-T}) option is not given,
7318 @command{cp} copies each @var{source} file to the specified directory,
7319 using the @var{source}s' names.
7322 Generally, files are written just as they are read. For exceptions,
7323 see the @option{--sparse} option below.
7325 By default, @command{cp} does not copy directories. However, the
7326 @option{-R}, @option{-a}, and @option{-r} options cause @command{cp} to
7327 copy recursively by descending into source directories and copying files
7328 to corresponding destination directories.
7330 When copying from a symbolic link, @command{cp} normally follows the
7331 link only when not copying
7332 recursively. This default can be overridden with the
7333 @option{--archive} (@option{-a}), @option{-d}, @option{--dereference}
7334 (@option{-L}), @option{--no-dereference} (@option{-P}), and
7335 @option{-H} options. If more than one of these options is specified,
7336 the last one silently overrides the others.
7338 When copying to a symbolic link, @command{cp} follows the
7339 link only when it refers to an existing regular file.
7340 However, when copying to a dangling symbolic link, @command{cp}
7341 refuses by default, and fails with a diagnostic, since the operation
7342 is inherently dangerous. This behavior is contrary to historical
7343 practice and to @acronym{POSIX}.
7344 Set @env{POSIXLY_CORRECT} to make @command{cp} attempt to create
7345 the target of a dangling destination symlink, in spite of the possible risk.
7346 Also, when an option like
7347 @option{--backup} or @option{--link} acts to rename or remove the
7348 destination before copying, @command{cp} renames or removes the
7349 symbolic link rather than the file it points to.
7351 By default, @command{cp} copies the contents of special files only
7352 when not copying recursively. This default can be overridden with the
7353 @option{--copy-contents} option.
7355 @cindex self-backups
7356 @cindex backups, making only
7357 @command{cp} generally refuses to copy a file onto itself, with the
7358 following exception: if @option{--force --backup} is specified with
7359 @var{source} and @var{dest} identical, and referring to a regular file,
7360 @command{cp} will make a backup file, either regular or numbered, as
7361 specified in the usual ways (@pxref{Backup options}). This is useful when
7362 you simply want to make a backup of an existing file before changing it.
7364 The program accepts the following options. Also see @ref{Common options}.
7371 Preserve as much as possible of the structure and attributes of the
7372 original files in the copy (but do not attempt to preserve internal
7373 directory structure; i.e., @samp{ls -U} may list the entries in a copied
7374 directory in a different order).
7375 Try to preserve SELinux security context and extended attributes (xattr),
7376 but ignore any failure to do that and print no corresponding diagnostic.
7377 Equivalent to @option{-dR --preserve=all} with the reduced diagnostics.
7379 @itemx --attributes-only
7380 @opindex --attributes-only
7381 Preserve the specified attributes of the original files in the copy,
7382 but do not copy any data. See the @option{--preserve} option for
7383 controlling which attributes to copy.
7386 @itemx @w{@kbd{--backup}[=@var{method}]}
7389 @vindex VERSION_CONTROL
7390 @cindex backups, making
7391 @xref{Backup options}.
7392 Make a backup of each file that would otherwise be overwritten or removed.
7393 As a special case, @command{cp} makes a backup of @var{source} when the force
7394 and backup options are given and @var{source} and @var{dest} are the same
7395 name for an existing, regular file. One useful application of this
7396 combination of options is this tiny Bourne shell script:
7400 # Usage: backup FILE...
7401 # Create a @sc{gnu}-style backup of each listed FILE.
7403 cp --backup --force -- "$i" "$i"
7407 @item --copy-contents
7408 @cindex directories, copying recursively
7409 @cindex copying directories recursively
7410 @cindex recursively copying directories
7411 @cindex non-directories, copying as special files
7412 If copying recursively, copy the contents of any special files (e.g.,
7413 FIFOs and device files) as if they were regular files. This means
7414 trying to read the data in each source file and writing it to the
7415 destination. It is usually a mistake to use this option, as it
7416 normally has undesirable effects on special files like FIFOs and the
7417 ones typically found in the @file{/dev} directory. In most cases,
7418 @code{cp -R --copy-contents} will hang indefinitely trying to read
7419 from FIFOs and special files like @file{/dev/console}, and it will
7420 fill up your destination disk if you use it to copy @file{/dev/zero}.
7421 This option has no effect unless copying recursively, and it does not
7422 affect the copying of symbolic links.
7426 @cindex symbolic links, copying
7427 @cindex hard links, preserving
7428 Copy symbolic links as symbolic links rather than copying the files that
7429 they point to, and preserve hard links between source files in the copies.
7430 Equivalent to @option{--no-dereference --preserve=links}.
7436 When copying without this option and an existing destination file cannot
7437 be opened for writing, the copy fails. However, with @option{--force}),
7438 when a destination file cannot be opened, @command{cp} then removes it and
7439 tries to open it again. Contrast this behavior with that enabled by
7440 @option{--link} and @option{--symbolic-link}, whereby the destination file
7441 is never opened but rather is removed unconditionally. Also see the
7442 description of @option{--remove-destination}.
7444 This option is independent of the @option{--interactive} or
7445 @option{-i} option: neither cancels the effect of the other.
7447 This option is redundant if the @option{--no-clobber} or @option{-n} option is
7452 If a command line argument specifies a symbolic link, then copy the
7453 file it points to rather than the symbolic link itself. However,
7454 copy (preserving its nature) any symbolic link that is encountered
7455 via recursive traversal.
7458 @itemx --interactive
7460 @opindex --interactive
7461 When copying a file other than a directory, prompt whether to
7462 overwrite an existing destination file. The @option{-i} option overrides
7463 a previous @option{-n} option.
7469 Make hard links instead of copies of non-directories.
7472 @itemx --dereference
7474 @opindex --dereference
7475 Follow symbolic links when copying from them.
7476 With this option, @command{cp} cannot create a symbolic link.
7477 For example, a symlink (to regular file) in the source tree will be copied to
7478 a regular file in the destination tree.
7483 @opindex --no-clobber
7484 Do not overwrite an existing file. The @option{-n} option overrides a previous
7485 @option{-i} option. This option is mutually exclusive with @option{-b} or
7486 @option{--backup} option.
7489 @itemx --no-dereference
7491 @opindex --no-dereference
7492 @cindex symbolic links, copying
7493 Copy symbolic links as symbolic links rather than copying the files that
7494 they point to. This option affects only symbolic links in the source;
7495 symbolic links in the destination are always followed if possible.
7498 @itemx @w{@kbd{--preserve}[=@var{attribute_list}]}
7501 @cindex file information, preserving, extended attributes, xattr
7502 Preserve the specified attributes of the original files.
7503 If specified, the @var{attribute_list} must be a comma-separated list
7504 of one or more of the following strings:
7508 Preserve the file mode bits and access control lists.
7510 Preserve the owner and group. On most modern systems,
7511 only users with appropriate privileges may change the owner of a file,
7513 may preserve the group ownership of a file only if they happen to be
7514 a member of the desired group.
7516 Preserve the times of last access and last modification, when possible.
7517 On older systems, it is not possible to preserve these attributes
7518 when the affected file is a symbolic link.
7519 However, many systems now provide the @code{utimensat} function,
7520 which makes it possible even for symbolic links.
7522 Preserve in the destination files
7523 any links between corresponding source files.
7524 Note that with @option{-L} or @option{-H}, this option can convert
7525 symbolic links to hard links. For example,
7527 $ mkdir c; : > a; ln -s a b; cp -aH a b c; ls -i1 c
7532 Note the inputs: @file{b} is a symlink to regular file @file{a},
7533 yet the files in destination directory, @file{c/}, are hard-linked.
7534 Since @option{-a} implies @option{--preserve=links}, and since @option{-H}
7535 tells @command{cp} to dereference command line arguments, it sees two files
7536 with the same inode number, and preserves the perceived hard link.
7538 Here is a similar example that exercises @command{cp}'s @option{-L} option:
7540 $ mkdir b c; (cd b; : > a; ln -s a b); cp -aL b c; ls -i1 c/b
7546 Preserve SELinux security context of the file, or fail with full diagnostics.
7548 Preserve extended attributes of the file, or fail with full diagnostics.
7549 If @command{cp} is built without xattr support, ignore this option.
7550 If SELinux context, ACLs or Capabilities are implemented using xattrs,
7551 they are preserved by this option as well.
7553 Preserve all file attributes.
7554 Equivalent to specifying all of the above, but with the difference
7555 that failure to preserve SELinux security context or extended attributes
7556 does not change @command{cp}'s exit status. In contrast to @option{-a},
7557 all but @samp{Operation not supported} warnings are output.
7560 Using @option{--preserve} with no @var{attribute_list} is equivalent
7561 to @option{--preserve=mode,ownership,timestamps}.
7563 In the absence of this option, each destination file is created with the
7564 mode bits of the corresponding source file, minus the bits set in the
7565 umask and minus the set-user-ID and set-group-ID bits.
7566 @xref{File permissions}.
7568 @itemx @w{@kbd{--no-preserve}=@var{attribute_list}}
7569 @cindex file information, preserving
7570 Do not preserve the specified attributes. The @var{attribute_list}
7571 has the same form as for @option{--preserve}.
7575 @cindex parent directories and @command{cp}
7576 Form the name of each destination file by appending to the target
7577 directory a slash and the specified name of the source file. The last
7578 argument given to @command{cp} must be the name of an existing directory.
7579 For example, the command:
7582 cp --parents a/b/c existing_dir
7586 copies the file @file{a/b/c} to @file{existing_dir/a/b/c}, creating
7587 any missing intermediate directories.
7594 @opindex --recursive
7595 @cindex directories, copying recursively
7596 @cindex copying directories recursively
7597 @cindex recursively copying directories
7598 @cindex non-directories, copying as special files
7599 Copy directories recursively. By default, do not follow symbolic
7600 links in the source; see the @option{--archive} (@option{-a}), @option{-d},
7601 @option{--dereference} (@option{-L}), @option{--no-dereference}
7602 (@option{-P}), and @option{-H} options. Special files are copied by
7603 creating a destination file of the same type as the source; see the
7604 @option{--copy-contents} option. It is not portable to use
7605 @option{-r} to copy symbolic links or special files. On some
7606 non-@sc{gnu} systems, @option{-r} implies the equivalent of
7607 @option{-L} and @option{--copy-contents} for historical reasons.
7608 Also, it is not portable to use @option{-R} to copy symbolic links
7609 unless you also specify @option{-P}, as @acronym{POSIX} allows
7610 implementations that dereference symbolic links by default.
7612 @item --reflink[=@var{when}]
7613 @opindex --reflink[=@var{when}]
7616 @cindex copy on write
7617 Perform a lightweight, copy-on-write (COW) copy, if supported by the
7618 file system. Once it has succeeded, beware that the source and destination
7619 files share the same disk data blocks as long as they remain unmodified.
7620 Thus, if a disk I/O error affects data blocks of one of the files,
7621 the other suffers the same fate.
7623 The @var{when} value can be one of the following:
7627 The default behavior: if the copy-on-write operation is not supported
7628 then report the failure for each file and exit with a failure status.
7631 If the copy-on-write operation is not supported then fall back
7632 to the standard copy behaviour.
7635 This option is overridden by the @option{--link}, @option{--symbolic-link}
7636 and @option{--attributes-only} options, thus allowing it to be used
7637 to configure the default data copying behavior for @command{cp}.
7638 For example, with the following alias, @command{cp} will use the
7639 minimum amount of space supported by the file system.
7642 alias cp='cp --reflink=auto --sparse=always'
7645 @item --remove-destination
7646 @opindex --remove-destination
7647 Remove each existing destination file before attempting to open it
7648 (contrast with @option{-f} above).
7650 @item --sparse=@var{when}
7651 @opindex --sparse=@var{when}
7652 @cindex sparse files, copying
7653 @cindex holes, copying files with
7654 @findex read @r{system call, and holes}
7655 A @dfn{sparse file} contains @dfn{holes}---a sequence of zero bytes that
7656 does not occupy any physical disk blocks; the @samp{read} system call
7657 reads these as zeros. This can both save considerable disk space and
7658 increase speed, since many binary files contain lots of consecutive zero
7659 bytes. By default, @command{cp} detects holes in input source files via a crude
7660 heuristic and makes the corresponding output file sparse as well.
7661 Only regular files may be sparse.
7663 The @var{when} value can be one of the following:
7667 The default behavior: if the input file is sparse, attempt to make
7668 the output file sparse, too. However, if an output file exists but
7669 refers to a non-regular file, then do not attempt to make it sparse.
7672 For each sufficiently long sequence of zero bytes in the input file,
7673 attempt to create a corresponding hole in the output file, even if the
7674 input file does not appear to be sparse.
7675 This is useful when the input file resides on a file system
7676 that does not support sparse files
7677 (for example, @samp{efs} file systems in SGI IRIX 5.3 and earlier),
7678 but the output file is on a type of file system that does support them.
7679 Holes may be created only in regular files, so if the destination file
7680 is of some other type, @command{cp} does not even try to make it sparse.
7683 Never make the output file sparse.
7684 This is useful in creating a file for use with the @command{mkswap} command,
7685 since such a file must not have any holes.
7688 @optStripTrailingSlashes
7691 @itemx --symbolic-link
7693 @opindex --symbolic-link
7694 @cindex symbolic links, copying with
7695 Make symbolic links instead of copies of non-directories. All source
7696 file names must be absolute (starting with @samp{/}) unless the
7697 destination files are in the current directory. This option merely
7698 results in an error message on systems that do not support symbolic links.
7704 @optNoTargetDirectory
7710 @cindex newer files, copying only
7711 Do not copy a non-directory that has an existing destination with the
7712 same or newer modification time. If time stamps are being preserved,
7713 the comparison is to the source time stamp truncated to the
7714 resolutions of the destination file system and of the system calls
7715 used to update time stamps; this avoids duplicate work if several
7716 @samp{cp -pu} commands are executed with the same source and
7723 Print the name of each file before copying it.
7726 @itemx --one-file-system
7728 @opindex --one-file-system
7729 @cindex file systems, omitting copying to different
7730 Skip subdirectories that are on different file systems from the one that
7731 the copy started on.
7732 However, mount point directories @emph{are} copied.
7740 @section @command{dd}: Convert and copy a file
7743 @cindex converting while copying a file
7745 @command{dd} copies a file (from standard input to standard output, by
7746 default) with a changeable I/O block size, while optionally performing
7747 conversions on it. Synopses:
7750 dd [@var{operand}]@dots{}
7754 The only options are @option{--help} and @option{--version}.
7755 @xref{Common options}. @command{dd} accepts the following operands.
7761 Read from @var{file} instead of standard input.
7765 Write to @var{file} instead of standard output. Unless
7766 @samp{conv=notrunc} is given, @command{dd} truncates @var{file} to zero
7767 bytes (or the size specified with @samp{seek=}).
7769 @item ibs=@var{bytes}
7771 @cindex block size of input
7772 @cindex input block size
7773 Set the input block size to @var{bytes}.
7774 This makes @command{dd} read @var{bytes} per block.
7775 The default is 512 bytes.
7777 @item obs=@var{bytes}
7779 @cindex block size of output
7780 @cindex output block size
7781 Set the output block size to @var{bytes}.
7782 This makes @command{dd} write @var{bytes} per block.
7783 The default is 512 bytes.
7785 @item bs=@var{bytes}
7788 Set both input and output block sizes to @var{bytes}.
7789 This makes @command{dd} read and write @var{bytes} per block,
7790 overriding any @samp{ibs} and @samp{obs} settings.
7791 In addition, if no data-transforming @option{conv} option is specified,
7792 each input block is copied to the output as a single block,
7793 without aggregating short reads.
7795 @item cbs=@var{bytes}
7797 @cindex block size of conversion
7798 @cindex conversion block size
7799 @cindex fixed-length records, converting to variable-length
7800 @cindex variable-length records, converting to fixed-length
7801 Set the conversion block size to @var{bytes}.
7802 When converting variable-length records to fixed-length ones
7803 (@option{conv=block}) or the reverse (@option{conv=unblock}),
7804 use @var{bytes} as the fixed record length.
7806 @item skip=@var{blocks}
7808 Skip @var{blocks} @samp{ibs}-byte blocks in the input file before copying.
7810 @item seek=@var{blocks}
7812 Skip @var{blocks} @samp{obs}-byte blocks in the output file before copying.
7814 @item count=@var{blocks}
7816 Copy @var{blocks} @samp{ibs}-byte blocks from the input file, instead
7817 of everything until the end of the file.
7821 Do not print the overall transfer rate and volume statistics
7822 that normally make up the third status line when @command{dd} exits.
7824 @item conv=@var{conversion}[,@var{conversion}]@dots{}
7826 Convert the file as specified by the @var{conversion} argument(s).
7827 (No spaces around any comma(s).)
7834 @opindex ascii@r{, converting to}
7835 Convert @acronym{EBCDIC} to @acronym{ASCII},
7836 using the conversion table specified by @acronym{POSIX}.
7837 This provides a 1:1 translation for all 256 bytes.
7840 @opindex ebcdic@r{, converting to}
7841 Convert @acronym{ASCII} to @acronym{EBCDIC}.
7842 This is the inverse of the @samp{ascii} conversion.
7845 @opindex alternate ebcdic@r{, converting to}
7846 Convert @acronym{ASCII} to alternate @acronym{EBCDIC},
7847 using the alternate conversion table specified by @acronym{POSIX}.
7848 This is not a 1:1 translation, but reflects common historical practice
7849 for @samp{~}, @samp{[}, and @samp{]}.
7851 The @samp{ascii}, @samp{ebcdic}, and @samp{ibm} conversions are
7855 @opindex block @r{(space-padding)}
7856 For each line in the input, output @samp{cbs} bytes, replacing the
7857 input newline with a space and padding with spaces as necessary.
7861 Remove any trailing spaces in each @samp{cbs}-sized input block,
7862 and append a newline.
7864 The @samp{block} and @samp{unblock} conversions are mutually exclusive.
7867 @opindex lcase@r{, converting to}
7868 Change uppercase letters to lowercase.
7871 @opindex ucase@r{, converting to}
7872 Change lowercase letters to uppercase.
7874 The @samp{lcase} and @samp{ucase} conversions are mutually exclusive.
7877 @opindex swab @r{(byte-swapping)}
7878 @cindex byte-swapping
7879 Swap every pair of input bytes. @sc{gnu} @command{dd}, unlike others, works
7880 when an odd number of bytes are read---the last byte is simply copied
7881 (since there is nothing to swap it with).
7885 @cindex read errors, ignoring
7886 Continue after read errors.
7890 @cindex creating output file, avoiding
7891 Do not create the output file; the output file must already exist.
7895 @cindex creating output file, requiring
7896 Fail if the output file already exists; @command{dd} must create the
7899 The @samp{excl} and @samp{nocreat} conversions are mutually exclusive.
7903 @cindex truncating output file, avoiding
7904 Do not truncate the output file.
7907 @opindex sync @r{(padding with @acronym{ASCII} @sc{nul}s)}
7908 Pad every input block to size of @samp{ibs} with trailing zero bytes.
7909 When used with @samp{block} or @samp{unblock}, pad with spaces instead of
7914 @cindex synchronized data writes, before finishing
7915 Synchronize output data just before finishing. This forces a physical
7916 write of output data.
7920 @cindex synchronized data and metadata writes, before finishing
7921 Synchronize output data and metadata just before finishing. This
7922 forces a physical write of output data and metadata.
7926 @item iflag=@var{flag}[,@var{flag}]@dots{}
7928 Access the input file using the flags specified by the @var{flag}
7929 argument(s). (No spaces around any comma(s).)
7931 @item oflag=@var{flag}[,@var{flag}]@dots{}
7933 Access the output file using the flags specified by the @var{flag}
7934 argument(s). (No spaces around any comma(s).)
7936 Here are the flags. Not every flag is supported on every operating
7943 @cindex appending to the output file
7944 Write in append mode, so that even if some other process is writing to
7945 this file, every @command{dd} write will append to the current
7946 contents of the file. This flag makes sense only for output.
7947 If you combine this flag with the @samp{of=@var{file}} operand,
7948 you should also specify @samp{conv=notrunc} unless you want the
7949 output file to be truncated before being appended to.
7953 @cindex concurrent I/O
7954 Use concurrent I/O mode for data. This mode performs direct I/O
7955 and drops the @acronym{POSIX} requirement to serialize all I/O to the same file.
7956 A file cannot be opened in CIO mode and with a standard open at the
7962 Use direct I/O for data, avoiding the buffer cache.
7963 Note that the kernel may impose restrictions on read or write buffer sizes.
7964 For example, with an ext4 destination file system and a linux-based kernel,
7965 using @samp{oflag=direct} will cause writes to fail with @code{EINVAL} if the
7966 output buffer size is not a multiple of 512.
7970 @cindex directory I/O
7972 Fail unless the file is a directory. Most operating systems do not
7973 allow I/O to a directory, so this flag has limited utility.
7977 @cindex synchronized data reads
7978 Use synchronized I/O for data. For the output file, this forces a
7979 physical write of output data on each write. For the input file,
7980 this flag can matter when reading from a remote file that has been
7981 written to synchronously by some other process. Metadata (e.g.,
7982 last-access and last-modified time) is not necessarily synchronized.
7986 @cindex synchronized data and metadata I/O
7987 Use synchronized I/O for both data and metadata.
7991 @cindex nonblocking I/O
7992 Use non-blocking I/O.
7997 Do not update the file's access time.
7998 Some older file systems silently ignore this flag, so it is a good
7999 idea to test it on your files before relying on it.
8003 @cindex controlling terminal
8004 Do not assign the file to be a controlling terminal for @command{dd}.
8005 This has no effect when the file is not a terminal.
8006 On many hosts (e.g., @acronym{GNU}/Linux hosts), this option has no effect
8011 @cindex symbolic links, following
8012 Do not follow symbolic links.
8017 Fail if the file has multiple hard links.
8022 Use binary I/O. This option has an effect only on nonstandard
8023 platforms that distinguish binary from text I/O.
8028 Use text I/O. Like @samp{binary}, this option has no effect on
8033 Accumulate full blocks from input. The @code{read} system call
8034 may return early if a full block is not available.
8035 When that happens, continue calling @code{read} to fill the remainder
8037 This flag can be used only with @code{iflag}.
8041 These flags are not supported on all systems, and @samp{dd} rejects
8042 attempts to use them when they are not supported. When reading from
8043 standard input or writing to standard output, the @samp{nofollow} and
8044 @samp{noctty} flags should not be specified, and the other flags
8045 (e.g., @samp{nonblock}) can affect how other processes behave with the
8046 affected file descriptors, even after @command{dd} exits.
8050 @cindex multipliers after numbers
8051 The numeric-valued strings above (@var{bytes} and @var{blocks}) can be
8052 followed by a multiplier: @samp{b}=512, @samp{c}=1,
8053 @samp{w}=2, @samp{x@var{m}}=@var{m}, or any of the
8054 standard block size suffixes like @samp{k}=1024 (@pxref{Block size}).
8056 Any block size you specify via @samp{bs=}, @samp{ibs=}, @samp{obs=}, @samp{cbs=}
8057 should not be too large---values larger than a few megabytes
8058 are generally wasteful or (as in the gigabyte..exabyte case) downright
8059 counterproductive or error-inducing.
8061 Use different @command{dd} invocations to use different block sizes for
8062 skipping and I/O@. For example, the following shell commands copy data
8063 in 512 KiB blocks between a disk and a tape, but do not save or restore a
8064 4 KiB label at the start of the disk:
8067 disk=/dev/rdsk/c0t1d0s2
8070 # Copy all but the label from disk to tape.
8071 (dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape
8073 # Copy from tape back to disk, but leave the disk label alone.
8074 (dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk
8077 Sending an @samp{INFO} signal to a running @command{dd}
8078 process makes it print I/O statistics to standard error
8079 and then resume copying. In the example below,
8080 @command{dd} is run in the background to copy 10 million blocks.
8081 The @command{kill} command makes it output intermediate I/O statistics,
8082 and when @command{dd} completes normally or is killed by the
8083 @code{SIGINT} signal, it outputs the final statistics.
8086 $ dd if=/dev/zero of=/dev/null count=10MB & pid=$!
8087 $ kill -s INFO $pid; wait $pid
8088 3385223+0 records in
8089 3385223+0 records out
8090 1733234176 bytes (1.7 GB) copied, 6.42173 seconds, 270 MB/s
8091 10000000+0 records in
8092 10000000+0 records out
8093 5120000000 bytes (5.1 GB) copied, 18.913 seconds, 271 MB/s
8096 @vindex POSIXLY_CORRECT
8097 On systems lacking the @samp{INFO} signal @command{dd} responds to the
8098 @samp{USR1} signal instead, unless the @env{POSIXLY_CORRECT}
8099 environment variable is set.
8104 @node install invocation
8105 @section @command{install}: Copy files and set attributes
8108 @cindex copying files and setting attributes
8110 @command{install} copies files while setting their file mode bits and, if
8111 possible, their owner and group. Synopses:
8114 install [@var{option}]@dots{} [-T] @var{source} @var{dest}
8115 install [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8116 install [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8117 install [@var{option}]@dots{} -d @var{directory}@dots{}
8122 If two file names are given, @command{install} copies the first file to the
8126 If the @option{--target-directory} (@option{-t}) option is given, or
8127 failing that if the last file is a directory and the
8128 @option{--no-target-directory} (@option{-T}) option is not given,
8129 @command{install} copies each @var{source} file to the specified
8130 directory, using the @var{source}s' names.
8133 If the @option{--directory} (@option{-d}) option is given,
8134 @command{install} creates each @var{directory} and any missing parent
8135 directories. Parent directories are created with mode
8136 @samp{u=rwx,go=rx} (755), regardless of the @option{-m} option or the
8137 current umask. @xref{Directory Setuid and Setgid}, for how the
8138 set-user-ID and set-group-ID bits of parent directories are inherited.
8141 @cindex Makefiles, installing programs in
8142 @command{install} is similar to @command{cp}, but allows you to control the
8143 attributes of destination files. It is typically used in Makefiles to
8144 copy programs into their destination directories. It refuses to copy
8145 files onto themselves.
8147 @cindex extended attributes, xattr
8148 @command{install} never preserves extended attributes (xattr).
8150 The program accepts the following options. Also see @ref{Common options}.
8160 Compare each pair of source and destination files, and if the destination has
8161 identical content and any specified owner, group, permissions, and possibly
8162 SELinux context, then do not modify the destination at all.
8166 Ignored; for compatibility with old Unix versions of @command{install}.
8170 Create any missing parent directories of @var{dest},
8171 then copy @var{source} to @var{dest}.
8172 This option is ignored if a destination directory is specified
8173 via @option{--target-directory=DIR}.
8178 @opindex --directory
8179 @cindex directories, creating with given attributes
8180 @cindex parent directories, creating missing
8181 @cindex leading directories, creating missing
8182 Create any missing parent directories, giving them the default
8183 attributes. Then create each given directory, setting their owner,
8184 group and mode as given on the command line or to the defaults.
8186 @item -g @var{group}
8187 @itemx --group=@var{group}
8190 @cindex group ownership of installed files, setting
8191 Set the group ownership of installed files or directories to
8192 @var{group}. The default is the process's current group. @var{group}
8193 may be either a group name or a numeric group ID.
8196 @itemx --mode=@var{mode}
8199 @cindex permissions of installed files, setting
8200 Set the file mode bits for the installed file or directory to @var{mode},
8201 which can be either an octal number, or a symbolic mode as in
8202 @command{chmod}, with @samp{a=} (no access allowed to anyone) as the
8203 point of departure (@pxref{File permissions}).
8204 The default mode is @samp{u=rwx,go=rx,a-s}---read, write, and
8205 execute for the owner, read and execute for group and other, and with
8206 set-user-ID and set-group-ID disabled.
8207 This default is not quite the same as @samp{755}, since it disables
8208 instead of preserving set-user-ID and set-group-ID on directories.
8209 @xref{Directory Setuid and Setgid}.
8211 @item -o @var{owner}
8212 @itemx --owner=@var{owner}
8215 @cindex ownership of installed files, setting
8216 @cindex appropriate privileges
8217 @vindex root @r{as default owner}
8218 If @command{install} has appropriate privileges (is run as root), set the
8219 ownership of installed files or directories to @var{owner}. The default
8220 is @code{root}. @var{owner} may be either a user name or a numeric user
8223 @item --preserve-context
8224 @opindex --preserve-context
8226 @cindex security context
8227 Preserve the SELinux security context of files and directories.
8228 Failure to preserve the context in all of the files or directories
8229 will result in an exit status of 1. If SELinux is disabled then
8230 print a warning and ignore the option.
8233 @itemx --preserve-timestamps
8235 @opindex --preserve-timestamps
8236 @cindex timestamps of installed files, preserving
8237 Set the time of last access and the time of last modification of each
8238 installed file to match those of each corresponding original file.
8239 When a file is installed without this option, its last access and
8240 last modification times are both set to the time of installation.
8241 This option is useful if you want to use the last modification times
8242 of installed files to keep track of when they were last built as opposed
8243 to when they were last installed.
8249 @cindex symbol table information, stripping
8250 @cindex stripping symbol table information
8251 Strip the symbol tables from installed binary executables.
8253 @itemx --strip-program=@var{program}
8254 @opindex --strip-program
8255 @cindex symbol table information, stripping, program
8256 Program used to strip binaries.
8262 @optNoTargetDirectory
8268 Print the name of each file before copying it.
8270 @item -Z @var{context}
8271 @itemx --context=@var{context}
8275 @cindex security context
8276 Set the default SELinux security context to be used for any
8277 created files and directories. If SELinux is disabled then
8278 print a warning and ignore the option.
8286 @section @command{mv}: Move (rename) files
8290 @command{mv} moves or renames files (or directories). Synopses:
8293 mv [@var{option}]@dots{} [-T] @var{source} @var{dest}
8294 mv [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8295 mv [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8300 If two file names are given, @command{mv} moves the first file to the
8304 If the @option{--target-directory} (@option{-t}) option is given, or
8305 failing that if the last file is a directory and the
8306 @option{--no-target-directory} (@option{-T}) option is not given,
8307 @command{mv} moves each @var{source} file to the specified
8308 directory, using the @var{source}s' names.
8311 @command{mv} can move any type of file from one file system to another.
8312 Prior to version @code{4.0} of the fileutils,
8313 @command{mv} could move only regular files between file systems.
8314 For example, now @command{mv} can move an entire directory hierarchy
8315 including special device files from one partition to another. It first
8316 uses some of the same code that's used by @code{cp -a} to copy the
8317 requested directories and files, then (assuming the copy succeeded)
8318 it removes the originals. If the copy fails, then the part that was
8319 copied to the destination partition is removed. If you were to copy
8320 three directories from one partition to another and the copy of the first
8321 directory succeeded, but the second didn't, the first would be left on
8322 the destination partition and the second and third would be left on the
8325 @cindex extended attributes, xattr
8326 @command{mv} always tries to copy extended attributes (xattr), which may
8327 include SELinux context, ACLs or Capabilities.
8328 Upon failure all but @samp{Operation not supported} warnings are output.
8330 @cindex prompting, and @command{mv}
8331 If a destination file exists but is normally unwritable, standard input
8332 is a terminal, and the @option{-f} or @option{--force} option is not given,
8333 @command{mv} prompts the user for whether to replace the file. (You might
8334 own the file, or have write permission on its directory.) If the
8335 response is not affirmative, the file is skipped.
8337 @emph{Warning}: Avoid specifying a source name with a trailing slash,
8338 when it might be a symlink to a directory.
8339 Otherwise, @command{mv} may do something very surprising, since
8340 its behavior depends on the underlying rename system call.
8341 On a system with a modern Linux-based kernel, it fails with @code{errno=ENOTDIR}.
8342 However, on other systems (at least FreeBSD 6.1 and Solaris 10) it silently
8343 renames not the symlink but rather the directory referenced by the symlink.
8344 @xref{Trailing slashes}.
8346 The program accepts the following options. Also see @ref{Common options}.
8356 @cindex prompts, omitting
8357 Do not prompt the user before removing a destination file.
8359 If you specify more than one of the @option{-i}, @option{-f}, @option{-n}
8360 options, only the final one takes effect.
8365 @itemx --interactive
8367 @opindex --interactive
8368 @cindex prompts, forcing
8369 Prompt whether to overwrite each existing destination file, regardless
8371 If the response is not affirmative, the file is skipped.
8377 @opindex --no-clobber
8378 @cindex prompts, omitting
8379 Do not overwrite an existing file.
8381 This option is mutually exclusive with @option{-b} or @option{--backup} option.
8387 @cindex newer files, moving only
8388 Do not move a non-directory that has an existing destination with the
8389 same or newer modification time.
8390 If the move is across file system boundaries, the comparison is to the
8391 source time stamp truncated to the resolutions of the destination file
8392 system and of the system calls used to update time stamps; this avoids
8393 duplicate work if several @samp{mv -u} commands are executed with the
8394 same source and destination.
8400 Print the name of each file before moving it.
8402 @optStripTrailingSlashes
8408 @optNoTargetDirectory
8416 @section @command{rm}: Remove files or directories
8419 @cindex removing files or directories
8421 @command{rm} removes each given @var{file}. By default, it does not remove
8422 directories. Synopsis:
8425 rm [@var{option}]@dots{} [@var{file}]@dots{}
8428 @cindex prompting, and @command{rm}
8429 If the @option{-I} or @option{--interactive=once} option is given,
8430 and there are more than three files or the @option{-r}, @option{-R},
8431 or @option{--recursive} are given, then @command{rm} prompts the user
8432 for whether to proceed with the entire operation. If the response is
8433 not affirmative, the entire command is aborted.
8435 Otherwise, if a file is unwritable, standard input is a terminal, and
8436 the @option{-f} or @option{--force} option is not given, or the
8437 @option{-i} or @option{--interactive=always} option @emph{is} given,
8438 @command{rm} prompts the user for whether to remove the file.
8439 If the response is not affirmative, the file is skipped.
8441 Any attempt to remove a file whose last file name component is
8442 @file{.} or @file{..} is rejected without any prompting.
8444 @emph{Warning}: If you use @command{rm} to remove a file, it is usually
8445 possible to recover the contents of that file. If you want more assurance
8446 that the contents are truly unrecoverable, consider using @command{shred}.
8448 The program accepts the following options. Also see @ref{Common options}.
8456 Ignore nonexistent files and never prompt the user.
8457 Ignore any previous @option{--interactive} (@option{-i}) option.
8461 Prompt whether to remove each file.
8462 If the response is not affirmative, the file is skipped.
8463 Ignore any previous @option{--force} (@option{-f}) option.
8464 Equivalent to @option{--interactive=always}.
8468 Prompt once whether to proceed with the command, if more than three
8469 files are named or if a recursive removal is requested. Ignore any
8470 previous @option{--force} (@option{-f}) option. Equivalent to
8471 @option{--interactive=once}.
8473 @itemx --interactive [=@var{when}]
8474 @opindex --interactive
8475 Specify when to issue an interactive prompt. @var{when} may be
8479 @vindex never @r{interactive option}
8480 - Do not prompt at all.
8482 @vindex once @r{interactive option}
8483 - Prompt once if more than three files are named or if a recursive
8484 removal is requested. Equivalent to @option{-I}.
8486 @vindex always @r{interactive option}
8487 - Prompt for every file being removed. Equivalent to @option{-i}.
8489 @option{--interactive} with no @var{when} is equivalent to
8490 @option{--interactive=always}.
8492 @itemx --one-file-system
8493 @opindex --one-file-system
8494 @cindex one file system, restricting @command{rm} to
8495 When removing a hierarchy recursively, skip any directory that is on a
8496 file system different from that of the corresponding command line argument.
8499 This option is useful when removing a build ``chroot'' hierarchy,
8500 which normally contains no valuable data. However, it is not uncommon
8501 to bind-mount @file{/home} into such a hierarchy, to make it easier to
8502 use one's start-up file. The catch is that it's easy to forget to
8503 unmount @file{/home}. Then, when you use @command{rm -rf} to remove
8504 your normally throw-away chroot, that command will remove everything
8505 under @file{/home}, too.
8506 Use the @option{--one-file-system} option, and it will
8507 warn about and skip directories on other file systems.
8508 Of course, this will not save your @file{/home} if it and your
8509 chroot happen to be on the same file system.
8511 @itemx --preserve-root
8512 @opindex --preserve-root
8513 @cindex root directory, disallow recursive destruction
8514 Fail upon any attempt to remove the root directory, @file{/},
8515 when used with the @option{--recursive} option.
8516 This is the default behavior.
8517 @xref{Treating / specially}.
8519 @itemx --no-preserve-root
8520 @opindex --no-preserve-root
8521 @cindex root directory, allow recursive destruction
8522 Do not treat @file{/} specially when removing recursively.
8523 This option is not recommended unless you really want to
8524 remove all the files on your computer.
8525 @xref{Treating / specially}.
8532 @opindex --recursive
8533 @cindex directories, removing (recursively)
8534 Remove the listed directories and their contents recursively.
8540 Print the name of each file before removing it.
8544 @cindex files beginning with @samp{-}, removing
8545 @cindex @samp{-}, removing files beginning with
8546 One common question is how to remove files whose names begin with a
8547 @samp{-}. @sc{gnu} @command{rm}, like every program that uses the @code{getopt}
8548 function to parse its arguments, lets you use the @samp{--} option to
8549 indicate that all following arguments are non-options. To remove a file
8550 called @file{-f} in the current directory, you could type either:
8563 @opindex - @r{and Unix @command{rm}}
8564 The Unix @command{rm} program's use of a single @samp{-} for this purpose
8565 predates the development of the getopt standard syntax.
8570 @node shred invocation
8571 @section @command{shred}: Remove files more securely
8574 @cindex data, erasing
8575 @cindex erasing data
8577 @command{shred} overwrites devices or files, to help prevent even
8578 very expensive hardware from recovering the data.
8580 Ordinarily when you remove a file (@pxref{rm invocation}), the data is
8581 not actually destroyed. Only the index listing where the file is
8582 stored is destroyed, and the storage is made available for reuse.
8583 There are undelete utilities that will attempt to reconstruct the index
8584 and can bring the file back if the parts were not reused.
8586 On a busy system with a nearly-full drive, space can get reused in a few
8587 seconds. But there is no way to know for sure. If you have sensitive
8588 data, you may want to be sure that recovery is not possible by actually
8589 overwriting the file with non-sensitive data.
8591 However, even after doing that, it is possible to take the disk back
8592 to a laboratory and use a lot of sensitive (and expensive) equipment
8593 to look for the faint ``echoes'' of the original data underneath the
8594 overwritten data. If the data has only been overwritten once, it's not
8597 The best way to remove something irretrievably is to destroy the media
8598 it's on with acid, melt it down, or the like. For cheap removable media
8599 like floppy disks, this is the preferred method. However, hard drives
8600 are expensive and hard to melt, so the @command{shred} utility tries
8601 to achieve a similar effect non-destructively.
8603 This uses many overwrite passes, with the data patterns chosen to
8604 maximize the damage they do to the old data. While this will work on
8605 floppies, the patterns are designed for best effect on hard drives.
8606 For more details, see the source code and Peter Gutmann's paper
8607 @uref{http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html,
8608 @cite{Secure Deletion of Data from Magnetic and Solid-State Memory}},
8609 from the proceedings of the Sixth @acronym{USENIX} Security Symposium (San Jose,
8610 California, July 22--25, 1996).
8612 @strong{Please note} that @command{shred} relies on a very important assumption:
8613 that the file system overwrites data in place. This is the traditional
8614 way to do things, but many modern file system designs do not satisfy this
8615 assumption. Exceptions include:
8620 Log-structured or journaled file systems, such as those supplied with
8621 AIX and Solaris, and JFS, ReiserFS, XFS, Ext3 (in @code{data=journal} mode),
8622 BFS, NTFS, etc.@: when they are configured to journal @emph{data}.
8625 File systems that write redundant data and carry on even if some writes
8626 fail, such as RAID-based file systems.
8629 File systems that make snapshots, such as Network Appliance's NFS server.
8632 File systems that cache in temporary locations, such as NFS version 3
8636 Compressed file systems.
8639 In the particular case of ext3 file systems, the above disclaimer applies (and
8640 @command{shred} is thus of limited effectiveness) only in @code{data=journal}
8641 mode, which journals file data in addition to just metadata. In both
8642 the @code{data=ordered} (default) and @code{data=writeback} modes,
8643 @command{shred} works as usual. Ext3 journaling modes can be changed
8644 by adding the @code{data=something} option to the mount options for a
8645 particular file system in the @file{/etc/fstab} file, as documented in
8646 the mount man page (man mount).
8648 If you are not sure how your file system operates, then you should assume
8649 that it does not overwrite data in place, which means that shred cannot
8650 reliably operate on regular files in your file system.
8652 Generally speaking, it is more reliable to shred a device than a file,
8653 since this bypasses the problem of file system design mentioned above.
8654 However, even shredding devices is not always completely reliable. For
8655 example, most disks map out bad sectors invisibly to the application; if
8656 the bad sectors contain sensitive data, @command{shred} won't be able to
8659 @command{shred} makes no attempt to detect or report this problem, just as
8660 it makes no attempt to do anything about backups. However, since it is
8661 more reliable to shred devices than files, @command{shred} by default does
8662 not truncate or remove the output file. This default is more suitable
8663 for devices, which typically cannot be truncated and should not be
8666 Finally, consider the risk of backups and mirrors.
8667 File system backups and remote mirrors may contain copies of the
8668 file that cannot be removed, and that will allow a shredded file
8669 to be recovered later. So if you keep any data you may later want
8670 to destroy using @command{shred}, be sure that it is not backed up or mirrored.
8673 shred [@var{option}]@dots{} @var{file}[@dots{}]
8676 The program accepts the following options. Also see @ref{Common options}.
8684 @cindex force deletion
8685 Override file permissions if necessary to allow overwriting.
8688 @itemx -n @var{number}
8689 @itemx --iterations=@var{number}
8690 @opindex -n @var{number}
8691 @opindex --iterations=@var{number}
8692 @cindex iterations, selecting the number of
8693 By default, @command{shred} uses @value{SHRED_DEFAULT_PASSES} passes of
8694 overwrite. You can reduce this to save time, or increase it if you think it's
8695 appropriate. After 25 passes all of the internal overwrite patterns will have
8696 been used at least once.
8698 @item --random-source=@var{file}
8699 @opindex --random-source
8700 @cindex random source for shredding
8701 Use @var{file} as a source of random data used to overwrite and to
8702 choose pass ordering. @xref{Random sources}.
8704 @item -s @var{bytes}
8705 @itemx --size=@var{bytes}
8706 @opindex -s @var{bytes}
8707 @opindex --size=@var{bytes}
8708 @cindex size of file to shred
8709 Shred the first @var{bytes} bytes of the file. The default is to shred
8710 the whole file. @var{bytes} can be followed by a size specification like
8711 @samp{K}, @samp{M}, or @samp{G} to specify a multiple. @xref{Block size}.
8717 @cindex removing files after shredding
8718 After shredding a file, truncate it (if possible) and then remove it.
8719 If a file has multiple links, only the named links will be removed.
8725 Display to standard error all status updates as sterilization proceeds.
8731 By default, @command{shred} rounds the size of a regular file up to the next
8732 multiple of the file system block size to fully erase the last block of the file.
8733 Use @option{--exact} to suppress that behavior.
8734 Thus, by default if you shred a 10-byte regular file on a system with 512-byte
8735 blocks, the resulting file will be 512 bytes long. With this option,
8736 shred does not increase the apparent size of the file.
8742 Normally, the last pass that @command{shred} writes is made up of
8743 random data. If this would be conspicuous on your hard drive (for
8744 example, because it looks like encrypted data), or you just think
8745 it's tidier, the @option{--zero} option adds an additional overwrite pass with
8746 all zero bits. This is in addition to the number of passes specified
8747 by the @option{--iterations} option.
8751 You might use the following command to erase all trace of the
8752 file system you'd created on the floppy disk in your first drive.
8753 That command takes about 20 minutes to erase a ``1.44MB'' (actually
8757 shred --verbose /dev/fd0
8760 Similarly, to erase all data on a selected partition of
8761 your hard disk, you could give a command like this:
8764 shred --verbose /dev/sda5
8767 A @var{file} of @samp{-} denotes standard output.
8768 The intended use of this is to shred a removed temporary file.
8775 echo "Hello, world" >&3
8780 However, the command @samp{shred - >file} does not shred the contents
8781 of @var{file}, since the shell truncates @var{file} before invoking
8782 @command{shred}. Use the command @samp{shred file} or (if using a
8783 Bourne-compatible shell) the command @samp{shred - 1<>file} instead.
8788 @node Special file types
8789 @chapter Special file types
8791 @cindex special file types
8792 @cindex file types, special
8794 This chapter describes commands which create special types of files (and
8795 @command{rmdir}, which removes directories, one special file type).
8797 @cindex special file types
8799 Although Unix-like operating systems have markedly fewer special file
8800 types than others, not @emph{everything} can be treated only as the
8801 undifferentiated byte stream of @dfn{normal files}. For example, when a
8802 file is created or removed, the system must record this information,
8803 which it does in a @dfn{directory}---a special type of file. Although
8804 you can read directories as normal files, if you're curious, in order
8805 for the system to do its job it must impose a structure, a certain
8806 order, on the bytes of the file. Thus it is a ``special'' type of file.
8808 Besides directories, other special file types include named pipes
8809 (FIFOs), symbolic links, sockets, and so-called @dfn{special files}.
8812 * link invocation:: Make a hard link via the link syscall
8813 * ln invocation:: Make links between files.
8814 * mkdir invocation:: Make directories.
8815 * mkfifo invocation:: Make FIFOs (named pipes).
8816 * mknod invocation:: Make block or character special files.
8817 * readlink invocation:: Print value of a symlink or canonical file name.
8818 * rmdir invocation:: Remove empty directories.
8819 * unlink invocation:: Remove files via the unlink syscall
8823 @node link invocation
8824 @section @command{link}: Make a hard link via the link syscall
8827 @cindex links, creating
8828 @cindex hard links, creating
8829 @cindex creating links (hard only)
8831 @command{link} creates a single hard link at a time.
8832 It is a minimalist interface to the system-provided
8833 @code{link} function. @xref{Hard Links, , , libc,
8834 The GNU C Library Reference Manual}.
8835 It avoids the bells and whistles of the more commonly-used
8836 @command{ln} command (@pxref{ln invocation}).
8840 link @var{filename} @var{linkname}
8843 @var{filename} must specify an existing file, and @var{linkname}
8844 must specify a nonexistent entry in an existing directory.
8845 @command{link} simply calls @code{link (@var{filename}, @var{linkname})}
8848 On a @acronym{GNU} system, this command acts like @samp{ln --directory
8849 --no-target-directory @var{filename} @var{linkname}}. However, the
8850 @option{--directory} and @option{--no-target-directory} options are
8851 not specified by @acronym{POSIX}, and the @command{link} command is
8852 more portable in practice.
8854 If @var{filename} is a symbolic link, it is unspecified whether
8855 @var{linkname} will be a hard link to the symbolic link or to the
8856 target of the symbolic link. Use @command{ln -P} or @command{ln -L}
8857 to specify which behavior is desired.
8863 @section @command{ln}: Make links between files
8866 @cindex links, creating
8867 @cindex hard links, creating
8868 @cindex symbolic (soft) links, creating
8869 @cindex creating links (hard or soft)
8871 @cindex file systems and hard links
8872 @command{ln} makes links between files. By default, it makes hard links;
8873 with the @option{-s} option, it makes symbolic (or @dfn{soft}) links.
8877 ln [@var{option}]@dots{} [-T] @var{target} @var{linkname}
8878 ln [@var{option}]@dots{} @var{target}
8879 ln [@var{option}]@dots{} @var{target}@dots{} @var{directory}
8880 ln [@var{option}]@dots{} -t @var{directory} @var{target}@dots{}
8886 If two file names are given, @command{ln} creates a link to the first
8887 file from the second.
8890 If one @var{target} is given, @command{ln} creates a link to that file
8891 in the current directory.
8894 If the @option{--target-directory} (@option{-t}) option is given, or
8895 failing that if the last file is a directory and the
8896 @option{--no-target-directory} (@option{-T}) option is not given,
8897 @command{ln} creates a link to each @var{target} file in the specified
8898 directory, using the @var{target}s' names.
8902 Normally @command{ln} does not remove existing files. Use the
8903 @option{--force} (@option{-f}) option to remove them unconditionally,
8904 the @option{--interactive} (@option{-i}) option to remove them
8905 conditionally, and the @option{--backup} (@option{-b}) option to
8908 @cindex hard link, defined
8909 @cindex inode, and hard links
8910 A @dfn{hard link} is another name for an existing file; the link and the
8911 original are indistinguishable. Technically speaking, they share the
8912 same inode, and the inode contains all the information about a
8913 file---indeed, it is not incorrect to say that the inode @emph{is} the
8914 file. Most systems prohibit making a hard link to
8915 a directory; on those where it is allowed, only the super-user can do
8916 so (and with caution, since creating a cycle will cause problems to many
8917 other utilities). Hard links cannot cross file system boundaries. (These
8918 restrictions are not mandated by @acronym{POSIX}, however.)
8920 @cindex dereferencing symbolic links
8921 @cindex symbolic link, defined
8922 @dfn{Symbolic links} (@dfn{symlinks} for short), on the other hand, are
8923 a special file type (which not all kernels support: System V release 3
8924 (and older) systems lack symlinks) in which the link file actually
8925 refers to a different file, by name. When most operations (opening,
8926 reading, writing, and so on) are passed the symbolic link file, the
8927 kernel automatically @dfn{dereferences} the link and operates on the
8928 target of the link. But some operations (e.g., removing) work on the
8929 link file itself, rather than on its target. The owner and group of a
8930 symlink are not significant to file access performed through
8931 the link, but do have implications on deleting a symbolic link from a
8932 directory with the restricted deletion bit set. On the GNU system,
8933 the mode of a symlink has no significance and cannot be changed, but
8934 on some BSD systems, the mode can be changed and will affect whether
8935 the symlink will be traversed in file name resolution. @xref{Symbolic Links,,,
8936 libc, The GNU C Library Reference Manual}.
8938 Symbolic links can contain arbitrary strings; a @dfn{dangling symlink}
8939 occurs when the string in the symlink does not resolve to a file.
8940 There are no restrictions against creating dangling symbolic links.
8941 There are trade-offs to using absolute or relative symlinks. An
8942 absolute symlink always points to the same file, even if the directory
8943 containing the link is moved. However, if the symlink is visible from
8944 more than one machine (such as on a networked file system), the file
8945 pointed to might not always be the same. A relative symbolic link is
8946 resolved in relation to the directory that contains the link, and is
8947 often useful in referring to files on the same device without regards
8948 to what name that device is mounted on when accessed via networked
8951 When creating a relative symlink in a different location than the
8952 current directory, the resolution of the symlink will be different
8953 than the resolution of the same string from the current directory.
8954 Therefore, many users prefer to first change directories to the
8955 location where the relative symlink will be created, so that
8956 tab-completion or other file resolution will find the same target as
8957 what will be placed in the symlink.
8959 The program accepts the following options. Also see @ref{Common options}.
8970 @opindex --directory
8971 @cindex hard links to directories
8972 Allow users with appropriate privileges to attempt to make hard links
8974 However, note that this will probably fail due to
8975 system restrictions, even for the super-user.
8981 Remove existing destination files.
8984 @itemx --interactive
8986 @opindex --interactive
8987 @cindex prompting, and @command{ln}
8988 Prompt whether to remove existing destination files.
8994 If @option{-s} is not in effect, and the source file is a symbolic
8995 link, create the hard link to the file referred to by the symbolic
8996 link, rather than the symbolic link itself.
8999 @itemx --no-dereference
9001 @opindex --no-dereference
9002 Do not treat the last operand specially when it is a symbolic link to
9003 a directory. Instead, treat it as if it were a normal file.
9005 When the destination is an actual directory (not a symlink to one),
9006 there is no ambiguity. The link is created in that directory.
9007 But when the specified destination is a symlink to a directory,
9008 there are two ways to treat the user's request. @command{ln} can
9009 treat the destination just as it would a normal directory and create
9010 the link in it. On the other hand, the destination can be viewed as a
9011 non-directory---as the symlink itself. In that case, @command{ln}
9012 must delete or backup that symlink before creating the new link.
9013 The default is to treat a destination that is a symlink to a directory
9014 just like a directory.
9016 This option is weaker than the @option{--no-target-directory}
9017 (@option{-T}) option, so it has no effect if both options are given.
9023 If @option{-s} is not in effect, and the source file is a symbolic
9024 link, create the hard link to the symbolic link itself. On platforms
9025 where this is not supported by the kernel, this option creates a
9026 symbolic link with identical contents; since symbolic link contents
9027 cannot be edited, any file name resolution performed through either
9028 link will be the same as if a hard link had been created.
9034 Make symbolic links instead of hard links. This option merely produces
9035 an error message on systems that do not support symbolic links.
9041 @optNoTargetDirectory
9047 Print the name of each file after linking it successfully.
9051 @cindex hard links to symbolic links
9052 @cindex symbolic links and @command{ln}
9053 If @option{-L} and @option{-P} are both given, the last one takes
9054 precedence. If @option{-s} is also given, @option{-L} and @option{-P}
9055 are silently ignored. If neither option is given, then this
9056 implementation defaults to @option{-P} if the system @code{link} supports
9057 hard links to symbolic links (such as the GNU system), and @option{-L}
9058 if @code{link} follows symbolic links (such as on BSD).
9067 # Create link ../a pointing to a in that directory.
9068 # Not really useful because it points to itself.
9073 # Change to the target before creating symlinks to avoid being confused.
9079 # Hard coded file names don't move well.
9080 ln -s $(pwd)/a /some/dir/
9084 # Relative file names survive directory moves and also
9085 # work across networked file systems.
9086 ln -s afile anotherfile
9087 ln -s ../adir/afile yetanotherfile
9091 @node mkdir invocation
9092 @section @command{mkdir}: Make directories
9095 @cindex directories, creating
9096 @cindex creating directories
9098 @command{mkdir} creates directories with the specified names. Synopsis:
9101 mkdir [@var{option}]@dots{} @var{name}@dots{}
9104 @command{mkdir} creates each directory @var{name} in the order given.
9105 It reports an error if @var{name} already exists, unless the
9106 @option{-p} option is given and @var{name} is a directory.
9108 The program accepts the following options. Also see @ref{Common options}.
9113 @itemx --mode=@var{mode}
9116 @cindex modes of created directories, setting
9117 Set the file permission bits of created directories to @var{mode},
9118 which uses the same syntax as
9119 in @command{chmod} and uses @samp{a=rwx} (read, write and execute allowed for
9120 everyone) for the point of the departure. @xref{File permissions}.
9122 Normally the directory has the desired file mode bits at the moment it
9123 is created. As a @acronym{GNU} extension, @var{mode} may also mention
9124 special mode bits, but in this case there may be a temporary window
9125 during which the directory exists but its special mode bits are
9126 incorrect. @xref{Directory Setuid and Setgid}, for how the
9127 set-user-ID and set-group-ID bits of directories are inherited unless
9128 overridden in this way.
9134 @cindex parent directories, creating
9135 Make any missing parent directories for each argument, setting their
9136 file permission bits to the umask modified by @samp{u+wx}. Ignore
9137 existing parent directories, and do not change their file permission
9140 To set the file permission bits of any newly-created parent
9141 directories to a value that includes @samp{u+wx}, you can set the
9142 umask before invoking @command{mkdir}. For example, if the shell
9143 command @samp{(umask u=rwx,go=rx; mkdir -p P/Q)} creates the parent
9144 @file{P} it sets the parent's permission bits to @samp{u=rwx,go=rx}.
9145 To set a parent's special mode bits as well, you can invoke
9146 @command{chmod} after @command{mkdir}. @xref{Directory Setuid and
9147 Setgid}, for how the set-user-ID and set-group-ID bits of
9148 newly-created parent directories are inherited.
9154 Print a message for each created directory. This is most useful with
9157 @item -Z @var{context}
9158 @itemx --context=@var{context}
9162 @cindex security context
9163 Set the default SELinux security context to be used for created directories.
9170 @node mkfifo invocation
9171 @section @command{mkfifo}: Make FIFOs (named pipes)
9174 @cindex FIFOs, creating
9175 @cindex named pipes, creating
9176 @cindex creating FIFOs (named pipes)
9178 @command{mkfifo} creates FIFOs (also called @dfn{named pipes}) with the
9179 specified names. Synopsis:
9182 mkfifo [@var{option}] @var{name}@dots{}
9185 A @dfn{FIFO} is a special file type that permits independent processes
9186 to communicate. One process opens the FIFO file for writing, and
9187 another for reading, after which data can flow as with the usual
9188 anonymous pipe in shells or elsewhere.
9190 The program accepts the following option. Also see @ref{Common options}.
9195 @itemx --mode=@var{mode}
9198 @cindex modes of created FIFOs, setting
9199 Set the mode of created FIFOs to @var{mode}, which is symbolic as in
9200 @command{chmod} and uses @samp{a=rw} (read and write allowed for everyone)
9201 for the point of departure. @var{mode} should specify only file
9202 permission bits. @xref{File permissions}.
9204 @item -Z @var{context}
9205 @itemx --context=@var{context}
9209 @cindex security context
9210 Set the default SELinux security context to be used for created FIFOs.
9217 @node mknod invocation
9218 @section @command{mknod}: Make block or character special files
9221 @cindex block special files, creating
9222 @cindex character special files, creating
9224 @command{mknod} creates a FIFO, character special file, or block special
9225 file with the specified name. Synopsis:
9228 mknod [@var{option}]@dots{} @var{name} @var{type} [@var{major} @var{minor}]
9231 @cindex special files
9232 @cindex block special files
9233 @cindex character special files
9234 Unlike the phrase ``special file type'' above, the term @dfn{special
9235 file} has a technical meaning on Unix: something that can generate or
9236 receive data. Usually this corresponds to a physical piece of hardware,
9237 e.g., a printer or a disk. (These files are typically created at
9238 system-configuration time.) The @command{mknod} command is what creates
9239 files of this type. Such devices can be read either a character at a
9240 time or a ``block'' (many characters) at a time, hence we say there are
9241 @dfn{block special} files and @dfn{character special} files.
9243 @c mknod is a shell built-in at least with OpenBSD's /bin/sh
9244 @mayConflictWithShellBuiltIn{mknod}
9246 The arguments after @var{name} specify the type of file to make:
9251 @opindex p @r{for FIFO file}
9255 @opindex b @r{for block special file}
9256 for a block special file
9259 @c Don't document the `u' option -- it's just a synonym for `c'.
9260 @c Do *any* versions of mknod still use it?
9262 @opindex c @r{for character special file}
9263 @c @opindex u @r{for character special file}
9264 for a character special file
9268 When making a block or character special file, the major and minor
9269 device numbers must be given after the file type.
9270 If a major or minor device number begins with @samp{0x} or @samp{0X},
9271 it is interpreted as hexadecimal; otherwise, if it begins with @samp{0},
9272 as octal; otherwise, as decimal.
9274 The program accepts the following option. Also see @ref{Common options}.
9279 @itemx --mode=@var{mode}
9282 Set the mode of created files to @var{mode}, which is symbolic as in
9283 @command{chmod} and uses @samp{a=rw} as the point of departure.
9284 @var{mode} should specify only file permission bits.
9285 @xref{File permissions}.
9287 @item -Z @var{context}
9288 @itemx --context=@var{context}
9292 @cindex security context
9293 Set the default SELinux security context to be used for created files.
9300 @node readlink invocation
9301 @section @command{readlink}: Print value of a symlink or canonical file name
9304 @cindex displaying value of a symbolic link
9305 @cindex canonical file name
9306 @cindex canonicalize a file name
9310 @command{readlink} may work in one of two supported modes:
9316 @command{readlink} outputs the value of the given symbolic link.
9317 If @command{readlink} is invoked with an argument other than the name
9318 of a symbolic link, it produces no output and exits with a nonzero exit code.
9320 @item Canonicalize mode
9322 @command{readlink} outputs the absolute name of the given file which contains
9323 no @file{.}, @file{..} components nor any repeated separators
9324 (@file{/}) or symbolic links.
9329 readlink [@var{option}] @var{file}
9332 By default, @command{readlink} operates in readlink mode.
9334 The program accepts the following options. Also see @ref{Common options}.
9339 @itemx --canonicalize
9341 @opindex --canonicalize
9342 Activate canonicalize mode.
9343 If any component of the file name except the last one is missing or unavailable,
9344 @command{readlink} produces no output and exits with a nonzero exit
9345 code. A trailing slash is ignored.
9348 @itemx --canonicalize-existing
9350 @opindex --canonicalize-existing
9351 Activate canonicalize mode.
9352 If any component is missing or unavailable, @command{readlink} produces
9353 no output and exits with a nonzero exit code. A trailing slash
9354 requires that the name resolve to a directory.
9357 @itemx --canonicalize-missing
9359 @opindex --canonicalize-missing
9360 Activate canonicalize mode.
9361 If any component is missing or unavailable, @command{readlink} treats it
9367 @opindex --no-newline
9368 Do not output the trailing newline.
9378 Suppress most error messages.
9384 Report error messages.
9388 The @command{readlink} utility first appeared in OpenBSD 2.1.
9390 There is a @command{realpath} command on some systems
9391 which operates like @command{readlink} in canonicalize mode.
9396 @node rmdir invocation
9397 @section @command{rmdir}: Remove empty directories
9400 @cindex removing empty directories
9401 @cindex directories, removing empty
9403 @command{rmdir} removes empty directories. Synopsis:
9406 rmdir [@var{option}]@dots{} @var{directory}@dots{}
9409 If any @var{directory} argument does not refer to an existing empty
9410 directory, it is an error.
9412 The program accepts the following options. Also see @ref{Common options}.
9416 @item --ignore-fail-on-non-empty
9417 @opindex --ignore-fail-on-non-empty
9418 @cindex directory deletion, ignoring failures
9419 Ignore each failure to remove a directory that is solely because
9420 the directory is non-empty.
9426 @cindex parent directories, removing
9427 Remove @var{directory}, then try to remove each component of @var{directory}.
9428 So, for example, @samp{rmdir -p a/b/c} is similar to @samp{rmdir a/b/c a/b a}.
9429 As such, it fails if any of those directories turns out not to be empty.
9430 Use the @option{--ignore-fail-on-non-empty} option to make it so such
9431 a failure does not evoke a diagnostic and does not cause @command{rmdir} to
9432 exit unsuccessfully.
9438 @cindex directory deletion, reporting
9439 Give a diagnostic for each successful removal.
9440 @var{directory} is removed.
9444 @xref{rm invocation}, for how to remove non-empty directories (recursively).
9449 @node unlink invocation
9450 @section @command{unlink}: Remove files via the unlink syscall
9453 @cindex removing files or directories (via the unlink syscall)
9455 @command{unlink} deletes a single specified file name.
9456 It is a minimalist interface to the system-provided
9457 @code{unlink} function. @xref{Deleting Files, , , libc,
9458 The GNU C Library Reference Manual}. Synopsis:
9459 It avoids the bells and whistles of the more commonly-used
9460 @command{rm} command (@pxref{rm invocation}).
9463 unlink @var{filename}
9466 On some systems @code{unlink} can be used to delete the name of a
9467 directory. On others, it can be used that way only by a privileged user.
9468 In the GNU system @code{unlink} can never delete the name of a directory.
9470 The @command{unlink} command honors the @option{--help} and
9471 @option{--version} options. To remove a file whose name begins with
9472 @samp{-}, prefix the name with @samp{./}, e.g., @samp{unlink ./--help}.
9477 @node Changing file attributes
9478 @chapter Changing file attributes
9480 @cindex changing file attributes
9481 @cindex file attributes, changing
9482 @cindex attributes, file
9484 A file is not merely its contents, a name, and a file type
9485 (@pxref{Special file types}). A file also has an owner (a user ID), a
9486 group (a group ID), permissions (what the owner can do with the file,
9487 what people in the group can do, and what everyone else can do), various
9488 timestamps, and other information. Collectively, we call these a file's
9491 These commands change file attributes.
9494 * chgrp invocation:: Change file groups.
9495 * chmod invocation:: Change access permissions.
9496 * chown invocation:: Change file owners and groups.
9497 * touch invocation:: Change file timestamps.
9501 @node chown invocation
9502 @section @command{chown}: Change file owner and group
9505 @cindex file ownership, changing
9506 @cindex group ownership, changing
9507 @cindex changing file ownership
9508 @cindex changing group ownership
9510 @command{chown} changes the user and/or group ownership of each given @var{file}
9511 to @var{new-owner} or to the user and group of an existing reference file.
9515 chown [@var{option}]@dots{} @{@var{new-owner} | --reference=@var{ref_file}@} @var{file}@dots{}
9518 If used, @var{new-owner} specifies the new owner and/or group as follows
9519 (with no embedded white space):
9522 [@var{owner}] [ : [@var{group}] ]
9529 If only an @var{owner} (a user name or numeric user ID) is given, that
9530 user is made the owner of each given file, and the files' group is not
9533 @item owner@samp{:}group
9534 If the @var{owner} is followed by a colon and a @var{group} (a
9535 group name or numeric group ID), with no spaces between them, the group
9536 ownership of the files is changed as well (to @var{group}).
9539 If a colon but no group name follows @var{owner}, that user is
9540 made the owner of the files and the group of the files is changed to
9541 @var{owner}'s login group.
9544 If the colon and following @var{group} are given, but the owner
9545 is omitted, only the group of the files is changed; in this case,
9546 @command{chown} performs the same function as @command{chgrp}.
9549 If only a colon is given, or if @var{new-owner} is empty, neither the
9550 owner nor the group is changed.
9554 If @var{owner} or @var{group} is intended to represent a numeric user
9555 or group ID, then you may specify it with a leading @samp{+}.
9556 @xref{Disambiguating names and IDs}.
9558 Some older scripts may still use @samp{.} in place of the @samp{:} separator.
9559 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) does not
9560 require support for that, but for backward compatibility @acronym{GNU}
9561 @command{chown} supports @samp{.} so long as no ambiguity results.
9562 New scripts should avoid the use of @samp{.} because it is not
9563 portable, and because it has undesirable results if the entire
9564 @var{owner@samp{.}group} happens to identify a user whose name
9567 The @command{chown} command sometimes clears the set-user-ID or
9568 set-group-ID permission bits. This behavior depends on the policy and
9569 functionality of the underlying @code{chown} system call, which may
9570 make system-dependent file mode modifications outside the control of
9571 the @command{chown} command. For example, the @command{chown} command
9572 might not affect those bits when invoked by a user with appropriate
9573 privileges, or when the
9574 bits signify some function other than executable permission (e.g.,
9576 When in doubt, check the underlying system behavior.
9578 The program accepts the following options. Also see @ref{Common options}.
9586 @cindex changed owners, verbosely describing
9587 Verbosely describe the action for each @var{file} whose ownership
9596 @cindex error messages, omitting
9597 Do not print error messages about files whose ownership cannot be
9600 @itemx @w{@kbd{--from}=@var{old-owner}}
9602 @cindex symbolic links, changing owner
9603 Change a @var{file}'s ownership only if it has current attributes specified
9604 by @var{old-owner}. @var{old-owner} has the same form as @var{new-owner}
9606 This option is useful primarily from a security standpoint in that
9607 it narrows considerably the window of potential abuse.
9608 For example, to reflect a user ID numbering change for one user's files
9609 without an option like this, @code{root} might run
9612 find / -owner OLDUSER -print0 | xargs -0 chown -h NEWUSER
9615 But that is dangerous because the interval between when the @command{find}
9616 tests the existing file's owner and when the @command{chown} is actually run
9618 One way to narrow the gap would be to invoke chown for each file
9622 find / -owner OLDUSER -exec chown -h NEWUSER @{@} \;
9625 But that is very slow if there are many affected files.
9626 With this option, it is safer (the gap is narrower still)
9627 though still not perfect:
9630 chown -h -R --from=OLDUSER NEWUSER /
9634 @opindex --dereference
9635 @cindex symbolic links, changing owner
9637 Do not act on symbolic links themselves but rather on what they point to.
9638 This is the default.
9641 @itemx --no-dereference
9643 @opindex --no-dereference
9644 @cindex symbolic links, changing owner
9646 Act on symbolic links themselves instead of what they point to.
9647 This mode relies on the @code{lchown} system call.
9648 On systems that do not provide the @code{lchown} system call,
9649 @command{chown} fails when a file specified on the command line
9651 By default, no diagnostic is issued for symbolic links encountered
9652 during a recursive traversal, but see @option{--verbose}.
9654 @itemx --preserve-root
9655 @opindex --preserve-root
9656 @cindex root directory, disallow recursive modification
9657 Fail upon any attempt to recursively change the root directory, @file{/}.
9658 Without @option{--recursive}, this option has no effect.
9659 @xref{Treating / specially}.
9661 @itemx --no-preserve-root
9662 @opindex --no-preserve-root
9663 @cindex root directory, allow recursive modification
9664 Cancel the effect of any preceding @option{--preserve-root} option.
9665 @xref{Treating / specially}.
9667 @item --reference=@var{ref_file}
9668 @opindex --reference
9669 Change the user and group of each @var{file} to be the same as those of
9670 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9671 user and group of the symbolic link, but rather those of the file it
9678 Output a diagnostic for every file processed.
9679 If a symbolic link is encountered during a recursive traversal
9680 on a system without the @code{lchown} system call, and @option{--no-dereference}
9681 is in effect, then issue a diagnostic saying neither the symbolic link nor
9682 its referent is being changed.
9687 @opindex --recursive
9688 @cindex recursively changing file ownership
9689 Recursively change ownership of directories and their contents.
9692 @xref{Traversing symlinks}.
9695 @xref{Traversing symlinks}.
9698 @xref{Traversing symlinks}.
9707 # Change the owner of /u to "root".
9710 # Likewise, but also change its group to "staff".
9713 # Change the owner of /u and subfiles to "root".
9718 @node chgrp invocation
9719 @section @command{chgrp}: Change group ownership
9722 @cindex group ownership, changing
9723 @cindex changing group ownership
9725 @command{chgrp} changes the group ownership of each given @var{file}
9726 to @var{group} (which can be either a group name or a numeric group ID)
9727 or to the group of an existing reference file. Synopsis:
9730 chgrp [@var{option}]@dots{} @{@var{group} | --reference=@var{ref_file}@} @var{file}@dots{}
9733 If @var{group} is intended to represent a
9734 numeric group ID, then you may specify it with a leading @samp{+}.
9735 @xref{Disambiguating names and IDs}.
9737 The program accepts the following options. Also see @ref{Common options}.
9745 @cindex changed files, verbosely describing
9746 Verbosely describe the action for each @var{file} whose group actually
9755 @cindex error messages, omitting
9756 Do not print error messages about files whose group cannot be
9760 @opindex --dereference
9761 @cindex symbolic links, changing owner
9763 Do not act on symbolic links themselves but rather on what they point to.
9764 This is the default.
9767 @itemx --no-dereference
9769 @opindex --no-dereference
9770 @cindex symbolic links, changing group
9772 Act on symbolic links themselves instead of what they point to.
9773 This mode relies on the @code{lchown} system call.
9774 On systems that do not provide the @code{lchown} system call,
9775 @command{chgrp} fails when a file specified on the command line
9777 By default, no diagnostic is issued for symbolic links encountered
9778 during a recursive traversal, but see @option{--verbose}.
9780 @itemx --preserve-root
9781 @opindex --preserve-root
9782 @cindex root directory, disallow recursive modification
9783 Fail upon any attempt to recursively change the root directory, @file{/}.
9784 Without @option{--recursive}, this option has no effect.
9785 @xref{Treating / specially}.
9787 @itemx --no-preserve-root
9788 @opindex --no-preserve-root
9789 @cindex root directory, allow recursive modification
9790 Cancel the effect of any preceding @option{--preserve-root} option.
9791 @xref{Treating / specially}.
9793 @item --reference=@var{ref_file}
9794 @opindex --reference
9795 Change the group of each @var{file} to be the same as that of
9796 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9797 group of the symbolic link, but rather that of the file it refers to.
9803 Output a diagnostic for every file processed.
9804 If a symbolic link is encountered during a recursive traversal
9805 on a system without the @code{lchown} system call, and @option{--no-dereference}
9806 is in effect, then issue a diagnostic saying neither the symbolic link nor
9807 its referent is being changed.
9812 @opindex --recursive
9813 @cindex recursively changing group ownership
9814 Recursively change the group ownership of directories and their contents.
9817 @xref{Traversing symlinks}.
9820 @xref{Traversing symlinks}.
9823 @xref{Traversing symlinks}.
9832 # Change the group of /u to "staff".
9835 # Change the group of /u and subfiles to "staff".
9840 @node chmod invocation
9841 @section @command{chmod}: Change access permissions
9844 @cindex changing access permissions
9845 @cindex access permissions, changing
9846 @cindex permissions, changing access
9848 @command{chmod} changes the access permissions of the named files. Synopsis:
9851 chmod [@var{option}]@dots{} @{@var{mode} | --reference=@var{ref_file}@} @var{file}@dots{}
9854 @cindex symbolic links, permissions of
9855 @command{chmod} never changes the permissions of symbolic links, since
9856 the @command{chmod} system call cannot change their permissions.
9857 This is not a problem since the permissions of symbolic links are
9858 never used. However, for each symbolic link listed on the command
9859 line, @command{chmod} changes the permissions of the pointed-to file.
9860 In contrast, @command{chmod} ignores symbolic links encountered during
9861 recursive directory traversals.
9863 A successful use of @command{chmod} clears the set-group-ID bit of a
9864 regular file if the file's group ID does not match the user's
9865 effective group ID or one of the user's supplementary group IDs,
9866 unless the user has appropriate privileges. Additional restrictions
9867 may cause the set-user-ID and set-group-ID bits of @var{mode} or
9868 @var{ref_file} to be ignored. This behavior depends on the policy and
9869 functionality of the underlying @code{chmod} system call. When in
9870 doubt, check the underlying system behavior.
9872 If used, @var{mode} specifies the new file mode bits.
9873 For details, see the section on @ref{File permissions}.
9874 If you really want @var{mode} to have a leading @samp{-}, you should
9875 use @option{--} first, e.g., @samp{chmod -- -w file}. Typically,
9876 though, @samp{chmod a-w file} is preferable, and @command{chmod -w
9877 file} (without the @option{--}) complains if it behaves differently
9878 from what @samp{chmod a-w file} would do.
9880 The program accepts the following options. Also see @ref{Common options}.
9888 Verbosely describe the action for each @var{file} whose permissions
9897 @cindex error messages, omitting
9898 Do not print error messages about files whose permissions cannot be
9901 @itemx --preserve-root
9902 @opindex --preserve-root
9903 @cindex root directory, disallow recursive modification
9904 Fail upon any attempt to recursively change the root directory, @file{/}.
9905 Without @option{--recursive}, this option has no effect.
9906 @xref{Treating / specially}.
9908 @itemx --no-preserve-root
9909 @opindex --no-preserve-root
9910 @cindex root directory, allow recursive modification
9911 Cancel the effect of any preceding @option{--preserve-root} option.
9912 @xref{Treating / specially}.
9918 Verbosely describe the action or non-action taken for every @var{file}.
9920 @item --reference=@var{ref_file}
9921 @opindex --reference
9922 Change the mode of each @var{file} to be the same as that of @var{ref_file}.
9923 @xref{File permissions}.
9924 If @var{ref_file} is a symbolic link, do not use the mode
9925 of the symbolic link, but rather that of the file it refers to.
9930 @opindex --recursive
9931 @cindex recursively changing access permissions
9932 Recursively change permissions of directories and their contents.
9939 @node touch invocation
9940 @section @command{touch}: Change file timestamps
9943 @cindex changing file timestamps
9944 @cindex file timestamps, changing
9945 @cindex timestamps, changing file
9947 @command{touch} changes the access and/or modification times of the
9948 specified files. Synopsis:
9951 touch [@var{option}]@dots{} @var{file}@dots{}
9954 @cindex empty files, creating
9955 Any @var{file} argument that does not exist is created empty, unless
9956 option @option{--no-create} (@option{-c}) or @option{--no-dereference}
9957 (@option{-h}) was in effect.
9959 A @var{file} argument string of @samp{-} is handled specially and
9960 causes @command{touch} to change the times of the file associated with
9963 @cindex permissions, for changing file timestamps
9964 If changing both the access and modification times to the current
9965 time, @command{touch} can change the timestamps for files that the user
9966 running it does not own but has write permission for. Otherwise, the
9967 user must own the files.
9969 Although @command{touch} provides options for changing two of the times---the
9970 times of last access and modification---of a file, there is actually
9971 a standard third one as well: the inode change time. This is often
9972 referred to as a file's @code{ctime}.
9973 The inode change time represents the time when the file's meta-information
9974 last changed. One common example of this is when the permissions of a
9975 file change. Changing the permissions doesn't access the file, so
9976 the atime doesn't change, nor does it modify the file, so the mtime
9977 doesn't change. Yet, something about the file itself has changed,
9978 and this must be noted somewhere. This is the job of the ctime field.
9979 This is necessary, so that, for example, a backup program can make a
9980 fresh copy of the file, including the new permissions value.
9981 Another operation that modifies a file's ctime without affecting
9982 the others is renaming. In any case, it is not possible, in normal
9983 operations, for a user to change the ctime field to a user-specified value.
9984 Some operating systems and file systems support a fourth time: the
9985 birth time, when the file was first created; by definition, this
9986 timestamp never changes.
9989 Time stamps assume the time zone rules specified by the @env{TZ}
9990 environment variable, or by the system default rules if @env{TZ} is
9991 not set. @xref{TZ Variable,, Specifying the Time Zone with @env{TZ},
9992 libc, The GNU C Library Reference Manual}.
9993 You can avoid ambiguities during
9994 daylight saving transitions by using @sc{utc} time stamps.
9996 The program accepts the following options. Also see @ref{Common options}.
10001 @itemx --time=atime
10002 @itemx --time=access
10006 @opindex atime@r{, changing}
10007 @opindex access @r{time, changing}
10008 @opindex use @r{time, changing}
10009 Change the access time only.
10014 @opindex --no-create
10015 Do not warn about or create files that do not exist.
10018 @itemx --date=@var{time}
10022 Use @var{time} instead of the current time. It can contain month names,
10023 time zones, @samp{am} and @samp{pm}, @samp{yesterday}, etc. For
10024 example, @option{--date="2004-02-27 14:19:13.489392193 +0530"}
10025 specifies the instant of time that is 489,392,193 nanoseconds after
10026 February 27, 2004 at 2:19:13 PM in a time zone that is 5 hours and 30
10027 minutes east of @acronym{UTC}. @xref{Date input formats}.
10028 File systems that do not support high-resolution time stamps
10029 silently ignore any excess precision here.
10033 @cindex BSD @command{touch} compatibility
10034 Ignored; for compatibility with BSD versions of @command{touch}.
10037 @itemx --no-dereference
10039 @opindex --no-dereference
10040 @cindex symbolic links, changing time
10042 Attempt to change the timestamps of a symbolic link, rather than what
10043 the link refers to. When using this option, empty files are not
10044 created, but option @option{-c} must also be used to avoid warning
10045 about files that do not exist. Not all systems support changing the
10046 timestamps of symlinks, since underlying system support for this
10047 action was not required until @acronym{POSIX} 2008. Also, on some
10048 systems, the mere act of examining a symbolic link changes the access
10049 time, such that only changes to the modification time will persist
10050 long enough to be observable. When coupled with option @option{-r}, a
10051 reference timestamp is taken from a symbolic link rather than the file
10055 @itemx --time=mtime
10056 @itemx --time=modify
10059 @opindex mtime@r{, changing}
10060 @opindex modify @r{time, changing}
10061 Change the modification time only.
10063 @item -r @var{file}
10064 @itemx --reference=@var{file}
10066 @opindex --reference
10067 Use the times of the reference @var{file} instead of the current time.
10068 If this option is combined with the @option{--date=@var{time}}
10069 (@option{-d @var{time}}) option, the reference @var{file}'s time is
10070 the origin for any relative @var{time}s given, but is otherwise ignored.
10071 For example, @samp{-r foo -d '-5 seconds'} specifies a time stamp
10072 equal to five seconds before the corresponding time stamp for @file{foo}.
10073 If @var{file} is a symbolic link, the reference timestamp is taken
10074 from the target of the symlink, unless @option{-h} was also in effect.
10076 @item -t [[@var{cc}]@var{yy}]@var{mmddhhmm}[.@var{ss}]
10077 Use the argument (optional four-digit or two-digit years, months,
10078 days, hours, minutes, optional seconds) instead of the current time.
10079 If the year is specified with only two digits, then @var{cc}
10080 is 20 for years in the range 0 @dots{} 68, and 19 for years in
10081 69 @dots{} 99. If no digits of the year are specified,
10082 the argument is interpreted as a date in the current year.
10083 Note that @var{ss} may be @samp{60}, to accommodate leap seconds.
10087 @vindex _POSIX2_VERSION
10088 On older systems, @command{touch} supports an obsolete syntax, as follows.
10089 If no timestamp is given with any of the @option{-d}, @option{-r}, or
10090 @option{-t} options, and if there are two or more @var{file}s and the
10091 first @var{file} is of the form @samp{@var{mmddhhmm}[@var{yy}]} and this
10092 would be a valid argument to the @option{-t} option (if the @var{yy}, if
10093 any, were moved to the front), and if the represented year
10094 is in the range 1969--1999, that argument is interpreted as the time
10095 for the other files instead of as a file name.
10096 This obsolete behavior can be enabled or disabled with the
10097 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
10098 conformance}), but portable scripts should avoid commands whose
10099 behavior depends on this variable.
10100 For example, use @samp{touch ./12312359 main.c} or @samp{touch -t
10101 12312359 main.c} rather than the ambiguous @samp{touch 12312359 main.c}.
10107 @chapter Disk usage
10111 No disk can hold an infinite amount of data. These commands report
10112 how much disk storage is in use or available, report other file and
10113 file status information, and write buffers to disk.
10116 * df invocation:: Report file system disk space usage.
10117 * du invocation:: Estimate file space usage.
10118 * stat invocation:: Report file or file system status.
10119 * sync invocation:: Synchronize memory and disk.
10120 * truncate invocation:: Shrink or extend the size of a file.
10124 @node df invocation
10125 @section @command{df}: Report file system disk space usage
10128 @cindex file system disk usage
10129 @cindex disk usage by file system
10131 @command{df} reports the amount of disk space used and available on
10132 file systems. Synopsis:
10135 df [@var{option}]@dots{} [@var{file}]@dots{}
10138 With no arguments, @command{df} reports the space used and available on all
10139 currently mounted file systems (of all types). Otherwise, @command{df}
10140 reports on the file system containing each argument @var{file}.
10142 Normally the disk space is printed in units of
10143 1024 bytes, but this can be overridden (@pxref{Block size}).
10144 Non-integer quantities are rounded up to the next higher unit.
10146 @cindex disk device file
10147 @cindex device file, disk
10148 If an argument @var{file} is a disk device file containing a mounted
10149 file system, @command{df} shows the space available on that file system
10150 rather than on the file system containing the device node (i.e., the root
10151 file system). @sc{gnu} @command{df} does not attempt to determine the disk usage
10152 on unmounted file systems, because on most kinds of systems doing so
10153 requires extremely nonportable intimate knowledge of file system
10156 The program accepts the following options. Also see @ref{Common options}.
10164 @cindex automounter file systems
10165 @cindex ignore file systems
10166 Include in the listing dummy file systems, which
10167 are omitted by default. Such file systems are typically special-purpose
10168 pseudo-file-systems, such as automounter entries.
10170 @item -B @var{size}
10171 @itemx --block-size=@var{size}
10173 @opindex --block-size
10174 @cindex file system sizes
10175 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10176 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10180 @cindex grand total of disk size, usage and available space
10181 Print a grand total of all arguments after all arguments have
10182 been processed. This can be used to find out the total disk size, usage
10183 and available space of all listed devices.
10189 Equivalent to @option{--si}.
10195 @cindex inode usage
10196 List inode usage information instead of block usage. An inode (short
10197 for index node) contains information about a file such as its owner,
10198 permissions, timestamps, and location on the disk.
10202 @cindex kibibytes for file system sizes
10203 Print sizes in 1024-byte blocks, overriding the default block size
10204 (@pxref{Block size}).
10205 This option is equivalent to @option{--block-size=1K}.
10211 @cindex file system types, limiting output to certain
10212 Limit the listing to local file systems. By default, remote file systems
10217 @cindex file system space, retrieving old data more quickly
10218 Do not invoke the @code{sync} system call before getting any usage data.
10219 This may make @command{df} run significantly faster on systems with many
10220 disks, but on some systems (notably SunOS) the results may be slightly
10221 out of date. This is the default.
10224 @itemx --portability
10226 @opindex --portability
10227 @cindex one-line output format
10228 @cindex @acronym{POSIX} output format
10229 @cindex portable output format
10230 @cindex output format, portable
10231 Use the @acronym{POSIX} output format. This is like the default format except
10236 The information about each file system is always printed on exactly
10237 one line; a mount device is never put on a line by itself. This means
10238 that if the mount device name is more than 20 characters long (e.g., for
10239 some network mounts), the columns are misaligned.
10242 The labels in the header output line are changed to conform to @acronym{POSIX}.
10245 The default block size and output format are unaffected by the
10246 @env{DF_BLOCK_SIZE}, @env{BLOCK_SIZE} and @env{BLOCKSIZE} environment
10247 variables. However, the default block size is still affected by
10248 @env{POSIXLY_CORRECT}: it is 512 if @env{POSIXLY_CORRECT} is set, 1024
10249 otherwise. @xref{Block size}.
10256 @cindex file system space, retrieving current data more slowly
10257 Invoke the @code{sync} system call before getting any usage data. On
10258 some systems (notably SunOS), doing this yields more up to date results,
10259 but in general this option makes @command{df} much slower, especially when
10260 there are many or very busy file systems.
10262 @item -t @var{fstype}
10263 @itemx --type=@var{fstype}
10266 @cindex file system types, limiting output to certain
10267 Limit the listing to file systems of type @var{fstype}. Multiple
10268 file system types can be specified by giving multiple @option{-t} options.
10269 By default, nothing is omitted.
10272 @itemx --print-type
10274 @opindex --print-type
10275 @cindex file system types, printing
10276 Print each file system's type. The types printed here are the same ones
10277 you can include or exclude with @option{-t} and @option{-x}. The particular
10278 types printed are whatever is supported by the system. Here are some of
10279 the common names (this list is certainly not exhaustive):
10284 @cindex @acronym{NFS} file system type
10285 An @acronym{NFS} file system, i.e., one mounted over a network from another
10286 machine. This is the one type name which seems to be used uniformly by
10289 @item 4.2@r{, }ufs@r{, }efs@dots{}
10290 @cindex Linux file system types
10291 @cindex local file system types
10292 @opindex 4.2 @r{file system type}
10293 @opindex ufs @r{file system type}
10294 @opindex efs @r{file system type}
10295 A file system on a locally-mounted hard disk. (The system might even
10296 support more than one type here; Linux does.)
10298 @item hsfs@r{, }cdfs
10299 @cindex CD-ROM file system type
10300 @cindex High Sierra file system
10301 @opindex hsfs @r{file system type}
10302 @opindex cdfs @r{file system type}
10303 A file system on a CD-ROM drive. HP-UX uses @samp{cdfs}, most other
10304 systems use @samp{hsfs} (@samp{hs} for ``High Sierra'').
10307 @cindex PC file system
10308 @cindex DOS file system
10309 @cindex MS-DOS file system
10310 @cindex diskette file system
10312 An MS-DOS file system, usually on a diskette.
10316 @item -x @var{fstype}
10317 @itemx --exclude-type=@var{fstype}
10319 @opindex --exclude-type
10320 Limit the listing to file systems not of type @var{fstype}.
10321 Multiple file system types can be eliminated by giving multiple
10322 @option{-x} options. By default, no file system types are omitted.
10325 Ignored; for compatibility with System V versions of @command{df}.
10330 Failure includes the case where no output is generated, so you can
10331 inspect the exit status of a command like @samp{df -t ext3 -t reiserfs
10332 @var{dir}} to test whether @var{dir} is on a file system of type
10333 @samp{ext3} or @samp{reiserfs}.
10336 @node du invocation
10337 @section @command{du}: Estimate file space usage
10340 @cindex file space usage
10341 @cindex disk usage for files
10343 @command{du} reports the amount of disk space used by the specified files
10344 and for each subdirectory (of directory arguments). Synopsis:
10347 du [@var{option}]@dots{} [@var{file}]@dots{}
10350 With no arguments, @command{du} reports the disk space for the current
10351 directory. Normally the disk space is printed in units of
10352 1024 bytes, but this can be overridden (@pxref{Block size}).
10353 Non-integer quantities are rounded up to the next higher unit.
10355 If two or more hard links point to the same file, only one of the hard
10356 links is counted. The @var{file} argument order affects which links
10357 are counted, and changing the argument order may change the numbers
10358 that @command{du} outputs.
10360 The program accepts the following options. Also see @ref{Common options}.
10368 Show counts for all files, not just directories.
10370 @itemx --apparent-size
10371 @opindex --apparent-size
10372 Print apparent sizes, rather than disk usage. The apparent size of a
10373 file is the number of bytes reported by @code{wc -c} on regular files,
10374 or more generally, @code{ls -l --block-size=1} or @code{stat --format=%s}.
10375 For example, a file containing the word @samp{zoo} with no newline would,
10376 of course, have an apparent size of 3. Such a small file may require
10377 anywhere from 0 to 16 KiB or more of disk space, depending on
10378 the type and configuration of the file system on which the file resides.
10379 However, a sparse file created with this command:
10382 dd bs=1 seek=2GiB if=/dev/null of=big
10386 has an apparent size of 2 GiB, yet on most modern
10387 systems, it actually uses almost no disk space.
10393 Equivalent to @code{--apparent-size --block-size=1}.
10395 @item -B @var{size}
10396 @itemx --block-size=@var{size}
10398 @opindex --block-size
10400 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10401 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10407 @cindex grand total of disk space
10408 Print a grand total of all arguments after all arguments have
10409 been processed. This can be used to find out the total disk usage of
10410 a given set of files or directories.
10413 @itemx --dereference-args
10415 @opindex --dereference-args
10416 Dereference symbolic links that are command line arguments.
10417 Does not affect other symbolic links. This is helpful for finding
10418 out the disk usage of directories, such as @file{/usr/tmp}, which
10419 are often symbolic links.
10421 @c --files0-from=FILE
10422 @filesZeroFromOption{du,, with the @option{--total} (@option{-c}) option}
10428 Equivalent to @option{--dereference-args} (@option{-D}).
10432 @cindex kibibytes for file sizes
10433 Print sizes in 1024-byte blocks, overriding the default block size
10434 (@pxref{Block size}).
10435 This option is equivalent to @option{--block-size=1K}.
10438 @itemx --count-links
10440 @opindex --count-links
10441 @cindex hard links, counting in @command{du}
10442 Count the size of all files, even if they have appeared already (as a
10446 @itemx --dereference
10448 @opindex --dereference
10449 @cindex symbolic links, dereferencing in @command{du}
10450 Dereference symbolic links (show the disk space used by the file
10451 or directory that the link points to instead of the space used by
10456 @cindex mebibytes for file sizes
10457 Print sizes in 1,048,576-byte blocks, overriding the default block size
10458 (@pxref{Block size}).
10459 This option is equivalent to @option{--block-size=1M}.
10462 @itemx --no-dereference
10464 @opindex --no-dereference
10465 @cindex symbolic links, dereferencing in @command{du}
10466 For each symbolic links encountered by @command{du},
10467 consider the disk space used by the symbolic link.
10469 @item -d @var{depth}
10470 @item --max-depth=@var{depth}
10471 @opindex -d @var{depth}
10472 @opindex --max-depth=@var{depth}
10473 @cindex limiting output of @command{du}
10474 Show the total for each directory (and file if --all) that is at
10475 most MAX_DEPTH levels down from the root of the hierarchy. The root
10476 is at level 0, so @code{du --max-depth=0} is equivalent to @code{du -s}.
10485 @opindex --summarize
10486 Display only a total for each argument.
10489 @itemx --separate-dirs
10491 @opindex --separate-dirs
10492 Normally, in the output of @command{du} (when not using @option{--summarize}),
10493 the size listed next to a directory name, @var{d}, represents the sum
10494 of sizes of all entries beneath @var{d} as well as the size of @var{d} itself.
10495 With @option{--separate-dirs}, the size reported for a directory name,
10496 @var{d}, is merely the @code{stat.st_size}-derived size of the directory
10501 @cindex last modified dates, displaying in @command{du}
10502 Show time of the most recent modification of any file in the directory,
10503 or any of its subdirectories.
10505 @itemx --time=ctime
10506 @itemx --time=status
10509 @opindex ctime@r{, show the most recent}
10510 @opindex status time@r{, show the most recent}
10511 @opindex use time@r{, show the most recent}
10512 Show the most recent status change time (the @samp{ctime} in the inode) of
10513 any file in the directory, instead of the modification time.
10515 @itemx --time=atime
10516 @itemx --time=access
10518 @opindex atime@r{, show the most recent}
10519 @opindex access time@r{, show the most recent}
10520 Show the most recent access time (the @samp{atime} in the inode) of
10521 any file in the directory, instead of the modification time.
10523 @item --time-style=@var{style}
10524 @opindex --time-style
10526 List timestamps in style @var{style}. This option has an effect only if
10527 the @option{--time} option is also specified. The @var{style} should
10528 be one of the following:
10531 @item +@var{format}
10533 List timestamps using @var{format}, where @var{format} is interpreted
10534 like the format argument of @command{date} (@pxref{date invocation}).
10535 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
10536 @command{du} to list timestamps like @samp{2002-03-30 23:45:56}. As
10537 with @command{date}, @var{format}'s interpretation is affected by the
10538 @env{LC_TIME} locale category.
10541 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
10542 format with nanosecond precision, e.g., @samp{2002-03-30
10543 23:45:56.477817180 -0700}. This style is equivalent to
10544 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
10547 List @acronym{ISO} 8601 date and time in minutes, e.g.,
10548 @samp{2002-03-30 23:45}. These timestamps are shorter than
10549 @samp{full-iso} timestamps, and are usually good enough for everyday
10550 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
10553 List @acronym{ISO} 8601 dates for timestamps, e.g., @samp{2002-03-30}.
10554 This style is equivalent to @samp{+%Y-%m-%d}.
10558 You can specify the default value of the @option{--time-style} option
10559 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
10560 the default style is @samp{long-iso}. For compatibility with @command{ls},
10561 if @env{TIME_STYLE} begins with @samp{+} and contains a newline,
10562 the newline and any later characters are ignored; if @env{TIME_STYLE}
10563 begins with @samp{posix-} the @samp{posix-} is ignored; and if
10564 @env{TIME_STYLE} is @samp{locale} it is ignored.
10567 @itemx --one-file-system
10569 @opindex --one-file-system
10570 @cindex one file system, restricting @command{du} to
10571 Skip directories that are on different file systems from the one that
10572 the argument being processed is on.
10574 @item --exclude=@var{pattern}
10575 @opindex --exclude=@var{pattern}
10576 @cindex excluding files from @command{du}
10577 When recursing, skip subdirectories or files matching @var{pattern}.
10578 For example, @code{du --exclude='*.o'} excludes files whose names
10581 @item -X @var{file}
10582 @itemx --exclude-from=@var{file}
10583 @opindex -X @var{file}
10584 @opindex --exclude-from=@var{file}
10585 @cindex excluding files from @command{du}
10586 Like @option{--exclude}, except take the patterns to exclude from @var{file},
10587 one per line. If @var{file} is @samp{-}, take the patterns from standard
10592 @cindex NFS mounts from BSD to HP-UX
10593 On BSD systems, @command{du} reports sizes that are half the correct
10594 values for files that are NFS-mounted from HP-UX systems. On HP-UX
10595 systems, it reports sizes that are twice the correct values for
10596 files that are NFS-mounted from BSD systems. This is due to a flaw
10597 in HP-UX; it also affects the HP-UX @command{du} program.
10602 @node stat invocation
10603 @section @command{stat}: Report file or file system status
10606 @cindex file status
10607 @cindex file system status
10609 @command{stat} displays information about the specified file(s). Synopsis:
10612 stat [@var{option}]@dots{} [@var{file}]@dots{}
10615 With no option, @command{stat} reports all information about the given files.
10616 But it also can be used to report the information of the file systems the
10617 given files are located on. If the files are links, @command{stat} can
10618 also give information about the files the links point to.
10620 @mayConflictWithShellBuiltIn{stat}
10625 @itemx --dereference
10627 @opindex --dereference
10628 @cindex symbolic links, dereferencing in @command{stat}
10629 Change how @command{stat} treats symbolic links.
10630 With this option, @command{stat} acts on the file referenced
10631 by each symbolic link argument.
10632 Without it, @command{stat} acts on any symbolic link argument directly.
10635 @itemx --file-system
10637 @opindex --file-system
10638 @cindex file systems
10639 Report information about the file systems where the given files are located
10640 instead of information about the files themselves.
10641 This option implies the @option{-L} option.
10644 @itemx --format=@var{format}
10646 @opindex --format=@var{format}
10647 @cindex output format
10648 Use @var{format} rather than the default format.
10649 @var{format} is automatically newline-terminated, so
10650 running a command like the following with two or more @var{file}
10651 operands produces a line of output for each operand:
10653 $ stat --format=%d:%i / /usr
10658 @itemx --printf=@var{format}
10659 @opindex --printf=@var{format}
10660 @cindex output format
10661 Use @var{format} rather than the default format.
10662 Like @option{--format}, but interpret backslash escapes,
10663 and do not output a mandatory trailing newline.
10664 If you want a newline, include @samp{\n} in the @var{format}.
10665 Here's how you would use @option{--printf} to print the device
10666 and inode numbers of @file{/} and @file{/usr}:
10668 $ stat --printf='%d:%i\n' / /usr
10677 @cindex terse output
10678 Print the information in terse form, suitable for parsing by other programs.
10682 The valid @var{format} directives for files with @option{--format} and
10683 @option{--printf} are:
10686 @item %a - Access rights in octal
10687 @item %A - Access rights in human readable form
10688 @item %b - Number of blocks allocated (see @samp{%B})
10689 @item %B - The size in bytes of each block reported by @samp{%b}
10690 @item %C - The SELinux security context of a file, if available
10691 @item %d - Device number in decimal
10692 @item %D - Device number in hex
10693 @item %f - Raw mode in hex
10694 @item %F - File type
10695 @item %g - Group ID of owner
10696 @item %G - Group name of owner
10697 @item %h - Number of hard links
10698 @item %i - Inode number
10699 @item %m - Mount point (See note below)
10700 @item %n - File name
10701 @item %N - Quoted file name with dereference if symbolic link
10702 @item %o - I/O block size
10703 @item %s - Total size, in bytes
10704 @item %t - Major device type in hex
10705 @item %T - Minor device type in hex
10706 @item %u - User ID of owner
10707 @item %U - User name of owner
10708 @item %w - Time of file birth, or @samp{-} if unknown
10709 @item %W - Time of file birth as seconds since Epoch, or @samp{0}
10710 @item %x - Time of last access
10711 @item %X - Time of last access as seconds since Epoch
10712 @item %y - Time of last modification
10713 @item %Y - Time of last modification as seconds since Epoch
10714 @item %z - Time of last change
10715 @item %Z - Time of last change as seconds since Epoch
10718 The @samp{%W}, @samp{%X}, @samp{%Y}, and @samp{%Z} formats accept a
10719 precision preceded by a period to specify the number of digits to
10720 print after the decimal point. For example, @samp{%.9X} outputs the
10721 last access time to nanosecond precision. If a period is given but no
10722 precision, @command{stat} uses the estimated precision of the file
10723 system. When discarding excess precision, time stamps are truncated
10724 toward minus infinity.
10728 $ stat -c '[%015Y]' /usr
10731 $ stat -c '[%15Y]' /usr
10733 $ stat -c '[%-15Y]' /usr
10736 $ stat -c '[%.3Y]' /usr
10738 $ stat -c '[%.Y]' /usr
10739 [1288929712.114951]
10742 The mount point printed by @samp{%m} is similar to that output
10743 by @command{df}, except that:
10746 stat does not dereference symlinks by default
10747 (unless @option{-L} is specified)
10749 stat does not search for specified device nodes in the
10750 file system list, instead operating on them directly
10753 stat outputs the alias for a bind mounted file, rather than
10754 the initial mount point of its backing device.
10755 One can recursively call stat until there is no change in output,
10756 to get the current base mount point
10759 When listing file system information (@option{--file-system} (@option{-f})),
10760 you must use a different set of @var{format} directives:
10763 @item %a - Free blocks available to non-super-user
10764 @item %b - Total data blocks in file system
10765 @item %c - Total file nodes in file system
10766 @item %d - Free file nodes in file system
10767 @item %f - Free blocks in file system
10768 @item %i - File System ID in hex
10769 @item %l - Maximum length of file names
10770 @item %n - File name
10771 @item %s - Block size (for faster transfers)
10772 @item %S - Fundamental block size (for block counts)
10773 @item %t - Type in hex
10774 @item %T - Type in human readable form
10778 Time stamps are listed according to the time zone rules specified by
10779 the @env{TZ} environment variable, or by the system default rules if
10780 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
10781 with @env{TZ}, libc, The GNU C Library Reference Manual}.
10786 @node sync invocation
10787 @section @command{sync}: Synchronize data on disk with memory
10790 @cindex synchronize disk and memory
10792 @cindex superblock, writing
10793 @cindex inodes, written buffered
10794 @command{sync} writes any data buffered in memory out to disk. This can
10795 include (but is not limited to) modified superblocks, modified inodes,
10796 and delayed reads and writes. This must be implemented by the kernel;
10797 The @command{sync} program does nothing but exercise the @code{sync} system
10800 @cindex crashes and corruption
10801 The kernel keeps data in memory to avoid doing (relatively slow) disk
10802 reads and writes. This improves performance, but if the computer
10803 crashes, data may be lost or the file system corrupted as a
10804 result. The @command{sync} command ensures everything in memory
10805 is written to disk.
10807 Any arguments are ignored, except for a lone @option{--help} or
10808 @option{--version} (@pxref{Common options}).
10813 @node truncate invocation
10814 @section @command{truncate}: Shrink or extend the size of a file
10817 @cindex truncating, file sizes
10819 @command{truncate} shrinks or extends the size of each @var{file} to the
10820 specified size. Synopsis:
10823 truncate @var{option}@dots{} @var{file}@dots{}
10826 @cindex files, creating
10827 Any @var{file} that does not exist is created.
10829 @cindex sparse files, creating
10830 @cindex holes, creating files with
10831 If a @var{file} is larger than the specified size, the extra data is lost.
10832 If a @var{file} is shorter, it is extended and the extended part (or hole)
10833 reads as zero bytes.
10835 The program accepts the following options. Also see @ref{Common options}.
10842 @opindex --no-create
10843 Do not create files that do not exist.
10848 @opindex --io-blocks
10849 Treat @var{size} as number of I/O blocks of the @var{file} rather than bytes.
10851 @item -r @var{rfile}
10852 @itemx --reference=@var{rfile}
10854 @opindex --reference
10855 Base the size of each @var{file} on the size of @var{rfile}.
10857 @item -s @var{size}
10858 @itemx --size=@var{size}
10861 Set or adjust the size of each @var{file} according to @var{size}.
10862 @multiplierSuffixesNoBlocks{size}
10864 @var{size} may also be prefixed by one of the following to adjust
10865 the size of each @var{file} based on their current size:
10867 @samp{+} => extend by
10868 @samp{-} => reduce by
10869 @samp{<} => at most
10870 @samp{>} => at least
10871 @samp{/} => round down to multiple of
10872 @samp{%} => round up to multiple of
10880 @node Printing text
10881 @chapter Printing text
10883 @cindex printing text, commands for
10884 @cindex commands for printing text
10886 This section describes commands that display text strings.
10889 * echo invocation:: Print a line of text.
10890 * printf invocation:: Format and print data.
10891 * yes invocation:: Print a string until interrupted.
10895 @node echo invocation
10896 @section @command{echo}: Print a line of text
10899 @cindex displaying text
10900 @cindex printing text
10901 @cindex text, displaying
10902 @cindex arbitrary text, displaying
10904 @command{echo} writes each given @var{string} to standard output, with a
10905 space between each and a newline after the last one. Synopsis:
10908 echo [@var{option}]@dots{} [@var{string}]@dots{}
10911 @mayConflictWithShellBuiltIn{echo}
10913 The program accepts the following options. Also see @ref{Common options}.
10914 Options must precede operands, and the normally-special argument
10915 @samp{--} has no special meaning and is treated like any other
10921 Do not output the trailing newline.
10925 @cindex backslash escapes
10926 Enable interpretation of the following backslash-escaped characters in
10935 produce no further output
10951 the eight-bit value that is the octal number @var{nnn}
10952 (zero to three octal digits)
10954 the eight-bit value that is the octal number @var{nnn}
10955 (one to three octal digits)
10957 the eight-bit value that is the hexadecimal number @var{hh}
10958 (one or two hexadecimal digits)
10963 @cindex backslash escapes
10964 Disable interpretation of backslash escapes in each @var{string}.
10965 This is the default. If @option{-e} and @option{-E} are both
10966 specified, the last one given takes effect.
10970 @vindex POSIXLY_CORRECT
10971 If the @env{POSIXLY_CORRECT} environment variable is set, then when
10972 @command{echo}'s first argument is not @option{-n} it outputs
10973 option-like arguments instead of treating them as options. For
10974 example, @code{echo -ne hello} outputs @samp{-ne hello} instead of
10975 plain @samp{hello}.
10977 @acronym{POSIX} does not require support for any options, and says
10978 that the behavior of @command{echo} is implementation-defined if any
10979 @var{string} contains a backslash or if the first argument is
10980 @option{-n}. Portable programs can use the @command{printf} command
10981 if they need to omit trailing newlines or output control characters or
10982 backslashes. @xref{printf invocation}.
10987 @node printf invocation
10988 @section @command{printf}: Format and print data
10991 @command{printf} does formatted printing of text. Synopsis:
10994 printf @var{format} [@var{argument}]@dots{}
10997 @command{printf} prints the @var{format} string, interpreting @samp{%}
10998 directives and @samp{\} escapes to format numeric and string arguments
10999 in a way that is mostly similar to the C @samp{printf} function.
11000 @xref{Output Conversion Syntax,, @command{printf} format directives,
11001 libc, The GNU C Library Reference Manual}, for details.
11002 The differences are listed below.
11004 @mayConflictWithShellBuiltIn{printf}
11009 The @var{format} argument is reused as necessary to convert all the
11010 given @var{argument}s. For example, the command @samp{printf %s a b}
11014 Missing @var{argument}s are treated as null strings or as zeros,
11015 depending on whether the context expects a string or a number. For
11016 example, the command @samp{printf %sx%d} prints @samp{x0}.
11020 An additional escape, @samp{\c}, causes @command{printf} to produce no
11021 further output. For example, the command @samp{printf 'A%sC\cD%sF' B
11022 E} prints @samp{ABC}.
11025 The hexadecimal escape sequence @samp{\x@var{hh}} has at most two
11026 digits, as opposed to C where it can have an unlimited number of
11027 digits. For example, the command @samp{printf '\x07e'} prints two
11028 bytes, whereas the C statement @samp{printf ("\x07e")} prints just
11033 @command{printf} has an additional directive, @samp{%b}, which prints its
11034 argument string with @samp{\} escapes interpreted in the same way as in
11035 the @var{format} string, except that octal escapes are of the form
11036 @samp{\0@var{ooo}} where @var{ooo} is 0 to 3 octal digits.
11037 If a precision is also given, it limits the number of bytes printed
11038 from the converted string.
11041 Numeric arguments must be single C constants, possibly with leading
11042 @samp{+} or @samp{-}. For example, @samp{printf %.4d -3} outputs
11046 @vindex POSIXLY_CORRECT
11047 If the leading character of a numeric argument is @samp{"} or @samp{'}
11048 then its value is the numeric value of the immediately following
11049 character. Any remaining characters are silently ignored if the
11050 @env{POSIXLY_CORRECT} environment variable is set; otherwise, a
11051 warning is printed. For example, @samp{printf "%d" "'a"} outputs
11052 @samp{97} on hosts that use the @acronym{ASCII} character set, since
11053 @samp{a} has the numeric value 97 in @acronym{ASCII}.
11058 A floating-point argument must use a period before any fractional
11059 digits, but is printed according to the @env{LC_NUMERIC} category of the
11060 current locale. For example, in a locale whose radix character is a
11061 comma, the command @samp{printf %g 3.14} outputs @samp{3,14} whereas
11062 the command @samp{printf %g 3,14} is an error.
11066 @command{printf} interprets @samp{\@var{ooo}} in @var{format} as an octal number
11067 (if @var{ooo} is 1 to 3 octal digits) specifying a character to print,
11068 and @samp{\x@var{hh}} as a hexadecimal number (if @var{hh} is 1 to 2 hex
11069 digits) specifying a character to print.
11074 @cindex ISO/IEC 10646
11076 @command{printf} interprets two character syntaxes introduced in
11077 @acronym{ISO} C 99:
11078 @samp{\u} for 16-bit Unicode (@acronym{ISO}/@acronym{IEC} 10646)
11079 characters, specified as
11080 four hexadecimal digits @var{hhhh}, and @samp{\U} for 32-bit Unicode
11081 characters, specified as eight hexadecimal digits @var{hhhhhhhh}.
11082 @command{printf} outputs the Unicode characters
11083 according to the @env{LC_CTYPE} locale. Unicode characters in the ranges
11084 U+0000...U+009F, U+D800...U+DFFF cannot be specified by this syntax, except
11085 for U+0024 ($), U+0040 (@@), and U+0060 (@`).
11087 The processing of @samp{\u} and @samp{\U} requires a full-featured
11088 @code{iconv} facility. It is activated on systems with glibc 2.2 (or newer),
11089 or when @code{libiconv} is installed prior to this package. Otherwise
11090 @samp{\u} and @samp{\U} will print as-is.
11092 The only options are a lone @option{--help} or
11093 @option{--version}. @xref{Common options}.
11094 Options must precede operands.
11096 The Unicode character syntaxes are useful for writing strings in a locale
11097 independent way. For example, a string containing the Euro currency symbol
11100 $ env printf '\u20AC 14.95'
11104 will be output correctly in all locales supporting the Euro symbol
11105 (@acronym{ISO}-8859-15, UTF-8, and others). Similarly, a Chinese string
11108 $ env printf '\u4e2d\u6587'
11112 will be output correctly in all Chinese locales (GB2312, BIG5, UTF-8, etc).
11114 Note that in these examples, the @command{printf} command has been
11115 invoked via @command{env} to ensure that we run the program found via
11116 your shell's search path, and not a shell alias or a built-in function.
11118 For larger strings, you don't need to look up the hexadecimal code
11119 values of each character one by one. @acronym{ASCII} characters mixed with \u
11120 escape sequences is also known as the JAVA source file encoding. You can
11121 use GNU recode 3.5c (or newer) to convert strings to this encoding. Here
11122 is how to convert a piece of text into a shell script which will output
11123 this text in a locale-independent way:
11126 $ LC_CTYPE=zh_CN.big5 /usr/local/bin/printf \
11127 '\u4e2d\u6587\n' > sample.txt
11128 $ recode BIG5..JAVA < sample.txt \
11129 | sed -e "s|^|/usr/local/bin/printf '|" -e "s|$|\\\\n'|" \
11136 @node yes invocation
11137 @section @command{yes}: Print a string until interrupted
11140 @cindex repeated output of a string
11142 @command{yes} prints the command line arguments, separated by spaces and
11143 followed by a newline, forever until it is killed. If no arguments are
11144 given, it prints @samp{y} followed by a newline forever until killed.
11146 Upon a write error, @command{yes} exits with status @samp{1}.
11148 The only options are a lone @option{--help} or @option{--version}.
11149 To output an argument that begins with
11150 @samp{-}, precede it with @option{--}, e.g., @samp{yes -- --help}.
11151 @xref{Common options}.
11155 @chapter Conditions
11158 @cindex commands for exit status
11159 @cindex exit status commands
11161 This section describes commands that are primarily useful for their exit
11162 status, rather than their output. Thus, they are often used as the
11163 condition of shell @code{if} statements, or as the last command in a
11167 * false invocation:: Do nothing, unsuccessfully.
11168 * true invocation:: Do nothing, successfully.
11169 * test invocation:: Check file types and compare values.
11170 * expr invocation:: Evaluate expressions.
11174 @node false invocation
11175 @section @command{false}: Do nothing, unsuccessfully
11178 @cindex do nothing, unsuccessfully
11179 @cindex failure exit status
11180 @cindex exit status of @command{false}
11182 @command{false} does nothing except return an exit status of 1, meaning
11183 @dfn{failure}. It can be used as a place holder in shell scripts
11184 where an unsuccessful command is needed.
11185 In most modern shells, @command{false} is a built-in command, so when
11186 you use @samp{false} in a script, you're probably using the built-in
11187 command, not the one documented here.
11189 @command{false} honors the @option{--help} and @option{--version} options.
11191 This version of @command{false} is implemented as a C program, and is thus
11192 more secure and faster than a shell script implementation, and may safely
11193 be used as a dummy shell for the purpose of disabling accounts.
11195 Note that @command{false} (unlike all other programs documented herein)
11196 exits unsuccessfully, even when invoked with
11197 @option{--help} or @option{--version}.
11199 Portable programs should not assume that the exit status of
11200 @command{false} is 1, as it is greater than 1 on some
11201 non-@acronym{GNU} hosts.
11204 @node true invocation
11205 @section @command{true}: Do nothing, successfully
11208 @cindex do nothing, successfully
11210 @cindex successful exit
11211 @cindex exit status of @command{true}
11213 @command{true} does nothing except return an exit status of 0, meaning
11214 @dfn{success}. It can be used as a place holder in shell scripts
11215 where a successful command is needed, although the shell built-in
11216 command @code{:} (colon) may do the same thing faster.
11217 In most modern shells, @command{true} is a built-in command, so when
11218 you use @samp{true} in a script, you're probably using the built-in
11219 command, not the one documented here.
11221 @command{true} honors the @option{--help} and @option{--version} options.
11223 Note, however, that it is possible to cause @command{true}
11224 to exit with nonzero status: with the @option{--help} or @option{--version}
11225 option, and with standard
11226 output already closed or redirected to a file that evokes an I/O error.
11227 For example, using a Bourne-compatible shell:
11230 $ ./true --version >&-
11231 ./true: write error: Bad file number
11232 $ ./true --version > /dev/full
11233 ./true: write error: No space left on device
11236 This version of @command{true} is implemented as a C program, and is thus
11237 more secure and faster than a shell script implementation, and may safely
11238 be used as a dummy shell for the purpose of disabling accounts.
11240 @node test invocation
11241 @section @command{test}: Check file types and compare values
11244 @cindex check file types
11245 @cindex compare values
11246 @cindex expression evaluation
11248 @command{test} returns a status of 0 (true) or 1 (false) depending on the
11249 evaluation of the conditional expression @var{expr}. Each part of the
11250 expression must be a separate argument.
11252 @command{test} has file status checks, string operators, and numeric
11253 comparison operators.
11255 @command{test} has an alternate form that uses opening and closing
11256 square brackets instead a leading @samp{test}. For example, instead
11257 of @samp{test -d /}, you can write @samp{[ -d / ]}. The square
11258 brackets must be separate arguments; for example, @samp{[-d /]} does
11259 not have the desired effect. Since @samp{test @var{expr}} and @samp{[
11260 @var{expr} ]} have the same meaning, only the former form is discussed
11266 test @var{expression}
11268 [ @var{expression} ]
11273 @mayConflictWithShellBuiltIn{test}
11275 If @var{expression} is omitted, @command{test} returns false.
11276 If @var{expression} is a single argument,
11277 @command{test} returns false if the argument is null and true otherwise. The argument
11278 can be any string, including strings like @samp{-d}, @samp{-1},
11279 @samp{--}, @samp{--help}, and @samp{--version} that most other
11280 programs would treat as options. To get help and version information,
11281 invoke the commands @samp{[ --help} and @samp{[ --version}, without
11282 the usual closing brackets. @xref{Common options}.
11284 @cindex exit status of @command{test}
11288 0 if the expression is true,
11289 1 if the expression is false,
11290 2 if an error occurred.
11294 * File type tests:: -[bcdfhLpSt]
11295 * Access permission tests:: -[gkruwxOG]
11296 * File characteristic tests:: -e -s -nt -ot -ef
11297 * String tests:: -z -n = !=
11298 * Numeric tests:: -eq -ne -lt -le -gt -ge
11299 * Connectives for test:: ! -a -o
11303 @node File type tests
11304 @subsection File type tests
11306 @cindex file type tests
11308 These options test for particular types of files. (Everything's a file,
11309 but not all files are the same!)
11313 @item -b @var{file}
11315 @cindex block special check
11316 True if @var{file} exists and is a block special device.
11318 @item -c @var{file}
11320 @cindex character special check
11321 True if @var{file} exists and is a character special device.
11323 @item -d @var{file}
11325 @cindex directory check
11326 True if @var{file} exists and is a directory.
11328 @item -f @var{file}
11330 @cindex regular file check
11331 True if @var{file} exists and is a regular file.
11333 @item -h @var{file}
11334 @itemx -L @var{file}
11337 @cindex symbolic link check
11338 True if @var{file} exists and is a symbolic link.
11339 Unlike all other file-related tests, this test does not dereference
11340 @var{file} if it is a symbolic link.
11342 @item -p @var{file}
11344 @cindex named pipe check
11345 True if @var{file} exists and is a named pipe.
11347 @item -S @var{file}
11349 @cindex socket check
11350 True if @var{file} exists and is a socket.
11354 @cindex terminal check
11355 True if @var{fd} is a file descriptor that is associated with a
11361 @node Access permission tests
11362 @subsection Access permission tests
11364 @cindex access permission tests
11365 @cindex permission tests
11367 These options test for particular access permissions.
11371 @item -g @var{file}
11373 @cindex set-group-ID check
11374 True if @var{file} exists and has its set-group-ID bit set.
11376 @item -k @var{file}
11378 @cindex sticky bit check
11379 True if @var{file} exists and has its @dfn{sticky} bit set.
11381 @item -r @var{file}
11383 @cindex readable file check
11384 True if @var{file} exists and read permission is granted.
11386 @item -u @var{file}
11388 @cindex set-user-ID check
11389 True if @var{file} exists and has its set-user-ID bit set.
11391 @item -w @var{file}
11393 @cindex writable file check
11394 True if @var{file} exists and write permission is granted.
11396 @item -x @var{file}
11398 @cindex executable file check
11399 True if @var{file} exists and execute permission is granted
11400 (or search permission, if it is a directory).
11402 @item -O @var{file}
11404 @cindex owned by effective user ID check
11405 True if @var{file} exists and is owned by the current effective user ID.
11407 @item -G @var{file}
11409 @cindex owned by effective group ID check
11410 True if @var{file} exists and is owned by the current effective group ID.
11414 @node File characteristic tests
11415 @subsection File characteristic tests
11417 @cindex file characteristic tests
11419 These options test other file characteristics.
11423 @item -e @var{file}
11425 @cindex existence-of-file check
11426 True if @var{file} exists.
11428 @item -s @var{file}
11430 @cindex nonempty file check
11431 True if @var{file} exists and has a size greater than zero.
11433 @item @var{file1} -nt @var{file2}
11435 @cindex newer-than file check
11436 True if @var{file1} is newer (according to modification date) than
11437 @var{file2}, or if @var{file1} exists and @var{file2} does not.
11439 @item @var{file1} -ot @var{file2}
11441 @cindex older-than file check
11442 True if @var{file1} is older (according to modification date) than
11443 @var{file2}, or if @var{file2} exists and @var{file1} does not.
11445 @item @var{file1} -ef @var{file2}
11447 @cindex same file check
11448 @cindex hard link check
11449 True if @var{file1} and @var{file2} have the same device and inode
11450 numbers, i.e., if they are hard links to each other.
11456 @subsection String tests
11458 @cindex string tests
11460 These options test string characteristics. You may need to quote
11461 @var{string} arguments for the shell. For example:
11467 The quotes here prevent the wrong arguments from being passed to
11468 @command{test} if @samp{$V} is empty or contains special characters.
11472 @item -z @var{string}
11474 @cindex zero-length string check
11475 True if the length of @var{string} is zero.
11477 @item -n @var{string}
11478 @itemx @var{string}
11480 @cindex nonzero-length string check
11481 True if the length of @var{string} is nonzero.
11483 @item @var{string1} = @var{string2}
11485 @cindex equal string check
11486 True if the strings are equal.
11488 @item @var{string1} != @var{string2}
11490 @cindex not-equal string check
11491 True if the strings are not equal.
11496 @node Numeric tests
11497 @subsection Numeric tests
11499 @cindex numeric tests
11500 @cindex arithmetic tests
11502 Numeric relational operators. The arguments must be entirely numeric
11503 (possibly negative), or the special expression @w{@code{-l @var{string}}},
11504 which evaluates to the length of @var{string}.
11508 @item @var{arg1} -eq @var{arg2}
11509 @itemx @var{arg1} -ne @var{arg2}
11510 @itemx @var{arg1} -lt @var{arg2}
11511 @itemx @var{arg1} -le @var{arg2}
11512 @itemx @var{arg1} -gt @var{arg2}
11513 @itemx @var{arg1} -ge @var{arg2}
11520 These arithmetic binary operators return true if @var{arg1} is equal,
11521 not-equal, less-than, less-than-or-equal, greater-than, or
11522 greater-than-or-equal than @var{arg2}, respectively.
11529 test -1 -gt -2 && echo yes
11531 test -l abc -gt 1 && echo yes
11534 @error{} test: integer expression expected before -eq
11538 @node Connectives for test
11539 @subsection Connectives for @command{test}
11541 @cindex logical connectives
11542 @cindex connectives, logical
11544 The usual logical connectives.
11550 True if @var{expr} is false.
11552 @item @var{expr1} -a @var{expr2}
11554 @cindex logical and operator
11555 @cindex and operator
11556 True if both @var{expr1} and @var{expr2} are true.
11558 @item @var{expr1} -o @var{expr2}
11560 @cindex logical or operator
11561 @cindex or operator
11562 True if either @var{expr1} or @var{expr2} is true.
11567 @node expr invocation
11568 @section @command{expr}: Evaluate expressions
11571 @cindex expression evaluation
11572 @cindex evaluation of expressions
11574 @command{expr} evaluates an expression and writes the result on standard
11575 output. Each token of the expression must be a separate argument.
11577 Operands are either integers or strings. Integers consist of one or
11578 more decimal digits, with an optional leading @samp{-}.
11579 @command{expr} converts
11580 anything appearing in an operand position to an integer or a string
11581 depending on the operation being applied to it.
11583 Strings are not quoted for @command{expr} itself, though you may need to
11584 quote them to protect characters with special meaning to the shell,
11585 e.g., spaces. However, regardless of whether it is quoted, a string
11586 operand should not be a parenthesis or any of @command{expr}'s
11587 operators like @code{+}, so you cannot safely pass an arbitrary string
11588 @code{$str} to expr merely by quoting it to the shell. One way to
11589 work around this is to use the @sc{gnu} extension @code{+},
11590 (e.g., @code{+ "$str" = foo}); a more portable way is to use
11591 @code{@w{" $str"}} and to adjust the rest of the expression to take
11592 the leading space into account (e.g., @code{@w{" $str" = " foo"}}).
11594 You should not pass a negative integer or a string with leading
11595 @samp{-} as @command{expr}'s first argument, as it might be
11596 misinterpreted as an option; this can be avoided by parenthesization.
11597 Also, portable scripts should not use a string operand that happens to
11598 take the form of an integer; this can be worked around by inserting
11599 leading spaces as mentioned above.
11601 @cindex parentheses for grouping
11602 Operators may be given as infix symbols or prefix keywords. Parentheses
11603 may be used for grouping in the usual manner. You must quote
11604 parentheses and many operators to avoid the shell evaluating them,
11607 When built with support for the GNU MP library, @command{expr} uses
11608 arbitrary-precision arithmetic; otherwise, it uses native arithmetic
11609 types and may fail due to arithmetic overflow.
11611 The only options are @option{--help} and @option{--version}. @xref{Common
11612 options}. Options must precede operands.
11614 @cindex exit status of @command{expr}
11618 0 if the expression is neither null nor 0,
11619 1 if the expression is null or 0,
11620 2 if the expression is invalid,
11621 3 if an internal error occurred (e.g., arithmetic overflow).
11625 * String expressions:: + : match substr index length
11626 * Numeric expressions:: + - * / %
11627 * Relations for expr:: | & < <= = == != >= >
11628 * Examples of expr:: Examples.
11632 @node String expressions
11633 @subsection String expressions
11635 @cindex string expressions
11636 @cindex expressions, string
11638 @command{expr} supports pattern matching and other string operators. These
11639 have higher precedence than both the numeric and relational operators (in
11640 the next sections).
11644 @item @var{string} : @var{regex}
11645 @cindex pattern matching
11646 @cindex regular expression matching
11647 @cindex matching patterns
11648 Perform pattern matching. The arguments are converted to strings and the
11649 second is considered to be a (basic, a la GNU @code{grep}) regular
11650 expression, with a @code{^} implicitly prepended. The first argument is
11651 then matched against this regular expression.
11653 If the match succeeds and @var{regex} uses @samp{\(} and @samp{\)}, the
11654 @code{:} expression returns the part of @var{string} that matched the
11655 subexpression; otherwise, it returns the number of characters matched.
11657 If the match fails, the @code{:} operator returns the null string if
11658 @samp{\(} and @samp{\)} are used in @var{regex}, otherwise 0.
11660 @kindex \( @r{regexp operator}
11661 Only the first @samp{\( @dots{} \)} pair is relevant to the return
11662 value; additional pairs are meaningful only for grouping the regular
11663 expression operators.
11665 @kindex \+ @r{regexp operator}
11666 @kindex \? @r{regexp operator}
11667 @kindex \| @r{regexp operator}
11668 In the regular expression, @code{\+}, @code{\?}, and @code{\|} are
11669 operators which respectively match one or more, zero or one, or separate
11670 alternatives. SunOS and other @command{expr}'s treat these as regular
11671 characters. (@acronym{POSIX} allows either behavior.)
11672 @xref{Top, , Regular Expression Library, regex, Regex}, for details of
11673 regular expression syntax. Some examples are in @ref{Examples of expr}.
11675 @item match @var{string} @var{regex}
11677 An alternative way to do pattern matching. This is the same as
11678 @w{@samp{@var{string} : @var{regex}}}.
11680 @item substr @var{string} @var{position} @var{length}
11682 Returns the substring of @var{string} beginning at @var{position}
11683 with length at most @var{length}. If either @var{position} or
11684 @var{length} is negative, zero, or non-numeric, returns the null string.
11686 @item index @var{string} @var{charset}
11688 Returns the first position in @var{string} where the first character in
11689 @var{charset} was found. If no character in @var{charset} is found in
11690 @var{string}, return 0.
11692 @item length @var{string}
11694 Returns the length of @var{string}.
11696 @item + @var{token}
11698 Interpret @var{token} as a string, even if it is a keyword like @var{match}
11699 or an operator like @code{/}.
11700 This makes it possible to test @code{expr length + "$x"} or
11701 @code{expr + "$x" : '.*/\(.\)'} and have it do the right thing even if
11702 the value of @var{$x} happens to be (for example) @code{/} or @code{index}.
11703 This operator is a @acronym{GNU} extension. Portable shell scripts should use
11704 @code{@w{" $token"} : @w{' \(.*\)'}} instead of @code{+ "$token"}.
11708 To make @command{expr} interpret keywords as strings, you must use the
11709 @code{quote} operator.
11712 @node Numeric expressions
11713 @subsection Numeric expressions
11715 @cindex numeric expressions
11716 @cindex expressions, numeric
11718 @command{expr} supports the usual numeric operators, in order of increasing
11719 precedence. These numeric operators have lower precedence than the
11720 string operators described in the previous section, and higher precedence
11721 than the connectives (next section).
11729 @cindex subtraction
11730 Addition and subtraction. Both arguments are converted to integers;
11731 an error occurs if this cannot be done.
11737 @cindex multiplication
11740 Multiplication, division, remainder. Both arguments are converted to
11741 integers; an error occurs if this cannot be done.
11746 @node Relations for expr
11747 @subsection Relations for @command{expr}
11749 @cindex connectives, logical
11750 @cindex logical connectives
11751 @cindex relations, numeric or string
11753 @command{expr} supports the usual logical connectives and relations. These
11754 have lower precedence than the string and numeric operators
11755 (previous sections). Here is the list, lowest-precedence operator first.
11761 @cindex logical or operator
11762 @cindex or operator
11763 Returns its first argument if that is neither null nor zero, otherwise
11764 its second argument if it is neither null nor zero, otherwise 0. It
11765 does not evaluate its second argument if its first argument is neither
11770 @cindex logical and operator
11771 @cindex and operator
11772 Return its first argument if neither argument is null or zero, otherwise
11773 0. It does not evaluate its second argument if its first argument is
11776 @item < <= = == != >= >
11783 @cindex comparison operators
11785 Compare the arguments and return 1 if the relation is true, 0 otherwise.
11786 @code{==} is a synonym for @code{=}. @command{expr} first tries to convert
11787 both arguments to integers and do a numeric comparison; if either
11788 conversion fails, it does a lexicographic comparison using the character
11789 collating sequence specified by the @env{LC_COLLATE} locale.
11794 @node Examples of expr
11795 @subsection Examples of using @command{expr}
11797 @cindex examples of @command{expr}
11798 Here are a few examples, including quoting for shell metacharacters.
11800 To add 1 to the shell variable @code{foo}, in Bourne-compatible shells:
11803 foo=`expr $foo + 1`
11806 To print the non-directory part of the file name stored in
11807 @code{$fname}, which need not contain a @code{/}:
11810 expr $fname : '.*/\(.*\)' '|' $fname
11813 An example showing that @code{\+} is an operator:
11821 expr abc : 'a\(.\)c'
11823 expr index abcdef cz
11826 @error{} expr: syntax error
11827 expr index + index a
11833 @chapter Redirection
11835 @cindex redirection
11836 @cindex commands for redirection
11838 Unix shells commonly provide several forms of @dfn{redirection}---ways
11839 to change the input source or output destination of a command. But one
11840 useful redirection is performed by a separate command, not by the shell;
11841 it's described here.
11844 * tee invocation:: Redirect output to multiple files or processes.
11848 @node tee invocation
11849 @section @command{tee}: Redirect output to multiple files or processes
11852 @cindex pipe fitting
11853 @cindex destinations, multiple output
11854 @cindex read from stdin and write to stdout and files
11856 The @command{tee} command copies standard input to standard output and also
11857 to any files given as arguments. This is useful when you want not only
11858 to send some data down a pipe, but also to save a copy. Synopsis:
11861 tee [@var{option}]@dots{} [@var{file}]@dots{}
11864 If a file being written to does not already exist, it is created. If a
11865 file being written to already exists, the data it previously contained
11866 is overwritten unless the @option{-a} option is used.
11868 A @var{file} of @samp{-} causes @command{tee} to send another copy of
11869 input to standard output, but this is typically not that useful as the
11870 copies are interleaved.
11872 The program accepts the following options. Also see @ref{Common options}.
11879 Append standard input to the given files rather than overwriting
11883 @itemx --ignore-interrupts
11885 @opindex --ignore-interrupts
11886 Ignore interrupt signals.
11890 The @command{tee} command is useful when you happen to be transferring a large
11891 amount of data and also want to summarize that data without reading
11892 it a second time. For example, when you are downloading a DVD image,
11893 you often want to verify its signature or checksum right away.
11894 The inefficient way to do it is simply:
11897 wget http://example.com/some.iso && sha1sum some.iso
11900 One problem with the above is that it makes you wait for the
11901 download to complete before starting the time-consuming SHA1 computation.
11902 Perhaps even more importantly, the above requires reading
11903 the DVD image a second time (the first was from the network).
11905 The efficient way to do it is to interleave the download
11906 and SHA1 computation. Then, you'll get the checksum for
11907 free, because the entire process parallelizes so well:
11910 # slightly contrived, to demonstrate process substitution
11911 wget -O - http://example.com/dvd.iso \
11912 | tee >(sha1sum > dvd.sha1) > dvd.iso
11915 That makes @command{tee} write not just to the expected output file,
11916 but also to a pipe running @command{sha1sum} and saving the final
11917 checksum in a file named @file{dvd.sha1}.
11919 Note, however, that this example relies on a feature of modern shells
11920 called @dfn{process substitution}
11921 (the @samp{>(command)} syntax, above;
11922 @xref{Process Substitution,,Process Substitution, bashref,
11923 The Bash Reference Manual}.),
11924 so it works with @command{zsh}, @command{bash}, and @command{ksh},
11925 but not with @command{/bin/sh}. So if you write code like this
11926 in a shell script, be sure to start the script with @samp{#!/bin/bash}.
11928 Since the above example writes to one file and one process,
11929 a more conventional and portable use of @command{tee} is even better:
11932 wget -O - http://example.com/dvd.iso \
11933 | tee dvd.iso | sha1sum > dvd.sha1
11936 You can extend this example to make @command{tee} write to two processes,
11937 computing MD5 and SHA1 checksums in parallel. In this case,
11938 process substitution is required:
11941 wget -O - http://example.com/dvd.iso \
11942 | tee >(sha1sum > dvd.sha1) \
11943 >(md5sum > dvd.md5) \
11947 This technique is also useful when you want to make a @emph{compressed}
11948 copy of the contents of a pipe.
11949 Consider a tool to graphically summarize disk usage data from @samp{du -ak}.
11950 For a large hierarchy, @samp{du -ak} can run for a long time,
11951 and can easily produce terabytes of data, so you won't want to
11952 rerun the command unnecessarily. Nor will you want to save
11953 the uncompressed output.
11955 Doing it the inefficient way, you can't even start the GUI
11956 until after you've compressed all of the @command{du} output:
11959 du -ak | gzip -9 > /tmp/du.gz
11960 gzip -d /tmp/du.gz | xdiskusage -a
11963 With @command{tee} and process substitution, you start the GUI
11964 right away and eliminate the decompression completely:
11967 du -ak | tee >(gzip -9 > /tmp/du.gz) | xdiskusage -a
11970 Finally, if you regularly create more than one type of
11971 compressed tarball at once, for example when @code{make dist} creates
11972 both @command{gzip}-compressed and @command{bzip2}-compressed tarballs,
11973 there may be a better way.
11974 Typical @command{automake}-generated @file{Makefile} rules create
11975 the two compressed tar archives with commands in sequence, like this
11976 (slightly simplified):
11979 tardir=your-pkg-M.N
11980 tar chof - "$tardir" | gzip -9 -c > your-pkg-M.N.tar.gz
11981 tar chof - "$tardir" | bzip2 -9 -c > your-pkg-M.N.tar.bz2
11984 However, if the hierarchy you are archiving and compressing is larger
11985 than a couple megabytes, and especially if you are using a multi-processor
11986 system with plenty of memory, then you can do much better by reading the
11987 directory contents only once and running the compression programs in parallel:
11990 tardir=your-pkg-M.N
11991 tar chof - "$tardir" \
11992 | tee >(gzip -9 -c > your-pkg-M.N.tar.gz) \
11993 | bzip2 -9 -c > your-pkg-M.N.tar.bz2
11999 @node File name manipulation
12000 @chapter File name manipulation
12002 @cindex file name manipulation
12003 @cindex manipulation of file names
12004 @cindex commands for file name manipulation
12006 This section describes commands that manipulate file names.
12009 * basename invocation:: Strip directory and suffix from a file name.
12010 * dirname invocation:: Strip last file name component.
12011 * pathchk invocation:: Check file name validity and portability.
12012 * mktemp invocation:: Create temporary file or directory.
12016 @node basename invocation
12017 @section @command{basename}: Strip directory and suffix from a file name
12020 @cindex strip directory and suffix from file names
12021 @cindex directory, stripping from file names
12022 @cindex suffix, stripping from file names
12023 @cindex file names, stripping directory and suffix
12024 @cindex leading directory components, stripping
12026 @command{basename} removes any leading directory components from
12027 @var{name}. Synopsis:
12030 basename @var{name} [@var{suffix}]
12033 If @var{suffix} is specified and is identical to the end of @var{name},
12034 it is removed from @var{name} as well. Note that since trailing slashes
12035 are removed prior to suffix matching, @var{suffix} will do nothing if it
12036 contains slashes. @command{basename} prints the result on standard
12039 @c This test is used both here and in the section on dirname.
12040 @macro basenameAndDirname
12041 Together, @command{basename} and @command{dirname} are designed such
12042 that if @samp{ls "$name"} succeeds, then the command sequence @samp{cd
12043 "$(dirname "$name")"; ls "$(basename "$name")"} will, too. This works
12044 for everything except file names containing a trailing newline.
12046 @basenameAndDirname
12048 @acronym{POSIX} allows the implementation to define the results if
12049 @var{name} is empty or @samp{//}. In the former case, @acronym{GNU}
12050 @command{basename} returns the empty string. In the latter case, the
12051 result is @samp{//} on platforms where @var{//} is distinct from
12052 @var{/}, and @samp{/} on platforms where there is no difference.
12054 The only options are @option{--help} and @option{--version}. @xref{Common
12055 options}. Options must precede operands.
12063 basename /usr/bin/sort
12066 basename include/stdio.h .h
12070 @node dirname invocation
12071 @section @command{dirname}: Strip last file name component
12074 @cindex directory components, printing
12075 @cindex stripping non-directory suffix
12076 @cindex non-directory suffix, stripping
12078 @command{dirname} prints all but the final slash-delimited component of
12079 @var{name}. Slashes on either side of the final component are also
12080 removed. If the string contains no slash, @command{dirname} prints
12081 @samp{.} (meaning the current directory). Synopsis:
12087 @var{name} need not be a file name, but if it is, this operation
12088 effectively lists the directory that contains the final component,
12089 including the case when the final component is itself a directory.
12091 @basenameAndDirname
12093 @acronym{POSIX} allows the implementation to define the results if
12094 @var{name} is @samp{//}. With @acronym{GNU} @command{dirname}, the
12095 result is @samp{//} on platforms where @var{//} is distinct from
12096 @var{/}, and @samp{/} on platforms where there is no difference.
12098 The only options are @option{--help} and @option{--version}. @xref{Common
12106 # Output "/usr/bin".
12107 dirname /usr/bin/sort
12108 dirname /usr/bin//.//
12115 @node pathchk invocation
12116 @section @command{pathchk}: Check file name validity and portability
12119 @cindex file names, checking validity and portability
12120 @cindex valid file names, checking for
12121 @cindex portable file names, checking for
12123 @command{pathchk} checks validity and portability of file names. Synopsis:
12126 pathchk [@var{option}]@dots{} @var{name}@dots{}
12129 For each @var{name}, @command{pathchk} prints an error message if any of
12130 these conditions is true:
12134 One of the existing directories in @var{name} does not have search
12135 (execute) permission,
12137 The length of @var{name} is larger than the maximum supported by the
12140 The length of one component of @var{name} is longer than
12141 its file system's maximum.
12144 A nonexistent @var{name} is not an error, so long a file with that
12145 name could be created under the above conditions.
12147 The program accepts the following options. Also see @ref{Common options}.
12148 Options must precede operands.
12154 Instead of performing checks based on the underlying file system,
12155 print an error message if any of these conditions is true:
12159 A file name is empty.
12162 A file name contains a character outside the @acronym{POSIX} portable file
12163 name character set, namely, the ASCII letters and digits, @samp{.},
12164 @samp{_}, @samp{-}, and @samp{/}.
12167 The length of a file name or one of its components exceeds the
12168 @acronym{POSIX} minimum limits for portability.
12173 Print an error message if a file name is empty, or if it contains a component
12174 that begins with @samp{-}.
12176 @item --portability
12177 @opindex --portability
12178 Print an error message if a file name is not portable to all @acronym{POSIX}
12179 hosts. This option is equivalent to @samp{-p -P}.
12183 @cindex exit status of @command{pathchk}
12187 0 if all specified file names passed all checks,
12191 @node mktemp invocation
12192 @section @command{mktemp}: Create temporary file or directory
12195 @cindex file names, creating temporary
12196 @cindex directory, creating temporary
12197 @cindex temporary files and directories
12199 @command{mktemp} manages the creation of temporary files and
12200 directories. Synopsis:
12203 mktemp [@var{option}]@dots{} [@var{template}]
12206 Safely create a temporary file or directory based on @var{template},
12207 and print its name. If given, @var{template} must include at least
12208 three consecutive @samp{X}s in the last component. If omitted, the template
12209 @samp{tmp.XXXXXXXXXX} is used, and option @option{--tmpdir} is
12210 implied. The final run of @samp{X}s in the @var{template} will be replaced
12211 by alpha-numeric characters; thus, on a case-sensitive file system,
12212 and with a @var{template} including a run of @var{n} instances of @samp{X},
12213 there are @samp{62**@var{n}} potential file names.
12215 Older scripts used to create temporary files by simply joining the
12216 name of the program with the process id (@samp{$$}) as a suffix.
12217 However, that naming scheme is easily predictable, and suffers from a
12218 race condition where the attacker can create an appropriately named
12219 symbolic link, such that when the script then opens a handle to what
12220 it thought was an unused file, it is instead modifying an existing
12221 file. Using the same scheme to create a directory is slightly safer,
12222 since the @command{mkdir} will fail if the target already exists, but
12223 it is still inferior because it allows for denial of service attacks.
12224 Therefore, modern scripts should use the @command{mktemp} command to
12225 guarantee that the generated name will be unpredictable, and that
12226 knowledge of the temporary file name implies that the file was created
12227 by the current script and cannot be modified by other users.
12229 When creating a file, the resulting file has read and write
12230 permissions for the current user, but no permissions for the group or
12231 others; these permissions are reduced if the current umask is more
12234 Here are some examples (although note that if you repeat them, you
12235 will most likely get different file names):
12240 Create a temporary file in the current directory.
12247 Create a temporary file with a known suffix.
12249 $ mktemp --suffix=.txt file-XXXX
12251 $ mktemp file-XXXX-XXXX.txt
12256 Create a secure fifo relative to the user's choice of @env{TMPDIR},
12257 but falling back to the current directory rather than @file{/tmp}.
12258 Note that @command{mktemp} does not create fifos, but can create a
12259 secure directory in which the fifo can live. Exit the shell if the
12260 directory or fifo could not be created.
12262 $ dir=$(mktemp -p "$@{TMPDIR:-.@}" -d dir-XXXX) || exit 1
12264 $ mkfifo "$fifo" || @{ rmdir "$dir"; exit 1; @}
12268 Create and use a temporary file if possible, but ignore failure. The
12269 file will reside in the directory named by @env{TMPDIR}, if specified,
12270 or else in @file{/tmp}.
12272 $ file=$(mktemp -q) && @{
12273 > # Safe to use $file only within this block. Use quotes,
12274 > # since $TMPDIR, and thus $file, may contain whitespace.
12275 > echo ... > "$file"
12281 Act as a semi-random character generator (it is not fully random,
12282 since it is impacted by the contents of the current directory). To
12283 avoid security holes, do not use the resulting names to create a file.
12293 The program accepts the following options. Also see @ref{Common options}.
12300 @opindex --directory
12301 Create a directory rather than a file. The directory will have read,
12302 write, and search permissions for the current user, but no permissions
12303 for the group or others; these permissions are reduced if the current
12304 umask is more restrictive.
12310 Suppress diagnostics about failure to create a file or directory. The
12311 exit status will still reflect whether a file was created.
12317 Generate a temporary name that does not name an existing file, without
12318 changing the file system contents. Using the output of this command
12319 to create a new file is inherently unsafe, as there is a window of
12320 time between generating the name and using it where another process
12321 can create an object by the same name.
12324 @itemx --tmpdir[=@var{dir}]
12327 Treat @var{template} relative to the directory @var{dir}. If
12328 @var{dir} is not specified (only possible with the long option
12329 @option{--tmpdir}) or is the empty string, use the value of
12330 @env{TMPDIR} if available, otherwise use @samp{/tmp}. If this is
12331 specified, @var{template} must not be absolute. However,
12332 @var{template} can still contain slashes, although intermediate
12333 directories must already exist.
12335 @item --suffix=@var{suffix}
12337 Append @var{suffix} to the @var{template}. @var{suffix} must not
12338 contain slash. If @option{--suffix} is specified, @var{template} must
12339 end in @samp{X}; if it is not specified, then an appropriate
12340 @option{--suffix} is inferred by finding the last @samp{X} in
12341 @var{template}. This option exists for use with the default
12342 @var{template} and for the creation of a @var{suffix} that starts with
12347 Treat @var{template} as a single file relative to the value of
12348 @env{TMPDIR} if available, or to the directory specified by
12349 @option{-p}, otherwise to @samp{/tmp}. @var{template} must not
12350 contain slashes. This option is deprecated; the use of @option{-p}
12351 without @option{-t} offers better defaults (by favoring the command
12352 line over @env{TMPDIR}) and more flexibility (by allowing intermediate
12357 @cindex exit status of @command{mktemp}
12361 0 if the file was created,
12366 @node Working context
12367 @chapter Working context
12369 @cindex working context
12370 @cindex commands for printing the working context
12372 This section describes commands that display or alter the context in
12373 which you are working: the current directory, the terminal settings, and
12374 so forth. See also the user-related commands in the next section.
12377 * pwd invocation:: Print working directory.
12378 * stty invocation:: Print or change terminal characteristics.
12379 * printenv invocation:: Print environment variables.
12380 * tty invocation:: Print file name of terminal on standard input.
12384 @node pwd invocation
12385 @section @command{pwd}: Print working directory
12388 @cindex print name of current directory
12389 @cindex current working directory, printing
12390 @cindex working directory, printing
12393 @command{pwd} prints the name of the current directory. Synopsis:
12396 pwd [@var{option}]@dots{}
12399 The program accepts the following options. Also see @ref{Common options}.
12406 If the contents of the environment variable @env{PWD} provide an
12407 absolute name of the current directory with no @samp{.} or @samp{..}
12408 components, but possibly with symbolic links, then output those
12409 contents. Otherwise, fall back to default @option{-P} handling.
12414 @opindex --physical
12415 Print a fully resolved name for the current directory. That is, all
12416 components of the printed name will be actual directory names---none
12417 will be symbolic links.
12420 @cindex symbolic links and @command{pwd}
12421 If @option{-L} and @option{-P} are both given, the last one takes
12422 precedence. If neither option is given, then this implementation uses
12423 @option{-P} as the default unless the @env{POSIXLY_CORRECT}
12424 environment variable is set.
12426 @mayConflictWithShellBuiltIn{pwd}
12431 @node stty invocation
12432 @section @command{stty}: Print or change terminal characteristics
12435 @cindex change or print terminal settings
12436 @cindex terminal settings
12437 @cindex line settings of terminal
12439 @command{stty} prints or changes terminal characteristics, such as baud rate.
12443 stty [@var{option}] [@var{setting}]@dots{}
12444 stty [@var{option}]
12447 If given no line settings, @command{stty} prints the baud rate, line
12448 discipline number (on systems that support it), and line settings
12449 that have been changed from the values set by @samp{stty sane}.
12450 By default, mode reading and setting are performed on the tty line
12451 connected to standard input, although this can be modified by the
12452 @option{--file} option.
12454 @command{stty} accepts many non-option arguments that change aspects of
12455 the terminal line operation, as described below.
12457 The program accepts the following options. Also see @ref{Common options}.
12464 Print all current settings in human-readable form. This option may not
12465 be used in combination with any line settings.
12467 @item -F @var{device}
12468 @itemx --file=@var{device}
12471 Set the line opened by the file name specified in @var{device} instead of
12472 the tty line connected to standard input. This option is necessary
12473 because opening a @acronym{POSIX} tty requires use of the @code{O_NONDELAY} flag to
12474 prevent a @acronym{POSIX} tty from blocking until the carrier detect line is high if
12475 the @code{clocal} flag is not set. Hence, it is not always possible
12476 to allow the shell to open the device in the traditional manner.
12482 @cindex machine-readable @command{stty} output
12483 Print all current settings in a form that can be used as an argument to
12484 another @command{stty} command to restore the current settings. This option
12485 may not be used in combination with any line settings.
12489 Many settings can be turned off by preceding them with a @samp{-}.
12490 Such arguments are marked below with ``May be negated'' in their
12491 description. The descriptions themselves refer to the positive
12492 case, that is, when @emph{not} negated (unless stated otherwise,
12495 Some settings are not available on all @acronym{POSIX} systems, since they use
12496 extensions. Such arguments are marked below with ``Non-@acronym{POSIX}'' in their
12497 description. On non-@acronym{POSIX} systems, those or other settings also may not
12498 be available, but it's not feasible to document all the variations: just
12504 * Control:: Control settings
12505 * Input:: Input settings
12506 * Output:: Output settings
12507 * Local:: Local settings
12508 * Combination:: Combination settings
12509 * Characters:: Special characters
12510 * Special:: Special settings
12515 @subsection Control settings
12517 @cindex control settings
12523 @cindex two-way parity
12524 Generate parity bit in output and expect parity bit in input.
12530 @cindex even parity
12531 Set odd parity (even if negated). May be negated.
12538 @cindex character size
12539 @cindex eight-bit characters
12540 Set character size to 5, 6, 7, or 8 bits.
12545 Send a hangup signal when the last process closes the tty. May be
12551 Use two stop bits per character (one if negated). May be negated.
12555 Allow input to be received. May be negated.
12559 @cindex modem control
12560 Disable modem control signals. May be negated.
12564 @cindex hardware flow control
12565 @cindex flow control, hardware
12566 @cindex RTS/CTS flow control
12567 Enable RTS/CTS flow control. Non-@acronym{POSIX}. May be negated.
12572 @subsection Input settings
12574 @cindex input settings
12575 These settings control operations on data received from the terminal.
12580 @cindex breaks, ignoring
12581 Ignore break characters. May be negated.
12585 @cindex breaks, cause interrupts
12586 Make breaks cause an interrupt signal. May be negated.
12590 @cindex parity, ignoring
12591 Ignore characters with parity errors. May be negated.
12595 @cindex parity errors, marking
12596 Mark parity errors (with a 255-0-character sequence). May be negated.
12600 Enable input parity checking. May be negated.
12604 @cindex eight-bit input
12605 Clear high (8th) bit of input characters. May be negated.
12609 @cindex newline, translating to return
12610 Translate newline to carriage return. May be negated.
12614 @cindex return, ignoring
12615 Ignore carriage return. May be negated.
12619 @cindex return, translating to newline
12620 Translate carriage return to newline. May be negated.
12624 @cindex input encoding, UTF-8
12625 Assume input characters are UTF-8 encoded. May be negated.
12629 @kindex C-s/C-q flow control
12630 @cindex XON/XOFF flow control
12631 Enable XON/XOFF flow control (that is, @kbd{CTRL-S}/@kbd{CTRL-Q}). May
12638 @cindex software flow control
12639 @cindex flow control, software
12640 Enable sending of @code{stop} character when the system input buffer
12641 is almost full, and @code{start} character when it becomes almost
12642 empty again. May be negated.
12646 @cindex uppercase, translating to lowercase
12647 Translate uppercase characters to lowercase. Non-@acronym{POSIX}. May be
12648 negated. Note ilcuc is not implemented, as one would not be able to issue
12649 almost any (lowercase) Unix command, after invoking it.
12653 Allow any character to restart output (only the start character
12654 if negated). Non-@acronym{POSIX}. May be negated.
12658 @cindex beeping at input buffer full
12659 Enable beeping and not flushing input buffer if a character arrives
12660 when the input buffer is full. Non-@acronym{POSIX}. May be negated.
12665 @subsection Output settings
12667 @cindex output settings
12668 These settings control operations on data sent to the terminal.
12673 Postprocess output. May be negated.
12677 @cindex lowercase, translating to output
12678 Translate lowercase characters to uppercase. Non-@acronym{POSIX}. May be
12679 negated. (Note ouclc is not currently implemented.)
12683 @cindex return, translating to newline
12684 Translate carriage return to newline. Non-@acronym{POSIX}. May be negated.
12688 @cindex newline, translating to crlf
12689 Translate newline to carriage return-newline. Non-@acronym{POSIX}. May be
12694 Do not print carriage returns in the first column. Non-@acronym{POSIX}.
12699 Newline performs a carriage return. Non-@acronym{POSIX}. May be negated.
12703 @cindex pad instead of timing for delaying
12704 Use fill (padding) characters instead of timing for delays. Non-@acronym{POSIX}.
12709 @cindex pad character
12710 Use @acronym{ASCII} @sc{del} characters for fill instead of
12711 @acronym{ASCII} @sc{nul} characters. Non-@acronym{POSIX}.
12717 Newline delay style. Non-@acronym{POSIX}.
12724 Carriage return delay style. Non-@acronym{POSIX}.
12730 @opindex tab@var{n}
12731 Horizontal tab delay style. Non-@acronym{POSIX}.
12736 Backspace delay style. Non-@acronym{POSIX}.
12741 Vertical tab delay style. Non-@acronym{POSIX}.
12746 Form feed delay style. Non-@acronym{POSIX}.
12751 @subsection Local settings
12753 @cindex local settings
12758 Enable @code{interrupt}, @code{quit}, and @code{suspend} special
12759 characters. May be negated.
12763 Enable @code{erase}, @code{kill}, @code{werase}, and @code{rprnt}
12764 special characters. May be negated.
12768 Enable non-@acronym{POSIX} special characters. May be negated.
12772 Echo input characters. May be negated.
12778 Echo @code{erase} characters as backspace-space-backspace. May be
12783 @cindex newline echoing after @code{kill}
12784 Echo a newline after a @code{kill} character. May be negated.
12788 @cindex newline, echoing
12789 Echo newline even if not echoing other characters. May be negated.
12793 @cindex flushing, disabling
12794 Disable flushing after @code{interrupt} and @code{quit} special
12795 characters. May be negated.
12799 @cindex case translation
12800 Enable input and output of uppercase characters by preceding their
12801 lowercase equivalents with @samp{\}, when @code{icanon} is set.
12802 Non-@acronym{POSIX}. May be negated.
12806 @cindex background jobs, stopping at terminal write
12807 Stop background jobs that try to write to the terminal. Non-@acronym{POSIX}.
12814 Echo erased characters backward, between @samp{\} and @samp{/}.
12815 Non-@acronym{POSIX}. May be negated.
12821 @cindex control characters, using @samp{^@var{c}}
12822 @cindex hat notation for control characters
12823 Echo control characters in hat notation (@samp{^@var{c}}) instead
12824 of literally. Non-@acronym{POSIX}. May be negated.
12830 Echo the @code{kill} special character by erasing each character on
12831 the line as indicated by the @code{echoprt} and @code{echoe} settings,
12832 instead of by the @code{echoctl} and @code{echok} settings. Non-@acronym{POSIX}.
12838 @subsection Combination settings
12840 @cindex combination settings
12841 Combination settings:
12848 Same as @code{parenb -parodd cs7}. May be negated. If negated, same
12849 as @code{-parenb cs8}.
12853 Same as @code{parenb parodd cs7}. May be negated. If negated, same
12854 as @code{-parenb cs8}.
12858 Same as @code{-icrnl -onlcr}. May be negated. If negated, same as
12859 @code{icrnl -inlcr -igncr onlcr -ocrnl -onlret}.
12863 Reset the @code{erase} and @code{kill} special characters to their default
12870 @c This is too long to write inline.
12872 cread -ignbrk brkint -inlcr -igncr icrnl -ixoff
12873 -iuclc -ixany imaxbel opost -olcuc -ocrnl onlcr
12874 -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0
12875 ff0 isig icanon iexten echo echoe echok -echonl
12876 -noflsh -xcase -tostop -echoprt echoctl echoke
12880 and also sets all special characters to their default values.
12884 Same as @code{brkint ignpar istrip icrnl ixon opost isig icanon}, plus
12885 sets the @code{eof} and @code{eol} characters to their default values
12886 if they are the same as the @code{min} and @code{time} characters.
12887 May be negated. If negated, same as @code{raw}.
12894 -ignbrk -brkint -ignpar -parmrk -inpck -istrip
12895 -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany
12896 -imaxbel -opost -isig -icanon -xcase min 1 time 0
12900 May be negated. If negated, same as @code{cooked}.
12904 Same as @option{-icanon}. May be negated. If negated, same as
12909 @cindex eight-bit characters
12910 Same as @code{-parenb -istrip cs8}. May be negated. If negated,
12911 same as @code{parenb istrip cs7}.
12915 Same as @option{-parenb -istrip -opost cs8}. May be negated.
12916 If negated, same as @code{parenb istrip opost cs7}.
12920 Same as @option{-ixany}. Non-@acronym{POSIX}. May be negated.
12924 Same as @code{tab0}. Non-@acronym{POSIX}. May be negated. If negated, same
12931 Same as @code{xcase iuclc olcuc}. Non-@acronym{POSIX}. May be negated.
12932 (Used for terminals with uppercase characters only.)
12936 Same as @code{echoe echoctl echoke}.
12940 Same as @code{echoe echoctl echoke -ixany intr ^C erase ^? kill C-u}.
12945 @subsection Special characters
12947 @cindex special characters
12948 @cindex characters, special
12950 The special characters' default values vary from system to system.
12951 They are set with the syntax @samp{name value}, where the names are
12952 listed below and the value can be given either literally, in hat
12953 notation (@samp{^@var{c}}), or as an integer which may start with
12954 @samp{0x} to indicate hexadecimal, @samp{0} to indicate octal, or
12955 any other digit to indicate decimal.
12957 @cindex disabling special characters
12958 @kindex u@r{, and disabling special characters}
12959 For GNU stty, giving a value of @code{^-} or @code{undef} disables that
12960 special character. (This is incompatible with Ultrix @command{stty},
12961 which uses a value of @samp{u} to disable a special character. GNU
12962 @command{stty} treats a value @samp{u} like any other, namely to set that
12963 special character to @key{U}.)
12969 Send an interrupt signal.
12973 Send a quit signal.
12977 Erase the last character typed.
12981 Erase the current line.
12985 Send an end of file (terminate the input).
12993 Alternate character to end the line. Non-@acronym{POSIX}.
12997 Switch to a different shell layer. Non-@acronym{POSIX}.
13001 Restart the output after stopping it.
13009 Send a terminal stop signal.
13013 Send a terminal stop signal after flushing the input. Non-@acronym{POSIX}.
13017 Redraw the current line. Non-@acronym{POSIX}.
13021 Erase the last word typed. Non-@acronym{POSIX}.
13025 Enter the next character typed literally, even if it is a special
13026 character. Non-@acronym{POSIX}.
13031 @subsection Special settings
13033 @cindex special settings
13038 Set the minimum number of characters that will satisfy a read until
13039 the time value has expired, when @option{-icanon} is set.
13043 Set the number of tenths of a second before reads time out if the minimum
13044 number of characters have not been read, when @option{-icanon} is set.
13046 @item ispeed @var{n}
13048 Set the input speed to @var{n}.
13050 @item ospeed @var{n}
13052 Set the output speed to @var{n}.
13056 Tell the tty kernel driver that the terminal has @var{n} rows. Non-@acronym{POSIX}.
13059 @itemx columns @var{n}
13062 Tell the kernel that the terminal has @var{n} columns. Non-@acronym{POSIX}.
13068 Print the number of rows and columns that the kernel thinks the
13069 terminal has. (Systems that don't support rows and columns in the kernel
13070 typically use the environment variables @env{LINES} and @env{COLUMNS}
13071 instead; however, GNU @command{stty} does not know anything about them.)
13072 Non-@acronym{POSIX}.
13076 Use line discipline @var{n}. Non-@acronym{POSIX}.
13080 Print the terminal speed.
13083 @cindex baud rate, setting
13084 Set the input and output speeds to @var{n}. @var{n} can be one of: 0
13085 50 75 110 134 134.5 150 200 300 600 1200 1800 2400 4800 9600 19200
13086 38400 @code{exta} @code{extb}. @code{exta} is the same as 19200;
13087 @code{extb} is the same as 38400. Many systems, including GNU/Linux,
13088 support higher speeds. The @command{stty} command includes support
13105 4000000 where the system supports these.
13106 0 hangs up the line if @option{-clocal} is set.
13110 @node printenv invocation
13111 @section @command{printenv}: Print all or some environment variables
13114 @cindex printing all or some environment variables
13115 @cindex environment variables, printing
13117 @command{printenv} prints environment variable values. Synopsis:
13120 printenv [@var{option}] [@var{variable}]@dots{}
13123 If no @var{variable}s are specified, @command{printenv} prints the value of
13124 every environment variable. Otherwise, it prints the value of each
13125 @var{variable} that is set, and nothing for those that are not set.
13127 The program accepts the following option. Also see @ref{Common options}.
13135 @cindex exit status of @command{printenv}
13139 0 if all variables specified were found
13140 1 if at least one specified variable was not found
13141 2 if a write error occurred
13145 @node tty invocation
13146 @section @command{tty}: Print file name of terminal on standard input
13149 @cindex print terminal file name
13150 @cindex terminal file name, printing
13152 @command{tty} prints the file name of the terminal connected to its standard
13153 input. It prints @samp{not a tty} if standard input is not a terminal.
13157 tty [@var{option}]@dots{}
13160 The program accepts the following option. Also see @ref{Common options}.
13170 Print nothing; only return an exit status.
13174 @cindex exit status of @command{tty}
13178 0 if standard input is a terminal
13179 1 if standard input is not a terminal
13180 2 if given incorrect arguments
13181 3 if a write error occurs
13185 @node User information
13186 @chapter User information
13188 @cindex user information, commands for
13189 @cindex commands for printing user information
13191 This section describes commands that print user-related information:
13192 logins, groups, and so forth.
13195 * id invocation:: Print user identity.
13196 * logname invocation:: Print current login name.
13197 * whoami invocation:: Print effective user ID.
13198 * groups invocation:: Print group names a user is in.
13199 * users invocation:: Print login names of users currently logged in.
13200 * who invocation:: Print who is currently logged in.
13204 @node id invocation
13205 @section @command{id}: Print user identity
13208 @cindex real user and group IDs, printing
13209 @cindex effective user and group IDs, printing
13210 @cindex printing real and effective user and group IDs
13212 @command{id} prints information about the given user, or the process
13213 running it if no user is specified. Synopsis:
13216 id [@var{option}]@dots{} [@var{username}]
13219 @vindex POSIXLY_CORRECT
13220 By default, it prints the real user ID, real group ID, effective user ID
13221 if different from the real user ID, effective group ID if different from
13222 the real group ID, and supplemental group IDs.
13223 In addition, if SELinux
13224 is enabled and the @env{POSIXLY_CORRECT} environment variable is not set,
13225 then print @samp{context=@var{c}}, where @var{c} is the security context.
13227 Each of these numeric values is preceded by an identifying string and
13228 followed by the corresponding user or group name in parentheses.
13230 The options cause @command{id} to print only part of the above information.
13231 Also see @ref{Common options}.
13238 Print only the group ID.
13244 Print only the group ID and the supplementary groups.
13250 Print the user or group name instead of the ID number. Requires
13251 @option{-u}, @option{-g}, or @option{-G}.
13257 Print the real, instead of effective, user or group ID. Requires
13258 @option{-u}, @option{-g}, or @option{-G}.
13264 Print only the user ID.
13271 @cindex security context
13272 Print only the security context of the current user.
13273 If SELinux is disabled then print a warning and
13274 set the exit status to 1.
13280 @macro primaryAndSupplementaryGroups{cmd,arg}
13281 Primary and supplementary groups for a process are normally inherited
13282 from its parent and are usually unchanged since login. This means
13283 that if you change the group database after logging in, @command{\cmd\}
13284 will not reflect your changes within your existing login session.
13285 Running @command{\cmd\} with a \arg\ causes the user and group
13286 database to be consulted afresh, and so will give a different result.
13288 @primaryAndSupplementaryGroups{id,user argument}
13290 @node logname invocation
13291 @section @command{logname}: Print current login name
13294 @cindex printing user's login name
13295 @cindex login name, printing
13296 @cindex user name, printing
13299 @command{logname} prints the calling user's name, as found in a
13300 system-maintained file (often @file{/var/run/utmp} or
13301 @file{/etc/utmp}), and exits with a status of 0. If there is no entry
13302 for the calling process, @command{logname} prints
13303 an error message and exits with a status of 1.
13305 The only options are @option{--help} and @option{--version}. @xref{Common
13311 @node whoami invocation
13312 @section @command{whoami}: Print effective user ID
13315 @cindex effective user ID, printing
13316 @cindex printing the effective user ID
13318 @command{whoami} prints the user name associated with the current
13319 effective user ID. It is equivalent to the command @samp{id -un}.
13321 The only options are @option{--help} and @option{--version}. @xref{Common
13327 @node groups invocation
13328 @section @command{groups}: Print group names a user is in
13331 @cindex printing groups a user is in
13332 @cindex supplementary groups, printing
13334 @command{groups} prints the names of the primary and any supplementary
13335 groups for each given @var{username}, or the current process if no names
13336 are given. If more than one name is given, the name of each user is
13338 the list of that user's groups and the user name is separated from the
13339 group list by a colon. Synopsis:
13342 groups [@var{username}]@dots{}
13345 The group lists are equivalent to the output of the command @samp{id -Gn}.
13347 @primaryAndSupplementaryGroups{groups,list of users}
13349 The only options are @option{--help} and @option{--version}. @xref{Common
13355 @node users invocation
13356 @section @command{users}: Print login names of users currently logged in
13359 @cindex printing current usernames
13360 @cindex usernames, printing current
13362 @cindex login sessions, printing users with
13363 @command{users} prints on a single line a blank-separated list of user
13364 names of users currently logged in to the current host. Each user name
13365 corresponds to a login session, so if a user has more than one login
13366 session, that user's name will appear the same number of times in the
13375 With no @var{file} argument, @command{users} extracts its information from
13376 a system-maintained file (often @file{/var/run/utmp} or
13377 @file{/etc/utmp}). If a file argument is given, @command{users} uses
13378 that file instead. A common choice is @file{/var/log/wtmp}.
13380 The only options are @option{--help} and @option{--version}. @xref{Common
13386 @node who invocation
13387 @section @command{who}: Print who is currently logged in
13390 @cindex printing current user information
13391 @cindex information, about current users
13393 @command{who} prints information about users who are currently logged on.
13397 @command{who} [@var{option}] [@var{file}] [am i]
13400 @cindex terminal lines, currently used
13402 @cindex remote hostname
13403 If given no non-option arguments, @command{who} prints the following
13404 information for each user currently logged on: login name, terminal
13405 line, login time, and remote hostname or X display.
13409 If given one non-option argument, @command{who} uses that instead of
13410 a default system-maintained file (often @file{/var/run/utmp} or
13411 @file{/etc/utmp}) as the name of the file containing the record of
13412 users logged on. @file{/var/log/wtmp} is commonly given as an argument
13413 to @command{who} to look at who has previously logged on.
13417 If given two non-option arguments, @command{who} prints only the entry
13418 for the user running it (determined from its standard input), preceded
13419 by the hostname. Traditionally, the two arguments given are @samp{am
13420 i}, as in @samp{who am i}.
13423 Time stamps are listed according to the time zone rules specified by
13424 the @env{TZ} environment variable, or by the system default rules if
13425 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
13426 with @env{TZ}, libc, The GNU C Library Reference Manual}.
13428 The program accepts the following options. Also see @ref{Common options}.
13436 Same as @samp{-b -d --login -p -r -t -T -u}.
13442 Print the date and time of last system boot.
13448 Print information corresponding to dead processes.
13454 Print a line of column headings.
13460 List only the entries that correspond to processes via which the
13461 system is waiting for a user to login. The user name is always @samp{LOGIN}.
13465 Attempt to canonicalize hostnames found in utmp through a DNS lookup. This
13466 is not the default because it can cause significant delays on systems with
13467 automatic dial-up internet access.
13471 Same as @samp{who am i}.
13477 List active processes spawned by init.
13483 Print only the login names and the number of users logged on.
13484 Overrides all other options.
13489 @opindex --runlevel
13490 Print the current (and maybe previous) run-level of the init process.
13494 Ignored; for compatibility with other versions of @command{who}.
13500 Print last system clock change.
13505 After the login time, print the number of hours and minutes that the
13506 user has been idle. @samp{.} means the user was active in the last minute.
13507 @samp{old} means the user has been idle for more than 24 hours.
13518 @opindex --writable
13519 @cindex message status
13520 @pindex write@r{, allowed}
13521 After each login name print a character indicating the user's message status:
13524 @samp{+} allowing @code{write} messages
13525 @samp{-} disallowing @code{write} messages
13526 @samp{?} cannot find terminal device
13534 @node System context
13535 @chapter System context
13537 @cindex system context
13538 @cindex context, system
13539 @cindex commands for system context
13541 This section describes commands that print or change system-wide
13545 * date invocation:: Print or set system date and time.
13546 * arch invocation:: Print machine hardware name.
13547 * nproc invocation:: Print the number of processors.
13548 * uname invocation:: Print system information.
13549 * hostname invocation:: Print or set system name.
13550 * hostid invocation:: Print numeric host identifier.
13551 * uptime invocation:: Print system uptime and load.
13554 @node date invocation
13555 @section @command{date}: Print or set system date and time
13558 @cindex time, printing or setting
13559 @cindex printing the current time
13564 date [@var{option}]@dots{} [+@var{format}]
13565 date [-u|--utc|--universal] @c this avoids a newline in the output
13566 [ MMDDhhmm[[CC]YY][.ss] ]
13570 Invoking @command{date} with no @var{format} argument is equivalent to invoking
13571 it with a default format that depends on the @env{LC_TIME} locale category.
13572 In the default C locale, this format is @samp{'+%a %b %e %H:%M:%S %Z %Y'},
13573 so the output looks like @samp{Thu Mar @ 3 13:47:51 PST 2005}.
13576 Normally, @command{date} uses the time zone rules indicated by the
13577 @env{TZ} environment variable, or the system default rules if @env{TZ}
13578 is not set. @xref{TZ Variable,, Specifying the Time Zone with
13579 @env{TZ}, libc, The GNU C Library Reference Manual}.
13581 @findex strftime @r{and @command{date}}
13582 @cindex time formats
13583 @cindex formatting times
13584 If given an argument that starts with a @samp{+}, @command{date} prints the
13585 current date and time (or the date and time specified by the
13586 @option{--date} option, see below) in the format defined by that argument,
13587 which is similar to that of the @code{strftime} function. Except for
13588 conversion specifiers, which start with @samp{%}, characters in the
13589 format string are printed unchanged. The conversion specifiers are
13595 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
13596 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
13597 * Literal conversion specifiers:: %[%nt]
13598 * Padding and other flags:: Pad with zeros, spaces, etc.
13599 * Setting the time:: Changing the system clock.
13600 * Options for date:: Instead of the current time.
13602 * Date input formats:: Specifying date strings.
13604 * Examples of date:: Examples.
13607 @node Time conversion specifiers
13608 @subsection Time conversion specifiers
13610 @cindex time conversion specifiers
13611 @cindex conversion specifiers, time
13613 @command{date} conversion specifiers related to times.
13617 hour (@samp{00}@dots{}@samp{23})
13619 hour (@samp{01}@dots{}@samp{12})
13621 hour (@samp{ 0}@dots{}@samp{23}).
13622 This is a @acronym{GNU} extension.
13624 hour (@samp{ 1}@dots{}@samp{12}).
13625 This is a @acronym{GNU} extension.
13627 minute (@samp{00}@dots{}@samp{59})
13629 nanoseconds (@samp{000000000}@dots{}@samp{999999999}).
13630 This is a @acronym{GNU} extension.
13632 locale's equivalent of either @samp{AM} or @samp{PM};
13633 blank in many locales.
13634 Noon is treated as @samp{PM} and midnight as @samp{AM}.
13636 like @samp{%p}, except lower case.
13637 This is a @acronym{GNU} extension.
13639 locale's 12-hour clock time (e.g., @samp{11:11:04 PM})
13641 24-hour hour and minute. Same as @samp{%H:%M}.
13642 This is a @acronym{GNU} extension.
13644 @cindex epoch, seconds since
13645 @cindex seconds since the epoch
13646 @cindex beginning of time
13647 seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC.
13648 Leap seconds are not counted unless leap second support is available.
13649 @xref{%s-examples}, for examples.
13650 This is a @acronym{GNU} extension.
13652 second (@samp{00}@dots{}@samp{60}).
13653 This may be @samp{60} if leap seconds are supported.
13655 24-hour hour, minute, and second. Same as @samp{%H:%M:%S}.
13657 locale's time representation (e.g., @samp{23:13:48})
13659 @w{@acronym{RFC} 2822/@acronym{ISO} 8601} style numeric time zone
13660 (e.g., @samp{-0600} or @samp{+0530}), or nothing if no
13661 time zone is determinable. This value reflects the numeric time zone
13662 appropriate for the current time, using the time zone rules specified
13663 by the @env{TZ} environment variable.
13664 The time (and optionally, the time zone rules) can be overridden
13665 by the @option{--date} option.
13666 This is a @acronym{GNU} extension.
13668 @w{@acronym{RFC} 3339/@acronym{ISO} 8601} style numeric time zone with
13669 @samp{:} (e.g., @samp{-06:00} or @samp{+05:30}), or nothing if no time
13670 zone is determinable.
13671 This is a @acronym{GNU} extension.
13673 Numeric time zone to the nearest second with @samp{:} (e.g.,
13674 @samp{-06:00:00} or @samp{+05:30:00}), or nothing if no time zone is
13676 This is a @acronym{GNU} extension.
13678 Numeric time zone with @samp{:} using the minimum necessary precision
13679 (e.g., @samp{-06}, @samp{+05:30}, or @samp{-04:56:02}), or nothing if
13680 no time zone is determinable.
13681 This is a @acronym{GNU} extension.
13683 alphabetic time zone abbreviation (e.g., @samp{EDT}), or nothing if no
13684 time zone is determinable. See @samp{%z} for how it is determined.
13688 @node Date conversion specifiers
13689 @subsection Date conversion specifiers
13691 @cindex date conversion specifiers
13692 @cindex conversion specifiers, date
13694 @command{date} conversion specifiers related to dates.
13698 locale's abbreviated weekday name (e.g., @samp{Sun})
13700 locale's full weekday name, variable length (e.g., @samp{Sunday})
13702 locale's abbreviated month name (e.g., @samp{Jan})
13704 locale's full month name, variable length (e.g., @samp{January})
13706 locale's date and time (e.g., @samp{Thu Mar @ 3 23:05:25 2005})
13708 century. This is like @samp{%Y}, except the last two digits are omitted.
13709 For example, it is @samp{20} if @samp{%Y} is @samp{2000},
13710 and is @samp{-0} if @samp{%Y} is @samp{-001}.
13711 It is normally at least two characters, but it may be more.
13713 day of month (e.g., @samp{01})
13715 date; same as @samp{%m/%d/%y}
13717 day of month, space padded; same as @samp{%_d}
13719 full date in @acronym{ISO} 8601 format; same as @samp{%Y-%m-%d}.
13720 This is a good choice for a date format, as it is standard and
13721 is easy to sort in the usual case where years are in the range
13723 This is a @acronym{GNU} extension.
13725 year corresponding to the @acronym{ISO} week number, but without the century
13726 (range @samp{00} through @samp{99}). This has the same format and value
13727 as @samp{%y}, except that if the @acronym{ISO} week number (see
13729 to the previous or next year, that year is used instead.
13730 This is a @acronym{GNU} extension.
13732 year corresponding to the @acronym{ISO} week number. This has the
13733 same format and value as @samp{%Y}, except that if the @acronym{ISO}
13735 @samp{%V}) belongs to the previous or next year, that year is used
13737 It is normally useful only if @samp{%V} is also used;
13738 for example, the format @samp{%G-%m-%d} is probably a mistake,
13739 since it combines the ISO week number year with the conventional month and day.
13740 This is a @acronym{GNU} extension.
13744 day of year (@samp{001}@dots{}@samp{366})
13746 month (@samp{01}@dots{}@samp{12})
13748 day of week (@samp{1}@dots{}@samp{7}) with @samp{1} corresponding to Monday
13750 week number of year, with Sunday as the first day of the week
13751 (@samp{00}@dots{}@samp{53}).
13752 Days in a new year preceding the first Sunday are in week zero.
13754 @acronym{ISO} week number, that is, the
13755 week number of year, with Monday as the first day of the week
13756 (@samp{01}@dots{}@samp{53}).
13757 If the week containing January 1 has four or more days in
13758 the new year, then it is considered week 1; otherwise, it is week 53 of
13759 the previous year, and the next week is week 1. (See the @acronym{ISO} 8601
13762 day of week (@samp{0}@dots{}@samp{6}) with 0 corresponding to Sunday
13764 week number of year, with Monday as first day of week
13765 (@samp{00}@dots{}@samp{53}).
13766 Days in a new year preceding the first Monday are in week zero.
13768 locale's date representation (e.g., @samp{12/31/99})
13770 last two digits of year (@samp{00}@dots{}@samp{99})
13772 year. This is normally at least four characters, but it may be more.
13773 Year @samp{0000} precedes year @samp{0001}, and year @samp{-001}
13774 precedes year @samp{0000}.
13778 @node Literal conversion specifiers
13779 @subsection Literal conversion specifiers
13781 @cindex literal conversion specifiers
13782 @cindex conversion specifiers, literal
13784 @command{date} conversion specifiers that produce literal strings.
13796 @node Padding and other flags
13797 @subsection Padding and other flags
13799 @cindex numeric field padding
13800 @cindex padding of numeric fields
13801 @cindex fields, padding numeric
13803 Unless otherwise specified, @command{date} normally pads numeric fields
13804 with zeros, so that, for
13805 example, numeric months are always output as two digits.
13806 Seconds since the epoch are not padded, though,
13807 since there is no natural width for them.
13809 As a @acronym{GNU} extension, @command{date} recognizes any of the
13810 following optional flags after the @samp{%}:
13814 (hyphen) Do not pad the field; useful if the output is intended for
13817 (underscore) Pad with spaces; useful if you need a fixed
13818 number of characters in the output, but zeros are too distracting.
13820 (zero) Pad with zeros even if the conversion specifier
13821 would normally pad with spaces.
13823 Use upper case characters if possible.
13825 Use opposite case characters if possible.
13826 A field that is normally upper case becomes lower case, and vice versa.
13830 Here are some examples of padding:
13833 date +%d/%m -d "Feb 1"
13835 date +%-d/%-m -d "Feb 1"
13837 date +%_d/%_m -d "Feb 1"
13841 As a @acronym{GNU} extension, you can specify the field width
13842 (after any flag, if present) as a decimal number. If the natural size of the
13843 output of the field has less than the specified number of characters,
13844 the result is written right adjusted and padded to the given
13845 size. For example, @samp{%9B} prints the right adjusted month name in
13846 a field of width 9.
13848 An optional modifier can follow the optional flag and width
13849 specification. The modifiers are:
13853 Use the locale's alternate representation for date and time. This
13854 modifier applies to the @samp{%c}, @samp{%C}, @samp{%x}, @samp{%X},
13855 @samp{%y} and @samp{%Y} conversion specifiers. In a Japanese locale, for
13856 example, @samp{%Ex} might yield a date format based on the Japanese
13860 Use the locale's alternate numeric symbols for numbers. This modifier
13861 applies only to numeric conversion specifiers.
13864 If the format supports the modifier but no alternate representation
13865 is available, it is ignored.
13868 @node Setting the time
13869 @subsection Setting the time
13871 @cindex setting the time
13872 @cindex time setting
13873 @cindex appropriate privileges
13875 If given an argument that does not start with @samp{+}, @command{date} sets
13876 the system clock to the date and time specified by that argument (as
13877 described below). You must have appropriate privileges to set the
13878 system clock. The @option{--date} and @option{--set} options may not be
13879 used with such an argument. The @option{--universal} option may be used
13880 with such an argument to indicate that the specified date and time are
13881 relative to Coordinated Universal Time rather than to the local time
13884 The argument must consist entirely of digits, which have the following
13897 first two digits of year (optional)
13899 last two digits of year (optional)
13904 The @option{--set} option also sets the system clock; see the next section.
13907 @node Options for date
13908 @subsection Options for @command{date}
13910 @cindex @command{date} options
13911 @cindex options for @command{date}
13913 The program accepts the following options. Also see @ref{Common options}.
13917 @item -d @var{datestr}
13918 @itemx --date=@var{datestr}
13921 @cindex parsing date strings
13922 @cindex date strings, parsing
13923 @cindex arbitrary date strings, parsing
13926 @opindex next @var{day}
13927 @opindex last @var{day}
13928 Display the date and time specified in @var{datestr} instead of the
13929 current date and time. @var{datestr} can be in almost any common
13930 format. It can contain month names, time zones, @samp{am} and @samp{pm},
13931 @samp{yesterday}, etc. For example, @option{--date="2004-02-27
13932 14:19:13.489392193 +0530"} specifies the instant of time that is
13933 489,392,193 nanoseconds after February 27, 2004 at 2:19:13 PM in a
13934 time zone that is 5 hours and 30 minutes east of @acronym{UTC}.@*
13935 Note: input currently must be in locale independent format. E.g., the
13936 LC_TIME=C below is needed to print back the correct date in many locales:
13938 date -d "$(LC_TIME=C date)"
13940 @xref{Date input formats}.
13942 @item -f @var{datefile}
13943 @itemx --file=@var{datefile}
13946 Parse each line in @var{datefile} as with @option{-d} and display the
13947 resulting date and time. If @var{datefile} is @samp{-}, use standard
13948 input. This is useful when you have many dates to process, because the
13949 system overhead of starting up the @command{date} executable many times can
13952 @item -r @var{file}
13953 @itemx --reference=@var{file}
13955 @opindex --reference
13956 Display the date and time of the last modification of @var{file},
13957 instead of the current date and time.
13964 @opindex --rfc-2822
13965 Display the date and time using the format @samp{%a, %d %b %Y %H:%M:%S
13966 %z}, evaluated in the C locale so abbreviations are always in English.
13970 Fri, 09 Sep 2005 13:51:39 -0700
13973 This format conforms to
13974 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt, Internet
13975 @acronym{RFCs} 2822} and
13976 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc822.txt, 822}, the
13977 current and previous standards for Internet email.
13979 @item --rfc-3339=@var{timespec}
13980 @opindex --rfc-3339=@var{timespec}
13981 Display the date using a format specified by
13982 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt, Internet
13983 @acronym{RFC} 3339}. This is a subset of the @acronym{ISO} 8601
13984 format, except that it also permits applications to use a space rather
13985 than a @samp{T} to separate dates from times. Unlike the other
13986 standard formats, @acronym{RFC} 3339 format is always suitable as
13987 input for the @option{--date} (@option{-d}) and @option{--file}
13988 (@option{-f}) options, regardless of the current locale.
13990 The argument @var{timespec} specifies how much of the time to include.
13991 It can be one of the following:
13995 Print just the full-date, e.g., @samp{2005-09-14}.
13996 This is equivalent to the format @samp{%Y-%m-%d}.
13999 Print the full-date and full-time separated by a space, e.g.,
14000 @samp{2005-09-14 00:56:06+05:30}. The output ends with a numeric
14001 time-offset; here the @samp{+05:30} means that local time is five
14002 hours and thirty minutes east of @acronym{UTC}. This is equivalent to
14003 the format @samp{%Y-%m-%d %H:%M:%S%:z}.
14006 Like @samp{seconds}, but also print nanoseconds, e.g.,
14007 @samp{2005-09-14 00:56:06.998458565+05:30}.
14008 This is equivalent to the format @samp{%Y-%m-%d %H:%M:%S.%N%:z}.
14012 @item -s @var{datestr}
14013 @itemx --set=@var{datestr}
14016 Set the date and time to @var{datestr}. See @option{-d} above.
14023 @opindex --universal
14024 @cindex Coordinated Universal Time
14026 @cindex Greenwich Mean Time
14029 Use Coordinated Universal Time (@acronym{UTC}) by operating as if the
14030 @env{TZ} environment variable were set to the string @samp{UTC0}.
14032 Universal Time is often called ``Greenwich Mean Time'' (@sc{gmt}) for
14033 historical reasons.
14037 @node Examples of date
14038 @subsection Examples of @command{date}
14040 @cindex examples of @command{date}
14042 Here are a few examples. Also see the documentation for the @option{-d}
14043 option in the previous section.
14048 To print the date of the day before yesterday:
14051 date --date='2 days ago'
14055 To print the date of the day three months and one day hence:
14058 date --date='3 months 1 day'
14062 To print the day of year of Christmas in the current year:
14065 date --date='25 Dec' +%j
14069 To print the current full month name and the day of the month:
14075 But this may not be what you want because for the first nine days of
14076 the month, the @samp{%d} expands to a zero-padded two-digit field,
14077 for example @samp{date -d 1may '+%B %d'} will print @samp{May 01}.
14080 To print a date without the leading zero for one-digit days
14081 of the month, you can use the (@acronym{GNU} extension)
14082 @samp{-} flag to suppress
14083 the padding altogether:
14086 date -d 1may '+%B %-d
14090 To print the current date and time in the format required by many
14091 non-@acronym{GNU} versions of @command{date} when setting the system clock:
14094 date +%m%d%H%M%Y.%S
14098 To set the system clock forward by two minutes:
14101 date --set='+2 minutes'
14105 To print the date in @acronym{RFC} 2822 format,
14106 use @samp{date --rfc-2822}. Here is some example output:
14109 Fri, 09 Sep 2005 13:51:39 -0700
14112 @anchor{%s-examples}
14114 To convert a date string to the number of seconds since the epoch
14115 (which is 1970-01-01 00:00:00 UTC), use the @option{--date} option with
14116 the @samp{%s} format. That can be useful in sorting and/or graphing
14117 and/or comparing data by date. The following command outputs the
14118 number of the seconds since the epoch for the time two minutes after the
14122 date --date='1970-01-01 00:02:00 +0000' +%s
14126 If you do not specify time zone information in the date string,
14127 @command{date} uses your computer's idea of the time zone when
14128 interpreting the string. For example, if your computer's time zone is
14129 that of Cambridge, Massachusetts, which was then 5 hours (i.e., 18,000
14130 seconds) behind UTC:
14133 # local time zone used
14134 date --date='1970-01-01 00:02:00' +%s
14139 If you're sorting or graphing dated data, your raw date values may be
14140 represented as seconds since the epoch. But few people can look at
14141 the date @samp{946684800} and casually note ``Oh, that's the first second
14142 of the year 2000 in Greenwich, England.''
14145 date --date='2000-01-01 UTC' +%s
14149 An alternative is to use the @option{--utc} (@option{-u}) option.
14150 Then you may omit @samp{UTC} from the date string. Although this
14151 produces the same result for @samp{%s} and many other format sequences,
14152 with a time zone offset different from zero, it would give a different
14153 result for zone-dependent formats like @samp{%z}.
14156 date -u --date=2000-01-01 +%s
14160 To convert such an unwieldy number of seconds back to
14161 a more readable form, use a command like this:
14164 # local time zone used
14165 date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
14166 1999-12-31 19:00:00 -0500
14169 Or if you do not mind depending on the @samp{@@} feature present since
14170 coreutils 5.3.0, you could shorten this to:
14173 date -d @@946684800 +"%F %T %z"
14174 1999-12-31 19:00:00 -0500
14177 Often it is better to output UTC-relative date and time:
14180 date -u -d '1970-01-01 946684800 seconds' +"%Y-%m-%d %T %z"
14181 2000-01-01 00:00:00 +0000
14187 @node arch invocation
14188 @section @command{arch}: Print machine hardware name
14191 @cindex print machine hardware name
14192 @cindex system information, printing
14194 @command{arch} prints the machine hardware name,
14195 and is equivalent to @samp{uname -m}.
14199 arch [@var{option}]
14202 The program accepts the @ref{Common options} only.
14207 @node nproc invocation
14208 @section @command{nproc}: Print the number of available processors
14211 @cindex Print the number of processors
14212 @cindex system information, printing
14214 Print the number of processing units available to the current process,
14215 which may be less than the number of online processors.
14216 If this information is not accessible, then print the number of
14217 processors installed. If the @env{OMP_NUM_THREADS} environment variable is
14218 set, then it will determine the returned value. The result is guaranteed to be
14219 greater than zero. Synopsis:
14222 nproc [@var{option}]
14225 The program accepts the following options. Also see @ref{Common options}.
14231 Print the number of installed processors on the system, which may
14232 be greater than the number online or available to the current process.
14233 The @env{OMP_NUM_THREADS} environment variable is not honored in this case.
14235 @item --ignore=@var{number}
14237 If possible, exclude this @var{number} of processing units.
14244 @node uname invocation
14245 @section @command{uname}: Print system information
14248 @cindex print system information
14249 @cindex system information, printing
14251 @command{uname} prints information about the machine and operating system
14252 it is run on. If no options are given, @command{uname} acts as if the
14253 @option{-s} option were given. Synopsis:
14256 uname [@var{option}]@dots{}
14259 If multiple options or @option{-a} are given, the selected information is
14260 printed in this order:
14263 @var{kernel-name} @var{nodename} @var{kernel-release} @var{kernel-version}
14264 @var{machine} @var{processor} @var{hardware-platform} @var{operating-system}
14267 The information may contain internal spaces, so such output cannot be
14268 parsed reliably. In the following example, @var{release} is
14269 @samp{2.2.18ss.e820-bda652a #4 SMP Tue Jun 5 11:24:08 PDT 2001}:
14273 @result{} Linux dum 2.2.18 #4 SMP Tue Jun 5 11:24:08 PDT 2001 i686 unknown unknown GNU/Linux
14277 The program accepts the following options. Also see @ref{Common options}.
14285 Print all of the below information, except omit the processor type
14286 and the hardware platform name if they are unknown.
14289 @itemx --hardware-platform
14291 @opindex --hardware-platform
14292 @cindex implementation, hardware
14293 @cindex hardware platform
14294 @cindex platform, hardware
14295 Print the hardware platform name
14296 (sometimes called the hardware implementation).
14297 Print @samp{unknown} if the kernel does not make this information
14298 easily available, as is the case with Linux kernels.
14304 @cindex machine type
14305 @cindex hardware class
14306 @cindex hardware type
14307 Print the machine hardware name (sometimes called the hardware class
14313 @opindex --nodename
14316 @cindex network node name
14317 Print the network node hostname.
14322 @opindex --processor
14323 @cindex host processor type
14324 Print the processor type (sometimes called the instruction set
14325 architecture or ISA).
14326 Print @samp{unknown} if the kernel does not make this information
14327 easily available, as is the case with Linux kernels.
14330 @itemx --operating-system
14332 @opindex --operating-system
14333 @cindex operating system name
14334 Print the name of the operating system.
14337 @itemx --kernel-release
14339 @opindex --kernel-release
14340 @cindex kernel release
14341 @cindex release of kernel
14342 Print the kernel release.
14345 @itemx --kernel-name
14347 @opindex --kernel-name
14348 @cindex kernel name
14349 @cindex name of kernel
14350 Print the kernel name.
14351 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) calls this
14352 ``the implementation of the operating system'', because the
14353 @acronym{POSIX} specification itself has no notion of ``kernel''.
14354 The kernel name might be the same as the operating system name printed
14355 by the @option{-o} or @option{--operating-system} option, but it might
14356 differ. Some operating systems (e.g., FreeBSD, HP-UX) have the same
14357 name as their underlying kernels; others (e.g., GNU/Linux, Solaris)
14361 @itemx --kernel-version
14363 @opindex --kernel-version
14364 @cindex kernel version
14365 @cindex version of kernel
14366 Print the kernel version.
14373 @node hostname invocation
14374 @section @command{hostname}: Print or set system name
14377 @cindex setting the hostname
14378 @cindex printing the hostname
14379 @cindex system name, printing
14380 @cindex appropriate privileges
14382 With no arguments, @command{hostname} prints the name of the current host
14383 system. With one argument, it sets the current host name to the
14384 specified string. You must have appropriate privileges to set the host
14388 hostname [@var{name}]
14391 The only options are @option{--help} and @option{--version}. @xref{Common
14397 @node hostid invocation
14398 @section @command{hostid}: Print numeric host identifier
14401 @cindex printing the host identifier
14403 @command{hostid} prints the numeric identifier of the current host
14404 in hexadecimal. This command accepts no arguments.
14405 The only options are @option{--help} and @option{--version}.
14406 @xref{Common options}.
14408 For example, here's what it prints on one system I use:
14415 On that system, the 32-bit quantity happens to be closely
14416 related to the system's Internet address, but that isn't always
14421 @node uptime invocation
14422 @section @command{uptime}: Print system uptime and load
14425 @cindex printing the system uptime and load
14427 @command{uptime} prints the current time, the system's uptime, the
14428 number of logged-in users and the current load average.
14430 If an argument is specified, it is used as the file to be read
14431 to discover how many users are logged in. If no argument is
14432 specified, a system default is used (@command{uptime --help} indicates
14433 the default setting).
14435 The only options are @option{--help} and @option{--version}.
14436 @xref{Common options}.
14438 For example, here's what it prints right now on one system I use:
14442 14:07 up 3:35, 3 users, load average: 1.39, 1.15, 1.04
14445 The precise method of calculation of load average varies somewhat
14446 between systems. Some systems calculate it as the average number of
14447 runnable processes over the last 1, 5 and 15 minutes, but some systems
14448 also include processes in the uninterruptible sleep state (that is,
14449 those processes which are waiting for disk I/O). The Linux kernel
14450 includes uninterruptible processes.
14452 @node SELinux context
14453 @chapter SELinux context
14455 @cindex SELinux context
14456 @cindex SELinux, context
14457 @cindex commands for SELinux context
14459 This section describes commands for operations with SELinux
14463 * chcon invocation:: Change SELinux context of file
14464 * runcon invocation:: Run a command in specified SELinux context
14467 @node chcon invocation
14468 @section @command{chcon}: Change SELinux context of file
14471 @cindex changing security context
14472 @cindex change SELinux context
14474 @command{chcon} changes the SELinux security context of the selected files.
14478 chcon [@var{option}]@dots{} @var{context} @var{file}@dots{}
14479 chcon [@var{option}]@dots{} [-u @var{user}] [-r @var{role}] [-l @var{range}] [-t @var{type}] @var{file}@dots{}
14480 chcon [@var{option}]@dots{} --reference=@var{rfile} @var{file}@dots{}
14483 Change the SELinux security context of each @var{file} to @var{context}.
14484 With @option{--reference}, change the security context of each @var{file}
14485 to that of @var{rfile}.
14487 The program accepts the following options. Also see @ref{Common options}.
14492 @itemx --no-dereference
14494 @opindex --no-dereference
14495 @cindex no dereference
14496 Affect symbolic links instead of any referenced file.
14498 @item --reference=@var{rfile}
14499 @opindex --reference
14500 @cindex reference file
14501 Use @var{rfile}'s security context rather than specifying a @var{context} value.
14506 @opindex --recursive
14507 Operate on files and directories recursively.
14510 @xref{Traversing symlinks}.
14513 @xref{Traversing symlinks}.
14516 @xref{Traversing symlinks}.
14523 Output a diagnostic for every file processed.
14525 @item -u @var{user}
14526 @itemx --user=@var{user}
14529 Set user @var{user} in the target security context.
14531 @item -r @var{role}
14532 @itemx --role=@var{role}
14535 Set role @var{role} in the target security context.
14537 @item -t @var{type}
14538 @itemx --type=@var{type}
14541 Set type @var{type} in the target security context.
14543 @item -l @var{range}
14544 @itemx --range=@var{range}
14547 Set range @var{range} in the target security context.
14553 @node runcon invocation
14554 @section @command{runcon}: Run a command in specified SELinux context
14557 @cindex run with security context
14560 @command{runcon} runs file in specified SELinux security context.
14564 runcon @var{context} @var{command} [@var{args}]
14565 runcon [ -c ] [-u @var{user}] [-r @var{role}] [-t @var{type}] [-l @var{range}] @var{command} [@var{args}]
14568 Run @var{command} with completely-specified @var{context}, or with
14569 current or transitioned security context modified by one or more of @var{level},
14570 @var{role}, @var{type} and @var{user}.
14572 If none of @option{-c}, @option{-t}, @option{-u}, @option{-r}, or @option{-l}
14573 is specified, the first argument is used as the complete context.
14574 Any additional arguments after @var{command}
14575 are interpreted as arguments to the command.
14577 With neither @var{context} nor @var{command}, print the current security context.
14579 The program accepts the following options. Also see @ref{Common options}.
14587 Compute process transition context before modifying.
14589 @item -u @var{user}
14590 @itemx --user=@var{user}
14593 Set user @var{user} in the target security context.
14595 @item -r @var{role}
14596 @itemx --role=@var{role}
14599 Set role @var{role} in the target security context.
14601 @item -t @var{type}
14602 @itemx --type=@var{type}
14605 Set type @var{type} in the target security context.
14607 @item -l @var{range}
14608 @itemx --range=@var{range}
14611 Set range @var{range} in the target security context.
14615 @cindex exit status of @command{runcon}
14619 126 if @var{command} is found but cannot be invoked
14620 127 if @command{runcon} itself fails or if @var{command} cannot be found
14621 the exit status of @var{command} otherwise
14624 @node Modified command invocation
14625 @chapter Modified command invocation
14627 @cindex modified command invocation
14628 @cindex invocation of commands, modified
14629 @cindex commands for invoking other commands
14631 This section describes commands that run other commands in some context
14632 different than the current one: a modified environment, as a different
14636 * chroot invocation:: Modify the root directory.
14637 * env invocation:: Modify environment variables.
14638 * nice invocation:: Modify niceness.
14639 * nohup invocation:: Immunize to hangups.
14640 * stdbuf invocation:: Modify buffering of standard streams.
14641 * su invocation:: Modify user and group ID.
14642 * timeout invocation:: Run with time limit.
14646 @node chroot invocation
14647 @section @command{chroot}: Run a command with a different root directory
14650 @cindex running a program in a specified root directory
14651 @cindex root directory, running a program in a specified
14653 @command{chroot} runs a command with a specified root directory.
14654 On many systems, only the super-user can do this.@footnote{However,
14655 some systems (e.g., FreeBSD) can be configured to allow certain regular
14656 users to use the @code{chroot} system call, and hence to run this program.
14657 Also, on Cygwin, anyone can run the @command{chroot} command, because the
14658 underlying function is non-privileged due to lack of support in MS-Windows.}
14662 chroot @var{option} @var{newroot} [@var{command} [@var{args}]@dots{}]
14663 chroot @var{option}
14666 Ordinarily, file names are looked up starting at the root of the
14667 directory structure, i.e., @file{/}. @command{chroot} changes the root to
14668 the directory @var{newroot} (which must exist) and then runs
14669 @var{command} with optional @var{args}. If @var{command} is not
14670 specified, the default is the value of the @env{SHELL} environment
14671 variable or @command{/bin/sh} if not set, invoked with the @option{-i} option.
14672 @var{command} must not be a special built-in utility
14673 (@pxref{Special built-in utilities}).
14675 The program accepts the following options. Also see @ref{Common options}.
14676 Options must precede operands.
14680 @itemx --userspec=@var{user}[:@var{group}]
14681 @opindex --userspec
14682 By default, @var{command} is run with the same credentials
14683 as the invoking process.
14684 Use this option to run it as a different @var{user} and/or with a
14685 different primary @var{group}.
14687 @itemx --groups=@var{groups}
14689 Use this option to specify the supplementary @var{groups} to be
14690 used by the new process.
14691 The items in the list (names or numeric IDs) must be separated by commas.
14695 Here are a few tips to help avoid common problems in using chroot.
14696 To start with a simple example, make @var{command} refer to a statically
14697 linked binary. If you were to use a dynamically linked executable, then
14698 you'd have to arrange to have the shared libraries in the right place under
14699 your new root directory.
14701 For example, if you create a statically linked @command{ls} executable,
14702 and put it in @file{/tmp/empty}, you can run this command as root:
14705 $ chroot /tmp/empty /ls -Rl /
14708 Then you'll see output like this:
14713 -rwxr-xr-x 1 0 0 1041745 Aug 16 11:17 ls
14716 If you want to use a dynamically linked executable, say @command{bash},
14717 then first run @samp{ldd bash} to see what shared objects it needs.
14718 Then, in addition to copying the actual binary, also copy the listed
14719 files to the required positions under your intended new root directory.
14720 Finally, if the executable requires any other files (e.g., data, state,
14721 device files), copy them into place, too.
14723 @cindex exit status of @command{chroot}
14727 125 if @command{chroot} itself fails
14728 126 if @var{command} is found but cannot be invoked
14729 127 if @var{command} cannot be found
14730 the exit status of @var{command} otherwise
14734 @node env invocation
14735 @section @command{env}: Run a command in a modified environment
14738 @cindex environment, running a program in a modified
14739 @cindex modified environment, running a program in a
14740 @cindex running a program in a modified environment
14742 @command{env} runs a command with a modified environment. Synopses:
14745 env [@var{option}]@dots{} [@var{name}=@var{value}]@dots{} @c
14746 [@var{command} [@var{args}]@dots{}]
14750 Operands of the form @samp{@var{variable}=@var{value}} set
14751 the environment variable @var{variable} to value @var{value}.
14752 @var{value} may be empty (@samp{@var{variable}=}). Setting a variable
14753 to an empty value is different from unsetting it.
14754 These operands are evaluated left-to-right, so if two operands
14755 mention the same variable the earlier is ignored.
14757 Environment variable names can be empty, and can contain any
14758 characters other than @samp{=} and @acronym{ASCII} @sc{nul}.
14759 However, it is wise to limit yourself to names that
14760 consist solely of underscores, digits, and @acronym{ASCII} letters,
14761 and that begin with a non-digit, as applications like the shell do not
14762 work well with other names.
14765 The first operand that does not contain the character @samp{=}
14766 specifies the program to invoke; it is
14767 searched for according to the @env{PATH} environment variable. Any
14768 remaining arguments are passed as arguments to that program.
14769 The program should not be a special built-in utility
14770 (@pxref{Special built-in utilities}).
14772 Modifications to @env{PATH} take effect prior to searching for
14773 @var{command}. Use caution when reducing @env{PATH}; behavior is
14774 not portable when @env{PATH} is undefined or omits key directories
14775 such as @file{/bin}.
14777 In the rare case that a utility contains a @samp{=} in the name, the
14778 only way to disambiguate it from a variable assignment is to use an
14779 intermediate command for @var{command}, and pass the problematic
14780 program name via @var{args}. For example, if @file{./prog=} is an
14781 executable in the current @env{PATH}:
14784 env prog= true # runs 'true', with prog= in environment
14785 env ./prog= true # runs 'true', with ./prog= in environment
14786 env -- prog= true # runs 'true', with prog= in environment
14787 env sh -c '\prog= true' # runs 'prog=' with argument 'true'
14788 env sh -c 'exec "$@@"' sh prog= true # also runs 'prog='
14791 @cindex environment, printing
14793 If no command name is specified following the environment
14794 specifications, the resulting environment is printed. This is like
14795 specifying the @command{printenv} program.
14797 For some examples, suppose the environment passed to @command{env}
14798 contains @samp{LOGNAME=rms}, @samp{EDITOR=emacs}, and
14799 @samp{PATH=.:/gnubin:/hacks}:
14804 Output the current environment.
14806 $ env | LC_ALL=C sort
14809 PATH=.:/gnubin:/hacks
14813 Run @command{foo} with a reduced environment, preserving only the
14814 original @env{PATH} to avoid problems in locating @command{foo}.
14816 env - PATH="$PATH" foo
14820 Run @command{foo} with the environment containing @samp{LOGNAME=rms},
14821 @samp{EDITOR=emacs}, and @samp{PATH=.:/gnubin:/hacks}, and guarantees
14822 that @command{foo} was found in the file system rather than as a shell
14829 Run @command{nemacs} with the environment containing @samp{LOGNAME=foo},
14830 @samp{EDITOR=emacs}, @samp{PATH=.:/gnubin:/hacks}, and
14831 @samp{DISPLAY=gnu:0}.
14833 env DISPLAY=gnu:0 LOGNAME=foo nemacs
14837 Attempt to run the program @command{/energy/--} (as that is the only
14838 possible path search result); if the command exists, the environment
14839 will contain @samp{LOGNAME=rms} and @samp{PATH=/energy}, and the
14840 arguments will be @samp{e=mc2}, @samp{bar}, and @samp{baz}.
14842 env -u EDITOR PATH=/energy -- e=mc2 bar baz
14848 The program accepts the following options. Also see @ref{Common options}.
14849 Options must precede operands.
14855 @item -u @var{name}
14856 @itemx --unset=@var{name}
14859 Remove variable @var{name} from the environment, if it was in the
14864 @itemx --ignore-environment
14867 @opindex --ignore-environment
14868 Start with an empty environment, ignoring the inherited environment.
14872 @cindex exit status of @command{env}
14876 0 if no @var{command} is specified and the environment is output
14877 125 if @command{env} itself fails
14878 126 if @var{command} is found but cannot be invoked
14879 127 if @var{command} cannot be found
14880 the exit status of @var{command} otherwise
14884 @node nice invocation
14885 @section @command{nice}: Run a command with modified niceness
14889 @cindex scheduling, affecting
14890 @cindex appropriate privileges
14892 @command{nice} prints or modifies a process's @dfn{niceness},
14893 a parameter that affects whether the process is scheduled favorably.
14897 nice [@var{option}]@dots{} [@var{command} [@var{arg}]@dots{}]
14900 If no arguments are given, @command{nice} prints the current niceness.
14901 Otherwise, @command{nice} runs the given @var{command} with its
14902 niceness adjusted. By default, its niceness is incremented by 10.
14904 Niceness values range at least from @minus{}20 (process has high priority
14905 and gets more resources, thus slowing down other processes) through 19
14906 (process has lower priority and runs slowly itself, but has less impact
14907 on the speed of other running processes). Some systems
14908 may have a wider range of nicenesses; conversely, other systems may
14909 enforce more restrictive limits. An attempt to set the niceness
14910 outside the supported range is treated as an attempt to use the
14911 minimum or maximum supported value.
14913 A niceness should not be confused with a scheduling priority, which
14914 lets applications determine the order in which threads are scheduled
14915 to run. Unlike a priority, a niceness is merely advice to the
14916 scheduler, which the scheduler is free to ignore. Also, as a point of
14917 terminology, @acronym{POSIX} defines the behavior of @command{nice} in
14918 terms of a @dfn{nice value}, which is the nonnegative difference
14919 between a niceness and the minimum niceness. Though @command{nice}
14920 conforms to @acronym{POSIX}, its documentation and diagnostics use the
14921 term ``niceness'' for compatibility with historical practice.
14923 @var{command} must not be a special built-in utility (@pxref{Special
14924 built-in utilities}).
14926 @mayConflictWithShellBuiltIn{nice}
14928 The program accepts the following option. Also see @ref{Common options}.
14929 Options must precede operands.
14932 @item -n @var{adjustment}
14933 @itemx --adjustment=@var{adjustment}
14935 @opindex --adjustment
14936 Add @var{adjustment} instead of 10 to the command's niceness. If
14937 @var{adjustment} is negative and you lack appropriate privileges,
14938 @command{nice} issues a warning but otherwise acts as if you specified
14941 For compatibility @command{nice} also supports an obsolete
14942 option syntax @option{-@var{adjustment}}. New scripts should use
14943 @option{-n @var{adjustment}} instead.
14947 @cindex exit status of @command{nice}
14951 0 if no @var{command} is specified and the niceness is output
14952 125 if @command{nice} itself fails
14953 126 if @var{command} is found but cannot be invoked
14954 127 if @var{command} cannot be found
14955 the exit status of @var{command} otherwise
14958 It is sometimes useful to run a non-interactive program with reduced niceness.
14961 $ nice factor 4611686018427387903
14964 Since @command{nice} prints the current niceness,
14965 you can invoke it through itself to demonstrate how it works.
14967 The default behavior is to increase the niceness by @samp{10}:
14978 The @var{adjustment} is relative to the current niceness. In the
14979 next example, the first @command{nice} invocation runs the second one
14980 with niceness 10, and it in turn runs the final one with a niceness
14984 $ nice nice -n 3 nice
14988 Specifying a niceness larger than the supported range
14989 is the same as specifying the maximum supported value:
14992 $ nice -n 10000000000 nice
14996 Only a privileged user may run a process with lower niceness:
15000 nice: cannot set niceness: Permission denied
15002 $ sudo nice -n -1 nice
15007 @node nohup invocation
15008 @section @command{nohup}: Run a command immune to hangups
15011 @cindex hangups, immunity to
15012 @cindex immunity to hangups
15013 @cindex logging out and continuing to run
15016 @command{nohup} runs the given @var{command} with hangup signals ignored,
15017 so that the command can continue running in the background after you log
15021 nohup @var{command} [@var{arg}]@dots{}
15024 If standard input is a terminal, it is redirected from
15025 @file{/dev/null} so that terminal sessions do not mistakenly consider
15026 the terminal to be used by the command. This is a @acronym{GNU}
15027 extension; programs intended to be portable to non-@acronym{GNU} hosts
15028 should use @samp{nohup @var{command} [@var{arg}]@dots{} </dev/null}
15032 If standard output is a terminal, the command's standard output is appended
15033 to the file @file{nohup.out}; if that cannot be written to, it is appended
15034 to the file @file{$HOME/nohup.out}; and if that cannot be written to, the
15035 command is not run.
15036 Any @file{nohup.out} or @file{$HOME/nohup.out} file created by
15037 @command{nohup} is made readable and writable only to the user,
15038 regardless of the current umask settings.
15040 If standard error is a terminal, it is normally redirected to the same file
15041 descriptor as the (possibly-redirected) standard output.
15042 However, if standard output is closed, standard error terminal output
15043 is instead appended to the file @file{nohup.out} or
15044 @file{$HOME/nohup.out} as above.
15046 To capture the command's output to a file other than @file{nohup.out}
15047 you can redirect it. For example, to capture the output of
15051 nohup make > make.log
15054 @command{nohup} does not automatically put the command it runs in the
15055 background; you must do that explicitly, by ending the command line
15056 with an @samp{&}. Also, @command{nohup} does not alter the
15057 niceness of @var{command}; use @command{nice} for that,
15058 e.g., @samp{nohup nice @var{command}}.
15060 @var{command} must not be a special built-in utility (@pxref{Special
15061 built-in utilities}).
15063 The only options are @option{--help} and @option{--version}. @xref{Common
15064 options}. Options must precede operands.
15066 @cindex exit status of @command{nohup}
15070 125 if @command{nohup} itself fails, and @env{POSIXLY_CORRECT} is not set
15071 126 if @var{command} is found but cannot be invoked
15072 127 if @var{command} cannot be found
15073 the exit status of @var{command} otherwise
15076 If @env{POSIXLY_CORRECT} is set, internal failures give status 127
15080 @node stdbuf invocation
15081 @section @command{stdbuf}: Run a command with modified I/O stream buffering
15084 @cindex standard streams, buffering
15085 @cindex line buffered
15087 @command{stdbuf} allows one to modify the buffering operations of the
15088 three standard I/O streams associated with a program. Synopsis:
15091 stdbuf @var{option}@dots{} @var{command}
15094 Any additional @var{arg}s are passed as additional arguments to the
15097 The program accepts the following options. Also see @ref{Common options}.
15101 @item -i @var{mode}
15102 @itemx --input=@var{mode}
15105 Adjust the standard input stream buffering.
15107 @item -o @var{mode}
15108 @itemx --output=@var{mode}
15111 Adjust the standard output stream buffering.
15113 @item -e @var{mode}
15114 @itemx --error=@var{mode}
15117 Adjust the standard error stream buffering.
15121 The @var{mode} can be specified as follows:
15126 Set the stream to line buffered mode.
15127 In this mode data is coalesced until a newline is output or
15128 input is read from any stream attached to a terminal device.
15129 This option is invalid with standard input.
15132 Disable buffering of the selected stream.
15133 In this mode data is output immediately and only the
15134 amount of data requested is read from input.
15137 Specify the size of the buffer to use in fully buffered mode.
15138 @multiplierSuffixesNoBlocks{size}
15142 NOTE: If @var{command} adjusts the buffering of its standard streams
15143 (@command{tee} does for e.g.) then that will override corresponding settings
15144 changed by @command{stdbuf}. Also some filters (like @command{dd} and
15145 @command{cat} etc.) don't use streams for I/O, and are thus unaffected
15146 by @command{stdbuf} settings.
15148 @cindex exit status of @command{stdbuf}
15152 125 if @command{stdbuf} itself fails
15153 126 if @var{command} is found but cannot be invoked
15154 127 if @var{command} cannot be found
15155 the exit status of @var{command} otherwise
15159 @node su invocation
15160 @section @command{su}: Run a command with substitute user and group ID
15163 @cindex substitute user and group IDs
15164 @cindex user ID, switching
15165 @cindex super-user, becoming
15166 @cindex root, becoming
15168 @command{su} allows one user to temporarily become another user. It runs a
15169 command (often an interactive shell) with the real and effective user
15170 ID, group ID, and supplemental groups of a given @var{user}. Synopsis:
15173 su [@var{option}]@dots{} [@var{user} [@var{arg}]@dots{}]
15176 @cindex passwd entry, and @command{su} shell
15178 @flindex /etc/passwd
15179 If no @var{user} is given, the default is @code{root}, the super-user.
15180 The shell to use is taken from @var{user}'s @code{passwd} entry, or
15181 @file{/bin/sh} if none is specified there. If @var{user} has a
15182 password, @command{su} prompts for the password unless run by a user with
15183 effective user ID of zero (the super-user).
15189 @cindex login shell
15190 By default, @command{su} does not change the current directory.
15191 It sets the environment variables @env{HOME} and @env{SHELL}
15192 from the password entry for @var{user}, and if @var{user} is not
15193 the super-user, sets @env{USER} and @env{LOGNAME} to @var{user}.
15194 By default, the shell is not a login shell.
15196 Any additional @var{arg}s are passed as additional arguments to the
15199 @cindex @option{-su}
15200 GNU @command{su} does not treat @file{/bin/sh} or any other shells specially
15201 (e.g., by setting @code{argv[0]} to @option{-su}, passing @option{-c} only
15202 to certain shells, etc.).
15205 @command{su} can optionally be compiled to use @code{syslog} to report
15206 failed, and optionally successful, @command{su} attempts. (If the system
15207 supports @code{syslog}.) However, GNU @command{su} does not check if the
15208 user is a member of the @code{wheel} group; see below.
15210 The program accepts the following options. Also see @ref{Common options}.
15213 @item -c @var{command}
15214 @itemx --command=@var{command}
15217 Pass @var{command}, a single command line to run, to the shell with
15218 a @option{-c} option instead of starting an interactive shell.
15225 @cindex file name pattern expansion, disabled
15226 @cindex globbing, disabled
15227 Pass the @option{-f} option to the shell. This probably only makes sense
15228 if the shell run is @command{csh} or @command{tcsh}, for which the @option{-f}
15229 option prevents reading the startup file (@file{.cshrc}). With
15230 Bourne-like shells, the @option{-f} option disables file name pattern
15231 expansion (globbing), which is not likely to be useful.
15239 @c other variables already indexed above
15242 @cindex login shell, creating
15243 Make the shell a login shell. This means the following. Unset all
15244 environment variables except @env{TERM}, @env{HOME}, and @env{SHELL}
15245 (which are set as described above), and @env{USER} and @env{LOGNAME}
15246 (which are set, even for the super-user, as described above), and set
15247 @env{PATH} to a compiled-in default value. Change to @var{user}'s home
15248 directory. Prepend @samp{-} to the shell's name, intended to make it
15249 read its login startup file(s).
15253 @itemx --preserve-environment
15256 @opindex --preserve-environment
15257 @cindex environment, preserving
15258 @flindex /etc/shells
15259 @cindex restricted shell
15260 Do not change the environment variables @env{HOME}, @env{USER},
15261 @env{LOGNAME}, or @env{SHELL}. Run the shell given in the environment
15262 variable @env{SHELL} instead of the shell from @var{user}'s passwd
15263 entry, unless the user running @command{su} is not the super-user and
15264 @var{user}'s shell is restricted. A @dfn{restricted shell} is one that
15265 is not listed in the file @file{/etc/shells}, or in a compiled-in list
15266 if that file does not exist. Parts of what this option does can be
15267 overridden by @option{--login} and @option{--shell}.
15269 @item -s @var{shell}
15270 @itemx --shell=@var{shell}
15273 Run @var{shell} instead of the shell from @var{user}'s passwd entry,
15274 unless the user running @command{su} is not the super-user and @var{user}'s
15275 shell is restricted (see @option{-m} just above).
15279 @cindex exit status of @command{su}
15283 125 if @command{su} itself fails
15284 126 if subshell is found but cannot be invoked
15285 127 if subshell cannot be found
15286 the exit status of the subshell otherwise
15289 @cindex wheel group, not supported
15290 @cindex group wheel, not supported
15292 @subsection Why GNU @command{su} does not support the @samp{wheel} group
15294 (This section is by Richard Stallman.)
15298 Sometimes a few of the users try to hold total power over all the
15299 rest. For example, in 1984, a few users at the MIT AI lab decided to
15300 seize power by changing the operator password on the Twenex system and
15301 keeping it secret from everyone else. (I was able to thwart this coup
15302 and give power back to the users by patching the kernel, but I
15303 wouldn't know how to do that in Unix.)
15305 However, occasionally the rulers do tell someone. Under the usual
15306 @command{su} mechanism, once someone learns the root password who
15307 sympathizes with the ordinary users, he or she can tell the rest. The
15308 ``wheel group'' feature would make this impossible, and thus cement the
15309 power of the rulers.
15311 I'm on the side of the masses, not that of the rulers. If you are
15312 used to supporting the bosses and sysadmins in whatever they do, you
15313 might find this idea strange at first.
15316 @node timeout invocation
15317 @section @command{timeout}: Run a command with a time limit
15321 @cindex run commands with bounded time
15323 @command{timeout} runs the given @var{command} and kills it if it is
15324 still running after the specified time interval. Synopsis:
15327 timeout [@var{option}] @var{duration} @var{command} [@var{arg}]@dots{}
15330 @var{command} must not be a special built-in utility (@pxref{Special
15331 built-in utilities}).
15333 The program accepts the following options. Also see @ref{Common options}.
15334 Options must precede operands.
15337 @item -k @var{duration}
15338 @itemx --kill-after=@var{duration}
15340 @opindex --kill-after
15341 Ensure the monitored @var{command} is killed by also sending a @samp{KILL}
15342 signal, after the specified @var{duration}. Without this option, if the
15343 selected signal proves not to be fatal, @command{timeout} does not kill
15346 @item -s @var{signal}
15347 @itemx --signal=@var{signal}
15350 Send this @var{signal} to @var{command} on timeout, rather than the
15351 default @samp{TERM} signal. @var{signal} may be a name like @samp{HUP}
15352 or a number. Also see @xref{Signal specifications}.
15356 @var{duration} is an integer followed by an optional unit:
15358 @samp{s} for seconds (the default)
15359 @samp{m} for minutes
15363 A duration of 0 disables the associated timeout.
15365 @cindex exit status of @command{timeout}
15369 124 if @var{command} times out
15370 125 if @command{timeout} itself fails
15371 126 if @var{command} is found but cannot be invoked
15372 127 if @var{command} cannot be found
15373 the exit status of @var{command} otherwise
15377 @node Process control
15378 @chapter Process control
15380 @cindex processes, commands for controlling
15381 @cindex commands for controlling processes
15384 * kill invocation:: Sending a signal to processes.
15388 @node kill invocation
15389 @section @command{kill}: Send a signal to processes
15392 @cindex send a signal to processes
15394 The @command{kill} command sends a signal to processes, causing them
15395 to terminate or otherwise act upon receiving the signal in some way.
15396 Alternatively, it lists information about signals. Synopses:
15399 kill [-s @var{signal} | --signal @var{signal} | -@var{signal}] @var{pid}@dots{}
15400 kill [-l | --list | -t | --table] [@var{signal}]@dots{}
15403 @mayConflictWithShellBuiltIn{kill}
15405 The first form of the @command{kill} command sends a signal to all
15406 @var{pid} arguments. The default signal to send if none is specified
15407 is @samp{TERM}. The special signal number @samp{0} does not denote a
15408 valid signal, but can be used to test whether the @var{pid} arguments
15409 specify processes to which a signal could be sent.
15411 If @var{pid} is positive, the signal is sent to the process with the
15412 process ID @var{pid}. If @var{pid} is zero, the signal is sent to all
15413 processes in the process group of the current process. If @var{pid}
15414 is @minus{}1, the signal is sent to all processes for which the user has
15415 permission to send a signal. If @var{pid} is less than @minus{}1, the signal
15416 is sent to all processes in the process group that equals the absolute
15417 value of @var{pid}.
15419 If @var{pid} is not positive, a system-dependent set of system
15420 processes is excluded from the list of processes to which the signal
15423 If a negative @var{pid} argument is desired as the first one, it
15424 should be preceded by @option{--}. However, as a common extension to
15425 @acronym{POSIX}, @option{--} is not required with @samp{kill
15426 -@var{signal} -@var{pid}}. The following commands are equivalent:
15435 The first form of the @command{kill} command succeeds if every @var{pid}
15436 argument specifies at least one process that the signal was sent to.
15438 The second form of the @command{kill} command lists signal information.
15439 Either the @option{-l} or @option{--list} option, or the @option{-t}
15440 or @option{--table} option must be specified. Without any
15441 @var{signal} argument, all supported signals are listed. The output
15442 of @option{-l} or @option{--list} is a list of the signal names, one
15443 per line; if @var{signal} is already a name, the signal number is
15444 printed instead. The output of @option{-t} or @option{--table} is a
15445 table of signal numbers, names, and descriptions. This form of the
15446 @command{kill} command succeeds if all @var{signal} arguments are valid
15447 and if there is no output error.
15449 The @command{kill} command also supports the @option{--help} and
15450 @option{--version} options. @xref{Common options}.
15452 A @var{signal} may be a signal name like @samp{HUP}, or a signal
15453 number like @samp{1}, or an exit status of a process terminated by the
15454 signal. A signal name can be given in canonical form or prefixed by
15455 @samp{SIG}. The case of the letters is ignored, except for the
15456 @option{-@var{signal}} option which must use upper case to avoid
15457 ambiguity with lower case option letters. For a list of supported
15458 signal names and numbers see @xref{Signal specifications}.
15463 @cindex delaying commands
15464 @cindex commands for delaying
15466 @c Perhaps @command{wait} or other commands should be described here also?
15469 * sleep invocation:: Delay for a specified time.
15473 @node sleep invocation
15474 @section @command{sleep}: Delay for a specified time
15477 @cindex delay for a specified time
15479 @command{sleep} pauses for an amount of time specified by the sum of
15480 the values of the command line arguments.
15484 sleep @var{number}[smhd]@dots{}
15488 Each argument is a number followed by an optional unit; the default
15489 is seconds. The units are:
15502 Historical implementations of @command{sleep} have required that
15503 @var{number} be an integer, and only accepted a single argument
15504 without a suffix. However, GNU @command{sleep} accepts
15505 arbitrary floating point numbers (using a period before any fractional
15508 The only options are @option{--help} and @option{--version}. @xref{Common
15511 @c sleep is a shell built-in at least with Solaris 11's /bin/sh
15512 @mayConflictWithShellBuiltIn{sleep}
15517 @node Numeric operations
15518 @chapter Numeric operations
15520 @cindex numeric operations
15521 These programs do numerically-related operations.
15524 * factor invocation:: Show factors of numbers.
15525 * seq invocation:: Print sequences of numbers.
15529 @node factor invocation
15530 @section @command{factor}: Print prime factors
15533 @cindex prime factors
15535 @command{factor} prints prime factors. Synopses:
15538 factor [@var{number}]@dots{}
15539 factor @var{option}
15542 If no @var{number} is specified on the command line, @command{factor} reads
15543 numbers from standard input, delimited by newlines, tabs, or spaces.
15545 The @command{factor} command supports only a small number of options:
15549 Print a short help on standard output, then exit without further
15553 Print the program version on standard output, then exit without further
15557 Factoring the product of the eighth and ninth Mersenne primes
15558 takes about 30 milliseconds of CPU time on a 2.2 GHz Athlon.
15561 M8=`echo 2^31-1|bc` ; M9=`echo 2^61-1|bc`
15562 /usr/bin/time -f '%U' factor $(echo "$M8 * $M9" | bc)
15563 4951760154835678088235319297: 2147483647 2305843009213693951
15567 Similarly, factoring the eighth Fermat number @math{2^{256}+1} takes
15568 about 20 seconds on the same machine.
15570 Factoring large numbers is, in general, hard. The Pollard Rho
15571 algorithm used by @command{factor} is particularly effective for
15572 numbers with relatively small factors. If you wish to factor large
15573 numbers which do not have small factors (for example, numbers which
15574 are the product of two large primes), other methods are far better.
15576 If @command{factor} is built without using GNU MP, only
15577 single-precision arithmetic is available, and so large numbers
15578 (typically @math{2^{64}} and above) will not be supported. The single-precision
15579 code uses an algorithm which is designed for factoring smaller
15585 @node seq invocation
15586 @section @command{seq}: Print numeric sequences
15589 @cindex numeric sequences
15590 @cindex sequence of numbers
15592 @command{seq} prints a sequence of numbers to standard output. Synopses:
15595 seq [@var{option}]@dots{} @var{last}
15596 seq [@var{option}]@dots{} @var{first} @var{last}
15597 seq [@var{option}]@dots{} @var{first} @var{increment} @var{last}
15600 @command{seq} prints the numbers from @var{first} to @var{last} by
15601 @var{increment}. By default, each number is printed on a separate line.
15602 When @var{increment} is not specified, it defaults to @samp{1},
15603 even when @var{first} is larger than @var{last}.
15604 @var{first} also defaults to @samp{1}. So @code{seq 1} prints
15605 @samp{1}, but @code{seq 0} and @code{seq 10 5} produce no output.
15606 Floating-point numbers
15607 may be specified (using a period before any fractional digits).
15609 The program accepts the following options. Also see @ref{Common options}.
15610 Options must precede operands.
15613 @item -f @var{format}
15614 @itemx --format=@var{format}
15615 @opindex -f @var{format}
15616 @opindex --format=@var{format}
15617 @cindex formatting of numbers in @command{seq}
15618 Print all numbers using @var{format}.
15619 @var{format} must contain exactly one of the @samp{printf}-style
15620 floating point conversion specifications @samp{%a}, @samp{%e},
15621 @samp{%f}, @samp{%g}, @samp{%A}, @samp{%E}, @samp{%F}, @samp{%G}.
15622 The @samp{%} may be followed by zero or more flags taken from the set
15623 @samp{-+#0 '}, then an optional width containing one or more digits,
15624 then an optional precision consisting of a @samp{.} followed by zero
15625 or more digits. @var{format} may also contain any number of @samp{%%}
15626 conversion specifications. All conversion specifications have the
15627 same meaning as with @samp{printf}.
15629 The default format is derived from @var{first}, @var{step}, and
15630 @var{last}. If these all use a fixed point decimal representation,
15631 the default format is @samp{%.@var{p}f}, where @var{p} is the minimum
15632 precision that can represent the output numbers exactly. Otherwise,
15633 the default format is @samp{%g}.
15635 @item -s @var{string}
15636 @itemx --separator=@var{string}
15637 @cindex separator for numbers in @command{seq}
15638 Separate numbers with @var{string}; default is a newline.
15639 The output always terminates with a newline.
15642 @itemx --equal-width
15643 Print all numbers with the same width, by padding with leading zeros.
15644 @var{first}, @var{step}, and @var{last} should all use a fixed point
15645 decimal representation.
15646 (To have other kinds of padding, use @option{--format}).
15650 You can get finer-grained control over output with @option{-f}:
15653 $ seq -f '(%9.2E)' -9e5 1.1e6 1.3e6
15659 If you want hexadecimal integer output, you can use @command{printf}
15660 to perform the conversion:
15663 $ printf '%x\n' `seq 1048575 1024 1050623`
15669 For very long lists of numbers, use xargs to avoid
15670 system limitations on the length of an argument list:
15673 $ seq 1000000 | xargs printf '%x\n' | tail -n 3
15679 To generate octal output, use the printf @code{%o} format instead
15682 On most systems, seq can produce whole-number output for values up to
15683 at least @math{2^{53}}. Larger integers are approximated. The details
15684 differ depending on your floating-point implementation, but a common
15685 case is that @command{seq} works with integers through @math{2^{64}},
15686 and larger integers may not be numerically correct:
15689 $ seq 18446744073709551616 1 18446744073709551618
15690 18446744073709551616
15691 18446744073709551616
15692 18446744073709551618
15695 Be careful when using @command{seq} with outlandish values: otherwise
15696 you may see surprising results, as @command{seq} uses floating point
15697 internally. For example, on the x86 platform, where the internal
15698 representation uses a 64-bit fraction, the command:
15701 seq 1 0.0000000000000000001 1.0000000000000000009
15704 outputs 1.0000000000000000007 twice and skips 1.0000000000000000008.
15709 @node File permissions
15710 @chapter File permissions
15713 @include parse-datetime.texi
15717 @node Opening the software toolbox
15718 @chapter Opening the Software Toolbox
15720 An earlier version of this chapter appeared in
15721 @uref{http://www.linuxjournal.com/article.php?sid=2762, the
15722 @cite{What's GNU?} column of @cite{Linux Journal}, 2 (June, 1994)}.
15723 It was written by Arnold Robbins.
15726 * Toolbox introduction:: Toolbox introduction
15727 * I/O redirection:: I/O redirection
15728 * The who command:: The @command{who} command
15729 * The cut command:: The @command{cut} command
15730 * The sort command:: The @command{sort} command
15731 * The uniq command:: The @command{uniq} command
15732 * Putting the tools together:: Putting the tools together
15736 @node Toolbox introduction
15737 @unnumberedsec Toolbox Introduction
15739 This month's column is only peripherally related to the GNU Project, in
15740 that it describes a number of the GNU tools on your GNU/Linux system and how they
15741 might be used. What it's really about is the ``Software Tools'' philosophy
15742 of program development and usage.
15744 The software tools philosophy was an important and integral concept
15745 in the initial design and development of Unix (of which Linux and GNU are
15746 essentially clones). Unfortunately, in the modern day press of
15747 Internetworking and flashy GUIs, it seems to have fallen by the
15748 wayside. This is a shame, since it provides a powerful mental model
15749 for solving many kinds of problems.
15751 Many people carry a Swiss Army knife around in their pants pockets (or
15752 purse). A Swiss Army knife is a handy tool to have: it has several knife
15753 blades, a screwdriver, tweezers, toothpick, nail file, corkscrew, and perhaps
15754 a number of other things on it. For the everyday, small miscellaneous jobs
15755 where you need a simple, general purpose tool, it's just the thing.
15757 On the other hand, an experienced carpenter doesn't build a house using
15758 a Swiss Army knife. Instead, he has a toolbox chock full of specialized
15759 tools---a saw, a hammer, a screwdriver, a plane, and so on. And he knows
15760 exactly when and where to use each tool; you won't catch him hammering nails
15761 with the handle of his screwdriver.
15763 The Unix developers at Bell Labs were all professional programmers and trained
15764 computer scientists. They had found that while a one-size-fits-all program
15765 might appeal to a user because there's only one program to use, in practice
15770 difficult to write,
15773 difficult to maintain and
15777 difficult to extend to meet new situations.
15780 Instead, they felt that programs should be specialized tools. In short, each
15781 program ``should do one thing well.'' No more and no less. Such programs are
15782 simpler to design, write, and get right---they only do one thing.
15784 Furthermore, they found that with the right machinery for hooking programs
15785 together, that the whole was greater than the sum of the parts. By combining
15786 several special purpose programs, you could accomplish a specific task
15787 that none of the programs was designed for, and accomplish it much more
15788 quickly and easily than if you had to write a special purpose program.
15789 We will see some (classic) examples of this further on in the column.
15790 (An important additional point was that, if necessary, take a detour
15791 and build any software tools you may need first, if you don't already
15792 have something appropriate in the toolbox.)
15794 @node I/O redirection
15795 @unnumberedsec I/O Redirection
15797 Hopefully, you are familiar with the basics of I/O redirection in the
15798 shell, in particular the concepts of ``standard input,'' ``standard output,''
15799 and ``standard error''. Briefly, ``standard input'' is a data source, where
15800 data comes from. A program should not need to either know or care if the
15801 data source is a disk file, a keyboard, a magnetic tape, or even a punched
15802 card reader. Similarly, ``standard output'' is a data sink, where data goes
15803 to. The program should neither know nor care where this might be.
15804 Programs that only read their standard input, do something to the data,
15805 and then send it on, are called @dfn{filters}, by analogy to filters in a
15808 With the Unix shell, it's very easy to set up data pipelines:
15811 program_to_create_data | filter1 | ... | filterN > final.pretty.data
15814 We start out by creating the raw data; each filter applies some successive
15815 transformation to the data, until by the time it comes out of the pipeline,
15816 it is in the desired form.
15818 This is fine and good for standard input and standard output. Where does the
15819 standard error come in to play? Well, think about @command{filter1} in
15820 the pipeline above. What happens if it encounters an error in the data it
15821 sees? If it writes an error message to standard output, it will just
15822 disappear down the pipeline into @command{filter2}'s input, and the
15823 user will probably never see it. So programs need a place where they can send
15824 error messages so that the user will notice them. This is standard error,
15825 and it is usually connected to your console or window, even if you have
15826 redirected standard output of your program away from your screen.
15828 For filter programs to work together, the format of the data has to be
15829 agreed upon. The most straightforward and easiest format to use is simply
15830 lines of text. Unix data files are generally just streams of bytes, with
15831 lines delimited by the @acronym{ASCII} @sc{lf} (Line Feed) character,
15832 conventionally called a ``newline'' in the Unix literature. (This is
15833 @code{'\n'} if you're a C programmer.) This is the format used by all
15834 the traditional filtering programs. (Many earlier operating systems
15835 had elaborate facilities and special purpose programs for managing
15836 binary data. Unix has always shied away from such things, under the
15837 philosophy that it's easiest to simply be able to view and edit your
15838 data with a text editor.)
15840 OK, enough introduction. Let's take a look at some of the tools, and then
15841 we'll see how to hook them together in interesting ways. In the following
15842 discussion, we will only present those command line options that interest
15843 us. As you should always do, double check your system documentation
15844 for the full story.
15846 @node The who command
15847 @unnumberedsec The @command{who} Command
15849 The first program is the @command{who} command. By itself, it generates a
15850 list of the users who are currently logged in. Although I'm writing
15851 this on a single-user system, we'll pretend that several people are
15856 @print{} arnold console Jan 22 19:57
15857 @print{} miriam ttyp0 Jan 23 14:19(:0.0)
15858 @print{} bill ttyp1 Jan 21 09:32(:0.0)
15859 @print{} arnold ttyp2 Jan 23 20:48(:0.0)
15862 Here, the @samp{$} is the usual shell prompt, at which I typed @samp{who}.
15863 There are three people logged in, and I am logged in twice. On traditional
15864 Unix systems, user names are never more than eight characters long. This
15865 little bit of trivia will be useful later. The output of @command{who} is nice,
15866 but the data is not all that exciting.
15868 @node The cut command
15869 @unnumberedsec The @command{cut} Command
15871 The next program we'll look at is the @command{cut} command. This program
15872 cuts out columns or fields of input data. For example, we can tell it
15873 to print just the login name and full name from the @file{/etc/passwd}
15874 file. The @file{/etc/passwd} file has seven fields, separated by
15878 arnold:xyzzy:2076:10:Arnold D. Robbins:/home/arnold:/bin/bash
15881 To get the first and fifth fields, we would use @command{cut} like this:
15884 $ cut -d: -f1,5 /etc/passwd
15885 @print{} root:Operator
15887 @print{} arnold:Arnold D. Robbins
15888 @print{} miriam:Miriam A. Robbins
15892 With the @option{-c} option, @command{cut} will cut out specific characters
15893 (i.e., columns) in the input lines. This is useful for input data
15894 that has fixed width fields, and does not have a field separator. For
15895 example, list the Monday dates for the current month:
15897 @c Is using cal ok? Looked at gcal, but I don't like it.
15908 @node The sort command
15909 @unnumberedsec The @command{sort} Command
15911 Next we'll look at the @command{sort} command. This is one of the most
15912 powerful commands on a Unix-style system; one that you will often find
15913 yourself using when setting up fancy data plumbing.
15916 command reads and sorts each file named on the command line. It then
15917 merges the sorted data and writes it to standard output. It will read
15918 standard input if no files are given on the command line (thus
15919 making it into a filter). The sort is based on the character collating
15920 sequence or based on user-supplied ordering criteria.
15923 @node The uniq command
15924 @unnumberedsec The @command{uniq} Command
15926 Finally (at least for now), we'll look at the @command{uniq} program. When
15927 sorting data, you will often end up with duplicate lines, lines that
15928 are identical. Usually, all you need is one instance of each line.
15929 This is where @command{uniq} comes in. The @command{uniq} program reads its
15930 standard input. It prints only one
15931 copy of each repeated line. It does have several options. Later on,
15932 we'll use the @option{-c} option, which prints each unique line, preceded
15933 by a count of the number of times that line occurred in the input.
15936 @node Putting the tools together
15937 @unnumberedsec Putting the Tools Together
15939 Now, let's suppose this is a large ISP server system with dozens of users
15940 logged in. The management wants the system administrator to write a program that will
15941 generate a sorted list of logged in users. Furthermore, even if a user
15942 is logged in multiple times, his or her name should only show up in the
15945 The administrator could sit down with the system documentation and write a C
15946 program that did this. It would take perhaps a couple of hundred lines
15947 of code and about two hours to write it, test it, and debug it.
15948 However, knowing the software toolbox, the administrator can instead start out
15949 by generating just a list of logged on users:
15959 Next, sort the list:
15962 $ who | cut -c1-8 | sort
15969 Finally, run the sorted list through @command{uniq}, to weed out duplicates:
15972 $ who | cut -c1-8 | sort | uniq
15978 The @command{sort} command actually has a @option{-u} option that does what
15979 @command{uniq} does. However, @command{uniq} has other uses for which one
15980 cannot substitute @samp{sort -u}.
15982 The administrator puts this pipeline into a shell script, and makes it available for
15983 all the users on the system (@samp{#} is the system administrator,
15984 or @code{root}, prompt):
15987 # cat > /usr/local/bin/listusers
15988 who | cut -c1-8 | sort | uniq
15990 # chmod +x /usr/local/bin/listusers
15993 There are four major points to note here. First, with just four
15994 programs, on one command line, the administrator was able to save about two
15995 hours worth of work. Furthermore, the shell pipeline is just about as
15996 efficient as the C program would be, and it is much more efficient in
15997 terms of programmer time. People time is much more expensive than
15998 computer time, and in our modern ``there's never enough time to do
15999 everything'' society, saving two hours of programmer time is no mean
16002 Second, it is also important to emphasize that with the
16003 @emph{combination} of the tools, it is possible to do a special
16004 purpose job never imagined by the authors of the individual programs.
16006 Third, it is also valuable to build up your pipeline in stages, as we did here.
16007 This allows you to view the data at each stage in the pipeline, which helps
16008 you acquire the confidence that you are indeed using these tools correctly.
16010 Finally, by bundling the pipeline in a shell script, other users can use
16011 your command, without having to remember the fancy plumbing you set up for
16012 them. In terms of how you run them, shell scripts and compiled programs are
16015 After the previous warm-up exercise, we'll look at two additional, more
16016 complicated pipelines. For them, we need to introduce two more tools.
16018 The first is the @command{tr} command, which stands for ``transliterate.''
16019 The @command{tr} command works on a character-by-character basis, changing
16020 characters. Normally it is used for things like mapping upper case to
16024 $ echo ThIs ExAmPlE HaS MIXED case! | tr '[:upper:]' '[:lower:]'
16025 @print{} this example has mixed case!
16028 There are several options of interest:
16032 work on the complement of the listed characters, i.e.,
16033 operations apply to characters not in the given set
16036 delete characters in the first set from the output
16039 squeeze repeated characters in the output into just one character.
16042 We will be using all three options in a moment.
16044 The other command we'll look at is @command{comm}. The @command{comm}
16045 command takes two sorted input files as input data, and prints out the
16046 files' lines in three columns. The output columns are the data lines
16047 unique to the first file, the data lines unique to the second file, and
16048 the data lines that are common to both. The @option{-1}, @option{-2}, and
16049 @option{-3} command line options @emph{omit} the respective columns. (This is
16050 non-intuitive and takes a little getting used to.) For example:
16072 The file name @file{-} tells @command{comm} to read standard input
16073 instead of a regular file.
16075 Now we're ready to build a fancy pipeline. The first application is a word
16076 frequency counter. This helps an author determine if he or she is over-using
16079 The first step is to change the case of all the letters in our input file
16080 to one case. ``The'' and ``the'' are the same word when doing counting.
16083 $ tr '[:upper:]' '[:lower:]' < whats.gnu | ...
16086 The next step is to get rid of punctuation. Quoted words and unquoted words
16087 should be treated identically; it's easiest to just get the punctuation out of
16091 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | ...
16094 The second @command{tr} command operates on the complement of the listed
16095 characters, which are all the letters, the digits, the underscore, and
16096 the blank. The @samp{\n} represents the newline character; it has to
16097 be left alone. (The @acronym{ASCII} tab character should also be included for
16098 good measure in a production script.)
16100 At this point, we have data consisting of words separated by blank space.
16101 The words only contain alphanumeric characters (and the underscore). The
16102 next step is break the data apart so that we have one word per line. This
16103 makes the counting operation much easier, as we will see shortly.
16106 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16107 > tr -s ' ' '\n' | ...
16110 This command turns blanks into newlines. The @option{-s} option squeezes
16111 multiple newline characters in the output into just one. This helps us
16112 avoid blank lines. (The @samp{>} is the shell's ``secondary prompt.''
16113 This is what the shell prints when it notices you haven't finished
16114 typing in all of a command.)
16116 We now have data consisting of one word per line, no punctuation, all one
16117 case. We're ready to count each word:
16120 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16121 > tr -s ' ' '\n' | sort | uniq -c | ...
16124 At this point, the data might look something like this:
16137 The output is sorted by word, not by count! What we want is the most
16138 frequently used words first. Fortunately, this is easy to accomplish,
16139 with the help of two more @command{sort} options:
16143 do a numeric sort, not a textual one
16146 reverse the order of the sort
16149 The final pipeline looks like this:
16152 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16153 > tr -s ' ' '\n' | sort | uniq -c | sort -n -r
16162 Whew! That's a lot to digest. Yet, the same principles apply. With six
16163 commands, on two lines (really one long one split for convenience), we've
16164 created a program that does something interesting and useful, in much
16165 less time than we could have written a C program to do the same thing.
16167 A minor modification to the above pipeline can give us a simple spelling
16168 checker! To determine if you've spelled a word correctly, all you have to
16169 do is look it up in a dictionary. If it is not there, then chances are
16170 that your spelling is incorrect. So, we need a dictionary.
16171 The conventional location for a dictionary is @file{/usr/dict/words}.
16172 On my GNU/Linux system,@footnote{Redhat Linux 6.1, for the November 2000
16173 revision of this article.}
16174 this is a sorted, 45,402 word dictionary.
16176 Now, how to compare our file with the dictionary? As before, we generate
16177 a sorted list of words, one per line:
16180 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16181 > tr -s ' ' '\n' | sort -u | ...
16184 Now, all we need is a list of words that are @emph{not} in the
16185 dictionary. Here is where the @command{comm} command comes in.
16188 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16189 > tr -s ' ' '\n' | sort -u |
16190 > comm -23 - /usr/dict/words
16193 The @option{-2} and @option{-3} options eliminate lines that are only in the
16194 dictionary (the second file), and lines that are in both files. Lines
16195 only in the first file (standard input, our stream of words), are
16196 words that are not in the dictionary. These are likely candidates for
16197 spelling errors. This pipeline was the first cut at a production
16198 spelling checker on Unix.
16200 There are some other tools that deserve brief mention.
16204 search files for text that matches a regular expression
16207 count lines, words, characters
16210 a T-fitting for data pipes, copies data to files and to standard output
16213 the stream editor, an advanced tool
16216 a data manipulation language, another advanced tool
16219 The software tools philosophy also espoused the following bit of
16220 advice: ``Let someone else do the hard part.'' This means, take
16221 something that gives you most of what you need, and then massage it the
16222 rest of the way until it's in the form that you want.
16228 Each program should do one thing well. No more, no less.
16231 Combining programs with appropriate plumbing leads to results where
16232 the whole is greater than the sum of the parts. It also leads to novel
16233 uses of programs that the authors might never have imagined.
16236 Programs should never print extraneous header or trailer data, since these
16237 could get sent on down a pipeline. (A point we didn't mention earlier.)
16240 Let someone else do the hard part.
16243 Know your toolbox! Use each program appropriately. If you don't have an
16244 appropriate tool, build one.
16247 As of this writing, all the programs we've discussed are available via
16248 anonymous @command{ftp} from: @*
16249 @uref{ftp://gnudist.gnu.org/textutils/textutils-1.22.tar.gz}. (There may
16250 be more recent versions available now.)
16252 None of what I have presented in this column is new. The Software Tools
16253 philosophy was first introduced in the book @cite{Software Tools}, by
16254 Brian Kernighan and P.J. Plauger (Addison-Wesley, ISBN 0-201-03669-X).
16255 This book showed how to write and use software tools. It was written in
16256 1976, using a preprocessor for FORTRAN named @command{ratfor} (RATional
16257 FORtran). At the time, C was not as ubiquitous as it is now; FORTRAN
16258 was. The last chapter presented a @command{ratfor} to FORTRAN
16259 processor, written in @command{ratfor}. @command{ratfor} looks an awful
16260 lot like C; if you know C, you won't have any problem following the
16263 In 1981, the book was updated and made available as @cite{Software Tools
16264 in Pascal} (Addison-Wesley, ISBN 0-201-10342-7). Both books are
16265 still in print and are well worth
16266 reading if you're a programmer. They certainly made a major change in
16267 how I view programming.
16269 The programs in both books are available from
16270 @uref{http://cm.bell-labs.com/who/bwk, Brian Kernighan's home page}.
16271 For a number of years, there was an active
16272 Software Tools Users Group, whose members had ported the original
16273 @command{ratfor} programs to essentially every computer system with a
16274 FORTRAN compiler. The popularity of the group waned in the middle 1980s
16275 as Unix began to spread beyond universities.
16277 With the current proliferation of GNU code and other clones of Unix programs,
16278 these programs now receive little attention; modern C versions are
16279 much more efficient and do more than these programs do. Nevertheless, as
16280 exposition of good programming style, and evangelism for a still-valuable
16281 philosophy, these books are unparalleled, and I recommend them highly.
16283 Acknowledgment: I would like to express my gratitude to Brian Kernighan
16284 of Bell Labs, the original Software Toolsmith, for reviewing this column.
16286 @node GNU Free Documentation License
16287 @appendix GNU Free Documentation License
16291 @node Concept index
16298 @c Local variables:
16299 @c texinfo-column-for-description: 32