3 @setfilename coreutils.info
4 @settitle @sc{gnu} Coreutils
9 @include constants.texi
11 @c Define new indices.
15 @c Put everything in one index (arbitrarily chosen to be the concept index).
25 * Coreutils: (coreutils). Core GNU (file, text, shell) utilities.
26 * Common options: (coreutils)Common options. Common options.
27 * File permissions: (coreutils)File permissions. Access modes.
28 * Date input formats: (coreutils)Date input formats.
31 @c FIXME: the following need documentation
32 @c * [: (coreutils)[ invocation. File/string tests.
33 @c * pinky: (coreutils)pinky invocation. FIXME.
35 @dircategory Individual utilities
37 * arch: (coreutils)arch invocation. Print machine hardware name.
38 * base64: (coreutils)base64 invocation. Base64 encode/decode data.
39 * basename: (coreutils)basename invocation. Strip directory and suffix.
40 * cat: (coreutils)cat invocation. Concatenate and write files.
41 * chcon: (coreutils)chcon invocation. Change SELinux CTX of files.
42 * chgrp: (coreutils)chgrp invocation. Change file groups.
43 * chmod: (coreutils)chmod invocation. Change file permissions.
44 * chown: (coreutils)chown invocation. Change file owners/groups.
45 * chroot: (coreutils)chroot invocation. Specify the root directory.
46 * cksum: (coreutils)cksum invocation. Print POSIX CRC checksum.
47 * comm: (coreutils)comm invocation. Compare sorted files by line.
48 * cp: (coreutils)cp invocation. Copy files.
49 * csplit: (coreutils)csplit invocation. Split by context.
50 * cut: (coreutils)cut invocation. Print selected parts of lines.
51 * date: (coreutils)date invocation. Print/set system date and time.
52 * dd: (coreutils)dd invocation. Copy and convert a file.
53 * df: (coreutils)df invocation. Report file system disk usage.
54 * dir: (coreutils)dir invocation. List directories briefly.
55 * dircolors: (coreutils)dircolors invocation. Color setup for ls.
56 * dirname: (coreutils)dirname invocation. Strip last file name component.
57 * du: (coreutils)du invocation. Report on disk usage.
58 * echo: (coreutils)echo invocation. Print a line of text.
59 * env: (coreutils)env invocation. Modify the environment.
60 * expand: (coreutils)expand invocation. Convert tabs to spaces.
61 * expr: (coreutils)expr invocation. Evaluate expressions.
62 * factor: (coreutils)factor invocation. Print prime factors
63 * false: (coreutils)false invocation. Do nothing, unsuccessfully.
64 * fmt: (coreutils)fmt invocation. Reformat paragraph text.
65 * fold: (coreutils)fold invocation. Wrap long input lines.
66 * groups: (coreutils)groups invocation. Print group names a user is in.
67 * head: (coreutils)head invocation. Output the first part of files.
68 * hostid: (coreutils)hostid invocation. Print numeric host identifier.
69 * hostname: (coreutils)hostname invocation. Print or set system name.
70 * id: (coreutils)id invocation. Print user identity.
71 * install: (coreutils)install invocation. Copy and change attributes.
72 * join: (coreutils)join invocation. Join lines on a common field.
73 * kill: (coreutils)kill invocation. Send a signal to processes.
74 * link: (coreutils)link invocation. Make hard links between files.
75 * ln: (coreutils)ln invocation. Make links between files.
76 * logname: (coreutils)logname invocation. Print current login name.
77 * ls: (coreutils)ls invocation. List directory contents.
78 * md5sum: (coreutils)md5sum invocation. Print or check MD5 digests.
79 * mkdir: (coreutils)mkdir invocation. Create directories.
80 * mkfifo: (coreutils)mkfifo invocation. Create FIFOs (named pipes).
81 * mknod: (coreutils)mknod invocation. Create special files.
82 * mktemp: (coreutils)mktemp invocation. Create temporary files.
83 * mv: (coreutils)mv invocation. Rename files.
84 * nice: (coreutils)nice invocation. Modify niceness.
85 * nl: (coreutils)nl invocation. Number lines and write files.
86 * nohup: (coreutils)nohup invocation. Immunize to hangups.
87 * nproc: (coreutils)nproc invocation. Print the number of processors.
88 * od: (coreutils)od invocation. Dump files in octal, etc.
89 * paste: (coreutils)paste invocation. Merge lines of files.
90 * pathchk: (coreutils)pathchk invocation. Check file name portability.
91 * pr: (coreutils)pr invocation. Paginate or columnate files.
92 * printenv: (coreutils)printenv invocation. Print environment variables.
93 * printf: (coreutils)printf invocation. Format and print data.
94 * ptx: (coreutils)ptx invocation. Produce permuted indexes.
95 * pwd: (coreutils)pwd invocation. Print working directory.
96 * readlink: (coreutils)readlink invocation. Print referent of a symlink.
97 * rm: (coreutils)rm invocation. Remove files.
98 * rmdir: (coreutils)rmdir invocation. Remove empty directories.
99 * runcon: (coreutils)runcon invocation. Run in specified SELinux CTX.
100 * seq: (coreutils)seq invocation. Print numeric sequences
101 * sha1sum: (coreutils)sha1sum invocation. Print or check SHA-1 digests.
102 * sha2: (coreutils)sha2 utilities. Print or check SHA-2 digests.
103 * shred: (coreutils)shred invocation. Remove files more securely.
104 * shuf: (coreutils)shuf invocation. Shuffling text files.
105 * sleep: (coreutils)sleep invocation. Delay for a specified time.
106 * sort: (coreutils)sort invocation. Sort text files.
107 * split: (coreutils)split invocation. Split into fixed-size pieces.
108 * stat: (coreutils)stat invocation. Report file(system) status.
109 * stdbuf: (coreutils)stdbuf invocation. Modify stdio buffering.
110 * stty: (coreutils)stty invocation. Print/change terminal settings.
111 * su: (coreutils)su invocation. Modify user and group ID.
112 * sum: (coreutils)sum invocation. Print traditional checksum.
113 * sync: (coreutils)sync invocation. Synchronize memory and disk.
114 * tac: (coreutils)tac invocation. Reverse files.
115 * tail: (coreutils)tail invocation. Output the last part of files.
116 * tee: (coreutils)tee invocation. Redirect to multiple files.
117 * test: (coreutils)test invocation. File/string tests.
118 * timeout: (coreutils)timeout invocation. Run with time limit.
119 * touch: (coreutils)touch invocation. Change file timestamps.
120 * tr: (coreutils)tr invocation. Translate characters.
121 * true: (coreutils)true invocation. Do nothing, successfully.
122 * truncate: (coreutils)truncate invocation. Shrink/extend size of a file.
123 * tsort: (coreutils)tsort invocation. Topological sort.
124 * tty: (coreutils)tty invocation. Print terminal name.
125 * uname: (coreutils)uname invocation. Print system information.
126 * unexpand: (coreutils)unexpand invocation. Convert spaces to tabs.
127 * uniq: (coreutils)uniq invocation. Uniquify files.
128 * unlink: (coreutils)unlink invocation. Removal via unlink(2).
129 * uptime: (coreutils)uptime invocation. Print uptime and load.
130 * users: (coreutils)users invocation. Print current user names.
131 * vdir: (coreutils)vdir invocation. List directories verbosely.
132 * wc: (coreutils)wc invocation. Line, word, and byte counts.
133 * who: (coreutils)who invocation. Print who is logged in.
134 * whoami: (coreutils)whoami invocation. Print effective user ID.
135 * yes: (coreutils)yes invocation. Print a string indefinitely.
139 This manual documents version @value{VERSION} of the @sc{gnu} core
140 utilities, including the standard programs for text and file manipulation.
142 Copyright @copyright{} 1994-1996, 2000-2010 Free Software Foundation, Inc.
145 Permission is granted to copy, distribute and/or modify this document
146 under the terms of the GNU Free Documentation License, Version 1.3 or
147 any later version published by the Free Software Foundation; with no
148 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
149 Texts. A copy of the license is included in the section entitled ``GNU
150 Free Documentation License''.
155 @title @sc{gnu} @code{Coreutils}
156 @subtitle Core GNU utilities
157 @subtitle for version @value{VERSION}, @value{UPDATED}
158 @author David MacKenzie et al.
161 @vskip 0pt plus 1filll
174 @cindex core utilities
175 @cindex text utilities
176 @cindex shell utilities
177 @cindex file utilities
180 * Introduction:: Caveats, overview, and authors
181 * Common options:: Common options
182 * Output of entire files:: cat tac nl od base64
183 * Formatting file contents:: fmt pr fold
184 * Output of parts of files:: head tail split csplit
185 * Summarizing files:: wc sum cksum md5sum sha1sum sha2
186 * Operating on sorted files:: sort shuf uniq comm ptx tsort
187 * Operating on fields:: cut paste join
188 * Operating on characters:: tr expand unexpand
189 * Directory listing:: ls dir vdir dircolors
190 * Basic operations:: cp dd install mv rm shred
191 * Special file types:: mkdir rmdir unlink mkfifo mknod ln link readlink
192 * Changing file attributes:: chgrp chmod chown touch
193 * Disk usage:: df du stat sync truncate
194 * Printing text:: echo printf yes
195 * Conditions:: false true test expr
197 * File name manipulation:: dirname basename pathchk mktemp
198 * Working context:: pwd stty printenv tty
199 * User information:: id logname whoami groups users who
200 * System context:: date arch nproc uname hostname hostid uptime
201 * SELinux context:: chcon runcon
202 * Modified command invocation:: chroot env nice nohup stdbuf su timeout
203 * Process control:: kill
205 * Numeric operations:: factor seq
206 * File permissions:: Access modes
207 * Date input formats:: Specifying date strings
208 * Opening the software toolbox:: The software tools philosophy
209 * GNU Free Documentation License:: Copying and sharing this manual
210 * Concept index:: General index
213 --- The Detailed Node Listing ---
217 * Exit status:: Indicating program success or failure
218 * Backup options:: Backup options
219 * Block size:: Block size
220 * Signal specifications:: Specifying signals
221 * Disambiguating names and IDs:: chgrp and chown owner and group syntax
222 * Random sources:: Sources of random data
223 * Target directory:: Target directory
224 * Trailing slashes:: Trailing slashes
225 * Traversing symlinks:: Traversing symlinks to directories
226 * Treating / specially:: Treating / specially
227 * Standards conformance:: Standards conformance
229 Output of entire files
231 * cat invocation:: Concatenate and write files
232 * tac invocation:: Concatenate and write files in reverse
233 * nl invocation:: Number lines and write files
234 * od invocation:: Write files in octal or other formats
235 * base64 invocation:: Transform data into printable data
237 Formatting file contents
239 * fmt invocation:: Reformat paragraph text
240 * pr invocation:: Paginate or columnate files for printing
241 * fold invocation:: Wrap input lines to fit in specified width
243 Output of parts of files
245 * head invocation:: Output the first part of files
246 * tail invocation:: Output the last part of files
247 * split invocation:: Split a file into fixed-size pieces
248 * csplit invocation:: Split a file into context-determined pieces
252 * wc invocation:: Print newline, word, and byte counts
253 * sum invocation:: Print checksum and block counts
254 * cksum invocation:: Print CRC checksum and byte counts
255 * md5sum invocation:: Print or check MD5 digests
256 * sha1sum invocation:: Print or check SHA-1 digests
257 * sha2 utilities:: Print or check SHA-2 digests
259 Operating on sorted files
261 * sort invocation:: Sort text files
262 * shuf invocation:: Shuffle text files
263 * uniq invocation:: Uniquify files
264 * comm invocation:: Compare two sorted files line by line
265 * ptx invocation:: Produce a permuted index of file contents
266 * tsort invocation:: Topological sort
268 @command{ptx}: Produce permuted indexes
270 * General options in ptx:: Options which affect general program behavior
271 * Charset selection in ptx:: Underlying character set considerations
272 * Input processing in ptx:: Input fields, contexts, and keyword selection
273 * Output formatting in ptx:: Types of output format, and sizing the fields
274 * Compatibility in ptx:: The @acronym{GNU} extensions to @command{ptx}
278 * cut invocation:: Print selected parts of lines
279 * paste invocation:: Merge lines of files
280 * join invocation:: Join lines on a common field
282 Operating on characters
284 * tr invocation:: Translate, squeeze, and/or delete characters
285 * expand invocation:: Convert tabs to spaces
286 * unexpand invocation:: Convert spaces to tabs
288 @command{tr}: Translate, squeeze, and/or delete characters
290 * Character sets:: Specifying sets of characters
291 * Translating:: Changing one set of characters to another
292 * Squeezing:: Squeezing repeats and deleting
296 * ls invocation:: List directory contents
297 * dir invocation:: Briefly list directory contents
298 * vdir invocation:: Verbosely list directory contents
299 * dircolors invocation:: Color setup for @command{ls}
301 @command{ls}: List directory contents
303 * Which files are listed:: Which files are listed
304 * What information is listed:: What information is listed
305 * Sorting the output:: Sorting the output
306 * Details about version sort:: More details about version sort
307 * General output formatting:: General output formatting
308 * Formatting the file names:: Formatting the file names
312 * cp invocation:: Copy files and directories
313 * dd invocation:: Convert and copy a file
314 * install invocation:: Copy files and set attributes
315 * mv invocation:: Move (rename) files
316 * rm invocation:: Remove files or directories
317 * shred invocation:: Remove files more securely
321 * link invocation:: Make a hard link via the link syscall
322 * ln invocation:: Make links between files
323 * mkdir invocation:: Make directories
324 * mkfifo invocation:: Make FIFOs (named pipes)
325 * mknod invocation:: Make block or character special files
326 * readlink invocation:: Print value of a symlink or canonical file name
327 * rmdir invocation:: Remove empty directories
328 * unlink invocation:: Remove files via unlink syscall
330 Changing file attributes
332 * chown invocation:: Change file owner and group
333 * chgrp invocation:: Change group ownership
334 * chmod invocation:: Change access permissions
335 * touch invocation:: Change file timestamps
339 * df invocation:: Report file system disk space usage
340 * du invocation:: Estimate file space usage
341 * stat invocation:: Report file or file system status
342 * sync invocation:: Synchronize data on disk with memory
343 * truncate invocation:: Shrink or extend the size of a file
347 * echo invocation:: Print a line of text
348 * printf invocation:: Format and print data
349 * yes invocation:: Print a string until interrupted
353 * false invocation:: Do nothing, unsuccessfully
354 * true invocation:: Do nothing, successfully
355 * test invocation:: Check file types and compare values
356 * expr invocation:: Evaluate expressions
358 @command{test}: Check file types and compare values
360 * File type tests:: File type tests
361 * Access permission tests:: Access permission tests
362 * File characteristic tests:: File characteristic tests
363 * String tests:: String tests
364 * Numeric tests:: Numeric tests
366 @command{expr}: Evaluate expression
368 * String expressions:: + : match substr index length
369 * Numeric expressions:: + - * / %
370 * Relations for expr:: | & < <= = == != >= >
371 * Examples of expr:: Examples of using @command{expr}
375 * tee invocation:: Redirect output to multiple files or processes
377 File name manipulation
379 * basename invocation:: Strip directory and suffix from a file name
380 * dirname invocation:: Strip last file name component
381 * pathchk invocation:: Check file name validity and portability
382 * mktemp invocation:: Create temporary file or directory
386 * pwd invocation:: Print working directory
387 * stty invocation:: Print or change terminal characteristics
388 * printenv invocation:: Print all or some environment variables
389 * tty invocation:: Print file name of terminal on standard input
391 @command{stty}: Print or change terminal characteristics
393 * Control:: Control settings
394 * Input:: Input settings
395 * Output:: Output settings
396 * Local:: Local settings
397 * Combination:: Combination settings
398 * Characters:: Special characters
399 * Special:: Special settings
403 * id invocation:: Print user identity
404 * logname invocation:: Print current login name
405 * whoami invocation:: Print effective user ID
406 * groups invocation:: Print group names a user is in
407 * users invocation:: Print login names of users currently logged in
408 * who invocation:: Print who is currently logged in
412 * arch invocation:: Print machine hardware name
413 * date invocation:: Print or set system date and time
414 * nproc invocation:: Print the number of processors
415 * uname invocation:: Print system information
416 * hostname invocation:: Print or set system name
417 * hostid invocation:: Print numeric host identifier
418 * uptime invocation:: Print system uptime and load
420 @command{date}: Print or set system date and time
422 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
423 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
424 * Literal conversion specifiers:: %[%nt]
425 * Padding and other flags:: Pad with zeros, spaces, etc.
426 * Setting the time:: Changing the system clock
427 * Options for date:: Instead of the current time
428 * Date input formats:: Specifying date strings
429 * Examples of date:: Examples
433 * chcon invocation:: Change SELinux context of file
434 * runcon invocation:: Run a command in specified SELinux context
436 Modified command invocation
438 * chroot invocation:: Run a command with a different root directory
439 * env invocation:: Run a command in a modified environment
440 * nice invocation:: Run a command with modified niceness
441 * nohup invocation:: Run a command immune to hangups
442 * stdbuf invocation:: Run a command with modified I/O buffering
443 * su invocation:: Run a command with substitute user and group ID
444 * timeout invocation:: Run a command with a time limit
448 * kill invocation:: Sending a signal to processes.
452 * sleep invocation:: Delay for a specified time
456 * factor invocation:: Print prime factors
457 * seq invocation:: Print numeric sequences
461 * Mode Structure:: Structure of file mode bits
462 * Symbolic Modes:: Mnemonic representation of file mode bits
463 * Numeric Modes:: File mode bits as octal numbers
464 * Directory Setuid and Setgid:: Set-user-ID and set-group-ID on directories
468 * General date syntax:: Common rules
469 * Calendar date items:: 19 Dec 1994
470 * Time of day items:: 9:20pm
471 * Time zone items:: @sc{est}, @sc{pdt}, @sc{gmt}
472 * Day of week items:: Monday and others
473 * Relative items in date strings:: next tuesday, 2 years ago
474 * Pure numbers in date strings:: 19931219, 1440
475 * Seconds since the Epoch:: @@1078100502
476 * Specifying time zone rules:: TZ="America/New_York", TZ="UTC0"
477 * Authors of get_date:: 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.
1500 @itemx --squeeze-blank
1502 @opindex --squeeze-blank
1503 @cindex squeezing empty lines
1504 Suppress repeated adjacent empty lines; output just one empty line
1509 Equivalent to @option{-vT}.
1514 @opindex --show-tabs
1515 Display TAB characters as @samp{^I}.
1519 Ignored; for @acronym{POSIX} compatibility.
1522 @itemx --show-nonprinting
1524 @opindex --show-nonprinting
1525 Display control characters except for LFD and TAB using
1526 @samp{^} notation and precede characters that have the high bit set with
1531 On systems like MS-DOS that distinguish between text and binary files,
1532 @command{cat} normally reads and writes in binary mode. However,
1533 @command{cat} reads in text mode if one of the options
1534 @option{-bensAE} is used or if @command{cat} is reading from standard
1535 input and standard input is a terminal. Similarly, @command{cat}
1536 writes in text mode if one of the options @option{-bensAE} is used or
1537 if standard output is a terminal.
1544 # Output f's contents, then standard input, then g's contents.
1547 # Copy standard input to standard output.
1552 @node tac invocation
1553 @section @command{tac}: Concatenate and write files in reverse
1556 @cindex reversing files
1558 @command{tac} copies each @var{file} (@samp{-} means standard input), or
1559 standard input if none are given, to standard output, reversing the
1560 records (lines by default) in each separately. Synopsis:
1563 tac [@var{option}]@dots{} [@var{file}]@dots{}
1566 @dfn{Records} are separated by instances of a string (newline by
1567 default). By default, this separator string is attached to the end of
1568 the record that it follows in the file.
1570 The program accepts the following options. Also see @ref{Common options}.
1578 The separator is attached to the beginning of the record that it
1579 precedes in the file.
1585 Treat the separator string as a regular expression. Users of @command{tac}
1586 on MS-DOS/MS-Windows should note that, since @command{tac} reads files in
1587 binary mode, each line of a text file might end with a CR/LF pair
1588 instead of the Unix-style LF.
1590 @item -s @var{separator}
1591 @itemx --separator=@var{separator}
1593 @opindex --separator
1594 Use @var{separator} as the record separator, instead of newline.
1602 @section @command{nl}: Number lines and write files
1605 @cindex numbering lines
1606 @cindex line numbering
1608 @command{nl} writes each @var{file} (@samp{-} means standard input), or
1609 standard input if none are given, to standard output, with line numbers
1610 added to some or all of the lines. Synopsis:
1613 nl [@var{option}]@dots{} [@var{file}]@dots{}
1616 @cindex logical pages, numbering on
1617 @command{nl} decomposes its input into (logical) pages; by default, the
1618 line number is reset to 1 at the top of each logical page. @command{nl}
1619 treats all of the input files as a single document; it does not reset
1620 line numbers or logical pages between files.
1622 @cindex headers, numbering
1623 @cindex body, numbering
1624 @cindex footers, numbering
1625 A logical page consists of three sections: header, body, and footer.
1626 Any of the sections can be empty. Each can be numbered in a different
1627 style from the others.
1629 The beginnings of the sections of logical pages are indicated in the
1630 input file by a line containing exactly one of these delimiter strings:
1641 The two characters from which these strings are made can be changed from
1642 @samp{\} and @samp{:} via options (see below), but the pattern and
1643 length of each string cannot be changed.
1645 A section delimiter is replaced by an empty line on output. Any text
1646 that comes before the first section delimiter string in the input file
1647 is considered to be part of a body section, so @command{nl} treats a
1648 file that contains no section delimiters as a single body section.
1650 The program accepts the following options. Also see @ref{Common options}.
1654 @item -b @var{style}
1655 @itemx --body-numbering=@var{style}
1657 @opindex --body-numbering
1658 Select the numbering style for lines in the body section of each
1659 logical page. When a line is not numbered, the current line number
1660 is not incremented, but the line number separator character is still
1661 prepended to the line. The styles are:
1667 number only nonempty lines (default for body),
1669 do not number lines (default for header and footer),
1671 number only lines that contain a match for the basic regular
1672 expression @var{bre}.
1673 @xref{Regular Expressions, , Regular Expressions, grep, The GNU Grep Manual}.
1677 @itemx --section-delimiter=@var{cd}
1679 @opindex --section-delimiter
1680 @cindex section delimiters of pages
1681 Set the section delimiter characters to @var{cd}; default is
1682 @samp{\:}. If only @var{c} is given, the second remains @samp{:}.
1683 (Remember to protect @samp{\} or other metacharacters from shell
1684 expansion with quotes or extra backslashes.)
1686 @item -f @var{style}
1687 @itemx --footer-numbering=@var{style}
1689 @opindex --footer-numbering
1690 Analogous to @option{--body-numbering}.
1692 @item -h @var{style}
1693 @itemx --header-numbering=@var{style}
1695 @opindex --header-numbering
1696 Analogous to @option{--body-numbering}.
1698 @item -i @var{number}
1699 @itemx --line-increment=@var{number}
1701 @opindex --line-increment
1702 Increment line numbers by @var{number} (default 1).
1704 @item -l @var{number}
1705 @itemx --join-blank-lines=@var{number}
1707 @opindex --join-blank-lines
1708 @cindex empty lines, numbering
1709 @cindex blank lines, numbering
1710 Consider @var{number} (default 1) consecutive empty lines to be one
1711 logical line for numbering, and only number the last one. Where fewer
1712 than @var{number} consecutive empty lines occur, do not number them.
1713 An empty line is one that contains no characters, not even spaces
1716 @item -n @var{format}
1717 @itemx --number-format=@var{format}
1719 @opindex --number-format
1720 Select the line numbering format (default is @code{rn}):
1724 @opindex ln @r{format for @command{nl}}
1725 left justified, no leading zeros;
1727 @opindex rn @r{format for @command{nl}}
1728 right justified, no leading zeros;
1730 @opindex rz @r{format for @command{nl}}
1731 right justified, leading zeros.
1735 @itemx --no-renumber
1737 @opindex --no-renumber
1738 Do not reset the line number at the start of a logical page.
1740 @item -s @var{string}
1741 @itemx --number-separator=@var{string}
1743 @opindex --number-separator
1744 Separate the line number from the text line in the output with
1745 @var{string} (default is the TAB character).
1747 @item -v @var{number}
1748 @itemx --starting-line-number=@var{number}
1750 @opindex --starting-line-number
1751 Set the initial line number on each logical page to @var{number} (default 1).
1753 @item -w @var{number}
1754 @itemx --number-width=@var{number}
1756 @opindex --number-width
1757 Use @var{number} characters for line numbers (default 6).
1765 @section @command{od}: Write files in octal or other formats
1768 @cindex octal dump of files
1769 @cindex hex dump of files
1770 @cindex ASCII dump of files
1771 @cindex file contents, dumping unambiguously
1773 @command{od} writes an unambiguous representation of each @var{file}
1774 (@samp{-} means standard input), or standard input if none are given.
1778 od [@var{option}]@dots{} [@var{file}]@dots{}
1779 od [-abcdfilosx]@dots{} [@var{file}] [[+]@var{offset}[.][b]]
1780 od [@var{option}]@dots{} --traditional [@var{file}] [[+]@var{offset}[.][b] [[+]@var{label}[.][b]]]
1783 Each line of output consists of the offset in the input, followed by
1784 groups of data from the file. By default, @command{od} prints the offset in
1785 octal, and each group of file data is a C @code{short int}'s worth of input
1786 printed as a single octal number.
1788 If @var{offset} is given, it specifies how many input bytes to skip
1789 before formatting and writing. By default, it is interpreted as an
1790 octal number, but the optional trailing decimal point causes it to be
1791 interpreted as decimal. If no decimal is specified and the offset
1792 begins with @samp{0x} or @samp{0X} it is interpreted as a hexadecimal
1793 number. If there is a trailing @samp{b}, the number of bytes skipped
1794 will be @var{offset} multiplied by 512.
1796 If a command is of both the first and second forms, the second form is
1797 assumed if the last operand begins with @samp{+} or (if there are two
1798 operands) a digit. For example, in @samp{od foo 10} and @samp{od +10}
1799 the @samp{10} is an offset, whereas in @samp{od 10} the @samp{10} is a
1802 The program accepts the following options. Also see @ref{Common options}.
1806 @item -A @var{radix}
1807 @itemx --address-radix=@var{radix}
1809 @opindex --address-radix
1810 @cindex radix for file offsets
1811 @cindex file offset radix
1812 Select the base in which file offsets are printed. @var{radix} can
1813 be one of the following:
1823 none (do not print offsets).
1826 The default is octal.
1828 @item -j @var{bytes}
1829 @itemx --skip-bytes=@var{bytes}
1831 @opindex --skip-bytes
1832 Skip @var{bytes} input bytes before formatting and writing. If
1833 @var{bytes} begins with @samp{0x} or @samp{0X}, it is interpreted in
1834 hexadecimal; otherwise, if it begins with @samp{0}, in octal; otherwise,
1836 @multiplierSuffixes{bytes}
1838 @item -N @var{bytes}
1839 @itemx --read-bytes=@var{bytes}
1841 @opindex --read-bytes
1842 Output at most @var{bytes} bytes of the input. Prefixes and suffixes on
1843 @code{bytes} are interpreted as for the @option{-j} option.
1845 @item -S @var{bytes}
1846 @itemx --strings[=@var{bytes}]
1849 @cindex string constants, outputting
1850 Instead of the normal output, output only @dfn{string constants}: at
1851 least @var{bytes} consecutive @acronym{ASCII} graphic characters,
1852 followed by a zero byte (@acronym{ASCII} @sc{nul}).
1853 Prefixes and suffixes on @code{bytes} are interpreted as for the
1856 If @var{n} is omitted with @option{--strings}, the default is 3.
1859 @itemx --format=@var{type}
1862 Select the format in which to output the file data. @var{type} is a
1863 string of one or more of the below type indicator characters. If you
1864 include more than one type indicator character in a single @var{type}
1865 string, or use this option more than once, @command{od} writes one copy
1866 of each output line using each of the data types that you specified,
1867 in the order that you specified.
1869 Adding a trailing ``z'' to any type specification appends a display
1870 of the @acronym{ASCII} character representation of the printable characters
1871 to the output line generated by the type specification.
1875 named character, ignoring high-order bit
1877 @acronym{ASCII} character or backslash escape,
1890 The type @code{a} outputs things like @samp{sp} for space, @samp{nl} for
1891 newline, and @samp{nul} for a zero byte. Only the least significant
1892 seven bits of each byte is used; the high-order bit is ignored.
1893 Type @code{c} outputs
1894 @samp{ }, @samp{\n}, and @code{\0}, respectively.
1897 Except for types @samp{a} and @samp{c}, you can specify the number
1898 of bytes to use in interpreting each number in the given data type
1899 by following the type indicator character with a decimal integer.
1900 Alternately, you can specify the size of one of the C compiler's
1901 built-in data types by following the type indicator character with
1902 one of the following characters. For integers (@samp{d}, @samp{o},
1903 @samp{u}, @samp{x}):
1916 For floating point (@code{f}):
1928 @itemx --output-duplicates
1930 @opindex --output-duplicates
1931 Output consecutive lines that are identical. By default, when two or
1932 more consecutive output lines would be identical, @command{od} outputs only
1933 the first line, and puts just an asterisk on the following line to
1934 indicate the elision.
1937 @itemx --width[=@var{n}]
1940 Dump @code{n} input bytes per output line. This must be a multiple of
1941 the least common multiple of the sizes associated with the specified
1944 If this option is not given at all, the default is 16. If @var{n} is
1945 omitted, the default is 32.
1949 The next several options are shorthands for format specifications.
1950 @sc{gnu} @command{od} accepts any combination of shorthands and format
1951 specification options. These options accumulate.
1957 Output as named characters. Equivalent to @samp{-t a}.
1961 Output as octal bytes. Equivalent to @samp{-t o1}.
1965 Output as @acronym{ASCII} characters or backslash escapes. Equivalent to
1970 Output as unsigned decimal two-byte units. Equivalent to @samp{-t u2}.
1974 Output as floats. Equivalent to @samp{-t fF}.
1978 Output as decimal ints. Equivalent to @samp{-t dI}.
1982 Output as decimal long ints. Equivalent to @samp{-t dL}.
1986 Output as octal two-byte units. Equivalent to @option{-t o2}.
1990 Output as decimal two-byte units. Equivalent to @option{-t d2}.
1994 Output as hexadecimal two-byte units. Equivalent to @samp{-t x2}.
1997 @opindex --traditional
1998 Recognize the non-option label argument that traditional @command{od}
1999 accepted. The following syntax:
2002 od --traditional [@var{file}] [[+]@var{offset}[.][b] [[+]@var{label}[.][b]]]
2006 can be used to specify at most one file and optional arguments
2007 specifying an offset and a pseudo-start address, @var{label}.
2008 The @var{label} argument is interpreted
2009 just like @var{offset}, but it specifies an initial pseudo-address. The
2010 pseudo-addresses are displayed in parentheses following any normal
2017 @node base64 invocation
2018 @section @command{base64}: Transform data into printable data
2021 @cindex base64 encoding
2023 @command{base64} transforms data read from a file, or standard input,
2024 into (or from) base64 encoded form. The base64 encoded form uses
2025 printable @acronym{ASCII} characters to represent binary data.
2029 base64 [@var{option}]@dots{} [@var{file}]
2030 base64 --decode [@var{option}]@dots{} [@var{file}]
2033 The base64 encoding expands data to roughly 133% of the original.
2034 The format conforms to
2035 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc4648.txt, RFC 4648}.
2037 The program accepts the following options. Also see @ref{Common options}.
2042 @itemx --wrap=@var{cols}
2046 @cindex column to wrap data after
2047 During encoding, wrap lines after @var{cols} characters. This must be
2050 The default is to wrap after 76 characters. Use the value 0 to
2051 disable line wrapping altogether.
2057 @cindex Decode base64 data
2058 @cindex Base64 decoding
2059 Change the mode of operation, from the default of encoding data, to
2060 decoding data. Input is expected to be base64 encoded data, and the
2061 output will be the original data.
2064 @itemx --ignore-garbage
2066 @opindex --ignore-garbage
2067 @cindex Ignore garbage in base64 stream
2068 When decoding, newlines are always accepted.
2069 During decoding, ignore unrecognized bytes,
2070 to permit distorted data to be decoded.
2077 @node Formatting file contents
2078 @chapter Formatting file contents
2080 @cindex formatting file contents
2082 These commands reformat the contents of files.
2085 * fmt invocation:: Reformat paragraph text.
2086 * pr invocation:: Paginate or columnate files for printing.
2087 * fold invocation:: Wrap input lines to fit in specified width.
2091 @node fmt invocation
2092 @section @command{fmt}: Reformat paragraph text
2095 @cindex reformatting paragraph text
2096 @cindex paragraphs, reformatting
2097 @cindex text, reformatting
2099 @command{fmt} fills and joins lines to produce output lines of (at most)
2100 a given number of characters (75 by default). Synopsis:
2103 fmt [@var{option}]@dots{} [@var{file}]@dots{}
2106 @command{fmt} reads from the specified @var{file} arguments (or standard
2107 input if none are given), and writes to standard output.
2109 By default, blank lines, spaces between words, and indentation are
2110 preserved in the output; successive input lines with different
2111 indentation are not joined; tabs are expanded on input and introduced on
2114 @cindex line-breaking
2115 @cindex sentences and line-breaking
2116 @cindex Knuth, Donald E.
2117 @cindex Plass, Michael F.
2118 @command{fmt} prefers breaking lines at the end of a sentence, and tries to
2119 avoid line breaks after the first word of a sentence or before the last
2120 word of a sentence. A @dfn{sentence break} is defined as either the end
2121 of a paragraph or a word ending in any of @samp{.?!}, followed by two
2122 spaces or end of line, ignoring any intervening parentheses or quotes.
2123 Like @TeX{}, @command{fmt} reads entire ``paragraphs'' before choosing line
2124 breaks; the algorithm is a variant of that given by Donald E. Knuth
2125 and Michael F. Plass in ``Breaking Paragraphs Into Lines'',
2126 @cite{Software---Practice & Experience} @b{11}, 11 (November 1981),
2129 The program accepts the following options. Also see @ref{Common options}.
2134 @itemx --crown-margin
2136 @opindex --crown-margin
2137 @cindex crown margin
2138 @dfn{Crown margin} mode: preserve the indentation of the first two
2139 lines within a paragraph, and align the left margin of each subsequent
2140 line with that of the second line.
2143 @itemx --tagged-paragraph
2145 @opindex --tagged-paragraph
2146 @cindex tagged paragraphs
2147 @dfn{Tagged paragraph} mode: like crown margin mode, except that if
2148 indentation of the first line of a paragraph is the same as the
2149 indentation of the second, the first line is treated as a one-line
2155 @opindex --split-only
2156 Split lines only. Do not join short lines to form longer ones. This
2157 prevents sample lines of code, and other such ``formatted'' text from
2158 being unduly combined.
2161 @itemx --uniform-spacing
2163 @opindex --uniform-spacing
2164 Uniform spacing. Reduce spacing between words to one space, and spacing
2165 between sentences to two spaces.
2168 @itemx -w @var{width}
2169 @itemx --width=@var{width}
2170 @opindex -@var{width}
2173 Fill output lines up to @var{width} characters (default 75). @command{fmt}
2174 initially tries to make lines about 7% shorter than this, to give it
2175 room to balance line lengths.
2177 @item -p @var{prefix}
2178 @itemx --prefix=@var{prefix}
2179 Only lines beginning with @var{prefix} (possibly preceded by whitespace)
2180 are subject to formatting. The prefix and any preceding whitespace are
2181 stripped for the formatting and then re-attached to each formatted output
2182 line. One use is to format certain kinds of program comments, while
2183 leaving the code unchanged.
2191 @section @command{pr}: Paginate or columnate files for printing
2194 @cindex printing, preparing files for
2195 @cindex multicolumn output, generating
2196 @cindex merging files in parallel
2198 @command{pr} writes each @var{file} (@samp{-} means standard input), or
2199 standard input if none are given, to standard output, paginating and
2200 optionally outputting in multicolumn format; optionally merges all
2201 @var{file}s, printing all in parallel, one per column. Synopsis:
2204 pr [@var{option}]@dots{} [@var{file}]@dots{}
2208 By default, a 5-line header is printed at each page: two blank lines;
2209 a line with the date, the file name, and the page count; and two more
2210 blank lines. A footer of five blank lines is also printed.
2211 The default @var{page_length} is 66
2212 lines. The default number of text lines is therefore 56.
2213 The text line of the header takes the form
2214 @samp{@var{date} @var{string} @var{page}}, with spaces inserted around
2215 @var{string} so that the line takes up the full @var{page_width}. Here,
2216 @var{date} is the date (see the @option{-D} or @option{--date-format}
2217 option for details), @var{string} is the centered header string, and
2218 @var{page} identifies the page number. The @env{LC_MESSAGES} locale
2219 category affects the spelling of @var{page}; in the default C locale, it
2220 is @samp{Page @var{number}} where @var{number} is the decimal page
2223 Form feeds in the input cause page breaks in the output. Multiple form
2224 feeds produce empty pages.
2226 Columns are of equal width, separated by an optional string (default
2227 is @samp{space}). For multicolumn output, lines will always be truncated to
2228 @var{page_width} (default 72), unless you use the @option{-J} option.
2230 column output no line truncation occurs by default. Use @option{-W} option to
2231 truncate lines in that case.
2233 The following changes were made in version 1.22i and apply to later
2234 versions of @command{pr}:
2235 @c FIXME: this whole section here sounds very awkward to me. I
2236 @c made a few small changes, but really it all needs to be redone. - Brian
2237 @c OK, I fixed another sentence or two, but some of it I just don't understand.
2242 Some small @var{letter options} (@option{-s}, @option{-w}) have been
2243 redefined for better @acronym{POSIX} compliance. The output of some further
2244 cases has been adapted to other Unix systems. These changes are not
2245 compatible with earlier versions of the program.
2248 Some @var{new capital letter} options (@option{-J}, @option{-S}, @option{-W})
2249 have been introduced to turn off unexpected interferences of small letter
2250 options. The @option{-N} option and the second argument @var{last_page}
2251 of @samp{+FIRST_PAGE} offer more flexibility. The detailed handling of
2252 form feeds set in the input files requires the @option{-T} option.
2255 Capital letter options override small letter ones.
2258 Some of the option-arguments (compare @option{-s}, @option{-e},
2259 @option{-i}, @option{-n}) cannot be specified as separate arguments from the
2260 preceding option letter (already stated in the @acronym{POSIX} specification).
2263 The program accepts the following options. Also see @ref{Common options}.
2267 @item +@var{first_page}[:@var{last_page}]
2268 @itemx --pages=@var{first_page}[:@var{last_page}]
2269 @c The two following @opindex lines evoke warnings because they contain `:'
2270 @c The `info' spec does not permit that. If we use those lines, we end
2271 @c up with truncated index entries that don't work.
2272 @c @opindex +@var{first_page}[:@var{last_page}]
2273 @c @opindex --pages=@var{first_page}[:@var{last_page}]
2274 @opindex +@var{page_range}
2275 @opindex --pages=@var{page_range}
2276 Begin printing with page @var{first_page} and stop with @var{last_page}.
2277 Missing @samp{:@var{last_page}} implies end of file. While estimating
2278 the number of skipped pages each form feed in the input file results
2279 in a new page. Page counting with and without @samp{+@var{first_page}}
2280 is identical. By default, counting starts with the first page of input
2281 file (not first page printed). Line numbering may be altered by @option{-N}
2285 @itemx --columns=@var{column}
2286 @opindex -@var{column}
2288 @cindex down columns
2289 With each single @var{file}, produce @var{column} columns of output
2290 (default is 1) and print columns down, unless @option{-a} is used. The
2291 column width is automatically decreased as @var{column} increases; unless
2292 you use the @option{-W/-w} option to increase @var{page_width} as well.
2293 This option might well cause some lines to be truncated. The number of
2294 lines in the columns on each page are balanced. The options @option{-e}
2295 and @option{-i} are on for multiple text-column output. Together with
2296 @option{-J} option column alignment and line truncation is turned off.
2297 Lines of full length are joined in a free field format and @option{-S}
2298 option may set field separators. @option{-@var{column}} may not be used
2299 with @option{-m} option.
2305 @cindex across columns
2306 With each single @var{file}, print columns across rather than down. The
2307 @option{-@var{column}} option must be given with @var{column} greater than one.
2308 If a line is too long to fit in a column, it is truncated.
2311 @itemx --show-control-chars
2313 @opindex --show-control-chars
2314 Print control characters using hat notation (e.g., @samp{^G}); print
2315 other nonprinting characters in octal backslash notation. By default,
2316 nonprinting characters are not changed.
2319 @itemx --double-space
2321 @opindex --double-space
2322 @cindex double spacing
2323 Double space the output.
2325 @item -D @var{format}
2326 @itemx --date-format=@var{format}
2327 @cindex time formats
2328 @cindex formatting times
2329 Format header dates using @var{format}, using the same conventions as
2330 for the command @samp{date +@var{format}}; @xref{date invocation}.
2331 Except for directives, which start with
2332 @samp{%}, characters in @var{format} are printed unchanged. You can use
2333 this option to specify an arbitrary string in place of the header date,
2334 e.g., @option{--date-format="Monday morning"}.
2336 @vindex POSIXLY_CORRECT
2338 The default date format is @samp{%Y-%m-%d %H:%M} (for example,
2339 @samp{2001-12-04 23:59});
2340 but if the @env{POSIXLY_CORRECT} environment variable is set
2341 and the @env{LC_TIME} locale category specifies the @acronym{POSIX}
2342 locale, the default is @samp{%b %e %H:%M %Y} (for example,
2343 @samp{Dec@ @ 4 23:59 2001}.
2346 Time stamps are listed according to the time zone rules specified by
2347 the @env{TZ} environment variable, or by the system default rules if
2348 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
2349 with @env{TZ}, libc, The GNU C Library Reference Manual}.
2351 @item -e[@var{in-tabchar}[@var{in-tabwidth}]]
2352 @itemx --expand-tabs[=@var{in-tabchar}[@var{in-tabwidth}]]
2354 @opindex --expand-tabs
2356 Expand @var{tab}s to spaces on input. Optional argument @var{in-tabchar} is
2357 the input tab character (default is the TAB character). Second optional
2358 argument @var{in-tabwidth} is the input tab character's width (default
2366 @opindex --form-feed
2367 Use a form feed instead of newlines to separate output pages. This does
2368 not alter the default page length of 66 lines.
2370 @item -h @var{header}
2371 @itemx --header=@var{header}
2374 Replace the file name in the header with the centered string @var{header}.
2375 When using the shell, @var{header} should be quoted and should be
2376 separated from @option{-h} by a space.
2378 @item -i[@var{out-tabchar}[@var{out-tabwidth}]]
2379 @itemx --output-tabs[=@var{out-tabchar}[@var{out-tabwidth}]]
2381 @opindex --output-tabs
2383 Replace spaces with @var{tab}s on output. Optional argument @var{out-tabchar}
2384 is the output tab character (default is the TAB character). Second optional
2385 argument @var{out-tabwidth} is the output tab character's width (default
2391 @opindex --join-lines
2392 Merge lines of full length. Used together with the column options
2393 @option{-@var{column}}, @option{-a -@var{column}} or @option{-m}. Turns off
2394 @option{-W/-w} line truncation;
2395 no column alignment used; may be used with
2396 @option{--sep-string[=@var{string}]}. @option{-J} has been introduced
2397 (together with @option{-W} and @option{--sep-string})
2398 to disentangle the old (@acronym{POSIX}-compliant) options @option{-w} and
2399 @option{-s} along with the three column options.
2402 @item -l @var{page_length}
2403 @itemx --length=@var{page_length}
2406 Set the page length to @var{page_length} (default 66) lines, including
2407 the lines of the header [and the footer]. If @var{page_length} is less
2408 than or equal to 10, the header and footer are omitted, as if the
2409 @option{-t} option had been given.
2415 Merge and print all @var{file}s in parallel, one in each column. If a
2416 line is too long to fit in a column, it is truncated, unless the @option{-J}
2417 option is used. @option{--sep-string[=@var{string}]} may be used.
2419 some @var{file}s (form feeds set) produce empty columns, still marked
2420 by @var{string}. The result is a continuous line numbering and column
2421 marking throughout the whole merged file. Completely empty merged pages
2422 show no separators or line numbers. The default header becomes
2423 @samp{@var{date} @var{page}} with spaces inserted in the middle; this
2424 may be used with the @option{-h} or @option{--header} option to fill up
2425 the middle blank part.
2427 @item -n[@var{number-separator}[@var{digits}]]
2428 @itemx --number-lines[=@var{number-separator}[@var{digits}]]
2430 @opindex --number-lines
2431 Provide @var{digits} digit line numbering (default for @var{digits} is
2432 5). With multicolumn output the number occupies the first @var{digits}
2433 column positions of each text column or only each line of @option{-m}
2434 output. With single column output the number precedes each line just as
2435 @option{-m} does. Default counting of the line numbers starts with the
2436 first line of the input file (not the first line printed, compare the
2437 @option{--page} option and @option{-N} option).
2438 Optional argument @var{number-separator} is the character appended to
2439 the line number to separate it from the text followed. The default
2440 separator is the TAB character. In a strict sense a TAB is always
2441 printed with single column output only. The TAB width varies
2442 with the TAB position, e.g., with the left @var{margin} specified
2443 by @option{-o} option. With multicolumn output priority is given to
2444 @samp{equal width of output columns} (a @acronym{POSIX} specification).
2445 The TAB width is fixed to the value of the first column and does
2446 not change with different values of left @var{margin}. That means a
2447 fixed number of spaces is always printed in the place of the
2448 @var{number-separator} TAB. The tabification depends upon the output
2451 @item -N @var{line_number}
2452 @itemx --first-line-number=@var{line_number}
2454 @opindex --first-line-number
2455 Start line counting with the number @var{line_number} at first line of
2456 first page printed (in most cases not the first line of the input file).
2458 @item -o @var{margin}
2459 @itemx --indent=@var{margin}
2462 @cindex indenting lines
2464 Indent each line with a margin @var{margin} spaces wide (default is zero).
2465 The total page width is the size of the margin plus the @var{page_width}
2466 set with the @option{-W/-w} option. A limited overflow may occur with
2467 numbered single column output (compare @option{-n} option).
2470 @itemx --no-file-warnings
2472 @opindex --no-file-warnings
2473 Do not print a warning message when an argument @var{file} cannot be
2474 opened. (The exit status will still be nonzero, however.)
2476 @item -s[@var{char}]
2477 @itemx --separator[=@var{char}]
2479 @opindex --separator
2480 Separate columns by a single character @var{char}. The default for
2481 @var{char} is the TAB character without @option{-w} and @samp{no
2482 character} with @option{-w}. Without @option{-s} the default separator
2483 @samp{space} is set. @option{-s[char]} turns off line truncation of all
2484 three column options (@option{-COLUMN}|@option{-a -COLUMN}|@option{-m}) unless
2485 @option{-w} is set. This is a @acronym{POSIX}-compliant formulation.
2488 @item -S@var{string}
2489 @itemx --sep-string[=@var{string}]
2491 @opindex --sep-string
2492 Use @var{string} to separate output columns. The @option{-S} option doesn't
2493 affect the @option{-W/-w} option, unlike the @option{-s} option which does. It
2494 does not affect line truncation or column alignment.
2495 Without @option{-S}, and with @option{-J}, @command{pr} uses the default output
2497 Without @option{-S} or @option{-J}, @command{pr} uses a @samp{space}
2498 (same as @option{-S"@w{ }"}). @option{--sep-string} with no
2499 @samp{=@var{string}} is equivalent to @option{--sep-string=""}.
2502 @itemx --omit-header
2504 @opindex --omit-header
2505 Do not print the usual header [and footer] on each page, and do not fill
2506 out the bottom of pages (with blank lines or a form feed). No page
2507 structure is produced, but form feeds set in the input files are retained.
2508 The predefined pagination is not changed. @option{-t} or @option{-T} may be
2509 useful together with other options; e.g.: @option{-t -e4}, expand TAB characters
2510 in the input file to 4 spaces but don't make any other changes. Use of
2511 @option{-t} overrides @option{-h}.
2514 @itemx --omit-pagination
2516 @opindex --omit-pagination
2517 Do not print header [and footer]. In addition eliminate all form feeds
2518 set in the input files.
2521 @itemx --show-nonprinting
2523 @opindex --show-nonprinting
2524 Print nonprinting characters in octal backslash notation.
2526 @item -w @var{page_width}
2527 @itemx --width=@var{page_width}
2530 Set page width to @var{page_width} characters for multiple text-column
2531 output only (default for @var{page_width} is 72). @option{-s[CHAR]} turns
2532 off the default page width and any line truncation and column alignment.
2533 Lines of full length are merged, regardless of the column options
2534 set. No @var{page_width} setting is possible with single column output.
2535 A @acronym{POSIX}-compliant formulation.
2537 @item -W @var{page_width}
2538 @itemx --page_width=@var{page_width}
2540 @opindex --page_width
2541 Set the page width to @var{page_width} characters. That's valid with and
2542 without a column option. Text lines are truncated, unless @option{-J}
2543 is used. Together with one of the three column options
2544 (@option{-@var{column}}, @option{-a -@var{column}} or @option{-m}) column
2545 alignment is always used. The separator options @option{-S} or @option{-s}
2546 don't affect the @option{-W} option. Default is 72 characters. Without
2547 @option{-W @var{page_width}} and without any of the column options NO line
2548 truncation is used (defined to keep downward compatibility and to meet
2549 most frequent tasks). That's equivalent to @option{-W 72 -J}. The header
2550 line is never truncated.
2557 @node fold invocation
2558 @section @command{fold}: Wrap input lines to fit in specified width
2561 @cindex wrapping long input lines
2562 @cindex folding long input lines
2564 @command{fold} writes each @var{file} (@option{-} means standard input), or
2565 standard input if none are given, to standard output, breaking long
2569 fold [@var{option}]@dots{} [@var{file}]@dots{}
2572 By default, @command{fold} breaks lines wider than 80 columns. The output
2573 is split into as many lines as necessary.
2575 @cindex screen columns
2576 @command{fold} counts screen columns by default; thus, a tab may count more
2577 than one column, backspace decreases the column count, and carriage
2578 return sets the column to zero.
2580 The program accepts the following options. Also see @ref{Common options}.
2588 Count bytes rather than columns, so that tabs, backspaces, and carriage
2589 returns are each counted as taking up one column, just like other
2596 Break at word boundaries: the line is broken after the last blank before
2597 the maximum line length. If the line contains no such blanks, the line
2598 is broken at the maximum line length as usual.
2600 @item -w @var{width}
2601 @itemx --width=@var{width}
2604 Use a maximum line length of @var{width} columns instead of 80.
2606 For compatibility @command{fold} supports an obsolete option syntax
2607 @option{-@var{width}}. New scripts should use @option{-w @var{width}}
2615 @node Output of parts of files
2616 @chapter Output of parts of files
2618 @cindex output of parts of files
2619 @cindex parts of files, output of
2621 These commands output pieces of the input.
2624 * head invocation:: Output the first part of files.
2625 * tail invocation:: Output the last part of files.
2626 * split invocation:: Split a file into fixed-size pieces.
2627 * csplit invocation:: Split a file into context-determined pieces.
2630 @node head invocation
2631 @section @command{head}: Output the first part of files
2634 @cindex initial part of files, outputting
2635 @cindex first part of files, outputting
2637 @command{head} prints the first part (10 lines by default) of each
2638 @var{file}; it reads from standard input if no files are given or
2639 when given a @var{file} of @option{-}. Synopsis:
2642 head [@var{option}]@dots{} [@var{file}]@dots{}
2645 If more than one @var{file} is specified, @command{head} prints a
2646 one-line header consisting of:
2649 ==> @var{file name} <==
2653 before the output for each @var{file}.
2655 The program accepts the following options. Also see @ref{Common options}.
2660 @itemx --bytes=@var{k}
2663 Print the first @var{k} bytes, instead of initial lines.
2664 However, if @var{k} starts with a @samp{-},
2665 print all but the last @var{k} bytes of each file.
2666 @multiplierSuffixes{k}
2669 @itemx --lines=@var{k}
2672 Output the first @var{k} lines.
2673 However, if @var{k} starts with a @samp{-},
2674 print all but the last @var{k} lines of each file.
2675 Size multiplier suffixes are the same as with the @option{-c} option.
2683 Never print file name headers.
2689 Always print file name headers.
2693 For compatibility @command{head} also supports an obsolete option syntax
2694 @option{-@var{count}@var{options}}, which is recognized only if it is
2695 specified first. @var{count} is a decimal number optionally followed
2696 by a size letter (@samp{b}, @samp{k}, @samp{m}) as in @option{-c}, or
2697 @samp{l} to mean count by lines, or other option letters (@samp{cqv}).
2698 Scripts intended for standard hosts should use @option{-c @var{count}}
2699 or @option{-n @var{count}} instead. If your script must also run on
2700 hosts that support only the obsolete syntax, it is usually simpler to
2701 avoid @command{head}, e.g., by using @samp{sed 5q} instead of
2707 @node tail invocation
2708 @section @command{tail}: Output the last part of files
2711 @cindex last part of files, outputting
2713 @command{tail} prints the last part (10 lines by default) of each
2714 @var{file}; it reads from standard input if no files are given or
2715 when given a @var{file} of @samp{-}. Synopsis:
2718 tail [@var{option}]@dots{} [@var{file}]@dots{}
2721 If more than one @var{file} is specified, @command{tail} prints a
2722 one-line header consisting of:
2725 ==> @var{file name} <==
2729 before the output for each @var{file}.
2731 @cindex BSD @command{tail}
2732 @sc{gnu} @command{tail} can output any amount of data (some other versions of
2733 @command{tail} cannot). It also has no @option{-r} option (print in
2734 reverse), since reversing a file is really a different job from printing
2735 the end of a file; BSD @command{tail} (which is the one with @option{-r}) can
2736 only reverse files that are at most as large as its buffer, which is
2737 typically 32 KiB@. A more reliable and versatile way to reverse files is
2738 the @sc{gnu} @command{tac} command.
2740 The program accepts the following options. Also see @ref{Common options}.
2745 @itemx --bytes=@var{k}
2748 Output the last @var{k} bytes, instead of final lines.
2749 However, if @var{k} starts with a @samp{+}, start printing with the
2750 @var{k}th byte from the start of each file, instead of from the end.
2751 @multiplierSuffixes{k}
2754 @itemx --follow[=@var{how}]
2757 @cindex growing files
2758 @vindex name @r{follow option}
2759 @vindex descriptor @r{follow option}
2760 Loop forever trying to read more characters at the end of the file,
2761 presumably because the file is growing.
2762 If more than one file is given, @command{tail} prints a header whenever it
2763 gets output from a different file, to indicate which file that output is
2766 There are two ways to specify how you'd like to track files with this option,
2767 but that difference is noticeable only when a followed file is removed or
2769 If you'd like to continue to track the end of a growing file even after
2770 it has been unlinked, use @option{--follow=descriptor}. This is the default
2771 behavior, but it is not useful if you're tracking a log file that may be
2772 rotated (removed or renamed, then reopened). In that case, use
2773 @option{--follow=name} to track the named file, perhaps by reopening it
2774 periodically to see if it has been removed and recreated by some other program.
2775 Note that the inotify-based implementation handles this case without
2776 the need for any periodic reopening.
2778 No matter which method you use, if the tracked file is determined to have
2779 shrunk, @command{tail} prints a message saying the file has been truncated
2780 and resumes tracking the end of the file from the newly-determined endpoint.
2782 When a file is removed, @command{tail}'s behavior depends on whether it is
2783 following the name or the descriptor. When following by name, tail can
2784 detect that a file has been removed and gives a message to that effect,
2785 and if @option{--retry} has been specified it will continue checking
2786 periodically to see if the file reappears.
2787 When following a descriptor, tail does not detect that the file has
2788 been unlinked or renamed and issues no message; even though the file
2789 may no longer be accessible via its original name, it may still be
2792 The option values @samp{descriptor} and @samp{name} may be specified only
2793 with the long form of the option, not with @option{-f}.
2795 The @option{-f} option is ignored if
2796 no @var{file} operand is specified and standard input is a FIFO or a pipe.
2797 Likewise, the @option{-f} option has no effect for any
2798 operand specified as @samp{-}, when standard input is a FIFO or a pipe.
2802 This option is the same as @option{--follow=name --retry}. That is, tail
2803 will attempt to reopen a file when it is removed. Should this fail, tail
2804 will keep trying until it becomes accessible again.
2808 This option is useful mainly when following by name (i.e., with
2809 @option{--follow=name}).
2810 Without this option, when tail encounters a file that doesn't
2811 exist or is otherwise inaccessible, it reports that fact and
2812 never checks it again.
2814 @itemx --sleep-interval=@var{number}
2815 @opindex --sleep-interval
2816 Change the number of seconds to wait between iterations (the default is 1.0).
2817 During one iteration, every specified file is checked to see if it has
2819 Historical implementations of @command{tail} have required that
2820 @var{number} be an integer. However, GNU @command{tail} accepts
2821 an arbitrary floating point number (using a period before any
2824 @itemx --pid=@var{pid}
2826 When following by name or by descriptor, you may specify the process ID,
2827 @var{pid}, of the sole writer of all @var{file} arguments. Then, shortly
2828 after that process terminates, tail will also terminate. This will
2829 work properly only if the writer and the tailing process are running on
2830 the same machine. For example, to save the output of a build in a file
2831 and to watch the file grow, if you invoke @command{make} and @command{tail}
2832 like this then the tail process will stop when your build completes.
2833 Without this option, you would have had to kill the @code{tail -f}
2837 $ make >& makerr & tail --pid=$! -f makerr
2840 If you specify a @var{pid} that is not in use or that does not correspond
2841 to the process that is writing to the tailed files, then @command{tail}
2842 may terminate long before any @var{file}s stop growing or it may not
2843 terminate until long after the real writer has terminated.
2844 Note that @option{--pid} cannot be supported on some systems; @command{tail}
2845 will print a warning if this is the case.
2847 @itemx --max-unchanged-stats=@var{n}
2848 @opindex --max-unchanged-stats
2849 When tailing a file by name, if there have been @var{n} (default
2850 n=@value{DEFAULT_MAX_N_UNCHANGED_STATS_BETWEEN_OPENS}) consecutive
2851 iterations for which the file has not changed, then
2852 @code{open}/@code{fstat} the file to determine if that file name is
2853 still associated with the same device/inode-number pair as before.
2854 When following a log file that is rotated, this is approximately the
2855 number of seconds between when tail prints the last pre-rotation lines
2856 and when it prints the lines that have accumulated in the new log file.
2857 This option is meaningful only when following by name.
2860 @itemx --lines=@var{k}
2863 Output the last @var{k} lines.
2864 However, if @var{k} starts with a @samp{+}, start printing with the
2865 @var{k}th line from the start of each file, instead of from the end.
2866 Size multiplier suffixes are the same as with the @option{-c} option.
2874 Never print file name headers.
2880 Always print file name headers.
2884 For compatibility @command{tail} also supports an obsolete usage
2885 @samp{tail -[@var{count}][bcl][f] [@var{file}]}, which is recognized
2886 only if it does not conflict with the usage described
2887 above. This obsolete form uses exactly one option and at most one
2888 file. In the option, @var{count} is an optional decimal number optionally
2889 followed by a size letter (@samp{b}, @samp{c}, @samp{l}) to mean count
2890 by 512-byte blocks, bytes, or lines, optionally followed by @samp{f}
2891 which has the same meaning as @option{-f}.
2893 @vindex _POSIX2_VERSION
2894 On older systems, the leading @samp{-} can be replaced by @samp{+} in
2895 the obsolete option syntax with the same meaning as in counts, and
2896 obsolete usage overrides normal usage when the two conflict.
2897 This obsolete behavior can be enabled or disabled with the
2898 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
2901 Scripts intended for use on standard hosts should avoid obsolete
2902 syntax and should use @option{-c @var{count}[b]}, @option{-n
2903 @var{count}}, and/or @option{-f} instead. If your script must also
2904 run on hosts that support only the obsolete syntax, you can often
2905 rewrite it to avoid problematic usages, e.g., by using @samp{sed -n
2906 '$p'} rather than @samp{tail -1}. If that's not possible, the script
2907 can use a test like @samp{if tail -c +1 </dev/null >/dev/null 2>&1;
2908 then @dots{}} to decide which syntax to use.
2910 Even if your script assumes the standard behavior, you should still
2911 beware usages whose behaviors differ depending on the @acronym{POSIX}
2912 version. For example, avoid @samp{tail - main.c}, since it might be
2913 interpreted as either @samp{tail main.c} or as @samp{tail -- -
2914 main.c}; avoid @samp{tail -c 4}, since it might mean either @samp{tail
2915 -c4} or @samp{tail -c 10 4}; and avoid @samp{tail +4}, since it might
2916 mean either @samp{tail ./+4} or @samp{tail -n +4}.
2921 @node split invocation
2922 @section @command{split}: Split a file into fixed-size pieces
2925 @cindex splitting a file into pieces
2926 @cindex pieces, splitting a file into
2928 @command{split} creates output files containing consecutive sections of
2929 @var{input} (standard input if none is given or @var{input} is
2930 @samp{-}). Synopsis:
2933 split [@var{option}] [@var{input} [@var{prefix}]]
2936 By default, @command{split} puts 1000 lines of @var{input} (or whatever is
2937 left over for the last section), into each output file.
2939 @cindex output file name prefix
2940 The output files' names consist of @var{prefix} (@samp{x} by default)
2941 followed by a group of characters (@samp{aa}, @samp{ab}, @dots{} by
2942 default), such that concatenating the output files in traditional
2943 sorted order by file name produces
2944 the original input file. If the output file names are exhausted,
2945 @command{split} reports an error without deleting the output files
2948 The program accepts the following options. Also see @ref{Common options}.
2952 @item -l @var{lines}
2953 @itemx --lines=@var{lines}
2956 Put @var{lines} lines of @var{input} into each output file.
2958 For compatibility @command{split} also supports an obsolete
2959 option syntax @option{-@var{lines}}. New scripts should use @option{-l
2960 @var{lines}} instead.
2963 @itemx --bytes=@var{size}
2966 Put @var{size} bytes of @var{input} into each output file.
2967 @multiplierSuffixes{size}
2970 @itemx --line-bytes=@var{size}
2972 @opindex --line-bytes
2973 Put into each output file as many complete lines of @var{input} as
2974 possible without exceeding @var{size} bytes. Individual lines longer than
2975 @var{size} bytes are broken into multiple files.
2976 @var{size} has the same format as for the @option{--bytes} option.
2978 @item -a @var{length}
2979 @itemx --suffix-length=@var{length}
2981 @opindex --suffix-length
2982 Use suffixes of length @var{length}. The default @var{length} is 2.
2985 @itemx --numeric-suffixes
2987 @opindex --numeric-suffixes
2988 Use digits in suffixes rather than lower-case letters.
2992 Write a diagnostic just before each output file is opened.
2999 @node csplit invocation
3000 @section @command{csplit}: Split a file into context-determined pieces
3003 @cindex context splitting
3004 @cindex splitting a file into pieces by context
3006 @command{csplit} creates zero or more output files containing sections of
3007 @var{input} (standard input if @var{input} is @samp{-}). Synopsis:
3010 csplit [@var{option}]@dots{} @var{input} @var{pattern}@dots{}
3013 The contents of the output files are determined by the @var{pattern}
3014 arguments, as detailed below. An error occurs if a @var{pattern}
3015 argument refers to a nonexistent line of the input file (e.g., if no
3016 remaining line matches a given regular expression). After every
3017 @var{pattern} has been matched, any remaining input is copied into one
3020 By default, @command{csplit} prints the number of bytes written to each
3021 output file after it has been created.
3023 The types of pattern arguments are:
3028 Create an output file containing the input up to but not including line
3029 @var{n} (a positive integer). If followed by a repeat count, also
3030 create an output file containing the next @var{n} lines of the input
3031 file once for each repeat.
3033 @item /@var{regexp}/[@var{offset}]
3034 Create an output file containing the current line up to (but not
3035 including) the next line of the input file that contains a match for
3036 @var{regexp}. The optional @var{offset} is an integer.
3037 If it is given, the input up to (but not including) the
3038 matching line plus or minus @var{offset} is put into the output file,
3039 and the line after that begins the next section of input.
3041 @item %@var{regexp}%[@var{offset}]
3042 Like the previous type, except that it does not create an output
3043 file, so that section of the input file is effectively ignored.
3045 @item @{@var{repeat-count}@}
3046 Repeat the previous pattern @var{repeat-count} additional
3047 times. The @var{repeat-count} can either be a positive integer or an
3048 asterisk, meaning repeat as many times as necessary until the input is
3053 The output files' names consist of a prefix (@samp{xx} by default)
3054 followed by a suffix. By default, the suffix is an ascending sequence
3055 of two-digit decimal numbers from @samp{00} to @samp{99}. In any case,
3056 concatenating the output files in sorted order by file name produces the
3057 original input file.
3059 By default, if @command{csplit} encounters an error or receives a hangup,
3060 interrupt, quit, or terminate signal, it removes any output files
3061 that it has created so far before it exits.
3063 The program accepts the following options. Also see @ref{Common options}.
3067 @item -f @var{prefix}
3068 @itemx --prefix=@var{prefix}
3071 @cindex output file name prefix
3072 Use @var{prefix} as the output file name prefix.
3074 @item -b @var{suffix}
3075 @itemx --suffix=@var{suffix}
3078 @cindex output file name suffix
3079 Use @var{suffix} as the output file name suffix. When this option is
3080 specified, the suffix string must include exactly one
3081 @code{printf(3)}-style conversion specification, possibly including
3082 format specification flags, a field width, a precision specifications,
3083 or all of these kinds of modifiers. The format letter must convert a
3084 binary integer argument to readable form; thus, only @samp{d}, @samp{i},
3085 @samp{u}, @samp{o}, @samp{x}, and @samp{X} conversions are allowed. The
3086 entire @var{suffix} is given (with the current output file number) to
3087 @code{sprintf(3)} to form the file name suffixes for each of the
3088 individual output files in turn. If this option is used, the
3089 @option{--digits} option is ignored.
3091 @item -n @var{digits}
3092 @itemx --digits=@var{digits}
3095 Use output file names containing numbers that are @var{digits} digits
3096 long instead of the default 2.
3101 @opindex --keep-files
3102 Do not remove output files when errors are encountered.
3105 @itemx --elide-empty-files
3107 @opindex --elide-empty-files
3108 Suppress the generation of zero-length output files. (In cases where
3109 the section delimiters of the input file are supposed to mark the first
3110 lines of each of the sections, the first output file will generally be a
3111 zero-length file unless you use this option.) The output file sequence
3112 numbers always run consecutively starting from 0, even when this option
3123 Do not print counts of output file sizes.
3129 Here is an example of its usage.
3130 First, create an empty directory for the exercise,
3137 Now, split the sequence of 1..14 on lines that end with 0 or 5:
3140 $ seq 14 | csplit - '/[05]$/' '@{*@}'
3146 Each number printed above is the size of an output
3147 file that csplit has just created.
3148 List the names of those output files:
3155 Use @command{head} to show their contents:
3180 @node Summarizing files
3181 @chapter Summarizing files
3183 @cindex summarizing files
3185 These commands generate just a few numbers representing entire
3189 * wc invocation:: Print newline, word, and byte counts.
3190 * sum invocation:: Print checksum and block counts.
3191 * cksum invocation:: Print CRC checksum and byte counts.
3192 * md5sum invocation:: Print or check MD5 digests.
3193 * sha1sum invocation:: Print or check SHA-1 digests.
3194 * sha2 utilities:: Print or check SHA-2 digests.
3199 @section @command{wc}: Print newline, word, and byte counts
3203 @cindex character count
3207 @command{wc} counts the number of bytes, characters, whitespace-separated
3208 words, and newlines in each given @var{file}, or standard input if none
3209 are given or for a @var{file} of @samp{-}. Synopsis:
3212 wc [@var{option}]@dots{} [@var{file}]@dots{}
3215 @cindex total counts
3216 @command{wc} prints one line of counts for each file, and if the file was
3217 given as an argument, it prints the file name following the counts. If
3218 more than one @var{file} is given, @command{wc} prints a final line
3219 containing the cumulative counts, with the file name @file{total}. The
3220 counts are printed in this order: newlines, words, characters, bytes,
3221 maximum line length.
3222 Each count is printed right-justified in a field with at least one
3223 space between fields so that the numbers and file names normally line
3224 up nicely in columns. The width of the count fields varies depending
3225 on the inputs, so you should not depend on a particular field width.
3226 However, as a @acronym{GNU} extension, if only one count is printed,
3227 it is guaranteed to be printed without leading spaces.
3229 By default, @command{wc} prints three counts: the newline, words, and byte
3230 counts. Options can specify that only certain counts be printed.
3231 Options do not undo others previously given, so
3238 prints both the byte counts and the word counts.
3240 With the @option{--max-line-length} option, @command{wc} prints the length
3241 of the longest line per file, and if there is more than one file it
3242 prints the maximum (not the sum) of those lengths. The line lengths here
3243 are measured in screen columns, according to the current locale and
3244 assuming tab positions in every 8th column.
3246 The program accepts the following options. Also see @ref{Common options}.
3254 Print only the byte counts.
3260 Print only the character counts.
3266 Print only the word counts.
3272 Print only the newline counts.
3275 @itemx --max-line-length
3277 @opindex --max-line-length
3278 Print only the maximum line lengths.
3280 @macro filesZeroFromOption{cmd,withTotalOption,subListOutput}
3281 @itemx --files0-from=@var{file}
3282 @opindex --files0-from=@var{file}
3283 @c This is commented out to avoid a texi2dvi failure.
3284 @c texi2dvi (GNU Texinfo 4.11) 1.104
3285 @c @cindex including files from @command{\cmd\}
3286 Disallow processing files named on the command line, and instead process
3287 those named in file @var{file}; each name being terminated by a zero byte
3288 (@acronym{ASCII} @sc{nul}).
3289 This is useful \withTotalOption\
3290 when the list of file names is so long that it may exceed a command line
3292 In such cases, running @command{\cmd\} via @command{xargs} is undesirable
3293 because it splits the list into pieces and makes @command{\cmd\} print
3294 \subListOutput\ for each sublist rather than for the entire list.
3295 One way to produce a list of @acronym{ASCII} @sc{nul} terminated file names is with @sc{gnu}
3296 @command{find}, using its @option{-print0} predicate.
3297 If @var{file} is @samp{-} then the @acronym{ASCII} @sc{nul} terminated file names
3298 are read from standard input.
3300 @filesZeroFromOption{wc,,a total}
3302 For example, to find the length of the longest line in any @file{.c} or
3303 @file{.h} file in the current hierarchy, do this:
3306 find . -name '*.[ch]' -print0 |
3307 wc -L --files0-from=- | tail -n1
3315 @node sum invocation
3316 @section @command{sum}: Print checksum and block counts
3319 @cindex 16-bit checksum
3320 @cindex checksum, 16-bit
3322 @command{sum} computes a 16-bit checksum for each given @var{file}, or
3323 standard input if none are given or for a @var{file} of @samp{-}. Synopsis:
3326 sum [@var{option}]@dots{} [@var{file}]@dots{}
3329 @command{sum} prints the checksum for each @var{file} followed by the
3330 number of blocks in the file (rounded up). If more than one @var{file}
3331 is given, file names are also printed (by default). (With the
3332 @option{--sysv} option, corresponding file names are printed when there is
3333 at least one file argument.)
3335 By default, @sc{gnu} @command{sum} computes checksums using an algorithm
3336 compatible with BSD @command{sum} and prints file sizes in units of
3339 The program accepts the following options. Also see @ref{Common options}.
3345 @cindex BSD @command{sum}
3346 Use the default (BSD compatible) algorithm. This option is included for
3347 compatibility with the System V @command{sum}. Unless @option{-s} was also
3348 given, it has no effect.
3354 @cindex System V @command{sum}
3355 Compute checksums using an algorithm compatible with System V
3356 @command{sum}'s default, and print file sizes in units of 512-byte blocks.
3360 @command{sum} is provided for compatibility; the @command{cksum} program (see
3361 next section) is preferable in new applications.
3366 @node cksum invocation
3367 @section @command{cksum}: Print CRC checksum and byte counts
3370 @cindex cyclic redundancy check
3371 @cindex CRC checksum
3373 @command{cksum} computes a cyclic redundancy check (CRC) checksum for each
3374 given @var{file}, or standard input if none are given or for a
3375 @var{file} of @samp{-}. Synopsis:
3378 cksum [@var{option}]@dots{} [@var{file}]@dots{}
3381 @command{cksum} prints the CRC checksum for each file along with the number
3382 of bytes in the file, and the file name unless no arguments were given.
3384 @command{cksum} is typically used to ensure that files
3385 transferred by unreliable means (e.g., netnews) have not been corrupted,
3386 by comparing the @command{cksum} output for the received files with the
3387 @command{cksum} output for the original files (typically given in the
3390 The CRC algorithm is specified by the @acronym{POSIX} standard. It is not
3391 compatible with the BSD or System V @command{sum} algorithms (see the
3392 previous section); it is more robust.
3394 The only options are @option{--help} and @option{--version}. @xref{Common
3400 @node md5sum invocation
3401 @section @command{md5sum}: Print or check MD5 digests
3405 @cindex 128-bit checksum
3406 @cindex checksum, 128-bit
3407 @cindex fingerprint, 128-bit
3408 @cindex message-digest, 128-bit
3410 @command{md5sum} computes a 128-bit checksum (or @dfn{fingerprint} or
3411 @dfn{message-digest}) for each specified @var{file}.
3413 Note: The MD5 digest is more reliable than a simple CRC (provided by
3414 the @command{cksum} command) for detecting accidental file corruption,
3415 as the chances of accidentally having two files with identical MD5
3416 are vanishingly small. However, it should not be considered truly
3417 secure against malicious tampering: although finding a file with a
3418 given MD5 fingerprint, or modifying a file so as to retain its MD5 are
3419 considered infeasible at the moment, it is known how to produce
3420 different files with identical MD5 (a ``collision''), something which
3421 can be a security issue in certain contexts. For more secure hashes,
3422 consider using SHA-1 or SHA-2. @xref{sha1sum invocation}, and
3423 @ref{sha2 utilities}.
3425 If a @var{file} is specified as @samp{-} or if no files are given
3426 @command{md5sum} computes the checksum for the standard input.
3427 @command{md5sum} can also determine whether a file and checksum are
3428 consistent. Synopsis:
3431 md5sum [@var{option}]@dots{} [@var{file}]@dots{}
3434 For each @var{file}, @samp{md5sum} outputs the MD5 checksum, a flag
3435 indicating a binary or text input file, and the file name.
3436 If @var{file} contains a backslash or newline, the
3437 line is started with a backslash, and each problematic character in
3438 the file name is escaped with a backslash, making the output
3439 unambiguous even in the presence of arbitrary file names.
3440 If @var{file} is omitted or specified as @samp{-}, standard input is read.
3442 The program accepts the following options. Also see @ref{Common options}.
3450 @cindex binary input files
3451 Treat each input file as binary, by reading it in binary mode and
3452 outputting a @samp{*} flag. This is the inverse of @option{--text}.
3453 On systems like @acronym{GNU} that do not distinguish between binary
3454 and text files, this option merely flags each input file as binary:
3455 the MD5 checksum is unaffected. This option is the default on systems
3456 like MS-DOS that distinguish between binary and text files, except
3457 for reading standard input when standard input is a terminal.
3461 Read file names and checksum information (not data) from each
3462 @var{file} (or from stdin if no @var{file} was specified) and report
3463 whether the checksums match the contents of the named files.
3464 The input to this mode of @command{md5sum} is usually the output of
3465 a prior, checksum-generating run of @samp{md5sum}.
3466 Each valid line of input consists of an MD5 checksum, a binary/text
3467 flag, and then a file name.
3468 Binary files are marked with @samp{*}, text with @samp{ }.
3469 For each such line, @command{md5sum} reads the named file and computes its
3470 MD5 checksum. Then, if the computed message digest does not match the
3471 one on the line with the file name, the file is noted as having
3472 failed the test. Otherwise, the file passes the test.
3473 By default, for each valid line, one line is written to standard
3474 output indicating whether the named file passed the test.
3475 After all checks have been performed, if there were any failures,
3476 a warning is issued to standard error.
3477 Use the @option{--status} option to inhibit that output.
3478 If any listed file cannot be opened or read, if any valid line has
3479 an MD5 checksum inconsistent with the associated file, or if no valid
3480 line is found, @command{md5sum} exits with nonzero status. Otherwise,
3481 it exits successfully.
3485 @cindex verifying MD5 checksums
3486 This option is useful only when verifying checksums.
3487 When verifying checksums, don't generate an 'OK' message per successfully
3488 checked file. Files that fail the verification are reported in the
3489 default one-line-per-file format. If there is any checksum mismatch,
3490 print a warning summarizing the failures to standard error.
3494 @cindex verifying MD5 checksums
3495 This option is useful only when verifying checksums.
3496 When verifying checksums, don't generate the default one-line-per-file
3497 diagnostic and don't output the warning summarizing any failures.
3498 Failures to open or read a file still evoke individual diagnostics to
3500 If all listed files are readable and are consistent with the associated
3501 MD5 checksums, exit successfully. Otherwise exit with a status code
3502 indicating there was a failure.
3508 @cindex text input files
3509 Treat each input file as text, by reading it in text mode and
3510 outputting a @samp{ } flag. This is the inverse of @option{--binary}.
3511 This option is the default on systems like @acronym{GNU} that do not
3512 distinguish between binary and text files. On other systems, it is
3513 the default for reading standard input when standard input is a
3520 @cindex verifying MD5 checksums
3521 When verifying checksums, warn about improperly formatted MD5 checksum lines.
3522 This option is useful only if all but a few lines in the checked input
3530 @node sha1sum invocation
3531 @section @command{sha1sum}: Print or check SHA-1 digests
3535 @cindex 160-bit checksum
3536 @cindex checksum, 160-bit
3537 @cindex fingerprint, 160-bit
3538 @cindex message-digest, 160-bit
3540 @command{sha1sum} computes a 160-bit checksum for each specified
3541 @var{file}. The usage and options of this command are precisely the
3542 same as for @command{md5sum}. @xref{md5sum invocation}.
3544 Note: The SHA-1 digest is more secure than MD5, and no collisions of
3545 it are known (different files having the same fingerprint). However,
3546 it is known that they can be produced with considerable, but not
3547 unreasonable, resources. For this reason, it is generally considered
3548 that SHA-1 should be gradually phased out in favor of the more secure
3549 SHA-2 hash algorithms. @xref{sha2 utilities}.
3552 @node sha2 utilities
3553 @section sha2 utilities: Print or check SHA-2 digests
3560 @cindex 224-bit checksum
3561 @cindex 256-bit checksum
3562 @cindex 384-bit checksum
3563 @cindex 512-bit checksum
3564 @cindex checksum, 224-bit
3565 @cindex checksum, 256-bit
3566 @cindex checksum, 384-bit
3567 @cindex checksum, 512-bit
3568 @cindex fingerprint, 224-bit
3569 @cindex fingerprint, 256-bit
3570 @cindex fingerprint, 384-bit
3571 @cindex fingerprint, 512-bit
3572 @cindex message-digest, 224-bit
3573 @cindex message-digest, 256-bit
3574 @cindex message-digest, 384-bit
3575 @cindex message-digest, 512-bit
3577 The commands @command{sha224sum}, @command{sha256sum},
3578 @command{sha384sum} and @command{sha512sum} compute checksums of
3579 various lengths (respectively 224, 256, 384 and 512 bits),
3580 collectively known as the SHA-2 hashes. The usage and options of
3581 these commands are precisely the same as for @command{md5sum}.
3582 @xref{md5sum invocation}.
3584 Note: The SHA384 and SHA512 digests are considerably slower to
3585 compute, especially on 32-bit computers, than SHA224 or SHA256.
3588 @node Operating on sorted files
3589 @chapter Operating on sorted files
3591 @cindex operating on sorted files
3592 @cindex sorted files, operations on
3594 These commands work with (or produce) sorted files.
3597 * sort invocation:: Sort text files.
3598 * shuf invocation:: Shuffle text files.
3599 * uniq invocation:: Uniquify files.
3600 * comm invocation:: Compare two sorted files line by line.
3601 * ptx invocation:: Produce a permuted index of file contents.
3602 * tsort invocation:: Topological sort.
3606 @node sort invocation
3607 @section @command{sort}: Sort text files
3610 @cindex sorting files
3612 @command{sort} sorts, merges, or compares all the lines from the given
3613 files, or standard input if none are given or for a @var{file} of
3614 @samp{-}. By default, @command{sort} writes the results to standard
3618 sort [@var{option}]@dots{} [@var{file}]@dots{}
3621 @command{sort} has three modes of operation: sort (the default), merge,
3622 and check for sortedness. The following options change the operation
3629 @itemx --check=diagnose-first
3632 @cindex checking for sortedness
3633 Check whether the given file is already sorted: if it is not all
3634 sorted, print a diagnostic containing the first out-of-order line and
3635 exit with a status of 1.
3636 Otherwise, exit successfully.
3637 At most one input file can be given.
3640 @itemx --check=quiet
3641 @itemx --check=silent
3644 @cindex checking for sortedness
3645 Exit successfully if the given file is already sorted, and
3646 exit with status 1 otherwise.
3647 At most one input file can be given.
3648 This is like @option{-c}, except it does not print a diagnostic.
3654 @cindex merging sorted files
3655 Merge the given files by sorting them as a group. Each input file must
3656 always be individually sorted. It always works to sort instead of
3657 merge; merging is provided because it is faster, in the case where it
3662 @cindex sort stability
3663 @cindex sort's last-resort comparison
3664 A pair of lines is compared as follows:
3665 @command{sort} compares each pair of fields, in the
3666 order specified on the command line, according to the associated
3667 ordering options, until a difference is found or no fields are left.
3668 If no key fields are specified, @command{sort} uses a default key of
3669 the entire line. Finally, as a last resort when all keys compare
3670 equal, @command{sort} compares entire lines as if no ordering options
3671 other than @option{--reverse} (@option{-r}) were specified. The
3672 @option{--stable} (@option{-s}) option disables this @dfn{last-resort
3673 comparison} so that lines in which all fields compare equal are left
3674 in their original relative order. The @option{--unique}
3675 (@option{-u}) option also disables the last-resort comparison.
3679 Unless otherwise specified, all comparisons use the character collating
3680 sequence specified by the @env{LC_COLLATE} locale.@footnote{If you
3681 use a non-@acronym{POSIX} locale (e.g., by setting @env{LC_ALL}
3682 to @samp{en_US}), then @command{sort} may produce output that is sorted
3683 differently than you're accustomed to. In that case, set the @env{LC_ALL}
3684 environment variable to @samp{C}. Note that setting only @env{LC_COLLATE}
3685 has two problems. First, it is ineffective if @env{LC_ALL} is also set.
3686 Second, it has undefined behavior if @env{LC_CTYPE} (or @env{LANG}, if
3687 @env{LC_CTYPE} is unset) is set to an incompatible value. For example,
3688 you get undefined behavior if @env{LC_CTYPE} is @code{ja_JP.PCK} but
3689 @env{LC_COLLATE} is @code{en_US.UTF-8}.}
3691 @sc{gnu} @command{sort} (as specified for all @sc{gnu} utilities) has no
3692 limit on input line length or restrictions on bytes allowed within lines.
3693 In addition, if the final byte of an input file is not a newline, @sc{gnu}
3694 @command{sort} silently supplies one. A line's trailing newline is not
3695 part of the line for comparison purposes.
3697 @cindex exit status of @command{sort}
3701 0 if no error occurred
3702 1 if invoked with @option{-c} or @option{-C} and the input is not sorted
3703 2 if an error occurred
3707 If the environment variable @env{TMPDIR} is set, @command{sort} uses its
3708 value as the directory for temporary files instead of @file{/tmp}. The
3709 @option{--temporary-directory} (@option{-T}) option in turn overrides
3710 the environment variable.
3712 The following options affect the ordering of output lines. They may be
3713 specified globally or as part of a specific key field. If no key
3714 fields are specified, global options apply to comparison of entire
3715 lines; otherwise the global options are inherited by key fields that do
3716 not specify any special options of their own. In pre-@acronym{POSIX}
3717 versions of @command{sort}, global options affect only later key fields,
3718 so portable shell scripts should specify global options first.
3723 @itemx --ignore-leading-blanks
3725 @opindex --ignore-leading-blanks
3726 @cindex blanks, ignoring leading
3728 Ignore leading blanks when finding sort keys in each line.
3729 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3730 can change this. Note blanks may be ignored by your locale's collating
3731 rules, but without this option they will be significant for character
3732 positions specified in keys with the @option{-k} option.
3735 @itemx --dictionary-order
3737 @opindex --dictionary-order
3738 @cindex dictionary order
3739 @cindex phone directory order
3740 @cindex telephone directory order
3742 Sort in @dfn{phone directory} order: ignore all characters except
3743 letters, digits and blanks when sorting.
3744 By default letters and digits are those of @acronym{ASCII} and a blank
3745 is a space or a tab, but the @env{LC_CTYPE} locale can change this.
3748 @itemx --ignore-case
3750 @opindex --ignore-case
3751 @cindex ignoring case
3752 @cindex case folding
3754 Fold lowercase characters into the equivalent uppercase characters when
3755 comparing so that, for example, @samp{b} and @samp{B} sort as equal.
3756 The @env{LC_CTYPE} locale determines character types.
3757 When used with @option{--unique} those lower case equivalent lines are
3758 thrown away. (There is currently no way to throw away the upper case
3759 equivalent instead. (Any @option{--reverse} given would only affect
3760 the final result, after the throwing away.))
3763 @itemx --general-numeric-sort
3764 @itemx --sort=general-numeric
3766 @opindex --general-numeric-sort
3768 @cindex general numeric sort
3770 Sort numerically, using the standard C function @code{strtold} to convert
3771 a prefix of each line to a long double-precision floating point number.
3772 This allows floating point numbers to be specified in scientific notation,
3773 like @code{1.0e-34} and @code{10e100}.
3774 The @env{LC_NUMERIC} locale determines the decimal-point character.
3775 Do not report overflow, underflow, or conversion errors.
3776 Use the following collating sequence:
3780 Lines that do not start with numbers (all considered to be equal).
3782 NaNs (``Not a Number'' values, in IEEE floating point arithmetic)
3783 in a consistent but machine-dependent order.
3787 Finite numbers in ascending numeric order (with @math{-0} and @math{+0} equal).
3792 Use this option only if there is no alternative; it is much slower than
3793 @option{--numeric-sort} (@option{-n}) and it can lose information when
3794 converting to floating point.
3797 @itemx --human-numeric-sort
3798 @itemx --sort=human-numeric
3800 @opindex --human-numeric-sort
3802 @cindex human numeric sort
3804 Sort numerically, as per the @option{--numeric-sort} option below, and in
3805 addition handle IEC or SI suffixes like MiB, MB etc (@ref{Block size}).
3806 Note a mixture of IEC and SI suffixes is not supported and will
3807 be flagged as an error. Also the numbers must be abbreviated uniformly.
3808 I.E. values with different precisions like 6000K and 5M will be sorted
3812 @itemx --ignore-nonprinting
3814 @opindex --ignore-nonprinting
3815 @cindex nonprinting characters, ignoring
3816 @cindex unprintable characters, ignoring
3818 Ignore nonprinting characters.
3819 The @env{LC_CTYPE} locale determines character types.
3820 This option has no effect if the stronger @option{--dictionary-order}
3821 (@option{-d}) option is also given.
3827 @opindex --month-sort
3829 @cindex months, sorting by
3831 An initial string, consisting of any amount of blanks, followed
3832 by a month name abbreviation, is folded to UPPER case and
3833 compared in the order @samp{JAN} < @samp{FEB} < @dots{} < @samp{DEC}.
3834 Invalid names compare low to valid names. The @env{LC_TIME} locale
3835 category determines the month spellings.
3836 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3840 @itemx --numeric-sort
3841 @itemx --sort=numeric
3843 @opindex --numeric-sort
3845 @cindex numeric sort
3847 Sort numerically. The number begins each line and consists
3848 of optional blanks, an optional @samp{-} sign, and zero or more
3849 digits possibly separated by thousands separators, optionally followed
3850 by a decimal-point character and zero or more digits. An empty
3851 number is treated as @samp{0}. The @env{LC_NUMERIC}
3852 locale specifies the decimal-point character and thousands separator.
3853 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3856 Comparison is exact; there is no rounding error.
3858 Neither a leading @samp{+} nor exponential notation is recognized.
3859 To compare such strings numerically, use the
3860 @option{--general-numeric-sort} (@option{-g}) option.
3863 @itemx --version-sort
3865 @opindex --version-sort
3866 @cindex version number sort
3867 Sort by version name and number. It behaves like a standard sort,
3868 except that each sequence of decimal digits is treated numerically
3869 as an index/version number. (@xref{Details about version sort}.)
3875 @cindex reverse sorting
3876 Reverse the result of comparison, so that lines with greater key values
3877 appear earlier in the output instead of later.
3880 @itemx --random-sort
3881 @itemx --sort=random
3883 @opindex --random-sort
3886 Sort by hashing the input keys and then sorting the hash values.
3887 Choose the hash function at random, ensuring that it is free of
3888 collisions so that differing keys have differing hash values. This is
3889 like a random permutation of the inputs (@pxref{shuf invocation}),
3890 except that keys with the same value sort together.
3892 If multiple random sort fields are specified, the same random hash
3893 function is used for all fields. To use different random hash
3894 functions for different fields, you can invoke @command{sort} more
3897 The choice of hash function is affected by the
3898 @option{--random-source} option.
3906 @item --compress-program=@var{prog}
3907 Compress any temporary files with the program @var{prog}.
3909 With no arguments, @var{prog} must compress standard input to standard
3910 output, and when given the @option{-d} option it must decompress
3911 standard input to standard output.
3913 Terminate with an error if @var{prog} exits with nonzero status.
3915 White space and the backslash character should not appear in
3916 @var{prog}; they are reserved for future use.
3918 @filesZeroFromOption{sort,,sorted output}
3920 @item -k @var{pos1}[,@var{pos2}]
3921 @itemx --key=@var{pos1}[,@var{pos2}]
3925 Specify a sort field that consists of the part of the line between
3926 @var{pos1} and @var{pos2} (or the end of the line, if @var{pos2} is
3927 omitted), @emph{inclusive}.
3929 Each @var{pos} has the form @samp{@var{f}[.@var{c}][@var{opts}]},
3930 where @var{f} is the number of the field to use, and @var{c} is the number
3931 of the first character from the beginning of the field. Fields and character
3932 positions are numbered starting with 1; a character position of zero in
3933 @var{pos2} indicates the field's last character. If @samp{.@var{c}} is
3934 omitted from @var{pos1}, it defaults to 1 (the beginning of the field);
3935 if omitted from @var{pos2}, it defaults to 0 (the end of the field).
3936 @var{opts} are ordering options, allowing individual keys to be sorted
3937 according to different rules; see below for details. Keys can span
3940 Example: To sort on the second field, use @option{--key=2,2}
3941 (@option{-k 2,2}). See below for more notes on keys and more examples.
3942 See also the @option{--debug} option to help determine the part
3943 of the line being used in the sort.
3946 Highlight the portion of each line used for sorting.
3948 @item --batch-size=@var{nmerge}
3949 @opindex --batch-size
3950 @cindex number of inputs to merge, nmerge
3951 Merge at most @var{nmerge} inputs at once.
3953 When @command{sort} has to merge more than @var{nmerge} inputs,
3954 it merges them in groups of @var{nmerge}, saving the result in
3955 a temporary file, which is then used as an input in a subsequent merge.
3957 A large value of @var{nmerge} may improve merge performance and decrease
3958 temporary storage utilization at the expense of increased memory usage
3959 and I/0. Conversely a small value of @var{nmerge} may reduce memory
3960 requirements and I/0 at the expense of temporary storage consumption and
3963 The value of @var{nmerge} must be at least 2. The default value is
3964 currently 16, but this is implementation-dependent and may change in
3967 The value of @var{nmerge} may be bounded by a resource limit for open
3968 file descriptors. The commands @samp{ulimit -n} or @samp{getconf
3969 OPEN_MAX} may display limits for your systems; these limits may be
3970 modified further if your program already has some files open, or if
3971 the operating system has other limits on the number of open files. If
3972 the value of @var{nmerge} exceeds the resource limit, @command{sort}
3973 silently uses a smaller value.
3975 @item -o @var{output-file}
3976 @itemx --output=@var{output-file}
3979 @cindex overwriting of input, allowed
3980 Write output to @var{output-file} instead of standard output.
3981 Normally, @command{sort} reads all input before opening
3982 @var{output-file}, so you can safely sort a file in place by using
3983 commands like @code{sort -o F F} and @code{cat F | sort -o F}.
3984 However, @command{sort} with @option{--merge} (@option{-m}) can open
3985 the output file before reading all input, so a command like @code{cat
3986 F | sort -m -o F - G} is not safe as @command{sort} might start
3987 writing @file{F} before @command{cat} is done reading it.
3989 @vindex POSIXLY_CORRECT
3990 On newer systems, @option{-o} cannot appear after an input file if
3991 @env{POSIXLY_CORRECT} is set, e.g., @samp{sort F -o F}. Portable
3992 scripts should specify @option{-o @var{output-file}} before any input
3995 @item --random-source=@var{file}
3996 @opindex --random-source
3997 @cindex random source for sorting
3998 Use @var{file} as a source of random data used to determine which
3999 random hash function to use with the @option{-R} option. @xref{Random
4006 @cindex sort stability
4007 @cindex sort's last-resort comparison
4009 Make @command{sort} stable by disabling its last-resort comparison.
4010 This option has no effect if no fields or global ordering options
4011 other than @option{--reverse} (@option{-r}) are specified.
4014 @itemx --buffer-size=@var{size}
4016 @opindex --buffer-size
4017 @cindex size for main memory sorting
4018 Use a main-memory sort buffer of the given @var{size}. By default,
4019 @var{size} is in units of 1024 bytes. Appending @samp{%} causes
4020 @var{size} to be interpreted as a percentage of physical memory.
4021 Appending @samp{K} multiplies @var{size} by 1024 (the default),
4022 @samp{M} by 1,048,576, @samp{G} by 1,073,741,824, and so on for
4023 @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}. Appending
4024 @samp{b} causes @var{size} to be interpreted as a byte count, with no
4027 This option can improve the performance of @command{sort} by causing it
4028 to start with a larger or smaller sort buffer than the default.
4029 However, this option affects only the initial buffer size. The buffer
4030 grows beyond @var{size} if @command{sort} encounters input lines larger
4033 @item -t @var{separator}
4034 @itemx --field-separator=@var{separator}
4036 @opindex --field-separator
4037 @cindex field separator character
4038 Use character @var{separator} as the field separator when finding the
4039 sort keys in each line. By default, fields are separated by the empty
4040 string between a non-blank character and a blank character.
4041 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4044 That is, given the input line @w{@samp{ foo bar}}, @command{sort} breaks it
4045 into fields @w{@samp{ foo}} and @w{@samp{ bar}}. The field separator is
4046 not considered to be part of either the field preceding or the field
4047 following, so with @samp{sort @w{-t " "}} the same input line has
4048 three fields: an empty field, @samp{foo}, and @samp{bar}.
4049 However, fields that extend to the end of the line,
4050 as @option{-k 2}, or fields consisting of a range, as @option{-k 2,3},
4051 retain the field separators present between the endpoints of the range.
4053 To specify @acronym{ASCII} @sc{nul} as the field separator,
4054 use the two-character string @samp{\0}, e.g., @samp{sort -t '\0'}.
4056 @item -T @var{tempdir}
4057 @itemx --temporary-directory=@var{tempdir}
4059 @opindex --temporary-directory
4060 @cindex temporary directory
4062 Use directory @var{tempdir} to store temporary files, overriding the
4063 @env{TMPDIR} environment variable. If this option is given more than
4064 once, temporary files are stored in all the directories given. If you
4065 have a large sort or merge that is I/O-bound, you can often improve
4066 performance by using this option to specify directories on different
4067 disks and controllers.
4073 @cindex uniquifying output
4075 Normally, output only the first of a sequence of lines that compare
4076 equal. For the @option{--check} (@option{-c} or @option{-C}) option,
4077 check that no pair of consecutive lines compares equal.
4079 This option also disables the default last-resort comparison.
4081 The commands @code{sort -u} and @code{sort | uniq} are equivalent, but
4082 this equivalence does not extend to arbitrary @command{sort} options.
4083 For example, @code{sort -n -u} inspects only the value of the initial
4084 numeric string when checking for uniqueness, whereas @code{sort -n |
4085 uniq} inspects the entire line. @xref{uniq invocation}.
4087 @macro zeroTerminatedOption
4089 @itemx --zero-terminated
4091 @opindex --zero-terminated
4092 @cindex process zero-terminated items
4093 Delimit items with a zero byte rather than a newline (@acronym{ASCII} @sc{lf}).
4094 I.E. treat input as items separated by @acronym{ASCII} @sc{nul}
4095 and terminate output items with @acronym{ASCII} @sc{nul}.
4096 This option can be useful in conjunction with @samp{perl -0} or
4097 @samp{find -print0} and @samp{xargs -0} which do the same in order to
4098 reliably handle arbitrary file names (even those containing blanks
4099 or other special characters).
4101 @zeroTerminatedOption
4105 Historical (BSD and System V) implementations of @command{sort} have
4106 differed in their interpretation of some options, particularly
4107 @option{-b}, @option{-f}, and @option{-n}. @sc{gnu} sort follows the @acronym{POSIX}
4108 behavior, which is usually (but not always!) like the System V behavior.
4109 According to @acronym{POSIX}, @option{-n} no longer implies @option{-b}. For
4110 consistency, @option{-M} has been changed in the same way. This may
4111 affect the meaning of character positions in field specifications in
4112 obscure cases. The only fix is to add an explicit @option{-b}.
4114 A position in a sort field specified with @option{-k} may have any
4115 of the option letters @samp{MbdfghinRrV} appended to it, in which case no
4116 global ordering options are inherited by that particular field. The
4117 @option{-b} option may be independently attached to either or both of
4118 the start and end positions of a field specification, and if it is
4119 inherited from the global options it will be attached to both.
4120 If input lines can contain leading or adjacent blanks and @option{-t}
4121 is not used, then @option{-k} is typically combined with @option{-b} or
4122 an option that implicitly ignores leading blanks (@samp{MghnV}) as otherwise
4123 the varying numbers of leading blanks in fields can cause confusing results.
4125 If the start position in a sort field specifier falls after the end of
4126 the line or after the end field, the field is empty. If the @option{-b}
4127 option was specified, the @samp{.@var{c}} part of a field specification
4128 is counted from the first nonblank character of the field.
4130 @vindex _POSIX2_VERSION
4131 @vindex POSIXLY_CORRECT
4132 On older systems, @command{sort} supports an obsolete origin-zero
4133 syntax @samp{+@var{pos1} [-@var{pos2}]} for specifying sort keys.
4134 The obsolete sequence @samp{sort +@var{a}.@var{x} -@var{b}.@var{y}}
4135 is equivalent to @samp{sort -k @var{a+1}.@var{x+1},@var{b}} if @var{y}
4136 is @samp{0} or absent, otherwise it is equivalent to @samp{sort -k
4137 @var{a+1}.@var{x+1},@var{b+1}.@var{y}}.
4139 This obsolete behavior can be enabled or disabled with the
4140 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4141 conformance}); it can also be enabled when @env{POSIXLY_CORRECT} is
4142 not set by using the obsolete syntax with @samp{-@var{pos2}} present.
4144 Scripts intended for use on standard hosts should avoid obsolete
4145 syntax and should use @option{-k} instead. For example, avoid
4146 @samp{sort +2}, since it might be interpreted as either @samp{sort
4147 ./+2} or @samp{sort -k 3}. If your script must also run on hosts that
4148 support only the obsolete syntax, it can use a test like @samp{if sort
4149 -k 1 </dev/null >/dev/null 2>&1; then @dots{}} to decide which syntax
4152 Here are some examples to illustrate various combinations of options.
4157 Sort in descending (reverse) numeric order.
4164 Sort alphabetically, omitting the first and second fields
4165 and the blanks at the start of the third field.
4166 This uses a single key composed of the characters beginning
4167 at the start of the first nonblank character in field three
4168 and extending to the end of each line.
4175 Sort numerically on the second field and resolve ties by sorting
4176 alphabetically on the third and fourth characters of field five.
4177 Use @samp{:} as the field delimiter.
4180 sort -t : -k 2,2n -k 5.3,5.4
4183 Note that if you had written @option{-k 2n} instead of @option{-k 2,2n}
4184 @command{sort} would have used all characters beginning in the second field
4185 and extending to the end of the line as the primary @emph{numeric}
4186 key. For the large majority of applications, treating keys spanning
4187 more than one field as numeric will not do what you expect.
4189 Also note that the @samp{n} modifier was applied to the field-end
4190 specifier for the first key. It would have been equivalent to
4191 specify @option{-k 2n,2} or @option{-k 2n,2n}. All modifiers except
4192 @samp{b} apply to the associated @emph{field}, regardless of whether
4193 the modifier character is attached to the field-start and/or the
4194 field-end part of the key specifier.
4197 Sort the password file on the fifth field and ignore any
4198 leading blanks. Sort lines with equal values in field five
4199 on the numeric user ID in field three. Fields are separated
4203 sort -t : -k 5b,5 -k 3,3n /etc/passwd
4204 sort -t : -n -k 5b,5 -k 3,3 /etc/passwd
4205 sort -t : -b -k 5,5 -k 3,3n /etc/passwd
4208 These three commands have equivalent effect. The first specifies that
4209 the first key's start position ignores leading blanks and the second
4210 key is sorted numerically. The other two commands rely on global
4211 options being inherited by sort keys that lack modifiers. The inheritance
4212 works in this case because @option{-k 5b,5b} and @option{-k 5b,5} are
4213 equivalent, as the location of a field-end lacking a @samp{.@var{c}}
4214 character position is not affected by whether initial blanks are
4218 Sort a set of log files, primarily by IPv4 address and secondarily by
4219 time stamp. If two lines' primary and secondary keys are identical,
4220 output the lines in the same order that they were input. The log
4221 files contain lines that look like this:
4224 4.150.156.3 - - [01/Apr/2004:06:31:51 +0000] message 1
4225 211.24.3.231 - - [24/Apr/2004:20:17:39 +0000] message 2
4228 Fields are separated by exactly one space. Sort IPv4 addresses
4229 lexicographically, e.g., 212.61.52.2 sorts before 212.129.233.201
4230 because 61 is less than 129.
4233 sort -s -t ' ' -k 4.9n -k 4.5M -k 4.2n -k 4.14,4.21 file*.log |
4234 sort -s -t '.' -k 1,1n -k 2,2n -k 3,3n -k 4,4n
4237 This example cannot be done with a single @command{sort} invocation,
4238 since IPv4 address components are separated by @samp{.} while dates
4239 come just after a space. So it is broken down into two invocations of
4240 @command{sort}: the first sorts by time stamp and the second by IPv4
4241 address. The time stamp is sorted by year, then month, then day, and
4242 finally by hour-minute-second field, using @option{-k} to isolate each
4243 field. Except for hour-minute-second there's no need to specify the
4244 end of each key field, since the @samp{n} and @samp{M} modifiers sort
4245 based on leading prefixes that cannot cross field boundaries. The
4246 IPv4 addresses are sorted lexicographically. The second sort uses
4247 @samp{-s} so that ties in the primary key are broken by the secondary
4248 key; the first sort uses @samp{-s} so that the combination of the two
4252 Generate a tags file in case-insensitive sorted order.
4255 find src -type f -print0 | sort -z -f | xargs -0 etags --append
4258 The use of @option{-print0}, @option{-z}, and @option{-0} in this case means
4259 that file names that contain blanks or other special characters are
4261 by the sort operation.
4263 @c This example is a bit contrived and needs more explanation.
4265 @c Sort records separated by an arbitrary string by using a pipe to convert
4266 @c each record delimiter string to @samp{\0}, then using sort's -z option,
4267 @c and converting each @samp{\0} back to the original record delimiter.
4270 @c printf 'c\n\nb\n\na\n'|perl -0pe 's/\n\n/\n\0/g'|sort -z|perl -0pe 's/\0/\n/g'
4274 Use the common @acronym{DSU, Decorate Sort Undecorate} idiom to
4275 sort lines according to their length.
4278 awk '@{print length, $0@}' /etc/passwd | sort -n | cut -f2- -d' '
4281 In general this technique can be used to sort data that the @command{sort}
4282 command does not support, or is inefficient at, sorting directly.
4285 Shuffle a list of directories, but preserve the order of files within
4286 each directory. For instance, one could use this to generate a music
4287 playlist in which albums are shuffled but the songs of each album are
4291 ls */* | sort -t / -k 1,1R -k 2,2
4297 @node shuf invocation
4298 @section @command{shuf}: Shuffling text
4301 @cindex shuffling files
4303 @command{shuf} shuffles its input by outputting a random permutation
4304 of its input lines. Each output permutation is equally likely.
4308 shuf [@var{option}]@dots{} [@var{file}]
4309 shuf -e [@var{option}]@dots{} [@var{arg}]@dots{}
4310 shuf -i @var{lo}-@var{hi} [@var{option}]@dots{}
4313 @command{shuf} has three modes of operation that affect where it
4314 obtains its input lines. By default, it reads lines from standard
4315 input. The following options change the operation mode:
4323 @cindex command-line operands to shuffle
4324 Treat each command-line operand as an input line.
4326 @item -i @var{lo}-@var{hi}
4327 @itemx --input-range=@var{lo}-@var{hi}
4329 @opindex --input-range
4330 @cindex input range to shuffle
4331 Act as if input came from a file containing the range of unsigned
4332 decimal integers @var{lo}@dots{}@var{hi}, one per line.
4336 @command{shuf}'s other options can affect its behavior in all
4341 @item -n @var{lines}
4342 @itemx --head-count=@var{count}
4344 @opindex --head-count
4345 @cindex head of output
4346 Output at most @var{count} lines. By default, all input lines are
4349 @item -o @var{output-file}
4350 @itemx --output=@var{output-file}
4353 @cindex overwriting of input, allowed
4354 Write output to @var{output-file} instead of standard output.
4355 @command{shuf} reads all input before opening
4356 @var{output-file}, so you can safely shuffle a file in place by using
4357 commands like @code{shuf -o F <F} and @code{cat F | shuf -o F}.
4359 @item --random-source=@var{file}
4360 @opindex --random-source
4361 @cindex random source for shuffling
4362 Use @var{file} as a source of random data used to determine which
4363 permutation to generate. @xref{Random sources}.
4365 @zeroTerminatedOption
4381 might produce the output
4391 Similarly, the command:
4394 shuf -e clubs hearts diamonds spades
4408 and the command @samp{shuf -i 1-4} might output:
4418 These examples all have four input lines, so @command{shuf} might
4419 produce any of the twenty-four possible permutations of the input. In
4420 general, if there are @var{n} input lines, there are @var{n}! (i.e.,
4421 @var{n} factorial, or @var{n} * (@var{n} - 1) * @dots{} * 1) possible
4422 output permutations.
4427 @node uniq invocation
4428 @section @command{uniq}: Uniquify files
4431 @cindex uniquify files
4433 @command{uniq} writes the unique lines in the given @file{input}, or
4434 standard input if nothing is given or for an @var{input} name of
4438 uniq [@var{option}]@dots{} [@var{input} [@var{output}]]
4441 By default, @command{uniq} prints its input lines, except that
4442 it discards all but the first of adjacent repeated lines, so that
4443 no output lines are repeated. Optionally, it can instead discard
4444 lines that are not repeated, or all repeated lines.
4446 The input need not be sorted, but repeated input lines are detected
4447 only if they are adjacent. If you want to discard non-adjacent
4448 duplicate lines, perhaps you want to use @code{sort -u}.
4449 @xref{sort invocation}.
4452 Comparisons honor the rules specified by the @env{LC_COLLATE}
4455 If no @var{output} file is specified, @command{uniq} writes to standard
4458 The program accepts the following options. Also see @ref{Common options}.
4463 @itemx --skip-fields=@var{n}
4465 @opindex --skip-fields
4466 Skip @var{n} fields on each line before checking for uniqueness. Use
4467 a null string for comparison if a line has fewer than @var{n} fields. Fields
4468 are sequences of non-space non-tab characters that are separated from
4469 each other by at least one space or tab.
4471 For compatibility @command{uniq} supports an obsolete option syntax
4472 @option{-@var{n}}. New scripts should use @option{-f @var{n}} instead.
4475 @itemx --skip-chars=@var{n}
4477 @opindex --skip-chars
4478 Skip @var{n} characters before checking for uniqueness. Use a null string
4479 for comparison if a line has fewer than @var{n} characters. If you use both
4480 the field and character skipping options, fields are skipped over first.
4482 @vindex _POSIX2_VERSION
4483 On older systems, @command{uniq} supports an obsolete option syntax
4485 This obsolete behavior can be enabled or disabled with the
4486 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4487 conformance}), but portable scripts should avoid commands whose
4488 behavior depends on this variable.
4489 For example, use @samp{uniq ./+10} or @samp{uniq -s 10} rather than
4490 the ambiguous @samp{uniq +10}.
4496 Print the number of times each line occurred along with the line.
4499 @itemx --ignore-case
4501 @opindex --ignore-case
4502 Ignore differences in case when comparing lines.
4508 @cindex repeated lines, outputting
4509 Discard lines that are not repeated. When used by itself, this option
4510 causes @command{uniq} to print the first copy of each repeated line,
4514 @itemx --all-repeated[=@var{delimit-method}]
4516 @opindex --all-repeated
4517 @cindex all repeated lines, outputting
4518 Do not discard the second and subsequent repeated input lines,
4519 but discard lines that are not repeated.
4520 This option is useful mainly in conjunction with other options e.g.,
4521 to ignore case or to compare only selected fields.
4522 The optional @var{delimit-method} tells how to delimit
4523 groups of repeated lines, and must be one of the following:
4528 Do not delimit groups of repeated lines.
4529 This is equivalent to @option{--all-repeated} (@option{-D}).
4532 Output a newline before each group of repeated lines.
4533 With @option{--zero-terminated} (@option{-z}), use a zero
4534 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4537 Separate groups of repeated lines with a single newline.
4538 With @option{--zero-terminated} (@option{-z}), use a zero
4539 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4540 This is the same as using @samp{prepend}, except that
4541 no delimiter is inserted before the first group, and hence
4542 may be better suited for output direct to users.
4545 Note that when groups are delimited and the input stream contains
4546 two or more consecutive blank lines, then the output is ambiguous.
4547 To avoid that, filter the input through @samp{tr -s '\n'} to replace
4548 each sequence of consecutive newlines with a single newline.
4550 This is a @sc{gnu} extension.
4551 @c FIXME: give an example showing *how* it's useful
4557 @cindex unique lines, outputting
4558 Discard the first repeated line. When used by itself, this option
4559 causes @command{uniq} to print unique lines, and nothing else.
4562 @itemx --check-chars=@var{n}
4564 @opindex --check-chars
4565 Compare at most @var{n} characters on each line (after skipping any specified
4566 fields and characters). By default the entire rest of the lines are
4569 @zeroTerminatedOption
4576 @node comm invocation
4577 @section @command{comm}: Compare two sorted files line by line
4580 @cindex line-by-line comparison
4581 @cindex comparing sorted files
4583 @command{comm} writes to standard output lines that are common, and lines
4584 that are unique, to two input files; a file name of @samp{-} means
4585 standard input. Synopsis:
4588 comm [@var{option}]@dots{} @var{file1} @var{file2}
4592 Before @command{comm} can be used, the input files must be sorted using the
4593 collating sequence specified by the @env{LC_COLLATE} locale.
4594 If an input file ends in a non-newline
4595 character, a newline is silently appended. The @command{sort} command with
4596 no options always outputs a file that is suitable input to @command{comm}.
4598 @cindex differing lines
4599 @cindex common lines
4600 With no options, @command{comm} produces three-column output. Column one
4601 contains lines unique to @var{file1}, column two contains lines unique
4602 to @var{file2}, and column three contains lines common to both files.
4603 Columns are separated by a single TAB character.
4604 @c FIXME: when there's an option to supply an alternative separator
4605 @c string, append `by default' to the above sentence.
4610 The options @option{-1}, @option{-2}, and @option{-3} suppress printing of
4611 the corresponding columns (and separators). Also see @ref{Common options}.
4613 Unlike some other comparison utilities, @command{comm} has an exit
4614 status that does not depend on the result of the comparison.
4615 Upon normal completion @command{comm} produces an exit code of zero.
4616 If there is an error it exits with nonzero status.
4618 @macro checkOrderOption{cmd}
4619 If the @option{--check-order} option is given, unsorted inputs will
4620 cause a fatal error message. If the option @option{--nocheck-order}
4621 is given, unsorted inputs will never cause an error message. If
4622 neither of these options is given, wrongly sorted inputs are diagnosed
4623 only if an input file is found to contain unpairable lines. If an
4624 input file is diagnosed as being unsorted, the @command{\cmd\} command
4625 will exit with a nonzero status (and the output should not be used).
4627 Forcing @command{\cmd\} to process wrongly sorted input files
4628 containing unpairable lines by specifying @option{--nocheck-order} is
4629 not guaranteed to produce any particular output. The output will
4630 probably not correspond with whatever you hoped it would be.
4632 @checkOrderOption{comm}
4637 Fail with an error message if either input file is wrongly ordered.
4639 @item --nocheck-order
4640 Do not check that both input files are in sorted order.
4644 @item --output-delimiter=@var{str}
4645 Print @var{str} between adjacent output columns,
4646 rather than the default of a single TAB character.
4648 The delimiter @var{str} may not be empty.
4652 @node ptx invocation
4653 @section @command{ptx}: Produce permuted indexes
4657 @command{ptx} reads a text file and essentially produces a permuted index, with
4658 each keyword in its context. The calling sketch is either one of:
4661 ptx [@var{option} @dots{}] [@var{file} @dots{}]
4662 ptx -G [@var{option} @dots{}] [@var{input} [@var{output}]]
4665 The @option{-G} (or its equivalent: @option{--traditional}) option disables
4666 all @sc{gnu} extensions and reverts to traditional mode, thus introducing some
4667 limitations and changing several of the program's default option values.
4668 When @option{-G} is not specified, @sc{gnu} extensions are always enabled.
4669 @sc{gnu} extensions to @command{ptx} are documented wherever appropriate in this
4670 document. For the full list, see @xref{Compatibility in ptx}.
4672 Individual options are explained in the following sections.
4674 When @sc{gnu} extensions are enabled, there may be zero, one or several
4675 @var{file}s after the options. If there is no @var{file}, the program
4676 reads the standard input. If there is one or several @var{file}s, they
4677 give the name of input files which are all read in turn, as if all the
4678 input files were concatenated. However, there is a full contextual
4679 break between each file and, when automatic referencing is requested,
4680 file names and line numbers refer to individual text input files. In
4681 all cases, the program outputs the permuted index to the standard
4684 When @sc{gnu} extensions are @emph{not} enabled, that is, when the program
4685 operates in traditional mode, there may be zero, one or two parameters
4686 besides the options. If there are no parameters, the program reads the
4687 standard input and outputs the permuted index to the standard output.
4688 If there is only one parameter, it names the text @var{input} to be read
4689 instead of the standard input. If two parameters are given, they give
4690 respectively the name of the @var{input} file to read and the name of
4691 the @var{output} file to produce. @emph{Be very careful} to note that,
4692 in this case, the contents of file given by the second parameter is
4693 destroyed. This behavior is dictated by System V @command{ptx}
4694 compatibility; @sc{gnu} Standards normally discourage output parameters not
4695 introduced by an option.
4697 Note that for @emph{any} file named as the value of an option or as an
4698 input text file, a single dash @kbd{-} may be used, in which case
4699 standard input is assumed. However, it would not make sense to use this
4700 convention more than once per program invocation.
4703 * General options in ptx:: Options which affect general program behavior.
4704 * Charset selection in ptx:: Underlying character set considerations.
4705 * Input processing in ptx:: Input fields, contexts, and keyword selection.
4706 * Output formatting in ptx:: Types of output format, and sizing the fields.
4707 * Compatibility in ptx::
4711 @node General options in ptx
4712 @subsection General options
4717 @itemx --traditional
4718 As already explained, this option disables all @sc{gnu} extensions to
4719 @command{ptx} and switches to traditional mode.
4722 Print a short help on standard output, then exit without further
4726 Print the program version on standard output, then exit without further
4734 @node Charset selection in ptx
4735 @subsection Charset selection
4737 @c FIXME: People don't necessarily know what an IBM-PC was these days.
4738 As it is set up now, the program assumes that the input file is coded
4739 using 8-bit @acronym{ISO} 8859-1 code, also known as Latin-1 character set,
4740 @emph{unless} it is compiled for MS-DOS, in which case it uses the
4741 character set of the IBM-PC@. (@sc{gnu} @command{ptx} is not known to work on
4742 smaller MS-DOS machines anymore.) Compared to 7-bit @acronym{ASCII}, the set
4743 of characters which are letters is different; this alters the behavior
4744 of regular expression matching. Thus, the default regular expression
4745 for a keyword allows foreign or diacriticized letters. Keyword sorting,
4746 however, is still crude; it obeys the underlying character set ordering
4752 @itemx --ignore-case
4753 Fold lower case letters to upper case for sorting.
4758 @node Input processing in ptx
4759 @subsection Word selection and input processing
4764 @itemx --break-file=@var{file}
4766 This option provides an alternative (to @option{-W}) method of describing
4767 which characters make up words. It introduces the name of a
4768 file which contains a list of characters which can@emph{not} be part of
4769 one word; this file is called the @dfn{Break file}. Any character which
4770 is not part of the Break file is a word constituent. If both options
4771 @option{-b} and @option{-W} are specified, then @option{-W} has precedence and
4772 @option{-b} is ignored.
4774 When @sc{gnu} extensions are enabled, the only way to avoid newline as a
4775 break character is to write all the break characters in the file with no
4776 newline at all, not even at the end of the file. When @sc{gnu} extensions
4777 are disabled, spaces, tabs and newlines are always considered as break
4778 characters even if not included in the Break file.
4781 @itemx --ignore-file=@var{file}
4783 The file associated with this option contains a list of words which will
4784 never be taken as keywords in concordance output. It is called the
4785 @dfn{Ignore file}. The file contains exactly one word in each line; the
4786 end of line separation of words is not subject to the value of the
4790 @itemx --only-file=@var{file}
4792 The file associated with this option contains a list of words which will
4793 be retained in concordance output; any word not mentioned in this file
4794 is ignored. The file is called the @dfn{Only file}. The file contains
4795 exactly one word in each line; the end of line separation of words is
4796 not subject to the value of the @option{-S} option.
4798 There is no default for the Only file. When both an Only file and an
4799 Ignore file are specified, a word is considered a keyword only
4800 if it is listed in the Only file and not in the Ignore file.
4805 On each input line, the leading sequence of non-white space characters will be
4806 taken to be a reference that has the purpose of identifying this input
4807 line in the resulting permuted index. For more information about reference
4808 production, see @xref{Output formatting in ptx}.
4809 Using this option changes the default value for option @option{-S}.
4811 Using this option, the program does not try very hard to remove
4812 references from contexts in output, but it succeeds in doing so
4813 @emph{when} the context ends exactly at the newline. If option
4814 @option{-r} is used with @option{-S} default value, or when @sc{gnu} extensions
4815 are disabled, this condition is always met and references are completely
4816 excluded from the output contexts.
4818 @item -S @var{regexp}
4819 @itemx --sentence-regexp=@var{regexp}
4821 This option selects which regular expression will describe the end of a
4822 line or the end of a sentence. In fact, this regular expression is not
4823 the only distinction between end of lines or end of sentences, and input
4824 line boundaries have no special significance outside this option. By
4825 default, when @sc{gnu} extensions are enabled and if @option{-r} option is not
4826 used, end of sentences are used. In this case, this @var{regex} is
4827 imported from @sc{gnu} Emacs:
4830 [.?!][]\"')@}]*\\($\\|\t\\| \\)[ \t\n]*
4833 Whenever @sc{gnu} extensions are disabled or if @option{-r} option is used, end
4834 of lines are used; in this case, the default @var{regexp} is just:
4840 Using an empty @var{regexp} is equivalent to completely disabling end of
4841 line or end of sentence recognition. In this case, the whole file is
4842 considered to be a single big line or sentence. The user might want to
4843 disallow all truncation flag generation as well, through option @option{-F
4844 ""}. @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
4847 When the keywords happen to be near the beginning of the input line or
4848 sentence, this often creates an unused area at the beginning of the
4849 output context line; when the keywords happen to be near the end of the
4850 input line or sentence, this often creates an unused area at the end of
4851 the output context line. The program tries to fill those unused areas
4852 by wrapping around context in them; the tail of the input line or
4853 sentence is used to fill the unused area on the left of the output line;
4854 the head of the input line or sentence is used to fill the unused area
4855 on the right of the output line.
4857 As a matter of convenience to the user, many usual backslashed escape
4858 sequences from the C language are recognized and converted to the
4859 corresponding characters by @command{ptx} itself.
4861 @item -W @var{regexp}
4862 @itemx --word-regexp=@var{regexp}
4864 This option selects which regular expression will describe each keyword.
4865 By default, if @sc{gnu} extensions are enabled, a word is a sequence of
4866 letters; the @var{regexp} used is @samp{\w+}. When @sc{gnu} extensions are
4867 disabled, a word is by default anything which ends with a space, a tab
4868 or a newline; the @var{regexp} used is @samp{[^ \t\n]+}.
4870 An empty @var{regexp} is equivalent to not using this option.
4871 @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
4874 As a matter of convenience to the user, many usual backslashed escape
4875 sequences, as found in the C language, are recognized and converted to
4876 the corresponding characters by @command{ptx} itself.
4881 @node Output formatting in ptx
4882 @subsection Output formatting
4884 Output format is mainly controlled by the @option{-O} and @option{-T} options
4885 described in the table below. When neither @option{-O} nor @option{-T} are
4886 selected, and if @sc{gnu} extensions are enabled, the program chooses an
4887 output format suitable for a dumb terminal. Each keyword occurrence is
4888 output to the center of one line, surrounded by its left and right
4889 contexts. Each field is properly justified, so the concordance output
4890 can be readily observed. As a special feature, if automatic
4891 references are selected by option @option{-A} and are output before the
4892 left context, that is, if option @option{-R} is @emph{not} selected, then
4893 a colon is added after the reference; this nicely interfaces with @sc{gnu}
4894 Emacs @code{next-error} processing. In this default output format, each
4895 white space character, like newline and tab, is merely changed to
4896 exactly one space, with no special attempt to compress consecutive
4897 spaces. This might change in the future. Except for those white space
4898 characters, every other character of the underlying set of 256
4899 characters is transmitted verbatim.
4901 Output format is further controlled by the following options.
4905 @item -g @var{number}
4906 @itemx --gap-size=@var{number}
4908 Select the size of the minimum white space gap between the fields on the
4911 @item -w @var{number}
4912 @itemx --width=@var{number}
4914 Select the maximum output width of each final line. If references are
4915 used, they are included or excluded from the maximum output width
4916 depending on the value of option @option{-R}. If this option is not
4917 selected, that is, when references are output before the left context,
4918 the maximum output width takes into account the maximum length of all
4919 references. If this option is selected, that is, when references are
4920 output after the right context, the maximum output width does not take
4921 into account the space taken by references, nor the gap that precedes
4925 @itemx --auto-reference
4927 Select automatic references. Each input line will have an automatic
4928 reference made up of the file name and the line ordinal, with a single
4929 colon between them. However, the file name will be empty when standard
4930 input is being read. If both @option{-A} and @option{-r} are selected, then
4931 the input reference is still read and skipped, but the automatic
4932 reference is used at output time, overriding the input reference.
4935 @itemx --right-side-refs
4937 In the default output format, when option @option{-R} is not used, any
4938 references produced by the effect of options @option{-r} or @option{-A} are
4939 placed to the far right of output lines, after the right context. With
4940 default output format, when the @option{-R} option is specified, references
4941 are rather placed at the beginning of each output line, before the left
4942 context. For any other output format, option @option{-R} is
4943 ignored, with one exception: with @option{-R} the width of references
4944 is @emph{not} taken into account in total output width given by @option{-w}.
4946 This option is automatically selected whenever @sc{gnu} extensions are
4949 @item -F @var{string}
4950 @itemx --flac-truncation=@var{string}
4952 This option will request that any truncation in the output be reported
4953 using the string @var{string}. Most output fields theoretically extend
4954 towards the beginning or the end of the current line, or current
4955 sentence, as selected with option @option{-S}. But there is a maximum
4956 allowed output line width, changeable through option @option{-w}, which is
4957 further divided into space for various output fields. When a field has
4958 to be truncated because it cannot extend beyond the beginning or the end of
4959 the current line to fit in, then a truncation occurs. By default,
4960 the string used is a single slash, as in @option{-F /}.
4962 @var{string} may have more than one character, as in @option{-F ...}.
4963 Also, in the particular case when @var{string} is empty (@option{-F ""}),
4964 truncation flagging is disabled, and no truncation marks are appended in
4967 As a matter of convenience to the user, many usual backslashed escape
4968 sequences, as found in the C language, are recognized and converted to
4969 the corresponding characters by @command{ptx} itself.
4971 @item -M @var{string}
4972 @itemx --macro-name=@var{string}
4974 Select another @var{string} to be used instead of @samp{xx}, while
4975 generating output suitable for @command{nroff}, @command{troff} or @TeX{}.
4978 @itemx --format=roff
4980 Choose an output format suitable for @command{nroff} or @command{troff}
4981 processing. Each output line will look like:
4984 .xx "@var{tail}" "@var{before}" "@var{keyword_and_after}" "@var{head}" "@var{ref}"
4987 so it will be possible to write a @samp{.xx} roff macro to take care of
4988 the output typesetting. This is the default output format when @sc{gnu}
4989 extensions are disabled. Option @option{-M} can be used to change
4990 @samp{xx} to another macro name.
4992 In this output format, each non-graphical character, like newline and
4993 tab, is merely changed to exactly one space, with no special attempt to
4994 compress consecutive spaces. Each quote character: @kbd{"} is doubled
4995 so it will be correctly processed by @command{nroff} or @command{troff}.
5000 Choose an output format suitable for @TeX{} processing. Each output
5001 line will look like:
5004 \xx @{@var{tail}@}@{@var{before}@}@{@var{keyword}@}@{@var{after}@}@{@var{head}@}@{@var{ref}@}
5008 so it will be possible to write a @code{\xx} definition to take care of
5009 the output typesetting. Note that when references are not being
5010 produced, that is, neither option @option{-A} nor option @option{-r} is
5011 selected, the last parameter of each @code{\xx} call is inhibited.
5012 Option @option{-M} can be used to change @samp{xx} to another macro
5015 In this output format, some special characters, like @kbd{$}, @kbd{%},
5016 @kbd{&}, @kbd{#} and @kbd{_} are automatically protected with a
5017 backslash. Curly brackets @kbd{@{}, @kbd{@}} are protected with a
5018 backslash and a pair of dollar signs (to force mathematical mode). The
5019 backslash itself produces the sequence @code{\backslash@{@}}.
5020 Circumflex and tilde diacritical marks produce the sequence @code{^\@{ @}} and
5021 @code{~\@{ @}} respectively. Other diacriticized characters of the
5022 underlying character set produce an appropriate @TeX{} sequence as far
5023 as possible. The other non-graphical characters, like newline and tab,
5024 and all other characters which are not part of @acronym{ASCII}, are merely
5025 changed to exactly one space, with no special attempt to compress
5026 consecutive spaces. Let me know how to improve this special character
5027 processing for @TeX{}.
5032 @node Compatibility in ptx
5033 @subsection The @sc{gnu} extensions to @command{ptx}
5035 This version of @command{ptx} contains a few features which do not exist in
5036 System V @command{ptx}. These extra features are suppressed by using the
5037 @option{-G} command line option, unless overridden by other command line
5038 options. Some @sc{gnu} extensions cannot be recovered by overriding, so the
5039 simple rule is to avoid @option{-G} if you care about @sc{gnu} extensions.
5040 Here are the differences between this program and System V @command{ptx}.
5045 This program can read many input files at once, it always writes the
5046 resulting concordance on standard output. On the other hand, System V
5047 @command{ptx} reads only one file and sends the result to standard output
5048 or, if a second @var{file} parameter is given on the command, to that
5051 Having output parameters not introduced by options is a dangerous
5052 practice which @sc{gnu} avoids as far as possible. So, for using @command{ptx}
5053 portably between @sc{gnu} and System V, you should always use it with a
5054 single input file, and always expect the result on standard output. You
5055 might also want to automatically configure in a @option{-G} option to
5056 @command{ptx} calls in products using @command{ptx}, if the configurator finds
5057 that the installed @command{ptx} accepts @option{-G}.
5060 The only options available in System V @command{ptx} are options @option{-b},
5061 @option{-f}, @option{-g}, @option{-i}, @option{-o}, @option{-r}, @option{-t} and
5062 @option{-w}. All other options are @sc{gnu} extensions and are not repeated in
5063 this enumeration. Moreover, some options have a slightly different
5064 meaning when @sc{gnu} extensions are enabled, as explained below.
5067 By default, concordance output is not formatted for @command{troff} or
5068 @command{nroff}. It is rather formatted for a dumb terminal. @command{troff}
5069 or @command{nroff} output may still be selected through option @option{-O}.
5072 Unless @option{-R} option is used, the maximum reference width is
5073 subtracted from the total output line width. With @sc{gnu} extensions
5074 disabled, width of references is not taken into account in the output
5075 line width computations.
5078 All 256 bytes, even @acronym{ASCII} @sc{nul} bytes, are always read and
5079 processed from input file with no adverse effect, even if @sc{gnu} extensions
5080 are disabled. However, System V @command{ptx} does not accept 8-bit characters,
5081 a few control characters are rejected, and the tilde @kbd{~} is also rejected.
5084 Input line length is only limited by available memory, even if @sc{gnu}
5085 extensions are disabled. However, System V @command{ptx} processes only
5086 the first 200 characters in each line.
5089 The break (non-word) characters default to be every character except all
5090 letters of the underlying character set, diacriticized or not. When @sc{gnu}
5091 extensions are disabled, the break characters default to space, tab and
5095 The program makes better use of output line width. If @sc{gnu} extensions
5096 are disabled, the program rather tries to imitate System V @command{ptx},
5097 but still, there are some slight disposition glitches this program does
5098 not completely reproduce.
5101 The user can specify both an Ignore file and an Only file. This is not
5102 allowed with System V @command{ptx}.
5107 @node tsort invocation
5108 @section @command{tsort}: Topological sort
5111 @cindex topological sort
5113 @command{tsort} performs a topological sort on the given @var{file}, or
5114 standard input if no input file is given or for a @var{file} of
5115 @samp{-}. For more details and some history, see @ref{tsort background}.
5119 tsort [@var{option}] [@var{file}]
5122 @command{tsort} reads its input as pairs of strings, separated by blanks,
5123 indicating a partial ordering. The output is a total ordering that
5124 corresponds to the given partial ordering.
5138 will produce the output
5149 Consider a more realistic example.
5150 You have a large set of functions all in one file, and they may all be
5151 declared static except one. Currently that one (say @code{main}) is the
5152 first function defined in the file, and the ones it calls directly follow
5153 it, followed by those they call, etc. Let's say that you are determined
5154 to take advantage of prototypes, so you have to choose between declaring
5155 all of those functions (which means duplicating a lot of information from
5156 the definitions) and rearranging the functions so that as many as possible
5157 are defined before they are used. One way to automate the latter process
5158 is to get a list for each function of the functions it calls directly.
5159 Many programs can generate such lists. They describe a call graph.
5160 Consider the following list, in which a given line indicates that the
5161 function on the left calls the one on the right directly.
5167 tail_file pretty_name
5168 tail_file write_header
5170 tail_forever recheck
5171 tail_forever pretty_name
5172 tail_forever write_header
5173 tail_forever dump_remainder
5176 tail_lines start_lines
5177 tail_lines dump_remainder
5178 tail_lines file_lines
5179 tail_lines pipe_lines
5181 tail_bytes start_bytes
5182 tail_bytes dump_remainder
5183 tail_bytes pipe_bytes
5184 file_lines dump_remainder
5188 then you can use @command{tsort} to produce an ordering of those
5189 functions that satisfies your requirement.
5192 example$ tsort call-graph | tac
5212 @command{tsort} detects any cycles in the input and writes the first cycle
5213 encountered to standard error.
5215 Note that for a given partial ordering, generally there is no unique
5216 total ordering. In the context of the call graph above, the function
5217 @code{parse_options} may be placed anywhere in the list as long as it
5218 precedes @code{main}.
5220 The only options are @option{--help} and @option{--version}. @xref{Common
5226 * tsort background:: Where tsort came from.
5229 @node tsort background
5230 @subsection @command{tsort}: Background
5232 @command{tsort} exists because very early versions of the Unix linker processed
5233 an archive file exactly once, and in order. As @command{ld} read each object
5234 in the archive, it decided whether it was needed in the program based on
5235 whether it defined any symbols which were undefined at that point in
5238 This meant that dependencies within the archive had to be handled
5239 specially. For example, @code{scanf} probably calls @code{read}. That means
5240 that in a single pass through an archive, it was important for @code{scanf.o}
5241 to appear before read.o, because otherwise a program which calls
5242 @code{scanf} but not @code{read} might end up with an unexpected unresolved
5243 reference to @code{read}.
5245 The way to address this problem was to first generate a set of
5246 dependencies of one object file on another. This was done by a shell
5247 script called @command{lorder}. The GNU tools don't provide a version of
5248 lorder, as far as I know, but you can still find it in BSD
5251 Then you ran @command{tsort} over the @command{lorder} output, and you used the
5252 resulting sort to define the order in which you added objects to the archive.
5254 This whole procedure has been obsolete since about 1980, because
5255 Unix archives now contain a symbol table (traditionally built by
5256 @command{ranlib}, now generally built by @command{ar} itself), and the Unix
5257 linker uses the symbol table to effectively make multiple passes over
5260 Anyhow, that's where tsort came from. To solve an old problem with
5261 the way the linker handled archive files, which has since been solved
5265 @node Operating on fields
5266 @chapter Operating on fields
5269 * cut invocation:: Print selected parts of lines.
5270 * paste invocation:: Merge lines of files.
5271 * join invocation:: Join lines on a common field.
5275 @node cut invocation
5276 @section @command{cut}: Print selected parts of lines
5279 @command{cut} writes to standard output selected parts of each line of each
5280 input file, or standard input if no files are given or for a file name of
5284 cut @var{option}@dots{} [@var{file}]@dots{}
5287 In the table which follows, the @var{byte-list}, @var{character-list},
5288 and @var{field-list} are one or more numbers or ranges (two numbers
5289 separated by a dash) separated by commas. Bytes, characters, and
5290 fields are numbered starting at 1. Incomplete ranges may be
5291 given: @option{-@var{m}} means @samp{1-@var{m}}; @samp{@var{n}-} means
5292 @samp{@var{n}} through end of line or last field. The list elements
5293 can be repeated, can overlap, and can be specified in any order; but
5294 the selected input is written in the same order that it is read, and
5295 is written exactly once.
5297 The program accepts the following options. Also see @ref{Common
5302 @item -b @var{byte-list}
5303 @itemx --bytes=@var{byte-list}
5306 Select for printing only the bytes in positions listed in
5307 @var{byte-list}. Tabs and backspaces are treated like any other
5308 character; they take up 1 byte. If an output delimiter is specified,
5309 (see the description of @option{--output-delimiter}), then output that
5310 string between ranges of selected bytes.
5312 @item -c @var{character-list}
5313 @itemx --characters=@var{character-list}
5315 @opindex --characters
5316 Select for printing only the characters in positions listed in
5317 @var{character-list}. The same as @option{-b} for now, but
5318 internationalization will change that. Tabs and backspaces are
5319 treated like any other character; they take up 1 character. If an
5320 output delimiter is specified, (see the description of
5321 @option{--output-delimiter}), then output that string between ranges
5324 @item -f @var{field-list}
5325 @itemx --fields=@var{field-list}
5328 Select for printing only the fields listed in @var{field-list}.
5329 Fields are separated by a TAB character by default. Also print any
5330 line that contains no delimiter character, unless the
5331 @option{--only-delimited} (@option{-s}) option is specified.
5332 Note @command{cut} does not support specifying runs of whitespace as a
5333 delimiter, so to achieve that common functionality one can pre-process
5334 with @command{tr} like:
5336 tr -s '[:blank:]' '\t' | cut -f@dots{}
5339 @item -d @var{input_delim_byte}
5340 @itemx --delimiter=@var{input_delim_byte}
5342 @opindex --delimiter
5343 With @option{-f}, use the first byte of @var{input_delim_byte} as
5344 the input fields separator (default is TAB).
5348 Do not split multi-byte characters (no-op for now).
5351 @itemx --only-delimited
5353 @opindex --only-delimited
5354 For @option{-f}, do not print lines that do not contain the field separator
5355 character. Normally, any line without a field separator is printed verbatim.
5357 @item --output-delimiter=@var{output_delim_string}
5358 @opindex --output-delimiter
5359 With @option{-f}, output fields are separated by @var{output_delim_string}.
5360 The default with @option{-f} is to use the input delimiter.
5361 When using @option{-b} or @option{-c} to select ranges of byte or
5362 character offsets (as opposed to ranges of fields),
5363 output @var{output_delim_string} between non-overlapping
5364 ranges of selected bytes.
5367 @opindex --complement
5368 This option is a @acronym{GNU} extension.
5369 Select for printing the complement of the bytes, characters or fields
5370 selected with the @option{-b}, @option{-c} or @option{-f} options.
5371 In other words, do @emph{not} print the bytes, characters or fields
5372 specified via those options. This option is useful when you have
5373 many fields and want to print all but a few of them.
5380 @node paste invocation
5381 @section @command{paste}: Merge lines of files
5384 @cindex merging files
5386 @command{paste} writes to standard output lines consisting of sequentially
5387 corresponding lines of each given file, separated by a TAB character.
5388 Standard input is used for a file name of @samp{-} or if no input files
5410 paste [@var{option}]@dots{} [@var{file}]@dots{}
5413 The program accepts the following options. Also see @ref{Common options}.
5421 Paste the lines of one file at a time rather than one line from each
5422 file. Using the above example data:
5425 $ paste -s num2 let3
5430 @item -d @var{delim-list}
5431 @itemx --delimiters=@var{delim-list}
5433 @opindex --delimiters
5434 Consecutively use the characters in @var{delim-list} instead of
5435 TAB to separate merged lines. When @var{delim-list} is
5436 exhausted, start again at its beginning. Using the above example data:
5439 $ paste -d '%_' num2 let3 num2
5450 @node join invocation
5451 @section @command{join}: Join lines on a common field
5454 @cindex common field, joining on
5456 @command{join} writes to standard output a line for each pair of input
5457 lines that have identical join fields. Synopsis:
5460 join [@var{option}]@dots{} @var{file1} @var{file2}
5463 Either @var{file1} or @var{file2} (but not both) can be @samp{-},
5464 meaning standard input. @var{file1} and @var{file2} should be
5465 sorted on the join fields.
5468 Normally, the sort order is that of the
5469 collating sequence specified by the @env{LC_COLLATE} locale. Unless
5470 the @option{-t} option is given, the sort comparison ignores blanks at
5471 the start of the join field, as in @code{sort -b}. If the
5472 @option{--ignore-case} option is given, the sort comparison ignores
5473 the case of characters in the join field, as in @code{sort -f}.
5475 The @command{sort} and @command{join} commands should use consistent
5476 locales and options if the output of @command{sort} is fed to
5477 @command{join}. You can use a command like @samp{sort -k 1b,1} to
5478 sort a file on its default join field, but if you select a non-default
5479 locale, join field, separator, or comparison options, then you should
5480 do so consistently between @command{join} and @command{sort}.
5481 If @samp{join -t ''} is specified then the whole line is considered which
5482 matches the default operation of sort.
5484 If the input has no unpairable lines, a @acronym{GNU} extension is
5485 available; the sort order can be any order that considers two fields
5486 to be equal if and only if the sort comparison described above
5487 considers them to be equal. For example:
5504 @checkOrderOption{join}
5508 @item the join field is the first field in each line;
5509 @item fields in the input are separated by one or more blanks, with leading
5510 blanks on the line ignored;
5511 @item fields in the output are separated by a space;
5512 @item each output line consists of the join field, the remaining
5513 fields from @var{file1}, then the remaining fields from @var{file2}.
5516 The program accepts the following options. Also see @ref{Common options}.
5520 @item -a @var{file-number}
5522 Print a line for each unpairable line in file @var{file-number} (either
5523 @samp{1} or @samp{2}), in addition to the normal output.
5526 Fail with an error message if either input file is wrongly ordered.
5528 @item --nocheck-order
5529 Do not check that both input files are in sorted order. This is the default.
5531 @item -e @var{string}
5533 Replace those output fields that are missing in the input with
5538 Treat the first line of each input file as a header line. The header lines will
5539 be joined and printed as the first output line. If @option{-o} is used to
5540 specify output format, the header line will be printed according to the
5541 specified format. The header lines will not be checked for ordering even if
5542 @option{--check-order} is specified. Also if the header lines from each file
5543 do not match, the heading fields from the first file will be used.
5546 @itemx --ignore-case
5548 @opindex --ignore-case
5549 Ignore differences in case when comparing keys.
5550 With this option, the lines of the input files must be ordered in the same way.
5551 Use @samp{sort -f} to produce this ordering.
5553 @item -1 @var{field}
5555 Join on field @var{field} (a positive integer) of file 1.
5557 @item -2 @var{field}
5559 Join on field @var{field} (a positive integer) of file 2.
5561 @item -j @var{field}
5562 Equivalent to @option{-1 @var{field} -2 @var{field}}.
5564 @item -o @var{field-list}
5565 Construct each output line according to the format in @var{field-list}.
5566 Each element in @var{field-list} is either the single character @samp{0} or
5567 has the form @var{m.n} where the file number, @var{m}, is @samp{1} or
5568 @samp{2} and @var{n} is a positive field number.
5570 A field specification of @samp{0} denotes the join field.
5571 In most cases, the functionality of the @samp{0} field spec
5572 may be reproduced using the explicit @var{m.n} that corresponds
5573 to the join field. However, when printing unpairable lines
5574 (using either of the @option{-a} or @option{-v} options), there is no way
5575 to specify the join field using @var{m.n} in @var{field-list}
5576 if there are unpairable lines in both files.
5577 To give @command{join} that functionality, @acronym{POSIX} invented the @samp{0}
5578 field specification notation.
5580 The elements in @var{field-list}
5581 are separated by commas or blanks.
5582 Blank separators typically need to be quoted for the shell. For
5583 example, the commands @samp{join -o 1.2,2.2} and @samp{join -o '1.2
5584 2.2'} are equivalent.
5586 All output lines---including those printed because of any -a or -v
5587 option---are subject to the specified @var{field-list}.
5590 Use character @var{char} as the input and output field separator.
5591 Treat as significant each occurrence of @var{char} in the input file.
5592 Use @samp{sort -t @var{char}}, without the @option{-b} option of
5593 @samp{sort}, to produce this ordering. If @samp{join -t ''} is specified,
5594 the whole line is considered, matching the default operation of sort.
5595 If @samp{-t '\0'} is specified then the @acronym{ASCII} @sc{nul}
5596 character is used to delimit the fields.
5598 @item -v @var{file-number}
5599 Print a line for each unpairable line in file @var{file-number}
5600 (either @samp{1} or @samp{2}), instead of the normal output.
5607 @node Operating on characters
5608 @chapter Operating on characters
5610 @cindex operating on characters
5612 This commands operate on individual characters.
5615 * tr invocation:: Translate, squeeze, and/or delete characters.
5616 * expand invocation:: Convert tabs to spaces.
5617 * unexpand invocation:: Convert spaces to tabs.
5622 @section @command{tr}: Translate, squeeze, and/or delete characters
5629 tr [@var{option}]@dots{} @var{set1} [@var{set2}]
5632 @command{tr} copies standard input to standard output, performing
5633 one of the following operations:
5637 translate, and optionally squeeze repeated characters in the result,
5639 squeeze repeated characters,
5643 delete characters, then squeeze repeated characters from the result.
5646 The @var{set1} and (if given) @var{set2} arguments define ordered
5647 sets of characters, referred to below as @var{set1} and @var{set2}. These
5648 sets are the characters of the input that @command{tr} operates on.
5649 The @option{--complement} (@option{-c}, @option{-C}) option replaces
5651 complement (all of the characters that are not in @var{set1}).
5653 Currently @command{tr} fully supports only single-byte characters.
5654 Eventually it will support multibyte characters; when it does, the
5655 @option{-C} option will cause it to complement the set of characters,
5656 whereas @option{-c} will cause it to complement the set of values.
5657 This distinction will matter only when some values are not characters,
5658 and this is possible only in locales using multibyte encodings when
5659 the input contains encoding errors.
5661 The program accepts the @option{--help} and @option{--version}
5662 options. @xref{Common options}. Options must precede operands.
5667 * Character sets:: Specifying sets of characters.
5668 * Translating:: Changing one set of characters to another.
5669 * Squeezing:: Squeezing repeats and deleting.
5673 @node Character sets
5674 @subsection Specifying sets of characters
5676 @cindex specifying sets of characters
5678 The format of the @var{set1} and @var{set2} arguments resembles
5679 the format of regular expressions; however, they are not regular
5680 expressions, only lists of characters. Most characters simply
5681 represent themselves in these strings, but the strings can contain
5682 the shorthands listed below, for convenience. Some of them can be
5683 used only in @var{set1} or @var{set2}, as noted below.
5687 @item Backslash escapes
5688 @cindex backslash escapes
5690 The following backslash escape sequences are recognized:
5708 The character with the value given by @var{ooo}, which is 1 to 3
5714 While a backslash followed by a character not listed above is
5715 interpreted as that character, the backslash also effectively
5716 removes any special significance, so it is useful to escape
5717 @samp{[}, @samp{]}, @samp{*}, and @samp{-}.
5722 The notation @samp{@var{m}-@var{n}} expands to all of the characters
5723 from @var{m} through @var{n}, in ascending order. @var{m} should
5724 collate before @var{n}; if it doesn't, an error results. As an example,
5725 @samp{0-9} is the same as @samp{0123456789}.
5727 @sc{gnu} @command{tr} does not support the System V syntax that uses square
5728 brackets to enclose ranges. Translations specified in that format
5729 sometimes work as expected, since the brackets are often transliterated
5730 to themselves. However, they should be avoided because they sometimes
5731 behave unexpectedly. For example, @samp{tr -d '[0-9]'} deletes brackets
5734 Many historically common and even accepted uses of ranges are not
5735 portable. For example, on @acronym{EBCDIC} hosts using the @samp{A-Z}
5736 range will not do what most would expect because @samp{A} through @samp{Z}
5737 are not contiguous as they are in @acronym{ASCII}.
5738 If you can rely on a @acronym{POSIX} compliant version of @command{tr}, then
5739 the best way to work around this is to use character classes (see below).
5740 Otherwise, it is most portable (and most ugly) to enumerate the members
5743 @item Repeated characters
5744 @cindex repeated characters
5746 The notation @samp{[@var{c}*@var{n}]} in @var{set2} expands to @var{n}
5747 copies of character @var{c}. Thus, @samp{[y*6]} is the same as
5748 @samp{yyyyyy}. The notation @samp{[@var{c}*]} in @var{string2} expands
5749 to as many copies of @var{c} as are needed to make @var{set2} as long as
5750 @var{set1}. If @var{n} begins with @samp{0}, it is interpreted in
5751 octal, otherwise in decimal.
5753 @item Character classes
5754 @cindex character classes
5756 The notation @samp{[:@var{class}:]} expands to all of the characters in
5757 the (predefined) class @var{class}. The characters expand in no
5758 particular order, except for the @code{upper} and @code{lower} classes,
5759 which expand in ascending order. When the @option{--delete} (@option{-d})
5760 and @option{--squeeze-repeats} (@option{-s}) options are both given, any
5761 character class can be used in @var{set2}. Otherwise, only the
5762 character classes @code{lower} and @code{upper} are accepted in
5763 @var{set2}, and then only if the corresponding character class
5764 (@code{upper} and @code{lower}, respectively) is specified in the same
5765 relative position in @var{set1}. Doing this specifies case conversion.
5766 The class names are given below; an error results when an invalid class
5778 Horizontal whitespace.
5787 Printable characters, not including space.
5793 Printable characters, including space.
5796 Punctuation characters.
5799 Horizontal or vertical whitespace.
5808 @item Equivalence classes
5809 @cindex equivalence classes
5811 The syntax @samp{[=@var{c}=]} expands to all of the characters that are
5812 equivalent to @var{c}, in no particular order. Equivalence classes are
5813 a relatively recent invention intended to support non-English alphabets.
5814 But there seems to be no standard way to define them or determine their
5815 contents. Therefore, they are not fully implemented in @sc{gnu} @command{tr};
5816 each character's equivalence class consists only of that character,
5817 which is of no particular use.
5823 @subsection Translating
5825 @cindex translating characters
5827 @command{tr} performs translation when @var{set1} and @var{set2} are
5828 both given and the @option{--delete} (@option{-d}) option is not given.
5829 @command{tr} translates each character of its input that is in @var{set1}
5830 to the corresponding character in @var{set2}. Characters not in
5831 @var{set1} are passed through unchanged. When a character appears more
5832 than once in @var{set1} and the corresponding characters in @var{set2}
5833 are not all the same, only the final one is used. For example, these
5834 two commands are equivalent:
5841 A common use of @command{tr} is to convert lowercase characters to
5842 uppercase. This can be done in many ways. Here are three of them:
5845 tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
5847 tr '[:lower:]' '[:upper:]'
5851 But note that using ranges like @code{a-z} above is not portable.
5853 When @command{tr} is performing translation, @var{set1} and @var{set2}
5854 typically have the same length. If @var{set1} is shorter than
5855 @var{set2}, the extra characters at the end of @var{set2} are ignored.
5857 On the other hand, making @var{set1} longer than @var{set2} is not
5858 portable; @acronym{POSIX} says that the result is undefined. In this situation,
5859 BSD @command{tr} pads @var{set2} to the length of @var{set1} by repeating
5860 the last character of @var{set2} as many times as necessary. System V
5861 @command{tr} truncates @var{set1} to the length of @var{set2}.
5863 By default, @sc{gnu} @command{tr} handles this case like BSD @command{tr}.
5864 When the @option{--truncate-set1} (@option{-t}) option is given,
5865 @sc{gnu} @command{tr} handles this case like the System V @command{tr}
5866 instead. This option is ignored for operations other than translation.
5868 Acting like System V @command{tr} in this case breaks the relatively common
5872 tr -cs A-Za-z0-9 '\012'
5876 because it converts only zero bytes (the first element in the
5877 complement of @var{set1}), rather than all non-alphanumerics, to
5881 By the way, the above idiom is not portable because it uses ranges, and
5882 it assumes that the octal code for newline is 012.
5883 Assuming a @acronym{POSIX} compliant @command{tr}, here is a better way to write it:
5886 tr -cs '[:alnum:]' '[\n*]'
5891 @subsection Squeezing repeats and deleting
5893 @cindex squeezing repeat characters
5894 @cindex deleting characters
5896 When given just the @option{--delete} (@option{-d}) option, @command{tr}
5897 removes any input characters that are in @var{set1}.
5899 When given just the @option{--squeeze-repeats} (@option{-s}) option,
5900 @command{tr} replaces each input sequence of a repeated character that
5901 is in @var{set1} with a single occurrence of that character.
5903 When given both @option{--delete} and @option{--squeeze-repeats}, @command{tr}
5904 first performs any deletions using @var{set1}, then squeezes repeats
5905 from any remaining characters using @var{set2}.
5907 The @option{--squeeze-repeats} option may also be used when translating,
5908 in which case @command{tr} first performs translation, then squeezes
5909 repeats from any remaining characters using @var{set2}.
5911 Here are some examples to illustrate various combinations of options:
5916 Remove all zero bytes:
5923 Put all words on lines by themselves. This converts all
5924 non-alphanumeric characters to newlines, then squeezes each string
5925 of repeated newlines into a single newline:
5928 tr -cs '[:alnum:]' '[\n*]'
5932 Convert each sequence of repeated newlines to a single newline:
5939 Find doubled occurrences of words in a document.
5940 @c Separate the following two "the"s, so typo checkers don't complain.
5941 For example, people often write ``the @w{}the'' with the repeated words
5942 separated by a newline. The Bourne shell script below works first
5943 by converting each sequence of punctuation and blank characters to a
5944 single newline. That puts each ``word'' on a line by itself.
5945 Next it maps all uppercase characters to lower case, and finally it
5946 runs @command{uniq} with the @option{-d} option to print out only the words
5952 | tr -s '[:punct:][:blank:]' '[\n*]' \
5953 | tr '[:upper:]' '[:lower:]' \
5958 Deleting a small set of characters is usually straightforward. For example,
5959 to remove all @samp{a}s, @samp{x}s, and @samp{M}s you would do this:
5965 However, when @samp{-} is one of those characters, it can be tricky because
5966 @samp{-} has special meanings. Performing the same task as above but also
5967 removing all @samp{-} characters, we might try @code{tr -d -axM}, but
5968 that would fail because @command{tr} would try to interpret @option{-a} as
5969 a command-line option. Alternatively, we could try putting the hyphen
5970 inside the string, @code{tr -d a-xM}, but that wouldn't work either because
5971 it would make @command{tr} interpret @code{a-x} as the range of characters
5972 @samp{a}@dots{}@samp{x} rather than the three.
5973 One way to solve the problem is to put the hyphen at the end of the list
5980 Or you can use @samp{--} to terminate option processing:
5986 More generally, use the character class notation @code{[=c=]}
5987 with @samp{-} (or any other character) in place of the @samp{c}:
5993 Note how single quotes are used in the above example to protect the
5994 square brackets from interpretation by a shell.
5999 @node expand invocation
6000 @section @command{expand}: Convert tabs to spaces
6003 @cindex tabs to spaces, converting
6004 @cindex converting tabs to spaces
6006 @command{expand} writes the contents of each given @var{file}, or standard
6007 input if none are given or for a @var{file} of @samp{-}, to standard
6008 output, with tab characters converted to the appropriate number of
6012 expand [@var{option}]@dots{} [@var{file}]@dots{}
6015 By default, @command{expand} converts all tabs to spaces. It preserves
6016 backspace characters in the output; they decrement the column count for
6017 tab calculations. The default action is equivalent to @option{-t 8} (set
6018 tabs every 8 columns).
6020 The program accepts the following options. Also see @ref{Common options}.
6024 @item -t @var{tab1}[,@var{tab2}]@dots{}
6025 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6028 @cindex tab stops, setting
6029 If only one tab stop is given, set the tabs @var{tab1} spaces apart
6030 (default is 8). Otherwise, set the tabs at columns @var{tab1},
6031 @var{tab2}, @dots{} (numbered from 0), and replace any tabs beyond the
6032 last tab stop given with single spaces. Tab stops can be separated by
6033 blanks as well as by commas.
6035 For compatibility, GNU @command{expand} also accepts the obsolete
6036 option syntax, @option{-@var{t1}[,@var{t2}]@dots{}}. New scripts
6037 should use @option{-t @var{t1}[,@var{t2}]@dots{}} instead.
6043 @cindex initial tabs, converting
6044 Only convert initial tabs (those that precede all non-space or non-tab
6045 characters) on each line to spaces.
6052 @node unexpand invocation
6053 @section @command{unexpand}: Convert spaces to tabs
6057 @command{unexpand} writes the contents of each given @var{file}, or
6058 standard input if none are given or for a @var{file} of @samp{-}, to
6059 standard output, converting blanks at the beginning of each line into
6060 as many tab characters as needed. In the default @acronym{POSIX}
6061 locale, a @dfn{blank} is a space or a tab; other locales may specify
6062 additional blank characters. Synopsis:
6065 unexpand [@var{option}]@dots{} [@var{file}]@dots{}
6068 By default, @command{unexpand} converts only initial blanks (those
6069 that precede all non-blank characters) on each line. It
6070 preserves backspace characters in the output; they decrement the column
6071 count for tab calculations. By default, tabs are set at every 8th
6074 The program accepts the following options. Also see @ref{Common options}.
6078 @item -t @var{tab1}[,@var{tab2}]@dots{}
6079 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6082 If only one tab stop is given, set the tabs @var{tab1} columns apart
6083 instead of the default 8. Otherwise, set the tabs at columns
6084 @var{tab1}, @var{tab2}, @dots{} (numbered from 0), and leave blanks
6085 beyond the tab stops given unchanged. Tab stops can be separated by
6086 blanks as well as by commas. This option implies the @option{-a} option.
6088 For compatibility, GNU @command{unexpand} supports the obsolete option syntax,
6089 @option{-@var{tab1}[,@var{tab2}]@dots{}}, where tab stops must be
6090 separated by commas. (Unlike @option{-t}, this obsolete option does
6091 not imply @option{-a}.) New scripts should use @option{--first-only -t
6092 @var{tab1}[,@var{tab2}]@dots{}} instead.
6098 Also convert all sequences of two or more blanks just before a tab stop,
6099 even if they occur after non-blank characters in a line.
6106 @node Directory listing
6107 @chapter Directory listing
6109 This chapter describes the @command{ls} command and its variants @command{dir}
6110 and @command{vdir}, which list information about files.
6113 * ls invocation:: List directory contents.
6114 * dir invocation:: Briefly ls.
6115 * vdir invocation:: Verbosely ls.
6116 * dircolors invocation:: Color setup for ls, etc.
6121 @section @command{ls}: List directory contents
6124 @cindex directory listing
6126 The @command{ls} program lists information about files (of any type,
6127 including directories). Options and file arguments can be intermixed
6128 arbitrarily, as usual.
6130 For non-option command-line arguments that are directories, by default
6131 @command{ls} lists the contents of directories, not recursively, and
6132 omitting files with names beginning with @samp{.}. For other non-option
6133 arguments, by default @command{ls} lists just the file name. If no
6134 non-option argument is specified, @command{ls} operates on the current
6135 directory, acting as if it had been invoked with a single argument of @samp{.}.
6138 By default, the output is sorted alphabetically, according to the locale
6139 settings in effect.@footnote{If you use a non-@acronym{POSIX}
6140 locale (e.g., by setting @env{LC_ALL} to @samp{en_US}), then @command{ls} may
6141 produce output that is sorted differently than you're accustomed to.
6142 In that case, set the @env{LC_ALL} environment variable to @samp{C}.}
6143 If standard output is
6144 a terminal, the output is in columns (sorted vertically) and control
6145 characters are output as question marks; otherwise, the output is listed
6146 one per line and control characters are output as-is.
6148 Because @command{ls} is such a fundamental program, it has accumulated many
6149 options over the years. They are described in the subsections below;
6150 within each section, options are listed alphabetically (ignoring case).
6151 The division of options into the subsections is not absolute, since some
6152 options affect more than one aspect of @command{ls}'s operation.
6154 @cindex exit status of @command{ls}
6159 1 minor problems (e.g., failure to access a file or directory not
6160 specified as a command line argument. This happens when listing a
6161 directory in which entries are actively being removed or renamed.)
6162 2 serious trouble (e.g., memory exhausted, invalid option, failure
6163 to access a file or directory specified as a command line argument
6164 or a directory loop)
6167 Also see @ref{Common options}.
6170 * Which files are listed::
6171 * What information is listed::
6172 * Sorting the output::
6173 * Details about version sort::
6174 * General output formatting::
6175 * Formatting file timestamps::
6176 * Formatting the file names::
6180 @node Which files are listed
6181 @subsection Which files are listed
6183 These options determine which files @command{ls} lists information for.
6184 By default, @command{ls} lists files and the contents of any
6185 directories on the command line, except that in directories it ignores
6186 files whose names start with @samp{.}.
6194 In directories, do not ignore file names that start with @samp{.}.
6199 @opindex --almost-all
6200 In directories, do not ignore all file names that start with @samp{.};
6201 ignore only @file{.} and @file{..}. The @option{--all} (@option{-a})
6202 option overrides this option.
6205 @itemx --ignore-backups
6207 @opindex --ignore-backups
6208 @cindex backup files, ignoring
6209 In directories, ignore files that end with @samp{~}. This option is
6210 equivalent to @samp{--ignore='*~' --ignore='.*~'}.
6215 @opindex --directory
6216 List just the names of directories, as with other types of files, rather
6217 than listing their contents.
6218 @c The following sentence is the same as the one for -F.
6219 Do not follow symbolic links listed on the
6220 command line unless the @option{--dereference-command-line} (@option{-H}),
6221 @option{--dereference} (@option{-L}), or
6222 @option{--dereference-command-line-symlink-to-dir} options are specified.
6225 @itemx --dereference-command-line
6227 @opindex --dereference-command-line
6228 @cindex symbolic links, dereferencing
6229 If a command line argument specifies a symbolic link, show information
6230 for the file the link references rather than for the link itself.
6232 @itemx --dereference-command-line-symlink-to-dir
6233 @opindex --dereference-command-line-symlink-to-dir
6234 @cindex symbolic links, dereferencing
6235 Do not dereference symbolic links, with one exception:
6236 if a command line argument specifies a symbolic link that refers to
6237 a directory, show information for that directory rather than for the
6239 This is the default behavior when no other dereferencing-related
6240 option has been specified (@option{--classify} (@option{-F}),
6241 @option{--directory} (@option{-d}),
6243 @option{--dereference} (@option{-L}), or
6244 @option{--dereference-command-line} (@option{-H})).
6246 @item --group-directories-first
6247 @opindex --group-directories-first
6248 Group all the directories before the files and then sort the
6249 directories and the files separately using the selected sort key
6250 (see --sort option).
6251 That is, this option specifies a primary sort key,
6252 and the --sort option specifies a secondary key.
6253 However, any use of @option{--sort=none}
6254 (@option{-U}) disables this option altogether.
6256 @item --hide=PATTERN
6257 @opindex --hide=@var{pattern}
6258 In directories, ignore files whose names match the shell pattern
6259 @var{pattern}, unless the @option{--all} (@option{-a}) or
6260 @option{--almost-all} (@option{-A}) is also given. This
6261 option acts like @option{--ignore=@var{pattern}} except that it has no
6262 effect if @option{--all} (@option{-a}) or @option{--almost-all}
6263 (@option{-A}) is also given.
6265 This option can be useful in shell aliases. For example, if
6266 @command{lx} is an alias for @samp{ls --hide='*~'} and @command{ly} is
6267 an alias for @samp{ls --ignore='*~'}, then the command @samp{lx -A}
6268 lists the file @file{README~} even though @samp{ly -A} would not.
6270 @item -I @var{pattern}
6271 @itemx --ignore=@var{pattern}
6273 @opindex --ignore=@var{pattern}
6274 In directories, ignore files whose names match the shell pattern
6275 (not regular expression) @var{pattern}. As
6276 in the shell, an initial @samp{.} in a file name does not match a
6277 wildcard at the start of @var{pattern}. Sometimes it is useful
6278 to give this option several times. For example,
6281 $ ls --ignore='.??*' --ignore='.[^.]' --ignore='#*'
6284 The first option ignores names of length 3 or more that start with @samp{.},
6285 the second ignores all two-character names that start with @samp{.}
6286 except @samp{..}, and the third ignores names that start with @samp{#}.
6289 @itemx --dereference
6291 @opindex --dereference
6292 @cindex symbolic links, dereferencing
6293 When showing file information for a symbolic link, show information
6294 for the file the link references rather than the link itself.
6295 However, even with this option, @command{ls} still prints the name
6296 of the link itself, not the name of the file that the link points to.
6301 @opindex --recursive
6302 @cindex recursive directory listing
6303 @cindex directory listing, recursive
6304 List the contents of all directories recursively.
6309 @node What information is listed
6310 @subsection What information is listed
6312 These options affect the information that @command{ls} displays. By
6313 default, only file names are shown.
6319 @cindex hurd, author, printing
6320 List each file's author when producing long format directory listings.
6321 In GNU/Hurd, file authors can differ from their owners, but in other
6322 operating systems the two are the same.
6328 @cindex dired Emacs mode support
6329 With the long listing (@option{-l}) format, print an additional line after
6333 //DIRED// @var{beg1} @var{end1} @var{beg2} @var{end2} @dots{}
6337 The @var{begn} and @var{endn} are unsigned integers that record the
6338 byte position of the beginning and end of each file name in the output.
6339 This makes it easy for Emacs to find the names, even when they contain
6340 unusual characters such as space or newline, without fancy searching.
6342 If directories are being listed recursively (@option{-R}), output a similar
6343 line with offsets for each subdirectory name:
6346 //SUBDIRED// @var{beg1} @var{end1} @dots{}
6349 Finally, output a line of the form:
6352 //DIRED-OPTIONS// --quoting-style=@var{word}
6356 where @var{word} is the quoting style (@pxref{Formatting the file names}).
6358 Here is an actual example:
6361 $ mkdir -p a/sub/deeper a/sub2
6363 $ touch a/sub/deeper/file
6364 $ ls -gloRF --dired a
6367 -rw-r--r-- 1 0 Jun 10 12:27 f1
6368 -rw-r--r-- 1 0 Jun 10 12:27 f2
6369 drwxr-xr-x 3 4096 Jun 10 12:27 sub/
6370 drwxr-xr-x 2 4096 Jun 10 12:27 sub2/
6374 drwxr-xr-x 2 4096 Jun 10 12:27 deeper/
6378 -rw-r--r-- 1 0 Jun 10 12:27 file
6382 //DIRED// 48 50 84 86 120 123 158 162 217 223 282 286
6383 //SUBDIRED// 2 3 167 172 228 240 290 296
6384 //DIRED-OPTIONS// --quoting-style=literal
6387 Note that the pairs of offsets on the @samp{//DIRED//} line above delimit
6388 these names: @file{f1}, @file{f2}, @file{sub}, @file{sub2}, @file{deeper},
6390 The offsets on the @samp{//SUBDIRED//} line delimit the following
6391 directory names: @file{a}, @file{a/sub}, @file{a/sub/deeper}, @file{a/sub2}.
6393 Here is an example of how to extract the fifth entry name, @samp{deeper},
6394 corresponding to the pair of offsets, 222 and 228:
6397 $ ls -gloRF --dired a > out
6398 $ dd bs=1 skip=222 count=6 < out 2>/dev/null; echo
6402 Note that although the listing above includes a trailing slash
6403 for the @samp{deeper} entry, the offsets select the name without
6404 the trailing slash. However, if you invoke @command{ls} with @option{--dired}
6405 along with an option like @option{--escape} (aka @option{-b}) and operate
6406 on a file whose name contains special characters, notice that the backslash
6411 $ ls -blog --dired 'a b'
6412 -rw-r--r-- 1 0 Jun 10 12:28 a\ b
6414 //DIRED-OPTIONS// --quoting-style=escape
6417 If you use a quoting style that adds quote marks
6418 (e.g., @option{--quoting-style=c}), then the offsets include the quote marks.
6419 So beware that the user may select the quoting style via the environment
6420 variable @env{QUOTING_STYLE}. Hence, applications using @option{--dired}
6421 should either specify an explicit @option{--quoting-style=literal} option
6422 (aka @option{-N} or @option{--literal}) on the command line, or else be
6423 prepared to parse the escaped names.
6426 @opindex --full-time
6427 Produce long format directory listings, and list times in full. It is
6428 equivalent to using @option{--format=long} with
6429 @option{--time-style=full-iso} (@pxref{Formatting file timestamps}).
6433 Produce long format directory listings, but don't display owner information.
6439 Inhibit display of group information in a long format directory listing.
6440 (This is the default in some non-@sc{gnu} versions of @command{ls}, so we
6441 provide this option for compatibility.)
6449 @cindex inode number, printing
6450 Print the inode number (also called the file serial number and index
6451 number) of each file to the left of the file name. (This number
6452 uniquely identifies each file within a particular file system.)
6455 @itemx --format=long
6456 @itemx --format=verbose
6459 @opindex long ls @r{format}
6460 @opindex verbose ls @r{format}
6461 In addition to the name of each file, print the file type, file mode bits,
6462 number of hard links, owner name, group name, size, and
6463 timestamp (@pxref{Formatting file timestamps}), normally
6464 the modification time. Print question marks for information that
6465 cannot be determined.
6467 Normally the size is printed as a byte count without punctuation, but
6468 this can be overridden (@pxref{Block size}). For example, @option{-h}
6469 prints an abbreviated, human-readable count, and
6470 @samp{--block-size="'1"} prints a byte count with the thousands
6471 separator of the current locale.
6473 For each directory that is listed, preface the files with a line
6474 @samp{total @var{blocks}}, where @var{blocks} is the total disk allocation
6475 for all files in that directory. The block size currently defaults to 1024
6476 bytes, but this can be overridden (@pxref{Block size}).
6477 The @var{blocks} computed counts each hard link separately;
6478 this is arguably a deficiency.
6480 The file type is one of the following characters:
6482 @c The commented-out entries are ones we're not sure about.
6490 character special file
6492 high performance (``contiguous data'') file
6496 door (Solaris 2.5 and up)
6498 @c semaphore, if this is a distinct file type
6502 @c multiplexed file (7th edition Unix; obsolete)
6504 off-line (``migrated'') file (Cray DMF)
6506 network special file (HP-UX)
6510 port (Solaris 10 and up)
6512 @c message queue, if this is a distinct file type
6516 @c shared memory object, if this is a distinct file type
6518 @c typed memory object, if this is a distinct file type
6520 @c whiteout (4.4BSD; not implemented)
6522 some other file type
6525 @cindex permissions, output by @command{ls}
6526 The file mode bits listed are similar to symbolic mode specifications
6527 (@pxref{Symbolic Modes}). But @command{ls} combines multiple bits into the
6528 third character of each set of permissions as follows:
6532 If the set-user-ID or set-group-ID bit and the corresponding executable bit
6536 If the set-user-ID or set-group-ID bit is set but the corresponding
6537 executable bit is not set.
6540 If the restricted deletion flag or sticky bit, and the
6541 other-executable bit, are both set. The restricted deletion flag is
6542 another name for the sticky bit. @xref{Mode Structure}.
6545 If the restricted deletion flag or sticky bit is set but the
6546 other-executable bit is not set.
6549 If the executable bit is set and none of the above apply.
6555 Following the file mode bits is a single character that specifies
6556 whether an alternate access method such as an access control list
6557 applies to the file. When the character following the file mode bits is a
6558 space, there is no alternate access method. When it is a printing
6559 character, then there is such a method.
6561 GNU @command{ls} uses a @samp{.} character to indicate a file
6562 with an SELinux security context, but no other alternate access method.
6564 A file with any other combination of alternate access methods
6565 is marked with a @samp{+} character.
6568 @itemx --numeric-uid-gid
6570 @opindex --numeric-uid-gid
6571 @cindex numeric uid and gid
6572 @cindex numeric user and group IDs
6573 Produce long format directory listings, but
6574 display numeric user and group IDs instead of the owner and group names.
6578 Produce long format directory listings, but don't display group information.
6579 It is equivalent to using @option{--format=long} with @option{--no-group} .
6585 @cindex disk allocation
6586 @cindex size of files, reporting
6587 Print the disk allocation of each file to the left of the file name.
6588 This is the amount of disk space used by the file, which is usually a
6589 bit more than the file's size, but it can be less if the file has holes.
6591 Normally the disk allocation is printed in units of
6592 1024 bytes, but this can be overridden (@pxref{Block size}).
6594 @cindex NFS mounts from BSD to HP-UX
6595 For files that are NFS-mounted from an HP-UX system to a BSD system,
6596 this option reports sizes that are half the correct values. On HP-UX
6597 systems, it reports sizes that are twice the correct values for files
6598 that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX;
6599 it also affects the HP-UX @command{ls} program.
6608 @cindex security context
6609 Display the SELinux security context or @samp{?} if none is found.
6610 When used with the @option{-l} option, print the security context
6611 to the left of the size column.
6616 @node Sorting the output
6617 @subsection Sorting the output
6619 @cindex sorting @command{ls} output
6620 These options change the order in which @command{ls} sorts the information
6621 it outputs. By default, sorting is done by character code
6622 (e.g., @acronym{ASCII} order).
6628 @itemx --time=status
6631 @opindex ctime@r{, printing or sorting by}
6632 @opindex status time@r{, printing or sorting by}
6633 @opindex use time@r{, printing or sorting files by}
6634 If the long listing format (e.g., @option{-l}, @option{-o}) is being used,
6635 print the status change time (the @samp{ctime} in the inode) instead of
6636 the modification time.
6637 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6638 or when not using a long listing format,
6639 sort according to the status change time.
6643 @cindex unsorted directory listing
6644 @cindex directory order, listing by
6645 Primarily, like @option{-U}---do not sort; list the files in whatever
6646 order they are stored in the directory. But also enable @option{-a} (list
6647 all files) and disable @option{-l}, @option{--color}, and @option{-s} (if they
6648 were specified before the @option{-f}).
6654 @cindex reverse sorting
6655 Reverse whatever the sorting method is---e.g., list files in reverse
6656 alphabetical order, youngest first, smallest first, or whatever.
6662 @opindex size of files@r{, sorting files by}
6663 Sort by file size, largest first.
6669 @opindex modification time@r{, sorting files by}
6670 Sort by modification time (the @samp{mtime} in the inode), newest first.
6674 @itemx --time=access
6678 @opindex use time@r{, printing or sorting files by}
6679 @opindex atime@r{, printing or sorting files by}
6680 @opindex access time@r{, printing or sorting files by}
6681 If the long listing format (e.g., @option{--format=long}) is being used,
6682 print the last access time (the @samp{atime} in the inode).
6683 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6684 or when not using a long listing format, sort according to the access time.
6690 @opindex none@r{, sorting option for @command{ls}}
6691 Do not sort; list the files in whatever order they are
6692 stored in the directory. (Do not do any of the other unrelated things
6693 that @option{-f} does.) This is especially useful when listing very large
6694 directories, since not doing any sorting can be noticeably faster.
6697 @itemx --sort=version
6700 @opindex version@r{, sorting option for @command{ls}}
6701 Sort by version name and number, lowest first. It behaves like a default
6702 sort, except that each sequence of decimal digits is treated numerically
6703 as an index/version number. (@xref{Details about version sort}.)
6706 @itemx --sort=extension
6709 @opindex extension@r{, sorting files by}
6710 Sort directory contents alphabetically by file extension (characters
6711 after the last @samp{.}); files with no extension are sorted first.
6716 @node Details about version sort
6717 @subsection Details about version sort
6719 Version sorting handles the fact that file names frequently include indices or
6720 version numbers. Standard sorting usually does not produce the order that one
6721 expects because comparisons are made on a character-by-character basis.
6722 Version sorting is especially useful when browsing directories that contain
6723 many files with indices/version numbers in their names:
6727 abc.zml-1.gz abc.zml-1.gz
6728 abc.zml-12.gz abc.zml-2.gz
6729 abc.zml-2.gz abc.zml-12.gz
6732 Version-sorted strings are compared such that if @var{ver1} and @var{ver2}
6733 are version numbers and @var{prefix} and @var{suffix} (@var{suffix} matching
6734 the regular expression @samp{(\.[A-Za-z~][A-Za-z0-9~]*)*}) are strings then
6735 @var{ver1} < @var{ver2} implies that the name composed of
6736 ``@var{prefix} @var{ver1} @var{suffix}'' sorts before
6737 ``@var{prefix} @var{ver2} @var{suffix}''.
6739 Note also that leading zeros of numeric parts are ignored:
6743 abc-1.007.tgz abc-1.01a.tgz
6744 abc-1.012b.tgz abc-1.007.tgz
6745 abc-1.01a.tgz abc-1.012b.tgz
6748 This functionality is implemented using gnulib's @code{filevercmp} function,
6749 which has some caveats worth noting.
6752 @item @env{LC_COLLATE} is ignored, which means @samp{ls -v} and @samp{sort -V}
6753 will sort non-numeric prefixes as if the @env{LC_COLLATE} locale category
6754 was set to @samp{C}.
6755 @item Some suffixes will not be matched by the regular
6756 expression mentioned above. Consequently these examples may
6757 not sort as you expect:
6765 abc-1.2.3.4.x86_64.rpm
6766 abc-1.2.3.x86_64.rpm
6770 @node General output formatting
6771 @subsection General output formatting
6773 These options affect the appearance of the overall output.
6778 @itemx --format=single-column
6781 @opindex single-column @r{output of files}
6782 List one file per line. This is the default for @command{ls} when standard
6783 output is not a terminal.
6786 @itemx --format=vertical
6789 @opindex vertical @r{sorted files in columns}
6790 List files in columns, sorted vertically. This is the default for
6791 @command{ls} if standard output is a terminal. It is always the default
6792 for the @command{dir} program.
6793 @sc{gnu} @command{ls} uses variable width columns to display as many files as
6794 possible in the fewest lines.
6796 @item --color [=@var{when}]
6798 @cindex color, distinguishing file types with
6799 Specify whether to use color for distinguishing file types. @var{when}
6800 may be omitted, or one of:
6803 @vindex none @r{color option}
6804 - Do not use color at all. This is the default.
6806 @vindex auto @r{color option}
6807 @cindex terminal, using color iff
6808 - Only use color if standard output is a terminal.
6810 @vindex always @r{color option}
6813 Specifying @option{--color} and no @var{when} is equivalent to
6814 @option{--color=always}.
6815 Piping a colorized listing through a pager like @command{more} or
6816 @command{less} usually produces unreadable results. However, using
6817 @code{more -f} does seem to work.
6821 @itemx --indicator-style=classify
6824 @opindex --indicator-style
6825 @cindex file type and executables, marking
6826 @cindex executables and file type, marking
6827 Append a character to each file name indicating the file type. Also,
6828 for regular files that are executable, append @samp{*}. The file type
6829 indicators are @samp{/} for directories, @samp{@@} for symbolic links,
6830 @samp{|} for FIFOs, @samp{=} for sockets, @samp{>} for doors,
6831 and nothing for regular files.
6832 @c The following sentence is the same as the one for -d.
6833 Do not follow symbolic links listed on the
6834 command line unless the @option{--dereference-command-line} (@option{-H}),
6835 @option{--dereference} (@option{-L}), or
6836 @option{--dereference-command-line-symlink-to-dir} options are specified.
6839 @itemx --indicator-style=file-type
6840 @opindex --file-type
6841 @opindex --indicator-style
6842 @cindex file type, marking
6843 Append a character to each file name indicating the file type. This is
6844 like @option{-F}, except that executables are not marked.
6846 @item --indicator-style=@var{word}
6847 @opindex --indicator-style
6848 Append a character indicator with style @var{word} to entry names,
6853 Do not append any character indicator; this is the default.
6855 Append @samp{/} for directories. This is the same as the @option{-p}
6858 Append @samp{/} for directories, @samp{@@} for symbolic links, @samp{|}
6859 for FIFOs, @samp{=} for sockets, and nothing for regular files. This is
6860 the same as the @option{--file-type} option.
6862 Append @samp{*} for executable regular files, otherwise behave as for
6863 @samp{file-type}. This is the same as the @option{-F} or
6864 @option{--classify} option.
6869 Print file sizes in 1024-byte blocks, overriding the default block
6870 size (@pxref{Block size}).
6871 This option is equivalent to @option{--block-size=1K}.
6874 @itemx --format=commas
6877 @opindex commas@r{, outputting between files}
6878 List files horizontally, with as many as will fit on each line,
6879 separated by @samp{, } (a comma and a space).
6882 @itemx --indicator-style=slash
6884 @opindex --indicator-style
6885 @cindex file type, marking
6886 Append a @samp{/} to directory names.
6889 @itemx --format=across
6890 @itemx --format=horizontal
6893 @opindex across@r{, listing files}
6894 @opindex horizontal@r{, listing files}
6895 List the files in columns, sorted horizontally.
6898 @itemx --tabsize=@var{cols}
6901 Assume that each tab stop is @var{cols} columns wide. The default is 8.
6902 @command{ls} uses tabs where possible in the output, for efficiency. If
6903 @var{cols} is zero, do not use tabs at all.
6905 @c FIXME: remove in 2009, if Apple Terminal has been fixed for long enough.
6906 Some terminal emulators (at least Apple Terminal 1.5 (133) from Mac OS X 10.4.8)
6907 do not properly align columns to the right of a TAB following a
6908 non-@acronym{ASCII} byte. If you use such a terminal emulator, use the
6909 @option{-T0} option or put @code{TABSIZE=0} in your environment to tell
6910 @command{ls} to align using spaces, not tabs.
6913 @itemx --width=@var{cols}
6917 Assume the screen is @var{cols} columns wide. The default is taken
6918 from the terminal settings if possible; otherwise the environment
6919 variable @env{COLUMNS} is used if it is set; otherwise the default
6925 @node Formatting file timestamps
6926 @subsection Formatting file timestamps
6928 By default, file timestamps are listed in abbreviated form. Most
6929 locales use a timestamp like @samp{2002-03-30 23:45}. However, the
6930 default @acronym{POSIX} locale uses a date like @samp{Mar 30@ @ 2002}
6931 for non-recent timestamps, and a date-without-year and time like
6932 @samp{Mar 30 23:45} for recent timestamps.
6934 A timestamp is considered to be @dfn{recent} if it is less than six
6935 months old, and is not dated in the future. If a timestamp dated
6936 today is not listed in recent form, the timestamp is in the future,
6937 which means you probably have clock skew problems which may break
6938 programs like @command{make} that rely on file timestamps.
6941 Time stamps are listed according to the time zone rules specified by
6942 the @env{TZ} environment variable, or by the system default rules if
6943 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
6944 with @env{TZ}, libc, The GNU C Library Reference Manual}.
6946 The following option changes how file timestamps are printed.
6949 @item --time-style=@var{style}
6950 @opindex --time-style
6952 List timestamps in style @var{style}. The @var{style} should
6953 be one of the following:
6958 List timestamps using @var{format}, where @var{format} is interpreted
6959 like the format argument of @command{date} (@pxref{date invocation}).
6960 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
6961 @command{ls} to list timestamps like @samp{2002-03-30 23:45:56}. As
6962 with @command{date}, @var{format}'s interpretation is affected by the
6963 @env{LC_TIME} locale category.
6965 If @var{format} contains two format strings separated by a newline,
6966 the former is used for non-recent files and the latter for recent
6967 files; if you want output columns to line up, you may need to insert
6968 spaces in one of the two formats.
6971 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
6972 format with nanosecond precision, e.g., @samp{2002-03-30
6973 23:45:56.477817180 -0700}. This style is equivalent to
6974 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
6976 This is useful because the time output includes all the information that
6977 is available from the operating system. For example, this can help
6978 explain @command{make}'s behavior, since @acronym{GNU} @command{make}
6979 uses the full timestamp to determine whether a file is out of date.
6982 List @acronym{ISO} 8601 date and time in minutes, e.g.,
6983 @samp{2002-03-30 23:45}. These timestamps are shorter than
6984 @samp{full-iso} timestamps, and are usually good enough for everyday
6985 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
6988 List @acronym{ISO} 8601 dates for non-recent timestamps (e.g.,
6989 @samp{2002-03-30@ }), and @acronym{ISO} 8601 month, day, hour, and
6990 minute for recent timestamps (e.g., @samp{03-30 23:45}). These
6991 timestamps are uglier than @samp{long-iso} timestamps, but they carry
6992 nearly the same information in a smaller space and their brevity helps
6993 @command{ls} output fit within traditional 80-column output lines.
6994 The following two @command{ls} invocations are equivalent:
6999 ls -l --time-style="+%Y-%m-%d $newline%m-%d %H:%M"
7000 ls -l --time-style="iso"
7005 List timestamps in a locale-dependent form. For example, a Finnish
7006 locale might list non-recent timestamps like @samp{maalis 30@ @ 2002}
7007 and recent timestamps like @samp{maalis 30 23:45}. Locale-dependent
7008 timestamps typically consume more space than @samp{iso} timestamps and
7009 are harder for programs to parse because locale conventions vary so
7010 widely, but they are easier for many people to read.
7012 The @env{LC_TIME} locale category specifies the timestamp format. The
7013 default @acronym{POSIX} locale uses timestamps like @samp{Mar 30@
7014 @ 2002} and @samp{Mar 30 23:45}; in this locale, the following two
7015 @command{ls} invocations are equivalent:
7020 ls -l --time-style="+%b %e %Y$newline%b %e %H:%M"
7021 ls -l --time-style="locale"
7024 Other locales behave differently. For example, in a German locale,
7025 @option{--time-style="locale"} might be equivalent to
7026 @option{--time-style="+%e. %b %Y $newline%e. %b %H:%M"}
7027 and might generate timestamps like @samp{30. M@"ar 2002@ } and
7028 @samp{30. M@"ar 23:45}.
7030 @item posix-@var{style}
7032 List @acronym{POSIX}-locale timestamps if the @env{LC_TIME} locale
7033 category is @acronym{POSIX}, @var{style} timestamps otherwise. For
7034 example, the @samp{posix-long-iso} style lists
7035 timestamps like @samp{Mar 30@ @ 2002} and @samp{Mar 30 23:45} when in
7036 the @acronym{POSIX} locale, and like @samp{2002-03-30 23:45} otherwise.
7041 You can specify the default value of the @option{--time-style} option
7042 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
7043 the default style is @samp{locale}. @acronym{GNU} Emacs 21.3 and
7044 later use the @option{--dired} option and therefore can parse any date
7045 format, but if you are using Emacs 21.1 or 21.2 and specify a
7046 non-@acronym{POSIX} locale you may need to set
7047 @samp{TIME_STYLE="posix-long-iso"}.
7049 To avoid certain denial-of-service attacks, timestamps that would be
7050 longer than 1000 bytes may be treated as errors.
7053 @node Formatting the file names
7054 @subsection Formatting the file names
7056 These options change how file names themselves are printed.
7062 @itemx --quoting-style=escape
7065 @opindex --quoting-style
7066 @cindex backslash sequences for file names
7067 Quote nongraphic characters in file names using alphabetic and octal
7068 backslash sequences like those used in C.
7072 @itemx --quoting-style=literal
7075 @opindex --quoting-style
7076 Do not quote file names. However, with @command{ls} nongraphic
7077 characters are still printed as question marks if the output is a
7078 terminal and you do not specify the @option{--show-control-chars}
7082 @itemx --hide-control-chars
7084 @opindex --hide-control-chars
7085 Print question marks instead of nongraphic characters in file names.
7086 This is the default if the output is a terminal and the program is
7091 @itemx --quoting-style=c
7093 @opindex --quote-name
7094 @opindex --quoting-style
7095 Enclose file names in double quotes and quote nongraphic characters as
7098 @item --quoting-style=@var{word}
7099 @opindex --quoting-style
7100 @cindex quoting style
7101 Use style @var{word} to quote file names and other strings that may
7102 contain arbitrary characters. The @var{word} should
7103 be one of the following:
7107 Output strings as-is; this is the same as the @option{-N} or
7108 @option{--literal} option.
7110 Quote strings for the shell if they contain shell metacharacters or would
7111 cause ambiguous output.
7112 The quoting is suitable for @acronym{POSIX}-compatible shells like
7113 @command{bash}, but it does not always work for incompatible shells
7116 Quote strings for the shell, even if they would normally not require quoting.
7118 Quote strings as for C character string literals, including the
7119 surrounding double-quote characters; this is the same as the
7120 @option{-Q} or @option{--quote-name} option.
7122 Quote strings as for C character string literals, except omit the
7123 surrounding double-quote
7124 characters; this is the same as the @option{-b} or @option{--escape} option.
7126 Quote strings as for C character string literals, except use
7127 surrounding quotation marks appropriate for the
7130 @c Use @t instead of @samp to avoid duplicate quoting in some output styles.
7131 Quote strings as for C character string literals, except use
7132 surrounding quotation marks appropriate for the locale, and quote
7133 @t{`like this'} instead of @t{"like
7134 this"} in the default C locale. This looks nicer on many displays.
7137 You can specify the default value of the @option{--quoting-style} option
7138 with the environment variable @env{QUOTING_STYLE}. If that environment
7139 variable is not set, the default value is @samp{literal}, but this
7140 default may change to @samp{shell} in a future version of this package.
7142 @item --show-control-chars
7143 @opindex --show-control-chars
7144 Print nongraphic characters as-is in file names.
7145 This is the default unless the output is a terminal and the program is
7151 @node dir invocation
7152 @section @command{dir}: Briefly list directory contents
7155 @cindex directory listing, brief
7157 @command{dir} is equivalent to @code{ls -C
7158 -b}; that is, by default files are listed in columns, sorted vertically,
7159 and special characters are represented by backslash escape sequences.
7161 @xref{ls invocation, @command{ls}}.
7164 @node vdir invocation
7165 @section @command{vdir}: Verbosely list directory contents
7168 @cindex directory listing, verbose
7170 @command{vdir} is equivalent to @code{ls -l
7171 -b}; that is, by default files are listed in long format and special
7172 characters are represented by backslash escape sequences.
7174 @node dircolors invocation
7175 @section @command{dircolors}: Color setup for @command{ls}
7179 @cindex setup for color
7181 @command{dircolors} outputs a sequence of shell commands to set up the
7182 terminal for color output from @command{ls} (and @command{dir}, etc.).
7186 eval "`dircolors [@var{option}]@dots{} [@var{file}]`"
7189 If @var{file} is specified, @command{dircolors} reads it to determine which
7190 colors to use for which file types and extensions. Otherwise, a
7191 precompiled database is used. For details on the format of these files,
7192 run @samp{dircolors --print-database}.
7194 To make @command{dircolors} read a @file{~/.dircolors} file if it
7195 exists, you can put the following lines in your @file{~/.bashrc} (or
7196 adapt them to your favorite shell):
7200 test -r $d && eval "$(dircolors $d)"
7204 @vindex SHELL @r{environment variable, and color}
7205 The output is a shell command to set the @env{LS_COLORS} environment
7206 variable. You can specify the shell syntax to use on the command line,
7207 or @command{dircolors} will guess it from the value of the @env{SHELL}
7208 environment variable.
7210 The program accepts the following options. Also see @ref{Common options}.
7215 @itemx --bourne-shell
7218 @opindex --bourne-shell
7219 @cindex Bourne shell syntax for color setup
7220 @cindex @command{sh} syntax for color setup
7221 Output Bourne shell commands. This is the default if the @env{SHELL}
7222 environment variable is set and does not end with @samp{csh} or
7231 @cindex C shell syntax for color setup
7232 @cindex @command{csh} syntax for color setup
7233 Output C shell commands. This is the default if @code{SHELL} ends with
7234 @command{csh} or @command{tcsh}.
7237 @itemx --print-database
7239 @opindex --print-database
7240 @cindex color database, printing
7241 @cindex database for color setup, printing
7242 @cindex printing color database
7243 Print the (compiled-in) default color configuration database. This
7244 output is itself a valid configuration file, and is fairly descriptive
7245 of the possibilities.
7252 @node Basic operations
7253 @chapter Basic operations
7255 @cindex manipulating files
7257 This chapter describes the commands for basic file manipulation:
7258 copying, moving (renaming), and deleting (removing).
7261 * cp invocation:: Copy files.
7262 * dd invocation:: Convert and copy a file.
7263 * install invocation:: Copy files and set attributes.
7264 * mv invocation:: Move (rename) files.
7265 * rm invocation:: Remove files or directories.
7266 * shred invocation:: Remove files more securely.
7271 @section @command{cp}: Copy files and directories
7274 @cindex copying files and directories
7275 @cindex files, copying
7276 @cindex directories, copying
7278 @command{cp} copies files (or, optionally, directories). The copy is
7279 completely independent of the original. You can either copy one file to
7280 another, or copy arbitrarily many files to a destination directory.
7284 cp [@var{option}]@dots{} [-T] @var{source} @var{dest}
7285 cp [@var{option}]@dots{} @var{source}@dots{} @var{directory}
7286 cp [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
7291 If two file names are given, @command{cp} copies the first file to the
7295 If the @option{--target-directory} (@option{-t}) option is given, or
7296 failing that if the last file is a directory and the
7297 @option{--no-target-directory} (@option{-T}) option is not given,
7298 @command{cp} copies each @var{source} file to the specified directory,
7299 using the @var{source}s' names.
7302 Generally, files are written just as they are read. For exceptions,
7303 see the @option{--sparse} option below.
7305 By default, @command{cp} does not copy directories. However, the
7306 @option{-R}, @option{-a}, and @option{-r} options cause @command{cp} to
7307 copy recursively by descending into source directories and copying files
7308 to corresponding destination directories.
7310 When copying from a symbolic link, @command{cp} normally follows the
7311 link only when not copying
7312 recursively. This default can be overridden with the
7313 @option{--archive} (@option{-a}), @option{-d}, @option{--dereference}
7314 (@option{-L}), @option{--no-dereference} (@option{-P}), and
7315 @option{-H} options. If more than one of these options is specified,
7316 the last one silently overrides the others.
7318 When copying to a symbolic link, @command{cp} follows the
7319 link only when it refers to an existing regular file.
7320 However, when copying to a dangling symbolic link, @command{cp}
7321 refuses by default, and fails with a diagnostic, since the operation
7322 is inherently dangerous. This behavior is contrary to historical
7323 practice and to @acronym{POSIX}.
7324 Set @env{POSIXLY_CORRECT} to make @command{cp} attempt to create
7325 the target of a dangling destination symlink, in spite of the possible risk.
7326 Also, when an option like
7327 @option{--backup} or @option{--link} acts to rename or remove the
7328 destination before copying, @command{cp} renames or removes the
7329 symbolic link rather than the file it points to.
7331 By default, @command{cp} copies the contents of special files only
7332 when not copying recursively. This default can be overridden with the
7333 @option{--copy-contents} option.
7335 @cindex self-backups
7336 @cindex backups, making only
7337 @command{cp} generally refuses to copy a file onto itself, with the
7338 following exception: if @option{--force --backup} is specified with
7339 @var{source} and @var{dest} identical, and referring to a regular file,
7340 @command{cp} will make a backup file, either regular or numbered, as
7341 specified in the usual ways (@pxref{Backup options}). This is useful when
7342 you simply want to make a backup of an existing file before changing it.
7344 The program accepts the following options. Also see @ref{Common options}.
7351 Preserve as much as possible of the structure and attributes of the
7352 original files in the copy (but do not attempt to preserve internal
7353 directory structure; i.e., @samp{ls -U} may list the entries in a copied
7354 directory in a different order).
7355 Try to preserve SELinux security context and extended attributes (xattr),
7356 but ignore any failure to do that and print no corresponding diagnostic.
7357 Equivalent to @option{-dR --preserve=all} with the reduced diagnostics.
7360 @itemx @w{@kbd{--backup}[=@var{method}]}
7363 @vindex VERSION_CONTROL
7364 @cindex backups, making
7365 @xref{Backup options}.
7366 Make a backup of each file that would otherwise be overwritten or removed.
7367 As a special case, @command{cp} makes a backup of @var{source} when the force
7368 and backup options are given and @var{source} and @var{dest} are the same
7369 name for an existing, regular file. One useful application of this
7370 combination of options is this tiny Bourne shell script:
7374 # Usage: backup FILE...
7375 # Create a @sc{gnu}-style backup of each listed FILE.
7377 cp --backup --force -- "$i" "$i"
7381 @item --copy-contents
7382 @cindex directories, copying recursively
7383 @cindex copying directories recursively
7384 @cindex recursively copying directories
7385 @cindex non-directories, copying as special files
7386 If copying recursively, copy the contents of any special files (e.g.,
7387 FIFOs and device files) as if they were regular files. This means
7388 trying to read the data in each source file and writing it to the
7389 destination. It is usually a mistake to use this option, as it
7390 normally has undesirable effects on special files like FIFOs and the
7391 ones typically found in the @file{/dev} directory. In most cases,
7392 @code{cp -R --copy-contents} will hang indefinitely trying to read
7393 from FIFOs and special files like @file{/dev/console}, and it will
7394 fill up your destination disk if you use it to copy @file{/dev/zero}.
7395 This option has no effect unless copying recursively, and it does not
7396 affect the copying of symbolic links.
7400 @cindex symbolic links, copying
7401 @cindex hard links, preserving
7402 Copy symbolic links as symbolic links rather than copying the files that
7403 they point to, and preserve hard links between source files in the copies.
7404 Equivalent to @option{--no-dereference --preserve=links}.
7410 When copying without this option and an existing destination file cannot
7411 be opened for writing, the copy fails. However, with @option{--force}),
7412 when a destination file cannot be opened, @command{cp} then removes it and
7413 tries to open it again. Contrast this behavior with that enabled by
7414 @option{--link} and @option{--symbolic-link}, whereby the destination file
7415 is never opened but rather is removed unconditionally. Also see the
7416 description of @option{--remove-destination}.
7418 This option is independent of the @option{--interactive} or
7419 @option{-i} option: neither cancels the effect of the other.
7421 This option is redundant if the @option{--no-clobber} or @option{-n} option is
7426 If a command line argument specifies a symbolic link, then copy the
7427 file it points to rather than the symbolic link itself. However,
7428 copy (preserving its nature) any symbolic link that is encountered
7429 via recursive traversal.
7432 @itemx --interactive
7434 @opindex --interactive
7435 When copying a file other than a directory, prompt whether to
7436 overwrite an existing destination file. The @option{-i} option overrides
7437 a previous @option{-n} option.
7443 Make hard links instead of copies of non-directories.
7446 @itemx --dereference
7448 @opindex --dereference
7449 Follow symbolic links when copying from them.
7450 With this option, @command{cp} cannot create a symbolic link.
7451 For example, a symlink (to regular file) in the source tree will be copied to
7452 a regular file in the destination tree.
7457 @opindex --no-clobber
7458 Do not overwrite an existing file. The @option{-n} option overrides a previous
7459 @option{-i} option. This option is mutually exclusive with @option{-b} or
7460 @option{--backup} option.
7463 @itemx --no-dereference
7465 @opindex --no-dereference
7466 @cindex symbolic links, copying
7467 Copy symbolic links as symbolic links rather than copying the files that
7468 they point to. This option affects only symbolic links in the source;
7469 symbolic links in the destination are always followed if possible.
7472 @itemx @w{@kbd{--preserve}[=@var{attribute_list}]}
7475 @cindex file information, preserving, extended attributes, xattr
7476 Preserve the specified attributes of the original files.
7477 If specified, the @var{attribute_list} must be a comma-separated list
7478 of one or more of the following strings:
7482 Preserve the file mode bits and access control lists.
7484 Preserve the owner and group. On most modern systems,
7485 only users with appropriate privileges may change the owner of a file,
7487 may preserve the group ownership of a file only if they happen to be
7488 a member of the desired group.
7490 Preserve the times of last access and last modification, when possible.
7491 On older systems, it is not possible to preserve these attributes
7492 when the affected file is a symbolic link.
7493 However, many systems now provide the @code{utimensat} function,
7494 which makes it possible even for symbolic links.
7496 Preserve in the destination files
7497 any links between corresponding source files.
7498 Note that with @option{-L} or @option{-H}, this option can convert
7499 symbolic links to hard links. For example,
7501 $ mkdir c; : > a; ln -s a b; cp -aH a b c; ls -i1 c
7506 Note the inputs: @file{b} is a symlink to regular file @file{a},
7507 yet the files in destination directory, @file{c/}, are hard-linked.
7508 Since @option{-a} implies @option{--preserve=links}, and since @option{-H}
7509 tells @command{cp} to dereference command line arguments, it sees two files
7510 with the same inode number, and preserves the perceived hard link.
7512 Here is a similar example that exercises @command{cp}'s @option{-L} option:
7514 $ mkdir b c; (cd b; : > a; ln -s a b); cp -aL b c; ls -i1 c/b
7520 Preserve SELinux security context of the file, or fail with full diagnostics.
7522 Preserve extended attributes of the file, or fail with full diagnostics.
7523 If @command{cp} is built without xattr support, ignore this option.
7524 If SELinux context, ACLs or Capabilities are implemented using xattrs,
7525 they are preserved by this option as well.
7527 Preserve all file attributes.
7528 Equivalent to specifying all of the above, but with the difference
7529 that failure to preserve SELinux security context or extended attributes
7530 does not change @command{cp}'s exit status. In contrast to @option{-a},
7531 all but @samp{Operation not supported} warnings are output.
7534 Using @option{--preserve} with no @var{attribute_list} is equivalent
7535 to @option{--preserve=mode,ownership,timestamps}.
7537 In the absence of this option, each destination file is created with the
7538 mode bits of the corresponding source file, minus the bits set in the
7539 umask and minus the set-user-ID and set-group-ID bits.
7540 @xref{File permissions}.
7542 @itemx @w{@kbd{--no-preserve}=@var{attribute_list}}
7543 @cindex file information, preserving
7544 Do not preserve the specified attributes. The @var{attribute_list}
7545 has the same form as for @option{--preserve}.
7549 @cindex parent directories and @command{cp}
7550 Form the name of each destination file by appending to the target
7551 directory a slash and the specified name of the source file. The last
7552 argument given to @command{cp} must be the name of an existing directory.
7553 For example, the command:
7556 cp --parents a/b/c existing_dir
7560 copies the file @file{a/b/c} to @file{existing_dir/a/b/c}, creating
7561 any missing intermediate directories.
7568 @opindex --recursive
7569 @cindex directories, copying recursively
7570 @cindex copying directories recursively
7571 @cindex recursively copying directories
7572 @cindex non-directories, copying as special files
7573 Copy directories recursively. By default, do not follow symbolic
7574 links in the source; see the @option{--archive} (@option{-a}), @option{-d},
7575 @option{--dereference} (@option{-L}), @option{--no-dereference}
7576 (@option{-P}), and @option{-H} options. Special files are copied by
7577 creating a destination file of the same type as the source; see the
7578 @option{--copy-contents} option. It is not portable to use
7579 @option{-r} to copy symbolic links or special files. On some
7580 non-@sc{gnu} systems, @option{-r} implies the equivalent of
7581 @option{-L} and @option{--copy-contents} for historical reasons.
7582 Also, it is not portable to use @option{-R} to copy symbolic links
7583 unless you also specify @option{-P}, as @acronym{POSIX} allows
7584 implementations that dereference symbolic links by default.
7586 @item --reflink[=@var{when}]
7587 @opindex --reflink[=@var{when}]
7590 @cindex copy on write
7591 Perform a lightweight, copy-on-write (COW) copy.
7592 Copying with this option can succeed only on some file systems.
7593 Once it has succeeded, beware that the source and destination files
7594 share the same disk data blocks as long as they remain unmodified.
7595 Thus, if a disk I/O error affects data blocks of one of the files,
7596 the other suffers the exact same fate.
7598 The @var{when} value can be one of the following:
7602 The default behavior: if the copy-on-write operation is not supported
7603 then report the failure for each file and exit with a failure status.
7606 If the copy-on-write operation is not supported then fall back
7607 to the standard copy behaviour.
7611 @item --remove-destination
7612 @opindex --remove-destination
7613 Remove each existing destination file before attempting to open it
7614 (contrast with @option{-f} above).
7616 @item --sparse=@var{when}
7617 @opindex --sparse=@var{when}
7618 @cindex sparse files, copying
7619 @cindex holes, copying files with
7620 @findex read @r{system call, and holes}
7621 A @dfn{sparse file} contains @dfn{holes}---a sequence of zero bytes that
7622 does not occupy any physical disk blocks; the @samp{read} system call
7623 reads these as zeros. This can both save considerable disk space and
7624 increase speed, since many binary files contain lots of consecutive zero
7625 bytes. By default, @command{cp} detects holes in input source files via a crude
7626 heuristic and makes the corresponding output file sparse as well.
7627 Only regular files may be sparse.
7629 The @var{when} value can be one of the following:
7633 The default behavior: if the input file is sparse, attempt to make
7634 the output file sparse, too. However, if an output file exists but
7635 refers to a non-regular file, then do not attempt to make it sparse.
7638 For each sufficiently long sequence of zero bytes in the input file,
7639 attempt to create a corresponding hole in the output file, even if the
7640 input file does not appear to be sparse.
7641 This is useful when the input file resides on a file system
7642 that does not support sparse files
7643 (for example, @samp{efs} file systems in SGI IRIX 5.3 and earlier),
7644 but the output file is on a type of file system that does support them.
7645 Holes may be created only in regular files, so if the destination file
7646 is of some other type, @command{cp} does not even try to make it sparse.
7649 Never make the output file sparse.
7650 This is useful in creating a file for use with the @command{mkswap} command,
7651 since such a file must not have any holes.
7654 @optStripTrailingSlashes
7657 @itemx --symbolic-link
7659 @opindex --symbolic-link
7660 @cindex symbolic links, copying with
7661 Make symbolic links instead of copies of non-directories. All source
7662 file names must be absolute (starting with @samp{/}) unless the
7663 destination files are in the current directory. This option merely
7664 results in an error message on systems that do not support symbolic links.
7670 @optNoTargetDirectory
7676 @cindex newer files, copying only
7677 Do not copy a non-directory that has an existing destination with the
7678 same or newer modification time. If time stamps are being preserved,
7679 the comparison is to the source time stamp truncated to the
7680 resolutions of the destination file system and of the system calls
7681 used to update time stamps; this avoids duplicate work if several
7682 @samp{cp -pu} commands are executed with the same source and
7689 Print the name of each file before copying it.
7692 @itemx --one-file-system
7694 @opindex --one-file-system
7695 @cindex file systems, omitting copying to different
7696 Skip subdirectories that are on different file systems from the one that
7697 the copy started on.
7698 However, mount point directories @emph{are} copied.
7706 @section @command{dd}: Convert and copy a file
7709 @cindex converting while copying a file
7711 @command{dd} copies a file (from standard input to standard output, by
7712 default) with a changeable I/O block size, while optionally performing
7713 conversions on it. Synopses:
7716 dd [@var{operand}]@dots{}
7720 The only options are @option{--help} and @option{--version}.
7721 @xref{Common options}. @command{dd} accepts the following operands.
7727 Read from @var{file} instead of standard input.
7731 Write to @var{file} instead of standard output. Unless
7732 @samp{conv=notrunc} is given, @command{dd} truncates @var{file} to zero
7733 bytes (or the size specified with @samp{seek=}).
7735 @item ibs=@var{bytes}
7737 @cindex block size of input
7738 @cindex input block size
7739 Set the input block size to @var{bytes}.
7740 This makes @command{dd} read @var{bytes} per block.
7741 The default is 512 bytes.
7743 @item obs=@var{bytes}
7745 @cindex block size of output
7746 @cindex output block size
7747 Set the output block size to @var{bytes}.
7748 This makes @command{dd} write @var{bytes} per block.
7749 The default is 512 bytes.
7751 @item bs=@var{bytes}
7754 Set both input and output block sizes to @var{bytes}.
7755 This makes @command{dd} read and write @var{bytes} per block,
7756 overriding any @samp{ibs} and @samp{obs} settings.
7757 In addition, if no data-transforming @option{conv} option is specified,
7758 each input block is copied to the output as a single block,
7759 without aggregating short reads.
7761 @item cbs=@var{bytes}
7763 @cindex block size of conversion
7764 @cindex conversion block size
7765 @cindex fixed-length records, converting to variable-length
7766 @cindex variable-length records, converting to fixed-length
7767 Set the conversion block size to @var{bytes}.
7768 When converting variable-length records to fixed-length ones
7769 (@option{conv=block}) or the reverse (@option{conv=unblock}),
7770 use @var{bytes} as the fixed record length.
7772 @item skip=@var{blocks}
7774 Skip @var{blocks} @samp{ibs}-byte blocks in the input file before copying.
7776 @item seek=@var{blocks}
7778 Skip @var{blocks} @samp{obs}-byte blocks in the output file before copying.
7780 @item count=@var{blocks}
7782 Copy @var{blocks} @samp{ibs}-byte blocks from the input file, instead
7783 of everything until the end of the file.
7787 Do not print the overall transfer rate and volume statistics
7788 that normally make up the third status line when @command{dd} exits.
7790 @item conv=@var{conversion}[,@var{conversion}]@dots{}
7792 Convert the file as specified by the @var{conversion} argument(s).
7793 (No spaces around any comma(s).)
7800 @opindex ascii@r{, converting to}
7801 Convert @acronym{EBCDIC} to @acronym{ASCII},
7802 using the conversion table specified by @acronym{POSIX}.
7803 This provides a 1:1 translation for all 256 bytes.
7806 @opindex ebcdic@r{, converting to}
7807 Convert @acronym{ASCII} to @acronym{EBCDIC}.
7808 This is the inverse of the @samp{ascii} conversion.
7811 @opindex alternate ebcdic@r{, converting to}
7812 Convert @acronym{ASCII} to alternate @acronym{EBCDIC},
7813 using the alternate conversion table specified by @acronym{POSIX}.
7814 This is not a 1:1 translation, but reflects common historical practice
7815 for @samp{~}, @samp{[}, and @samp{]}.
7817 The @samp{ascii}, @samp{ebcdic}, and @samp{ibm} conversions are
7821 @opindex block @r{(space-padding)}
7822 For each line in the input, output @samp{cbs} bytes, replacing the
7823 input newline with a space and padding with spaces as necessary.
7827 Remove any trailing spaces in each @samp{cbs}-sized input block,
7828 and append a newline.
7830 The @samp{block} and @samp{unblock} conversions are mutually exclusive.
7833 @opindex lcase@r{, converting to}
7834 Change uppercase letters to lowercase.
7837 @opindex ucase@r{, converting to}
7838 Change lowercase letters to uppercase.
7840 The @samp{lcase} and @samp{ucase} conversions are mutually exclusive.
7843 @opindex swab @r{(byte-swapping)}
7844 @cindex byte-swapping
7845 Swap every pair of input bytes. @sc{gnu} @command{dd}, unlike others, works
7846 when an odd number of bytes are read---the last byte is simply copied
7847 (since there is nothing to swap it with).
7851 @cindex read errors, ignoring
7852 Continue after read errors.
7856 @cindex creating output file, avoiding
7857 Do not create the output file; the output file must already exist.
7861 @cindex creating output file, requiring
7862 Fail if the output file already exists; @command{dd} must create the
7865 The @samp{excl} and @samp{nocreat} conversions are mutually exclusive.
7869 @cindex truncating output file, avoiding
7870 Do not truncate the output file.
7873 @opindex sync @r{(padding with @acronym{ASCII} @sc{nul}s)}
7874 Pad every input block to size of @samp{ibs} with trailing zero bytes.
7875 When used with @samp{block} or @samp{unblock}, pad with spaces instead of
7880 @cindex synchronized data writes, before finishing
7881 Synchronize output data just before finishing. This forces a physical
7882 write of output data.
7886 @cindex synchronized data and metadata writes, before finishing
7887 Synchronize output data and metadata just before finishing. This
7888 forces a physical write of output data and metadata.
7892 @item iflag=@var{flag}[,@var{flag}]@dots{}
7894 Access the input file using the flags specified by the @var{flag}
7895 argument(s). (No spaces around any comma(s).)
7897 @item oflag=@var{flag}[,@var{flag}]@dots{}
7899 Access the output file using the flags specified by the @var{flag}
7900 argument(s). (No spaces around any comma(s).)
7902 Here are the flags. Not every flag is supported on every operating
7909 @cindex appending to the output file
7910 Write in append mode, so that even if some other process is writing to
7911 this file, every @command{dd} write will append to the current
7912 contents of the file. This flag makes sense only for output.
7913 If you combine this flag with the @samp{of=@var{file}} operand,
7914 you should also specify @samp{conv=notrunc} unless you want the
7915 output file to be truncated before being appended to.
7919 @cindex concurrent I/O
7920 Use concurrent I/O mode for data. This mode performs direct I/O
7921 and drops the @acronym{POSIX} requirement to serialize all I/O to the same file.
7922 A file cannot be opened in CIO mode and with a standard open at the
7928 Use direct I/O for data, avoiding the buffer cache.
7929 Note that the kernel may impose restrictions on read or write buffer sizes.
7930 For example, with an ext4 destination file system and a linux-based kernel,
7931 using @samp{oflag=direct} will cause writes to fail with @code{EINVAL} if the
7932 output buffer size is not a multiple of 512.
7936 @cindex directory I/O
7938 Fail unless the file is a directory. Most operating systems do not
7939 allow I/O to a directory, so this flag has limited utility.
7943 @cindex synchronized data reads
7944 Use synchronized I/O for data. For the output file, this forces a
7945 physical write of output data on each write. For the input file,
7946 this flag can matter when reading from a remote file that has been
7947 written to synchronously by some other process. Metadata (e.g.,
7948 last-access and last-modified time) is not necessarily synchronized.
7952 @cindex synchronized data and metadata I/O
7953 Use synchronized I/O for both data and metadata.
7957 @cindex nonblocking I/O
7958 Use non-blocking I/O.
7963 Do not update the file's access time.
7964 Some older file systems silently ignore this flag, so it is a good
7965 idea to test it on your files before relying on it.
7969 @cindex controlling terminal
7970 Do not assign the file to be a controlling terminal for @command{dd}.
7971 This has no effect when the file is not a terminal.
7972 On many hosts (e.g., @acronym{GNU}/Linux hosts), this option has no effect
7977 @cindex symbolic links, following
7978 Do not follow symbolic links.
7983 Fail if the file has multiple hard links.
7988 Use binary I/O. This option has an effect only on nonstandard
7989 platforms that distinguish binary from text I/O.
7994 Use text I/O. Like @samp{binary}, this option has no effect on
7999 Accumulate full blocks from input. The @code{read} system call
8000 may return early if a full block is not available.
8001 When that happens, continue calling @code{read} to fill the remainder
8003 This flag can be used only with @code{iflag}.
8007 These flags are not supported on all systems, and @samp{dd} rejects
8008 attempts to use them when they are not supported. When reading from
8009 standard input or writing to standard output, the @samp{nofollow} and
8010 @samp{noctty} flags should not be specified, and the other flags
8011 (e.g., @samp{nonblock}) can affect how other processes behave with the
8012 affected file descriptors, even after @command{dd} exits.
8016 @cindex multipliers after numbers
8017 The numeric-valued strings above (@var{bytes} and @var{blocks}) can be
8018 followed by a multiplier: @samp{b}=512, @samp{c}=1,
8019 @samp{w}=2, @samp{x@var{m}}=@var{m}, or any of the
8020 standard block size suffixes like @samp{k}=1024 (@pxref{Block size}).
8022 Use different @command{dd} invocations to use different block sizes for
8023 skipping and I/O@. For example, the following shell commands copy data
8024 in 512 KiB blocks between a disk and a tape, but do not save or restore a
8025 4 KiB label at the start of the disk:
8028 disk=/dev/rdsk/c0t1d0s2
8031 # Copy all but the label from disk to tape.
8032 (dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape
8034 # Copy from tape back to disk, but leave the disk label alone.
8035 (dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk
8038 Sending an @samp{INFO} signal to a running @command{dd}
8039 process makes it print I/O statistics to standard error
8040 and then resume copying. In the example below,
8041 @command{dd} is run in the background to copy 10 million blocks.
8042 The @command{kill} command makes it output intermediate I/O statistics,
8043 and when @command{dd} completes normally or is killed by the
8044 @code{SIGINT} signal, it outputs the final statistics.
8047 $ dd if=/dev/zero of=/dev/null count=10MB & pid=$!
8048 $ kill -s INFO $pid; wait $pid
8049 3385223+0 records in
8050 3385223+0 records out
8051 1733234176 bytes (1.7 GB) copied, 6.42173 seconds, 270 MB/s
8052 10000000+0 records in
8053 10000000+0 records out
8054 5120000000 bytes (5.1 GB) copied, 18.913 seconds, 271 MB/s
8057 @vindex POSIXLY_CORRECT
8058 On systems lacking the @samp{INFO} signal @command{dd} responds to the
8059 @samp{USR1} signal instead, unless the @env{POSIXLY_CORRECT}
8060 environment variable is set.
8065 @node install invocation
8066 @section @command{install}: Copy files and set attributes
8069 @cindex copying files and setting attributes
8071 @command{install} copies files while setting their file mode bits and, if
8072 possible, their owner and group. Synopses:
8075 install [@var{option}]@dots{} [-T] @var{source} @var{dest}
8076 install [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8077 install [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8078 install [@var{option}]@dots{} -d @var{directory}@dots{}
8083 If two file names are given, @command{install} copies the first file to the
8087 If the @option{--target-directory} (@option{-t}) option is given, or
8088 failing that if the last file is a directory and the
8089 @option{--no-target-directory} (@option{-T}) option is not given,
8090 @command{install} copies each @var{source} file to the specified
8091 directory, using the @var{source}s' names.
8094 If the @option{--directory} (@option{-d}) option is given,
8095 @command{install} creates each @var{directory} and any missing parent
8096 directories. Parent directories are created with mode
8097 @samp{u=rwx,go=rx} (755), regardless of the @option{-m} option or the
8098 current umask. @xref{Directory Setuid and Setgid}, for how the
8099 set-user-ID and set-group-ID bits of parent directories are inherited.
8102 @cindex Makefiles, installing programs in
8103 @command{install} is similar to @command{cp}, but allows you to control the
8104 attributes of destination files. It is typically used in Makefiles to
8105 copy programs into their destination directories. It refuses to copy
8106 files onto themselves.
8108 @cindex extended attributes, xattr
8109 @command{install} never preserves extended attributes (xattr).
8111 The program accepts the following options. Also see @ref{Common options}.
8121 Compare each pair of source and destination files, and if the destination has
8122 identical content and any specified owner, group, permissions, and possibly
8123 SELinux context, then do not modify the destination at all.
8127 Ignored; for compatibility with old Unix versions of @command{install}.
8131 Create any missing parent directories of @var{dest},
8132 then copy @var{source} to @var{dest}.
8133 This option is ignored if a destination directory is specified
8134 via @option{--target-directory=DIR}.
8139 @opindex --directory
8140 @cindex directories, creating with given attributes
8141 @cindex parent directories, creating missing
8142 @cindex leading directories, creating missing
8143 Create any missing parent directories, giving them the default
8144 attributes. Then create each given directory, setting their owner,
8145 group and mode as given on the command line or to the defaults.
8147 @item -g @var{group}
8148 @itemx --group=@var{group}
8151 @cindex group ownership of installed files, setting
8152 Set the group ownership of installed files or directories to
8153 @var{group}. The default is the process's current group. @var{group}
8154 may be either a group name or a numeric group ID.
8157 @itemx --mode=@var{mode}
8160 @cindex permissions of installed files, setting
8161 Set the file mode bits for the installed file or directory to @var{mode},
8162 which can be either an octal number, or a symbolic mode as in
8163 @command{chmod}, with @samp{a=} (no access allowed to anyone) as the
8164 point of departure (@pxref{File permissions}).
8165 The default mode is @samp{u=rwx,go=rx,a-s}---read, write, and
8166 execute for the owner, read and execute for group and other, and with
8167 set-user-ID and set-group-ID disabled.
8168 This default is not quite the same as @samp{755}, since it disables
8169 instead of preserving set-user-ID and set-group-ID on directories.
8170 @xref{Directory Setuid and Setgid}.
8172 @item -o @var{owner}
8173 @itemx --owner=@var{owner}
8176 @cindex ownership of installed files, setting
8177 @cindex appropriate privileges
8178 @vindex root @r{as default owner}
8179 If @command{install} has appropriate privileges (is run as root), set the
8180 ownership of installed files or directories to @var{owner}. The default
8181 is @code{root}. @var{owner} may be either a user name or a numeric user
8184 @item --preserve-context
8185 @opindex --preserve-context
8187 @cindex security context
8188 Preserve the SELinux security context of files and directories.
8189 Failure to preserve the context in all of the files or directories
8190 will result in an exit status of 1. If SELinux is disabled then
8191 print a warning and ignore the option.
8194 @itemx --preserve-timestamps
8196 @opindex --preserve-timestamps
8197 @cindex timestamps of installed files, preserving
8198 Set the time of last access and the time of last modification of each
8199 installed file to match those of each corresponding original file.
8200 When a file is installed without this option, its last access and
8201 last modification times are both set to the time of installation.
8202 This option is useful if you want to use the last modification times
8203 of installed files to keep track of when they were last built as opposed
8204 to when they were last installed.
8210 @cindex symbol table information, stripping
8211 @cindex stripping symbol table information
8212 Strip the symbol tables from installed binary executables.
8214 @itemx --strip-program=@var{program}
8215 @opindex --strip-program
8216 @cindex symbol table information, stripping, program
8217 Program used to strip binaries.
8223 @optNoTargetDirectory
8229 Print the name of each file before copying it.
8231 @item -Z @var{context}
8232 @itemx --context=@var{context}
8236 @cindex security context
8237 Set the default SELinux security context to be used for any
8238 created files and directories. If SELinux is disabled then
8239 print a warning and ignore the option.
8247 @section @command{mv}: Move (rename) files
8251 @command{mv} moves or renames files (or directories). Synopses:
8254 mv [@var{option}]@dots{} [-T] @var{source} @var{dest}
8255 mv [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8256 mv [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8261 If two file names are given, @command{mv} moves the first file to the
8265 If the @option{--target-directory} (@option{-t}) option is given, or
8266 failing that if the last file is a directory and the
8267 @option{--no-target-directory} (@option{-T}) option is not given,
8268 @command{mv} moves each @var{source} file to the specified
8269 directory, using the @var{source}s' names.
8272 @command{mv} can move any type of file from one file system to another.
8273 Prior to version @code{4.0} of the fileutils,
8274 @command{mv} could move only regular files between file systems.
8275 For example, now @command{mv} can move an entire directory hierarchy
8276 including special device files from one partition to another. It first
8277 uses some of the same code that's used by @code{cp -a} to copy the
8278 requested directories and files, then (assuming the copy succeeded)
8279 it removes the originals. If the copy fails, then the part that was
8280 copied to the destination partition is removed. If you were to copy
8281 three directories from one partition to another and the copy of the first
8282 directory succeeded, but the second didn't, the first would be left on
8283 the destination partition and the second and third would be left on the
8286 @cindex extended attributes, xattr
8287 @command{mv} always tries to copy extended attributes (xattr), which may
8288 include SELinux context, ACLs or Capabilities.
8289 Upon failure all but @samp{Operation not supported} warnings are output.
8291 @cindex prompting, and @command{mv}
8292 If a destination file exists but is normally unwritable, standard input
8293 is a terminal, and the @option{-f} or @option{--force} option is not given,
8294 @command{mv} prompts the user for whether to replace the file. (You might
8295 own the file, or have write permission on its directory.) If the
8296 response is not affirmative, the file is skipped.
8298 @emph{Warning}: Avoid specifying a source name with a trailing slash,
8299 when it might be a symlink to a directory.
8300 Otherwise, @command{mv} may do something very surprising, since
8301 its behavior depends on the underlying rename system call.
8302 On a system with a modern Linux-based kernel, it fails with @code{errno=ENOTDIR}.
8303 However, on other systems (at least FreeBSD 6.1 and Solaris 10) it silently
8304 renames not the symlink but rather the directory referenced by the symlink.
8305 @xref{Trailing slashes}.
8307 The program accepts the following options. Also see @ref{Common options}.
8317 @cindex prompts, omitting
8318 Do not prompt the user before removing a destination file.
8320 If you specify more than one of the @option{-i}, @option{-f}, @option{-n}
8321 options, only the final one takes effect.
8326 @itemx --interactive
8328 @opindex --interactive
8329 @cindex prompts, forcing
8330 Prompt whether to overwrite each existing destination file, regardless
8332 If the response is not affirmative, the file is skipped.
8338 @opindex --no-clobber
8339 @cindex prompts, omitting
8340 Do not overwrite an existing file.
8342 This option is mutually exclusive with @option{-b} or @option{--backup} option.
8348 @cindex newer files, moving only
8349 Do not move a non-directory that has an existing destination with the
8350 same or newer modification time.
8351 If the move is across file system boundaries, the comparison is to the
8352 source time stamp truncated to the resolutions of the destination file
8353 system and of the system calls used to update time stamps; this avoids
8354 duplicate work if several @samp{mv -u} commands are executed with the
8355 same source and destination.
8361 Print the name of each file before moving it.
8363 @optStripTrailingSlashes
8369 @optNoTargetDirectory
8377 @section @command{rm}: Remove files or directories
8380 @cindex removing files or directories
8382 @command{rm} removes each given @var{file}. By default, it does not remove
8383 directories. Synopsis:
8386 rm [@var{option}]@dots{} [@var{file}]@dots{}
8389 @cindex prompting, and @command{rm}
8390 If the @option{-I} or @option{--interactive=once} option is given,
8391 and there are more than three files or the @option{-r}, @option{-R},
8392 or @option{--recursive} are given, then @command{rm} prompts the user
8393 for whether to proceed with the entire operation. If the response is
8394 not affirmative, the entire command is aborted.
8396 Otherwise, if a file is unwritable, standard input is a terminal, and
8397 the @option{-f} or @option{--force} option is not given, or the
8398 @option{-i} or @option{--interactive=always} option @emph{is} given,
8399 @command{rm} prompts the user for whether to remove the file.
8400 If the response is not affirmative, the file is skipped.
8402 Any attempt to remove a file whose last file name component is
8403 @file{.} or @file{..} is rejected without any prompting.
8405 @emph{Warning}: If you use @command{rm} to remove a file, it is usually
8406 possible to recover the contents of that file. If you want more assurance
8407 that the contents are truly unrecoverable, consider using @command{shred}.
8409 The program accepts the following options. Also see @ref{Common options}.
8417 Ignore nonexistent files and never prompt the user.
8418 Ignore any previous @option{--interactive} (@option{-i}) option.
8422 Prompt whether to remove each file.
8423 If the response is not affirmative, the file is skipped.
8424 Ignore any previous @option{--force} (@option{-f}) option.
8425 Equivalent to @option{--interactive=always}.
8429 Prompt once whether to proceed with the command, if more than three
8430 files are named or if a recursive removal is requested. Ignore any
8431 previous @option{--force} (@option{-f}) option. Equivalent to
8432 @option{--interactive=once}.
8434 @itemx --interactive [=@var{when}]
8435 @opindex --interactive
8436 Specify when to issue an interactive prompt. @var{when} may be
8440 @vindex never @r{interactive option}
8441 - Do not prompt at all.
8443 @vindex once @r{interactive option}
8444 - Prompt once if more than three files are named or if a recursive
8445 removal is requested. Equivalent to @option{-I}.
8447 @vindex always @r{interactive option}
8448 - Prompt for every file being removed. Equivalent to @option{-i}.
8450 @option{--interactive} with no @var{when} is equivalent to
8451 @option{--interactive=always}.
8453 @itemx --one-file-system
8454 @opindex --one-file-system
8455 @cindex one file system, restricting @command{rm} to
8456 When removing a hierarchy recursively, skip any directory that is on a
8457 file system different from that of the corresponding command line argument.
8459 This option is useful when removing a build ``chroot'' hierarchy,
8460 which normally contains no valuable data. However, it is not uncommon
8461 to bind-mount @file{/home} into such a hierarchy, to make it easier to
8462 use one's start-up file. The catch is that it's easy to forget to
8463 unmount @file{/home}. Then, when you use @command{rm -rf} to remove
8464 your normally throw-away chroot, that command will remove everything
8465 under @file{/home}, too.
8466 Use the @option{--one-file-system} option, and it will
8467 warn about and skip directories on other file systems.
8468 Of course, this will not save your @file{/home} if it and your
8469 chroot happen to be on the same file system.
8471 @itemx --preserve-root
8472 @opindex --preserve-root
8473 @cindex root directory, disallow recursive destruction
8474 Fail upon any attempt to remove the root directory, @file{/},
8475 when used with the @option{--recursive} option.
8476 This is the default behavior.
8477 @xref{Treating / specially}.
8479 @itemx --no-preserve-root
8480 @opindex --no-preserve-root
8481 @cindex root directory, allow recursive destruction
8482 Do not treat @file{/} specially when removing recursively.
8483 This option is not recommended unless you really want to
8484 remove all the files on your computer.
8485 @xref{Treating / specially}.
8492 @opindex --recursive
8493 @cindex directories, removing (recursively)
8494 Remove the listed directories and their contents recursively.
8500 Print the name of each file before removing it.
8504 @cindex files beginning with @samp{-}, removing
8505 @cindex @samp{-}, removing files beginning with
8506 One common question is how to remove files whose names begin with a
8507 @samp{-}. @sc{gnu} @command{rm}, like every program that uses the @code{getopt}
8508 function to parse its arguments, lets you use the @samp{--} option to
8509 indicate that all following arguments are non-options. To remove a file
8510 called @file{-f} in the current directory, you could type either:
8523 @opindex - @r{and Unix @command{rm}}
8524 The Unix @command{rm} program's use of a single @samp{-} for this purpose
8525 predates the development of the getopt standard syntax.
8530 @node shred invocation
8531 @section @command{shred}: Remove files more securely
8534 @cindex data, erasing
8535 @cindex erasing data
8537 @command{shred} overwrites devices or files, to help prevent even
8538 very expensive hardware from recovering the data.
8540 Ordinarily when you remove a file (@pxref{rm invocation}), the data is
8541 not actually destroyed. Only the index listing where the file is
8542 stored is destroyed, and the storage is made available for reuse.
8543 There are undelete utilities that will attempt to reconstruct the index
8544 and can bring the file back if the parts were not reused.
8546 On a busy system with a nearly-full drive, space can get reused in a few
8547 seconds. But there is no way to know for sure. If you have sensitive
8548 data, you may want to be sure that recovery is not possible by actually
8549 overwriting the file with non-sensitive data.
8551 However, even after doing that, it is possible to take the disk back
8552 to a laboratory and use a lot of sensitive (and expensive) equipment
8553 to look for the faint ``echoes'' of the original data underneath the
8554 overwritten data. If the data has only been overwritten once, it's not
8557 The best way to remove something irretrievably is to destroy the media
8558 it's on with acid, melt it down, or the like. For cheap removable media
8559 like floppy disks, this is the preferred method. However, hard drives
8560 are expensive and hard to melt, so the @command{shred} utility tries
8561 to achieve a similar effect non-destructively.
8563 This uses many overwrite passes, with the data patterns chosen to
8564 maximize the damage they do to the old data. While this will work on
8565 floppies, the patterns are designed for best effect on hard drives.
8566 For more details, see the source code and Peter Gutmann's paper
8567 @uref{http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html,
8568 @cite{Secure Deletion of Data from Magnetic and Solid-State Memory}},
8569 from the proceedings of the Sixth @acronym{USENIX} Security Symposium (San Jose,
8570 California, July 22--25, 1996).
8572 @strong{Please note} that @command{shred} relies on a very important assumption:
8573 that the file system overwrites data in place. This is the traditional
8574 way to do things, but many modern file system designs do not satisfy this
8575 assumption. Exceptions include:
8580 Log-structured or journaled file systems, such as those supplied with
8581 AIX and Solaris, and JFS, ReiserFS, XFS, Ext3 (in @code{data=journal} mode),
8582 BFS, NTFS, etc.@: when they are configured to journal @emph{data}.
8585 File systems that write redundant data and carry on even if some writes
8586 fail, such as RAID-based file systems.
8589 File systems that make snapshots, such as Network Appliance's NFS server.
8592 File systems that cache in temporary locations, such as NFS version 3
8596 Compressed file systems.
8599 In the particular case of ext3 file systems, the above disclaimer applies (and
8600 @command{shred} is thus of limited effectiveness) only in @code{data=journal}
8601 mode, which journals file data in addition to just metadata. In both
8602 the @code{data=ordered} (default) and @code{data=writeback} modes,
8603 @command{shred} works as usual. Ext3 journaling modes can be changed
8604 by adding the @code{data=something} option to the mount options for a
8605 particular file system in the @file{/etc/fstab} file, as documented in
8606 the mount man page (man mount).
8608 If you are not sure how your file system operates, then you should assume
8609 that it does not overwrite data in place, which means that shred cannot
8610 reliably operate on regular files in your file system.
8612 Generally speaking, it is more reliable to shred a device than a file,
8613 since this bypasses the problem of file system design mentioned above.
8614 However, even shredding devices is not always completely reliable. For
8615 example, most disks map out bad sectors invisibly to the application; if
8616 the bad sectors contain sensitive data, @command{shred} won't be able to
8619 @command{shred} makes no attempt to detect or report this problem, just as
8620 it makes no attempt to do anything about backups. However, since it is
8621 more reliable to shred devices than files, @command{shred} by default does
8622 not truncate or remove the output file. This default is more suitable
8623 for devices, which typically cannot be truncated and should not be
8626 Finally, consider the risk of backups and mirrors.
8627 File system backups and remote mirrors may contain copies of the
8628 file that cannot be removed, and that will allow a shredded file
8629 to be recovered later. So if you keep any data you may later want
8630 to destroy using @command{shred}, be sure that it is not backed up or mirrored.
8633 shred [@var{option}]@dots{} @var{file}[@dots{}]
8636 The program accepts the following options. Also see @ref{Common options}.
8644 @cindex force deletion
8645 Override file permissions if necessary to allow overwriting.
8648 @itemx -n @var{number}
8649 @itemx --iterations=@var{number}
8650 @opindex -n @var{number}
8651 @opindex --iterations=@var{number}
8652 @cindex iterations, selecting the number of
8653 By default, @command{shred} uses @value{SHRED_DEFAULT_PASSES} passes of
8654 overwrite. You can reduce this to save time, or increase it if you think it's
8655 appropriate. After 25 passes all of the internal overwrite patterns will have
8656 been used at least once.
8658 @item --random-source=@var{file}
8659 @opindex --random-source
8660 @cindex random source for shredding
8661 Use @var{file} as a source of random data used to overwrite and to
8662 choose pass ordering. @xref{Random sources}.
8664 @item -s @var{bytes}
8665 @itemx --size=@var{bytes}
8666 @opindex -s @var{bytes}
8667 @opindex --size=@var{bytes}
8668 @cindex size of file to shred
8669 Shred the first @var{bytes} bytes of the file. The default is to shred
8670 the whole file. @var{bytes} can be followed by a size specification like
8671 @samp{K}, @samp{M}, or @samp{G} to specify a multiple. @xref{Block size}.
8677 @cindex removing files after shredding
8678 After shredding a file, truncate it (if possible) and then remove it.
8679 If a file has multiple links, only the named links will be removed.
8685 Display to standard error all status updates as sterilization proceeds.
8691 By default, @command{shred} rounds the size of a regular file up to the next
8692 multiple of the file system block size to fully erase the last block of the file.
8693 Use @option{--exact} to suppress that behavior.
8694 Thus, by default if you shred a 10-byte regular file on a system with 512-byte
8695 blocks, the resulting file will be 512 bytes long. With this option,
8696 shred does not increase the apparent size of the file.
8702 Normally, the last pass that @command{shred} writes is made up of
8703 random data. If this would be conspicuous on your hard drive (for
8704 example, because it looks like encrypted data), or you just think
8705 it's tidier, the @option{--zero} option adds an additional overwrite pass with
8706 all zero bits. This is in addition to the number of passes specified
8707 by the @option{--iterations} option.
8711 You might use the following command to erase all trace of the
8712 file system you'd created on the floppy disk in your first drive.
8713 That command takes about 20 minutes to erase a ``1.44MB'' (actually
8717 shred --verbose /dev/fd0
8720 Similarly, to erase all data on a selected partition of
8721 your hard disk, you could give a command like this:
8724 shred --verbose /dev/sda5
8727 A @var{file} of @samp{-} denotes standard output.
8728 The intended use of this is to shred a removed temporary file.
8735 echo "Hello, world" >&3
8740 However, the command @samp{shred - >file} does not shred the contents
8741 of @var{file}, since the shell truncates @var{file} before invoking
8742 @command{shred}. Use the command @samp{shred file} or (if using a
8743 Bourne-compatible shell) the command @samp{shred - 1<>file} instead.
8748 @node Special file types
8749 @chapter Special file types
8751 @cindex special file types
8752 @cindex file types, special
8754 This chapter describes commands which create special types of files (and
8755 @command{rmdir}, which removes directories, one special file type).
8757 @cindex special file types
8759 Although Unix-like operating systems have markedly fewer special file
8760 types than others, not @emph{everything} can be treated only as the
8761 undifferentiated byte stream of @dfn{normal files}. For example, when a
8762 file is created or removed, the system must record this information,
8763 which it does in a @dfn{directory}---a special type of file. Although
8764 you can read directories as normal files, if you're curious, in order
8765 for the system to do its job it must impose a structure, a certain
8766 order, on the bytes of the file. Thus it is a ``special'' type of file.
8768 Besides directories, other special file types include named pipes
8769 (FIFOs), symbolic links, sockets, and so-called @dfn{special files}.
8772 * link invocation:: Make a hard link via the link syscall
8773 * ln invocation:: Make links between files.
8774 * mkdir invocation:: Make directories.
8775 * mkfifo invocation:: Make FIFOs (named pipes).
8776 * mknod invocation:: Make block or character special files.
8777 * readlink invocation:: Print value of a symlink or canonical file name.
8778 * rmdir invocation:: Remove empty directories.
8779 * unlink invocation:: Remove files via the unlink syscall
8783 @node link invocation
8784 @section @command{link}: Make a hard link via the link syscall
8787 @cindex links, creating
8788 @cindex hard links, creating
8789 @cindex creating links (hard only)
8791 @command{link} creates a single hard link at a time.
8792 It is a minimalist interface to the system-provided
8793 @code{link} function. @xref{Hard Links, , , libc,
8794 The GNU C Library Reference Manual}.
8795 It avoids the bells and whistles of the more commonly-used
8796 @command{ln} command (@pxref{ln invocation}).
8800 link @var{filename} @var{linkname}
8803 @var{filename} must specify an existing file, and @var{linkname}
8804 must specify a nonexistent entry in an existing directory.
8805 @command{link} simply calls @code{link (@var{filename}, @var{linkname})}
8808 On a @acronym{GNU} system, this command acts like @samp{ln --directory
8809 --no-target-directory @var{filename} @var{linkname}}. However, the
8810 @option{--directory} and @option{--no-target-directory} options are
8811 not specified by @acronym{POSIX}, and the @command{link} command is
8812 more portable in practice.
8814 If @var{filename} is a symbolic link, it is unspecified whether
8815 @var{linkname} will be a hard link to the symbolic link or to the
8816 target of the symbolic link. Use @command{ln -P} or @command{ln -L}
8817 to specify which behavior is desired.
8823 @section @command{ln}: Make links between files
8826 @cindex links, creating
8827 @cindex hard links, creating
8828 @cindex symbolic (soft) links, creating
8829 @cindex creating links (hard or soft)
8831 @cindex file systems and hard links
8832 @command{ln} makes links between files. By default, it makes hard links;
8833 with the @option{-s} option, it makes symbolic (or @dfn{soft}) links.
8837 ln [@var{option}]@dots{} [-T] @var{target} @var{linkname}
8838 ln [@var{option}]@dots{} @var{target}
8839 ln [@var{option}]@dots{} @var{target}@dots{} @var{directory}
8840 ln [@var{option}]@dots{} -t @var{directory} @var{target}@dots{}
8846 If two file names are given, @command{ln} creates a link to the first
8847 file from the second.
8850 If one @var{target} is given, @command{ln} creates a link to that file
8851 in the current directory.
8854 If the @option{--target-directory} (@option{-t}) option is given, or
8855 failing that if the last file is a directory and the
8856 @option{--no-target-directory} (@option{-T}) option is not given,
8857 @command{ln} creates a link to each @var{target} file in the specified
8858 directory, using the @var{target}s' names.
8862 Normally @command{ln} does not remove existing files. Use the
8863 @option{--force} (@option{-f}) option to remove them unconditionally,
8864 the @option{--interactive} (@option{-i}) option to remove them
8865 conditionally, and the @option{--backup} (@option{-b}) option to
8868 @cindex hard link, defined
8869 @cindex inode, and hard links
8870 A @dfn{hard link} is another name for an existing file; the link and the
8871 original are indistinguishable. Technically speaking, they share the
8872 same inode, and the inode contains all the information about a
8873 file---indeed, it is not incorrect to say that the inode @emph{is} the
8874 file. Most systems prohibit making a hard link to
8875 a directory; on those where it is allowed, only the super-user can do
8876 so (and with caution, since creating a cycle will cause problems to many
8877 other utilities). Hard links cannot cross file system boundaries. (These
8878 restrictions are not mandated by @acronym{POSIX}, however.)
8880 @cindex dereferencing symbolic links
8881 @cindex symbolic link, defined
8882 @dfn{Symbolic links} (@dfn{symlinks} for short), on the other hand, are
8883 a special file type (which not all kernels support: System V release 3
8884 (and older) systems lack symlinks) in which the link file actually
8885 refers to a different file, by name. When most operations (opening,
8886 reading, writing, and so on) are passed the symbolic link file, the
8887 kernel automatically @dfn{dereferences} the link and operates on the
8888 target of the link. But some operations (e.g., removing) work on the
8889 link file itself, rather than on its target. The owner and group of a
8890 symlink are not significant to file access performed through
8891 the link, but do have implications on deleting a symbolic link from a
8892 directory with the restricted deletion bit set. On the GNU system,
8893 the mode of a symlink has no significance and cannot be changed, but
8894 on some BSD systems, the mode can be changed and will affect whether
8895 the symlink will be traversed in file name resolution. @xref{Symbolic Links,,,
8896 libc, The GNU C Library Reference Manual}.
8898 Symbolic links can contain arbitrary strings; a @dfn{dangling symlink}
8899 occurs when the string in the symlink does not resolve to a file.
8900 There are no restrictions against creating dangling symbolic links.
8901 There are trade-offs to using absolute or relative symlinks. An
8902 absolute symlink always points to the same file, even if the directory
8903 containing the link is moved. However, if the symlink is visible from
8904 more than one machine (such as on a networked file system), the file
8905 pointed to might not always be the same. A relative symbolic link is
8906 resolved in relation to the directory that contains the link, and is
8907 often useful in referring to files on the same device without regards
8908 to what name that device is mounted on when accessed via networked
8911 When creating a relative symlink in a different location than the
8912 current directory, the resolution of the symlink will be different
8913 than the resolution of the same string from the current directory.
8914 Therefore, many users prefer to first change directories to the
8915 location where the relative symlink will be created, so that
8916 tab-completion or other file resolution will find the same target as
8917 what will be placed in the symlink.
8919 The program accepts the following options. Also see @ref{Common options}.
8930 @opindex --directory
8931 @cindex hard links to directories
8932 Allow users with appropriate privileges to attempt to make hard links
8934 However, note that this will probably fail due to
8935 system restrictions, even for the super-user.
8941 Remove existing destination files.
8944 @itemx --interactive
8946 @opindex --interactive
8947 @cindex prompting, and @command{ln}
8948 Prompt whether to remove existing destination files.
8954 If @option{-s} is not in effect, and the source file is a symbolic
8955 link, create the hard link to the file referred to by the symbolic
8956 link, rather than the symbolic link itself.
8959 @itemx --no-dereference
8961 @opindex --no-dereference
8962 Do not treat the last operand specially when it is a symbolic link to
8963 a directory. Instead, treat it as if it were a normal file.
8965 When the destination is an actual directory (not a symlink to one),
8966 there is no ambiguity. The link is created in that directory.
8967 But when the specified destination is a symlink to a directory,
8968 there are two ways to treat the user's request. @command{ln} can
8969 treat the destination just as it would a normal directory and create
8970 the link in it. On the other hand, the destination can be viewed as a
8971 non-directory---as the symlink itself. In that case, @command{ln}
8972 must delete or backup that symlink before creating the new link.
8973 The default is to treat a destination that is a symlink to a directory
8974 just like a directory.
8976 This option is weaker than the @option{--no-target-directory}
8977 (@option{-T}) option, so it has no effect if both options are given.
8983 If @option{-s} is not in effect, and the source file is a symbolic
8984 link, create the hard link to the symbolic link itself. On platforms
8985 where this is not supported by the kernel, this option creates a
8986 symbolic link with identical contents; since symbolic link contents
8987 cannot be edited, any file name resolution performed through either
8988 link will be the same as if a hard link had been created.
8994 Make symbolic links instead of hard links. This option merely produces
8995 an error message on systems that do not support symbolic links.
9001 @optNoTargetDirectory
9007 Print the name of each file after linking it successfully.
9011 @cindex hard links to symbolic links
9012 @cindex symbolic links and @command{ln}
9013 If @option{-L} and @option{-P} are both given, the last one takes
9014 precedence. If @option{-s} is also given, @option{-L} and @option{-P}
9015 are silently ignored. If neither option is given, then this
9016 implementation defaults to @option{-P} if the system @code{link} supports
9017 hard links to symbolic links (such as the GNU system), and @option{-L}
9018 if @code{link} follows symbolic links (such as on BSD).
9027 # Create link ../a pointing to a in that directory.
9028 # Not really useful because it points to itself.
9033 # Change to the target before creating symlinks to avoid being confused.
9039 # Hard coded file names don't move well.
9040 ln -s $(pwd)/a /some/dir/
9044 # Relative file names survive directory moves and also
9045 # work across networked file systems.
9046 ln -s afile anotherfile
9047 ln -s ../adir/afile yetanotherfile
9051 @node mkdir invocation
9052 @section @command{mkdir}: Make directories
9055 @cindex directories, creating
9056 @cindex creating directories
9058 @command{mkdir} creates directories with the specified names. Synopsis:
9061 mkdir [@var{option}]@dots{} @var{name}@dots{}
9064 @command{mkdir} creates each directory @var{name} in the order given.
9065 It reports an error if @var{name} already exists, unless the
9066 @option{-p} option is given and @var{name} is a directory.
9068 The program accepts the following options. Also see @ref{Common options}.
9073 @itemx --mode=@var{mode}
9076 @cindex modes of created directories, setting
9077 Set the file permission bits of created directories to @var{mode},
9078 which uses the same syntax as
9079 in @command{chmod} and uses @samp{a=rwx} (read, write and execute allowed for
9080 everyone) for the point of the departure. @xref{File permissions}.
9082 Normally the directory has the desired file mode bits at the moment it
9083 is created. As a @acronym{GNU} extension, @var{mode} may also mention
9084 special mode bits, but in this case there may be a temporary window
9085 during which the directory exists but its special mode bits are
9086 incorrect. @xref{Directory Setuid and Setgid}, for how the
9087 set-user-ID and set-group-ID bits of directories are inherited unless
9088 overridden in this way.
9094 @cindex parent directories, creating
9095 Make any missing parent directories for each argument, setting their
9096 file permission bits to the umask modified by @samp{u+wx}. Ignore
9097 existing parent directories, and do not change their file permission
9100 To set the file permission bits of any newly-created parent
9101 directories to a value that includes @samp{u+wx}, you can set the
9102 umask before invoking @command{mkdir}. For example, if the shell
9103 command @samp{(umask u=rwx,go=rx; mkdir -p P/Q)} creates the parent
9104 @file{P} it sets the parent's permission bits to @samp{u=rwx,go=rx}.
9105 To set a parent's special mode bits as well, you can invoke
9106 @command{chmod} after @command{mkdir}. @xref{Directory Setuid and
9107 Setgid}, for how the set-user-ID and set-group-ID bits of
9108 newly-created parent directories are inherited.
9114 Print a message for each created directory. This is most useful with
9117 @item -Z @var{context}
9118 @itemx --context=@var{context}
9122 @cindex security context
9123 Set the default SELinux security context to be used for created directories.
9130 @node mkfifo invocation
9131 @section @command{mkfifo}: Make FIFOs (named pipes)
9134 @cindex FIFOs, creating
9135 @cindex named pipes, creating
9136 @cindex creating FIFOs (named pipes)
9138 @command{mkfifo} creates FIFOs (also called @dfn{named pipes}) with the
9139 specified names. Synopsis:
9142 mkfifo [@var{option}] @var{name}@dots{}
9145 A @dfn{FIFO} is a special file type that permits independent processes
9146 to communicate. One process opens the FIFO file for writing, and
9147 another for reading, after which data can flow as with the usual
9148 anonymous pipe in shells or elsewhere.
9150 The program accepts the following option. Also see @ref{Common options}.
9155 @itemx --mode=@var{mode}
9158 @cindex modes of created FIFOs, setting
9159 Set the mode of created FIFOs to @var{mode}, which is symbolic as in
9160 @command{chmod} and uses @samp{a=rw} (read and write allowed for everyone)
9161 for the point of departure. @var{mode} should specify only file
9162 permission bits. @xref{File permissions}.
9164 @item -Z @var{context}
9165 @itemx --context=@var{context}
9169 @cindex security context
9170 Set the default SELinux security context to be used for created FIFOs.
9177 @node mknod invocation
9178 @section @command{mknod}: Make block or character special files
9181 @cindex block special files, creating
9182 @cindex character special files, creating
9184 @command{mknod} creates a FIFO, character special file, or block special
9185 file with the specified name. Synopsis:
9188 mknod [@var{option}]@dots{} @var{name} @var{type} [@var{major} @var{minor}]
9191 @cindex special files
9192 @cindex block special files
9193 @cindex character special files
9194 Unlike the phrase ``special file type'' above, the term @dfn{special
9195 file} has a technical meaning on Unix: something that can generate or
9196 receive data. Usually this corresponds to a physical piece of hardware,
9197 e.g., a printer or a disk. (These files are typically created at
9198 system-configuration time.) The @command{mknod} command is what creates
9199 files of this type. Such devices can be read either a character at a
9200 time or a ``block'' (many characters) at a time, hence we say there are
9201 @dfn{block special} files and @dfn{character special} files.
9203 @c mknod is a shell built-in at least with OpenBSD's /bin/sh
9204 @mayConflictWithShellBuiltIn{mknod}
9206 The arguments after @var{name} specify the type of file to make:
9211 @opindex p @r{for FIFO file}
9215 @opindex b @r{for block special file}
9216 for a block special file
9219 @c Don't document the `u' option -- it's just a synonym for `c'.
9220 @c Do *any* versions of mknod still use it?
9222 @opindex c @r{for character special file}
9223 @c @opindex u @r{for character special file}
9224 for a character special file
9228 When making a block or character special file, the major and minor
9229 device numbers must be given after the file type.
9230 If a major or minor device number begins with @samp{0x} or @samp{0X},
9231 it is interpreted as hexadecimal; otherwise, if it begins with @samp{0},
9232 as octal; otherwise, as decimal.
9234 The program accepts the following option. Also see @ref{Common options}.
9239 @itemx --mode=@var{mode}
9242 Set the mode of created files to @var{mode}, which is symbolic as in
9243 @command{chmod} and uses @samp{a=rw} as the point of departure.
9244 @var{mode} should specify only file permission bits.
9245 @xref{File permissions}.
9247 @item -Z @var{context}
9248 @itemx --context=@var{context}
9252 @cindex security context
9253 Set the default SELinux security context to be used for created files.
9260 @node readlink invocation
9261 @section @command{readlink}: Print value of a symlink or canonical file name
9264 @cindex displaying value of a symbolic link
9265 @cindex canonical file name
9266 @cindex canonicalize a file name
9270 @command{readlink} may work in one of two supported modes:
9276 @command{readlink} outputs the value of the given symbolic link.
9277 If @command{readlink} is invoked with an argument other than the name
9278 of a symbolic link, it produces no output and exits with a nonzero exit code.
9280 @item Canonicalize mode
9282 @command{readlink} outputs the absolute name of the given file which contains
9283 no @file{.}, @file{..} components nor any repeated separators
9284 (@file{/}) or symbolic links.
9289 readlink [@var{option}] @var{file}
9292 By default, @command{readlink} operates in readlink mode.
9294 The program accepts the following options. Also see @ref{Common options}.
9299 @itemx --canonicalize
9301 @opindex --canonicalize
9302 Activate canonicalize mode.
9303 If any component of the file name except the last one is missing or unavailable,
9304 @command{readlink} produces no output and exits with a nonzero exit
9305 code. A trailing slash is ignored.
9308 @itemx --canonicalize-existing
9310 @opindex --canonicalize-existing
9311 Activate canonicalize mode.
9312 If any component is missing or unavailable, @command{readlink} produces
9313 no output and exits with a nonzero exit code. A trailing slash
9314 requires that the name resolve to a directory.
9317 @itemx --canonicalize-missing
9319 @opindex --canonicalize-missing
9320 Activate canonicalize mode.
9321 If any component is missing or unavailable, @command{readlink} treats it
9327 @opindex --no-newline
9328 Do not output the trailing newline.
9338 Suppress most error messages.
9344 Report error messages.
9348 The @command{readlink} utility first appeared in OpenBSD 2.1.
9350 There is a @command{realpath} command on some systems
9351 which operates like @command{readlink} in canonicalize mode.
9356 @node rmdir invocation
9357 @section @command{rmdir}: Remove empty directories
9360 @cindex removing empty directories
9361 @cindex directories, removing empty
9363 @command{rmdir} removes empty directories. Synopsis:
9366 rmdir [@var{option}]@dots{} @var{directory}@dots{}
9369 If any @var{directory} argument does not refer to an existing empty
9370 directory, it is an error.
9372 The program accepts the following options. Also see @ref{Common options}.
9376 @item --ignore-fail-on-non-empty
9377 @opindex --ignore-fail-on-non-empty
9378 @cindex directory deletion, ignoring failures
9379 Ignore each failure to remove a directory that is solely because
9380 the directory is non-empty.
9386 @cindex parent directories, removing
9387 Remove @var{directory}, then try to remove each component of @var{directory}.
9388 So, for example, @samp{rmdir -p a/b/c} is similar to @samp{rmdir a/b/c a/b a}.
9389 As such, it fails if any of those directories turns out not to be empty.
9390 Use the @option{--ignore-fail-on-non-empty} option to make it so such
9391 a failure does not evoke a diagnostic and does not cause @command{rmdir} to
9392 exit unsuccessfully.
9398 @cindex directory deletion, reporting
9399 Give a diagnostic for each successful removal.
9400 @var{directory} is removed.
9404 @xref{rm invocation}, for how to remove non-empty directories (recursively).
9409 @node unlink invocation
9410 @section @command{unlink}: Remove files via the unlink syscall
9413 @cindex removing files or directories (via the unlink syscall)
9415 @command{unlink} deletes a single specified file name.
9416 It is a minimalist interface to the system-provided
9417 @code{unlink} function. @xref{Deleting Files, , , libc,
9418 The GNU C Library Reference Manual}. Synopsis:
9419 It avoids the bells and whistles of the more commonly-used
9420 @command{rm} command (@pxref{rm invocation}).
9423 unlink @var{filename}
9426 On some systems @code{unlink} can be used to delete the name of a
9427 directory. On others, it can be used that way only by a privileged user.
9428 In the GNU system @code{unlink} can never delete the name of a directory.
9430 The @command{unlink} command honors the @option{--help} and
9431 @option{--version} options. To remove a file whose name begins with
9432 @samp{-}, prefix the name with @samp{./}, e.g., @samp{unlink ./--help}.
9437 @node Changing file attributes
9438 @chapter Changing file attributes
9440 @cindex changing file attributes
9441 @cindex file attributes, changing
9442 @cindex attributes, file
9444 A file is not merely its contents, a name, and a file type
9445 (@pxref{Special file types}). A file also has an owner (a user ID), a
9446 group (a group ID), permissions (what the owner can do with the file,
9447 what people in the group can do, and what everyone else can do), various
9448 timestamps, and other information. Collectively, we call these a file's
9451 These commands change file attributes.
9454 * chgrp invocation:: Change file groups.
9455 * chmod invocation:: Change access permissions.
9456 * chown invocation:: Change file owners and groups.
9457 * touch invocation:: Change file timestamps.
9461 @node chown invocation
9462 @section @command{chown}: Change file owner and group
9465 @cindex file ownership, changing
9466 @cindex group ownership, changing
9467 @cindex changing file ownership
9468 @cindex changing group ownership
9470 @command{chown} changes the user and/or group ownership of each given @var{file}
9471 to @var{new-owner} or to the user and group of an existing reference file.
9475 chown [@var{option}]@dots{} @{@var{new-owner} | --reference=@var{ref_file}@} @var{file}@dots{}
9478 If used, @var{new-owner} specifies the new owner and/or group as follows
9479 (with no embedded white space):
9482 [@var{owner}] [ : [@var{group}] ]
9489 If only an @var{owner} (a user name or numeric user ID) is given, that
9490 user is made the owner of each given file, and the files' group is not
9493 @item owner@samp{:}group
9494 If the @var{owner} is followed by a colon and a @var{group} (a
9495 group name or numeric group ID), with no spaces between them, the group
9496 ownership of the files is changed as well (to @var{group}).
9499 If a colon but no group name follows @var{owner}, that user is
9500 made the owner of the files and the group of the files is changed to
9501 @var{owner}'s login group.
9504 If the colon and following @var{group} are given, but the owner
9505 is omitted, only the group of the files is changed; in this case,
9506 @command{chown} performs the same function as @command{chgrp}.
9509 If only a colon is given, or if @var{new-owner} is empty, neither the
9510 owner nor the group is changed.
9514 If @var{owner} or @var{group} is intended to represent a numeric user
9515 or group ID, then you may specify it with a leading @samp{+}.
9516 @xref{Disambiguating names and IDs}.
9518 Some older scripts may still use @samp{.} in place of the @samp{:} separator.
9519 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) does not
9520 require support for that, but for backward compatibility @acronym{GNU}
9521 @command{chown} supports @samp{.} so long as no ambiguity results.
9522 New scripts should avoid the use of @samp{.} because it is not
9523 portable, and because it has undesirable results if the entire
9524 @var{owner@samp{.}group} happens to identify a user whose name
9527 The @command{chown} command sometimes clears the set-user-ID or
9528 set-group-ID permission bits. This behavior depends on the policy and
9529 functionality of the underlying @code{chown} system call, which may
9530 make system-dependent file mode modifications outside the control of
9531 the @command{chown} command. For example, the @command{chown} command
9532 might not affect those bits when invoked by a user with appropriate
9533 privileges, or when the
9534 bits signify some function other than executable permission (e.g.,
9536 When in doubt, check the underlying system behavior.
9538 The program accepts the following options. Also see @ref{Common options}.
9546 @cindex changed owners, verbosely describing
9547 Verbosely describe the action for each @var{file} whose ownership
9556 @cindex error messages, omitting
9557 Do not print error messages about files whose ownership cannot be
9560 @itemx @w{@kbd{--from}=@var{old-owner}}
9562 @cindex symbolic links, changing owner
9563 Change a @var{file}'s ownership only if it has current attributes specified
9564 by @var{old-owner}. @var{old-owner} has the same form as @var{new-owner}
9566 This option is useful primarily from a security standpoint in that
9567 it narrows considerably the window of potential abuse.
9568 For example, to reflect a user ID numbering change for one user's files
9569 without an option like this, @code{root} might run
9572 find / -owner OLDUSER -print0 | xargs -0 chown -h NEWUSER
9575 But that is dangerous because the interval between when the @command{find}
9576 tests the existing file's owner and when the @command{chown} is actually run
9578 One way to narrow the gap would be to invoke chown for each file
9582 find / -owner OLDUSER -exec chown -h NEWUSER @{@} \;
9585 But that is very slow if there are many affected files.
9586 With this option, it is safer (the gap is narrower still)
9587 though still not perfect:
9590 chown -h -R --from=OLDUSER NEWUSER /
9594 @opindex --dereference
9595 @cindex symbolic links, changing owner
9597 Do not act on symbolic links themselves but rather on what they point to.
9598 This is the default.
9601 @itemx --no-dereference
9603 @opindex --no-dereference
9604 @cindex symbolic links, changing owner
9606 Act on symbolic links themselves instead of what they point to.
9607 This mode relies on the @code{lchown} system call.
9608 On systems that do not provide the @code{lchown} system call,
9609 @command{chown} fails when a file specified on the command line
9611 By default, no diagnostic is issued for symbolic links encountered
9612 during a recursive traversal, but see @option{--verbose}.
9614 @itemx --preserve-root
9615 @opindex --preserve-root
9616 @cindex root directory, disallow recursive modification
9617 Fail upon any attempt to recursively change the root directory, @file{/}.
9618 Without @option{--recursive}, this option has no effect.
9619 @xref{Treating / specially}.
9621 @itemx --no-preserve-root
9622 @opindex --no-preserve-root
9623 @cindex root directory, allow recursive modification
9624 Cancel the effect of any preceding @option{--preserve-root} option.
9625 @xref{Treating / specially}.
9627 @item --reference=@var{ref_file}
9628 @opindex --reference
9629 Change the user and group of each @var{file} to be the same as those of
9630 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9631 user and group of the symbolic link, but rather those of the file it
9638 Output a diagnostic for every file processed.
9639 If a symbolic link is encountered during a recursive traversal
9640 on a system without the @code{lchown} system call, and @option{--no-dereference}
9641 is in effect, then issue a diagnostic saying neither the symbolic link nor
9642 its referent is being changed.
9647 @opindex --recursive
9648 @cindex recursively changing file ownership
9649 Recursively change ownership of directories and their contents.
9652 @xref{Traversing symlinks}.
9655 @xref{Traversing symlinks}.
9658 @xref{Traversing symlinks}.
9667 # Change the owner of /u to "root".
9670 # Likewise, but also change its group to "staff".
9673 # Change the owner of /u and subfiles to "root".
9678 @node chgrp invocation
9679 @section @command{chgrp}: Change group ownership
9682 @cindex group ownership, changing
9683 @cindex changing group ownership
9685 @command{chgrp} changes the group ownership of each given @var{file}
9686 to @var{group} (which can be either a group name or a numeric group ID)
9687 or to the group of an existing reference file. Synopsis:
9690 chgrp [@var{option}]@dots{} @{@var{group} | --reference=@var{ref_file}@} @var{file}@dots{}
9693 If @var{group} is intended to represent a
9694 numeric group ID, then you may specify it with a leading @samp{+}.
9695 @xref{Disambiguating names and IDs}.
9697 The program accepts the following options. Also see @ref{Common options}.
9705 @cindex changed files, verbosely describing
9706 Verbosely describe the action for each @var{file} whose group actually
9715 @cindex error messages, omitting
9716 Do not print error messages about files whose group cannot be
9720 @opindex --dereference
9721 @cindex symbolic links, changing owner
9723 Do not act on symbolic links themselves but rather on what they point to.
9724 This is the default.
9727 @itemx --no-dereference
9729 @opindex --no-dereference
9730 @cindex symbolic links, changing group
9732 Act on symbolic links themselves instead of what they point to.
9733 This mode relies on the @code{lchown} system call.
9734 On systems that do not provide the @code{lchown} system call,
9735 @command{chgrp} fails when a file specified on the command line
9737 By default, no diagnostic is issued for symbolic links encountered
9738 during a recursive traversal, but see @option{--verbose}.
9740 @itemx --preserve-root
9741 @opindex --preserve-root
9742 @cindex root directory, disallow recursive modification
9743 Fail upon any attempt to recursively change the root directory, @file{/}.
9744 Without @option{--recursive}, this option has no effect.
9745 @xref{Treating / specially}.
9747 @itemx --no-preserve-root
9748 @opindex --no-preserve-root
9749 @cindex root directory, allow recursive modification
9750 Cancel the effect of any preceding @option{--preserve-root} option.
9751 @xref{Treating / specially}.
9753 @item --reference=@var{ref_file}
9754 @opindex --reference
9755 Change the group of each @var{file} to be the same as that of
9756 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9757 group of the symbolic link, but rather that of the file it refers to.
9763 Output a diagnostic for every file processed.
9764 If a symbolic link is encountered during a recursive traversal
9765 on a system without the @code{lchown} system call, and @option{--no-dereference}
9766 is in effect, then issue a diagnostic saying neither the symbolic link nor
9767 its referent is being changed.
9772 @opindex --recursive
9773 @cindex recursively changing group ownership
9774 Recursively change the group ownership of directories and their contents.
9777 @xref{Traversing symlinks}.
9780 @xref{Traversing symlinks}.
9783 @xref{Traversing symlinks}.
9792 # Change the group of /u to "staff".
9795 # Change the group of /u and subfiles to "staff".
9800 @node chmod invocation
9801 @section @command{chmod}: Change access permissions
9804 @cindex changing access permissions
9805 @cindex access permissions, changing
9806 @cindex permissions, changing access
9808 @command{chmod} changes the access permissions of the named files. Synopsis:
9811 chmod [@var{option}]@dots{} @{@var{mode} | --reference=@var{ref_file}@} @var{file}@dots{}
9814 @cindex symbolic links, permissions of
9815 @command{chmod} never changes the permissions of symbolic links, since
9816 the @command{chmod} system call cannot change their permissions.
9817 This is not a problem since the permissions of symbolic links are
9818 never used. However, for each symbolic link listed on the command
9819 line, @command{chmod} changes the permissions of the pointed-to file.
9820 In contrast, @command{chmod} ignores symbolic links encountered during
9821 recursive directory traversals.
9823 A successful use of @command{chmod} clears the set-group-ID bit of a
9824 regular file if the file's group ID does not match the user's
9825 effective group ID or one of the user's supplementary group IDs,
9826 unless the user has appropriate privileges. Additional restrictions
9827 may cause the set-user-ID and set-group-ID bits of @var{mode} or
9828 @var{ref_file} to be ignored. This behavior depends on the policy and
9829 functionality of the underlying @code{chmod} system call. When in
9830 doubt, check the underlying system behavior.
9832 If used, @var{mode} specifies the new file mode bits.
9833 For details, see the section on @ref{File permissions}.
9834 If you really want @var{mode} to have a leading @samp{-}, you should
9835 use @option{--} first, e.g., @samp{chmod -- -w file}. Typically,
9836 though, @samp{chmod a-w file} is preferable, and @command{chmod -w
9837 file} (without the @option{--}) complains if it behaves differently
9838 from what @samp{chmod a-w file} would do.
9840 The program accepts the following options. Also see @ref{Common options}.
9848 Verbosely describe the action for each @var{file} whose permissions
9857 @cindex error messages, omitting
9858 Do not print error messages about files whose permissions cannot be
9861 @itemx --preserve-root
9862 @opindex --preserve-root
9863 @cindex root directory, disallow recursive modification
9864 Fail upon any attempt to recursively change the root directory, @file{/}.
9865 Without @option{--recursive}, this option has no effect.
9866 @xref{Treating / specially}.
9868 @itemx --no-preserve-root
9869 @opindex --no-preserve-root
9870 @cindex root directory, allow recursive modification
9871 Cancel the effect of any preceding @option{--preserve-root} option.
9872 @xref{Treating / specially}.
9878 Verbosely describe the action or non-action taken for every @var{file}.
9880 @item --reference=@var{ref_file}
9881 @opindex --reference
9882 Change the mode of each @var{file} to be the same as that of @var{ref_file}.
9883 @xref{File permissions}.
9884 If @var{ref_file} is a symbolic link, do not use the mode
9885 of the symbolic link, but rather that of the file it refers to.
9890 @opindex --recursive
9891 @cindex recursively changing access permissions
9892 Recursively change permissions of directories and their contents.
9899 @node touch invocation
9900 @section @command{touch}: Change file timestamps
9903 @cindex changing file timestamps
9904 @cindex file timestamps, changing
9905 @cindex timestamps, changing file
9907 @command{touch} changes the access and/or modification times of the
9908 specified files. Synopsis:
9911 touch [@var{option}]@dots{} @var{file}@dots{}
9914 @cindex empty files, creating
9915 Any @var{file} argument that does not exist is created empty, unless
9916 option @option{--no-create} (@option{-c}) or @option{--no-dereference}
9917 (@option{-h}) was in effect.
9919 A @var{file} argument string of @samp{-} is handled specially and
9920 causes @command{touch} to change the times of the file associated with
9923 @cindex permissions, for changing file timestamps
9924 If changing both the access and modification times to the current
9925 time, @command{touch} can change the timestamps for files that the user
9926 running it does not own but has write permission for. Otherwise, the
9927 user must own the files.
9929 Although @command{touch} provides options for changing two of the times---the
9930 times of last access and modification---of a file, there is actually
9931 a standard third one as well: the inode change time. This is often
9932 referred to as a file's @code{ctime}.
9933 The inode change time represents the time when the file's meta-information
9934 last changed. One common example of this is when the permissions of a
9935 file change. Changing the permissions doesn't access the file, so
9936 the atime doesn't change, nor does it modify the file, so the mtime
9937 doesn't change. Yet, something about the file itself has changed,
9938 and this must be noted somewhere. This is the job of the ctime field.
9939 This is necessary, so that, for example, a backup program can make a
9940 fresh copy of the file, including the new permissions value.
9941 Another operation that modifies a file's ctime without affecting
9942 the others is renaming. In any case, it is not possible, in normal
9943 operations, for a user to change the ctime field to a user-specified value.
9944 Some operating systems and file systems support a fourth time: the
9945 birth time, when the file was first created; by definition, this
9946 timestamp never changes.
9949 Time stamps assume the time zone rules specified by the @env{TZ}
9950 environment variable, or by the system default rules if @env{TZ} is
9951 not set. @xref{TZ Variable,, Specifying the Time Zone with @env{TZ},
9952 libc, The GNU C Library Reference Manual}.
9953 You can avoid ambiguities during
9954 daylight saving transitions by using @sc{utc} time stamps.
9956 The program accepts the following options. Also see @ref{Common options}.
9962 @itemx --time=access
9966 @opindex atime@r{, changing}
9967 @opindex access @r{time, changing}
9968 @opindex use @r{time, changing}
9969 Change the access time only.
9974 @opindex --no-create
9975 Do not warn about or create files that do not exist.
9978 @itemx --date=@var{time}
9982 Use @var{time} instead of the current time. It can contain month names,
9983 time zones, @samp{am} and @samp{pm}, @samp{yesterday}, etc. For
9984 example, @option{--date="2004-02-27 14:19:13.489392193 +0530"}
9985 specifies the instant of time that is 489,392,193 nanoseconds after
9986 February 27, 2004 at 2:19:13 PM in a time zone that is 5 hours and 30
9987 minutes east of @acronym{UTC}. @xref{Date input formats}.
9988 File systems that do not support high-resolution time stamps
9989 silently ignore any excess precision here.
9993 @cindex BSD @command{touch} compatibility
9994 Ignored; for compatibility with BSD versions of @command{touch}.
9997 @itemx --no-dereference
9999 @opindex --no-dereference
10000 @cindex symbolic links, changing time
10002 Attempt to change the timestamps of a symbolic link, rather than what
10003 the link refers to. When using this option, empty files are not
10004 created, but option @option{-c} must also be used to avoid warning
10005 about files that do not exist. Not all systems support changing the
10006 timestamps of symlinks, since underlying system support for this
10007 action was not required until @acronym{POSIX} 2008. Also, on some
10008 systems, the mere act of examining a symbolic link changes the access
10009 time, such that only changes to the modification time will persist
10010 long enough to be observable. When coupled with option @option{-r}, a
10011 reference timestamp is taken from a symbolic link rather than the file
10015 @itemx --time=mtime
10016 @itemx --time=modify
10019 @opindex mtime@r{, changing}
10020 @opindex modify @r{time, changing}
10021 Change the modification time only.
10023 @item -r @var{file}
10024 @itemx --reference=@var{file}
10026 @opindex --reference
10027 Use the times of the reference @var{file} instead of the current time.
10028 If this option is combined with the @option{--date=@var{time}}
10029 (@option{-d @var{time}}) option, the reference @var{file}'s time is
10030 the origin for any relative @var{time}s given, but is otherwise ignored.
10031 For example, @samp{-r foo -d '-5 seconds'} specifies a time stamp
10032 equal to five seconds before the corresponding time stamp for @file{foo}.
10033 If @var{file} is a symbolic link, the reference timestamp is taken
10034 from the target of the symlink, unless @option{-h} was also in effect.
10036 @item -t [[@var{cc}]@var{yy}]@var{mmddhhmm}[.@var{ss}]
10037 Use the argument (optional four-digit or two-digit years, months,
10038 days, hours, minutes, optional seconds) instead of the current time.
10039 If the year is specified with only two digits, then @var{cc}
10040 is 20 for years in the range 0 @dots{} 68, and 19 for years in
10041 69 @dots{} 99. If no digits of the year are specified,
10042 the argument is interpreted as a date in the current year.
10043 Note that @var{ss} may be @samp{60}, to accommodate leap seconds.
10047 @vindex _POSIX2_VERSION
10048 On older systems, @command{touch} supports an obsolete syntax, as follows.
10049 If no timestamp is given with any of the @option{-d}, @option{-r}, or
10050 @option{-t} options, and if there are two or more @var{file}s and the
10051 first @var{file} is of the form @samp{@var{mmddhhmm}[@var{yy}]} and this
10052 would be a valid argument to the @option{-t} option (if the @var{yy}, if
10053 any, were moved to the front), and if the represented year
10054 is in the range 1969--1999, that argument is interpreted as the time
10055 for the other files instead of as a file name.
10056 This obsolete behavior can be enabled or disabled with the
10057 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
10058 conformance}), but portable scripts should avoid commands whose
10059 behavior depends on this variable.
10060 For example, use @samp{touch ./12312359 main.c} or @samp{touch -t
10061 12312359 main.c} rather than the ambiguous @samp{touch 12312359 main.c}.
10067 @chapter Disk usage
10071 No disk can hold an infinite amount of data. These commands report
10072 how much disk storage is in use or available, report other file and
10073 file status information, and write buffers to disk.
10076 * df invocation:: Report file system disk space usage.
10077 * du invocation:: Estimate file space usage.
10078 * stat invocation:: Report file or file system status.
10079 * sync invocation:: Synchronize memory and disk.
10080 * truncate invocation:: Shrink or extend the size of a file.
10084 @node df invocation
10085 @section @command{df}: Report file system disk space usage
10088 @cindex file system disk usage
10089 @cindex disk usage by file system
10091 @command{df} reports the amount of disk space used and available on
10092 file systems. Synopsis:
10095 df [@var{option}]@dots{} [@var{file}]@dots{}
10098 With no arguments, @command{df} reports the space used and available on all
10099 currently mounted file systems (of all types). Otherwise, @command{df}
10100 reports on the file system containing each argument @var{file}.
10102 Normally the disk space is printed in units of
10103 1024 bytes, but this can be overridden (@pxref{Block size}).
10104 Non-integer quantities are rounded up to the next higher unit.
10106 @cindex disk device file
10107 @cindex device file, disk
10108 If an argument @var{file} is a disk device file containing a mounted
10109 file system, @command{df} shows the space available on that file system
10110 rather than on the file system containing the device node (i.e., the root
10111 file system). @sc{gnu} @command{df} does not attempt to determine the disk usage
10112 on unmounted file systems, because on most kinds of systems doing so
10113 requires extremely nonportable intimate knowledge of file system
10116 The program accepts the following options. Also see @ref{Common options}.
10124 @cindex automounter file systems
10125 @cindex ignore file systems
10126 Include in the listing dummy file systems, which
10127 are omitted by default. Such file systems are typically special-purpose
10128 pseudo-file-systems, such as automounter entries.
10130 @item -B @var{size}
10131 @itemx --block-size=@var{size}
10133 @opindex --block-size
10134 @cindex file system sizes
10135 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10136 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10140 @cindex grand total of disk size, usage and available space
10141 Print a grand total of all arguments after all arguments have
10142 been processed. This can be used to find out the total disk size, usage
10143 and available space of all listed devices.
10149 Equivalent to @option{--si}.
10155 @cindex inode usage
10156 List inode usage information instead of block usage. An inode (short
10157 for index node) contains information about a file such as its owner,
10158 permissions, timestamps, and location on the disk.
10162 @cindex kibibytes for file system sizes
10163 Print sizes in 1024-byte blocks, overriding the default block size
10164 (@pxref{Block size}).
10165 This option is equivalent to @option{--block-size=1K}.
10171 @cindex file system types, limiting output to certain
10172 Limit the listing to local file systems. By default, remote file systems
10177 @cindex file system space, retrieving old data more quickly
10178 Do not invoke the @code{sync} system call before getting any usage data.
10179 This may make @command{df} run significantly faster on systems with many
10180 disks, but on some systems (notably SunOS) the results may be slightly
10181 out of date. This is the default.
10184 @itemx --portability
10186 @opindex --portability
10187 @cindex one-line output format
10188 @cindex @acronym{POSIX} output format
10189 @cindex portable output format
10190 @cindex output format, portable
10191 Use the @acronym{POSIX} output format. This is like the default format except
10196 The information about each file system is always printed on exactly
10197 one line; a mount device is never put on a line by itself. This means
10198 that if the mount device name is more than 20 characters long (e.g., for
10199 some network mounts), the columns are misaligned.
10202 The labels in the header output line are changed to conform to @acronym{POSIX}.
10205 The default block size and output format are unaffected by the
10206 @env{DF_BLOCK_SIZE}, @env{BLOCK_SIZE} and @env{BLOCKSIZE} environment
10207 variables. However, the default block size is still affected by
10208 @env{POSIXLY_CORRECT}: it is 512 if @env{POSIXLY_CORRECT} is set, 1024
10209 otherwise. @xref{Block size}.
10216 @cindex file system space, retrieving current data more slowly
10217 Invoke the @code{sync} system call before getting any usage data. On
10218 some systems (notably SunOS), doing this yields more up to date results,
10219 but in general this option makes @command{df} much slower, especially when
10220 there are many or very busy file systems.
10222 @item -t @var{fstype}
10223 @itemx --type=@var{fstype}
10226 @cindex file system types, limiting output to certain
10227 Limit the listing to file systems of type @var{fstype}. Multiple
10228 file system types can be specified by giving multiple @option{-t} options.
10229 By default, nothing is omitted.
10232 @itemx --print-type
10234 @opindex --print-type
10235 @cindex file system types, printing
10236 Print each file system's type. The types printed here are the same ones
10237 you can include or exclude with @option{-t} and @option{-x}. The particular
10238 types printed are whatever is supported by the system. Here are some of
10239 the common names (this list is certainly not exhaustive):
10244 @cindex @acronym{NFS} file system type
10245 An @acronym{NFS} file system, i.e., one mounted over a network from another
10246 machine. This is the one type name which seems to be used uniformly by
10249 @item 4.2@r{, }ufs@r{, }efs@dots{}
10250 @cindex Linux file system types
10251 @cindex local file system types
10252 @opindex 4.2 @r{file system type}
10253 @opindex ufs @r{file system type}
10254 @opindex efs @r{file system type}
10255 A file system on a locally-mounted hard disk. (The system might even
10256 support more than one type here; Linux does.)
10258 @item hsfs@r{, }cdfs
10259 @cindex CD-ROM file system type
10260 @cindex High Sierra file system
10261 @opindex hsfs @r{file system type}
10262 @opindex cdfs @r{file system type}
10263 A file system on a CD-ROM drive. HP-UX uses @samp{cdfs}, most other
10264 systems use @samp{hsfs} (@samp{hs} for ``High Sierra'').
10267 @cindex PC file system
10268 @cindex DOS file system
10269 @cindex MS-DOS file system
10270 @cindex diskette file system
10272 An MS-DOS file system, usually on a diskette.
10276 @item -x @var{fstype}
10277 @itemx --exclude-type=@var{fstype}
10279 @opindex --exclude-type
10280 Limit the listing to file systems not of type @var{fstype}.
10281 Multiple file system types can be eliminated by giving multiple
10282 @option{-x} options. By default, no file system types are omitted.
10285 Ignored; for compatibility with System V versions of @command{df}.
10290 Failure includes the case where no output is generated, so you can
10291 inspect the exit status of a command like @samp{df -t ext3 -t reiserfs
10292 @var{dir}} to test whether @var{dir} is on a file system of type
10293 @samp{ext3} or @samp{reiserfs}.
10296 @node du invocation
10297 @section @command{du}: Estimate file space usage
10300 @cindex file space usage
10301 @cindex disk usage for files
10303 @command{du} reports the amount of disk space used by the specified files
10304 and for each subdirectory (of directory arguments). Synopsis:
10307 du [@var{option}]@dots{} [@var{file}]@dots{}
10310 With no arguments, @command{du} reports the disk space for the current
10311 directory. Normally the disk space is printed in units of
10312 1024 bytes, but this can be overridden (@pxref{Block size}).
10313 Non-integer quantities are rounded up to the next higher unit.
10315 If two or more hard links point to the same file, only one of the hard
10316 links is counted. The @var{file} argument order affects which links
10317 are counted, and changing the argument order may change the numbers
10318 that @command{du} outputs.
10320 The program accepts the following options. Also see @ref{Common options}.
10328 Show counts for all files, not just directories.
10330 @itemx --apparent-size
10331 @opindex --apparent-size
10332 Print apparent sizes, rather than disk usage. The apparent size of a
10333 file is the number of bytes reported by @code{wc -c} on regular files,
10334 or more generally, @code{ls -l --block-size=1} or @code{stat --format=%s}.
10335 For example, a file containing the word @samp{zoo} with no newline would,
10336 of course, have an apparent size of 3. Such a small file may require
10337 anywhere from 0 to 16 KiB or more of disk space, depending on
10338 the type and configuration of the file system on which the file resides.
10339 However, a sparse file created with this command:
10342 dd bs=1 seek=2GiB if=/dev/null of=big
10346 has an apparent size of 2 GiB, yet on most modern
10347 systems, it actually uses almost no disk space.
10353 Equivalent to @code{--apparent-size --block-size=1}.
10355 @item -B @var{size}
10356 @itemx --block-size=@var{size}
10358 @opindex --block-size
10360 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10361 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10367 @cindex grand total of disk space
10368 Print a grand total of all arguments after all arguments have
10369 been processed. This can be used to find out the total disk usage of
10370 a given set of files or directories.
10373 @itemx --dereference-args
10375 @opindex --dereference-args
10376 Dereference symbolic links that are command line arguments.
10377 Does not affect other symbolic links. This is helpful for finding
10378 out the disk usage of directories, such as @file{/usr/tmp}, which
10379 are often symbolic links.
10381 @c --files0-from=FILE
10382 @filesZeroFromOption{du,, with the @option{--total} (@option{-c}) option}
10388 Equivalent to @option{--dereference-args} (@option{-D}).
10392 @cindex kibibytes for file sizes
10393 Print sizes in 1024-byte blocks, overriding the default block size
10394 (@pxref{Block size}).
10395 This option is equivalent to @option{--block-size=1K}.
10398 @itemx --count-links
10400 @opindex --count-links
10401 @cindex hard links, counting in @command{du}
10402 Count the size of all files, even if they have appeared already (as a
10406 @itemx --dereference
10408 @opindex --dereference
10409 @cindex symbolic links, dereferencing in @command{du}
10410 Dereference symbolic links (show the disk space used by the file
10411 or directory that the link points to instead of the space used by
10416 @cindex mebibytes for file sizes
10417 Print sizes in 1,048,576-byte blocks, overriding the default block size
10418 (@pxref{Block size}).
10419 This option is equivalent to @option{--block-size=1M}.
10422 @itemx --no-dereference
10424 @opindex --no-dereference
10425 @cindex symbolic links, dereferencing in @command{du}
10426 For each symbolic links encountered by @command{du},
10427 consider the disk space used by the symbolic link.
10429 @item --max-depth=@var{depth}
10430 @opindex --max-depth=@var{depth}
10431 @cindex limiting output of @command{du}
10432 Show the total for each directory (and file if --all) that is at
10433 most MAX_DEPTH levels down from the root of the hierarchy. The root
10434 is at level 0, so @code{du --max-depth=0} is equivalent to @code{du -s}.
10443 @opindex --summarize
10444 Display only a total for each argument.
10447 @itemx --separate-dirs
10449 @opindex --separate-dirs
10450 Normally, in the output of @command{du} (when not using @option{--summarize}),
10451 the size listed next to a directory name, @var{d}, represents the sum
10452 of sizes of all entries beneath @var{d} as well as the size of @var{d} itself.
10453 With @option{--separate-dirs}, the size reported for a directory name,
10454 @var{d}, is merely the @code{stat.st_size}-derived size of the directory
10459 @cindex last modified dates, displaying in @command{du}
10460 Show time of the most recent modification of any file in the directory,
10461 or any of its subdirectories.
10463 @itemx --time=ctime
10464 @itemx --time=status
10467 @opindex ctime@r{, show the most recent}
10468 @opindex status time@r{, show the most recent}
10469 @opindex use time@r{, show the most recent}
10470 Show the most recent status change time (the @samp{ctime} in the inode) of
10471 any file in the directory, instead of the modification time.
10473 @itemx --time=atime
10474 @itemx --time=access
10476 @opindex atime@r{, show the most recent}
10477 @opindex access time@r{, show the most recent}
10478 Show the most recent access time (the @samp{atime} in the inode) of
10479 any file in the directory, instead of the modification time.
10481 @item --time-style=@var{style}
10482 @opindex --time-style
10484 List timestamps in style @var{style}. This option has an effect only if
10485 the @option{--time} option is also specified. The @var{style} should
10486 be one of the following:
10489 @item +@var{format}
10491 List timestamps using @var{format}, where @var{format} is interpreted
10492 like the format argument of @command{date} (@pxref{date invocation}).
10493 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
10494 @command{du} to list timestamps like @samp{2002-03-30 23:45:56}. As
10495 with @command{date}, @var{format}'s interpretation is affected by the
10496 @env{LC_TIME} locale category.
10499 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
10500 format with nanosecond precision, e.g., @samp{2002-03-30
10501 23:45:56.477817180 -0700}. This style is equivalent to
10502 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
10505 List @acronym{ISO} 8601 date and time in minutes, e.g.,
10506 @samp{2002-03-30 23:45}. These timestamps are shorter than
10507 @samp{full-iso} timestamps, and are usually good enough for everyday
10508 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
10511 List @acronym{ISO} 8601 dates for timestamps, e.g., @samp{2002-03-30}.
10512 This style is equivalent to @samp{+%Y-%m-%d}.
10516 You can specify the default value of the @option{--time-style} option
10517 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
10518 the default style is @samp{long-iso}. For compatibility with @command{ls},
10519 if @env{TIME_STYLE} begins with @samp{+} and contains a newline,
10520 the newline and any later characters are ignored; if @env{TIME_STYLE}
10521 begins with @samp{posix-} the @samp{posix-} is ignored; and if
10522 @env{TIME_STYLE} is @samp{locale} it is ignored.
10525 @itemx --one-file-system
10527 @opindex --one-file-system
10528 @cindex one file system, restricting @command{du} to
10529 Skip directories that are on different file systems from the one that
10530 the argument being processed is on.
10532 @item --exclude=@var{pattern}
10533 @opindex --exclude=@var{pattern}
10534 @cindex excluding files from @command{du}
10535 When recursing, skip subdirectories or files matching @var{pattern}.
10536 For example, @code{du --exclude='*.o'} excludes files whose names
10539 @item -X @var{file}
10540 @itemx --exclude-from=@var{file}
10541 @opindex -X @var{file}
10542 @opindex --exclude-from=@var{file}
10543 @cindex excluding files from @command{du}
10544 Like @option{--exclude}, except take the patterns to exclude from @var{file},
10545 one per line. If @var{file} is @samp{-}, take the patterns from standard
10550 @cindex NFS mounts from BSD to HP-UX
10551 On BSD systems, @command{du} reports sizes that are half the correct
10552 values for files that are NFS-mounted from HP-UX systems. On HP-UX
10553 systems, it reports sizes that are twice the correct values for
10554 files that are NFS-mounted from BSD systems. This is due to a flaw
10555 in HP-UX; it also affects the HP-UX @command{du} program.
10560 @node stat invocation
10561 @section @command{stat}: Report file or file system status
10564 @cindex file status
10565 @cindex file system status
10567 @command{stat} displays information about the specified file(s). Synopsis:
10570 stat [@var{option}]@dots{} [@var{file}]@dots{}
10573 With no option, @command{stat} reports all information about the given files.
10574 But it also can be used to report the information of the file systems the
10575 given files are located on. If the files are links, @command{stat} can
10576 also give information about the files the links point to.
10578 @mayConflictWithShellBuiltIn{stat}
10583 @itemx --dereference
10585 @opindex --dereference
10586 @cindex symbolic links, dereferencing in @command{stat}
10587 Change how @command{stat} treats symbolic links.
10588 With this option, @command{stat} acts on the file referenced
10589 by each symbolic link argument.
10590 Without it, @command{stat} acts on any symbolic link argument directly.
10593 @itemx --file-system
10595 @opindex --file-system
10596 @cindex file systems
10597 Report information about the file systems where the given files are located
10598 instead of information about the files themselves.
10601 @itemx --format=@var{format}
10603 @opindex --format=@var{format}
10604 @cindex output format
10605 Use @var{format} rather than the default format.
10606 @var{format} is automatically newline-terminated, so
10607 running a command like the following with two or more @var{file}
10608 operands produces a line of output for each operand:
10610 $ stat --format=%d:%i / /usr
10615 @itemx --printf=@var{format}
10616 @opindex --printf=@var{format}
10617 @cindex output format
10618 Use @var{format} rather than the default format.
10619 Like @option{--format}, but interpret backslash escapes,
10620 and do not output a mandatory trailing newline.
10621 If you want a newline, include @samp{\n} in the @var{format}.
10622 Here's how you would use @option{--printf} to print the device
10623 and inode numbers of @file{/} and @file{/usr}:
10625 $ stat --printf='%d:%i\n' / /usr
10634 @cindex terse output
10635 Print the information in terse form, suitable for parsing by other programs.
10639 The valid @var{format} directives for files with @option{--format} and
10640 @option{--printf} are:
10643 @item %a - Access rights in octal
10644 @item %A - Access rights in human readable form
10645 @item %b - Number of blocks allocated (see @samp{%B})
10646 @item %B - The size in bytes of each block reported by @samp{%b}
10647 @item %d - Device number in decimal
10648 @item %D - Device number in hex
10649 @item %f - Raw mode in hex
10650 @item %F - File type
10651 @item %g - Group ID of owner
10652 @item %G - Group name of owner
10653 @item %h - Number of hard links
10654 @item %i - Inode number
10655 @item %n - File name
10656 @item %N - Quoted file name with dereference if symbolic link
10657 @item %o - I/O block size
10658 @item %s - Total size, in bytes
10659 @item %t - Major device type in hex
10660 @item %T - Minor device type in hex
10661 @item %u - User ID of owner
10662 @item %U - User name of owner
10663 @item %x - Time of last access
10664 @item %X - Time of last access as seconds since Epoch
10665 @item %y - Time of last modification
10666 @item %Y - Time of last modification as seconds since Epoch
10667 @item %z - Time of last change
10668 @item %Z - Time of last change as seconds since Epoch
10671 When listing file system information (@option{--file-system} (@option{-f})),
10672 you must use a different set of @var{format} directives:
10675 @item %a - Free blocks available to non-super-user
10676 @item %b - Total data blocks in file system
10677 @item %c - Total file nodes in file system
10678 @item %d - Free file nodes in file system
10679 @item %f - Free blocks in file system
10680 @item %i - File System ID in hex
10681 @item %l - Maximum length of file names
10682 @item %n - File name
10683 @item %s - Block size (for faster transfers)
10684 @item %S - Fundamental block size (for block counts)
10685 @item %t - Type in hex
10686 @item %T - Type in human readable form
10690 Time stamps are listed according to the time zone rules specified by
10691 the @env{TZ} environment variable, or by the system default rules if
10692 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
10693 with @env{TZ}, libc, The GNU C Library Reference Manual}.
10698 @node sync invocation
10699 @section @command{sync}: Synchronize data on disk with memory
10702 @cindex synchronize disk and memory
10704 @cindex superblock, writing
10705 @cindex inodes, written buffered
10706 @command{sync} writes any data buffered in memory out to disk. This can
10707 include (but is not limited to) modified superblocks, modified inodes,
10708 and delayed reads and writes. This must be implemented by the kernel;
10709 The @command{sync} program does nothing but exercise the @code{sync} system
10712 @cindex crashes and corruption
10713 The kernel keeps data in memory to avoid doing (relatively slow) disk
10714 reads and writes. This improves performance, but if the computer
10715 crashes, data may be lost or the file system corrupted as a
10716 result. The @command{sync} command ensures everything in memory
10717 is written to disk.
10719 Any arguments are ignored, except for a lone @option{--help} or
10720 @option{--version} (@pxref{Common options}).
10725 @node truncate invocation
10726 @section @command{truncate}: Shrink or extend the size of a file
10729 @cindex truncating, file sizes
10731 @command{truncate} shrinks or extends the size of each @var{file} to the
10732 specified size. Synopsis:
10735 truncate @var{option}@dots{} @var{file}@dots{}
10738 @cindex files, creating
10739 Any @var{file} that does not exist is created.
10741 @cindex sparse files, creating
10742 @cindex holes, creating files with
10743 If a @var{file} is larger than the specified size, the extra data is lost.
10744 If a @var{file} is shorter, it is extended and the extended part (or hole)
10745 reads as zero bytes.
10747 The program accepts the following options. Also see @ref{Common options}.
10754 @opindex --no-create
10755 Do not create files that do not exist.
10760 @opindex --io-blocks
10761 Treat @var{size} as number of I/O blocks of the @var{file} rather than bytes.
10763 @item -r @var{rfile}
10764 @itemx --reference=@var{rfile}
10766 @opindex --reference
10767 Set the size of each @var{file} to the same size as @var{rfile}.
10769 @item -s @var{size}
10770 @itemx --size=@var{size}
10773 Set the size of each @var{file} to this @var{size}.
10774 @multiplierSuffixesNoBlocks{size}
10776 @var{size} may also be prefixed by one of the following to adjust
10777 the size of each @var{file} based on their current size:
10779 @samp{+} => extend by
10780 @samp{-} => reduce by
10781 @samp{<} => at most
10782 @samp{>} => at least
10783 @samp{/} => round down to multiple of
10784 @samp{%} => round up to multiple of
10792 @node Printing text
10793 @chapter Printing text
10795 @cindex printing text, commands for
10796 @cindex commands for printing text
10798 This section describes commands that display text strings.
10801 * echo invocation:: Print a line of text.
10802 * printf invocation:: Format and print data.
10803 * yes invocation:: Print a string until interrupted.
10807 @node echo invocation
10808 @section @command{echo}: Print a line of text
10811 @cindex displaying text
10812 @cindex printing text
10813 @cindex text, displaying
10814 @cindex arbitrary text, displaying
10816 @command{echo} writes each given @var{string} to standard output, with a
10817 space between each and a newline after the last one. Synopsis:
10820 echo [@var{option}]@dots{} [@var{string}]@dots{}
10823 @mayConflictWithShellBuiltIn{echo}
10825 The program accepts the following options. Also see @ref{Common options}.
10826 Options must precede operands, and the normally-special argument
10827 @samp{--} has no special meaning and is treated like any other
10833 Do not output the trailing newline.
10837 @cindex backslash escapes
10838 Enable interpretation of the following backslash-escaped characters in
10847 produce no further output
10863 the eight-bit value that is the octal number @var{nnn}
10864 (zero to three octal digits)
10866 the eight-bit value that is the octal number @var{nnn}
10867 (one to three octal digits)
10869 the eight-bit value that is the hexadecimal number @var{hh}
10870 (one or two hexadecimal digits)
10875 @cindex backslash escapes
10876 Disable interpretation of backslash escapes in each @var{string}.
10877 This is the default. If @option{-e} and @option{-E} are both
10878 specified, the last one given takes effect.
10882 @vindex POSIXLY_CORRECT
10883 If the @env{POSIXLY_CORRECT} environment variable is set, then when
10884 @command{echo}'s first argument is not @option{-n} it outputs
10885 option-like arguments instead of treating them as options. For
10886 example, @code{echo -ne hello} outputs @samp{-ne hello} instead of
10887 plain @samp{hello}.
10889 @acronym{POSIX} does not require support for any options, and says
10890 that the behavior of @command{echo} is implementation-defined if any
10891 @var{string} contains a backslash or if the first argument is
10892 @option{-n}. Portable programs can use the @command{printf} command
10893 if they need to omit trailing newlines or output control characters or
10894 backslashes. @xref{printf invocation}.
10899 @node printf invocation
10900 @section @command{printf}: Format and print data
10903 @command{printf} does formatted printing of text. Synopsis:
10906 printf @var{format} [@var{argument}]@dots{}
10909 @command{printf} prints the @var{format} string, interpreting @samp{%}
10910 directives and @samp{\} escapes to format numeric and string arguments
10911 in a way that is mostly similar to the C @samp{printf} function.
10912 @xref{Output Conversion Syntax,, @command{printf} format directives,
10913 libc, The GNU C Library Reference Manual}, for details.
10914 The differences are listed below.
10916 @mayConflictWithShellBuiltIn{printf}
10921 The @var{format} argument is reused as necessary to convert all the
10922 given @var{argument}s. For example, the command @samp{printf %s a b}
10926 Missing @var{argument}s are treated as null strings or as zeros,
10927 depending on whether the context expects a string or a number. For
10928 example, the command @samp{printf %sx%d} prints @samp{x0}.
10932 An additional escape, @samp{\c}, causes @command{printf} to produce no
10933 further output. For example, the command @samp{printf 'A%sC\cD%sF' B
10934 E} prints @samp{ABC}.
10937 The hexadecimal escape sequence @samp{\x@var{hh}} has at most two
10938 digits, as opposed to C where it can have an unlimited number of
10939 digits. For example, the command @samp{printf '\x07e'} prints two
10940 bytes, whereas the C statement @samp{printf ("\x07e")} prints just
10945 @command{printf} has an additional directive, @samp{%b}, which prints its
10946 argument string with @samp{\} escapes interpreted in the same way as in
10947 the @var{format} string, except that octal escapes are of the form
10948 @samp{\0@var{ooo}} where @var{ooo} is 0 to 3 octal digits.
10949 If a precision is also given, it limits the number of bytes printed
10950 from the converted string.
10953 Numeric arguments must be single C constants, possibly with leading
10954 @samp{+} or @samp{-}. For example, @samp{printf %.4d -3} outputs
10958 @vindex POSIXLY_CORRECT
10959 If the leading character of a numeric argument is @samp{"} or @samp{'}
10960 then its value is the numeric value of the immediately following
10961 character. Any remaining characters are silently ignored if the
10962 @env{POSIXLY_CORRECT} environment variable is set; otherwise, a
10963 warning is printed. For example, @samp{printf "%d" "'a"} outputs
10964 @samp{97} on hosts that use the @acronym{ASCII} character set, since
10965 @samp{a} has the numeric value 97 in @acronym{ASCII}.
10970 A floating-point argument must use a period before any fractional
10971 digits, but is printed according to the @env{LC_NUMERIC} category of the
10972 current locale. For example, in a locale whose radix character is a
10973 comma, the command @samp{printf %g 3.14} outputs @samp{3,14} whereas
10974 the command @samp{printf %g 3,14} is an error.
10978 @command{printf} interprets @samp{\@var{ooo}} in @var{format} as an octal number
10979 (if @var{ooo} is 1 to 3 octal digits) specifying a character to print,
10980 and @samp{\x@var{hh}} as a hexadecimal number (if @var{hh} is 1 to 2 hex
10981 digits) specifying a character to print.
10986 @cindex ISO/IEC 10646
10988 @command{printf} interprets two character syntaxes introduced in
10989 @acronym{ISO} C 99:
10990 @samp{\u} for 16-bit Unicode (@acronym{ISO}/@acronym{IEC} 10646)
10991 characters, specified as
10992 four hexadecimal digits @var{hhhh}, and @samp{\U} for 32-bit Unicode
10993 characters, specified as eight hexadecimal digits @var{hhhhhhhh}.
10994 @command{printf} outputs the Unicode characters
10995 according to the @env{LC_CTYPE} locale. Unicode characters in the ranges
10996 U+0000...U+009F, U+D800...U+DFFF cannot be specified by this syntax, except
10997 for U+0024 ($), U+0040 (@@), and U+0060 (@`).
10999 The processing of @samp{\u} and @samp{\U} requires a full-featured
11000 @code{iconv} facility. It is activated on systems with glibc 2.2 (or newer),
11001 or when @code{libiconv} is installed prior to this package. Otherwise
11002 @samp{\u} and @samp{\U} will print as-is.
11004 The only options are a lone @option{--help} or
11005 @option{--version}. @xref{Common options}.
11006 Options must precede operands.
11008 The Unicode character syntaxes are useful for writing strings in a locale
11009 independent way. For example, a string containing the Euro currency symbol
11012 $ env printf '\u20AC 14.95'
11016 will be output correctly in all locales supporting the Euro symbol
11017 (@acronym{ISO}-8859-15, UTF-8, and others). Similarly, a Chinese string
11020 $ env printf '\u4e2d\u6587'
11024 will be output correctly in all Chinese locales (GB2312, BIG5, UTF-8, etc).
11026 Note that in these examples, the @command{printf} command has been
11027 invoked via @command{env} to ensure that we run the program found via
11028 your shell's search path, and not a shell alias or a built-in function.
11030 For larger strings, you don't need to look up the hexadecimal code
11031 values of each character one by one. @acronym{ASCII} characters mixed with \u
11032 escape sequences is also known as the JAVA source file encoding. You can
11033 use GNU recode 3.5c (or newer) to convert strings to this encoding. Here
11034 is how to convert a piece of text into a shell script which will output
11035 this text in a locale-independent way:
11038 $ LC_CTYPE=zh_CN.big5 /usr/local/bin/printf \
11039 '\u4e2d\u6587\n' > sample.txt
11040 $ recode BIG5..JAVA < sample.txt \
11041 | sed -e "s|^|/usr/local/bin/printf '|" -e "s|$|\\\\n'|" \
11048 @node yes invocation
11049 @section @command{yes}: Print a string until interrupted
11052 @cindex repeated output of a string
11054 @command{yes} prints the command line arguments, separated by spaces and
11055 followed by a newline, forever until it is killed. If no arguments are
11056 given, it prints @samp{y} followed by a newline forever until killed.
11058 Upon a write error, @command{yes} exits with status @samp{1}.
11060 The only options are a lone @option{--help} or @option{--version}.
11061 To output an argument that begins with
11062 @samp{-}, precede it with @option{--}, e.g., @samp{yes -- --help}.
11063 @xref{Common options}.
11067 @chapter Conditions
11070 @cindex commands for exit status
11071 @cindex exit status commands
11073 This section describes commands that are primarily useful for their exit
11074 status, rather than their output. Thus, they are often used as the
11075 condition of shell @code{if} statements, or as the last command in a
11079 * false invocation:: Do nothing, unsuccessfully.
11080 * true invocation:: Do nothing, successfully.
11081 * test invocation:: Check file types and compare values.
11082 * expr invocation:: Evaluate expressions.
11086 @node false invocation
11087 @section @command{false}: Do nothing, unsuccessfully
11090 @cindex do nothing, unsuccessfully
11091 @cindex failure exit status
11092 @cindex exit status of @command{false}
11094 @command{false} does nothing except return an exit status of 1, meaning
11095 @dfn{failure}. It can be used as a place holder in shell scripts
11096 where an unsuccessful command is needed.
11097 In most modern shells, @command{false} is a built-in command, so when
11098 you use @samp{false} in a script, you're probably using the built-in
11099 command, not the one documented here.
11101 @command{false} honors the @option{--help} and @option{--version} options.
11103 This version of @command{false} is implemented as a C program, and is thus
11104 more secure and faster than a shell script implementation, and may safely
11105 be used as a dummy shell for the purpose of disabling accounts.
11107 Note that @command{false} (unlike all other programs documented herein)
11108 exits unsuccessfully, even when invoked with
11109 @option{--help} or @option{--version}.
11111 Portable programs should not assume that the exit status of
11112 @command{false} is 1, as it is greater than 1 on some
11113 non-@acronym{GNU} hosts.
11116 @node true invocation
11117 @section @command{true}: Do nothing, successfully
11120 @cindex do nothing, successfully
11122 @cindex successful exit
11123 @cindex exit status of @command{true}
11125 @command{true} does nothing except return an exit status of 0, meaning
11126 @dfn{success}. It can be used as a place holder in shell scripts
11127 where a successful command is needed, although the shell built-in
11128 command @code{:} (colon) may do the same thing faster.
11129 In most modern shells, @command{true} is a built-in command, so when
11130 you use @samp{true} in a script, you're probably using the built-in
11131 command, not the one documented here.
11133 @command{true} honors the @option{--help} and @option{--version} options.
11135 Note, however, that it is possible to cause @command{true}
11136 to exit with nonzero status: with the @option{--help} or @option{--version}
11137 option, and with standard
11138 output already closed or redirected to a file that evokes an I/O error.
11139 For example, using a Bourne-compatible shell:
11142 $ ./true --version >&-
11143 ./true: write error: Bad file number
11144 $ ./true --version > /dev/full
11145 ./true: write error: No space left on device
11148 This version of @command{true} is implemented as a C program, and is thus
11149 more secure and faster than a shell script implementation, and may safely
11150 be used as a dummy shell for the purpose of disabling accounts.
11152 @node test invocation
11153 @section @command{test}: Check file types and compare values
11156 @cindex check file types
11157 @cindex compare values
11158 @cindex expression evaluation
11160 @command{test} returns a status of 0 (true) or 1 (false) depending on the
11161 evaluation of the conditional expression @var{expr}. Each part of the
11162 expression must be a separate argument.
11164 @command{test} has file status checks, string operators, and numeric
11165 comparison operators.
11167 @command{test} has an alternate form that uses opening and closing
11168 square brackets instead a leading @samp{test}. For example, instead
11169 of @samp{test -d /}, you can write @samp{[ -d / ]}. The square
11170 brackets must be separate arguments; for example, @samp{[-d /]} does
11171 not have the desired effect. Since @samp{test @var{expr}} and @samp{[
11172 @var{expr} ]} have the same meaning, only the former form is discussed
11178 test @var{expression}
11180 [ @var{expression} ]
11185 @mayConflictWithShellBuiltIn{test}
11187 If @var{expression} is omitted, @command{test} returns false.
11188 If @var{expression} is a single argument,
11189 @command{test} returns false if the argument is null and true otherwise. The argument
11190 can be any string, including strings like @samp{-d}, @samp{-1},
11191 @samp{--}, @samp{--help}, and @samp{--version} that most other
11192 programs would treat as options. To get help and version information,
11193 invoke the commands @samp{[ --help} and @samp{[ --version}, without
11194 the usual closing brackets. @xref{Common options}.
11196 @cindex exit status of @command{test}
11200 0 if the expression is true,
11201 1 if the expression is false,
11202 2 if an error occurred.
11206 * File type tests:: -[bcdfhLpSt]
11207 * Access permission tests:: -[gkruwxOG]
11208 * File characteristic tests:: -e -s -nt -ot -ef
11209 * String tests:: -z -n = !=
11210 * Numeric tests:: -eq -ne -lt -le -gt -ge
11211 * Connectives for test:: ! -a -o
11215 @node File type tests
11216 @subsection File type tests
11218 @cindex file type tests
11220 These options test for particular types of files. (Everything's a file,
11221 but not all files are the same!)
11225 @item -b @var{file}
11227 @cindex block special check
11228 True if @var{file} exists and is a block special device.
11230 @item -c @var{file}
11232 @cindex character special check
11233 True if @var{file} exists and is a character special device.
11235 @item -d @var{file}
11237 @cindex directory check
11238 True if @var{file} exists and is a directory.
11240 @item -f @var{file}
11242 @cindex regular file check
11243 True if @var{file} exists and is a regular file.
11245 @item -h @var{file}
11246 @itemx -L @var{file}
11249 @cindex symbolic link check
11250 True if @var{file} exists and is a symbolic link.
11251 Unlike all other file-related tests, this test does not dereference
11252 @var{file} if it is a symbolic link.
11254 @item -p @var{file}
11256 @cindex named pipe check
11257 True if @var{file} exists and is a named pipe.
11259 @item -S @var{file}
11261 @cindex socket check
11262 True if @var{file} exists and is a socket.
11266 @cindex terminal check
11267 True if @var{fd} is a file descriptor that is associated with a
11273 @node Access permission tests
11274 @subsection Access permission tests
11276 @cindex access permission tests
11277 @cindex permission tests
11279 These options test for particular access permissions.
11283 @item -g @var{file}
11285 @cindex set-group-ID check
11286 True if @var{file} exists and has its set-group-ID bit set.
11288 @item -k @var{file}
11290 @cindex sticky bit check
11291 True if @var{file} exists and has its @dfn{sticky} bit set.
11293 @item -r @var{file}
11295 @cindex readable file check
11296 True if @var{file} exists and read permission is granted.
11298 @item -u @var{file}
11300 @cindex set-user-ID check
11301 True if @var{file} exists and has its set-user-ID bit set.
11303 @item -w @var{file}
11305 @cindex writable file check
11306 True if @var{file} exists and write permission is granted.
11308 @item -x @var{file}
11310 @cindex executable file check
11311 True if @var{file} exists and execute permission is granted
11312 (or search permission, if it is a directory).
11314 @item -O @var{file}
11316 @cindex owned by effective user ID check
11317 True if @var{file} exists and is owned by the current effective user ID.
11319 @item -G @var{file}
11321 @cindex owned by effective group ID check
11322 True if @var{file} exists and is owned by the current effective group ID.
11326 @node File characteristic tests
11327 @subsection File characteristic tests
11329 @cindex file characteristic tests
11331 These options test other file characteristics.
11335 @item -e @var{file}
11337 @cindex existence-of-file check
11338 True if @var{file} exists.
11340 @item -s @var{file}
11342 @cindex nonempty file check
11343 True if @var{file} exists and has a size greater than zero.
11345 @item @var{file1} -nt @var{file2}
11347 @cindex newer-than file check
11348 True if @var{file1} is newer (according to modification date) than
11349 @var{file2}, or if @var{file1} exists and @var{file2} does not.
11351 @item @var{file1} -ot @var{file2}
11353 @cindex older-than file check
11354 True if @var{file1} is older (according to modification date) than
11355 @var{file2}, or if @var{file2} exists and @var{file1} does not.
11357 @item @var{file1} -ef @var{file2}
11359 @cindex same file check
11360 @cindex hard link check
11361 True if @var{file1} and @var{file2} have the same device and inode
11362 numbers, i.e., if they are hard links to each other.
11368 @subsection String tests
11370 @cindex string tests
11372 These options test string characteristics. You may need to quote
11373 @var{string} arguments for the shell. For example:
11379 The quotes here prevent the wrong arguments from being passed to
11380 @command{test} if @samp{$V} is empty or contains special characters.
11384 @item -z @var{string}
11386 @cindex zero-length string check
11387 True if the length of @var{string} is zero.
11389 @item -n @var{string}
11390 @itemx @var{string}
11392 @cindex nonzero-length string check
11393 True if the length of @var{string} is nonzero.
11395 @item @var{string1} = @var{string2}
11397 @cindex equal string check
11398 True if the strings are equal.
11400 @item @var{string1} != @var{string2}
11402 @cindex not-equal string check
11403 True if the strings are not equal.
11408 @node Numeric tests
11409 @subsection Numeric tests
11411 @cindex numeric tests
11412 @cindex arithmetic tests
11414 Numeric relational operators. The arguments must be entirely numeric
11415 (possibly negative), or the special expression @w{@code{-l @var{string}}},
11416 which evaluates to the length of @var{string}.
11420 @item @var{arg1} -eq @var{arg2}
11421 @itemx @var{arg1} -ne @var{arg2}
11422 @itemx @var{arg1} -lt @var{arg2}
11423 @itemx @var{arg1} -le @var{arg2}
11424 @itemx @var{arg1} -gt @var{arg2}
11425 @itemx @var{arg1} -ge @var{arg2}
11432 These arithmetic binary operators return true if @var{arg1} is equal,
11433 not-equal, less-than, less-than-or-equal, greater-than, or
11434 greater-than-or-equal than @var{arg2}, respectively.
11441 test -1 -gt -2 && echo yes
11443 test -l abc -gt 1 && echo yes
11446 @error{} test: integer expression expected before -eq
11450 @node Connectives for test
11451 @subsection Connectives for @command{test}
11453 @cindex logical connectives
11454 @cindex connectives, logical
11456 The usual logical connectives.
11462 True if @var{expr} is false.
11464 @item @var{expr1} -a @var{expr2}
11466 @cindex logical and operator
11467 @cindex and operator
11468 True if both @var{expr1} and @var{expr2} are true.
11470 @item @var{expr1} -o @var{expr2}
11472 @cindex logical or operator
11473 @cindex or operator
11474 True if either @var{expr1} or @var{expr2} is true.
11479 @node expr invocation
11480 @section @command{expr}: Evaluate expressions
11483 @cindex expression evaluation
11484 @cindex evaluation of expressions
11486 @command{expr} evaluates an expression and writes the result on standard
11487 output. Each token of the expression must be a separate argument.
11489 Operands are either integers or strings. Integers consist of one or
11490 more decimal digits, with an optional leading @samp{-}.
11491 @command{expr} converts
11492 anything appearing in an operand position to an integer or a string
11493 depending on the operation being applied to it.
11495 Strings are not quoted for @command{expr} itself, though you may need to
11496 quote them to protect characters with special meaning to the shell,
11497 e.g., spaces. However, regardless of whether it is quoted, a string
11498 operand should not be a parenthesis or any of @command{expr}'s
11499 operators like @code{+}, so you cannot safely pass an arbitrary string
11500 @code{$str} to expr merely by quoting it to the shell. One way to
11501 work around this is to use the @sc{gnu} extension @code{+},
11502 (e.g., @code{+ "$str" = foo}); a more portable way is to use
11503 @code{@w{" $str"}} and to adjust the rest of the expression to take
11504 the leading space into account (e.g., @code{@w{" $str" = " foo"}}).
11506 You should not pass a negative integer or a string with leading
11507 @samp{-} as @command{expr}'s first argument, as it might be
11508 misinterpreted as an option; this can be avoided by parenthesization.
11509 Also, portable scripts should not use a string operand that happens to
11510 take the form of an integer; this can be worked around by inserting
11511 leading spaces as mentioned above.
11513 @cindex parentheses for grouping
11514 Operators may be given as infix symbols or prefix keywords. Parentheses
11515 may be used for grouping in the usual manner. You must quote
11516 parentheses and many operators to avoid the shell evaluating them,
11519 When built with support for the GNU MP library, @command{expr} uses
11520 arbitrary-precision arithmetic; otherwise, it uses native arithmetic
11521 types and may fail due to arithmetic overflow.
11523 The only options are @option{--help} and @option{--version}. @xref{Common
11524 options}. Options must precede operands.
11526 @cindex exit status of @command{expr}
11530 0 if the expression is neither null nor 0,
11531 1 if the expression is null or 0,
11532 2 if the expression is invalid,
11533 3 if an internal error occurred (e.g., arithmetic overflow).
11537 * String expressions:: + : match substr index length
11538 * Numeric expressions:: + - * / %
11539 * Relations for expr:: | & < <= = == != >= >
11540 * Examples of expr:: Examples.
11544 @node String expressions
11545 @subsection String expressions
11547 @cindex string expressions
11548 @cindex expressions, string
11550 @command{expr} supports pattern matching and other string operators. These
11551 have higher precedence than both the numeric and relational operators (in
11552 the next sections).
11556 @item @var{string} : @var{regex}
11557 @cindex pattern matching
11558 @cindex regular expression matching
11559 @cindex matching patterns
11560 Perform pattern matching. The arguments are converted to strings and the
11561 second is considered to be a (basic, a la GNU @code{grep}) regular
11562 expression, with a @code{^} implicitly prepended. The first argument is
11563 then matched against this regular expression.
11565 If the match succeeds and @var{regex} uses @samp{\(} and @samp{\)}, the
11566 @code{:} expression returns the part of @var{string} that matched the
11567 subexpression; otherwise, it returns the number of characters matched.
11569 If the match fails, the @code{:} operator returns the null string if
11570 @samp{\(} and @samp{\)} are used in @var{regex}, otherwise 0.
11572 @kindex \( @r{regexp operator}
11573 Only the first @samp{\( @dots{} \)} pair is relevant to the return
11574 value; additional pairs are meaningful only for grouping the regular
11575 expression operators.
11577 @kindex \+ @r{regexp operator}
11578 @kindex \? @r{regexp operator}
11579 @kindex \| @r{regexp operator}
11580 In the regular expression, @code{\+}, @code{\?}, and @code{\|} are
11581 operators which respectively match one or more, zero or one, or separate
11582 alternatives. SunOS and other @command{expr}'s treat these as regular
11583 characters. (@acronym{POSIX} allows either behavior.)
11584 @xref{Top, , Regular Expression Library, regex, Regex}, for details of
11585 regular expression syntax. Some examples are in @ref{Examples of expr}.
11587 @item match @var{string} @var{regex}
11589 An alternative way to do pattern matching. This is the same as
11590 @w{@samp{@var{string} : @var{regex}}}.
11592 @item substr @var{string} @var{position} @var{length}
11594 Returns the substring of @var{string} beginning at @var{position}
11595 with length at most @var{length}. If either @var{position} or
11596 @var{length} is negative, zero, or non-numeric, returns the null string.
11598 @item index @var{string} @var{charset}
11600 Returns the first position in @var{string} where the first character in
11601 @var{charset} was found. If no character in @var{charset} is found in
11602 @var{string}, return 0.
11604 @item length @var{string}
11606 Returns the length of @var{string}.
11608 @item + @var{token}
11610 Interpret @var{token} as a string, even if it is a keyword like @var{match}
11611 or an operator like @code{/}.
11612 This makes it possible to test @code{expr length + "$x"} or
11613 @code{expr + "$x" : '.*/\(.\)'} and have it do the right thing even if
11614 the value of @var{$x} happens to be (for example) @code{/} or @code{index}.
11615 This operator is a @acronym{GNU} extension. Portable shell scripts should use
11616 @code{@w{" $token"} : @w{' \(.*\)'}} instead of @code{+ "$token"}.
11620 To make @command{expr} interpret keywords as strings, you must use the
11621 @code{quote} operator.
11624 @node Numeric expressions
11625 @subsection Numeric expressions
11627 @cindex numeric expressions
11628 @cindex expressions, numeric
11630 @command{expr} supports the usual numeric operators, in order of increasing
11631 precedence. These numeric operators have lower precedence than the
11632 string operators described in the previous section, and higher precedence
11633 than the connectives (next section).
11641 @cindex subtraction
11642 Addition and subtraction. Both arguments are converted to integers;
11643 an error occurs if this cannot be done.
11649 @cindex multiplication
11652 Multiplication, division, remainder. Both arguments are converted to
11653 integers; an error occurs if this cannot be done.
11658 @node Relations for expr
11659 @subsection Relations for @command{expr}
11661 @cindex connectives, logical
11662 @cindex logical connectives
11663 @cindex relations, numeric or string
11665 @command{expr} supports the usual logical connectives and relations. These
11666 have lower precedence than the string and numeric operators
11667 (previous sections). Here is the list, lowest-precedence operator first.
11673 @cindex logical or operator
11674 @cindex or operator
11675 Returns its first argument if that is neither null nor zero, otherwise
11676 its second argument if it is neither null nor zero, otherwise 0. It
11677 does not evaluate its second argument if its first argument is neither
11682 @cindex logical and operator
11683 @cindex and operator
11684 Return its first argument if neither argument is null or zero, otherwise
11685 0. It does not evaluate its second argument if its first argument is
11688 @item < <= = == != >= >
11695 @cindex comparison operators
11697 Compare the arguments and return 1 if the relation is true, 0 otherwise.
11698 @code{==} is a synonym for @code{=}. @command{expr} first tries to convert
11699 both arguments to integers and do a numeric comparison; if either
11700 conversion fails, it does a lexicographic comparison using the character
11701 collating sequence specified by the @env{LC_COLLATE} locale.
11706 @node Examples of expr
11707 @subsection Examples of using @command{expr}
11709 @cindex examples of @command{expr}
11710 Here are a few examples, including quoting for shell metacharacters.
11712 To add 1 to the shell variable @code{foo}, in Bourne-compatible shells:
11715 foo=`expr $foo + 1`
11718 To print the non-directory part of the file name stored in
11719 @code{$fname}, which need not contain a @code{/}:
11722 expr $fname : '.*/\(.*\)' '|' $fname
11725 An example showing that @code{\+} is an operator:
11733 expr abc : 'a\(.\)c'
11735 expr index abcdef cz
11738 @error{} expr: syntax error
11739 expr index + index a
11745 @chapter Redirection
11747 @cindex redirection
11748 @cindex commands for redirection
11750 Unix shells commonly provide several forms of @dfn{redirection}---ways
11751 to change the input source or output destination of a command. But one
11752 useful redirection is performed by a separate command, not by the shell;
11753 it's described here.
11756 * tee invocation:: Redirect output to multiple files or processes.
11760 @node tee invocation
11761 @section @command{tee}: Redirect output to multiple files or processes
11764 @cindex pipe fitting
11765 @cindex destinations, multiple output
11766 @cindex read from stdin and write to stdout and files
11768 The @command{tee} command copies standard input to standard output and also
11769 to any files given as arguments. This is useful when you want not only
11770 to send some data down a pipe, but also to save a copy. Synopsis:
11773 tee [@var{option}]@dots{} [@var{file}]@dots{}
11776 If a file being written to does not already exist, it is created. If a
11777 file being written to already exists, the data it previously contained
11778 is overwritten unless the @option{-a} option is used.
11780 A @var{file} of @samp{-} causes @command{tee} to send another copy of
11781 input to standard output, but this is typically not that useful as the
11782 copies are interleaved.
11784 The program accepts the following options. Also see @ref{Common options}.
11791 Append standard input to the given files rather than overwriting
11795 @itemx --ignore-interrupts
11797 @opindex --ignore-interrupts
11798 Ignore interrupt signals.
11802 The @command{tee} command is useful when you happen to be transferring a large
11803 amount of data and also want to summarize that data without reading
11804 it a second time. For example, when you are downloading a DVD image,
11805 you often want to verify its signature or checksum right away.
11806 The inefficient way to do it is simply:
11809 wget http://example.com/some.iso && sha1sum some.iso
11812 One problem with the above is that it makes you wait for the
11813 download to complete before starting the time-consuming SHA1 computation.
11814 Perhaps even more importantly, the above requires reading
11815 the DVD image a second time (the first was from the network).
11817 The efficient way to do it is to interleave the download
11818 and SHA1 computation. Then, you'll get the checksum for
11819 free, because the entire process parallelizes so well:
11822 # slightly contrived, to demonstrate process substitution
11823 wget -O - http://example.com/dvd.iso \
11824 | tee >(sha1sum > dvd.sha1) > dvd.iso
11827 That makes @command{tee} write not just to the expected output file,
11828 but also to a pipe running @command{sha1sum} and saving the final
11829 checksum in a file named @file{dvd.sha1}.
11831 Note, however, that this example relies on a feature of modern shells
11832 called @dfn{process substitution}
11833 (the @samp{>(command)} syntax, above;
11834 @xref{Process Substitution,,Process Substitution, bashref,
11835 The Bash Reference Manual}.),
11836 so it works with @command{zsh}, @command{bash}, and @command{ksh},
11837 but not with @command{/bin/sh}. So if you write code like this
11838 in a shell script, be sure to start the script with @samp{#!/bin/bash}.
11840 Since the above example writes to one file and one process,
11841 a more conventional and portable use of @command{tee} is even better:
11844 wget -O - http://example.com/dvd.iso \
11845 | tee dvd.iso | sha1sum > dvd.sha1
11848 You can extend this example to make @command{tee} write to two processes,
11849 computing MD5 and SHA1 checksums in parallel. In this case,
11850 process substitution is required:
11853 wget -O - http://example.com/dvd.iso \
11854 | tee >(sha1sum > dvd.sha1) \
11855 >(md5sum > dvd.md5) \
11859 This technique is also useful when you want to make a @emph{compressed}
11860 copy of the contents of a pipe.
11861 Consider a tool to graphically summarize disk usage data from @samp{du -ak}.
11862 For a large hierarchy, @samp{du -ak} can run for a long time,
11863 and can easily produce terabytes of data, so you won't want to
11864 rerun the command unnecessarily. Nor will you want to save
11865 the uncompressed output.
11867 Doing it the inefficient way, you can't even start the GUI
11868 until after you've compressed all of the @command{du} output:
11871 du -ak | gzip -9 > /tmp/du.gz
11872 gzip -d /tmp/du.gz | xdiskusage -a
11875 With @command{tee} and process substitution, you start the GUI
11876 right away and eliminate the decompression completely:
11879 du -ak | tee >(gzip -9 > /tmp/du.gz) | xdiskusage -a
11882 Finally, if you regularly create more than one type of
11883 compressed tarball at once, for example when @code{make dist} creates
11884 both @command{gzip}-compressed and @command{bzip2}-compressed tarballs,
11885 there may be a better way.
11886 Typical @command{automake}-generated @file{Makefile} rules create
11887 the two compressed tar archives with commands in sequence, like this
11888 (slightly simplified):
11891 tardir=your-pkg-M.N
11892 tar chof - "$tardir" | gzip -9 -c > your-pkg-M.N.tar.gz
11893 tar chof - "$tardir" | bzip2 -9 -c > your-pkg-M.N.tar.bz2
11896 However, if the hierarchy you are archiving and compressing is larger
11897 than a couple megabytes, and especially if you are using a multi-processor
11898 system with plenty of memory, then you can do much better by reading the
11899 directory contents only once and running the compression programs in parallel:
11902 tardir=your-pkg-M.N
11903 tar chof - "$tardir" \
11904 | tee >(gzip -9 -c > your-pkg-M.N.tar.gz) \
11905 | bzip2 -9 -c > your-pkg-M.N.tar.bz2
11911 @node File name manipulation
11912 @chapter File name manipulation
11914 @cindex file name manipulation
11915 @cindex manipulation of file names
11916 @cindex commands for file name manipulation
11918 This section describes commands that manipulate file names.
11921 * basename invocation:: Strip directory and suffix from a file name.
11922 * dirname invocation:: Strip last file name component.
11923 * pathchk invocation:: Check file name validity and portability.
11924 * mktemp invocation:: Create temporary file or directory.
11928 @node basename invocation
11929 @section @command{basename}: Strip directory and suffix from a file name
11932 @cindex strip directory and suffix from file names
11933 @cindex directory, stripping from file names
11934 @cindex suffix, stripping from file names
11935 @cindex file names, stripping directory and suffix
11936 @cindex leading directory components, stripping
11938 @command{basename} removes any leading directory components from
11939 @var{name}. Synopsis:
11942 basename @var{name} [@var{suffix}]
11945 If @var{suffix} is specified and is identical to the end of @var{name},
11946 it is removed from @var{name} as well. Note that since trailing slashes
11947 are removed prior to suffix matching, @var{suffix} will do nothing if it
11948 contains slashes. @command{basename} prints the result on standard
11951 @c This test is used both here and in the section on dirname.
11952 @macro basenameAndDirname
11953 Together, @command{basename} and @command{dirname} are designed such
11954 that if @samp{ls "$name"} succeeds, then the command sequence @samp{cd
11955 "$(dirname "$name")"; ls "$(basename "$name")"} will, too. This works
11956 for everything except file names containing a trailing newline.
11958 @basenameAndDirname
11960 @acronym{POSIX} allows the implementation to define the results if
11961 @var{name} is empty or @samp{//}. In the former case, @acronym{GNU}
11962 @command{basename} returns the empty string. In the latter case, the
11963 result is @samp{//} on platforms where @var{//} is distinct from
11964 @var{/}, and @samp{/} on platforms where there is no difference.
11966 The only options are @option{--help} and @option{--version}. @xref{Common
11967 options}. Options must precede operands.
11975 basename /usr/bin/sort
11978 basename include/stdio.h .h
11982 @node dirname invocation
11983 @section @command{dirname}: Strip last file name component
11986 @cindex directory components, printing
11987 @cindex stripping non-directory suffix
11988 @cindex non-directory suffix, stripping
11990 @command{dirname} prints all but the final slash-delimited component of
11991 a string (presumably a file name, but also works on directories). Synopsis:
11997 If @var{name} is a single component, @command{dirname} prints @samp{.}
11998 (meaning the current directory).
12000 @basenameAndDirname
12002 @acronym{POSIX} allows the implementation to define the results if
12003 @var{name} is @samp{//}. With @acronym{GNU} @command{dirname}, the
12004 result is @samp{//} on platforms where @var{//} is distinct from
12005 @var{/}, and @samp{/} on platforms where there is no difference.
12007 The only options are @option{--help} and @option{--version}. @xref{Common
12015 # Output "/usr/bin".
12016 dirname /usr/bin/sort
12023 @node pathchk invocation
12024 @section @command{pathchk}: Check file name validity and portability
12027 @cindex file names, checking validity and portability
12028 @cindex valid file names, checking for
12029 @cindex portable file names, checking for
12031 @command{pathchk} checks validity and portability of file names. Synopsis:
12034 pathchk [@var{option}]@dots{} @var{name}@dots{}
12037 For each @var{name}, @command{pathchk} prints an error message if any of
12038 these conditions is true:
12042 One of the existing directories in @var{name} does not have search
12043 (execute) permission,
12045 The length of @var{name} is larger than the maximum supported by the
12048 The length of one component of @var{name} is longer than
12049 its file system's maximum.
12052 A nonexistent @var{name} is not an error, so long a file with that
12053 name could be created under the above conditions.
12055 The program accepts the following options. Also see @ref{Common options}.
12056 Options must precede operands.
12062 Instead of performing checks based on the underlying file system,
12063 print an error message if any of these conditions is true:
12067 A file name is empty.
12070 A file name contains a character outside the @acronym{POSIX} portable file
12071 name character set, namely, the ASCII letters and digits, @samp{.},
12072 @samp{_}, @samp{-}, and @samp{/}.
12075 The length of a file name or one of its components exceeds the
12076 @acronym{POSIX} minimum limits for portability.
12081 Print an error message if a file name is empty, or if it contains a component
12082 that begins with @samp{-}.
12084 @item --portability
12085 @opindex --portability
12086 Print an error message if a file name is not portable to all @acronym{POSIX}
12087 hosts. This option is equivalent to @samp{-p -P}.
12091 @cindex exit status of @command{pathchk}
12095 0 if all specified file names passed all checks,
12099 @node mktemp invocation
12100 @section @command{mktemp}: Create temporary file or directory
12103 @cindex file names, creating temporary
12104 @cindex directory, creating temporary
12105 @cindex temporary files and directories
12107 @command{mktemp} manages the creation of temporary files and
12108 directories. Synopsis:
12111 mktemp [@var{option}]@dots{} [@var{template}]
12114 Safely create a temporary file or directory based on @var{template},
12115 and print its name. If given, @var{template} must include at least
12116 three consecutive @samp{X}s in the last component. If omitted, the template
12117 @samp{tmp.XXXXXXXXXX} is used, and option @option{--tmpdir} is
12118 implied. The final run of @samp{X}s in the @var{template} will be replaced
12119 by alpha-numeric characters; thus, on a case-sensitive file system,
12120 and with a @var{template} including a run of @var{n} instances of @samp{X},
12121 there are @samp{62**@var{n}} potential file names.
12123 Older scripts used to create temporary files by simply joining the
12124 name of the program with the process id (@samp{$$}) as a suffix.
12125 However, that naming scheme is easily predictable, and suffers from a
12126 race condition where the attacker can create an appropriately named
12127 symbolic link, such that when the script then opens a handle to what
12128 it thought was an unused file, it is instead modifying an existing
12129 file. Using the same scheme to create a directory is slightly safer,
12130 since the @command{mkdir} will fail if the target already exists, but
12131 it is still inferior because it allows for denial of service attacks.
12132 Therefore, modern scripts should use the @command{mktemp} command to
12133 guarantee that the generated name will be unpredictable, and that
12134 knowledge of the temporary file name implies that the file was created
12135 by the current script and cannot be modified by other users.
12137 When creating a file, the resulting file has read and write
12138 permissions for the current user, but no permissions for the group or
12139 others; these permissions are reduced if the current umask is more
12142 Here are some examples (although note that if you repeat them, you
12143 will most likely get different file names):
12148 Create a temporary file in the current directory.
12155 Create a temporary file with a known suffix.
12157 $ mktemp --suffix=.txt file-XXXX
12159 $ mktemp file-XXXX-XXXX.txt
12164 Create a secure fifo relative to the user's choice of @env{TMPDIR},
12165 but falling back to the current directory rather than @file{/tmp}.
12166 Note that @command{mktemp} does not create fifos, but can create a
12167 secure directory in which the fifo can live. Exit the shell if the
12168 directory or fifo could not be created.
12170 $ dir=$(mktemp -p "$@{TMPDIR:-.@}" -d dir-XXXX) || exit 1
12172 $ mkfifo "$fifo" || @{ rmdir "$dir"; exit 1; @}
12176 Create and use a temporary file if possible, but ignore failure. The
12177 file will reside in the directory named by @env{TMPDIR}, if specified,
12178 or else in @file{/tmp}.
12180 $ file=$(mktemp -q) && @{
12181 > # Safe to use $file only within this block. Use quotes,
12182 > # since $TMPDIR, and thus $file, may contain whitespace.
12183 > echo ... > "$file"
12189 Act as a semi-random character generator (it is not fully random,
12190 since it is impacted by the contents of the current directory). To
12191 avoid security holes, do not use the resulting names to create a file.
12201 The program accepts the following options. Also see @ref{Common options}.
12208 @opindex --directory
12209 Create a directory rather than a file. The directory will have read,
12210 write, and search permissions for the current user, but no permissions
12211 for the group or others; these permissions are reduced if the current
12212 umask is more restrictive.
12218 Suppress diagnostics about failure to create a file or directory. The
12219 exit status will still reflect whether a file was created.
12225 Generate a temporary name that does not name an existing file, without
12226 changing the file system contents. Using the output of this command
12227 to create a new file is inherently unsafe, as there is a window of
12228 time between generating the name and using it where another process
12229 can create an object by the same name.
12232 @itemx --tmpdir[=@var{dir}]
12235 Treat @var{template} relative to the directory @var{dir}. If
12236 @var{dir} is not specified (only possible with the long option
12237 @option{--tmpdir}) or is the empty string, use the value of
12238 @env{TMPDIR} if available, otherwise use @samp{/tmp}. If this is
12239 specified, @var{template} must not be absolute. However,
12240 @var{template} can still contain slashes, although intermediate
12241 directories must already exist.
12243 @item --suffix=@var{suffix}
12245 Append @var{suffix} to the @var{template}. @var{suffix} must not
12246 contain slash. If @option{--suffix} is specified, @var{template} must
12247 end in @samp{X}; if it is not specified, then an appropriate
12248 @option{--suffix} is inferred by finding the last @samp{X} in
12249 @var{template}. This option exists for use with the default
12250 @var{template} and for the creation of a @var{suffix} that starts with
12255 Treat @var{template} as a single file relative to the value of
12256 @env{TMPDIR} if available, or to the directory specified by
12257 @option{-p}, otherwise to @samp{/tmp}. @var{template} must not
12258 contain slashes. This option is deprecated; the use of @option{-p}
12259 without @option{-t} offers better defaults (by favoring the command
12260 line over @env{TMPDIR}) and more flexibility (by allowing intermediate
12265 @cindex exit status of @command{mktemp}
12269 0 if the file was created,
12274 @node Working context
12275 @chapter Working context
12277 @cindex working context
12278 @cindex commands for printing the working context
12280 This section describes commands that display or alter the context in
12281 which you are working: the current directory, the terminal settings, and
12282 so forth. See also the user-related commands in the next section.
12285 * pwd invocation:: Print working directory.
12286 * stty invocation:: Print or change terminal characteristics.
12287 * printenv invocation:: Print environment variables.
12288 * tty invocation:: Print file name of terminal on standard input.
12292 @node pwd invocation
12293 @section @command{pwd}: Print working directory
12296 @cindex print name of current directory
12297 @cindex current working directory, printing
12298 @cindex working directory, printing
12301 @command{pwd} prints the name of the current directory. Synopsis:
12304 pwd [@var{option}]@dots{}
12307 The program accepts the following options. Also see @ref{Common options}.
12314 If the contents of the environment variable @env{PWD} provide an
12315 absolute name of the current directory with no @samp{.} or @samp{..}
12316 components, but possibly with symbolic links, then output those
12317 contents. Otherwise, fall back to default @option{-P} handling.
12322 @opindex --physical
12323 Print a fully resolved name for the current directory. That is, all
12324 components of the printed name will be actual directory names---none
12325 will be symbolic links.
12328 @cindex symbolic links and @command{pwd}
12329 If @option{-L} and @option{-P} are both given, the last one takes
12330 precedence. If neither option is given, then this implementation uses
12331 @option{-P} as the default unless the @env{POSIXLY_CORRECT}
12332 environment variable is set.
12334 @mayConflictWithShellBuiltIn{pwd}
12339 @node stty invocation
12340 @section @command{stty}: Print or change terminal characteristics
12343 @cindex change or print terminal settings
12344 @cindex terminal settings
12345 @cindex line settings of terminal
12347 @command{stty} prints or changes terminal characteristics, such as baud rate.
12351 stty [@var{option}] [@var{setting}]@dots{}
12352 stty [@var{option}]
12355 If given no line settings, @command{stty} prints the baud rate, line
12356 discipline number (on systems that support it), and line settings
12357 that have been changed from the values set by @samp{stty sane}.
12358 By default, mode reading and setting are performed on the tty line
12359 connected to standard input, although this can be modified by the
12360 @option{--file} option.
12362 @command{stty} accepts many non-option arguments that change aspects of
12363 the terminal line operation, as described below.
12365 The program accepts the following options. Also see @ref{Common options}.
12372 Print all current settings in human-readable form. This option may not
12373 be used in combination with any line settings.
12375 @item -F @var{device}
12376 @itemx --file=@var{device}
12379 Set the line opened by the file name specified in @var{device} instead of
12380 the tty line connected to standard input. This option is necessary
12381 because opening a @acronym{POSIX} tty requires use of the @code{O_NONDELAY} flag to
12382 prevent a @acronym{POSIX} tty from blocking until the carrier detect line is high if
12383 the @code{clocal} flag is not set. Hence, it is not always possible
12384 to allow the shell to open the device in the traditional manner.
12390 @cindex machine-readable @command{stty} output
12391 Print all current settings in a form that can be used as an argument to
12392 another @command{stty} command to restore the current settings. This option
12393 may not be used in combination with any line settings.
12397 Many settings can be turned off by preceding them with a @samp{-}.
12398 Such arguments are marked below with ``May be negated'' in their
12399 description. The descriptions themselves refer to the positive
12400 case, that is, when @emph{not} negated (unless stated otherwise,
12403 Some settings are not available on all @acronym{POSIX} systems, since they use
12404 extensions. Such arguments are marked below with ``Non-@acronym{POSIX}'' in their
12405 description. On non-@acronym{POSIX} systems, those or other settings also may not
12406 be available, but it's not feasible to document all the variations: just
12412 * Control:: Control settings
12413 * Input:: Input settings
12414 * Output:: Output settings
12415 * Local:: Local settings
12416 * Combination:: Combination settings
12417 * Characters:: Special characters
12418 * Special:: Special settings
12423 @subsection Control settings
12425 @cindex control settings
12431 @cindex two-way parity
12432 Generate parity bit in output and expect parity bit in input.
12438 @cindex even parity
12439 Set odd parity (even if negated). May be negated.
12446 @cindex character size
12447 @cindex eight-bit characters
12448 Set character size to 5, 6, 7, or 8 bits.
12453 Send a hangup signal when the last process closes the tty. May be
12459 Use two stop bits per character (one if negated). May be negated.
12463 Allow input to be received. May be negated.
12467 @cindex modem control
12468 Disable modem control signals. May be negated.
12472 @cindex hardware flow control
12473 @cindex flow control, hardware
12474 @cindex RTS/CTS flow control
12475 Enable RTS/CTS flow control. Non-@acronym{POSIX}. May be negated.
12480 @subsection Input settings
12482 @cindex input settings
12483 These settings control operations on data received from the terminal.
12488 @cindex breaks, ignoring
12489 Ignore break characters. May be negated.
12493 @cindex breaks, cause interrupts
12494 Make breaks cause an interrupt signal. May be negated.
12498 @cindex parity, ignoring
12499 Ignore characters with parity errors. May be negated.
12503 @cindex parity errors, marking
12504 Mark parity errors (with a 255-0-character sequence). May be negated.
12508 Enable input parity checking. May be negated.
12512 @cindex eight-bit input
12513 Clear high (8th) bit of input characters. May be negated.
12517 @cindex newline, translating to return
12518 Translate newline to carriage return. May be negated.
12522 @cindex return, ignoring
12523 Ignore carriage return. May be negated.
12527 @cindex return, translating to newline
12528 Translate carriage return to newline. May be negated.
12532 @cindex input encoding, UTF-8
12533 Assume input characters are UTF-8 encoded. May be negated.
12537 @kindex C-s/C-q flow control
12538 @cindex XON/XOFF flow control
12539 Enable XON/XOFF flow control (that is, @kbd{CTRL-S}/@kbd{CTRL-Q}). May
12546 @cindex software flow control
12547 @cindex flow control, software
12548 Enable sending of @code{stop} character when the system input buffer
12549 is almost full, and @code{start} character when it becomes almost
12550 empty again. May be negated.
12554 @cindex uppercase, translating to lowercase
12555 Translate uppercase characters to lowercase. Non-@acronym{POSIX}. May be
12556 negated. Note ilcuc is not implemented, as one would not be able to issue
12557 almost any (lowercase) Unix command, after invoking it.
12561 Allow any character to restart output (only the start character
12562 if negated). Non-@acronym{POSIX}. May be negated.
12566 @cindex beeping at input buffer full
12567 Enable beeping and not flushing input buffer if a character arrives
12568 when the input buffer is full. Non-@acronym{POSIX}. May be negated.
12573 @subsection Output settings
12575 @cindex output settings
12576 These settings control operations on data sent to the terminal.
12581 Postprocess output. May be negated.
12585 @cindex lowercase, translating to output
12586 Translate lowercase characters to uppercase. Non-@acronym{POSIX}. May be
12587 negated. (Note ouclc is not currently implemented.)
12591 @cindex return, translating to newline
12592 Translate carriage return to newline. Non-@acronym{POSIX}. May be negated.
12596 @cindex newline, translating to crlf
12597 Translate newline to carriage return-newline. Non-@acronym{POSIX}. May be
12602 Do not print carriage returns in the first column. Non-@acronym{POSIX}.
12607 Newline performs a carriage return. Non-@acronym{POSIX}. May be negated.
12611 @cindex pad instead of timing for delaying
12612 Use fill (padding) characters instead of timing for delays. Non-@acronym{POSIX}.
12617 @cindex pad character
12618 Use @acronym{ASCII} @sc{del} characters for fill instead of
12619 @acronym{ASCII} @sc{nul} characters. Non-@acronym{POSIX}.
12625 Newline delay style. Non-@acronym{POSIX}.
12632 Carriage return delay style. Non-@acronym{POSIX}.
12638 @opindex tab@var{n}
12639 Horizontal tab delay style. Non-@acronym{POSIX}.
12644 Backspace delay style. Non-@acronym{POSIX}.
12649 Vertical tab delay style. Non-@acronym{POSIX}.
12654 Form feed delay style. Non-@acronym{POSIX}.
12659 @subsection Local settings
12661 @cindex local settings
12666 Enable @code{interrupt}, @code{quit}, and @code{suspend} special
12667 characters. May be negated.
12671 Enable @code{erase}, @code{kill}, @code{werase}, and @code{rprnt}
12672 special characters. May be negated.
12676 Enable non-@acronym{POSIX} special characters. May be negated.
12680 Echo input characters. May be negated.
12686 Echo @code{erase} characters as backspace-space-backspace. May be
12691 @cindex newline echoing after @code{kill}
12692 Echo a newline after a @code{kill} character. May be negated.
12696 @cindex newline, echoing
12697 Echo newline even if not echoing other characters. May be negated.
12701 @cindex flushing, disabling
12702 Disable flushing after @code{interrupt} and @code{quit} special
12703 characters. May be negated.
12707 @cindex case translation
12708 Enable input and output of uppercase characters by preceding their
12709 lowercase equivalents with @samp{\}, when @code{icanon} is set.
12710 Non-@acronym{POSIX}. May be negated.
12714 @cindex background jobs, stopping at terminal write
12715 Stop background jobs that try to write to the terminal. Non-@acronym{POSIX}.
12722 Echo erased characters backward, between @samp{\} and @samp{/}.
12723 Non-@acronym{POSIX}. May be negated.
12729 @cindex control characters, using @samp{^@var{c}}
12730 @cindex hat notation for control characters
12731 Echo control characters in hat notation (@samp{^@var{c}}) instead
12732 of literally. Non-@acronym{POSIX}. May be negated.
12738 Echo the @code{kill} special character by erasing each character on
12739 the line as indicated by the @code{echoprt} and @code{echoe} settings,
12740 instead of by the @code{echoctl} and @code{echok} settings. Non-@acronym{POSIX}.
12746 @subsection Combination settings
12748 @cindex combination settings
12749 Combination settings:
12756 Same as @code{parenb -parodd cs7}. May be negated. If negated, same
12757 as @code{-parenb cs8}.
12761 Same as @code{parenb parodd cs7}. May be negated. If negated, same
12762 as @code{-parenb cs8}.
12766 Same as @code{-icrnl -onlcr}. May be negated. If negated, same as
12767 @code{icrnl -inlcr -igncr onlcr -ocrnl -onlret}.
12771 Reset the @code{erase} and @code{kill} special characters to their default
12778 @c This is too long to write inline.
12780 cread -ignbrk brkint -inlcr -igncr icrnl -ixoff
12781 -iuclc -ixany imaxbel opost -olcuc -ocrnl onlcr
12782 -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0
12783 ff0 isig icanon iexten echo echoe echok -echonl
12784 -noflsh -xcase -tostop -echoprt echoctl echoke
12788 and also sets all special characters to their default values.
12792 Same as @code{brkint ignpar istrip icrnl ixon opost isig icanon}, plus
12793 sets the @code{eof} and @code{eol} characters to their default values
12794 if they are the same as the @code{min} and @code{time} characters.
12795 May be negated. If negated, same as @code{raw}.
12802 -ignbrk -brkint -ignpar -parmrk -inpck -istrip
12803 -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany
12804 -imaxbel -opost -isig -icanon -xcase min 1 time 0
12808 May be negated. If negated, same as @code{cooked}.
12812 Same as @option{-icanon}. May be negated. If negated, same as
12817 @cindex eight-bit characters
12818 Same as @code{-parenb -istrip cs8}. May be negated. If negated,
12819 same as @code{parenb istrip cs7}.
12823 Same as @option{-parenb -istrip -opost cs8}. May be negated.
12824 If negated, same as @code{parenb istrip opost cs7}.
12828 Same as @option{-ixany}. Non-@acronym{POSIX}. May be negated.
12832 Same as @code{tab0}. Non-@acronym{POSIX}. May be negated. If negated, same
12839 Same as @code{xcase iuclc olcuc}. Non-@acronym{POSIX}. May be negated.
12840 (Used for terminals with uppercase characters only.)
12844 Same as @code{echoe echoctl echoke}.
12848 Same as @code{echoe echoctl echoke -ixany intr ^C erase ^? kill C-u}.
12853 @subsection Special characters
12855 @cindex special characters
12856 @cindex characters, special
12858 The special characters' default values vary from system to system.
12859 They are set with the syntax @samp{name value}, where the names are
12860 listed below and the value can be given either literally, in hat
12861 notation (@samp{^@var{c}}), or as an integer which may start with
12862 @samp{0x} to indicate hexadecimal, @samp{0} to indicate octal, or
12863 any other digit to indicate decimal.
12865 @cindex disabling special characters
12866 @kindex u@r{, and disabling special characters}
12867 For GNU stty, giving a value of @code{^-} or @code{undef} disables that
12868 special character. (This is incompatible with Ultrix @command{stty},
12869 which uses a value of @samp{u} to disable a special character. GNU
12870 @command{stty} treats a value @samp{u} like any other, namely to set that
12871 special character to @key{U}.)
12877 Send an interrupt signal.
12881 Send a quit signal.
12885 Erase the last character typed.
12889 Erase the current line.
12893 Send an end of file (terminate the input).
12901 Alternate character to end the line. Non-@acronym{POSIX}.
12905 Switch to a different shell layer. Non-@acronym{POSIX}.
12909 Restart the output after stopping it.
12917 Send a terminal stop signal.
12921 Send a terminal stop signal after flushing the input. Non-@acronym{POSIX}.
12925 Redraw the current line. Non-@acronym{POSIX}.
12929 Erase the last word typed. Non-@acronym{POSIX}.
12933 Enter the next character typed literally, even if it is a special
12934 character. Non-@acronym{POSIX}.
12939 @subsection Special settings
12941 @cindex special settings
12946 Set the minimum number of characters that will satisfy a read until
12947 the time value has expired, when @option{-icanon} is set.
12951 Set the number of tenths of a second before reads time out if the minimum
12952 number of characters have not been read, when @option{-icanon} is set.
12954 @item ispeed @var{n}
12956 Set the input speed to @var{n}.
12958 @item ospeed @var{n}
12960 Set the output speed to @var{n}.
12964 Tell the tty kernel driver that the terminal has @var{n} rows. Non-@acronym{POSIX}.
12967 @itemx columns @var{n}
12970 Tell the kernel that the terminal has @var{n} columns. Non-@acronym{POSIX}.
12976 Print the number of rows and columns that the kernel thinks the
12977 terminal has. (Systems that don't support rows and columns in the kernel
12978 typically use the environment variables @env{LINES} and @env{COLUMNS}
12979 instead; however, GNU @command{stty} does not know anything about them.)
12980 Non-@acronym{POSIX}.
12984 Use line discipline @var{n}. Non-@acronym{POSIX}.
12988 Print the terminal speed.
12991 @cindex baud rate, setting
12992 Set the input and output speeds to @var{n}. @var{n} can be one of: 0
12993 50 75 110 134 134.5 150 200 300 600 1200 1800 2400 4800 9600 19200
12994 38400 @code{exta} @code{extb}. @code{exta} is the same as 19200;
12995 @code{extb} is the same as 38400. Many systems, including GNU/Linux,
12996 support higher speeds. The @command{stty} command includes support
13013 4000000 where the system supports these.
13014 0 hangs up the line if @option{-clocal} is set.
13018 @node printenv invocation
13019 @section @command{printenv}: Print all or some environment variables
13022 @cindex printing all or some environment variables
13023 @cindex environment variables, printing
13025 @command{printenv} prints environment variable values. Synopsis:
13028 printenv [@var{option}] [@var{variable}]@dots{}
13031 If no @var{variable}s are specified, @command{printenv} prints the value of
13032 every environment variable. Otherwise, it prints the value of each
13033 @var{variable} that is set, and nothing for those that are not set.
13035 The program accepts the following option. Also see @ref{Common options}.
13043 @cindex exit status of @command{printenv}
13047 0 if all variables specified were found
13048 1 if at least one specified variable was not found
13049 2 if a write error occurred
13053 @node tty invocation
13054 @section @command{tty}: Print file name of terminal on standard input
13057 @cindex print terminal file name
13058 @cindex terminal file name, printing
13060 @command{tty} prints the file name of the terminal connected to its standard
13061 input. It prints @samp{not a tty} if standard input is not a terminal.
13065 tty [@var{option}]@dots{}
13068 The program accepts the following option. Also see @ref{Common options}.
13078 Print nothing; only return an exit status.
13082 @cindex exit status of @command{tty}
13086 0 if standard input is a terminal
13087 1 if standard input is not a terminal
13088 2 if given incorrect arguments
13089 3 if a write error occurs
13093 @node User information
13094 @chapter User information
13096 @cindex user information, commands for
13097 @cindex commands for printing user information
13099 This section describes commands that print user-related information:
13100 logins, groups, and so forth.
13103 * id invocation:: Print user identity.
13104 * logname invocation:: Print current login name.
13105 * whoami invocation:: Print effective user ID.
13106 * groups invocation:: Print group names a user is in.
13107 * users invocation:: Print login names of users currently logged in.
13108 * who invocation:: Print who is currently logged in.
13112 @node id invocation
13113 @section @command{id}: Print user identity
13116 @cindex real user and group IDs, printing
13117 @cindex effective user and group IDs, printing
13118 @cindex printing real and effective user and group IDs
13120 @command{id} prints information about the given user, or the process
13121 running it if no user is specified. Synopsis:
13124 id [@var{option}]@dots{} [@var{username}]
13127 @vindex POSIXLY_CORRECT
13128 By default, it prints the real user ID, real group ID, effective user ID
13129 if different from the real user ID, effective group ID if different from
13130 the real group ID, and supplemental group IDs.
13131 In addition, if SELinux
13132 is enabled and the @env{POSIXLY_CORRECT} environment variable is not set,
13133 then print @samp{context=@var{c}}, where @var{c} is the security context.
13135 Each of these numeric values is preceded by an identifying string and
13136 followed by the corresponding user or group name in parentheses.
13138 The options cause @command{id} to print only part of the above information.
13139 Also see @ref{Common options}.
13146 Print only the group ID.
13152 Print only the group ID and the supplementary groups.
13158 Print the user or group name instead of the ID number. Requires
13159 @option{-u}, @option{-g}, or @option{-G}.
13165 Print the real, instead of effective, user or group ID. Requires
13166 @option{-u}, @option{-g}, or @option{-G}.
13172 Print only the user ID.
13179 @cindex security context
13180 Print only the security context of the current user.
13181 If SELinux is disabled then print a warning and
13182 set the exit status to 1.
13188 @macro primaryAndSupplementaryGroups{cmd,arg}
13189 Primary and supplementary groups for a process are normally inherited
13190 from its parent and are usually unchanged since login. This means
13191 that if you change the group database after logging in, @command{\cmd\}
13192 will not reflect your changes within your existing login session.
13193 Running @command{\cmd\} with a \arg\ causes the user and group
13194 database to be consulted afresh, and so will give a different result.
13196 @primaryAndSupplementaryGroups{id,user argument}
13198 @node logname invocation
13199 @section @command{logname}: Print current login name
13202 @cindex printing user's login name
13203 @cindex login name, printing
13204 @cindex user name, printing
13207 @command{logname} prints the calling user's name, as found in a
13208 system-maintained file (often @file{/var/run/utmp} or
13209 @file{/etc/utmp}), and exits with a status of 0. If there is no entry
13210 for the calling process, @command{logname} prints
13211 an error message and exits with a status of 1.
13213 The only options are @option{--help} and @option{--version}. @xref{Common
13219 @node whoami invocation
13220 @section @command{whoami}: Print effective user ID
13223 @cindex effective user ID, printing
13224 @cindex printing the effective user ID
13226 @command{whoami} prints the user name associated with the current
13227 effective user ID. It is equivalent to the command @samp{id -un}.
13229 The only options are @option{--help} and @option{--version}. @xref{Common
13235 @node groups invocation
13236 @section @command{groups}: Print group names a user is in
13239 @cindex printing groups a user is in
13240 @cindex supplementary groups, printing
13242 @command{groups} prints the names of the primary and any supplementary
13243 groups for each given @var{username}, or the current process if no names
13244 are given. If more than one name is given, the name of each user is
13246 the list of that user's groups and the user name is separated from the
13247 group list by a colon. Synopsis:
13250 groups [@var{username}]@dots{}
13253 The group lists are equivalent to the output of the command @samp{id -Gn}.
13255 @primaryAndSupplementaryGroups{groups,list of users}
13257 The only options are @option{--help} and @option{--version}. @xref{Common
13263 @node users invocation
13264 @section @command{users}: Print login names of users currently logged in
13267 @cindex printing current usernames
13268 @cindex usernames, printing current
13270 @cindex login sessions, printing users with
13271 @command{users} prints on a single line a blank-separated list of user
13272 names of users currently logged in to the current host. Each user name
13273 corresponds to a login session, so if a user has more than one login
13274 session, that user's name will appear the same number of times in the
13283 With no @var{file} argument, @command{users} extracts its information from
13284 a system-maintained file (often @file{/var/run/utmp} or
13285 @file{/etc/utmp}). If a file argument is given, @command{users} uses
13286 that file instead. A common choice is @file{/var/log/wtmp}.
13288 The only options are @option{--help} and @option{--version}. @xref{Common
13294 @node who invocation
13295 @section @command{who}: Print who is currently logged in
13298 @cindex printing current user information
13299 @cindex information, about current users
13301 @command{who} prints information about users who are currently logged on.
13305 @command{who} [@var{option}] [@var{file}] [am i]
13308 @cindex terminal lines, currently used
13310 @cindex remote hostname
13311 If given no non-option arguments, @command{who} prints the following
13312 information for each user currently logged on: login name, terminal
13313 line, login time, and remote hostname or X display.
13317 If given one non-option argument, @command{who} uses that instead of
13318 a default system-maintained file (often @file{/var/run/utmp} or
13319 @file{/etc/utmp}) as the name of the file containing the record of
13320 users logged on. @file{/var/log/wtmp} is commonly given as an argument
13321 to @command{who} to look at who has previously logged on.
13325 If given two non-option arguments, @command{who} prints only the entry
13326 for the user running it (determined from its standard input), preceded
13327 by the hostname. Traditionally, the two arguments given are @samp{am
13328 i}, as in @samp{who am i}.
13331 Time stamps are listed according to the time zone rules specified by
13332 the @env{TZ} environment variable, or by the system default rules if
13333 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
13334 with @env{TZ}, libc, The GNU C Library Reference Manual}.
13336 The program accepts the following options. Also see @ref{Common options}.
13344 Same as @samp{-b -d --login -p -r -t -T -u}.
13350 Print the date and time of last system boot.
13356 Print information corresponding to dead processes.
13362 Print a line of column headings.
13368 List only the entries that correspond to processes via which the
13369 system is waiting for a user to login. The user name is always @samp{LOGIN}.
13373 Attempt to canonicalize hostnames found in utmp through a DNS lookup. This
13374 is not the default because it can cause significant delays on systems with
13375 automatic dial-up internet access.
13379 Same as @samp{who am i}.
13385 List active processes spawned by init.
13391 Print only the login names and the number of users logged on.
13392 Overrides all other options.
13397 @opindex --runlevel
13398 Print the current (and maybe previous) run-level of the init process.
13402 Ignored; for compatibility with other versions of @command{who}.
13408 Print last system clock change.
13413 After the login time, print the number of hours and minutes that the
13414 user has been idle. @samp{.} means the user was active in the last minute.
13415 @samp{old} means the user has been idle for more than 24 hours.
13426 @opindex --writable
13427 @cindex message status
13428 @pindex write@r{, allowed}
13429 After each login name print a character indicating the user's message status:
13432 @samp{+} allowing @code{write} messages
13433 @samp{-} disallowing @code{write} messages
13434 @samp{?} cannot find terminal device
13442 @node System context
13443 @chapter System context
13445 @cindex system context
13446 @cindex context, system
13447 @cindex commands for system context
13449 This section describes commands that print or change system-wide
13453 * date invocation:: Print or set system date and time.
13454 * arch invocation:: Print machine hardware name.
13455 * nproc invocation:: Print the number of processors.
13456 * uname invocation:: Print system information.
13457 * hostname invocation:: Print or set system name.
13458 * hostid invocation:: Print numeric host identifier.
13459 * uptime invocation:: Print system uptime and load.
13462 @node date invocation
13463 @section @command{date}: Print or set system date and time
13466 @cindex time, printing or setting
13467 @cindex printing the current time
13472 date [@var{option}]@dots{} [+@var{format}]
13473 date [-u|--utc|--universal] @c this avoids a newline in the output
13474 [ MMDDhhmm[[CC]YY][.ss] ]
13478 Invoking @command{date} with no @var{format} argument is equivalent to invoking
13479 it with a default format that depends on the @env{LC_TIME} locale category.
13480 In the default C locale, this format is @samp{'+%a %b %e %H:%M:%S %Z %Y'},
13481 so the output looks like @samp{Thu Mar @ 3 13:47:51 PST 2005}.
13484 Normally, @command{date} uses the time zone rules indicated by the
13485 @env{TZ} environment variable, or the system default rules if @env{TZ}
13486 is not set. @xref{TZ Variable,, Specifying the Time Zone with
13487 @env{TZ}, libc, The GNU C Library Reference Manual}.
13489 @findex strftime @r{and @command{date}}
13490 @cindex time formats
13491 @cindex formatting times
13492 If given an argument that starts with a @samp{+}, @command{date} prints the
13493 current date and time (or the date and time specified by the
13494 @option{--date} option, see below) in the format defined by that argument,
13495 which is similar to that of the @code{strftime} function. Except for
13496 conversion specifiers, which start with @samp{%}, characters in the
13497 format string are printed unchanged. The conversion specifiers are
13503 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
13504 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
13505 * Literal conversion specifiers:: %[%nt]
13506 * Padding and other flags:: Pad with zeros, spaces, etc.
13507 * Setting the time:: Changing the system clock.
13508 * Options for date:: Instead of the current time.
13510 * Date input formats:: Specifying date strings.
13512 * Examples of date:: Examples.
13515 @node Time conversion specifiers
13516 @subsection Time conversion specifiers
13518 @cindex time conversion specifiers
13519 @cindex conversion specifiers, time
13521 @command{date} conversion specifiers related to times.
13525 hour (@samp{00}@dots{}@samp{23})
13527 hour (@samp{01}@dots{}@samp{12})
13529 hour (@samp{ 0}@dots{}@samp{23}).
13530 This is a @acronym{GNU} extension.
13532 hour (@samp{ 1}@dots{}@samp{12}).
13533 This is a @acronym{GNU} extension.
13535 minute (@samp{00}@dots{}@samp{59})
13537 nanoseconds (@samp{000000000}@dots{}@samp{999999999}).
13538 This is a @acronym{GNU} extension.
13540 locale's equivalent of either @samp{AM} or @samp{PM};
13541 blank in many locales.
13542 Noon is treated as @samp{PM} and midnight as @samp{AM}.
13544 like @samp{%p}, except lower case.
13545 This is a @acronym{GNU} extension.
13547 locale's 12-hour clock time (e.g., @samp{11:11:04 PM})
13549 24-hour hour and minute. Same as @samp{%H:%M}.
13550 This is a @acronym{GNU} extension.
13552 @cindex epoch, seconds since
13553 @cindex seconds since the epoch
13554 @cindex beginning of time
13555 seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC.
13556 Leap seconds are not counted unless leap second support is available.
13557 @xref{%s-examples}, for examples.
13558 This is a @acronym{GNU} extension.
13560 second (@samp{00}@dots{}@samp{60}).
13561 This may be @samp{60} if leap seconds are supported.
13563 24-hour hour, minute, and second. Same as @samp{%H:%M:%S}.
13565 locale's time representation (e.g., @samp{23:13:48})
13567 @w{@acronym{RFC} 2822/@acronym{ISO} 8601} style numeric time zone
13568 (e.g., @samp{-0600} or @samp{+0530}), or nothing if no
13569 time zone is determinable. This value reflects the numeric time zone
13570 appropriate for the current time, using the time zone rules specified
13571 by the @env{TZ} environment variable.
13572 The time (and optionally, the time zone rules) can be overridden
13573 by the @option{--date} option.
13574 This is a @acronym{GNU} extension.
13576 @w{@acronym{RFC} 3339/@acronym{ISO} 8601} style numeric time zone with
13577 @samp{:} (e.g., @samp{-06:00} or @samp{+05:30}), or nothing if no time
13578 zone is determinable.
13579 This is a @acronym{GNU} extension.
13581 Numeric time zone to the nearest second with @samp{:} (e.g.,
13582 @samp{-06:00:00} or @samp{+05:30:00}), or nothing if no time zone is
13584 This is a @acronym{GNU} extension.
13586 Numeric time zone with @samp{:} using the minimum necessary precision
13587 (e.g., @samp{-06}, @samp{+05:30}, or @samp{-04:56:02}), or nothing if
13588 no time zone is determinable.
13589 This is a @acronym{GNU} extension.
13591 alphabetic time zone abbreviation (e.g., @samp{EDT}), or nothing if no
13592 time zone is determinable. See @samp{%z} for how it is determined.
13596 @node Date conversion specifiers
13597 @subsection Date conversion specifiers
13599 @cindex date conversion specifiers
13600 @cindex conversion specifiers, date
13602 @command{date} conversion specifiers related to dates.
13606 locale's abbreviated weekday name (e.g., @samp{Sun})
13608 locale's full weekday name, variable length (e.g., @samp{Sunday})
13610 locale's abbreviated month name (e.g., @samp{Jan})
13612 locale's full month name, variable length (e.g., @samp{January})
13614 locale's date and time (e.g., @samp{Thu Mar @ 3 23:05:25 2005})
13616 century. This is like @samp{%Y}, except the last two digits are omitted.
13617 For example, it is @samp{20} if @samp{%Y} is @samp{2000},
13618 and is @samp{-0} if @samp{%Y} is @samp{-001}.
13619 It is normally at least two characters, but it may be more.
13621 day of month (e.g., @samp{01})
13623 date; same as @samp{%m/%d/%y}
13625 day of month, space padded; same as @samp{%_d}
13627 full date in @acronym{ISO} 8601 format; same as @samp{%Y-%m-%d}.
13628 This is a good choice for a date format, as it is standard and
13629 is easy to sort in the usual case where years are in the range
13631 This is a @acronym{GNU} extension.
13633 year corresponding to the @acronym{ISO} week number, but without the century
13634 (range @samp{00} through @samp{99}). This has the same format and value
13635 as @samp{%y}, except that if the @acronym{ISO} week number (see
13637 to the previous or next year, that year is used instead.
13638 This is a @acronym{GNU} extension.
13640 year corresponding to the @acronym{ISO} week number. This has the
13641 same format and value as @samp{%Y}, except that if the @acronym{ISO}
13643 @samp{%V}) belongs to the previous or next year, that year is used
13645 It is normally useful only if @samp{%V} is also used;
13646 for example, the format @samp{%G-%m-%d} is probably a mistake,
13647 since it combines the ISO week number year with the conventional month and day.
13648 This is a @acronym{GNU} extension.
13652 day of year (@samp{001}@dots{}@samp{366})
13654 month (@samp{01}@dots{}@samp{12})
13656 day of week (@samp{1}@dots{}@samp{7}) with @samp{1} corresponding to Monday
13658 week number of year, with Sunday as the first day of the week
13659 (@samp{00}@dots{}@samp{53}).
13660 Days in a new year preceding the first Sunday are in week zero.
13662 @acronym{ISO} week number, that is, the
13663 week number of year, with Monday as the first day of the week
13664 (@samp{01}@dots{}@samp{53}).
13665 If the week containing January 1 has four or more days in
13666 the new year, then it is considered week 1; otherwise, it is week 53 of
13667 the previous year, and the next week is week 1. (See the @acronym{ISO} 8601
13670 day of week (@samp{0}@dots{}@samp{6}) with 0 corresponding to Sunday
13672 week number of year, with Monday as first day of week
13673 (@samp{00}@dots{}@samp{53}).
13674 Days in a new year preceding the first Monday are in week zero.
13676 locale's date representation (e.g., @samp{12/31/99})
13678 last two digits of year (@samp{00}@dots{}@samp{99})
13680 year. This is normally at least four characters, but it may be more.
13681 Year @samp{0000} precedes year @samp{0001}, and year @samp{-001}
13682 precedes year @samp{0000}.
13686 @node Literal conversion specifiers
13687 @subsection Literal conversion specifiers
13689 @cindex literal conversion specifiers
13690 @cindex conversion specifiers, literal
13692 @command{date} conversion specifiers that produce literal strings.
13704 @node Padding and other flags
13705 @subsection Padding and other flags
13707 @cindex numeric field padding
13708 @cindex padding of numeric fields
13709 @cindex fields, padding numeric
13711 Unless otherwise specified, @command{date} normally pads numeric fields
13712 with zeros, so that, for
13713 example, numeric months are always output as two digits.
13714 Seconds since the epoch are not padded, though,
13715 since there is no natural width for them.
13717 As a @acronym{GNU} extension, @command{date} recognizes any of the
13718 following optional flags after the @samp{%}:
13722 (hyphen) Do not pad the field; useful if the output is intended for
13725 (underscore) Pad with spaces; useful if you need a fixed
13726 number of characters in the output, but zeros are too distracting.
13728 (zero) Pad with zeros even if the conversion specifier
13729 would normally pad with spaces.
13731 Use upper case characters if possible.
13733 Use opposite case characters if possible.
13734 A field that is normally upper case becomes lower case, and vice versa.
13738 Here are some examples of padding:
13741 date +%d/%m -d "Feb 1"
13743 date +%-d/%-m -d "Feb 1"
13745 date +%_d/%_m -d "Feb 1"
13749 As a @acronym{GNU} extension, you can specify the field width
13750 (after any flag, if present) as a decimal number. If the natural size of the
13751 output of the field has less than the specified number of characters,
13752 the result is written right adjusted and padded to the given
13753 size. For example, @samp{%9B} prints the right adjusted month name in
13754 a field of width 9.
13756 An optional modifier can follow the optional flag and width
13757 specification. The modifiers are:
13761 Use the locale's alternate representation for date and time. This
13762 modifier applies to the @samp{%c}, @samp{%C}, @samp{%x}, @samp{%X},
13763 @samp{%y} and @samp{%Y} conversion specifiers. In a Japanese locale, for
13764 example, @samp{%Ex} might yield a date format based on the Japanese
13768 Use the locale's alternate numeric symbols for numbers. This modifier
13769 applies only to numeric conversion specifiers.
13772 If the format supports the modifier but no alternate representation
13773 is available, it is ignored.
13776 @node Setting the time
13777 @subsection Setting the time
13779 @cindex setting the time
13780 @cindex time setting
13781 @cindex appropriate privileges
13783 If given an argument that does not start with @samp{+}, @command{date} sets
13784 the system clock to the date and time specified by that argument (as
13785 described below). You must have appropriate privileges to set the
13786 system clock. The @option{--date} and @option{--set} options may not be
13787 used with such an argument. The @option{--universal} option may be used
13788 with such an argument to indicate that the specified date and time are
13789 relative to Coordinated Universal Time rather than to the local time
13792 The argument must consist entirely of digits, which have the following
13805 first two digits of year (optional)
13807 last two digits of year (optional)
13812 The @option{--set} option also sets the system clock; see the next section.
13815 @node Options for date
13816 @subsection Options for @command{date}
13818 @cindex @command{date} options
13819 @cindex options for @command{date}
13821 The program accepts the following options. Also see @ref{Common options}.
13825 @item -d @var{datestr}
13826 @itemx --date=@var{datestr}
13829 @cindex parsing date strings
13830 @cindex date strings, parsing
13831 @cindex arbitrary date strings, parsing
13834 @opindex next @var{day}
13835 @opindex last @var{day}
13836 Display the date and time specified in @var{datestr} instead of the
13837 current date and time. @var{datestr} can be in almost any common
13838 format. It can contain month names, time zones, @samp{am} and @samp{pm},
13839 @samp{yesterday}, etc. For example, @option{--date="2004-02-27
13840 14:19:13.489392193 +0530"} specifies the instant of time that is
13841 489,392,193 nanoseconds after February 27, 2004 at 2:19:13 PM in a
13842 time zone that is 5 hours and 30 minutes east of @acronym{UTC}.@*
13843 Note: input currently must be in locale independent format. E.g., the
13844 LC_TIME=C below is needed to print back the correct date in many locales:
13846 date -d "$(LC_TIME=C date)"
13848 @xref{Date input formats}.
13850 @item -f @var{datefile}
13851 @itemx --file=@var{datefile}
13854 Parse each line in @var{datefile} as with @option{-d} and display the
13855 resulting date and time. If @var{datefile} is @samp{-}, use standard
13856 input. This is useful when you have many dates to process, because the
13857 system overhead of starting up the @command{date} executable many times can
13860 @item -r @var{file}
13861 @itemx --reference=@var{file}
13863 @opindex --reference
13864 Display the date and time of the last modification of @var{file},
13865 instead of the current date and time.
13872 @opindex --rfc-2822
13873 Display the date and time using the format @samp{%a, %d %b %Y %H:%M:%S
13874 %z}, evaluated in the C locale so abbreviations are always in English.
13878 Fri, 09 Sep 2005 13:51:39 -0700
13881 This format conforms to
13882 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt, Internet
13883 @acronym{RFCs} 2822} and
13884 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc822.txt, 822}, the
13885 current and previous standards for Internet email.
13887 @item --rfc-3339=@var{timespec}
13888 @opindex --rfc-3339=@var{timespec}
13889 Display the date using a format specified by
13890 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt, Internet
13891 @acronym{RFC} 3339}. This is a subset of the @acronym{ISO} 8601
13892 format, except that it also permits applications to use a space rather
13893 than a @samp{T} to separate dates from times. Unlike the other
13894 standard formats, @acronym{RFC} 3339 format is always suitable as
13895 input for the @option{--date} (@option{-d}) and @option{--file}
13896 (@option{-f}) options, regardless of the current locale.
13898 The argument @var{timespec} specifies how much of the time to include.
13899 It can be one of the following:
13903 Print just the full-date, e.g., @samp{2005-09-14}.
13904 This is equivalent to the format @samp{%Y-%m-%d}.
13907 Print the full-date and full-time separated by a space, e.g.,
13908 @samp{2005-09-14 00:56:06+05:30}. The output ends with a numeric
13909 time-offset; here the @samp{+05:30} means that local time is five
13910 hours and thirty minutes east of @acronym{UTC}. This is equivalent to
13911 the format @samp{%Y-%m-%d %H:%M:%S%:z}.
13914 Like @samp{seconds}, but also print nanoseconds, e.g.,
13915 @samp{2005-09-14 00:56:06.998458565+05:30}.
13916 This is equivalent to the format @samp{%Y-%m-%d %H:%M:%S.%N%:z}.
13920 @item -s @var{datestr}
13921 @itemx --set=@var{datestr}
13924 Set the date and time to @var{datestr}. See @option{-d} above.
13931 @opindex --universal
13932 @cindex Coordinated Universal Time
13934 @cindex Greenwich Mean Time
13937 Use Coordinated Universal Time (@acronym{UTC}) by operating as if the
13938 @env{TZ} environment variable were set to the string @samp{UTC0}.
13940 Universal Time is often called ``Greenwich Mean Time'' (@sc{gmt}) for
13941 historical reasons.
13945 @node Examples of date
13946 @subsection Examples of @command{date}
13948 @cindex examples of @command{date}
13950 Here are a few examples. Also see the documentation for the @option{-d}
13951 option in the previous section.
13956 To print the date of the day before yesterday:
13959 date --date='2 days ago'
13963 To print the date of the day three months and one day hence:
13966 date --date='3 months 1 day'
13970 To print the day of year of Christmas in the current year:
13973 date --date='25 Dec' +%j
13977 To print the current full month name and the day of the month:
13983 But this may not be what you want because for the first nine days of
13984 the month, the @samp{%d} expands to a zero-padded two-digit field,
13985 for example @samp{date -d 1may '+%B %d'} will print @samp{May 01}.
13988 To print a date without the leading zero for one-digit days
13989 of the month, you can use the (@acronym{GNU} extension)
13990 @samp{-} flag to suppress
13991 the padding altogether:
13994 date -d 1may '+%B %-d
13998 To print the current date and time in the format required by many
13999 non-@acronym{GNU} versions of @command{date} when setting the system clock:
14002 date +%m%d%H%M%Y.%S
14006 To set the system clock forward by two minutes:
14009 date --set='+2 minutes'
14013 To print the date in @acronym{RFC} 2822 format,
14014 use @samp{date --rfc-2822}. Here is some example output:
14017 Fri, 09 Sep 2005 13:51:39 -0700
14020 @anchor{%s-examples}
14022 To convert a date string to the number of seconds since the epoch
14023 (which is 1970-01-01 00:00:00 UTC), use the @option{--date} option with
14024 the @samp{%s} format. That can be useful in sorting and/or graphing
14025 and/or comparing data by date. The following command outputs the
14026 number of the seconds since the epoch for the time two minutes after the
14030 date --date='1970-01-01 00:02:00 +0000' +%s
14034 If you do not specify time zone information in the date string,
14035 @command{date} uses your computer's idea of the time zone when
14036 interpreting the string. For example, if your computer's time zone is
14037 that of Cambridge, Massachusetts, which was then 5 hours (i.e., 18,000
14038 seconds) behind UTC:
14041 # local time zone used
14042 date --date='1970-01-01 00:02:00' +%s
14047 If you're sorting or graphing dated data, your raw date values may be
14048 represented as seconds since the epoch. But few people can look at
14049 the date @samp{946684800} and casually note ``Oh, that's the first second
14050 of the year 2000 in Greenwich, England.''
14053 date --date='2000-01-01 UTC' +%s
14057 An alternative is to use the @option{--utc} (@option{-u}) option.
14058 Then you may omit @samp{UTC} from the date string. Although this
14059 produces the same result for @samp{%s} and many other format sequences,
14060 with a time zone offset different from zero, it would give a different
14061 result for zone-dependent formats like @samp{%z}.
14064 date -u --date=2000-01-01 +%s
14068 To convert such an unwieldy number of seconds back to
14069 a more readable form, use a command like this:
14072 # local time zone used
14073 date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
14074 1999-12-31 19:00:00 -0500
14077 Or if you do not mind depending on the @samp{@@} feature present since
14078 coreutils 5.3.0, you could shorten this to:
14081 date -d @@946684800 +"%F %T %z"
14082 1999-12-31 19:00:00 -0500
14085 Often it is better to output UTC-relative date and time:
14088 date -u -d '1970-01-01 946684800 seconds' +"%Y-%m-%d %T %z"
14089 2000-01-01 00:00:00 +0000
14095 @node arch invocation
14096 @section @command{arch}: Print machine hardware name
14099 @cindex print machine hardware name
14100 @cindex system information, printing
14102 @command{arch} prints the machine hardware name,
14103 and is equivalent to @samp{uname -m}.
14107 arch [@var{option}]
14110 The program accepts the @ref{Common options} only.
14115 @node nproc invocation
14116 @section @command{nproc}: Print the number of available processors
14119 @cindex Print the number of processors
14120 @cindex system information, printing
14122 Print the number of processing units available to the current process,
14123 which may be less than the number of online processors.
14124 If this information is not accessible, then print the number of
14125 processors installed. If the @env{OMP_NUM_THREADS} environment variable is
14126 set, then it will determine the returned value. The result is guaranteed to be
14127 greater than zero. Synopsis:
14130 nproc [@var{option}]
14133 The program accepts the following options. Also see @ref{Common options}.
14139 Print the number of installed processors on the system, which may
14140 be greater than the number online or available to the current process.
14141 The @env{OMP_NUM_THREADS} environment variable is not honored in this case.
14143 @item --ignore=@var{number}
14145 If possible, exclude this @var{number} of processing units.
14152 @node uname invocation
14153 @section @command{uname}: Print system information
14156 @cindex print system information
14157 @cindex system information, printing
14159 @command{uname} prints information about the machine and operating system
14160 it is run on. If no options are given, @command{uname} acts as if the
14161 @option{-s} option were given. Synopsis:
14164 uname [@var{option}]@dots{}
14167 If multiple options or @option{-a} are given, the selected information is
14168 printed in this order:
14171 @var{kernel-name} @var{nodename} @var{kernel-release} @var{kernel-version}
14172 @var{machine} @var{processor} @var{hardware-platform} @var{operating-system}
14175 The information may contain internal spaces, so such output cannot be
14176 parsed reliably. In the following example, @var{release} is
14177 @samp{2.2.18ss.e820-bda652a #4 SMP Tue Jun 5 11:24:08 PDT 2001}:
14181 @result{} Linux dum 2.2.18 #4 SMP Tue Jun 5 11:24:08 PDT 2001 i686 unknown unknown GNU/Linux
14185 The program accepts the following options. Also see @ref{Common options}.
14193 Print all of the below information, except omit the processor type
14194 and the hardware platform name if they are unknown.
14197 @itemx --hardware-platform
14199 @opindex --hardware-platform
14200 @cindex implementation, hardware
14201 @cindex hardware platform
14202 @cindex platform, hardware
14203 Print the hardware platform name
14204 (sometimes called the hardware implementation).
14205 Print @samp{unknown} if the kernel does not make this information
14206 easily available, as is the case with Linux kernels.
14212 @cindex machine type
14213 @cindex hardware class
14214 @cindex hardware type
14215 Print the machine hardware name (sometimes called the hardware class
14221 @opindex --nodename
14224 @cindex network node name
14225 Print the network node hostname.
14230 @opindex --processor
14231 @cindex host processor type
14232 Print the processor type (sometimes called the instruction set
14233 architecture or ISA).
14234 Print @samp{unknown} if the kernel does not make this information
14235 easily available, as is the case with Linux kernels.
14238 @itemx --operating-system
14240 @opindex --operating-system
14241 @cindex operating system name
14242 Print the name of the operating system.
14245 @itemx --kernel-release
14247 @opindex --kernel-release
14248 @cindex kernel release
14249 @cindex release of kernel
14250 Print the kernel release.
14253 @itemx --kernel-name
14255 @opindex --kernel-name
14256 @cindex kernel name
14257 @cindex name of kernel
14258 Print the kernel name.
14259 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) calls this
14260 ``the implementation of the operating system'', because the
14261 @acronym{POSIX} specification itself has no notion of ``kernel''.
14262 The kernel name might be the same as the operating system name printed
14263 by the @option{-o} or @option{--operating-system} option, but it might
14264 differ. Some operating systems (e.g., FreeBSD, HP-UX) have the same
14265 name as their underlying kernels; others (e.g., GNU/Linux, Solaris)
14269 @itemx --kernel-version
14271 @opindex --kernel-version
14272 @cindex kernel version
14273 @cindex version of kernel
14274 Print the kernel version.
14281 @node hostname invocation
14282 @section @command{hostname}: Print or set system name
14285 @cindex setting the hostname
14286 @cindex printing the hostname
14287 @cindex system name, printing
14288 @cindex appropriate privileges
14290 With no arguments, @command{hostname} prints the name of the current host
14291 system. With one argument, it sets the current host name to the
14292 specified string. You must have appropriate privileges to set the host
14296 hostname [@var{name}]
14299 The only options are @option{--help} and @option{--version}. @xref{Common
14305 @node hostid invocation
14306 @section @command{hostid}: Print numeric host identifier
14309 @cindex printing the host identifier
14311 @command{hostid} prints the numeric identifier of the current host
14312 in hexadecimal. This command accepts no arguments.
14313 The only options are @option{--help} and @option{--version}.
14314 @xref{Common options}.
14316 For example, here's what it prints on one system I use:
14323 On that system, the 32-bit quantity happens to be closely
14324 related to the system's Internet address, but that isn't always
14329 @node uptime invocation
14330 @section @command{uptime}: Print system uptime and load
14333 @cindex printing the system uptime and load
14335 @command{uptime} prints the current time, the system's uptime, the
14336 number of logged-in users and the current load average.
14338 If an argument is specified, it is used as the file to be read
14339 to discover how many users are logged in. If no argument is
14340 specified, a system default is used (@command{uptime --help} indicates
14341 the default setting).
14343 The only options are @option{--help} and @option{--version}.
14344 @xref{Common options}.
14346 For example, here's what it prints right now on one system I use:
14350 14:07 up 3:35, 3 users, load average: 1.39, 1.15, 1.04
14353 The precise method of calculation of load average varies somewhat
14354 between systems. Some systems calculate it as the average number of
14355 runnable processes over the last 1, 5 and 15 minutes, but some systems
14356 also include processes in the uninterruptible sleep state (that is,
14357 those processes which are waiting for disk I/O). The Linux kernel
14358 includes uninterruptible processes.
14360 @node SELinux context
14361 @chapter SELinux context
14363 @cindex SELinux context
14364 @cindex SELinux, context
14365 @cindex commands for SELinux context
14367 This section describes commands for operations with SELinux
14371 * chcon invocation:: Change SELinux context of file
14372 * runcon invocation:: Run a command in specified SELinux context
14375 @node chcon invocation
14376 @section @command{chcon}: Change SELinux context of file
14379 @cindex changing security context
14380 @cindex change SELinux context
14382 @command{chcon} changes the SELinux security context of the selected files.
14386 chcon [@var{option}]@dots{} @var{context} @var{file}@dots{}
14387 chcon [@var{option}]@dots{} [-u @var{user}] [-r @var{role}] [-l @var{range}] [-t @var{type}] @var{file}@dots{}
14388 chcon [@var{option}]@dots{} --reference=@var{rfile} @var{file}@dots{}
14391 Change the SELinux security context of each @var{file} to @var{context}.
14392 With @option{--reference}, change the security context of each @var{file}
14393 to that of @var{rfile}.
14395 The program accepts the following options. Also see @ref{Common options}.
14400 @itemx --no-dereference
14402 @opindex --no-dereference
14403 @cindex no dereference
14404 Affect symbolic links instead of any referenced file.
14406 @item --reference=@var{rfile}
14407 @opindex --reference
14408 @cindex reference file
14409 Use @var{rfile}'s security context rather than specifying a @var{context} value.
14414 @opindex --recursive
14415 Operate on files and directories recursively.
14418 @xref{Traversing symlinks}.
14421 @xref{Traversing symlinks}.
14424 @xref{Traversing symlinks}.
14431 Output a diagnostic for every file processed.
14433 @item -u @var{user}
14434 @itemx --user=@var{user}
14437 Set user @var{user} in the target security context.
14439 @item -r @var{role}
14440 @itemx --role=@var{role}
14443 Set role @var{role} in the target security context.
14445 @item -t @var{type}
14446 @itemx --type=@var{type}
14449 Set type @var{type} in the target security context.
14451 @item -l @var{range}
14452 @itemx --range=@var{range}
14455 Set range @var{range} in the target security context.
14461 @node runcon invocation
14462 @section @command{runcon}: Run a command in specified SELinux context
14465 @cindex run with security context
14468 @command{runcon} runs file in specified SELinux security context.
14472 runcon @var{context} @var{command} [@var{args}]
14473 runcon [ -c ] [-u @var{user}] [-r @var{role}] [-t @var{type}] [-l @var{range}] @var{command} [@var{args}]
14476 Run @var{command} with completely-specified @var{context}, or with
14477 current or transitioned security context modified by one or more of @var{level},
14478 @var{role}, @var{type} and @var{user}.
14480 If none of @option{-c}, @option{-t}, @option{-u}, @option{-r}, or @option{-l}
14481 is specified, the first argument is used as the complete context.
14482 Any additional arguments after @var{command}
14483 are interpreted as arguments to the command.
14485 With neither @var{context} nor @var{command}, print the current security context.
14487 The program accepts the following options. Also see @ref{Common options}.
14495 Compute process transition context before modifying.
14497 @item -u @var{user}
14498 @itemx --user=@var{user}
14501 Set user @var{user} in the target security context.
14503 @item -r @var{role}
14504 @itemx --role=@var{role}
14507 Set role @var{role} in the target security context.
14509 @item -t @var{type}
14510 @itemx --type=@var{type}
14513 Set type @var{type} in the target security context.
14515 @item -l @var{range}
14516 @itemx --range=@var{range}
14519 Set range @var{range} in the target security context.
14523 @cindex exit status of @command{runcon}
14527 126 if @var{command} is found but cannot be invoked
14528 127 if @command{runcon} itself fails or if @var{command} cannot be found
14529 the exit status of @var{command} otherwise
14532 @node Modified command invocation
14533 @chapter Modified command invocation
14535 @cindex modified command invocation
14536 @cindex invocation of commands, modified
14537 @cindex commands for invoking other commands
14539 This section describes commands that run other commands in some context
14540 different than the current one: a modified environment, as a different
14544 * chroot invocation:: Modify the root directory.
14545 * env invocation:: Modify environment variables.
14546 * nice invocation:: Modify niceness.
14547 * nohup invocation:: Immunize to hangups.
14548 * stdbuf invocation:: Modify buffering of standard streams.
14549 * su invocation:: Modify user and group ID.
14550 * timeout invocation:: Run with time limit.
14554 @node chroot invocation
14555 @section @command{chroot}: Run a command with a different root directory
14558 @cindex running a program in a specified root directory
14559 @cindex root directory, running a program in a specified
14561 @command{chroot} runs a command with a specified root directory.
14562 On many systems, only the super-user can do this.@footnote{However,
14563 some systems (e.g., FreeBSD) can be configured to allow certain regular
14564 users to use the @code{chroot} system call, and hence to run this program.
14565 Also, on Cygwin, anyone can run the @command{chroot} command, because the
14566 underlying function is non-privileged due to lack of support in MS-Windows.}
14570 chroot @var{option} @var{newroot} [@var{command} [@var{args}]@dots{}]
14571 chroot @var{option}
14574 Ordinarily, file names are looked up starting at the root of the
14575 directory structure, i.e., @file{/}. @command{chroot} changes the root to
14576 the directory @var{newroot} (which must exist) and then runs
14577 @var{command} with optional @var{args}. If @var{command} is not
14578 specified, the default is the value of the @env{SHELL} environment
14579 variable or @command{/bin/sh} if not set, invoked with the @option{-i} option.
14580 @var{command} must not be a special built-in utility
14581 (@pxref{Special built-in utilities}).
14583 The program accepts the following options. Also see @ref{Common options}.
14584 Options must precede operands.
14588 @itemx --userspec=@var{user}[:@var{group}]
14589 @opindex --userspec
14590 By default, @var{command} is run with the same credentials
14591 as the invoking process.
14592 Use this option to run it as a different @var{user} and/or with a
14593 different primary @var{group}.
14595 @itemx --groups=@var{groups}
14597 Use this option to specify the supplementary @var{groups} to be
14598 used by the new process.
14599 The items in the list (names or numeric IDs) must be separated by commas.
14603 Here are a few tips to help avoid common problems in using chroot.
14604 To start with a simple example, make @var{command} refer to a statically
14605 linked binary. If you were to use a dynamically linked executable, then
14606 you'd have to arrange to have the shared libraries in the right place under
14607 your new root directory.
14609 For example, if you create a statically linked @command{ls} executable,
14610 and put it in @file{/tmp/empty}, you can run this command as root:
14613 $ chroot /tmp/empty /ls -Rl /
14616 Then you'll see output like this:
14621 -rwxr-xr-x 1 0 0 1041745 Aug 16 11:17 ls
14624 If you want to use a dynamically linked executable, say @command{bash},
14625 then first run @samp{ldd bash} to see what shared objects it needs.
14626 Then, in addition to copying the actual binary, also copy the listed
14627 files to the required positions under your intended new root directory.
14628 Finally, if the executable requires any other files (e.g., data, state,
14629 device files), copy them into place, too.
14631 @cindex exit status of @command{chroot}
14635 125 if @command{chroot} itself fails
14636 126 if @var{command} is found but cannot be invoked
14637 127 if @var{command} cannot be found
14638 the exit status of @var{command} otherwise
14642 @node env invocation
14643 @section @command{env}: Run a command in a modified environment
14646 @cindex environment, running a program in a modified
14647 @cindex modified environment, running a program in a
14648 @cindex running a program in a modified environment
14650 @command{env} runs a command with a modified environment. Synopses:
14653 env [@var{option}]@dots{} [@var{name}=@var{value}]@dots{} @c
14654 [@var{command} [@var{args}]@dots{}]
14658 Operands of the form @samp{@var{variable}=@var{value}} set
14659 the environment variable @var{variable} to value @var{value}.
14660 @var{value} may be empty (@samp{@var{variable}=}). Setting a variable
14661 to an empty value is different from unsetting it.
14662 These operands are evaluated left-to-right, so if two operands
14663 mention the same variable the earlier is ignored.
14665 Environment variable names can be empty, and can contain any
14666 characters other than @samp{=} and @acronym{ASCII} @sc{nul}.
14667 However, it is wise to limit yourself to names that
14668 consist solely of underscores, digits, and @acronym{ASCII} letters,
14669 and that begin with a non-digit, as applications like the shell do not
14670 work well with other names.
14673 The first operand that does not contain the character @samp{=}
14674 specifies the program to invoke; it is
14675 searched for according to the @env{PATH} environment variable. Any
14676 remaining arguments are passed as arguments to that program.
14677 The program should not be a special built-in utility
14678 (@pxref{Special built-in utilities}).
14680 Modifications to @env{PATH} take effect prior to searching for
14681 @var{command}. Use caution when reducing @env{PATH}; behavior is
14682 not portable when @env{PATH} is undefined or omits key directories
14683 such as @file{/bin}.
14685 In the rare case that a utility contains a @samp{=} in the name, the
14686 only way to disambiguate it from a variable assignment is to use an
14687 intermediate command for @var{command}, and pass the problematic
14688 program name via @var{args}. For example, if @file{./prog=} is an
14689 executable in the current @env{PATH}:
14692 env prog= true # runs 'true', with prog= in environment
14693 env ./prog= true # runs 'true', with ./prog= in environment
14694 env -- prog= true # runs 'true', with prog= in environment
14695 env sh -c '\prog= true' # runs 'prog=' with argument 'true'
14696 env sh -c 'exec "$@@"' sh prog= true # also runs 'prog='
14699 @cindex environment, printing
14701 If no command name is specified following the environment
14702 specifications, the resulting environment is printed. This is like
14703 specifying the @command{printenv} program.
14705 For some examples, suppose the environment passed to @command{env}
14706 contains @samp{LOGNAME=rms}, @samp{EDITOR=emacs}, and
14707 @samp{PATH=.:/gnubin:/hacks}:
14712 Output the current environment.
14714 $ env | LC_ALL=C sort
14717 PATH=.:/gnubin:/hacks
14721 Run @command{foo} with a reduced environment, preserving only the
14722 original @env{PATH} to avoid problems in locating @command{foo}.
14724 env - PATH="$PATH" foo
14728 Run @command{foo} with the environment containing @samp{LOGNAME=rms},
14729 @samp{EDITOR=emacs}, and @samp{PATH=.:/gnubin:/hacks}, and guarantees
14730 that @command{foo} was found in the file system rather than as a shell
14737 Run @command{nemacs} with the environment containing @samp{LOGNAME=foo},
14738 @samp{EDITOR=emacs}, @samp{PATH=.:/gnubin:/hacks}, and
14739 @samp{DISPLAY=gnu:0}.
14741 env DISPLAY=gnu:0 LOGNAME=foo nemacs
14745 Attempt to run the program @command{/energy/--} (as that is the only
14746 possible path search result); if the command exists, the environment
14747 will contain @samp{LOGNAME=rms} and @samp{PATH=/energy}, and the
14748 arguments will be @samp{e=mc2}, @samp{bar}, and @samp{baz}.
14750 env -u EDITOR PATH=/energy -- e=mc2 bar baz
14756 The program accepts the following options. Also see @ref{Common options}.
14757 Options must precede operands.
14763 @item -u @var{name}
14764 @itemx --unset=@var{name}
14767 Remove variable @var{name} from the environment, if it was in the
14772 @itemx --ignore-environment
14775 @opindex --ignore-environment
14776 Start with an empty environment, ignoring the inherited environment.
14780 @cindex exit status of @command{env}
14784 0 if no @var{command} is specified and the environment is output
14785 125 if @command{env} itself fails
14786 126 if @var{command} is found but cannot be invoked
14787 127 if @var{command} cannot be found
14788 the exit status of @var{command} otherwise
14792 @node nice invocation
14793 @section @command{nice}: Run a command with modified niceness
14797 @cindex scheduling, affecting
14798 @cindex appropriate privileges
14800 @command{nice} prints or modifies a process's @dfn{niceness},
14801 a parameter that affects whether the process is scheduled favorably.
14805 nice [@var{option}]@dots{} [@var{command} [@var{arg}]@dots{}]
14808 If no arguments are given, @command{nice} prints the current niceness.
14809 Otherwise, @command{nice} runs the given @var{command} with its
14810 niceness adjusted. By default, its niceness is incremented by 10.
14812 Niceness values range at least from @minus{}20 (process has high priority
14813 and gets more resources, thus slowing down other processes) through 19
14814 (process has lower priority and runs slowly itself, but has less impact
14815 on the speed of other running processes). Some systems
14816 may have a wider range of nicenesses; conversely, other systems may
14817 enforce more restrictive limits. An attempt to set the niceness
14818 outside the supported range is treated as an attempt to use the
14819 minimum or maximum supported value.
14821 A niceness should not be confused with a scheduling priority, which
14822 lets applications determine the order in which threads are scheduled
14823 to run. Unlike a priority, a niceness is merely advice to the
14824 scheduler, which the scheduler is free to ignore. Also, as a point of
14825 terminology, @acronym{POSIX} defines the behavior of @command{nice} in
14826 terms of a @dfn{nice value}, which is the nonnegative difference
14827 between a niceness and the minimum niceness. Though @command{nice}
14828 conforms to @acronym{POSIX}, its documentation and diagnostics use the
14829 term ``niceness'' for compatibility with historical practice.
14831 @var{command} must not be a special built-in utility (@pxref{Special
14832 built-in utilities}).
14834 @mayConflictWithShellBuiltIn{nice}
14836 The program accepts the following option. Also see @ref{Common options}.
14837 Options must precede operands.
14840 @item -n @var{adjustment}
14841 @itemx --adjustment=@var{adjustment}
14843 @opindex --adjustment
14844 Add @var{adjustment} instead of 10 to the command's niceness. If
14845 @var{adjustment} is negative and you lack appropriate privileges,
14846 @command{nice} issues a warning but otherwise acts as if you specified
14849 For compatibility @command{nice} also supports an obsolete
14850 option syntax @option{-@var{adjustment}}. New scripts should use
14851 @option{-n @var{adjustment}} instead.
14855 @cindex exit status of @command{nice}
14859 0 if no @var{command} is specified and the niceness is output
14860 125 if @command{nice} itself fails
14861 126 if @var{command} is found but cannot be invoked
14862 127 if @var{command} cannot be found
14863 the exit status of @var{command} otherwise
14866 It is sometimes useful to run a non-interactive program with reduced niceness.
14869 $ nice factor 4611686018427387903
14872 Since @command{nice} prints the current niceness,
14873 you can invoke it through itself to demonstrate how it works.
14875 The default behavior is to increase the niceness by @samp{10}:
14886 The @var{adjustment} is relative to the current niceness. In the
14887 next example, the first @command{nice} invocation runs the second one
14888 with niceness 10, and it in turn runs the final one with a niceness
14892 $ nice nice -n 3 nice
14896 Specifying a niceness larger than the supported range
14897 is the same as specifying the maximum supported value:
14900 $ nice -n 10000000000 nice
14904 Only a privileged user may run a process with lower niceness:
14908 nice: cannot set niceness: Permission denied
14910 $ sudo nice -n -1 nice
14915 @node nohup invocation
14916 @section @command{nohup}: Run a command immune to hangups
14919 @cindex hangups, immunity to
14920 @cindex immunity to hangups
14921 @cindex logging out and continuing to run
14924 @command{nohup} runs the given @var{command} with hangup signals ignored,
14925 so that the command can continue running in the background after you log
14929 nohup @var{command} [@var{arg}]@dots{}
14932 If standard input is a terminal, it is redirected from
14933 @file{/dev/null} so that terminal sessions do not mistakenly consider
14934 the terminal to be used by the command. This is a @acronym{GNU}
14935 extension; programs intended to be portable to non-@acronym{GNU} hosts
14936 should use @samp{nohup @var{command} [@var{arg}]@dots{} </dev/null}
14940 If standard output is a terminal, the command's standard output is appended
14941 to the file @file{nohup.out}; if that cannot be written to, it is appended
14942 to the file @file{$HOME/nohup.out}; and if that cannot be written to, the
14943 command is not run.
14944 Any @file{nohup.out} or @file{$HOME/nohup.out} file created by
14945 @command{nohup} is made readable and writable only to the user,
14946 regardless of the current umask settings.
14948 If standard error is a terminal, it is normally redirected to the same file
14949 descriptor as the (possibly-redirected) standard output.
14950 However, if standard output is closed, standard error terminal output
14951 is instead appended to the file @file{nohup.out} or
14952 @file{$HOME/nohup.out} as above.
14954 To capture the command's output to a file other than @file{nohup.out}
14955 you can redirect it. For example, to capture the output of
14959 nohup make > make.log
14962 @command{nohup} does not automatically put the command it runs in the
14963 background; you must do that explicitly, by ending the command line
14964 with an @samp{&}. Also, @command{nohup} does not alter the
14965 niceness of @var{command}; use @command{nice} for that,
14966 e.g., @samp{nohup nice @var{command}}.
14968 @var{command} must not be a special built-in utility (@pxref{Special
14969 built-in utilities}).
14971 The only options are @option{--help} and @option{--version}. @xref{Common
14972 options}. Options must precede operands.
14974 @cindex exit status of @command{nohup}
14978 125 if @command{nohup} itself fails, and @env{POSIXLY_CORRECT} is not set
14979 126 if @var{command} is found but cannot be invoked
14980 127 if @var{command} cannot be found
14981 the exit status of @var{command} otherwise
14984 If @env{POSIXLY_CORRECT} is set, internal failures give status 127
14988 @node stdbuf invocation
14989 @section @command{stdbuf}: Run a command with modified I/O stream buffering
14992 @cindex standard streams, buffering
14993 @cindex line buffered
14995 @command{stdbuf} allows one to modify the buffering operations of the
14996 three standard I/O streams associated with a program. Synopsis:
14999 stdbuf @var{option}@dots{} @var{command}
15002 Any additional @var{arg}s are passed as additional arguments to the
15005 The program accepts the following options. Also see @ref{Common options}.
15009 @item -i @var{mode}
15010 @itemx --input=@var{mode}
15013 Adjust the standard input stream buffering.
15015 @item -o @var{mode}
15016 @itemx --output=@var{mode}
15019 Adjust the standard output stream buffering.
15021 @item -e @var{mode}
15022 @itemx --error=@var{mode}
15025 Adjust the standard error stream buffering.
15029 The @var{mode} can be specified as follows:
15034 Set the stream to line buffered mode.
15035 In this mode data is coalesced until a newline is output or
15036 input is read from any stream attached to a terminal device.
15037 This option is invalid with standard input.
15040 Disable buffering of the selected stream.
15041 In this mode data is output immediately and only the
15042 amount of data requested is read from input.
15045 Specify the size of the buffer to use in fully buffered mode.
15046 @multiplierSuffixesNoBlocks{size}
15050 NOTE: If @var{command} adjusts the buffering of its standard streams
15051 (@command{tee} does for e.g.) then that will override corresponding settings
15052 changed by @command{stdbuf}. Also some filters (like @command{dd} and
15053 @command{cat} etc.) don't use streams for I/O, and are thus unaffected
15054 by @command{stdbuf} settings.
15056 @cindex exit status of @command{stdbuf}
15060 125 if @command{stdbuf} itself fails
15061 126 if @var{command} is found but cannot be invoked
15062 127 if @var{command} cannot be found
15063 the exit status of @var{command} otherwise
15067 @node su invocation
15068 @section @command{su}: Run a command with substitute user and group ID
15071 @cindex substitute user and group IDs
15072 @cindex user ID, switching
15073 @cindex super-user, becoming
15074 @cindex root, becoming
15076 @command{su} allows one user to temporarily become another user. It runs a
15077 command (often an interactive shell) with the real and effective user
15078 ID, group ID, and supplemental groups of a given @var{user}. Synopsis:
15081 su [@var{option}]@dots{} [@var{user} [@var{arg}]@dots{}]
15084 @cindex passwd entry, and @command{su} shell
15086 @flindex /etc/passwd
15087 If no @var{user} is given, the default is @code{root}, the super-user.
15088 The shell to use is taken from @var{user}'s @code{passwd} entry, or
15089 @file{/bin/sh} if none is specified there. If @var{user} has a
15090 password, @command{su} prompts for the password unless run by a user with
15091 effective user ID of zero (the super-user).
15097 @cindex login shell
15098 By default, @command{su} does not change the current directory.
15099 It sets the environment variables @env{HOME} and @env{SHELL}
15100 from the password entry for @var{user}, and if @var{user} is not
15101 the super-user, sets @env{USER} and @env{LOGNAME} to @var{user}.
15102 By default, the shell is not a login shell.
15104 Any additional @var{arg}s are passed as additional arguments to the
15107 @cindex @option{-su}
15108 GNU @command{su} does not treat @file{/bin/sh} or any other shells specially
15109 (e.g., by setting @code{argv[0]} to @option{-su}, passing @option{-c} only
15110 to certain shells, etc.).
15113 @command{su} can optionally be compiled to use @code{syslog} to report
15114 failed, and optionally successful, @command{su} attempts. (If the system
15115 supports @code{syslog}.) However, GNU @command{su} does not check if the
15116 user is a member of the @code{wheel} group; see below.
15118 The program accepts the following options. Also see @ref{Common options}.
15121 @item -c @var{command}
15122 @itemx --command=@var{command}
15125 Pass @var{command}, a single command line to run, to the shell with
15126 a @option{-c} option instead of starting an interactive shell.
15133 @cindex file name pattern expansion, disabled
15134 @cindex globbing, disabled
15135 Pass the @option{-f} option to the shell. This probably only makes sense
15136 if the shell run is @command{csh} or @command{tcsh}, for which the @option{-f}
15137 option prevents reading the startup file (@file{.cshrc}). With
15138 Bourne-like shells, the @option{-f} option disables file name pattern
15139 expansion (globbing), which is not likely to be useful.
15147 @c other variables already indexed above
15150 @cindex login shell, creating
15151 Make the shell a login shell. This means the following. Unset all
15152 environment variables except @env{TERM}, @env{HOME}, and @env{SHELL}
15153 (which are set as described above), and @env{USER} and @env{LOGNAME}
15154 (which are set, even for the super-user, as described above), and set
15155 @env{PATH} to a compiled-in default value. Change to @var{user}'s home
15156 directory. Prepend @samp{-} to the shell's name, intended to make it
15157 read its login startup file(s).
15161 @itemx --preserve-environment
15164 @opindex --preserve-environment
15165 @cindex environment, preserving
15166 @flindex /etc/shells
15167 @cindex restricted shell
15168 Do not change the environment variables @env{HOME}, @env{USER},
15169 @env{LOGNAME}, or @env{SHELL}. Run the shell given in the environment
15170 variable @env{SHELL} instead of the shell from @var{user}'s passwd
15171 entry, unless the user running @command{su} is not the super-user and
15172 @var{user}'s shell is restricted. A @dfn{restricted shell} is one that
15173 is not listed in the file @file{/etc/shells}, or in a compiled-in list
15174 if that file does not exist. Parts of what this option does can be
15175 overridden by @option{--login} and @option{--shell}.
15177 @item -s @var{shell}
15178 @itemx --shell=@var{shell}
15181 Run @var{shell} instead of the shell from @var{user}'s passwd entry,
15182 unless the user running @command{su} is not the super-user and @var{user}'s
15183 shell is restricted (see @option{-m} just above).
15187 @cindex exit status of @command{su}
15191 125 if @command{su} itself fails
15192 126 if subshell is found but cannot be invoked
15193 127 if subshell cannot be found
15194 the exit status of the subshell otherwise
15197 @cindex wheel group, not supported
15198 @cindex group wheel, not supported
15200 @subsection Why GNU @command{su} does not support the @samp{wheel} group
15202 (This section is by Richard Stallman.)
15206 Sometimes a few of the users try to hold total power over all the
15207 rest. For example, in 1984, a few users at the MIT AI lab decided to
15208 seize power by changing the operator password on the Twenex system and
15209 keeping it secret from everyone else. (I was able to thwart this coup
15210 and give power back to the users by patching the kernel, but I
15211 wouldn't know how to do that in Unix.)
15213 However, occasionally the rulers do tell someone. Under the usual
15214 @command{su} mechanism, once someone learns the root password who
15215 sympathizes with the ordinary users, he or she can tell the rest. The
15216 ``wheel group'' feature would make this impossible, and thus cement the
15217 power of the rulers.
15219 I'm on the side of the masses, not that of the rulers. If you are
15220 used to supporting the bosses and sysadmins in whatever they do, you
15221 might find this idea strange at first.
15224 @node timeout invocation
15225 @section @command{timeout}: Run a command with a time limit
15229 @cindex run commands with bounded time
15231 @command{timeout} runs the given @var{command} and kills it if it is
15232 still running after the specified time interval. Synopsis:
15235 timeout [@var{option}] @var{duration} @var{command} [@var{arg}]@dots{}
15238 @var{command} must not be a special built-in utility (@pxref{Special
15239 built-in utilities}).
15241 The program accepts the following options. Also see @ref{Common options}.
15242 Options must precede operands.
15245 @item -k @var{duration}
15246 @itemx --kill-after=@var{duration}
15248 @opindex --kill-after
15249 Ensure the monitored @var{command} is killed by also sending a @samp{KILL}
15250 signal, after the specified @var{duration}. Without this option, if the
15251 selected signal proves not to be fatal, @command{timeout} does not kill
15254 @item -s @var{signal}
15255 @itemx --signal=@var{signal}
15258 Send this @var{signal} to @var{command} on timeout, rather than the
15259 default @samp{TERM} signal. @var{signal} may be a name like @samp{HUP}
15260 or a number. Also see @xref{Signal specifications}.
15264 @var{duration} is an integer followed by an optional unit:
15266 @samp{s} for seconds (the default)
15267 @samp{m} for minutes
15271 A duration of 0 disables the associated timeout.
15273 @cindex exit status of @command{timeout}
15277 124 if @var{command} times out
15278 125 if @command{timeout} itself fails
15279 126 if @var{command} is found but cannot be invoked
15280 127 if @var{command} cannot be found
15281 the exit status of @var{command} otherwise
15285 @node Process control
15286 @chapter Process control
15288 @cindex processes, commands for controlling
15289 @cindex commands for controlling processes
15292 * kill invocation:: Sending a signal to processes.
15296 @node kill invocation
15297 @section @command{kill}: Send a signal to processes
15300 @cindex send a signal to processes
15302 The @command{kill} command sends a signal to processes, causing them
15303 to terminate or otherwise act upon receiving the signal in some way.
15304 Alternatively, it lists information about signals. Synopses:
15307 kill [-s @var{signal} | --signal @var{signal} | -@var{signal}] @var{pid}@dots{}
15308 kill [-l | --list | -t | --table] [@var{signal}]@dots{}
15311 @mayConflictWithShellBuiltIn{kill}
15313 The first form of the @command{kill} command sends a signal to all
15314 @var{pid} arguments. The default signal to send if none is specified
15315 is @samp{TERM}. The special signal number @samp{0} does not denote a
15316 valid signal, but can be used to test whether the @var{pid} arguments
15317 specify processes to which a signal could be sent.
15319 If @var{pid} is positive, the signal is sent to the process with the
15320 process ID @var{pid}. If @var{pid} is zero, the signal is sent to all
15321 processes in the process group of the current process. If @var{pid}
15322 is @minus{}1, the signal is sent to all processes for which the user has
15323 permission to send a signal. If @var{pid} is less than @minus{}1, the signal
15324 is sent to all processes in the process group that equals the absolute
15325 value of @var{pid}.
15327 If @var{pid} is not positive, a system-dependent set of system
15328 processes is excluded from the list of processes to which the signal
15331 If a negative @var{pid} argument is desired as the first one, it
15332 should be preceded by @option{--}. However, as a common extension to
15333 @acronym{POSIX}, @option{--} is not required with @samp{kill
15334 -@var{signal} -@var{pid}}. The following commands are equivalent:
15343 The first form of the @command{kill} command succeeds if every @var{pid}
15344 argument specifies at least one process that the signal was sent to.
15346 The second form of the @command{kill} command lists signal information.
15347 Either the @option{-l} or @option{--list} option, or the @option{-t}
15348 or @option{--table} option must be specified. Without any
15349 @var{signal} argument, all supported signals are listed. The output
15350 of @option{-l} or @option{--list} is a list of the signal names, one
15351 per line; if @var{signal} is already a name, the signal number is
15352 printed instead. The output of @option{-t} or @option{--table} is a
15353 table of signal numbers, names, and descriptions. This form of the
15354 @command{kill} command succeeds if all @var{signal} arguments are valid
15355 and if there is no output error.
15357 The @command{kill} command also supports the @option{--help} and
15358 @option{--version} options. @xref{Common options}.
15360 A @var{signal} may be a signal name like @samp{HUP}, or a signal
15361 number like @samp{1}, or an exit status of a process terminated by the
15362 signal. A signal name can be given in canonical form or prefixed by
15363 @samp{SIG}. The case of the letters is ignored, except for the
15364 @option{-@var{signal}} option which must use upper case to avoid
15365 ambiguity with lower case option letters. For a list of supported
15366 signal names and numbers see @xref{Signal specifications}.
15371 @cindex delaying commands
15372 @cindex commands for delaying
15374 @c Perhaps @command{wait} or other commands should be described here also?
15377 * sleep invocation:: Delay for a specified time.
15381 @node sleep invocation
15382 @section @command{sleep}: Delay for a specified time
15385 @cindex delay for a specified time
15387 @command{sleep} pauses for an amount of time specified by the sum of
15388 the values of the command line arguments.
15392 sleep @var{number}[smhd]@dots{}
15396 Each argument is a number followed by an optional unit; the default
15397 is seconds. The units are:
15410 Historical implementations of @command{sleep} have required that
15411 @var{number} be an integer, and only accepted a single argument
15412 without a suffix. However, GNU @command{sleep} accepts
15413 arbitrary floating point numbers (using a period before any fractional
15416 The only options are @option{--help} and @option{--version}. @xref{Common
15419 @c sleep is a shell built-in at least with Solaris 11's /bin/sh
15420 @mayConflictWithShellBuiltIn{sleep}
15425 @node Numeric operations
15426 @chapter Numeric operations
15428 @cindex numeric operations
15429 These programs do numerically-related operations.
15432 * factor invocation:: Show factors of numbers.
15433 * seq invocation:: Print sequences of numbers.
15437 @node factor invocation
15438 @section @command{factor}: Print prime factors
15441 @cindex prime factors
15443 @command{factor} prints prime factors. Synopses:
15446 factor [@var{number}]@dots{}
15447 factor @var{option}
15450 If no @var{number} is specified on the command line, @command{factor} reads
15451 numbers from standard input, delimited by newlines, tabs, or spaces.
15453 The @command{factor} command supports only a small number of options:
15457 Print a short help on standard output, then exit without further
15461 Print the program version on standard output, then exit without further
15465 Factoring the product of the eighth and ninth Mersenne primes
15466 takes about 30 milliseconds of CPU time on a 2.2 GHz Athlon.
15469 M8=`echo 2^31-1|bc` ; M9=`echo 2^61-1|bc`
15470 /usr/bin/time -f '%U' factor $(echo "$M8 * $M9" | bc)
15471 4951760154835678088235319297: 2147483647 2305843009213693951
15475 Similarly, factoring the eighth Fermat number @math{2^{256}+1} takes
15476 about 20 seconds on the same machine.
15478 Factoring large numbers is, in general, hard. The Pollard Rho
15479 algorithm used by @command{factor} is particularly effective for
15480 numbers with relatively small factors. If you wish to factor large
15481 numbers which do not have small factors (for example, numbers which
15482 are the product of two large primes), other methods are far better.
15484 If @command{factor} is built without using GNU MP, only
15485 single-precision arithmetic is available, and so large numbers
15486 (typically @math{2^{64}} and above) will not be supported. The single-precision
15487 code uses an algorithm which is designed for factoring smaller
15493 @node seq invocation
15494 @section @command{seq}: Print numeric sequences
15497 @cindex numeric sequences
15498 @cindex sequence of numbers
15500 @command{seq} prints a sequence of numbers to standard output. Synopses:
15503 seq [@var{option}]@dots{} @var{last}
15504 seq [@var{option}]@dots{} @var{first} @var{last}
15505 seq [@var{option}]@dots{} @var{first} @var{increment} @var{last}
15508 @command{seq} prints the numbers from @var{first} to @var{last} by
15509 @var{increment}. By default, each number is printed on a separate line.
15510 When @var{increment} is not specified, it defaults to @samp{1},
15511 even when @var{first} is larger than @var{last}.
15512 @var{first} also defaults to @samp{1}. So @code{seq 1} prints
15513 @samp{1}, but @code{seq 0} and @code{seq 10 5} produce no output.
15514 Floating-point numbers
15515 may be specified (using a period before any fractional digits).
15517 The program accepts the following options. Also see @ref{Common options}.
15518 Options must precede operands.
15521 @item -f @var{format}
15522 @itemx --format=@var{format}
15523 @opindex -f @var{format}
15524 @opindex --format=@var{format}
15525 @cindex formatting of numbers in @command{seq}
15526 Print all numbers using @var{format}.
15527 @var{format} must contain exactly one of the @samp{printf}-style
15528 floating point conversion specifications @samp{%a}, @samp{%e},
15529 @samp{%f}, @samp{%g}, @samp{%A}, @samp{%E}, @samp{%F}, @samp{%G}.
15530 The @samp{%} may be followed by zero or more flags taken from the set
15531 @samp{-+#0 '}, then an optional width containing one or more digits,
15532 then an optional precision consisting of a @samp{.} followed by zero
15533 or more digits. @var{format} may also contain any number of @samp{%%}
15534 conversion specifications. All conversion specifications have the
15535 same meaning as with @samp{printf}.
15537 The default format is derived from @var{first}, @var{step}, and
15538 @var{last}. If these all use a fixed point decimal representation,
15539 the default format is @samp{%.@var{p}f}, where @var{p} is the minimum
15540 precision that can represent the output numbers exactly. Otherwise,
15541 the default format is @samp{%g}.
15543 @item -s @var{string}
15544 @itemx --separator=@var{string}
15545 @cindex separator for numbers in @command{seq}
15546 Separate numbers with @var{string}; default is a newline.
15547 The output always terminates with a newline.
15550 @itemx --equal-width
15551 Print all numbers with the same width, by padding with leading zeros.
15552 @var{first}, @var{step}, and @var{last} should all use a fixed point
15553 decimal representation.
15554 (To have other kinds of padding, use @option{--format}).
15558 You can get finer-grained control over output with @option{-f}:
15561 $ seq -f '(%9.2E)' -9e5 1.1e6 1.3e6
15567 If you want hexadecimal integer output, you can use @command{printf}
15568 to perform the conversion:
15571 $ printf '%x\n' `seq 1048575 1024 1050623`
15577 For very long lists of numbers, use xargs to avoid
15578 system limitations on the length of an argument list:
15581 $ seq 1000000 | xargs printf '%x\n' | tail -n 3
15587 To generate octal output, use the printf @code{%o} format instead
15590 On most systems, seq can produce whole-number output for values up to
15591 at least @math{2^{53}}. Larger integers are approximated. The details
15592 differ depending on your floating-point implementation, but a common
15593 case is that @command{seq} works with integers through @math{2^{64}},
15594 and larger integers may not be numerically correct:
15597 $ seq 18446744073709551616 1 18446744073709551618
15598 18446744073709551616
15599 18446744073709551616
15600 18446744073709551618
15603 Be careful when using @command{seq} with outlandish values: otherwise
15604 you may see surprising results, as @command{seq} uses floating point
15605 internally. For example, on the x86 platform, where the internal
15606 representation uses a 64-bit fraction, the command:
15609 seq 1 0.0000000000000000001 1.0000000000000000009
15612 outputs 1.0000000000000000007 twice and skips 1.0000000000000000008.
15617 @node File permissions
15618 @chapter File permissions
15621 @include getdate.texi
15625 @node Opening the software toolbox
15626 @chapter Opening the Software Toolbox
15628 An earlier version of this chapter appeared in
15629 @uref{http://www.linuxjournal.com/article.php?sid=2762, the
15630 @cite{What's GNU?} column of @cite{Linux Journal}, 2 (June, 1994)}.
15631 It was written by Arnold Robbins.
15634 * Toolbox introduction:: Toolbox introduction
15635 * I/O redirection:: I/O redirection
15636 * The who command:: The @command{who} command
15637 * The cut command:: The @command{cut} command
15638 * The sort command:: The @command{sort} command
15639 * The uniq command:: The @command{uniq} command
15640 * Putting the tools together:: Putting the tools together
15644 @node Toolbox introduction
15645 @unnumberedsec Toolbox Introduction
15647 This month's column is only peripherally related to the GNU Project, in
15648 that it describes a number of the GNU tools on your GNU/Linux system and how they
15649 might be used. What it's really about is the ``Software Tools'' philosophy
15650 of program development and usage.
15652 The software tools philosophy was an important and integral concept
15653 in the initial design and development of Unix (of which Linux and GNU are
15654 essentially clones). Unfortunately, in the modern day press of
15655 Internetworking and flashy GUIs, it seems to have fallen by the
15656 wayside. This is a shame, since it provides a powerful mental model
15657 for solving many kinds of problems.
15659 Many people carry a Swiss Army knife around in their pants pockets (or
15660 purse). A Swiss Army knife is a handy tool to have: it has several knife
15661 blades, a screwdriver, tweezers, toothpick, nail file, corkscrew, and perhaps
15662 a number of other things on it. For the everyday, small miscellaneous jobs
15663 where you need a simple, general purpose tool, it's just the thing.
15665 On the other hand, an experienced carpenter doesn't build a house using
15666 a Swiss Army knife. Instead, he has a toolbox chock full of specialized
15667 tools---a saw, a hammer, a screwdriver, a plane, and so on. And he knows
15668 exactly when and where to use each tool; you won't catch him hammering nails
15669 with the handle of his screwdriver.
15671 The Unix developers at Bell Labs were all professional programmers and trained
15672 computer scientists. They had found that while a one-size-fits-all program
15673 might appeal to a user because there's only one program to use, in practice
15678 difficult to write,
15681 difficult to maintain and
15685 difficult to extend to meet new situations.
15688 Instead, they felt that programs should be specialized tools. In short, each
15689 program ``should do one thing well.'' No more and no less. Such programs are
15690 simpler to design, write, and get right---they only do one thing.
15692 Furthermore, they found that with the right machinery for hooking programs
15693 together, that the whole was greater than the sum of the parts. By combining
15694 several special purpose programs, you could accomplish a specific task
15695 that none of the programs was designed for, and accomplish it much more
15696 quickly and easily than if you had to write a special purpose program.
15697 We will see some (classic) examples of this further on in the column.
15698 (An important additional point was that, if necessary, take a detour
15699 and build any software tools you may need first, if you don't already
15700 have something appropriate in the toolbox.)
15702 @node I/O redirection
15703 @unnumberedsec I/O Redirection
15705 Hopefully, you are familiar with the basics of I/O redirection in the
15706 shell, in particular the concepts of ``standard input,'' ``standard output,''
15707 and ``standard error''. Briefly, ``standard input'' is a data source, where
15708 data comes from. A program should not need to either know or care if the
15709 data source is a disk file, a keyboard, a magnetic tape, or even a punched
15710 card reader. Similarly, ``standard output'' is a data sink, where data goes
15711 to. The program should neither know nor care where this might be.
15712 Programs that only read their standard input, do something to the data,
15713 and then send it on, are called @dfn{filters}, by analogy to filters in a
15716 With the Unix shell, it's very easy to set up data pipelines:
15719 program_to_create_data | filter1 | ... | filterN > final.pretty.data
15722 We start out by creating the raw data; each filter applies some successive
15723 transformation to the data, until by the time it comes out of the pipeline,
15724 it is in the desired form.
15726 This is fine and good for standard input and standard output. Where does the
15727 standard error come in to play? Well, think about @command{filter1} in
15728 the pipeline above. What happens if it encounters an error in the data it
15729 sees? If it writes an error message to standard output, it will just
15730 disappear down the pipeline into @command{filter2}'s input, and the
15731 user will probably never see it. So programs need a place where they can send
15732 error messages so that the user will notice them. This is standard error,
15733 and it is usually connected to your console or window, even if you have
15734 redirected standard output of your program away from your screen.
15736 For filter programs to work together, the format of the data has to be
15737 agreed upon. The most straightforward and easiest format to use is simply
15738 lines of text. Unix data files are generally just streams of bytes, with
15739 lines delimited by the @acronym{ASCII} @sc{lf} (Line Feed) character,
15740 conventionally called a ``newline'' in the Unix literature. (This is
15741 @code{'\n'} if you're a C programmer.) This is the format used by all
15742 the traditional filtering programs. (Many earlier operating systems
15743 had elaborate facilities and special purpose programs for managing
15744 binary data. Unix has always shied away from such things, under the
15745 philosophy that it's easiest to simply be able to view and edit your
15746 data with a text editor.)
15748 OK, enough introduction. Let's take a look at some of the tools, and then
15749 we'll see how to hook them together in interesting ways. In the following
15750 discussion, we will only present those command line options that interest
15751 us. As you should always do, double check your system documentation
15752 for the full story.
15754 @node The who command
15755 @unnumberedsec The @command{who} Command
15757 The first program is the @command{who} command. By itself, it generates a
15758 list of the users who are currently logged in. Although I'm writing
15759 this on a single-user system, we'll pretend that several people are
15764 @print{} arnold console Jan 22 19:57
15765 @print{} miriam ttyp0 Jan 23 14:19(:0.0)
15766 @print{} bill ttyp1 Jan 21 09:32(:0.0)
15767 @print{} arnold ttyp2 Jan 23 20:48(:0.0)
15770 Here, the @samp{$} is the usual shell prompt, at which I typed @samp{who}.
15771 There are three people logged in, and I am logged in twice. On traditional
15772 Unix systems, user names are never more than eight characters long. This
15773 little bit of trivia will be useful later. The output of @command{who} is nice,
15774 but the data is not all that exciting.
15776 @node The cut command
15777 @unnumberedsec The @command{cut} Command
15779 The next program we'll look at is the @command{cut} command. This program
15780 cuts out columns or fields of input data. For example, we can tell it
15781 to print just the login name and full name from the @file{/etc/passwd}
15782 file. The @file{/etc/passwd} file has seven fields, separated by
15786 arnold:xyzzy:2076:10:Arnold D. Robbins:/home/arnold:/bin/bash
15789 To get the first and fifth fields, we would use @command{cut} like this:
15792 $ cut -d: -f1,5 /etc/passwd
15793 @print{} root:Operator
15795 @print{} arnold:Arnold D. Robbins
15796 @print{} miriam:Miriam A. Robbins
15800 With the @option{-c} option, @command{cut} will cut out specific characters
15801 (i.e., columns) in the input lines. This is useful for input data
15802 that has fixed width fields, and does not have a field separator. For
15803 example, list the Monday dates for the current month:
15805 @c Is using cal ok? Looked at gcal, but I don't like it.
15816 @node The sort command
15817 @unnumberedsec The @command{sort} Command
15819 Next we'll look at the @command{sort} command. This is one of the most
15820 powerful commands on a Unix-style system; one that you will often find
15821 yourself using when setting up fancy data plumbing.
15824 command reads and sorts each file named on the command line. It then
15825 merges the sorted data and writes it to standard output. It will read
15826 standard input if no files are given on the command line (thus
15827 making it into a filter). The sort is based on the character collating
15828 sequence or based on user-supplied ordering criteria.
15831 @node The uniq command
15832 @unnumberedsec The @command{uniq} Command
15834 Finally (at least for now), we'll look at the @command{uniq} program. When
15835 sorting data, you will often end up with duplicate lines, lines that
15836 are identical. Usually, all you need is one instance of each line.
15837 This is where @command{uniq} comes in. The @command{uniq} program reads its
15838 standard input. It prints only one
15839 copy of each repeated line. It does have several options. Later on,
15840 we'll use the @option{-c} option, which prints each unique line, preceded
15841 by a count of the number of times that line occurred in the input.
15844 @node Putting the tools together
15845 @unnumberedsec Putting the Tools Together
15847 Now, let's suppose this is a large ISP server system with dozens of users
15848 logged in. The management wants the system administrator to write a program that will
15849 generate a sorted list of logged in users. Furthermore, even if a user
15850 is logged in multiple times, his or her name should only show up in the
15853 The administrator could sit down with the system documentation and write a C
15854 program that did this. It would take perhaps a couple of hundred lines
15855 of code and about two hours to write it, test it, and debug it.
15856 However, knowing the software toolbox, the administrator can instead start out
15857 by generating just a list of logged on users:
15867 Next, sort the list:
15870 $ who | cut -c1-8 | sort
15877 Finally, run the sorted list through @command{uniq}, to weed out duplicates:
15880 $ who | cut -c1-8 | sort | uniq
15886 The @command{sort} command actually has a @option{-u} option that does what
15887 @command{uniq} does. However, @command{uniq} has other uses for which one
15888 cannot substitute @samp{sort -u}.
15890 The administrator puts this pipeline into a shell script, and makes it available for
15891 all the users on the system (@samp{#} is the system administrator,
15892 or @code{root}, prompt):
15895 # cat > /usr/local/bin/listusers
15896 who | cut -c1-8 | sort | uniq
15898 # chmod +x /usr/local/bin/listusers
15901 There are four major points to note here. First, with just four
15902 programs, on one command line, the administrator was able to save about two
15903 hours worth of work. Furthermore, the shell pipeline is just about as
15904 efficient as the C program would be, and it is much more efficient in
15905 terms of programmer time. People time is much more expensive than
15906 computer time, and in our modern ``there's never enough time to do
15907 everything'' society, saving two hours of programmer time is no mean
15910 Second, it is also important to emphasize that with the
15911 @emph{combination} of the tools, it is possible to do a special
15912 purpose job never imagined by the authors of the individual programs.
15914 Third, it is also valuable to build up your pipeline in stages, as we did here.
15915 This allows you to view the data at each stage in the pipeline, which helps
15916 you acquire the confidence that you are indeed using these tools correctly.
15918 Finally, by bundling the pipeline in a shell script, other users can use
15919 your command, without having to remember the fancy plumbing you set up for
15920 them. In terms of how you run them, shell scripts and compiled programs are
15923 After the previous warm-up exercise, we'll look at two additional, more
15924 complicated pipelines. For them, we need to introduce two more tools.
15926 The first is the @command{tr} command, which stands for ``transliterate.''
15927 The @command{tr} command works on a character-by-character basis, changing
15928 characters. Normally it is used for things like mapping upper case to
15932 $ echo ThIs ExAmPlE HaS MIXED case! | tr '[:upper:]' '[:lower:]'
15933 @print{} this example has mixed case!
15936 There are several options of interest:
15940 work on the complement of the listed characters, i.e.,
15941 operations apply to characters not in the given set
15944 delete characters in the first set from the output
15947 squeeze repeated characters in the output into just one character.
15950 We will be using all three options in a moment.
15952 The other command we'll look at is @command{comm}. The @command{comm}
15953 command takes two sorted input files as input data, and prints out the
15954 files' lines in three columns. The output columns are the data lines
15955 unique to the first file, the data lines unique to the second file, and
15956 the data lines that are common to both. The @option{-1}, @option{-2}, and
15957 @option{-3} command line options @emph{omit} the respective columns. (This is
15958 non-intuitive and takes a little getting used to.) For example:
15980 The file name @file{-} tells @command{comm} to read standard input
15981 instead of a regular file.
15983 Now we're ready to build a fancy pipeline. The first application is a word
15984 frequency counter. This helps an author determine if he or she is over-using
15987 The first step is to change the case of all the letters in our input file
15988 to one case. ``The'' and ``the'' are the same word when doing counting.
15991 $ tr '[:upper:]' '[:lower:]' < whats.gnu | ...
15994 The next step is to get rid of punctuation. Quoted words and unquoted words
15995 should be treated identically; it's easiest to just get the punctuation out of
15999 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | ...
16002 The second @command{tr} command operates on the complement of the listed
16003 characters, which are all the letters, the digits, the underscore, and
16004 the blank. The @samp{\n} represents the newline character; it has to
16005 be left alone. (The @acronym{ASCII} tab character should also be included for
16006 good measure in a production script.)
16008 At this point, we have data consisting of words separated by blank space.
16009 The words only contain alphanumeric characters (and the underscore). The
16010 next step is break the data apart so that we have one word per line. This
16011 makes the counting operation much easier, as we will see shortly.
16014 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16015 > tr -s ' ' '\n' | ...
16018 This command turns blanks into newlines. The @option{-s} option squeezes
16019 multiple newline characters in the output into just one. This helps us
16020 avoid blank lines. (The @samp{>} is the shell's ``secondary prompt.''
16021 This is what the shell prints when it notices you haven't finished
16022 typing in all of a command.)
16024 We now have data consisting of one word per line, no punctuation, all one
16025 case. We're ready to count each word:
16028 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16029 > tr -s ' ' '\n' | sort | uniq -c | ...
16032 At this point, the data might look something like this:
16045 The output is sorted by word, not by count! What we want is the most
16046 frequently used words first. Fortunately, this is easy to accomplish,
16047 with the help of two more @command{sort} options:
16051 do a numeric sort, not a textual one
16054 reverse the order of the sort
16057 The final pipeline looks like this:
16060 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16061 > tr -s ' ' '\n' | sort | uniq -c | sort -n -r
16070 Whew! That's a lot to digest. Yet, the same principles apply. With six
16071 commands, on two lines (really one long one split for convenience), we've
16072 created a program that does something interesting and useful, in much
16073 less time than we could have written a C program to do the same thing.
16075 A minor modification to the above pipeline can give us a simple spelling
16076 checker! To determine if you've spelled a word correctly, all you have to
16077 do is look it up in a dictionary. If it is not there, then chances are
16078 that your spelling is incorrect. So, we need a dictionary.
16079 The conventional location for a dictionary is @file{/usr/dict/words}.
16080 On my GNU/Linux system,@footnote{Redhat Linux 6.1, for the November 2000
16081 revision of this article.}
16082 this is a sorted, 45,402 word dictionary.
16084 Now, how to compare our file with the dictionary? As before, we generate
16085 a sorted list of words, one per line:
16088 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16089 > tr -s ' ' '\n' | sort -u | ...
16092 Now, all we need is a list of words that are @emph{not} in the
16093 dictionary. Here is where the @command{comm} command comes in.
16096 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16097 > tr -s ' ' '\n' | sort -u |
16098 > comm -23 - /usr/dict/words
16101 The @option{-2} and @option{-3} options eliminate lines that are only in the
16102 dictionary (the second file), and lines that are in both files. Lines
16103 only in the first file (standard input, our stream of words), are
16104 words that are not in the dictionary. These are likely candidates for
16105 spelling errors. This pipeline was the first cut at a production
16106 spelling checker on Unix.
16108 There are some other tools that deserve brief mention.
16112 search files for text that matches a regular expression
16115 count lines, words, characters
16118 a T-fitting for data pipes, copies data to files and to standard output
16121 the stream editor, an advanced tool
16124 a data manipulation language, another advanced tool
16127 The software tools philosophy also espoused the following bit of
16128 advice: ``Let someone else do the hard part.'' This means, take
16129 something that gives you most of what you need, and then massage it the
16130 rest of the way until it's in the form that you want.
16136 Each program should do one thing well. No more, no less.
16139 Combining programs with appropriate plumbing leads to results where
16140 the whole is greater than the sum of the parts. It also leads to novel
16141 uses of programs that the authors might never have imagined.
16144 Programs should never print extraneous header or trailer data, since these
16145 could get sent on down a pipeline. (A point we didn't mention earlier.)
16148 Let someone else do the hard part.
16151 Know your toolbox! Use each program appropriately. If you don't have an
16152 appropriate tool, build one.
16155 As of this writing, all the programs we've discussed are available via
16156 anonymous @command{ftp} from: @*
16157 @uref{ftp://gnudist.gnu.org/textutils/textutils-1.22.tar.gz}. (There may
16158 be more recent versions available now.)
16160 None of what I have presented in this column is new. The Software Tools
16161 philosophy was first introduced in the book @cite{Software Tools}, by
16162 Brian Kernighan and P.J. Plauger (Addison-Wesley, ISBN 0-201-03669-X).
16163 This book showed how to write and use software tools. It was written in
16164 1976, using a preprocessor for FORTRAN named @command{ratfor} (RATional
16165 FORtran). At the time, C was not as ubiquitous as it is now; FORTRAN
16166 was. The last chapter presented a @command{ratfor} to FORTRAN
16167 processor, written in @command{ratfor}. @command{ratfor} looks an awful
16168 lot like C; if you know C, you won't have any problem following the
16171 In 1981, the book was updated and made available as @cite{Software Tools
16172 in Pascal} (Addison-Wesley, ISBN 0-201-10342-7). Both books are
16173 still in print and are well worth
16174 reading if you're a programmer. They certainly made a major change in
16175 how I view programming.
16177 The programs in both books are available from
16178 @uref{http://cm.bell-labs.com/who/bwk, Brian Kernighan's home page}.
16179 For a number of years, there was an active
16180 Software Tools Users Group, whose members had ported the original
16181 @command{ratfor} programs to essentially every computer system with a
16182 FORTRAN compiler. The popularity of the group waned in the middle 1980s
16183 as Unix began to spread beyond universities.
16185 With the current proliferation of GNU code and other clones of Unix programs,
16186 these programs now receive little attention; modern C versions are
16187 much more efficient and do more than these programs do. Nevertheless, as
16188 exposition of good programming style, and evangelism for a still-valuable
16189 philosophy, these books are unparalleled, and I recommend them highly.
16191 Acknowledgment: I would like to express my gratitude to Brian Kernighan
16192 of Bell Labs, the original Software Toolsmith, for reviewing this column.
16194 @node GNU Free Documentation License
16195 @appendix GNU Free Documentation License
16199 @node Concept index
16206 @c Local variables:
16207 @c texinfo-column-for-description: 32