3 @setfilename coreutils.info
4 @settitle @sc{gnu} Coreutils
9 @include constants.texi
11 @c Define new indices.
15 @c Put everything in one index (arbitrarily chosen to be the concept index).
25 * Coreutils: (coreutils). Core GNU (file, text, shell) utilities.
26 * Common options: (coreutils)Common options. Common options.
27 * File permissions: (coreutils)File permissions. Access modes.
28 * Date input formats: (coreutils)Date input formats.
31 @c FIXME: the following need documentation
32 @c * [: (coreutils)[ invocation. File/string tests.
33 @c * pinky: (coreutils)pinky invocation. FIXME.
35 @dircategory Individual utilities
37 * arch: (coreutils)arch invocation. Print machine hardware name.
38 * base64: (coreutils)base64 invocation. Base64 encode/decode data.
39 * basename: (coreutils)basename invocation. Strip directory and suffix.
40 * cat: (coreutils)cat invocation. Concatenate and write files.
41 * chcon: (coreutils)chcon invocation. Change SELinux CTX of files.
42 * chgrp: (coreutils)chgrp invocation. Change file groups.
43 * chmod: (coreutils)chmod invocation. Change file permissions.
44 * chown: (coreutils)chown invocation. Change file owners/groups.
45 * chroot: (coreutils)chroot invocation. Specify the root directory.
46 * cksum: (coreutils)cksum invocation. Print POSIX CRC checksum.
47 * comm: (coreutils)comm invocation. Compare sorted files by line.
48 * cp: (coreutils)cp invocation. Copy files.
49 * csplit: (coreutils)csplit invocation. Split by context.
50 * cut: (coreutils)cut invocation. Print selected parts of lines.
51 * date: (coreutils)date invocation. Print/set system date and time.
52 * dd: (coreutils)dd invocation. Copy and convert a file.
53 * df: (coreutils)df invocation. Report file system disk usage.
54 * dir: (coreutils)dir invocation. List directories briefly.
55 * dircolors: (coreutils)dircolors invocation. Color setup for ls.
56 * dirname: (coreutils)dirname invocation. Strip last file name component.
57 * du: (coreutils)du invocation. Report on disk usage.
58 * echo: (coreutils)echo invocation. Print a line of text.
59 * env: (coreutils)env invocation. Modify the environment.
60 * expand: (coreutils)expand invocation. Convert tabs to spaces.
61 * expr: (coreutils)expr invocation. Evaluate expressions.
62 * factor: (coreutils)factor invocation. Print prime factors
63 * false: (coreutils)false invocation. Do nothing, unsuccessfully.
64 * fmt: (coreutils)fmt invocation. Reformat paragraph text.
65 * fold: (coreutils)fold invocation. Wrap long input lines.
66 * groups: (coreutils)groups invocation. Print group names a user is in.
67 * head: (coreutils)head invocation. Output the first part of files.
68 * hostid: (coreutils)hostid invocation. Print numeric host identifier.
69 * hostname: (coreutils)hostname invocation. Print or set system name.
70 * id: (coreutils)id invocation. Print user identity.
71 * install: (coreutils)install invocation. Copy and change attributes.
72 * join: (coreutils)join invocation. Join lines on a common field.
73 * kill: (coreutils)kill invocation. Send a signal to processes.
74 * link: (coreutils)link invocation. Make hard links between files.
75 * ln: (coreutils)ln invocation. Make links between files.
76 * logname: (coreutils)logname invocation. Print current login name.
77 * ls: (coreutils)ls invocation. List directory contents.
78 * md5sum: (coreutils)md5sum invocation. Print or check MD5 digests.
79 * mkdir: (coreutils)mkdir invocation. Create directories.
80 * mkfifo: (coreutils)mkfifo invocation. Create FIFOs (named pipes).
81 * mknod: (coreutils)mknod invocation. Create special files.
82 * mktemp: (coreutils)mktemp invocation. Create temporary files.
83 * mv: (coreutils)mv invocation. Rename files.
84 * nice: (coreutils)nice invocation. Modify niceness.
85 * nl: (coreutils)nl invocation. Number lines and write files.
86 * nohup: (coreutils)nohup invocation. Immunize to hangups.
87 * nproc: (coreutils)nproc invocation. Print the number of processors.
88 * od: (coreutils)od invocation. Dump files in octal, etc.
89 * paste: (coreutils)paste invocation. Merge lines of files.
90 * pathchk: (coreutils)pathchk invocation. Check file name portability.
91 * pr: (coreutils)pr invocation. Paginate or columnate files.
92 * printenv: (coreutils)printenv invocation. Print environment variables.
93 * printf: (coreutils)printf invocation. Format and print data.
94 * ptx: (coreutils)ptx invocation. Produce permuted indexes.
95 * pwd: (coreutils)pwd invocation. Print working directory.
96 * readlink: (coreutils)readlink invocation. Print referent of a symlink.
97 * rm: (coreutils)rm invocation. Remove files.
98 * rmdir: (coreutils)rmdir invocation. Remove empty directories.
99 * runcon: (coreutils)runcon invocation. Run in specified SELinux CTX.
100 * seq: (coreutils)seq invocation. Print numeric sequences
101 * sha1sum: (coreutils)sha1sum invocation. Print or check SHA-1 digests.
102 * sha2: (coreutils)sha2 utilities. Print or check SHA-2 digests.
103 * shred: (coreutils)shred invocation. Remove files more securely.
104 * shuf: (coreutils)shuf invocation. Shuffling text files.
105 * sleep: (coreutils)sleep invocation. Delay for a specified time.
106 * sort: (coreutils)sort invocation. Sort text files.
107 * split: (coreutils)split invocation. Split into pieces.
108 * stat: (coreutils)stat invocation. Report file(system) status.
109 * stdbuf: (coreutils)stdbuf invocation. Modify stdio buffering.
110 * stty: (coreutils)stty invocation. Print/change terminal settings.
111 * su: (coreutils)su invocation. Modify user and group ID.
112 * sum: (coreutils)sum invocation. Print traditional checksum.
113 * sync: (coreutils)sync invocation. Synchronize memory and disk.
114 * tac: (coreutils)tac invocation. Reverse files.
115 * tail: (coreutils)tail invocation. Output the last part of files.
116 * tee: (coreutils)tee invocation. Redirect to multiple files.
117 * test: (coreutils)test invocation. File/string tests.
118 * timeout: (coreutils)timeout invocation. Run with time limit.
119 * touch: (coreutils)touch invocation. Change file timestamps.
120 * tr: (coreutils)tr invocation. Translate characters.
121 * true: (coreutils)true invocation. Do nothing, successfully.
122 * truncate: (coreutils)truncate invocation. Shrink/extend size of a file.
123 * tsort: (coreutils)tsort invocation. Topological sort.
124 * tty: (coreutils)tty invocation. Print terminal name.
125 * uname: (coreutils)uname invocation. Print system information.
126 * unexpand: (coreutils)unexpand invocation. Convert spaces to tabs.
127 * uniq: (coreutils)uniq invocation. Uniquify files.
128 * unlink: (coreutils)unlink invocation. Removal via unlink(2).
129 * uptime: (coreutils)uptime invocation. Print uptime and load.
130 * users: (coreutils)users invocation. Print current user names.
131 * vdir: (coreutils)vdir invocation. List directories verbosely.
132 * wc: (coreutils)wc invocation. Line, word, and byte counts.
133 * who: (coreutils)who invocation. Print who is logged in.
134 * whoami: (coreutils)whoami invocation. Print effective user ID.
135 * yes: (coreutils)yes invocation. Print a string indefinitely.
139 This manual documents version @value{VERSION} of the @sc{gnu} core
140 utilities, including the standard programs for text and file manipulation.
142 Copyright @copyright{} 1994-1996, 2000-2010 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 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
2824 When @command{tail} uses inotify, this polling-related option is ignored.
2826 @itemx --pid=@var{pid}
2828 When following by name or by descriptor, you may specify the process ID,
2829 @var{pid}, of the sole writer of all @var{file} arguments. Then, shortly
2830 after that process terminates, tail will also terminate. This will
2831 work properly only if the writer and the tailing process are running on
2832 the same machine. For example, to save the output of a build in a file
2833 and to watch the file grow, if you invoke @command{make} and @command{tail}
2834 like this then the tail process will stop when your build completes.
2835 Without this option, you would have had to kill the @code{tail -f}
2839 $ make >& makerr & tail --pid=$! -f makerr
2842 If you specify a @var{pid} that is not in use or that does not correspond
2843 to the process that is writing to the tailed files, then @command{tail}
2844 may terminate long before any @var{file}s stop growing or it may not
2845 terminate until long after the real writer has terminated.
2846 Note that @option{--pid} cannot be supported on some systems; @command{tail}
2847 will print a warning if this is the case.
2849 @itemx --max-unchanged-stats=@var{n}
2850 @opindex --max-unchanged-stats
2851 When tailing a file by name, if there have been @var{n} (default
2852 n=@value{DEFAULT_MAX_N_UNCHANGED_STATS_BETWEEN_OPENS}) consecutive
2853 iterations for which the file has not changed, then
2854 @code{open}/@code{fstat} the file to determine if that file name is
2855 still associated with the same device/inode-number pair as before.
2856 When following a log file that is rotated, this is approximately the
2857 number of seconds between when tail prints the last pre-rotation lines
2858 and when it prints the lines that have accumulated in the new log file.
2859 This option is meaningful only when polling (i.e., without inotify)
2860 and when following by name.
2863 @itemx --lines=@var{k}
2866 Output the last @var{k} lines.
2867 However, if @var{k} starts with a @samp{+}, start printing with the
2868 @var{k}th line from the start of each file, instead of from the end.
2869 Size multiplier suffixes are the same as with the @option{-c} option.
2877 Never print file name headers.
2883 Always print file name headers.
2887 For compatibility @command{tail} also supports an obsolete usage
2888 @samp{tail -[@var{count}][bcl][f] [@var{file}]}, which is recognized
2889 only if it does not conflict with the usage described
2890 above. This obsolete form uses exactly one option and at most one
2891 file. In the option, @var{count} is an optional decimal number optionally
2892 followed by a size letter (@samp{b}, @samp{c}, @samp{l}) to mean count
2893 by 512-byte blocks, bytes, or lines, optionally followed by @samp{f}
2894 which has the same meaning as @option{-f}.
2896 @vindex _POSIX2_VERSION
2897 On older systems, the leading @samp{-} can be replaced by @samp{+} in
2898 the obsolete option syntax with the same meaning as in counts, and
2899 obsolete usage overrides normal usage when the two conflict.
2900 This obsolete behavior can be enabled or disabled with the
2901 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
2904 Scripts intended for use on standard hosts should avoid obsolete
2905 syntax and should use @option{-c @var{count}[b]}, @option{-n
2906 @var{count}}, and/or @option{-f} instead. If your script must also
2907 run on hosts that support only the obsolete syntax, you can often
2908 rewrite it to avoid problematic usages, e.g., by using @samp{sed -n
2909 '$p'} rather than @samp{tail -1}. If that's not possible, the script
2910 can use a test like @samp{if tail -c +1 </dev/null >/dev/null 2>&1;
2911 then @dots{}} to decide which syntax to use.
2913 Even if your script assumes the standard behavior, you should still
2914 beware usages whose behaviors differ depending on the @acronym{POSIX}
2915 version. For example, avoid @samp{tail - main.c}, since it might be
2916 interpreted as either @samp{tail main.c} or as @samp{tail -- -
2917 main.c}; avoid @samp{tail -c 4}, since it might mean either @samp{tail
2918 -c4} or @samp{tail -c 10 4}; and avoid @samp{tail +4}, since it might
2919 mean either @samp{tail ./+4} or @samp{tail -n +4}.
2924 @node split invocation
2925 @section @command{split}: Split a file into pieces.
2928 @cindex splitting a file into pieces
2929 @cindex pieces, splitting a file into
2931 @command{split} creates output files containing consecutive or interleaved
2932 sections of @var{input} (standard input if none is given or @var{input}
2933 is @samp{-}). Synopsis:
2936 split [@var{option}] [@var{input} [@var{prefix}]]
2939 By default, @command{split} puts 1000 lines of @var{input} (or whatever is
2940 left over for the last section), into each output file.
2942 @cindex output file name prefix
2943 The output files' names consist of @var{prefix} (@samp{x} by default)
2944 followed by a group of characters (@samp{aa}, @samp{ab}, @dots{} by
2945 default), such that concatenating the output files in traditional
2946 sorted order by file name produces the original input file (except
2947 @option{-r}). If the output file names are exhausted, @command{split}
2948 reports an error without deleting the output files that it did create.
2950 The program accepts the following options. Also see @ref{Common options}.
2954 @item -l @var{lines}
2955 @itemx --lines=@var{lines}
2958 Put @var{lines} lines of @var{input} into each output file.
2960 For compatibility @command{split} also supports an obsolete
2961 option syntax @option{-@var{lines}}. New scripts should use @option{-l
2962 @var{lines}} instead.
2965 @itemx --bytes=@var{size}
2968 Put @var{size} bytes of @var{input} into each output file.
2969 @multiplierSuffixes{size}
2972 @itemx --line-bytes=@var{size}
2974 @opindex --line-bytes
2975 Put into each output file as many complete lines of @var{input} as
2976 possible without exceeding @var{size} bytes. Individual lines longer than
2977 @var{size} bytes are broken into multiple files.
2978 @var{size} has the same format as for the @option{--bytes} option.
2980 @item -n @var{chunks}
2981 @itemx --number=@var{chunks}
2985 Split @var{input} to @var{chunks} output files where @var{chunks} may be:
2988 @var{n} generate @var{n} files based on current size of @var{input}
2989 @var{k}/@var{n} only output @var{k}th of @var{n} to stdout
2990 l/@var{n} generate @var{n} files without splitting lines
2991 l/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
2992 r/@var{n} like @samp{l} but use round robin distribution
2993 r/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
2996 Any excess bytes remaining after dividing the @var{input}
2997 into @var{n} chunks, are assigned to the last chunk.
2998 Any excess bytes appearing after the initial calculation are discarded
2999 (except when using @samp{r} mode).
3001 All @var{n} files are created even if there are fewer than @var{n} lines,
3002 or the @var{input} is truncated.
3004 For @samp{l} mode, chunks are approximately @var{input} size / @var{n}.
3005 The @var{input} is partitioned into @var{n} equal sized portions, with
3006 the last assigned any excess. If a line @emph{starts} within a partition
3007 it is written completely to the corresponding file. Since lines
3008 are not split even if they overlap a partition, the files written
3009 can be larger or smaller than the partition size, and even empty
3010 if a line is so long as to completely overlap the partition.
3012 For @samp{r} mode, the size of @var{input} is irrelevant,
3013 and so can be a pipe for example.
3015 @item -a @var{length}
3016 @itemx --suffix-length=@var{length}
3018 @opindex --suffix-length
3019 Use suffixes of length @var{length}. The default @var{length} is 2.
3022 @itemx --numeric-suffixes
3024 @opindex --numeric-suffixes
3025 Use digits in suffixes rather than lower-case letters.
3028 @itemx --elide-empty-files
3030 @opindex --elide-empty-files
3031 Suppress the generation of zero-length output files. This can happen
3032 with the @option{--number} option if a file is (truncated to be) shorter
3033 than the number requested, or if a line is so long as to completely
3034 span a chunk. The output file sequence numbers, always run consecutively
3035 even when this option is specified.
3040 @opindex --unbuffered
3041 Immediately copy input to output in @option{--number r/...} mode,
3042 which is a much slower mode of operation.
3046 Write a diagnostic just before each output file is opened.
3053 @node csplit invocation
3054 @section @command{csplit}: Split a file into context-determined pieces
3057 @cindex context splitting
3058 @cindex splitting a file into pieces by context
3060 @command{csplit} creates zero or more output files containing sections of
3061 @var{input} (standard input if @var{input} is @samp{-}). Synopsis:
3064 csplit [@var{option}]@dots{} @var{input} @var{pattern}@dots{}
3067 The contents of the output files are determined by the @var{pattern}
3068 arguments, as detailed below. An error occurs if a @var{pattern}
3069 argument refers to a nonexistent line of the input file (e.g., if no
3070 remaining line matches a given regular expression). After every
3071 @var{pattern} has been matched, any remaining input is copied into one
3074 By default, @command{csplit} prints the number of bytes written to each
3075 output file after it has been created.
3077 The types of pattern arguments are:
3082 Create an output file containing the input up to but not including line
3083 @var{n} (a positive integer). If followed by a repeat count, also
3084 create an output file containing the next @var{n} lines of the input
3085 file once for each repeat.
3087 @item /@var{regexp}/[@var{offset}]
3088 Create an output file containing the current line up to (but not
3089 including) the next line of the input file that contains a match for
3090 @var{regexp}. The optional @var{offset} is an integer.
3091 If it is given, the input up to (but not including) the
3092 matching line plus or minus @var{offset} is put into the output file,
3093 and the line after that begins the next section of input.
3095 @item %@var{regexp}%[@var{offset}]
3096 Like the previous type, except that it does not create an output
3097 file, so that section of the input file is effectively ignored.
3099 @item @{@var{repeat-count}@}
3100 Repeat the previous pattern @var{repeat-count} additional
3101 times. The @var{repeat-count} can either be a positive integer or an
3102 asterisk, meaning repeat as many times as necessary until the input is
3107 The output files' names consist of a prefix (@samp{xx} by default)
3108 followed by a suffix. By default, the suffix is an ascending sequence
3109 of two-digit decimal numbers from @samp{00} to @samp{99}. In any case,
3110 concatenating the output files in sorted order by file name produces the
3111 original input file.
3113 By default, if @command{csplit} encounters an error or receives a hangup,
3114 interrupt, quit, or terminate signal, it removes any output files
3115 that it has created so far before it exits.
3117 The program accepts the following options. Also see @ref{Common options}.
3121 @item -f @var{prefix}
3122 @itemx --prefix=@var{prefix}
3125 @cindex output file name prefix
3126 Use @var{prefix} as the output file name prefix.
3128 @item -b @var{suffix}
3129 @itemx --suffix=@var{suffix}
3132 @cindex output file name suffix
3133 Use @var{suffix} as the output file name suffix. When this option is
3134 specified, the suffix string must include exactly one
3135 @code{printf(3)}-style conversion specification, possibly including
3136 format specification flags, a field width, a precision specifications,
3137 or all of these kinds of modifiers. The format letter must convert a
3138 binary unsigned integer argument to readable form. The format letters
3139 @samp{d} and @samp{i} are aliases for @samp{u}, and the
3140 @samp{u}, @samp{o}, @samp{x}, and @samp{X} conversions are allowed. The
3141 entire @var{suffix} is given (with the current output file number) to
3142 @code{sprintf(3)} to form the file name suffixes for each of the
3143 individual output files in turn. If this option is used, the
3144 @option{--digits} option is ignored.
3146 @item -n @var{digits}
3147 @itemx --digits=@var{digits}
3150 Use output file names containing numbers that are @var{digits} digits
3151 long instead of the default 2.
3156 @opindex --keep-files
3157 Do not remove output files when errors are encountered.
3160 @itemx --elide-empty-files
3162 @opindex --elide-empty-files
3163 Suppress the generation of zero-length output files. (In cases where
3164 the section delimiters of the input file are supposed to mark the first
3165 lines of each of the sections, the first output file will generally be a
3166 zero-length file unless you use this option.) The output file sequence
3167 numbers always run consecutively starting from 0, even when this option
3178 Do not print counts of output file sizes.
3184 Here is an example of its usage.
3185 First, create an empty directory for the exercise,
3192 Now, split the sequence of 1..14 on lines that end with 0 or 5:
3195 $ seq 14 | csplit - '/[05]$/' '@{*@}'
3201 Each number printed above is the size of an output
3202 file that csplit has just created.
3203 List the names of those output files:
3210 Use @command{head} to show their contents:
3235 @node Summarizing files
3236 @chapter Summarizing files
3238 @cindex summarizing files
3240 These commands generate just a few numbers representing entire
3244 * wc invocation:: Print newline, word, and byte counts.
3245 * sum invocation:: Print checksum and block counts.
3246 * cksum invocation:: Print CRC checksum and byte counts.
3247 * md5sum invocation:: Print or check MD5 digests.
3248 * sha1sum invocation:: Print or check SHA-1 digests.
3249 * sha2 utilities:: Print or check SHA-2 digests.
3254 @section @command{wc}: Print newline, word, and byte counts
3258 @cindex character count
3262 @command{wc} counts the number of bytes, characters, whitespace-separated
3263 words, and newlines in each given @var{file}, or standard input if none
3264 are given or for a @var{file} of @samp{-}. Synopsis:
3267 wc [@var{option}]@dots{} [@var{file}]@dots{}
3270 @cindex total counts
3271 @command{wc} prints one line of counts for each file, and if the file was
3272 given as an argument, it prints the file name following the counts. If
3273 more than one @var{file} is given, @command{wc} prints a final line
3274 containing the cumulative counts, with the file name @file{total}. The
3275 counts are printed in this order: newlines, words, characters, bytes,
3276 maximum line length.
3277 Each count is printed right-justified in a field with at least one
3278 space between fields so that the numbers and file names normally line
3279 up nicely in columns. The width of the count fields varies depending
3280 on the inputs, so you should not depend on a particular field width.
3281 However, as a @acronym{GNU} extension, if only one count is printed,
3282 it is guaranteed to be printed without leading spaces.
3284 By default, @command{wc} prints three counts: the newline, words, and byte
3285 counts. Options can specify that only certain counts be printed.
3286 Options do not undo others previously given, so
3293 prints both the byte counts and the word counts.
3295 With the @option{--max-line-length} option, @command{wc} prints the length
3296 of the longest line per file, and if there is more than one file it
3297 prints the maximum (not the sum) of those lengths. The line lengths here
3298 are measured in screen columns, according to the current locale and
3299 assuming tab positions in every 8th column.
3301 The program accepts the following options. Also see @ref{Common options}.
3309 Print only the byte counts.
3315 Print only the character counts.
3321 Print only the word counts.
3327 Print only the newline counts.
3330 @itemx --max-line-length
3332 @opindex --max-line-length
3333 Print only the maximum line lengths.
3335 @macro filesZeroFromOption{cmd,withTotalOption,subListOutput}
3336 @itemx --files0-from=@var{file}
3337 @opindex --files0-from=@var{file}
3338 @c This is commented out to avoid a texi2dvi failure.
3339 @c texi2dvi (GNU Texinfo 4.11) 1.104
3340 @c @cindex including files from @command{\cmd\}
3341 Disallow processing files named on the command line, and instead process
3342 those named in file @var{file}; each name being terminated by a zero byte
3343 (@acronym{ASCII} @sc{nul}).
3344 This is useful \withTotalOption\
3345 when the list of file names is so long that it may exceed a command line
3347 In such cases, running @command{\cmd\} via @command{xargs} is undesirable
3348 because it splits the list into pieces and makes @command{\cmd\} print
3349 \subListOutput\ for each sublist rather than for the entire list.
3350 One way to produce a list of @acronym{ASCII} @sc{nul} terminated file names is with @sc{gnu}
3351 @command{find}, using its @option{-print0} predicate.
3352 If @var{file} is @samp{-} then the @acronym{ASCII} @sc{nul} terminated file names
3353 are read from standard input.
3355 @filesZeroFromOption{wc,,a total}
3357 For example, to find the length of the longest line in any @file{.c} or
3358 @file{.h} file in the current hierarchy, do this:
3361 find . -name '*.[ch]' -print0 |
3362 wc -L --files0-from=- | tail -n1
3370 @node sum invocation
3371 @section @command{sum}: Print checksum and block counts
3374 @cindex 16-bit checksum
3375 @cindex checksum, 16-bit
3377 @command{sum} computes a 16-bit checksum for each given @var{file}, or
3378 standard input if none are given or for a @var{file} of @samp{-}. Synopsis:
3381 sum [@var{option}]@dots{} [@var{file}]@dots{}
3384 @command{sum} prints the checksum for each @var{file} followed by the
3385 number of blocks in the file (rounded up). If more than one @var{file}
3386 is given, file names are also printed (by default). (With the
3387 @option{--sysv} option, corresponding file names are printed when there is
3388 at least one file argument.)
3390 By default, @sc{gnu} @command{sum} computes checksums using an algorithm
3391 compatible with BSD @command{sum} and prints file sizes in units of
3394 The program accepts the following options. Also see @ref{Common options}.
3400 @cindex BSD @command{sum}
3401 Use the default (BSD compatible) algorithm. This option is included for
3402 compatibility with the System V @command{sum}. Unless @option{-s} was also
3403 given, it has no effect.
3409 @cindex System V @command{sum}
3410 Compute checksums using an algorithm compatible with System V
3411 @command{sum}'s default, and print file sizes in units of 512-byte blocks.
3415 @command{sum} is provided for compatibility; the @command{cksum} program (see
3416 next section) is preferable in new applications.
3421 @node cksum invocation
3422 @section @command{cksum}: Print CRC checksum and byte counts
3425 @cindex cyclic redundancy check
3426 @cindex CRC checksum
3428 @command{cksum} computes a cyclic redundancy check (CRC) checksum for each
3429 given @var{file}, or standard input if none are given or for a
3430 @var{file} of @samp{-}. Synopsis:
3433 cksum [@var{option}]@dots{} [@var{file}]@dots{}
3436 @command{cksum} prints the CRC checksum for each file along with the number
3437 of bytes in the file, and the file name unless no arguments were given.
3439 @command{cksum} is typically used to ensure that files
3440 transferred by unreliable means (e.g., netnews) have not been corrupted,
3441 by comparing the @command{cksum} output for the received files with the
3442 @command{cksum} output for the original files (typically given in the
3445 The CRC algorithm is specified by the @acronym{POSIX} standard. It is not
3446 compatible with the BSD or System V @command{sum} algorithms (see the
3447 previous section); it is more robust.
3449 The only options are @option{--help} and @option{--version}. @xref{Common
3455 @node md5sum invocation
3456 @section @command{md5sum}: Print or check MD5 digests
3460 @cindex 128-bit checksum
3461 @cindex checksum, 128-bit
3462 @cindex fingerprint, 128-bit
3463 @cindex message-digest, 128-bit
3465 @command{md5sum} computes a 128-bit checksum (or @dfn{fingerprint} or
3466 @dfn{message-digest}) for each specified @var{file}.
3468 Note: The MD5 digest is more reliable than a simple CRC (provided by
3469 the @command{cksum} command) for detecting accidental file corruption,
3470 as the chances of accidentally having two files with identical MD5
3471 are vanishingly small. However, it should not be considered secure
3472 against malicious tampering: although finding a file with a given MD5
3473 fingerprint is considered infeasible at the moment, it is known how
3474 to modify certain files, including digital certificates, so that they
3475 appear valid when signed with an MD5 digest.
3476 For more secure hashes, consider using SHA-2. @xref{sha2 utilities}.
3478 If a @var{file} is specified as @samp{-} or if no files are given
3479 @command{md5sum} computes the checksum for the standard input.
3480 @command{md5sum} can also determine whether a file and checksum are
3481 consistent. Synopsis:
3484 md5sum [@var{option}]@dots{} [@var{file}]@dots{}
3487 For each @var{file}, @samp{md5sum} outputs the MD5 checksum, a flag
3488 indicating a binary or text input file, and the file name.
3489 If @var{file} contains a backslash or newline, the
3490 line is started with a backslash, and each problematic character in
3491 the file name is escaped with a backslash, making the output
3492 unambiguous even in the presence of arbitrary file names.
3493 If @var{file} is omitted or specified as @samp{-}, standard input is read.
3495 The program accepts the following options. Also see @ref{Common options}.
3503 @cindex binary input files
3504 Treat each input file as binary, by reading it in binary mode and
3505 outputting a @samp{*} flag. This is the inverse of @option{--text}.
3506 On systems like @acronym{GNU} that do not distinguish between binary
3507 and text files, this option merely flags each input file as binary:
3508 the MD5 checksum is unaffected. This option is the default on systems
3509 like MS-DOS that distinguish between binary and text files, except
3510 for reading standard input when standard input is a terminal.
3514 Read file names and checksum information (not data) from each
3515 @var{file} (or from stdin if no @var{file} was specified) and report
3516 whether the checksums match the contents of the named files.
3517 The input to this mode of @command{md5sum} is usually the output of
3518 a prior, checksum-generating run of @samp{md5sum}.
3519 Each valid line of input consists of an MD5 checksum, a binary/text
3520 flag, and then a file name.
3521 Binary files are marked with @samp{*}, text with @samp{ }.
3522 For each such line, @command{md5sum} reads the named file and computes its
3523 MD5 checksum. Then, if the computed message digest does not match the
3524 one on the line with the file name, the file is noted as having
3525 failed the test. Otherwise, the file passes the test.
3526 By default, for each valid line, one line is written to standard
3527 output indicating whether the named file passed the test.
3528 After all checks have been performed, if there were any failures,
3529 a warning is issued to standard error.
3530 Use the @option{--status} option to inhibit that output.
3531 If any listed file cannot be opened or read, if any valid line has
3532 an MD5 checksum inconsistent with the associated file, or if no valid
3533 line is found, @command{md5sum} exits with nonzero status. Otherwise,
3534 it exits successfully.
3538 @cindex verifying MD5 checksums
3539 This option is useful only when verifying checksums.
3540 When verifying checksums, don't generate an 'OK' message per successfully
3541 checked file. Files that fail the verification are reported in the
3542 default one-line-per-file format. If there is any checksum mismatch,
3543 print a warning summarizing the failures to standard error.
3547 @cindex verifying MD5 checksums
3548 This option is useful only when verifying checksums.
3549 When verifying checksums, don't generate the default one-line-per-file
3550 diagnostic and don't output the warning summarizing any failures.
3551 Failures to open or read a file still evoke individual diagnostics to
3553 If all listed files are readable and are consistent with the associated
3554 MD5 checksums, exit successfully. Otherwise exit with a status code
3555 indicating there was a failure.
3561 @cindex text input files
3562 Treat each input file as text, by reading it in text mode and
3563 outputting a @samp{ } flag. This is the inverse of @option{--binary}.
3564 This option is the default on systems like @acronym{GNU} that do not
3565 distinguish between binary and text files. On other systems, it is
3566 the default for reading standard input when standard input is a
3573 @cindex verifying MD5 checksums
3574 When verifying checksums, warn about improperly formatted MD5 checksum lines.
3575 This option is useful only if all but a few lines in the checked input
3583 @node sha1sum invocation
3584 @section @command{sha1sum}: Print or check SHA-1 digests
3588 @cindex 160-bit checksum
3589 @cindex checksum, 160-bit
3590 @cindex fingerprint, 160-bit
3591 @cindex message-digest, 160-bit
3593 @command{sha1sum} computes a 160-bit checksum for each specified
3594 @var{file}. The usage and options of this command are precisely the
3595 same as for @command{md5sum}. @xref{md5sum invocation}.
3597 Note: The SHA-1 digest is more secure than MD5, and no collisions of
3598 it are known (different files having the same fingerprint). However,
3599 it is known that they can be produced with considerable, but not
3600 unreasonable, resources. For this reason, it is generally considered
3601 that SHA-1 should be gradually phased out in favor of the more secure
3602 SHA-2 hash algorithms. @xref{sha2 utilities}.
3605 @node sha2 utilities
3606 @section sha2 utilities: Print or check SHA-2 digests
3613 @cindex 224-bit checksum
3614 @cindex 256-bit checksum
3615 @cindex 384-bit checksum
3616 @cindex 512-bit checksum
3617 @cindex checksum, 224-bit
3618 @cindex checksum, 256-bit
3619 @cindex checksum, 384-bit
3620 @cindex checksum, 512-bit
3621 @cindex fingerprint, 224-bit
3622 @cindex fingerprint, 256-bit
3623 @cindex fingerprint, 384-bit
3624 @cindex fingerprint, 512-bit
3625 @cindex message-digest, 224-bit
3626 @cindex message-digest, 256-bit
3627 @cindex message-digest, 384-bit
3628 @cindex message-digest, 512-bit
3630 The commands @command{sha224sum}, @command{sha256sum},
3631 @command{sha384sum} and @command{sha512sum} compute checksums of
3632 various lengths (respectively 224, 256, 384 and 512 bits),
3633 collectively known as the SHA-2 hashes. The usage and options of
3634 these commands are precisely the same as for @command{md5sum}.
3635 @xref{md5sum invocation}.
3637 Note: The SHA384 and SHA512 digests are considerably slower to
3638 compute, especially on 32-bit computers, than SHA224 or SHA256.
3641 @node Operating on sorted files
3642 @chapter Operating on sorted files
3644 @cindex operating on sorted files
3645 @cindex sorted files, operations on
3647 These commands work with (or produce) sorted files.
3650 * sort invocation:: Sort text files.
3651 * shuf invocation:: Shuffle text files.
3652 * uniq invocation:: Uniquify files.
3653 * comm invocation:: Compare two sorted files line by line.
3654 * ptx invocation:: Produce a permuted index of file contents.
3655 * tsort invocation:: Topological sort.
3659 @node sort invocation
3660 @section @command{sort}: Sort text files
3663 @cindex sorting files
3665 @command{sort} sorts, merges, or compares all the lines from the given
3666 files, or standard input if none are given or for a @var{file} of
3667 @samp{-}. By default, @command{sort} writes the results to standard
3671 sort [@var{option}]@dots{} [@var{file}]@dots{}
3674 @command{sort} has three modes of operation: sort (the default), merge,
3675 and check for sortedness. The following options change the operation
3682 @itemx --check=diagnose-first
3685 @cindex checking for sortedness
3686 Check whether the given file is already sorted: if it is not all
3687 sorted, print a diagnostic containing the first out-of-order line and
3688 exit with a status of 1.
3689 Otherwise, exit successfully.
3690 At most one input file can be given.
3693 @itemx --check=quiet
3694 @itemx --check=silent
3697 @cindex checking for sortedness
3698 Exit successfully if the given file is already sorted, and
3699 exit with status 1 otherwise.
3700 At most one input file can be given.
3701 This is like @option{-c}, except it does not print a diagnostic.
3707 @cindex merging sorted files
3708 Merge the given files by sorting them as a group. Each input file must
3709 always be individually sorted. It always works to sort instead of
3710 merge; merging is provided because it is faster, in the case where it
3715 @cindex sort stability
3716 @cindex sort's last-resort comparison
3717 A pair of lines is compared as follows:
3718 @command{sort} compares each pair of fields, in the
3719 order specified on the command line, according to the associated
3720 ordering options, until a difference is found or no fields are left.
3721 If no key fields are specified, @command{sort} uses a default key of
3722 the entire line. Finally, as a last resort when all keys compare
3723 equal, @command{sort} compares entire lines as if no ordering options
3724 other than @option{--reverse} (@option{-r}) were specified. The
3725 @option{--stable} (@option{-s}) option disables this @dfn{last-resort
3726 comparison} so that lines in which all fields compare equal are left
3727 in their original relative order. The @option{--unique}
3728 (@option{-u}) option also disables the last-resort comparison.
3732 Unless otherwise specified, all comparisons use the character collating
3733 sequence specified by the @env{LC_COLLATE} locale.@footnote{If you
3734 use a non-@acronym{POSIX} locale (e.g., by setting @env{LC_ALL}
3735 to @samp{en_US}), then @command{sort} may produce output that is sorted
3736 differently than you're accustomed to. In that case, set the @env{LC_ALL}
3737 environment variable to @samp{C}. Note that setting only @env{LC_COLLATE}
3738 has two problems. First, it is ineffective if @env{LC_ALL} is also set.
3739 Second, it has undefined behavior if @env{LC_CTYPE} (or @env{LANG}, if
3740 @env{LC_CTYPE} is unset) is set to an incompatible value. For example,
3741 you get undefined behavior if @env{LC_CTYPE} is @code{ja_JP.PCK} but
3742 @env{LC_COLLATE} is @code{en_US.UTF-8}.}
3744 @sc{gnu} @command{sort} (as specified for all @sc{gnu} utilities) has no
3745 limit on input line length or restrictions on bytes allowed within lines.
3746 In addition, if the final byte of an input file is not a newline, @sc{gnu}
3747 @command{sort} silently supplies one. A line's trailing newline is not
3748 part of the line for comparison purposes.
3750 @cindex exit status of @command{sort}
3754 0 if no error occurred
3755 1 if invoked with @option{-c} or @option{-C} and the input is not sorted
3756 2 if an error occurred
3760 If the environment variable @env{TMPDIR} is set, @command{sort} uses its
3761 value as the directory for temporary files instead of @file{/tmp}. The
3762 @option{--temporary-directory} (@option{-T}) option in turn overrides
3763 the environment variable.
3765 The following options affect the ordering of output lines. They may be
3766 specified globally or as part of a specific key field. If no key
3767 fields are specified, global options apply to comparison of entire
3768 lines; otherwise the global options are inherited by key fields that do
3769 not specify any special options of their own. In pre-@acronym{POSIX}
3770 versions of @command{sort}, global options affect only later key fields,
3771 so portable shell scripts should specify global options first.
3776 @itemx --ignore-leading-blanks
3778 @opindex --ignore-leading-blanks
3779 @cindex blanks, ignoring leading
3781 Ignore leading blanks when finding sort keys in each line.
3782 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3783 can change this. Note blanks may be ignored by your locale's collating
3784 rules, but without this option they will be significant for character
3785 positions specified in keys with the @option{-k} option.
3788 @itemx --dictionary-order
3790 @opindex --dictionary-order
3791 @cindex dictionary order
3792 @cindex phone directory order
3793 @cindex telephone directory order
3795 Sort in @dfn{phone directory} order: ignore all characters except
3796 letters, digits and blanks when sorting.
3797 By default letters and digits are those of @acronym{ASCII} and a blank
3798 is a space or a tab, but the @env{LC_CTYPE} locale can change this.
3801 @itemx --ignore-case
3803 @opindex --ignore-case
3804 @cindex ignoring case
3805 @cindex case folding
3807 Fold lowercase characters into the equivalent uppercase characters when
3808 comparing so that, for example, @samp{b} and @samp{B} sort as equal.
3809 The @env{LC_CTYPE} locale determines character types.
3810 When used with @option{--unique} those lower case equivalent lines are
3811 thrown away. (There is currently no way to throw away the upper case
3812 equivalent instead. (Any @option{--reverse} given would only affect
3813 the final result, after the throwing away.))
3816 @itemx --general-numeric-sort
3817 @itemx --sort=general-numeric
3819 @opindex --general-numeric-sort
3821 @cindex general numeric sort
3823 Sort numerically, using the standard C function @code{strtold} to convert
3824 a prefix of each line to a long double-precision floating point number.
3825 This allows floating point numbers to be specified in scientific notation,
3826 like @code{1.0e-34} and @code{10e100}.
3827 The @env{LC_NUMERIC} locale determines the decimal-point character.
3828 Do not report overflow, underflow, or conversion errors.
3829 Use the following collating sequence:
3833 Lines that do not start with numbers (all considered to be equal).
3835 NaNs (``Not a Number'' values, in IEEE floating point arithmetic)
3836 in a consistent but machine-dependent order.
3840 Finite numbers in ascending numeric order (with @math{-0} and @math{+0} equal).
3845 Use this option only if there is no alternative; it is much slower than
3846 @option{--numeric-sort} (@option{-n}) and it can lose information when
3847 converting to floating point.
3850 @itemx --human-numeric-sort
3851 @itemx --sort=human-numeric
3853 @opindex --human-numeric-sort
3855 @cindex human numeric sort
3857 Sort numerically, first by numeric sign (negative, zero, or positive);
3858 then by @acronym{SI} suffix (either empty, or @samp{k} or @samp{K}, or
3859 one of @samp{MGTPEZY}, in that order; @pxref{Block size}); and finally
3860 by numeric value. For example, @samp{1023M} sorts before @samp{1G}
3861 because @samp{M} (mega) precedes @samp{G} (giga) as an @acronym{SI}
3862 suffix. This option sorts values that are consistently scaled to the
3863 nearest suffix, regardless of whether suffixes denote powers of 1000
3864 or 1024, and it therefore sorts the output of any single invocation of
3865 the @command{df}, @command{du}, or @command{ls} commands that are
3866 invoked with their @option{--human-readable} or @option{--si} options.
3867 The syntax for numbers is the same as for the @option{--numeric-sort}
3868 option; the @acronym{SI} suffix must immediately follow the number.
3871 @itemx --ignore-nonprinting
3873 @opindex --ignore-nonprinting
3874 @cindex nonprinting characters, ignoring
3875 @cindex unprintable characters, ignoring
3877 Ignore nonprinting characters.
3878 The @env{LC_CTYPE} locale determines character types.
3879 This option has no effect if the stronger @option{--dictionary-order}
3880 (@option{-d}) option is also given.
3886 @opindex --month-sort
3888 @cindex months, sorting by
3890 An initial string, consisting of any amount of blanks, followed
3891 by a month name abbreviation, is folded to UPPER case and
3892 compared in the order @samp{JAN} < @samp{FEB} < @dots{} < @samp{DEC}.
3893 Invalid names compare low to valid names. The @env{LC_TIME} locale
3894 category determines the month spellings.
3895 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3899 @itemx --numeric-sort
3900 @itemx --sort=numeric
3902 @opindex --numeric-sort
3904 @cindex numeric sort
3906 Sort numerically. The number begins each line and consists
3907 of optional blanks, an optional @samp{-} sign, and zero or more
3908 digits possibly separated by thousands separators, optionally followed
3909 by a decimal-point character and zero or more digits. An empty
3910 number is treated as @samp{0}. The @env{LC_NUMERIC}
3911 locale specifies the decimal-point character and thousands separator.
3912 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3915 Comparison is exact; there is no rounding error.
3917 Neither a leading @samp{+} nor exponential notation is recognized.
3918 To compare such strings numerically, use the
3919 @option{--general-numeric-sort} (@option{-g}) option.
3922 @itemx --version-sort
3924 @opindex --version-sort
3925 @cindex version number sort
3926 Sort by version name and number. It behaves like a standard sort,
3927 except that each sequence of decimal digits is treated numerically
3928 as an index/version number. (@xref{Details about version sort}.)
3934 @cindex reverse sorting
3935 Reverse the result of comparison, so that lines with greater key values
3936 appear earlier in the output instead of later.
3939 @itemx --random-sort
3940 @itemx --sort=random
3942 @opindex --random-sort
3945 Sort by hashing the input keys and then sorting the hash values.
3946 Choose the hash function at random, ensuring that it is free of
3947 collisions so that differing keys have differing hash values. This is
3948 like a random permutation of the inputs (@pxref{shuf invocation}),
3949 except that keys with the same value sort together.
3951 If multiple random sort fields are specified, the same random hash
3952 function is used for all fields. To use different random hash
3953 functions for different fields, you can invoke @command{sort} more
3956 The choice of hash function is affected by the
3957 @option{--random-source} option.
3965 @item --compress-program=@var{prog}
3966 Compress any temporary files with the program @var{prog}.
3968 With no arguments, @var{prog} must compress standard input to standard
3969 output, and when given the @option{-d} option it must decompress
3970 standard input to standard output.
3972 Terminate with an error if @var{prog} exits with nonzero status.
3974 White space and the backslash character should not appear in
3975 @var{prog}; they are reserved for future use.
3977 @filesZeroFromOption{sort,,sorted output}
3979 @item -k @var{pos1}[,@var{pos2}]
3980 @itemx --key=@var{pos1}[,@var{pos2}]
3984 Specify a sort field that consists of the part of the line between
3985 @var{pos1} and @var{pos2} (or the end of the line, if @var{pos2} is
3986 omitted), @emph{inclusive}.
3988 Each @var{pos} has the form @samp{@var{f}[.@var{c}][@var{opts}]},
3989 where @var{f} is the number of the field to use, and @var{c} is the number
3990 of the first character from the beginning of the field. Fields and character
3991 positions are numbered starting with 1; a character position of zero in
3992 @var{pos2} indicates the field's last character. If @samp{.@var{c}} is
3993 omitted from @var{pos1}, it defaults to 1 (the beginning of the field);
3994 if omitted from @var{pos2}, it defaults to 0 (the end of the field).
3995 @var{opts} are ordering options, allowing individual keys to be sorted
3996 according to different rules; see below for details. Keys can span
3999 Example: To sort on the second field, use @option{--key=2,2}
4000 (@option{-k 2,2}). See below for more notes on keys and more examples.
4001 See also the @option{--debug} option to help determine the part
4002 of the line being used in the sort.
4005 Highlight the portion of each line used for sorting.
4006 Also issue warnings about questionable usage to stderr.
4008 @item --batch-size=@var{nmerge}
4009 @opindex --batch-size
4010 @cindex number of inputs to merge, nmerge
4011 Merge at most @var{nmerge} inputs at once.
4013 When @command{sort} has to merge more than @var{nmerge} inputs,
4014 it merges them in groups of @var{nmerge}, saving the result in
4015 a temporary file, which is then used as an input in a subsequent merge.
4017 A large value of @var{nmerge} may improve merge performance and decrease
4018 temporary storage utilization at the expense of increased memory usage
4019 and I/0. Conversely a small value of @var{nmerge} may reduce memory
4020 requirements and I/0 at the expense of temporary storage consumption and
4023 The value of @var{nmerge} must be at least 2. The default value is
4024 currently 16, but this is implementation-dependent and may change in
4027 The value of @var{nmerge} may be bounded by a resource limit for open
4028 file descriptors. The commands @samp{ulimit -n} or @samp{getconf
4029 OPEN_MAX} may display limits for your systems; these limits may be
4030 modified further if your program already has some files open, or if
4031 the operating system has other limits on the number of open files. If
4032 the value of @var{nmerge} exceeds the resource limit, @command{sort}
4033 silently uses a smaller value.
4035 @item -o @var{output-file}
4036 @itemx --output=@var{output-file}
4039 @cindex overwriting of input, allowed
4040 Write output to @var{output-file} instead of standard output.
4041 Normally, @command{sort} reads all input before opening
4042 @var{output-file}, so you can safely sort a file in place by using
4043 commands like @code{sort -o F F} and @code{cat F | sort -o F}.
4044 However, @command{sort} with @option{--merge} (@option{-m}) can open
4045 the output file before reading all input, so a command like @code{cat
4046 F | sort -m -o F - G} is not safe as @command{sort} might start
4047 writing @file{F} before @command{cat} is done reading it.
4049 @vindex POSIXLY_CORRECT
4050 On newer systems, @option{-o} cannot appear after an input file if
4051 @env{POSIXLY_CORRECT} is set, e.g., @samp{sort F -o F}. Portable
4052 scripts should specify @option{-o @var{output-file}} before any input
4055 @item --random-source=@var{file}
4056 @opindex --random-source
4057 @cindex random source for sorting
4058 Use @var{file} as a source of random data used to determine which
4059 random hash function to use with the @option{-R} option. @xref{Random
4066 @cindex sort stability
4067 @cindex sort's last-resort comparison
4069 Make @command{sort} stable by disabling its last-resort comparison.
4070 This option has no effect if no fields or global ordering options
4071 other than @option{--reverse} (@option{-r}) are specified.
4074 @itemx --buffer-size=@var{size}
4076 @opindex --buffer-size
4077 @cindex size for main memory sorting
4078 Use a main-memory sort buffer of the given @var{size}. By default,
4079 @var{size} is in units of 1024 bytes. Appending @samp{%} causes
4080 @var{size} to be interpreted as a percentage of physical memory.
4081 Appending @samp{K} multiplies @var{size} by 1024 (the default),
4082 @samp{M} by 1,048,576, @samp{G} by 1,073,741,824, and so on for
4083 @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}. Appending
4084 @samp{b} causes @var{size} to be interpreted as a byte count, with no
4087 This option can improve the performance of @command{sort} by causing it
4088 to start with a larger or smaller sort buffer than the default.
4089 However, this option affects only the initial buffer size. The buffer
4090 grows beyond @var{size} if @command{sort} encounters input lines larger
4093 @item -t @var{separator}
4094 @itemx --field-separator=@var{separator}
4096 @opindex --field-separator
4097 @cindex field separator character
4098 Use character @var{separator} as the field separator when finding the
4099 sort keys in each line. By default, fields are separated by the empty
4100 string between a non-blank character and a blank character.
4101 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4104 That is, given the input line @w{@samp{ foo bar}}, @command{sort} breaks it
4105 into fields @w{@samp{ foo}} and @w{@samp{ bar}}. The field separator is
4106 not considered to be part of either the field preceding or the field
4107 following, so with @samp{sort @w{-t " "}} the same input line has
4108 three fields: an empty field, @samp{foo}, and @samp{bar}.
4109 However, fields that extend to the end of the line,
4110 as @option{-k 2}, or fields consisting of a range, as @option{-k 2,3},
4111 retain the field separators present between the endpoints of the range.
4113 To specify @acronym{ASCII} @sc{nul} as the field separator,
4114 use the two-character string @samp{\0}, e.g., @samp{sort -t '\0'}.
4116 @item -T @var{tempdir}
4117 @itemx --temporary-directory=@var{tempdir}
4119 @opindex --temporary-directory
4120 @cindex temporary directory
4122 Use directory @var{tempdir} to store temporary files, overriding the
4123 @env{TMPDIR} environment variable. If this option is given more than
4124 once, temporary files are stored in all the directories given. If you
4125 have a large sort or merge that is I/O-bound, you can often improve
4126 performance by using this option to specify directories on different
4127 disks and controllers.
4129 @item --parallel=@var{n}
4131 @cindex multithreaded sort
4132 Limit the number of sorts run in parallel to @var{n}. By default,
4133 @var{n} is set to the number of available processors, and values
4134 greater than that are reduced to that limit. Also see
4135 @ref{nproc invocation}.
4141 @cindex uniquifying output
4143 Normally, output only the first of a sequence of lines that compare
4144 equal. For the @option{--check} (@option{-c} or @option{-C}) option,
4145 check that no pair of consecutive lines compares equal.
4147 This option also disables the default last-resort comparison.
4149 The commands @code{sort -u} and @code{sort | uniq} are equivalent, but
4150 this equivalence does not extend to arbitrary @command{sort} options.
4151 For example, @code{sort -n -u} inspects only the value of the initial
4152 numeric string when checking for uniqueness, whereas @code{sort -n |
4153 uniq} inspects the entire line. @xref{uniq invocation}.
4155 @macro zeroTerminatedOption
4157 @itemx --zero-terminated
4159 @opindex --zero-terminated
4160 @cindex process zero-terminated items
4161 Delimit items with a zero byte rather than a newline (@acronym{ASCII} @sc{lf}).
4162 I.E. treat input as items separated by @acronym{ASCII} @sc{nul}
4163 and terminate output items with @acronym{ASCII} @sc{nul}.
4164 This option can be useful in conjunction with @samp{perl -0} or
4165 @samp{find -print0} and @samp{xargs -0} which do the same in order to
4166 reliably handle arbitrary file names (even those containing blanks
4167 or other special characters).
4169 @zeroTerminatedOption
4173 Historical (BSD and System V) implementations of @command{sort} have
4174 differed in their interpretation of some options, particularly
4175 @option{-b}, @option{-f}, and @option{-n}. @sc{gnu} sort follows the @acronym{POSIX}
4176 behavior, which is usually (but not always!) like the System V behavior.
4177 According to @acronym{POSIX}, @option{-n} no longer implies @option{-b}. For
4178 consistency, @option{-M} has been changed in the same way. This may
4179 affect the meaning of character positions in field specifications in
4180 obscure cases. The only fix is to add an explicit @option{-b}.
4182 A position in a sort field specified with @option{-k} may have any
4183 of the option letters @samp{MbdfghinRrV} appended to it, in which case no
4184 global ordering options are inherited by that particular field. The
4185 @option{-b} option may be independently attached to either or both of
4186 the start and end positions of a field specification, and if it is
4187 inherited from the global options it will be attached to both.
4188 If input lines can contain leading or adjacent blanks and @option{-t}
4189 is not used, then @option{-k} is typically combined with @option{-b} or
4190 an option that implicitly ignores leading blanks (@samp{Mghn}) as otherwise
4191 the varying numbers of leading blanks in fields can cause confusing results.
4193 If the start position in a sort field specifier falls after the end of
4194 the line or after the end field, the field is empty. If the @option{-b}
4195 option was specified, the @samp{.@var{c}} part of a field specification
4196 is counted from the first nonblank character of the field.
4198 @vindex _POSIX2_VERSION
4199 @vindex POSIXLY_CORRECT
4200 On older systems, @command{sort} supports an obsolete origin-zero
4201 syntax @samp{+@var{pos1} [-@var{pos2}]} for specifying sort keys.
4202 The obsolete sequence @samp{sort +@var{a}.@var{x} -@var{b}.@var{y}}
4203 is equivalent to @samp{sort -k @var{a+1}.@var{x+1},@var{b}} if @var{y}
4204 is @samp{0} or absent, otherwise it is equivalent to @samp{sort -k
4205 @var{a+1}.@var{x+1},@var{b+1}.@var{y}}.
4207 This obsolete behavior can be enabled or disabled with the
4208 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4209 conformance}); it can also be enabled when @env{POSIXLY_CORRECT} is
4210 not set by using the obsolete syntax with @samp{-@var{pos2}} present.
4212 Scripts intended for use on standard hosts should avoid obsolete
4213 syntax and should use @option{-k} instead. For example, avoid
4214 @samp{sort +2}, since it might be interpreted as either @samp{sort
4215 ./+2} or @samp{sort -k 3}. If your script must also run on hosts that
4216 support only the obsolete syntax, it can use a test like @samp{if sort
4217 -k 1 </dev/null >/dev/null 2>&1; then @dots{}} to decide which syntax
4220 Here are some examples to illustrate various combinations of options.
4225 Sort in descending (reverse) numeric order.
4232 Run no more that 4 sorts concurrently, using a buffer size of 10M.
4235 sort --parallel=4 -S 10M
4239 Sort alphabetically, omitting the first and second fields
4240 and the blanks at the start of the third field.
4241 This uses a single key composed of the characters beginning
4242 at the start of the first nonblank character in field three
4243 and extending to the end of each line.
4250 Sort numerically on the second field and resolve ties by sorting
4251 alphabetically on the third and fourth characters of field five.
4252 Use @samp{:} as the field delimiter.
4255 sort -t : -k 2,2n -k 5.3,5.4
4258 Note that if you had written @option{-k 2n} instead of @option{-k 2,2n}
4259 @command{sort} would have used all characters beginning in the second field
4260 and extending to the end of the line as the primary @emph{numeric}
4261 key. For the large majority of applications, treating keys spanning
4262 more than one field as numeric will not do what you expect.
4264 Also note that the @samp{n} modifier was applied to the field-end
4265 specifier for the first key. It would have been equivalent to
4266 specify @option{-k 2n,2} or @option{-k 2n,2n}. All modifiers except
4267 @samp{b} apply to the associated @emph{field}, regardless of whether
4268 the modifier character is attached to the field-start and/or the
4269 field-end part of the key specifier.
4272 Sort the password file on the fifth field and ignore any
4273 leading blanks. Sort lines with equal values in field five
4274 on the numeric user ID in field three. Fields are separated
4278 sort -t : -k 5b,5 -k 3,3n /etc/passwd
4279 sort -t : -n -k 5b,5 -k 3,3 /etc/passwd
4280 sort -t : -b -k 5,5 -k 3,3n /etc/passwd
4283 These three commands have equivalent effect. The first specifies that
4284 the first key's start position ignores leading blanks and the second
4285 key is sorted numerically. The other two commands rely on global
4286 options being inherited by sort keys that lack modifiers. The inheritance
4287 works in this case because @option{-k 5b,5b} and @option{-k 5b,5} are
4288 equivalent, as the location of a field-end lacking a @samp{.@var{c}}
4289 character position is not affected by whether initial blanks are
4293 Sort a set of log files, primarily by IPv4 address and secondarily by
4294 time stamp. If two lines' primary and secondary keys are identical,
4295 output the lines in the same order that they were input. The log
4296 files contain lines that look like this:
4299 4.150.156.3 - - [01/Apr/2004:06:31:51 +0000] message 1
4300 211.24.3.231 - - [24/Apr/2004:20:17:39 +0000] message 2
4303 Fields are separated by exactly one space. Sort IPv4 addresses
4304 lexicographically, e.g., 212.61.52.2 sorts before 212.129.233.201
4305 because 61 is less than 129.
4308 sort -s -t ' ' -k 4.9n -k 4.5M -k 4.2n -k 4.14,4.21 file*.log |
4309 sort -s -t '.' -k 1,1n -k 2,2n -k 3,3n -k 4,4n
4312 This example cannot be done with a single @command{sort} invocation,
4313 since IPv4 address components are separated by @samp{.} while dates
4314 come just after a space. So it is broken down into two invocations of
4315 @command{sort}: the first sorts by time stamp and the second by IPv4
4316 address. The time stamp is sorted by year, then month, then day, and
4317 finally by hour-minute-second field, using @option{-k} to isolate each
4318 field. Except for hour-minute-second there's no need to specify the
4319 end of each key field, since the @samp{n} and @samp{M} modifiers sort
4320 based on leading prefixes that cannot cross field boundaries. The
4321 IPv4 addresses are sorted lexicographically. The second sort uses
4322 @samp{-s} so that ties in the primary key are broken by the secondary
4323 key; the first sort uses @samp{-s} so that the combination of the two
4327 Generate a tags file in case-insensitive sorted order.
4330 find src -type f -print0 | sort -z -f | xargs -0 etags --append
4333 The use of @option{-print0}, @option{-z}, and @option{-0} in this case means
4334 that file names that contain blanks or other special characters are
4336 by the sort operation.
4338 @c This example is a bit contrived and needs more explanation.
4340 @c Sort records separated by an arbitrary string by using a pipe to convert
4341 @c each record delimiter string to @samp{\0}, then using sort's -z option,
4342 @c and converting each @samp{\0} back to the original record delimiter.
4345 @c printf 'c\n\nb\n\na\n'|perl -0pe 's/\n\n/\n\0/g'|sort -z|perl -0pe 's/\0/\n/g'
4349 Use the common @acronym{DSU, Decorate Sort Undecorate} idiom to
4350 sort lines according to their length.
4353 awk '@{print length, $0@}' /etc/passwd | sort -n | cut -f2- -d' '
4356 In general this technique can be used to sort data that the @command{sort}
4357 command does not support, or is inefficient at, sorting directly.
4360 Shuffle a list of directories, but preserve the order of files within
4361 each directory. For instance, one could use this to generate a music
4362 playlist in which albums are shuffled but the songs of each album are
4366 ls */* | sort -t / -k 1,1R -k 2,2
4372 @node shuf invocation
4373 @section @command{shuf}: Shuffling text
4376 @cindex shuffling files
4378 @command{shuf} shuffles its input by outputting a random permutation
4379 of its input lines. Each output permutation is equally likely.
4383 shuf [@var{option}]@dots{} [@var{file}]
4384 shuf -e [@var{option}]@dots{} [@var{arg}]@dots{}
4385 shuf -i @var{lo}-@var{hi} [@var{option}]@dots{}
4388 @command{shuf} has three modes of operation that affect where it
4389 obtains its input lines. By default, it reads lines from standard
4390 input. The following options change the operation mode:
4398 @cindex command-line operands to shuffle
4399 Treat each command-line operand as an input line.
4401 @item -i @var{lo}-@var{hi}
4402 @itemx --input-range=@var{lo}-@var{hi}
4404 @opindex --input-range
4405 @cindex input range to shuffle
4406 Act as if input came from a file containing the range of unsigned
4407 decimal integers @var{lo}@dots{}@var{hi}, one per line.
4411 @command{shuf}'s other options can affect its behavior in all
4416 @item -n @var{lines}
4417 @itemx --head-count=@var{count}
4419 @opindex --head-count
4420 @cindex head of output
4421 Output at most @var{count} lines. By default, all input lines are
4424 @item -o @var{output-file}
4425 @itemx --output=@var{output-file}
4428 @cindex overwriting of input, allowed
4429 Write output to @var{output-file} instead of standard output.
4430 @command{shuf} reads all input before opening
4431 @var{output-file}, so you can safely shuffle a file in place by using
4432 commands like @code{shuf -o F <F} and @code{cat F | shuf -o F}.
4434 @item --random-source=@var{file}
4435 @opindex --random-source
4436 @cindex random source for shuffling
4437 Use @var{file} as a source of random data used to determine which
4438 permutation to generate. @xref{Random sources}.
4440 @zeroTerminatedOption
4456 might produce the output
4466 Similarly, the command:
4469 shuf -e clubs hearts diamonds spades
4483 and the command @samp{shuf -i 1-4} might output:
4493 These examples all have four input lines, so @command{shuf} might
4494 produce any of the twenty-four possible permutations of the input. In
4495 general, if there are @var{n} input lines, there are @var{n}! (i.e.,
4496 @var{n} factorial, or @var{n} * (@var{n} - 1) * @dots{} * 1) possible
4497 output permutations.
4502 @node uniq invocation
4503 @section @command{uniq}: Uniquify files
4506 @cindex uniquify files
4508 @command{uniq} writes the unique lines in the given @file{input}, or
4509 standard input if nothing is given or for an @var{input} name of
4513 uniq [@var{option}]@dots{} [@var{input} [@var{output}]]
4516 By default, @command{uniq} prints its input lines, except that
4517 it discards all but the first of adjacent repeated lines, so that
4518 no output lines are repeated. Optionally, it can instead discard
4519 lines that are not repeated, or all repeated lines.
4521 The input need not be sorted, but repeated input lines are detected
4522 only if they are adjacent. If you want to discard non-adjacent
4523 duplicate lines, perhaps you want to use @code{sort -u}.
4524 @xref{sort invocation}.
4527 Comparisons honor the rules specified by the @env{LC_COLLATE}
4530 If no @var{output} file is specified, @command{uniq} writes to standard
4533 The program accepts the following options. Also see @ref{Common options}.
4538 @itemx --skip-fields=@var{n}
4540 @opindex --skip-fields
4541 Skip @var{n} fields on each line before checking for uniqueness. Use
4542 a null string for comparison if a line has fewer than @var{n} fields. Fields
4543 are sequences of non-space non-tab characters that are separated from
4544 each other by at least one space or tab.
4546 For compatibility @command{uniq} supports an obsolete option syntax
4547 @option{-@var{n}}. New scripts should use @option{-f @var{n}} instead.
4550 @itemx --skip-chars=@var{n}
4552 @opindex --skip-chars
4553 Skip @var{n} characters before checking for uniqueness. Use a null string
4554 for comparison if a line has fewer than @var{n} characters. If you use both
4555 the field and character skipping options, fields are skipped over first.
4557 @vindex _POSIX2_VERSION
4558 On older systems, @command{uniq} supports an obsolete option syntax
4560 This obsolete behavior can be enabled or disabled with the
4561 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4562 conformance}), but portable scripts should avoid commands whose
4563 behavior depends on this variable.
4564 For example, use @samp{uniq ./+10} or @samp{uniq -s 10} rather than
4565 the ambiguous @samp{uniq +10}.
4571 Print the number of times each line occurred along with the line.
4574 @itemx --ignore-case
4576 @opindex --ignore-case
4577 Ignore differences in case when comparing lines.
4583 @cindex repeated lines, outputting
4584 Discard lines that are not repeated. When used by itself, this option
4585 causes @command{uniq} to print the first copy of each repeated line,
4589 @itemx --all-repeated[=@var{delimit-method}]
4591 @opindex --all-repeated
4592 @cindex all repeated lines, outputting
4593 Do not discard the second and subsequent repeated input lines,
4594 but discard lines that are not repeated.
4595 This option is useful mainly in conjunction with other options e.g.,
4596 to ignore case or to compare only selected fields.
4597 The optional @var{delimit-method} tells how to delimit
4598 groups of repeated lines, and must be one of the following:
4603 Do not delimit groups of repeated lines.
4604 This is equivalent to @option{--all-repeated} (@option{-D}).
4607 Output a newline before each group of repeated lines.
4608 With @option{--zero-terminated} (@option{-z}), use a zero
4609 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4612 Separate groups of repeated lines with a single newline.
4613 With @option{--zero-terminated} (@option{-z}), use a zero
4614 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4615 This is the same as using @samp{prepend}, except that
4616 no delimiter is inserted before the first group, and hence
4617 may be better suited for output direct to users.
4620 Note that when groups are delimited and the input stream contains
4621 two or more consecutive blank lines, then the output is ambiguous.
4622 To avoid that, filter the input through @samp{tr -s '\n'} to replace
4623 each sequence of consecutive newlines with a single newline.
4625 This is a @sc{gnu} extension.
4626 @c FIXME: give an example showing *how* it's useful
4632 @cindex unique lines, outputting
4633 Discard the first repeated line. When used by itself, this option
4634 causes @command{uniq} to print unique lines, and nothing else.
4637 @itemx --check-chars=@var{n}
4639 @opindex --check-chars
4640 Compare at most @var{n} characters on each line (after skipping any specified
4641 fields and characters). By default the entire rest of the lines are
4644 @zeroTerminatedOption
4651 @node comm invocation
4652 @section @command{comm}: Compare two sorted files line by line
4655 @cindex line-by-line comparison
4656 @cindex comparing sorted files
4658 @command{comm} writes to standard output lines that are common, and lines
4659 that are unique, to two input files; a file name of @samp{-} means
4660 standard input. Synopsis:
4663 comm [@var{option}]@dots{} @var{file1} @var{file2}
4667 Before @command{comm} can be used, the input files must be sorted using the
4668 collating sequence specified by the @env{LC_COLLATE} locale.
4669 If an input file ends in a non-newline
4670 character, a newline is silently appended. The @command{sort} command with
4671 no options always outputs a file that is suitable input to @command{comm}.
4673 @cindex differing lines
4674 @cindex common lines
4675 With no options, @command{comm} produces three-column output. Column one
4676 contains lines unique to @var{file1}, column two contains lines unique
4677 to @var{file2}, and column three contains lines common to both files.
4678 Columns are separated by a single TAB character.
4679 @c FIXME: when there's an option to supply an alternative separator
4680 @c string, append `by default' to the above sentence.
4685 The options @option{-1}, @option{-2}, and @option{-3} suppress printing of
4686 the corresponding columns (and separators). Also see @ref{Common options}.
4688 Unlike some other comparison utilities, @command{comm} has an exit
4689 status that does not depend on the result of the comparison.
4690 Upon normal completion @command{comm} produces an exit code of zero.
4691 If there is an error it exits with nonzero status.
4693 @macro checkOrderOption{cmd}
4694 If the @option{--check-order} option is given, unsorted inputs will
4695 cause a fatal error message. If the option @option{--nocheck-order}
4696 is given, unsorted inputs will never cause an error message. If
4697 neither of these options is given, wrongly sorted inputs are diagnosed
4698 only if an input file is found to contain unpairable lines. If an
4699 input file is diagnosed as being unsorted, the @command{\cmd\} command
4700 will exit with a nonzero status (and the output should not be used).
4702 Forcing @command{\cmd\} to process wrongly sorted input files
4703 containing unpairable lines by specifying @option{--nocheck-order} is
4704 not guaranteed to produce any particular output. The output will
4705 probably not correspond with whatever you hoped it would be.
4707 @checkOrderOption{comm}
4712 Fail with an error message if either input file is wrongly ordered.
4714 @item --nocheck-order
4715 Do not check that both input files are in sorted order.
4719 @item --output-delimiter=@var{str}
4720 Print @var{str} between adjacent output columns,
4721 rather than the default of a single TAB character.
4723 The delimiter @var{str} may not be empty.
4727 @node ptx invocation
4728 @section @command{ptx}: Produce permuted indexes
4732 @command{ptx} reads a text file and essentially produces a permuted index, with
4733 each keyword in its context. The calling sketch is either one of:
4736 ptx [@var{option} @dots{}] [@var{file} @dots{}]
4737 ptx -G [@var{option} @dots{}] [@var{input} [@var{output}]]
4740 The @option{-G} (or its equivalent: @option{--traditional}) option disables
4741 all @sc{gnu} extensions and reverts to traditional mode, thus introducing some
4742 limitations and changing several of the program's default option values.
4743 When @option{-G} is not specified, @sc{gnu} extensions are always enabled.
4744 @sc{gnu} extensions to @command{ptx} are documented wherever appropriate in this
4745 document. For the full list, see @xref{Compatibility in ptx}.
4747 Individual options are explained in the following sections.
4749 When @sc{gnu} extensions are enabled, there may be zero, one or several
4750 @var{file}s after the options. If there is no @var{file}, the program
4751 reads the standard input. If there is one or several @var{file}s, they
4752 give the name of input files which are all read in turn, as if all the
4753 input files were concatenated. However, there is a full contextual
4754 break between each file and, when automatic referencing is requested,
4755 file names and line numbers refer to individual text input files. In
4756 all cases, the program outputs the permuted index to the standard
4759 When @sc{gnu} extensions are @emph{not} enabled, that is, when the program
4760 operates in traditional mode, there may be zero, one or two parameters
4761 besides the options. If there are no parameters, the program reads the
4762 standard input and outputs the permuted index to the standard output.
4763 If there is only one parameter, it names the text @var{input} to be read
4764 instead of the standard input. If two parameters are given, they give
4765 respectively the name of the @var{input} file to read and the name of
4766 the @var{output} file to produce. @emph{Be very careful} to note that,
4767 in this case, the contents of file given by the second parameter is
4768 destroyed. This behavior is dictated by System V @command{ptx}
4769 compatibility; @sc{gnu} Standards normally discourage output parameters not
4770 introduced by an option.
4772 Note that for @emph{any} file named as the value of an option or as an
4773 input text file, a single dash @kbd{-} may be used, in which case
4774 standard input is assumed. However, it would not make sense to use this
4775 convention more than once per program invocation.
4778 * General options in ptx:: Options which affect general program behavior.
4779 * Charset selection in ptx:: Underlying character set considerations.
4780 * Input processing in ptx:: Input fields, contexts, and keyword selection.
4781 * Output formatting in ptx:: Types of output format, and sizing the fields.
4782 * Compatibility in ptx::
4786 @node General options in ptx
4787 @subsection General options
4792 @itemx --traditional
4793 As already explained, this option disables all @sc{gnu} extensions to
4794 @command{ptx} and switches to traditional mode.
4797 Print a short help on standard output, then exit without further
4801 Print the program version on standard output, then exit without further
4809 @node Charset selection in ptx
4810 @subsection Charset selection
4812 @c FIXME: People don't necessarily know what an IBM-PC was these days.
4813 As it is set up now, the program assumes that the input file is coded
4814 using 8-bit @acronym{ISO} 8859-1 code, also known as Latin-1 character set,
4815 @emph{unless} it is compiled for MS-DOS, in which case it uses the
4816 character set of the IBM-PC@. (@sc{gnu} @command{ptx} is not known to work on
4817 smaller MS-DOS machines anymore.) Compared to 7-bit @acronym{ASCII}, the set
4818 of characters which are letters is different; this alters the behavior
4819 of regular expression matching. Thus, the default regular expression
4820 for a keyword allows foreign or diacriticized letters. Keyword sorting,
4821 however, is still crude; it obeys the underlying character set ordering
4827 @itemx --ignore-case
4828 Fold lower case letters to upper case for sorting.
4833 @node Input processing in ptx
4834 @subsection Word selection and input processing
4839 @itemx --break-file=@var{file}
4841 This option provides an alternative (to @option{-W}) method of describing
4842 which characters make up words. It introduces the name of a
4843 file which contains a list of characters which can@emph{not} be part of
4844 one word; this file is called the @dfn{Break file}. Any character which
4845 is not part of the Break file is a word constituent. If both options
4846 @option{-b} and @option{-W} are specified, then @option{-W} has precedence and
4847 @option{-b} is ignored.
4849 When @sc{gnu} extensions are enabled, the only way to avoid newline as a
4850 break character is to write all the break characters in the file with no
4851 newline at all, not even at the end of the file. When @sc{gnu} extensions
4852 are disabled, spaces, tabs and newlines are always considered as break
4853 characters even if not included in the Break file.
4856 @itemx --ignore-file=@var{file}
4858 The file associated with this option contains a list of words which will
4859 never be taken as keywords in concordance output. It is called the
4860 @dfn{Ignore file}. The file contains exactly one word in each line; the
4861 end of line separation of words is not subject to the value of the
4865 @itemx --only-file=@var{file}
4867 The file associated with this option contains a list of words which will
4868 be retained in concordance output; any word not mentioned in this file
4869 is ignored. The file is called the @dfn{Only file}. The file contains
4870 exactly one word in each line; the end of line separation of words is
4871 not subject to the value of the @option{-S} option.
4873 There is no default for the Only file. When both an Only file and an
4874 Ignore file are specified, a word is considered a keyword only
4875 if it is listed in the Only file and not in the Ignore file.
4880 On each input line, the leading sequence of non-white space characters will be
4881 taken to be a reference that has the purpose of identifying this input
4882 line in the resulting permuted index. For more information about reference
4883 production, see @xref{Output formatting in ptx}.
4884 Using this option changes the default value for option @option{-S}.
4886 Using this option, the program does not try very hard to remove
4887 references from contexts in output, but it succeeds in doing so
4888 @emph{when} the context ends exactly at the newline. If option
4889 @option{-r} is used with @option{-S} default value, or when @sc{gnu} extensions
4890 are disabled, this condition is always met and references are completely
4891 excluded from the output contexts.
4893 @item -S @var{regexp}
4894 @itemx --sentence-regexp=@var{regexp}
4896 This option selects which regular expression will describe the end of a
4897 line or the end of a sentence. In fact, this regular expression is not
4898 the only distinction between end of lines or end of sentences, and input
4899 line boundaries have no special significance outside this option. By
4900 default, when @sc{gnu} extensions are enabled and if @option{-r} option is not
4901 used, end of sentences are used. In this case, this @var{regex} is
4902 imported from @sc{gnu} Emacs:
4905 [.?!][]\"')@}]*\\($\\|\t\\| \\)[ \t\n]*
4908 Whenever @sc{gnu} extensions are disabled or if @option{-r} option is used, end
4909 of lines are used; in this case, the default @var{regexp} is just:
4915 Using an empty @var{regexp} is equivalent to completely disabling end of
4916 line or end of sentence recognition. In this case, the whole file is
4917 considered to be a single big line or sentence. The user might want to
4918 disallow all truncation flag generation as well, through option @option{-F
4919 ""}. @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
4922 When the keywords happen to be near the beginning of the input line or
4923 sentence, this often creates an unused area at the beginning of the
4924 output context line; when the keywords happen to be near the end of the
4925 input line or sentence, this often creates an unused area at the end of
4926 the output context line. The program tries to fill those unused areas
4927 by wrapping around context in them; the tail of the input line or
4928 sentence is used to fill the unused area on the left of the output line;
4929 the head of the input line or sentence is used to fill the unused area
4930 on the right of the output line.
4932 As a matter of convenience to the user, many usual backslashed escape
4933 sequences from the C language are recognized and converted to the
4934 corresponding characters by @command{ptx} itself.
4936 @item -W @var{regexp}
4937 @itemx --word-regexp=@var{regexp}
4939 This option selects which regular expression will describe each keyword.
4940 By default, if @sc{gnu} extensions are enabled, a word is a sequence of
4941 letters; the @var{regexp} used is @samp{\w+}. When @sc{gnu} extensions are
4942 disabled, a word is by default anything which ends with a space, a tab
4943 or a newline; the @var{regexp} used is @samp{[^ \t\n]+}.
4945 An empty @var{regexp} is equivalent to not using this option.
4946 @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
4949 As a matter of convenience to the user, many usual backslashed escape
4950 sequences, as found in the C language, are recognized and converted to
4951 the corresponding characters by @command{ptx} itself.
4956 @node Output formatting in ptx
4957 @subsection Output formatting
4959 Output format is mainly controlled by the @option{-O} and @option{-T} options
4960 described in the table below. When neither @option{-O} nor @option{-T} are
4961 selected, and if @sc{gnu} extensions are enabled, the program chooses an
4962 output format suitable for a dumb terminal. Each keyword occurrence is
4963 output to the center of one line, surrounded by its left and right
4964 contexts. Each field is properly justified, so the concordance output
4965 can be readily observed. As a special feature, if automatic
4966 references are selected by option @option{-A} and are output before the
4967 left context, that is, if option @option{-R} is @emph{not} selected, then
4968 a colon is added after the reference; this nicely interfaces with @sc{gnu}
4969 Emacs @code{next-error} processing. In this default output format, each
4970 white space character, like newline and tab, is merely changed to
4971 exactly one space, with no special attempt to compress consecutive
4972 spaces. This might change in the future. Except for those white space
4973 characters, every other character of the underlying set of 256
4974 characters is transmitted verbatim.
4976 Output format is further controlled by the following options.
4980 @item -g @var{number}
4981 @itemx --gap-size=@var{number}
4983 Select the size of the minimum white space gap between the fields on the
4986 @item -w @var{number}
4987 @itemx --width=@var{number}
4989 Select the maximum output width of each final line. If references are
4990 used, they are included or excluded from the maximum output width
4991 depending on the value of option @option{-R}. If this option is not
4992 selected, that is, when references are output before the left context,
4993 the maximum output width takes into account the maximum length of all
4994 references. If this option is selected, that is, when references are
4995 output after the right context, the maximum output width does not take
4996 into account the space taken by references, nor the gap that precedes
5000 @itemx --auto-reference
5002 Select automatic references. Each input line will have an automatic
5003 reference made up of the file name and the line ordinal, with a single
5004 colon between them. However, the file name will be empty when standard
5005 input is being read. If both @option{-A} and @option{-r} are selected, then
5006 the input reference is still read and skipped, but the automatic
5007 reference is used at output time, overriding the input reference.
5010 @itemx --right-side-refs
5012 In the default output format, when option @option{-R} is not used, any
5013 references produced by the effect of options @option{-r} or @option{-A} are
5014 placed to the far right of output lines, after the right context. With
5015 default output format, when the @option{-R} option is specified, references
5016 are rather placed at the beginning of each output line, before the left
5017 context. For any other output format, option @option{-R} is
5018 ignored, with one exception: with @option{-R} the width of references
5019 is @emph{not} taken into account in total output width given by @option{-w}.
5021 This option is automatically selected whenever @sc{gnu} extensions are
5024 @item -F @var{string}
5025 @itemx --flac-truncation=@var{string}
5027 This option will request that any truncation in the output be reported
5028 using the string @var{string}. Most output fields theoretically extend
5029 towards the beginning or the end of the current line, or current
5030 sentence, as selected with option @option{-S}. But there is a maximum
5031 allowed output line width, changeable through option @option{-w}, which is
5032 further divided into space for various output fields. When a field has
5033 to be truncated because it cannot extend beyond the beginning or the end of
5034 the current line to fit in, then a truncation occurs. By default,
5035 the string used is a single slash, as in @option{-F /}.
5037 @var{string} may have more than one character, as in @option{-F ...}.
5038 Also, in the particular case when @var{string} is empty (@option{-F ""}),
5039 truncation flagging is disabled, and no truncation marks are appended in
5042 As a matter of convenience to the user, many usual backslashed escape
5043 sequences, as found in the C language, are recognized and converted to
5044 the corresponding characters by @command{ptx} itself.
5046 @item -M @var{string}
5047 @itemx --macro-name=@var{string}
5049 Select another @var{string} to be used instead of @samp{xx}, while
5050 generating output suitable for @command{nroff}, @command{troff} or @TeX{}.
5053 @itemx --format=roff
5055 Choose an output format suitable for @command{nroff} or @command{troff}
5056 processing. Each output line will look like:
5059 .xx "@var{tail}" "@var{before}" "@var{keyword_and_after}" "@var{head}" "@var{ref}"
5062 so it will be possible to write a @samp{.xx} roff macro to take care of
5063 the output typesetting. This is the default output format when @sc{gnu}
5064 extensions are disabled. Option @option{-M} can be used to change
5065 @samp{xx} to another macro name.
5067 In this output format, each non-graphical character, like newline and
5068 tab, is merely changed to exactly one space, with no special attempt to
5069 compress consecutive spaces. Each quote character: @kbd{"} is doubled
5070 so it will be correctly processed by @command{nroff} or @command{troff}.
5075 Choose an output format suitable for @TeX{} processing. Each output
5076 line will look like:
5079 \xx @{@var{tail}@}@{@var{before}@}@{@var{keyword}@}@{@var{after}@}@{@var{head}@}@{@var{ref}@}
5083 so it will be possible to write a @code{\xx} definition to take care of
5084 the output typesetting. Note that when references are not being
5085 produced, that is, neither option @option{-A} nor option @option{-r} is
5086 selected, the last parameter of each @code{\xx} call is inhibited.
5087 Option @option{-M} can be used to change @samp{xx} to another macro
5090 In this output format, some special characters, like @kbd{$}, @kbd{%},
5091 @kbd{&}, @kbd{#} and @kbd{_} are automatically protected with a
5092 backslash. Curly brackets @kbd{@{}, @kbd{@}} are protected with a
5093 backslash and a pair of dollar signs (to force mathematical mode). The
5094 backslash itself produces the sequence @code{\backslash@{@}}.
5095 Circumflex and tilde diacritical marks produce the sequence @code{^\@{ @}} and
5096 @code{~\@{ @}} respectively. Other diacriticized characters of the
5097 underlying character set produce an appropriate @TeX{} sequence as far
5098 as possible. The other non-graphical characters, like newline and tab,
5099 and all other characters which are not part of @acronym{ASCII}, are merely
5100 changed to exactly one space, with no special attempt to compress
5101 consecutive spaces. Let me know how to improve this special character
5102 processing for @TeX{}.
5107 @node Compatibility in ptx
5108 @subsection The @sc{gnu} extensions to @command{ptx}
5110 This version of @command{ptx} contains a few features which do not exist in
5111 System V @command{ptx}. These extra features are suppressed by using the
5112 @option{-G} command line option, unless overridden by other command line
5113 options. Some @sc{gnu} extensions cannot be recovered by overriding, so the
5114 simple rule is to avoid @option{-G} if you care about @sc{gnu} extensions.
5115 Here are the differences between this program and System V @command{ptx}.
5120 This program can read many input files at once, it always writes the
5121 resulting concordance on standard output. On the other hand, System V
5122 @command{ptx} reads only one file and sends the result to standard output
5123 or, if a second @var{file} parameter is given on the command, to that
5126 Having output parameters not introduced by options is a dangerous
5127 practice which @sc{gnu} avoids as far as possible. So, for using @command{ptx}
5128 portably between @sc{gnu} and System V, you should always use it with a
5129 single input file, and always expect the result on standard output. You
5130 might also want to automatically configure in a @option{-G} option to
5131 @command{ptx} calls in products using @command{ptx}, if the configurator finds
5132 that the installed @command{ptx} accepts @option{-G}.
5135 The only options available in System V @command{ptx} are options @option{-b},
5136 @option{-f}, @option{-g}, @option{-i}, @option{-o}, @option{-r}, @option{-t} and
5137 @option{-w}. All other options are @sc{gnu} extensions and are not repeated in
5138 this enumeration. Moreover, some options have a slightly different
5139 meaning when @sc{gnu} extensions are enabled, as explained below.
5142 By default, concordance output is not formatted for @command{troff} or
5143 @command{nroff}. It is rather formatted for a dumb terminal. @command{troff}
5144 or @command{nroff} output may still be selected through option @option{-O}.
5147 Unless @option{-R} option is used, the maximum reference width is
5148 subtracted from the total output line width. With @sc{gnu} extensions
5149 disabled, width of references is not taken into account in the output
5150 line width computations.
5153 All 256 bytes, even @acronym{ASCII} @sc{nul} bytes, are always read and
5154 processed from input file with no adverse effect, even if @sc{gnu} extensions
5155 are disabled. However, System V @command{ptx} does not accept 8-bit characters,
5156 a few control characters are rejected, and the tilde @kbd{~} is also rejected.
5159 Input line length is only limited by available memory, even if @sc{gnu}
5160 extensions are disabled. However, System V @command{ptx} processes only
5161 the first 200 characters in each line.
5164 The break (non-word) characters default to be every character except all
5165 letters of the underlying character set, diacriticized or not. When @sc{gnu}
5166 extensions are disabled, the break characters default to space, tab and
5170 The program makes better use of output line width. If @sc{gnu} extensions
5171 are disabled, the program rather tries to imitate System V @command{ptx},
5172 but still, there are some slight disposition glitches this program does
5173 not completely reproduce.
5176 The user can specify both an Ignore file and an Only file. This is not
5177 allowed with System V @command{ptx}.
5182 @node tsort invocation
5183 @section @command{tsort}: Topological sort
5186 @cindex topological sort
5188 @command{tsort} performs a topological sort on the given @var{file}, or
5189 standard input if no input file is given or for a @var{file} of
5190 @samp{-}. For more details and some history, see @ref{tsort background}.
5194 tsort [@var{option}] [@var{file}]
5197 @command{tsort} reads its input as pairs of strings, separated by blanks,
5198 indicating a partial ordering. The output is a total ordering that
5199 corresponds to the given partial ordering.
5213 will produce the output
5224 Consider a more realistic example.
5225 You have a large set of functions all in one file, and they may all be
5226 declared static except one. Currently that one (say @code{main}) is the
5227 first function defined in the file, and the ones it calls directly follow
5228 it, followed by those they call, etc. Let's say that you are determined
5229 to take advantage of prototypes, so you have to choose between declaring
5230 all of those functions (which means duplicating a lot of information from
5231 the definitions) and rearranging the functions so that as many as possible
5232 are defined before they are used. One way to automate the latter process
5233 is to get a list for each function of the functions it calls directly.
5234 Many programs can generate such lists. They describe a call graph.
5235 Consider the following list, in which a given line indicates that the
5236 function on the left calls the one on the right directly.
5242 tail_file pretty_name
5243 tail_file write_header
5245 tail_forever recheck
5246 tail_forever pretty_name
5247 tail_forever write_header
5248 tail_forever dump_remainder
5251 tail_lines start_lines
5252 tail_lines dump_remainder
5253 tail_lines file_lines
5254 tail_lines pipe_lines
5256 tail_bytes start_bytes
5257 tail_bytes dump_remainder
5258 tail_bytes pipe_bytes
5259 file_lines dump_remainder
5263 then you can use @command{tsort} to produce an ordering of those
5264 functions that satisfies your requirement.
5267 example$ tsort call-graph | tac
5287 @command{tsort} detects any cycles in the input and writes the first cycle
5288 encountered to standard error.
5290 Note that for a given partial ordering, generally there is no unique
5291 total ordering. In the context of the call graph above, the function
5292 @code{parse_options} may be placed anywhere in the list as long as it
5293 precedes @code{main}.
5295 The only options are @option{--help} and @option{--version}. @xref{Common
5301 * tsort background:: Where tsort came from.
5304 @node tsort background
5305 @subsection @command{tsort}: Background
5307 @command{tsort} exists because very early versions of the Unix linker processed
5308 an archive file exactly once, and in order. As @command{ld} read each object
5309 in the archive, it decided whether it was needed in the program based on
5310 whether it defined any symbols which were undefined at that point in
5313 This meant that dependencies within the archive had to be handled
5314 specially. For example, @code{scanf} probably calls @code{read}. That means
5315 that in a single pass through an archive, it was important for @code{scanf.o}
5316 to appear before read.o, because otherwise a program which calls
5317 @code{scanf} but not @code{read} might end up with an unexpected unresolved
5318 reference to @code{read}.
5320 The way to address this problem was to first generate a set of
5321 dependencies of one object file on another. This was done by a shell
5322 script called @command{lorder}. The GNU tools don't provide a version of
5323 lorder, as far as I know, but you can still find it in BSD
5326 Then you ran @command{tsort} over the @command{lorder} output, and you used the
5327 resulting sort to define the order in which you added objects to the archive.
5329 This whole procedure has been obsolete since about 1980, because
5330 Unix archives now contain a symbol table (traditionally built by
5331 @command{ranlib}, now generally built by @command{ar} itself), and the Unix
5332 linker uses the symbol table to effectively make multiple passes over
5335 Anyhow, that's where tsort came from. To solve an old problem with
5336 the way the linker handled archive files, which has since been solved
5340 @node Operating on fields
5341 @chapter Operating on fields
5344 * cut invocation:: Print selected parts of lines.
5345 * paste invocation:: Merge lines of files.
5346 * join invocation:: Join lines on a common field.
5350 @node cut invocation
5351 @section @command{cut}: Print selected parts of lines
5354 @command{cut} writes to standard output selected parts of each line of each
5355 input file, or standard input if no files are given or for a file name of
5359 cut @var{option}@dots{} [@var{file}]@dots{}
5362 In the table which follows, the @var{byte-list}, @var{character-list},
5363 and @var{field-list} are one or more numbers or ranges (two numbers
5364 separated by a dash) separated by commas. Bytes, characters, and
5365 fields are numbered starting at 1. Incomplete ranges may be
5366 given: @option{-@var{m}} means @samp{1-@var{m}}; @samp{@var{n}-} means
5367 @samp{@var{n}} through end of line or last field. The list elements
5368 can be repeated, can overlap, and can be specified in any order; but
5369 the selected input is written in the same order that it is read, and
5370 is written exactly once.
5372 The program accepts the following options. Also see @ref{Common
5377 @item -b @var{byte-list}
5378 @itemx --bytes=@var{byte-list}
5381 Select for printing only the bytes in positions listed in
5382 @var{byte-list}. Tabs and backspaces are treated like any other
5383 character; they take up 1 byte. If an output delimiter is specified,
5384 (see the description of @option{--output-delimiter}), then output that
5385 string between ranges of selected bytes.
5387 @item -c @var{character-list}
5388 @itemx --characters=@var{character-list}
5390 @opindex --characters
5391 Select for printing only the characters in positions listed in
5392 @var{character-list}. The same as @option{-b} for now, but
5393 internationalization will change that. Tabs and backspaces are
5394 treated like any other character; they take up 1 character. If an
5395 output delimiter is specified, (see the description of
5396 @option{--output-delimiter}), then output that string between ranges
5399 @item -f @var{field-list}
5400 @itemx --fields=@var{field-list}
5403 Select for printing only the fields listed in @var{field-list}.
5404 Fields are separated by a TAB character by default. Also print any
5405 line that contains no delimiter character, unless the
5406 @option{--only-delimited} (@option{-s}) option is specified.
5407 Note @command{cut} does not support specifying runs of whitespace as a
5408 delimiter, so to achieve that common functionality one can pre-process
5409 with @command{tr} like:
5411 tr -s '[:blank:]' '\t' | cut -f@dots{}
5414 @item -d @var{input_delim_byte}
5415 @itemx --delimiter=@var{input_delim_byte}
5417 @opindex --delimiter
5418 With @option{-f}, use the first byte of @var{input_delim_byte} as
5419 the input fields separator (default is TAB).
5423 Do not split multi-byte characters (no-op for now).
5426 @itemx --only-delimited
5428 @opindex --only-delimited
5429 For @option{-f}, do not print lines that do not contain the field separator
5430 character. Normally, any line without a field separator is printed verbatim.
5432 @item --output-delimiter=@var{output_delim_string}
5433 @opindex --output-delimiter
5434 With @option{-f}, output fields are separated by @var{output_delim_string}.
5435 The default with @option{-f} is to use the input delimiter.
5436 When using @option{-b} or @option{-c} to select ranges of byte or
5437 character offsets (as opposed to ranges of fields),
5438 output @var{output_delim_string} between non-overlapping
5439 ranges of selected bytes.
5442 @opindex --complement
5443 This option is a @acronym{GNU} extension.
5444 Select for printing the complement of the bytes, characters or fields
5445 selected with the @option{-b}, @option{-c} or @option{-f} options.
5446 In other words, do @emph{not} print the bytes, characters or fields
5447 specified via those options. This option is useful when you have
5448 many fields and want to print all but a few of them.
5455 @node paste invocation
5456 @section @command{paste}: Merge lines of files
5459 @cindex merging files
5461 @command{paste} writes to standard output lines consisting of sequentially
5462 corresponding lines of each given file, separated by a TAB character.
5463 Standard input is used for a file name of @samp{-} or if no input files
5485 paste [@var{option}]@dots{} [@var{file}]@dots{}
5488 The program accepts the following options. Also see @ref{Common options}.
5496 Paste the lines of one file at a time rather than one line from each
5497 file. Using the above example data:
5500 $ paste -s num2 let3
5505 @item -d @var{delim-list}
5506 @itemx --delimiters=@var{delim-list}
5508 @opindex --delimiters
5509 Consecutively use the characters in @var{delim-list} instead of
5510 TAB to separate merged lines. When @var{delim-list} is
5511 exhausted, start again at its beginning. Using the above example data:
5514 $ paste -d '%_' num2 let3 num2
5525 @node join invocation
5526 @section @command{join}: Join lines on a common field
5529 @cindex common field, joining on
5531 @command{join} writes to standard output a line for each pair of input
5532 lines that have identical join fields. Synopsis:
5535 join [@var{option}]@dots{} @var{file1} @var{file2}
5538 Either @var{file1} or @var{file2} (but not both) can be @samp{-},
5539 meaning standard input. @var{file1} and @var{file2} should be
5540 sorted on the join fields.
5543 Normally, the sort order is that of the
5544 collating sequence specified by the @env{LC_COLLATE} locale. Unless
5545 the @option{-t} option is given, the sort comparison ignores blanks at
5546 the start of the join field, as in @code{sort -b}. If the
5547 @option{--ignore-case} option is given, the sort comparison ignores
5548 the case of characters in the join field, as in @code{sort -f}.
5550 The @command{sort} and @command{join} commands should use consistent
5551 locales and options if the output of @command{sort} is fed to
5552 @command{join}. You can use a command like @samp{sort -k 1b,1} to
5553 sort a file on its default join field, but if you select a non-default
5554 locale, join field, separator, or comparison options, then you should
5555 do so consistently between @command{join} and @command{sort}.
5556 If @samp{join -t ''} is specified then the whole line is considered which
5557 matches the default operation of sort.
5559 If the input has no unpairable lines, a @acronym{GNU} extension is
5560 available; the sort order can be any order that considers two fields
5561 to be equal if and only if the sort comparison described above
5562 considers them to be equal. For example:
5579 @checkOrderOption{join}
5583 @item the join field is the first field in each line;
5584 @item fields in the input are separated by one or more blanks, with leading
5585 blanks on the line ignored;
5586 @item fields in the output are separated by a space;
5587 @item each output line consists of the join field, the remaining
5588 fields from @var{file1}, then the remaining fields from @var{file2}.
5591 The program accepts the following options. Also see @ref{Common options}.
5595 @item -a @var{file-number}
5597 Print a line for each unpairable line in file @var{file-number} (either
5598 @samp{1} or @samp{2}), in addition to the normal output.
5601 Fail with an error message if either input file is wrongly ordered.
5603 @item --nocheck-order
5604 Do not check that both input files are in sorted order. This is the default.
5606 @item -e @var{string}
5608 Replace those output fields that are missing in the input with
5613 Treat the first line of each input file as a header line. The header lines will
5614 be joined and printed as the first output line. If @option{-o} is used to
5615 specify output format, the header line will be printed according to the
5616 specified format. The header lines will not be checked for ordering even if
5617 @option{--check-order} is specified. Also if the header lines from each file
5618 do not match, the heading fields from the first file will be used.
5621 @itemx --ignore-case
5623 @opindex --ignore-case
5624 Ignore differences in case when comparing keys.
5625 With this option, the lines of the input files must be ordered in the same way.
5626 Use @samp{sort -f} to produce this ordering.
5628 @item -1 @var{field}
5630 Join on field @var{field} (a positive integer) of file 1.
5632 @item -2 @var{field}
5634 Join on field @var{field} (a positive integer) of file 2.
5636 @item -j @var{field}
5637 Equivalent to @option{-1 @var{field} -2 @var{field}}.
5639 @item -o @var{field-list}
5640 Construct each output line according to the format in @var{field-list}.
5641 Each element in @var{field-list} is either the single character @samp{0} or
5642 has the form @var{m.n} where the file number, @var{m}, is @samp{1} or
5643 @samp{2} and @var{n} is a positive field number.
5645 A field specification of @samp{0} denotes the join field.
5646 In most cases, the functionality of the @samp{0} field spec
5647 may be reproduced using the explicit @var{m.n} that corresponds
5648 to the join field. However, when printing unpairable lines
5649 (using either of the @option{-a} or @option{-v} options), there is no way
5650 to specify the join field using @var{m.n} in @var{field-list}
5651 if there are unpairable lines in both files.
5652 To give @command{join} that functionality, @acronym{POSIX} invented the @samp{0}
5653 field specification notation.
5655 The elements in @var{field-list}
5656 are separated by commas or blanks.
5657 Blank separators typically need to be quoted for the shell. For
5658 example, the commands @samp{join -o 1.2,2.2} and @samp{join -o '1.2
5659 2.2'} are equivalent.
5661 All output lines---including those printed because of any -a or -v
5662 option---are subject to the specified @var{field-list}.
5665 Use character @var{char} as the input and output field separator.
5666 Treat as significant each occurrence of @var{char} in the input file.
5667 Use @samp{sort -t @var{char}}, without the @option{-b} option of
5668 @samp{sort}, to produce this ordering. If @samp{join -t ''} is specified,
5669 the whole line is considered, matching the default operation of sort.
5670 If @samp{-t '\0'} is specified then the @acronym{ASCII} @sc{nul}
5671 character is used to delimit the fields.
5673 @item -v @var{file-number}
5674 Print a line for each unpairable line in file @var{file-number}
5675 (either @samp{1} or @samp{2}), instead of the normal output.
5682 @node Operating on characters
5683 @chapter Operating on characters
5685 @cindex operating on characters
5687 This commands operate on individual characters.
5690 * tr invocation:: Translate, squeeze, and/or delete characters.
5691 * expand invocation:: Convert tabs to spaces.
5692 * unexpand invocation:: Convert spaces to tabs.
5697 @section @command{tr}: Translate, squeeze, and/or delete characters
5704 tr [@var{option}]@dots{} @var{set1} [@var{set2}]
5707 @command{tr} copies standard input to standard output, performing
5708 one of the following operations:
5712 translate, and optionally squeeze repeated characters in the result,
5714 squeeze repeated characters,
5718 delete characters, then squeeze repeated characters from the result.
5721 The @var{set1} and (if given) @var{set2} arguments define ordered
5722 sets of characters, referred to below as @var{set1} and @var{set2}. These
5723 sets are the characters of the input that @command{tr} operates on.
5724 The @option{--complement} (@option{-c}, @option{-C}) option replaces
5726 complement (all of the characters that are not in @var{set1}).
5728 Currently @command{tr} fully supports only single-byte characters.
5729 Eventually it will support multibyte characters; when it does, the
5730 @option{-C} option will cause it to complement the set of characters,
5731 whereas @option{-c} will cause it to complement the set of values.
5732 This distinction will matter only when some values are not characters,
5733 and this is possible only in locales using multibyte encodings when
5734 the input contains encoding errors.
5736 The program accepts the @option{--help} and @option{--version}
5737 options. @xref{Common options}. Options must precede operands.
5742 * Character sets:: Specifying sets of characters.
5743 * Translating:: Changing one set of characters to another.
5744 * Squeezing:: Squeezing repeats and deleting.
5748 @node Character sets
5749 @subsection Specifying sets of characters
5751 @cindex specifying sets of characters
5753 The format of the @var{set1} and @var{set2} arguments resembles
5754 the format of regular expressions; however, they are not regular
5755 expressions, only lists of characters. Most characters simply
5756 represent themselves in these strings, but the strings can contain
5757 the shorthands listed below, for convenience. Some of them can be
5758 used only in @var{set1} or @var{set2}, as noted below.
5762 @item Backslash escapes
5763 @cindex backslash escapes
5765 The following backslash escape sequences are recognized:
5783 The character with the value given by @var{ooo}, which is 1 to 3
5789 While a backslash followed by a character not listed above is
5790 interpreted as that character, the backslash also effectively
5791 removes any special significance, so it is useful to escape
5792 @samp{[}, @samp{]}, @samp{*}, and @samp{-}.
5797 The notation @samp{@var{m}-@var{n}} expands to all of the characters
5798 from @var{m} through @var{n}, in ascending order. @var{m} should
5799 collate before @var{n}; if it doesn't, an error results. As an example,
5800 @samp{0-9} is the same as @samp{0123456789}.
5802 @sc{gnu} @command{tr} does not support the System V syntax that uses square
5803 brackets to enclose ranges. Translations specified in that format
5804 sometimes work as expected, since the brackets are often transliterated
5805 to themselves. However, they should be avoided because they sometimes
5806 behave unexpectedly. For example, @samp{tr -d '[0-9]'} deletes brackets
5809 Many historically common and even accepted uses of ranges are not
5810 portable. For example, on @acronym{EBCDIC} hosts using the @samp{A-Z}
5811 range will not do what most would expect because @samp{A} through @samp{Z}
5812 are not contiguous as they are in @acronym{ASCII}.
5813 If you can rely on a @acronym{POSIX} compliant version of @command{tr}, then
5814 the best way to work around this is to use character classes (see below).
5815 Otherwise, it is most portable (and most ugly) to enumerate the members
5818 @item Repeated characters
5819 @cindex repeated characters
5821 The notation @samp{[@var{c}*@var{n}]} in @var{set2} expands to @var{n}
5822 copies of character @var{c}. Thus, @samp{[y*6]} is the same as
5823 @samp{yyyyyy}. The notation @samp{[@var{c}*]} in @var{string2} expands
5824 to as many copies of @var{c} as are needed to make @var{set2} as long as
5825 @var{set1}. If @var{n} begins with @samp{0}, it is interpreted in
5826 octal, otherwise in decimal.
5828 @item Character classes
5829 @cindex character classes
5831 The notation @samp{[:@var{class}:]} expands to all of the characters in
5832 the (predefined) class @var{class}. The characters expand in no
5833 particular order, except for the @code{upper} and @code{lower} classes,
5834 which expand in ascending order. When the @option{--delete} (@option{-d})
5835 and @option{--squeeze-repeats} (@option{-s}) options are both given, any
5836 character class can be used in @var{set2}. Otherwise, only the
5837 character classes @code{lower} and @code{upper} are accepted in
5838 @var{set2}, and then only if the corresponding character class
5839 (@code{upper} and @code{lower}, respectively) is specified in the same
5840 relative position in @var{set1}. Doing this specifies case conversion.
5841 The class names are given below; an error results when an invalid class
5853 Horizontal whitespace.
5862 Printable characters, not including space.
5868 Printable characters, including space.
5871 Punctuation characters.
5874 Horizontal or vertical whitespace.
5883 @item Equivalence classes
5884 @cindex equivalence classes
5886 The syntax @samp{[=@var{c}=]} expands to all of the characters that are
5887 equivalent to @var{c}, in no particular order. Equivalence classes are
5888 a relatively recent invention intended to support non-English alphabets.
5889 But there seems to be no standard way to define them or determine their
5890 contents. Therefore, they are not fully implemented in @sc{gnu} @command{tr};
5891 each character's equivalence class consists only of that character,
5892 which is of no particular use.
5898 @subsection Translating
5900 @cindex translating characters
5902 @command{tr} performs translation when @var{set1} and @var{set2} are
5903 both given and the @option{--delete} (@option{-d}) option is not given.
5904 @command{tr} translates each character of its input that is in @var{set1}
5905 to the corresponding character in @var{set2}. Characters not in
5906 @var{set1} are passed through unchanged. When a character appears more
5907 than once in @var{set1} and the corresponding characters in @var{set2}
5908 are not all the same, only the final one is used. For example, these
5909 two commands are equivalent:
5916 A common use of @command{tr} is to convert lowercase characters to
5917 uppercase. This can be done in many ways. Here are three of them:
5920 tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
5922 tr '[:lower:]' '[:upper:]'
5926 But note that using ranges like @code{a-z} above is not portable.
5928 When @command{tr} is performing translation, @var{set1} and @var{set2}
5929 typically have the same length. If @var{set1} is shorter than
5930 @var{set2}, the extra characters at the end of @var{set2} are ignored.
5932 On the other hand, making @var{set1} longer than @var{set2} is not
5933 portable; @acronym{POSIX} says that the result is undefined. In this situation,
5934 BSD @command{tr} pads @var{set2} to the length of @var{set1} by repeating
5935 the last character of @var{set2} as many times as necessary. System V
5936 @command{tr} truncates @var{set1} to the length of @var{set2}.
5938 By default, @sc{gnu} @command{tr} handles this case like BSD @command{tr}.
5939 When the @option{--truncate-set1} (@option{-t}) option is given,
5940 @sc{gnu} @command{tr} handles this case like the System V @command{tr}
5941 instead. This option is ignored for operations other than translation.
5943 Acting like System V @command{tr} in this case breaks the relatively common
5947 tr -cs A-Za-z0-9 '\012'
5951 because it converts only zero bytes (the first element in the
5952 complement of @var{set1}), rather than all non-alphanumerics, to
5956 By the way, the above idiom is not portable because it uses ranges, and
5957 it assumes that the octal code for newline is 012.
5958 Assuming a @acronym{POSIX} compliant @command{tr}, here is a better way to write it:
5961 tr -cs '[:alnum:]' '[\n*]'
5966 @subsection Squeezing repeats and deleting
5968 @cindex squeezing repeat characters
5969 @cindex deleting characters
5971 When given just the @option{--delete} (@option{-d}) option, @command{tr}
5972 removes any input characters that are in @var{set1}.
5974 When given just the @option{--squeeze-repeats} (@option{-s}) option,
5975 @command{tr} replaces each input sequence of a repeated character that
5976 is in @var{set1} with a single occurrence of that character.
5978 When given both @option{--delete} and @option{--squeeze-repeats}, @command{tr}
5979 first performs any deletions using @var{set1}, then squeezes repeats
5980 from any remaining characters using @var{set2}.
5982 The @option{--squeeze-repeats} option may also be used when translating,
5983 in which case @command{tr} first performs translation, then squeezes
5984 repeats from any remaining characters using @var{set2}.
5986 Here are some examples to illustrate various combinations of options:
5991 Remove all zero bytes:
5998 Put all words on lines by themselves. This converts all
5999 non-alphanumeric characters to newlines, then squeezes each string
6000 of repeated newlines into a single newline:
6003 tr -cs '[:alnum:]' '[\n*]'
6007 Convert each sequence of repeated newlines to a single newline:
6014 Find doubled occurrences of words in a document.
6015 @c Separate the following two "the"s, so typo checkers don't complain.
6016 For example, people often write ``the @w{}the'' with the repeated words
6017 separated by a newline. The Bourne shell script below works first
6018 by converting each sequence of punctuation and blank characters to a
6019 single newline. That puts each ``word'' on a line by itself.
6020 Next it maps all uppercase characters to lower case, and finally it
6021 runs @command{uniq} with the @option{-d} option to print out only the words
6027 | tr -s '[:punct:][:blank:]' '[\n*]' \
6028 | tr '[:upper:]' '[:lower:]' \
6033 Deleting a small set of characters is usually straightforward. For example,
6034 to remove all @samp{a}s, @samp{x}s, and @samp{M}s you would do this:
6040 However, when @samp{-} is one of those characters, it can be tricky because
6041 @samp{-} has special meanings. Performing the same task as above but also
6042 removing all @samp{-} characters, we might try @code{tr -d -axM}, but
6043 that would fail because @command{tr} would try to interpret @option{-a} as
6044 a command-line option. Alternatively, we could try putting the hyphen
6045 inside the string, @code{tr -d a-xM}, but that wouldn't work either because
6046 it would make @command{tr} interpret @code{a-x} as the range of characters
6047 @samp{a}@dots{}@samp{x} rather than the three.
6048 One way to solve the problem is to put the hyphen at the end of the list
6055 Or you can use @samp{--} to terminate option processing:
6061 More generally, use the character class notation @code{[=c=]}
6062 with @samp{-} (or any other character) in place of the @samp{c}:
6068 Note how single quotes are used in the above example to protect the
6069 square brackets from interpretation by a shell.
6074 @node expand invocation
6075 @section @command{expand}: Convert tabs to spaces
6078 @cindex tabs to spaces, converting
6079 @cindex converting tabs to spaces
6081 @command{expand} writes the contents of each given @var{file}, or standard
6082 input if none are given or for a @var{file} of @samp{-}, to standard
6083 output, with tab characters converted to the appropriate number of
6087 expand [@var{option}]@dots{} [@var{file}]@dots{}
6090 By default, @command{expand} converts all tabs to spaces. It preserves
6091 backspace characters in the output; they decrement the column count for
6092 tab calculations. The default action is equivalent to @option{-t 8} (set
6093 tabs every 8 columns).
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 @cindex tab stops, setting
6104 If only one tab stop is given, set the tabs @var{tab1} spaces apart
6105 (default is 8). Otherwise, set the tabs at columns @var{tab1},
6106 @var{tab2}, @dots{} (numbered from 0), and replace any tabs beyond the
6107 last tab stop given with single spaces. Tab stops can be separated by
6108 blanks as well as by commas.
6110 For compatibility, GNU @command{expand} also accepts the obsolete
6111 option syntax, @option{-@var{t1}[,@var{t2}]@dots{}}. New scripts
6112 should use @option{-t @var{t1}[,@var{t2}]@dots{}} instead.
6118 @cindex initial tabs, converting
6119 Only convert initial tabs (those that precede all non-space or non-tab
6120 characters) on each line to spaces.
6127 @node unexpand invocation
6128 @section @command{unexpand}: Convert spaces to tabs
6132 @command{unexpand} writes the contents of each given @var{file}, or
6133 standard input if none are given or for a @var{file} of @samp{-}, to
6134 standard output, converting blanks at the beginning of each line into
6135 as many tab characters as needed. In the default @acronym{POSIX}
6136 locale, a @dfn{blank} is a space or a tab; other locales may specify
6137 additional blank characters. Synopsis:
6140 unexpand [@var{option}]@dots{} [@var{file}]@dots{}
6143 By default, @command{unexpand} converts only initial blanks (those
6144 that precede all non-blank characters) on each line. It
6145 preserves backspace characters in the output; they decrement the column
6146 count for tab calculations. By default, tabs are set at every 8th
6149 The program accepts the following options. Also see @ref{Common options}.
6153 @item -t @var{tab1}[,@var{tab2}]@dots{}
6154 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6157 If only one tab stop is given, set the tabs @var{tab1} columns apart
6158 instead of the default 8. Otherwise, set the tabs at columns
6159 @var{tab1}, @var{tab2}, @dots{} (numbered from 0), and leave blanks
6160 beyond the tab stops given unchanged. Tab stops can be separated by
6161 blanks as well as by commas. This option implies the @option{-a} option.
6163 For compatibility, GNU @command{unexpand} supports the obsolete option syntax,
6164 @option{-@var{tab1}[,@var{tab2}]@dots{}}, where tab stops must be
6165 separated by commas. (Unlike @option{-t}, this obsolete option does
6166 not imply @option{-a}.) New scripts should use @option{--first-only -t
6167 @var{tab1}[,@var{tab2}]@dots{}} instead.
6173 Also convert all sequences of two or more blanks just before a tab stop,
6174 even if they occur after non-blank characters in a line.
6181 @node Directory listing
6182 @chapter Directory listing
6184 This chapter describes the @command{ls} command and its variants @command{dir}
6185 and @command{vdir}, which list information about files.
6188 * ls invocation:: List directory contents.
6189 * dir invocation:: Briefly ls.
6190 * vdir invocation:: Verbosely ls.
6191 * dircolors invocation:: Color setup for ls, etc.
6196 @section @command{ls}: List directory contents
6199 @cindex directory listing
6201 The @command{ls} program lists information about files (of any type,
6202 including directories). Options and file arguments can be intermixed
6203 arbitrarily, as usual.
6205 For non-option command-line arguments that are directories, by default
6206 @command{ls} lists the contents of directories, not recursively, and
6207 omitting files with names beginning with @samp{.}. For other non-option
6208 arguments, by default @command{ls} lists just the file name. If no
6209 non-option argument is specified, @command{ls} operates on the current
6210 directory, acting as if it had been invoked with a single argument of @samp{.}.
6213 By default, the output is sorted alphabetically, according to the locale
6214 settings in effect.@footnote{If you use a non-@acronym{POSIX}
6215 locale (e.g., by setting @env{LC_ALL} to @samp{en_US}), then @command{ls} may
6216 produce output that is sorted differently than you're accustomed to.
6217 In that case, set the @env{LC_ALL} environment variable to @samp{C}.}
6218 If standard output is
6219 a terminal, the output is in columns (sorted vertically) and control
6220 characters are output as question marks; otherwise, the output is listed
6221 one per line and control characters are output as-is.
6223 Because @command{ls} is such a fundamental program, it has accumulated many
6224 options over the years. They are described in the subsections below;
6225 within each section, options are listed alphabetically (ignoring case).
6226 The division of options into the subsections is not absolute, since some
6227 options affect more than one aspect of @command{ls}'s operation.
6229 @cindex exit status of @command{ls}
6234 1 minor problems (e.g., failure to access a file or directory not
6235 specified as a command line argument. This happens when listing a
6236 directory in which entries are actively being removed or renamed.)
6237 2 serious trouble (e.g., memory exhausted, invalid option, failure
6238 to access a file or directory specified as a command line argument
6239 or a directory loop)
6242 Also see @ref{Common options}.
6245 * Which files are listed::
6246 * What information is listed::
6247 * Sorting the output::
6248 * Details about version sort::
6249 * General output formatting::
6250 * Formatting file timestamps::
6251 * Formatting the file names::
6255 @node Which files are listed
6256 @subsection Which files are listed
6258 These options determine which files @command{ls} lists information for.
6259 By default, @command{ls} lists files and the contents of any
6260 directories on the command line, except that in directories it ignores
6261 files whose names start with @samp{.}.
6269 In directories, do not ignore file names that start with @samp{.}.
6274 @opindex --almost-all
6275 In directories, do not ignore all file names that start with @samp{.};
6276 ignore only @file{.} and @file{..}. The @option{--all} (@option{-a})
6277 option overrides this option.
6280 @itemx --ignore-backups
6282 @opindex --ignore-backups
6283 @cindex backup files, ignoring
6284 In directories, ignore files that end with @samp{~}. This option is
6285 equivalent to @samp{--ignore='*~' --ignore='.*~'}.
6290 @opindex --directory
6291 List just the names of directories, as with other types of files, rather
6292 than listing their contents.
6293 @c The following sentence is the same as the one for -F.
6294 Do not follow symbolic links listed on the
6295 command line unless the @option{--dereference-command-line} (@option{-H}),
6296 @option{--dereference} (@option{-L}), or
6297 @option{--dereference-command-line-symlink-to-dir} options are specified.
6300 @itemx --dereference-command-line
6302 @opindex --dereference-command-line
6303 @cindex symbolic links, dereferencing
6304 If a command line argument specifies a symbolic link, show information
6305 for the file the link references rather than for the link itself.
6307 @itemx --dereference-command-line-symlink-to-dir
6308 @opindex --dereference-command-line-symlink-to-dir
6309 @cindex symbolic links, dereferencing
6310 Do not dereference symbolic links, with one exception:
6311 if a command line argument specifies a symbolic link that refers to
6312 a directory, show information for that directory rather than for the
6314 This is the default behavior when no other dereferencing-related
6315 option has been specified (@option{--classify} (@option{-F}),
6316 @option{--directory} (@option{-d}),
6318 @option{--dereference} (@option{-L}), or
6319 @option{--dereference-command-line} (@option{-H})).
6321 @item --group-directories-first
6322 @opindex --group-directories-first
6323 Group all the directories before the files and then sort the
6324 directories and the files separately using the selected sort key
6325 (see --sort option).
6326 That is, this option specifies a primary sort key,
6327 and the --sort option specifies a secondary key.
6328 However, any use of @option{--sort=none}
6329 (@option{-U}) disables this option altogether.
6331 @item --hide=PATTERN
6332 @opindex --hide=@var{pattern}
6333 In directories, ignore files whose names match the shell pattern
6334 @var{pattern}, unless the @option{--all} (@option{-a}) or
6335 @option{--almost-all} (@option{-A}) is also given. This
6336 option acts like @option{--ignore=@var{pattern}} except that it has no
6337 effect if @option{--all} (@option{-a}) or @option{--almost-all}
6338 (@option{-A}) is also given.
6340 This option can be useful in shell aliases. For example, if
6341 @command{lx} is an alias for @samp{ls --hide='*~'} and @command{ly} is
6342 an alias for @samp{ls --ignore='*~'}, then the command @samp{lx -A}
6343 lists the file @file{README~} even though @samp{ly -A} would not.
6345 @item -I @var{pattern}
6346 @itemx --ignore=@var{pattern}
6348 @opindex --ignore=@var{pattern}
6349 In directories, ignore files whose names match the shell pattern
6350 (not regular expression) @var{pattern}. As
6351 in the shell, an initial @samp{.} in a file name does not match a
6352 wildcard at the start of @var{pattern}. Sometimes it is useful
6353 to give this option several times. For example,
6356 $ ls --ignore='.??*' --ignore='.[^.]' --ignore='#*'
6359 The first option ignores names of length 3 or more that start with @samp{.},
6360 the second ignores all two-character names that start with @samp{.}
6361 except @samp{..}, and the third ignores names that start with @samp{#}.
6364 @itemx --dereference
6366 @opindex --dereference
6367 @cindex symbolic links, dereferencing
6368 When showing file information for a symbolic link, show information
6369 for the file the link references rather than the link itself.
6370 However, even with this option, @command{ls} still prints the name
6371 of the link itself, not the name of the file that the link points to.
6376 @opindex --recursive
6377 @cindex recursive directory listing
6378 @cindex directory listing, recursive
6379 List the contents of all directories recursively.
6384 @node What information is listed
6385 @subsection What information is listed
6387 These options affect the information that @command{ls} displays. By
6388 default, only file names are shown.
6394 @cindex hurd, author, printing
6395 List each file's author when producing long format directory listings.
6396 In GNU/Hurd, file authors can differ from their owners, but in other
6397 operating systems the two are the same.
6403 @cindex dired Emacs mode support
6404 With the long listing (@option{-l}) format, print an additional line after
6408 //DIRED// @var{beg1} @var{end1} @var{beg2} @var{end2} @dots{}
6412 The @var{begn} and @var{endn} are unsigned integers that record the
6413 byte position of the beginning and end of each file name in the output.
6414 This makes it easy for Emacs to find the names, even when they contain
6415 unusual characters such as space or newline, without fancy searching.
6417 If directories are being listed recursively (@option{-R}), output a similar
6418 line with offsets for each subdirectory name:
6421 //SUBDIRED// @var{beg1} @var{end1} @dots{}
6424 Finally, output a line of the form:
6427 //DIRED-OPTIONS// --quoting-style=@var{word}
6431 where @var{word} is the quoting style (@pxref{Formatting the file names}).
6433 Here is an actual example:
6436 $ mkdir -p a/sub/deeper a/sub2
6438 $ touch a/sub/deeper/file
6439 $ ls -gloRF --dired a
6442 -rw-r--r-- 1 0 Jun 10 12:27 f1
6443 -rw-r--r-- 1 0 Jun 10 12:27 f2
6444 drwxr-xr-x 3 4096 Jun 10 12:27 sub/
6445 drwxr-xr-x 2 4096 Jun 10 12:27 sub2/
6449 drwxr-xr-x 2 4096 Jun 10 12:27 deeper/
6453 -rw-r--r-- 1 0 Jun 10 12:27 file
6457 //DIRED// 48 50 84 86 120 123 158 162 217 223 282 286
6458 //SUBDIRED// 2 3 167 172 228 240 290 296
6459 //DIRED-OPTIONS// --quoting-style=literal
6462 Note that the pairs of offsets on the @samp{//DIRED//} line above delimit
6463 these names: @file{f1}, @file{f2}, @file{sub}, @file{sub2}, @file{deeper},
6465 The offsets on the @samp{//SUBDIRED//} line delimit the following
6466 directory names: @file{a}, @file{a/sub}, @file{a/sub/deeper}, @file{a/sub2}.
6468 Here is an example of how to extract the fifth entry name, @samp{deeper},
6469 corresponding to the pair of offsets, 222 and 228:
6472 $ ls -gloRF --dired a > out
6473 $ dd bs=1 skip=222 count=6 < out 2>/dev/null; echo
6477 Note that although the listing above includes a trailing slash
6478 for the @samp{deeper} entry, the offsets select the name without
6479 the trailing slash. However, if you invoke @command{ls} with @option{--dired}
6480 along with an option like @option{--escape} (aka @option{-b}) and operate
6481 on a file whose name contains special characters, notice that the backslash
6486 $ ls -blog --dired 'a b'
6487 -rw-r--r-- 1 0 Jun 10 12:28 a\ b
6489 //DIRED-OPTIONS// --quoting-style=escape
6492 If you use a quoting style that adds quote marks
6493 (e.g., @option{--quoting-style=c}), then the offsets include the quote marks.
6494 So beware that the user may select the quoting style via the environment
6495 variable @env{QUOTING_STYLE}. Hence, applications using @option{--dired}
6496 should either specify an explicit @option{--quoting-style=literal} option
6497 (aka @option{-N} or @option{--literal}) on the command line, or else be
6498 prepared to parse the escaped names.
6501 @opindex --full-time
6502 Produce long format directory listings, and list times in full. It is
6503 equivalent to using @option{--format=long} with
6504 @option{--time-style=full-iso} (@pxref{Formatting file timestamps}).
6508 Produce long format directory listings, but don't display owner information.
6514 Inhibit display of group information in a long format directory listing.
6515 (This is the default in some non-@sc{gnu} versions of @command{ls}, so we
6516 provide this option for compatibility.)
6524 @cindex inode number, printing
6525 Print the inode number (also called the file serial number and index
6526 number) of each file to the left of the file name. (This number
6527 uniquely identifies each file within a particular file system.)
6530 @itemx --format=long
6531 @itemx --format=verbose
6534 @opindex long ls @r{format}
6535 @opindex verbose ls @r{format}
6536 In addition to the name of each file, print the file type, file mode bits,
6537 number of hard links, owner name, group name, size, and
6538 timestamp (@pxref{Formatting file timestamps}), normally
6539 the modification time. Print question marks for information that
6540 cannot be determined.
6542 Normally the size is printed as a byte count without punctuation, but
6543 this can be overridden (@pxref{Block size}). For example, @option{-h}
6544 prints an abbreviated, human-readable count, and
6545 @samp{--block-size="'1"} prints a byte count with the thousands
6546 separator of the current locale.
6548 For each directory that is listed, preface the files with a line
6549 @samp{total @var{blocks}}, where @var{blocks} is the total disk allocation
6550 for all files in that directory. The block size currently defaults to 1024
6551 bytes, but this can be overridden (@pxref{Block size}).
6552 The @var{blocks} computed counts each hard link separately;
6553 this is arguably a deficiency.
6555 The file type is one of the following characters:
6557 @c The commented-out entries are ones we're not sure about.
6565 character special file
6567 high performance (``contiguous data'') file
6571 door (Solaris 2.5 and up)
6573 @c semaphore, if this is a distinct file type
6577 @c multiplexed file (7th edition Unix; obsolete)
6579 off-line (``migrated'') file (Cray DMF)
6581 network special file (HP-UX)
6585 port (Solaris 10 and up)
6587 @c message queue, if this is a distinct file type
6591 @c shared memory object, if this is a distinct file type
6593 @c typed memory object, if this is a distinct file type
6595 @c whiteout (4.4BSD; not implemented)
6597 some other file type
6600 @cindex permissions, output by @command{ls}
6601 The file mode bits listed are similar to symbolic mode specifications
6602 (@pxref{Symbolic Modes}). But @command{ls} combines multiple bits into the
6603 third character of each set of permissions as follows:
6607 If the set-user-ID or set-group-ID bit and the corresponding executable bit
6611 If the set-user-ID or set-group-ID bit is set but the corresponding
6612 executable bit is not set.
6615 If the restricted deletion flag or sticky bit, and the
6616 other-executable bit, are both set. The restricted deletion flag is
6617 another name for the sticky bit. @xref{Mode Structure}.
6620 If the restricted deletion flag or sticky bit is set but the
6621 other-executable bit is not set.
6624 If the executable bit is set and none of the above apply.
6630 Following the file mode bits is a single character that specifies
6631 whether an alternate access method such as an access control list
6632 applies to the file. When the character following the file mode bits is a
6633 space, there is no alternate access method. When it is a printing
6634 character, then there is such a method.
6636 GNU @command{ls} uses a @samp{.} character to indicate a file
6637 with an SELinux security context, but no other alternate access method.
6639 A file with any other combination of alternate access methods
6640 is marked with a @samp{+} character.
6643 @itemx --numeric-uid-gid
6645 @opindex --numeric-uid-gid
6646 @cindex numeric uid and gid
6647 @cindex numeric user and group IDs
6648 Produce long format directory listings, but
6649 display numeric user and group IDs instead of the owner and group names.
6653 Produce long format directory listings, but don't display group information.
6654 It is equivalent to using @option{--format=long} with @option{--no-group} .
6660 @cindex disk allocation
6661 @cindex size of files, reporting
6662 Print the disk allocation of each file to the left of the file name.
6663 This is the amount of disk space used by the file, which is usually a
6664 bit more than the file's size, but it can be less if the file has holes.
6666 Normally the disk allocation is printed in units of
6667 1024 bytes, but this can be overridden (@pxref{Block size}).
6669 @cindex NFS mounts from BSD to HP-UX
6670 For files that are NFS-mounted from an HP-UX system to a BSD system,
6671 this option reports sizes that are half the correct values. On HP-UX
6672 systems, it reports sizes that are twice the correct values for files
6673 that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX;
6674 it also affects the HP-UX @command{ls} program.
6683 @cindex security context
6684 Display the SELinux security context or @samp{?} if none is found.
6685 When used with the @option{-l} option, print the security context
6686 to the left of the size column.
6691 @node Sorting the output
6692 @subsection Sorting the output
6694 @cindex sorting @command{ls} output
6695 These options change the order in which @command{ls} sorts the information
6696 it outputs. By default, sorting is done by character code
6697 (e.g., @acronym{ASCII} order).
6703 @itemx --time=status
6706 @opindex ctime@r{, printing or sorting by}
6707 @opindex status time@r{, printing or sorting by}
6708 @opindex use time@r{, printing or sorting files by}
6709 If the long listing format (e.g., @option{-l}, @option{-o}) is being used,
6710 print the status change time (the @samp{ctime} in the inode) instead of
6711 the modification time.
6712 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6713 or when not using a long listing format,
6714 sort according to the status change time.
6718 @cindex unsorted directory listing
6719 @cindex directory order, listing by
6720 Primarily, like @option{-U}---do not sort; list the files in whatever
6721 order they are stored in the directory. But also enable @option{-a} (list
6722 all files) and disable @option{-l}, @option{--color}, and @option{-s} (if they
6723 were specified before the @option{-f}).
6729 @cindex reverse sorting
6730 Reverse whatever the sorting method is---e.g., list files in reverse
6731 alphabetical order, youngest first, smallest first, or whatever.
6737 @opindex size of files@r{, sorting files by}
6738 Sort by file size, largest first.
6744 @opindex modification time@r{, sorting files by}
6745 Sort by modification time (the @samp{mtime} in the inode), newest first.
6749 @itemx --time=access
6753 @opindex use time@r{, printing or sorting files by}
6754 @opindex atime@r{, printing or sorting files by}
6755 @opindex access time@r{, printing or sorting files by}
6756 If the long listing format (e.g., @option{--format=long}) is being used,
6757 print the last access time (the @samp{atime} in the inode).
6758 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6759 or when not using a long listing format, sort according to the access time.
6765 @opindex none@r{, sorting option for @command{ls}}
6766 Do not sort; list the files in whatever order they are
6767 stored in the directory. (Do not do any of the other unrelated things
6768 that @option{-f} does.) This is especially useful when listing very large
6769 directories, since not doing any sorting can be noticeably faster.
6772 @itemx --sort=version
6775 @opindex version@r{, sorting option for @command{ls}}
6776 Sort by version name and number, lowest first. It behaves like a default
6777 sort, except that each sequence of decimal digits is treated numerically
6778 as an index/version number. (@xref{Details about version sort}.)
6781 @itemx --sort=extension
6784 @opindex extension@r{, sorting files by}
6785 Sort directory contents alphabetically by file extension (characters
6786 after the last @samp{.}); files with no extension are sorted first.
6791 @node Details about version sort
6792 @subsection Details about version sort
6794 Version sorting handles the fact that file names frequently include indices or
6795 version numbers. Standard sorting usually does not produce the order that one
6796 expects because comparisons are made on a character-by-character basis.
6797 Version sorting is especially useful when browsing directories that contain
6798 many files with indices/version numbers in their names:
6802 abc.zml-1.gz abc.zml-1.gz
6803 abc.zml-12.gz abc.zml-2.gz
6804 abc.zml-2.gz abc.zml-12.gz
6807 Version-sorted strings are compared such that if @var{ver1} and @var{ver2}
6808 are version numbers and @var{prefix} and @var{suffix} (@var{suffix} matching
6809 the regular expression @samp{(\.[A-Za-z~][A-Za-z0-9~]*)*}) are strings then
6810 @var{ver1} < @var{ver2} implies that the name composed of
6811 ``@var{prefix} @var{ver1} @var{suffix}'' sorts before
6812 ``@var{prefix} @var{ver2} @var{suffix}''.
6814 Note also that leading zeros of numeric parts are ignored:
6818 abc-1.007.tgz abc-1.01a.tgz
6819 abc-1.012b.tgz abc-1.007.tgz
6820 abc-1.01a.tgz abc-1.012b.tgz
6823 This functionality is implemented using gnulib's @code{filevercmp} function,
6824 which has some caveats worth noting.
6827 @item @env{LC_COLLATE} is ignored, which means @samp{ls -v} and @samp{sort -V}
6828 will sort non-numeric prefixes as if the @env{LC_COLLATE} locale category
6829 was set to @samp{C}.
6830 @item Some suffixes will not be matched by the regular
6831 expression mentioned above. Consequently these examples may
6832 not sort as you expect:
6840 abc-1.2.3.4.x86_64.rpm
6841 abc-1.2.3.x86_64.rpm
6845 @node General output formatting
6846 @subsection General output formatting
6848 These options affect the appearance of the overall output.
6853 @itemx --format=single-column
6856 @opindex single-column @r{output of files}
6857 List one file per line. This is the default for @command{ls} when standard
6858 output is not a terminal.
6861 @itemx --format=vertical
6864 @opindex vertical @r{sorted files in columns}
6865 List files in columns, sorted vertically. This is the default for
6866 @command{ls} if standard output is a terminal. It is always the default
6867 for the @command{dir} program.
6868 @sc{gnu} @command{ls} uses variable width columns to display as many files as
6869 possible in the fewest lines.
6871 @item --color [=@var{when}]
6873 @cindex color, distinguishing file types with
6874 Specify whether to use color for distinguishing file types. @var{when}
6875 may be omitted, or one of:
6878 @vindex none @r{color option}
6879 - Do not use color at all. This is the default.
6881 @vindex auto @r{color option}
6882 @cindex terminal, using color iff
6883 - Only use color if standard output is a terminal.
6885 @vindex always @r{color option}
6888 Specifying @option{--color} and no @var{when} is equivalent to
6889 @option{--color=always}.
6890 Piping a colorized listing through a pager like @command{more} or
6891 @command{less} usually produces unreadable results. However, using
6892 @code{more -f} does seem to work.
6896 @itemx --indicator-style=classify
6899 @opindex --indicator-style
6900 @cindex file type and executables, marking
6901 @cindex executables and file type, marking
6902 Append a character to each file name indicating the file type. Also,
6903 for regular files that are executable, append @samp{*}. The file type
6904 indicators are @samp{/} for directories, @samp{@@} for symbolic links,
6905 @samp{|} for FIFOs, @samp{=} for sockets, @samp{>} for doors,
6906 and nothing for regular files.
6907 @c The following sentence is the same as the one for -d.
6908 Do not follow symbolic links listed on the
6909 command line unless the @option{--dereference-command-line} (@option{-H}),
6910 @option{--dereference} (@option{-L}), or
6911 @option{--dereference-command-line-symlink-to-dir} options are specified.
6914 @itemx --indicator-style=file-type
6915 @opindex --file-type
6916 @opindex --indicator-style
6917 @cindex file type, marking
6918 Append a character to each file name indicating the file type. This is
6919 like @option{-F}, except that executables are not marked.
6921 @item --indicator-style=@var{word}
6922 @opindex --indicator-style
6923 Append a character indicator with style @var{word} to entry names,
6928 Do not append any character indicator; this is the default.
6930 Append @samp{/} for directories. This is the same as the @option{-p}
6933 Append @samp{/} for directories, @samp{@@} for symbolic links, @samp{|}
6934 for FIFOs, @samp{=} for sockets, and nothing for regular files. This is
6935 the same as the @option{--file-type} option.
6937 Append @samp{*} for executable regular files, otherwise behave as for
6938 @samp{file-type}. This is the same as the @option{-F} or
6939 @option{--classify} option.
6944 Print file sizes in 1024-byte blocks, overriding the default block
6945 size (@pxref{Block size}).
6946 This option is equivalent to @option{--block-size=1K}.
6949 @itemx --format=commas
6952 @opindex commas@r{, outputting between files}
6953 List files horizontally, with as many as will fit on each line,
6954 separated by @samp{, } (a comma and a space).
6957 @itemx --indicator-style=slash
6959 @opindex --indicator-style
6960 @cindex file type, marking
6961 Append a @samp{/} to directory names.
6964 @itemx --format=across
6965 @itemx --format=horizontal
6968 @opindex across@r{, listing files}
6969 @opindex horizontal@r{, listing files}
6970 List the files in columns, sorted horizontally.
6973 @itemx --tabsize=@var{cols}
6976 Assume that each tab stop is @var{cols} columns wide. The default is 8.
6977 @command{ls} uses tabs where possible in the output, for efficiency. If
6978 @var{cols} is zero, do not use tabs at all.
6980 @c FIXME: remove in 2009, if Apple Terminal has been fixed for long enough.
6981 Some terminal emulators (at least Apple Terminal 1.5 (133) from Mac OS X 10.4.8)
6982 do not properly align columns to the right of a TAB following a
6983 non-@acronym{ASCII} byte. If you use such a terminal emulator, use the
6984 @option{-T0} option or put @code{TABSIZE=0} in your environment to tell
6985 @command{ls} to align using spaces, not tabs.
6988 @itemx --width=@var{cols}
6992 Assume the screen is @var{cols} columns wide. The default is taken
6993 from the terminal settings if possible; otherwise the environment
6994 variable @env{COLUMNS} is used if it is set; otherwise the default
7000 @node Formatting file timestamps
7001 @subsection Formatting file timestamps
7003 By default, file timestamps are listed in abbreviated form, using
7004 a date like @samp{Mar 30@ @ 2002} for non-recent timestamps, and a
7005 date-without-year and time like @samp{Mar 30 23:45} for recent timestamps.
7006 This format can change depending on the current locale as detailed below.
7008 A timestamp is considered to be @dfn{recent} if it is less than six
7009 months old, and is not dated in the future. If a timestamp dated
7010 today is not listed in recent form, the timestamp is in the future,
7011 which means you probably have clock skew problems which may break
7012 programs like @command{make} that rely on file timestamps.
7015 Time stamps are listed according to the time zone rules specified by
7016 the @env{TZ} environment variable, or by the system default rules if
7017 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
7018 with @env{TZ}, libc, The GNU C Library Reference Manual}.
7020 The following option changes how file timestamps are printed.
7023 @item --time-style=@var{style}
7024 @opindex --time-style
7026 List timestamps in style @var{style}. The @var{style} should
7027 be one of the following:
7032 List timestamps using @var{format}, where @var{format} is interpreted
7033 like the format argument of @command{date} (@pxref{date invocation}).
7034 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
7035 @command{ls} to list timestamps like @samp{2002-03-30 23:45:56}. As
7036 with @command{date}, @var{format}'s interpretation is affected by the
7037 @env{LC_TIME} locale category.
7039 If @var{format} contains two format strings separated by a newline,
7040 the former is used for non-recent files and the latter for recent
7041 files; if you want output columns to line up, you may need to insert
7042 spaces in one of the two formats.
7045 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
7046 format with nanosecond precision, e.g., @samp{2002-03-30
7047 23:45:56.477817180 -0700}. This style is equivalent to
7048 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
7050 This is useful because the time output includes all the information that
7051 is available from the operating system. For example, this can help
7052 explain @command{make}'s behavior, since @acronym{GNU} @command{make}
7053 uses the full timestamp to determine whether a file is out of date.
7056 List @acronym{ISO} 8601 date and time in minutes, e.g.,
7057 @samp{2002-03-30 23:45}. These timestamps are shorter than
7058 @samp{full-iso} timestamps, and are usually good enough for everyday
7059 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
7062 List @acronym{ISO} 8601 dates for non-recent timestamps (e.g.,
7063 @samp{2002-03-30@ }), and @acronym{ISO} 8601 month, day, hour, and
7064 minute for recent timestamps (e.g., @samp{03-30 23:45}). These
7065 timestamps are uglier than @samp{long-iso} timestamps, but they carry
7066 nearly the same information in a smaller space and their brevity helps
7067 @command{ls} output fit within traditional 80-column output lines.
7068 The following two @command{ls} invocations are equivalent:
7073 ls -l --time-style="+%Y-%m-%d $newline%m-%d %H:%M"
7074 ls -l --time-style="iso"
7079 List timestamps in a locale-dependent form. For example, a Finnish
7080 locale might list non-recent timestamps like @samp{maalis 30@ @ 2002}
7081 and recent timestamps like @samp{maalis 30 23:45}. Locale-dependent
7082 timestamps typically consume more space than @samp{iso} timestamps and
7083 are harder for programs to parse because locale conventions vary so
7084 widely, but they are easier for many people to read.
7086 The @env{LC_TIME} locale category specifies the timestamp format. The
7087 default @acronym{POSIX} locale uses timestamps like @samp{Mar 30@
7088 @ 2002} and @samp{Mar 30 23:45}; in this locale, the following two
7089 @command{ls} invocations are equivalent:
7094 ls -l --time-style="+%b %e %Y$newline%b %e %H:%M"
7095 ls -l --time-style="locale"
7098 Other locales behave differently. For example, in a German locale,
7099 @option{--time-style="locale"} might be equivalent to
7100 @option{--time-style="+%e. %b %Y $newline%e. %b %H:%M"}
7101 and might generate timestamps like @samp{30. M@"ar 2002@ } and
7102 @samp{30. M@"ar 23:45}.
7104 @item posix-@var{style}
7106 List @acronym{POSIX}-locale timestamps if the @env{LC_TIME} locale
7107 category is @acronym{POSIX}, @var{style} timestamps otherwise. For
7108 example, the @samp{posix-long-iso} style lists
7109 timestamps like @samp{Mar 30@ @ 2002} and @samp{Mar 30 23:45} when in
7110 the @acronym{POSIX} locale, and like @samp{2002-03-30 23:45} otherwise.
7115 You can specify the default value of the @option{--time-style} option
7116 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
7117 the default style is @samp{locale}. @acronym{GNU} Emacs 21.3 and
7118 later use the @option{--dired} option and therefore can parse any date
7119 format, but if you are using Emacs 21.1 or 21.2 and specify a
7120 non-@acronym{POSIX} locale you may need to set
7121 @samp{TIME_STYLE="posix-long-iso"}.
7123 To avoid certain denial-of-service attacks, timestamps that would be
7124 longer than 1000 bytes may be treated as errors.
7127 @node Formatting the file names
7128 @subsection Formatting the file names
7130 These options change how file names themselves are printed.
7136 @itemx --quoting-style=escape
7139 @opindex --quoting-style
7140 @cindex backslash sequences for file names
7141 Quote nongraphic characters in file names using alphabetic and octal
7142 backslash sequences like those used in C.
7146 @itemx --quoting-style=literal
7149 @opindex --quoting-style
7150 Do not quote file names. However, with @command{ls} nongraphic
7151 characters are still printed as question marks if the output is a
7152 terminal and you do not specify the @option{--show-control-chars}
7156 @itemx --hide-control-chars
7158 @opindex --hide-control-chars
7159 Print question marks instead of nongraphic characters in file names.
7160 This is the default if the output is a terminal and the program is
7165 @itemx --quoting-style=c
7167 @opindex --quote-name
7168 @opindex --quoting-style
7169 Enclose file names in double quotes and quote nongraphic characters as
7172 @item --quoting-style=@var{word}
7173 @opindex --quoting-style
7174 @cindex quoting style
7175 Use style @var{word} to quote file names and other strings that may
7176 contain arbitrary characters. The @var{word} should
7177 be one of the following:
7181 Output strings as-is; this is the same as the @option{-N} or
7182 @option{--literal} option.
7184 Quote strings for the shell if they contain shell metacharacters or would
7185 cause ambiguous output.
7186 The quoting is suitable for @acronym{POSIX}-compatible shells like
7187 @command{bash}, but it does not always work for incompatible shells
7190 Quote strings for the shell, even if they would normally not require quoting.
7192 Quote strings as for C character string literals, including the
7193 surrounding double-quote characters; this is the same as the
7194 @option{-Q} or @option{--quote-name} option.
7196 Quote strings as for C character string literals, except omit the
7197 surrounding double-quote
7198 characters; this is the same as the @option{-b} or @option{--escape} option.
7200 Quote strings as for C character string literals, except use
7201 surrounding quotation marks appropriate for the
7204 @c Use @t instead of @samp to avoid duplicate quoting in some output styles.
7205 Quote strings as for C character string literals, except use
7206 surrounding quotation marks appropriate for the locale, and quote
7207 @t{`like this'} instead of @t{"like
7208 this"} in the default C locale. This looks nicer on many displays.
7211 You can specify the default value of the @option{--quoting-style} option
7212 with the environment variable @env{QUOTING_STYLE}. If that environment
7213 variable is not set, the default value is @samp{literal}, but this
7214 default may change to @samp{shell} in a future version of this package.
7216 @item --show-control-chars
7217 @opindex --show-control-chars
7218 Print nongraphic characters as-is in file names.
7219 This is the default unless the output is a terminal and the program is
7225 @node dir invocation
7226 @section @command{dir}: Briefly list directory contents
7229 @cindex directory listing, brief
7231 @command{dir} is equivalent to @code{ls -C
7232 -b}; that is, by default files are listed in columns, sorted vertically,
7233 and special characters are represented by backslash escape sequences.
7235 @xref{ls invocation, @command{ls}}.
7238 @node vdir invocation
7239 @section @command{vdir}: Verbosely list directory contents
7242 @cindex directory listing, verbose
7244 @command{vdir} is equivalent to @code{ls -l
7245 -b}; that is, by default files are listed in long format and special
7246 characters are represented by backslash escape sequences.
7248 @node dircolors invocation
7249 @section @command{dircolors}: Color setup for @command{ls}
7253 @cindex setup for color
7255 @command{dircolors} outputs a sequence of shell commands to set up the
7256 terminal for color output from @command{ls} (and @command{dir}, etc.).
7260 eval "`dircolors [@var{option}]@dots{} [@var{file}]`"
7263 If @var{file} is specified, @command{dircolors} reads it to determine which
7264 colors to use for which file types and extensions. Otherwise, a
7265 precompiled database is used. For details on the format of these files,
7266 run @samp{dircolors --print-database}.
7268 To make @command{dircolors} read a @file{~/.dircolors} file if it
7269 exists, you can put the following lines in your @file{~/.bashrc} (or
7270 adapt them to your favorite shell):
7274 test -r $d && eval "$(dircolors $d)"
7278 @vindex SHELL @r{environment variable, and color}
7279 The output is a shell command to set the @env{LS_COLORS} environment
7280 variable. You can specify the shell syntax to use on the command line,
7281 or @command{dircolors} will guess it from the value of the @env{SHELL}
7282 environment variable.
7284 The program accepts the following options. Also see @ref{Common options}.
7289 @itemx --bourne-shell
7292 @opindex --bourne-shell
7293 @cindex Bourne shell syntax for color setup
7294 @cindex @command{sh} syntax for color setup
7295 Output Bourne shell commands. This is the default if the @env{SHELL}
7296 environment variable is set and does not end with @samp{csh} or
7305 @cindex C shell syntax for color setup
7306 @cindex @command{csh} syntax for color setup
7307 Output C shell commands. This is the default if @code{SHELL} ends with
7308 @command{csh} or @command{tcsh}.
7311 @itemx --print-database
7313 @opindex --print-database
7314 @cindex color database, printing
7315 @cindex database for color setup, printing
7316 @cindex printing color database
7317 Print the (compiled-in) default color configuration database. This
7318 output is itself a valid configuration file, and is fairly descriptive
7319 of the possibilities.
7326 @node Basic operations
7327 @chapter Basic operations
7329 @cindex manipulating files
7331 This chapter describes the commands for basic file manipulation:
7332 copying, moving (renaming), and deleting (removing).
7335 * cp invocation:: Copy files.
7336 * dd invocation:: Convert and copy a file.
7337 * install invocation:: Copy files and set attributes.
7338 * mv invocation:: Move (rename) files.
7339 * rm invocation:: Remove files or directories.
7340 * shred invocation:: Remove files more securely.
7345 @section @command{cp}: Copy files and directories
7348 @cindex copying files and directories
7349 @cindex files, copying
7350 @cindex directories, copying
7352 @command{cp} copies files (or, optionally, directories). The copy is
7353 completely independent of the original. You can either copy one file to
7354 another, or copy arbitrarily many files to a destination directory.
7358 cp [@var{option}]@dots{} [-T] @var{source} @var{dest}
7359 cp [@var{option}]@dots{} @var{source}@dots{} @var{directory}
7360 cp [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
7365 If two file names are given, @command{cp} copies the first file to the
7369 If the @option{--target-directory} (@option{-t}) option is given, or
7370 failing that if the last file is a directory and the
7371 @option{--no-target-directory} (@option{-T}) option is not given,
7372 @command{cp} copies each @var{source} file to the specified directory,
7373 using the @var{source}s' names.
7376 Generally, files are written just as they are read. For exceptions,
7377 see the @option{--sparse} option below.
7379 By default, @command{cp} does not copy directories. However, the
7380 @option{-R}, @option{-a}, and @option{-r} options cause @command{cp} to
7381 copy recursively by descending into source directories and copying files
7382 to corresponding destination directories.
7384 When copying from a symbolic link, @command{cp} normally follows the
7385 link only when not copying
7386 recursively. This default can be overridden with the
7387 @option{--archive} (@option{-a}), @option{-d}, @option{--dereference}
7388 (@option{-L}), @option{--no-dereference} (@option{-P}), and
7389 @option{-H} options. If more than one of these options is specified,
7390 the last one silently overrides the others.
7392 When copying to a symbolic link, @command{cp} follows the
7393 link only when it refers to an existing regular file.
7394 However, when copying to a dangling symbolic link, @command{cp}
7395 refuses by default, and fails with a diagnostic, since the operation
7396 is inherently dangerous. This behavior is contrary to historical
7397 practice and to @acronym{POSIX}.
7398 Set @env{POSIXLY_CORRECT} to make @command{cp} attempt to create
7399 the target of a dangling destination symlink, in spite of the possible risk.
7400 Also, when an option like
7401 @option{--backup} or @option{--link} acts to rename or remove the
7402 destination before copying, @command{cp} renames or removes the
7403 symbolic link rather than the file it points to.
7405 By default, @command{cp} copies the contents of special files only
7406 when not copying recursively. This default can be overridden with the
7407 @option{--copy-contents} option.
7409 @cindex self-backups
7410 @cindex backups, making only
7411 @command{cp} generally refuses to copy a file onto itself, with the
7412 following exception: if @option{--force --backup} is specified with
7413 @var{source} and @var{dest} identical, and referring to a regular file,
7414 @command{cp} will make a backup file, either regular or numbered, as
7415 specified in the usual ways (@pxref{Backup options}). This is useful when
7416 you simply want to make a backup of an existing file before changing it.
7418 The program accepts the following options. Also see @ref{Common options}.
7425 Preserve as much as possible of the structure and attributes of the
7426 original files in the copy (but do not attempt to preserve internal
7427 directory structure; i.e., @samp{ls -U} may list the entries in a copied
7428 directory in a different order).
7429 Try to preserve SELinux security context and extended attributes (xattr),
7430 but ignore any failure to do that and print no corresponding diagnostic.
7431 Equivalent to @option{-dR --preserve=all} with the reduced diagnostics.
7433 @itemx --attributes-only
7434 @opindex --attributes-only
7435 Preserve the specified attributes of the original files in the copy,
7436 but do not copy any data. See the @option{--preserve} option for
7437 controlling which attributes to copy.
7440 @itemx @w{@kbd{--backup}[=@var{method}]}
7443 @vindex VERSION_CONTROL
7444 @cindex backups, making
7445 @xref{Backup options}.
7446 Make a backup of each file that would otherwise be overwritten or removed.
7447 As a special case, @command{cp} makes a backup of @var{source} when the force
7448 and backup options are given and @var{source} and @var{dest} are the same
7449 name for an existing, regular file. One useful application of this
7450 combination of options is this tiny Bourne shell script:
7454 # Usage: backup FILE...
7455 # Create a @sc{gnu}-style backup of each listed FILE.
7457 cp --backup --force -- "$i" "$i"
7461 @item --copy-contents
7462 @cindex directories, copying recursively
7463 @cindex copying directories recursively
7464 @cindex recursively copying directories
7465 @cindex non-directories, copying as special files
7466 If copying recursively, copy the contents of any special files (e.g.,
7467 FIFOs and device files) as if they were regular files. This means
7468 trying to read the data in each source file and writing it to the
7469 destination. It is usually a mistake to use this option, as it
7470 normally has undesirable effects on special files like FIFOs and the
7471 ones typically found in the @file{/dev} directory. In most cases,
7472 @code{cp -R --copy-contents} will hang indefinitely trying to read
7473 from FIFOs and special files like @file{/dev/console}, and it will
7474 fill up your destination disk if you use it to copy @file{/dev/zero}.
7475 This option has no effect unless copying recursively, and it does not
7476 affect the copying of symbolic links.
7480 @cindex symbolic links, copying
7481 @cindex hard links, preserving
7482 Copy symbolic links as symbolic links rather than copying the files that
7483 they point to, and preserve hard links between source files in the copies.
7484 Equivalent to @option{--no-dereference --preserve=links}.
7490 When copying without this option and an existing destination file cannot
7491 be opened for writing, the copy fails. However, with @option{--force}),
7492 when a destination file cannot be opened, @command{cp} then removes it and
7493 tries to open it again. Contrast this behavior with that enabled by
7494 @option{--link} and @option{--symbolic-link}, whereby the destination file
7495 is never opened but rather is removed unconditionally. Also see the
7496 description of @option{--remove-destination}.
7498 This option is independent of the @option{--interactive} or
7499 @option{-i} option: neither cancels the effect of the other.
7501 This option is redundant if the @option{--no-clobber} or @option{-n} option is
7506 If a command line argument specifies a symbolic link, then copy the
7507 file it points to rather than the symbolic link itself. However,
7508 copy (preserving its nature) any symbolic link that is encountered
7509 via recursive traversal.
7512 @itemx --interactive
7514 @opindex --interactive
7515 When copying a file other than a directory, prompt whether to
7516 overwrite an existing destination file. The @option{-i} option overrides
7517 a previous @option{-n} option.
7523 Make hard links instead of copies of non-directories.
7526 @itemx --dereference
7528 @opindex --dereference
7529 Follow symbolic links when copying from them.
7530 With this option, @command{cp} cannot create a symbolic link.
7531 For example, a symlink (to regular file) in the source tree will be copied to
7532 a regular file in the destination tree.
7537 @opindex --no-clobber
7538 Do not overwrite an existing file. The @option{-n} option overrides a previous
7539 @option{-i} option. This option is mutually exclusive with @option{-b} or
7540 @option{--backup} option.
7543 @itemx --no-dereference
7545 @opindex --no-dereference
7546 @cindex symbolic links, copying
7547 Copy symbolic links as symbolic links rather than copying the files that
7548 they point to. This option affects only symbolic links in the source;
7549 symbolic links in the destination are always followed if possible.
7552 @itemx @w{@kbd{--preserve}[=@var{attribute_list}]}
7555 @cindex file information, preserving, extended attributes, xattr
7556 Preserve the specified attributes of the original files.
7557 If specified, the @var{attribute_list} must be a comma-separated list
7558 of one or more of the following strings:
7562 Preserve the file mode bits and access control lists.
7564 Preserve the owner and group. On most modern systems,
7565 only users with appropriate privileges may change the owner of a file,
7567 may preserve the group ownership of a file only if they happen to be
7568 a member of the desired group.
7570 Preserve the times of last access and last modification, when possible.
7571 On older systems, it is not possible to preserve these attributes
7572 when the affected file is a symbolic link.
7573 However, many systems now provide the @code{utimensat} function,
7574 which makes it possible even for symbolic links.
7576 Preserve in the destination files
7577 any links between corresponding source files.
7578 Note that with @option{-L} or @option{-H}, this option can convert
7579 symbolic links to hard links. For example,
7581 $ mkdir c; : > a; ln -s a b; cp -aH a b c; ls -i1 c
7586 Note the inputs: @file{b} is a symlink to regular file @file{a},
7587 yet the files in destination directory, @file{c/}, are hard-linked.
7588 Since @option{-a} implies @option{--preserve=links}, and since @option{-H}
7589 tells @command{cp} to dereference command line arguments, it sees two files
7590 with the same inode number, and preserves the perceived hard link.
7592 Here is a similar example that exercises @command{cp}'s @option{-L} option:
7594 $ mkdir b c; (cd b; : > a; ln -s a b); cp -aL b c; ls -i1 c/b
7600 Preserve SELinux security context of the file, or fail with full diagnostics.
7602 Preserve extended attributes of the file, or fail with full diagnostics.
7603 If @command{cp} is built without xattr support, ignore this option.
7604 If SELinux context, ACLs or Capabilities are implemented using xattrs,
7605 they are preserved by this option as well.
7607 Preserve all file attributes.
7608 Equivalent to specifying all of the above, but with the difference
7609 that failure to preserve SELinux security context or extended attributes
7610 does not change @command{cp}'s exit status. In contrast to @option{-a},
7611 all but @samp{Operation not supported} warnings are output.
7614 Using @option{--preserve} with no @var{attribute_list} is equivalent
7615 to @option{--preserve=mode,ownership,timestamps}.
7617 In the absence of this option, each destination file is created with the
7618 mode bits of the corresponding source file, minus the bits set in the
7619 umask and minus the set-user-ID and set-group-ID bits.
7620 @xref{File permissions}.
7622 @itemx @w{@kbd{--no-preserve}=@var{attribute_list}}
7623 @cindex file information, preserving
7624 Do not preserve the specified attributes. The @var{attribute_list}
7625 has the same form as for @option{--preserve}.
7629 @cindex parent directories and @command{cp}
7630 Form the name of each destination file by appending to the target
7631 directory a slash and the specified name of the source file. The last
7632 argument given to @command{cp} must be the name of an existing directory.
7633 For example, the command:
7636 cp --parents a/b/c existing_dir
7640 copies the file @file{a/b/c} to @file{existing_dir/a/b/c}, creating
7641 any missing intermediate directories.
7648 @opindex --recursive
7649 @cindex directories, copying recursively
7650 @cindex copying directories recursively
7651 @cindex recursively copying directories
7652 @cindex non-directories, copying as special files
7653 Copy directories recursively. By default, do not follow symbolic
7654 links in the source; see the @option{--archive} (@option{-a}), @option{-d},
7655 @option{--dereference} (@option{-L}), @option{--no-dereference}
7656 (@option{-P}), and @option{-H} options. Special files are copied by
7657 creating a destination file of the same type as the source; see the
7658 @option{--copy-contents} option. It is not portable to use
7659 @option{-r} to copy symbolic links or special files. On some
7660 non-@sc{gnu} systems, @option{-r} implies the equivalent of
7661 @option{-L} and @option{--copy-contents} for historical reasons.
7662 Also, it is not portable to use @option{-R} to copy symbolic links
7663 unless you also specify @option{-P}, as @acronym{POSIX} allows
7664 implementations that dereference symbolic links by default.
7666 @item --reflink[=@var{when}]
7667 @opindex --reflink[=@var{when}]
7670 @cindex copy on write
7671 Perform a lightweight, copy-on-write (COW) copy, if supported by the
7672 file system. Once it has succeeded, beware that the source and destination
7673 files share the same disk data blocks as long as they remain unmodified.
7674 Thus, if a disk I/O error affects data blocks of one of the files,
7675 the other suffers the same fate.
7677 The @var{when} value can be one of the following:
7681 The default behavior: if the copy-on-write operation is not supported
7682 then report the failure for each file and exit with a failure status.
7685 If the copy-on-write operation is not supported then fall back
7686 to the standard copy behaviour.
7689 This option is overridden by the @option{--link}, @option{--symbolic-link}
7690 and @option{--attributes-only} options, thus allowing it to be used
7691 to configure the default data copying behavior for @command{cp}.
7692 For example, with the following alias, @command{cp} will use the
7693 minimum amount of space supported by the file system.
7696 alias cp='cp --reflink=auto --sparse=always'
7699 @item --remove-destination
7700 @opindex --remove-destination
7701 Remove each existing destination file before attempting to open it
7702 (contrast with @option{-f} above).
7704 @item --sparse=@var{when}
7705 @opindex --sparse=@var{when}
7706 @cindex sparse files, copying
7707 @cindex holes, copying files with
7708 @findex read @r{system call, and holes}
7709 A @dfn{sparse file} contains @dfn{holes}---a sequence of zero bytes that
7710 does not occupy any physical disk blocks; the @samp{read} system call
7711 reads these as zeros. This can both save considerable disk space and
7712 increase speed, since many binary files contain lots of consecutive zero
7713 bytes. By default, @command{cp} detects holes in input source files via a crude
7714 heuristic and makes the corresponding output file sparse as well.
7715 Only regular files may be sparse.
7717 The @var{when} value can be one of the following:
7721 The default behavior: if the input file is sparse, attempt to make
7722 the output file sparse, too. However, if an output file exists but
7723 refers to a non-regular file, then do not attempt to make it sparse.
7726 For each sufficiently long sequence of zero bytes in the input file,
7727 attempt to create a corresponding hole in the output file, even if the
7728 input file does not appear to be sparse.
7729 This is useful when the input file resides on a file system
7730 that does not support sparse files
7731 (for example, @samp{efs} file systems in SGI IRIX 5.3 and earlier),
7732 but the output file is on a type of file system that does support them.
7733 Holes may be created only in regular files, so if the destination file
7734 is of some other type, @command{cp} does not even try to make it sparse.
7737 Never make the output file sparse.
7738 This is useful in creating a file for use with the @command{mkswap} command,
7739 since such a file must not have any holes.
7742 @optStripTrailingSlashes
7745 @itemx --symbolic-link
7747 @opindex --symbolic-link
7748 @cindex symbolic links, copying with
7749 Make symbolic links instead of copies of non-directories. All source
7750 file names must be absolute (starting with @samp{/}) unless the
7751 destination files are in the current directory. This option merely
7752 results in an error message on systems that do not support symbolic links.
7758 @optNoTargetDirectory
7764 @cindex newer files, copying only
7765 Do not copy a non-directory that has an existing destination with the
7766 same or newer modification time. If time stamps are being preserved,
7767 the comparison is to the source time stamp truncated to the
7768 resolutions of the destination file system and of the system calls
7769 used to update time stamps; this avoids duplicate work if several
7770 @samp{cp -pu} commands are executed with the same source and
7777 Print the name of each file before copying it.
7780 @itemx --one-file-system
7782 @opindex --one-file-system
7783 @cindex file systems, omitting copying to different
7784 Skip subdirectories that are on different file systems from the one that
7785 the copy started on.
7786 However, mount point directories @emph{are} copied.
7794 @section @command{dd}: Convert and copy a file
7797 @cindex converting while copying a file
7799 @command{dd} copies a file (from standard input to standard output, by
7800 default) with a changeable I/O block size, while optionally performing
7801 conversions on it. Synopses:
7804 dd [@var{operand}]@dots{}
7808 The only options are @option{--help} and @option{--version}.
7809 @xref{Common options}. @command{dd} accepts the following operands.
7815 Read from @var{file} instead of standard input.
7819 Write to @var{file} instead of standard output. Unless
7820 @samp{conv=notrunc} is given, @command{dd} truncates @var{file} to zero
7821 bytes (or the size specified with @samp{seek=}).
7823 @item ibs=@var{bytes}
7825 @cindex block size of input
7826 @cindex input block size
7827 Set the input block size to @var{bytes}.
7828 This makes @command{dd} read @var{bytes} per block.
7829 The default is 512 bytes.
7831 @item obs=@var{bytes}
7833 @cindex block size of output
7834 @cindex output block size
7835 Set the output block size to @var{bytes}.
7836 This makes @command{dd} write @var{bytes} per block.
7837 The default is 512 bytes.
7839 @item bs=@var{bytes}
7842 Set both input and output block sizes to @var{bytes}.
7843 This makes @command{dd} read and write @var{bytes} per block,
7844 overriding any @samp{ibs} and @samp{obs} settings.
7845 In addition, if no data-transforming @option{conv} option is specified,
7846 each input block is copied to the output as a single block,
7847 without aggregating short reads.
7849 @item cbs=@var{bytes}
7851 @cindex block size of conversion
7852 @cindex conversion block size
7853 @cindex fixed-length records, converting to variable-length
7854 @cindex variable-length records, converting to fixed-length
7855 Set the conversion block size to @var{bytes}.
7856 When converting variable-length records to fixed-length ones
7857 (@option{conv=block}) or the reverse (@option{conv=unblock}),
7858 use @var{bytes} as the fixed record length.
7860 @item skip=@var{blocks}
7862 Skip @var{blocks} @samp{ibs}-byte blocks in the input file before copying.
7864 @item seek=@var{blocks}
7866 Skip @var{blocks} @samp{obs}-byte blocks in the output file before copying.
7868 @item count=@var{blocks}
7870 Copy @var{blocks} @samp{ibs}-byte blocks from the input file, instead
7871 of everything until the end of the file.
7875 Do not print the overall transfer rate and volume statistics
7876 that normally make up the third status line when @command{dd} exits.
7878 @item conv=@var{conversion}[,@var{conversion}]@dots{}
7880 Convert the file as specified by the @var{conversion} argument(s).
7881 (No spaces around any comma(s).)
7888 @opindex ascii@r{, converting to}
7889 Convert @acronym{EBCDIC} to @acronym{ASCII},
7890 using the conversion table specified by @acronym{POSIX}.
7891 This provides a 1:1 translation for all 256 bytes.
7894 @opindex ebcdic@r{, converting to}
7895 Convert @acronym{ASCII} to @acronym{EBCDIC}.
7896 This is the inverse of the @samp{ascii} conversion.
7899 @opindex alternate ebcdic@r{, converting to}
7900 Convert @acronym{ASCII} to alternate @acronym{EBCDIC},
7901 using the alternate conversion table specified by @acronym{POSIX}.
7902 This is not a 1:1 translation, but reflects common historical practice
7903 for @samp{~}, @samp{[}, and @samp{]}.
7905 The @samp{ascii}, @samp{ebcdic}, and @samp{ibm} conversions are
7909 @opindex block @r{(space-padding)}
7910 For each line in the input, output @samp{cbs} bytes, replacing the
7911 input newline with a space and padding with spaces as necessary.
7915 Remove any trailing spaces in each @samp{cbs}-sized input block,
7916 and append a newline.
7918 The @samp{block} and @samp{unblock} conversions are mutually exclusive.
7921 @opindex lcase@r{, converting to}
7922 Change uppercase letters to lowercase.
7925 @opindex ucase@r{, converting to}
7926 Change lowercase letters to uppercase.
7928 The @samp{lcase} and @samp{ucase} conversions are mutually exclusive.
7931 @opindex swab @r{(byte-swapping)}
7932 @cindex byte-swapping
7933 Swap every pair of input bytes. @sc{gnu} @command{dd}, unlike others, works
7934 when an odd number of bytes are read---the last byte is simply copied
7935 (since there is nothing to swap it with).
7939 @cindex read errors, ignoring
7940 Continue after read errors.
7944 @cindex creating output file, avoiding
7945 Do not create the output file; the output file must already exist.
7949 @cindex creating output file, requiring
7950 Fail if the output file already exists; @command{dd} must create the
7953 The @samp{excl} and @samp{nocreat} conversions are mutually exclusive.
7957 @cindex truncating output file, avoiding
7958 Do not truncate the output file.
7961 @opindex sync @r{(padding with @acronym{ASCII} @sc{nul}s)}
7962 Pad every input block to size of @samp{ibs} with trailing zero bytes.
7963 When used with @samp{block} or @samp{unblock}, pad with spaces instead of
7968 @cindex synchronized data writes, before finishing
7969 Synchronize output data just before finishing. This forces a physical
7970 write of output data.
7974 @cindex synchronized data and metadata writes, before finishing
7975 Synchronize output data and metadata just before finishing. This
7976 forces a physical write of output data and metadata.
7980 @item iflag=@var{flag}[,@var{flag}]@dots{}
7982 Access the input file using the flags specified by the @var{flag}
7983 argument(s). (No spaces around any comma(s).)
7985 @item oflag=@var{flag}[,@var{flag}]@dots{}
7987 Access the output file using the flags specified by the @var{flag}
7988 argument(s). (No spaces around any comma(s).)
7990 Here are the flags. Not every flag is supported on every operating
7997 @cindex appending to the output file
7998 Write in append mode, so that even if some other process is writing to
7999 this file, every @command{dd} write will append to the current
8000 contents of the file. This flag makes sense only for output.
8001 If you combine this flag with the @samp{of=@var{file}} operand,
8002 you should also specify @samp{conv=notrunc} unless you want the
8003 output file to be truncated before being appended to.
8007 @cindex concurrent I/O
8008 Use concurrent I/O mode for data. This mode performs direct I/O
8009 and drops the @acronym{POSIX} requirement to serialize all I/O to the same file.
8010 A file cannot be opened in CIO mode and with a standard open at the
8016 Use direct I/O for data, avoiding the buffer cache.
8017 Note that the kernel may impose restrictions on read or write buffer sizes.
8018 For example, with an ext4 destination file system and a linux-based kernel,
8019 using @samp{oflag=direct} will cause writes to fail with @code{EINVAL} if the
8020 output buffer size is not a multiple of 512.
8024 @cindex directory I/O
8026 Fail unless the file is a directory. Most operating systems do not
8027 allow I/O to a directory, so this flag has limited utility.
8031 @cindex synchronized data reads
8032 Use synchronized I/O for data. For the output file, this forces a
8033 physical write of output data on each write. For the input file,
8034 this flag can matter when reading from a remote file that has been
8035 written to synchronously by some other process. Metadata (e.g.,
8036 last-access and last-modified time) is not necessarily synchronized.
8040 @cindex synchronized data and metadata I/O
8041 Use synchronized I/O for both data and metadata.
8045 @cindex nonblocking I/O
8046 Use non-blocking I/O.
8051 Do not update the file's access time.
8052 Some older file systems silently ignore this flag, so it is a good
8053 idea to test it on your files before relying on it.
8057 @cindex controlling terminal
8058 Do not assign the file to be a controlling terminal for @command{dd}.
8059 This has no effect when the file is not a terminal.
8060 On many hosts (e.g., @acronym{GNU}/Linux hosts), this option has no effect
8065 @cindex symbolic links, following
8066 Do not follow symbolic links.
8071 Fail if the file has multiple hard links.
8076 Use binary I/O. This option has an effect only on nonstandard
8077 platforms that distinguish binary from text I/O.
8082 Use text I/O. Like @samp{binary}, this option has no effect on
8087 Accumulate full blocks from input. The @code{read} system call
8088 may return early if a full block is not available.
8089 When that happens, continue calling @code{read} to fill the remainder
8091 This flag can be used only with @code{iflag}.
8095 These flags are not supported on all systems, and @samp{dd} rejects
8096 attempts to use them when they are not supported. When reading from
8097 standard input or writing to standard output, the @samp{nofollow} and
8098 @samp{noctty} flags should not be specified, and the other flags
8099 (e.g., @samp{nonblock}) can affect how other processes behave with the
8100 affected file descriptors, even after @command{dd} exits.
8104 @cindex multipliers after numbers
8105 The numeric-valued strings above (@var{bytes} and @var{blocks}) can be
8106 followed by a multiplier: @samp{b}=512, @samp{c}=1,
8107 @samp{w}=2, @samp{x@var{m}}=@var{m}, or any of the
8108 standard block size suffixes like @samp{k}=1024 (@pxref{Block size}).
8110 Any block size you specify via @samp{bs=}, @samp{ibs=}, @samp{obs=}, @samp{cbs=}
8111 should not be too large---values larger than a few megabytes
8112 are generally wasteful or (as in the gigabyte..exabyte case) downright
8113 counterproductive or error-inducing.
8115 Use different @command{dd} invocations to use different block sizes for
8116 skipping and I/O@. For example, the following shell commands copy data
8117 in 512 KiB blocks between a disk and a tape, but do not save or restore a
8118 4 KiB label at the start of the disk:
8121 disk=/dev/rdsk/c0t1d0s2
8124 # Copy all but the label from disk to tape.
8125 (dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape
8127 # Copy from tape back to disk, but leave the disk label alone.
8128 (dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk
8131 Sending an @samp{INFO} signal to a running @command{dd}
8132 process makes it print I/O statistics to standard error
8133 and then resume copying. In the example below,
8134 @command{dd} is run in the background to copy 10 million blocks.
8135 The @command{kill} command makes it output intermediate I/O statistics,
8136 and when @command{dd} completes normally or is killed by the
8137 @code{SIGINT} signal, it outputs the final statistics.
8140 $ dd if=/dev/zero of=/dev/null count=10MB & pid=$!
8141 $ kill -s INFO $pid; wait $pid
8142 3385223+0 records in
8143 3385223+0 records out
8144 1733234176 bytes (1.7 GB) copied, 6.42173 seconds, 270 MB/s
8145 10000000+0 records in
8146 10000000+0 records out
8147 5120000000 bytes (5.1 GB) copied, 18.913 seconds, 271 MB/s
8150 @vindex POSIXLY_CORRECT
8151 On systems lacking the @samp{INFO} signal @command{dd} responds to the
8152 @samp{USR1} signal instead, unless the @env{POSIXLY_CORRECT}
8153 environment variable is set.
8158 @node install invocation
8159 @section @command{install}: Copy files and set attributes
8162 @cindex copying files and setting attributes
8164 @command{install} copies files while setting their file mode bits and, if
8165 possible, their owner and group. Synopses:
8168 install [@var{option}]@dots{} [-T] @var{source} @var{dest}
8169 install [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8170 install [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8171 install [@var{option}]@dots{} -d @var{directory}@dots{}
8176 If two file names are given, @command{install} copies the first file to the
8180 If the @option{--target-directory} (@option{-t}) option is given, or
8181 failing that if the last file is a directory and the
8182 @option{--no-target-directory} (@option{-T}) option is not given,
8183 @command{install} copies each @var{source} file to the specified
8184 directory, using the @var{source}s' names.
8187 If the @option{--directory} (@option{-d}) option is given,
8188 @command{install} creates each @var{directory} and any missing parent
8189 directories. Parent directories are created with mode
8190 @samp{u=rwx,go=rx} (755), regardless of the @option{-m} option or the
8191 current umask. @xref{Directory Setuid and Setgid}, for how the
8192 set-user-ID and set-group-ID bits of parent directories are inherited.
8195 @cindex Makefiles, installing programs in
8196 @command{install} is similar to @command{cp}, but allows you to control the
8197 attributes of destination files. It is typically used in Makefiles to
8198 copy programs into their destination directories. It refuses to copy
8199 files onto themselves.
8201 @cindex extended attributes, xattr
8202 @command{install} never preserves extended attributes (xattr).
8204 The program accepts the following options. Also see @ref{Common options}.
8214 Compare each pair of source and destination files, and if the destination has
8215 identical content and any specified owner, group, permissions, and possibly
8216 SELinux context, then do not modify the destination at all.
8220 Ignored; for compatibility with old Unix versions of @command{install}.
8224 Create any missing parent directories of @var{dest},
8225 then copy @var{source} to @var{dest}.
8226 This option is ignored if a destination directory is specified
8227 via @option{--target-directory=DIR}.
8232 @opindex --directory
8233 @cindex directories, creating with given attributes
8234 @cindex parent directories, creating missing
8235 @cindex leading directories, creating missing
8236 Create any missing parent directories, giving them the default
8237 attributes. Then create each given directory, setting their owner,
8238 group and mode as given on the command line or to the defaults.
8240 @item -g @var{group}
8241 @itemx --group=@var{group}
8244 @cindex group ownership of installed files, setting
8245 Set the group ownership of installed files or directories to
8246 @var{group}. The default is the process's current group. @var{group}
8247 may be either a group name or a numeric group ID.
8250 @itemx --mode=@var{mode}
8253 @cindex permissions of installed files, setting
8254 Set the file mode bits for the installed file or directory to @var{mode},
8255 which can be either an octal number, or a symbolic mode as in
8256 @command{chmod}, with @samp{a=} (no access allowed to anyone) as the
8257 point of departure (@pxref{File permissions}).
8258 The default mode is @samp{u=rwx,go=rx,a-s}---read, write, and
8259 execute for the owner, read and execute for group and other, and with
8260 set-user-ID and set-group-ID disabled.
8261 This default is not quite the same as @samp{755}, since it disables
8262 instead of preserving set-user-ID and set-group-ID on directories.
8263 @xref{Directory Setuid and Setgid}.
8265 @item -o @var{owner}
8266 @itemx --owner=@var{owner}
8269 @cindex ownership of installed files, setting
8270 @cindex appropriate privileges
8271 @vindex root @r{as default owner}
8272 If @command{install} has appropriate privileges (is run as root), set the
8273 ownership of installed files or directories to @var{owner}. The default
8274 is @code{root}. @var{owner} may be either a user name or a numeric user
8277 @item --preserve-context
8278 @opindex --preserve-context
8280 @cindex security context
8281 Preserve the SELinux security context of files and directories.
8282 Failure to preserve the context in all of the files or directories
8283 will result in an exit status of 1. If SELinux is disabled then
8284 print a warning and ignore the option.
8287 @itemx --preserve-timestamps
8289 @opindex --preserve-timestamps
8290 @cindex timestamps of installed files, preserving
8291 Set the time of last access and the time of last modification of each
8292 installed file to match those of each corresponding original file.
8293 When a file is installed without this option, its last access and
8294 last modification times are both set to the time of installation.
8295 This option is useful if you want to use the last modification times
8296 of installed files to keep track of when they were last built as opposed
8297 to when they were last installed.
8303 @cindex symbol table information, stripping
8304 @cindex stripping symbol table information
8305 Strip the symbol tables from installed binary executables.
8307 @itemx --strip-program=@var{program}
8308 @opindex --strip-program
8309 @cindex symbol table information, stripping, program
8310 Program used to strip binaries.
8316 @optNoTargetDirectory
8322 Print the name of each file before copying it.
8324 @item -Z @var{context}
8325 @itemx --context=@var{context}
8329 @cindex security context
8330 Set the default SELinux security context to be used for any
8331 created files and directories. If SELinux is disabled then
8332 print a warning and ignore the option.
8340 @section @command{mv}: Move (rename) files
8344 @command{mv} moves or renames files (or directories). Synopses:
8347 mv [@var{option}]@dots{} [-T] @var{source} @var{dest}
8348 mv [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8349 mv [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8354 If two file names are given, @command{mv} moves the first file to the
8358 If the @option{--target-directory} (@option{-t}) option is given, or
8359 failing that if the last file is a directory and the
8360 @option{--no-target-directory} (@option{-T}) option is not given,
8361 @command{mv} moves each @var{source} file to the specified
8362 directory, using the @var{source}s' names.
8365 @command{mv} can move any type of file from one file system to another.
8366 Prior to version @code{4.0} of the fileutils,
8367 @command{mv} could move only regular files between file systems.
8368 For example, now @command{mv} can move an entire directory hierarchy
8369 including special device files from one partition to another. It first
8370 uses some of the same code that's used by @code{cp -a} to copy the
8371 requested directories and files, then (assuming the copy succeeded)
8372 it removes the originals. If the copy fails, then the part that was
8373 copied to the destination partition is removed. If you were to copy
8374 three directories from one partition to another and the copy of the first
8375 directory succeeded, but the second didn't, the first would be left on
8376 the destination partition and the second and third would be left on the
8379 @cindex extended attributes, xattr
8380 @command{mv} always tries to copy extended attributes (xattr), which may
8381 include SELinux context, ACLs or Capabilities.
8382 Upon failure all but @samp{Operation not supported} warnings are output.
8384 @cindex prompting, and @command{mv}
8385 If a destination file exists but is normally unwritable, standard input
8386 is a terminal, and the @option{-f} or @option{--force} option is not given,
8387 @command{mv} prompts the user for whether to replace the file. (You might
8388 own the file, or have write permission on its directory.) If the
8389 response is not affirmative, the file is skipped.
8391 @emph{Warning}: Avoid specifying a source name with a trailing slash,
8392 when it might be a symlink to a directory.
8393 Otherwise, @command{mv} may do something very surprising, since
8394 its behavior depends on the underlying rename system call.
8395 On a system with a modern Linux-based kernel, it fails with @code{errno=ENOTDIR}.
8396 However, on other systems (at least FreeBSD 6.1 and Solaris 10) it silently
8397 renames not the symlink but rather the directory referenced by the symlink.
8398 @xref{Trailing slashes}.
8400 The program accepts the following options. Also see @ref{Common options}.
8410 @cindex prompts, omitting
8411 Do not prompt the user before removing a destination file.
8413 If you specify more than one of the @option{-i}, @option{-f}, @option{-n}
8414 options, only the final one takes effect.
8419 @itemx --interactive
8421 @opindex --interactive
8422 @cindex prompts, forcing
8423 Prompt whether to overwrite each existing destination file, regardless
8425 If the response is not affirmative, the file is skipped.
8431 @opindex --no-clobber
8432 @cindex prompts, omitting
8433 Do not overwrite an existing file.
8435 This option is mutually exclusive with @option{-b} or @option{--backup} option.
8441 @cindex newer files, moving only
8442 Do not move a non-directory that has an existing destination with the
8443 same or newer modification time.
8444 If the move is across file system boundaries, the comparison is to the
8445 source time stamp truncated to the resolutions of the destination file
8446 system and of the system calls used to update time stamps; this avoids
8447 duplicate work if several @samp{mv -u} commands are executed with the
8448 same source and destination.
8454 Print the name of each file before moving it.
8456 @optStripTrailingSlashes
8462 @optNoTargetDirectory
8470 @section @command{rm}: Remove files or directories
8473 @cindex removing files or directories
8475 @command{rm} removes each given @var{file}. By default, it does not remove
8476 directories. Synopsis:
8479 rm [@var{option}]@dots{} [@var{file}]@dots{}
8482 @cindex prompting, and @command{rm}
8483 If the @option{-I} or @option{--interactive=once} option is given,
8484 and there are more than three files or the @option{-r}, @option{-R},
8485 or @option{--recursive} are given, then @command{rm} prompts the user
8486 for whether to proceed with the entire operation. If the response is
8487 not affirmative, the entire command is aborted.
8489 Otherwise, if a file is unwritable, standard input is a terminal, and
8490 the @option{-f} or @option{--force} option is not given, or the
8491 @option{-i} or @option{--interactive=always} option @emph{is} given,
8492 @command{rm} prompts the user for whether to remove the file.
8493 If the response is not affirmative, the file is skipped.
8495 Any attempt to remove a file whose last file name component is
8496 @file{.} or @file{..} is rejected without any prompting.
8498 @emph{Warning}: If you use @command{rm} to remove a file, it is usually
8499 possible to recover the contents of that file. If you want more assurance
8500 that the contents are truly unrecoverable, consider using @command{shred}.
8502 The program accepts the following options. Also see @ref{Common options}.
8510 Ignore nonexistent files and never prompt the user.
8511 Ignore any previous @option{--interactive} (@option{-i}) option.
8515 Prompt whether to remove each file.
8516 If the response is not affirmative, the file is skipped.
8517 Ignore any previous @option{--force} (@option{-f}) option.
8518 Equivalent to @option{--interactive=always}.
8522 Prompt once whether to proceed with the command, if more than three
8523 files are named or if a recursive removal is requested. Ignore any
8524 previous @option{--force} (@option{-f}) option. Equivalent to
8525 @option{--interactive=once}.
8527 @itemx --interactive [=@var{when}]
8528 @opindex --interactive
8529 Specify when to issue an interactive prompt. @var{when} may be
8533 @vindex never @r{interactive option}
8534 - Do not prompt at all.
8536 @vindex once @r{interactive option}
8537 - Prompt once if more than three files are named or if a recursive
8538 removal is requested. Equivalent to @option{-I}.
8540 @vindex always @r{interactive option}
8541 - Prompt for every file being removed. Equivalent to @option{-i}.
8543 @option{--interactive} with no @var{when} is equivalent to
8544 @option{--interactive=always}.
8546 @itemx --one-file-system
8547 @opindex --one-file-system
8548 @cindex one file system, restricting @command{rm} to
8549 When removing a hierarchy recursively, skip any directory that is on a
8550 file system different from that of the corresponding command line argument.
8553 This option is useful when removing a build ``chroot'' hierarchy,
8554 which normally contains no valuable data. However, it is not uncommon
8555 to bind-mount @file{/home} into such a hierarchy, to make it easier to
8556 use one's start-up file. The catch is that it's easy to forget to
8557 unmount @file{/home}. Then, when you use @command{rm -rf} to remove
8558 your normally throw-away chroot, that command will remove everything
8559 under @file{/home}, too.
8560 Use the @option{--one-file-system} option, and it will
8561 warn about and skip directories on other file systems.
8562 Of course, this will not save your @file{/home} if it and your
8563 chroot happen to be on the same file system.
8565 @itemx --preserve-root
8566 @opindex --preserve-root
8567 @cindex root directory, disallow recursive destruction
8568 Fail upon any attempt to remove the root directory, @file{/},
8569 when used with the @option{--recursive} option.
8570 This is the default behavior.
8571 @xref{Treating / specially}.
8573 @itemx --no-preserve-root
8574 @opindex --no-preserve-root
8575 @cindex root directory, allow recursive destruction
8576 Do not treat @file{/} specially when removing recursively.
8577 This option is not recommended unless you really want to
8578 remove all the files on your computer.
8579 @xref{Treating / specially}.
8586 @opindex --recursive
8587 @cindex directories, removing (recursively)
8588 Remove the listed directories and their contents recursively.
8594 Print the name of each file before removing it.
8598 @cindex files beginning with @samp{-}, removing
8599 @cindex @samp{-}, removing files beginning with
8600 One common question is how to remove files whose names begin with a
8601 @samp{-}. @sc{gnu} @command{rm}, like every program that uses the @code{getopt}
8602 function to parse its arguments, lets you use the @samp{--} option to
8603 indicate that all following arguments are non-options. To remove a file
8604 called @file{-f} in the current directory, you could type either:
8617 @opindex - @r{and Unix @command{rm}}
8618 The Unix @command{rm} program's use of a single @samp{-} for this purpose
8619 predates the development of the getopt standard syntax.
8624 @node shred invocation
8625 @section @command{shred}: Remove files more securely
8628 @cindex data, erasing
8629 @cindex erasing data
8631 @command{shred} overwrites devices or files, to help prevent even
8632 very expensive hardware from recovering the data.
8634 Ordinarily when you remove a file (@pxref{rm invocation}), the data is
8635 not actually destroyed. Only the index listing where the file is
8636 stored is destroyed, and the storage is made available for reuse.
8637 There are undelete utilities that will attempt to reconstruct the index
8638 and can bring the file back if the parts were not reused.
8640 On a busy system with a nearly-full drive, space can get reused in a few
8641 seconds. But there is no way to know for sure. If you have sensitive
8642 data, you may want to be sure that recovery is not possible by actually
8643 overwriting the file with non-sensitive data.
8645 However, even after doing that, it is possible to take the disk back
8646 to a laboratory and use a lot of sensitive (and expensive) equipment
8647 to look for the faint ``echoes'' of the original data underneath the
8648 overwritten data. If the data has only been overwritten once, it's not
8651 The best way to remove something irretrievably is to destroy the media
8652 it's on with acid, melt it down, or the like. For cheap removable media
8653 like floppy disks, this is the preferred method. However, hard drives
8654 are expensive and hard to melt, so the @command{shred} utility tries
8655 to achieve a similar effect non-destructively.
8657 This uses many overwrite passes, with the data patterns chosen to
8658 maximize the damage they do to the old data. While this will work on
8659 floppies, the patterns are designed for best effect on hard drives.
8660 For more details, see the source code and Peter Gutmann's paper
8661 @uref{http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html,
8662 @cite{Secure Deletion of Data from Magnetic and Solid-State Memory}},
8663 from the proceedings of the Sixth @acronym{USENIX} Security Symposium (San Jose,
8664 California, July 22--25, 1996).
8666 @strong{Please note} that @command{shred} relies on a very important assumption:
8667 that the file system overwrites data in place. This is the traditional
8668 way to do things, but many modern file system designs do not satisfy this
8669 assumption. Exceptions include:
8674 Log-structured or journaled file systems, such as those supplied with
8675 AIX and Solaris, and JFS, ReiserFS, XFS, Ext3 (in @code{data=journal} mode),
8676 BFS, NTFS, etc.@: when they are configured to journal @emph{data}.
8679 File systems that write redundant data and carry on even if some writes
8680 fail, such as RAID-based file systems.
8683 File systems that make snapshots, such as Network Appliance's NFS server.
8686 File systems that cache in temporary locations, such as NFS version 3
8690 Compressed file systems.
8693 In the particular case of ext3 file systems, the above disclaimer applies (and
8694 @command{shred} is thus of limited effectiveness) only in @code{data=journal}
8695 mode, which journals file data in addition to just metadata. In both
8696 the @code{data=ordered} (default) and @code{data=writeback} modes,
8697 @command{shred} works as usual. Ext3 journaling modes can be changed
8698 by adding the @code{data=something} option to the mount options for a
8699 particular file system in the @file{/etc/fstab} file, as documented in
8700 the mount man page (man mount).
8702 If you are not sure how your file system operates, then you should assume
8703 that it does not overwrite data in place, which means that shred cannot
8704 reliably operate on regular files in your file system.
8706 Generally speaking, it is more reliable to shred a device than a file,
8707 since this bypasses the problem of file system design mentioned above.
8708 However, even shredding devices is not always completely reliable. For
8709 example, most disks map out bad sectors invisibly to the application; if
8710 the bad sectors contain sensitive data, @command{shred} won't be able to
8713 @command{shred} makes no attempt to detect or report this problem, just as
8714 it makes no attempt to do anything about backups. However, since it is
8715 more reliable to shred devices than files, @command{shred} by default does
8716 not truncate or remove the output file. This default is more suitable
8717 for devices, which typically cannot be truncated and should not be
8720 Finally, consider the risk of backups and mirrors.
8721 File system backups and remote mirrors may contain copies of the
8722 file that cannot be removed, and that will allow a shredded file
8723 to be recovered later. So if you keep any data you may later want
8724 to destroy using @command{shred}, be sure that it is not backed up or mirrored.
8727 shred [@var{option}]@dots{} @var{file}[@dots{}]
8730 The program accepts the following options. Also see @ref{Common options}.
8738 @cindex force deletion
8739 Override file permissions if necessary to allow overwriting.
8742 @itemx -n @var{number}
8743 @itemx --iterations=@var{number}
8744 @opindex -n @var{number}
8745 @opindex --iterations=@var{number}
8746 @cindex iterations, selecting the number of
8747 By default, @command{shred} uses @value{SHRED_DEFAULT_PASSES} passes of
8748 overwrite. You can reduce this to save time, or increase it if you think it's
8749 appropriate. After 25 passes all of the internal overwrite patterns will have
8750 been used at least once.
8752 @item --random-source=@var{file}
8753 @opindex --random-source
8754 @cindex random source for shredding
8755 Use @var{file} as a source of random data used to overwrite and to
8756 choose pass ordering. @xref{Random sources}.
8758 @item -s @var{bytes}
8759 @itemx --size=@var{bytes}
8760 @opindex -s @var{bytes}
8761 @opindex --size=@var{bytes}
8762 @cindex size of file to shred
8763 Shred the first @var{bytes} bytes of the file. The default is to shred
8764 the whole file. @var{bytes} can be followed by a size specification like
8765 @samp{K}, @samp{M}, or @samp{G} to specify a multiple. @xref{Block size}.
8771 @cindex removing files after shredding
8772 After shredding a file, truncate it (if possible) and then remove it.
8773 If a file has multiple links, only the named links will be removed.
8779 Display to standard error all status updates as sterilization proceeds.
8785 By default, @command{shred} rounds the size of a regular file up to the next
8786 multiple of the file system block size to fully erase the last block of the file.
8787 Use @option{--exact} to suppress that behavior.
8788 Thus, by default if you shred a 10-byte regular file on a system with 512-byte
8789 blocks, the resulting file will be 512 bytes long. With this option,
8790 shred does not increase the apparent size of the file.
8796 Normally, the last pass that @command{shred} writes is made up of
8797 random data. If this would be conspicuous on your hard drive (for
8798 example, because it looks like encrypted data), or you just think
8799 it's tidier, the @option{--zero} option adds an additional overwrite pass with
8800 all zero bits. This is in addition to the number of passes specified
8801 by the @option{--iterations} option.
8805 You might use the following command to erase all trace of the
8806 file system you'd created on the floppy disk in your first drive.
8807 That command takes about 20 minutes to erase a ``1.44MB'' (actually
8811 shred --verbose /dev/fd0
8814 Similarly, to erase all data on a selected partition of
8815 your hard disk, you could give a command like this:
8818 shred --verbose /dev/sda5
8821 A @var{file} of @samp{-} denotes standard output.
8822 The intended use of this is to shred a removed temporary file.
8829 echo "Hello, world" >&3
8834 However, the command @samp{shred - >file} does not shred the contents
8835 of @var{file}, since the shell truncates @var{file} before invoking
8836 @command{shred}. Use the command @samp{shred file} or (if using a
8837 Bourne-compatible shell) the command @samp{shred - 1<>file} instead.
8842 @node Special file types
8843 @chapter Special file types
8845 @cindex special file types
8846 @cindex file types, special
8848 This chapter describes commands which create special types of files (and
8849 @command{rmdir}, which removes directories, one special file type).
8851 @cindex special file types
8853 Although Unix-like operating systems have markedly fewer special file
8854 types than others, not @emph{everything} can be treated only as the
8855 undifferentiated byte stream of @dfn{normal files}. For example, when a
8856 file is created or removed, the system must record this information,
8857 which it does in a @dfn{directory}---a special type of file. Although
8858 you can read directories as normal files, if you're curious, in order
8859 for the system to do its job it must impose a structure, a certain
8860 order, on the bytes of the file. Thus it is a ``special'' type of file.
8862 Besides directories, other special file types include named pipes
8863 (FIFOs), symbolic links, sockets, and so-called @dfn{special files}.
8866 * link invocation:: Make a hard link via the link syscall
8867 * ln invocation:: Make links between files.
8868 * mkdir invocation:: Make directories.
8869 * mkfifo invocation:: Make FIFOs (named pipes).
8870 * mknod invocation:: Make block or character special files.
8871 * readlink invocation:: Print value of a symlink or canonical file name.
8872 * rmdir invocation:: Remove empty directories.
8873 * unlink invocation:: Remove files via the unlink syscall
8877 @node link invocation
8878 @section @command{link}: Make a hard link via the link syscall
8881 @cindex links, creating
8882 @cindex hard links, creating
8883 @cindex creating links (hard only)
8885 @command{link} creates a single hard link at a time.
8886 It is a minimalist interface to the system-provided
8887 @code{link} function. @xref{Hard Links, , , libc,
8888 The GNU C Library Reference Manual}.
8889 It avoids the bells and whistles of the more commonly-used
8890 @command{ln} command (@pxref{ln invocation}).
8894 link @var{filename} @var{linkname}
8897 @var{filename} must specify an existing file, and @var{linkname}
8898 must specify a nonexistent entry in an existing directory.
8899 @command{link} simply calls @code{link (@var{filename}, @var{linkname})}
8902 On a @acronym{GNU} system, this command acts like @samp{ln --directory
8903 --no-target-directory @var{filename} @var{linkname}}. However, the
8904 @option{--directory} and @option{--no-target-directory} options are
8905 not specified by @acronym{POSIX}, and the @command{link} command is
8906 more portable in practice.
8908 If @var{filename} is a symbolic link, it is unspecified whether
8909 @var{linkname} will be a hard link to the symbolic link or to the
8910 target of the symbolic link. Use @command{ln -P} or @command{ln -L}
8911 to specify which behavior is desired.
8917 @section @command{ln}: Make links between files
8920 @cindex links, creating
8921 @cindex hard links, creating
8922 @cindex symbolic (soft) links, creating
8923 @cindex creating links (hard or soft)
8925 @cindex file systems and hard links
8926 @command{ln} makes links between files. By default, it makes hard links;
8927 with the @option{-s} option, it makes symbolic (or @dfn{soft}) links.
8931 ln [@var{option}]@dots{} [-T] @var{target} @var{linkname}
8932 ln [@var{option}]@dots{} @var{target}
8933 ln [@var{option}]@dots{} @var{target}@dots{} @var{directory}
8934 ln [@var{option}]@dots{} -t @var{directory} @var{target}@dots{}
8940 If two file names are given, @command{ln} creates a link to the first
8941 file from the second.
8944 If one @var{target} is given, @command{ln} creates a link to that file
8945 in the current directory.
8948 If the @option{--target-directory} (@option{-t}) option is given, or
8949 failing that if the last file is a directory and the
8950 @option{--no-target-directory} (@option{-T}) option is not given,
8951 @command{ln} creates a link to each @var{target} file in the specified
8952 directory, using the @var{target}s' names.
8956 Normally @command{ln} does not remove existing files. Use the
8957 @option{--force} (@option{-f}) option to remove them unconditionally,
8958 the @option{--interactive} (@option{-i}) option to remove them
8959 conditionally, and the @option{--backup} (@option{-b}) option to
8962 @cindex hard link, defined
8963 @cindex inode, and hard links
8964 A @dfn{hard link} is another name for an existing file; the link and the
8965 original are indistinguishable. Technically speaking, they share the
8966 same inode, and the inode contains all the information about a
8967 file---indeed, it is not incorrect to say that the inode @emph{is} the
8968 file. Most systems prohibit making a hard link to
8969 a directory; on those where it is allowed, only the super-user can do
8970 so (and with caution, since creating a cycle will cause problems to many
8971 other utilities). Hard links cannot cross file system boundaries. (These
8972 restrictions are not mandated by @acronym{POSIX}, however.)
8974 @cindex dereferencing symbolic links
8975 @cindex symbolic link, defined
8976 @dfn{Symbolic links} (@dfn{symlinks} for short), on the other hand, are
8977 a special file type (which not all kernels support: System V release 3
8978 (and older) systems lack symlinks) in which the link file actually
8979 refers to a different file, by name. When most operations (opening,
8980 reading, writing, and so on) are passed the symbolic link file, the
8981 kernel automatically @dfn{dereferences} the link and operates on the
8982 target of the link. But some operations (e.g., removing) work on the
8983 link file itself, rather than on its target. The owner and group of a
8984 symlink are not significant to file access performed through
8985 the link, but do have implications on deleting a symbolic link from a
8986 directory with the restricted deletion bit set. On the GNU system,
8987 the mode of a symlink has no significance and cannot be changed, but
8988 on some BSD systems, the mode can be changed and will affect whether
8989 the symlink will be traversed in file name resolution. @xref{Symbolic Links,,,
8990 libc, The GNU C Library Reference Manual}.
8992 Symbolic links can contain arbitrary strings; a @dfn{dangling symlink}
8993 occurs when the string in the symlink does not resolve to a file.
8994 There are no restrictions against creating dangling symbolic links.
8995 There are trade-offs to using absolute or relative symlinks. An
8996 absolute symlink always points to the same file, even if the directory
8997 containing the link is moved. However, if the symlink is visible from
8998 more than one machine (such as on a networked file system), the file
8999 pointed to might not always be the same. A relative symbolic link is
9000 resolved in relation to the directory that contains the link, and is
9001 often useful in referring to files on the same device without regards
9002 to what name that device is mounted on when accessed via networked
9005 When creating a relative symlink in a different location than the
9006 current directory, the resolution of the symlink will be different
9007 than the resolution of the same string from the current directory.
9008 Therefore, many users prefer to first change directories to the
9009 location where the relative symlink will be created, so that
9010 tab-completion or other file resolution will find the same target as
9011 what will be placed in the symlink.
9013 The program accepts the following options. Also see @ref{Common options}.
9024 @opindex --directory
9025 @cindex hard links to directories
9026 Allow users with appropriate privileges to attempt to make hard links
9028 However, note that this will probably fail due to
9029 system restrictions, even for the super-user.
9035 Remove existing destination files.
9038 @itemx --interactive
9040 @opindex --interactive
9041 @cindex prompting, and @command{ln}
9042 Prompt whether to remove existing destination files.
9048 If @option{-s} is not in effect, and the source file is a symbolic
9049 link, create the hard link to the file referred to by the symbolic
9050 link, rather than the symbolic link itself.
9053 @itemx --no-dereference
9055 @opindex --no-dereference
9056 Do not treat the last operand specially when it is a symbolic link to
9057 a directory. Instead, treat it as if it were a normal file.
9059 When the destination is an actual directory (not a symlink to one),
9060 there is no ambiguity. The link is created in that directory.
9061 But when the specified destination is a symlink to a directory,
9062 there are two ways to treat the user's request. @command{ln} can
9063 treat the destination just as it would a normal directory and create
9064 the link in it. On the other hand, the destination can be viewed as a
9065 non-directory---as the symlink itself. In that case, @command{ln}
9066 must delete or backup that symlink before creating the new link.
9067 The default is to treat a destination that is a symlink to a directory
9068 just like a directory.
9070 This option is weaker than the @option{--no-target-directory}
9071 (@option{-T}) option, so it has no effect if both options are given.
9077 If @option{-s} is not in effect, and the source file is a symbolic
9078 link, create the hard link to the symbolic link itself. On platforms
9079 where this is not supported by the kernel, this option creates a
9080 symbolic link with identical contents; since symbolic link contents
9081 cannot be edited, any file name resolution performed through either
9082 link will be the same as if a hard link had been created.
9088 Make symbolic links instead of hard links. This option merely produces
9089 an error message on systems that do not support symbolic links.
9095 @optNoTargetDirectory
9101 Print the name of each file after linking it successfully.
9105 @cindex hard links to symbolic links
9106 @cindex symbolic links and @command{ln}
9107 If @option{-L} and @option{-P} are both given, the last one takes
9108 precedence. If @option{-s} is also given, @option{-L} and @option{-P}
9109 are silently ignored. If neither option is given, then this
9110 implementation defaults to @option{-P} if the system @code{link} supports
9111 hard links to symbolic links (such as the GNU system), and @option{-L}
9112 if @code{link} follows symbolic links (such as on BSD).
9121 # Create link ../a pointing to a in that directory.
9122 # Not really useful because it points to itself.
9127 # Change to the target before creating symlinks to avoid being confused.
9133 # Hard coded file names don't move well.
9134 ln -s $(pwd)/a /some/dir/
9138 # Relative file names survive directory moves and also
9139 # work across networked file systems.
9140 ln -s afile anotherfile
9141 ln -s ../adir/afile yetanotherfile
9145 @node mkdir invocation
9146 @section @command{mkdir}: Make directories
9149 @cindex directories, creating
9150 @cindex creating directories
9152 @command{mkdir} creates directories with the specified names. Synopsis:
9155 mkdir [@var{option}]@dots{} @var{name}@dots{}
9158 @command{mkdir} creates each directory @var{name} in the order given.
9159 It reports an error if @var{name} already exists, unless the
9160 @option{-p} option is given and @var{name} is a directory.
9162 The program accepts the following options. Also see @ref{Common options}.
9167 @itemx --mode=@var{mode}
9170 @cindex modes of created directories, setting
9171 Set the file permission bits of created directories to @var{mode},
9172 which uses the same syntax as
9173 in @command{chmod} and uses @samp{a=rwx} (read, write and execute allowed for
9174 everyone) for the point of the departure. @xref{File permissions}.
9176 Normally the directory has the desired file mode bits at the moment it
9177 is created. As a @acronym{GNU} extension, @var{mode} may also mention
9178 special mode bits, but in this case there may be a temporary window
9179 during which the directory exists but its special mode bits are
9180 incorrect. @xref{Directory Setuid and Setgid}, for how the
9181 set-user-ID and set-group-ID bits of directories are inherited unless
9182 overridden in this way.
9188 @cindex parent directories, creating
9189 Make any missing parent directories for each argument, setting their
9190 file permission bits to the umask modified by @samp{u+wx}. Ignore
9191 existing parent directories, and do not change their file permission
9194 To set the file permission bits of any newly-created parent
9195 directories to a value that includes @samp{u+wx}, you can set the
9196 umask before invoking @command{mkdir}. For example, if the shell
9197 command @samp{(umask u=rwx,go=rx; mkdir -p P/Q)} creates the parent
9198 @file{P} it sets the parent's permission bits to @samp{u=rwx,go=rx}.
9199 To set a parent's special mode bits as well, you can invoke
9200 @command{chmod} after @command{mkdir}. @xref{Directory Setuid and
9201 Setgid}, for how the set-user-ID and set-group-ID bits of
9202 newly-created parent directories are inherited.
9208 Print a message for each created directory. This is most useful with
9211 @item -Z @var{context}
9212 @itemx --context=@var{context}
9216 @cindex security context
9217 Set the default SELinux security context to be used for created directories.
9224 @node mkfifo invocation
9225 @section @command{mkfifo}: Make FIFOs (named pipes)
9228 @cindex FIFOs, creating
9229 @cindex named pipes, creating
9230 @cindex creating FIFOs (named pipes)
9232 @command{mkfifo} creates FIFOs (also called @dfn{named pipes}) with the
9233 specified names. Synopsis:
9236 mkfifo [@var{option}] @var{name}@dots{}
9239 A @dfn{FIFO} is a special file type that permits independent processes
9240 to communicate. One process opens the FIFO file for writing, and
9241 another for reading, after which data can flow as with the usual
9242 anonymous pipe in shells or elsewhere.
9244 The program accepts the following option. Also see @ref{Common options}.
9249 @itemx --mode=@var{mode}
9252 @cindex modes of created FIFOs, setting
9253 Set the mode of created FIFOs to @var{mode}, which is symbolic as in
9254 @command{chmod} and uses @samp{a=rw} (read and write allowed for everyone)
9255 for the point of departure. @var{mode} should specify only file
9256 permission bits. @xref{File permissions}.
9258 @item -Z @var{context}
9259 @itemx --context=@var{context}
9263 @cindex security context
9264 Set the default SELinux security context to be used for created FIFOs.
9271 @node mknod invocation
9272 @section @command{mknod}: Make block or character special files
9275 @cindex block special files, creating
9276 @cindex character special files, creating
9278 @command{mknod} creates a FIFO, character special file, or block special
9279 file with the specified name. Synopsis:
9282 mknod [@var{option}]@dots{} @var{name} @var{type} [@var{major} @var{minor}]
9285 @cindex special files
9286 @cindex block special files
9287 @cindex character special files
9288 Unlike the phrase ``special file type'' above, the term @dfn{special
9289 file} has a technical meaning on Unix: something that can generate or
9290 receive data. Usually this corresponds to a physical piece of hardware,
9291 e.g., a printer or a disk. (These files are typically created at
9292 system-configuration time.) The @command{mknod} command is what creates
9293 files of this type. Such devices can be read either a character at a
9294 time or a ``block'' (many characters) at a time, hence we say there are
9295 @dfn{block special} files and @dfn{character special} files.
9297 @c mknod is a shell built-in at least with OpenBSD's /bin/sh
9298 @mayConflictWithShellBuiltIn{mknod}
9300 The arguments after @var{name} specify the type of file to make:
9305 @opindex p @r{for FIFO file}
9309 @opindex b @r{for block special file}
9310 for a block special file
9313 @c Don't document the `u' option -- it's just a synonym for `c'.
9314 @c Do *any* versions of mknod still use it?
9316 @opindex c @r{for character special file}
9317 @c @opindex u @r{for character special file}
9318 for a character special file
9322 When making a block or character special file, the major and minor
9323 device numbers must be given after the file type.
9324 If a major or minor device number begins with @samp{0x} or @samp{0X},
9325 it is interpreted as hexadecimal; otherwise, if it begins with @samp{0},
9326 as octal; otherwise, as decimal.
9328 The program accepts the following option. Also see @ref{Common options}.
9333 @itemx --mode=@var{mode}
9336 Set the mode of created files to @var{mode}, which is symbolic as in
9337 @command{chmod} and uses @samp{a=rw} as the point of departure.
9338 @var{mode} should specify only file permission bits.
9339 @xref{File permissions}.
9341 @item -Z @var{context}
9342 @itemx --context=@var{context}
9346 @cindex security context
9347 Set the default SELinux security context to be used for created files.
9354 @node readlink invocation
9355 @section @command{readlink}: Print value of a symlink or canonical file name
9358 @cindex displaying value of a symbolic link
9359 @cindex canonical file name
9360 @cindex canonicalize a file name
9364 @command{readlink} may work in one of two supported modes:
9370 @command{readlink} outputs the value of the given symbolic link.
9371 If @command{readlink} is invoked with an argument other than the name
9372 of a symbolic link, it produces no output and exits with a nonzero exit code.
9374 @item Canonicalize mode
9376 @command{readlink} outputs the absolute name of the given file which contains
9377 no @file{.}, @file{..} components nor any repeated separators
9378 (@file{/}) or symbolic links.
9383 readlink [@var{option}] @var{file}
9386 By default, @command{readlink} operates in readlink mode.
9388 The program accepts the following options. Also see @ref{Common options}.
9393 @itemx --canonicalize
9395 @opindex --canonicalize
9396 Activate canonicalize mode.
9397 If any component of the file name except the last one is missing or unavailable,
9398 @command{readlink} produces no output and exits with a nonzero exit
9399 code. A trailing slash is ignored.
9402 @itemx --canonicalize-existing
9404 @opindex --canonicalize-existing
9405 Activate canonicalize mode.
9406 If any component is missing or unavailable, @command{readlink} produces
9407 no output and exits with a nonzero exit code. A trailing slash
9408 requires that the name resolve to a directory.
9411 @itemx --canonicalize-missing
9413 @opindex --canonicalize-missing
9414 Activate canonicalize mode.
9415 If any component is missing or unavailable, @command{readlink} treats it
9421 @opindex --no-newline
9422 Do not output the trailing newline.
9432 Suppress most error messages.
9438 Report error messages.
9442 The @command{readlink} utility first appeared in OpenBSD 2.1.
9444 There is a @command{realpath} command on some systems
9445 which operates like @command{readlink} in canonicalize mode.
9450 @node rmdir invocation
9451 @section @command{rmdir}: Remove empty directories
9454 @cindex removing empty directories
9455 @cindex directories, removing empty
9457 @command{rmdir} removes empty directories. Synopsis:
9460 rmdir [@var{option}]@dots{} @var{directory}@dots{}
9463 If any @var{directory} argument does not refer to an existing empty
9464 directory, it is an error.
9466 The program accepts the following options. Also see @ref{Common options}.
9470 @item --ignore-fail-on-non-empty
9471 @opindex --ignore-fail-on-non-empty
9472 @cindex directory deletion, ignoring failures
9473 Ignore each failure to remove a directory that is solely because
9474 the directory is non-empty.
9480 @cindex parent directories, removing
9481 Remove @var{directory}, then try to remove each component of @var{directory}.
9482 So, for example, @samp{rmdir -p a/b/c} is similar to @samp{rmdir a/b/c a/b a}.
9483 As such, it fails if any of those directories turns out not to be empty.
9484 Use the @option{--ignore-fail-on-non-empty} option to make it so such
9485 a failure does not evoke a diagnostic and does not cause @command{rmdir} to
9486 exit unsuccessfully.
9492 @cindex directory deletion, reporting
9493 Give a diagnostic for each successful removal.
9494 @var{directory} is removed.
9498 @xref{rm invocation}, for how to remove non-empty directories (recursively).
9503 @node unlink invocation
9504 @section @command{unlink}: Remove files via the unlink syscall
9507 @cindex removing files or directories (via the unlink syscall)
9509 @command{unlink} deletes a single specified file name.
9510 It is a minimalist interface to the system-provided
9511 @code{unlink} function. @xref{Deleting Files, , , libc,
9512 The GNU C Library Reference Manual}. Synopsis:
9513 It avoids the bells and whistles of the more commonly-used
9514 @command{rm} command (@pxref{rm invocation}).
9517 unlink @var{filename}
9520 On some systems @code{unlink} can be used to delete the name of a
9521 directory. On others, it can be used that way only by a privileged user.
9522 In the GNU system @code{unlink} can never delete the name of a directory.
9524 The @command{unlink} command honors the @option{--help} and
9525 @option{--version} options. To remove a file whose name begins with
9526 @samp{-}, prefix the name with @samp{./}, e.g., @samp{unlink ./--help}.
9531 @node Changing file attributes
9532 @chapter Changing file attributes
9534 @cindex changing file attributes
9535 @cindex file attributes, changing
9536 @cindex attributes, file
9538 A file is not merely its contents, a name, and a file type
9539 (@pxref{Special file types}). A file also has an owner (a user ID), a
9540 group (a group ID), permissions (what the owner can do with the file,
9541 what people in the group can do, and what everyone else can do), various
9542 timestamps, and other information. Collectively, we call these a file's
9545 These commands change file attributes.
9548 * chgrp invocation:: Change file groups.
9549 * chmod invocation:: Change access permissions.
9550 * chown invocation:: Change file owners and groups.
9551 * touch invocation:: Change file timestamps.
9555 @node chown invocation
9556 @section @command{chown}: Change file owner and group
9559 @cindex file ownership, changing
9560 @cindex group ownership, changing
9561 @cindex changing file ownership
9562 @cindex changing group ownership
9564 @command{chown} changes the user and/or group ownership of each given @var{file}
9565 to @var{new-owner} or to the user and group of an existing reference file.
9569 chown [@var{option}]@dots{} @{@var{new-owner} | --reference=@var{ref_file}@} @var{file}@dots{}
9572 If used, @var{new-owner} specifies the new owner and/or group as follows
9573 (with no embedded white space):
9576 [@var{owner}] [ : [@var{group}] ]
9583 If only an @var{owner} (a user name or numeric user ID) is given, that
9584 user is made the owner of each given file, and the files' group is not
9587 @item owner@samp{:}group
9588 If the @var{owner} is followed by a colon and a @var{group} (a
9589 group name or numeric group ID), with no spaces between them, the group
9590 ownership of the files is changed as well (to @var{group}).
9593 If a colon but no group name follows @var{owner}, that user is
9594 made the owner of the files and the group of the files is changed to
9595 @var{owner}'s login group.
9598 If the colon and following @var{group} are given, but the owner
9599 is omitted, only the group of the files is changed; in this case,
9600 @command{chown} performs the same function as @command{chgrp}.
9603 If only a colon is given, or if @var{new-owner} is empty, neither the
9604 owner nor the group is changed.
9608 If @var{owner} or @var{group} is intended to represent a numeric user
9609 or group ID, then you may specify it with a leading @samp{+}.
9610 @xref{Disambiguating names and IDs}.
9612 Some older scripts may still use @samp{.} in place of the @samp{:} separator.
9613 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) does not
9614 require support for that, but for backward compatibility @acronym{GNU}
9615 @command{chown} supports @samp{.} so long as no ambiguity results.
9616 New scripts should avoid the use of @samp{.} because it is not
9617 portable, and because it has undesirable results if the entire
9618 @var{owner@samp{.}group} happens to identify a user whose name
9621 The @command{chown} command sometimes clears the set-user-ID or
9622 set-group-ID permission bits. This behavior depends on the policy and
9623 functionality of the underlying @code{chown} system call, which may
9624 make system-dependent file mode modifications outside the control of
9625 the @command{chown} command. For example, the @command{chown} command
9626 might not affect those bits when invoked by a user with appropriate
9627 privileges, or when the
9628 bits signify some function other than executable permission (e.g.,
9630 When in doubt, check the underlying system behavior.
9632 The program accepts the following options. Also see @ref{Common options}.
9640 @cindex changed owners, verbosely describing
9641 Verbosely describe the action for each @var{file} whose ownership
9650 @cindex error messages, omitting
9651 Do not print error messages about files whose ownership cannot be
9654 @itemx @w{@kbd{--from}=@var{old-owner}}
9656 @cindex symbolic links, changing owner
9657 Change a @var{file}'s ownership only if it has current attributes specified
9658 by @var{old-owner}. @var{old-owner} has the same form as @var{new-owner}
9660 This option is useful primarily from a security standpoint in that
9661 it narrows considerably the window of potential abuse.
9662 For example, to reflect a user ID numbering change for one user's files
9663 without an option like this, @code{root} might run
9666 find / -owner OLDUSER -print0 | xargs -0 chown -h NEWUSER
9669 But that is dangerous because the interval between when the @command{find}
9670 tests the existing file's owner and when the @command{chown} is actually run
9672 One way to narrow the gap would be to invoke chown for each file
9676 find / -owner OLDUSER -exec chown -h NEWUSER @{@} \;
9679 But that is very slow if there are many affected files.
9680 With this option, it is safer (the gap is narrower still)
9681 though still not perfect:
9684 chown -h -R --from=OLDUSER NEWUSER /
9688 @opindex --dereference
9689 @cindex symbolic links, changing owner
9691 Do not act on symbolic links themselves but rather on what they point to.
9692 This is the default.
9695 @itemx --no-dereference
9697 @opindex --no-dereference
9698 @cindex symbolic links, changing owner
9700 Act on symbolic links themselves instead of what they point to.
9701 This mode relies on the @code{lchown} system call.
9702 On systems that do not provide the @code{lchown} system call,
9703 @command{chown} fails when a file specified on the command line
9705 By default, no diagnostic is issued for symbolic links encountered
9706 during a recursive traversal, but see @option{--verbose}.
9708 @itemx --preserve-root
9709 @opindex --preserve-root
9710 @cindex root directory, disallow recursive modification
9711 Fail upon any attempt to recursively change the root directory, @file{/}.
9712 Without @option{--recursive}, this option has no effect.
9713 @xref{Treating / specially}.
9715 @itemx --no-preserve-root
9716 @opindex --no-preserve-root
9717 @cindex root directory, allow recursive modification
9718 Cancel the effect of any preceding @option{--preserve-root} option.
9719 @xref{Treating / specially}.
9721 @item --reference=@var{ref_file}
9722 @opindex --reference
9723 Change the user and group of each @var{file} to be the same as those of
9724 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9725 user and group of the symbolic link, but rather those of the file it
9732 Output a diagnostic for every file processed.
9733 If a symbolic link is encountered during a recursive traversal
9734 on a system without the @code{lchown} system call, and @option{--no-dereference}
9735 is in effect, then issue a diagnostic saying neither the symbolic link nor
9736 its referent is being changed.
9741 @opindex --recursive
9742 @cindex recursively changing file ownership
9743 Recursively change ownership of directories and their contents.
9746 @xref{Traversing symlinks}.
9749 @xref{Traversing symlinks}.
9752 @xref{Traversing symlinks}.
9761 # Change the owner of /u to "root".
9764 # Likewise, but also change its group to "staff".
9767 # Change the owner of /u and subfiles to "root".
9772 @node chgrp invocation
9773 @section @command{chgrp}: Change group ownership
9776 @cindex group ownership, changing
9777 @cindex changing group ownership
9779 @command{chgrp} changes the group ownership of each given @var{file}
9780 to @var{group} (which can be either a group name or a numeric group ID)
9781 or to the group of an existing reference file. Synopsis:
9784 chgrp [@var{option}]@dots{} @{@var{group} | --reference=@var{ref_file}@} @var{file}@dots{}
9787 If @var{group} is intended to represent a
9788 numeric group ID, then you may specify it with a leading @samp{+}.
9789 @xref{Disambiguating names and IDs}.
9791 The program accepts the following options. Also see @ref{Common options}.
9799 @cindex changed files, verbosely describing
9800 Verbosely describe the action for each @var{file} whose group actually
9809 @cindex error messages, omitting
9810 Do not print error messages about files whose group cannot be
9814 @opindex --dereference
9815 @cindex symbolic links, changing owner
9817 Do not act on symbolic links themselves but rather on what they point to.
9818 This is the default.
9821 @itemx --no-dereference
9823 @opindex --no-dereference
9824 @cindex symbolic links, changing group
9826 Act on symbolic links themselves instead of what they point to.
9827 This mode relies on the @code{lchown} system call.
9828 On systems that do not provide the @code{lchown} system call,
9829 @command{chgrp} fails when a file specified on the command line
9831 By default, no diagnostic is issued for symbolic links encountered
9832 during a recursive traversal, but see @option{--verbose}.
9834 @itemx --preserve-root
9835 @opindex --preserve-root
9836 @cindex root directory, disallow recursive modification
9837 Fail upon any attempt to recursively change the root directory, @file{/}.
9838 Without @option{--recursive}, this option has no effect.
9839 @xref{Treating / specially}.
9841 @itemx --no-preserve-root
9842 @opindex --no-preserve-root
9843 @cindex root directory, allow recursive modification
9844 Cancel the effect of any preceding @option{--preserve-root} option.
9845 @xref{Treating / specially}.
9847 @item --reference=@var{ref_file}
9848 @opindex --reference
9849 Change the group of each @var{file} to be the same as that of
9850 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9851 group of the symbolic link, but rather that of the file it refers to.
9857 Output a diagnostic for every file processed.
9858 If a symbolic link is encountered during a recursive traversal
9859 on a system without the @code{lchown} system call, and @option{--no-dereference}
9860 is in effect, then issue a diagnostic saying neither the symbolic link nor
9861 its referent is being changed.
9866 @opindex --recursive
9867 @cindex recursively changing group ownership
9868 Recursively change the group ownership of directories and their contents.
9871 @xref{Traversing symlinks}.
9874 @xref{Traversing symlinks}.
9877 @xref{Traversing symlinks}.
9886 # Change the group of /u to "staff".
9889 # Change the group of /u and subfiles to "staff".
9894 @node chmod invocation
9895 @section @command{chmod}: Change access permissions
9898 @cindex changing access permissions
9899 @cindex access permissions, changing
9900 @cindex permissions, changing access
9902 @command{chmod} changes the access permissions of the named files. Synopsis:
9905 chmod [@var{option}]@dots{} @{@var{mode} | --reference=@var{ref_file}@} @var{file}@dots{}
9908 @cindex symbolic links, permissions of
9909 @command{chmod} never changes the permissions of symbolic links, since
9910 the @command{chmod} system call cannot change their permissions.
9911 This is not a problem since the permissions of symbolic links are
9912 never used. However, for each symbolic link listed on the command
9913 line, @command{chmod} changes the permissions of the pointed-to file.
9914 In contrast, @command{chmod} ignores symbolic links encountered during
9915 recursive directory traversals.
9917 A successful use of @command{chmod} clears the set-group-ID bit of a
9918 regular file if the file's group ID does not match the user's
9919 effective group ID or one of the user's supplementary group IDs,
9920 unless the user has appropriate privileges. Additional restrictions
9921 may cause the set-user-ID and set-group-ID bits of @var{mode} or
9922 @var{ref_file} to be ignored. This behavior depends on the policy and
9923 functionality of the underlying @code{chmod} system call. When in
9924 doubt, check the underlying system behavior.
9926 If used, @var{mode} specifies the new file mode bits.
9927 For details, see the section on @ref{File permissions}.
9928 If you really want @var{mode} to have a leading @samp{-}, you should
9929 use @option{--} first, e.g., @samp{chmod -- -w file}. Typically,
9930 though, @samp{chmod a-w file} is preferable, and @command{chmod -w
9931 file} (without the @option{--}) complains if it behaves differently
9932 from what @samp{chmod a-w file} would do.
9934 The program accepts the following options. Also see @ref{Common options}.
9942 Verbosely describe the action for each @var{file} whose permissions
9951 @cindex error messages, omitting
9952 Do not print error messages about files whose permissions cannot be
9955 @itemx --preserve-root
9956 @opindex --preserve-root
9957 @cindex root directory, disallow recursive modification
9958 Fail upon any attempt to recursively change the root directory, @file{/}.
9959 Without @option{--recursive}, this option has no effect.
9960 @xref{Treating / specially}.
9962 @itemx --no-preserve-root
9963 @opindex --no-preserve-root
9964 @cindex root directory, allow recursive modification
9965 Cancel the effect of any preceding @option{--preserve-root} option.
9966 @xref{Treating / specially}.
9972 Verbosely describe the action or non-action taken for every @var{file}.
9974 @item --reference=@var{ref_file}
9975 @opindex --reference
9976 Change the mode of each @var{file} to be the same as that of @var{ref_file}.
9977 @xref{File permissions}.
9978 If @var{ref_file} is a symbolic link, do not use the mode
9979 of the symbolic link, but rather that of the file it refers to.
9984 @opindex --recursive
9985 @cindex recursively changing access permissions
9986 Recursively change permissions of directories and their contents.
9993 @node touch invocation
9994 @section @command{touch}: Change file timestamps
9997 @cindex changing file timestamps
9998 @cindex file timestamps, changing
9999 @cindex timestamps, changing file
10001 @command{touch} changes the access and/or modification times of the
10002 specified files. Synopsis:
10005 touch [@var{option}]@dots{} @var{file}@dots{}
10008 @cindex empty files, creating
10009 Any @var{file} argument that does not exist is created empty, unless
10010 option @option{--no-create} (@option{-c}) or @option{--no-dereference}
10011 (@option{-h}) was in effect.
10013 A @var{file} argument string of @samp{-} is handled specially and
10014 causes @command{touch} to change the times of the file associated with
10017 @cindex permissions, for changing file timestamps
10018 If changing both the access and modification times to the current
10019 time, @command{touch} can change the timestamps for files that the user
10020 running it does not own but has write permission for. Otherwise, the
10021 user must own the files.
10023 Although @command{touch} provides options for changing two of the times---the
10024 times of last access and modification---of a file, there is actually
10025 a standard third one as well: the inode change time. This is often
10026 referred to as a file's @code{ctime}.
10027 The inode change time represents the time when the file's meta-information
10028 last changed. One common example of this is when the permissions of a
10029 file change. Changing the permissions doesn't access the file, so
10030 the atime doesn't change, nor does it modify the file, so the mtime
10031 doesn't change. Yet, something about the file itself has changed,
10032 and this must be noted somewhere. This is the job of the ctime field.
10033 This is necessary, so that, for example, a backup program can make a
10034 fresh copy of the file, including the new permissions value.
10035 Another operation that modifies a file's ctime without affecting
10036 the others is renaming. In any case, it is not possible, in normal
10037 operations, for a user to change the ctime field to a user-specified value.
10038 Some operating systems and file systems support a fourth time: the
10039 birth time, when the file was first created; by definition, this
10040 timestamp never changes.
10043 Time stamps assume the time zone rules specified by the @env{TZ}
10044 environment variable, or by the system default rules if @env{TZ} is
10045 not set. @xref{TZ Variable,, Specifying the Time Zone with @env{TZ},
10046 libc, The GNU C Library Reference Manual}.
10047 You can avoid ambiguities during
10048 daylight saving transitions by using @sc{utc} time stamps.
10050 The program accepts the following options. Also see @ref{Common options}.
10055 @itemx --time=atime
10056 @itemx --time=access
10060 @opindex atime@r{, changing}
10061 @opindex access @r{time, changing}
10062 @opindex use @r{time, changing}
10063 Change the access time only.
10068 @opindex --no-create
10069 Do not warn about or create files that do not exist.
10072 @itemx --date=@var{time}
10076 Use @var{time} instead of the current time. It can contain month names,
10077 time zones, @samp{am} and @samp{pm}, @samp{yesterday}, etc. For
10078 example, @option{--date="2004-02-27 14:19:13.489392193 +0530"}
10079 specifies the instant of time that is 489,392,193 nanoseconds after
10080 February 27, 2004 at 2:19:13 PM in a time zone that is 5 hours and 30
10081 minutes east of @acronym{UTC}. @xref{Date input formats}.
10082 File systems that do not support high-resolution time stamps
10083 silently ignore any excess precision here.
10087 @cindex BSD @command{touch} compatibility
10088 Ignored; for compatibility with BSD versions of @command{touch}.
10091 @itemx --no-dereference
10093 @opindex --no-dereference
10094 @cindex symbolic links, changing time
10096 Attempt to change the timestamps of a symbolic link, rather than what
10097 the link refers to. When using this option, empty files are not
10098 created, but option @option{-c} must also be used to avoid warning
10099 about files that do not exist. Not all systems support changing the
10100 timestamps of symlinks, since underlying system support for this
10101 action was not required until @acronym{POSIX} 2008. Also, on some
10102 systems, the mere act of examining a symbolic link changes the access
10103 time, such that only changes to the modification time will persist
10104 long enough to be observable. When coupled with option @option{-r}, a
10105 reference timestamp is taken from a symbolic link rather than the file
10109 @itemx --time=mtime
10110 @itemx --time=modify
10113 @opindex mtime@r{, changing}
10114 @opindex modify @r{time, changing}
10115 Change the modification time only.
10117 @item -r @var{file}
10118 @itemx --reference=@var{file}
10120 @opindex --reference
10121 Use the times of the reference @var{file} instead of the current time.
10122 If this option is combined with the @option{--date=@var{time}}
10123 (@option{-d @var{time}}) option, the reference @var{file}'s time is
10124 the origin for any relative @var{time}s given, but is otherwise ignored.
10125 For example, @samp{-r foo -d '-5 seconds'} specifies a time stamp
10126 equal to five seconds before the corresponding time stamp for @file{foo}.
10127 If @var{file} is a symbolic link, the reference timestamp is taken
10128 from the target of the symlink, unless @option{-h} was also in effect.
10130 @item -t [[@var{cc}]@var{yy}]@var{mmddhhmm}[.@var{ss}]
10131 Use the argument (optional four-digit or two-digit years, months,
10132 days, hours, minutes, optional seconds) instead of the current time.
10133 If the year is specified with only two digits, then @var{cc}
10134 is 20 for years in the range 0 @dots{} 68, and 19 for years in
10135 69 @dots{} 99. If no digits of the year are specified,
10136 the argument is interpreted as a date in the current year.
10137 Note that @var{ss} may be @samp{60}, to accommodate leap seconds.
10141 @vindex _POSIX2_VERSION
10142 On older systems, @command{touch} supports an obsolete syntax, as follows.
10143 If no timestamp is given with any of the @option{-d}, @option{-r}, or
10144 @option{-t} options, and if there are two or more @var{file}s and the
10145 first @var{file} is of the form @samp{@var{mmddhhmm}[@var{yy}]} and this
10146 would be a valid argument to the @option{-t} option (if the @var{yy}, if
10147 any, were moved to the front), and if the represented year
10148 is in the range 1969--1999, that argument is interpreted as the time
10149 for the other files instead of as a file name.
10150 This obsolete behavior can be enabled or disabled with the
10151 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
10152 conformance}), but portable scripts should avoid commands whose
10153 behavior depends on this variable.
10154 For example, use @samp{touch ./12312359 main.c} or @samp{touch -t
10155 12312359 main.c} rather than the ambiguous @samp{touch 12312359 main.c}.
10161 @chapter Disk usage
10165 No disk can hold an infinite amount of data. These commands report
10166 how much disk storage is in use or available, report other file and
10167 file status information, and write buffers to disk.
10170 * df invocation:: Report file system disk space usage.
10171 * du invocation:: Estimate file space usage.
10172 * stat invocation:: Report file or file system status.
10173 * sync invocation:: Synchronize memory and disk.
10174 * truncate invocation:: Shrink or extend the size of a file.
10178 @node df invocation
10179 @section @command{df}: Report file system disk space usage
10182 @cindex file system disk usage
10183 @cindex disk usage by file system
10185 @command{df} reports the amount of disk space used and available on
10186 file systems. Synopsis:
10189 df [@var{option}]@dots{} [@var{file}]@dots{}
10192 With no arguments, @command{df} reports the space used and available on all
10193 currently mounted file systems (of all types). Otherwise, @command{df}
10194 reports on the file system containing each argument @var{file}.
10196 Normally the disk space is printed in units of
10197 1024 bytes, but this can be overridden (@pxref{Block size}).
10198 Non-integer quantities are rounded up to the next higher unit.
10200 @cindex disk device file
10201 @cindex device file, disk
10202 If an argument @var{file} is a disk device file containing a mounted
10203 file system, @command{df} shows the space available on that file system
10204 rather than on the file system containing the device node (i.e., the root
10205 file system). @sc{gnu} @command{df} does not attempt to determine the disk usage
10206 on unmounted file systems, because on most kinds of systems doing so
10207 requires extremely nonportable intimate knowledge of file system
10210 The program accepts the following options. Also see @ref{Common options}.
10218 @cindex automounter file systems
10219 @cindex ignore file systems
10220 Include in the listing dummy file systems, which
10221 are omitted by default. Such file systems are typically special-purpose
10222 pseudo-file-systems, such as automounter entries.
10224 @item -B @var{size}
10225 @itemx --block-size=@var{size}
10227 @opindex --block-size
10228 @cindex file system sizes
10229 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10230 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10234 @cindex grand total of disk size, usage and available space
10235 Print a grand total of all arguments after all arguments have
10236 been processed. This can be used to find out the total disk size, usage
10237 and available space of all listed devices.
10243 Equivalent to @option{--si}.
10249 @cindex inode usage
10250 List inode usage information instead of block usage. An inode (short
10251 for index node) contains information about a file such as its owner,
10252 permissions, timestamps, and location on the disk.
10256 @cindex kibibytes for file system sizes
10257 Print sizes in 1024-byte blocks, overriding the default block size
10258 (@pxref{Block size}).
10259 This option is equivalent to @option{--block-size=1K}.
10265 @cindex file system types, limiting output to certain
10266 Limit the listing to local file systems. By default, remote file systems
10271 @cindex file system space, retrieving old data more quickly
10272 Do not invoke the @code{sync} system call before getting any usage data.
10273 This may make @command{df} run significantly faster on systems with many
10274 disks, but on some systems (notably SunOS) the results may be slightly
10275 out of date. This is the default.
10278 @itemx --portability
10280 @opindex --portability
10281 @cindex one-line output format
10282 @cindex @acronym{POSIX} output format
10283 @cindex portable output format
10284 @cindex output format, portable
10285 Use the @acronym{POSIX} output format. This is like the default format except
10290 The information about each file system is always printed on exactly
10291 one line; a mount device is never put on a line by itself. This means
10292 that if the mount device name is more than 20 characters long (e.g., for
10293 some network mounts), the columns are misaligned.
10296 The labels in the header output line are changed to conform to @acronym{POSIX}.
10299 The default block size and output format are unaffected by the
10300 @env{DF_BLOCK_SIZE}, @env{BLOCK_SIZE} and @env{BLOCKSIZE} environment
10301 variables. However, the default block size is still affected by
10302 @env{POSIXLY_CORRECT}: it is 512 if @env{POSIXLY_CORRECT} is set, 1024
10303 otherwise. @xref{Block size}.
10310 @cindex file system space, retrieving current data more slowly
10311 Invoke the @code{sync} system call before getting any usage data. On
10312 some systems (notably SunOS), doing this yields more up to date results,
10313 but in general this option makes @command{df} much slower, especially when
10314 there are many or very busy file systems.
10316 @item -t @var{fstype}
10317 @itemx --type=@var{fstype}
10320 @cindex file system types, limiting output to certain
10321 Limit the listing to file systems of type @var{fstype}. Multiple
10322 file system types can be specified by giving multiple @option{-t} options.
10323 By default, nothing is omitted.
10326 @itemx --print-type
10328 @opindex --print-type
10329 @cindex file system types, printing
10330 Print each file system's type. The types printed here are the same ones
10331 you can include or exclude with @option{-t} and @option{-x}. The particular
10332 types printed are whatever is supported by the system. Here are some of
10333 the common names (this list is certainly not exhaustive):
10338 @cindex @acronym{NFS} file system type
10339 An @acronym{NFS} file system, i.e., one mounted over a network from another
10340 machine. This is the one type name which seems to be used uniformly by
10343 @item 4.2@r{, }ufs@r{, }efs@dots{}
10344 @cindex Linux file system types
10345 @cindex local file system types
10346 @opindex 4.2 @r{file system type}
10347 @opindex ufs @r{file system type}
10348 @opindex efs @r{file system type}
10349 A file system on a locally-mounted hard disk. (The system might even
10350 support more than one type here; Linux does.)
10352 @item hsfs@r{, }cdfs
10353 @cindex CD-ROM file system type
10354 @cindex High Sierra file system
10355 @opindex hsfs @r{file system type}
10356 @opindex cdfs @r{file system type}
10357 A file system on a CD-ROM drive. HP-UX uses @samp{cdfs}, most other
10358 systems use @samp{hsfs} (@samp{hs} for ``High Sierra'').
10361 @cindex PC file system
10362 @cindex DOS file system
10363 @cindex MS-DOS file system
10364 @cindex diskette file system
10366 An MS-DOS file system, usually on a diskette.
10370 @item -x @var{fstype}
10371 @itemx --exclude-type=@var{fstype}
10373 @opindex --exclude-type
10374 Limit the listing to file systems not of type @var{fstype}.
10375 Multiple file system types can be eliminated by giving multiple
10376 @option{-x} options. By default, no file system types are omitted.
10379 Ignored; for compatibility with System V versions of @command{df}.
10384 Failure includes the case where no output is generated, so you can
10385 inspect the exit status of a command like @samp{df -t ext3 -t reiserfs
10386 @var{dir}} to test whether @var{dir} is on a file system of type
10387 @samp{ext3} or @samp{reiserfs}.
10390 @node du invocation
10391 @section @command{du}: Estimate file space usage
10394 @cindex file space usage
10395 @cindex disk usage for files
10397 @command{du} reports the amount of disk space used by the specified files
10398 and for each subdirectory (of directory arguments). Synopsis:
10401 du [@var{option}]@dots{} [@var{file}]@dots{}
10404 With no arguments, @command{du} reports the disk space for the current
10405 directory. Normally the disk space is printed in units of
10406 1024 bytes, but this can be overridden (@pxref{Block size}).
10407 Non-integer quantities are rounded up to the next higher unit.
10409 If two or more hard links point to the same file, only one of the hard
10410 links is counted. The @var{file} argument order affects which links
10411 are counted, and changing the argument order may change the numbers
10412 that @command{du} outputs.
10414 The program accepts the following options. Also see @ref{Common options}.
10422 Show counts for all files, not just directories.
10424 @itemx --apparent-size
10425 @opindex --apparent-size
10426 Print apparent sizes, rather than disk usage. The apparent size of a
10427 file is the number of bytes reported by @code{wc -c} on regular files,
10428 or more generally, @code{ls -l --block-size=1} or @code{stat --format=%s}.
10429 For example, a file containing the word @samp{zoo} with no newline would,
10430 of course, have an apparent size of 3. Such a small file may require
10431 anywhere from 0 to 16 KiB or more of disk space, depending on
10432 the type and configuration of the file system on which the file resides.
10433 However, a sparse file created with this command:
10436 dd bs=1 seek=2GiB if=/dev/null of=big
10440 has an apparent size of 2 GiB, yet on most modern
10441 systems, it actually uses almost no disk space.
10447 Equivalent to @code{--apparent-size --block-size=1}.
10449 @item -B @var{size}
10450 @itemx --block-size=@var{size}
10452 @opindex --block-size
10454 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10455 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10461 @cindex grand total of disk space
10462 Print a grand total of all arguments after all arguments have
10463 been processed. This can be used to find out the total disk usage of
10464 a given set of files or directories.
10467 @itemx --dereference-args
10469 @opindex --dereference-args
10470 Dereference symbolic links that are command line arguments.
10471 Does not affect other symbolic links. This is helpful for finding
10472 out the disk usage of directories, such as @file{/usr/tmp}, which
10473 are often symbolic links.
10475 @c --files0-from=FILE
10476 @filesZeroFromOption{du,, with the @option{--total} (@option{-c}) option}
10482 Equivalent to @option{--dereference-args} (@option{-D}).
10486 @cindex kibibytes for file sizes
10487 Print sizes in 1024-byte blocks, overriding the default block size
10488 (@pxref{Block size}).
10489 This option is equivalent to @option{--block-size=1K}.
10492 @itemx --count-links
10494 @opindex --count-links
10495 @cindex hard links, counting in @command{du}
10496 Count the size of all files, even if they have appeared already (as a
10500 @itemx --dereference
10502 @opindex --dereference
10503 @cindex symbolic links, dereferencing in @command{du}
10504 Dereference symbolic links (show the disk space used by the file
10505 or directory that the link points to instead of the space used by
10510 @cindex mebibytes for file sizes
10511 Print sizes in 1,048,576-byte blocks, overriding the default block size
10512 (@pxref{Block size}).
10513 This option is equivalent to @option{--block-size=1M}.
10516 @itemx --no-dereference
10518 @opindex --no-dereference
10519 @cindex symbolic links, dereferencing in @command{du}
10520 For each symbolic links encountered by @command{du},
10521 consider the disk space used by the symbolic link.
10523 @item -d @var{depth}
10524 @item --max-depth=@var{depth}
10525 @opindex -d @var{depth}
10526 @opindex --max-depth=@var{depth}
10527 @cindex limiting output of @command{du}
10528 Show the total for each directory (and file if --all) that is at
10529 most MAX_DEPTH levels down from the root of the hierarchy. The root
10530 is at level 0, so @code{du --max-depth=0} is equivalent to @code{du -s}.
10539 @opindex --summarize
10540 Display only a total for each argument.
10543 @itemx --separate-dirs
10545 @opindex --separate-dirs
10546 Normally, in the output of @command{du} (when not using @option{--summarize}),
10547 the size listed next to a directory name, @var{d}, represents the sum
10548 of sizes of all entries beneath @var{d} as well as the size of @var{d} itself.
10549 With @option{--separate-dirs}, the size reported for a directory name,
10550 @var{d}, is merely the @code{stat.st_size}-derived size of the directory
10555 @cindex last modified dates, displaying in @command{du}
10556 Show time of the most recent modification of any file in the directory,
10557 or any of its subdirectories.
10559 @itemx --time=ctime
10560 @itemx --time=status
10563 @opindex ctime@r{, show the most recent}
10564 @opindex status time@r{, show the most recent}
10565 @opindex use time@r{, show the most recent}
10566 Show the most recent status change time (the @samp{ctime} in the inode) of
10567 any file in the directory, instead of the modification time.
10569 @itemx --time=atime
10570 @itemx --time=access
10572 @opindex atime@r{, show the most recent}
10573 @opindex access time@r{, show the most recent}
10574 Show the most recent access time (the @samp{atime} in the inode) of
10575 any file in the directory, instead of the modification time.
10577 @item --time-style=@var{style}
10578 @opindex --time-style
10580 List timestamps in style @var{style}. This option has an effect only if
10581 the @option{--time} option is also specified. The @var{style} should
10582 be one of the following:
10585 @item +@var{format}
10587 List timestamps using @var{format}, where @var{format} is interpreted
10588 like the format argument of @command{date} (@pxref{date invocation}).
10589 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
10590 @command{du} to list timestamps like @samp{2002-03-30 23:45:56}. As
10591 with @command{date}, @var{format}'s interpretation is affected by the
10592 @env{LC_TIME} locale category.
10595 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
10596 format with nanosecond precision, e.g., @samp{2002-03-30
10597 23:45:56.477817180 -0700}. This style is equivalent to
10598 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
10601 List @acronym{ISO} 8601 date and time in minutes, e.g.,
10602 @samp{2002-03-30 23:45}. These timestamps are shorter than
10603 @samp{full-iso} timestamps, and are usually good enough for everyday
10604 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
10607 List @acronym{ISO} 8601 dates for timestamps, e.g., @samp{2002-03-30}.
10608 This style is equivalent to @samp{+%Y-%m-%d}.
10612 You can specify the default value of the @option{--time-style} option
10613 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
10614 the default style is @samp{long-iso}. For compatibility with @command{ls},
10615 if @env{TIME_STYLE} begins with @samp{+} and contains a newline,
10616 the newline and any later characters are ignored; if @env{TIME_STYLE}
10617 begins with @samp{posix-} the @samp{posix-} is ignored; and if
10618 @env{TIME_STYLE} is @samp{locale} it is ignored.
10621 @itemx --one-file-system
10623 @opindex --one-file-system
10624 @cindex one file system, restricting @command{du} to
10625 Skip directories that are on different file systems from the one that
10626 the argument being processed is on.
10628 @item --exclude=@var{pattern}
10629 @opindex --exclude=@var{pattern}
10630 @cindex excluding files from @command{du}
10631 When recursing, skip subdirectories or files matching @var{pattern}.
10632 For example, @code{du --exclude='*.o'} excludes files whose names
10635 @item -X @var{file}
10636 @itemx --exclude-from=@var{file}
10637 @opindex -X @var{file}
10638 @opindex --exclude-from=@var{file}
10639 @cindex excluding files from @command{du}
10640 Like @option{--exclude}, except take the patterns to exclude from @var{file},
10641 one per line. If @var{file} is @samp{-}, take the patterns from standard
10646 @cindex NFS mounts from BSD to HP-UX
10647 On BSD systems, @command{du} reports sizes that are half the correct
10648 values for files that are NFS-mounted from HP-UX systems. On HP-UX
10649 systems, it reports sizes that are twice the correct values for
10650 files that are NFS-mounted from BSD systems. This is due to a flaw
10651 in HP-UX; it also affects the HP-UX @command{du} program.
10656 @node stat invocation
10657 @section @command{stat}: Report file or file system status
10660 @cindex file status
10661 @cindex file system status
10663 @command{stat} displays information about the specified file(s). Synopsis:
10666 stat [@var{option}]@dots{} [@var{file}]@dots{}
10669 With no option, @command{stat} reports all information about the given files.
10670 But it also can be used to report the information of the file systems the
10671 given files are located on. If the files are links, @command{stat} can
10672 also give information about the files the links point to.
10674 @mayConflictWithShellBuiltIn{stat}
10679 @itemx --dereference
10681 @opindex --dereference
10682 @cindex symbolic links, dereferencing in @command{stat}
10683 Change how @command{stat} treats symbolic links.
10684 With this option, @command{stat} acts on the file referenced
10685 by each symbolic link argument.
10686 Without it, @command{stat} acts on any symbolic link argument directly.
10689 @itemx --file-system
10691 @opindex --file-system
10692 @cindex file systems
10693 Report information about the file systems where the given files are located
10694 instead of information about the files themselves.
10695 This option implies the @option{-L} option.
10698 @itemx --format=@var{format}
10700 @opindex --format=@var{format}
10701 @cindex output format
10702 Use @var{format} rather than the default format.
10703 @var{format} is automatically newline-terminated, so
10704 running a command like the following with two or more @var{file}
10705 operands produces a line of output for each operand:
10707 $ stat --format=%d:%i / /usr
10712 @itemx --printf=@var{format}
10713 @opindex --printf=@var{format}
10714 @cindex output format
10715 Use @var{format} rather than the default format.
10716 Like @option{--format}, but interpret backslash escapes,
10717 and do not output a mandatory trailing newline.
10718 If you want a newline, include @samp{\n} in the @var{format}.
10719 Here's how you would use @option{--printf} to print the device
10720 and inode numbers of @file{/} and @file{/usr}:
10722 $ stat --printf='%d:%i\n' / /usr
10731 @cindex terse output
10732 Print the information in terse form, suitable for parsing by other programs.
10736 The valid @var{format} directives for files with @option{--format} and
10737 @option{--printf} are:
10740 @item %a - Access rights in octal
10741 @item %A - Access rights in human readable form
10742 @item %b - Number of blocks allocated (see @samp{%B})
10743 @item %B - The size in bytes of each block reported by @samp{%b}
10744 @item %C - The SELinux security context of a file, if available
10745 @item %d - Device number in decimal
10746 @item %D - Device number in hex
10747 @item %f - Raw mode in hex
10748 @item %F - File type
10749 @item %g - Group ID of owner
10750 @item %G - Group name of owner
10751 @item %h - Number of hard links
10752 @item %i - Inode number
10753 @item %m - Mount point (See note below)
10754 @item %n - File name
10755 @item %N - Quoted file name with dereference if symbolic link
10756 @item %o - I/O block size
10757 @item %s - Total size, in bytes
10758 @item %t - Major device type in hex
10759 @item %T - Minor device type in hex
10760 @item %u - User ID of owner
10761 @item %U - User name of owner
10762 @item %w - Time of file birth, or @samp{-} if unknown
10763 @item %W - Time of file birth as seconds since Epoch, or @samp{0}
10764 @item %x - Time of last access
10765 @item %X - Time of last access as seconds since Epoch
10766 @item %y - Time of last modification
10767 @item %Y - Time of last modification as seconds since Epoch
10768 @item %z - Time of last change
10769 @item %Z - Time of last change as seconds since Epoch
10772 The @samp{%W}, @samp{%X}, @samp{%Y}, and @samp{%Z} formats accept a
10773 precision preceded by a period to specify the number of digits to
10774 print after the decimal point. For example, @samp{%.3X} outputs the
10775 last access time to millisecond precision. If a period is given but no
10776 precision, @command{stat} uses 9 digits, so @samp{%.X} is equivalent to
10777 @samp{%.9X}. When discarding excess precision, time stamps are truncated
10778 toward minus infinity.
10782 $ stat -c '[%015Y]' /usr
10785 $ stat -c '[%15Y]' /usr
10787 $ stat -c '[%-15Y]' /usr
10790 $ stat -c '[%.3Y]' /usr
10792 $ stat -c '[%.Y]' /usr
10793 [1288929712.114951834]
10796 The mount point printed by @samp{%m} is similar to that output
10797 by @command{df}, except that:
10800 stat does not dereference symlinks by default
10801 (unless @option{-L} is specified)
10803 stat does not search for specified device nodes in the
10804 file system list, instead operating on them directly
10807 stat outputs the alias for a bind mounted file, rather than
10808 the initial mount point of its backing device.
10809 One can recursively call stat until there is no change in output,
10810 to get the current base mount point
10813 When listing file system information (@option{--file-system} (@option{-f})),
10814 you must use a different set of @var{format} directives:
10817 @item %a - Free blocks available to non-super-user
10818 @item %b - Total data blocks in file system
10819 @item %c - Total file nodes in file system
10820 @item %d - Free file nodes in file system
10821 @item %f - Free blocks in file system
10822 @item %i - File System ID in hex
10823 @item %l - Maximum length of file names
10824 @item %n - File name
10825 @item %s - Block size (for faster transfers)
10826 @item %S - Fundamental block size (for block counts)
10827 @item %t - Type in hex
10828 @item %T - Type in human readable form
10832 Time stamps are listed according to the time zone rules specified by
10833 the @env{TZ} environment variable, or by the system default rules if
10834 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
10835 with @env{TZ}, libc, The GNU C Library Reference Manual}.
10840 @node sync invocation
10841 @section @command{sync}: Synchronize data on disk with memory
10844 @cindex synchronize disk and memory
10846 @cindex superblock, writing
10847 @cindex inodes, written buffered
10848 @command{sync} writes any data buffered in memory out to disk. This can
10849 include (but is not limited to) modified superblocks, modified inodes,
10850 and delayed reads and writes. This must be implemented by the kernel;
10851 The @command{sync} program does nothing but exercise the @code{sync} system
10854 @cindex crashes and corruption
10855 The kernel keeps data in memory to avoid doing (relatively slow) disk
10856 reads and writes. This improves performance, but if the computer
10857 crashes, data may be lost or the file system corrupted as a
10858 result. The @command{sync} command ensures everything in memory
10859 is written to disk.
10861 Any arguments are ignored, except for a lone @option{--help} or
10862 @option{--version} (@pxref{Common options}).
10867 @node truncate invocation
10868 @section @command{truncate}: Shrink or extend the size of a file
10871 @cindex truncating, file sizes
10873 @command{truncate} shrinks or extends the size of each @var{file} to the
10874 specified size. Synopsis:
10877 truncate @var{option}@dots{} @var{file}@dots{}
10880 @cindex files, creating
10881 Any @var{file} that does not exist is created.
10883 @cindex sparse files, creating
10884 @cindex holes, creating files with
10885 If a @var{file} is larger than the specified size, the extra data is lost.
10886 If a @var{file} is shorter, it is extended and the extended part (or hole)
10887 reads as zero bytes.
10889 The program accepts the following options. Also see @ref{Common options}.
10896 @opindex --no-create
10897 Do not create files that do not exist.
10902 @opindex --io-blocks
10903 Treat @var{size} as number of I/O blocks of the @var{file} rather than bytes.
10905 @item -r @var{rfile}
10906 @itemx --reference=@var{rfile}
10908 @opindex --reference
10909 Base the size of each @var{file} on the size of @var{rfile}.
10911 @item -s @var{size}
10912 @itemx --size=@var{size}
10915 Set or adjust the size of each @var{file} according to @var{size}.
10916 @multiplierSuffixesNoBlocks{size}
10918 @var{size} may also be prefixed by one of the following to adjust
10919 the size of each @var{file} based on their current size:
10921 @samp{+} => extend by
10922 @samp{-} => reduce by
10923 @samp{<} => at most
10924 @samp{>} => at least
10925 @samp{/} => round down to multiple of
10926 @samp{%} => round up to multiple of
10934 @node Printing text
10935 @chapter Printing text
10937 @cindex printing text, commands for
10938 @cindex commands for printing text
10940 This section describes commands that display text strings.
10943 * echo invocation:: Print a line of text.
10944 * printf invocation:: Format and print data.
10945 * yes invocation:: Print a string until interrupted.
10949 @node echo invocation
10950 @section @command{echo}: Print a line of text
10953 @cindex displaying text
10954 @cindex printing text
10955 @cindex text, displaying
10956 @cindex arbitrary text, displaying
10958 @command{echo} writes each given @var{string} to standard output, with a
10959 space between each and a newline after the last one. Synopsis:
10962 echo [@var{option}]@dots{} [@var{string}]@dots{}
10965 @mayConflictWithShellBuiltIn{echo}
10967 The program accepts the following options. Also see @ref{Common options}.
10968 Options must precede operands, and the normally-special argument
10969 @samp{--} has no special meaning and is treated like any other
10975 Do not output the trailing newline.
10979 @cindex backslash escapes
10980 Enable interpretation of the following backslash-escaped characters in
10989 produce no further output
11005 the eight-bit value that is the octal number @var{nnn}
11006 (zero to three octal digits)
11008 the eight-bit value that is the octal number @var{nnn}
11009 (one to three octal digits)
11011 the eight-bit value that is the hexadecimal number @var{hh}
11012 (one or two hexadecimal digits)
11017 @cindex backslash escapes
11018 Disable interpretation of backslash escapes in each @var{string}.
11019 This is the default. If @option{-e} and @option{-E} are both
11020 specified, the last one given takes effect.
11024 @vindex POSIXLY_CORRECT
11025 If the @env{POSIXLY_CORRECT} environment variable is set, then when
11026 @command{echo}'s first argument is not @option{-n} it outputs
11027 option-like arguments instead of treating them as options. For
11028 example, @code{echo -ne hello} outputs @samp{-ne hello} instead of
11029 plain @samp{hello}.
11031 @acronym{POSIX} does not require support for any options, and says
11032 that the behavior of @command{echo} is implementation-defined if any
11033 @var{string} contains a backslash or if the first argument is
11034 @option{-n}. Portable programs can use the @command{printf} command
11035 if they need to omit trailing newlines or output control characters or
11036 backslashes. @xref{printf invocation}.
11041 @node printf invocation
11042 @section @command{printf}: Format and print data
11045 @command{printf} does formatted printing of text. Synopsis:
11048 printf @var{format} [@var{argument}]@dots{}
11051 @command{printf} prints the @var{format} string, interpreting @samp{%}
11052 directives and @samp{\} escapes to format numeric and string arguments
11053 in a way that is mostly similar to the C @samp{printf} function.
11054 @xref{Output Conversion Syntax,, @command{printf} format directives,
11055 libc, The GNU C Library Reference Manual}, for details.
11056 The differences are listed below.
11058 @mayConflictWithShellBuiltIn{printf}
11063 The @var{format} argument is reused as necessary to convert all the
11064 given @var{argument}s. For example, the command @samp{printf %s a b}
11068 Missing @var{argument}s are treated as null strings or as zeros,
11069 depending on whether the context expects a string or a number. For
11070 example, the command @samp{printf %sx%d} prints @samp{x0}.
11074 An additional escape, @samp{\c}, causes @command{printf} to produce no
11075 further output. For example, the command @samp{printf 'A%sC\cD%sF' B
11076 E} prints @samp{ABC}.
11079 The hexadecimal escape sequence @samp{\x@var{hh}} has at most two
11080 digits, as opposed to C where it can have an unlimited number of
11081 digits. For example, the command @samp{printf '\x07e'} prints two
11082 bytes, whereas the C statement @samp{printf ("\x07e")} prints just
11087 @command{printf} has an additional directive, @samp{%b}, which prints its
11088 argument string with @samp{\} escapes interpreted in the same way as in
11089 the @var{format} string, except that octal escapes are of the form
11090 @samp{\0@var{ooo}} where @var{ooo} is 0 to 3 octal digits.
11091 If a precision is also given, it limits the number of bytes printed
11092 from the converted string.
11095 Numeric arguments must be single C constants, possibly with leading
11096 @samp{+} or @samp{-}. For example, @samp{printf %.4d -3} outputs
11100 @vindex POSIXLY_CORRECT
11101 If the leading character of a numeric argument is @samp{"} or @samp{'}
11102 then its value is the numeric value of the immediately following
11103 character. Any remaining characters are silently ignored if the
11104 @env{POSIXLY_CORRECT} environment variable is set; otherwise, a
11105 warning is printed. For example, @samp{printf "%d" "'a"} outputs
11106 @samp{97} on hosts that use the @acronym{ASCII} character set, since
11107 @samp{a} has the numeric value 97 in @acronym{ASCII}.
11112 A floating-point argument must use a period before any fractional
11113 digits, but is printed according to the @env{LC_NUMERIC} category of the
11114 current locale. For example, in a locale whose radix character is a
11115 comma, the command @samp{printf %g 3.14} outputs @samp{3,14} whereas
11116 the command @samp{printf %g 3,14} is an error.
11120 @command{printf} interprets @samp{\@var{ooo}} in @var{format} as an octal number
11121 (if @var{ooo} is 1 to 3 octal digits) specifying a character to print,
11122 and @samp{\x@var{hh}} as a hexadecimal number (if @var{hh} is 1 to 2 hex
11123 digits) specifying a character to print.
11128 @cindex ISO/IEC 10646
11130 @command{printf} interprets two character syntaxes introduced in
11131 @acronym{ISO} C 99:
11132 @samp{\u} for 16-bit Unicode (@acronym{ISO}/@acronym{IEC} 10646)
11133 characters, specified as
11134 four hexadecimal digits @var{hhhh}, and @samp{\U} for 32-bit Unicode
11135 characters, specified as eight hexadecimal digits @var{hhhhhhhh}.
11136 @command{printf} outputs the Unicode characters
11137 according to the @env{LC_CTYPE} locale. Unicode characters in the ranges
11138 U+0000...U+009F, U+D800...U+DFFF cannot be specified by this syntax, except
11139 for U+0024 ($), U+0040 (@@), and U+0060 (@`).
11141 The processing of @samp{\u} and @samp{\U} requires a full-featured
11142 @code{iconv} facility. It is activated on systems with glibc 2.2 (or newer),
11143 or when @code{libiconv} is installed prior to this package. Otherwise
11144 @samp{\u} and @samp{\U} will print as-is.
11146 The only options are a lone @option{--help} or
11147 @option{--version}. @xref{Common options}.
11148 Options must precede operands.
11150 The Unicode character syntaxes are useful for writing strings in a locale
11151 independent way. For example, a string containing the Euro currency symbol
11154 $ env printf '\u20AC 14.95'
11158 will be output correctly in all locales supporting the Euro symbol
11159 (@acronym{ISO}-8859-15, UTF-8, and others). Similarly, a Chinese string
11162 $ env printf '\u4e2d\u6587'
11166 will be output correctly in all Chinese locales (GB2312, BIG5, UTF-8, etc).
11168 Note that in these examples, the @command{printf} command has been
11169 invoked via @command{env} to ensure that we run the program found via
11170 your shell's search path, and not a shell alias or a built-in function.
11172 For larger strings, you don't need to look up the hexadecimal code
11173 values of each character one by one. @acronym{ASCII} characters mixed with \u
11174 escape sequences is also known as the JAVA source file encoding. You can
11175 use GNU recode 3.5c (or newer) to convert strings to this encoding. Here
11176 is how to convert a piece of text into a shell script which will output
11177 this text in a locale-independent way:
11180 $ LC_CTYPE=zh_CN.big5 /usr/local/bin/printf \
11181 '\u4e2d\u6587\n' > sample.txt
11182 $ recode BIG5..JAVA < sample.txt \
11183 | sed -e "s|^|/usr/local/bin/printf '|" -e "s|$|\\\\n'|" \
11190 @node yes invocation
11191 @section @command{yes}: Print a string until interrupted
11194 @cindex repeated output of a string
11196 @command{yes} prints the command line arguments, separated by spaces and
11197 followed by a newline, forever until it is killed. If no arguments are
11198 given, it prints @samp{y} followed by a newline forever until killed.
11200 Upon a write error, @command{yes} exits with status @samp{1}.
11202 The only options are a lone @option{--help} or @option{--version}.
11203 To output an argument that begins with
11204 @samp{-}, precede it with @option{--}, e.g., @samp{yes -- --help}.
11205 @xref{Common options}.
11209 @chapter Conditions
11212 @cindex commands for exit status
11213 @cindex exit status commands
11215 This section describes commands that are primarily useful for their exit
11216 status, rather than their output. Thus, they are often used as the
11217 condition of shell @code{if} statements, or as the last command in a
11221 * false invocation:: Do nothing, unsuccessfully.
11222 * true invocation:: Do nothing, successfully.
11223 * test invocation:: Check file types and compare values.
11224 * expr invocation:: Evaluate expressions.
11228 @node false invocation
11229 @section @command{false}: Do nothing, unsuccessfully
11232 @cindex do nothing, unsuccessfully
11233 @cindex failure exit status
11234 @cindex exit status of @command{false}
11236 @command{false} does nothing except return an exit status of 1, meaning
11237 @dfn{failure}. It can be used as a place holder in shell scripts
11238 where an unsuccessful command is needed.
11239 In most modern shells, @command{false} is a built-in command, so when
11240 you use @samp{false} in a script, you're probably using the built-in
11241 command, not the one documented here.
11243 @command{false} honors the @option{--help} and @option{--version} options.
11245 This version of @command{false} is implemented as a C program, and is thus
11246 more secure and faster than a shell script implementation, and may safely
11247 be used as a dummy shell for the purpose of disabling accounts.
11249 Note that @command{false} (unlike all other programs documented herein)
11250 exits unsuccessfully, even when invoked with
11251 @option{--help} or @option{--version}.
11253 Portable programs should not assume that the exit status of
11254 @command{false} is 1, as it is greater than 1 on some
11255 non-@acronym{GNU} hosts.
11258 @node true invocation
11259 @section @command{true}: Do nothing, successfully
11262 @cindex do nothing, successfully
11264 @cindex successful exit
11265 @cindex exit status of @command{true}
11267 @command{true} does nothing except return an exit status of 0, meaning
11268 @dfn{success}. It can be used as a place holder in shell scripts
11269 where a successful command is needed, although the shell built-in
11270 command @code{:} (colon) may do the same thing faster.
11271 In most modern shells, @command{true} is a built-in command, so when
11272 you use @samp{true} in a script, you're probably using the built-in
11273 command, not the one documented here.
11275 @command{true} honors the @option{--help} and @option{--version} options.
11277 Note, however, that it is possible to cause @command{true}
11278 to exit with nonzero status: with the @option{--help} or @option{--version}
11279 option, and with standard
11280 output already closed or redirected to a file that evokes an I/O error.
11281 For example, using a Bourne-compatible shell:
11284 $ ./true --version >&-
11285 ./true: write error: Bad file number
11286 $ ./true --version > /dev/full
11287 ./true: write error: No space left on device
11290 This version of @command{true} is implemented as a C program, and is thus
11291 more secure and faster than a shell script implementation, and may safely
11292 be used as a dummy shell for the purpose of disabling accounts.
11294 @node test invocation
11295 @section @command{test}: Check file types and compare values
11298 @cindex check file types
11299 @cindex compare values
11300 @cindex expression evaluation
11302 @command{test} returns a status of 0 (true) or 1 (false) depending on the
11303 evaluation of the conditional expression @var{expr}. Each part of the
11304 expression must be a separate argument.
11306 @command{test} has file status checks, string operators, and numeric
11307 comparison operators.
11309 @command{test} has an alternate form that uses opening and closing
11310 square brackets instead a leading @samp{test}. For example, instead
11311 of @samp{test -d /}, you can write @samp{[ -d / ]}. The square
11312 brackets must be separate arguments; for example, @samp{[-d /]} does
11313 not have the desired effect. Since @samp{test @var{expr}} and @samp{[
11314 @var{expr} ]} have the same meaning, only the former form is discussed
11320 test @var{expression}
11322 [ @var{expression} ]
11327 @mayConflictWithShellBuiltIn{test}
11329 If @var{expression} is omitted, @command{test} returns false.
11330 If @var{expression} is a single argument,
11331 @command{test} returns false if the argument is null and true otherwise. The argument
11332 can be any string, including strings like @samp{-d}, @samp{-1},
11333 @samp{--}, @samp{--help}, and @samp{--version} that most other
11334 programs would treat as options. To get help and version information,
11335 invoke the commands @samp{[ --help} and @samp{[ --version}, without
11336 the usual closing brackets. @xref{Common options}.
11338 @cindex exit status of @command{test}
11342 0 if the expression is true,
11343 1 if the expression is false,
11344 2 if an error occurred.
11348 * File type tests:: -[bcdfhLpSt]
11349 * Access permission tests:: -[gkruwxOG]
11350 * File characteristic tests:: -e -s -nt -ot -ef
11351 * String tests:: -z -n = !=
11352 * Numeric tests:: -eq -ne -lt -le -gt -ge
11353 * Connectives for test:: ! -a -o
11357 @node File type tests
11358 @subsection File type tests
11360 @cindex file type tests
11362 These options test for particular types of files. (Everything's a file,
11363 but not all files are the same!)
11367 @item -b @var{file}
11369 @cindex block special check
11370 True if @var{file} exists and is a block special device.
11372 @item -c @var{file}
11374 @cindex character special check
11375 True if @var{file} exists and is a character special device.
11377 @item -d @var{file}
11379 @cindex directory check
11380 True if @var{file} exists and is a directory.
11382 @item -f @var{file}
11384 @cindex regular file check
11385 True if @var{file} exists and is a regular file.
11387 @item -h @var{file}
11388 @itemx -L @var{file}
11391 @cindex symbolic link check
11392 True if @var{file} exists and is a symbolic link.
11393 Unlike all other file-related tests, this test does not dereference
11394 @var{file} if it is a symbolic link.
11396 @item -p @var{file}
11398 @cindex named pipe check
11399 True if @var{file} exists and is a named pipe.
11401 @item -S @var{file}
11403 @cindex socket check
11404 True if @var{file} exists and is a socket.
11408 @cindex terminal check
11409 True if @var{fd} is a file descriptor that is associated with a
11415 @node Access permission tests
11416 @subsection Access permission tests
11418 @cindex access permission tests
11419 @cindex permission tests
11421 These options test for particular access permissions.
11425 @item -g @var{file}
11427 @cindex set-group-ID check
11428 True if @var{file} exists and has its set-group-ID bit set.
11430 @item -k @var{file}
11432 @cindex sticky bit check
11433 True if @var{file} exists and has its @dfn{sticky} bit set.
11435 @item -r @var{file}
11437 @cindex readable file check
11438 True if @var{file} exists and read permission is granted.
11440 @item -u @var{file}
11442 @cindex set-user-ID check
11443 True if @var{file} exists and has its set-user-ID bit set.
11445 @item -w @var{file}
11447 @cindex writable file check
11448 True if @var{file} exists and write permission is granted.
11450 @item -x @var{file}
11452 @cindex executable file check
11453 True if @var{file} exists and execute permission is granted
11454 (or search permission, if it is a directory).
11456 @item -O @var{file}
11458 @cindex owned by effective user ID check
11459 True if @var{file} exists and is owned by the current effective user ID.
11461 @item -G @var{file}
11463 @cindex owned by effective group ID check
11464 True if @var{file} exists and is owned by the current effective group ID.
11468 @node File characteristic tests
11469 @subsection File characteristic tests
11471 @cindex file characteristic tests
11473 These options test other file characteristics.
11477 @item -e @var{file}
11479 @cindex existence-of-file check
11480 True if @var{file} exists.
11482 @item -s @var{file}
11484 @cindex nonempty file check
11485 True if @var{file} exists and has a size greater than zero.
11487 @item @var{file1} -nt @var{file2}
11489 @cindex newer-than file check
11490 True if @var{file1} is newer (according to modification date) than
11491 @var{file2}, or if @var{file1} exists and @var{file2} does not.
11493 @item @var{file1} -ot @var{file2}
11495 @cindex older-than file check
11496 True if @var{file1} is older (according to modification date) than
11497 @var{file2}, or if @var{file2} exists and @var{file1} does not.
11499 @item @var{file1} -ef @var{file2}
11501 @cindex same file check
11502 @cindex hard link check
11503 True if @var{file1} and @var{file2} have the same device and inode
11504 numbers, i.e., if they are hard links to each other.
11510 @subsection String tests
11512 @cindex string tests
11514 These options test string characteristics. You may need to quote
11515 @var{string} arguments for the shell. For example:
11521 The quotes here prevent the wrong arguments from being passed to
11522 @command{test} if @samp{$V} is empty or contains special characters.
11526 @item -z @var{string}
11528 @cindex zero-length string check
11529 True if the length of @var{string} is zero.
11531 @item -n @var{string}
11532 @itemx @var{string}
11534 @cindex nonzero-length string check
11535 True if the length of @var{string} is nonzero.
11537 @item @var{string1} = @var{string2}
11539 @cindex equal string check
11540 True if the strings are equal.
11542 @item @var{string1} != @var{string2}
11544 @cindex not-equal string check
11545 True if the strings are not equal.
11550 @node Numeric tests
11551 @subsection Numeric tests
11553 @cindex numeric tests
11554 @cindex arithmetic tests
11556 Numeric relational operators. The arguments must be entirely numeric
11557 (possibly negative), or the special expression @w{@code{-l @var{string}}},
11558 which evaluates to the length of @var{string}.
11562 @item @var{arg1} -eq @var{arg2}
11563 @itemx @var{arg1} -ne @var{arg2}
11564 @itemx @var{arg1} -lt @var{arg2}
11565 @itemx @var{arg1} -le @var{arg2}
11566 @itemx @var{arg1} -gt @var{arg2}
11567 @itemx @var{arg1} -ge @var{arg2}
11574 These arithmetic binary operators return true if @var{arg1} is equal,
11575 not-equal, less-than, less-than-or-equal, greater-than, or
11576 greater-than-or-equal than @var{arg2}, respectively.
11583 test -1 -gt -2 && echo yes
11585 test -l abc -gt 1 && echo yes
11588 @error{} test: integer expression expected before -eq
11592 @node Connectives for test
11593 @subsection Connectives for @command{test}
11595 @cindex logical connectives
11596 @cindex connectives, logical
11598 The usual logical connectives.
11604 True if @var{expr} is false.
11606 @item @var{expr1} -a @var{expr2}
11608 @cindex logical and operator
11609 @cindex and operator
11610 True if both @var{expr1} and @var{expr2} are true.
11612 @item @var{expr1} -o @var{expr2}
11614 @cindex logical or operator
11615 @cindex or operator
11616 True if either @var{expr1} or @var{expr2} is true.
11621 @node expr invocation
11622 @section @command{expr}: Evaluate expressions
11625 @cindex expression evaluation
11626 @cindex evaluation of expressions
11628 @command{expr} evaluates an expression and writes the result on standard
11629 output. Each token of the expression must be a separate argument.
11631 Operands are either integers or strings. Integers consist of one or
11632 more decimal digits, with an optional leading @samp{-}.
11633 @command{expr} converts
11634 anything appearing in an operand position to an integer or a string
11635 depending on the operation being applied to it.
11637 Strings are not quoted for @command{expr} itself, though you may need to
11638 quote them to protect characters with special meaning to the shell,
11639 e.g., spaces. However, regardless of whether it is quoted, a string
11640 operand should not be a parenthesis or any of @command{expr}'s
11641 operators like @code{+}, so you cannot safely pass an arbitrary string
11642 @code{$str} to expr merely by quoting it to the shell. One way to
11643 work around this is to use the @sc{gnu} extension @code{+},
11644 (e.g., @code{+ "$str" = foo}); a more portable way is to use
11645 @code{@w{" $str"}} and to adjust the rest of the expression to take
11646 the leading space into account (e.g., @code{@w{" $str" = " foo"}}).
11648 You should not pass a negative integer or a string with leading
11649 @samp{-} as @command{expr}'s first argument, as it might be
11650 misinterpreted as an option; this can be avoided by parenthesization.
11651 Also, portable scripts should not use a string operand that happens to
11652 take the form of an integer; this can be worked around by inserting
11653 leading spaces as mentioned above.
11655 @cindex parentheses for grouping
11656 Operators may be given as infix symbols or prefix keywords. Parentheses
11657 may be used for grouping in the usual manner. You must quote
11658 parentheses and many operators to avoid the shell evaluating them,
11661 When built with support for the GNU MP library, @command{expr} uses
11662 arbitrary-precision arithmetic; otherwise, it uses native arithmetic
11663 types and may fail due to arithmetic overflow.
11665 The only options are @option{--help} and @option{--version}. @xref{Common
11666 options}. Options must precede operands.
11668 @cindex exit status of @command{expr}
11672 0 if the expression is neither null nor 0,
11673 1 if the expression is null or 0,
11674 2 if the expression is invalid,
11675 3 if an internal error occurred (e.g., arithmetic overflow).
11679 * String expressions:: + : match substr index length
11680 * Numeric expressions:: + - * / %
11681 * Relations for expr:: | & < <= = == != >= >
11682 * Examples of expr:: Examples.
11686 @node String expressions
11687 @subsection String expressions
11689 @cindex string expressions
11690 @cindex expressions, string
11692 @command{expr} supports pattern matching and other string operators. These
11693 have higher precedence than both the numeric and relational operators (in
11694 the next sections).
11698 @item @var{string} : @var{regex}
11699 @cindex pattern matching
11700 @cindex regular expression matching
11701 @cindex matching patterns
11702 Perform pattern matching. The arguments are converted to strings and the
11703 second is considered to be a (basic, a la GNU @code{grep}) regular
11704 expression, with a @code{^} implicitly prepended. The first argument is
11705 then matched against this regular expression.
11707 If the match succeeds and @var{regex} uses @samp{\(} and @samp{\)}, the
11708 @code{:} expression returns the part of @var{string} that matched the
11709 subexpression; otherwise, it returns the number of characters matched.
11711 If the match fails, the @code{:} operator returns the null string if
11712 @samp{\(} and @samp{\)} are used in @var{regex}, otherwise 0.
11714 @kindex \( @r{regexp operator}
11715 Only the first @samp{\( @dots{} \)} pair is relevant to the return
11716 value; additional pairs are meaningful only for grouping the regular
11717 expression operators.
11719 @kindex \+ @r{regexp operator}
11720 @kindex \? @r{regexp operator}
11721 @kindex \| @r{regexp operator}
11722 In the regular expression, @code{\+}, @code{\?}, and @code{\|} are
11723 operators which respectively match one or more, zero or one, or separate
11724 alternatives. SunOS and other @command{expr}'s treat these as regular
11725 characters. (@acronym{POSIX} allows either behavior.)
11726 @xref{Top, , Regular Expression Library, regex, Regex}, for details of
11727 regular expression syntax. Some examples are in @ref{Examples of expr}.
11729 @item match @var{string} @var{regex}
11731 An alternative way to do pattern matching. This is the same as
11732 @w{@samp{@var{string} : @var{regex}}}.
11734 @item substr @var{string} @var{position} @var{length}
11736 Returns the substring of @var{string} beginning at @var{position}
11737 with length at most @var{length}. If either @var{position} or
11738 @var{length} is negative, zero, or non-numeric, returns the null string.
11740 @item index @var{string} @var{charset}
11742 Returns the first position in @var{string} where the first character in
11743 @var{charset} was found. If no character in @var{charset} is found in
11744 @var{string}, return 0.
11746 @item length @var{string}
11748 Returns the length of @var{string}.
11750 @item + @var{token}
11752 Interpret @var{token} as a string, even if it is a keyword like @var{match}
11753 or an operator like @code{/}.
11754 This makes it possible to test @code{expr length + "$x"} or
11755 @code{expr + "$x" : '.*/\(.\)'} and have it do the right thing even if
11756 the value of @var{$x} happens to be (for example) @code{/} or @code{index}.
11757 This operator is a @acronym{GNU} extension. Portable shell scripts should use
11758 @code{@w{" $token"} : @w{' \(.*\)'}} instead of @code{+ "$token"}.
11762 To make @command{expr} interpret keywords as strings, you must use the
11763 @code{quote} operator.
11766 @node Numeric expressions
11767 @subsection Numeric expressions
11769 @cindex numeric expressions
11770 @cindex expressions, numeric
11772 @command{expr} supports the usual numeric operators, in order of increasing
11773 precedence. These numeric operators have lower precedence than the
11774 string operators described in the previous section, and higher precedence
11775 than the connectives (next section).
11783 @cindex subtraction
11784 Addition and subtraction. Both arguments are converted to integers;
11785 an error occurs if this cannot be done.
11791 @cindex multiplication
11794 Multiplication, division, remainder. Both arguments are converted to
11795 integers; an error occurs if this cannot be done.
11800 @node Relations for expr
11801 @subsection Relations for @command{expr}
11803 @cindex connectives, logical
11804 @cindex logical connectives
11805 @cindex relations, numeric or string
11807 @command{expr} supports the usual logical connectives and relations. These
11808 have lower precedence than the string and numeric operators
11809 (previous sections). Here is the list, lowest-precedence operator first.
11815 @cindex logical or operator
11816 @cindex or operator
11817 Returns its first argument if that is neither null nor zero, otherwise
11818 its second argument if it is neither null nor zero, otherwise 0. It
11819 does not evaluate its second argument if its first argument is neither
11824 @cindex logical and operator
11825 @cindex and operator
11826 Return its first argument if neither argument is null or zero, otherwise
11827 0. It does not evaluate its second argument if its first argument is
11830 @item < <= = == != >= >
11837 @cindex comparison operators
11839 Compare the arguments and return 1 if the relation is true, 0 otherwise.
11840 @code{==} is a synonym for @code{=}. @command{expr} first tries to convert
11841 both arguments to integers and do a numeric comparison; if either
11842 conversion fails, it does a lexicographic comparison using the character
11843 collating sequence specified by the @env{LC_COLLATE} locale.
11848 @node Examples of expr
11849 @subsection Examples of using @command{expr}
11851 @cindex examples of @command{expr}
11852 Here are a few examples, including quoting for shell metacharacters.
11854 To add 1 to the shell variable @code{foo}, in Bourne-compatible shells:
11857 foo=`expr $foo + 1`
11860 To print the non-directory part of the file name stored in
11861 @code{$fname}, which need not contain a @code{/}:
11864 expr $fname : '.*/\(.*\)' '|' $fname
11867 An example showing that @code{\+} is an operator:
11875 expr abc : 'a\(.\)c'
11877 expr index abcdef cz
11880 @error{} expr: syntax error
11881 expr index + index a
11887 @chapter Redirection
11889 @cindex redirection
11890 @cindex commands for redirection
11892 Unix shells commonly provide several forms of @dfn{redirection}---ways
11893 to change the input source or output destination of a command. But one
11894 useful redirection is performed by a separate command, not by the shell;
11895 it's described here.
11898 * tee invocation:: Redirect output to multiple files or processes.
11902 @node tee invocation
11903 @section @command{tee}: Redirect output to multiple files or processes
11906 @cindex pipe fitting
11907 @cindex destinations, multiple output
11908 @cindex read from stdin and write to stdout and files
11910 The @command{tee} command copies standard input to standard output and also
11911 to any files given as arguments. This is useful when you want not only
11912 to send some data down a pipe, but also to save a copy. Synopsis:
11915 tee [@var{option}]@dots{} [@var{file}]@dots{}
11918 If a file being written to does not already exist, it is created. If a
11919 file being written to already exists, the data it previously contained
11920 is overwritten unless the @option{-a} option is used.
11922 A @var{file} of @samp{-} causes @command{tee} to send another copy of
11923 input to standard output, but this is typically not that useful as the
11924 copies are interleaved.
11926 The program accepts the following options. Also see @ref{Common options}.
11933 Append standard input to the given files rather than overwriting
11937 @itemx --ignore-interrupts
11939 @opindex --ignore-interrupts
11940 Ignore interrupt signals.
11944 The @command{tee} command is useful when you happen to be transferring a large
11945 amount of data and also want to summarize that data without reading
11946 it a second time. For example, when you are downloading a DVD image,
11947 you often want to verify its signature or checksum right away.
11948 The inefficient way to do it is simply:
11951 wget http://example.com/some.iso && sha1sum some.iso
11954 One problem with the above is that it makes you wait for the
11955 download to complete before starting the time-consuming SHA1 computation.
11956 Perhaps even more importantly, the above requires reading
11957 the DVD image a second time (the first was from the network).
11959 The efficient way to do it is to interleave the download
11960 and SHA1 computation. Then, you'll get the checksum for
11961 free, because the entire process parallelizes so well:
11964 # slightly contrived, to demonstrate process substitution
11965 wget -O - http://example.com/dvd.iso \
11966 | tee >(sha1sum > dvd.sha1) > dvd.iso
11969 That makes @command{tee} write not just to the expected output file,
11970 but also to a pipe running @command{sha1sum} and saving the final
11971 checksum in a file named @file{dvd.sha1}.
11973 Note, however, that this example relies on a feature of modern shells
11974 called @dfn{process substitution}
11975 (the @samp{>(command)} syntax, above;
11976 @xref{Process Substitution,,Process Substitution, bashref,
11977 The Bash Reference Manual}.),
11978 so it works with @command{zsh}, @command{bash}, and @command{ksh},
11979 but not with @command{/bin/sh}. So if you write code like this
11980 in a shell script, be sure to start the script with @samp{#!/bin/bash}.
11982 Since the above example writes to one file and one process,
11983 a more conventional and portable use of @command{tee} is even better:
11986 wget -O - http://example.com/dvd.iso \
11987 | tee dvd.iso | sha1sum > dvd.sha1
11990 You can extend this example to make @command{tee} write to two processes,
11991 computing MD5 and SHA1 checksums in parallel. In this case,
11992 process substitution is required:
11995 wget -O - http://example.com/dvd.iso \
11996 | tee >(sha1sum > dvd.sha1) \
11997 >(md5sum > dvd.md5) \
12001 This technique is also useful when you want to make a @emph{compressed}
12002 copy of the contents of a pipe.
12003 Consider a tool to graphically summarize disk usage data from @samp{du -ak}.
12004 For a large hierarchy, @samp{du -ak} can run for a long time,
12005 and can easily produce terabytes of data, so you won't want to
12006 rerun the command unnecessarily. Nor will you want to save
12007 the uncompressed output.
12009 Doing it the inefficient way, you can't even start the GUI
12010 until after you've compressed all of the @command{du} output:
12013 du -ak | gzip -9 > /tmp/du.gz
12014 gzip -d /tmp/du.gz | xdiskusage -a
12017 With @command{tee} and process substitution, you start the GUI
12018 right away and eliminate the decompression completely:
12021 du -ak | tee >(gzip -9 > /tmp/du.gz) | xdiskusage -a
12024 Finally, if you regularly create more than one type of
12025 compressed tarball at once, for example when @code{make dist} creates
12026 both @command{gzip}-compressed and @command{bzip2}-compressed tarballs,
12027 there may be a better way.
12028 Typical @command{automake}-generated @file{Makefile} rules create
12029 the two compressed tar archives with commands in sequence, like this
12030 (slightly simplified):
12033 tardir=your-pkg-M.N
12034 tar chof - "$tardir" | gzip -9 -c > your-pkg-M.N.tar.gz
12035 tar chof - "$tardir" | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12038 However, if the hierarchy you are archiving and compressing is larger
12039 than a couple megabytes, and especially if you are using a multi-processor
12040 system with plenty of memory, then you can do much better by reading the
12041 directory contents only once and running the compression programs in parallel:
12044 tardir=your-pkg-M.N
12045 tar chof - "$tardir" \
12046 | tee >(gzip -9 -c > your-pkg-M.N.tar.gz) \
12047 | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12053 @node File name manipulation
12054 @chapter File name manipulation
12056 @cindex file name manipulation
12057 @cindex manipulation of file names
12058 @cindex commands for file name manipulation
12060 This section describes commands that manipulate file names.
12063 * basename invocation:: Strip directory and suffix from a file name.
12064 * dirname invocation:: Strip last file name component.
12065 * pathchk invocation:: Check file name validity and portability.
12066 * mktemp invocation:: Create temporary file or directory.
12070 @node basename invocation
12071 @section @command{basename}: Strip directory and suffix from a file name
12074 @cindex strip directory and suffix from file names
12075 @cindex directory, stripping from file names
12076 @cindex suffix, stripping from file names
12077 @cindex file names, stripping directory and suffix
12078 @cindex leading directory components, stripping
12080 @command{basename} removes any leading directory components from
12081 @var{name}. Synopsis:
12084 basename @var{name} [@var{suffix}]
12087 If @var{suffix} is specified and is identical to the end of @var{name},
12088 it is removed from @var{name} as well. Note that since trailing slashes
12089 are removed prior to suffix matching, @var{suffix} will do nothing if it
12090 contains slashes. @command{basename} prints the result on standard
12093 @c This test is used both here and in the section on dirname.
12094 @macro basenameAndDirname
12095 Together, @command{basename} and @command{dirname} are designed such
12096 that if @samp{ls "$name"} succeeds, then the command sequence @samp{cd
12097 "$(dirname "$name")"; ls "$(basename "$name")"} will, too. This works
12098 for everything except file names containing a trailing newline.
12100 @basenameAndDirname
12102 @acronym{POSIX} allows the implementation to define the results if
12103 @var{name} is empty or @samp{//}. In the former case, @acronym{GNU}
12104 @command{basename} returns the empty string. In the latter case, the
12105 result is @samp{//} on platforms where @var{//} is distinct from
12106 @var{/}, and @samp{/} on platforms where there is no difference.
12108 The only options are @option{--help} and @option{--version}. @xref{Common
12109 options}. Options must precede operands.
12117 basename /usr/bin/sort
12120 basename include/stdio.h .h
12124 @node dirname invocation
12125 @section @command{dirname}: Strip last file name component
12128 @cindex directory components, printing
12129 @cindex stripping non-directory suffix
12130 @cindex non-directory suffix, stripping
12132 @command{dirname} prints all but the final slash-delimited component of
12133 @var{name}. Slashes on either side of the final component are also
12134 removed. If the string contains no slash, @command{dirname} prints
12135 @samp{.} (meaning the current directory). Synopsis:
12141 @var{name} need not be a file name, but if it is, this operation
12142 effectively lists the directory that contains the final component,
12143 including the case when the final component is itself a directory.
12145 @basenameAndDirname
12147 @acronym{POSIX} allows the implementation to define the results if
12148 @var{name} is @samp{//}. With @acronym{GNU} @command{dirname}, the
12149 result is @samp{//} on platforms where @var{//} is distinct from
12150 @var{/}, and @samp{/} on platforms where there is no difference.
12152 The only options are @option{--help} and @option{--version}. @xref{Common
12160 # Output "/usr/bin".
12161 dirname /usr/bin/sort
12162 dirname /usr/bin//.//
12169 @node pathchk invocation
12170 @section @command{pathchk}: Check file name validity and portability
12173 @cindex file names, checking validity and portability
12174 @cindex valid file names, checking for
12175 @cindex portable file names, checking for
12177 @command{pathchk} checks validity and portability of file names. Synopsis:
12180 pathchk [@var{option}]@dots{} @var{name}@dots{}
12183 For each @var{name}, @command{pathchk} prints an error message if any of
12184 these conditions is true:
12188 One of the existing directories in @var{name} does not have search
12189 (execute) permission,
12191 The length of @var{name} is larger than the maximum supported by the
12194 The length of one component of @var{name} is longer than
12195 its file system's maximum.
12198 A nonexistent @var{name} is not an error, so long a file with that
12199 name could be created under the above conditions.
12201 The program accepts the following options. Also see @ref{Common options}.
12202 Options must precede operands.
12208 Instead of performing checks based on the underlying file system,
12209 print an error message if any of these conditions is true:
12213 A file name is empty.
12216 A file name contains a character outside the @acronym{POSIX} portable file
12217 name character set, namely, the ASCII letters and digits, @samp{.},
12218 @samp{_}, @samp{-}, and @samp{/}.
12221 The length of a file name or one of its components exceeds the
12222 @acronym{POSIX} minimum limits for portability.
12227 Print an error message if a file name is empty, or if it contains a component
12228 that begins with @samp{-}.
12230 @item --portability
12231 @opindex --portability
12232 Print an error message if a file name is not portable to all @acronym{POSIX}
12233 hosts. This option is equivalent to @samp{-p -P}.
12237 @cindex exit status of @command{pathchk}
12241 0 if all specified file names passed all checks,
12245 @node mktemp invocation
12246 @section @command{mktemp}: Create temporary file or directory
12249 @cindex file names, creating temporary
12250 @cindex directory, creating temporary
12251 @cindex temporary files and directories
12253 @command{mktemp} manages the creation of temporary files and
12254 directories. Synopsis:
12257 mktemp [@var{option}]@dots{} [@var{template}]
12260 Safely create a temporary file or directory based on @var{template},
12261 and print its name. If given, @var{template} must include at least
12262 three consecutive @samp{X}s in the last component. If omitted, the template
12263 @samp{tmp.XXXXXXXXXX} is used, and option @option{--tmpdir} is
12264 implied. The final run of @samp{X}s in the @var{template} will be replaced
12265 by alpha-numeric characters; thus, on a case-sensitive file system,
12266 and with a @var{template} including a run of @var{n} instances of @samp{X},
12267 there are @samp{62**@var{n}} potential file names.
12269 Older scripts used to create temporary files by simply joining the
12270 name of the program with the process id (@samp{$$}) as a suffix.
12271 However, that naming scheme is easily predictable, and suffers from a
12272 race condition where the attacker can create an appropriately named
12273 symbolic link, such that when the script then opens a handle to what
12274 it thought was an unused file, it is instead modifying an existing
12275 file. Using the same scheme to create a directory is slightly safer,
12276 since the @command{mkdir} will fail if the target already exists, but
12277 it is still inferior because it allows for denial of service attacks.
12278 Therefore, modern scripts should use the @command{mktemp} command to
12279 guarantee that the generated name will be unpredictable, and that
12280 knowledge of the temporary file name implies that the file was created
12281 by the current script and cannot be modified by other users.
12283 When creating a file, the resulting file has read and write
12284 permissions for the current user, but no permissions for the group or
12285 others; these permissions are reduced if the current umask is more
12288 Here are some examples (although note that if you repeat them, you
12289 will most likely get different file names):
12294 Create a temporary file in the current directory.
12301 Create a temporary file with a known suffix.
12303 $ mktemp --suffix=.txt file-XXXX
12305 $ mktemp file-XXXX-XXXX.txt
12310 Create a secure fifo relative to the user's choice of @env{TMPDIR},
12311 but falling back to the current directory rather than @file{/tmp}.
12312 Note that @command{mktemp} does not create fifos, but can create a
12313 secure directory in which the fifo can live. Exit the shell if the
12314 directory or fifo could not be created.
12316 $ dir=$(mktemp -p "$@{TMPDIR:-.@}" -d dir-XXXX) || exit 1
12318 $ mkfifo "$fifo" || @{ rmdir "$dir"; exit 1; @}
12322 Create and use a temporary file if possible, but ignore failure. The
12323 file will reside in the directory named by @env{TMPDIR}, if specified,
12324 or else in @file{/tmp}.
12326 $ file=$(mktemp -q) && @{
12327 > # Safe to use $file only within this block. Use quotes,
12328 > # since $TMPDIR, and thus $file, may contain whitespace.
12329 > echo ... > "$file"
12335 Act as a semi-random character generator (it is not fully random,
12336 since it is impacted by the contents of the current directory). To
12337 avoid security holes, do not use the resulting names to create a file.
12347 The program accepts the following options. Also see @ref{Common options}.
12354 @opindex --directory
12355 Create a directory rather than a file. The directory will have read,
12356 write, and search permissions for the current user, but no permissions
12357 for the group or others; these permissions are reduced if the current
12358 umask is more restrictive.
12364 Suppress diagnostics about failure to create a file or directory. The
12365 exit status will still reflect whether a file was created.
12371 Generate a temporary name that does not name an existing file, without
12372 changing the file system contents. Using the output of this command
12373 to create a new file is inherently unsafe, as there is a window of
12374 time between generating the name and using it where another process
12375 can create an object by the same name.
12378 @itemx --tmpdir[=@var{dir}]
12381 Treat @var{template} relative to the directory @var{dir}. If
12382 @var{dir} is not specified (only possible with the long option
12383 @option{--tmpdir}) or is the empty string, use the value of
12384 @env{TMPDIR} if available, otherwise use @samp{/tmp}. If this is
12385 specified, @var{template} must not be absolute. However,
12386 @var{template} can still contain slashes, although intermediate
12387 directories must already exist.
12389 @item --suffix=@var{suffix}
12391 Append @var{suffix} to the @var{template}. @var{suffix} must not
12392 contain slash. If @option{--suffix} is specified, @var{template} must
12393 end in @samp{X}; if it is not specified, then an appropriate
12394 @option{--suffix} is inferred by finding the last @samp{X} in
12395 @var{template}. This option exists for use with the default
12396 @var{template} and for the creation of a @var{suffix} that starts with
12401 Treat @var{template} as a single file relative to the value of
12402 @env{TMPDIR} if available, or to the directory specified by
12403 @option{-p}, otherwise to @samp{/tmp}. @var{template} must not
12404 contain slashes. This option is deprecated; the use of @option{-p}
12405 without @option{-t} offers better defaults (by favoring the command
12406 line over @env{TMPDIR}) and more flexibility (by allowing intermediate
12411 @cindex exit status of @command{mktemp}
12415 0 if the file was created,
12420 @node Working context
12421 @chapter Working context
12423 @cindex working context
12424 @cindex commands for printing the working context
12426 This section describes commands that display or alter the context in
12427 which you are working: the current directory, the terminal settings, and
12428 so forth. See also the user-related commands in the next section.
12431 * pwd invocation:: Print working directory.
12432 * stty invocation:: Print or change terminal characteristics.
12433 * printenv invocation:: Print environment variables.
12434 * tty invocation:: Print file name of terminal on standard input.
12438 @node pwd invocation
12439 @section @command{pwd}: Print working directory
12442 @cindex print name of current directory
12443 @cindex current working directory, printing
12444 @cindex working directory, printing
12447 @command{pwd} prints the name of the current directory. Synopsis:
12450 pwd [@var{option}]@dots{}
12453 The program accepts the following options. Also see @ref{Common options}.
12460 If the contents of the environment variable @env{PWD} provide an
12461 absolute name of the current directory with no @samp{.} or @samp{..}
12462 components, but possibly with symbolic links, then output those
12463 contents. Otherwise, fall back to default @option{-P} handling.
12468 @opindex --physical
12469 Print a fully resolved name for the current directory. That is, all
12470 components of the printed name will be actual directory names---none
12471 will be symbolic links.
12474 @cindex symbolic links and @command{pwd}
12475 If @option{-L} and @option{-P} are both given, the last one takes
12476 precedence. If neither option is given, then this implementation uses
12477 @option{-P} as the default unless the @env{POSIXLY_CORRECT}
12478 environment variable is set.
12480 @mayConflictWithShellBuiltIn{pwd}
12485 @node stty invocation
12486 @section @command{stty}: Print or change terminal characteristics
12489 @cindex change or print terminal settings
12490 @cindex terminal settings
12491 @cindex line settings of terminal
12493 @command{stty} prints or changes terminal characteristics, such as baud rate.
12497 stty [@var{option}] [@var{setting}]@dots{}
12498 stty [@var{option}]
12501 If given no line settings, @command{stty} prints the baud rate, line
12502 discipline number (on systems that support it), and line settings
12503 that have been changed from the values set by @samp{stty sane}.
12504 By default, mode reading and setting are performed on the tty line
12505 connected to standard input, although this can be modified by the
12506 @option{--file} option.
12508 @command{stty} accepts many non-option arguments that change aspects of
12509 the terminal line operation, as described below.
12511 The program accepts the following options. Also see @ref{Common options}.
12518 Print all current settings in human-readable form. This option may not
12519 be used in combination with any line settings.
12521 @item -F @var{device}
12522 @itemx --file=@var{device}
12525 Set the line opened by the file name specified in @var{device} instead of
12526 the tty line connected to standard input. This option is necessary
12527 because opening a @acronym{POSIX} tty requires use of the @code{O_NONDELAY} flag to
12528 prevent a @acronym{POSIX} tty from blocking until the carrier detect line is high if
12529 the @code{clocal} flag is not set. Hence, it is not always possible
12530 to allow the shell to open the device in the traditional manner.
12536 @cindex machine-readable @command{stty} output
12537 Print all current settings in a form that can be used as an argument to
12538 another @command{stty} command to restore the current settings. This option
12539 may not be used in combination with any line settings.
12543 Many settings can be turned off by preceding them with a @samp{-}.
12544 Such arguments are marked below with ``May be negated'' in their
12545 description. The descriptions themselves refer to the positive
12546 case, that is, when @emph{not} negated (unless stated otherwise,
12549 Some settings are not available on all @acronym{POSIX} systems, since they use
12550 extensions. Such arguments are marked below with ``Non-@acronym{POSIX}'' in their
12551 description. On non-@acronym{POSIX} systems, those or other settings also may not
12552 be available, but it's not feasible to document all the variations: just
12558 * Control:: Control settings
12559 * Input:: Input settings
12560 * Output:: Output settings
12561 * Local:: Local settings
12562 * Combination:: Combination settings
12563 * Characters:: Special characters
12564 * Special:: Special settings
12569 @subsection Control settings
12571 @cindex control settings
12577 @cindex two-way parity
12578 Generate parity bit in output and expect parity bit in input.
12584 @cindex even parity
12585 Set odd parity (even if negated). May be negated.
12592 @cindex character size
12593 @cindex eight-bit characters
12594 Set character size to 5, 6, 7, or 8 bits.
12599 Send a hangup signal when the last process closes the tty. May be
12605 Use two stop bits per character (one if negated). May be negated.
12609 Allow input to be received. May be negated.
12613 @cindex modem control
12614 Disable modem control signals. May be negated.
12618 @cindex hardware flow control
12619 @cindex flow control, hardware
12620 @cindex RTS/CTS flow control
12621 Enable RTS/CTS flow control. Non-@acronym{POSIX}. May be negated.
12626 @subsection Input settings
12628 @cindex input settings
12629 These settings control operations on data received from the terminal.
12634 @cindex breaks, ignoring
12635 Ignore break characters. May be negated.
12639 @cindex breaks, cause interrupts
12640 Make breaks cause an interrupt signal. May be negated.
12644 @cindex parity, ignoring
12645 Ignore characters with parity errors. May be negated.
12649 @cindex parity errors, marking
12650 Mark parity errors (with a 255-0-character sequence). May be negated.
12654 Enable input parity checking. May be negated.
12658 @cindex eight-bit input
12659 Clear high (8th) bit of input characters. May be negated.
12663 @cindex newline, translating to return
12664 Translate newline to carriage return. May be negated.
12668 @cindex return, ignoring
12669 Ignore carriage return. May be negated.
12673 @cindex return, translating to newline
12674 Translate carriage return to newline. May be negated.
12678 @cindex input encoding, UTF-8
12679 Assume input characters are UTF-8 encoded. May be negated.
12683 @kindex C-s/C-q flow control
12684 @cindex XON/XOFF flow control
12685 Enable XON/XOFF flow control (that is, @kbd{CTRL-S}/@kbd{CTRL-Q}). May
12692 @cindex software flow control
12693 @cindex flow control, software
12694 Enable sending of @code{stop} character when the system input buffer
12695 is almost full, and @code{start} character when it becomes almost
12696 empty again. May be negated.
12700 @cindex uppercase, translating to lowercase
12701 Translate uppercase characters to lowercase. Non-@acronym{POSIX}. May be
12702 negated. Note ilcuc is not implemented, as one would not be able to issue
12703 almost any (lowercase) Unix command, after invoking it.
12707 Allow any character to restart output (only the start character
12708 if negated). Non-@acronym{POSIX}. May be negated.
12712 @cindex beeping at input buffer full
12713 Enable beeping and not flushing input buffer if a character arrives
12714 when the input buffer is full. Non-@acronym{POSIX}. May be negated.
12719 @subsection Output settings
12721 @cindex output settings
12722 These settings control operations on data sent to the terminal.
12727 Postprocess output. May be negated.
12731 @cindex lowercase, translating to output
12732 Translate lowercase characters to uppercase. Non-@acronym{POSIX}. May be
12733 negated. (Note ouclc is not currently implemented.)
12737 @cindex return, translating to newline
12738 Translate carriage return to newline. Non-@acronym{POSIX}. May be negated.
12742 @cindex newline, translating to crlf
12743 Translate newline to carriage return-newline. Non-@acronym{POSIX}. May be
12748 Do not print carriage returns in the first column. Non-@acronym{POSIX}.
12753 Newline performs a carriage return. Non-@acronym{POSIX}. May be negated.
12757 @cindex pad instead of timing for delaying
12758 Use fill (padding) characters instead of timing for delays. Non-@acronym{POSIX}.
12763 @cindex pad character
12764 Use @acronym{ASCII} @sc{del} characters for fill instead of
12765 @acronym{ASCII} @sc{nul} characters. Non-@acronym{POSIX}.
12771 Newline delay style. Non-@acronym{POSIX}.
12778 Carriage return delay style. Non-@acronym{POSIX}.
12784 @opindex tab@var{n}
12785 Horizontal tab delay style. Non-@acronym{POSIX}.
12790 Backspace delay style. Non-@acronym{POSIX}.
12795 Vertical tab delay style. Non-@acronym{POSIX}.
12800 Form feed delay style. Non-@acronym{POSIX}.
12805 @subsection Local settings
12807 @cindex local settings
12812 Enable @code{interrupt}, @code{quit}, and @code{suspend} special
12813 characters. May be negated.
12817 Enable @code{erase}, @code{kill}, @code{werase}, and @code{rprnt}
12818 special characters. May be negated.
12822 Enable non-@acronym{POSIX} special characters. May be negated.
12826 Echo input characters. May be negated.
12832 Echo @code{erase} characters as backspace-space-backspace. May be
12837 @cindex newline echoing after @code{kill}
12838 Echo a newline after a @code{kill} character. May be negated.
12842 @cindex newline, echoing
12843 Echo newline even if not echoing other characters. May be negated.
12847 @cindex flushing, disabling
12848 Disable flushing after @code{interrupt} and @code{quit} special
12849 characters. May be negated.
12853 @cindex case translation
12854 Enable input and output of uppercase characters by preceding their
12855 lowercase equivalents with @samp{\}, when @code{icanon} is set.
12856 Non-@acronym{POSIX}. May be negated.
12860 @cindex background jobs, stopping at terminal write
12861 Stop background jobs that try to write to the terminal. Non-@acronym{POSIX}.
12868 Echo erased characters backward, between @samp{\} and @samp{/}.
12869 Non-@acronym{POSIX}. May be negated.
12875 @cindex control characters, using @samp{^@var{c}}
12876 @cindex hat notation for control characters
12877 Echo control characters in hat notation (@samp{^@var{c}}) instead
12878 of literally. Non-@acronym{POSIX}. May be negated.
12884 Echo the @code{kill} special character by erasing each character on
12885 the line as indicated by the @code{echoprt} and @code{echoe} settings,
12886 instead of by the @code{echoctl} and @code{echok} settings. Non-@acronym{POSIX}.
12892 @subsection Combination settings
12894 @cindex combination settings
12895 Combination settings:
12902 Same as @code{parenb -parodd cs7}. May be negated. If negated, same
12903 as @code{-parenb cs8}.
12907 Same as @code{parenb parodd cs7}. May be negated. If negated, same
12908 as @code{-parenb cs8}.
12912 Same as @code{-icrnl -onlcr}. May be negated. If negated, same as
12913 @code{icrnl -inlcr -igncr onlcr -ocrnl -onlret}.
12917 Reset the @code{erase} and @code{kill} special characters to their default
12924 @c This is too long to write inline.
12926 cread -ignbrk brkint -inlcr -igncr icrnl -ixoff
12927 -iuclc -ixany imaxbel opost -olcuc -ocrnl onlcr
12928 -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0
12929 ff0 isig icanon iexten echo echoe echok -echonl
12930 -noflsh -xcase -tostop -echoprt echoctl echoke
12934 and also sets all special characters to their default values.
12938 Same as @code{brkint ignpar istrip icrnl ixon opost isig icanon}, plus
12939 sets the @code{eof} and @code{eol} characters to their default values
12940 if they are the same as the @code{min} and @code{time} characters.
12941 May be negated. If negated, same as @code{raw}.
12948 -ignbrk -brkint -ignpar -parmrk -inpck -istrip
12949 -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany
12950 -imaxbel -opost -isig -icanon -xcase min 1 time 0
12954 May be negated. If negated, same as @code{cooked}.
12958 Same as @option{-icanon}. May be negated. If negated, same as
12963 @cindex eight-bit characters
12964 Same as @code{-parenb -istrip cs8}. May be negated. If negated,
12965 same as @code{parenb istrip cs7}.
12969 Same as @option{-parenb -istrip -opost cs8}. May be negated.
12970 If negated, same as @code{parenb istrip opost cs7}.
12974 Same as @option{-ixany}. Non-@acronym{POSIX}. May be negated.
12978 Same as @code{tab0}. Non-@acronym{POSIX}. May be negated. If negated, same
12985 Same as @code{xcase iuclc olcuc}. Non-@acronym{POSIX}. May be negated.
12986 (Used for terminals with uppercase characters only.)
12990 Same as @code{echoe echoctl echoke}.
12994 Same as @code{echoe echoctl echoke -ixany intr ^C erase ^? kill C-u}.
12999 @subsection Special characters
13001 @cindex special characters
13002 @cindex characters, special
13004 The special characters' default values vary from system to system.
13005 They are set with the syntax @samp{name value}, where the names are
13006 listed below and the value can be given either literally, in hat
13007 notation (@samp{^@var{c}}), or as an integer which may start with
13008 @samp{0x} to indicate hexadecimal, @samp{0} to indicate octal, or
13009 any other digit to indicate decimal.
13011 @cindex disabling special characters
13012 @kindex u@r{, and disabling special characters}
13013 For GNU stty, giving a value of @code{^-} or @code{undef} disables that
13014 special character. (This is incompatible with Ultrix @command{stty},
13015 which uses a value of @samp{u} to disable a special character. GNU
13016 @command{stty} treats a value @samp{u} like any other, namely to set that
13017 special character to @key{U}.)
13023 Send an interrupt signal.
13027 Send a quit signal.
13031 Erase the last character typed.
13035 Erase the current line.
13039 Send an end of file (terminate the input).
13047 Alternate character to end the line. Non-@acronym{POSIX}.
13051 Switch to a different shell layer. Non-@acronym{POSIX}.
13055 Restart the output after stopping it.
13063 Send a terminal stop signal.
13067 Send a terminal stop signal after flushing the input. Non-@acronym{POSIX}.
13071 Redraw the current line. Non-@acronym{POSIX}.
13075 Erase the last word typed. Non-@acronym{POSIX}.
13079 Enter the next character typed literally, even if it is a special
13080 character. Non-@acronym{POSIX}.
13085 @subsection Special settings
13087 @cindex special settings
13092 Set the minimum number of characters that will satisfy a read until
13093 the time value has expired, when @option{-icanon} is set.
13097 Set the number of tenths of a second before reads time out if the minimum
13098 number of characters have not been read, when @option{-icanon} is set.
13100 @item ispeed @var{n}
13102 Set the input speed to @var{n}.
13104 @item ospeed @var{n}
13106 Set the output speed to @var{n}.
13110 Tell the tty kernel driver that the terminal has @var{n} rows. Non-@acronym{POSIX}.
13113 @itemx columns @var{n}
13116 Tell the kernel that the terminal has @var{n} columns. Non-@acronym{POSIX}.
13122 Print the number of rows and columns that the kernel thinks the
13123 terminal has. (Systems that don't support rows and columns in the kernel
13124 typically use the environment variables @env{LINES} and @env{COLUMNS}
13125 instead; however, GNU @command{stty} does not know anything about them.)
13126 Non-@acronym{POSIX}.
13130 Use line discipline @var{n}. Non-@acronym{POSIX}.
13134 Print the terminal speed.
13137 @cindex baud rate, setting
13138 Set the input and output speeds to @var{n}. @var{n} can be one of: 0
13139 50 75 110 134 134.5 150 200 300 600 1200 1800 2400 4800 9600 19200
13140 38400 @code{exta} @code{extb}. @code{exta} is the same as 19200;
13141 @code{extb} is the same as 38400. Many systems, including GNU/Linux,
13142 support higher speeds. The @command{stty} command includes support
13159 4000000 where the system supports these.
13160 0 hangs up the line if @option{-clocal} is set.
13164 @node printenv invocation
13165 @section @command{printenv}: Print all or some environment variables
13168 @cindex printing all or some environment variables
13169 @cindex environment variables, printing
13171 @command{printenv} prints environment variable values. Synopsis:
13174 printenv [@var{option}] [@var{variable}]@dots{}
13177 If no @var{variable}s are specified, @command{printenv} prints the value of
13178 every environment variable. Otherwise, it prints the value of each
13179 @var{variable} that is set, and nothing for those that are not set.
13181 The program accepts the following option. Also see @ref{Common options}.
13189 @cindex exit status of @command{printenv}
13193 0 if all variables specified were found
13194 1 if at least one specified variable was not found
13195 2 if a write error occurred
13199 @node tty invocation
13200 @section @command{tty}: Print file name of terminal on standard input
13203 @cindex print terminal file name
13204 @cindex terminal file name, printing
13206 @command{tty} prints the file name of the terminal connected to its standard
13207 input. It prints @samp{not a tty} if standard input is not a terminal.
13211 tty [@var{option}]@dots{}
13214 The program accepts the following option. Also see @ref{Common options}.
13224 Print nothing; only return an exit status.
13228 @cindex exit status of @command{tty}
13232 0 if standard input is a terminal
13233 1 if standard input is not a terminal
13234 2 if given incorrect arguments
13235 3 if a write error occurs
13239 @node User information
13240 @chapter User information
13242 @cindex user information, commands for
13243 @cindex commands for printing user information
13245 This section describes commands that print user-related information:
13246 logins, groups, and so forth.
13249 * id invocation:: Print user identity.
13250 * logname invocation:: Print current login name.
13251 * whoami invocation:: Print effective user ID.
13252 * groups invocation:: Print group names a user is in.
13253 * users invocation:: Print login names of users currently logged in.
13254 * who invocation:: Print who is currently logged in.
13258 @node id invocation
13259 @section @command{id}: Print user identity
13262 @cindex real user and group IDs, printing
13263 @cindex effective user and group IDs, printing
13264 @cindex printing real and effective user and group IDs
13266 @command{id} prints information about the given user, or the process
13267 running it if no user is specified. Synopsis:
13270 id [@var{option}]@dots{} [@var{username}]
13273 @vindex POSIXLY_CORRECT
13274 By default, it prints the real user ID, real group ID, effective user ID
13275 if different from the real user ID, effective group ID if different from
13276 the real group ID, and supplemental group IDs.
13277 In addition, if SELinux
13278 is enabled and the @env{POSIXLY_CORRECT} environment variable is not set,
13279 then print @samp{context=@var{c}}, where @var{c} is the security context.
13281 Each of these numeric values is preceded by an identifying string and
13282 followed by the corresponding user or group name in parentheses.
13284 The options cause @command{id} to print only part of the above information.
13285 Also see @ref{Common options}.
13292 Print only the group ID.
13298 Print only the group ID and the supplementary groups.
13304 Print the user or group name instead of the ID number. Requires
13305 @option{-u}, @option{-g}, or @option{-G}.
13311 Print the real, instead of effective, user or group ID. Requires
13312 @option{-u}, @option{-g}, or @option{-G}.
13318 Print only the user ID.
13325 @cindex security context
13326 Print only the security context of the current user.
13327 If SELinux is disabled then print a warning and
13328 set the exit status to 1.
13334 @macro primaryAndSupplementaryGroups{cmd,arg}
13335 Primary and supplementary groups for a process are normally inherited
13336 from its parent and are usually unchanged since login. This means
13337 that if you change the group database after logging in, @command{\cmd\}
13338 will not reflect your changes within your existing login session.
13339 Running @command{\cmd\} with a \arg\ causes the user and group
13340 database to be consulted afresh, and so will give a different result.
13342 @primaryAndSupplementaryGroups{id,user argument}
13344 @node logname invocation
13345 @section @command{logname}: Print current login name
13348 @cindex printing user's login name
13349 @cindex login name, printing
13350 @cindex user name, printing
13353 @command{logname} prints the calling user's name, as found in a
13354 system-maintained file (often @file{/var/run/utmp} or
13355 @file{/etc/utmp}), and exits with a status of 0. If there is no entry
13356 for the calling process, @command{logname} prints
13357 an error message and exits with a status of 1.
13359 The only options are @option{--help} and @option{--version}. @xref{Common
13365 @node whoami invocation
13366 @section @command{whoami}: Print effective user ID
13369 @cindex effective user ID, printing
13370 @cindex printing the effective user ID
13372 @command{whoami} prints the user name associated with the current
13373 effective user ID. It is equivalent to the command @samp{id -un}.
13375 The only options are @option{--help} and @option{--version}. @xref{Common
13381 @node groups invocation
13382 @section @command{groups}: Print group names a user is in
13385 @cindex printing groups a user is in
13386 @cindex supplementary groups, printing
13388 @command{groups} prints the names of the primary and any supplementary
13389 groups for each given @var{username}, or the current process if no names
13390 are given. If more than one name is given, the name of each user is
13392 the list of that user's groups and the user name is separated from the
13393 group list by a colon. Synopsis:
13396 groups [@var{username}]@dots{}
13399 The group lists are equivalent to the output of the command @samp{id -Gn}.
13401 @primaryAndSupplementaryGroups{groups,list of users}
13403 The only options are @option{--help} and @option{--version}. @xref{Common
13409 @node users invocation
13410 @section @command{users}: Print login names of users currently logged in
13413 @cindex printing current usernames
13414 @cindex usernames, printing current
13416 @cindex login sessions, printing users with
13417 @command{users} prints on a single line a blank-separated list of user
13418 names of users currently logged in to the current host. Each user name
13419 corresponds to a login session, so if a user has more than one login
13420 session, that user's name will appear the same number of times in the
13429 With no @var{file} argument, @command{users} extracts its information from
13430 a system-maintained file (often @file{/var/run/utmp} or
13431 @file{/etc/utmp}). If a file argument is given, @command{users} uses
13432 that file instead. A common choice is @file{/var/log/wtmp}.
13434 The only options are @option{--help} and @option{--version}. @xref{Common
13440 @node who invocation
13441 @section @command{who}: Print who is currently logged in
13444 @cindex printing current user information
13445 @cindex information, about current users
13447 @command{who} prints information about users who are currently logged on.
13451 @command{who} [@var{option}] [@var{file}] [am i]
13454 @cindex terminal lines, currently used
13456 @cindex remote hostname
13457 If given no non-option arguments, @command{who} prints the following
13458 information for each user currently logged on: login name, terminal
13459 line, login time, and remote hostname or X display.
13463 If given one non-option argument, @command{who} uses that instead of
13464 a default system-maintained file (often @file{/var/run/utmp} or
13465 @file{/etc/utmp}) as the name of the file containing the record of
13466 users logged on. @file{/var/log/wtmp} is commonly given as an argument
13467 to @command{who} to look at who has previously logged on.
13471 If given two non-option arguments, @command{who} prints only the entry
13472 for the user running it (determined from its standard input), preceded
13473 by the hostname. Traditionally, the two arguments given are @samp{am
13474 i}, as in @samp{who am i}.
13477 Time stamps are listed according to the time zone rules specified by
13478 the @env{TZ} environment variable, or by the system default rules if
13479 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
13480 with @env{TZ}, libc, The GNU C Library Reference Manual}.
13482 The program accepts the following options. Also see @ref{Common options}.
13490 Same as @samp{-b -d --login -p -r -t -T -u}.
13496 Print the date and time of last system boot.
13502 Print information corresponding to dead processes.
13508 Print a line of column headings.
13514 List only the entries that correspond to processes via which the
13515 system is waiting for a user to login. The user name is always @samp{LOGIN}.
13519 Attempt to canonicalize hostnames found in utmp through a DNS lookup. This
13520 is not the default because it can cause significant delays on systems with
13521 automatic dial-up internet access.
13525 Same as @samp{who am i}.
13531 List active processes spawned by init.
13537 Print only the login names and the number of users logged on.
13538 Overrides all other options.
13543 @opindex --runlevel
13544 Print the current (and maybe previous) run-level of the init process.
13548 Ignored; for compatibility with other versions of @command{who}.
13554 Print last system clock change.
13559 After the login time, print the number of hours and minutes that the
13560 user has been idle. @samp{.} means the user was active in the last minute.
13561 @samp{old} means the user has been idle for more than 24 hours.
13572 @opindex --writable
13573 @cindex message status
13574 @pindex write@r{, allowed}
13575 After each login name print a character indicating the user's message status:
13578 @samp{+} allowing @code{write} messages
13579 @samp{-} disallowing @code{write} messages
13580 @samp{?} cannot find terminal device
13588 @node System context
13589 @chapter System context
13591 @cindex system context
13592 @cindex context, system
13593 @cindex commands for system context
13595 This section describes commands that print or change system-wide
13599 * date invocation:: Print or set system date and time.
13600 * arch invocation:: Print machine hardware name.
13601 * nproc invocation:: Print the number of processors.
13602 * uname invocation:: Print system information.
13603 * hostname invocation:: Print or set system name.
13604 * hostid invocation:: Print numeric host identifier.
13605 * uptime invocation:: Print system uptime and load.
13608 @node date invocation
13609 @section @command{date}: Print or set system date and time
13612 @cindex time, printing or setting
13613 @cindex printing the current time
13618 date [@var{option}]@dots{} [+@var{format}]
13619 date [-u|--utc|--universal] @c this avoids a newline in the output
13620 [ MMDDhhmm[[CC]YY][.ss] ]
13624 Invoking @command{date} with no @var{format} argument is equivalent to invoking
13625 it with a default format that depends on the @env{LC_TIME} locale category.
13626 In the default C locale, this format is @samp{'+%a %b %e %H:%M:%S %Z %Y'},
13627 so the output looks like @samp{Thu Mar @ 3 13:47:51 PST 2005}.
13630 Normally, @command{date} uses the time zone rules indicated by the
13631 @env{TZ} environment variable, or the system default rules if @env{TZ}
13632 is not set. @xref{TZ Variable,, Specifying the Time Zone with
13633 @env{TZ}, libc, The GNU C Library Reference Manual}.
13635 @findex strftime @r{and @command{date}}
13636 @cindex time formats
13637 @cindex formatting times
13638 If given an argument that starts with a @samp{+}, @command{date} prints the
13639 current date and time (or the date and time specified by the
13640 @option{--date} option, see below) in the format defined by that argument,
13641 which is similar to that of the @code{strftime} function. Except for
13642 conversion specifiers, which start with @samp{%}, characters in the
13643 format string are printed unchanged. The conversion specifiers are
13649 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
13650 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
13651 * Literal conversion specifiers:: %[%nt]
13652 * Padding and other flags:: Pad with zeros, spaces, etc.
13653 * Setting the time:: Changing the system clock.
13654 * Options for date:: Instead of the current time.
13656 * Date input formats:: Specifying date strings.
13658 * Examples of date:: Examples.
13661 @node Time conversion specifiers
13662 @subsection Time conversion specifiers
13664 @cindex time conversion specifiers
13665 @cindex conversion specifiers, time
13667 @command{date} conversion specifiers related to times.
13671 hour (@samp{00}@dots{}@samp{23})
13673 hour (@samp{01}@dots{}@samp{12})
13675 hour (@samp{ 0}@dots{}@samp{23}).
13676 This is a @acronym{GNU} extension.
13678 hour (@samp{ 1}@dots{}@samp{12}).
13679 This is a @acronym{GNU} extension.
13681 minute (@samp{00}@dots{}@samp{59})
13683 nanoseconds (@samp{000000000}@dots{}@samp{999999999}).
13684 This is a @acronym{GNU} extension.
13686 locale's equivalent of either @samp{AM} or @samp{PM};
13687 blank in many locales.
13688 Noon is treated as @samp{PM} and midnight as @samp{AM}.
13690 like @samp{%p}, except lower case.
13691 This is a @acronym{GNU} extension.
13693 locale's 12-hour clock time (e.g., @samp{11:11:04 PM})
13695 24-hour hour and minute. Same as @samp{%H:%M}.
13696 This is a @acronym{GNU} extension.
13698 @cindex epoch, seconds since
13699 @cindex seconds since the epoch
13700 @cindex beginning of time
13701 seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC.
13702 Leap seconds are not counted unless leap second support is available.
13703 @xref{%s-examples}, for examples.
13704 This is a @acronym{GNU} extension.
13706 second (@samp{00}@dots{}@samp{60}).
13707 This may be @samp{60} if leap seconds are supported.
13709 24-hour hour, minute, and second. Same as @samp{%H:%M:%S}.
13711 locale's time representation (e.g., @samp{23:13:48})
13713 @w{@acronym{RFC} 2822/@acronym{ISO} 8601} style numeric time zone
13714 (e.g., @samp{-0600} or @samp{+0530}), or nothing if no
13715 time zone is determinable. This value reflects the numeric time zone
13716 appropriate for the current time, using the time zone rules specified
13717 by the @env{TZ} environment variable.
13718 The time (and optionally, the time zone rules) can be overridden
13719 by the @option{--date} option.
13720 This is a @acronym{GNU} extension.
13722 @w{@acronym{RFC} 3339/@acronym{ISO} 8601} style numeric time zone with
13723 @samp{:} (e.g., @samp{-06:00} or @samp{+05:30}), or nothing if no time
13724 zone is determinable.
13725 This is a @acronym{GNU} extension.
13727 Numeric time zone to the nearest second with @samp{:} (e.g.,
13728 @samp{-06:00:00} or @samp{+05:30:00}), or nothing if no time zone is
13730 This is a @acronym{GNU} extension.
13732 Numeric time zone with @samp{:} using the minimum necessary precision
13733 (e.g., @samp{-06}, @samp{+05:30}, or @samp{-04:56:02}), or nothing if
13734 no time zone is determinable.
13735 This is a @acronym{GNU} extension.
13737 alphabetic time zone abbreviation (e.g., @samp{EDT}), or nothing if no
13738 time zone is determinable. See @samp{%z} for how it is determined.
13742 @node Date conversion specifiers
13743 @subsection Date conversion specifiers
13745 @cindex date conversion specifiers
13746 @cindex conversion specifiers, date
13748 @command{date} conversion specifiers related to dates.
13752 locale's abbreviated weekday name (e.g., @samp{Sun})
13754 locale's full weekday name, variable length (e.g., @samp{Sunday})
13756 locale's abbreviated month name (e.g., @samp{Jan})
13758 locale's full month name, variable length (e.g., @samp{January})
13760 locale's date and time (e.g., @samp{Thu Mar @ 3 23:05:25 2005})
13762 century. This is like @samp{%Y}, except the last two digits are omitted.
13763 For example, it is @samp{20} if @samp{%Y} is @samp{2000},
13764 and is @samp{-0} if @samp{%Y} is @samp{-001}.
13765 It is normally at least two characters, but it may be more.
13767 day of month (e.g., @samp{01})
13769 date; same as @samp{%m/%d/%y}
13771 day of month, space padded; same as @samp{%_d}
13773 full date in @acronym{ISO} 8601 format; same as @samp{%Y-%m-%d}.
13774 This is a good choice for a date format, as it is standard and
13775 is easy to sort in the usual case where years are in the range
13777 This is a @acronym{GNU} extension.
13779 year corresponding to the @acronym{ISO} week number, but without the century
13780 (range @samp{00} through @samp{99}). This has the same format and value
13781 as @samp{%y}, except that if the @acronym{ISO} week number (see
13783 to the previous or next year, that year is used instead.
13784 This is a @acronym{GNU} extension.
13786 year corresponding to the @acronym{ISO} week number. This has the
13787 same format and value as @samp{%Y}, except that if the @acronym{ISO}
13789 @samp{%V}) belongs to the previous or next year, that year is used
13791 It is normally useful only if @samp{%V} is also used;
13792 for example, the format @samp{%G-%m-%d} is probably a mistake,
13793 since it combines the ISO week number year with the conventional month and day.
13794 This is a @acronym{GNU} extension.
13798 day of year (@samp{001}@dots{}@samp{366})
13800 month (@samp{01}@dots{}@samp{12})
13802 day of week (@samp{1}@dots{}@samp{7}) with @samp{1} corresponding to Monday
13804 week number of year, with Sunday as the first day of the week
13805 (@samp{00}@dots{}@samp{53}).
13806 Days in a new year preceding the first Sunday are in week zero.
13808 @acronym{ISO} week number, that is, the
13809 week number of year, with Monday as the first day of the week
13810 (@samp{01}@dots{}@samp{53}).
13811 If the week containing January 1 has four or more days in
13812 the new year, then it is considered week 1; otherwise, it is week 53 of
13813 the previous year, and the next week is week 1. (See the @acronym{ISO} 8601
13816 day of week (@samp{0}@dots{}@samp{6}) with 0 corresponding to Sunday
13818 week number of year, with Monday as first day of week
13819 (@samp{00}@dots{}@samp{53}).
13820 Days in a new year preceding the first Monday are in week zero.
13822 locale's date representation (e.g., @samp{12/31/99})
13824 last two digits of year (@samp{00}@dots{}@samp{99})
13826 year. This is normally at least four characters, but it may be more.
13827 Year @samp{0000} precedes year @samp{0001}, and year @samp{-001}
13828 precedes year @samp{0000}.
13832 @node Literal conversion specifiers
13833 @subsection Literal conversion specifiers
13835 @cindex literal conversion specifiers
13836 @cindex conversion specifiers, literal
13838 @command{date} conversion specifiers that produce literal strings.
13850 @node Padding and other flags
13851 @subsection Padding and other flags
13853 @cindex numeric field padding
13854 @cindex padding of numeric fields
13855 @cindex fields, padding numeric
13857 Unless otherwise specified, @command{date} normally pads numeric fields
13858 with zeros, so that, for
13859 example, numeric months are always output as two digits.
13860 Seconds since the epoch are not padded, though,
13861 since there is no natural width for them.
13863 As a @acronym{GNU} extension, @command{date} recognizes any of the
13864 following optional flags after the @samp{%}:
13868 (hyphen) Do not pad the field; useful if the output is intended for
13871 (underscore) Pad with spaces; useful if you need a fixed
13872 number of characters in the output, but zeros are too distracting.
13874 (zero) Pad with zeros even if the conversion specifier
13875 would normally pad with spaces.
13877 Use upper case characters if possible.
13879 Use opposite case characters if possible.
13880 A field that is normally upper case becomes lower case, and vice versa.
13884 Here are some examples of padding:
13887 date +%d/%m -d "Feb 1"
13889 date +%-d/%-m -d "Feb 1"
13891 date +%_d/%_m -d "Feb 1"
13895 As a @acronym{GNU} extension, you can specify the field width
13896 (after any flag, if present) as a decimal number. If the natural size of the
13897 output of the field has less than the specified number of characters,
13898 the result is written right adjusted and padded to the given
13899 size. For example, @samp{%9B} prints the right adjusted month name in
13900 a field of width 9.
13902 An optional modifier can follow the optional flag and width
13903 specification. The modifiers are:
13907 Use the locale's alternate representation for date and time. This
13908 modifier applies to the @samp{%c}, @samp{%C}, @samp{%x}, @samp{%X},
13909 @samp{%y} and @samp{%Y} conversion specifiers. In a Japanese locale, for
13910 example, @samp{%Ex} might yield a date format based on the Japanese
13914 Use the locale's alternate numeric symbols for numbers. This modifier
13915 applies only to numeric conversion specifiers.
13918 If the format supports the modifier but no alternate representation
13919 is available, it is ignored.
13922 @node Setting the time
13923 @subsection Setting the time
13925 @cindex setting the time
13926 @cindex time setting
13927 @cindex appropriate privileges
13929 If given an argument that does not start with @samp{+}, @command{date} sets
13930 the system clock to the date and time specified by that argument (as
13931 described below). You must have appropriate privileges to set the
13932 system clock. The @option{--date} and @option{--set} options may not be
13933 used with such an argument. The @option{--universal} option may be used
13934 with such an argument to indicate that the specified date and time are
13935 relative to Coordinated Universal Time rather than to the local time
13938 The argument must consist entirely of digits, which have the following
13951 first two digits of year (optional)
13953 last two digits of year (optional)
13958 The @option{--set} option also sets the system clock; see the next section.
13961 @node Options for date
13962 @subsection Options for @command{date}
13964 @cindex @command{date} options
13965 @cindex options for @command{date}
13967 The program accepts the following options. Also see @ref{Common options}.
13971 @item -d @var{datestr}
13972 @itemx --date=@var{datestr}
13975 @cindex parsing date strings
13976 @cindex date strings, parsing
13977 @cindex arbitrary date strings, parsing
13980 @opindex next @var{day}
13981 @opindex last @var{day}
13982 Display the date and time specified in @var{datestr} instead of the
13983 current date and time. @var{datestr} can be in almost any common
13984 format. It can contain month names, time zones, @samp{am} and @samp{pm},
13985 @samp{yesterday}, etc. For example, @option{--date="2004-02-27
13986 14:19:13.489392193 +0530"} specifies the instant of time that is
13987 489,392,193 nanoseconds after February 27, 2004 at 2:19:13 PM in a
13988 time zone that is 5 hours and 30 minutes east of @acronym{UTC}.@*
13989 Note: input currently must be in locale independent format. E.g., the
13990 LC_TIME=C below is needed to print back the correct date in many locales:
13992 date -d "$(LC_TIME=C date)"
13994 @xref{Date input formats}.
13996 @item -f @var{datefile}
13997 @itemx --file=@var{datefile}
14000 Parse each line in @var{datefile} as with @option{-d} and display the
14001 resulting date and time. If @var{datefile} is @samp{-}, use standard
14002 input. This is useful when you have many dates to process, because the
14003 system overhead of starting up the @command{date} executable many times can
14006 @item -r @var{file}
14007 @itemx --reference=@var{file}
14009 @opindex --reference
14010 Display the date and time of the last modification of @var{file},
14011 instead of the current date and time.
14018 @opindex --rfc-2822
14019 Display the date and time using the format @samp{%a, %d %b %Y %H:%M:%S
14020 %z}, evaluated in the C locale so abbreviations are always in English.
14024 Fri, 09 Sep 2005 13:51:39 -0700
14027 This format conforms to
14028 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt, Internet
14029 @acronym{RFCs} 2822} and
14030 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc822.txt, 822}, the
14031 current and previous standards for Internet email.
14033 @item --rfc-3339=@var{timespec}
14034 @opindex --rfc-3339=@var{timespec}
14035 Display the date using a format specified by
14036 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt, Internet
14037 @acronym{RFC} 3339}. This is a subset of the @acronym{ISO} 8601
14038 format, except that it also permits applications to use a space rather
14039 than a @samp{T} to separate dates from times. Unlike the other
14040 standard formats, @acronym{RFC} 3339 format is always suitable as
14041 input for the @option{--date} (@option{-d}) and @option{--file}
14042 (@option{-f}) options, regardless of the current locale.
14044 The argument @var{timespec} specifies how much of the time to include.
14045 It can be one of the following:
14049 Print just the full-date, e.g., @samp{2005-09-14}.
14050 This is equivalent to the format @samp{%Y-%m-%d}.
14053 Print the full-date and full-time separated by a space, e.g.,
14054 @samp{2005-09-14 00:56:06+05:30}. The output ends with a numeric
14055 time-offset; here the @samp{+05:30} means that local time is five
14056 hours and thirty minutes east of @acronym{UTC}. This is equivalent to
14057 the format @samp{%Y-%m-%d %H:%M:%S%:z}.
14060 Like @samp{seconds}, but also print nanoseconds, e.g.,
14061 @samp{2005-09-14 00:56:06.998458565+05:30}.
14062 This is equivalent to the format @samp{%Y-%m-%d %H:%M:%S.%N%:z}.
14066 @item -s @var{datestr}
14067 @itemx --set=@var{datestr}
14070 Set the date and time to @var{datestr}. See @option{-d} above.
14077 @opindex --universal
14078 @cindex Coordinated Universal Time
14080 @cindex Greenwich Mean Time
14083 Use Coordinated Universal Time (@acronym{UTC}) by operating as if the
14084 @env{TZ} environment variable were set to the string @samp{UTC0}.
14086 Universal Time is often called ``Greenwich Mean Time'' (@sc{gmt}) for
14087 historical reasons.
14091 @node Examples of date
14092 @subsection Examples of @command{date}
14094 @cindex examples of @command{date}
14096 Here are a few examples. Also see the documentation for the @option{-d}
14097 option in the previous section.
14102 To print the date of the day before yesterday:
14105 date --date='2 days ago'
14109 To print the date of the day three months and one day hence:
14112 date --date='3 months 1 day'
14116 To print the day of year of Christmas in the current year:
14119 date --date='25 Dec' +%j
14123 To print the current full month name and the day of the month:
14129 But this may not be what you want because for the first nine days of
14130 the month, the @samp{%d} expands to a zero-padded two-digit field,
14131 for example @samp{date -d 1may '+%B %d'} will print @samp{May 01}.
14134 To print a date without the leading zero for one-digit days
14135 of the month, you can use the (@acronym{GNU} extension)
14136 @samp{-} flag to suppress
14137 the padding altogether:
14140 date -d 1may '+%B %-d
14144 To print the current date and time in the format required by many
14145 non-@acronym{GNU} versions of @command{date} when setting the system clock:
14148 date +%m%d%H%M%Y.%S
14152 To set the system clock forward by two minutes:
14155 date --set='+2 minutes'
14159 To print the date in @acronym{RFC} 2822 format,
14160 use @samp{date --rfc-2822}. Here is some example output:
14163 Fri, 09 Sep 2005 13:51:39 -0700
14166 @anchor{%s-examples}
14168 To convert a date string to the number of seconds since the epoch
14169 (which is 1970-01-01 00:00:00 UTC), use the @option{--date} option with
14170 the @samp{%s} format. That can be useful in sorting and/or graphing
14171 and/or comparing data by date. The following command outputs the
14172 number of the seconds since the epoch for the time two minutes after the
14176 date --date='1970-01-01 00:02:00 +0000' +%s
14180 If you do not specify time zone information in the date string,
14181 @command{date} uses your computer's idea of the time zone when
14182 interpreting the string. For example, if your computer's time zone is
14183 that of Cambridge, Massachusetts, which was then 5 hours (i.e., 18,000
14184 seconds) behind UTC:
14187 # local time zone used
14188 date --date='1970-01-01 00:02:00' +%s
14193 If you're sorting or graphing dated data, your raw date values may be
14194 represented as seconds since the epoch. But few people can look at
14195 the date @samp{946684800} and casually note ``Oh, that's the first second
14196 of the year 2000 in Greenwich, England.''
14199 date --date='2000-01-01 UTC' +%s
14203 An alternative is to use the @option{--utc} (@option{-u}) option.
14204 Then you may omit @samp{UTC} from the date string. Although this
14205 produces the same result for @samp{%s} and many other format sequences,
14206 with a time zone offset different from zero, it would give a different
14207 result for zone-dependent formats like @samp{%z}.
14210 date -u --date=2000-01-01 +%s
14214 To convert such an unwieldy number of seconds back to
14215 a more readable form, use a command like this:
14218 # local time zone used
14219 date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
14220 1999-12-31 19:00:00 -0500
14223 Or if you do not mind depending on the @samp{@@} feature present since
14224 coreutils 5.3.0, you could shorten this to:
14227 date -d @@946684800 +"%F %T %z"
14228 1999-12-31 19:00:00 -0500
14231 Often it is better to output UTC-relative date and time:
14234 date -u -d '1970-01-01 946684800 seconds' +"%Y-%m-%d %T %z"
14235 2000-01-01 00:00:00 +0000
14241 @node arch invocation
14242 @section @command{arch}: Print machine hardware name
14245 @cindex print machine hardware name
14246 @cindex system information, printing
14248 @command{arch} prints the machine hardware name,
14249 and is equivalent to @samp{uname -m}.
14253 arch [@var{option}]
14256 The program accepts the @ref{Common options} only.
14261 @node nproc invocation
14262 @section @command{nproc}: Print the number of available processors
14265 @cindex Print the number of processors
14266 @cindex system information, printing
14268 Print the number of processing units available to the current process,
14269 which may be less than the number of online processors.
14270 If this information is not accessible, then print the number of
14271 processors installed. If the @env{OMP_NUM_THREADS} environment variable is
14272 set, then it will determine the returned value. The result is guaranteed to be
14273 greater than zero. Synopsis:
14276 nproc [@var{option}]
14279 The program accepts the following options. Also see @ref{Common options}.
14285 Print the number of installed processors on the system, which may
14286 be greater than the number online or available to the current process.
14287 The @env{OMP_NUM_THREADS} environment variable is not honored in this case.
14289 @item --ignore=@var{number}
14291 If possible, exclude this @var{number} of processing units.
14298 @node uname invocation
14299 @section @command{uname}: Print system information
14302 @cindex print system information
14303 @cindex system information, printing
14305 @command{uname} prints information about the machine and operating system
14306 it is run on. If no options are given, @command{uname} acts as if the
14307 @option{-s} option were given. Synopsis:
14310 uname [@var{option}]@dots{}
14313 If multiple options or @option{-a} are given, the selected information is
14314 printed in this order:
14317 @var{kernel-name} @var{nodename} @var{kernel-release} @var{kernel-version}
14318 @var{machine} @var{processor} @var{hardware-platform} @var{operating-system}
14321 The information may contain internal spaces, so such output cannot be
14322 parsed reliably. In the following example, @var{release} is
14323 @samp{2.2.18ss.e820-bda652a #4 SMP Tue Jun 5 11:24:08 PDT 2001}:
14327 @result{} Linux dum 2.2.18 #4 SMP Tue Jun 5 11:24:08 PDT 2001 i686 unknown unknown GNU/Linux
14331 The program accepts the following options. Also see @ref{Common options}.
14339 Print all of the below information, except omit the processor type
14340 and the hardware platform name if they are unknown.
14343 @itemx --hardware-platform
14345 @opindex --hardware-platform
14346 @cindex implementation, hardware
14347 @cindex hardware platform
14348 @cindex platform, hardware
14349 Print the hardware platform name
14350 (sometimes called the hardware implementation).
14351 Print @samp{unknown} if the kernel does not make this information
14352 easily available, as is the case with Linux kernels.
14358 @cindex machine type
14359 @cindex hardware class
14360 @cindex hardware type
14361 Print the machine hardware name (sometimes called the hardware class
14367 @opindex --nodename
14370 @cindex network node name
14371 Print the network node hostname.
14376 @opindex --processor
14377 @cindex host processor type
14378 Print the processor type (sometimes called the instruction set
14379 architecture or ISA).
14380 Print @samp{unknown} if the kernel does not make this information
14381 easily available, as is the case with Linux kernels.
14384 @itemx --operating-system
14386 @opindex --operating-system
14387 @cindex operating system name
14388 Print the name of the operating system.
14391 @itemx --kernel-release
14393 @opindex --kernel-release
14394 @cindex kernel release
14395 @cindex release of kernel
14396 Print the kernel release.
14399 @itemx --kernel-name
14401 @opindex --kernel-name
14402 @cindex kernel name
14403 @cindex name of kernel
14404 Print the kernel name.
14405 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) calls this
14406 ``the implementation of the operating system'', because the
14407 @acronym{POSIX} specification itself has no notion of ``kernel''.
14408 The kernel name might be the same as the operating system name printed
14409 by the @option{-o} or @option{--operating-system} option, but it might
14410 differ. Some operating systems (e.g., FreeBSD, HP-UX) have the same
14411 name as their underlying kernels; others (e.g., GNU/Linux, Solaris)
14415 @itemx --kernel-version
14417 @opindex --kernel-version
14418 @cindex kernel version
14419 @cindex version of kernel
14420 Print the kernel version.
14427 @node hostname invocation
14428 @section @command{hostname}: Print or set system name
14431 @cindex setting the hostname
14432 @cindex printing the hostname
14433 @cindex system name, printing
14434 @cindex appropriate privileges
14436 With no arguments, @command{hostname} prints the name of the current host
14437 system. With one argument, it sets the current host name to the
14438 specified string. You must have appropriate privileges to set the host
14442 hostname [@var{name}]
14445 The only options are @option{--help} and @option{--version}. @xref{Common
14451 @node hostid invocation
14452 @section @command{hostid}: Print numeric host identifier
14455 @cindex printing the host identifier
14457 @command{hostid} prints the numeric identifier of the current host
14458 in hexadecimal. This command accepts no arguments.
14459 The only options are @option{--help} and @option{--version}.
14460 @xref{Common options}.
14462 For example, here's what it prints on one system I use:
14469 On that system, the 32-bit quantity happens to be closely
14470 related to the system's Internet address, but that isn't always
14475 @node uptime invocation
14476 @section @command{uptime}: Print system uptime and load
14479 @cindex printing the system uptime and load
14481 @command{uptime} prints the current time, the system's uptime, the
14482 number of logged-in users and the current load average.
14484 If an argument is specified, it is used as the file to be read
14485 to discover how many users are logged in. If no argument is
14486 specified, a system default is used (@command{uptime --help} indicates
14487 the default setting).
14489 The only options are @option{--help} and @option{--version}.
14490 @xref{Common options}.
14492 For example, here's what it prints right now on one system I use:
14496 14:07 up 3:35, 3 users, load average: 1.39, 1.15, 1.04
14499 The precise method of calculation of load average varies somewhat
14500 between systems. Some systems calculate it as the average number of
14501 runnable processes over the last 1, 5 and 15 minutes, but some systems
14502 also include processes in the uninterruptible sleep state (that is,
14503 those processes which are waiting for disk I/O). The Linux kernel
14504 includes uninterruptible processes.
14506 @node SELinux context
14507 @chapter SELinux context
14509 @cindex SELinux context
14510 @cindex SELinux, context
14511 @cindex commands for SELinux context
14513 This section describes commands for operations with SELinux
14517 * chcon invocation:: Change SELinux context of file
14518 * runcon invocation:: Run a command in specified SELinux context
14521 @node chcon invocation
14522 @section @command{chcon}: Change SELinux context of file
14525 @cindex changing security context
14526 @cindex change SELinux context
14528 @command{chcon} changes the SELinux security context of the selected files.
14532 chcon [@var{option}]@dots{} @var{context} @var{file}@dots{}
14533 chcon [@var{option}]@dots{} [-u @var{user}] [-r @var{role}] [-l @var{range}] [-t @var{type}] @var{file}@dots{}
14534 chcon [@var{option}]@dots{} --reference=@var{rfile} @var{file}@dots{}
14537 Change the SELinux security context of each @var{file} to @var{context}.
14538 With @option{--reference}, change the security context of each @var{file}
14539 to that of @var{rfile}.
14541 The program accepts the following options. Also see @ref{Common options}.
14546 @itemx --no-dereference
14548 @opindex --no-dereference
14549 @cindex no dereference
14550 Affect symbolic links instead of any referenced file.
14552 @item --reference=@var{rfile}
14553 @opindex --reference
14554 @cindex reference file
14555 Use @var{rfile}'s security context rather than specifying a @var{context} value.
14560 @opindex --recursive
14561 Operate on files and directories recursively.
14564 @xref{Traversing symlinks}.
14567 @xref{Traversing symlinks}.
14570 @xref{Traversing symlinks}.
14577 Output a diagnostic for every file processed.
14579 @item -u @var{user}
14580 @itemx --user=@var{user}
14583 Set user @var{user} in the target security context.
14585 @item -r @var{role}
14586 @itemx --role=@var{role}
14589 Set role @var{role} in the target security context.
14591 @item -t @var{type}
14592 @itemx --type=@var{type}
14595 Set type @var{type} in the target security context.
14597 @item -l @var{range}
14598 @itemx --range=@var{range}
14601 Set range @var{range} in the target security context.
14607 @node runcon invocation
14608 @section @command{runcon}: Run a command in specified SELinux context
14611 @cindex run with security context
14614 @command{runcon} runs file in specified SELinux security context.
14618 runcon @var{context} @var{command} [@var{args}]
14619 runcon [ -c ] [-u @var{user}] [-r @var{role}] [-t @var{type}] [-l @var{range}] @var{command} [@var{args}]
14622 Run @var{command} with completely-specified @var{context}, or with
14623 current or transitioned security context modified by one or more of @var{level},
14624 @var{role}, @var{type} and @var{user}.
14626 If none of @option{-c}, @option{-t}, @option{-u}, @option{-r}, or @option{-l}
14627 is specified, the first argument is used as the complete context.
14628 Any additional arguments after @var{command}
14629 are interpreted as arguments to the command.
14631 With neither @var{context} nor @var{command}, print the current security context.
14633 The program accepts the following options. Also see @ref{Common options}.
14641 Compute process transition context before modifying.
14643 @item -u @var{user}
14644 @itemx --user=@var{user}
14647 Set user @var{user} in the target security context.
14649 @item -r @var{role}
14650 @itemx --role=@var{role}
14653 Set role @var{role} in the target security context.
14655 @item -t @var{type}
14656 @itemx --type=@var{type}
14659 Set type @var{type} in the target security context.
14661 @item -l @var{range}
14662 @itemx --range=@var{range}
14665 Set range @var{range} in the target security context.
14669 @cindex exit status of @command{runcon}
14673 126 if @var{command} is found but cannot be invoked
14674 127 if @command{runcon} itself fails or if @var{command} cannot be found
14675 the exit status of @var{command} otherwise
14678 @node Modified command invocation
14679 @chapter Modified command invocation
14681 @cindex modified command invocation
14682 @cindex invocation of commands, modified
14683 @cindex commands for invoking other commands
14685 This section describes commands that run other commands in some context
14686 different than the current one: a modified environment, as a different
14690 * chroot invocation:: Modify the root directory.
14691 * env invocation:: Modify environment variables.
14692 * nice invocation:: Modify niceness.
14693 * nohup invocation:: Immunize to hangups.
14694 * stdbuf invocation:: Modify buffering of standard streams.
14695 * su invocation:: Modify user and group ID.
14696 * timeout invocation:: Run with time limit.
14700 @node chroot invocation
14701 @section @command{chroot}: Run a command with a different root directory
14704 @cindex running a program in a specified root directory
14705 @cindex root directory, running a program in a specified
14707 @command{chroot} runs a command with a specified root directory.
14708 On many systems, only the super-user can do this.@footnote{However,
14709 some systems (e.g., FreeBSD) can be configured to allow certain regular
14710 users to use the @code{chroot} system call, and hence to run this program.
14711 Also, on Cygwin, anyone can run the @command{chroot} command, because the
14712 underlying function is non-privileged due to lack of support in MS-Windows.}
14716 chroot @var{option} @var{newroot} [@var{command} [@var{args}]@dots{}]
14717 chroot @var{option}
14720 Ordinarily, file names are looked up starting at the root of the
14721 directory structure, i.e., @file{/}. @command{chroot} changes the root to
14722 the directory @var{newroot} (which must exist) and then runs
14723 @var{command} with optional @var{args}. If @var{command} is not
14724 specified, the default is the value of the @env{SHELL} environment
14725 variable or @command{/bin/sh} if not set, invoked with the @option{-i} option.
14726 @var{command} must not be a special built-in utility
14727 (@pxref{Special built-in utilities}).
14729 The program accepts the following options. Also see @ref{Common options}.
14730 Options must precede operands.
14734 @itemx --userspec=@var{user}[:@var{group}]
14735 @opindex --userspec
14736 By default, @var{command} is run with the same credentials
14737 as the invoking process.
14738 Use this option to run it as a different @var{user} and/or with a
14739 different primary @var{group}.
14741 @itemx --groups=@var{groups}
14743 Use this option to specify the supplementary @var{groups} to be
14744 used by the new process.
14745 The items in the list (names or numeric IDs) must be separated by commas.
14749 Here are a few tips to help avoid common problems in using chroot.
14750 To start with a simple example, make @var{command} refer to a statically
14751 linked binary. If you were to use a dynamically linked executable, then
14752 you'd have to arrange to have the shared libraries in the right place under
14753 your new root directory.
14755 For example, if you create a statically linked @command{ls} executable,
14756 and put it in @file{/tmp/empty}, you can run this command as root:
14759 $ chroot /tmp/empty /ls -Rl /
14762 Then you'll see output like this:
14767 -rwxr-xr-x 1 0 0 1041745 Aug 16 11:17 ls
14770 If you want to use a dynamically linked executable, say @command{bash},
14771 then first run @samp{ldd bash} to see what shared objects it needs.
14772 Then, in addition to copying the actual binary, also copy the listed
14773 files to the required positions under your intended new root directory.
14774 Finally, if the executable requires any other files (e.g., data, state,
14775 device files), copy them into place, too.
14777 @cindex exit status of @command{chroot}
14781 125 if @command{chroot} itself fails
14782 126 if @var{command} is found but cannot be invoked
14783 127 if @var{command} cannot be found
14784 the exit status of @var{command} otherwise
14788 @node env invocation
14789 @section @command{env}: Run a command in a modified environment
14792 @cindex environment, running a program in a modified
14793 @cindex modified environment, running a program in a
14794 @cindex running a program in a modified environment
14796 @command{env} runs a command with a modified environment. Synopses:
14799 env [@var{option}]@dots{} [@var{name}=@var{value}]@dots{} @c
14800 [@var{command} [@var{args}]@dots{}]
14804 Operands of the form @samp{@var{variable}=@var{value}} set
14805 the environment variable @var{variable} to value @var{value}.
14806 @var{value} may be empty (@samp{@var{variable}=}). Setting a variable
14807 to an empty value is different from unsetting it.
14808 These operands are evaluated left-to-right, so if two operands
14809 mention the same variable the earlier is ignored.
14811 Environment variable names can be empty, and can contain any
14812 characters other than @samp{=} and @acronym{ASCII} @sc{nul}.
14813 However, it is wise to limit yourself to names that
14814 consist solely of underscores, digits, and @acronym{ASCII} letters,
14815 and that begin with a non-digit, as applications like the shell do not
14816 work well with other names.
14819 The first operand that does not contain the character @samp{=}
14820 specifies the program to invoke; it is
14821 searched for according to the @env{PATH} environment variable. Any
14822 remaining arguments are passed as arguments to that program.
14823 The program should not be a special built-in utility
14824 (@pxref{Special built-in utilities}).
14826 Modifications to @env{PATH} take effect prior to searching for
14827 @var{command}. Use caution when reducing @env{PATH}; behavior is
14828 not portable when @env{PATH} is undefined or omits key directories
14829 such as @file{/bin}.
14831 In the rare case that a utility contains a @samp{=} in the name, the
14832 only way to disambiguate it from a variable assignment is to use an
14833 intermediate command for @var{command}, and pass the problematic
14834 program name via @var{args}. For example, if @file{./prog=} is an
14835 executable in the current @env{PATH}:
14838 env prog= true # runs 'true', with prog= in environment
14839 env ./prog= true # runs 'true', with ./prog= in environment
14840 env -- prog= true # runs 'true', with prog= in environment
14841 env sh -c '\prog= true' # runs 'prog=' with argument 'true'
14842 env sh -c 'exec "$@@"' sh prog= true # also runs 'prog='
14845 @cindex environment, printing
14847 If no command name is specified following the environment
14848 specifications, the resulting environment is printed. This is like
14849 specifying the @command{printenv} program.
14851 For some examples, suppose the environment passed to @command{env}
14852 contains @samp{LOGNAME=rms}, @samp{EDITOR=emacs}, and
14853 @samp{PATH=.:/gnubin:/hacks}:
14858 Output the current environment.
14860 $ env | LC_ALL=C sort
14863 PATH=.:/gnubin:/hacks
14867 Run @command{foo} with a reduced environment, preserving only the
14868 original @env{PATH} to avoid problems in locating @command{foo}.
14870 env - PATH="$PATH" foo
14874 Run @command{foo} with the environment containing @samp{LOGNAME=rms},
14875 @samp{EDITOR=emacs}, and @samp{PATH=.:/gnubin:/hacks}, and guarantees
14876 that @command{foo} was found in the file system rather than as a shell
14883 Run @command{nemacs} with the environment containing @samp{LOGNAME=foo},
14884 @samp{EDITOR=emacs}, @samp{PATH=.:/gnubin:/hacks}, and
14885 @samp{DISPLAY=gnu:0}.
14887 env DISPLAY=gnu:0 LOGNAME=foo nemacs
14891 Attempt to run the program @command{/energy/--} (as that is the only
14892 possible path search result); if the command exists, the environment
14893 will contain @samp{LOGNAME=rms} and @samp{PATH=/energy}, and the
14894 arguments will be @samp{e=mc2}, @samp{bar}, and @samp{baz}.
14896 env -u EDITOR PATH=/energy -- e=mc2 bar baz
14902 The program accepts the following options. Also see @ref{Common options}.
14903 Options must precede operands.
14909 @item -u @var{name}
14910 @itemx --unset=@var{name}
14913 Remove variable @var{name} from the environment, if it was in the
14918 @itemx --ignore-environment
14921 @opindex --ignore-environment
14922 Start with an empty environment, ignoring the inherited environment.
14926 @cindex exit status of @command{env}
14930 0 if no @var{command} is specified and the environment is output
14931 125 if @command{env} itself fails
14932 126 if @var{command} is found but cannot be invoked
14933 127 if @var{command} cannot be found
14934 the exit status of @var{command} otherwise
14938 @node nice invocation
14939 @section @command{nice}: Run a command with modified niceness
14943 @cindex scheduling, affecting
14944 @cindex appropriate privileges
14946 @command{nice} prints or modifies a process's @dfn{niceness},
14947 a parameter that affects whether the process is scheduled favorably.
14951 nice [@var{option}]@dots{} [@var{command} [@var{arg}]@dots{}]
14954 If no arguments are given, @command{nice} prints the current niceness.
14955 Otherwise, @command{nice} runs the given @var{command} with its
14956 niceness adjusted. By default, its niceness is incremented by 10.
14958 Niceness values range at least from @minus{}20 (process has high priority
14959 and gets more resources, thus slowing down other processes) through 19
14960 (process has lower priority and runs slowly itself, but has less impact
14961 on the speed of other running processes). Some systems
14962 may have a wider range of nicenesses; conversely, other systems may
14963 enforce more restrictive limits. An attempt to set the niceness
14964 outside the supported range is treated as an attempt to use the
14965 minimum or maximum supported value.
14967 A niceness should not be confused with a scheduling priority, which
14968 lets applications determine the order in which threads are scheduled
14969 to run. Unlike a priority, a niceness is merely advice to the
14970 scheduler, which the scheduler is free to ignore. Also, as a point of
14971 terminology, @acronym{POSIX} defines the behavior of @command{nice} in
14972 terms of a @dfn{nice value}, which is the nonnegative difference
14973 between a niceness and the minimum niceness. Though @command{nice}
14974 conforms to @acronym{POSIX}, its documentation and diagnostics use the
14975 term ``niceness'' for compatibility with historical practice.
14977 @var{command} must not be a special built-in utility (@pxref{Special
14978 built-in utilities}).
14980 @mayConflictWithShellBuiltIn{nice}
14982 The program accepts the following option. Also see @ref{Common options}.
14983 Options must precede operands.
14986 @item -n @var{adjustment}
14987 @itemx --adjustment=@var{adjustment}
14989 @opindex --adjustment
14990 Add @var{adjustment} instead of 10 to the command's niceness. If
14991 @var{adjustment} is negative and you lack appropriate privileges,
14992 @command{nice} issues a warning but otherwise acts as if you specified
14995 For compatibility @command{nice} also supports an obsolete
14996 option syntax @option{-@var{adjustment}}. New scripts should use
14997 @option{-n @var{adjustment}} instead.
15001 @cindex exit status of @command{nice}
15005 0 if no @var{command} is specified and the niceness is output
15006 125 if @command{nice} itself fails
15007 126 if @var{command} is found but cannot be invoked
15008 127 if @var{command} cannot be found
15009 the exit status of @var{command} otherwise
15012 It is sometimes useful to run a non-interactive program with reduced niceness.
15015 $ nice factor 4611686018427387903
15018 Since @command{nice} prints the current niceness,
15019 you can invoke it through itself to demonstrate how it works.
15021 The default behavior is to increase the niceness by @samp{10}:
15032 The @var{adjustment} is relative to the current niceness. In the
15033 next example, the first @command{nice} invocation runs the second one
15034 with niceness 10, and it in turn runs the final one with a niceness
15038 $ nice nice -n 3 nice
15042 Specifying a niceness larger than the supported range
15043 is the same as specifying the maximum supported value:
15046 $ nice -n 10000000000 nice
15050 Only a privileged user may run a process with lower niceness:
15054 nice: cannot set niceness: Permission denied
15056 $ sudo nice -n -1 nice
15061 @node nohup invocation
15062 @section @command{nohup}: Run a command immune to hangups
15065 @cindex hangups, immunity to
15066 @cindex immunity to hangups
15067 @cindex logging out and continuing to run
15070 @command{nohup} runs the given @var{command} with hangup signals ignored,
15071 so that the command can continue running in the background after you log
15075 nohup @var{command} [@var{arg}]@dots{}
15078 If standard input is a terminal, it is redirected from
15079 @file{/dev/null} so that terminal sessions do not mistakenly consider
15080 the terminal to be used by the command. This is a @acronym{GNU}
15081 extension; programs intended to be portable to non-@acronym{GNU} hosts
15082 should use @samp{nohup @var{command} [@var{arg}]@dots{} </dev/null}
15086 If standard output is a terminal, the command's standard output is appended
15087 to the file @file{nohup.out}; if that cannot be written to, it is appended
15088 to the file @file{$HOME/nohup.out}; and if that cannot be written to, the
15089 command is not run.
15090 Any @file{nohup.out} or @file{$HOME/nohup.out} file created by
15091 @command{nohup} is made readable and writable only to the user,
15092 regardless of the current umask settings.
15094 If standard error is a terminal, it is normally redirected to the same file
15095 descriptor as the (possibly-redirected) standard output.
15096 However, if standard output is closed, standard error terminal output
15097 is instead appended to the file @file{nohup.out} or
15098 @file{$HOME/nohup.out} as above.
15100 To capture the command's output to a file other than @file{nohup.out}
15101 you can redirect it. For example, to capture the output of
15105 nohup make > make.log
15108 @command{nohup} does not automatically put the command it runs in the
15109 background; you must do that explicitly, by ending the command line
15110 with an @samp{&}. Also, @command{nohup} does not alter the
15111 niceness of @var{command}; use @command{nice} for that,
15112 e.g., @samp{nohup nice @var{command}}.
15114 @var{command} must not be a special built-in utility (@pxref{Special
15115 built-in utilities}).
15117 The only options are @option{--help} and @option{--version}. @xref{Common
15118 options}. Options must precede operands.
15120 @cindex exit status of @command{nohup}
15124 125 if @command{nohup} itself fails, and @env{POSIXLY_CORRECT} is not set
15125 126 if @var{command} is found but cannot be invoked
15126 127 if @var{command} cannot be found
15127 the exit status of @var{command} otherwise
15130 If @env{POSIXLY_CORRECT} is set, internal failures give status 127
15134 @node stdbuf invocation
15135 @section @command{stdbuf}: Run a command with modified I/O stream buffering
15138 @cindex standard streams, buffering
15139 @cindex line buffered
15141 @command{stdbuf} allows one to modify the buffering operations of the
15142 three standard I/O streams associated with a program. Synopsis:
15145 stdbuf @var{option}@dots{} @var{command}
15148 Any additional @var{arg}s are passed as additional arguments to the
15151 The program accepts the following options. Also see @ref{Common options}.
15155 @item -i @var{mode}
15156 @itemx --input=@var{mode}
15159 Adjust the standard input stream buffering.
15161 @item -o @var{mode}
15162 @itemx --output=@var{mode}
15165 Adjust the standard output stream buffering.
15167 @item -e @var{mode}
15168 @itemx --error=@var{mode}
15171 Adjust the standard error stream buffering.
15175 The @var{mode} can be specified as follows:
15180 Set the stream to line buffered mode.
15181 In this mode data is coalesced until a newline is output or
15182 input is read from any stream attached to a terminal device.
15183 This option is invalid with standard input.
15186 Disable buffering of the selected stream.
15187 In this mode data is output immediately and only the
15188 amount of data requested is read from input.
15191 Specify the size of the buffer to use in fully buffered mode.
15192 @multiplierSuffixesNoBlocks{size}
15196 NOTE: If @var{command} adjusts the buffering of its standard streams
15197 (@command{tee} does for e.g.) then that will override corresponding settings
15198 changed by @command{stdbuf}. Also some filters (like @command{dd} and
15199 @command{cat} etc.) don't use streams for I/O, and are thus unaffected
15200 by @command{stdbuf} settings.
15202 @cindex exit status of @command{stdbuf}
15206 125 if @command{stdbuf} itself fails
15207 126 if @var{command} is found but cannot be invoked
15208 127 if @var{command} cannot be found
15209 the exit status of @var{command} otherwise
15213 @node su invocation
15214 @section @command{su}: Run a command with substitute user and group ID
15217 @cindex substitute user and group IDs
15218 @cindex user ID, switching
15219 @cindex super-user, becoming
15220 @cindex root, becoming
15222 @command{su} allows one user to temporarily become another user. It runs a
15223 command (often an interactive shell) with the real and effective user
15224 ID, group ID, and supplemental groups of a given @var{user}. Synopsis:
15227 su [@var{option}]@dots{} [@var{user} [@var{arg}]@dots{}]
15230 @cindex passwd entry, and @command{su} shell
15232 @flindex /etc/passwd
15233 If no @var{user} is given, the default is @code{root}, the super-user.
15234 The shell to use is taken from @var{user}'s @code{passwd} entry, or
15235 @file{/bin/sh} if none is specified there. If @var{user} has a
15236 password, @command{su} prompts for the password unless run by a user with
15237 effective user ID of zero (the super-user).
15243 @cindex login shell
15244 By default, @command{su} does not change the current directory.
15245 It sets the environment variables @env{HOME} and @env{SHELL}
15246 from the password entry for @var{user}, and if @var{user} is not
15247 the super-user, sets @env{USER} and @env{LOGNAME} to @var{user}.
15248 By default, the shell is not a login shell.
15250 Any additional @var{arg}s are passed as additional arguments to the
15253 @cindex @option{-su}
15254 GNU @command{su} does not treat @file{/bin/sh} or any other shells specially
15255 (e.g., by setting @code{argv[0]} to @option{-su}, passing @option{-c} only
15256 to certain shells, etc.).
15259 @command{su} can optionally be compiled to use @code{syslog} to report
15260 failed, and optionally successful, @command{su} attempts. (If the system
15261 supports @code{syslog}.) However, GNU @command{su} does not check if the
15262 user is a member of the @code{wheel} group; see below.
15264 The program accepts the following options. Also see @ref{Common options}.
15267 @item -c @var{command}
15268 @itemx --command=@var{command}
15271 Pass @var{command}, a single command line to run, to the shell with
15272 a @option{-c} option instead of starting an interactive shell.
15279 @cindex file name pattern expansion, disabled
15280 @cindex globbing, disabled
15281 Pass the @option{-f} option to the shell. This probably only makes sense
15282 if the shell run is @command{csh} or @command{tcsh}, for which the @option{-f}
15283 option prevents reading the startup file (@file{.cshrc}). With
15284 Bourne-like shells, the @option{-f} option disables file name pattern
15285 expansion (globbing), which is not likely to be useful.
15293 @c other variables already indexed above
15296 @cindex login shell, creating
15297 Make the shell a login shell. This means the following. Unset all
15298 environment variables except @env{TERM}, @env{HOME}, and @env{SHELL}
15299 (which are set as described above), and @env{USER} and @env{LOGNAME}
15300 (which are set, even for the super-user, as described above), and set
15301 @env{PATH} to a compiled-in default value. Change to @var{user}'s home
15302 directory. Prepend @samp{-} to the shell's name, intended to make it
15303 read its login startup file(s).
15307 @itemx --preserve-environment
15310 @opindex --preserve-environment
15311 @cindex environment, preserving
15312 @flindex /etc/shells
15313 @cindex restricted shell
15314 Do not change the environment variables @env{HOME}, @env{USER},
15315 @env{LOGNAME}, or @env{SHELL}. Run the shell given in the environment
15316 variable @env{SHELL} instead of the shell from @var{user}'s passwd
15317 entry, unless the user running @command{su} is not the super-user and
15318 @var{user}'s shell is restricted. A @dfn{restricted shell} is one that
15319 is not listed in the file @file{/etc/shells}, or in a compiled-in list
15320 if that file does not exist. Parts of what this option does can be
15321 overridden by @option{--login} and @option{--shell}.
15323 @item -s @var{shell}
15324 @itemx --shell=@var{shell}
15327 Run @var{shell} instead of the shell from @var{user}'s passwd entry,
15328 unless the user running @command{su} is not the super-user and @var{user}'s
15329 shell is restricted (see @option{-m} just above).
15333 @cindex exit status of @command{su}
15337 125 if @command{su} itself fails
15338 126 if subshell is found but cannot be invoked
15339 127 if subshell cannot be found
15340 the exit status of the subshell otherwise
15343 @cindex wheel group, not supported
15344 @cindex group wheel, not supported
15346 @subsection Why GNU @command{su} does not support the @samp{wheel} group
15348 (This section is by Richard Stallman.)
15352 Sometimes a few of the users try to hold total power over all the
15353 rest. For example, in 1984, a few users at the MIT AI lab decided to
15354 seize power by changing the operator password on the Twenex system and
15355 keeping it secret from everyone else. (I was able to thwart this coup
15356 and give power back to the users by patching the kernel, but I
15357 wouldn't know how to do that in Unix.)
15359 However, occasionally the rulers do tell someone. Under the usual
15360 @command{su} mechanism, once someone learns the root password who
15361 sympathizes with the ordinary users, he or she can tell the rest. The
15362 ``wheel group'' feature would make this impossible, and thus cement the
15363 power of the rulers.
15365 I'm on the side of the masses, not that of the rulers. If you are
15366 used to supporting the bosses and sysadmins in whatever they do, you
15367 might find this idea strange at first.
15370 @node timeout invocation
15371 @section @command{timeout}: Run a command with a time limit
15375 @cindex run commands with bounded time
15377 @command{timeout} runs the given @var{command} and kills it if it is
15378 still running after the specified time interval. Synopsis:
15381 timeout [@var{option}] @var{duration} @var{command} [@var{arg}]@dots{}
15384 @var{command} must not be a special built-in utility (@pxref{Special
15385 built-in utilities}).
15387 The program accepts the following options. Also see @ref{Common options}.
15388 Options must precede operands.
15391 @item -k @var{duration}
15392 @itemx --kill-after=@var{duration}
15394 @opindex --kill-after
15395 Ensure the monitored @var{command} is killed by also sending a @samp{KILL}
15396 signal, after the specified @var{duration}. Without this option, if the
15397 selected signal proves not to be fatal, @command{timeout} does not kill
15400 @item -s @var{signal}
15401 @itemx --signal=@var{signal}
15404 Send this @var{signal} to @var{command} on timeout, rather than the
15405 default @samp{TERM} signal. @var{signal} may be a name like @samp{HUP}
15406 or a number. Also see @xref{Signal specifications}.
15410 @var{duration} is an integer followed by an optional unit:
15412 @samp{s} for seconds (the default)
15413 @samp{m} for minutes
15417 A duration of 0 disables the associated timeout.
15419 @cindex exit status of @command{timeout}
15423 124 if @var{command} times out
15424 125 if @command{timeout} itself fails
15425 126 if @var{command} is found but cannot be invoked
15426 127 if @var{command} cannot be found
15427 the exit status of @var{command} otherwise
15431 @node Process control
15432 @chapter Process control
15434 @cindex processes, commands for controlling
15435 @cindex commands for controlling processes
15438 * kill invocation:: Sending a signal to processes.
15442 @node kill invocation
15443 @section @command{kill}: Send a signal to processes
15446 @cindex send a signal to processes
15448 The @command{kill} command sends a signal to processes, causing them
15449 to terminate or otherwise act upon receiving the signal in some way.
15450 Alternatively, it lists information about signals. Synopses:
15453 kill [-s @var{signal} | --signal @var{signal} | -@var{signal}] @var{pid}@dots{}
15454 kill [-l | --list | -t | --table] [@var{signal}]@dots{}
15457 @mayConflictWithShellBuiltIn{kill}
15459 The first form of the @command{kill} command sends a signal to all
15460 @var{pid} arguments. The default signal to send if none is specified
15461 is @samp{TERM}. The special signal number @samp{0} does not denote a
15462 valid signal, but can be used to test whether the @var{pid} arguments
15463 specify processes to which a signal could be sent.
15465 If @var{pid} is positive, the signal is sent to the process with the
15466 process ID @var{pid}. If @var{pid} is zero, the signal is sent to all
15467 processes in the process group of the current process. If @var{pid}
15468 is @minus{}1, the signal is sent to all processes for which the user has
15469 permission to send a signal. If @var{pid} is less than @minus{}1, the signal
15470 is sent to all processes in the process group that equals the absolute
15471 value of @var{pid}.
15473 If @var{pid} is not positive, a system-dependent set of system
15474 processes is excluded from the list of processes to which the signal
15477 If a negative @var{pid} argument is desired as the first one, it
15478 should be preceded by @option{--}. However, as a common extension to
15479 @acronym{POSIX}, @option{--} is not required with @samp{kill
15480 -@var{signal} -@var{pid}}. The following commands are equivalent:
15489 The first form of the @command{kill} command succeeds if every @var{pid}
15490 argument specifies at least one process that the signal was sent to.
15492 The second form of the @command{kill} command lists signal information.
15493 Either the @option{-l} or @option{--list} option, or the @option{-t}
15494 or @option{--table} option must be specified. Without any
15495 @var{signal} argument, all supported signals are listed. The output
15496 of @option{-l} or @option{--list} is a list of the signal names, one
15497 per line; if @var{signal} is already a name, the signal number is
15498 printed instead. The output of @option{-t} or @option{--table} is a
15499 table of signal numbers, names, and descriptions. This form of the
15500 @command{kill} command succeeds if all @var{signal} arguments are valid
15501 and if there is no output error.
15503 The @command{kill} command also supports the @option{--help} and
15504 @option{--version} options. @xref{Common options}.
15506 A @var{signal} may be a signal name like @samp{HUP}, or a signal
15507 number like @samp{1}, or an exit status of a process terminated by the
15508 signal. A signal name can be given in canonical form or prefixed by
15509 @samp{SIG}. The case of the letters is ignored, except for the
15510 @option{-@var{signal}} option which must use upper case to avoid
15511 ambiguity with lower case option letters. For a list of supported
15512 signal names and numbers see @xref{Signal specifications}.
15517 @cindex delaying commands
15518 @cindex commands for delaying
15520 @c Perhaps @command{wait} or other commands should be described here also?
15523 * sleep invocation:: Delay for a specified time.
15527 @node sleep invocation
15528 @section @command{sleep}: Delay for a specified time
15531 @cindex delay for a specified time
15533 @command{sleep} pauses for an amount of time specified by the sum of
15534 the values of the command line arguments.
15538 sleep @var{number}[smhd]@dots{}
15542 Each argument is a number followed by an optional unit; the default
15543 is seconds. The units are:
15556 Historical implementations of @command{sleep} have required that
15557 @var{number} be an integer, and only accepted a single argument
15558 without a suffix. However, GNU @command{sleep} accepts
15559 arbitrary floating point numbers (using a period before any fractional
15562 The only options are @option{--help} and @option{--version}. @xref{Common
15565 @c sleep is a shell built-in at least with Solaris 11's /bin/sh
15566 @mayConflictWithShellBuiltIn{sleep}
15571 @node Numeric operations
15572 @chapter Numeric operations
15574 @cindex numeric operations
15575 These programs do numerically-related operations.
15578 * factor invocation:: Show factors of numbers.
15579 * seq invocation:: Print sequences of numbers.
15583 @node factor invocation
15584 @section @command{factor}: Print prime factors
15587 @cindex prime factors
15589 @command{factor} prints prime factors. Synopses:
15592 factor [@var{number}]@dots{}
15593 factor @var{option}
15596 If no @var{number} is specified on the command line, @command{factor} reads
15597 numbers from standard input, delimited by newlines, tabs, or spaces.
15599 The @command{factor} command supports only a small number of options:
15603 Print a short help on standard output, then exit without further
15607 Print the program version on standard output, then exit without further
15611 Factoring the product of the eighth and ninth Mersenne primes
15612 takes about 30 milliseconds of CPU time on a 2.2 GHz Athlon.
15615 M8=`echo 2^31-1|bc` ; M9=`echo 2^61-1|bc`
15616 /usr/bin/time -f '%U' factor $(echo "$M8 * $M9" | bc)
15617 4951760154835678088235319297: 2147483647 2305843009213693951
15621 Similarly, factoring the eighth Fermat number @math{2^{256}+1} takes
15622 about 20 seconds on the same machine.
15624 Factoring large numbers is, in general, hard. The Pollard Rho
15625 algorithm used by @command{factor} is particularly effective for
15626 numbers with relatively small factors. If you wish to factor large
15627 numbers which do not have small factors (for example, numbers which
15628 are the product of two large primes), other methods are far better.
15630 If @command{factor} is built without using GNU MP, only
15631 single-precision arithmetic is available, and so large numbers
15632 (typically @math{2^{64}} and above) will not be supported. The single-precision
15633 code uses an algorithm which is designed for factoring smaller
15639 @node seq invocation
15640 @section @command{seq}: Print numeric sequences
15643 @cindex numeric sequences
15644 @cindex sequence of numbers
15646 @command{seq} prints a sequence of numbers to standard output. Synopses:
15649 seq [@var{option}]@dots{} @var{last}
15650 seq [@var{option}]@dots{} @var{first} @var{last}
15651 seq [@var{option}]@dots{} @var{first} @var{increment} @var{last}
15654 @command{seq} prints the numbers from @var{first} to @var{last} by
15655 @var{increment}. By default, each number is printed on a separate line.
15656 When @var{increment} is not specified, it defaults to @samp{1},
15657 even when @var{first} is larger than @var{last}.
15658 @var{first} also defaults to @samp{1}. So @code{seq 1} prints
15659 @samp{1}, but @code{seq 0} and @code{seq 10 5} produce no output.
15660 Floating-point numbers
15661 may be specified (using a period before any fractional digits).
15663 The program accepts the following options. Also see @ref{Common options}.
15664 Options must precede operands.
15667 @item -f @var{format}
15668 @itemx --format=@var{format}
15669 @opindex -f @var{format}
15670 @opindex --format=@var{format}
15671 @cindex formatting of numbers in @command{seq}
15672 Print all numbers using @var{format}.
15673 @var{format} must contain exactly one of the @samp{printf}-style
15674 floating point conversion specifications @samp{%a}, @samp{%e},
15675 @samp{%f}, @samp{%g}, @samp{%A}, @samp{%E}, @samp{%F}, @samp{%G}.
15676 The @samp{%} may be followed by zero or more flags taken from the set
15677 @samp{-+#0 '}, then an optional width containing one or more digits,
15678 then an optional precision consisting of a @samp{.} followed by zero
15679 or more digits. @var{format} may also contain any number of @samp{%%}
15680 conversion specifications. All conversion specifications have the
15681 same meaning as with @samp{printf}.
15683 The default format is derived from @var{first}, @var{step}, and
15684 @var{last}. If these all use a fixed point decimal representation,
15685 the default format is @samp{%.@var{p}f}, where @var{p} is the minimum
15686 precision that can represent the output numbers exactly. Otherwise,
15687 the default format is @samp{%g}.
15689 @item -s @var{string}
15690 @itemx --separator=@var{string}
15691 @cindex separator for numbers in @command{seq}
15692 Separate numbers with @var{string}; default is a newline.
15693 The output always terminates with a newline.
15696 @itemx --equal-width
15697 Print all numbers with the same width, by padding with leading zeros.
15698 @var{first}, @var{step}, and @var{last} should all use a fixed point
15699 decimal representation.
15700 (To have other kinds of padding, use @option{--format}).
15704 You can get finer-grained control over output with @option{-f}:
15707 $ seq -f '(%9.2E)' -9e5 1.1e6 1.3e6
15713 If you want hexadecimal integer output, you can use @command{printf}
15714 to perform the conversion:
15717 $ printf '%x\n' `seq 1048575 1024 1050623`
15723 For very long lists of numbers, use xargs to avoid
15724 system limitations on the length of an argument list:
15727 $ seq 1000000 | xargs printf '%x\n' | tail -n 3
15733 To generate octal output, use the printf @code{%o} format instead
15736 On most systems, seq can produce whole-number output for values up to
15737 at least @math{2^{53}}. Larger integers are approximated. The details
15738 differ depending on your floating-point implementation, but a common
15739 case is that @command{seq} works with integers through @math{2^{64}},
15740 and larger integers may not be numerically correct:
15743 $ seq 18446744073709551616 1 18446744073709551618
15744 18446744073709551616
15745 18446744073709551616
15746 18446744073709551618
15749 Be careful when using @command{seq} with outlandish values: otherwise
15750 you may see surprising results, as @command{seq} uses floating point
15751 internally. For example, on the x86 platform, where the internal
15752 representation uses a 64-bit fraction, the command:
15755 seq 1 0.0000000000000000001 1.0000000000000000009
15758 outputs 1.0000000000000000007 twice and skips 1.0000000000000000008.
15763 @node File permissions
15764 @chapter File permissions
15767 @include parse-datetime.texi
15771 @node Opening the software toolbox
15772 @chapter Opening the Software Toolbox
15774 An earlier version of this chapter appeared in
15775 @uref{http://www.linuxjournal.com/article.php?sid=2762, the
15776 @cite{What's GNU?} column of @cite{Linux Journal}, 2 (June, 1994)}.
15777 It was written by Arnold Robbins.
15780 * Toolbox introduction:: Toolbox introduction
15781 * I/O redirection:: I/O redirection
15782 * The who command:: The @command{who} command
15783 * The cut command:: The @command{cut} command
15784 * The sort command:: The @command{sort} command
15785 * The uniq command:: The @command{uniq} command
15786 * Putting the tools together:: Putting the tools together
15790 @node Toolbox introduction
15791 @unnumberedsec Toolbox Introduction
15793 This month's column is only peripherally related to the GNU Project, in
15794 that it describes a number of the GNU tools on your GNU/Linux system and how they
15795 might be used. What it's really about is the ``Software Tools'' philosophy
15796 of program development and usage.
15798 The software tools philosophy was an important and integral concept
15799 in the initial design and development of Unix (of which Linux and GNU are
15800 essentially clones). Unfortunately, in the modern day press of
15801 Internetworking and flashy GUIs, it seems to have fallen by the
15802 wayside. This is a shame, since it provides a powerful mental model
15803 for solving many kinds of problems.
15805 Many people carry a Swiss Army knife around in their pants pockets (or
15806 purse). A Swiss Army knife is a handy tool to have: it has several knife
15807 blades, a screwdriver, tweezers, toothpick, nail file, corkscrew, and perhaps
15808 a number of other things on it. For the everyday, small miscellaneous jobs
15809 where you need a simple, general purpose tool, it's just the thing.
15811 On the other hand, an experienced carpenter doesn't build a house using
15812 a Swiss Army knife. Instead, he has a toolbox chock full of specialized
15813 tools---a saw, a hammer, a screwdriver, a plane, and so on. And he knows
15814 exactly when and where to use each tool; you won't catch him hammering nails
15815 with the handle of his screwdriver.
15817 The Unix developers at Bell Labs were all professional programmers and trained
15818 computer scientists. They had found that while a one-size-fits-all program
15819 might appeal to a user because there's only one program to use, in practice
15824 difficult to write,
15827 difficult to maintain and
15831 difficult to extend to meet new situations.
15834 Instead, they felt that programs should be specialized tools. In short, each
15835 program ``should do one thing well.'' No more and no less. Such programs are
15836 simpler to design, write, and get right---they only do one thing.
15838 Furthermore, they found that with the right machinery for hooking programs
15839 together, that the whole was greater than the sum of the parts. By combining
15840 several special purpose programs, you could accomplish a specific task
15841 that none of the programs was designed for, and accomplish it much more
15842 quickly and easily than if you had to write a special purpose program.
15843 We will see some (classic) examples of this further on in the column.
15844 (An important additional point was that, if necessary, take a detour
15845 and build any software tools you may need first, if you don't already
15846 have something appropriate in the toolbox.)
15848 @node I/O redirection
15849 @unnumberedsec I/O Redirection
15851 Hopefully, you are familiar with the basics of I/O redirection in the
15852 shell, in particular the concepts of ``standard input,'' ``standard output,''
15853 and ``standard error''. Briefly, ``standard input'' is a data source, where
15854 data comes from. A program should not need to either know or care if the
15855 data source is a disk file, a keyboard, a magnetic tape, or even a punched
15856 card reader. Similarly, ``standard output'' is a data sink, where data goes
15857 to. The program should neither know nor care where this might be.
15858 Programs that only read their standard input, do something to the data,
15859 and then send it on, are called @dfn{filters}, by analogy to filters in a
15862 With the Unix shell, it's very easy to set up data pipelines:
15865 program_to_create_data | filter1 | ... | filterN > final.pretty.data
15868 We start out by creating the raw data; each filter applies some successive
15869 transformation to the data, until by the time it comes out of the pipeline,
15870 it is in the desired form.
15872 This is fine and good for standard input and standard output. Where does the
15873 standard error come in to play? Well, think about @command{filter1} in
15874 the pipeline above. What happens if it encounters an error in the data it
15875 sees? If it writes an error message to standard output, it will just
15876 disappear down the pipeline into @command{filter2}'s input, and the
15877 user will probably never see it. So programs need a place where they can send
15878 error messages so that the user will notice them. This is standard error,
15879 and it is usually connected to your console or window, even if you have
15880 redirected standard output of your program away from your screen.
15882 For filter programs to work together, the format of the data has to be
15883 agreed upon. The most straightforward and easiest format to use is simply
15884 lines of text. Unix data files are generally just streams of bytes, with
15885 lines delimited by the @acronym{ASCII} @sc{lf} (Line Feed) character,
15886 conventionally called a ``newline'' in the Unix literature. (This is
15887 @code{'\n'} if you're a C programmer.) This is the format used by all
15888 the traditional filtering programs. (Many earlier operating systems
15889 had elaborate facilities and special purpose programs for managing
15890 binary data. Unix has always shied away from such things, under the
15891 philosophy that it's easiest to simply be able to view and edit your
15892 data with a text editor.)
15894 OK, enough introduction. Let's take a look at some of the tools, and then
15895 we'll see how to hook them together in interesting ways. In the following
15896 discussion, we will only present those command line options that interest
15897 us. As you should always do, double check your system documentation
15898 for the full story.
15900 @node The who command
15901 @unnumberedsec The @command{who} Command
15903 The first program is the @command{who} command. By itself, it generates a
15904 list of the users who are currently logged in. Although I'm writing
15905 this on a single-user system, we'll pretend that several people are
15910 @print{} arnold console Jan 22 19:57
15911 @print{} miriam ttyp0 Jan 23 14:19(:0.0)
15912 @print{} bill ttyp1 Jan 21 09:32(:0.0)
15913 @print{} arnold ttyp2 Jan 23 20:48(:0.0)
15916 Here, the @samp{$} is the usual shell prompt, at which I typed @samp{who}.
15917 There are three people logged in, and I am logged in twice. On traditional
15918 Unix systems, user names are never more than eight characters long. This
15919 little bit of trivia will be useful later. The output of @command{who} is nice,
15920 but the data is not all that exciting.
15922 @node The cut command
15923 @unnumberedsec The @command{cut} Command
15925 The next program we'll look at is the @command{cut} command. This program
15926 cuts out columns or fields of input data. For example, we can tell it
15927 to print just the login name and full name from the @file{/etc/passwd}
15928 file. The @file{/etc/passwd} file has seven fields, separated by
15932 arnold:xyzzy:2076:10:Arnold D. Robbins:/home/arnold:/bin/bash
15935 To get the first and fifth fields, we would use @command{cut} like this:
15938 $ cut -d: -f1,5 /etc/passwd
15939 @print{} root:Operator
15941 @print{} arnold:Arnold D. Robbins
15942 @print{} miriam:Miriam A. Robbins
15946 With the @option{-c} option, @command{cut} will cut out specific characters
15947 (i.e., columns) in the input lines. This is useful for input data
15948 that has fixed width fields, and does not have a field separator. For
15949 example, list the Monday dates for the current month:
15951 @c Is using cal ok? Looked at gcal, but I don't like it.
15962 @node The sort command
15963 @unnumberedsec The @command{sort} Command
15965 Next we'll look at the @command{sort} command. This is one of the most
15966 powerful commands on a Unix-style system; one that you will often find
15967 yourself using when setting up fancy data plumbing.
15970 command reads and sorts each file named on the command line. It then
15971 merges the sorted data and writes it to standard output. It will read
15972 standard input if no files are given on the command line (thus
15973 making it into a filter). The sort is based on the character collating
15974 sequence or based on user-supplied ordering criteria.
15977 @node The uniq command
15978 @unnumberedsec The @command{uniq} Command
15980 Finally (at least for now), we'll look at the @command{uniq} program. When
15981 sorting data, you will often end up with duplicate lines, lines that
15982 are identical. Usually, all you need is one instance of each line.
15983 This is where @command{uniq} comes in. The @command{uniq} program reads its
15984 standard input. It prints only one
15985 copy of each repeated line. It does have several options. Later on,
15986 we'll use the @option{-c} option, which prints each unique line, preceded
15987 by a count of the number of times that line occurred in the input.
15990 @node Putting the tools together
15991 @unnumberedsec Putting the Tools Together
15993 Now, let's suppose this is a large ISP server system with dozens of users
15994 logged in. The management wants the system administrator to write a program that will
15995 generate a sorted list of logged in users. Furthermore, even if a user
15996 is logged in multiple times, his or her name should only show up in the
15999 The administrator could sit down with the system documentation and write a C
16000 program that did this. It would take perhaps a couple of hundred lines
16001 of code and about two hours to write it, test it, and debug it.
16002 However, knowing the software toolbox, the administrator can instead start out
16003 by generating just a list of logged on users:
16013 Next, sort the list:
16016 $ who | cut -c1-8 | sort
16023 Finally, run the sorted list through @command{uniq}, to weed out duplicates:
16026 $ who | cut -c1-8 | sort | uniq
16032 The @command{sort} command actually has a @option{-u} option that does what
16033 @command{uniq} does. However, @command{uniq} has other uses for which one
16034 cannot substitute @samp{sort -u}.
16036 The administrator puts this pipeline into a shell script, and makes it available for
16037 all the users on the system (@samp{#} is the system administrator,
16038 or @code{root}, prompt):
16041 # cat > /usr/local/bin/listusers
16042 who | cut -c1-8 | sort | uniq
16044 # chmod +x /usr/local/bin/listusers
16047 There are four major points to note here. First, with just four
16048 programs, on one command line, the administrator was able to save about two
16049 hours worth of work. Furthermore, the shell pipeline is just about as
16050 efficient as the C program would be, and it is much more efficient in
16051 terms of programmer time. People time is much more expensive than
16052 computer time, and in our modern ``there's never enough time to do
16053 everything'' society, saving two hours of programmer time is no mean
16056 Second, it is also important to emphasize that with the
16057 @emph{combination} of the tools, it is possible to do a special
16058 purpose job never imagined by the authors of the individual programs.
16060 Third, it is also valuable to build up your pipeline in stages, as we did here.
16061 This allows you to view the data at each stage in the pipeline, which helps
16062 you acquire the confidence that you are indeed using these tools correctly.
16064 Finally, by bundling the pipeline in a shell script, other users can use
16065 your command, without having to remember the fancy plumbing you set up for
16066 them. In terms of how you run them, shell scripts and compiled programs are
16069 After the previous warm-up exercise, we'll look at two additional, more
16070 complicated pipelines. For them, we need to introduce two more tools.
16072 The first is the @command{tr} command, which stands for ``transliterate.''
16073 The @command{tr} command works on a character-by-character basis, changing
16074 characters. Normally it is used for things like mapping upper case to
16078 $ echo ThIs ExAmPlE HaS MIXED case! | tr '[:upper:]' '[:lower:]'
16079 @print{} this example has mixed case!
16082 There are several options of interest:
16086 work on the complement of the listed characters, i.e.,
16087 operations apply to characters not in the given set
16090 delete characters in the first set from the output
16093 squeeze repeated characters in the output into just one character.
16096 We will be using all three options in a moment.
16098 The other command we'll look at is @command{comm}. The @command{comm}
16099 command takes two sorted input files as input data, and prints out the
16100 files' lines in three columns. The output columns are the data lines
16101 unique to the first file, the data lines unique to the second file, and
16102 the data lines that are common to both. The @option{-1}, @option{-2}, and
16103 @option{-3} command line options @emph{omit} the respective columns. (This is
16104 non-intuitive and takes a little getting used to.) For example:
16126 The file name @file{-} tells @command{comm} to read standard input
16127 instead of a regular file.
16129 Now we're ready to build a fancy pipeline. The first application is a word
16130 frequency counter. This helps an author determine if he or she is over-using
16133 The first step is to change the case of all the letters in our input file
16134 to one case. ``The'' and ``the'' are the same word when doing counting.
16137 $ tr '[:upper:]' '[:lower:]' < whats.gnu | ...
16140 The next step is to get rid of punctuation. Quoted words and unquoted words
16141 should be treated identically; it's easiest to just get the punctuation out of
16145 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | ...
16148 The second @command{tr} command operates on the complement of the listed
16149 characters, which are all the letters, the digits, the underscore, and
16150 the blank. The @samp{\n} represents the newline character; it has to
16151 be left alone. (The @acronym{ASCII} tab character should also be included for
16152 good measure in a production script.)
16154 At this point, we have data consisting of words separated by blank space.
16155 The words only contain alphanumeric characters (and the underscore). The
16156 next step is break the data apart so that we have one word per line. This
16157 makes the counting operation much easier, as we will see shortly.
16160 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16161 > tr -s ' ' '\n' | ...
16164 This command turns blanks into newlines. The @option{-s} option squeezes
16165 multiple newline characters in the output into just one. This helps us
16166 avoid blank lines. (The @samp{>} is the shell's ``secondary prompt.''
16167 This is what the shell prints when it notices you haven't finished
16168 typing in all of a command.)
16170 We now have data consisting of one word per line, no punctuation, all one
16171 case. We're ready to count each word:
16174 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16175 > tr -s ' ' '\n' | sort | uniq -c | ...
16178 At this point, the data might look something like this:
16191 The output is sorted by word, not by count! What we want is the most
16192 frequently used words first. Fortunately, this is easy to accomplish,
16193 with the help of two more @command{sort} options:
16197 do a numeric sort, not a textual one
16200 reverse the order of the sort
16203 The final pipeline looks like this:
16206 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16207 > tr -s ' ' '\n' | sort | uniq -c | sort -n -r
16216 Whew! That's a lot to digest. Yet, the same principles apply. With six
16217 commands, on two lines (really one long one split for convenience), we've
16218 created a program that does something interesting and useful, in much
16219 less time than we could have written a C program to do the same thing.
16221 A minor modification to the above pipeline can give us a simple spelling
16222 checker! To determine if you've spelled a word correctly, all you have to
16223 do is look it up in a dictionary. If it is not there, then chances are
16224 that your spelling is incorrect. So, we need a dictionary.
16225 The conventional location for a dictionary is @file{/usr/dict/words}.
16226 On my GNU/Linux system,@footnote{Redhat Linux 6.1, for the November 2000
16227 revision of this article.}
16228 this is a sorted, 45,402 word dictionary.
16230 Now, how to compare our file with the dictionary? As before, we generate
16231 a sorted list of words, one per line:
16234 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16235 > tr -s ' ' '\n' | sort -u | ...
16238 Now, all we need is a list of words that are @emph{not} in the
16239 dictionary. Here is where the @command{comm} command comes in.
16242 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16243 > tr -s ' ' '\n' | sort -u |
16244 > comm -23 - /usr/dict/words
16247 The @option{-2} and @option{-3} options eliminate lines that are only in the
16248 dictionary (the second file), and lines that are in both files. Lines
16249 only in the first file (standard input, our stream of words), are
16250 words that are not in the dictionary. These are likely candidates for
16251 spelling errors. This pipeline was the first cut at a production
16252 spelling checker on Unix.
16254 There are some other tools that deserve brief mention.
16258 search files for text that matches a regular expression
16261 count lines, words, characters
16264 a T-fitting for data pipes, copies data to files and to standard output
16267 the stream editor, an advanced tool
16270 a data manipulation language, another advanced tool
16273 The software tools philosophy also espoused the following bit of
16274 advice: ``Let someone else do the hard part.'' This means, take
16275 something that gives you most of what you need, and then massage it the
16276 rest of the way until it's in the form that you want.
16282 Each program should do one thing well. No more, no less.
16285 Combining programs with appropriate plumbing leads to results where
16286 the whole is greater than the sum of the parts. It also leads to novel
16287 uses of programs that the authors might never have imagined.
16290 Programs should never print extraneous header or trailer data, since these
16291 could get sent on down a pipeline. (A point we didn't mention earlier.)
16294 Let someone else do the hard part.
16297 Know your toolbox! Use each program appropriately. If you don't have an
16298 appropriate tool, build one.
16301 As of this writing, all the programs we've discussed are available via
16302 anonymous @command{ftp} from: @*
16303 @uref{ftp://gnudist.gnu.org/textutils/textutils-1.22.tar.gz}. (There may
16304 be more recent versions available now.)
16306 None of what I have presented in this column is new. The Software Tools
16307 philosophy was first introduced in the book @cite{Software Tools}, by
16308 Brian Kernighan and P.J. Plauger (Addison-Wesley, ISBN 0-201-03669-X).
16309 This book showed how to write and use software tools. It was written in
16310 1976, using a preprocessor for FORTRAN named @command{ratfor} (RATional
16311 FORtran). At the time, C was not as ubiquitous as it is now; FORTRAN
16312 was. The last chapter presented a @command{ratfor} to FORTRAN
16313 processor, written in @command{ratfor}. @command{ratfor} looks an awful
16314 lot like C; if you know C, you won't have any problem following the
16317 In 1981, the book was updated and made available as @cite{Software Tools
16318 in Pascal} (Addison-Wesley, ISBN 0-201-10342-7). Both books are
16319 still in print and are well worth
16320 reading if you're a programmer. They certainly made a major change in
16321 how I view programming.
16323 The programs in both books are available from
16324 @uref{http://cm.bell-labs.com/who/bwk, Brian Kernighan's home page}.
16325 For a number of years, there was an active
16326 Software Tools Users Group, whose members had ported the original
16327 @command{ratfor} programs to essentially every computer system with a
16328 FORTRAN compiler. The popularity of the group waned in the middle 1980s
16329 as Unix began to spread beyond universities.
16331 With the current proliferation of GNU code and other clones of Unix programs,
16332 these programs now receive little attention; modern C versions are
16333 much more efficient and do more than these programs do. Nevertheless, as
16334 exposition of good programming style, and evangelism for a still-valuable
16335 philosophy, these books are unparalleled, and I recommend them highly.
16337 Acknowledgment: I would like to express my gratitude to Brian Kernighan
16338 of Bell Labs, the original Software Toolsmith, for reviewing this column.
16340 @node GNU Free Documentation License
16341 @appendix GNU Free Documentation License
16345 @node Concept index
16352 @c Local variables:
16353 @c texinfo-column-for-description: 32